Effect of oxidation treatment on the crack propagation rate of aerogels

Typical silica aerogels obtained by alcohol supercritical drying are hydrop hobic. After a low temperature heat treatment below 400 degreesC, they beco me hydrophilic. The temperature of this heat treatment may increase the net work connectivity by establishing new siloxane bonds. On the microscopic sc ale, this process induces a very small shrinkage during which the silica ba ckbone strengthens and approaches that of pure porous silica. Silica aeroge ls, whatever the nature of their surface, hydrophobic or hydrophilic, exhib it a stress corrosion effect as previously reported. A comparison of the su bcritical crack growth rates of differently heat treated aerogels is given. The experimental investigations are carried out under a controlled moistur e 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 corr osion domain that hydrophobic aerogels display a very low crack rate. This rate increases with the amount of surface silanols. For higher oxidation te mperatures, the number of surface silanols decreases inducing a lowering of the crack rate. (C) 2001 Elsevier Science B.V. All rights reserved.