Heat Treating Glass
Heat Treating Glass AnonymousWhen fabricating glass, it is usually vitally important to control the cooling of the fabricated pieces. Due to the brittle nature of ceramics, failure to remove internal stress in the glass either introduced during fabrication or due to uneven cooling will likely result in catastrophic structural failure of the piece. There are two basic types of heat treatments applied to glasses. In annealing, cooling is controlled in an effort to remove (or minimize) the internal stress in the glass. This is in contrast with tempering. In tempering, compressive stress is intentionally introduced into the surface of the piece as shown in the figure below. This compressive stress can prevent surface scratches and cracks from growing, which would likely fracture the glass.
Tempering: Compressive stress is intentionally introduced into the surface of the piece.
This image illustrates how controlled cooling of a hot material can suppress surface crack growth by inducing compressive stress. In the top section, the process is shown in three stages. Initially, the material is uniformly hot. As cooling begins, the outer layers (top and bottom) cool faster than the center, creating a temperature gradient. Once the material reaches room temperature, this gradient results in compressive stress on the surface and tensile stress in the middle. These compressive forces help prevent cracks from growing on the surface. The bottom section reinforces this concept by showing a close-up of surface cracks being compressed, visually demonstrating how this stress distribution effectively inhibits crack propagation. The image emphasizes the importance of thermal management in enhancing material durability.
In the next section, we will discuss sintering, which is very important for particulate forming of ceramics.