**2.2 Plasma spray coating**

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, 1992).

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 unit.

Ceramic Coating Applications and Research Fields for Internal Combustion Engines 203

Porosity is a property and a structural indicator of plasma spray coating. By utilizing high viscosity grains and high power plasma units, an intensive coating layer can be attained. Coating layers consisted from brittle and hard ceramic materials have high porosity rates. High porosity negatively affects material hardness which is a mechanical property of the material. While the least porosity layers have about 700 Vickers hardness, porous coating layers have about 300 Vickers hardness. 10 percent of the porosity after plasma spray coating is closed while rest of the porosity is open ones which combined with other defects in the structure because of insufficient fillings of blank areas among settled ceramic grains. Open porosities spoil mechanical properties of substance material by enabling corrosive sediments and gases to diffuse coating layer. On the other hand, spaces parallel to substance surface between layers negatively affect coating adhesion (Yaşar, 1997; Geçkinli, 1992).

Target surface must be rough, cleaned from oxides, oil, dirt and dust for making coating adhere to target surface. Surface roughness usually acquired by spraying an abrasive powder such as dust or alumina to target surface by a pressurized air. By coating base material having its surface prepared with a special binding material, target surface has a proper ground for ceramic coating. In addition to its binding property, binding layers can be used for reducing thermal expansion, protecting base material from effects of corrosion, gases and high temperatures. The most preferred binding material is NiAl. Work pieces which have their surface prepared for coating are placed perpendicular to plasma flame and fixed. Spray powders must hit to target surface perpendicularly to obtain an intensive and

Another important factor is powder size distribution in the spray. Very small grains in the plasma flame can easily reach plasma flame temperature, big grains however, adhere to target surface without being properly molten and make structure to be porous. Researches

Plasma spray coating can be conducted either in atmospheric conditions or in vacuum conditions. When it is done in vacuum conditions, plasma flame can expand to 20 cm and more intensive coatings can be obtained (Geçkinli, 1992). Fundamental elements and parameters affecting them in plasma spray coating are given in Table 3. One part of the process parameters which are determined for a specific coating application are depended to operator. To eliminate these parameters effecting coating quality, operating plasma gun with a robot arm or making plasma gun to move vertically and horizontally are proposed as

**3. Effects of ceramic coatings to internal combustion engine performance** 

To reduce damages occurring from high cycle temperatures, high cycle forces, sliding, erosion and corrosion on engine parts, several special techniques have been developed. Water cooling and thick combustion chamber walls had been utilized up to the end of Second World War to transfer excessive heat which material properties of combustion chamber construction materials such as cast iron can't bear. Later on, using low thermal conductivity materials such as glass and its' derivatives were considered. Despite low thermal conductivity, cost and low expansion rate, glass couldn't be used in internal combustion engines due to its' lacking strength. Using glass ceramic materials in engine parts was first seen at 1950s. In those days, ceramics used in spark plugs although low

good quality ceramic coating (Yaşar, 1997; Geçkinli, 1992).

show that grain sizes between 60 ±10 μm give good results.

solutions and applied.

Fig. 4. Plasma spray coating system

Fig. 5. Plasma spray gun

Direct current electrical arc is formed between electrode and nozzle in plasma spray coating gun. The inert gas (usually argon) and a little amount of hydrogen gas which is used to empower inert gas mixtures are sent to arc area of plasma gun and heated with electrical arc. Gas mixture temperature reaches to 8300 0C and it becomes ionized. Hence, high temperature plasma beam leaves from gun nozzle. In this system, ceramic grains are supplied to plasma beam as dispersible form. Grains molten by the hot gases are piled up on target surface and hardened. Argon/helium gas mixture increase gas flow and hence ceramic grains speed. Coating layer structure by the plasma spray coating contains equal axial thin solid grains. In some layers, an amorphous structure is attained because of fast solidification (Geçkinli, 1992).

Direct current electrical arc is formed between electrode and nozzle in plasma spray coating gun. The inert gas (usually argon) and a little amount of hydrogen gas which is used to empower inert gas mixtures are sent to arc area of plasma gun and heated with electrical arc. Gas mixture temperature reaches to 8300 0C and it becomes ionized. Hence, high temperature plasma beam leaves from gun nozzle. In this system, ceramic grains are supplied to plasma beam as dispersible form. Grains molten by the hot gases are piled up on target surface and hardened. Argon/helium gas mixture increase gas flow and hence ceramic grains speed. Coating layer structure by the plasma spray coating contains equal axial thin solid grains. In some layers, an amorphous structure is attained because of fast solidification (Geçkinli, 1992).

Fig. 4. Plasma spray coating system

Fig. 5. Plasma spray gun

Porosity is a property and a structural indicator of plasma spray coating. By utilizing high viscosity grains and high power plasma units, an intensive coating layer can be attained. Coating layers consisted from brittle and hard ceramic materials have high porosity rates. High porosity negatively affects material hardness which is a mechanical property of the material. While the least porosity layers have about 700 Vickers hardness, porous coating layers have about 300 Vickers hardness. 10 percent of the porosity after plasma spray coating is closed while rest of the porosity is open ones which combined with other defects in the structure because of insufficient fillings of blank areas among settled ceramic grains. Open porosities spoil mechanical properties of substance material by enabling corrosive sediments and gases to diffuse coating layer. On the other hand, spaces parallel to substance surface between layers negatively affect coating adhesion (Yaşar, 1997; Geçkinli, 1992).

Target surface must be rough, cleaned from oxides, oil, dirt and dust for making coating adhere to target surface. Surface roughness usually acquired by spraying an abrasive powder such as dust or alumina to target surface by a pressurized air. By coating base material having its surface prepared with a special binding material, target surface has a proper ground for ceramic coating. In addition to its binding property, binding layers can be used for reducing thermal expansion, protecting base material from effects of corrosion, gases and high temperatures. The most preferred binding material is NiAl. Work pieces which have their surface prepared for coating are placed perpendicular to plasma flame and fixed. Spray powders must hit to target surface perpendicularly to obtain an intensive and good quality ceramic coating (Yaşar, 1997; Geçkinli, 1992).

Another important factor is powder size distribution in the spray. Very small grains in the plasma flame can easily reach plasma flame temperature, big grains however, adhere to target surface without being properly molten and make structure to be porous. Researches show that grain sizes between 60 ±10 μm give good results.

Plasma spray coating can be conducted either in atmospheric conditions or in vacuum conditions. When it is done in vacuum conditions, plasma flame can expand to 20 cm and more intensive coatings can be obtained (Geçkinli, 1992). Fundamental elements and parameters affecting them in plasma spray coating are given in Table 3. One part of the process parameters which are determined for a specific coating application are depended to operator. To eliminate these parameters effecting coating quality, operating plasma gun with a robot arm or making plasma gun to move vertically and horizontally are proposed as solutions and applied.
