ENHANCEMENT OF OXIDATION SULFIDATION RESISTANCE OF TI AND TI-6AL-4V ALLOY BY HFN COATING

An attempt has been made to enhance the oxidation/sulphidation resista nce of Ti and Ti-6Al-4V alloy by the application of an HfN coating pro duced using Physical Vapour Deposition (PVD). The coated specimens wer e exposed to an atmosphere comprising high sulphur and low oxygen pote ntials (pS(2) similar to 10(-1) Pa and pO(2) similar to 10(-18) Pa) at 750 degrees C for periods up to 240 h. The corrosion kinetics were ob tained by a discontinuous gravimetric method. Linear and linear-parabo lic 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 deve loped beneath the inner layer of TiO2. Application of the HfN coating suppressed the formation of the duplex oxide scales. However, extensiv e cracking developed either parallel or perpendicular to the sample su rface, and severely compromised the protectiveness of the coating.