Sintering forces in equilibrium and non-equilibrium states during sintering of two particles

The sintering force was originally defined for an equilibrium state, in which the sintering contraction is balanced by an external force. The concept of sintering force can be expanded to non-equilibrium process of sintering by using Beere's definition of sintering force. The sintering of two spherical particles was simulated by using the Surface Evolver program. The shrinkage rate was approximately proportional to the sintering force in a non-equilibrium process. The sintering force in a non-equilibrium process was smaller than the force necessary to stop the shrinkage at equilibrium.     

Sintering forces in equilibrium and non-equilibrium states during sintering of two particles

The sintering force was originally defined for an equilibrium state, in which the sintering contraction is balanced by an external force. The concept of sintering force can be expanded to non-equilibrium process of sintering by using Beere’s definition of sintering force. The sintering of two spherical particles was simulated by using the Surface Evolver program. The shrinkage rate was approximately proportional to the sintering force in a non-equilibrium process. The sintering force in a non-equilibrium process was smaller than the force necessary to stop the shrinkage at equilibrium.     

Synthesis of Ti matrix composites reinforced with TiC particles: in situ synchrotron X-ray diffraction and modeling

The reaction tending toward thermodynamic equilibrium during the synthesis of Ti/TiC MMC prepared by the powder metallurgy route was studied by in situ synchrotron X-ray diffraction. The carbide composition was found to change rapidly from its initial stoichiometric value TiC_0.96 toward a substoichiometric value (TiC_0.57) corresponding to thermodynamic equilibrium with the C-saturated Ti matrix. The reaction rate is very fast, and the solid-state reaction is almost complete after only a few minutes at 1073 K (800 °C) for the smallest particles, whereas the rate-limiting step remains the particle size. In addition, modeling of the diffusion processes in MMCs, i.e., initial dissolution of particles and their trend toward equilibrium composition, was performed using three particles size classes and the calculations were performed using the ThermoCalc and Dictra package. First, dissolution of the smallest particles (10% of the initial TiC_0.96 particles) is expected to be achieved after only 1 s at 800 °C. Second, the change in TiC composition leads to an increase in the total amount of carbide in the composite from 16 to 19 mass%. The consequences on the industrial process of Ti/TiC MMC synthesis have also been considered. A typical industrial heat treatment of a MMC billet, 1 h at 900 °C, was modeled, and the results showing an increase in the total amount of carbide in the composite from 16 to 22 mass% are in rather good agreement with the experimental value (21 mass%). This highlights the potential of thermodynamic and kinetic modeling to help understand and optimize industrial processes for MMC synthesis.     

Effect of tempering on development of carbide particles in 2.7Cr-0.6Mo-0.3V steel

To study the influence of tempering conditions (823–1058 K, 9–3600 ks) on the phase transformations and changes of morphology, size and chemical composition of carbide particles in 2.7Cr-0.6Mo-0.3V steel, the methods of, electron diffraction and EDXS/STEM have been used. Three carbide types: M₃C, M₇C₃ and MC have been identified altogether, the last two of which are equilibrium ones under given conditions. Diagrams describing changes in chemical composition of carbide particles and the average size during tempering have been constructed. In the development of carbide chemical composition three stages have been recorded. The carbide particles grow continuously with the increasing of time and/or temperature of tempering.     

Densification process of TiCx–Ni composites formed by self-propagating high-temperature synthesis reaction

Densification behaviour of the TiC-Ni system formed by self-propagating high-temperature synthesis (SHS) were studied to develop the structural metal-matrix composites. The composites were prepared by two routes: (1) consolidation during the SHS reaction, and (2) consolidation process after the SHS reaction. The final phases of the stoichiometric reactant mixture of titanium and graphite with 50 wt% nickel produced by simultaneous combustion reactions, were titanium carbide in a nickel-rich solid solution containing carbon and titanium. The density of the products was relatively low, with a value of 90% theoretical density. In the second approach, liquid infiltration and liquid-phase sintering were applied for the titanium carbide-nickel mixture. Densification rates were reduced due to the excess carbon in the combustion products of titanium carbide. The densities of the liquid-phase sintered samples were more than 97% theoretical density. This revised version was published online in November 2006 with corrections to the Cover Date.     

Two-temperature non-chemical equilibrium analysis of Ar-CH4-O2 ICTP at reduced pressure

In present work, a two-temperature chemically non-equilibrium (2T-NCE) model has been studied for Ar-CH₄-O₂ inductively coupled thermal plasma (ICTP) at reduced pressure from the viewpoint of interaction between plasma and fuel combustion. Chemically non-equilibrium effects have been taken into account by solving mass conservation equations for each of 22 particles considering convection, diffusion and net production effects. Species density of each particle or simply particle composition was also derived from the mass conservation equation of each one taking the non chemical equilibrium effect into account. For the net production term, totally 196 chemical reactions were taken into account including dissociation, ionization and other chemical reactions and their backward reactions. Finally, heat profile due to individual reaction and the total contribution of all reactions to the heat generated has been observed.     

场激活燃烧合成碳化钨(Ⅰ)——场激活燃烧合成

研究了场激活下燃烧合成碳化钨,研究结果显示,只有当施加的场强超过临界值(1V·cm^-1),燃烧波才能蔓延下去.燃烧产物的特性与场强有关,当场强增加时,X射线衍射圈中WC相的衍射峰强度增强,表明碳在钨中的扩散随场强的增强而增大.钨颗粒的粒径大小和样品初始相对密度对燃烧温度和燃烧波蔓延速率的影响研究表明,随着钨颗粒的缩小,燃烧温度和燃烧波蔓延速率变大,而燃烧温度和燃烧波蔓延速率的最大值出现在一个合适的相对密度处.     

Role of mechanical activation in SHS synthesis of TiC

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of titanium carbide was investigated. The SHS experiments were performed on two compositions, Ti₅₀C₅₀ and Ti₇₀C₃₀, which define the homogeneity range of the TiC equilibrium carbide. Milling times were progressively increased up to the time at which a combustion-like process ignites spontaneously under milling. The combustion peak temperature, wave velocity, and ignition temperature were markedly influenced by the degree of mechanical activation of the reactants. In particular the ignition temperature was observed to decrease from a temperature corresponding to the melting point of Ti to 500°C. The apparent activation energy for propagation of the combustion wave was also determined (～100 kJ·mol^?1) and was found to be independent of both the degree of mechanical activation and the composition of the starting mixture.     

Effect of sintering temperature on the phase composition and microhardness of WC-8 wt % Co cemented carbide

The effect of sintering temperature (800–1600°C) on the phase composition, density, and microhardness of WC-8 wt % Co cemented carbide has been studied using x-ray diffraction, scanning electron microscopy, optical microscopy, and density measurements. The results indicate that, during sintering of the starting powder mixture, containing not only WC and Co but also the lower carbide W₂C and free carbon, W₂C reacts with cobalt metal to form Co₃W. At sintering temperatures from 900 to 1200°C, the reaction intermediate is the ternary carbide phase Co₆W₆C. During sintering at 1300°C, this phase reacts with carbon to form Co₃W₃C. Sintering at 1000°C and higher temperatures is accompanied by the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co(WC). The density and microhardness of the sintered samples have been measured as functions of sintering temperature, and the optimal sintering temperature has been determined.     

Effect of the method for producing $$\hbox {}$$–$$\hbox {Cr}_{3}\hbox {C}_{2}$$Cr3C2 bulk composites on the structure and properties

Copper–chromium carbide composites containing a carbide phase of 20–30 vol% were obtained with the use of solid- and liquid-phase mechanosyntheses, followed by magnetic pulse compaction (MPC) and spark plasma sintering. The morphology, structural-phase composition, density, hardness and electrical conductivity of the composites were investigated. The structure of composites obtained by MPC represents regions of copper matrix hardened by superfine carbide precipitates surrounded by a layer of chromium carbide. In the composites obtained by spark plasma sintering, the copper matrix hardened by superfine carbide precipitates was divided into areas surrounded by a copper–chromium layer. A composite obtained by the MPC of the powders synthesized using solid-phase mechanosynthesis (MS) (copper, chromium and graphite) had the highest values of Vickers microhardness (4.6 GPa) and Rockwell hardness (HRA 69). The best value of electrical conductivity (36% IACS) was achieved using liquid-phase MS (copper, chromium and xylene) and spark plasma sintering. Liquid-phase MS is the only way to synthesize the powder with a small amount of the carbide phase and without contamination.     

High-temperature liquid-phase synthesis and sintering of MoSi<Subscript>2</Subscript> powders

Regularities of synthesis and sintering of molybdenum disilicide are studied. The synthesis is carried out in the combustion mode (self-propagating high-temperature synthesis) with the use of high-caloric mixtures of molybdenum oxide with aluminum and silicon. The cast synthesis product (MoSi₂) is subjected to grinding and subsequent sintering. The studies show that the basic parameters that influence the process of synthesis and its regularities are gas pressure in the reaction vessel and sizes of silicon particles. The sintering process, microstructure, and characteristics of sintered samples may be controlled through varying the sintering temperature and ratios of the ZrO₂ and Y₂O₃ additives.     

The chemical behaviour of carbon fibres in magnesium base Mg-Al alloys

The chemical interaction between carbon fibres and Mg-rich Mg-Al alloys was studied at 723–1273 K using optical metallography, scanning electron microscopy and electron probe microanalysis. In a first stage, carbon fibres were heated at 723, 1000 and 1273 K with Mg-Al alloys of different compositions. Two carbide phases were identified at 1000 and 1273 K: an Al₄C₃ type phase with up to 6 wt.%Mg present in solid solution and a new ternary carbide with the chemical formula Al₂MgC₂ existing under two hexagonal crystalline varieties. In a second stage, the solid-liquid phase equilibria in the Al-C-Mg ternary system were experimentally established at 1000 K. At that temperature, all the Mg-Al alloys containing more than 19 ± 2 wt.%Al were observed to be in equilibrium with the Al₄C₃ type phase whereas Mg-Al alloys containing from 0.6 to 19 wt.%Al were found in equilibrium with the ternary carbide Al₂MgC₂. As for the Mg-Al alloys with an Al content lower than or equal to 0.6 ± 0.2 wt.%, they appeared to be in equilibrium with carbon. These thermodynamic principles being established, the extent, morphology and composition of the reaction zones formed in out of equilibrium conditions at the interface between carbon fibres and Mg-rich Mg-Al alloys were characterized. Attempts were made to determine the influence of different factors such as the fibre nature, the alloy composition, the heating time and the heating temperature on the formation and growth of the ternary carbide Al₂MgC₂. The observed changes were interpreted in terms of reaction mechanism and kinetics.     

Self-propagating high-temperature synthesis of titanium nitride with the participation of ammonium chloride

We have studied the combustion of titanium in nitrogen in the presence of ammonium chloride. It has been shown that the use of NH₄Cl in the self-propagating high-temperature synthesis of TiN considerably reduces the combustion temperature, prevents sintering of the synthesized titanium nitride particles, and increases their specific surface area. The synthesis products have the form of nanostructured titanium nitride particles which reproduce the shape of the starting titanium particles but consist of equiaxed titanium nitride grains ranging in size from 50 to 500 nm. We have obtained nanostructured titanium nitride powders ranging in specific surface area up to 80 m²/g.     

Self-propagating high-temperature synthesis of ultrafine and nanometer-sized TiC particles

The feasibility of preparing ultrafine and nanometer-sized titanium carbide particles by self-propagating high-temperature synthesis (SHS) has been studied. Data are presented on the structure formation of TiC powders during SHS with a reduction step. Basic to this process is an exothermic reaction between titanium dioxide, magnesium metal, and carbon. The effects of the composition of the starting mixture, relationship between its components, and the morphology and particle size of the starting TiO₂ powder on the particle size of the forming material have been investigated. The TiC powder was recovered from the sinter cake by chemical dispersion, a chemothermal treatment of the synthesis product in different solutions. The results demonstrate that treatment of the sinter cake with appropriate solutions removes impurities and causes imperfect intergranular layers to dissolve. As a result, the cake breaks down into homogeneous single-crystal particles. Subsequent treatment in different solutions further reduces the particle size of the powder. The effect of the composition of the dispersing solution on the particle size of the TiC powder has been studied. Our results made it possible to identify conditions for the preparation of titanium carbide powders containing up to 70% of particles less than 0.3 μm in size by SHS followed by chemical dispersion.     

碳化硅密封材料的性能及应用

根据机械密封对密封副材料的要求,综述了不同烧结方法制备的碳化硅材料的力学性能、耐腐蚀性能和摩擦学性能,由于碳化硅所具有的优异性能,作为密封材料可广泛应用于高压、高转速、高温、腐蚀介质、干摩擦以及含有固体颗粒的工况下,成为机械密封材料的理想选择。     

Fabrication of dense silicon carbide through aqueous slurries containing well-dispersed carbon as a sintering aid

Tetramethyl ammonium hydroxide (TMAOH) has no fluidizing effect on carbon particles, because of their hydrophobic surface properties. On the other hand, styrene/maleic copolymer (SM) enhanced the fluidity of two kinds of carbon slurries in the basic pH range, where the particle diameters of carbon {#}2650 and MA100 were 13 and 22 nm, respectively. High-density sintered bodies of silicon carbide containing carbon and boron carbide as sintering aids were obtained by controlling the pH of 10–11 with TMAOH and by addition of SM: i.e., 9.5 and 5.5 wt% of SM were added to #2650 and MA100, respectively, in the dry weight base of the carbon. The highest sintered density product with more than 98% was obtained by intimate dispersion of the finer carbon particles in the silicon carbide slurries.     

Microstructure and properties of SiC-whisker-reinforced Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics with calcium aluminate additions

Silicon-nitride-based ceramics containing Al₂O₃-CaO sintering aids and reinforced with silicon carbide whiskers have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and their microstructure, phase composition, and mechanical properties have been studied. The results indicate that the Si₃N₄ ceramic containing 15 wt % calcium aluminate additions and 10 wt % SiC fibers has a dense microstructure with a uniform distribution of skeletal and dendritic silicon carbide crystals. The observed variations in the morphology of the crystals are tentatively attributed to the secondary crystallization of silicon carbide from the eutectic calcium aluminate melt during cooling.     

WCCo/cBN composites produced by pulse plasma sintering method

WCCo/cBN composites have been considered as a next-generation material for use in cutting-tool edges, being characterized by an optimal combination of hardness and toughness. They can be used instead of WCCo/diamond composites in machining of iron-based materials. The major challenge in sintering these composites is to produce a well-bonded interface between the WCCo matrix and cBN particles. In this study, WCCo/cBN composites were fabricated by the pulse plasma sintering technique. The aim of this work is to obtain sintered parts with density near the theoretical value and with very good contact between the cBN particles and WCCo matrix. cBN/cemented carbide containing 30 vol.% of cBN particles was produced using a mixture of 6 and 12 wt.% Co-added WC powder, with WC grain size of 0.4 μm and cBN powder with grain size ranging from 4 to 40 μm. Scanning electron microscopy (SEM) observations of the microstructure and diffraction phase examinations did not show the presence of hBN phase. The specific heating conditions used to consolidate the material using high-current pulses hamper the transformation of cBN into hBN and ensure a strong bond between the cBN particles and the cemented carbide matrix. Fractures through the WCCo/cBN composite showed that only few cBN particles were torn out from the cemented carbide matrix, with most of them having been cleaved along the fracture plane. This provides evidence that the bond at the WCCo/cBN interface is mechanically strong. Composites sintered at temperature of 1,200 °C under pressure of 100 MPa for 5 min had density near the theoretical value. Increase of the sintering temperature to 1,200 °C resulted in an increase of the hardness to 2,330 HK1 for the WC6Co/cBN(1/3) composite and to 2,160 HK1 for the WC6Co/cBN(37/44) composite.     

In-situ high temparature X-ray diffraction studies of nanocrystalline iron carbides

Three samples of passivated ultrafine iron carbide particles, synthesized by laser induced pyrolysis of gaseous precursors were examined by in-situ high temperature XRD during and after high temperature (600°C) treatment under UHP He atmosphere. The results indicated that the particles sintered around 400°C, with the preservation of the carbide structure dependent on the precursor composition and the amount of retained oxygen from passivation and synthesis. Transformation of the carbide to α-Fe around 400°C was dependent on the precursor purity, temperature and type of gaseous exposure. Preservation of the carbide structure on heating was dependent on the purity of the carbide, and the concentration of oxygen and carbon in the particles. A topotactic transformation from Fe₇C₃ to Fe_0.98O was observed in the case of samples studied under UHP He, which was absent in the other two treatments and is discussed relative to the as-synthesized/passivated particles composition and structure. This topotaxy leads to the selective formation of γ-Fe₂O₃ from specific ultrafine particle carbides when treated with UHP He.     

The influence of high-energy impacts on the microstructure of synthesized metal ceramics

On the example of the metal-ceramic alloy of titanium carbide (TiC) with nickel-chromium (Ni-Cr) binder, the comparative analysis of the influence of different high-energy impacts on the dispersion of the internal structure and phase composition of the synthesized metal ceramics 70 vol % TiC + 30 vol % (Ni-Cr) has been performed for the first time (self-spreading high-temperature synthesis (SSHTS) under pressure, preliminary mechanical activation (MA) of metal components of the initial powder mixture titanium-carbon-nickel-chromium binder, subsequent MA of the whole powder mixture, and intense plastic deformation of the synthesis product). It has been demonstrated that, under intense plastic deformation with extrusion of the high-temperature synthesis product, there a metal-ceramic structure forms containing particles of the nanosized carbide phase of the stoichiometric composition.