US3533759A - Platinum matrix composites - Google Patents

Description

Oct. 13, 1970 TENSILE STRENGTH (psi x I03) F. HITTMAN PLATINUM MATRIX COMPOSITES Filed Dec. 8, 1966 PLATINUM- I5 VOL.% ALUMINA WHISKERS (CALCULATED) PLATINUM (EXPERIMENTAL) TEMPERATURE C) INVENTOR FRED HITTMAN ORNEYS United States Patent 3,533,759 PLATINUM MATRIX COMPOSITES Fred Hittrnan, Pikesville, Md., assiguor to Hittman Associates, Inc., Baltimore, Md., a corporation of Maryland Filed Dec. 8, 1966, Ser. No. 600,233 Int. Cl. B22f 7/08 US. Cl. 29132.2 4 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to high strength, high ductility and oxidation resistant platinum matrix, fiber or whisker reinforced composite materials, and, more particularly, this invention is directed to a class of platinum matrix structural materials having a combination of high strength, high ductility and oxidation resistance at both room and elevated temperatures that are greatly superior in characteristics to all presently known competitive materials.

It is known in the prior art to produce increased strength of metals for use at elevated temperatures by producing such metals by dispersion hardening. Such metals include materials consisting of a metallic matrix containing a large number of small, inert, hard particles which may be obtained by precipitation of a constituent of the alloy or mechanical distribution of an insoluble component. In the dispersion hardened materials, the load is borne by the metallic matrix, and the function of the dispersed phase is merely to impede the generation and motion of dislocations within the matrix, thereby permitting it to bear a high stress without any deformation. in this material, the total content of the dispersed phase is generally one percent or less.

Platinum is indeed a potentially most attractive and desirable matrix material, since it has a high melting point, a high ductility and ease of bonding to ceramics, and good oxidation resistance.

However, the prohibitive cost of platinum metal and its alloys and the availability of other ostensibly similar metal matrix composites which exhibit satisfactory strength and temperature characteristics had heretofore served to stifie research and development along the lines of high strength platinum matrix structural elements. In point of fact, silver composites strengthened by particles and whiskers are known to exhibit potentially useful tensile strengths of up to about 65,000 pounds per square inch at 0 F. and up to about 43,000 pounds per square inch at 1400 F. Compare Sutton et al., Metals Engineering Quarterly, February 1963, vol. 3, page 43.

It has now been surprisingly found that composite structural elements comprising a platinum metal or a platinum metal alloy matrix can be produced without a substantial increase in gross cost, the composite structural elements unexpectedly exhibit tensile strengths in excess of 135,000 pounds per square inch at 0 C. and in excess of 65,000 pounds per square inch at 1400 C.

Thus, an object of the present invention is that of providing the art with reinforced structural elements of such greatly improved strength characteristics at both room and elevated temperatures that many hitherto impracticable applications for composite materials are rendered feasible, both from economic and technical points of view.

It is yet another object of the instant invention to provide a high strength, high ductility and oxidation resistant platinum matrix composite material for use at substantially elevated temperatures or at room temperatures, and in which the cost of producing the material is substantially reduced by increasing the fiber or whisker content to higher levels and yet maintain, as well as increase, the

strength characteristics, the ductility and the oxidation resistance characteristics.

These and other objects, briefly stated, are accomplished according to the present invention, by providing a composite product of platinum and ceramic or other fibers or whiskers, or of platinum alloys and ceramic or other fibers or whiskers, comprising a metallic matrix of platinum metal or a platinum metal alloy such as platinum-rhodium and a dispersed fibrous or whisker component such as fibers or whiskers of A1 0 particularly a-AI O The fiber or whisker content may range from one volume percent to volume percent and may be randomly disposed or disposed in a given orientation or in several given orientations, depending upon the particular application and use of the product contemplated, and may be comprised of either continuous or discontinuous fibers or whiskers.

There is shown in the figure of drawing a characteristic curve for the behavior of a platinum, 15 volume percent A1 0 whisker composite of this invention as a function of temperature.

In the fiber or whisker reinforced composites of the present invention, as well as in those of the prior art, the load is borne primarily by the fibers orwhiskers. The matrix serves to transmit the stress to the fibers or whiskers in shear over a relatively large surface area, and thereby avoid any significant stress concentrations that might fracture the brittle fibers or whiskers. At ends or fractures in the fibers or whiskers, the ductile matrix will flow locally and distribute the load in the terminating fiber or whisker to adjacent fibers or whiskers, again over a reasonably long length of fiber or Whisker to prevent thereby propagation of the fracture to neighboring fibers or whiskers. In this way, the potentially very high strengths of refractory ceramic materials is approached in realization of the invention.

Strengths of ceramic whiskers and fibers have been measured to be as high as 4X 10 p.s.i. Table 1 lists representative values:

TABLE 1 Pounds per square inch Graphite whisker 0.35 to 3.0x 10 A1 0 whisker 0.16 to 2.60 10 BeO whisker 2.0 to 2.8 l0 SiC whisker 0.26 to 1.50 l0 B whisker 096x10 W wire 040x10 SiO fiber 0.20 10 It has been observed that strengths of composites may be interpolated between the strengths of the matrix and the fiber or whisker up to approximately 50 percent fiber or whisker content but additional fiber or whisker additions do not increase composite strength. It is desirable with platinum to use high contents of fibers or whiskers in order to reduce the cost of the composite. The difliculty in producing high fiber or whisker content composites lies in the problem of maintaining a continuous matrix in contact with all fiber or whisker surfaces at fiber or whisker contents. Unless each fiber or Whisker is completely surrounded by matrix, it cannot be properly loaded. It is possible to produce fully bonded composites with fiber or Whisker contents as high as 80 percent by applying the platinum metal to individual fibers or whiskers before consolidation. This may easily be done by suspending the fibers or whiskers in an aqueous platinizing solution and controlling the metal content by the weight increment on the fibers. Alternately, it may be accomplished by vapor deposition of the platinum metal onto the fibers or whiskers.

With the possibility of using a variety of fibers or whiskers having an appreciable range of strengths as shown in Table 1, it is desirable to be able to control the matrix strength as well in order to develop the optimum balance between components. This may be accomplished by alloying within the family of platinum-like metals. In the following reference is shown the mechanical properties of a family of platinum-rhodium alloys: Hill, J. S., Hot Tensile Properties of Platinum and Its Alloys, Englehard Industries, Inc., Technical Bulletin, June 1962, volume III, No. 1. Other platinum-type alloys are also feasible.

The reinforced platinum metal matrix composites of the present invention with fiber or whisker contents of from 1 to 80 volume percent can obviously be prepared according to any one of a number of conventional art techniques such as, for example, those disclosed in Sutton et al., Metals Engineering Quarterly, supra.

If, however, the intended application is such that a simple stress pattern exists, appreciable benefit can be derived by orienting the fibers or whiskers with the principal stress axes. As disclosed in copending application, Ser. No. 600,165, filed of even date herewith, now Pat. No. 3,432,295, this can be accomplished by preparing the composite as a mixture of discontinuous fibers or whiskers, matrix platinum metal or alloy in the form of powder, and an organic binder such as polyvinyl alcohol. If this mixture is extruded with a large area ratio, 100:1 or higher, e.g., extruded through a die provided with an orifice whose diameter is less than the lengths of the reinforcing fibers or whiskers, a high degree of fiber or whisker orientation is obtained within the extruded rods. This material can then be cut to length, and a large number of short sections staked into a hot pressing die in a parallel configuration, and a dense body with a high degree of orientation produced by hot pressing. A 10 volume percent A1 whisker-platinum metal matrix composite prepared in this manner by hot pressing at 1300 C. and under 2 t.s.i. for two hours, followed by sintering at 1250 C. in air for two hours, exhibited a tensile strength of 38,000 p.s.i., as compared to approximately 20,000 p.s.i. without whiskers. In like manner, there can be prepared additional material combinations of platinum matrix and 20, 30, 40 and 50 volume percent, respectively, of a whisker material as named above including A1 0 In a specific instance, four such additional combinations were made using SiO whiskers. In the event that a biaxial or a triaxial stress state exists in the application, a biaxial or triaxial fiber orientation can also be prepared. In the event that a high fiber or whisker content is required such that all of the matrix metal is applied to the fibers or whiskers before consolidation, it is possible to achieve a similar orientation by adding sutficient organic filler to the coated fiber or whisker to provide a suitable plastic mass for extrusion, removing the organic matter by thermal decomposition during final consolidation. It is apparent that numerous other deformation and consolidation processes can besubstituted forextrusion and hot pressing without changing the substance of the invention.

As additional reinforcing fibers or whiskers contemplated, reference is made to those disclosed in the hereinbefore mentioned copending application.

The strength obtainable at elevated temperatures is superior to all competitive metals. Oxidation resistance is of major importance in most high temperature applications, and the refractory metals, which offer the only potential strength competition to the newly invented composites, are notoriously poor in this respect. In the following references are shown the relative oxidation rates of the refractory metals as a class and the platinum metals as a class: Jaif, R. I., Refractory Metals, High Temperature Technology Conference, October 1959, Stanford Research Institute; and Phillips, W. L., Jr., Oxidation of Platinum Metals in Air, Trans. ASM 1964, 57, pp. 33-37. It is concluded that improvements of 5 or 6 orders 0 magnitude are possible.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the same is not to be limited to the specific embodiments thereof, except as defined in the appended claims.

What is claimed is:

1. A high strength, high ductility and oxidation resistant composite structure, comprising a matrix selected from the group consisting of platinum and a platinum alloy, and a reinforcing component selected from the group consisting of A1 0 fibers and A1 0 Whiskers dispersed within said matrix.

2. The composite structure of claim 1, wherein the matrix is platinum.

3. The composite structure of claim 1, wherein the matrix is platinum-rhodium.

4. The composite structure of claim 1, wherein the reinforcing component is oriented.

References Cited UNITED STATES PATENTS 3,005,876 4/1963 Alexander 29182.5 XR 3,282,658 11/1966 Wainer 29l83.5 3,421,862 1/1969 Shyne 205 XR OTHER REFERENCES Sabunas: Metal Fiber Composites in Product Engineering, 53060, pp. 57-61.

CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant Examiner US. or. X.R. 29 1s2.5, 191.2; 75 200, 206

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