Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Liquid phase sintering
Reexamination Certificate
1999-05-11
2001-03-13
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Liquid phase sintering
C419S010000, C419S012000, C419S013000, C419S014000, C419S019000, C264S650000, C264S658000
Reexamination Certificate
active
06200526
ABSTRACT:
Ceramics are typically known as low-density materials with high hardness and stiffness; however, their brittleness limits their usefulness. Furthermore, ceramics are typically formed by creating a densified compact that requires significant and expensive grinding to achieve a final shape due to the large amount of shrinkage that occurs during densification of the compact. Metals are typically non-brittle, non-breakable materials; however, they lack some of the desirable properties of the ceramics, such as high hardness and stiffness. Therefore, combining a ceramic with a metal can create a composite material that exhibits the properties of a ceramic and a metal.
Processes for making ceramic-metal composite articles using ceramic preforms are known to those skilled in the art. U.S. Pat. No. 5,308,422 discloses a process for making ceramic-metal composite articles involving forming layers of ceramic material, sintering the layers of ceramic material into a porous ceramic compact and then infiltrating the porous compact with a metal by immersing the porous body in a bath of molten metal. This process involves the uncontrolled infiltration of the metal into the ceramic compact which leads to increased finishing costs due to the regions of undesirable excess metal and phases formed on the surface(s) of the composite. An undesirable phase is a reaction phase of the chosen ceramic and metal which occurs at the infiltration interface. The reaction phase is chemically unstable or it can cause pullout damage to the surface of the infiltrated part upon machining. Pullout damage results from machining of the undesirable phase on the surface of the article with partial removal of the undesirable phase occurring which leads to pitting and defects in the surface of the article.
What is needed is a process for preparing complex-shaped ceramic-metal composite articles that require little or no finishing of the article after infiltration. What is needed is a process for controlling the infiltration of the metal into the ceramic body, such that the metal is restricted to certain regions within the article. What is needed is a process for preparing ceramic-metal composite article, wherein the undesirable regions of excess metal and undesirable phases on the surface(s) are limited and controlled. What is needed is a ceramic-metal composite article, wherein the undesirable regions of excess metal and undesirable phases on the surface(s) are limited and controlled.
The invention is a process for preparing complex-shaped, ceramic-metal composite articles, comprising:
a) contacting a non-wettable powder that is non-wetting to a metal to be used for infiltration with a shaped ceramic body to form a layer of the non-wettable powder on one or more surface of the shaped ceramic body, wherein the shaped ceramic body has a region where there is no layer of the non-wettable powder and
b) infiltrating the shaped ceramic body with the metal through the region or regions where there is no layer of the non-wettable powder, such that a complex-shaped ceramic-metal composite comprising one or more metal phases and one or more ceramic phases is formed, wherein the article has substantially the net shape of the shaped ceramic body and undesirable regions of excess metal on the surface and undesirable phases within the complex-shaped ceramic-metal composite article near the surface are located only in the region or regions where there is no layer of the non-wettable powder. The invention is also a complex-shaped ceramic-metal composite article with undesirable regions of excess metal and undesirable phases on the surface(s) of, or within, the article only where there is, or was, no layer of the non-wettable powder.
The process of the invention allows the preparation of complex-shaped ceramic-metal composite articles with undesirable regions of excess metal and undesirable phases on the surface(s) of, or within, the article only in the regions where there is, or was, no layer of non-wettable powder. The process of the invention allows the preparation of a complex-shaped ceramic-metal composite article which requires little or no machining of the surface(s) to achieve a finished article. A complex-shaped ceramic-metal composite article is prepared which contains few undesirable regions of excess metal and undesirable phases.
The process of the invention is used to prepare ceramic-metal composite articles of complex shape comprising one or more metal phases and one or more ceramic phases. The ceramic and metal are chosen such that the metal will wet and infiltrate the ceramic to form a ceramic-metal composite of varying phases. Furthermore, the non-wettable powder is chosen to prevent unwanted interaction between the ceramic and metal. The non-wettable powder limits the point of interface between the metal and ceramic, thereby preventing or limiting the formation of undesirable excess surface metal and phases. Therefore, the phases in the coated and reacted ceramic-metal system are controlled and desirable.
The complex-shaped ceramic-metal composite articles of the invention are articles with undesirable regions of excess metal and phases on the surface(s) of or within the article only in the areas where there is no layer of non-wettable powder. For example, if a shaped ceramic body is layered in non-wettable powder with a small part of the surface area unlayered, the infiltration of metal will occur through the unlayered portion. Once infiltration is finished, the unlayered portion of the surface will be the only part of the surface of the article to contain excess metal and undesirable phases. This greatly reduces the final machining and finishing costs since only the unlayered portion needs to be machined, versus a large fraction of the entire surface(s) of the article if conventional infiltration techniques are used. Also, a portion of the surface of the article may be unlayered and, thus, contain undesirable excess metal and phases but not need machining because that portion is not necessary to the usage of the article. Furthermore, by controlling infiltration and the areas of excess metal, undesirable phase formation can be controlled. By controlling undesirable phase formation, the stability of the phases is controlled along with machining costs. For example, in aluminum metal systems with carbon containing ceramics, uncontrolled infiltration leads to the formation of aluminum carbide and Al
4
BC at the aluminum-boron-carbide infiltration interface. Aluminum carbide on the surface of a ceramic-metal composite article will react with moisture in the atmosphere and cause corrosion of the surface of the article. Corrosion of the surface leads to an unfinished, rough surface which is undesirable for most applications. Furthermore, undesirable phases can be harder and more difficult to machine to a given smoothness than the surrounding desirable phases at the surface of an article. Therefore, when machining is performed on the article, the undesirable phases at surface of the article can break off and cause pullout damage leaving unwanted pits and craters in the surface of the machined article, thus, making the article unusable for its intended purpose.
The process can be used to prepare any shape article for which infiltration is desired. Preferably, the process is particularly effective to prepare thin ceramic-metal composite articles of complex shape. The complex-shaped ceramic-metal composite articles preferably comprise at least three phases. Preferably, each of the phases is present in an amount of at least 2 volume percent based on the volume of the multi-phased ceramic-metal material. The ceramic-metal composite article preferably has a residual free metal content of 2 volume percent or greater. The ceramic-metal composite article preferably has a residual free metal content of 75 volume percent or less, more preferably 50 volume percent or less, and even more preferably 25 volume percent or less.
The process of the invention may be utilized to produce ceramic-metal composite articles in which the metal infil
Fox Richard T.
Han Chan
Nilsson Robert T.
Pyzik Aleksander J.
Jenkins Daniel J.
The Dow Chemical Company
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