Effect of AlN-content on the microstructure and fracture toughness of hot-pressed and heat-treated LPS-SiC ceramics

The influence of additive content on the microstructural development of
hot-pressed and heat-treated LPS-SiC has been investigated using
AlN-Y2O3 mixtures at a molar ratio of 80:20, varying the total amount
from 5, 10, 15 to 20 wt.%. Specimen were hot-pressed at 1900 degrees C
for I It in nitrogen atmosphere under an applied pressure of 25 MPa and
subsequently heat-treated at 2000 degrees C for 1, 2, 4 and 8 h.
It has been found that the transformation rate of beta- into alpha-SiC
is retarded by higher AlN-contents and the formation of the 6H
alpha-SiC polytype is favored. Furthermore, grain growth during
annealing is also effectively inhibited. While hardness remained almost
unchanged, fracture toughness varied with additive content and/or
duration of the heat-treatment. Fracture toughness increased during the
first I or 2 It of annealing,depending on the AlN-content, and
diminishing for more prolonged treatments. The maximum fracture
toughness has been determined for samples containing 10 wt.% of
additives, hot-pressed and annealed during 1 h.