Study on the microstructures and properties of WC-12Co coatings deposited by HVOF and APS
Youqun
CNKI
Abstract
Thermally sprayed Tungsten carbide-cobalt coatings are widely used in aviation,aerospace and automotive industry because of the excellent performance such as high melting point, wear resistance, hardness. Air plasma spaying (APS) and high velocity oxygen fuel (HVOF) have been used for producing high quality carbide coatings. In this study, in order to improve the performance of the steel Q235, WC-12Co powder was sprayed using APS and HVOF processes. Microstructures and mechanical properties of the coatings deposited by those processes were analyzed and evaluated.
The coating properties are influenced not only by the used spray processes but also by the spray parameters. Firstly, parameters impacting on the properties of coating were studied. The proper parameters were obtained when using air plasma spray process.Secondly, microstructures and properties of the coatings deposited by APS and HVOF were comparatively investigated. Based on the results of the experiment designed according to the reference, the conclusions were drawn that the qualified coating was deposited using the parameters as follow: spraying current of 500A, hydrogen flow rate of 2.4L/min,powder feed rate of 50g/min, spraying distance of 100mm. Additionally, Microstructure of WC-12Co coatings deposited by APS and HVOF were evaluated by suing Scanning electron microscopy (SEM), X-ray diffraction (XRD). Meanwhile, mechanical properties of coatings such as micro-hardness, abrasion resistance and adhesive strength were tested.The microstructures and mechanical properties of the coatings deposited by those two different processes were compared. The coatings were deposited by APS and HVOF thermal spraying, hereafter named coating 1and coating 2, respectively. The results are as follows:
⑴ It can be seen that different spraying methods result in different phases. A large volume 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 is attributed to the WC decomposition into W2C and carbon, followed by oxidation process.
After comparison, the compounds which are mentioned above in the coating 1 are more than the compounds volume in the coating 2.
⑵ Using the metallographic method to test the porosity of those two coatings. The result shows that the porosities of coating 1and 2 are 5.0%and 2.4%. So, coating 2 is much denser than coating 1.
⑶ The surface micro-hardness data of coating 1 and 2 are 1083HV and 1486HV, and the cross section micro-hardness of coating1and 2 are 827HV and 1161HV, respectively.
⑷ The wear resistance properties of WC-12Co coatings were examined. The results show that the mass loss of coating 1 is more than that of the coating 2, the coating deposited by HVOF exhibited higher abrasion resistance.
⑸ Adhesive strengths of those two coatings were investigated. The adhesive strength of coating 1 and 2 are 30.0MPa and 55.9MPa, respectively.
All in all, it can be concluded that high velocity oxygen fuel (HVOF) thermal spraying is more suitable for WC-12Co coatings preparation, the high velocity of the flame shortens the time of interaction between the powder and air, low temperature limits WC decomposition. The high kinetic energy of HVOF ensures a good cohesion of the coatings.
Key words: Air plasma spray (APS), High velocity oxygen fuel (HVOF), WC-12Co coatings, Microstructures and properties
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