Transformation toughening

In the field of mechanical properties, strength and toughness are related as follows. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness, however, indicates how much energy a material can absorb before mechanical failure. Fracture toughness (denoted K_Ic ) is a property which describes the ability of a material with inherent microstructural flaws to resist fracture via crack growth and propagation. Methods have been devised to modify the yield strength, ductility, and fracture toughness of both crystalline and amorphous materials.

For example, the high strength of industrial steel results from the incorporation of carbon atoms into the iron lattice. Interstitial carbon atoms interfere with dislocation glide by preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of carbon anywhere from 0.2 - 2.1 % alters mechanical properties such as the hardness, ductility, and tensile strength of the resulting steel. Thus, steel with increased carbon content can be made harder and Stronger than Iron, but is also less ductile. The superior mechanical properties of brass, a binary alloy of copper and zinc, compared to its constituent elements is a result of the same type of "solution strengthening". Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strength.

Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present. This is one of the most important properties of any brittle material for virtually all design applications. If a material has a large value of fracture toughness it will probably undergo ductile fracture. Brittle fracture is very characteristic of most ceramic and glass-ceramic materials which typically exhibit low (and inconsistent) values of K_Ic.

Transformation toughening is one way of drastically improving the fracture toughness of an inherently brittle material. This is accomplished by introducing a mechanism (structural phase transformation) for triggering a uniform distribution of low level internal strain within the bulk of the material. This strain acts locally to oppose the stress field associated with crack propagation, thus annihilating it at the approaching crack tip front.

Introduction

Transformation-toughened zirconium oxide (TTZ) is an important high-strength, high-toughness ceramic that has been developed during the past 25 years. The development of transformation-toughened materials requires both control of chemical composition and manipulation of the material's microstructure at the submicron level.

Transformation toughening has emerged on the assumption that crystalline zirconia (ZrO₂) undergoes several structural phase transformations between room temperature and practical sintering (or firing) temperatures (1000-1500 °C). Thus, due to the volume restrictions induced by the glassy/ceramic matrix, metastable crystalline structures can become frozen in which impart an internal strain field surrounding each submicron zirconia inclusion upon cooling below the equilibrium transformation range. This enables a zirconia particle (or inclusion) to absorb the energy (or stress intensity factor) of an approaching crack tip front in its nearby vicinity, drastically increasing the fracture toughness.

Composites exhibiting the highest level of fracture toughness are typically made of a pure alumina or some silica-alumina (SiO₂ /Al₂O₃) matrix with tiny inclusions of zirconia (ZrO₂) dispersed as uniformly as possible within the solid matrix. (*Note: a wet chemical or colloidal approach is typically necessary in order to obtain a complete dispersion of the zirconia particles within the matrix, and thus establish compositional uniformity of the ceramic body before firing).

Structural transformation

The polymorphic (or structural) phase transformations (cubic → tetragonal → monoclinic) which occur in crystalline zirconium dioxide (or zirconia) can be described as follows. ZrO₂ has a monoclinic crystal system (see above) between room temperature and ~ 950°C. Above this critical temperature, zirconia converts to the tetragonal crystal system (see below). This transformation is accompanied by ~ 1% volume reduction (shrinkage) during heating and an equivalent volume increase (expansion) during cooling.

At a much higher temperature, the zirconia changes from tetragonal to a cubic crystal system (see below). With proper chemical additions and heat treatments, a microstructure can be achieved during cooling that consists of lens-shaped "precipitates" of tetragonal zirconia in cubic grains of zirconia. Normally, the tetragonal material would transform to the monoclinic form during cooling—but it must expand to do so. However, the high strength of the surrounding cubic zirconia prevents this expansion. So the tetragonal form is retained in a metastable state all the way down to room temperature. As a result, each tetragonal zirconia precipitate is under stress and full of strain energy that wants to be released—sort of like a balloon that has been stuffed into a box that is too small. As soon as the box is opened, the balloon is allowed to expand to its equilibrium condition and protrude from the box.

Toughening mechanism

The same thing happens for each tetragonal precipitate, either if a crack tries to form or if someone tries to break the ceramic by applying a load. The crack is analogous to opening the box. Tetragonal precipitates near the cracktip front are able to expand and transform back to their thermodynamically stable monoclinic form. This expansion presses against the crack tip front and prevents it from propagating. This is the physical mechanism which is responsible for the mechanical phenomenon of transformation toughening. Since it is similar in some ways to the toughening mechanism in certain forms of steel, TTZ was dubbed by its first discoverers as "ceramic steel".

Thus, pure ZrO₂ has a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at increasing temperatures. The volume expansion associated with each transformation induces very large stresses, and will cause pure ZrO₂ to crack upon cooling from high temperatures. Thus, for mechanically reliable products composed of pure ziconia, different oxides may added in order to stabilize the tetragonal and/or cubic phases upon cooling. However, if sufficient quantities of the metastable phase is present, then an applied stress in combination with the stress concentration at a crack tip can cause the metastable (e.g. tetragonal) phase to convert to a thermodynamically stable (e.g. monoclinic) phase, with the associated volume expansion. This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness of the material.

TTZ has been developed in a couple of different forms. The one described above is typically called partially stabilized zirconia (PSZ).The second form consists of nearly every crystallite or grain in the material being retained in the tetragonal form to room temperature so that each grain can transform instead of only the precipitates. This material is referred to as tetragonal zirconia polycrystal (TZP). Both types are mentioned because they have different properties, and one may be preferable for a specific application.

Transformation toughening was a breakthrough in achieving high-strength ceramic materials with a high value for the fracture toughness. For the first time, a ceramic material was available with an internal mechanism for actually inhibiting crack propagation. A crack in a normal ceramic travels all the way through the ceramic with little inhibition, resulting in immediate and brittle fracture and catastrophic failure. TTZ exhibits a fracture toughness (or resistance to crack propagation) which is 3-6 times higher than normal zirconia and most other ceramics. TIZ is so tough that it can be struck with a hammer or even fabricated into a hammer for driving nails. The stress intensity factor values for window glass (silica), transformation toughened alumina, and a typical iron/carbon steel range from 1 to 20 to 50 respectively.

Thus, the application of large shear stresses during fracture nucleates the transformation of a zirconia inclusion from the metastable phase. The subsequent volume expansion from the inclusion (via an increase in the height of the unit cell) introduces compressive stresses which therefore strengthen the matrix near the approaching crack tip front. Zirconia whiskers may be used expressly for this purpose.

Glass ceramics

The utilization of transformation toughening has generally been restricted to increasing the fracture resistance of polycrystalline ceramic materials. In fact, the use of transformation toughening has been accepted rapidly as a rational strategy for toughening all sorts of ceramics including oxides, nitrides, and carbides. Although a number of investigators have attempted to extend the concept to toughening glasses and glass-ceramics with tetragonal zirconia, few successful reports have been published. It has been argued that the approaches employed are inevitably limited primarily because they do not take into account the necessity of nucleating the transformation away from the crack tip itself. By concentrating on the nucleation event and using standard ceramic processing techniques, Clarke and Schwartz have demonstrated that transformation toughening can be used to increase the fracture toughness of a magnesium-aluminum-silicate (cordierite) glass ceramic.

Previous workers who have attempted to toughen glasses and glass ceramics by the use of zirconia have done so by precipitating the zirconia from the glass phase. There is considerable precedent for this approach, since this is the manner in which zirconia is known to be formed in its tetragonal form. These authors argue, however, that this approach suffers from a number of limitations if zirconia is incorporated for the purposes of transformation toughening of the materials. It is conjectured that perhaps the most important of the limitations is that the precipitated zirconia has a morphology ill suited for nucleation of the tetragonal-monoclinic transformation away from the crack tip itself. Nucleation is a necessary step in order to develop a zone of transformation having an appreciable extent around the crack.

The conventional way in which glasses incorporating zirconia have typically been prepared has been by dissolving the zirconia during the glass making step and then subsequently heat treating the glass to precipitate the zirconia as a crystalline phase. While this is a particularly convenient processing method, it suffers from four key limitations.

1) The limited solubility of zirconia in glasses restricts the volume fraction of transformable particles that can be attained in the material.

2) The chemical reaction with silica to form zircon in preference to zirconia limits the processing times and temperatures that can be employed in making the material.

3) There is little microstructural control over the distribution of the zirconia particles in a material.

4) The ability to nucleate the tetragonal-monoclinic phase transformation in the stress field of a propagating crack. The ease of nucleation is determined by the stress necessary to cause the transformation, and in turn determines the distance from the crack at which the transformation occurs. I.E. the easier the nucleation, the lower the stress and hence the larger the zone size.

To overcome some of these limitations, these authors developed an alternative approach that is suitable for making either a transformation-toughened glass or glass ceramic. The process is essentially a composite one in which zirconia powder and a powder having the desired glass-ceramic composition are mixed together and then densified, as in a standard ceramic process as distinct from a conventional glass-ceramic process. This approach allows for the use of a variable volume fraction of zirconia, one that is not dictated by the zirconia solubility in the glass but rather one governed by the extent of transformation toughening desired.

In addition, it enables a transformable tetragonal zirconia to be incorporated. It is presumed that this is due to the preservation of nucleating sites in the zirconia through the processing cycle. An additional advantage of the composite approach is that larger zirconia particles can more readily be mixed into the material than is possible by the precipitation method since, in the latter, the particle size is governed by diffusional growth and coarsening. This is also likely to be relevant to the ease of nucleation, since the probability of a particle containing a transformation nucleus is expected to scale with its size.

Finally, by altering the size and distribution of the zirconia particles in the mixture, a microstructure having spatially varying degrees of toughening can be fabricated. According to the temperature cycles employed, the samples could be processed to be either a glass containing zirconia particles or a polyphase ceramic with a dispersed zirconia phase. In the latter process the materials were given a conventional crystallization heat treatment for a prolonged period of time in order to complete crystallization of the glass phase to cordierite.

The fracture toughness data although indicate a clear dependence on the volume fraction of zirconia. Results indicate that this processing approach preserves the size of the zirconia particles close to the critical size required for crack-tip stress-induced transformation. Thus, the measured fracture toughness increases linearly with the volume fraction of zirconia, and using Raman Microprobe spectroscopy, it is shown that the transformation is associated with the formation and propagation of a crack.

See also

• Crystal structure
• Fracture mechanics
• Fracture toughness
• Ceramic engineering
• Diffusionless transformations
• Phase transformations in solids
• Plastic deformation in solids
• Polymorphism
• Zirconium dioxide

Further reading

• Onoda, G.Y., Jr. and Hench, L.L. Eds., Ceramic Processing Before Firing (Wiley & Sons, New York, 1979)
• Richerson, D., Modern Ceramic Engineering, (Marcel Dekker, New York, 1992)
• Porter, D.A. and Easterling, K.E., Phase Transformations in Metals and Alloys (Chapman & Hall, 1992)
• Khachaturyan, A.G., Theory of Structural Transformations in Solids (Dover Publications, 1983)