Microstructure of tribologically induced nanolayers produced at ultra-low wear rates

Wear in modern tribological systems is characterized by ultra-low rates in the order of a few nanometers per hour. Even under low wear conditions, morphology, crystalline arrangement, and chemical bonds of surface and subsurface are modified significantly and considerable plastic deformation occurs up to 1 μm depth. We characterize these changes with the help of tribometer experiments employing the radionuclide technique and transmission electron microscopy (TEM). The tribological conditions lead to complex and strongly inhomogeneous layered structures. The top zone of the subsurface has a nano-crystalline structure with an average grain size of about 10 nm containing ferrite, austenite, and cementite phases. Below the subsurface material consists of a work-hardened, fine-grained structure. Our experiments lead us to conclude that the observed combination of a nano-crystalline top layer and a work-hardened subsurface favours high wear resistance and low friction.