Studies on the development of protective coating on TZM alloy and its subsequent characterization

Abstract

TZM alloy having nominal composition of Mo–0.5Ti–0.08Zr–0.02C (wt%) has high melting temperature, superior creep strength at elevated temperature and excellent corrosion resistance against liquid metal environment such as zinc, lead, lead–bismuth eutectic, etc. Hence, TZM alloy is being considered as the prime candidate among various molybdenum-based alloys for application as heat resistant and structural materials in the areas of high temperature nuclear reactors, aerospace, power generation and chemical industries. However, TZM alloy has a propensity to undergo rapid oxidation at high temperature in air. Hence it needs protective coating before use. In the present investigation, therefore, attempts were made to develop in situ formation of protective coating of aluminide over TZM alloy using pack cementation coating technique. The TZM alloy was initially prepared in this laboratory from oxide intermediates of MoO 2, TiO 2, ZrO 2, etc. in presence of requisite amount of carbon, by alumino-thermic reduction smelting technique. The arc melted homogenized samples of TZM alloy substrate was then embedded in the chosen and intimately mixed pack composition consisting of inert matrix (Al 2O 3), coating material (Al) and activator (NH 4Cl) taken in the judicious proportion. The sealed charge packs contained in an alumina crucible were heated at temperatures of 850 °C and 1000 °C for 4–12 h. The desired coating was achieved by in situ deposition of active aluminium over TZM alloy and its subsequent diffusion into the substrate forming different aluminides, namely, Al 17Mo 4, Al 12Mo, Al 7Mo 4, Al 5Mo, Al 2 (MoO 4) 3 ,etc. An average coating thickness of 50 μm was achieved on each side of the TZM alloy surface. The aluminide coating was found to be granular in nature, uniform and adherent. It is also found to be adequately resistant to oxidation at elevated temperature and resistant to wear.