High-velocity oxy-fuel thermally sprayed CoNiCrAlY coatings on Ti-6Al-4V alloy: High cycle fatigue properties of coating

Mechanical response of HVOF-coated workpieces is important for the life assessment of coating adherence, particularly for the coated surfaces being exposed to hightemperature environments prior to mechanical tests. In the present study, fatigue tests are carried out for Ti-6Al-4V workpieces coated via HVOF spraying of AMDRY 995 (CoNiCrAlY) powders. In order to simulate the effect of high temperature on the fatigue response of coating, workpieces were heat treated at the annealing temperature of the alloy for 1 h. Surface morphology prior to and after fatigue testing is carried out using scanning electron microscopy (SEM) and optical microscopy, while energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) are carried out for metallurgical examinations. It is found that heat treatment results in early fatigue failure of the workpieces owing to thermally induced stresses developed at the coating-substrate interface. The metallurgical changes in the substrate material, and formation of brittle oxygen compounds and alumina particles left over from grit blasting at the coating-substrate material interface are responsible for the coating failure.