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Article

Influence of Aluminum Alloy Substrate Temperature on Microstructure and Corrosion Resistance of Cr/Ti Bilayer Coatings

by
Yuqi Wang
1,
Tao He
1,*,
Xiangyang Du
1,
Alexey Vereschaka
2,
Catherine Sotova
3,
Yang Ding
1,
Kang Chen
1,
Jian Li
1 and
Peiyu He
1
1
School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201600, China
2
Institute of Design and Technological Informatics of the Russian Academy of Sciences, 127994 Moscow, Russia
3
Department of High-Efficiency Machining Technologies, Moscow State University of Technology “STANKIN”, 127055 Moscow, Russia
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(8), 891; https://doi.org/10.3390/coatings15080891 (registering DOI)
Submission received: 13 July 2025 / Revised: 28 July 2025 / Accepted: 29 July 2025 / Published: 1 August 2025
(This article belongs to the Special Issue Innovative Coatings for Corrosion Protection of Alloy Surfaces)
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Abstract

Cr/Ti bilayer coatings were deposited on 7050 aluminum alloy via magnetron sputtering at substrate temperatures of room temperature (RT), 150 °C, and 300 °C to investigate temperature effects on microstructure, hardness, and corrosion resistance. All coatings exhibited Cr(110) and Ti(002) phases. Temperature significantly modulated corrosion resistance by altering pore density, grain boundary density, and passivation film composition. Increasing temperature from RT to 150 °C raised corrosion rates primarily due to increased pore density. Further increasing to 300 °C reduced corrosion rates mainly through decreased grain boundary density, while passivation film composition changes altered electrochemical reaction kinetics. Substrate-coating interface defect density primarily influenced hardness with minimal effect on corrosion. Consequently, the RT-deposited coating, despite lower hardness, demonstrated optimal corrosion resistance: polarization resistance (7.17 × 104 Ω·cm2), charge transfer resistance (12,400 Ω·cm2), and corrosion current density (2.47 × 10−7 A/cm2), the latter being two orders of magnitude lower than the substrate.

1. Introduction

Aluminum (Al) and its alloys possess high specific strength and excellent mechanical properties, making them widely used in aerospace, marine, and automotive applications [1,2,3]. However, susceptibility to localized corrosion remains a persistent challenge for these alloys [4,5]. Research efforts have explored strengthening methods like plastic deformation and heat treatment to enhance corrosion resistance. Zhang Junjie et al. [6] prepared 7075 aluminum alloy parts with a fine-grained microstructure through preheating treatment combined with subsequent equal-channel angular extrusion and cold heading composite deformation, significantly improving corrosion resistance. Li Jian et al. [7] designed a novel processing strategy combining equal-channel angular extrusion with interlayer aging. The aluminum alloys processed using this method exhibit a more negative corrosion potential and lower corrosion current density, with the maximum intergranular corrosion depth reduced to 38 μm. Nevertheless, intrinsic strengthening approaches offer limited improvements, necessitating alternative methods.
Protective coatings represent an effective means to enhance the corrosion resistance of metal substrates [8,9,10,11], with numerous studies focused on aluminum alloys [12,13,14,15,16]. However, long-term exposure to harsh environments can lead to coating degradation or defects, potentially causing undesirable pitting corrosion [17,18,19]. Such random corrosion spots can compromise the coating’s integrity and protective function. Consequently, optimizing coating microstructure and macroscopic properties is crucial for improving aluminum substrate corrosion resistance [12].
Various surface deposition techniques, including physical vapor deposition (PVD), chemical vapor deposition, micro-arc oxidation, and spraying, have been developed [20,21,22,23]. Magnetron sputtering (MS), a PVD technique, enables high deposition rates for metal, alloy, and nitride coatings. It allows precise control over coating thickness, composition, and microstructure by regulating target power, deposition temperature, and gas pressure [24,25]. However, MS processes can introduce microdefects [26], which facilitate the ingress of corrosive media. Upon reaching the substrate, these media initiate redox reactions, leading to substrate corrosion. Therefore, mitigating MS defects and optimizing coating microstructure are vital for enhancing corrosion resistance.
The substrate temperature has a significant impact on the microstructure and macro properties of the coating. Liang et al. [27] prepared (TiVCrZrHf)N thin films at different substrate temperatures and observed that the nitrogen content decreased with increasing substrate temperature, while the grain size first decreased and then increased. V. Pershin et al. [28] sprayed nickel coatings on stainless steel and cobalt alloy specimens and found that substrate temperature affects coating adhesion strength, porosity, and surface oxide layer thickness. Heating the substrate removes surface moisture and other volatile contaminants, promoting diffusion between the coating and substrate. Lai et al. [29] prepared a multi-component (AlCrTaTiZr)N coating and observed that as the substrate temperature increased, the nitrogen concentration decreased, and the grain size and lattice parameters decreased. J. Khakzadian et al. [30] evaluated the effect of substrate temperature on the deposition of NiCrAl coatings on steel substrates, finding that as substrate temperature increased, coating porosity decreased from 4.4% to 0.6%, and coating crystallinity increased. The above studies demonstrate that substrate temperature influences macro-properties such as adhesion strength and porosity by regulating microstructural parameters including elemental composition, grain size, crystallinity, and phase structure of the coating. Therefore, identifying the optimal substrate temperature is crucial for enhancing the corrosion resistance of the coating.
Although substrate temperature effects have been extensively studied, research specifically focused on aluminum alloy substrates remains limited. Therefore, this study investigates Cr/Ti bilayer coatings deposited via MS onto 7050 aluminum alloy substrates at three temperatures (room temperature, 150 °C, 300 °C). The aim is to elucidate the role of substrate temperature on coating microstructure and its subsequent impact on hardness and corrosion resistance, providing a theoretical basis for further improving aluminum alloy corrosion resistance.

2. Materials and Methods

2.1. Coating Preparation

Cr/Ti double-layer coatings with Cr layer as the transition layer and Ti layer as the corrosion-resistant layer were prepared on monocrystalline silicon wafers (111) and 7050 aluminum alloy substrate using a DC magnetron sputtering process under different substrate temperature conditions. The size of the cast 7050 aluminum alloy disc was Φ28 mm × 3 mm, and the surface size of the monocrystalline silicon substrate was 10 mm × 10 mm.
The 7050 aluminum alloy substrates were cut by an EDM wire cutter(Suzhou Industrial Park Jiangnan Saite CNC Equipment Co., Ltd., Suzhou, China), then sequentially sanded with 400-, 800-, 1200-, 1500-, and 2000-grit silicon carbide sandpaper and polished to a mirror finish with a polishing machine. Prior to deposition, the polished substrates were continuously ultrasonically cleaned with acetone and ethanol for 10 min and dried to remove organic impurities from the surface, and then all substrates were placed on a rotating stage in a vacuum chamber containing high-purity titanium and chromium targets (99.99% purity). The background vacuum was pumped to 8.0 × 10−4 Pa, and high-purity argon (99.999%) was used as the sputtering gas, with the flow rate accurately controlled at 100 sccm. For the deposition of the coatings, the Cr transition layer was deposited first at a power of 130 W for 30 min, and then the Ti resist layer was deposited at a power of 250 W for 90 min. The working air pressure was maintained at 0.15 Pa during the deposition process, and the substrate temperatures were precisely controlled at room temperature (RT, 25 ± 2 °C), 150 °C, and 300 °C by resistive heating, and the coated samples were labeled as TRT, T150, and T300, respectively. The specific deposition parameters for the above processes are shown in Table 1.

2.2. Characterization and Performance Testing

The cross-sectional morphology of the coating was analyzed using a scanning electron microscope (SEM, CIQTEK-5000X, Hefei Quantum Photonic Technology Co., Ltd., Hefei, China), and the elemental composition was estimated using an energy dispersive spectrometer (EDS, Oxford Instruments, Abingdon, UK). Phase identification was performed using an X-ray diffractometer (XRD, D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15405 nm). The testing conditions were as follows: Cu Kα radiation (λ = 0.15405 nm), acceleration voltage of 40 kV, scanning speed of 4°/min, and a diffraction angle (2θ) range of 20°–90°. Phase calibration was performed using Jade 6.5 software. The binding energy of the coating under monochromatic Al Kα irradiation was measured using an X-ray photoelectron spectrometer (XPS, ESCALAB QXi+, Thermo Fisher Scientific, Prague, Czech Republic), with a transmission energy of 30 eV and an energy step of 0.05 eV.
Nanoindentation testing was performed on aluminum alloy substrates and coatings using a nanoindenter (Bruker Hysitron TS 77, Bruker Nano Surfaces Division, Minneapolis, MN, USA) with a displacement resolution of 0.2 nm, a loading resolution of 75 nN, and a hold time of 3 s. At least five indentations were performed on each sample to obtain the average hardness value. Electrochemical tests were carried out in 3.5 wt.% NaCl solution (25 ± 0.5 °C) using a three-electrode system consisting of a saturated calomel reference electrode (SCE), a platinum sheet counter electrode and a working electrode with an exposed area of 1 cm2. The open-circuit potential of the samples was monitored for 60 min to obtain a stable potential prior to the kinetic potential polarization test. Then, an EIS test was performed at a frequency of 0.01 Hz–100 KHz and the kinetic potential polarization test was performed at a scan rate of 0.01 mV/s over a potential range of −1000 mV (vs. SCE) to −300 mV (vs. SCE).

3. Results

3.1. XRD Analysis

The X-ray diffractograms of the coatings are shown in Figure 1. The phase compositions of the three groups of coatings are similar, with only minor differences in the intensity of the diffraction peaks for the different phases. The strongest peak at 38.7° corresponds to the (002) facet of the Ti phase, while the (101) and (004) facets of the Ti phase at 40.3° and 82.6° are also detected. The diffraction peaks at 34.9° and 44.8° correspond to the (331) crystal face of the CrTi3O phase and the (110) crystal face of the Cr phase, respectively. The internal stresses generated by the magnetron sputtering process lead to the collapse of the coating lattice and the reduction of the crystal plane spacing, which results in the shift of the Cr diffraction peaks to a higher angle [31]. The high intensity of the diffraction peaks at the Ti (002) crystal plane suggests that the Ti layer has a (002) selective orientation.
It is noteworthy that the intensity of the diffraction peaks of the CrTi3O phase is higher in the T150 coating than in the other two groups, and considering the overall thinness of the coating and its low oxygen content, this relatively significant change represents more oxygen penetration into the T150 coating samples close to the interface between the coated Cr layer and the Ti layer. This phenomenon suggests that the substrate temperature may affect the number of pores formed in the coating during the deposition process, and these pores may have an impact on the corrosion resistance of the coating.

3.2. Microstructural and Compositional Analysis

Figure 2 illustrates the cross-sectional morphology of the coatings, and light and dark columnar structures can be observed inside the coatings, which is determined by the process characteristics of magnetron sputtering [32]. As shown in Figure 2, the coating with a substrate temperature of room temperature has the most obvious microdefects at the coating–substrate interface, and cracks at the interface can be clearly seen, which is due to the fact that coatings deposited at lower substrate temperatures are prone to microdefects due to the large stresses formed by particle bombardment [33]. However, coatings with substrate temperatures of 150 °C and 300 °C have a continuous, smooth and flat morphology without visible defects at the coating–substrate interface, which indicates that the substrate temperature has a significant effect on the bonding at the coating–substrate interface, a phenomenon that is attributed to the fact that as the deposition temperature increases, the deposited atoms have enough thermal energy to diffuse, resulting in a more homogeneous coating [34]. The measured thicknesses of the TRT and T150 coatings are close to each other, about 390 nm, while the T300 coating thickness is about 420 nm, which represents a relatively insignificant increase in coating thickness with increasing substrate temperature between room temperature and 150 °C, and a relatively more pronounced increase in coating thickness between 150 °C and 300 °C. The cross-section of the T300 coatings shows a pronounced banding pattern, which indicates that the grain size of the coatings increases with increasing deposition temperature, which is attributed to the fact that at higher substrate temperatures, the energy required for the growth of the Cr and Ti layer coatings is sufficient, resulting in a more crystalline coating with larger grain size [34]. In this study, the morphology of the Ti corrosion resistant layer is discussed in detail because the thickness of the Cr layer is very small and the top layer is the surface in direct contact with the corrosive medium, which directly determines the protective effect of the coating on the substrate. It should be emphasized that the underlying Cr layer, as the key transition layer between the aluminum alloy substrate and the coating, primarily serves to improve the adhesion between the coating and the substrate, reduce interface defects (such as pores and cracks), and provide a stable foundation for the top Ti layer. The interface condition of the Cr layer is critical to the integrity of the entire coating system.
The elemental compositions, as shown in Table 2, ranged from 61 to 62 wt.% titanium and 32 to 35 wt.% chromium for all coatings, indicating successful deposition of the chromium and titanium layers. With the change of substrate temperature, the content of titanium remained basically unchanged, indicating that the substrate temperature had no significant effect on the content of Ti. Impurities are mainly elemental oxygen, as Ti has a high passivation tendency, as demonstrated in the XPS tests below.

3.3. XPS Analysis

The elemental chemical states of the three coating systems were characterized and analyzed using XPS. The peak splitting analysis of the Ti 2p orbitals strictly follows the constrained peak splitting algorithm proposed by the electronic state database of transition metals established by Biesinger’s team [31,35], which systematically characterizes the characteristic binding energies and peak widths of Ti elements. This analysis provides a good basis for the quantitative analysis of multivalent transition metal oxides. The analysis also provides a general framework for the quantitative analysis of multivalent transition metal oxides [36].
Figure 3 displays the Ti 2p XPS spectra for the three sample groups. The broad peak of the T300 coating indicates that titanium exists in different oxidation states, but TiO2 is still the main component. The oxidation states were resolved by peak deconvolution using a Lorentzian–Gaussian (L–G) mixed function, following the constraints established by Biesinger et al. [35]. A positive binding energy shift in the Ti 2p peaks was detected for all samples, confirming the ubiquitous formation of Ti-O bonds [36]. Although the deposition process was conducted within an inert atmosphere, the detected Ti-O bonding signal may originate either from the co-deposition of the native oxide layer present on the target material surface or from a self-passivation layer formed upon post-deposition atmospheric exposure. Among them, the characteristic bimodal peaks of TiO2 dominate, confirming that the Ti layer forms a passivation film with TiO2 as the main component. It is worth noting that the Ti 2p spectrum of the T300 coating shows that its spin-orbit splitting has decreased from 5.7 eV to 5.4 eV, reflecting an increase in the proportion of low-valent titanium [37]. The increase in low-valent titanium, combined with the increase in substrate temperature to 300°C, indicates that in addition to TiO2, partially reduced titanium oxides such as Ti2O3 (Ti3+) or TiO (Ti2+) may have formed in the T300 coating. These low-valent oxides are generally less stable than TiO2 and may offer relatively poor protection. It has been shown that TiO2 films are chemically resistant, which are necessary requirements for composites and coatings [38], and TiO2-based films can be considered as coating materials for structural materials exposed to aggressive media [39], so that the TiO2 passivation films detected at all three substrate temperature conditions are beneficial for the corrosion resistance of Cr/Ti coatings.

3.4. Hardness Testing

Aluminum alloys in service in marine environments need to be considered for hardness in addition to corrosion, and the hardness and modulus of elasticity of the three sets of coatings are shown in Figure 4. The hardness of the coatings is affected by a combination of grain size, lattice structure, and coating thickness [40,41]. Due to the low hardness and elastic modulus of the Ti layer itself and the release of part of the residual stress during deposition, which further leads to a decrease in hardness and elastic modulus [42,43], the hardness of the TRT coating is relatively low. However, as the substrate temperature increases, the hardness of the coating shows a significant upward trend. The results of nanoindentation tests showed that the hardness of the TRT coatings was 1.95 GPa, the hardness of the T150 coatings was 2.25 GPa, and the hardness of the T300 coatings reached 2.90 GPa. The low hardness values of the TRT coatings showed a significant correlation with the high number of microdefects at the coatings–substrate interface.

3.5. Electrochemical Testing

Kinetic potential polarization analysis was performed to elucidate the corrosion mechanism and kinetics of the three sets of samples. Figure 5 illustrates the kinetic potential polarization curves of the coatings. The electrochemical parameters calculated from Figure 5 using Tafel extrapolation are listed in Table 3. Where Ecorr is the corrosion potential, Icorr is the corrosion current density, βa and βc are the anodic and cathodic slopes, respectively, while Rp is the polarization resistance, which can be calculated from the simplified Stern–Geary formula (Equation (1)) [44] as
R p = β a × β c 2.3 ( β a + β c ) I c o r r
In general, the thermodynamic parameter Ecorr shifted in the positive direction indicates that the samples have better resistance to the intrusion of corrosive media, while Icorr shifted in the negative direction indicates that the samples have lower corrosion rates [45,46]. Among all the samples, the T150 coating has the lowest Ecorr and the highest Icorr, indicating the highest tendency to corrode and the worst corrosion resistance, which suggests that increasing the substrate temperature up to 150 °C decreases the corrosion resistance of magnetron sputtered Cr/Ti coatings. However, when the substrate temperature is increased from 150 °C to 300 °C, the Ecorr of the T300 coating increases and the Icorr decreases. This indicates that increasing the substrate temperature from 150 °C to 300 °C improves the corrosion resistance of magnetron sputtered Cr/Ti coatings. Among them, the T300 coating has the largest Ecorr −0.922 V) and the TRT coating has the smallest Icorr (2.47 × 10−7 A·cm−2), indicating the better corrosion resistance of the TRT and T300 coatings. The Icorr of 7050 aluminum alloy in 3.5 wt.% NaCl solution was of the order of 10−5 A·cm−2 [47], and the Icorr of the TRT coating and T300 coating was reduced by two orders of magnitude compared to the substrate. The Icorr values of the three sets of coated samples show that the Cr/Ti bilayer coatings prepared by magnetron sputtering process can effectively improve the corrosion resistance of aluminum alloy substrates. The corrosion resistance of the composite coatings was further evaluated using Rp, which is related to the kinetics of the electrochemical reaction, which is inversely proportional to the corrosion rate [48,49]. The Rp value of TRT coating was 7.17 × 104 Ω·cm2, which was significantly higher than that of T150 coating (1.06 × 104 Ω·cm2) and T300 coating (2.41 × 104 Ω·cm2). The results showed that the combined performance of the T300 coating and TRT coating was better than that of the T150 coating for the three parameter values of Icorr, Ecorr and Rp.
In order to evaluate the corrosion resistance of the Cr/Ti coatings at three substrate temperatures, electrochemical impedance spectroscopy (EIS) measurements were carried out in 3.5 wt.% NaCl solution. The EIS curves of the different samples were fitted and analyzed using an equivalent circuit model to examine and evaluate the structural and electrochemical properties of the samples, and the relevant EIS curve fitting data are recorded in Table 4. The corresponding Nyquist, Bode, and phase angle diagrams for the three groups of coatings are shown in Figure 6. Generally, the impedance modulus at low-frequency 0.01 Hz (|Z|0.01Hz) is an important parameter to reveal the corrosion protection performance of the coatings [50], and coatings with higher values of |Z|0.01Hz are usually considered to have better protection performance [51]. From the fitted curves of the Bode diagrams (Figure 6b), it can be seen that the |Z|0.01Hz values of the Cr/Ti coatings firstly increase and then decrease with the increase in the substrate temperature, and the |Z|0.01Hz value of the T150 coatings reaches 7.87 × 103 Ω·cm2, whereas that of the T300 coatings has the smallest |Z|0.01Hz value. The trend of the |Z|0.01Hz values is different from the corrosion resistance of the coatings analyzed by the kinetic potential polarization test, and this difference reveals the complex corrosion mechanism of the coatings. For the Cr/Ti bilayer coating, the radius of the capacitive loop slightly decreased as the substrate temperature increased from room temperature to 150 °C, while a notably more pronounced reduction in the capacitive loop radius occurred upon further elevation of the substrate temperature to 300 °C. This indicates the emergence of a predominant factor within the 150 °C to 300 °C range that significantly compromised the corrosion resistance of the coating (Figure 6a).
As illustrated in Figure 6d, an equivalent circuit model [52,53] was employed to extract EIS parameters, where Rs denotes solution resistance; Rpore and CPEc represent the pore resistance and constant-phase element (CPE) of the intact coating, respectively; Rct and CPEdl correspond to charge transfer resistance and the non-ideal double-layer CPE, respectively; and n is the CPE exponent (n = 1 indicates ideal capacitive behavior [54]). Phase angle analysis confirmed conformity of all three coatings to this model, with fitted parameters for intact coatings compiled in Table 4. Significantly higher Rct values observed for TRT and T150 coatings versus T300 indicate greater charge transfer resistance, slower reaction kinetics, and consequently superior corrosion resistance [55].

4. Discussion

Both PDP and EIS tests showed that the TRT coating had the best corrosion resistance. However, the T150 and T300 coatings had different results in the two tests. This is because the two tests focus on different things, and the main factors affecting corrosion resistance vary across different temperature ranges. This discrepancy is attributed to the complex behavior of the corrosion system under different perturbation intensities, timescales, and measurement conditions [56,57]. The Icorr from polarization tests primarily reflects the total, macroscopic dissolution rate, whereas the Rct from EIS mainly characterizes the resistance to interfacial charge transfer. If the corrosion process is diffusion-controlled (e.g., oxygen diffusion), charge transfer becomes difficult, resulting in a potentially large Rct, but the overall corrosion rate may not be low due to diffusion limitations [58,59]. The diffusion of oxygen may originate from pores present within the coating. SEM images reveal a distinct columnar crystal structure in the coating, which is known to contain a high number of pores. However, due to the limited resolution of SEM images, it is not possible to quantitatively analyze the pore density within the coating. Nevertheless, XRD analysis indicates that the T150 coating contains a relatively higher proportion of CrTi3O phases, and the T150 coating exhibits a higher corrosion rate. This suggests that the T150 coating likely possesses a greater number of pores serving as oxygen diffusion pathways, which contain a higher concentration of Cl− corrosion sites [60,61], thereby resulting in a higher corrosion rate and larger Icorr. The established explanation for pore formation during PVD coating deposition is that following the initial nucleation stage, the coating grows as isolated islands. As the number of these islands increases, they coalesce and grow together, leaving voids between them. Although various techniques can minimize void density, complete elimination is generally unattainable, and voids are universally present in all types of coatings on all types of substrates [34]. Substrate temperature may influence the energy required for island coalescence and growth. In potentiodynamic polarization testing, the T300 coating exhibited a Icorr of 7.59 × 10−7 A·cm−2 and the most noble corrosion potential. This is attributed to its larger grain size and higher crystallinity. Existing research indicates that grain boundaries act as significant pathways for ionic corrosion; increased grain size reduces grain boundary density, thereby enhancing corrosion resistance [62]. This explains the lower corrosion rate of the T300 coating compared to the T150 coating. In addition, atoms deposited at high temperatures have sufficient thermal energy to diffuse, thereby promoting a more uniform interface between the coating and the substrate and reducing interface defects (such as pores and voids), which has a positive effect on improving corrosion resistance.
These defects can act as initiation points for corrosion; once exposed to a corrosive environment, they trigger and locally accelerate the corrosion process [63]. However, in this study, both the T150 and T300 coatings showed poorer corrosion resistance than the TRT coatings, despite the fact that the TRT coatings had the most pronounced coating–substrate defects, suggesting that interfacial defects are not a determining factor for the corrosion resistance of Cr/Ti coatings. In addition, the higher the substrate temperature, the higher the diffusion coefficient of the deposited particles. The rapid diffusion of reactive ions accelerates the coating deposition and thus increases the coating thickness [64]. The results of this experiment show that the thickness of the Cr/Ti coatings deposited by magnetron sputtering increases nonlinearly with increasing substrate temperature, and grows more rapidly under high temperature conditions. Studies have shown that coating thickness has a significant impact on corrosion resistance [65,66]. However, in this study, the TRT coating and T150 coating had similar thicknesses but exhibited significant differences in corrosion resistance, while the T300 coating, despite being thicker, did not show improved corrosion resistance compared to the TRT coating, indicating that corrosion resistance is almost unrelated to coating thickness. This is because the coating thickness in this study varied only within a narrow range of 390 nm (TRT, T150) to 420 nm (T300) (an increase of approximately 7.7%). For such submicron-level coating thicknesses, a slight increase in coating thickness has limited impact on increasing the effective path length for blocking the penetration of corrosive media (such as Cl− and H2O). The effect may be far smaller than that caused by changes in coating microstructure (such as pore density, grain boundary characteristics, and passivation film performance) or interface state. Therefore, it is necessary to investigate the key factors influencing corrosion resistance. Furthermore, the XPS results show that the titanium layers deposited at all substrate temperatures form passivation films composed mainly of TiO2. However, the T300 coatings show a tendency for the conversion of Ti4+ to a lower valence state. the formation of TiO2 passivation film has a positive effect on the corrosion resistance of the coatings, and the conversion of TiO2 to a lower valence metal oxide at high temperatures may be directly responsible for the decrease in the interfacial charge transfer resistance.
It should be noted that the microstructural inhomogeneity of the 7050-aluminum alloy substrate [67] affects its thermal properties. The thermal properties of the substrate influence deposition behavior [68], which in turn affects the coating’s microstructure; this aspect warrants further investigation. Interestingly, higher substrate temperature may improve the surface property uniformity of the aluminum alloy substrate and enhance the activity of surface layer particles. On the other hand, higher substrate temperature places both the coating and substrate in a heated state. Due to the different coefficients of thermal expansion of the coating and the substrate, thermal stresses are generated inside the coated specimen [64], which may negatively affect the corrosion resistance of the coating. Experiments have shown that TRT coatings have the best corrosion resistance, but their hardness values are reduced due to the presence of a large number of microdefects near the coating–substrate interface. It is important to note that the coatings prepared in this study are relatively thin (approximately 390–420 nm). When interpreting mechanical test results such as nanoindentation, this limitation must be taken into account. For coatings of such submicron thickness, the indentation depth in nanoindentation testing must be strictly controlled to be less than 10% of the coating thickness to avoid significant influence of substrate effects on measurement results (such as hardness and elastic modulus). Although efforts were made to adhere to this principle during testing, the intrinsic mechanical properties of the thin coating may still be subject to minor constraints or influences from the underlying substrate, particularly in the presence of interface defects or residual stresses. Therefore, the observed lower hardness of the TRT coating, although related to interface microdefects, may have its absolute value partially influenced by thin-film effects and substrate effects. It is worth noting that the trends in coating hardness and corrosion resistance affected by substrate temperature are different. This phenomenon essentially stems from their varying sensitivities to the types and locations of microstructural defects. Although this phenomenon appears contradictory, it actually reflects differences in the mechanisms by which different performance indicators of coatings are affected by substrate temperature.
Based on the results of this study, future research should focus on the following areas: (1) combining high-resolution TEM and atomic probe tomography techniques to investigate the atomic-level structure, elemental diffusion behavior, and their impact on performance and stability at the Cr/Ti interface and the coating/aluminum alloy substrate interface; (2) conduct a systematic study of the influence patterns and dominant mechanisms of coating thickness variations within the submicron to nanoscale range (e.g., <500 nm) on corrosion resistance and mechanical behavior; (3) explore methods such as composition gradient design, multi-layer structure construction, or post-processing techniques to enhance the interface bonding strength and hardness of TRT coatings while maintaining their excellent corrosion resistance, thereby achieving synergistic optimization of protective performance and mechanical properties.

5. Conclusions

This study investigated the impact of substrate temperature on the resulting structure and properties of PVD coatings. The following conclusions were drawn from this investigation:
(1)
Stability of phase structure: In the range of substrate temperature from room temperature to 300 °C, the Cr/Ti bilayer coatings deposited by magnetron sputtering all have Cr (110) and Ti (002) as the main phases.
(2)
Passivation film behavior: The titanium layer always forms a passivation film composed mainly of TiO2, which is the basis for the coating’s excellent corrosion resistance. The partial reduction reaction of TiO2 at 300 °C is correlated with the decrease in the electrochemical impedance value of the T300 coating.
(3)
The regulatory mechanism of substrate temperature on corrosion resistance: Within the temperature range of room temperature to 150 °C, the increase in pore density is the primary cause of the elevated corrosion rate; within the temperature range of 150 °C to 300 °C, the decrease in grain boundary density becomes the primary cause of the reduced corrosion rate. However, changing the passivation film composition from TiO2 to Ti2O3 or TiO reduces the electrochemical impedance.
(4)
There is a positive correlation between hardness values and substrate temperature: the higher the substrate temperature, the lower the stress level at the interface, the lower the defect density, and the higher the hardness value of the coating.
(5)
Coating properties: The TRT coating had a polarization resistance of 7.17 × 104 Ω·cm2, a charge transfer resistance of 12,400 Ω·cm2, and a corrosion current density of 2.47 × 10−7 A·cm−2. The corrosion current density is two orders of magnitude lower than that of the 7050-aluminum alloy substrate, demonstrating excellent corrosion resistance.

Author Contributions

Conceptualization, data curation, writing—original draft Y.W. and T.H.; methodology, X.D. and K.C.; methodology, resources, writing—review and editing, A.V., C.S. and J.L.; investigation, Y.D. and P.H.; supervision, T.H.; project administration, T.H.; funding acquisition, T.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (Grant No. 52275350), International Cooperation Research Platform Construction Project of Shanghai University of Engineering Science (Grant No. 0301006).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 00891 g001
Figure 1. XRD patterns of different coatings.
Figure 1. XRD patterns of different coatings.
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Figure 2. Cross-sectional morphology and EDS line sweeps of different coatings: (a) TRT; (b) T150; (c) T300.
Figure 2. Cross-sectional morphology and EDS line sweeps of different coatings: (a) TRT; (b) T150; (c) T300.
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Figure 3. Ti 2p profiles of different coatings: (a) TRT, (b) T150, (c) T300.
Figure 3. Ti 2p profiles of different coatings: (a) TRT, (b) T150, (c) T300.
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Figure 4. Hardness and modulus of elasticity of different coatings.
Figure 4. Hardness and modulus of elasticity of different coatings.
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Figure 5. Kinetic potential polarization curve data for different coatings.
Figure 5. Kinetic potential polarization curve data for different coatings.
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Figure 6. EIS test results for different coatings: (a) Nyquist plot (b) Bode plot; (c) phase angle plot; (d) equivalent circuit model.
Figure 6. EIS test results for different coatings: (a) Nyquist plot (b) Bode plot; (c) phase angle plot; (d) equivalent circuit model.
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Table 1. Sputtering conditions used to deposit Cr/Ti coatings.
Table 1. Sputtering conditions used to deposit Cr/Ti coatings.
Deposition Parameters
Base pressure (Pa)8.0 × 10−4
Etching time (min)10
Deposition temperature (°C)RT, 150 °C, 300 °C
Ar flow (sccm)100
Working pressure (Pa)0.15
Rotary Table Rotation Speed (rpm)10
Cr target power (W)130
Cr sputtering time (min)30
Ti target power (W)250
Ti sputtering time (min)90
Table 2. Results of cross-sectional EDS line scans.
Table 2. Results of cross-sectional EDS line scans.
SamplesCrTiImpurities
TRT32.561.75.8
T15035.061.13.9
T30033.361.65.1
Table 3. Fitting data of PDP curves of different samples in 3.5 % NaCl solution.
Table 3. Fitting data of PDP curves of different samples in 3.5 % NaCl solution.
SamplesEcorr (V vs. SCE)Icorr (A·cm−2)βa (mV·dec−1)−βc (mV·dec−1)Rp (Ω·cm2)
TRT−0.9312.47 × 10−7220507.17 × 104
T150−0.9511.44 × 10−6296401.06 × 104
T300−0.9227.59 × 10−7192542.41 × 104
Table 4. EIS parameters of TRT, T150, and T300 coated samples.
Table 4. EIS parameters of TRT, T150, and T300 coated samples.
SamplesRs
/(Ω·cm2)		CPEc
/(F·cm−2)		ncRpore
/(Ω·cm2)		CPEdl
/(F·cm−2)		ndlRct
/(Ω·cm2)
TRT69.89.86 × 10−40.89720203.37 × 10−40.85212,400
T15067.18.18 × 10−40.8926104.56 × 10−40.89611,900
T30069.37.92 × 10−40.89624904.83 × 10−40.8069140
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Wang, Y.; He, T.; Du, X.; Vereschaka, A.; Sotova, C.; Ding, Y.; Chen, K.; Li, J.; He, P. Influence of Aluminum Alloy Substrate Temperature on Microstructure and Corrosion Resistance of Cr/Ti Bilayer Coatings. Coatings 2025, 15, 891. https://doi.org/10.3390/coatings15080891

AMA Style

Wang Y, He T, Du X, Vereschaka A, Sotova C, Ding Y, Chen K, Li J, He P. Influence of Aluminum Alloy Substrate Temperature on Microstructure and Corrosion Resistance of Cr/Ti Bilayer Coatings. Coatings. 2025; 15(8):891. https://doi.org/10.3390/coatings15080891

Chicago/Turabian Style

Wang, Yuqi, Tao He, Xiangyang Du, Alexey Vereschaka, Catherine Sotova, Yang Ding, Kang Chen, Jian Li, and Peiyu He. 2025. "Influence of Aluminum Alloy Substrate Temperature on Microstructure and Corrosion Resistance of Cr/Ti Bilayer Coatings" Coatings 15, no. 8: 891. https://doi.org/10.3390/coatings15080891

APA Style

Wang, Y., He, T., Du, X., Vereschaka, A., Sotova, C., Ding, Y., Chen, K., Li, J., & He, P. (2025). Influence of Aluminum Alloy Substrate Temperature on Microstructure and Corrosion Resistance of Cr/Ti Bilayer Coatings. Coatings, 15(8), 891. https://doi.org/10.3390/coatings15080891

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