A METHOD FOR UNIFORM COATING OF A CERAMIC OXIDE FILM ON A PISTON

FIELD OF THE INVENTION

[001] The present invention generally relates to a method for uniform coating of a ceramic oxide film on a piston to improve a temperature stability, a wear resistance and fatigue resistance characteristics, and more particularly, but not exclusively, to the piston made from a chemical composition of Al-Si alloy that is partially, or wholly, coated with a ceramic oxide film to withstand high surface temperature.

BACKGROUND OF THE INVENTION

[002] A piston used in an internal combustion engine must satisfy several requirements to work properly. In an internal combustion engine, the piston acts as the movable end of a combustion chamber and convert a thermal energy into a mechanical energy. Hence, the piston must withstand an extreme surface temperatures associated with combustion chamber in the internal combustion engine. More particularly, the piston must be able to substantially maintain its shape after repeated exposure to extreme surface temperature and pressure. In other words, an ideal piston should exhibit limited "creep" or distortion. Creep is a measure of how much a particular material distorts or moves (i.e. creeps) when exposed to intense heat and pressure, without returning to its original position after the heat and pressure are removed. Other desirable characteristics for a piston include good hardness and wear resistant properties.

[003] Al and Al alloys are extremely desirable metals for manufacturing the piston because they are relatively inexpensive and have relatively low densities. However, Al and Al alloys have the drawbacks of being relatively soft and do not resist wear and abrasion very well. To overcome this drawback, the surface of Al and al alloys based piston is coated with a ceramic oxide film. The ceramic oxide film improves the wear resistance of the piston from abrasion and is to protect the surface of the piston from thermal degradation, oxidation and/or corrosion. Traditionally, hard anodizing process is used obtain ceramic oxide film on a surface of AI or Al-alloys based piston. The ceramic oxide film achieved through hard anodizing process is more porous and weekly adherent to substrate (i.e.) the bonding between ceramic oxide film and the surface of the Al or Al alloy based piston is easy to break. Later, a Micro Arc oxidation (MAO) process is used to form a ceramic oxide film on the surface of Al or Al alloy based piston. The MAO process results in good bonding efficiency between the surface of Al or Al alloy based piston and the ceramic oxide film.

[004] In an internal combustion engine, it is necessary to manufacture the piston with a protective coating or film, preferably ceramic oxide film on a top face and a bowl section of the piston. Hence, the temperature in the top face and bowl section of the piston gets increase in proportions to the performance of the engine. Similarly, the thickness of the protective coating or film should be uniform in the top face and the bowl section to withstand high temperature. In typical MAO process, the coating thickness at the top face and the bowl section of the piston cannot be maintained uniformly because the top face and the bowl section are having different topography. In addition, the thin porous outer layer is preferred in the ceramic oxide film. The thin porous outer layer provides high impendence to heat flow and accommodates more thermal stress thus results in a low thermal conductivity.

[005] Accordingly there is a need in the art to provide a solution to one or more of above said problems. The present invention solves one or more of these problems in a unique and economical manner.

SUMMARY OF THE INVENTION

[006] It is a feature of the present invention to provide a system and a method which substantially overcomes the one or more of the above mentioned disadvantages.

[007] It is the principal object of the present embodiment is relates to a method for coating a ceramic oxide film on a piston to withstand high surface temperature and to improve wear and fatigue resistance.

[008] Another object of the present embodiment is relates to the method for coating the ceramic oxide film with uniform thickness in a top face and bowl section of the piston.

[009] The present embodiment herein provides a method for coating the piston with a ceramic oxide film. The method includes (i) manufacturing the piston using chemical composition of Al-Si alloy and (ii) forming a ceramic oxide film on a predetermined portion of the piston by a Micro Arc Oxidation (MAO) coating process. The chemical composition of Al-Si alloy includes AI, Si and at least one material selected from Fe, Cu, Mn, Mg, Zn, Ni, Pb, Sn, and Ti. The piston is capable of withstanding high surface temperature in the range from 390 C to 500 C. The MAO coating process is processed for 40 to 80 minutes to form said ceramic oxide film with a uniform thickness from 75 to 150 microns. The thickness of the ceramic oxide film includes 80 % of said dense inner layer and 20 % of said porous outer layer. The predetermined portion includes a bowl and top face of the piston, and the ceramic oxide film formed on the bowl first and followed by the top face of the piston. The MAO process is carried out in two phases, in first phase, the piston is masked with a jig and a top cover to expose the bowl section of the piston. The bowl section is coated with the ceramic oxide film in the first phase. In the second phase, the top cover is removed to expose the top face of the piston. The piston with an exposed top face is processed under MAO process to coat the ceramic oxide film in top face of the piston.

BRIEF DISCRETION OF THE ACCOMPANYING DRAWINGS

[0010] The advantages and features of the invention will become more clearly apparent from the following description which refers to the accompanying drawings given as non-restrictive examples only and in which:

[0011] Figures 1A & IB illustrate a piston which is made using a chemical composition of Al-Si alloy in accordance to one embodiment herein;

[0012] Figures 2A through 2D illustrate a method for forming a ceramic oxide film on the piston of Figure 1 in accordance to one embodiment herein;

[0013] Figure 3 illustrates an outline of a cross sectional view of the piston with the ceramic oxide film having thick inner dense layer and thin outer porous layer in accordance to one embodiment herein; and

[0014] Figure 4 is a flow diagram that illustrates a method for manufacturing the piston of Figure 1 and a method for coating the ceramic oxide film on the piston in accordance to one embodiment herein.

DETAILED DISCRETION OF THE INVENTION

[0015] The present invention will be described herein below with reference to the accompanying drawings. A method for manufacturing an article with high temperature stability is described.

[0016] The following description is of exemplary embodiment of the invention only, and is not limit the scope, applicability or configuration of the invention. Rather, the following description is intended to provide a convenient illustration for x implementing various embodiments of the invention. As will become apparent, various changes may be made in the function and arrangement of the structural/operational features described in these embodiments without departing from the scope of the invention as set forth herein. It should be appreciated that the description herein may be adapted to be employed with alternatively configured devices having different shaped, components, and the like and still fall within the scope of the present invention. Thus the detailed description herein is presented for purposes of illustration only and not of limitation.

[0017] The present embodiment provides a method for manufacturing the article having high temperature stability, and improved wear resistance and fatigue strength, and more particularly, the embodiment provides the article made from a chemical composition of Al-Si alloy that is partially, or wholly, coated with ceramic oxide film. The article is referred herein and exemplified as a piston in the figures. However, the article may comprise any device capable of operating similar to piston.

[0018] Figures 1A & IB illustrate a piston 100 which is made using a chemical composition of Al-Si alloy in accordance to one embodiment herein. The piston 100 includes a top face 102, a bowl section 104, a ring grooves portion 106, a skirt portion 108, and a pin insert 110. Below is Table I which shows chemical composition of Al-Si alloy that can be used for manufacturing the piston 100. The table should in no way be construed as limiting.

TABLE I

PERCENTAGE BY WEIGHT OF COMPONENTS

[0019] Figures 2A through 2D illustrate a method for coating a ceramic oxide film on the piston 100 of Figure 1 in accordance to one embodiment herein. The ceramic oxide film coated over the predetermined portions i.e. the top face 102 and the bowl section 104 of the piston 100 to improve the temperature stability as well as other characteristics such as wear and fatigue resistance. Figure 2A illustrates the piston 100 inserted with a pin 202 in the pin insert 110. The pin 202 includes a pin hole 204. Figure 2B illustrates a perspective view of a jig 206 (e.g. first jig) which is configured or modeled or designed to cover the piston 100 with exposed top face 102 and bowl section 104. The jig 206 is made using nonmetallic, nonconductive and non-reactive material preferably nylon is used for making the jig 206.

[0020] The jig 206 includes a two half pieces 208A & 208B and each of the two half pieces 208A & 208B include a first connection arrangement 210, a second connection arrangement 212 and a third connection arrangement 214. The first connection arrangement 210 is provided in such a manner, to match the pin hole 204 of the pin 202 inserted in the pin insert 110. The second connection arrangement 212 and the third connection arrangement 214 are provided to tighten the jig 206 with the piston 100.

[0021] The coating of ceramic oxide film on the top face 102 and bowl section 104 of the piston 100 is carried out in two phases. In the first phase, the work piece 216 is constructed by enclosing the piston 100 with the two half pieces 208A & 208B of the jig 206 and a top cover 218 (as shown in Figure 2C). The top cover 218 (e.g. second jig) is modeled or designed to cover the top face 102 of the piston 100. In one embodiment, the top cover 218 is made of nylon material. In another embodiment, the top cover 218 is a part of the jig 206. The jig 206 and the top cover 218 are tightened with the piston 100 using a bolt through the second connection arrangement 212 and the third connection arrangement 214 provided in the jig 206. The electrical connection is given through the first connection arrangement 210 and the pin hole 204. The first work piece 216 includes the piston 100 and the jig 206 with exposed bowl section 104. The first work piece 206 and a rectangle shape aluminum piece are immersed fully in a reaction chamber of the typical MAO process arrangement. The first work piece 216 is connected to anode and the rectangle shape aluminum piece is connected to cathode poles of AC power supply. The coating characteristic such as desirable uniform thickness and the properties of thickness are controlled and/or adjustable by performing the MAO process under controlled parameters. The controlled parameters include a distance between two bodies in the reaction chamber, a current density, an electrical power supply, a reaction chamber temperature etc.

[0022] In the present embodiment, the distance between the first work piece 216 and rectangle shape aluminum piece are maintained at 65 mm in the reaction chamber throughout the MAO process. The constant current density of 0.3 A/cm2 is maintained throughout the MAO process. The electrical power supply is gradually increased from IV throughout the process until the final voltage 450V at the end of the MAO process. The temperature of the reaction chamber was maintained 10-20 degree C during the process by using chiller or heat exchanger. The MAO process is continued for 40 minutes to form the ceramic oxide film thickness of 75 microns. In this first phase, the bowl section 104 alone is coated with ceramic oxide film, since the other portions of the piston 100 is covered or masked with the jig 206 and the top cover 218. At the end of the process, the electrical power was switched off and the first work piece 216 was taken out and cleaned with fresh water and dried in warm air.

[0023] In the second phase, the top cover 218 is removed or detached from the first work piece 216 to obtain a second work piece 220 (as shown in Figure 2D). The second work piece 220 and rectangle shape aluminum piece are immersed again in the reaction chamber and the MAO process with the above controlled parameters is processed for 40 minutes. In this second phase, the top face 102 is coated by the ceramic oxide film with the thickness of 75 microns. The second work piece 220 is taken out and cleaned with fresh water and dried in warm air.

[0024] Figure 3 illustrates an outline of a cross sectional view 300 of the piston 100 with the ceramic oxide film having a thick inner dense layer 302 (e.g. inner dense and a thin outer porous layer 304 in accordance to one embodiment herein. The thickness of the ceramic oxide film in the piston 100 after performing MAO process, was observed that 58-60 microns dense (80% of coating thickness) and 14.5-15 microns porous in nature (20% of coating thickness) with a 3-6 number of pores per Square cm area and an average pore diameter of 6-11 microns approximately. The hardness of the ceramic oxide film was observed that 800-950HV0.2. The piston 100 with the ceramic oxide film is capable of withstanding the surface temperature of 500 C. The thickness of the ceramic oxide film can be increased up to 150 microns by processing the MAO process for 80 minutes in each of the two phases.

[0025] Figure 4 is a flow diagram 400 that illustrates a method for manufacturing the piston 100 of Figure 1 and a method for coating a ceramic oxide film on the piston 100 in accordance to one embodiment herein. In step 402, the piston is manufactured using the chemical composition of Al-Si alloy mentioned in table I. In step 404, a first work piece is obtained by enclosing the piston 100 with a jig and a top cover. The bowl section 104 of the piston 100 is exposed in the first work piece. In step 406, the MAO coating process is performed by immersing the first work piece and an aluminum piece in the reaction chamber to coat the bowl section 104 with the ceramic oxide film. In step 408, the first work piece is taken out and the top cover is removed to obtain a second work piece. In step 410, the MAO process is performed again by immersing the second work piece and the aluminum piece in the reaction chamber to coat the top face 102 with the ceramic oxide film.

[0026] The present embodiment facilitates to provide the piston with the ceramic oxide film on the top face 102 and the bowl section 104 of the piston 100 to withstand high temperature. Further the present embodiment the piston 100 has improved wear and fatigue resistance due to thin porous outer surface and a thick dense inner surface of the ceramic oxide film.

[0027] A rig test for evaluating a thermal shock (thermal resistance) of the piston ring was performed on the MAO coated piston 100 and a typical hard anodizing (HA) piston. The rig test was conducted in both hot and cold condition for 25 cycles. The test was conducted for 11 minutes for each cycle i.e. 6.5 minutes for each hot and cold condition. The piston skirt temperature kept as 220 degree C under hot condition (the temperature at the bowl section 104 can be more than 500 degree C) and 17 degree C under cold condition. The die penetration test was conducted after each cycle for detecting the crack in the piston. The test results showed that, there is no crack detected in the bowl section 104 of the MAO coated piston 100 after 25 cycles. However, the crack observed in the bowl section of the typical HA piston after 2-3 cycles. Thus, according to the embodiment, the MAO coated piston 100 showed superior degree of thermal resistance in comparison with the typical HA piston. Hence, the MAO coated piston 100 has better life than HA piston.

[0028] Several exemplary embodiments have thus been described. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiments be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

We claim:

1. A method for uniform coating of a ceramic oxide film on a piston, wherein said method comprising the steps of, forming said ceramic oxide film on a predetermined portion of said piston by a Micro Arc Oxidation (MAO) coating process, wherein said piston is made of using chemical composition of Al-Si alloy, said ceramic oxide film comprises a porous outer layer and a dense inner layer, and said chemical composition of Al-Si alloy comprises: Si, at a weight percentage from 7.5% to 15%; Fe, at a weight percentage from 0.1% to 0.9%; Cu, at a weight percentage from 1.5% to 4.5%; Mn, at a weight percentage maximum of 0.5%; Mg, at a weight percentage from 0.5% to 2.0%; Zn, at a weight percentage maximum of 0.5%; Ni, at a weight percentage from 0.5% to 3.5%; Pb, at a weight percentage maximum of 0.5%; Sn, at a weight percentage maximum of 0.5%; Ti, at a weight percentage maximum of 0.5%; and Al, at a remaining weight percentage.

2. The method as claimed in claim 1, wherein said MAO coating process is processed for 40 to 80 minutes to form said ceramic oxide film with an uniform thickness from 75 to 150 microns, wherein a current density for said MAO coating process is selected as 0.3 A/cm2.

3. The method as claimed in claim 2, wherein said uniform thickness of said ceramic oxide film comprises 80 % of said dense inner layer and 20 % of said porous outer layer.

4. The method as claimed in claim 1, wherein said porous outer layer comprises 3 to 6 pores per Sq. cm area with an average pore diameter of 6 to 11 microns.

5. The method as claimed in claim 1, wherein said predetermined portion comprises a bowl section and top face of said piston, and said ceramic oxide film formed on said bowl first and followed by said top face of said piston.

6. The method as claimed in claim 1, comprising: modeling a plurality of jigs to mask said piston expect said predetermined portion, and enclosing said piston with said plurality of jigs before forming said ceramic oxide film by said MAO coating process.

7. A method for uniform coating of a ceramic oxide film on a predetermined portion of an article, wherein said uniform coating comprising a dense inner layer and a porous outer layer on, said method comprising the steps of, forming said ceramic oxide film on said predetermined portion of said article by a Micro Arc Oxidation (MAO) coating process for 40 to 80 minutes to form said ceramic oxide film with an uniform thickness ranging from 75 to 150 microns, wherein said uniform thickness of said ceramic oxide film comprises 80 % of said dense inner layer and 20 % of said porous outer layer, said article is made of using a chemical composition of Al-Si alloy, and said chemical composition of Al-Si alloy comprises Al, Si and at least one material selected from Fe, Cu, Mn, Mg, Zn, Ni, Pb, Sn, Ti.

8. The method as claimed in claim 7, wherein said article comprises a piston, and said predetermined portion comprises a bowl section and a top face of said piston, and said ceramic oxide film formed on said bowl first and followed by said top face of said piston.

9. The method as claimed in claim 8, comprising: modeling a first jig to mask said piston expect said top face and said bowl section; modeling a second jig to mask to cover said top face of said piston; enclosing said article by said first jig and said second jig for exposing said bowl section; forming said ceramic oxide film on said bowl section by said MAO coating process; detaching said second jig from said piston to expose said top face; and forming said ceramic oxide film on said top face by said MAO coating process.

10. A system configured to perform the method claimed in claims 1-9.