Oxidation resistance of molybdenum silicon boride composite

Abstract

Molybdenum composites containing silicon and boron for environmental resistance are combined so as to minimize the silicon solid solution in the molybdenum phase. The composites include ratios of molybdenum, silicon, and boron to form three phase mixtures of molybdenum, A15 (Mo.sub.3Si), and T2 (Mo.sub.5SiB.sub.2) or molybdenum, SiO.sub.2, and T2 (Mo.sub.5SiB.sub.2). Beneficial additives, including manganese and strontium aluminosilicate, are included to improve the composite's properties. Manufacturing processes to produce these composites as either powders or solid parts are included.

Claims

1. The process of forming a molybdenum silicon boride reacted high density composite part which comprises: a. forming a slurry of molybdenum, silicon oxide, silicon nitride, boron nitride, and a liquid; b. spray drying the slurry to form a homogenous, unreacted powder mixture; c. reacting the homogeneous powder mixture in a reducing atmosphere at an elevated temperature to form a three phase composite powder of Mo.sub.ss, silicon oxide and T2 (Mo.sub.5SiB.sub.2), in which the Mo.sub.ss contains less than about 3 atomic percent silicon in solution in equilibrium with the T2; d. compacting the reacted composite powder; and e. sintering the compacted reacted composite powder at an elevated temperature and pressure to form a reacted high density composite part.

2. The process of claim 1 in which the reacted high density composite part has a volume percent of about 50% to about 65% Moss, the balance of the high density composite part being composed of silicon oxide and T2 in a ratio of silicon oxide to T2 of about 0.5 to about 3.5 on a volume basis.

3. The process of claim 1 in which one or more additives is included in the powders forming the slurry.

4. The process of claim 3 in which at least one additive is manganese in an amount less than about 2 atomic percent of the total composite.

5. The process of claim 4 in which at least one additive is manganese in an amount of about 0.5 to about 1.5 atomic percent of the total composite.

6. The process of claim 3 in which at least one additive is an alkaline earth aluminosilicate in an amount less than about 2 volume percent of the total composite.

7. The process of claim 6 in which at least one additive is an alkaline earth aluminosilicate in an amount of about 0.25 to about 1.75 volume percent of the total composite.

8. The process of claim 6 wherein the alkaline earth aluminosilicate is strontium aluminosilicate.

9. The process of claim 1 wherein the Moss in the reacted composite powder contains less than 1.5 atomic percent silicon in solution in equilibrium with the T2.

10. The process of claim 1 wherein the silicon nitride is Si.sub.3N.sub.4 and the boron nitride is BN.

11. The process of forming a molybdenum silicon boride reacted high density composite part which comprises: a. forming a slurry of molybdenum, silicon oxide, silicon nitride, boron nitride, and a liquid; b. spray drying the slurry to form a homogenous, unreacted powder mixture; c. compacting the reacted composite powder; and d. reacting the homogeneous powder mixture in a reducing atmosphere at an elevated temperature to form a three phase composite powder of Mo.sub.ss, silicon oxide and T2 (Mo.sub.5SiB.sub.2), in which the Mo.sub.ss contains less than about 3 atomic percent silicon in solution in equilibrium with the T2; e. sintering the compacted reacted composite powder at an elevated temperature and pressure to form a reacted high density composite part.

12. The process of claim 11 in which the reacted high density composite part has a volume percent of about 50% to about 65% Moss, the balance of the high density composite part being composed of silicon oxide and T2 in a ratio of silicon oxide to T2 of about 0.5 to about 3.5 on a volume basis.

13. The process of claim 11 in which one or more additives is included in the powders forming the slurry.

14. The process of claim 13 in which at least one additive is manganese in an amount less than about 2 atomic percent of the total composite.

15. The process of claim 14 in which at least one additive is manganese in an amount of about 0.5 to about 1.5 atomic percent of the total composite.

16. The process of claim 13 in which at least one additive is an alkaline earth aluminosilicate in an amount less than about 2 volume percent of the total composite.

17. The process of claim 16 in which at least one additive is an alkaline earth aluminosilicate in an amount of about 0.25 to about 1.75 volume percent of the total composite.

18. The process of claim 16 wherein the alkaline earth aluminosilicate is strontium aluminosilicate.

19. The process of claim 11 wherein the Moss in the reacted composite powder contains less than 1.5 atomic percent silicon in solution in equilibrium with the T2.

20. The process of claim 11 wherein the silicon nitride is Si.sub.3N.sub.4 and the boron nitride is BN.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram illustrating the fabricating and combining of intermetallic powder with molybdenum powder.

(2) FIG. 2 is a block diagram illustrating the combining process of Mo.sub.5SiB.sub.2 (T2), Mo.sub.3Si (A15), and molybdenum powders.

(3) FIG. 3A shows a phase diagram of molybdenum and silicon.

(4) FIG. 3B shows a closer view of the origin of the phase diagram in FIG. 3A.

(5) FIG. 4 is a schematic or notional representation of molybdenum composite fabricated using the described process.

(6) FIG. 5 is schematic or notional representation of a layered composite fabricated using the described process.

(7) FIG. 6 shows molybdenum combined with beneficial additions.

(8) FIG. 7 is a block diagram illustrating the combining process of T2, SiO.sub.2, and molybdenum powders.

(9) FIG. 8 is a portion of a ternary phase diagram for molybdenum, boron, and silicon at 1600 C.

(10) FIG. 9 is a block diagram illustrating the fabrication steps for producing a high density part.

(11) FIG. 10 is an alternate block diagram illustrating another embodiment of the fabrication steps for producing a high density part.

(12) FIG. 11 is an alternate block diagram illustrating another embodiment of the fabrication steps for producing a high density part

DETAILED DESCRIPTION OF THE DRAWINGS

(13) This invention relates to a low temperature process that controls the formation of molybdenum solid solution (Mo.sub.ss) and allows for fabrication of novel MoSiB composites not possible or envisioned by the prior art. Further disclosed is one or more additives that improve high temperature oxidation resistance, glass formation, ductility.

(14) Molybdenum composites containing silicon and boron for environmental resistance are combined so as to minimize the silicon solid solution in the molybdenum phase. The composites include ratios of molybdenum, silicon, and boron to form three phase mixtures of molybdenum, A15 (Mo.sub.3Si), and T2 (Mo.sub.5SiB.sub.2) or molybdenum, SiO.sub.2, and T2 (MosSiB.sub.2). Beneficial additives, including manganese and strontium aluminosilicate, are included to improve the composite's properties. Manufacturing processes to produce these composites as either powders or solid parts are included.

(15) One process allows for the separate fabrication of beneficial intermetallics or other components which are combined with pure molybdenum to form a ductile, high temperature, oxidation resistant composite.

(16) To achieve ductility at room temperature it is desirable to control the formation of molybdenum solid solution (Mo.sub.ss). This is not possible with prior art melt process or powder process which require temperatures in excess of 1500 C. to form the desirable intermetallics, for example Mo.sub.3Si (A15) and Mo.sub.5SiB.sub.2(T2). Typically molybdenum, silicon, boron, or compounds thereof are mixed and heated at a high temperature. During the process, an equilibrium distribution of molybdenum, silicon, and boron are naturally formed as defined by the molybdenum, silicon, boron ternary phase diagram. Mo.sub.ss is formed along with various beneficial intermetallics.

(17) The MoSiB fabrication method disclosed U.S. Patent Application Publication No. 2009/0011266 is relatively simple and low cost. However, it suffers from the same problem as other MoSiB fabrication methods in that during high temperature processing, above about 1500 C., molybdenum solid solution (Mo.sub.ss) of about 1.5 to 2.0 atomic percent of silicon is formed. This and other processes result in a MoSiB composition that has a ductile to brittle transition temperature (DBTT) of about 1200 C. This means that below 1200 C., the material is brittle. Additional process refinements and additives such as zirconium may reduce the DBTT from about 800 C. to 1000 C. However, because the MoSiB alloy is brittle at room temperature, it has limited commercial use. For example higher concentrations A15 and T2 with respect to the molybdenum matrix will result in higher strength and better oxidation resistance. Lower concentrations of these two phases with respect to the molybdenum matrix will result in a more ductile composite.

(18) In one embodiment, there is provided a process for controlling the formation of Mo.sub.ss by separately preparing particles of selected intermetallics and combining them with pure molybdenum as shown in FIG. 1. This allows for the selected intermetallics to be formed at a high temperature in step 101 and the intermetallics combined with molybdenum at a lower temperature in step 102. The steps of FIG. 1 may be accomplished using nitride precursor powder processing techniques as described herein.

(19) In a preferred embodiment, intermetallic phases T2 and A15 are formed in two separate process steps 103a and 103b as shown in FIG. 2. In one specific embodiment, both T2 and A15 are formed together in the same step. In the T2 103a forming process step of FIG. 2, submicron sized powders of silicon nitrides, boron nitrides, and molybdenum are combined in the presence of heat with the following reaction:

(20) $5\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4+2\mathrm{BN}.fwdarw.{\mathrm{Mo}}_5\mathrm{SiB}+\frac{5}{3}{N}_2$

(21) The composite precursors of T2 may be milled prior to the sintering action to break up agglomerates of the boron nitride powder, the silicon nitride powder, and the molybdenum powder. The silicon nitride powder and the molybdenum powder may be mixed with an organic or inorganic liquid such as acetone or water to form a suspension. An organic dispersant and binder, such as a methyl methacrylate polymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension. The suspension may be spray dried to form a homogenous powder mixture. The homogenous powder mixture is fired at a temperature sufficient to complete the reaction. This includes temperatures above 1100 C. with temperatures above 1500 C. being preferred.

(22) In the A15 forming step 103b of FIG. 2, submicron sized powders of silicon nitrides and molybdenum are combined in the presence of heat with the following reaction:

(23) $3\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4.fwdarw.{\mathrm{Mo}}_3\mathrm{Si}+\frac{2}{3}{N}_2$

(24) The composite precursor of A15 may be milled prior to the sintering action to break up agglomerates of the boron nitride powder, the silicon nitride powder and the molybdenum powder. The silicon nitride powder and the molybdenum powder may be mixed with a liquid (such water, acetone, or other organic or inorganic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate polymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension. The suspension may be spray dried to form a homogenous powder mixture. The homogenous powder mixture is fired at temperatures sufficient to complete the reaction. This includes temperatures above 1000 C. with temperatures above 1500 C. being preferred.

(25) In the combining process 104 of FIG. 2, T2, A15, and molybdenum powders are combined. This composition may be ball milled to break up agglomerates. The powder may be mixed with a liquid (such as water, acetone, or other organic or inorganic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension. The suspension may be spray dried to form a homogenous powder mixture. The powder mixture may be sintered at temperature below 1400 C., more preferably at or below 1300 C., which will greatly reduce the silicon concentration in the Mo.sub.ss present. The sintered powder may be subsequently processed to form parts.

(26) Alternatively, prior to sintering the spray dried powder may be pressed in step 105 to form a shape, sintered and Hot Isostatic Pressed in step 106. Sintering and Hot Isostatic Pressing at temperatures below 1400 C., more preferably at or below 1300 C., will greatly reduce the silicon concentration in the Mo.sub.ss present.

(27) FIG. 3A shows a phase diagram of molybdenum and silicon. FIG. 3B shows a closer view of the origin. It may be seen from these diagrams that if sintering temperatures are kept below about 1400 C., less than 3 atomic percent silicon, typically about 2 atomic percent silicon will form a solution with molybdenum. If the sintering temperatures are near or below 1350 C., less than 1.2 atomic percent silicon will form a solution with molybdenum.

(28) The resultant MoSiB solid composite is different from the prior art in that it is free of the equilibrium phase constraints imposed by simultaneously heating all the starting material to a high temperature, for example 1600 C. or greater. The resultant material of the process described in FIG. 2 is characterized by a continuous phase of pure molybdenum surrounding select intermetallics of A15 and T2. The silicon concentration in the Mo.sub.ss is minimized. The resulting material has strength and oxidation resistance at high temperature while exhibiting good ductility. The process allows for tailored compositions of MoSiB. Among other things, separately forming A15 and T2 allows for control of grain growth and particles size of these intermetallics. The components, A15, T2, and molybdenum may be combined in any beneficial ratio.

(29) Using the described process the envisioned composites include molybdenum matrix representing between about 50% and about 65% by volume the balance of the composite being composed of A15 and T2 in a ratio of A15 to T2 of about 0.5 to about 3.5 on a volume basis.

(30) Ratios of the three phases may be varied throughout a fabricated article to form a gradient. For example an article may be fabricated to include more A15 and/or T2 at the surface and less toward the center. This may be accomplished through spraying various lay ers of material characterized by different concentrations of A15, T2, and molybdenum onto an article. Other methods are possible.

(31) FIG. 4 is a schematic or notional representation of molybdenum composite fabricated using the described process. An essentially continuous matrix of molybdenum 107 surrounds intermetallic particles of T2 and A15, elements 108 and 109 as shown in FIG. 4.

(32) FIG. 5 is a schematic or notional representation of a layered composite fabricated using the described process. A continuous matrix of molybdenum 110 and a heavy concentration of T2 and A15, elements 111 and 112 are shown near the exterior surface with a reduced concentration of T2 and A15 toward the center. In this embodiment a higher concentration of T2 and A15 toward the exterior surface will promote oxygen resistance at the surface while a reduced concentration at the center of the composite will promote improved ductility.

Beneficial Additions

(33) In another embodiment as shown in FIG. 6, intermetallics 119, beneficial additives 113 are combined with molybdenum powder. In certain preferred embodiments, it is advantageous to add a reactive element such as titanium, zirconium, hafnium, and/or aluminum to the alloy to: (1) promote wetting of the borosilicate layer once it has formed, (2) raise the melting point of the borosilicate, (3) form a more refractory oxide layer below the initial borosilicate layer further impeding oxygen transport to the molybdenum matrix (4) strengthen the composite.

(34) Envisioned additives include but are not limited to: Al, C, Cr, Hf, Ir, Mo, Nb, Os, Re, Rh, Ru, Si, Ta, Ti, V, W, Zr, compositions thereof including but not limited to oxides, nitrides and carbides. Additional additives include fluxing agents including but not limited to, Na, Ca, Mg, Sr, Ba, Pd, Zn, and compositions thereof compositions thereof including but not limited to oxides, nitrides and carbides. In one embodiment, select transition metals are incorporated including, but not limited to, Sc, Mn, Fe, Co. Ni, Cu, Zn and compositions thereof including but not limited to oxides, nitrides and carbides.

(35) Other potential additives include selections from the alkali metals, alkaline earth metals, post transition metals, lanthanides, actinides, metalloids, and non-metals. Manganese added in small amounts of less than about 2 atomic percent, about 0.5 to about 1.5 atomic percent, of the composite have been shown to improve the overall all oxidation resistance of the MoSiB composition. Adding an alkaline earth aluminosilicate such as strontium aluminosilicate (1SrO-1Al.sub.2O.sub.3-2SiO.sub.2) in an amount less than about 2 volume percent, about 0.25 to about 1.75 volume percent, of the total composite or in combination with (Mn) has demonstrated good oxidation resistance.

(36) Another preferred embodiment is shown in the process of FIG. 7. This process is essentially identical to the process described in FIG. 2 except that A15 is replaced by SiO.sub.2.

(37) In the combining process of FIG. 7, T2, SiO.sub.2, and molybdenum powders are combined in step 115. This composition may be ball milled to break up agglomerates. The powder may be mixed with a liquid (such as water, acetone, or other organic or inorganic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension. The suspension may be spray dried to form a homogenous powder mixture.

(38) The spray dried powder may be pressed to form a shape in step 116 and sintered and Hot Isostatic Pressed in step 117. Temperatures below 1400 C., preferably at or below 1300 C., will reduce the silicon concentration in the Mo.sub.ss present.

(39) In the MoSiB system, the A15 phase provides silicon to form silica coating when exposed to high temperature. However A15 is a source of silicon to form Mo.sub.ss because it is silica and not silicon which is ultimately desired, the introduction of silica ab initio into the microstructure will allow the formation of a protective scale while avoiding the presence of the A15 and thus alleviating the formation of Mo.sub.ss. In addition to SiO.sub.2 other beneficial additions may be included as described in FIG. 6.

(40) FIG. 9 shows one method for producing MoSiB with reduced silicon content in the Mo.sub.ss phase. It comprises of the steps of forming MoSiB powder. Optionally the powder may be compacted and sintered to from a part or slug

(41) Step 120 comprises combining precursor powders which when heated will react to form MoSiB. These submicron powders include, but are not limited to: boron nitride (BN), silicon nitride (Si.sub.3N.sub.4), and molybdenum. These powders are added in such a ratio as to form beneficial amounts of T2 and A15 in a continuous matrix of molybdenum. T2 and A15 are formed in the presence of heat via the following reactions:

(42) $\begin{array}{c}\text{T2:}5\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4+2\mathrm{BN}.fwdarw.{\mathrm{Mo}}_5{\mathrm{SiB}}_2+\frac{5}{3}{N}_2\\ \text{A15:}3\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4.fwdarw.{\mathrm{Mo}}_3\mathrm{Si}+\frac{2}{3}{N}_2\end{array}$

(43) Other additives may be included in this step. Additives known in the prior art may be included to: promote wetting of the borosilicate layer once it has formed, raise the melting point of the borosilicate, form a more refractory oxide layer below the initial borosilicate layer further impeding oxygen transport to the molybdenum matrix, and strengthen the composite.

(44) Step 121 comprises forming a slurry. The precursor powders of Step 120 are dispersed or dissolved in a liquid (such as acetone, or other organic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension.

(45) Step 122 comprises milling the suspension to break up agglomerates of the boron nitride powder, the silicon nitride powder, and the molybdenum powder.

(46) Step 123 comprises spray drying the slurry to form a homogenous powder mixture.

(47) Step 124 comprises reaction sintering the homogenous powder in a reducing atmosphere, including but not limited to, hydrogen at a temperature at least below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. The resulting powder consists essentially of phases: T2, A15 and only trace amounts of silicon in the Mo.sub.ss. When fired at about 1400 C. there is 2% or fewer atoms of silicon in the Mo.sub.ss phase. When fired at about 1300 C. there is about 1.2% or fewer atoms of silicon in the Mo.sub.ss phase.

(48) Step 125 comprises storing the material in oxygen free atmosphere. At this stage, care should be taken to limit the exposure of this material to air. The high surface area of the powder is susceptible to oxidation. Viable storage methods include, but are not limited to, vacuum bagging.

(49) Step 126 comprises optionally milling the powder to break up large agglomerates formed as a result of sintered necks at particle-particle contact points.

(50) Optionally, a part or slug may be formed from the powder using standard powder processing methods. These include, but are not limited to, Step 127 and Step 128.

(51) Step 127 comprises optionally compacting the powder. This may be completed in an inert atmosphere. Potential compacting methods include cold isostatic pressing at above about 10,000 psi and temperatures below about 200 C. Vibratory methods may also be used to compact the powder into a mold or form.

(52) Step 128 comprises optionally sintering the powder in an inert or reducing environment at a temperature below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. To achieve a dense part, it is desirable to sinter under a pressure of about 10,000 psi or greater with a more preferable pressure of about 50,000 psi. The resulting sintered part is at least 98% of the 100% theoretical density and has substantially reduced silicon in the Mo.sub.ss phase. When fired at about 1400 C. there is 2% or less atoms of silicon in the Mo.sub.ss phase. When fired at 1300 C. there is about 1.2% or less atoms of silicon in the Mo.sub.ss phase.

(53) FIG. 10 shows another method for producing MoSiB with reduced silicon content in the Mo.sub.ss phase. It comprises of the steps of forming MoSiB powder. Optionally the powder may be compacted and sintered to from a part or slug

(54) Step 129 comprises combining precursor powders which when heated will react to form MoSiB. These submicron powders include, but are not limited to: boron nitride (BN), silicon nitride (Si.sub.3N.sub.4), and molybdenum. These powders are added in such a ratio as to form beneficial amounts of T2 and A15. T2 and A15 are formed in the presence of heat via the following reactions:

(55) $\begin{array}{c}\text{T2:}5\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4+2\mathrm{BN}.fwdarw.{\mathrm{Mo}}_5{\mathrm{SiB}}_2+\frac{5}{3}{N}_2\\ \text{A15:}3\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4.fwdarw.{\mathrm{Mo}}_3\mathrm{Si}+\frac{2}{3}{N}_2\end{array}$

(56) Other additives may be included in this step. Additives known in the prior art may be included to: promote wetting of the borosilicate layer once it has formed; raise the melting point of the borosilicate; form a more refractory oxide layer below the initial borosilicate layer further impeding oxyg en transport to the molybdenum matrix; and strengthen the composite.

(57) Step 130 comprises forming a slurry. The precursor powders of Step 129 are dispersed or dissolved in a liquid (such as acetone, or other organic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension.

(58) Step 131 comprises milling the suspension to break up agglomerates of the boron nitride powder, the silicon nitride powder, and the molybdenum powder.

(59) Step 132 comprises spray drying the slurry to form a homogenous powder mixture.

(60) Step 133 comprises reaction sintering the homogenous powder in a reducing atmosphere, including but not limited to, hydrogen at a temperature at least below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. The resulting powder consists essentially of phases: T2, A15 and only trace amounts of silicon in the Mo.sub.ss. When fired at 1 about 400 C. there is 2% or fewer atoms of silicon in the Mo.sub.ss phase. When fired at about 1300 C. there is about 1.2% or fewer atoms of silicon in the Mo.sub.ss phase.

(61) Step 134 comprises storing the material in oxygen free atmosphere. At this stage, care should be taken to limit the exposure of this material to air. The high surface area of the powder is susceptible to oxidation. Viable storage methods include, but are not limited to, vacuum bagging.

(62) Step 135 comprises combining precursor powders from step 134 with additional precursor powders which when heated will react to form MoSiB. These submicron powders include, but are not limited to: A15, T2, boron nitride (BN), silicon nitride (Si.sub.3N.sub.4), and molybdenum. These powders are added in such a ratio as to form beneficial amounts of T2 and A15 in a continuous matrix of molybdenum.

(63) Other additives may be included in this step. Additives known in the prior art may be included to: promote wetting of the borosilicate layer once it has formed; raise the melting point of the borosilicate; form a more refractory oxide layer below the initial borosilicate layer further impeding oxygen transport to the molybdenum matrix; and strengthen the composite.

(64) Step 136 comprises forming a slurry. The precursor powders of Step 135 are dispersed or dissolved in a liquid (such as acetone, or other organic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as steari c acid, may also be added to the suspension.

(65) Step 137 comprises milling the suspension to break up agglomerates of the A15 powder, the T2 powder, the boron nitride powder, the silicon nitride powder, and the molybdenum powder.

(66) Step 138 comprises spray drying the slurry to form a homogenous powder mixture.

(67) Optionally, a part or slug may be formed from the powder using standard powder processing methods. These include, but are not limited to, Step 139 and Step 140.

(68) Step 139 comprises optionally compacting the powder. This may be completed in an inert atmosphere. Potential compacting methods include cold isostatic pressing at above about 10,000 psi and temperatures below about 200 C. Vibratory methods may also be used to compact the powder into a mold or form.

(69) Step 140 comprises optionally sintering the powder in an inert or reducing environment at a temperature below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. To achieve a dense part, it is desirable to sinter under a pressure of about 10,000 psi or greater with a more preferable pressure of about 50,000 psi. The resulting sintered part is at least 98% of the 100% theoretical density and has substantially reduced silicon in the Mo.sub.ss phase. When fired at about 1400 C. there is 2% or less atoms of silicon in the Mo.sub.ss phase. When fired at 1300 C. there is about 1.2% or less atoms of silicon in the Mo.sub.ss phase.

(70) FIG. 11 shows another method for producing MoSiB with reduced silicon content in the Mo.sub.ss phase. It comprises of the steps of forming MoSiB powder. Optionally the powder may be compacted and sintered to from a part or slug

(71) Step 141 comprises combining precursor powders which when heated will react to form MoSiB. These submicron powders include, but are not limited to: boron nitride (BN), silicon nitride (Si.sub.3N.sub.4), and molybdenum. These powders are added in such a ratio as to form beneficial amounts of T2 and A15. T2 and A15 are formed in the presence of heat via the following reactions:

(72) $\begin{array}{c}\text{T2:}5\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4+2\mathrm{BN}.fwdarw.{\mathrm{Mo}}_5{\mathrm{SiB}}_2+\frac{5}{3}{N}_2\\ \text{A15:}3\mathrm{Mo}+\frac{1}{3}{\mathrm{Si}}_3{N}_4.fwdarw.{\mathrm{Mo}}_3\mathrm{Si}+\frac{2}{3}{N}_2\end{array}$

(73) Other additives may be included in this step. Additives known in the prior art may be included to: promote wetting of the borosilicate layer once it has formed; raise the melting point of the borosilicate; form a more refractory oxide layer below the initial borosilicate layer further impeding oxygen transport to the molybdenum matrix; and strengthen the composite.

(74) Step 142 comprises forming a slurry. The precursor powders of Step 129 are dispersed or dissolved in a liquid (such as acetone, or other organic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension.

(75) Step 143 comprises milling the suspension to break up agglomerates of the boron nitride powder, the silicon nitride powder, and the molybdenum powder.

(76) Step 144 comprises spray drying the slurry to form a homogenous powder mixture.

(77) Step 145 comprises reaction sintering the homogenous powder in a reducing atmosphere, including but not limited to, hydrogen at a temperature at least below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. The resulting powder consists essentially of phases: T2, A15 and only trace amounts of silicon in the Mo.sub.ss. When fired at 1 about 400 C. there is 2% or fewer atoms of silicon in the Mo.sub.ss phase. When fired at about 1300 C. there is about 1.2% or fewer atoms of silicon in the Mo.sub.ss phase.

(78) Step 146 comprises storing the material in oxygen free atmosphere. At this stage, care should be taken to limit the exposure of this material to air. The high surface area of the powder is susceptible to oxidation. Viable storage methods include, but are not limited to, vacuum bagging.

(79) Step 147 comprises combining precursor powders from step 146 with additional precursor powders which when heated will react to form MoSiB. These submicron powders include, but are not limited to: A15, T2, boron nitride (BN), silicon nitride (Si.sub.3N.sub.4), and molybdenum. These powders are added in such a ratio as to form beneficial amounts of T2 and A15 in a continuous matrix of molybdenum.

(80) Other additives may be included in this step. Additives known in the prior art may be included to: promote wetting of the borosilicate layer once it has formed; raise the melting point of the borosilicate; form a more refractory oxide layer below the initial borosilicate layer further impeding oxygen transport to the molybdenum matrix; and strengthen the composite.

(81) Step 148 comprises forming a slurry. The precursor powders of Step 135 are dispersed or dissolved in a liquid (such as acetone, or other organic liquid) to form a suspension. An organic dispersant and binder, such as a methyl methacrylate copolymer, may be added to the suspension. A lubricant, such as stearic acid, may also be added to the suspension.

(82) Step 149 comprises milling the suspension to break up agglomerates of the A15 powder, the T2 powder, the boron nitride powder, the silicon nitride powder, and the molybdenum powder.

(83) Step 150 comprises spray drying the slurry to form a homogenous powder mixture.

(84) Step 151 comprises sintering the powder in an inert or reducing environment at a temperature below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. To achieve a dense part, it is desirable to sinter under a pressure of about 10,000 psi or greater with a more preferable pressure of about 50,000 psi. The resulting sintered part is at least 98% of the 100% theoretical density and has substantially reduced silicon in the Mo.sub.ss phase. When fired at about 1400 C. there is 2% or less atoms of silicon in the Mo.sub.ss phase. When fired at 1300 C. there is about 1.2% or less atoms of silicon in the Mo.sub.ss phase.

(85) Optionally, a part or slug may be formed from the powder using standard powder processing methods. These include, but are not limited to, Step 152, Step 153 and Step 154.

(86) Step 152 comprises milling the suspension to break up agglomerates of the reacted MoSiB powder.

(87) Step 153 comprises optionally compacting the powder. This may be completed in an inert atmosphere. Potential compacting methods include cold isostatic pressing at above about 10,000 psi and temperatures below about 200 C. Vibratory methods may also be used to compact the powder into a mold or form.

(88) Step 154 comprises optionally sintering the powder in an inert or reducing environment at a temperature below about 1400 C. more preferably below about 1350 C. and even more preferably below about 1300 C. To achieve a dense part, it is desirable to sinter under a pressure of about 10,000 psi or greater with a more preferable pressure of about 50,000 psi. The resulting sintered part is at least 98% of the 100% theoretical density and has substantially reduced silicon in the Mo.sub.ss phase. When fired at about 1400 C. there is 2% or less atoms of silicon in the Mo.sub.ss phase. When fired at 1300 C. there is about 1.2% or less atoms of silicon in the Mo.sub.ss phase.

(89) The foregoing description of various preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims to be interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.