Sliding Friction Phenomena of a Ball-on-Disc Contact under Different Lubricating Conditions

Experiments were carried out to measure the friction of a ball-on-disc contact in a simple sliding experiment under different lubricating conditions using a Universal Micro Materials Tester (UMT-2). The measurements were also taken for the direct contact between the ball and the disc. The sliding speed was between 5.0 x 10(-6) m/s and 1.0 x 10(-2) m/s. The load of the contact ranged between 1 N and 40 N giving the nominal maximum Hertzian contact pressures of 0.59 GPa to 2.03 GPa. It was found that when the sliding speed was below 1.0 x 10(-5) m/s, the contact is in boundary lubrication and the measured friction coefficients for all the lubricants could be considered as independent of the sliding speed. It is suggested that the boundary film-contact interfacial slippage in the loaded zone of the asperity contact between the two surfaces is responsible for this friction phenomenon and the friction coefficient is determined by the boundary film-contact interfacial shear strength. Under the same load condition, the boundary film-contact interfacial shear strength for the 68 mechanical oil was found to be the highest; that for the 32 mechanical oil was higher than that for the PB2400 oil; while that for the purified and deionized water was the lowest and significantly lower than that for the other three lubricants. The friction coefficients measured for the direct contact between the ball and the disc were found to be close to those for water lubrication. When the sliding speed was above 1.0 x 10(-5) m/s, the friction phenomena, taking into account sliding speed variation, was found to be quite different for the different lubricants. For the applied loads, the measured friction coefficients can be considered as independent of the load. From this phenomenon, it is suggested that the asperity contacts formed between the coupled surfaces in the experiment are almost in full plastic deformation mode. (C) Koninklijke Brill NV, Leiden, 2011