Surface-Strengthening Mechanisms in Reaction-Bonded Si sub 3 N sub 4 and Subcritical Crack-Growth Behavior in Ceramics

摘要

Reported herein are results of a number of investigations on: (1) strength and fracture toughness of oxidized, reaction-bonded (RB) Si3N4, (2) evaluation of chemical-vapor-deposition (CVD)-coated reaction-bonded Si3N4 and characterization and properties of controlled-nucleation-thermochemical-deposited (CNTD) silicon carbide, (3) subcritical-crack-growth (SCG) behavior of hot-pressed (HP)Si3N4 under mixed-mode loading conditions and SCG behavior of oxidized HP Si3N4, and (4) SCG behavior of Lithium-Aluminum-Silicate (LAS) glass-ceramics. The oxidation of RB Si3N4 at 900 C increases the strength by as much as 25%, presumably by blunting the flaw tips associated with open surface pores. The increase in fracture toughness is approximately the same. The 1500 deg C oxidation, on the other hand, does not provide effective flaw blunting and, in addition, an external silica scale is formed which cracks during cooling. At intermediate temperatures, the post-oxidation strength behavior is intermediate. The CVD-coated RB Si3N4 exhibited a significant loss of strength, even though a uniform, adherent coating was produced. The strength of HP Si3N4 under mixed-mode loading conditions indicated that the effect of the shear component of applied stress is minimal and that the SCG mainly occurs in Mode I. The pre-oxidation of HP Si3N4 decreased the SCG kinetics measurably, presumably due to the depletion of Ca and Mg from the glassy intergranular phase. The SCG behavior of LAS glass-ceramic also indicated that Mode I is the predominant mode of SCG, even in the presence of Modes II and III. (Author)