Friction and wear behaviors of TiCN coating treated by R.F magnetron sputtering
H. Mechri1, 2,*, N. Saoula1, N.Madaoui1
1Division Milieux Ionisés, Centre de Développement des Technologies Avancées CDTA Cité du 20 août 1956, Baba Hassen, BP n°17, Alger, Algérie.
2Laboratoire des Sciences et de Génie des Matériaux. Faculté Génie mécanique et Génie des Procédées.
Université Des Sciences et de la Technologie Houari Boumediene. B.P 32, El Alia, Bab Ezzouar, Alger 16111, Algérie.
*E-mail: hmechri@cdta.dz Abstract— Titanium carbonitride (TiCN) coatings
combine high hardness and wear resistance, low friction, good corrosion resistance and high adhesion strength with substrate.
These unique properties make TiCN coatings a good solution for the applications requiring high abrasion and wear resistance. In this study, we present the effect of the negative bias applied to the substrate on the microstructure, friction and wear behaviors of the TiCN coating.
The elaboration of our films has been carried out by reactive magnetron sputtering (R.F) at 13,56MHz, using a gas mixture of methane, nitrogen and argon. The film deposition has been done on the XC38 steel substrates.
The deposited layers were characterized by XRD analysis, potentiodynamic polarization and tribometer.
Keywords—Thin films, Magnetron, Sputtering, Hard coating, TiCN, corrosion, friction and wear.
I. INTRODUCTION
Progress in industry requires the improvement of relevant materials properties (e.g., the wear and corrosion resistance) which can be achieved by coating using PVD methods [1,2]. It is recognized that Titanium carbonitride (TiCN) coatings combine high hardness and wear resistance, low friction, good corrosion resistance, good electrical and thermal conductivities, chemical inertness and high adhesion strength with substrate [3]. These properties make TiCN coatings a good solution for the applications requiring high abrasion and wear resistance.
Due to high abrasion and wear resistance, titanium carbonitride (TiCN) coating [4,5] properties make it more suitable coating for tribological applications. They have proven to be most effective in increasing tools durability.
There are many fabrication techniques for cermet coatings, such as physical vapor deposition (PVD) [6], chemical vapor deposition (CVD) [7] and laser cladding [8]. Magnetron sputtering has low levels of impurities and easy-control of deposition rate, thus it has been used for preparing hard coatings of TiCN films, which improves tool life [9].
Depending on sputtering conditions, this method also allows the production of coatings with various morphologies and crystallographic structures. A few studies on TiCN have been performed to investigate the effect of the sputtering conditions on the properties of TiCN coatings and the effect of bias
voltage is not sufficiently clarified and understood. In this context, we carry out a study on the effect of the substrate bias voltage on the properties of TiCxNy films prepared by RFreactive magnetron sputtering.
The aim of this paper is to investigate the effects of the substrate bias on the structure and properties of the TiCN thin films. The TiCN films were grown onto steel substrates by RF reactive magnetron sputtering from a pure titanium target in Ar-CH4-N2 gas mixture.
II. EXPERIMENTAL
The TiCxNy films are deposited onto steel substrates XC38 by rf (13.56 MHz) reactive magnetron sputtering.
Before deposition, the substrates were polished to mirror surface and then ultrasonically cleaned with acetone and ethanol and then air dried. The deposition chamber consists of a cylindrical stainless steel reactor of 230 mm diameter and 250 mm height. The sample holder was mounted at the midpoint of a circle planetary substrate holder (100 mm in diameter). The distance between the Ti target and the substrate holder was about 30 mm. The pressure control device consisted of a penning and baratron gauges.
Pure titanium target (99.99%) was used as a sputter target. Argon with high purity (99.99%) was used as the sputtering gas. The reactive gases were a mixture of nitrogen (99.99%) and methane (99.99%). After evacuating the chamber down to a pressure of 1.33 × 10−7 Pa, the gases were introduced into the chamber ( Ar: 8 sccm, N2: 4 sccm & CH4:
4 sccm). A set of TiCxNy samples was deposited on steel for measuring the mechanical properties of the films. During deposition, negative bias voltage was applied to the substrates and was varied from no-bias to −100 V. Although substrates were not heated, a temperature rise that didn’t exceed 200 °C was observed due to ion bombardment. The phase components were analyzed with X-ray diffraction using a CuKαradiation, performing 2θ scans from 30° to 90° with an accelerating voltage of 40 kV and filament current of 40 mA. Corrosion resistance was evaluated with an electrochemical workstation (PARSTAT4000) using 3.5% NaCl solution as corrosive media and Tafel polarization curves were recorded at scanning. Obtained data were automatically collected and analyzed using VersaStudio software. Prior to each measurement, the sample was exposed to corrosion test
solution for 6 h at open circuit potential to ensure the stabilization potential. During these measurements, a constant temperature of 25 °C was maintained.
Friction and wear tests were carried out using a CSM ball-on- disk tribometer as shown in Figure 1. The counter body used was an alumina ball of 6 mm diameter. A ball is placed in contact with the sample surface under a predetermined load which generates a friction force. The wear rates for the ball and the samples are calculated by determining the loss of volume during the test using equation 1. All tests were conducted at room temperature without lubrication, testing parameters are summarized in Table I.
Fig.1. Ball on disk system configuration where F is the applied normal force to the ball, (r) the ball diameter, (R) the wear track radius and (s) the disc rotation speed.
Vdisk= 2πR [r2 sin-1(d⁄2r)-(d⁄4) (4r2-d2) (1⁄2)] (1) Where d is the wear track width.
TABLE I :Tribological test parameters.
Parameter Value
Normal load (N) 1, 10
Sliding distance (laps) 1000
Sliding speed (cm.s-1) 10
Wear time (mn) 30
The constant wear track radius (mm) 3
III. RESULTSANDDISCUSSIONS A. Phases and Microstructure
Fig. 2 shows the X-ray diffraction (XRD) analysis performed on the TiCN films deposited under different negative substrate bias voltages. The patterns show that the structure of the TiCN films changes from an amorphous (at Vs
= 0 V) to a crystalline structure one (Vs = −50, −70 and −100 V). Good crystallinity was obtained with stronger intensity of TiCN (111) at −100 V, indicating a preferential growth orientation of (111) direction.
For texture determination, the texture index, Tchkl [10] , different diffracting sets of crystal plane hkl were obtained by using the integrated intensities, Ihkl,from the fitted Gaussian functions for the actual diffraction pattern, and relating them to the corresponding intensities of a randomly oriented sample according to the JCPDS files [11]. From the definition of Tc, a plane with a preferred orientation parallel hkl to sample surface has a texture index value >1.
Fig.2. XRD patterns of TiCN for unbiased and biased substrates.
In this work, the texture coefficient (TC) of TiCN films as a function of the bias voltage (Fig.3) was calculated from their respective XRD peaks by using the following formula [12]:
0 20 40 60 80 100
0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4
Tc
potentiel (V)
(111) (220)
Fig.3: The dependence of the film biased on texture.
Texture coefficient Tc hkl = (I hkl /I0, hkl)
/
(1/n) Σ1n (Ihkl=I0, hkl) In this work, hkl represents the (111) and (220) orientations.B. Tribological behavior
The wear behavior of the coatings mainly depends on the microstructure, hardness and binding strength, which are usually measured in terms of friction coefficient and wear mass loss. Tab II shows the friction coefficients of coatings with different biases under the load of 1 and 10 N.
It can be seen that the average friction coefficient of the sample under unbiased conditions (0 V) is the highest and the friction coefficient of TiCN films deposited at −70 V decreases by 47.19 % and 71.74 % compared with that of the unbiased sample under load of 1 and 10 N respectively.
TABLE II: Friction coefficient of TiCN coating.
Substrate bias
(V) Friction coefficient
Load 1N 10N
0 0.32 0,46
-30 0,15 0,25
-70 0,16 0,13
The wear volume loss of the coatings at different biases under 10 N normal loads is shown in Fig. 4. The film deposited under unbiased conditions (0 V) exhibited the worst performance in tribological tests.
-10 0 10 20 30 40 50 60 70 80
0,02 0,04 0,06 0,08 0,10 0,12
Wear Volume loss/mm3
substrate bias
10N
Fig. 4: Wear volume loss of TiCN under different normal loads
Figure 5 presents the wear surface of the coatings under different applied loads at different biases. As exhibited in Fig.
5(a) and (b), some apparent and slight grooves can be seen on the wear surface of TiCN coating under the load of 1 N, which is in conformity with the mechanism of mild wear. When the applied load increases to 10 N (Fig. 5 (d)), a large number of grooves which are parallel to the sliding direction on the wear surface can be observed and severe adhesion can be seen in the wear tracks of the coating.
Fig. 5: Optical micrographs of wear tracks for TiCN coatings under 1N at 0V (a), -70V (b), and for 10 N 0V (c), -70 V (d).
C. Electrochemical behavior
Potentiodynamic polarization curves were performed to investigate the corrosion resistance of uncoated steel (XC48) and coated samples Ti(C,N) with bias varied from 0 to −100V.
Fig. 6 summarizes the experimental results of these corrosion tests. The corrosion current density (Icorr) and the corrosion potential (Ecorr) were obtained by the intersection of the extrapolation of anodic and cathodic Tafel curves. The values for Ecorr and Icorr for these samples are summarized in Table III. As seen from the values presented, all the investigated samples of the coated steel showed high corrosion potential Ecorr, low corrosion current density Icorr and low corrosion rate compared to the uncoated samples. This demonstrates good corrosion resistance of the TiCN coated samples. The corrosion potential (Ecorr) of the uncoated steel is equivalent to that of the unbiased coated steel. For lower substrate bias voltage of −30 V, the film exhibited high corrosion rate and equivalent Icorr to the unbiased sample. The Icorr values determined from the biased samples (−70 V) are 0.0205 mA/cm2 which is approximately five orders lower than that of the uncoated steel (0.1140 mA/cm2).
Fig. 6: Tafel curves in NaCl for the uncoated steel and with TiCN coating deposited at unbias and biased substrates.
Table III : The electrochemical parameters of the XC48 steel and TiCN films in NaCl solution, 25 °C, (corrosion potential (Ecorr), corrosion current density (Icorr), polarization resistance (Rp), protective efficiency (Pe) of the coatings, and ba and bc are the anodic and cathodic Tafel slopes of the substrate, corrosion rate (Cr)).
Sample Ecorr (mV)
Icorr (mA/cm2 )
Rp (Ω·cm2 )
Ba (mV/dec)
Bc (mV/dec)
Pe (%) Cr (mpy)
XC48 −575.8 0.1140 86.11 37 −91.8 - -
0 V −557.7 0.0799 391.45 140.6 −791.9 29.91 2.556
−30 V −464.5 0.07449 356.07 81.2 63.637 34.66 8.4348
−70 V −366.1 0.0205 38,860 304.8 312.8 82.01 0.03030
−100 V −345.5 0.045533 18,848 60.8 100.681 60.05 0.50035
IV. CONCLUSION
The effect of negative substrate bias on structural, tribological and electrochemical properties of TiCN coatings produced by RF magnetron sputtering was investigated in this study. According to the XRD analysis, the cubic structure of TiCN was observed. The minimum wear rates and the lower
coefficient of friction of 0.13 were obtained for a bias voltage averaging −70 V. The best corrosion current density of coatings deposited at −70 V was 0.0205 mA/cm2, which were about 5 orders of magnitude less than that of the uncoated steel (0.1140 mA/cm2). These results show very promising mechanical properties of TiCN films, which suggest a potential use as wear-resistant films for various applications, which require high protective efficiency of up to 82%.
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