JOURNAL OF MATERIALS SCIENCE 27 (1992) 5470-5476 Hot dynamic consolidation of hard ceramics SHI-SHYAN SHANG, K. HOKAMOTO, M. A. MEYERS Materials Science Program, University of California, San Diego, La Jolla, CA 92093, USA Diamond and cubic boron nitride powders were shock compacted at high temperature (873 and 973 K) by using a planar impact system at 1.2 and 2.0 kms -1. Silicon, graphite or a mixture of titanium and carbon powders were added to enhance the bonding of these superhard materials. Hot-consolidated specimens exhibited fewer surface cracks as compared with the specimens shock consolidated at room temperature. Diamond compacts having microhardness values over 55 GPa were obtained by subjecting porous mixtures of diamond crystals (4-8 I~m) plus 15 wt% graphite (325 mesh) to an impact velocity of 1.2 kms -1 at 873 K. Well-consolidated c-BN samples, with microhardnesses (starting powders with 10-20 and 40-50 I~m) over 53 GPa were obtained. 1. Introduction It has been shown that shock compaction per se is a viable technique for soft materials. However, the high pressures required for compacting hard powders gen- erate tensile stresses which cannot be totally elimina- ted because of geometrical constraints. These tensile stresses act on the existing flaws and activate them, resulting in cracking. On the other hand, ductile ma- terials require lower pressures; thus the reflected pulse amplitudes are lower. The more ductile materials are also less flaw sensitive. Shock consolidation of ex- tremely hard non-oxide ceramic powders has already been attempted at room temperature by many re- searchers [1-6]. However, it has been widely reported that there remain two unsolved problems in the shock compaction technique. One is cracking of the com- pacts at both the microscopic and macroscopic levels. The other is a lack of uniformity in microstructure and mechanical properties within resulting compacts. At three recent workshops held in the United States [7], the Soviet Union [8], and Japan [9], cracking was identified as a major unresolved problem. These two problems tend to multiply with an increase in the shock pressure used. In order to alleviate these two problems, preheating and low shock pressures are desirable. Earlier experiments conducted by Wang et al. [10] on nickel-base superalloys indicated that preheating them to 500-700 ~ had a very positive effect on the mechanical properties of shock consoli- dated superalloys. Taniguchi and Kondo [11] and Ferreira et al. [12] successfully used higher starting temperatures to help shock consolidation. Therefore, with the purpose of minimizing the above problems, we tried to consolidate diamond and c-BN powders at high temperatures instead of room temperature. The objectives of this investigation were (1) to preheat the powders to eliminate or minimize cracks in resulting compacts, and (2) to use heat generated from exothermic reaction to help consolida- tion. In hot shock consolidation, the shock energy required to melt the powder surfaces is decreased while the powder strength is lower. The effect of particle size on the consolidated diamond and c-BN powders was examined by comparing the results with experiments conducted at room temperature [1-3, 6]. The effect of the addition of graphite, silicon or a mixture of titanium and carbon powders was also examined [2, 4, 13-15]. 2. Experimental procedure Three different sizes of natural diamond powders (4-8, 10-15 and 20-25 ~m) and two different sizes of c-BN powders (10-20 and 40-50 lam) were used as starting materials. The composition of the specimens is shown in Table I. The powders were pressed to 60% theoret- ical density into 5 mm thick and 12 mm diameter stainless steel or Inconel 718 capsules. The planar impact system developed by Akashi and Sawaoka [16] was used for the consolidation. A schematic illustration of this set-up is shown in Fig. la. Twelve capsules can be compacted simultan- eously. Fig. lb shows the set-up for the hot consolida- tion developed by Yu and Meyers [17]. The planar impact of the flyer plate on the system creates high- amplitude shock waves that transmit through the powders. The detonation is initiated from the deton- ator at the top of the set-up. The conical lens consists of explosives with two detonation velocities and gen- erates a planar wave in the main charge. The flyer plate is accelerated by the main charge and impacts the capsules, consolidating the powders. Two experiments were conducted. The capsules containing the powders were preheated to 873 and 973 K and shock compacted at impact velocities of 1.2 and 2.0 km s -z, respectively. The microstructure and crack distribution of the specimens were observed by optical microscopy and scanning electron microscopy, and the crystal structure was analysedlby X-ray diffractometry. 5470 0022-2461 9 1992 Chapman & Hall