Study On Laser Cladding Self-lubricating Wear-resistant Composite Coatings On Austenitic Stainless Steel

Due to its good mechanical and high chemical stability, austenitic stainless steel findsextensive applications in mechanical, chemical, medical and architectural as well asnuclear engineering areas. But it can’t be used as key tribological moving componentsbecause of its relatively lower hardness and wear resistance. Moreover, there are manytribological components working in high temperature aggressive environments whereliquid lubricants and greases cannot be used, demanding combination of high-temperaturewear, oxidation and self-lubricating properties. Fabricating a self-lubricating wear-resistantcoating with advanced surface technology on a tribological component surface is one ofmost efficient and economic ways to solve the problem.Laser cladding is fundamentally a type of coating technology that utilizes a focused ora defocused high-power laser beam to locally melt the thin surface layer of a substrate andthe added materials while at the same time forming a new layer of material with desiredproperties after solidification. A great variety of powder materials or powder mixtures canbe effectively deposited onto the substrates to achieve various properties such as wearresistance and corrosion resistance. The microstructure of the deposited layer is very fineand has the typical characteristic of rapid solidification, so it can achieve completemetallurgical bond at the interface.In order to improve the wear resistance of austenitic stainless steel and develop newsystems of advanced composite coating with good wear resistance and anti-friction, theauthor individually used NiCr/Cr₃C₂, NiCr/Cr₃C₂-15WS₂-15CaF₂andNiCr/Cr₃C₂-15WS₂-30CaF₂alloy powder as raw materials, fabricated self-lubricatingwear-resistant coating on austenitic stainless steel by laser cladding. Microstructures of thecoatings were characterized by X-ray diffractometer （XRD）, scanning electron microscope（SEM） and energy dispersive spectrometer （EDS）. Microhardness of the coatings wasmeasured by a microhardness tester. The friction and wear properties of the coatings under different temperatures and loads were investigated, and the corresponding wearmechanisms were analyzed.The results showed that a wear resistant coating reinforced with hard （Cr,Fe）₇C₃carbide and toughened by ductile γ-（Ni,Fe） solid solution was fabricated by laser claddingusing NiCr/Cr₃C₂as precursor. The thickness and average microhardness of the coating isabout0.8mm and850HV0.3, respectively, and the bonding to the substrate is metallurgical.The microstructure is dense and uniform. A self-lubricating wear-resistant coatingconsisting of （Cr,Fe）₇C₃/WC, γ-（Ni,Fe）, WS₂and CrS were obtained usingNiCr/Cr₃C₂-15WS₂-15CaF₂as precursor. The thickness and average microhardness of theself-lubricating wear-resistant composite coating is about0.6mm and620HV0.3,respectively. When the precursor changes to NiCr/Cr₃C₂-15WS₂-30CaF₂, the thickness ofthe coating is only about0.25mm.At room temperature, NiCr/Cr₃C₂coating exhibits superior wear resistance. Frictioncoefficient and wear rate of NiCr/Cr₃C₂-15WS₂-15CaF₂coating are slightly higher thanNiCr/Cr₃C₂coating but far lower than stainless steel substrate. With the increase oftemperature, both the friction coefficients and wear rates of NiCr/Cr₃C₂composite coatingand stainless steel substrate gradually increased while that of NiCr/Cr₃C₂-15WS₂-15CaF₂coating decreased. The lubricant phase spreading out onto the worn surface at300℃wasbeneficial to the friction-reducing and wear-resisting. Whereas, the lubricant phasecouldn’t provide effective lubricious effect due to its oxidation at600℃and the wear ratesof NiCr/Cr₃C₂coating and NiCr/Cr₃C₂-15WS₂-15CaF₂coating were very close. Both thefriction coefficient and wear rate of NiCr/Cr₃C₂-15WS₂-15CaF₂coating firstly decreasedand then slightly increased with the increasing normal load.In conclusion, laser cladding NiCr/Cr₃C₂-15WS₂-15CaF₂self-lubricating compositecoating possesses excellent wear resistance and anti-friction under moderate temperature（300℃） andmoderate load （5N）.