Characterization of TiCN Thin Films Deposited by Dc-Pulsed PACVD Using Methane Precursor

DIN 1.2606 (X4Cr5MoWSiV) steel is currently one of frequently used chromium-molybdenum tool steels, which contains high molybdenum content. This steel has various applications, such as hot forging dies, slotting tools, scissors, hot forging punches, etc. The mechanical, friction and wear properties of it have been extensively investigated as a function of tempering treatment and associated microstructure1,2. The PACVD technique is an appropriate method to deposit wear and corrosion resistant hard coatings which have excellent properties applicable for several applications. In comparison to PVD methods, it offers the possibility of homogeneous coating on work pieces with complicated shapes. Other advantages of PACVD method are high adhesion and good morphology of the layers. Tools deposited with hard coatings, like TiN, TiCN, TiC, have already been used successfully for many industrial application3-6. The PACVD process is affected by several process parameters, like discharge voltage, current density, gas pressure, gas composition and flow rate7-9. In the present research, the influence of the CH4/ (CH4+N2) gas flow ratio on characteristics of PACVD TiCN thin film on the hot – work tool steel has been investigated.


Introduction
steel is currently one of frequently used chromium-molybdenum tool steels, which contains high molybdenum content.This steel has various applications, such as hot forging dies, slotting tools, scissors, hot forging punches, etc.The mechanical, friction and wear properties of it have been extensively investigated as a function of tempering treatment and associated microstructure 1,2 .
The PACVD technique is an appropriate method to deposit wear and corrosion resistant hard coatings which have excellent properties applicable for several applications.In comparison to PVD methods, it offers the possibility of homogeneous coating on work pieces with complicated shapes.Other advantages of PACVD method are high adhesion and good morphology of the layers.Tools deposited with hard coatings, like TiN, TiCN, TiC, have already been used successfully for many industrial application [3][4][5][6] .The PACVD process is affected by several process parameters, like discharge voltage, current density, gas pressure, gas composition and flow rate [7][8][9] .
In the present research, the influence of the CH 4 / (CH 4 +N 2 ) gas flow ratio on characteristics of PACVD TiCN thin film on the hot -work tool steel has been investigated.

Experimental Procedure
TiCN coatings with compositional gradients were deposited on a X4Cr5MoWSiV hot-work tool steel substrate using a PACVD coating system equipped with a voltagecontrolled pulse generator.
During coating, process parameters such as gas flow ratio, wall temperature, voltage duration of pulse-on and pulse-off time and total pressure were monitored.H 2 , Ar, N 2 and CH 4 gases and TiCl 4 vapor were used as process gases for coating deposition.Total pressure was kept at 2 mbar and substrate temperature was controlled at 470°C to avoid exceeding the tempering temperature of the hot-work tool steel.Plasma nitriding was used as a pre-treatment to decrease hardness gradient between substrate and coating.
The processing parameters for the plasma nitriding are listed in Table 1.The N 2 /CH 4 gas flow ratio was defined CH 4 / (CH 4 +N 2 ).The crystalline structure of the coatings was determined by grazing incidence X-ray diffraction (GIXRD) in the continuous scanning mode using CuK α radiation (λ = 0.154056 nm).The full-width at half-maximum (FWHM) of the Bragg peaks is used to estimate the grain size based on Scherrer formula (Equation 1) 10 : Where D is grain size, β is the FWHM of the Bragg peak, and θ is the Bragg reflection angle.The chemical composition of the film was analyzed by x-ray photoelectron spectroscopy (XPS) with monochromated AlKα radiation at the pass energy of 1486.6 eV.The film morphology was studied by scanning electron microscopy (SEM, VEGA-TESCAN-XMU) and atomic force microscopy (AFM, DME-DS-95-50E).The Vickers hardness of the TiCN coatings was measured using a micro-hardness test, within the loading range of 50 g; five micro-hardness tests were performed for each sample to obtain the average values of the hardness.

Results and Discussions
Figure 1 illustrates the GIXRD patterns of the coatings deposited at 470 °C using the two gas mixtures.The (200) plane is revealed to be the preferred structure, deposited via kinetics-limited crystal growth 11 and under thermodynamically stable conditions because the (200) plane has the lowest energy surface in the TiCN crystal.This implies that the film structure is dense because the plane (200) orientation is the densely packed plane.The GIXRD pattern also indicates an exclusively face-center-cubic (FCC) structure which is closely related to Ti. Nitrogen and Carbon can be incorporated on the octahedral sites of both the hexagonal close-packed lattice of titanium and the FCC sublattice in TiCN and thus form TiC x N y .In stoichiometric TiCN, it is assumed that all the octahedral sites are filled with nitrogen and carbon 12 .Titanium carbonitride is a compound stoichiometry of which in TiC x N y can vary in the range of 0<x, y<1.Moreover, the peaks of the TiCN phase shift to lower angles 13 and are close to the TiC phase while the C relative contents reach around 45 at% measured by XPS.This is ascribed to the substitution of N atoms with the bigger C atoms in the solid solution.
Figure 1 also indicates that by increasing the flow ratio from 50% to 66%, the (220) plane is thermodynamically stable relative to the (200) plane and the broad peak of Only for a CH 4 / (CH 4 +N 2 ) gas flow ratio of 0.5, the colour of the TiCN layers is brown.At higher gas ratio it becomes increasingly dark blue.This is in conformation to the investigation reported by E. Erturk et al. 14 .
The later layers mentioned are not stoichiometric.When the gas flow ratio was %50, the C/N ratio was 0.43 with TiC 30 N 70 stoichiometric composition (Figure 1a).By increasing the gas flow ratio to %66, the C/N ratio changed to 2.33 and TiC 70 N 30 stoichiometric composition (Figure 1b).The TiC 30 N 70 possesses a face-centered cubic (FCC) lattice with a o =0.4258 nm and grain size 28.6 nm but the TiC 70 N 30 possesses a face-centered cubic (FCC) lattice with a o =0.4287 nm and grain size 44.37 nm, so lattice parameter decreased with increasing gas flow ratio.This is in conformation to the relation (Equation 2) proposed by Li Yuanbing et al for TiC x N 1-x [15] .0.4235 0.007 ( ) a x nm = + (2) The residual stresses of the samples were determined by X-ray measurements.The surface residual stresses of TiCN layers were -28.74 MPa for %66 and -23.42 MPa for %50.The residual stress values increase with increasing the flow ratio.
The crystallite percents were determined by X-ray diffraction.The crystallite percents of TiCN layers were %76 for % 66 gas flow ratio and %85 for % 50.Also films deposited at low CH 4 /N 2 ratios were much more crystallite than those deposited at high ratios.
Figure 2 presents the XPS spectra of Ti 2p, N 1s and C 1s of the TiCN film.In the Ti spectrum, two peaks of Ti 2p are found at 462.7 and 468.4 eV.The N 1s spectrum exhibits a weak peak at 394.6 eV, which corresponds to the N-Ti bonds.The weak electronegative of C element, involves in the TiCN film formation, would slightly increase the binding energies of Ti-N.Cheng et al. 16 have reported that the binding energy of the N 1s spectra of the TiCN films is lower than that of the TiN films.The C 1s spectrum consists of three peaks at binding energies of 281.5, 283.4 and 285.2 eV.The peak at 281.5 eV is assigned to the Ti-C bonds 17 .The peaks at 283.4 eV and 285.2 eV are assigned to the sp2-C and sp3-C bonds, respectively 18 .The C 1s binding energies of pure graphite (284.3 eV) and diamond (285.3 eV) measured under the same conditions are cited for comparison.
It is also conveyed from Figures 2 and 3 that the peak at 281.5 eV occupies a large fraction of the total C 1s spectrum compared to the other peaks, indicating that a large fraction of C atoms are bonded to Ti atoms, and a little amount of C atoms exist as amorphous carbon.That is in conformation to GIXRD pattern.
Figure 2a shows that chemical composition is TiC 0.426 N 0.544 O 0.03 for 50% and TiC 0.453 N 0.51 O 0.038 for 66% (Figure 3).Therefore the XPS spectra offer similar chemical composition for both cases while they were different from GIXRD patterns because the XPS spectra are very sensitive to surface analysis.
SEM images show the thickness of layer plus the rate of growth, both of them decreased by increasing gas flow from 6.57 μm to 4.83 μm.(Figure 4) Figure 5 shows the AFM images of the TiCN layers.High CH 4 gas flows led to a high surface roughness of the TiCN layers, as can be seen in the Figure 5b.A dense TiCN layer is grown in 50% but not in 66%, where the TiCN layer has a relatively rough structure.According to Quirós et al. 19 , increasing roughness could be due to the fact that the assisting ions lead to significant damage and preferential sputtering effects, mainly during the deposition of the TiCN film.The effects are rather significant, inducing rougher surfaces and disappearance of the previous texture.Figure 5 shows that films deposited at low CH 4 /N 2 ratios are much more homogeneous than those deposited at high ratios.Most of deposited particles size is around 55 nm at low CH 4 /N 2 ratios while it is approximately 90 nm at low CH 4 /N 2 ratios.The hardness values of TiCN coatings deposited at a temperature of 470 °C with various flow ratios are illustrated in Figure 6.A hardness value of 2038 HV 0.05 for the flow ratio of 50% to a hardness of 1896 HV 0.05 for the flow ratio of 66%.It means that the hardness of the TiCN coatings decreased as the flow ratio increased.The reduction of hardness can be attributed to two reasons.When the flow  ratio increases, the grain size of the TiCN coatings are increased (as shown in Figure 1), resulting a decrease in the hardness of TiCN coating.Furthermore, during the growth of the grain size, the chlorine atoms can diffuse into high angle grain boundaries, after exceeding the solubility limit of chlorine in the TiCN lattice 20 .Thus, the hardness of the thin films is decreased.

Conclusion
The following conclusions can be drawn from this research: 1-TiC 30 N 70 stoichiometric composition was obtained in 0.5 gas flow ratio and TiC 70 N 30 for 0.66 which were different from XPS results.2-XPS peaks indicate that a large fraction of C atoms are bonded to Ti atoms, and a low fraction of C atoms exist as amorphous carbon.
3-The films grown, using plasma-assisted processes at lower flow ratios were generally fine-grained and had a denser structure.4-It was found that the growth rate decreased with increasing the flow ratio.5-Finally, it was found that the surface hardness decreased with increasing the flow ratio due to decrease in grain size and diffusion of chlorine in grain boundary.

Figure 6 .
Figure 6.hardness values of TiCN coatings in various flow ratios.