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How do low thermal expansion and high thermal conductivity reduce thermal shock in tribological ceramics?
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advanced structural ceramics
Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.
In the context of tribological ceramics, heat generated during wear can lead to thermal shock, a phenomenon where rapid temperature changes cause surface layers to contract against inner layers, creating tensile stress and crack propagation.12
Low thermal expansion and high thermal conductivity mitigate this risk through the following mechanisms:
- Low Thermal Expansion: By utilizing materials with low thermal expansion coefficients, the physical dimensions of the ceramic change less significantly during temperature fluctuations. This reduction in dimensional change directly decreases the magnitude of the thermal stresses that would otherwise lead to cracking.1
- High Thermal Conductivity: Materials with high thermal conductivity are effective at dissipating heat away from the contact surface. By rapidly conducting heat into the bulk of the material, the ceramic prevents localized temperature spikes, thereby reducing the thermal gradients that drive thermal shock.1
Effectively managing these properties is essential for maintaining the structural integrity of ceramics used in high-friction environments, such as rotating shafts and valve seats, where heat generation is a constant factor.1