Mo-Si-B manufacture

Abstract

A method for controlling the formation of molybdenum solid solution in MoSiB composites which comprises processing at 1400 C. or less to minimize, if not prevent, the silicon from going into solid solution in the molybdenum.

Claims

1. The process of forming a molybdenum silicon boron composite which includes the steps of: combining precursor powders and heating to form MoSiB; forming a slurry of precursor powders and liquid; spray drying the slurry to form a homogenous powder mixture; sintering the homogenous powder in a reducing atmosphere; compacting the powder; and sintering the compacted powder.

2. The process of claim 1 in which the precursor powders include molybdenum, silicon nitride, and boron nitride.

3. The process of claim 1 in which the slurry is ball milled prior to spraying.

4. The process of claim 1 in which spray drying parameters of drying temperature and slurry feed rate are controlled to achieve a powder particle size below 100 microns.

5. The process of claim 4 in which the spray drying parameters of drying temperature and feed rate are controlled to achieve a powder particle size between 10 to 60 microns.

6. The process of claim 1 in which teh sintering of the homogeneous powder is at a temperature below 1400 C.

7. The process of claim 1 in which the sintering of the homogeneous powder is at a temperature below 1350 C.

8. The process of claim 1 in which the sintering of the homogeneous powder is at a temperature below 1300 C.

9. The process of claim 1 in which the reducing atmosphere is hydrogen.

10. The process of claim 1 in which the reducing atmosphere is carbon monoxide.

11. The process of claim 1 in which the sintened compacted powder is stored under vacuum.

12. The process of claim 1 in which the powder is milled prior to compacting.

13. The process of claim 1 in which the compacting and sintering are conducted in a reduced oxygen atmosphere.

14. The process of claim 1 in which the compacting is achieved through cold isostatic pressing at a pressure above 10,000 psi.

15. The process of claim 1 in which the sintering of the compacted powder is conducted at a temperature below 1400 C.

16. The process of claim 1 in which the sintering of the compacted powder is conducted at a temperature below 1300 C.

17. The process of claim 1 in which the sintering of the compacted powder is conducted under a pressure greater than 10,000 psi.

18. The process of claim 1 in which the sintering of the compacted powder is conducted under a pressure greater than 50,000 psi.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram illustrating the fabrication steps for producing a high density part or article of manufacture.

(2) FIG. 2 is a portion of a ternary phase diagram for molybdenum, boron, and silicon at 1600 C.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) FIG. 1 shows the method steps 101, 102, 103, 104, 105, 106, 107, 108 and 109 for producing MoSiB with reduced silicon content in the Mo.sub.ss phase. It consists of the steps of forming MoSiB powder. Optionally the powder may be compacted and sintered to from a part or slug

(4) Step 101 comprises combining precursor powders which when heated will react to form Mo-Si-B. These submicron powders include at least 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.

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

(6) Other additives may be included in this step. Additives known in the prior art may be included 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.

(7) Step 102 comprises forming a slurry. The precursor powders of Step 101 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, can be added to the suspension. A lubricant, such as stearic acid, can also be added to the suspension.

(8) Step 103 comprises optionally milling the suspension to break up agglomerates of the boron nitride powder, the silicon nitride powder and the molybdenum powder.

(9) Step 104 comprises spray drying the slurry to form a homogenous powder mixture.

(10) Step 105 comprises reaction sintering the homogenous powder in a reducing atmosphere including but not limited to hydrogen at a temperature at least below 1400 C. more preferably below 1350 C. and even more preferably below 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 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.

(11) Step 106 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.

(12) Step 107 comprises milling the powder to break up large agglomerates formed as a result of sintered necks at particle-particle contact points

(13) Optionally, a part or slug may be formed from the powder using standard powder processing methods. These include but are not limited to Step 108 and Step 109.

(14) Step 108 comprises compacting the powder. This may be done in an inert atmosphere. Potential compacting methods include cold isostatic pressing at above 10,000 psi and temperatures below 200 C. Vibratory methods may also be used to compact the powder into a mold or form.

(15) Step 109 comprises sintering the powder in an inert or reducing environment at a temperature below 1400 C. more preferably below 1350 C. and even more preferably below 1300 C. To achieve a dense part, it is desirable to sinter under pressure of 10,000 psi or greater with a more preferable pressure of 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 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.

(16) 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.