The mechanism of reaction sintering of iron-iron boride cermets

The process of reaction sintering of iron and boron in a vacuum has been analysed, as a result of which iron-iron boride cermets have been produced. They constitute a composite material in which iron boride, Fe₂B, with a hardness of about 1800 HV plays the role of the reinforcement, while iron-iron boride, Fe-Fe₂B, with a hardness of about 500 HV plays the role of the matrix. The eutectic filling the spaces between iron boride grains is composed of boron solid solution plates in iron with a hardness of around 250 HV, and iron boride, Fe₂B, plates with a hardness of approximately 1800 HV. The combination of such different materials results in attractive properties of the cermet thus produced: high total hardness (1500 HV), constant over the whole section, and ductility. The properties mentioned, resulting from the cermet structure, depend (apart from the chemical composition) on the phenomena occurring while sintering: the boron diffusion in iron, the formation of the liquid phase and the processes of dissolving powder components in it, and finally upon the crystallization of the boride phase from the liquid. These all determine the unique character of the reaction sintering of iron and boron. The mechanism of this process is reported.     

Properties of NbC-Co cermets obtained by spark plasma sintering

Sub-micrometer sized NbC-Co powder mixtures with 8, 12, 18 or 24.5 wt.% Co were consolidated by spark plasma sintering (SPS) for 2 min at 1200-1280 °C and 30-60 MPa. The optimum densification conditions were determined by analysing the dimensional change of the NbC-12 wt.% Co powder compact. SPS for 2 min at 1280 °C under a pressure of 60 MPa allowed full densification of the NbC-Co cermets with limited NbC grain growth. The microstructure is characterized as a highly interconnected NbC grain network with an inhomogeneously distributed Co binder. The Vickers hardness increased from 11.70 to 15.40 GPa whereas the fracture toughness decreased from 9.0 to 5.5 MPa m^1 / 2 with decreasing Co content from 24.5 to 8 wt.%.     

TiC-xNi金属陶瓷的燃烧合成及其热物理性能

利用自蔓延高温燃烧民结合机械厂压力方法制备了TiC-Ni金属陶瓷,研究了ＴiC-Ni金属陶瓷材料的密度,比热度、比热容、热传导以及热膨胀性质随温度和组成的变化关系,结果表明：ＴiC-Ni材料的密度在w(Ni)为２０％时致密性达到最高,材料的比定压热容在浊试温度区内呈线性增加;ＴiC-Ni材料的热扩散率随度的升高基本呈增加趋势,热扩散率随Ｎi含量的增加而增加,热导率的变化与热扩散率的变化有相似的规律,材料的热膨胀系数随温度的升高单调递增,温度上升相同情况下,材料的热膨胀系数随Ｎi含量的增加而增加。     

Synthesis of orthorhombic chromium boride by solid state reaction

Chromium boride is characterized by interesting properties, like high melting point, hardness, and corrosion and abrasion resistances. In this paper a novel synthesis of chromium boride micropaticles via a solid-state route at 600°C is reported. The X-ray diffraction pattern taken from the reaction product indicated that the product was orthorhombic chromium boride. The CrB particle size (about 1–2 μm) is confirmed by FESEM and TEM images. Solid state reactions that were carried out in sealed autoclave systems provide an alternative, convenient, and environmentally friendly pathway for the fabrication of CrB.     

Fracture characteristics of metal/intermetallic laminar composites produced by reaction sintering and hot pressing

Metal/intermetallic layered composites were formed by a process recently developed in which a self-propagating, high-temperature synthesis reaction was initiated at the interface between dissimilar metal foils. After the reaction, one of the metal foils was entirely consumed, resulting in a metal/intermetallic laminar composite. This study details the tensile fracture characteristics of these unique composites. Fracture mechanism and failure energy were controlled by varying the intermetallic-to-metal volume ratio. Failure initiated with the formation of cracks in the intermetallic layer. For high intermetallic-to-metal ratios, the intermetallic crack release energy was too great to prevent cracks from propagating through the metal layer and propagating the crack into the adjacent intermetallic layers, leading to a fast, low energy fracture. For lower intermetallic-to-metal ratios, the metal layers adsorbed the intermetallic crack release energy and blunted the propagating crack. Final failure resulted by ductile fracture of the metal layer after extensive intermetallic cracking.     

Titanium/aluminum diborides in composites produced in the cBN-TiC-Al system by reaction sintering under high pressure

It has been found by X-ray diffraction and structure analyses that in the reaction sintering of cubic boron nitride composites from the cBN+8 % Al+26 % TiC mixture at high pressure and temperature (4.2 GPa, 1750 K) in the binding ceramics composition in addition to AlN, there forms a Ti _x Al_1?x B_2y N_2(1?y) solid solution, in which titanium and aluminum atoms generate a skeleton, whose composition is close to the equimolar one and boron and nitrogen atoms are distributed randomly in graphite-like networks.     

Microstructure development during sintering and heat-treatment of cemented carbides and cermets

WC based cemented carbides and Ti(C, N) based cermets are used in cutting tool applications. They are produced by liquid phase sintering, but much of the microstructure is formed already during solid state sintering, before the eutectic temperature has been reached. Changes in the microstructure during the sintering process have been followed with microscopy and microanalysis, in particular SEM and TEM including energy filtered transmission electron microscopy (EFTEM). It was found that part of the hard phases are dissolved in the solid binder, transported by diffusion and re-precipitated onto undissolved hard grains with a composition given by the equilibrium conditions (“inner rim”). For vacuum sintering of nitrogen containing materials, this means a low nitrogen activity due to the open porosity at this stage. After the liquid has formed, further dissolution and re-precipitation occur, but now the porosity is closed so the “outer rim” is formed with the nitrogen activity of the material. The solid solution of the binder (often by tungsten) is determined primarily by the carbon activity in the liquid phase. During cooling after sintering, tungsten and other metals dissolved in the binder re-precipitate onto hard grains so that depleted zones around these are formed, whereas dissolved carbon and nitrogen have time to leave the binder almost completely. Sintering or post-sintering heat-treatment of nitrogen containing materials in an atmosphere with a lower or higher nitrogen activity than in the material results in the formation of surface zones with a composition different from the bulk (“gradient sintering”). In the former case, a tough zone enriched in binder and depleted of cubic carbides is created, which is beneficial if the material is going to be coated with a wear resistant layer. In the latter case a hard surface zone rich in cubic carbo-nitrides and depleted of WC and binder may be obtained, improving the wear resistance of the material.     

Physico-mechanical properties of cBN composites produced by a high-pressure reaction sintering of cubic boron nitride and aluminum owders

Physico-mechanical properties (fracture toughness, hardness and its dependence on the temperature) have been considered of samples of composites of the cBN-Al system produced at a pressure of 4.2 GPa and temperature of 1750 K with the variable aluminum content of the reaction mixture and time of sintering. The effect of the phase composition and real crystalline structure on the composite properties has been shown.     

Texture of transition-metal boride,nitride, and silicide films produced by ion deposition

We analyze preferential orientation in films of transition-metal borides, nitrides, and silicides produced by different ion deposition techniques. The texture of the films is shown to depend primarily on the energy of incident atoms, the structural state of the substrate, and the deposition temperature. Orientation relationships between the substrate and forming phases are determined.     

SiC matrix composites containing transition metal boride particulates internally synthesised by carbothermal reaction

SiC matrix composites reinforced with the various borides of the transition metals in group IV a-VI a, which were synthesized from the transition metal oxide, boron carbide and carbon mixed with SiC powder. Dense composites containing boride particulates of titanium, zirconium, niobium and chromium were prepared through reactive hot-pressing. The morphology of the internally synthesized boride particles reflected that of the starting oxide powders. SiC-NbB₂ composites with four-point flexural strength of 500 to 600 MPa and better oxidation resistance than SiC-TiB₂ were prepared even through pressureless sintering process. Pressureless-sintered and HIPed SiC-20 vol% NbB₂ exhibited the four-point flexural strength of 760 MPa at 20 °C and 820 MPa at 1400 °C.     

Multicomponent toughened ceramic materials obtained by reaction sintering

Fully dense zirconia toughened ceramics with a mullite matrix from the basis of information on the quaternary system ZrO₂-Al₂O₃-SiO₂-CaO, in a temperature range as low as 1425 to 1450° C, have been obtained by reaction sintering of zircon/alumina/calcium carbonate mixtures. The shrinkage, advance of reaction, microstructure and mechanical properties of different compositions are reported. The results are explained in terms of a transitory liquid phase that appears at temperatures lower than 1400° C.     

Multicomponent toughened ceramic materials obtained by reaction sintering

Very dense zirconia-toughened ceramics with a mullite matrix based on the quaternary system ZrO₂-Al₂O₃-SiO₂-MgO in the temperature range as low as 1450 to 1500° C have been obtained by reaction sintering of zircon/alumina/magnesia mixtures. The shrinkage, advance of reaction, microstructure, densification mechanism and mechanical properties are reported. The results are explained in terms of transitory and permanent liquids that appear at 1425° C and ~ 1450° C respectively.     

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.     

Phase composition of composite materials produced by high-pressure reaction sintering in the cubic boron nitride—diamond—aluminum system

X-ray diffraction analysis has been used to study the phase composition of composite materials produced by high pressure—high temperature (4.2 GPa, 1750 K) sintering of cBN and Al powders with diamond added to the reaction mixture. It has been shown that as a result of the reaction sintering depending on the relationship among the mixture components, in parallel with cBN and diamond, the composite materials may contain aluminum nitride, diboride, carboboride and carbide as well as solid solutions of boron and/or carbon based on the crystalline lattices of Al, AlN, and cBN. A possibility is shown of dispersion hardening of a composite providing the diamond content is below the threshold percolation. Along with diamond an increase in the resistance to abrasive wear of composites is responsible by the Al₃BC phase, which is located at the phase boundaries.     

Microstructure of cBN-diamond sintered compact prepared by reaction sintering

cBN-diamond composite sintered compacts (diamond content 15–70 wt %) were prepared by reaction sintering at 7–7.5 GPa and 1400–1700 °C for 10–30 min from the starting powder of the hBN-diamond system in the presence of 1 wt % NH₄NO₃ as a volatile catalyst. A fully dense sintered compact with 99% conversion from hBN to cBN was obtained at 7 GPa and 1700 °C after 30 min. An induced transformation from hBN to cBN seemed to occur on the surface of the added diamond seed crystals. Diamond seed crystals (about 30 wt %, grain size 0.2–1.5 m) were found to be well-dispersed in the reaction-bonded cBN matrix. The Vickers microhardness of the sintered compact was 5100 kg mm^–2. The contacts between diamond grains were observed in the sintered compacts containing diamond seed grains of more than 70 wt %. The toughness of the sintered compact tended to increase with decreasing diamond content and the grain size of seed crystals.