Effect of HPHT Sintering on Crystal Structure of NbC and TaC Carbides in PcBN Composites of cBN-NbC-Al and cBN-TaC-Al Systems

Ceramic-matrix composites based on cubic boron nitride (cBN) with binders, containing refractory compounds of transition metals and aluminum, are widely used for high-speed (300 – 500 m/min) and finishing metal processing of hardened alloy steels and heat resistant alloys. PcBN composite materials of the BL group obtained in the cBN–NbC–Al and cBN–TaC–Al systems (60:35:5 vol. % charge content) have good application prospects. The purpose of this work was to study the effect of high pressure – high temperature (HPHT) sintering conditions (7.7 GPa, 1600 – 2450 C) on the crystal structure of NbC and TaC carbides, which form together with aluminum the binder of the charge. As a result of a detailed X-ray diffraction study, it was shown that the original NaCl type crystal structure of these carbides is modified under barothermal exposure. Namely, an additional position for the placement of small atoms (nitrogen and/or carbon) is formed. During HPHT sintering, the interaction of NbC and TaC with Al atoms in the charge, as well as with the nitrogen flow formed during partial decomposition of cBN, results in the formation of solid solutions. At the same time, the dissolution of aluminum takes place in accordance with the type of Nb or Ta atoms substitution by it (up to 2.5 at.% Al). Accumulation of nitrogen atoms in the TaC structure occurs by filling vacancies in the carbon sublattice (dissolves up to 1 at.% N). In the NbC structure this process goes through interstitial of nitrogen atoms with their placement on the additional position of a modified NaCl-type structure (dissolves up to 5 at.% N). Besides, the carbon atom also partially moves from its basic positions. This accumulation of defects leads to a significant increase in the lattice parameter of NbC carbide (a relative increase of 0.28% for NbC versus 0.11% for TaC). It is shown that the carbide compositions can be described as Nb0,96Al0,04(C, N)1,17 and Ta0,95Al0,05(C, N)1. for composites of the cBN–NbC–Al and cBN–TaC–Al systems with the best operational characteristics (HPHT sintering at 7.7 GPa and 2150 C).