Flexural Creep Deformation of ZrB₂/SiC Ceramics in Oxidizing Atmosphere

Abstract

Flexural creep of ZrB2/0–50 vol% SiC ceramics was characterized in oxidizing atmosphere as a function of temperature (1200°–1500°C), stress (30–180 MPa), and SiC particle size (2 and 10 μm). Creep behavior showed strong dependence on SiC content and particle size, temperature and stress. The rate of creep increased with increasing SiC content, temperature, and stress and with decreasing SiC particle size, especially, at temperatures above 1300°C. The activation energy of creep showed linear dependence on the SiC content increasing from about 130 to 511 kJ/mol for ceramics containing 0 and 50 vol% 2‐μm SiC, respectively. The stress exponent was about 2 for ZrB2 containing 50 vol% SiC regardless of SiC particle size, which is an indication that the leading mechanism of creep for this composition is sliding of grain boundaries. Compared with that, the stress exponent is about 1 for ZrB2 containing 0–25vol% SiC, which is an indication that diffusional creep has a significant contribution to the mechanism of creep for these compositions. Cracking and grain shifting were observed on the tensile side of the samples containing 25 and 50 vol% SiC. Cracks propagate through the SiC phase confirming the assumption that grain‐boundary sliding of the SiC grains is the controlling creep mechanism in the ceramics containing 50 vol% SiC. The presence of stress, both compressive and tensile, in the samples enhanced oxidation.

Creep and Oxidation of Hafnium Diboride Based Ultra High Temperature Ceramics at 1500C

“…Upon the addition of 10% SiC, the minimum creep rate appears to increase by two orders of magnitude. This is consistent with the literature, which generally associates an increase in SiC content with an increase in creep rate, due to smaller grain sizes and sliding of SiC grains [17]. However, the addition of more SiC to 20% and 30% appears to progressively decrease the minimum creep rates.…”

Grain rotation and translation contribute substantially to creep of a zirconium diboride silicon carbide composite

“…The shift in activation energy with stress appears to scale with a shift in the dominant vacancy diffusion path. At lower stresses the activation energy begins to correlate with the range of ZrB 2 grain boundary activation energies following the ZrB 2 polycrystalline creep work of Talmy [49] and Kats [50] and the shear viscosity measurements conducted by Kuzenkova [51]. With increasing stress the measured activation energy correlates with those for ZrB 2 bulk diffusion.…”

Prediction of tensile power law creep constants from compression and bend data for ZrB2–20 vol% SiC composites at 1800 °C

“…No experimental tensile data are available in the literature for direct comparison. In fact, for ZrB 2 composites, only flexure and compression experiments were carried out in the literature [7,[18][19][20][21][22][23]. Additionally, we propose an enhanced specimen geometry for compression creep testing that allows higher creep strains to be reached prior to the onset of catastrophic specimen instability.…”

“…The outer-fiber strain of the flexure specimen (nominal dimension of 4 mm  4 mm  40 mm) was calculated from the tensile outer-fiber displacement δ based on linear elastic beam assumptions [7]:…”

“…Improved oxidation behavior as well as enhanced fracture strength and toughness can be achieved by addition of 20-30 vol% of SiC [2,7]. Creep behavior of these materials is particularly important for several applications such as re-entry vehicles and hypersonic aircraft, which involve exposure to high temperatures for extended periods of time [4,8].…”