CA2151931A1

CA2151931A1 - Metal nitride coated substrates

Info

Publication number: CA2151931A1
Application number: CA002151931A
Authority: CA
Inventors: Youming Xiao; Beng Jit Tan; Steven Lawrence Suib; Francis S. Galasso
Original assignee: Individual
Current assignee: Raytheon Technologies Corp
Priority date: 1992-12-17
Filing date: 1993-12-14
Publication date: 1994-06-23
Also published as: EP0674723A1; JPH08504888A; KR950704536A; WO1994013853A1

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 °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

~10 94/13853 2151 9 31 PCT/US93/12150 Description Metal Nitride Coated SubsLld~es Terhnir~l Field The present illvellliOIl is directed to a method for making metal nitride coated~ul)sLl~leS.

Bac~.uund Art In recent years, there has been ill~ r~illg interest in non-oxide ceramics, such as metal nitrides and carbides, that possess high ~.,,pe~du.e strength and corrosion r~ re.
10 Among these materials, ~h.."il""ll nitride (AIN) is especi~lly hll~olL;~l because of its unique physical properties. For example, AIN has a thermal conductivity close to that of metals and more than 10 times that of alumina (Al203), a coefficient of thermal eYp~n~ion co~ ble to silicon and silicon carbide, a high electrical leSi~livi~y, and Ill~hAl-irAl strength col,l~d~le to alumina ceramics.
Metal nitride powders can be made in various ways. For example, a metal oxide powder, such as Al203, zirconia (ZrO2), or titania CliO~), can be mixed with an excess of a c&,l,onaceuus powder and heated to a ~lll~el~lw`e above 1100C in a nitrogen-culllail~ g al~llo~lJh~lè. The metal nitride powder formed by this method is, however, mixed with unreaeled ca,l,ollaceous powder that detracts from the propelliæ of the metal nitride powder.
The um~ea~led c~l onaceous powder can be réluuved by oXi~li7in~ it at lelllperdluies between about 600C and about 700C. At these lelllpe~dLu,es, however, a portion of the metal nitride powder also can oxidize.
United States Patent 4,975,260 to Imai et al. teaches an AlL~ "~le method for making a metal nitride powder by reacting a met~l oxide or metal hydroxide powder with a gaseous mixture of ammonia (NH3) and a hydloc~bon at a lelll~eldlule ranging from 1300C to 1600C. Although this method is an iulpmûvélllelll over some prior art methods, it still leaves residual carbon in the metal nitride product. Moreover, it requires a t~nrer~tl-re of at least Japanese Abstract Vol. 14, Number 123 (C-698) [4066] 11iccl~ses the "Formation of ~IIlminllm Nitride Filrn" by dissolving a metal alk~ in an organic solvent and hydrolyzing s the alkoxide to form a solution. The substrate is then coated with the solution and heated to about 600-1000C in a nitrogen ~tmosphere~
The journal article by Joseph Keddie et al on the "Effect of Nitrifl~tion Rate on the Composition and Conductivity of Titanium Nitride Films Prepared from Sol-&el Titania"
teaches that varying the nitridation rate effects the oxygen content and ~1ectri~l resistivity of the film.
These prior art methods have several drawbacks. For exampLe, they either make a product that contains signifiç~nt ~m~-l-ntc of carbon or require relatively high h~llp.,.aLulcs.
Moreover, they cannot make a contin~lonC~ coating on a s~bstr~ For some applic~tinn.c, though, such coatings are highly desirable.
Therefore, what is needed in the industry is method of making metal nitride coatings.

Disclosure of the Invention The present invention is directed to a method of making metal nitride coatings.
One aspect of the invention includes a method of making a metal nitride coated substrate. A substrate is coated with an oxide of Al, Ti, or Zr or a hvdroxide of Al, Ti, or Zr 20 derived from a sol gel. The coated substrdte is heated to a reaction temperature of at least about 750C in a nonreactive atmfsph~re and cont~çted with a gaseous reactant mixture culll~liaillg 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 m~int~in~d at the reaction h~.~c.dLulc for a s~ffiri~nt time to convert the sol gel^derived coating to a metal 2 5 nitride coating.
Another aspect of the present invention includes a metal nitride coated snbstr~te made by the me~od described above.
These and other features and advantages of the present invention will become more apparent from the following description and ~-~cù,\.p~ yi..g drawing.

'4~E~D~D S/tEET

~ r r 2~ il 9 3 1 - 2a -Brief Description of the Drawing Figure 1 is a scllrlllAIic of a spray coating a~al_Lus useful with the method of the present invention.
Figure 2 is a s~h~nlAtic of a dip coating apparatus useful with the method of the S present invention.
Figure 3 is an optical micrograph of coated SiC yarns of the present invention.
Figure 4 is an x-ray ~ ction pattern for a AIN coating of the present invention.

EI~ S~lE~

b ' ' ' 2 1 5 1 9 3 1 Best Mode for Carrying Out the Invention The method of the present invention can make AIN, TiN, or ZrN coatings on a variet~r of substrates. The starting m~teri~1~ for the coatings include sol gel-derived coatings of an oxide or hydroxide of Al, Ti, or Zr, such as Al203, TiO2, ZrO7, Y203, Al(OH)3, Ti(OH)4, or Zr(OH)4. The r~nn~in~er of the application describes the method of the present invention in terms of making AIN coatings from sol gel AkO3. One skilled in the art, huw~ , will understand that the following description, with appropriate adjll~trn~nt~ to reaction L~1.1pe1dLu1~, applies e~ually to methods of making AlN, TiN, and ZrN from Al, Ti, and Zr oxide or hydroxide sol gel-derived coatings.
0 The substrate may be any material that does not react urith the AIN coating and that can withstand p-oces.,l.1g con~iti~n~. For example, the substrate may be a metal or ceramic article. ~It~rn~t~ly, the s~lbstr~t~ may be a fiber in the form of a ml~nofil~m~nt 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 monofil~mPnr~, such as BP fiber (British Petroleum, Cleveland, OH), and carbon-coated SiC yarrl7 such as C-Nicalon~ yarn (Nippon Carbon Co., Tokyo, Japan).
To coat the s~lbstr~te by the method of the present invention, an Al203 p1~,u~,or coating is applied to the snbstr~te The Alz03 p1~Cu1aOi coating is contacted with a gaseous reactant m-iyture at suitable reaction con~itil n~ to form an AIN coatirlg. The gaseous reactant mixture co111p1;3e3 a nitrogen source and a carbon source. If NH3 is the nitrogen source and CH~, is the carbon source, the reaction can be written as:
Al203 (s3 + 2 NH3 (~,) + 3 CH~ (g) ~ 2 AIN (s) + 3 CO (p,) + 9 H2 (g) The Al203 coating may be any contimloll~, hi~,h surface area Al~03 coating, such as ~-Al203 or Al203 made with a sol gel method. The sol gel Al203 may be made with any method known in the art. For example, ~1.. ;.. isopropoxide [Al(O-i-C3H7)3] may be Lap~laed in water, for example water heated to about 75C, in any suitable molar ratio of Al(O-i-C3EI7)3:H20 to make a sol. Good results have been obtained with molar ratios between about 1:100 and about l:lOOO. In general, the more Al(O-i-C3H7)3 in the sol, the more viscous the sol will be. As explained below, the p1c;r~ ratio of Al(O-i-C3H7)3:H20 depends on the substrate to be coated. Suitable Al(O-i-C3H7)3 may be puil.hased from CG.. 1.. 1~,~. ;al sources, in~ lin~ Alfa Products ~i t AMENDED SHEET

~0 94/13853 2 ~ 31 PCT/US93/12150 (Oanvers, MA). A small amount of HNO3 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 s~lffi~iently viscous to make a gel than can be applied to the ~ub~llale 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 sul,~L,d~e by any collvel.lionalmethod. 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 appata~Lls for ~layiug the sol from a spraying means 2 onto the substrate 4. The spraying means 2 can be any collvel,Liollal s~layillg e~ui~ nl. Preferably, the sol's viscosity will be 10 controlled so it is c~.u~dLible with the ~l~hlg means or other e4~ P~l used to apply the coating. The sol's viSC(jSily 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 sub~lld~e 4 or to particular portions of the substrate. In either case, the coating will prl,f~ly cover the surfaces to which it is applied llnifonnly. If desired, the s~lb~llaLe 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 col,v~llienl thic~nPs~ and can be applied as more than one coat.
Fibers and yarns are p~ert;,dl?ly coated by dipping the fibers or yarns into a sol. Fig.
2 shows a dip coating d~p~dlu5 in which a fiber (or yarn) 8 is dipped into a sol 10 in a suitable conL~iner 12. As the fiber 8 is withdrawn from the sol 10, a gel film 14 forms on the outside surfaces of the fiber. P~t;Ç~,~bly, the gel will be thin enough to coat each fiber in a yarn without bri~lging. Often, the coating on yarns will look bridged immPAi~tely after it is applied. If the sol's viscosiLy is prop~;~ly stol~teA the brici~ing will dis~peal as the gel dries. Good results have been obtained with sols made with Al(O-i-C3H7)3:H2O molar ratios of about 1:100 to about 1:1000.
After coating the sul)~llal~, the gel on the ~ubsliale may be allowed to dry for a sufficient time to partially convert it to Al2O3 and to allow it to be handled more easily. The coated sul,sL,ale may then be fired at a suitable temperature in a suitable atmosphere to complete the c.,l,vt;,~ion of the gel to Al203. The drying and/or firing Lelul,~,alule and time depend on the thi~n~ of the coating and the particular substrate. The coated substrate may be dried and/or fired at t~ e,~Lures ranging from room Lelll~elaLule to the temperature at ~WO 94/13853 2 ~ 519 ~1 PCT/US93/121~0 which the Al2O3 will be collvel~d to AIN for several minutes to several days. For example, the coated substrate may be dried at about 500C for about 30 min. If the ~ub~l,a~e will not react with air at the drying or firing telllpt;ldlUle, the ~ul~l-ale may be dried or fired in air.
If, ho~t;ver, the substrate will react with air, the su~slld~ may be dried fired in an inert 5 a~--os~h~e. Helium is a suitable al~l~Gs~here for firing SiC fibers.
After drying and/or firing the coated substrate, it may be heated to the reaction ~Lu~elalurt; at a moderate rate, for example, about 11.5C/min. Prer~ably, the rate of heating will be selected to prevent the coating from spalling off the ~ub;~lla~. In general, the desired reaction to AIN will occur at leLu~eLalules of at least about 850C. Subst~nti~lly 10 complete collvt;l~ion to AIN can be readily obtained at lelllpt;ldul~s less than 1300C, such as at lt;lll~eld~ui~s of about 1000C and greater, although temperatures of up to about 1600C
may be used. The reaction leLl~laluie, ll~er~çu~ may range from about 850C to about 1600C. Prereral)ly the reaction ~ulperalule will be less than about 1275C or 1299C. For example, the reaction ~eLupelalulc may be between about 1000C and about 1275C. Most prt;rt;l~ly, the reaction l~luyeldule will be about 1000C to about 1100C. The reaction pl~ uie is not critical and may be any conveniently oblail,able p-es~u,e.
To make TiN, a TiO2 or Ti(OH), coating can be reacted with a gaseous reactant mixture at t~ elalu-es of at least about 750C. Sl~hst~nti~lly complete collve ~ion to TiN
can be obtained at lt;Lupt; dlu-es greater than about 800C. A ZrO2 or Zr(OH)4 coating can 20 be reacted with a gaseous reactant mixture at l~ eldult;s of at least about 1050C to make ZrN. S~bs~ lly complete coL,vt;l~ion to ZrN can be obtained at temperatures greater than about 1100C. It may be desirable to make the TiN or ZrN coating~ of the present invention at ~eul~eraluies less than 1300C, for example at ~e~ r~ ies less than 1275C or 1299C.
Prert;l~bly, the coated ~ulJ~lldt~ will be pr~;hed1ed to the reaction ~Lul~ldlure in an 25 inert ~llllnsph~ore. For example, the substrate may be pr~ilealed in He, Ne, Ar, Kr, or Xe.
The inert ~I...o~h~e should be selected to avoid ~lldt# degradation. If the coated substrate was dried or fired in an inert ~tmosph~re, the same atmosrh~re may be used to heat - the sul,slldte to the reaction leLu~eralure.
Once the substrate reaches the reaction lellllJe;lalule~ the gaseous reactant mixture, 30 cou.~.isillg nillogen and carbon sources, is flowed into the reactor at a rate s~ffi~ient to achieve a desired N:C ratio. Reaction conditions are ...~ i--~ for a sufficient time to 3 96~. PCT/US93/1215û~

convert the Al2O3 coating to AIN. Reaction times of about 1 min to several days at reaction IGlll~lGlaLUiGS between about 850C and about 1200C have been found suitable to convert the Al2O3 to AlN. Longer times may be n~cp-sc~y with lower tGlll~)GrdlUlGS. Similar reaction times are suitable to produce TiN and ZrN. Once the AIN coating is formed, the gaseous S reactant mixture is shut off and a nol~lGa.;~ivt; ~ h~le 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 NH3, N2H2, or N2. Anllydloùs NH3 is the prefGllGd nitrogen source because it is readily available and easy to use. NH3 may 10 be ~ul`chas~d from many suppliers, inlhl-ling Aero All Gas Company (Hartford, CT).
PlGrGl~bly, the nillogell source will not contain any co.~ that would produce side reactions or ulllG,wise hl~GlrGlG with the nitritl~ti~n reaction.
The carbon source should be a carbon~ulailling compound that is a gæ at the reaction conditions. For example, the carbon source may be a hydLoc~ul,on or an amine.
15 Although any hydroca.l,on that is gæeous at reaction conditions may be used, alkanes having four or fewer carbon atoms are pr~rG"Gd because they are easier to handle. Similarly, amines having four or fewer carbon atoms, such æ methyl amine (CH3NH2), are pr,f~l.,d.
Most prGrGl~ly, the carbon source will be CH4 because it is eæy to obtain and work with.
CH4 may be purchæed from many suppliers, inf h-tling Aero All Gæ Co. Preferably, the 20 carbon source will not contain any co..l;~ that would produce side reactions or otherwise hl~elrGre with the nitridation reaction.
The nillu~Gll and carbon sources in the gæeous reactant mixture may be supplied from separate sources or from a premixed source. In either cæe, it is prGrG,~ble that they be mixed U~ l`Galll of the reactor. The ratios of N:Al203 and C:AI203 in the reactor are not 25 critical, although a~ t~o amounts of gæeous ,~ua~ should be used to convert the Al2O3 to AIN in a desired time. The ratio of nitrogen to carbon (N:C) in the gaseous reactant mixture is critical to ob~inmg a ~..h~ lly carbon-free product. To form such a product, the molar ratio of N:C in the gæeous reactant mixture should be at least about 15.
Preferably, the N:C ratio will be between about 15 and about 2000. Most prGrGi~bly, the 30 N:C ratio will be between about 30 and about 40.

2~5f931 -~! .

If desired, Hz may be added to the gaseous reactant mixture to expand the range of c~7n-~itionc under which substantially carbon-~ee AlN can be rnade. Any excess of H2 will ~cilitAte the conversion of the sol gel-deI'ived coating to the nitride coating. H2 may be obtained from many CO~ ,idl suppliers.
The following examples r~ AI~ the present invention without limiting the invention's broad scope.

Example 1 (SiC Yarn) An Al20~ sol gel was prepared by dispersin~ 14 g of Al(O-i-C3H7)3 in 190 ml of H20 - 10 to make a sol with an Al(O-i-C3H7)3:H20 molar ratio of 1:5~5. The sol was ~ ifi~d by adding about 2 ml of cnn~ Alrd HNO3 to bring its pH to about 3 and was allowed to sit overnight to thicker~ er 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 5 yarns. The yarns were dipped into the sol. After 5 minllt~, the yarns were slowly lt;uved from the sol and allowed to air dry at room temperature overnight. When first l~.llov~d from the sol, the gel coating the yarns looked like it bridged_that is, coated more than one fiber in the yarns rather than each fiber. A~er 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 500C in He. The temperablre in the reactor was held at 500C for 30 min to convert the gel to Al2O3. The reactor temperature was then raised to 1050C at a rate of 11.5C/rnin.
Once the reactor reached 1050C, the He was turned offand NH3 and C~, gases were flowed into the reactor at 400 ml/min and 30 ml/min, l~s~c~liv~ly. After allowing the reaction to proceed for 40 rnin, the NH; and CE~ gases were turned o and the yarns were cooled to room 2 5 temperature in He.
FXAIII;nAI;nn ofthe yarns af'cer the reaction showed they had a uniform coating about 2 ~Lm thiclc. Fig. 3, an optical micrograph od coated SiC yarn, shows that this coating did not bridge between fibers. Rather, it covered individual fibers. X-ray pho~o~ tron ~1 A~AEN~ED S~EET

r ~
- 21~193~

spectroscopy showed the coating was substantially carbon-free and completely cuuvt;ll~d to ~lN.

Example 2 (SiC Yarn) s E~ample 1 was repeated using a sol with an Al(O-i-C3H~)3:HzO ratio of 1:1000.
IILspection of the yarn 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 cuuv~ d to AlN with no substantial carbon deposition.

Example 3 0 (Steel Substrate) An Al2O3 sol gel was ~ d by di~,.aillg 2 g of Al(O-i-C3H~)3 in 100 ml of H20 to provide an Al(O-i-C3H,)3:H20 molar ratio of 1:150. The sol was ~ci-lified by adding about 0.5 ml of concentrated HNO3 to bring its pH to about 3. The sol was loaded into a sprayer and sprayed onto a flat, 5t~inless steel plate to form a uniform coating about 1 ~n thick The plate had been heated to about 150 C before spraying to drive off the water in the sol as it cont~cted the plate. The coated substrate was placed in a quartz reactor and heated to 500C in air to convert the gel to Al2O3. A He atmosphere was then established in the reactor and the reactor temperature was raised to 10~0C at a rate of 11.5C/min. Once the reactor reached 1050C, the He was tumed o~ and NH3 and CE~ gases were flowed into the reactor at 400 mL/min and 30 ml/min, l~ e~iliv~ly. After allowing the reaction to proceed for 40 min, the NH3 and CH4 gases were tumed off and the substrate was cooled to room ~ ,-aLul~ in He.
X-ray ph-~to.~ tron spectroscopy showed the coating was substantially carbon-free and consisted of AlN. Fig. 4 is an ~-ray ~iffi~ction pattern for an AIN coating on a steel substrate.
2 5 We claim: ~

AMENDED SHEET

Claims

1. A method of making an AlN 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 AlN coating;
whereby said AlN coating is substantially carbon-free.

2. The method of claim 1, wherein the nitrogen source comprises NH3 or N2H2.

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 Al(O-i-C3H7)3 in water to form an Al(O-i-C3H7)3/H2O sol, (b) acidifying the Al(O-i-C3H,)3/H2O sol, and (c) allowing the Al(O-i-C3H7)3/H2O 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 about1000°C to about 1275°C.

6. The method of claims 1, 2, 3, 4, or 5, wherein the molar ratio of nitrogen tocarbon in the gaseous reactant mixture is about 30 to about 40.

-9a-

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 NH3, the carbon source is CH4, 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 AlN 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;
whereby said coating is substantially carbon-free.

12. The method of claim 11, wherein the nitrogen source comprises NH3 or N2H2.

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.

- 10a -

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 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;
whereby said ZrN coating is substantially carbon free.

17. The method of claim 16, wherein the nitrogen source comprises NH3 or N2H2.

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.