Why does glass burst and ceramics not during rapid transitions from hot to cold?

- Feb 12, 2020-

Why does glass burst and ceramics not during rapid transitions from hot to cold?

 

Why the rapid cooling and bursting will explode, the knowledge about thermal stress and micro-cracks needs to be explained by surface compressive stress and micro-cracks.

 

The first thing to say is that the strength of tempered glass is not due to the existence of internal stress. The existence of internal stress is unfavorable to the glass and should be eliminated by annealing. It is said that the daily use of tempered glass is to form a laminated stress layer on the surface by heat treatment to offset some of the tensile stress generated in daily use and improve the strength of the glass. Secondly, regardless of the ingredients, the melting temperature and the firing temperature are all hooligans. The melting of daily-use glass can not reach 1500 ° C, which is a relatively pure glass at 1500 ° C, so manufacturers at a high temperature are losing money, so in fact, many fluxes are added to reduce the melting temperature.

 

Ceramics and glass are both brittle materials. It should be known that all brittle materials are very brittle, but not resistant to impact. A piece of glass, a piece of ceramic, you need to use a lot of force to fracture it, but if you want to fold it, you don't need a very strong force. A perfect material without defects, the strength is the strength of the chemical bond, but in practice due to various factors, there are always defects in the material, which makes the material strength on the basis of chemical bond strength greatly reduced. Under the action of external forces, a large stress concentration occurs near these microcracks, causing the cracks to propagate and the material to break. Cracking caused by rapid cooling and heating is actually caused by thermal stress. When a material is affected by a temperature gradient, every part of it is not thermally expanded and contracted simultaneously. This is because the conduction of heat in a material is a process that takes time. In this process, if the stress (ie, thermal stress) caused by the unevenness is greater than the critical strength required for the above failure, the material is naturally destroyed.

 

The main factors affecting the thermal stability of materials are the following:

First, the strength and elastic modulus of the material itself

Thermal conductivity of materials

Third, the thermal expansion coefficient of the material

Fourth, the surface thermal conductivity of the material

Five, material thickness

 

It is generally believed that the higher the strength / elastic modulus, the better the material toughness and the resistance to thermal shock. The strength of ceramics and glass is not low. The difference is that the elastic modulus of ordinary glass is greater than that of ceramic, meaning that the same stress The amount of glass change should be small. In other words, when the strain at which the material is broken is reached, the stress on the glass is smaller and more likely to be broken.

 

At the same temperature difference, a material with a small coefficient of thermal expansion produces less thermal stress.

 

The coefficient of thermal expansion is mainly determined by the structure. Glass is amorphous, that is, a looser structure than crystals, so the coefficient of thermal expansion is larger than that of ceramics. In summary, when subjected to the same temperature difference, the thermal stress generated by the glass is greater, and at the same time, the toughness of the glass is worse and it is more likely to break.