Thermal Processing Effects on the Adhesive Strength of PS304 High Temperature Solid Lubricant Coatings

The effects of post deposition heat treatments on the strength properties ofPS304, a plasma sprayed nickel-chrome based, high temperature solid lubricant coating are studied. Coating samples were exposed in air at temperatures from 432 to 650°C for up to 500 hr to promote residual stress relief, enhance particle to particle bonding and increase coating to substrate bond strength. Coating pull off strength was measured using a commercial adhesion tester. The as-deposited (untreated) samples either delaminated at the coating-substrate interface or failed internally (cohesive failure) at about 17 MP a. Samples heat treated at temperatures above 540°Cfor 100 hr or at 600°C or above for more than 24 hr exhibited strengths above 31 MPa. Coating failure occurred inside the body of the coating (cohesive failure) for nearly all of the heat-treated samples and only occasionally at the coating substrate interface (adhesive failure). A microstructural change is believed to be partially responsible for the coating strength increase. Increasing the heat treatment temperature, exposure time or both accelerate the heat treatment process.