Friction material using steel fibers

A friction material particularly useful for vehicle brakes comprises a friction material body including a plurality of steel fibers. Each steel fiber includes at least 0.07% by weight of sulphur, at least 0.10% by weight of phosphorus, or at least 0.15% by weight of both sulphur and phosphorus. The friction material exhibits high friction coefficients and less abrasion with respect to materials against which it bears as compared to conventional wear materials utilizing steel fibers.

FIELD OF INVENTION 
The present invention relates to a friction material for use in vehicle 
brakes, clutches, etc., such as those typically utilized in trucks and 
automobiles and, particularly, to a friction material using steel fibers 
suited for use as a disc or drum brake of a vehicle brake assembly. 
BACKGROUND AND SUMMARY OF INVENTION 
In general, friction materials for brakes are typically produced by 
combining together asbestos as a strength material, ceramic (inorganic) 
powders such as oxides and carbides as a friction coefficient (hereafter 
referred to as ".mu.")-increasing material, and graphite, soft metal, etc. 
as a friction-controlling material by the use of an organic binder 
(resin). Asbestos, however, has well known safety and hygienic 
disadvantages in the production or use thereof. Accordingly, it has been 
proposed to make friction materials for vehicle brakes asbestos-free by 
using steel fibers as the strength material in place of asbestos (see for 
example, Japanese Patent Application (CPI) Nos. 157673/80 and 111373/82 
which disclose friction materials using iron powder and steel fibers in 
place of asbestos). Thus, the use of steel fibers in friction materials 
is, in and of itself, known. 
It is necessary for the friction material however to exhibit a high 
friction coefficient (.mu.). Accordingly, for those friction materials 
using conventional steel fibers, various techniques are known to increase 
the friction material's friction coefficient. For example, the friction 
coefficient is increased by increasing the carbon content of the steel 
fibers so as to also increase the hardness of the resulting friction 
material. Additionally, the diameter of the steel fibers can be increased 
to thereby increase the friction coefficient as a result of its increased 
abrasive surface area with respect to the bearing surface of the brake 
drum, rotor or the like. Such a technique leads to a problem that the 
material of the brake drum etc. is seriously worn. 
A friction material using steel fibers has thus been needed which has a 
high friction coefficient while yet decreases the wear amount of the 
material of the brake drum, rotor and the like against which the friction 
material bears. 
In general, P (phosphorus) and S (sulfur) are impurities for steel, and are 
responsible for a reduction in its strength, toughness, workability, and 
so forth. Thus, it has been conventional wisdom in the production of 
steel, to decrease the phosphorus and sulfur contents as much as possible 
and such has been the case for steel fibers to be used in brake friction 
materials. In all conventional steel fibers, both the P and S contents are 
typically not more than 0.04% while the fiber's carbon content is between 
0.1 to 0.2%. 
It has been discovered, according to the present invention, that larger 
amounts of P and S in the steel fibers than have been conventionally 
utilized surprisingly increase the friction coefficient (.mu.) while at 
the same time, decrease the wear of the brake drum or rotor material. That 
is, as the P and S contents increase, the friction coefficient (.mu.) is 
also increased and the material against which the friction material bears 
is less worn as compared to conventional friction materials for similar 
purposes. 
The present invention thus overcomes the above defects of the conventional 
friction materials using conventional steel fibers and provides a friction 
material having a high friction coefficient and less abrasion of the 
material against which the friction material bears by using steel fibers 
containing at least 0.07% of phosphorus, at least 0.10% of sulfur, or at 
least 0.15% of both phosphorus and sulfur.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A further understanding of this invention will be obtained by reference to 
the following nonlimiting example. 
EXAMPLE 
A friction material was prepared by combining the following materials and 
forming the combined materials into a friction pad for an automobile disc 
brake: 
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Steel Fibers 45 wt. % 
Phenol Resin 8 wt. % 
BaSO.sub.4 10 wt. % 
Graphite 14 wt. % 
SiO.sub.2 1 wt. % 
Sb.sub.2 O.sub.3 
8 wt. % 
Kevlar .RTM. Pulp* 
7 wt. % 
(Aramide Fibers) 
Cu Powder 7 wt. % 
100 wt. % 
______________________________________ 
*Kevlar .RTM. is a Registered Trademark of E. I. DuPont deNemours & Co., 
Wilmington, Delaware. 
The friction pad was formed by pressing the above mixture at ambient 
temperature under a pressure of 350 kg/cm.sup.2 for one minute using a 
press. The pad was then molded in the press at 150.degree. C. under a 
pressure of 300 kg/cm.sup.2 for 10 minutes and then after-cured in a 
furnace at a temperature of 250.degree. C. for 15 hours. The friction pad 
thus obtained was ground to a desired thickness and then bonded to a 
metallic back plate to thereby produce a pad assembly. 
Steel fibers A-F having sulphur, phosphorus and carbon as constituents were 
utilized in the friction materials of this example as identified below in 
materials of this example as identified below in Table 1. 
TABLE 1 
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Components of Steel Fibers 
Fiber S P C 
No. (wt. %) (wt. %) (wt. %) 
______________________________________ 
A 0.005 0.015 0.15 
B 0.015 0.057 0.16 
C 0.075 0.061 0.15 
D 0.035 0.105 0.16 
E 0.061 0.092 0.16 
F 0.180 0.111 0.09 
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The pad assemblies having the friction materials respectively containing 
steel fibers A-F were tested by means of a dynamometer to simulate a 1500 
cc powered automobile. 
The following tests were performed. 
(a) Burnish: Each brake pad was applied 200 times at a deceleration rate of 
0.3 g* from 65 km/hr to stop with the pad temperature before braking being 
120.degree. C. 
FNT *The unit "g" herein denotes the acceleration of gravity--that is, 
approximately 9.8 meter/sec.sup.2. 
(b) Effectiveness Test: Each brake pad was tested five times at a 
deceleration rates of 0.6 g from 50 km/hr, 100 km/hr and 130 km/hr to 
stop, respectively. The pad temperature before each braking cycle was 
100.degree. C. and the friction coefficient for the pads was measured 
after each braking cycle. 
(c) Disc Rotor Abrasion Test: Each brake pad was applied 4000 times at a 
deceleration rate of 0.05 g from 50 km/hr to stop in order to measure the 
abrasiveness of the wear material of this invention upon a conventional 
cast iron brake disc rotor. The pad temperature before braking was 
50.degree. C. and the amount of the cast iron rotor which was abraded 
during braking was measured. 
The results of the above tests are shown in table 2 below. 
TABLE 2 
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(Test Results) 
Amount of Disc 
Rotor Material 
Friction Coefficient Worn Out 
Sample 50 km/H 100 km/H 130 km/H 
(.times. 10.sup.-3 m/m) 
______________________________________ 
A* 0.33 0.35 0.32 39 
B* 0.35 0.34 0.33 35 
C** 0.39 0.38 0.36 29 
D** 0.41 0.39 0.36 32 
E** 0.43 0.40 0.37 24 
F** 0.45 0.43 0.39 31 
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*Comparative Conventional Product 
**Product of Present Invention 
It can be seen from the above results that the products of the present 
invention have a high friction coefficient and less abrasion and attack on 
the disc rotor material as compared with the conventional products using 
steel fibers having low sulphur and phosphorus contents, and thus have 
excellent characteristics as friction materials. 
As described above in detail, the present invention uses steel fibers 
having high sulphur and phosphorus contents which have not been been 
thought of as being really useful for friction materials for vehicles 
using steel fibers. Thus the present invention is very effective in that 
it provides a friction material which is low cost, has a high friction 
coefficient, and less abrasion with respect to other materials.