Si-Al-N-O Multi-Layer Coatings with Increased Corrosion Resistance Deposited on Stainless Steel by Magnetron Sputtering

: This work studies single-layer (Al-Si-N) and multi-layer (Al-Si-N-O/Al-Si-O) coatings deposited by magnetron sputtering on stainless steel specimens (AISI 321), which can be used under aggressive conditions. The multi-layer coating consists of six alternating layers of Al-Si-N-O and Al-Si-O with a thickness of 0.9 µ m and 0.2 µ m, respectively. The structural-phase state and the chemical composition of the coatings were studied by transmission and scanning electron microscopy and XPS analysis. It was revealed that single-layer coatings are nanocrystalline and contain AlN and α -Si3N4 phases. Multi-layer coatings (Al-Si-N-O/Al-Si-O) are amorphous in each of the layers. The corrosion properties of substrate and coated specimens were investigated using a potentiostat in the 3.5 mg/l sea salt solution. It was found that corrosion resistance of stainless steel specimens with multi-layer coating is substantially (tenfold) higher compared with substrates and the specimens with single-layer coating.


Introduction
Metal construction and manufactured parts experience complex external loads during their operation, as they suffer from atmospheric factors, aggressive substances and temperature fluctuation-induced loads. A major condition for the durability and operation reliability of structural materials is high corrosion resistance [1,2]. Stainless steels are widely used as a construction material due to their corrosion resistance under a wide range of operation conditions. However, under certain conditions stainless steel materials may need coatings, since they-like other parts made from construction steels-suffer from corrosion and consequent fracture under aggressive conditions [1][2][3]. Steel corrosion is a grave industrial problem which economic and ecological consequences are known well [4]. The problem of increased corrosion resistance of stainless steels can be solved by alloying steel [5][6][7][8][9] or using corrosion-resistant coatings. Authors have proposed various materials [10][11][12][13][14][15], ranging from metal to ceramic as coatings. Coatings provide additional protection to metal surfaces and serve as a barrier to prevent contact with chemical compounds and corrosion-inducing materials. Coatings protect metal parts from destruction during wetting, salt deposition, oxidation and from the impact of various negative environmental and industrial factors. Anti-corrosion coatings provide maximum life of metal materials (along with such physicomechanical properties as wear resistance, adhesion, hardness, thermal stability) and widen their application range [10][11][12][13][14][15][16].
There are numerous methods to deposit such multifunctional coatings, such as electrochemical methods [17][18][19], thermal methods [20], sol-gel methods [21,22] or a combination of methods [23]. In our work, we used magnetron deposition which allows forming singleand multi-layer coatings with specific functional properties. Works [24,25] show that the most preferable coatings are multi-layer composite coatings that reduce bubble formation and delamination of a material when exposed to a corrosive environment. In addition, multi-layer composite coatings provide more heterogeneous interfaces which reduce the grain sizes and residual stresses under load [26,27].
Nitride ceramics Al-Si-N on a steel substrate possess high cracking resistance and thermal stability, hardness, wear resistance and increased tribomechanical properties [28][29][30][31][32]. The introduction of oxide intermediate Al-Si-O-based layers can increase the corrosion resistance of the coating, since the coating of this kind obtained by the authors [33] on aluminum had corrosion resistance. The ability of aluminium to form stable layers of Al 2 O 3 in oxygen-containing environment is well known [6,[34][35][36], therefore, the aluminum content in the surface layers increases the corrosion resistance of steel materials. Such layers have proven themselves in aggressive media at high temperatures, because aluminium oxide has outstanding mechanical properties and high chemical stability at high temperatures. Silicon oxide [15,37,38] in addition to chemical stability has high hardness and strength.
The authors of [39,40] obtained Al-Si-N and Al-O-N that possess increased physicomechanical and physicochemical properties. Nevertheless, the discussion of Al-Si-N is mainly focused on the mechanical and thermal properties of the coating; there is no description of the corrosion resistance of multi-layer Al-Si-N-O/Al-Si-O coatings. By applying two layers at the same time, one can enhance the effect of each of the layers. It is known [41] that the thickness of the coating has a negative effect due to an increase in stresses as the thickness of the layer increases. Thin coatings [42] while retaining their other qualities are elastic. In addition, nanocomposite coatings, according to the authors [43,44], have a complex of enhanced physicochemical and mechanical properties. Therefore, the main goal of present work is the investigation of the corrosion resistance of multi-layer Al-Si-N/Al-Si-O coatings in comparison to single-layer Al-Si-N coatings and substrate AISI 321 stainless steel, as well as the structural-phase state of the coatings.

Materials and Methods
The coating was applied to AISI 321 substrates, pre-mechanically polished with diamond paste to a roughness of Ra = 0.08 ± 0.008 µm. Then, the specimens were flushed with deionized water and ultrasonically washed in ethanol to remove residual lubrication of the surface and dried at room temperature.
The coating was formed by pulse magnetron sputtering of the AlSi x composite target in a vacuum chamber. The target consisted of aluminum and 20 ÷ 22% of silicon. The pulsed magnetron sputtering method prevented the target surface to be contaminated in the reactive gas environment of argon and oxygen during the deposition of the oxide layers of the coating. The samples were placed in the vacuum installation chamber in the position opposite to the magnetron. The distance was 60 mm ± 0.1mm. Vacuum was reached in the chamber by using a backing scroll pump and a high-vacuum cryogenic pump, providing a residual gas pressure of no more than 6 × 10 −4 Pa. Next, the gas mixture of nitrogen and argon was fed into the chamber, resulting in the deposition of an Al-Si-N layer. Then, without removing the samples from the vacuum chamber, the argon-nitrogen gas mixture was pumped out and the oxygen-argon gas mixture was injected, forming an Al-Si-O layer. Then the process was repeated for spraying Al-Si-N and Al-Si-O layers. Thus, a six-layer coating was obtained. The presence of oxygen in the Al-Si-N layer is due to the specifics of the process, when the oxygen which was adsorbed on the inner surface of the chamber during the formation of the Al-Si-O layer does not have time to be desorbed during the pumping time before the deposition of the Al-Si-N layer. In this case, the first layer was based on aluminium-silicon nitride Al-Si-N (it has higher adhesion to the substrate surface) [45]. The final coating consisted of three pairs of Al-Si-N-O/Al-Si-O layers. The formation modes are given in Table 1. The growth rate of the coating was~0.83 ± 0.04 nm/s. Before deposition, the surface was ion-bombarded by a beam of Ar + (I = 15 mA) over 5 min at an acceleration voltage of 10 kV. The total thickness of the coatings was 3.3 ± 0.2 µm.
The single-layer Al-Si-N coating was formed under the same conditions. The experimental series consisted of 12 samples with and without Al-Si-N-O/Al-Si-O and Al-Si-N coatings. At the same time, four samples were placed in the vacuum chamber and three samples were selected for corrosion testing.
The coatings were studied using an LEO EVO-50XVP scanning electron microscope (Zeiss, Germany) and a JEOL JEM-2100 transmission electron microscope (JEOL Ltd., Tokyo, Japan). The microdiffraction and X-ray microanalysis modes of the JEOL-2100 microscope were used to determine the phase and chemical composition of individual structural components and local micro-regions in the coating. The analysis used crosssection foils prepared by an Ion Slicer EM-09100IS device (JEOL Ltd., Tokyo, Japan). Since the coating consisted of several layers, the thickness of each layer was measured on the specimen's cross-section.
The elemental composition of the coating was analyzed by an energy dispersive X-ray microanalysis (EDXMA) attachment for an INCA-Energy electron microscope (Oxford Instruments, High Wycombe, UK). The phase composition of the steel specimens was determined by an DRON-7 X-ray diffractometer (UED-Lab, Petersburg, Russia).
The chemical composition of the surface layers was analyzed with X-ray photoelectronic spectroscopy (XPS) on a K-Alpha-Spectrometer (Thermo Scientific, Waltham, MA, USA) using an Al Kα monochromatic X-ray source (h ν = 1486.6 eV). The adjustment of the spectra for the curve was performed using Thermo Electron software (v. 5.938 Avantage) with interactive Shirley background subtraction.
The corrosion rate was measured by a three-electrode electrochemical cell and P40X potentiostat. Corrosion measurements were carried out in a 0.5 M NaCl solution using a three-electrode cell, where a Ag/AgCl electrode was used as a reference electrode, and stainless steel was used as an auxiliary electrode, with an area exceeding the area of the working electrode by more than 100 times (it was 400 cm 2 with an area of the working electrode of 1 cm 2 ). It was necessary to neglect overvoltage in current density calculations when constructing voltage dependences. Although a 0.5 mV/s rate is adopted, it is remarked that this selection has no provided substantial distortions in the polarization curves obtained [46][47][48]. In this sense, it is worth noted that potential scan rate has an important role in order to minimize the effects of distortion in Tafel slopes and corrosion current density analyses, as previously reported [46][47][48][49]. Figure 1a shows the cross-section of the surface layer of the formed multi-layer coating on the AISI 321 stainless steel. There are evident alternating layers: one with a thickness of 0.2 µm, the other one with that of 0.9 µm, totaling to 3.3 µm. Figure 1b Figure 1a shows the cross-section of the surface layer of the formed multi-layer coating on the AISI 321 stainless steel. There are evident alternating layers: one with a thickness of 0.2 µm, the other one with that of 0.9 µm, totaling to 3.3 µm. Figure 1b presents the elemental composition of the coating with the element distribution along the coating depth. The image differentiates well the alternating coating layers having different chemical composition. Al-Si-N-based layers contain about 12 at.% of silicon, 32 at.% of aluminium, 35 at% of nitrogen and 20 at.% of oxygen. Thus, the layer is an Al0.32Si0.12N0.32O0.24 silicon-aluminium oxynitride. In the case of Al-Si-O layer, the coating is a fairly stable oxide. This layer contains about 60 at.% of oxygen, 20 at.% of aluminium and 20 at.% of silicon, i.e., the approximate formula of the composition is Al0.2Si0.2O0.6. The oxygen consist in the Al-Si-N layer is due to the specifics of the process when oxygen is actively adsorbed on the surface during the Al-Si-O layer formation. The oxygen consist in the primary surface layer can be explained by two factors: the native film on the steel surface [50] associated with the absorption of atmosphere oxygen and the residual oxygen in the installation chamber, which did not have time to desorb during the pumping time before the Al-Si-N layer deposition. Oxygen, in our case, has a positive effect, which is discussed in more detail below in the article (structure, composition, corrosion mechanism).

Results and Discussion
To analyze the effect of structure on corrosion behavior of the material in more detail, the structure and phase composition of single-and multi-coatings were studied. Figure 2 presents the results of TEM-investigation of the microstructure and phase composition of surface layers in the cross-section of Al-Si-N single-layer coating depos- The oxygen consist in the Al-Si-N layer is due to the specifics of the process when oxygen is actively adsorbed on the surface during the Al-Si-O layer formation. The oxygen consist in the primary surface layer can be explained by two factors: the native film on the steel surface [50] associated with the absorption of atmosphere oxygen and the residual oxygen in the installation chamber, which did not have time to desorb during the pumping time before the Al-Si-N layer deposition. Oxygen, in our case, has a positive effect, which is discussed in more detail below in the article (structure, composition, corrosion mechanism).
To analyze the effect of structure on corrosion behavior of the material in more detail, the structure and phase composition of single-and multi-coatings were studied. Figure 2 presents the results of TEM-investigation of the microstructure and phase composition of surface layers in the cross-section of Al-Si-N single-layer coating deposited on the stainlesssteel specimen. The bright-field TEM-image of the surface layer of the coated specimen ( Figure 2a) clearly shows its nanocrystalline structure. The average transverse dimensions of AlN grains in the Al-SI-N coating is less than 15 nm. The microdiffraction pattern (Figure 3b) mainly shows a system of closed rings. The microdiffraction analysis showed that in the Al-Si-N coating, the AlN aluminium nitride phase forms with hexagonal densely packed crystal lattice. These data were confirmed by the results of X-ray studies in our previous work, where the α-Si3N4 phase was identified [51]. the coated specimen (Figure 2a) clearly shows its nanocrystalline structure. The average transverse dimensions of AlN grains in the Al-SI-N coating is less than 15 nm. The microdiffraction pattern (Figure 3b) mainly shows a system of closed rings. The microdiffraction analysis showed that in the Al-Si-N coating, the AlN aluminium nitride phase forms with hexagonal densely packed crystal lattice. These data were confirmed by the results of X-ray studies in our previous work, where the α-Si3N4 phase was identified [51].  The Al-Si-N-O layer starts forming the coating on the stainless-steel substrate; the thin layer of Al-Si-O finishes the coating. Figure 3a clearly shows the interface between the coating and the substrate. Obviously, the coating fills the surface cavities, which promotes high adhesion between the coating and the substrate and, correspondingly, increases the corrosion resistance of the coating.
The microdiffraction patterns obtained by TEM for each of the layers in transverse cross-section showed halos (total absence of reflexes) (Figure 3b,c). This speaks to both layers in the multi-layer coating to have an amorphous structure. The amorphization of the oxide layer proceeds due to the incorporation of Si atoms into the crystal lattice of Al2O3 [27]. The nitride layer transits into amorphous state as a result of introduction of transverse dimensions of AlN grains in the Al-SI-N coating is less than 15 nm. The microdiffraction pattern (Figure 3b) mainly shows a system of closed rings. The microdiffraction analysis showed that in the Al-Si-N coating, the AlN aluminium nitride phase forms with hexagonal densely packed crystal lattice. These data were confirmed by the results of X-ray studies in our previous work, where the α-Si3N4 phase was identified [51].   The Al-Si-N-O layer starts forming the coating on the stainless-steel substrate; the thin layer of Al-Si-O finishes the coating. Figure 3a clearly shows the interface between the coating and the substrate. Obviously, the coating fills the surface cavities, which promotes high adhesion between the coating and the substrate and, correspondingly, increases the corrosion resistance of the coating.
The microdiffraction patterns obtained by TEM for each of the layers in transverse cross-section showed halos (total absence of reflexes) (Figure 3b,c). This speaks to both layers in the multi-layer coating to have an amorphous structure. The amorphization of the oxide layer proceeds due to the incorporation of Si atoms into the crystal lattice of Al2O3 [27]. The nitride layer transits into amorphous state as a result of introduction of  Figure 3 shows the multi-layer coating cross-section on AISI 321 stainless steel substrate with clear substrate crystalline structure and coating structure without crystallites.
The Al-Si-N-O layer starts forming the coating on the stainless-steel substrate; the thin layer of Al-Si-O finishes the coating. Figure 3a clearly shows the interface between the coating and the substrate. Obviously, the coating fills the surface cavities, which promotes high adhesion between the coating and the substrate and, correspondingly, increases the corrosion resistance of the coating.
The microdiffraction patterns obtained by TEM for each of the layers in transverse cross-section showed halos (total absence of reflexes) (Figure 3b,c). This speaks to both layers in the multi-layer coating to have an amorphous structure. The amorphization of the oxide layer proceeds due to the incorporation of Si atoms into the crystal lattice of Al 2 O 3 [27]. The nitride layer transits into amorphous state as a result of introduction of oxygen ions into its crystal lattice the small amount of which presents after the change of gas environment in the vacuum chamber, as indicated by the authors [43,52] (during the process the microstructure evolves from crystalline to nanocrystalline and then to the amorphous microstructure). This is confirmed by the investigation of the elemental composition of coating layers in the multi-layer coating cross-section by EDXMA on the transmission electron microscope (Figure 1). Therefore, the first layer of the coating is the amorphous aluminium-silicon oxynitride. This compound possesses higher hardness as compared to conventional silicon nitride [43][44][45][46][47][48][49][50][51][52][53][54][55][56]. Products based on aluminium-silicon oxynitride are used in the conditions of high mechanical loads, thermal shock, aggressive chemical and abrasive media, which is conditioned by their high strength, chemical stability, corrosion and thermal stability. In the case of Al-Si-O layer, the formation of aluminium silicate is possible. It can form after simultaneous synthesis of aluminium and silicon oxides in magnetron discharge plasma [57]. This is possible due to amphiprotic properties of aluminium oxide. The molecular reaction is as follows: Al 2 O 3 + SiO 2 = Al 2 (SiO 3 ) 2 X-ray diagrams of the initial stainless-steel specimens and the steel specimen with multi-layer coating contain typical reflexes of γ-iron [51]. XRD analysis of the multi-layer coating has shown that within the measurement error, both curves almost coincide, i.e., the coating yields no diffraction reflections after irradiation by an X-ray beam. This confirms our assumption that both layers in the multi-layer coating have an amorphous structure. Since the corrosion rate decreases with decreasing total number of grain boundaries in metal, while in the amorphous material they are absent, the observed increase in the corrosion stability of steel specimens after coating deposition, especially multi-layer coatings, can be due to this reason [27].
The assumption of aluminium silicate (Al 2 (SiO 3 ) 2 ) formation in the multi-layer coating is confirmed by the XPS analysis of the specimen surface layer (Figure 4). The layer-bylayer oxygen spectrum at 470 nm from the coating surface has revealed oxygen peak shift towards lower energies. The oxygen concentration at the surface is three times higher, as compared to internal layers, which is conditioned by its transition from the Al-Si-O layer into the Al-Si-N-O layer, which correlates with the data received by EDXMA and presented in Figure 1b. amorphous microstructure). This is confirmed by the investigation of the elemental composition of coating layers in the multi-layer coating cross-section by EDXMA on the transmission electron microscope (Figure 1). Therefore, the first layer of the coating is the amorphous aluminium-silicon oxynitride. This compound possesses higher hardness as compared to conventional silicon nitride [43][44][45][46][47][48][49][50][51][52][53][54][55][56]. Products based on aluminium-silicon oxynitride are used in the conditions of high mechanical loads, thermal shock, aggressive chemical and abrasive media, which is conditioned by their high strength, chemical stability, corrosion and thermal stability. In the case of Al-Si-O layer, the formation of aluminium silicate is possible. It can form after simultaneous synthesis of aluminium and silicon oxides in magnetron discharge plasma [57]. This is possible due to amphiprotic properties of aluminium oxide. The molecular reaction is as follows: Al2O3 + SiO2 = Al2(SiO3)2 X-ray diagrams of the initial stainless-steel specimens and the steel specimen with multi-layer coating contain typical reflexes of γ-iron [51]. XRD analysis of the multi-layer coating has shown that within the measurement error, both curves almost coincide, i.e., the coating yields no diffraction reflections after irradiation by an X-ray beam. This confirms our assumption that both layers in the multi-layer coating have an amorphous structure. Since the corrosion rate decreases with decreasing total number of grain boundaries in metal, while in the amorphous material they are absent, the observed increase in the corrosion stability of steel specimens after coating deposition, especially multi-layer coatings, can be due to this reason [27].
The assumption of aluminium silicate (Al2(SiO3)2) formation in the multi-layer coating is confirmed by the XPS analysis of the specimen surface layer (Figure 4). The layer-by-layer oxygen spectrum at 470 nm from the coating surface has revealed oxygen peak shift towards lower energies. The oxygen concentration at the surface is three times higher, as compared to internal layers, which is conditioned by its transition from the Al-Si-O layer into the Al-Si-N-O layer, which correlates with the data received by EDXMA and presented in Figure 1b.   The presence of aluminium in the coating could not be recorded as individual spectra. Oxides and hydroxides of aluminium (Al 2 O 3 , Al(OH) 3 and AlOOH) are extremely difficult to differentiate by XPS. Unfortunately, energies in spectra of aluminium oxides and hydroxides overlap each other [34]. Layer-by-layer spectra of silicon are presented in Figure 5.
The presence of aluminium in the coating could not be recorded as individual spectra. Oxides and hydroxides of aluminium (Al2O3, Al(OH)3 and AlOOH) are extremely difficult to differentiate by XPS. Unfortunately, energies in spectra of aluminium oxides and hydroxides overlap each other [34]. Layer-by-layer spectra of silicon are presented in Figure 5. The analysis of silicon spectra has shown domination of the silicon oxide SiO2 (Е ≈ 103.1 V) on the surface; the content of Si-N (Е ≈ 101 V) on the surface is negligibly small, while at a depth of ~ 230 nm (Figure 5d), the content of Si-N becomes substantially higher than that of its oxide. The transition layer from Al-Si-O to Al-Si-N-O can be seen in Figure  5b and 5c when silicon oxide content decreases, while that of nitride compounds conversely increases. Such data on compositions of silicon with nitrogen were confirmed after the analysis of nitrogen spectra that has shown increased nitrogen content inside the coating (Figure 5f) in its absence on the surface. Therefore, XPS shows ( Figure 4) that on the coating surface, there is aluminium oxide which is absent in deeper layers. The analysis of the silicon spectra shows that aluminium oxide is present on the surface. Then, silicon nitride forms, which becomes prevalent at a depth of 470 nm, which promotes the deposition of the layers. These data correlate with the elemental analysis data received by EDXMA. The analysis of silicon spectra has shown domination of the silicon oxide SiO 2 (E ≈ 103.1 V) on the surface; the content of Si-N (E ≈ 101 V) on the surface is negligibly small, while at a depth of 230 nm (Figure 5d), the content of Si-N becomes substantially higher than that of its oxide. The transition layer from Al-Si-O to Al-Si-N-O can be seen in Figure 5b and 5c when silicon oxide content decreases, while that of nitride compounds conversely increases. Such data on compositions of silicon with nitrogen were confirmed after the analysis of nitrogen spectra that has shown increased nitrogen content inside the coating (Figure 5f) in its absence on the surface. Therefore, XPS shows ( Figure 4) that on the coating surface, there is aluminium oxide which is absent in deeper layers. The analysis of the silicon spectra shows that aluminium oxide is present on the surface. Then, silicon nitride forms, which becomes prevalent at a depth of 470 nm, which promotes the deposition of the layers. These data correlate with the elemental analysis data received by EDXMA.
The corrosion stability was studied on three batches of specimens: 1-without coating (three samples), 2-with single-layer Al-Si-N coating (three samples), and 3-with multilayer Al-Si-N-O/Al-Si-O coating (three samples). To assess the corrosion resistance of the coatings, polarization curves were plotted ( Figure 6). The scanning rate was 0.5 mVs −1 . The polarization curves were plotted using averaged values obtained for 3 similar specimens. The voltammetric curves of the coated specimens displace to the region of higher voltage and lower current density. The polarization characteristics of the specimens with Al-Si-N coating are initially similar to those of multi-layer coating; however, the polarization potential E of the specimens with Al-Si-N-O/Al-Si-O is higher and amounts to 375 mV, unlike the single-layer coating, where E = 63 mV, while the current density is appreciably lower. The corrosion current are I corr~8 .7 × 10 −6 A,~5.2 × 10 −5 A and~7.8 × 10 −5 A for Al-Si-N-O/Al-Si-O, Al-Si-N and stainless-steel, respectively. Moreover, there is no passivation region for the single-layer coating. For Al-Si-N-O/Al-Si-O specimens, the passivation region lies within 700-1250 mV. In this interval, the metal surface shifts to inactive passive state due to corrosion-preventing compounds. In our case, it included the formation of oxynitrides that were identified in the Al-Si-N-based coatings (shown below). resistance of the coatings, polarization curves were plotted ( Figure 6). The scanning rate was 0.5 mVs −1 . The polarization curves were plotted using averaged values obtained for 3 similar specimens. The voltammetric curves of the coated specimens displace to the region of higher voltage and lower current density. The polarization characteristics of the specimens with Al-Si-N coating are initially similar to those of multi-layer coating; however, the polarization potential E of the specimens with Al-Si-N-O/Al-Si-O is higher and amounts to 375 mV, unlike the single-layer coating, where E = 63 mV, while the current density is appreciably lower. The corrosion current are Icorr ~8.7 × 10 −6 A, ~5.2 × 10 −5 A and ~7.8 × 10 −5 A for Al-Si-N-O/Al-Si-O, Al-Si-N and stainless-steel, respectively. Moreover, there is no passivation region for the single-layer coating. For Al-Si-N-O/Al-Si-O specimens, the passivation region lies within 700-1250 mV. In this interval, the metal surface shifts to inactive passive state due to corrosion-preventing compounds. In our case, it included the formation of oxynitrides that were identified in the Al-Si-N-based coatings (shown below).
All these factors testify to increased corrosion resistance of stainless-steel specimens with Al-Si-N and Al-Si-N-O/Al-Si-O coatings, which ultimately increases for the multi-layer coating, as compared to uncoated specimens. Multi-layer coatings surpasses single-layer coatings in terms of decreased pore and decreased defect dimensions in every layer. The formation of interfaces between heterogeneous layers inhibits the columnar crystallites growth in the coating, which may inhibit the diffusion of atoms along the coating thickness and, correspondingly, impede corrosion processes. The increase in the number of layers may increase the effect. Therefore, the multiple layers of a coating may be a positive factor decreasing corrosion of steel All these factors testify to increased corrosion resistance of stainless-steel specimens with Al-Si-N and Al-Si-N-O/Al-Si-O coatings, which ultimately increases for the multilayer coating, as compared to uncoated specimens.
Multi-layer coatings surpasses single-layer coatings in terms of decreased pore and decreased defect dimensions in every layer. The formation of interfaces between heterogeneous layers inhibits the columnar crystallites growth in the coating, which may inhibit the diffusion of atoms along the coating thickness and, correspondingly, impede corrosion processes. The increase in the number of layers may increase the effect. Therefore, the multiple layers of a coating may be a positive factor decreasing corrosion of steel specimens [15,25,26]. The authors [43,[53][54][55][56] consider that nanocomposite coatings have undeniable advantages due to the nanostructuring of the applied coatings. Multi-layer coatings [58] create an additional barrier. This is explained by the fact that, due to the different chemical composition of each of the layers, barriers arise that prevent the development of a leading channel for the corrosive environment penetration (Figure 7). Single-layer coatings have more defects [7,59], which may be caused by the intergranular space instability [60]. Also, it may be caused by the development of cracks in the monolithic coating as a result of additional mechanical action. Or it may be caused by the active diffusion of chlorine and/or oxygen ions, coating dissolution, and the corrosion products diffusion [8,56]. The authors point out [41] that in such coatings, multiple channels for the penetration of a corrosive environment, and even pitting destruction of the base under the coating, often occur. space instability [60]. Also, it may be caused by the development of cracks in the monolithic coating as a result of additional mechanical action. Or it may be caused by the active diffusion of chlorine and/or oxygen ions, coating dissolution, and the corrosion products diffusion [8,56]. The authors point out [41] that in such coatings, multiple channels for the penetration of a corrosive environment, and even pitting destruction of the base under the coating, often occur.
The scheme of the mechanism of material behavior with multi-and single-layer coating after its placement into an aggressive environment containing chlorine anion (sea salt solution) is shown in Figure 7. Regarding the impact of the aggressive environment on the steel substrate with multi-layer coating, chlorine ions-before oxidation-have to pass six dense amorphous layers, three of which contain silicon and aluminium oxides (or synthesized silicon or aluminium silicate) which are corrosion-resistant agents, while the three remaining layers contain corrosion-resistant silicon-aluminium oxynitride (Figure 7a). The presence of such corrosion resistance is confirmed by the polarization curves having passivation region. In the case of single-layer crystalline coating (following earlier works [61]) containing silicon and aluminium nitrides, chlorine ions more easily penetrate to the metal substrate forming iron chlorides, which is confirmed by the absence of the passivation region on polarization curve. We assume that the oxidant penetrates along the intercrystalline void promoting further active damage to the material under the coating (Figure 7b) with its consequent delamination and disintegration. The polarization curves for single-layer coatings presented above confirm this, i.e., while for the initial period, there is a bias towards positive potentials, the following parts of the curves for single-layer coatings and for steel substrate coincide.

Conclusions
Magnetron sputtering allowed producing corrosion-resistant single-layer Al-Si-N-coatings and multi-layer Al-Si-N-O/Al-Si-O-coatings. We have then examined the structural-phase state and corrosion properties of the coatings. Electron microscopy made it possible to establish that the single-layer Al-Si-N-coating is a nanocrystalline The scheme of the mechanism of material behavior with multi-and single-layer coating after its placement into an aggressive environment containing chlorine anion (sea salt solution) is shown in Figure 7.
Regarding the impact of the aggressive environment on the steel substrate with multilayer coating, chlorine ions-before oxidation-have to pass six dense amorphous layers, three of which contain silicon and aluminium oxides (or synthesized silicon or aluminium silicate) which are corrosion-resistant agents, while the three remaining layers contain corrosion-resistant silicon-aluminium oxynitride (Figure 7a). The presence of such corrosion resistance is confirmed by the polarization curves having passivation region. In the case of single-layer crystalline coating (following earlier works [61]) containing silicon and aluminium nitrides, chlorine ions more easily penetrate to the metal substrate forming iron chlorides, which is confirmed by the absence of the passivation region on polarization curve. We assume that the oxidant penetrates along the intercrystalline void promoting further active damage to the material under the coating (Figure 7b) with its consequent delamination and disintegration. The polarization curves for single-layer coatings presented above confirm this, i.e., while for the initial period, there is a bias towards positive potentials, the following parts of the curves for single-layer coatings and for steel substrate coincide.

Conclusions
Magnetron sputtering allowed producing corrosion-resistant single-layer Al-Si-Ncoatings and multi-layer Al-Si-N-O/Al-Si-O-coatings. We have then examined the structuralphase state and corrosion properties of the coatings. Electron microscopy made it possible to establish that the single-layer Al-Si-N-coating is a nanocrystalline material that contains AlN phases with hexagonal densely packed crystal lattice and α-Si 3 N 4 phases. The multilayer Al-Si-O-N-based coating consists of alternating layers of Al-Si-N and Al-Si-O that have a structure close to amorphous one. Moreover, it should be noted that the coating growth conditions are the same in both cases, i.e., oxygen affects the structure of the coating.
The distribution of elements along the depth of Al-Si-N-O/Al-Si-O-coating obtained by the EDXMA method indicates that the layer of Al-Si-N-coating is an oxynitride that possesses both increased mechanical properties and enhanced corrosion resistance.
It was established that multi-layer Al-Si-N-O/Al-Si-O-coating has higher corrosion resistance vs substrate steel and single-layer Al-Si-N-layer, which is confirmed by the polarization curves demonstrating their higher polarization potential as compared to the uncoated specimens and an order of magnitude lower value of the current density for a multi-layer coating, and hence the dissolution rate. The increased corrosion stability of Al-Si-N-O/Al-Si-O multi-layer coating is connected with first, decreased total number of grain boundaries in metal and transition from crystalline state to nanocrystalline and further amorphous state. Second, Al-Si-N-based layer in multi-layer coating is represented by Al 0.32 Si 0.12 N 0.32 O 0.24 silicon-nitrogen oxynitride which has higher corrosion resistance. Third, the multi-layer coating surpasses single-layer coatings in terms of decreased pore and defect dimensions in every layer. The formation of interfaces between heterogeneous layers inhibits the growth of columnar crystallites in the coating, which may inhibit the diffusion of atoms along the coating thickness and, correspondingly, impede corrosion processes.
In our case, a synergistic effect is manifested due to several factors: there are six layers in the coating; coating is in the form of thin films; each of the layers has the amorphous structure; layers of different chemical composition alternate; the films contain the corrosionresistant components [62].

Conflicts of Interest:
The authors declare no conflict of interest.