Oxidation behavior of the CVD-Al2O3 coatings on tungsten substrate at 1000 degrees C

In this paper, TiN/TiCN/Al2O3 coating, as a classic coating system for cemented carbide tools, was prepared by chemical vapor deposition (CVD) on pure tungsten substrate for improved oxidation resistance. It is found that the oxidation weight gain per unit surface area of the coated sample is 7.45 mg/cm2 after 5 h oxidation at 1000 degrees C, significantly lower than that of 218.44 mg/cm2 in uncoated specimen. Due to the difference of thermal expansion coefficient, a network of microcracks were formed within the top Al2O3 layer during fabrication process. These microcracks provide a diffusion path for titanium ions and oxygen ions in the initial oxidation stage (0-1h), which produces TiO2 at cracking regions. As oxidation time prolongs (1-5 h), the oxidation of nitride layer beneath the microcracking region creates short-circuit oxygen diffusion paths towards the metal substrate. The generation of volcanic-like WO3 breaks the graded structure of CVD coating and expands the channel between oxygen and substrate. Nevertheless, the weight gain is still limited at this stage as the main area of Al2O3 coating is dense and protective. Finally, the oxidation kinetics accelerate at 5-10 h when coating failure occurs and severe breakaway oxidation appears in tungsten.