Piston of internal combustion engine

A piston for an internal combustion engine includes a piston base material including a pair of skirt portions opposed to each other in a radial direction of the piston base material. The piston includes a multiple-layer coating formed on at least one of surfaces of the pair of skirt portions. The piston further includes marks provided to respective layers of the multiple-layer coating at locations different from each other.

BACKGROUND OF THE INVENTION

The present invention relates to a piston of internal combustion engine, on which a multiple-layer coating is formed.

U.S. Patent Application Publication No. 2008/0060603 corresponding to Japanese Patent Application Publication No. 2008-56750 (hereinafter referred to as, patent document 1) discloses a previously-proposed piston of internal combustion engine for an automobile, as one method of improving an abrasion resistance or a seizing resistance of the piston.

In this technique, a surface of piston base material is coated by a lower-layer coating composite, and a surface of the lower-layer coating composite is coated by an upper-layer coating composite. The lower-layer coating composite includes an epoxy resin and a polyamide-imide resin as binding resins, and a polytetrafluoroethylene and a molybdenum disulfide as solid lubricants. The upper-layer coating composite includes the epoxy resin and the polyamide-imide resin as the binding resins, a boron nitride as the solid lubricant, and a silicon nitride and an alumina as hard particles. That is, a double-layer coating composite is formed in order to attain a superior abrasion resistance and to improve the seizing resistance and an initial fitting property.

SUMMARY OF THE INVENTION

However, in the technique disclosed by the patent document 1, the upper and lower layers of the double-layer coating composite are simply in an overcoated state. Hence, it cannot be recognized whether or not the upper and lower layers of the double-layer coating composite have been properly formed, from an outward appearance of the piston. Therefore, there is a risk that a piston including only single layer of coating is distributed as a piston product by mistake, so that a reliability of piston product becomes low.

It is an object of the present invention to provide a piston of an internal combustion engine, devised to enable to determine whether or not a predetermined multiple-layer coating has been formed on an outer surface of the piston, from an outer appearance of the piston.

According to one aspect of the present invention, there is provided a piston for an internal combustion engine, comprising: a piston base material including a pair of skirt portions opposed to each other in a radial direction of the piston base material; a multiple-layer coating formed on at least one of surfaces of the pair of skirt portions; and marks provided to respective layers of the multiple-layer coating at locations different from each other.

According to another aspect of the present invention, there is provided a piston for an internal combustion engine, comprising: a piston base material including a pair of skirt portions opposed to each other in a radial direction of the piston base material; and a multiple-layer coating formed on at least one of surfaces of the pair of skirt portions, wherein an upper layer of the multiple-layer coating is formed with a window portion, and a surface of the piston base material or a lower layer of the multiple-layer coating is exposed through the window portion.

According to still another aspect of the present invention, there is provided a piston for an internal combustion engine, comprising: a piston base material including a pair of skirt portions opposed to each other in a radial direction of the piston base material; a multiple-layer coating formed on at least one of surfaces of the pair of skirt portions, the multiple-layer coating containing a solid lubricant; and single-layer mark coatings provided at locations which are different from each other and which are away from the multiple-layer coating through a non-coated portion between the multiple-layer coating and each of the single-layer mark coatings.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of a piston of internal combustion engine according to the present invention will be explained in detail referring to the drawings. In the following embodiments, the piston is applied to a four-cycle gasoline engine.

First Embodiment

As shown inFIG. 2, a cylinder block2includes a cylinder wall-surface3formed substantially in a circularly cylindrical shape. The piston1is provided to be able to slide in contact with the cylinder wall-surface3. The piston1cooperates with the cylinder wall-surface3and a cylinder head (not shown) to define a combustion chamber4. The piston1is connected to a crankshaft (not shown) through a con-rod6. The con-rod6is connected with a piston pin5.

Whole of the piston1is integrally molded by an Al—Si-series aluminum alloy in AC8A (JIS: Japanese Industrial Standards). As shown inFIGS. 1,2and6A-6C, the piston1is formed approximately in a circularly cylindrical shape. The piston1includes a crown portion7, a pair of thrust-side skirt portion8and counter-thrust-side skirt portion9, and a pair of apron portions11and12. The crown portion7includes a crown surface7aon which the combustion chamber4is defined. The pair of thrust-side skirt portion8and counter-thrust-side skirt portion9are provided integrally on an outer circumferential edge of a lower end of the crown portion7. Each of the pair of thrust-side skirt portion8and counter-thrust-side skirt portion9is formed in a circular-arc shape in cross section. The pair of apron portions11and12are connected to circumferential both ends of the pair of skirt portions8and9through linking portions10.

The crown portion7is formed to be relatively thick and formed in a disc shape. Valve recesses7eand7fare formed in the crown surface7aof the crown portion7. Each of the valve recesses7eand7ffunctions to prevent an interference with an intake or exhaust valve. Three ring grooves7b,7cand7dare formed in an outer circumferential portion of the crown portion7. The three ring grooves7b,7cand7dhold three piston rings such as a pressure ring and an oil ring.

The both skirt portions8and9are located symmetrically with respect to an axis (a center line parallel to a piston moving direction) of the piston1, and are shaped like arc in cross section. In other words, the both skirt portions8and9are formed to be opposed to each other in a radial direction of the piston1. Almost whole of the both skirt portions8and9is formed to be relatively thin. When the piston1moves toward its bottom dead center at the time of expansion stroke and the like, the thrust-side skirt portion8is inclined to the cylinder wall-surface3to become in press-contact with the cylinder wall-surface3in relation to an angle of the con-rod6. On the other hand, when the piston1rises at the time of compression stroke and the like, the counter-thrust-side skirt portion9is inclined to the cylinder wall-surface3to become in press-contact with the cylinder wall-surface3in a counter direction. A load of this press contact of the thrust-side skirt portion8against the cylinder wall-surface3is larger than that of the counter-thrust-side skirt portion9against the cylinder wall-surface3because the thrust-side skirt portion8presses the cylinder wall-surface3by receiving a combustion pressure.

As shown inFIGS. 1 and 4A, a multiple-layer coating composite20has been applied to the thrust-side skirt portion8and the counter-thrust-side skirt portion9of the piston1. In this embodiment, the multiple-layer coating composite20has two (upper and lower) layers.

That is, the multiple-layer coating composite20includes an upper-layer coating composite21and a lower-layer coating composite22. The multiple-layer coating composite20is formed by using one or two selected from an epoxy resin, a polyimide resin and a polyamide-imide resin (PAI) which are superior in heat resistance, abrasion resistance (wear resistance) and adhesion property, as binding resins.

Specifically, the upper-layer coating composite21is set to include any one of the epoxy resin, the polyimide resin and the polyamide-imide resin (which are the binding resins) in a range from 5 to 50 wt %. Moreover, the upper-layer coating composite21is set to include a molybdenum disulfide (MoS2) in a range from 50 to 95 wt %, as a solid lubricant.

If the binding resin(s) accounts for a rate lower than 5 wt %, an adhesion between the upper-layer coating composite21and the lower-layer coating composite22is reduced due to a reduction of binding force. On the contrary, if the binding resin(s) accounts for a rate higher than 50 wt %, the solid lubricant is relatively decreased so that an initial fitting property (initial compatibility) is reduced.

The lower-layer coating composite22is set to include any one of the epoxy resin, the polyimide resin and the polyamide-imide resin (which are the binding resins) same as the upper-layer coating composite21, in a range higher than or equal to 50 wt %. Moreover, the lower-layer coating composite22is set to basically include one or more of a polytetrafluoroethylene (PTFE), the molybdenum disulfide (MoS2) and a graphite (GF), in a range lower than or equal to 50 wt % as the solid lubricant. The lower-layer coating composite22does not necessarily need to include the solid lubricant.

If the binding resin(s) accounts for a rate lower than 50 wt % in the lower-layer coating composite22, an adhesion between the lower-layer coating composite22and a piston base material (base member)1ais reduced. In a case that each of the solid lubricants is increasingly added to the binding resin PAI as shown inFIG. 5, an adhesive force is rapidly reduced when the solid lubricant exceeds 50 wt %, i.e., when the binding resin becomes lower than 50 wt %.

That is, the lower-layer coating composite22functions to secure the adhesion between the lower-layer coating composite22and the piston base material1a, and to secure the adhesion between the upper-layer coating composite21and the lower-layer coating composite22.

Accordingly, although the lower-layer coating composite22does not need to contain the solid lubricant, the solid lubricant(s) may be added to the lower-layer coating composite22within a rate capable of securing these adhesions, in order to improve a characteristic of the coating. If the polytetrafluoroethylene is lower than 15 wt % in the lower-layer coating composite22, a lubricity is reduced. On the other hand, if the polytetrafluoroethylene is higher than 30 wt %, an abrasion amount is increased.

Moreover, if the molybdenum disulfide as the solid lubricant accounts for a rate lower than 5 wt % in the lower-layer coating composite22, a seizing resistance is reduced. On the other hand, if the molybdenum disulfide accounts for a rate higher than 20 wt % in the lower-layer coating composite22, the abrasion amount is increased due to a reduction of strength of the coating.

Moreover, an improvement of the seizing resistance can be achieved by a synergistic effect of combination between the molybdenum disulfide and the graphite given as the solid lubricants.

That is, the lower-layer coating composite22can be formed by using the molybdenum disulfide and the graphite in addition to the polytetrafluoroethylene as the solid lubricants. In this case, it is preferable that a total rate of the molybdenum disulfide and the graphite ranges from 5 to 20 wt %, and a rate of the molybdenum disulfide ranges from 1 to 10 wt %.

This is because the above-mentioned improvement effect of seizing resistance by the synergistic effect cannot be obtained if the molybdenum disulfide is lower than 1 wt %, and the abrasion resistance is reduced if the molybdenum disulfide is higher than 10 wt %.

Moreover, a reason to set a content (contained amount) of the molybdenum disulfide and the like functioning as the solid lubricants of the upper-layer coating composite21in the range from 50 to 95 wt % is as follows. That is, from an experimental result as shown inFIG. 3A, the initial fitting property is reduced if the content of the solid lubricant(s) is lower than 50 wt %. On the other hand, if the content of the solid lubricant(s) is higher than 95 wt %, a content of the binding resin becomes lower than 5 wt %, so that the adhesion between the upper-layer coating composite21and the lower-layer coating composite22is reduced due to the reduction of binding force as mentioned above.

A method of adjusting the upper-layer coating composite21and the lower-layer coating composite22which constitute the multiple-layer coating composite20is as follows, for example. An organic solvent is mixed with the epoxy resin, the polyimide resin and the polyamide-imide resin which are the biding resins. Then, the solid lubricant(s) is added to this resin solution. Further, as needed basis, hard particles are added to this resin solution. Then, this solution is mixed and dispersed by use of a beads-mill or the like.

A total mixture amount of the hard particles, the binding resin and the solid lubricant(s) such as PTFE, MoS2and GF is equal to 100 wt % of the upper-layer coating composite21or the lower-layer coating composite22.

The upper-layer coating composite21and the lower-layer coating composite22which constitute the multiple-layer coating composite20according to the present invention are diluted by organic solvent, as needed basis. Then, the upper-layer coating composite21and the lower-layer coating composite22are applied to the piston base material1a, as a coating material.

That is, the lower-layer coating composite22and the upper-layer coating composite21are applied to an outer circumferential surface of (the thrust-side skirt portion8and the counter-thrust-side skirt portion9of) the piston base material1a, in this order. Then, the applied lower-layer coating composite22and upper-layer coating composite21are burned and cured to obtain the multiple-layer coating composite20.

The organic solvent which is used for the above-mentioned dilution has only to be able to dissolve the binding resin. That is, the organic solvent which is used in this embodiment is not limited to specified solvents.

A burning condition such as a burning temperature and a burning time is appropriately set. The lower-layer coating composite22and the upper-layer coating composite21can be properly burned even at a temperature lower than 200° C., and therefore, is applicable also to an aluminum-alloy base material of the piston1.

A film thickness (coating thickness) of the multiple-layer coating composite20can be appropriately selected. However, it is preferable that the film thickness of the multiple-layer coating composite20falls within a range from 5 to 40 μm, in consideration of an applying workability of the coating composite20and a cost for the coating composite20and the like.

A concrete method for applying the multiple-layer coating composite20on the surface of the piston base material1awill now be explained.

First Surface-Treatment Method

At first, oil and dirt of the surface of the piston base material1aare removed by a pretreatment such as a solvent degreasing and an alkaline degreasing.

Next, the lower-layer coating composite22is applied to (putted on) the surface of the piston base material1aby a known method such as an air spray and a screen printing. Subsequently, the upper-layer coating composite21is applied to (putted on) an upper surface of the lower-layer coating composite22.

Subsequently, the organic solvent is removed by drying. Then, the applied upper-layer coating composite21and lower-layer coating composite22are burned under a known condition, for example, for thirty minutes at 180° C. or for twenty minutes at 200° C. Thereby, the multiple-layer coating composite20constituted by the upper-layer coating composite21and the lower-layer coating composite22is formed.

Second Surface-Treatment Method

Another surface-treatment method is as follows. At first, oil and dirt are removed from the surface of the piston base material1aon which the multiple-layer coating composite20should be formed, by a pretreatment such as the solvent degreasing and the alkaline degreasing.

Then, the lower-layer coating composite22is applied to the surface of the piston base material1aby a known method such as the air spray and the screen printing. Then, the applied lower-layer coating composite22is burned under a known condition, for example, for thirty minutes at 180° C. or for twenty minutes at 200° C.

Subsequently, the piston base material1ais drawn (pulled) out from a burning furnace. Then, the upper-layer coating composite21is applied to an upper surface of the lower-layer coating composite22of the piston base material1aunder a condition where the piston base material1ahas a temperature ranging from 50 to 120° C. Subsequently, the piston base material1ais dried without burning, so that the multiple-layer coating composite20constituted by the upper-layer coating composite21and the lower-layer coating composite22is formed.

The multiple-layer coating composite according to the present invention is widely applicable to various sliding members for various intended uses under an environment of oil lubrication and an environment of dry lubrication. The epoxy resin, the polyimide resin and the polyamide-imide resin which are the biding resins for the multiple-layer coating composite are superior in adhesion property. Hence, these epoxy resin, polyimide resin and polyamide-imide resin are applicable to various materials (each constituting the base material) such as cast iron, steel and copper alloy in addition to various kinds of aluminum alloy materials, without being limited to specified kinds of base materials. In particular, it is favorable that the epoxy resin, the polyimide resin and the polyamide-imide resin are used for the piston1of internal combustion engine, especially, for the thrust-side skirt portion8and the counter-thrust-side skirt portion9of the piston1as in this embodiment.

Experimental Example

The following formulas are satisfied as to the piston1having a notch shape (track recess) in an outer circumferential surface of the piston1.
t2≧a−5 (μm)
t1≧2 (μm)

Wherein a denotes a height (depth) of the notch, t1denotes a film thickness of the lower-layer coating composite22, and t2denotes a film thickness of the upper-layer coating composite21. As the binding resins, the polyamide-imide resin (PAI) was used. The content (contained amount) of each of the graphite (GF), the molybdenum disulfide (MoS2) and the polytetrafluoroethylene (PTFE) given as the solid lubricants was varied from 0 wt % to 95 wt %.

The upper-layer coating composite and the lower-layer coating composite were adjusted as shown in the following table 1.

The organic solvent was added to and mixed with the upper-layer and lower-layer coating composites for so each of the samples No. 1 to No. 59 except the sample No. 21. Then, each of the mixed upper-layer and lower-layer coating composites of the samples No. 1 to No. 59 was dispersed by the beads-mill for thirty minutes, so that upper-layer coating paint and lower-layer coating paint for each of the samples No. 1 to No. 59 were obtained.

The lower-layer coating paint for each sample was applied on a test piece1ahaving a surface shape shown inFIGS. 4A-4Cwhich was made of aluminum alloy in AC8A, so as to cause the entire lower-layer coating to have a film thickness ranging from 3 to 6 μm. Then, the lower-layer coating paint applied on the test piece1awas burned for 30 minutes at 190° C.

Subsequently, the upper-layer coating paint for each sample was applied on (the lower-layer coating of) the test piece1aso as to cause the entire upper-layer coating to have a film thickness ranging from 5 to 11 μm. Then, the upper-layer coating paint applied on the test piece1awas dried by air drying without the burning.

Alternatively, the lower-layer coating paint applied on the test piece1amay be dried by forced drying. In this case, the upper-layer coating paint applied on (the lower-layer coating of) the test piece1ais burned for 30 minutes at 190° C.

As to the obtained samples No. 1 to 20 which had undergone the double-layer surface treatment and as to the sample No. 21 which had undergone no surface treatment, friction coefficients were measured by a chip-on-ring-type friction and abrasion tester under a lubricating environment where a slip speed is equal to 2 m/sec (meters per second), a contact material1aFC250 (JIS), a slip distance is equal to 600 m, a surface pressure is equal to 1.3 MPa, and a drip amount of engine oil is equal to 5 mg/min (milligrams per minute).

FIG. 3Bis a graph showing that result, i.e., a relation between the friction coefficient and the notch height. As is clear fromFIG. 3B, the friction coefficient is determined uniquely by the notch height. The friction coefficient becomes a lowest constant value when the notch height is lower than or equal to 5 μm.

That is, in order to obtain a low friction coefficient, it is effective to quickly abrade (wear) the upper-layer coating. Therefore, as is clear fromFIG. 3A, the upper-layer coating composite21is easy to abrade when the upper-layer coating composite21contains the solid lubricant at a rate higher than or equal to 50 wt %. In particular, this is most effective in the case that the upper-layer coating composite21contains the molybdenum disulfide (MoS2) as the solid lubricant. On the other hand, this becomes less effective in the case that the upper-layer coating composite21contains the graphite (GF) as the solid lubricant, and in the case that the upper-layer coating composite21contains the polytetrafluoroethylene (PTFE) as the solid lubricant.

FIG. 4Ashows the first embodiment according to the present invention. In the case shown byFIG. 4A, the upper-layer coating composite21was formed to have a composition easy to abrade. In this case, the notch height is changed from C0of original state to c1, by the sliding. On the other hand,FIG. 4Cshows a case where the surface treatment has not been done. In this case, the notch height was changed from a0of original state to a1, by the sliding. Since it is apparent that the aluminum alloy is more difficult to abrade than the coating constituted by the solid lubricant(s) and the binding resin, a relation of a1>c1is satisfied.

Accordingly, the low friction can be obtained in this embodiment.FIG. 4Bshows an earlier technology. In this case, the notch height was changed from b0of original state to b1, by the sliding. The single coating of this earlier technology is easier to abrade than the aluminum alloy and more difficult to abrade than the upper-layer coating ofFIG. 4A. Hence, a relation of a1>b1>c1is satisfied. Therefore, in this embodiment according to the present invention, a lower friction than the case of earlier technology is attained.

For example, in the case ofFIG. 4C, the notch height was abraded by 1 μm and hence changed from a0=10 μm to a1=9 μm by the sliding. In this case, the friction coefficient was extremely large to have a rate of 162% (see sample No. 21), with respect to a friction-coefficient value in the case of upper-layer coating that contains the solid lubricant of 0 wt % (see sample No. 1). Contrary to this, in the case of upper-layer coating that contains the MoS2of 75 wt %, the notch height was abraded by 7 μm by the sliding and hence changed from c0=10 μm to c1=3 μm. In this case, the friction coefficient was extremely small to have a rate of 60% (see sample No. 6), with respect to the friction-coefficient value in the case of upper-layer coating that contains the solid lubricant of 0 wt % (see sample No. 1).

Moreover, as understood byFIG. 4B, in the case that the biding resin exceeds 50 wt % of the upper-layer coating as disclosed by the Patent Document 1, i.e., in the case that the solid lubricant is lower than 50 wt % of the upper-layer coating; the abrasion is not promoted. Therefore, in such cases (see samples No. 2, No. 3, No. 8, No. 9 and No. 20), the friction coefficient can be lower than in the case of no surface treatment, but is higher than in this embodiment according to the present invention.

If the lower-layer coating is set to contain the solid lubricant(s) accounting for 50 wt % or more of the lower-layer coating (see examples No. 22 to No. 40), the adhesion property between the lower-layer coating and the piston base material1ais reduced as shown inFIG. 5. Hence, the lower-layer coating that contains the solid lubricant(s) accounting for 50 wt % or more is inappropriate in terms of practical utility.

Therefore, the lower-layer coating in this embodiment according to the present invention is set to ensure the adhesion to the piston base material1aand also to ensure the adhesion to the upper-layer coating containing the solid lubrication of 50 wt % or more.

As the solid lubricant for the lower-layer coating containing the binding resin of PAI, each content of the molybdenum disulfide (MoS2), the graphite (GF) and the polytetrafluoroethylene (PTFE) was varied from 15 wt % through 30 wt %, 50 wt %, 60 wt % and 75 wt % to 95 wt % under a condition that the upper-layer coating was constituted by the binding resin of 5 wt % and the molybdenum disulfide of 95 wt % which does not secure the adhesion to the aluminum-alloy base material. The adhesive forces of these various samples (see samples No. 41 to No. 59) were measured. As a result, it is found that the adhesive force can be ensured by causing the lower-layer coating to contain the solid lubricant at a rate lower than or equal to 50 wt %, even if the upper-layer coating so composite which does not ensure the adhesion property is used.

As explained above, the adhesion property of the lower-layer coating composite22to the piston base material1asuperior in this embodiment. Moreover, is since the content (contained amount) of the molybdenum disulfide (MoS2) is set at the range from 50 wt % to 95 wt % as the solid lubricant of the upper-layer coating composite21, the initial fitting property (initial compatibility) is superior when the outer circumferential surfaces of the thrust-side skirt portion8and the counter-thrust-side skirt portion9of the piston1slide on the cylinder wall-surface3. That is, a surface of the upper-layer coating composite21is abraded in a short time to quickly form a smooth sliding surface of the upper-layer coating composite21, so that a superior initial fitting property can be obtained instantly.

In this embodiment, the single lower-layer coating composite22and the single upper-layer coating composite21are applied to both the skirt portions8and9of piston1in a double-layered state, as explained above. The piston1in this embodiment according to the present invention includes a means for checking (judging) whether or not these lower-layer coating composite22and upper-layer coating composite21have been applied without mistake. Both of the lower-layer coating composite22and the upper-layer coating composite21have a color close to black.

In more specifically, as shown inFIG. 6A, the lower-layer coating composite22is formed with a first window portion (not-coated portion)30located on each of the skirt portions8and9. That is, the first window portion30is provided substantially at a circumferentially center location of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9and at a lower location (counter-crown-side portion) of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9, when the lower-layer coating composite22is applied to the thrust-side skirt portion8and the counter-thrust-side skirt portion9. This first window portion30is formed substantially in a relatively-small square shape, and is provided only to the lower-layer coating composite22. It is noted that the first window portion30corresponds to a mark according to the present invention.

On the other hand, as shown inFIG. 6B, the upper-layer coating composite21is formed with a second window portion (not-coated portion)31located on each of the skirt portions8and9. That is, the second window portion31is provided substantially at a circumferentially center location of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9and at a upper location (crown-side portion) of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9, when the upper-layer coating composite21is applied to (the lower-layer coating composite22of) the thrust-side skirt portion8and the counter-thrust-side skirt portion9. This second window portion31is formed substantially in a relatively small square shape in the same manner as the first window portion30, and is provided only to the upper-layer coating composite21. A forming location of the second window portion31is shifted from a forming location of the first window portion30. That is, the second window portion31does not overlap with the first window portion30in the radial direction of the piston1(i.e., when viewed from a radially outer side of the piston1). It is noted that is the second window portion31corresponds to the mark according to the present invention.

Accordingly, for example, if the upper-layer coating composite21has not been applied by mistake after the lower-layer coating composite22was applied to the piston base material1a, the surface of the piston base material1ais exposed through the first window portion30as shown byFIG. 6A. In this case, since the piston base material1ais the aluminum alloy, the surface of the piston base material1ahas a color near silver. Hence, a working person can recognize and determine that only the lower-layer coating composite22has been applied, by visual perception.

For example, if the lower-layer coating composite22has not been applied by mistake although the upper-layer coating composite21has been applied to the piston base material1a, the surface of the piston base material1ais exposed through the second window portion31as shown byFIG. 6B. Hence, a working person can recognize and determine that only the upper-layer coating composite21has been applied, from the second window portion31by visual check.

Moreover, if both of the lower-layer coating composite22and upper-layer coating composite21have been properly applied; the first window portion30is coated by the upper-layer coating composite21, and the lower-layer coating composite22is exposed through the second window portion31as a background of the second window portion31, as shown inFIG. 6C. That is, these first and second window portions30and31are closed or filled by the coating composites21and22, and become almost black as a whole. Hence, a working person can recognize and determine that both of the lower-layer coating composite22and the upper-layer coating composite21have been formed.

Therefore, a piston including only single layer of coating can be prevented from being distributed as a piston product by error. Hence, a reliability of product can be enhanced.

The first and second window portions30and31are provided in a region of each skirt portion8or9which has a relatively less-frequent or weak slide contact with the cylinder wall-surface3. Hence, the first and second window portions30and31are little influenced by friction so that a generation of abrasion can be suppressed. Moreover, a freedom degree of design of each skirt portion8or9in an axial direction of the piston1becomes high.

Moreover, the lower-layer coating composite22and the upper-layer coating composite21have filled the first and second window portions30and31in the case that both of the upper-layer coating composite21and the lower-layer coating composite22have been properly applied. Hence, the generation of abrasion and the like can be suppressed even if a slight influence of friction is caused at the first and second window portions30and31.

Moreover, the first and second window portions30and31are formed together when the lower-layer coating composite22and the upper-layer coating composite21are applied and formed. Hence, a forming operation of the first and second window portions30and31is very easy without requiring any special equipment. Therefore, a rise in cost can also be suppressed.

Although the error recognition and determination using the first and second window portions30and31are done by the visual check of working person in the above explanation, the structure according to this embodiment is not limited to this. For example, the error recognition and determination using the first and second window portions30and31can be mechanically done by means of a camera or the like. Therefore, the first and second window portions30and31according to this embodiment are applicable also to an operation of automated production line.

Second Embodiment

FIGS. 7A to 7Care views showing a second embodiment according to the present invention. Also in the second embodiment, the two of upper-layer and lower-layer coating composites21and22are applied on the piston base material1aas the multiple-layer coating. As shown inFIGS. 7A and 7B, a first window portion (not-coated portion)32is provided to the lower-layer coating composite22as a mark of the lower-layer coating composite22, and a second window portion (not-coated portion)33is provided to the upper-layer coating composite21as a mark of the upper-layer coating composite21. That is, the lower-layer coating composite22is formed with the mark of the first window portion32, and the upper-layer coating composite21is formed with the mark of the second window portion33. Each of the first window portion32and the second window portion33is formed in a circular shape. In the same manner as the first embodiment, a forming location of the first window portion32is shifted from a forming location of the second window portion33in the up-down direction (i.e., in the axial direction of piston1). Thereby, the first window portion32does not overlap with the second window portion33in the radial direction of the piston1(i.e., when viewed from a radially outer side of the piston1).

Accordingly, if only one of the upper-layer coating composite21and the lower-layer coating composite22has been applied to the piston base material1a, only one of the first window portion32and the second window portion33has been formed. In this case, the surface of the piston base material1ais exposed through the first window portion32or the second window portion33as shown inFIGS. 7A and 7B. Since the piston base material1ais the aluminum alloy, the surface of the piston base material1ahas a color near silver. Hence, a working person can recognize and determine that only one of the upper-layer coating composite21and the lower-layer coating composite22has been applied, by visual perception.

Therefore, a piston including only single layer of coating can be prevented from being distributed as a piston product by mistake. Hence, the reliability of product can be enhanced.

Moreover, if both of the lower-layer coating composite22and upper-layer coating composite21have been applied; the first window portion32is coated by the upper-layer coating composite21, and the lower-layer coating composite22is exposed through the second window portion33, as shown inFIG. 7C. That is, these first and second window portions32and33are closed or filled by the coating composites21and22, and become almost black as a whole. Hence, a working person can recognize and determine that both of the lower-layer coating composite22and the upper-layer coating composite21have been applied, by vision.

The other operations and effects in the second embodiment are similar as those in the first embodiment.

Third Embodiment

FIGS. 8A to 8Care views showing a third embodiment according to the present invention. Also in the third embodiment, the two of upper-layer and lower-layer coating composites21and22are applied on the piston base material1a. As shown inFIG. 8A, a first window portion34is provided to the lower-layer coating composite22as a mark of the lower-layer coating composite22. The first window portion34is located at circumferentially one end part of each of the skirt portions8and9and is located at an upper end part (crown-side end part) of each of the skirt portions8and9. The first window portion34is formed in a small circle shape. Moreover, as shown inFIG. 8B, a second window portion35is provided to the upper-layer coating composite21as a mark of the upper-layer coating composite21. The second window portion35is located at circumferentially one end part of each of the skirt portions8and9and is located approximately at an is axially center part of each of the skirt portions8and9. The second window portion35is formed in a small circle shape. A forming location of the first window portion34is different from a forming location of the second window portion35in the up-down direction (i.e., in the axial direction of piston1). Thereby, the first window portion34does not overlap with the second window portion35in the radial direction of the piston1(i.e., when viewed from a radially outer side of the piston1).

Accordingly, if only one of the upper-layer coating composite21and the lower-layer coating composite22has been applied to the piston base material1a, only one of the first window portion34and the second window portion35has been formed. In this case, the surface of the piston base material1aexposed through the first window portion34or the second window portion35as shown inFIGS. 8A and 8B. Since the piston base material1athe aluminum alloy, the surface of the piston base material1ahas a color close to silver. Hence, a working person can recognize and determine that only one of the upper-layer coating composite21and the lower-layer coating composite22has been applied, by vision.

As a result, a piston including only single layer of coating can be prevented from being distributed as a piston product by mistake. Hence, the reliability of product can be improved.

Moreover, if both of the lower-layer coating composite22and upper-layer coating composite21have been applied; the first window portion34is covered by the upper-layer coating composite21, and the lower-layer coating composite22is exposed through the second window portion35, as shown inFIG. 8C. That is, these first and second window portions34and35are closed or filled by the coating composites21and22, and become almost black as a whole. Hence, a working person can recognize and determine that both of the lower-layer coating composite22and the upper-layer coating composite21have been applied, by vision. Therefore, also in the third embodiment, operations and effects similar as the first and second embodiments can be obtained.

In the third embodiment, the first and second window portions34and35are provided in a region of each skirt portion8or9which conducts a less-frequent or weak slide contact with the cylinder wall-surface3. Moreover, sizes of the first and second window portions34and35are sufficiently small as compared with those of the second embodiment. Hence, the first and second window portions34and35are little influenced by friction so that the generation of abrasion can be further suppressed. Moreover, the freedom degree of design of each skirt portion8or9in the axial direction is high.

Fourth Embodiment

FIGS. 9A to 9Care views showing a fourth embodiment according to the present invention. Basic structure of the fourth embodiment is same as the first embodiment. A first window portion36is formed in the lower-layer coating composite22, and a second window portion37is formed in the upper-layer coating composite21. Each of the first and second window portions36and37is formed substantially in a square shape. In this fourth embodiment, the forming locations of the first and second window portions36and37are different from those of the first embodiment.

That is, as shown inFIGS. 9A and 9B, the first and second window portions36and37are set substantially to have an imaginary common axial line (same axis) X passing through both centers of the first and second window portions36and37parallel to the axial direction of piston1. In other words, the first and second window portions36and37are formed at an approximately identical location with each other, relative to the circumferential direction of the piston1. The first and second window portions36and37are provided substantially at a circumferentially center location of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9. Moreover, the first and second window portions36and37are formed to be closer to a center of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9relative to the axial direction, as compared with those of the first embodiment. That is, the first and second window portions36and37are closer to each other in the axial direction than the case of the first embodiment. Hence, when both of the lower-layer coating composite22and upper-layer coating composite21have been applied, a lower part of the first window portion36overlaps with an upper part of the second window portion37to define a third window portion38as shown inFIG. 9C. This third window portion38is formed in a strip shape extending in the circumferential direction of the thrust-side skirt portion8or the counter-thrust-side skirt portion9. In this case, the aluminum-alloy surface of the piston base material1aalways exposed through the third window portion38. Moreover, an area (size) of the third window portion38is sufficiently small, and a forming location of the third window portion38is set in a region which has little slide contact with the cylinder wall-surface3.

Since the first and second window portions36and37are formed respectively in the lower-layer coating composite22and upper-layer coating composite21as mentioned above, a piston including only single layer of coating can be prevented from being distributed as a piston product by mistake. Hence, a reliability of product can be enhanced, in the same manner as the above respective embodiments. Moreover, there is little influence of friction when the thrust-side skirt portion8and the counter-thrust-side skirt portion9slide in contact with the cylinder wall-surface3, so that the generation of abrasion can be suppressed.

Moreover, after the lower-layer coating composite22and upper-layer coating composite21were applied, a radial size (diameter) between the thrust-side skirt portion8and the counter-thrust-side skirt portion9can be accurately measured by a micrometer or the like by using the radially-opposed pair of third window portions38and38of both the skirt portions8and9, i.e., by using both the exposed surfaces of the piston base material1a.

Forming locations of both of the first and second window portions36and37do not necessarily need to be set with high precision relative to the circumferential and axial directions. That is, the first and second window portions36and37have only to function as the marks of the lower-layer coating composite22and upper-layer coating composite21and also to form the third window portion38securing its size necessary to measure the length between the both skirt portions8and9even if the shape of third window portion38is somewhat deformed.

Fifth Embodiment

FIGS. 10A to 10Care views showing a fifth embodiment according to the present invention. Basic structure of the fifth embodiment is same as the fourth embodiment. However, in the fifth embodiment, each of first and second window portions39and40is formed in a circular shape.

That is, as shown inFIGS. 10A and 10B, the first and second window portions39and40are set substantially to have an imaginary common axial line (same axis) X passing through both centers of the first and second window portions39and40parallel to the axial direction of piston1. In other words, the first and second window portions39and40are formed at an approximately identical location with each other, relative to the circumferential direction of the piston1. The first and second window portions39and40are provided substantially at a circumferentially center location of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9. Moreover, the first and second window portions36and37are formed to be closer to the center of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9relative to the axial direction, as compared with those of the second embodiment. That is, the first and second window portions39and40are closer to each other in the axial direction than the case of the second embodiment. Hence, when both of the lower-layer coating composite22and upper-layer coating composite21have been applied, a lower part of the first window portion39overlaps with an upper part of the second window portion40to define a third window portion41as shown inFIG. 10C. This third window portion41is formed in a narrow elliptical shape extending in the circumferential direction of the thrust-side skirt portion8and the counter-thrust-side skirt portion9. In this case, the aluminum-alloy surface of the piston base material1aalways exposed through the third window portion41. Moreover, an area of the third window portion41is sufficiently small, and a forming location of the third window portion41is set in a region which has little slide contact with the cylinder wall-surface3.

Since the first and second window portions39and40are formed respectively in the lower-layer coating composite22and the upper-layer coating composite21as mentioned above, a piston including only single layer of coating can be prevented from being distributed as a piston product by mistake. Hence, a reliability of product can be enhanced, in the same manner as the above respective embodiments. Moreover, there is little influence of friction when the thrust-side skirt portion8and the counter-thrust-side skirt portion9slide in contact with the cylinder wall-surface3, so that the generation of abrasion can be suppressed.

Moreover, after the lower-layer coating composite22and upper-layer coating composite21were applied, the radial size (diameter) between the thrust-side skirt portion8and the counter-thrust-side skirt portion9can be accurately measured by the micrometer or the like by using the radially-opposed pair of third window portions41and41of the skirt portions8and9, i.e., by using both the exposed surfaces of the piston base material1a.

Sixth Embodiment

FIGS. 11A to 11Care views showing a sixth embodiment according to the present invention. Although the marks are defined by the window portions in the above respective embodiments, first and second marks42and43are provided respectively at locations circumferentially outside the lower-layer coating composite22and the upper-layer coating composite21in the sixth embodiment. The first mark42is formed of the same components (i.e., has the same composition of materials) as the lower-layer coating composite22, and the second mark43is formed of the same components (i.e., has the same composition) as the upper-layer coating composite21.

As shown inFIG. 11A, the first mark42is formed as a single-layer coating, together when the lower-layer coating composite22is formed. As shown inFIG. 11B, the second mark43is formed as a single-layer coating, together when the upper-layer coating composite21is formed. Each of the first and second marks42and43is shaped like a small rectangle. The first and second marks42and43are located on a circumferentially one end side of each of the skirt portions8and9and are located in a circumferentially-right region (ofFIGS. 11A to 11C) outside the lower-layer coating composite22and the upper-layer coating composite21. That is, the first and second marks42and43are formed on an outer surface of a connecting region between the skirt portion8or9and the apron portion11or12. The first and second marks42and43are respectively away from the lower-layer coating composite22and the upper-layer coating composite21in the circumferential direction of piston1, to have a slight clearance (non-coated portion) C between the mark42or43and the coating composite22or21as shown inFIG. 11A.

As shown inFIG. 11A, the first mark42is located on an upper side of (a circumferential end portion of) the lower-layer coating composite22, relative to the axial direction of piston1. On the other hand, as shown inFIG. 11B, the second mark43is located on a lower side from a center of (a circumferential end portion of) the upper-layer coating composite21, relative to the axial direction of piston1. Moreover, as shown inFIG. 11C, the first and second marks42and43are away from each other in the axial direction to have a slight axial clearance S therebetween.

Therefore, in this embodiment, if the upper-layer coating composite21has not been applied by mistake after the lower-layer coating composite22was applied to the piston base material1a, only the first mark42has been formed as shown inFIG. 11A. Hence, a working person can recognize and determine that only the lower-layer coating composite22has been applied, by visual check.

If the lower-layer coating composite22has not been applied by mistake although the upper-layer coating composite21has been applied to the piston base material1a, only the second mark43has been formed as shown byFIG. 11B. Hence, a working person can recognize and determine that only the upper-layer coating composite21has been applied, from the second mark43by vision.

Moreover, if both of the lower-layer coating composite22and the upper-layer coating composite21have been applied, the first mark42and the second mark43have been formed in upper-and-lower alignment (axial alignment) as shown inFIG. 11C. Hence, a working person can recognize and determine that both of the lower-layer coating composite22and the upper-layer coating composite21have been applied. In particular, since the first mark42and the second mark43are apart from the lower-layer coating composite22and the upper-layer coating composite21by the clearance C, the existence of each of the first and second marks42and43is easy to recognize from the appearance of the piston1. Moreover, since the first and second marks42and43are apart from each other by the large clearance S in the axial direction, it becomes clearer whether the lower-layer coating composite22and/or the upper-layer coating composite21have not yet been applied to the piston base material1a.

Therefore, a piston including only single layer of coating can be prevented from being distributed as a piston product by mistake. Hence, the reliability of product can be enhanced, in the same manner as the above respective embodiments.

The first mark42and the second mark43are provided in a region which has a relatively less-frequent or weak slide contact with the cylinder wall-surface3. Hence, the first mark42and the second mark43are little influenced by friction so that a generation of abrasion can be suppressed. Moreover, since the first mark42and the second mark43are formed in the region which does not directly correspond to the skirt portions8and9, a freedom degree of design of the skirt portions8and9and the marks42and43is high in the axial direction.

Moreover, the first mark42is formed concurrently together with the lower-layer coating composite22, and the second mark43is formed concurrently together with the upper-layer coating composite21, as mentioned above. Hence, a forming operation for the first mark42and the second mark43is very easy without requiring any special equipment. Therefore, the rise in cost can also be suppressed.

Seventh Embodiment

FIGS. 12A to 12Care views showing a seventh embodiment according to the present invention. In the seventh embodiment, the forming locations of the first and second marks42and43is brought closer to each other in the axial direction as shown inFIGS. 12A and 12B. If only one of the lower-layer coating composite22and the upper-layer coating composite21has been applied by error, only one of the first and second marks42and43has already been formed. If both of the lower-layer coating composite22and the upper-layer coating composite21have been applied, both the marks42and43have been combined with each other in the up-down direction (i.e., the axial direction) so that an axially-elongated rectangular mark is formed as a whole as shown inFIG. 12C.

Accordingly, operations and effects similar as the fifth embodiment can be obtained. In addition, since the mutually-combined first and second marks42and43form the elongated rectangular shape, a visibility for the working person becomes more favorable to enable a quick recognition or determination.

Also in the seventh embodiment, the first and second marks42and43are apart from the lower-layer coating composite22and the upper-layer coating composite21by the clearance C. Hence, also from this point of view, the recognition by vision is easy in the same manner as the fifth embodiment.

Eighth Embodiment

FIGS. 13A to 13Care views showing an eighth embodiment according to the present invention. In the eighth embodiment, the forming locations of the first and second marks42and43are changed from those of the sixth and seventh embodiments. In the eighth embodiment, the first mark42is formed to be continuous with (i.e., to be connected with) one circumferential edge of the lower-layer coating composite22, and the second mark43is formed to be continuous with (i.e., to be connected with) one circumferential edge of the upper-layer coating composite21. That is, when the lower-layer coating composite22is applied to the surface of the skirt portion8or9, the first mark42is concurrently formed so as to be continuous with the lower-layer coating composite22. The first mark42protrudes from the circumferential end (right end ofFIG. 13A) of the lower-layer coating composite22in the circumferential direction. On the other hand, when the upper-layer coating composite21is applied to the surface of (the lower-layer coating composite22of) the skirt portion8or9, the second mark43is concurrently formed so as to be continuous with the upper-layer coating composite21. The second mark43protrudes from the circumferential end (right end ofFIG. 13A) of the upper-layer coating composite21in the circumferential direction. There is a clearance S between both of the first and second marks42and43as shown inFIG. 13C.

In the eighth embodiment, operations and effects similar as the above respective embodiments can be obtained. In addition, since the first and second marks42and43are continuous with the lower-layer coating composite22and the upper-layer coating composite21, the forming operation for the first and second marks42and43is easy. Moreover, the visibility for the working person is favorable because of the existence of the clearance S.

Ninth Embodiment

FIGS. 14A to 14Care views showing a ninth embodiment according to the present invention. Basic structure in the ninth embodiment is similar as the eighth embodiment. The first and second marks42and43are formed concurrently with the application of the lower-layer coating composite22and the upper-layer coating composite21in the same manner as the eighth embodiment. In this ninth embodiment, the forming locations of the first and second marks42and43are moved toward a center portion of (a circumferential end portion of) the coating composite22or21in the axial direction of piston1. Accordingly, if both of the lower-layer coating composite22and the upper-layer coating composite21have been applied without mistake, the first and second marks42and43are combined with each other from the up and down directions (i.e., in the axial direction) as shown inFIG. 14C.

Therefore, also in this ninth embodiment, operations and effects similar as the eighth embodiment can be obtained. In addition, since the first and second marks42and43are connected with each other in the axial direction so that an axially-elongated rectangular mark is formed, the visibility for the working person is favorable.

Tenth Embodiment

FIGS. 15A to 15Care views showing a tenth embodiment according to the present invention. In the tenth embodiment, rectangular first and second marks44and45are provided on upper end portions of the skirt portions8and9. That is, the rectangular first mark44is formed to be continuous with an upper edge (crown-side edge) of the lower-layer coating composite22. On the other hand, the rectangular second mark45is formed to be continuous with an upper edge (crown-side edge) of the upper-layer coating composite21.

The first mark44is located substantially at a circumferentially-center portion of the upper edge of the lower-layer coating composite22. On the other hand, the second mark45is located on a right side (ofFIGS. 15A to 15C) beyond a circumferentially-center portion of the upper edge of the upper-layer coating composite21. Thereby, both of the first and second marks44and45are arranged in right-and-left alignment (circumferential alignment) to have a predetermined clearance between the first and second marks44and45.

Therefore, operations and effects similar as the ninth embodiment and the like can be obtained.

Eleventh Embodiment

FIGS. 16A to 16Care views showing an eleventh embodiment according to the present invention. In the eleventh embodiment, rectangular first and second marks44and45are provided on lower end sides of the skirt portions8and9. That is, the rectangular first mark44is formed to be continuous with a lower edge (counter-crown-side edge) of the lower-layer coating composite22. On the other hand, the rectangular second mark45is formed to be continuous with a lower edge (counter-crown-side edge) of the upper-layer coating composite21.

The first mark44is located substantially at a circumferentially-center portion of the lower edge of the lower-layer coating composite22. On the other hand, the second mark45is located on the right side (ofFIGS. 15A to 15C) beyond a circumferentially-center portion of the lower edge of the upper-layer coating composite21. Thereby, both of the first and second marks44and45are arranged in right-and-left alignment (circumferential alignment) to have a predetermined clearance between the so first and second marks44and45.

Therefore, operations and effects similar as the ninth embodiment and the like can be obtained.

According to the tenth and eleventh embodiments, the forming locations of the first and second marks44and45may be set to cause the first and second marks44and45to be combined with each other in the circumferential direction when both of the lower-layer coating composite22and the upper-layer coating composite21are applied.

Twelfth Embodiment

FIGS. 17A to 17Care views showing a twelfth embodiment according to the present invention. Although the multiple-layer coating composite20is constituted by two layers of the lower-layer coating composite22and the upper-layer coating composite21in the above respective embodiments, a multiple-layer coating composite in the twelfth embodiment is constituted by three layers or more.

In this twelfth embodiment, as shown inFIG. 17A, first-layer to sixth-layer coating composites46ato46fare applied to the outer surface of each of the thrust-side skirt portion8and the counter-thrust-side skirt portion9. As shown inFIG. 17A, a plurality of window portions47ato47fare formed in the first-layer to sixth-layer coating composites46ato46f. Each of the plurality of window portions47ato47fis formed as a circumferentially-elongated mark, and is located in a circumferentially end portion (right side ofFIG. 17) of each of the first-layer to sixth-layer coating composites46ato46f.

These window portions47ato47fare located in the circumferentially end side (right side) of each of the first-layer to sixth-layer coating composites46ato46f, and arranged axially in a row at even intervals. Each layer of the first-layer to sixth-layer coating composites46ato46fhas five windows (6-1) selected from the window portions47ato47f. Each of the first-layer to sixth-layer coating composites46ato46fis exposed to the outside of the piston1through the corresponding window portion given from the window portions47ato47f.

That is, as shown inFIG. 17A, the first-layer coating composite46aincludes the second to sixth window portions47bto47fformed in second to sixth steps (lines) without including the first window portion47aof a top step (line). The second-layer coating composite46bincludes the first window portion47aformed in the top step and the third to sixth window portions47cto47fformed in the third to sixth steps without including the second window portion47b. The third-layer coating composite46cincludes the first and second window portions47aand47band the fourth to sixth window portions47dto47fwithout including the third window portion47c. The forth-layer coating composite46dincludes the first to third window portions47ato47cand the fifth and sixth window portions47eand47fwithout including the fourth window portion47d. The fifth-layer coating composite46eincludes the first to fourth window portions47ato47dand the sixth window portion47fwithout including the fifth window portion47e. The sixth-layer coating composite46fincludes the first to fifth window portions47ato47ewithout including the sixth window portion47fof a bottom step (lowest line).

Accordingly, for example, if the application of the fourth-layer coating composite46dwas forgotten, the fourth window portion47dhas been formed in all of the other coating composites46a-46c,46eand46f. Hence, as shown inFIG. 17B, the aluminum-alloy surface of the piston base material1aexposed to the outside through all of the fourth window portions47dbecause each fourth window portion47dhas not been closed or filled.

Therefore, the working person can recognize and determine that the fourth-layer coating composite46dhas not been applied, by visibly recognizing the aluminum-alloy surface exposed from the fourth window portion47d. As a result, a piston product having its coating failure can be sufficiently prevented from being distributed, in the same manner as the above embodiments. Hence, the reliability of product is enhanced.

If all of the first-layer to sixth-layer coating composites46ato46fhave been applied without mistake, all of the window portions47ato47fhave been closed or filled by the first-layer to sixth-layer coating composites46ato46fso that no window portion (having the color near silver) is left as shown inFIG. 17C. Hence, the working person can visibly recognize that all of the first-layer to sixth-layer coating composites46ato46fhave been applied without mistake.

Thirteenth Embodiment

FIGS. 18A to 18Care views showing a thirteenth embodiment according to the present invention. In the thirteenth embodiment, the multiple-layer coating composite which is applied on the outer surfaces of the thrust-side skirt portion8and the counter-thrust-side skirt portion9is constituted by four layers48ato48d. As shown inFIG. 18A, first to fourth four marks49ato49dwhich are elongated in the circumferential direction are formed on circumferential edges (on right edges ofFIG. 18A) of the first-layer to fourth-layer coating composites48ato48d, to be continuous with the first-layer to fourth-layer coating composites48ato48d.

The first to fourth marks49ato49dare respectively formed integrally with the circumferential (right) edges of the first-layer to fourth-layer coating composites48ato48d. The first to fourth marks49ato49dare arranged in a row in the axial direction (up-down direction). There is no space between adjacent two of the first to fourth marks49ato49din the axial direction. That is, forming locations of the first to fourth marks49ato49dare set to combine the first to fourth marks49ato49dintegrally with one another (from upper and lower directions) as axially-extending one mark when all of the first-layer to fourth-layer coating composites48ato48dare properly applied, as shown inFIG. 18C.

In detail, as shown inFIG. 18A, the first mark49ais formed integrally with the circumferential edge of the first-layer coating composite48aat an axially upmost portion (a top-step portion) of the first-layer coating composite48a. The second mark49bis formed integrally with the circumferential edge of the second-layer coating composite48bat a second-step portion which is axially shifted in the lower direction from the location of the first mark49aby an axial width of each mark. In the same manner, the third mark49cis formed integrally with the circumferential edge of the third-layer coating composite48cat a third-step portion which is axially shifted in the lower direction from the location of the second mark49bby the axial width of each mark. In the same manner, the is fourth mark49dis formed integrally with the circumferential edge of the fourth-layer coating composite48dat a bottom-step portion which is axially shifted in the lower direction from the location of the third mark49cby the axial width of each mark.

Accordingly, for example, if only the third-layer coating composite48chas not been applied by mistake, the first and second marks49aand49band the fourth mark49dhave been formed except the third mark49c. Hence, the combined shape of the marks49a,49band49dis in a state cut (chipped) by the location S of the third mark49c, as shown inFIG. 18B. Hence, the aluminum-alloy surface of the piston base material1aexposed to the outside through the location S of the third mark49c.

Therefore, the working person can recognize and determine that the third-layer coating composite48chas not been applied, by visibly recognizing the aluminum-alloy surface exposed from the cutout location S of the third mark49c. As a result, a piston product having its failure can be sufficiently prevented from being distributed, in the same manner as the above embodiments. Hence, the reliability of product is enhanced.

Fourteenth Embodiment

FIGS. 19A to 19Care views showing a fourteenth embodiment according to the present invention. In the fourteenth embodiment, the structure according to the thirteenth embodiment is further developed. In the fourteenth embodiment, the multiple-layer coating composite which is applied on the outer surfaces of the thrust-side skirt portion8and the counter-thrust-side skirt portion9is constituted by a lot of layers50ato50n. As shown inFIG. 19A, first to nth marks51ato51nwhich are elongated in the circumferential direction are formed on circumferential edges (on right edges ofFIG. 19A) of the first-layer to nth-layer coating composites50ato50n. In the same manner as the thirteenth embodiment, the first to nth marks51ato51nare integrally formed respectively with the first-layer to nth-layer coating composites50ato50n.

As shown inFIG. 19C, the forming locations of the first to nth marks51ato51nare set to arrange the first to nth marks51ato51nin a row in the axial direction (up-down direction) when all of the first-layer to nth-layer coating composites50ato50nhave been applied. At this time, there is a predetermined clearance C between adjacent two of the first to nth marks51ato51nin the axial direction, as shown inFIG. 19C.

Accordingly, for example, if only the third-layer coating composite50chas not been applied by mistake, the first and second marks51aand51band the fourth to nth marks51dto51nhave been formed except the third mark51c. Hence, the combined shape of the marks51a,51band51d-51nis in a state cut (chipped) by the location S of the third mark51c, as shown inFIG. 19B. Hence, the aluminum-alloy surface of the piston base material1ais exposed to the outside through the location S of the third mark51c.

Therefore, the working person can recognize and determine that the third-layer coating composite50chas not been applied, by visibly recognizing the aluminum-alloy surface exposed from the cutout location S of the third mark51c. As a result, a piston product having its failure can be sufficiently prevented from being distributed, in the same manner as the above embodiments. Hence, the reliability of product is enhanced.

As explained above, even in the case that the multiple-layer coating includes three or more layers as in the twelfth to fourteenth embodiments, it can be judged whether or not the multiple-layer coating has been properly formed, by use of the marks (window portions).

Fifteenth Embodiment

FIGS. 20A to 20CandFIG. 21Aare views showing a fifteenth embodiment according to the present invention. In the fifteenth embodiment, forming locations of the first and second marks42and43are same as the eighth embodiment shown byFIG. 13. However, in the fifteenth embodiment, when the lower-layer coating composite22is applied to the thrust-side skirt portion8and the counter-thrust-side skirt portion9of the piston1together with the first mark42, a plurality of black-dot coating portions (partly coating portions)52each formed of the same components (i.e., having the same composition of materials) as the lower-layer coating composite22are dispersed or scattered within a region53in which the second mark43of the upper-layer coating composite21is scheduled to be formed. That is, the plurality of black-dot coating portions52are provided as a part of the lower-layer coating composite22.

That is, at first, the lower-layer coating composite22and the first mark42are applied to the surfaces of both skirt portions8and9of the piston1by the above-mentioned method. At the same time, the plurality of black-dot coating portions52constituting a part of the lower-layer coating composite22are applied to a location to which the second mark43of the upper-layer coating composite21will be applied in a next process. Thereby, whole (aggregate) of the black-dot coating portions52defines a partly-coating-portion forming region53as shown inFIG. 20A.

As shown inFIG. 21A, each of the black-dot coating portions52is formed in a small circle shape having its diameter approximately equal to a few millimeters. The black-dot coating portions52are dispersed from one another to have a predetermined distance between adjacent two of the black-dot coating portions52. Thereby, whole of the black-dot coating portions52forms the partly-coating-portion forming region53which is a bit larger than outer dimensions of the second mark43. The partly-coating-portion forming region53is formed substantially in a square shape including polka dots. The surface of the piston base material1a, i.e., the surface of aluminum alloy is exposed through spaces given between the black-dot coating portions52.

A total overlapping area between the black-dot coating portions52and the second mark43can be set within a range from 10% to 85% of an area of the second mark43. That is, the black-dot coating portions52can be formed to cause parts of the second mark43which completely overlap with (i.e., cover) the black-dot coating portions52in a thickness direction of the multiple-layer coating composite20, to have its area falling within a range between 10% and 85% of the area of the second mark43. In this example according to the fifteenth embodiment, the total overlapping area is set approximately at 50% of the area of the second mark43.

Therefore, in the fifteenth embodiment, a piston product including only single layer of coating can be prevented from being distributed by mistake, because of the existence of the marks42and43, in the same manner as the above embodiments. Moreover, when the upper-layer coating composite21is applied to the upper surface of the lower-layer coating composite22, the second mark43is concurrently bound to the black-dot coating portions52in addition to the surface of the piston base material1aexposed through the spaces between the black-dot coating portions52in the partly-coating-portion forming region53. Hence, the second mark43can be strongly bound to the piston1in the partly-coating-portion forming region53.

That is, the second mark43is bound with (adheres to) the surface of the piston base material1aand is also bound with the scattered black-dot coating portions52, in the partly-coating-portion forming region53. The second mark43is strongly bound to the respective black-dot coating portions52on the principle that the upper-layer coating composite21is strongly bound to the lower-layer coating composite22. Accordingly, the second mark43is prevented from being carelessly detached from the surface of the piston1after the upper-layer coating composite21was formed on the lower-layer coating composite22. As a result, by using the above-mentioned marks42and43, it can be determined whether or not the multiple-layer coating composite20includes only a single layer, always with certainty.

Moreover, since the partly-coating-portion forming region53is formed more largely than the area (dimensions) of the second mark43, the second mark43is not formed outside the partly-coating-portion forming region53.

Other Patterns of Partly Coating Portion52

Although the black-dot coating portions52each shaped like a small circle have been explained as shown inFIGS. 20A-20CandFIG. 21A, various shapes and patterns can be employed as the partly coating portion(s)52. For example, as shown inFIG. 21B, each of the partly coating portions52may be formed in a small square shape instead of the small circular shape. Alternatively, each of the partly coating portions52may be formed in a small triangular shape. Moreover, as shown inFIG. 21C, the partly coating portion52may be formed in a lattice shape (e.g., a square grid shape) within the partly-coating-portion forming region53, instead of the plurality of dots. As shown inFIG. 21D, the partly coating portion52can also be formed in a square-cross-hatched shape.

The various kinds of partly coating portions52are applicable to all of the embodiments explained above. In the first to fifth embodiments shown inFIGS. 6A to 10C, the partly coating portion(s)52can be formed within the first window portion30,32,34,36or39.

Moreover, in the sixth to eleventh embodiments as shown inFIGS. 11A to 16C, the partly coating portion(s)52can be formed within a region (the partly-coating-portion forming region53) in which the second mark43or45of the upper-layer coating composite21is scheduled to be formed. In this case, the partly coating portion(s)52are formed concurrently when the lower-layer coating composite22is applied to the piston base material1a.

Moreover, in the twelfth embodiment as shown inFIGS. 17A to 17C, for example if the fifth-layer coating composite46ehas a low adhesion property (low binding property) against the piston base material1abut has a high adhesion property against the third-layer coating composite46c, the partly coating portion(s)52is formed within the fifth window portion47eof the third-layer coating composite46cthat has the plurality of window portions because the fifth-layer coating composite46eis scheduled to cover the fifth window portion47e.

Moreover, in the thirteenth embodiment as shown inFIGS. 18A to 18C, for example if the fourth-layer coating composite48dhas a low adhesion property against the piston base material1abut has a high adhesion property against the second-layer coating composite48b, the partly coating portion(s)52is formed (to be continuous) with the second-layer coating composite48bat a location corresponding to the forming location of the fourth mark49dof the fourth-layer coating composite48d.

Moreover, in the fourteenth embodiment as shown inFIGS. 19A to 19C, for example if the nth-layer coating composite50nhas a low adhesion property against the piston base material1abut has a high adhesion property against the mth-layer coating composite50m, the partly coating portion(s)52is formed (to be continuous) with the mth-layer coating composite50mat a location corresponding to the forming location of the nth mark51nof the nth-layer coating composite50n.

Therefore, in the case that the partly coating portion(s)52is formed in the window portion30,32, . . . or in the partly-coating-portion forming region53in the above respective embodiments, operations and effects similar as the fifteenth embodiment can be obtained.

Although the invention has been described above with reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. For example, the above-mentioned materials (components) constituting each of the upper-layer coating composite21and the lower-layer coating composite22in the first embodiment can be changed to the other material(s).

Some technical structures obtainable from the above embodiments according to the present invention will now be listed with their advantageous effects.

[a] A piston for an internal combustion engine, comprising: a piston base material (1a) including a pair of skirt portions (8,9) opposed to each other in a radial direction of the piston base material (1a); a multiple-layer coating (20) formed on at least one of surfaces of the pair of skirt portions (8,9); and marks (30-37,39,40,42-45,47a-47f,49a-49d,51a-51n) provided to respective layers (21,22,46a-46f,48a-48d,50a-50n) of the multiple-layer coating (20) at locations different from each other.

[b] A piston for an internal combustion engine, comprising: a piston base material (1a) including a pair of skirt portions (8,9) opposed to each other in a radial direction of the piston base material (1a); and a multiple-layer coating (20) formed on at least one of surfaces of the pair of skirt portions (8,9), wherein an upper layer (21,46b-46f) of the multiple-layer coating (20) is formed with a window portion (33,35,37,40,47a-47f), and a surface of the piston base material (1a) or a lower layer (22,46a-46e) of the multiple-layer coating (20) is exposed through the window portion (33,35,37,40,47a-47f).

[c] A piston for an internal combustion engine, comprising: a piston base material (1a) including a pair of skirt portions (8,9) opposed to each other in a radial direction of the piston base material (1a); a multiple-layer coating (20) formed on at least one of surfaces of the pair of skirt portions (8,9), the multiple-layer coating (20) containing a solid lubricant; and single-layer mark coatings (42-43) provided at locations which are different from each other and which are away from the multiple-layer coating (20) through a non-coated portion (C) between the multiple-layer coating (20) and each of the single-layer mark coatings (42-43).

Accordingly, as an advantageous effect, for example, it can be determined whether or not the predetermined multiple-layer coating has been formed, by a visual check of the appearance of the piston by use of the marks (window portions).

[d] The piston as described in the above item [a], wherein the marks (34,35,42,43,47a-47f,49a-49d,51a-51n) corresponding to the respective layers (21,22,46a-46f,48a-48d,50a-50n) of the multiple-layer coating (20) are located at least on one circumferential side of the skirt portion (8,9).

According to this structure, since the marks are provided at a skirt portion's part at which the skirt portion less-frequently or weakly slides in contact with the cylinder wall-surface, the marks are little influenced by friction so that the generation of abrasion can be suppressed. Moreover, the freedom degree of design of the skirt portion in the axial direction of piston is high.

[e] The piston as described in the above item [a], wherein each of the marks has the same composition as the corresponding layer of the multiple-layer coating (20), and the marks (44,45) are located on an upper or lower side of the skirt portion (8,9) relative to an axial direction of the piston.

According to this structure, since the marks are provided at a skirt portion's part at which the skirt portion less-frequently or weakly slides in contact with the cylinder wall-surface, the marks are little influenced by friction so that the generation of abrasion can be suppressed. Moreover, the freedom degree of design of the skirt portion in the axial direction of piston is high.

[f] The piston as described in the above item [a], wherein each of the marks has the same composition as the corresponding layer of the multiple-layer coating (20), and each of the marks (42-45,49a-49d,51a-51n) is formed to be continuous with the corresponding layer of the multiple-layer coating (20).

According to this structure, since the respective marks are continuous with the corresponding layers, the forming operation of the marks becomes easy. Moreover, the marks can be formed even if there is only a small space for forming the marks.

[g] The piston as described in the above item [a], wherein the multiple-layer coating (20) includes a lower-layer coating composite (22) coating a surface of the piston base material (1a) and an upper-layer coating composite (21) coating an upper surface of the lower-layer coating composite (22), each of the lower-layer coating composite (22) and the upper-layer coating composite (21) contains at least one of a polyamide-imide resin, a polyimide resin and an epoxy resin which are binding resins, the lower-layer coating composite (22) contains a solid lubricant including at least one of a graphite and a molybdenum disulfide, a content of the solid lubricant of the lower-layer coating composite (22) is lower than or equal to 50 wt % of the lower-layer coating composite (22), the upper-layer coating composite (21) contains a solid lubricant including one or both of the graphite and the molybdenum disulfide, and a content of the solid lubricant of the upper-layer coating composite (21) falls within a range from 50 to 95 wt % of the upper-layer coating composite (21).

According to this structure, a high adhesion property between the piston base material and the lower-layer coating composite is secured, and the upper-layer coating composite contains the solid lubricant including one or both of the graphite and the molybdenum disulfide. Moreover, the content of the solid lubricant of the upper-layer coating composite falls within a range from 50 to 95 wt % of the upper-layer coating composite. Therefore, the initial fitting property is superior when the outer circumferential surface of the piston slides on the cylinder wall-surface. That is, the surface of the upper-layer coating composite is abraded in a short time so that a smooth sliding surface of the piston is quickly formed. Hence, a superior initial fitting property can be obtained.

[h] The piston as described in the above item [b], wherein the lower layer (22,46a-46e) of the multiple-layer coating (20) is exposed from an entire region of the window portion (33,35,37,40,47a-47e).

According to this structure, since the lower layer of the multiple-layer coating is exposed to the outside of piston from the entire window portion, the reduction of abrasion resistance due to friction can be inhibited when sliding in contact with the cylinder wall-surface.

[i] The piston as described in the above item [b], wherein the lower layer (22) of the multiple-layer coating (20) is exposed from one part of the window portion (37,40), the surface of the piston base material (1a) is exposed from another part of the window portion (37,40), and the another part of the window portion (37,40) is located substantially at a circumferential center of the skirt portion (8,9).

According to this structure, after the multiple-layer coating was applied, the radial size (diameter) between the both skirt portions can be measured by a micrometer or the like by using both the exposed surfaces of the piston base material.

[j] The piston as described in the above item [b], wherein one layer (46f) of the multiple-layer coating (20) includes at least the window portions (47a-47e) having a number obtained by subtracting 1 from a number of layers of the multiple-layer coating (20), and different layers of the multiple-layer coating (20) are exposed respectively from the window portions (47a-47e) of the one of the multiple-layer coating (20).

According to this structure, it can be judged whether or not the respective layers have been formed, by checking the corresponding window portions, even if the multiple-layer coating is constituted by two or more layers.

[k] The piston as described in the above item [j], wherein each layer of the multiple-layer coating (20) includes the window portions (47a-47f) having the number obtained by subtracting 1 from the number of layers of the multiple-layer coating (20), and each of a plurality of upper layers (46b-46f) of the multiple-layer coating (20) which are applied on an outer surface of a lowest layer (46a) of the multiple-layer coating (20) locates its window portions (47a-47f) so as to cover only one of the window portions (47b-47f) of the lowest layer (46a).

This application is based on prior Japanese Patent Applications No. 2010-145981 filed on Jun. 28, 2010 and No. 2011-63502 filed on Mar. 23, 2011. The entire contents of these Japanese Patent Applications are hereby incorporated by reference.

The scope of the invention is defined with reference to the following claims.