CA1181264A - Heat-resistant spring made of fiber-reinforced metallic composite material - Google Patents

Abstract

HEAT-RESISTANT SPRING MADE OF
FIBER-REINFORCED METALLIC COMPOSITE MATERIAL
Abstract of the Disclosure The present invention relates to a spring useful for various parts or elements of machines and apparatuses.
The spring is made of a fiber-reinforced metallic com-posite material comprising a matrix metal (e.g. a metal having a melting point of not higher than 1,700°C) and reinforcing inorganic fibers having a high modulus of elasticity and high strength, selected from ceramic fibers and metallic fibers, particularly aluimina or alumina-silica fibers containing 100 to 72 % by weight of alumina, preferably 98 to 75 % by weight, and 0 to 28 % by weight of silica, preferably 2 to 25 % by weight, and having sub-stantially no .alpha.-alumina reflection in X-ray diffraction.
The spring of the present invention is light in weight and has far greater heat resistance and mechanical properties in comparison with conventional metallic or non-metallic springs, and hence is particularly valuable from the standpoint of saving energy and saving resources.

Description

The present invention relates to a heat-resistant spring made of a fiber-reinforced metal (hereinafter referred to as "FRM").
Springs are commonly used as parts or elements of various machines and apparatuses and are produced by forming an elastic material, i.e. one capable of absorbing an external force as elastic energy, into an appropriate shape. Springs are roughly c~assified into (1) metallic springs (e.g. steel springs, non-ferrous metallic springs, etc.) and (2) non-metallic springs (e.g. rubber springs, fluidic springs, etc.) There are six major types of spring classified accord-ing to shape, e.g. as shown in McGraw-Hill, Encyclopedia of Science & Technology, 1960, Vol. 13, pp. 14-17. Leaf springs and coil springs are two of these types.

In order to select the most suitable spring for a particular utility, various factors, such as the elastic coefficient, elastic limit, fatigue strength, heat re-sistance, corrosion resistance, fabrication quality, or coefficient of thermal expansion of the material, should be taken into consideration.
Examples oE known metallic springs are steel' springs such as springs made of carbon steel or alloyed steel.
These steel springs are suitable for practical use in view of their excellent fabrication quality and are used in various shapes. However, steel springs have the drawbacks that their density is too large, such as 8 - 9 g/cm3, and the strength and elasticity thereof are signiicantly decreased at high temperatures. For example, carbon steel $~

springs have a working temperature limit of 180C, and even stainless steel springs have a working limit at about 310~C (cf. "Spring Design'l 2nd Ed., edited by Spring Technique Research Committee in Japan, issued by Maruzen, 1963, page 9). Various non-ferrous metal springs have been used in order to improve the heat resistance of - la -f .3 ~ ; L~

springs, such as copper alloyed spring, cobalt- or nickel-base spring. However, these non-ferrous metallic springs still show decreased strength and modulus of elasticity at higher temperatures. Thus, a light, heat-resistant spring material having a large specific strength (i.e. strength/
density) and a large specific modulus of elasticity ~i.e.
modulus of elasticity/density) has not yet been obtained.
Moreover, the conventional metall c springs usually have fatigue breaking stresses which decrease rapidly at re-peating times of 106 in fatigue tests, and hence theyare of reduced practical value. Besides, carbon steel springs are also inferior in terms of corrosion resist-ance, and steel springs show unfavorably decreased absorption energy at a comparatively low temperature, such as at temperatures lower than the transition tem-perature of steel, because of rapid breaking thereof, that is, they show so-called brittleness at low temperature.
(cf. "Springs" 2nd Ed., edited by Spring Technique Research Committee in Japan, issued by Maruzen, 1970, page 278~.

Examples of known non-metallic springs, are rubber springs, fluidic springs, and further, fiber-reinforced resin (FRP) springs. (cf. Japanese Patent Publication (unexamined) Nos. 33962/1977, 33963/1977, 3~161/1977, 36250/1977, and 56252/1977). In the case o~ springs made oE FRP, springs made of thermosetting or thermoplastic resin rein~orced with gLass fibers or carbon fibers are known, for example. FRP is comparatively easily formed and is light in weight and has desirable mechanical pro-perties, but it suffers from the severe disadvantage o low heat resistance. For example, a spring made of polyimide resin reinforced by glass fibers has a heat resistance of lower than 300C.
It has recently become a major concern that energy and resourGes in the world will be exhausted in the near future, and hence, it is necessary to save energy and resources in various fields. From this viewpoint, it has become desirable to reduce the weight of various parts or elements of various transportation vehicles such as automobiles, airplanes, railway cars, and further various other machines and apparatuses. Moreover, from the standpoint of saving energy by enhancing efficiency and imparting high performance to machines and appara-tuses, it is advantageous to improve the heat resistance of the parts and elements. By weight-saving or improve-ment of heat resistance of the parts or elements, not only the parts or elements themselves but also whole machines or apparatuses or whole systems into which the parts or elements are incorporated will be improved. Accordingly, if springs having good heat resistance ti.e. springs having mechanical properties at a low or high temperature similar to those at room temperature) and also having a light weight can be obtained, it will result in a revolu-tion in the machine and related industries.
Thus, an object of the present invention is to provide a novel fiber-reinEorced metallic composite material suit-able for springs having good mechanical properties and good heat resistance.
From these viewpoints, the present inventors have intensively researched the improvement of spring materials in order to eliminate the drawbacks of the conventional metallic springs and non-metallic springs materials (e.g.
FRP). As a result, it has been found that springs having good mechanical properties and good heat resistance can be obtained from a fiber-reinforced metallic composite material (FRM).
~ ccording to the invention there is provided a leaf spring or coil spring made of a fiber-reinforced metallic composite material comprising a matrix metal and a reinforcement of inorganic fibers having a tensile modulus of 1,000 t/cm2 or more and a tensile strength of 10 t/cm2 or more, the inorganic fibers being ceramic fibers selected from alumina fiber, alumina-silica fiber, carbon fiber, graphite fiber, silicon carbide fiber and zirconia boron fiber or metallic fibers selected from tungsten fiber and stainless steel fiber, the maximum fiber content of the composite material being 68 % by volume when the composite material is obtained by uni-directionally arranging continuous fibers or long fibers and being 45 % by volume when the composite material is obtained by arranging at random short fibers, the fiber diameter being 0.6 to 400 ~m for alumina or alumina-silica fiber, and when the composite material comprises contin-uous fibers or long fibers, the fibers being arranged in fiber bundles having of from 1 to 200,000 filaments.
The invention, at least in the preferred forms, thus provides a novel light spring made of a fiber-reinforced metallic composite material, which has good heat-resistance, i.e. dynamic and mechanical properties such as;strength and modulus at elongation, bending or compression, and in which the fatigue strength of the spring material is not deteriorated even at a low or high temperature as well as at room temerature.

. ~, FRM materials have been produced by way of trial for the purpose of reinforcements for single metals such as lead, aluminum, copper, nickel or titanium and alloys of these metals. Although these FRM materials are similar in form to FRP, the process for their production is still developing, and a practically useful FRM has not been produced until now. FRM materials are different from FRP in their reinforcing mechanism, interfacial reaction between the fibers and the matrix, and their breaking mechanism due to impact or fatigue, and hence, springs made of FRM are different in quality from springs m~de of FR~.
Among the FRM materials, the novel alumina or alumina-silica fiber-reinforced metallic composite materials as - 4a -3~

described hereinafter has characteristics such that a composite structure can be formed without reaction between the fibers and metal and it is light in weight in addition to having good mechanical properties and heat resistance.
Thus, alumina or alumina-silica fiber-reinforced composite materials are particularly superior to the conventional spring materials and are useful for the production of springs for various machine elements in various industries, e.g. spaceships, atomic power machines and transportation 10 vehicles.
The reinforcing fibers used in the present invention include ceramics fibers or whiskers (e.g. fibers or whiskers of alumina, silica, alumina-silica, carbon, graphite, silicon carbide, zirconia, or boron), metallic fibers (e.g. tungsten fibers, stainless steel fibers), and iron whiskers, and also include fibers coated with metals or ceramics (e.g. boron/silicon carbide fiber) which are produced by coating the fiber surfaces by means of (1) flame spray coating (plasma spray coating), (2) electro-deposition coating (electroplating, chemical plating), (3)deposition coating (e.g. vacuum deposition, chemical vapor deposition, sputtering, ion plating).
~ hese reinforcement fibers used for FRM have, however, some drawbacks. For example, boron fibers have a high strength, but they are inferior in flexibility because of their large fiber diameter of about 100 um, and further, in the case oE a matrix of aluminum alloy, a boron com-pound is easily produced at the interface of the fibers and matrix at a high temperature, which results in lower-ing of strength of the FRM. In order to eliminate thelatter drawback~ boron fibers are usually coated with silicon carbide, by which the undesirable reaction at the interface is prevented, but it is still unsatisfactory~
Although carbon fibers also have excellent strength and elasticity, in the case of an aluminum alloy matrix, a brittle layer of A14C3 is formed at the interface of the fibers and matrix, which results in reduction of strength of the composite rnaterial. Moreover, since carbon Eibers have a high electrical conductivity, gal-vanic corrosion occurs at the interface of the fibers and matrix to result in a reduction of strength of the composite material. Accordingly, this composite material has an inferior resistance against corrosion, i.e. in-ferior resistance to saline solutions. Besides, since carbon fibers have a poor wetting effect with aluminum in the liquid phase, attempts have been made to coat the surface of carbon fibers with metals or ceramics in order to give them good wetting with the matrix metal and also to inhibit the undesirable reaction at the interface of the fibers and matrix, as mentioned above. This has been achieved to some extent, but it is very difficult and requires a carefully controlled technique to ensure uniform coating of carbon fibers with metals or ceramics because carbon fibers are too fine, i.e. they have a fiber diameter of ]0 ~m or less. Metallic fibers, e.g. stain~
less steel fibers, have comparatively larger diameters e.g. B to 15 ~m on average and have good flexibility~ but they have a large specific weight, e.g. about 8 g/cm3, and hence are not suitable for weight-saving FRM. More-over, in the case of mo]ten aluminum matrix, the metallic fibers easily react with the matrix to result in a reduc-tion of strength of the composite material.

Under the circumstances, the inventors attempted to find other combinations of various metals with various fibers in order to provide a composite material having desirable properties. In the selection of the most suitable combination of fibers and metals, it is pre-ferable to avoid combining a fiber and a metal which react significantly at their interface, e.g. a combination of E
glass fibers with aluminum or aluminum alloys. Even in such an undesirable combination, if the undesirable reac-tion at the interface thereof can be inhibited by coatingthe surface of the fibers with metals or ceramics, the combination may also be acceptable. It is also prefer-able to avoid selecting a combination wherein the fiber decreases its mechanical properties significantl~ (e.g.
strength, modulus of elasticity) at a temerature range of around the melting point of the matrix metals. From these viewpoints, it is preferable to combine an aluminum matrix with a fiber selected from alumina fibers, alumina-silica fibers, and boron fibers coated with silicon carbide.

Among these reinforcing fibers, the following alumina or alumina silica fiber has a good effect for reinforcing metals.
The alumina or alumina-silica fibers can be obtained in the form of continuous fibers having good handling properties (different from whiskers), and have greater flexibility than boron fibers and also have a large oxi-dation resistance even at a high temperature, which can not be observed in the ease of boron fibers and carbon fibers. Accordingly, the alumina or alumina-silica fibers are easily used for the produetion of metallic composite material. Moreover, the alumina or alumina-siliea fibers do not easily react with various metals and have excellent mechanical properties and hence show excellent specific strength and specific modulus of elasticity within a wide range of temperature from low temperature to a high tem-perature, and they can give composite materials having excellent creep characteristics at high temperature, fatigue properties and impact resistance with various matrix metals.
The alumina fiber or alumina-silica fiber desirably contains 100 to 72 % by weight, preferably 98 to 75 ~ by weight, of alumina (A1203) and O to 28 % by weight, preferably 2 to 25 % by weight, of silica (SiO2). Within the stated content of silica and not more than 10 % by weight, preferably not more than 5 % by weight, based on the total weight of the fiber, the silica may be replaced by one or more kinds of oxides of various elements selec-ted from lithium, beryllium, boron, sodium, magnesium, phosphorus, potassium, calcium, ~itanium, chromium, manqanese, yttrium, zirconium, lanthanum, tun~sten, and barium.
The alumina fibers and alumina-silica fibers prefer-ably show no a-alumina reflection upon X-ray diffraction.
Generally~ when an inorganic fibers is heated and calcined at a high temperature, the fiber-forming inorganic mater-ials crystallize into small grains, and because of ease of fracture between the crystallized grains, the strength of the fiber decreases significantly. According to the pre-sent inventors' study, the growth of crystalline grains is characterized by the appearance of the a-alumina re-Election in X-ray diffraction. Accordingly, the alumina fibers and alumina-silica fibers should preferably be ,f ~

prepared so that no ~-alumina reflection appears in X-ray diffraction.
The alumina fibers and alumina-silica fibers have the following excellent properties suitable for producing reinforced composite materials. These fibers have a high tensile strength, e.g. 10 t/cm2 or more, and high tensile modulus, e.g. 1,000 t/cm2 or more. Since they are com-posed of stable oxides, they do not deteriorate even when exposed to air at a high temperature, e.g. l,000C or higher, for a long period of time. Since they contain predominantly alumina, they are stable and are substan-tially non-reactive with various molten metals. Since the crystalline grains are not large, they are easily wettable with various metals. Moreover, the fibers have a density of about 2.5 to 3.5 g/cm3 and are light in weight. These properties may vary depending on the silica content of the fibers, and it has been found by the present inventors that the fibers have the most preferable properties when the silica content is not more than 28 % by weight, and is preferably in the range of 2 to 25 % by weight.
The alumina fibers and alumina-silica fibers can be prepared by various methods. For example, a viscous solution containing an aluminum~compound (e.g. alumina sol, aluminum salt, etc.), a silicon compound (e.g. silica sol, ethyl silicate, etc.) and an organic high molecular compound (e.g. polyethylene oxide, polyvinyl alcohol, etc.) may be spun to give a precursor fiber, and then the precursor fiber thus obtained is calcined in air at a temperature lower than the temperature at which the -alumina reflection appears in X-ray diffraction. Al-ternatively, the fibers may be prepared by immersing an organic fiber into a solution containing an aluminum com-pound and a silicon compound and then calcining in air.
The most suitable alumina fiber and alumina-silica fiber can be prepared by the method as disclosed in U.S.
Patent 4,101,615 issued on July 18, 1978 to Sumitomo Chemical Company (corresponding to Canadian Patent 1,01~,718). Tha-t is, a solution containing a poly-aluminoxane and desirably a silicon compound is spun to give a precursor fiber, and the precursor fiber is calcined in air. The calcining should be done at a temperature at maximum which no ~-alumina reflection is observed in X-ray diffraction.
In order to improve the properties of the resulting alumina or alumina-silica fibers, it is preferable to add to the spinning solution a small amount of one or more kinds of the compounds containing an element such as lithium, beryllium, boron, sodium, magnesium, phosphorus, potassium, calcium, titanium, chromium, manganese, yttrium, zirconium, barium, lanthanum, or tungsten.
The alumina or alumina-silica fibers obtained by the method as described in U.S. ~atent 4,101,615 referred to above have a fiber diameter of 0.6 to ~00~ , a tensile st.rength of 10 to 30 t/cm2, a tensile modulus of 1,000 to 3,000 t/cm~ and are stable in air at a temperature of 1,000C or higher for a long period of time. Such eibers are the most suitable for the production of the desired fiber-reinforced metallic composite material.
It is effective to coat the surface of the alumina or alumina-silica fibers with metals, e.g. nickel or titanium, or ceramics as mentioned above, by which the reaction between the fibers and metal is controlled and the wettability between them is also improved, and thereby the reinforcing effect of the fibers is enhanced.
In order to obtain various fiber-reinforced metallic composite materials suitable for particular utilities, the alumina or alumina-silica fiber may be used together with other inorganic fibers e.g. boron fibers, graphite fibers, whiskers or metallic fibers e.g. stainless steel fibers.
The fibers may be in various shapes e.g. continuous fibers, long fibers, short fibers or whiskers. However, in the case of short fibers or whiskers, they should pre-ferably have an aspect ratio (i.e. ratio of fiber length to fiber diameter) of lO or more, preferably 50 or more, in view of the mechanism of composite forming.
The number of filament in the fiber bundle is not critical, and a wide range of numbers of filaments, i.e.
from a monofilament (single) to 200,000 filaments (like carbon fibers) may be applicable. It has been found, however, that it is particularly preferable to use a fiber bundle containing 30,000 filaments or less in order to ensure uniform impregnation of the matrix into the fibers.
The matrix metal to be reinforced with the fibers should preferably have a mqlting polnt of not higher than 1,700C in view of the red~ction of strength at high temperatures of the fibers. Suitable examples of the metals to be used as the matrix are single metals e.g. beryllium, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, titanium, manganese, iron, cobalt, nickel, palladium, copper, silver, gold, zinc, cadmium, aluminum, gallium, indium, thallium, silicon, germanium, tin, lead, bismuth, selenium, or tellurium;
and alloys of various metals e.g. copper-zinc alloys (e.g. brass), copper-tin alloys (e.g. phosphor bronze~, copper-tin-phosphorus alloys, aluminum-copper alloys (e.g. duralumin), aluminium-magnesium-copper alloys, aluminum-zinc alloys, aluminum-silicon alloys, nickel-aluminum alloys, or nickel-aluminum-copper alloys. The matrix of these metals may contain a few or several %
by weight of one or more other elements e.g. chromium, titanium, zirconium, magnesium, tin, or lithium in order to improve the wettability thereof or to inhibit the reaction between the fiber and matrix metal.
The most suitable metal for the matrix should be selected for each case in view of the following factors.
When the composite material should have the greatest strength with a light weight, magnesium, aluminum or alloys thereof (e.g. aluminum-magnesium-copper-manganese alloy such as duralumin) are preferable, and when the composite material should have particularly high heat resistance, copper, nickel, titanium, cobalt or alloys of these metals are preferable. For instance, alumina fiber-reinforced aluminum or aluminum alloy advantageously has no brittleness at low temperatures, unlike steel.
Moreover, from the standpoint of ease of production of FRM, aluminum-silicon (12 ~) alloys (e.g. silimin) are preferable because of the good fluidity of the molten metal.
With the increase of the content by volume of inorganic fibers or alumina or alumina-silica fibers in the composite material, the strength and modulus of elasticity of the composite material is favorably in-creased. However, the content by volume of fibers isat maximum 68 % in the case of a composite material obtained by unidirectionally arranging continuous fibers or long fibers, and 45 ~ in the case of composite material obtained by arranging short fibers at random. When the content by volume of the fiber is over the above maximum value, the composite material tends to have a decreased tensile strength and bending strength.
Spring materials of the fiber-reinforced metallic composite material can be produced by conventional methods which are applicable to the production of FRM materials, for examplej (1) a liquid phase method e.g. a molten metal infiltration method, (2) a solid phase method e.g. diffusion bonding, (3) a powder metallurgical technique (e.g~ sinter-ing, welding), (4) deposition (e.g. plasma spraying, elec-trodeposition, chemical vapor deposition), and ~5) a plastic processing method (e.g. extrusion, hot rolling). For example, a material for leaf springs can be produced by the above methods (1), (2), (3), (4) and rolling (5). Besides, a coil spring can be produced by forming the composite material into a wire by the hot extrusion (5) and then forming the wire into the desired coil shape. Other springs having various shapes can also be produced by applying the c~nventional metal processing techniques to the FRM spring materials obtained by the above methods.
According to the present invention, at least in its preferred forms, the following various characteristics can be achieved.
The springs made of the fiber-reinforced metallic composite material are lighter and have greater mechan-ical properties in comparison with conventional metallic springs. The FRM springs are superior to the conventional metallic springs and FRP springs in that they have far greater heat resistance from low temperature to high temperature. The upper limit of temperature at which there is sufficient heat resistance is just below the melting point of the matrix metal, at which the matrix metal looses almost all the mechanical properties thereof. For instance, in the case of pure aluminum (melting point: 660C), the spring can be used even at 600 - 620C while maintaining its strength as at room temperature, unless it is used for a long period of time.
Moreover, the FRM spring material of the present inven-tion is superior in its modulus of elasticity. For example, when using fibers having a tensile modulus of more than 4,000 t/cm2, e.g. boron fibers, silicon carbide fibers, coated boron Eibers, tungsten fibers or alumina whiskers, an FRM spring material having a fiber conten~ of 50 % by volume shows far greater modulus of elasticity than that of the conventional metallic spring materials (1,900 - 2,100 t/cm2).
The composite material obtained by reinforcing a metal having a melting point of not higher than 1,700C with the alumina or alumina-silica fibers has the following char-acteristics. The alumina or alumina~siLica fibers are obtained in the form of continuous fibers which are easily handled, unlike whiskers, and have far greater Elexibility than boron fibers and also have an excellent oxidation resistance even at high temperatures which is not observed in boron fibers and carbon Eibers. Accordingly, a com-posite material using the alumina or alumina-silica fiber can easily be produced. Moreover, the alumina or alumina-silica fibers are substantially non-reactive with various metals and have good mechanical properties, and hence, the composite material obtained therefrom has a large specific strength, large specific modulus of elasticity over a wide range of temperatures of from room temperature to high temperature and also has good creep characteristics at high temperatures and fatigue properties.
Thus, the present invention can provide a spring which is light in weight and has good heat resistance, high strength (high specific strength) and high modulus of elasticity (high specific modulus of elasticity) over a wide range of temperatures of from low temperature to high temperature. According to the present invention, not only machine parts or elements (springs) are made lighter, but also the machines or apparatuses and whole systems in a spaceship, atomic power machines, natural gas tanks, auto-mobiles, and the like can be improved, which results in saving energy and saving resources.
The present invention is illustrated by the following Examples but is not limited thereto.
Reference is made in the Examples to the accompanying drawings, in which:
Fig. 1 and Fig. 2 are graphs showing the specific strength and specific elas~ticity modulus, respectively, against temperature of samples.according to embodiments of the invention and conventional samples;
Fig. 3 shows an apparatus as used in a testing method;
and Fig. 4 is a graph showing the results of testing carried out in the apparatus of Fig. 3.
Example 1 A bundle of continuous alumina~-silica fibers [A12O3: 85 % by weight, SiO2: 15 % by weight, average fiber diameter 17 ~m, number of filaments: 200, density:

3.05 g/cm3, tensile strength: 22.3 t/cm2 (length of gauge for measurement: 20 mm), elasticity modulus: 2,350 t/cm j was wound onto a mandre`-l to form a single layer, in parallel and in the same pitch. The mandrel wound with the bundle of fibers was immersed in a suspension of aluminum powder (purity: 99.9 %l average particle size:
5 ~m) in methyl ethyl ketone and then dried at room tem-perature. The resulting mandrel was further immersed in a suspension of aluminum powder (average particle size:
44 ~m) in a resin solution, and air-dried.
The composite material thus obtained was cut open into a sheet-like form and further cut to a size to fit a hot pressing mold. Twenty sheets of the composite material were unidirectionally piled up within the hot pressi~g mold. Hot pressing was carried out by first heating under a vacuum at 500C for 30 minutes to evaporate the solvent and to decompose the polymer and then pressing the sample under a pressure of 50 kg/cm2 at 665C for 1 hour under a vacuum or under an atmosphere of an inert gas, so that the sheets were bonded to each other and the matrix metal was sufficiently impregnated among -the fibers to give a plain plate of FRM material 150 mm square and 2.1 mm thick. This FRM material had a-fiber content of 51 ~ by volume, which was confirmed by melting the matrix metal.
A test sample for measurement oE tensile strenyth and bending strength was prepared by cutting the FRM plate obtained above. According to the measurements at room temperature, it had a specific weight: 2.9 g/cm2, tensile strength~ 102 kg/mm2, bending strength: 143 kg/mm2, and modulus of elasticity: 1.45 x 104 kg/mm2. Thus, this FRM ma-terial had a specific strength (tensile strength/density of about 5 times that of stainless steel and a specific modulus of elastirity (modulus of elasticity/density) of about double that of Inconel 600 (trade mark) which is a representative example of a heat resistant alloy. Thus, this FRM material had excellent properties not present in conventional spring materials even at room temperature.
A test sample for measurement o~ fatigue un~er tension was prepared from the above FRM material. The fatigue strength was measured by a SERVOPULSER EHF-5 (trade mark) type fatig~é tester (made by Shimadzu Seisakusho) under the following conditions of temprature: 25CI repeating frequency: 30 Hz, output wave pattern: sine wave (load control). By varying the amplitude o~ the repeating stress (S), the number (N) of repetitions until the test sample was broken because of fatigue was measured, and an S-N curve was drawn based on the resulting data. It is clear from the test results that when the sample was broken due to fatigue at N = 107, the stress was extremely high such as 69 - 73 % of the static tens;le strength. This characteristic is not seen in the case of conventional composite metals alld alloys.
By observing the section of breaking with tension and bending with an electron microscope, it was confirmed that the fibers were uniformly distributed in the matrix and there were extr~mely few voids.
_ mple 2 In the same manner as described in Example 1, a plate of a composite material of alumina-silica fiber/Al (purity: 99.9 %) was produced. The FRM material had a Eiber content of 49 % by volume. Ten test samples for measurement of tensile strength at high temperature were prepared by cutting the composite material plate. Each ~; .

sample had a width of lO mm, a length of 66 mm and a thickness of 1.5 - 2.3 mm.
The tensile strength at a high ten~perature was measured by using a test sample waisted on 18 mmR at the center to a width of 4 - 6 mm. The modulus of elasticity at high temperature was measured by bonding a strain gauge KH-3-G3 (made by Kyowa Dengyo) to the sample while raising the temperature and changing the load at a certain tem-perature. Heating was carried out with a high-frequency induction f~rnace provided in the tensile tester with con-trolling the temperature within the range of 600C ~ 5C.
The temperature dependence of these specific tensile strength and specific modulus of elasticity was compared with those of other conventional metals and alloys. The results are shown in the accompanying Figure l and Figure

2. Figure 1 shows the temperature dependence of specific tensile strength of the alumina-silica fiber-reinforced aluminum composite material of Example 2 together with the data of conventional composite material prepared by using other conventional metals and alloys. In Figure l, (l) is the data of Hastelloy X (trade mark) (2) is the data of stainless steel, (3) is the data of pure titaniuml and (4) is the data of ultra super duralurnin, and m.p. means thernelting point of aluminum. Figure 2 shows the temperature depen-dence of specific modulus of elasticity of FRM of Example 2, wherein (l) is the data of Inconel 600 (trade mark) (2) is the data of Hastelloy X (trade mark) and (3) is the data of pure aluminum. ~s is clear Erom these results, the composite material of the present invention has novel characteristics making it suitable for use as a spring material having light weight and excellent heat resistance.

5, .. ....

Example 3 In the same manner as described in Example 1, a bundle of continuous alumina-silica fibers was wound onto a mandrel in a single layer. While rotating the mandrel, aluminum powder (purity: 99.9 %, average particle size:
5 ~m) was plasma-sprayed onto the surface of alumina-silica fibers with a plasma spraying device (6MR-630 type equipped with an electric power supplier, made by Metco Co.). The plasma spraying was carried out in a gaseous mixture of argon and hydrogen (flowing ratio: 30 : 1) at a spraying distance of 22 cm for 80 seconds. After remov-ing the sheet from the mandrel, plasma spraying was also carried out on the reverse surface for 30 seconds in the same manner as described above. The resulting sheet-like composite material had an average thickness of 0.32 mm~
The sheet-like composite material was cut to a si~e of 10 mm x 66 mm, and 21 sheets were unidirectionally piled up within a curved carbon hot pressing mold (length of arc: 68 mm, width: 10 mm, curvature radius: 90 mm)~ Hot pressing was carried out by pressing at a pressure of 50 kg/cm2 at 670C under an argon atmosphere for 30 minutes. After cooling, a curved plate of alumina-silica fiber-reinforced aluminum composite material having a thickness of 1.33 mm and a content of fiber of 50 ~ by volume was obtained. This was used as a machine element, a so~called leaE spring.
A load (W) was repeatedly added to the curved FRM
plate obtained above as shown in the accompanying Figure

3 wherein (1) is the curved FRM plate, and the deflection (~) at the loading point at the middle of the plate was measured. As a result, a spring coefficient was obtained with good reproducibility. That is, in case of the FRM

plate of a width: b, a thickness: h, a curvature radius r, and the length of A,B: Q, and both end points A and B
being a movable supporting point and the friction between the plate and the supporting point being ignored, the relation between the load weight (W) and the deflection (~3 at the central area of the plate, i.e. the spring coefficient, is theoxetically shown by the following equation:
W 1 Ebh3 ~ ~ Q3 (1) wherein 3 3 sin ~ - 8 sin o/2 + (2 - cos ~). 9 = - . 3 16 sin 3/2 ~ = 2 sin 1 ( ~2r) t3) E = EfVf + Em(l - Vf) (4) E means a modulus of elasticity, f means a fiber, m means a matri~ metal, and Vf means a content by volume of the fiber.
In accordance with the above equation (1), the theo-retical load - deflection curve is drawn based on the date obtained above~ The curve is shown in a dotted line in the accompanying Figure 4. The curve based on the data is also shown in Figure 1 is a solid line. The theoretical value shown by the dotted line corresponds well with the found value shown in the solid line. In this case, the composite materlal had a specific modulus of elasticity of 5.0 x 108 cm and a bending strength of 102 kg/mm2 Example 4 Carbon fibers T-300 (made by Toray Co., averaye fiber diameter: 6.9 ~, number of filaments: 3,000, tensile strength: 27 t/cm2, tensile modulusO 2,500 t/cm2) were electroplated with copper in an electrolytic cell con-taining 200 g/liter of copper sulphate and 50 g/liter ~_, - 20 -of sulphuric acid under the conditions of electrolytic temperature of 20C, a current density of 0.5 A/dm2 and a period of time: 5 to 10 minutes, by which the surface of carbon fibers were uniformly coated with copper to a thick-ness of 0.6 ~m on the average. ~his was confirmed by observing the product with a scanning electron microscope and a X-ray photoelectron spectrophotometer (ESCA 650B, made by Shimadzu Seisakusho - DuPont).

The carbon fibers coated with copper were cut in a length of 2;5 mm with a roving cutter. The resulting fibers were provided within a mold having a cavity of diameter: 8 mm~ and length: 150 mm, so that the fiber content became 30 ~ by volume. Molten Al-Cu (33 %) alloy was added to the mold in a vacuum at 615C. After maintaining the temperature for 1 minute, the product was immediately hardened with water to give a rod-like, copper-coated carbon fiber-reinforced Al-Cu alloy com-posite material. The resulting composite material was preheated at 455C for 30 minutes and was subjected to hot-extrusion molding under the conditions of extrusion rate: 5~0 mm/minute and extrusion pressure: 2,000 kg/cm2 to give a copper-coated carbon fiber-reinforced Al-Cu alloy composite material in the form of a wire having a diameter of 3.7 mm~. The distribution of fibers was checked by abrading the wire parallel to the axis. As a result, it was confirmed that the fibers were arranged in the direction of the extrusion axis. Moreover, when the wire immediately after being extruded from the extrusion nozzle was passed through a curved, stainless steel heating guide and was air-cooled, a coil spring having an outer diameter of 6() mm~ and a pitch interval of 5 mm was obtained.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A leaf spring or coil spring made of a fiber-reinforced metallic composite material comprising a matrix metal and a reinforcement of inorganic fibers having a tensile modulus of 1,000 t/cm2 or more and a tensile strength of 10 t/cm2 or more, the inorganic fibers being ceramic fibers selected from alumina fiber, alumina-silica fiber, carbon fiber, graphite fiber, silicon carbide fiber and zirconia boron fiber or metallic fibers selected from tungsten fiber and stainless steel fiber, the maximum fiber content of the composite material being 68 % by volume when the composite material is obtained by uni-directionally arranging continuous fibers or long fibers and being 45 % by volume when the composite material is obtained by arranging at random short fibers, the fiber diameter being 0.6 to 400 µm for alumina or alumina-silica fiber, and when the composite material comprises contin-uous fibers or long fibers, the fibers being arranged in fiber bundles having of from 1 to 200,000 filaments.

2. A spring as claimed in claim 1, wherein the matrix metal has a melting point of not higher than 1,700°C and the inorganic fiber reinforcement comprises alumina or alumina-silica fiber containing from 100 to 72 % by weight Al2O3 and from 0 to 28 % by weight SiO2 and having substantially no .alpha.-alumina reflection when examined by X-ray diffraction.

3. A spring as claimed in claim 2, wherein the fiber is alumina-silica fiber containing from 98 to 75 % by weight Al2O3 and from 2 to 25 % by weight SiO2.

4. A spring as claimed in claim 2 or claim 3, wherein the alumina-silica fiber contains one or more of oxides of lithium, beryllium, boron, sodium, magnesium, phosphorus, potassium, calcium, titanium, chromium, manganese, yttrium, zirconium, lanthanum, tungsten or barium in an amount within the content of SiO2 and of not more than 10 % by weight based on the total weight of the fiber.