One of the most common failure modes for a friction interface is the accumulation of glaze on the friction material surface. Until recently, our analysis of glaze chemistry has always been consistent with the degradation of detergent, antiwear and extreme pressure additives in the oil. These additive degradation products are readily identified by the presence of Ca, P, S and Zn in an EDS analysis. In these cases the loss of friction performance, characterized by a gradual fade in friction coefficient and the concomitant development of a negative friction-speed gradient, is directly related to the loss of surface porosity due to the accumulation of glaze on the friction material surface.
Over the past few years, the drive for better fuel economy in passenger cars has led to the introduction of lower viscosity oils possessing high viscosity index and shear stability. We have observed that these fluids also can lead to glaze accumulation on the friction surface. However, for these new oils, under certain conditions, the glaze is a consequence of the viscosity modifier additives. The association of the glaze to this additive type is more difficult to establish due to the absence of unambiguous indicator elements. To correctly identify the additive chemistry responsible for this glaze we have used Evolved Gas Analysis; a technique involving the volatilization of the glaze and subsequent identification using a coupled Gas Chromatograph - Mass Spectroscopy technique.
In this paper we contrast the glaze chemistry arising from conventional fluids (traditional ATF viscosity) with that of the newer, lower viscosity fluids. We provide data suggesting that the failure mode is not only related to the accumulated loss of surface porosity but also the deactivation of the friction surface (loss of surface active sites). We then discuss how this new fluid chemistry influences the formulation of new friction materials.
