Toughness is a very important property in metals, but there exists some controversy over whether toughness measurements on ceramic samples actually tell much about the material. Toughness data for ceramics is often annoyingly inconsistent, and strength and toughness do not always respond in the same manner to changes in microstructure or interfacial properties.
An illustration of this lack of a relationship between the properties is shown by a recent study of reinforced fused silica that found no increase in toughness (measured by single edge. notch beam tests) as a result of fiber addition to silica despite the fact that work-of-fracture (as determined by triangular notched beam tests) showed a dramatic increase.
That having been said, there are many materials scientists working hard at improving this elusive property in ceramics. One material to particularly benefit from this work is zirconia. In its partially stabilized, transformation toughened form, zirconia gains a 3-fold strength and toughness advantage over unmodified ZrO, and it is now a leading high-performance structural material.
In transformation toughened materials (ZrO, Alo, and SiN matrices, all with a dispersed ZrO tetragonal phase) a phase transformation actually occurs in the region of applied stress within the material that absorbs energy at the tip of the advancing crack arresting its propagation and significantly boosting both strength and toughness.
In the engineering of ceramic composites for toughness, a number of other strategies can be pursued, among them matrix microcracking, fiber debonding or crack deflection, and fiber pullout. If matrix microcracking can be limited to the stress field of a primary crack, energy will be dissipated without promoting further crack growth. The ideal materials for this approach are those with strong fibers securely bonded in a relatively weak matrix. In these materials the microcracks will be arrested by the reinforcing fibers, preventing further growth. Cracks are deflected in strong-matrix composites containing weakly bonded but strong fibers by the mechanism of fiber debonding.
Interfacial tension caused by differences in the thermal expansion between matrix and fibers in a composite can also provide a toughening effect. If the matrix is placed in tension as the composite is cooled from fabrication temperature (expansion coefficient of the matrix is greater than that df the fibers), matrix microcracking and the attendant primary
crack attenuation will be promoted. If the difference in thermal expansion favors the fibers, fiber debonding in the stress field of a crack will be the toughening mechanism.
A “hybrid composite” approach using both large and small diameter fibers has been studied as a way to maximize both strength and toughness. A low concentration of large diameter fibers enhances the strength of such a material, while the small fibers provide toughness. It has been found that the two types of fibers function somewhat cooperatively to both suppress and stabilize matrix cracks.
Toughness values for ceramics*
| Material | Comments | Toughness Kc (MPa m1/2) |
|---|---|---|
| Fe | Medium-strength steel | 50.0 |
| NaCI | Monocrystal | 0.4 |
| Soda-lime glass + + | Amorphous | 0.74 DCB |
| Aluminosilicate glass | Amorphous | 0.91 DCB |
| ZnSe | Vapor-deposited | 0.9 |
| WC | Co-bonded | 13.0 |
| ZnS | Vapor-deposited | 1.0 |
Si N | Hot-pressed | 5.0 |
Al o | MgO-doped | 4.0 |
| Alo (sapphire) | Monocrystal | 2.1 |
| SiC | Hot-pressed | 4.0 |
| SiC-ZrO | Hot-pressed + | 5.0 |
| MgF2 | Hot-pressed | 0.9 |
| MgO | Hot-pressed | 1.2 |
| ByC | Hot-pressed | 6.0 |
| Si | Monocrystal | 0.6 |
| ZrO | Ca-stabilized | 7.6 DCB |
20% ZrO, 14% mullite by weight, ZrO present in monoclinic form: no transformation toughening.
+ + Commercial sheet glass.
*Double torsion measurement technique, except where double cantilever beam test (DCB) indicated.
Ceramic matrix composite toughening concepts
| Concept | Basic requirements | Status of verification and modeling |
|---|---|---|
| 1. Modulus transfer of load from matrix  to fibers | Ef > E , preferably by a factor of > 2. | Verified, reasonable modeling. |
| 2. Pretesting of fibers and matrix |  , so axial tensile stresses in fibers < their fracture stress to give reasonable compressive axial stress in matrix. | Not verified. Basic modeling not expected to be difficult. |
| 3. Crack impeding second phases | Fracture toughness of fibers (or particles > local matrix so crack is either arrested or bow out, i.e., gives line tension effects between fibers or particles. | Arrest impractical. Line tension modeling, but uncertain verification. |
| 4. Fiber pull-out | Fiber (or elongated particles) have high enough transerse fracture toughness os failure occurs along fiber-matrix interface | Limited verification and modeling. |
| 5. Crack deflection or multiplication | Sufficiently weak fiber (or particle) matrix interfaces, or appropriate mismatch of properties, and particles (fibers) and use of appropriate particles (fiber sizes). | Limited verification, no modeling Some verification, possible modeling developing. |
| 6. Phase transformation toughening | Second phase paricles (fibers) increase one or more dimensions by shear or volume expansion so  V >O. | Verified with ZrO particles, modeling developing. |