Creep, Strength, Expansion, and Elastic Moduli of Sintered BeO As a Function of Grain Size, Porosity, and Grain Orientation

Abstract

Physical properties are presented for extruded and sintered B e 0 from 25" to 1400°C as a function of porosity (0 to 15%), grain size (5 to loop), and grain orientation (random to 80% preferred). T h e elastic constants and linear thermal expansion are sensitive to the degree of preferred grain orientation. Measurements on polycrystalline specimens permitted calculation of the anisotropy in single crystals of B e 0 for these properties. Modulus of rupture data are treated in terms of the Knudsen equation, and compressive creep data have been used to estimate the diffusion coefficient controlling strain rate at 1200°C.

A review of the processing, composition, and temperature-dependent mechanical and thermal properties of dielectric technical ceramics

“…Hot pressing of BeO typically results in higher densification for a given final grain size than pressureless sintering, and this results in higher strength. For pressureless sintering, an optimal compromise between densification and grain size is typically found in the grain size range 5-20 µm, with relative density ≥ 95% [49,59]. Even still, the typical flexural strength of BeO is lower than that of the other ceramics considered in this study, with some RT values as low as 100-150 MPa reported [49,52].…”

“…The strength of BeO typically increases above room temperature, reaching a strength maximum between 500-1200°C, and then decreases rapidly at higher temperatures [54,57,59,150]. However, the maxima are not always observed.…”

“…Porosity, or high additive or impurity contents, which can dominate the failure mode, can reduce or eliminate the strength maxima, giving a strength plateau at intermediate temperatures [150]. Fryxell and Chandler [59] reported the increase in strength from room temperature to the strength maximum at intermediate temperature to be larger, and occur at higher temperature, with increasing grain size. Carniglia et al [54] ascribed the strength minima observed around room temperature in their three-point flexural strength tests of hot pressed BeO to chemisorbed moisture.…”

“…Extrusion leads to some preferential grain orientation, the c-axis tending to align with the extrusion axis [58,59], leading to some anisotropy in the sintered body [57]. BeO may be hot pressed [54], although this is less commercially prevalent than sintering.…”

“…Some polycrystalline ceramics exhibit preferential grain orientation, typically resulting from processing such as extrusion, hot pressing or tape-casting, leading to macroscopically anisotropic thermal expansion. BeO ceramics have been variously reported with macroscopically isotropic [58] and anisotropic [59] thermal expansion coefficients. The COTE of many but not all specimens of vitreous silica reported are very slightly anisotropic [99].…”