Study On Synthesis Of WC-Al₂O₃Nanopowders And Its Subsequent Hot Pressing

Cemented carbide with superior hardness and strength, high elastic modulus and low thermal expansion coefficient, has been playing an important part in the areas of cutting tools, drilling and mining equipments, wear resistance parts and precision molds. Among hard alloys, WC-Co composites find the widest applications; The WC element is the assurance of high hardness, and the Co element is the guarantee of its fracture toughness. However, Co is rare, expensive and non-renewable strategic resource. Meanwhile, due to its excellent physical, chemical and mechanical properties, the demand of Co in ceramics, food additive, catalyst, batteries, electronic components and other aspects is increasing rapidly. Moreover, because of the low melting point and high chemical activity of Co, it will be soften and react with other elements easily, deteriorating the hardness and corrosion resistance of WC-Co composite. Therefore, it is of great significance and urgency to consolidate new WC matrix composite with cheap raw materials but high hardness and strength to instead of the WC-Co composite.In this paper, Al2O3was chose as the substitute of Co, and WC-Al2O3composite had been consolidated through high energy ball milling and the following hot pressing. The content ratio of elements and the hot pressing parameters were systematically analyzed, the phase transformation of Al2O3during ball milling and hot pressing process, the thermodynamic of as milled powders, the hot pressing schedules and the toughening mechanisms had been investigated in details. Some significant original results are as follows:Firstly, WC-Al2O3composites (WA1) were prepared by high energy ball milling and the following hot pressing. The tungsten carbide (WC) and commercial alumina (Al2O3) powders composed of amorphous Al2O3, boehmite (AlOOH) and χ-Al2O3were used as the starting materials. XRD was used to analyze the phase transformation during the ball milling and hot pressing process. The optimum hot pressing schedule, optimum content of Al2O3and the toughening mechanisms were clarified according to the investigation of microstructure and mechanical properties of the hot pressed bulk samples. The results showed that the powder size decreased and there was no phase transformation during the ball milling process, while during the hot pressing process, the amorphous and transitional Al2O3transformed to γ-Al2O3at961℃, and completely to α-Al2O3at1100℃. The WA1composites combined a relative density of97.98%and an excellent Vickers hardness of18.65GPa with an acceptable fracture toughness of10.43MPa-m1/2when hot pressed at1540℃for90min with a pressure of39.6MPa The second phase toughening mechanism, residual stress toughening mechanism and phase transformation of Al2O3were main toughening mechanisms in WA1composites. The crack bridging, crack deflection and the generation of secondary crack and lateral crack were the main reasons for the high fracture toughness and strength of WA1composites.Secondly, WC-40vol.%α-Al2O3composites (WA2) were hot pressed at1540℃for90min under the pressure of39.6MPa. The toughening effects of phase transformation of Al2O3were investigated by comparison of microstructure and mechanical properties of WAI and WA2. The results showed that a relative density of98.38%and a maximum hardness of16.55GPa, combining a fracture toughness of8.52MPa-m1/2with an improved flexural strength of881.35MPa were obtained for the WA2composites. The relative density and the flexure strength of WA2were higher than that of WA1due to the residual pores induced by the water generated during the phase transformation of Al2O3in WA1; The decarburization of WC in WA1was suppressed may be attributed to the lowered energy provided for it, because many of the system energy was consumed by the phase transformation of Al2O3. The refined microstructure and higher hardness and fracture toughness were due to the decreased volume of Al2O3particles during the phase transformation.Thirdly, to improve the mechanical properties of WA1and WA2, influence of MgO and CeO2additives on the microstructure and mechanical properties, and the mechanisms were investigated. The results showed that adding MgO and CeO2to WA1composite did not improve its mechanical properties; while for the WA2composite, when the MgO and CeO2were added separately, the optimum content was0.1wt.%, they could promote the microstructural refinement and improve the interface coherence of the WC matrix and Al2O3leading to the enhancement of the mechanical properties. Trace MgO mainly acted as an effective grain growth inhibitor for the WC-Al2O3composites and trace CeO2could suppress the decarburization of WC as well due to the unique properties of rare earth elements such as high surface activity and large ionic radius. The synergistic effect of0.05wt.%MgO and0.05wt.%CeO2added in WC-40vol.%α-Al2O3composites resulted in the achievement of a relative density of99.04%with an excellent Vickers hardness of18.18GPa, combining a fracture toughness of10.14MPa-m1/2with an acceptable flexural strength of1158.38MPa.Fourthly, two step sintering (TSS) regimes were designed according to the densification rate and the grain growth rate of WC-40vol.%α-Al2O3composite. Microstructure and mechanical properties of WC-40voI.%α-Al2O3samples under each regime with different T1,, T2and t2were compared and analyzed to obtain the optimum TSS regime, the densification and toughening mechanisms of the composites under TSS regimes were discussed. Then WC-40vol.%α-Al2O3-0.05wt.%MgO-0.05wt.%CeO2powders were hot pressed under the optimum TSS regime to investigate the synergistic effect of additives and TSS regime. The results showed that the mechanical properties of samples under TSS4regime (T1=1600℃, T2=1450℃for6h) were improved compared with samples consolidated under CS1regime (1540℃for90min). WC-40vol.%α-Al2O3-0.05wt.%MgO-0.05wt.%CeO2powders hot pressed under TSS4regime achieved a relative density of99.42%TD with a grain size of2.92μm, combining an excellent Vickers hardness of19.22GPa, a fracture toughness of11.21MPa-m1/2with an acceptable flexural strength of1236.78MPa. The flexural strength was comparable with that of the WC-(3-8)wt.%Co cemented hard alloys (1150-1650MPa).Fifthly, the optimum TSS regime of WC-40vol.%Al2O3composite with WC and amorphous Al2O3was investigated. Influence of Al2O3phase transformation and TSS regimes on the microstructure and mechanical properties of this WC-40vol.%Al2O3composite were illustrated. The results showed that Al2O3phase transformation had been completed when the sample was hot pressed under the first step sintering (1600℃for3min), resulting in the smaller particle size of Al2O3beneficial for the refined microstructure. When the as milled WC-40vol.%Al2O3powders were hot pressed under TSS4regime, a relative density of99%and a grain size of2.38μm were obtained, and an excellent Vickers hardness of19.71GPa was achieved, combining a fracture toughness of12MPa-m1/2with an acceptable flexural strength of1285.03MPa. Compared with the nearly full densified samples consolidated under CS1regime (1540℃for90min), the grain size decreased (2.79μm for the CS1sample), the Vickers hardness, fracture toughness and the flexural strength were all improved (18.65GPa,10.43MPa·m1/2,756.34MPa for the CS, sample) due to the refined microstructure and the transgranular fracture mode.In this paper, amorphous Al2O3and α-Al2O3were used as the substitute of Co. WC-Al2O3composites were consolidated through high energy ball milling, the following hot pressing and two step hot pressing. Modern analytical technologies were used respectively to investigate the phases, thermodynamics and microstructure of the samples. The densification and toughening effects, as well as the influence of additives were illustrated. This paper has made a solid foundation for the further investigation and application of WC-Al2O3composites.