Coated clutch separator plates

Abstract: A clutch separator plate (24) utilized in a clutch or brake pack (10) for an automotive transmission for vehicles, with the plates being internally or externally splined to be operatively connected to the rotatable and/or stationary members (12,14) in a torsional coupling, wherein the separator plate surfaces (44) are treated by a suitable surface treatment, such as lapping, and then sputter coated with titanium nitride or a similar material (46) to provide a surface texture which enhances the frictional characteristics of the plate but does not substantially increase the wear of the lining of interleaved clutch plates (16).

Description: This invention relates to clutch separator plates, such as utilized in clutch disc packs. Multiple disc clutches and brakes generally include a clutch disc pack of the kind comprising clutch plates having friction linings and interleaved with clutch separator plates. Such a clutch disc pack is is known from JP-A-61041023. More particularly, in JP-A-61041023, one or both of the clutch and separator plates of a friction clutch are formed of austenite steel material whose surface is subjected to a salt-bath nitriding treatment, thus providing a non-magnetic coating to the plates and enabling the apparatus to be used in an environment subjected to magnetic fields which would, otherwise, adversely affect the operation of the friction clutch. Prior efforts to improve the frictional characteristics of a clutch or brake employed in a drive train to transmit torque in an automatic transmission for an automotive vehicle or the like, the clutch or brake employing a clutch disc pack of the kind described, have principally resided in the improvement of the friction lining for the clutch plates, and substantially no attention has been paid to the frictional characteristics of the separator plates themselves. One problem of the separator plates is in wear of the separator plate surfaces due to the frictional characteristics of the clutch plates. Less wear of the separator plates would provide more of a "steady state" rotating member and therefore consistent performance. There also is a need for higher friction to support current higher torque applications. More recently where the friction linings for the clutch plates have been formed of a paper-based friction material, the separator plates should not be formed of a material or have a surface texture which would cause undue wear upon the friction lining. As the clutch plates have friction linings secured to the opposite faces thereof, the separator plates are conventionally finished by a tumbling process to have relatively smooth surfaces engaging with the friction lining material. To enhance the frictional characteristics of the steel plates when used in combination with the friction lined clutch plates, it has been suggested that the surface be altered to change the characteristics of the steel plates. Various surface preparation procedures where tried, such as sand blasting, shot peening and knurling; however, the resulting surface either did not enhance the coefficient of friction or caused undue wear and/or damage to the friction lining of the mating clutch disc faces. This, in turn, would shorten the useful life of the clutch plates in the clutch pack for the automatic transmission. Also, coating of the separator plates was attempted using a wear-resistant material, such as sputter coating of the steel plates with titanium nitride or similar metallic nitrides; materials known for their wear resistant properties for tool bits, saw blades, etc. Although a slight increase in the frictional characteristics may have been detected, this improvement was clearly offset by a noticeable increase in the wear on the friction linings. In accordance with the present invention as claimed, a clutch disc pack of the aforementioned kind is characterised in that a separator plate has at least one face provided with a surface texture in the arithmetic average (Ra) range of 0.0762 to 0.02286 micrometres (3 to 9 microinches) covered by a thin metallic nitride coating which coated surface texture provides an increased coefficient of friction between the separator plate and the cooperating clutch plate without substantially increasing the frictional wear of the friction lining of the clutch plate. In a preferred embodiment, the separator plate is a steel plate whose opposite faces are treated to provide the aforesaid surface texture, and subsequently to enhance the hardness and wearability of the surfaces a thin coating of titanium nitride was applied to the treated surfaces so as not to disturb or affect the characteristics of the surface finish. With the coated surface having the desired texture, the wear of the mating friction linings is found to be the same or slightly reduced compared with when used with tumbled steel separator plates, but the coefficient of friction is significantly increased. Preferably, the opposite surfaces of the separator plate are treated by lapping to provide a surface texture in the aforementioned range, and a titanium nitride coating having a thickness equal to or less than 1.5 microns is sputter-coated or deposited by other similar processes upon the lapped surfaces. For best results, the lapping is carried out using a diamond lapping compound. As an alternative, the surface treatment may be carried out by chemical means rather than by mechanical lapping. In one practical embodiment of the invention, one set of the clutch and separator plates has interior splines to engage a rotatable member, and the other set of plates has exterior peripheral splines to engage either a second rotatable member or a stationary member to form a torsional coupling or brake, respectively. Thus, one set of plates is splined to an encompassing clutch or brake drum while the other set of plates is splined on to a central shaft or ring. Attention is also drawn as an example of the prior art to FR-A-2402532. In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which: FIGURE 1 is a partial cross sectional view through a clutch disc pack showing the friction and separator plates; FIGURE 2 is an enlarged partial cross sectional view of a coated separator plate; FIGURE 3 is a graph comparing normal separator plates with titanium nitride-coated plates for initial dynamic friction (relative fast speed); FIGURE 4 is a graph similar to FIGURE 3, but comparing the median dynamic frictions (intermittent speed); FIGURE 5 is a graph similar to FIGURE 3, but comparing the final dynamic frictions (slower speed); FIGURE 6 is a graph similar to FIGURE 3, but comparing the static friction; and FIGURES 7 through 12 are photographs of various surface treatments for the clutch separator plates. Referring first to FIGURE 1, there is shown a portion of the upper half of a conventional clutch pack 10 utilized in a clutch or brake for the automatic transmission of an automotive vehicle comprising a metal drum 12 formed of cast iron, steel or powdered metal and mounted for rotation on suitable bearings (not shown), and an internal metal hub or shaft 14 also formed of one of the above materials and suitably mounted for rotation. The drum 12 is adapted to be driven by a torque apply member and the hub or shaft 14 is adapted to be drivingly connected to a load to be driven, although the driving and driven portions could be reversed. Between the drum and hub are located a plurality of annular metal plates comprising alternating friction discs 16 and separator plates 24; the friction discs 16 having friction linings 18 on the opposite surfaces thereof and internal teeth 20 in mesh with external splines or ribs 22 on the exterior surface of the hub 14, and the separator plates 24 having external teeth 26 meshing with the internal splines or ribs 28 on the interior surface of the drum 12. Although shown as operatively connected to a rotating member 12, the separator plates 24 could be connected to a suitable stationary member as where the coupling is applied to a brake rather than a clutch in the transmission. An annular backing plate 30 has teeth 32 on its periphery meshing with the drum splines 28 and is retained axially through a retaining ring 34 mounted in an internal annular groove 36 in the drum. An annular apply piston 38 is mounted to slide axially within the drum and is normally urged leftward (as seen in FIGURE 1) to a disengaged position by clutch release springs or other suitable release means (not shown). Fluid under pressure is admitted into the chamber 40 at the left hand end of the piston 38 to urge the piston towards the clutch plates 16 to squeeze the clutch plates 16 having the opposed friction facings 18 and the separator plates 24 together between the piston and the backing plate 30. With the plates 16, 24 engaged, a power path is established for torque transmittal between drum 12 and the hub 14. An annular seal 42 in an external groove in the periphery of the piston prevents leakage from the apply chamber 40 past the piston 38. When the fluid in the chamber is allowed to exhaust or exit, the return means moves the piston to the left to its disengaged position to disengage the clutch. FIGURE 2 discloses a portion of a single separator plate 24 utilized in the clutch pack separating a pair of adjacent clutch plates 16 having suitable linings 18 thereon. The opposed surfaces 44 of the separator plate formed of a suitable metal, such as steel, have been treated in a novel manner to enhance their frictional characteristics in cooperation with the clutch plates that come into engagement with them. It is this novel treatment that is the subject of the present invention. This treatment is a two-stage process that generally includes a surface finish operation followed by the suitable deposition of a material 46, such as titanium nitride, onto the lapped surface. Laboratory tests have shown that the metal mating surface and coating is a very important property in determining the friction and wear of clutch linings. With the objective of increasing friction for higher torque (or lighter weight) and no increase in wear of the clutch lining, it was found that standard resin-saturated paper-based linings running against diamond lapped 0.0762 to 0.2286 micrometres (3 to 9 microinches Ra) 1035 steel gave very high friction and essentially no increase in wear of the clutch lining. However, with conventionally tumbled 1035 steel having a titanium nitride sputter coating alone, the wear of the clutch lining increased; and with lapping alone, there was substantially no lining wear, but the steel separator plate would gradually wear as the test progressed. The lapping alone will provide an increase in the friction of the clutch. In combining the lapping with the sputter coating, it is believed that the coating of titanium nitride stabilizes the surface treatment of lapping the steel plate (locks in the texture) and provides chemical resistance to the automatic transmission fluid. This surface finish with titanium nitride was found to provide the optimum texture which allowed fluid suspension/support in the contact area. It appeared that the titanium nitride coating preserved the surface texture from the lapping operation in its initial state. A similar increase in friction was observed for a surface treatment of lapping with 14.5 micron aluminum oxide, although the clutch lining wear was slightly higher than for the diamond lapping. Other surface finishes, such as knurling, sandblasting, or shot peening, either did not increase the coefficient of friction or, if friction increased, the wear of the friction material also increased, such as shown in Table I. Surface roughness and profile were measured before and after testing using a Gould Profilometer. In most instances, both wear and surface roughness increased. Friction and wear tests were run in the laboratory with a modified Shell 4-Ball Tester under conditions of continuous slip at 1910 rpm and at a temperature of 93°C (200°F) in a Texaco TL8570 automatic transmission fluid (ATF) for a thirty minute break-in period. This was followed by continuous slip conditions at ten speed intervals from 1910 to 280 rpm over a period of approximately one hour (total test time of approximately one and one-half hours). For these tests, the friction material was SD1777 with 8.3 bar (120 psi) loading, a contact area of 2.02cm² (0.393 square inches) and a mean radius of 0.0127m (one-half inch). Surface Treatment No Coating TiN Coating % Change in Fric. Wear Mils % Change in Fric. Wear Mils Std Tumbled 0 1.0 3 1.6 Knurled -8 3.4 -2 1.6 Sandblast -12 23.4 -6 26.9 Scotch Brite -1 0.8 0 1.3 50% HCl -4 0.9 Diamond Lap (6 micron) 32 5.3 7 1.6 Lapmaster Compound 1700 - 14.5 micron Al₂O₃ -6 0.8 9 2.4 Diamond Lap (9 micron) 62 8.1 9 1.9 Shot Peen #1 -4 0.9 5 5.5 Shot Peen #1 & Tumbled -8 1.0 -3 2.0 Shot Peen #1 & Tumbled/Annealed -8 1.0 -3 1.9 A second type of testing involving single plate tests were run at 93°C (200°F) and 8.3 bar (120 psi) in Texaco TL8570 ATF; the test consisting of 50 cycles at 1000 rpm engagement speed, followed by 150 cycles at 2000 rpm engagement speed, and finally 10 cycles at 1000 rpm. The contact area was 51.89cm² (8.45 square inches) and the mean radius was 68.83mm (2.71 inches). Friction was plotted for the 10th, 50th, 60th, 200th and 210th cycles. For a comparison of materials, three readings were taken on the 200th cycle (initial, midpoint and final speed) as the friction material engaged the mating plate and slowed down. Each run was given a grade by multiplying the initial friction by 2, midpoint by 1.5 and final friction by 1, and summing the three values as seen in Table II. The column in this table labeled % is the percent increase in friction or grade over the standard SD1777 control value. The column Fc/Fa is the ratio of static friction over dynamic friction, which for ease of shifting should be less than 1.1. Mating plate surfaces were coated with various thicknesses of titanium nitride , and all coatings increased friction with the highest (13%) at 1.5 microns thick titanium nitride with no increase in the Fc/Fa ratio. The coatings were applied using the high rate reactive sputtering process covered by U. S. Patent No. 4,428,811. Prior to coating, all samples were cleaned in acetone, alcohol and freon TES (vapor degreased). Once in the sputtering chamber, the samples were given an rf sputter etch just before coating for five minutes at a power level of 1.5 kilowatts and an argon pressure of 8 mTorr. The coatings were done in the stationary mode where the samples were moved quickly under the target and held there until the desired thickness was reached. During reactive sputter deposition, the power to the D.C. magnetron cathode was typically 10 kilowatts, and the deposition rate for titanium nitride was 500 nanometers/min. MATERIAL % CHANGE IN FRICTION GRADE Fc/Fa SD1777 Control 0 1.1 Aluminum Oxide Implant (lining) 6 1.2 Silicone Implant (lining) 6 1.2 Experimental Fluid 1 (no additive) -12 .8 Experimental Fluid 2 (no friction modifier) 8 1.3 58% Silicone SR80M lining 7 1.6 0.5 micron Titanium Nitride (plate) 7 1.1 1.5 micron Titanium Nitride (plate) 13 1.1 Additional single plate tests were run to evaluate five different surface treatments prior to a titanium nitride coating on a separator plate (see Table III). Standard production linings (SD1777) and Exxon 1975 ATF at 93°C (200°F) were used. One hundred inertia stops were performed on each lining/separator plate combination. In an inertia stop, the flywheel of the single plate tester is brought up to 2500 rpm, then declutched from the drive system, and a 6.9 bar (100 psi) load applied until the flywheel has stopped. Dynamic friction coefficients and wear were recorded, with the friction values shown being for median dynamic friction (intermittent speed). TiN Coated Separator Plate Surface Finish Dynamic Friction At 100th Cycle Lining Wear, mils As rec'd (uncoated) 0.150 0.5 As rec'd (coated) 0.165 1.5 Six micron compound 0.175 0.7 No. 6 plate 0.175 1.1 Roto lap 0.180 2.0 No. 10 plate 0.170 7.3 All titanium nitride coated separator plates with prior surface treatment have the highest friction magnitudes during the first few cycles or inertia stops; then they tend to stabilize at a lower value. This stabilization effect seems to be attributed to a change in the lining condition because surface measurements of the coated separator plates indicate no change. Coated separator plate surfaces can be thought of as being in a "steady state". The initial dynamic friction (early running stages) magnitudes ranging from 0.190 to 0.270 for the five different surface finishes tested may be prolonged if the lining could maintain its initial surface character. It was found that a 20% increase in the dynamic friction coefficient can be obtained without an increase of lining wear if the separator plate is treated by lapping or a similar process prior to coating. Higher friction coefficients are needed to meet demands of high torque capacity transmissions and possibly improve compatibility with new generations of automatic transmission fluids. Optimum performance appears to be obtained with a 6 micron lap compound before coating with titanium nitride, which resulted in a 0.175 dynamic coefficient of friction and 0.7 mils of lining wear. This is substantially higher friction than is produced by standard steel separator plates with no appreciable increase in wear. An increased area of contact from this 6 micron compound lapped surface may provide the proper sized cavities for automatic transmission fluid to balance shear and support phenomenon. Titanium nitride sputtered on an "an received" separator plate increased friction to 0.165, but when this coating is coupled with the proper surface treatment, performance modifications are more pronounced. Other materials or coatings may provide increased performance provided the surface texture is substantially identical to the 6 micron compound surface and can be maintained. FIGURES 3 through 6 illustrate graphs comparing normal steel separator plates with those that have been lapped and coated with titanium nitride at initial dynamic friction, median dynamic friction and final friction coefficients, as well as static friction shown in FIGURE 6. These comparisons were made in a SAE No. 2 machine for a full pack test using three clutch plates having six SD1777X facings, a Toyota JWS-2343 ATF and a temperature in the range of 100° to 110°C (212° to 230°F) at a high speed of 3590 rpm. The kinetic energy of clutch engagement was 24,135J (17,800 foot pounds), a contact area of 43.6cm² (6.764 square inches), a mean radius of 63.16mm (2.29 inches) and an inertia of 0.252 foot pounds seconds squared. In all of these four examples, the coefficient of friction of the titanium nitride sample with proper surface texture was far greater than those of the normal uncoated "as received" separator plates. FIGURES 7 through 12 provide photomicrographs of the surface texture of the various samples of separator plate in both an "as received", surface lapped and titanium nitride coated condition. More specifically, FIGURE 7 discloses an uncoated steel 1035 plate that has been tested in a clutch environment and shows substantial scratching. FIGURE 8 discloses a titanium nitride coated separator plate as received which has been tumbled and tested; showing substantially no wear. FIGURE 9 discloses a titanium nitride coated separator plate which was lapped with a No. 6 lapping plate prior to coating. This plate shows shallow scratches and is less rough than the next plate of FIGURE 10. FIGURE 10 discloses a titanium nitride coated plate which has been lapped with a No. 10 lapping plate before coating. This plate surface is rougher than that of the No. 6 lapping plate and shows evidence of deep scratches. FIGURE 11 discloses a separator plate which has the surface treated with a RotoLap machine and then coated with titanium nitride. The surface of this plate evidences only faint scratches, but has increased bumpiness. FIGURE 12 shows a separator plate which has been lapped with a number 6 lapping compound and then coated with titanium nitride. After testing, this plate shows the smoothest surface with no scratches and is the preferred plate for the present invention. Another possible surface treatment mode which would produce the desired surface having a finish of 0.1016 to 0.1270 micrometres (4 to 5 microinches)(Ra) is a tumbling operation in a ceramic media. Obviously , although diamond lapping has been found to give the best results so far, other lapping compounds could achieve a surface finish with subsequent titanium nitride coating to enhance the frictional characteristics desired for the application.

Claims: A clutch disc pack comprising clutch plates (16) having friction linings (18) and interleaved with separator plates (24), characterised in that a separator plate (24) has at least one face (44) provided with a surface texture in the arithmetic average (Ra) range of 0.0762 to 0.2286 micrometres (3 to 9 microinches) covered with a thin metallic nitride coating (46) which coated surface texture provides an increased coefficient of friction between the separator plate (24) and the cooperating clutch plate (16) without substantially increasing the frictional wear of the friction lining (18) of the clutch plate (16). A clutch disc pack as claimed in claim 1, wherein the thin metallic nitride coating has a thickness equal to or less than 1.5 microns. A clutch disc pack as claimed in claim 1 or claim 2, wherein the surface texture of the at least one face (44) of each separator plate (24) is achieved by lapping prior to the at least one face (44) being covered with the thin metallic nitride coating (46). A clutch disc pack as claimed in claim 3, wherein the lapping is performed using a diamond lapping compound. A clutch disc pack as claimed in any of the preceding claims, wherein the metallic nitride coating (46) is a titanium nitride coating (46). A clutch disc pack as claimed in any of the preceding claims, wherein the metallic nitride coating (46) is applied to the at least one face (44) of the separator plate (24) by a sputter coating process. A clutch disc pack as claimed in any of the preceding claims, wherein the friction linings (18) are of a resin-based paper friction material. A clutch disc pack as claimed in any of the preceding claims, wherein the wear of the coated separator plate (24) in use is in the order of less than 0.0508 micrometres (2 microinches). A clutch disc pack as claimed in any of the preceding claims, wherein one set (16) of the clutch (16) and separator (24) plates is splined to one rotatable member (14) and the other set (24) of the clutch (16) and separator (24) plates is splined to a second rotatable (12) or stationary member to form a torsional coupling or brake, respectively.

Inventors: YESNIK MARC A, YESNIK, MARC A.
Applicants: BORG WARNER AUTOMOTIVE, BORG-WARNER AUTOMOTIVE TRANSMISSION AND ENGINE COMPONENTS CORPORATION
Application year: 1991
Main classification: F16D13
IPC codes: C23C14, F16D65, F16D25, F16D13, F16D69