Skip to main content

Selective tribological optimisation of fluid kinetics and efficiency by laser surface structuring

Final Report Summary - STOKES (Selective tribological optimisation of fluid kinetics and efficiency by laser surface structuring)

Executive Summary:
The project aimed on the optimisation of hydraulic units by means of a laser process. This laser process modifies the conditions of high stressed contact surfaces in order to enhance the tribological performance of the entire system. The addressed hydraulic systems are products of SMEs from all across Europe. Thereby the project’s overall objective was to strengthen the European SMEs by improving their technological resources and enhancing their products value. This objective should be reached by a significant increase of the efficiency of the pumps, which at last leads to more economic operating pumps with extended life time.

A consortium had been established, which covers the laser supply and technique as well as the surface preparation technology. Manufacturers of hydraulic parts were members of the consortium in order to close the technological range. Two powerful RTD performers could have been gained, which are specialized on the laser processing on the one hand and on tribology on the other hand.

The first period of the project has been used for installing the necessary manufacturing systems, the laser structuring machine on the one hand and the tribological test systems on the other hand. Within the second period, which is in focus here, the experimental work on laser micro structuring and tribological investigations has been conducted. Main focus were the design and manufacturing of micro structure elements on different samples for tribometer tests and the investigations on the influence of those micro structures on friction. Therefore demonstrator systems have been selected from the production portfolio of the participating suppliers, such as pump components (for rotor pump and gear pump) and rubber seals. On basis of the design and working conditions of those demonstrators test samples were designed.
A block-on-ring friction test system for radial bearing applications has been installed, a thrust-washer test system for axial bearing applications, a pin-on-disc system for applications with plane contact and a special test system for investigating life time and sealing performance of seals. The according test samples to be laser structured were pins and discs (thrust-washer and pin-on-disc system) as well as cylindrical shafts (block-on-ring). These components were made from steel and aluminum – according to the underlying real systems – and have been laser structured at accessible surface areas. In opposite NBR rubber seals were laser structured, since a real part test system has been utilized, which offered avoiding simplified sample testing. In addition to the very different material, the important contact areas to be structured were located in the inner of the circular shaped seals. Ablation behavior of the various materials under ultra-short pulse laser irradiation has been studied and strategies have been developed enabling the laser treatment of each sample geometry.
In accordance to experience published by several researchers within the last years, simple micro geometry elements, such as dimples, have been designed as well as more complex structure elements, exhibiting asymmetries, which were aligned to or against a main stress direction. Thus, a so called “single effect” has been investigated, caused by each micro geometry element in opposite to the “collective effect”, caused by the entity of micro structure elements.
Unlike to surface laser structuring of large continuous surfaces areas, like e.g. design structures, tribological micro structures consist of repeating single elements. In order to enable automated manufacturing, a software tool has been developed, which is capable of combining laser tool path data for single micro geometry elements with work piece macro geometry. Hence, program data for laser structuring systems can be obtained automated, while control commands for laser sources can be adapted according to utilized systems.
By comparing short pulse and ultra-short pulse laser process and systems, an economic evaluation has been conducted in order to supply integration concept for the demonstrator manufacturing chains.



Project Context and Objectives:
Economic losings caused by wear and friction are still tremendous, just in Germany the losings are amounted to 100 bn € p.a. for Europe the losses exceed 400 bn €. Investigations within the last years have shown that laser manufactured structures can exert considerable influence on the tribological behavior of surfaces. Besides hydrodynamic effects, which can improve friction, the ability of the structures to store lubricant lead to the maintenance of a lubrication film.

As the state of the art techniques for laser surface structuring, particularly for tribological applications are mainly on an a R&D level, the production technology is in need of adequate manufacturing techniques. Main topics in this field of research are the inevitable pre- and post-treatment steps of current laser surface structuring techniques as well as the high process durations. The overall goal of this project was to solve both of those tasks by the development and realization of a process technology, which enables the process chain integrated laser surface structuring of hydraulic parts. The project aimed to cover a defined segment of a growing market and the technological achievements will offer the participating SMEs promising options of upgrading their product values. In addition to the direct improvement of single systems by the investigations on demonstration parts within the project, the high transferability of the technique to further products should enable the value enhancement of whole product classes. This can offer the possibility of a strong enhancement of the total product output.


The S&T objectives of the project were:
1. The development of an ultra-short pulse laser surface structuring (USPLSS) process in order to obtain the possibility of manufacturing complex micro structures on the surface of free formed surfaces.
2. The verification of the tribological functionality of these laser manufactured surface micro structures and such the supply of a comprehensive technological overview of micro structures and their influence on tribological performance. The functional surfaces should include the entire manufacturing history by including the initial surface of the samples into the tribological investigations.
3. The derivation of concept requirements for integration of the laser process into production lines in order to supply a basis for utilization in mass production.


Project Results:
The first period of the project has been used for installing the necessary manufacturing systems, the laser structuring machine on the one hand and the tribological test systems on the other hand. Within the second period, the experimental work on laser micro structuring and tribological investigations has been conducted. Main focus were the design and manufacturing of micro structure elements on different samples for tribometer tests and the investigations on the influence of those micro structures on friction. Therefore demonstrator systems have been selected from the production portfolio of the participating suppliers, such as pump components (for rotor pump and gear pump) and rubber seals. On basis of the design and working conditions of those demonstrators test samples were designed.

Laser ablation studies were carried out in order to determine process limits and manufacturing strategies for a large range of micro structures. The laser structuring was conducted on a modified 5-axes-milling machine. A picosecond laser (λ=532 nm, 1064 nm) has been installed into the machine together with an entire beam guiding system, including beam shaping lenses, mirrors and a galvanometer scanner. Additionally the laser scanner offers 2 high speed galvanometer axes (X,Y) and one optomechanical (Z) axis. For the purpose of structuring complex parts in the project, the laser structuring system has been upgraded with tilt- and turning axes. Thus a multi-axes (5 mechanical + 3 optical + 1 virtual) laser structuring was enabled.

The recent works contained the design of the CAD/CAM strategy for laser structuring. A software tool has been developed, which conducts the distribution and alignment of the single structures an surface areas. These surfaces are being reduced to basic geometric bodies, such as cylinders, spheres, cones or parts of those. In this case the positioning of the single structures can be conducted procedure based. For general tasks an additional option fort the single structure distribution is designed and will be implemented. An additional strategy for complex shaped surfaces has been defined: Complex shaped functional surfaces can be subdivided into almost equilateral triangles using triangulation algorithms. The corners of those triangles define the position of the single structure element on the surface. By this means structure elements can be distributed on arbitrary surfaces homogenously, although this alignment does not follow a certain pattern.

The easiest standard method for forming micro dimples is by laser ablation is to bring single laser pulses on the surface at the target location. In case of SP laser ablation, micro dimples of several microns depth and several tens of microns diameter can be formed with only one pulse. According to the results from ablation investigations, this method is not applicable for processing steel with USP laser. A modified approach therefore has been developed in the present work. If the laser expels a number of pulses while focused on a defined destination without movement, a sequence of pulses hit the same area leading to a multi-pulse ablation structure. The shape of this structure is defined by the laser beam diameter and intensity distribution. In easy words, this approach is based on the before investigated line ablation without lateral movement. In this approach the dimple depth can be adjusted by the laser power and the number of pulses brought on one location. The number of pulses is a result of laser on time and pulse repetition rate frep. The diameter can be varied by varying the working distance and thus the effective laser beam diameter. It became obvious during the experiments that this approach can be used to produce dimples with a diameter similar to the beam diameter, which is 15 to 20 µm. Dimples over 30 µm did not fulfill tolerance criteria, meaning that the shape was not regular enough. Based on these results general methods for dimple formation have been investigated.

1. Spiral design:
By programming an Archimedic spiral, which is characterized by the radius growing proportional to the revolution angle, dimple geometries with selectable diameter can be obtained. The laser beam thereat is led from the inner to the outer, so that the last path defines the outer contour. This approach has the advantage of many reversal points – one at each end of a linear path. Since the laser beam is moved by a galvanometer scanner, high velocities of several meters per second are applicable. Such high velocities require extremely high acceleration rates at the beginning and end of each path, leading to a distortion of the target geometry. The effect can be avoided using low beam velocities – this however is against the overall goal of an efficient production. Thus it has been confirmed to use the spiral strategy. The depths can be varied not only by varying laser power but also by varying the beam velocity and/or the number of cycles.

After the investigations on steel it turned out that the spiral strategy is not feasible for NBR, because the ablation rate is that high, that the minimum ablation depth is much higher than the desired one. Figure 25 shows a dimple formed using the spiral method at lowest laser power and 200 mm/s. The effective ablation volume in general can be reduced by increasing the laser beam velocity. However there are upper limits coming from the curvature of the path and the resulting deviation due to small radii at high velocities. A separate study on NBR has been performed to find out these limits.

A square shape has been ablated at low power, so the resulting line is sufficient sharp. It became clear that with increasing laser beam velocity the accuracy of the contour decreases. Sharp corners, as they were programmed here, are the highest challenge for a continuous movement. Already at velocities exceeding 500 mm/s the corners loose the sharp contour. However up to 1000 mm/s the shape itself is acceptable. At significant higher velocities even the linear slope is not in tolerance. At smaller structures, as required for micro dimples, it turned out that velocities higher than 200 mm/s are not feasible. Thus it was concluded to limit the beam velocity at 200 mm/s for micro dimple formation.

2. Dot composition:
To cope with the high ablation rate at lowest laser power a separate manufacturing strategy has been designed, based on the above described standard method for small dimples without laser beam movement. With controlled laser-on time frames small ablation depths can be reached at the irradiated location “dot”. By combining several of these dots a larger connected dimple can be formed.. By adjusting the distance between the single dots, the resulting dimple radius can be varied. The depth can be adjusted by laser power or by the number of single pulses in each dot, which means by the duration of the laser-on time frame. There is a limit of lateral distance between the pulses, above which the resulting dimples exhibit irregular shape. Dimples up to 100 µm could be formed, which was sufficient for the present work.

3. Complex structures:
In addition to the study on high symmetric dimples complex structures should be investigated. Target was the study on micro cavities, which exhibit a symmetry axis with defined orientation to the stress condition. The target application in this project is a hydrodynamic supported shaft, in which the stress has a main direction, like a gear shaft in gear pumps. The idea than is to form cavities which are capable of supporting the stress endurance in one specific direction.
The manufacturing of these cavities has been conducted similar to the spiral based dimples. Of course a higher programming effort is necessary. The cavities have been designed semi-manually leading to single programs for each type and size of cavity. The programing has taken into account the results from the results of the two methods presented in the previous chapters. Three kinds of cavities have been manufactured: Elliptical, triangular and rectangular ones. In addition the rectangular cavities have been formed with inclined ground level.


A block-on-ring friction test system for radial bearing applications has been installed, a thrust-washer test system for axial bearing applications, a pin-on-disc system for applications with plane contact and a special test system for investigating life time and sealing performance of seals. The according test samples to be laser structured were pins and discs (thrust-washer and pin-on-disc system) as well as cylindrical shafts (block-on-ring). These components were made from steel and aluminum – according to the underlying real systems – and have been laser structured at accessible surface areas. In opposite NBR rubber seals were laser structured, since a real part test system has been utilized, which offered avoiding simplified sample testing. In addition to the very different material, the important contact areas to be structured were located in the inner of the circular shaped seals. Ablation behavior of the various materials under ultra-short pulse laser irradiation has been studied and strategies have been developed enabling the laser treatment of each sample geometry.

In accordance to experience published by several researchers within the last years, simple micro geometry elements, such as dimples, have been designed as well as more complex structure elements, exhibiting asymmetries, which were aligned to or against a main stress direction. Thus, a so called “single effect” has been investigated, caused by each micro geometry element in opposite to the “collective effect”, caused by the entity of micro structure elements.
Unlike to surface laser structuring of large continuous surfaces areas, like e.g. design structures, tribological micro structures consist of repeating single elements. In order to enable automated manufacturing, a software tool has been developed, which is capable of combining laser tool path data for single micro geometry elements with work piece macro geometry. Hence, program data for laser structuring systems can be obtained automated, while control commands for laser sources can be adapted according to utilized systems.
By comparing short pulse and ultra-short pulse laser process and systems, an economic evaluation has been conducted in order to supply integration concept for the demonstrator manufacturing chains.

Potential Impact:
The potential impact of the project's results result mainly from the disseminated output, which is in detail described in Section A. Oral presentation have been held at conferences with highest impact and reputation in the fields of machine tools, tribology and laser proseccing. Some results and project contents already could be published. More results will be published soon. Among these were/are:

Press release in 2012 by Fraunhofer

"Laser Structuring for Tribological Performance Enhancement", published in LaserTechnik Journal (Wiley) 2012

Demonstration activities regarding laser machine, process and manufacturing strategies, already introduced at "Aachener Werkzeugmaschinenkolloquium 2011" and in more detail (going to be) presented in "Aachener Werkzeugmaschinenkolloquium 2014".

"Funktionsgerechte Strukturen auf Werkzeugen und Formen mittels abtragender Laserverfahren" (engl.: "functional designed structures on tools and dies by laser ablation techniques". Oral presentation at congress »Photonische Prozessketten – die Revolution in der Produktion?« 2012, Frankfurt

"Enhancing surfaces’ friction performance – micro structures in the context of surface conditions". Future presentation at conference: WFriction, wear and wear prediction 2014" in Karlsruhe, Germany. Already registerd and abstract published.

Exploitation:
The exploitation will be conducted in the near future by close partnership between the consortium participants. Based on project results the supplied integration conecpts will be evaluated. If possible, test systems will be applied to a single step of production. Knowledge of micro structure performance ans well as the software tool can be used by the partners to enhance performance of a wide spectrum of products.
List of Websites:
Web. http://www.laserage.ie/Section_2.html

Andreas Janssen
Tel: +49 241 8904 446
Email: andreas.janssen@ipt.fraunhofer.de

Dr. Leo Sexton
Tel.: +353 (0)87 608 0919
Email: leo@laserage.ie
www.LaserAge.ie

Dr. Amaya Igartua
Tel. 0034680656085
E-mail: amaya.igartua@tekniker.es