The Forms and Phases of Zirconia Engineering Ceramics that Lead to High Strength and Toughness

Engineering ceramics such as zirconia (ZrO₂), alumina (Al₂O₃), silicon nitride (Si₃N₄), silicon carbide (SiC) etc. have been used more frequently in engineering applications as engineers and designers appreciate their superior properties compared to more traditional materials. However, the properties of these materials are different to those of materials like tools steels and must be understood before they can be used to replace their metallic counterparts.

Engineering ceramics such zirconia typically exhibit improved wear and corrosion resistance and superior high temperature stability when compared to most metals and have been found in many instances to bring performance and life expectancy improvements. With increasing awareness of its beneficial properties it is being used more often in both new equipment as well as being retrofitted to older equipment.

In the following sections, we provide information to give you a better understanding of zirconia’s structure and how this relates to its excellent properties.

The Crystalline Forms of Zirconia

Pure zirconia can exist in three crystallographic forms, cubic, tetragonal and monoclinic. All of these phases are variants on the cubic fluorite structure. The range of stability for each of these phases is shown below in figure 1.

Figure 1. Ranges of Stability for the Crystallographic Phases of Zirconia

The Tetragonal to Monoclinic Transformation of Pure Zirconia

The tetragonal to monoclinic transformation is of great technological significance, due to the martensitic nature of the reaction and the accompanying 3-5% volume expansion. The martensitic transition is significant as it is similar to the hardening mechanism in steels and it’s ability to harness the volume expansion of the structure that leads to the interesting properties of high strength and toughness displayed by many zirconia engineering ceramics.

The large volume and shape deformations, which occur through the martensitic transformation, set up large strains in the structure. These strains cannot be relieved by diffusion, instead they are accommodated by elastic or plastic deformation of the surrounding matrix.

Martensitic reactions are usually athermal; that is, they occur only when the temperature is changing. The behavior can be explained as follows: the increase in strain energy occurring on transformation opposes further transformation and consequently, the chemical driving force for the reaction has to be increased for the reaction to proceed. Generally this is accomplished by undercooling below the temperature defining the start of the martensitic reaction, commonly referred to as the Ms Temperature.

Stablizing High Temperature Phases

The addition of oxides soluble in zirconia (CaO, MgO, Y 2O 3) lowers the tetragonal to monoclinic (t-m) and cubic to tetragonal (c-t) transformation temperatures. These additions are therefore said to stabilize the high temperature phases. The amount of alloying oxide required to produce stabilization is determined from the relevant phase diagram. The phase diagram for the zirconia-yttria system is given in figure 2. below. Any composition which is fired in the cubic phase field and retains a wholly cubic structure on cooling is stated to be “fully stabilized”.

Retention of the tetragonal phase at room temperature is also feasible, providing the t-m transformation is inhibited. This can be achieved by a combination of fine powders, matrix constraint and stabilizing additions.

Figure 2. Phase Diagram for the Zirconia Yttria System.

How Does the Martensite Transformation Start in Yttria -Tetragonal Zirconia Polycrystals

Although it is agreed that the martensitic transformation in zirconia is nucleation controlled, different models for the nucleation controlled martensitic reaction have been developed, based on classical and non-classical processes.

In Y-TZP's nucleation has been observed to occur at grain corners and has been shown to be easier in faceted intergranular grains compared to spherical intragranular grains of equivalent dimensions.

Such behavior supports the theory of a “stress assisted” transformation mechanism. In simple terms the stress that is seen within the material being subject to a propagating crack is often sufficient to initiate the martensitic transformation, the subsequent volume expansion is in effect a “crack stopping” force and leads to the high values of strength and toughness recorded for these materials.

Zirconia “Alloy” Systems

The characteristic features of the three basic Transformation Toughened Zirconia (TTZ) microstructures are summarised in figure 3.

Figure 3. Transformation Toughened Zirconia Schematic Microstructures (the size of the tetragonal particle is denoted by dimension d).

Type I Microstructure

This structure is typical of a conventional partially stabilised zirconia (PSZ), produced by additions of 8-10 mol% MgO (Mg-PSZ). After sintering in the cubic phase field at temperatures between 1650-1850 oC, and followed by controlled cooling, the microstructure is one of large cubic grains (50-100 m), within which are dispersed coherent tetragonal precipitates. To optimize their transformability under stress the precipitates are then coarsened, by ageing at a temperature between 1100 and 1450°C.

Type II Microstructure

Characteristic of Tetragonal Zirconia Polycrystals (TZP), this fine grained, predominantly tetragonal microstructure is produced by additions of 2-4 mol% Y₂O₃ (Y-TZP) or 9-14 mol% CeO₂ (Ce-TZP). To enhance densification at the relatively low temperatures necessary to inhibit grain growth, impurities such as alumina and silica are tolerated in the form of an amorphous grain boundary film.

Type III Microstructure

The use of dispersed partially stabilized zirconia particles in a non zirconia matrix leads to the microstructure shown. Particles are incorporated either intergranularly (type IIIa) or intragranularly (type IIIb) by a variety of processing methods. Several parent matrices have been used (Alumina, Mullite, Silicon Nitride) in combination with varying amounts of zirconia. The volume fraction, distribution and size of the particles have to be carefully controlled in order to ensure optimum mechanical properties.

Zirconia Engineering Ceramics

The use of these phase transformations and microstructures has lead over the last 30 years to the development of a range of Zirconia Engineering Ceramics with properties that are of great interest to the engineer who wishes to reduce wear and corrosion in industrial processes, but who also wishes to use an Engineering Ceramic with high strength and toughness.

Insaco

If you are considering an engineering application that you feel may be suited to zirconia then Insaco with 65 years of experience is second to none.

They have many decades of experience on machining and polishing zirconia materials for a wide range of applications and have been involved in many of the applications above when they were in their infancy.

Presently they have an 85,000 ft² facility with more than 300 machine tools capable of grinding and polishing glass ceramics and other ceramic materials to useful geometries and tolerances.

Reference

• "Fabrication and Wear of Yttria Tetragonal Polycrystals", Ian Birkby, University of Leeds Ph.D. Thesis, 1994.

This information has been sourced, reviewed and adapted from materials provided by INSACO Inc.

For more information on this source, please visit INSACO Inc.