Study On Plasma Spraying Process And Properties Of CoMoCrSi Coating On The Surface Of GH907 Superalloy

CoMoCrSi alloy coatings exhibit excellent corrosion–and oxidation–resistances together with high abrasive–wear resistance and anti–scratching ability,which can be used at elevated temperatures up to 800^oC due to its high hardness and operating temperatures.The addition of excessive molybdenum and silicon is beneficial to the formation of Laves phase such as CoMoSi intermetallic compound,which is uniformly distributed into the cobalt matrix.These Laves phases have a high melting point,a high hardness and a low friction coefficient,which is attributed to excellent wear resistance and good comprehensive properties of CoMoCrSi alloy coatings at elevated temperatures.Generally,CoMoCrSi alloy coatings are fabricated by high velocity oxygen fuel spraying?HVOF?,plasma spraying and plasma transferred arc hardfacing,which can be widely used for surface strengthening and repairing of wear–resistant components such as high pressure turbine casing,turbine seal ring,outer ring of high pressure guide,undercarriage,bearing,piston ring,camshaft,extruder cylinder,turbine blade,high and low pressure nozzle for a variety of applications,especially in aviation,automotive,metallurgy and mechanical industries.CoMoCrSi alloy coatings exhibit low coefficients of friction in sliding against different materials such as superalloys,low alloy steel,tool steel,bronze and graphite especially under the conditions of boundary lubrication or dry friction.CoMoCrSi alloy coatings as important surface modification or repair materials for key components can prolong its lifetime and promote its reuse to effectively save energy and resources.Based on the orthogonal experimental design,the present work fabricated CoMoCrSi alloy coatings on GH907 superalloy by plasma spraying,and then investigated the influences of plasma gun structure,spray voltage,current,spray distance and powder feed rate on morphology and properties of CoMoCrSi coatings.The plasma spraying process for CoMoCrSi coatings on GH907 superalloy is optimized by tailoring technological parameters,property evaluation and automatic plasma spraying experiments on simulated components.The microstructure,microhardness,adhesive strength and tribological properties of plasma–sprayed CoMoCrSi coatings were investigated by means of optical microscopy,JSM–6500scanning electron microscopy,D/max2200PC X–ray diffractometer,HXZ–1000microhardness tester,JJ–50KN universal tensile testing machine and UMT–3multifunctional friction and wear tester to further finalize the plasma spray process of coatings or components for verifying tests of practical applications.CoMoCrSi coating fabricated by TriplexPro 210 plasma gun exhibits better comprehensive properties than those obtained by both FH–80 and SX–80 plasma guns.The electrode structure of TriplexPro 210 plasma gun is closely related to the velocity and temperature of plasma jet,and directly determines the porosity and bonding strength of CoMoCrSi coating with the substrate.The plasma spray technological parameters for the TriplexPro 210 plasma gun are optimized to be spray current of520 A,voltage of 95 V,spray distance of 120 mm and powder feed rate of 30 g/min.Plasma–sprayed CoMoCrSi coating consists mainly of CoMoSi,Co₂Mo₃ and Co₇Mo₆phases,and exhibits lamellar structure from the view of coating cross–section.Plasma sprayed CoMoCrSi coatings fabricated by FH–80 and SX–80 plasma guns have an average surface hardness of HR15N 72 and HR15N 78,respectively.However,CoMoCrSi coating fabricated by TriplexPro 210 plasma gun exhibits an average surface hardness of HR15N 81.Plasma sprayed CoMoCrSi coatings fabricated by FH–80 and SX–80 plasma guns have an adhesive strength of 21.1 MPa and 24.4 MPa,respectively.However,CoMoCrSi coating fabricated by TriplexPro210 plasma gun exhibits an average adhesive strength of 42.7 MPa.The TCI image processing software is used to measure the porosities of different CoMoCrSi coatings.The measured porosity of CoMoCrSi coating fabricated by TriplexPro 210 plasma gun is about 4.02%,which is much lower than those of 6.82%and 7.16%obtained by FH–80 and SX–80 plasma guns,respectively.CoMoCrSi coating fabricated by TriplexPro 210 plasma gun has an average friction coefficient of 0.2 and a weight loss of 9 mg.However,plasma sprayed CoMoCrSi coatings fabricated by FH–80 and SX–80 plasma guns have average friction coefficients of 0.25 and 0.45,and average weight loss of 19 mg and 24 mg,respectively.Obviously,CoMoCrSi coating fabricated by TriplexPro 210 plasma gun exhibits a relatively low coefficient of friction and small weight loss under identical wear condition,which is mainly attributed to its low porosity,high microhardness and high adhesive strength.The corresponding wear mechanisms are adhesive wear,microplowing and brittle delamination.