New Method for Synthesis of Coatings of Molybdenum, Tungsten, Their Carbides and Composites

Rhushikesh V. Godbole, Mahendra A. More, Anil S. Gupte, Vijay P. Godbole

Center for Nano-Materials and Quantum Systems (CNQS), Department of Physics, University of Pune, Pune, India.
Technical Director, Poona Shims Pvt. Ltd., Pune, India.

DOI: 10.4236/ojsta.2013.22010                

Abstract

In this paper we report, for the first time, a new approach for synthesis of high quality faceted microcrystalline coatings of molybdenum (Mo), tungsten (W), their carbides and composites. These studies are carried out using Hot Filament Chemical Vapor Deposition (HF-CVD) method wherein parent materials (Mo and/or W) are taken in the form of wires (~0.5 mmdia) and are heated to a high temperature (T_F ~ 1500 - 2000 C), in ambient of “oxygen (O₂) diluted hydrogen (H₂) gas”. Due to high filament temperature (T_F), a series of pyrolytic reactions take place. Firstly, the gasification of wire material (Mo and/or W) occurs in the form of its oxide. The oxide molecules reach the substrate which is kept underneath the filament assembly. Secondly, molecular hydrogen gets dissociated into atomic hydrogen and subsequently reaches the substrate to react with oxide molecules, finally leading to the precipitation of a pure metal. This method can also be used, in situ, to convert metallic coatings into their carbides and/or composites. The method offers many other attractive features, which can not be rendered by the conventionalCVD/PVDmethods. The results are discussed in terms of temperature induced “Red-ox” reactions.

Keywords

CVD Coatings; Pyrolysis; Ceramics; Refractories

1. Introduction

Since materials play a central role in different technologies, there has been a tremendous increase in the activity of “materials research”. In view of a new trend of “collapsing dimentions” in modern technologies, the subject of “materials coatings” has received a prime importance to form new materials. Novelty of these new materials refers to a multi-parametric restrictions on different physical/chemical properties, so as to be suitable in given type of application/s. To design and develop such special coatings, a major imoprtance has also been to the methods of fabrication of such coatings. Over last few years, substantial research efforts have been and are being expended in this subject. Various types of newer physical/ chemical methods have been devised for synthesis of coatings of various types. On one hand, some of these methods are now matured enough to routinely achieve the objective, while on the other hand newer methods/ concepts are still being proposed to synthesize required types of materials coatings.

The refractory metals particularly molybdenum (Mo), tungsten (W) and also their compounds viz. carbides, oxides etc. form a special class of “materials “coatings. These coatings exhibit properties useful for many technological applications [1-3]. In view of their high melting points, these materials are thermally very stable and hence can be used for high temperature applications [4]. Because of high erosion resistance, these coatings are useful for protection against high-energy radiations [5].

The feature of high “chemical corrosion resistance” makes them useful in corrosion science. These materials coatings also exhibit lower diffusivities towards different atomic species. So coatings of these materials can be used as diffusion barriers [6,7] as well as thermal barriers [8]. The resistance to elecro-migration phenomenon as well as appropriate electrical resitivities makes them useful for device technologies for fabrication of schottky barriers, interconnect lines and back contacts to solar cells etc. [9-11]. These materials also show enhanced “electron emission” and hence these are useful for developments of electron emission devices [12]. The higher values of modulus and hardness attract their applications in wear control in mechanical industry [13-16]. The oxides of these materials have applications for gas sensing [17] as well as electro chromic devices [18,19].

Because of high melting points, however, conventional resistive evaporation method is not possible for the fabrication of coatings of these materials. The conventional CVD method and the sputtering method can be used but both these methods pose problems such as 1) requirement of hazardous organic precursors; 2) limitations on area of deposition; 3) limited growth rates; 4) complicated system set up etc. A need is, thus, felt to use newer methodology which could avoid these problems and yield easier and efficient fabrication of thicker, high quality coatings of these materials.

In this brief report, we describe for the first time, a simple and novel approach to synthesize high quality, faceted, micro-crystalline coatings of molybdenum (Mo), tungsten (W), their carbides and composites. This method is primarily based upon “Hot Filament Chemical Vapor Deposition (HF-CVD)” technique. This new method has many attractive features, which are not rendered by conventional CVD/PVD methods. The present method is 1) very simple and efficient; 2) it offers an easy control on different process parameters which could lead to a different morphological features of the resultant coatings; 3) since the method is based on HF-CVD technique, it requires a simple system set up. More importantly; 4) the method does not require use of any hazardous chemical precursors; 5) the method renders much larger growth rates ( typically more than ~10 mm/hour or even higher) than those offered by conventional CVD/ PVD methods and hence; 6) it is possible to achieve much thicker coatings over practicable time scales. Finally; 7) the method is capable of obtaining coatings on much larger areas. In this paper, we have demonstrated that it is just a question of appropriately controlling the process parameters, to achieve all these objectives.

2. Experimental

The HF-CVD set up [20,21] used in the present studies has been indegeniously designed and developed in our laboratory. It consists of a water cooled stainless steel chamber (size ~30 cm dia ´ 25 cm height) having a number of ports to mount different accessory components such as high current feed throughs for filament heating, a gas mixing unit and input gas lines, the filament and the substrate holders along with controls for X-Y movement and rotational movement etc. The small pieces (typical size ~ 10 mm ´ 10 mm) of <100> and <111> silicon (Si), high purity (99.99%) polycrystalline alumina (α-Al₂O₃), copper (Cu), tool steel (TS), stainless steel (SS) etc. are used as substrates. The samples are cleaned by using standard procedures and are used for deposition experiments. The pure (99.99%) gases such as oxygen (O₂) and/or methane (CH₄) are diluted in excess amount of hydrogen (H₂) and/or argon (Ar) gas and then flown over the heated filament assembly of molybdenum (Mo) and/or tungsten (W) wires. For the deposition of pure metal coating, the gas composition H₂:O₂ = 100:3 sccm is used. The filament temperatures are adjusted, using Pyrometer, over the range 1500˚C - 2000˚C by passing a suitable amount of current. The gas flow rates are controlled by using mass flow controllers (MFCs). The microprocessor based combination of “auto-throttle valve and a Capacitance diaphram gauge” is used to control the chamber pressure. It is typically adjusted over the range 20 - 100 Torr depending upon the particular type of experiment. The deposition time is varied over 10 min - 120 min. Subsequent to depostion, the specimens are characterized by using methods such as conventional and glancing angle X-ray diffraction (XRD), field emmision scanning electron microscopy (FE-SEM), energy dispersive analysis of X-rays (EDX), Resistivity measurements etc. to get an information on morphological, structural, and compositional features of resultant coatings. The studies using additional microscopic characterization tools such as transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning tunneling Microscopy (STM), X-ray photo-electron spectroscpy (XPS), nanoindentation are also in progress in our laboratory and these results will be published separatly.

3. Results and Discussion

In a given deposition experiment, the different types of substrates are kept “side-by-side” on the substrate holder so that these get processed at almost identical processing conditions. So it is meaningful to compare the morphologies of coatings, on different types of substrates.

Figure 1 shows the FE-SEM micrographs, at different magnifications, corresponding to coatings obtained using Mo-filament (Figures 1(a) and (b)) and using W-filament (Figures 1(c) and (d)) on <111> silicon substrates. These coatings are obtained using nominally adjusted process parameters, viz. the composition of gas mixture H₂:O₂ = 100:10 sccm, flow rate of gas mixture ~ 110 sccm, the filament temperature (T_F) = 1800 C, the substrate temperature (T_S) = 500 C, the chamber pressure = 30 Torr, and the deposition time 20 min. As can be seen from these micrographs, both the types of coatings exhibit a cauliflower type of morphology. The EDX and glancing angle X-ray diffraction measurements are also carried out on both these types of coatings. These results exhibit simillar results in the context of structural and compositional features. The EDX measurements, as shown in Figure 1(e), refers to the Mo-coatings which reveals that the coating consists of ~30 at % of oxygen and ~70 at % of molybdenum. While the glancing angle X-ray diffraction result, as shown in Figure 1(f), for coating obtained using W-filament reveals formation of highly disordered (very broad

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	D. R. Linde, “Handbook of Chemistry and Physics,” 78th Edition, CRC Press, New York, 1998, pp. 12-144, 12-192-194.
[2]	E. Lassner and W.-D. Schubert “Tungsten: Properties, Chemistry, and Technology of the Element, Alloys, and Chemical Compounds,” Springer, Berlin, 1999, pp. 86, 114-118.
[3]	C. K Gupta, “Extractive Metallurgy of Molybdenum,” 1st Edition, CRC Press, New York, 1992.
[4]	C. Porosnicu1, M. Osiac, E. Osiac and C. P. Lungu, Physics AUC, Vol. 20, Pt. 1, 2010, pp. 57-63.
[5]	I. Smid, M. Akiba, G. Vieider and L. Plochl, “Development of Tungsten Armor and Bonding to Copper for PlasmaInteractive Components,” Journal of Nuclear Materials, Vol. 258-263, Pt. 1, 1998, pp. 160-172.
[6]	K. M. Chang, T. H. Yeh, I. C. Deng and C. W. Shih, “Amorphouslike Chemical Vapor Deposited Tungsten Diffusion Barrier for Copper Metallization and Effects of Nitrogen Addition,” Journal of Applied Physics, Vol. 82, No. 3, 1997, pp. 1469-1475. doi:10.1063/1.365925
[7]	J. S. Becker and R. G. Gordon, “Diffusion Barrier Properties of Tungsten Nitride Films Grown by Atomic Layer Deposition from Bis(Tert-Butylimido)Bis(Dimethylamido) Tungsten and Ammonia,” Applied Physics Letters, Vol. 82, No. 14, 2003, 7 Pages. doi:10.1063/1.1565699
[8]	N. P. Padture, et al., “Towards Durable Thermal Barrier Coatings with Novel Microstructures Deposited by Solution-Precursor Plasma Spray,” Acta Materialia, Vol. 49, No. 12, 2001, pp. 2251-2257. doi:10.1016/S1359-6454(01)00130-6
[9]	M. Tabbal, M. Meunier, R. Izquierdo, B. Beau and A. Yelon, “Laser-Chemical Vapor Deposition of W Schottky Contacts on GaAs Using WF6 and SiH4,” Journal of Applied Physics, Vol. 81, No. 10, 1997, pp. 6607-6611. doi:10.1063/1.365199
[10]	K. M. Chang, T. H. Yeh, I. C. Deng and C. W. Shih, “Amorphouslike Chemical Vapor Deposited Tungsten Diffusion Barrier for Copper Metallization and Effects of Nitrogen Addition,” Journal of Applied Physics, Vol. 82, No. 3, 1997, pp. 1469-1475. doi:10.1063/1.365925
[11]	K. Aryal, H. Khatri, R. W. Collins and S. Marsillac, “In Situ and ex Situ Studies of Molybdenum Thin Films Deposited by rf and dc Magnetron Sputtering as a Back Contact for CIGS Solar Cells,” International Journal of Photo Energy, Vol. 2012, 2012, Article ID: 723714, 7 Pages.
[12]	A. A. Rouse, J. B. Bemhard, E. D. Sosa and D. E. Golden, “Field Emission from Molybdenum Carbide,” Applied Physics Letters, Vol. 76, No. 18, 2000, pp. 2583-2585. doi:10.1063/1.126415
[13]	E. Weigert, M. Humbert, Z. J. Mellinger, Q. Ren, T. P. Beebe, L. Bao and J. Chen, “Physical Vapor Deposition Synthesis of Tungsten Monocarbide (WC) Thin Films on Different Carbon Substrates,” Journal of Vacuum Science and Technology, Vol. 26, No. 1, 2008, pp. 23-28. doi:10.1116/1.2806941
[14]	V. Fox, J. Hampshire and D. Teer, “MoS2/Metal Composite Coatings Deposited by Closed-Field Unbalanced Magnetron Sputtering: Tribological Properties and Industrial Uses,” Surface Coating Technology, Vol. 112, No. 1-3, 1999, pp. 118-122. doi:10.1016/S0257-8972(98)00798-1
[15]	O. Smorygo, S. Voronin, P. Bertrand and I. Smurov, “Fabrication of Thick Molybdenum Disulphide Coatings by Thermal-Diffusion Synthesis,” Tribology Letters, Vol. 17, No. 4, 2004, pp. 723-726. doi:10.1007/s11249-004-8079-8
[16]	H. Zhang and D. Y. Li, “Effects of Sputtering Condition on Tribological Properties of Tungsten Coatings,” Wear, Vol. 255, No. 7, 2003, pp. 924-932.
[17]	P. S. Pokhre, C. Simion, V. Teodorescu, N. Barsan and U. Weimar, “Synthesis, Mechanism, and Gas-Sensing Application of Surfactant Tailored Tungsten Oxide Nanostructures,” Advanced Functional Materials, Vol. 19, No. 11, 2009, pp. 1767-1774. doi:10.1002/adfm.200801171
[18]	Y. Djaoued, S. Balaji and R. Brüning, “Electrochromic Devices Based on Porous Tungsten Oxide Thin Films,” Journal of Nanomaterials, Vol. 2012, 2012, Article ID: 674168, 9 Pages.
[19]	C. A. Ellefson, O. Marin-Flores, S. Ha and M. G. Norton, “Synthesis and Applications of Molybdenum (IV) Oxide,” Journal of Materials Science, Vol. 47, No. 5, 2012, pp. 2057-2071.
[20]	R. F. Davis, “Diamond Films and Coatings,” Noyes Publications, New Jersey, 1993.
[21]	K. E. Spear and J. P. Dismukes, “Synthetic Diamond: Emerging CVD Science and Technology,” A Wiley-Interscience Publication, John Wiley & Sons, Inc., New York, 1994.
[22]	International Center for Diffraction Data (PCPDFWIN), “v 1.3, JCPDS-Card nos. 04-0806 (for W), 42-1120 (for Mo), 45-0167 for WO3-x, 41-0905 for WO3, 47-1319 for β-W, 25-1047 (for WC), 35-0787 (for α-Mo2C) and 35-0776 (for α-W2C),” 1997.
[23]	V. P. Godbole, R. J. Narayan, X. Zu, J. Narayan and J. Sankar, “Diamond Films and Composites on Cobalt— Chromium Alloys,” Material Science and Engineering: B, Vol. 58, No. 3, 1999, pp. 251-257. doi:10.1016/S0921-5107(98)00430-9
[24]	V. P. Godbole, K. Jagannadhem and J. Narayan, “Nucleation and Growth of Diamond Films on Aluminum Nitride Coated Nickel,” Applied Physics Letters, Vol. 67, No. 9, 1995, pp. 1322-1324. doi:10.1063/1.114527
[25]	V. P. Godbole and J. Narayan, “Synthesis of Diamond Films on Hastelloy,” Journal of Material Research, Vol. 7, No. 10, 1992, pp. 2758-2790. doi:10.1557/JMR.1992.2785
[26]	V. P. Godbole and J. Narayan, “Nucleation and Growth of Diamond on FeSi2/Si Substrates by Hot Filament Chemical Vapor Deposition,” Journal of Applied Physics, Vol. 71, No. 10, 1992, pp. 4944-4948. doi:10.1063/1.351380
[27]	J. Narayan, V. P. Godbole, G. Matera and R. K. Singh, Journal of Applied Physics, Vol. 71, No. 2.