THE INFLUENCE OF COATING THICKNESS ON LUBRICATED ROLLING-CONTACT FATIGUE LIFE

The effect of coating thickness on the fatigue life of TiN-coated rollers under lubricated contact was investigated with a twin-disk machine. TiN films of various thicknesses (0.25-5-mu-m) were deposited on AISI 4118 steel rollers by high rate reactive sputtering. The ratio of the spalled area (A(sp)) to the observed area (A(ob)) was used as the fatigue damage index for different coating thicknesses at selected cycles. The experimental results revealed that coating thicknesses of 0.25-mu-m give the best fatigue resistance. On the basis of the test data, fatigue life charts of coated contact are established. The behavior of spalling initiation is described according to the stress state and the coating thickness. The relatively thick coatings have larger initial spalls. The process of spalling propagation can be illustrated by the progressive measurement of spalling areas as related to length of rolling contact cycles. The spalling propagation curve shows a slow propagation mode followed by a fast growth to contact failure.