Nickel sulphide inclusions in glass: an example of microcracking induced by a volumetric expanding phase change

Microcracking about inclusions of nickel sulphide in soda-lime glass has been studied. These cracks are nucleated by the volumetric expansion that accompanies the- phase transformation in nickel sulphide. The-phase is metastable and, upon transformation to the-phase, is accompanied by a volumetric expansion of about 4%. A simple generalized fracture mechanical analysis of the cracking about such inclusions is developed. Firstly, crack initiation about an inclusion is considered; this is then extended to consider inclusions within a thermally tempered plate. The critical dimensions of the inclusion to initiate microcracking in an annealed specimen and to cause spontaneous fracture of a tempered plate are predicted. Observations of nickel sulphide inclusions in annealed and tempered specimens support the predictions. The implications of this work with regard to the addition of fine dispersions of zirconia to ceramics are pointed out.