Sliding friction of a ball-on-plane contact with low surface hardness and different bulk lubricant temperatures

摘要

The friction coefficients of a ball-on-plane contact in simple sliding lubricated by different lubricants were measured for the provided standard hard steel ball and the soft plate made of the Q235 steel with the surface hardness of HRA 45-47 (Rockwell hardness A scale) by using a Universal Micro Materials Tester (UMT-2). The measurements were also respectively taken for the direct contact between the ball and the plate and for 32 mechanical oil with bulk oil temperatures of 26, 70 and 110°C. In the experimental conditions, the contact was in boundary lubrication. It was found that the measured friction coefficients can be considered as independent both of the contact load and the sliding speed; the reduction of the hardness of the softer contact surface significantly increased the friction coefficient. These show that the asperity contacts formed between the 2 contact surfaces actually carrying the load were in fully plastic deformation. This confirms the occurrence of the plastic deformation of the asperity contacts in a practically mixed lubrication. The boundary film-contact interfacial slippage should be responsible for the independence of the friction coefficient on the sliding speed. It was also indicated that the boundary lubricating film may be locally ruptured in some asperity contacts for high bulk lubricant temperatures.