Fundamental Study On Supersonic Plasma Sprayed Zirconia Coatings On Copper Plate For Continuous Casting Mold

In the processing of continuous casting of steel, the mold is described as the heartof continuous casting machine. Hence the performance and service lifespan of mold arecrucial factors which influence the surface quality, cost of casting slab and efficiency ofproduction. Previous researches and practical applications revealed that thermal cracksand thermal deformation at the meniscus and severe wear at the bottom of the mould arethe main failure modes of the mold. In addition, other factors such as high-temperatureoxidation, chemical corrosion caused by mold flux, cavitation erosion and scratchcaused by width adjusting on-line also contribute to the failure of mold. According tothe relatively statistics, the annual expense for replacement and maintenance of moldwas about2billion Yuan in the main top steel making enterprises in China, while theprimary mode to failure of mold is wear. Therefore, research and development of thelong-lifespan mold has become the key factor to improve surface quality of slab, reducecost of semis and promote the equipment level of continuous casting machine.Plasma spray is a complex process which heats and ionizes the introduced gases bynon-transferred arc generated between anode and cathode to melt sprayed materials intomolten droplets, and to inject the droplets by plasma jet at a particular velocity toimpact and adhere to the target substrate, therefore leading to formation of coatings. Thesprayed coatings can endow the inner surface of the mould with wear resistance,anti-chemical corrosion, high-temperature oxidation resistance and electrical andthermal insulation, radiation-proof, so as one of the most widely used surfacestrengthening and modification technologies, plasma spraying has been successfullyapplied in many important application fields such as aerocraft, gas-turbine, offshore oildrilling, metallurgy, mining and nuclear energy. For providing an alternative for mold materials,8%yttria partially stabilizedzirconia (8YPSZ)-CoNiCrAlY coatings were deposited onto pure copper substrate byHEPJet-high efficiency supersonic atmosphere plasma spray system. The phasecomposition, microstructure, surface roughness and porosity of the as-sprayed8YPSZcoating were characterized by XRD, SEM, color3D laser scanning microscope andimage software. Furthermore，the bonding strength, thermal shock resistance and wearresistance were determined by tensile experiment, thermal shock experiment and wearexperiment at the room temperature under dry friction conditions. Subsequently, basedon the investigation of the effect of spraying parameters (such as spraying power,secondary gas flow rate, and powder feeding rate) on microhardness, property andsurface roughness of the8YPSZ coating, the spraying parameters were optimized.The results show that the8YPSZ coating consists in the exclusivenon-transferrable t’-ZrO2phase and exhibits a bimodal microstructure composed ofwell-melted regions and partially-melted particles. The as-sprayed coating possesses thesurface roughness (Ra) value of6.475μm. Columnar crystals with size of2-5μm wasobserved to grow perpendicular to lamella plane by and large in the fracture structure ofas-sprayed coating, co-existing with some fine equiaxed grains and loose structureresulted from partially-melted particles. However, pores and a certain amount ofmicro-cracks were observed in the cross-section of8YPSZ coating. The ceramic topcoat has a porosity of1.2%. The thermal shock experiment from500℃,650℃and800℃to room temperature showed thatthe as-sprayed coatings pilled after135,58and28thermal cycles, respectively. The failure of coatings samples is mainly attributedto the larger thickness and thermal stress on the hemisphere dome of samples. However,the main body of coatings samples was even free of macro-cracks and the surfacemicro-cracks were observed to stay above the bond coat. Also, there existed a growingtendency in micro-crack ratio with the increment of thermal shock temperature. Thefriction and wear test against GCr15counterpart revealed that compared with coppersubstrate, the8YPSZ coating had a smaller range of friction coefficient fluctuating from0.42to0.50. Moreover, its wear volume was only719.7×10-6mm3and its mechanismmainly displayed wear of the GCr15counterpart. While in the case of anothercounterpart (corundum), the friction coefficient of the8YPSZ coating increased to 0.50-0.60, its wear volume was as high as12550.8×10-6mm3and its mechanics wasabrasive wear resulting from brittle fracture. Nevertheless, the chemical hardeningprocess had a significant effect on micro-hardness and the chemical hardened coatingsample exhibited better tribological property including smaller wear volume of647.5×10-6mm3and degree of brittle fracture. The mechanism of chemical hardenedcoating sample was still abrasive wear resulting from brittle fracture. Themicro-hardness, porosity and surface roughness of the as-sprayed coating had a similartrend of increasing initially and decreasing with the increment of spraying power, thesecondary gas flow rate and powder feeding rate. Based on the optimization of processof parameters, the porosity was0.8%and the adhesion strength with the substrateamounted to above32MPa.