GB1603449A - Method for the formation of hard deposits - Google Patents

Description

(54) A METHOD FOR THE FORMATION OF HARD DEPOSITS (71) We, CHEMETAL CORPORATION, a corporation organised and existing under the laws of the State of California, United States of America, of 10258 Norris Avenue, Pacoima, State of California, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- This invention relates to an improvement in and modification of our invention disclosed and claimed in Specification No 1465819. More particularly, the invention relates to the production of deposits on substrates, as coatings, or the production of free standing objects made from a deposit after removal of a substrate. The deposits of the invention have physical characteristics which are substantially improved over those presently known to those skilled in the art.

In our Patent Specification No 1465819 there is described and claimed a method of producing a hard metal deposit on a substrate surface comprising; providing a gaseous, volatile halide of a Group IVA, VA, or VIA metal; reacting the volatile halide to effect the deposition onto the substrate surface of a compound of the metal which is in a liquid phase, the reaction being with oxygen or an oxygen-containing substance when the metal used is tungsten or molybdenum, or the reaction being with a reducing agent in the absence of oxygen when the metal used is other than tungsten or molybdenum; and reacting the liquid phase deposit on the substrate surface with one or more gases containing boron, carbon, or silicon and, when the metal used is tungsten or molybdenum, with hydrogen at a volumetric ratio of hydrogen to metal halide of not greater than the stoichiometric volume ratio (as hereinbefore defined) to produce a hard metal deposit containing the metal in combination with boron. carbon or silicon.

The present invention provides a method for depositing a hard alloy, which method comprises providing a volatile gaseous halide of a metal or semi-metal, reducing the volatile halide to effect a liquid phase deposition of an intermediate compound of the metal or semi-metal onto a substrate, and thermochemically reacting the liquid phase deposit on the substrate in the presence of an alloying agent to produce the hard alloy, not being a method as disclosed or claimed in Specification No 1465819.

Very generally, the method of the invention comprises providing a volatile halide of a metal or semi-metal, an alloy of which is to be deposited on the substrate. The term alloy is used in its broadest sense herein, and is meant to include solid solutions, chemical compounds, or mixtures of solid solutions and chemical compounds.

The volatile halide is then reduced in the presence of a substrate in such a way that deposition occurs on the substrate of a compound of the metal or semi-metal which is in a liquid phase. The term substrate is used herein in its broadest sense and is intended to include any form upon which the coating is deposited, whether subsequently used in the bonded condition or dispensed with after deposition such as a mandrel or die. The liquid phase is then thermochemi cally reacted in the presence of an alloying agent. The deposited liquid may produce the solid phase alloy on the substrate by pyrolysis or by reaction with the gaseous environment. The deposited liquid contains at least one alloying agent but may contain more, and indeed all the alloying agents may be present in the liquid.

The preferred alloying agents contain one or more of the elements boron, carbon, silicon, or nitrogen, to produce the corresponding boride, carbide, silicide, or nitride of the metal or semi-me al. G. ; ygen may be one of the alloying agents when more than one alloying agent is used.

The term"metal or semi-metal"as used herein means any element capable of forming a volatile halide which will react with a reducing agent to produce an intermediate compound having the proper vapor pressure and melting temperature such that this intermediate compound may in turn be deposited on the substrate or mandrel as a liquid. The reaction to produce this intermediate and relatively non-volatile compound must be relatively fast. The intermediate compound must then be able to be converted chemically on the surface of the substrate by pyrolysis or by reaction with a gaseous species to the desired solid hard deposit by a reaction on the heated surface of the substrate.

Exemplary of the metals for use in practising the present invention are aluminium and certain of the transition metals of Groups IVb, Vb, and VIb, namely, titanium, zirconium, and hafnium of Group IVb, vanadium, columbium, and tantalum of Group Vb, and chromium, molybdenum and tungsten of Group VIb.

-The preferred semi-metals for use in practising the present invention are boron and silicon. These elements generally behave as semi-metals but when silicon is reacted with an alloying agent comprised of carbon, nitrogen, or boron, and when boron is reacted with an alloying agent comprised of carbon, or nitrogen, they behave in the same way as metals and produce a hard"alloy"as contemplated by the present invention.

The alloying agents contemplated for use in practising the present invention may be in elemental form, such as boron, carbon, silicon, or nitrogen, or may be compounds thereof from which the alloying element may be derived for reaction with the metal. Thus ammonia, NH3, may be utilized as a source of nitrogen and hydrocarbons may be utilized as a source of carbon. Exemplary of such compounds are diborane, Bd or boron trichloride as a source of boron.

In addition, the alloying elements may be included within a compound containing the metal or semi-metal such as methyltrichlorosilane providing that it is possible to produce the necessary intermediate and relatively non-volatile compound therefrom. The volatile metal or semi-metal halides may, themselves, be made from said metals or semimetals in their elemental form by reaction with a halogen or can be made from compounds containing certain metals. For example, silicon tetrachloride can be made by reaction of silicon with chlorine, or may be made by the action of chlorine on silicon carbide. Titanium tetrachloride can, in like manner, be made from either titanium or titanium carbide or nitride. These volatile metal halides should be capable of being reduced to a lower oxidation state to produce the liquid intermediate. For example, silicon tetrachloride appears to be reduced to a polymeric material having a silicon to chloride ratio of about 1: 2.6.

It will be appreciated that certain processes for the production of hard deposits, using particular combinations of the above metals (titanium, zirconium, hafnium, vanadium, columbium, tantalum, chromium, molybdenum and tungsten) and alloying agents (gases containing boron, carbon and silicon) are described and claimed in Patent Specification No 1465819. No claim is made herein to any process claimed by that prior specifi- cation. Suitable apparatus for carrying out the method of the invention is shown in Figure I of the drawings accompanying Specification No 1465819, to which the reader is referred, and references herein to "Figure 1"should be construed accordingly.

The method of the present invention, although similar to chemical vapor deposition, is not truly that. The method of the invention employs a deposition apparatus essentially similar to a chemical vapor deposition apparatus; however, the apparatus is operated in such a manner that the typical chemical vapor deposition process does not take place.

In accordance with the method of this invention, a sequence of events is made to take place which is different from what has been believed desirable by those skilled in the art. It has been discovered that superior deposits can be produced by causing a chemical reaction off the surface of the substrate resulting in an intermediate product which is deposited on the substrate or mandrel in a liquid phase, and by further reacting the liquid phase on the substrate to form the desired solid phase.

Such reactions are possible with compounds of a number of metals and semimetals as previously discussed. The chemical characteristic required of said metal and semi-metal compounds is that the volatile halide of the metal portion of the compound must be able to be reacted with a substitution agent or a reducing agent to produce an intermediate metallic compound having the proper vapor pressure and melting temperature such that this intermediate compound may in turn be deposited on the substrate or mandrel as a liquid. The reaction to produce this intermediate and relatively non-volatile compound must be relatively fast. The intermediate compound is then thermochemically reacted on the surface of the substrate and in the presence of an alloying agent to produce the desired hard alloy.

The substance for causing the reaction which produces the liquid phase may be introduced from the source 69 of Figure I or may be provided in particulate form as indicated at 75 suspended within the porous basket 41. In either case, there is occurring within the chamber 21 and spaced from the surface of the substrate 11, a reduction or disproportionation rection which produces a lower halide of the metal. The chamber wall is kept at a sufficiently high temperature to prevent condensation of the lower halide on the wall.

This lower halide may polymerize to cause the deposition on the substrate of the inter mediate liquid phase, or may in some cases react with the additional gases in the system to do so. The gases containing the alloying agent may contribute to the formation of the intermediate liquid or may react with the liquid after its deposition to form the solid deposit. The gaseous species containing the alloying agent, as aforesaid, are introduced into the reactor at a region which is down stream from the additional reduction reac tion such as through a tube from the source 71. Most frequently a reaction occurs which produces an observable fog or halo on the substrate which is observable during the process and which can usually be noted in connection with the deposition of the liquid of the substrate surface.

If the liquid phase contains a sufficient amount of the alloying element, it may be convered to the solid by pyrolysis, or may react with the gases in the environment to form the desired solid deposit of hard alloy or compound. Alternatively, the liquid may thermochemically react with the alloying agent as part of the surrounding gaseous environment to form the solid deposit.

Although this mechanism is not entirely understood, it is apparent that this reaction of the liquid phase, although slower than the reaction to form the liquid, occurs relatively rapidly by comparison with an all gaseous reaction, thereby contributing to higher efficiency at a greater deposition rate.

It has been determined that if the method of the invention is practised, deposition can be effected at substantially higher rates at lower temperatures than with conventional chemical vapor deposition.

Although not always essential, the reactions above described are preferably carried out in the presence of hydrogen gas. A flow of hydrogen in the reactor has typically resulted in much better process operation and more rapid deposition rates. Where hydrogen is used as the reducing agent in the initial reduction of the metal halide, the hydrogen is, of course, already present in the reactor. The reducing agent in bed 75 in Figure I may also be chips of the metal corresponding to the volatile halide to be reduced, i. e., titanium chips to reduce titanium tetrachloride, and when it is, the hydrogen may be introduced through the tube from the source 73.

The method of the invention using the two step deposition reaction, wherein a liquid is first deposited and then reacted in the gaseous environment to form the desired hard metal deposit, is similar for all of the metal or semi-metal compounds cited. Among the metals and semi-metals previously described, however, there is a difference in the thermochemical reactions possible. Of these, only molybdenum and tungsten allow for the removal of oxygen from a metal compound by hydrogen due to, of course, the relative free energies of the metal oxides, or metal oxy halides and the free energy of water. The favorable thermocouple relationships allow, therefore, that the intermediate liquid phase deposit in the case of molybdenum or tungsten may be, and preferably is, an oxygen bearing compound, since a number of such compounds have relatively broad liquid ranges.

In the case, however, of the other metals, aluminum titanium, zirconium, hafnium, vanadium, columbium, tantalum and chromium, and of the non-metals, silicon and boron, any oxygen introduced to form a liquid is irreversibly retained in the eventual deposit. Such addition is useful if the desired final deposit is to contain oxygen or contains aluminum, silicon or boron. It is, therefore, desirable from the point of view of maximum hardness and strength, but not obligatory, that the liquid deposit be formed from a lower halide of the metal species of interest.

In any case, however, the method of the invention depends uniquely on the formation of such a liquid deposit as a precursor to the solid hard metal deposit.

In practising the method of the invention without the introduction of oxygen, the preliminary reduction preferably occurs at a temperature not less than 700 C. The preliminary reduction may be effected by passing the halide through a particulate matter such as metal chips as previously described, or by simply passing the halide with a reductant gas through a heated zone.

In dealing with these active metal and semi-metal elements, suitable alloying species to produce hard deposits are made from the group consisting of carbon, silicon boron, or nitrogen. The alloying agent may also be oxygen in those instances where two or more alloying agents are used.

In the case of producing tungsten or molybdenum carbide, using the method of the invention, a more convenient way of causing the preliminary reaction to occur is possible. The volatile halide of the metal is reacted with a gaseous substance containing oxygen. Elemental oxygen may be used or oxygen bearing volatile compounds. In fact, it is most convenient to use a material which contains both carbon and oxygen since, when employed under proper conditions, it can serve as both the source of oxygen for the intermediate liquid product and the source of carbon to form the final hard alloy solid deposit. The suitable reacting substances, therefore, may be elemental oxygen, water, carbon monoxide, or other volatile compouns containing both carbon and oxygen.

Examples of the last are alcools such as methyl and ethyl alcohols, ketones such as acetone, ethers such as ethyl ether, and ethylene oxide. In order to assure that a liquid phase is first deposited on the substrate, the deposition temperature is preferably held not greater than 1000 C. Using the oxygen from one of the above reactants to form the liquid phase deposition, the conversion to the solid hard alloy is accomplished by reacting this liquid phase with hydrogen and one or more of the carbon bearing gases.

The ratio of hydrogen relative to the metal or semi-metal halide must not be high since this tends to foster conventional chemical vapor deposition, i. e., direct deposition of the solid from the gas phase. Preferably, the ratio of hydrogen to the volatile halide of the metal or semi-metal should not exceed stoichiometric dimensions. The total volume of the carbon bearing gases relative to the volatile halide of the metal or semi-metal serves to control the proportion of carbon in the solid deposit. It is preferred, for the best operation of the process, that the volume ratio of carbon monoxide to the volatile halide of the metal or semi-metal should not exceed unity. If another source of carbon is used, the ratio is determined by the number of carbon atoms in the gas to establish equivalence to carbon monoxide.

The resultant thermochemically deposited product consists of a hard metal or semimetal alloy free of columnar grains.

The method of the invention is applicable to the production of coatings which exhibit extreme hardness, e. g. in excess of 4000 Vickers hardness number. These deposits are useful as coatings, and may be made so thin as to produce a negligible change in the substrate dimension. From a commercial point of view the coatings of principal interest are titanium with non-metallic or semi-metallic elements. The method is, however, applicable as well to the deposition of hard compounds of aluminium, zirconium, hafnium, vanadium, columbium, tantalum and chromium. The non-metallic or semimetallic elements used are generally carbon, boron, nitrogen and silicon.

Silicon or boron may be used in the role of a semi-metal alloyed with carbon or nitrogen or may be used as a non-metal alloyed with other metal species discussed herein. Alloys may be simple binary alloys or may be more complex alloys containing more than one metallic species and more than one nonmetallic species. Oxygen may be added to the alloys to produce certain desirable properties as in the case of alloys containing aluminium, nirtrogen and oxygen or silicon, aluminium, oxygen and nitrogen.

These coatings may be applied to many different substrates such as graphite; refractory ceramics, such as oxides; cemented tungsten carbides; refractory metal, such as tungsten, molybdenum, titanium, and even iron, nickel or cobalt base materials. In the case of low expansion coefficient materials the coating is regularly applied directly as an overlayer, in other words, a build-up on the surface with no pre-treatment of the surface required. In the case of the iron, nickel and cobalt base materials, wherein the expansion coefficient of the coating is vastly different from that of the substrate, it is frequently necessary to pretreat the substrate with a diffusion coating first. Other diffusion coatings which would perform a similar function would be silicon, carbon or nitrogen. The diffusion coating may be made by one of two well known chemical vapor deposition methods, pack-cementation or a flowing gas system. The latter is preferred for reasons which will become apparent during the description set forth below.

The invention is illustrated by the following Examples, all of which were carried out using the apparatus of Figure I of Specification No 1465819.

Example 1. Deposits of fine grain silicon carbide were made by the following method: Silicon tetrachloride at a flow rate of 300 ml. per minute was mixed with a hydrogen stream of at least 300 ml. per min. and this mixture passed through a preliminary heated zone of the reaction chamber such as to heat the mixture to 600 C. A stream of propane at 68 ml. per min. was then added to the stream and the mixed gases passed over a tungsten wire maintained at 1150 C. The total pressure was 500 Torr. Silicon carbide was deposited on the wire at a rate of 0.25 microns per hour. This silicon carbide had an average grain size of 0.05 microns, a hardness of 4200 HV500, and a Rupture Modulus in bending of 2400 kPa. The as-deposited surface was extremely smooth and the general appearance, vitreous. X-ray diffraction indicated, however, that the material was pure silicon carbide. Cooled portions of the chamber were covered with a yellow viscuous liquid which contained about 23% silicon and 77% chloride by weight. The test was repeated using identical conditions except without preheating the mixed silicon tetrachloride and hydrogen stream. The silicon carbide was deposited at the same rate except coarse columnar grains resulted in the deposit and the Rupture Modulus of the material was 725 kPa. The surface topography now showed a rough crystalline surface.

Example 2. The experiment of the above example was repeated using trichlorosilane as a source of silicon. Part temperature was held at I 150 C and the total pressure at 250 torr. Similar fine grain deposits of silicon carbide resulted in nearly identical strength and hardness at a deposition rate of 0.5 microns per hour.

The experiment was repeated using methyltrichlorosilane as a source of silicon and omitting the propane from the gas stream with the same results. In all cases, the yellow viscuous liquid was observed. The experiment was repeated with methyltrichlorosilane without preheating the mixed hydrogen and methyltrichlorosilane stream. No yellow viscuous liquid was observed. Deposits were all coarse columnar morphology, typical of chemical vapor deposits. The Rupture Modulus in bending was 860 MPa.

Example 3. Using an experimental arrangement identical to that used for silicon carbide deposits, the following experiment was conducted. A mixture of silicon tetrachloride at 275 ml. per minute with a like flow of hydrogen was premixed and heated to 600 C. 100 ml. per minute of ammonia having been preheated to the same temperature was then added down-stream of the heater and the total mixture passed over a graphite rod heated to 1250 C. The total pressure in the chamber was 75 Torr. A deposit of 0.5 microns thickness of silicon nitride was made on the graphite rod in three hours. The silicon nitride deposit was composed of crystallites of less than 0.1 microns with a smooth botryoidal surface topography. The Rupture Modulus of the material was 1030 kPa, and the hardness 3800 HV5"0.

The experiment was repeated without preheating the gas streams and the resultant deposit consiste of poorly bonded, cracked, coarse grain crystals, too weak to make strength or hardness measurements.

Example 4. The experiment of Example 3 was repeated using the following conditions: 92 ml. per minute of aluminum chloride was mixed with 300 ml. per minute of hydrogen. The total pressure in the chamber was 300 Torr. The mixed stream of the aluminum chloride and hydrogen was heated to 600 C and then mixed with a preheated stream of 50 ml. per minute of ammonia. At a substrate temperature of 1060 C, a deposition rate of 0.25 microns per hour was achieved. The resulting deposit was smooth, having average grain size of 0. I microns. The X-ray diffraction showed the material to be stoichiometric aluminum nitride. The hardness of the deposit was 1600 HV50".

Example 5. The above experiment (Example 4) was repeated under identical conditions except that 50 ml. per minute of carbon dioxide was added to the preheated mixed stream. A similar fine grain deposit resulted except that the hardness was increased to 2400 HVsK,, The deposit is believed to be an alloy containing Al, O and N.

Example 6. An experiment was conducted in which a stream of 140 ml. per minute of silicon tetrachloride was mixed with a like amount of hydrogen preheated to 700 C and thence mixed with a preheated stream of 70 ml. per minute of ammonia plus 15 ml. per minute of oxygen. The graphite rod part temperature was held at 1400'C.

The total pressure was 50 Torr. A deposit of Si2N2O was made at a rate of 0.5 mm. per hour. Deposits had grain size of less than 0.5 microns with a Rupture Modulus in bending of 1240 kPa.

Example 7. An experiment was conducted in which a stream of 600 ml. per minute silicon tetrachloride, 100 ml. per minute of aluminium trichloride, 800 ml. of hydrogen were preheated to 700 C, and subsequently mixed with a preheated stream of 100 ml. per minute of ammonia and 20 ml. per minute of oxygen. The total pressure in the chamber was 50 Torr. The graphite rod substrate was held at 1325 C. A dense finegrain, coherent deposit containing silicon, aluminum, oxygen, and nitrogen was made at a rate of 0.25 ml. per minute. The deposit showed an X-ray diffraction pattern of beta silicon nitride and was, therefore, presumed to be a material, reported by several investigators, as beta prime sialon.

In practising the present invention, best results in terms of producing a hard deposit and reproducibility of results were achieved using the halides of aluminium, titanium, silicon and hafnium as the source of the metallic component of the hard metal alloy deposited on the substrate. The halides of columbium, molybdenum and boron were also found to work as the metallic component but not as well as aluminum, titanium, silicon or hafnium. Results using the halides of zirconium and tantalum were not always uniform but are nevertheless covered by the present invention to the extent that they are not covered by the claims of Specification No 1465819. Results using the halides of zirconium and tantalum and the halides of colum bium, molybdenum and boron were improved when the halide is reacted with a silicon halide to produce a hard metal alloy on the substrate.

It may therefore be seen that the invention provides an improved method for producing a coated substrate, as well as improved quality coated substrates. By providing an intermediate liquid phase on the surface of the substrate being coated, pyrolyzing this liquid or reacting a gas therewith to produce the final coating composition, the structure of the coating composition is such as to provide superior physical qualities. The parameters necessary to carry out the method of the invention are readily determinable by those skilled in the art from the information contained herein combined with that con tained in"Techniques of Metals Research" R. F. Bunshah, Ed., Intersciences Publishers, Div. of J. Wylie and Sons, New York, N. Y., 1968, Volume 1, Chapter 33.

Various modifications of the invention in addition to those shown and described herein will becorne apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.

Claims (16)

In our Specification No 1465819 there is described and claimed a method of producing a hard metal deposit on a substrate surface comprising; providing a gaseous, volatile halide of a Group IVA, VA, or VIA metal; reacting the volatile halide to effect the deposition onto the substrate surface of a compound of the metal which is in a liquid phase, the reaction being with oxygen or an oxygen-containing substance when the metal used is tungsten or molybdenum, or the reaction being with a reducing agent in the absence of oxygen when the metal used is other than tungsten or molybdenum; and reacting the liquid phase deposit on the substrate surface with one or more gases containing boron, carbon or silicon and, when the metal used is tungsten or molybdenum, with hydrogen at a volumetric ratio of hydrogen to metal halide of not greater than the stoichiometric volume ratio (as hereinbefore defined) to produce a hard metal deposit containing the metal in combination with boron, carbon or silicon. We make no claim in this specification to the invention claimed in any claim of Specification No 1465819. SUBJECT TO THE FOREGOING DIS CLAIMER, WHAT WE CLAIM IS :

1. A method for depositing a hard alloy, which method comprises providing a volatile gaseous halide of a metal or semi-metal, reducing the volatile halide to effect a liquid phase deposition of an intermediate compound of the metal or semi-metal onto a substrate, and thermochemically reacting the liquid phase deposit on the substrate in the presence of an alloying agent to produce the hard alloy.

2. A method as claimed in claim I wherein the volatile gaseous halide is a halide of aluminium.

3. A method as claimed in claim I or claim 2, wherein the hard alloy consists essentially of aluminium nitride.

4. A method as claimed in claim I or claim 2, wherein the hard alloy consists essentially of aluminium, oxygen and nitrogen.

5. A method as claimed in claim 1, wherein the volatile gaseous halide is a halide of boron or silicon.

6. A method as claimed in claim 5, wherein a silicon halide is reacted with an alloying agent which comprises of carbon, nitrogen or boron.

7. A method as claimed in claim 5, wherein a boron halide is reacted with an alloying agent which comprises of carbon or nitrogen.

8. A method as claimed in claim 5, wherein the hard alloy consists essentially of silicon, nitrogen and oxygen.

9. A method as claimed in any one of the preceding claims wherein the deposition is carried out in the presence of hydrogen.

10. A method as claimed in any one of the preceding claims, which method comprises additionally placing a substrate within a chemical vapour deposition reactor, introducing a flow of the gaseous volatile halide into the reactor, introducing a reducing agent into the reactor for reacting with the volatile halide to effect the deposition on the substrate surface of a halogen compound of the metal or semi-metal which is in a liquid phase, and introducing one or more gases comprising an alloying agent to produce a reaction in the liquid phase on the substrate surface to remove the halogen and to produce the desired hard deposit on the substrate surface.

11. A method as claimed in claim 10, wherein all the reactants are introduced into the reactor simultaneously.

12. A method as claimed in any one of the preceding claims, wherein the deposition is carried out in the presence of hydrogen.

13. A method as claimed in any one of the preceding claims wherein the surface of the substrate is pre-treated to provide a diffusion coating thereon comprising a compound of boron, silicon, carbon or nitrogen.

14. A method of producing a hard metal deposit on a substrate surface as claimed in claim I substantially as hereinbefore described in any one of the Examples.

15. A coated substrate when produced by a method as claimed in any one of the preceding claims.

16. A coated substrate as claimed in claim 18, wherein the product is a free standing product.