The chemical vapor deposition of TiB2 from diborane

Thin Solid Films, 72 (1980) 511-516 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

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THE C H E M I C A L VAPOR DEPOSITION OF TiB 2 F R O M DIBORANE* H. O. PIERSON AND A. W. MULLENDORE Sandia National Laboratories, Albuquerque, N.M. 87185 (U.S.A.) (Received April 8, 1980; accepted April 23, 1980)

In this paper we describe an experimental study of the chemical vapor deposition of titanium diborid e on graphite using the reaction of TiCI 4 with B2H 6 in a hydrogen atmosphere in the temperature range 600-900 °C. Dense and adherent coatings were obtained varying in composition from boron rich at 600 °C (TiBa.12) to stoichiometric above 700 °C. There was a gr~R!ual increase in crystaUite size with increasing deposition temperature. Chlorine tended to remain incorporated in the deposit (1.86 at.~ at 600 °C decreasing to 0.51~ at 900 °C). The coatings were very hard (HV = 3715 kgfmm-2). 1. INTRODUCTION TiB 2 is a very stable hard refractory material with good chemical resistance. It is a promising candidate for many engineering applications where conditions of erosion, corrosion and high temperature are found. It is often used in the form of coatings by chemical vapor deposition (CVD) using the hydrogen reduction of the chlorides. The reaction is as follows: TiC14 + 2BC13 + 5H 2 ~ TiB 2 + 10HC1

(1)

Under optimum conditions, excellent coatings may be produced 1-4. However, there are two inherent disadvantages: (1) deposition temperatures of 900 °C or higher are required; (2) a large amount of HCI is produced which is detrimental to many metallic substrates. Another source of boron is diborane (B2H6) which has been used successfully in previous studies to produce boron coatings at low temperatures 5, 6. A potentially lower tieposition temperature and a reduction in HCI byproducts make the CVD of TiB 2 by the B2H6-TiC14 reaction seem attractive. In this paper we describe an experimental study of ttris reaction. 2. CHEMICAL VAPOR DEPOSITION CONSIDERATIONS The overall reaction is as follows: TiCI,+B2H6 --* TiB2 + 4 H C I + H 2

(2)

* Paper presented at the International Conferenceon MetallurgicalCoatings, San Diego, California, U.S.A., April 21-25, 1980.

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PIERSON, A. W. MULLENDORE

There are many possible intermediate reactions such as the formation of lower chlorides (e.g. TIC13) or higher solid boranes. A better theoretical understanding of this reaction will not be reached until more complete thermodynamic and kinetic analyses have been carried out. The changes in the free energy of formation for reactions (1) and (2) are shown in Fig. 1. Reaction (2) is thermodynamically feasible at room temperature. However, CVD processes are governed to a large extent by kinetic factors and the thermodynamic predictions must be viewed accordingly. 3. EXPERIMENTAL

3.1. Equipment All experiments were conducted in the quartz reactor shown in Fig. 2. Heat was supplied by a 450 kHz r.f. generator. The temperature was controlled using a P t ( P t - 1 0 ~ R h ) thermocouple sheathed in a quartz tube and inserted in a graphite susceptor. All substrate temperatures reported here are corrected optical surface temperatures and were reproducible to + 5 °C. TIC14 was metered by a syringe pump and an evaporator maintained at 250 °C. Argon was used as a carrier gas for the TIC14 vapor. B2H 6 was metered directly into the reactor through a mass flowmeter. HYDROGEN i

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Fig. 1. Changesin freeenergyof formation:reaction(1),TiC14+ 2BC13+ 5H2 --, TiB2+ 10HCI;reaction (2), TiCI4 + B2H6 ---,TiB2 + 4HC1+ H2. Fig. 2. CVD reactor.

3.2. Substrates The substrates were graphite disks 2.5 cm in diameter and 0.5 cm high of Poco AXF-5Q (Pure Oil Co.); this material was chosen because of its high density and purity and the good match of its thermal expansion with that of TiB2. Previous experience had shown that a light roughening of the surface improved adhesion 4. This was done by blasting with 150 pm glass beads at low pressure (2.75 × 105 Pa). The blasting was followed by ultrasonic cleaning in acetone and alcohol and drying in vacuum at 200 °C for 1 h.

CVD OF TiB 2 FROM DIBORANE

513

3.3. Chemical vapor deposition procedures B2H 6 is a very toxic gas which, in the correct proportions, burns explosively in air. It must be handled with adequate safety precautions 7. The graphite disks were heated to the desired temperatures (500-950 °C in 50 °C increments) in an argon atmosphere. The reactant gases were then introduced with the following typical flow rates: B2H6, 38.7 ml min-1; TIC14, 0.19 ml min-1; hydrogen, 600 ml min- 1; argon (to the TiC14 vaporizer), 100 ml min- 1. These flows correspond to stoichiometric ratios of the reactants (except for hydrogen which was in excess of stoichiometry). The purity of the reactants is shown in Table I. The average duration of the deposition was 1 h. The samples were cooled to room temperature in flowing hydrogen. .~ TABLE I PURITY OF REACTANTS

Reactant

Impurity

H2 Ar TiC14 B2H 6

Total, 0.5 ppm H2, 0.1 ppm; 02, 0.1 ppm Fe, I ppm; Si, 20 ppm N2, 0.1%; H2, 3.2% B4HIo , 0.64%; BsH 11, 0.076%

3.4. Analysis The coatings were analyzed by optical metallography and X-ray diffraction. The boron and titanium contents were measured using induction-coupled plasma spectroscopy and electron microprobe impurities using emission spectroscopy. The hardness was determined using a Vickers microhardness tester with a load of 25 gf. The adhesion to the substrate was evaluated in a pulsed electron beam described elsewhere 4. 4. RESULTS AND DISCUSSION

4.1. Transport reaction The deposition rates of reactions (1) and (2) as a function of temperature and under similar gas flow conditions are shown in Fig. 3. The deposition rate of the B2H 6 reaction (2) is considerably greater than that of the BC13 reaction (1) and it proceeds at a lower temperature in accordance with thermodynamic calculations (Fig. 1). Reaction (1) is accompanied by side reactions, leading to the formation of subchlori~les such as TiC13 and higher boranes. These solid compounds deposited on the cooler portion of the reactor are oxidized in contact with air after opening the reactor. Below 600 °C the deposition rate was very low and the coatings were soft and lacked cohesion. Above 900 °C there was a tendency to form powdery coatings through vapor phase nucleation. 4.2. Structure and composition X-ray diffraction patterns of the coatings were obtained at each deposition temperature. The 500 °C coating shows only graphite lines (the substrate), indicating an essentially amorphous deposit and low titanium content. At higher deposition

514

H . O . PIERSON, A. W. MULLENDORE

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temperatures the coatings all gave patterns corresponding to the hexagonal TiB 2 structure. The line breadths for the (10.0) reflection at half-maximum intensity (minus instrumental broadening) and the Scherrer crystallite sizes D = 2/(B cos 0) are given in Table II. TABLE II LINE BREADTH AND CRYSTALLITE SIZE

Deposition temperature (°C)

Line breadth B (deg)

Crystallite size D (A)

600 700 800 850 900

1.5 1.3 0.5 0.5 0.35

60 70 180 180 260

As expected, a gradual increase in crystallite size is noted with increasing deposition temperature. The ratio of intensity of the two strongest peaks (I(lo.o~:I(lo.1)) increased with increasing deposition temperature and indicated a pronounced (10.0) preferred orientation at 900 °C. The compositional data obtained by both induction-coupled spectroscopy and by the electron microprobe method are summarized in Table III. The results from the two methods correlate well. At deposition temperatures of 700 °C and above, the deposits were essentially stoichiometric TiB 2 whereas, below 700 °C, an increasing amount of free boron was noted with decreasing temperature (in these experiments the ratio of boron to boron plus titanium remained constant at 0.66). These experimental results are different from those obtained with reaction (1) where free boron was obtained only when the ratio of boron to boron plus titanium was greater than 0.65 and was essentially independent of the deposition temperature s . The relatively low deposition temperatures studied tend to promote the retention of chlorine in the TiB 2 coatings. Chlorine analyses were performed on metallographic sections of the deposits. The average of these determinations for each deposition temperature', together with the values obtained after annealing for 1 h at 1000 °C, are given in Table IV. In the as-deposited condition the chlorine content was highest in the 700 °C deposit but was reduced by a factor of 11 by annealing. The coating developed uniform microporosity as a result. The other

CVD OF TiB 2 FROM DIBORANE

515

deposits showed much lower chlorine contents which were not reduced by annealing. Additional experiments using a large excess of hydrogen did not reduce the chlorine contents nor did the use of high purity hydrogen, argon or a leak-tight reactor. Further studies are under way to determine the reason for the incorporation and stabilization of chlorine in the deposits. TABLE III VALUES OBTAINED FOR x IN TiB x

Deposition temperature (°C)

Electron microprobe

Induction-coupled plasma spectroscopy

500 600 700 800 850 900

3.12+0.70 2.57 + 0.04 2.02 - 0.015 1.93 ---0.02 2.15-t-0.015 2.15 + 0 . 0 1 5

-2.53 - 0.04 1.99 + 0.03 2.01 -+ 0.08 2.14+0.06 2.05 + 0 . 0 5

Deposition temperature (°C)

Cl 2 as deposited (at.%)

Cl 2 after 1000 °C anneal (at.%)

600 700 800 850 900

1.86 3.25 2.03 0.78 0.51

1.94 0.28 -0.86 0.45

T A B L E IV CHLORINE CONTENT OF TiB 2

4.3. Properties of the coating Microhardness tests on unetched cross sections of TiB 2 gave a hardness of 2893 kgf m m - 2 (HV 25) (a = 88.7) for coatings deposited at 600 °C and 3715 kgf m m - 2 (HV 25) (a = 255) for coatings at 900 °C. This increase in hardness may be due to the greater homogeneity and crystallinity of the 900 °C coating. The value of 3715 kgf m m - 2 is in good agreement with other reported values a. 5. CONCLUSIONS

TiB 2 coatings may be successfully produced by the reaction of TiC14 with B 2 H 6. This reaction compares favorably with the hydrogen coreduction of the chlorides since it proceeds at lower temperatures, has a higher deposition rate and produces less HC1. These factors should make the deposition of TiB 2 coatings practical over a wider selection of substrates. ACKNOWLEDGMENT

This work was supported by the Division of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC04-76-DP00789.

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H.O. PIERSON, A. W. MULLENDORE

REFERENCES T.M. Besmann and K. E. Spear, J. Electrochem. Soc., 124 (5) (1977) 786-797. T. Takahashi and H. Kamiya, J. Cryst. Growth, 26 (1974) 203-209. H.O. Pierson and E. Randich, Thin Solid Films, 54 (1978) 119-128. H.O. Pierson, E. Randich and D. M. Mattox, J. Less-Common Met., 67 (1979) 381-388. H. O. Pierson and A. W. Mullendore, Proc. 7th Int. Conf. on Chemical Vapor Deposition, Electrochemical Society, Princeton, New Jersey, 1979, pp. 360-367. 6 P. Casadesus, C. Frantz and M. Gantois, Metall. Trans. A, 10 (1979) 1739-1743. 7 Diborane, Tech. Bull., May 1976, Callery Chemical Co., Callery, Pennsylvania. 8 G. Blandenet, Y. Lagarde, J. Morlevat and G. Uny, Proc. 6th Int. Conf. on Chemical Vapor Deposition, Atlanta, Georgia, 1977, Electrochemical Society, Princeton, New Jersey, 1977, pp. 330-348.

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