Mechanical Properties of Ceramics and Composites

(as in directionally solidified eutectics) matrices that have no equivalence of grain structure themselves, λ defines the same basic physical dimension as G does in a dense monolithic ceramic, i.e. G defines the average separation of nearest neighbor nonadajecent grains, just as λ defines the average separation of particles or eutectic lamellae or rods. Thus the correlation of strength with λ in the latter cases is analogous to the correlation of strength with G in the large G region for monolithic ceramics, which lends support to its use in such cases. However, broader use in more complex structures such as those in crystallized glasses is uncertain, as is similar use of D, since both are related to each other, and both in turn to φ, with varying correlations with basic physical properties such as E, as was discussed earlier for results of Hing and McMillan [16].

Turning now to evaluation, the overall need is for broader consideration of possible contributing mechanisms, e.g. as was addressed above, and broader microstructural and especially property and behavior measurement, with much more attention to self-consistency. Thus, at the minimum, besides flexure strength and toughness measurements, Young’s modulus should be determined as a function of composite parameters, with preferably both the initial modulus being measured, e.g. by flexural resonance or ultrasonic techniques, as well as of possible changes in E as a function of loading to mechanical failure to detect stress-induced microcracking. Acoustic emission, internal friction, and damping can also be useful in this regard. Different toughness tests should be compared, especially on the basis of crack sizes to critical microstructural scales, e.g. grain and particle sizes as well as particle separations. Hardness testing, especially over a range of loads and with careful examination of indent cracking and compressive strength testing, again possibly accompanied by acoustic emission, can both indicate microcracking or debonding of second phase particles. Flexural strength testing for stressing parallel and then perpendicular to the machining direction can be a valuable tool for probing whether dispersed particles are constraining flaw sizes in the spacing between particles, since in such cases the anisotropy of strength with machining direction should disappear, e.g. as shown as a function of G in monolithic ceramics (Fig. 3.33). Testing with different strain rates, with biaxial loading, and with artificially introduced flaws can also be of use. Often of greater benefit is testing at higher and lower temperatures, for E as well as toughness and strength. Testing at lower temperatures reduces possible environmental effects on microcracking and crack growth and increases expansion mismatch stresses, while testing at higher temperatures reduces the latter stress and environmental effects. Other tests may also be valuable, e.g. electrical and thermal conductivities of the bulk or especially of the tensile surface may be good indicators of microcracking or debonding.

Testing of a broader range of composite microstructures and better characterization of them is also a critical need. Thus tests of a sufficient range of D values to determine the impact of this on strength and related properties is

important. Further, since there can be various correlations between microstructural parameters and properties without clear proof of causation, two steps are important. The first is evaluating properties against different microstructural parameters, i.e. G, D, λ, and φ to see if there are any unique correlations, and checking all correlations for self-consistency among various properties and possible mechanisms. The second, and especially important, but often neglected step is substantial fracture surface examination, especially, but not exclusively, seeking fracture origins. General examination typically gives a much more accurate picture of the microstructural extremes that may play a role in, or dominate, the composite behavior, and may indicate the relevance of some mechanisms, e.g. bridging is favored more by interrather than transgranular fracture. Crack interactions with dispersed particles may also be shown on fracture surfaces (especially but not exclusively) with glass or single crystal matrices (e.g. the latter in eutectic composites). However, identification of fracture origins, and especially quantitative analysis of them, e.g. of flaw sizes and implied fracture toughnesses, can be immensely valuable.

VI. SUMMARY AND CONCLUSIONS

While strengths often increase with the volume fraction second phase, in which case they typically go through a maximum, as toughness generally does, there are a number of cases where strength decreases, i.e. shows opposite dependence from typical toughness tests. Further, many composites show opposite strength dependence on key microstructural parameters such as particle size from that of toughness, which reinforce and more clearly show basic toughness and strength differences attributed to differences in crack size effects.

This review of composite strength behavior reinforces previous evidence for composite composition effects on machining flaw sizes. It was further shown that composites often exhibit the same two-branch strength–D-1/2 dependence as the strength–G-1/2 dependence for monolithic ceramics as a result of the same underlying impact of composition and microstructure on machining flaw sizes controlling strength. Thus there is a finer particle branch where strengths show variable but limited decreases in strength with increasing particle size due to the flaw sizes being > D, but being affected some by the impact on the particles on the local Young’s modulus, and especially hardness and toughness controlling flaw formation. On the other hand, there is a larger particle branch where the sizes of the particles (or clusters of them) are about that of the flaw size or greater so that the particle size becomes the flaw size, e.g. due to associated machining flaws, with resultant greater strength dependence on D.

The above flaw mechanisms, given attention because of their lack of recognition, are however only part of the picture, since there are commonly other mechanisms that are also operative to varying degrees, but which vary for

different types of composites. Thus transforming toughening clearly commonly, but not necessarily always, plays an important role in zirconia toughened composites, as does ductile elongation of metal particles in ceramic–metal composites, while pullout is often indicated in whisker composites but is more uncertain. In particulate composites, crack deflection, bridging, and branching may play some role, but effects of stress-induced or spontaneous microcracking and effects on flaw sizes are important, often probably more so in such composites, and can also be factors in the above composites. Effects on flaw sizes may occur via dispersed particles constraining flaw sizes (mainly when there are limited particle–matrix mismatch stresses), or much more generally via effects on machining flaws from E, H, and K, or via residual surface compressive stresses. Another important factor is matrix grain size, which is found to be much more pervasive than originally generally thought and can impact all types of composites with polycrystalline matrices and is probably important in nanocomposites.

Finally, a number of recommendations have been made to improve documentation and understanding of the mechanical behavior of ceramic composites. These include a broader range of the type of tests, e.g. more strength, toughness, and Young’s modulus testing, complemented by other evaluations, e.g. for acoustic emission and internal friction or damping, as well as over a broader range of temperatures. Broader evaluation of microstructural parameters and evaluation of the consistency of various correlations is also recommended, since correlation does not necessarily mean causation, and microstructural parameters are interrelated. A key factor that needs much more use and correlation with both microstructural aspects of failure and evaluation of mechanisms is fractography, both of the general fracture surfaces and for fracture origin determination and evaluation.

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