Development of Oxide-Free Oxide Materials under Spark-Plasma Sintering Conditions of a Mixture of Oxide-Free Components
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Vol. 61, No. 1, May, 2020
DEVELOPMENT OF OXIDE-FREE OXIDE MATERIALS UNDER SPARK-PLASMA SINTERING CONDITIONS OF A MIXTURE OF OXIDE-FREE COMPONENTS AND VARIOUS METAL POWDER ADDITIVES A. V. Hmelov1,2 Translated from Novye Ogneupory, No. 2, pp. 17 – 25, February, 2020.
Original article submitted December 16, 2019. The effects of W and Mo power additives during spark-plasma sintering of compositions at compression force 60 MPa at 1200 – 1600°C on the phase composition, microstructure, crystalline-phase grain size, relative density, linear shrinkage, physicomechanical properties, and the linear correlation of the elasticity modulus and fracture toughness of mullite–ZrB2–b-SiC–TaC samples were studied. The synthesized ZrB2, b-SiC, and TaC powders were characterized by extensive crystallization of ZrB2, b-SiC, and TaCcub, respectively. Sintered samples with W and Mo powder additives showed extensive mullite formation and more active crystallization of b-SiC, (Zr, Ta)B, C, b-WC, and b-WB than (Ta, Zr)C, B, b-MoC, b-MoB, and MoB2 at 1200 – 1600°C. The microstructure of the sample with the Mo additive at 1500°C was more uniformly and densely sintered and crystalline; contained fewer pores and weakly sintered areas; and was more reinforced at the boundaries of oxide-free oxide and oxide-free crystalline phases. This sample had characteristically smaller grain sizes of the crystalline phases at 1400 – 1600°C and showed uniform growth of the relative density, linear shrinkage, and physicomechanical properties; higher crack resistance (with propagation of microcracks along a meandering path), and a better linear correlation of the elasticity modulus and fracture toughness at 1200 – 1600°C. Keywords: mullite–ZrB2–b-SiC–TaC–W, mullite–ZrB2–b-SiC–TaC–Mo, solid solutions (Zr, Ta)B, C and (Ta, Zr)C, B, spark-plasma sintering.
Sintering of two and more oxide-free powders occurs mainly via solid-state reactions with minimal transition of the components into a viscous state or none at all, irregular sintering of the particles, and filling of pores by matter with increasing temperature and compression force [1 – 3]. Furthermore, sintering of oxide-free powders causes compatibility problems, especially in sintered mixtures of multicomponent powders because of strong covalent bonds and large differences in the diffusion coefficients in oxide-free powders [3, 4]. As a result, a heterogeneous microstructure with many weakly sintered and incompletely consolidated boundary areas of oxide-free crystalline phases forms and causes brittleness, reduced friction resistance with development of microcracks at the boundary areas of the oxide-free 1 2
crystalline phases, and decreased the physicomechanical properties [1 – 4]. Various traditional approaches, e.g., addition to the mixture being sintered of oxide-free components of Al2O3 and SiO2 oxide powders [5], Y2O3 powder forming low-melting eutectics with the starting components [6], and tetragonal-cubic ZrO2 [7] stimulating uniform and more complete diffusion in the viscous, liquid, and solid ph
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