MCrAlY Coatings for the Protection of Gamma Titanium Aluminide

摘要

MCrAlY (M=Ni or Ni,Co) coatings are largely used on nickel-based alloys substrates for many years to provide a protective barrier against oxidation and corrosion at high temperature (800-1300°C). To assume this role they must be adherent to the substrate and chemically/mechanically compatible under cyclic temperature and stress conditions. In this study NiCrAlY (atmospheric plasma spraying) and NiCoCrAlYTa (vaccum plasma spraying) have been investigated on TiAl substrate because of similar work conditions (high temperature oxido-corrosive atmosphere). After performing post-heat treatment, such coatings have been tested under two types of thermal shock to evaluate mechanical compatibility: four times of the cycle 800°C/24h-free air cooling-150°C/1h under 50 MPa tensile stress and five times of the cycle 800°C/1 h followed by air pulsed quenching to 200°C under the same stress and cyclic air exposure at 825°C during 500h for chemical compatibility. Tested specimens have been examinated by both optical and scanning electron microscopy (SEM) in cross section whereas chemical and phase analysis were carried by X-ray spectrometry (EDX) and diffractometry (XRD) respectively. Specimens exposed to creep under thermal shock conditions exhibit a similar behaviour for all the coating types, with a well mechanical compatibility between coating and substrate. No superficial microcracks or interfacial delamination between coating and TiAl substrate have been observed after thermal shock, but hard and brittle diffusion layers due to Ni or (Ni, Co) migration have been formed at the materials interface (100h exposure). Nevertheless, after 500h of cyclic air exposure at 825°C, an interfacial decohesion can occur which can be assigned to the formation of many pores at the interface.