In recent years there have been a number of sensational reports in the media about “glass cancer” and “spontaneous glass fracture”, with stories relating to “glass raining down from highrise buildings”.
These stories refer to incidents where toughened glass windows shatter without warning. Although it is only recently that this so-called “spontaneous” failure of toughened glass has come to public attention, it has been known about since 1960. These failures are due to the presence of nickel sulphide inclusions. In fact, nickel sulphide inclusions in glass are quite rare. In a typical glass batch there will be one 5 µg inclusion per tonne of glass (an average concentration of 5 parts in 1012).
Though rare, the nickel sulphide inclusions are very troublesome and potentially dangerous when present in toughened glass. The reason for all the trouble is a delayed phase transformation in nickel sulphide. Nickel sulphide crystals have a high temperature and a low temperature form. The dense crystal form at high temperature swells on cooling to make a less dense crystal form at low temperatures.
In ordinary annealed glass nickel sulphide inclusions do not cause problems because the transformation occurs as the glass is cooled slowly during manufacture. However, the transformation is sluggish and when glass is rapidly cooled as part of the toughening process, the nickel sulphide remains trapped in its high temperature form until some years later when its transformation breaks the glass.
A characteristic “spontaneous” fracture pattern in a toughened glass window. The nickel sulphide inclusion is found at the initiation of failure (at the centre of the butterfly pattern, arrowed).
The manufacture of toughened glass and generation of unstable nickel sulphide
Flat glass is toughened in an oven that resembles a giant toaster. The glass is transported on rollers and is rolled back and forth inside the oven and heated to a temperature of between 600 and 700 °C until it becomes soft. The glass in its softened state is rolled out of the oven into an air shower where both sides of the glass are cooled rapidly. Glass has low thermal conductivity so that the inside of the glass remains hot and soft while the two outer surfaces of the glass cool, solidify and then contract due to thermal contraction (thermal expansion in reverse).
After this the inside gradually cools, solidifies and contracts. Because the outer surfaces are already cold when the inner region begins to solidify, contraction in the inner region squeezes the outer surfaces. In the finished glass the regions near the outer surfaces experience high compressive forces which are balanced by high tensile forces generated at the inner region. The toughening process produces a safety glass that is also very strong.
Nickel sulphide inclusions found inside intact glass. There is significant cracking in the glass adjacent to the inclusion, but at the time of photography these cracks were sub-critical.
Nickel sulphide is in its high temperature form at above 380 °C and should revert to the low temperature form during cooling to room temperature, but in toughened glass does not do so because the transformation is sluggish and because of the rapid cooling rate required by the toughening process. Swain found that the high to low temperature transformation results in a 4% expansion of nickel sulphide, so that inclusions larger than 60 µm in diameter can generate potentially dangerous cracks in the surrounding glass. On the positive side, external and internal stresses impart valuable properties to toughened glass. The compressive stress at the surfaces closes up surface cracks and increases the glass strength by a factor of 3 to 5 times. The internal tensile stress ensures that, if the glass should be broken, then stress release causes the glass to shatter safely into small blocks (fracture dice).
However, if a nickel sulphide inclusion is present in the tensile zone there is a down side and the internal tensile stress becomes an “Achilles heel” for toughened glass. The problem is that swelling of the nickel sulphide inclusions does generate cracks in the glass and any small crack in the tensile zone will cause catastrophic failure. The sluggish property of the transformation results in a delay between toughening (which generates the unstable inclusion) and glass failure. The rates of failure are hard to predict since both rate of failure and length of time delay vary from location to location. In some installations all failures occur within 5 years, but in many cases failures continue for 10 years or more after installation.
Nickel sulphide inclusions found on fracture surfaces of glass that failed by “spontaneous fracture”.
Towards fixing the problem: characterisation, testing and detection
More than 45 years has elapsed since the cause of spontaneous fracture in toughened glass was first established. Since then there have been a number of attempts to eliminate the problem. Three different types of approach have been adopted; which are: characterisation, destructive tests, and detection.
Scanning electron microscopy images of nickel sulphide inclusions found on a fracture surface. The inclusion on the left is quite large (400 µm in diameter), the inclusion on the right is relatively small (85 µm).
Characterisation is a study of a material; what it looks like, what is made of, what kinds of impurities and trace elements does it contain, and how is it situated in relation to its surrounds. By characterisation of the nickel sulphide and other impurities in the glass we can hope to find out something about how the nickel sulphide came to be there and perhaps figure out how to remove it. We know that sulphur is added as a component in glass manufacture but the sources of nickel are unknown. Characterisation, therefore, is focussed on identification of sources of nickel.
There are three possible sources of nickel; the raw materials, the materials used in storage and handling of raw materials, and contamination coming into the glass melt via fire bricks and the burners. As a result of some early characterisation work it was recognised that fuel oil contains traces of nickel and because of this glass processing tanks which originally used fuel oil burners have been converted to natural gas. The conversion to natural gas and modifications in handling of raw materials has improved the situation with nickel sulphide but has not eliminated the problem. There have been some recent discoveries made in relation to sources of nickel and these will be discussed further below.
A destructive test called “heat soaking” has been developed for detection and removal of glass which contains nickel sulphide inclusions. In a heat-soak test the toughened glass is stacked inside a hot box and reheated to 290 °C for a few hours. The heat-soak process induces the inclusions to transform to their low temperature form which causes the problem windows to fail. The advantage in using this process in that the heat-soak removes more than 95% of problem windows and does not significantly reduce the temper of the glass. The disadvantage of the process is that it adds extra cost because of the need for extra processing, and because it is destructive. All shaping, hole-drilling and finishing of the glass must be done before toughening, so the loss of windows in a destructive test represents a loss of time spent in preparing the glass. In addition, because the sheets are stacked close together on the hot box, the failure of one sheet of glass will often damage adjacent glass as well.