Tribological Investigations of Cobalt-Based Alloys in Metal-on-Metal Contacts Using a Reciprocating Cylinder-on-Flat Apparatus with Bovine Serum Lubricants

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Abstract
Retrieval studies have shown higher wear of metal-on-metal hip implants, on average, than found in simulator testing. The investigation of a simplified metal-on- metal contact under conditions of boundary lubrication might provide some insight into this behavior. Thus, the purpose of the present study was to perform exploratory tribological investigations using a linear reciprocating cylinder-on-flat apparatus under such conditions. Wrought cobalt-based alloys of high and low carbon content were tested in four lubricants, most containing bovine serum, for 3600 or 36000 cycles. The motion was sinusoidal with a stroke length of 13 mm. The load per cylinder was 50.5 N giving an average contact stress similar to metal-metal hip implants in simulators and in vivo. Some elementary analyses of lubrication conditions and the influence of mechanical vibrations caused by the tangential dynamics of the apparatus were presented. The main results consisted of friction and wear measurements. In the limited number of experiments performed, it was observed that alloy carbon content did not influence friction or wear for up to 36000 cycles, nor did additives in the bovine serum lubricant designed to discourage calcium-type deposits on the surfaces. Wear factors were calculated and provided an approximate prediction of some simulator wear in the initial period. Increasing the bovine serum content of the lubricant caused the friction to rise significantly thus suggesting a role of serum elements in metal-on-metal tribology. As a consequence of the work reported here, the application of cobalt-based alloys with low carbon content was encouraged and the use of the additives to suppress surface deposits was recommended in simulator testing.

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