Papers by Keyword: Spark Plasma Sintering (SPS)

Abstract: Ceramic/ metal functionally graded materials (FGMs) have been promising to sustain coating structures working under super high temperature as well as high temperature gradient conditions. Compositional gradients in the FGMs can be engineered according to functional performance requirements. This study aims to fabricate Al₂O₃/ZrO₂/SUS304 hybrid FGMs with continuous compositional gradient manners using a combination of centrifugal slurry methods and spark plasma sintering (SPS). The compositional gradients in the FGMs were investigated on microstructures with elemental distributions and hardness on the cross sections of the FGMs. It was demonstrated that the compositions of ZrO₂ and SUS304 continuously varied in the FGMs, while Al₂O₃ resided only on ZrO₂-rich sides, which can effectively enhance the fracture toughness of the ZrO₂-rich layer. Ball milling treatments can make the Al₂O₃ layer more formed in the ZrO₂-rich layer. With increasing the amount of Al₂O₃, the Al₂O₃ layer resided closer to the top of the ZrO₂ surfaces in the FGMs subject to ball milling treatments, which can prevent the crack propagation from the ZrO₂ top surfaces.

Abstract: Ceramic-metal functionally graded materials (FGMs) are advantageous to two dissimilar materials joined directly together, which includes smoothing of thermal stress distributions, minimization or elimination of stress concentrations and singularities at the interface corners and increase in bonding strength. In this study, ZrO₂/ 304 stainless steel (SUS304) FGMs with continuous gradient manners, not stepwise manners, were fabricated by a combination of centrifugal slurry methods and spark plasma sintering (SPS). The size and surface smoothness of the powders of SUS304 highly affected formation of compositional gradient patterns in the FGMs. Effects of ball milling time and ball sizes on such conditions of the powders as well as compositional gradients in the FGMs were investigated by microstructure observations with element analysis and hardness probing on the cross sections of the FGMs.

Abstract: Recently much interest has been attracted in replacing heavy metals with light metals such as aluminum to reduce weight to improve fuel efficiency of automobiles. However, weight reduction by light metals often leads to decrease in strength, which causes problems in terms of safety and sustainability. This study aims to produce carbon nanotube (CNT) reinforced aluminum (Al) matrix composites with high performance using a combination of spark plasma sintering (SPS) and hot rolling techniques. Dry ball milling and SPS followed by hot rolling were conducted to uniformly disperse CNT in Al matrix. The microstructures of the composites including dispersibility of CNT were observed using a scanning electron microscope (SEM) and transmission electron microscope (TEM). Anisotropic mechanical behavior was investigated through Vickers hardness and tensile tests. The experimental results demonstrated that the post-sintering hot rolling can highly enhance the tensile strength of the composites.

Abstract: Functionally graded materials (FGMs) are multi-phase composites to be engineered with gradual spatial variations of constituents. Centrifugal techniques enable to have a continuous gradient manner in FGMs. In this study, zirconia (ZrO₂)/ 304 stainless steel (SUS304) FGMs were fabricated using centrifugal slurry methods with powders of ZrO₂ and SUS304 mixed with a dispersant, ammonium polycarboxylic acid (PCA). The green body samples produced by centrifugal techniques were consolidated using spark plasma sintering (SPS). The fabricated samples were investigated on their microstructures and contents using scanning electron microscope (SEM) with EDX and micro Vickers and nanoindentation hardness. The results demonstrated that controlling of the continuous gradient were possibly achieved using the combination of the current techniques.

Abstract: Ceramics/ metal functionally graded materials (FGMs) have been attracting much attention as composite materials that are intended for application to high-temperature structures. This study fucuses on toughening of ceramics and their matrix composites, which are building blocks of the FGMs. Partially stabilized zirconia (PSZ) with toughening additives such as Ni particles, Inconel fibers, SiC particles, SiC whiskers and a-alumina (Al₂O₃) particles were fabricated by spark plasma sintering (SPS). Toughening mechanisms were investigated based on their micro and macro-scale mechanical properties and microstructures. Disk bending tests, producing balanced biaxial loading conditions, were conducted probing bending strength at macro scales, while Vickers hardness tests were carried out to estimate fracture toughness and examine toughening mechanisms at micro scales. The results demonstrated that toughness of ZrO2 increased by adding Ni particles, Inconel fibers and Al₂O₃ particles. Addition of Al₂O₃ particles highly enhanced disk bending strength and deformability.

Abstract: Spark plasma sintering can be carried out at a low temperature with short heating, holding and cooling times, which allows for the fabrication of non-equilibrium materials. In this article, metal-matrix-composite fabrication with thermally unstable reinforcements by spark plasma sintering is described.

Abstract: This study aims to fabricate SiC whisker (w)/ particle (p)-reinforced magnesium (Mg) composites with enhanced mechanical properties using spark plasma sintering (SPS) methods. It has been confirmed that dispersing state of SiCw can be improved by addition of SiCp. However, due to presence of voids and cracks between the oxide layers, surrounding SiCw/p, and Mg matrix in the composites, SiCw with SiCp cannot contribute to enhance the bending strength of Mg matrix. This issue can be tackled by adding low melting point metals such as Sn into the composites to fill the defects in the composites.

Abstract: BN-Si₃N₄ composite ceramic wave-transparent materials with excellent mechanical properties were prepared by spark plasma sintering (SPS) using h-BN and α-Si₃N₄ powders as raw materials, Al₂O₃ and Y₂O₃ as sintering aids. The influence of sintering pressure on density and mechanical properties of BN-Si₃N₄ composite ceramics were studied. The phases were observed by X-ray diffraction (XRD), and the microstructures were identified by scanning electron microscopy (SEM). The results showed that with the sintering pressure increases, the relative density, bending strength and fracture toughness of the composite ceramics were significantly increased, and the porosity decreased rapidly. The effects of pressure on the properties of the composite ceramics was not significant at >40MPa, so 40MPa is optimal for the composite ceramics to gain good overall performance, i.e. the relative density was 89.1%, the porosity was 2.3%, the bending strength reached 215.4 MPa, and the fracture toughness was 3.1/MPa·m^1/2.

Abstract: In the present paper, the ZrB₂/h-BN multiphase ceramics were fabricated by SPS (spark plasma sintering) technology at lower sintering temperature using h-BN, ZrO₂, AlN and Si as raw materials and B₂O₃ as a sintering aid. The phase constitution and microstructure of specimens were analyzed by XRD and SEM. Moreover, the effects of different sintering pressures on the densification, microstructure and mechanical properties of ZrB₂/h-BN multiphase ceramics were also systematically investigated. The results show that the ZrB₂ was obtained through solid phase reaction at different sintering pressures, and increasing sintering pressure could accelerate the formation of ZrB₂ phase. As the sintering pressure increasing, the fracture strength and toughness of the sintered samples had a similar increasing tendency as the relative density. The better comprehensive properties were obtained at given sintering pressure of 50 MPa, and the relative density, fracture strength and toughness reached about 93.4%, 321 MPa and 3.3 MPa·m^1/2, respectively. The SEM analysis shows that the h-BN grains were fine and uniform, and the effect of sintering pressure on grain size was inconspicuous. The distribution of grain is random cross array, and the fracture texture was more obvious with the increase of sintering pressure. The fracture mode of sintered samples remained intergranular fracture mechanism as sintering pressure changed, and the grain refinement, grain pullout and crack deflection helped to increase the mechanical properties.

Abstract: The early-stage sintering behaviours of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramics during spark plasma sintering (SPS) was investigated using different pressure and heating regimes.It was found that dependent neither on pressure value (20~100 MPa) nor heating rates higher than 50 °C/min, the maximum densification rate had always been observed at rather similar ~78% of theoretical density (TD), where the grain growth was rather moderate. A novel intensive-particle-rearrangement mechanism was proposed to dominate the rapid densification of early-stage SPS process, by which yielded the considerable faster densification rate than those achievable by diffusion-related processes.Present findings showed the possibility of particle rearrangement in high density compacts and the effects of classic particle rearrangement should be re-evaluated in nanoceramic sintering.