JPH0233669B2 - SERAMITSUKUSEMENTOOYOBIFUKUGOSERAMITSUKUZAIRYOOSEIZOSURUHOHO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the joining of individual ceramic components or pieces to form composite ceramic articles. Although the invention is specifically described with respect to articles fabricated from sintered or unsintered silicon carbide components, other carbide materials such as titanium carbide or tungsten carbide or mixtures can be used as the ceramic component. will be understood.

Silicon carbide has long been known for its hardness, strength, and excellent resistance to oxidation and corrosion.
Silicon carbide has a low coefficient of expansion, good heat transfer properties, and retains high strength at high temperatures. Recently, techniques have been developed to produce high density silicon carbide materials by sintering silicon carbide powder under substantially pressureless conditions. High-density silicon carbide material is used for turbines,
It has been found useful in the machining of heat exchanger units, pumps and other equipment or tool components that are subject to severe corrosion or wear, particularly in operations carried out at high temperatures.

Ceramic articles or components thereof may be formed or shaped by various casting or molding processes. The present invention is first prepared using any suitable molding or shaping process well known in the art, such as cold pressing, isostatic molding, slip casting, extrusion, injection or transfer molding or tape casting. can form a component of

In many molding or shaping operations, one component is formed by one molding method and another or other component is formed by a different molding method, and then the components are joined to form a composite article. It is desirable or economical to do so. In some cases, it is not practical to cast or mold the entire article as a unit. In such cases, the components are formed separately and then combined to form a complex article of complex shape or structure.

The invention relates to purely unsintered metal carbides or green bodies, purely sintered metal carbides,
or a mixture of sintered and unsintered metal carbides.

Silicon carbide bodies with high density and high strength are produced by sintering granular silicon carbide in the form of articles. More recently, the technology of pressureless sintering of silicon carbide has been applied to full-scale commercial processes. In such processes, a green body of green silicon carbide is formed by molding or shaping a mixture of particulate silicon carbide, excess carbon, and a sintering aid. The formed green body is sintered under substantially pressureless conditions at a temperature of about 2050° to about 2100° C. for a period of about 20 minutes to about 30 minutes to produce a sintered silicon carbide article.

The sintering process produces a product that has a shaped shape but slightly smaller dimensions due to shrinkage during the sintering process. Various compounds of boron or beryllium have been found useful as sintering or densification aids. Such adjuvants typically include boron or beryllium added to the ceramic material powder in an amount ranging from about 0.3% to about 5.0% by weight, based on the weight of the mixture. The sintering aid may be in the form of elemental boron or beryllium or in the form of boron-containing or beryllium-containing compounds. Boron is a preferred additive because of its handling and performance. Boron is commonly used in the form of boron carbide. Examples of boron-containing silicon carbide powders are described in U.S. Patent Application No. 584,226, filed June 5, 1975, and U.S. Pat. put in. See U.S. Pat. No. 4,172,109 regarding Berylius.

The present invention provides methods for using sintered or unsintered silicon carbide bodies as components of articles having complex shapes or as components of composite articles having surfaces or portions of varying chemical or physical properties.

According to the invention, the composite ceramic article comprises:
_{Mo2B5 , MoB2} , _TiB2 , _GeB2 , _ZrB2 , _SmB6 ,
It can be manufactured by joining separate components of ceramic material with a finely divided metal boride cement or brazing compound such as NbB ₂ , HfB, VB ₂ , WB ₂ or TaB ₂ .

If the metal carbide parts or components to be joined are sintered, the metal boride cement should have a melting point within 150°C of but below the sintering temperature of the metal carbide. To be elected. For example, if the components to be joined are sintered silicon carbide, the metal boride cement is within 150° of the sintering temperature of silicon carbide, approximately 2150°C, but below the sintering temperature. Selected to have a low melting point.

If the metal carbide parts or components to be joined are unsintered or unsintered components are to be joined to sintered components, the metal boride cement should be joined. It is chosen to have a melting point slightly above the sintering temperature of the metal carbide component.

The metal boride cements of the present invention can be prepared by using metal hydroxides and boron, by using stoichiometric amounts of finely divided metal and boron mixtures, or by using metal hydroxides and boron, or by using metal hydroxides and boron. , may be produced in situ by using a mixture of carbon or a carbon source and boron.

Molybdenum borides are preferred for use as metal boride cements, and Mo ₂ B ₅ is particularly preferred for use with sintered silicon carbide. Molybdenum boride is compatible with sintered silicon carbide in terms of chemistry and melting point, and _Mo2B5 is particularly compatible with sintered alpha silicon carbide in terms of coefficient of _thermal expansion. Average coefficient of thermal expansion (CTE) of silicon carbide and _Mo2B5 sintered over the same temperature _range
are as follows: Sintered alpha silicon carbide: 4.32 x 10 ^-6 cm/cm°C Mo ₂ B ₅ : 5.0 x 10 ^-6 cm/cm°C The CTE for beta silicon carbide is approximately less than that for alpha silicon carbide. 1-2% less. in contrast,
The CTE for MoB ₂ is 7.74×10 ^-6 cm/cm°C.

Theoretical stress analysis indicates that, in the most rigorous case, the coefficient of thermal expansion of the cement used for sintered alpha or beta (or mixed phase) silicon carbide is 2.5 x 10 ^-6 cm/cm°C.
It should be between 6.5 x 10 ^-6 cm/cm°C. This proves that _Mo2B5 is _an excellent bonding material for sintered silicon carbide.

The melting point of the metal boride cement of the present invention can be improved by the addition of carbon or by a eutectic solution with a low melting point.
can be lowered by mixing low melting point metal borides to form solutions).

The cement of the invention can be used to produce composite articles by placing metal borides in powder form on the surface of at least one of the components to be joined. Preferably the metal boride cement is mixed with a temporary binder to better disperse the cement or cement components and hold the components together and aligned before furnacing. (aligned). Examples of suitable binders are waxes such as paraffin, mineral waxes and vegetable waxes, thermoplastic resins such as styrene, acrylic resins, ethylcellulose, ABS (acrylonitrile-butadiene-styrene), hydroxypropylcellulose, high-density and low-density polyethylene,
polyethylene oxide, cellulose acetate, nylon,
Ethylene acrylic acid copolymer, cellulose acetate butyrate, polystyrene, polybutylene,
Polysulfones, polyethylene glycols and polyethylene oxides, rubbers such as tragacanth, cellulose-containing materials such as methylcellulose, or in some applications thermosetting resins such as phenols. The components to be joined are pressed together and excess cement or cement and binder is removed. Preferably, the components are clamped and, if appropriate, the temporary binder is allowed to dry or harden. The composite is then fired.

If the components to be joined are all green metal carbide materials or include at least one green metal carbide component, the brazing or cementing process may include the metal carbide component. This is done under sintering conditions.

If the components to be joined are sintered metal carbide materials, brazing or cementing is carried out at a sintering temperature slightly lower than the sintering temperature of the metal carbide material. Generally, temperatures within the range of about 150°C from the sintering temperature applied for a period of about 20 to 30 minutes are suitable for use.

Sintering of the metal carbide component is preferably carried out under inert conditions. As used herein,
"Inert conditions" means conditions in which neither the metal carbide materials nor the metal boride cement react chemically to any substantial degree with each other or with the firing atmosphere. Vacuum is a useful inert atmosphere, and an atmosphere of an inert gas such as argon is extremely useful.

Sintered silicon carbide is the preferred ceramic material. Sintered silicon carbide can be in alpha or beta phase. Such ceramic materials may consist essentially entirely of silicon carbide in the alpha phase, for example 95% by weight, or they may contain a mixture of various forms of silicon carbide. Other ceramic materials, such as titanium carbide or tungsten carbide, can be used to produce composite articles having surfaces or portions of varying physical or chemical properties depending on the requirements or use of the composite article.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION The present invention relates to cement for joining ceramic materials. Cement is made of metal borides, such as Mo ₂ B ₅ , MoB ₂ , TiB ₂ , GeB ₂ , ZrB ₂ ,
Consists of SmB ₆ , NbB ₂ , HfB, VB ₂ , TaB ₂ or mixtures thereof. Most preferred and best adapted for single use is Mo ₂ B ₅ . The most preferred metal carbide ceramic material is SiC.

Preferably, at least one of the surfaces to be joined is coated or buttered with a temporary binder mixture consisting of a metal boride in a temporary binder. The binder mixture is approximately 0.08 mm to approx.
Suitably applied as a 0.16 mm thick coating or layer.
The metal boride content of the mixture preferably ranges from about 60% to about 90% by weight. The amount of metal boride is spreadable or pliable
It can be varied to obtain a mixture. Typically, mixtures containing more than about 80% by weight metal borides are not sufficiently pliable to provide the necessary spreading or layering. Mixtures containing less than about 60% by weight metal borides do not consistently produce the desired mechanically strong joints. Useful temporary binders are organic pyrolyzable materials such as waxes, thermosets, rubbers, polyvinyl alcohol, methylcellulose, thermoplastics, or mixtures thereof. A relatively inexpensive temporary binder that is particularly useful is methylcellulose. Normally,
The temporary binder is chosen so that there is little, less than about 4.0%, carbon char remaining. Except where the boride cement is formed in situ using a metal carbide starting material, a boron source, and a carbon source. In such cases, the phenolic resin is suitable for use as a temporary binder that also supplies the carbon source material.

After coating, the surfaces of the components to be joined are pressed firmly together and excess temporary binder mixture is removed from around the joined areas.
Generally, after the pressing operation, the components to be joined are spaced apart by about 0.5 microns to about 1.0 microns of cement. Preferably, the components are clamped or held in the joined position by other suitable retention means. If appropriate, the temporary binder is then dried or cured while holding the components in bonded position. Thereafter, the retaining means can be removed and the components fired to produce the desired composite article.

Although the metal boride cement of the present invention functions to provide a physical bond between the components, it is within a range of about 150°C, more preferably about 50°C, from the sintering temperature of the metal carbide components to be joined. A thin layer of metal boride or a mixture thereof having a melting point within and in the form of a layer of metal boride sandwich between two thinner layers of solid solution of metal carbide. The cement of the present invention also flows outward and wicks around the joint, forming a smooth thin coating of cement in solid solution with metal carbides. It is assumed that a wicking effect demonstrates a very good wetting of the metal carbide components by the cement of the invention and provides an additionally strengthened joint.

A cement of green silicon carbide containing a carbon source and a sintering aid is expected to function substantially as a cement in joining two components of green silicon carbide. However, such joints are not mechanically strong and are subject to fracture when exposed to mechanical shock.

The following examples are illustrative and should not be construed as limiting the invention. Unless otherwise indicated, parts are parts by weight and temperatures are in degrees Celsius.

EXAMPLE Green Components Two 1.27 cm diameter rods of green silicon carbide were formed as green bodies. One end of each rod is
Approximately 1.6 mm of a temporary bonding mixture containing approximately 80% by weight Mo ₂ B ₅ in a matrix of methylcellulose in an aqueous slurry was coated. Then attach the rod end to
Press together with enough force to force out some of the mixture, clamp in place, and remove excess
Remove _{the Mo2B5} -methylcellulose mixture.
The silicon carbide rod is then fired by heating the rod to temporarily remove the water from the binder and then firing in an argon atmosphere at a temperature of about 2150° for a period of about 25 minutes.

Once cooled, the bonded rods were tested by mechanical impact and were found to be structurally strong composites.

EXAMPLES Sintered Components Two 1.27 cm diameter rods of sintered silicon carbide were coated with a similar temporary bonding mixture in the same manner as described in the Examples. The rods were fired in an argon atmosphere at a temperature of approximately 2100 °C for a period of approximately 25 minutes. The cooled, bonded rods were tested by mechanical impact and were found to form into a structurally strong composite.

EXAMPLES Sintered and Unsintered Components One 2.54 cm diameter rod of unsintered silicon carbide and one 1.27 cm diameter rod of sintered silicon carbide were prepared in the same manner as described in the examples. coated with a temporary bonding mixture. The rod was then calcined in an argon atmosphere at a temperature of about 2150° for a period of about 25 minutes.
The resulting composite sintered product was tested by mechanical impact and was found to be structurally strong.

EXAMPLE In-Situ Cement Formation Two 1.27 cm diameter rods of sintered silicon carbide were coated with a mixture of powdered Mo and boron in stoichiometric proportions in an aqueous slurry of methylcellulose. The rods were coated and fired in the manner of the examples. The in-situ produced _Mo2B5 joints were tested by mechanical impact and were found to be _structurally strong.

It will be obvious that this invention is not limited to the specific description set forth in the examples and specification, and that various modifications may be made by those skilled in the art without departing from the spirit and scope of the invention. Probably.

Claims (1)

[Claims] 1 (a) The surface of at least one component to be joined is made of Mo ₂ B ₅ , MoB ₂ , TiB ₂ , GeB ₂ ,
₍ b) pressing the surfaces to be joined together; and _{( c )} sintering _. a method of joining separate metal carbide components to form a composite article, comprising the steps of: forming a composite sintered article; 2. The method according to claim 1, wherein the metal carbide is silicon carbide. 3. The method according to claim ₂ , wherein the metal carbide in step (a) is _Mo2B5 . 4 Metal borides include wax, thermoplastic resin, rubber, polyvinyl alcohol, methyl cellulose,
2. The method of claim 1, wherein the composition is dispersed in a temporary binder selected from the group of thermosetting resins or mixtures thereof. 5. The method of claim 4, wherein the temporary binder is methylcellulose. 6. Claim 4, wherein the temporary binder mixture contains from about 50% to about 90% by weight of metal borides.
The method described in section. 7. The method of claim 1, wherein the coating of step (a) is about 0.08 mm to about 0.15 mm thick. 8. The method of claim 1, wherein at least one of the components to be joined consists of an unsintered metal carbide, and the sintering step is carried out at the sintering temperature of the metal carbide. 9. The method according to claim 8, wherein the metal carbide is silicon carbide. 10. The method according to claim ₉ , wherein the metal boride is _Mo2B5 . 11. The components to be joined are in sintered form and the sintering is carried out at a temperature below the sintering temperature of the metal carbide but within 150° C. of the sintering temperature. Method described. 12. The method according to claim 11, wherein the metal carbide is silicon carbide. 13. The method according to claim ₁₂ , wherein the metal boride is _Mo2B5 . 14. The method of claim 1, wherein the metal boride is formed in situ. 15 The metal boride is powdered boron and Mo,
15. The method of claim 14, wherein the method is produced in situ from a powdered metal selected from the group of Ti, Ge, Zr, Sm, Nb, Hf, V, W, Ta or mixtures thereof. 16. The method according to claim 15, wherein the metal carbide is SiC. 17 Claim 15 in which the metal is Mo
The method described in section. 18 The metal boride is formed in situ from powdered boron and powdered metal hydride, and the metal in the metal hydride is Mo, Ti, Ge, Zr, Sm, Nb,
15. The method according to claim 14, wherein the method is selected from the group of Hf, V, W, Ta or mixtures thereof. 19 The metal boride is formed from powdered boron, a powdered metal oxide, and a carbon source, and the metal in the metal oxide is Mo, Ti, Ge, Zr, Sm, Nb, Hf,
15. The method of claim 14, wherein the material is selected from V, W, Ta or mixtures thereof. 20. The method according to claim 19, wherein the carbon source is a phenolic resin.