Creep

Using the strain rate data shown in Fig. 12, the activation energy for creep in mono­clinic zirconia has been found to be Qc ~ 330-360 kJ mol-1 [48, 49]. The measured stress exponent, n, from equation:

Creep

Creep

(3)

was found to be 1.7 by Roddy et al. [48] and 2.3-2.5 by Yoshida et al. [49]. In this equa­tion, A is a constant, G is the shear modulus, b is the Burger’s vector, d is the grain size,

Creep

Fig. 12 Stress dependence of monoclinic zirconia during creep deformation (adapted from Roddy et al. [48] and Yoshida et al. [49])

s is the applied stress, p is the grain size exponent, n is the stress exponent, and D is the diffusion coefficient. The value by Roddy et al. is intermediate between diffusional creep (n = 1) and superplastic deformation (n = 2), but closer to superplastic deformation, whereas the values by Yoshida et al. are higher than both, but close to the stress exponent for superplastic deformation. The grain size exponent, p, was found to be 2.8 by Roddy et al., which is closer to p = 3 for Coble creep (lattice diffusion) than p = 2 for superplastic or Nabarro-Herring creep (grain boundary diffusion). However, Yoshida et al. found values between 2.4 and 2.5. From the exponents found in both studies, it is likely that creep deformation in monoclinic zirconia is due to superplastic deformation.