Effect of oxidation treatment on the crack propagation rate of aerogels

Typical silica aerogels obtained by alcohol supercritical drying are hydrophobic. After a low temperature heat treatment below 400°C, they become hydrophilic. The temperature of this heat treatment may increase the network connectivity by establishing new siloxane bonds. On the microscopic scale, this process induces a very small shrinkage during which the silica backbone strengthens and approaches that of pure porous silica. Silica aerogels, whatever the nature of their surface, hydrophobic or hydrophilic, exhibit a stress corrosion effect as previously reported. A comparison of the subcritical crack growth rates of differently heat treated aerogels is given. The experimental investigations are carried out under a controlled moisture of 50% RH using the double cleavage drilled compression (DCDC) technique. The crack length is optically measured within the range 10−10–10−5 m s−1. For a given stress intensity factor KI, we observe in the stress corrosion domain that hydrophobic aerogels display a very low crack rate. This rate increases with the amount of surface silanols. For higher oxidation temperatures, the number of surface silanols decreases inducing a lowering of the crack rate.