Eutectics and their models, sintered composites, in systems of refractory materials

Abstract

Eutectics and high-disperse systems of noninteracting or thermodynamically incompatible components are considered for their potential use in the design of a wide range of new heterophase ceramics with well-developed interphase boundaries ("coarse-conglomerate" models for eutectics) and their physicomechanical properties and structure-sensitive parameters are analyzed.Oxygen-free refractory compounds -metalloids and covalent-bonded species -recently have been gaining the ever-increasing application in the development of new ceramic materials with superior physicomechanical properties intended for service under heavy-duty conditions in machine building, nuclear power engineering, aircraft engineering, and special technologies. In the second half of the past century, much effort has been put into the study of properties and the development of technologies for these materials. An important achievement in the field of refractory oxygen-free materials was the development of sintering methods such as hot pressing (HP), isostatic hot pressing (IHP) and the theoretical background of a so-called activated sintering.Progress in technology has always been a stimulus for the development of new materials which, in design at least, would combine a set of properties rarely (if ever) found in a single material; a route to successful solution of the problem is provided by developing composite (heterophase) materials. To that effect, a simple and efficient means is the ceramic powder technology. At present, a range of versions of this technology have been proposed which permit the production of complicated ceramic components from precursor powders of different particle size gradation -from 10 to 10 6 nm.State diagrams of n-component systems describing the conditions (temperature and concentration, both varying over a wide range) for refractory materials provide the physicochemical basis for designing composite materials. At present, the number of thoroughly studied ternary state diagrams Me¢-X-Me¢ or Me¢-X¢-X² is rather limited; for the most part, there are data on phase relations and structure of individual isothermal or polythermal cross-sections (here Me refers to transition metals, Al, Si, etc., and X refers to B, C, N, and O).In recent decades, data have been reported in the literature on the interaction between refractory materials of different class; in other words, the structure of polythermal crosssections Me d ¢X¢-Me d¢ X² in ternary systems Me d ¢-X¢-X² (for example, Me d C-Me d B 2 , Me IV N-Me IV B 2 , Me d C-SiC, etc.).Based on generalized data from the literature [1] and applying a trial tetrahedration method to quaternary (yet-unstudied) systems Me¢-Me¢¢-X¢-X¢¢ and Me-X¢-X¢¢-X¢¢¢, a classification for quasi-binary systems has been proposed that can help lay down the guidelines for the preparation of heterophase materials for various applications (Fig. 1). It is important to indicate that the majority of these systems can be described by state diagrams of the eutectic type. So, the systems, and SiC-Me d C [9] are eut...

Densification Behavior and Microstructure Evolution of Hot-Pressed HfB2

“…H afnium diboride is a Group IV metal diboride that belongs to the family of ultra‐high temperature ceramics known for their unusual combination of thermophysical properties. (Me) IV B 2 (Me IV =Ti, Zr, Hf) ceramics may find use in nuclear reactors, microelectronic substrates, and aerospace applications that require properties such as high electrical conductivity, thermal conductivity, elastic modulus, and hardness 1–4 . Although the Group IV diborides exhibit significant metallic bonding between their hexagonally packed metal atoms, their strong covalent bonding resulting from interstitial boron atoms inhibits diffusional transport.…”

Nonoxide High-Melting Point Compounds as Materials for Extreme Conditions

“…), and experimentally determined by describing the phase equilibria in the systems Ме'-Ме"-Х', Ме'-Ме"-Х'-Х", Ме'-Х'-Х"-Х"' (Ме = Ме d , Ме f , Al, Si; Х = B, C, N, Si). Based on our original research and published data, we have elsewhere [7] proposed a classification of the quasi-binary systems that describes the co-existing systems of high-melting compounds (such as Ме'-Ме"Х, Ме'Х-Ме'Х", etc., e.g. Ме'-Ме'C, Ме'-Ме"В 2 etc., МеС-МеВ 2 , МеB 2 -МеSi 2 , SiC(B 4 C)-Me d B 2 etc.…”

“…Hot pressing (HP), hot isostatic pressing (HIP), or spark plasma sintering (SPS) processes, where the externally applied pressure enhances the driving force for densification, enable to decrease sintering temperature [4][5][6]. On the other hand, in multiphase (composite) materials the sintering temperature may be significantly reduced through eutectic reaction between various components of the system, enabling the use of pressureless sintering technology [7]. Implementation of this approach requires in-depth understanding of the phase equilibria in the high-melting systems, as well as a detailed analysis of the chemical and physical processes that accompany sintering.…”

Oxidation resistance in structural ceramics based on refractory materials

“…3b ), an oxide film 20 -30 mm thick was formed on the surface. Because of the diffusion of oxygen depthwise, an oxynitride inner layer with a density of 6.88 -6.91 g/cm 3 [7] might have formed to provide cohesive bonding for the oxide film over the entire surface.…”

“…Heterogeneous systems of eutectic composition built up by ultradisperse high-hardness oxygen-free materials were shown to exhibit an effect of microplasticity superior to that in individual compounds [7]. The effect of superplasticity can be used to advantage under conditions of rapid thermocycling when the generated thermal stresses are removed owing to the involvement of boundary microplastic deformation.…”