COUPLING

The durability and aesthetics of ceramic products depend on the thermomechanical compatibility between their constituent layers. At the Ceramic Coatings Center (CRC), we understand that the origin of some critical defects lies in the coupling stresses that develop between the body (support) and the glaze during the manufacturing process. When engobes, enamels, or granules are present, these elements also affect the stress state of the resulting products.

Stresses arise during firing when the product layers are rigid and exhibit incompatible movements. Therefore, to avoid problems, it is necessary to monitor:

Differences in Thermal Expansion (CET): The incompatibility between the linear thermal expansion coefficients (CET) of the glaze and the support is the primary cause of stresses, which manifest themselves during the cooling phase of firing.

Differences in Fusibility: In engobes, enamels, and less fusible glazes, the layers are rigid even in the firing zone. In this case, the differences in shrinkage between the mass and these layers are also sources of stress.

The result of this incompatibility is the development of opposing stresses in the product structure, caused by tensile and compressive stresses.

The lack of proper adjustment of these stresses developed during firing can lead to serious defects, such as:

Cracking: Occurs when the glaze is subjected to tensile stresses. This usually happens when the coefficient of thermal expansion of the glaze is greater than that of the substrate. Cracking can also be delayed, manifesting itself months or years after installation. In this case, the Moisture Expansion (ME) of the porous substrate or the shrinkage of the fixing mortar converts the initial compressive stress of the glaze into tensile stress, causing failure.

Warping: The differences in shrinkage between glaze and substrate result in concave or convex curvatures in the fired tile. If the substrate shrinks more than the glaze, the piece acquires a convex curvature and the glaze is under compression. If the glaze shrinks excessively, the curvature becomes concave, and the glaze is under tension, increasing the risk of cracking.

Shivering: Results from excessive compression of the glaze, which can exceed the bond strength at the interface with the substrate.

At CRC, we are equipped to estimate coupling stresses and curvatures in multilayer products.

Our services include:

Experimental Determination: We use a combination of analytical techniques to understand the stresses generated by each layer of the product.

Improvement Proposals: Based on the quantification of stresses, we propose adjustments to the formulation of the body, engobe, and glaze, as well as optimizations in firing cycles, to ensure better results.