Method of manufacturing a refractory article and a...

Compositions: ceramic – Ceramic compositions – Refractory

Reexamination Certificate

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C228S262100, C228S262700, C228S262710, C029S025010, C501S087000, C501S096100

Reexamination Certificate

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06319869

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of forming a refractory article including the step of forming a refractory material powder into an intermediate body having a shape and size corresponding to the desired shape and size of the article, and an refractory article manufactured by this method.
BACKGROUND OF THE INVENTION
Refractory articles made of carbides or comprising carbide components are known to have good properties when used under high temperature conditions. It is therefore desirable to use constructional materials comprising carbide components for heat resistant materials; erosion-resistant materials for electrical technology, which can be used under operating conditions in air; materials for high-temperature heat storage; materials for ablation heat-reflecting systems; and abrasion-resistant and tribotechnical materials. However, it is a problem to produce such articles having a desired shape, especially if the articles shall have a complex form.
U.S. Pat. No. 3,189,472 and U.S. Pat. No. 3,205,043 disclose a method of manufacturing a composite refractory article. This method comprises the steps of mixing of a silicon carbide powder and a carbonaceous material, subsequent forming of an intermediate product by pressing and filling the product containing the silicon carbide and the carbon with silicon. The last step was realized with the use of silicon vapor or molten silicon at a temperature exceeding the melting-point of the silicon.
While penetrating the intermediate product, the silicon is bonded into a secondary silicon carbide using the carbon from the product. By the known method a composite refractory article is obtained. Such an article has a high level of residual stress, moreover it is prone to cracking. U.S. Pat. No. 3,725,015 discloses another method of manufacturing a composite refractory article. The article produced thereby is based on at least one refractory compound. The known method includes the following steps:
mixing of a refractory material powder with a carbonaceous substance;
forming from the mixture obtained an intermediate product of a necessary form and size by pressing;
heating of the obtained intermediate product in order to allocate carbon from the carbonaceous substance;
filling said intermediate product with a molten metal or a mixture of metals, containing 74-99% vol. of at least a metal chosen from the group consisting of: Si, Cr, Fe, Ni, Ti and 1-25% vol. of metal (mixture) chosen from the group, consisting of: Al, Cu, Co, Fe and their mixtures, 0-24% vol. of a metal, which constitutes the metal portion of the refractory material.
This known method does not eliminate residual stress in the product completely, though the level is reduced. Articles having a complicated shape cannot be produced with accuracy by this method due to great shrinkage of the article during the heat treatment. The article produced by this method can have a closed porosity. Such a kind of porosity makes it difficult for a molten metal (mixture) to penetrate into the intermediate product. A complicated equipment for maintaining necessary high temperatures is needed to perform this method.
An article produced by said method represents at least a triple system, consisting of a sintered refractory compound from the group including boron carbide, boron silicide, titanium boride, titanium carbide, zirconium carbide, zirconium boride, silicon nitride, beryllium carbide, boron carbide, their mixtures and an alloy, consisting of at least two metals. A residue volume of space between particles of the article is filled up with said alloy, one of its metal components being the same as the metal forming the refractory material, and another one chosen from the group consisting of: aluminium, copper, cobolt, iron, and their mixtures.
An article produced by said method has a porosity of 10-40% vol.; the space between the article particles which is occupied by the metal carbide is 5-35% vol. The rest 5-35% vol. of said space is occupied by the alloy.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for manufacturing a refractory article having a desired shape in a simple way and with simple and inexpensive equipment. Moreover, thermal stresses and microcracking in the article produced should be minimized and an open porosity should be existant throughout the whole volume of the article. Finishing of the article produced should also be minimized.
A further object of the invention is to provide composite refractory articles having high physico-mechanical properties, high level of electric conductivity, heat capacity, hardness and abrasion resistance; being suitable for use in several technical areas, also at high temperatures and even at temperatures higher than the melting point of the metal phase filling the pores of an intermediate body produced by the method according the invention.
This object is accomplished by a method of forming a refractory article including the step of forming a refractory material powder into an intermediate body having a shape and size corresponding to the desired shape and size of the article, characterized by chosing a carbide-forming metal or alloy as material for the intermediate body, exposing the intermediate body formed to a gaseous hydrocarbon or a mixture of hydrocarbons at a temperature exceeding the decomposition temperature for the hydrocarbon or hydro carbons until the mass of the intermediate body has increased by at least 3%, and thereafter exposing the intermediate body to a temperature of 1000-1700° C. in an inert atmosphere. The body formed by this method consists of a continuous spatial skeleton of carbide material having open pores, such a material in itself being useful as a heat-resistant structural material. Such a porous carbide skeleton material could also be used for filters, catalyst substrates, and various types of electrochemical electrodes. Moreover, such a material is excellent as a starting material for producing a composite refractory article by filling the open pores with different metals or metal alloys in order to create a composite refractory article having certain desired properties, such as a high electrical conductivity or a low friction coefficient. The last heating step facilitates such an metal infiltration.
In a first embodiment the intermediate body formed is exposed to a gaseous hydrocarbon or a mixture of hydrocarbons at a temperature exceeding the decomposition temperature for the hydrocarbon or hydrocarbons until the mass of the intermediate body has increased by 25%, at the most. The carbide-forming metal is preferably selected from the IV,V or VI group of the Periodic Mendeleyev System of Elements, more preferably from the group of Ti,Zr,Hf,V,Nb,Ta,Cr,Mo and W. It is also possible to use other carbide-forming metals, such as Al, and carbide-forming alloys. Forming of the intermediate body is preferably made by pressing.
Alternatively the intermediate body is formed by slip, slurry or tape casting.
The intermediate body is formed to have a porosity of 10-80% vol., preferably 20-60% vol. and more preferably 25-50% vol.
In the first embodiment the intermediate body is formed with an uniform porosity throughout the body volume.
In a second embodiment the intermediate body is formed with a different porosity in different parts of the body volume.
In both embodiments the step of exposing the intermediate body formed to a gaseous hydrocarbon or a mixture of hydrocarbons consists of exposing the intermediate body formed to a natural gas at a temperature of 750-950° C. or to a gas or a mixture of gases from the group of acetylene, methane, ethane, propane, pentane, hexane, benzene and their derivatives at a temperature of 550-1200° C.
Both embodiments could advantageously include the further step of saturating the carbide skeleton body with a molten metal comprising at least one metal from or an alloy based on at least one metal from the group consisting of Ag,Cu,Ga,Ti,Ni,Fe, and Co. Before the saturating step the carbide skeleton body is heated to

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