Enhancement of oxidation/sulphidation resistance of Ti and Ti6Al4V alloy by HfN coating

An attempt has been made to enhance the oxidation/sulphidation resistance of Ti and Ti6Al4V alloy by the application of an HfN coating produced using Physical Vapour Deposition (PVD). The coated specimens were exposed to an atmosphere comprising high sulphur and low oxygen potentials (pS2 ∼ 10−1 Pa and pO2 ∼ 10−18 Pa) at 750 °C for periods up to 240 h. The corrosion kinetics were obtained by a discontinuous gravimetric method. Linear and linear-parabolic kinetics were recorded for Ti and for Ti6Al4V alloy respectively. The HfN-coated Ti and Ti6Al4V specimens showed enhanced corrosion resistance. The scale formed on the uncoated Ti specimen consisted of a double layer of TiO2 with an underlying TiS2 film on the substrate. The exposed Ti6Al4V alloy also contained a double layer of TiO2 but with Al2O3 precipitated in the external portion of the outer layer of the TiO2, whilst a layer of Al2S3 and TiS2 with vanadium sulphide developed beneath the inner layer of TiO2. Application of the HfN coating suppressed the formation of the duplex oxide scales. However, extensive cracking developed either parallel or perpendicular to the sample surface, and severely compromised the protectiveness of the coating.