**1.1.1. Zirconia (ZrO2)**

Zirconia can be found in three crystal structure as it can be seen in Fig. 2. These are monolithic (m), tetragonal (t) and cubic (c) structures. Monolithic structure is stable between room temperature and 1170 0C while it turns to tetragonal structure above 1170 0C. Tetragonal structure is stable up to 2379 0C and above this temperature, the structure turns to cubic structure.

Ceramic Coating Applications and Research Fields for Internal Combustion Engines 199

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.

Magnesia is the most abundant one in refractory oxides and its melting point is 2800 0C. Its thermal expansion rate is very high. It can be reduced easily at high temperatures and evaporate at 2300-2400 0C. At high temperature levels, magnesia has resistance to mineral acids, acid gases, neutral salts and moisture. When contacted to carbon, it is stable up to 1800 0C. It rapidly reacts with carbons and carbides over 2000 0C. The most important

Melting point of alumina is about 2000 0C. It is the most durable refractory material to mechanical loads and chemical materials at middle temperature levels. Relatively low melting point limits its application. It doesn't dissolve in water and mineral acids and basis if adequately calcined. Raw alumina can be found as corundum with silicates as well as compounds of bauxide, diaspore, cryolit, silimanite, kyanite, nephelite and many other minerals. As its purety rises, it becomes resistant to temperature, wear and electricity.

Beryllia has a high resistance to reduction and thermal stability and its melting point is 2550 0C. It is the most resistant oxide to reduction with carbon at higher temperatures. Thermal resistance is very high though its electrical conductivity is very low. Mechanical properties of beryllia are steady till 1600 0C and it is one of the oxides that has high compression strength at this temperature. An important amount of beryllium oxide acquired from beryl. It is a favourable refractory material for molten metals owing to its resistance to chemical

Ceramic coatings which are applied to reduce heat transfer are divided into two groups. Generally, up to 0,5 mm coatings named as thin coatings and thick coatings are up to 5-6 mm. Thin ceramic coatings are used in gas turbines, piston tops, cylinder heads and valves of otto and diesel engines. At the beginning of ceramic coatings to low heat rejection engines, thick monolithic ceramic coatings were applied to engine parts. Later, it was understood that these coatings are not appropriate for diesel engine operation conditions.

There are a lot of types and system for ceramic and other material coatings. Most important

**2. Ceramic coating applications in internal combustion engines** 

Thus, new approaches were started to develop (Yaşar, 1997; Kamo et. al., 1989).

minerals of magnesia are magnesite, asbestos, talc, dolomite and spinel.

**1.1.2 Yttria (Y2O3)** 

**1.1.3 Magnesia (MgO)** 

**1.1.4 Alumina (Al2O3)** 

**1.1.5 Beryllia** 

ones are;

materials (Geçkinli, 1992).

Its structure is cubic very refractory.

Fig. 2. Cubic, tetragonal and monolithic zirconia

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

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;



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

Table 2. Partially stabilized zirconia types and properties
