Preparation Of MAX-CBN Composites By Spark Plasma Sintering

The MAX phases, a group of nano-layered ternary compounds, have unusualproperties associated with metals and ceramics. Like metals, they are good thermal andelectrical conductors, relatively soft, thermal shock resistant and easily machinable. Likeceramics, they display a set of typical properties including high elastic modulus, lowdensity, high yield strength and good oxidation resistance. Ti2AlN belongs to MAX phases.Recently, research work mainly concentrates on Ti2AlN thin films. However, thefabrication of single phase, bulk dense samples of Ti2AlN has still remained difficult. Thus,single-phase bulk Ti2AlN prepared by spark plasma sintering (SPS) is one of thehighlights of this paper. In spite of having the unusual properties, the single-phase bulkTi2AlN ceramic has some weakness which is the obstacle of its application, just like lowhardness and weak corrosion resistance. Cubic boron nitride (cBN) is the hardest materialafter diamond, and there have been some reports on cBN and MAX phases composites. Inthis paper, a new composite was fabricated with the mixed powder of Ti2AlN and cBN asraw materials by spark plasma sintering. The effects of pressure, sintering temperature andcBN content on phase composition, microstructure and hardness are considered.Discussing the change of the starting materials of Ti2AlN and cBN in sintering process,the formation mechanism of the phases of AlN and TiB2is mainly studied.In light of the major objectives above and after a series of experiments based on anextensive survey of literature, the following conclusions were drawn:Bulk Ti2AlN materials with high purity could be fabricated by spark plasma sintering(SPS) with the start molar ratio n(Ti):n(Al):n(TiN)=1.0:1.0:1.0at1300℃and50MPa inargon atmosphere. The density of the bulk Ti2AlN ceramic was4.23g/cm3reaching98.1%of the theoretical density. Ti2AlN phase was well developed with a close and lamellatedstructure; the grains were plate-like having the sizes of4~6μm and8~10μm in thicknessand elongated dimension, respectively.Through the analysis of the effects of pressure, sintering temperature and cBN content on phase composition, microstructure and hardness, we arrive the followingreview. To some extent, pressure can inhibit the decomposition of Ti2AlN by discouragingthe diffusion of Al, but the primary function of pressure is to make sintered bodies moredense. When the volume content of cBN is more than10%, the compactness and hardnesswill decline. With a rise of sintering temperature, more Al diffuse from Ti2AlN, moreTi2AlN and Ti4AlN3are decomposed into TiN and Al, cBN content decline by reactingwith Al and TiN to form TiB2and TiN.During preparation of composites with the raw materials of the mixture powder ofTi2AlN and cBN, Ti2AlN begins to decomposition at1000℃and completely decomposesat1300℃; Ti4AlN3is formed at1100℃but Ti4AlN3becomes unstable and decomposes attemperatures above1200℃. Thus, cBN can promote the decomposition of Ti2AlN andTi4AlN3.Analysis of the XRD patterns and EDS result of composites revealed that Ti4AlN3and TiN are primarily in starting position of Ti2AlN, and the interphases, AlN and TiB2,are around cBN particles, the possible reaction is as follow:2BN(s)3Al(l) TiN(s)3AlN(s) TiB2(s)...