Antibacterial and Film Characteristics of Copper-Doped Diamond-like Carbon Films via Sputtering Using a Mixed Target of Copper and Graphite

Abstract

Copper-doped diamond-like carbon films (Cu-DLC) are effective antibacterial materials and are fabricated using different techniques. By controlling the ratio of the graphite and diamond structures as well as the hydrogen bonds, the biocompatibility, chemical stability, wear resistance, and high hardness of Cu-DLC can be regulated. In this study, three types of Cu-DLC films were deposited on SUS304 substrates using Ar-sputtering with mixed targets comprising different C/Cu ratios. The films’ structures, surface, and antibacterial properties were investigated using electron probe microanalysis, Raman and X-ray photoelectron spectroscopy, atomic force microscopy, and ball-on-disk tests. The Cu concentration in the Cu-DLC films increased with an increase in its content in the target; however, no significant differences were observed in the Raman spectra. The surface composition, roughness, and dynamic friction coefficients were similar across all Cu-DLC films, which displayed smoothness and friction properties similar to those of standard DLC films without Cu. The antibacterial activity (R value) was evaluated as per ISO 22196. Although DLC films exhibited no antibacterial activity (R < 2), all the prepared Cu-DLC films displayed good antibacterial activity (R ≥ 2). The proposed deposition process facilitated Cu-DLC coating, thus promoting its use in the healthcare fields.

1. Introduction

Diamond-like carbon (DLC) films are amorphous carbon films that contain the graphite structure (sp2) and diamond structure (sp3) of carbon, as well as hydrogen bonds. By controlling the ratio of these structures, various excellent film properties, such as biocompatibility, chemical stability, wear resistance, and high hardness can be obtained [1]. Therefore, DLC is used as a means of surface modification for various industrial products, such as cutting tools, hard disks, and PET bottles [2,3]. Furthermore, the addition of elements such as copper, silver, zinc, fluorine, and nitrogen endows DLC films with attractive functions specific to the introduced element while maintaining their excellent properties [4,5]. These films are also used in hygienic materials.

Owing to the heightened awareness of the microbial threat over the past few years, the demand for hygiene resilience and, hence, for hygienic materials is growing, particularly in medical settings and residential environments [6]. In general, Cu ions have a microbicidal effect; thus, alloys containing Cu have applications in hygiene products. However, its surface changes significantly upon oxidation, owing to its high surface activity. Thus, concerns abound regarding the long-term sustainability of its surface functionality. To maintain a stable surface, Cu has been incorporated into chemically stable DLC films using various deposition processes, thus producing Cu-containing DLC (Cu-DLC) films. Typical methods for fabricating Cu-DLC films are as follows: (a) the dual magnetron sputtering method, which uses two types of solid raw materials; (b) the reactive sputtering method, which uses hydrocarbon gas and solid raw materials; and (c) the cathode arc method, which uses a DC and pulse dual excitation source [7,8,9,10,11]. Recently, Ohta et al. fabricated Cu-DLC with a controlled Cu content ranging between 0.3 and 40 at%, which resulted in E. coli inactivation at Cu contents ≥ 0.3 at%. In addition, the antibacterial properties improved as the Cu content increased [7]. Cu-DLC can, therefore, be applied for the surface modification of medical equipment and everyday items, such as handrails and doorknobs.

However, the existing deposition methods considered use two types of raw materials and excitation sources, and to obtain the desired film, it is necessary to control multiple parameters. Although this has the advantage of precisely controlling the film quality, it requires skilled engineers. For process simplification, we developed a C/Cu mixed target that uses C and Cu as a single solid raw material and investigated the Cu-DLC coating using our original Ar sputtering method with this mixed target. Generally, the film structure and surface properties of DLC coatings are significantly affected by the deposition method [2]. Additionally, the quality of the target raw material affects the physical properties of the sputtered thin films [12]. Furthermore, the surface composition of Cu-DLC films affects the amount of Cu ions released and their antiviral properties [13].

Therefore, the properties of the Cu-DLC films produced using our unique sputtering method and their effectiveness as a hygienic material must be investigated and proven.

In this study, three mixed targets with different C/Cu ratios were developed. Using these targets, we produced Cu-DLC films with different Cu contents on SUS304 plates via sputtering. The structural, surface, and antibacterial properties of the Cu-DLC samples were evaluated and facilitated the commercialization of the fabricated films using the proposed deposition process. The establishment of the Cu-DLC coating technology is expected to further strengthen hygiene management, particularly in healthcare and residential environments.

2. Experimental

2.1. Preparation of Cu-DLC Films

Three types of target materials (Toshima Seisakusho, Saitama, Japan) with different C/Cu ratios were developed to adjust the Cu content of the Cu-DLC film. The mixed targets were manufactured by mixing C and Cu powders in ratios of 30%, 40%, and 50% Cu, followed by sintering. The higher the C concentration, the higher the sintering temperature, but if the C concentration in the target exceeds 70%, the sintering temperature exceeds the melting point of Cu, which then melts. Therefore, the lower limit of the Cu concentration in the C/Cu mixed target was set to 30% to prevent Cu from melting. Cu-DLC films were prepared on SUS304 substrates via magnetron sputtering (NPS-330, Nanotec, Chiba, Japan) with Ar gas, employing three types of mixed target materials. The Cu-DLC films were labeled Cu-DLC (A), Cu-DLC (B), and Cu-DLC (C) in the order of the Cu ratio in the C/Cu mixed target (30%, 40%, and 50%, respectively). In addition, DLC and Cu films were produced using C and Cu targets, respectively, for comparison with the Cu-DLC films.

Figure 1 shows a schematic of the experimental apparatus, and

Table 1. Deposition conditions for each sample.

Sample	C:Cu Target Ratio	Ar Gas Flow Rate [sccm]	Gas Pressure [Pa]	Bias Voltage [V]	Bias Current [A]	Deposition Time [min]
DLC	100:0	20	3.7 × 10−1	530	1.44	180
Cu-DLC (A)	70:30	20	3.5 × 10−1	540	1.61	80
Cu-DLC (B)	60:40	20	3.6 × 10−1	530	1.92	60
Cu-DLC (C)	50:50	20	3.6 × 10−1	530	1.84	40
Cu	0:100	20	3.7 × 10−1	390	2.40	13

lists the deposition conditions. It should be noted that the structures around the chamber and cathode electrode in the figure were grounded, and the distance between the substrate (anode) and target (cathode) was ~100 mm. In addition, the anode was a rotating system (3.6 rpm) to enable uniform deposition on the three-dimensional structure. The potential on the substrate side can be freely adjusted, enabling the application of grounding, floating, and negative biases. Ground potential was applied to inhibit arcing during bias application. The size of the SUS304 base material (standard test piece; Kanagawa, Japan) used was adjusted as appropriate for each evaluation item. Before deposition, ultrasonic cleaning was performed in acetone and ethanol for 10 min each to remove contaminants from the substrate. In the sputtering process, the chamber was first evacuated to a gas pressure of 3.0 × 10⁻³ Pa. Subsequently, Ar gas (gas flow rate 8 sccm, purity 99.9995%) was introduced to adjust the gas pressure to approximately 3.5 × 10⁻¹ Pa, and sputtering deposition was initiated at a controlled voltage level. The discharge voltage (bias voltage) applied to the substrate side of all films, except the Cu film, was controlled at approximately 530 V. However, the discharge voltage applied to the Cu film was adjusted to 390 V because of abnormal discharge problems. During the deposition process, the deposition time was adjusted to achieve a target film thickness of 1000 nm. Figure 2 shows photographs of each sample with dimensions of 10 mm × 10 mm × 1 mm and an SUS304 substrate with no deposited material. The thickness of each sample was measured using a tactile step meter (SURFTEST, Mitutoyo, Kanagawa, Japan), with N = 3. The deposition conditions are listed in Table 1.

2.2. Structure of Cu-DLC Film

The structures of the DLC and Cu-DLC films were evaluated using electron probe microanalysis (EPMA, JXA-8230, JEOL, Tokyo, Japan), X-ray photoelectron spectroscopy (XPS; ESCA-3400, Shimadzu, Kyoto, Japan), and laser Raman spectroscopy (Renishaw inVia Qontor TDO, Renishaw, Gloucestershire, UK). EPMA confirmed the presence of C, Cu, and O in the Cu-DLC film. The EPMA measurement conditions were as follows: acceleration voltage = 15 kV, beam current = 100 nA, spot diameter = 10 mm, and total count = 3.

In addition, XPS depth profile analysis was performed to confirm the profile of the Cu content in the Cu-DLC film in the depth direction. In this XPS analysis, a conical X-ray source (MgKα source, 10 mA, 10 kV, spot diameter, φ = 6 mm) was used to confirm the metal components (Fe 2p, Ni 2p, Cr 2p) contained in SUS, as well as C 1s, Cu 2p, and O 1s. The etching was performed for 40 cycles and an etching time of 100 s. In this experiment, we focused on Cu-DLC (C), which is assumed to contain the highest Cu content in the Cu-DLC film.

The measurement conditions for Raman spectroscopy were as follows: laser power = 3.75 mW, laser wavelength = 532 nm, exposure time = 1 s, and 50 integrations. Previous research has shown that the Raman spectra of amorphous carbon films contain a D peak (at approximately 1350 cm⁻¹), owing to the disordered structure and a G peak (at approximately 1550 cm⁻¹) owing to the graphite structure [1]. In this experiment, we used Igor Pro 9 software (WaveMetrics, Portland, OR, USA) to separate the waveforms of these peaks using a Gaussian function and calculated the ID/IG intensity ratio and G peak position from the peak areas.

2.3. Surface Properties of Cu-DLC Films

The surface composition, roughness, and friction properties of the samples were evaluated. The surface composition (chemical bonding state) of each sample was analyzed using the XPS. In this XPS measurement, C 1s, Cu 2p, and O 1s XPS narrow spectra were obtained. In this case, the maximum peak of the C 1s spectrum, shifted to 284.6 eV, was normalized to 1. Furthermore, using Igor software, the C 1s peak was deconvoluted into C-C sp², C-C sp³, C-O, C=O, and O=C-O bonds, and the ratio of the area of each bond was derived [14]. In this case, the Shirley function was used as the background.

The surface roughness of each sample was evaluated using atomic force microscopy (AFM; JSPM-5200, Tokyo, JEOL) in contact mode, with N = 3. The AFM measurement conditions included a scanning size of 10 μm × 10 μm and a scanning speed = 333.33 μs; moreover, the average root mean square roughness (Rq: RMS) was calculated.

The friction characteristics of each sample were evaluated using a ball-on-disk tester (TRIBOMETER PIN DISK; Anton Paar, Graz, Steiermark, Australia) at room temperature (N = 1). The measurement conditions for the ball-on-disk test were as follows: ball material, SUS304 (Φ = 6 mm), load = 3 N, linear velocity = 5 cm/s, sliding radius = 3 mm, sliding distance = 200 m, and measurement time = 4000 s.

2.4. Antibacterial Properties of Cu-DLC Films

The antibacterial activities of the DLC samples, Cu-DLC samples, and SUS304 substrate were tested in accordance with ISO 22196 [15]. The presence or absence of antibacterial activity was determined by calculating the antibacterial activity of the samples against that of a standard sample (control) without antibacterial activity. Cu films that generally exhibited antibacterial activity were excluded from this evaluation, which used the base material SUS304 as the control. The test bacterium was Staphylococcus aureus (NBRC12732), which is frequently used in antibacterial activity tests. The concentration of the inoculum was adjusted to approximately 4–6 × 10⁵ cfu/mL. The procedure for evaluating the antibacterial properties was as follows: the test bacteria (0.4 mL) was added dropwise onto each sample (50 mm × 50 mm × 1 mm), which was then covered with a reinforced polyethylene film (40 mm × 40 mm × 0.09 mm). The samples were then incubated at 35 ± 1 °C at a relative humidity of ≥90% for 24 h. After incubation, the bacteria present in each sample were collected. The number of viable bacteria per cm² was then measured and the antibacterial activity was calculated using Equation (1) [16]:

R = log(A/U),

(1)

where R is the antibacterial activity, U is average number of viable bacteria per cm² after 24 h in the control (SUS304), and A is the average number of viable bacteria per cm² after 24 h in each sample. If the R value of the target sample exceeds 2.0, the sample is considered to have antibacterial properties.

2.5. Release of Copper from Cu-DLC Films in Wet Environments

To investigate the release of Cu from the film, which is an important factor in the expression of antibacterial properties by Cu-DLC films, high-frequency inductively coupled plasma optical emission spectrometry (ICP-OES, ICPE-9820, Kyoto, SHIMADZU) was performed. In this analysis, the Cu-DLC sample was immersed in pure water for 24 h, assuming a wet environment such as an antibacterial test, and the amount of Cu contained in the pure water was confirmed. The analysis conditions were set at a high-frequency output of 1.2 kW and with measurements taken in triplicate, and a calibration curve was used to derive the results. To confirm the amount of Cu contained in the Cu-DLC film after immersion in water, EPMA was performed. In these analyses, Cu-DLC (A), which was assumed to have the lowest amount of Cu, and Cu-DLC (C), which was assumed to have the highest amount of Cu, were investigated. In this experiment, we calculated the difference in Cu content before and after immersion in water (reduction in Cu content: ΔCu content).

3. Results and Discussion

3.1. Structural Change in Cu-DLC Film with Different C/Cu Target Ratios

Table 2. Film structure of each sample.

Sample	Film Thickness (nm)	Deposition Rate (nm/min)	EPMA			Raman
			C (at%)	Cu (at%)	O (at%)	D-Peak Position (cm−1)	G-Peak Position (cm−1)	ID/IG Intensity Ratio
DLC	1132	6.3	86.2	-	13.8	1393.0 ± 1.1	1568.5 ± 0.4	1.44
Cu-DLC (A)	944	11.8	53.0	40.0	7.0	1386.7 ± 1.7	1563.2 ± 0.9	1.88
Cu-DLC (B)	1287	21.5	44.2	47.3	8.5	1391.8 ± 2.1	1562.7 ± 1.3	2.04
Cu-DLC (C)	1058	26.5	34.9	54.1	11.0	1395.7 ± 1.6	1566.9 ± 0.9	1.97
Cu	930	71.5	-	96.9	3.1	-	-	-

lists the film thickness and deposition rate of each sample, the concentration of elements in the film, and the Raman spectrum analysis. The film thickness of each sample was ~930–1287 nm, and was generally 1000 nm, as intended. All the Cu-DLC films produced in this experiment were stable and uniform on the SUS304 substrate, with no color irregularities or peeling (Figure 2). We additionally confirmed that the three types of Cu-DLC films had a jet-black color that was more similar to that of the DLC films than that of the yellowish hue of Cu. Furthermore, we verified that the deposition rate increased with the amount of Cu in the target. This is likely because the electrical resistance decreased and the discharge current increased as the Cu content increased.

The Cu concentration in each Cu-DLC film increased as the Cu ratio of the C/Cu mixed target increased to 40.0, 47.3, and 54.1 at%, respectively, demonstrating that the Cu concentration in the Cu-DLC film can be controlled by adjusting the C/Cu ratio of the mixed target. Interestingly, the Cu concentration in the Cu-DLC films ranged between 5 and 10 at%, higher than the Cu content in the C/Cu mixed target used in this experiment. This may be due to the differences in the sputtering rates of C and Cu in Ar sputtering or irregularities in the mixing of C and Cu during the target preparation process [5]. Insufficient mixing of C and Cu may reduce the continuity of the Cu content in the depth direction of the film. Because Cu is highly reactive toward oxygen, we assume that the ratio of Cu content will differ greatly between the surface of the Cu-DLC film, which is exposed to oxygen molecules after sputtering, and the interior of the film, which is not exposed to oxygen.

Figure 3 shows the XPS depth profile of Cu-DLC (C). The profile represents the depth information of the Cu-DLC film as the etching time increases. When the etching time reaches approximately 3200 s, Fe, Cr, and Ni are detected, suggesting that the etching has reached the substrate. Focusing on the Cu content in the depth direction of the Cu-DLC film, approximately 58 at% Cu was continuously present in the Cu-DLC film in the area except near the Cu-DLC top surface and the substrate interface. This continuity in the depth direction of the Cu film suggests that the elements in the Cu/C mixed target are mixed to a certain extent. The amount of Cu in the Cu-DLC film compared to that in the target is strongly influenced by the element-specific sputtering rate. When using a Cu/C mixed target to produce Cu-DLC films, as in this study, the Cu content in the Cu/C mixed target should be lower than the desired content. The amount of Cu estimated by XPS is slightly higher than the amount of Cu detected by EPMA. It was confirmed that the Cu content on the Cu-DLC surface exposed to molecular oxygen is clearly lower than that in the interior of the film, owing to the more advanced oxidation of the surface. Because the amount of Cu detected by EPMA is an average over the entire film, it seems reasonable that the amount of Cu estimated by the XPS depth profile would be slightly higher.

Figure 4 shows the Raman spectra of the DLC and Cu-DLC samples. G and D peaks, which are characteristic of amorphous carbon films, were observed in the Raman spectra of all samples. A comparison of the Raman spectra of DLC and each Cu-DLC indicates that the positions of the G and D peaks are not shifted, but the ID/IG ratio is higher for Cu-DLC than that for DLC. In general, this ratio increases as the sp² content increases [17,18]. Therefore, the sp² bond content likely increased with the addition of Cu to the DLC film. No clear trend was observed in the three types of Cu-DLC films, and the ID/IG ratios were equivalent. Dai et al. investigated the film structure in relation to the Cu concentration in Cu-DLC films deposited using a hybrid ion beam system consisting of an anode-type linear ion source and a direct current (DC) magnetron sputtering source. The ID/IG ratio decreased rapidly as the Cu concentration in the Cu-DLC increased from 0.74 to 1.93 at%, and then it increased gradually as the Cu concentration further increased. Furthermore, the TEM observations show that the Cu-DLC film structure evolved into three forms. In the first form with a Cu concentration < 2 at%, Cu dissolved into the amorphous structure of the DLC. In the second form with a Cu concentration approximately 20–30 at%, an amorphous composite structure formed with Cu amorphous nanoclusters embedded in the carbon matrix. In the third form with a Cu concentration approximately 40–50 at%, the Cu formed a nano-composite structure with Cu nanocrystals embedded in the amorphous carbon matrix [19]. Previous reports suggest that the Cu-DLCs we produced exhibited a nanocomposite structure in which Cu nanocrystals were formed but did not significantly affect the structure of the amorphous carbon within the range of Cu concentrations used in this study. Therefore, the ID/IG ratios of the three types of Cu-DLC films used in this study were similar.

3.2. Change in Surface Properties of Cu-DLC Film for C/Cu Mixed Target Ratio

Table 3. Surface properties of each sample.

Sample	Surface Composition			C 1s Curve Fitting Area					Surface Roughness [nm]	Friction Coefficient
	C (at%)	Cu (at%)	O (at%)	C-C sp2	C-C sp3	C-O	C=O	O=C-O
DLC	86.8	-	13.3	0.58	0.14	0.09	0.11	0.03	3.36 ± 0.96	0.14 ± 0.02
Cu-DLC (A)	46.7	27.9	25.4	0.59	0.17	0.05	0.10	0.02	4.29 ± 0.12	0.16 ± 0.02
Cu-DLC (B)	52.0	26.4	21.6	0.48	0.25	0.05	0.09	0.01	4.02 ± 0.15	0.17 ± 0.03
Cu-DLC (C)	53.2	22.4	24.5	0.49	0.20	0.08	0.11	0.03	5.97 ± 0.11	0.16 ± 0.02
Cu	-	62.9	37.1	-	-	-	-	-	2.36 ± 0.05	0.70 ± 0.20

lists the surface composition, surface roughness, and dynamic friction coefficient of each sample, as determined by XPS. The C 1s and Cu 2p spectra of each sample are shown in Figure 5, AFM images are shown in Figure 6, and the results of the ball-on-disk test are shown in Figure 7.

Analysis of the surface composition of each sample using XPS showed that the Cu was clearly detected in all Cu-DLC films that used a C/Cu mixed target, as confirmed by EPMA. In addition, the shapes of the Cu 2p spectra of the Cu-DLC films and Cu films were very similar, suggesting that the chemical bonding state of Cu was maintained in the Cu-DLC films. However, compared with the Cu content of the Cu-DLC films estimated by EPMA, the Cu content on the surface of each Cu-DLC film, as determined via XPS, was lower, likely because the Cu on the Cu-DLC surface reacted with oxygen and moisture in the air, causing surface oxidation, as indicated in the XPS depth profile. This is demonstrated via the binding peaks derived from CuO that are observed in the Cu 2p spectrum [20]. Comparison of the Cu-DLC films revealed a slight decrease in the Cu concentration on the Cu-DLC surface as the Cu concentration in the target increased. This trend contradicted that observed using EPMA. Possible causes include simple in-plane variation and the adhesion of organic matter to the film surface; however, the nanocrystallization of the Cu contained in the Cu-DLC film may affect the surface layer (for example, by causing Cu to aggregate in the film interior). In addition, the C 1s spectrum waveform separation displayed no clear trend in the binding ratio between the Cu-DLC samples, which supported the Raman analysis. As the film structure of DLC strongly affected the surface state, this trend is considered reasonable, that is, no clear difference was observed among the binding ratios of the samples [17].

The surface roughness of each sample was a few nanometers, and we confirmed that the smoothness of the SUS304 base substrate was maintained. The surface roughness of the samples was within the range of variation, and no significant differences were observed. The surface roughness listed in Table 3 is the average value ± standard deviation. In addition, no cracks or pinholes were observed at the microscale in the AFM images.

The dynamic friction coefficient of each sample is expressed as the average value ± standard deviation when the ball-on-disk test is performed for 4000 s. The dynamic friction coefficient of the base material SUS304 was 0.82 ± 0.10, and all the Cu-DLC samples maintained good sliding properties equivalent to that of DLC. When comparing these Cu-DLC samples, the measurement variation was within the acceptable range, and no significant difference was observed in the Cu concentration range (~40.0–54.1 at%) of the Cu-DLC. The sliding properties of Cu-DLC may be affected by the film thickness and the base substrate material, in addition to the amount of Cu in the film; therefore, further investigations are planned [21].

3.3. Antimicrobial Properties of Cu-DLC Films with Different C/Cu Mixed Target Ratios

Figure 8 and Figure 9 show the antibacterial activities of the Cu-DLC films. Figure 8 presents the antibacterial activity values of the DLC samples against Staphylococcus aureus; DLC, Cu-DLC (A), Cu-DLC (B), and Cu-DLC (C) are 0.7 ± 0.02, 3.9 ± 0.50, 4.2 ± 0.00, and 4.2 ± 0.00, respectively, proving that all the Cu-DLC samples produced in this experiment have strong antibacterial properties (R ≥ 2). Figure 9 shows a photograph of the bacteria collected from each sample after being transferred to a Petri dish (Φ = 90 mm). Bacteria (white spots) were observed in the DLC and control samples. The antibacterial effect of Cu-DLC is supported by the observed activity of Cu-DLC produced using other deposition processes [7,10,13]. The Cu present on the surface and inside the Cu-DLC film is released from the film as Cu ions, and the growth of bacteria is inhibited by the reactions of these Cu ions [7]. Ohta et al. found that even Cu-DLC with a very low Cu content (0.3 at%) exhibits antibacterial effects, and that complete sterilization is possible when the Cu content is ≥10 at%. Thus, it is reasonable to assume that Cu-DLCs with a relatively high Cu concentration, as produced in this study, will have good antibacterial properties.

To verify whether the Cu-DLC films produced in this experiment also have the same Cu release effect, we confirmed the amount of Cu released and the amount reduced in the film under wet conditions, such as in antibacterial tests, using two Cu-DLC samples with different Cu contents (Cu-DLC (A) and Cu-DLC (C)) (

Table 4. Amount of Cu released from Cu-DLC and amount of Cu reduction in the film under wet conditions.

Sample	EPMA	ICP-OES
	ΔCu Content (at%)	Cu Release Amount (ppm)
Cu-DLC (A)	1.3	0.37
Cu-DLC (C)	1.7	0.56

). We demonstrated that Cu in the order of ppm is released from the Cu-DLC film based on ICP-OES analysis. In addition, the Cu-DLC (C) film, which contained more Cu, showed a slightly greater decrease in Cu content and a slightly higher Cu release rate than the Cu-DLC (A) film. Additionally, the relationship between the amount of Cu on the outermost surface of the Cu-DLC film and the amount released was contradictory, suggesting that the amount present in the interior of the Cu-DLC film had a strong influence. In addition, when we focus on the decrease in its amount in the film, the change is only a few at%, so we speculate that the Cu present in a certain fixed surface layer is involved in the release, without having a significant effect on the bulk structure of the Cu-DLC film. In the future, we plan to clarify the lifespan of the antibacterial effect of Cu-DLC by confirming the history of Cu release through long-term testing. In addition, in this study, we confirmed the amount of Cu in the Cu-DLC film before and after immersion in pure water using model-based experiments; however, to accurately determine the release of Cu upon immersion in water, the samples must also be evaluated after the antibacterial test. Furthermore, because the surface morphology of the Cu-DLC may change during the release of Cu, we also plan to conduct a detailed investigation of the changes in the surface morphology and the mechanical properties of Cu-DLC before and after Cu release (i.e., antimicrobial tests).

Our findings provide important basic data demonstrating the usefulness of Cu-DLC films produced using the deposition process with the proposed Cu/C mixed target. Although additional verification, such as the long-term stability, influence of the film thickness, and influence of negative bias on the substrate, is required, this Cu-DLC coating technology is expected to be applied as a surface modification method to enhance hygiene management.

4. Conclusions

In this study, three types of Cu-DLC films were prepared on SUS304 substrates using a unique Ar-sputtering method with mixed targets containing different C/Cu ratios. The structure, surface properties, and antibacterial properties of these Cu-DLC films were investigated. The Cu concentration in the Cu-DLC films increased in accordance with the target composition; however, no significant change was observed in the shape of the Raman spectra of these Cu-DLC films. Similarly, although no significant differences were observed in the surface composition, surface roughness, or kinetic friction coefficient of these Cu-DLC films, all the Cu-DLC films maintained the same excellent smoothness and friction characteristics as those of the DLC films, and all Cu-DLC films exhibited good antibacterial properties. The deposition process we have proposed (Cu/C mixed target) will facilitate Cu-DLC coating and we hope that it will promote the use of Cu-DLC coating in the medical and healthcare fields.