Fabrication and Properties of Plasma-Sprayed Al 2 O 3 /ZrO 2 Composite Coatings
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N. Dejang, A. Limpichaipanit, A. Watcharapasorn, S. Wirojanupatump, P. Niranatlumpong, and S. Jiansirisomboon (Submitted December 25, 2010; in revised form May 11, 2011) Al2O3/xZrO2 (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed a-Al2O3 and nano-sized monoclinic-ZrO2 powders. Microstructural investigation showed that the coatings comprised well-separated Al2O3 and ZrO2 lamellae, pores, and partially molten particles. The coating comprised mainly of metastable c-Al2O3 and tetragonal-ZrO2 with trace of original a-Al2O3 and monoclinic-ZrO2 phases. The effect of ZrO2 addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO2 improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.
Keywords
Al2O3/ZrO2 coating, mechanical properties, nanocomposite, plasma spray, wear behavior
1. Introduction Plasma spray is a well-known technique used to prepare coatings for various applications, including aerospace and automobile applications. In plasma spray, solid particles, which are melted or partially melted in the plasma jet, are accelerated to reach a substrate (after their flights). During the impact on the substrate, droplets of molten particles form splats which solidify in a few microseconds. The coating resulting from the layer of splats has a lamellar structure (Ref 1). Solidification of the molten droplets is relatively fast and most often leads to the formation of metastable or amorphous phases (Ref 2, 3). It is well known that engineering ceramics exhibit properties such as high chemical stability, corrosion, and wear resistances. Aluminum oxide or Alumina (Al2O3) is an important ceramic which is widely used in various industrial applications, which require high wear and erosion resistance, corrosion protection and thermal insulation (Ref 4, 5). Alumina has two main important phases, namely
N. Dejang, A. Limpichaipanit, A. Watcharapasorn, and S. Jiansirisomboon, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand; S. Wirojanupatump, Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand; and P. Niranatlumpong, National Metal and Materials Technology Center, National Science and Technology Development Agency, Pathumthani 12121, Thailand. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
a (alpha) and c (gamma). During the plasma spraying process, Al2O3 undergoes crystallographic modifications and this affects the properties of the coatings. The stable and hard a phase transforms to the softer c phase because of the lower nucleation energy of this compound. It is well known that the mechanical properties and wear behavior of the coatings are usually strongly dependent on their microstructures such as phase composition, grain size, porosity, and second-phase particle distribution. Mic
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