Phase transformation of NiCrBSi–WC and NiBSi–WC arc sprayed coatings

The coating microstructures of electric arc sprayed NiCrBSi–WC and NiBSi–WC coatings were investigated. It was found that, for NiCrBSi–WC coating, the resulting microstructure consists of NiCr, NiCrW solid solutions and WC/W2C as major phases. For NiBSi–WC coating, the major phases are Ni, NiW solid solution and WC/W2C. Some B is present within the coatings as inclusions. The amount of WC/W2C was reduced in both coatings due to the formation of the NiCrW and NiW solid solutions containing a large amount of W. Precipitation of W-rich phase from NiCrW and NiW solutions was also observed but the transformation is restricted by the fast cooling rate characteristic of the spraying process. There is more dissolution of WC/W2C into the matrix in NiBSi–WC coating resulting in a greater reduction in the microhardness of this coating. Wear test results, however, show that even though the NiBSi–WC coating possesses lower microhardness, it is more effective against dry sliding wear than the NiCrBSi–WC coating, owing to better metallurgical bonding between the matrix and the carbide as a result of WC/W2C dissolution.