Lubricating film thickness measurements with bovine serum

Lubricant film thickness measurements were made for bovine serum solutions under steady state rolling and sliding. The effect of low (30 MPa) and high contact pressures (200 MPa) was examined. In the high pressure rolling tests BS initially formed films 5–50 nm thick over the speed range. However, in subsequent speed sweeps, a relatively speed independent film of 40–50 nm developed. In some cases thick (up to 100 nm) films were formed at low speeds; this behaviour was considered representative of high-viscosity surface layers rather than of solid films. At the end of each test residual boundary films of 9–19 nm were measured under static loading. These are attributed to the multilayer adsorption of protein molecules and will provide surface protection during stance or on initiation of gait. The results at low pressure showed that much thicker films (∼60–80 nm) were formed over the same speed range. Again thicker films were formed at the lower speeds. There was significant scatter in the film thickness results, possibly due to the inherent nature of the fluid, which is an inhomogeneous biological solution. The film thickness/speed behaviour was not representative of a simple Newtonian fluid and this has considerable implications for the development of predictive film thickness models and new designs of artificial hip joints.