Papers by Keyword: SHS

Abstract: Porous TiC-TiB₂-NiAl composites have been prepared through self-propagating high-temperature synthesis (SHS) of 3Ti + B₄C + x(Ni+Al) → TiC + 2TiB₂ + xNiAl using Ni, Al, Ti, B₄C powders as raw materials. The effects of Ni and Al content on phase constitution and pore structure of the obtained porous TiC-TiB₂-NiAl composites have been investigated. XRD results show that the final composites of Ti+B₄C+Ni+Al powders consist of TiC, TiB₂ and NiAl, while only TiC and TiB₂ are observed without using Ni+Al powders in the reaction system. NiAl additives play a significant role in controlling reaction behavior and constituents of the final products. TiC, TiB₂ and NiAl are homogenously distributed in porous TiC-TiB₂-NiAl composites, and TiC shows spherical shape, TiB₂ exhibits hexagon or long strip shape while NiAl shows the irregular shape distributed between TiB₂ and TiC particles. The prepared TiC-TiB₂-NiAl composites contain a large amount of pores with the aperture concentrating between 20-80μm and the porosity was about 35%, NiAl additives make the pores distributed uniformly in the matrix. The more NiAl additives, the more obvious of melting trace on the matrix skeleton, some spherical particles adhering on pore wall gradually submerge in matrix and combine with it closely. The NiAl additives increase the compressive strength of TiC-TiB₂ and for TiC-TiB₂-NiAl composites, the fracture mode of TiB₂ and TiC is brittle fracture, while NiAl mainly occur transgranular fracture.

Abstract: This paper presents a fabricating process and microstructural features of TiB₂+TiC duplex particulates reinforced surface composite with carbon steel matrix. The effect of ferrous-titanium addition on microstructures is investigated and analyzed. The results showed that the surface compound layer do not exhibit at lower addition, but appear at higher addition. With increasing ferrous-titanium addition, the concentration and size of the synthesized particles at the surface compound layer are decreased, but the tendency of uniform distribution increased.

Abstract: A composite was produced from initial powder mixture of B4C (70 wt.%) and Al (30 wt. %) with WC-Co additives introduced during ball-milling and acting as catalysts by self-propagating high-temperature synthesis and followed by heat treatment of raw samples under gaseous nitrogen flow at 650, 800, 1000, 1150 and 1450 °C, respectively. Formation of different new superhard phases was detected via XRD investigation and analysis of microstructures. Micromechanical properties were tested by nanoindentation. The tribological behavior in dry sliding conditions of the composite was investigated using the ball-on-disk technique against alumina balls. The friction coefficient of the composite increased and wear rate decreased with formation of c-BC2N, c-BN, B13C2, W2B5, Al3BC, AlN, etc. contents during heat treatment at increased temperatures.

Abstract: Copper oxide and pure chromium being as raw materials,CuCr contact composites can be fabricated through the self-propagating high-temperature synthsis technology,then the microstructure of the samples was observed under a metallographic microscope and the effects of different chromium content on hardness and electric conductivity of CuCr alloys were studied.The experiment's results showed that the SHS reaction of CuO-Cr-Al system could react drastcally,give off a lot of heat and CuCr alloy could be obtained simply as well as with low cost. With the increase of the chromium content, the hardness of the CuCr alloy was elevated, but the electric conductivity was decreased.

Abstract: TiB₂ is a superhard, high-temperature and high corrosion resistant material and it is under consideration for tungsten-free cutting tools and high temperature structural applications. Although such a covalent compound requires significantly elevated temperature for the consolidation, great exothermicity of TiB₂ synthesis by means of SHS (Self-propagating High-temperature Synthesis) can be “ïn situ” utilized. In this study, TiB₂-based composites are fabricated from titanium, boron and binder metal. In order to optimize consolidation process and improve fracture toughness of the products, three types of binder, based on cobalt, nickel or copper were investigated. In respect to hardness, limited amount of binder, 5, 10 or 15 vol.% respectively, were applied; each time 5 vol.% of Ti addition for reaction with boron completeness was used. The TiB₂ based composites were fabricated from elements in one process by means of the SHS process combined with p-HIP (pseudo-hot isostatic pressing) method. The raw elemental powders were homogenized by wet mixing using ball milling technique. Dried mixtures were pressed into a compact, coiled by heating element and then exposed to the SHS-p-HIP process. After SHS initiation, the compact was pressed pseudo-isostatically under pressure of 190MPa for 5 min. The sintering additives and their concentrations significantly affected the consolidation process as well as the properties of composites. The highest hardness was obtained for samples sintered with cobalt, containing intermetallic binder. However, elemental metal binder was detected as a main component for samples sintered with copper. The relative density, SEM microstructure, phase composition and hardness are compared in this study.

Abstract: Aluminum nitride has been acknowledged as an important industrial material because of its unique combination of high thermal conductivity and high electrical resistivity. Although there have been several excellent reviews on the synthesis of aluminum nitride, little has been mentioned on the combustion synthesis of AlN. In this work, the combustion synthesis methods for synthesis of AlN are reviewed and classified according to the phases involved in the reaction, the types of reactants (and additives) used and the design principles for the synthesis processes. Problems critical to combustion synthesis of AlN to obtain products with high product yields and low impurity contents are summarized and discussed.

Abstract: The combustion stability of the self-propagating thermit reactions was analyzed, and the influence factors on combustion temperature and combustion speed were discussed in detail. Especially, a complex combustion system had an advantage in balancing the reaction heat. Through the welding experiment and result discussion, the control strategy for heat was verified in the complex combustion system of (CuO+Al)+( CuO+CaSi). The results indicated that the optimization of particle size and content rate about oxidizing agent, reducing agent and additive components had a significant effect in stable combustion, heat uniformity, beautiful forming and so on.

Abstract: Barium ferrites hollow microspheres were prepared by self-propagating high-temperature synthesis self-reactive flame spraying method. The droplets enter in the cooling medium and solidified rapidly so that the gas can not escape to remain holes inside the products. The powder samples were used for characterization. The microstructure and crystal structure were studied by SEM, EDS and XRD. The results show that the obtained particles are hollow microspheres. They are spherical or ellipsoidal shapes from nanometer dimension to several micrometers. The phase structure of composite powders is the mixtures of barium ferrite and Fe₂O₃.

Abstract: The titanium diboride reinforced titanium nitride-based ceramic cutting tool composite materials were fabricated by a new technology namely in situ synthesis followed by hot-pressed sintering. The experimental results showed that the highest density and the best mechanical properties of the composite material were obtained when the content of Ni was 5wt%. X-ray diffraction (XRD) was used to identify the phases of the composite. Microstructures of the composite were observed by means of scanning electron microscope (SEM). It is concluded that the sintered material was pure, no harmful reaction products was found; with the increase of Ni content, the proportion of short rod-like TiB₂ increased, fracture toughness of the composite materials were improved because lots of TiB₂ were pulled out; bridging phenomenon of short rod-like particles and metal particles were observed in the crack propagating process. The main toughening mechanisms of the composites were deflection and bridging of the crack as well as grains pulling out.

Abstract: Hot dynamic densification method was developed by combining self-propagating high temperature synthesis (SHS) with explosively shock powder compaction technique. This method is extremely short time processing. The main purpose in this study is to perform from synthesis to densification of TiB₂-TiN system high temperature ceramic composites and TiB₂-TiNi-Cu system functionally graded materials (FGMs) in one step. In TiN-TiB₂ ceramic composites, they showed up to 95% of relative density. It was appeared by TEM observations that both the two phases joined tightly each other. The FGMs also were produced by the same technique. They indicated no interlayer exfoliation and no macro cracks after thermal shock tests from 973 K to room temperature. It was shown that thermoelastic property of intermetallic TiNi phase as intermediate layer between ceramics and metal layers operated effectively.