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Prediction of the life time behaviour for c/c-sic tubes as high and ultrahigh temperature heat exchangers (HITHEX)


Tensile tests on SCHUNK and DLR materials developed and on the BoraSiC coating system were performed to characterise the material behaviour from room temperature up to 1550°C. Medium term creep tests at 1250°C up to 1000 hours on flat specimen with different additional BoraSiC coatings were performed. Up to now these materials show at temperatures up to 1250°C no distinctive creep behaviour. One SCHUNK material was investigated under dynamic tension/tension loading. No significant change in strain, young's modulus or crack density was detected up to 100.000 cycles. By means of internal pressure tests the gas permeability and burst behaviour at ambient temperature were determined on SKT and DLR material. Investigation on tubes with two open ends and tubes with flange and end cap were performed to characterise the pure tube material and also the influence of the flange and the geometry of the end cap on the burst behaviour.
The improvement of the filament winding is tailored to reduce the damage caused by shrinking effects by an optimised fibre pattern as well as selection of C-fibres for reinforcement. After completion of this subtask, the achieved progress can be directly introduced in the existing manufacturing technology for commercial products at Schunk. In particular, filament winded parts with high wall thickness up to 50mm or more show in practice an unacceptable high rate of wasted parts. More than 100,000 Euros can be saved per year by avoiding such extreme delamination defects as observed in the beginning of the project. Furthermore, all improvements in the reliability of the winding processes can be immediately used for commercial productions. This is valid for gas pipes, heating tubes, load carrying parts and high temperature drive shafts. The in-situ joining technique offers new methods for load introductions. The mechanical potential can be determined by tests developed within HITHEX by the DLR in Stuttgart.
IMP-CNRS prepared mullite coatings by low pressure CVD in a hot wall reactor on CMC (C/C-SiC) substrates in order to protect them again oxidation at high temperature. The CMC samples were small plates (7x7x3mm3) supplied by SKT and DLR. An experimental study of the deposition conditions was carried out in order to improve the resistance to high temperature air resistance of the coated samples. The oxidation resistance of these coatings was tested by means thermo-gravimetric “in situ” measurements in the range 1350-1600°C. The cracks formation on the coatings during the cooling stage of the oxidation cycles result in weight losses due to the carbon combustion within the CMC samples. Two layers coating made of an intermediate porous mullite layer and an outer dense mullite layer were tested seem to show some improvement. In addition, SiO2-rich mullite layers showed also encouraging results.
Setting up of a new test rig for isothermal oxidation has been obtained; its capability is to be operated up to 1500°C.
The anisotropy of fibre reinforced materials such as ceramic matrix composites requires a determination of their thermophysical properties in all directions (e.g. in fibre, perpendicular to fibre direction). The manufacturing process of such materials often results in geometry dependent properties, e.g. the process leading to tubular samples often does not allow manufacturing flat test samples with comparable properties. Up to now, most test methods for thermal conductivity / diffusivity, thermal expansion, emissivity only allow the testing flat or otherwise simple shaped samples (e.g. cylinders for CTE measurements). The test methods developed are no longer limited to those geometries, e.g. the direct measurement of arched samples is now possible, allowing an application-relevant determination of key properties in a geometry that is relevant for the real-case application scenario. This is achieved by specific sample preparation, modified, sample holders and the application of mathematical correction of geometry effects. The application of these test methods is mainly in (but not limited to) the wide field of anisotropic materials, namely fibre reinforced / composite materials.
Microstructural characterisation of C-C/SiC materials available in the project (provided by SKT and DLR) have been carried out before and after high temperature oxidation and erosion testing. Results obtained can be applied to the same materials used as base ones for similar components applied in the construction of power generation plants.
The availability of an easy and reliable non-destructive testing method for ceramic matrix composites is a prerequisite for their widespread application. Failure may occur due to faults or damages in the manufacturing process; their detection is essential before a component’s integration into a complex system. An ultrasonic inspection method with some attractive advantages compared to other NDT methods has been developed and application-tested; it is fast, reliable, cost-efficient, applicable for more complex geometries, e.g. tubes. A high resolution 4-axis ultrasonic testing device is being used. In principle, there are no limitations to the component size, only the scannable thickness and the minimum detectable defect size are limited, mainly by the material’s porosity and the presence of delaminations in the structure, i.e. by potentially failure-relevant faults. Both flat and tubular specimens with a (wall) thickness of up to 10 mm were successfully tested. The technology could also be used for NDT of fibre reinforced polymers, metal matrix composites and other high end materials that are currently difficult to evaluate, but need a reliable quality control for application in safety-relevant fields where a failure might cause significant material damage or even damage to people’s health and life.
A set of ageing tests on all the SCHUNK and DLR materials developed and on the BoraSiC coating system were performed to evaluate the loss of weight and the decrease of the flexural strength. Additional high temperature, up to 1500°C, flexural and compression on C-Ring short term tests were performed. The strength results for filament winded materials at RT were evaluated by Weibull statistical analysis, finding relatively high Weibull moduli. The coating seems to be, generally, suitable for medium term oxidation environment. The presence of delaminations and voids inside the bulk of the CMCs render the strength and oxidation results quite spread. Some materials show a brittle failure mode (es. DLR BE5) in all testing conditions. A set of interlaminar shear strength (ILSS) by compression of notched specimens tests were performed on Schunk fabric based materials. The shear strength is quite high. The slow crck growth (SCG, dynamic fatigue) tests indicate that the CMC material isn’t significantly sensitive to sub-critical crack propagation, and there is no significant environmental influence on MOR at RT, in air. No significant cyclic fatigue damage is measurable up to 100000 cycles and at max. load below 70% of static MOR of the CMC. The damage at higher loads close to the failure can be indicated by change of the stiffness and increase of accumulated strain. Fatigue limits seems to be at high % of MOR. The cyclic fatigue damage on instrumented flanged and end capped tubes is negligible at pressure close to rupture, since the failure is mainly determined by strength limit at geometric discontinuities (flanges). The SCG and cyclic fatigue results were used to set up a life time prediction model of the tested materials.
Most of the sol/gel coating procedures enable only layer thickness of some nanometers per coating step. A method was developed within HITHEX, which offers the possibility to increase the coating thickness per step up to 500 micrometers. Such a technology is of industrial interest. Coatings with glass sealing layers can be realized in a one step or two-step procedure saving costs and time. Further improvements of gas tightness and density of the layers are necessary. It is expected that these problems can be solved by an optimisation of the sintering post-treatments.
PANOX end-caps and PANOX tubes were developed and joined together. Initial attempts to needle separately produced parts resulted in a weak joint. After several modifications, a method was develooped where the layers that build up the end caps were directly needled onto the tubes. Eight to ten cross shaped pieces were cut out of a PANOX felt and laid up on an antistatic foam mandrel (covered with a latex cap) to build up the end-cap. A poly-glycol-ester was used to keep the felt layers closely together. Additionally, a latex cap was pulled over the structure to hold the pieces together. Subsequently the felt pieces were needled directly onto the PANOX tube with a self developed device that pushed a single needle 120 times per minute into the pre-form and moved the needle along the whole end-cap. When the needle had reached the end of the sample, the sample was turned by a motor a little bit. This process was repeated until the whole end-cap was needled. One advantage of this new method is that the joint between end-cap and tube is much stronger (compared to making end-cap and tube separately and joining both pieces thereafter). A second advantage is that the production time can be significantly reduced. A possible application is the production of PANOX tubes with end-caps.
Rapid thermal cycling under oxidising conditions is a critical test for materials, which are intrinsically vulnerable to oxidation even when elaborate steps are taken to protect them. The manufacturing process of the composites of the HITHEX project inevitably leads to microcracks thereby making it necessary to develop advanced coatings to seal the surface and prevent ingress of oxidising species. Even the advanced coatings of this project have limited ability to achieve this objective, due to thermal property mismatches. IE-JRC has developed a rapid test to discriminate and rank the different coating/material combinations using a furnace designed to accomodate eventually complex environments. The furnace operates by transporting test materials into and out of the hot zone whilst maintaining the test environment. Dwell time at high temperature [1250°C, up to 1500°C is possible] of a single cycle was 5h, allowing a significant time for the oxidation protection to achieve its designed aim. Up to 30 cycles were used with maximum heating rates of 20K/sec. This test is complementary to the complex corrosion test.
Biomass and wastes are potential fuels to be used in plants for power and heat generation. Starting from a general study aimed at the evaluation of availability of residuals both from agriculture and from industrial processes and from municipal waste, a limited number of non-standard fuels has been identified. In particular, wood and municipal RDF have been pointed out as fuels for Externally Fired Cycles (EFC).
IE-JRC has developed facilities required for the measurement of residual stresses at elevated temperatures in monolithic and/or composite material specimens, of which at least one of its phases is of crystalline nature, based on novel neutron diffraction techniques. Two custom made furnaces have been developed for this purpose operating at inert atmosphere and vacuum respectively so as to avoid oxidation of investigated specimens. They both allow for temperature monitoring and control through 3 thermo- couples equally spaced along the gauge length. The first furnace (F1) operates up to 1200°C based on a Kanthal heating element and offers a specimen cavity of 50mm diameter and 100-mm height. The second (F2) operates up to 1450°C based on a Molybdenum heating element and is suitable for tubular specimens only. F1 is made of Alumina (Al2O3) and F2 is made of thin Quartz (SiO2) with a few layers of Sigraflex as thermal insulation. The chosen materials for these furnaces allow for minimum attenuation of the intensity of the incoming and diffracted neutron beams, which is crucial for performance of such testing. These furnaces have been procured for the testing of tubular CMCs relevant to the investigations of the HITHEX project. After completion of HITHEX the equipment will be available for high temperature testing of metallic alloys welded specimens in relation to structural integrity assessment of power plant components in the context of future activities at the HFR. Both furnaces have a relatively small outer diameter, which makes them particularly useful for such neutron diffraction measurements. This is a particular strength of furnace F2, which has only about two thirds of the diameter of F1. The large specimen cavity is an extraordinary feature of F1. There is hardly any other neutron facility in the world where residual stress testing at high temperatures could be performed in specimens of similar size. The restriction to tubular specimens is a disadvantage of furnace F2. Nevertheless, the concept of the quartz glass furnace has significant potential for further development. With the necessary design modifications such a furnace could equally well be used for bulky specimens. Furthermore, such a furnace can be procured at relatively low cost.
Externally fired cycles (EFC) for power and heat production have been studied and compared also with conventional and competing technologies. Thermodynamic analyses have been conducted: reference cases simulating different working conditions and representative of typical annual operation have been considered. The Indirect Fired Gas Turbine in simple cycle has been considered as a convenient solution for plant size equal or greater than 26MWth.
The mechanical behaviour of C/C-SiC low cost composites has been investigated in coupon and component (tube) scales at different temperatures ranging from 1250°C to 1400°C. Of primary interest was the investigation-characterisation of material’s creep behaviour and lifetime prediction starting from coupons and scaling up to components. It is suggested that creep is not the main damage mechanism for the loads, timescales and temperatures under consideration. Other mechanisms seem to play more dominant role like matrix cracking, fibre failure, and interfacial sliding. A model that captures the essential features of the exhibited behaviour is proposed and currently under development. For tube-like components where stress state is more complicated than coupons, damage state was assessed via strain measurements and experimental results are currently being processed. Based on the experimental results and the analysis that has been done so far, it is possible to construct a “roadmap” with guidelines and best-practice guidelines to aid the designer’s task when utilising such materials. Furthermore an existence of such a life-prediction model and design tool is very important for the industry. In its final version this tool will take the form of software that will be connected with other state of the art CAE tolls, like FEM codes, and will make the design of CMC components an automated interactive procedure.
CMC tubes with a length up to 900mm can be manufactured at DLR via the resin transfer moulding (RTM) technique. The tube fabrication functions with pressures of up to 20bar. The curing time depends mainly on the used phenolic-based resin system. The method starts with dry carbon fabrics which are wound on a steel core and which were then inserted in a mould chamber. External pressure is applied on the chamber, so that the resin can infiltrate in the fabric. The technique is advantageous for the manufacture of small tubes and sophisticated prototypes. The tubes exhibit high strength. The tubes could be pyrolysed without deformations. The final in-situ joining allows the production of one end closed tubes (end cap) with a flange. The final tubular dimensions (length, wall thickness, dimensions of flange and end cap) meet the requirements defined by the end-user.
The Oxidation Protection of CVD based coating systems on Carbon/Carbon and CMC (C/C-SiC) was improved by applying a plasma sprayed cordierite sealing layer (done by DLR). This combination has shown the best thermal shock resistance of all multilayers developed and/or tested within the HITHEX and the previous projects. Long-term oxidation tests are running to prove, if the system possesses a high potential for oxidation protection. Short-term oxidation resistance under extreme thermal shock conditions can be guaranteed with the new developed combined coating system.
IE-JRC has developed the methodology of complex corrosion testing on a laboratory scale to achieve the following technical objectives: - To test at high temperature, up to 1300°C, although the methodology and equipment is capable of increasing this to 1500°C, under quasi-isothermal conditions in a simulated combustion environment containing 6vol% water vapour, the oxidation/corrosion resistant properties of a wide range of ceramic composite materials. The test cycle is 100h at high temperature, heating and cooling being achieved by control of the furnace. - Because the test materials of this project are exceptionally sensitive to oxidation, the heating and cooling was carried out under an inert argon atmosphere to avoid oxidation damage during the non-isothermal part of the cycle. Total exposure times were up to 2000h and materials were withdrawn from testing after the recognisable breakdown of the oxidation/corrosion protection coating, as measured by weight loss during the inter-cycle down time. This test is complementary to the thermocyclic oxidation test, both of which taken together allow a ranking of different materials and protective coating systems.
The structural component, a heat exchanger tube, was manufactured in hard felt and fully converted to SiSiC. The procedure was successfully applied on different test tubes up to 300mm in length. Further process optimisation from viewpoint of design and homogenity of conversion are still necessary. The optimisation of the design, contributes to improved load capacity of the tubes.
Schunk started within the HITHEX project to manufacture tubes with carbon cloths via a winding technique. The materials were characterized and can be further developed and improved based on the test results. The winding of carbon cloths offers the potential of a high speed manufacturing technique for cylindrical composites with bidirectional reinforcement. Such reinforcements are required in case of internal pressure loads. The technique can be used for CFRP-as well as C/C-and CMC-components. Further improvements are necessary to use the full potential of the properties of carbon cloth. Reliable processes are feasible by an investment in an optimised winding-up device, which will not be realized within the project. The technique can be used after completion of the project and contributes to a broader manufacturing technology.
The radial strength test according to ISO 2739 can be applied for CMC as well as for CFRP (reinforced polymers) tubes. The test is independent from the wall thickness of the tube. Already tubular specimen with a length of 10mm, are sufficient to determine the radial strength by a tube compression test. The test delivers comparable strength results, which are used as quality assurance for the manufacturing process of CMC and CFRP tubes. The method was already successfully tested for three applications. The method cannot be used to determine material properties due to the complex loading conditions for the crushing test. The test can be simply applied on all cylindrical fibre reinforced tube samples.
Pressure testing has been conducted on C-C/SiC tubes available in the project (from SKT and DLR).
BoraSiC® coatings are a sandwich CVD coating system composed of SiC/B4C/SiC. The coating procedure was the first time applied on structural components like tubes with flange and end caps. Such a CVD coating has shown a good self healing potential under isothermal oxidation conditions combined with crack closing effects. Furthermore, the oxidation resistance at temperatures below 1000 °C was superior to all other systems applied on carbon/carbon based CMCs. The thermocyclic oxidation behaviour was tested under complex corrosion conditions. The system has shown a good potential. However, in presence of moisture the life time is limited by an evaporation of the formed boron oxide in the cracks.
Starting from the innovative non-reactive BraSiC® filler alloys developed previously by CEA, joints have been produced using several most suitable BraSiC® grades. These joints remain non-reactive against the CMC and the infiltration of the braze in the composite is limited. Depending on the process conditions, the properties can be tailored to match different requirements: decrease the global CTE of the joint to limit cracking and thus increase strength and tightness.
Ceramic Matrix Composites (CMCs) are highly attractive materials for a multitude of advances applications. Carbon fibres as reinforcing component gives them unique strength and a tailorable failure behaviour due to the well-defined fibre-matrix interface that is still an unsolved problem for other types of fibres. The vulnerability of carbon fibres by oxygen at elevated temperatures limits their application, unless an oxidation protection coating is applied. Various systems are available on the market and described in literature and patents. The thermal mismatch between coating and bulk material introduces severe stresses that are most severe under high and very high thermal gradients. ARCS has developed a testing method that does allow to assess the stability and lifetime of a coating under high thermal loads that are to be regarded as thermal shocks, where a typical heating/cooling rate may be in the range of some ten to a few hundred Kelvin per second. These severe conditions may not be predominant in standard operation, but might occur under some operation conditions. These results are required for CMC coating application in safety-relevant fields where a failure under non-standard operation conditions might cause significant material damage or even damage to people’s health and life.
Process and functional requirements for the HTHE e.g. entrance and exit temperatures, pressure of working gases, maximum pressure losses etc. The further outcome has been the identification of properties of structural bayonet tube materials.
Isothermal oxidation testing of C-C/SiC materials available in the project (from SKT and DLR) has been conducted on form flat samples at 950 and 1250°C.