Erosion of composite ceramics

The theoretical basis to describe solid-particle erosion of monolithic ceramics is well developed. In many cases, the models can account for the impact velocity, impact angle and erodent-size dependencies of the steady-state erosion rate. In addition, the models account for effects of materials parameters such as fracture toughness and hardness. Steady-state erosion measurements on a wide variety of composite ceramics, including SiC whisker-reinforced Al[sub 2]O[sub 3], Si[sub 3]N[sub 4] containing Si[sub 3]N[sub 4] or SiC whiskers, Y[sub 2]O[sub 3]-stabilized ZrO[sub 2] reinforced with SiC whiskers, and duplex-microstructure Si[sub 3]N[sub 4] have been reported. The theories developed for monolithic ceramics are, however, less successful in describing the results for composites

Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta

In previous studies on a number of under- and overdoped high temperature superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta}, the transition temperature T_c has been found to change with time in a manner which depends on the sample's detailed temperature and pressure history. This relaxation behavior in T_c is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly under- and overdoped samples ($T_c\simeq 40 - 50 K$) with an activation energy $E_{A}(1 bar) \simeq 0.8 - 0.9 eV$. For overdoped HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of +11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an anomalous result for high-T_c superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa

Degradation of structural ceramics by erosion

Materials wastage by solid-particle erosion can be severe and can limit lifetimes. This paper will review the theoretical description of solid-particle erosion in brittle materials, which is well-developed for monolithic ceramics. The models can usually account for effects from the principal projectile properties of size, impact velocity, and impact angle. Materials parameters such as fracture toughness and hardness can be included. Steady-state erosion measurements on a wide variety of ceramics, ranging from Si single crystals to SiC-whisker-reinforced Al{sub 2}O{sub 3}, are reviewed and compared with the models. It is believed that R-curve behavior and/or particle fragmentation is responsible for discrepancies between theory and experimental results for composite ceramics. In addition, the theories make no attempt to describe threshold or incubation effects

HIGH-TEMPERATURE DEFORMATION OF CUBIC OXIDES

Les courbes contrainte-déformation de MgO et NiO ont un taux d'écrouissage θ qui décroît linéairement quand la contrainte augmente jusqu'à un état stationnaire (θ=0) alors que la courbe pour CoO présente un crochet de traction et un taux d'écrouissage négatif qui augmente jusqu'à s'annuler quand l'état stationnaire est atteint. Ces résultats prédisent des formes de courbes de fluage similaires pour NiO et MgO, différentes de celle obtenue pour CoO. Finalement, la contrainte τs de l'état stationnaire divisée par le module de cisaillement est très semblable pour les trois oxydes, NiO présentant la plus grande valeur. La dépendance en température de τs est donnée pour les trois oxydes par la loi : log τs = B - T/A, où A et B sont des constantes.The stress-strain curves for MgO and NiO have a work-hardening rate θ that decreases linearly with increasing stress to a steady state (θ=0) while the curve for CoO has an upper yield point and a work-hardening rate that is negative but increases to zero when steady state is achieved. These results predict that the shapes of the creep curves for NiO and MgO are similar, and different from those obtained for CoO. Finally, the steady-state stress τs divided by the shear modulus is very similar for all three oxides, with NiO having the largest value. The functional dependence of τs on temperature T for all three oxides is given by log τs = B - T/A, where B and A are constants

Creep of SiC-Whisker reinforced Si3N4

The mechanical behaviour of a composite of Si3N4 (with 3 % MgO) containing 0-20 % SiC whiskers has been investigated by compressive creep experiments (100-300 MPa) at temperatures between 1 250 °C and 1 500 °C. No significant influence of the SiC whiskers on the creep behaviour could be detected up to about 5 % of strain. Two temperature regions could be identified : a) at high temperature the creep deformation occurs at a quasiconstant strain rate. Extended cavitation in the triple junctions occurs. b) At low temperatures, creep rates rapidly decrease due to a strain hardening coefficient much more important than for higher temperatures. In the microstructure, cavities and microcracks along grain boundaries and fiber-matrix interfaces were observed. Dislocations play an insignificant role for the deformation in both temperatures regions.Le comportement mécanique d'un composite de Si3N4 (avec 3 % MgO) contenant de 0 à 20 % de fibres de SiC est étudié par des essais de fluage en compression (100-300 MPa) entre 1 250 et 1 500 °C. Les fibres de SiC n'influencent pas le comportement du fluage jusqu'à des déformations de l'ordre de 0,05. On peut distinguer deux régimes de fluage : un régime à haute température où la déformation se fait à vitesse quasi constante. A basse température la vitesse de déformation diminue rapidement à cause d'un coefficient de durcissement beaucoup plus élevé qu'à haute température. La microstructure des échantillons non-déformés et déformés révèle que les dislocations jouent un rôle négligeable dans les conditions de déformation décrites. Les cavités observées aux points triples et les fissures aux joints de grains et aux interfaces entre matrice et fibres indiquent plutôt un mécanisme de déformation par cavitation et microfissuration

Solid-particle erosion of an Al{sub 2}O{sub 3}-SiC-TiC composite

An electrodischarge-machinable Al{sub 2}O{sub 3}-SiC-TiC composite developed by Industrial Ceramic Technology, Inc., has a high fracture toughness, 9.6{+-}0.6 MPm{sup 1/2}, as measured by indentation, and a Vickers hardness of 20.3{+-}0.6 GPa. The composite`s resistance to solid-particle erosion was measured for 143-{mu}m dia SiC particles impacting at 20-90{degree} angles and 50-100 m/s velocities. Erosion rate exhibited a maximum for normal incidence, and the erosion resistance was better than that of commercial Al{sub 2}O{sub 3}. SEM indicated that material wastage was by a combination of brittle fracture and microplasticity

Plastic deformation of alumina reinforced with SiC whiskers

Addition of small amounts of stiff reinforcement (SiC whiskers) to a polycrystalline AL{sub 2}O{sub 3} matrix partially inhibits grain boundary sliding because of an increase in threshold stress. When the concentration of whiskers is high enough, a pure diffusional mechanism takes over the control of plastic deformation of the composites. For higher whisker loadings, the materials creep properties depend on a microstructural feature different from the nominal grain size. A tentative correlation of this effective microstructural parameter with the spacing between the whiskers was established through a model