On the tribology of the MAX phases and their composites during dry sliding: A review

By now, it is fairly well established that the layered hexagonal MAX phases are thermodynamically stable nanolaminates displaying unusual and sometimes unique properties. These phases are so-called because they possess a Mn + 1AXn chemistry, where n is 1, 2, or 3, M is an early transition metal element, A is an A-group element and X is C or N. They are highly damage tolerant, thermal shock resistant, readily machinable, and with Vickers hardness values of 2–8 GPa, are anomalously soft for transition metal carbides and nitrides. Some of them display a ductile–brittle transition at temperatures > 1000 °C, while retaining decent mechanical properties at these elevated temperatures. Moreover, their layered nature suggests they may have excellent promise as solid lubricant materials. Recently, first generation MAX Phase based composites shafts were successfully tested against Ni-based superalloy at 50,000 rpm from RT till 550 °C during thermal cycling in a foil bearing rig. This study further demonstrates the potential of MAX Phases and their composites in different tribological applications. The main objective of this review is to present recent progress, and consequently develop a comprehensive understanding about the tribological behavior of MAX Phases and their composites. We are also proposing a way of classifying the different tribofilms to understand the complex tribological behavior of these solids over a wide range of different experimental conditions.