| Abstract: |
Nano sized NiCrAlOY coatings are considered to be quite useful to improve the properties of bond coat, such as better adhesion and spalling resistance. However, applying a nano-sized powder by conventional spraying process is not very successful, due to large extent of agglomeration and grain growth, which takes away all the advantages of nano particles. Application of nano powders by wet injection method has been found to be more useful but is relatively cumbersome. Alternative technique of introducing nano powders is by blending other nano particles along with with the NiCrAlOY powders. The present work demonstrates that introduction of inert nano particles such as alumina/yttria in conventional powders, results in tremendous improvement in the characteristics of the bond coat.
Oxidation behaviour of NiCrAlY powder, blended with nano and micro sized Al2O3 and Y2O3 was studied to understand the effect of nano oxide powder dispersion. The blended powders were applied on the IN 738 by HVOF technique. The present work compares the oxidation behavior of NiCrAlY dispersed with nano and micro Y2O3, Al2O3 oxide. Coated samples were characterised by XRD, SEM/EDAX in terms of surface composition, scale cross section and the identification of different phases. The oxidation tests were carried out at 1223K, 1323K, 1423K in air. Oxidation kinetcs were determined from these tests which infer that at 1223 K and 1323 K, nano yittria addition, in fact, resulted in higher oxidation rate, while nano alumina addition resulted in lower oxidation rate. The effect was more pronounced at 1423 K, where the nano and micro size alumina, yittria addition, resulted in bringing down the oxidation rate considerably.
Oxidation kinects for different coatings at one temperature show deviation from parabolic behaviour. The oxide scale with nano and micro size alumina and yittria dispersed particle comprised of NiCr2O4, ±,³,¸-Al2O3, YAlO3, Cr2O3, NiAl2O4, NiO as emerging peaks. The mechanism of blending, the effect of tempertare and types of nano particle used has been studies.
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