Aluminum composite material and method of producing the same

An aluminum composite material has a surface structure in which a part of lubricative granules projects by 2 μm to 25 μm from the surface of the aluminum alloy base material. The lubricative property of the lubricative granules is utilized sufficiently, and the abrasion of the aluminum alloy base material can be prevented. Further, according to a manufacturing method of the aluminum composite material where the surface of the aluminum alloy base material is eroded with a etching solution, a level of erosion of the aluminum alloy base material can be easily adjusted and a surface structure from which the lubricative granules project can be formed sufficiently.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an aluminum composite material and a method of producing the same; wherein lubricative granules are provided at least on a base material surface of an aluminum alloy base material.

(2) Description of the Background Art

An aluminum alloy has been optimally used in devices including such as automobiles, electric appliances, electronic parts, and precious measurement equipment because of its lightness and malleability. Nevertheless, the application of the aluminum alloy is limited to a sliding portion of structures because of low resistance to abrasion. Accordingly, various composite materials such as graphite and activated charcoal with superior lubricating property have been added to a base surface to promote resistance to abrasion.

A composition in which an activated charcoal and ceramics such as alumina particles and alumina fiber are dispersed in an aluminum alloy material was disclosed in Japanese Laid Open Patent Publication No. S58-81.948. Further, a method of manufacturing the composite material was disclosed in Japanese Laid Open Patent Publication No. H6-240305 in which a compact, which was obtained by dehydration or de-alcoholization after alumina short fiber and graphite were mixed with water or alcohol, and an aluminum alloy were made into a complex.

The aluminum composite material as produced above is generally cut mechanically in accordance with the type of application. By the mechanical processing, the lubricative granules exposed on the surface of the aluminum composite material are grinded off. Accordingly, when such an aluminum composite material is used as a sliding member, the lubricative granules are not exposed on the surface or only a very small amount exists, and accordingly, an initial sliding ability or sliding property at a low surface pressure could not be adequately utilized. Accordingly, the aluminum composite materials disclosed in the above JP58-81948 or JP6-240305 could not utilize a desired sliding property even as a sliding member of a specific structure produced by a simple machinery process.

SUMMARY OF THE INVENTION

According to the invention, an aluminum composite material which can achieve an adequate sliding property as a sliding portion material and a method of manufacturing the material are disclosed.

According to an implementation of the invention, in the aluminum composite material, the lubricative granules are connected at least to the base material surface of the aluminum alloy base material. The aluminum composite material includes the surface structure in which a part of the lubricative granules projects by 2 μm to 25 μm from the aluminum alloy base material. When such an aluminum composite material is used for the sliding portion material, the lubricative granules projecting from the surface of the aluminum composite material contact the sliding counter material, and accordingly, the contact of the aluminum alloy base material to the sliding counter material can be prevented, and specifically, any abrasion of the aluminum alloy base material because of burning can be prevented. Further, in accordance with the surface structure in which the lubricative granules are projected to the surface, in the initial sliding with sliding counter material and the sliding with a low surface pressure, an excellent sliding property can be obtained.

In many cases, such lubricative granules projecting from the aluminum composite material surface are pulverized by friction with the counter material and reduced to powder. Thus, the powder of the lubricative granules are dispersed and exist on the sliding surface against sliding counter material, and accordingly a superior lubricative action can be achieved and the aluminum alloy base material can prevent burning from occurring. In order to perform the lubricative action adequately, the size of the projected portion of the lubricative granules from the surface of the aluminum base material is in a range of 2 μm to 25 μm. If the projected portion of the lubricative granules is smaller than 2 μm, the powder pulverized by friction with the sliding counter material becomes minimized, and accordingly, the sliding counter material might contact the aluminum alloy base material and the occurrence of burning cannot be sufficiently prevented. Further if the projected portion is small, the amount of powder pulverized by the friction is so small that the lubricative action cannot be sufficiently achieved. On the other hand, if the projected portion is bigger than 25 μm, the powder pulverized by the friction with the sliding counter material is large, and the gap between the aluminum composite material and the sliding counter material is widened, and accordingly, impurity such as debris can easily accumulated and the ability to lubricate may be degraded.

According to another implementation of the invention, the projected portion of the lubricative granules projecting from the surface of the aluminum alloy base material is larger than the surface roughness of the aluminum alloy base material, and is in a range of less than 50% of the average particle diameter of the lubricative granules. If the size of the projected portion of the lubricative granules is smaller than the surface roughness of the aluminum alloy base material, the sliding counter material would contact the surface of the aluminum alloy material, and therefore, the aluminum alloy material would be easily burned and lubricating ability of the lubricative granules could not be achieved. Further, if the size of the projected portion is bigger than 50% of the average particle diameter of the granules, the lubricative granules could easily drop off from the surface of the aluminum base material. Accordingly, it is highly possible that the lubricative granules could drop off before being used as a sliding portion material during transportation or installation, and accordingly, the aluminum composite material could not be used to suitably take advantage of its sliding property. Thus, according to the composition, the lubricative granules can adequately utilize the sliding property between the aluminum composite material and the lubricative counter material. The aluminum composite material can have an excellent sliding property.

According to another implementation, the lubricative granules are graphite. Accordingly, an excellent lubricating ability of graphite can be utilized and the aluminum composite material can have an excellent sliding property.

According to still another implementation, the lubricative granules are activated charcoal. Accordingly, an excellent lubricating ability of activated charcoal can be utilized and the aluminum composite material can have an excellent sliding property.

Further, according to another implementation, the method of producing the aluminum composite material includes steps of eroding the surface of the aluminum alloy material with a specific etching solution after mechanically working to form a specific form, and, after eroding, executing a surface finishing process to form a surface structure with lubricative granules projecting 2 μm to 25 μm from the surface of the aluminum alloy base material. In such a surface finishing process, by eroding the aluminum alloy base material on the surface with the specific etching solution without eroding or damaging the lubricative granules, a level of erosion of the aluminum alloy base material can be relatively easily adjusted. Thus, the adequate surface structure of the aluminum composite material can be easily formed with the lubricative granules projecting 2 μm to 25 μm from the surface and dotting substantially evenly thereon. Further, the aluminum composite material formed in accordance with such a manufacturing method can have an excellent sliding property. The etching solution can erode the aluminum alloy material to give a desired surface structure.

According to another implementation, in the manufacturing method, the eroding level of the aluminum alloy base material with the etching solution is bigger than the surface roughness of the aluminum alloy base material and in the range of less than 50% of the average particle diameter of the lubricative granules. The eroding level of the aluminum alloy base material can be relatively easily set within the range by such producing method, and accordingly the surface structure being able to perform the abovementioned excellent sliding property can be adequately and easily formed.

According to another implementation, in the manufacturing method, the etching solution is an aqueous sodium hydroxide solution. According to the manufacturing method, the aluminum alloy base material of the surface can be eroded in a relatively short period without eroding the lubricative granules, and accordingly, the surface structure comprising the projected portion of the lubricative granules can be adequately formed.

DETAINED DESCRIPTION OF THE INVENTION

The inventor explains the embodiments of the present invention referring to the drawings.

Referring toFIGS. 3 and 4, the aluminum composite material (not shown) according to an embodiment of the invention starts as a pre-form1by a pre-form forming process shown inFIG. 1. A hot solution3of an aluminum alloy base material2is impregnated in the pre-form1by the aluminum impregnating process inFIG. 2. A unified form of a composite element4of the layering structure body comprises the aluminum composite layer12and the aluminum alloy layer13. Further, after machine-processing to produce a specific form with a milling machine, by eroding the aluminum alloy base material2of the surface of the aluminum composite layer12with a sodium hydroxide aqueous solution (etching solution)16, the composite element4is produced. In the following, the details of each process are explained.

Referring toFIG. 1(a), an alumina fiber5and powdery graphite6were mixed by stirring using a rabble arm31in water in a specific vessel. Then, alumina-sol7as an inorganic binder was added to the aqueous solution in which the alumina fiber5and the graphite6were being mixed. Wherein, the alumina fiber5having approximately 3 μm of the average particle diameter and 50 cc/5 gf of the average length, and a chemical composition of Al2O3(approximately 95%)/SiO2(approximately 5%) was employed, the graphite6having approximately 40 μm of particle diameter and chemical composition of C (97%)/Al2O3and SiO2(approximately 3%) were employed. Further, the alumina-sol employed was Al2O3(approximately 11%). Further, since the alumina fiber is generally complexly intertwined, the average length was defined quantitatively by volume per unit weight.

And then, the aqueous mixture solution8of the alumina fiber5, the powdery graphite6and the alumina-sol7were transferred to the suction forming means22. The suction forming means22is connected to the vacuum pump23, as shown inFIG. 1(b), and draws water of the aqueous solution8by the vacuum pump23through a filter24. Accordingly, the dehydrated forming base material9in which the graphite6was almost evenly dispersed and coagulated on the alumina fiber5was obtained. And then, the dehydrated base material was taken out from the vacuum forming means22and dried sufficiently (not shown in Fig.).

Referring toFIG. 1(c), the dehydrated forming base material was installed on the table33in the heating furnace25. The inside of the heating furnace25was kept under vacuum condition at 1×10−3Torr by the vacuum pump23. Then, while argon gas at the rate of 5 cc/minute was constantly flown, the furnace was heated up to approximately 1000° C. which was maintained for 2 hours, and then cooled down (not shown in Fig.) to room temperature to give the desired pre-form1. In addition, during cooling, the argon gas was flown continuously until the temperature was cooled down sufficiently. Further, the argon gas over-flown from the inside of the furnace was exhausted to the outside of the furnace through a leak valve32. Accordingly, the processes to form the pre-form were carried out step-by-step. Herein, besides argon gas, inactive gas such as helium gas, and reduction gas such as hydrogen gas and nitrogen gas can be employed. Further, the vacuum condition can be employed. In accordance with such atmosphere in the inside of the heating furnace25, the burning-shrinking (contraction) of the pre-form1can be adequately prevented without sintering the graphite6or reacting with the alumina fiber5.

Further, the hot solution3of the aluminum alloy base material2(JIS AC8A) was impregnated in the pre-form1formed in the abovementioned pre-form forming process by pressure-casting. A hydraulic press-machine30shown inFIG. 2was employed for the pressure-casting. A extrusive portion26is installed under part of the hydraulic press-machine30; after casting, by moving the extrusive portion26to upper position, a nesting28in the inside of a metal mold27installed on the extrusive portion26can be removed from the metal mold27. As shown inFIG. 2(a), the nesting28was installed in the inside of the metal mold28and the pre-form1pre-heated by approximately 550° C. was set to the nesting28. Then the specific amount of the hot solution3of the aluminum alloy base material2of approximately 750° C. was added to the top portion of the pre-form1. Then, referringFIG. 2(b), by direct pressing the hot solution3from the top direction with a punch29of the hydraulic press-machine30, the aluminum composite layer12impregnating the hot solution3in the pre-form1and the aluminum alloy layer13comprising the aluminum alloy base material2were produced in the unified form. Then, as shown inFIG. 2(c), the nesting28was removed from the metal mold27by the extrusive portion26, and the composite element4comprising the aluminum alloy base material2and the aluminum composite layer12in which the alumina fiber5and the graphite6were existing as a mixture was obtained. A volumetric content (%) of the graphite6in the composite element4was 15%, a volumetric content of the alumina fiber5was 6.5%. The rest of the volume of aluminum composite layer was the aluminum alloy base material.

Thus, the formed composite element4was machine-processed using a milling machine to provide the processed composite material10having a desired sliding portion material. (Not shown in Fig.) Referring toFIG. 7(a) andFIG. 8(a), wherein, the surface structure14of the aluminum composite layer12composing the processed composite material10was formed by the machine-processing.

In the following, the surface finishing process is a principal portion according to an embodiment of the invention. The surface of the aluminum composite layer12of the processed composite material10was eroded with a sodium hydroxide aqueous solution16(an etching solution) as shown inFIG. 3. Referring toFIG. 4(a) toFIG. 4(b), the magnified view at A-part inFIG. 3, the aluminum alloy base material2on the surface of the aluminum composite layer12was eroded with the sodium hydroxide aqueous solution16and a part of the graphite6was projected from the surface of the aluminum alloy base material2. The concentration of the sodium hydroxide aqueous solution16was approximately 15% and was stored at approximately 40° C. The erosion time when the sodium hydroxide aqueous solution16eroded the surface of the aluminum composite layer12was approximately 20 seconds. By such surface finishing process, the aluminum composite material, not shown, comprising the surface structure15was obtained; wherein the part of the graphite6was projected from the surface of the aluminum alloy base material2.

The surface structure15of the aluminum composite layer12of the aluminum composite material as shown in the surface photo ofFIG. 5(a) and the rough sketch ofFIG. 6, shows the structure in which the graphite6projecting from the surface of the aluminum alloy base material2. The size of the projected graphite6was measured and was approximately 15 μm in average. Further, as comparative example, in the surface structure14of the processed composite material10as shown in the surface photo ofFIG. 7(a) and the rough sketch figure ofFIG. 8(a), the graphite6which was caved in from the surface of the aluminum alloy base material was confirmed. Further, the cutting damage17because of the machine-processing using a milling machine was confirmed in the surface structure of the processed composite material. Specifically, according to the surface structure14, it was considered that the graphite existing on the surface of the composite element4was cut off by machine-processing and accordingly the structure had no graphite6exposed on the surface.

The sliding property of the aluminum composite material and the processed composite material10was tested by a sliding property test.

In the sliding property test, each specific board shape composite material test piece was set in the motor oil 10W-30 for automobile. Each specific tube-shape chrome steel SCr420 (JIS G 4104) rotating at the rotating rate of 2 m/sec was burdened with surface pressure of 50 MPa to the surface of each aluminum composite layer and when the sliding distance reached to 2400 m, the abrasion property was measured. The abrasion property was obtained by measuring weight change (mg) of the aluminum composite material, the processed composite material, and the sliding counter material.

The results of the above sliding property test are shown inFIG. 9. The weight change of the aluminum composite material was −0.20 mg and there was no weight change for the sliding counter material. In contrast, the weight change of the processed composite material10was −11.00 mg and the weight of the sliding counter material increased by 0.50 mg. The weight decrease of the aluminum composite material and the processed composite material10were considered due to the burning-on of the aluminum alloy base material12and abrasion. Specifically, in case of the aluminum composite material with almost no weight loss, the lubricating action of the graphite6projected to the surface was adequately performed. In contrast, the weight loss of the processed composite materials was large, and the lubricating ability of the graphite6was not performed. Further, the weight increase of the sliding counter material is considered due to that the aluminum alloy base material2burning onto the sliding counter material. Accordingly, from these, the amount of the aluminum alloy base material2was obtained, and the excellent sliding property of the aluminum composite material of the invention was understandable.

After the sliding property test, the surface structure of the aluminum composite material15and the surface structure14of the processed composite material10were compared using the surface photos and rough sketches. Referring toFIG. 5(b) andFIG. 6(b), even after sliding property test, the surface of the aluminum composite material surface had no sliding damage or indication of dropout of the graphite6. Accordingly, it was considered that the projected part on the surface or the pulverized powder performed the lubricative action and burning of the aluminum alloy base material2onto the surface was not generated. In contrast, referring toFIG. 7(a) andFIG. 8(b), on the surface of the processed composite material10, the trace18of dropout of the graphite6was observed, and relatively large sliding damage19was also observed. Accordingly, in the processed composite material10, it was observed that burning of the aluminum alloy base material2onto its surface was generated due to sliding with the sliding counter material. Thus, the aluminum composite material according to the embodiment of the invention has an excellent sliding property in accordance with the graphite6and can be optimally applied to various sliding materials.

According to the above embodiment, the aluminum composite material employing the graphite6as lubricative granules is disclosed, but also the activated charcoal can be employed. The results of the sliding property test of the aluminum composite material when the activated charcoal was employed are shown also inFIG. 9.

When the aluminum composite material and the processed composite material were compared in case of activated charcoal, the change of the aluminum composite material was −0.15 mg, so that it was extremely small in comparison with −9.50 mg for the processed composite material10′. As well as the above embodiment using the graphite, even when the activated charcoal was employed as lubricative granules, the aluminum composite material which has an excellent sliding property can be formed. Further, for the aluminum composite material using activated charcoal, the manufacturing method and test method are the same except the activated charcoal employed instead of the graphite6as described in the above embodiment and the explanation is omitted. On the other hand, for lubricative granules, others such as molybdenum sulfide and BN can be employed.

In the above surface finishing process, as the erosion condition for the sodium hydroxide aqueous solution16, in the case of the sodium hydroxide aqueous solution16being kept at 40° C., by immersing for 20 to 30 seconds, the aluminum alloy base material2on the surface of the processed composite material10can be optimally eroded. Further, in the case of the sodium hydroxide aqueous solution16being kept at 20° C., the immersing time of 50 to 60 seconds is optimal condition. If an immersing time is longer, the surface of the aluminum alloy base material2becomes rough and accordingly the lubricative granules cannot be projected evenly and cannot sufficiently utilize the lubricative ability of the lubricative granules. Further, if the immersing time is short, the aluminum alloy base material on the surface cannot be eroded sufficiently, and accordingly, the lubricative ability of the lubricative granules cannot be utilized.

In the above surface finishing process, the sodium hydroxide aqueous solution was employed as an etching solution, but other such as a hydrofluoric acid solution, a mixed solution of a sodium hydroxide aqueous solution and hydrofluoric acid, and a mixed solution of hydrogen chloride and a hydrofluoric acid solution can be employed with an adequate concentration. According to these, the surface of the aluminum alloy base material2can be eroded without eroding the graphite6. Further, in case of use of such etching solution, conditions such as temperature of aqueous solution, and erosion time should be set up adequately.

In the surface finishing process according to the embodiment of the invention, after forming the pre-form1, in addition to the composite element4produced by including the hot solution3of the aluminum alloy base material2, can be applied to other composite material produced by using various method such as a method for the element produced by adding the lubricative granules directly to the hot solution3of the aluminum alloy base material2.