METAL NITRIDE COATED SUBSTRATES

Abstract: A metal nitride coated substrate can be made by coating the substrate with an oxide of Al, Ti, or Zr or a hydroxide of Al, Ti, or Zr derived from a sol gel. The coated substrate is heated to a reaction temperature of at least about 850 DEG C and contacted with a gaseous reactant mixture comprising a nitrogen source and a carbon source. The molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15. The coated substrate is maintained at the reaction temperature for a sufficient time to convert the sol gel-derived coating to a metal nitride coating.

Description: Description Metal Nitride Coated Substrates Technical Field The present invention is directed to a method for making metal mtride coated substrates. Background Art In recent years, there has been increasing interest in non-oxide ceramics, such as metal nitrides and carbides, that possess high temperature strength and corrosion resistance. Among these materials, aluminum nitride (A1N) is especially important because of its unique physical properties. For example, A1N has a thermal conductivity close to that of metals and more than 10 times that of alumina (Al 2 O 3 ), a coefficient of thermal expansion comparable to silicon and silicon carbide, a high electrical resistivity, and mechanical strength comparable to alumina ceramics. Metal nitride powders can be made in various ways. For example, a metal oxide powder, such as Al 2 O 3 , zirconia (ZrOj), or titania (TiOz), can be mixed with an excess of a carbonaceous powder and heated to a temperature above 1100°C in a nitrogen-containing atmosphere. The metal nitride powder formed by this method is, however, mixed with unreacted carbonaceous powder that detracts from the properties of the metal mtride powder. The unreacted carbonaceous powder can be removed by oxidizing it at temperatures between about 600 °C and about 700 °C. At these temperatures, however, a portion of the metal nitride powder also can oxidize. United States Patent 4,975,260 to Imai et al. teaches an alternate method for making a metal nitride powder by reacting a metal oxide or metal hydroxide powder with a gaseous mixture of ammonia (NH 3 ) and a hydrocarbon at a temperature ranging from 1300°C to 1600 °C. Although this method is an improvement over some prior art methods, it still leaves residual carbon in the metal nitride product. Moreover, it requires a temperature of at least 1300°C. These prior art methods have several drawbacks. For example, they either make a product that contains significant amounts of carbon or require relatively high temperatures. Moreover, they cannot make a continuous coating on a substrate. For some applications, though, such coatings are highly desirable. Therefore, what is needed in the industry is method of making metal mtride coatings. Disclosure of the Invention The present invention is directed to a method of making metal mtride coatings. One aspect of the invention includes a method of making a metal mtride coated substrate. A substrate is coated with an oxide of Al, Ti, or Zr or a hydroxide of Al, Ti, or Zr derived from a sol gel. The coated substrate is heated to a reaction temperature of at least about 850 °C in a nonreactive atmosphere and contacted with a gaseous reactant mixture comprising a nitrogen source and a carbon source. The molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15. The coated substrate is maintained at the reaction temperature for a sufficient time to convert the sol gel-derived coating to a metal nitride coating. Another aspect of the present invention includes a metal mtride coated substrate made by the method described above. These and other features and advantages of the present invention will become more apparent from the following description and accompanying drawing. Brief Description of the Drawing Figure 1 is a schematic of a spray coating apparatus useful with the method of the present invention. Figure 2 is a schematic of a dip coating apparatus useful with the method of the present invention. Figure 3 is an optical micrograph of coated SiC yarns of the present invention. Figure 4 is an x-ray diffraction pattern for a A1N coating of the present invention. Best Mode for Carrying Out the Invention The method of the present invention can make AIN, TiN, ZrN, or YN coatings on a variety of substrates. The starting materials for the coatings include sol gel-derived coatings of an oxide or hydroxide of Al, Ti, Zr, or Y, such as Al 2 O 3 , TiO 2 , ZrO 2 , Y 2 O 3 , Al(OH) 3 , Ti(OH) 4 , Zr(OH) 4 , or Y(OH) 3 . The remainder of the application describes the method of the present invention in terms of making AIN coatings from sol gel Al 2 O 3 . One skilled in the art, however, will understand that the following description, with appropriate adjustments to reaction temperature, applies equally to methods of making AIN, TiN, ZrN, and YN from Al, Ti, Zr, and Y oxide or hydroxide sol gel-derived coatings. The substrate may be any material that does not react with the AIN coating and that can withstand processing conditions. For example, the substrate may be a metal or ceramic article. Alternately, the substrate may be a fiber in the form of a monofϊlament or a yarn. If the substrate is a yarn, the coating should be applied so it coats each fiber, rather than bridging between two or more fibers. Suitable fibers include SiC monofilaments, such as BP fiber (British Petroleum, Cleveland, OH), and carbon-coated SiC yarn, such as C-Nicalon ® yarn (Nippon Carbon Co., Tokyo, Japan). To coat the substrate by the method of the present invention, an Al 2 O 3 precursor coating is applied to the substrate. The Al 2 O 3 precursor coating is contacted with a gaseous reactant mixture at suitable reaction conditions to form an AIN coating. The gaseous reactant mixture comprises a nitrogen source and a carbon source. If NH 3 is the nitrogen source and CH 4 is the carbon source, the reaction can be written as: Al 2 O 3 (s) + 2 NH 3 (g) + 3 CH 4 (g) → 2 AIN (s) + 3 CO (g) + 9 H 2 (g) The Al 2 O 3 coating may be any continuous, high surface area Al 2 O 3 coating, such as γ-Al 2 O 3 or Al 2 O 3 made with a sol gel method. Preferably, the coating will be made with a sol gel method. The sol gel Al 2 O 3 may be made with any method known in the art. For example, aluminum isopropoxide [AKO-Z- H^] may be dispersed in water, for example water heated to about 75°C, in any suitable molar ratio of Al(O-ι-C 3 H 7 ) 3 :H 2 O to make a sol. Good results have been obtained with molar ratios between about 1:100 and about 1:1000. In general, the more Al(O- -C 3 H 7 ) 3 in the sol, the more viscous the sol will be. As explained below, the preferred ratio of AlCO-Z- H^HzO depends on the substrate to be coated. Suitable A O-i- H,);, may be purchased from commercial sources, including Alfa Products (Danvers, MA). A small amount of HNO 3 or other acid may be added to the sol to initiate reaction. For example, the initial pH of the sol can be about 3. The acidified sol may be allowed to sit until it is sufficiently viscous to make a gel than can be applied to the substrate. For example, the sol may be allowed to sit for up to 24 hr or 48 hr. The sol gel-derived coating may be applied to a substrate by any conventional method. For example, the sol gel coating may be applied to a metal or ceramic article by spray coating, dip coating, spin coating, or any other suitable method. Fig. 1 shows an apparatus for spraying the sol from a spraying means 2 onto the substrate 4. The spraying means 2 can be any conventional spraying equipment. Preferably, the sol's viscosity will be controlled so it is compatible with the spraying means or other equipment used to apply the coating. The sol's viscosity can be controlled by varying the amount of water in it or by adjusting its acidity. The sol gel coating may be applied to the entire substrate 4 or to particular portions of the substrate. In either case, the coating will preferably cover the surfaces to which it is applied uniformly. If desired, the substrate 4 can be heated, for example by placing it on a hot plate 6, before it is coated so the water in the sol evaporates rapidly. This produces a more even coating. The coating may be applied to any convenient thickness and can be applied as more than one coat. Fibers and yarns are preferably coated by dipping the fibers or yarns into a sol. Fig. 2 shows a dip coating apparatus in which a fiber (or yarn) 8 is dipped into a sol 10 in a suitable container 12. As the fiber 8 is withdrawn from the sol 10, a gel film 14 forms on the outside surfaces of the fiber. Preferably, the gel will be thin enough to coat each fiber in a yarn without bridging. Often, the coating on yarns will look bridged immediately after it is applied. If the sol's viscosity is properly selected, the bridging will disappear as the gel dries. Good results have been obtained with sols made with Al(O- -C 3 H 7 ) 3 :H 2 O molar ratios of about 1 : 100 to about 1 : 1000. After coating the substrate, the gel on the substrate may be allowed to dry for a sufficient time to partially convert it to Al 2 O 3 and to allow it to be handled more easily. The coated substrate may then be fired at a suitable temperature in a suitable atmosphere to complete the conversion of the gel to Al 2 O 3 . The drying and/or firing temperature and time depend on the thickness of the coating and the particular substrate. The coated substrate may be dried and/or fired at temperatures ranging from room temperature to the temperature at which the Al 2 O 3 will be converted to AIN for several minutes to several days. For example, the coated substrate may be dried at about 500°C for about 30 min. If the substrate will not react with air at the drying or firing temperature, the substrate may be dried or fired in air. If, however, the substrate will react with air, the substrate may be dried fired in an inert atmosphere. Helium is a suitable atmosphere for firing SiC fibers. After drying and/or firing the coated substrate, it may be heated to the reaction temperature at a moderate rate, for example, about 11.5°C/min. Preferably, the rate of heating will be selected to prevent the coating from spalling off the substrate. In general, the desired reaction to AIN will occur at temperatures of at least about 850 C C. Substantially complete conversion to AIN can be readily obtained at temperatures less than 1300°C, such as at temperatures of about 1000 C C and greater, although temperatures of up to about 1600°C may be used. The reaction temperature, therefore, may range from about 850°C to about 1600°C. Preferably the reaction temperature will be less than about 1275°C or 1299°C. For example, the reaction temperature may be between about 1000°C and about 1275°C. Most preferably, the reaction temperature will be about 1000°C to about 1100°C. The reaction pressure is not critical and may be any conveniently obtainable pressure. To make TiN, a TiO 2 or Ti(OH) 4 coating can be reacted with a gaseous reactant mixture at temperatures of at least about 750 °C. Substantially complete conversion to TiN can be obtained at temperatures greater than about 800 °C. A ZrO 2 or Zr(OH) 4 coating can be reacted with a gaseous reactant mixture at temperatures of at least about 1050 °C to make ZrN. Substantially complete conversion to ZrN can be obtained at temperatures greater than about 1100 °C. It may be desirable to make the TiN or ZrN coatings of the present invention at temperatures less than 1300°C, for example at temperatures less than 1275°C or 1299°C. Preferably, the coated substrate will be preheated to the reaction temperature in an inert atmosphere. For example, the substrate may be preheated in He, Ne, Ar, Kr, or Xe. The inert atmosphere should be selected to avoid substrate degradation. If the coated substrate was dried or fired in an inert atmosphere, the same atmosphere may be used to heat the substrate to the reaction temperature. Once the substrate reaches the reaction temperature, the gaseous reactant mixture, comprising nitrogen and carbon sources, is flowed into the reactor at a rate sufficient to achieve a desired N:C ratio. Reaction conditions are maintained for a sufficient time to convert the Al 2 O 3 coating to AIN. Reaction times of about 1 min to several days at reaction temperatures between about 850°C and about 1200°C have been found suitable to convert the Al 2 O 3 to AIN. Longer times may be necessary with lower temperatures. Similar reaction times are suitable to produce TiN and ZrN. Once the AIN coating is formed, the gaseous reactant mixture is shut off and a nonreactive atmosphere is established in the reactor. The reactor and product are then cooled at a convenient rate. The nitrogen source may be any reactive nitrogen compound that is a gas at the reaction conditions. For example, the nitrogen source may be NH 3 , N 2 H 2 , or N 2 . Anhydrous NH 3 is the preferred nitrogen source because it is readily available and easy to use. NH 3 may be purchased from many suppliers, including Aero All Gas Company (Hartford, CT). Preferably, the nitrogen source will not contain any contaminants that would produce side reactions or otherwise interfere with the nitridation reaction. The carbon source should be a carbon-containing compound that is a gas at the reaction conditions. For example, the carbon source may be a hydrocarbon or an amine. Although any hydrocarbon that is gaseous at reaction conditions may be used, alkanes having four or fewer carbon atoms are preferred because they are easier to handle. Similarly, amines having four or fewer carbon atoms, such as methyl amine (CH 3 NH 2 ), are preferred. Most preferably, the carbon source will be CH 4 because it is easy to obtain and work with. CH 4 may be purchased from many suppliers, including Aero All Gas Co. Preferably, the carbon source will not contain any contaminants that would produce side reactions or otherwise interfere with the nitridation reaction. The nitrogen and carbon sources in the gaseous reactant mixture may be supplied from separate sources or from a premixed source. In either case, it is preferable that they be mixed upstream of the reactor. The ratios of N:Al 2 O 3 and C:Al 2 O 3 in the reactor are not critical, although adequate amounts of gaseous reactants should be used to convert the A^O;, to AIN in a desired time. The ratio of nitrogen to carbon (N:C) in the gaseous reactant mixture is critical to obtaining a substantially carbon-free product. To form such a product, the molar ratio of N:C in the gaseous reactant mixture should be at least about 15. Preferably, the N:C ratio will be between about 15 and about 2000. Most preferably, the N:C ratio will be between about 30 and about 40. If desired, H 2 may be added to the gaseous reactant mixture to expand the range of conditions under which substantially carbon-free AIN can be made. Any excess of H 2 will facilitate the conversion of the sol gel-derived coating to the nitride coating. H 2 may be obtained from many commercial suppliers. The following examples demonstrate the present invention without limiting the invention's broad scope. Example 1 (SiC Yam) An Al j O j sol gel was prepared by dispersing 14 g of A O-t-CjH,)., in 190 ml of H 2 O to make a sol with an Al(O- -C 3 H 7 ) 3 :H 2 O molar ratio of 1:525. The sol was acidified by adding about 2 ml of concentrated HNO 3 to bring its pH to about 3 and was allowed to sit overnight to thicken. After sitting overnight, the sol's pH had rising to about 6. Several C- Nicalon ® carbon coated SiC yarns (Nippon Carbon Co., Tokyo, Japan) were cut to about 3.5 cm length and taped to the bottom of a bottle. The bottle served as a convenient holder for the yarns. The yarns were dipped into the sol. After 5 minutes, the yarns were slowly removed from the sol and allowed to air dry at room temperature overnight. When first removed from the sol, the gel coating the yarns looked like it bridged— hat is, coated more than one fiber in the yarns rather than each fiber. After drying, however, the gel coating was observed to coat the individual fibers without bridging. The coated yarns were place in a quartz reactor and heated to 500° C in He. The temperature in the reactor was held at 500° C for 30 min to convert the gel to Al^. The reactor temperature was then raised to 1050°C at a rate of 11.5°C/min. Once the reactor reached 1050°C, the He was turned off and NH 3 and CH 4 gases were flowed into the reactor at 400 ml/min and 30 ml/min, respectively. This created a N:C ratio of 13.33 in the reactor. After allowing the reaction to proceed for 40 min, the NH 3 and CH 4 gases were turned off and the yarns were cooled to room temperature in He. Examination of the yarns after the reaction showed they had a uniform coating about 2 μm thick. Fig. 3, an optical micrograph od coated SiC yam, shows that this coating did not bridge between fibers. Rather, it covered individual fibers. X-ray photoelectron spectroscopy showed the coating was substantially carbon-free and completely converted to AIN. Example 2 (SiC Yam) Example 1 was repeated using a sol with an Al(O-t-C 3 H 7 ) 3 :H 2 O ratio of 1:1000. Inspection of the ya after the gel coating was converted to AIN showed that individual fibers were covered with a very thin coating. There was no evidence of bridging. Analysis showed the coating was completely converted to AIN with no substantial carbon deposition. Example 3 (Steel Substrate) An Al 2 O 3 sol gel was prepared by dispersing 2 g of Al O-t- H^ in 100 ml of H 2 0 to provide an Al(O- -C 3 H 7 ) 3 :H 2 O molar ratio of 1 : 150. The sol was acidified by adding about 0.5 ml of concentrated HNO 3 to bring its pH to about 3. The sol was loaded into a sprayer and sprayed onto a flat, stainless steel plate to form a uniform coating about 1 μm thick. The plate had been heated to about 150 °C before spraying to drive off the water in the sol as it contacted the plate. The coated substrate was placed in a quartz reactor and heated to 500°C in air to convert the gel to Al 2 O 3 . A He atmosphere was then established in the reactor and the reactor temperature was raised to 1050°C at a rate of 11.5°C/min. Once the reactor reached 1050°C, the He was tumed off and NH 3 and CH 4 gases were flowed into the reactor at 400 ml/min and 30 ml/min, respectively. This created a N:C ratio of 13.33 in the reactor. After allowing the reaction to proceed for 40 min, the NH 3 and CH 4 gases were tumed off and the substrate was cooled to room temperature in He. X-ray photoelectron spectroscopy showed the coating was substantially carbon-free and consisted of AIN. Fig. 4 is an x-ray diffraction pattern for an AIN coating on a steel substrate. We claim:

Claims: Claims 1. A method of making an AIN coated substrate, comprising the steps of: (a) coating a substrate with aluminum oxide or aluminum hydroxide derived from a sol gel, (b) heating the coated substrate to a reaction temperature of at least about 850 °C in a nonreactive atmosphere, (c) contacting the heated, coated substrate with a gaseous reactant mixture comprising a nitrogen source and a carbon source, wherein the molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15, and (d) maintaining the coated substrate at the reaction temperature for a sufficient time to convert the sol gel-derived coating to an AIN coating. 2. The method of claim 1, wherein the nitrogen source comprises NH 3 or N 2 H 2 . 3. The method of claims 1 or 2, wherein the carbon source comprises a hydrocarbon that is a gas at the reaction temperature or an amine that is a gas at the reaction temperature. 4. The method of claims 1 , 2, or 3, wherein the sol gel is made by a method comprising the steps of: (a) dispersing in water to form an Al(O-z-C,H 7 ) 3 /H 2 O sol, (b) acidifying the Al(O- -C 3 H 7 ) 3 /H 2 O sol, and (c) allowing the Al(O-/-C 3 H 7 ) 3 /H 2 O sol to sit for a sufficient time to form a gel. 5. The method of claims 1 , 2, 3, or 4, wherein the reaction temperature is about 1000°C to about 1275 °C. 6. The method of claims 1, 2, 3, 4, or 5, wherein the molar ratio of nitrogen to carbon in the gaseous reactant mixture is about 30 to about 40. 7. The method of claim 1, 2, 3, 4, 5, or 6, wherein the coated substrate is maintained at the reaction temperature for at least about 40 min. 8. The method of claim 1, wherein the reaction temperature is between about 1000°C and about 1100°C, the nitrogen source is NH 3 , the carbon source is CH 4 , the molar ratio of nitrogen to carbon is about 15 to about 2000, and the coated substrate is maintained at the reaction temperature for at least about 40 min. 9. An AIN coated substrate made by the method of claims 1, 2, 3, 4, 5, 6, 7, or 8. 10. The coated substrate of claim 9, wherein the substrate comprises a SiC yarn or monofilament. 11. A method of making a TiN coated substrate, comprising the steps of: (a) coating a substrate with titanium oxide or titanium hydroxide derived from a sol gel, (b) heating the coated substrate to a reaction temperature of at least about 750 °C in a nonreactive atmosphere, (c) contacting the heated, coated substrate with a gaseous reactant mixture comprising a nitrogen source and a carbon source, wherein the molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15, and (d) maintaining the coated substrate at the reaction temperature for a sufficient time to convert the sol gel-derived coating to a TiN coating. 12. The method of claim 11, wherein the nitrogen source comprises NH 3 or N 2 H 2 . 13. The method of claims 11, or 12, wherein the carbon source comprises a hydrocarbon that is a gas at the reaction temperature or an amine that is a gas at the reaction temperature. 14. The method of claims 11, 12, or 13, wherein the reaction temperature is greater than about 800°C and the conversion of the sol gel-derived coating to TiN is substantially complete.. 15. A TiN coated substrate made by the method of claims 11, 12, 13, or 14. 16. A method of making a ZrN coated substrate, comprising the steps of: (a) coating a substrate with zirconium oxide or zirconium hydroxide derived from a sol gel, (b) heating the coated substrate to a reaction temperature of at least about 1050 C C in a nonreactive atmosphere, (c) contacting the heated, coated substrate with a gaseous reactant mixture comprising a nitrogen source and a carbon source, wherein the molar ratio of nitrogen to carbon in the gaseous reactant mixture is at least about 15, and (d) maintaining the coated substrate at the reaction temperature for a sufficient time to convert the sol gel-coated coating to a ZrN coating. 17. The method of claim 16, wherein the nitrogen source comprises NH 3 or N 2 H 2 . 18. The method of claims 16 or 17, wherein the carbon source comprises a hydrocarbon that is a gas at the reaction temperature or an amine that is a gas at the reaction temperature. 19. The method of claims 16, 17, or 18, wherein the reaction temperature is greater than about 1100°C and the conversion of the sol gel-derived coating to ZrN is substantially complete. 20. A ZrN coated substrate made by the method of claims 16, 17, 18, or 19.

Inventors: GALASSO FRANCIS S, SUIB STEVEN LAWRENCE, TAN BENG JIT, XIAO YOUMING, GALASSO, FRANCIS, S., SUIB, STEVEN, LAWRENCE, TAN, BENG, JIT, XIAO, YOUMING
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1993
Main classification: C23C18
IPC codes: C04B41, C23C18, C23C24, C23C28