| The ZrB2-SiC composite coating was prepared on the 310S heat-resistant stainless steel substrate by plasma spraying. The preparation process of the coating was optimized. The spraying distance of nano-agglomerated powder and conventional micronized commercial ZrB2-SiC powder was optimized. The optimum proportion of SiC in ZrB2-SiC composite coating was determined,XRD and SEM were used to analyze the organization and the microstructure of the nano-agglomerated powder and conventional micronized commercial powder coating. The bonding strength of coatings were determinated by tensile;Characterization of high temperature oxidation resistance of coatings were tested by static high temperature oxidation test; Oxygen acetylene high temperature ablation method was used to test the performance of the coating under the oxygen acetylene flame; The effects of nano agglomerated powder（n-ZS） and conventional commercial powder （m-ZS） on the morphology,mechanical and physical properties of the prepared coatings were compared.The spraying distance of 50mm-250mm was tested by using nano agglomerated powder （n-ZS） and conventional micron powder （m-ZS）,respectively. The results show that when the spraying distance is too close, it will lead incomplete melting of powder, combined with the substrate is not strong, more powder were floating on the surface of the coating; At the same time, this will make the plasma jet flow to the substrate material, and the quality of the coating will be affected; The spraying distance is too far, which results in the decrease of the velocity and kinetic energy of the powder when it reaches the substrate, and the powders can not form a complete occlusion with the substrate. The sprayed coating was smooth and uniform and the thickness was precisely, showing a better spraying deposition when m-ZS and n-ZS coatings were sprayed in the range of 60-130 mm and 65-140 mm respectively.The microstructure of the coating and the melting state of the powder were analyzed under this range, it is concluded that n-ZS and m-ZS coatings are sprayed at a distance of 100mm, the pore and micro cracks on the surface are the least, and the density of the coating is the highest.When the doping ratio of SiC is 25 wt %, the powder particles have higher spreading degree on the surface of the bonding layer, and the pores and micro cracks were significantly reduced compared with the other 4 kinds of coatings,the surface morphology of ZrB2-SiC composite coating is the most compact at the SiC is 25 wt %. The micro hardness of the coating increases with the increase of SiC content, but the Z25S coating had the highest bonding strength,reaching to 45.0MPa; After high temperature oxidation, the mass gain of Z25S coating was lower than that of the other 4 coatings, and the oxidation products produced on the surface of the coating are less, the oxidation degree is low.Therefore, the optimum SiC content of the ZrB2-SiC coating was determined to be 25 wt%.Using the optimum SiC ratio and the optimal spraying distance, m-ZS and n-ZS coating were sprayed, respectively. The preparation of the coating of n-ZS powder was better after spraying, and the m-ZS powder was more oxidized under the high temperature plasma flame. Comparing with n-ZS coating, there was more pore and micro cracks on the surface of the m-ZS coating, and some powder particle were incomplete melting, which made the density was significantly lower than that n-ZS coating. After 50h.1100℃static high temperature oxidation, both of the coating had mass gain, and some micro cracks and other defects appeared, but the mass gain were less than the substrate, which shows that the ZS coating has protective effect of high temperature oxidation resistance on 310S stainless steel. The mass gain of m-ZS coating was higher than that of n-ZS coating, at the same time, the surface and cross section of the coating appeared large scale oxide,ZrB2 phase in m-ZS coating has been transformed into ZrO2 phase, which shows the m-ZS coating had higher oxidation degree, and the reason for the higher weight gain was demonstrated. It is further explained that the high temperature oxidation resistance of nano agglomerated powder coating is higher than that of conventional micron coating.After oxygen acetylene flame ablation, both of the coating were pulverized,the m-ZS coating ablation duration was longer,reaching to 600s,compared with the n-ZS coating increased about 12 times. The combination of the m-ZS coating and the substrate is still good after ablation. Forming a ZrO2 particles as the skeleton,SiO2,ZrSiO4 and other fluid oxides as the filler of the reaction layer on the surface; Only in the central area due to the mechanical erosion of the flame flow and the formation of holes, and then react with the adhesive layer to generate needle-like θ-Al2O3; Glassy SiO2,ZrSiO4 and other oxides to the periphery of the hole segregation, embedded into the interior skeleton and the surface of ZrO2. After 53s high temperature ablation, the n-ZS coating occurrence of part of the crack peeling, resulting in the failure of the coating. Although the central area of the n-ZS coating surface is not burned, the oxide produced by the reaction is washed away from the coating by the ablated flame flow due to the presence of relatively small nanocrystals, and the surface is left with a large amount of ZrO2; The accumulation of ZrO2 particles and the absence of a fluid oxide filling transition lead to the rapid cracking and peeling of the coating, and the protection of the substrate material is lost. Therefore,the high temperature ablation-resistance ability of the m-ZS coating is stronger than n-ZS coating. |
