Cyclic-oxidation resistance of protective silicide layers on titanium

Titanium was powder siliconized and gas nitrided, in order to improve its cyclic-oxidation resistance. Siliconizing was performed in a pure-silicon powder at temperatures in the range of 800 - 1100 degrees C for 3 - 48 h. Gas nitriding was carried out in pure N-2 at 1100 degrees C/ 12 h. Cyclic-oxidation experiments with the siliconized and nitrided samples were conducted in air at 850 and 950 degrees C for up to 560 h. It was found that the siliconized layers grew according to the parabolic law with the activation energy for siliconizing ES being 47.2 kJ mol(-1). Powder siliconizing at 900 - 1100 degrees C/ 3 h produced multi-phase layers, in which Ti5Si3 silicide predominated The siliconizing temperature of 800 degrees C/ 3 h appeared to be insufficient, because it led to a non-uniform surface layer with a slight protective effect. The nitrided layers were composed of titanium nitride TiN and alpha-Ti( N) intestitial solid solution. Measurement of the oxidation kinetics revealed that the titanium siliconized at 900 1100 degrees C/ 3 h oxidized much more slowly than pure Ti, Ti - 6Al - 4V alloy and nitrided titanium. Microstructural investigation revealed the complex substructure of the scales on the siliconized samples which was composed of rutile+silica, rutile and nitrogen-rich sub-layers. The mechanism of high-temperature cyclic oxidation of the siliconized and nitrided titanium is discussed.