**1.1 Advanced technology ceramics**

Ceramics have been used since nearly at the beginning of low heat rejection engines. These materials have lower weight and heat conduction coefficient comparing with materials in conventional engines (Gataowski, 1990). Nowadays, important developments have been achieved in quantity and quality of ceramic materials. Also new materials named as "advanced technology ceramics" have been produced in the last quarter of 20th century. Advantages of advanced technology ceramics can be listed as below;


196 Ceramic Coatings – Applications in Engineering

Combustion characteristics is the most important factors which affect exhaust emissions, engine power output, fuel consumption, vibration and noise. In diesel engines, combustion characteristics depended on ignition delay at a high rate (Balc, 1983). Ignition delay is determined mostly by temperature and pressure of compressed air in combustion chamber. Conventional diesel engines have lower temperature and pressure of compressed air just because engine cooling system soaks considerable heat energy during compression to protect conventional combustion chamber materials. When the lost heat energy, useful work are taken into account, the idea of coating combustion chambers with low heat conduction and high temperature resistant materials leads to thermal barrier coated engines (also known as low heat rejection engines). Thermal barrier coated engines can be thought as a step to adiabatic engines. To achieve this aim, ceramic is a preferred alternative. Thermal barrier coating is mostly done by ceramic coating of combustion chamber, cylinder heads and intake/exhaust valves. If cylinder walls are intended to be coated, a material should be selected which has proper thermal dilatation and wear resistance. Some ceramic materials

Exhaust gas temperature changing between 400-600 0C for conventional diesel engines while it is between 700-900 0C for thermal barrier coated engine. This temperature value reaches to 1100 0C in turbocharged engines. When exhaust gas temperatures reaches these high levels, residual hydrocarbons and carbon monoxides in the exhaust gases are oxidized and exhaust emission are become less pollutant regarding hydrocarbons and carbon monoxide. In Figure 1, energy balance diagrams for conventional diesel engine and ceramic coated engine are given (Büyükkaya, 1994). Beside these advantages of ceramic coated low heat rejection engines, mechanical improvements also gained by light weight ceramic materials. By their high temperature resistance and light weight, moving parts of the engine have more duration owing to low inertia and stable geometry of the parts. Bryzik and Kamo (1983) reported 35% reduction in engine dimensions and 17% reduction in fuel consumption with a

Fig. 1. Energy balance illustration for conventional engine and ceramic coated engine

Ceramics have been used since nearly at the beginning of low heat rejection engines. These materials have lower weight and heat conduction coefficient comparing with materials in

have self lubrication properties up to 870 0C (Hocking et. al., 1989).

thermal barrier coated engine design in a military tank.

**1.1 Advanced technology ceramics** 


Advanced technology ceramics consist of pure oxides such as alumina (Al2O3), Zirconia (ZrO2), Magnesia (MgO), Berillya (BeO) and non oxide ones. Some advanced technology ceramic properties are given in Table 1.


Table 1. Some advanced technology ceramic properties

Zirconia has an important place among coating materials with its application areas and properties essential to itself. The most important property of zirconia is its high temperature resistance considering ceramic coating application in internal combustion engines. Ceramics containing zirconia have high melting points and they are durable against thermal shocks. They have also good corrosion and erosion resistances. They are used in diesel engines and turbine blades to reduce heat transfer.
