Microstructure and mechanical properties of Al2O3–Al composite coatings deposited by plasma spraying
Introduction
Thermal spraying process has been used successfully to produce a range of protective coatings for wear, erosion and heat resistance, as well as restoration of worn parts [1], [2], [3]. Especially, oxide ceramics such as Al2O3 ceramic coatings, having superior hardness, chemical stability and refractory character, are commonly utilized to resist wear by friction and solid particle erosion [4], [5]. As a surface modification technique, atmospheric plasma spraying (APS) has been well-established to deposit various coatings [1], [6]. However, plasma-sprayed coatings built up from the successively immediate solidification of the liquid or partially melted droplets onto target substrate typically present weak interface between splats and irregular reticula of microcracks and pores running through it [3], [7]. The porosity and weak interface adversely affect the wear property and the cracks allow corrosive substance in the environment to attach the protective coating. In addition, these ceramic coatings are also vulnerable to thermal fatigue and delamination under mechanical load for its intrinsic brittleness. So, the application of plasma-sprayed Al2O3 coating are restricted in many industrial fields such as wear and erosion resistance, especially under increasingly severe conditions by the combination of intrinsic brittleness and microstructural defects [4], [8]. In recent years, numerous studies have been conducted to improve the microstructure and resulting performance of plasma-sprayed ceramic coatings by incorporating metallic second phases into ceramic matrices [9], [10]. Dong et al. [9] reported that the composite coatings prepared by plasma spraying Fe2O3–Al self-reaction composite powders possessed multiphase metal and ceramic coexistence, which significantly decreased the brittleness and increased the wear resistance of coatings even when the load was up to 490 N. In Chwa et al.'s [10] study, plasma-sprayed nanostructured TiO2–Al composite coatings were prepared, and the results obtained from experimental work showed that the Al addition effectively improved the deposition efficiency and mechanical properties of coatings including toughness and wear resistance.
In this work, aluminum, having excellent ductility and thermal conductivity, was added to prepare Al2O3–Al composite coating. The present study is related to detailed characterization of microstructure of Al2O3 and Al2O3–Al composite coatings deposited by plasma spraying, and to understand the effect of Al additive on the microstructure, mechanical properties and the tribological behaviors of coatings under different degree of severity using block-on-ring configuration.
Section snippets
Spray powder preparation
The commercially available Al2O3 powders with an average particle size of 33.5 μm were used as a feedstock in the present study. The raw feedstock has the purity >99.0 wt.% of Al2O3 component. And also, fine aluminum powders with an average particle size of 1–2 μm were used as an additive to prepare the other feedstock of Al2O3–Al composite. The composite powders with a content of around 5% Al were directly mechanically mixed in a rotary-vibration mill, polyvinyl alcohol being used as a binder,
Characterization of spraying powders
Fig. 3 shows the morphologies of the as-received feedstock powders. It can be seen that the Al2O3 particles, showing an irregular and angular morphology, were nearly enwrapped well by fine Al particles, which is confirmed by the internal morphology of Al2O3–Al composite powders, as shown in Fig. 4. And also, the result of particle size distribution, as indicated in Fig. 5 by a laser scatter particle size analyzer, shows no significant difference in these two feedstock powders.
Fig. 6 presents
Conclusions
Al2O3 and Al2O3–Al composite coatings were deposited by plasma spraying. The Al additive effects and the correlation between coating microstructure, mechanical properties and wear resistance were investigated. The following points were found:
- 1.
The addition of Al on Al2O3 particle surface is beneficial to decrease the splashing extent of the Al/Al2O3 composite droplet and increases its deposition efficiency. And the dense microstructure and developed inter-splat contact can be obtained for the
Acknowledgements
The authors wish to express their thanks to Ms. Zhenglan Lu and Weijun Qian for mechanical test and SEM analysis, and Dr. Xuanyong Liu and Youtao Xie for their constructive proposals and effective discussions during performing the present study.
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