US3539192A - Plasma-coated piston rings - Google Patents

Description

nited States Patent [73] Assignee Ramsey Corporation St. Louis, Missouri a corporation of Ohio [54] PLASMA-COATED PISTON RINGS 9 Claims, 8 Drawing Figs.

[52] US. Cl 277/224; 117/93.1; 277/216, 277/235, 277/236 [51] Int. Cl Fl6j 9/00 [50] Field oi'Search 277/223,

224, MD, 216, 235A, 234, 235, 236; 29/182.2, 182.3. 182.7, 182.8, 183; 117/105,105.2,

3,322,513 5/1967 Corbett 29/1 82.7 3,407,866 10/1968 Sawchuk 29/182.7X FOREIGN PATENTS 441,812 1/193 6 Great Britain 277/235A Primary ExaminerLaverne D. Geiger Assistant ExaminerJeffrey S. Mednick Attorney-Hill, Sherman, Meroni, Gross and Simpson ABSTRACT: Piston rings, including compression and oil control rings, for internal combustion engine pistons, having a bearing face of an alloy formed in situ on the ring from a plasma jetstream. The alloy is composed of refractory metal carbides such as tungsten carbide in solid solution with another metal, such as cobalt, to provide a hard wear phase with the carbide particles relatively free from sharp edges and corners and a somewhat softer matrix phase composed of metals such as nickel, chromium, boron and aluminum. The coating is very hard and refractory, possesses a higher tensile strength than heretofore used piston ring facings, does not scuff, has improved abrasive wear resistance, and operates compatibly with the engine cylinders. The carbides in the coating will not pull out in operation of the ring because they are in solid solution and do not have a sharp particulate form. The nickel, chromium and boron in the alloy provide binders improving the mechanical strength of the coating and the hardness of the matrix.

1 PLASMA-COATED PI'sToN RlNGS BACKGROUND OF THE INVENTION 1. Field 'of the Invention This invention pertains to the packing ring or piston ring art and to the provision of bearing faces on piston rings which will resist adhesive wear, abrasive wear, and corrosive wear encountered in high-compression, high-speed, and high-temperature operating internal combustion engines without unduly wearing the engine cylinder.

2. Description of the Prior Art Piston rings, including compression rings and oil control' rings, coated with hard-facing metal with good scuff-resisting properties are disclosed in the following U.S. Pats. to Roy D. Anderson, No. 2,905,512, issued Sept 22, "1959; Melvin W. Marien, No. 3,133,739, issued May 19, 1964; Melvin W. Ma'rien, No. 3,133,341, issued May 19, 1964; Donald J. Mayhew, et al., No. 3,281,156, issued Oct. 25, 1966.

While the name spray applied molybdenum hard-facing material disclosed in these patents afforded the heretofore best known performance for piston rings in high-compression, high-temperature operating internal combustion engines, engine builders continue to increase compression ratios,'operating temperature ranges,a'nd speed requirements and continue to demand even greater perfection inpiston ring performance. While it was kno'wn'tha't-increasing the hardness of the facing metal on the pistonrings would enhance thewea'r resistance of the ring, metalsor alloys "more refractory than molybdenum were found to induce engine'cylinder wear and to haveinsufficient mechanical strength to withstand high-speed high-compression operation. Best heretofore known thermally applied molybdenum pistonringfacings have a tensilestrength of approximately 9,000 psi or less.

Attempts to provide piston rings with refractory facings composed of refractory metal carbid'es'such as chromium carbides, tungsten carbides,'and silicon carbides have heretofore been unsuccessful because the-carbides appear as sharp-edged or globular particles which pull out of the coating in operation inthe engine causing high piston ring and cylinder wear.

SUMMARY The presentinvention now provides hard-faced piston"rings, giving greater performance'in high-speed, high-compression, high-temperature operating engines than heretofore known piston rings. The rings of this invention are coated with a plasma jet appliedrefractory metal carbide alloy formed in situ on the ring. Suitable refractory metal carbides include the carbides of tungsten, titanium, tantalum,-columbium, molybdenum, vanadium, chromium, zirconium, hafnium, silicon and boron.

In general the 'term refractory'metal carbide as used herein means acarbideof-a'metalor'metalloid having a melting point above about 3,000F. and -'a hardness above about l,500'Vickers DPN (diamond pene'tration'numbefl with a'40 gram load, referred toas40 DPN, The'secarbideshave very low solubility in cobaltwhichis used in the pro'duction ofsintered carbides for-formingbodies of high hardness and compressive strength. According to "this inventionfhoweven'the carbides are placed in solution with cobalt by virtue of the very high temperatures obtainable in 'theplasrna jetstream.

The carbide-facing material 'of'this invention contrary to previously tested carbide-facing materials for'piston rings,'has

surprisingly eliminatedheretofore encountered-scuffing and does not appreciably wear the cylinder liner even when operated under conditions which cause heretofore known carbide-faced piston rings to scuff, adversely wear the cylinder liner and disintegrate. The plasmajet-applied 'carbide'coatings of this invention, by having the carbides' in solution in the alloy, eliminate 'the"heretofore encountered pullout problem causing the scuffingandwear.

The carbide contentof the'a'lloyshould'also'be controlled to insure retention of the refractoryrnetal carbide "in solution with the cobalt and to providean alloy ofsufficient strength to 1 refractory metal carbides thereon.

withstand the thermal bimetal expansion forces combined with the mechanical stresses encountered in the engine. Further, it is highly desirable to provide a facing material which can be finished by grinding on conventional silicon carbide and aluminum oxide-grinding wheels and excessive amounts of the refractory metal carbide in the alloy will provide a facing material which is too hard to finish by conventional grinding methods. Therefore, in general, the refractory metal carbide content of the powder mixture fed to the plasma jetstream to produce the in situ formed hard-facing material on the piston ring should be between about 25 to 55 percent by weight of the powder.

In accordance with the preferred embodiment of this invention, ferrous metal compression rings composed of conventionally cast nodular .iron of about 3% percent carbon content by weight, thin rail rings for oil control assemblies composed of carbon steel such as S.A.E. 1070, and the like base metal rings, are coated from a plasma jetstream, receiving a powder of the following composition:

25 to 55 percent 'by weight tungsten carbide 4 to 8 percent by weight cobalt 25 to 45 percent by weight nickel 3 to 7percent by weight chromium l to 7 percent by weight aluminum 0 to 3 percent by weight boron Balance substantially iron.

The tungsten carbide content of the powder may be admixed with or replacedby other carbides such as'the carbides of metals and metalloids from the group'including titanium,

tantalum, columbium, molybdenum, vanadium, chromium,

zirconium, hafnium,:silicon and boron.

Theplasmajet'has a fuel gas preferably composed of a mixture of nitrogen and hydrogen and an inert carrier gas,

matrix is composed'principally of nickel and chromium and thealuminum is in'the form of metallic aluminum'or aluminum oxide. This alloy has a melting point of approximately 5,000F., has about five times the wear resistance'of the heretofore used molybdenum or chromium hard-facing metals and reduces bore wear and scuffing far beyond the best results obtained with previously tested hard carbide-faced piston rings. The alloy has excellent mechanical strength and shock 'resistanceunder :a wide range of severe temperature conditions. Tensile strengths in :excess of 15,000 p.s.i. have been obtained.

It is then an object of this invention to provide improved pistonrrings with hard-faced 'bearingzsurfaces composed'ofla Jplasma jet applied refractoryimetal carbide alloy.

lt isalso an object ofthis invention to provide a piston ring coating containing refractory metal carbides in solution inthe facing alloy, reducingor eliminating the possibility of carbide pullout during engine operation.

A further object of this'invention is-to provide rings with temperature and-wear-resisting, hard-facing refractory metal carbide alloysw'hich will not unduly wearan engine cylinder and which have agreaterstrength.under wide .temperature "ranges than heretofore known'hard facings onpiston rings.

Another object ofvthis inventionis to increase the operating parameters of pistonrings byproviding plasma-jet coatings'of Another object of this invention isto provide piston rings with hard-facing alloys which are formed'in situ on thering by 'a 'plasma jet from a powder containing up to '55'percent 'by "weight of ar'efractory'metal carbide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side elevational view, with parts in cross section, of an engine piston and cylinder assembly, wherein the piston has ring grooves equipped with compression and oil control rings each having a bearing face engaging the cylinder which is composed of in situ formed plasma jet applied carbide alloys, according to this invention.

FIG. 2 is an enlarged fragmentary cross-sectional view of the top compression ring in the piston of FIG. 1.

FIG. 3 is a view similar to FIG. 2, but illustrating the second compression ring in the piston of FIG. 1.

FIG. 4 is a view similar to FIG. 2, but illustrating the oil control ring in the third ring groove ofthe piston of FIG. 1.

FIG. 5 is a view similar to FIG. 2, but illustrating the oil control ring in the fourth ring groove of the piston of FIG. 1.

FIG. 6 is an elevational view of an arbor of piston rings being plasma jet coated in accordance with this invention.

FIG. 7 is a greatly enlarged fragmentary cross-sectional view of a compression piston ring having a bearing face band of carbide alloy, bonded in a peripheral groove of the ring in accordance with this invention.

FIG. 8 is a greatly enlarged somewhat diagrammatic view illustrating the manner in which a piston ring coated with prior used hard-facing metal will scuff under severe operating conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The piston and cylinder assembly 10 of FIG. I illustrates generally a conventional four-ring groove internal combustion engine piston, operating in an engine cylinder. The assembly 10 includes apiston l1 and an engine cylinder 12 with a bore 13, receiving the piston 11. The piston 11 has a head 14 with a ring band 15 having four peripheral ring grooves 16, 17, 18 and 19 therearound. The top ring groove 16 has a split solid cast iron compression or fire piston ring therein. The second ring groove 17 has a split solid second compression ring 21 somewhat wider than the ring 20. The third .ring groove 18 carries a two-piece oil control ring assembly 22. The fourth or bottom ring groove 19 carries a three-piece oil control ring assembly 23.

As shown in FIG. 2, the top compression or fire ring 20 has a main body 24 composed of cast iron, preferably nodular gray iron, with a carbon content of about 3 /2 percent by weight. The outer periphery 25 of this ring is covered with a plasma jet applied carbide alloy coating 26.

As shown in FIG. 3, the second compression ring 21 has a main body 27 composed of the same type of cast iron as the body 24 of the ring 20. The outer periphery 28 of the body 27 is inclined upwardly and inwardly from the bottom edge of the ring, and a peripheral grove 29 is formed around this inclined periphery. The groove 29 is filled with the carbide alloy 26.

As shown in FIG. 4, the oil control ring assembly22 in the third ring groove 18 is composed ofa one-piece flexible channel ring 30 and a sheet metal expander ring 31, having legs extending into the channel for expanding the ring 30. The ring 30 and the expander are more fully described in the aforesaid Mayhew et al. U.S. Pat. No. 3,28l,l56.

The one-piece oil control ring 30 has a pair of axially spaced, radially projecting beads 32. The peripheries of these beads 32 are coated with the coating 26.

In FIG. 5, the oil control ring assembly 23 includes a resilient spacer-expander ring 33 supporting and expanding split thin rail rings 34. The assembly 33 is of the type disclosed in the aforesaid Marien U.S. Pat. No. 2,817,564. The outer peripheries of the rail of rings 34 are coated with the coating 26, according to this invention.

From the above description, it will be understood that the bearing faces of each of the compression and oil control rings 20, 21, 22, and 23 are coated with the carbide refractory alloy according to this invention. These bearing faces 26 ride on and sealingly engage the bore 13 of the engine cylinder 12, and the rings are compressed in the bore 13, so as to expand tightly against the bore wall, and maintain a good sealing sliding engagement therewith.

As shown in FIG. 6, the coatings 26 are applied on the rings as for example on the grooved rings 21 by stacking a plurality of the rings on an arbor 35, with the rings compressed so that their split ends will be in abutment. The arbor clamping the stack of rings in their closed, contracted position, may be mounted in a lathe and the peripheries of the rings machined to form the grooves 29 therearound. The outer peripheries of the rings 21 on the arbor are then coated with the coatings 26 from a plasma jet spray gun 36. The gun 36 includes an insulated casing such as Nylon 37, from which projects a rear electrode 38, the projection of which is adjustably controlled by a screw nob 39. The front face of the casing receives a front electrode 40. The casing 37 and electrode 40 are hollow and water-jacketed so that coolant may circulate therethrough from an inlet 41 to an outlet 42. Plasma jet gas is fed through an inlet 43 into the chamber provided by the casing 37 and the electrode 40 to flow around the electrode 3 8.

The front end of the electrode 40 provides a nozzle outlet 44 for the plasma flame and the ingredients to form the alloy of the coating 26 are fed to this nozzle through a powder inlet 45,just in advance of the discharge outlet of the nozzle.

A plasma composed of ionized gas is produced by passing the plasma gas from the inlet 42 through an electric are established between the electrodes 38 and 40. This plasma gas is nonoxidizing and may be composed of nitrogen and hydrogen with nitrogen, argon or helium as a carrier. The plasma flame exuding from the nozzle 44 draws the alloyforming powder therewith by aspiration and subjects the powder ingredients to such high temperatures as to cause them to alloy. The jetstream carries the alloy into the bottom of the groove-29 of each piston ring and fills the groove.

The preferred powder fed to the powder inlet 44 of the gun 36 is composed of tungsten carbide, cobalt, nickel, chromium, boron and aluminum, in the proportions indicated herein above, with a preferred powder mixture of the following composition:

40 percent by weight tungsten carbide 6 percent by weight cobalt 36.5 percent by weight nickel 6 percent by weight chromium 1 percent by weight boron 3 percent by weight aluminum Balance substantially iron, with minor amounts of silicon and carbon.

The preferred deposited coating 26 is a tungsten carbide alloy wherein the tungsten carbide ingredient is bound in a fused and alloyed matrix of the nickel, chromium and boron. The alloy 26 as illustrated in FIG. 7 is actually formed in situ in the groove 29, and is bonded to the base body 24 of the ring along a diffused interface or welded zone 46. This interface, or zone 46, is composed of the materials of the alloy 26 and the material of the ring body 24.

The alloy 26 has a hexagonal close pack crystalline structure in a matrix composed principally of nickel and chromium.

This alloy has three principal phases, the hardest phase being composed of tungsten carbide in solution with the cobalt and having a Vickers hardness of 2,900 to 4,000 DPM (diamond penetration number) with a 40 gram load (40 DPN). In this phase the carbides are well in solution and are not sharp. The second phase is also composed of tungsten carbide and cobalt with the carbides in solution but has a particle hardness in the range of 2,100 to 3,200 Vickers, 40 DPN. The third phase is a matrix phase with a Vickers particle hardness in the range 900 to 1,200, 40 DPN. This phase is composed principally of nickel and chromium with boron, if present, uniformly distributed in the matrix. A fourth phase comprising only about 4 percent by volume of the final coating consists principally of aluminum and has a hardness of approximately 500 Vickers 40 DPN.

The tungsten carbides in the preferred alloy consist principally of W2C and (WCo)2C and may be considered to have the following formula:

During the jet spray application, it is desired to maintain a temperature in the groove 29 such that will prevent excessive melting and burning away of the body metal 24. For this end result, the arbor of rings is preferably cooled with an external blast of inert gas impinging on both sides of the jet flame. It is desired to keep temperatures of the rings 21 in the arbor around 400F. or less. It is not necessary to provide any subsequent heat treatment for the plasma jet coated rings other than allowing the rings to air cool.

The powder fed to the inlet 45 is metered preferably with the aid of an aspiring gas, vibration, mechanical gearing etc. All of the powder is completely melted and penetrates into the center cone of the plasma jet flame.

To get the carbides in the solution with the cobalt it is not only important to use the very high temperatures available in the plasma jetstream but the so-called spray parameters of the powder fed to the stream are important since if the tungsten carbide content of the powder is appreciably over 55 percent by weight it is very difficult to retain the carbides in solution and if the carbide content is appreciably below 25 percent by weight, wear and scuffing of the cylinder occurs. Thus an alloy having a tungsten. carbide content of 66 percent, a nickelchromium-boron binder content of 18 percent, and a nickel aluminide content of 7 percent, was found to cause excessive cylinder wear and quickly scuffed. The material was brittle and flaked during tests. An alloy with a tungsten carbide content of 88 percent could not be finished on a conventional-grinding wheel. An alloy with a tungsten carbide content of 22 percent was found to quickly scuff and to increase the bore wear beyond the amounts encountered with the alloys of this invention.

The scuffing action is illustrated in FIG. 8, wherein the coating 26a filling the groove 29 in the piston ring 24 is illustrated as having an adhesive affinity for the wall of the engine bore 13 along the area 48 of sealing engagement withthe bore wall. This adhesion and the brittleness of the coating 26a causes the metal in the groove 29 to break away along lines of fracture indicated at 49. This produces the scuffing effect on the bearing face and destroys not only the sealing efficiency of the bearing face but also causes abrasion of the cylinder bore 13.

The coatings 26 of this invention are less porous than the heretofore'known flame-sprayed molybdenum coatings. For example where such flame-sprayed molybdenum coatings have a porosity in the range of l5 to 30 percent, the coatings 26 only have a porosity around 7 percent. This provides much greater corrosion resistance in the bearing face.

Since the softening points of the in situ formed alloy coatings 26 of this invention are over 1,900F., the bearing facev can of course withstand much higher temperatures than the prior known piston ring coating materials.

The alloys of the coatings 26 also have about 5 times the wear resistance of the heretofore used molybdenum and chromium hard-facing material, and therefore much thinner coatings can be used. It has been found that coating thicknesses of .002 inches in oil control ring assemblies and from .002 to .004 inches in compression rings are quite satisfactory. This of course reduces the expense of producing the rings.

The rings of this invention have been severely tested both in actual high-performance diesel engine operation and have also been subjected to thermal stress and oxidation resistance tests. The tungsten carbide alloy coatings of this invention withstood thermal stress tests which consisted of heating the piston rings with the coating in the stressed condition to 1,800F. for hours. Post test photornicrographic examination showed this coating to withstand this test, whereas previousl yused molybdenum coatings will not withstand this test at 750 The alloys of the coatings 26 can be ground with standard dressing wheels even though they have a superficial hardness far in excess of the heretofore known hard facing materials for piston rings. Thus, the alloys of the coatings 26 were found to have a hardness of more than 1,500 kilograms per square millimeter on the Vickers scale, whereas the best heretofore known hard-facing materials for piston rings had a Vickers hardness around 1,000 kg. sq. mm.

The provision of the alloy coatings 26 in a groove to form a band around the periphery of the piston ring 21, for example, utilizes the body metal of the ring as a land alongside of the groove to form an initial quick break-in surface for the ring, as described in the aforesaid Marien U.S. Pat. No. 3,133,739. The inclined periphery of the ring 21 may be formed by grinding or by torsional twisting of the ring in use in the ring groove, as described in the Marien patent.

It will be understood that, while tungsten carbide is the preferred refractory metal carbide, it may be replaced in whole or in part with any one or more of the hereinabove mentioned refractory metal carbides. The refractory metal carbide content may vary considerably as long as the carbides are in solution in the alloy and are relatively free from sharp edges and comers which induce pullout during operation. For practical purposes the refractory metal carbide content will be between 25 to 55 percent by weight of the powder fed to the plasma jetstream since these parameters insure the scuff-resistance, mechanical strength, and finishing ability of the al- 10y. The starting powder preferably has the carbides present as individual grains sintered with the cobalt so that in the preferred embodiment the starting powder would have sintered grains containing 40 percent by weight of tungsten carbide and 6 percent by weight of cobalt.

From the above descriptions, it will therefore be understood that this invention now provides piston rings coated with hardfacing' metals which will give better performance in engine operation than heretofore known.

I claim:

1. A piston ring having a ferrous metal body and a coating on the bearing face of the ring, said coating being composed of at least one carbide of a-substance from the group consisting of refractory metals and metalloids in solution with cobalt and alloyed in a matrix containing principally nickel and chromium, said carbide being free from sharp edges and corners, and said alloy being bound to said body along a diffused interface zone composed of materials of said alloy and said body. 16

2. A piston ring in accordance with claim 1 in which the carbide has a melting point above 3,000F. and a hardness above 1,500 Vickers 40 DPN.

3. A piston ring in accordance with claim 2 in which the carbide is tungsten carbide and' is present in the alloy mix in amounts not appreciably below 25 percent and not appreciably above 55 percent by weight.

4. A piston ring in accordance with claim 2 in which the coating alloy has a tensile strength in excess of 15,000 p.s.i.

DPN, a second phase having a hardness of 2,000 to 3,500

Vickers 40 DPN,

5. A piston ring in accordance with claim 1 in which the v coating has a three-phase structure, one phase having a hardness range between 2,900 and 4,000 Vickers 40 DPN, a second phase having a hardness of 2,000 to 3,500 Vickers 40 DPN, and a matrix phase having a hardness of 500 to 1,500 Vickers 40 DPN.

6. A piston ring in accordance with claim 1 in which the alloy has a hexagonal close pack crystalline structure.

7. A piston ring in accordance with claim 1 in which the coating contains W C(WCO)2C.

8. A piston ring in accordance with claim 1 in which said coating is formed on the hearing face by suspending a powdered mixture of the materials which form the coating in a plasma at a temperature substantially above the melting temperature of said mixture, forming a jetstream of the plasma with the mixture is suspension and applying the resulting jetstream to the bearing face of said ring while maintaining it at a temperature suitable to deposit thereon a coating of said carbide in solution with cobalt in said alloy matrix.

9. A piston ring in accordance with claim 1 wherein the coating on the bearing face is applied from a plasmajetstream and the alloy is formed in situ on the ring.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3, Dated November 10,

Inventor(s) F.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 56, "l500" should be --1500--;

Column 4, line 5, after "rail" cancel "of";

Column 5, line 47, after "scuffed" insert (period);

Column 5, line 47, before "brittle" insert --quite--;

Claim 4, column 6, lines 73-75, cancel "DPN, a second phase havi1 hardness of 2,000 to 3, 500 Vickers 40 DPN,

Claim 7, column 7, line 10, cancel "W C(WCO)2C" and insert --W C(WC0) C--.

Signed and sealed this 16th day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Pat FORM PO-105O (ID-69) nr- .mn-

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