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Article

The Role of Period Modulation on the Structure, Composition and Mechanical Properties of Nanocomposite Multilayer TiAlSiN/AlSiN Coatings

by
Stefan Kolchev
1,
Lilyana Kolaklieva
1,*,
Vasiliy Chitanov
1,
Tetiana Cholakova
1,
Ekaterina Zlatareva
1,
Daniela Kovacheva
2,
Genoveva Atanasova
2 and
Roumen Kakanakov
1
1
Central Laboratory of Applied Physics, Bulgarian Academy of Sciences, 61 St. Petersburg Blvd., 4000 Plovdiv, Bulgaria
2
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(9), 1546; https://doi.org/10.3390/coatings13091546
Submission received: 27 July 2023 / Revised: 18 August 2023 / Accepted: 26 August 2023 / Published: 4 September 2023
(This article belongs to the Special Issue Superhard Coating Materials and Deposition Technology)
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Abstract

:
This paper presents the results of the investigation of a multilayer TiAlSiN/AlSiN coating. A novel coating architecture with a period consisting of nanocomposite sublayers of TiAlSiN and AlSiN was developed. We discovered that the combination of a harder sublayer with a more elastic one allows for obtaining a suitable combination of superhardness and enhanced toughness. The coating was deposited by cathodic arc technology. The EDS, XRD, and XRS analyses revealed that the nanocomposite structure is composed of TiAlSiN and AlSiN nanocrystallites, with sizes of 12–13 nm and 4–5 nm, respectively. The nanograin phase is incorporated in an amorphous Si3N4 matrix. The achieved structure causes the presence of four factors contributing to the hardness increase: nanocomposition, solid solution, refinement hardening, and the formation of many interfaces. An instrumented indentation test was used to investigate the mechanical properties. The developed coating possesses a superhardness of 49.5 GPa and a low elastic modulus of 430 GPa, resulting in an improved elastic strain resistance of 0.11, a plastic deformation resistance of 0.58 GPa, and an elastic recovery of 68%. These results imply that the developed coating combines high stability with mechanical degradation under external influence and provides an improved ability to absorb energy at deformation before fracture, and high elastic recovery. The investigation of the effect of the period modulation on the structure, composition, and mechanical properties of the nanocomposite multilayer TiAlSiN/AlSiN coating showed that the superhardness was due to the nanocomposite and solid solution hardening rather than the increased number of interfaces. The demonstrated combination of superhardness with high elasticity and improved toughness determines the developed nanocomposite TiAlSiN/AlSiN coating as very suitable for industrial applications such as high speed and dry machining.

Graphical Abstract

1. Introduction

Among the different types of coatings used in modern industry, Ti-based ones are mainly applied for the improvement of the work performance of tools, machine details, and elements in order to extend their lifetime and corrosion resistance [1,2]. Titanium nitride, the first developed Ti-based coating, has been widely used until now, because of its high hardness and wear resistance [3]. The main disadvantage of that coating is its low-temperature stability (<500 °C). In order to overcome this drawback and meet the increased needs of the industry, approaches such as a change of the composition and/or structure and architecture of the coatings were applied.
The addition of alloyed elements such as Al, C, V, Cr, Si, B leads to the formation of ternary or quaternary compounds with enhanced mechanical properties, temperature stability, and wear resistance [4,5,6,7,8,9,10]. TiAlN coating gained a large research interest as it is very suitable for dry machining applications [11,12,13] due to its high hardness [14] and good wear resistance [15,16]. Furthermore, at temperatures of 800–1000 °C, a spinodal decomposition of TiAlN into cubic TiN-rich and AlN-rich domains takes place [17,18,19], which causes increased thermal stability [19]. The formation of quaternary compounds by the addition of different elements to TiAlN improves its performance. Thus, enhancement of the high thermal wear resistance was achieved by the alloying of V and the formation of V2O5 at increased temperatures, which acts as a self-lubricant [20,21]. The addition of Nb and Zr causes improvement of the mechanical and thermal properties, as well [22,23].
The presence of Si as a solution and/or amorphous phase in TiAlSiN coatings also enhances its properties [24,25]. Significant strengthening of the Ti-based coating performance was achieved by the development of the nc-TiAlSiN nanocomposite [26]. Its structure of nanocrystals of TiAlN embedded in a SiNx amorphous matrix ensures a very high hardness (>40 GPa) [27] and improved oxidation stability (>900 °C) [28,29], which determines the TiAlSiN nanocomposite as very appropriate for the high-speed and dry machining industries [30]. The effect of the Si content on the hardness of the TiAlSiN nanocomposites was well investigated [29,31,32,33,34,35]. There was reported a very high hardness of 55 GPa at a 9% Si content, which was attributed to the formation of a nanocomposite structure wherein nanocrystallites are embedded in an amorphous matrix of SiNx [31].
Mostly, the industrial applications of coatings with very high hardness are limited due to their brittleness. The enhanced hardness could be caused by the compressive residual stress arising out of the ion bombardment during deposition. The latter is characteristic of the most technological techniques used for coating deposition. Contrariwise to the effect on the hardness, the residual stress reduces the coating toughness [36,37] and, consequently, the wear resistance. However, industrial applications such as high-speed and dry machining require very hard coatings with enhanced toughness. A multilayer architecture is a very promising approach to overcoming the nanocomposite brittleness, and coatings have been developed combining superhardness with enhanced elasticity, i.e., high toughness, and consequently, high wear resistance. Multilayers consisting of periodically alternating sublayers enable the obtaining of a coating with desired properties by a suitable selection of composition, structure, and thickness of the sublayers. Furthermore, two phenomena in the multilayer architecture benefit the multilayer advantages. The existence of many interfaces in the multilayers hinders the dislocation movement, as they act as storage sites for the defects [38], and thus facilitate the high hardness. Moreover, the crack propagation direction is changed at the interface [39,40], causing the toughness improvement. Among the wide variety of reported multilayers, coatings with a nanocomposite TiAlSiN sublayer of the period are rather limited [41,42,43,44,45,46]. In most of the reported coatings, Ti is the basic element in the second sublayer. The latter could be a Ti interlayer [44], or binary (TiN [41]), ternary (TiAlN [42] or AlTiN [45]), or quaternary (TiAlVN [28]) compounds. The dependence of the mechanical and tribological properties [28,41,42,43,45], thermal stability [28,47], and oxidation stability [47] on the coating composition, microstructure, and period thickness were investigated in detail, showing the advantages of the multilayer architecture. Multilayer coatings with non-Ti-based film as a second sublayer are very few [43]. The hardness of the reported multilayered coatings does not exceed 36–38 GPa, and the elasticity varied from 0.07 to 0.09.
To the best of our knowledge, multilayer coatings combining very high hardness and enhanced toughness composed of superhard sublayers are not reported. The presented work aimed at the development and investigation of a multilayer superhard coating with high toughness intended for industrial applications. The combination of the advantages of nanocomposites and multilayers is a very promising approach for achieving this purpose. Consequently, a multilayer coating with a period of nanocomposites with very high hardness and high elasticity should be developed. The TiAlSiN nanocomposite is a very suitable sublayer of such coating architecture due to its good mechanical and thermal properties. Our previous studies, [48,49], have revealed that the use of a nanocomposite AlSiN as a second sublayer in the multilayer architecture benefits the improvement of the coating strength, toughness, and high thermal stability. Accordingly, a coating consisting of TiAlSiN and AlSiN superhard nanocomposite sublayers was designed and developed. In order to obtain the optimal coating architecture, the effect of the period modulation on the structure, composition, mechanical properties, and toughness of the developed superhard TiAlSiN/AlSiN multilayer coatings was investigated and is presented.

2. Materials and Methods

2.1. Coating Preparation

Sets of specimens including single-layer referent nanocomposite coatings, sl-TiAlSiN and sl-AlSiN, and two multilayer TiAlSiN/AlSiN structures were prepared. In order to investigate the influence of the period modulation on the mechanical and structural properties, one of the multilayer structures was created with 3 periods of modulation (named ml3-TiAlSiN/AlSiN) and the other one had 30 periods (named ml30-TiAlSiN/AlSiN). The investigated coatings were obtained on two types of substrates. The first group contained 20 mm in diameter and 5 mm thick high-speed stainless steel (HSS DIN 1.3343) coupons, which were used for the investigation of the mechanical properties. The second group contained 10 mm × 10 mm square stainless steel DIN 1.4541 plates with a thickness of 1 mm used for investigation of the composition and structure. The substrate surface was preliminary wet cleaned in an ultrasonic bath with an alkaline solution followed by plentiful rinsing in de-ionized water and drying at 130 °C.
The coatings were deposited by cathodic arc evaporation in a piece of equipment, a Platit π80+ with a LARC® (LAteral Rotating Cathode) system (Figure 1a), consisting of two cathodes, Ti (99.99 wt.%) and alloyed AlSi (82 at. % Al, 18 at. % Si). A three-axis planetary system was used to obtain uniform coating thickness. Immediately, additional surface treatment was performed with Ti ions in an Ar discharge at a bias voltage of 1000 V before the coating deposition. Nitrogen with a purity of 99.9999% was used as a reactive gas. The coatings were deposited at a pressure in the range of 9 × 10−1 Pa to 4 Pa, depending on the required composition.
The investigated coatings consisted of a consequently deposited Ti contact layer ensuring good adhesion to the substrate, TiN transition layers, and a TiAlSiN gradient layer, followed by the main structure of the coating. The main structure was formed by alternating periods with TiAlSiN and AlSiN sublayers. The composition of the period was changed by variation of the cathode current. During the deposition of the TiAlSiN sublayer, the current of the Ti and Al + Si cathodes was kept at 130 A. The AlSiN sublayer was formed at IAlSi = 160 A and ITi = 0 A. The bias voltages were maintained at –45 V and –90 V for the TiAlSiN and AlSiN sublayers, respectively. The coating architecture was completed with a top TiAlSiN layer. The referent sl-TiAlSiN and sl-AlSiN coatings were deposited with the same technological conditions as the corresponding sublayers. The multilayer coatings and referent nanocomposites were deposited at a constant temperature of 500 °C. After coating deposition, two-hour annealing at 525 °C in a nitrogen ambience was performed to complete the formation of the nanocomposite structure.

2.2. Characterization Methods

The coating thickness was determined by a compact calotest CAT2/ball-cratering thickness tester (Anton Paar, Graz, Austria). Diamond suspension with a grain size of 0.5 μm was applied to the ball to form a spherical crater by a rotating steel ball in a diameter of 20 mm. Determination of the diameter of the grinding image was carried out under an optical microscope (Anton Paar Tritec SA, Corcelles, Switzerland) with a magnification of 100×. The total coating thickness and the sublayer thickness were calculated by the specialized Video Software Version 9.0.12 of the calotest equipment. Obtaining a magnified imprint on the PC screen allows precise calculation of the results [50,51]. This methodology applies to coatings with thicknesses between 0.1 and 50 μm. The period thickness and sublayer thickness were determined by SEM from the cross section of the multilayer coatings.
The mechanical properties were evaluated by nanoindentation within the loading interval of 5–500 mN using a Compact Platform CPX (MHT/NHT) system (CSM Instruments, Anton Paar, Austria) equipped with a certificated diamond Berkovich indenter. The hardness, elastic modulus, penetration depth, and stiffness were determined from the load–displacement curves by the Oliver and Pharr method [52].
The coating surface and cross section were observed on a Hitachi SU 5000 Schottky field emission scanning electron microscope (Hitachi, Japan) (Figure 1b). Secondary electron (SE) and backscattered electron (BSE) images were taken to characterize samples by topology and element composition. Energy-dispersive X-ray spectroscopy (EDS) was used for element analyses in point and linear modes by an energy-dispersive X-ray microanalysis system (Thermo Scientific, Madison, WI, USA). The cross sections were obtained by 100 μm etching of the sample edge in an ion milling machine, IM 4000 Plus (Hitachi, Tokyo, Japan). The etching was performed using an ion gun in an argon ambiance at an accelerating voltage, Uacc = 6 kV, and a discharge voltage of 1.5 kV. During etching, a constant argon flow of 0.1 sccm was maintained, the swing of the holder was ±30°, and a speed of 30 reciprocations/min was maintained. A powder XRD pattern was collected in the 10–90° 2θ range in a standard Bragg–Brentano focusing geometry with CuKα in a Bruker D8 Advance diffractometer using a LynxEye detector. The phase composition was determined using EVA software and the ICDD-PDF2 database. The Thopas-4.2 program was used so that the unit cell parameters and crystallite size of the phases were calculated. The XPS measurements were carried out on an AXIS Supra electron spectrometer (Kratos Analytical Ltd., Manchester, UK) using AlKα radiation with a photon energy of 1486.6 eV. The energy calibration was performed by normalizing the C1s line of adsorbed adventitious hydrocarbons to 284.6 eV. The binding energies (BE) were determined with an accuracy of ±0.1 eV and the deconvolution of the peaks was performed using the commercial data-processing software ESCApeTM of Kratos Analytical Ltd.

3. Results and Discussion

3.1. Thickness Determination

The images from the thickness measurement by the calotest are presented in Figure 2. The abraded craters are easily visible, as two areas could be distinguished in them, the inner circle corresponding to the erased part of the substrate and the outer ring related to the coating. Total thicknesses of 3.50 μm and 3.21 μm were determined for the referent coatings sl-TiAlSiN (Figure 2e) and sl-AlSiN (Figure 2f), respectively. These results are comparable with the thickness of the sl-TiAlSiN (3.14 µm) and sl-AlSiN (3.39 µm) monolayers obtained by SEM from their cross sections (not presented here).
The multilayer architecture of the ml3-TiAlSiN/AlSiN coating (Figure 2a) is distinctly expressed in the calotest image. Accordingly, the ml3-TiAlSiN/AlSiN coating has a total thickness of 3.81 μm. SEM measurements on the coating cross section were performed in order that more accurate information about the period and sublayer’s thickness could be obtained. Figure 2b presents the cross section of the ml3-TiAlSiN/AlSiN coating. Its total thickness, as measured by SEM, was 3.97 µm, which is in good agreement with the thickness obtained by the calotest. The thickness of the TiAlSiN sublayer was 412 nm, while the AlSiN sublayer was 836 nm thick. These values determine the period thickness of 1.248 µm. A very close period dimension of 1 μm was measured by calotest.
Due to the limited resolution of the calotest microscope, it was impossible for the multilayer structure in the impression image of the ml30-TiAlSiN/AlSiN coating to be distinguished (Figure 2c). The total coating thicknesses of 3.30 μm and 3.37 µm were determined by calotest and SEM, respectively. The period thickness, as measured by SEM (Figure 2d), was 123 nm. The thicknesses of 44 nm and 79 nm were determined for TiAlSiN and AlSiN sublayers, respectively.

3.2. Composition and Structure

The element composition of the coating surface and in the depth was investigated by EDS analysis in Point ID and Linear Scan modes. Figure 3 presents the SEM images of the surfaces of the multilayer and referent coatings. It can be seen that the surfaces are similar regarding their features. Groove traces originating from the substrate are visible on the surfaces of all coatings. The bright particles are associated with solidified molten metal droplets, which is characteristic of the cathodic arc evaporation technique. However, craters, the other feature attributed to the technology used, are not observed. The absence of craters on the coatings’ surfaces presumes that the kinetic energy of the droplets adhered to the surface is sufficient to prevent their peeling off. Therefore, the element content of the surface was determined on two characteristic areas: a matrix and droplets. The average values obtained from measurements in several points are presented in Table 1.
We discovered that the element content of the matrix and droplets in each coating does not differ essentially. Unusually, the nitrogen content in the droplets is high, which might be caused by the well-balanced technological parameters, and as a result, the molten metal droplets adhere on the surface as stoichiometric or sub-stoichiometric nitride. Expectably, the element concentration of matrix/droplets in both multilayer coatings is very similar because the top layer in them is the same TiAlSiN top layer. The small carbon concentration detected in the surfaces is contaminations due to air exposure.
The element composition of the multilayer and referent coatings was investigated by an EDS analysis in a Linear Scan mode through their cross sections. Figure 4 presents the cross-section images and corresponding EDS results of the investigated coatings. The coatings’ architecture is well expressed in the images. The bright and dark layers in the cross section correspond to the TiAlSiN and AlSiN sublayers of the period. The morphology of the cross sections revealed a dense structure, as no features of columnar growth were observed. The element concentration and composition of the investigated coatings are presented in Table 2. All coatings have a (Ti + Al + Si)/N ratio of 1 ± 0.04, which defines their composition as stoichiometric. Further, the Si/Al ratio is 0.22 ± 0.02 for the referent sl-AlSiN coating and the corresponding sublayers of the multilayer coatings. This result indicates that the Al atoms do not hamper the incorporation of the Si atoms into the surface. Hence, no re-sputtering of the Si atoms takes place during the deposition of the AlSiN layers despite the higher energy of the Al atoms. The high Si concentration is a prerequisite for the grain size refinement, as well as the segregation around the nanograins and the formation of a SiNx amorphous phase. Thus, the formation of a nanocomposite structure in each sublayer is quite achievable. We found that both multilayer coatings have almost the same element composition. This result indicates that the element composition could be excluded as a factor influencing the mechanical properties of the multilayer coatings.
XRD results are presented in Figure 5. The substrate shows typical peaks of stainless steel (SS) corresponding to the PDF card #00-031-0619. The cubic unit cell parameter of the SS phase was a = 0.3560(1) nm and the mean crystallite size was about 47 nm. A slightly preferred orientation along the <111> direction was observed. Along the peaks from the substrate, the diffraction pattern of the multilayer coatings contains two phases, fcc-TiN and hcp-AlN, belonging to the coating film. The most intensive peak detected in both multilayer coatings, at 2θ of 42.51° is associated with a <200> reflection of the fcc-TiN phase. Its high intensity suggests a preferred orientation of the crystallites of this phase along the <200> direction. This peak is slightly shifted to the lower angles as compared to the standard peak of TiN (PDF 00-038-1420), indicating a change of the unit cell parameters of the TiN phase by partial substitution of the Ti atoms by the Al (or Si) ones with a smaller atomic radius [53]. Hence, an fcc-(TiAl(Si))N phase was identified in the multilayer coatings. The replacement of the Ti atoms was confirmed by the calculated unit cell parameters a = 0.4187(7) nm and a = 0.4186(9) nm for ml3-TiAlSiN/AlSiN and ml30-TiAlSiN/AlSiN, respectively, which values are smaller than that of TiN (a = 0.4241(7) nm). The addition of Si to the coating composition causes a change in the preferred crystal growth plane from perpendicular to the <111> direction to perpendicular to the <200> direction in the TiAlSiN films [35]. The <111> peak intensity decreases as much as the Si content is increased. When the silicon content exceeds 4 at. %, an amorphous Si3N4 matrix, in which TiAlN nanocrystallites are incorporated, is formed. However, with a Si content of as high as 22 at. %, no crystallization was observed [35]. The XRD results show that the change in the modulation period does not lead to a change in the diffraction peak positions but affects their intensity. It is noticeable that the peak at 42.51° of the ml30-TiAlSiN/AlSiN coating is slightly broader and with lower intensity than those of the 3-period coating. This is due to a decrease in the sublayers’ thickness in the 30-period coating, which hampers the grain growth. Similar behavior was observed by A. Getachew and T. Wang for AlTiN/AlTiSiN multilayers [45]. The peak broadening could be caused by the crystallite size refinement due to the Si presence in the coatings’ composition and/or the micro-strains in the crystal lattice [47]. The mean crystallite size of about 12 nm was determined for the ml3-TiAlSiN/AlSiN coating, and of 13 nm for the ml30-TiAlSiN/AlSiN one. As far as the crystalline size in both multilayers not differing significantly, it could be supposed that the slight peak broadening in the 30-period coating is due to the micro-strains that should be higher in the 30-period coating because of the larger number of interfaces.
The second phase determined in the multilayer coatings was a hexagonal, wurtzite-type AlN with its most indicative peak with indexes of the <110> plane positioned at 59.15° and 59.21° for the ml3-TiAlSiN/AlSiN and ml30-TiAlSiN/AlSiN coatings, respectively. This peak is slightly shifted to the lower 2θ values as compared to the standard hcp-AlN peak (PDF 00-025-1133), implying that Si atoms with a smaller atomic radius might partially substitute for the Al ones in the AlN-lattice-forming AlSiN solid solution [54]. Because of the higher Si content (>7%), the peaks of the hexagonal phase in the 2θ range of 31° to 38° are not easily visible. They are expressed as a hump with very low intensity, indicating the existence of an amorphous phase [55]. The unit cell parameters of a = 0.314(1) nm and c = 0.508(2) nm and mean crystalline size of 5 nm were determined for ml3-TiAlSiN/AlSiN. The cell parameters of the ml30-TiAlSiN/AlSiN coating were calculated to be a = 0.3139(7) nm and c = 0.5168(7) nm. A crystallite size of about 4 nm was found. It should be noted that due to the very small crystallite size, leading to the peak broadening, these lattice parameters were determined with insufficient accuracy. The presence of a small amorphous component was also detected. As a whole, the obtained values of the cell parameters in both multilayers are a little higher than that of the standard AlN. Most probably, this discrepancy is due to the tendency of the lattice of the hexagonal phase to match the lattice of the cubic phase in the coating, or to the incorporation of Si.
The evaluation of the quantities of the two phases resulted in approximately 28 mass% Ti(Al)N and 72 mass% Al(Si)N in the ml3-TiAlSiN/AlSiN coating and about 15 mass% Ti(Al)N and 85 mass% Al(Si)N in the ml30-TiAlSiN/AlSiN coating. The domination of the Al(Si)N phase in multilayer coatings is consistent with the results obtained from EDS measurements.
The presence of individual SiNx or silicide phases were not observed, which indicates that Si may either partially substitute for Ti or Al, thus forming solid solution phases, or be presented in the amorphous part.
Figure 5 shows that the XRD spectra of the referent coatings do not differ essentially from that of the multilayers. The XRD pattern of the sl-TiAlSiN coating contains TiAlN as a main phase (> 70 mass %), with unit cell parameter a = 0.4189(9) nm and a mean crystallite size of about 17nm. The preferred orientation of the TiN phase along the <200> direction is also observed for this coating. The second-phase AlN with a very low crystallite size (< 4 nm) contributes mainly to the amorphous part of the diffraction pattern, and its unit cell parameters could not be determined properly. The XRD patterns of the sl-AlSiN coating indicate that the film crystallizes in a hexagonal AlN type. The diffraction peaks could be assigned to the hcp-(AlSi)N phase [56]. In contrast to the corresponding sublayers of the multilayer structures, the peaks in the 2θ range of 31° to 38° are more expressed. A value of 5 nm for the crystallite size was determined in the sl-AlSiN coating. Similarly, in the multilayers, no Si-N phase was detected in the single-layer referent coatings, which indicates that SiNx might exist in the film in an amorphous state.
The XRD analysis of the developed coatings revealed that a nanocomposite structure was formed and grain refinement was achieved as a result of the high Si content. Further, solid solution TiAl(Si)N and AlSiN compounds were formed. The presence of these prerequisites is indicative that the developed multilayer coatings consist of nanocomposite sublayers. The latter is a precondition for high hardness stability at high temperatures.
The compound composition of the multilayer and referent single coatings was identified by XPS analysis. The measurements were performed on the coating surfaces after 10 min etching by Ar+ ions. It should be noticed that the top layer of the referent and multilayer coatings is TiAlSiN deposited with the same technological regime. Therefore, the results collected from the measurement of their surfaces provided information about the chemical bonding state and chemical composition of the TiAlSiN sublayer of the multilayers and the sl-TiAlSiN referent coating. Expectably, the obtained XPS spectra do not differ in respect to the binding energy position of the constituent elements. The high-resolution Ti2p, Al2p, Si2p, and N1s spectra are presented in Figure 6. The Ti2p peak is expressed by spin doublet peaks, Ti2p3/2 and Ti2p1/2, centered at 455.8 eV and 461.3 eV, respectively (Figure 6a). The Ti2p peak was deconvoluted into three double peaks. The deconvolution shows that the Ti2p3/2 spectrum consists of peaks with core levels at 455.5 eV, 457.5 eV, and 459.6 eV. The 455.5 eV peak corresponds to the bonds in TiAlN [57], while that at 457.5 eV is associated with TiOXNY [58]. The least intensive peak, with BE 459.6 eV, can be attributed to TiO2. The presence of oxygen in the spectra is caused by surface contamination due to air exposure. The Al2p spectra are also deconvoluted into three peaks (Figure 6b). The most intensive peak corresponding to the binding energy of 74.5 eV was recognized as an Al-N bond in AlN [58]. The peak positioned at a lower BE of 73.6 eV is assigned to TiAlN or non-stoichiometric AlNx [57]. Because the presence of TiAlN was identified by the Ti2p spectrum, it could be assumed that this peak of the Al2p spectrum corresponds to the TiAlN solid solution. The peak positioned at BE 75.6 relates to the Al-O bond in Al2O3 [58].
The deconvolution of the N1s spectrum revealed that it is composed of two peaks centered at 397.3 eV and 398.5 eV, determining two nitride compounds (Figure 6c). The more intensive peak was found at BE 397.3 eV, which position associates with nitrogen bonds in the compounds TiN (397.2 eV) [59] and AlN (397.3 eV) [60]. The existence of these complexes of nitrides and the N1s spectrum indicates that the formation of a TiAlN solid solution is assumed, which is consistent with the results from the Ti2p spectrum and XRD analysis. The peak located at BE 398.5 eV corresponds to Si-N bonds in Si3N4 [61]. The Si2p core level is positioned at a binding energy of 101.8 eV, associated with Si-N bonds in Si3N4 [61] (Figure 6d). No other peaks related to Si were detected in the Si2p spectrum, which implies that other silicides do not exist in the coatings developed. As no crystalline Si-containing phase was detected by XRD analysis, it could be assumed that Si3N4 is in an amorphous state.
The spectra taken from the surface of the sl-AlSiN coating are representative of the composition of the AlSiN sublayers of the multilayers and the referent sl-AlSiN coating, which were deposited with the same technological regime. The positions of the Si2p peaks coincide with that of the quaternary TiAlSiN compound (Figure 6c,d), implying the same boundary state and chemical compound. The N1s spectrum is composed of peaks related to AlN and Si3N4. The deconvolution of the Al2p peak reveals that it is composed of non-stoichiometric AlNx, stoichiometric AlN, and aluminum oxides. No peaks of the Ti2p spectrum were found. Hence, two types of dominated bonds, Al-N and Si-N, were detected in the AlSiN sublayers of the multilayers and the referent sl-AlSiN.
The results of the XRD and XPS analyses revealed that the developed multilayer coatings have a period consisting of nanocomposite sublayers. They are composed of TiAlN and AlSiN nanograins embedded in an amorphous matrix of Si3N4. The modulation of the period does not affect the coating structure and composed phases. However, the increase of the period modulation creates a larger number of interfaces and thus hinders the coherent crystallite growth, and this could cause an increase in the micro-strain. The quantitative evaluation of the element concentrations (EDS), obtained phases (XRD), and compounds (XPS) showed that the Al-N bonds prevailed in the developed multilayer coatings, indicating improved elasticity [48].
The XPS analysis showed that three types of bonds (Ti-N, Al-N, and Si-N) exist in the developed coatings. The stronger Ti-N and weaker Al-N bonds make the main contributions to the coating’s hardness and elasticity. Their presence implies that the developed multilayers may combine high hardness with improved toughness [48].

3.3. Mechanical Properties

The as-measured load–displacement curves of the multilayers and single referent coatings, obtained at 30 mN loading, are presented in Figure 7. The loading and uploading parts are smooth, without any pop-in occurrence. The negligible waviness is expressed at small (<40 mN) loading forces and is due to the higher surface roughness characteristic for coatings deposited by cathodic arc technology. It was not observed at higher loading corresponding to a penetration depth of almost 50% of the coating thickness. This result implies that no transformation from elastic to plastic deformation occurs even at high loading forces, which indicates that no interaction between the grain bonders, an amorphous Si3N4 phase, and interfaces happens [41]. The latter presumes that the developed TiAlSiN/AlSiN multilayer coatings are more stable against plastic deformation than the TiN/TiAlSiN multilayers [41]. The comparison of the load–displacement curves regarding their stiffness and the maximum displacement into the coating suggests that the ml3-TiAlSiN/AlSiN coating possesses the highest hardness. This suggestion is confirmed by the hardness’s dependence on the penetration depth, presented in Figure 8. Typically, the hardness of all coatings tends to gradually decrease with an increase in the indentation depth. The maximum hardness of the coatings was measured at between 200 nm and 400 nm, which interval lies within the commonly accepted 10% range of the total coating thickness, where the substrate influence is negligible. Further, the hardness decreased gradually with an increase in the loading, and, respectively, the indentation depth. The decrease in the hardness in the coating depth could be caused by different factors, such as substrate influence, indentation size effect, and the formation of micro-cracks [62]. The decrease is more strongly expressed in the multilayer coatings, whereas the hardness of the single layers decreases more slowly. This behavior could be attributed to the change in the period composition, resulting in an alteration of the hardness inside the coating period. The smaller period thickness the thinner the gradient area, or it completely disappears and the interfaces become sharper. This may cause a more rapid change in the nanohardness. These changes follow the period modulation through the coating thickness.
The multilayer coatings exhibited a higher hardness as compared to the single ones. (Figure 9). Hardnesses of 49.5 GPa and 43.4 GPa were determined for ml3-TiAlSiN/AlSiN and ml30-TiAlSiN/AlSiN coatings, respectively. The hardnesses measured for the referent sl-TiAlSiN and sl-AlSiN coatings were 41.5 GPa and 40.6 GPa, respectively. The obtained hardness values determine the developed coatings as superhard. The nanocomposite hardness depends on different factors, such as the type of the chosen materials, i.e., chemical bonds, solution hardening, Hall–Petch strengthening, and the formation of a stable nanocomposite structure [35,63]. The introduction of silicon in the composition of the developed coatings has an essential role in the formation of a nanocomposite structure and the enhancement of their hardness. The effect of Si incorporation has been widely investigated and it is well known that it leads to several hardening mechanisms, depending on the percentage content. Thus, a Si content of up to 4 at. % causes a grain size decrease. The refined grains hamper the dislocation movement, yielding the hardness enhancement [64], e.g., refined hardening. Furthermore, the introduction of a small Si quantity supports the formation of a solid solution Ti(Al,Si)N phase in Ti-Al-based coatings, which also leads to a hardness increase. The increase of Si in the coating composition to over 4 at. % and up to 10 at. % causes the formation of a thin (~1 nm) amorphous Si3N4 phase around the nanocrystallites. The increased cohesive interface between the Ti(Al,Si)N and Si3N4 phases hinders the grain boundary sliding [61], which also contributes to the hardness enhancement. The further increase of Si in the composition leads to an increase in the tissue phase and a decrease in the hardness. The EDS analysis revealed that in the developed coatings, the Si content varied from 4.02 at. % to 8.80 at. %. As it was obtained from the XRD and XPS analyses, this quantity causes the formation of TiAlN and AlSiN solid solutions with grain sizes of 12–17 nm and 4–5 nm, respectively, and an amorphous Si3N4 phase. Consequently, three factors determine the superhardness of the developed coatings: refined hardening, solid solution hardening, and the formation of a nanocomposite structure. These factors are decisive for overcoming the influence of the softening effect of the wurtzite-type AlN phase, even in the sl-AlSiN coating.
The hardness of the multilayer coatings exceeds that of the single-layer ones and the values obtained by the mechanical mixing rule [65]. According to it, the hardness of the ml3-TiAlSiN/AlSiN coating should be 41.03 GPa, and of the ml30-TiAlSiN/AlSiN coating, 41.05 GPa, which values differ significantly from the measured ones. The discrepancy is 8.5 GPa and 2.4 GPa for the coatings with 3 periods and 30 periods of modulation, respectively. It provides information about the hardness enhancement in the multilayer structure. The latter is attributed to the existence of many interfaces, which generate significant micro-stress [47]. As much as the micro-stress increases, the grain size decreases. Therefore, it is understood that the increase in period modulation causes an increase in hardness. However, the study of the mechanical properties showed that the hardness of the ml3-TiAlSiN/AlSiN coating is higher. The same effect was reported for TiAlSiN/VSiN multilayers [43]. The hardness decrease with the increase in the period modulation was explained by the interference of the shear stress by the adjacent interfaces, which results in the application of an opposite image force on the dislocation, leading to a decrease in the maximum sheared stress, and, respectively, the hardness, in the coatings with a very small period [43]. Despite the sublayers’ thicknesses in the ml30-TiAlSiN/AlSiN coating being 44 nm and 79 nm, they could not be considered very thin. Consequently, the shielding effect of the neighboring interface could not explain the observed lower hardness. We suggest that the observed effect relates to the coating composition. As XRD analysis revealed, the quantity of the softer wurtzite phase in the ml30-TiAlSiN/AlSiN coating is 18% higher than that in the ml3-TiAlSiN/AlSiN one, which presumes lower hardness. Indeed, the measured hardness of ml30-TiAlSiN/AlSiN coating is 14% lower than that of the multilayer coating with three periods of modulation.
Hardness is a very important property of the coatings for their industrial applications. The higher the hardness, the higher the resistance to plastic deformation. However, superhardness usually relates to a high elastic modulus, which makes the coating brittle. In order for this contradiction to be avoided, a low elastic modulus is required to distribute the external loads over a wider area. Elastic moduli of 437 GPa, 430 GPa, 403 GPa, and 365 GPa were determined for the sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings, respectively (Figure 9a). The obtained values could be considered low for superhard coatings. Our previous investigation showed that the existence of a phase with a weaker bond in the coating composition causes a decrease in the elastic modulus. The lower the bond strength, the lower the elastic modulus of the material is [48]. In the investigated coatings, such a bond is Al-N (297 ± 96 kJ/mol). The amount of the AlN phase increased in the multilayer coatings, toward the sl-TiAlSiN one, and the elastic modulus decreased, as well. Expectably, it is lowest in the sl-AlSiN coating. The combination of superhardness with a low elastic modulus indicates improved coating toughness. The ratios H/E* and H3/E*2 were introduced as a measure of the coating’s mechanical properties, as they are indicative of the tribological properties. In these ratios, E* is an effective elastic modulus calculated at a Poisson ratio of ν = 0.25 [43]. The H/E* ratio is a measure of the elastic strain resistance of the coating before failure. It is accepted that at H/E* ≥ 0.1, a coating has a low coefficient of friction and good wear resistance. The H3/E*2 ratio gives information about the coating’s resistance to plastic deformation. Figure 9b shows that the multilayer architecture significantly improves the toughness of the ml3-TiAlSiN/AlSiN and ml30-TiAlSiN/AlSiN coatings, as compared to the sl-TiAlSiN one. These coatings also possess a high elastic recovery of between 65% and 68%, which demonstrates their ability to recover after external influence.
The developed multilayers are compared with the reported ones, which have a TiAlSiN sublayer in the period. The values of the main parameters characterizing the mechanical properties are presented in Table 3. It should be noted that despite their superhardness, the developed multilayers possess a quite low elastic modulus. It is comparable with the elastic modulus of the coatings with a lower hardness of 35–37 GPa. The achieved significant decrease in the elastic modulus allows an improvement of their toughness. Further, the multilayer architecture and the nanocomposite structure of the sublayers decelerate the propagation of cracks originating from external influence and thus contribute to the brittleness decrease [66].

4. Conclusions

This work presents the results from the investigation of the developed nanocomposite multilayer TiAlSiN/AlSiN coating. The composition, structure, and mechanical properties were studied thoroughly by EDS, XRD, and XPS analyses, and an instrumented indentation test. The period of the multilayer is composed of TiAlSiN and AlSiN sublayers. The XRD analysis revealed that both sublayers have a nanocomposite structure consisting of two phases, nanograins and an amorphous phase. The nanograin phase is formed by TiAlSiN and AlSiN crystallites with sizes of 12–13 nm and 4–5 nm, respectively. The nanograins are incorporated in a Si3N4 amorphous matrix, which is formed because of the high Si content (7.62 at. %) in the coating composition. The high Si content causes grain size refinement as well. Thus, due to the appropriate multilayer design and the developed proper technology, a structure combining the advantages of the nanocomposites and multilayers was achieved. As EDS, XDR, and XPS analyses showed, the developed coating structure combines factors such as grain refinement, solid solution formation, a nanocomposite structure, and a multilayer architecture, which benefit the superhardness being obtained. Furthermore, the predominant presence (72 mass %) of the weaker Al-N bonds in the nanocomposites improved the coating elasticity. These results indicate that a very suitable structure for a combination of superhardness with enhanced elasticity was developed. The latter was confirmed by the results of the mechanical properties investigation. The developed TiAlSiN/AlSiN coating with three periods of modulation possesses a superhardness of 49.5 GPa and a low elastic modulus of 430 GPa. This achievement results in an improved elastic strain resistance H/E* = 0.11, plastic deformation resistance H3/E*2 = 0.58 GPa, and a high elastic recovery of 68%, which indicates improved toughness. We found that the coating superhardness was due to the nanocomposite and solid solution hardening rather than the increased number of interfaces. Therefore, the increase in the period modulation in the TiAlSiN/AlSiN coating does not cause an increase in the hardness.
The obtained results indicate that the developed coating demonstrates high stability to mechanical degradation under external influence, the ability to absorb energy at deformation before fracture, and high elastic recovery. Consequently, the nanocomposite multilayer TiAlSiN/AlSiN coating combines high elasticity and improved toughness with superhardness. These features determine it as very appropriate for industrial applications such as high-speed and dry machining. Therefore, its thermal and wear resistance will be subjected to subsequent investigations.

Author Contributions

Conceptualization, L.K. and R.K.; Formal analysis, S.K., V.C. and T.C.; Investigation, L.K., S.K., D.K., G.A., V.C., T.C. and E.Z.; Methodology, L.K. and R.K.; Project administration, L.K. and R.K.; Resources, R.K.; Supervision, L.K.; Validation, L.K., R.K. and D.K.; Visualization, L.K., and E.Z.; Writing—original draft, S.K., L.K., D.K. and G.A.; Writing—review and editing, L.K., R.K., and D.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the European Regional Development Fund within the OP “Science and Education for Smart Growth 2014–2020”, Project CoE “National center of mechatronics and clean technologies”, No. BG05M2OP001-1.001-0008-C01.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are not publicly available due to issues related to proprietary rights.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photos of the equipment used for: (a) coating deposition (Platit π80+) and (b) element composition measurement (Hitachi SU 5000).
Figure 1. Photos of the equipment used for: (a) coating deposition (Platit π80+) and (b) element composition measurement (Hitachi SU 5000).
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Coatings 13 01546 g002aCoatings 13 01546 g002b
Figure 2. Images of the coating’s thickness measurements: (a) calotest image of ml3-TiAlSiN/AlSiN; (b) cross section of ml3-TiAlSiN/AlSiN; (c) calotest image of ml30-TiAlSiN/AlSiN; (d) cross section of ml30-TiAlSiN/AlSiN; (e) calotest image of sl-TiAlSiN; (f) calotest image of sl-AlSiN.
Figure 2. Images of the coating’s thickness measurements: (a) calotest image of ml3-TiAlSiN/AlSiN; (b) cross section of ml3-TiAlSiN/AlSiN; (c) calotest image of ml30-TiAlSiN/AlSiN; (d) cross section of ml30-TiAlSiN/AlSiN; (e) calotest image of sl-TiAlSiN; (f) calotest image of sl-AlSiN.
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Figure 3. SEM images of the coatings’ surfaces: (a) ml3-TiAlSiN/AlSiN; (b) ml30-TiAlSiN/AlSiN; (c) sl-TiAlSiN; (d) sl-AlSiN.
Figure 3. SEM images of the coatings’ surfaces: (a) ml3-TiAlSiN/AlSiN; (b) ml30-TiAlSiN/AlSiN; (c) sl-TiAlSiN; (d) sl-AlSiN.
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Figure 4. SEM images of the cross section and EDS linear scan graphs of (a) ml3-TiAlSiN/AlSiN; (b) ml30-TiAlSiN/AlSiN; (c) sl-TiAlSiN; (d) sl-AlSiN. Element spectrum: red—N, green—Al, blue—Si, yellow—Ti.
Figure 4. SEM images of the cross section and EDS linear scan graphs of (a) ml3-TiAlSiN/AlSiN; (b) ml30-TiAlSiN/AlSiN; (c) sl-TiAlSiN; (d) sl-AlSiN. Element spectrum: red—N, green—Al, blue—Si, yellow—Ti.
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Figure 5. XRD pattern of 3-period and 30-period multilayer TiAlSiN/AlSiN coatings and referent coatings sl-TiAlSiN and sl-AlSiN.
Figure 5. XRD pattern of 3-period and 30-period multilayer TiAlSiN/AlSiN coatings and referent coatings sl-TiAlSiN and sl-AlSiN.
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Figure 6. XPS spectra of 3-period and 30-period multilayer TiAlSiN/AlSiN coatings and referent coatings sl-TiAlSiN and sl-AlSi: (a) Ti2p, (b) Al2p, (c) N1s, and (d) Si2p.
Figure 6. XPS spectra of 3-period and 30-period multilayer TiAlSiN/AlSiN coatings and referent coatings sl-TiAlSiN and sl-AlSi: (a) Ti2p, (b) Al2p, (c) N1s, and (d) Si2p.
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Figure 7. Typical loading–unloading curves of the sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings.
Figure 7. Typical loading–unloading curves of the sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings.
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Figure 8. Dependence of the hardness on the indentation depth of the sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings.
Figure 8. Dependence of the hardness on the indentation depth of the sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings.
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Figure 9. Comparison of the mechanical properties of sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings: (a) hardness and an elastic modulus; (b) elastic strain to failure and resistance to plastic deformation; (c) elastic recovery.
Figure 9. Comparison of the mechanical properties of sl-TiAlSiN, ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, and sl-AlSiN coatings: (a) hardness and an elastic modulus; (b) elastic strain to failure and resistance to plastic deformation; (c) elastic recovery.
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Table 1. Element composition of the surface of ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, sl-TiAlSiN, and sl-AlSiN coatings.
Table 1. Element composition of the surface of ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, sl-TiAlSiN, and sl-AlSiN coatings.
CoatingElement Concentration, (at. %)
TiAlSiNC
ml3-TiAlSiN/AlSiN
Matrix14.0128.806.8348.233.13
Droplet15.3723.075.3352.433.80
ml30-TiAlSiN/AlSiN
Matrix15.8230.457.1843.553.00
Droplet15.5027.706.7046.703.40
sl-TiAlSiN
Matrix28.0019.004.3545.302.72
Droplet27.0018.504.4046.953.15
sl-AlSiN-
Matrix-43.2711.5941.074.07
Droplet-40.0411.0444.444.48
Table 2. Element composition of the surfaces of ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, sl-TiAlSiN, and sl-AlSiN coatings.
Table 2. Element composition of the surfaces of ml3-TiAlSiN/AlSiN, ml30-TiAlSiN/AlSiN, sl-TiAlSiN, and sl-AlSiN coatings.
CoatingElement Concentration, (at. %)Composition
TiAlSiN
ml3-TiAlSiN/AlSiN14.25 ± 0.2929.10± 0.447.62 ± 0.1949.20 ± 1.53Ti0.28Al0.58Si0.14N
ml30-TiAlSiN/AlSiN14.00 ± 0.2829.00 ± 0.447.50 ± 0.1949.48 ± 1.53Ti0.28Al0.57Si0.15N
sl-TiAlSiN28.45 ± 0.5717.45 ± 0.264.02 ± 0.1049.48 ± 1.53Ti0.57Al0.35Si0.09N
sl-AlSiN-41.55 ± 0.628.45 ± 0.2150.00 ± 1.55Al0.83Si0.17N
Table 3. Comparison of the mechanical properties of multilayers with a TiAlSiN sublayer of the period.
Table 3. Comparison of the mechanical properties of multilayers with a TiAlSiN sublayer of the period.
CoatingsH (GPa)E (GPa)H/E*H3/E*2 (GPa)We (%)Ref.
ml3-TiAlSiN/AlSiN49.54300.110.5868This study
ml30-TiAlSiN/AlSiN43.54030.100.4565This study
TiAlN/TiAlSiN35.6473.2---[25]
TiAlVN/TiAlSiN35.2498.5---[28]
TiAlN/TiAlSiN20.8----[42]
TiN/TiAlSiN26.3366.50.07170.13538.799[41]
TiAlSiN/VSiN29.07259.78-0.36†-[43]
TiAlSiN/Ti312770.112 †--[44]
CrAlN/TiAlSiN35.4377.60.094 †0.258 -[46]
TiAlTaN/TiAlSiN36.7445.9---[47]
† The marked data are calculated by an elastic modulus E.
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Kolchev, S.; Kolaklieva, L.; Chitanov, V.; Cholakova, T.; Zlatareva, E.; Kovacheva, D.; Atanasova, G.; Kakanakov, R. The Role of Period Modulation on the Structure, Composition and Mechanical Properties of Nanocomposite Multilayer TiAlSiN/AlSiN Coatings. Coatings 2023, 13, 1546. https://doi.org/10.3390/coatings13091546

AMA Style

Kolchev S, Kolaklieva L, Chitanov V, Cholakova T, Zlatareva E, Kovacheva D, Atanasova G, Kakanakov R. The Role of Period Modulation on the Structure, Composition and Mechanical Properties of Nanocomposite Multilayer TiAlSiN/AlSiN Coatings. Coatings. 2023; 13(9):1546. https://doi.org/10.3390/coatings13091546

Chicago/Turabian Style

Kolchev, Stefan, Lilyana Kolaklieva, Vasiliy Chitanov, Tetiana Cholakova, Ekaterina Zlatareva, Daniela Kovacheva, Genoveva Atanasova, and Roumen Kakanakov. 2023. "The Role of Period Modulation on the Structure, Composition and Mechanical Properties of Nanocomposite Multilayer TiAlSiN/AlSiN Coatings" Coatings 13, no. 9: 1546. https://doi.org/10.3390/coatings13091546

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