Effect of aluminising on high temperature oxidation resistance of TiAI cOmpounds

Abstract

Intermetallic compounds of TiAl were aluminised by pack cementation in the temperature range 700–900°C with a powder mixture of aluminium, NH4Cl, and Al203 under a flow of argon gas. The coating products and oxidised products formed on the TiAl substrate were investigated by optical microscopy, X-ray diffraction, and scanning electron microscopy. Single layers of TiAl3 were formed on the substrate of all the aluminised specimens. Through thickness cracks and pores were often observed inside the coating layers. High quality coating layers (7sim;30 μm) containing a very small amount of microcracks and pores were obtained by a treatment of 800°C for 3 h. The average surface hardness of the aluminised specimens (∼1010 HV(25 mg)) was much higher than that of the TiAl (∼396 HV (25 mg)), thus improving the wear resistance. In particular, the aluminising significantly improved the high temperature oxidation resistance. After the high temperature oxidation tests, four sublayers, i.e. α-Al203, TiAl2, TiAl with a high aluminium content, and TiAl with a low aluminium content, from the surface, were formed on the substrate. These four sub layers contributed to a significant improvement in the high temperature oxidation resistance.