Different proportions of α-Al2O3 and pure Al powders were coated onto AZ91D magnesium alloy substrates by cold gas dynamic spray. The microstructure and morphologies of the coatings were observed by scanning electron microscope. The friction and wear properties were tested by a ball-on-disk wear tester. It was found that the interfaces between grains and substrates formed close boundaries. It is revealed that the composite coatings could increase the friction or wear properties of the coatings. It was observed that the wear of coatings was converted from adhesive wear into abrasive wear with α-Al2O3 particles increasing and that the adhesive wear accompanied with abrasive wear would increase the wear rate of coatings. 1. Introduction The electroless plating and plating process will seriously cause detrimental effects to the environment and the human health. Therefore, research for alternatives of electroless plating and plating has been widely carried out in the world [1, 2]. Cold gas dynamic sprayed (CGDS) metal and cermet coatings are adopted increasingly in industry as an alternative for electroless plating and plating since such coating technique can provide a superb friction and wear resistance. CGDS, as a new technique for the surface coating fabrication, has been developed rapidly in recent years and attracted more and more scientists’ interests. In CGDS process, micro-particles are accelerated to supersonic velocities and impact onto a substrate surface; the coating is formed by plastic deformation of sprayed particles in a solid state during impact in cold spraying. In this technique process, the sprayed particles at speed over 500–1200？m/s [3–12] and at gas temperatures up to around 300？900？K can be deposited onto a wide range of substrate materials. The temperature of sprayed particles prior to impact is much lower than their melting point, and then spray materials experience little microstructure change, oxidation, or decomposition. Figure 1 shows the schematic drawing of CGDS system. Figure 1: Schematic illustration of CGDS. In this paper, α-Al2O3 and Al powders were sprayed onto the surface of magnesium alloy by CGDS. The microstructure and morphology of the coating, friction, and wear properties were studied; the phase and structures of the coating were also researched. 2. Experiments 2.1. Experimental Materials and Instruments As-cast AZ91D magnesium alloy, the most widely used material up to now, was applied as substrates which were cut into 25？mm in diameter and 5？mm in depth. All the substrates were ground with SiC paper down to grit
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