Question

Explain the following observations: During the sliding of a cylindrical steel pin against a steel disc in a pin-on-disc test in air, the friction coefficient is initially ~0.7, but then falls to ~0.5 as the test proceeds. At the end of the test the wear track on the disc is rough and back in color, with a fine black powder on the disc surface outside the wear track An identical test is conducted in vacuum. The friction coefficient remains high through the test, the wear volumes are much greater, and the wear track on the disc surface after the test is very rough and silvery in color Another identical test is conducted at 400 C (high temperature) in air. At the end of the test, the wear track on the disc is black in color and very smooth and shiny. No debris on the disc surface outside the wear track are observed.

Answer 1

- To estimate the coefficient of baiction (cof) and the wear mechanum of tribological surfaces, pin-on-dis sliding were test is a commonly used tei bological surfaces charac recization technique At the begining of the test, the measured wellrent of friction in high and as further procuds, cof deops. This behavious may have impact on feu aspects as followers in 4 It is typical to friction measuements. 2. It can be atteibuted to the change in Sueface topography 4 Chemical reactions taking place until the system comes to à steady state. This steady state cof in finally reported. During this test, wear debris is deposited on dic surface as fine black powdee. This test track looks lough as the material is eroded from the disc.

2 cor decreases with an increase in relative humidity as this tei bological test is affected by moisture or humidity present in the air. Hence, cof is higher in case of pin-on-disc test in vacuur Because of this, the weal volume is also higher than in air. Therefore, the wear track looks very cough and silvery. 3 At Gigher temperatures, the weal mechanism changes to adhesive mear because it is affected by temperature. There was no debeis on the disc surface possibly due to sintering of the weae debeis to foem compact dayees under the effect of high local compressive force. No debris on the disc sueface outside the wear track are obseewed at lowee temperature as the formation of this compact layer is influenced by temperature.