Si_3N_4 和 SiC 陶瓷材料的压痕断裂行为和断裂韧性测量

本文以热压 Si_3N_4和反应烧结 Si_3N_4为对象,研究其在 Vickers 钻石压头的作用下,相应于压痕断裂行为而发出的声发射特征。发现,除了与压痕特征参数的发展相应的准静态声发射之外,尚有三个以上突然增强的发射讯号,分别与径向裂纹的成核、径向裂纹突发至试样表面和裂纹分岔,以及侧向裂纹的萌生等效应相关。与一般力学测试法相比,据据 Evans 表达式(7),以 Vickers 压痕裂纹技术测得的 K_(IC)值具有的负偏差是,热压 Si_3N_4:-25.1%;反应烧结 Si_3M_4:-21.6%;热压 SiC:-29.2%。以含有弹性模量项的 Evans 表达式(8)取代式(7),从而得到上述材料的 K_(IC)值偏差分别从-25.1%降至-13.9%;从-21.6%降至-12.3%和从-29.2%降至-17.9%。作为从终点烧结温度淬冷至室温的必然结果,在热压陶瓷材料中存在的热残余应力似乎助长了热压 Si_3N_4和热压 SiC 陶瓷的 K_(IC)值偏差。

Indentation Fracture Behaviours and Measurement of Fracture Toughness of Si_3N_4 and SiC Ceramics

The feature of acoustic emission in response to indentation fracture behavioursunder the action of Vickers diamond indenter for hot-pressed Si_3N_4 and reaction-bondedSi_3N_4 ceramics has been investigated.It has been found that apart from the quasi-staticacoustic emission in response to the development of the indentation characteristicparameter,there are more than three suddenly intensive emission signals related toradial crack nucleation,radial crack pop into the surface of the specimen and crackbranch,as well as lateral crack initiation respectively.In comparison with the normal mechanical method,on the basis of the Evans'expression(7),the Vickers indentation crack determination shows the values of K_(IC)with a negative deviation of -25.1% for hot-pressed Si_3N_4,of -21.6% for reaction-bonded Si_3N_4 and of -29.2% for hot-pressed SiC.With the Evans' expression(8),containing elasticity modulus of materials,to replace the expression (7),the devia-tion of K_(IC) values decreases from -25.1% to-13.9%,from -21.6% to -12.3%and from -29.2% to -17.9% respectively for those materials mentioned above.Thepresence of thermal residual stresses in hot-pressed ceramics,as a result of quenchingfrom the ultimate sintering temperature down to room temperature,seems to intensifythe deviation of K_(IC) values of hot-pressed Si_3N_4 and SiC ceramics.