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.