Wear-resistant surface composite materials and method for producing same

Abstract

Materials, such as metals, alloys and other products are provided with improved wear-resistant surfaces comprised of a hard monocarbide phase in the form of filaments within the material matrix.

Parent Case Text

This application is a continuation-in-part of U.S. application Ser. No. 270,241 entitled "Improved Wear-Resistant Surface Composite Materials and Method for Producing Same" filed July 10, 1972, by P. S. Kotval, H. Hatwell and F. P. Gortsema, now abandoned.

Description

This invention relates to improved wear resistant surface composite materials and to a method for their production. In one aspect the invention relates to metals, particularly aluminum alloys with an especially wear-resistant "duplex" surface comprising a large area fraction of hard refractory metal monocarbide phases. In a further aspect, the invention relates to the use of refractory metal monocarbide phases in the form of fibers, felts, tapes and woven fabrics and a method for compositing these materials in the surface of otherwise wear-prone materials such as aluminum alloys, to impart exceptionally high wear-resistance to the composite surface.

As is well known, many materials such as metals, alloys and other products exhibit poor wear properties and as a consequence, the use of such materials is severely restricted in applications where wear-resistance is an important requirement. In the prior art, attempts at improving the poor wear-resistance of materials, such as aluminum and its alloys, have been focused on two principal methods: (a) the use of anodizing treatments which permit the formation of a hrad oxide film on the surface of aluminum, and (b) the use of surface coating treatments, such as plasma-spraying, flame-plating and similar methods which involve the deposition of a wear-resistance phase or phases on the surface of aluminum alloys. These state-of-the-art methods go some way toward fulfilling the requirements of improved wear-resistance for aluminum alloys. However, both methods suffer from limitations. The films created by anodizing treatments are mechanically weak and the enchancement of wear-resistance is not maintained once rupture of the film occurs. Further, there is a physical limitation of the thickness of the films which can be created on aluminum alloys. The methods involving the deposition of various hard phases to improve wear behaviour suffer from the disadvantage that they are not readily adaptable to cases where mechanical precision is necessary in the finished part and also have the disadvantage of a propensity (under certain conditions) toward separation of the wear-resistant coating from the substrate.

In marked contrast to both types of the above-mentioned prior art methods of improving wear resistance, the present invention teaches that the entire surface of the alloy need not be covered by the wear-improving phases. Instead, the utilization of the hard refractory mono-carbide phases, for example in the form of a woven tape permits the final surface to have a composite surface structure wherein up to 80 percent of the surface area is comprised of regions of the hard mononcarbide phase embedded within the matrix of the alloy. the fibrous nature of the wear-improving phase permits a continuous presence of the hard phase even in conditions of abrasive wear. Thus, the present invention represents a marked improvement over the prior art methods (e.g., plasma-sprayed powders) where a particle of hard phase, once removed during abrasive wear, ceases to provide any wear-improving role.

Accordingly, one or more of the following objects will be achieved by the practice of this invention. It is an object of this invention to provide improved wear resistant surface composite materials and to a method for their production. A further object of the invention is to provide materials with a duplex composite surface which exhibit exceptionally improved wear resistance in comparison to the untreated material surface. Another object of the invention is to employ refractory metal carbides in the form of filaments to provide a duplex composite surface having improved wear-resistance. A still further object of this invention is to provide aluminum alloys with improved wear-resistance surfaces. Another object is to provide non-metallic materials such as epoxy and phenolic resins which have improved wear-resistant surfaces. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein disclosed.

In its broad aspect this invention relates to improved wear-resistant composite materials and to a process for their preparation. The composites are characterized by a surface which has a duplex composite structure comprised of (a) a matrix material of poor wear resistance and (b) not less than about 50 percent area fraction and up to about 90 percent area fraction of a hard fibrous monocarbide phase. The monocarbide phase is represented by the general formula MC wherein M is a metal from the group tantalum, titanium or tungsten and C is carbon and is in the form of filaments of between about 2 and about 15 microns in diameter being present at the composite surface as sections of varying orientations, the remainder of the surface being comprised of matrix material.

By the term "filaments" as employed throughout the specification and appended claims is meant woven fabrics, tapes, felts, chopped fibers, and the like composed of the metallic monocarbides. These metallic monocarbides are prepared by the process disclosed in U.S. Pat. No. 3,403,008.

It has been observed that a wide variety of materials which are normally characterized by poor wear-resistance, can have their wear-resistance markedly improved by the teachings of the instant invention. Illustrative materials include, among others, aluminum, silver, copper, and their alloys, non-metallic matrices, such as epoxy and phenolic resins, and the like. For example, aluminum alloys which normally exhibit poor wear-resistant properties include those based on the aluminum-silicon alloy system particularly hyper-eutectic combinations which are commonly used as casting grade aluminum alloys. Also included are silver and silver containing alloys such as those used for brazing applications, copper and its alloys as well as other matrices. As previously indicated the invention is also applicable to other non-metallic matrices such as epoxy resins and phenolic resins.

The ability to produce the materials of this invention with exceptionally enhanced wear-resistance results from the discovery that when filaments of the monocarbides of the aforementioned refractory metals are in contact with materials such as aluminum and its alloys in the molten state, the filaments are rapidly "wetted" by the molten metal. A feature of this wetting reaction is the formation of intermetallic phases at the interfaces between the individual fibers and filaments of the monocarbide phase and the adjacent metallic matrix, the intermetallic phase providing a good cohesive bond between the matric and the monocarbide filaments.

In those cases where the natural characteristics of the matrix allow only limited wetting of the monocarbide phase, it may be necessary to first create a thin metallic coating such as copper, aluminum or nickel by electrodeposition or vapor deposition on the surface of the filaments and thereafter to proceed with the process of this invention with the matrix of choice.

In practice, the material of this invention can be conveniently prepared by a variety of methods. For example, the ready wettability of the textile form of the refractory metal monocarbides by materials such as aluminum and its alloys lends itself to a method of embedding the wear-resistance-imparting monocarbide into the material directly during the casting operation. For example, the mold can be lined with the monocarbide filaments and the normal casting operation carried out. As previously indicated the filaments can be employed in a variety of forms. The particular configuration of the mold may, in part, determine the preferred form of the filaments.

In many instances, it has been observed that filaments in the form of tapes are convenient for lining the interior surfaces of the mold. The thickness of the tape is not necessarily critical as long as sufficient metallic monocarbide filaments are present to improve the surface wear-resistant properties. In practice it has been observed that marked improvements are obtained when the surface composite structure is comprised of not less than about 50 percent area fraction and up to about 90 percent area fraction of the hard fibrous monocarbide phase.

As previously indicated the present invention can provide mechanically precise castings which have improved wear-resistance. Inasmuch as the metallic monocarbides are placed in the mold prior to the matrix material, the overall configuration of the cast article has the same dimensions as the cast article without the monocarbides.

The filaments employed in this invention are also useful for imparting improved wear-resistance to articles prepared by hot pressing powdered matrix materials. For example a tape, felt, woven fabric, or chopped fibers of the metallic monocarbide can be placed on one or more surfaces of a die, powdered matrix material added, and the composite hot pressed to form the desired article.

The present invention thus provides a simple and convenient technique for imparting improved wear-resistance to surfaces of a variety of materials. Without any complicated processing steps, other than those involved in normal material fabrication, a wear-resistant composite duplex surface for the material can be directly created.

The following examples are illustrative:

EXAMPLE I

A tape of satin-weave woven fabric comprised of tantalum carbide prepared by the process disclosed in U.S. Pat. No. 3,403,008 was used to create a composite surface on 6061 aluminum alloy. The filaments comprising the tape were 5.mu. in diameter.

The apparent molecular weight of the tantalum carbide was 191.4, showing that the material of the woven fabric approximated closely ideal stoichiometric TaC (molecular weight 192.96). A tape, 0.5 inches wide by 3.0 inches long, was placed in an alumina crucible and a bar of 6061 aluminum alloy was placed over the tape. The crucible and contents were held in an argon atmosphere furnace at 700.degree.C for 5 minutes. Upon cooling, the as-cast 6061 alloy was found to have the said tape embedded fully within the lower surface of the ingot resulting in a composite surface. Using part of the above sample and utilizing the technique of thin foil transmission electron microscopy, electron diffraction patterns were obtained from the region of the interface between the TaC filaments and the 6061 alloy matrix. The patterns revealed that in addition to the face-centered-cubic 6061 matrix and the face-centerd-cubic 6061 matrix and the face-centered-cubic TaC fibers, there was a tetragonal Al.sub.3 Ta intermetallic phase formed at the fiber/matrix interface, thus providing evidence of good bond formation.

Blocks 1/4 inch wide .times. 5/8 inch long .times. 3/8 inch deep were cut from the sample of 6061 alloy with the composite matrix plus TaC fiber surface. The composite surface was tested for wear-resistance under varying loads in a standard "Alpha" model wear-testing machine. In this test the test surface is subject to wear against a 4620 steel ring hardened to a hardness of R.sub.c = 58 to 63. The relative wear parameter of test material is given by measuring the volume of the scar created on the surface by wear test. The results obtained are set forth in Table I below:

Table I __________________________________________________________________________ Material Lubricant Load Wear Scar Volume __________________________________________________________________________ Cast 6061 5606 A hydraulic fluid 30 lbs. 6.5 .times. 10.sup..sup.-6 cm.sup.3 alloy with 75 area % Tac fibers 5606 A hydraulic fluid 180 lbs. 2372 .times. 10.sup..sup.-6 cm.sup.3 Untreated 5606 A hydraulic fluid 30 lbs. 5998 .times. 10.sup..sup.-6 cm.sup.3 aluminum alloy surface 5606 A hydraulic fluid 180 lbs. too high to measure __________________________________________________________________________

The wear scar volumes of the composite surface of the present invention are compared in Table I with the values for wear scar volumes for a typical aluminum alloy. It is evident that the composite surface of the present invention has wear characteristics which represent a thousand-fold improvement over the wear characteristics of conventional aluminum alloy surfaces.

EXAMPLE II

A tape of satin weave woven fabric comprising essentially of tantalum carbide was positioned into the bottom of a mold into which a Buehler No. 20-8133-001 Epoxy Resin was cast. Upon curing the resultant Epoxy block was found to have one surface with a duplex composite microstructure comprising .about. 75 area% TaC in Epoxy matrix. Blocks 1/4 inch wide, 5/8 inch long and 4/8 inch deep were cut from these Expoxy blocks with "matrix plus TaC fiber" surface. The composite surface was tested for wear-resistance under varying loads in a standard Dow-Corning Alpha model Wear-testing Machine. In this test the test surface is subjected to wear against a 4620 steel ring hardened to a R.sub.c = 58 to 63. The relative wear parameters for the composite surface of an Epoxy Matrix, with varying area fractions of the fibrous TaC phase is compared with parameter for the bare Epoxy block in Table II.

______________________________________ Material Lubricant Load Wear Scar Volume ______________________________________ Epoxy with Press Fluid 30 lbs. 105 .times. 10.sup..sup.-6 cm.sup.3 75 area%. TaC tape Epoxy with Press Fluid 30 lbs. 162 .times. 10.sup..sup.-6 cm.sup.3 60 area%. TaC tape Epoxy Press Fluid 30 lbs. 1798 .times. 10.sup..sup.-6 cm.sup.3 (untreated) ______________________________________

EXAMPLE III

Comparisons were made between the wear resistance of the surface composite materials of this invention and known materials using standard wear tests.

The wear data was obtained using a LFW-1 model Wear-Testing Machine manufactured by the Dow Corning Corporation. As described in ASTM Standard Test Method D 2714-68, all tests were carried out on stationary rectangular (1/4 inch wide .times. 5/8 inch long .times. 3/8 inch deep) test blocks pressed, with a pre-determined load, against a rotating ring. The wear properties measured were: (i) Volume of Wear Scar on the test surface of wear block material; (ii) Weight change of the mating ring; and (iii) the friction force measured at intervals during the test. The wear data listed in Table III were obtained under the following conditions of testing: Mating Ring: 4620 Steel; R.sub.c =58-62; Surface finish=8-12 micro inches Lubricant: Mobil 5606-A fluid Load: 30 lbs. Wear Speed: 180 r.p.m. (Ring diameter = 1.3775 in.) Total revs: 5400 revs.

As is evident from Table III, a Surface Composite comprising 75 area percent of TaC phase within a 6061 alloy matrix represents an improvement of up to a thousand-fold in the measured values of wear scar volume compared to the bare 6061 alloy. Compared to the wear-resistance of even the hypereutectic Al-18 weight percent Si alloy, the wear properties of Surface Composite materials represent an increase of between 10- and 30-fold.

In the measurement of wear behavior it is necessary to take into account the total "system" wear; i.e., the wear measured on the test block as well as the wear measured on the "mating" rotating member. Bearing this in mind, it is significant to compare state-of-the-art wear-resistant materials with the Surface Composite materials of this invention. In Table III are included two such state-of-the-art "bulk" composite materials produced via conventional powder metallurgy techniques: "Ferro-TiC" developed by Chromalloy Corporation and the Al + graphite composite developed by Toyo Kogyo Co. as a wear-resistant rotor apex seal for the "Mazda" automobile's rotary-piston engine. The Ferro-TiC material provides good wear-resistance but is very abrasive vis-a-vis the wear of the mating ring. Ring weight loss values for tests with Ferro-TiC are a factor of two higher than the comparable values for tests with Surface Composite materials in 6061 and 2024 alloy matrices. The Al + graphite composite material shows higher values for both Ring Weight Loss and Wear Scar Volume when compared to the Surface Composite Materials thus clearly indicating the superior wear behavior of the materials of this invention.

It should be noted that in addition to the wear properties of the Surface Composite materials being superior to the state-of-the-art materials, the fabrication of Surface Composite materials does not require any major modification of either the technology or the economics of conventional practice for casting aluminum alloys.

In contrast, the state-of-the-art wear-resistant composites mentioned hereinbefore, are fabricated by relatively more expensive processing involving sintering, hot pressing and the like.

Table III __________________________________________________________________________ Material Tested Wear Scar Ring Weight Coefficient Volume Loss of Friction __________________________________________________________________________ (X10.sup.-.sup.6 cm.sup.3) (mg) (at 5400 revs.) 1. Surface Composite 6 to 40 .16 - .7 .12 - .15 (75 area percent TaC satin Weave Textile in 6061 aluminum alloy matrix). 2. Surface Composite 32 .15 .11 (75 area percent TiC bias woven tape in 6061 aluminum alloy matrix). 3. Surface Composite 8 to 30 .12 - .25 .12 - .15 (75 percent TaC Satin Weave Textile in a 2024 alloy matrix). 4. Surface Composite (75 4 .9 .12 area percent TaC Satin Weave textile in a Co- Cr-Ni alloy matrix). 5. Surface Composite 9.5 .81 .11 - .12 (75 area percent TaC Satin Weave textile in Al-18w/o Si alloy matrix). 6. 6061 aluminum alloy 5998 weight gain .066 7. Al-18w/o Si alloy 380 .46 .12 8. Al + graphite 124- 131 .65-1.01 .133 composite (Toyo Kogyo) 9. Ferro TiC (Chromalloy) 2-7 1.28 .133 __________________________________________________________________________

Although the invention has been illustrated by the preceding examples it is not to be construed as being limited to the materials employed therein, but rather the invention encompasses the generic area as hereinbefore disclosed. Various modifications and embodiments of this invention can be made without departing from the spirit and scope thereof.

Claims

We claim:

1. An improved wear-resistant surface composite material, the surface of which has a duplex composite structure comprised of (a) a matrix material of poor wear-resistance and (b) not less than about 50 percent area fraction and up to about 90 percent area fraction of a hard fibrous monocarbide phase, said monocarbide phase being represented by the general formula MC wherein M is a metal selected from the group consisting of tantalum, titanium or tungsten and C is carbon, said monocarbide phase being in the form of filaments between 2 and about 15 microns in diameter and being present at the composite surface as sections of varying orientations, the remainder of the surface being comprised of the matrix of the said material.

2. The composite of claim 1 wherein said matrix material is aluminum.

3. The composite material of claim 1 wherein said matrix material is an aluminum alloy.

4. The composite material of claim 1 wherein said matrix material is a casting grade aluminum-silicon hypereutectic alloy composition.

5. The composite material of claim 1 wherein M of said general formula is tantalum.

6. The composite material of claim 1 wherein M of said general formula is titanium.

7. The composite material of claim 1 wherein M of said general formula is tungsten.

8. The composite material of claim 1 wherein said filaments are present as a woven fabric.

9. The composite material of claim 1 wherein said filaments are present as a felt.

10. The composite material of claim 1 wherein said filaments are present as chopped fibers.

11. The composite material of claim 9 wherein said woven fabric is present as a tape.