Values between 2.4 and 5.1 MPa Vm have been measured for CVD p-SiC, depending on the test technique employed and grain size. Fracture tough­ness of CVD SiC increases slightly at elevated tem­peratures. It does not exceed 6 MPaVm.

2.12.2.1.4 Fracture strength

As is usual with brittle ceramics, fracture data exhibit a significant scatter, as flaws that have a random distribution induce fracture. An important conse­quence is that the fracture stress is not an intrinsic characteristic. It is, instead, a statistical variable, which depends on several factors including the test method, the size of test specimens, and the number of test specimens.24 Therefore, a universal reference value of fracture strength cannot be recommended.

It is widely accepted that the Weibull model satis­factorily describes the statistical distribution of fail­ure strengths: (s/s0)mdV /V„ where P is the probability of failure, s is the stress, s0 is the scale factor, m is the Weibull modulus, V is the volume of specimen, and V0 is a reference volume (1 m3 is generally used); m reflects the scatter in data, and s0 is related to the mean value of the strength.

The strength data for a given geometry and stress state can be determined using eqn [4]. However, m, s0, and V0 must be available. It is important to note that the estimate of s0 depends on V0.24 It will be substantially different if V0 = 1 m3 or 1 mm3. This dependence is ignored in most publications, even in the work by Snead and coworkers23 in which a number of s0 values are reported. When V0 is not given, the estimate of s0 is meaningless. The strength cannot be determined safely. Unfortunately, reliable s0 values (characteristic strength in a few papers) cannot be recommended here until the authors have completed their papers. The values of Weibull modulus of CVD SiC at room temperature reported in Snead etal23 span a large range, from 2 to 12. The following values were measured using tensile tests on CVI SiC/SiC mini­composites: m = 6.1, s0 = 10.5 MPa (V0 = 1 m3).25,26

2.12.2.1.5 Thermal creep23

Primary and secondary creep deformations have been reported in the literature for CVD SiC (high-purity and polycrystalline p-SiC). Creep in SiC is highly depen­dent on the crystallographic orientation. The loading orientation of 45° from the CVD growth axis is the direction in which the most prominent creep strain is observed. A review of creep behaviors of stoichiometric CVD SiC has been provided by Davis and Carter.27

Primary creep of CVD SiC occurs immediately upon loading and tends to saturate with time. The primary creep strain generally obeys the following relationship:

ec = Ap (s/G)n(t/t)p [5]

where Ap, p, and t are creep parameters, and t is the time elapsed. n = 1.63, Ap = 29, p = 0.081, and t = 0.0095 s for the temperature of 1923 K. These para­meters are for the loading orientation of 45° from the CVD growth axis. In severe conditions, primary creep strain in the CVD SiC can reach as high as 1%.

Steady-state creep rates for polycrystalline mate­rials have been measured only above 1673 K, when the stress axis is 45° inclined from the deposition direction; temperatures as high as 2023 K are required when the stress axis is parallel to the depo­sition direction. The strain rate is given by a power — law creep equation:

de/dt = As(s/G)nexp(—Q / k^T) [6]

where As = 2.0 x 103, n = 2.3, Q= 174kJmol—1 (acti­vation energy), s is the applied stress, G is the shear modulus, and kb is the Boltzmann constant.