Richard Warren

Zirconia as a nucleating agent in a yttria-alumina-silica glass

Abstract The crystallization behaviour of a ZrO2-containing glass and a ZrO2-free glass in the Y2O3-Al2O3-SiO2 system prepared at 1700 °C were compared in order to study the influence of ZrO2 on the nucleation and growth processes. Techniques used included SEM, DTA and XRD analysis. The microstructural development of the ZrO2-containing material during crystallization was more complex than that of the ZrO2-free material. In the crystallization treatments no phase separation prior to crystallization could be observed and no precursor crystalline phase was observed to form in the ZrO2-containing glass. Thus the added zirconia could be considered to act as a growth modifier rather than a catalyst nucleating agent.

Creep properties of single crystal oxides evaluated by a Larson-Miller procedure

Abstract A collation of published results of compressive and tensile creep studies of single and binary oxide single crystals is presented. For the purposes of comparison the results were normalised by means of a Larson-Miller procedure. This method proved to be effective in providing a ranking of the oxides in terms of their creep resistance. Binary oxides with complex crystal structures exhibited the highest creep strengths; no direct correlation with the melting point could be found.

Mechanical alloying of alumina-yttria powder mixtures

Abstract Mechanical alloying has been used to prepare powder mixtures of alumina and yttria as a means to create composites with a dominant matrix phase together with small particles of a dispersed second phase. The yttria–alumina system, containing five possible phases, has the potential for creating eight combinations of matrix and dispersed phases. Here compositions designed to give YAlO 3 (YA) dispersed in Y 3 Al 5 O 12 (Y 3 A 5 i.e. YAG) or Y 4 Al 2 O 9 (Y 2 A) were studied. After milling with steel tools for times up to 8 h, the powders were subjected to thermal cycles up to 1500°C during which the phase evolution was monitored using X-ray diffractometry (including high-temperature XRD) and differential thermal analysis. During milling the original crystal structures were quickly broken down, in some cases partially replaced by an intermediate structure after milling. Upon subsequent heating the milled mixtures crystallized to give the expected phases, YA in Y 3 A 5 and YA in Y 2 A respectively, but the reaction route was seen to be different depending on the amount of amorphization of the yttria. Contamination by iron was seen to affect the phase distribution and the lattice parameters.

Nitridation study of reaction-bonded silicon nitride in situ by high temperature X-ray diffraction

Abstract The reaction-bonded silicon nitride (RBSN) nitriding process has been studied using a high temperature X-ray diffractometer (HT-XRD) under isothermal conditions in the temperature interval 1300–1400 °C. With HT-XRD, the nitridation reaction and phases formed could be monitored almost instantaneously at temperature. The experimentally observed kinetics of the nitriding reaction were found to be in fair agreement with a theoretical model which predicts that the nitriding reaction occurs predominantly by Knudsen diffusion of nitrogen molecules through channels in a layer of growing Si 3 N 4 . However, no single rate law is likely to describe the whole nitridation process. Observation of the microstructure after nitridation indicates that the process occurs partly by reaction of Si vapour with nitrogen gas but that inward diffusion of nitrogen into particles also contributes significantly to the overall nitridation. The phase analysis showed that α-Si 3 N 4 formation predominates over β-Si 3 N 4 formation but the proportion of β-Si 3 N 4 increases as nitridation continues.

Elevated temperature fracture behavior of monolithic and SiCw-reinforced silicon nitride under quasi-static loads

Abstract Fracture behavior of a monolithic and SiC w -reinforced Si 3 N 4 over the 1000–1550 °C temperature range with monotonically increasing loads was investigated. Peaks in the fracture initiation toughness, K c , were found to occur owing to the brittle-to-ductile transition in the fracture behavior. This transition is associated with the onset of viscous deformation of the secondary intergranular amorphous phase present in the microstructure and the concomitant relaxation of the crack-tip stresses. The brittle-to-ductile transition temperature (BDTT) depends on the loading rate. Damage accumulation in terms of nucleation and growth of cavities ahead of the crack-tip promote stable crack growth at temperatures higher than the BDTT. The length of the subcritical crack increases with increasing temperature. Experiments involving the introduction of stable cracks at elevated temperatures followed by room-temperature fracture toughness testing reveal that shielding owing to the crack-wake bridging increases the apparent fracture resistance above the BDTT. Micromechanisms of the brittle-to-ductile transition and subcritical crack growth at elevated temperatures in the Si 3 N 4 ceramics were discussed.

Stresses developed in reaction-bonded ceramics

Abstract A physical model is presented that predicts the stress distribution created in a particle during its reaction with a surrounding reactant to form a uniform layer of reaction product on its surface, when the reaction involves a volume change. The results of the model are applied specifically to the case of silicon reacting with nitrogen to form Si 3 N 4 . The model predicts the generation of a high, tensile hydrostatic stress in the Si core as well as high tensile radial stress and compressive tangential stress in the nitride layer. Although the model is restricted to elastic deformation only and therefore predicts unrealistically high stresses in some cases, the results are anyway of relevance in the consideration of possible non-elastic processes such as creep and fracture and also in assessing the possible effect of stress on the reaction equilibrium. It is predicted that the nitride reaction layer would fracture during the nitridation process. A second model is also presented predicting the residual stresses arising during cooling of a partially reacted particle as a result of the difference in thermal expansion of the reactant core and the reaction product layer. In the case of the reaction of silicon to silicon nitride these thermal expansion mismatch stresses are significant but small compared to the stresses due to the chemical reaction. ©

A microstructural investigation of the mechanisms of tensile creep deformation in an Al2O3/SiCw composite

Abstract The tensile creep behaviour of an SiC w (25%) reinforced alumina composite was investigated using scanning electron microscopy (SEM) and transmission electron microscopy and automatic image analysis in SEM. The creep tests were carried out in air in the ranges 1100–1300 °C and 11–67 MPa. Each creep test was performed at a constant temperature. The material had a stress exponent of about three for all temperatures and an approximate activation energy of 650 kJ mol −1 . The creep resistance of this composite is poorer than that of similar composites studied earlier. Microstructural examination revealed the microstructure to be extremely inhomogeneous consisting of spherical whisker-rich clusters (20–100 μm) surrounded/separated by Al 2 O 3 rich rims (10 μm). The secondary creep rate is dominated by a damage accumulation process namely cavitation and crack growth in both the SiC clusters and the Al 2 O 3 rims. Final fracture seems to occur through the alumina rich regions. The lower creep resistance of this composite compared to that of similar composites is attributed primarily to the inhomogeneity of the as-received material.

Tensile Creep of Alumina and SiC Whisker Reinforced Alumina

The present work is a report of an experimental study of the tensile creep of a polycrystalline alumina and a SiC whisker reinforced alumina composite. The focus of the work was on the composite since, as far as the authors are aware, no studies of the tensile creep of this type of material had been reported earlier. Differences in the grain size, processing and origin of the two materials inevitably clouds the interpretation of comparisons made between them; however, the study of the alumina, a more familiar material, also functioned as a means to gauge the experimental approach.