Recently, metal on metal hip prostheses have been revived following excellent clinical results. However, the risk of wear diseases has not been removed yet. Fluid film formation is necessary for long term reliability of a total hip prosthesis because solid contact of the prosthesis induces serious wear. However, the fluid film can be only formed under limited conditions together with an appropriate shape design. Radial clearance is also an important factor because the radius has an upper limit in the joint space. A radial clearance that is too small induces severe contact, whereas decreasing radial clearance enhances squeezed film formation. The most appropriate radial clearance currently remains unclear. We prepared some specimens with high accuracy of polishing, which were balls and cups of the Co-Cr-Mo alloy with a common radius of 16 mm and several radial clearances, 0.15-95.8 μm. Their lubricating ability was evaluated by a friction test using a pendulum machine. The frictional coefficient was a local minimum where the radial clearance was 20-30 μm. It was found that appropriate radial clearance of a total hip prosthesis with hard on hard material must exist within this range.
This paper presents the theoretical study of squeeze film lubrication between porous parallel stepped plates with couple stress fluids. The lubricant with additives in the film region and also in the porous region in modeled as Stokes couple stress fluid. The modified Reynolds equation is derived for porous parallel stepped plates with couple stress fluids. The closed form expressions are obtained. According to the results obtained, the influence of couple stresses enhances the squeeze film pressure, load carrying capacity and decreases the response time as compared to the classical Newtonian-lubricant case. The load carrying capacity decreases as the step height increases.
The scuffing process was in-situ observed in a sliding contact between a rotating sapphire disc and a stationary steel ball under dry condition and lubricated condition. The in-situ observations indicate that under both dry condition and lubricated condition scuffing is initiated as a result of the accumulation by wear particles. To further understand the role of wear particles in scuffing initiation, wear particles and contact area from sliding tests terminated prior to scuffing initiation and also just after scuffing initiation were examined. The shape and composition of the wear particles both prior to and after scuffing initiation were obtained. The profile, composition, and micro-hardness of the steel surface prior to scuffing initiation were also obtained with special interests on properties of the particle agglomerates formed on the steel surface. The role of wear particles in scuffing initiation was discussed based on these observations.
The tribological properties of self-assembled monolayers (SAMs) on Au were investigated with a focus on the effects of the structure and end groups of the molecules upon the frictional properties of the modified surfaces of the SAMs. Three kinds of organothiols were used: HS-(CH2)11-(O-CH2-CH2)3-OCH3: EG3OMe, CH2-(CH2)15-SH: HDT, and OH-(CH2)16-SH: HDTOH. The SAM surfaces were characterized by contact angle measurements of pure water, X-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. The frictional properties were examined using a pin-on-plate tribometer. The friction coefficients of the SAM surfaces were similar to the bare Au surface in the high normal load region. In the low normal load region, HDT SAMs exhibited lower friction coefficients than HDTOH SAMs. The end groups affected the friction coefficients for the same chain structures. The friction coefficients of the EG3OMe SAMs were lower than those of HDT SAMs. The HDT SAMs were thought to have well-organized structures. On the other hand, the EG3OMe SAMs probably had more flexible structures than the HDT SAMs because of their ethylene glycol chains, indicating that the chain structures as well as the end groups affected the friction coefficients of the SAMs.
Some of high-pressure rheology of solidified lubricating greases and their base oils were estimated up to 3.5 GPa by metal microsphere deformation analysis in a diamond-anvil pressure cell (DAC) under the quasi-static compression condition. From aluminum sphere deformation in the greases, the solidification pressures of lithium greases were obtained and were almost the same as those of their base oils. Whereas, the solidification pressures of urea greases were slightly higher than those of their base oils and lithium greases. The solidification pressure was about 0.6 GPa for the traction grease, whereas it was 1.5-1.8 GPa for both PAO greases and MAC greases (space lubricants). Estimated traction coefficients from copper sphere deformation were 0.06-0.07 at 0.6 GPa-2.5 GPa for the traction grease and 0.03-0.04 around 3 GPa for both PAO greases and MAC greases and few difference was found out between greases and their base oils for all the lubricants.