K. R. McKinney

Fracture energy of Si3N4

The fracture energy of Si3N4 made by hot pressing, reaction sintering, and chemical vapour deposition (CVD) was studied. Extrapolation of fracture energies to zero additive or porosity levels, as well as analysis of CVD Si3N4 all indicate an intrinsic fracture energy of 20–30J m−2. Higher fracture energies in dense bodies with increasing additive content, or in some more porous bodies (relative to expected porosity dependence) are associated with crack branching. In dense bodies such branching may arise due to micro-cracking from combined effects of crack tip stresses and mismatch stresses due to differences in properties, especially thermal expansion, between Si3N4 and the additive or its reaction products. In porous bodies such branching appears to be due to spatial distribution of pores.

Slow Crack Growth in Polycrystalline Ceramics

It is now well known that slow crack growth occurs in glasses at stress levels well below that which will cause immediate fractures. Work performed on polycrystalline ceramics1,2 has shown that many undergo delayed failure in a manner similar to glasses and some slow crack growth studies have been reported3–5 Evans6 recently showed a correlation between slow crack propagation in an alumina and its delayed failure characteristics. He also showed that the time-to-fail of the alumina under a given stress could be determined through knowledge of crack velocity as a function of the stress intensity factor, KI.

Strength and toughness of jade and related natural fibrous materials

Room temperature mechanical properties measured for natural fibrous “jade” materials with random fibre orientations were similar to strengths (e.g. ∼ 100 MPa) and toughnesses,KIC, (e.g. ∼3M Pa m1/2) in other studies. However, nearly three- and five-fold higher values were found respectively for strength and fracture toughness of “jade” with highly aligned fibres for crack propagation perpendicular to the fibres. Further, the results indicate significantly higher strength and toughness with decreasing fibre diameter and increasing aspect ratio, and an accompanying increase in intrafibre fracture. However, failure was predominantly catastrophic in character for all fibre orientations, indicating some material (i.e. matrix) is necessary for non-catastrophic failure as found in fibre composites.

Oxidation weight gain and strength degradation of Si3 N4 with various additives

Oxidation behaviour and strength degradation, due to long-term high temperature exposure in air, of Si3N4 with MgO, ZrO2 or Y2O3, as densification aids have been studied. It was found that the weight change and strength degradation in the specimens depend largely on the kind of densification aid. Strength degradation, which generally occurred and was greatest at higher additive levels, was related to the generation of pits in the specimen surface. Possible mechanisms for pit formation are discussed.

Fracture and fracture toughness of stoichiometric MgAl2O4 crystals at room temperature

The fracture toughness and path of stoichiometric spinel (MgAl2O4) crystals were determined at 22 °C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl2O4 fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1) the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (∼1.2 MPa m1/2), (2) fractography of machining flaw fracture origins was the most effective source of KIC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especially for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especially for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).

Slow crack growth in Si3N4 at room temperature

Tests of Si3N4 hot pressed with various types and levels of oxide additives show evidence of room temperature slow crack growth in delayed failure tests (using natural flaws), but not in fracture mechanics (e.g. DCB or DT) tests consistent with more limited literature data for these two types of tests. Neither type of test showed slow crack growth in either CVD Si3N4 or RSSN. Further the fracture mode in the latter two materials was essentially all transgranular, while it was predominantly (e.g. 80%) intergranular in the hot-pressed materials. It was thus postulated that (1) the oxide grain boundary phase is responsible for slow crack growth and; (2) varying distribution of the oxide boundary phase and grain boundary character result in sufficient boundaries not susceptible to slow crack growth to pin cracks with macroscopic crack lengths (i.e. as in DCB and DT tests). Both the much smaller crack front lengths and the large number of small (natural, e.g. machining) flaws allows some of these small flaws to grow to critical size, thus leading to delayed failure in the hot-pressed materials.

Strength and Toughness Measurements of Ceramic Fiber Composites

A recent modification of the applied moment double cantilever beam (AMDCB) fracture toughness test is described. This test, the L-arm AMDCB test; is used to measure the fracture toughness of UTRC Compglas unidirectional and 0/90 crossply ceramic fiber-ceramic matrix composites. It is shown that the test permits characterization of the fracture toughness and fracture behavior of such high toughness and highly anisotropic materials. The measured KIC values, for the high toughness directions, were found to be as high as 60 MPa·m1/2, with indications of a change in fracture mode with increase crack velocity.

Strength, Toughness And Fracture Initiation Of Fluoride Glasses

A ZBLAL fluoride glass produced at NRL was characterized mechanically both in bulk and fiber forms. Fracture toughness measurements gave Ki values of 0.38 ± 0.05 MPa.m1/2, about 1/2 that of Si02 glasses. Flexural strength of machined bars of the current ZBLAL glasses was about 60MPa, about 80% of similarly finished silicate glasses. While the fluoride glass has lower strength than Si02 glass, it had much less slow crack growth, which should give it a longer useful life over a range of stresses. Although the average tensile strength of fibers from initial fiber drawing trials was only about 170 MPa, fractographic examinations indicated that the useful strength of this fiber could be raised considerably with improved processing.