Cementitious composite friction compositions

Abstract

Friction materials intended to be used as brake shoes, brake pads, and other brake components, comprising a composite in which one or more friction modifiers is embedded in a high strength cement matrix.The matrix material exhibits a flexural strength of at least 1,500 psi, a compressive strength of at least about 15,000 psi and a flexural modulus of about 1.times.10.sup.6 psi.

Description

This invention relates to friction compositions which are generally comprised of friction modifiers such as particles and fillers embedded in a friction binder or matrix.

More particularly, the invention relates to cementitious composite friction materials which provide high performance, asbestos-free, relatively inexpensive compositions suitable for use as friction elements in such applications as brake shoes and disc brake pads and clutch plate faces for cars and light trucks, shoes and discs for railway use, and blocks, shoes or discs for heavy duty use and for aviation use.

Still more particularly, the invention relates to compositions comprising high strength inorganic cement binder materials and friction modifying filler materials or particles and to the manner in which such compositions are prepared.

Brake shoes composed of various fillers bonded by low strength inorganic cement binders have been described in the following United States Patents:

______________________________________Buell 484,404 issued October 18, 1892Laeufer et al 818,833 issued April 24, 1906Laeufer et al 909,617 issued January 12, 1909Norman 943,157 issued December 14, 1909Newman 1,019,989 issued March 12, 1912Steinbaugh 1,076,325 issued October 12, 1913Reid 1,205,482 issued November 21, 1916Laeufer 1,724,718 issued August 13, 1929______________________________________ and others of a similar nature.

With the adoption of synthetic resin binders and asbestos fillers, further development of such cement bonded friction materials appears to have been discontinued.

The art is in need of improved friction materials having performance and cost characteristics that exceed the properties and performance of existing friction materials. Desired improvements include higher heat stability, improved processability, machining and handling and lower cost.

An object of the invention is to provide a composition that satisfies the foregoing needs.

Another object of this invention is to provide a friction material which retains its effectiveness at the elevated temperatures encountered in normal service and which permits more extended service use as compared with friction materials in which the principal binder is a phenolic resin, such as those described in the prior art, for example in U.S. Pat. No. 3,879,338.

A further object of this invention is to provide a friction material having a matrix with a flexural strength in excess of 1,500 psi, a compressive strength in excess of 15,000 psi and a flexural modulus of about 1.0 million psi which when modified with suitable friction modifiers produces an acceptable braking effect with acceptable wear of the braking surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects will be pointed out or will become apparent from the description which follows taken in conjunction with the drawings in which:

FIGS. 1A and 1B are flow sheets depicting two alternative processes for preparing the materials and

FIGS. 2, 3 and 4 are test results for three of the brake compositions illustrative of the present invention and set forth in the examples which follow.

SUMMARY OF THE INVENTION

These and other objects of the invention are accomplished by the compositions of the invention which comprise:

(1) a high strength cement matrix material, and

(2) one or more tribological additives which control the friction, wear, lubrication, and noise characteristics of the resulting composition.

DESCRIPTION OF EMBODIMENTS

Each of the constituents of the compositions of the invention and their preparation will now be described.

(1) Cement Matrix Materials

The friction binder useful in the present invention is a high strength cement, that is a cement which in the final composite exhibits a flexural strength of at least 1,500 psi, a compressive strength of about 15,000 psi or more and a flexural modulus of about 1.0 million psi.

A wide variety of inorganic cements may be used to develop the high strength matrix material in the composites of the present invention. Brake linings have been produced in which the tribological additives were embedded in matrix materials based on one or more of the following cements:

1. Silica Based Cements, e.g. Portland Cements

2. High Alumina Cements

3. Polymer Treated Cements (polymer modified cements, polymer impregnated cements)

4. Other mixtures of Cement forming Ceramic Oxides e.g. Lime, Silica, Alumina, Magnesia, Phosphate

5. Phosphate Based Cements

Preferred high strength cements suitable for the present invention and their preparation are described in the following U.S. Pat. Nos. 4,353,748, 4,410,366, 4,482,385, 4,501,830, 4,505,753, the disclosures of which are incorporated herein by reference and in numerous other patents and publications.

The matrix materials useful in this invention also include the phosphate bonded matrix which may comprises the following:

______________________________________ Weight %______________________________________Alumina Cement 25- 75Calcined Alumina 8- 35Sodium Hexametaphosphate 8- 25Magnesium Oxide 0- 66Water 8- 25______________________________________

In addition to cement such as Portland Cement or High Alumina cement, the matrix material may include the additives disclosed in the five above noted U.S. Patents. For example, as described in Scheetz, U.S. Pat. No. 4,505,753 the cement is preferably Class H Portland Cement, mixed with Min-U-Sil.sup.R, silica fume and a superplasticizer. As described in U.S. Pat. No. 4,410,366, the cement is a high alumina cement and a water dispersable or water soluble polymer or copolymer.

More specifically, useful matrix components include the following in the indicated proportions, it being understood that the total comprises about 60% of the composite:

______________________________________ Broad Narrow______________________________________Class H Portland Cement 0- 40 15- 35Min-U-Sil .RTM. Silica 0- 20 13- 17Silica Fume 0- 10 3- 7Mighty 150 (Superplasticizer) 0- 3 2- 2Water 2- 20 5- 15Alumina Cement 0- 50 20- 45Polyvinyl Alcohol/Acetate 0- 25 4- 17Calcined Alumina 0- 30 4- 20Sodium Hexametaphosphate 0- 25 10- 20Magnesium Oxide 0- 50 10- 50Polyethyleneimine 0- 5 0.5- 2.0Wollastonite 0- 50 20- 30Polyvinylpyrrolidone 0- 10 2- 7______________________________________

Tribological Additives

Virtually every manufacturer of brake linings has its own recipe or formulation for the composition it manufactures but in general these are comprised of the following principal classes of ingredients:

(1) Fibers for reinforcement, for wear and heat resistance and for control of friction performance;

(2) Dry lubricants to control friction coefficient, lower the wear and provide lubricity to the composition;

(3) Inorganic fillers such as inorganic powders and/or metal particles to improve wear resistance and thermal properties and to control frictional characteristics; and

(4) Various organic fillers: e.g. rubber to control squeal and noise, and cashew nut shell resin for processing and desired friction qualities.

(1) Fibers

Fibers are a principal constituent in the friction modifying portion of the composition of this invention. A preferred material for the fibers is chopped steel wool, commercially available in short lengths (0.050"-0.5"). Various steels can be used but for reasons of availability and economy, wires or ribbons of plain carbon steels like SAE 1020 are suitable. Other metal fibers, such as copper or brass, mineral fibers such as glass, or synthetic resin fibers such as aramid fibers may be used in place of some or all of the steel fiber component. When steel fibers are used, the steel fibers usually comprise between 20% and 60% by weight of the friction composition.

(2) Dry lubricants

One or more of a variety of finely divided dry lubricants may be present in the composition including: graphite, petroleum coke, carbon black, molybdenum disulfide or sulfur. Suitable graphites are commercially available as synthetic graphite in powder form (-20 mesh). Suitable petroleum coke, carbon black and other dry lubricants are also commercially available. When used, the foregoing materials are usually present in amounts between 2.5 and 15% by weight of the friction composition.

(3) Inorganic Fillers

Other materials which may be included in the compositions of the present invention include both soft and hard inorganic filler materials and include relatively hard materials such as Al.sub.2 O.sub.3, feldspar, MgO, SiO.sub.2, SiC, and softer fillers such as barytes (BaSO.sub.4), kaolin, talc, mica, iron oxide and litharge to provide better thermal conductivity and enchanced wear resistance, the composition may also include finely divided metallic materials such as iron, steel, copper, brass, zinc and the like e.g. as described in Rhee et al (BENDIX) U.S. Pat. No. 3,835,118, issued Sept. 10, 1974. Amounts of such materials up to about 25% by weight of the friction composition have been found beneficial in the friction materials of the present invention.

(4) Organic Fillers

A small amount of powdered nitrile rubber may be included to reduce squeal and other undesired noise in service. Powders of other rubbers such as SBR or butadiene rubbers can be substituted for the nitrile rubber. Another organic filler which may be present in the compositions of this invention intended for service as brake linings is cashew nut shell resin, described in U.S. Pat. No. 4,178,278. Other organic fillers known in the art may be provided for specific benefits. Up to about 5 weight percent of organic filler is generally employed based on the weight of friction composition.

The friction materials of this invention may be cast from the above components or may be formulated as described in the following description which is intended to illustrate one mode of practicing the present invention and is not intended to limit the same. It will be understood that depending on the specific formulation, minor variations may be made in the procedure followed in preparing specimens for testing.

All of the dry ingredients except for the fibers are dry blended. The fibers are added slowly while the mixture is being blended. After a suitable interval of mixing the material has been dry blended. Blending may be performed under vacuum, if desired. The liquids are then added and thoroughly mixed with the blended solids, again under vacuum if desired, until the material has become homogenous, and the desired consistency has been obtained. After extrusion, the material is cut, pressed or rolled into the desired shapes and thicknesses.

The extruded object is wrapped to prevent moisture loss, and placed between two platens to retain its shape while it is being cured.

After these procedures the material is tested for its flexural strength, modulus, and frictional properties.

EXAMPLES

The compositions of the following examples were prepared according to the above described procedure and cured as noted and then tested. The examples illustrate the invention but are not intended to limit it. In this specification and claims, unless specified otherwise, all parts and percentages are by weight and temperatures are in degrees celsius.

EXAMPLE 1

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialClass H Portland Cement 29.1 24.7Min-U-Sil .RTM. Silica 14.2 14.5Silica Fume 4.0 4.8Mighty 150 (Superplasticizer) 0.7 1.6Water 9.1 24.2Friction ModifiersSteel Fiber 29.4 10.1Graphite 9.1 11.1Petroleum Coke 1.1 1.5Nitrile Rubber 1.1 2.5Cashew Nut Shell Resin 2.3 5.0Weight Ratio, Modifier 43:57to MatrixCuring 24 hours at 100% humidity, 20.degree. C. 24 hours at 100% humidity, 60.degree. C. 48 hours at ambient conditions 17 hours at 200.degree. C.Flexural Strength 3250 psiAfter cureModulus after cure 1.5 .times. 10.sup.6 psi______________________________________

When tested in a "F.A.S.T" tester, the cured composition exhibited braking properties which were adequate for many applications.

EXAMPLE 2

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialAlumina Cement 30.5 9.5Polyvinyl Alcohol/Acetate 6.0 13.2Calcined Alumina 10.3 6.8Water 11.9 31.4Friction ModifiersSteel Fiber 28.1 9.5Molybdenum Disulfide 5.8 3.2Fused Magnesium Oxide 2.2 1.6Barytes 2.2 1.3Nitrile Rubber 1.1 2.4Cashew Nut Shell Resin 2.2 4.7Weight Ratio, Modifier 41:59to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 125.degree. C. 3 hours oven drying at 175.degree. C.______________________________________

When tested in a "F.A.S.T" tester, the cured composition exhibited braking properties which were adequate for many applications.

EXAMPLE 3

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialAlumina Cement 28.2 21.6Polyvinyl Alcohol/Acetate 11.1 22.0Calcined Alumina 9.5 5.6Water 11.0 26.1Friction ModifiersSteel Fiber 26.0 7.9Graphite 8.4 9.0Petroleum Coke 1.0 1.2Nitrile Rubber 1.0 2.1Carbon Black 3.8 4.6Weight Ratio, Modifier 40:60to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 125.degree. C. 3 hours oven drying at 175.degree. C.Flexural Strength 2680 psiAfter cureModulus after cure 1.0 .times. 10.sup.6 psi______________________________________

FIG. 2 is a graph showing the braking properties of this composition when tested in a "F.A.S.T" tester.

When tested on a 1985 automobile, the cured composition exhibited braking properties adequate for the car.

EXAMPLE 4

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialSodium Hexametaphosphate 14.6 16.0Alumina Cement (Secar 71) 28.3 25.0Magnesium Oxide 14.1 10.7Water 8.8 24.0Friction ModifiersSteel Fiber 22.2 7.8Graphite 7.2 8.9Petroleum Coke 0.9 1.2Nitrile Rubber 0.9 1.9Carbon Black 3.3 4.5Weight Ratio, Modifier 34:66to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 80.degree. C. 15 hours oven drying at 125.degree. C. 15 hours oven drying at 200.degree. C. 15 hours oven drying at 400.degree. C.Flexural Strength 3270 psiAfter cure______________________________________

EXAMPLE 5

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialSodium Hexametaphosphate 15.3 16.1Alumina Cement 27.5 23.3Calcined Alumina 9.2 6.1Water 11.5 30.3Friction ModifiersSteel Fiber 23.5 7.9Graphite 7.6 9.1Petroleum Coke 0.9 1.2Nitrile Rubber 0.9 2.0Carbon Black 3.5 4.2Weight Ratio, Modifier 36:64to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 80.degree. C. 15 hours oven drying at 125.degree. C. 15 hours oven drying at 400.degree. C.Flexural Strength 2700 psiAfter cure______________________________________

FIG. 3 shows the results of a test of this composition as a brake pad in a "F.A.S.T" tester.

EXAMPLE 6

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialSodium Hexametaphosphate 15.1 16.1Alumina Cement 27.1 23.9Calcined Alumina 9.1 6.1Water 11.4 30.3Friction ModifiersSteel Fiber 23.1 7.9Graphite 7.5 9.1Petroleum Coke 0.9 1.2Silicon Carbide 2.4 2.0Carbon Black 3.5 4.2Weight Ratio, Modifier 37:63to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 80.degree. C. 15 hours oven drying at 125.degree. C. 4 hours oven drying at 400.degree.C.Flexural Strength 2170 psiAfter cure______________________________________

FIG. 4 shows the results of a test of this composition as a brake pad in a "F.A.S.T" tester.

EXAMPLE 7

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialSodium Hexametaphosphate 15.3 16.1Alumina Cement (Secar 71) 27.5 23.3Calcined Alumina 9.2 6.1Water 11.5 30.3Friction ModifiersSteel Fiber 23.5 7.9Graphite 7.6 9.1Petroleum Coke 0.9 1.2Nitrile Rubber 0.9 2.0Carbon Black 3.5 4.2Weight Ratio, Modifier 36:64to MatrixImpregnationImpregnation with high temperature NOVALAC EPOXYCuring 24 hours at ambient conditions 15 hours oven drying at 80.degree. C. 15 hours oven drying at 125.degree. C. 15 hours oven drying at 400.degree. C. Impregnation at 200.degree. C.Flexural Strength 8530 psiAfter cure______________________________________

EXAMPLE 8

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialSodium Hexametaphosphate 12.5 14.0Polyethyleneimine 1.1 2.6Magnesium Oxide (Coarse) 38.3 29.8Magnesium Oxide (Fine) 5.0 3.9Water 8.9 24.9Friction ModifiersSteel Fiber 22.1 7.9Graphite 7.1 9.1Petroleum Coke 0.8 1.2Nitrile Rubber 0.8 2.0Carbon Black 3.3 4.6Weight Ratio, Modifier 34:66to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 80.degree. C. 15 hours oven drying at 125.degree. C.Flexural Strength 4030 psiAfter cure______________________________________

EXAMPLE 9

______________________________________ Weight Volume Percent Percent______________________________________Matrix MaterialAlumina Cement 27.2 21.0Wollastonite 26.1 21.1Polyvinylpyrrolidone 5.4 10.6Water 10.8 25.3Friction ModifiersMullite 30.0 22.0Weight Ratio, Modifier 30:60to MatrixCuring 24 hours at ambient conditions 15 hours oven drying at 80.degree. C. 15 hours oven drying at 200.degree. C. Fired to 1450.degree. C.Flexural Strength 12150 psiAfter cure______________________________________

In the compositions of the invention, the friction modifier and matrix can be used in the following weight proportions:

______________________________________ Weight % Broad Preferred Volume %______________________________________Matrix 40- 80 55- 70 70- 78Friction Modifiers 60- 20 45- 30 30- 22______________________________________

More specifically the proportions of the various friction modifiers when present are as follows, it being understood that the total comprises about 40% of the composite:

______________________________________ Broad Narrow (Weight %) (Weight %)______________________________________Steel Fiber 0- 35 20- 30Graphite 0- 15 5- 10Petroleum Coke 0- 5 0.5- 2Cashew Nut Shell Resin 0- 5 2- 3Molybdenum Disulphide 0- 15 5- 8Magnesium Oxide 0- 5 2- 3Barytes 0- 5 2- 3Nitrile Rubber 0- 5 0.5- 15Carbon Black 0- 10 2- 5Silicon Carbide 0- 5 2- 3Mullite 0- 40 25- 35______________________________________

Claims

1. In a friction composition comprising a hydraulic cement matrix and friction modifying substance, the improvements which comprise utilizing as the matrix a high strength hydraulic cement which when cured exhibits a compressive strength of at least 15,000 psi and a flexural strength of at least 1,500 psi, and utilizing as the friction modifying substance at least one of each of the following classes of materials: fibers, dry lubricants and fillers; which composition better retains its effectiveness at the temperatures encountered in normal service and permits more extended service use as compared with friction materials in which the matrix is a phenolic resin or a lower strength hydraulic cement.

2. The composition of claim 1 wherein the relative proportion of matrix to friction-modifying substance is from about 70:30 to 55:45 by weight.

3. The composition of claim 1 wherein the friction-modifying substance comprises steel fibers, graphite, petroleum coke, nitrile rubber and cashew nut shell resin.

4. The composition of claim 1 wherein the friction-modifying substance comprises steel fibers, graphite, petroleum coke, nitrile rubber and carbon black.

5. The composition of claim 1 wherein the friction-modifying substance comprises steel fibers, nitrile rubber, cashew nut shell resin, molybdenum disulfide, magnesium oxide and barytes.

6. The composition of claim 1 wherein the relative proportion of matrix to friction-modifying substance is from about 78:22 to 70:30 by volume.

7. A friction element produced by curing the composition of claim 1.

8. A friction element produced by curing the composition of claim 3.

9. A friction element produced by curing the composition of claim 4.

10. A friction element produced by curing the composition of claim 5.