SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.     

Numerical modeling of liquid-phase infiltration in the process of sintering ceramic composites

The paper presents a numerical model for infiltration of porous preforms of ceramic materials in the process of sintering. The mathematical model of infiltration is based on the Richards equation and Van Genuchten formulas describing permeability properties of the porous structure and the infiltrant. A finite-element method and stable implicit finite-difference time scheme are applied for discretization of the Richards equation. Based on the outcome of solving test problems, a comparison of the numerical results with the results obtained by other researchers and with the experimental data, the numerical model has been proved adequate. The paper provides some findings of computer modeling of infiltration of liquid silicon into porous silicon carbide preforms of various geometries and dimensions.     

Rapid fabrication of in situ TiC particulates reinforced Fe-based composites by spark plasma sintering

TiC particulates reinforced Fe-based composites have been fabricated using ferrotitanium and carbon black powders with the combination of in situ and spark plasma sintering (SPS) technique. The sintering and densification behaviors were investigated. The results show that when the composite was sintered at 1150 °C for 5 min, the maximum relative density and hardness are 99.2% and 83.2 HRA, respectively. The phase evolution during sintering indicates that the in situ reaction occurs evidently between 850 °C and 1050 °C. The microstructure investigation demonstrates that with the rapid in situ SPS technique, fine TiC particulates with a size of ~ 1 μm are homogeneously distributed in the matrix.     

Fe-based metallic glass particles reinforced Al-7075 matrix composites prepared by spark plasma sintering

Metallic glass (MG) reinforced aluminum matrix composites (AMCs) have attracted the interest of many researchers in the past few years. In this study, Fe₅₀Cr₂₅Mo₉B₁₃C₃ metallic glass (FMG) particles reinforced 7075 aluminum matrix (Al-7075) composites were prepared by spark plasma sintering (SPS) technique. The microstructure of the composites showed good interface bonding between the FMG particles and the matrix. The micro-hardness of the composite with 30 vol% FMG particles reached 160.63 HV, which was increased by 30% compared with that of Al-7075 (119.3 HV). The ultimate compression strength (UCS) of the composite was also improved significantly from 596 MPa for Al-7075 matrix to 749 MPa for the composite reinforced with 30 vol% FMG particles, and the compression strain of the composite reached 22%. These results indicate that the mechanical properties of the composites can be enhanced by adding high volume fraction FMG particles. The enhancement of the strength is resulted from multiple strengthening mechanisms, and the main contributions come from the thermal mismatch and grain refinement.     

Fracture in ceramic-reinforced metal matrix composites based on high-speed steel

A series of metal matrix composites based upon M3/2 high-speed steel was produced by a powder metal sintering route. Hard ceramic titanium carbide or niobium carbide additions and a self-lubricant in the form of manganese sulfide, were added as a basis for achieving improved wear resistance and reduced friction. After sintering, the composites were given a full standard high-speed steel heat treatment and subjected to mechanical tests. All three particulate additions had a deleterious effect on three-point bend strength, particularly in the case of MnS addition, mainly due to the ease of initiating cracks at or near to the particulate additions. Bend strengths were further reduced by the simultaneous addition of both MnS and either TiC or NbC especially when a high volume fraction of approximately 25 vol % MnS was added. Single, low volume fraction (8%) additions, of TiC, NbC, or MnS, had little effect on fracture toughness and KIC values were comparable to those found in the baseline M3/2 steel. Slight improvements in fracture toughness shown to occur in the 7.74% NbC composites were attributed to energy dissipation caused by the effects of crack branching during crack propagation. Composites with the higher volume fraction additions of MnS and ceramic carbide gave poor fracture toughness by forming MnS/carbide clusters which provided an easy path for crack propagation. © 1998 Chapman & Hall     

Investigation of liquid-phase sintering by image analysis

The liquid-phase sintering of austenitic stainless steel alloyed with boron was studied by means of image analysis. By evaluating fractional porosity, the amount of liquid constituent, grain size, and pore morphology, the effect of boron content and sintering time on the sintering process was determined and discussed on the basis of the classic theory of liquid-phase sintering. Image analysis proves to be a powerful tool for optimizing the liquidphase sintering process.     

On the control of the liquid-phase distribution in multi-material assemblies processed by liquid-phase sintering

Proper control of composition gradients in components processed by liquid-phase sintering requires the control of the migration of the liquid phase during sintering. A model considering isotropic interface energies has been developed for expressing the variation of the solid–liquid interface area with liquid volume fraction during the final stage of liquid-phase sintering (i.e. when residual porosity has disappeared). The model allows the computation of the driving forces for shape accommodation as a function of the dihedral angle ψ, liquid volume fraction, u, particle volume, V_p, and average particle coordination, n_c. It particularly enlightens the coupling between the effects of ψ and of n_c. The model allows a new insight into the control of microstructural evolution during liquid-phase sintering of assemblies of dual-phase materials with different average particle size. Taking ψ = 0° for the WC–Co system, the model was found to predict with a good accuracy the distribution of Co at equilibrium in assemblies made by heat-treating at 1400 °C under protective atmosphere stacks of cylinders of WC–Co cermets differing in mean WC particle size and cobalt content.     

液相烧结碳化硅喷雾造粒工艺控制

阐述了液相烧结SiC陶瓷喷雾造粒过程中的相关制备工艺,研究了浆料的特性、干燥温度、喷雾压力等因素对粉体性能的影响.并通过实验总结出一套合理的工艺参数,制备出球形实心颗粒粉料.粉料的松装密度0.88 g/cm3、休止角33.5°、平均粒径50μm,具有良好的流动性及快速填充性,可直接用于干压成型.     

Control of properties of PZT ceramics during liquid-phase sintering

A PZT ceramic modified by 0.4Pb-(1–x)(B₂O₃)–x(GeO₂) glass, and glass-ceramics modified by V₂O₅, have been obtained. The influence of the chemical composition of the glasses on the mechanical strength and dielectric parameters of these ceramics is investigated. The mechanical and electrical parameters of PZT ceramics are considerably improved by small additions of glass ( 1 mass %) of suitable composition. During heat treatment, the mobility of the domain walls may be changed as a result of the interaction of smelt glass with the surface layer of the crystallites (resulting from a change of V _Pb ^t" vacancy concentration in grains and decreased mechanical tension on the grain boundaries). By introducing the glass, we can decrease the porosity of the ceramics and thus decrease the attenuation of surface acoustic waves in piezoelectric filters. The origin of the liquid phase during sintering of the PZT glass-ceramics prevents PbO evaporation. This determines the preservation of the solution's stoichiometry. The glass addition leads to a decrease in the sintering temperature of PZT, which simplifies the technological process. The glass ceramics are obtained by common ceramic technology which is used for commercial piezoceramic production. The main technological problem is choosing the chemical composition and concentration of the glass and thermal process parameters.     

Microstructural development during the liquid-phase sintering of VC-Co alloys

A study was made of the microstructural changes that occur during the liquid-phase sintering of VC-Co alloys. It was found that considerable growth of the carbide grains takes place and that this growth can be described by the equation: $$\bar d^3 - \bar d_o^3 = Kt$$ The value of the rate constant,K, At 1450°C was found to be in good agreement with that predicted by a modified Wagner analysis for a solution/precipitation process controlled by diffusion through the liquid. The observed variations in the growth rate both with temperature, between 1380 and 1500°C, and with the volume fraction of the liquid phase are also consistent with this theory. The contiguity of the carbide grains at the start of sintering is relatively high and is affected by the pre-sintering history of the specimen. During the early stages of sintering, however, the contiguity falls rapidly to a low value which subsequently remains relatively constant. Observations were made which suggest that the carbide does not form a continuous skeleton during sintering.     

Direct observation of liquid-phase sintering in the system iron-copper

The hot-stage of a scanning electron microscope has been used to observe liquid-phase sintering in the system iron-copper. The densification behaviour of compacts of Fe and Cu particles were determined. The influence of particle size of both components and the amount of liquid phase developed were investigated. In samples with about 20 vol % liquid phase, the densification kinetics as observed by direct observation shows that no rearrangement takes place. In samples with 40 vol % liquid phase and particle sizes of 10 to 20 µm, some rearrangement was observed.     

The gravitational effects on low solid-volume fraction liquid-phase sintering

The gravity effects on liquid-phase sintering (LPS) were experimentally examined by varying the solid- and liquid-phase density difference from 0–9 g cm^–3. The overall solid-volume fraction was held at 0.30 to allow free settling of solid grains in the liquid matrix. The solid grains were found to settle and pack to form a solid skeleton with a volume fraction of solid that reflects the density difference.     

Direct observation of liquid-phase sintering in the system tungsten carbide-cobalt

The hot-stage of a scanning electron microscope has been used to observe liquid-phase sintering in the system tungsten carbide-cobalt. Densification behaviour and the mechanism for the first, fast stage of sintering have been determined; the influence of particle size and the amount of liquid phase has been investigated. In all samples the densification kinetics is that of a rearrangement process; direct observation confirmed this result.     

Pressureless sintering of AlN-SiC composites

SiC-AlN composites have been successfully pressureless sintered by using commercial SiC and AlN powders with the optimum amount of sintering aid. The important parameters during pressureless sintering, including the amount and type of sintering aids, sintering temperature, sintering period and packing powder have been studied. Yttria was found to be a better sintering aid than alumina or calcia. The yttria sintering aid reacts with AlN and SiC powders and forms a Y-Al-Si-O-N grain-boundary phase to assist densification during pressureless sintering. With 2 wt% yttria, SiC-AlN composites can be pressureless sintered to high density at 2050–2100 °C for 2 h under the firing conditions where alpha-pp packing powder is used during firing. The microstructure and phases of the composites were characterized by using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffractometry.     

Mechanics of powders in the initial rearrangement stage of liquid-phase sintering

The mechanical response of powders in the initial rearrangement stage of liquid-phase sintering (LPS), where capillary force is the driving force for densification, is discussed in this paper. The model which is described in detail elsewhere [Xu, K., Mehrabadi, M.M., 1997, A micromechanical model for the initial rearrangement stage of liquid-phase sintering, Mech. Mater. 25, 137–157.] can be used to determine the dependence of the overall volume change on such factors as local liquid volume, uniformity of liquid distribution, contact angle, initial particle distance, initial confining pressure, particle size and viscosity. Here, the discussion is limited to the influence of macroscopic liquid volume ratio on the volume change behavior of the powder system. The results obtained are in qualitative agreement with the observation.     

Superconducting wires of Cu-Nb3Sn: the liquid-phase sintering method

A new method aimed at obtaining Cu-Nb₃Sn wires in a simple and low cost way is introduced. A powder mixture of Cu-30 wt% (Nb-H)-0.3 wt% Al was degassed and heat treated (T=1200° C) under vacuum, leading to soft, highly dense Cu-Nb sintered samples. Tin diffusion and reaction resulted from external and internal processes. In the latter case cores of tin-rich alloy were used, but an intrinsic drawback related to the occurrence of hard -phase precipitates can be identified. The critical temperature, critical current density, and normal electrical resistivity at 19 K are analysed and correlated with the other parameters (e.g. reaction time and Cu/SC ratio).     

铁基非晶条带-玻璃纤维混杂复合材料力学特性

针对铁基非晶条带-玻璃纤维混杂增强树脂基复合材料,研究了表面处理、热处理对非晶条带力学性能的影响,在此基础上选取了适宜的树脂基体,制备了混杂复合材料,测试了基本力学性能并分析了破坏模式。结果表明：酸蚀表面处理对条带的拉伸性能影响很小,但改变了条带的表面形貌和表面能,从而提高了条带与树脂的粘结性能;混杂复合材料纵向拉伸弹性模量符合混合定律,横向拉伸弹性模量主要由非晶条带贡献,并且非晶条带的承载对混杂复合材料的横向拉伸强度起到了一定的作用;弯曲破坏和剪切破坏均产生受压侧纤维层与非晶条带的分层以及纤维断裂。