Formation of diffusion cells in LPPS MCrAlY coatings

Abstract

The formation of diffusionally isolated regions in air plasma-sprayed, APS, MCrAlY bond coat or overlay coatings has been established. The ‘diffusion cells’ experience greatly accelerated depletion of aluminium and an early onset of breakaway oxidation associated with the morphology and extent of the internal oxides. In this paper the effect of internal oxides produced in low pressure plasma sprayed, LPPS, CoNiCrAlY coatings are presented. Isothermal tests in laboratory air at 1200°C were conducted with post-test characterisation performed using scanning electron microscopy and energy dispersive X-ray analysis. A parabolic correlation was found between TGO growth rate and time at temperature. Importantly, compositional profiles of cross sections through the coatings demonstrated that no barriers to diffusion occurred throughout the body of the coating. What has been found in the LPPS coatings is a mechanism whereby diffusion cell formation is possible at sites where splat boundaries containing internal oxides intersect the outer surface. Gas access is possible at these sites and, with increasing time at temperature, formation of a continuous internal oxide layer forms. The sequence of events at these sites follows that of the diffusion cell model presented earlier and a prediction of time to failure is made.