Fundamentals of Spontaneous Breakage Mechanism Caused by Nickel Sulfide

Andreas Kasper

As an introduction to "newcomers", the poster points out the mechanism of spontaneous failure of tempered glass caused by NiS stones.

During the glass melting process, and during the tempering process, the high temperature modifi cation (á-NiS) forms, stable above 390°C. It does not transform during the processes because of too high cooling speeds: it is "frozen in".

The volume of the â-NiS is by about 2.2% to 4% higher than that of the á-NiS. The á to â transformation puts an internal spot-wise stress on the glass. Pressure causes the initiation of small semicircular flaws around the inclusion. These are stable until a certain critical size has been reached, depending on the inclusion's environmental stress situation inside the glass.

At room temperature, the á to â transformation is slow. It takes months to years. A spontaneous break is happening when the inclusion's growth oversteps a critical limit.

An incubation period is typical for NiS breakage. The retardation may range from a few months to a couple of years. It is caused by the different thermal dilatation coefficients of glass and NiS.

The so-called Heat Soak (HS) Process, recently proposed as an European Standard (prEN14179-1), was developed to overcome the problem with spontaneous breakage. It is a destroying test, eliminating glasses bearing critical NiS inclusions, by a 2 hour's heat treatment at (290 ± 10)°C.

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The Authors

Dr. Andreas Kasper Dr. Saint-Gobain Profession: ChemistOccupation: Head of project group "chemistry of glass melting"Responsibilities:Chemical analysis of glass and raw materials; environmental measurements (emissions of glass tanks); s... Read more