Partner und Internationale Organisationen
(Englisch)
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AUTh, NETCOM, BAM-LTFI, VIOLEX-BIC S.A., TEER COATINGS LTD, ISA JOBIN-YVON S.A
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Abstract
(Englisch)
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The objectives of this project were to develop and optimize coatings for different applications and, in order to achieve the optimum coating properties and the highest reproducibility, an in-situ spectral ellipsometry system was to be developed and adapted to the requirements of the coating systems. The ellipsometer should be implemented to the coating facilities.
The applications of the partners investigated in the frame of this project were coatings on razor blades, crystal quartz oscillators and bearings and seals. Since the requirements were quite different, a range of coatings were applied and investigated.
Sulzer's work
Two coating systems were investigated: TiNx films on quartz and stainless steel substrates and a graded DLC coating with Ti, TiN, TiCN and TiC interlayers on quartz and stainless steel. No coating was deposited on cermet substrates as mentioned in the proposal, because of problems with the reproducibility of the system. The graded DLC build-up corresponds well to the Thornton model. At low bias and low pressure a columnar structure was observed. By increasing the bias, a denser and harder coating could be produced due to the incorporated residual stress. Preferentially at high bias levels spallation occurred. The TiN coatings behave similar to the graded DLC. A correlation between the OEM-setting and the Ti/N ratio was not observed. At low bias levels the TiOx in the upper most layer was high due to the 'open' structure. A high adhesion was measured. To further improve the performance of the coating, a new nanolaminated system was investigated. Based on the multilayer concept, TiN/NbN coatings were deposited. Characterisations have shown extremely high Young's modulus, hardness and critical load. These multilayer coatings have been investigated and further optimised, some of these results will be presented later. In the final stage of the project, SiC-coatings with high hardness could be produced in a statistical screening experiment. The coatings were characterised by the following methods: dynamic microhardness measurement, scratch test, ball crater method, glow discharge optical emission spectroscopy GDOS and SEM.
Wear tests of coated bearings under service conditions were conducted with different coatings. After a short time of running fretting occurred on the DLC-coated part. New tests with a polymeric counterpart showed that depending on of the roughness three different wear mechanism could be distinguished. The main problem was the heat transfer away from the contact zone. Due to the high temperatures the polymeric counterpart had melted at the surface.
Corrosive wear tests on TiN coatings were carried out. Surface analytical investigations (AES, SEM, AFM) showed that both surface morphology and composition of TiN are affected by the applied potential and rubbing. Further tribocorrosion experiments showed the beneficial role of a Ti interlayer in improving bonding and corrosion behaviour of coated systems.
Parts of the small, oil-free, reciprocating, high pressure compressor, developed by Sulzer Burckhardt for NGV-applications (natural gas for vehicles), were coated with MoST and Graphit-iC coatings. The parts were 20 pistons and 10 cylinder sleeves of the fourth stage compressor head. The adhesion of both coating types on the pistons was good. In contrast, the coating on the inner side of the cylinder sleeves showed signs of spalling. The latter was attributed to lack of uniformity in film thickness associated with small sleeve tolerance. As it turned out, it is rather difficult to coat cylinder sleeves with sufficient adhesion and coating uniformity and homogeneity with PVD technology. This is attributed to the unfavourable aspect ratio of inner circumference to cylinder length. Therefore, these inner surfaces had to be coated with DLC by plasma enhanced CVD.
The MoST-coatings exhibited only disappointing performance in natural gas. The test of a MoST coated piston running against a-DLC coated cylinder was stopped after 1500 hours due to performance loss of the compressor. Wear enhanced by tribooxidation was observed. The same coating of the piston sliding against an uncoated steel cylinder scuffed after only 20 minutes.
A piston coated with Graphit-iC is still being tested under real conditions on a dedicated compressor in Sulzer's laboratories. The piston has run against a DLC cylinder sleeve produced by plasma enhanced CVD for about 3000 hours without a break. This bench test will be continued until the performance of the compressor decreases.
One of the major aims of this project has been the development of an in-situ ellipsometer and the implementation of this device to Sulzer's PVD-facility. The in-situ spectroscopic ellipsometry has reached a stage of development which makes application possible in universities and institutes, where the appropriate know-how and expertise is available. Also, certain industrial production processes, where the substrates to be coated offer the required flat and smooth surface, e. g. in microelectronics, and where the process and coating parameters do not change too often, can benefit from the process monitoring and control provided by in-situ ellipsometry.
For most PVD-coating facilities, where many different substrates with different shapes and surface properties are treated each day, a number of difficulties and problems are not yet solved. Some of these problems, e.g. alignment of apparatus and samples, proved to be serious obstacles for the implementation of in-situ spectroscopic ellipsometry on these facilities.
Unfortunately, this turned out to be the case with Sulzer's facility. Therefore, instead of the implementation of the in-situ ellipsometer to this facility and in agreement with the project co-ordinator, a feasibility study was conducted to confirm this opinion and to identify areas where further development is necessary.
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