Tribology Online Vol. 10(2015) No. 1

Oxidation degradation of gamma-irradiated UHMWPE (Ultra-high Molecular Weight Polyethylene) is a well-known problem related to the failure of total joint replacement. The purpose of this study is to investigate the effects of oxidation on the mechanical and wear properties and to identify the main causes of the change of wear mechanisms of crosslinked UHMWPE (50 kGy and 100 kGy) after long-term shelf-ageing. The techniques used were FTIR mapping and micro-indentation test, which were performed on the same cross-section of UHMWPE specimen. Three distinct regions (the surface region, the more oxidized subsurface region, and the less oxidized center region) were prepared to carry out the differential scanning calorimetry (DSC) and multi-directional wear tests. The worn surfaces were checked by optical microscopy and scanning electron microscopy to reveal the wear mechanisms. The experimental results showed that the micro-hardness, elastic modulus and crystallinity increased with the increase of the oxidation index. The wear resistance deteriorated when the oxidation index increased. The slope of specific wear rate against oxidation index of the 100 kGy sample is higher than that of the 50 kGy sample. Our results may indicate that, when the oxidation index is lower than the critical threshold, the wear mechanism is mainly dependent on crosslinking density; when the oxidation index is higher than the critical threshold, the oxidation behavior plays an important role in the change of wear mechanisms.

A Magnetohydrodynamic effect on porous composite slider bearing lubricated with couple stress fluids is numerically analyzed. A modified Reynolds equation has been derived taking into magnetic field in the couple stress fluid lubrication of wide porous composite slider bearing. The closed forms of expressions are obtained for the non-dimensional fluid film pressure, load carrying capacity, frictional force and coefficient of friction. It found that when a magnetic field is applied to each particle experience force which causes the colloidal homogeneous suspension to move en masse significant MHD effects causes the fluid pressure, load carrying capacity, frictional force and coefficient friction. The effects of permeability parameter are depicted in the various figures. The results are compared with nonmagnetic case.

Gas Phase Lubrication (GPL) is used here to lubricate counterparts of a tribological system with a partial pressure of a gas. Because the rheological contribution (Hydrodynamic and EHL) of gas is practically negligible, the role of tribochemical reactions is of prime importance to achieve a good performance. Different sites of the contact zone are subjected to chemical reactions: inside-contact acid-base reactions, plasma-induced reactions in the vicinity of the contact area and nascent surfaces in the extended part of the worn surface. Here we highlight the interest of GPL in modeling boundary lubrication.

Today, in the world of materials the focus is on the development of metal matrix composites (MMC’s), particularly aluminium based composites due to its widespread applications in aerospace and automobile industries. To develop light weight MMC’s, carbon fibres in both continuous and dis-continuous forms are used as reinforcements. The processing and characterization of mechanical and adhesive wear behaviour of various composites are widely practiced by various researchers around the world. However, solid erosion wear of continuous carbon fibre reinforced MMC’s has not yet been reported. In the light of the above, the present work focuses on characteristics of erosive behaviour of Al6061-carbon fibre rods composites developed by casting route. Carbon fibre rods of 4 mm and 6 mm diameters were surface sensitized to achieve electroless nickel coating followed by copper electroplating on the nickel coated rods and were used as the reinforcements to develop the composites. An air jet erosion rig made according to ASTM G76 standard configuration was used to conduct the tests. The effect of carbon fibre reinforcement, test duration and angle of impingement of the erodent on the erosion wear behaviour of the developed will be discussed at length.

Considering the influence of the fluid-inertia item on dynamic transmission characteristics during soft-start of hydro-viscous drive (HVD), theoretical models were derived with the aim of revealing the effect of engagement pressure, lubricant viscosity, viscosity-temperature characteristics, surface roughness and the ratio of inner and outer radius of plates on total torque and viscous torque. The results indicate that the increase of engagement pressure and lubricant viscosity shortens soft-start time and increases peak total torque in the stage of mixed lubrication. The decrease of surface roughness and effective contact area of plates can both shorten soft-start time. And yet two cases of time to reach peak total torque are in different stages. Fluid-inertia item included can help to avoid fluctuation of the total torque while increasing lubricant viscosity or reducing surface roughness. It can increase soft-start time obviously when considering viscosity-temperature characteristics. In short, the research reveals the influence rules of the fluid-inertia item on dynamic transmission characteristics of HVD; additionally, fluid-inertia item included contributes to reduce oscillation of the total torque and the decline in efficiency. Therefore, the research results lay a theoretical foundation for the design of high-power HVD device.

This paper describes an experimental study of starvation and flow behavior in grease-lubricated elastohydrodynamic contact. Rolling tests were conducted with four grease samples with different thickeners and base oils in a conventional ball-on-disk test rig. The EHL central film thickness was determined with SLIM, the Spacer Layer Imaging Method, and grease flow around the conjunction and the flow pattern on the track of the disk specimen were observed with CCD cameras. The grease track included some thickener deposited on the track and a corrugated fingerlike flow pattern outward from the center of the track. It was found that the flow pattern varied with grease type and test conditions, and that the average interval between fingers decreased with the entrainment speed. At higher speeds, inlet starvation occurred and the finger pattern was deformed and gradually ruptured. The speed of the finger-loss was higher than the speed at which starvation started. It was also found that the starvation speed was greater with greases having lower apparent viscosity at lower shear rate and higher apparent viscosity at high shear rate. This implied that the replenishment and entrainment of the greases depended on the viscosity characteristics of the greases.

3 different base-stocks (poly-α-olefin, polypropylene glycol and unsaturated TMP ester) and additionally formulations with 2 different additives (1 mass% ZnDTP or 1 mass% dibenzyl disulfide (DBDS)) were used as lubricants to evaluate the tribological properties of A6061-T6 treated by means of plasma electrolytic oxidation in a SRV tribometer (T_oil = 80°C, F_N = 50 N, ν = 50 Hz, 2 mm stroke, 2 hrs). The base oils of PPG and the TMP-ester had better lubricities than the PAO with/without the additives. ZnDTP tend to reduce wear, but when added to PAO and PPG increased the friction. The unsaturated TMP-ester +1 mass% DBDS showed 45% lower friction than PAO + ZnDTP (reference), whereas PPG + DBDS had 30% lower friction.

In the starved lubrication regime, the film thickness can be much less than the fully flooded film thickness. The film thickness reduction is related to the inlet meniscus distance. The inlet meniscus and the film thickness can be computed for a given inlet film thickness distribution which is generally non-uniform. The inlet film thickness distribution, which is essentially the outlet film thickness distribution of the previous contact, is affected by the surface tension. In this paper, a modified Coyne-Elrod boundary condition is used to calculate the inlet film thickness distribution, taking the surface tension into account. A numerical analysis of EHL point contacts is carried out and solutions of the inlet meniscus and the film thickness are presented. At lower capillary numbers, side bands formed by reflow in the outlet region can replenish the contact owing to reflow in the inlet region. At higher capillary numbers, the side bands move away from the center of the track and the inlet distance decreases. This is a main cause of starvation in EHL point contacts. A good agreement is found between the numerical solutions and experimental results.