Materials Research Bulletin 48 (2013) 2766–2770 High-speed deposition of titanium carbide coatings by laser-assisted metal–organic CVD Yansheng Gong a, Rong Tu b,*, Takashi Goto c a Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China b State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China c Institute for Materials Research, Tohoku University, Aoba-ku, 2-1-1 Katahira, Sendai 980-8577, Japan A R T I C L E I N F O Article history: Received 2 November 2012 Received in revised form 4 February 2013 Accepted 25 March 2013 Available online 1 April 2013 Keywords: A. Carbides B. Laser deposition C. High speed deposition D. Electron microscopy E. Surface properties A B S T R A C T A semiconductor laser-assisted chemical vapor deposition (LCVD) of titanium carbide (TiCx) coatings on Al2O3 substrate using tetrakis (diethylamido) titanium (TDEAT) and C2H2 as source materials were investigated. The influences of laser power (PL) and pre-heating temperature (Tpre) on the microstructure and deposition rate of TiCx coatings were examined. Single phase of TiCx coatings were obtained at PL = 100–200 W. TiCx coatings had a cauliflower-like surface and columnar cross section. TiCx coatings in the present study had the highest Rdep (54 mm/h) at a relative low Tdep than those of conventional CVD- TiCx coatings. The highest volume deposition rate (Vdep) of TiCx coatings was about 4.7 � 10�12 m3 s�1, which had 3–105 times larger deposition area and 1–4 order lower laser density than those of previous LCVD using CO2, Nd:YAG and argon ion laser. � 2013 Elsevier Ltd. All rights reserved. Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u 1. Introduction Titanium carbide (TiCx) coatings exhibit very high melting point and thermal stability, high hardness and excellent wear resistance, low coefficient of friction, high electrical and thermal conductivi- ties [1,2], which can be used as wear resistant coatings for cutting tools and inserts, thermal barrier coatings in fusion reactors and diffusion barrier in semiconductor technology [3,4]. Thin films of titanium carbide have been grown from the vapor phase using several techniques, either chemical or physical [5,6]. The need for lower deposition temperature together with the need for local deposition led to those investigations using laser-assisted chemical vapor deposition (LCVD) [7]. It is well known that LCVD technique can be divided into two main categories depending on whether the laser beam interacts with the reactant gases or the substrate material. In photolytic LCVD the laser beam is absorbed by the reactant gases, which undergo photo dissociation, whereas in the pyrolytic LCVD the substrate is heated locally by the laser beam and the chemical reaction is thermally induced [8]. A few researches on the LCVD of TiCx coatings on different substrate materials by employing different types of lasers have previously been published in the literature [9–12]. High-powered CO2 laser, which is often called pyrolytic LCVD, has often been applied to CVD of TiCx coatings, however, due to a small beam size of laser (usually * Corresponding author. Tel.: +86 27 87499409; fax: +86 27 87879468. E-mail address:
[email protected] (R. Tu). 0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.03.039 less than several mm) and nonuniform radial distribution of intensity (namely Gaussian distribution), LCVD using CO2 laser would not be appropriate for uniform coatings on large-scaled substrates [13]. Nd:YAG laser and argon ion laser have also been applied for preparing CVD-TiCx films, intending to prepare thin films under high deposition temperature (1273–2030 K) for small deposition areas [11,14]. Since the heated area is small and the temperature varies across the laser spot on the substrate surface, temperature measurements are very difficult to perform in focused laser beam in the previous LCVD-TiCx reports. In the present study, a semiconductor laser with a wavelength of 808 nm, which has the wavelength in-between CO2 laser and excimer laser, was first used with the goal of preparing wide-area TiCx coatings with high deposition rate at a low deposition temperature. Among common hydrocarbon sources, carbon is most readily dissociated from C2H2 and least readily from CH4 [15]. Thus, the use of C2H2 and organometallic precursor could potentially reduce deposition temperatures significantly. Differ- ences in deposition parameters, mainly laser power (PL) and pre- heating temperature (Tpre), on the composition, microstructure, and deposition rate of TiCx coatings achieved with these sources were explored. The effect of laser for the deposition of TiCx coatings was also discussed. 2. Experimental TiCx coatings were prepared on Al2O3 substrates by a semiconductor (InGaAlAs) laser assisted CVD with a spot area of http://dx.doi.org/10.1016/j.materresbull.2013.03.039 mailto:
[email protected] http://www.sciencedirect.com/science/journal/00255408 http://dx.doi.org/10.1016/j.materresbull.2013.03.039 Fig. 1. Schematic diagram of LCVD deposition temperature. Table 1 Laser characteristics and deposition parameters of TiCx coatings prepared by LCVD. Laser characteristics Deposition condition Source: InGaAlAs Mode: continuous wave mode Wave length: 808 nm Power, PL: 100–200 W Total pressure in chamber, Ptot Substrate Pre-heating temperature, Tpre TDEAT vaporizer temperature Gas line temperature Nozzle temperature Flow rate of C2H2 Flow rate of Ar Nozzle-to-substrate distance Deposition time 200 Pa Al2O3 (Nikkato, 99.99%, a-phase) 293–873 K 403–413 K 423 K 423 K 1.32 � 10�6m3 s�1 1.65 � 10�6m3 s�1 23 mm 1.2 ks Fig. 2. XRD patterns of TiCx coatings prepared at Tpre = 298 K, Ptot = 200 Pa and PL = 100–200 W. Y. Gong et al. / Materials Research Bulletin 48 (2013) 2766–2770 2767 about 3 cm2 in a cold-wall chamber. TDEAT (C16H40N4Ti, Aldrich, 99.999%) and C2H2 (Japan Air Gas, 99.9995%) were used as the source materials. The vaporization temperature of TDEAT (Tvap) was controlled at 403–413 K and the vapor was carried by Ar (99.99%) gas into the chamber. The gas delivery line was heated to prevent condensation of the precursor. The laser power (PL) was changed from 100 to 200 W and the pre-heating temperature (Tpre) ranged from 293 to 873 K. The total pressure in the chamber (Ptot) was fixed at 200 Pa. A thermocouple was inserted into a slot in the substrate (2 mm in depth) to measure the deposition temperature (Tdep). The Tdep in the present study could be controlled by the combination of Tpre and PL, which was shown in Fig. 1. Table 1 summarizes the characters of semiconductor laser and deposition parameters for preparing TiCx coatings by LCVD. The crystal structure was examined by X-ray diffraction (u/2u scan) with Cu Ka radiation (Rigaku, RAD-2C). The lattice parameters were calculated from a program based on a least squares analysis. Ex situ X-ray photoelectron spectroscopy (XPS) was utilized to determine the bonding character of the coatings using Al Ka radiation (1486.6 eV) as the X-rays source. The surfaces of the coatings were sputtered using an Ar ion bombardment for 300 s (4 keV, current density 10 mA/cm2) to remove contamination from the surface. The residual pressure in the chamber of the XPS system was about 2 � 10�6 Pa before the ion bombardment. Scanning electron microscopy (SEM, Hitachi, S- 3100H) was used to observe the surface morphology and cross section. The deposition rate (Rdep) was calculated from the film thickness and the deposition time. 3. Results and discussion Fig. 2 shows the effect of PL on the X-ray diffraction (XRD) patterns of TiCx coatings at Tpre = 298 K. The standard XRD patterns of TiC and Al2O3 substrate were also shown in Fig. 1 for comparison. TiCx coatings in a single phase were obtained at PL > 100 W, and the crystallity of TiCx coatings improved with increasing PL. The lattice parameter of TiCx coatings prepared at PL = 200 W was 0.4324 nm based on an average from the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) diffraction peaks, consistent with a previously reported value of 0.4327 nm for stoichiometric TiC [16]. However, care must be taken in using this as the only evidence for stoichiometery since the lattice parameter for TiC does not vary monotonically with C/Ti ratio [15]. The XPS scan spectra exhibiting the chemical structural and bonding states in the TiCx coating is shown in Fig. 3. Dominant signals of Ti and C were detected. Except the dominant signals, the low signal intensity of O and Ar was also detected. The presence of oxygen can be understood either as the result of surface adsorption/oxidation of the samples as they are exposed to atmosphere while being transferred from the deposition chamber to the XPS system, or it can be present in the bulk of the films due to the residual pressure in the deposition chamber [19]. The signal of Ar can be attributed to the selection of Ar ion as the bombarding ions for the XPS analysis. Nitrides were not detected with XPS indicating that significant concentrations of nitride from TDEAT precursor were not incorporated into the films. The detected oxygen content was about 5 at.% after ion erosion. Fig. 4 shows the typical XPS spectra for C 1s region from TiCx coating at PL = 150 W, Tdep = 1063 K. The binding energy of carbon Fig. 4. XPS spectra for C 1s region from TiCx coating prepared at Tpre = 298 K, PL = 150 W, Tdep = 1063 K. Fig. 3. XPS survey scan spectrum of the TiCx coating prepared at Tpre = 298 K, PL = 150 W, Tdep = 1063 K. Fig. 5. XPS spectra for Ti 2p region from TiCx coating prepared at Tpre = 298 K, PL = 150 W, Tdep = 1063 K. Y. Gong et al. / Materials Research Bulletin 48 (2013) 2766–27702768 in the C–Ti bond is 282.0 eV indicating that the coating consists of TiC [17,18]. A shoulder was observed on the C 1s peak around the binding energy of 284 eV. Similar shoulders have been reported previously in TiC over a range of energies from 283.5 to 285 eV that has been associated with the presence of carbon in the carbon matrix [15]. The large size of this shoulder in the present study Fig. 6. Surface and cross-sectional SEM images of TiCx coatings prepared at Tpre = 29 Tdep = 1063 K; (d) PL = 200 W, Tdep = 1173 K. might have been attributed to the excess carbon concentration in TiCx coatings. The XPS spectrum for the Ti 2p region, shown in Fig. 5, displays the familiar 2p3/2 and 2p1/2 binding energies [19], again confirming the presence of Ti-C bonding. The Ti 2p region has peaks at the expected Ti-C binding energies of 455 and 461 eV indicating the existence of TiCx phase in the deposited coating. However, quantitative measurements of the TiC composition could not be performed because of the carbon in the carbon matrix. Fig. 6 shows the surface and cross-sectional morphologies of TiCx coatings prepared at PL = 100–200 W, Ptot = 200 Pa and Tpre = 298 K, where Tdep changed from 938 to 1173 K. At PL = 100– 200 W, the TiCx coatings had a cauliflower-like surface micro- structure and the cauliflower-like grain size was around 2 mm. The density of TiCx coatings increased with increasing PL. The TiCx coatings exhibited a dense columnar cross-sectional microstruc- ture and good adhesion to the substrate. With increasing PL, the Rdep, calculated from SEM micrographs, increased from 10 to 48 mm/h. Fig. 7 shows the effect of Tpre and PL on the Tdep at Ptot = 200 Pa. The Tdep increased with increasing Tpre and PL in the range from 938 to 1248 K. Since there are several combinations of PL and Tpre to determine Tdep, we can identify the effect of PL on the features of TiCx coatings at the same Tdep. A comparison of the surface morphologies of TiCx coatings prepared at the same Tdep with different PL and Tpre is presented in Fig. 8. At Tdep = 1095 K, as shown in Fig. 8 (a) and (b), the size of cauliflower-like grains at PL = 100 W was greater than that at PL = 150 W. At Tdep = 1190 K, 8 K and (a) PL = 100 W, Tdep = 938 K; (b) PL = 125 W, Tdep = 973 K; (c) PL = 150 W, Fig. 7. Effect of Tpre and PL on the deposition temperature of TiCx coatings. Fig. 9. Comparison of Rdep between the present work and previously reported values of CVD TiCx coatings. TDEAT + C2H2 system: present work; TiCl4 + CH4 system: [20], [22]; TiCl4 + C3H8 system: [21]; TiCl4 + CCl4 system: [23]; Cp2TiCl2 system [24]. Y. Gong et al. / Materials Research Bulletin 48 (2013) 2766–2770 2769 the TiCx coating at PL = 200 W had more dense surface microstruc- ture and smaller cauliflower-like grains than that at PL = 150 W even at the same Tdep. Since the TiCx coatings prepared at the same Tdep with different PL showed different microstructures as demonstrated in Fig. 8, the photolytic effect of the laser on the TiCx coatings can be evidently observed. Fig. 9 demonstrates the effect of Tdep on the Rdep of TiCx coatings in the Arrhenius format. The Rdep values by conventional CVD reported in the literatures [20–24] were compared with those in the present study. The Rdep in the present study showed the maximum value of 54 mm/h at Tdep = 1193 K, PL = 200 W and Tpre = 473 K. The Rdep of LCVD TiCx coatings were 2–50 times greater than those of films prepared by conventional CVD with TiCl4–C3H8 and TiCl4–CH4 systems. This indicates a different Fig. 8. SEM surface images of TiCx coatings prepared at (a) Tdep = 1095 K, PL = 150 W, Tp Tpre = 473 K; (d) Tdep = 1190 K, PL = 150 W, Tpre = 873 K. deposition mechanism in LCVD of TiCx coatings compared with conventional hot/cold wall CVD. Faster deposition rates for LCVD compared to conventional CVD have been attributed to enhanced mass transport of the source gases because of convective currents around the three dimensional hot zone [14,25]. Although the CO2 laser assisted CVD showed a relative higher deposition rate in thickness than that in the present study, the difference of the lasers is interested to be discussed. Table 2 summarized the characteristics of TiCx coatings prepared by LCVD in the present study and those of literatures [7,12–15,26]. TiCx coatings prepared by LCVD using CO2 laser (often called pyrolytic LCVD) with TiCl4 precursor had a high Rdep (288 mm/h) with low re = 473 K; (b) Tdep = 1095 K, PL = 100 W, Tpre = 873 K; (c) Tdep = 1190 K, PL = 200 W, Table 2 Comparison of the characters of TiCx coatings prepared by LCVD. Laser Wave-length (nm) Power (W) Spot area (m2) Laser power density (W m�2) Titanium precursor Tdep (K) Vdep (m 3 s�1) Ref. CO2 10,600 125–250 1 � 10�4 0.8–1.6 � 106 TiCl4 1173–1673 1.6 � 10�12 [26] CO2 10,600 100 1 � 10�5 0.1–1.7 � 108 TiCl4 1673 1.4 � 10�13 [7] CO2 10,600 135–250 1 � 10�4 0.9–1.9 � 106 TiCl4 1273–1673 8 � 10�13 [13] CO2 10,600 400 1 � 10�6 5–10 � 106 TiCl4 1473–2673 1 � 10�13 [12] Nd:YAG 1060 300 3 � 10�6 9.5 � 105 TiCl4 1273–1773 1.8 � 10�13 [15] Argon ion 514.5 0.9–4.5 1 � 10�9 4.2–5.7 � 105 TiCl4 1880–2030 2 � 10�14 [14] InGaAlAs 808 100–200 3 � 10�4 3.2–6.4 � 105 TDEAT 938–1248 4.7 � 10�12 This study Fig. 10. Relationship between laser density and volume deposition rate of TiCx coatings prepared by LCVD. Y. Gong et al. / Materials Research Bulletin 48 (2013) 2766–27702770 deposition area (10�4–10�6 m2) and high deposition temperature (1173–2673 K), whereas TiCx coatings with a relative low Rdep was obtained by LCVD using an Argon ion and Nd:YAG laser, which had a lower deposition area (10�6–10�9 m2) and highest deposition temperature (1273–2030 K) using TiCl4 as a precursor [14,15]. The semiconductor laser used in the present study has a wavelength at 808 nm, which is an intermediate wavelength between CO2 laser and the excimer laser. Pyrolytic and photolytic effects might have contributed in the present LCVD, thus TiCx coatings with high Vdep were obtained with large deposition area (3 � 10�4 m2) and low deposition temperature (938–1248 K) in the present study. Fig. 10 demonstrates the relationship between the volume deposition rate (deposited volume per time) and the laser density of LCVD-TiCx coatings. In the previous reports, the deposition area was far lower than that of the present study, which had a 1 to 4 order lower laser density. In the previous LCVD using CO2 laser, significant high deposition rate in thickness (288 mm/h) were reported [7,12], however due to the lower deposition area, the volume deposition rates were lower and the laser densities were much higher than that of in the present study. The TiCx coating for engineering applications particularly for cutting tools has been conducted by either PVD, thermal CVD or plasma CVD. This study first reported the possibility of LCVD applying on the preparation of TiCx coating as engineering materials. 4. Conclusions TiCx coatings in an almost single phase were prepared by LCVD employing a semiconductor (InGaAlAs) laser using TDEAT and C2H2 as the source materials. Lattice parameters of TiCx coatings were close to those of the stoichiometry at PL = 200 W. TiCx coatings showed a columnar cross section and a dense cauliflower- like surface texture. The highest deposition rate in thickness (Rdep) was 54 mm/h obtained at Tdep = 1193 K. The highest volume deposition rate (Vdep) of TiCx coatings was about 4.7 � 10�12 m3 s�1, which had a 3 to 105 times larger deposition area and a 1 to 4 order lower laser density than those of previous LCVD using CO2, Nd:YAG and Argon ion laser. Acknowledgements This work was financially supported by Global COE Program ‘‘Materials Integration (International Center of Education and Research), Tohoku University’’, and was also supported by the 111 Project (B13035), State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Tech- nology) and Fundamental Research Founds for National University China University of Geosciences (Wuhan) CUG120118. References [1] D.W. Flaherty, R.A. 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