| Thermal spraying WC-Co cermet coating is receiving more and more attention because ofits high hardness, excellent wear resistance and good toughness. High velocity oxy-fuelspraying is a favorable method compared with air plasma spray processes and explosionspraying processes due to its higher velocity and lower flame temperatures.For these reasons,thermal spraying WC-Co cermet coating is widely used in aerospace, automotive, metallurgy,electric power, papermaking and other fields to improve the wear resistance of many types ofengineering components or to repair worn parts. However,one or several phases of W2C, metalW and even other Co-W-C amorphous phases can inevitably be generated in the as-sprayedcoating as a result of the decarburization of WC during spraying.This extra phase formation isdetrimental to the performance of the WC-Co coating. Compared with WC-12Co cermetcoating, WC-17Co cermet coating has the better coating-substrate strength, the lower residualstress in the coating and the more extensive potential in engineering application. However,there is few studies related to it so far. In this paper, WC-17Co cermet coating was prepared onthe42CrMo steel by High Velocity Oxygen Fuel (HVOF) spraying. The influence of heattreatment on the phase, microstructure, wear-resistance and corrosion resistance of coating wasanalyzed by means of SEM, EDS, XRD and other instruments. The conclusions are as follows.With the increase of heat treatment temperature, W2C and W phases of as-sprayedWC-17Co cermet coating are reduced to form the Co3W3C and Co6W6C phase. After the heattreatment at low temperature, the release of residual compressive stress expanded the voids ofcoating, which formed the larger pores; after the heat treatment at900℃, as the high-densityCo6W6C phase precipitated from the coating, the locals volume was shrinked, and the numberof pores is reduced.With the increase of heat treatment temperature, the hardness of the coating is firstlyincreased and then decreased. After the heat treatment at700℃, the phases such as Co3W3Cand Co6W6C gradually appear in the WC-17Co cermet coating with the reduction of W2C phase. At the same time, the highest hardness is found for the WC-17Co coating with theincrease of WC phase. When the heat treatment is above900℃, the hardness of WC-17Cocermet coating is reduced with the reason that the more Co6W6C phase precipitated from thesolid state which caused grain growth. The other reason may be that the phases such as W2Cand WC in the cermet coating are reduced.With the increase of heat treatment temperature, the friction coefficient of WC-17Cocoating is firstly increased and then decreased.The lowest friction coefficient is found afterheat treatment temperature at900℃.The as-sprayed WC-17Co cermet coating has the worst sliding-wear resistance. After theWC-17Co coating were heat treated at temperature of500℃,700℃,900℃and1100℃,sliding-wear resistance is improved significantly. After heat treatment at900℃, Co6W6Cprecipitates from the solid state of the coating with fine crystalline grains and a uniformdistribution, which resulted in the lowest wear loss and the best sliding-wear resistance.After heat treatment at500℃, the WC-17Co cermet coating has the lowest weight lossafter288h immersion in the H2SO4solution, which showed the best corrosion resistance.When the temperature of heat treatment was above500℃, W2C and Co-W-C amorphousphases were reduced to form the Co3W3C and Co6W6C, which increases galvanic corrosion atthe carbide-substrate interface of WC-17Co coating in H2SO4solution and which resulted inthe decrease of corrosion resistance of WC-17Co coating.After heat treatment at500℃, the WC-17Co cermet coating has the highest corrosionpotential in the electrochemical test. |
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