Strength of brittle materials in moderately corrosive environments

Abstract

The strengths of four brittle materials-cordierite glass ceramic, fused silica, silicon, and polycrystalline alumina were measured after exposure to weakly corrosive water and moderately corrosive buffered HF (BHF) solution. Exposure to water did not alter the strengths in subsequent inert strength tests and decreased the strengths in reactive strength tests. Exposure to BHF increased the strengths in both tests, but only after an incubation exposure time. Prior to the incubation time, the BHF had the same effect as water, suggesting that the bond rupture kinetics were unaffected.Examination of strength-controlling indentation flaws after the incubation time showed clear corrosive effects on the flaw geometry indicative of reductions in the indentation residual stress fields. The implication is that moderately corrosive environments increase the strength or lifetime of a brittle component by reducing the crack driving force via flaw alteration and do not, as perhaps expected, decrease the strength or lifetime through enhanced chemical reactivity.

Long‐term ceramic reliability analysis including the crack‐velocity threshold and the “bathtub” curve

“…Unsurprisingly, the complicating factors required numerical solutions for strength predictions. A subsequent experimental work omitted microstructural effects and showed that constant stressing‐rate loading and long‐term zero‐stress aging in moderately corrosive environments could significantly affect reliability predictions through flaw alteration effects. Another work simplified consideration even further by omitting all effects of microstructure, contact‐flaw alteration, and crack‐velocity thresholds, to demonstrate a method for short‐term reliability prediction in reactive environments for components under constant applied stress containing contact flaws.…”

Thermal activation effects in crack propagation and reliability of fused silica

“…In constant stress studies, the failure time diverges as the applied stress approaches an analogous lower bound. For failure controlled by contact flaws, as is common in such tests, the lower bound reactive strengths are related to the upper bound inert strengths by the ratio (2 γ /2 γ 0 ) 2/3 , where 2 γ 0 is the surface energy of the material in an inert environment . For fused silica 2 γ 0 = 8.8 J m −2 and hence the strength ratio should be about 0.25 to 0.43 using the 2 γ estimates from above.…”

A simple method of short‐term mechanical reliability prediction for ceramics in reactive environments

“…Toughening and the formation of traction zones is minimized in materials with small or absent microstructural features, again similar to glasses. A material that meets these criteria, to be used here as an example, is a fine‐grained cordierite glass‐ceramic, similar to that studied earlier . The second two effects (3 and 4), thresholds and corrosion, depend on the mechanical and chemical environment.…”

“…Short failure times also minimize corrosion effects, as do environments that are reactive but do not remove material through dissolution. The second two criteria are met here by loading the samples in water to obtain failure times of 1 hour (threshold and corrosion effects are observed in cordierite in about 10 hours).…”

“…The material used here was a cordierite (2MgO·2Al 2 O 3 ·5SiO 2 ) glass‐ceramic, similar, but slightly denser after sintering (0.995 ± 0.004 relative density measured by Archimedes' method) to that used in previous strength studies (0.990 ± 0.004 relative density) (unless otherwise stated, all quantities and symbols reported here are given as mean ± standard deviation of experimental measurements). The initial samples were in the form of discs with diameter 31.8 mm and thickness 2.1 mm, and inert strength tests were conducted on the discs.…”