Mechanical properties of a low-pressure-plasma- applied Co-Cr-Al-Y coating

Abstract

The low pressure plasma spray process was used to deposit coatings of a Co- 29Cr-6A1-1Y alloy onto IN-738 test specimens and to make free-standing deposits from which specimens were machined. Tensile tests, stress rupture tests and high cycle fatigue test were run on the coated specimens from room temperature to 870°C and results showed no significant effect of the coating process. A strain-to-coating-cracking test was also run on coated specimens and a coating ductile-to-brittle transition was found between 760 and 870°C. Free-standing Co- 29Cr-6Al-1Y tensile specimens were machined from the thick low-pressure- plasma-sprayed deposits. The tensile properties of these deposits were measured from room temperature to 980°C and results showed that the tensile strength of the deposits ranged from 50% above to 100% below the strength of cast IN-738, while elongation went from 0% to 130%. It is shown that the low pressure plasma process produced clean low defect coatings which perform well in mechanical cycling. However, it is also shown that the Co-29Cr-6Al-1Y coating, at low temperatures (less than 540°C), has relatively low ductilities which could cause early cracking in high strain level testing or service conditions. Some scanning electron microscopy replica and metallographic evaluations are also presented to illustrate the tensile cracking behavior of the coating.