Special features of intergranular boundaries formed in shockwave sintering powders of sphaleritic boron nitride

Shockwave (SW) sintering powders of ceramic materials (including super hard) has been attracting the attention of the investigators as an alternative method of using dense polycrystalline objects competing with conventional sintering, hot pressing, and quasistatic high-pressure treatment. Considerable difficulties, formed in developing the methods of SW sintering, are associated with insufficient information available on the processes taking place in ceramics and super hard materials (SHM) under shockwave loading [1]. One of the most important and, at the same time, least examined is the problem of the formation of grain boundaries (GB). The currently available data are mainly of the hypothetical nature and many of them assume heating of the particles under the SW effect to and above the melting points [2-4]. Until recently, the problems of formation of the grain boundaries in polycrystals of SHM under high static pressures and SW effects have not been specially studied in the literature. One can mention only the studies [5-7] concerned with the structure formation in single- and two-phase polycrystals based on diamond and dense modification of the boron nitride. The currently available information on the structure of polycrystals of the SHM make it possible to assume that conventional GBs, i.e., similar to equilibrium grain boundaries in metals, start to form in BNsp polycrystals under the conditions of primary crystallization at temperatures above 2000°C. The real boundaries, formed in SW sintering, has not been studied although their state is expected to have a strong effect on the mechanical properties of the resultant polycrystals. In this work the results are presented of examining the structural features of the GBs, formed in SW sintering of BNsp powders with a grain size of 40-28/xm. The powders were sintered in cylindrical storage vessels. Shockwaves were generated during sliding detonation of trinitrotoluene. The parameters of the detonation wave were: velocity 6.9 km/sec, pressure 18 GPa. To measure the parameters of shockwaves in the powders we varied the thickness of the vessel wall at a constant internal diameter thus resulting in different degrees of sintering. The density of polycrystals in all cases was 95-97 % of the theoretical value. The degree and uniformity of sintering was evaluated from the hardness of polycrystals at the edge of the specimen and in its center. Transmission electron microscopy was used to examine the fracture surfaces of polycrystals and also their structure. As indicated by Table 1, the resultant polycrystals may be divided, according to the sintering conditions, into poorly (regime 1), satisfactorily (regime 2), and efficiently sintered (regime 3). Examination of the real structure and special features of the failure of selected polycrystals and also the data obtained in our previous work on similar objects [8, 9] make it possible to assume that the formation of the grain boundaries under the SW effect is preceded by the processes of displacement of the particles of the initial powder, their brittle failure, and plastic deformation, ensuring formation of dense contacts amongst the particles. These processes are accompanied by the formation of the substructure of the particles and the surface relief differing from the initial state. The resultant grain boundaries reflect these changes to a certain degree. In polycrystals sintered in the regime 1, failure in compacting in regions with large pores (including separation of fine particles from the surface of large ones) leads to the appearance of boundary finely dispersed interlayers, and the surface relief of the particles is highly developed and clearly visible on the fractographs (Fig. la). Formation of the boundaries with intensification of the SW effect is accompanied initially by 'refining' and then smoothing of this relief. In transition to regime 2 smoothing of the large projection is accompanied by an increase of the number of fine irregularities of the submicron size. Figures la and 3a show fractographs of failure of different groups of BNsp crystals. A typical feature is that the relief, formedin stages of surface failure, is smoothed out but does not disappear under allSW sintering conditions and the morphology