Alumina critical flaws

It is most unlikely that matrix grain refinement alone can explain the high fracture strengths that have been reported. Using the reported maximum fracture strengths (o = 1100 MPa)12 and the nominal fracture toughness of alumina (A"Ic = 3.5 MPa.m1/2), the critical flaw size, (c), can be estimated from... 

Thermal shock testing of an alumina/20 vol.% SiC whisker composite showed no decrease in flexural strength with temperature transients up to 900°C.33 Monolithic alumina, on the other hand, shows significant decreases in flexural strength with temperature changes of >400°C. The improvement is a result of interaction between the SiC whiskers and thermal-shock induced cracks in the matrix, which prevents coalescence of the cracks into critical flaws. 

The experimental observation that there exists a critical thickness above which cracking occurs cannot easily be explained. Brinker [1] discusses a theory which explains that very thin layers can bear much larger stresses because critical cracks carmot be formed unless a certain critical thickness is surpassed. This thickness is estimated to be equal to or less than 1 pm and Brinker comes to the conclusion that thicker films will always crack. This is certainly not the case for alumina, titania and zirconia films for which much larger (alumina) to larger (titania) thicknesses are observed. As shown in Table 8.2 critical thicknesses of a few pm in single-step dip-coated films occur and critical flaws are smaller than this thickness and so can be present. 

The speed of sound in an elastic medium is a function of elastic modulus and density as follows v = ElpY, where p is velocity, E is modulus, and p is density. Because ceramics are stiffer and less dense than metals, the speed of sound in ceramics is greater than in metals. For example, the speed of sound in iron is 5.13 mm/ ps, compared to 9.78 mm/ps in alumina. This means that, at the same frequency, the wavelength is greater in a ceramic and the resolution essentially halved. This difficulty and the fact that critical flaws in ceramics are much smaller than those in metals mean that frequencies between 10 and 100 MHz are required for ultrasonic evaluation of ceramics. 

The alumina scale shown in Figs. 7c and d failed certainly by the buckling mechanism. Within the spalled regions on the specimen surface, large interfacial voids were detected which represent interfacial flaws where the scale buckles during scale loading. A mechanical stability analysis yields the critical equi-biaxial stress, orh. when buckling occurs [7]... 

Shelty s Criteria. The critical crack size relative to the size of the stressconcentrating defect is the key parameter determining the effect of stress-state on flaw severity. The relative critical crack size is a parameter of the material. In commercial aluminas it is 2 to 3 times the pore size. In RBSN it is much smaller. (D.K. Shetty et al, in Fracture Mechanics of Ceramics 5, Bradt et al. Plenum 1983, p 531). See... 

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