Mechanical Properties Of Graphene Nanosheets Toughened Composite Coatings Fabricated By Plasma Spray

Al₂O₃ coating can be used to improve the surface properties of metal workpiece due to its outstanding high temperature,corrosion,wear,radiation resistance and electrical insulation.However,the intrinsic brittleness of Al₂O₃ restricts its excellent performance and practical application under harsh service conditions.Hence,introducing one of toughening agents into alumina coatings has been widely recognized to be an effective approach in materials community.Graphene nanosheets?GNSs?possess extraordinary mechanical properties,and therefore GNSs were proposed to toughen Al₂O₃ ceramic coating which fabricated through plasma spray in this research.GNSs were incorporated in submicron Al₂O₃powder matrix through spray drying for the feed stock powder materials.And then alumina composite coatings were fabricated by atmospheric plasma spraying,in which the added contents of the composite coatings were 0.5wt.%GNSs and 1.0wt.%GNSs,respectively.Al₂O₃,0.5wt.%GNS/Al₂O₃ and 1.0wt.%GNS/Al₂O₃ coatings were prepared by atmospheric plasma spraying.X-ray diffraction and Raman spectra were used for phase analysis of the composite coating,and scanning electron microscope was applied to microstructure observation of the reconstituted composite powders and the cross section of the as-sprayed coatings.The mechanical properties such as Vickers-hardness,fracture toughness,elastic modulus and sliding wear properties at room temperature were tested by instrumented micro-indentation,micro-hardness and ball-on-disc instruments.Moreover,toughening mechanism and sliding wear mechanism of GNS/Al₂O₃ composite coatings were discussed.The results of X-ray diffraction and Raman spectra showed that no phase transformation was observed in the raw materials during spray drying process.The microstructure observation indicated that GNSs were generally dispersed well in the alumina matrix.And the GNSs toughening phase were preferentially distributed along the splat-interfaces with the direction which parallel to the surface of the coatings or splats.Mechanical properties of the composite coating were investigated by using instrumented micro-indentation test,micro-hardness and ball-on-disc instruments.The results indicated comprehensive mechanical performance of 0.5wt.%GNS/Al₂O₃coating was the best among the three samples.Compared to pure Al₂O₃,0.5wt.%GNS/Al₂O₃ exhibited 24%,10%and 39%improvement in the toughness,elastic modulus and hardness,respectively.The toughness with 1wt.%GNSs addition was improved by only 5%.The toughening mechanisms of the composite coating include such toughening ways as pull-out,crack deflection and bridging.The addition of GNSs enhanced the friction and wear resistance of composite coatings.The wear mechanisms of Al₂O₃ coating are severe brittle fracture,lamellar exfoliation and abrasive wear.0.5wt.%GNS/Al₂O₃ coating and 1.0 wt.%GNS/Al₂O₃ have similar frictional features,the wear mechanisms of composite coatings include medium brittle fracture,exfoliation,abrasive wear and tribofilms formation of GNSs.The results in this paper show that a small amount of GNSs is more beneficial to the mechanical properties.