| Yttria stabilized zirconia (YSZ) coating has excellent thermal barrier properties, good stability and abrasion resistance at high temperature; therefore it has been widely used in the gas turbine and some components operating in high-temperature condition. The thermal shock resistance property and other aspects of ZrO2 coatings in the atmospheric environment were studied by many researchers. But whether such thermal barrier coatings can be used in the surface of the casting die under the conditions of short-time exposed to molten iron had few reportsed. Therefore, it is very important to study the thermal shock resistance property of the ZrO2 coating under the conditions of heating in molten iron and air-cooled.The NiCoCrAlY2O3 coatings of different thickness and ZrO2 ceramic coatings were prepared on the surface of 304 stainless steel and Q235 steel by plasma spray, the influence of different substrate and different thickness of bond coatings to the thermal shock life were studied after thermal shock. Scanning electron microscopy were used to observe the changes in the microstructure of YSZ coatings before and after thermal shock, to study the failure process of coating during thermal shock; X-ray diffraction was used to analyse the crystal strueture of the as spraied coatings and after thermal shock; Energy dispersive X-Ray was used to analyse the diffusion of elements of coating after dipping in molten iron thermal shock. Moreover, Graded coatings of metal-ceramic were used to reduce the stress generated in thermal shock, improve the bonding strength and thermal shock resistance properties. The NiCoCrAlY2O3-ZrO2 graded coatings were prepared on the surface of Q235 steel by plasma spray, then bonding strength, porosity, hardness and other properties of the graded coatings were measured. Comparison with duplex coating, the thermal shock life of gradient coatings were studyedThe experimental results indicated that:Under the thermal shock conditions of heated in furnace at 1250℃and then cooled by blowing air, the thermal shock life of coating of relatively low coefficient of thermal expansion carbon steel as the base is larger than stainless steel as the base. When the thickness of bonding layer is less than 90μm, the thermal shock life of the coating shown a direct proportional relationship to the thickness of bonding layer; when the thickness of bonding layer is greater than 90μm and less than 130μm, the growth of thermal shock life of the coating doesn't change significantly as the bonding layer thickness increasing. When the thickness of bonding layer is greater than 130μm, the thermal shock life of coating almost doesn't increase with the bonding layer thickness changes.There are some pores and cracks uniform distributed in the ZrO2 as sprayed coatings. The crystal strueture of the as spraied ZrO2 ceramic layer are tetragonal (t) and cubic phase (c). As the number of thermal shock increasing, some of the tetragonal and cubic ZrO2 changed into monoclinic phase (m), microcracks in the ceramic layer grown by the way of connecting irregular pores, finally there is a main crack formed near the side of the ceramic layer through the junction of bonding layer and ceramic layer.Through the test of dipping molten iron thermal shock, a large number of cracks generated in the ceramic layer were discovered. As the the number of thermal shock increasing, in the partial region of ceramic layer, the crack in all directions interweaved together. Elements in molten iron and ceramic elements interdiffused after dipping molten iron thermal shock, the diffusion depth more than 60μm. The Ca in the iron slag and the Zr and O in the ceramic layer formed a new phase.Compared with duplex coatings, the bond strength and thermal shock life of the NiCoCrAlY2O3-ZrO2 graded coating has been enhanced, and the thermal shock life increased nearly 20%. Transition layer applied easing the composition difference and reduce the thermal stress concentration when thermal shock, delayed the growing of cracks in the junction of bonding layer and ceramic layer. Therefore, graded coatings of metal-ceramic have better thermal shock resistance.
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