Key Engineering Materials Vol. 395

Abstract: The influence of ignition parameters for energy efficient processing of high temperature non-oxide ceramics by the micropyretic synthesis route is studied numerically in this article. The simulation results show that a lower ignition power leads to longer ignition time to initiate reactions. An increase in the ignition time also increases the length of pre-heating zone before propagating, which further changes the initiate propagation velocity and oscillatory frequency of the temperature variations. Such changes in the initiate propagation velocity and temperature variations result in inhomogeneous structures at the ignition spot. The simulation also indicates that using a higher power to ignite the micropyretic reactions can lower the ignition time and further prevent the inhomogeneous structures from being formed at the ignition spot. However, more heat loss is noted to occur due to a high temperature gradient and the energy required to ignite the reaction. The numerical calculation indicates that there is a 20 % increase in the required energy and a 90% decrease in the required time to ignite the specimen when the ignition power is increased from 87.5 kJ/(g･s) to 962.5 kJ/(g ･s). In addition, the effect of the individual material property on ignition is also investigated.

Abstract: Over the years, the self-propagating high-temperature synthesis (SHS) has become an interesting research field to prepare a large numbers of advanced materials. Recently, the demands for high temperature advanced ceramics have further intensified the research on SHS for efficient material preparation. Several reviews, large numbers of papers and patents on various aspects of material production by SHS are available in literature. These are scattered and it is desirable to have a comprehensive review of the literatures that not only helps the researchers but also guide the beginners in this area. In this paper, we have emphasized our contributions on synthesis of various advanced high temperature ceramics, the borides, carbides, oxides and their composites by SHS processes. Several advantages and disadvantages of the SHS technique for advanced high temperature (HT) materials are highlighted. The preparation of nano-sized powders and finegrained in-situ high temperature ceramic composites through SHS is specially mentioned.

Abstract: Bulk ceramics are usually obtained from sintering of powders, involving both densification and growth. The kinetics of grain growth are examined with a view to producing bulk nanoceramics. The high temperature deformation and failure of oxide ceramics are also examined with reference to diffusion processes.

Abstract: A comparative study has been carried out on the mechanical properties at room temperature, thermal shock and ablation resistance as well as oxidation behaviour of ZrB2-20SiC, ZrB2-20SiC-5Si3N4 and ZrB2-20ZrC-20SiC-5Si3N4 (amounts represent volume percent) composites. Fracture toughness has been determined using either three-point bend tests on single edge notch bend specimens, or by indentation technique. Addition of Si3N4 as sintering aid leads to enhancement in flexural strength and fracture toughness in the composite without ZrC. The specimens were subjected to thermal shock by quenching from temperatures in the range of 800o- 1200oC to ice cold water, and to ablation by exposure to oxy-acetylene flame at 2200oC. The composite having ZrC as constituent, exhibits the highest resistance to damage due to thermal shock and ablation, while the ZrB2-SiC composite shows the least change in mass during ablation. On the other hand, thermogravimetric experiments from room temperature to 1300oC have shown that the presence of ZrC is detrimental for oxidation resistance. Hence, the constituents of the composites need to be selected on the basis of the nature of application. The results of this study show that the investigated ZrB2 based composites bear the potential for multiple use thermal protection of reentry type space vehicles.

Abstract: Transition metal borides, carbides and nitrides are candidates for very high temperature applications. A review of various processing techniques to fabricate dense monolithic and composite materials is presented. In particular, we focus on reactive hot pressing (RHP) which allows synthesis and densification to be achieved simultaneously. We report the RHP of composites in the Ti-B-N, Zr-B-C and Zr-B-Si-C systems using the reactions of Ti/BN, Zr/B4C (Si, SiC particulate) powder mixtures at moderate pressures and temperatures. Substantial reductions in processing temperature may be realized from those in excess of 1800°C down to as low as 1200°C by exploiting a combination of transient liquid phases, plasticity in a non-stoichiometric ZrC and enhanced transport in a sub-micron microstructure.

Abstract: In view of potentiality of titanium diboride (TiB2) materials for high temperature applications, this paper presents the state of art on the processing, microstructure and properties of TiB2 ceramics. TiB2 ceramics are of interest for applications such as cutting tools, wear resistant parts, armor material and electrode materials in metal smelting because of their excellent combination of properties including high hardness, elastic modulus, better strength to weight ratio, wear resistance, good thermal and electrical conductivity. However, such broader applications of the monolithic TiB2 are inhibited by factors like high sintering temperature and poor toughness. Hence, research efforts are directed towards processing TiB2 at lower sintering temperature as well as to enhance the properties with the use of various metallic and non-metallic sinter-additives. In the above perspective, we review the existing literature in this article. In addition, our recent research results obtained with TiB2-TiSi2 materials are also presented. A review of research results revealed that a combination of room temperature properties, i.e. maximum Vickers hardness of 31 GPa and indentation toughness of 11 MPa m1/2 and flexural strength of 810 MPa is obtainable with optimally sintered TiB2. More importantly, a maximum hardness of 9 GPa (at 900oC) and flexural strength of 471 MPa can be retained upto 1200oC. From the perspective of oxidation resistance, TiB2 samples exhibit parabolic oxidation kinetics below 1000oC as result of the formation of TiO2 (s), and B2O3 (l) and linear oxidation kinetics above 1000oC in the presence of crystalline TiO2 and volatile B2O3.

Abstract: The focus of this research article is on the requirement, preparation and application of an improved material system composed of boron and carbon. These are known as boron rich boron carbides. The ability to form BRBC other than widely studied B4C composition through solid state reactive processes; hold an appeal owing to their potential for a variety of application in tribological, refractory, ballistic, nuclear energy, aerospace and other manufacturing industries. Study of the boron-carbon phase diagram, combined with the available literatures on ‘low yield’ vapor deposition processes and boron doping of B4C to prepare BRBC provided the impetus for this investigation on BRBC through solid state reactive processes, mainly micropyretic and plasma. This article summaries the ‘high yield’ experimental studies carried out for obtaining BRBC and their encouraging performance with respect to existing B4C composition based products.

Abstract: Refractories are used in a variety of processing industries including the ceramic, steel, aluminum, metal casting and heat treatment industries. Refractories provide thermal insulation, and do so by providing stagnant or "dead" gas space, namely, they contain a large volume fraction of voids. The prime criterion for material selection is refractoriness (i.e. use temperature) and the dimensional stability. One key property required for insulating refractory qualification is the service temperature limit (STL), which is related to composition, sinterability at use temperature, sintering temperature, and void volume. During the past ten years nano-pore and nano-scale fractal refractories have become available which are possibly significantly less toxic when compared to fibrous refractories. The materials used in fractal refractories are discussed in this article. Apart from use as high temperature thermal insulators the new class of materials are also finding use in a variety of products and applications of structural components such as nano-pore high performance coatings, sensors, filters and membranes used in the electronics, aeronautics, space, energy, and biomedical engineering fields.

Abstract: Studies were carried out on microstructure evolution and mechanical behavior of an Al matrix–nanoscale Al2O3 particulate-reinforced composite. The thermal stability of the composite, evaluated by heat treating specimens at temperatures from 300 to 600 °C for times varying from 1 to 100 hours, revealed that the nano-sized (30-100 nm) Al2O3 particles present in the as-received/ascast material coalesced into larger particles, but with sizes still in the 100 to 500 nm range. Despite the coarsening of the particles, high hardness was retained. The tensile properties of both the as-cast DSC material and those thermally soaked for 500 hours at a number of temperatures were evaluated. The results showed that the yield strength was quite high (283 MPa) at room temperature and decreased nearly linearly with temperature, though values as high as 110 MPa were obtained at 400oC. Thermal soaking did not have a detrimental effect on strength. Although the macroscopic ductility of both unsoaked and soaked materials remained quite low over the entire temperature range, SEM observations of the fracture surfaces provided substantial evidence for high localized plasticity as manifested by stretching, tearing and void formation in the Al matrix around the oxide particles. Possible strengthening mechanisms, including grain size reduction, Orowan bypass and forest hardening, were considered and modeled. Good agreement between the calculated and experimental strengths was obtained, and majority of the strengthening at room temperature was found to come from forest hardening (i.e, increase in dislocation density caused by the thermal expansion mismatch between Al and Al2O3), with secondary contributions from the Orowan mechanism. TEM observations provided confirmatory evidence for these mechanisms. The decrease in strength at higher temperatures was attributed to a diminishing contribution from forest hardening due to recovery processes.