Brittle to ductile transition of tribomaterial in relation to wear response at high temperatures

A. Viat; G. Guillonneau; S. Fouvry; G. Kermouche; S. Sao Joao; J. Wehrs; J. Michler; J. F. Henne

Wear 392-393 (2017) 60-68

The third body formed in a contact between HS25 cobalt-based superalloy versus ceramic under fretting wear (small reciprocating displacements) was investigated. This tribomaterial, commonly called “glaze layer”, was created from nanosized, compacted and sintered wear debris and adheres on both rubbed parts. The glaze layer was investigated both from tribological and rheological points of view. In terms of tribology, the glaze layer was found stable above 450 °C, providing low friction and very low wear rate in the interface. To study the mechanical properties of the third body, micropillars have been FIB-machined within the glaze layer and compressed between 25 °C and 500 °C. Low temperature testing showed a brittle and hard behavior for the glaze layer which was confirmed by nanoindentation. By contrast, glaze layer at 500 °C evidenced a perfect ductile response with high strain rate during fretting and no brittleness. Made of 10–20 nm nanocrystals embedded in a ceramic-metallic amorphous matrix, the glaze layer is put forward in the brittle to ductile transition: the amorphous matrix may act as a metallic glass. This allows the correlation between glaze layer ability to plastically accommodate the strain without fracturing processes, thus without being damaged through debris generation. The ejected debris flow is stopped and the wear rate becomes negligible.