Zirconia, alumina and composite ceramics
Ceramics based on zirconia (ZrO2) partially stabilized with yttria (Y2О3) (yttria-stabilized zirconia, YSZ) have outstanding strength and fracture toughness, as compared to other structural ceramics, while retaining good wear and corrosion resistance.
Zirconia is a refractory compound with atomic bonds of predominantly ionic nature. It exists in three crystallographic modifications: cubic, tetragonal and monoclinic. High strength and fracture toughness of zirconia are based on the mechanism of phase transformation (polymorph transformation) of the metastable tetragonal modification into a stable monoclinic one.
Low thermal conductivity of ZrO2 hinders heat dissipation in tribological applications. High coefficient of thermal expansion (CTE) of zirconia makes it possible to join zirconia parts with metal or steel parts having similar CTE.
Disadvantages of YSZ ceramics include degradation of its mechanical performance when subjected to moisture effects at temperatures under 300°С. This phenomenon is known as low-temperature degradation (LTD).
Composite alumina-YSZ ceramics (Al2O3 - ZrO2(Y2О3)) are less subject to LTD. These ceramics have better hardness than pure zirconia, due to the contribution of the ultrahard alumina phase. The same is true for thermal conductivity of composite ceramics.
Alumina (Al2O3) based ceramics possesses high hardness, relatively low strength, and high Young modulus. Alumina is resistant to corrosion in most organic and inorganic acids and salts. The major limitation of the otherwise good physical and mechanical performance of alumina ceramics is its poor fracture toughness, lowest among structural ceramics.
General applications for oxide ceramic wear parts include friction pairs (slide bearings) for pump engineering, stop valving parts, face seals and valves, lining and grinding media for mills, crucibles for melting of precious metals, etc.
|ZrO2(Y2O3)||Al2О3 - ZrO2||Al2O3|
|Density, g/cm³||6,0 - 6,05||4,5 - 4,6||3,8 - 3,9|
|Transverse rupture strength, MPa||750 - 1050||550 - 750||300 - 350|
|Young modulus, GPa||200 - 210||310 - 340||370 - 380|
|Vickers’ hardness, GPa||12 - 13||15 - 17||19 - 21|
|Impact toughness, kJ•mˆ(-2)||8,0 - 10,0||6,0 - 8,0||3,0 - 3,5|
|Thermal conductivity, W mˆ(-1) x Кˆ(-1)||2 - 3||20 - 22||25 - 30|
|Coefficient of thermal expansion (CTE), 10ˆ(-6) x Кˆ(-1)||10,0 - 11,0||8,5 - 9,5||8,0 - 9,0|