Oxidation and Corrosion Behavior of Nanolaminated MAX-Phase Ti 2 AlC Film Synthesized by High-Power Impulse Magnetron Sputtering and Annealing

1School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea 2Department of Applied Hybrid Materials, School of Convergence Science, Pusan National University, Busan 609-735, Republic of Korea 3Global Frontier Center for Hybrid Interface Materials, Pusan National University, Busan 609-735, Republic of Korea 4Department of Advanced Materials Science and Engineering, Kangwon National University, Chuncheon 24341, Republic of Korea


Introduction
M +1 AX n phases (MAX phases), one kind of nanolaminated carbides or nitrides (M: early transition metal, A: an element of group IIIA or IVA, and X: C and/or N) [1][2][3][4], have gained significant attention on the theoretical and experimental studies since they were discovered in 1960s [5,6].They possess excellent and combined properties of metals and ceramics, including readily machinability, good thermal and electrical conductivity, high strength and elastic modulus, good oxidation, and corrosion resistance [7,8].These properties are attributed to their unique high-order crystalline structures.The M-X octahedral layer and close-packed A-group layer alternatively stack along with the c-direction in the layered hexagonal symmetry structure (group P6 3 /mmc) of MAX phases.M-X is a strong covalent bonding which is similar to the binary carbide or nitride and displays the ceramic properties, while the M-A is a weaker bond which displays the metallic properties [9][10][11][12].The combined properties suggest the potential applications in elevated-temperature structural devices.
Most of the bulk-form MAX phase has been synthesized through hot-isostatic-pressing (HIP) [13][14][15] and spark plasma sintering (SPS) [16][17][18] by sintering a mixed powder of elements or compounds at a very high temperature.MAXphase thin films have been synthesized by various methods such as magnetron sputtering [19][20][21], pulsed cathodic arc [22,23], and chemical vapor deposition [24][25][26][27].Highpower impulse magnetron sputtering (HiPIMS), which has programmable pulsed power applying on the cathode, is a promising physical vapor deposition approach for film deposition.It utilizes extremely high current densities in short pulse at lower duty cycle (on-off time ratio <10%) [28].Zhang et al. reported synthesis of MAX-phase Ti 2 AlN films from a MAX-phase target by using HiPIMS technique [29].The films exhibited excellent oxidation and corrosion properties.Field et al. reported the synthesis of MAX-phase Cr 2 AlC films by a combinatorial approach, especially comparing the HiPIMS and DC magnetron technique under similar conditions on the Cr target [30].The HiPIMS coating generally exhibited lower resistivity than the conventional DC magnetron sputtering (DCMS) coating.Ti 2 AlC, as one representative example of Al containing MAX phases, has shown remarkable antioxidation and anticorrosion properties.In particular its film form, as an oxidation and corrosion protective layer, has attracted great attention in recent years.Abdulkadhim et al. got the TiC x /Al bilayer thin films to investigate the influence of the C content on the products at different annealing temperatures [31]. ≤ 0.7 is the appropriate ratio for Al intercalation into the TiC x to form the MAX phase.Wang et al. obtained the Ti 2 AlC thin films through cosputtering of titanium, aluminum, and carbon target at 500 ∘ C and then annealing at 800 ∘ C and investigated the oxidation behavior of the films [32].Frodelius et al. obtained the MAX-phase Ti 2 AlC through cosputtering the MAX-phase Ti 2 AlC and the Ti target, the Ti target was added to balance the excess of the C [33].More recently, Feng et al. obtained the MAX-phase Ti 2 AlC from the Ti 50 Al 50 target in Ar/CH 4 atmosphere with CH 4 as the reactive gas at room temperature and annealing at 800 ∘ C and investigated the oxidation behaviors of the MAX-phase films [34].However, the mechanism for the superior oxidation and corrosion properties of MAX-phase Ti 2 AlC films still needed to be further investigated.In this study, we synthesized MAXphase Ti 2 AlC films using HiPIMS technique from a MAXphase Ti 2 AlC compound target and postannealing process.The microstructure, oxidation, and corrosion properties of the MAX-phase films were systematically investigated.As a comparison, the oxidation and corrosion measurements of the as-deposited amorphous matrix film were also conducted.And the mechanism for the superior antioxidation and anticorrosion properties of MAX-phase Ti 2 AlC thin films was further discussed.

Sample Preparation.
A MAX-phase Ti 2 AlC target (diameter: 80 mm and thickness: 10 mm) was synthesized by spark plasma sintering (SPS) at 1100 ∘ C under 30 MPa in vacuum.Electron probe microanalysis (EPMA) showed that the atomic ratio was Ti : Al : C : O = 47.29 : 22.01 : 25.27 : 5.43.X-ray diffraction (XRD) results demonstrated that the Ti 2 AlC target was mainly Ti 2 AlC MAX phase.Prior to deposition, the polycrystalline Al 2 O 3 substrates and stainless steel (SUS304) substrates were ultrasonically cleaned in acetone and alcohol for 20 min.Argon ion bombardment was applied at −800 V bias for 5 min to further clean the substrates.The amorphous matrix Ti-Al-C films were grown at room temperature using a HiPIMS coating system (HIPIMS+ power, Hauzer Techno Coating BV) in the argon (Ar) atmosphere.The film thickness was determined by the deposition time.Table 1 shows the pulsing parameters of the HiPIMS and deposition conditions of the amorphous matrix Ti-Al-C films.The MAX-phase Ti 2 AlC films were obtained by annealing the as-deposited amorphous matrix Ti-Al-C films in a tube furnace (DTF-80600-PTFS, Dae Heung Science) at 800 ∘ C for 1 h in Ar (99.9999%) atmosphere.

Film Characterization.
The film thickness was measured using a scanning electron microscopy (FE-SEM, Hitachi, S-4800).Electron probe microanalysis was carried out to determine the elementary composition (EPMA, Shimadzu, EPMA-1600).X-ray diffractometer (XRD, D8, ADVANCE Cu K, 40 kV, 40 mA) was used to characterize the phase structure of the films.The morphology of the films was investigated by the scanning electron microscope (SEM, Hitachi, S-4800, 15 KV).A high resolution field emission transmission electron microscope (FE-TEM, JEOL, JEM-2100F HR) operated at 200 kV was conducted to further examine the microstructure.The focused ion beam (FIB) technique was used for TEM sample preparation.substrate, were conducted at 800 ∘ C in a muffle furnace in static air atmosphere for 1 h to 50 h, respectively.The XRD and TEM analyses were used to analyze the products of the oxidation.The surface and cross section morphology and the thickness of the oxidation layers were observed by SEM and TEM analyses.In order to demonstrate the influence of the oxidation onto the compositions of the films, the SEMline profiling and the TEM-mapping of energy dispersive X-ray spectrometer (EDX) were also carried out.Thermalgravimetric analysis (TGA) was carried out by using a TGA instrument (STA409PC, NETZSCH) at temperatures from room temperature to 1300 ∘ C in the flowing gas mixture of air and N 2 (air : N 2 = 35 : 15) with an empty pure alumina crucible as an inert reference.The mass gain of the films during heating was then recorded.
The corrosion behaviors of the SUS304 specimens coated with and without films were studied by the potential dynamic polarization (PDP) curves (potentiostat/galvanostat, VersaS-TAT 4) in 3.5 wt.% sodium chloride (NaCl) solution at room temperature (RT).The prepared samples, platinum (Pt) mesh, and the saturated calomel electrode (SCE) were used as the working electrode, the counter electrode, and the reference electrode, respectively.The electrode potential range of the measurement was from −1 V to 1 V and the open-circuit corrosion potential was at a scan rate of 1 mV/s.2, EPMA measurements were utilized to demonstrate the compositions of the Ti 2 AlC target and the Ti-Al-C films.There was a little composition deviation between the films and the target.Compared with the target, the excess of the Al atoms of the as-deposited films should be determined by the longer meanfree-path () compared to the titanium atoms ( Al > 1.3 Ti ) due to the smaller size of the aluminum atoms [35].The loss of the Al atoms during annealing at 800 ∘ C should be related to the lower vapor pressure of Al.In other MAX phases, for example, the Ga-containing MAX phase, the influence of the vapor pressure also effected the amounts of the Ga atoms [36].At high temperature, the "A" content was temperature dependent and the evaporation of the "A" element became the dominant factors to influence the compositions of the film.Similar results have also been observed in many MAXphase studies [8,20,[37][38][39][40].The incorporation of small amount of oxygen was mainly coming from the impurity of the target fabrication and the film deposition process.3.1.2.Structure.Figure 1 shows the XRD analyses of the MAX-phase Ti 2 AlC target and as-deposited and annealed Ti-Al-C films on SUS304 substrate, respectively.It was observed that the target was mainly composed of MAXphase Ti 2 AlC.No obvious diffraction peaks in as-deposited films were detected, which indicated that the as-deposited films were amorphous.In contrast, after annealing at 800 ∘ C, the amorphous matrix films transformed to polycrystalline MAX-phase films mainly composed of MAX-phase Ti 2 AlC, with (100), (103), and (110) planes being identified at 34.02 ∘ , 39.55 ∘ , and 60.90 ∘ , respectively.The transformation at high temperature indicated that high atom activity was required to form the high ordered MAX phase.

Composition. As shown in Table
Figure 2 shows the SEM images and line-scan EDX analyses of the as-deposited Ti-Al-C and MAX-phase Ti 2 AlC films, respectively.As can be found from the cross section (Figures 2(c) and 2(d)), two homogeneous films with a thickness of about 2 m were formed.Meanwhile, no distinctive crystal was observed from the Ti-Al-C film (Figures 2(a The characteristic microstructure of the amorphous matrix Ti-Al-C and the MAX-phase Ti 2 AlC film is also investigated by high resolution TEM analyses.Figure 3 shows the selected-area electron diffraction (SAED) patterns as well as the HR-TEM images of the amorphous matrix Ti-Al-C and MAX-phase Ti 2 AlC film.The SAED pattern and the HR-TEM images shown in Figure 3(b) demonstrated that the structure of the Ti-Al-C film was fine-grained polycrystalline; we defined it as an amorphous matrix phase (the grains are too small to be visible in XRD). Figure 3(d) shows typical region was oriented along the [111] zone axis.The oxide layer was confirmed to have a hexagonal aluminium oxide with a formula (Al 2 O 3 ) 5.333 (cubic, space group Fd-3m, ICSD ID: 11569) [41].EDX point scan indicates that the chemistry composition of the cross point is 37.43% Al, 62.43% O, and Ti 0.14% in atomic ratio as shown in Figure 5(c).
Figure 6 shows the surface and cross-sectional SEM morphologies and line-scan EDX analyses of the films on Al 2 O 3 substrates after isothermal oxidation at 800 ∘ C for 5 h, respectively.An oxide layer was formed on the surface of both films.The oxide layer on the amorphous matrix Ti-Al-C film exhibited loose morphology full of cracks, which could not prevent the attack of oxygen into the film.It was further proved by the cross-sectional SEM morphologies and its line-scan EDX analysis that oxygen existed throughout the depth of amorphous matrix film after the oxidation, while a continuous and homogenous oxide layer with dense microstructure was formed on the MAX-phase Ti 2 AlC film.Also the line-scan EDX analysis shows that the oxide layer was Al-rich and the oxygen signal below the oxide layer was significantly lower than that of the amorphous matrix Ti-Al-C after oxidation, which indicated that the dense oxide layer acted as a good barrier for oxygen diffusion into the MAXphase films during oxidation.
TEM test and EDX mapping were also performed on the oxide layer to explore the structure of the films.To further characterize the oxidation behavior of the MAX-phase Ti 2 AlC films, the time-dependent oxidation test at 800 ∘ C was performed.Figure 8 shows the XRD diffraction patterns of the MAX-phase Ti 2 AlC films after oxidation for 1, 3, 5, 10, 20, 30, and 50 h at 800 ∘ C, respectively.It was observed that no peaks corresponding to TiO 2 were detected before 20 h, which indicated that the dense Al 2 O 3 layer formed on the film surface performed good protection against oxidation diffusion before 20 h.And further increase of oxidation time to 50 h led to significant increase of intensity of the TiO 2 .Figure 9 shows the surface and cross-sectional SEM    morphologies and the line-scan EDX analysis of the films on Al 2 O 3 substrates after isothermal oxidation at 800 ∘ C at various times.For the sample after oxidation for 1 h, no oxide layer was observed.As the oxidation time increases to 30 h, obvious increase of the dense oxide layer could be seen, which was mainly composed of Al 2 O 3 .After the MAX-phase film oxidized for over 50 h, the surface of the film became much rougher and needle-like crystals appeared which represented the TiO 2 starting to form on the film surface, while the film below the oxide layer remained being well protected.This result is in good agreement with the XRD data in Figure 8.
Figure 10 shows the thickness increase of the oxide layer formed on film surface and the mass gain of the amorphous matrix Ti-Al-C and MAX-phase Ti 2 AlC films as a function of the oxidation time and oxidation temperature, respectively.As seen in Figure 10(a), the amorphous matrix Ti-Al-C films exhibited oxidation behavior following the parabolic oxidation law while the MAX-phase Ti 2 AlC films followed the logarithmic oxidation law due to the phase structure difference.The TGA test was carried out to investigate the oxidation temperature effect on the amorphous matrix Ti-Al-C films and MAX-phase Ti 2 AlC films and to seek their failure point (the temperature where the film lost protective function) against oxidation.As shown in Figure 10(b), a greater mass gain rate was observed for the Ti-Al-C amorphous matrix films, as compared with MAX-phase film, and it exhibited a significant increase at temperature at about 800 ∘ C, which meant the failure point for the amorphous matrix Ti-Al-C film was 800 ∘ C, while the MAX-phase Ti 2 AlC films exhibited a continuous increase of mass gain, with no obvious changing point that could be found.In this study, the thickness of the MAX-phase Ti 2 AlC film was only 2 m.The amount of the Al was limited.Correspondingly, in bulk-form MAX phase, the Al atoms are very sufficient, so the protection of the Al is much more significant than the film form [42][43][44].Thus, thicker MAX-phase film is preferred for oxidation resistance.
Because of the special nanolaminated microstructure of the MAX-phase Ti 2 AlC, the metallic-like Ti-Al bonds are relatively weaker than the covalent Ti-C bonds; the aluminum atoms have higher activity and higher migration rate than the titanium atoms.Therefore, at the initial stage of the oxidation, the aluminum atoms can migrate out to form a relatively high density Al 2 O 3 layer, which can prohibit the attack of oxygen atoms to the inward of the films.So it provided a more effective protection to the substrate.For the amorphous matrix Ti-Al-C film, the activity of aluminum atoms is lower.The oxidized surface of the film is a mixture of the Al 2 O 3 and TiO 2 which are much loose and porous; the attack of oxygen into the film proceeds with no effective obstacles, which means it cannot be a helpful protection against the attack of the oxygen compared with the pure and high density Al 2 O 3 layer.

Corrosion Behavior.
Figure 11 shows the polarization curves of the bare SUS304 substrate, MAX-phase Ti 2 AlC, and the amorphous matrix Ti-Al-C coated samples in 3.5 wt.% NaCl aqueous solution, respectively.The corrosion current density ( corr ), calculated by using Tafel equation [45], and corrosion potentials ( corr ) were listed in Table 3.  corr significantly decreased after by applying films, which revealed that the substrate was effectively protected from the corrosive media.Compared with the sample coated by amorphous matrix Ti-Al-C, the MAX-phase Ti 2 AlC coated sample exhibits lower  corr of 5.82 × 10 −8 A/cm 2 and higher  corr of −0.301 V, indicating better corrosion resistance and protective efficiency.and 12(f)), which indicated that it suffered much less corrosion attack and performed the best corrosion resistance during the corrosion process.It was considered that the high active Al atoms could readily migrate out to the surface of the film to form a high density Al 2 O 3 oxide layer, which passivated the surface and prevented further corrosion behavior during the chemical attack due to the special atomic arrangement of the MAX phase, in which Al-C bonds were relatively weaker.Thus, the MAX-phase Ti 2 AlC film provided a more effective protection compared with the amorphous matrix Ti-Al-C film.

Conclusion
In this study, nanolaminated MAX-phase Ti 2 AlC thin films were fabricated by high-power impulse magnetron sputtering (HiPIMS) from a Ti 2 AlC MAX-phase target.The amorphous matrix Ti-Al-C films were deposited at room temperature, while the MAX-phase Ti 2 AlC films were obtained through annealing process of the as-deposited amorphous films.The microstructure, oxidation resistance, and corrosion resistance of the amorphous matrix Ti-Al-C and MAX-phase Ti 2 AlC films were comparatively investigated.The MAXphase Ti 2 AlC films exhibited less mass gain rate and thinner thickness of the oxide layer compared with the amorphous matrix Ti-Al-C film.And the amorphous matrix Ti-Al-C films exhibited oxidation behavior following the parabolic oxidation law, while the MAX-phase Ti 2 AlC films followed the logarithmic oxidation law.Meanwhile the MAX-phase Ti 2 AlC films showed higher corrosion potential value ( corr ) and lower corrosion current density value ( corr ) compared with the amorphous films, which indicated better corrosion resistance.The mechanism for superior antioxidation and anticorrosion properties of MAX-phase films was related to a quick formation of stable and high density of Al 2 O 3 layer on the films due to fast outmigration of aluminum atoms, which acted as a good barrier layer for oxygen diffusion to prevent further oxidation.

2. 3 .
Film Performance.Isothermal oxidation tests for both amorphous matrix Ti-Al-C films and MAX-phase Ti 2 AlC films, which were deposited on polycrystalline Al 2 O 3 Journal of Nanomaterials 3

Figure 1 :
Figure 1: XRD patterns of the MAX-phase Ti 2 AlC target, the amorphous matrix film Ti-Al-C, and the MAX-phase Ti 2 AlC films on SUS304 substrate.
) and 2(b)) indicating that the structure of the Ti-Al-C film was a representative amorphous matrix phase, while a fine and submicro equiaxed crystal structure has been observed in the cross section SEM image of the MAX-phase Ti 2 AlC (Figures2(b) and 2(d)).In addition, Figures2(e) and 2(f) show the line-scan EDX analyses of the amorphous matrix Ti-Al-C film and MAX-phase Ti 2 AlC film, respectively.

Figure 2 :Figure 3 :
Figure 2: SEM images and line-scan EDX analyses of the (a, c, e) amorphous matrix Ti-Al-C films and (b, d, f) MAX-phase Ti 2 AlC films on the Al 2 O 3 substrate.

Figure 4 :Figure 5 :
Figure 4: XRD analyses of the films before and after isothermal oxidation at 800 ∘ C for 5 h: (a) amorphous matrix Ti-Al-C films and (b) MAX-phase Ti 2 AlC films on polycrystalline Al 2 O 3 substrate.

Figures 7 (
Figures 7(b)-7(d) indicates the even distributions of the Al and Ti atoms, suggesting a simultaneous oxidation of the Al and Ti (the formation of the Al-Ti-O compound) on the amorphous matrix Ti-Al-C film, while, in Figures 7(f)-7(h), there was a Al-rich region on the top of the film, indicating the preferred and rapid Al atoms migration of the MAX-phase Ti 2 AlC film.To further characterize the oxidation behavior of the MAX-phase Ti 2 AlC films, the time-dependent oxidation test

Figure 6 :
Figure 6: SEM images and line-scan EDX analyses of the films after isothermal oxidation: (a, b, c) amorphous matrix Ti-Al-C films at 800 ∘ C for 5 h and (d, e, f) MAX-phase Ti 2 AlC film at 800 ∘ C for 5 h on polycrystalline Al 2 O 3 substrate.

Figure 7 :
Figure 7: Microstructure and chemistry of the films after isothermal oxidation.(a) Low magnification TEM image of amorphous matrix Ti-Al-C films at 800 ∘ C for 5 h polycrystalline Al 2 O 3 substrate.The EDX mapping of the (b) Al, (c) Ti, and (d) O. (e) Low magnification TEM image of MAX-phase Ti 2 AlC film at 800 ∘ C for 5 h on polycrystalline Al 2 O 3 substrate.The EDX mapping of the (f) Al, (g) Ti, and (h) O.

Figure 8 :Figure 9 :
Figure 8: X-ray diffraction patterns of MAX-phase Ti 2 AlC films after oxidation at 800 ∘ C at various time on polycrystalline Al 2 O 3 substrate.

Figure 10 :
Figure 10: (a) The thickness of the oxide layers formed on amorphous matrix Ti-Al-C and MAX-phase Ti 2 AlC films after oxidation at 800 ∘ C as a function of oxidation time and (b) TGA analyses of the amorphous matrix Ti-Al-C and MAX-phase Ti 2 AlC films after the oxidation from RT to 1300 ∘ C at 15 K/min heating rate in the flowing Ar : N 2 = 35 : 15 atmosphere on polycrystalline Al 2 O 3 substrate.

Figure 11 :Figure 12 :
Figure 11: The potentiodynamic polarization curves of the amorphous matrix Ti-Al-C, MAX-phase Ti 2 AlC films, and the SUS304 substrate in the 3.5 wt.% NaCl aqueous solution.

Figure 12
shows the surface SEM images of the MAXphase Ti 2 AlC, the amorphous matrix Ti-Al-C deposited on the SUS304, and bare SUS304 substrate after the corrosion behavior in 3.5 wt.% NaCl aqueous solution.Obvious corrosive failure, with totally open pits over all the film, could be observed on the surface of bare SUS304 substrate and amorphous matrix Ti-Al-C deposited sample (Figures 12(a)-12(d)).However, no obvious pitting damage and pores were observed on the surface of the MAXphase Ti 2 AlC coated sample (Figures 12(e )

Table 1 :
The parameters of the HiPIMS and the deposition conditions of the films.  and   are the power of the peak and average target.(ii)   is the current of the average target in a pulse length.(iii)   is the voltage of the average target.(iv)   is the pulse current density of the peak target.

Table 2 :
The chemistry compositions of the MAX-phase Ti 2 AlC target and the films at various temperature.

Table 3 :
Corrosion analyses from potential dynamic polarization curves in the 3.5 wt.% NaCl aqueous solution.