SE1650532A1 - Piston body - Google Patents

Abstract

The present invention relates a piston body (12) comprising a coating structure (11) on a surface adapted to be on the side to be exposed to a combustion gas produced in an internal combustion engine (2). The coating structure (11) comprises a first layer (12c) of plasma sprayed thermal barrier material comprising a ceramic material and an outermost second layer (12d) of plasma sprayed metallic material. The invention also relates to a process for producing the coating structure (11) on a surface of a piston body (12). Through the present invention, heat losses can be reduced.(Fig. 3)

Description

Piston body TECHNICAL FIELD The present invention relates to a piston body for an internal combustion engine, a process forproducing a coating structure on a surface of the piston body, a piston body obtained, aninternal combustion engine comprising the piston body and a vehicle comprising the internal combustion engine.

BACKGROUND ART internal combustion ertgiries, such as diesel engines or ottra engines, are used in several typesof applications and vehicles, for example, in heavy vehicles, sutth trucl power plants, such as electric power plants including a diesel generator, and in locornotiyes.

An internal combustion engine typicaily coinprises an engine block with at least one cylinder,A piston is arranged to move inside the cylinder and it seals one end of the cylinder such that acornbiistiori chamher is forineti between the piston, the cylinder and a cylinder iteacl arrangedat the top end ofthe cylinder. Fuel is irijected irito the combustion charnber, vaporizeti, ignitedand cornbiisted. The gas pressure front the cornhustiort exerts a force on the piston such titatit rnoves. The reciprocating rnotioit oltne piston is then translated into a rotationai rnoverneittof a cranltshaft. The piston rnoves between a top dead centre and a bottorn dead centre,yyhere the top dead centre is nearest to the cylinder ltead and the hottoin dead centre is theopposite position furthest away front the cylinder head. The combustion is performed yyhenthe piston is close to or at the top dead centre. As the piston is pushed towards the bottomdead centre hy the combustion gases, the combustion gases expand and as a result their Éëfïlllïêïälfiiaiïë äilfj QlÉSSllFë ClëCfEiêSëS.

A piston in an internal ttonthiistioit engine is of course subject to very high 'teinperatures The rnore heat that is l 2 therefore desirabie to achieve and maihtain as high teinperature as possible and thusrnihihiize heat losses from the combustion Chamber. in the prior art there are solutions forminimizing the heat losses from the combustion Chamber. For example, document US4254621discioses a cast rhetai rriember of an internal cornhustion engine Which cerhprises a heatihsuiatirtg layer cornprising a metal body which has a rserous structure. However, the ttorousstructure is brittie, and therefore there is stiii a need te improve the soiutiens for rriinirriizirigheat losses frorn a coinbiistiort chamber. Especiaily, there is a need for solutions that aresimple and improve the efficiency and the fuel consumption of the internal combustion engine.

SUMMARY OF THE INVENTION lt is an object of the present invention to improve the efficiency of an internal combustionengine and thus for example to decrease the fuel consumption of an internal combustionengine. More specifically, it is an object of the invention to reduce heat losses in a combustion engine.

Further it is an object ofthe invention to provide a piston construction for an internal combustion engine which increases the efficiency of the internal combustion engine. lt is a further object to provide a simple and robust solution for minimizing heat losses from a combustion engine.

The objects above are achieved by providing a piston body comprising a coating structureaccording to the present invention and as defined in the appended claims. The piston bodycomprises a coating structure on a surface adapted to be on the side to be exposed to acombustion gas produced in an internal combustion engine and the coating structurecomprises a first layer of plasma sprayed thermal barrier material comprising a ceramicmaterial and an outermost second layer of plasma sprayed metallic material. The coatingstructure on the surface of the piston body may be provided by a process according to thepresent invention, which process comprises the steps of: i) providing a piston body; 3 ii) optionally providing a layer of a bonding agent on the surface of the pistonbody; iii) providing a thermal spray device arranged to spray a first layer of a thermalbarrier material comprising a ceramic material to the surface of the pistonbody or on the layer ofthe bonding agent; iv) feeding a process gas and the thermal barrier material to the thermal spraydevice and operating the thermal spray device so as to form a coating sprayplume which is propelled towards the surface of the piston body, wherein thecoating spray plume comprises the process gas and at least partially molten thermal barrier material and thus provide a first layer of the coating structure;v) optionally cooling the first layer;vi) optionally providing a layer of a bonding agent on the first layer; vii) feeding a process gas and a metallic material to a thermal spray device, andoperating the thermal spray device to form a coating spray plume which ispropelled towards the surface of the piston body, wherein the coating sprayplume comprises the process gas and at least partially molten metallic material and thus provides a second layer of the coating structure.

Further, the piston body comprising the surface coating according to the present invention is produced according to the process, as generally defined above and as further defined below. lt has been found that the piston body comprising a coating structure according to the presentinvention reduces effectively heat losses through the piston body. Thereby the efficiency ofthe internal combustion engine is improved. Also, for example fuel consumption of an internalcombustion engine can be reduced. Further, since the first layer of ceramic material and thesecond layer of metallic material are plasma sprayed, or sprayed by means of a thermal spraydevice, the coating structure will be robust, smooth and dense. The outermost metallic layerwill also provide a plain surface having reflective properties. The metallic outermost layer willnot be translucent to radiation having long wavelength, whereas a ceramic thermal barrier coating could be translucent. Thus, a synergistic improved effect in thermal insulation 4properties is obtained through the plasma sprayed first layer and the plasma sprayed second layer.

According to another feature, in the piston body or the process for producing a coatingstructure on a surface of a piston body, the thermal barrier material additionally comprisesmetallic particles. The ceramic particles may comprise metallic oxide particles of zirconiumoxide optionally stabilized with yttrium or alumina-zirconium composite particles. The metallicparticles that suitably constitute the outermost metallic layer could be for example stainlesssteel particles. This ensures that the coated piston body will have excellent anti-corrosion and anti-wear properties.

The metallic material in the second outermost layer preferably comprises or consists of analuminium alloy, chromium alloy, nickel-containing alloy or a nickel-chromium-containingalloy. Such metals or metallic materials have excellent anti-wear and anti-corrosion propertiesand reflective properties, whereby both excellent thermal and mechanical strength properties are obtained.

The thickness of the first layer of the coating structure is suitably from 50 to 1200 um. Thethickness of the second layer of the coating structure is suitably from 10 to 800 um. Suchthicknesses provide excellent thermal properties while the coating structure will be easy toobtain and will have excellent mechanical anti-wear properties. The second layer is alsoreferred to as of a metallic top coat. To further improve the adhesion of the first layer to thepiston body and to the second outermost metallic layer, the coating structure may compriseadditional layers of bonding agent. A layer of the bonding agent may be provided on thesurface of the piston body, e.g. in step ii) of the process and additionally or alternatively onthe first layer. The thickness of the layer of bonding agent obtained in steps ii) or vi) is from 10 to 500 um.

The thermal spray device is preferably a plasma spray device. This allows for high productiontemperatures whereby the coating materials in the first and second layer respectively maymelt. Thereby a decreased surface roughness is obtained and thus improved reflective properties and thermal properties are also obtained. 5According to a further aspect of the present invention, a piston body for an internalcombustion engine adapted to be arranged in a cylinder block of the internal combustionengine such that the piston together with a cylinder head and a cylinder wall delimits acombustion chamber is provided. The surface of the piston that is exposed to a combustiongas produced in the internal combustion engine comprises a coating structure as produced bythe process as defined above. The piston body is suitably a forged piston body. The pistonbody could alternatively be cast metal piston body or friction welded. The piston body maycomprise curved convex and concave portions, i.e. the piston body may comprise a complexouter surface contour. Since the piston body is coated by means of a thermal spray device, e.g.plasma spray device, it is possible to reach all parts of the surface and obtain a coatingstructure having essentially the same thickness over the whole surface. Thereby, excellent thermal properties may be obtained.

According to an aspect an internal combustion engine is provided. The internal combustion engine comprises the piston body as described above.

Further, the present invention also relates to a vehicle comprising an internal combustion engine as defined above.

The piston body, the process for producing a coating structure on a surface of a piston bodyand the piston body thus obtained, the internal combustion engine and the vehicle may bemodified within the scope of the appended claims in accordance with the detailed description below with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention drawings illustrating an example of thepresent invention are provided. Further objects and advantages of the invention will bedescribed more closely below in the detailed description. ln the appended drawings: Fig. 1 schematically illustrates a vehicle according to an embodiment of the invention; 6 Fig. 2 schematically illustrates an internal combustion engine according to anembodiment of the invention; Fig. 3 schematically illustrates a vertical cross-section of a part of a cylinder blockincluding a piston body according to an embodiment ofthe invention; Fig. 4 schematically shows a coating structure on a piston body according anembodiment; Fig. 5 schematically illustrates a plasma spray device, which is representative ofthermal spray devices; and Fig. 6 shows a flow chart of a process according to the present invention.

DETAILED DESCRIPTION The invention is now described with reference to the piston body, the process for producing acoating structure on a surface of a piston body and the piston body thus obtained, the internal combustion engine and the vehicle as generally described above.

The piston body ofthe invention is suitably adapted to be arranged inside a cylinder of acylinder block. The piston body is further adapted to be received in the cylinder defined bywalls forming a cylindrical space, and the piston body is arranged to be movable inside thecylinder in the direction of a centre axis of the cylinder. The cylinder is sealed at the lower endby the piston and at the upper end by a cylinder head. The piston body comprises a number ofpiston rings which ensure that combustion gases do not leak out from the cylinder. Also, thepiston body may comprise curved convex and concave portions, i.e. the piston body maycomprise a complex outer surface contour. The cylinder is adapted to be arranged such thatthe upper end surface of the cylinder wall faces the cylinder head. A sealing device may bearranged between the cylinder head and the cylinder wall. The wall is suitably shaped as an essentially circular cylinder with the centre axis in the longitudinal direction of the cylinder.

By providing the piston body with a coating structure according to the present invention, therate of heat transfer through the piston body may be decreased. The durability ofthe pistonbody is greatly affected by the high temperatures created in the combustion chamber. The end of the piston body that is not exposed to the combustion gases is therefore arranged so 7 that a cooling arrangement with cooling fluid can get access to the piston body and cool theunderside of the piston body that is not exposed to the combustion gases. ln this way thepiston ring temperature and piston body temperature can be kept below critical values. Suchcooling arrangement may however affect the rate of heat transfer through the piston body.The more heat is kept in the combustion chamber, the greater work on the crankshaft. lt istherefore desirable to maintain as high temperature as possible as long as possible and thus tominimize heat losses from the combustion chamber. Therefore, the surface of the piston bodyis provided with an insulating ceramic material layer, as in the present invention. Coolingarrangements in the pistons bottom side will typically cause a high temperature difference.The rate of heat transfer through the piston body is thereby increased and the temperature ofthe gases in the combustion chamber is rapidly decreased. When the temperature ofthegases in the combustion chamber is decreased, the efficiency of the internal combustionengine is decreased and the fuel consumption is thus disadvantageously affected. Byproviding a coating structure according to the present invention, both thermal insulation andreflective surface which is not translucent to radiation with long wavelength and with highmechanical durability are obtained. Therefore the heat from the combustion chamber will bebetter and more effectively kept inside the combustion chamber. ln this way heat losses from the piston body are minimized.

Any thermal barrier material known in the art could be used as the first layer in the coatingstructure of the present invention. The thermal-coating compositions that are suitable dependon the desired properties. For instance, improved thermal, wear and corrosion properties canbe obtained. Pure ceramic materials of mixtures with metals, alloys, oxides, nitrides,composites, carbides, or mixtures thereof can be used. The thermal-coating compositions areusually provided in powder form with particle sizes preferably in the range of 10-100 pm. An example of a commercial thermal barrier material is Metco 204-XCL ®.

The metal particles have a diameter of about 5 pm to about 45 pm. The ceramic composite particles have a diameter of about 5 pm to about 38 pm.

The invention will now be further described with reference to the appended drawings. 8 Figure 1 schernaticaliy snovvs a side view of a vehicle l in tvhicn an internal combustion engine2 cornpršsing a piston according to the inventioh can be used. The vehicle 1 comprises aninternai combustion engine 2 connected to a gearbox 4. The gearbox 4 is also connected tothe driving wheels 6 of the vehicie 1 through an output snaft of the gearbox (not shown). Thevehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle 1 may alternatively be a passenger car.

Figure 2 scnernatically shows an example of an internal combustion engine 2 and the functionthereof is herein explained. The internal combustion engine 2 is suitably arranged in a vehicleaccording to Figure 1. The internal combustion engine 2 comprises a cylinder block 8 with atleast one cylinder comprising a cylinder wall 10, at least one piston body 12, also referred to asa piston 12, and at least one cylinder head 14. A combustion chamber 16 is delimited by thecylinder wal|10, the cylinder head 14 and the piston 12. The cylinder defined by the wall1Omay comprise a cylinder liner which is suitably removably arranged in the cylinder block 8 suchthat it may be replaced. The piston 12 is movably arranged within the cylinder for performingstrokes. The piston 12 is connected to a crankshaft 18 via a connecting rod 19 such that thereciprocating motion of the piston results in a rotational motion of the crankshaft 18. Thecrankshaft 18 is connected to a flywheel 20 which may be arranged in connection with agearbox 4 such that torque may be transferred to driving wheels of a vehicle 1. The cylinderhead 14 may comprise for example fuel injectors 22 for injecting fuel into the cylinder forcombustion, inlet and exhaust ducts (not shown), cooling ducts (not shown) and inlet and exhaust valves (not shown).

The internal combustion engine 2 may be adapted to provide a four stroke cycle. A strokerefers to the full travel of the piston 12 along the longitudinal extension of the cylinder wall10, in either direction. For a complete four stroke cycle the crankshaft 18 will turn tworevolutions. The position of the piston 12 closest to the cylinder head 14 is known as the topdead centre TDC and the position of the piston 12 farthest away from the cylinder head 14 isknown as the bottom dead centre BDC. A four stroke cycle comprises a first stroke (intake)which begins with the piston at the top dead centre TDC and intake valves are open. When thepiston moves towards the bottom dead centre BDC the volume of the combustion chamber 16 increases and air or air-fuel mixture enters the combustion chamber. During the second stroke 9 (compression) the inlet and the exhaust valves are closed and the air or the air-fuel mixture inthe combustion chamber is compressed as the piston 12 moves towards the top dead centreTDC. Just before the piston 12 reaches the top dead centre TDC ignition begins. The pressureof the combustion gases pushes the piston 12 towards the bottom dead centre BDC in thethird stroke (combustion), wherein mechanical work is produced. As the piston 12 movestowards the bottom dead centre BDC the combustion gases expand and the pressure and thetemperature of the combustion gases is thereby decreased. During the fourth stroke (exhaust)the exhaust valve is open and the combustion gas is thereby expelled when the piston 12moves towards the top dead centre TDC. At the end of the fourth stroke the exhaust valve closes, the intake valve opens and the sequence may thereby be repeated in the next cycle.

The durability of the piston body 12 is greatly affecteci bg; the ltigh temperatures created in thecornbustioit chamber 16 and the end of the piston body 12 that is not exposed to thecombustion gases is therefore arranged so that a cooling arrangement with cooling fluid canget access to the piston body 12 and cool the underside of the piston body 12 that is notexposed to the combustion gases. ln this way the piston ring temperature and piston bodytemperature can be kept below critical values. Such cooling arrangement may however affectthe rate of heat transfer through the piston body and increase the heat losses from thecombustion gases to the piston surface. Therefore, the more heat is kept in the combustionchamber 16, the greater work on the crankshaft 20. lt is therefore desirable to maintain a hightemperature as long as possible and thus to minimize the heat losses from the combustionchamber 16. The piston 12 according to the present invention therefore comprises a surfacecoating comprising a thermal barrier coating of plasma sprayed ceramic heat-insulating material and a plasma sprayed top coat of metal or a metal alloy.

Fig. 3 schematically illustrates a vertical cross-section of a part of a cylinder block 8 of aninternal combustion engine 2 according to an embodiment of the invention. A piston body 12is arranged to be movable within a cylinder defined by the cylinder wall 10. The piston body12 has curved convex and concave portions. The cylinder can be arranged in the cylinder block8 ofan internal combustion engine 2 as shown in Fig. 2. The cylinder wall 10 has an upper end26 and a lower end 28, and the cylinder wall 10 extends longitudinally between the upper end 26 and the lower end 28. The upper end 26 has an upper end surface 32 extending essentially perpendicularly to the longitudinal extension ofthe cylinder wall 10. The upper end surface 32is thus adapted to face a cylinder head 14. The cylinder head 14 is herein illustratedcomprising fuel injectors 22 for injecting fuel into the combustion chamber 16, an inlet valve 42 and an exhaust valve 44.

As previously described, the durability of the piston body 12 is greatly affected by the hightemperatures created in the combustion chamber 16. At the same time it is desirable thatheat losses through the piston body 12 are minimized. To minimize the heat transfer to thepiston surface 13, i.e. the surface adapted to be on the side to be exposed to a combustion gasproduced in an internal combustion engine, the end 15 of the piston body not exposed to thecombustion gases is therefore arranged so that a cooling fluid can get access to the pistonbody via cooling passages 40 and 41 and cool the cylinder wall 10 and the underside of thepiston body 12. ln this way e.g. the temperature of the piston ring 17 and piston body 12 canbe kept below critical values. Such cooling passages 40, 41 however increase the rate of heattransfer and thus heat losses through the cylinder wall 10 and the piston body 12. The rate ofheat transfer through the piston body 12 depends for example on the difference intemperature on both sides ofthe piston body 12. The low temperature at the end 15 ofthepiston body not exposed to the combustion gases caused by the cooling arrangement willresult in a high temperature difference. The low temperature may be obtained for example bymeans of an oil cooling gallery. Thus, the rate of heat transfer through the piston 12 isthereby increased and the temperature of the gases in the combustion chamber 16 is rapidlydecreased. When the temperature ofthe gases in the combustion chamber 16 is decreased,the efficiency of the internal combustion engine 2 is decreased and the fuel consumption isthus impaired. By providing a surface coating structure 11 according to the present inventionand as schematically illustrated in Fig.4, heat losses through the piston body 12 are minimized.The coating structure is robust, smooth and dense and covers the surface ofthe piston body inan even manner, since the first and second layer of the coating structure are provided by means of a thermal spray device, preferably by means of a plasma spray device.

Fig. 4 shows a coating structure 11 which comprises a first layer 12c of plasma sprayedthermal barrier material comprising a ceramic material and an outermost second layer 12d of plasma sprayed metallic material. Since both coating layers are plasma sprayed the roughness 11 of the surface coating can be minimized and denser and more even coating layers can beprovided. Thereby, also the chemical resistivity can be improved. To increase adhesion ofthefirst layer 12c to the piston body 12 and to the second layer 12d, a layer 12a of bonding agentmay be provided on the surface of the piston body 12 and/or a layer 12b of bonding agentmay be provided on the first layer. The bonding agent may be any known bonding agent in the art, such as a commercial product Amdry 962 ® (NiCrAlY).

Fig. 5 shows a plasma spray device 50, which is representative for a thermal spray device. Thethermal spray device 50 typically comprises at least one process gas inlet 51, a thermal coatingcomposition inlet 52, and an outlet orifice 53. There may be more than one inlet or the inletmay surround the outlet orifice 53. A process gas is fed to the thermal spray device 50 andheated as described below. At the same time, the thermal barrier material is fed to the thermal spray device 50 and heated.

The thermal barrier material and the metallic material, i.e. the coating materials in the coatingstructure 11, are most commonly fed to the thermal spray device as a powder, but e.g. themetallic material may also be fed as a wire (combustion wire spraying). lf the materials are fedas a powder, a carrier gas such as nitrogen or argon can be used to assist the feeding.Depending on the technique used, the thermal barrier material and/or the metallic materialmay be mixed with the process gas either prior to heating or after heating, and either within the interior ofthe device or in direct proximity to the outlet orifice.

The manner of heating of the process gas and the coating materials differs depending on thethermal spray technique used. ln most techniques, the process gas is first heated and this heatis then transferred to the coating materials, at least partially melting the coating materials. lfthe process gas is a fuel/oxidant blend it is heated by combustion. Fuels known for applicationin thermal spray coating include acetylene, propane, propylene, hydrogen, natural gas andkerosene. Known oxidants include oxygen and air. The process gas may also be heated byionization of the process gas, forming a plasma. The process gas may also be heated indirectlyby bringing it into contact with a heated thermal coating composition, such as in electric arc wire spray. 12ln the combustion wire spray process the metallic material formed as a wire is fedconcentrically through the spray device into an oxygen-fuel flame, where it is melted. Themelted material is atomized and directed towards the substrate surface by the addition of a compressed air feed.

The combustion powder spray process is similar to the combustion wire process. However, thecoating material is fed as a powder, meaning that a wider range ofthermal barrier materials or metallic materials can be used, since not all materials can be manufactured in wire form. ln electric arc wire spraying, an arc is formed by the contact of two oppositely charged coatingmaterial feeds formed as wires, leading to melting at the contact location. A compressed air feed then atomizes the melted wire material and directs it towards the substrate. ln HVOF spraying, a supersonic jet of gas and coating material is formed by the combustion ofa liquid or gaseous fuel in oxygen. The coating particle impact velocities on the substrate aremuch higher than compared to conventional flame spraying, resulting in improved coating characteristics.

The thermal spray device according to the present invention is preferably a plasma spray device.

Returning to Fig. 5, plasma spraying utilizes a chamber with one or more cathodes 54(electrodes) and an anode 55 (nozzle). With process gases flowing through the chamber, directcurrent power is applied to the cathode 54, which arcs to the anode 55. The arc ionizes the gasmolecules to form a plasma plume. As the unstable plasma ions recombine back to the gaseous state, a large amount of thermal energy is released.

The coating material is fed into the hot process gas plume by a carrier gas, commonly nitrogenor argon. The coating material inlet 52 is either integrated into the outlet nozzle or ispositioned externally in direct proximity to the outlet orifice 53. The coating material isentrained in the process gas plume where it is at least partially melted and propelled towards the piston body to form the coating.

The process gases typically used are argon, hydrogen, nitrogen and helium, either individuallyor in mixtures of two, or even three of these gases. The gases used in combination with the current applied to the electrode controls the amount of energy produced. Since gas flows and 13the applied current can be accurately controlled, reproducible and predictable coating results can be obtained.

The plasma spray process can be performed in an open atmosphere, where it is termedatmospheric plasma spraying (APS) or in a controlled atmosphere such as under vacuum (VPS)or under low-pressure (LPPS). A controlled atmosphere process results in less oxidation of the coating particles, and thus a higher quality coating, but is significantly more expensive. ln addition, the shape and bore size of the nozzle, the point and angle that the coatingmaterial is injected into the plume, as well as the distance of the gun to the target surface are also controlled. This provides a high degree of flexibility to develop reproducible parameters The temperature in the thermal spray plume depends on a number of factors, primary amongthe being the thermal spray technique used. The plume temperature can range from about2600 °C for HVOF spraying to 12000-16000 °C for plasma spraying. The coating particles achieve a temperature from about 1200 °C to 2700 °C for single cathode plasma spraying.

The temperature of the piston body being coated can be controlled by regulating a number ofparameters in the arrangement, such as the distance of the plasma gun to the piston body andthe relative motion of the various parts of the arrangement. Cooling air jets focused on thesubstrate can also be used. By these techniques, the temperature of the piston body can be kept at a controlled temperature in the range of about 40 °C to about 260 °C. ln Fig. 6 a flow chart showing the steps of the process for producing a coating structure 11 ona surface of a piston body 12, the surface being adapted to be on the side to be exposed to acombustion gas produced in an internal combustion engine 2, the coating structurecomprising a first layer 12c of a ceramic thermal barrier material and an outermost second layer 12d of metallic material, the process comprising the steps of: i) providing a piston body 12; ii) optionally providing a layer 12a of bonding agent on the surface of the pistonbody 12; iii) providing a thermal spray device 50 arranged to spray a first layer 12c ofthermal barrier material comprising a ceramic material to the surface 13 of the piston body 12 or on the layer 12a; 14 iv) feeding a process gas and the thermal barrier material to the thermal spraydevice 50 and operating the thermal spray device so as to form a coatingspray plume which is prope||ed towards the surface of the piston body 12,wherein the coating spray plume comprises the process gas and at leastpartially molten thermal barrier material and thus provide a first layer 12c ofthe coating structure 11; v) optionally cooling the first layer 12c; vi) optionally providing a layer 12b of bonding agent on the first layer 12c; vii) feeding a process gas and a metallic material to a thermal spray device 50,and operating the thermal spray device to form a coating spray plume whichis prope||ed towards the surface of the piston body 12, wherein the coatingspray plume comprises the process gas and at least partially molten metallic material and thus provides a second layer 12d of the coating structure 11. lt should be understood that the above description of embodiments has been made in orderto exemplify the invention, and that alternative solutions will be obvious for a person skilled inthe art, however without departing from the scope of the invention as defined in the appended claims supported by the description and the drawings.

Claims (1)

1. CLAll\/IS Piston body (12) comprising a coating structure (11) on a surface (13) adapted to be on the side to be exposed to a combustion gas produced in an internal combustion engine (2), characterized in that the coating structure (11) comprises a first layer (12c) of plasma sprayed thermal barrier material comprising a ceramic material and an outermost second layer (12d) of plasma sprayed metallic material. Process for producing a coating structure (11) on a surface of a piston body (12), the surface (13) being adapted to be on the side to be exposed to a combustion gas produced in an internal combustion engine (2), the coating structure (11) comprising a first layer (12c) of a ceramic thermal barrier material and an outermost second layer (12d) of metallic material, the process comprising the steps of: vi) vii) providing a piston body (12); optionally providing a layer (12a) of a bonding agent on the surface of thepiston body (12); providing a thermal spray device (50) arranged to spray the first layer (12c) ofa thermal barrier material comprising a ceramic material to the surface of thepiston body (12) or on the layer (12a) of the bonding agent; feeding a process gas and the thermal barrier material to the thermal spraydevice (50) and operating the thermal spray device so as to form a coatingspray plume which is propelled towards the surface of the piston body,wherein the coating spray plume comprises the process gas and at leastpartially molten thermal barrier material and thus provide a first layer (12c)of the coating structure (11); optionally cooling the first layer (12c); optionally providing a layer (12b) of a bonding agent on the first layer (12c);feeding a process gas and a metallic material to a thermal spray device (50),and operating the thermal spray device to form a coating spray plume whichis propelled towards the surface of the piston body, wherein the coatingspray plume comprises the process gas and at least partially molten metallic material and thus provides a second layer (12d) of the coating structure (11). 10. 11. 12. 13. 16 The piston body according to claim 1 or the process according to claim 2, whereinthe thermal barrier material additionally comprises metallic particles. The piston body according to claim 1 or the process according to claim 2 or 3,wherein the ceramic particles comprise metallic oxide particles of zirconium oxideoptionally stabilized with yttrium or alumina-zirconium composite particles. The piston body according to claim 1 or the process according to any of claims 2 to 4,wherein the metallic material in the second outermost layer (12d) comprises orconsists of an aluminium alloy, chromium alloy, nickel-containing alloy or a nickel-chromium-containing alloy. The piston body according to claim 1 or the process according to any of claims 2 to 5,wherein the thickness of the first layer (12c) of the coating structure (11) is from 50to 1200 um. The piston body according to claim 1 or the process according to any of claims 2 to 6,wherein the thickness of the second layer (12d) of the coating structure (11) is from10 to 800 pm. The process according to any of claims 2 to 7, wherein the thickness of the layer(12a, 12b) of bonding agent obtained in steps ii) or vi) is from 10 to 500 pm. The process according to any one of the preceding claims, wherein the thermal spraydevice (50) is a plasma spray device. Piston body (12) for an internal combustion engine (2) adapted to be arranged in acylinder block (8) of the internal combustion engine (2) such that the piston togetherwith a cylinder head (14) and a cylinder wall (10) delimits a combustion chamber(16), characterized in that a surface (13) of the piston body that is exposed to acombustion gas produced in the internal combustion engine (2) comprises a coatingstructure as produced by the process according to any one of claims 2 to 9. The piston body of any of claims 10, wherein the piston body (12) comprises curvedconvex and concave portions. An internal combustion engine (2) comprising the piston body (12) according to anyone of claims 10 or 11. A vehicle (1) comprising an internal combustion engine (2) according to claim 12.