Patent Application
20090011266
1. Field of the Invention
The present invention relates to composite materials and, more specifically, to a composite material including molybdenum, molybdenum silicides and molybdenum silicon boride.
2. Description of the Prior Art
Nickel super alloy turbine blades in modern jet engines operate at temperatures approaching 1150° C. Using technologies such as thermal barrier coatings and elaborate cooling schemes, such engine temperatures may be pushed as high as 1500° C. Dramatic increases in the performance of such engines can be gained by operating at higher temperatures, but as more elaborate systems are employed to cool the airfoils, engine efficiency is reduced.
Refractory metals such as molybdenum have high melting points and excellent high temperature mechanical properties; however, the oxidation resistance of these metals is typically poor. Although molybdenum (Mo) by itself can not be used in high-temperature oxidizing environments, compounds of molybdenum and silicon (Si) are known for their excellent oxidation resistance due to the formation of a protective silicate glass. Adding boron (B) improves oxidation resistance by decreasing the viscosity of the glass and promoting better surface coverage. A 1600° C. isothermal section of a Mo—Si—B ternary phase diagram 100 is shown in
A variety of methods for producing Mo—Si—B alloys have emerged. To achieve high strength and fracture toughness, the alloys must be processed in a manner that creates a continuous molybdenum matrix. In addition, a fine dispersion of the Mo—Si—B intermetallic phases is necessary to maintain a continuous protective glass layer. Much of the research has focused on melt-based processing or consolidation of pre-alloyed powders formed by inert gas atomization. Molybdenum has the highest melting point of the three phases in the alloy, causing primary solidification of the molybdenum solid solution. The resulting microstructures produced by these methods are coarse grained with isolated molybdenum regions.
Unfortunately, existing composites employing these phases have grain structures that are too large for many practical applications. Unlike many other structural alloys, Mo—Si—B alloys do not lend themselves to microstructural improvement by heat treating. Their compositional homogeneity over a wide temperature range eliminates the possibility for microstructural reformation via phase transformations. Thus, this system is similar to a ceramic, in that the initial synthesis methods largely dictate the final microstructure.
Powder metallurgy methods may provide an opportunity for microstructure control. However, existing methods of creating Mo—Si—B composite materials by powder processing routes have met with little success. Impurity levels are difficult to control because fine silicon and boron powders are prone to oxidation during processing. Also, the segregation of carbon and oxygen at grain boundaries is known increase the ductile to brittle transition temperature (DBTT) of molybdenum. High residual oxygen levels of 3000 ppm for alloys produced by existing mechanical alloying of elemental powders leads to a significant quantity of silica inclusions in the composite. A glass phase present in the bulk microstructure may also harm high temperature creep resistance.
Therefore, there is a need for a method of producing a composite that includes Molybdenum, A15 and T2 while minimizing surface oxidation.
Therefore, there is a need for a composite that includes Molybdenum, A15 and T2 with small particle sizes.
Therefore, there is a need for highly dense composite structures that include Molybdenum, A15 and T2.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is a method of making a molybdenum, molybdenum silicide and molybdenum silicon boride composite material, in which a boron nitride powder, a silicon nitride powder and a molybdenum powder are mixed to form a composite precursor. The composite precursor is sintered in an atmosphere consisting essentially of hydrogen and an inert gas to form a sintered material. The sintered material is hot isostatic pressed to form the composite material into a final shape.
In another aspect, the invention is a composite material having an outer surface and including a metallic phase continuous molybdenum matrix, a molybdenum silicide intermetallic phase, a molybdenum silicon boride intermetallic phase and a borosilicate glass layer. The metallic phase continuous molybdenum matrix has an average grain size of less than 4.0 microns. The molybdenum silicide intermetallic phase has an average grain size of less than 2.5 microns and suspended in the metallic phase continuous molybdenum matrix. The molybdenum silicon boride intermetallic phase has an average grain size of less than 2.0 microns and suspended in the metallic phase continuous molybdenum matrix. The borosilicate glass layer covers at least a portion of the outer surface.
In yet another aspect, the invention is a mechanical structure that has an outer surface. The mechanical structure includes a composite material that includes a molybdenum silicide intermetallic phase, a molybdenum silicon boride intermetallic phase, and a metallic phase continuous molybdenum solid solution matrix. The composite material has been cold isostatic pressed from Mo, Si3N4 and BN, into a form of the mechanical structure, sintered and hot isostatic pressed so that the composite material has a sintered density that is greater than 94% of theoretical density. A borosilicate glass layer covers at least a portion of the outer surface of the structure.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.”
As shown in
The homogenous powder mixture is cold isostatically pressed 206 to form a green body. (It is understood that the term “green body” is a term commonly used in the ceramic arts, referring to an un-fired ceramic structure. This term does not connote any specific color.) A graph 400 relating green density to pressing pressure, as shown in
After the hot isostatic pressing, the final shape may be exposed to an atmosphere including oxygen at a temperature greater than 1000° C., thereby forming 212 a borosilicate glass layer over at least a portion of an outer surface of the final shape. This borosilicate glass reduces surface oxidation of the shape.
The resulting composite material will have a metallic phase of a continuous molybdenum matrix in which the average grain size is less than 4.0 microns, a molybdenum silicide intermetallic phase (such as A15) in which the average grain size is less than 2.5 microns and that is dispersed in the continuous molybdenum matrix, and a molybdenum silicon boride intermetallic phase (such as T2) in which the average grain size is less than 2.0 microns and that is dispersed in the continuous molybdenum matrix. In the composite, silicon is concentrated in a range of between 1 wt. % to 5 wt. % and boron is concentrated in a range of between ½ wt. % to 2 wt. %. This process also results in the final shape having a sintered density greater than 94% of theoretical density and preferably greater than 99% of theoretical density.
Also, a thin borosilicate glass layer covers at least a portion (and preferably all) of the outer surface. In an alternative embodiment, aluminum may be added to the composite, so that it includes up to 10 wt. % (up to 5 wt. % in most embodiments) of aluminum.
In one embodiment, as shown in
A micrograph 500 of the spray dried powder prior to shape formation is shown in
Employing the above-disclosed methods results in the synthesis and control of the intermetallic phases as fine, uniform dispersions in a continuous molybdenum matrix using powder metallurgical methods. Submicron molybdenum, Si3N4 and BN powders are reacted to form Mo—Si—B alloys. The covalent nitrides are stable in oxidizing environments up to 150° C. to 200° C., allowing for fine particle processing without the formation of silicon and boron oxides. At high temperatures both nitrides have high equilibrium nitrogen partial pressures and small free energies that promote formation of A15 and T2. The intermetallic phases are formed by the following reactions:
3 Mo+1/3 Si3N4→Mo3Si+2/3 N2
5 Mo+1/3 Si3N4+2BN→Mo5SiB2+5/3 N2
The resulting composites have low impurity levels and have microstructures with a fine dispersion of intermetallics in a continuous molybdenum matrix. This processing route allows for microstructural control through adjustments in processing, raw materials and firing parameters. The method disclosed herein uses common powder processing techniques which are standard industry practice, making the process relatively inexpensive and viable for scale up.
Reactant powders for Mo—Si—B alloys used in one experimental embodiment are listed in the following table:
Powder selection criteria were based on high purities and low oxygen contents, as well as being commercially available in large quantities. Submicron powders were chosen to lower sintering temperatures and to maintain a fine grain sizes after firing.
A homogenous dispersion of the starting powders helped to achieve a fine dispersion of the intermetallics phases in the final microstructure. The powders were mixed in acetone with 3 wt. % of low molecular weight methyl methacrylate copolymer, Elvacite 2008 (available from Lucite International), which was added as a dispersant and binder. The Elvacite 2008 resin burns out cleanly during firing by breaking down to the monomer and evaporating away, leading to low residual carbon levels. Stearic acid at 0.3 wt. % was added as a powder lubricant to reduce density gradients in the pressed compacts. The mixtures were milled with Al2O3 media for 30 minutes on a commercial paint shaker to break up agglomerates and improve dispersion. The slurries were then spray dried in a small laboratory spray dryer (a BÜCHI Model 190) to maintain the fine dispersion of the starting powders. The spray dried powders were screened to separate the spherical granules, which give a more uniform fill and reduce density gradients in dry pressing operations. Powder compacts were uniaxially pressed to 480 MPa in a ½″ die using an automated hydraulic press to ensure constant conditions for the loading rate and hold time.
The resulting powder compacts were fired in a sealed atmosphere 1600° C. tube furnace. The firing profile was computer controlled and monitored by a thermocouple placed directly above the center of the tube. Samples were heated at 3° C./min with a six hour hold at temperature. Oxygen concentrations in the samples were minimized by firing in a reducing atmosphere of Ar-10% H2. Hydrogen reduces molybdenum oxides, leading to low residual oxygen levels. Ultra-high purity grade gases (available from Airgas Products) had measured oxygen levels of 45 ppm and the inlet gas stream was purified using a titanium gettering furnace which reduced the measured oxygen content in the furnace outlet to value of 0.0 ppm.
Impurity levels of oxygen, carbon and nitrogen were measured for a Mo-3 Si-1B wt. % sample fired at 1600° C. for six hours. The measured oxygen values of 300-400 ppm were lower than reported previously for Mo—Si—B alloys produced using powder metallurgy. Low oxygen levels are important for limiting silica inclusions in the microstructure.
Pellets with varying Mo contents were fired between 1200° C. and 1600° C. with six hour holds at each set point. The densities of the fired samples were measured using the Archimedes method. The higher molybdenum content samples initially densified at a more rapid rate. The difference in sintering behavior may be due to the higher volume fraction of the molybdenum phase. After reaction, further densification of the alloy may be inhibited by the presence of the intermetallic phases. Ultimately, the compositions achieved relative densities of 95% of theoretical. At the maximum furnace temperature of 1600° C., densities have not leveled off and increasing the sintering temperature would result in higher densities. However, this would also lead to increased grain growth.
To achieve densities approaching theoretical while maintaining a fine grain size, hot-isostatic pressing (HIP) was used after sintering. For samples above about 94% theoretical density the discontinuous porosity allows for HIPing without encapsulation. Mo-3Si-1B wt. % samples were pre-fired at 1600° C. and then HIPed at 1400° C. and 207 MPa for six hours (using a press available from American Isostatic Presses, Columbus, Ohio). The average density of the samples increased from 94.1 to 99.4% of theoretical with no significant grain growth.
In reaction studies, Molybdenum, Si3N4 and BN powders were combined to yield stoichiometric mixtures of the A15 and T2 phases. Samples were heated in Ar-5% H2 at 3° C./min to match the firing conditions used in the furnace. Weight loss occurred up to 885° C. due to evaporation of molybdenum trioxide or by reduction of oxide. The onset of the reactions with the nitrides is evidenced by a second stage of weight loss due to the evolution of nitrogen from the mixtures. The A15 precursor mixture began reacting at 1193° C. and the T2 precursor mixture began reacting at 1140° C.
The formation of Mo—Si—B alloys by reaction of molybdenum, Si3N4 and BN powders has been demonstrated to produce Mo-T2-A15 composites with pressureless sintered densities greater than 94% of theoretical density. Use of the nitrides allows for fine particle processing both in forming steps with organic additives and sintering without oxidation. Overall impurity contents were maintained at low levels. The methods described provide a means for creating these materials in a much less complex and expensive manner than has been previously demonstrated. The intermetallic phases improve high-temperature creep resistance and provide oxidation resistance by forming a protective borosilicate glass surface layer.
It should be noted that by: (1) controlling particle size and shape of new material; (2) controlling heat treatment temperature and time; and (3) employing hot isostatic conditions, reduction of Mo, A15 and T2 grain sizes to 0.1 microns may be achieved while achieving densities of greater than 99% of theoretical.
The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/947,503, filed Jul. 2, 2007, the entirety of which is hereby incorporated herein by reference.
This invention was made with support from the U.S. government under grant number NAVAIR N00421-041-0002, awarded by the Office of Naval Research. The government may have certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60947503 | Jul 2007 | US |
