Mocvd of Yttria Stabilized Zirconia on Co-Cr
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ABSTRACT Thin films of yttria stabilized zirconia (YSZ) were deposited by metal organic chemical vapor deposition (MOCVD) on cobalt-chromium (Co-Cr) alloys used for orthopaedic implants. TMHD derivatives were preferred for the metalorganic sources over the less expensive fluorinated beta-diketonates as the latter left up to 1% fluorine in the film, as determined by Auger spectroscopy. Crystalline cubic structure of these YSZ films was verified by X-ray diffraction. Columnar grain growth gave a rough surface on thicker YSZ films, which increased wear of the mating polyethylene component during simulation tests. YSZ films under one micron thick were mirror smooth and reduced wear of the polymer component by 70% compared to uncoated Co-Cr. Film adhesion was excellent. BACKGROUND More than 300,000 artificial joint replacements are performed in the United States each year.' The vast majority employ an ultra-high molecular weight polyethylene (UHMWPE) component articulating with a metal (usually Co-Cr or Ti-6A1-4V alloy) component. The UHMWPE is subject to wear. A high release rate of wear debris causes adverse tissue reactions, pain, reduces joint mobility, and eventually failure of the prothesis.' 3' 4 It is the object of this research to reduce this UHMWPE wear through the use of an improved friction reducing coating on the metal part of the joint. Zirconia was chosen for the coating because it is biocompatible and has lubricity properties. 5 Solid zirconia ceramics used for some protheses are heat treated to produce the tetragonal phase, which has improved mechanical properties (toughness or fracture strength) in bulk form. For reduction of surface wear, the harder but brittle cubic phase is preferred. Chemical vapor deposition (CVD) was used as the coating technique because it can uniformly cover intricate shapes, and because we have shown it possible to deposit cubic phase yttria stabilized zirconia at reduced temperatures that would not alter the mechanical properties of the substrate metal.6
PROCEDURE Chemical vapor deposition of yttria stabilized zirconia (YSZ) at Spire was described in reference 6. For this application, an experimental matrix was constructed to test the effect of varying experimental parameters on the deposition rate of the zirconia film, the ratio of yttria to zirconia, and the surface morphology. Response surface methodology was used to design the test matrix. 7
89 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
The part to be coated was placed in the high temperature zone of a hot wall reactor (tube furnace) which was purged, then heated to either 500 or 750'C. The samples were rotated slowly to improve uniformity. Gases were injected in one end, facing the sample, and exhausted by vacuum pump at the opposite end. Total mass flow of gas into the chamber was either one or three standard liters per minute (slpm) and the pressure was maintained at 5 torr. Argon was used as a carrier gas for the metalorganic source compounds and as a diluent. The oxidizer was either pure
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