BRAKE ROTORS/DRUMS AND BRAKE PADS

BACKGROUND OF THE INVENTION

TECHNICAL FIELD:

This invention relates to brakes used on heavy vehicles such as aircraft, trucks, trains, and, more particularly, to a structural fiber reinforced ceramic matrix composite material adapted for high temperature brake use for the entirety of components of a brake system or as brake pads which can be used in the normal manner for brake pads. It also relates to a method of integrally molding fiber reinforced ceramic matrix composite brake components and attaching them to the surfaces of metal brake parts.

BACKGROUND ART: Any vehicle that moves typically is provided with a brake system with which to stop it. The lighter the combined stopping weight, the fewer the problems involved in designing a brake system which will last for an extended period of time and then be easy and inexpensive to replace or renovate. Thus, a vehicle such as a bicycle can be fitted with small rubber pads that squeeze and grip the rims of the wheels which will last virtually forever and which can be replaced in a few minutes at little expense.

When one gets to the mass of an automobile, which may contain a number of passengers, frictional heat build-up during stopping becomes a problem to be considered. Most automobiles today employ a so-called caliper disk brake on at least the front wheels since during stopping the weight of the vehicle is moved forward to the front wheels due to the force of inertia. Disk brakes as depicted in Figure 1 have good stopping power for various reasons. A rotor 10 carries the wheel (not shown) on a shaft 12. As the wheel rotates, the rotor 10 rotates in combination with it. The rotor 10 is disposed between a pair of calipers 14 having brake pads 16 thereon. To stop the automobile, hydraulic pressure is used to move the calipers 14 together until the rotor 10 is squeezed under pressure

between the pads 16. The calipers 1 are attached to the frame of the automobile and cannot rotate. The pads 16 are of a high friction material that resists deterioration and wear under fairly high temperature conditions. Thus, when the rotor 10 is squeezed by the calipers 14, a high frictional stopping force is applied to the rotor 10, bringing the automobile to a stop. Since the pads 16 are flat and contact the flat sides of the rotor 10, the entire area of the pads 16 contacts the rotor 10 to impart the stopping forces. This is in contrast to so-called "drum" brakes wherein the shoe carrying the pad is a circular arc which is supposed to match and fit the inside of a cylindrical brake drum. If there is a mismatch, only small parts of the pad actually rub on the drum. And, if there is a lot of frictional force generating a lot of frictional heat, the drum can be warped out of shape from the heat. With the disk brake, by contrast, the rotor 10 is in the air stream passing under the automobile, is thicker, and is therefore usually able to dissipate any heat build-up that takes place. And, even if minor warping should take place, the calipers are usually in a floating mounting that can follow the resultant wobble of the rotor.

To further prevent any damage to the surface of the rotor 10, the prior art suggests, as shown in Figure 2, facing the rotor 10 with a monolithic ceramic coating 18, which may or may not work for its intended purpose within the environment of an automobile. It definitely would not work for a braking system environment such as that addressed by the present invention.

When it comes to stopping an airplane, the braking system is an entirely different story. Particularly with a so-called "jumbo" jet carrying hundreds of passengers plus their baggage and freight in addition to the weight of the airplane itself, designing a successful braking system is a major undertaking. The prior art is depicted in Figure 3 in simplified form. There are a plurality of rotors 10' carrying shafts 12 which, in turn, carry the wheels (not shown) of the airplane. The rotors 10' are stacked with a plurality of stators 20 into a stack 22. While only two rotors 10' are shown, it is for simplicity only and many rotors 10' and stators 20 may be in the stack 22 of a typical airplane brake. The stack 22 is disposed in the hub of a wheel. The stators 20 are fixed and do not rotate while the rotors 10' rotate

in combination with the wheels. To apply the brake and stop the airplane, hydraulic pressure is applied which causes the stack 22 to be compressed together thereby squeezing the rotating rotors 10' between the fixed stators 20.

With smaller aircraft, the above-described brake construction was not a problem and worked well for its intended purpose. With the advent of large jets (both commercial and military) the frictional forces and attendant heat build-up soon became a major factor. This is particularly true with an aborted take-off, or with non-normal braking, which can result in the complete destruction of the entire brake stack 22. Modern jet brakes are of three types - all steel (rotors and pads), or steel rotors with sintered pads, and carbon/carbon rotors and pads. With an all steel brake system, both the stators 20 and rotors 10" are made of high quality steel specifically designed for the purpose. The steel/steel brakes develop good internal frictional forces. This is necessary in order to stop the airplane. If friction is removed, there is no heat build-up; but, there is also no stopping force created. Under the conditions of a normal stop, the brakes are applied in such a manner that they can dissipate the heat generated before it becomes a problem. Also, the so-called "jet brake" created by reversing the thrust of the jet engines is used to slow the airplane so that the brakes do not have to do all the work. The airplane is never brought completely to a stop from landing speed so that the rotors 10' and stators 20 are separated in the stack 22 as the heat created in them from frictional forces is dissipated.

In an aborted take-off, the airplane has attained a high ground speed which may be close to that required for take-off. At the last moment, the decision is made to abort, i.e. cancel the take-off. The only thing available to bring the airplane to a complete stop before the end of the runway is the brakes. To accomplish this, the pilot must "stand on" the brakes, i.e. fully apply them and hold them until the plane stops. The result is a heat build-up that cannot be successfully dissipated in time. The rotors 10' and stators 20 literally become so hot that as soon as the plane stops (or perhaps sooner), they weld together. Moreover, the built up heat travels to the surrounding structure and the wheels and may even start those on fire. If the brakes seize before the airplane stops, the rubber

wheels drag instead of rotating thereby quickly wearing through them whereupon they burst, causing the supporting structure to drag on the ground and maybe collapse. In short, it can be something that is going to require extensive repair of the airplane before it will be able to fly again.

To solve the above-described problem, carbon/carbon brakes were developed and are in use in the prior art. Carbon/carbon brakes have a number of problems - they provide low friction characteristics until they get hot, they are porous and therefore can be contaminated by de-icing or other fluids, they oxidize at a temperature similar to that realized during "heavy taxi-ing", they generate corrosive dust, and they are very expensive and time consuming to make. The carbon rotors 10' and carbon stators 20 are created by an infiltration process that is very expensive and takes a long time to accomplish. The cold stack 22 has a very low coefficient of friction and will not stop the plane. Thus, when first taxiing, the pilot must periodically apply the brakes to cause sufficient heat build-up such that the airplane can be stopped with the brakes when the need arises. Should the need arise before sufficient friction has built up, the airplane cannot be stopped quickly. Because the problem of break seizure is eliminated, most airlines and the military presently use the carbon/carbon brakes despite their shortcomings.

Truck, train, and racing applications could also utilize a better breaking system providing lighter weight and longer endurance than current technology braking materials.

Wherefore, it is an object of this invention to provide a seizure-resistant stack type brake system for aircraft and the like which is low cost and easily repairable.

It is another object of this invention to provide a seizure-resistant stack type brake system for aircraft and the like which has a high coefficient of friction even when cold.

It is still another object of this invention to provide a seizure-resistant stack type brake system for aircraft and the like which employs brake pads which can be replaced without having to replace the entire stack of rotors and stators .

It is yet another object of the present invention to provide a brake rotor/drum/pad material that is resistant to destruction in any application involving high frictional braking forces and extremely high generated heat.

It is a further object of the present invention to provide a brake rotor material that is resistant to destruction in any application involving high frictional braking forces and extremely high generated heat.

Other objects and benefits of this invention will become apparent from the description which follows hereinafter when read in conjunction with the drawing figures which accompany it.

DISCLOSURE OF THE INVENTION

The foregoing objects have been achieved by a method of forming a high temperature and wear resistant brake pad and of attaching it to a brake part comprising the steps of, forming a brake rotor/stator or pad of a structural fiber reinforced ceramic matrix composite material comprising a generic fiber system and an erosion resistant/friction-producing material disposed throughout a fired polymer-derived ceramic resin; riveting or bonding the brake pad to a surface of the brake part; and, smoothing a contacting braking surface of the brake pad as necessary.

The preferred method of making rotors and drums includes the steps of, choosing the polymer-derived ceramic resin from silicon-carboxyl resin or alumina silicate resin; and, using a generic fiber system from alumina, Nextel 312, Nextel 440, Nextel 510, Nextel 550, silicon nitride, silicon carbide, HPZ, graphite, carbon, and peat.

Optionally to achieve a tougher material, the method includes the additional step of disposing an interface material over fibers of the generic fiber system thereby preventing the fired polymer-derived ceramic resin from adhering directly to the fibers. Where employed, that step comprises disposing the interface material as a few microns thickness of carbon, silicon nitride, silicon carbide, and/or boron nitride.

The method also can include the step of disposing up to about 60 percent by volume of alumina, mullite, silica, silicon carbide, titania, silicon nitride, boron nitride, or an equivalent material, or any combination thereof up to a total volume of about 60 percent, throughout fibers of the generic fiber system at least adjacent the contacting braking surface. Different combinations allow for tailoring of hardness and friction coefficients of the material and thus provides a varying "feel" to the user. However, disposing a total of approximately 25 percent by volume of alumina and/or mullite is preferred.

The method may, if appropriate to the physiology of the generic fiber system, include the step of disposing fibers of the generic fiber system adjacent the contacting braking surface parallel to the contacting braking surface. It may also include disposing the fibers of the generic fiber system adjacent the contacting braking surface along circular arc segments and radial lines with respect to a center of rotation of a brake component to contact the contacting braking surface.

As an alternate to riveting or adhesively bonding if a suitable temperature resistant adhesive is developed, the invention also includes a method of forming a high temperature and wear resistant brake pad and of attaching it to a brake part comprising the steps of, forming a brake pad having attaching members extending therefrom of a structural fiber reinforced ceramic matrix composite material comprising a generic fiber system and an erosion resistant/friction-producing material disposed throughout a fired polymer-derived ceramic resin; placing the brake pad in a mold for the brake part with the attaching members extending into a portion of the mold to be filled with metal forming the brake part;

and, filling the mold with molten metal to form the brake part and capture the attaching members therein.

That method may also include the additional steps of removing the brake part from the mold; machining and finishing the brake part as necessary; and, smoothing a contacting braking surface of the brake pad as necessary.

Additionally, the option exists to fabricate rotors and pads for automobile scale brake applications from the polymer-derived ceramic composite system that require no metal reinforcement. The brake pad consisting only of the ceramic material is pressed between the brake caliper and the brake rotor. The brake rotor can also be an all ceramic material bolted or pinned directly to the wheel hub, or the rotor can be made of traditional steels like current technology rotors or preferably metal matrix composites (such as ALCAN's F3S2OS alloy) for additional wear resistance when running against ceramic brake pads.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified front view drawing of a prior art caliper disk brake of the type used on automobiles.

Figure 2 shows the prior art disk brake of Figure 1 with a monolithic ceramic coating on the faces of the rotor as suggested in the prior art.

Figure 3 is a simplified front view drawing of a prior art aircraft brake.

Figure 4 is an enlarged and partially cutaway drawing of an aircraft brake according to the present invention in a first embodiment.

Figure 5 is an enlarged drawing of an aircraft brake according to the present invention in a second embodiment.

Figures 6 through 7 depict the method of relining the brake components of Figure 5 according to the present invention.

Figure 8 depicts a method of permanently attaching a FRCMC brake pad to a metal rotor, stator, or shoe by casting it in place.

BEST MODES FOR CARRYING OUT THE INVENTION

According to one aspect of the present invention directed specifically to aircraft brakes as depicted in Figure 4, the parts of an aircraft brake stack 22 are made of a structural fiber reinforced ceramic composite matrix (FRCMC) material modified specifically for brake use. In this regard, the FRCMC brake material of this invention is akin to the rotors and stators being made entirely of a quasi-sintered brake pad material able to withstand the heat and frictional forces of aircraft braking. That is, the FRCMC brake material of this invention comprises a fiber impregnated ceramic material which also includes friction-producing elements. Such a structural FRCMC material exhibits high breakage resistance and is particular applicability to use for parts in high temperature applications. The FRCMC material employs any of several polymer-derived ceramic resins commercially available such as silicon-carboxyl resin (sold by Allied Signal under the trade name Blackglas) or alumina silicate resin (sold by Applied Poleramics under the product designation C02) combined with a generic fiber system such as, but not limited to, alumina, Nextel 312, Nextel 440, Nextel 510, Nextel 550, silicon nitride, silicon carbide, HPZ, graphite, carbon, and peat. To accomplish the objectives of the invention, the fiber system is first coated to 0.1-5.0 microns thickness with an interface material such as carbon, silicon nitride, silicon carbide, and boron nitride, or multiple layers of one or more of these interfacial materials. The interface material prevents the resin from adhering directly to the fibers of the fiber system. Thus, when the resin has been converted into a ceramic, there is

a slight play between the ceramic and fibers imparting the desired qualities to the final FRCMC. The method of forming the breakage resistant structural FRCMC parts generally entails the steps of coating the fibers of a generic fiber system including one or more of the aforementioned fibers with the interfacing material(s) , mixing the coated fibers with one or more of the pre-ceramic polymer resin, forming the resin containing the coated fibers into a desired part, and firing the part at a temperature and for a time which converts the polymer resin to a ceramic.

As described in a co-pending application entitled REDUCING WEAR BETWEEN STRUCTURAL FIBER REINFORCED CERAMIC MATRIX COMPOSITE AUTOMOTIVE

ENGINE PARTS IN SLIDING CONTACTING RELATIONSHIP, serial number filed on even date herewith, the surface of the structural FRCMC material can be treated to reduce wear/erosion in such applications by applying an erosion-resistant coating. Specifically, it was disclosed that:

The contacting surfaces of the structural fiber reinforced ceramic matrix composite component employing a woven or non-woven cloth mat of fibers are covered with an erosion-resistant coating which bonds tightly to the wearing surface of the FRCMC structures. For this purpose, the erosion-resistant coating preferably comprises mullite (i.e. alumina silicate AI₂Si ), alumina (i.e. AI₂O3), or equivalent, applied via a plasma spray, generally according to techniques well known to those of ordinary skill in the art.

The erosion-resistant coating is applied as follows. Prior to the application of the erosion-resistant coating, all holes any other machining is completed. Upon the completion of the machining processes, if any, all sharp edges on the surface of the part are knocked down. If the part has been machined, it is placed in an oven for a time and temperature adequate to assure "burn off' of any of the cutting lubricants used in the machining process. (Typically 2Hrs A 700°F, but is lubricant dependent.)

The key is getting the erosion-resistant coating to bond to the FRCMC structure. If the surface of the FRCMC structure is not properly prepared, the erosion-resistant coating can simply flake off and provide no long-term protection. In the preferred approach, the surface of the FRCMC structure is lightly grit-blasted to form small divots within the ceramic matrix of the FRCMC structure. It is also believed that the light grit blasting exposes hairs or whiskers on the exposed fiber of the generic fiber system which the erosion-resistant coating can grip and adhere thereto. Typical grit blasting that has proved successful is 100 grit @ 20 PSI.

According to a second possible approach, the surface of the FRCMC structure can be provided with a series of thin, shallow, regularly-spaced grooves similar to fine "threads" of a nut or bolt which the erosion-resistant coating can mechanical lock into. Essentially, the surface is scored to provide a roughened surface instead of a smooth surface. The depth, width, and spacing of the grooves is not critical and can be determined for each part or component without undue experimentation. In general, the grooves should be closely spaced so as to minimize any large smooth areas of the surface where there is a potential for the erosion-resistant coating to lose its adhesion and flake off. Thus, over-grooving would be preferable to under-grooving the surface with the exception that over-grooving requires the application of additional wear material to provide a smooth wear surface after final grinding. The grooves should be shallow so as to provide a mechanical locking area for the erosion-resistant coating without reducing the structural strength of the underlying FRCMC structure to any appreciable degree. After surface preparation, the part is cleaned by using clean dry compressed air and then loaded in an appropriate holding fixture for the plasma spray process. Direct air blowers are used to cool the opposite side of the part during the application of the erosion-resistant coating.

The plasma sprayed erosion-resistant coating is then applied using a deposition rate set to 5 grams per minute or more. The holding fixture speed, plasma gun movement rate across the surface, and spray width are set to achieve a barber pole spray pattern with 50% overlap. The spray gun is set relative to the sprayed surface from 0.1 inches to 3

inches away. Particle sizes used for this process range from 170 to 400 mesh. Enough material is applied to allow for finish machining. After the application of the erosion-resistant coating, the coated surface is smoothed out with diamond paper or an appropriate form tool (commercial grade diamond tools recommended) to achieve the final surface contour.

Alternatively, the plasma sprayed coating can be applied and then the part with the erosion-resistant coating attached can be further reinfiltrated with the pre-ceramic polymer resin and then converted to a ceramic state. The result is an additional toughening of the coating by essentially incorporating the coating into the mixed or combined ceramic matrix composite formed from the combination of the FRCMC and a ceramic matrix reinforced monolithic wear coating integrally bound together by the common ceramic matrix.

It is also noted that the erosion-resistant coating can also be applied via , a "print and fire" thin film deposition technique, or a "wet spray" technique, as desired, in accordance with well known processes. Regardless, any of the above-described coating processes can be employed to provide the friction-producing elements on the brake parts of the present invention.

Surface wear can also be reduced by not employing the generic fiber system in the form of a woven cloth mat at least adjacent the surface; but rather, by employing free fibers that are substantially parallel to the surface. For a rotating brake system, it would appear that the best orientation would be for the fibers to lie along circular arc segments or radial lines with respect to the rotational center of the part.

While such coating materials would probably work for brake components, having the necessary strength and wear resistance, it is preferred to modify the composition to increase the frictional coefficient of the material and therefore its braking efficiency while, at the same time, improving its total wear or erosion resistance in the manner of sintered brake shoes rather than providing erosion resistance only at the surface such that it is lost

after the surface is worn away from use. To this end, it is preferred that the friction-inducing, erosion-resistant material is mixed in with the resin/fiber mixture prior to part formation and firing. The wear resistant material particles may also be dispersed throughout the material after the first or second "firing" via conventional "SOL-GEL" techniques. This mixture can include up to about 60 percent by volume of alumina, mullite, silica, silicon carbide, titania, silicon nitride, boron nitride, or an equivalent material, or any combination thereof up to a total volume of about 60 percent, throughout fibers of the generic fiber system at least adjacent the contacting braking surface. Different combinations allow for tailoring of hardness and friction coefficients of the material and thus provides a varying "feel" to the user. However, disposing a total of approximately 25 percent by volume of alumina and/or mullite is preferred.

Thus, according to the preferred embodiment of the present invention, the steps of constructing a brake part such as the rotors 10' and stators 20 of Figure 4 comprise mixing approximately 25 percent by volume of alumina and/or mullite powder, or an equivalent, with the fiber system; mixing the powder-coated fiber system with the resin; forming the part with the resin mixture; and firing the resultant part at a temperature in the neighborhood of 1 ,800 F as suggested by the manufacturer to convert the resin into a ceramic. Alternatively, the fibers can be coated first with the above-described interface material before the alumina/mullite powder is added if an end structure having the qualities that the interface material adds is desired for the particular application.

Because it has qualities that conventional sintered brake pads and the like do not have such as heat and wear/erosion resistance under extreme temperatures, the above-described FRCMC brake material of this invention can be made into a replaceable brake pad for use in any brake system employing brake pads. Such an aircraft brake stack 22' is depicted in Figure 5. The conventional steel rotors 10' and stators 20 have brake pads 16' according to the present invention covering their braking surfaces. Thus, instead of having to replace the rotors 10' and stators 20 as required with the present

steel/steel and carbon/carbon brakes, the brakes can be "relined' in the manner of automotive brakes thereby greatly reducing the time and cost involved.

Originally, automotive brake pads were riveted to the brake shoes carrying them. More recently with the advent of appropriate adhesives, so-called "bonded" brake pads have been employed wherein the pads are attached with adhesive to the shoes. This reduces the incidence of the rivets "scoring" the brake drums or brake rotors when the brake pads wear down too far past the point of needing replacement. With the bonded brake pads, the pads must wear to the point that the shoe itself is contacting the drum or rotor before damage can occur. Unfortunately, there is no commercially available adhesive or bonding method that can attach the brake pads 16' to the rotors 10' and stators 20 and resist the temperatures generated during braking. Thus, an alternate method of bonding the pads 16' to the rotors 10' and stators 20 must be employed.

One approach as depicted in Figures 6 and 7 and adapted for "relining" the rotors

10' and stators 20 is to employ rivets 26. As depicted in Figure 6, the old pad 16' is first removed and discarded. As depicted in Figure 7, the new pad 16' is then positioned on the rotor 10' and riveted in place with rivets 26. Following the riveting, the surface 28 of the pad 16' can be ground smooth and parallel if necessary.

In another approach not intended for relining but lower production costs as depicted in Figure 8, the pads 16" are provided with gripping members 30 extending from the rear thereof. Since the pads 16" are able to withstand extremes of temperature including that of molten metal, the rotor 10' (or stator 20) can be cast onto the pad(s) 16". The pads 16" are placed within a mold 34 for the part to be cast (rotor 10' or stator 20). The molten metal 32 is then poured into the mold 34 and allowed to harden. The finished part is then removed and machined or otherwise finished as necessary.

One can also fabricate rotors and pads for automobile scale brake applications from the polymer-derived ceramic composite system that require no metal reinforcement.

The brake pad consisting only of the ceramic material is pressed between the brake caliper and the brake rotor. The brake rotor can also be an all ceramic material bolted or pinned directly to the wheel hub, or the rotor can be made of traditional steels like current technology rotors or preferably metal matrix composites (such as ALCAN's F3S2OS alloy) for additional wear resistance when running against ceramic brake pads.

And finally, the complete rotor and stator can be made from FRCMC without the need for any metal support structure and thereby eliminating the problems associated with the bonding and/or riveting of friction materials to structural members of the brake assembly.