颗粒增韧陶瓷裂纹扩展微观过程

MICROPROCESSES OF CRACK PROPAGATION IN PARTICULATE TOUGHENING CERAMICS

对于第二相颗粒增韧的复相陶瓷，颗粒热膨胀系数α＿ｐ与基体热膨胀系数α＿ｍ之间的匹配关系是决定增韧效果的主要因素。当两相弹性模量相当，颗粒粒径小于应力诱导微开裂的粒径，且α＿ｐ＞α＿ｍ时，残余热应力场的存在将会使扩展的裂纹在颗粒周围基体中产生较大的裂纹偏转，由此产生明显的增韧效果；而当α＿ｐ＜α＿ｍ时，扩展的裂纹将首先达到两相的界面，此时裂纹有可能沿两相界面偏转，也有可能穿过颗粒，这取决于颗粒的表面能、粒径、形状、取向及两相界面结合状况，即使发生偏转，因偏转的程度较小，两相界面断裂能低于基体断裂能，偏转增韧的幅度较小。但是，当第二相颗粒为纳米级并进入基体颗粒内部形成纳米复合材料时，也将获得较好的增韧效果；第二相颗粒的引入能在一定程度上阻止基体晶粒的长大，使材料的强度提高，但对某些系统（如Ａｌ＿２Ｏ＿３／ＳｉＣ＿（ｎｐ））并不一定能改善材料的韧性，基至有可能抵消一部分颗粒增韧效果。

The mismatch between thermal expansion coefficients α_p and α_m is the main factor affecting thetoughening of particulate toughened ceramics,If the matrix and second phase particles in the composites havesimilar elastic medule and α_p>α_m ,the crack seriously deflects in matrix around particles due to thermal stressesand therefore the toughness is improved. On the other hand,if α_p<α_m,the propagating crack reaches the interfaceof the two-phase boundary first,and then deflects either along the phase boundary or crosses the particle, con-trolled by surface energy,size,shape and orientation of the particle and the strength of the two-phase boundary,even if it deflects,increase of toughness is minor because of minor extent of deflection and smaller fracture energyof the two-phase boundary.Distinct toughening effect can also be obtained as the size of the second phase parti-cles reduces to nanometer. Addition of second phase particles can enchance the strength of the composites inhabit-ing the growth of matrix grains generally. But it may not improve the toughness for some system(for example,Al_2O_3/SiC_(np)),even partially offseting the particulate toughening effect.