A Comparison of Alumina and Zirconia Engineering Ceramics

Alumina and Zirconia are two of the most commonly applied oxide Engineering Ceramics.

Although they are often used in similar industrial applications they have unique properties that should be taken into account when looking to apply either material into an engineering application.

Key Property Differences

It’s beyond the scope of this article to delve into all the property differences between all the various grades of Alumina and Zirconia ceramics, so this review will focus on the primary engineering grades of Alumina (99%+) and Zirconia (Y-TZP and Mg-PSZ). However, the data tables set out below provide indicative mechanical and physical property data for the most common grades of Alumina and Zirconia.

Table 1. Typical properties for commercial grades of alumina.

Property

Alumina Grade

86%

94%

97.5%

99.5%

99.9%

99% recry.*

Saph**

Density (g.cm-3)

3.5

3.7

3.78

3.89

3.9

3.9

3.985

Dielectric Constant

8.5

9.2

9.5

9 – 10.1

9 – 10.1

7.5 – 10.5

Dielectric Strength
(kV.mm-1)

28

30 - 43

10 - 35

10 - 35

17

Volume Resistivity Ohm.cm

>1014

>1014

>1014

>1014

>1014

>1014

>1016

Thermal Conductivity
(W.m-1.K-1)

15

20

24

26

28-35

28-35

41.9

Thermal Expansion Coefficient
(20-1000°C x10-6K-1)

7

7.6

8.1

8.3

8

8

5.8

Specific Heat (J.K-1.kg-1)

920

900

850

753

Compressive Strength (MPa)

1800

2000

1750 - 2500

2200 - 2600

2200 - 2600

2100

Modulus of Rupture (MPa)

250

330

262

320 - 400

260

Hardness
(Vickers kg.f.mm-2)

1500 - 1600

1500 - 1650

1500 - 1650

2500 - 3000

* recrystallised ** Sapphire

Source: https://www.azom.com/article.aspx?ArticleID=52

A Data table for a range of zirconia grades is provided below:

Table 2. Typical properties for commercial grades of zirconia.

Property

Y-TZP

Ce-TZP

ZTA

Mg-PSZ

3Y20A

Density (g.cm-3)

6.05

6.15

4.15

5.75

5.51

Hardness (HV30)

1350

900

1600

1020

1470

Bend Str. (MPa)

1000

350

500

800

2400

Compressive Str. (MPa)

2000

-

-

2000

-

Young’s Modulus (GPa)

205

215

380

205

260

Poisson’s Ratio

0.3

-

-

0.23

-

Fracture Toughness (MPa.m-1/2)

9.5

15-20

4-5

8-15

6

Thermal Exp. Co-Eff (x10-6 °C-1)

10

8

8

10

9.4

Thermal Conductivity (W.m-1.K-1)

2

2

23

1.8

3

Source https://www.azom.com/article.aspx?ArticleID=940

To aid in the understanding of which material should be used and where, the following sections provide answers to the common alumina versus zirconia materials selection questions. Obviously, these are general in nature and are intended only as a guide. For detailed advice on your application. The sales engineers at Insaco will be happy to offer their experiences and opinions. E-mail [email protected] or call +1 215 536 3500.

Alumina is Harder than Zirconia So It Must be More Wear Resistant?

Although it’s commonly accepted that hardness equates with wear resistance, it’s not always the case. In sliding wear environments, hard counterfaces that do not interact with each other are an advantage. The engineering grades of Alumina are typically 25-50% harder than zirconia grade so in sliding wear environments or pure abrasive wear, where third body abrasive wear particles are present, alumina often outperforms Zirconia.

However, in erosive wear environments such as those caused by an abrasive slurry impacting a wear part such as an oilfield valve, Zirconia can be the best performer as it’s high toughness reduces the spread of impact cracks and the microfracture of the surface which generate erosive wear debris and surface damage.

When running an engineering ceramic against a dissimilar material it’s not always the case that the softer material performs poorly. Y-TZP running against Y-TZP has been shown to be a very poor wear surface combination, whereas, Y-TZP running against cast iron provides a better overall wear performance than the harder, alumina/cast iron combination.

Although its beyond this article to delve too deep into the tribology of ceramic interfaces, in Zirconia on Zirconia contact it’s the low thermal conductivity of the Zirconia that can be a negative factor, as the frictional heat generated in like on like sliding does not dissipate from the surface and the surface hardness decreases with a follow on increase in wear related damage.

Zirconia is Much Stronger than Alumina. Does That Mean It’s Always Going to Be The Safer Option?

Most engineering grades of zirconia are significantly stronger and tougher than high purity aluminas in bending or tension, often by a factor of 4 or more. However, don’t forget that bending strength is very different to compressive strength and depending on the application, if the component is subjected to high compressive stresses, perhaps paying for the additional strength of a Zirconia is simply a waste of money.

It’s also important to recognize that if the component is subjected to elevated temperatures, say greater than 1000°C, the bending strength of zirconia drops quite markedly to a similar level as Alumina, so once again, a simple glance at the specification sheet and a comparison of room temperature bending strengths may not always provide the optimum solution.

The Lower Thermal Expansion of Alumina Means My Components Will Be More Stable?

At first glance, perhaps a material with a relatively low thermal expansion coefficient is considered to be an attractive option in relation to dimensional stability. However, in most engineering systems engineering ceramics have to interface with metallic components and assemblies and often these are primarily steel components. In such situations, the thermal expansion coefficient of Zirconia can be attractive as it’s much closer to steel than Alumina and consequently, the whole assembly is more stable as temperatures fluctuate.

Which is The Most Corrosion Resistant Alumina or Zirconia?

Again, this is a very difficult question that can only be answered on a case-by-case basis by reference to an expert manufacturer. The combination of corrosive agent, chemical purity and temperature lead to a wide range of performance characteristics that are further varied by the individual composition and grade of Alumina or Zirconia. Do not think that because a material has excellent resistance to concentrated sulphuric acid it will perform well in steam at 200°C.

Zirconia is Much More Expensive Than Alumina?

As in many walks of life the price paid is only one facet of a good deal. It is true that in terms of raw material costs, as supplied to the ceramic manufacturer, alumina is generally much cheaper than Zirconia. However, if the requirement is for a precision engineered component, it’s possible that over 50% of the final cost may ultimately be attributable to machining costs which would be essentially the same for either material.

Equally, if the component in question can be produced in high volume without any final machining, either via injection molding or die pressing then it’s highly likely that Alumina would be the cheapest option.

A Final Word on Material Selection

Selecting the optimum material for a specific application always involves a decision tree constructed from a series of compromises. It is far too simplistic to say material A is always better than material B, because one must take into account the application, the environment and the available budget. The use of bare material specifications can also be dangerous as it can be all too easy to miss the key variable or combination of variables that are key to selecting the optimum material solution.

As is often the case, it’s always good to talk to an expert and if you are considering an engineering application that you feel may be suited to zirconia or alumina, why not consult someone like Insaco – www.insaco.com they have many decades of experience on selecting and machining engineering ceramics and also quartz and sapphire for a wide range of industrial applications.

Insaco

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

They have many decades of experience on machining and polishing Alumina or 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.

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