Temperature activated self-lubrication in CrN/Mo2N nanolayer coatings

a r t i c l e i n f o Keywords: Lubricous oxides Hard coatings Lubricious oxides High temperature lubrication High temperature friction Temperature activated lubrication Multilayer coatings CrN/Mo 2 N In-situ lubrication Nanolayers Multilayered coatings Tool coatings Self lubrication Multi-layers Magnetron Sputtering Reactive sputtering CrN Mo 2 N CrN/Mo 2 N multilayers were deposited and TEM, SEM, EDS, XPS and TGA studies were carried out. TEM studies show crystalline layers with sharp interfaces. TGA studies indicate that the CrN protects the underlying Mo 2 N layers from oxidation. On varying the periodicity the hardness of the films are in the 20–30 GPa regime, within the rule of mixtures of its constituents. Friction tests at high temperature shows the films to have a temperature activated self lubrication mechanism. Similar friction tests in a near nitrogen atmosphere shows the same temperature activated lubrication mechanism operating with the friction being in a lower regime. MoO 3 is isolated as the predominant oxides phase that evolves from the surface and at temperatures in excess of 400 °C acts as an in situ lubricant. Hard coatings for cutting tools have been in use for many decades , [1–6], and have evolved from the initial, simple nitride and oxide coatings (such as TiN and Al 2 O 3) applied by CVD and PVD in the 1970s, to more complex, high-performance alloy nitrides (such as TiAlN) employed today. More recently, the interest in green manufacturing has spurred development of coatings for dry machining, [7–10]. If coatings can effectively tolerate high temperatures or can help to reduce them, then coolant usage can be reduced. If the friction level due to the cutting process can be minimized, then lubricant usage can be reduced along with power requirements. In an earlier study the authors [11] characterized the deposition conditions and bulk properties of CrN and Mo 2 N coatings and their multilayers. In this paper the intent is to describe our work to develop and characterize multilayered, CrN/Mo 2 N coatings of different ratio and bi-layer peri-odicities. The intent is also to characterize the tribological behaviour of these coatings at high temperature and prove that CrN/Mo 2 N coatings do form hard coatings that are self lubricating at high temperatures. It is proposed to use the materials in a nano-layered structure that will be hard, tough, adherent, and lubricious at the service temperature (b1000 °C). The basic system …