High-Temperature Mechanical and Tribological Performance of W-DLC Coating with Cr interlayer on X40CrMoV5-1 Hot Work Tool Steel

Abstract

Diamond-like carbon (DLC) coatings enhance tool wear resistance across various temperatures. The sp³/sp² bond ratio within DLC significantly impacts its properties and thermal stability. Elevated temperatures can alter DLC’s structure, while metallic elements and interlayers like chromium can modify its microstructure and performance. To evaluate the potential of W-DLC coatings with a chromium interlayer on 40CrMoV5-1 hot work tool steel under elevated temperatures, mechanical and tribological properties were assessed at room temperature and at temperatures of 100, 200, 300, 400, and 500 °C. Nanoindentation revealed a maximum hardness of 14.1 ± 1.3 GPa for the coating deposited at room temperature, attributed to a high sp³ content confirmed by Raman spectroscopy. Hardness decreased to 9.3 ± 1.0 GPa at 400 °C due to graphitization. The elastic modulus remained relatively constant across all temperatures. Tribological tests indicated a low coefficient of friction (CoF) of 0.15 at room temperature, increasing to 0.35 at 100 °C. The CoF further rose to 0.5 at 200 °C, coinciding with increased graphitization. However, the CoF reduced to 0.45 and 0.35 at 400 °C and 500 °C, respectively, likely due to the formation of a WO₃ tribo-film and the protective effect of the chromium interlayer.

1. Introduction

In the production process of tools, their excellent functionality in connection with a long service life is important. To achieve this combination, the tools must have high-quality surfaces. The requirements for machining operations are constantly increasing due to factors that include the development of new difficult-to-machine materials, the demand for ever higher productivity with the lowest possible production costs, and strict environmental requirements associated with production processes. An increase in cutting speed leads to an increase in tool wear [1]. From the point of view of the tool’s functionality, the properties of the thin coating are important, such as hardness, resistance to friction and wear, and thermal and chemical stability. In order to reduce tool wear and prevent breakage, a hard coating is often applied to the tool surface. This coating primarily protects the cutting edge, but it also reduces its temperature by reducing friction between the chip and the rake face of a tool [2,3].

There are multiple types of coatings and their deposition technologies, which improve the wear resistance of the alloy substrate. For example, the authors in [4] studied Co-based alloy/WC/CaF₂ composite coatings based on Co, deposited on a TC21 titanium alloy substrate using laser cladding. Changing the chemical composition of the coating and increasing the laser power resulted in improved wear resistance by reducing the coefficient of friction (CoF). The laser cladding technique achieved several-times-higher wear resistance of Cr-Al-C composite coatings compared to the 316 stainless steel on which the coating was deposited [5]. The chemical composition of the coating, the properties of the substrate, and the technological parameters of the deposition process are key factors for the production of thin wear-resistant coatings with high hardness, adhesion to the substrate, and thermal stability for the required applications.

It has been proven in several studies [6,7,8,9] that the application of hard physical vapor deposition (PVD) coatings reduces friction between the tool and machined material, thereby preserving the tool’s cutting properties for a longer duration. One common PVD coating is diamond-like carbon (DLC). The designation DLC includes a group of coatings that differ in chemical composition and structure. A common feature is the proportion of graphite sp² and diamond sp³ components in the structure. When the diamond sp³ component is dominant, one can expect a higher hardness and a higher level of internal stress, which also leads to the deterioration of the coating’s adhesion [10]. DLC can be deposited on most tool materials, enabling dry and semi-dry machining that has a positive impact on the environment [11]. At high temperatures, however, DLC coatings undergo oxidation, graphitization, and dehydrogenation and lose their lubricating ability or even peel off from the substrate. This results in the creation of the so-called transfer layer and wear debris on the sliding surfaces of the counterparts [12]. The friction and wear of DLC coatings at room temperature (RT) and elevated temperatures depends on the chemical structure of the coatings, substrate material, contact pressure, and operating temperature [13]. The ratio of sp²/sp³ components is the main factor affecting the mechanical and tribological properties of DLC coatings. The value of the sp²/sp³ ratio depends mainly on the deposition conditions [14]. Raman spectroscopy is a non-destructive analysis for the characterization of crystalline, nanocrystalline, and amorphous carbon. Tuinstra and Koenig [15] obtained Raman spectra of monocrystalline and microcrystalline graphite samples. For monocrystalline graphite, the line at 1575 cm⁻¹ was attributed to the Raman active E_2g2 mode of graphite—referred to as the G peak for graphite. For microcrystalline graphite, they found two lines centered at 1575 cm⁻¹ and 1355 cm⁻¹. Wada and Solin [16] reported that the E_2g2 mode of graphite has a Raman cross-section that is 50 times larger than the “diamond band” at 1332 cm⁻¹. In the studies, the 1355 cm⁻¹ mode is referred to as the D peak disordered.

Among metallic materials, W and Cr are mainly used to dope wear-resistant DLC coating with improved mechanical properties [17,18]. As reported by the authors in [19,20], the formation of WO₃ led to low CoF values. At the same time, a significant improvement in the tribological properties of the WC-DLC coating applied by magnetron sputtering at a temperature of 200 °C was detected, due to the formation of a continuously compacted tribo-film of WO₃. The values of the coefficient of friction, according to the results in the aforementioned work, were in the range of 0.46–0.54 in the temperature interval 200–300 °C; for 400 °C, it was 0.07.

One limitation hindering the wider use of DLC coatings is their low adhesion to the substrate. To address this, the application of interlayers made of Cr, W, Ti, Si, or Al can significantly increase the adhesion of DLC coatings to the substrate [21].

The aim of this study was to evaluate the mechanical and tribological properties of DLC coatings doped with tungsten. We focused mainly on the thermal stability of the prepared DLC coatings. Specifically, using Raman spectroscopy, we investigated the influence of the heating temperature in the range of 100–500 °C during tribo-testing on changes in the coating composition and on the magnitude and course of the coefficient of friction. The influence of W doping and the presence of a Cr interlayer on tribological properties at elevated temperatures were investigated. This study investigated the effect of material transfer, the presence of a Cr interlayer, the formation of a WO₃ tribo-film, and the composition of the W-DLC coating on its tribological properties at high temperatures.

2. Materials and Methods

A substrate made of X40CrMoV5-1 steel was used for DLC coating deposition. According to EN ISO 4957: 2000 [22], it is a steel for hot work, with very good mechanical properties and high wear resistance even at higher temperatures. After heat treatment, it reaches a hardness of 52 HRC. The chemical composition of the steel is shown in

Table 1. Chemical composition of steel X40CrMoV5-1 EN ISO 4957 [20] (in wt. %).

Steel	Element Content (wt. %)
W302	C	Mn	Si	Cr	Mo	V
	0.38	0.4	0.4	5.0	2.8	0.65

.

The WC-C:H coating (W-DLC) was deposited on the steel surface by reactive magnetron sputtering in a vacuum environment with the presence of acetylene (the source of C and H in the coating) using a WC cathode and an applied bias on the substrate. This process formed a hydrogenated DLC coating alloyed with tungsten on the surface. Chromium (Cr) was used as an interlayer. The technological conditions of the PVD process are listed in

Table 2. Technological parameters of the deposition PVD process.

Coating	Technological Parameters
	Substrate	Substrate Temperature (°C)	Cathodic Current (A)	Substrate Bias (V)	Pressure (Pa)
W-DLC	X40CrMoV5-1	100	0.6–0.8	50–150	0.3–0.6

.

A Surftest SJ-301 profilometer (Mitutoyo, Kawasaki, Japan) was used to evaluate the microgeometry of the substrate surface and the DLC coating. The measurement was carried out on 5 sections (each 5 × 1.25 mm), for a total length of 6.25 mm.

The morphology and structure of the coated samples were observed in cross-sections and on the surface using a ZEISS AURIGA COMPACT (Carl Zeiss, Oberkochen, Germany) scanning electron microscope at an accelerating voltage of 5 kV. SEM analysis of the DLC coating surfaces after the tribo-test was conducted using a ZEISS EVO MA15 scanning electron microscope at an accelerating voltage of 20 kV.

An Anton Paar NHT² (Anton Paar, Graz, Austria) device was used to measure hardness H (in GPa) and elastic modulus E (in GPa). A Berkovich indenter (Antor Paar TriTec SA, Corcelles, Switzerland) was used, with a maximum load of 2 mN (in order to maintain the penetration depth of maximum 10% of the thickness of the DLC coating). The number of indents for each sample was 16 (4 × 4 matrix). The evaluation of the measurements was carried out according to the Oliver and Pharr method for samples at temperatures of 23 °C (RT), 100 °C, 200 °C, 300 °C, 400 °C and 500 °C. In all measurements, the load lasted for a maximum value of 20 s, the endurance at maximum load was 5 s, and unloading took 20 s.

The tribological properties of the DLC coating at the test temperatures were determined using a ball-on-disc test on a Bruker UMT2 microtribometer (Bruker, Billerica, MA, USA). A Si₃N₄ ball (diameter 6.35 mm) with a 5 N load was used as the pin under a rotation speed of 100 rpm. The total test track length was 500 m with a track radius of 10 mm. This resulted in 8000 cycles.

Raman spectroscopy was performed with an XploRA ONE spectrometer (Horiba Yvon Jobin, Palaiseau, France) with a 50 mW (original laser power) 532 nm laser source. Spectral data were collected using a 100× microscope objective over the range of 100–3000 cm⁻¹ with a 10 s acquisition time, 5 accumulations, and 10% of original laser power.

The phase analysis of the W-DLC coating was performed using X-ray diffraction (XRD) with a Philips X’Pert Pro laboratory X-ray diffractometer (Philips, Amsterdam, The Netherlands) in Bragg–Brentano (reflection) mode. The diffractometer was operated at 40 kV and 50 mA, using a Cu Kα radiation source (λ = 1.54184 Å). The measurements were conducted with a scan step of 0.03°, a time per step of 60 s, and a 2θ range of 10–120°. For phase analysis, QualX software was used.

3. Results

3.1. Surface Microgeometry of Steel Substrate before Coating and W-DLC Coating

The values of the microgeometric characteristics of the surface of the X40CrMoV5-1 steel substrate before coating and with W-DLC coating, obtained with a touch profilometer, are shown in

Table 3. Microgeometric characteristics.

Parameter	Substrate	W-DLC Coating
Ra (µm)	0.01 ± 0.001	0.02 ± 0.005
Rz (µm)	0.11 ± 0.001	0.18 ± 0.048
Rsm (µm)	6.00 ± 0.447	10.00 ± 2.563
Rpc (-/cm)	1636.00 ± 102.039	1084.40 ± 273.023

Ra—mean arithmetic deviation of the profile; R_z—the largest height of the profile; Rs_m—spacing parameter/mean width of profile elements; R_pc—average number of vertices per unit length.

.

The microgeometry of the polished steel substrate exhibited lower roughness values compared to the applied DLC coating. During reactive magnetron sputtering, the bias voltage applied to the substrate accelerated the influx of ionized coating atoms. This process also involved a resputter effect, where atoms were dislodged from the growing coating. These dislodged ions then aggregated under the action of surface energy and formed macroparticles. This effect contributed to an increase in the surface roughness of the coating [23].

3.2. SEM EDS Analysis of Coated Samples

Figure 1a shows a W-DLC coating surface with embedded macroparticles. If the energy of the sputtered atoms is insufficient, carbon atoms or alloys with limited mobility accumulate during coating growth and contribute to the formation of clusters or macroparticles on the surface [24]. As reported by Khan et al. [25], the wear rate of DLC coatings increases with increasing coating roughness, with the dominant wear mechanism changing from adhesive to coating fragment formation. Podgornik et al. [26] suggest that polishing the substrate to a roughness (Ra) of less than 0.05 µm before DLC coating formation creates prerequisites not only for excellent wear resistance but also for increased corrosion resistance due to the absence of features like grooves and other local irregularities on the surface.

Determining the thickness of the DLC coating using SEM after cross-sectioning the sample (Figure 1b) was important for measuring the coating’s mechanical properties (hardness H and elastic modulus E). According to [27], the depth of the indentation into the coating should not exceed 1/10th of the coating thickness for accurate measurement. Based on the image in Figure 1, the DLC coating thickness was determined to be around 1.1 µm and the Cr interlayer thickness was determined to be approximately 1 µm.

The EDS SEM map in Figure 2a–d shows the distribution of the main analyzed elements. As can be seen, W and C were present in the coating structure, while Cr was part of the interlayer. The thickness of the intermediate layer was comparable to the thickness of the W-DLC coating itself.

3.3. Nanoindentation Measurements of Hardness and Elastic Modulus Measurements on Samples at Room Temperature and Elevated Temperatures

After the nanoindentation test on the Anton Paar NHT2 device, hardness (H) and elastic modulus (E) were determined on the DLC-coated samples at RT and at elevated temperatures of 100 °C, 200 °C, 300 °C, and 400 °C. The measured values were processed and evaluated using the Oliver and Pharr method [28], which assumes elastic–plastic loading and purely elastic unloading.

Surface roughness affects the contact area size throughout the measurement, but this error is most pronounced at small indentation depths. The surface’s roughness also affects the determination of the indenter’s zero point. According to the ISO/DIS 14577-1.2 standard [27], the indentation depth should be h > 20 Ra.

The measured nanoindentation values are presented graphically in Figure 3. Based on the dependence of hardness in the figure, we can conclude that at room temperature (without thermal influence), the DLC coating had an average hardness of 14.1 ± 1.3 GPa. As stated in study [10], the hardness of DLC coatings depends primarily on the sp²/sp³ component ratio. According to [29], coatings with this hardness are classified as “harder” (with an approximate 40% sp² component). At 200 °C, a drop in hardness to an average value of 13.0 ± 1.0 GPa was observed, followed by further reductions to 12.0 ± 1.0 GPa at 300 °C. As noted in works [30,31], a graphitization process occurs on the coating’s surface above 250 °C, which explains the decrease in H values. At 400 °C and 500 °C, the hardness decreased to 9.4 ± 1.0 GPa and 9.3 ± 1.0 GPa, respectively.

The elastic modulus (E) exhibited a relatively constant trend across the heating temperatures (Figure 3). The average values of E ranged from 169.9 ± 21.2 GPa at 100 °C to 170.5 ± 7.3 GPa at 400 °C and 169.5 ± 9.3 GPa at 500 °C. As stated in [32], Young’s modulus remains almost constant up to 400 °C. Additionally, a higher H/E ratio indicates greater resistance to plastic deformation in the coating [33]. Several authors, such as [29,34], have investigated the H/E ratio dependence for various DLC coatings with different sp² and sp³ proportions. They found that a higher sp² content in the coating significantly reduces residual stress, resulting in lower hardness and higher plasticity.

3.4. Raman Spectroscopy of DLC Coatings at Room Temperature and at Elevated Temperatures

Raman spectroscopy, a non-destructive method, was used to characterize DLC coatings. It was performed on a W-DLC coating at ambient temperature and after heating to 100–500 °C, as shown in Figure 4, to evaluate the nature of its carbon bond structure. The Raman spectrum was fitted to curves using two Gaussian functions, peaking in the disordered (D band) and graphitic (G band) modes, according to [35].

In Figure 4, it can be seen that two Raman peaks were observed at ~1355 cm⁻¹ and ~1560 cm⁻¹, which are characteristic of DLC coatings and are labeled as the D band and G band [36].

An important factor in evaluating the mechanical properties of DLC coatings using Raman spectroscopy is the determination of the I_D/I_G ratios and the G band shift. As the temperature increased, the I_D/I_G ratio increased from 2.6 to 3.8, and the G band shifted from 1564 cm⁻¹ to 1594 cm⁻¹. The increase in I_D/I_G and the shift of the G band to higher wavenumbers with increasing temperature can be attributed to the structural change of the coating, with the conversion of part of the sp³ bonds to sp² in the form of graphitic clusters or layers [37]. As the authors further state, at temperatures above 400 °C, a decrease in the I_D/I_G ratio is possible, suggesting the initiation of the transformation of nanocrystalline graphite to perfect graphite. In our case, a drop in I_D/I_G from 3.8 to 3.4 was recorded. This possible transformation could be correlated with the sharp decrease in the hardness H of the W-DLC coating we tested (Figure 3).

A higher I_D/I_G ratio indicates a higher level of structural disorder and defects, while a lower ratio indicates fewer defects and a more ordered structure. In our case, the formation of defects in the carbon structure increased with increasing temperature. Similarly, in the work [38], the authors showed that the I_D/I_G ratio, which is a measure of the total sp³ content in DLC coatings and is related to mechanical properties such as hardness H and elastic modulus E, changes were related to changes in the abundance of the sp² component.

Raman analysis showed that the formation of WO₃ started at 400 °C, with the observation of peaks at 133, 270, 715, and 808 cm⁻¹. These peaks are commonly attributed to tungsten oxide (WO₃) and correspond to the bending vibration of the bridging oxygen of W–O–W bonds (first two peaks) and stretching vibration (latter two). The characteristic peak at 133 cm⁻¹ is the formation vibration of O–O bonds [39].

To confirm the presence of WO₃ in the W-DLC coating after heating to 500 °C, XRD analysis was performed on the coating at room temperature in reflection mode (Figure 5). The analyzed coating consisted of three phases: the substrate material (represented by iron (Fe)), with a space group of Im-3m and a lattice parameter of 2.867 Å (COD No. 9008536); the interlayer of chromium, with a hexagonal structure, a space group of P63/mmc, and lattice parameters of a = b = 2.722 Å and c = 4.427 Å (COD No. 9008493); and the formed WO₃, with a monoclinic structure, space group P121/c1, and lattice parameters of a = 7.688 Å, b = 7.539 Å, c = 10.515 Å, α = γ = 90°, and β = 136.06° (COD No. 2311041). By comparing the theoretical diffraction data of WO₃ with the obtained XRD spectrum, it is evident that the W-DLC coating, particularly the WO₃ layer, is highly textured.

3.5. Tribo-Test of the DLC Coating at the Tested Temperatures

To evaluate the behavior of the coefficient of friction (CoF) for the W-DLC coating, a tribo-test was conducted using 8000 cycles and a ball-on-disc path radius of 10 mm, as shown in Figure 6. The CoF curves at room temperature and 100 °C are relatively stable, reaching average values of 0.15 and 0.35, respectively. According to [18], W-DLC coatings can maintain good friction resistance between 25 and 200 °C.

In this work, tribo-tests at 200–300 °C showed an increase in CoF with significant fluctuations. The CoF values ranged from 0.5 to 0.65 during these tests. The addition of tungsten (W) to the DLC coating increases hardness and provides a good balance between mechanical and tribological properties for W-DLC coatings with a hardness range of 10 to 15 GPa [31]. As the authors further explain, the presence of an sp³ component on the surface contributes to good tribological properties at temperatures between 100 and 200 °C.

The increase in CoF at higher temperatures is related to the graphitization of carbon. This transformation occurs above 200 °C, where the diamond sp³ component transforms to the graphitic sp² component. Additionally, wear increases above 250 °C due to friction between wear particles generated during sliding contact. Large asperities tend to wear away significantly during sliding, leading to a noticeable increase in fluctuations in the friction curve.

As can be seen in the CoF curve for 300 °C in Figure 6, where abrasive particles are still present, the change in the coefficient of friction can be attributed to the combined effect of increased graphitization and slight oxidation of tungsten to form WO₃. Tribo-tests of the W-DLC coating at 400 °C or 500 °C showed improved tribological properties, with the CoF decreasing to 0.45 and 0.35, respectively, and exhibiting a more stable course. A similar trend was observed by the authors in [36,40]. They also found that a decrease in CoF can be observed at these temperatures due to the formation of WO₃ as the high-temperature oxidation product. As noted by the mentioned authors, friction-induced transformation leads to the dominance of tungsten oxide in the newly formed friction layer at 400 and 500 °C, resulting in a decrease in the friction coefficient.

3.6. SEM Analysis of the Surface of DLC Coatings after the Tribo-Test at Room Temperature and at a Temperature of 300–500 °C

The surface wear mechanism after tribo-tests at different temperatures was investigated using SEM.

Figure 7a,b shows the surface of the W-DLC coating after the tribo-test at room temperature. The images reveal a faint tribological wear track with minimal impact on the overall surface morphology of the W-DLC coating. However, localized degradation is observed around embedded macroparticles that were dislodged from the surface during the test by the pin. After the tribo-test, fragments of the material can be seen in the areas indicated by arrows in Figure 7b. EDS SEM analysis of these fragments (

Table 4. Quantitative EDS SEM analysis (in wt. %) at the location of the tribo-track after the tribo-test at room temperature.

Element	Spectrum 13	Spectrum 14	Spectrum 15	Spectrum 16	Spectrum 17	Spectrum 18
W	5.6	53.7	51.5	54.2	58.3	56.3
C	10.0	30.3	30.2	30.5	20.8	21.9
Cr	15.6	10.9	9.9	10.6	18.3	19.4
O	21.4	3.0	5.5	2.6	-	-
Fe	26.8	2.2	2.6	2.1	1.4	2.4
Si	14.8	-	-	-	-	-
N	5.8	-	-	-	-	-

) confirms the presence of elements essential for the coating’s formation. Spectrum 13 in Table 4 indicates a higher concentration of Fe (likely from the substrate) and O (suggesting oxide formation) in the locations of localized degradation of macroparticles. Silicon (Si) and nitrogen (N) were likely detected from the Si₃N₄ tribo-tester counterpart.

Figure 8a,b show the surface of the W-DLC coating after the tribo-test at 300 °C. The tribo-track in Figure 8 exhibits areas with higher roughness, created by the movement of material over the surface, leading to the exposure of the substrate. Quantitative EDS SEM analysis in

Table 5. Quantitative EDS SEM analysis (in wt. %) at the location of the tribo-track from Figure 8 after tribo-test at a temperature of 300 °C.

Element	Spectrum 24	Spectrum 25	Spectrum 26	Spectrum 27	Spectrum 28
W	54.2	2.7	2.4	-	2.0
C	31.7	3.4	4.6	5.1	4.1
Cr	10.7	26.8	26.7	16.9	27.6
O	2.3	32.3	22.7	8.4	21.6
Fe	1.1	32.0	41.4	67.0	42.7
Si	-	2.2	1.6	2.2	1.5
N	-	0.5	0.7	0.3	0.5

reveals a higher proportion of Fe (Spectrum 25–28).

This observation is further supported by the EDS SEM map of the analyzed elements in Figure 9a–f. The tribo-track in Figure 9f reveals the exposure of the substrate. Based on the distribution of Fe and O elements in Figure 9e,f, the formation of Fe oxides on the surface is likely. Conversely, the elements W and C, which constitute the W-DLC coating, are present on the surface in minimal quantities, as shown in Table 5 (Spectrum 25 and 26) and Figure 9b,c.

The oxidation reaction is accompanied by the wear process, while oxides are formed on the entire surface of the wear track. It is widely known that the oxide layer plays a beneficial role in wear behavior, for self-lubrication and self-protection of the worn surface, as confirmed by the authors in paper [41].

The documented surface of the tribo-track of the W-DLC coating after the tribo-test at a temperature of 400 °C is shown in Figure 10a,b. In this case, areas with a more embossed surface were observed as well. Figure 10 shows areas with a more embossed surface similar to the tribo-track morphology after the 300 °C test (Figure 8).

Quantitative EDS SEM analysis showed that there are areas in the tribo-track with a low proportion of coating material elements (W, C) and a higher proportion of Cr (Spectrum 33, 34, and 37–40;

Table 6. Quantitative EDS SEM analysis (in wt. %) at the location of the tribo-track from Figure 10a after the tribo-test at a temperature of 400 °C.

Element	Spectrum 32	Spectrum 33	Spectrum 34	Spectrum 35	Spectrum 36	Spectrum 37
W	53.6	2.8	2.2	7.0	60.3	2.7
C	27.3	3.6	3.9	3.7	20.4	4.1
Cr	15.6	19.5	17.3	25.2	9.3	20.4
O	2.9	19.0	24.5	29.5	9.0	28.8
Fe	0.6	53.6	49.3	31.6	1.0	41.0
Si	-	1.4	2.1	2.5	-	2.5
N	-	-	0.7	0.5	-	0.4

and

Table 7. Quantitative EDS SEM analysis (in wt. %) at the location of the tribo-track from Figure 10b after the tribo-test at a temperature of 400 °C.

Element	Spectrum 38	Spectrum 39	Spectrum 40
W	3.2	2.0	2.8
C	3.3	3.7	3.2
Cr	13.5	15.6	31.1
O	20.4	26.2	26.5
Fe	57.2	49.7	34.2
Si	1.2	2.2	2.1

). These areas are also found at the edge of the track observed in Spectrum 35 in Table 6.

This observation was also confirmed by the EDS maps of the analyzed elements in Figure 11a–f. The area of the tribo-track is mainly formed by Cr (Figure 11d) and Fe oxides (Figure 11e,f).

Figure 12 shows the surface of the W-DLC coating after the tribo-test at a temperature of 500 °C. The surface of the track exhibits an embossed appearance, with visible layering of material. Abrasive and adhesive wear mechanisms were observed, with adhesive wear being dominant. Quantitative EDS SEM analysis in

Table 8. Quantitative EDS SEM analysis (in wt. %) at the location of the tribo-track after the tribo-test at a temperature of 500 °C; Spectrum 4–7 from Figure 12a.

Element	Spectrum 4	Spectrum 5	Spectrum 6	Spectrum 7
W	34.9	32.1	6.2	18.0
C	4.5	5.4	9.2	7.0
Cr	36.6	39.8	83.5	63.7
O	19.5	18.6	-	10.1
Fe	4.5	4.1	1.1	1.2

shows that the coating material (Spectrum 4 and 5 in Table 8) remained bonded to the surface, while also containing a high proportion of Cr from the interlayer. Along the edges, where the surface is less embossed, a dominant proportion of Cr from the interlayer was found. Based on the quantitative EDS SEM analysis in

Table 9. Quantitative EDS SEM analysis (in wt. %) at the location of the tribo-track after the tribo-test at a temperature of 500 °C; Spectrum 8–12 from Figure 12b *.

Element	Spectrum 8	Spectrum 10	Spectrum 11	Spectrum 12
W	35.7	38.7	25.5	10.8
C	21.1	8.1	15.0	10.2
Cr	31.2	35.3	55.1	78.1
O	10.8	16.0	3.3	-
Fe	1.2	1.8	1.1	0.9

* Spectrum 9 is not listed in Table 9. It was an impurity on the surface of the coating.

, Cr was found to be the element with the highest proportion in the tribo-layer (Spectrum 8–12), followed by the coating material. The analysis also revealed a higher oxygen content in the tribo-track after the 500 °C test (Spectrum 4, 5, and 7 in Table 8; Spectrum 8 in Table 9). Based on the Raman spectroscopy results (Section 3.7), it can be concluded that tungsten oxide was part of the surface of the coating.

In places on the surface where no big groove and large delamination were visible, it was possible to assume that there was no significant plastic deformation and abrasive wear. The authors of [42] also had a similar experience with the wear mechanism at elevated temperatures, but they were studying composite coatings.

The EDS SEM map of analyzed elements in Figure 13a–f shows that the distribution of tungsten is similar to oxygen in several areas. Therefore, the presence of tungsten oxide can be assumed. This is also confirmed by Raman spectroscopy in Figure 14. Along the edges of the track, the highest proportion of Fe and oxygen is visible, indicating the presence of Fe oxides (Figure 13e,f). The distribution of Cr from the interlayer across most of the surface of the tribo-track is very interesting (Figure 13d). Notably, the distribution of Cr (from the Cr interlayer) is uniform across most of the tribo-track area, indicating minimal degradation of the Cr interlayer. A connection between W and C from the coating, which were incorporated into the Cr interlayer within the tribo-track, is also evident.

While various sources in the literature, such as [19,20,36], attribute the enhanced tribological properties of W-DLC coatings at 400 and 500 °C to the presence of a WO₃ tribo-film, our findings suggest that the presence of a sufficiently thick (approximately 1.0 μm) Cr interlayer plays a significant role in improving tribological performance.

3.7. Raman Spectroscopy of W-DLC Coatings at Room Temperature and at Elevated Temperatures after Tribo-Test

Raman spectroscopy confirmed the graphitization of the material inside the tribo-track at various temperatures (Figure 14). The two worn surfaces showed a difference in the I_D/I_G peak ratio and the G band position. It is well known that the maximum value of the I_D/I_G ratio indicates the extent of graphitization in DLC coatings [43], which is associated with an increase in defects within the carbon structure. In our case, after the tribo-test, the I_D/I_G ratio increased from 2.95 to 3.78, and the G band shifted from 1567 cm⁻¹ to 1590 cm⁻¹.

The WO₃ tribo-film formed at temperatures of 300 °C and 500 °C likely contributed to the decrease in the CoF to 0.35, exhibiting a stable course throughout the test. As highlighted in [36], sliding contact at 400 °C or 500 °C can induce the formation of a layer rich in tungsten oxide. At a temperature of 300 °C, the presence of peaks characteristic of iron oxides suggests partial wear in the steel substrate and the formation of Fe oxides.

Figure 14. Raman spectra of W-DLC coating at ambient temperature and provides 100–500 °C after tribo-test.

Figure 14. Raman spectra of W-DLC coating at ambient temperature and provides 100–500 °C after tribo-test.

3.8. A Friction Mechanism Diagram

This mechanism illustrates the wear process of the W-DLC coating with a Cr interlayer at increasing temperatures. This process involved changes in surface microgeometry and the formation of a tribo-oxide film (Figure 15).

4. Conclusions

In this study, the mechanical and tribological properties of W-DLC coatings were tested up to a temperature of 500 °C. Raman spectroscopy was used to investigate the structure and chemical composition of the samples.

Instrumented hardness decreased due to thermal effects on the W-DLC coated samples. Hardness values declined from 14.0 ± 1.0 GPa at 100 °C to 9.3 ± 1.0 GPa at 500 °C. This trend is attributed to the graphitization process and an increase in the sp² component within the coating.

The elastic modulus exhibited a relatively constant behavior across the tested temperatures. Values ranged from 165.5 ± 21.2 GPa at 100 °C to 169.5 ± 9.3 GPa at 500 °C.

The coefficient of friction remained stable at room temperature and 100 °C, and it reached an average value of 0.15 and 0.35, respectively. However, a significant increase and fluctuation in CoF were observed at 200–300 °C, ranging from 0.5 to 0.65. The tribo-tests at 400 °C and 500 °C showed improved tribological properties, with a decrease in CoF to 0.45 and 0.35, respectively, and a more stable course. This improvement is attributed to the presence of a high-quality Cr interlayer and the formation of the high-temperature oxidation product, WO₃.

Raman spectroscopy of the W-DLC coatings before the tribo-test confirmed the shift of the G band from a value of 1564 cm⁻¹ to 1594 cm⁻¹ depending on the increasing temperature. The change in the I_D/I_G ratio indicated the increase in defects in the carbon structure. The shift of the G band from a value of 1567 cm⁻¹ to 1590 cm⁻¹ was also detected after the tribo-test, as well as the increase in the I_D/I_G ratio from a value of 2.95 to a value of 3.78. Raman analysis also showed the presence of tungsten oxide on the W-DLC coating at higher temperatures for samples before and after the tribo-test.

The improvement in tribological properties at temperatures of 400 and 500 °C with advancing wear time could be attributed, in addition to the presence of tungsten oxides, to the Cr interlayer with sufficient thickness before grinding into the substrate.

Our findings are expected to be helpful in guiding the development of element-doped DLC coatings to achieve superior performance characteristics for high-temperature applications.