Failure of Steel Couples Under Boundary Lubrication: Influence of Steel Composition, Microstructure, and Hardness

If oil-lubricated sliding or rolling couples are overloaded, separating oil films can be broken through so that the friction coefficient increases severely. Thereby an instantaneous failure of tribosystems can be caused. This type of failure was investigated by means of an Amsler-wear-test-machine with two contacting and rotating disks that were immersed into a tempered oil bath. The load was increased in steps until failure was indicated by an increase of the coefficient of friction. The results reveal a strong influence of the chemical composition and of the microstruture of the steels while hardness does not show an influence. Failure is restricted if protective reaction layers can form on the stressed surfaces. As most of the alloying elements reduce the formation of such layers failure load is lowered by them. Unalloyed ferrite is able to form thick reaction layers causing a high failure load. Less reactive components of the microstructure like cementite, martensite, and above all austenite decrease the failure load.