高压烧结致密非晶SiBCN块体陶瓷的组织结构演化与力学性能

Microstructural evolution and mechanical performance of high-pressure sintered dense amorphous SiBCN monoliths

非晶SiBCN陶瓷是一类独特的结构材料,具有低比重、高比强度、优异的高温损伤容限等特殊结构和性能,因此在高温防热结构部件上极具应用潜力。通过合理的结构与化学成分协同设计,可探索陶瓷形貌/微观结构演化及断裂行为的基本特征,从而进一步提高其力学性能,以满足实际应用需求。因此,文章以石墨、六方氮化硼、立方硅和硼等元素粉末为原料,提出了采用机械合金化结合高压烧结技术(1 000°C/3~5 GPa/30 min)制备致密非晶Si2ByC2N(y=1.5~4)块体陶瓷的方法。通过XRD、SEM、TEM、TG等表征手段,研究了烧结压力诱导该系非晶陶瓷的组织结构演化、相变及热稳定性,并对其力学性能,特别是断裂行为进行了详细讨论。结果表明,提高烧结压力促使陶瓷基体由完全非晶态向晶态转变,部分块体陶瓷由大量非晶相、少量c-Si和/或t-BN(C)纳米晶相组成,显示出依赖于硼含量的物相组成。高压烧结有效地促进了陶瓷的烧结致密化,导致材料内自由体积的湮灭和"河流状"断裂形貌的产生。随着烧结压力的提高,陶瓷材料的体积密度、纳米硬度和杨氏模量单调增加。在相同烧结条件下,硼含量的增加削弱了非晶Si2ByC2N(y=1.5~4)块体陶瓷的力学性能和热稳定性。1 000°C/5 GPa/30 min烧结制备的致密非晶Si2B1.5C2N块体陶瓷的体积密度、纳米硬度和杨氏模量分别为2.69 g/cm3、33.6±2.2GPa和414.2±16.5 GPa。

Amorphous SiBCN ceramics are a unique class of structure materials with potential applications as high-temperature heat-resistant structural components owing to their combination of special structure and properties of low specific weight,high specific strength and excellent damage tolerance at elevated temperatures.However,it is still in need for further mechanical properties improvements for actual demands by rational co-design of architecture and chemistry as well as understanding the basic features of morphological/microstructural evolution and failure behavior.Thus in this attempt,to target the dense amorphous Si2ByC2N(y=1.5~4)monoliths with high mechanical properties,an attractive way of combinatorial mechanical alloying and high-pressure sintering technique(1 000℃/3~5 GPa/30 min)was proposed using elemental powders of graphite,hexagonal BN,cubic Si and boron as raw materials.The sintering pressure induced microstructural evolution,phase transformation and thermal stability have been investigated by XRD,SEM,TEM and TG measurements,and mechanical performance,especially the fracture behavior was also discussed in details.Results clearly presented that an increase in sintering pressure promoted the phase transformation of completely amorphous matrix to a hybrid structure of substantial amorphous phases and few nanocrystals of metal c-Si and/or t-BN(C)for some monoliths,implying boron-content depending phase composition.High-pressure sintering effectively promoted the densification leading to free volume annihilation and the river-like fracture morphology occurrence.A monotonous increase of the bulk density,nano hardness and Young’s modulus as a function of the sintering pressure was observed.Under the same sintering conditions,the increase in boron content weakened the mechanical properties and thermal stability of the amorphous Si2ByC2N(y=1.5~4)monoliths.Dense amorphous Si2B1.5C2N monoliths consolidated at 1 000℃/5 GPa/30 min showed the best performance with the bulk density,nano hardness and Young’s modulus obtaining at 2.69 g/cm3,33.6±2.2 GPa and 414.2±16.5 GPa,respectively.