US 3910734 A
An apex seal construction in a rotary internal combustion engine is disclosed as well as a method of making same. The apex seal is carried within a slot of the rotor and is urged by gas pressure for maintaining sealing contact between a side of said slot and the rotor housing. The apex seal has a supporting body formed of a light weight metallic material, such as aluminum, and a wear-resistant coating applied by plasma jet as a substantial envelope thereabout, the combined heat of the plasma as well as the force of impact of the jet combine to provide an improved adherency between the coating and light weight body. Immediately subsequent to plasma jet deposition, the coated apex seal is subjected to a quenching medium to develop a hardness of at least 40 Rc, uniformly throughout the bulk of the material the coating having a controlled porosity of 2-5% and a thickness in the range of 15-25 mils.
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Description (OCR text may contain errors)
United States Patent Telang Oct. 7, 1975 COMPOSITE APEX SEAL  Appl. No.: 390,133
Primary Examiner--C. .l. l-Iusar Assistant Examiner0. T. Sessions Attorney, Agent, or FirmJoseph W. Malleck; Keith L. Zerschling [5 7] ABSTRACT An apex seal construction in a rotary internal combustion engine is disclosed as well as a method of making same. The apex seal is carried within a slot of the rotor and is urged by gas pressure for maintaining sealing contact between a side of said slot and the rotor housing. The apex seal has a supporting body formed of a light weight metallic material, such as aluminum, and a wear-resistant coating applied by plasma jet as a substantial envelope thereabout, the combined heat of the plasma as well as the force of impact of the jet combine to provide an improved adherency between the coating and light weight body. Immediately subsequent to plasma jet deposition, the coated apex seal is subjected to a quenching medium to develop a hardness of at least 40 R uniformly throughout the bulk of the material the coating having a controlled porosity of 25% and a thickness in the range of 15-25 mils.
4 Claims, 6 Drawing Figures U.S. Patent 0a. 7,1975 Sheet 1 of2 3,910,734
FIG.3 22 PIC-3.4
COMPOSITE APEX SEAL BACKGROUND OF THE INVENTION Chatter marks are consistently experienced on the inner wall of the rotor housing for a rotary engine constructed according to prior art knowledge. At higher speeds of rotation of the piston, this is particularly notable and eventually result in an ineffective sealing between the rotor and the rotor housing because the sealing strips are unable to conform to the convolutions of such chatter marks. The chatter marks result in part from unusually high dynamic loading at certain zones of the epitrochoid path. The rotor, being eccentrically mounted, imparts inertial energy to the seal element to move not only radially outwardly but transversely in a slot of the rotor housing. Gas pressure promotes a force to cause the sealing strip to engage a side of the slot as well as engage the rotor housing surface. As the dynamic loading is accentuated in certain zones, chatter marks appear as a result of gouging of the surface of the rotor housing.
There is a high rate of abrasion between the inner epitrochoid surface of the rotor housing and apex seals on the rotor. To overcome this, abrasion resistant coatings or layers have been applied to the apex seals. Difficulties are encountered in formulating the abrasion resistant material since it must withstand the high operating temperature conditions in the engine and there must be an economical, efficient process for applying the coating. Most importantly, when the engine is in operation, the abrasion resistant layer can easily separate from its substrate, breaking away due to insufficient bond.
One possible way known to the prior art of increasing the abrasion resistance of the seal is to electrolytically apply a layer of nickel containing embedded particles of silicon carbide. However, electrolytic deposition results in uneven coating thickness due to the sharply turned configuration of the crown of an apex seal. The prior art has also used ceramic materials applied particularly by a flame spray technique. But a ceramic layer applied by flame spraying is brittle and has a low coefficient of thermal expansion resulting in thermal stresses during operation of the engine. Attempts to improve the adhesion between flame sprayed materials has comprised (a) undercutting slots, (b) using intermediate layers to promote a metallurgical bond therebetween, both of which is expensive and time consuming and has not resulted in optimum adhesion.
SUMMARY OF THE INVENTION A primary object of this invention is to provide an improved apex seal construction for use on the rotor of a rotary internal combustion engine.
Another object of the invention is to provide a composite apex seal construction that has unprecedented adherence between a wear-resistant coating and the supporting substrate.
Other objects of the invention include provision of a wear-resistant coating as part of the composite apex construction as in the foregoing objects, the wear resistant coating having limited porosity resulting from the uniqueness of deposition and the wear-resistant portion having a higher level of hardness at elevated temperatures than related coatings of the prior art. The composite apex seal construction is characterized by a very low modulus of the elasticity thereby cooperating to subject the wear-resistant coating to less stress during engine operation.
Yet still another object of the invention is to provide a unique method for fabricating a composite seal construction, the method comprising: (a) Defining a metallic supporting portion for an apex seal particularly comprised of an alloy of aluminum or a denser material hollowed for weight reduction; (b) Subjecting the supporting portion to a plasma spray of admixed powders consisting preferably of martensitic stainless steel and nickel-based alloy powders, each of generally the same hardness; (The powders are introduced to a plasma gun having a diverging supersonic type nozzle, the powders are subjected to a temperature of at least 2800F and impelled by the force of the plasma jet to deform in an irregular manner with less porosity than experienced by the prior art). (c) Immediately upon deposition of said plasma impelled powder, the body and coating are subjected to a quenching medium effective to develop a hardness level of at least R 80. The force of the plasma jet results in a mechanical bond between the coating and supporting portion which is at least 8,500 psi. as determined by an epoxy adhesion test.
SUMMARY OF THE DRAWINGS FIG. 1 is an elevational view of a rotary internal combustion enginehaving a portion broken away to illus trate the orientation of the side housings relative to the rotor housings and against which the apex seal construction must operate;
FIG. 2 is a sectional view taken substantially along line 2--2 of FIG. 1;
FIG. 3 is an enlarged central sectional view of an apex seal construction embodying the principles of this invention;
FIG. 4 is a broken side-elevational view of the construction shown in FIG. 3;
FIG. 5 is a schematic illustration of the microstructure of the coating system of the apex seal construction;
FIG. 6 is a schematic layout diagram of a plasma spraying apparatus illustrating the method of depositing the coating system of this invention.
DETAILED DESCRIPTION Turning now to FIGS. 1 and 2, the invention pertains to an apex seal construction 10 which is of the type adapted to fit within slots 11 defined within the apices of a rotor mounted for planetary movement within an epitrochoidally shaped housing chamber 12. The apex seal construction serves to separate the space between the triangulated rotor 12 and. the surrounding housing walls into three variable volume chambers a, b and c. To experience a four-cycle engine operation in each of the variable volume chambers, during a single revolution of the rotor, the efficiency of the apex seal construction becomes critical.
The structure defining the epitrochoidally shaped chamber 13 is comprised of side housings 14 and 15, spaced apart a distance slightly larger than the width of the narrow rotor. The side walls are connected by a rotor housing 16 at the outer extremity which carries an internally facing epitrochoidally shaped wall 16a. Each of the side housings are provided with a wearresistant coating system 17, the coating extending throughout the entire area of the side walls which do come in contact with the ends of the apex seal construction during operation. The rotor housing has a coating system effective to resist wear from engagement with the apex seal construction. Each of these coatings on the housing portions may be comprised of materials such as electrolytically deposited nickel with embedded silicon carbide particles. The coating system should have a minimum hardness level of at least 30 R to work compatibly with the metallurgical properties of the apex seal construction disclosed herein.
The apex seal of this invention particularly comprises a supporting portion which is defined as an elongated strip effective to span the width of the combustion chamber for providing a continuous sliding seal engagement between the epitrochoid wall of the rotor housing and a side of a slot extending across an apex of the rotor. As shows in FIG. 4, the elongated supporting portion 20 may have an undercut portion 21 defining feet 22 and 23 at the opposite ends of the elongation portions of the feed fitting within corner seals 24 which are effective to promote a proper seal at the juncture between side seals 25 and the apex seals. The undercut portions facilitate shift of gas pressure forces operating upon the underside of the apex seal to move the seal to one or the other side of the slot for affecting the appropriate seal at different zones of the epitrochoid path.
The supporting 20 is preferably constituted of a cast material such as aluminum having a specific gravity of about 2.6 and a modulus of elasticity of about 11-12 million p.s.i. It is important that the selected material for defining the supporting portion, be of light weight, be resistant to the environmental conditions of temperature and corrosion experienced within a rotary engine, and have a low modulus of elasticity so that there will be more flexing of the coating superimposed on this supporting portion (the more rigid supporting portion will result in higher stresses on the coating to provide a surface contact as opposed to a line contact). A particularly desirable aluminum for this purpose is type 2,618 or K01 which has high elevated temperature strength properties. It contains an analysis of copper, 0.62% silver, 0.29% titanium, and the balance aluminum. The aluminum alloy is subjected to solution heat treatment conditions for 16 hours at 995F and quenched in cold water to 60F, then heated to 3l0370F for 5-2O hours.
The as-cast surface of the supporting portion is relatively rough which facilitates bonding to the coating applied thereover. Other material may be selected for the supporting portion, such as steel, which has been hollowed to reduce the average weight throughout the volume normally enclosed by the supporting portion. In any event the materials should have a specific gravity for purposes of the full volume of its configuration of no greater than 3.0 and have a modulus of elasticity no less than 12 million p.s.i.
A wear-resistant coating 27 is applied to the supporting portion and is adapted to substantially envelope the supporting portion leaving only the bottom surface 28 exposed. The coating is a plasma sprayed admixture of metal bonded refractory powders and should have a low content of dry lubricants. Preferably an admixture of martensitic stainless steel and nickel-based alloy powders can be used, both being of generally equal hardness. Another is iron and titanium carbide. The critical importance is the use of the plasma spray technique according to the parameters herein. The admixed powders which form the chemicalanalysis of the coating, should be subjected to a temperature of at least 2,800F and subjected to a jet velocity which approaches sonic and in some embodiments is supersonic, such as Mach II. The adhesive bond resulting from the combination of temperature and jet velocity promote an adhesion for metal bonded refractories which will be at least 8,500 psi. as determined by an epoxy adhesion test. Such a test requires that a predetermined area of the coating be coated with an epoxy joint, the joint is coupled to a mechanical device for applying stress. The epoxy bond itself has an adhesion greater than the expected bond between the coating and substrate. Prior art coatings carried out according to other spray techniques have a typically maximum adhesion quality of about 4,0005,5OO p.s.i. as determined again by the epoxy adhesion test.
Equally important, is the denser coating that results from using the technique herein. By virtue of the increased velocity of the jet stream carrying the heated particles, the particles impact with considerably greater force resulting in distortion of the particles and greater elimination of voids that may occur in normally sprayed coatings. Instead of the typical 6l5% porosity that may occur in normal sprayed coatings, the density herein is uniformly in the range of 25% which is critically important to control oil permeation in the coating. More than 5% porosity will lead to excess flooding and interference of sealing efficiency of the apex seal.
To obtain the combined virtues of high wearresistance, strong adhesion and a denser coating, certain parameters must be observed. For example, the particle size of the admixed metal and refractories powders should be in the range of 44-74 microns, this being a relatively fine powder. An improved surface finish is produced and has a surface finish in the range of 400 r.m.s. Subsequent polishing to obtain a 4-8 r.m.s. finish surface is facilitated. Additionally, the density is improved as a result of controlled particle size and can reach -98% theoretical density. The deposition rate of the powders should be at least 10 lbs. per hour thereby increasing the efficiency of the deposition technique; the increased deposition rate results from the higher jet velocity and increased temperature. The
selected temperature of the plasma jet should be arranged so that the particles are substantially heated to solution conditions thereby allowing for a phase transformation in passing through the plasma gun. This permits increased hardness when the coating is quenched immediately after deposition. One metal bonded refractory composition; spray coated according to this invention appears in FIG. 5; martensitic stainless steel and nickel-based alloy were selected as the admixed powders. Some martensitic stainless steel particles, instead of having a perfectly globular shape, assume a striated configuration; the nickel-based alloy, being somewhat larger in particle shape, assumes a highly irregular configuration. Both configurations combine to provide a good binding and inter-locking between the particles of the coating. The voids (which constitute porosity) in the coating are substantially less. The rough as-cast aluminum substrate is somewhat heated by the impacting particles to provide a limited degree of alloying between certain of the adjacent particles. This is a self-contained heat treatment as a result of this invention which produces a lower oxygen content in the coating.
Method A particularly useful method of applying the coating of this invention is to provide forat least sonic and preferably supersonic plasma spray. A Mach II stream may be used to obtain particle velocities which are considerably higher resulting in increased coating density and bond strength. The higher density results because of the associated higher kinetic energy of the particles. The bond strength of the coating also is increased due to the increased mechanical energy expenditure at impact resulting in distortion and interlocking of the particles. The plasma jet velocity is increased by the use of an exit throat insert which has a straight bore and a diverging section. As in conventional chemical spray methods, the principle of the plasma process is that energy, both thermal and kinetic, is imparted to the injected particles by the plasma stream. Since this energy transfer is basically collisional, both the temperature and velocity of any entrained particle are relative to but lower than the properties of the plasma stream. In order to obtain a near molten state and an optimum exit velocity of the injected particles, not only the physical properties and parameters of the plasma stream, but also those of the injected particulates have to be considered.
In subsonic plasma spray systems, the temperature and density of the plasma stream are generally varied to maintain the required temperature level 'of an injected spray powder. This is accomplished by either independent or simultaneous adjustment of arc powder and arc gas flow. As the ambient pressure is fixed at one atmosphere, this adjustment results in either increasing or decreasing the plasma stream exit velocity due to the pressure variations upstream of the exit throat. But if the maximum arc chamber pressure is redesigned so as to operate considerably above three atmospheres (29.4 p.s.i.) and if the arc gas or gas mixture is redesigned so as to have an increased enthalpy abot 1,000 b.t.u./lb., and if the exit throat has not only a straight section but also a diverging section, the exit velocity of the plasma jet can be increased to supersonic conditions. As an example, to obtain supersonic Mach II exit conditions at one atmosphere of ambient pressure, an arc chamber pressure of 6.5 to 8 atmospheres (81 to 103 p.s.i.g.) and an argon plasma stream with an enthalpy ranging from 1,500 to 4,000 b.t.u./lb. in required. The nozzle throat to exit area ratio is matched to the required pressure ratio for these conditions. This prevents flow shock formation and creates a fully expanded plasma stream closely sized to the exit diameter of the nozzle. Therefore, the heated and accelerated particles remain confined within the stream from the nozzle exit to the workpiece. Arc power levels of at least 80 kw must be utilized to obtain the arc gas pressure conditions required.
Turning now to FIG. 6, a typical schematic arrangement of an apparatus for the described method is shown. The plasma gun contains a gas arc chamber 31 having an exit throat which particularly has a straight bore section 33 and a diverging section 34. The gassupply is introduced at the left hand portion of the gas chamber 31 and an arc is created across the chamber by virtue of an arc power supply 36. The metallic and refractory powders are introduced to the gun from a power feeder 37 and carried to a preheating tube 38 which is powered by a powder preheat supply 39; the powder is then conveyed to a precise location in the exit throat by way of passage 40 which is slightly angled (at 4lwith referenceto a center-line 42 of the passage) and enters the exit throat precisely at the juncture of the straight bore se'c'tiorrand divergingsection, The stream from the plasmagun is directed at an apex seal supporting portion 45 which is carried on a movable support 46. The entire workpiece, as well as the plasma jet, is enclosed within a chamber 44 evacuated by way of appropriate mechanical equipment 47. The Workpiece is maintained at a specific electrical potential by way of a transferred are power supply 48 so as to receive plasma spray particles.
I claim as my invention:
1. An apex seal for a rotary internal combustion engine and having surfaces to be sealed, comprising:
a. a supporting portion defined to extend between said surfaces and is comprised of a metal consisting essentially of aluminum and having a specific gravity no greater than 3.0 and. a modulus of elasticity no less than 12 million p.s.i.,
b. a wear-resistant coating substantially enveloping said supporting portion and having a thickness between 0.0l5 inches and 0.025 inches, said coating being comprised of an admixture of two different types of abrasion resistant particles having a particle size no greater than 74- microns, said particles being self-fused to each other providing limited porosity of 25%, said coating being mechanically locked to said supporting portion with a uniform adhesive force no less than 8,500 p.s.i. as determined by an epoxy test procedure, said adhesive force resulting from the impaction of said abrasion resistant particles against said supporting portion by a plasma jet having a flow velocity in excess of Mach I.
2. An apex seal as in claim 1, in which the hardness of said as-deposited coating is at least 40 R and has a hardness value at an operating temperature of about 350F of at least R 40.
3. An apex sea] as in claim 1, in which the asdeposited surface roughness of said coating is no greater than 300 r.m.s.
4. In a rotary piston engine having a rotor mounted for planetary movement within an enclosure of said engine, the rotor carrying at least one seal element within a slot at an apex thereof, said seal being adapted to be urged by gas pressure within said enclosure to seal against the side of said slot and also against said enclosure, the combination system comprising:
a. a coating system for said enclosure consisting of a metal bonded refractory having a hardness of at least 40 R b. an apex seal construction having a supporting portion adapted to extend between said slot and the coating system of said enclosure, said supporting portion consisting essentially of aluminum and having a curved crown adapted to facilitate sliding engagement with said enclosure coating system, and a continuous wear-resistant coating having a thickness less than 0.025 inch, said coating substantially enveloping said supporting portion and being mechanically bonded to and enveloping at least the full crown of said supporting portion with an adhesive force of at least 8,500 p.s.i. as determined by an epoxy test procedure, szaid wear-resistant coating consisting essentially of an admixture of a metal and a refractory powder with the particle size of said powders not exceeding 74 microns, said powders being self-fusedto each other and impacted after having been subjected to a temperature in the with each other and to said supporting portion to range of 2,800F and impelled by a plasma jet possess an improved matallurgical union, the coatstream having a velocity of at least Mach I.
ing being the result of self-fusion of the particles