CrC·CaF2—A PVD stratified coating for friction and wear under heavy load at high temperature

Abstract

This study is to enable a cobalt superalloy to work at 700°C in air under a normal load of 5500 N corresponding to a nominal pressure of 25 MPa. Preliminary experience shows that the cobalt superalloy does not have sufficient mechanical and tribological properties to support this load under sliding conditions. In fact, both the applied normal load and the induced tangential force rapidly damage the sliding surface. So it is necessary to modify its surface properties. The present study used a stratified coating with a hard layer made of chromium carbide (CrC) and a soft layer made of calcium fluoride (CaF2) for improving the alloy's tribological properties. All the coatings were deposited by PVD. The CrC layer has a thickness of 3 μm, and the CaF2 varies from 0 to 3 μm. In order to assess the tribological properties of the coatings, a reference couple consisting of a triboalloy PS212 to PS212 as well as three couples consisting of CrC to CrC, CrC·CaF2(0.6 μm) to CrC·CaF2(0.6 μm) and CrC·CaF2(3 μm) to CrC·CaF2(3 μm) have been tested at 500 and 700°C at ISMCM-CESTI using a specific high temperature tribometer FHTO (Frictiographe à Haute Température en atmosphère Oxydante). These tests have revealed that, by comparison to PS212, the stratified coatings have a slightly lower friction coefficient (0.28–0.32 instead of 0.35) and a significantly lower wear rate (30×10−6 mm3/Nm instead of 1000×10−6 mm3/Nm).