| The traditional caibide mainly is tungsten cobalt composites,in which Co has a good wettability and adhesion to improve the mechanical properties of composite material. As a novel metal, reserves of Co are extremely limited. In addition, Co will be oxidation at high temperature. Thus, it is of great significance to find a new material with both superior hardness and fracture toughness for simple cost of manufacturing, whose raw material is easy of access, as the substitute material for the commonly used.As early as the sixties and seventies of the last century, some researchers proposed the ideas that addition Fe、 Ni in cemented carbide instead of Co, the results show that the corrosion of material was imporved, but the mechanical performance was less than WC-Co alloy because of some factors limit of Fe、 Ni. In recent years, some researchers proposed to study the material used TiC、 ZrO2、 MgO and made some research.Al2O3belonds to ceramic material.Amorphous Al2O3takes place phase transformation during the sintering process and the phase change process is not reversible. The transition between χ-Al2O3、κ-Al2O3、γ-Al2O3、δ-Al2O3、θ-Al2O3is only the position change between atoms, dose not destroy the chemical bonds. But the phase change from γ-Al2O3to a-Al2O3needs destroy the chemical bonds because it is a nucleation and growth process. There is no chemical reaction between WC and Al2O3and Al2O3has lower melting point than WC, but the thermal expansion coefficient of two material is similar(WC is3.84×10-6/K、Al2O3is7.0×10-6/K). Although many researchers study the composites of WC-Al2O3, but the powders fabrication and sintering the WC and amorphous Al2O3conposites has not been reported.Hot-pressing sintering (HPS) is an effective sintering method in powder metallurgy. The characteristics of the hot pressing are that the mixed powder is synchronously heated and pressed to be shaped. Its equipment is relatively simple and can achieve products with high density and good properties. On the other hand, without the addition of molding agent, the method can decrease impurity in the cementation process.The study mainly includes:1. To determine the phase transformation process of amorphous Al2O3during the sintering process. WC-Al2O3powders have been fabricated by mechanical alloying method. Microstructure of powder is characterized by X-ray diffraction (XRD) and SEM. Then thermal analysis the as-milled powders by Netzsch409P simultaneous thermal analyzer, find the thermal change point of amorphous Al2O3during sintering process, select these thermal change point as the sintering temperature and sintere as-milled powders. The phase of as-sintered bulk are characterized by X-ray diffraction(XRD).The phase transformation process of amorphous Al2O3during the sintering process is:amorphous Al2O3'γ-Al2O3'α-Al2O3and at temperature of1100the amorphous Al2O3completely transform α-Al2O3.2. WC-Al2O3powders have been fabricated by the same mechanical alloying method. These as-milled composite powders were hot pressing sintered in a graphite die with multiple parameter circulation. Microstructure and phase of as-sintered bulk are characterized by XRD and SEM. The density of the specimens and the hardness are measured and determined. Then the fracture toughness are calculated and estimated by indentation cracks produced by the hardness measuring. Result shows that WC-Al2O3composites powder can be compacted in the condition of1540℃(sintering temperature),90min (keeping time),39.6MPa (pressure), and the relative density of the as-sintered bulk is97.98%, Vickers hardness is18.65GPa, fracture toughness is10.43MPa-m1/2. WC-Al2O3composites with the performance of combination high hardness and toughness is due to two-phase uniformly distributed,high density, crack deflection and crack bridging appearancing the crack propagation.3. These as-milled composite powders added in different contents of Y2O3were hot pressing sintered in a graphite die in the condition of1540℃,90min,39.6MPa. Microstructure and phase of as-sintered bulk are characterized by XRD and SEM. The density of the specimens and the hardness are measured and determined. Then the fracture toughness are calculated and estimated by indentation cracks produced by the hardness measuring.The bulk adding0.25%Y3O3has wonderful mechanical properties, the density、haedness and fracture toughness are98.95%、19.31GPa and10.85MPa·m1/2, respectively. The addition of Y2O3can accelerate the sintering WC-Al2O3composite and increase the density, enhance the particulate dispersion homogeneity and the matrix/particulate interfacial coherence, inhibit the coarsening of particle and grain. |
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