WO1996032358A1

WO1996032358A1 - Ceramic, intermetallic or metal ceramic composites with a reduced susceptibility to pesting

Info

Publication number: WO1996032358A1
Application number: PCT/US1995/004417
Authority: WO
Inventors: Jainagesh A. Sekhar; Ming Fu
Original assignee: Micropyretics Heaters International
Priority date: 1995-04-11
Filing date: 1995-04-11
Publication date: 1996-10-17
Also published as: AU2245395A

Abstract

A reduced pest ceramic, intermetallic or metal ceramic composite is disclosed including a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon, and at least 0.5 percent by weight excess added elemental silicon than that required for formation of the compound. Also disclosed is a reduced pest, ceramic intermetallic or metal ceramic composite including ternary compounds and mixtures thereof selected from the group consisting of (ZAIxSiy), where Z is an element and where x is an integer and y is a whole number.

Description

CERAMIC. INTERMETALLIC OR METAL CERAMIC COMPOSITES WITH A REDUCED SUSCEPTIBILITY TO PESTING

FIELD OF THE INVENTION

The present invention relates in general to ceramic, intermetallic or metal ceramic composites. More specifically, this invention relates to a reduced pest (the term "pest" as used herein connotes mechanical disintegration at temperatures under 1000°C), ceramic, intermetallic or metal ceramic products. Even more specifically, this invention relates to reduced pest ceramic, intermetallic or metal ceramic composites comprising a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon and mixtures thereof and further comprising at least 2 percent by weight excess added elemental silicon than that required for formation of the compound and added silica. This invention also relates to a reduced pest ceramic, intermetallic or metal ceramic composite comprising ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element and where x is an integer and y is a whole number.

BACKGROUND OF THE INVENTION

It is well known that the excellent high temperature oxidation and corrosion resistance properties demonstrated by molybdenum disilicide (MoSi₂), makes it one of the most suited ingredient for use in high temperature applications, such as heating elements and protective coatings. MoSi₂ has the following properties that make it an ideal metal-ceramic for use in applications such as top glass cooking stoves, which utilize a radiant plate placed under a glass ceramic transparent top with the heating element comprising an electrically heated body, supported by an insulating base: (1 ) The resistivity of MoSi₂ increases with temperature. (2) The resistivity-temperature curve for a MoSi₂ heating element is very steep, with the resistivity ratio at 20°C to 1500°C, being about 10. (3) The long time working temperature of MoSi₂ elements is well above 1350°C. Hence when a MoSi₂ heating element is connected to a constant voltage source, the power required will initially be high at low temperatures. As heating progresses, the power (current x voltage) required decreases as the radiant body temperature increases. The above described resistance - temperature characteristics thus enable a MoSi₂ heating element to be heated to above 1350°C immediately when the power is turned on. Additionally, because of its good thermal stability and electrical conductivity, MoSi₂ finds application in VLSI technology. Other applications for MoSi₂ include abrasives, cutting tools, polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron alternators; shape-memory alloys; steel melting additives; electrodes for electrolysis of corrosive media; ceramic filters used for filtering undesirable particles from a molten metal before the metal is poured into a pattern or mold; porous ceramic articles used as substrates for catalysts, for removal of contaminants from exhaust gases, for collecting impurities from highly corrosive acids and chemicals; in kiln furniture; aerospace applications; and in structural materials in low density load bearing structures.

However, the low-temperature oxidation and degradation of

MoSi₂ and MoSi₂-containing ceramic or metal ceramic composites, also referred to as "pest" continues to be a problem, especially in non-dense (porous), MoSi₂ containing ceramic or metal ceramic composites. Thus, there is a need for MoSi₂-containing ceramic or metal ceramic composites which are not susceptible to pest formation.

In Mechanism ofMoSi₂pest during low temperature oxidation, J. Mater. Res., Vol. 8, No. 1 , Jan 1993, there is proposed a theory of how pest is formed in MoSi₂ ceramic or metal ceramic composites. The references disclosed in this article form an excellent collection of general material regarding MoSi₂ and include a few articles about pest formation.

USP 4,704,372 to Watanabe et al. relates to a high-strength molybdenum silicide-based ceramic material and a process for producing such ceramic material. A powder mixture obtained by adding to titanium carbonitride 0-100% by weight of a specific metal boride compound is added to powder of molybdenum suicide used as base component, and the obtained mixed powder composition is sintered. Both strength and density of molybdenum suicide are said to be increased while maintaining its low-temperature sinterabilitγ. It is recognized in this patent that the sintered body of MoSi₂ alone is low in strength and also extremely brittle at normal temperature, so that the industrial use of MoSi₂ has been limited to electrical resistance heating elements.

A Study of pest oxidation in polycrystalline MoSi₂, J. Mater. Res. Vol. 7, No. 10, Oct. 1992, concludes that inclusion of lower than the required stoichiometric amount of silicon required to form MoSi₂ leads to a greater incidence of pest. It is also concluded that a slight stoichiometric excess of silicon leads to reduced pest formation. However, there is no disclosure or suggestion of the desirability of adding silica to the MoSi₂ to further (and greatly) reduce its susceptibility to pest. In Low Temperature Oxidation of Molybdenum Disilicides, Metallurgical Transactions A, Vol. 23A, June 1992 (pages 1763- 1772), it is suggested that molybdenum disilicide which has been oxidized at high temperatures to form a protective layer of SiO₂, shows a reduced susceptibility to pest when used after formation of the SiO₂ layer, at low temperatures. There is no disclosure or suggestion of forming an aluminum oxide containing protective layer.

Co-pending application USSN 07/847,782 ('782) provides a novel technique to make electrical heating elements which may be used up to 1900° C. New methods are also provided for manufacturing ceramic composites, which may be used as both electrical heating elements and oxidation resistant materials. There are also provided in the '782 application, compositions for manufacturing the above mentioned ceramic composites and heating elements. The '782 application is incorporated by reference into the present application. While the '782 application provides methods for the manufacture of electrical heating elements and the like, there is no suggestion that this method alone, i.e. micropyretic synthesis would eliminate, reduce or in any way affect pest in MoSi₂ containing composites. In fact, the '782 application does not even mention pest problems in MoSi₂ containing composites. This application is incorporated by reference herein.

Other patents disclosing molybdenum disilicide containing ceramic or metal ceramic composites are: USP 5,376,421 , Dec. 27, 1994; USP 5,374,342, Dec. 20, 1994; USP 5,364,513, Nov. 15, 1994; USP 5,364,442, Nov. 15, 1994; USP 5,340,448, Aug. 23, 1994; USP 5,340,014, Aug. 23, 1994; USP 5,316,718, May 31 , 1994; USP 5,310,476, May 10, 1994; USP 5,279,737, Jan. 18,

1994; USP 5,217,583, Jun. 8, 1993; USP 5,188,678, Feb. 23, 1993; USP 5, 127,969, Jul. 7, 1992; and USP 5,1 10,688, May 5, 1992. All these patents are incorporated by reference herein in their entirety.

One way of reducing the likelihood of formation of pest is by reducing the porosity of the composite. Methods of reducing porosity are described in copending applications 08/027,710, 08/085,313 and 08/242,362. However, porosity may be important for certain applications, including use of the composites in filtration and in separation of gas and liquid mixtures.

The only recent US patent referring to low temperature brittleness of MoSi₂ containing products (or pest), is USP 4,704,372, discussed above. While there may be prior art documents which disclose MoSi₂ containing products with excess silicon, not a single prior art document was located which disclosed or even suggested the use of added silica in combination with excess silicon in the manner disclosed and claimed in the invention. The generic or accidental disclosure of MoSi₂ containing products with excess silicon by itself, does not in any way diminish the patentability of added-silica-in-combination-with-excess-siliconaspectofthepresent invention, it is not possible for one skilled in the art to review the prior art and conclude that pest in composites may be reduced or eliminated by the addition of excess silica and silicon.

With respect to the inclusion of ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element and where x is an integer and y is a whole number, in ceramic, intermetallic and metal-ceramic compounds for reducing pest, not a single prior art source could be located, which disclosed or even suggested the desirability of the same. Thus, there is a need for, and the present invention provides, ceramic, intermetallic or metal-ceramic composites, with a reduced susceptibility to pest formation, without reducing the porosity of the composites.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a ceramic, intermetallic or metal ceramic composite, with a reduced susceptibility to pest formation.

In accordance with the first aspect of the present invention, there is provided a reduced pest ceramic, intermetallic or metal ceramic composite comprising a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon and mixtures thereof, the ceramic, intermetallic or metal ceramic composite further comprising: at least 0.5 percent by weight excess added elemental silicon than that required for formation of said compound; and added silica.

In accordance with a second aspect of the present invention there is provided a reduced pest, ceramic, intermetallic or metal ceramic composite comprising ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element and where x is an integer and γ is a whole number.

Preferably, Z is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, W. Also preferably, the composites further comprise quartenary and higher order compounds related to said ternary compounds and alloying additions including dopants and mixtures thereof.

In accordance with a third aspect of the present invention, there is provided a method of reducing pest formation in a ceramic, intermetallic or metal ceramic composite comprising a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon and mixtures thereof, comprising: adding to said composite, at least 0.5 percent by weight excess elemental silicon than that required for formation of said compound; and adding silica to said composite.

In accordance with a fourth aspect of the present invention, there is provided a method of reducing pest in ceramic, intermetallic or metal ceramic composites, comprising: including in said composite, ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element where x is an integer and y is a whole number. Preferably, Z is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, W.

Also preferably, the composites further comprise quartenary and higher order compounds related to said ternary compounds and alloying additions including dopants and mixtures thereof.

In accordance with a fifth aspect of the present invention, there is provided a method of reducing pest in ceramic, intermetallic or metal ceramic composites by providing a layer, said layer not comprising predominantly of silica. Preferably, the layer comprises aluminum oxide.

These and other objects and aspects of the present invention will become apparent from the following description of the preferred embodiments.

DESCRIPTION OF THE DRAWING

Figure 1 is a photomicrograph of a MoAI₂Si₂ composite according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several manufacturing technologies are available for the manufacture of the ceramic, intermetallic or metal ceramic composites of the present invention. These technologies include micropyretic synthesis (as defined in USP 5,188,678 and in copending application serial number 07,847,782, also known as combustion synthesis or CS or self-propagating high temperature synthesis or SHS), powder metallurgy, casting, extrusion, rolling, forging, vapor deposition, droplet deposition techniques, machining, electrodeposition and combinations thereof. Clearly, micropyretic synthesis is the preferred technique, as it requires no energy input, relatively little labor and allows the entire manufacturing process to be carried in relatively few processing steps.

Following are disclosed compositions which result in ceramic, intermetallic or metal ceramic composites with reduced susceptibility to pesting. Ceramic or metal ceramic composites using these particular compositions are manufactured using the technique disclosed in USSN 07/847,782, but may be manufactured using any of the techniques discussed above and by those techniques disclosed in the patents and patent applications discussed above. It will be understood that neither the compositions nor the methods limit the present invention.

Final products were prepared in accordance with the following non-limiting examples:

1 . Added-silica-in-combination-with-excess-silicon:

Two commercial MoSi₂-based heating elements manufactured by Micropyretics Heaters International, Inc. (MHI), were compared for their pesting behavior. Both elements included excess silicon than that required for the formation of MoSi₂. About 0.5 weight percent excess silicon was included. One of the elements contained added SiO₂ in the amount of 0.5 weight percent. When tested for pest formation by exposing the elements simultaneously to 500°C, it was found that the element containing added silica had no pest deposit on the surface, i.e. there was no mechanical disintegration of the element to form MoO₃ or the like. On the other hand, the element containing only excess silicon did show some yellow pest deposit. Products such as the one containing added silica are currently manufactured by MHI and sold under the trademark No- Pest™ and HR 1550SI.

Further experimentation revealed that the decrease in pest was greatly enhanced at 1.5 weight percent added SiO₂. It was preferred that the SiO₂ be in the form of quartz.

2. Ternary compounds

Separate from the development of the added-silica-in- combination-with-excess-silicon aspect of the present invention, it has been discovered, quite unexpectedly, that addition of ternary compounds selected from the group consisting of (ZAI_xSi_y), where Z is an element where x is an integer and y is a whole number, in small amounts to ceramic, intermetallic and metal ceramic composites with a susceptibility to pesting, dramatically reduced or even eliminated the pesting. Preferably, Z is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, W. The preferred ternary compound is MoAl_xSi_y or molybdenum aluminide with silicon. 10 weight percent added MoAl_xSi_y was sufficient to observe the reduction in pest formation. Greater the ternary compound, better the pest behavior. Thus a pure ternary compound showed no pesting at all.

To form MoAl_xSi_yf elemental Mo, Si and Al powders were used as reactants in the following proportions: Mo: 63(g)

Si: 30(g)

Al: 7(g)

The reactant powders were first dry-mixed in a plastic jar for 30 minutes using a Spex Industries 800 Mixer/Mill. The well mixed powders were then uniaxially pressed in a 1 -inch-diameter stainless steel die to form disk shaped green compacts. Micropyretic synthesis was then conducted. A micrograph of the resulting product is shown in Figure 1. Such products are currently manufactured by MHI and sold under the trademark GPF-21™. MoAI₂Si₂ was identified by X-ray diffraction pattern comparison.

MoAI₂Si₂ has a hexagonal (C40-tγpe) which is different from the body centered tetragonal (C1 1 b-type) crystal of MoSi₂, for example.

Therefore, the X-ray diffraction patterns of these two phases are very different and MoAI₂Si₂ was easily identified. MoAI₂Si₂ was tested for pest formation at 400, 500 and 600°C. Pesting was not detected at all these temperatures.

3. Protective laver not comprising predominantly of silica, preferably comprising aluminum oxide

Another surprising discovery was the reduction and/or prevention of pest in ceramic, intermetallic or metal ceramic composites by providing a layer not comprising predominantly of silica. Silica layers are usually and typically formed on ceramic, intermetallic or metal ceramic composites and are protective of the composites. The MoAI₂Si₂ composites, described above, were tested after use at high temperatures and it was found that a layer had been formed on the surface of the composites not comprising predominantly of silica. The better (lower pesting) protective layers were found to comprise aluminum oxide. Aluminum oxide layer containing composites were also found to afford very high temperature stability rarely found in ceramic, intermetallic or metal ceramic composites, at temperatures of about 2100°C. Such products are currently manufactured by MHI and sold under the trademark GPF-21™. High temperature stability at temperatures of about 2000°C, has heretofore been afforded only by lanthanum chromite and zirconia.

Thus, it is apparent that there has been provided, in accordance with the invention, low or no pest ceramic, intermetallic or metal ceramic products, which fully satisfy the objects and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.

Claims

1. A reduced pest ceramic, intermetallic or metal ceramic composite comprising a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon and mixtures thereof, the ceramic, intermetallic or metal ceramic composite further comprising: at least 0.5 percent by weight excess added elemental silicon than that required for formation of said compound; and added silica.

2. The composites of claim 1 , wherein said elemental silicon is present in an amount which is at least 5% by weight in excess and wherein said pest is eliminated.

3. Use of the composites of claim 1 in applications selected from the group consisting of heating elements, products used in VLSI technology, abrasives, cutting tools, polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron alternators; shape-memory alloys; steel melting additives; electrodes for electrolysis of corrosive media; ceramic filters used for filtering undesirable particles from a molten metal before the metal is poured into a pattern or mold; porous ceramic articles used as substrates for catalysts, for removal of contaminants from exhaust gases, for collecting impurities from highly corrosive acids and chemicals; in kiln furniture; aerospace applications; in structural materials in low density load bearing structures; and combinations of these applications.

4. The composite of claim 1 wherein said compound is molybdenum disilicide. 5. A method of reducing pest formation in a ceramic, intermetallic or metal ceramic composite comprising a compound selected from the group consisting of compounds between molybdenum and silicon, tungsten and silicon and mixtures thereof, comprising: adding to said composite, at least 0.

5 percent by weight excess elemental silicon than that required for formation of said compound; and adding silica to said composite.

6. The method of claim 5, wherein at least 5 percent by weight excess elemental silicon is added.

7. Use of the composites of claim 4 or 5, in applications selected from the group consisting of heating elements, products used in VLSI technology, abrasives, cutting tools, polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron alternators; shape-memory alloys; steel melting additives; electrodes for electrolysis of corrosive media; ceramic filters used for filtering undesirable particles from a molten metal before the metal is poured into a pattern or mold; porous ceramic articles used as substrates for catalysts, for removal of contaminants from exhaust gases, for collecting impurities from highly corrosive acids and chemicals; in kiln furniture; aerospace applications; in structural materials in low density load bearing structures; and combinations of these applications.

8. The method of claim 5, wherein said compound is molybdenum disilicide.

9. Use of the composite of claim 1 , at temperatures above 100°C.

10. The use of claim 9, in power generation equipment, automobiles, aircraft engines, spacecraft engines, truck engines, trucks, buses, general off-road transportation units and railroad engines.

1 1 . A reduced pest, ceramic, intermetallic or metal ceramic composite comprising ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element and where x is an integer and y is a whole number.

12. The composite of claim 1 1 , wherein Z is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, W.

13. The composite of claim 1 1 , further comprising quartenary and higher order compounds related to said ternary compounds and alloying additions including dopants and mixtures thereof.

14. The composite of claim 12, wherein said compound is MoAl_xSi_y.

15. Use of the composite of claim 12 as a heating element or a heating device including terminals.

16. The composite of claim 1 1 , including a protective layer, said layer not comprising predominantly of silica.

17. The protective layer of claim 16, said layer having formed during use of said composite.

18. The composite of claim 16, wherein said layer comprises aluminum oxide.

19. Use of the composite of claim 1 1 in high temperature applications above 1800°C.

20. Use of the composite of claim 18 in applications up to 2100°C.

21. The composite of claim 12, further comprising compounds selected from the gt up consisting of other compounds of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, zirconium, niobium, molybdenum, tungsten, and mixtures thereof, and compounds of aluminum.

22. Use of the composite of claim 1 1 , in applications selected from the group consisting of heating elements, products used in VLSI technology, abrasives, cutting tools, polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron alternators; shape-memory alloys; steel melting additives; electrodes for electrolysis of corrosive media; ceramic filters used for filtering undesirable particles from a molten metal before the metal is poured into a pattern or mold; porous ceramic articles used as substrates for catalysts, for removal of contaminants from exhaust gases, for collecting impurities from highly corrosive acids and chemicals; in kiln furniture; aerospace applications; in structural materials in low density load bearing structures; and combinations of these applications.

23. A method of reducing pest in ceramic, intermetallic or metal ceramic composites, comprising: including in said composite, ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element where x is an integer and y is a whole number.

24. The method of claim 23, wherein Z is an element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, W.

25. The method of claim 24, further comprising: including in said composite quartenarγ and higher order compounds related to said ternary compounds and alloying additions including dopants and mixtures thereof.

26. The method of claim 23, further comprising: adding to said composite, at least 0.5 percent by weight excess elemental silicon than that required for formation of said compounds; and adding silica to said composite.

27. The method of claim 26, wherein at least 5 percent by weight excess elemental silicon is added.

28. The composites of claims 1 or 1 1 , having been manufactured by a method selected from the group consisting of micropyretic synthesis, powder metallurgy, casting, extrusion, rolling, forging, vapor deposition, droplet deposition techniques, machining, electrodeposition and combinations thereof.

29. A method of reducing pest in ceramic, intermetallic or metal ceramic composites by providing a layer, said layer not comprising predominantly of silica.

30. The method of claim 29, wherein said layer comprises aluminum oxide.

31. The method of claim 29, wherein said composites comprise ternary compounds and mixtures thereof selected from the group consisting of (ZAI_xSi_y), where Z is an element and where x is an integer and y is a whole number.

32. The method of claim 31 , wherein Z is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, W.

33. The method of claim 32, further comprising: including in said composites quartenary compounds related to said ternary compounds and dopants.

34. The method of claim 32, wherein said ternary compound is Mo(AISi)₂.