WC Matrix Composites Toughened By ZrO₂Particles Fabricated By Spark Plasma Sintering

Due to good transverse rupture strength and high hardness, cemented carbides werewidely used in civil and military fields. Among cemented carbides, carbide cobalt alloysystem is one of the most significant cemented carbide in the research and application.Unfortunately, a major disadvantage of WC Co alloy is the low corrosion resistance of thecobalt binder. This problem can be avoided by reducing as much as possible the amount ofbinder phase or finding another attractive binder. Meanwhile, many experimental resultsindicate that ZrO₂can be used as an alternative binder and ZrO-2toughening cementedcarbides matrix composites are of great interest. Therefore, the major focus in the presentresearch are to develop WC matrix composites with ZrO₂binder via spark plasma sintering（SPS）, and the possibility of synthesizing nanocrystalline WC and （W,Ti）C is alsoinvestigated SPS of mechanically alloyed elemental mixed powders. The obtained researchresults are as follows:（1） The effects of sintering temperature, pressure, and holding time onmicrostructure and mechanical properties of sintered WC10wt.%ZrO₂composites werestudied. When the sintering temperature was relative low （e.g.1300C）, or without theholding time, the added m ZrO₂did not fully transformed into t ZrO₂. On the other hand,higher sintering temperature or longer holding time also promoted the grains growth andreduced the effect of the dominant toughening mechanism because a trace amount of t ZrO₂phase transformed back into m ZrO₂. When the sintering temperature ranking from1500to1600C, the transformation of tetragonal （t） to monoclinic （m） ZrO₂phase was fully and theinterface of WC matrix and ZrO₂was combined completely. The SEM micrographs of theWC10ZrO₂fractured specimens indicated that the fracture mode was clearly intergranularin nature. With the increased sintering temperature, the grains size and the fracturetoughness increased; the transverse rupture strength, Vickers hardness increased firstlyamong1300C to1600C, and then decreased above1600C. Higher pressure had littleeffect on the grains size and the fracture toughness, but is helpful to get better mechanicalproperties under lower sintering temperature. With the increased holding times, the relativedensity and the fracture toughness increased basically; the transverse rupture strength increased firstly and then decreased and finally increased; the Vickers hardness increasedfirst and then decreased. The WC matrix composit exhibit the best comprehensivemechanical properties fabricated at1600C and holding for5min at30MPa. It has atransverse rupture strength of1714.6MPa, a Vickers hardness of19.67GPa and a fracturetoughness of6.85MPa m1/2, respectively.（2） In order to make the m ZrO₂fully transformed into t ZrO₂at low temperatureand prevent grains growth at high temperature, two kinds of stage holding sintering processwere selected to carry out experiments. One is heated to lower temperature and holding for10min and then heated to the target temperature. Another is heated to a temperature （about100C lower than the target temperature） and holding for10min and then heated to thetarget temperature. The results indicated that stage holding sintering processes are helpful toimprovement of the mechanical properties of the sintered WC10wt.%ZrO₂composites.Among these two sintering processes, the later is better than the former. The reason is thatthe later can avoid grains growth at high temperature, and also fully complete phasetransformation of ZrO₂at low temperature. However, the former with holding at lowertemperature has little contribution to mechanical properties because of no fully phasetransformation of ZrO₂. The WC matrix composit fabricated at1500C and holding for10min and then heated to1600C has the best comprehensive mechanical properties. It hasa transverse rupture strength of1870.72MPa, a Vickers hardness of19.94GPa and a fracturetoughness of8.42MPa m1/2, respectively.（3） By compare analysis of WC matrix composites with6wt.%,10wt.%and15wt.%ZrO₂, it was found that the toughness of WC matrix composites was enhanced withthe increased ZrO₂content, because the pinning effect exerted by the zirconia particles wasmore obviously. However, the hardness was decreased with the increased ZrO₂content.（4） The possibility of synthesizing nanocrystalline WC and （W, Ti）C wasinvestigated by SPS of mechanically alloyed elemental mixed powders of W, Ti and C.When the rotational speed was200r/min and the milled time was up to100h, wet millingwas failure in forming WC from milled W and C mixed powders. Other milling experimentalso generated WxTi1-xinstead of directly forming （W, Ti）C or TiC. Besides, the graphite Ctransformed into amorphous phase surrounding the formed WxTi1-x. WC and （W, Ti）C was successfully prepared via in situ reactions by SPS at sintering temperatures of1000℃and1050℃using the milled elemental mixture powders. However, when the sinteringtemperature was up to1800℃, the decarbonization of WC during sintering resulted information of W2C phase in the WC sample. Wet milling also had the ability to make themixture powder particles well distributed and homogeneous purified.