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Preliminary tribological evaluation of nanostructured diamond coatings against ultra-high molecular weight polyethylene.

BACKGROUND
Some loss of joint prostheses has been attributed to osteolytic loosening associated with debris from wear of polyethylene articulating against metal alloys. Reduced polyethylene wear has been reported with ceramics serving as an alternative counterface.


METHODS
Nanostructured Diamond (NSD) coatings were deposited onto Ti6Al4V by microwave plasma-assisted chemical vapor deposition, with both hydrogen-rich (H-NSD) and helium-rich (He-NSD) feedgas mixtures. Pin-on-disk wear tests of polyethylene against NSD and CoCr were performed in serum lubrication at body temperature. Scanning electron microscopy was used to examine surface morphology, and nanoindentation was used to determine hardness and modulus of the polyethylene wear surfaces. Raman spectroscopy, surface roughness, and wettability analyses of the NSD coatings were performed.


RESULTS
Raman spectroscopy confirmed sp(2) and sp(3) bonded carbon in the NSD coatings. No significant differences in wear factors were found between polyethylene on H-NSD, He-NSD, and CoCr, despite higher roughness and friction coefficients for the He-NSD and H-NSD coatings, compared with CoCr. Contact angles for the diamond coatings were reduced following the wear tests, indicating that these surfaces became more hydrophilic. Numerous small protuberances were observed on pins articulated against CoCr, and a single, large protuberance was observed in polyethylene-on-NSD. These features were conjectured to be reconsolidated polyethylene particles. Nanoindentation modulus and hardness of the worn polyethylene surfaces were lower for polyethylene-on-diamond than for polyethylene-on-CoCr.


CONCLUSIONS
As a counterface to polyethylene, NSD-coated Ti6Al4V produced wear factors comparable to CoCr in the present pin-on-disk tests, a promising step towards its use in joint replacement bearing applications.