Key Engineering Materials Vols. 280-283

Abstract: A two-step method was presented to fabricate Ti3AlC2-based ceramics. The first step was to synthesize a reactive powder, which mainly contains Ti2AlC, by pressureless sintering (PLS) in argon at 1500°C for 10 mins with a heating rate of 20°C×min-1. The second step was to hot press the powder to a bulk material at 1300°C for 60 mins. The second step not only densified the powder, but also promoted the phase transformation from Ti2AlC to Ti3AlC2. The ceramic sample prepared by this method consists of Ti3AlC2, Ti2AlC and a little amount of TiC. It has excellent mechanical properties: density is 4.26 ± 0.02 g×cm-3, flexural strength is 664.4 ± 90 MPa, Vickers hardness is 6.4 ± 0.5 GPa, and fracture toughness (KIC) is 9.9 ± 0.5 MPa m1/2. The so high values of strength and fracture toughness are never reported in any literature about this material, as far as we aware.

Abstract: A new method to synthesize high-purity Ti3AlC2 bulk material by hot-pressing was presented through adding a little amount of B2O3 as a low-temperature aid. The elemental Ti, Al, and active carbon powders were used as raw materials with a molar ratio of 3:1.2:2.The effect of sintering temperature on the purity of Ti3AlC2 was investigated at the range of 1200°C to 1500°C. XRD and SEM were used to characterize the product synthesized. High-purity Ti3AlC2 bulk materials, in which almost no TiC phase could be found, were synthesized by hot-pressing at 1400°C and 1500°C for 1h in flow Ar atmosphere. The roles of B2O3 addition were considered as weakening the thermal explosive reaction in the mixture of elemental Ti, Al, and active carbon and increasing the purity of Ti3AlC2 in addition.

Abstract: Creep and stress relaxation of Ti3AlC2 were investigated using three-point bending tests at 800-1200°C under various load levels. The results show that the creep rate significantly increases with increasing temperature in the rang of 1000-1200°C. Subcritical crack growth during the creep process was found to be the main failure mechanism, i.e., the stress intensity factor increases with the creep-induced crack growth and results in the ultimate fracture. The lower limit of stress relaxation was considered as the threshold value of zero-creep stresses, and the ratio of the threshold stress to the applied stress was defined to be a parameter of creep resistance for estimating deformation behavior at high temperature. SEM examination confirmed that the creep failure in Ti3AlC2 was governed by such a damage evolution: cavitation ® crack initiation ® crack extension ® fracture.

Abstract: Synthesis method of Zr2Al3C5 via solid state reaction between Al, ZrC and carbon powder was studied. Al-ZrC-C compact with equivalent mol ratio of Zr2Al3C5 was heated up to 1600oC in Ar atmosphere for 1 h and 4 h but ZrC phase still existed as major phase with very small amount of Zr2Al3C5. Because ZrC started to oxidize at low temperature under very low oxygen partial pressure, the same mol ratio of Al-ZrC-C compact was heated at 1600oC in vacuum for 1 h as parallel test. After firing in vacuum, some carbon still exist with small amount of AlZrC2 occurred with Zr2Al3C5 as a main phase, but no ZrC was found. Different result from firing in Ar atmosphere and in vacuum had been discussed here. It was believed that very small amount of impurities in Ar had some effect on the formation of Al-Zr-Ccompound. The effect of very small amount of impurities in Ar was studied by thermal analysis (DTA/TG) and XRD. It was found that very small amount of impurities in Ar haseffect on the reaction between Al, ZrC and carbon by diffusion through the surface and formZr-C-O-N solid solution. This solid solution cannot differentiate from ZrC by XRD. With help ofthermal analysis method (DTA/TG), Zr-C-O-N solid solution can be differentiated from ZrC.Therefore, synthesis of Al-Zr-C compound should be done in vacuum. Zr2Al3C5 can be prepared from mixture of Al-ZrC-C with excess amount of Al at 1600oC for 1 h.

Abstract: 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.

Abstract: Cubic boron nitride(CBN) is a superhard materials with many advantages and many uses. Vitrified bond CBN grinding tool is a promising abrasive tool of high performance used for high speed, high efficiency, high precision grinding with lower grinding cost and less environment pollution. Sintering of vitrified bond CBN grinding tool was investigated in this paper. The results showed that practical sintering temperature of this tool was much lower than the initial oxidation temperature of CBN particle measured by comprehensive thermal analysis. The upper limit of sintering temperature should be determined by taking account of the thermal analyzing results, heating process of CBN and its change in strength and structure. Within the sintering temperature range of the vitrified bond, relatively higher sintering temperature was beneficial to the strength of bond bridge and the holding strength between bond and CBN abrasive particles. CBN tool sintered at relatively lower temperature tended to fracture through the bond bridge, while the one sintered at higher temperature tended to fracture along the boundary between CBN abrasive grain and vitrified bond.

Abstract: In this paper, the performances of CBN abrasives with and without Ti coatings and their changes during heat treatment were investigated under the same conditions. The results showed that the single particle compressive strength and the impact toughness for CBN abrasives with Ti coatings were lower than these of CBN abrasives without Ti coating. Ti coating on the surface of CBN abrasive particles oxidized and formed TiO2 with discontinuous structure at above 650°C during the firing process. The poor oxidation resistance of Ti coating and the discontinuous structure of its oxidation product could not protect CBN abrasives from heat damage, therefore could not improve properties of CBN abrasives.

Abstract: AlN fibers were synthesized by carbothermal reduction using Al2O3 fibers as starting material. The N content of AlN fibers synthesized at 1780°C was more than 31%. The morphology and microstructure of AlN fibers were studied with XRD, SEM and TG-DSC. The fibers containing small AlN grains were pipe-shaped, in which the particle size is about 1µ, diameter 4 ~ 6µ, length/diameter ratio 20 ~ 30. The main reaction process is as follows: Firstly, alumina is reduced by carbon to lower-valent aluminum oxide, which further reacts with nitrogen and forms AlN. Secondly, when the alumina fibers are coated by AlN layer, the reaction takes place simultaneously on both interfaces of the AlN layer.

Abstract: AlN whiskers have been successfully synthesized by carbothermal reduction. The thermodynamics and growth kinetics of AlN whiskers were studied at 1600°C using CaCO3 as a catalyst. The research indicated that AlN whiskers are more easily nucleated from the liquid phase than at the surface of solid phase. AlN whiskers are nucleated by VLS mechanism and the liquid, which plays a dominant role in the VLS mechanism, is formed by Al-Ca interphases, such as CaO×2Al2O3 and CaO×6Al2O3. Kinetic studies suggest that the catalyst reacts with Al2O3 to form a low melting point eutectic (1390°C). The liquid phase formed at this low melting point eutectic provides good conditions for nucleation of AlN whiskers. At the synthesis temperature, the liquid phase vaporizes, thus creating suitable conditions for the subsequent growing of whiskers by the VLS mechanism. This growing mechanism conforms to thermodynamics and a lot of proof indicates that screw dislocations play an important role in the process of the whiskers' formation.

Abstract: The sintering behavior of AlN powders with different particle sizes and specific surface areas was investigated in the present work. 4.5-8.0mass% of the as-synthesized sintering aid from the Li2O-Y2O3-CaO system was added to these AlN powders, and they were then fired at 1400-1650oC for 6h. At the initial stage of sintering the fine AlN powder, it was recognized that sintering of primary particles occurred in agglomerations. With an increase of firing temperature, particles were rearranged and grains grew rapidly at the final stage. On contrary, in sintering of samples using the coarse AlN powder, the densification increased with raising sintering temperatures slowly, and the sintering temperature higher than 1650oC was required to obtain densified specimens.