The secrets of Zirconia ceramic bearings, toughness

10/02/2021 Posted in ceramic bearings, Industry from Francesco Madaro: R&D Manager, Ingegneria dei Materiali


In this article we will talk about zirconia ceramic bearings, especially toughening.

To understand the potential of ceramic bearings, we will start from the material, zirconium oxide up to the toughening process.

Zirconium oxide

Zirconium (Zr) is a metal with atomic number 40. It was first discovered in 1789. The material has a density of 6.49 g / cm³, a melting point of 1852 ° C and a boiling point of 3580 ° C. It has a hexagonal crystal structure and is greyish in color. Zr is not found in nature in its pure state. It can be found in combination with silicate oxide under the mineral name Zircon (ZrO2 x SiO2) or as a free oxide (Zirconia, ZrO2) under the mineral name Baddeleyite.

 One of its first applications was in the dental and biomedical fields, now they are also widely used in industrial applications. These minerals cannot be used as primary materials in dentistry due to the impurities of various metallic elements that affect the color and due to natural radionuclides such as urania and thoria, which make them radioactive. Complex and time-consuming processes that result in effective separation of these elements are required to produce pure zirconia powders. After purification, the material produced can be used as a ceramic biomaterial.

Zirconium oxide or Zirconia, ZrO2 is a polymorphic material and comes in three forms: monoclinic, tetragonal and cubic. The monoclinic phase is stable at ambient temperatures up to 1150 ° C, the tetragonal phase at temperatures of 1150-2200 ° C and the cubic phase at temperatures above 2200 ° C, while the liquid phase is formed above 2680 ° C. The transition from one phase to another is associated with significant volume variations: for example, by heating the zirconia above 1150 ° C the transformation of the structure from monoclinic to tetragonal is obtained with a volume reduction of 5%. Conversely, a volume increase of 3% - 4% is observed during the cooling process (Figure 1).

Possible crystalline structures of Zirconia at different temperatures

Figure 1. Possible crystalline structures of Zirconia at different temperatures.

Stabilization of Zirconia

During a cooling process, zirconium oxide undergoes a phase transformation process as described in Figure 1, from cubic to tetragonal to monoclinic. The volume change associated with this transformation would lead to a breakage of the Zirconia, or at least to its excessive friability, which makes it impossible to use pure zirconium oxide in many applications, especially in bearings. To overcome this type of problem, some materials are added to stabilize the cubic phase of the zirconia at room temperature. These materials are called stabilizers and the related product called Stabilized Zirconia. If enough stabilizer is added, the cubic phase can be stabilized completely (fully stabilized zirconia). Otherwise, if smaller relative quantities of stabilizer are used, a partially stabilized zirconia can be obtained, in which there is a certain controlled percentage of tetragonal phase. In other words, partially stabilized zirconia is a fine dispersion of metastable tetragonal zirconia particles (capable of converting into monoclinic in the presence of perturbations), in a cubic matrix.

For example, if you want to stabilize the cubic and tetragonal phase with Yttria, relative percentages greater than 7 mol% lead to complete stabilization of cubic zirconia, while relative percentages between 2 and 6 mol% of Yttria give a partially stabilized zirconia, in which 5-10% of finely dispersed tetragonal zirconia persists in the cubic matrix.

The other commonly used zirconia stabilizers include ceria CeO2, calcium CaO and magnesia MgO etc. The most common and effective stabilizer is Y2O3 yttria. The stabilized zirconia is consequently referred to as yttria stabilized zirconia, ceria stabilized zirconia, calcium stabilized zirconia, magnesia stabilized zirconia.

Toughening of Zirconia

Stabilization mechanism and effects on mechanical properties: in a structural application, when a micro-crack encounters a tetragonal particle, the concentration of mechanical stress on the apex of the crack triggers the tetragonal à monoclinic transformation with consequent increase in volume, putting the same area affected by the transformation, slowing down or blocking the propagation of the crack, as described in Figure 2.

This process is known as the toughening mechanism, being toughness the ability of a material to absorb mechanical energy before breaking.

Crystalline transformation that introduces pressure on the crack, stopping its advance.

Figure 2. Crystalline transformation that introduces pressure on the crack, stopping its advance.

The toughening mechanisms:

Although the toughening phenomenon has been supported by experimental tests and it is the reason why stabilized zirconia is one of the most reliable structural ceramic materials even in the world of bearings, the mechanisms that lead to this toughening are not entirely clear. Specifically, there are at least two theories that explain the increase in toughness:

- The martensitic transformation of the tetragonal à monoclinic metastable phase is responsible for the absorption of elastic energy involved in the path of the fracture.

- Nucleation and growth of micro-cracks in the ceramic matrix, which cause an increase in energy necessary for the fracture to propagate. These micro-cracks are likely to arise before the component is stressed.

- The scientific community is oriented towards thinking that both contributions occur simultaneously.

Another very interesting aspect of toughening by transformation is linked to the generation of surface compression stresses during cooling, which give greater mechanical resistance to the ceramic component, just as occurs in tempered glass.

The surface layer can be further stressed by compression through finishing operations (grinding, sandblasting), in which any surface defects are rendered inactive, and in addition the area immediately affected by abrasion is in compression and therefore resistant to the propagation of any fractures.

As shown in Figure 3a, the finely dispersed tetragonal phase at the edges of the cubic phase transforms into a monoclinic phase by cooling (small blackened grains). If we proceed with targeted machining, we can increase the toughened area because we promote the tetragonal  monoclinic transformation more in depth.

Surface toughening

Tetragonal phase (small grains) at the edges of the cubic phase (larger grains).

Tetragonal grains transformed into monoclines (blackened grains) under cooling, with surface toughening.

Deeper toughened area thanks to mechanical processing.

 3. Surface toughening (a) high temperature free surface, (b) surface affected by tetragonamonoclinic transformation due to cooling (c) surface with greater toughened area due to mechanical machining.

Figure 4 shows the Zirconia powder with the naked eye (a) and the same powder observed under a scanning electron microscope (SEM), highlighting the sub-micrometric dimensions of the particles that will then form the agglomerates, and then the grains in the phase of sintering (firing).

Zirconia powderZirconia powder under microscope






Figure 4. Zirconia powder (a), and zoom under the SEM microscope (b).

As you can see, the choice of the best ceramic bearing in terms of mechanical resistance is also influenced by the selection of the ceramic component. Enter the world of bearings with our team to find the best solution for your applications.

New call-to-action

Francesco Madaro: R&D Manager, Ingegneria dei Materiali

“The study and development of materials for structural, electronic, energy and biomedical applications offer me every day the privilege of discovering little secrets of their behavior and adapting them to the applications in which they perform best. Ceramic materials are certainly my specialty, and ceramic bearings promise the most interesting performances in many applications ".

New call-to-action



17/09/2021 Posted in ceramic bearings, Industry

Ceramic bearings used in Industrial Ovens: the Tabo case

In this article we will talk about the use of ceramic bearings in industrial furnaces, the Tabo case.

15/09/2021 Posted in ceramic bearings, Industry

Ceramic Bearing For High-Performance Bicycles

In this article we will talk about ceramic bearing applications for high-performance bicycles

01/09/2021 Posted in ceramic bearings, Industry

Ceramic Bearings vs. Steel Bearings: Costs And Longevity

For several years, industrial electric motors have been typically made to run on steel ball bearings.

contact us