**1.1.2 Yttria (Y2O3)**

198 Ceramic Coatings – Applications in Engineering


Usually cracks and fractures are observed during changing phases because of 8% volume difference while transition to tetragonal structure from monolithic structure. To avoid this and make zirconia stable in cubic structure at room temperature, alkaline earth elements such as CaO (calcium oxide), MgO (magnesia), Y2O3 (yttria) and oxides of rare elements are added to zirconia. Zirconia based ceramic materials stabilized with yttria have better properties comparing with Zirconia based ceramic materials which are stabilized by

Mechanical properties of cubic structure zirconia are weak. Transition from tetragonal zirconia to monolithic zirconia occurs at lower temperatures between 850-1000 0C and this transition has some characteristics similar to martensitic transition characteristics which are observed in tempered steels. In practice, partially stabilized cubic zirconia (PSZ) which contains monolithic and tetragonal phases as sediments, is preferred owing to its improved mechanical properties and importance of martensitic transition. Partially stabilized zirconia has been commercially categorized since early 70s. Table 2. contains partially stabilized

zirconia types and their properties. Structural properties of these materials are;

**Elasticity Module (GPa)** 

ZN20: Is developed for thermal shocks. Contains (m) phase.

Table 2. Partially stabilized zirconia types and properties

Zt35: Contains 20% (t) phase in cubic matrix. Particle dimensions are about 60-70 μm.

Ca/Mg-PZS Zt35 200 4,8 1300 9,8x10-6 Mg-PZS ZN40 200 8,1 1200 9,8 Mg-PZS ZN50 200 9 900 7 Y-PZS ZN100 190 9,7 - 9,3 Mg-PZS ZN20 180 3,5 - 5,5

ZN50: Particle dimensions are about 60-70 μm and a thin film (m) phase lays on the

**Fracture Toughness (MPa m1/2)** 

**Vickers Hardness (HVat 22 0C)**

**Expansion Coefficient (22-1000 0C)** 

Fig. 2. Cubic, tetragonal and monolithic zirconia

ZN40: Contains 40-50% (t) phase.

borders of particles.

**Material Code** 

magnesia and calcium oxide (Yaşar, 1997; Geçkinli, 1992).

Melting point of yttria is 2410 0C. It is very stable in the air and cannot be reduced easily. It can be dissolved in acids and absorbs CO2. It is used in Nerst lambs as filament by alloyed with zirconia and thoria in small quantities. When added to zirconia, it stabilizes the material in cubic structure. Primary yttria minerals are gadolinite, xenotime and fergusonite. Its structure is cubic very refractory.
