W. Z. Zhu

Microstructure and mechanical properties of a Si3N4/Al2O3 nanocomposite

A nanocomposite material fabricated by hot pressing in the form of nanometre-sized Si3N4 particles dispersed in an Al2O3 matrix has been shown to exhibit enhanced mechanical properties compared with monolithic matrix material. It was observed by transmission electron microscopy (TEM) for the first time that the alumina grains were in the shape of elongated columns with aspect ratios in the range 2.5–4. The presence of liquid phase during sintering was found to be responsible for the appearance of columnar grains. Regular hexagon-shaped larger β′-Sialon grains formed during sintering were mainly situated at grain boundaries of the matrix material while irregular smaller dispersoids were trapped within the alumina grains. The improvement in the mechanical properties of the nanocomposite is attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “self-reinforcement effect” arising from the elongated columnar grains of the matrix, as well as the “pinning effect” due to the existence of intergranular β′-sialon particles. It was revealed that the trapped particles have an α-Al2O3 structure with partial sites of aluminium and oxygen atoms substituted by silicon and nitrogen atoms, which is also likely to lead to the strengthening of the composite.

Synthesis of nanometer-sized SiC powders through chemical vapour deposition II. Effect of C2H4/SiH4 mole ratio

Abstract In this paper, the effect of the C 2 H 4 /SiH 4 mole ratio on the particle size, composition and stoichiometry of nanometer-sized SiC powders has been explored. It is revealed that particle size is a strong function of the C 2 H 4 /SiH 4 mole ratio and powders having an average size as small as 13 nm are obtained when the parameter is selected to be 1.2, for which the underlying mechanism has been elucidated. Furthermore, it is found that C 2 H 4 /SiH 4 mole ratio also exerts a crucial effect upon the composition and stoichiometry of product powders. By optimizing some pertinent parameters such as temperature and C 2 H 4 /SiH 4 mole ratio, the chemical reactions responsible for the synthesis of SiC powders could be coordinated and the impurities within the powders were thus minimized.

Kinetics of isothermal transition from tetragonal to monoclinic phase in ZrO2(2 mol%Y2O3) ceramic

Abstract In this paper the kinetics of the isothermal tetragonal to monoclinic transformation in ZrO2(2 mol%Y2O3) ceramic have been investigated by means of high temperature X-ray diffractometry. The present experimental results have been compared with previous results obtained by dilation experiments. It is found that the time-temperature-transformation curve for T → M transformation is ‘C’-shaped with 300 °C as the nose temperature, which is similar to the isothermal kinetics character revealed by the thermal expansion method. The incubation period is greatly shortened due to the fact that high temperature X-ray diffraction (XRD) virtually reflects the kinetics data of T → M transformation occurring within the surface layer, several micrometers thick. In addition, the presence of pre-M phase, formed prior to holding, also plays an autocatalytic part in shortening the incubation period. The activation energy for transformation which is regressed to be 29.69 kJ mol−1 is almost equivalent to that calculated through dilation experiment. Thermodynamic calculation has shown that the critical nucleation energy barrier for surface nucleation is significantly decreased. The proposal that T → M transformation in ZrO2(Y2O3) ceramics possesses both displacive and nondisplacive features due to the existence of oxygen vacancies is put forward.

Synthesis of nanometer-sized SiC powders through chemical vapour deposition III. Effect of overall cell pressure

Abstract In this paper, the effect of overall cell pressure on the particle size, size distribution, phase and chemical composition, shape and crystallinity of SiC powders synthesized through chemical vapour deposition using a conventional heated furnace tube has been studied by means of transformation electron microscopy, X-ray diffraction (XRD), chemical analysis, X-ray photoelectron spectroscopy and IR absorption spectroscopy. Experimental results reveal that both particle size and crystallite size increase with increasing cell pressure due to the enhanced degree of supersaturation with respect to the chemical reaction. The composition of SiC powders can either be affected by cell pressure or remain unchanged, depending on the C 2 H 4 /SiH 4 mole ratio chosen. The shape of the powders is nearly spherical and the crystal form turns out to be β -SiC as evidenced by XRD analysis.

Effect of cooling rate on tetragonal to monoclinic transformation in hot pressed ZrO2(Y2O3) ceramics

It is well documented that the tetragonal (T) to monoclinic (M) transition in either pure zirconia or partially stabilized zirconia is the origin of toughening in that resistance to the propagation of cracks can be greatly enhanced by the concurrent appearance of the stress field of the transformation. In the present paper, the effect of cooling rate on the T {yields} M phase transformation in yttria-containing zirconia and its resultant mechanical properties have been studied by means of thermal expansion analysis. Both the T {yields} M and M {yields} T transformations are affected by the cooling and heating rates, respectively. The amount of M-phase decreases with increasing cooling rate. T {yields} M transition occurring within the interior part of specimen can be completely inhibited by the cooling rate of 100 C/min for ZrO{sub 2}(2mol% Y{sub 2}O{sub 3}) ceramic sintered at 1,600 C. The start point and end point of the T {yields} M transformation decreases and increases, respectively, with increasing cooling rate. Both the start point and end point of the M {yields} T transformation increase with increasing cooling rate. The divergence between the results of X-ray diffraction and the thermal expansion analysis has been rationalized in terms of themorexa0» both internal and external factors, namely, preferential sites of surface for the formation of the M-phase and limited sensitivity of measurement of the thermal expansion apparatus. Both the water-cooled and air-cooled specimens show much improved mechanical properties regardless of the sintering temperatures or yttria content because of the relatively higher T-phase fraction retained to room temperature.«xa0less

Synthesis of nanometer-sized SiC powders through chemical vapour deposition: I. Effect of reaction temperature

Nanometer-sized SiC powders have been synthesized through chemical vapour deposition using self-designed setup. The features of the powders have been examined by X-ray diffraction, transmission electron microscopy, infrared ray absorption spectrum and chemical analysis, with emphasis intentionally put on the effect of reaction temperature. It is disclosed that SiC powders with narrow size distribution, spherical shape, high purity, excellent crystallinity and as fine as 9 nm being the smallest average size that can be synthesized. Both the particle size and grain size increase with increasing temperatures. The highest purity is obtained when the reaction occurs at 1300°C.

Ageing behaviour of t'-phase in a hot-pressed ZrO2(4 mol%Y2O3) ceramic

The ageing behaviour of unequilibrium tetragonal (t′) phase and its resultant effect on the mechanical properties of hot-pressed ZrO2(4 mol% Y2O3) ceramic have been investigated by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Experimental results show that t′-phase which is the product of diffusionless transformation from cubic (c) phase during rapid cooling after sintering is unstable when aged in a temperature range of 1400–1600 °C for up to 80 h in that it decomposes diffusionally into equilibrium tetragonal(t) phase and c-phase. Yttria contents of phases formed during decomposition are basically in agreement with those indicated by phase diagram. The stability of t′-phase characterized by the existence of anti-phase domain microstructure under the microscopic dark field image is significantly associated with the tetragonality(c/a) measured by XRD and the larger the tetragonality, the more unstable the t′-phase. Metastable precipitates of t-phase are triggered by applied stress to transform to monoclinic (m) phase during which the fracture toughness is enhanced and transformability of t-phase is critically dependent upon the solute content as well as size. It is found that when t′- and m-phase coexist with adequate fractions of c- and t-phase, the fracture toughness of the aged specimen demonstrates a peak value that moves to shorter ageing times with increasing temperature while the Vickers hardness decreases monotonically with ageing time regardless of ageing temperature due to grain growth.