metals Article
Microstructure, Residual Stress, Corrosion and Wear Resistance of Vacuum Annealed TiCN/TiN/Ti Films Deposited on AZ31 Haitao Li 1,2, *, Shoufan Rong 1 , Pengfei Sun 1 and Qiang Wang 2, * 1 2
*
College of Material Science and Engineering, Jiamusi University, Jiamusi 154007, China;
[email protected] (S.R.);
[email protected] (P.S.) College of Material Science and Engineering, Jilin University, Changchun 130022, China Correspondence:
[email protected] (H.L.);
[email protected] (Q.W.); Tel.: +86-431-8509-4375 (H.L. & Q.W.); Fax: +86-431-8509-0802 (H.L. & Q.W.)
Academic Editors: Vijayaraghavan Venkatesh, Raman Singh and Vinod Kumar Received: 7 November 2016; Accepted: 20 December 2016; Published: 29 December 2016
Abstract: Composite titanium carbonitride (TiCN) thin films deposited on AZ31 by DC/RF magnetron sputtering were vacuum annealed at different temperatures. Vacuum annealing yields the following on the structure and properties of the films: the grain grows and the roughness increases with an increase of annealing temperature, the structure changes from polycrystalline to single crystal, and the distribution of each element becomes more uniform. The residual stress effectively decreases compared to the as-deposited film, and their corrosion resistance is much improved owing to the change of structure and fusion of surface defects, whereas the wear-resistance is degraded due to the grain growth and the increase of surface roughness under a certain temperature. Keywords: vacuum annealing; TiCN composite film; element distribution; structure; corrosion and wear
1. Introduction Magnesium alloys are one of the lightest modern structural materials, which have broad prospect and are widely used due to their good physical and mechanical properties, including high strength–weight ratio and specific stiffness, high resistance to shocks and vibration loads, good conductivity of heat and electricity, good electromagnetic shielding and recycling potential [1–5]. However, the poor corrosion resistance and wear resistance have restricted the application of magnesium alloys. In order to solve these two problems, the most effective method is to form a coating on the surface between the magnesium alloy and its environment. At present, various techniques are utilized for depositing the desired coatings on the magnesium alloys to improve corrosion resistance and wear resistance, such as micro-arc oxidation and arc ion plating. Of these techniques, magnetron sputtering is a favorable green technology for magnesium alloy, with the characteristics of low temperature and high speed deposition. Titanium carbonitride (TiCN) thin film presents good corrosion resistance and wear resistance, thermodynamic stability, high hardness and good chemical inertness [6–8]. However, the residual stress exists in the film, which may reduce the adhesion between film and substrate [9]. In this paper, the nano-composite TiCN/TiN/Ti films were fabricated by DC/RF magnetron sputtering, and the residual stress of nano-composite TiCN/TiN/Ti film was effectively reduced by vacuum annealing. The microstructure, elements distribution, corrosion behavior and wear resistance of annealed TiCN/TiN/Ti film were systematic investigated with respect to the application of Mg alloys, such as biological degradation of magnesium alloy.
Metals 2017, 7, 5; doi:10.3390/met7010005
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2. Experimental Section 2.1. Sample Deposition The AZ31 substrates (Zn: 0.75 wt %; Al: 3.09 wt %; Mn: 0.28 wt %; Mg: balance wt %) with dimensions of 20 mm × 20 mm × 3 mm were ground with SiC emery paper up to #2000, polished with Al2 O3 paste and cleaned by rinsing in ultrasonic bath of acetone, alcohol and DI water before deposition [10–12]. TiCN/TiN/Ti composite films were prepared by application of the unbalanced magnetron sputtering of DC and RF to sputter a titanium target (purity ≥ 99.99 wt %), and the base pressure of the chamber was 2 × 10−3 Pa. A thin Ti buffer layer was produced firstly to avoid a failure caused by the physical difference between the ceramic film and the substrate, using the parameters: bias voltage of −45 V, total pressure of 0.5 Pa, and DC target current of 0.4 A. Afterwards, a TiN film was deposited in a gaseous mixture of Ar (99.99%) and N2 (99.99%), and a TiCN coating was deposited by reactive magnetron sputtering in an Ar, N2 and C2 H2 (99.99%) mixed atmosphere. The bias voltage, total pressure and target current or power were kept constant at −45 V, 0.5 Pa, 0.4 A (DC) and 160 W (RF, 13.56 MHz), respectively. The gas flow ratio of Ar/N2 for preparation of the middle TiN layer was 20/5, and the gas flow ratio of Ar/N2 /C2 H2 for preparation of the outer TiCN layer was 20/5/3. After deposition, vacuum annealing was carried out on the composite films at 250 ◦ C, 300 ◦ C and 350 ◦ C for 60 min, respectively. 2.2. Microstructure and Residual Stress The microstructure of as-deposited film and annealed films were characterized by transmission electron microscopy (TEM, Tecani-F20, FEI, Hillsboro, OR, USA) operating at 200 kV accelerating voltage and glancing angle X-ray diffraction (GAXRD) thanks to a D8 diffractometer (Bruker, Karlsruhe, Germany) operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5406 Å) [13–15]. Details of the residual stress calculation can be found in our previous paper [16]. The surface and cross-section were observed by field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Tokyo, Japan). 2.3. Composition, Thickness and Property The chemical composition and element distribution of the coatings were determined by X-ray Fluorescence (XRF, Shimadzu-1800, Shimadzu, Kyoto, Japan) [16,17] as follows: tube power of 40 KV and 95 mA, aperture of 10 mm for element content, and 0.5 mm for element distribution. Thickness of film is also investigated by XRF with the same condition as composition. The potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS) were performed in a 3.5 wt % NaCl solution to evaluate the corrosion property employed by electrochemical workstation (Princeton VersaSTAT3, Ametek, Berwyn, IL, USA). The potential was referred to a saturated calomel electrode (SCE); the counter electrode was a platinum sheet and working electrode was samples. Prior to measurement, the specimens were exposed to the corrosion solution at open circuit potential for 20 min to ensure the stabilization potential [12,18,19], then the polarization scan was executed at a rate of 1 mV/S at room temperature, EIS measurements were conducted from 10 KHz down to 0.1 Hz using 10 mV amplitude sinusoidal perturbations. All the electrochemical tests were repeated twice to obtain reproducibility of results. The corrosion morphology was observed by SEM (JSM-6360LV, JEOL, Tokyo, Japan), equipped with energy dispersive spectroscopy (EDS-Falcon, EDAX Inc., Mahwah, NJ, USA). A pin-on-disk tribometer was used to serve dry rotating friction at room temperature under a load of 0.1 Kg. A 3-mm diameter GCr15 (HRC 61) ball was selected as the counter body to create a wear track on the coatings surface. The rotational speed of the electrical machine was 300 rpm.
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3. Results Results and and Discussion Discussion 3. 3.1. Microstructure Microstructure 3.1.
TiCN(222)
substrate(112) TiN(311)
TiCN(220)
Substrate(102)
substrate(110)
TiN(111)
350℃
TiCN(200)
Substrate(100) TiCN(111)
Intensity
(a)
TiCN(311)
Figure 11 shows shows the the XRD XRD patterns patterns of of annealed annealed films filmsand andelectrochemical electrochemicaltest testfilms. films. As As shown shown in in Figure Figure 2a, both as-deposited film and annealed films show a similar set of peaks consisting of TiN Figure 2a, both as-deposited film and annealed films show a similar set of peaks consisting of TiN and TiCN TiCN two twophase, phase,but butthe theannealed annealed films films exhibit exhibit sharper sharper reflections reflections with with aa lower lower Full Full Width Width at at and half maximum (FWHM), and the peak width decreases with an increase of annealing temperature. half maximum (FWHM), and the peak width decreases with an increase of annealing temperature. Moreover,the theannealed annealedfilms filmsshow showtwo two strong peaks associate to (111) (220) planes. reason Moreover, strong peaks associate to (111) andand (220) planes. TheThe reason for for this crystalline behavior is related to a larger number of oriented crystallites in (111) and (220) this crystalline behavior is related to a larger number of oriented crystallites in (111) and (220) directions; directions;the therefore, oriented crystallites more constructive reflections for these therefore, oriented the crystallites generate moregenerate constructive reflections for these crystallographic crystallographic planes, whichincrease produceinaintensity strong increase in intensity The peaks of observed substrate planes, which produce a strong signal. The peaks of signal. substrate can also be can also be observed due to the fact that the maximum thickness of the film is only detected by XRF due to the fact that the maximum thickness of the film is only detected by XRF as 1.83 µm (shown in as 1.83 μm (shown in Table 1). Another physical effect exhibited in the XRD patterns after vacuum Table 1). Another physical effect exhibited in the XRD patterns after vacuum annealing is a slight shift annealing a slight shift of the peak positions to higher is probably due to the decrease of the peakispositions to higher angles. This is probably dueangles. to the This decrease in the compressive residual in the compressive residual stress. It can be seen in Figure 1b, after electrochemical test, the stress. It can be seen in Figure 1b, after electrochemical test, the corrosion products are Mg(OH) 2 corrosion products are Mg(OH) 2 and TiO 2 . and TiO2 .
300℃ 250℃
as-deposited
0 20
30
40
50
60
70
80
2theta
TiO2
MgO
AZ31 TiCN+TiO2 Mg(OH)2
AZ31
TiCN+TiO
TiO2 Mg(OH)2
Intensity
Mg(OH)2
AZ31 TiCN
TiN
AZ31
(b)
350℃
300℃
0
250℃
20
30
40
50
60
70
80
90
2 theta Figure of of (a)(a) annealed films (b) after executed electrochemical test. Figure 1.1.X-ray X-rayDiffraction Diffraction(XRD) (XRD)patterns patterns annealed films (b) after executed electrochemical test.
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Figure Highresolution resolution transmission transmission electron electron microscopy Figure 2. 2.High microscopy(HRTEM) (HRTEM)and andInverse InverseFast FastFourier Fourier Transform (IFFT) images of (a,b) as-deposited film; (c,d) annealed film at 300 °C. ◦ Transform (IFFT) images of (a,b) as-deposited film; (c,d) annealed film at 300 C. Table 1. Element content values and thickness of deposited film and annealing films at different Table 1. Element content values and thickness of deposited film and annealing films at different temperature by X-ray Fluorescence (XRF) (wt %). temperature by X-ray Fluorescence (XRF) (wt %).
Element Contents Thickness (μm) Contents C TiElement N Thickness (µm) Temperature (◦ C) As-deposited Ti 46.2634 30.7397 22.9969C 1.75 N 48.2358 30.7397 28.2041 23.5601 1.77 1.75 As-deposited250 46.2634 22.9969 300 47.9233 28.2041 28.8855 23.1912 1.79 1.77 250 48.2358 23.5601 350 48.0995 28.8855 28.5277 23.3728 1.83 1.79 300 47.9233 23.1912 Temperature (°C)
350
48.0995
28.5277
23.3728
1.83
Figure 2a reveals the high resolution transmission electron microscopy (HRTEM) micrograph of as-deposited film, where the inset is a corresponding selected area electron diffraction pattern Figure 2a reveals the high resolution transmission electron microscopy (HRTEM) micrograph (SAEDP). The plan view HRTEM exhibits a dense microstructure with no porosity, and the SAEDP of as-deposited film, where the inset is a corresponding selected area electron diffraction pattern displays the multi-ring pattern of the polycrystalline phases with small crystalline grains, which is (SAEDP). The plan view HRTEM exhibits a dense microstructure with no porosity, and the SAEDP confirmed the cubic TiCN phase that is composed of diffracted plane indices at (111), (200), (220), displays the multi-ring pattern of the polycrystalline phases with small crystalline grains, which is and (311) indexes. The preferred orientation is (111) and the lattice fringes with a(111) is 2.468 Å . confirmed the cubic TiCN phase that is composed of diffracted plane indices at (111), (200), (220), and Figure 2b shows an inverse fast Fourier transform (IFFT) image of as-deposited film. It revealed that (311) indexes. The preferred (111) and the lattice fringes with that a(111)were is 2.468 Å. Figure the lattice distorted severelyorientation and a smallisamount of dislocations were found marked by the 2b shows an inverse fast Fourier (IFFT) image ofand as-deposited film. Itindicated revealed the thatexistence the lattice ‘‘T’’ symbol in Figure 2b. Thetransform generation of dislocation lattice distortion distorted severely and a small amount of dislocations were found that were marked by the “T” of residual stress in the film. Figure 2c reveals the HRTEM image of annealed film at 300 °C.symbol The in annealed Figure 2b.film The exhibits generation of dislocation and lattice distortion indicated the existence residual a regular arrangement of single crystal diffraction spots withof bigger ◦ C. The annealed film stress in the film. Figure 2c reveals the HRTEM image of annealed film at 300plane crystalline grains, and the cubic TiCN phase that is composed of diffracted indices at (111), exhibits regular of single crystal diffraction crystalline grains, and (220), aand (311)arrangement indexes is also confirmed. Figure 2d spots showswith an bigger IFFT image of annealed film,the cubic TiCN phase that is composed of diffracted plane indices at (111), (220), and indexes is also revealing that the lattice distorted slightly and all the dislocation disappeared after(311) annealing. confirmed. Figure 2d shows an IFFT image of annealed film, revealing that the lattice distorted slightly and all the dislocation disappeared after annealing.
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3.2. Composition Composition and and Thickness Thickness 3.2. Table 11 lists lists element element content content and and thickness thickness of of as-deposited as-deposited film film and and annealed annealed films films investigated investigated Table by XRF. It can be observed that vacuum annealing has little influence on chemical composition by XRF. It can be observed that vacuum annealing has little influence on chemical composition content. content. the of increase of annealing temperature, the thickness coating increases, which is the With the With increase annealing temperature, the thickness of coatingofincreases, which is the result of result of grain growth in a certain temperature. grain growth in a certain temperature. Figure 33 shows shows element element distribution distribution of of as-deposited as-deposited film film and and annealed annealed film film at at 300 300 ◦°C. As seen seen Figure C. As from the the figure, figure, the the elements elements distribute distribute unevenly unevenly before before annealing, annealing, but but they they become become more more uniform uniform from after annealing. The center of film is nearly unitary, only a partial region on the edge presents lower after annealing. The center of film is nearly unitary, only a partial region on the edge presents lower or or higher values. It indicates that vacuum annealing has strong effect on element distribution higher values. It indicates that vacuum annealing has strong effect on element distribution because because of the elements’ diffusion andgrowth grain growth a certain temperature, which can prompt of the elements’ diffusion and grain under under a certain temperature, which can prompt the the uniformity of element distribution. However, element distribution does not linearly change uniformity of element distribution. However, element distribution does not linearly change withwith the the increase of annealing temperature; the state of element distribution at 300 °C is significantly ◦ increase of annealing temperature; the state of element distribution at 300 C is significantly better betterthe than the other. than other.
(a) Ti
C
N
Ti
C
N
High
Low
(b) High
Low
Figure 3. 3. Elements Elements distribution distribution images images of of films films obtained obtained by by XRF XRF measurement: measurement: (a) (a) as-deposited as-deposited film film Figure ◦ (b) annealed film at 300 °C. (b) annealed film at 300 C.
3.3. Residual Residual Stress Stress 3.3. Figure 44shows shows residual stress curves according to Glancing Angle X-ray diffraction Figure thethe residual stress curves according to Glancing Angle X-ray diffraction (GAXRD) (GAXRD) by different grazing incident angles. It demonstrates that the residual compressive stress by different grazing incident angles. It demonstrates that the residual compressive stress can be can be effectively reduced by vacuum annealing. The decreasing residual stress during annealing, effectively reduced by vacuum annealing. The decreasing residual stress during annealing, measured measured by XRD peak shifts, bearsresemblance a close resemblance to the decreasing peak discussed widths discussed by XRD peak shifts, bears a close to the decreasing peak widths earlier, earlier, indicating that stress relaxation occurrs by defect annihilation processes. The magnitude of indicating that stress relaxation occurrs by defect annihilation processes. The magnitude of the the residual stress level progressively decreases with increase of annealing temperature; the residual stress level progressively decreases with increase of annealing temperature; the minimum minimum value of residual stress 80.65when ± 19 MPa whenat annealing at 350 °C. The residual stress value of residual stress is 80.65 ± 19is MPa annealing 350 ◦ C. The residual stress in thin film in is thin film is composed of the structural stress and the thermal stress; the structural stress is mainly composed of the structural stress and the thermal stress; the structural stress is mainly caused by lattice caused by and lattice mismatch andIn growth defects. the process annealing, the defects in film grains can be mismatch growth defects. the process of In annealing, theof defects in film can be reduced, reduced, and changes crystal structure changestemperature, under a certain temperature, the thickness of grow andgrains crystalgrow structure under a certain the thickness of coating increases, coating increases, part of stress releases and balances, which all makes the residual stress decrease part of stress releases and balances, which all makes the residual stress decrease effectively. effectively.
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Figure 4.4.Residual Residual stress curves of annealed performed different temperature (a) Figure stress curves of annealed coatingscoatings performed at differentattemperature (a) as-deposited as-deposited film; (b) 250 °C; (c) 300 °C; (d) 350 °C. ◦ ◦ ◦ film; (b) 250 C; (c) 300 C; (d) 350 C.
3.4. Morphology and EDS Analysis 3.4. Morphology and EDS Analysis Surface morphology and corrosion morphology with EDS analysis of annealed films and Surface morphology and corrosion morphology with EDS analysis of annealed films and electrochemical films were examined, as shown in Figure 5. The surface of deposited film is dense electrochemical films were examined, as shown in Figure 5. The surface of deposited film is dense and and uniform. From the EDS result, the values of C, N and Ti are 23.74, 31.18 and 45.08 wt %, uniform. From the EDS result, the values of C, N and Ti are 23.74, 31.18 and 45.08 wt %, respectively, respectively, which are consistent with XRF testing result. Figure 5a–e indicates that after annealing, which are consistent with XRF testing result. Figure 5a–e indicates that after annealing, the grain has the grain has increased to some extent, the films are more compact and uniform, the defects between increased to some extent, the films are more compact and uniform, the defects between the inner layer the inner layer interfaces are greatly reduced and the films present an obvious island growth interfaces are greatly reduced and the films present an obvious island growth pattern, the growth pattern, the growth tendency in (111) preferential orientation is weakened, and the columnar crystal tendency in (111) preferential orientation is weakened, and the columnar crystal is effectively refined. is effectively refined. With an increase of annealing temperature, the thickness, roughness and the With an increase of annealing temperature, the thickness, roughness and the average grain diameter of average grain diameter of the film all increase. the film all increase. After the electrochemical experiment, some cracks, pits and pores were found on the surface of After the electrochemical experiment, some cracks, pits and pores were found on the surface of as-deposited film and annealed films. The coating with annealing temperature of 300 ◦°C has better as-deposited film and annealed films. The coating with annealing temperature of 300 C has better protection without any corrosion crack due to its best density and uniformity. As also shown from protection without any corrosion crack due to its best density and uniformity. As also shown from Figure 5, the Mg content was observed by EDS in the annealed film, which was derived from Figure 5, the Mg content was observed by EDS in the annealed film, which was derived from substrate substrate diffusion under a certain temperature. The value of Ti is the highest and O is the lowest in diffusion under a certain temperature. The value of Ti is the highest and O is the lowest in the annealed the annealed film at 300 °C compared to 250 °C and 350 °C, which suggests that the film annealed at film at 300 ◦ C compared to 250 ◦ C and 350 ◦ C, which suggests that the film annealed at 300 ◦ C forms 300 °C forms relatively small amount of corrosion products and has the best protection. relatively small amount of corrosion products and has the best protection.
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Figure 5. Cont.
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Figure 5. (a,a Figure 5. (a,a11)) Surface Surface morphology, morphology, corrosion corrosion morphology morphology and and EDS EDS analysis analysis of of as-deposited as-deposited film; film; ◦ C; (b,b11)) surface and EDS analysis of annealed film film at 300at°C; 1) (b,b surfacemorphology, morphology,corrosion corrosionmorphology morphology and EDS analysis of annealed 300(c,c ◦(d) surface morphology, corrosion morphology and EDS analysis of annealed film at 350 °C; (c,c ) surface morphology, corrosion morphology and EDS analysis of annealed film at 350 C; 1 cross-section of as-deposited film; (e) (e) cross-section of annealed filmfilm at 300 °C.◦ C. (d) cross-section of as-deposited film; cross-section of annealed at 300
3.5. 3.5. Corrosion Corrosion Behavior Behavior Figure 6a shows shows the the polarization polarization curves curves of of samples. samples. Corrosion Figure 6a Corrosion current current density density can can be be directly directly derived from the cathodic polarization curves by Tafel extrapolation, the values of corrosion derived from the cathodic polarization curves by Tafel extrapolation, the values of corrosion potential potential Ecorr and corrosion current density Icorr are shown in Table 2. In view of the fact that E corr and corrosion current density Icorr are shown in Table 2. In view of the fact that magnesium magnesium alloys are to annealing, the annealed AZ31 also investigated. It is that clearboth that alloys are sensitive tosensitive annealing, the annealed AZ31 was alsowas investigated. It is clear both the annealed annealed AZ31 decrease anodiccurrent currentdensities densitiesand and increase increase the the annealed filmsfilms and and annealed AZ31 decrease thethe anodic the anode potential compared to that of the unannealed samples. The E corr of the annealed AZ31 is anode potential compared to that of the unannealed samples. The Ecorr of the annealed AZ31 −4 A/cm2 − −4 increased from −1.562V to V−1.448V, andV,the Icorrthe is decreased from 8.15 × 108.15 × 210to is increased from −1.562 to −1.448 and Icorr is decreased from × 10 to4 1.03 A/cm 2 A/cm×. 10 The Ecorr of2 . the coated is increased −1.562 V from to −1.151 V and −4 A/cm 1.03 The Ecorr sample of the coated samplefrom is increased −1.562 V toincreased −1.151 Vfrom and −1.151 V to −1.062 V after vacuum annealing at 300 °C. The I corr of◦ the coated sample is decreased increased from −1.151 V to −1.062 V after vacuum annealing at 300 C. The Icorr of the coated sample −4 A/cm2 to 7.65 2 and decreased 2 from 8.15 × 10 10−7 A/cm × 10−7 A/cm 4.69 × 10 A/cm22 2 to 7.65 2 and7.65 −7−7A/cm is decreased from 8.15 × 10−4 ×A/cm × 10−7 A/cmfrom decreased from to 7.65 × 10 after vacuum at 300 °C. This indicates that◦ C. theThis corrosion resistance the coated samples to 4.69 × 10−7annealing A/cm2 after vacuum annealing at 300 indicates that the of corrosion resistance of and annealed samples are improved in the 3.5 wt % NaCl solution, and annealed coatings improved the coated samples and annealed samples are improved in the 3.5 wt % NaCl solution, and annealed more than as-deposited coatings improved morecoating. than as-deposited coating.
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Figure Theresults results of electrochemical test forand AZ31 and with coatings withannealing differenttemperature. annealing Figure 6.6.The of electrochemical test for AZ31 coatings different temperature. (a) Potentiodynamic polarization curves; (b) Nyquist plots; (c) Bode plots of vs. (a) Potentiodynamic polarization curves; (b) Nyquist plots; (c) Bode plots of |Z| vs. frequency; |Z| (d) The frequency; (d) The equivalent circuit. equivalent circuit. Table potentiodynamic polarization polarization curves. curves. Table 2. 2. Electrochemical Electrochemical data data of of samples samples obtained obtained from from potentiodynamic
Samples Corrosion Potential Ecorr (V) Current Density Icorr (A·cm−2) −2 Corrosion Potential Ecorr (V) Current Density Uncoated AZ31 −1.562 8.15 × 10−4 Icorr (A·cm ) −4 Uncoated AZ31 −1.562 Annealing AZ31 −1.448 1.03 ×8.15 10−4× 10 −4 Annealing AZ31 −1.448 Deposited film −1.151 7.65 ×1.03 10−7× 10 Deposited film −1.151 7.65−7× 10−7 250 °C film −1.138 6.42 ×6.42 10 × 10−7 250 ◦ C film −1.138 ◦ 300 °C film −1.062 4.69 × 10−7× 10−7 300 C film −1.062 4.69 ◦ 350C°C film −1.084 6.08 ×6.08 10−7× 10−7 350 film −1.084 Figure 6b shows a typical Nyquist plot, the Nyquist plot employed to evaluate the degradation process of samples. Asashown Figure plot, 6b, the impedance (Zr) and imaginary impedance (Zi) of Figure 6b shows typicalin Nyquist thereal Nyquist plot employed to evaluate the degradation annealed AZ31 is higher than the unannealed substrate, and annealed coatings are much higher than process of samples. As shown in Figure 6b, the real impedance (Zr) and imaginary impedance (Zi) of that of as-deposited coating and annealed AZ31. There is a common feature on the Nyquist plots of annealed AZ31 is higher than the unannealed substrate, and annealed coatings are much higher than all coatings, a single semicircle in the feature measured frequency thatannealed of as-deposited coating andcapacitive annealed AZ31. Thereexisting is a common on the Nyquistrange. plots ofThe all sizes of the semicircles of annealed coatings are bigger than the as-deposited film, andsizes the annealed coatings, a single capacitive semicircle existing in the measured frequency range. The semicircle size of of annealed film at 300 are °C bigger is bigger than of 250 °C film, and 350 larger radius of the semicircles annealed coatings than thethat as-deposited and°C. theThe semicircle size of of annealed coatings proves that the coatings after vacuum annealing possess a higher ◦ ◦ ◦ annealed film at 300 C is bigger than that of 250 C and 350 C. The larger radius of annealedcorrosion coatings resistance, the coating at 300 °C possesses the highest corrosion resistance property proves thatand the coatings afterannealed vacuum annealing possess a higher corrosion resistance, and the coating due to the largest radius of the curvature. ◦ annealed at 300 C possesses the highest corrosion resistance property due to the largest radius of Figure 6c shows the corresponding Bode plots of all samples. The model values of impedance the curvature. (|Z|) are often used to evaluate the stability of passive film. The impedance evolution in low frequency range presents the response of the double layer at the interface of AZ31 substrate and the Samples
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Figure 6c shows the corresponding Bode plots of all samples. The model values of impedance (|Z|) are often used to evaluate the stability of passive film. The impedance evolution in low frequency range presents the response of the double layer at the interface of AZ31 substrate and the corrosion media. The presentation of a decrease of impedance values in high frequency range because of induction was the result of the onset of the corrosion process. The higher the value of Z modulus, the more stable the passive film is. It can be seen that the values of impedance of the annealed AZ31 and annealed coatings are bigger than unannealed AZ31 and as-deposited film, and the film annealed at 300 ◦ C shows the biggest impedance at the low frequency among all samples. The bare AZ31 is the smallest and represents the coating annealed at 300 ◦ C possesses a best corrosion resistance among all samples. Figure 6d gives the equivalent circuits, which are typical of a metallic material with a defective coating layer, and Table 3 shows the fitting data of EIS plot. In the equivalent circuit, Rs represents the aqueous impedance, originating from the ohmic contribution of the electrolyte solution between working and reference electrodes; Rct represents the charge transfer resistance and CPEdl represents the charging electric double layer capacitance; Rf represents film blocking effect, CPEf represents the capacitance of TiCN film. As seen from Table 3, the CPEf is the minimum and Rct, Rf values are the largest when the film annealed at 300 ◦ C, which indicates that the film annealed at 300 ◦ C has the best anti-corrosion performance. Table 3. Fitting results of electrochemical impedance spectroscopy (EIS) plot of annealed films.
Samples
Rs (ohm·cm2 )
CPEf − (cm 2 ·s−n ·Ω)
n
Rct (ohm·cm2 )
CPEdl (cm−2 ·s−n ·Ω)
Rf (ohm·cm2 )
250 ◦ C 300 ◦ C 350 ◦ C
88.63 95.35 99.84
4.58 × 10−6 2.05 × 10−6 4.19 × 10−6
0.79 0.79 0.78
5164 5859 5362
3.76 × 10−5 1.35 × 10−5 3.42 × 10−5
898 1156 1009
The improvement of corrosion resistance after annealing can be described as follows: before annealing, when TiCN films subjected to NaCl solution, chloride ions penetrate the film through some small defects and columnar crystal structure, the exposed area will experience anodic dissolution, the corrosive compounds are formed, which will usually extend laterally along the interface between the film and the substrate [1,20]. After annealing, the films become denser owing to the atoms rearrangement and the reduction of defects and interspaces. The annealing coatings with tiny defects and fine columnar crystal structure can effectively prevent the diffusion/penetration of the chloride ions to the films and AZ31, which enhances the corrosion resistance of as-deposited film and substrate. 3.6. Tribological Behavior Figure 7a reveals the friction coefficient–time relation for the as-deposited film and annealed films. As seen from Figure 7a, during the testing, the friction coefficients of all specimens are stable and possess similar features: the friction coefficients first increase to the maximum value, and then reach the relative steady wear stage after about 50 s. It is found that the friction coefficient of the annealed film increases from 0.26 (as-deposited) to 0.35 (350 ◦ C) due to the increase of surface roughness when more surface diffusion effects are occurring during the film nucleation and growth under a certain temperature, as can be seen in Figure 5. Figure 7b shows wear loss (rate) for the as-deposited film and annealed films. As demonstrated in the figure, the annealed films caused a decrease of the wear resistance compared to that of as-deposited film. The as-deposited film presents relatively slighter wear, compared with the annealed films’ apparent higher friction coefficients and wear loss rate. Moreover, it is observed that the wear resistance of 350 ◦ C annealed film is weaker than the annealed films of 300 ◦ C and 250 ◦ C. The main reason that the 350 ◦ C annealed film has weaker wear resistance, as mentioned previously, is that 350 ◦ C annealed film possesses the biggest grain size and surface roughness.
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(a)
0.0016
(b)
0.0014
3
0.5
0.0018
Wear loss rate (mm /Nm)
Friction Coefficient
0.6
0.4 350℃ 300℃ 250℃
0.3
as-deposited
0.2 0
200
400
600
Time (S)
800
1000
1200
0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000
as-deposited
250℃
300℃
350℃
Figure 7.7. (a) (a) Friction Friction coefficients coefficients of of as-deposited as-deposited film film and and annealed annealed films; films; (b) (b) wear wear loss loss (rates) (rates) of of Figure as-deposited film and annealed films. as-deposited film and annealed films.
Figure 7b shows wear loss (rate) for the as-deposited film and annealed films. As demonstrated 4. Conclusions in the figure, the annealed films caused a decrease of the wear resistance compared to that of In summary, theThe TiCN/TiN/Ti multilayer coatingsrelatively were deposited AZ31 compared via DC/RFwith reactive as-deposited film. as-deposited film presents slighteronwear, the magnetron sputtering. After vacuum annealing, the composite films exhibit sharper reflections with annealed films’ apparent higher friction coefficients and wear loss rate. Moreover, it is observed that athe lower FWHM, the peak width decreasesfilm withisan increase of annealing temperature, the °C SAEDP image wear resistance of 350 °C annealed weaker than the annealed films of 300 and 250 °C. changes from polycrystalline ring single crystal diffraction spots, the contentpreviously, values of The main reason that the 350 diffraction °C annealed filmtohas weaker wear resistance, as mentioned chemical composition change slightly, and elements more uniformly. Vacuum annealing is that 350 °C annealed film possesses the the biggest graindistribute size and surface roughness. makes the grain grow and structure refine, and the volume of defects and interspaces reduce, which leads to an effective reduction of residual stress of annealed films and a more compact structure of 4. Conclusions the coatings. The polarization curves and EIS suggested that the corrosion resistance of annealed In summary, the TiCN/TiN/Ti multilayer coatings were deposited on AZ31 via DC/RF reactive film has been improved. However, the increase of friction coefficient and wear loss rate indicated magnetron sputtering. After vacuum annealing, the composite films exhibit sharper reflections with that annealing decreases the wear resistance performance due to the grain growth and the increase of a lower FWHM, the peak width decreases with an increase of annealing temperature, the SAEDP surface roughness under a certain temperature. Vacuum annealing is an effective technology for films image changes from polycrystalline diffraction ring to single crystal diffraction spots, the content to reduce residual stress and improve the corrosion resistance, and the best annealing temperature values of chemical composition change slightly, and the elements distribute more uniformly. was found to be 300 ◦ C for this TiCN/TiN/Ti coating in our work. Vacuum annealing makes the grain grow and structure refine, and the volume of defects and interspaces reduce, which leads effective reduction of residual stressExperiment of annealed filmsbelongs and a Acknowledgments: The authors wishtotoan acknowledge the assistance of Engineering Center to College of Materials science and engineering of Jiamusi University, China, for providing tests of XRD and more compact structure of the coatings. The polarization curves and EIS suggested that XRF. the The authors wish to acknowledge the financial support of the Key project of Jiamusi University (12Z1201521) of corrosion resistance of annealed film has been improved. However, the increase of friction Heilongjiang province, China. coefficient and wear loss rate indicated that annealing decreases the wear resistance performance Author Contributions: Haitao Li and Shoufan Rong conceived and designed the experiments; Haitao Li due to the growth and Sun the and increase of surface roughness underLiawrote certain temperature. performed the grain experiments; Pengfei Qiang Wang analyzed the data; Haitao the paper. Vacuum annealing is an effective technology for films to reduce residual stress and improve the Conflicts of Interest: The authors declare no conflict of interest. corrosion resistance, and the best annealing temperature was found to be 300 °C for this TiCN/TiN/Ti coating in our work. 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