Fatigue and tensile behaviour of Ti+TiN+Ti+TiVN multilayer nitride films coated on AZ91 magnesium alloy by closed field unbalanced magnetron sputtering

Despite their extensive use in the automotive and aerospace industries, Mg and Mg alloys, which are light metals, exhibit low fatigue and tensile strength. In this study, transition metal-nitride (TMN) multilayer coatings (Ti+TiN+Ti+TiVN) were coated twice on AZ91 Mg alloy using a Confined Field Unbalanced Magnetron Sputtering (CFUBMS) system to increase fatigue and tensile strength. The structural properties of the films were analyzed by using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometry (EDS) methods, and the mechanical properties were analyzed by rotating bending fatigue and tensile testing machines. Ti+TiN+Ti+TiVN multilayer nitride surface coatings on AZ91 Mg alloys showed a dense and columnar microstructure and according to XRD results (111) was the preferred orientation with the dominant peak. The fatigue limit value of the AZ91 base material was fixed at 60.46 MPa, while it increased to 68.48 MPa after being coated with multilayer nitride. Along with the multilayer nitride coating, the tensile strength increased from 169.98 MPa to 175.43 MPa. As a result, the multilayer hard nitride coating with low surface roughness, which fills the defects, notches, and voids on the surface of the AZ91 base material, increased the fatigue and tensile strength in parallel. Based on the outcomes of the research, the literature has been enriched with an innovative approach through the enhancement of fatigue and tensile strengths by applying a CFUBMS coating to lightweight metals and alloys, such as AZ91, especially in the transportation industry where lightness and dynamic load resistance are essential.


Introduction
Magnesium alloys have a wide range of applications, especially in the automotive, aerospace, and biomedical fields, owing to their high strength, low density, and superior biocompatibility [1].However, there is a need for improvement in the corrosion, creep, and fatigue resistance of these alloys.For these reasons, various surface coating techniques are employed [2].
Many methods such as inorganic conversion coatings, plasma electrolytic oxidation, PVD, or thermal spraying have been applied for the surface coating of magnesium alloys [3].PVD produces higher-quality surface coatings compared to other conventional surface treatment methods.Magnesium alloys have been coated with various PVD coatings such as TiN, AlN, and ZrN [4].
Coatings deposited via PVD technologies seem to be a solution to overcome disadvantages such as weak corrosion and wear resistance of magnesium alloys [5].Despite extensive research, a PVD coating that effectively maintains the corrosion resistance of the compound, without considerably diminishing it when compared to pure magnesium, has not been identified so far.Among the PVD techniques used to produce thin and hard coating films, the Closed Field Unbalanced Magnetron Balanced Sputtering method is widely utilized due to its many advantages, such as high-speed deposition, controlled phase composition and microstructure, and low foreign matter accumulation [6,7].
The flexibility of the PVD method in producing excellent coatings with an outstanding level of control at the nanometric level provides superior properties.Among these coatings, nitride-based Ti, Cr, Al, and V transition metals provide high wear resistance, and excellent thermal and oxidation stability [8,9].For this purpose, multilayered coatings produced in different combinations such as TiN, TiAlN, TiCrN, TiVN and TiNbN have been developed [10][11][12][13].
Among these coating films, TiN is the most widely used hard coating to improve mechanical properties in industrial areas [14].Vanadium nitride (VN) is an alternative material for coating applications because it is a promising material for coating applications with mechanical properties such as good thermodynamic stability, low friction, high corrosion resistance, and high electrical conductivity [15,16].The mechanical and fatigue properties of AZ magnesium alloys produced by various techniques have been reported in many studies in the literature [17][18][19][20][21].The authors have shown that metallurgical factors and microstructure are the determining factors for fatigue life and higher strain amplitudes of the alloys [17][18][19][20][21][22][23].
There are also studies focused on the low cycle fatigue (LCF) and high cycle fatigue (HCF) behavior of magnesium alloys, especially at room temperature (RT) [24][25][26][27] .Very few studies in the literature use various thin film coating techniques to improve the mechanical (tensile) and fatigue properties of AZ Mg alloys.
Diab et al (2017) investigated the effect of pure aluminum cold spray coating on corrosion and corrosion fatigue of extruded AZ31B magnesium alloy.They observed a significant improvement in fatigue properties due to cold spraying compared to pure aluminum [28].
In the study by Karabudak (2023), the high-cycle fatigue and tensile strength of AZ91 Mg alloy coated with MAO-epoxy duplex coating were examined.The study concluded that the MAO-Epoxy duplex coating, possessing a homogenous and compact structure, significantly enhanced the fatigue limit/life, yield strength, ultimate tensile strength, and elongation percentage of the alloy [29].
Karabudak et al (2023) examined the effect of surface roughness on the fatigue life of AZ31 Mg alloy by coating MAO at 500 and 700 Hz frequencies.Then, in the direction of decreasing surface roughness by coating epoxy on MAO, MAO (500 Hz)< MAO (700 Hz)< AZ31 base< MAO (500 Hz)-Epoxy< MAO (700 Hz)-Epoxy fatigue limit values were obtained [30].
The CFUBMS technique among PVD methods provides superior properties due to its flexibility in producing coatings with excellent control at the nanometric level.Among these coatings, multilayer coatings have been developed using combinations of nitride-based transition metals such as Ti, Cr, Al, and V with TiN, TiAlN, TiCrN, TiVN, and TiNbN.There is no numerically defined standard for the number of layers in the literature.Upon reviewing studies on magnesium and its alloys coated using the Physical Vapor Deposition (PVD) method, it has been observed that properties such as corrosion, wear, and adhesion have improved with the application of various types of coatings.Altun and Sen (2006) investigated the application of multi-layered AlN (AlN + AlN + AlN) and AlN + TiN coatings on AZ91 magnesium alloy using DC magnetron sputtering, a PVD technique.Their study focused on the impact of these coatings on the corrosion behavior of the AZ91 alloy.The findings revealed that the PVD coatings deposited on the AZ91 magnesium alloy significantly enhanced its corrosion resistance [31].
In his 2020 research, Kim produced a Zn thin film on an AZ91D substrate utilizing thermally-electron activated ion plating, an environmentally friendly adaptation of the PVD technique.This study found that the Zn thin film significantly enhanced the corrosion resistance of the AZ91D alloy [32].
Allasi et al (2023) coated AZ91D Mg alloy with ZrO2 and ZrN ceramics by physical PVD.They observed wear mechanisms such as abrasion, delamination, thermal softening and oxidation.As a result, it has been shown that PVD coating increases the wear resistance of AZ91D Mg alloy [33].
The review of the literature reveals no prior studies evaluating tensile and fatigue tests on AZ91 base material coated with such a multilayer using the CFUBMS system, highlighting the original contribution of this work.This study aims to investigate the fatigue and tensile strength properties of AZ91 Mg alloy coated with Ti+TiN +Ti+TiVN multilayer.

Experimental procedures
AZ91 Mg alloy with a chemical composition of Al 8.84%, Zn 0.61%, Mn 0.18%, Si 0.02%, Cu 0.005%, and Mg balance by weight was cut according to the dimensions in figure 1.The specimens subjected to fatigue and tensile tests were ground using 800 and 1200-mesh SiC sandpaper and cleaned with alcohol.Ti+TiN+Ti+TiVN multilayer nitride coatings deposited by CFUBMS method were subjected to fatigue and tensile tests.Ti+TiN +Ti+ TiVN films (figure 1) were deposited using a CFUBMS system manufactured by Teer Coatings Ltd Argon (99.9%) sputter gas with Ti and V targets was used to produce the coatings.
The patented PVD system (PLASMAG 550) by Teer Coating Ltd was used to coat AZ91 base material with multilayer Ti+TiN+Ti+TiVN thin films.In the coating process, one titanium target (99.95%pure) and one vanadium target (99.95%pure) were used.These targets were connected to a pulsed DC power supply.Initially, an ion cleaning process was applied to the base materials for 40 min, during which the negative voltage applied to the base materials was set to 800V.Subsequently, to enhance the adhesion between the base material and the coating, a titanium interlayer was applied.During this phase, 2.5 A was applied to the titanium target for 10 min, while the negative voltage of the base material was maintained at 250 V.Then, the process moved to the TiN layer; during this 15-minute phase, N 2 gas was introduced to facilitate the formation of the nitride phase.The N 2 gas flow was maintained constant at 4 sccm.Titanium was again targeted with 2.5 A for 10 min, and during the 15-minute process for the TiVN layer, 2.5 A and 1.5 A were applied to the titanium and vanadium targets respectively.The negative voltage applied to the base materials was fixed at 100V.This procedure was repeated twice to achieve an eight-layer coating.Throughout the entire coating process, the working pressure was maintained at 0.26 Pa, the frequency at 100 kHz, and the pulse duration was set at 2 ms (μs) [10,11,34,35].
Multilayer coatings have been developed in the literature using various combinations of nitride-based transition metals such as Ti, Cr, Al, and V, with TiN, TiAlN, TiCrN, TiVN, and TiNbN.However, no studies have been found that specifically define each layer in these coatings.In our study, we have implemented an eightlayer structure, and each layer has been distinctly defined and described [36][37][38].
Tensile tests were carried out at room temperature using Shimadzu AGS-X universal testing machine at a speed of 0.5 mm min −1 .High cycle fatigue tests were performed on an R.R. Moore type rotary bending fatigue tester with a rotation speed of 3000 rpm.For AZ91 base material and multilayer nitride-coated specimens, at least three fatigue tests were performed at one stress level following ASTM [39] standard to obtain more accurate values (figure 2(a)).Six tensile test specimens were prepared for each of the AZ91 base materials and multilayer nitride coated specimens according to ASTM E8/E8M-13a standard [40] (figure 2(b)).
XRD analyses were conducted using a Rigaku 2200 Dmax diffractometer equipped with a CuKα (λ = 1.5404) radiation source, with measurement values in the scanning range of 20°to 90°and at a scanning speed of 2°/min.The chemical composition of the multi-layered Nitride films was determined by EDS, and the morphology of the fatigue-tensile fracture surfaces was investigated using a SEM; Jeol-6400.The surface roughness (Ra) values of the coatings were determined using a Mahr brand surface profilometer.

XRD and SEM-EDS analysis
The microstructure of AZ91 Mg alloy consists of α-Mg dendrites (matrix) and interdendritic eutectic β-phase (Mg17Al12) [38,39].The cross-sectional morphologies and surface SEM images of the Ti, TiN, and TiVN multilayered Nitride coatings are shown in figure 3. The multi-layered coating has exhibited a dense and columnar microstructure, and the thickness value obtained from the cross-sectional SEM image is 345.3 nm.At a sampling In the 2θ = 20°-90°scattering range, XRD diffraction peaks for the AZ91 Mg alloy show that α-Mg and Mg17Al12 are the prominent components [40].In the Ti, TiN, and TiVN multi-layered Nitride coatings, highdensity diffraction peaks (111), ( 200), (220), and (222) for TiN have been identified.For TiVN coating, the main diffraction peaks are identified as (111) and (311), and the preferred orientation may vary depending on the substrate materials [13,14,[41][42][43].After applying Ti, TiN, and TiVN multi-layered Nitride coatings, a decrease in the intensity of α-Mg and Mg17Al12 peaks is observed (figure 4).
According to the energy dispersive X-ray (EDAX) results in figure 5, the Ti+TiN+Ti+TiVN multi-layered coating contains 10.04% N, 39.13% Ti, and 15.36% V by weight.The other components are the alloy elements of the base material AZ91.

Fatigue test
In this study, applying Ti+TiN+Ti+TiVN multilayer nitride coatings on the AZ91 magnesium alloy has been observed to increase the fatigue strength by 13.26%.Before coating, the average fatigue limit was 60.46 MPa, which increased to 68.48 MPa after coating.Figure 6 displays the S-N curves for the uncoated AZ91 base material and samples with multilayer nitride coating.The load was applied until the sample failed or completed 1.0E+07 cycles.The endurance limit of the AZ91 alloy was 60 ± 10 MPa before coating and rose to 70 ± 10 MPa after applying the multilayer nitride coating.
The literature indicates that nitride coatings such as TiN, CrN, TiAlN, ZrN, TiZrN, and TiCrN have been applied using the PVD technique to enhance the fatigue strength of various metals and alloys [41][42][43][44] .The mechanical properties and adhesion strength of these coatings can be further improved by adding interlayers [34].Additionally, residual stresses caused by cathodic arc deposition (CAD) can be reduced with an optimized design of the coating interlayer [45,46] Studies have shown that TiN and TiVN films, when applied to different alloys, significantly affect their structural and mechanical properties.[35,47].In a study by Uslu et al (2015), TiN, TiVN, and TiN/VN coatings were applied to AZ91D magnesium alloy using the RF magnetron sputter method [48].Adding Vanadium (V) particularly enhanced surface roughness, strength, and adhesion strength [48].It is noted that surface roughness is directly related to fatigue damage and initiates cracks under fatigue conditions.Therefore, scientific studies have tried to improve fatigue strength by enhancing the rough structure with various methods.The studies by Çiçek et al (2017) and Smolik and his colleagues (2019) also demonstrated that TiVN films, especially compared to TiNbN and Cr-CrN/(CrN-VN) coatings, offer better fatigue resistance [38,47].When examining the fatigue crack initiation areas of the coated samples, surface defects and nodular defects of the AZ91 base material were identified.These surface flaws lead to stress accumulation, increasing the rate of fatigue and thus causing the material to damage more quickly.The study noted that the low surface roughness, averaging Ra = 1.651 μm, caused a delayed start of the fatigue crack and a lower rate of fatigue.In samples with multilayer coating, if the initiation of the fatigue crack occurs in the base material beneath the layer, the contribution of the coating to fatigue strength will be limited.

Fatigue fracture surfaces
A fractographic investigation was performed on uncoated and multilayer nitride-coated AZ91 specimens to determine the fatigue life initiation behavior.In AZ91 specimens, similar to the literature, fatigue cracking started at notches, cracks, and surface/sub-surface defects on the specimen surface and merged with secondary cracks with crack propagation and was characterized by line-like features [24,49].Ting and Lawrence (1993) calculated many shrinkage voids (1.2% by volume) in AZ91 material.Therefore, crack initiation in voids should be considered as a possible event [50].
Similar observations regarding crack initiation for AZ91 material were also reported by Mayer et al [51].If the number of cycles that can cause crack initiation is very low, the pores act as initiation sites for fatigue initiation.At low strain amplitude values, the increasing number of cycles controls crack initiation.In both cases, roughness has a negative effect on fatigue life.Since the Ti, TiN and TiVN multilayer Nitride coating shows a dense and columnar microstructure in SEM image results, it is concluded that there are fewer notches, cracks, and defects that may cause crack initiation on the surface compared to the uncoated AZ91 material.Therefore, the fatigue limit value and strength are better.The lower value of surface roughness also supports this conclusion.Rettberg et al (2012) found that in AZ91 alloy, the fatigue crack propagates in the plane parallel to the stress axis, starting in the pores close to the primary crack, and the crack path is not affected by α-Mg dendrites, interdendritic structures, and eutectic structures.It was concluded that, in contrast to the typical crack growth direction perpendicular to the stress axis, some cracks grow parallel to the stress axis due to the repulsive factor that allows cracks to connect with nearby pores [49].It was noted (figure 7) that step-like surfaces were visible on the fracture surface of uncoated AZ91 and multilayer nitride-coated test specimens.

Tensile test
The comparative tensile test curves of AZ91 base material and multilayer nitride-coated specimens are shown in figure 8.These results were obtained for six specimens from each material.In the graph in figure 8, the stressstrain curve obtained for each specimen is shown, along with an average curve depicted in color.The maximum tensile strength of the AZ91 base material is 169.98 MPa, while the maximum tensile strength of the nitridecoated material is 175.43 MPa.With the coating, the elasticity modulus of the base material increased from 26 GPa to 34 GPa.Additionally, the elongation value increased from 13.08% to 14.66%.The standard deviation of the tensile curves for the uncoated specimens was calculated as 0.97, and for the coated specimens, it was 1.09  (figure 10).As a result, the multilayer nitride-coated specimens showed an increase in tensile strength and elongation values compared to the AZ91 base material.
When the coated specimens were subjected to tensile stresses greater than the yield strength (160 MPa) of the AZ91 base material, evaluated in parallel with fatigue, the coating's homogeneous, well-adhering, high-strength, columnar, and dense properties allowed the coating to maintain its integrity for a long time during the tensile  test, thus improving the tensile performance.The delayed onset of plastic deformation and the parallel delay in crack initiation, nucleation, and propagation in the coating suggest that the initiation time of cracks in the base material is prolonged, and the crack density is likely reduced.These results support similar studies in the  literature [52,53].For example, Shugurov and Kuzminov (2021) examined the effects of multilayer architecture and Ta alloying on the mechanical performance of Ti-Al-N coatings and obtained similar results [52].Amer et al (2022) reviewed in situ mechanical testing of coatings and noted that multilayer coatings could improve performance [53].
In conclusion, the application of a Ti+TiN+Ti+TiVN thin hard coating film has been evaluated to increase tensile strength, thereby extending the crack initiation time.Concurrently, the use of multiple coatings has increased strain.These results demonstrate that coating applications are an effective method to improve the mechanical properties of materials like AZ91 Mg alloy.

Tensile fracture surfaces
Figures 9(a) and (b) display the SEM images of the tensile fracture surfaces of the AZ91 base material and the multi-layered nitride-coated samples, respectively.The SEM observations revealed that the initiation of tensile cracks starts from the surface of the samples or near-surface defects such as pores and inclusions.The propagation of tensile cracks can be characterized by more randomly oriented line-like features, combined with tearing ridges and secondary cracks along interfaces.
Figure 10 presents the scalar results obtained from fatigue and tensile tests.As can be seen from the graph, there is a significant increase in the material's fatigue strength, tensile strength, and modulus of elasticity after the application of the multi-layered Nitride coating.

Conclusions
In this study, Ti+TiN+Ti+Ti+TiVN multilayer nitride coatings were coated on AZ91 base materials.Fatigue and tensile tests were performed on AZ91 and multilayer nitride-coated samples and strength values were compared.
Multilayer Nitride coating thickness was measured as 345.3 nm and surface roughness as 1.651 μm.The preferential orientation of (111), which is the dominant peak in the coating showing a dense and columnar microstructure, indicates the formation of B1-NaCl crystal.
For AZ91 base material, fatigue, and tensile crack initiation occur at surface nicks, cracks, or voids.Fatigue crack propagation is characterized by randomly oriented and line-like features combined with tear ridges and secondary cracks along the interfaces.Similarly, in the Nitride coating, the crack initiates from the surface roughness and propagates similarly to the base material.
The fatigue strength and tensile strength increased in parallel with the multilayer hard nitride coating filling the defects, notches, and voids on the surface of the base material with low surface roughness.The fatigue limit value of the AZ91 base material increased from 60.46 MPa to 68.49MPa, an increase of 13.26%.The multilayer nitride coating increased the tensile strength value of the base material from 169.98 MPa to 175.43 MPa.

Figure 3 .
Figure 3.The surface and cross-sectional SEM images for multi-layered Ti+TiN+Ti+TiVN films.