Editing Zirconium dioxide (section)

==Engineering properties <!--linked from 'Yttria-stabilized zirconia'-->==
[[File:Zirconium dioxide ZrO2 bearing balls.jpg|thumb|left|Bearing balls]]
Zirconium dioxide is one of the most studied [[ceramic]] materials. {{chem2|ZrO2}} adopts a [[monoclinic crystal system|monoclinic]] [[crystal structure]] at room temperature and transitions to [[tetragonal]] and [[cubic crystal system|cubic]] at higher temperatures. The change of volume caused by the structure transitions from tetragonal to monoclinic to cubic induces large stresses, causing it to crack upon cooling from high temperatures.<ref>{{cite journal |last1=Platt |first1=P. |last2=Frankel |first2=P. |last3=Gass |first3=M. |last4=Howells |first4=R. |last5=Preuss |first5=M. |title=Finite element analysis of the tetragonal to monoclinic phase transformation during oxidation of zirconium alloys |journal=Journal of Nuclear Materials |date=November 2014 |volume=454 |issue=1–3 |pages=290–297 |doi=10.1016/j.jnucmat.2014.08.020 |bibcode=2014JNuM..454..290P |doi-access=free}}</ref> When the zirconia is [[doping (semiconductor)|blended with]] some other oxides, the tetragonal and/or cubic phases are stabilized. Effective dopants include [[magnesium oxide]] (MgO), [[yttrium(III) oxide|yttrium oxide]] ({{chem2|Y2O3}}, yttria), [[calcium oxide]] ({{chem2|CaO}}), and [[cerium(III) oxide]] ({{chem2|Ce2O3}}).<ref name=evans>{{cite journal |author=Evans, A.G. |author2=Cannon, R.M. |title=Toughening of brittle solids by martensitic transformations  |journal=Acta Metall. |volume=34 |page=761 |year=1986 |doi=10.1016/0001-6160(86)90052-0 |url=https://zenodo.org/record/1253774}}</ref>

Zirconia is often more useful in its phase 'stabilized' state. Upon heating, zirconia undergoes disruptive phase changes. By adding small percentages of yttria, these phase changes are eliminated, and the resulting material has superior thermal, mechanical, and electrical properties. In some cases, the tetragonal phase can be [[metastable]]. If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the [[stress concentration]] at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion. This phase transformation can then put the crack into compression, retarding its growth, and enhancing the [[fracture toughness]]. This mechanism, known as [[toughening#Transformation toughening|transformation toughening]], significantly extends the reliability and lifetime of products made with stabilized zirconia.<ref name=evans/><ref>{{cite journal |author=Porter, D.L. |author2=Evans, A.G. |author3=Heuer, A.H. |title=Transformation toughening in PSZ |journal=Acta Metall. |volume=27 |page=1649 |year=1979 |doi=10.1016/0001-6160(79)90046-4}}</ref>

The {{chem2|ZrO2}} [[band gap]] is dependent on the phase (cubic, tetragonal, monoclinic, or amorphous) and preparation methods, with typical estimates from 5–7&nbsp;eV.<ref>{{cite journal |first=Jane P. |last=Chang |author2=You-Sheng Lin |author3=Karen Chu  |title=Rapid thermal chemical vapor deposition of zirconium oxide for metal–oxide–semiconductor field effect transistor application |journal=[[Journal of Vacuum Science and Technology B]] |volume=19|issue=5 |pages=1782–1787 |year=2001 |doi=10.1116/1.1396639|bibcode=2001JVSTB..19.1782C }}</ref>

A special case of zirconia is that of [[tetragonal polycrystalline zirconia|tetragonal zirconia polycrystal]], or TZP, which is indicative of polycrystalline zirconia composed of only the metastable tetragonal phase.