Richard E. Tressler

Recent developments in fibers and interphases for high temperature ceramic matrix composites

Abstract The long term durability of CMCs is limited by two factors: (1) creep and rupture of the fibers, and (2) environmental degradation of the constituents, primarily in the nonoxide composites. Oxide CMCs are limited by the creep resistance of the fiber at this stage of development and by interphase concepts that are not yet mature. Nonoxide fibers have been developed with excellent creep resistance relative to oxide fibers, but oxidation of the interphase and the interface, particularly at intermediate temperatures, causes embrittlement of these composites. This effect is particularly severe when matrix cracks are present and under cyclic loading conditions.

Prediction of the Strength of Ceramic Tubular Components: Part I—Analysis

The objective of this paper is to develop the analytical background for test methodologies that will enable accurate prediction of the strength distribution of ceramic tubular components from the strength distributions of simple specimens. Four simple specimen configurations and two tubular configurations were selected for this purpose. The simple specimen configurations were (1) four-point bend, (2) C-ring tested in compression, (3) C-ring tested in tension, and (4) O-ring tested in diametral compression. In addition, a short tube tested by axially compressing rubber inside the tube and a long tube subjected to internal pressure were analyzed. These specimen configurations were for the most part selected in a tubular shape in order to simulate the shape of tubular structural components. The prediction of the strength distribution of one specimen from that of another was based on Weibull statistical theory. Effective volume and area expressions, necessary for failure prediction, were derived for these specimen configurations.

Stress relaxation of a soda lime silicate glass below the glass transition temperature

Stress relaxation is an important effect in the ion-exchange procedure of glasses, as it controls the stress profile and the strength. Creep and stress relaxation tests have been performed to study the viscoelastic behavior of soda-lime silicate glass at typical ion-exchange temperatures. The experimental data of these tests can be fitted well by the Burger model and a comparison between the viscosity data from both tests was made. The strain and temperature dependences of the stress relaxation process were studied and the glass exhibited a non-linear viscoelastic behavior and an anomalous temperature dependence. In addition, it was found there is a relationship between the glass density and the stress relaxation behavior.

Microstructural instability and the resultant strength of SiCO (Nicalon) and SiNCO (HPZ) fibres

Abstract Microstructural changes and the related strength degradation were investigated for SiCO Nicalon and SiNCO HPZ fibres during heat treatments in air and argon at 1000–1400 °C for 0.5 to 90 h. While the as-received Nicalon fibre contains β-SiC and carbon nanocrystals in an amorphous SiO 1.12 C 0.44 phase, the HPZ fibre is completely amorphous. It is also inhomogeneous in surface composition compared with that of the bulk. The time-dependent strength degradation of the Nicalon fibre in argon is related to the gradual decomposition of the SiO 1.12 C 0.44 phase from the surface which produces surface defects, β-SiC grain growth and intergranular porosity. The strength degradation of the HPZ fibre results from surface crystallization into α-SiO 2 , Si 2 N 2 O and β-SiC. On the other hand, the HPZ fibre core — which has composition close to SiN 1.02 C 0.23 — shows structural stability for all heat treatment conditions.

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al2O3, Al2O3-ZrO2) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10−8−7.70 × 10−6s−1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol−1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.

Microstructural evolution of sol-gel-derived phosphosilicate gel with heat treatment

The microstructure of phosphosilicate gel prepared by a sol-gel process was investigated as a function of the heat-treatment temperature. Crystalline phases, Si3(PO4)4 and SiP2O7, were identified in the heat-treated phosphosilicate gel containing 56 mol% P2O5. It was found from the Fourier transform infrared spectra that phosphorus enters into the copolymer structure at ∼ 600 °C. The specific surface area of the gel increased with the heat-treatment temperature. An increase of the density of open pores with the heat-treatment temperature was observed in the secondary electron micrographs.

A preliminary study of precipitation in Ti4+-doped polycrystalline alumina

Precipitation processes in two titanium-doped aluminas (0.14 and 0.60 cation % titanium) have been examined using a variety of analytical electron microscopy techniques. The results strongly suggest that, for an ageing temperature of 1573 K, rutile is the only precipitate formed in the 0.14 cation % titanium material. However, evidence for the precipitation of both rutile and Β-Al2TiO5 (the latter at triple junctions) was obtained in the 0.60 cation % titanium sample.

High temperature mechanical behavior of a chemically vapor deposited beta silicon carbide

Abstract Mechanical properties of a pure and completely dense polycrystalline beta silicon carbide material fabricated via chemical vapor deposition (CVD) were evaluated. Properties varied extensively from one batch to another, mainly due to the presence of residual stresses formed during and after the deposition process. Unreacted silicon and carbon were detected in materials processed in a hot wall reactor. Free silicon was observed primarily along the column boundaries of silicon carbides produced in a hot wall reactor. The presence of the silicon resulted in severe degradation of properties above 1400°C. One batch of material which was fabricated in a cold wall reactor had the highest strength at 1400°C and free silicon was detected only locally and in small amounts in this material. Flaw healing and partial relief of tensile residual stress due to oxidation and increased ductility of the silicon phase at high temperatures improved the short-term strenght of the VCD silicon carbide at elevated temperatures.

Oxygen penetration into silicon carbide ceramics during oxidation

Abstract The penetration of oxygen into polycrystalline silicon carbide ceramics, in advance of the oxide/substrate interface, during oxidation for 1–100 hrs at 1200–1400°C was studied using SIMS and TEM techniques. Fully dense hot pressed ceramics containing aluminum additives, with and without an oxide grain boundary phase and CVD silucon carbide exhibited sharp interfaces. Sintered silicon carbides with boron and carbon additives (∼ 97% dense) and aluminum carbide additive (∼ 90% dense) exhibited a region of oxygen penetration ∼10–15 μm in depth beneath the oxides scale, the depth of which was insensitive to the time and temperature of oxidation. The amorphous oxide phase in this zone was located at three and four grain junctions but the two grain junctions were unaffected in this zone by oxidation. This oxygen affected region, which is responsible for the slow crack growth susceptibility of these ceramics after oxydation, results from gaseous oxygen penetration along interconnected or nearly interconnected pores and oxidation of impurity laden channels and SiC surfaces. The depth of penetration is presumably limited by closure of the channels by the oxidation products.

Interfacial behavior of microcomposites during creep at elevated temperatures

Abstract The high temperature interfacial behavior of SiC/C/SiC microcomposites was investigated. The interfacial sliding resistance dropped slightly from a room temperature value of 10 MPa with increasing temperature up to 1300°C in argon. The interfacial shear stress was shown to remain constant during the creep of microcomposites at 1200–1300°C and 200–450 MPa in argon. For creep in air, the interfacial shear stress increases at long exposure times.