Influence of Cr-Al-Si-N and DLC-Si Thin Coatings on Wear Resistance of Titanium Alloy Samples with Different Surface Conditions

Abstract

The influence of Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si thin coatings deposited on titanium alloy (Ti-Al-Zr-Sn-Nb system) samples with different surface reliefs on wear resistance under abrasion and fretting conditions was investigated. The influence of coatings on the initial microrelief after finishing milling and lapping with micro-grained abrasive was studied by profilometry. The Martens hardness (H) and the elastic modulus (E) were determined through nanoindentation. The H/E ratio was 0.08, 0.09, and 0.13, respectively. The adhesion bond strength and H/E ratio relationship was revealed using a scratch testing analysis. Volumetric wear after 20 min of abrasive exposure was reduced by 11, 25, and 31 times for Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings compared to uncoated ones after milling and by 15, 32, and 35 times after lapping. Volumetric wear under fretting conditions was reduced by 1.8 and 4 times for Cr-Al-Si-N coating after milling and lapping. It was reduced by tens of times for DLC-Si coating and by hundreds of times for Cr-Al-Si-N/DLC-Si coating. The Cr-Al-Si-N/DLC-Si coating (a thickness of 3.1 ± 0.15/2.0 ± 0.1 µm) is characterized by the best combination of hardness (24 ± 1 GPa), elastic modulus (185 ± 8 GPa), and friction coefficient (0.04–0.05 after milling and 0.1 after lapping) and ensures maximum wear resistance under a wide range of loads. The novelty of the work is that those coatings were not practically under study concerning the deposition on the titanium alloy regarding typical mechanical loads such as abrasive and fretting wear but are of interest to the aviation and aerospace industry.

1. Introduction

Titanium alloys are currently one of the most popular construction materials in the industry. They are widely used to manufacture aerospace and automotive parts, elements of gas and oil refining systems [1,2], reactor parts of power plants [3,4], parts for the transport sector [5], implantology products [6,7], etc.

The widespread use of titanium alloys is associated with unique properties such as high corrosion resistance, specific strength, thermal stability, and relatively low density, significantly reducing the weight of manufactured parts [8,9,10,11]. However, relatively low wear resistance and surface hardness characterize titanium alloys. It complicates their application for parts working under conditions of intensive frictional wear [12,13,14]. The natural oxide films formed on titanium alloys are easily destroyed by friction owing to high loads at the contact point, which leads to adhesion of the mating surfaces [15,16]. In operation, high corrosion resistance due to surface passivation distinguishes titanium alloys. However, this feature simultaneously decreases friction wear resistance due to the continuous destruction and removal of oxide films. The oxide film-free surfaces of titanium alloys are prone to adhesion at the contact areas of the counter bodies. It leads to a high probability of wedging of parts of the friction unit [17,18,19,20]. In addition to the abovementioned, high wear intensity is promoted by other features: increased elastic deformation due to low elastic modulus (compared to steel), low thermal conductivity, and so on [21,22].

Considering the increased demand for titanium alloys in the industry and the limitations of practical application in friction units, numerous scientific groups conduct research. Their main objective is to increase resistance to frictional wear of the surface layer of titanium alloy parts. Among the investigated technological approaches, the most widespread is the processes of chemical-thermal treatment, consisting of diffusion saturation of the surface layer of titanium alloy parts with various elements [22,23,24,25,26,27] and coating deposition processes by various techniques such as detonation, ion-plasma, laser powder cladding, and so on [28,29,30,31,32].

The expressed effect of the coating deposition is possible only when coatings exhibit a complex of properties, such as a high strength of the adhesive bond with the titanium alloy and a low friction coefficient that ensures increased surface layer hardness of the part, and the coatings are not excessively brittle [33,34,35,36]. Thin coatings based on diamond-like carbon (DLC) are very promising for this task. They have excellent mechanical and tribological characteristics [37,38,39,40,41]. However, DLC coatings have negative characteristics such as a high level of residual stresses and insufficient strength of the adhesive bond with the substrate when coatings are deposited to materials with low hardness and when their thickness exceeds 2.5 μm. The deposition of various sublayers before DLC coating has become widely used to eliminate this negative effect. Ceramic coatings of the Cr-Al-Si-N type may be used as sublayers, which various researchers recognize now as extremely promising [42,43,44,45,46].

The authors of the present work investigated the possibility of using silicon-doped DLC coatings (DLC-Si) to increase the wear resistance of ceramic and hard alloy parts under intensive mechanical loads. The effectiveness of a Cr-Al-Si-N sublayer deposition prior to DLC-Si coatings was evaluated, and a significant contribution was demonstrated [47,48,49,50,51]. However, titanium alloys principally vary in physical, mechanical, and structural characteristics from previously studied materials and require independent investigation.

It should be noted that most studies previously carried out by scientific groups in the field of DLC coatings on titanium alloys relate to evaluating their effectiveness for protection against the impact of aggressive media. Elements of natural and technological environments interact chemically with the surface layer of titanium alloy parts and cause a loss of their performance properties [52,53,54,55,56,57]. In such conditions, DLC coatings show outstanding results for surface layer degradation protection of various biomedical constructions, bipolar plates for fuel cells, injection nozzles, and other titanium alloy parts.

Today, parts made of titanium alloys with DLC coatings operating under conditions of intensive frictional wear still need to be studied as objects of research. In addition, the influence of the surface layer condition of titanium alloy on which DLC coatings are deposited on the tribological characteristics of the samples was out of the focus of the research.

The present research aims to study the effect of Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si thin coatings deposited on titanium alloy samples with different surface conditions on wear resistance under abrasive and fretting wear environments.

The novelty of the work lies in the research of the influence of the mentioned coatings on the roughness, H/E ratio, and wear resistance under abrasive and fretting wear of the titanium alloy samples after two finishing techniques, such as milling and lapping. Those coatings deposited on the titanium alloy surface with different reliefs were not practically the focus of attention of the research group before but have prospects for the industrial applications.

This work aims to fill the research gap related to the study of those coatings deposited on the titanium alloy samples for a subject of mechanical loads, such as abrasive and fretting wear, that are of interest to the aviation and aerospace industry.

2. Materials and Methods, and Equipment

2.1. Characteristics of Titanium Alloy Samples

The titanium alloy of the Ti-Al-Zr-Sn-Nb system (BT18U grade) produced by TITAN-SPA LLC (Podolsk, Russia) was used as a material for research. The chemical composition of BT18U alloy and the basic physical and mechanical characteristics are provided in

Table 1. Chemical composition of BT18U titanium alloy samples.

Material	Elements Composition, % Mass
	Ti	Al	Zr	Sn	Nb	Mo	Si	Admixtures
BT18U	84.5	6.8	4.2	2.1	1.2	0.8	0.25	0.15

and

Table 2. Physical and mechanical properties of BT18U titanium alloy samples.

Material	Tensile Strength, MPa	Impact Toughness, kJ/m2	Relative Elongation, %	Hardness HRC
BT18U	1010 ± 40	230 ± 6	9 ± 2	34 ± 2

. The specified titanium alloy belongs to high-strength pseudo-α-alloys, the structure of which contains α-phase and a slight amount of β-phase (up to 5% mass).

BT18U alloy is widely applied in the industry [58,59]; therefore, it was selected as the research object. Samples for experimental studies were cut out of a square section bar of BT18U material. The samples were rectangular parallelepiped shaped with a width of 20.0 mm and a height of 5.5 mm (Figure 1).

Two titanium alloy pretreatment techniques were applied to estimate the influence of the surface layer condition of the samples on the wear resistance of coatings deposited subsequently:

-

finishing milling with GC1745 carbide end mills at a cutting speed of 60 m/min, feed rate of 0.08 mm/tooth, and depth of 0.2 mm (CTX 1250TC milling machining center, DMG, Bielefeld, Germany);

-

abrasive machining with lapping wheels using a suspension of silicon carbide micro-powders of grain size F800 at a cutting speed of 3 m/s (Lapmaster 24 lapping and polishing machine, Lapmaster Wolters, Mt Prospect, IL, USA).

2.2. Cr-Al-Si-N and DLC-Si Coatings Deposition Technique for Titanium Alloy Samples

2.2.1. Equipment and Technology of Coating Deposition

Preliminary cleaning of the surface of titanium alloy samples before coating deposition was carried out in the ultrasonic tank model Stegler 22DT (Shenzhen Bestman Instrument Co., Ltd., Shenzhen, China) at a temperature of 80 °C and a frequency of 40 kHz.

Coating deposition on titanium alloy samples was achieved using a technological unit model π311 (Platit AG, Selzach, Switzerland) [60]. The unit was equipped with arc and glow discharge plasma generation systems. The technological cycles embedded in the control programs of the plant control system were used as rational.

The unit allows for deposition in one technological cycle: ceramic coatings based on refractory metal nitrides (using vaporization of cathode materials to form the metal component of the plasma); DLC coatings (using gas-phase deposition in the chemical reaction and decomposition of gas mixture components).

In gas-phase deposition, the chemical reaction occurs near the sample surfaces, resulting in the deposition of DLC coating. Hydrogen and silicon atoms are contained along with carbon atoms in the structure of DLC coatings [61,62,63]. The general view and internal design of the technological unit are shown in Figure 2. Three variants of coatings were deposited on samples made of BT18U titanium alloy with different states of the surface layer: Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si.

2.2.2. Deposition of Cr-Al-Si-N Coatings

Deposition of Cr-Al-Si-N coatings includes sequential execution of the following procedures:

-

pumping the vacuum chamber by pumps to a pressure of 0.03 Pa and heating with heaters for 60 min to a temperature of 500 °C;

-

cleaning of the sample surface with Ar ions for 20 min at a chamber temperature of ~500 °C, a pressure of 1.2 Pa, and a bias voltage of −650 V;

-

cleaning of the surface of the sample by metal ions for 20 min at a chamber temperature of ~500 °C, pressure of 2.2 Pa, bias voltage of −800 V, and Al-Si cathode current of 90 A;

-

Cr-Al-Si-N coating deposition in a gas mixture of 10% vol. Ar and 90% vol. N₂ for 60 min at a chamber temperature of ~500 °C, pressure of 0.9 Pa, bias voltage of −80 V, and current on Cr and Al-Si cathodes of 90 A.

When after Cr-Al-Si-N coating deposition of DLC-Si coating is envisaged, the vacuum chamber is filled with a gas mixture containing 16% vol. Si(CH₃)₄, 6% vol. Ar, and 78% vol. N₂ to form a gradient layer 5 min before the process completion.

2.2.3. Deposition of DLC-Si and Cr-Al-Si-N/DLC-Si Coatings

Deposition of DLC-Si coatings includes sequential execution of the following procedures where the first three stages repeat the stages of Cr-Al-Si-N coating deposition, and then they are as follows:

-

the vacuum chamber is filled with a gas mixture containing 16% vol. Si(CH₃)₄, 6% vol. Ar, and 78% vol. N₂ to form a gradient layer 5 min before the process completion;

-

DLC-Si coating deposition at the temperature in a vacuum chamber of up to 180 °C in a gas mixture of 3% vol. Si(CH₃)₄, 52% vol. Ar, and 45% vol. C₂H₂ for 110 min at a pressure of 1.5 Pa and a bias voltage of −500 V.

If the deposition of a Cr-Al-Si-N sublayer before DLC-Si coating is not envisaged, the final procedure (DLC-Si coating deposition) is immediately started after pumping out the chamber and cleaning with Ar ions.

2.3. Evaluation of Physical and Mechanical Characteristics of Coatings

Investigation of the cross-section of titanium alloy samples with three types of coatings and taking a high spatial resolution surface layer image were provided on a scanning electron microscope Mira 3 (TESCAN JSC, Brno, Czech Republic).

Hardness on the Martens scale and elastic modulus of thin coatings were evaluated on the Nano Hardness Tester (CSM Instruments SA, Peseux, Switzerland). The nanoindentation scheme using a Berkovich diamond indenter as a three-sided pyramid was applied. Nanoindentation was carried out during tests under conditions of indenter loading and unloading with programmed speed. The loading–unloading time was 50 s, and the applied load was 4.0 mN. The value of the applied load and the corresponding indenter penetration depth were chosen based on the condition of about 15% of the coating thickness [49]. The experimental data were processed using CSEM instruments software (version 4.0) based on the algorithm of Oliver W.C. and Pharr G.M [64,65].

The surface profile parameters of the experimental samples were evaluated on an optical profilometer Dektak XT (Bruker, Billerica, MA, USA). Topological three-dimensional images were obtained using a highly sensitive contact stylus moved over the surface of the sample under study.

Studies of adhesive bond strength of thin coatings with titanium alloy samples were conducted on a Nanovea Mechanical Testing M1 device (Nanovea, Irvine, CA, USA). The scratch testing method with the acoustic emission signal spectrum recording was applied.

Testing was performed with a Rockwell indenter in the form of a diamond cone with a radius at the apex of 100 µm and an angle of 120°. Five scratches were formed on each sample under a linearly increasing load. The indenter displacement path along the sample surface was 10 mm, and the maximum applied load was 25 N. As a result of the testing, the critical failure loads, which determine the degradation character of the coating, were evaluated by acoustic-emission signal level and visual scratch analysis: critical loads LC1 and LC2, identifying the processes of cohesive failure (cracking) and subsequent adhesive failure of the coating, respectively. In addition, the critical load LC3, identifying the complete detachment of the coating from the specimen, was also determined. A scanning electron microscope PHENOM G2 PRO (Phenom-World BV, Eindhoven, The Netherlands), equipped with an energy-dispersive X-ray spectroscopy system, was used to assess coating degradation visually. SEM images were taken in the 6.0–6.25 mm area from the start of the indenter scratching path, where the critical load LC3 was recorded for the samples with coatings under study. The elemental composition of the coating degradation areas was determined by semi-quantitative EDS analysis.

2.4. Evaluation of Wear Resistance of Coatings Deposited on Titanium Alloy

2.4.1. Resistance to Abrasion Wear

Calowear Abrasion Tester (CSM Instruments, Switzerland) was used to evaluate the abrasion resistance of the surface layer of the samples. In the test process (Figure 3), an Al₂O₃-based abrasive suspension is introduced into the contact area between the coated sample and a 25.4 mm diameter hardened steel ball rotating at 949 rpm with the applied force of 0.2 N. Under these conditions, a wear spot in the form of a spherical well is formed on the surface of the titanium alloy sample. The volumetric wear data at different moments and the “exposure time—volume abrasive wear” relationships were obtained through contact scanning of the wear spot of the surface with a diamond tip on a Dektak XT profilometer. The volumetric wear was calculated using the equations presented in [66].

2.4.2. Resistance to Fretting Wear

Fretting wear is a specific process of local material destruction in the contact area between two materials under load. The contact area is subject to slight relative movement due to vibration or other forces.

A modernized friction machine of model 1401 (Moscow Aviation Institute, Moscow, Russia) was used to evaluate the sample’s surface layer resistance to wear under fretting conditions (mechanical wear of counter bodies under conditions of small oscillatory relative displacements) [67]. The equipment allows for modeling the conditions of reciprocating fretting wear (Figure 4). The friction machine contains an electromagnetic vibrator, which provides reciprocating movements of the test sample, and a loading system in the form of a lever, which is equilibrated by a balance. The latter transmits to the contact area normal load, which is regulated by weights of different masses. The registration and control system of parameters contains a sinusoidal signal amplifier, a piezoelectric force sensor, a signal controller, and a laser movement sensor.

During the experiments under the conditions of reciprocating movement of the counter bodies, the scheme “sphere-plane” was realized. In this case, the plane was a sample made of titanium alloys with various coatings, and the sphere was a ball of 10 mm in diameter from Al₂O₃.

During the experiments, the displacement frequency was 20 Hz in all cases. Other experimental conditions varied for different conditions of the surface layer of titanium alloy samples and were as follows:

• Relative displacement of counter bodies of 60 and 100 µm.
• Normal force in contact of 1 and 20 N.
• A number of friction cycles of 100,000 and 300,000 (for samples after finishing milling) and 25,000 (for samples after lapping with micro-grained abrasive).

Evaluation of wear spots and measurement of wear profiles of the samples’ surface layer was carried out with the Olympus LEXT OLS 5000 confocal microscope manufactured by Olympus Corporation (Tokyo, Japan). All samples were degreased with ethyl alcohol before the experiments.

3. Results

3.1. Microstructure and Properties of Coatings

Figure 5 shows SEM images of a cross-sectional view of BT18U titanium alloy samples with three coatings variants, Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si, deposited on the surface after lapping with micro-grained abrasive. The thicknesses of the Cr-Al-Si-N coating and sub-layer measured 3.2 ± 0.15 μm and 3.0 ± 0.15 μm, respectively. The thicknesses of the single-layer DLC coating and external DLC layer were 2.3 ± 0.1 μm and 2.3 ± 0.1 μm, respectively.

Table 3. Physical and mechanical properties of BT18U titanium alloy samples with different coatings.

Coating Composition	Applied Load, mN	Martens Hardness, GPa	Elastic Modulus, GPa	Coating Thickness, μm
Cr-Al-Si-N	4.0	30 ± 3	370 ± 7	3.2 ± 0.15
DLC-Si		23 ± 2	248 ± 7	2.3 ± 0.1
Cr-Al-Si-N/DLC-Si		24 ± 1	185 ± 8	3.1 ± 0.15/2.0 ± 0.1

shows the experimental data on the Martens hardness and elastic modulus of Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings obtained by nanoindentation. It is noteworthy that when the external DLC layer is deposited on the Cr-Al-Si-N sublayer, the elastic modulus decreases significantly (up to 160 GPa) compared with the single-layer DLC coating (238 GPa) deposited on the titanium alloy BT18U.

Figure 6 shows that the titanium alloy samples’ surface layer conditions differed significantly after applying different pretreatment techniques. After finishing milling, the R_a roughness parameter was 0.55 μm, and the total height of the microroughness profile (R_t parameter) was 3.5 μm (Figure 6a). For the samples after lapping with micro-grained abrasive, R_a was 0.03 μm, and R_t was 0.49 μm (Figure 6b).

Figure 7 shows the results of surface profilometry of samples from BT18U titanium alloy with three variants of coatings (Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si) deposited on the surface after finishing milling and lapping with micro-grained abrasive. It can be seen that the values of the total height of the profile (R_t parameter) after the deposition of coatings on the milled surface change insignificantly (±10%–20%) compared to the uncoated samples (

Table 4. Surface layer profile parameters of BT18U titanium alloy samples with different coatings deposited after finishing milling and after lapping with micro-grained abrasive.

Group of Samples	Sample Types	Parameter Values of the Surface Layer Profile
		Total Height of the Profile Rt, μm	Arithmetic Mean Deviation of the Assessed Profile Ra, μm
After finishing milling	BT18U	3.5	0.55
	With Cr-Al-Si-N coating	3.8	0.26
	With DLC-Si coating	2.8	0.2
	With Cr-Al-Si-N/DLC-Si coating	3.6	0.32
After lapping with micro-grained abrasive	BT18U	0.49	0.03
	With Cr-Al-Si-N coating	1.6	0.09
	With DLC-Si coating	1.9	0.07
	With Cr-Al-Si-N/DLC-Si coating	1.7	0.08

). The effect of coating deposition on the change of arithmetic mean deviation of the assessed profile (R_a parameter) is more significant: a 40%–60% decrease is observed compared with uncoated samples (Table 4). At the deposition of coatings on the surface after lapping with micro-grained abrasive, all coatings are characterized by a significant increase in R_t and R_a parameters compared with uncoated samples (Table 4).

3.2. Adhesion Bond Strength of Coatings

Figure 8 shows experimental data on destructive load forces (LC1, LC2, and LC3) in scratch testing of titanium alloy samples with Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings. The lowest adhesive bond strength among the studied coatings is observed in samples with DLC-Si coatings (the first signs of cohesive destruction are observed at the LC1 of less than 3–4 N). At a subsequent increase of the load up to 5 N (LC2), the destruction of DLC-Si coatings occurred by the mechanism of adhesive delamination (Figure 8). In the scratching of DLC-Si coatings deposited on the surface after lapping with micro-grained abrasive, cracks and significant areas of delamination are observed along the scratch edges (Figure 9). Total delamination of DLC-Si coatings is observed at the level of critical load LC3 of about 10 N. EDS analysis of the elemental composition of the DLC-Si coating destructive area in scratch testing (

Table 5. EDS analysis results of destruction area elemental composition in scratch testing of titanium alloy samples with different coatings deposited after lapping with micro-grained abrasive.

Coatings	Element Number	Element Symbol	Element Name	Atomic Concentration, %	Atomic Concentration Error, % 1	Weight Concentration, %	Weight Concentration Error, % 2
Cr-Al-Si-N	22	Ti	Titanium	49.69	0.25	70.31	0.26
	8	O	Oxygen	34.10	1.71	16.12	0.60
	7	N	Nitrogen	5.62	0.28	3.02	0.09
	13	Al	Aluminum	7.63	0.04	6.52	0.02
	40	Zr	Zirconium	2.96	0.01	4.03	0.03
DLC-Si	22	Ti	Titanium	48.12	0.24	69.89	0.25
	8	O	Oxygen	33.02	1.65	15.6	0.57
	7	N	Nitrogen	5.38	0.27	3.04	0.08
	13	Al	Aluminum	6.76	0.03	5.98	0.02
	6	C	Carbon	4.5	0.23	2.21	0.06
	40	Zr	Zirconium	2.22	0.01	3.28	0.02
Cr-Al-Si-N/DLC-Si	8	O	Oxygen	23.16	1.16	15.25	0.40
	7	N	Nitrogen	21.72	1.09	13.02	0.33
	24	Cr	Chromium	20.96	0.10	33.87	0.12
	13	Al	Aluminum	20.06	0.10	21.90	0.06
	22	Ti	Titanium	5.22	0.03	8.89	0.03
	14	Si	Silicon	4.99	0.02	5.05	0.02
	6	C	Carbon	3.89	0.19	2.02	0.05

¹ Estimated measurement error of the light elements such as oxygen, nitrogen, and carbon is at 5 at.% [68]. For the polished samples of ceramics, the estimated measurement error should not exceed 0.5 at.% [69]; ² calculated for convenience: $Wt.\%=\frac{At.\%·At.Wt.}{\sum _{i=1}^n{At.\%}_i·{At.Wt.}_i}$, where At.Wt. is the atomic weight and At.Wt._i is the atomic weight of i-th element, where $i=1\dots n$.

) revealed only the presence of the base material (BT18U titanium alloy) with a small content of carbon component. These results are typical for DLC-Si coatings deposited on the surfaces after finishing milling and lapping with micro-grained abrasive.

Signs of cohesive destructive (LC1) of Cr-Al-Si-N coatings are observed at loads of 6 and 7 N for coatings deposited on the surface after finishing milling and after lapping with micro-grained abrasive, respectively (Figure 8). The condition of the surface on which the Cr-Al-Si-N coating is deposited significantly affected the LC2 load value that characterized adhesion destruction. LC2 was 7 N for coatings deposited on the surface after milling and 12 N after lapping. Scratch testing of Cr-Al-Si-N coatings revealed a significant amount of microcracks and delamination areas along the scratch boundaries, which were located perpendicular to the direction of indenter movement (Figure 9). Complete delamination of Cr-Al-Si-N coatings is observed at the level of the critical load LC3 of 16–17 N (for two variants of the surface condition). EDS analysis of the elemental composition of the Cr-Al-Si-N coating destruction area in scratch testing (Table 5) revealed only the presence of the base material (BT18U titanium alloy).

The highest adhesive bond strength of the coatings deposited on titanium alloy was observed for samples with Cr-Al-Si-N/DLC-Si coatings. The first signs of cohesive destruction were observed at loads of 6 and 8 N for coatings deposited on the surface after finishing milling and after lapping with micro-grained abrasive, respectively (Figure 8). The onset of adhesion destruction for two-layered coatings shifted to higher values and was 9 N (after milling) and 15 N (after lapping). Complete coating delamination was observed at the critical load LC3 of 18 N and 20 N for the coatings deposited after milling and after lapping, respectively.

The pronounced differences in the SEM images of the internal surfaces of the scratches performed in the area of 6.0–6.25 mm from the start of the indenter scratching path under critical loads after delamination of the different coatings (Figure 9) are noteworthy. For samples with Cr-Al-Si-N and DLC-Si coatings, the scratch surfaces have distinct longitudinal traces of plastically deformed base material (titanium alloy) characterized by relatively low density and high plasticity. A different character is observed for the Cr-Al-Si-N/DLC-Si coated samples: no pronounced plastic deformation traces were detected. EDS analysis of the elemental composition of the Cr-Al-Si-N/DLC-Si coating destruction area in scratch testing (Table 5) also revealed significant differences. The content of only a small amount of titanium (about 8% of weight concentration) was revealed, which is 8 times less than for samples with Cr-Al-Si-N and DLC-Si coatings. The internal surface of scratches after testing of Cr-Al-Si-N/DLC-Si coated samples contains a significant weight concentration of the following elements: chromium (33.8%), aluminum (21.9%), nitrogen (13%), and some amount of silicon (5%). The characteristic appearance of scratches and the results of EDS analysis of the samples with Cr-Al-Si-N/DLC-Si coating testify to the preservation of a significant amount of sublayer Cr-Al-Si-N on the surface of titanium alloy when the external DLC-Si coating was delaminated. Thus, complete delamination of the sublayer was not observed in the studied range of loads (up to 25 N) for the Cr-Al-Si-N sublayer.

3.3. Wear Resistance of Titanium Alloy Samples with Coatings

3.3.1. Wear Resistance under Abrasive Exposure

Figure 10 and Figure 11 show 3D profiles of wear spots in the shape of wells on titanium alloy samples with different coatings (the wear profile was evaluated after 20 min of abrasive exposure). Regardless of the surface condition on which the coatings were deposited (Figure 10—after finishing milling, and Figure 11—after lapping with micro-grained abrasive), the uncoated BT18U material shows intensive abrasive wear with the formation of high-volume wear areas. Deposition of Cr-Al-Si-N coatings significantly reduces the wear intensity, while DLC-Si and Cr-Al-Si-N/DLC-Si coatings demonstrated maximum effectiveness (abrasive resistance).

The wear kinetics and quantitative values of volumetric wear for titanium alloy samples with different coatings are presented in

Table 6. Kinetics of volumetric wear progression under abrasive exposure on titanium alloy samples with different coatings.

Group of Samples	Exposure Time, min	Volume Abrasive Wear, ×103 μm3
		Uncoated Samples	Cr-Al-Si-N Coating	DLC-Si Coating	Cr-Al-Si-N/DLC-Si Coating
Coatings deposited after finishing milling	5	8454	2753	324	177
	10	12,485	3631	1271	1149
	15	18,182	4846	1377	1475
	20	62,831	5452	2459	1986
Coatings deposited after lapping with micro-grained abrasive	5	6767	913	205	188
	10	19,739	1996	459	434
	15	26,360	2641	803	774
	20	64,322	4214	1993	1809

. For uncoated samples, a characteristic dynamic of abrasive wear was observed: volumetric wear increases rapidly after 15 min of exposure for two surface layer conditions. Another behavior was observed for the coated samples: the volumetric wear increases monotonically with increased abrasive exposure time (Table 6).

After 20 min of abrasive exposure, volumetric wear was reduced by 11, 25, and 31 times for Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings, respectively, deposited on the surface after finishing milling compared with uncoated BT18U samples. The reduction in volumetric wear was 15, 32, and 35 times for Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings, respectively, deposited on the surface after lapping with micro-grained abrasive compared to uncoated BT18U samples.

3.3.2. Wear Resistance under Fretting

Figure 12a shows the dependences of the friction coefficients of titanium alloy samples with different coatings deposited on the surface after finishing milling on the number of cycles under fretting conditions. At a relative displacement of 60 µm and a normal force in contact of 1 N, the friction coefficient curves for uncoated samples (BT18U) and for the samples with Cr-Al-Si-N coating exhibited a large amplitude throughout the test period, and the average value of the friction coefficient was 0.37–0.4.

The friction coefficient curves for samples with DLC-Si and Cr-Al-Si-N/DLC-Si coatings demonstrated a monotonic character under the abovementioned test regimes. The average value of the friction coefficients was 0.2 and 0.15, respectively. Additionally, tests were performed at a relative counter bodies displacement of 100 μm and a normal force in contact of 20 N (Figure 12b) to reveal the behavior of DLC-based coatings under more severe conditions. The obtained data show that the character of the friction coefficient development curve for the samples with DLC-Si coatings had a noticeable amplitude at increased loads. The average value of the friction coefficient was 0.04 at the initial testing stage, which rather sharply increased up to 0.25 after 120,000 cycles and then gradually began to decrease with characteristic surges.

Cr-Al-Si-N/DLC-Si coating demonstrated the monotonic character of friction curves even under intensive loads. The average value of the friction coefficient was 0.04–0.05 during the complete testing period. A surge appeared on the friction coefficient curve only after 270,000 cycles.

Figure 13 shows the dependence of the friction coefficient of titanium alloy samples with different coatings deposited on the surface after lapping with micro-grained abrasive on the number of cycles under fretting conditions. At a relative displacement of 60 µm and a normal force in contact of 1 N, the friction coefficient curves have distinct hollows for uncoated samples (BT18U) during the test period. The average value of the friction coefficient was 0.44. A slightly lower value of the friction coefficient was found for Cr-Al-Si-N coating samples (about 0.4). Still, its development curve on time exhibited an unstable character with numerous hollows. The friction coefficient curves for samples with DLC-Si and Cr-Al-Si-N/DLC-Si coatings have a monotonic character with an average value of friction coefficients of 0.1. However, the friction coefficient of DLC-Si coating began to increase after 21,000 cycles, doubling (up to 0.2) at 25,000 cycles.

Optical images of wear spots of titanium alloy samples with different coatings after fretting tests are shown in Figure 14 for coatings deposited after finishing milling and Figure 15 for coatings deposited after lapping with micro-grained abrasive.

Table 7. Volumetric wear values of titanium alloy samples with different coatings under fretting conditions.

Group of Samples	Number of Cycles	Load, N	Offset, μm	Volume Abrasive Wear, ×103 μm3
				Uncoated Samples	Cr-Al-Si-N Coating	DLC-Si Coating	Cr-Al-Si-N/DLC-Si Coating
Coatings deposited after finishing milling	100,000	1	60	1239 (min)	673 (min)	13 (min)	1.1 (min)
				1429 (max)	780 (max)	17 (max)	1.8 (max)
	300,000	20	100	–	–	10,939 (min)	458 (min)
				–	–	13,820 (max)	746 (max)
Coatings deposited after lapping with micro-grained abrasive	25,000	1	60	568 (min)	156 (min)	5 (min)	1.1 (min)
				825 (max)	182 (max)	9 (max)	1.3 (max)

shows the quantitative values of volumetric wear of titanium alloy samples with different coatings after fretting tests.

The results (Table 7) demonstrate that coatings deposited on the surface after milling allow a considerable effect under fretting conditions. Cr-Al-Si-N coating reduces the volumetric wear by 1.8 times, DLC-Si coating by tens of times, and Cr-Al-Si-N/DLC-Si coating by hundreds of times compared with uncoated BT18U titanium alloy samples. In the case of deposition of coatings on titanium alloy samples after lapping with micro-grained abrasive, the achieved effect is also significant—by 4 times for Cr-Al-Si-N coating, by tens of times for DLC-Si coating, and by hundreds of times for Cr-Al-Si-N/DLC-Si coating compared with uncoated samples.

4. Discussion

The experimental data presented in Table 3 indicate that the deposition of coatings of the selected composition greatly increases the initial hardness (34 HRC) of titanium alloy samples. The average values of achieved hardness for Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings are 30, 23, and 24 GPa, respectively. The revealed increase in hardness should provide a higher resistance of the titanium alloy surface layer to plastic deformations [70,71]. However, high hardness values of coatings do not guarantee their high operational properties. A no less significant indicator of coatings is their elastic modulus, whose value for coatings should be close to the value of the base material to reduce stresses at the “titanium alloy—coating” interface [72]. As follows from Table 3, the average values of elastic moduli are 370, 248, and 185 GPa for Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings, respectively. Considering that the BT18U titanium alloy has an elastic modulus of about 100–110 GPa [73], the Cr-Al-Si-N/DLC-Si coating is closest to it. An important result is noteworthy: when a DLC-Si coating is deposited to a titanium alloy, its elastic modulus is 228 GPa, and, when deposited to a previously deposited Cr-Al-Si-N ceramic sublayer, it decreases to 180 GPa. This observation correlates with the data of the previously published work [74], which proves that the use of buffer layers based on refractory metals makes it possible to reduce the elastic modulus of DLC coatings by reducing the stress at the interface and improving adhesion to a softer material (confirmed by the scratch test carried out in the present study).

The physical and mechanical characteristics of the three coatings obtained by nanoindentation (Table 3) make it possible to estimate an important parameter, the ratio of hardness (H) to elasticity modulus (E), which characterizes the elastic deformation of coatings before failure [75]. Practice shows that the best resistance to applied loads has hard coatings that provide a higher H/E ratio [76,77]. The specified values of H/E were 0.08, 0.09, and 0.13 for the three investigated coatings: Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si, respectively. Thus, the DLC-Si coating deposited under the same technological regimes but on different base materials (BT18U alloy and Cr-Al-Si-N sublayer) has different H/E ratio values. The recommended H/E ratio is 0.1 for hard thin coatings [78].

Analysis of experimental data and comparison of morphological patterns of titanium alloy samples before coating (Figure 6) and after coating (Figure 7) reveals the following regularities. The coatings duplicate the specific microrelief (irregularities and grooves) formed on the surface of titanium alloy samples during finishing milling in many respects. In addition, the surface of titanium alloy with coatings contains characteristic defects associated with the technological features of the coating deposition processes [79,80]. The observed decrease in the R_a roughness parameter (Table 4) allows us to assume some smoothing of the rough surface and conclude that less coating material is deposited on the surface protrusions than in the recesses. When the coatings are deposited on a surface after lapping with micro-grain abrasive, their morphological relief is represented mainly by a set of traditional structural features described in [81,82]: microdroplets (for the Cr-Al-Si-N sublayer) and isolated carbon particles having a spherical shape (for the DLC-Si outer layer). At the same time, the R_a roughness parameter increases (Table 4). Thus, the parameters of the micro-drop component and spherical particles largely determine the surface roughness of the samples with coatings deposited after lapping.

The adhesion bond strength values of different coatings deposited on the titanium alloy samples (Figure 8) correlate well with the H/E values. From a thermodynamic aspect, the destruction of the interface between two dissimilar bodies is defined as the amount of energy that should be expended to create free surfaces [83]. It can be assumed that the lower the stresses at the interface between the “soft” titanium alloy and the “hard” coating, the more energy is required for complete delamination of the coating. The values of maximum tangential stresses, which lead to delamination of coatings under the influence of external loads, can be reduced by reducing the elastic modulus of the coating. This is confirmed by the empirical dependence of the stresses arising in the plane of adhesive contact under the external load, which can cause adhesive destruction of the coating. [84,85]. Because of the above, it is logical that the Cr-Al-Si-N/DLC-Si coating, which has the elastic modulus closest to titanium alloy and the best H/E ratio, demonstrated higher LC2 and LC3 loads, indicating adhesive destruction and delamination of the coating from the samples. In addition, the two-layered Cr-Al-Si-N/DLC-Si coating structure provides an additional internal interface that ensures the dissipation of energy accumulation under load. The trajectory of cracks formed under load in Cr-Al-Si-N/DLC-Si coating is deflected at the interface between the external and internal layers. This assumption is confirmed by microscopic analysis of the destruction area after scratch testing (Figure 9) and EDS analysis of elemental composition (Table 5). The obtained results indicate that elements of the Cr-Al-Si-N internal layer are present significantly on the surface of the titanium alloy sample after delamination of the DLC-Si external layer.

Analysis of the influence of the samples’ initial state after milling and lapping with a micro-grained abrasive on the adhesion strength of various coatings (Figure 8) allows us to draw the following conclusions. A more defective surface layer negatively impacts the ability of coatings to resist cohesive and adhesion destructions and delamination during scratch testing. This can be explained by the adhesion value that depends on the presence and the number of bonds between the contacting bodies. In turn, the number of bonds is determined by the area of actual contact between the coating and the substrate (titanium alloy), which decreases in the presence of numerous grooves and irregularities. In addition, the irregular formation of coatings in depressions and protrusions can serve as crack nucleators and preferential locations for coating destruction [86].

The experimental data obtained when testing titanium alloy samples with different coatings under abrasive wear conditions (Figure 10 and Figure 11) allow us to conclude that the hardness of the surface layer is an important but not decisive characteristic. The Cr-Al-Si-N ceramic coating, with an average hardness of 30 GPa, resists abrasive wear significantly worse than the DLC-Si and Cr-Al-Si-N/DLC-Si coatings, with an average hardness of 23 and 24 GPa, respectively. The substantially lower values of volumetric wear under the abrasion exposure for DLC-based coatings (Table 6) can be explained by the significant differences in friction conditions of DLC-Si and Cr-Al-Si-N/DLC-Si coatings compared to a Cr-Al-Si-N coating.

The dependences of the friction coefficient change in time under fretting conditions demonstrate (Figure 12 and Figure 13) that Cr-Al-Si-N coating and uncoated titanium alloy samples demonstrate unstable friction coefficient evolution regardless of the surface layer condition (after finishing milling and after lapping with a micro-grained abrasive). For samples after lapping with a micro-grained abrasive, the deposition of a Cr-Al-Si-N coating slightly reduces the average friction coefficient (by 10%). For samples after finishing, no changes are observed. The revealed high amplitude (instability) of the friction coefficient for BT18U alloy and Cr-Al-Si-N coating results from alternating processes of adhesive grasping of contact surfaces and destruction of “bridges” of adhesive bonds. This indirectly indicates the absence of a strong interface film and the intense adhesion of the surfaces during contact.

DLC-Si and Cr-Al-Si-N/DLC-Si deposited on titanium alloy show a drastically different character of the friction coefficient variation with testing time under fretting conditions (Figure 12 and Figure 13). For DLC coatings deposited on a defective surface layer, the character of the friction coefficient variation is monotonic, and its average values are 0.15 and 0.2 under fretting conditions for DLC-Si and Cr-Al-Si-N/DLC-Si coatings, respectively (Figure 12a). Under more intense loading conditions in fretting (Figure 12b), the average friction coefficient for DLC coatings is slightly reduced (to less than 0.05). However, a sharp increase in the friction coefficient is observed after 100,000 cycles for the single-layer DLC-Si coating, which appears to be due to local ruptures of the coating, loss of its lubricating ability, outcrop of the base material (titanium alloy), and changes in the tribo-contact area. Cr-Al-Si-N/DLC-Si coating with higher adhesive bond strength and the best “hardness–elastic modulus” ratio for a longer time provides high lubricating ability and stable conditions in the tribo-contact area. A similar behavior is observed for DLC coatings deposited on samples with higher surface layer quality (after lapping with micro-grained abrasive) (Figure 13). After 21,000 fretting cycles, local destructions occur in the DLC-Si coating, and the friction coefficient increases rather sharply, while the Cr-Al-Si-N/DLC-Si coating continues to provide antifriction functions.

The above-described behavior patterns of different coatings ultimately influence the wear intensity of BT18U titanium alloy samples under fretting conditions. The optical images of wear spots of samples with the different surface conditions after finishing milling (Figure 13) and after lapping with micro-grained abrasive (Figure 14) demonstrate the effect achieved by Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si coatings. The main indicator for evaluating the effectiveness of the deposited coatings at the interaction of counter-bodies under fretting conditions is the volumetric wear of samples made of BT18U titanium alloy with coatings (Table 7). The outstanding results demonstrated by Cr-Al-Si-N/DLC-Si coatings (hundreds of times less volumetric wear compared to uncoated samples) are explained by the following. Under fretting conditions, a high amount of wear and significant damage to uncoated samples occurs with a minimal number of cycles and friction distance. In addition, the normal force is quite high, and the slip amplitude is minimal, which makes it much more difficult to remove wear products from the contact zone, which can further intensify wear. The presence of a Cr-Al-Si-N/DLC-Si coating on the surface, which exhibits a high hardness, a relatively low elastic modulus, and a reduced friction coefficient, ensures the long-term presence of a strong interfacial film in the contact zone that protects titanium alloy samples from destruction and reduces the amount of wear products.

5. Conclusions

(1)

The condition of the surface layer of titanium alloy samples on which thin film coatings are deposited significantly influences the physical and mechanical properties of coatings and the wear resistance of the coated material under external loads. When coatings are deposited on a surface with numerous irregularities and grooves, the coatings replicate the characteristic microrelief. At the same time, they provide multiple increases in wear resistance under abrasive exposure and fretting wear at the expense of a principal change in the conditions of adhesive and frictional interaction in the tribo-contact area.

(2)

When selecting thin film coatings for deposition on sufficiently “soft” titanium alloy along with high hardness, they should have an elastic modulus similar to the base material to reduce stresses at the “titanium alloy coating” interface and provide the highest adhesive bond strength under external loads.

(3)

Multilayer coatings increase adhesive bond strength with titanium alloy samples by creating additional internal interfaces and deflecting the trajectory of cracks generated under the external load. In scratch testing, the delamination character along the boundary between the outer and inner layers of the coating was confirmed by the example of the Cr-Al-Si-N/DLC-Si coating.

(4)

The best perspectives for increase of wear resistance of titanium alloy samples under conditions of abrasive exposure and fretting wear can be provided by coatings with a minimum friction coefficient, which exhibit high lubricating ability for a prolonged period and, therefore, stable conditions in the tribo-contact area at the expense of minimization of adhesion bonding.

(5)

The results of complex comparative research of three coatings, Cr-Al-Si-N, DLC-Si, and Cr-Al-Si-N/DLC-Si, deposited on titanium alloy samples demonstrated an outstanding effect of the Cr-Al-Si-N/DLC-Si coating in increasing wear resistance under conditions of abrasive exposure and fretting wear. The mentioned effect is achieved by high hardness, relatively low elastic modulus, and a reduced friction coefficient, which ensures prolonged availability of the strong boundary film in the tribo-contact area, protecting the surface layer of titanium alloy from destruction and minimizing the number of wear products.

(6)

The obtained positive results of the laboratory condition testing may allow us to proceed to the next stage: testing the coated experimental parts on specialized stands. The next level of this study would provide an even deeper understanding of the operating mechanisms since the acting loads and the configuration of the parts’ contact surfaces are as close as possible to real operating conditions.