Development of laser-cladding layers containing nano-Al 2 O 3 particles for wear-resistance materials
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INTRODUCTION
TRIBOLOGICAL behavior has been given considerable attention in the synthesis, fabrication, and application of a novel material. In particular, as one of the most common failure or damage modes of a material, wear plays a crucial role in the service life of a component or a part. In recent years, to fabricate wear-resistant materials and to reduce fabricating costs, the powder metallurgy method is used widely in various industries (e.g., automotive gears). However, because of the existence of pores, the mechanical properties, such as hardness and wear resistance, have not been satisfactory. Moreover, such pores in the developed materials often act as sites of generation and collection of wear debris and hence result in surface catastrophic cracks.[1–4] To improve the wear resistance, one strategy is to use ceramic or intermetallic compound powders to produce composites.[5–8] However, unexpected shortcomings (e.g., embrittlement) in such composites usually occur. In addition, so far, other methods, such as electroless coating[9,10] and flame spray processing,[11,12,13] have not been very effective. It is therefore necessary to develop novel materials or techniques to solve this issue. Over the past decades, composite coatings produced by laser cladding have shown great promise in material surface hardening because of their high microhardness, excellent wear resistance, and good corrosion resistance.[14–19] It is noticed that substrate materials include steel, Al-Mg alloy, Mg alloy, etc., whereas reinforcements are mainly ceramic particles, such as Al2O3, TiC, and TiO2, etc. However, the application of powder metallurgy materials as substrates, in particular, the use of nanomaterials or nanoparticles as rein-
forcements of laser cladding layer, has never been reported in the literature. The aim of the present work is to develop a thick hardened layer containing nano-Al2O3 particles on the surfaces of iron-based powder metallurgy materials by means of laser cladding in order to improve the wear resistance. The effects of nano-Al2O3 content and load on wear resistance of surface laser cladding layer were investigated. II.
EXPERIMENTAL
A. Materials Iron-based powder metallurgy materials, 40 3 20 3 15 mm in size, were used as substrates. Nano-Al2O3 (provided by Zhejiang Hongcheng Materials Science and Technology Ltd., Hangzhou, China) and Fe powder were used as the raw materials of the laser surface coating. The surface morphology is shown in Figure 1. Rosin-alcohol solution is selected as the binder. The content of nanoalumina with a crystal structure of a-Al2O3 is 99.99 pct. The size of nanoparticles is 80 to 180 nm and the specific surface area is 14 6 5 m2/g. The energy spectrum of this nanoalumina is shown in Figure 2. The pure Fe powder contains 98 pct Fe and is 5 to 200 mm in size. On the surface of Fe powder, there are many microcavities (as shown in Figure 1). When Fe powder is mixed with nanoalumina by agitation, nanoalumina can enter easily into the microcavity, which can prevent nanoalumina from
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