Characterization of Nanostructured TiC Thin Films Synthesized by TVA (Thermionic Vacuum Arc) Method

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1 Journal of Materials Science and Engineering A 2 (1) (2012) D DAVID PUBLISHING Characterization of Nanostructured TiC Thin Films Synthesized by TVA (Thermionic Vacuum Arc) Victor Ciupina 1, Rodica Vladoiu 1, Petrica Popov 1, Virginia Dinca 1, Mirela Contulov 1, Aurelia Mandes 1 and Cristian Petrica Lungu 2 1. Department of Physics, Ovidius University, Constanta , Romania 2. National Institute for Lasers, Plasma and Radiation Physics, Bucharest , Romania Received: September 07, 2011 / Accepted: October 06, 2011 / Published: January 10, Abstract: Nanocrystalline titanium carbide (TiC) thin films were prepared by an original method based on Thermoionic Vacuum Arc (TVA) plasma. Characterization of the obtained titanium carbide thin films has been made by Transmission Electron Microscope (TEM) with high resolution (1,4 Å) and Atomic Force Microscopy (AFM). The AFM measurements have proved the smoothness of the deposited films with peak to valley roughness in the range of nm. Key words: Titanium carbide thin films, Thermoionic Vacuum Arc, AFM, HRTEM. 1. Introduction Titanium carbide (TiC) hard films are considered as high technological materials due to their unique characteristics that have made them of particular interest in a wide variety of applications [1, 2]. Titanium carbide exhibits high hardness values, high melting point and thermal stability, low friction coefficient, high thermal and electrical conductivity [3, 4]. TiC has a relative low density (ρ = 4.91 g/cm 3 ), very high melting point (T m = 3,340 K), high hardness (HV = GPa), high modulus (Y = GPa), and good thermal stability. Because of their special properties they are increasingly used as wear and corrosion-resistant films on cutting tools and diffusion barrier in semiconductor technology [5]. Moreover, its high melting point makes it also a promising material to be used as first wall material in fusion reactors. Due to these characteristics it is essential to study the properties of these thin films and recognize how the relative parameters affect the material structures for Corresponding author: Rodica Vladoiu, Assoc. Prof., research field: thin films plasma deposition, rvladoiu@univ-ovidius.ro. advanced applications. In this work, we have explored the properties of titanium carbide thin films deposited by TVA method of TiC powder as an alternative to other conventional deposition techniques (sputtering, CVD, etc.). TVA is an original method for deposition of high purity thin films suitable for nanostructured film synthesis of any solid materials, even with high melting point as carbon, tungsten, tantalum pentaoxide, but also complex combinations of them [6-10]. 2. Experimental Set-up One of the most important characteristics of TVA method is the presence of energetic ions in the pure carbon and titanium vapor plasmas. The experimental set-up is shown in Fig. 1, it can be noticed that the cathode is a heated tungsten filament of 1.5 mm mounted inside of the Wehnelt cylinder. The role of this cylinder is crucial because it focuses the electrons on the anode surface. The bombarding ions on the substrate are just the ions of the depositing material

2 Characterization of Nanostructured TiC Thin Films Synthesized by TVA (Thermionic Vacuum Arc) 17 Fig. 1 Schematical diagram of the experimental set-up. (carbon and titanium) contained in the crucible: the anode. Moreover, the energy of ions can be fully controlled from outside of the discharge vessel via the control of the arc voltage drop value. It results that during film preparation, the growing thin film is bombarded by energetic ions with an established value of the energy of ions. Due to the accelerated electrons from the cathode heating the anode, a continuous evaporation of the anode material is established. With a further increase of the applied voltage, a bright discharge is suddenly established in the interelectrodic space. As samples we used glass and silicon wafers mounted on the holder at a distance of 150 mm from the anode. The system has a shutter that protects the samples until the deposition parameters are achieved. Once the discharge is stable, the shutter is removed and the deposition starts directly on the substrates. The base pressure in the deposition chamber was Pa. The filament of the cathode was externally heated for 30 min with a current intensity of 43 A (I f ). TiC plasma was ignited at a breakdown voltage of 2.32 kv, the anodic current being of 750 ma (Fig. 2). Due to the high energy dissipated in the unit volume plasma, the material is strongly dispersed and moreover, is completely droplets free. The TVA discharge is controlled by the cathodic electron beam and there are important operation parameters like the arc voltage and the arc current due to the getter properties of titanium vapors. The film structure depends on the influence of the ion energy emitted by the cathode [11], and was characterized by Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM) and by Talystep profilometer. Fig. 2 Volt-ampere characteristic of the Thermionic Vacuum Arc.

3 18 Characterization of Nanostructured TiC Thin Films Synthesized by TVA (Thermionic Vacuum Arc) 3. Results and Discussion TEM images were obtained on CM120ST microscope working at 100 kv with a magnification of 1.4 M and a resolution of 1.4 Å. A great advantage of the transmission electron microscope is in the capability to observe, by adjusting the electron lenses, both electron microscope images (information in real space) and diffraction patterns (information in reciprocal space) for the same region. In order to perform TEM investigations, the samples were prepared using a quick method described in Ref. [12]. The method is inexpensive and provides a good working area with clean sample, without chemical or mechanical artefacts. TEM images at different resolutions (100 nm and 20 nm) are shown in Fig. 3 and the grain size distribution is listed in the table from below. The deposited thin films are nanostructured and the size of nanocrystalline structures according to Feret diameter is about nm. The statistical function presented in the table indicates a mean diameter of grain size about nm. The electron diffraction diagrams confirm the presence of crystalline structure in the deposited thin films. In Fig. 4 are presented the selected area electron diffraction (SAED) image analysis with the cubic structure of the TiC formed crystals (SG: Fm 3 m, a = nm, Vc = 81.07, Vm = 12.21) [13]. As can be revealed from the left side table, there is a good Fig. 3 TEM images of films and table with statistical function of grains.

4 Characterization of Nanostructured TiC Thin Films Synthesized by TVA (Thermionic Vacuum Arc) 19 Fig. 4 SAED image of the TiC film (a = 43,280). A: table with experimental data, B: TiC CPDS card data. concordance between the experimental data (A) with the compared data (B) from Ref. [13]. Thickness of the deposited TiC films deposited on glass substrate was measured by Talystep profilometer, ranging in the values of approximately 100 nm (Fig. 5). The topography of the films was studied by AFM carried out with AFM XE-150 Park System. The XY motorized sample stage is optimized for both small and large sample placement (Fig. 6). The AFM measurements have proved the smoothness of the deposited films with peak to valley roughness in the range of nm, while the Ten-point mean roughness (Sz), the sum of the average peak among 5 tallest peaks and the average valley between 5 lowest valleys, is 35 nm. According to the Table 1, from a total area of 30 μm 2, Fig. 5 Thickness analyses of TiC thin film.

5 20 Characterization of Nanostructured TiC Thin Films Synthesized by TVA (Thermionic Vacuum Arc) Fig. 6 AFM analyses of TiC thin film. Table 1 Results of the TiC films roughness investigation. Total area St. 31,467 μm 2 Roughness avg. Sa. 10,668 nm Root mean square Sq. 17,597 nm Ten points height Sz. 352,126 nm Range Sr. 375,652 nm the arithmetical mean roughness (Sa) is about 1 nm, giving a promising hopes for applications where complete flat surfaces at nanometric scale is required, as coating tools with wear resistant films. 4. Conclusions Though the present results have an exploratory character, TiC thin films obtained for the first time have been successfully synthesized by TVA technology as deposition method. From the SAED patterns were identified the structure and the related profile: cubic structure of TiC (Fm 3 m) a = nm with the mean crystallite size of nm. TEM and AFM images revealed high uniformity and smoothness in mean range of 1.1 nm, in this way offering convincing advantages for emerging applications and allowing the creating of adherent and wear resistant films. Acknowledgments This work was supported by the strategic grant POSDRU/89/1.5/S/58852, Project Postdoctoral programme for training scientific researchers cofinanced by the European Social Found within the Sectorial Operational Program Human Resources Development and by the Romanian Ministry of Education and Research, under project CAPACIF /2008. References [1] B.E. Jacobson, C.V. Deshpandey, H.J. Doerr, A.A. Karim, R.F. Bunshah, Microstructure and hardness of Ti(C, N) coatings on steel prepared by the activated reactive evaporation technique, Thin Solid Films 118 (1984) [2] T.H. Fang, S.R. Jian, D.S. Chuu, Nanomechanical properties of TiC, TiN and TiCN thin films using scanning probe microscopy and nanoindentation, Applied Surface Science 228 (2004) [3] S.Q. Wang, L.H. Allen, Thermal stability of α titanium in contact with titanium nitride, J. Appl. Phys. 79 (1996) [4] H.M. Gabriel, K.H. Kloos, Morphology and structure of ion-plated TiN, TiC and Ti(C, N) coatings, Thin Solid Films 118 (1984) 243. [5] M.L.F. Parames, O. Conde, Growth of TiC films by thermal laser-assisted chemical vapour deposition, Applied Surface Science (1997) [6] H. Ehrich, J. Schuhmann, G. Musa, A. Popescu, I. Mustata, Adhesive metal films obtained by thermionic vacuum arc (TVA) deposition, Thin Solid Films 333 (1998) 95.

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