Method of Nitriding, Carburizing, or Oxidizing Refractory Metal Articles Using Microwaves

This invention relates to a method of nitriding. More particularly, this invention relates to a method of nitriding metal and metalloid articles.

The nitrides of many metals and metalloids have high melting points and are resistant to oxidation. Currently, powders of these nitrides are consolidated into a body, then the body is hot-pressed or hot-isostatic-pressed at high temperatures and pressures into a dense body. Operations of this type require expensive equipment and usually long periods of time. Also, bodies are frequently in a highly-stressed state after this type of heating and pressing operation.

The object of the subject invention is to provide a method for nitriding refractory metal and metalloid articles to form dense, crack-free bodies of metal or metalloid nitrides, composites of these nitrides, coatings of these nitrides, and nitrides of metal and metalloid articles having a density less then theoretical density.

In accordance with one aspect of the present invention, a new and improved method of nitriding a refractory-nitride forming metal or metalloid article comprises the following steps:

Step 1. A consolidated metal or metalloid article having an intimately contacting enwrapment of a ceramic insulating material is provided within a microwave oven.

Step 2. A nitrogen containing atmosphere is introduced into the microwave oven.

Step 3. The metal or metalloid article is heated by microwave energy within the microwave oven containing the nitrogen containing atmosphere to a temperature sufficient to react the metal or metalloid with the nitrogen by applying a microwave energy within the microwave oven.

Step 4. The metal or metalloid article is maintained at the temperature for a period sufficient to convert the metal or metalloid to a metal nitride or a metalloid nitride.

In accordance with another aspect of the present invention, a new and improved method of providing a nitride coating on a metal or metalloid article comprises the following steps:

Step 1. A consolidated metal or metalloid article having an intimately contacting enwrapment of a ceramic insulating material is provided within a microwave oven.

Step 2. A nitrogen containing atmosphere is introduced into the microwave oven.

Step 3. The metal or metalloid article is heated by microwave energy within the microwave oven containing the nitrogen containing atmosphere to a temperature sufficient to react the metal or metalloid with the nitrogen by applying a microwave energy within the microwave oven.

Step 4. The metal or metalloid article is maintained at the temperature for a period sufficient to form a coating of metal nitride or metalloid nitride on the article of metal or metalloid.

In accordance with another aspect of the present invention, a new and improved method of providing an oxide coating on a refractory-oxide-forming metal or metalloid article comprises the following steps:

Step 1. A consolidated refractory-oxide-forming metal or metalloid article having an intimately contacting enwrapment of a ceramic insulating material is provided within a microwave oven.

Step 2. An atmosphere containing oxygen is introduced into the microwave oven.

Step 3. The refractory-oxide-forming metal or metalloid article is heated by microwave energy within the microwave oven containing the atmosphere to a temperature sufficient to react the refractory-oxide-forming metal or metalloid with the oxygen contained in the atmosphere by applying a microwave energy within the microwave oven.

Step 4. The refractory-oxide-forming metal or metalloid article is maintained at the temperature for a period sufficient to form a coating of oxide on the article of the refractory-oxide-forming metal or metalloid.

In accordance with another aspect of the present invention, a new and improved method of providing a carbide coating on a refractory-carbide-forming metal or metalloid article comprises the following steps:

Step 1. A consolidated refractory-carbide-forming metal or metalloid article having an intimately contacting enwrapment of a ceramic insulating material is provided within a microwave oven.

Step 2. A carbon containing atmosphere is introduced into the microwave oven.

Step 3. The refractory-carbide-forming metal or metalloid article is heated by microwave energy within the microwave oven containing the carbon containing atmosphere to a temperature sufficient to react the refractory-carbide-forming metal or metalloid with the methane contained in the atmosphere by applying a microwave energy within the microwave oven.

Step 4. The refractory-carbide-forming metal or metalloid article is maintained at the temperature for a period sufficient to form a carbide coating on the article of the refractory-carbide-forming metal or metalloid.

In accordance with another aspect of the present invention, a new and improved method of providing a metal carbo-nitride coating on a refractory-"carbo"-nitride-forming metal or metalloid article comprises the following steps:

Step 1. A consolidated refractory-"carbo"-nitride-forming metal or metalloid article having an intimately contacting enwrapment of a ceramic insulating material is provided within a microwave oven.

Step 2. A carbon and nitrogen containing atmosphere is introduced into the microwave oven.

Step 3. The refractory-"carbo"-nitride-forming metal or metalloid article is heated by microwave energy within the microwave oven containing the carbon and nitrogen containing atmosphere to a temperature sufficient to react the refractory-"carbo"-nitride-forming metal or metalloid with the methane and nitrogen contained in the atmosphere by applying a microwave energy within the microwave oven.

Step 4. The refractory-"carbo"-nitride-forming metal or metalloid article is maintained at the temperature for a period sufficient to form a carbo-nitride coating on the article of refractory-"carbo"-nitride-forming metal or metalloid.

The new and improved method of the present invention for fabricating of refractory-nitride forming metal or metalloid dense articles, composites and coatings comprises the following operations (where refractory-nitride-forming "metal" refers to Ti, Zr, Hf, Nb, Ta, and Al; and where "metalloid" refers to refractory-nitride-forming nonmetals such as Si and B):

1) consolidating (i.e., pressed powder) at least one material selected from the group consisting of refractory metals, refractory metal nitrides, aluminum, silicon, and boron into a selected configuration;

2) surrounding the configuration with an intimately contacting enwrapment of a ceramic aggregate of granular material having an average particle size ranging from 2 .mu.m to +100 mesh and selected from the group consisting of alumina, calcia, magnesia, yttria, and silicon nitride in which the granular materials are electrical and thermal insulators and essentially noncouplers with microwaves to prevent electrical sparking and to provide thermal insulation for the configuration; and

3) heating the enwrapped configuration with microwaves in a nitrogen atmosphere or a nitrogen containing atmosphere such as NH.sub.3 or mixtures of N.sub.2 --H.sub.2, NH.sub.3 --H.sub.2, N.sub.2 --Ar, or N.sub.2 -inert gas to a desired temperature for conversion of any non-nitride portion in the enwrapped material to a nitride and densification of the formed nitrides.

In a demonstration of the subject invention, a powder mixture containing 50 wt % niobium, 20 wt. % tungsten and 30 wt. % titanium nitride was pressed into a disc having a diameter and a thickness of 1 inch. The disc was surrounded with grit of yttria having a particle size ranging from 150 to 425 microns in a encasement of alumina brick that had been arranged in a microwave oven. The oven was equipped with a standard-energy (1.6 kW) and standard frequency (2.45 GHz) microwave unit.

In the heating operation, the disc was exposed to the maximum energy level of 1.6 kW for 100 minutes in an environment of flowing nitrogen. Then, the power switch on the oven was moved to the "off" position and the disc was cooled to ambient temperature in the flowing nitrogen. The cooled disc was removed from the arrangement in the oven and easily brushed free of the yttria granules.

The disc was visually examined for cracks, then the physical characteristics of the disc were determined.

The tests indicated that the disc was completely sintered and crack free. Standard mercury intrusion porosimetry techniques indicated a real density of 7.68 g/cc, an open porosity of 40% and a closed porosity of 4%.

Analyses indicated that the disc contained 53.5% of niobium nitride, 27.9% of titanium nitride and 18.6% of tungsten.

In another demonstration of the subject invention, a powder mixture containing 50 wt. % niobium, 20 wt. % tungsten and 30 wt. % titanium nitride was pressed into a disc having a diameter and thickness of 1 inch. The disc was surrounded in alumina grit having a particle size ranging from 150 to 425 microns in an encasement of alumina brick, except alumina grit was used instead of yttria granules. This arrangement was assembled in a microwave oven, and the metals contained in the disc were converted to a nitride of the metals and sintered as described in Example I.

The processed disc was visually examined for cracks, then the physical characteristics of the disc were determined. These tests indicated that the disc was completely sintered and crack-free. Standard mercury intrusion porosimetry techniques indicated a real density of 7.7 g/cc, an open porosity of 42% and a closed porosity of 4%. Analyses indicated that the disc contained 53 wt. % of niobium nitride, 28 wt. % titanium nitride and 19 wt. % of tungsten.

A powder mixture having a 99% purity and 325-mesh particles was pressed into a disc. The powder mixture contained 88.1 wt. % elemental silicon, 9.5 wt. % yttrium oxide and 2.4 wt. % aluminum oxide. Also, the pressed disc had a diameter of 1 inch and a thickness of 0.5 inch. In preparation for the heating operation, the disc was surrounded in yttria grit with particle sizes ranging from 150 to 425 microns inside a case of alumina brick. This arrangement was assembled inside a microwave oven. The pressed disc was converted to a nitride composite and sintered by the procedure described in EXAMPLE I.

Visual examination indicated that the processed disc was free of cracks. The processed disc was designed to convert to a composition consisting of 92.5 wt. % silicon nitride, 6.0 wt. % yttrium oxide and 1.5 wt. % aluminum oxide, with a theoretical density of 3.2 g/cc. Dimensional measurements indicated the density of the processed disc was 70% of the theoretical density.

Control of the temperatures (1400.degree. to 1875.degree. C.) and time period of exposure (10 to 100 minutes) is required to obtain full density. However, this experiment indicated that ceramic compositions with a silicon nitride base can be synthesized by the subject development.

A silicon powder consisting of greater than 99.95% purity was pressed into a cylindrical compact having approximate dimensions of 2 inches diameter and 1.75 inches in length. The sample was placed in a boron nitride crucible containing silicon nitride--2 wt. % yttria powder having an average particle size of approximately 2 .mu.m. An alumina fiber board was placed around the crucible. This arrangement was placed in a microwave oven and the oven filled with nitrogen gas.

The compact temperature was monitored by a thermocouple and heated to 1400.degree. C. oven for about 23 hr. The compact was converted to greater than 78% silicon nitride.

A powder mixture consisting of 86.9 wt. % silicon (>99.95%), 9.8 wt. % yttria and 3.3 wt. % alumina were blended together and pressed into a compact and placed in a crucible as described in Example IV and the arrangement as described in Example IV was placed in the oven. The oven was filled with argon gas. The temperature of the compact was then raised to approximately 1000.degree. C. Nitrogen was then flowed into the microwave furnace and the temperature raised to 1400.degree. C. The total treating time in the microwave oven was about 24 hr. The compact was converted to greater than 75% silicon nitride.

Synthesis of silicon nitride by direct reaction of a silicon bar with nitrogen is known in the art. However, the use of powder and of microwave radiation for heating the compacted powder is believed to be new. It was not known that elemental silicon could be heated to a temperature sufficient for conversion to silicon nitride with microwave radiation.

A sample of -325 mesh powder of niobium--1 wt. % zirconium (Nb-lZr) alloy was pressed into a disc having a diameter of 1 inch and a thickness of 0.5 inch. The disc was arranged in a microwave oven as described in EXAMPLE I, then exposed to the maximum energy level (1.6 kw) for 90 minutes in flowing argon. The environment of the furnace was changed from flowing argon to flowing nitrogen, then heated for 5 minutes in the nitrogen environment at the maximum energy level. The disc was cooled in the nitrogen environment and removed from the microwave oven.

Visual inspection indicated that the processed disc was free of cracks. Microscopic examination indicated that the surface of the processed disc was heavily nitrided and the interior of the processed disc metallic. A major portion of the surface nitride was identified as zirconium nitride and the composition beneath the nitride layer rapidly graded to a mixture of niobium and zirconium nitrides. The depth of the nitrided layer ranged from 1 to 2 mm.

It is believed that this method of coating surfaces could be used for coating surfaces with other compounds by using other reactive gases. For example, a surface could be coated with a refractory-oxide-forming metal or metalloid (where refractory-oxide-forming "metal" refers to Zr, Hf, Y, Sc and the rare earths, U, Th, Ti, Al, Cr; and "metalloid: refers to Si) by flowing oxygen (air) through the furnace. Similarly, a surface could be coated with a refractory-carbide-forming metal or metalloid (where refractory-carbide-forming "metal" refers to Ti, Zr, Hf, Nb, Ta, V, Cr, Mo, and W; and "metalloid" refers to Si and B) by flowing a carbon containing atmosphere such as methane, acetylene, butane, or mixtures thereof with argon, helium or hydrogen through the furnace. Also, surfaces could be coated with a refractory-carbo-nitride-forming metal or metalloid (where refractory-"carbo"-nitride-forming "metal" refers to Ti, Zr, Hf, Nb, and Ta; and "metalloid" refers to Si and B) by flowing a gas mixture of carbon containing gas such as methane, acetylene, butane or mixtures thereof with argon, helium, hydrogen or ammonia and nitrogen through the furnace. In a similar manner to these examples of forming nitrides, oxides and carbides, any mixed phase (such as the "carbo" nitrides illustrated) can be produced: oxycarbides and oxynitrides (i.e., Si-O-N) can be produced and used as mixed-metal phases (i.e., Si-Al-O-N). This coating technique could be very useful for coating items such as crucibles, drill bits and cutting tools with special coatings which have desired properties.

Refractory metal composites of refractory metals, silicon and aluminum are rapidly and efficiently converted to the nitride and densified in one operation. Also, poor microwave couplers, such as silicon nitride, can be relatively sintered from the microwave-coupling elemental silicon and formed into dense composites.

It is believed that the subject development is not limited to the preparation and densification of refractory nitrides. Other intermetallic materials such as the borides, carbides and silicides could be easily and efficiently prepared and densified with the subject method by simply selecting the proper materials and conditions.

In the subject invention, the production of silicon nitride in a gaseous reaction with the required heat provided by a flux of microwave radiation is believed to be a significant contribution to the state of the art. In other applications, materials such as intermetallics, composites and coatings can be produced by the subject method. The combination of materials in a composite can be varied by including reactive and nonreactive components in a composite. A hard ceramic coating of a material can be formed on drill bits, cutting tools, and crucibles; and surfaces of articles can be coated with intermetallic mixtures such as the carbo-nitrides.

Shown in Tables I, II, and III are the conditions of processing and the data obtained for numerous examples of compacts of elemental silicon powder (alone or with the typical additions of appropriate amounts of yttrium oxide [or yttrium nitrate] to yield 6 wt. % Y.sub.2 O.sub.3 and aluminum oxide [or aluminum nitrate] to yield 1.5% Al.sub.2 O.sub.3) being microwave-treated in nitrogen atmosphere to yield silicon nitride. For the microwave-reaction of silicon given in Tables I, II, and III, either a 1.6Kw fixed power or a 0 to 6Kw variable power, standard frequency (2.45 GHz) microwave applicator was used. With specimens up to several inches thickness and over four inches diameter, experiments showed that the nitridation was uniform; the nitride was distributed evenly throughout the compacts. Iron oxide (which is typically added as a nitriding catalyst in conventional nitriding operations) was tested in the standard 3 to 5 wt. % level; the nitriding was about the same with or without this iron oxide addition. This is very important, since the iron oxide addition lowers the melting point of the grain boundary mixed oxide phase, thus lowering the high-temperature utility of the resultant silicon nitride. Thus, microwaved silicon nitride has a decided advantage over conventional reaction-bonded silicon nitride since the microwaved material does not require the detrimental iron oxide addition.

Also, it has been shown quantitatively that microwave heating enhances the nitridation of silicon to produce reaction-bonded silicon nitride (or "RBSN"). With microwave nitridation, only about 12 hours is required at 1350.degree. C. to get approximately 95% conversion of the silicon metal to silicon nitride; whereas, conventional nitridation would require around 160 hours to accomplish the same degree of conversion to silicon nitride. In other words, the microwave nitriding takes less than one-tenth the time of conventional nitriding.

RBSN is important since the dimension of the part essentially stays the same before and after nitriding, an unusual feature of the reactive nitriding of silicon--to yield a dimensionally constant part. The dimensions of the un-nitrided silicon compact are essentially the same as the dimensions of the nitrided part; thus, by this process near-net-shape parts of silicon nitride can be produced.

The fact that microwave formation of silicon is so efficient may result from the enhanced reaction of the nitrogen at the grain-to-grain surfaces. Microwave energy is typically absorbed preferentially in the grain boundaries; micro-arcing may occur in the grain boundary regions. In any case, the nitriding of silicon is greatly enhanced. Silicon nitride (this is, completely nitrided silicon) is not a good microwave absorber. Thus, it is remarkable that silicon metal heats up readily and also readily converts to silicon nitride when a nitrogen atmosphere is utilized.

The subject invention offers investigators of inter-metallics a rapid and efficient method for preparation and densification of these materials and compositions of them.

While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.