Preparation and oxidation behavior of B4C–Ni and B4C–TiB2–TiC–Ni composite coatings produced by an HVOF process
Introduction
Oxidation is one of the most important degradation mechanisms for materials that operating in air or oxygen atmospheres at high temperatures. The oxidation rate, determines the rate of material consumption that generally controlled by solid state diffusion through the oxide scale. Many components, however, experience cyclic oxidation in applications. Typically, some oxide spallation may occur during cooling, resulting in loss of the protective diffusion barrier. During reheating, the oxidation rate increases in the spalled regions [1]. It has been recognized [2] that the oxide spallation is more likely to occur when the oxide layer is placed under lateral compression rather than tension stresses. In most oxide systems, the differences in the thermal expansion coefficients that creates compressive stresses, are developed upon cooling from the oxidation temperature.
Advanced thermal spray processes such as plasma spraying, high velocity oxy-fuel (HVOF) etc. are usually used to make coatings on the surface of engineering materials to improve their high temperature oxidation resistance [3]. In the HVOF method, the thermal degradation of carbide and boride compounds is decreased during the spraying, because of lower flame temperature and shorter dwell time of the powder particles in comparison with other thermal spray processes [4]. Moreover, the coatings with lower porosity (2 vol%) can be obtained by this process [5] and therefore, the HVOF coatings will exhibit more oxidation resistance [6].
The use of the Protective coatings is required for better performance of critical airfoil components in gas turbines. The success of a coating at high temperature application is determined by its capability to resist oxidation, avoid cracking and limit diffusion between coating and substrate [7]. The mechanical properties of the coating are also important in high temperature cyclic oxidation [8].
Ceramic matrix composites containing TiC and TiB2 have been extensively studied due to the unique properties such as high melting temperature, low density, superior thermal and chemical stability, excellent wear resistance and high fracture toughness [9]. The oxidation behavior of these hard materials plays an important role in industrial applications, because they are frequently exposed to oxidative atmospheres at high temperature during their service as wear and corrosion resistant films. Multicomponent coatings containing different metallic and non-metallic elements leading to the further improvement of coating properties as comparison with single phase coatings [10]. Telle [11] reported that B4C–SiC composites synthesized by hot pressing exhibited better oxidation resistance than single phase B4C.
Murthy et al. [12] studied oxidation behavior of TiB2–MoSi2–CrB2 composite. They reported that, the oxidation behavior of the composite obeyed the parabolic law at 750 and cubic law at 850 °C. Protective oxide layers of TiO2, Cr2O3 and SiO2 were detected on the surface of the oxidized specimens. This composite indicated more oxidation resistant in comparison with the composite containing either CrB2 or MoSi2 only.
Lotfi [6] studied high temperature oxidation behavior of HVOF sprayed TiB2 cermet coating. Lotfi reported that, HVOF sprayed Ni(Cr)–TiB2 coating indicated high oxidation resistance. The oxide scale was thin, and was adherent to the coating up to 900 °C with no cracking. The predominant phase in the oxide scale was rutile (TiO2). The oxidation behavior of TiB2–WSi2 composite was studied by Murthy et al. [13]. Parabolic behavior of oxidation was observed and with increasing WSi2 content the oxidation rate decreased. This composite exhibited superior oxidation resistance in comparison with other TiB2 composites containing MoSi2, TiSi2, CrB2 and CrSi2.
Qin et al. [14] reported that the oxidation kinetics of in situ synthesized (TiB–TiC)/Ti–Al composites at temperatures between 823 and 923 K in air for 300 h basically obeyed a parabolic rate law. The formed oxide scales indicated excellent spallation resistance under all testing conditions, implying that growth and thermal stresses generated upon heating and cooling periods had been effectively released.
In this study, B4C–Ni and B4C–TiB2–TiC–Ni composite coatings were thermally sprayed by an HVOF technique and the oxidation behavior of these coatings was investigated at three different temperatures by TGA experiments.
Section snippets
Starting materials and MA process
Ti, B4C and Ni powders, with purity higher than 99, 98 and 99% respectively, were used as the starting materials. The Ti powder particles were irregular in shape with a size distribution of 50–100 μm and B4C particles had a uniform angular shape with a size distribution of 150–200 μm. The Ni powder had a branch structure from very fine particles with a size lower than 10 μm. To produce B4C–TiB2–TiC composite powder, Ti and B4C powders were mixed according to Reaction (1) and the MA process was
Synthesis and characterization of feed-stock powders
B4C-20 vol% Ni feed-stock powder was synthesized by MA of B4C and Ni powders. Fig. 1 shows XRD patterns of B4C–Ni powder mixture. As can be seen (Fig. 1a), after MA of this powder mixture for 6 h only B4C and Ni peaks were observed, indicating during the MA process no reaction was occurred between B4C and Ni powders and the MA just led to the mixing of powders. In order to study thermal behavior, heat treatment of powder mixture was done at 1200 °C for 1 h. After heat treatment (Fig. 1b), Ni peaks
Conclusions
B4C–Ni and B4C–TiB2–TiC–Ni composite coatings were successfully synthesized by HVOF of milled composite powders. Cyclic oxidation behavior of coatings was studied by TGA experiments and the following results were obtained:
- 1.
B4C–TiB2–TiC–Ni coating had fine microstructure, while the coarse microstructure was seen in B4C–Ni coating. The distribution of ceramic hard particles throughout the coating was more homogeneous in B4C–TiB2–TiC–Ni coating in comparison with B4C–Ni coating.
- 2.
During TGA
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