Accession Number:

ADA109087

Title:

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

Descriptive Note:

Final rept. 1 Aug 77-30 Sep 81

Corporate Author:

SYSTEMS RESEARCH LABS INC DAYTON OH RESEARCH APPLICATIONS DIV

Personal Author(s):

Report Date:

1981-11-01

Pagination or Media Count:

79.0

Abstract:

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 HPSi3N4 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

Subject Categories:

  • Ceramics, Refractories and Glass
  • Mechanics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE