Experimental investigations of the hardness, facture toughness, and elastic characteristics (longitudinal velocity of sound waves, Young's modulus, and Poisson coefficient) of almost poreless polycrystalline ceramic materials based on corundum, higher borides of aluminum and calcium, boron and silicon carbides, boron (cubic) and aluminum nitrides, tungsten-free TiB2-based solid alloy, cordierite ceramics, ceramics from a mixture of copper titanate and copper aluminate, and metalloceramic materials of the BaC -SiC -AI -Si system are described. It is established that a linear correlation between the hardness and the velocity of sound waves exists for ceramic and metalloceramic materials. It is shown that ceramic materials can be classified in accordance with their rigidity and capacity for absorbing the energy of ballistic impact, the value of the Poisson coefficient, and a combined criterion comprising hardness, fracture toughness, Young's modulus, and the longitudinal velocity of sound waves. The physicomechanical properties of cermets in the B4C -SiC -AI -Si system can be controlled by changing the amount of the metallic phase and the annealing time.Despite the large number of investigations on the structure and physicomechanical characteristics of ceramic materials (CM), the properties of many of them, especially those of materials with low density, have not been investigated. These materials are used as components of composite organoceramic armor [1], and interest in them has increased noticeably in recent years. The main requirements imposed on them are a minimum density for a specific combination of physicomechanical characteristics (hardness, velocity of longitudinal elastic vibrations, and fracture tou~aness) and a sufficiently low cost.It has been established that the efficiency of ceramic materials in absorption and scattering of the elastic energy of an impact can be evaluated by the criterion cr c /Kic [2] or HGE/K 2 [3], which can also be represented as pC 4/K 2 with allowance for the linear correlation between H and Ct in many very hard ceramic materials [4]. Here H is the hardness, C l is the velocity of longitudinal vibrations (the core velocity of sound), E is Yotmg's modulus, KI~ is the 271fracture toughness, p is the density, and t~ c is the ultimate compressive strength.In the present work we generalize the results of an experimental investigation of the hardness, elastic characteristics, and fracture toughness of light superhard polycrystalline single-phase ceramic materials (B4C, CaB 6, BN, A1BI2, AIN), composite CM based on corundum (from 99 to 78% A1203 ), and composite CM based on BgC in the B -C -A1 and B -C -Si -Al systems. Specimens of sin~e-phase composite CM were prepared by hot pressing at 30 -40 MPa (1900 with the exception of the CM based on cubic BN, which was sintered under a pressure of 7 -8 GPa.CM based on A1203 (with a grain size of 3 -8 ~tm) were prepared by semidry pressing and $intering at 1350-1750°C, and composite CM of the B-Al-C and B-A1-Si-C systems were prepared by ...

The acoustic nondestructive method of determining such important indices of refractory parts as open porosity, apparent density, and compressive strength has obtained wide use in the refractory industry [1][2][3]. The method is based on the correlation relationships between these indices and the frequency of characteristic oscillations of the part. In inspection the frequency of characteristic oscillations of the part and its weight are measured and then the index is determined using previously developed regression equations or nomograms.Measurement of the frequency of characteristic oscillations is possible under conditions of induced (resonant method) or free (impact method) oscillations. With the use of induced oscillations the frequency of characteristic oscillations is called the resonant frequency of the part (from the method of its measurement).In the domestic refractory industry instruments based on the method of induced oscillations are used. Oscillations the frequency of which is changed smoothly are excited in the part by an external source. At the moment of agreement of this frequency with the frequency of characteristic oscillations of the part resonance, characterized by a sharp increase in the amplitude of oscillations, occurs. The frequency of oscillations of the external source at the moment of resonance corresponds to the value of the frequency of characteristic oscillations (resonant frequency) of the part.With the use of the method of free oscillations oscillations are excited in the part by an impact. After finish of the excitation stage the part produces damping oscillations at the base frequency (frequency of characteristic oscillations), which is recorded by an instrument. Despite a number of advantages of the method of free oscillations (lower system error, measuring simplicity, etc.) until recently its practical use has been restricted by the absence of industrial instruments. The development of electronics and digital technology has created the prerequisites for the development of appropriate instruments.Abroad the Grindosonic instrument (Belgium) has found wide use for inspection of ceramic, structural, abrasive, and other parts [4]. In particular, in the Federal Republic of Germany this instrument is used for inspection of refractory parts [5]. In our country production use of the free oscillation method is related to the development by the All-Union ScientificResearch Institute for Abrasives and Grinding of the Zvuk-202 instrument [6].The functional plan of the Zvuk-202 instrument is shown in Fig. i. With application by the striker 1 of an impact on the part being inspected 2, which is located on the soft base 3 (porolon, foam rubber, etc.), mechanical oscillations occur in it and are received and converted into an electrical signal by the microphone 4. The signal is amplified by the amplifier 5 and delivered to the tuneable filter, with which the frequency of characteristic oscillations, measured by the frequency meter 7, is separated. The frequenCy meter is triggered by the synchro...