Spontaneous cracking of thoughened glass

Large fronts of tempered safety glasses are well known design features in modern architecture. For these thoughened glasses spontaneous cracks and shattering are known problems that can occur even years after fitting. Already the optical image of a spontaneous fracture provides important information on the damage mechanism to the glass expert. However, a clear determination of the reason for a crack formation of a thoughened glass requires more detailed investigations with analytical methods like SEM/EDX

Figure 1 shows an optical image of a spontaneous crack with a visible particulate inclusion in the centre. A material contrast image acquired with a Scanning Electron Microscope is given in figure 2 and provides information on the particle dimension (ca. 200 µm) and morphology. The observed image contrast between the particle and the surrounding safty glass also indicates that the particle mainly consists of elements which are heavier than Si. A detailed chemical characterisation of the particle composition can be obtained by means of Energy Dispersive X-ray Spectroscopy (EDX).A comparison of figures 3a and b shows that the analysed particle mainly consists of nickel sulphide which leads to the corresponding element signals in the EDX spectrum.

EDX thus confirms NiS inclusions (Millerite) which are a known as reason for spontaneous fractures of toughened safty glass since the 60tes. The fracture is caused by a slow volume increase of nickel sulphide particles due to a so-called allotropic transformation. This NiS transformation process can be forced with a Heat Soak Test at the end glass production.