Toughening Behavior in Sic‐Whisker‐Reinforced Alumina

Abstract

The development of hot-pressed Sic-whisker-reinforced alumina has resulted in composites with fracture toughness values -8.7 MPa.m"2 at 20 vol% S i c . Whisker orientation during processing leads to anisotropy in both fracture toughness and fracture strength. Fracture strengths are also limited by the ability to disperse the S i c whiskers; however, use of bothfine alumina powders and ultrasonic dispersion techniques yields strengths =800 MPa. R, Prewo and Brennan' demonstrated that the fracture toughness of glass-ceramics can be increased substantially by reinforcing them with continuous high-strength S i c fibers. In that study, notched-beam KIc values of up to =20 MPa-m"' (where the notch is perpendicular to the fiber axes in uniaxially aligned fiber compositions) and flexure strengths of 4 5 0 MPa were obtained for a borosilicate-matrix Sic-fiber composite. As pointed out by Rice' and S h e t t~,~ load transfer to long, high-strength fibers requires fiber-to-matrix Young's modulus (E) ratios of greater than two, which is the case in the Prewo and Brennan' study. Crack deflection4 as well as fiber or whisker pullout3 toughening processes can occur in composites containing short, strong whiskers where the whisker-to-matrix modulus ratios are somewhat lower. The potential for toughening by fiber reinforcement processes becomes very attractive if short, strong whiskers could be incorporated into various polycrystalline ceramics by conventional powder processing techniques. Clearly, the degree of toughening that can be achieved in ceramics by incorporating short, strong whiskers needs to be assessed. With this in mind, the fracture behavior was examined for the case of polycrystalline alumina containing highstrength S i c whiskers, where the fiber-tomatrix modulus ratio is ==I .5.* RESULTSAND DISCUSSION Fully dense, fine-grained ( s 4 p m ) alumina matrix composites were obtained by hot-pressing a mixture of alumina pow-CONTRIBUT[NC Eo[ToR-R. J. STOKES

Comparison of short and longer term loading on the creep behaviour of alumina-silicon carbide whisker composites

“…No bimodal creep behaviour was observed and the stress exponent, n, was found to be -1. 5. The activation energy, Q, was found to be 370k50 kJ/mol.…”

“…The creep rate has been observed to increase by a factor of -3 to 5 [lo] in air as compared to an inert environment. The remaining difference should thus be due to the different duration of the creep tests.…”

High Temperature Engineering Ceramics

“…8 of this Volume). It was reported that the bending strength and the fracture toughness were significantly improved by these additions, as shown in Table 10-26 (Becher and Wei, 1984;Wei and Becher, 1985). These composites also showed excellent creep resistance.…”

“…Typical data for commercial grades are given in Table 10-6 in comparison with alumina ceramics. Al 2 O 3 -SiC whisker composites have been developed for improved strength and toughness (Becher and Wei, 1984;Wei and Becher, 1985), and are discussed in Sec. 10.6.3.…”

R‐Curve Behavior of Silicon Nitride Ceramic Reinforced with Silicon Carbide Platelets

“…4,5,[13][14][15] High strength, high elastic modulus, and chemical inertness at elevated temperature made the SiC most suitable for the reinforcing agent. In many systems, including Si 3 N 4 , substantial improvements in fracture toughness and slow-crack-growth resistance were obtained by reinforcement with SiC whiskers.…”