1. Introduction
In order to meet the demands of rapidly advancing technology, it is critical that frictionally interacting components have sufficient tribological, mechanical, and corrosion-resistant properties for enhanced reliability and service life. These properties are strongly dependent on the condition of the surface layer. The properties of the component surface layer are determined by the choice of method and finishing parameters [
1,
2,
3,
4].
Depending on the intended use of the components, various mechanical treatments are used, such as grinding, sandblasting, polishing, and surface modification by the deposition of coatings [
4,
5]. The extended life of the components is a constantly evolving topic [
6]. Improvements in functional properties are achieved through the use of conventional and unconventional technologies, which include laser and electrical discharge machining, electrochemical and chemical deposition, additive technologies (3D printing) [
7], and modern methods of coating deposition by vacuum techniques [
8,
9,
10], such as CVD and PVD gas-phase deposition [
11], often further assisted by plasma [
11,
12,
13], or ALD atomic layer deposition [
14]. The main advantage of these methods is the improvement obtained in the material properties without changing its dimensions. The coating thickness ranges from 100 nm to 5 µm.
In addition to ensuring enhanced friction and wear performance, coatings serve decorative and protective purposes [
12,
13].
The cutting tool industry has become an essential market for protective coatings. Their important functional characteristics are high hardness, wear resistance, and chemical inertness. These properties are inherent in diamond-like carbon (DLC) coatings consisting of bonds such as sp3, characteristic of diamond; sp2, characteristic of graphite; and sp1 with α and β carbine structures [
15,
16,
17]. Due to their low friction coefficient, corrosion resistance and biocompatibility, DLC coatings are used in medicine for hip, knee, or shoulder implants, blood pumps, heart valves, or artificial heart components [
18,
19,
20,
21,
22].
Various types of interlayers and doping with metallic or non-metallic elements are used to improve the adhesion and functional properties of coatings (
Figure 1).
Doping DLC coatings improve adhesion, friction-wear, electrical and thermal resistance, biocompatibility, surface energy, and reduced internal stresses [
23]. Among the elements most commonly chosen for doping, tungsten enhances tribological properties at elevated temperatures.
Figure 1.
Effects of chemical elements on DLC coatings [
24].
Figure 1.
Effects of chemical elements on DLC coatings [
24].
Yetim et al. [
25] studied the effect of doping a DLC coating with Ti, Al, and V on structural, mechanical, and tribological properties. The sp3/sp2 bond ratio increased, as did hardness, due to the formation of hard carbides; moreover, the doping reduced residual stresses and increased adhesion strength, hardness, and tribological wear resistance.
Bai et al. [
26] evaluated the mechanical and tribological properties of three types of diamond-like DLC coatings, undoped and doped with Si and W, in friction-wear tests under dry friction in reciprocating motion. They also evaluated the effect of sliding velocity on the adhesion and wear of the coatings. The dominant wear mechanism of Si-DLC coatings was oxidation, which increased with increasing sliding speed. The degree of the graphitization of undoped and W-doped DLC increased with sliding speed, resulting in a lower coefficient of friction and reduced wear rate. The wear rate of W-DLC coatings with an oxidized surface increased considerably at a medium sliding speed. At high sliding speeds, this increase was smaller.
Piotrowska et al. [
27] examined a-C:H:Si coating applied to the surface of Ti13Nb13Zr titanium alloy subjected to different treatments. Polished (Ra = 0.05 µm) or sandblasted specimens (Ra = 1.41 µm) were compared, microscopic observations, thickness, adhesion, wettability, and surface topography were measured, and tribological tests were performed. The tribological tests were conducted in reciprocating motion under dry friction and lubrication with the cutting fluid. The adhesion of coatings applied to polished surfaces was higher than on sandblasted surfaces. For the a-C:H:Si deposited coating, lower friction coefficient values and linear and volumetric wear values were obtained on the polished specimen compared to the a-C:H:Si coating applied to the sandblasted disc. In addition, the cutting fluid was found to reduce the friction coefficients of the coating applied to the sandblasted specimen compared to the polished specimen without the coating by about 94%. Microscopic observations of wear traces allowed the authors to identify wear mechanisms; in the case of Ti13Nb13Zr, it was tribochemical wear by oxidation, while scratching and micro-scratching dominated in the case of coatings. The authors of [
28] presented the results of a study of the tribological properties of diamond-like DLC coatings doped with tungsten deposited on 100Cr6 steel. The hardness of the DLC coating was determined using a microhardness tester. Friction-wear tests were carried out using a ball-on-disc tribometer at loads of 10 N, 25 N, and 50 N under dry friction. The lowest value of the friction coefficient was obtained at a load of 50 N. It was shown that DLC a-C:H:W type prepared by the PVD technique can be used in non-lubricated tribological systems operating under high loads.
Kadam and his team [
29] compared the wear of AISI 4140 alloy steel nitrided without a coating and with a WC/C (a-C:H:W) coating applied by physical vapor deposition. They performed friction-wear tests on a pin-on-disc tribotester under dry friction conditions. After nitriding, the WC/C coating on 4140 steel showed significantly less surface wear than the alloy steel without coating. The results were correlated with the properties determined from the tribological and mechanical characterization.
The authors [
30] compared the Cr-GLC coating (made of graphite-like carbon doped with chromium) with the Cr-DL coating (made of diamond-like carbon doped with chromium), which they applied to the Al-Si alloy. They performed tribological tests both dry and with base oil lubrication. They observed that the coefficient of friction under lubricated conditions was reduced by 40% compared to dry friction. They concluded that the Cr-DLC coating was characterized by greater anti-wear resistance under dry and lubricated friction conditions than the Cr-GLC coating, by 15% and over 60%, respectively.
Humphrey et al. [
28] focused on comparing the impact of DLC coating on the performance of electric vehicle transmissions compared to a standard steel automotive gear set and a polished steel set and improving the performance of electric car transmissions in extra-urban and urban driving. They found lower viscosity for DLC-coated gears compared to steel gears. DLC coatings will save 1.1 km of range in extra-urban driving and 0.9 km in urban driving.
Podgornik [
31] investigated the differences in the behavior of the WC/a-C:H:W coating under conditions of technically dry friction and various lubricants. The tests used PAO, diesel, and gasoline as lubricants and were carried out in various combinations: steel/steel, steel/W-DLC, and steel/DLC. He confirmed that DLC coatings achieve better tribological results during technically dry friction or boundary lubrication than with lubrication. The best tribological properties were obtained for the steel/DLC combination. The use of DLC coatings extends the life of the components used five times.
Kržan et al. investigated [
32] the influence of the chemical composition of the lubricant on the tribological properties of a tungsten-doped diamond-like coating during oil lubrication in reciprocating motion. They applied a load of 10 N, increasing to a load of 50 N. The tests lasted 1000 s. The lowest value of the friction coefficient was recorded when using rapeseed oil without additives. It was found that the selected additives were not effective enough to maintain the specified anti-wear boundary layer. The surface of the abrasion mark after lubrication with oils with a very low content of impurities was covered with a tungsten-rich tribofilm.
The present study examined a-C:H:W coatings applied by PVD. The results of tribological tests performed at loads of 10 N and 50 N under lubrication with Swisscool 3000 cutting fluid were compared. Surface structure, mechanical properties, and wettability were also evaluated. There is currently no work reported on the use of Swisscool 3000 in the study of DLC coatings.
3. Results
3.1. Surface Morphology
Figure 5a shows an image of the a-C:H:W coating microstructure at a magnification of ×2000 against the X-ray characteristic spectrum, and
Figure 5b shows the X-ray characteristic spectrum for tungsten inclusions.
Table 4 shows the obtained chemical composition of the tested coating.
An example X-ray characteristic spectrum was made by averaging the analysis of three different tungsten inclusions. The a-C:H:W coating obtained by physical vapor deposition (PVD) was characterized by a heterogeneous structure. Microstructure studies indicated the presence of tungsten inclusions on the surface of the a-C:H:W coating in the form of beads ranging from 2 µm to 5 µm in diameter.
3.2. Coating Thickness
Figure 6 shows the linear elemental distribution, and
Figure 7 shows the SEM image of the cross-sections along with the thickness measurement. The thickness of the coating was determined from observations in three areas.
Linear analysis of the chemical composition of the a-C:H:W coating showed that the near-surface layer consisted of tungsten and carbon. At a depth of about 2.9 µm from the surface, chromium was observed, constituting the interlayer. It was applied to ensure adequate adhesion of the coating to the metallic substrate. The analysis also indicated the presence of iron from the substrate.
The PVD process resulted in a coating thickness of 3.06 ± 0.1 μm and an interlayer thickness of 0.32–0.48 μm.
3.3. Hardness
Figure 8 presents the load–unload curve as a function of indenter penetration recorded during the hardness test for the reference material and the coating.
Table 5 summarizes the most important mechanical parameters of the tested materials.
The tests show that the instrumental hardness for the coating was 11.72 GPa, which was almost three-fold higher than for the substrate material. The plastic work for the coating was about 73% lower than for 100Cr6 steel, and the elastic work was about 71% lower for the reference specimen than for the coating.
3.4. Contact Angle
Figure 9 shows sample images of droplets of the measuring fluids, and
Figure 10 summarizes the average values of wetting angles for the tested surfaces. Measurements were made with demineralized water and the cutting fluid used during tribological tests.
The test results indicated that both 100Cr6 steel and a-C:H:W coating had good wettability. This is evidenced by the obtained values of wetting angles below 90° for both test fluids used (demineralized water and cutting fluid). The average value of the wetting angle with demineralized water was about 27% smaller for the coating than for 100Cr6 steel. The a-C:H:W coating wetting angle for the cutting fluid was about 28% smaller than for the reference material.
3.5. Tribological Tests
Figure 11 and
Figure 12 illustrate example waveform of friction coefficients and average values of this parameter.
Figure 13 and
Figure 14 present example waveforms of linear wear and its average values.
The results of friction-wear tests indicated that 100Cr6 steel exhibited more turbulent friction coefficient plots, both for tests conducted with the 10 N load and 50 N load. In addition, it was observed that for the 10 N load, the coated surface had a lower friction coefficient value of 0.1, which was about 29% lower compared to that of the 100Cr6 steel. For the load of 50 N, the lowest friction coefficient value was obtained for the a-C:H:W coating of 0.11; it was about 15% lower than the value obtained for 100Cr6 steel. It was observed that as the load increased, the coefficient of friction decreased by about 7% for the steel and increased by about 9% for the a-C:H:W coating.
A more stable character of the linear wear changes was observed for the a-C:H:W coating, both at 10 N and 50 N load. After a friction distance of 500 m for a load of 50 N, there was an apparent increase in linear wear for the coating, reaching higher values than 100Cr6 steel. This was most likely due to the more intensive counter surface wear, namely the 100Cr6 steel ball. This required further study using confocal microscopy to determine the wear of the specimens and counter surfaces at the friction nodes analyzed.
When the friction node was loaded with a force of 10 N, approximately 10% lower values of average linear wear were obtained for 100Cr6 steel compared to the a-C:H:W coating. The higher value of the linear wear of the a-C:H:W coating—100Cr6 steel friction node (39.46 μm)—is most likely due to the greater wear of the counter surface, i.e., the ball. When the friction node was loaded with a force of 50 N, the values were similar but slightly smaller for the coating.
3.6. Evaluation of Surface Morphology after Tribological Tests
No wear traces were observed on the a-C:H:W-coated disc under a load of 10 N, while a circular wear trace with a diameter of about 800 µm was measured on the counter specimen. An abrasion trace with a width of 400 µm was found on the 100Cr6 steel and a 450 µm trace on the ball. It was about 44% smaller than that on the ball with the coating after friction.
A 100 µm wear trace was recorded on the a-C:H:W-coated specimen at a load of 50 N, and a 900 µm wear trace was recorded on the counter specimen. Also, traces with a width of 550 µm were observed on 100Cr6 steel. On the ball, it took an irregular shape with a diameter of about 400 µm. It was more than 55% smaller than during friction with the coating. At the same time, the wear trace of the a-C:H:W coating was more than 81% smaller than that for 100Cr6 steel, demonstrating its viability for improving wear resistance. After friction-wear tests, local cracks and chipping were observed on the surface of the coating. Regardless of the applied load in the case of steel discs and balls, the dominant wear mechanism was abrasive wear with furrowing and micro-scratching, and local material transfers.
3.7. Assessment of Surface Geometric Structure of Samples
Figure 23 and
Figure 24 show axonometric images and examples of surface profiles after tests with 10 N and 50 N loads.
Table 6 summarizes the volumetric wear of the balls after tribological tests for both loads.
After tribological tests performed at a load of 10 N, no wear trace was observed on the a-C:H:W coating. This proves the high wear resistance of this material. Wear trace with a width of 400 µm and indentations of 1.3 µm were measured on the 100Cr6 steel specimens.
Figure 25 and
Figure 26 show optical images of the ball wear traces after tribological tests under 10 N and 50 N loads.
At 50 N, the wear trace for 100Cr6 steel was about three-fold wider and deeper compared to the a-C:H:W coating, indicating the higher wear resistance of the tungsten-doped diamond-like coating.
Table 6 shows the volumetric wear of the balls after friction-wear tests.
The tabulated results show higher ball wear after tests with the a-C:H:W coating. At loads of 10 N and 50 N, the wear of the ball in contact with the uncoated steel disc was 11-fold and 4-fold lower, respectively, than in contact with the coated steel disc.
Figure 27 shows the average values of the wear trace depth and area after tribological tests. Due to the lack of a visible wear trace on the coating at a load of 10 N, it is not included in the graph below.
The smallest average value of the wear depth was recorded for the a-C:H:W coating at a load of 50 N. It was about 55% smaller compared to the uncoated steel disc. As for the average wear area values, at the same load, the smallest value was also obtained for the a-C:H:W coating. The average wear trace area for 100Cr6 steel was about 15-fold higher than that of the steel disc with the coating applied to it.