The formation of a cobalt-based glaze layer at high temperature: A layered structure

Abstract

The purpose of this study is to investigate the protective glaze layer structure formed during high temperature fretting wear process. The studied material is a cobalt-based alloy with a Cr-Ni-W solid solution (HS25) fretted against an alumina sample. It appears that a protective third body is spontaneously created at the interface for temperatures above 400 °C. The excellent tribological properties of the glaze layer leads to a stabilized friction coefficient of 0.3 and a quasi no-wear regime.Microstructure observations at the micro- and nanoscale were performed and revealed that the high temperature glaze layer is not homogeneous but rather composed of three layers with different grain sizes and chemical compositions. The investigations show that the effective glaze layer, the layer able to resist to fretting wear, is a thin layer made of nanocrystalline cobalt oxides. However, the chemical analysis reveals that before the formation of the effective glaze layer, the interface displays a high content of chromium oxides. Hence, a chemical segregation happens leading to the formation of a cobalt-rich effective glaze layer on the top of the former chromium-rich layer. In the light of these results, a chemical mechanism involving the consumption of chromium during the running-in wear stage is proposed to describe the glaze layer formation.