Final Report Summary - NATURAL (Standardised metrology of Nano-sTrUctuRed CoAtings with Low surface energy)
Executive Summary:
There is a growing range of applications that can benefit from the use of nanostructured coatings. The functional performance of a surface is intimately linked with its structure. The ability to characterise nanostructured coatings is therefore an essential part of their future uptake. However, the ability to examine and characterise at the nano-scale is currently limited to sophisticated, time-consuming, laboratory-based equipment. The standards body (ISO TC229) recognises that there are no current procedures that relate the functional performance of a surface or coating to its nanostructure. The NATURAL project has developed methods that allow rapid evaluation of surfaces at the nanoscale and correlate the measured surface structure with functional performance. This will enable new methods for lifetime determination and facilitate coatings evaluation in the field. The project focussed on the development of surface profilometry methods to allow the rapid resolution of surfaces at the nanoscale. Data generated was correlated with the physical and physio-chemical characteristics of surfaces to allow their rapid, reliable and accurate assessment. The change of the surface nano-morphology and the functional performance were related to allow the estimation of durability and to estimate coating lifetime.
As part of the project, work was done to identify a suitable polyurethane resin to which nanoparticles could be added to give the specific properties required to improve coating performance. Three different resins were trialled; an accepted industrial product and two model resins. Three different nanoparticles were chosen as candidate fillers, each having specific properties. A spray coating process was developed and optimised to achieve increased coating quality in terms of finish, thickness, homogeneity and ease of application. A number of samples were produced for lab-based characterisation. Various analytical techniques were employed to characterise the coatings, including nanoindentation, FTIR, confocal microscopy, adhesion, SEM, coefficient of friction and XPS. The coatings were also applied to wind turbine blade sections for wind tunnel testing. Both lab and wind tunnel samples underwent ageing by biofouling, particle and sand erosion and water erosion. Wind tunnel testing was performed to determine the effect of surface conditions such as roughness and ageing on the boundary flow characteristics.
Characterisation activities found that higher nanoparticle loading gave rise to high surface roughness whilst some of the lower loaded coatings exhibited gas bubble inclusions but maintained durability. Biofouling saw the coating fail much sooner under water droplet erosion and loss of some surface tension. Assessment of the effects of ageing on flow characteristics found that the time to transition of laminar to turbulent flow was not affected by micro-roughness or ageing by biofouling, but only where the roughness, Ra is <8μm and biofouling is <10% surface coverage. Using this correlation between surface characteristics and flow behaviour, a relationship was identified and an algorithm produced to link the surface roughness, surface energy and severity of biofouling to the onset of turbulent flow. This will be implemented in to the device and can also be used to assess surface conditions and predict service lifetime.
A portable profiler was designed and built. The device incorporates the ability to measure fluorescence for biofouling, surface energy by drop shape analysis for wettability, and a laser scanning device for surface profilometry and roughness measurements. The device can generate and capture real-time data whilst being able to transmit data remotely to the operator to make assessments in the field.
Finally, based on the outcomes of the project, a series of recommendations were made to take forward for the development of a standard which will aim to demonstrate a protocol for nanostructured coatings on wind-turbines (and similar applications). The standard will include steps to assess the surface in line with the methods employed in NATURAL. It will also go some way to proposing a suitable device(s) to aid the prediction of coating lifetime and performance.
Project Context and Objectives:
The driving purpose: The rise of the new nano-technology age has led to a broad range of nano-structured coatings which are increasingly being used in critical applications. The characterisation of nano-structured coatings is essential to understanding and improving their performance. The ability to analyse sub-micron features or nano/micro contaminants is the enabling step to increasing durability in a number of industrial applications, such as marine, aviation and wind blades. Nano-structuring has proved it importance in anti-fouling (ice, insect, biofouling), low friction and boundary layer control applications.
Erosion of surfaces of aircraft during routine flight has been claimed to increase fuel consumption and CO2 emissions by almost 5% (1). For wind turbines, ice build-up can cause a loss of 5-10% of electricity production; in sub-tropical warmer environment, fouling of the blade by insects can reduce power production from wind turbines by 50% (2). Conventional coatings and surface treatments have not been successful in reducing these effects. Fouling of ships by marine organisms is believed to increase fuel consumption by 40% and conventional biocidal approaches to addressing this issue are a substantial concern due to their biological effects.
New coatings and surface treatments with nano-structured materials and topographies are generally believed to offer the potential to reduce erosion rates and fouling, and therefore have significant and beneficial effects on fuel consumption and carbon footprints. Such advantages of nano-coatings stem from both the ability to tailor roughness levels, and more importantly from the ability to modify the energy of the underlying surface.
Controlling turbulence transition in boundary layers offers considerable advantages to numerous industrial applications. Using nano-structured coatings, the flow-surface interaction can be engineered to delay the transition of flow to turbulence, and hence reduce flow forces, thereby saving costs for structures such as aerofoils and blades.
The sustainability of a coating is linked to its lifetime. With time and environmental exposure, unwanted change in the nano-structure frequently reduces performance by two major mechanisms:
- Surface fouling: external material (eg dust/oil, ice, biofouling such as insects, barnacles and algae) builds up on the surface, presenting an inferior surface, hence reducing performance (Figure. 1).
- Surface erosion (wear, corrosion): weathering including UV degradation, abrasion and friction cause surface erosion of the coating, leading to loss of material over time, hence a loss of performance associated with loss of the active component and change in surface morphology. (Figure 1).
To improve the quality of nano-structured coatings, a greater understanding of structure at the nano-level, corresponding mechanical properties at the nano-scale and the influence of these on macro-scale behaviour is needed. Especially, mechanical characterisation of nano-phases is important for understanding the effect of the nano-structure on the real-life performance of the coating. Laboratory-based techniques such as nano-indentation and AFM are proven to be successful for characterising nano-structured coatings on a small length scale but specimen preparation is time-consuming and relatively expensive. Moreover, some other techniques are truly destructive and time consuming (e.g. cross-section microscopy) and may require operation in a vacuum chamber (e.g. TEM, SEM), that implies the use of a small section of the coating rather than a large flat area.
Project Objectives
The NATURAL project focussed on developing real-world, practical characterisation methods for nano-structured coatings with a low surface energy for anti-fouling applications and/or for low friction applications.
The main standardisation bodies, (CEN, ISO) have recognised that uncontrolled change of the nano-structure frequently results in a loss of performance, thus, the first aim of NATURAL is to relate surface nano-morphology to the functional performance of surfaces and to enable the development of new methods for lifetime determination and, ultimately, prediction.
The NATURAL project integrates research organisations and industry into a coordinated and interdisciplinary research programme, incorporating all the necessary elements from nano-materials technology selection and engineering through to evaluation and end-user trials.
• The consortium identified candidate nano-structured coatings both in the commercial market and the research community (including those which had already been developed by members of the consortium). These candidates were used as model systems, providing a range of nano-structures and surface chemistries.
• The consortium gathered data on these selected coatings, using high resolution AFM and nano-indentation techniques.
• Conventional tribological methods were used to provide a baseline of mechanical performance.
• Novel, short wavelength, high resolution laser profiling methods were developed to allow rapid surface profiling of large areas.
• Correlations between the fine nano-scale (AFM, nano-indentation), coarser nano-scale (laser profiling) and the macro-scale (tribology, fluid flow) were established to form the basis for a new standard.
• The effect of ageing under representative conditions was monitored using methods developed within the project to allow the key structural and functional modifications to be interrelated. This was used to develop a preliminary model for durability and lifetime prediction and also determine the effect of wear on performance.
The second aim of the project was to overcome the problem of inspecting the coatings in-situ, since access may be difficult or hazardous and the commercial impact of sampling or extended shut-down for laboratory testing may be significant. The NATURAL project addressed this aim by designing and building a prototype portable device based on laser profiling but with the capability for determining surface energy. This device will allow the user to obtain fast information on the morphology of the surface and correlate it to studies undertaken in the laboratory.
The NATURAL partnership aimed to develop a simple, robust, rapid and standardised methodology that allows the nano-scale structure of a surface to be measured in the field and then relate this to its mechanical and tribological properties at the micro-scale and hence performance on the macro-scale.
Specifically, the overall objectives of the NATURAL project were to:
• Develop understanding of practical characterisation methods for nano-structured coatings with a focus on low energy coatings for anti-fouling applications and/or for low friction applications such that the results can be interpreted in a meaningful fashion in a real-world application.
• Develop a standardised method to determine the lifetime of nanostructured coatings.
• Develop a prototype nano-characterisation measurement system capable of non-destructive evaluation in the field.
• Monitor the mentioned surface conditions of nanostructured coatings in operating conditions, such as in a wind turbine blade.
Project Results:
Coating Development
The main objectives for the development of nanostructured coatings were first, to identify a host resin system for the addition of nanoparticle fillers. The nanoparticles (NP) will improve overall performance in terms of hydrophobicity, wear and erosion performance, impact resistance, anti-fouling and anti-icing (Figure 3). A commercially recognised and accepted resin (M1) was initially used but due to the complexity of its chemistry proved difficult to mix with the nanoparticles. A second model system (PU) was then selected but was dismissed due to problems with a short pot life and rapid gelling. The third resin (PU2) offered best performance and was chosen as the candidate resin. Three different nanoparticles were selected, each having preferential properties: super-hydrophobicity, anti-icing, abrasion resistance, anti-fouling etc.
Fabrication was done by spray method optimised by LUR for improved coating deposition and final quality (Figure 5). Each of the NPs posed their own challenges which increased the efforts required to optimise the spray process. The TWI NP’s produced rough, thick coatings that were considerably uneven. The TNO were thin and smooth as desired but generated bubbles. The LUR coating was also good but too had issues with bubble formation. Nevertheless, sample preparation was successfully accomplished despite various issues regarding suitable PU matrix selection, optimum nanoparticle incorporation and fine-tuning of the spray gun parameters.
Following successful feedback the characterisation and validation exercises, coating of large plates was complete. The plates will be used for wind tunnel testing.
Characterisation of Coatings
Initially, coatings were applied to flat substrates for characterisation and validation. Following this, samples were artificially aged to mimic real life exposure. The surface and near surface properties of coatings used for wind turbine blades were studied in detail. These coatings are typically polyurethane. The aim was to compare the commercial polyurethane only coatings with those coats developed which contain nano-particles and nano-structure. As discussed above, the three resin systems were trialled. Findings from the chosen system, PU2, are detailed here.
Microscopy revealed noticeable differences between the different variants. Figure 5 is shows the variable difference in surface condition of pristine samples. PU2-TWI is notably rough compared to the other three samples. This is consistent with roughness measurements taken by confocal microscopy (Figure 6). (PU2-P: Ra=0.92μm; PU2-LUR: Ra=0.46μm; PU2-TNO: Ra=0.31μm; PU2-TWI: Ra=8.92μm).
Chemical surface analysis using X-ray photo-electron spectroscopy (XPS) revealed the M1 un-aged sample showed Ca, Mg, Cl, S, Si, Al peaks. These are all consistent with both contamination on the surface and also fillers such as talc and metal oxides being present in the coating. XPS spectra of the bare, un-aged PU2 sample showed that as well as elemental peaks associated with polyurethane (C, N, O) there was also some silicon present. Closer examination of the peak background revealed that this silicon was a contamination layer lying on top of the polyurethane.
The elemental compositions of the PU2 samples – bare and with addition of nanoparticles are detailed in Table2. The results were comparable to the types of nanoparticles used for each of the variants; TNO samples should contain fluorinated functionalised particles but no fluorine is found in the top 10 nm. The LUR samples should contain aluminium oxide based nanoparticles but no aluminium was found. TWI samples contain silica based nanoparticles and higher silicon was seen in this sample compared to the other samples. Hence, only the TWI sample contains nanoparticles at the surface, the others show no nanoparticles at the surface and hence the nanoparticles are covered by an over-layer of polyurethane.
Contact angle measurements for the pristine surfaces are shown in Figure 7. The surface energy of LUR and TWI samples were marginally higher than those determined for the bare PU and TNO samples.
Nanoindentation results of pristine samples (Table 3) show that the resin with TWI nanoparticles had the highest hardness but it should be noted that the coating also had the roughest surface. Overall, the LUR variant had a reduction in surface hardness compared to the unmodified LPU2-P resin.
Adhesion testing was carried out by TWI found adhesion between the coating and substrate was strongest for the base LPU2-P and LPU2-TWI-P coatings as the failure mode was adhesive. The LUR and TNO variants failed due to bonding failure, i.e. the complete separation between the dolly and the coating in all six tests of each coating.
Tubular impact tests found that the coatings withstood impact more than the substrates that they were deposited on. For example, the LPU2-LUR-P coating failed at 1000 mm-kg but the substrate began to fail at <300 mm-kg. Looking at the impact test results overall, the TNO variation was found to be less resilient than the base resin on its own. The LUR variant was the toughest coating.
SEM analysis of the surface and cross-sections of pristine coatings with and without nanoparticles are shown in Figure 8. Looking at the images of the top surfaces, the bare coatings (LPU2) have a relatively smooth, even surface, LPU2-LUR-P was similar but with the presence of small pores. The TNO variant exhibited the most uniform coating with minimal defects whilst the TWI coating is evidently rough and uneven with many large pores on the surface.
Analysis of the cross-sections through the coating thickness correlated with the images of the top surfaces. The bare LPU2-P exhibited a fairly even coating with few voids or defects. The LUR coating had several large particles at the surface with visible porosity. The TNO was comparatively uniform with few imperfections and exhibiting sparsely distributed nanoparticles. The TWI variant was comparatively thick, more than double the thickness of the LPU2-P coating; nanoparticles were infrequent but little to no through-thickness porosity could be found.
Ageing
Aging of the surfaces was performed to simulate conditions experienced by wind turbines at sea. The main types of exposure were identified as biofouling, particle erosion (abrasive wear) and water erosion.
Particle Aging by TNO
The test conditions used for the sand erosion wear rate determination and aging procedures are:
1.Abrasive: Sand particles,
• grain size: d50 = 0.25 mm, range: 0.2 –0.4 mm,
• hardness: 950 HV,
• shape: rounded particles,
2.Particle impact velocity: 65 ± 5 m/s,
3.Amount of abrasive: 500 mg/min,
4.Stand-off distance: 150 mm,
5.Angle of impact: 45 and 90°,
6.Area of erosion: Ø20 mm (314 mm2)
7.Environment: Air.
8.Test duration: 75 min, interrupted for specimen weighing each 15 min.
Some of the conditions used in this test, abrasive type and size and amount of abrasive, have been derived from MIL-STD-810G (2008), a test method standard of the US Department of Defencee for environmental engineering considerations.
Particle Aging by NPL
Use RT particle erosion system
Use large stand-off distance
100 g aliquot 220 µm diameter sand
Produced scar ~50 mm across
Samples tested:
LPU2_P scar depth max ~12 µm
LPU2_TNO_P scar depth max ~5 µm
LPU2_LUR_P scar depth max 14 µm
Water droplet aging by TNO
Test conditions
The following test conditions were used:
• Nozzle type: rounded, Ø0.45 mm, CN = 0.963
• Drop velocity, vd = 100 m/s (water pressure pw = 80 bar),
• Drop size, dd = 0.1 to 0.06 mm,
• Amount of water: 0.95 l/min,
• Stand of distance, SOD: 100 mm (standard),
• Angle of impact: 90°.
The droplet velocity in the test was derived from an offshore wind turbine application, type XE128-5MW of XEMC-Darwind, (The Netherlands). The datasheet for this wind turbine gives:
- Power: 5 MW
- Rotor diameter: 128 m
- Speed: variable (nominal 15 rpm)
This means that the nominal tip velocity of the blades is 101 m/s.
The mean gravitational velocity of water drops varies, depending on the size, normally between 5 and 8 m/s. This means that during the upward movement of the blades the impact velocity is between 106 to 109 m/s and in downward direction between 93 and 96 m/s.
A value of ~100 m/s has been used in the performed droplet erosion experiments.
The key test criteria for a potential material and/or coating surface, for a certain set of test condition, are:
• Incubation period, Ip (min),
• Erosion rate, Re (mm/min),
• Time to coating failure, If (min).
Figure 9a shows a schematic of the key test criteria for a coating material determined with droplet erosion tests.
Water droplet aging by NPL (Figure 9b)
Designed and built at NPL
In vacuum – needed for high tip speeds (max 500 ms-1)
Ø 1.2 mm nozzle, nominally Ø 2 mm diameter droplets
Single droplet stream
Particulate erosion aged LPU2-P
Particulate erosion aged LPU2-TNO-P
Particulate erosion aged LPU2-LUR-P
All on 8 mm wide dovetail specimens
Test Conditions
Chamber pressure with jet running: 20 mbar
1438 RPM
0.5 m arm radius
(~80 ms-1 tip speed)
Up to 120 minutes’ duration (1-5 minute increments).
Biofouling
Samples were exposed to sub-aerial fouling with natural seawater (NSW) for 3weeks. The natural seawater was sprayed as droplets inside the mist chamber as indicated in Error! Reference source not found. 9. Temperature in the cabinet was 25± 2°C and the following spraying cycle was used:
- NSW mist + light = 14h
- Darkness, no NSW = 10h
- Season: summer
4 glass slides were exposed as a control for biofouling. Status of biofouling was checked twice a week by the use of the epifluorescence microscopy.
Aging by biofouling was measured with fluorescence imaging to measure the degree of surface coverage. Average coverage of the bare resin samples (LPU2P-B) was found to be slightly higher than the glass slides used as reference samples. The average coverage of the TWI samples with 40% nanoparticles added was comparatively higher at 10% (Table 5).
Aging by droplet erosion was measured by time to failure (Figure 10). Results found that the time to failure was the longest for the TNO samples and in general where the nanoparticle loading was around 1% and certainly no more than 3%. The same was found for the pure resin where time to failure was the lowest. Water droplet erosion carried out by NPL also found similar results. The figure on the left shows how the TWI nanoparticle loaded samples (LPU2-TWI-P) fail within in a matter of minutes, whereas the bare PU2, TNO and LUR coatings take much longer to fail
Sand erosion was performed and measured by surface roughness (Figure 11). Particle erosion was performed at two angles of attack (40O and 90O). Results show a significant increase in roughness for all three samples at 90O. However, the LUR coating shows the most drastic deterioration of surface condition across both angles of attack. Sand erosion tests by particle erosion were performed by particle erosion. Bare resin, LPU2-TNO and LPU2-LUR samples were tested. LPU2-P.
XPS analysis of the biofouled samples are shown in the table below. The effects of biofouling show chemistry on the surface typical of salts from sea water and biomaterial growth (Table 7).
Droplet erosion testing before and after aging by biofouling, shown in Table 8, gives evidence of a decrease in coating lifetime for aged samples compared to that seen for un-aged samples.
Contact angle and surface energy measurements show that there was no significant decrease in surface energy; the TWI sample actually exhibited a slight increase.
Nanoindentation of the biofouled samples show that there is a negligible increase in hardness from the pristine LPU2 to the biofouled LPU2 coating, 0.20GPa and 0.26GPa respectively. The TWI biofouled surface presents a decrease of the hardness; 0.32GPa for the pristine and 0.24GPa for the biofouled coating (Table 9).
Wind Tunnel Testing
In order to test the different nanomaterial based coatings, 11 flat plate samples were manufactured using fibreglass as the substrate material. Six of these panels are tested in their pristine condition, (without abrasion or fouling) two were biofouled in a special environmental chamber, and three were subject to abrasion using sand-blasting. One of the pristine panels (WFG-P) was not coated to obtain smooth surface. This panel achieves the baseline flow condition: a zero-pressure-gradient (ZPG) laminar boundary layer flow over the entire length. This way the flow field over remaining five pristine nano-coatings can be compared to the baseline flow condition. Similarly, the flow field over abraded and biofouled samples could be compared their pristine counterparts. The overview of samples and a short description is given in Table 10.
The wind tunnel facility belonging to the Department of Mechanical Engineering at the Technical University of Denmark (DTU) was used for the tests. The facility was modified for the requirements of the blade trials by Dantec. The main features of the wind tunnel are listed as follows:
• Closed-loop subsonic facility with a nominal maximum flow speed of 60m/s.
• Measured background turbulence intensity level of 0.15% at 20m/s.
• Nominal test section dimensions are 0.3x0.3x0.75m with an increasing cross sectional area along the streamwise direction to compensate for developing boundary layers.
• The temperature of the flow cannot be controlled during the experiments. As a result, the air temperature in the tunnel rises from 22°C to 26°C during the course of the experiments.
• Two interchangeable test sections are available which enables a quick change to a different experimental setup. One of these test sections was modified to meet the boundary layer measurement requirements imposed by Project NATURAL. These requirements include:
o Automated measurement of laminar, transitional and turbulent boundary layer profiles.
o Angle of attack adjustment from outside the test section.
o Zero-pressure-gradient alignment during tunnel operation.
o Quick replacement of test panels.
o Keeping the alignment between panels.
o Minimum flow disturbance and vibrations due to probe support equipment; and
o Real-time normalisation of boundary layer velocity using the freestream velocity.
• An automated positioning system in the x, y and z axis allows for fine control. The minimum step size in each direction is 6.25µm with a positioning accuracy of 1µm. The hotwire sensor is located on the y axis (Figure 12).
To control the angle of attack, the system was carefully designed to incorporate special fixtures and guide rails. The blades were mounted in two free rotating carousels. The angular orientation of the panel with respect to the incoming flow can be changed by rotating a micrometre screw on the top of the test section. The angle of attack for experimental work was maintained at zero (Figure 13).
A hot wire sensor set-up was used for CTA measurement in the wind tunnel. These probes were streamlined for minimum disturbance. They are designed such that they are shaped aerodynamically. They are supported by the arm on the y-axis and intentionally kept short to minimise any vibrations (Figure 14).
The constant-temperature anemometer (CTA) system was manufactured by Dantec Dynamics (DD, Skovlunde Denmark). The system consists of a StreamLine Pro Frame and Controller with three CTA velocity channels; StreamLine Pro Automatic Calibrator with Nozzle I; a Pitch/Yaw – Roll (PYR) manipulator to perform velocity calibrations; StreamWare Pro software; and a custom-developed LabVIEW program. The measured voltages were transferred to the computer via a National Instruments 4-channel simultaneous sampling differential USB A/D converter. The new 55P15 boundary layer probe and the 55P11 freestream probe are positioned at multiple grid points using a computer-controlled 3-axis traversing system. Both probes are mounted on the traversing mechanism using the hotwire support arm.
Wind Tunnel Results
In order to test the different nanomaterial-based coatings, 11 flat plate samples were manufactured using fibreglass as the substrate material. Six of these panels are tested in their pristine condition, (without abrasion or fouling), two were biofouled and three were subject to abrasion using sand-blasting. One of the pristine panels (WFG-P) was not coated to obtain a smooth surface. This panel achieves the baseline flow condition: a zero-pressure-gradient (ZPG) laminar boundary layer flow over the entire length. This way the flow field over remaining five pristine nano-coatings can be compared to the baseline flow condition. Similarly, the flow field over abraded and biofouled samples could be compared their pristine counterparts. The overview of samples and a short description is given in Table 11.
The average streamwise velocity profile and the root mean square of streamwise velocity fluctuation measured at several cross sections of the testing section show that any change in boundary flow, and in fact, is almost identical for all of the plates regardless of the coating type. Figure 15 shows a similar boundary layer velocity profile. Thus, the application of nano-particles did not cause any change to the boundary layer development/transition from laminar to turbulent. However, a boundary flow fluctuation plot (Figure 16) shows that plate WPU2-TWI-NC exhibits a high fluctuation, or high turbulence, due to the roughness introduced by the nano-particles for this particular plate.
Boundary flow measurements found that the effect of biofouling and/or abrasion had little to no effect on the flow characteristics. There was no noticeable difference in the onset of turbulence from that described above. The only exception was for the TWI- biofouled coating (WPU2-TWI-NC-B) which exhibited the transition from laminar to turbulent flow much earlier. This suggests that significant surface roughness combined with the increased level of biofouling reported previously has an effect on boundary flow (Figure 17).
Correlation of coating characteristics with boundary flow
Key parameters identifying relationships between the functional performance of nano-surface and flow turbulence were reported. This included a comparison of data from aged and pristine samples, to find existing surface-flow and surface-functional performance correlations that identify the effect of aging. The surface characterisation will help identify new standards for surface quality assessment.
To summarise, the following findings were reported:
-The boundary flow characteristics are not affected enough by surface roughness to compromise the turbine efficiency unless a rough surface has been suitably covered by biofouling (>10% surface coverage).
-Surfaces with Ra ≤1 will be susceptible to ~6% (average) biofouling coverage (up to 12%) and surface energy of <49.5 N/m.
-Surfaces with Ra ≥ 1.1 will be susceptible to ~10% biofouling (3.9%) and surface energy >50 N/m.
Algorithm Building
The algorithm identifies key inputs and generates the desired outputs through the multifunctional surface analyser. Analytical relations from coating characterisation were integrated to facilitate a measurement process that characterises “in-situ” quality of nano surfaces. The correlation between surface-flow parameters would be programmed in the novel surface profiler device.
An algorithm was presented which allows measurements of the surface roughness and biofouled surface area of a wind turbine blade to be related to the boundary flow of air over the blade surface. The algorithm may be used to predict the onset of turbulent air flow over the blades, which is an undesirable phenomenon limiting the performance of the turbine. The algorithm is based on experimental data obtained previously, and basic turbulent flow theory. The algorithmic procedure may be summarised as follows:
1.To begin, the shear velocity, u ̅, is to be determined from the linear velocity flow gradient within the laminar sub-layer:
u ̅=√(v├ ∂u/∂y┤|_(y=0) ),
where v=1.3×〖10〗^(-5) m^2 s^(-1) is the kinematic viscosity of air.
2.With knowledge of the shear velocity, one can compute the laminar sub-layer thickness:
δ_s=5v/u ̅ .
3.If the surface roughness is less than the laminar sub-layer thickness, R_a<δ_s, boundary flow is not influenced by surface roughness.
4.If the surface roughness exceeds the laminar sub-layer thickness, R_a>δ_s, turbulent flow above the sub-layer will be modified, however, this bound is not satisfied for any of NATURAL samples. Therefore the algorithm does not need to include surface roughness.
5.The influence of fractional biofouled surface area, f_b, on the onset of turbulence is accounted for through a logarithmic fit to the turbulent velocity profile, with a gradient which is quadratic in f_b:
u(y)=(-46.85〖f_b〗^2+2.57f_b+0.424) ln[y/y_0 ],
where y_0 is the value of y at which the flow velocity would vanish if the above turbulent flow equation were extrapolated to u=0. Setting y=δ_s in this expression determines the onset of turbulence.
Development and Validation of a Prototype Profiler
Profiler Design
A portable profiler design was produced and built to specification. Currently there are is no one single device that can obtain multiple different measurements in a single pass. The final device produced under the NATURAL project possess the ability to measure fluorescence for biofouling, surface energy by drop shape analysis for wettability, and a laser scanning device for surface profilometry and roughness measurements. The device can generate and capture real-time data whilst being able to transmit data remotely to the operator to make assessments in the field. Whilst the design can not be reported at present the outline specifications are shown below.
Lab and Field testing
Lab Testing
Lab validation trials were performed to validate the operation of the individual components both on their own and individually. A sample produced using the LUR nanoparticles in the coating (LPU2-LUR) was used as the test surface. Figure 18 exhibits an image of the feedback signal where a scan has been initiated. The 3D intensity will be displayed on the right whilst an arsine wave feeds into one of the ADC channels using the signal generator, representing the Z axis (Depth).
Field Testing
Following successful trials of the profiler with lab based samples, the device was tested on actual blade surfaces. Features on samples with real geometries and defects were analysed for validation (Figure 19). The prototype has also been configured to be able to stream live data to a mobile phone. Figure 20 demonstrates the code that is transmitted via Bluetooth to an Android mobile device.
The fluorescence camera (Figure 21) was able to capture and record images in real time for the operator to monitor. Areas of biofouling were successfully identified.
Standardisation
A thorough literature search was performed on each of the key criteria for optimisation, monitoring and evaluation of nanostructured coatings for wind turbine blades. This was done with the following points in mind: biofouling, contact angle and surface energy, surface roughness and boundary flow. It was concluded that there were no specific standards or recommendations available that could be applied the evaluation of wind turbine blades. A list of recommendations was made as follows:
1. To assess the condition of a coating in the following order:
a. Visual inspection macroscopic assessment
b. Surface roughness measurement by laser interferometry. (NB: The surface wettability should be known for a pristine surface using the same method.)
c. Assessment of biofouling by fluorescence imaging to determine the percentage area affected.
i. Where areas are identified, re-do surface roughness.
d. To perform drop shape analysis for surface wettability and surface energy assessment. (NB: The surface wettability should be known for a pristine surface.)
2. A fluorescence camera should be used for assessment of biofouling.
3. A laser interferometer should be used to record 3-dimensional surface profiles and configured to calculate surface roughness from the profile obtained.
4. A drop shape analyser capable of measuring water contact angle measurements on inclined surfaces should be used.
5. The profiler devices should be self-supporting and attachable to the wind turbine blade in order to carry out full assessment of the surface without human interference.
Further work is needed to combine these recommendations with the specifications for a suitable profiler and its parameters.
Potential Impact:
Socio-economic Impact and the Wider Societal Implications
• Benefits to users
o Rapid analytical methodology
o Portable instrumentation allows for ease of use
o Remote operation = Improved safety
o Unique, multi-technology, single measurement
o Minimum skill required to interpret results
o Reduced costs of repair to components
o Improved deposition techniques developed in the project provide significant enhancements in coating quality.
• Benefits to the public
o Enhanced operational performance = enhanced energy production
-This will ultimately lead to lower energy bills
-Increase competitiveness through competition from energy providers
-Increased efficiency means reduced pollution through the potential to rely less on non-renewable energy production
• Industry benefits
o The NATURAL project offers the following key achievements
-Real time monitoring methods
-Low-cost analytical instrumentation
-Understanding of relationship between nanostructured surfaces with roughness, surface energy and fouling conditions and their effect on flow boundary behaviour
-Recommendation for standardisation for the correct and proper procedures for surface characterisation of nanostructured coatings. This will ultimately lead to increased lifetime performance
-Nanostructured coatings offer the opportunity to use lighter metals such as aluminium or titanium in place of heavier metals with improved wear and anti-fouling properties. For example, automotive and aerospace. This will result in weight reduction and so, reduced fuel efficiency and pollution.
-Improved understanding of nanoparticles and their impact on several properties.
-Improved durability means less down-time to allow for servicing and repairs
-Improved hydrophobicity, anti-fouling and wear properties
-Opportunity to transfer the technology to other industries such as aerospace, automotive, oil and gas and defence.
-Increased competitiveness by reducing labour time and materials cost
Potential Impact and Main Dissemination and Exploitation Activities
-Dissemination Activity
*Seminars/congresses
Project participants have attended industrial specialized congresses or other relevant congresses to present publicly the preliminary project results. All these actions can be used to render NATURAL visible to the broader public. This will raise awareness about the NATURAL goals amongst both the specialized audiences and the general public (Table 15).
*Published papers
Below are listed the publications (papers, poster, communications to conferences) made by the partners, in the frame of the NATURAL project.
•Title: Nanoscale Mechanical Metrology for Industrial Process and Products
Author/s: A. Rana, G. Durand & A. Gunner
Partner: TWI
•Title: Device and strategy for surface energy measurement
Author/s: Noemí Domínguez
Partner: SO
•Title: Water drop profile measurement by using interferometry with incoherent light; preliminary results
Author/s: N. Domínguez, J. Arasa, C. Garcia
Partner: SO
•Title: Nanoscale roughness and bias in step height measurements by atomic force microscopy.
Author/s: Charles Clifford
Partner: NPL
-Exploitation
The key exploitable results at the start of the project are listed Table 16.
During the course of the project, an updated plan for the use and dissemination of foreground (PUDF) was completed to outline the two key exploitable results, including market analysis to understand risks and intervention needed to help commercially exploit the technologies (Table 17).
A benchmarking analysis was performed in order to locate the appropriate market segment for the technology developed during the project. It was concluded that the combination of profilometry measurements and surface energy measurements in a single device was a viable option to generate a competitive product. A real lack of commercial, portable devices was also detected. Accordingly, the technology developed during the project has coalesced into two viable, different devices. The partners involved in their development (on the one hand, SO, and on the other, AMT and SG) are currently initiating the patenting process for both technologies.
List of Websites:
One of the main dissemination tools of NATURAL is the project website :
http://www.natural-project.eu
The web is divided in two areas (Figure 22).
a) Public area: these pages aim at giving a public description of the project, objectives, partners, events and results (when publishable). In addition, the different project partners have provided details about the NATURAL project and have included links to the project website in their own sites;
b) Restricted area: this space is linked to PROCEMM, and thus dedicated to partners for internal dissemination and communication, with specific tools for exchanging and sharing data, files and all relevant project information. This area is accessible for every partner using a specific username and password
Corporate image
A NATURAL logotype and its different versions to be used according to the type of dissemination document have been prepared, as shown in Figure 23.
There is a growing range of applications that can benefit from the use of nanostructured coatings. The functional performance of a surface is intimately linked with its structure. The ability to characterise nanostructured coatings is therefore an essential part of their future uptake. However, the ability to examine and characterise at the nano-scale is currently limited to sophisticated, time-consuming, laboratory-based equipment. The standards body (ISO TC229) recognises that there are no current procedures that relate the functional performance of a surface or coating to its nanostructure. The NATURAL project has developed methods that allow rapid evaluation of surfaces at the nanoscale and correlate the measured surface structure with functional performance. This will enable new methods for lifetime determination and facilitate coatings evaluation in the field. The project focussed on the development of surface profilometry methods to allow the rapid resolution of surfaces at the nanoscale. Data generated was correlated with the physical and physio-chemical characteristics of surfaces to allow their rapid, reliable and accurate assessment. The change of the surface nano-morphology and the functional performance were related to allow the estimation of durability and to estimate coating lifetime.
As part of the project, work was done to identify a suitable polyurethane resin to which nanoparticles could be added to give the specific properties required to improve coating performance. Three different resins were trialled; an accepted industrial product and two model resins. Three different nanoparticles were chosen as candidate fillers, each having specific properties. A spray coating process was developed and optimised to achieve increased coating quality in terms of finish, thickness, homogeneity and ease of application. A number of samples were produced for lab-based characterisation. Various analytical techniques were employed to characterise the coatings, including nanoindentation, FTIR, confocal microscopy, adhesion, SEM, coefficient of friction and XPS. The coatings were also applied to wind turbine blade sections for wind tunnel testing. Both lab and wind tunnel samples underwent ageing by biofouling, particle and sand erosion and water erosion. Wind tunnel testing was performed to determine the effect of surface conditions such as roughness and ageing on the boundary flow characteristics.
Characterisation activities found that higher nanoparticle loading gave rise to high surface roughness whilst some of the lower loaded coatings exhibited gas bubble inclusions but maintained durability. Biofouling saw the coating fail much sooner under water droplet erosion and loss of some surface tension. Assessment of the effects of ageing on flow characteristics found that the time to transition of laminar to turbulent flow was not affected by micro-roughness or ageing by biofouling, but only where the roughness, Ra is <8μm and biofouling is <10% surface coverage. Using this correlation between surface characteristics and flow behaviour, a relationship was identified and an algorithm produced to link the surface roughness, surface energy and severity of biofouling to the onset of turbulent flow. This will be implemented in to the device and can also be used to assess surface conditions and predict service lifetime.
A portable profiler was designed and built. The device incorporates the ability to measure fluorescence for biofouling, surface energy by drop shape analysis for wettability, and a laser scanning device for surface profilometry and roughness measurements. The device can generate and capture real-time data whilst being able to transmit data remotely to the operator to make assessments in the field.
Finally, based on the outcomes of the project, a series of recommendations were made to take forward for the development of a standard which will aim to demonstrate a protocol for nanostructured coatings on wind-turbines (and similar applications). The standard will include steps to assess the surface in line with the methods employed in NATURAL. It will also go some way to proposing a suitable device(s) to aid the prediction of coating lifetime and performance.
Project Context and Objectives:
The driving purpose: The rise of the new nano-technology age has led to a broad range of nano-structured coatings which are increasingly being used in critical applications. The characterisation of nano-structured coatings is essential to understanding and improving their performance. The ability to analyse sub-micron features or nano/micro contaminants is the enabling step to increasing durability in a number of industrial applications, such as marine, aviation and wind blades. Nano-structuring has proved it importance in anti-fouling (ice, insect, biofouling), low friction and boundary layer control applications.
Erosion of surfaces of aircraft during routine flight has been claimed to increase fuel consumption and CO2 emissions by almost 5% (1). For wind turbines, ice build-up can cause a loss of 5-10% of electricity production; in sub-tropical warmer environment, fouling of the blade by insects can reduce power production from wind turbines by 50% (2). Conventional coatings and surface treatments have not been successful in reducing these effects. Fouling of ships by marine organisms is believed to increase fuel consumption by 40% and conventional biocidal approaches to addressing this issue are a substantial concern due to their biological effects.
New coatings and surface treatments with nano-structured materials and topographies are generally believed to offer the potential to reduce erosion rates and fouling, and therefore have significant and beneficial effects on fuel consumption and carbon footprints. Such advantages of nano-coatings stem from both the ability to tailor roughness levels, and more importantly from the ability to modify the energy of the underlying surface.
Controlling turbulence transition in boundary layers offers considerable advantages to numerous industrial applications. Using nano-structured coatings, the flow-surface interaction can be engineered to delay the transition of flow to turbulence, and hence reduce flow forces, thereby saving costs for structures such as aerofoils and blades.
The sustainability of a coating is linked to its lifetime. With time and environmental exposure, unwanted change in the nano-structure frequently reduces performance by two major mechanisms:
- Surface fouling: external material (eg dust/oil, ice, biofouling such as insects, barnacles and algae) builds up on the surface, presenting an inferior surface, hence reducing performance (Figure. 1).
- Surface erosion (wear, corrosion): weathering including UV degradation, abrasion and friction cause surface erosion of the coating, leading to loss of material over time, hence a loss of performance associated with loss of the active component and change in surface morphology. (Figure 1).
To improve the quality of nano-structured coatings, a greater understanding of structure at the nano-level, corresponding mechanical properties at the nano-scale and the influence of these on macro-scale behaviour is needed. Especially, mechanical characterisation of nano-phases is important for understanding the effect of the nano-structure on the real-life performance of the coating. Laboratory-based techniques such as nano-indentation and AFM are proven to be successful for characterising nano-structured coatings on a small length scale but specimen preparation is time-consuming and relatively expensive. Moreover, some other techniques are truly destructive and time consuming (e.g. cross-section microscopy) and may require operation in a vacuum chamber (e.g. TEM, SEM), that implies the use of a small section of the coating rather than a large flat area.
Project Objectives
The NATURAL project focussed on developing real-world, practical characterisation methods for nano-structured coatings with a low surface energy for anti-fouling applications and/or for low friction applications.
The main standardisation bodies, (CEN, ISO) have recognised that uncontrolled change of the nano-structure frequently results in a loss of performance, thus, the first aim of NATURAL is to relate surface nano-morphology to the functional performance of surfaces and to enable the development of new methods for lifetime determination and, ultimately, prediction.
The NATURAL project integrates research organisations and industry into a coordinated and interdisciplinary research programme, incorporating all the necessary elements from nano-materials technology selection and engineering through to evaluation and end-user trials.
• The consortium identified candidate nano-structured coatings both in the commercial market and the research community (including those which had already been developed by members of the consortium). These candidates were used as model systems, providing a range of nano-structures and surface chemistries.
• The consortium gathered data on these selected coatings, using high resolution AFM and nano-indentation techniques.
• Conventional tribological methods were used to provide a baseline of mechanical performance.
• Novel, short wavelength, high resolution laser profiling methods were developed to allow rapid surface profiling of large areas.
• Correlations between the fine nano-scale (AFM, nano-indentation), coarser nano-scale (laser profiling) and the macro-scale (tribology, fluid flow) were established to form the basis for a new standard.
• The effect of ageing under representative conditions was monitored using methods developed within the project to allow the key structural and functional modifications to be interrelated. This was used to develop a preliminary model for durability and lifetime prediction and also determine the effect of wear on performance.
The second aim of the project was to overcome the problem of inspecting the coatings in-situ, since access may be difficult or hazardous and the commercial impact of sampling or extended shut-down for laboratory testing may be significant. The NATURAL project addressed this aim by designing and building a prototype portable device based on laser profiling but with the capability for determining surface energy. This device will allow the user to obtain fast information on the morphology of the surface and correlate it to studies undertaken in the laboratory.
The NATURAL partnership aimed to develop a simple, robust, rapid and standardised methodology that allows the nano-scale structure of a surface to be measured in the field and then relate this to its mechanical and tribological properties at the micro-scale and hence performance on the macro-scale.
Specifically, the overall objectives of the NATURAL project were to:
• Develop understanding of practical characterisation methods for nano-structured coatings with a focus on low energy coatings for anti-fouling applications and/or for low friction applications such that the results can be interpreted in a meaningful fashion in a real-world application.
• Develop a standardised method to determine the lifetime of nanostructured coatings.
• Develop a prototype nano-characterisation measurement system capable of non-destructive evaluation in the field.
• Monitor the mentioned surface conditions of nanostructured coatings in operating conditions, such as in a wind turbine blade.
Project Results:
Coating Development
The main objectives for the development of nanostructured coatings were first, to identify a host resin system for the addition of nanoparticle fillers. The nanoparticles (NP) will improve overall performance in terms of hydrophobicity, wear and erosion performance, impact resistance, anti-fouling and anti-icing (Figure 3). A commercially recognised and accepted resin (M1) was initially used but due to the complexity of its chemistry proved difficult to mix with the nanoparticles. A second model system (PU) was then selected but was dismissed due to problems with a short pot life and rapid gelling. The third resin (PU2) offered best performance and was chosen as the candidate resin. Three different nanoparticles were selected, each having preferential properties: super-hydrophobicity, anti-icing, abrasion resistance, anti-fouling etc.
Fabrication was done by spray method optimised by LUR for improved coating deposition and final quality (Figure 5). Each of the NPs posed their own challenges which increased the efforts required to optimise the spray process. The TWI NP’s produced rough, thick coatings that were considerably uneven. The TNO were thin and smooth as desired but generated bubbles. The LUR coating was also good but too had issues with bubble formation. Nevertheless, sample preparation was successfully accomplished despite various issues regarding suitable PU matrix selection, optimum nanoparticle incorporation and fine-tuning of the spray gun parameters.
Following successful feedback the characterisation and validation exercises, coating of large plates was complete. The plates will be used for wind tunnel testing.
Characterisation of Coatings
Initially, coatings were applied to flat substrates for characterisation and validation. Following this, samples were artificially aged to mimic real life exposure. The surface and near surface properties of coatings used for wind turbine blades were studied in detail. These coatings are typically polyurethane. The aim was to compare the commercial polyurethane only coatings with those coats developed which contain nano-particles and nano-structure. As discussed above, the three resin systems were trialled. Findings from the chosen system, PU2, are detailed here.
Microscopy revealed noticeable differences between the different variants. Figure 5 is shows the variable difference in surface condition of pristine samples. PU2-TWI is notably rough compared to the other three samples. This is consistent with roughness measurements taken by confocal microscopy (Figure 6). (PU2-P: Ra=0.92μm; PU2-LUR: Ra=0.46μm; PU2-TNO: Ra=0.31μm; PU2-TWI: Ra=8.92μm).
Chemical surface analysis using X-ray photo-electron spectroscopy (XPS) revealed the M1 un-aged sample showed Ca, Mg, Cl, S, Si, Al peaks. These are all consistent with both contamination on the surface and also fillers such as talc and metal oxides being present in the coating. XPS spectra of the bare, un-aged PU2 sample showed that as well as elemental peaks associated with polyurethane (C, N, O) there was also some silicon present. Closer examination of the peak background revealed that this silicon was a contamination layer lying on top of the polyurethane.
The elemental compositions of the PU2 samples – bare and with addition of nanoparticles are detailed in Table2. The results were comparable to the types of nanoparticles used for each of the variants; TNO samples should contain fluorinated functionalised particles but no fluorine is found in the top 10 nm. The LUR samples should contain aluminium oxide based nanoparticles but no aluminium was found. TWI samples contain silica based nanoparticles and higher silicon was seen in this sample compared to the other samples. Hence, only the TWI sample contains nanoparticles at the surface, the others show no nanoparticles at the surface and hence the nanoparticles are covered by an over-layer of polyurethane.
Contact angle measurements for the pristine surfaces are shown in Figure 7. The surface energy of LUR and TWI samples were marginally higher than those determined for the bare PU and TNO samples.
Nanoindentation results of pristine samples (Table 3) show that the resin with TWI nanoparticles had the highest hardness but it should be noted that the coating also had the roughest surface. Overall, the LUR variant had a reduction in surface hardness compared to the unmodified LPU2-P resin.
Adhesion testing was carried out by TWI found adhesion between the coating and substrate was strongest for the base LPU2-P and LPU2-TWI-P coatings as the failure mode was adhesive. The LUR and TNO variants failed due to bonding failure, i.e. the complete separation between the dolly and the coating in all six tests of each coating.
Tubular impact tests found that the coatings withstood impact more than the substrates that they were deposited on. For example, the LPU2-LUR-P coating failed at 1000 mm-kg but the substrate began to fail at <300 mm-kg. Looking at the impact test results overall, the TNO variation was found to be less resilient than the base resin on its own. The LUR variant was the toughest coating.
SEM analysis of the surface and cross-sections of pristine coatings with and without nanoparticles are shown in Figure 8. Looking at the images of the top surfaces, the bare coatings (LPU2) have a relatively smooth, even surface, LPU2-LUR-P was similar but with the presence of small pores. The TNO variant exhibited the most uniform coating with minimal defects whilst the TWI coating is evidently rough and uneven with many large pores on the surface.
Analysis of the cross-sections through the coating thickness correlated with the images of the top surfaces. The bare LPU2-P exhibited a fairly even coating with few voids or defects. The LUR coating had several large particles at the surface with visible porosity. The TNO was comparatively uniform with few imperfections and exhibiting sparsely distributed nanoparticles. The TWI variant was comparatively thick, more than double the thickness of the LPU2-P coating; nanoparticles were infrequent but little to no through-thickness porosity could be found.
Ageing
Aging of the surfaces was performed to simulate conditions experienced by wind turbines at sea. The main types of exposure were identified as biofouling, particle erosion (abrasive wear) and water erosion.
Particle Aging by TNO
The test conditions used for the sand erosion wear rate determination and aging procedures are:
1.Abrasive: Sand particles,
• grain size: d50 = 0.25 mm, range: 0.2 –0.4 mm,
• hardness: 950 HV,
• shape: rounded particles,
2.Particle impact velocity: 65 ± 5 m/s,
3.Amount of abrasive: 500 mg/min,
4.Stand-off distance: 150 mm,
5.Angle of impact: 45 and 90°,
6.Area of erosion: Ø20 mm (314 mm2)
7.Environment: Air.
8.Test duration: 75 min, interrupted for specimen weighing each 15 min.
Some of the conditions used in this test, abrasive type and size and amount of abrasive, have been derived from MIL-STD-810G (2008), a test method standard of the US Department of Defencee for environmental engineering considerations.
Particle Aging by NPL
Use RT particle erosion system
Use large stand-off distance
100 g aliquot 220 µm diameter sand
Produced scar ~50 mm across
Samples tested:
LPU2_P scar depth max ~12 µm
LPU2_TNO_P scar depth max ~5 µm
LPU2_LUR_P scar depth max 14 µm
Water droplet aging by TNO
Test conditions
The following test conditions were used:
• Nozzle type: rounded, Ø0.45 mm, CN = 0.963
• Drop velocity, vd = 100 m/s (water pressure pw = 80 bar),
• Drop size, dd = 0.1 to 0.06 mm,
• Amount of water: 0.95 l/min,
• Stand of distance, SOD: 100 mm (standard),
• Angle of impact: 90°.
The droplet velocity in the test was derived from an offshore wind turbine application, type XE128-5MW of XEMC-Darwind, (The Netherlands). The datasheet for this wind turbine gives:
- Power: 5 MW
- Rotor diameter: 128 m
- Speed: variable (nominal 15 rpm)
This means that the nominal tip velocity of the blades is 101 m/s.
The mean gravitational velocity of water drops varies, depending on the size, normally between 5 and 8 m/s. This means that during the upward movement of the blades the impact velocity is between 106 to 109 m/s and in downward direction between 93 and 96 m/s.
A value of ~100 m/s has been used in the performed droplet erosion experiments.
The key test criteria for a potential material and/or coating surface, for a certain set of test condition, are:
• Incubation period, Ip (min),
• Erosion rate, Re (mm/min),
• Time to coating failure, If (min).
Figure 9a shows a schematic of the key test criteria for a coating material determined with droplet erosion tests.
Water droplet aging by NPL (Figure 9b)
Designed and built at NPL
In vacuum – needed for high tip speeds (max 500 ms-1)
Ø 1.2 mm nozzle, nominally Ø 2 mm diameter droplets
Single droplet stream
Particulate erosion aged LPU2-P
Particulate erosion aged LPU2-TNO-P
Particulate erosion aged LPU2-LUR-P
All on 8 mm wide dovetail specimens
Test Conditions
Chamber pressure with jet running: 20 mbar
1438 RPM
0.5 m arm radius
(~80 ms-1 tip speed)
Up to 120 minutes’ duration (1-5 minute increments).
Biofouling
Samples were exposed to sub-aerial fouling with natural seawater (NSW) for 3weeks. The natural seawater was sprayed as droplets inside the mist chamber as indicated in Error! Reference source not found. 9. Temperature in the cabinet was 25± 2°C and the following spraying cycle was used:
- NSW mist + light = 14h
- Darkness, no NSW = 10h
- Season: summer
4 glass slides were exposed as a control for biofouling. Status of biofouling was checked twice a week by the use of the epifluorescence microscopy.
Aging by biofouling was measured with fluorescence imaging to measure the degree of surface coverage. Average coverage of the bare resin samples (LPU2P-B) was found to be slightly higher than the glass slides used as reference samples. The average coverage of the TWI samples with 40% nanoparticles added was comparatively higher at 10% (Table 5).
Aging by droplet erosion was measured by time to failure (Figure 10). Results found that the time to failure was the longest for the TNO samples and in general where the nanoparticle loading was around 1% and certainly no more than 3%. The same was found for the pure resin where time to failure was the lowest. Water droplet erosion carried out by NPL also found similar results. The figure on the left shows how the TWI nanoparticle loaded samples (LPU2-TWI-P) fail within in a matter of minutes, whereas the bare PU2, TNO and LUR coatings take much longer to fail
Sand erosion was performed and measured by surface roughness (Figure 11). Particle erosion was performed at two angles of attack (40O and 90O). Results show a significant increase in roughness for all three samples at 90O. However, the LUR coating shows the most drastic deterioration of surface condition across both angles of attack. Sand erosion tests by particle erosion were performed by particle erosion. Bare resin, LPU2-TNO and LPU2-LUR samples were tested. LPU2-P.
XPS analysis of the biofouled samples are shown in the table below. The effects of biofouling show chemistry on the surface typical of salts from sea water and biomaterial growth (Table 7).
Droplet erosion testing before and after aging by biofouling, shown in Table 8, gives evidence of a decrease in coating lifetime for aged samples compared to that seen for un-aged samples.
Contact angle and surface energy measurements show that there was no significant decrease in surface energy; the TWI sample actually exhibited a slight increase.
Nanoindentation of the biofouled samples show that there is a negligible increase in hardness from the pristine LPU2 to the biofouled LPU2 coating, 0.20GPa and 0.26GPa respectively. The TWI biofouled surface presents a decrease of the hardness; 0.32GPa for the pristine and 0.24GPa for the biofouled coating (Table 9).
Wind Tunnel Testing
In order to test the different nanomaterial based coatings, 11 flat plate samples were manufactured using fibreglass as the substrate material. Six of these panels are tested in their pristine condition, (without abrasion or fouling) two were biofouled in a special environmental chamber, and three were subject to abrasion using sand-blasting. One of the pristine panels (WFG-P) was not coated to obtain smooth surface. This panel achieves the baseline flow condition: a zero-pressure-gradient (ZPG) laminar boundary layer flow over the entire length. This way the flow field over remaining five pristine nano-coatings can be compared to the baseline flow condition. Similarly, the flow field over abraded and biofouled samples could be compared their pristine counterparts. The overview of samples and a short description is given in Table 10.
The wind tunnel facility belonging to the Department of Mechanical Engineering at the Technical University of Denmark (DTU) was used for the tests. The facility was modified for the requirements of the blade trials by Dantec. The main features of the wind tunnel are listed as follows:
• Closed-loop subsonic facility with a nominal maximum flow speed of 60m/s.
• Measured background turbulence intensity level of 0.15% at 20m/s.
• Nominal test section dimensions are 0.3x0.3x0.75m with an increasing cross sectional area along the streamwise direction to compensate for developing boundary layers.
• The temperature of the flow cannot be controlled during the experiments. As a result, the air temperature in the tunnel rises from 22°C to 26°C during the course of the experiments.
• Two interchangeable test sections are available which enables a quick change to a different experimental setup. One of these test sections was modified to meet the boundary layer measurement requirements imposed by Project NATURAL. These requirements include:
o Automated measurement of laminar, transitional and turbulent boundary layer profiles.
o Angle of attack adjustment from outside the test section.
o Zero-pressure-gradient alignment during tunnel operation.
o Quick replacement of test panels.
o Keeping the alignment between panels.
o Minimum flow disturbance and vibrations due to probe support equipment; and
o Real-time normalisation of boundary layer velocity using the freestream velocity.
• An automated positioning system in the x, y and z axis allows for fine control. The minimum step size in each direction is 6.25µm with a positioning accuracy of 1µm. The hotwire sensor is located on the y axis (Figure 12).
To control the angle of attack, the system was carefully designed to incorporate special fixtures and guide rails. The blades were mounted in two free rotating carousels. The angular orientation of the panel with respect to the incoming flow can be changed by rotating a micrometre screw on the top of the test section. The angle of attack for experimental work was maintained at zero (Figure 13).
A hot wire sensor set-up was used for CTA measurement in the wind tunnel. These probes were streamlined for minimum disturbance. They are designed such that they are shaped aerodynamically. They are supported by the arm on the y-axis and intentionally kept short to minimise any vibrations (Figure 14).
The constant-temperature anemometer (CTA) system was manufactured by Dantec Dynamics (DD, Skovlunde Denmark). The system consists of a StreamLine Pro Frame and Controller with three CTA velocity channels; StreamLine Pro Automatic Calibrator with Nozzle I; a Pitch/Yaw – Roll (PYR) manipulator to perform velocity calibrations; StreamWare Pro software; and a custom-developed LabVIEW program. The measured voltages were transferred to the computer via a National Instruments 4-channel simultaneous sampling differential USB A/D converter. The new 55P15 boundary layer probe and the 55P11 freestream probe are positioned at multiple grid points using a computer-controlled 3-axis traversing system. Both probes are mounted on the traversing mechanism using the hotwire support arm.
Wind Tunnel Results
In order to test the different nanomaterial-based coatings, 11 flat plate samples were manufactured using fibreglass as the substrate material. Six of these panels are tested in their pristine condition, (without abrasion or fouling), two were biofouled and three were subject to abrasion using sand-blasting. One of the pristine panels (WFG-P) was not coated to obtain a smooth surface. This panel achieves the baseline flow condition: a zero-pressure-gradient (ZPG) laminar boundary layer flow over the entire length. This way the flow field over remaining five pristine nano-coatings can be compared to the baseline flow condition. Similarly, the flow field over abraded and biofouled samples could be compared their pristine counterparts. The overview of samples and a short description is given in Table 11.
The average streamwise velocity profile and the root mean square of streamwise velocity fluctuation measured at several cross sections of the testing section show that any change in boundary flow, and in fact, is almost identical for all of the plates regardless of the coating type. Figure 15 shows a similar boundary layer velocity profile. Thus, the application of nano-particles did not cause any change to the boundary layer development/transition from laminar to turbulent. However, a boundary flow fluctuation plot (Figure 16) shows that plate WPU2-TWI-NC exhibits a high fluctuation, or high turbulence, due to the roughness introduced by the nano-particles for this particular plate.
Boundary flow measurements found that the effect of biofouling and/or abrasion had little to no effect on the flow characteristics. There was no noticeable difference in the onset of turbulence from that described above. The only exception was for the TWI- biofouled coating (WPU2-TWI-NC-B) which exhibited the transition from laminar to turbulent flow much earlier. This suggests that significant surface roughness combined with the increased level of biofouling reported previously has an effect on boundary flow (Figure 17).
Correlation of coating characteristics with boundary flow
Key parameters identifying relationships between the functional performance of nano-surface and flow turbulence were reported. This included a comparison of data from aged and pristine samples, to find existing surface-flow and surface-functional performance correlations that identify the effect of aging. The surface characterisation will help identify new standards for surface quality assessment.
To summarise, the following findings were reported:
-The boundary flow characteristics are not affected enough by surface roughness to compromise the turbine efficiency unless a rough surface has been suitably covered by biofouling (>10% surface coverage).
-Surfaces with Ra ≤1 will be susceptible to ~6% (average) biofouling coverage (up to 12%) and surface energy of <49.5 N/m.
-Surfaces with Ra ≥ 1.1 will be susceptible to ~10% biofouling (3.9%) and surface energy >50 N/m.
Algorithm Building
The algorithm identifies key inputs and generates the desired outputs through the multifunctional surface analyser. Analytical relations from coating characterisation were integrated to facilitate a measurement process that characterises “in-situ” quality of nano surfaces. The correlation between surface-flow parameters would be programmed in the novel surface profiler device.
An algorithm was presented which allows measurements of the surface roughness and biofouled surface area of a wind turbine blade to be related to the boundary flow of air over the blade surface. The algorithm may be used to predict the onset of turbulent air flow over the blades, which is an undesirable phenomenon limiting the performance of the turbine. The algorithm is based on experimental data obtained previously, and basic turbulent flow theory. The algorithmic procedure may be summarised as follows:
1.To begin, the shear velocity, u ̅, is to be determined from the linear velocity flow gradient within the laminar sub-layer:
u ̅=√(v├ ∂u/∂y┤|_(y=0) ),
where v=1.3×〖10〗^(-5) m^2 s^(-1) is the kinematic viscosity of air.
2.With knowledge of the shear velocity, one can compute the laminar sub-layer thickness:
δ_s=5v/u ̅ .
3.If the surface roughness is less than the laminar sub-layer thickness, R_a<δ_s, boundary flow is not influenced by surface roughness.
4.If the surface roughness exceeds the laminar sub-layer thickness, R_a>δ_s, turbulent flow above the sub-layer will be modified, however, this bound is not satisfied for any of NATURAL samples. Therefore the algorithm does not need to include surface roughness.
5.The influence of fractional biofouled surface area, f_b, on the onset of turbulence is accounted for through a logarithmic fit to the turbulent velocity profile, with a gradient which is quadratic in f_b:
u(y)=(-46.85〖f_b〗^2+2.57f_b+0.424) ln[y/y_0 ],
where y_0 is the value of y at which the flow velocity would vanish if the above turbulent flow equation were extrapolated to u=0. Setting y=δ_s in this expression determines the onset of turbulence.
Development and Validation of a Prototype Profiler
Profiler Design
A portable profiler design was produced and built to specification. Currently there are is no one single device that can obtain multiple different measurements in a single pass. The final device produced under the NATURAL project possess the ability to measure fluorescence for biofouling, surface energy by drop shape analysis for wettability, and a laser scanning device for surface profilometry and roughness measurements. The device can generate and capture real-time data whilst being able to transmit data remotely to the operator to make assessments in the field. Whilst the design can not be reported at present the outline specifications are shown below.
Lab and Field testing
Lab Testing
Lab validation trials were performed to validate the operation of the individual components both on their own and individually. A sample produced using the LUR nanoparticles in the coating (LPU2-LUR) was used as the test surface. Figure 18 exhibits an image of the feedback signal where a scan has been initiated. The 3D intensity will be displayed on the right whilst an arsine wave feeds into one of the ADC channels using the signal generator, representing the Z axis (Depth).
Field Testing
Following successful trials of the profiler with lab based samples, the device was tested on actual blade surfaces. Features on samples with real geometries and defects were analysed for validation (Figure 19). The prototype has also been configured to be able to stream live data to a mobile phone. Figure 20 demonstrates the code that is transmitted via Bluetooth to an Android mobile device.
The fluorescence camera (Figure 21) was able to capture and record images in real time for the operator to monitor. Areas of biofouling were successfully identified.
Standardisation
A thorough literature search was performed on each of the key criteria for optimisation, monitoring and evaluation of nanostructured coatings for wind turbine blades. This was done with the following points in mind: biofouling, contact angle and surface energy, surface roughness and boundary flow. It was concluded that there were no specific standards or recommendations available that could be applied the evaluation of wind turbine blades. A list of recommendations was made as follows:
1. To assess the condition of a coating in the following order:
a. Visual inspection macroscopic assessment
b. Surface roughness measurement by laser interferometry. (NB: The surface wettability should be known for a pristine surface using the same method.)
c. Assessment of biofouling by fluorescence imaging to determine the percentage area affected.
i. Where areas are identified, re-do surface roughness.
d. To perform drop shape analysis for surface wettability and surface energy assessment. (NB: The surface wettability should be known for a pristine surface.)
2. A fluorescence camera should be used for assessment of biofouling.
3. A laser interferometer should be used to record 3-dimensional surface profiles and configured to calculate surface roughness from the profile obtained.
4. A drop shape analyser capable of measuring water contact angle measurements on inclined surfaces should be used.
5. The profiler devices should be self-supporting and attachable to the wind turbine blade in order to carry out full assessment of the surface without human interference.
Further work is needed to combine these recommendations with the specifications for a suitable profiler and its parameters.
Potential Impact:
Socio-economic Impact and the Wider Societal Implications
• Benefits to users
o Rapid analytical methodology
o Portable instrumentation allows for ease of use
o Remote operation = Improved safety
o Unique, multi-technology, single measurement
o Minimum skill required to interpret results
o Reduced costs of repair to components
o Improved deposition techniques developed in the project provide significant enhancements in coating quality.
• Benefits to the public
o Enhanced operational performance = enhanced energy production
-This will ultimately lead to lower energy bills
-Increase competitiveness through competition from energy providers
-Increased efficiency means reduced pollution through the potential to rely less on non-renewable energy production
• Industry benefits
o The NATURAL project offers the following key achievements
-Real time monitoring methods
-Low-cost analytical instrumentation
-Understanding of relationship between nanostructured surfaces with roughness, surface energy and fouling conditions and their effect on flow boundary behaviour
-Recommendation for standardisation for the correct and proper procedures for surface characterisation of nanostructured coatings. This will ultimately lead to increased lifetime performance
-Nanostructured coatings offer the opportunity to use lighter metals such as aluminium or titanium in place of heavier metals with improved wear and anti-fouling properties. For example, automotive and aerospace. This will result in weight reduction and so, reduced fuel efficiency and pollution.
-Improved understanding of nanoparticles and their impact on several properties.
-Improved durability means less down-time to allow for servicing and repairs
-Improved hydrophobicity, anti-fouling and wear properties
-Opportunity to transfer the technology to other industries such as aerospace, automotive, oil and gas and defence.
-Increased competitiveness by reducing labour time and materials cost
Potential Impact and Main Dissemination and Exploitation Activities
-Dissemination Activity
*Seminars/congresses
Project participants have attended industrial specialized congresses or other relevant congresses to present publicly the preliminary project results. All these actions can be used to render NATURAL visible to the broader public. This will raise awareness about the NATURAL goals amongst both the specialized audiences and the general public (Table 15).
*Published papers
Below are listed the publications (papers, poster, communications to conferences) made by the partners, in the frame of the NATURAL project.
•Title: Nanoscale Mechanical Metrology for Industrial Process and Products
Author/s: A. Rana, G. Durand & A. Gunner
Partner: TWI
•Title: Device and strategy for surface energy measurement
Author/s: Noemí Domínguez
Partner: SO
•Title: Water drop profile measurement by using interferometry with incoherent light; preliminary results
Author/s: N. Domínguez, J. Arasa, C. Garcia
Partner: SO
•Title: Nanoscale roughness and bias in step height measurements by atomic force microscopy.
Author/s: Charles Clifford
Partner: NPL
-Exploitation
The key exploitable results at the start of the project are listed Table 16.
During the course of the project, an updated plan for the use and dissemination of foreground (PUDF) was completed to outline the two key exploitable results, including market analysis to understand risks and intervention needed to help commercially exploit the technologies (Table 17).
A benchmarking analysis was performed in order to locate the appropriate market segment for the technology developed during the project. It was concluded that the combination of profilometry measurements and surface energy measurements in a single device was a viable option to generate a competitive product. A real lack of commercial, portable devices was also detected. Accordingly, the technology developed during the project has coalesced into two viable, different devices. The partners involved in their development (on the one hand, SO, and on the other, AMT and SG) are currently initiating the patenting process for both technologies.
List of Websites:
One of the main dissemination tools of NATURAL is the project website :
http://www.natural-project.eu
The web is divided in two areas (Figure 22).
a) Public area: these pages aim at giving a public description of the project, objectives, partners, events and results (when publishable). In addition, the different project partners have provided details about the NATURAL project and have included links to the project website in their own sites;
b) Restricted area: this space is linked to PROCEMM, and thus dedicated to partners for internal dissemination and communication, with specific tools for exchanging and sharing data, files and all relevant project information. This area is accessible for every partner using a specific username and password
Corporate image
A NATURAL logotype and its different versions to be used according to the type of dissemination document have been prepared, as shown in Figure 23.
