Aging-Induced Microstructural Transformations and Performance Enhancement of Cr/DLC Coatings on ECAP-7075 Aluminum Alloy

Abstract

This study systematically investigates the effects of aging treatment (AT) on the microstructure and properties of Cr/DLC coatings deposited via cathodic arc ion plating onto the surface of ECAP-7075 aluminum alloy. Utilizing a comprehensive approach combining performance tests (nanoindentation, nanoscratch testing, dynamic polarization analysis) with characterization tests (scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy), the synergistic effects of equal channel angular pressing (ECAP) and aging treatment(AT) were elucidated. The results demonstrate that the combined ECAP and AT significantly enhance the coating’s performance. Specifically, AT promotes the precipitation of η’ phase within the 7075 aluminum alloy substrate, increases the size of Cr₇C₃ crystallites in the Cr-based interlayer, improves the crystallinity of the Cr₇C₃ phase on the (060) or (242) crystal planes, and elevates the sp³-C/sp²-C ratio in the diamond-like carbon(DLC) top layer, leading to partial healing of defects and a denser overall coating structure. These microstructural transformations, induced by AT, result in substantial improvements in the mechanical properties (hardness reaching 5.2 GPa, bond strength achieving 15.1 N) and corrosion resistance (corrosion potential increasing to -0.698 V) of the Cr/DLC-coated ECAP-7075 aluminum alloy. This enhanced combination of properties makes these coatings particularly well-suited for high-performance aerospace components requiring both wear resistance and corrosion protection in demanding environments.

1. Introduction

Aluminum alloys, with their excellent strength-to-density ratio and elastic modulus-to-density ratio, are widely used in the manufacture of lightweight, high-rigidity automotive and aerospace components [1]. However, their inherent lack of hardness can lead to severe wear under extreme conditions (such as low temperatures and lack of lubrication) [2], and their insufficient corrosion resistance has long been a key issue limiting their widespread application [3,4]. Therefore, enhancing the mechanical and corrosion properties of aluminum alloys in complex environments is crucial for extending their service life [5,6]. Applying protective coatings is an effective means of improving the surface hardness and corrosion resistance of aluminum alloys [7].

Diamond-like carbon (DLC) coatings, as a series of metastable amorphous carbon materials with unique sp²/sp³ bond hybridization [8], not only exhibit high wear resistance and chemical stability [9], but also form an effective barrier to prevent corrosive media from directly contacting the substrate, demonstrating excellent resistance to acidic and alkaline environments [10,11,12]. Physical vapor deposition (PVD) technology can precisely control the thickness and properties of DLC coatings [13], making it a good choice for depositing DLC coatings. However, the application of DLC coatings on metal substrates is often limited by their insufficient bonding strength with the substrate [14]. Low adhesion is mainly caused by differences in the physical/mechanical properties between the coating and the substrate. Additionally, bond angle deviations caused by sp²/sp³ hybridization lead to high internal stress. Aluminum alloy substrates have lower hardness and elastic modulus compared to other metal substrates, and are significantly lower than DLC coatings. The stress distribution generated by DLC coatings can significantly transfer into the softer aluminum alloy substrate. Therefore, it is essential to carefully match the mechanical properties of the coating and substrate to avoid excessive stress concentration at the coating itself or the interface, which could lead to cracking or delamination [15,16].

Doping with heteroatoms [17,18,19,20] or constructing multilayer structures [21] can enhance toughness, reduce internal stress, improve adhesion, and enhance thermal stability. Heteroatoms can be incorporated into the carbon network in the form of carbides or elements, altering the atomic bonding structure and hybridization state [22]. Viswanathan et al.’s research report [23] indicates that incorporating Cr into DLC coatings can enhance corrosion resistance, adhesion to substrate, and tribological properties. Additionally, DLC coatings with a Cr as an interlayer [24,25] improve the bond strength between the DLC coating and the substrate, preventing cracking or delamination under wear and corrosion conditions. Based on the above research findings, this study developed a Cr/DLC bilayer coating. The use of Cr as a transition layer solves the problem of low bonding strength caused by mismatched mechanical properties between the DLC coating and the aluminum substrate. Furthermore, the diffusion of Cr into the DLC coating optimizes its internal structure, thereby improving both mechanical properties and corrosion resistance.

Equal channel angular pressing (ECAP) is currently one of the most mature severe plastic deformation processes, capable of generating ultra-fine grains in aluminium alloys [26], reducing the mechanical property differences between aluminium alloys and hard coatings, and enhancing bond strength. Systematic research has been conducted on optimising aluminium alloy performance through ECAP. Zhang et al. [27] enhanced the corrosion resistance of 7075 aluminium alloy components by combining preheating treatment with ECAP and cold heading (CU) composite deformation; Xu et al. [28] improved the hardness uniformity of 6061 aluminium alloy through multi-pass ECAP processing at room temperature; Li et al. [29] proposed a new strategy combining ECAP strengthening with aging treatment (AT) to optimise microstructure and balance strength and corrosion resistance. The combination of ECAP and AT not only enhances the inherent properties of aluminium alloys but also plays a crucial role in determining the bonding mechanism between Cr layer particles and the substrate [30].

AT significantly influences the mechanical and corrosion properties of materials by regulating microstructure and composition [31,32,33]. Previous studies have shown that different AT schemes significantly affect the grain size of the matrix, dislocation density, and precipitation phase kinetics. Zhang et al. [34] found that the calculated dislocation density of 7075 aluminium alloy treated with ECAP and CU reached a maximum value of 2.6 × 10¹⁵ m⁻². The dislocation density then gradually decreased with increasing aging temperature and time. Microstructural defects introduced by ECAP-CU composite deformation accelerate precipitate nucleation and growth. Within certain limits, the precipitate size and volume fraction increase with increasing post-aging time and temperature. Jia et al. [35] found that, after ECAP processing of the aluminum alloy, precipitation of the η phase is suppressed, and the apparent activation energy of the η′ phase decreases with dislocation accumulation, thereby shortening the peak-aging time. During aging, the concurrent annihilation of dislocations and phase precipitation leads to a competition between softening and strengthening in the mechanical response. Investigating the coupled effects of AT on the microstructure and overall performance of both the aluminum substrate and its coating represents a highly significant research direction. The AT scheme adopted in this study involves holding the coating at 120 °C for 12 h after deposition. This is because 120 °C falls within the optimal precipitation temperature range for the η’ strengthening phase (MgZn₂ metastable phase) of 7-series aluminium alloys. At this temperature, the nucleation density of the η’ phase is high, and the coarsening rate is low, thereby enhancing the strengthening effect of the precipitated phase. Additionally, since the dislocations and subgrain boundaries generated by ECAP provide rapid diffusion pathways, enhancing the migration rate of solute atoms, the aging time was selected as half of the peak aging time [29]. Based on this, two processes were developed: ECAP + coating (EC) and ECAP + coating + aging (ECA). These two processes correspond to two sets of samples: EC samples and ECA samples. Performance tests were conducted on both sets of samples to verify whether the combination of ECAP and AT could enhance the mechanical properties and corrosion resistance of the Cr/DLC coating. Microstructural characterization was performed on both sets of samples to analyze the effect of aging on the microstructure of the Cr/DLC coating on ECAP-7075 aluminum alloy and to investigate the fundamental reasons behind the superior performance of the ECA samples, providing theoretical guidance for the deposition process of hard coatings on aluminum alloys.

2. Materials and Methods

2.1. Matrix Treatment and Coating Deposition

The initial material was cast 7075 aluminum alloy. Both samples were subjected to a two-stage homogenization treatment, specifically: they were held at 465 °C for 24 h in a high-vacuum heating furnace (SGL-1700C, Shanghai Ju Jing Precision Instrument Manufacturing Co., Ltd., Shanghai, China), followed by holding at 475 °C for 4 h. After the holding period, the aluminum alloy was rapidly immersed in cold water. This process can be considered as solution treatment.

Both samples must undergo ECAP. ECAP processing was performed on a four-column vertical hydraulic press (Y32 series, Nantong Metalforming Equipment Co., Ltd., Nantong, China). The extrusion die had a channel diameter of Φ10 mm, with inner and outer angles of 120° and 30°, respectively. The extrusion speed during the process was 3 mm/s, and MoS₂ was used as a lubricant to reduce the impact of friction on alloy forming and improve processing efficiency.

To study the effect of aging treatment on the microstructure of the coating, it is necessary to ensure that the thickness and other parameters of the two sets of coatings are identical. The deposition processes for the two sets of coatings are identical. The substrate was a polished 7075 aluminum alloy with dimensions of 10 mm × 5 mm × 5 mm. Prior to deposition, the substrate was ultrasonically cleaned in anhydrous ethanol for 30 min. It was then dried with nitrogen and placed in a vacuum physical vapor deposition (PVD) equipment (GSV-10L4, Shenyang Kejing Automation Equipment Co., Ltd., Shenyang, China). The vacuum chamber temperature was maintained at 120 °C, and the pressure was reduced to 3 × 10⁻³ Pa. Under a bias voltage of −100 V, the substrate surface was bombarded with argon ions for 30 min to remove surface impurities. Finally, under a bias voltage of −300 V, a transition layer of Cr and a wear-resistant layer of DLC were deposited for 30 min each.

After the coating deposition is complete, the ECA samples must undergo AT. The ECA samples were placed in a high-vacuum heating furnace (SGL-1700C, Shanghai Ju Jing Precision Instrument Manufacturing Co., Ltd., Shanghai, China) and maintained at 120 °C for 12 h for AT. The temperature fluctuation range of the high-vacuum heating furnace is ±1 °C, the heating rate of the sample is 10 °C/min, and the initial heating temperature is 20 °C.

2.2. Performance Testing

Hardness and elastic modulus tests of the coating were conducted using a microhardness tester (MHVD-1000IS, Shanghai Yanrun Optical Technology Co., Ltd., Shanghai, China). During testing, a load of 300 gf was applied, with a holding time of 15 s. To ensure the accuracy of the measurement data, all hardness test specimens were ground and polished prior to testing to minimize the influence of residual stress and surface roughness on the hardness values.

Electrochemical polarization tests were conducted on the samples using an electrochemical workstation (CHI600F, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). A conventional three-electrode cell was employed, with a platinum plate as the counter electrode, Ag/AgCl as the reference electrode, and the alloy sample as the working electrode. A 3.5 wt% NaCl solution was used as the electrolyte to investigate the electrochemical corrosion behavior of the alloy. Potential polarization tests were conducted using a saturated calomel electrode as the reference, with a scanning rate of 0.001 V/s between −1.0 V and −0.5 V, and corrosion parameters were obtained using the Tafel extrapolation method.

The adhesion strength between the Cr/DLC coating and the ECAP-7075 aluminum alloy substrate was tested using a nanoindentometer (RST 300, CSM Instruments, Peseux, Switzerland). During the test, a diamond indenter was subjected to a linearly increasing load and scratched across the coating surface until the coating failed by delamination. The test conditions were as follows: a linearly increasing load ranging from 0.02 N to 30 N, a scratch speed of 6 mm/min, and a scratch length of 5–6 mm.

2.3. Microstructural Characterization

The surface and cross-sectional morphology of the coating were analyzed using a scanning electron microscope (SEM, EVO 18, Carl Zeiss Microscopy GmbH, Oberkochen, Germany), and the elemental composition was estimated using an energy dispersive spectrometer (EDS, X-MaxN 20, Oxford Instruments NanoAnalysis, High Wycombe, 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°. Additionally, the binding energy of the coating under monochromatic Al (Kα = 1486.6 eV) irradiation was measured using an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, East Grinstead, UK), with an energy resolution of 0.45 eV.

3. Results

3.1. Hardness

Figure 1 shows the hardness of Cr/DLC coatings prepared using two different processes. In nanoindentation testing, to reduce the impact of single-point measurement deviation, three positions are randomly selected for measurement on each sample, and the average value is used as the final hardness value. The hardness of the ECA sample (5.24 ± 0.08 GPa) is higher than that of the EC sample (4.69 ± 0.17 GPa), indicating that the AT process can further improve the hardness of ECAP alloys. The strengthening mechanism of the combination of AT and ECAP may be attributed to the combined effects of precipitation hardening and residual compressive stress [36].

3.2. Adhesion Analysis

Figure 2 shows the optical morphology of Cr/DLC coatings under different processes. The curve represents the variation in friction signals with critical load during scratch testing. As the critical load increases, the Cr/DLC coating on the aluminum alloy substrate begins to crack under the pressure of the indenter, resulting in enhanced fluctuations in the friction signal. This corresponds to the low critical load Lc1 at the onset of cracking; As the load continues to increase, the Cr/DLC coating is completely destroyed, and the friction signal exhibits an inflection point. At this point, the coating completely peels off until the substrate is exposed, with the white area in the scratch indicating complete separation between the Cr/DLC coating and the substrate. This phenomenon is a typical characteristic of adhesion failure of DLC coatings on softer substrates, corresponding to the high critical load Lc2 when the substrate and coating separate. Lc2 is typically used as an indicator of the bonding strength of the Cr/DLC coating. Analysis shows that the Lc1 values for samples C, EC, and ECA are 2.4 N, 3.6 N, and 4.1 N, respectively, while the Lc2 values are 11.4 N, 12.4 N, and 15.1 N, respectively. The measurement results indicate that ECAP can enhance the strength and load-bearing capacity of the substrate, mitigate the mismatch in mechanical properties between the coating and the aluminum substrate, and facilitate the bonding between the hard coating and the soft substrate. Subsequent AT can further improve the bonding strength.

3.3. Dynamic Polarization Analysis

Figure 3 shows the dynamic polarization curves of Cr/DLC coatings under different processes to elucidate the corrosion mechanisms and kinetics of the two sets of samples. Prior to polarization testing, an open-circuit potential test of 1800 s was conducted to ensure that the potential difference between the working electrode and the reference electrode reached a stable state. The electrochemical parameters calculated from Figure 3 using the Tafel extrapolation method are listed in

Table 1. Electrochemical polarization test parameters for different samples.

Samples	Ecorr (V vs SCE)	Icorr (A·cm−2)	βa (mV·dec−1)	−βc (mV·dec−1)
Substrate	−1.287	6.5 × 10−5	181.77	86.42
EC	−0.723	2.0 × 10−6	11	596
ECA	−0.698	4.2 × 10−6	30	270

. E_corr denotes the corrosion potential, I_corr represents the corrosion current density, and β_a and β_c denote the anodic and cathodic slopes, respectively.

In terms of electronegativity, E_corr reflects the relative tendency of a material to attract electrons in a corrosive environment. A more positive E_corr value corresponds to a higher effective electronegativity of the surface, indicating a greater resistance to electron loss and a reduced tendency to undergo corrosion. This implies better ability to hinder the penetration of corrosive species. A lower value of I_corr indicates a lower corrosion rate of the sample [37,38]. The corrosion current densities of EC and ECA samples are of the same order of magnitude, indicating similar corrosion rates, both of which are lower than that of the aluminum substrate [39], demonstrating that the Cr/DLC dual-layer coating prepared by the cathodic vacuum arc process can effectively enhance the corrosion resistance of the aluminum alloy substrate. Through a comparative analysis of corrosion potentials, it can be observed that AT reduces the thermodynamic tendency for corrosion in ECAP samples. This can be attributed to the prolonged AT increasing the density of the Cr/DLC coating, reducing the diffusion pathways between the corrosion medium and the base alloy, and thereby inhibiting the oxidation and reduction reactions occurring at the coating/solution interface during the cathodic reaction [40].

3.4. Microstructure and Morphology

Figure 4 shows the SEM surface images of EC and ECA samples treated and un-treated with AT. The surfaces of the two untreated samples exhibit poor uniformity, and such a relatively rough surface is commonly observed in DLC coatings [41]. The surface quality of the EC sample is very poor, with light gray impurities distributed throughout the entire surface of the DLC layer, which may reduce the coating’s corrosion resistance and wear resistance [42]. The bright white spots on the surfaces of both groups of samples are due to the lower thickness of the intermediate layer, formed as circular microdroplets during the cathodic arc physical vapor deposition process [43]. Due to the operation of the PVD system, microdroplets embedded in the DLC coating surface are a common phenomenon. Both sets of samples exhibited noticeable pores, which can lead to corrosion current leakage and reduce corrosion resistance [44]. The pore density of the EC samples was significantly higher than that of the ECA samples, causing the corrosion medium to penetrate the coating from multiple sites simultaneously. The superposition of corrosion pathways within the coating accelerates corrosion. Notably, the pore size of the ECA samples increased significantly, making it easier for the corrosion medium to penetrate and penetrate deeper into the coating. This phenomenon of reduced pore number and increased pore size is primarily attributed to the thermal activation effect during the AT process. At the aging temperature, enhanced atomic diffusion significantly promotes interface migration within the coating. This accelerated interface migration drives the coalescence of small pores [45,46,47]. The reduction in pore count accounts for the elevated E_corr values in ECA samples, whilst the increase in pore size explains the rise in Icorr values.

Figure 5 displays cross-sectional images of the coating in both the overall view (Figure 5a,b) and the region adjacent to the substrate (Figure 5c,d). Figure 5a,b can be used to estimate the coating thickness, showing that the bilayer coating consists sequentially of a Cr transition layer and a DLC layer along the growth direction. The thicknesses of both coatings are essentially identical, with the Cr transition layer measuring approximately 0.3 μm and the diamond-like carbon (DLC) layer approximately 5.4 μm, indicating that the deposition rate of the DLC layer is significantly higher than that of the Cr layer. After eliminating thickness as a variable, this study focuses on investigating the effect of aging treatment (AT) on the microstructure of the coating.

Figure 5c,d show the cross-sectional morphology of the coating near the aluminum substrate region. Both the EC and ECA samples exhibit significant irregular morphologies and defects, including intrinsic coating defects and those induced by wire cutting. The formation of wire-cutting defects is attributed to differences in material removal mechanisms between the soft metal substrate and the hard DLC layer during precision cutting and polishing processes [48]. In the aluminum substrate of the ECA sample, a large number of spherical precipitated particles can be observed. This is because the high dislocation density generated by equal-channel angular pressing (ECAP) on the Al (111) crystal plane promotes the precipitation of nanoscale η’ phases during the aging treatment, thereby enhancing the corrosion resistance of the substrate [29].

Additionally, a pronounced plastic deformation zone is observed near the interface between the Cr layer and the DLC layer. Within this plastic deformation zone, precipitates are formed in both EC and ECA samples, with larger precipitate sizes observed in the ECA samples. Studies have shown that aging treatment influences element enrichment and precipitation behavior in the coating; by attenuating element enrichment effects, it facilitates the formation of larger precipitates [31]. Furthermore, during aging treatment, precipitates preferentially form in high-energy regions [49], reflecting the accumulation of internal stress within the plastic deformation zone.

In the EC samples, localized micro-delamination and interfacial debonding are observed at the Cr/DLC interface. In contrast, the ECA samples exhibit improved interfacial bonding, which can be attributed to the aging treatment promoting the diffusion of Cr elements into the DLC layer, thereby stabilizing sp² and sp³ bonds [50]. In the non-plastic deformation region above the plastic deformation zone, the defect density in EC samples is higher than that in ECA samples, due to enhanced atomic diffusion during aging treatment, which promotes grain boundary healing [51]. The interfaces between the Cr layer and the aluminum substrate in both EC and ECA samples demonstrate tight bonding, indicating that ECAP treatment of the aluminum substrate increases its hardness, thereby enhancing the bonding strength between the Cr layer and the substrate. When metal atoms are deposited on the aluminum substrate, intermetallic compounds may form at the interface, and localized temperature rise induced by adiabatic shear deformation also contributes to interfacial bonding [52].

Figure 6 shows the EDS line scan spectrum, illustrating the elemental distribution corresponding to each layer. The EDS spectrum reveals that some Cr atoms have diffused into the aluminum matrix, indicating a good state of diffusion bonding.

Table 2. Atomic percentages of C, O, Al, and Cr in different samples.

Samples	C	O	Al	Cr
EC	63.31	3.17	25.59	7.93
ECA	68.47	1.22	23.09	7.22

lists the atomic percentages of various elements within the ‘scanning regions’ of both sample sets. The DLC layers of both sets of samples exhibit a relatively dense structure with no significant columnar crystal structure observed, indicating that the current of the C target material was sufficiently high, resulting in a high plasma density and a densely structured DLC layer.

3.5. XRD Analysis

Figure 7 shows the XRD patterns of the EC and ECA samples. Due to insufficient crystallinity, no distinct diffraction signals for the DLC phase were detected. The observed diffraction peaks correspond to the Cr₂AlC phase and chromium carbide phase, whose formation indicates interdiffusion and interface reactions between the Cr layer, DLC layer, and substrate components during the cathodic arc ion plating process. Figure 5 reveals numerous gray spherical particles dispersed within the DLC coating, which is the typical morphology of the Cr₇C₃ phase in chromium-doped DLC coatings. Chromium carbide exists in various stoichiometric forms as a ceramic material (e.g., Cr₃C₂, Cr₇C₃, and Cr₂₃C₆), but XRD only detected the Cr₇C₃ and Cr₂₃C₆ phases, while the Cr₃C₂ phase was not identified due to its low intensity. Notably, the (080) crystal plane is located at 76.8°, and the positive shift in the diffraction angle toward 78.4° may be due to Al atoms solid-solving into the Cr₇C₃ lattice to form (Cr,Al)₇C₃ [53]. Since the radius of Al atoms (143 pm) is larger than that of Cr atoms (130 pm), this increases the unit cell volume, causing a positive shift in the diffraction angle. The diffraction peak at 82.5° likely corresponds to the (640) crystal plane of Cr₂₃C₆. Its positive shift relative to the standard peak position may be due to the presence of the Cr₂₃C₆ phase in the plastic deformation region near the interface between the Cr layer and the DLC layer, which is influenced by residual stress, leading to a shift in the diffraction angle. Regardless of whether AT is present, the binary carbide Cr₇C₃ remains the dominant phase. It can be observed that AT alters the preferred orientation of the Cr₇C₃ phase. In the ECA sample, the peak intensities of the (112), (041), and (080) crystal planes have decreased, while the overlapping peak intensities of the (060) and (242) crystal planes have significantly increased. The (060)/(242) overlapping peaks in the ECA sample exhibit narrow peak shapes and strong signals, indicating that the crystallinity of the Cr₇C₃ phase on these two crystal planes is very high. The increase in the size and number of greyish-white spherical particles in the ECA sample in Figure 6 further confirms this finding. The hardness value of the Cr₇C₃ phase is 18.3 GPa [54], and the high hardness of the Cr/DLC coating is likely attributed to the formation of this phase. Additionally, AT also reduces the crystallinity of Cr₂₃C₆, with its signal being nearly undetectable in the ECA samples.

In addition to distinct chromium carbides, both samples exhibit weak diffraction peaks corresponding to the Cr₂AlC phase, the formation of which requires a Cr/C molar ratio of approximately 1.72–1.93 and a Cr/Al molar ratio of about 1.42–2.03 [55,56]. The formation mechanisms for this phase potentially involve: (1) the reaction Al + CrCₓ → Cr₂AlC, or (2) a lattice transformation (Cr, Al)Cₓ → Cr₂AlC driven by thermally activated diffusion [57]. Notably, aging treatment (AT) slightly attenuated the intensity of the Cr₂AlC peaks. This observation suggests that mechanism (2) is not the primary route for Cr₂AlC phase formation. The binary Cr₇C₃ phase has a more negative Gibbs free energy than the ternary Cr₂AlC phase, leading to preferential reaction of carbon with Cr in the DLC layer. This thermodynamic advantage limits the nucleation and growth of the Cr₂AlC phase, resulting in its content being lower than that of Cr₇C₃. Another hypothesis for the reduced peak intensity of the Cr₂AlC phase in ECA samples is that in the Cr₂AlC lattice, the Al–Cr bond strength is lower than the Cr–C bond strength [58], conferring higher mobility to aluminum atoms. AT can enhance aluminum atom migration, disrupting the stability of Cr₂AlC and thereby reducing its peak intensity. Although XRD cannot resolve all phases, metastable CrC or Cr₂Al phases may exist in the system [59].

3.6. XPS Analysis

As shown in Figure 8, the chemical bonding state of the Cr/DLC coating on EC and ECA samples was analyzed quantitatively using XPS. The XPS spectra were analyzed after Shirley background subtraction. The content of C atoms with sp² and sp³ hybridization was determined by analyzing the C1s peak [60]. Carbon-oxygen bonding was detected in the Cr/DLC coatings of both EC and ECA samples, indicating the presence of functional groups. The C1s spectrum was fitted using a Gaussian-Lorentzian mixed function [61] and decomposed into six peaks at 283.0, 284.2, 284.8, 285.8, 287.65, and 288.7 eV, corresponding to the C–Cr peak, sp²-C peak, sp³-C peak, C–O peak, C=O peak, and O–C=O peak [62,63,64]. The relative percentages of the chemical bonds were quantified based on the ratio of the integrated area of each deconvoluted peak to the total area of all peaks after background subtraction. The relatively low C–O, C=O, and O–C=O signals can be attributed to oxygen adsorbed on the surface [65]. Compared to the strongest peak caused by C–C bonds, the intensity of the C–Cr peak is lower. The source of the C–Cr signal could be either locally Cr–rich droplets or inclusions, or it could originate from the uniform bulk diffusion of Cr elements. However, no C–Cr signal was detected in the Cr2p spectrum, which increases the possibility that the C–Cr signal detected in the C1s spectrum originated from Cr-rich droplets or Cr-containing inclusions. Cr atoms diffusing into the DLC layer, in addition to bonding with C atoms to form carbides, may exhibit catalytic activity during the diffusion process, promoting the aggregation of sp²-hybridized carbon into aromatic rings [66]. The sp³-C:sp²-C ratio of the Cr/DLC coating in EC and ECA samples is 1.00 and 1.31, respectively, with the sp³ proportion increasing in ECA samples, effectively enhancing the nano-hardness of the Cr/DLC coating and thereby reducing the friction coefficient [67,68]. XPS analysis showed that the full width at half maximum (FWHM) of the sp³-C peak and sp²-C peak in the EC sample were 1.25 and 1.14, respectively, with the sp³-C peak being slightly wider. After AT of the ECA sample, the FWHM of both peaks narrowed slightly to 1.16 and 1.13, respectively. The regulation of the sp³-C:sp²-C ratio within the Cr/DLC coating by AT is a key factor influencing its mechanical properties.

4. Discussion

Through XRD combined with EDS line scan results, it can be observed that elemental interdiffusion and interface reactions occur between the substrate, Cr layer, and DLC layer, resulting in the presence of chromium–carbon compounds within the coating. The Cr₇C₃ phase in the EC sample exhibits multiple preferred orientations. AT promotes the formation of a single preferred orientation of Cr₇C₃ (060) or (242) crystal planes. The uniform arrangement of the crystal lattice reduces residual stress within the coating, thereby enhancing mechanical properties. Additionally, AT may promote the transformation of Cr₂AlC to Cr₇C₃. AT may increase the migration rate of Al atoms, thereby preventing the formation of a stable Cr₂AlC crystal lattice. It is known that the hardness of Cr₂AlC is lower than that of chromium carbides [69], so regulating the content of Cr₂AlC and Cr₇C₃ is also a key factor in determining the hardness of the coating. SEM and XRD analysis jointly show that the size and crystallinity of the Cr₇C₃ phase in ECA samples are higher than those in EC samples, which is an important reason for the increased hardness of ECA samples. Another important factor contributing to the increased hardness of ECA samples is the elevated sp³-C/sp²-C ratio in the DLC layer. Controlling the content of sp²-hybridized and sp³-hybridized carbon atoms is an important method for controlling coating hardness. Additionally, AT reduces the porosity of the coating surface and the defect density within the coating, resulting in a more dense and uniform internal structure. Additionally, there is one more point to note regarding the XPS test. In this paper, the C1s peak at 283.0 eV is attributed to the C–Cr bond. However, under the Al Kα excitation source, the XPS penetration depth of carbon-based thin films is approximately 7 nm, indicating that the observed C–Cr signal primarily originates from the near-surface region of the DLC coating. The cross-sectional EDS scan results of the coating show that Cr elements are still distributed in regions outside the Cr transition layer. However, due to the spatial resolution and detection limit limitations of SEM-EDS, as well as the absence of a distinct C–Cr bond signal in the Cr 2p spectrum, the origin of this signal cannot be determined. This signal may originate from locally enriched Cr droplets or inclusions, or from the uniform bulk diffusion of Cr elements. However, no discernible signal corresponding to C–Cr bonds was detected in the Cr 2p spectrum. Therefore, the C–Cr signal observed in the C 1s spectrum is less likely to originate from the uniform bulk diffusion of Cr from the Cr interlayer to the DLC layer. This observation is consistent with the microstructural characteristics wherein Cr-rich droplets and Cr-containing inclusions are confined to localized regions. Such spatially restricted distribution accounts for the absence of distinct C–Cr bonding features in the Cr 2p spectrum.

According to the dynamic potential polarization analysis, the ECA sample exhibits the most positive E_corr value, suggesting a higher effective electronegativity at the surface and the lowest thermodynamic tendency to lose electrons and undergo corrosion. The I_corr values for both the EC and ECA samples were significantly lower than those of the aluminium substrate, demonstrating that both EC and ECA samples possess excellent kinetic corrosion resistance. Based on the electrochemical test results, the higher E_corr of the ECA sample, which reflects an increased electronegativity, may be attributed to the reduction in pore number, while the higher I_corr may be related to the increase in pore size. AT can also improve the density of the DLC coating both internally and on the surface, as AT allows deposited atoms sufficient thermal energy to diffuse [70]. A dense structure can suppress the formation of corrosion pathways and reduce the number of Cl⁻ corrosion sites within the coating [71,72]. The poorer surface quality of EC samples leads to the adsorption of more impurities, which is also one of the reasons for the reduced corrosion resistance and mechanical properties of EC samples. Additionally, the ECA samples exhibit a large amount of uneven η’ phase precipitation in the substrate, which enhances the corrosion resistance of the aluminum substrate. However, both EC and ECA samples have numerous bright, circular microdroplets distributed on their surfaces, indicating that AT cannot resolve the issue of microdroplets formation during the cathodic arc physical vapor deposition process. Eliminating microdroplets to improve coating corrosion resistance remains a future research direction.

5. Conclusions

This study investigated the regulatory mechanisms of the microstructure and properties of Cr/DLC coatings deposited on the surface of 7075 aluminum alloy using ECAP and AT techniques, combined with performance testing and microstructural characterization. The following conclusions were drawn:

(1)

ECAP enhances the strength and load-bearing capacity of the 7075 aluminum alloy substrate, mitigating the mismatch in mechanical properties between the aluminum substrate and the Cr transition layer. The Cr transition layer is tightly bonded to the substrate via diffusion bonding.

(2)

AT promotes the precipitation of η’ phase, thereby enhancing the corrosion resistance of the aluminum substrate. However, it also leads to an increase in the pore diameter of the Cr/DLC coating, resulting in a slight reduction in the coating’s corrosion resistance. Additionally, AT promotes the transformation of the Cr₇C₃ hard phase from a polycrystalline orientation to a single orientation along the (060) or (242) crystal planes; increases the size of the Cr₇C₃ phase; elevates the sp³-C/sp²-C ratio; and improves the compactness of the DLC layer. These microstructural evolutions are beneficial for enhancing the mechanical properties of the Cr/DLC coating.

(3)

The combination of ECAP and AT processes enables the Cr/DLC coating deposited on 7075 aluminum alloy to achieve a hardness of 5.2 GPa, an adhesion strength of 15.1 N, and a corrosion potential of −0.698 V. However, this process cannot improve the adhesion state between the Cr layer and the DLC layer, and there are obvious plastic deformation regions near the interface.

Further research should be directed towards optimizing the interfacial integrity between the Cr transition layer and the DLC top layer, potentially through tailored interlayer designs or novel deposition parameters, to eliminate plastic deformation regions and further enhance the overall durability and long-term reliability of these high-performance coated alloys for demanding applications.