CORROSION AND ADHERENCE OF STABILIZED ZRO2 COATINGS AT HIGH-TEMPERATURES

ZrO2 coatings partially stabilized by yttria and/or alumina were prepared by reactive d.c. magnetron sputtering. Coatings about 6 mu m thick were deposited on high temperature Ni-based alloy (Inconel 617) with the aim of protecting the steel against corrosion (oxidation and sulphuration) at 1000 degrees C. The tensile thermal coating stress during thermal cycling was controlled by introducing an intrinsic compressive stress of about 1.7 x 10(9) N m(-2) during the deposition procedure. Owing to this stress control, the coatings remained undamaged (without visible cracks and delamination) during rapid thermal cycling at heating and cooling rates of about 9 K s(-1) and during prolonged exposure to atmospheric pressure. Nevertheless, owing to the high diffusity of oxygen in ZrO2, some oxidation of the substrate was unavoidable for the ZrO2-Y2O3 coatings. Substrate oxidation could be significantly diminished by the use of ZrO2-4 wt.%Y2O3-15 wt.%Al2O3 coatings. The corrosion of the protected substrates followed a parabolic law with a parabolic corrosion constant of about k(p)=9x10(-10)kg(2)m(-4) s(-1)(K-p=9x10(-12) g(2)cm(-4)s(-1)). The adherence of the as-deposited coating was found to be a critical load of about 22 N as determined by scratch testing. A 1000 degrees C heat treatment at atmospheric pressure did not alter the adherence of the ZrO2-4 wt.%Y2O3-15 wt.%Al2O3 on the growing Cr2O3 layer. The adherence of the growing Cr2O3 layer on the substrate was always superior to the adherence of the physical vapour deposition layer on the corrosion layer. These results and scanning electron microscopy studies showed the capability of stabilized ZrO2 to protect high temperature steel in gas mixtures typical for gas turbines.