Combined Effects of DLC Coating and Surface Texturing on Seizure and Friction in Reciprocating Sliding

Abstract

Surface texturing is designed to improve the functional properties of machine elements by generating dimples on the surface contacted. Friction and wear resistance can also be improved by creating diamond-like carbon (DLC) coatings. These two techniques were combined to extend the lifetime of the elements and minimise friction in reciprocating conformal sliding contact. This work is functionally important for assemblies operating under high normal loads. Experiments were carried out in initially lubricated reciprocating sliding contact using an Optimol SRV 5 tribotester in the flat-on-flat configuration. The disc samples were untextured, laser textured, and DLC-coated untextured and textured. The combination of DLC coating and surface texturing caused an enhancement of the tribological performance of the sliding pair compared to that of untextured discs with and without DLC coating and textured discs without DLC coating. The DLC coating of the untextured disc caused a growth in the lifetime of a friction pair by a factor of 2.4. Seizure resistance also increased due to surface texturing of the steel disc for pit area ratios of 9 and 13%. Combining surface texturing with pit area ratios of 3 and 9% and DLC coating led to a decrease in the coefficients of friction of sliding pairs compared to only textured and coated discs. The DLC coating caused a decrease in the wear of the disc sample and reduction in wear levels of the counter samples in comparison to those of textured discs without DLC coatings.

1. Introduction

Well-lubricated and long-lasting surfaces are important in industry because they reduce friction and wear, which helps equipment run reliably, leading to increased productivity and efficiency. Improvements in tribological behaviour can be achieved by surface texturing and DLC coating.

Surface texturing relies on generating dimples (oil pockets) or grooves on machine elements in sliding and rolling contacts. This technique leads to an improvement in friction, especially under mixed or boundary lubrication [1,2]. Surface texturing can also result in wear improvement [3]. Lu and Wood [4] reviewed surface texturing applications in mechanical engineering (cutting tools, piston rings and cylinder liners, sealing, and journal bearings). Surface texturing can also be used to improve the quality and adhesion of anti-wear and low-friction coatings [5,6].

Surface texturing can also cause a decrease in oil demand, leading to an improvement in seizure resistance [7]. The growth of the oil film lifetime caused by surface texturing was previously studied. Andersson et al. [8] prolonged the life of the oil film by a factor of up to eight by combining the selection of pit area ratio (density of oil pockets) and depth of the dimple under conditions of mixed lubrication. Hu and Lu [9] achieved an increase in the lifetime of the oil film by a factor of nine. Duarte et al. [10] proved that a cross-like pattern caused an increase in the lifetime of the oil film by a factor of 16 under mixed lubrication. Rosenkranz et al. [11] created periodic cross-like patterns in stainless-steel samples. Due to surface texturing, the oil film lifetime increased by a factor of 130 compared to the untextured surface in the pin-on-disc configuration in rotational sliding during mixed lubrication. Koszela et al. obtained a growth in the lifetime of a steel–bronze assembly in the ring-on-block configuration by a factor of five [12].

Researchers carried out seizure resistance tests by increasing loads. Kuroiwa et al. [13] improved the seizure resistance of a lead-free copper alloy in contact with ductile iron in the ring-on-ring configuration under an incrementally increased load. The texture specimens presented superior anti-seizure properties in comparison to those of untextured surfaces. The influence of the array of dimples was substantial. Galda et al. [14] found that surface texturing of a steel surface in contact with cast-iron in the block-on-ring configuration with increasing normal load could lead to increased resistance to seizure. Oil pocket shape and orientation were very important factors. Fan and Zhong [15] improved the seizure resistance of cast-iron cylinder liners by creating dimples. The experiment was carried out by increasing the load under starved lubrication in reciprocating motion.

Creating wear-resistant coatings can lead to a reduction in tribology losses [16]. Diamond-like carbon (DLC) coatings are characterised by low wear and high frictional resistance [17]. The applications of DLC coatings frequently led to an improvement in seizure resistance. Iwata et al. [18,19] reduced friction and improved seizure resistance by applying DLC coating to the crank journal. Usui et al. [20] achieved a significant improvement in the anti-seizure characteristic of DLC-coated piston pins compared to uncoated pins. Wu et al. [21] obtained a substantial increase in the scuffing limit temperature by coating gears with DLC compared to the grinding state.

Surface texturing can be combined with DLC coating to improve tribological properties. Arslan et al. [22] achieved an improvement in the tribological performance of a DLC coating by surface texturing in boundary reciprocating sliding. This improvement was obtained for specific sizes of dimples (diameter and depth). Shum et al. [23] obtained decreases in friction and wear for the combination of DLC coating and surface texturing for appropriate dimple density and depth of the dimple under reciprocal lubricated conditions compared to untextured DLC coatings. Oil pockets enhanced lubricant retention and were traps for wear particles. Amanov et al. [24] achieved an improvement in the tribological behaviour of a Si–DLC coating by surface texturing under lubricated reciprocating sliding at several temperatures (from room temperature to 200 °C). Mishra and Penchaliah [25] found that, under all lubrication regimes, coated cylinder samples with oil pockets had better frictional behaviour than untextured coated samples in reciprocating motion. Koskinen et al. [26] experimentally studied the influence of DLC coatings and surface texturing in contact with a steel ball on the coefficient of friction (COF) in reciprocating lubricated sliding. Friction decreased as oil capacity increased. Ding et al. [27] improved the tribological behaviour of DLC coatings under water-lubricated conditions by surface texturing. The creation of dimples resulted in a reduction in the COF and an extension of coating lifetime. Dufils et al. [28] analysed the combined effects of surface texturing and DLC coatings on the tribological behaviour of the thermoplastic polymer PEEK in reciprocating sliding. Coated PEEK textures, characterised by shallow dimples and a low pit area ratio, caused reduced wear and friction in distilled water in comparison to smooth DLC-coated PEEK. Koszela et al. [29] studied the effects of cylinder surface texturing and DLC coating on the functional performances of internal combustion motorcycle engines. The best behaviour was achieved for textured cylinders with a DLC coating (an increase in maximum speed and effective power). Vallinot et al. [30] studied the individual and combined effects of DLC coating and surface texturing in the lubricated pin-on-disc configuration. The combination of DLC coating and surface texturing could cause a decrease in friction compared to the behaviours of only coated disc samples, especially under severe operating conditions.

It is obvious from the analysis of the literature that the combined effects of DLC coating and surface texturing on the seizure and the friction resistances under high normal loads and conformal contact under starved lubrication have not been experimentally investigated yet. However, similar instigations were performed for lower load conditions [25,29].

This study aims to answer the following question: Can the combination of DLC coating and surface texturing cause improvement in tribological behaviour of sliding elements under hard working conditions (high normal load, starved lubrication, and reciprocating motion)? The positive effect of surface texturing on friction reduction can disappear as a result of wear. The application of a DLC coating to textured surfaces can prolong this effect. The combined effect of surface texturing and DLC coating can also improve the seizure resistance of sliding elements in difficult working conditions. This technique can be used for internal combustion engine cylinder liners and valve guides.

2. Materials and Methods

The tests were performed in an initially lubricated reciprocating motion using an Optimol SRV 5 tribotester in the disc-on-disc configuration (Figure 1). A small disc (counter sample; object 2 in Figure 1) of 21 mm diameter and 5 mm thickness co-acted with a disc (sample; object 4 in Figure 1) of 25 mm diameter and 7.9 mm thickness. The contact was conformal with a ring shape, with an outer diameter of 20 mm and an inner diameter of 17 mm. All the counter samples (small discs) were untextured; however, the samples (discs) were in different configurations, such as textured and untextured, DLC-coated untextured, and DLC-coated textured. During the tests, 3 different samples were tested for each configuration. The roughness amplitude of the untextured disc samples and the counter sample, defined by the Ra parameter, was 0.09–0.14 µm. This parameter also characterises the roughness of the base surface of samples free of dimples (for both DLC-coated and uncoated samples).

The dimples were created using a SpeedMarker 300 laser engraver (Trotec, Marchtrenk, Austria).

Table 1. Texturing parameters.

Parameter	Values
Laser power	20 W (100%)
Focal length	254 mm
Diameter of the beam focusing area	64 µm
Pulse duration	1.5 ns
Pulse repetition rate	820 kHz
Feed speed	200 mm/s
Laser path determination type	Wobble (spiral path around designated shape)
Laser power	20 W (100%)

presents the texturing parameters. The engraver was equipped with an ytterbium fibre laser. Due to the laser characteristics, the beam parameters were set to minimalise material buildups around the oil pocket. No additional polishing was needed before coating with DLC or before the tribological tests with the uncoated samples. The assumed pit area ratios were achieved by changing the diameters of the circular dimples.

Laser surface texturing was performed after grinding and before DLC coating. BALINIT^® DLC coatings were applied by the Oerlikon Balzers firm (Balzers, Liechtenstein). Material a-C:H was used for coatings with PACVD coating process technology at a temperature smaller than 250 °C. The hardness of the coating according to ISO 14577 [31] was 15–24 GPa. The maximum available working temperature was 300 °C. The application of a DLC coating is ideal for extreme operating conditions, such as high loads, high and very low sliding speeds under starved lubrication, and, occasionally, dry contact conditions. The coated surface protects against excessive abrasion and adhesive wear under high loads.

The displacement frequency was 20 Hz, the stroke was set to 3 mm, and the normal load was set to 1000 N. The pressure in the contact area was 11.47 MPa. Discs and small discs from steel 42CrMo4 with a hardness of 44 ± 2 HRC were tested. The initial temperature was 30 °C. Before each test, a drop of mineral oil L-AN-46 (0.08 ± 0.01 mL) was placed on the inner side of the contact zone. The amount of oil was sufficient to fill all the lubrication pockets in the contact zone. The kinematic viscosity of this oil at 40 °C was 46.0 mm²/s, the kinematic viscosity at 100 °C was 6.66 mm²/s, and the viscosity index was 96. The combined impacts of DLC coating and surface texturing were previously studied in reciprocating motion [22,23,24,25,26,27,28,29,30]. Therefore, the experiment was carried out in this configuration. The conformal contact was chosen because of the possible application of the results of this research (cylinder liners and valve guides). Testing parameters were selected to obtain difficult working conditions leading to seizures.

The duration of the experiment was 2 h when the seizure did not occur and was stopped when the seizure occurred (COF reached 0.3 value). COF was calculated as the average value of the maximum friction coefficients (in reversal points) of each cycle after every second. The friction coefficients under mixed lubrication [32] were analysed. The surface textures of the disc samples before and after the tribologic tests were measured using a Talysurf CCI Lite white light interferometer, with an objective 5×. The measuring area of 3.29 mm × 3.29 mm contained 1024 × 1024 data points. The measured surfaces were levelled only; digital filtration was not used. Worn surfaces were also analysed by Phenom ProX scanning electron microscopy (SEM) (Phenom-World BV, Eindhoven, The Netherlands).

Circular oil pockets were created on the surfaces of the disc samples. Three types of textured surfaces were formed. The pit area ratios were 3, 9, and 13%, which correspond to dimple diameters of 0.2, 0.3, and 0.4 mm and to maximum depths of 30, 20, and 15 µm, respectively. Differences in depth and diameter resulted from different volumes of single oil pockets, which were 0.0005, 0.001, and 0.0015 mm³, respectively. The distances between the centres of each dimple were the same in all cases. The pit area ratio was calculated as the ratio of the area occupied by the dimples to the entire surface area. Figure 2 presents pseudo-colour views and extracted profiles of textured disc surfaces.

Figure 3 presents photos of untextured and selected textured coated and uncoated surfaces. One can see that the DLC coating made the colours of the samples darker. However, the DLC coating had negligible effects on surface textures.

3. Results and Discussion

Figure 4 shows the friction coefficients versus time for assemblies having untextured samples without and with DLC coatings. Non-coated disc surfaces had a high tendency to seizure. The COF increased to values near 0.2, and then for all samples, it increased considerably. For two repetitions of tests, the coefficients of friction had values higher than 0.3, and the tests were stopped. For the third case, the COF increased to 0.28 and then decreased to 0.16, so the test was not completed.

The introduction of a DLC coating improved seizure resistance. For two test repetitions, the friction coefficients were between 0.1 and 0.13; in the remaining case, the friction coefficients were between 0.15 and, finally, 0.17. Generally, DLC coating of the disc caused a growth of the friction pair lifetime by a factor of 2.4 (ratio of the test duration to mean time of seizure point) and caused a reduction in the coefficient of friction compared to the assembly with the uncoated disc.

Figure 5 presents the friction coefficient versus time for the textured disc sample with a pit area ratio of 3% without and with DLC coatings. For two uncoated samples, the coefficients of friction after initial variations gained a value of 0.15 after 300 s. The seizure occurred for one sliding pair with an uncoated disc surface after 2300 s. For the second repetition, after 400 s, the coefficient of friction was rather stable. For the third sliding pair containing textured surfaces without DLC coating, the coefficient of friction reached 0.13 after 200 s and then decreased to a value close to 0.1.

Sliding pairs with coated textured discs after initial fluctuation obtained smaller values compared to sliding pairs with uncoated surfaces up to a test duration of 5300 s. During sliding, for two test repetitions, the coefficients of friction were rather stable. For the third repetition of the test, the COF, after reaching a value of 0.12 in 200 s, decreased as the test progressed. The final friction coefficients for the assemblies having coated textured disc samples were between 0.03 and 0.12.

Figure 6 presents a friction coefficient as a function of time for the assemblies that had textured disc samples with a pit area ratio of 9%. Surface texturing without DLC coating did not cause a seizure. For two test repetitions, the COF, after initial fluctuation, obtained a value of 0.155 after 1000 s and then slowly increased. For the third repetition, the COF obtained a value of 0.15 after 300 s and slowly decreased.

Different results were obtained for the textured disc surfaces with DLC coatings. After initial fluctuations, the friction coefficients stabilised after 200 s and obtained values of about 0.11. Then, after 1500 s, they decreased and obtained final values between 0.05 and 0.075.

Figure 7 presents a friction coefficient as a function of time for the assemblies with textured disc samples for a pit area ratio of 13%. For two repetitions of tests, the assemblies with textured discs, after initial instabilities, obtained a friction coefficient of 0.165 after 300 s, and from this time, the friction coefficients slowly decreased. For the other repetition, the friction coefficients, after 200 s, obtained a value of 0.15 and then decreased.

Different situations occurred for the textured disc surfaces with DLC coatings. Before 2800 s, similar to the textured surfaces with other pit area ratios, the friction coefficient was smaller compared to only textured disc surfaces. For one repetition of the test, from this time, the friction coefficient slowly decreased. For two other repetitions, the friction coefficients increased and gained final values between 0.15 and 0.165.

Figure 8 presents the mean values and error bars of the COF in the initial period after friction stabilisation (between 1000 and 2000 s), in the final test part (the last 100 s), and in the entire tests (after 1000 s); in the last two cases when a seizure occurred, the coefficients of friction before sharp increases were taken into account. The effects of surface texturing and DLC coatings on the final values of the coefficient of friction were assumed to be the most important; therefore, the initial 1000 s was not analysed. The behaviours of the friction pairs after friction stabilisation (between 1000 and 2000 s) were studied because of friction reduction due to DLC coatings obtained for all used samples.

In the initial test part (Figure 8a), surface texturing in all analysed cases caused decreases in COF; however, these reductions were low (up to 7%) compared to the untextured samples. The DLC coating caused a decrease in the COF of approximately 20% compared to the untextured sample without the DLC coating. A higher friction reduction was achieved by the combination of surface texturing and DLC coating compared to only surface texturing, and the highest decrease was 38%. This performance was obtained for the textured samples with all pit area ratios analysed.

A decrease in the COF due to surface texturing was also achieved in the final parts of the tests (Figure 8b) for all textured sliding pairs of approximately 20%. Application of the DLC coating to an untextured disc sample caused a reduction in the COF of 28%. In contrast to the initial test part, the application of a DLC coating to textured surfaces caused a decrease in the COF only for samples with pit area ratios of 3 and 9%. The highest decrease in the final friction coefficients was near 60% for a pit area ratio of 9%. The non-beneficial impact of surface texturing on resistance to motion was caused by an increase in friction for two repetitions of the test after 1600 s (Figure 7).

The application of surface texturing also caused a decrease in the friction coefficient of approximately 20% in all tests (Figure 8c). The use of DLC coating for an untextured surface also caused a decrease in the COF of 20%. Similar to the final test part (Figure 8b), the DLC coating did not cause a substantial reduction in the COF for the sliding pair containing a textured disc sample with a dimple density of 13%. For the other textured assemblies, the reduction in the COF was about 50%.

Figure 9 presents photos of sliding pairs after the seizure process, with marked corresponding areas where the seizure occurred. In all the cases analysed, the seizure occurred on one side of the disc samples. Tracks of seizure are visible both on the disc sample and counter sample. The smallest scars of the textured sample and counter sample can be seen (Figure 9e,f).

Figure 10 presents examples of non-seizured sliding pairs after the tribologic tests. Wear tracks are not visible in the untextured disc sample with the DLC coating (Figure 10a). However, one can see wear traces on the steel counter sample (Figure 10b). The textured disc without the DLC coating with a pit area ratio of 3% (Figure 10c) showed wear tracks, which were also visible on the counter sample (Figure 10d). For the textured disc sample without the DLC coating with an oil pocket density of 13% (Figure 10e), the wear tracks were more uniform compared to those obtained for the pit area ratio of 3% (Figure 10c). However, the counter samples shown in Figure 10d,f look similar. Wear tracks are not visible in the textured disc samples for a pit area ratio of 3% with the DLC coating shown in Figure 10g. A similar view occurred on a textured disc with a DLC coating for a pit area ratio of 9%. However, wear tracks are visible on the DLC-coated textured disc with a pit area ratio of 13% (Figure 10i). It is difficult to quantitatively compare the wear volumes of the disc surfaces; the wear depended only on the truncation of the highest roughness peaks. The DLC coating caused a decrease in the wear of the disc samples. Unlike the counter sample that co-acted with an untextured disc with the DLC coating (Figure 10a), the counter samples that co-acted with textured discs with pit area ratios of 3 and 13% with DLC coatings do not show wear tracks (Figure 10h,j). In the case of the non-textured samples covered with DLC, the high hardness of the applied DLC layer caused increased wear of the counter sample. The use of surface texturing without the DLC coating resulted in wear traces that corresponded to the array of lubrication pockets. However, the combination of the DLC layer and laser texturing did not cause such effects.

Figure 11 presents isometric views of the disc surfaces after seizure and views of disc surfaces not subjected to seizure after the tests. Figure 11a,c,e,g show surfaces without the DLC coating after the test; the other graphs presented in Figure 11 are surfaces that are DLC coated. The untextured surface without DLC coatings presented in Figure 11a corresponds to the disc shown in Figure 9a. The seizure process resulted in the formation of valleys and peaks created by adhesion, and consequently, the height of the roughness increased compared to the ground surfaces, for which the maximum amplitude was approximately 2 µm. The textured surface with a pit area ratio of 3% without the DLC coating presented in Figure 11c corresponds to the disc shown in Figure 9e. In this case, the seizure resulted in the formation of new peaks and an increase in the height of roughness, which was lower compared to the untextured surface presented in Figure 11a. Among the surfaces shown in Figure 11a,c, the seizure area was the smallest for the last-mentioned surface. For the surfaces not subjected to seizure, wear tracks were visible on the textured surface with a pit area ratio of 9% without the DLC coating (Figure 11e) and not visible on the textured surface with an oil pocket density of 13% without the DLC coating (Figure 11g). The destruction of the DLC coating was visible on the textured surface with the highest pit area ratio of the 13% around the edge of oil pocket (arrow in Figure 11f). A similar removal of the DLC layer was observed in other parts of the coated disc sample with a pit area ratio of 9%. In all cases, use of a DLC-coated surface caused low non-measurable wear, except for DLC coating destruction around the biggest oil pockets. The use of the surface with the highest pit area ratio of 13% also resulted in non-measurable wear. However, the COF was higher in this case. The low wear was probably due to the largest volume of oil pockets with a pit ratio of 13% and the ability to capture the wear products efficiently.

Figure 12 presents views of the disc sample surfaces before and after the tribological tests obtained using the Phenom ProX SEM microscope.

The results obtained with the Phenom ProX SEM microscope show that wear of DLC-coated textured surfaces occurred at the outer area close to the oil pocket. Typical elemental analysis on the textured DLC-coated surfaces before the tribologic tests showed more than 96% of the carbon weight concentration, as in point 1 in Figure 12a. The scars of the greatest wear were visible on the textured DLC-coated surface with the highest dimple density of 13% (and of the highest dimple diameter of 0.4 mm) in regions that corresponded to the zone affected by heat by laser texturing outside of the oil pocket. The behaviour of the sample with a pit area ratio of 13% was shown because of the highest wear of the DLC-coated disc. Elemental analysis performed with a SEM microscope shows that the base material was at point 1 in Figure 12b (79% Fe; 14% C; 6% O of the weight concentration) with some oxidations. The regions at point 2 (inside oil pocket) and point 3 (outside oil pocket) show similar and high carbon concentrations (around 99% of weight) typical for regions coated with DLC. In some cases, regions, as in measurement point 4, were visible with a highly oxidised area inside the oil pocket. In this region, the elemental concentration was around 69% C, 17% O, 13% Fe of the weight concentration. This suggests that, during wear, regions that were highly oxidised had low adherence to the DLC layer. In places of wear around the oil pockets, the DLC coating was removed. This phenomenon could occur due to two mechanisms. Perhaps the cohesion between the heat-affected zone and the DLC layer was lower than that in other regions. As a result of the large pit area ratio and the high load, stress was probably concentrated on the edges of the oil pockets. The impact of limited lubricant in the contact region due to the highest oil pocket volumes was probably smaller because the amount of oil was sufficient to fill all dimples in the contact zone, and the effect of pit area ratio on the coefficient of friction of only textured surfaces was marginal.

Working conditions near reversal points under oscillation, high load, and the limited oil volume may lead to starved lubrication conditions. Laser texturing of the disc surface caused growth in the seizure resistance in the steel–steel initially lubricated sliding pair in reciprocating motion in such difficult circumstances. The lifetime of the friction pair increased 2.4 times due to DLC coating and surface texturing. However, this increase could probably be longer (tests were stopped after a duration of 2 h). This effect was evident mainly for textured discs of pit area ratios of 9 and 13%, and for a dimple density of 3%, seizure occurred for one sliding pair. The increase in friction pair lifetime by surface texturing was also found in works [5,6,7,8,9,10,11,12]. Surface texturing also caused an increase in seizure resistance in [13,14,15]. However, the authors of the cited articles applied dimples with pit area ratios greater than 3%. The tendency to seizure can be reduced by breaking the contact between two surfaces. This behaviour is hardly possible using a textured surface with a very small pit area ratio of 3%.

The application of the DLC coating on the disc surface led to a significant increase in the seizure resistance of the sliding assembly compared to that of the ground disc. DLC coating of the textured disc surface with the smallest ratio also led to an improvement in the anti-seizure characteristic. The seizure did not occur for sliding pairs with DLC-coated discs. The authors of articles [18,19,20,21] also found that the use of DLC coating caused growth of seizure resistance of machine elements. The DLC coating of the untextured disc surface also led to a reduction in the coefficient of friction compared to the uncoated disc. Friction reduction was also achieved by DLC coating of the crank journal [18,19].

Combining surface texturing with DLC coating could also cause decreases in the COF of sliding assemblies compared to only textured discs. This impact was visible in the initial portions of the tests for all textured assemblies, for the lowest oil pocket density of 3% before 5300 s, for the highest oil pocket density of 13% before 2800 s, and for the medium oil pocket density of 9% for the entire test. The further frictional behaviour was influenced by the pit area ratios of the disc samples. For the textured discs without the DLC coating, the friction force after initial fluctuation slowly decreased or obtained stable values for the smallest and highest pit area ratios. When the density of the dimples was 9%, the COF slowly increased with time for two repetitions or was constant. For the same pit area ratio of 9% combined with the DLC coating, the friction coefficient decreased as the test progressed. Therefore, a considerable decrease in the COF was obtained in the final test portion—up to 60%. When the pit area ratio of 3% was combined with the DLC coating for one test repetition, the COF decreased with time, while for other courses, the friction coefficient increased after 5000 s. Therefore, the final COF was smaller for the combination of surface texturing with DLC coating compared with only surface texturing; however, the friction reduction for the pit area ratio of 3% was lower compared to the oil pocket density of 9%. The beneficial effect of surface texturing and DLC coating on friction reduction was presumably caused by negligible wear of the co-acting pair. For a combination of a 13% dimple density and DLC coating, the friction coefficient increased after 1800 s for two test repetitions. This increase was probably related to higher disc wear compared to other DLC-coated textured disc samples. The removal of the DLC coating around the dimples with the largest diameter for a very high normal load was presumably caused by the high load stresses and low cohesion between the heat-affected zone and the DLC layer. This behaviour led to similar values of the final friction coefficients for textured sliding pairs with and without DLC coatings, and beneficial effects of DLC coating on the frictional behaviour of textured assemblies were found only for oil pocket densities of 3 and 9%. For pit area ratios of 3 and 9%, applications of DLC coating and surface texturing caused a friction decrease compared to only DLC coating. Friction reductions due to surface texturing combined with DLC coating in relation to DLC coating were also achieved in works [22,23,24,25,27,28,29]; however, the normal loads in the cited studies were smaller than in this research.

DLC coatings are applied to increase wear resistance [17]. In this research, wear levels of co-acting parts were qualitatively assessed. DLC coatings caused a reduction in disc wear for textured and untextured sliding parts. For the untextured assembly, wear of the only counter sample was visible. Similarly to [25], untextured and textured DLC samples typically did not show wear marks. In [30], no deformation of oil pockets occurred for textured DLC samples. Among the DLC coated textured discs, wear tracks and the removal of the DLC layer around the edges of the dimples were only found for a pit area ratio of 13% (Figure 11e and Figure 12b). DLC coatings led to a reduction in the wear levels of the counter samples compared to those acted with textured discs without DLC coatings. The authors of References [22,23,24,27,28] also obtained improvement in wear resistance by combining DLC coating and surface texturing compared to DLC coating. On the basis of the analysis of surfaces using a white light interferometer and scanning electron microscope, it was found that the wear of the samples and the counter samples not subjected to seizure had an abrasive character.

4. Conclusions

This experiment was carried out to study the effects of laser texturing and DLC coating on seizure and friction of the sliding pair under conformal reciprocating conditions with a high normal load. The following conclusions were obtained:

• The combination of surface texturing and DLC coating led to improved tribological performance of the lubricated assembly in reciprocating conformal sliding contact in comparison to untextured discs with and without DLC coating and textured discs without DLC coating.
• The DLC coating of the disc surface caused an improvement in seizure resistance in the steel–steel initially lubricated sliding pair in reciprocating motion. The lifetime of the friction pair increased at least 2.4 times due to DLC coating.
• The seizure resistance also increased due to the surface texturing of the steel discs. This effect was evident mainly for pit area ratios of 9 and 13%.
• Combining surface texturing and DLC coating could cause decreases in the coefficients of friction of sliding assemblies compared to only textured discs and to only DLC-coated samples. This effect was visible for disc pit area ratios of 3 and 9%.
• The DLC coating caused disc wear reductions for textured and untextured sliding parts. For the untextured assembly, wear of the only counter sample was visible. Wear of the disc sample and removal of the DLC layer around the edges of the dimples were found for the largest pit area ratio of 13%. DLC coatings caused reduction in wear levels of the counter samples compared to those co-acted with textured discs without DLC coatings.
• The small pit area ratio should be avoided under hard working conditions in steel-to-steel contact due to seizure because of the low possibility of breaking the contact between the sliding surfaces.
• Removal of the DLC layer around the large-diameter dimples, caused by the high load stresses and low cohesion between the heat-affected zone and the DLC layer, can lead to friction growth. Use of smaller types of oil pockets is recommended in combination with DLC coating.
• This work is functionally important, especially for assemblies operated under high normal loads. Research results can be used for various applications, such as cylinder liners and valve guides from internal combustion engines. Further research will be focused on tribological research in conditions similar to those of industrial applications.