microwave heating (24 and 30 GHz) (MWH) and conventional heating (CH). A gyrotron system is used for MWH and electric furnaces for CH. The samples are prepared by sintering of ultrafine powder mixtures at 1000-1300°C (MWH) and 1700°C (CH). Mercury porometry is used to determine the mean pore size and specific surface area of the samples. The Barus-Bechgold method is employed to find the maximum pore contraction. A quantitative microscopic analysis of the samples is carried out. The content of solid solution is determined using x-ray analysis. The volume and local shrinkage is calculated. Relationships between the local and volume shrinkage, pore structure parameters, and amounts of solid solutions for different heating conditions are shown. The results reveal different diffusion processes in sintering of powder systems with complete miscibility of components under MWH and CH.The faster compaction of monophase and multiphase powder bodies when sintered under microwave heating (MWH) is attributed by many researchers to more intensive mass transfer. Interdiffusion proceeds in multiphase powder bodies along with compaction. It is assumed that diffusion proceeds faster in microwave heating not only because of temperature but also because of the microwave field as such, i.e., its nonthermal effect on the mass transfer during sintering [1]. It is reported, for example, in [2] that the activation energy of oxygen diffusion in alumina decreases by 40% in MWH. The nonthermal effect of the microwave field on the mass transfer in ion crystals has been confirmed by theory and experiment [3].

The nitridation of an AlB 2 precursor in a controlled nitrogen flow is examined with the aim of obtaining AlN-BN composite powders. It is shown that superfine powders with uniformly distributed AlN and BN phases and AlB 12 admixture form over the temperature range 1000-1400ºC. The powders do not require further grinding. The morphology and particle size of the powders and the microstructure and properties of hot-pressed samples are studied. The developed material exhibits better mechanical properties (as compared with known ABN composites): in particular, R bm = = 235 MPa and HV = 14.2 ± 1.3 GPa. The high bending strength and hardness are due to the finegrained structure of the samples and the strengthening effect of superfine (≤1 µm) inclusions of aluminum dodecaboride.

The effect of the metal-layer structure and of the interface between the ceramic and metal components on the heat resistance of multilayer ceramic-metal composites is examined. The use of metal fabric layers substantially enhances the heat resistance of composites. The adverse effect of the residual porosity and, sometimes, poor adhesion between the ceramic and metal phases on heat resistance decreases owing to the skeleton structure of metal layers. The skeleton structure of metal layers promotes crack branching and retards the formation of the main crack.Heat resistance is an important characteristic of many types of ceramics, which is the capability of a material to withstand, without failure, thermal stresses caused by temperature change during heating or cooling. Metal-based ceramic materials often exhibit low thermal expansion coefficient, low heat conductivity, high elastic modulus, ultimate strength over a wide range, and low plasticity; hence, they have low heat resistance.One of the ways to improve the heat resistance of ceramics is to produce composite materials by introducing metal fibers into the ceramic base. Heat resistance is also enhanced because mechanical stresses are better distributed and hardening fibers restrain cracking of the ceramic matrix [1]. Layered metal ceramic composites, in which metal fabric and ceramic layers alternate, may serve as an example [2].It was previously shown that such multilayer composites exhibited high fracture toughness though the samples had significant (to 24-25%) residual porosity [3]. Fracture toughness grows substantially especially when mesh layers are made of metal fabric. When such samples are pressed, ceramic powder fills the fabric cells and the resulting metal ceramic layer has a skeleton-like structure. Such a developed skeleton structure of layered composites weakens the adverse effect of residual porosity on the mechanical properties. This paper examines how the structure of metal layers and the interface between ceramic and metal layers influence the heat resistance of layered metal ceramic composites.