Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Sintering which includes a chemical reaction
Reexamination Certificate
1998-02-20
2001-02-27
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Sintering which includes a chemical reaction
C419S010000, C419S012000, C419S013000, C419S014000, C419S019000, C264S649000
Reexamination Certificate
active
06193928
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a process for manufacturing ceramic metal composite bodies, to the ceramic metal composite bodies and to their use.
In many technological fields, for example, for tribological applications, highly wear resistant ceramic metal composite bodies with good frictional properties are required. Applications may be at room temperature as well as the high-temperature range. Such ceramic metal composite bodies are manufactured by means of a process in the case of which a liquid-phase infiltration of ceramic pre-bodies by a molten metal and an exchange reaction coupled therewith take place. The metallic constituents can be charged as an external molten metal as well as an internal molten metal, which is obtained even before the molten infiltration/exchange reaction (SI/AR), by the metal phase being present in the ceramic phase in a distributed manner.
Processes for manufacturing metal ceramic composite bodies by means of the infiltration of porous ceramic bodies have been described in the literature. U.S. Pat. No. 5,535,857 claims the manufacturing of a metal ceramic brake disk by way of the infiltration of a porous SiC pre-body. For this ceramic body, the SiC powder is pressed into the required shape and presintered so that open pore channels remain. The porous disk is then infiltrated by means of an aluminum alloy, whereby a metal-reinforced ceramic matrix is created. During infiltration, the metal carries out no reaction with the matrix so that the temperature stability of the material depends on the reinforcing matrix. In the case of an aluminum infiltration, this means that the application limit of the material is 400° C.
Another process also describes the infiltration of a ceramic pre-body by means of aluminum (U.S. Pat. No. 4,988,645). In this case, the ceramic body is manufactured by way of an SHS reaction (SHS-reaction: Self-propagating high-temperature synthesis is the ignition of a reactive mixture), in which case the reaction maintains itself and supplies the desired ceramic matrix as a reaction product.
In U.S. Pat. No. 4,033,400, the infiltration of a porous ceramic body with a liquid metal is claimed, in which case the matrix consists of Si
3
N
4
and the metal is composed of an Al-alloy. In this case also, it is clearly important that no reaction is to take place between the matrix and the metal.
The firm Lanxide Technology also claims a number of materials which were produced by way of metal infiltration (for example, European Patent Documents EP-B-0 368 785 and EP-B-0 368 784). These patents essentially claim new process steps, as, for example, the targeted oxidation of the ceramic pre-body.
So far, all patents have had in common that no reaction infiltration has taken place. One exception, in this case, was Patent U.S. Pat. No. 4,585,618 in which the infiltrated metal (aluminum) reacts with the matrix.
It is an object of this invention to produce a reinforced TiB
2
/Al
2
O
3
ceramic material for electrolysis cells. For this purpose, a TiO
2
/B
2
O
3
/Al-mixture is infiltrated by aluminum. The infiltration time is 100 hours. The reaction product consists of TiB
2
/Al
2
O
3
/Al, in which case, Al
3
Ti is also found on the surface which is not desirable.
The manufacturing of ceramic metal composite bodies presents the problem of requiring high process temperatures. This is connected with high costs because the furnaces and reaction vessels are exposed to high wear. If inert components are charged, high temperature and long reaction times must absolutely be avoided because, under these conditions, they are subjected to a serious corrosion by the molten metal. High melting temperatures occur, for example, when the metals titanium, zirconium, boron, vanadium and chromium are used. When low-melting alloys or metals are used, the maximal application temperature of the composite body will be at the low melting temperature of the alloy.
In addition, the use of external molten phases results in various process-related problems which are caused by the fact that no stationary reaction conditions exist. The penetration of the molten metal from the outside begins an exchange reaction. As a result, a reaction front pushes through the ceramic pre-body. Because of the exchange reaction, the front of the molten phase builds up reaction products and loses reactive metals. Component-shape-dependent and size-dependent concentration gradients therefore extend through the composite material. These concentration gradients can be reduced only by a very long aging at high temperatures. The gradients of the composition also lead to discontinuous physical and mechanical characteristics. In addition to the non-uniform distribution of the phases, the gradients may also result in different stoichiometric compounds with different physical or chemical characteristics.
Another problem is clogging of the pores by reaction products, which prevents a further penetration of the molten metal. Particularly in the case of large components, the edge areas are exposed to the molten metal significantly longer, whereby much reaction product is deposited there, whereas the internal areas come only briefly in contact with the molten metal. A precise checking of the porosity and of the pore size of the ceramic pre-body must therefore take place. A pore radius which decreases radially from the outside toward the inside would be ideal.
Finally, the infiltration may take place incompletely as a function of the type and composition of the porous ceramic pre-body, of the molten metal or of the implementation of the process so that an interfering residual porosity remains in the composite body. This problem occurs particularly when an internal molten metal is used because no additional material can be supplied to the porous pre-body made of the metal and the ceramic material.
Reference is made at this point to the applicant's patent application, German Patent Document DE 197 06 926 and U.S. Provisional Application Serial No. 60/040,496 with the title “for Manufacturing Ceramic Metal Compound Bodies and Use Thereof,” which was filed with the same priority date as the present patent application.
It is therefore another object of this invention to develop a process for manufacturing ceramic metal composite bodies in which the above-mentioned disadvantages are avoided.
According to the invention, this object is achieved by a process for manufacturing ceramic metal composite bodies which has the following process steps:
producing a reaction mass from 40 to 95% by volume ceramics fraction and from 5 to 60% by volume of at least two-high-temperature melting metals as well as optionally inert components, the composition of the metals being selected such that a low-melting eutectic phase of a reactive and less reactive metal is formed;
heating the reaction mass under inert conditions to a temperature which permits the formation of a molten phase and maintaining the temperature until an exchange reaction has concluded.
In an embodiment of the invention the ceramics fraction of the reaction mass is used in bulk as a powder mixture or is used as a pressed part from a powder mixture; or as presintered porous mixed ceramics from a mixture of nitrides, carbides, silicides or borides of the elements silicon, titanium, vanadium, chromium, iron, aluminum, boron or magnesium.
According to the invention, it is provided that the ceramics fraction has an open porosity of from 5 to 70%.
It is within the scope of the invention that the metals in the reaction mass in the form of a metal alloy are used in bulk as metal powder; or are used as compressed-powder charges or as coarse pieces, which contain at least two of the elements silicon, tin, titanium, zirconium, aluminum, boron, indium, magnesium, calcium, vanadium, chromium, iron, cobalt, nickel, and copper.
Furthermore, it is preferred that the ceramics fraction of the reaction mass contains as inert components 20 to 80% by volume ceramic reinforcing components in the form of short fibers, fibrous tissue, fiber mesh, platel
Rauscher Steffen
Scheydecker Michael
Tschirge Tanja
Weisskopf Karl
Zimmermann-Chopin Rainer
Daimler-Chrysler AG
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Jenkins Daniel J.
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