Filtern
Dokumenttyp
Sprache
- Englisch (6)
Referierte Publikation
- ja (6) (entfernen)
Schlagworte
- Brake (1)
- Coating (1)
- Coating, debris (1)
- Corundum (1)
- Cross-sectional transmission electron microscopy (XTEM) (1)
- Cytokines (1)
- DLC (1)
- Dispersing behavior of particles (1)
- Electron microscopy (1)
- Friction layer (1)
Ti/TiN multilayer coatings with multilayer periods in the range 550 nm and a final thickness of 2 µm were deposited on steel substrates by cyclic modulation of nitrogen gas flow into the chamber of a PVD sputtering device. Coating characterization was performed by cross-sectional transmission electron microscopy, glancing-angle X-ray diffraction and instrumental indentation testing. Individual a-titanium and titanium nitride layers were always observed, although for the finer microstructures, the TiN layers were thicker than the Ti layers by a factor three. The plastic hardness of the films increased steadily with decreasing layer spacing, following a HallPetch relationship. Finally, a hardness value of 42 GPa was reached, which is similar to that of a thick TiN monolayer, prepared under the same coating conditions.
Tribological screening tests (simple, reciprocating ball-on-flat tests) were performed with the objective to identify an appropriate coating for the articulating surfaces of artificial hip joints whose acetabular cups and femoral stems are made from Ti6Al4V alloy, which is appreciated for its light weight, good biocompatibility and elastic properties similar to those of natural bone. Standard coatings like TiN or CrN performed better than more complicated multi-layer systems, though not as good as different types of amorphous carbon coatings, generally referred to as diamond-like carbon or DLC coatings. Among the latter, hydrogenated amorphous carbon (a-C:H) displayed the best properties, especially if the hydrogen content was increased by reducing the bias voltage during PA-CVD-deposition.
The optimised a-C:H coating revealed the most promising wear behaviour under the applied testing conditions, i.e. the increase of linear wear with the number of cycles was close to zero. Regarding the materials examined in this study, correlation of wear with mechanical properties obtained by nano-indentation revealed that high hardness was not an adequate criterion for selecting appropriate coatings. A high ratio of hardness and elastic modulus (H/E) proved to be more important. Microstructural and micro-analytical investigations revealed transformation of TiN and CrN to TiO2 and Cr2O3, respectively, and amorphous carbon was, at least partly, transformed to graphite. Furthermore, incorporation of Al2O3 from the ball was observed at a very fine scale. The wear debris of favourable coatings always formed agglomerates of nano-scale particles. It was shown that commercial nano-particles of Al2O3, Cr2O3 and carbon black are comparable to particles generated by the tribological tests. However, it is uncertain whether they are comparable to those formed during simulator studies or in vivo. Furthermore, the performance of the favourable coating has to be tested in a hip joint simulator before its potential for application in prostheses can be assessed.
An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO2), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO2-x Magnéli phases and/or graphite.
The chemical and microstructural changes occurring during braking simulation tests at the surface of a conventional brake pad material were investigated mainly by scanning and transmission electron microscopy and surface analytical techniques. It can be shown that patches of a third body material develop, comprising a compositional mix of all constituents of the pad and iron oxides from the disk. Milled debris particles still have the crystal structure of barite, the major phase of the pad material, but the grain size is reduced drastically to the nanometer scale. The major wear mechanism is delamination of filler particles from the organic binder, supported by local degradation of the phenolic resin during asperity heating. Quartz crystals are preserved thereby adopting the function of primary contact areas.