Mechanical Properties of Ceramics and Composites

VII. SUMMARY

There are reasonable models (including bounding approaches) for predicting elastic properties of many composites, especially those with equiaxed particulate fillers. Uncertainties exist primarily as the particle morphology and contiguity change, and increased preferred orientation occurs, especially with single crystal particles, platelets, and whiskers, that may require attention to their crystal orientation in the body texture and their crystalline anisotropy. These all pose uncertainties for modeling as well as for (generally inadequate) characterization. However, specialized models for the elastic properties of fiber composites may indicate a modeling approach for some composites with substantial orientation of dispersed particles, platelets, and whiskers.

While some intergranular fracture of matrices and dispersed particles, platelets, and whiskers occurs, greater transgranular fracture of one and commonly both phases occurs in most ceramic composites. Though this may reflect better bonding of boundaries due to common higher processing temperatures for composites than corresponding monolithic ceramics, no detailed information is available on possible mechanisms, and better quantification of results is needed. Slow crack growth can clearly occur in composites, especially those with matrices, e.g. silicate glasses, susceptible to SCG, but dispersed particles, especially with suitable bonding to the matrix, can reduce SCG by both increasing the overall stress intensity needed for SCG as well as actually pinning the crack, e.g. as shown in some glass–metal particle composites (Sec. IV).

Toughness almost invariably increases with increasing volume fraction of dispersed phase in composites with any degree of investigation, regardless of physical morphology of the dispersed phase. Some studies have been conducted to high enough dispersed phase contents to show that toughening effects typically reach maxima at φ = 0.3–0.5. Less investigated and thus more uncertain are the effects of dispersed phase dimensions on the extent of toughening, but several studies show that increased dimensions increase toughness. In such cases, a maximum toughness as a function of size is again generally expected and indicated in a few more detailed studies. Such data is particularly important not only for composite design and processing but also for understanding mechanisms, since such dependence on the dimensions of the dispersed phase typically results in opposite trends of toughness and tensile strength, as did grain size dependence in monolithic ceramics. Composite toughness often also increases with matrix grain size, paralleling such effects in monolithic ceramics, again indicating substantial toughness–strength discrepancies in composites. These discrepancies are discussed extensively in Chap. 9 for composites and in Chap. 3 for grain size effects in monolithic ceramics. An important factor in many composites and a critical one in some is orientation of the dispersed phase, and to a lesser extent that of the matrix. Again limited experiments indicating important changes in crack

propagation as a function of crack velocity and history indicate that important effects can occur in composites in this widely neglected area, as shown by tests of Jessen and Lewis [81–84] on glass–metal particle composites (Sec. V.F).

Turning to mechanisms, much remains to be established, with it being probable that most composites have more than one active mechanism involved, with their relative importance shifting as the material or microstructural parameters and the mechanical property of interest change. Thus, as noted earlier, mechanisms controlling large scale crack effects may be different from those controlling normal strength. However, while quantitative models are generally only guidelines, some mechanisms appear to be established or probable for certain composites, with fiber pullout being a dominant factor in continuous fiber composites. Similarly, bridging in the crack wake region by ductile metal particles, fibers, or filaments seems established as a major factor in toughness of many ceramic metal composites. Wake bridging appears to be a common factor in increased large crack toughness in many ceramic composites, and whisker pullout may be a factor in at least some whisker composites, e.g. those with Al2O3 matrices. However, three issues question such wake toughness effects as the general mechanisms controlling strengths of composites, the first being the frequent basic toughness–strength discrepancies. Second is the increasing number of questions of the accuracy and validity of such large crack effects for strength, including the generally neglected issue of how such phenomena change as a function of extent and velocity of crack propagation. The third is that, as is shown in Chap. 9, much composite strength dependence appears to arise due to microstructural effects on machining flaw character and size, as was shown for monolithic ceramics (e.g. Fig. 3.1).

NOTE

Since completion of this chapter work of Swearengen et al. [216] on composites of up to 40 v/o of spherical polycrystalline Al2O3 particles ( 25 m dia.) in various borosilicate glasses with thermal expansions ranging from 4 ppm/°C lower to 4 ppm/°C greater than that of the alumina was found. They showed E increasing linearly as φ increased, for a total increase of 100%. Using a vibrational technique to mark developing fracture surfaces they showed that cracks interacted with particles as outlined in Fig. 8.9, the marking technique showing significant crack velocity changes as cracks approached particles. They found similar increases in toughness whether the glass had higher or lower expansion than the alumina particles. (The contrast to results of Davidge and Green [97] where bodies with ThO2 particles in a glass matrix with 2 ppm/°C expansion > that of ThO2 were fractured may reflect effects of the much larger ThO2 particles relative to the alumina particles here.) The toughness increases were ap-

proximately linear with λ-1, but this is also consistent with toughness increasing with E as φ increases, though greater increases in K versus E by 50% may reflect some additional mechanism over that due simply to increased E.

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