Room and high temperature tribological performance of CrAlN(Ag) coatings: The influence of Ag additions

The present study aims to investigate the effect of the addition of Ag on the tribological performance of a CrAlN coating system, at room and high temperatures (600 °C). The addition of Ag, in general, has been seen beneficial in lowering the coefficient of friction (COF) at room and high temperatures. Independent of the decrease of COF with Ag additions, the specific wear rate was degraded. The only exception was the CrAlNAg9 coating with 8.6 at.% of Ag which exhibited the lowest specific rate at room temperature while maintaining its comparable wear resistance properties with that of reference CrAlN coating at 600 °C. The CrAlNAg9 coating also demonstrated coefficient of friction (COF) of 0.16 and 0.26 at room temperature and 600 °C, respectively. Formation of Ag and AgCrO2-rich tribolayer during sliding at high temperature was responsible for lubricious properties of CrAlN(Ag) coatings. However, doping the coatings with small content of Ag (around 2.4 at.%) was not effective in decreasing the COF, nor the specific wear rate under high temperatures. On the other hand, high concentration of Ag (11.8 at.% and above) had a detrimental influence on mechanical properties and poor resistance to oxidation at elevated temperature compromising both friction and wear properties of the CrAlNAg12 and CrAlNAg16 coatings.
S. Gangopadhyay would like to thank the Department of Science and Technology, Government of India for funding the research work under the bilateral collaborative project between India and Portugal (Project No. DST/INT/Portugal/P-14/2017). This research is also sponsored by FEDER Funds through Portugal 2020 (PT2020), by the Competitiveness and Internationalization Operational Program (COMPETE 2020) and national founds through the Portuguese Foundation for Science and Technology (FCT), under the projects: SMARTLUB – ref. “POCI-01-0145- FEDER-031807”, MCTool21- ref.: “POCI-01-0247-FEDER-045940” and CEMMPRE - ref. “UIDB/00285/2020”. Bilateral collaborative project between Portugal and India (Project number 441.00 INDIA) is also acknowledged.