Research On Preparation Of Alumina-Zirconia Toughened Tungsten Carbide Composites By Spark Plasma Sintering

WC-Co cemented carbides have been widely used in manufacturing,road construction and national defense industries due to their superior properties such as high hardness,wear resistant and low thermal expansion coefficient.Cobalt?Co?is added as a metallic binder phase to significantly improve fracture toughness and overall strength of the WC matrix.This Co binder however will get oxidized and softened easily at high temperatures,which limits the high temperature applications of WC-Co cemented carbides.In our previous research,particle toughening Al2O3 and transformation toughening ZrO₂ ceramic particles were employed to replace Co binder,producing WC-Al2O3 composites with high hardness but low fracture toughness and WC-ZrO₂ composites with high fracture toughness but low hardness respectively.To mitigate this trade-off,both Al2O3 and ZrO₂ were introduced into WC ceramics to obtain both high hardness and high fracture toughness in this research.On the other hand,the Al2O3-ZrO₂ oxide eutectic ceramics are promising high temperature structural materials and they can be formed at a eutectic point of 1860°C which approaches to the sintering temperature to prepare WC-Al2O3 or WC-ZrO₂ composites by spark plasma sintering?SPS?.However,up to date,there are no articles reporting whether Al2O3-ZrO₂ oxide eutectics can be fabricated by SPS or not.Therefore the behaviour of Al2O3-ZrO₂ with eutectic ratio fabricated by SPS was necessarily investigated to understand its influence on WC composites fabricated by SPS as well.Then a range of Al2O3-ZrO₂ with eutectic ratio into WC composites was researched to figure out the effect of eutectic constituent on the relationship between sintering behavior,phase composition,microstructural envolution and mechanical properties.Guided by our previous work,an optimized ratio of Al2O3-Zr O₂ was introduced into WC ceramics.The dependence of mechanical properties and microstructure on sintering temperature was investigated in detail and the mechanisms to toughen and strengthen were identified and explored.The Al2O3-ZrO₂?Y2O3?eutectic oxide ceramic can be successfully fabricated via SPS and the melting point is lower than the eutectic temperature in Al2O3-ZrO₂ binary phase diagram.When sintered at 1600°C,the morphology of the specimen presents typical lamellar eutectic microstructure.While sintered at 1500°C,the morphology of the bulk specimen possesses transitional microstructure including continuous-net-shaped t-ZrO₂ and isolated ?-Al2O3.When sintered at 1400°C,the morphology of the specimen exhibits a sintering microstructure in which ?-Al2O3 and t-ZrO₂ disperse disorderly and inhomogeneously.The x wt.% Al2O3-ZrO₂ with a eutectic ration?x=1,3,6,9,12?eutectic oxide into WC composites were fabricates by SPS at 1600 °C.As the fraction of Al2O3-ZrO₂?eutectic ration?increases from 1 wt.% to 12 wt.% into WC,all the composite specimens produce Al2O3-ZrO₂ liquid eutectics and get densified gradually.The size of eutectic Al2O3-ZrO₂ colony grows with an increasing content of itself,while the required temperature to melt it falls from ¹600°C to ¹550°C.The solution re-precipitation of WC grains facilitates their growth,while the pinning of second phase particles inhibits their growth.Hence,the size of WC matrix firstly increases then declines due to the competition of the two foregoing mechanisms.The hardness of WC-x wt.% Al2O3-ZrO₂?eutectic ration??x=1,3,6,9,12?goes up to its peak at x=6 wt.% then drops as the eutectic content increases,while the fracture toughness shows a generally escalating trend,other than the x= 3 wt.% specimen.In the morphology of the x=3 wt.% specimen,WC grain grows up to an abnormal size?>200?m?and embeds Al2O3-ZrO₂ colony group in itself,leading to many defects in it.When a crack arrives,it will not extend along but go through the defected WC grain directly.In consequence,the x=3 wt.% specimen exhibits poorest mechanical properties.In the microstructures of specimens sintered at the temperatures from 1400 to 1600°C,the second phase of Al2O3-Zr O₂ change its microstructural morphology from sintering arrangement to casting arrangement.The second phase of Al2O3-ZrO₂ shows a disorder and inhomogeneous dispersion of sintering microstructure at 1400°C,a transitional microstructure at1500°C,and a typical lamellar eutectic microstructure at 1600°C.When the sintering temperature rises from 1600 to 1650°C,the lamellar eutectic colonies somewhere coalesce and grow to a large size rapidly.Combined with our previous work and the results of the first two chapters,WC-2.8 wt% Al2O3-6.8 wt% ZrO₂ composites without any metallic binders were optimized and fabricated by spark plasma sintering over a temperature range from 1350 to 1600 °C to get rid of a trade-off between hardness and fracture toughness.WC grains grow slowly as sintering temperature increases from 1350 to 1500 °C.However,growth accelerates once the sintering temperature reaches 1600 °C due to the formation of an Al2O3-ZrO₂ liquid eutectic.The grain size of WC matrix is responsible for the hardness of this composite,while the dispersion of Al2O3-ZrO₂ group effects the fracture toughness.The composite sintered at 1450 °C exhibits a combination of high hardness?21.0 GPa?,generated by fine-grained WC,and remarkable fracture toughness?8.5·MPa m1/2?,resulting from the homogeneous dispersion of secondary phase particles.Furthermore,the toughening mechanism of the WC-2.8 wt% Al2O3-6.8 wt% Zr O₂ composite involves crack deflection,crack bridging,a transformation from intergranular to transgranular fracture of the WC,and a phase transformation of ZrO₂ induced by the stress in front of the crack-tip.