  An original setup combining a very stable loading stage, an atomic force
microscope and an environmental chamber, allows to obtain very stable
sub-critical fracture propagation in oxide glasses under controlled
environment, and subsequently to finely characterize the nanometric roughness
properties of the crack surfaces. The analysis of the surface roughness is
conducted both in terms of the classical root mean square roughness to compare
with the literature, and in terms of more physically adequate indicators
related to the self-affine nature of the fracture surfaces. Due to the
comparable nanometric scale of the surface roughness, the AFM tip size and the
instrumental noise, a special care is devoted to the statistical evaluation of
the metrologic properties. The 2 roughness amplitude of several oxide glasses
was shown to decrease as a function of the stress intensity factor, to be quite
insensitive to the relative humidity and to increase with the degree of
heterogeneity of the glass. The results are discussed in terms of several
modeling arguments concerning the coupling between crack propagation,
material's heterogeneity, crack tip plastic deformation and water diffusion at
the crack tip. A synthetic new model is presented combining the predictions of
a model by Wiederhorn et al. [1] on the effect of the material's heterogeneity
on the crack tip stresses with the self-affine nature of the fracture surfaces.

Soft Condensed Matter