Lubricants, Volume 4, Issue 4 (December 2016) – 4 articles

It is crucial that orthopaedic implant materials are tested in conditions that replicate the natural body’s environment as closely as possible. Bovine serum is currently recommended for use by the International Organisation for Standardisation (ISO) for the wear testing of these implant materials, however, the rheological properties of bovine serum do not match fully those of the body’s natural lubricant, synovial fluid. This study looks at a potential alternative to bovine serum for the testing of orthopaedic implant materials; 0.5% gellan gum fluid gel. Wear tests using multidirectional motion were conducted on ultra-high molecular weight polyethylene (UHMWPE) pins rubbing against stainless steel plates. Roughness measurements were performed during testing along with particle analysis of the testing lubricant. At two million cycles (equivalent to 121.3 km of sliding), the mean wear factor for the four UHMWPE pins was 0.25 (standard deviation (SD) 0.03) × 10⁻⁶ mm³/Nm and there was no evidence of any transfer film on the plate surfaces. The wear factor produced by 0.5% gellan gum fluid gel was lower than that measured in previous studies using bovine serum as the lubricant but greater than the wear factor shown in published work using a similar alternative lubricant (sodium alginate mixed with gellan gum). Work on the development of a suitable alternative lubricant to bovine serum will continue. Full article

Lubricating greases show a structural degradation due to friction stress. The shear process dissipates energy. This results in a breakdown of the thickener structure, heat and entropy generation. Tribo-systems are energy driven systems. The stressed lubricating grease is modeled as a subsystem and presents an open thermodynamic system. Investigations were made to obtain more information about the correlation of system entropy and structural degradation of a lubricating grease. Experimental studies were done to estimate the role in terms of entropy transport for the open system. The degradation-entropy theorem was applied with the help of an empirical model to describe the correlation between degradation process and entropy production for the special case of a closed and stationary system. Full article

Finely dispersed copper nanoparticles were added as an additive to fully-formulated engine oils. The copper additive was in colloidal form, with an inner core of Cu²⁺ atoms covered by surfactants to form stable reverse micelles that are completely dispersible in the base oil. The tribological process to form protective films at the metal surface is comprised of three phases. Phase I can be considered a physical process involving the build-up of polar molecules by absorption to produce a friction modifier film, whereas phases II and III have to be treated as mechanochemical processes comprising a combination of redox reactions and a third body formation. The tribological performance was investigated using atomic force microscopy, a microtribometer, a pin-on-disk tribometer in combination with continuous and high-resolution wear measurements with radionuclide technique, and high pressure stressing in a thrust roller bearing test rig. In addition, the nanostructure of the additive was characterized by atomic force microscopy. Finally, the chemical composition of the metal surface was analyzed using photoelectron spectroscopy. Full article

The articulation performance of silicon nitride against conventional and highly cross-linked polyethylene, as well as for self-mated silicon nitride bearings, was examined in a series of standard hip simulation studies. Wear rates for polyethylene liners against silicon nitride femoral heads were consistent with reported literature, although higher than cobalt chromium controls. Excessive protein precipitation was a confounding factor in interpretation of the wear data. Post wear-test Raman spectroscopy of the cross-linked polyethylene liners showed no oxidative degradation. Wear of self-mated silicon nitride was found to be essentially zero and indistinguishable from alumina controls using continuously orbital hip simulation for up to three million cycles. However, introduction of an alternative loading profile from three to five million cycles, including a stop-dwell-start sequence, significantly increased wear for two of six silicon nitride couples. This behavior is associated with formation and disruption of a gelatinous silicic acid tribochemical film, and is consistent with a recurrent transition from fluid-film to boundary lubrication. Overall, these results suggest that silicon nitride articulation against dissimilar counterface surfaces (e.g., highly cross-linked polyethylene) is preferred. Full article