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Diamond-like carbon coatings may decrease implant wear, therefore, they are helping to reduce aseptic loosening and increase service life of total knee arthroplasties (TKAs). This two-part study addresses the development of such coatings for ultrahigh molecular weight polyethylene (UHMWPE) tibial inlays as well as cobalt-chromium-molybdenum (CoCr) and titanium (Ti64) alloy femoral components. While the deposition of a pure (a-C:H) and tungsten-doped hydrogen-containing amorphous carbon coating (a-C:H:W) as well as the detailed characterization of mechanical and adhesion properties were the subject of Part I, the tribological behavior is studied in Part II. Pin-on-disk tests are performed under artificial synovial fluid lubrication. Numerical elastohydrodynamic lubrication modeling is used to show the representability of contact conditions for TKAs and to assess the influence of coatings on lubrication conditions. The wear behavior is characterized by means of light and laser scanning microscopy, Raman spectroscopy, scanning electron microscopy and particle analyses. Although the coating leads to an increase in friction due to the considerably higher roughness, especially the UHMWPE wear is significantly reduced up to a factor of 49% (CoCr) and 77% (Ti64). Thereby, the coating shows continuous wear and no sudden failure or spallation of larger wear particles. This demonstrated the great potential of amorphous carbon coatings for knee replacements.

Diamond-like carbon (DLC) coatings have the potential to reduce implant wear and thus to contribute to avoiding premature failure and increase service life of total knee replacements (TKAs). This two-part study addresses the development of such coatings for ultrahigh molecular weight polyethylene (UHMWPE) tibial inlays as well as cobalt–chromium–molybdenum (CoCr) and titanium (Ti64) alloy femoral components. While a detailed characterization of the tribological behavior is the subject of part II, part I focusses on the deposition of pure (a-C:H) and tungsten-doped hydrogen-containing amorphous carbon coatings (a-C:H:W) and the detailed characterization of their chemical, cytological, mechanical and adhesion behavior. The coatings are fabricated by physical vapor deposition (PVD) and display typical DLC morphology and composition, as verified by focused ion beam scanning electron microscopy and Raman spectroscopy. Their roughness is higher than that of the plain substrates. Initial screening with contact angle and surface tension as well as in vitro testing by indirect and direct application indicate favorable cytocompatibility. The DLC coatings feature excellent mechanical properties with a substantial enhancement of indentation hardness and elastic modulus ratios. The adhesion of the coatings as determined in modified scratch tests can be considered as sufficient for the use in TKAs.

Depositing MoS2 coatings for industrial applications involves rotating the samples during the PVD magnetron sputtering process. Here, we show that a 3-fold substrate rotation, along a large target–substrate distance given by the deposition unit, introduces porosity inside the coatings. The mechanical properties and wear behavior strongly correlate with the degree of porosity, which, in turn, depends on the temperature and the rotational speed of the substrate. Ball-on-disk tests and nanoindentation wear experiments show a consistent change in tribological behavior; first, a compaction of the porous structure dominates, followed by wear of the compacted material. Compaction was the main contributor to the volume loss during the running-in process. Compared to a dense coating produced without substrate rotation, the initially porous coatings showed lower hardness and a distinct running-in behavior. Tribological lifetime experiments showed good lubrication performance after compaction

Depositing MoS2 coatings for industrial applications involves rotating the samples during the PVD magnetron sputtering process. Here, we show that a 3-fold substrate rotation, along a large target–substrate distance given by the deposition unit, introduces porosity inside the coatings. The mechanical properties and wear behavior strongly correlate with the degree of porosity, which, in turn, depends on the temperature and the rotational speed of the substrate. Ball-on-disk tests and nanoindentation wear experiments show a consistent change in tribological behavior, first, a compaction of the porous structure dominates, followed by wear of the compacted material. Compaction was the main contributor to the volume loss during the running-in process. Compared to a dense coating produced without substrate rotation, the initially porous coatings showed lower hardness and a distinct running-in behavior. Tribological lifetime experiments showed good lubrication performance after compaction.