Study On Microstructures And Properties Of WC-12Co Coatings Deposited By HVOF And APS

Thermally sprayed Tungsten carbide-cobalt coatings are widely used in aviation,aerospace and automotive industry because of the excellent performance such as highmelting point, wear resistance, hardness. Air plasma spaying (APS) and high velocityoxygen fuel (HVOF) have been used for producing high quality carbide coatings. In thisstudy, in order to improve the performance of the steel Q235, WC-12Co powder wassprayed using APS and HVOF processes. Microstructures and mechanical properties of thecoatings deposited by those processes were analyzed and evaluated.The coating properties are influenced not only by the used spray processes but also bythe spray parameters. Firstly, parameters impacting on the properties of coating werestudied. The proper parameters were obtained when using air plasma spray process.Secondly, microstructures and properties of the coatings deposited by APS and HVOF werecomparatively investigated. Based on the results of the experiment designed according tothe reference, the conclusions were drawn that the qualified coating was deposited usingthe parameters as follow: spraying current of500A, hydrogen flow rate of2.4L/min,powder feed rate of50g/min, spraying distance of100mm. Additionally, Microstructure ofWC-12Co coatings deposited by APS and HVOF were evaluated by suing Scanningelectron microscopy (SEM), X-ray diffraction (XRD). Meanwhile, mechanical propertiesof coatings such as micro-hardness, abrasion resistance and adhesive strength were tested.The microstructures and mechanical properties of the coatings deposited by those twodifferent processes were compared. The coatings were deposited by APS and HVOFthermal spraying, hereafter named coating1and coating2, respectively. The results are asfollows:⑴It can be seen that different spraying methods result in different phases. A largevolume of WC and Co which is from original powder is still present in the coatings. A lot of W2C and small amount of Co6W6C phases are also observed in the coatings. This isattributed to the WC decomposition into W2C and carbon, followed by oxidation process.After comparison, the compounds which are mentioned above in the coating1are morethan the compounds volume in the coating2.⑵Using the metallographic method to test the porosity of those two coatings. Theresult shows that the porosities of coating1and2are5.0%and2.4%. So, coating2is muchdenser than coating1.⑶The surface micro-hardness data of coating1and2are1083HV and1486HV, andthe cross section micro-hardness of coating1and2are827HV and1161HV, respectively.⑷The wear resistance properties of WC-12Co coatings were examined. The resultsshow that the mass loss of coating1is more than that of the coating2, the coatingdeposited by HVOF exhibited higher abrasion resistance.⑸Adhesive strengths of those two coatings were investigated. The adhesive strength ofcoating1and2are30.0MPa and55.9MPa, respectively.All in all, it can be concluded that high velocity oxygen fuel (HVOF) thermal sprayingis more suitable for WC-12Co coatings preparation, the high velocity of the flame shortensthe time of interaction between the powder and air, low temperature limits WCdecomposition. The high kinetic energy of HVOF ensures a good cohesion of the coatings.