THERMAL FATIGUE DEGRADATION OF AN OVERLAY COATING

The degradation of a 92 mum-thick NiCoCrAlY overlay coating applied with electron beam physical vapor deposition to single crystals of a nickel base superalloy (CMSX-3) was studied, by subjecting thin disk-shaped specimens to 6000 thermal cycles with different rates of heating (from 520-degrees-C to 1090-degrees-C on 4.5 s, 6 s and 20 s) and cooling (to 520-degrees-C in 7 s and 15 s). Diffusion during coating deposition and the subsequent heat treatments to allow for further substrate precipitate growth and coating-substrate interdiffusion gave a 10 mum beta-alpha-gamma transition zone between the coating and substrate, which grew to 80 mum and transformed into a single gamma phase after cycling for the most severe history (DELTAepsilon(coat)el + DELTAepsilon(coat)creep = 0.83%). A 100 h isothermal test at 1090-degrees-C revealed little thinning of the coating, no interface voids, and no cracking. Cyclic degradation consisted of severe cracking, which in some cases propagated well into the substrate, and critically depended on the calculated coating strain ranges and oxide coating stresses. Two different crack initiation processes were observed in the absence of initial surface defects (pits). At normal cooling rates (15 s), cracks appeared to originate from interface (Kirkendall) voids that grew from the original coating-substrate interface towards the coating surface. These voids were associated with calculated coating strain ranges of 0.57% and above. At high cooling rates (7 s), giving maximum calculated oxide stresses of 218 MPa rather than 116 MPa, there was early cracking of the protective oxide scale with cracks initiating at the coating surface.