Thermal shock and thermal fatigue in ceramic materials

Sammanfattning: This thesis concerns thermal stress fracture in ceramicmaterials. The work describes a new technique for determiningthe thermal shock resistance of brittle materials as well asexperimental results and thermal stress simulations.A new indentation-quench method to the determine theresistance of ceramics to thermal shock and thermal cycling isdescribed. The method defines a critical temperaturedifference, ΔTc, from the amount of extension and propagation ofsmall Vickers cracks. Statistically significant data areobtained by studying a large number of cracks in each specimen.Few test specimens are thus required. Another advantage withthe method is that no specific specimen shape is required. Themethod can therefore be used directly on complex shapedcomponents. This is important since the thermal shockresistance of a material is highly dependent on its size andshape.The new method is used to determine the thermal shockresistance for several materials. The materials are rankedaccording to their value of ΔTc. The ranking from quenches into water at roomtemperature is compared with that from quenches into boilingwater. The results show that the thermal conductivity of thematerials becomes of large importance when the materials aresubjected to slower quenching rates in boiling water, while thevalues of ΔTcis more dependent on the mechanical properties forquenches into water at room temperature. The resistance to tenthermal cycles close to the ΔTcof the materials is investigated. Thesecomparisons show that milder quenching conditions gives alarger crack extension. Milder quenches give longer durationtimes of the thermal stresses. The cracks will thus have longertimes to find weak paths in the microstructure. Furthermore itis shown that a finely distributed toughening phase ispreferable in thermal cycling conditions.Thermal stresses are simulated in flat plates subjected tosymmetric cooling in order to estimate the effect of severalimportant thermal shock variables. The simulated conditions aresimilar to those met by alumina in quench tests. A thermaldiffusivity, with negative temperature dependence is found toincrease the thermal stresses significantly for thinner platesizes, whereas the effects for thicker plates are small. Theeffect of the statistical distribution of fracture strengths isshown to give a larger scatter in thermal shock resistance forthin specimens than for thick ones.Keywords:Thermal shock, thermal stress, thermalfatigue, indentation, SiC-whisker-reinforced alumina, waterquench, cooling rate, thermal diffusivity.