WO1992016325A1

WO1992016325A1 - Methods for producing ceramic-metal composites from ceramic and metal powders

Info

Publication number: WO1992016325A1
Application number: PCT/US1992/001044
Authority: WO
Inventors: Paul D. Williams; Aleksander J. Pyzik
Original assignee: The Dow Chemical Company
Priority date: 1991-03-19
Filing date: 1992-02-07
Publication date: 1992-10-01

Abstract

Ceramic-metal composite devices are fabricated from an initial mixture of ceramic powder and metal powder. The mixture is shaped into a preform compact and additional metal is placed on the peripheral surface of the compact. The preform compact and additional metal are placed in a furnace and heated to at least the wetting temperature of the metal. The molten metal intrudes the preform compact and fills voids of the compact. This intrusion and filling process is aided by the metal from the metal powder which wets the ceramic powder. Rapid and extensive intrusion and filling are achieved. Large ceramic-metal composite devices can be made by this technique.

Description

METHODS FOR PRODUCING CERAMIC-METAL COMPOSITES FROM CERAMIC AND METAL POWDERS

This invention relates to methods for making ceramic-metal composite materials or cermets. It also relates to the composite objects that result from the practice of these methods.

Ceramic articles and devices are used for a wide variety of purposes. They have useful properties such as strength, durability, and resistance to chemical attack. They are lightweight and can withstand wide temperature environments without losing physical integrity. They can also be molded, machined and shaped into diverse forms while maintaining their strength and integrity.

A group of these articles and devices are formed from composites of ceramic material and metal that are known as cermets. Articles and devices made from cermets have distinct material properties and forming characteristics. They combine the hardness of a ceramic with the ductility of a metal in a lightweight structure. The combination of ceramics and metals into composite materials allows desirable characteristics of these two substances to be retained together with advantageous unique properties that provide the superiority of these composite materials. Examples of articles and devices with distinctive properties include bearings, seals, cutting tools and armor plates.

Until now, these ceramic-metal composite materials or cermets have been made by taking a porous ceramic article and incorporating the metal into the interstices of the porous article until all the space that formerly constituted the pores and interstices is filled with an appropriate metal. This process necessarily involves heating the porous ceramic article and metal past the melting point of the metal to allow the molten metal to flow into the pores and interstices of the ceramic article by wicking or capillary action. This procedure has disadvantages such as the amount of time the porous ceramic article and metal must be kept at an elevated temperature to ensure that molten metal flows into, and fills, the pores and interstices. A related disadvantage is that dimensions of the ceramic article are limited because the flow of molten metal becomes progressively slower as it proceeds toward the interior of the ceramic article. In practice, it is difficult to produce thick objects because this flow essentially stops within a few centimeters of the surface of the article.

To alleviate these molten metal flow problems, the porous ceramic article can be washed or wetted with water or an organic material to increase the wicking or capillary action. This procedure decreases the surface tension between the porous ceramic article and the molten metal. The depth of molten metal penetration is not,however, significantly increased when an organic wetting material is used. The increased capillary flow rate, though significant, is overshadowed by other factors. For example, residual organic material can chemically react with the metal and alter its properties. In addition, the organic material is an undesirable third component in a ceramic-metal matrix even if it does not chemically react with the metal.

One aspect of the present invention is a method for forming a solid composite of a ceramic and metal comprising the steps of: (a) mixing ceramic powder and metal powder together; (b) contacting a peripheral surface of the mixture of step (a) with a metal, wherein the quantity of said contacting metal is sufficient to fill the voids in said mixture when said mixture and said contacting metal are heated to at least the wetting temperature of said metal powder and said contacting metal; (c) heating said mixture and said contacting metal of step (b) to at least the wetting temperature of said metal powder and said contacting metal, whereby molten metal intrudes voids in said mixture; and (d)cooling the intruded mixture of step (c) until said molten metal solidifies, thereby forming said solid composite.

An alternative embodiment of the present invention is a method for forming a solid composite of a ceramic material and metal comprising the steps of: (a) mixing ceramic powder and metal powder together; (b) molding the mixture of step (a) into a specified shape; (c) heating the shaped mixture of step (b) until said metal powder wets said ceramic or vaporizes; (d) cooling the heated material of step (c) until the wetting or vaporized metal solidifies, thereby forming a shaped ceramic-metal unit; (e) contacting a peripheral surface of the cooled ceramic-metal unit of step (d) with a metal, wherein the quantity of said contacting metal is sufficient to fill the voids in said unit when said contacting metal is heated to at least wetting temperature; (f) heating said unit and said contacting metal to at least the wetting temperature of said contacting metal, whereby molten metal intrudes voids in said unit until said intrusion ceases; and (g) cooling the intruded unit of step (f) until said molten metal solidifies, thereby forming said solid composite.

The ceramic-metal composite objects formed by these methods are also a subject of this invention. These objects incorporate metal from a metal powder and also from metal that intrudes the voids in the initial mixture of ceramic powder and metal powder. The metal intrusion occurs for large ceramic-metal composite objects as well as for smaller composite objects. For example, the metal intrusion and filling can exceed one inch (2.5 centimeter (cm)) of composite object dimension measured from the point of initial metal intrusion.

The present invention has at least five advantages. First, relatively large ceramic-metal composite objects can be fabricated using these methods. Second, these objects can be initially formed in a general shape and then refined into a more complex shape before the final metal intrusion and filling is completed. Third, the intrusion and filling process rapidly proceeds to conclusion. Fourth, most of the voids in the ceramic-metal composite are filled. This provides maximum structural integrity of the composite object. Fifth, more than one metal or metal alloy can be incorporated into the ceramic-metal composite. The ceramic-metal composite objects of this invention have uniform physical properties throughout their structure. They can be quickly fabricated and their size and shape is not limited by a lack of metal intrusion.

In this invention, the ceramic material is initially in the form of a powder. The ceramic material is desirably a carbide such as BjiC, SiC, TiC, Mo₂C or WC; a boride such as TiB₂ or SiBg; a suicide such as MoSi₂; a nitride such as AIN, SigNi_j or TiN; or an oxide such as Al₂0 or Si0₂. The ceramic powder must also be wettable by at least one of the metals of the invention. The ceramic powder and metal must be chosen so they are compatible in the sense that the metal can wet the ceramic.

Once the ceramic powder is chosen, it is mixed with a suitable metal powder. The metal can be any composition provided it interacts properly with (wets) the ceramic material to form the cermet when the powders are heated to at least the wetting temperature of the cermet metal(s) and subsequently cooled. Proper ceramic-metal interaction is attained whenever the consequent cermet has the desired properties of integrity, strength and durability. Examples of metals that form suitable powders include aluminum, magnesium, silicon, titanium, copper, nickel and alloys of these metals.

The ceramic powder desirably occupies between H0% and 905. of the volume of the mixture. The relative proportions of ceramic powder and metal powder in the powder mixture depend on the relative granular sizes of the constituent powders. Typically, the size of the etal powder will be 2-5 times that of the ceramic powder. There should be enough metal powder to spread, or vaporize, and partially coat the surfaces of the ceramic powder particles when the powder mixture is heated to at least the wetting temperature of the metal constituting the metal powder. In general, the amount of metal powder should be between 0.1 % and 105& of the dry volume of the ceramic and metal powder mixture. Preferably, the amount of metal powder should be within a range of 0.55- and 45. of the dry volume of the powder mixture under these conditions. The ceramic powder makes up the balance of the powder mixtures.

The dry ceramic and metal powders are mixed together to form a relatively homogeneous mixture. The mixing can be accomplished by a conventional technique such as dry tumbling, shaking or milling. Alternatively, the ceramic and metal powders can be dispersed in a liquid medium and then stirred or similarly mixed until the desired blending of these powders is achieved.

The ceramic and metal powder mixture is then placed in a suitable container, such as a crucible or, more usually, formed into a desired shape. This shaping can be accomplished by a variety of techniques such as forming the mixture into a preform (forming "greenware"), also known as a preform compact. Conventional additives, such as dispersants, binders, press lubricants or other such processing aids, may be added to the powder mixture before it is formed into the desired shape. The dry powder mixture can be formed into the desired shape by conventional procedures such as uniaxial pressing or isostatic pressing. The mixed powder dispersions can be formed into a desired shape by conventional techniques such as filtration or "slip" casting, injection molding or clamp molding.

Once the preform compact is formed, the liquid dispersion medium, dispersant, binder, press lubricant or other such processing aid should be removed to ensure successful completion of the subsequent steps in the method of the invention. Residual liquid dispersion medium or processing aids may interfere with proper metal intrusion or the bonding of ceramic to metal that occurs in the subsequent steps. The liquid dispersion medium or processing aids can be removed by any suitable technique such as vacuum evaporation or heating. For example, the preform compact can be heated in a nonoxidizing atmosphere to 70-6O0°C. This temperature will vaporize the liquid based materials away from the powder mixture without melting the metal powder.

In the next step, metal is placed adjacent to, and usually in contact with, the ceramic and metal powder mixture or the preform compact. The amount of metal is desirably at least enough to fill the voids in the preform compact when the metal is heated at least to its wetting temperature and allowed to flow into, or intrude, the voids. The placed metal can be of any suitable form such as powder, shavings or pieces. Since the powder mixture, or the preform compact, and placed metal are to be subsequently heated to at least the metal wetting temperature, a container that can withstand such heating should be used to hold the powder mixture or the preform compact and the placed metal. An appropriate container is a refractory crucible since it can be used in a metallurgical furnace. The metal that is placed adjacent to the preform compact usually has the same composition as the metal powder. The metals need not, however, be identical. The placed metal must simply wet and intrude the voids in the powder mixture or the preform compact. If the metals are dissimilar, the heating is at least to the highest of the metal wetting temperatures. The temperature is maintained for a period of time sufficient to allow the metal(s) to reach the appropriate wetting temperature and intrude into the voids in the powder mixture or greenware.

The temperature at which a metal "wets" is at or above the melting temperature of that metal. For purposes of this invention, wetting occurs when the contact angle of molten metal is less than 90° where the molten metal contacts the surface of, for example, a preform compact. The contact angle is preferably less than 45°.

Ceramic-metal material can be formed when metal from the metal powder vaporizes and atoms of this metal adhere to the ceramic material. The wetting properties of this material can differ from the wetting properties of the ceramic material alone. A ceramic-metal material that is ceramic material coated with a film of metal can have wetting properties that differ from both the ceramic material alone and the ceramic material with adherent metal atoms. The wetting temperature of the placed metal may also be lower for a ceramic-metal material than for the ceramic material alone.

Wetting between the placed metal and the ceramic material having metal atoms adhered thereto, the ceramic material that is coated with a metal film, or the ceramic-metal material typically occurs as intrusion of metal into the voids proceeds. The metal powder of the powder mixture or the preform compact usually wets the ceramic material or vaporizes so it can coat the ceramic particles. This wetting of ceramic surfaces by the metal of the metal powder enhances the intrusion of the wetting, molten placed metal into the voids. The conditions for enhanced intrusion are, therefore, (1) changing the wetting properties of the ceramic surface by distribution of the metal from the metal powder onto the ceramic surface by wetting with, or vaporization and condensation of, the metal powder, and (2) wetting of the ceramic or ceramic-metal material by the placed metal. These conditions guide the choice of the metals constituting the metal powder and placed metal.

It is better for the powdered metal to wet or vaporize either before or simultaneously with the wetting of the placed metal. This allows the ceramic material to wet either before or as the placed metal wets and flows into the voids of the powder mixture or the compact. The placed metal may have a higher wetting temperature than the powdered metal provided intrusion occurs once the powdered metal has wet the surface of the ceramic material.

It is possible, in this invention, that at least some of the voids are intruded by and filled with metal of the metal powder. In these instances, wetting by the placed metal does not need to occur since the wetting and intrusion of the ceramic or ceramic-metal is accomplished with the metal of the metal powder.

The powdered metal should have wetting properties that favor intrusion of the molten placed metal. Some metals inherently infiltrate materials better than other metals. The former metals should be used to formulate the metal powders. For example, powdered aluminum could be used in the powder mixture. Its presence will enhance the intrusion of placed copper, a metal that normally does not readily intrude. As an alternative, aluminum alloys that infiltrate readily could be used as the powdered metal and aluminum alloys that infiltrate more slowly could be used as the placed metal.

When the powder mixture, or the preform compact, and the placed metal are present in a proper container, the container is put into a furnace, oven or similar device and heated to at least the wetting temperature of the metal(s). The powdered metal is heated sufficiently to substantially coat the surfaces of the ceramic material and allow the molten placed metal to wet the preform and intrude pores and voids of the preform compact. For aluminum metal powder and additionally placed metal and boron carbide or silicon carbide preforms, the temperature is typically held between 1000°C and 1300°C, preferably between 1100° and 1200°. The temperature is preferably maintained until intrusion ceases and the voids are substantially filled. Since the wetting or vaporized powdered metal coats or wets the surface of the ceramic material throughout the extent of the preform compact, intrusion and void filling occurs rapidly and thoroughly. The intrusion and filling are normally complete within one hour even for objects which have a thickness dimension exceeding 2 inches (5.1 cm).

The wetting temperature and the intrusion and filling time vary with the particular ceramic and metal combination and other operating conditions. Some ceramic materials wet more readily with certain metals than with other metals. Conversely, some metals more readily wet certain ceramic materials than other ceramic materials. In addition, the wetting, intrusion and filling times depend on the maximum temperature attained, time at maximum temperature and environmental pressure for the intrusion and filling process. Higher temperatures cause more rapid diffusion of the metal to occur. Lower pressures increase the likelihood of movement of metal to ceramic surface. Higher pressures can be used to help force the metal into the powder mixture or preform compact. Longer times at maximum operating temperature increase the probability of more complete void filling. These conditions can be varied to achieve the ceramic-metal composite with the desired characteristics.

The time for intrusion and void filling is preferably minimized because the properties of the cermet change when temperatures are maintained at elevated levels. In addition, the microstructure of the cermet goes through phase transitions at elevated temperatures. The temperature induced phase changes are continual and generally detrimental to the properties sought in a cermet. The phases do not remain uniform throughout the cermet as the phase transitions occur.

The melted or vaporized powdered metal preparation of the preform also allows intrusion and void filling to proceed to a greater extent than previously attainable by fabrication methods prior to this invention. The metal-wetted ceramic materials do not significantly impede the flow of the added metal as it intrudes the preform. The rapid and more extensive penetration of the added metal in the intrusion process of this invention yields relatively large, elaborately designed cermets with uniform material properties. For example, aluminum intrusion and void filling of an aluminum powder and either boron carbide or silicon carbide powder preform that is 2 inches (5.1 cm) thick is completed in less than 15 minutes.

The heating procedures in this invention are usually performed under nonoxidizing conditions. If the heating is done when oxygen is present in the atmosphere surrounding the preform, the metal(s) can quickly become oxidized. That is, an oxide coat can form on the surface of the molten metal(s). This oxide coat can impede the wetting of the ceramic material surfaces and also impede the intrusion process. Void filling is less rapid or complete when an oxide coat is present. In addition, the oxidized metal has different, and usually deleterious, properties upon bonding to the ceramic material or other metal atoms in the cermet. For these reasons, nonoxidizing conditions are normally used when the preform compact is heated. These conditions can be achieved either by performing the heating procedure in an evacuated furnace or by flushing the preform compact with a nonoxidizing gas, such as nitrogen or argon, as the compact is heated. Vacuum conditions are also usually beneficial for metal infiltration and intrusion because the time at maximum operating temperature is shortened. Typically, the pressure is maintained at 50- 200 mtorr (6.7 to 26.7 Pascals).

After the metal intrusion and void filling are complete, the object is allowed to cool. The resultant cooled object is a cermet that can be further processed by conventional fabrication procedures if desired.

A modification of the just described method for producing a cermet can be performed without sacrificing the properties sought in the cermet. After the mixed ceramic and metal powders are formed into a preform compact, and before additional metal is placed adjacent to the compact, the compact can be heated at least to the wetting temperature of the metal powder to wet or vaporize the powdered metal in order to substantially coat the surfaces of the ceramic material. This temperature is usually maintained only for a period of time sufficient to allow movement of the wetting or vaporized metal to reach an equilibrium. This time period is normally only several minutes in duration. A time period of 15 to 45 minutes produces satisfactory results. This heating procedure is usually performed under nonoxidizing conditions to prevent the formation of a metal oxide surface. The heated object is then allowed to cool so that the molten metal solidifies.

The resultant object has more structural rigidity than the original preform compact since the solidified metal provides a solid, albeit somewhat fragile, framework. The cooled object can be handled with less caution than the preform compact. This object can be subjected to operations, such as milling or grinding, to modify or refine the form of the object to more precise tolerances than can be achieved for the preform compact.

The object can then undergo the additional metal placement followed by the intrusion and void filling procedures to form the desired cermet. In these latter procedures, the object is reheated until the metal that coats the ceramic material surfaces once again wets these surfaces for the subsequent additional metal intrusion and void filling.

If the preform compact is formed with the use of a liquid dispersion medium or other liquid processing aid, this liquid medium or processing aid can be removed from the compact as it is being heated to the melting point of the metal of the metal powder. Since the liquid medium or processing aid usually has a boiling point that is lower than the melting point of the powdered metal, the liquid will evaporate away from the preform compact as the temperature of the compact is being raised to the melting point of the powdered metal. If necessary, the temperature can be held at the boiling point of the liquid medium or processing aid until all the liquid has evaporated before the temperature is raised to the melting point of the powdered metal.

The cermets of this invention are made from a ceramic powder and metal powder mixture into which additional metal is intruded. The intrusion and concomitant void filling is accomplished quickly and extensively. Within one hour, the intrusion and void filling is a completed, even for large cermets. Since the elevated temperatures, at which metal intrusion occurs, are maintained for a minimum amount of time, the cermets have relatively uniform aterial properties. These cermets are also subjects of the present invention. The present invention is illustrated by the following example which is not intended to be limiting in any way.

EXAMPLE

Two 1 cm x 1 cm x 6 cm bars were preformed by the slip cast technique. The bars were positioned side by side in an aluminum nitride lined graphite crucible such that their long dimensions were vertical. One bar, designated the control sample, had a volume fraction of 80% B4C powder and 20% pore or void volume. The other bar, designated the experimental sample, had a volume fraction of 16% B4C, 45. Al metal powder and 20% pore or void volume. The bottom of the crucible around the bars was filled with 10.7g of Al metal pieces (1145 alloy). The crucible was loaded into a graphite element electrode vacuum furnace and pumped down to less than 100 mtorr pressure (13-3 Pascals). The furnace was filled with argon and again pumped down. The furnace enclosure was maintained at 100-200 mtorr (13-3-26.6 Pascals) for the duration of the subsequent heating and cooling cycle. The crucible was heated rapidly to 800°C and held for 5 minutes. It was then heated at a heating rate of 8°C/min to 1130°C and held at this temperature for 15 minutes. The furnace was allowed to cool to about 100°C. The crucible was exposed to air and removed from the furnace. The control sample had been intruded with Al metal to a height of approximately 2 cm. In contrast, the experimental bar, containing 4% Al powder prior to intrusion, had filled to its full height of 6 cm.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiment of the invention described herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

1. A method for forming a solid composite of a ceramic and metal comprising the steps of:

(a) mixing ceramic powder and metal powder together;

(b) contacting a peripheral surface of the mixture of step (a) with a metal, wherein the quantity of said contacting metal is sufficient to fill the voids in said mixture when said mixture and said contacting metal are heated to at least the wetting temperature of said metal powder and said contacting metal;

(c) heating said mixture and said contacting metal of step (b) to at least the wetting temperature of said metal powder and said contacting metal, whereby molten metal intrudes voids in said mixture; and

(d) cooling the intruded mixture of step (c) until said molten metal solidifies, thereby forming said solid composite.

2. A method as claimed in Claim 1 wherein the mixture of step (a) is formed into a preform compact before said contacting metal of step (b) is brought into contact with a contacting peripheral surface of the mixture.

3. A method for forming a solid composite of a ceramic material and metal comprising the steps of:

(a) mixing ceramic powder and metal powder together;

(b) molding the mixture of step (a) into a specified shape;

(c) heating the shaped mixture of step (b) until said metal powder wets said ceramic or vaporizes;

(d) cooling the heated material of step (c) until the wetting or vaporized metal solidifies, thereby forming a shaped ceramic-metal unit;

(e) contacting a peripheral surface of the cooled ceramic-metal unit of step (d) with a metal, wherein the quantity of said contacting metal is sufficient to fill the voids in said unit when said contacting metal is heated to at least wetting temperature;

( ) heating said unit and said contacting metal to at least the wetting temperature of said contacting metal, whereby molten metal intrudes voids in said unit until said intrusion ceases; and

(g) cooling the intruded unit of step (f) until said molten metal solidifies, thereby forming said solid composite.

4. A method as claimed in Claim 3 wherein the cooled ceramic-metal unit of step (d) is reshaped before said contacting metal of step (e) is brought into contact with a peripheral surface.

5. A method as claimed in any one of the preceding claims wherein heating is performed either in a vacuum or in a nonoxidizing atmosphere.

6. A method as claimed in any one of the preceding claims wherein the ceramic powder is selected from borides, carbides, suicides, nitrides and oxides and the metal powder and contacting metal are separately selected from aluminum, magnesium, silicon, titanium, copper, nickel, and alloys thereof.

7. A method as claimed in any one of the preceding claims wherein the wetting temperature of the metal powder does not exceed the wetting temperature of the contacting metal.

8. A method as claimed in any one of the preceding claims wherein the amount of metal powder is between 0.15. and 105. by volume of the total amount of the mixture of said ceramic powder and said metal powder.

9. A solid ceramic-metal composite as formed by the method of any one of Claims 1-8.

10. A solid ceramic-metal composite as prepared by the method of any one of Claims 1-8 wherein the ceramic is B2jC and the metal is aluminum.