GB2306350A

GB2306350A - Wear-resistant parts, particularly the piston ring of an internal combustion engine

Info

Publication number: GB2306350A
Application number: GB9622507A
Authority: GB
Inventors: Kazuhiko Ichimura
Original assignee: Riken Corp
Current assignee: Riken Corp
Priority date: 1995-10-30
Filing date: 1996-10-29
Publication date: 1997-05-07
Anticipated expiration: 2016-10-29
Also published as: KR970021701A; CN1159511A; GB9622507D0; DE19646524A1; GB2306350B; KR100235836B1

Abstract

Wear-resistant parts, particularly a piston ring, which have a laminate structure comprising a substrate, a first coating formed on the substrate by phosphatizing, and, a second coating formed on the first coating and containing a solid lubricant, are known but have poor durability. The durability is improved by forming clearances between the crystal grains of the phosphate on its top surface part, which clearances have width and depth to embed particles of the solid lubricant in the clearances.

Description

Wear-Resistant Parts, Particularly the Piston Ring of an Internal Combustion Engine BACKGROUND OF INVENTION 1. Field of Invention The present invention is related to wear-resistant parts, particularly to wear-resistant parts which are brought into contact with and impinge on an aluminum alloy. More particularly, the present invention is related to a piston ring which has a wear-resistant surface appropriate to be used in an assembly with an aluminum-alloy piston of an internal combustion engine.

2. Description of Related Arts Along with the recent trend to increase power and rotation of an internal combustion engine, the pressure and thermal load applied to the piston rings, particularly the first piston ring, are increased. Since aluminum alloy is widely used for the piston of an automobile internal-combustion engine, the adhesive wear described hereinbelow becomes liable to occur.

Referring to Fig. 11, a four-cycle gasoline engine of an automobile, particularly a piston and its circumferential parts are illustrated. The four-cycle engine undergoes four cycles consisting of suction, compression, combustion and exhaustion during the two reciprocating processes of the piston 11, which are repeated. A crank shaft, which is hidden behind the counterweight 18 and does not appear in the drawing, is rotated via the connecting rod 17 by means of the piston 11 which reciprocates as described above.

The cycle shown in Fig. 11 is the final stage of the suction cycle, in which the suction valve is kept closed; and further, the gas mixture, which has been introduced into a cylinder bore xia the suction manifold 29 and then the suction port 22 of a cylinder head 21, is compressed by the piston 11 being upward displaced. Upon energization of the plug 24, the gas mixture being compressed combusts in an explosive manner and causes the downward displacement of the piston 11.

The first and second piston rings 12 and 13, respectively, provide a gas-tight contact between the piston 11 and the cylinder liner 15, while the oil-control ring 14 adjusts the supply of lubricating oil to an appropriate amount. These rings 12, 13 and 14 are successively arranged in the downward direction and are fitted in the ring grooves of the piston ring 11. Among these piston rings, particularly the first pressure ring 12 is exposed to the high temperature of the combustion gas.

The suction valve 23 is kept open only in the suction cycle.

The exhaust valve, which is hidden by the suction valve 23 and hence does not appear in Fig. 11, is kept open only in the exhaust cycle. The valves are opened and closed by a locker arm 27 which is coupled to and driven together with the cam 28. The valves are constantly forced by the valve springs 26 to displace in a closing direction.

Referring to Figs. 9 and 10, which are enlarged views of the piston rings and the piston, the ring grooves lla, 11b and 11c are formed on the piston 11. The first pressure ring 12, the second pressure ring 13 and the oil control ring 14 are fitted, respectively, in the ring grooves lla, 11b and llc, in such a manner that a slight clearance is formed between the rings and both sides of the ring grooves.

Generally, the wear-resistant parts should have such properties that the wear thereof is small;, further, the wear of the opposite member brought into slidable contact with said members is small. The "wear-resistant" herein, therefore, indicates specifically for the piston ring that its wear on the upper and lower surfaces is small, while the wear of the ring grooves is small. The "wear-resistance" required for the piston ring will be described in detail with reference to Figs.

9 and 10.

When the piston 11 is displaced downward, the first and second pressure rings 12 and 13 abut against and impinge on the upper surface of the ring grooves lia and llb, respectively.

On the other hand, when the piston 11 is displaced upward, the first and second pressure rings 12 and 13 abut against and impinge on the lower surface of the ring grooves 11a and llb, respectively. Since such abutting and impinging action repeats at a frequency proportional to the number of rotations of an engine, the piston rings are subjected to more severe impact under a higher number of rotations of the engine. While the piston 11 reciprocates, the first and second pressure rings 12, 13 are irregurally displaced around the piston's circumference in the ring grooves 11a, llb. The first and second pressure rings 12, 13 are, therefore, subjected not only to impinging contact but to sliding contact.

Furthermore, particularlly the first pressure ring 12 is exposed to the high temperature of the explosion gas. The environment to which the first pressure ring 12 is exposed is, therefore, very severe.

The aluminum alloy of the piston softens due to the high temperature, is scraped off by the piston rings and deposits on the first pressure ring 12. The so-called adhesive wear occurs, therefore, between the first pressure ring 12 and the aluminum alloy of the piston 11. The wear of the piston 11 at its ring grooves thus advances and the so-called "blow by" is incurred, that is, the combustion gas is blown through via the ring grooves. The power of the engine is, therefore, diminished.

Heretofore, conteremeasures undertaken to solve the above described problem of the piston have been to subject the piston to anodic oxidation to form a hard alumite coating, while countermeasures undertaken to solve the above problems of the piston ring have been to subject the piston ring to chemical conversion phosphatizing, particularly a manganese phosphate coating. The anodic oxidation treatment is expensive. The manganese phosphate coating is effective for preventing adhesive wear in the initial operation period of an engine, but is not expected to maintain its effectiveness over an extended period of operation due to its low durability.

Japanese Unexamined Utility Model Publication No. 6082,552 discloses to subject at least the upper and lower surfaces of a piston ring to chemical conversion phosphatizing and subsequently to formation of a heat-resistant and wearresistant resin coating with a solid lubricant. Japanese Unexamined Utility Model Publication No. 1-307,568 discloses to subject the upper and lower surfaces of a piston ring to nitriding and subsequently to formation of a heat-resistant and wear-resistant resin coating with a solid lubricant. The solid lubricant incorporated in the heat-resistant and wear-resistant coating disclosed in these publications may wear off or peel off, to the extent that the sliding performance of the coating is lost, thus incurring adhesive wear.

SUMMARY OF THE INVENTION It is an object of the present invention to provide the wear-resistant parts, particularly a piston ring, the wear of which is small, which wear off the opposite contact parts slightly, and which exhibit excellent wear resistance over an extended period of time.

In accordance with the objects of the present invention, there is provided wear-resistant parts which have a laminate structure comprising a substrate, a first coating formed on the substrate by a chemical conversion phosphatizing, and a second coating formed on the first coating and containing a solid lubricant, characterized in that clearances are formed between the crystal grains of the chemical conversion phosphate on its top surface part and such clearances have width and depth to embed particles of the solid lubricant in the clearances. Preferably, most of the particles of the solid lubricant are embedded in the clearances.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably, the chemical conversion phosphate is manganese phosphate, because minute clearances having large depth/width ratio can be formed on the top surface of the first coating.

Average diameter of the solid-lubricant particles is preferably from 1 to 2 pm, and the second coating is preferably from 3 to 13 lum thick, because the solid lubricant can be embedded in the clearances in a high proportion. The thickness of the second coating is measured by observing the cross-section of the second coating by an optical microscope and determining its thickness from the lowest position of the minute unevennesses of the chemical conversion phosphate.

According to the embodiment described above, the second coating consists of the particles of molybdenum disulfide or the like having an average diameter of from 1 to 2 pm and heatresistant and wear-resistant resin, in which the molybdenum disulfide or the like is contained and dispersed. This second coating durably maintains good lubricating performance and low-friction performance. Besides the molybdenum disulfide, tungsten disulfide, boron nitride (hBN), graphite and known agents in tribology can be used as the solid lubricant.

The wear-resistant members according to the present invention are appropriate as the members being in slidable contact with the aluminum alloy. Particularly, the first and second coatings according to the present invention are applied on at least the upper and lower surfaces of a piston ring. In this embodiment, satisfactorily durable wear-resistance can be attained without subjecting the aluminum alloy to anodic oxidation. The anodic oxidation may, however, be applied to the aluminum alloy but does not cause serious wear of a piston ring.

Polyamide imide and the other binder resins, such as epoxy and polyimide resins, as well as polytetrafluoroethylene (PTFE) can be used as the binder of the solid lubricant.

Heretofore, the chemical conversion phosphate of the first layer disclosed in Japanese Unexamined Utility Model Publication No. 60-82,552 is used mainly for anchoring and bonding the heatresistant and wear-resistant resin and hence enhancing the adhesion property of such resin. The known chemical conversion phosphate is not effective for preventing the removal of the solid lubricant contained in such resin from the surface of such resin layer. Along with the wear of the heat-resistant and wear-resistant resin, adhesive wear therefore occurs on the surface of such resin coating.

Contrary to the prior art, the size of solid-lubricant particles and the clearances between the crystals of chemical conversion phosphatizing, which is an underlying layer, are taken into consideration. That is, when the phosphate crystals formed by a certain kind of chemical conversion phosphatizing grow from a substrate, and finally, when the growth completes, minute unevennesses are formed on the top surface of the phosphate layer. The solid-lubricant particles are embedded in the clearances between the crystals of the underlying layer, provided that the size of the solid lubricant particles is considerably smaller than the diameter or width and depth of the intercrystaline clearances.When the surface layer constructed as above wears off with the lapse of time, the solid lubricant particles maintain their lubricating function until the wear proceeds into the vicinity of the substrate, because the solid-lubricant particles virtually are not separated from the bonding resin layer. It is, therefore, possible according to the present invention to prevent the seizure wear from occurring for a longer period than by the prior art and thus to enhance durability of the surface layer of the wear-resistant parts.

Regarding the crystal morphology of the phosphate coating by chemical conversion, leaflette form or rectangular parallelepiped are shown in Japanese Examined Patent Publication 3-25,514. It is, however, difficult to embed the solid-lubricant particles between the crystals of this publication. Even if possible, the proportion of solid-lubricant particles embedded seems to be extremely small.

According to a specific method for forming the second coating, particles of the solid lubricant, heat-resistant and wear-resistant resin and its solvent are thoroughly mixed together and then dried to vaporize the solvent. The heat-resistant and wear-resistant resin is baked so as to interlock the resin. The second coating is preferably of such thickness that the concavities of the phosphate crystals of the underlying layer are completely covered by the second coating, desirably from 3 to 13 lum.

Since the present invention is constructed as described hereinabove, a preferential relationship exsits between the diameter of the solid-lubricant particles and the intercrystalline clearances of the underlying chemical-conversion phosphate layer. That is, since the solid-lubricant particles must be embedded in the intercrystalline clearances, a finer particle size is more advantageous. However, when the average particle size is finer than 1 tum, the solid lubricant particles may coagulate in the heat-resistant and wear-resistant resin used as the binder at the coating step. In this case, the solidlubricant particles may coagulate in the binder resin, and the diameter of the coagulated particles become so enlarged that the particles cannot enter the intercrystalline clearances of the underlying layer.

On the other hand, when the average diameter is more than 2 pm, the proportion of the solid-lubricant particles entering the intercrystalline clearances of the underlying layer is so lowered that their separation from the second coating becomes liable to occur. In addition, when the average diameter is more than 2 pm, the solid-lubricant particles become unstable in the second coating. For example, the proportion of the solid-lubricant particles which are not covered by the binder but protrude out of the surface of the second coating increases.

Impact force is concentrated on the top end of the protruding solid-lubricant particles during the repeated, vigorous collision against and sliding on the opposed member, with the result that the solid-lubricant particles are liable to separate from the second coating. In the extreme case, not only the solid-lubricant particles but also the entire second coating with such particles and resin may occasionally peel off from the wear-resistant parts.

A manganese phosphate coating is advisable to form the intercrystalline clearances, in which the solid-lubricant particles with an average diameter of from 1 to 2 pm can enter.

Since the manganese phosphate grows on a steel substrate in the form of pyramid crystals, the solid-lubricant particles are liable to enter the intercrystalline clearances. Furthermore, the heat-resistant and wear-resistant resin is well bonded on such crystals.

When the average particle diameter of solid lubricant is from 1 to 2 Vm, the second coating layer wears out from its surface uniformly, while preventing localized wear. Furthermore, since the wear of the second coating layer is suppressed to the lowest level, the wear-resistant member according to the present invention can be stably used over a long period of time. For example, polyamide-imide resin is recommended as the binder resin. The proportion of the solid-lubricant, such as molybdenum disulfide, relative to the total of it and the binder is preferably from 40 to 60 weight %, so as to balance the low friction- and lubricating- properties and the adhesion properties. Application of the wear-resistant surface layer according to the present invention is recommended on the wear-resistant members which are brought into contact with an aluminum-alloy part.

The present invention is described hereinafter with reference to several drawings.

BRIEF DESCRIPTION OF DRAWINGS Figure 1 is an enlarged cross sectional view of a pressurepiston ring.

Figures 2 and 3 are photographs of an SEM image of the chemical conversion manganese phosphate coating.

Figure 4 is a photograph of an SEM image of the solid lubricant layer (the second coating layer) of Example 3.

Figure 5 is a photograph of an SEM image of the solid lubricant layer (the second coating layer) of Example 6.

Figure 6 is a cross-sectional view of the wear tester and illustrates its essential parts.

Figure 7 is a graph showing the wear amounts in the inventive examples in comparison with those of the comparative examples.

Figure 8 is a photograph of an SEM sketch of the solid lubricant layer after the wear test in Example 3.

Figure 9 is an enlarged view of piston rings, ring grooves and a piston and illustrates the positional relationship between the piston rings and the ring grooves during downward displacement of the piston.

Figure 10 is an enlarged view of piston rings, ring grooves and a piston and illustrates the positional relationship between the piston rings and the ring grooves during upward displacement of the piston.

Fig. 11 is a cross-sectional view of the piston and circumferential parts of a gasoline engine.

Now, referring to Figs. 2 and 3, the crystal grains of manganese phosphate 31, 32 formed by chemical conversion are shown by SEM images. On the top surface of the first coating the manganese phosphate crystals in polyhedral shapes are arranged. The ridges of the polyhedral crystals are directed upward and a number of the clearances, which are slightly less than 10 pm, are formed between the upward directed ridges. The present invention is hereinafter described with references to the examples, in which the present invention is applied to a piston ring.

EXAMPLES First, details which are common to all the examples are described.

Referring to Fig. 1, an enlarged first piston ring is illustrated. The first piston ring 8 has a nominal diameter of 78 mm, width (B) of 1.5 mm and thickness (T) of 2.8 mm. The substrate 1 is made of oil-tempered silicon-chromium steel for the valve-spring use, which steel corresponds to SWOSC-V under JIS designation. The manganese phosphate layer (the first coating 2) is formed on the substrate 1 except for the outer peripheral surface. On the first coating 2 is formed the second coating 3 with the binder resin (polyamide imide) and the particles of molybdenum disulfide being dispersed in the binder resin.

On the outer peripheral surface of the substrate is formed the hard chromium-plating layer 4 being the slidable contact with the cylinder liner denoted as 15 in Figs. 9 and 10. Instead of the hard chromium-plating layer 4, a nitride layer or a flame-sprayed layer of Mo or the like may be formed. The second pressure ring may have the same construction as in Fig. 1.

The first pressure ring 8 was manufactured by the following method. Note that there are given descriptions of both the typical values employed in the examples and the preferred range of values applicable to the present invention.

The above-mentioned oil-tempered SWOSC-V, which has been worked into a wire having a rectangular cross section of 1.5 mm x 2.8 mm, is plastically worked into a ring shape having a nominal diameter of 78 mm in the free state. First, the hard chromium plating layer 4 is formed by the conventional method on the outer peripheral surface of the ring-shaped wire material.

This material is then subjected to the following chemical conversion phosphatizing under the following procedure (1) (4) to form the manganese phosphate layer (the first coating 1), while masking the hard chromium-plating layer 4.

(1) First, the substrate is dipped in a 16% aqueous solution of the sodium-hydroxide and sodium-carbonate mixture (temperature - from 50 to 90"C, typically 90"C) for 3 to 7 minutes, typically 3 minutes so as to degrease the substrate 1.

(2) The substrate 1 is then dipped in 5% aqueous solution of a sulfuric-acid and phosphoric-acid mixture in the ratio of 4 : 6 (the temperature - from 15 to 35"C, typically 300C) so as to clean the surface of the substrate.

(3) Subsequently, the substrate is subjected to surface conditioning with the titanium colloid using a surface conditioning agent for the manganese phosphate coating available under the trade name of PP=VMA, B (Nihon Parkerizing Co., Ltd.) (temperature - from 25 to 500C, typically from 450C) for 30 to 70 seconds (typically 60 seconds).

(4) The substrate is subsequently subjected to the chemical conversion phosphatizing to form manganese phosphate using an agent available under the trade name of PF=MS (Nihon Parkerizing Co., Ltd.) at a temperature of from 82 to 850C for 2 to 5 minutes (typically 5 minutes). The total acid (TA) of the phosphatizing liquid is from 21 to 25 points, typically 23 points. The free acid is from 4 to 7 points, typically 6 points. The iron content is from 0.4 to 3 g/L (typically 1 g/L). The total acid and free acid is analyzed by using titrants.

The lubricating layer (second coating 3) is then formed by applying the polyamide-imide binder with the dispersed molybdenum disulfide particles on the upper and lower surfaces of the piston rings. The amount of molybdenum disulfide is 50 % by weight.

Finally, the hard chromium-plating layer 4 on the outer peripheral surface of the piston rings is lapping-finished.

The pressure piston rings shown in Fig. 1 is thus completed.

Examples 1 - 7.

As shown in Table 1 below, the average particle diameter of molybdenum disulfide is varied in Examples 1 through 6, i.e., the inventive examples of piston rings. In addition, the molybdenum disulfide particles are replaced with the graphite particles having average particle size of 1.5 pm and dispersed in 50 % by weight in Example 7. The other features of Example are the same as those of Examples 1 through 6.

Comparative Example 1 For the comparison purpose, the phosphatizing agent to form a zinc phosphate coating, which is available under the name of Palbond 880 (Nihon Parkerizing Co., Ltd.), is used.

The other items are the same as in Example 1.

Table 1 Solid Lubricant Average Particle Diameter (ym) Example 1 MoS2 0.8 Example 2 MoS2 1.0 Example 3 MoS2 1.5 Example 4 MoS2 2.0 MoS2 2.2 Example 5 MoS2 2.2 Example 6 Mop 8 Example 7 Graphite 1.5 Comparative MoS2 0.8 Example 1 Referring to Fig. 4, an SEM image of the solid lubricant layer (the second coating layer) of Example 3 is shown, while referring to Fig. 5 an SEM image of the solid lubricant layer (the second coating layer) of Example 6 is shown. In Figs. 4 and 5, the binder resin (polyamide imide) is denoted by 20, and the molybdenum disulfide particles 10 are denoted by 10.

They, 10 and 20, are embedded between the manganese phosphate crystals.

The wear-resistance test of the piston rings produced by the above procedure was carried out to evaluate the relationship between the materials of the piston rings and the sliding of the piston rings in piston grooves of a piston.

Referring to Fig. 6, a wear-tester comprises a vertically moving part 5 for tapping and a rotary part 6 which is opposite to the vertically moving part 5. The vertically moving part 5 is vertically driven by a motor via a crank mechanism (not shown). The rotating part 6 is secured to the rotary vertical shaft 6a and driven by a motor via the shaft 6a. The rotary part 6 can be continuously rotated or be rotated in an inverse direction. A disc 7 made of aluminum alloy for a piston is fixed horizontally to the lower side of the vertically moving part 5. A heater is mounted in the interior of the vertically moving part 5 and heats the aluminum alloy to any predetermined temperature. A piston ring 8 is inserted and horizontally fixed on the upper side or the rotary part 6 and is arranged opposite to the disc 7.

By the construction as described above, the vertically moving disc 7 driven by the crank mechanism is tapped on the rotating piston-ring 8 driven on the rotary part 6. The tester shown in Fig.6 reproduces the collision and sliding of the piston rings and ring grooves, in which the piston rings are fitted.

The tapping load can be optionally set by means of the hydraulic mechanism (not shown) which is provided for the rotary part 6. The tapping frequency (per time) and the rotating speed can also be optionally set.

The pressure rings 8 treated and the disc 7 made of aluminum alloy for piston (AC8A-T 6) were mounted in the tester.

The wear test was carried out under the following conditions.

Lubrication: dry Tapping Load: 150 kgf Tapping Frequency: eight times per second Rotating Speed: Continuous one-directional, 2.0 mm/second Temperature of Disc: 2800C The results of the test are shown in Fig. 7.

As is apparent from Fig. 7, the piston rings according to the inventive examples exhibit both less piston-ring wear and less disc-wear (wear of the piston) than all the comparative examples. The durability attained by the present invention is improved over that of the comparative examples.

The durability is outstanding when the average particle diameter of the solid lubricant is from 1 to 2 pm.

After the wear test of Example 3, the surface of the piston ring was observed by SEM. The SEM image is shown in Fig. 8.

In Fig. 8, between the crystal grains 9 of manganese phosphate, which appear light gray, the intercrystalline clearances are formed and appear dark gray. Most of the molybdenum disulfide particles 10, which appear lustrous black, enter and fill the intercrystalline clearances 9. Contrary to this, since the molybdenum-disulfide particles used in Example 6 are coarser than those of Example 3, the proportion of particles embedded in the intercrystalline clearances 9 is smaller in Example 6 than in Example 3. In other words, the majority of molybdenum-disulfide particles are merely bonded by the binder.

The lubricating layer partly peels off from the underlying manganese phosphate layer, although very slightly, in Example 6. However, the wear-resistance is improved in Example 6 as compared with Comparative Examples.

Various modifications can be made to the examples hereinabove, provided that such modifications lie within the technical concept of the present invention.

Claims

1. Wear-resistant parts, particularly a piston ring, which have a laminate structure comprising a substrate (1), a first coating (2) formed on the substrate (1) by a chemical conversion phosphatizing, and, a second coating (3) formed on the first coating (2) and containing a solid lubricant, characterized in that clearances are formed between the crystal grains (31, 32) of the chemical conversion phosphate on its top surface part and have width and depth to embed particles of the solid lubricant (10, 31, 32) in the clearances.

2. Wear-resistant parts according to claim 1, wherein most of the particles of the solid lubricant (10, 31, 32) embedded in the clearances.

3. Wear-resistant parts according to claim 1 or 2, wherein said first coating (2) formed by a chemical conversion phosphatizing consists of manganese phosphate.

4. Wear-resistant parts according to claim any one of claims 1 through 3, wherein an average diameter of said particles of the solid lubricant (10, 31, 32) is from 1 to 2 pm.

5. Wear-resistant parts according to claim any one of claims 1 through 4, wherein said solid lubricant is molybdenum disulfide.

6. Wear-resistant parts according to claim any one of claims 1 through 5, wherein they are brought into a slidable contact with aluminum alloy which may be anodic oxidized.

7. Wear-resistant parts according to claim any one of claims 1 through 6, wherein said first coating (2) and said second coating (3) are formed on an upper surface and a lower surface of a piston ring (8) to be mounted in a piston (11) made of an aluminum alloy.

8. Wear-resistant parts according to claim any one of claims 2 through 7, wherein said substrate (1) has been subjected to surface conditioning by titanium colloid prior to said chemical conversion phosphatizing.

9. Wear-resistant parts according to claim any one of claims 4 through 8, wherein said second coating (3) has a thickness of from 3 to 13 pm.

10. Wear resistant parts substantially as hereinbefore described with reference to Figs. 1 to 8 of the accompanying drawings.