This paper describes an experimental study of energy dissipation at the normal contact of solids by plastic deformation of surface asperities and adhesion. Experiments were conducted in which ball and plate made of AISI440C were cyclically pressed against each other in helium, hydrogen gas and air. It was found that energy dissipation occurred with no evidence of adhesion, and the plastic deformation of surface asperities was a main factor of the energy dissipation. Energy dissipated in the first contact, and it continued in the subsequent contact cycles. This implied that the asperities did not shakedown in the first contact under macroscopically elastic contact. Larger energy dissipation in the first contact led to larger energy dissipation in the repeated contacts. It was concluded from these results that substantial energy dissipation occurred by plastic deformation of surface asperities under normal loads that exceeded the shakedown limit.
In order to clarify the sliding friction phenomena between a diamond slider and a copper specimen in the air, a series of the molecular dynamics simulations and the friction experiments using a friction force microscope were examined. In the simulation model, both the adhesion effects obtained through the nanocontact experiments in the air and the effects of the oxide film generated in the uppermost specimen surface were considered by using a simple method with just controlling the dissociation energy in the Morse potential function. From the comparisons between the simulation and the experimental results, it is confirmed that they have a good similarities in the friction coefficient, in particular, in its sudden increase when the oxide film is broken by wear. The breaking mechanism of the oxide film originating from the relatively higher hardness of the oxide film is also clarified by observing the slip deformation state of the copper specimen.
The present study describes the tribological properties of bronze which contains a micro-sized sulfide phase consisting of copper, sulfur and iron. Bronze without a sulfide phase was fabricated through a sintering process, and was prepared for comparison purposes. The tribological properties were evaluated using a face-to-face-type testing apparatus under dry conditions. The counter specimens were three chromium alloy steel balls with a flat surface. Results showed that the friction coefficient of the sulfide-containing bronze was lower, and anti-seizure properties significantly greater, than the bronze without sulfide. Furthermore, the friction coefficient was low and stable until the end of experiment. The transfer layer on the friction surface was different depending on the initial composition of the disc surface. It is inferred that frictional resistance decreases when the transfer layer contains sulfide components on the dry tribological properties. Therefore, it is to be concluded that the dispersion of a micro-sized sulfide-iron-copper phase into bronze is an effective means of reducing the occurrence of seizures in the bronze.
The lubrication performance of polyisobutylene which is used as a lubricant for bolt tightening, was investigated in order to understand the lubrication mechanism during the tightening process. Ball on plate type spinning friction tests were conducted, and three kind of polyisobutylene(PIB-L, M, H) in which the highest viscosity is PIB-H, followed by PIB-M and PIB-L, and polyphenyl ether(5P4E) were used as lubricants in this study. Results show that the friction coefficient of PIB at initial stage of the tests at 400 MPa, depends on the viscosity, and that of PIB-H is the highest in the lubricants. It is also shown the friction coefficient increase with increasing contact pressure, and that of PIB-L, PIB-H and 5P4E at 600 MPa is greater than that of PIB-M. Large wear is observed on the contact surface after the test. In the spinning test, the thickness of entrapment oil film decrease with time, and the decreasing rate in 5P4E is greater than that in PIB-M. These results suggest that the lubrication film formed by PIB-M has an ability to prevent for wear and seizure at the condition with high contact pressure, such as the contact of screw tightening.
Engineering plastic materials like polyamide and polyacetal are widely used for worm wheels and other tribological elements under EHL conditions with grease; typical examples are found in electrical power steering systems and power window mechanisms. Low elastic modulus of those materials makes their contact pressure much lower than steel-to-steel contact, and EHL of those contacts is called “soft EHL”. The current concern of this technology is higher reliability and lower friction losses, which mean maintaining sufficient EHL film thickness and reducing traction in tribological terms. The present work tries to measure soft EHL film thickness and traction of oil and grease by an optical interferometry system, in which a steel ball was made contact with a transparent polycarbonate disk having a similar elasticity as the above-cited engineering plastic materials.The results show that prediction is possible by using the analyses by Dong and Qian for the film thickness and by Muraki and Kimura for the traction coefficient.
It is well known that a main cause of tightening error on the torque control method is changes of the frictional coefficients in thread surfaces and bearing surfaces in bolt tightening. It has been shown in the previous study that the frictional coefficients depend on surface roughness of contact surface, lubrication conditions and bolt configuration errors. The frictional coefficients can be changed by the bolt configuration errors such as a bend of bolt shank and inclination of bolt bearing surface, if large wear and seizure do not occur on the lubricated surfaces.In this study, an influence of the squareness error of bolt bearing surface on the tightening accuracy has been investigated. Results clearly showed that the squareness error of bearing surface causes variations of torque coefficients and the clamping force error. It was shown by FEM analysis that the variation of torque coefficient between bearing surfaces is caused by change of an apparent frictional coefficient. Tightening accuracy also depended on the squareness error of bearing surface.