i v.* ^r ^v.* * .. • *W .V /.c^.% ^.-^L^V t ,o*..--,.*o ^ »' 4* v^4r % I™ . t • o "*%. 1 > • s% ■■■ ^d* S °*. -i . » • A * 4* -^ • . » * A ^°^ ;• > u ^k -: ^d« °i, ' • • • .<* <* ♦_ x0 % ».LaJ* * > ♦ 0>V 3 +JtMj^K m Vj, ^V xO V .•**£' ^6 x« * p •!!£?• >,■•• 7. •',*•' «G^ *o, *< v »!«.i% % x*° *^~'* - ft**** * «? ^ « ^ ,$< '* .0° iTn.* x0 >*-.litf# ^ v % ^ ^^ JCJ 8928 Bureau of Mines Information Circular/1983 .; 2'c1983 Chemical Vapor Deposition of Group IVB, VB, and VIB Elements With Nonmetals A Literature Review By H. O. McDonald and J. B. Stephenson UNITED STATES DEPARTMENT OF THE INTERIOR ■j^faf &*ti^- &sx>^- ~j p^ Information Circular 8928 \ Chemical Vapor Deposition of Group IVB, VB, and VIB Elements With Nonmetals A Literature Review By H. O. McDonald and J. B. Stephenson UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Horton, Director ^z°t> # This publication has been cataloged as follows: McDonald, H, 4 (Hector 0.) Chemical vapor deposition of group IVB, VB, and VIB elements with nonmetals: a literature review. (Information circular / Bureau of Mines : 8928) Bibliography: p. ]4-29. Supt. of Docs, no.: I 28.27:8928. 1. Vapor-plating. I. Stephenson, J. B. (J ames Blake), 1942- ll. Title. 111. Series: Information' circular 'United States. Bureau of Mines) ; 8928. TN295.U4 [TS695] 622s [671, 7'35 1 83-600000 CONTENTS Page ■to VAbstract 1 Introduction 2 Group IVB metals (Ti, Zr, Hf) 2 Titanium boride 2 Zirconium boride 3 Titanium carbide 3 Zirconium carbide 5 Hafnium carbide 5 Titanium carbonitride 5 Titanium nitride 6 K Zirconium and haf nium nitrides 7 Miscellaneous compounds 7 oup VB metals [V, Nb(Cb), Ta] 9 Miscellaneous vanadium compounds 9 Niobium and tantalum borides 9 Niobium and tantalum carbides 9 Niobium and tantalum nitrides 9 Miscellaneous compounds 10 Group VIB metals (Cr, Mo, W) *. 10 Chromium carbide 10 Molybdenum carbide 11 Tungsten carbide 12 Molybdenum and tungsten borides and silicides 12 Conclusions 13 References 14 TABLES 1 . Some CVD reactions for group IVB elements 8 2. Some CVD reactions for group VB elements 11 3. Some CVD reactions for group VIB elements 13 - UNIT OF MEASURE ABBREVIATIONS USED IN. THIS REPORT atm atmosphere min minute ° C degree Celsius mL milliliter cm centimeter mm millimeter eV electron volt urn micrometer hr hour mole gram mole of material Hz hertz, reciprocal s econd pet percent kcal kilocalorie sec second kg kilogram torr millimeter of Hg pressure mA milliampere CHEMICAL VAPOR DEPOSITION OF GROUP IVB, VB, AND VIB ELEMENTS WITH NONMETALS A Literature Review By H. 0, McDonald ' and J. B. Stephenson 2 ABSTRACT The Bureau of Mines reviewed the chemical vapor deposition (CVD) lit- erature on the nonmetal binary and ternary compounds of the group IVB, VB, and VIB elements, with emphasis directed to the following nonmetals: B, C, N, 0, and Si. This review examines each of these binary and se- lected ternary compounds of the group IVB, VB, and VIB elements as coat- ings and gives some of their preparative methods, uses, and properties. A total of 259 references were found for these compounds of the nine elements. This review was utilized in the Bureau's research to provide abrasion-, erosion-, and corrosion-resistant coatings in order to con- serve critical metals and protect various metallic surfaces in metallur- gical, mining, and energy conversion systems. 1 Research chemist, Rolla Research Center, Bureau of Mines, Rolla, Mo.; associate professor of chemistry, University of Missouri — Rolla, Rolla, Mo. ^Research chemist, Rolla Research Center, Bureau of Mines, Rolla, Mo. INTRODUCTION Chemical vapor deposition (CVD) can be defined as a system in which one or more gaseous substances react on a heated sub- strate to form a compound or an element. CVD coatings have assumed a vital role in expanding the horizons of materials con- servation; CVD coatings have a signifi- cant influence on material properties, providing improved corrosion resistance, electrical contact resistance, reflectiv- ity, color, abrasion resistance, erosion resistance, and solderability , or a de- crease in the coefficient of friction. Preparation of semiconductor and super- conductor materials relies heavily on CVD technology. Using CVD coatings can re- duce the use of critical and strategic materials — while retaining improvements in desired material performance — in a wide variety of applications. CVD research by the Bureau of Mines has been conducted to minimize the consumption of strategic and critical materials in the manufacture of erosion-, abrasion-, and corrosion-resistant com- ponents used in metallurgical, mining, and energy conversion systems. Test re- sults were recently reported for one CVD- coated ball valve seat prepared during previous Bureau research ( 205 ) . 3 The Bureau, a pioneer in the prepara- tion of CVD tungsten, is reviewing the literature relating to the deposition of abrasion-, erosion-, and corrosion- resistant coatings of the group IVB, VB, and VIB elements and compounds. A literature review of the group IVB, VB, and VIB elements was published in 1979 ( 133 ) . This present review brings to- gether many of the references that have appeared since about 1966 on the binary and selected ternary compounds of the group IVB, VB, and VIB elements with B, C, N, 0, and Si. Several CVD reviews have been pub- lished that give some of the deposition techniques, as well as the properties of the deposited metals and some of the binary compounds. One such article, by Archer (_5 ) , concerns a few metals and some metalloids. Broszeit and Ga- briel (31) have reviewed CVD techniques for protective coating and treatment for tools and structural parts. Perry and Archer ( 162-163 ) have surveyed wear- resistant coatings and some techniques of CVD. Yee ( 258 ) has made quite an exten- sive review of the use of CVD for pro- tective coatings. In addition to the review articles, there have been eight international con- ferences on CVD (21_-23, 42, 61_, _197, 203 , 252 ) , which will not be covered in gen- eral here. In this review, the CVD literature will be considered for each group IVB, VB, and VIB metal, by periodic family, fol- lowed in each section by discussion of some of the methods of preparation, uses, and properties of the nonmetal deposits. GROUP IVB METALS (Ti, Zr, Hf) TITANIUM BORIDE Titanium diboride (TiB 2 ) is usually de- posited by the reaction of TiCl4 with BCI3 and H 2 at temperatures varying from 850° to 1,400° C and near 1 atm total pressure ( 130 , 167 , 225 ) . Pierson and Randich have shown that TiB2 can be de- posited on Ta and stainless steels at ^Underlined numbers in parentheses re- fer to items in the list of references at the end of this report. temperatures of 850° to 1,100° C, yield- ing surfaces with good erosion resistance and Knoop hardness of about 3,300 kg mm -2 ( 175 ). Takahashi, Sugiyama, and Suzuki grew TiB 2 fibers using a gas mixture of TiCl 4 , BCI3, H 2 , and Ar in an alternating current (ac) discharge ( 232 ) . Under these conditions, the reaction could be accomplished at the lower temperatures of 300° to 700° C. Maximum growth was ob- tained at 400° C and discharge currents of 0.4 to 0.6 mA. Other investigators studied the growth of TiB 2 whiskers on quartz (146). They found that if small amounts of Au, Pt, or Pd were painted onto the substrates, the whisker growth was improved. Some single crystals of TiB 2 grown on Ta wire were demonstra- ted ( 212 ); however, the addition of HC1 was essential, as well as temperatures of 1,700° to 1,900° C. Several investi- gators studied the formation of TiB 2 on graphite substrates (1_8, 178-179 , 237 ). Besmann and Spear have published ther- modynamic ( 16 ) and kinetic (17) studies of the CVD of TiB 2 . Their results indi- cated that TiCl 3 , as well as HBC1 2 and HC1, was also present (16). They ob- tained a simple linear rate expression that was used to calculate an activation energy of 40±12 kcal mole -1 ( 15 , 17 ) for the deposition. Randich and Gerlach have published a method for calculating the phase diagram for the Ti-B-Cl-H system ( 188-189 ). Pierson and Randich ( 176-177 ) investi- gated the interaction of TiB 2 with the substrate. They found the substrate should not be deformed or transformed at temperatures up to 1,100° C, nor should it react with the byproducts of the reac- tion, in particular HC1. There was in some cases a thermal expansion difference that had to be corrected by the use of an interlayer of Ni or Cu. The substrates that could be used were Mo, W, Ta, Kovar, and high-Cr steels, as well as WC and TiC. These investigators found that a boride interlayer, usually M 3 B or M 2 B, was formed, possibly by diffusion. Here M could be a pure metal or an alloy. Ultrasound was employed to prepare thick films of TiB 2 on low carbon steel ( 220 ) . Pierson and Mullendore have reported the preparation of TiB 2 using B 2 H 6 instead of BCI3 ( 174 ) . Dense and adherent coatings were obtained using B 2 H 6 with TiCl 4 and H 2 at 600° to 900° C on graphite substrates. Bonetti, Comte, and Hintermann (28) used metal borohydride compounds to boronize metals. In this process, Ti(BH 4 ) 5 was produced in situ by the re- action of TiCl4 with LiBH 4 in an air-free system at low temperatures. The thermal decomposition of Ti(BH 4 ) 3 at tempera- tures of 300° to 500° C yielded yellow metallic deposits, which were not found to be suitable at the present stage of this technology (28) . Good wear-resistant coatings of tita- nium boronitride (TiB 2 + x N y ) have recently been reported ( 170-171 ) . These coat- ings were produced by the action of TiCl 4 and BC1 3 with N 2 and H 2 at 1,150° to 1,450° C and pressures of 10 to 20 torr ( 171 ). The cubic boronitrides with an atomic ratio B/(B + N) of less than 0.75 had microhardness values up to 2,600 kg mm" 2 (170). ZIRCONIUM BORIDE Whiskers of zirconium boride (ZrB 2 ) were formed on quartz substrates at tem- peratures of 1,000° to 1,200° C from a mixture of ZrCl 4 , BC1 3 , H 2» and Ar ( 145 ). Randich synthesized some alloy borides of the form (Ti, Zr)B 2 ( 185-186 ). These gave Vickers hardness values of approxi- mately 3,700 kg mm" 2 for (Ti, Zr)B 2 and 2,200 kg mm" 2 for ZrB 2 . These alloys were prepared by the H 2 reduction of the metal chloride and BCI3 ^ n tne tem- perature range 800° to 1,100° C, with graphite as the substrate. Takahashi and Kamiya ( 224 ) studied the system Tii_ x Zr x B 2 , where x = 1 to 0. They found that increasing the temperature increased the deposition rate, but the rate was not dependent on total flow rate. The tem- perature range was 900° to 1,400° C, and dense uniform deposits resulted when the metal halide partial pressures were high compared with the partial pressure of BCI3. TITANIUM CARBIDE Titanium carbide (TiC) formation tech- nology is far more advanced than that for TiB 2 . Titanium carbide can be formed by the H 2 reduction of TiCl 4 on graphite at 1,000° to 1,900° C (50) or by the action of H 2 on a mixture of TiCl 4 and a suit- able hydrocarbon at temperatures ranging from 850° to 1,350° C ( 228 , 230 , 233). The hydrocarbon can be propane ( 228 , 233 ), methane (46), isopentane ( 230 ) , and ethane, or ethene ( 230 ). Even car- bon tetrachloride (CC1 4 ) can be used as the source of C (38, 152, 190). Toulene ( 138 , 180 ) and benzene ( 139 ) have been employed to coat steel as well as several hard alloys with TiC. Several patents have been granted for processes to coat manufactured objects ( 137 ) , gun barrels ( 253 ) , and even composite sub- strates ( 209 ) . Two patents are concerned with codepositing TiC with ductile metals such as Co and Ni ( 38 , 253 ) , which is ac- complished by introducing CoCl 2 or NiCl 2 as vapors with a carrier gas such as Ar or He. Pearce and Marek ( 161 ) have given ex- perimental and thermodynamic data to in- dicate that C needs to be present to re- duce TiCl 4 with H 2 efficiently. This was also experimentally verified by Aggour, Fitzer, and Schlichting (1). There have been several thermodynamic equilibrium treatments involving the Ti- Cl-H-C system over several temperature ranges ( 14 , 43 , 113 , 127 ) . In addition to these equilibrium treatments, there have been several rate studies reported ( 107 , 208 , 211 ). Kato, Yasunaga, and Tamari reported a growth rate of 1.2 x 10 cm sec - for TiC grown from the re- duction of TiCl 4 and methane with H 2 at 1,360° C (107). The TiC whiskers grown on graphite were in the [111] direction. Stjernberg, Gass, and Hintermann ( 208 ) have reported that the rate of deposition of TiC is proportional to the methane concentration and inversely proportional to the HC1 concentration at high HC1 con- centrations. They used the Langmuir- Hinshelwood mechanism to explain their experimental data. Subrahmanyam, Lahiri, and Abraham ( 211 ) have shown that the ob-> served rate of formation of TiC from a mixture of TiCl 4 , toluene, and H 2 is really a combination of a chemical reac- tion rate and a diffusion-controlled rate. When large flow rates are used, a plot of the logarithm of deposition rate versus the reciprocal of the absolute temperature produces a linear plot, com- pared with low flow rates, which give a nonlinear plot. This nonlinearity is due to diffusion rates. A thermodynamic approach has been published concerning the deposition of nonstoichiometric car- bides of Ti (245). Titanium carbide has been vapor- deposited onto cemented carbide sub- strates (65, 122 , 210 , 240 ) and onto sin- tered hard carbide substrates ( 121 ). In one case, the interface between the CVD TiC and cemented carbide substrate ( 204 ) was examined by scanning electron micros- copy as well as Auger electron spectros- copy. There was evidence of C loss from the substrate during the formation of the TiC coating. Lee and Richman ( 122 ) found that the presence of air or water in the coating system changed the growth rate as well as the coating structure be- cause of the fine particles of Ti0 2 that were formed. Karp, Filip, and Gibas ( 103 ) found that TiC growth oc- curred in the [111] direction on sintered carbide substrates. The wear resistance of steels has been improved by coating with TiC ( 44 , 93 , 165 ) . There is a review article without references by Yamakishi ( 256 ) on the TiC treatment of steel, and an article on the industrial applications of TiC coat- ings on steel ( 166 ) . Several patents have been granted (41, 79, JL40, 182 , 215) that are concerned with the deposi- tion of TiC or the apparatus for its deposition. TiC and TiN coatings reduce fric- tion ( 75 , 183 ) or strengthen the sur- face ( 202 ) and are also used as decora- tive coatings ( 200 ) . Schintlmeister and Pacher ( 198-199 ) have discussed several of these applications and have predicted a great future for these coatings. Bonetti (27) has given some hardness val- ues as well as thermal expansion coef- ficients for TiC on various cemented carbide substrates. The use of lasers (3_, 131 ) and of plas- mas (48-49) for the deposition of TiC has been applied with success. The future of laser chemical vapor deposition (LCVD) seems secure because improved control of heating and cooling rates can be obtained and cleaner surfaces are exposed to the coating process. ZIRCONIUM CARBIDE Some of the main uses of CVD ZrC are as coatings for cutting tools ( 112 , 255 ) and for Th0 2 spheres (85) and as reactor fuel particles (54, 157 ). HAFNIUM CARBIDE Zirconium carbide (ZrC) is usually formed by the reduction of an appropriate zirconium halide with H 2 and a suitable hydrocarbon. This can be done at tem- peratures of 800° to 1,200° C ( 229 , 236 ). Using methane, ZrCl 4 , and H 2 , Tamari and Kato ( 236 ) found that ZrC grew preferen- tially in the [100] direction with side planes generally {l00}. The halide can be generated in situ by the action of a halogenation agent such as methylene chloride upon Zr sponge at 600° C ( 191 ) , or it can be obtained by the sublimation of ZrCl 4 at 210° to 310° C (235). In one method, ZrCl 4 was fed into a fluidized- bed reactor as a fine powder (86). Ikawa and Iwamoto ( 98 ) employed methyl iodide vapor on Zr sponge to produce Zrl 4 . The methyl iodide was reacted with the Zr at 400° to 800° C, and then ZrC was formed at temperatures above 1,000° C in a sepa- rate reaction zone ( 97 , 99 ) or by addi- tion of H 2 at 1,100° C (98). Recently, Ikawa (95) produced ZrC by first reacting Br 2 with Zr sponge at 600° C and then reacting methane and H 2 at approximately 1,400° C with the ZrBr 4 . The effect of the gas composition on the deposition of ZrC has been reported ( 156 ) . Most of the ZrC obtained by CVD methods is produced from ZrCl 4 reacting with H 2 and methane, as is evidenced by several investigations ( 39 , 96 , 184 , 196 , 250 ). Samoilenko and Pereselentseva ( 196 ) de- posited ZrC on W wire at temperatures of 1,300° to 1,500° C and obtained good growth rates with an activation energy of 21 kcal mole -1 . Ikawa ( 96 ) found that for good ZrC formation both H 2 and the hydrocarbon must be present. Ambartsumyan and Babich (4) found that ZrC formed on graphite by the reaction of ZrCl 4 and H 2 obeys the rate equation: V = 4.30 x 10 3 exp (-1660/T) um min -1 Hafnium carbide (HfC) is produced by CVD from the chloride or the iodide with a H 2 and hydrocarbon mixture. Hertz, Spitz, and Besson (70-71) studied the conditions for forming HfC from HfCl 4 , H 2 , and methane. They found that stoi- chiometric HfC could be deposited (70) at temperatures of 1,200° to 1,500° C and at methane-to-HfCl 4 ratios of 0.25 to 4.5. Hertz, Spitz, and Besson (72) have re- ported that a maximum deposition rate of approximately 350 um hr -1 at 1,500° C could be obtained when the H 2 -to-methane ratio was approximately 30 and the methane-to-HfCl 4 ratio was approximately 2. The presence of free C lowered the coating adherence (71). In general, a large excess of H 2 is employed to yield hard adhesive coatings (69). At least two U.S. patents ( 29 , 62 ) concerning HfC have been granted, of which one is for coating SiC fibers that are used to re- inforce certain composites (62). There have been at least five Japanese patents (56-58, 77-78) that report the preparation of HfC from the action of Hfl 4 with a suitable hydrocarbon. In these methods, I 2 and Hf are reacted first to form Hfl 4 at 200° to 600° C; care is required to prevent the decompo- sition of the Hfl 4 (57). The hydrocar- bon, propane or butane (56), and the Hfl 4 are then reacted on the substrate at temperatures of 800° to 1,250° C (58). These patents were for HfC coatings on cutting tools. TITANIUM CARBONITRIDE Since both TiC and TiN offer good cor- rosion, abrasion, and erosion protection, it would seem that titanium carbonitride (TiCxNj.x) might offer better protection. With this idea in mind, several inves- tigators have studied the carbonitride system (40, 172-173, 198). Denker (40) studied the mechanical, chemical, and electrical properties of monocarbides, mononitrides , and monoxides of several cubic systems. It was Denker's evalua- tion of the materials that predicted the carbonitrides would be useful as coating materials, particularly for the element Ti. Titanium carbonitrides are prepared by the action of TiCl 4 with H 2 , N 2 , and methane at approximately 900° C ( 37 , 195 , 201 , 206-207 ), Instead of methane, eth- ane, propane (25) , chlorobenzene, pyr- idine ( 251 ) , or CC1 4 ( 154 ) can be used. Takahashi and Itoh ( 219 ) used an ultra- sonic field to form carbonitride films 30 to 120 urn thick. Reactants of propane or methane along with H 2 , N 2 , and TiCl 4 were used to produce the carbonitride films. The investigators found the Vick- ers microhardness increased from 1,850 to 3,600 kg mm -2 as x changed from to 1 in the formula TiC x Ni_ x . They also found that films produced in an ultrasonic field were more adherent than those pro- duced without it. Bitzer and Lohmann (19-20) patented a process for preparing diffusion coatings at 800° to 1,400° C, using suitable or- ganic compounds such as cyanuryl chlo- ride, acetonitrile, propionitrile, or tetracyanoethylene as the source of C and N. In these cases, the substrate was Ti or Ti alloy and Ar was the carrier gas for the organic compound. Bloom (24) employed trimethyl amine along with TiCl 4 and H 2 at 550° to 750° C to form carboni- tride coatings on steel. Yaws and Wake- field ( 257 ) have reported on a scaled-up system that used the amine and TiCl4 at temperatures of 600° to 700° C. TITANIUM NITRIDE Titanium nitride (TiN) is generally de- posited by the action of H 2 and N 2 with TiCl4 at temperatures ranging from 700° to 1,400° C (2, 104-105 , 216 ). Kato and Tamari ( 104 ) studied the crystal growth of TiN on graphite, and found the growth rate nearly proportional to the square root of the H 2 partial pressure. They also found the TiN to grow in the [111] direction preferentially. High frequency discharge conditions have been used to obtain TiN as a powder ( 241 ). Peter- son ( 168 ) has reported on the role of the partial pressure of TiCl 4 in the pro- duction of TiN. He found that low TiCl 4 partial pressures produced columnar grains, whereas higher partial pressures resulted in randomly oriented grains. In addition, the lower partial pressure re- sulted in a faster coating rate. One Japanese patent ( 100 ) involves the coating of W or Mo alloys with TiN. Sadahiro, Cho, and Yamaya ( 194 ) stud- ied the effect of temperature and gas composition on the deposition of TiN onto cemented carbides. Okamoto and Umezawa investigated the coating of mild steel with TiN, TiC, or Ti ( 158 ). They found Vickers hardnesses of 1,600 to 1,800 kg mm -2 for the TiN coatings and 1,800 to 3,600 kg mm" 2 for the TiC coat- ings. Takahashi and Itoh ( 218 ) obtained TiN coatings with Vickers hardnesses of 1,600 to 2,000 kg mm -2 when the deposi- tion was conducted in an ultrasonic field. In addition, the film had a strong <200> orientation. Some investi- gators who studied the growth rate of TiN on Mo wire have suggested that the mech- anism of growth was surface controlled in the early stages ( 142 ) . The linear growths were on (100) planes and were cubic single crystals. The CVD tempera- tures used in this study were between 1,600° and 2,200° C, with a gas flow ratio of 2N 2 to TiCl 4 of 0.7 to 1.0. Kagawa ( 101 ) investigated the deposi- tion of TiN using TiBr 4 instead of TiCl 4 . When N 2 was used as the carrier gas, the substrate temperatures needed to be greater than 1,260° C, but slightly lower temperatures could be employed when a mixture of H 2 and N 2 was the carrier gas. Bo jar ski, Wokulaska, and Wokulska (26) grew TiN whiskers on W substrates by the reaction of TiCl 4 with N 2 and H 2 at tem- peratures from 1,200° to 1,450° C. The crystal growth was found to be in the [001] direction with well-formed pyra- mid cube tips. Some organometallic compounds have been utilized with the object of producing good deposits at lower temperatures. One such compound was the liquid titanium tetrakis (dimethylamide) , Ti[N(CH 3 ) 2 ]4 ( 213 ). However, a temperature of 800° C was necessary for good TiN formation be- cause lower temperatures were not suf- ficient for the Ti and N to interact and combine. When titanium tetrakis (di- ethylamide) was decomposed at 10" 2 torr and 350° to 650° C on ceramic substrates, a phase that was shown to be Ti(CN) was formed ( 120 ) . During the decomposition, H 2 , methane, ethane, and ethene were formed. The thermal decomposition of metal coordination compounds of Ti with 2,2'-bipyridine (bipy) has been patented (34). When 2 and N 2 were introduced with the Ti(bipy)3 compound, TiN was said to be deposited. The compound was sublimed at 250° to 400° C at 10" 4 to 10" torr and then decomposed on the sub- strate at about 500° C. Hintermann (74) has reported on the coating of bearing surfaces with TiC or Ti(CN) for use in places where high wear resistance is needed. Deposition of TiN on steel substrates has been accomplished by the action of a radio frequency discharge upon a mixture of TiCl 4 with N 2 mixture ( 116 ) . 850° to 950 { or with an N 2 and H 2 Lower temperatures of C could be used. ZIRCONIUM AND HAFNIUM NITRIDES Zirconium nitride (ZrN) is usually de- posited by the CVD process at tempera- tures of 950° to 1,300° C from the gas mixture of ZrCl 4 , H 2 , and N 2 ( 51 , 141 , 221 ) . The best conditions were those in which the N 2 -to-ZrCl 4 ratio was greater than 1 and in which there was at least 40 mole pet H 2 in the gas mixture (221). Whiskers were grown at the higher tem- peratures and were usually in the <100> orientation ( 141 , 221 ) . If various impu- rities are coated on the substrate, whisker growth can be improved. The most effective impurities are the metals Ni, Pd, Pt, Fe, and Mn ( 106 , 144 ). Kato and Tamari ( 106 ) found the growth direction <100> to generally occur. In a study of the kinetics of ZrN formation from the gas phase, the reaction rate was found to change from first order to zero order in ZrCl 4 with increasing ZrCl 4 concentration, and an activation energy of 39 kcal mole -1 was calculated (52). There have been at least two thermody- namic studies reported ( 114 , 246 ) . Hafnium nitride (HfN) can be prepared in the same manner as is ZrN, except that HfCl 4 is generated in situ by the reac- tion of HC1 upon Hf at 700° C (63-64). It is quite likely that the lower chlo- rides of Hf are also formed ( 64 ) . In ad- dition to coating W wires (63) , HfN has been used to coat carbide tools for machining steel ( 192 ) . MISCELLANEOUS COMPOUNDS There are several types of compounds formed by CVD of the group IVB metals in addition to those discussed above, in particular the silicides, oxides, and sulfides. The production of TiSi or TiSi 2 has been achieved by the reaction of TiCl 4 with SiCl 4 , using H 2 gas in ex- cess ( 117-118 , 153 ) . In most cases , a graphite substrate was employed at tem- peratures of 900° to 1,300° C. Some thermodynamic calculations have been re- ported ( 117 ) . One German patent is listed ( 59 ) that describes the use of lower temperatures in a vacuum apparatus. Nickl, Schweitzer, and Luxenberg studied the system Ti-Si-C up to temperatures of 1,200° C using TiCl 4 , SiCl 4 , CC1 4 , and H 2 ( 155 ) . They reported that the ternary phases Ti 3 SiC 2 or Ti 5 Si3C x were deposited at normal pressures. At a temperature of 150° C, thin films of Ti0 2 can be produced by the CVD pro- cess involving H 2 vapor and tetraisopro- pyl titanate ( 53 , 66 ) . Powdered Ti0 2 as anatase was produced by the vapor phase reaction of 2 with TiCl 4 by Suyama and Kato ( 214 ). Thin films of Ti0 2 were pro- duced by the vapor pyrolysis of ethyl titanate on glass substrates a at a tem- perature of 445° C ( 234 ) . Titanium oxy- carbide (Ti0 0# 5C 0# 5) was produced by re- acting TiCl 4 , H 2 , C0 2 , CO, and methane in a reaction tube ( 111 ) . The oxy carbide was used to increase cemented carbide tool life. Films of Zr0 2 and Hf0 2 were prepared by the thermal decomposition of Zr or Hf 3~diketonate compounds in the gas phase with 2 (13). Crystals and whiskers of TiS 2 ( 143 ) and ZrS 2 ( 149 ) were produced by the vapor deposition of TiCl4 or ZrCl 4 reacting with H 2 S on quartz substrates. The nor- mal temperature was 400° to 850° C for TiS 2 and 800° C for ZrS 2 . In addition, Motojima, Takahashi, and Sugiyama (150) formed zirconium phosphide (ZrP) whiskers at 900° to 1,300° C with a mixture of ZrCl 4 , PC1 3 , and H 2 . A general summary of some of the reac- tions for the preparation of the group IVB metal compounds is given in table 1. TABLE 1. - Some CVD reactions for group IVB elements Reaction Vaporization Substrate temperature, °C temperature, 25- 60 850-1,400 200-250 900-1,300 25- 60 850-1,360 25- 60 1,200-1,600 25- 60 850-1,000 ^lO-SlO NAp 200-250 800-1,200 ] 600 NAp NAp -1,400 300 1,200-1,500 ^00-600 NAp NAp 800-1,250 25- 60 -900 25- 60 700-1,400 NAp 950-1,300 ^00 NAp NAp 900-1,300 TiCl^ + 2BC1 3 + 5H 2 * TiB 2 + 10HC1. ZrCl 4 + 2BC1 3 + 5H 2 ■*■ ZrB 2 + 10HC1, H 2 TiCl^ + CR\ — ► TiC + 4HC1, TiCl 4 + CCI4 + 4H 2 ->• TiC + 8HC1. STiCl^ + C 3 H 8 + 2H 2 -► 3TiC + 12HC1, Zr + 2CH 2 C1 2 ♦ ZrCl 4 + pyrolysis products, H 2 ZrCl^ + CR\ — ► ZrC + 4HC1, Zr + 2Br 2 ♦ ZrBr 4 , H 2 ZrBr 4 + CHt, -* ZrC + 4HBr, HfClu + CHl H 2 HfC + 4HC1, Hf + 2I 2 ■»■ Hfl 4 3HfI 1+ + C 3 H 8 ■► 3HfC + 8HI + 2I 2 , H 2 2TiCl 4 + 2CR\ + N 2 — ► 2Ti(CN) + 8HC1, 2HCI4 + 4H 2 + N 2 + 2TiN + 8HC1, 2ZrCl 4 + 4H 2 + N 2 -»• 2ZrN + 8HC1, 2Hf + 2xHCl * 2HfCl x + xH 2 2 ...., 2HfCl t+ + N 2 + 4H 2 > 2HfN + 8HC1, NAp Not applicable. *In situ. 2 Here x = 2, 3, 4. GROUP VB METALS [V, Nb(Cb), Ta] MISCELLANEOUS VANADIUM COMPOUNDS There are few CVD processes for the element V in the literature. Vanadium carbide (VC) was produced by the gas phase reaction of VCI2 with methane at 1,050° to 1,150° C (47) for the treatment of low carbon metal working tools. The VC1 2 was usually prepared in situ by the action of Cl 2 or HC1 upon V or ferro- vanadium ( 162 ) . In addition, Kieffer, Fister, and Heidler ( 109 ) deposited a titanium-vanadium nitride [(Ti,V)N) coat- ing on cemented carbides at 1,100° C, us- ing a mixture of TiCl4 , VCI4 , N 2 , and H 2 . Fine VN powder was produced by the action of VCI4 with NH 3 , H 2 , and N 2 at 700° to 1,200° C (80). There is a published process to form thin films of V 2 5 by the CVD reaction of V0C1 3 with H 2 vapor at room temperature ( 134 , 217) . NIOBIUM AND TANTALUM BORIDES Niobium diboride (NbB 2 ) was prepared by the reaction of NbCl 5 and BC1 3 with H 2 at temperatures of 950° to 1,200° C (148). The NbCl 5 and BC1 3 were prepared in situ by the action of Cl 2 on Nb foil and B 4 C at 500° and 800° C, respectively (148). In e similar manner, Motojima and Sugi- yama ( 147 ) deposited TaB 2 on quartz sub- strates at temperatures between 900° and 1,300° C. In this process, TaCl 5 was prepared by chlorination of Ta sponge at 500° C, and BCI3 was prepared from B 4 C at 800° C. These investigators found that the flow rates were low and quite criti- cal for diboride formation and that the reaction was quite dependent upon the HC1 concentration. Armas (8) conducted a thermodynamic in- vestigation to determine if TaB 2 and NbB 2 could be vapor-deposited in the absence of H 2 . By using NbBr 5 or TaRv^ with BBr 3 , Armas and Combescure ( 10 ) deposited NbB 2 and TaB 2 in the temperature range 1,000° to 1,700° C at pressures from 10" 2 to 2 torr. Good hexagonal crystals were produced at 1,400° C and a pressure of 2.5xl0 -2 torr (12) . Armas, Combescure, and Trombe ( 11 ) also employed a solar furnace to produce similar results. Randich ( 187 ) vapor-deposited TaB 2 onto several substrates at temperatures of 500° to 1,000° C, using TaCl 5 and B 2 H 6 with good success. The coating hardness was found to be temperature dependent with values around 2,500 kg mm -2 produced at temperatures above 600° C. NIOBIUM AND TANTALUM CARBIDES Niobium carbide (NbC) is generally vapor-deposited at temperatures of 900° to 1,200° C from the reaction of NbCl 5 with methane ( 32 , 55). Coatings of NbC on steel yielded a hardness of 2,900 kg mm -2 when a methane-to-NbCl 5 gas ratio of 0.5:1 was used ( 136 ) . Hydrogen was also used to reduce a mixture of NbCl5 and CCI4 at temperatures of 1,200° to 1,600° C ( 129 , 169 ). Several patents have been granted for processes that react the metal halide with methane ( 135 , 151 ) or CCI4 ( 242 ). Tantalum carbide (TaC) can be prepared by the reduction of TaCl 5 and CC1 4 with H 2 at temperatures of 850° to 1,300° C (242). Takahashi and Sugiyama ( 226 ) employed an ac discharge to produce TaC from a mixture of TaCl 5 , H 2 , and propylene at temperatures of 400° to 600° C. Thick-wall tubes (up to 2.5 mm thick) of NbC were deposited on graphite tubes using NbCl 5 and methane (32) . There was also a continuous CVD process reported for coating W filaments with TaC (67) . The ac discharge method was also employed to produce fibrous NbC and NbN at temperatures of 300° to 700° C, using N 2 (231). NbCl 5 , H 2 , and propane or NIOBIUM AND TANTALUM NITRIDES Niobium nitride (NbN) can be prepared by the vapor deposition of a mixture of NbCl 5 , N 2 , and H 2 at substrate tempera- tures of 800° to 1,300° C (110). Recent- ly, the growth parameters and crystal morphology were investigated (223) . The 10 investigators found that a gas mixture with an N 2 -to-NbCl 5 ratio greater than 45 and with an H 2 flow rate of 3.5 mL sec -1 at 1,350° C produced the single nitride phase of NbN. At lower temperatures other phases were obtained. The phases Nb 2 N and Nb 4 N 3 were also identified along with NbN. The NbN crystal growth was preferentially in the <111> orienta- tion ( 223 ) . Instead of N 2 , NH3 or hydrazine can be used to produce Nb 2 N or NbN as a fine powder using an H 2 plasma source (33) . Use of NbF 5 instead of NbCl 5 was shown to produce NbN on a Mo substrate at about 900° C ( 193 ). The source of nitrogen in this case was N 2 , and in addition, a large excess of both H 2 and N 2 was needed. Tantalum nitride (TaN) was also vapor- deposited from the gas phase mixture of TaCl 5 , N 2 , and H 2 at temperatures of 700° to 1,300° C (73, 222 ). The substrates were cemented carbides ( 73 ) and Si ( 222 ) . Both Ta 2 N and TaN were found in the films produced. The Vickers microhard- ness values ranged from 1,200 to 2,200 kg mm ( 222 ) . Ammonia has been employed as the N source at temperatures of 700° to 1,300° C at atmospheric pres- sure ( 119 ) . The major portion of the TaN produced was of the face-centered cubic variety. Both TaN and NbN were deposited at tem- peratures of 300° to 500° C by thermally decomposing tantalum or niobium pentakis (dimethylamide) ( 213 ) . Use of N 2 or H 2 as the carrier gas was found to be satis- factory. The decomposition product was identified as NbN, but the TaN was not completely identified. MISCELLANEOUS COMPOUNDS Several binary Nb superconducting com- pounds have been prepared by conven- tional CVD. Among these are Nb 3 Ge (30), Nb 3 Ga (247), Nb 3 Sn (7, 244 ), and recent- ly, Nb 3 Si ( 160 , 254 ). In general, the chlorides of Nb and the corresponding binary element are produced in situ at temperatures of 250° to 350° C (30, 160 ). These compounds are not covered in this review, as their major use is in the electronics industry. Tietjin ( 239 ) has published a review that addresses this area, and there is also a book by Vossen and Werner ( 249 ) concerning the produc- tion of thin films. The reaction of NbCl 5 with SiCl 4 in the presence of H 2 was difficult to con- trol ( 160 ) , as generally Nb5Si 3 and me- tallic Nb were formed instead of Nb 3 Si. Both NbSi 2 and TaSi 2 were deposited by the action of NbCl 5 or TaCl 5 with SiCl4 and H 2 at temperatures of 700° to 1,400° C ( 108 ). The disilicides as coat- ings were reported to have good oxidation resistance properties up to temperatures of 1,700° C (108). In addition to the compounds mentioned, films of Ta 2 05 have been produced for semiconductor devices ( 102 ) , as well as thin films of LiNb0 3 for optical de- vices (36). A summary of the reactions and conditions for the group VB metal compounds is given in table 2. GROUP VIB METALS (Cr, Mo, W) CHROMIUM CARBIDE The formation of chromium carbide (Cr 3 C 2 or CryC 3 ) onto steel is usually accomplished by gas chromizing ( 258 ) . The steel parts with about 1 pet C are treated with CrCl 2 and H 2 at 900° to 1,000° C, with some methane added to aid in the carbide formation (45) . The parts can be hardened to Vickers hardness values of 3,800 to 4,200 kg mm" 2 . In addition to the chromizing process, car- bide coatings can be produced by the de- composition or pyrolysis of organometal- lic compounds of Cr. One such process used dicumene chromium at 450° to 650° C to produce Cr7C 3 coatings on stainless steel turbine blades (60). Another process involved the decomposition of chromium bis(ethylbenzene) in a vacuum at TABLE 2. - Some CVD reactions for group VB elements 11 Reaction V + 2HC1 -► VC1 2 + H2 1 VC1 2 + CR h + VC + 2HC1 + H 2 , 2VCl lt + 2NH 3 + H 2 ■»• 2VN + 8HC1, 2Nb + 5C1 2 + 2NbCl 5 B^C + 8C1 2 + 4BC1 3 + CCli, 2NbCl 5 + 4BC1 3 + 11H 2 ->- 2NbB 2 + 22HC1... 2Ta + 5C1 2 + 2TaCl 5 B^C + 8C1 2 -► 4BC1 3 + CC1 4 2TaCl 5 + 4BC1 3 + 11H 2 > 2TaB 2 + 22HC1... 2TaCl 5 + 2B 2 H 6 ->• 2TaB 2 + 10HC1 + H 2 , 2NbCl 5 + 2CH\ + 2NbC + 8HC1 + Cl 2 , 2NbCl 5 + 2001,+ + 9H 2 -► 2NbC + 18HC1, 2TaCl 5 + 2001^ + 9H 2 -► 2TaC + 18HC1, 6NbCl 5 + 7H 2 + 2C 3 H 8 > 6NbC + 30HC1 3 2NbCl 5 + 5H 2 + N 2 + 2NbN + 10HC1 3 2NbCl 5 + N 2 + 5H 2 + 2NbN + 10HC1, 2TaCl 5 + N 2 + 5H 2 ->• 2TaN + 10HC1. Vaporization Substrate temperature, °C temperature, 2 ~900- 1,000 NAp NAp 1,050-1,150 -400 700-1,200 2 500 NAp 2 800 NAp NAp 950-1,200 2 500 NAp 2 800 NAp 200- 300 900-1,300 200- 300 500-1,000 200- 300 900-1,200 200- 300 1,200-1,600 200- 300 850-1,300 200- 300 300- 800 200- 300 300- 800 200- 300 800-2,300 200- 300 700-2,300 NAp Not applicable. The partial pressure of VC1 2 varies directly as P 2 HCl/ p H 7 when both HC1 and H 2 are present. 2 In situ. 3 An ac discharge of 0.05 to 3.0 mA and 60 Hz frequency. substrate temperatures 300° to 350° C ( 126 ) . Some care must be employed to prevent deposition of the metal along with the metal carbide ( 133 ) . There have been several articles that discuss the wear-resistant coating of CryC3 on steel (90-92) , as well as corrosion-resistant Cr7C3 coatings on bearings and some cut- ting tools (76, 164). MOLYBDENUM CARBIDE Molybdenum carbide (Mo 2 C) can best be deposited on steel at temperatures of 400° to 1,000° C by the reaction of MoF 6 with benzene and H 2 ( 258 ) . Recently, Hojo, Tajika, and Kato produced Mo 2 C as a fine powder by the reaction of MoCl 4 , H 2 , and methane at 800° to 1,400° C (83-84). 12 In this process, the MoCl 4 was produced in situ at 500° to 600° C from the action of Cl 2 on Mo. Films of Mo 2 C can be de- posited on glass from the thermal decom- position of Mo(C0) fi (248). At a pressure of 10" J torr and temperatures of 170° to 350° C, the deposition rate of M02C was increased by using 600-eV electrons at 3 to 5 mA cm . Microspheres have been coated with M02C by the pyrolysis of Mo(C0) 6 for use as laser fusion tar- gets ( 132 ) . Even an ac discharge method has been employed to produce M02C or W 2 C from a mixture of isobutane, H 2 , and the respective metal chloride at 360° to 480° C at atmospheric pressure ( 227 ) . TUNGSTEN CARBIDE There have been more CVD processes de- veloped for the production of tungsten carbide (WC) than for Mo 2 C. One of the earliest methods involved the pyroly- sis of W(C0) 6 , at temperatures of 900° to 1,100° C, using an inert carrier gas (94) . The thermodynamics of the de- composition of W(C0)6 has been reported by Komorova, Lavrin, and Imris ( 115 ) , who used the experimental data of others to show that the WC and W 2 C came from the decomposition steps and not by a recom- bination reaction. Coatings of WC have been deposited on different substrates such as tools and costume jewelry by the reaction of WF 6 with H 2 and a suitable hydrocarbon. The hydrocarbon can be ethene ( 159 ) , benzene ( 35 , 123 ) , toluene ( 6_) , or xylene (6^. At tempera- tures up to 550° C, W 2 C can be obtained from benzene, toluene, or xylene (6^. A mixture of H 2 and CO has also been employed with WFs at a temperature of 925° C and 300 torr (88-89, 238 ). In- stead of WF5 , WC16 can De used with H 2 and methane to form WC coatings on ce- mented carbide tool tips ( 243 ) or ultra- fine WC powder (81-82). The WC powders are produced at temperatures of 1,000° to 1,400° C (81). Wear-resistant coatings of WC were applied to substrates of Cu, Cu alloys, and Al ( 259 ) , as well as to forms for molds for molding elasto- mers ( 181 ) . A controlled nucleation pro- cess that gives the surface improved wear properties was developed (87) . MOLYBDENUM AND TUNGSTEN BORIDES AND SILICIDES Armas, in addition to depositing the borides of Nb and Ta, also studied the vapor deposition of Mo and W borides (8- 10) . Deposition was accomplished by the thermal decomposition of a mixture of M0CI5 or WCls with BBr3, using con- centrated solar radiation as the heat source. The borides are of three compo- sition types: M 2 B, MB, and M 2 B 5 (M = Mo or W). One patent for electrical contact layers has reported that Mo and W borides were deposited from a mixture of BCI3 and H 2 with the respective metal chloride at 1,800° to 2,000° C (68). The method of Armas (8-10) can be used at temperatures of 1,400° to 1,600° C, without H 2 ; how- ever, BBr3 i s more expensive chan BC1 3 . Molybdenum disilicide (MoSi 2 ) has been reported as being deposited from a mix- ture of M0CI5, SiCl 4 , and H 2 , using Ar to control the deposition rate ( 128 ) . The deposition of tungsten silicide (WSi 2 ) was accomplished by the reaction of WF6 with SiH 4 at substrate temperatures of 600° to 800° C ( 124-125 ), and the best fine-grained structure was obtained at 800° C. By carefully controlling the ratio of the two flow rates, WSi 2 can be formed without the species, W, Si, and W 5 Si3. A summary of some representative equa- tions with conditions for the preparation of the group VIB metal compounds is given in table 3. 13 TABLE 3. - Some CVD reactions for group VIB elements Reaction Vaporization temperature, °C Substrate temperature, °C 7Cr[C 6 H 5 CH(CH 3 ) 2 ]2 ♦ c ryC 3 + pyrolysis -195 450- 650 35- 50 400-1,000 Mo + 2C1 2 ■»■ MoCl^ 1 500-600 NAp 25-100 NAp 2MOC11+ + CHi* + 2H2 •*■ M02C + 8HC1 800-1,000 2Mo(C0)g ■* Mo2 c + 10CO + C0 2 900-1,000 2W(C0>6 ■*■ W 2 C + 10CO + C0 2 -144 650- 900 2WF 6 + C 2 H6 + 3H 2 + 2WC + 12HF 18- 35 18- 35 400- 900 12WF 6 + C 6 H 6 + 33H 2 + 6W 2 C + 72HF 400- 900 WF 6 + CO + 4H 2 -► WC + H 2 + 6HF 2 18- 35 300 925 1,400-1,600 300 1,000-1,700 2WC1 6 + 2BBr 3 ♦ 2WB + 6C1 2 + 3Br 2 3 140-150 140-150 1,400-1,600 1,000-1,700 2MoCl 5 + 4SiCli t + 13H 2 + 2MoSi 2 + 26HC1.... -160 700-1,400 WF 6 + 25111^ ■»■ WSi 2 + 6HF + H 2 18- 35 600- 800 NAp Not applicable. *In situ. 2 Low pressure of 300 torr, 3 Low pressure of 10~ 2 to 2 torr and use of solar furnace. CONCLUSIONS Risks of periodic shortages of critical and strategic materials continue to exist as virtually all of the Western World is dependent on imports of critical mineral raw materials. Critical materials can be conserved through the use of alloys with lower strategic metal content and im- proved abrasion, erosion, and corrosion resistance on the surface. Concentrating the critical materials on substrate sur- faces by chemical vapor deposition (CVD) can provide their needed properties al- though the materials are not present in the substrates. In addition, synergistic effects can result from combining coating and substrate properties. Through selec- tive use of coatings on low-grade sub- strates, it may also be possible to re- duce costs while conserving critical materials. 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