Microstructure and thermal shock behavior of CoMoCrSi coatings prepared by HVOF and APS

CoMoCrSi coatings with different thickness were prepared by high velocity oxy-fuel spraying (HVOF) and atmospheric plasma spraying (APS), respectively. Variation of microstructure and phase constitution the coatings were investigated by SEM and XRD, respectively. Thermal shock performance of the coatings was evaluated by heating (500 °C and 900 °C) and water quenching method. Compared with APS coatings, the as-sprayed HVOF coatings exhibited denser microstructure and higher hardness. The phase and microstructure of the coatings did not change significantly after 50 thermal cycles at 500 °C. On the contrary, spallation was observed in APS coatings with thickness of ∼350 μm during 900 °C thermal shock cycles. Obvious oxidation appeared in all of the coatings after 900 °C thermal shock. Meanwhile, segmentation cracks perpendicular to the coating surface were found in the edge of 900 °C thermal shocked HVOF coatings (both ∼350 μm and ∼130 μm) and APS coatings (∼350 μm). Element interdiffusion was not detected between the substrate and coating after 50 thermal shocks at 900 °C or 500 °C. After the thermal shock at 500 °C, the microhardness of the ∼130 μm coating did not change greatly while the microhardness of the ∼350 μm coating was greatly improved. After the thermal shock at 900 °C, the microhardness of the coating increased by about 40 % due to the reduction of defects in the coating and the generation of hard phases. At the same time, the migration of elements and the oxidation and recrystallization of some phases may increase the hardness of the coating.