Guo Jun Zhang

Boron carbide and nitride as reactants for in situ synthesis of boride-containing ceramic composites

Abstract Using boron carbide (B 4 C) and boron nitride (here refers to hexagonal BN) as reactants, a series of boride-containing ceramic composites, mainly in the present work zirconium diboride-containing composites including zirconium diboride–zirconium carbide (ZrB 2 –ZrC), zirconium diboride–zirconium nitride (ZrB 2 –ZrN), zirconium diboride–silicon carbide (ZrB 2 –SiC) and zirconium diboride–aluminum nitride (ZrB 2 –AlN) were prepared by in situ reactive hot pressing. The features and the development mechanisms of the composite microstructures were characterized and modeled. The obtained zirconium diboride-containing composites demonstrated high bending strength. In addition, some general problems such as transformation between B 4 C and BN and thermodynamics of using B 4 C and BN as reactants were also briefly discussed.

In situ Si3N4 SiC-BN composites: preparation, microstructures and properties

Abstract Boron nitride composites in Si–B–C–N system have been widely studied and applied in industry in these decades. In the present investigation, in situ reaction process was used to fabricate a new kind of silicon nitride-silicon carbide-boron nitride (Si 3 N 4 –SiC–BN) composites according to a previously proposed reaction of Si 3 N 4 , B 4 C, and C with extra addition of Si 3 N 4 in the starting powder mixture. The composites were produced by hot pressing at 1800xa0°C in a nitrogen atmosphere. The obtained composites demonstrated relatively high density and high bending strength over those of conventionally-processed ones, although with increasing BN content the densification behavior became poorer especially for composites with BN contents higher than 25 vol.%. The growth of rodlike β-Si 3 N 4 grains was suppressed in the composites. Different from the previously reported SiC–BN in situ composites in which the strength almost kept unchanged when the BN content was lower than 35 vol.%, the strength of the present Si 3 N 4 –SiC–BN composites decreased monotonously with increase in BN content due to the decreased relative density and limited grain growth of rodlike β-Si 3 N 4 grains.

SiAlON-Boron Nitride Porous Composites: In Situ Synthesis, Microstructure and Properties

Sialon-boron nitride porous composites were prepared based on the in situ n itridation of AlB2 to form AlN and BN in Si 3N4 and Al2O3 in this work. The fabricated composites by pressureless sintering at 1800 °C for 2 h under N2 atmosphere of 8 atm show closed type pores and demonstrate relatively high bending strength by comparison with tha of conventionally processed composites. In addition, due to the large volume expansion of 226% during the nit ridation process of AlB2, Sialon-BN composites with low-shrinkage in the reaction synthesizi ng process are obtained. Introduction Comparing to the sintering processes of mechanically mixed c omponent phases, ceramic composites fabricated from various reaction processes demonstrate s ome unique features such as special microstructures, high chemical and microstructural st ability at high temperature, and even cheap starting materials [1]. Many reactions were designed in our previous work to produce various ceramic composites, for example, Al 2O3-MoSi2 from controlled oxidized MoSi 2 plus Al [2], Mo(Al, Si)2-SiC from MoSi 2-Al-C [3], TiB2-TiN from BN-Ti [4], AlN-TiB 2 from BN-Ti-Al [5], and SiC-TiB2 (or ZrB2) from B4C-Si-Ti (or Zr) [6-8]. BN ceramics, which shows very low elast ic modulus among ceramic materials, has been widely used as the sec ond phase to reduce the elastic modulus and improve the thermal shock resistance of monolithic ceramic materials, such as in Si3N4-BN and Sialon-BN [9-11], SiC-BN [12], AlN-BN[13] and Alon-BN[14] compos ite . Unfortunately, the strength of BN-containing composites generally decreases with increasing the BN content compared to the monolithic ceramics. One of the main re sons is that the densification behavior of BN composite becomes bad with the addition of BN due to the poor s interability of BN. The other is that because BN is weak, BN agglomerates or large BN particles/platelets that are difficult to avoid their existence in starting powders may act a s fr cture flaws. This means that BN dispersoids with fine particle size and homogeneous distribution are the k y factors to obtain high strength composites. In situ formation of fine and homogeneously distribut ed BN in matrix through reactions is an attractive way to solve this problem. There are some reports on in situ reaction synthesis of BN composites. For example, composites of SiO 2-BN, Al2O3-BN and mullite-BN from the in situ reactions of Si 3N4 + B2O3, AlN + B2O3 or Si3N4 + AlN + B2O3, respectively [15]. The authors formerly proposed some novel reactions to in situ synt he ize various nonoxide-boron nitride (Nobn) composites. SiC-BN composites were either pr epared from the reaction between Si 3N4, B4C and C or Si 3N4, B2O3 (or H3BO3) and C [1,16,17]. We found that the in Key Engineering Materials Online: 2003-04-15 ISSN: 1662-9795, Vol. 237, pp 123-128 doi:10.4028/www.scientific.net/KEM.237.123

Effects of Pore Morphology on the Fabrication and Mechanical Properties of Porous Si3N4 Ceramics

Porous Si3N4 ceramics with different pore morphology have been fabricated, utilizing either organic whiskers or starch as the fugitive agents, through slip-casting and die-pressing technique, respectively. The obtained porous ceramics have rod shaped or equiaxial pore morphology, originated from there two kinds of pore forming agents. The mechanical properties were investigated. The strength decreased considerably when small amount of whiskers were added, however, further increase in the whisker content only cause a moderate decrease of the strength. Gas permeability were measured for the samples with high whisker content of 60 vol% (corresponding to porosity of about 45% in the sintered bodies), and was compared with the counterpart contained the same porosity in which pores were equiaxial. The flexural strength of the samples with these two types of fugitive particles was almost the same, but the permeability of samples with rod-shaped pores were much higher than that with equiaxial pores, which can be understood in terms of a short pass model.

Strengthening Effect of In-Situ Dispersed Hexagonal Boron Nitride in Ceramic Composites

High strength particulate ceramic composites are in general reinforced by strong dispersoids, such as strong ceramic particles (SiC, TiB2, ZrO2, et al) and strong metallic particles (Mo, W, et al). In this work high strength ceramic composites with in-situ synthesized hexagonal boron nitride (h-BN) have been prepared and characterized. As an example, we manufactured mullite-BN composites by reactive hot pressing (RHP) using aluminum borates (9Al2O3·2B2O3 and 2Al2O3·B2O3) and silicon nitride as starting materials. The obtained material RHPed at 1800°C showed a strength of 540 MPa, which was 1.64 times higher than that of the monolithic mullite ceramics. TEM observation revealed that the composite had an isotropic microstructure with a fine mullite matrix grain size of less than 1 μm and a nano-sized h-BN platelets of about 200 nm in length and 60∼80 nm in thickness. The high strength was suggested to be from the reduced matrix grain size and the small toughening effect by the h-BN platelets. In addition, this kind of ceramic composite demonstrates low Young’s modulus that is beneficial to the thermal/mechanical shock resistance, and excellent machinability.

Reaction Synthesized Boron Nitride-Containing Composites (BNCC)

Based on the proposed inorganic reactions a series of high performance hexagonal boron nitride-containing composites (BNCC), include SiC-BN, Si3N4-SiC-BN, SiAlON-BN, AlN-BN, Al2O3-BN, AlON-BN and mullite-BN, have been prepared via reactive hot pressing or pressureless reactive sintering. Various boron-bearing components such as B, B4C, AlB2, SiB4, SiB6, B2O3 or H3BO3, 9Al2O3×2B2O3 (9A2B) and 2Al2O3×B2O3 (2AB) are used as the boron source. On the other hand, nitrogen gas or solid state nitirgen-bearing metal nitrides such as Si3N4 and AlN can be used as the nitrogen source. The in situ synthesized composites demonstrated homogeneous and isotropical microstructures with very fine (nano-sized) BN platelets or their agglomerates distributed in the matrixes. These composites showed high strength, low elasticity and improved strain tolerance. In this article the reaction design, thermodynamics, reaction mechanisms, reactive hot pressing or pressureless reactive sintering, microstructures and mechanical properties will be discussed.