Microstructure and mechanical properties of Al₂O₃–Al composite coatings deposited by plasma spraying

Abstract

Al₂O₃ and Al₂O₃–Al composite coatings were prepared by plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction (XRD), while optical microscopy (OM) and scanning electron microscopy (SEM) were employed to investigate the morphology of impacted droplets, polished and fractured surface, and the element distribution in terms of wavelength-dispersive spectrometer (WDS). Mechanical properties including microhardness, adhesion and bending strength, fracture toughness and sliding wear rate were evaluated. The results indicated that the addition of Al into Al₂O₃ was beneficial to decrease the splashing of impinging droplets and to increase the deposition efficiency. The Al₂O₃–Al composite coating exhibited homogeneously dispersed pores and the co-sprayed Al particles were considered to be distributed in the splat boundary. Compared with Al₂O₃ coating, the composite coating showed slightly lower hardness, whereas the coexistence of metal Al phase and Al₂O₃ ceramic phase effectively improved the toughness, strength and wear resistance of coatings.

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.