Localised oxidation defects in TiAlN/CrN superlattice structured hard coatings grown by cathodic arc/ unbalanced magnetron deposition on various substrate materials

TiAlN/CrN superlattice coatings show excellent surface oxidation resistance up to 900°C, with an as deposited hardness maximum of HK 3400 (bi-layer 3.8 nm). Coated M2 high speed steel and cemented carbide samples have been examined by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) after heat treatment in air between 600°C and 1000°C to study the influence of the oxidation behaviour. X-ray diffraction (XRD) and thermogravimetric (TG) measurements were performed on the uncoated substrates to gain an overall picture of the oxidation stability of the plain substrate materials at high temperatures. Globular oxides were observed on coated cemented carbide at 700°C, while on M2 high speed steel similar oxides did not form below 800°C. The small globular oxides were gradually replaced by spherical oxides mainly formed out of substrate material. Initially oxidation starts at the bottom of pores or craters produced by small droplets or by growth defects grown on the droplets. The droplets were deposited during the cathodic arc metal ion step whereas the growth defects developed during film growth. Some detached growth defects formed craters through which oxidation products formed from the substrate material. Elements from the substrate penetrated to the coating surface at temperatures far below the actual oxidation temperature of TiAlN/CrN (900°C), namely at temperatures as low as 700°C (on cemented carbide). On M2 and stainless steel SS304, this type of localised oxidation was observed at temperatures equal to and above 900°C.