Research On Preparation Of Ceramic Particle/Whisker Toughened WC Composites By Spark Plasma Sintering

WC-Co cemented carbides have been widely used as cutting tools and wear-resistantcomponents due to a singular combination of mechanical properties. The addition of Cometallic binder phase to the WC which possesses extremely high hardness and excellentcorrosion-resistance as well as high temperature performance, increases the strength andtoughness of the composite considerably. However, the metallic binders in cemented carbidesare deleterious on hardness and corrosion-resistance, and also be an obstacle for theapplication at elevated temperature where it does soften. Pure WC can be densified by somenewly developing sintering techniques such as spark plasma sintering （SPS）, but the lowtoughness of which limits the application of it. To improve the toughness of WC withoutmetallic binder phase, attention should be attracted to the toughening methods used intraditional ceramics.In this study binderless WC were sintered at different SPS conditions with focus on thedensification behavior, phase constitution, microstructure and mechanical properties of thematerial. In order to elevate the fracture toughness of the binderless WC,transformation-toughening by partially stabilized ZrO₂（PSZ） and particle toughening byAl₂O₃were adopted, as well as whisker toughening by in-situ grown elongated β-Si₃N₄grains, which have not been investigated to date. Moreover,“two-step sintering” wasextended and successfully applied to preparation of WC-Si₃N₄composites with largeelongated β-Si₃N₄grains in a fine WC matrix. In addition, effects of VC and Cr₃C₂additionson the material systems mentioned above were also studied. The main research results are asfollows.（1） Pure WC, WC-x wt.%ZrO₂（x=1，2，3，6，8，10） composites, WC-x vol.%Al₂O₃（x=0，6.8，12.4，16.7，43.6） composites, and WC-x wt.%Si₃N₄（x=1，3，6，8，10，12，15） composites are fabricated by SPS. The addition of ZrO₂/Al₂O₃/Si₃N₄（93wt.%Si₃N₄-6wt.%Y₂O₃-1wt.%Al₂O₃） to WC facilitates sintering of the composites.（2） After SPSed, most of the tetragonal ZrO₂remain until room temperature, and theZrO₂-particles are homogeneously dispersed in the WC matrix. As the ZrO₂content increasesfrom0to10wt.%, the hardness of the WC-ZrO₂composites decreases from²4GPa to¹8 GPa, and the fracture toughness of the WC-ZrO₂composites increases from⁶MPa m^1/2to¹0.6MPa m^1/2. The transformation toughening is believed to be the most importanttoughening mechanism. For the WC-8wt.%ZrO₂composites sintered at1500¹700°C, theaverage size of WC and ZrO₂grains increases with sintering temperature, however, theaddition of VC and Cr₃C₂significantly suppresses the growth of WC and ZrO₂grains. Themicrostructure coarsening at elevated temperatures causes degradation in hardness, whereasthe fracture toughness seems insensitive to the coarseness of microstructure. TheWCVCr-8ZrO₂specimen sintered at1600°C for holding5min using fine WC powder （⁰.2μm） with VC and Cr₃C₂possesses hardness and fracture toughness of22.20GPa and11.40MPa·m^1/2respectively. For the WC-ZrO₂composites, the Palmqvist cracks are typicallyobserved in the high ZrO₂-content grade composites （≥6wt.%）, while the median cracks,corresponding to the half-penny cracks system, are involved for the low ZrO₂-content grade（≤3wt.%）.（3） The toughening effect exerted by Al₂O₃particles is not significant, and the fracturetoughness of the WC-Al₂O₃composites reaches6.49MPa·m^1/2only. The tougheningmechanisms include the crack-bridging by Al₂O₃particles and the local change in fracturemode from intergranular to transgranular.（4） For the WC-Si₃N₄composites sintered with a heating rate of100°C/min, fasttransformation of Si₃N₄from α to β and the β-Si₃N₄grain fast-growth resulted from“dynamic ripening” happen at above1700°C, accompanied with WC-grain fast-growth. Byexploiting the difference in kinetics between WC grain-growth and β-Si₃N₄grain-growththrough sintering at1550¹600°C for holding a long time, the separation of β-Si₃N₄grain-growth from WC-grain fast-growth and the suppression of WC-grain fast-growth areachieved. The WC-10wt.%Si₃N₄specimen after heated to1700°C and treated at1600°C for30min （two-step sintering） obtains large elongated β-Si₃N₄grains in fine WC matrix. For thedense WC-10wt.%Si₃N₄specimen, the hardness increases against the WC-grain size, andthe fracture toughness increases with the amount of elongated β-Si₃N₄grains. The majortoughening mechanisms are found to be elongated Si₃N₄grain-pullout and crack-bridging byelongated Si₃N₄grain. For the two-step sintered WC-x wt.%Si₃N₄（x=1¹5） composites,the α→β-Si₃N₄transition rate and the fracture toughness increase with the Si₃N₄content, and then decrease as the Si₃N₄content exceeds10wt.%. As the Si₃N₄content reaches10wt.%,the α→β-Si₃N₄transition rate reaches¹00%, and the hardness and fracture toughness of thespecimen reaches17.65GPa and10.94MPa·m^1/2respectively. Addition of VC and Cr₃C₂donot hinder the transformation of α to β-Si₃N₄or the growth of elongated β-Si₃N₄grains, butshows an inhibitory effect against WC grain growth, which results in higher hardness for theWC-Si₃N₄composites. The WC-10wt.%Si₃N₄specimen containing VC and Cr₃C₂sinteredat1900°C without holding possesses hardness and fracture toughness of17.43GPa and10.07MPa·m^1/2.