On the Growth Rate of Tribomaterial in Bovine Serum Lubricated Sliding Contacts

Abstract

Considering total hip arthroplasty, so-called tribolayers (aka tribomaterial), consist of carbonaceous material from the periprosthetic joint fluid or bovine serum mixed with nanometer size metal and oxide wear particles. Currently, its growth sequence and rate are unknown. Thus, smooth surfaces of low-Carbon (LC-) vs. high-Carbon (HC-)CoCrMo (Cobalt-Chromium-Molybdenum) alloys have been worn in a conforming contact under bovine serum lubrication by means of a pin-on-ball wear tester. These tests were interrupted at certain numbers of cycles in order to weigh the specimens, characterize the topography, and investigate the wear appearances. In addition, after cleaning in ethanol and anionic detergent, before-and-after comparison rendered the weight of the tribomaterial. This revealed that, during run-in, the specimens gained weight by generating tribomaterial. Afterwards the loss of material surpassed the generation of new tribomaterial and a steady weight-loss was measured. Topography measurements were used as input data for contact mechanics calculations. Apparently the incipient, locally high contact stresses accelerated tribochemical reactions. After run-in, the contact situation changes and leads to a much smaller generation rate. This paper provides information about the growth sequence and rate of such tribomaterial formation. It further highlights the significance of highly localized contact stress as an important factor for tribomaterial generation.

In-situ generated tribomaterial in metal/metal contacts: Current understanding and future implications for implants

“…Grain boundaries act as obstacles for shear and lead to overfolding. MoM laboratory experiments also support these findings, that most of the tribomaterial is generated during run-in [66]. During steady state this nanostructured composite layer is worn by surface fatigue brought about either by blunt asperities (delamination), or by rotating nanosize wear debris (indentation), or it reaches its critical thickness and spalls off as it is known for tribochemical reactions.…”

“…Obviously the loading history of any specific small spot within the contact area that is analysed by high-resolution techniques must be known. The extreme localization of tribological events at small wear rates [66, 95, 96] renders different appearances within very small geometrical scales.…”