| Description |
R-curve behavior of four different silicon carbide (SiC) materials was assessed using an indentation/strength technique. Two developmental grades, with similar microstructures and chemistries but different fracture modes, were compared with commercially available SiC-N (Cercom, Inc., Vista, CA) and solid-state PS-5000 (Morgan AM&T). The long-crack fracture toughness, measured using single-edge precracked beam (SEPB) technique, was 88% higher for material A (SEPB of 6.4 MPavm), which showed a highly intergranular fracture mode compared to B (SEPB of 3.4 MPavm), which was mainly transgranular. The higher SEPB toughness was associated with a modest increase in average strength (25%) and a significant increase in Weibull modulus (11 to 32). Short-crack fracture toughness measured with the indentation method used fracture strength and crack lengths at fracture to assess R-curve behavior. A new method for determining 6, which characterizes the residual driving force of the plastic zones based on stable growth of indentation cracks from the initial (co) to instability (c*) lengths, was developed. Material A exhibited a rising crack-growth resistance behavior compared to material B, which had mainly flat crack-growth resistance. Commercial SiC-N also showed R-curve behavior whereas PS-5000 had no Rcurve. The concept of a crack-stability map was developed for materials with R-curve behavior. Transformation-toughened Ce02 doped ZrC>2 (Ce-TZP) was shown fifteen years ago to have both R-curve behavior and high reliability. These data were revisited by considering the interaction between the crack-driving force and the rising resistance to such growth. Three distinct crack-growth regimes occur for this material. Unstable crack growth is followed by a region of stable crack growth, which leads to a second condition for unstable crack growth. Conditions for each of the three regions are discussed. Extremely high Weibull values (m>50) are possible for ceramic materials with large regions of stable crack growth. It is shown that the ratio of the fracture toughness at the initiation of crack growth (K0) to the steady-state fracture toughness (Kx) determines the behavior of materials with R-curve behavior. For materials like SiC, where Ko/Kx is greater than 0.197, it is nearly impossible to grow cracks from natural flaws because the cracks grow unstably. For Ce-TZP, however, since Ko/Kx is less than 0.197, a region of stable crack growth allows nearly invariant strength, which results in truly damage resistant ceramics. |
