US 3846162 A
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United States Patent Q 3,846,162 METAL CARBONITRIDE COATINGS John Allison Bloom, Dallas, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex. No Drawing. Filed Oct. 21, 1968, Ser. No. 769,385 Int. Cl. C23c 11/14, 13/00 US. Cl. 117-106 R 4 Claims ABSTRACT OF THE DISCLOSURE Metal carbonitride coatings, for example, titanium carbonitride coatings, are applied to substrates by heating the substrates to the proper reaction temperature and thereafter contacting the substrates with a vaporous stream containing a reactant compound which consists essentially of carbon, nitrogen, hydrogen, and the metal, for example, tetrakis(dimethylamino)titanium.
This invention relates to coatings. In another aspect, this invention relates to vapor-deposited coatings of metal carbonitrides.
It has been found desirable to apply very hard, durable and oxidation-resistant coatings to the surfaces of various objects such as missile nose cones, machine tools, turbine blades, and the like.
Thus, it is often desirable to utilize the basic properties of a material, but to protect the material from exposure to the environment within which it must function. As mentioned in U.S. Pat. 2,972,556, carbon and graphite articles have exceptionally good thermal and electrical properties. However, in some applications where the thermal characteristics of the material could be utilized, such as in a missile nose cone, the material oxidizes in the air under the high temperatures to which it is subjected. The material must, for such application, be coated with a more oxygen-resistant material such as a metal carbide, a metal nitride, or both, as described in US. Pat. 2,972,556, mentioned above.
Also, in the production of machine tools, for example, it is often more economical to form a steel in the desired shape and coat with a harder material such as tungsten carbide or titanium carbide. The diflicult and expensive step of machining a dense, solid block of tungsten carbide or titanium carbide into a desired shape is thus avoided.
It is known that coatings such as titanium carbide can be applied to a substrate such as metal by exposing the surface of the metal to a gaseous stream of titanium tetrachloride and methane. The metal is usually heated to a temperature between 900 C. and 1200 C. and upon contacting the metal surface, the materials within the gaseous stream react to form titanium carbide which will adhere to the surface of the metal. More specific details of this reaction may be found in US. Pat. 2,962,388 and the description of equipment suitable for applying hard, dense coatings may be found in US. Pat. 2,884,894.
One of the problems encountered in coating metal with titanium carbide by the above-described process is the loss of temper in the metal. Generally, metals, and in particular steel, are first hardened by elevating the steel to about 1000 C. and then quickly quenched. The steel after quenching is tempered by elevating the temperature to about 500 C. to 600 C. thus reducing its brittleness. If a hardened and tempered steel is then reheated to a temperature between 900 C. and 1200 C. to permit the application of a coat of titanium carbide, the hardness and temper of the steel is lost during the reheating process. If the steel, after application of the coating, is quenched to again harden the steel, the coating may be damaged as the steel will change in size during the quenching process which can rupture the coating, create a rough- 3,846,162 Patented Nov. 5, 1974 ness in the coating or cause it to eventually peel from the surface of the steel. Thus, not only is the coating damaged, but the steel is also weakened and does not provide as strong a support for the coating necessitating that the coating be thicker to withstand the forces to which it may be subjected.
Recently a process has been developed for coating substrates with a solid-solution carbonitride of a metal selected from silicon, boron, and transition metals in groups IVB, VB and VIB of the Periodic Table, for example, titanium carbonitride. This recently developed process is described in co-pending patent application Ser. No. 694,390 filed Dec. 29, 1967, now abandoned. This process can occur either at low temperatures, thus permitting the application of a hard coat to a metal without loss of hardness and temper which has been imparted to the metal by previous heating steps, or at higher temperatures for materials having compatible thermal behavior in any step required after the coating operation. Not only can the metal carbonitride exhibit greater hardness than materials such as titanium carbide or titanium nitride, but the deposition rate obtainable with the metal carbonitride is from about 2 to 10 times that of titanium carbide, for example. This process includes the steps of heating the substrate to at least the decomposition temperature of the reactants (generally from about 400 C. to about 1200 C.) and then passing a gaseous stream containing the reactants over the substrate to thereby yield the reactants at the temperature of the body to permit the reaction of the metal, carbon and nitrogen, thereby forming a solid solution of the metal carbonitride on the body. The reactants generally include a metal halide, e.g., titanium tetrachloride, molecular nitrogen and/or an easily decomposable nitrogen-containing compound, an easily decomposable carbon-containing compound (alternatively, an easily decomposable nitrogenand carbon-containing compound can be used), and molecular hydrogen as a reducing agent.
The metal carbonitride coating applied by this recently developed method is a solid-solution material having the metal, carbon and nitrogen within a single phase crystal lattice. In addition to the great hardness of the material, the strong bonding present gives a relatively large surface energy to the material. This large surface energy is believed to render the material less likely to wet and adhere to the molten materials such as glass, metals or alloys, after it is applied to a substrate.
Even though this process for applying coatings of metallic carbonitrides yields improved products which are superior to the coatings heretofore known in the art, the process produces substantial amounts of halogencontaining contaminants such as solid halogen-containing compounds which can deleteriously effect the coating. Additionally, acid by-products such as HCl are formed which can deleteriously effect the coating and the substrate by destructive etching during deposition. The flow of the reactants over the substrate must be controlled so that the by-products are removed as they are formed adjacent the substrate. However, it is generally very difficult to remove all of the acid by-product before destructive etching results during the deposition, particularly when the substrate being coated is a metal such as steel.
Therefore, one object of this invention is to provide an improved process for coating a substrate with a metal carbonitride by vapor-phase deposition in the absence of the conventional contaminating halogen-containing byproducts.
Another object of this invention is to provide an improved process for coating a substrate with a metal carbonitride by vapor-phase deposition wherein the danger of destructive etching of the coating and/or the substrate by acid byproduct is eliminated.
According to this invention, a solid-solution carbonitride of a metal selected from boron, silicon, and the transition metals in groups IVB, VB and VIB of the Periodic Table is firmly coated on a substrate by vaporphase deposition with the use of a reactant compound consisting essentially of carbon, nitrogen, hydrogen, and the metal. The substrate is initially heated to a temperature at which the reactant compound will decompose, and then the reactant is passed, in the vapor phase, over the substrate. The resulting decomposition of the compound on the substrate will yield carbon, nitrogen and metal atoms in the reactive state and permit the reaction thereof to form the solid-solution layer of the metal carbonitride on the surface of the heated substrate.
The reactant compounds which can be used in the practice of this invention include organic compounds which consist essentially of carbon, nitrogen, hydrogen and the metal, and will decompose at the deposition temperature, generally from the temperature within the range of from at least about 400 C. to about 1200 C. or higher. A preferred group of compounds are represented by the formula [(R) N],,Me wherein Me is selected from silicon, boron, and the transition metals in groups IVB, VB and VIB of the Periodic Table as set forth on page B-2 of the Handbook of Chemistry and Physics, Chemical Rubber Company, 45th Edition (1964), n is a valence of Me, and R is selected from hydrogen and hydrocarbon radicals having from 1 to about 18 carbon atoms, for example, alkyl, cycloalkyl, aryl, aralkyl, and provided that at least one of the R groups is one of said hydrocarbon radicals.
An even more preferred group of reactants are the compounds of the above-identified generic formula wherein Me is titanium, n is 4, and R is selected from phenyl and alkyl groups having from 1 to about carbon atoms. Some specific examples of these preferred reactants include tetrakis(dimethylamino)titanium, tetrakis(diethylamino)titanium, tetrakis(dipentylamino)titanium, tetrakis (dioctylamino titanium, tetrakis (diphenylamino titanium, and the like.
The contact of the substrate with vaporous reactant can occur in conventional equipment. It is preferred that the substrate be suspended in a reaction chamber and heated to a temperature at which the vaporous reactant compound will decompose when contacted therewith. Thus, the reaction temperature will vary depending upon the particular reactant compound utilized but will generally fall within the range of about 400 C. to about 1200 C. or higher. Next, a vaporous stream containing the reactant compound is passed over the heated substrate whereby the deposition of the metal carbonitride on the substrate occurs. The rate of flow of the reactant compound over the substrate is generally not critical and can be adjusted as desired in the particular coating operation. It is preferred that the vaporous reactant compound be suspended within a carrier gas which is nondeleterious to the reaction, such as, for example, nitrogen and/or hydrogen.
Various substrates can be coated by the process of this invention including ferrous metals, titanium, ceramic materials, and refractory materials such as tungsten, molybdenum, niobium, and tantalum.
A better understanding of this invention can be obtained by referring to the following illustrative example.
Example A graphite specimen substrate being approximately /2 inch by inch by 40 mils is supported on a graphite pedestal intermediate the ends of the cylindrical steel reactor having a 6 inch I.D. An inlet conduit having a 1 inch I.D. communicates through the top of the steel reactor and is positioned to discharge vaporous reactants at a point about 1 inch above the top of the substrate. The bottom of the reactor is fitted with an outlet conduit to permit exhaustion of gases admitted to and generated within the reactor in the coating process. Beneath the substrate is positioned a conventional resistance-heating device. A temperature-sensing device is positioned directly above the substrate. The reaction chamber is evacuated and flushed with nitrogen, and the substrate is heated to about 1000 C. Next vaporous tetrakis (dimethylamino) titanium at a rate of 0.1 liters per minute and nitrogen carrier gas at a rate of 5 liters per minute are passed through the inlet conduit. The reactant stream is allowed to contact the substrate for about 15 minutes to about 2 hours. This contact will yield a hard coating of titanium carbonitride on the substrate.
While specific terms have been used in describing some preferred embodiments of this invention, given for illustrative purposes only, and are not intended as limitations upon the invention.
What is claimed is:
1. A process for coating a substrate with a solid solution carbonitride of a metal selected from silicon, boron, and the transition metals in Groups IVB, VB and VIB of the Periodic Table comprising:
heating said substrate to a temperature of 400 to 1200 C.; and
contacting the heated substrate, in the absence of halogen, with a vaporous organic compound having the generic formula [(R) N],,Me wherein Me is one of said metals, n is a valence of Me, and R is selected from hydrogen and hydrocarbon radicals having from 1 to about 18 carbon atoms, provided at least one R group is at least one of said hydrocarbon radicals.
2. The method of Claim 1 wherein said compound is tetrakis( dimethylamino) titanium.
3. The method of Claim 1 wherein said compound is tetrakis diethylamino) titanium.
4. The method of Claim 1 wherein said compound is tetrakis (diphenylamino titanium.
References Cited UNITED STATES PATENTS 3,356,618 12/1967 Rich et al. 117-106 CX 3,382,113 5/1968 Ebert et al l17106X 3,386,866 6/1968 Ebert et al 117-106 X 3,432,330 3/1969 Diefendorf 1l746 OTHER REFERENCES Deitschrift Jtir anorganische und allgemeine Chemie, Vol. 198 (1931), p. 260-261.
GEORGE F. LESMES, Primary Examiner W. E. BALL, Assistant Examiner US. Cl. X.R. 1l7106 C, 46 CG