Elevated temperature tribological assessment of Ni-based cermet self-lubricating coatings deposited by cold spray

Abstract

Solid lubricant coatings provide reduced friction and wear resistance and hence finding widespread application in harsh environmental conditions, such as gas turbine, aircraft, and internal combustion engines. It is apparent that single lubricant fails to offer effective lubrication over wide temperature range. Hence, two or more solid lubricants are mutually incorporated to achieve the appropriate lubrication by their synergy under harsh service conditions. The current investigation is being carried out to evaluate the synergistic impact of different solid lubricants viz. silver (Ag), molybdenum disulfide (MoS2), hexagonal boron nitride (h-BN) and chromium oxide (Cr2O3) on the frictional properties of coatings from room temperature (RT) to 800 °C. Cold spraying process has gathered extensive acceptance for the development of metallic and metallic-ceramic coatings. The current study elucidated the tribological characteristics of cold sprayed nickel-based high temperature lubrication coatings, namely, Ni-Al-MoS2-Ag (NB0), Ni-Al-MoS2-Ag-5 wt% h-BN (NB5), Ni-Al-MoS2-Ag-7.5 wt% h-BN (NB7.5), Ni-Al-MoS2-Ag-10 wt% h-BN (NB10) and Ni-Al-MoS2-Ag-Cr2O3–7.5 wt% h-BN (NB7.5 + 7 wt% Cr2O3) against Al2O3 ball in an operative testing regime from (RT-800 °C) using ball-on-disc tribometer. Results indicated that NB7.5+ 7 wt% Cr2O3 coating exhibited remarkable self-lubricating and wear resistance properties as compared to other coatings. However, NB7.5+ 7 wt% Cr2O3 revealed slightly higher coefficient of friction (COF) in compare to NB7.5 coating till 400 °C, and beyond that it showed a decreasing trend and lesser COF in compare to other coatings. The observed wear mechanisms at different temperatures were discussed in detail after analyzing the morphologies of worn surface. The pivotal role played by the participating solid lubricants were also discussed in detail which help in to provide the continuous compacted layers at the worn surfaces at elevated temperature.