BRAKE ELEMENT

Abstract: In a brake the friction surface (39) of a brake element is a refractory metal carbide. The brake element has a moulded carbon structure (41) for mechanical and structural strength and the friction surface or surfaces (39) thereon are converted to refractory metal carbide. The refractory metal carbide is preferably silicon carbide and the depth ( d ) of conversion is preferably from 0.38mm to 3.8mm (0.015 inches to 0.15 inches).

Description: The invention relates to a brake element. A wide variety of brake materials are presently available for meeting the needs of today's vehicle brakes. One of the most demanding brake applications requiring use of materials with the best available properties is in the field of aircraft brakes. Aircraft brakes must be lightweight, while having high static and dynamic friction characteristics. They must produce consistent and smooth braking while demanding only relatively low pressure to produce the desired braking. The life of the brake material must be long enough to prevent constant changing of the brakes. Among the materials presently used for such brakes are organic substances, copper, steel and carbon. One type of carbon aircraft brake is made from carbon/carbon composites. Carbon brakes are formed by a heated mould process into discs having very light weight and a relatively low coefficient of friction. The faces of the discs serve as the friction surfaces. Because carbon discs have a low coefficient of friction, the demands of aircraft brakes require that such discs be arranged in multiple disc brake systems so that sufficiently high braking ability is achieved. These brakes also suffer from inconsistent friction characteristics. Particularly, the dynamic coefficient of friction of carbon brakes is erratic at low speeds and the static coefficient of friction is erratic over broad temperature ranges. Patent Specification GB-A-1 402 807 discloses a braking element formed from a cylindrical graphite substrate by pyrolytic deposition from an injected mixture of methyl trichlorosilane, methane, argon and hydrogen, the greatest thickness of the formed microcomposite being 26 mils and containing about 25 volume percent of needle like crystalline aciculae of silicon carbide embedded in laminar layers of pyrolytic graphite. According to the invention there is provided a brake element for use in a brake system and having a friction surface thereon, comprising: a carbon structure for providing the mechanical and structural properties of the brake element; and a brake friction surface disposed on the carbon structure and comprising a refractory metal carbide material formed by converting carbon of the carbon structure to refractory metal carbide; characterised in that the refractory metal carbide surface has a depth of from about 0.38mm to about 3.8mm (about 0.015 inches to about 0.15 inches). Such a brake element can be long lasting, have consistent friction characteristics, and be light in weight. Refractory metal carbide friction surfaces together provide surprisingly improved braking and wear characteristics. Preferably the refractory metal carbide comprises silicon carbide and where the silicon carbide has a penetration thickness from the surface of the disc of about 1.52mm (about 0.060 inches). The brake elements are preferably formed in the shape of discs with one or both faces of the discs converted to silicon carbide and serving as friction surfaces. Conventional configurations of carbon brake discs are suitable and may be converted by forming the friction surfaces of silicon carbide. The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which:- Figure 1 is a cross sectional view of a brake system incorporating a brake element according to the invention; Figure 2 is a side view of a braking element shown in Figure 1; Figure 3 is a cross sectional view of a portion of the braking element of Figure 2; and Figures 4 to 6 illustrate properties of brake elements of a brake system according to the invention. It has long been known that refractory metal carbides such as silicon carbide are very hard and heat resistant. However, it is also very well known that such refractory metal carbides have very high coefficients of friction. For example, some types of sandpaper and skid resistant floor mats have silicon carbide surfaces because of the high coefficient of friction and cutting ability of the silicon carbide. These properties of cutting and skid resistance would cause one not to use refractory metal carbides in a brake mechanism. Although it is often desirable to have high coefficients of friction in a brake mechanism, it is also required that the brake mechanism not grab or bind because of inconsistent friction characteristics. Prior to this invention it has generally been thought that refractory metal carbides could not function as brake surfaces since it has been assumed that the material would have extremely inconsistent friction characteristics as the hard, sharp edges of the carbide material interlocked. Refractory metal carbides have also not been used as brake surfaces since it has been assumed that the cutting action of such materials would quickly wear the brake into an unusable condition. The cutting ability of materials such as silicon carbide which make them useful for sandpaper and floor mats was thought to be totally inconsistent with the long wear required for brake mechanisms. Another conceptual problem with utilizing refractory metal carbides for brake surfaces is that, prior to the present invention, how to form the refractory metal carbide into a useable brake surface was not readily apparent. Sandpaper and floor mats are granular silicon carbide which has been glued to a mat surface. Such a process would not be suitable to create a brake surface since the glue would break down at the high temperatures and stresses resulting from a vehicle brake mechanism operation. Solid silicon carbide materials, on the other hand, do not have sufficient structural strength to serve as a brake material. Despite all of these conceptual difficulties presented by conventional wisdom in the brake art, the brake of the invention utilizes a refractory metal carbide friction surface. In fact, a refractory metal carbide brake surface provides relatively consistent friction characteristics, relatively long wear, and is relatively easy to manufacture. The refractory metal carbide brake surface of the present invention can provide much improved braking characteristics in certain brake applications. Referring now to Figure 1, a brake mechanism 11 is shown in cross sectional view. The sectional view is taken along a line through the centre of a wheel shaft 13. The mechanism 11 is of a kind which will be suitable for use in an aircraft. However, it is believed that the present invention would be suitable as a brake mechanism with all types of vehicles if suitably adapted using the knowledge well known in the braking art. Mounted for rotation on the shaft 13 is a wheel 15. Journal bearings 17 support the wheel 15 for the rotation on the shaft 13. A flange 19 extends axially inwardly from the wheel 15 to retain radially outer rotating brake disc elements 21. Thus, the rotating brake disc elements 21 rotate with the wheel 15. A brake housing assembly 23 is fixed to the shaft 13 and does not rotate with the wheel 15. Instead, the brake housing assembly 23 and the shaft 13 are fixed with respect to the vehicle or vehicle strut 25 on which they are used. Extending axially outwardly toward the wheel 15 from the brake housing assembly 23 is a back plate 27. The back plate 27 has fixed to it radially inner stationary brake disc elements 29. The three stationary brake disc elements 29 are spaced to receive the two radially outer brake disc elements 21 therebetween. As the wheel 15 rotates the outer brake disc elements 21 rotate therewith. The stationary brake disc elements 29 are held stationary in the housing 23. A hydraulic cylinder 31 is mounted in the housing assembly 23 and is actuated from the cabin of the aircraft by hydraulic pressure. As pressure is applied to the brake cylinder 31, pistons within the brake cylinder move axially to press the stationary and rotating discs 21 and 29 together. This produces a braking action by preventing the wheel 15 from rotating or braking the rotation of the wheel 15. This generates the braking action which decelerates or holds the vehicle. The construction of the brake mechanism described above is essentially conventional. The improvement of the brake mechanism of the present invention is in the construction of the brake disc elements 21 and 29. A portion of an element 21 is shown in more detail in Figures 2 and 3. It has long been known that one possible material for brake disc elements is carbon/carbon composite. Examples of materials which are utilized to make the carbon brake disc elements of the prior art art rayon, PAN and pitch base carbon fibre. An example of pitch base carbon fibre is named P55 and sold by Amoco, and an example of prepreged carbon fibre is named karbon #663 and made by Fiberite. The invention can utilize a carbon material such as these to form a core onto which a refractory metal carbide surface is formed. Carbon fibre such as mentioned above can be moulded into a desired disc shape utilizing a pressurized mould. Such processes for moulding a carbon brake are well known and can be utilized to form a core. For example, the fibre could be loaded into the mould and then subjected to a temperature of 163°C (325°F) and a pressure of 6.8 x 10⁶ Pa (1000 psi) for approximately two hours to form a conventional type of disc. The disc is then removed from the mould and carbonized at elevated temperature under nitrogen. The carbon is then machined to a desired oversized shape. The carbon core can then be densified utilizing a controlled mixture of methane to form pyrolytic carbon in the interstices of the carbon material. This is a conventional process known as chemical vapour deposition. For example, methane can be introduced over the machined carbonized disc at an elevated temperature of 1100°C and a reduced pressure of 15mm of mercury. Following densification, the core is machined to a final core shape and size. At this point, the carbon brake disc is of a conventional nature. The friction faces of the carbon core resulting from the above process can then be converted to a stable, refractory metal carbide of a predetermined depth and of a predetermined nature. Refractory metals which can form stable, refractory metal carbides are silicon, boron, tungsten, hafnium, zirconium, titanium, uranium, vanadium, niobium, tantalum, chromium, molybdenum, thorium and plutonium. However, silicon is most preferred. To convert the friction faces, a refractory metal powder such as silicon powder is placed in contact with the friction face surface of the carbon disc. It is desirable to have the metal powder be at least 50 mesh and more preferably 200 mesh in order to react properly with the carbon surface. With the refractory metal powder in contact with the friction face surface of the disc, the disc and powder are heated to the range of approximately 1400°C to approximately 2000°C under argon gas at one atmosphere. More preferably it is heated to the range of 1700°C to 1900°C. Most preferred is 1800°C. This treatment causes the powder to melt, wet and diffuse into the surface of the carbon core. The treatment continues for approximately 3 hours so that a proper conversion of the carbon to refractory metal carbide will occur. The timing must be gauged so that refractory metal does not remain in an unconverted state to any significant extend in the carbon disc; that is the metal on and in the carbon disc is converted to carbide metal. The depth of the conversion is controlled by the porosity of the carbon core surface on which the refractory metal powder is placed. This porosity can be adjusted by the densification process and by grinding away the surface to expose the pores after densification. Depth of conversion or penetration depth is important since the conversion to refractory metal carbide changes the mechanical properties and too great a depth will produce a brake disc which is unusable because the mechanical and structural properties of the thick metal carbide will cause it to crack or fracture under normal braking stresses. However, too little depth of carbide will not produce a surface capable of sufficient wear. Although complete conversion of the friction face of the disc to refractory metal carbide is preferred, it is also possible to utilize the above process to achieve a surface which is harder than carbon but not as hard as the refractory metal carbide. Thus, by restricting the depth of the metal penetration and the density of the metal penetration, an alloy can be created. This alloy characteristic can be adjusted by the porosity of the carbon disc prior to conversion. In turn, the porosity can be controlled by the carbon vapour deposition process and the machining process. In this manner a friction surface with intermediate friction properties and sturdier structural properties can be formed. Referring now to Figure 2, a rotating brake disc element 21 is illustrated. Notches 33 are formed in the outer hub of the disc 21 and are reinforced with metal clips (not shown), attached to the disc 21 through holes 35. Teeth 37 extending inwardly from the flange 19 of the wheel 15 extend into the notches 33 to retain the discs 29 against rotation while allowing the discs to move axially in a braking operation. Figure 3 shows a portion of a cross-section of the brake element 21 taken along line 3-3 of Figure 2. The friction surface 39 of the disc 21 has been converted to silicon carbide while the centre or core 41 and the edges (all non-friction surfaces) of the disc 21 are not converted and remain hardened, moulded carbon fibre. The penetration depth d of the refractory metal carbide is preferably in the range of about 0.38mm to about 3.8mm (about .015 inches to about 0.15 inches). Most preferably, the depth is about 1.52mm (about .060 inches). These depths produce a Iong wearing disc surface of refractory metal carbide while retaining the mechanical properties of the carbon fibre for the disc as a whole. The total axial thickness of the disc 21 is, for example, 12.7mm (0.5 inches) and the diameter is, for example, 280mm (11 inches). These dimensions are not critical as long as the mechanical properties are not compromised by making them too small. Figures 4 to 6 illustrate properties of a brake disc and brake system constructed according to the description above with silicon carbide friction surfaces. Figure 4 illustrates the tested properties of such a silicon carbide-silicon carbide brake test compared to a brake test using a conventional carbon-conventional carbon brake of the same dimensions. This test illustrates that a relatively constant dynamic coefficient of friction at both high and low speeds can be achieved. In each test the wheel was rotated at selected speeds with an inertia of 4794kg (10,570 pounds). The brake pressure to stop the wheel with a deceleration of 1.83m (six feet) per second was measured for each speed. The results illustrated in Figure show that conventional carbon requires a consistently higher brake pressure from 8 to 64km (5 to 40 miles) per hour and the range of pressure required is greater. The test illustrated in Figure 5 is of a similar silicon carbide brake of the present invention as the Figure 4 silicon carbide brake but tests for static coefficient of friction. A range of brake pressure was applied to both a new brake and a brake which had been stopped 100 times. The static coefficient of pressure was measured for each and is illustrated in Figure 5. This shows that the static coefficient of friction of the brake of the invention is relatively constant in both old and new brakes and that many stops do not change the static coefficient of friction greatly. The test illustrated in Figure 6 is of a brake of the present invention as described with the Figures 4 and 5 tests. In this test dynamic coefficient of friction is measured for repeated stops at 8, 16, 24 and 64km (5, 10, 15 and 40 miles) per hour with an inertia of 4794kg (10,570 pounds) and a 1.83m (6 feet)/sec. deceleration. The measured coefficient for each stop is plotted to show the range at each speed.

Claims: A brake element for use in a brake system and having a friction surface thereon, comprising: a carbon structure (41) for providing the mechanical and structural properties of the brake element; and a brake friction surface (39) disposed on the carbon structure (41) and comprising a refractory metal carbide material formed by converting carbon of the carbon structure to refractory metal carbide; characterised in that the refractory metal carbide surface (39) has a depth ( d ) of from about 0.38mm to about 3.8mm (about 0.015 inches to about 0.15 inches). A brake element according to claim 1, wherein the refractory metal carbide comprises silicon carbide. A brake element according to claim 2, wherein the silicon carbide surface has a depth ( d ) of from about 0.38mm to about 3.8mm (about 0.15 inches to about 0.15 inches). A brake element according to claim 3, wherein the silicon carbide surface has a depth of about 1.52mm (about 0.06 inches). A brake system incorporating; first and second brake elements (21, 29) according to any one of claims 1 to 4, and; a brake mechanism (11) for holding and operatively pressing the first (21) and second (29) brake elements together such that the refractory metal carbide friction surfaces of the elements bear on each other in a braking action.

Inventors: FROBERG, ROBERT W., GRIDER, BRUCE A., MILLER, OTTO C., PRATT, THOMAS E.
Applicants: PARKER HANNIFIN CORPORATION
Application year: 1988
Main classification: F16D69
IPC codes: F16D55, F16D69, F16D65, C04B41