Behaviour and phase transformations of nickel sulphide inclusions in glass melts

Nickel sulphide inclusions with the NiS composition can cause spontaneous failure of thermally toughened glass. During annealing a phase transformation from high to low-temperature phase takes place, which has a higher volume. This phase transformation, which is very slow at room temperature, generates internal stress in the glass, and after some months up to several years spontaneous fracture can occur. To obtain more information about the nickel sulphide behaviour, NiS and Ni3S2 were prepared and investigated in melts of different glass types at various temperatures and melting times. Analysis of the nickel sulphides was done before and after treatment in the melts. Knowing that additives modify the thermodynamics and kinetics of dissolution behaviour, experiments were carried out with 5 mol% Fe or 5 mol% Sn added to Ni3S2. The oxidation of the intermediate phases (Ni,Fe)9S8 and Ni3Sn2S2 was preferred over the oxidation of the pure nickel sulphides. Additionally, the possible forming process of nickel sulphide in glass was tested. The normally formed phase was Ni3S2, but some additional complex metallic phases were also found, depending on the glass type.