**1. Introduction**

194 Ceramic Coatings – Applications in Engineering

Tombs, N. C.; Croft, W. J. & Mattraw, H. C. (1963). Preparation and Properties of Chromium

Wood, G. J. et al. (1982). Mechanism of oxidation of the crystallographic shear phase Ti4O7,

Borate, *Inorg. Chem. 2*, Vol. 4 (1963), pp. 872–873

*Philosophical Magazine A*, Vol. 46, (1982), No. 1, pp. 75-86

Research for decreasing costs and consumed fuel in internal combustion engines and technological innovation studies have been continuing. Engine efficiency improvement efforts via constructional modifications are increased today; for instance, parallel to development of advanced technology ceramics, ceramic coating applications in internal combustion engines grow rapidly. To improve engine performance, fuel energy must be converted to mechanical energy at the most possible rate. Coating combustion chamber with low heat conducting ceramic materials leads to increasing temperature and pressure in internal combustion engine cylinders. Hence, an increase in engine efficiency should be observed.

Ceramic coatings applied to diesel engine combustion chambers are aimed to reduce heat which passes from in-cylinder to engine cooling system. Engine cooling systems are planned to be removed from internal combustion engines by the development of advanced technology ceramics. One can expect that engine power can be increased and engine weight and cost can be decreased by removing cooling system elements (coolant pump, ventilator, water jackets and radiators etc.) (Gataowski, 1990; Schwarz et. al. 1993).

Initiation of the engine can be easier like shortened ignition delay in ceramic coated diesel engines due to increased temperature after compression because of low heat rejection. More silent engine operation can be obtained considering less detonation and noise causing from uncontrolled combustion. Engine can be operated at lower compression ratios due to shortened ignition delay. Thus better mechanical efficiency can be obtained and fuel economy can be improved (Büyükkaya et. al., 1997).

Another important topic from the view point of internal combustion engines is exhaust emissions. Increased combustion chamber temperature of ceramic coated internal combustion engines causes a decrease in soot and carbon monoxide emissions. When increased exhaust gases temperature considered, it is obvious that turbocharging and consequently total thermal efficiency of the engine is increased.

Ceramic Coating Applications and Research Fields for Internal Combustion Engines 197

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.

Advanced technology ceramics consist of pure oxides such as alumina (Al2O3), Zirconia (ZrO2), Magnesia (MgO), Berillya (BeO) and non oxide ones. Some advanced technology

> **Strength (MPa)**

SiO2 500 2,2 48 7,2 0,5 650

Al2O3 2050 3,96 250-300 36-40 4,5 1300

ZrO2 2700 5,6 113-130 17-25 6-9 1200

SiC 3000 3,2 310 40-44 3,4 2800

Si3N4 1900 3,24 410 30-70 5 1300

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

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

**Elasticity Module (GPa)** 

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

**Hardness (kg/mm2)** 

Advantages of advanced technology ceramics can be listed as below;

**Density (g/cm3)** 

Table 1. Some advanced technology ceramic properties

Can be found as raw material form in environment

Resistant to high temperatures

Low heat conduction coefficient

ceramic properties are given in Table 1.

**Melting Temperature (0C)**

turbine blades to reduce heat transfer.

**1.1.1. Zirconia (ZrO2)** 

to cubic structure.

High compression strength (Çevik, 1992)

 High chemical stability High hardness values

Low densities

**Material** 

Resistant to wear

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

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 thermal barrier coated engine design in a military tank.

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