Preparation and Healing of Cracks in Al2O3 - MgAION Composite

The Al2 O3 - MgAION specimens with dimensions of Ф20 mm × 10 mm and 3 mm × 4 mm× 36 mm were prepared using corundum grains （ w （ Al2 O3 ） 〉 98%, particle size： 3-1 mm, ≤1 mm, ≤85μm and ≤3μm, respectively） and MgAlON （particle size ≤ 2 μm ） as starting materials, PVA as binder, designing 3 formulations containing aggregate and 4 special formulations without aggregate, pressing under 150 MPa and keeping for 5 s, firing at 1 500 ℃ for 2 h in nitrogen under normal pressure, and the cracks were prepared on the specimens by thermal shock method （air cooling at 1 100 ℃ for 10 cycles） and by Vickers method （29. 4, 49. 0 and 98. 0 N, keeping for 25 s）, then holding at 1 100 ℃ and 1 550 ℃ for 6 h in nitrogen atmosphere for cracks healing, respectively. The influences of crack prefabricated method, grain composition, pressure of Vickers and pressure holding time on the shape and dimension of cracks were studied. Effects of healing temperature on healing degree and MOR of specimens were investigated and the crack healing rate was calculated. The results showed that： （1） the cracks of MgAlON specimens prepared by thermal shock method under normal pressure were of irregular shape and uncertain length; （2） the cracks prepared by Vickers method was approximate diamond-shape without slender cracks emitting from every vertex angle; but if composite specimens were fabricated by particles of several microns, the crack size, shape and its site, especially that obtained under 98. 0 N for 25 s, could be controlled exactly under a Vickers hardometer, which made the crack healing research easy to conduct; （3） after healing treatment at 1 100 ℃ or 1 500℃, the cracks became shallower and smaller even disappeared, the strength of the specimen was recovered finally; （4） a formulation for crack heal ing rate expressed by the cracked capacity was： V/ t=1/3 Cv^2（1＋ctg θ ） Cv/ t.