D. W. Seo, K. Ogawa, T. Shoji, Tohoku University, Sendai, Japan; S. Murata, Murata Boring Technology Research Co., Ltd., Shizuoka, Japan
Commercially available CoNiCrAlY and CoCrAlY powders of several different particle size ranges were vacuum-plasma sprayed (VPS) on a nickel alloy substrate. Coatings were oxidized isothermally at a 1000°C for different time periods in order to form the thermally grown oxide (TGO) layer. The oxidation behavior at processing resulting in the oxide content inside sprayed coatings was investigated. The oxygen contents in coatings and supplied powders were estimated by energy dispersive X-ray analysis (EDAX) and electron spectroscopy of chemical analysis (ESCA). It was found that the oxygen contents in the powders and coatings depended on the particle size range. With decreasing in particle size range, especially within lower diameter level (<25 μm), the oxygen content in the as-sprayed coatings increased exponentially. And the TGO formations in coatings were considerably influenced by exposure time, particle size range and the composition. The oxide scale and surface roughness increased proportional to the particle size range. It can be also found that coatings deposited by small particle size presented a lamellar structure which corresponded to a well melting condition before deposition. From research results, when the small particles are used, the in-flight oxidation determines the oxygen content in the coating. When large particles are used, the post-impact oxidation controls the oxygen content in the coating.
Summary: Thermal spray coatings are deposited in an ambient atmosphere or vacuum chamber. Although VPS coat is deposited inside vacuum, oxygen can penetrate into the flame during spraying process, in particular, high velocity oxygen-fuel (HVOF) spraying. This causes the spray materials to be exposed directly to an oxidizing atmosphere. Consequently, oxidation occurs in materials which are sensitive to oxidation, such as metals or carbides. This oxidation significantly influences the phase composition, microstructure, properties and performance of sprayed coatings. Metal oxides are grown on the lamellar interface. The oxides are brittle and have different thermal expansion coefficients than that of the metal, the inclusion of which may cause the spalling of the coating. Moreover, the inclusion of oxides in the MCrAlY coating will degrade the resistance of the coating to sulfur and vanadium, etc., under high temperature corrosion. The presence of the oxides in steel coating also affects its mechanical properties. However, some coating properties can be improved by metal oxides in sprayed coatings. A typical example is the improved wear resistance. The deposited oxides increase also the hardness of the coating. Therefore, it is important to understand the oxidizing behavior of spray materials at spraying. Generally, it might be considered that the oxidizing occurs through two steps: (i) during the particle in-flight period; (ii) at the period shortly after the droplet impacts on substrate. Which step controls the oxide in the coating is not clear yet. Based on some experimental results, the oxidation during in-flight was limited, and the oxidation after splatting would determine the oxide in the coating. However, from the distribution of oxides at both surfaces of a splat that resulted from a completely molten droplet, the argument seems more reasonable that the oxidation takes place mostly during in-flight and the oxidation after splatting is more limited. Therefore, the results of different studies disagree with each other and there exists a clear discrepancy between the results of different investigators. The oxide content in the as-sprayed coating depends on the spraying technique, spraying parameters and starting material compositions. It can be considered that such a difference in grain size of spray powder may be a cause responsible for the notable difference in coating oxygen content and oxidation behavior. Also, it may be considered that the properties of VPS process also might be affected by particle size, even though a vacuum process suppresses better oxidation of metallic coats during spraying than the APS or HVOF.
Therefore, in the present study, the CoNi- and Co-CrAlY powders of different particle size range were used to deposit the coatings and the influence of particle size on the mechanical and oxidation behavior was investigated with increasing exposure time. With the quantitative measurement of the oxygen contents, the effect of particle size on the oxidation behavior was examined. The objective of the present work is to characterize the influence of powder size range with exposure time on the vacuum plasma sprayed metallic coating properties of CoCrAlY, NiCrAlY, CoNiCrAlY and NiCoCrAlY on a Ni-based superalloy namely Inconel 601 as it is essential for the evaluation of the coatings for specific applications. In the present paper, four typical kinds of vacuum plasma sprayed MCrAlY coatings were selected for comparative study. Further, oxidation resistance of these coatings has also been evaluated under isothermal conditions at a temperature of 1000°C and the brief results for the same have also been given in this paper.
