Study On Mechanical Behavior Of Porous Si₃N₄ Ceramics At High Temperature

Porous silicon nitride ceramics are widely used in the aerospace field,not only for high-temperature applications,but also for withstanding the thermal shock and thermal cycling caused by drastic temperature changes.Therefore,when designing materials,it is necessary to consider not only high-temperature resistance but also the ability to withstand thermal shock.As a result,studying and predicting the fracture behavior of porous silicon nitride ceramics under high-temperature conditions,as well as the oxidation and thermal shock resistance,is particularly important for the safety evaluation of materials during service.This paper systematically studies the high-temperature mechanical behavior of porous Si₃N₄ceramics from three aspects:high-temperature strength,high-temperature thermal shock resistance,and high-temperature oxidation behavior.Experimental results show that the high-temperature strength of porous Si₃N₄ceramics with different porosities at 800-1200°C first decreases and then increases,with the lowest strength at 1000°C,which should be used with caution.Within a certain range of porosity,as the porosity increases,the thermal shock damage resistance of the material improves.The higher the porosity,the narrower the cracks in the samples after thermal shock,indicating that the thermal shock resistance of high-porosity materials is stronger.For porous Si₃N₄ceramics with high porosity（35%-50%）,their activation energy is not affected by porosity;the oxidation behavior of high-porosity Si₃N₄ceramics at 800-1300°C conforms to the oxidation kinetics equation.The research results are of great significance for the application of porous silicon nitride ceramics in the aerospace field.In addition,this study conducted an in-depth analysis of the high-temperature oxidation mechanism of porous Si₃N₄materials and proposed that the oxidation behavior of porous Si₃N₄materials is mainly controlled by two mechanisms as the temperature changes:at the low-temperature stage（800-1200°C）,it is dominated by oxygen ion diffusion,with an activation energy of 155.88k J·mol^-1;at the high-temperature stage（1200-1300°C）,due to the quartz crystal transformation-induced cracks,it is co-dominated by oxygen molecule diffusion and oxygen ion diffusion,with an activation energy of 129.8k J·mol^-1.