The effects of substrate material and thermal processing atmosphere on the strength of PS304: A high temperature solid lubricant coating

PS304, a plasma spray deposited solid lubricant coating developed for high temperature sliding contacts was deposited on nine different substrate metals, heat treated at 650degreesC in either air or argon and subsequently tested for strength using a commercially available pull-off adhesion test. Some samples were examined metallographically to help elucidate and explain the results. As-deposited coatings exhibit pull-off strengths typically between 16 and 20 MPa with failure occurring (cohesively) within the coating. Heat treatment in argon at 650degreesC results in a slight increase in coating (cohesive) strength of about 30 percent to 21 to 27 MPa. Heat treatment in air at 650degreesC results in a draman. c increase in strength to over 30 MPa, exceeding the strength of the epoxy used in the pull test. Cross section metallographic analyses show that no microstructural coating changes occur following the argon heat treatments, however, exposure to air at 650degreesC gives rise to the formation of a second chromium-rich phase precipitate within the PS304 NiCr constituent which provides a strengthening effect and a slight (similar to5 percent) coating thickness increase. Subsequent heat treatments do not result in any further coating changes. Based upon these studies, PS304 is a suitable coating for use on a wide variety of high temperature substrates and must be heat treated following deposition to enhance strength and ensure dimensional stability.