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

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

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

Ceramic Coating Applications and Research Fields for Internal Combustion Engines 201

In this method, micro-pulverized powder alloys are sprayed to target surface in oxyacetylene flame by oxygen vacuum. It is called cold coating because flame temperature is about 3300 0C and target surface is about 200 0C during coating process. Adherence is mechanical. Coating layer thickness is changed 0,5 to 2,5 mm according to shape of work piece. Using highly alloyed and self lubricant NiCrBSi materials as coating powder and making materials which are not produced in rod or wire shapes possible for coating are the main advantages of this method. Powder flame spray systems are proper for spraying primarily ceramics and metals and cermets (metals and ceramic oxide alloys) as coating materials. Bearing supports, axle and shaft pivots, compressor pistons, cam shafts, bushes, rings and sleeves, hydraulic cylinders and pistons can be coated by this very method (Yaşar,

Wire flame spray coating method is applied by spraying a wire shaped metal which has a melting point below flame temperature to coated surface. It can be used for metal spray materials and metal surfaces. Coating material wire is molten by oxygen and gas fuel flame after passing from the coating gun nozzle. Acetylene, propane and hydrogen are used for gas fuel. Relatively low equipment costs, high spray speeds and adjustment property according to wire diameters are the advantages of this system. Lower coating intensity and adherence strength comparing with other methods can be told as disadvantages of the method. Bearing supports, hydraulic piston pins, various bearings, shafts, wearing surfaces of axles, piston segments, synchromesh, crank shafts, clutch pressure plates can be coated with wire flame spray coating systems (Yaşar, 1997;

Plasma is a dense gas which has equal number of electron and positive ion and generally named as fourth state of the matter. This method has two primary priorities; It can provide very high temperatures that can melt all known materials and provides better heat transfer than other materials. High operating temperature of plasma spray coating, gives opportunity to operate with metals and alloys having high melting points. Also using plasma spray coating in inert surroundings is another positive side of the method. Oxidation problem of the subject material is reduced due to inert gas usage in plasma spray such as argon, hydrogen and nitrogen. All materials that are produced in powder form and having a specific grain size can be used in this method (Yaşar, 1997; Geçkinli,

The main objective in plasma spraying is to constitute a thin layer that has high protection value over a non expensive surface. The process is applied as spraying coating material in powder form molten in ionized gas rapidly to coated surface. Plasma spray coating system is shown in Fig. 4. The spraying gun is illustrated in Fig. 5. The system primarily consists of power unit, powder supply unit, gas supply unit, cooling system, spraying gun and control

**2.1.1 Powder flame spray coatings** 

1997; Anonymous, 2004).

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1992).

unit.

**2.2 Plasma spray coating** 

**2.1.2 Wire flame spray coatings** 


Material conglomerations can be avoided by reducing erosion-corrosion, friction-wear, using ceramics as well as improving heat insulation. Non the less, these methods are proper for very thin coatings except thermal spray coatings. Thin layer coatings are successfully used in gas turbine industry, coating turbine and stator blades and combustion rooms. For thick layer coatings like diesel engines, plasma spray and flame spray coatings are generally utilized (Kamo et. al., 1989).
