Rolling contact fatigue (RCF) testings of thrust ball bearing under boundary lubrication condition (low lambda condition) are conducted while changing test conditions of load, rotational speed and combination of surface roughnesses for bearing component parts. The RCF test results and measurements of surface roughnesses of before and after the tests show that the raceway surface with larger roughness promotes the surface initiated failure of the other component. In addition, they demonstrate that the behavior of change in surface roughness during rolling contact depends on the test conditions and influences RCF life accordingly. These results suggest that RCF life of rolling bearing used under low lambda conditions should be estimated considering not only analytical relationship between repeated stress in subsurface and surface roughness during RCF, but also experimental database of the running-in behavior depending on RCF conditions.
Presence of micrometer-scale textures on a surface can modify the friction properties of that surface. In this study, we investigated the fundamental tribological properties of Cu-Sn porous sintered metal with the rounded rectangle dimple texturing under hydrodynamic lubrications. The results of block-on-disk tribological tests and in-situ oil flow observations revealed that the presence of dimple texturing on the sliding surface increases the friction coefficient of porous specimens. From the visualization of oil effused regions on porous surfaces, it was observed that the position of the maximum peak of oil film pressure distribution is shifted toward the outlet direction of oil flow as a result of dimple texturing. In case of porous sintered metal, addition of dimple texturing enhanced the absorbance of lubricating oil into the interconnected pores and caused a reduction in the oil film thickness.
In the tribology of mechanical elements using lubricating oil, verification of the dynamic surface pressure distribution during sliding operation is necessary in order to elucidate the sliding condition. The same also applies to the piston rings which are the sliding part of compression reciprocating device. Although some verification case in static condition or limited sliding area has been studied, there are many unknown aspects in this field. The thin-film sensor, since it has small influence on the shape of the sliding surface, is widely used as a measurement method of the sliding surface pressure between two different surfaces, however this method has never been applied to the piston ring in the past. In this study, various film deposition methods on the piston ring of the thin-film sensor were examined, and the thin-film pressure sensor with a total thickness of 4.6 μm was successfully formed on the sliding surface by sputtering. The surface pressure distribution of the sliding part was measured using the reciprocating sliding tester by this thin-film pressure sensor, and the measurement values were in good agreement with the mixed lubrication model between the piston ring and the cylinder liner.
Viscosity index improvers are polymeric additives that are commonly used to control the viscometrics of lubricants. Nevertheless, there have been few studies on the molecular size of these additives, which is the origin of their viscosity-increasing effects. We herein show the fundamental differences of the topological features of two types of viscosity index improver molecules: olefin copolymers and polyalkylmethacrylates. The hydrodynamic radii of these viscosity index improvers were estimated from their intrinsic viscosity with varying molecular weights. The measured relationships between the hydrodynamic radii and molecular weight were compared to the theories of polymer chain mechanics. The behaviors of olefin copolymers likely correspond to the classical theory of polymer chains with excluded-volume effects. Polyalkylmethacrylates showed features of both an ideal chain and an excluded-volume chain, depending on the temperature, molecular weight, and side chain heterogeneity. The hydrodynamic radii of olefin copolymers decreased with increasing temperature independently of their molecular weight. Meanwhile, the hydrodynamic radii of polyalkylmethacrylates increased with increasing temperature in a molecular weight-dependent manner. The coil expansion of polyalkylmethacrylates might result from the extension of an effective bond angle and transition from an ideal to an excluded-volume chain.
For predicting the sliding wear in Pin-on-Disc (PoD) tribometer contact, a numerical wear simulation technique is presented in this paper. It is based on the Finite element method (FEM) which incorporates the Archard's wear law and the UMESHMOTION subroutine for calculating the wear depth for a 3-D PoD tribometer contact. FEM is utilized in solving the 3-D contact problem. The geometry is updated using the UMESHMOTION subroutine coupled with the Augmented Lagrangian-Eulerian (ALE) remeshing technique of ABAQUS. But, a significant disadvantage of FEM wear prediction is the enormous computational time required for performing 3-D analysis. Hence, to minimize the computational time, an approximation technique is introduced which accounts for the contact pressure evolution at the contact region. It decreased the simulation time and also preserved the accuracy of 3-D wear prediction. Finally, the results obtained from the simulations are compared with the experiments for brass-on-bearing steel PoD contact. An accuracy of 98.81% was obtained for the 10N and 83.10% for the 30N load.
To improve the transmission efficiency of a continuously variable transmission (CVT) system, it is effective to form a tribofilm with higher friction coefficient. Therefore, understanding of the correlation between the tribofilm formation and friction coefficient in a CVT fluid has been a great deal. In this study, the tribofilm formation on a bearing steel ball surface under the simulated additive oil was investigated. We used ball-on-disk tribological test and optical microscopy imaging to observe the temporal changes in the tribofilm growth. Atomic force microscopy and energy-dispersive X-ray spectrometry analyses revealed that the tribofilm consisted of pad-like structures derived by tricresyl phosphate and calcium sulfonate additives. A binary imaging analysis showed that the total area of the pad-like structure was a key parameter determining the friction coefficient. The pads were repeatedly desorbed and reformed during the sliding test. These results improve the design of future CVT systems and lubricants.
We have seen small black spots or craters on the surface of worn machine parts. They are considered to be caused by discharges of static electricity. However, there is no clear evidence of being caused by discharges of static electricity, as nobody can see the phenomena when discharges are caused there. Using TEM (Transmission Electron Microscope) and SEM (scanning electron microscope), the authors examined the black spots which were found on the lubricated surfaces of machine components and the ones which were artificially caused by spark discharges on a surface of a test material. The results of this study showed that the grains of black spots on the lubricated surface of worn machine parts were almost identical to those which were artificially caused by discharges of static electricity. Moreover, melted wear debris that might have been formed during spark discharge, was found in an oil filter, showing that there was high temperature melting the metal.
The temperature dependence of friction coefficient of lubricant oils containing MoDTC was evaluated using a ball on disk tribometer at constant temperatures of 25, 40, 60 and 80°C. In the experiments, the friction coefficient gradually decreased and tended to become lower at steady state. The decreasing slope of the friction coefficient was steeper and the lower friction coefficient at the steady state was obtained at a higher oil temperature. The friction coefficient was also controlled by the concentration of MoDTC in oil. Considering that the results of lubricating tests supported the successive processes involving production and wear of MoS2, the friction coefficient can be controlled by the coverage of MoS2 which is determined by a balance of formation and wear of MoS2. An analysis formula was established based on kinetic assessment of the process in order to obtain the Arrhenius plots of the test results. There were two regions which had different slopes in the plot. The activation energies were 70 and 4 kJ/mol for the low and the high temperature regions, respectively. This result suggested that MoS2 production from MoDTC was reaction rate-limiting at low temperature and transport ratelimiting at high temperature.
Concentrated polymer brushes (CPBs) are a class of thin polymer films that are composed of polymer chains densely grafted to substrate surfaces. Due to their excellent tribological performance derived from the highly stretched conformation of the polymer chains in suitable solvents, CPBs are expected to be applicable as tribological coatings in sliding components. However, a long swelling time is required to obtain fully swelled CPBs in solvents. To overcome this issue, we propose a simple method involving thermal treatment of CPBs in solvents. Our results showed that this method can dramatically reduce the swelling time of CPBs from more than 48 h to less than 3 min. Moreover, thermal-treatment-CPBs (TT-CPBs) can provide an extremely low friction coefficient (0.002) under the boundary lubrication regime at the TT-CPB/steel tribo-pair interface. All our results suggest that the thermal treatment method is a practical method that will allow the industrial application of CPBs.