Oxidation behavior of molybdenum nitride coatings

Molybdenum nitride coatings were deposited onto H13 hot working tool steel and alumina substrates by arc PVD. The coatings were characterized with respect to their mechanical and structural properties. Oxidation behavior of the coatings on alumina substrates was investigated by TG/DTA. Coated H13 substrates were oxidized in a tube furnace under atmospheric conditions at specific temperatures determined from TG/DTA tests of the same coating on alumina substrate. The oxide scale morphology was characterized by scanning electron microscopy and X-ray diffraction. Resistance to oxidation of Mo-N coatings is poor because of the non-protective nature of their oxides and the volatility of the oxides at moderately high temperatures. The oxides formed during the oxidation were determined as MoO₂ and MoO₃. The oxidation reaction product MoO₃ is a volatile oxide and its evaporation reaction starts at after 550 °C. In addition, decomposition of Mo₂N starts at 681 °C and the decomposition reaction product is metallic molybdenum, which also oxidizes to volatile MoO₃.