Previous Article in Journal
α-Amino Isobutyric Acid-Derived Silacyclopentane Complexes with Penta- and Hexacoordinate Si Atoms
Previous Article in Special Issue
Modification Strategies and Microstructure–Property Relationships of 6xxx and 8xxx Aluminum Alloy Conductors for Wires and Cables
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Crystals
Volume 16
Issue 6
10.3390/cryst16060390
crystals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Investigation of the Mechanical Properties of Cr/CrN/CrAlN Hard Coating Deposited on Special AlSiMgCu Alloy

by
Vasiliy Chitanov
1,2,*,
Boyan Dochev
2,3,*,
Desislava Dimova
2,3,
Ekaterina Zlatareva
1,2,
Stefan Kolchev
1,2,
Tetiana Cholakova
1,
Denis Faik
1,
Lilyana Kolaklieva
1,2,
Roumen Kakanakov
1 and
Teodor Solakov
3
1
Central Laboratory of Applied Physics, Bulgarian Academy of Sciences, 61 St. Petersburg Blvd., 4000 Plovdiv, Bulgaria
2
Center of Competence “Smart Mechatronic, Eco-and Energy-Saving Systems and Technologies”, Hadji Dimitar str., 5300 Gabrovo, Bulgaria
3
Department of Mechanics, Faculty of Mechanical Engineering, Technical University of Sofia, Plovdiv Branch, 4000 Plovdiv, Bulgaria
*
Authors to whom correspondence should be addressed.
Crystals 2026, 16(6), 390; https://doi.org/10.3390/cryst16060390 (registering DOI)
Submission received: 30 April 2026 / Revised: 3 June 2026 / Accepted: 11 June 2026 / Published: 14 June 2026
(This article belongs to the Special Issue Advances in High-Performance Alloys)
Browse Figures
Versions Notes

Abstract

In this work, a non-standardized hypereutectic aluminum–silicon alloy AlSi21Cu5MgCr intended for the automotive industry is presented. The modification of the alloy is performed with the conventional modifier phosphorus in an amount of 0.04 wt%. The applied metallurgical treatment is the basis for the obtained modified structure. It has been established that after conducting the T6 heat treatment, the free silicon crystals are reduced to 26.9 µm, and the eutectic silicon crystals are spherical in shape and have dimensions not exceeding 8 µm. The macrohardness of the studied alloy is 168.5HV10/10, a value significantly higher than that required for this type of alloy, which is in the range of 95 ÷ 137 HV (90 ÷ 130 HB). The microhardness of the α-phase in the composition of the eutectic is 154 µHV50/10, which indicates that after quenching a saturated solid solution was fixed, and during the artificial aging process secondary strengthening phases were formed and separated. A CrAlN hard coating was deposited on the alloy surface. The mechanical properties of the coating were characterized by a hardness of 14 GPa, whereas the AlSi21Cu5MgCr substrate had a hardness of 2 GPa. The results showed considerable improvement of the hardness of the new alloy and well-tuned elastic–plastic properties. The obtained adhesive properties are compatible with this class of materials. The composition of the CrAlN hard coating is homogeneously distributed on the alloy surface and the morphology is improved. The investigations showed that CrAlN hard coatings could successfully be applied for the modification of the surface of AlSIMgCu alloys.

1. Introduction

Aluminum–silicon (Al-Si) alloys occupy a leading position among casting materials. Eutectic and hypereutectic alloys are used for casting pistons for internal combustion engines due to their low weight, good casting properties, dimensional stability, heat resistance and wear resistance. During operation, pistons undergo various cyclic and thermomechanical loads, which requires the two-component alloys to be alloyed with various chemical elements to give them the necessary properties [1,2]. Magnesium is one of the main alloying elements in the Al-Si system. Its influence is expressed in increasing the mechanical properties of the alloys without deteriorating their corrosion resistance. Interacting with silicon, magnesium forms the intermetallic phase Mg2Si, forming the Al–Si–Mg2Si eutectic, which has a microhardness of 600 μHV20/10. Magnesia also participates in the α-solid solution. Another major alloying element is copper, which is used to improve the strength (formation of the CuAl2 phase), machinability and heat resistance of alloys. Due to its high price and conflicting opinions about the positive influence of nickel on the structure and properties of this type of alloys, its use as an alloying element is debatable. In order to improve the mechanical properties, heat resistance and structural characteristics of alloys, alloying with Mn, Cr, Co, Mo, V and Zr has also been used [3,4,5,6,7,8]. Each of these elements has a positive influence (spheroidization) on the harmful iron-containing phases. It is known that the mechanical properties of piston aluminum–silicon alloys depend both on the size, amount and distribution of eutectic silicon and on the amount, size and shape of the primary silicon crystals [9,10,11,12,13,14,15,16]. This necessitates the application of modifying treatment in order to influence the crystals of free and eutectic silicon, and, depending on the type of alloy, Na is used for eutectic and hypoeutectic compositions, and P for the hypereutectic ones. By conducting metallurgical treatment, the mechanical properties of these alloys are improved, which expands their range of application [17]. To obtain the necessary complex of physical and mechanical properties, Al-Si alloys are subjected to the T6 heat treatment [18,19,20,21]. Of particular importance for obtaining structures that provide the necessary mechanical and operational properties of the alloys are the operating parameters of the hardening regimes and subsequent artificial aging. The highest mechanical properties are revealed by alloys subjected to artificial aging at temperatures in the range of 150 ÷ 200 °C. This is explained by the formation of dispersed particles of the intermediate (metastable) phase θ′, having a tetragonal crystal lattice and a coherent connection with the α-phase. With increasing temperature (200 ÷ 400 °C), coagulation of θ′, formation of the stable θ phase and disruption of the coherent connection between the α-phase and the θ-phase are observed.
The investigation of the mechanical properties of Al-Si alloys shows hardness under 1 GPa [22,23]. The lower hardness and insufficient wear resistance of aluminum and its alloys restrict their broader application in various fields. To overcome this limits surface protective coatings are applied. Aluminum alloys are exposed to aggressive corrosion reactions and surface protective layers like hard coatings are applied [24]. A review is made of the recent research reports of preparing wear-resistant coatings through one-step methods (such as anodic oxidation, micro-arc oxidation, cold spraying, plasma spraying, and electrodeposition) and two-step methods (anodic oxidation and physical vapor deposition, micro-arc oxidation and sealing, magnetron sputtering, and plasma nitriding) for Al-based alloys. Among the known methods magnetron sputtering is characterized with superior sputtering and deposition rates, higher ionization efficiency, and sustained discharge at lower operating pressures and voltages [25]. CrN coatings offer perfect protection against sticking and surface scratching of aluminum alloy tools [7]. The thickness of the CrN coating influences the corrosion resistance [26]. Sputtering of Cr/CrN coatings with different nitrogen composition led to different microstructure and protective properties and investigation is necessary for their optimization [27]. The addition of Al element could significantly enhance the mechanical properties, oxidation and corrosion resistance of CrN coatings. CrAlN is considered as the most promising nitride coating material for high-temperature applications and has great potential as protective coating for Al alloy components [28]. Deposition of thin layers at temperatures under 200 °C through magnetron sputtering in a closed-field configuration led to a high-quality, uniform and dense morphology [29]. The mechanical properties of CrAlN thin layers deposited on special AlSiCu alloys with this technology are still not well researched.
About 23% of carbon dioxide emissions occur due to road transport. This requires a focus on finding new technologies that lead to carbon-free mobility. In recent years, ammonia has been gaining increasing popularity as an alternative fuel for internal combustion engines. The main technical problems when using ammonia fuel include its high auto-ignition (combustion) temperature, which reaches 650 °C, as well as a high compression ratio between 35:1 and 100:1 for the successful ignition of ammonia without the use of other fuel. Ammonia and the intermediate products obtained during its combustion are corrosively aggressive to the aluminum alloys used to produce the piston–cylinder groups for internal combustion engines. The solution of these problems and meeting the specific operating conditions of ICEs with ammonia fuel is a priority for the development of new promising materials. Such materials can be new non-standardized Al-Si alloys with increased mechanical properties, with a coating applied to them with high refractoriness, hardness and wear resistance.
The aim of the presented study is to evaluate both the hardness and elastic modulus of a hard CrAlN coating deposited through unbalanced close-field magnetron sputtering, as well as its microstructure and adhesion strength on a specially designed AlSi21Cu5MgCr alloy. The hardness measurements have to provide information about the protective properties of the layer and the ameliorated quality of the alloy. The modulus of elasticity has to show the similarity of the elastic properties of the alloy and coating. The adhesion measurements will present the coating’s strength relative to the substrate under progressive loading. The microstructure and the elemental composition will reveal the nature of both materials and whether their protective layer will improve the physical properties of the substrate alloy. All the investigated data will generalize if CrAlN thin layers are appropriate for protection of the new developed AlSi21Cu5MgCr alloy and for improving its mechanical and physical properties.

2. Materials and Methods

The object of this study is the non-standardized hypereutectic aluminum–silicon alloy AlSi21Cu5MgCr with the chemical composition shown in Table 1. The alloy was melted in a laboratory electric resistance furnace under a layer of covering–refining flux with a composition of 10 KCl: 50 NaCl: 10 Na3AlF6, in an amount of 0.5 wt% relative to the mass of the melt, modified with 0.04 wt% P, introduced into the melt through the CuP10 ligature at a temperature of 830 °C, at which temperature the alloy was degassed by purging with argon for 3 min.
After casting, the experimental castings were subjected to the T6 heat treatment (quenching with subsequent artificial aging). The recommended heating temperature for homogenizing the structure of this type of alloys is 20–50 °C lower than the temperature of the solidus line of the system (577 °C). Heating to temperatures of 20–30 °C below this temperature is risky due to the possibility of obtaining defects from overheating and burning of grain boundaries. Therefore, a heating temperature of 510–515 °C was chosen for conducting the technological process, which is high enough for the complete dissolution of the primary intermetallics formed in the crystallization process in the α-solid solution. Usually, the holding time at the selected working temperature is within 4–8 h, and for this reason, the holding time chosen by us at the working temperature is 6 h and 30 min. To obtain a saturated solid solution, the quenching was carried out in a cooling medium—water with a temperature of 20 °C. The artificial aging process was carried out at a working temperature of 190 °C, with the holding time at this temperature being 12 h. The microstructure of the alloys was studied using a Leica DM ILM microscope (Wetzlar, Germany) with the help of software and a module for grain measurement and phase analysis. The microsections were wet ground on sandpaper with the numbers 240, 320, 400, 600, 800 and 1000 and mechanically polished with diamond paste and lubricant. The microstructure of the samples was developed with Keller’s reagent (PACE Technologies, Tucson, AZ, USA) (1 part HF, 1.5 parts HCl, 2.5 parts HNO3, 95 parts H2O). The macrohardness of the studied alloy and the microhardness of the α-solid solution were measured. After the heat treatment, test specimens were fabricated for the application of Cr/CrN/CrAlN coating.
The semi-industrial equipment UDP850/4 shown in Figure 1 is situated at the Central Laboratory of Applied Physics. It was used for the deposition of Cr/CrN/CrAlN hard coatings. Standard rectangular Cr (99.99%) and two Al (99.99%) targets were sputtered. One Si target was working at a safety current setpoint of 0.5 A, insignificantly sputtering Si. The experimental AlSi21Cu5MgCr alloy was pre-cleaned in a special alkaline solution and subsequently rinsed in deionised water.
The base pressure was 3.8 × 10−3 Pa. Argon (Ar) was used as a working inert gas with a flow rate of 25 sccm. An optical emission monitoring (OEM) with a feedback control system, based on the metal chromium emission line was used for control of the nitrogen flow. During the deposition of the coating, pulse direct current was applied to the corresponding targets. The rotation speed of the sample holder was fixed at 5 rpm. Supporting −70 V bias voltage was applied to the substrate. The substrate temperature was in the range of 140–215 °C. The temperature was controlled by plasma and the setpoint of the carbon heater. The deposition process started after Ar plasma cleaning with a bias voltage of −500 V for 30 min. The bias was controlled in the Pulse DC regime (pulse frequency f 250 × 103 Hz and pause 2.5 × 10−6 s). A Cr adhesion layer was deposited onto the substrates. A CrN transition layer was firstly deposited with gradually increasing nitrogen flow and after that the power of the Al targets gradually reached the setpoint value. The gradually introduced interlayers are necessary to ensure the smooth transition between the soft substrate and the harder protective layer, as well as the proper adhesion of the coating with the substrate. The main CrAlN layer was deposition time was 90 min. The deposition process parameters are summarized in Table 2.
Nanoindentation was used to characterize nanohardness. Using Anton Paar equipment (Anton Paar, Graz, Austria) indentations were made under normal loads of 10, 20, 30, 40, 50 and 100 mN. The values of 30–100 N show the influence of the substrate hardness on the complex hardness in depth. The load of 20 mN was chosen as representative for the characterization. The measured hardness at this load is slightly influenced by the roughness of the sample surface and by the substrate hardness. The penetration depth at 20 mN is around the 10% value (235 nm) of the coating thickness. To guarantee substrate-independent value at lower penetration depth a control nanoindentation at 10 mN was made corresponding to penetrating under 9% of the coating thickness. Five measurements were made at 20 mN load. The highest and lowest results were eliminated and the average value of the other three measurements was taken as a representative value. The Poisson ratio of the coating used for the calculations was 0.25.
Scratch test was used to characterize the adhesion with Anton Paar micro-indentation equipment (Anton Paar, Graz, Austria). A progressive scratch up to 30 N was applied. The length was 1mm and the speed was 10 N/min.
The thickness of the coatings was determined using a compact Calotest CAT2 (Anton Paar, Graz, Austria) equipped with a light microscopy (Anton Paar Tritec SA, Corcelles, Switzerland) with a magnification of 10×. A depression in the shape of a spherical crater (calotte) was abraded into the sample surface. The rotation speed was 1200 rpm. A steel ball with a diameter of 20 mm was used. The coating thickness was calculated using specialized Video Software Version 9.0.12 after the examination of the obtained crater. Five measurements were done in different surface positions, one in the center and four at the controversy positions. The average value and the standard deviations were calculated.
Scanning electron microscopy SU-5000 (Hitachi, Tokyo, Japan) and energy-dispersive X-ray (Thermo Scientific, Waltham, MA, USA) spectroscopy analyses were used to investigate the morphology of the sample surface and to identify the element composition within the total surface and several rectangular subareas. Surface images were obtained in secondary electrons (morphology contrast) and backscattered electrons (density contrast) at an operating voltage of 5 kV and 15 kV, respectively.

3. Results

The structure of the studied alloy AlSi21Cu5MgCr modified with 0.04 wt%P before being subjected to heat treatment is shown in Figure 2a. For the studied alloy, five different representative microstructural fields were analyzed. In each field, the linear dimensions of the visible primary separated silicon crystals, as well as of the eutectic silicon, were measured. For crystals with approximately equilibrium or irregular morphology, the size was determined by a characteristic conditional diameter. For crystals with a more pronounced rectangular or elongated shape, the two main dimensional characteristics were measured—length and width, and their average value was used as a representative size of the individual crystal. The values obtained from all measured fields were statistically processed, and the average size of the silicon crystals was determined as the arithmetic mean value. The primary silicon crystals are evenly distributed in the observed plane of the metallographic section, they are in the form of plates with rounded corners with a conditional average diameter of 32.4 µm. The silicon crystals in the composition of the eutectic are in the form of needles with an average linear size of 8.6 µm as they are located in dendritic crystals of the α-solid solution. After heat treatment, a decrease in the size of the free silicon crystals to a conditional average diameter of 26.9 µm was registered. No significant difference in the size of the eutectic silicon crystals (8 µm) was observed, but they changed their shape—from plate-like to spheroidized (Figure 2b).
The AlSi21Cu5MgCr alloy before heat treatment showed macrohardness of 128.5 HV10/10 and microhardness of 122 µHV50/10. After heat treatment, the measured values of both parameters increased, with the macrohardness reaching 168.5 HV10/10 and the microhardness of the α-phase reaching 154 µHV50/10. The macrohardness value of the alloy is significantly higher than that required for this type of alloy: 95 ÷ 137 HV (90 ÷ 130 HB) [30]. This shows that in the heating process before quenching, the alloying elements are dissolved in the α-solid solution and after quenching a saturated solid solution is fixed. In the process of artificial aging, strengthening phases have formed and separated, which contribute to the increased microhardness and macrohardness values of the alloy.
An image of the calotte, obtained by the calottes in the center of the sample surface is shown in Figure 3. The average value of the measured total thickness D in the different five surface locations of the Cr/CrN/CrAlN coating is 2.35 microns. The measured values of the thickness with 5% error, the calculated average value and the standard deviation STD are shown in Table 3. The adhesive and transition Cr/CrN layers have thickness of 0.67 microns and the main CrAlN layer of 1.69 microns.
Figure 4a and Figure 5a and Table 4 present the morphology and composition of the AlSiMgCu alloy as total surface spectrum and within the chosen rectangular subareas. The surface of the substrate is uneven with lighter and darker areas. The amount of magnesium in the alloy composition is 1.12 wt%, and the results of the conducted EDS show 26.2–35 at% in certain areas. This is due to the so-called liquation (chemical inhomogeneity) in the volume of the casting or in the crystals themselves. The mechanism of increasing the mechanical properties of aluminum alloys is due to the formation and separation of secondary intermetallics (including the magnesium-containing ones). Their distribution in the volume of the solid alloy is a process that is almost impossible to control and this is the reason for the obtained results. The elemental composition varies over the measured areas and deviates from the total spectrum. The surface morphology of the investigated areas of the CrAlN coating is shown in Figure 4b and Figure 5b. The elemental composition is given in Table 5. The coating morphology is compact and equally distributed. The grain geometry is floury type, well packed and with small intergranular pores. The elemental composition estimated in the three different rectangular subareas shows values close to this of the total spectrum of the sample coating surface. This result reveals that in addition to the protection function the coating equalizes the surface material properties.
The hardness of the CrAlN layer and the substrate is compared in Figure 6a. The hardness of the AlSiMgCu alloy is around 2 GPa that is a low value typical for Al-based alloys but higher from the hardness of AlSi21Cu5MgCr alloys developed and characterized by other researchers [31,32]. The measured hardness of the CrAlN hard coating is seven times higher than the AlSiMgCu alloy. The measured nanohardness of the coating according to the 10% depth rule of the coating thickness, where the substrate does not influence the result, is about 14 GPa. With an increase in the indentation loads and the penetration depth, the hardness decreases gradually to about 5 GPa, which is also a higher value than the substrate hardness. Depending on the Al composition CrAlN coatings could have hardness over 30 GPa [33] when deposited on high-speed steel or WC. When deposited by sputtering on AlSi-based alloys, the increased hardness could reach four [34] and seven [35] times higher values in comparison with the substrate. With other PVD methods where the ionization energy is higher the hardness could reach even 30 GPa [28]. As the obtained result is about seven times higher than the substrate one the CLFUBMS technique could be accepted as competitive with the other PVD sputtering methods.
Figure 6b shows the modulus of elasticity in dependance on the penetration depth. At a penetration depth where the substrate does not influence the result, the measured values are between 135 GPa and 170 GPa. With an increase in the penetration depth, the modulus of elasticity decreases to 110–120 GPa. The modulus of elasticity of the AlSiMgCu alloy is about 110 GPa. The close results for both the nitride coating and the substrate alloy indicate good similarity in the elastic properties and further support the smooth transition between the materials. The modulus of elasticity of CrAlN hard coatings is in the range of 300–520 GPa [36]. Data for modulus of elasticity of CrAlN hard coating on AlSi21Cu5MgCr alloy are limited. The coatings deposited by CAD PVD technology show values between 210 GPa and 280 GPa in dependence on the nitrogen flow [28].
Figure 7 shows the trends of the scratch test including normal force Fn[N] (progressive during the test), tangential force Ft[N] (the resistance of the material against the indenter), penetration depth Pd[μm] (displacement of the indenter measuring elastic recovery), acoustic emission AE [%] (the noise during the measurement), and coefficient of friction against the diamond indenter μ[−] (determined as the ratio Ft/Fn). After the scratch test the surface is panoramically observed with an objective of ×20 magnification and the picture is also included. The different failure modes are identified with objective with magnification ×100. The pictures correspond to the loads where the failure mode appears. The failure modes are classified according to schematic representation of the scratch channel obtained during the scratch test [37]. During the scratch test the classified critical loads are generalized in three LC groups. LC1 defines the cohesive failures including the lowest critical load. It characterized the tensile and conformal cracks where the coating remains fully adhered to the substrate. LC2 defines the stage at which the coating has first delamination and spallation with exposing small patches of the underlying substrate, LC3 show the load at which the coating is fully removed leaving the substrate completely bare. The first surface cohesive failures appeared like lateral cracks at LC1 load of 5 N. At the load of 10 N their size increased and the first single arc tensile crack was observed under the microscope. Increasing the load with 3 N lead to seria of arc tensile cracks. After the LC2 load of 20 N the cohesive failure started to transform to adhesive edge thin film spalling and partly coating break-through. This process went until the end of the scratch test where the higher loads of 30 N will transform LC2 to the fully exposer LC3 load. The acoustic emission AE signal was not smooth and the picks have maximum height of 10% [38]. The penetration depth Pd is changing its value without any sudden drops or picks. The friction force Ft has linearly increasing trend at the beginning followed by not straight and with small deviations trend reaching the value of about 10 N. These are related to the process of cohesive and adhesive failures. The friction coefficient trend followed the Ft one. The measured value of the friction coefficient against the diamond indenter was about 0.3 at the end of the scratch test. The adhesion test of the CAD PVD CrAlN coating on Al alloys showed that the initial cracks started at loads of about 7 N. Depending on the nitrogen pressure during the deposition process the fully delamination of the coating was in the loads range of 10–16 N. This lower adhesive strength is explained with the internal stress of the coating especially at higher nitrogen content [28]. The magnetron sputtered CrAlN hard coating on Si-Al alloys showed adhesion strength of 2.3 N [34] and 2.5 N [35]. The comparison of the results shows that CLFUBMS technique allows deposition of CrAlN hard coatings on AlSi based alloys with improved adhesion strength. The adhesion of DLC-based films and W-C:H films on Si-Mg alloys shown critial loads with crack formation Lc1—2.8–3.5 N, chiping of the coating along the track sides Lc2 6.8–8.3 N, chipping of the coating across the entire track width Lc3 9.2–16.1 N and conitnius delamination of the coating LDealm 11.2–17.1 N [39]. The adhesion of Cr-based coatings on AlSI4140 substrates shown that Lc1 is 8.5 N for Cr/(Cr,N) layer, 5 N for Cr/CrN/Cr(Al,N) layer and 1.3 N for CrN/Cr(Al,N) layers. The point at which the damage become continuous the force Lc2 is 11 N, 7.8 N and 2.5 N correspondingly showing that the multilayer structure and the interface design re playing a dominant role in adhesion properties [40]. Further optimization of the adhesion properties of the research alloy could be made with multilayer Cr-based coatings based on the developed CrAlN hard coating.

4. Discussion

A non-standardized piston hypereutectic alloy AlSi21Cu5MgCr has been developed, in the composition of which no nickel has been added. This element is critical due to its high price, and the effect of its use is entirely on the heat resistance of the alloys. To improve the operational properties, a PVD coating Cr/CrN/CrAlN has been applied to the alloy. After carrying out the metallurgical treatment of the alloy and the subsequent heat treatment, a structure with modified free silicon crystals was obtained, which has the dimensions of 26.9 µm, fully meeting the recommended critical size of ≈30 µm. The eutectic silicon crystals are spheroidized and the have dimensions of 8 µm, which in turn ignores their notching effect on the matrix, i.e., they do not act as stress concentrators. All this is the basis for obtaining increased mechanical properties of the studied alloy AlSi21Cu5MgCr.
One of the many advantages of applying hard coatings through unbalanced magnetron sputtering is the possibility of working at lower substrate temperatures, which is of great importance for aluminum alloys. Aluminum alloys obtain the highest mechanical performance when the artificial aging process is carried out at temperatures in the range of 150 ÷ 200 °C. This is explained by the formation of dispersed particles of the intermediate (metastable) phase θ′, having a tetragonal crystal lattice and a coherent connection with the α-phase. With increasing temperature (200 ÷ 400 °C) the coagulation of θ′ is observed, the formation of the stable θ phase and the disruption of the coherent connection between the α-phase and the θ-phase. This leads to a decrease in the mechanical and deterioration of the operational properties of the alloys. The Cr/CrN/CrAlN coating was applied to test bodies of the studied alloy at a substrate temperature in the range of 140–215 °C. At this operating temperature, diffusion processes in the substrate should not occur, which would negatively affect its structure and properties. The applied coating has a thickness of 2.35 µm, i.e., a value in the range typical for industrial coatings (2–5 µm). The morphology of the coating is compact and evenly distributed, and no deviations in the elemental composition are observed. This in turn is of extremely important importance, because when studying the elemental composition of the alloy, a deviation was registered in different areas. This phenomenon is typical for this type of alloys, because the distribution of the formed secondary intermetallics obtained in the process of artificial aging is uneven in the volume of the alloy. The applied coating has a hardness of 14 GPa, i.e., a value about seven times higher than that of the studied AlSi21Cu5MgCr alloy. With increasing loads and penetration depth, a gradual decrease in the hardness of the coating in depth was recorded and reached a value of about 5 GPa, which is also higher than the hardness of the studied substrate. The elastic modulus of the applied coating has measured values in the range of 135 GPa and 170 GPa. With increasing penetration depth, the elastic modulus decreases to 110 ÷ 120 GPa, values identical to the elastic modulus of the AlSi21Cu5MgCr alloy, which is about 110 GPa. The close values of the elastic moduli of the nitride coating and the studied alloy are an indication of good similarity in elastic properties and further support the smooth transition between the materials.
Currently, various technological methods are used to improve the performance properties of the working surfaces of parts made of conventional aluminum alloys [41,42,43,44,45,46]. A pilot study is presented to develop functional structures for a new generation of internal combustion engines operating on eco-fuels. At present, in the literature we have studied, there is no data on the study of such a system (hypereutectic Al-Si alloy/nitride coating), which also determines the novelty of the present study. A non-standardized hypereutectic aluminum–silicon alloy has been developed, which has increased mechanical properties as a result of the used unconventional combination of alloying elements and a nitride coating has been applied to it in order to improve its operational properties. The system thus created also implies its use in friction pairs operating at elevated temperatures and at the same time subjected to cyclic thermomechanical loads. The industrial application is in the direction of using the presented system in engines operating with alternative eco-fuels, but also as an alternative to the systems previously used in certain engines (racing cars): pistons made of plastically deformed aluminum alloys with a DLS coating applied to them.

5. Conclusions

New AlSIMgCu alloy was developed. The alloy surface was modified with the deposi-tion of CrAlN hard coating. The investigation of the mechanical properties showed considerable improvement of the hardness of the new alloy and well-tuned elastic–plastic properties. The adhesion strength and the coefficient of friction are compatible but further experimental work is necessary for it to became ready for real industrial applications. The addition of Al to CrN layer improves the hardness of the binary coating. The deposited CrAlN hard coating has the Al/Al+Cr ratio of around 10% and is characterized with seven times higher hardness than the alloy substrate. The similarity in the coating and substrate modulus of elasticity confirms that the composition of the coating is properly chosen to improve the elastic–plastic properties of the alloy. The adhesion strength is compatible with that of the same class of materials. The cohesive and adhesive failures require further optimization of the adhesion of the CrAlN coating to the AlSIMgCu substrate.

Author Contributions

Conceptualization, V.C., B.D., L.K. and R.K.; methodology, V.C., B.D., L.K. and R.K.; software, D.D., E.Z., S.K., T.C., D.F. and T.S.; validation, D.D., E.Z., S.K., T.C., D.F. and T.S.; formal analysis, V.C. and B.D.; investigation, V.C. and B.D.; resources, L.K. and R.K.; data curation, L.K. and R.K.; writing—original draft preparation, D.D., E.Z., S.K., T.C., D.F. and T.S.; writing—review and editing, V.C. and B.D.; visualization, D.D., E.Z., S.K., T.C., D.F. and T.S.; supervision, V.C.; project administration, B.D.; funding acquisition, V.C., B.D. and D.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the European Regional Development Fund under the Operational Program “Scientific Research, Innovation and Digitization for Smart Transformation 2021–2027”, Project CoC “Smart Mechatronics, Eco- and Energy Saving Systems and Technologies”, BG16RFPR002-1.014-0005.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed at the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Li, Z.; Samuel, A.M.; Samuel, F.H.; Ravindran, C.; Valtierra, S. Effect of alloying elements on the segregation and dissolution of CuAl2 phase in Al-Si-Cu 319 alloys. J. Mater. Sci. 2003, 38, 1203–1218. [Google Scholar] [CrossRef]
  2. Gowri, S.; Samuel, F.H. Effect of alloying elements on the solidification characteristics and microstructure of Al- Si- Cu- Mg- Fe 380 alloy. Metall. Mater. Trans. A 1994, 25, 437–448. [Google Scholar] [CrossRef]
  3. Yang, Y.; Yu, K.; Li, Y.; Zhao, D.; Liu, X. Evolution of nickel-rich phases in Al–Si–Cu–Ni–Mg piston alloys with different Cu additions. Mater. Des. 2012, 33, 220–225. [Google Scholar] [CrossRef]
  4. Jiang, L.; Zhao, Z.; Wang, G. The Effects on Stability and Electronic Structure of Si-Segregated θ′Al Interface Systems in Al-Cu Alloys. Coatings 2024, 14, 879. [Google Scholar] [CrossRef]
  5. Mao, F.; Wei, S.; Ou, L.; Zhang, C.; Chen, C.; Wang, X.; Cao, Z. Different Influences of Rare Earth Eu Addition on Primary Si Refinement in Hypereutectic Al–Si Alloys with Varied Purity. Materials 2019, 12, 3505. [Google Scholar] [CrossRef]
  6. Xiao, L.; Yu, H.; Qin, Y.; Liu, G.; Peng, Z.; Tu, X.; Su, H.; Xiao, Y.; Zhong, Q.; Wang, S.; et al. Microstructure and Mechanical Properties of Cast Al-Si-Cu-Mg-Ni-Cr Alloys: Effects of Time and Temperature on Two-Stage Solution Treatment and Ageing. Materials 2023, 16, 2675. [Google Scholar] [CrossRef]
  7. Ishimaru, H. Chromium-nitride coating on the aluminum alloy flange. In Proceedings of the Workshop on First Wall Coating, Nagoya, Japan, 28 November 1980; Report number IPPJ-551. 1982; pp. 219–228. Available online: https://inis.iaea.org/records/cf4z0-xqs30 (accessed on 14 April 2026).
  8. Šćepanović, J.; Asanović, V.; Radonjić, D.; Vuksanović, D.; Herenda, S.; Korać, F.; Bikić, F. Mechanical properties and corrosion behaviour of Al-Si alloys for IC engine. J. Serb. Chem. Soc. 2019, 84, 503–516. [Google Scholar] [CrossRef]
  9. Zhang, M.; Tian, Y.; Zheng, X.; Zhang, Y.; Chen, L.; Wang, J. Research Progress on Multi-Component Alloying and Heat Treatment of High Strength and Toughness Al–Si–Cu–Mg Cast Aluminum Alloys. Materials 2023, 16, 1065. [Google Scholar] [CrossRef]
  10. Hu, Z.; Huo, Q.; Chen, Y.; Liu, M.; Chen, X. Improving Mechanical Property of Hyper-Eutectic Al-Si Alloys via Regulating the Microstructure by Rheo-Die-Casting. Metals 2023, 13, 968. [Google Scholar] [CrossRef]
  11. Xia, F.; Dong, X.; Wang, J.; Duan, H.; Ma, Z.; Liang, M. Microstructural Evolution and Tensile Properties of Al-Si Piston Alloys during Long-Term Thermal Exposure. Metals 2024, 14, 535. [Google Scholar] [CrossRef]
  12. Noga, P.; Cech, R.; Dvořák, P.; Janeček, M.; Buršík, J. Microstructure and Mechanical Properties of Rapidly Solidified Al–Si Alloys. Materials 2023, 16, 5223. [Google Scholar] [CrossRef] [PubMed]
  13. Bibik, N.; Metel, A.; Cherenda, N.; Sotova, C.; Astashynski, V.; Kuzmitski, A.; Melnik, Y.; Vereschaka, A. Structure of Eutectic Al-Si Alloy Subjected to Compression Plasma Flow Impact. Metals 2024, 14, 1415. [Google Scholar] [CrossRef]
  14. Singhal, V.; Shelly, D.; Babbar, A.; Lee, S.-Y.; Park, S.-J. Review of Wear and Mechanical Characteristics of Al-Si Alloy Matrix Composites Reinforced with Natural Minerals. Lubricants 2024, 12, 350. [Google Scholar] [CrossRef]
  15. Chandra, V.S.; Krishna, K.S.V.B.R.; Ravi, M.; Sivaprasad, K.; Dhanasekaran, S.; Prashanth, K.G. Mechanical and Tribological Behavior of Gravity and Squeeze Cast Novel Al-Si Alloy. Metals 2022, 12, 194. [Google Scholar] [CrossRef]
  16. Callegari, B.; Lima, T.N.; Coelho, R.S. The Influence of Alloying Elements on the Microstructure and Properties of Al-Si-Based Casting Alloys: A Review. Metals 2023, 13, 1174. [Google Scholar] [CrossRef]
  17. Alshmri, F. Lightweight material: Aluminium high silicon alloys in the automotive industry. Adv. Mater. Res. 2013, 774–776, 1271–1276. [Google Scholar] [CrossRef]
  18. Shi, Z.; Yan, P.; Yan, B. Solidification Mechanism of Microstructure of Al-Si-Cu-Ni Alloy Manufactured by Laser Powder Bed Fusion and Mechanical Properties Effect. Metals 2024, 14, 586. [Google Scholar] [CrossRef]
  19. Kang, N.; El Mansori, M. A new insight on induced-tribological behaviour of hypereutectic Al-Si alloys manufactured by selective laser melting. Tribol. Int. 2020, 149, 105751. [Google Scholar] [CrossRef]
  20. Herrera-Hernández, H.; Mandujano Ruiz, A.; González Morán, C.O.; Miranda Hernández, J.G.; Flores Cuautle, J.D.J.A.; Morales Hernández, J.; Hernández Casco, I. Pitting Corrosion Mechanism on A390 Al–Si Alloy. Materials 2024, 17, 3044. [Google Scholar] [CrossRef] [PubMed]
  21. Zhao, J.; Zhao, T.; Zhang, Y.; Zhang, Z.; Chen, Z.; Wang, J.; Chen, M. Corrosion Behavior of the 2024 Aluminum Alloy in the Atmospheric Environment of the South China Sea Islands. Coatings 2024, 14, 331. [Google Scholar] [CrossRef]
  22. Malleswararao, N.D.; Niranjan, I.N. Mechanical characterization of rapid solidified Alsicumg alloys by new CRSS casting method under T6 condition. IJITEE 2019, 8, 2897–2902. [Google Scholar] [CrossRef]
  23. Sekhar, A.P.; Nandy, S.; Ray, K.K.; Das, D. Hardness—Yield strength relation of Al-Mg-Si Alloys. IOP Conf. Ser. Mater. Sci. Eng. 2018, 388, 1–6. [Google Scholar] [CrossRef]
  24. Kwolek, P. Hard anodic coatigns on aluminum alloys. Adv. Manuf. Sci. Technol. 2017, 41, 35–46. [Google Scholar]
  25. Jia, B.; Ren, R.; Zhang, H.; Man, T.; Cui, X.; Liu, T.; Zhao, T.; Luhovskyi, Y.; Nong, Z. Research advancements of wear-resistant coatings fabricated on aluminum and its alloys. Coatings 2025, 15, 750. [Google Scholar] [CrossRef]
  26. Barranco, J.; Barreras, F.; Lozano, A.; Maza, M. Influence of CrN-coating thickness on the corrosion resistance behaviour of aluminium-based bipolar plates. J. Power Sources 2011, 196, 4283–4289. [Google Scholar] [CrossRef]
  27. Li, X.; Jiang, X.; Li, W.; Zhou, X.; Wang, Y.; Chen, Y.; Xiong, C.; You, F.; Li, X.; Liu, L. Effect of different N contents on CrN coatings of aluminium alloy bipolar plates for proton exchange membrane fuel cells. Ceram. Int. 2025, 51, 3546–3558. [Google Scholar] [CrossRef]
  28. Cao, H.; Yang, J.; Li, Y.; Ren, L.; Qi, F.; Zhao, N.; Zhou, Y.; Li, B.; Ouyang, X. Effect of nitrogen pressure on the microstructure, mechanical and electrochemical properties of CrAlN coatings deposited by filter cathode vacuum arc. Ceram. Int. 2022, 48, 36570–36584. [Google Scholar] [CrossRef]
  29. Kelly, P.; Arnell, R.; Ahmed, W.; Afzal, A. Novel emgineering coatings produced by close-field unbalanced magnetron sputtering. Mater. Des. 1996, 17, 215–219. [Google Scholar] [CrossRef]
  30. Mahle GmbH. Piston materials. In Pistons and Engine Testing. ATZ/MTZ-Fachbuch; Vieweg+Teubner Verlag: Wiesbaden, Germany, 2012. [Google Scholar]
  31. Wu, C.-T.; Lee, S.-L.; Hsieh, M.-H.; Lin, J.-C. Effects of Cu content on microstructure and mechanical properties of Al-14.5Si-0.5Mg alloy. Mater. Charact. 2010, 61, 1074–1079. [Google Scholar] [CrossRef]
  32. Dong, X.; Amirkhanlou, S.; Ji, S. Formation of strength platform in cast Al-Si-Mg-Cu alloys. Sci. Rep. 2019, 9, 9582. [Google Scholar] [CrossRef]
  33. Aouadi, K.; Nouveau, C.; Besnard, A.; TIili, B.; Montagne, A.; Chafra, M. Characterization of CrAlN films synthesized by DC reactive magnetron sputtering. In International Conference on Advances in Mechanical Engineering and Mechanics; Lecture Notes in Mechanical Engineering; Springer International Publishing: Cham, Switzerland, 2021. [Google Scholar] [CrossRef]
  34. Mehran, Q.; Bushora, A.; Fazal, M. Evaluation of CrALN multilayered coatings deposited by PVD magnetron sputtering. J. Adhes. Sci. Technol. 2015, 29, 2076–2089. [Google Scholar] [CrossRef]
  35. Mehran, Q.; Bushora, A.; Fazal, M. Characteristics of PVD CrAlN thin film on Al-SI piston alloy. In Proceedings of Malaysian International Tribology Conference; Malaysian Tribology Society: Kuala Lumpur, Malaysia, 2015; pp. 234–235. [Google Scholar]
  36. Benlatreche, Y.; Nouveau, C.; Aknouche, H.; Imhoff, L.; Martin, N.; Gavoille, J.; Rousselot, C.; Rauch, J.-Y.; Pilloud, D. Physical and mechanical properties of CrALN and CrSiN ternary systems for wood machining applications. Plasma Process. Polym. 2009, 6, S113–S117. [Google Scholar] [CrossRef]
  37. Lepicka, M.; Gradzka-Dahlke, M. The initial evaluation of performance of hard anti-wear coatings deposited on metallic substrates: Thickness, mechanical properties and adhesion measurements—A brief review. Rev. Adv. Mater. Sci. 2019, 58, 50–65. [Google Scholar] [CrossRef]
  38. Ding, Y.; He, T.; Du, X.; Vereschaka, A.; Sotova, C.; Li, J.; Wang, Y.; He, P. Influence of nitrogen pressure on microstructure, mechanical, and tribocorrosion properties of (Zr,Cr,Al)N coatings on aluminum alloy. Tribol. Int. 2026, 218, 111720. [Google Scholar] [CrossRef]
  39. Bonilauri, M.F.; Emanuele, G.; Bolelli, G.; Alessandro, B.; Cerri, E. Adhesion of DLC-based coatings on LPBF Al-Si-Mg alloys: Influence of substrate hardness. Surf. Coat. Technol. 2026, 523, 133203. [Google Scholar] [CrossRef]
  40. Tlili, B.; Nouveau, C.; Benlatreche, Y.; Mustapha, N.; Lambertin, M. Hardness and scratch response of PVD multilayer coatings. In 3ème Congrès International Conception et Modélisation des Systèmes Mécaniques, Hammamet, Tunisia, 16–18 March 2009; HAL Open Science: Lyon, France, 2009; 7p, Available online: https://hal.science/hal-00908722v1 (accessed on 14 April 2026).
  41. Zhu, H.; Li, J. Advancements in corrosion protection for aerospace aluminum alloys through surface treatment. Int. J. Electrochem. Sci. 2024, 19, 100487. [Google Scholar] [CrossRef]
  42. Staszuk, M. Application of PVD and ALD Methods for Surface Treatment of Al-Si-Cu Alloys. Solid State Phenom. 2019, 293, 97–109. [Google Scholar] [CrossRef]
  43. Roy, P.; Sarangi, S.K.; Ghosh, A.; Chattopadhyay, A.K. Machinability study of pure aluminium and Al–12% Si alloys against uncoated and coated carbide inserts. Int. J. Refract. Met. Hard Mater. 2009, 27, 535–544. [Google Scholar] [CrossRef]
  44. Fayed, S.M.; Chen, D.; Li, S.; Zhou, Y.; Wang, H.; Sadawy, M.M. Corrosion behavior and passive stability of multilayer DLC-Si coatings. Surf. Coat. Technol. 2022, 431, 128001. [Google Scholar] [CrossRef]
  45. Tański, T.; Lukaszkowicz, K. Structure and properties of PVD coatings deposited on aluminium alloys. Surf. Eng. 2012, 28, 598–604. [Google Scholar] [CrossRef]
  46. Tański, T.; Labisz, K.; Dobrzański, L.A.; Wiśniowski, M.; Matysiak, W. TEM microstructure investigations of aluminium alloys used as coating substrate. Arch. Mater. Sci. Eng. 2013, 59, 82–92. [Google Scholar]
Crystals 16 00390 g001
Figure 1. Scheme of the close-field unbalanced magnetron sputtering deposition process.
Figure 1. Scheme of the close-field unbalanced magnetron sputtering deposition process.
Crystals 16 00390 g001
Crystals 16 00390 g002
Figure 2. Structure of the alloy AlSi21Cu5MgCr: (a) before heat treatment T6, (b) after heat treatment T6 and artificial aging at 190 °C/12h.
Figure 2. Structure of the alloy AlSi21Cu5MgCr: (a) before heat treatment T6, (b) after heat treatment T6 and artificial aging at 190 °C/12h.
Crystals 16 00390 g002
Crystals 16 00390 g003
Figure 3. The calotte on the Cr/CrN/CrAlN hard coating with the sublayers defined with the color circles.
Figure 3. The calotte on the Cr/CrN/CrAlN hard coating with the sublayers defined with the color circles.
Crystals 16 00390 g003
Crystals 16 00390 g004
Figure 4. SEM images of the surface of: (a) AlSi21Cu5MgCr alloy substrate; (b) Cr/CrN/CrAlN hard coating.
Figure 4. SEM images of the surface of: (a) AlSi21Cu5MgCr alloy substrate; (b) Cr/CrN/CrAlN hard coating.
Crystals 16 00390 g004
Crystals 16 00390 g005
Figure 5. SEM images of the subareas of: (a) AlSi21Cu5MgCr alloy substrate (b) Cr/CrN/CrAlN hard coating. The characterized subareas areas are numbered and marked in color rectangles.
Figure 5. SEM images of the subareas of: (a) AlSi21Cu5MgCr alloy substrate (b) Cr/CrN/CrAlN hard coating. The characterized subareas areas are numbered and marked in color rectangles.
Crystals 16 00390 g005
Crystals 16 00390 g006
Figure 6. (a) Hardness and (b) modulus of elasticity of the AlSi21Cu5MgCr alloy substrate and Cr/CrN/CrAlN hard coating.
Figure 6. (a) Hardness and (b) modulus of elasticity of the AlSi21Cu5MgCr alloy substrate and Cr/CrN/CrAlN hard coating.
Crystals 16 00390 g006
Crystals 16 00390 g007aCrystals 16 00390 g007b
Figure 7. Plot of the scratch test of the Cr/CrN/CrAlN hard coating on the AlSi21Cu5MgCr substrate with panorama picture and failure modes directed with arrows at different loads and the dependences Ft/Fn and AE/Pd.
Figure 7. Plot of the scratch test of the Cr/CrN/CrAlN hard coating on the AlSi21Cu5MgCr substrate with panorama picture and failure modes directed with arrows at different loads and the dependences Ft/Fn and AE/Pd.
Crystals 16 00390 g007aCrystals 16 00390 g007b
Table 1. Chemical composition of the investigated alloy AlSi21Cu5MgCr wt%.
Table 1. Chemical composition of the investigated alloy AlSi21Cu5MgCr wt%.
AlloySiCuCrMgNiMnFeAl
AlSi21Cu5MgCr21.015.290.371.120.0050.070.19rest
Table 2. Process parameters.
Table 2. Process parameters.
LayerIcr
[A]		IAl
[A]		Pulse RegimeOEM
[%]		N2
[sccm]		T
[°C]
Ƒ
[KHz]		Pause
[ns]
Cr80.51501500--140–160
CrN80.515015008016160–180
CrAlN83.515015007027180–215
Table 3. Thickness of CrAlN layer measured with the calotest.
Table 3. Thickness of CrAlN layer measured with the calotest.
PositionD [μm]
Center2.35 ± 0.12
Edge 12.33 ± 0.12
Edge 22.36 ± 0.12
Edge 32.34 ± 0.12
Edge 42.35 ± 0.12
Average2.35 ± 0.12
STD0.01
Table 4. Composition of the substrate layer.
Table 4. Composition of the substrate layer.
Substrate AlSi21Cu5MgCr Alloy
ElementAl
[at. %]		Si
[at. %]		Mg
[at. %]		Cu
[at. %]		C
[at. %]		O
[at. %]
Spectrum79.26.55.82.54.11.9
Area 1352526.36.44.62.7
Area 22130.2357.94.61.3
Area 390.30.81.11.84.21.8
Area 491.20.80.41.64.41.6
Table 5. Composition of CrAlN layer.
Table 5. Composition of CrAlN layer.
CrAlN Layer
ElementCr
[at. %]		Al
[at. %]		N
[at. %]		Si
[at. %]		O
[at. %]		C
[at. %]
Spectrum51.75.433.60.53.75.1
Area 152.55.433.40.53.94.3
Area 252.35.433.00.63.75.0
Area 351.85.532.60.63.65.9
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Chitanov, V.; Dochev, B.; Dimova, D.; Zlatareva, E.; Kolchev, S.; Cholakova, T.; Faik, D.; Kolaklieva, L.; Kakanakov, R.; Solakov, T. Investigation of the Mechanical Properties of Cr/CrN/CrAlN Hard Coating Deposited on Special AlSiMgCu Alloy. Crystals 2026, 16, 390. https://doi.org/10.3390/cryst16060390

AMA Style

Chitanov V, Dochev B, Dimova D, Zlatareva E, Kolchev S, Cholakova T, Faik D, Kolaklieva L, Kakanakov R, Solakov T. Investigation of the Mechanical Properties of Cr/CrN/CrAlN Hard Coating Deposited on Special AlSiMgCu Alloy. Crystals. 2026; 16(6):390. https://doi.org/10.3390/cryst16060390

Chicago/Turabian Style

Chitanov, Vasiliy, Boyan Dochev, Desislava Dimova, Ekaterina Zlatareva, Stefan Kolchev, Tetiana Cholakova, Denis Faik, Lilyana Kolaklieva, Roumen Kakanakov, and Teodor Solakov. 2026. "Investigation of the Mechanical Properties of Cr/CrN/CrAlN Hard Coating Deposited on Special AlSiMgCu Alloy" Crystals 16, no. 6: 390. https://doi.org/10.3390/cryst16060390

APA Style

Chitanov, V., Dochev, B., Dimova, D., Zlatareva, E., Kolchev, S., Cholakova, T., Faik, D., Kolaklieva, L., Kakanakov, R., & Solakov, T. (2026). Investigation of the Mechanical Properties of Cr/CrN/CrAlN Hard Coating Deposited on Special AlSiMgCu Alloy. Crystals, 16(6), 390. https://doi.org/10.3390/cryst16060390

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Article metric data becomes available approximately 24 hours after publication online.
Back to TopTop