Assessment of AlTiN/TiSiXN Coating Characteristics in Selected Tribological Systems

Abstract

This study examines the effect of an AlTiN/TiSiXN two-layer coating on the tribological performance of HS6-5-2C steel under dry friction conditions. Tribological assessments were conducted using a tribometer and a calotester with a ball-on-disc configuration, involving HS6-5-2C steel discs (both uncoated and coated with AlTiN/TiSiXN) and 100Cr6 steel balls. Analyses, including surface topography, microstructure, and chemical composition, were performed utilising confocal microscopy, atomic force microscopy, and scanning electron microscopy with energy dispersive spectroscopy. The hardness and elastic modulus of the coating and substrate were determined through nanoindentation techniques. The coating exhibited a hardness of approximately 38 GPa and high elasticity, substantially enhancing the tribological characteristics of the system. Notably, the coated specimens exhibited friction coefficients approximately 10% lower than those of the uncoated steel, while wear on the coated discs was reduced by more than 90% in comparison to their uncoated counterparts. Wear rate evaluations of the counter-samples indicated a slightly increased wear of the balls—approximately 21%—when in contact with the coated discs, which can be attributed to the high hardness of the coating. These results substantiate the superior efficacy of the AlTiN/TiSiXN coating in improving wear resistance and reducing friction.

1. Introduction

Friction occurs between the surfaces of interacting components in all mechanical systems, inevitably causing wear and material damage to machine parts and devices. Wear serves as an indicator of the lifetime of systems or their components. Additionally, resistance to motion accounts for approximately 33% of losses in mechanical systems [1]. The application of thin, hard coatings has proven to be an effective method for protecting and extending the lifespan of cutting tools, dies, and forming tools [2,3]. These coatings should possess high hardness, low coefficient of friction, and resistance to wear, corrosion, and oxidation. Those that meet these criteria are widely utilised on working elements subjected to high, constant, and variable loads [2].

Hard, thin coatings are produced using PVD and CVD techniques [2,3,4]. The selection of the coating deposition method depends on specific properties tailored for a particular application. Should a thick coating [3,5] deposited at elevated temperatures [3] be necessary, chemical vapour deposition (CVD) is an effective technique. Such coatings are suitable for applications involving significant wear, such as turning [3,6] and they also resist high-temperature abrasion [5]. When a thinner coating is required, characterised by lower residual compressive stresses [3,6], fine grains, a less developed surface, and an absence of cracking [3], the physical vapour deposition (PVD) method is preferred [3,6]. Coatings produced via PVD are renowned for their high hardness, wear resistance, corrosion resistance, and favourable fatigue properties [7], rendering them suitable for use on milling cutters. Multilayer coatings deposited through PVD magnetron sputtering demonstrate high oxidation resistance and exceptional hardness. Conversely, coatings fabricated using the cathodic arc plasma evaporation method are distinguished by their superior adhesive properties [2].

Specific techniques, deposition parameters, and dopants such as Ti, Si, Al, N, and C [2] influence the properties of coatings. Adding silicon (Si) is an effective way to enhance hardness and oxidation resistance [8]. In turn, titanium (Ti) provides high hardness, good wear [9,10] and corrosion resistance [9,10,11], while adding aluminium (Al) greatly improves mechanical properties and oxidation resistance [12]. Furthermore, metal nitrides contribute to high hardness, high melting temperature, chemical stability, wear and corrosion resistance, and strong adhesion to the substrate [13].

Recent advancements in coating technologies have facilitated the widespread implementation of dry machining, which entails machining with minimal cooling and lubrication [14]. This development is driven by environmental concerns [14,15] and the reduction in production costs [14]. The application of one or more hard metallic or non-metallic coatings, characterised by excellent anti-wear properties, to a relatively soft substrate material—such as a cutting tool—substantially improves the tool’s mechanical performance [10,16,17]. The incorporation of aluminium into TiN coatings results in AlTiN coatings that are harder and exhibit enhanced resistance to wear and oxidation at elevated temperatures. The mechanical properties and crystalline structure of AlTiN coatings are dependent on the aluminium content, which, when precisely calibrated, facilitates the formation of a hard and inert film at the tool-chip interface during the machining process. This film not only inhibits further oxidation of the coating but also acts as a thermal barrier [10,16]. Furthermore, the AlTiN coating may be combined with other coatings to establish a multilayer system, such as the integration of the hard AlTiN layer with softer coatings. Each layer offers distinct advantages, thereby producing a composite coating with enhanced overall performance. The softer coating, often referred to as a self-lubricating coating, demonstrates exceptional lubricating properties that reduce friction between the workpiece and the tool, while also mitigating built-up edge and chip packing. As a result, the surface quality of the machined component is improved, and the lifespan of the tool is increased [16]. The addition of aluminium (Al) to TiN results in the formation of a TiAlN coating, which exhibits improved oxidation resistance due to the formation of a protective Al₂O₃ layer at elevated temperatures [18]. The TiSiN coating is highly regarded for its exemplary mechanical properties [8], including high hardness [19], a high modulus of elasticity, a low coefficient of friction [20], and significant resistance to high-temperature oxidation, wear, erosion [19], and corrosion [20,21]. Veprek et al. [22,23] demonstrated that the TiSiN coating exhibits resistance to crack formation and maintains integrity under oxidation at temperatures surpassing 800 °C [6,10].

Vattanaprateep et al. [24] performed a comparative analysis of TiSiN, DLC, and TiSiN/DLC coatings, concluding that the TiSiN coating has the highest modulus of elasticity.

Haja Syeddu Masooth et al. [25] evaluated the effect of coatings applied to HSS high-speed steel) drills. They compared single-layer coatings TiN and AlCrN, as well as the two-layer coating AlTiN + TiSiXN (Durana). They investigated the surface roughness of the workpiece, an AA6061 composite containing C and ZrO₂. Drilling was performed on the prepared sample using a CNC machining centre. The results showed that the AlTiN + TiSiXN coating notably improved the surface quality of the workpiece, with the lowest surface roughness, Ra, reaching 1.666 µm.

Das et al. [26] determined the service life of a cemented carbide tool coated with DURANA (AlTiN/TiSiXN) and investigated the lubrication and cooling performance using minimum quantity lubrication based on ionic liquids. Two ionic liquids were used: 1-methyl-3-butylimidazolium tetrafluoroborate (BMIM BF4) and 1-methyl-3-butylimidazolium hexafluorophosphate (BMIM PF6). The workpiece material was biomedical-grade stainless steel. They evaluated the following parameters: cutting and thrust forces, flank and crater wear, surface assessment (roughness, morphology, residual stress, microhardness), chip morphology, and temperature. The BMIM PF6 IL-based coolant exhibited better tool wear resistance, higher viscosity, and higher thermal conductivity at a minimal contact angle compared to the BMIM BF4-based lubricant.

The objective of this study was to determine the tribological properties of the two-layer coating under conditions of unlubricated friction. The choice of this coating was motivated by its innovative nature and the limited number of studies focused on it. The evaluation involved measuring linear wear on a pin-on-disc tribometer during sliding friction and volumetric wear after ball-cratering testing, which could represent a new approach in studying this particular material coating.

2. Materials and Methods

2.1. Materials

For the tribological tests, HS6-5-2C steel discs (uncoated) and AlTiN/TiSiXN-coated discs were used. HS6-5-2C steel is recognised for its high ductility (elongation: 33%) [27], impact strength (Izod impact, unnotched: 67 J), and abrasion resistance (abrasion loss in hardened state, ASTM G65: 25.8 mm³) [28]. Its hardness after quenching and tempering at 550 °C is 65 HRC [14], and in the softened state it is 207–269 HB.

Table 1. Composition of HS6-5-2C steel [15,29].

Element	C	Mn	Si	P	S	Cr	Ni	Mo	W	V	Co	Cu
%	0.82–0.92	≥0.40	≥0.50	≥0.03	≥0.03	3.50–4.50	≥0.40	4.50–5.50	6–7	1.70–2.10	≥0.50	≥0.30

lists the chemical components of HS6-5-2C steel [15,29].

The AlTiN/TiSiXN coating was applied to the steel discs. This coating was selected because there are limited reports in the literature describing its tribological properties. According to the manufacturer, Oerlikon Balzers, AlTiN/TiSiXN has excellent tribological properties, high hardness, oxidation resistance, and thermal stability. Consequently, it is used on cutting tools for machining titanium, nickel alloys, stainless steel, and hardened steel, among other applications. The coating was applied using arc coating technology at a temperature below 600 °C. It is brown in colour and, according to the manufacturer’s data, its hardness is 38 +/− 5 GPa (instrumentally measured in according with ISO 14577 [30]). The AlTiN/TiSiXN coating can operate at temperatures up to 1100 °C [31].

The counter-samples were balls made of 100Cr6 steel with a diameter of 6 mm. 100Cr6 steel is used for rolling elements such as balls, rollers, and raceways. It is characterised by excellent wear and fatigue resistance, as well as stable elasticity and microstructure at high temperatures [14].

Table 2. Composition of 100Cr6 steel [32].

Element	C	Mn	Si	P	S	Cr	Cu
%	0.93–1.05	0.25–0.45	0.15–0.35	<0.025	<0.03	1.35–1.60	<0.30

shows the chemical composition of the product.

2.2. Research Methodology

Both coated and uncoated samples were examined for their surface structure using a Leica DCM8 confocal microscope (Leica Geneva, Switzerland). Additionally, a Dimension Icon atomic force microscope (Veeco—now Bruker, New York, NY, USA) was employed for analysis. The chemical composition of the AlTiN/TiSiXN coating was analysed using a scanning electron microscope equipped with a Phenom XL EDS analyzer (PhenomWorld, Eindhoven, The Netherlands). The hardness of both the coating and the substrate was measured using an NHT2 nanohardness tester (Anton Paar, Baden, Switzerland), with a Berkovich indenter pressed into the materials at a load of 20 mN.

Wear testing was carried out with a calotester (Compact Industrial, Combo Calotest, Anton Paar, Corcelles, Switzerland), as shown in Figure 1a. The tests were performed under the following conditions:

• duration: 2000 s,
• speed: 3000 rpm,
• inclination angle of the ball to the disc: 30°,
• ball diameters: 20 mm and 30 mm,
• conditions: dry, without abrasive materials.

Additionally, friction and wear assessments were conducted using a TRB³ tribotester (Anton Paar, Baden, Switzerland) in a ball-disc configuration with rotational motion. The setup is illustrated in Figure 1b. The tests followed these parameters:

• load P = 10 N,
• sliding speed v = 0.1 m/s,
• friction distance s = 1000 m,
• friction pairs: HS6-5-2C steel disc without coating and 100Cr6 steel ball, and a coated HS6-5-2C steel disc with an AlTiN/TiSiXN coating plus a 100Cr6 steel ball,
• ball diameter: 6 mm,
• lubrication: none.

After these tests, wear tracks were observed using a confocal microscope. The coating thickness was measured, and its chemical composition was examined with a scanning electron microscope equipped with an EDS detector. In addition, the Hertz unit pressures of balls with diameters of:

• 6 mm (tribological tests—TRB3 tester),
• 20 mm and 30 mm (abrasion tests—calotester).

The gravitational force of the ball on the sample in the calotester was calculated using the formula:

$${\mathrm{F}}_{\mathrm{g}}=\mathrm{c}\mathrm{o}\mathrm{s}\mathsf{\alpha }·\mathrm{m}·\mathrm{g}$$

(1)

where:

α—angle of the ball’s inclination with respect to the disc,

m—mass of ball,

g—acceleration of gravity.

The Hertz unit pressures were calculated using the formulas for contact area pressure, p (2), contact curvature radius, a (3), ball indentation depth, δ (4) and effective (reduced) Young’s modulus, K (5).

$$\mathrm{p}={\displaystyle \frac{6·\mathrm{F}·{\mathrm{K}}^2}{(3.14·3·{\mathrm{R}}^2{)}^{\frac{1}{3}}}}$$

(2)

$$\mathrm{a}={\displaystyle \frac{6·\mathrm{F}·{\mathrm{R}}^2}{(4·\mathrm{K}{)}^{\frac{1}{3}}}}$$

(3)

$$\mathsf{\delta }={\displaystyle \frac{{\mathrm{a}}^2}{\mathrm{R}}}$$

(4)

$$\mathrm{K}={\displaystyle \frac{1}{\frac{\left(1-{\mathsf{\vartheta }}_1^2\right)}{{\mathrm{E}}_1}+\frac{\left(1-{\mathsf{\vartheta }}_2^2\right)}{{\mathrm{E}}_2}}}$$

(5)

where:

F—normal load,

R—radius of the ball,

$\mathsf{\vartheta }$—Poisson’s ratio,

E—Young’s modulus,

K—effective (reduced) Young’s modulus.

After conducting tribological tests, linear wear, friction coefficient, and the wear track regions on the sample and counter-sample were observed. The wear tracks were measured using a confocal microscope. The height and volume of wear tracks were calculated from the diameters of the ball wear tracks using Formulas (6) and (7):

$${\mathrm{V}}_{\mathrm{b}\mathrm{a}\mathrm{l}\mathrm{l}}={\displaystyle \frac{1}{3}}\mathsf{\pi }{\mathrm{h}}^2\left(3\mathrm{R}-\mathrm{h}\right)$$

(6)

$$\mathrm{h}=\mathrm{R}-\sqrt{{\mathrm{R}}^2-{\mathrm{r}}^2}$$

(7)

where:

r—radius of the wear mark,

h—height of area used.

The chemical composition at the wear regions was also analysed using an SEM equipped with an EDS microanalyzer.

3. Results

3.1. Surface Morphology

Figure 2 shows isometric images and primary profiles of the discs.

When comparing steel discs with and without coating, it was observed that the uncoated disc had a less developed surface. Elevations and depressions of approximately 0.2 µm were noted. After applying the coating, the roughness increased, with maximum elevations averaging around 0.8 µm.

Observations of the geometric structure of the coated sample surface were also made using an atomic force microscope, and the results are presented in Figure 3.

The images show that the uncoated sample had a surface with slight elevations and depressions of 0.05 µm and 0.02 µm, respectively. In contrast, the coated disc has higher elevations and depressions of 0.8 µm and 0.4 µm, respectively. Analysis of the Abbott-Firestone curve clearly demonstrates that the uncoated disc had a more homogeneous structure [33]. The coated sample exhibited a more developed surface topography, as confirmed by three-dimensional topographic maps, primary profiles, material share curves (Abbott-Firestone) (Figure 3b), and amplitude parameters of the geometric surface structure. This type of microgeometry may contribute to higher friction coefficient values in the initial phase of the tribological test. Furthermore, when comparing the height histogram and the Abbott–Firestone curve, it was observed that the surface of the coated disc contains more profound topographic valleys compared to the surface of the uncoated disc. This allows for the storage of lubricant [34] through the formation of so-called “lubricant pockets” [35].

The chemical composition of the samples tested was also analysed using a scanning electron microscope with EDS (Figure 4).

The results of tests on uncoated and coated samples showed that the chemical composition is consistent with the actual content of elements in HS6-5-2C steel and AlTiN/TiSiXN coating. In Figure 4a, which presents the HS6-5-2C steel, distinct light and dark areas can be clearly observed. The lighter regions correspond to tungsten carbides, while the darker areas represent the matrix—tempered martensite. Figure 4b shows the AlTiN/TiSiXN coating, where a homogeneous matrix is visible—the grey area with lighter inclusions of secondary phases (nitrides), and darker regions corresponding to titanium oxides.

3.2. Hardness

The hardness of both the coating and substrate was also measured. The results, which display the change in force with indentation depth for the substrate and coating, are shown in Figure 5.

Table 3. Summary of nanohardness results for the substrate—HS6-5-2C steel and coating—AlTiN/TiSiXN.

Parameter	Substrate Material HS6-5-2C Steel	Coating AlTiN/TiSiXN
Instrumental hardness HIT (GPa)	3.68 ± 0.52	43.68 ± 3.29
Vickers hardness, HVIT, VICKERS	341.06 ± 32.59	4045.46 ± 304.45
Young’s modulus EIT (GPa)	211.94 ± 5.59	471.49 ± 27.16
Maximum penetration depth hm (nm)	775.41 ± 32.34	284.71 ± 6.15
Elastic work, Welast (%)	13.67 ± 7.39	50.63 ± 2.84
Plastic work, Wplast (%)	86.33 ± 5.71	49.37 ± 10.51

summarises the nanohardness measurements for both the substrate and coating.

A comparison of the force variation graphs with penetration depth indicates that the most significant indentations occur on the uncoated disc—HS6-5-2C steel (see Figure 5a). Furthermore, an analysis of the load-unload curve reveals that both the coating and the indentation following nanohardness testing are predominantly elastic, whereas the substrate—HS6-5-2C steel—exhibits ductile properties. This observation is corroborated by the results presented in Table 3, which demonstrate that the AlTiN/TiAlNSiXN coating has more than 90% higher hardness and over 73% higher elasticity relative to the steel substrate. These attributes are particularly advantageous for coatings employed in tribological applications, as the energy imparted by the applied load is chiefly dissipated through elastic deformation of the coating material [36]. The substrate displays increased ductility, as reflected by a 42% higher ductile work value.

3.3. Abrasion Wear and Tribological Tests

Table 4. Masses and gravitational forces of balls used in abrasion tests (calotester).

Parameter	Ball Diameter (mm)
	20	30
Mass (kg)	0.03	0.11
Gravitional force (N)	0.27	0.93

lists the weights of balls with diameters of 20 mm and 30 mm, along with the calculated values of their gravitational forces.

The larger diameter balls were heavier, so their gravitational force was also greater (Table 4).

Hertz unit pressures were calculated for balls with diameters of 6 mm, 20 mm, and 30 mm, and the results are summarised in

Table 5. Hertzian contact pressures for balls with diameters of 6 mm (tribometer), 20 mm and 30 mm (calotester).

Hertzian Contact Pressures	Steel Disc HS6-5-2C			Disc with AlTiN/TiSiXN Coating
	Ball Diameter			Ball Diameter
	6 mm	20 mm	30 mm	6 mm	20 mm	30 mm
p (MPa)	1 425	198	227	1 749	235	269
a (mm)	0.058	0.027	0.046	0.052	0.025	0.042
d (mm)	0.0011	0.00007	0.00014	0.0009	0.00006	0.00012

.

The analysis of the test results demonstrated that the highest values of pressure in the contact area (p), contact curvature radius (a), and ball indentation depth (d) were observed with a ball of 6 mm diameter, utilised during tribological testing on the TRB3 apparatus. When compared to the uncoated disc, a higher-pressure value was recorded in the steel-steel contact involving the AlTiN/TiSiXN coating. It was also noted that lower values of parameters p, a, and d were recorded for the 20 mm diameter ball in comparison to the 30 mm diameter ball.

Following friction and wear testing, diagrams illustrating the relationship between the coefficient of friction and friction distance (Figure 6) and linear wear and friction distance (Figure 7) were generated.

In the comparison of friction coefficients (see Figure 4), lower values were observed for the disc with the AlTiN/TiSiXN coating. Although, during the initial phase of testing, the friction coefficient was higher for the coated disc than for the uncoated disc—an outcome influenced by its more developed surface microgeometry (refer to Figure 3b)—the analysis of linear wear (see Figure 5) indicated a lower value for the uncoated steel disc; however, this trend was not consistently stable. Conversely, a higher value was recorded for the disc with the AlTiN/TiSiXN coating. The higher value was due to the coating being harder than the steel disc. Linear wear is the sum of the wear of the sample (disc) and the counter-sample (ball). In this case, the ball wore more than the coated disc.

Figure 8 presents the average coefficient of friction values for the tested friction pairs.

The calculated average values of the friction coefficient were 0.58 for the coated disc and 0.71 for the uncoated steel disc. The standard deviation was 0.01 for the coated sample and 0.09 for the steel sample, respectively.

3.4. Assessment of Surface Geometric Structure of Samples After Tribological Tests

To better illustrate the individual wear of the discs and balls, a confocal microscope was used for microscopic analysis in the next stage of the research. Figure 9 and Figure 10 present isometric images and original profiles of uncoated (Figure 9) and AlTiN/TiSiXN-coated steel discs (Figure 10), as well as balls after tribological testing under dry friction conditions.

An analysis of the isomeric images and primary profiles of the wear tracks on the discs and balls indicated reduced disc wear for the coated sample. Build-ups of approximately 4 µm were observed on the coated disc (Figure 10, build-ups shown in green). The uncoated steel disc demonstrated both build-ups of approximately 13 µm and indentations exceeding 5 µm (Figure 9, indentations shown in red). Upon comparing the wear of the balls, a narrower wear track was observed under friction conditions with the uncoated disc, whereas it was more than 20% wider in contact with the disc coated with the AlTiN/TiSiXN layer.

Table 6. Wear of the balls after tribological tests.

Friction Pair and Sliding Conditions	r (mm)	h (mm)	V (mm3)
HS6-5-2C/100Cr6   TTS	0.92	0.14	0.19
AlTiN/TiSiXN/100Cr6   TTS	1.17	0.24	0.52

shows the ball wear values after the tribological tests.

The wear height and volume for the balls exhibited approximately 50% lower values when tested against the uncoated steel disc.

Table 7. The roughness parameters of the discs and balls in the unworn area and within the wear track.

Surface Roughness Parameters		Unworn			After TTS
		HS6-5-2C	AlTiN/TiSiXN	Ball	HS6-5-2C	Kula	AlTiN/TiSiXN	Ball
Sq	µm	0.09	0.36	0.30	3.71	8.17	1.69	6.05
Ssk	-	−0.03	0.78	0.22	−0.09	0.20	−0.11	0.23
Sku	-	2.80	11.39	6.33	3.12	1.98	2.71	2.42
Sp	µm	0.76	6.93	3.04	27.83	130.68	9.02	129.71
Sv	µm	0.32	2.70	2.77	19.57	19.96	6.68	21.81
Sz	µm	1.08	9.63	5.81	47.40	150.65	15.69	151.52
Sa	µm	0.07	0.27	0.23	2.94	7.09	1.39	5.15

presents the roughness parameters of the discs and balls used in the study. These parameters were measured in areas both outside and within the wear track.

In the comparison of the roughness parameters after the friction tests, the coated disc exhibited a smoother surface than the uncoated disc, with all roughness parameters showing lower values. For the ball tested in contact with the coated disc, lower values were also recorded for the following parameters compared to the ball tested against the uncoated steel disc: Sq (root mean square deviation of surface roughness), Sp (maximum peak height of the surface), and Sa (arithmetic mean deviation of surface roughness). This indicates that a surface with smaller elevations was achieved. The Sp value for the uncoated steel disc was 75% lower than for the coated disc. This result confirms the presence of numerous, larger elevations on the surface of the coated sample (Figure 3b). Conversely, after the friction tests, the Sp value at the wear track on the coated sample was more than 60% lower compared to the uncoated steel sample. This reduction signifies the presence of only small micro-irregularities or elevations in the wear track of the coated disc (Figure 10 (bottom)) [37].

3.5. Evaluation of Surface Morphology After Tribological Tests

Additionally, SEM and EDS analyses were used to determine the chemical composition at the wear tracks. The results of this investigation are presented in Figure 11.

The results from the chemical composition analysis of the discs and balls in the wear track of the steel friction pair (Figure 11a) indicated material transfer from the disc to the ball. This transfer was evidenced by the presence of tungsten (W) on the ball, an element found only in the composition of the HS6-5-2C steel disc. Similarly, in the second friction pair (an AlTiN/TiSiXN-coated disc against a 100Cr6 steel ball, Figure 11b), reciprocal material transfer was observed: from the ball to the disc and from the disc to the ball. This is confirmed by the presence of iron (Fe) on the surface of the coated disc and aluminium (Al) on the ball. A small amount of abrasive wear was also observed on the coated disc. Furthermore, observations of the wear on the balls in both friction pairs revealed visible grooves on the ball paired with the uncoated disc, while the disc surface showed build-ups, wear areas, and localised material pull-outs.

The build-ups formed on the coated disc were analysed to ascertain their chemical composition. The results of the linear analysis and distribution mapping are compiled in Figure 12 and Figure 13.

The linear analysis (Figure 12) in the area without wear shows the elemental composition of the coating, mainly featuring Ti, N, and Si. In contrast, in the worn area, the content of iron and oxygen increases. Within the wear track, there is an apparent increase in iron content at the location of a build-up. This observation confirms material transfer from the counter-body (steel ball) to the sample (coated disc). This phenomenon can also be seen in the elemental mapping presented in Figure 13.

The elemental distribution analysis revealed the presence of oxygen and iron within the wear track. Iron originates from the substrate, while oxygen comes from the formation of an oxide layer. In contrast, the unworn area of the coating showed the presence of titanium, nitrogen, and silicon—elements constituting the coating. Aluminium was detected throughout the entire investigated area, both within the wear track and in the unworn region. The formed oxide layer plays a protective role by reducing wear and providing a barrier against corrosion, thereby enhancing the durability of the coating under tribological conditions.

3.6. Assessment of Surface Geometric Structure of Samples After Abrasion Wear Tests

In addition to tribological testing, the AlTiN/TiSiXN coating was studied for wear using a calotest. Figure 14 shows isometric images and primary profiles of the steel discs after the ball crater test.

Figure 15 shows the wear tracks on the HS6-5-2C steel disc without coating after wear tests conducted using a ball-cratering tests (calotest).

Figure 16 shows isometric images and primary profiles of the discs with AlTiN/TiSiXN coating after the ball crater test.

Figure 17 presents the wear tracks on the disc coated with AlTiN/TiSiXN after wear testing using a ball-on-disc tribotester.

The comparison of the wear regions following the abrasion wear test indicated a narrower, shallower region (Figure 16b) and a reduced wear volume (Figure 17b) when the ball with a larger diameter was employed.

Figure 18 shows the wear volumes of the uncoated and coated steel discs after the ball-cratering tests (calotest).

When comparing the wear volumes of the discs after tests conducted with the calotest, it was observed that the highest wear occurred for the steel disc in contact with a 30 mm diameter ball. Conversely, the lowest wear volume was recorded for the coated disc in contact with a 30 mm diameter ball. The comparison of the uncoated and coated discs in contact with a 20 mm and 30 mm diameter ball showed similar wear volume values.

3.7. Evaluation of Surface Morphology After Abrasion Wear Tests

The coating thickness and chemical composition were also examined at the wear track generated after ball-cratering testing, with the results presented in Figure 19, Figure 20 and Figure 21.

The wear track of the coating was measured using an optical microscope, and the thickness was determined with image analysis software. Based on three measurements, the average thickness was 3.08 µm with a standard deviation of 0.13 µm.

After the wear test on the ball-on-disc tribometer, two rings of different colours and a silver-coloured central area—corresponding to the substrate—were clearly visible on the disc (the area between the red circle). The two rings represent the successive coating layers (the first one between the red and green circles, and the second one between the green and violet circles). This indicates that the wear process occurred in stages, gradually removing the coating layers and eventually exposing the underlying substrate.

Figure 20 and Figure 21 present the elemental mapping analysis of the coated disc after the wear test on the ball-on-disc tribometer, using balls with diameters of 20 mm and 30 mm, respectively.

The elemental distribution mapping after the ball cratering tests (20 mm and 30 mm diameter balls) showed substrate-derived elements in the wear track area: iron, chromium, molybdenum, and tungsten. The first coating layer (i.e., the ring surrounding the wear track) was composed of aluminium, titanium, and nitrogen, corresponding to the AlTiN top layer. The second coating (i.e., the area outside the wear track) had the following chemical composition: titanium, nitrogen, and silicon, corresponding to the TiSiXN layer. Additionally, substrate-derived elements were still visible: tungsten, chromium, molybdenum, iron, and vanadium.

The results of the elemental mapping made it possible to identify the sequence of the duplex coating layers: the AlTiN layer is located on the surface, while the TiSiXN layer lies beneath it, closer to the substrate.

4. Conclusions

The study on the influence of the AlTiN/TiSiXN double-layer coating on the tribological properties of HS6-5-2C steel demonstrated its effectiveness under dry friction conditions. Both coated and uncoated discs and counter-samples (100Cr6 steel balls) were analysed. Based on the results, the following conclusions were drawn.

• Enhancement of mechanical properties
  The AlTiN/TiSiXN coating exhibits superior mechanical characteristics, which are crucial for its protective function. Nanoindentation and force-penetration depth analyses confirm the coating’s high hardness and excellent elastic properties, particularly in the top TiSiN layer.
  The combination of high hardness and elasticity effectively restricts plastic deformation and chipping, allowing the material to temporarily absorb load energy as elastic strain. This mechanism is vital for reducing local stresses and lowering the risk of permanent damage under heavy loads. The coating maintained its integrity throughout intense dry friction tests, demonstrating its inherent stability and abrasion resistance.
• Improvement in tribological performance
  The coating provides a significant enhancement in wear resistance and friction characteristics compared to the uncoated steel. The coated discs exhibited a wear area more than 90% smaller than the uncoated steel samples, which was further validated by ball cratering tests (calotest) showing nearly 50% less wear on the coated surfaces.
  The coating successfully reduced the friction coefficient and stabilised the friction process, which minimises energy losses.
  Wear analysis confirms that the coating acts as an effective protective barrier. The coated discs showed virtually no material loss or chipping; localised iron-containing build-ups from the counter-sample were found. Conversely, the uncoated steel experienced significant wear and material transfer (tungsten dominant), while the coated system saw iron transfer from the softer steel ball counter-sample to the harder coating surface.
• Potential industrial applications
  The demonstrated performance makes the AlTiN/TiSiXN coating a highly effective modification for industrial applications. Given its exceptional performance in reducing wear and friction under heavy loads, the coating is highly suited for cutting tools and other machine components that operate under high friction and intense mechanical stress. This application ensures increased operational durability and efficiency.