Pore Formation Mechanism of WC-10Co4Cr Coatings Based on Collected In-flight Particles and Individual Splat

The supersonic plasma spray equipment was utilized to fabricate WC-10Co4Cr coatings. Four spraying parameters with different spraying powers were employed to obtain coatings with different porous structure. The liquid nitrogen was used to freezing in-flight particles with different molten status. The individual splat was deposited on mirror polished stainless steel at preheating temperature of about 200 degrees C. The scanning electron microscopy (SEM) was used to observe the morphologies of coating, raw spraying powder, freezing particle, and individual splat. The field emission microscopy (FEM) was utilized to observe the microstructure of coating, while the energy dispersive spectrometer (EDS) was used to analyze the composition. The results show that with increment of spraying power, the large size pores kept decreasing. With 60 kW spraying power, a few thermal cracks appeared within the coating due to the heat accumulation. Micro hardness of the coatings increased firstly and then decreased with the increment of spraying power. Solidified morphologies of individual splat could be distinguishe as four types, namely, partially melted type, fragmented type, bubble-like type, as well as flower-like type. Pores within the WC-10Co4Cr were mainly formed by shadowing effect of solidified coatings and substrate, gas effect of the micro valley, insufficient wetting of droplet, as well as the pore migration within the liquid phase of the splat.