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TaN coatings deposited by DC magnetron sputtering
Surface & Coatings Technology 378 (2019) 124941
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Mechanical and tribological properties of nanostructured TiAlN/TaN coatings deposited by DC magnetron sputtering
T
Elbert Contreras Romeroa, , Abel Hurtado Macíasb, Juan Méndez Nonellb,c, Oscar Solís Cantob, Maryory Gómez Boteroa ⁎
a
Centro de Investigación, Innovación y Desarrollo de Materiales -CIDEMAT-, Universidad de Antioquia UdeA, Calle 70 N° 52-21, Medellín, Colombia Centro de Investigación en Materiales Avanzados -CIMAV-, Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31136 Chihuahua, Mexico c Cinvestav Saltillo, Av. Industria Metalúrgica 1062, Ramos Arizpe, Coahuila 25900, Mexico b
ARTICLE INFO
ABSTRACT
Keywords: Nanoscale multilayer coatings Tribological performance Mechanical performance FIB analysis
In this research, nanoscale TiAlN/TaN multilayer coatings was deposited by DC magnetron sputtering, controlling the substrate rotation speed in order to obtain different periods and to evaluate their effect on mechanical and tribological properties. FE-SEM images showed multilayer architecture of coatings with larger bilayer period, TEM analysis were necessary to observe the architecture for coatings with periods below 25 nm. Mechanical properties exhibited a parallel behavior. Hardness, Young's modulus and recovery percentage increase progressively as the bilayer period of multilayer coatings decreases. Regarding to the residual stresses, a significant reduction of residual stress was observed for multilayer coatings compare to monolayer constituent monolayers, decreasing the residual stress from −8 GPa up to −1 GPa. By FIB, it was possible to study the deformation mechanisms in nanoindentation tests; inter-columnar shear was the main mechanisms observed. The increase in critical load of coatings was one of the most relevant results obtained in the investigation, it was possible to increase two times the critical load of multilayer coatings with ʌ = 10 nm, compare to TiAlN and TaN monolayer coatings. With Regard to tribological properties, it was clearly observe the influence of multilayer architecture and the bilayer period. TiAlN/TaN coatings with ʌ = 5 mn exhibited friction coefficients lower than 0.2 and wear rates 2 time lower than the bare substrate. The optical images suggest the presence of tribooxidation reactions and with EDS it was possible to confirm the oxidation of coatings.
1. Introduction In recent decades, TiAlN coatings have become an indispensable alternative in the protection of machining tools and engineering components due to their mechanical and tribological properties, and good oxidation resistance, especially at high temperatures [1–4]. However, the main disadvantage associated with hard monolayer coatings has been their poor toughness [5,6]. This is the reason why scientific work and materials research on coatings for industrial applications has focused on the search for strategies to allow the continuous improvement of the mechanical and tribological performance of coatings. In this regard, the combination of two or more materials in multilayer architecture to develop coatings with higher performance than conventional monolayers has become one of the most highly-studied strategies in recent years. Significantly increased hardness, decreased friction coefficients and wear rates, increased adhesion (a product of the decrease in residual stresses), and resistance to high temperatures
⁎
and corrosion, have been the main advantages of coatings with multilayer architecture [7–9]. The interfaces act as barriers against the movement of dislocations and the propagation of cracks, and as thermal barriers, as well as promoting stress relaxation, these being the main reasons for the improvement in the relevant properties. Based on this, it could be assumed that the number of interfaces should be increased as much as possible. However, several studies have shown an optimal number of interfaces where there is a significant increase in the relevant properties, and this optimum is intrinsic to the materials and the deposition conditions. Therefore, research and development regarding multilayers has focused on the systematic study of new combinations of materials. Many combinations have been used to improve the performance of TiAlN coatings. TiAlN/CrN coatings have shown increased thermal stability even at 800 °C [7]. TiAlN/TiAl coatings exhibit hardness of up to 30 GPa, an increase of up to 60% compared to their constituent monolayers, as well as a decrease in friction coefficients of up to 0.3 and
Corresponding author. E-mail address: [email protected] (E. Contreras Romero).
https://doi.org/10.1016/j.surfcoat.2019.124941 Received 9 April 2019; Received in revised form 16 August 2019; Accepted 23 August 2019 Available online 31 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 378 (2019) 124941
E. Contreras Romero, et al.
an increase in adhesion of up to 50% [10]. Many other combinations have, like those previously mentioned, shown significant improvements in mechanical and tribological performance. TaN coatings, due to their multifunctional properties, are subject to increasing interest as an alternative to conventional MexN (Me = Ti, Cr, Mo, etc.) coatings in different fields of application. Ta–N is a widely used material for producing hard coatings, wear resistant layers, thin film resistors, and diffusion barriers in integrated circuits, among other applications [11]. The main reason for the improvement in the properties of multilayer coatings is the interruption of the columnar growth of the coatings, grain growth and the formation of an interface that acts as a barrier to movement in the dislocations, the propagation of cracks and the diffusion of elements along the coating. Therefore, the main investigations have focused on the study that has the number of interfaces (i.e. bilayer period) in the mechanical, tribological, thermal, corrosion properties, among others. Shi et al. report the increase in hardness and adhesion of NbN-NbB2/NbN coatings as a function of the bi-layer period [12]. Kim et al. report an increase in hardness with values higher than 45 GPa, friction coefficients lower than 0.2 and a significant decrease in the wear rate of CrN/TiAlN coatings as a function of the bilayer period [13], it is important to note that the investigations have focused on obtaining coatings at a nanoscale (below 100 nm). Given the above mentioned properties of (Ti,Al)N and TaN coatings, the combination of these materials in a TiAlN/TaN multilayer architecture could result in coatings with enhanced mechanical, tribological, and thermal properties with multifunctional applications. Furthermore, in the literature there is a lack of research of the mechanical and tribological properties of these two materials in a multilayer architecture. For these reasons, the measurements, evaluations, and discussions in this work constitute an important contribution for future research on the development of nanostructured materials of this type.
Ti-Al and Ta targets, respectively. Prior to the deposition of these monolayer coatings, a tantalum adhesion layer (~80 nm) was deposited with a power of 1450 W, an argon flow of 30 sccm and substrate rotation speed of 12 rpm, for 5 min. Once the adhesion layer had been deposited, a 7 sccm nitrogen flow was applied for 3 h for the formation of the respective nitrides. Five (Ti,Al)N/TaN multilayer coatings were deposited for 2 h, obtaining an average thickness of 1500 nm and bilayer numbers n = 23, 31, 60, 150 and 300, using substrate rotation speeds of 0.2, 0.5, 1.0, 3.0 and 4.0 rpm, respectively. A simple one-fold rotation system was used to rotate the samples, and a frequency converter was used to control the substrate speed rotation. All other process parameters were the same as those used for the deposition of the monolayer coatings. The influence of the bilayer period of the nanolayered coatings on the mechanical and tribological properties was studied. 2.2. Coatings characterization In order to observe the coating morphology, transverse images were taken using a JEOL JSM-7401F field emission scanning electron microscope (FE- SEM). Transmission electron microscopy (TEM) was also performed using JEOL JEM-2200FS equipment in order to identify the microstructure and interfaces of the coatings at a nanometric scale. The cross-sectional sample preparation was performed using JEOL JEM9320 FIB focused ion beam (FIB) equipment, both for TEM analysis and for the cross-sectional nanoindentation analysis. Mechanical properties such as hardness, Young modulus, and recovery percentages of the TiAlN/TaN coatings were measured using an Agilent G200 nanoindenter with an XP Berkovich diamond tip and a radius of curvature of 20 ± 5 nm. The nanoindenter was calibrated using a standard fused silica sample (13 GPa). The test parameters of the area function were: C0 = 24.02, C1 = −179.21, C2 = 6703.01, C3 = −25,333.40 and C5 = 18,810.3. The mechanical properties were measured using the Oliver and Pharr method [14], maximum load of 2 mN, Poisson ratio of υ = 0.22 and holding time of 2 s. In order to evaluate the deformation mechanisms present during the nanoindentation tests, it was necessary to increase the indentation load to 10 mN, in order to obtain a penetration depth close to 500 nm. The adhesion of the coating to the substrates was evaluated by progressive load scratch tests. The scratch test was measured according to ASTM C1624 [15]. Scratch tracks with a length of 10 mm were generated with a scratch tester (Revetest CSM, Switzerland) utilizing a Rockwell C diamond indenter with 200 μm radius. The normal force was linearly increased from 0 to 100 N, the load rate was 1 N/mm and the scratch time was 60 s. The tribological properties of the films were evaluated by a CSM Instruments tribometer with a rotating ball-on-disc setup. A WC-6wt %Co bearing was used as a ball counterpart with a diameter of 6 mm. Relative velocity of 0.04 m/s, normal load of 1 N, wear track radius of 0.2 cm and 1200 cycles were set as testing conditions. The tests were carried out at 298 ± 2 K, in ambient air, at a relative humidity ranging from 40 to 50%. In order to calculate the average friction coefficient, the first step was to determine the limit between the initial regime and the steady regime, selecting the 400 cycles as the inflection point between both regimes. Then, the friction coefficient data was selected only in steady regime, i.e. from 400 to 1200 cycles. The wear rate was calculated by volume loss (eq. 1), using a Bruker DektakXT profilometer to calculate the area worn in the wear tracks. The worn area was always measured at the end of each test, i.e. after 1200 cycles, which in meters is equivalent to 15 m. In addition, the profilometer was used to measure the radius of curvature of silicon substrates with dimensions of 3 cm × 1 cm, before and after the coating process, in order to calculate the residual stresses of the coatings using Stoney's equation.
2. Experimental procedure 2.1. Coatings deposition Five nanolayered TiAlN/TaN coatings were deposited onto AISI M2 substrates with dimensions of 1.6 cm diameter and 0.3 cm thickness, using a direct current magnetron sputtering system. Additionally, TiAlN and TaN monolayer coatings were deposited for comparison purposes. Heat treatment was carried out in order to increase the hardness of the substrates. This consisted of preheating of the substrates at 1100 K, followed by an austenization at 1470 K and then oil quenching at 823 K. A tempering process with slow heating to 823 K was carried out immediately after quenching. The final hardness of the AISI M2 substrates was 64 ± 2 HRC (~7 GPa). The substrate was polished using SiC emery paper with different grit sizes, and then polished to a mirror finish with 1 μm diamond paste. Finally, an ultrasonic cleaning process was carried out on the substrate using a 3:4 solution of ethanol:acetone for 0.5 h, in order to remove any contamination from the polish. A magnetron sputtering system with dimensions of 550 × 600 × 800 mm3, designed and built by the research team, was used for the coating deposition process. Planar targets of Ta (99.9 wt%) and Ti-Al (50–50 wt%) with dimensions of 500 × 100 × 6 mm3 were used, and were placed facing one another. The chamber was evacuated to a background pressure below 10−3 Pa. Once the background pressure was reached, the targets were subjected to a plasma cleaning process using an argon flow of 40 sccm and chamber pressure of 3 Pa. The cleaning power was 500 W for both the Ti-Al and Ta targets, and the temperature was 523 K. Finally, the same plasma cleaning process was carried out on the substrates, using a bias voltage of −700 V and a duty cycle of 80 μs/10 μs. For the deposition of the TaN and TiAlN monolayer coatings, a mixture of gases (Ar/N2) with 19% nitrogen was used. Deposition conditions were as follows: temperature of 523 K, working pressure of 0.45 Pa, bias voltage of −70 V, and power of 1700 W and 1450 W for 2
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3. Results
Table 1 Setup parameters of TiAlN, TaN and TiAlN/TaN coatings: substrate speed rotation, bilayer number and bilayer period. Deposited coatings TiAlN TaN TiAlN/TaN TiAlN/TaN TiAlN/TaN TiAlN/TaN TiAlN/TaN
#1 #2 #3 #4 #5
Substrate speed rotation (rpm) 12 12 0.2 0.5 1.0 3.0 4.0
Bilayer number 1 1 23 31 60 150 300
Table 1 presents information on the TiAlN and TaN monolayer coatings and the five TiAlN/TaN multilayer coatings, together with the substrate speed rotation used to deposit the coatings, the bilayer number, and the experimental bilayer period. It should be noted that, as expected, as the substrate rotation speed increases, the number of times that the substrates passes in front of each of the targets increases (in less time), and therefore the bilayer number increases and the thickness of the bilayer period decreases. Cross-sectional FIB cuts of TiAlN, TaN monolayer coatings and representative multilayer TiAlN/TaN coatings with bilayer period ʌ = 65 nm (0.2 rpm) and ʌ = 48 nm (0.5 rpm) are shown in Fig. 1. It is possible to observe homogenous and compact coatings without delamination between the coating and substrate. Fig. 1 shows each of the constituent layers of the coatings studied (adhesion layer, coating, gold and carbon). The AISI M2 steel substrate is observed at the base, followed in turn by a pure tantalum adhesion layer (~100 nm). Next, the respective coating (TaN, TiAlN, TiAlN/TaN) and a gold layer in order to improve the conductivity of the samples. Finally, a carbon layer acting
Bilayer period (nm) 800⁎ 1500⁎ 65 48 25 10 5
* Thickness of the monolayer coatings.
k=
A. 2. . r l. d
(1)
where A is the calculated worn area in the wear track by profilometry, r is the radius of the wear track, l is the load applied in the tribological test and d is the total distance in the tribological test.
Fig. 1. Cross sectional FE-SEM image of monolayer a) TaN, b) TiAlN and TiAlN/TaN multilayer coatings with c) 0.2 rpm and d) 0.5 rpm.
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Fig. 2. TEM Analysis of representative TiAlN/TaN multilayer coating deposited with 1.0 rpm, a) Low magnified image STEM mode with HAADF coatings, b) and c) EDS line scan in TiAlN/TaN in STEM-HAADF mode, d) calculated bilayer period of TiAlN/TaN, e) HRTEM image in TiAlN/TaN coatings, f) and g) Fast Fourier Transform analysis in TaN and TiAlN coatings respectively.
as a protective layer that prevents unwanted erosion in FIB cutting. The cross-sectional images show TaN coatings (Fig. 1a) with thickness around 1500 nm, while the TiAlN coatings (Fig. 1b) exhibit thickness around 800 nm. It is important to remember that both monolayer coatings was deposited during 3 h. The difference in the thickness of the TaN and TiAlN coatings has been attributed in previous works to the low sputtering rate of the Ti-Al target resulting from the formation of intermetallic TiAl3 and to the difference in formation energies between TiAlN and TaN [16]. In Fig. 1c, TiAlN/TaN multilayer architecture of the coating deposited at 0.2 rpm is observed, with thickness ~1500 nm, number of bilayers n = 23 and ʌ = 65 nm. Meanwhile, Fig. 1d shows the coating deposited at 0.5 rpm where, similarly to the coatings deposited at 0.2 rpm, TiAlN/TaN multilayer architecture was identified, thickness of ~1500 nm, bilayer number n = 31 and ʌ = 48 nm. In these coatings, it was not possible to observe the multilayer architecture, reason why TEM analysis was used to calculate the period of these coatings. The representative study of the TiAlN/TaN deposited at 1.0 rpm is presented in Fig. 2. Fig. 2 shows the structure, morphology and multilayer architecture of TiAlN/TaN coatings deposited with 1.0 rpm. Fig. 2a shows the cross
section and at low-magnified image which was prepared by FIB. The calculated bilayer period of TiAlN/TaN coatings was approximately ʌ = 25 nm as shown in Fig. 2d. In order to corroborate the chemical composition and distinguish if there is an interphase between TiAlN and TaN coatings, it was analyzed in a region with more magnification in mode STEM High-Angle Annular Dark Field (HAADF) as shown in the Fig. 2b. In this figure it can be clearly observed how the TaN layer is the brightest due to in STEM mode the heaviest elements such as Ta are highlighted. Additionally, a linear EDS scan analysis was performed in order to see the modulation of the chemical composition of each coating as shown in Fig. 2c. According to the EDS analysis, it is clearly observed the modulation of the Ta (blue spectrum line), Ti (green spectrum line) and Al (yellow spectrum line) elements. Whereas the Ta increase the Al and Ti decrease. It validate the chemical composition and the formation of a multilayer architecture. Moreover, in the Fig. 2e it is observed the identification of the TiAlN and TaN layer, a small inter-diffusion zone can be clearly observed, suggesting a non-zero interface between TaN and TiAlN constituent coatings in HRTEM mode. Fig. 4f and g show the analysis of the Fast Furrier Transform (FFT) in a region marked with a square on each coating in HRTEM mode. 4
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Fig. 3. Nanomechanical properties of nanolayered TiAlN/TaN coatings measured by nanoindentation: a) load-unload curves, b) recovery percentage of coatings after nanoindentation tests, c) hardness and young modulus, d) H/⁎E and H3/*E2 ratios.
Fig. 4. Cross-sectional FE-SEM image of nanoindentation footprints of TiAlN and TaN monolayer coatings.
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Fig. 5. Representative cross-sectional FE-SEM image of nanoindentation footprints of TiAlN/TaN nanolayered coatings with ʌ = 65 nm and ʌ = 48 nm.
Performing an analysis of the interatomic distances of the FFT, the planes corresponding to each of the phases were determined. These figures clearly show how the crystals are oriented. TaN layer has face centered cubic (fcc) with (2,2,0) and (2,-2,0) planes with the interatomic distances of 1.5312 Å agree PDF reference code 01-089-5196. While the TiAlN layer has face centered cubic (fcc) too with (1,1,1) and (1,-1,1) planes with the interatomic distances of 2.450 Å agree PDF reference code 00-031-1403. This interatomic distance is slightly greater than TiN, due to the interstitial aluminum incorporation within the TiN lattice, forming a (Ti,Al)N solid solution. Nanomechanical properties of the TiAlN/TaN multilayer coatings were studied by nanoindentation, obtained results are presented in Fig. 3. Fig. 3a shows the loading-unloading curves of multilayer coatings deposited with different bilayer periods. The curves obtained are characteristic of elastoplastic behavior, with a progressive increase in the load to 2 mN and a maximum depth of 73 nm for the coatings with ʌ = 65 nm. It is possible to observe a progressive reduction in penetration depth to h = 59 nm as the bilayer period of the coatings reduces to 10 nm. Finally, a slight increase in penetration depth is observed for the coatings deposited with ʌ = 5 nm. As previously mentioned the curves indicate that the coatings were subjected to an elastoplastic deformation. This means that part of the total energy involved in the nanoindentation test is recovered as elastic energy, and another part is dissipated as plastic energy (associated with the plastic deformation of the material during the indentation). Calculating the area below each of the loading curves (total energy applied) and unloading curves (elastic energy) it is possible to determine the percentage of plastic energy dissipated during the test for each of the coatings [17,18]. Recovery percentages of TiAlN/TaN are shown in Fig. 3b, it is possible to observe a progressive increase in the recovery percentages of the TiAlN/TaN multilayer coatings as the coating period decreases. TiAlN/TaN coatings with ʌ = 65 nm showed a recovery percentage of 42%, while the recovery percentage of coatings with ʌ = 10 nm and ʌ = 25 nm it is around 56%. In addition, it is important to note that coatings deposited with 10, 25 and 48 nm exhibited higher recovery percentages than the TiAlN and TaN monolayer coatings. Fig. 3c shows the hardness and Young's modulus values calculated from the loading-unloading curves using the Oliver and Pharr
method. A progressive increase in hardness and Young's modulus of the TiAlN/TaN multilayer coatings is observed as the coating period decreases, reaching values of H = 38 GPa and E = 472 GPa for TiAlN/ TAN coatings deposited with ʌ = 10 nm. However, multilayer coatings with ʌ = 5 nm, a sudden fall in both properties up to 35% is observed. The increase in hardness and Young's modulus of multilayer coatings can be attributed to the increase in the number of interfaces, which act as barriers for the propagation of dislocations. However, as the bilayer period continued to decrease, a sudden decrease in hardness and young modulus was observed. According to Chu and Barnett, the main explanation for this phenomenon is that the decrease of the bilayer period below 5 nm leads an interface mixture which leads to lower modulation amplitude [19]. Considering the solubility of tantalum within the TiAlN matrix, it is possible to affirm that this phenomenon may be the main reason for the decrease in the hardness and Young's modulus of coatings with a bilayer period below 5 nm. While mechanical and tribological performance improves as the
Fig. 6. Residual stresses of TiAlN/TaN multilayer coatings, deposited with different bilayer period, measured by the curvature method and Stoney equation. 6
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period is reduced (increasing the number of interfaces), Holleck reported that the number of interfaces (known as interface volume) plays a fundamental role in the properties of multilayer coatings. Moreover, Holleck reported that all materials have an optimum number of interfaces where their performance is improved significantly, this optimum in ceramic multilayer coatings is generally between 100 and 500 [20]. TiBCN/CNx [8], TiN/VN [21], TiAlN/CrAlN [22] and Ti0.33Al0.67N/CrN [23] are some examples that have been studied the influence of bilayer period on mechanical and tribological performance. Additionally, optimum number of layers for mechanical performance of the coatings was identified. In the present work, optimal hardness and Young's modulus are observed for the coatings between 10 and 20 nm, considering the coatings thickness of 1500 nm, the optimal interface volume is between 75 and 150. The results of the H/⁎E and H3/⁎E2 ratios, associated with elastic strain to failure and resistance to plastic deformation respectively, are presented in Fig. 3d. Different investigations have identified a limit of H/⁎E = 0.1 for coatings with high resistance to plastic deformation and low elastic strain to failure [18,24–26]. In Fig. 3d, it can be observed that the H/⁎E ratios of the TiAlN/TaN multilayer coatings progressively increase as the period reduces. The coatings with ʌ = 25 nm and ʌ = 10 nm exhibited values above 0.1, suggesting high elastic strain to failure. Parallel behavior presents the relationship H3/⁎E2, authors report that this relationship can be a parameter to predict the tribological behavior of coatings. In the tribology results obtained (Fig. 8), the
correlation between both properties will be deepened. Fig. 4a shows the lamella of the monolayer TiAlN and TaN coatings, extracted and milled by FIB, observed transversally in FE-SEM. The cross section of two indentations can clearly be observed at 5000×, with a separation of 15 μm to avoid influence between the indentations. In Fig. 4b (20,000×) the cross section of one indentation is shown. It is possible to identify the adhesion layer, the TiAlN coating, gold layer and protective carbon layer. The elastoplastic deformation induced in both the TiAlN and M2 substrate can be observed, with evidence of inter-columnar shearing across both the coating and the adhesion layer, dissipating once the ductile substrate is reached. In Fig. 4c at (50,000×) the inter-columnar shearing can be observed more clearly, with no evidence of lateral, inclined or curved cracks. With regard to the indentations made to the TaN coatings, in Fig. 4d (5000×) two indentations with separation of 15 μm is observed once again. In Fig. 4e (20,000×) one of the indentations is observed, with characteristic pyramid shape in the top of the coating. Inter-columnar shearing is observed, with greater intensity compared to the TiAlN coatings. Finally, in Fig. 4f (50,000×) it can be observed in greater detail that the only deformation mechanism present is inter-columnar shearing, with evidence of characteristic slip in the coating-substrate interface, which is progressive until reaching the center of the indentation. Representative images of the indentations made to the multilayer coatings are presented in Fig. 5. In Fig. 5a (5000×) two indentations made to the coatings deposited with period ʌ = 65 nm are observed.
Fig. 7. Optical and FE-SEM image of the scratch up to Lc3 of nanolayered TiAlN/TaN coatings. 7
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Fig. 8. a) Lc3, H/⁎E and H3/⁎E2 in function of the bilayer period of multilayer coatings and b) Lc3 vs H/⁎E and H3/⁎E2 and its respective cubic fitting.
Each of the constituent layers and the pyramid-shaped indentations are clearly observed in the surface of the coatings. In Fig. 5b (20,000×), only inter-columnar dislocations can be observed as a failure mechanism, as in the case of the TaN coatings. However, a reduction in the number of dislocations present is observed. In Fig. 5c (50,000×), it can be observed that the inter-columnar dislocation leads to transverse shearing of the TiAlN/TaN multilayer, increasing the number of dislocated layers as this reaches the center of the indentation, which is consistent with the force concentration during the nanoindentation test. Fig. 5d, e and f present the indentations of the coatings with ʌ = 48 nm. Similarly to the TiAlN/TaN coatings with ʌ = 65 nm, the multilayer structure is observed, although there are significant differences in the deformation mechanism. In Fig. 5e (20,000×) no evidence of any type of inter-columnar shearing or lateral, inclined or curved cracks is observed. In the top of the surface in Fig. 5f the pyramid shape of the Berkovich indentation is clearly observed, while in the coatingsubstrate interface the indentation footprint shape changes from pyramid to semi-circular. Based on what is observed, it can be suggested that the energy associated to the nanoindentation test is dissipated progressively between interfaces as it advances across the coating. The curvature method together with the Stoney's equation has been widely used to measure the residual stresses of coatings deposited by sputtering; Fig. 6 shows the residual stresses of TiAlN, TaN monolayer coatings and TiAlN/TaN multilayer coatings. Both monolayer coatings exhibited high residual stress, −8,69 GPa for TaN coatings and −7,01 GPa for TiAlN. The negative sign by convention has been adapted to indicate that the stresses are compressive. Regarding to TiAlN/TaN multilayer coating it is possible to observe a significant reduction in residual stress, the reduction is close to 50% for TiAlN/TaN deposited with ʌ = 65 nm (−3.98 GPa). Additionally, there is a clearly trend towards a reduction in residual stress as the bilayer period decreases. Each of the interfaces acts as a stress dissipater, relaxing the structure and decreasing the residual stresses, therefore, as the number of interfaces increases (decrease in the bilayer period) the decrease in residual stresses becomes evident [27–29]. Fig. 7 shows optical and scanning electron microscopy of the tracks obtained in the scratch tests of the monolayer TiAlN and TaN coatings and multilayer TiAlN/TaN coatings. It can be observed that all the multilayer coatings exhibit higher critical loads compare to the monolayer coatings. Additionally, it can be seen that TaN coatings showed a critical load twice as high as the TiAlN monolayer, probably due to this coating having greater chemical affinity with the Ta adhesion film than TiAlN. The TiAlN and TaN monolayer coatings exhibited Lc3 critical loads of 8.9 N and 18.1 N, respectively. With regard to the multilayer TiAlN/ TaN coatings, a progressive increase in the adherence of the coatings as the period decreases can be observed, with Lc3 values of 37.1 N for the coatings deposited with ʌ = 10 nm, falling to 31 N when the period is
reduced to ʌ = 5 nm. At low loads, plastic deformation of the substratecoating system occurs. The most important aspects to highlight are as follows: i) the significant increase in adherence of the multilayer coatings by up to 90% in comparison with the TaN monolayer and, ii) the clearly correlation between the adherence of the TiAlN/TaN coatings and their mechanical properties (recovery percentage, H/E ratio and H3/⁎E2). In all scratches, sudden failure of the coatings is observed as the Lc3 critical load is reached. Propagation of compressive, semicircular spallation is observed from the center of the scratch line to the outside, leaving the substrate completely bare even in areas outside the edges of the scratch. Based on the optical evidence and on the ASTM C 1424-05 standard, the main failure mechanism in the coatings appears to be compressive wedging-spallation. This is typical of the brittle fracture generally present in hard coatings deposited on hard substrates such as HSS, as a result of the compressive force generated ahead of the indenter as it advances [30,31]. The Lc3 values has been plotted together with H/⁎E and H3/⁎E2 (Fig. 8a), in addition has been plotted the Lc3 vs H/⁎E and H3/⁎E2, respectively (Fig. 8b), in order to find a mathematical correlation between the properties. As expected, the Lc3 values showed the same trend obtained for H/⁎E and H3/⁎E2 (Fig. 8a). This correlation is congruent if it is considered that, during the scratch test, the coatings are subjected to a shear stresses, these stresses induce a plastic deformation in the material, therefore, as the ratio of the values H/⁎E and H3/⁎E2 increases, coatings with greater resistance to shear stresses are expected and consequently an increase in adhesion is observed. Additionally, the critical load Lc3 vs H/⁎E and H3/⁎E2 was plotted in order to find a
Fig. 9. Friction records of monolayer TiAlN, TaN and TiAlN/TaN multilayer coatings deposited with different bilayer period. 8
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steady regime around 300 cycles is reached. Other reports also show friction coefficients between 0.6 and 0.8 for TiAlN, depending on the counterbody used and the speed at which the test is performed [32,33]. The exception is the coating with ʌ = 5 nm, which retains a low friction coefficient throughout the test. The monolayer coatings show greater fluctuation in friction values than the multilayer coatings, suggesting that during these tests, contact between the tribological surfaces was more chemically and mechanically stable. The average value of the friction coefficients and wear rates in steady state are presented in Fig. 10. All the multilayer coatings showed friction coefficients lower than the monolayer coatings, showing a reduction in friction coefficient as the period of the multilayers decreased, with the lowest friction coefficient of 0.22 obtained in the multilayer with ʌ = 5 nm. Considering that tribological tests consist mainly in the measurement of the tangent force, which, when divided the normal load, is obtained the friction coefficient. The decrease in friction coefficients is mainly associated with the increase in the number of interfaces, which help dissipate the energy associated with the tribological test [34,35]. The wear rates exhibit parallel behavior to the friction coefficients, exhibiting lower values as the multilayer period decreases, with the lowest value of 1.8 × 10−4 mm3/Nm obtained in the coating with ʌ = 5 nm, which corresponds to a value 10 times lower compared to the monolayer coatings. All the multilayer coatings presented wear rates one order of magnitude lower than those of the monolayer coatings do. Although the best mechanical properties were presented by the coating with a ʌ = 10 nm, the coating showed the best tribological properties with ʌ = 5 nm. In the 80's Tsui et al. introduce the Jhonson's equation suggesting that the H3/⁎E2 ratio can be a useful parameter for predicting the plastic deformation resistance of coatings [36]. However, in recent years multiple investigations have been reported that differ from the one proposed by Tsui, arguing that the decrease of Young's modulus goes against the search of the increase in toughness. Additionally, Young's modulus and hardness are directly correlated properties and it is not trivial to modify one without affecting the other. In the TiAlN/ TaN multilayer coatings studied in this research, there is no direct or
Fig. 10. Friction coefficients in steady regime and wear rates of TiAlN/TaN multilayer coatings deposited with different bilayer periods.
mathematical expression that adjusts the correlation between the respective properties. A cubic correlation was found between each of the properties, the correlation between the critical load Lc3 and the H/⁎E ratio can be expressed using the cubic function y = − 3.7105 + 0.4611x − 0.0179x2 + 2.2933x10−4x3, with an R2 adjustment of 0.9730, suggesting a high fitting between experimental and theoretical values. In the other hand, the correlation between Lc3 and H3/⁎E2 ratio can be expressed using the cubic function y = − 0.2654 + 0.0408x − 0.0016x2 + 1.9766x10−5x3, with an R2 adjustment of 0.9851. Fig. 9 shows the friction records for the multilayer and monolayer systems. Excluding TiAlN coatings, initially, in the first 130 cycles, all coatings show low friction coefficients (0.2) similar to TaN coatings, undoubtedly because the last layer of the multilayer systems is TaN, while the TiAlN monolayer starts with a higher value (0.4). These values increase suddenly and quickly (after 50 cycles) up to 0.8 once the
Fig. 11. Optical images of wear tracks of TiAlN/TaN multilayer coatings deposited with a) ʌ = 65 nm, b) ʌ = 25 nm, c) ʌ = 10 nm y d) ʌ = 5 nm. 9
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indirect correlation between the H3/⁎E2 ratio and the tribological behavior of the coatings. Authors such as Abad et al. have reported the same phenomenon in TiBC coatings [25]. Therefore, it is important to note that the concept associated with H3/*E2 ratio is not universal, should be used with caution and should be studied further. The optical microscopy images of the multilayer coatings reveal the presence of remaining products adhering on the wear tracks of the coatings (Fig. 11). Fig. 11a shows the wear track of TiAlN/TaN coatings with ʌ = 65 nm. It is possible to observe the presence of debris on each external side of the wear track, additionally, the adhesion of debris formed within the wear track was observed. The presence of litmus colors suggests tribo-oxidative reactions between TiAlN/TaN coatings and WC-Co counterbody. Similar behavior can be seen in Fig. 11b (ʌ = 48 nm), however, there is a decrease in debris adhesion at the external wear track, while the size and number of debris adhered within the wear track increases. In Fig. 11c, it is possible to identify the formation of grooves in the TiAlN/TaN coatings with ʌ = 10 nm, suggesting abrasive wear as the main wear mechanism. In the SEM images of the wear tracks shown in Fig. 12, wear debris adhering to the tribological wear track is observed. In Fig. 12a and b, it is possible to identify flake-like debris, this kind of debris are generated during the delamination process due to plastic shear strain accumulation at the wearing surfaces. Two selected EDS in different zones and a mapping was carried out to study the debris adhered on the tribological wear tracks. Fig. 12d shown the selected areas analyzed by EDS, area 1
onto the debris and area 2 outside them. In Fig. 12e, the oxygen distribution obtained in the EDX mapping is shown. Two zones are clearly observed, one with high oxygen concentration corresponding to area 1 associated with the adhered debris and second one with a low oxygen concentration corresponding to area 2, free of debris. Fig. 12f shows the EDS spectra corresponding to each of the studied areas. Additionally, the element atomic percentages is shown. Congruent with the mapping, area 1 shows high oxygen content, with percentages higher than 50 at.%, with which it is possible to affirm that the debris formed during the tribological test correspond to oxides. On the other hand, in area 2, it can be seen that the oxygen concentration is very low (5%) so it is possible to affirm that this zone corresponds to the coating and that the tribological test has not yet exposed the substrate. Based on the SEM and optical microscopy images, it can be said that the increase in the period caused a parallel increase in the formation of tribofilms on the tribological wear track, resulting in higher friction coefficients as the contact with the tribological surface began to be governed by oxides formed with oxygen and humidity in the atmosphere. Other authors have found similar behavior in monolayer and multilayer AlCrN, TiAlN, TiN, TiAlVN, TiAlTaN and AlN system [37]. The tribological behavior of these nitride coatings could be strongly influenced by the presence of humidity at room temperature, as the continuous formation of oxides in sliding contact can result the removal of material by abrasive mechanisms, as occurred in the multilayer coating with ʌ = 10 nm, showed in Fig. 11c [32–38].
Fig. 12. SEM images of wear tracks of TiAlN/TaN multilayer coatings deposited with a) ʌ = 65 nm, b) ʌ = 10 nm, c) ʌ = 5 nm, d) zoom of selected area in the wear track of coatings with ʌ = 10 nm, e) EDS mapping of oxygen in the selected zone and f) the atomic composition of selected zone. 10
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4. Conclusions
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Stueber, H. Holleck, H. Leiste, K. Seemann, S. Ulrich, C. Ziebert, Concepts for the design of advanced nanoscale PVD multilayer protective thin films, J. Alloys Compd. 483 (2009) 321–333, https://doi.org/10.1016/j.jallcom.2008.08.133. [22] H.C. Barshilia, B. Deepthi, K.S. Rajam, K.P. Bhatti, S. Chaudhary, Growth and characterization of TiAlN∕CrAlN superlattices prepared by reactive direct current magnetron sputtering, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 27 (2009) 29, https://doi.org/10.1116/1.3013858. [23] S.-S. Ahn, K.-S. Oh, T.-J. Chung, J.-K. Park, Hardness and oxidation resistance of Ti0.33Al0.67N/CrN nano-multilayered superlattice coatings, J. Korean Ceram. Soc. 56 (2019) 49–55, https://doi.org/10.4191/kcers.2019.56.1.08. [24] J. Musil, Hard nanocomposite coatings: thermal stability, oxidation resistance and toughness, Surf. Coat. Technol. 207 (2012) 50–65, https://doi.org/10.1016/j. surfcoat.2012.05.073. [25] M.D. Abad, J.C. Sánchez-López, M. Brizuela, A. 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It was possible to modify the deposition of the TiAlN/TaN coatings using only the rotation speed of the substrates, by progressively decreasing the bilayer period as the rotation speed of the substrates increased. As expected, in coatings with larger bilayer periods it was possible to identify the multilayer architecture using FIB for transversal cuts and FE-SEM for the cross-section. For coatings below ʌ = 25 nm, TEM analysis was necessary to calculate the period of the coatings. All multilayer coatings exhibited a defined structure between layers, however, through TEM it was possible to observe that the coatings presented a non-zero interface. This means that there was an interdiffusion zone between each of the layers. With regard to the mechanical properties of the coatings, the recovery percentages, hardness, and Young's modulus exhibit parallel behavior. All properties increase progressively as the period of the coatings decreases. However, there is an optimum point between 10 and 25 nm after which the properties decrease suddenly. Similar behavior was shown for residual stresses, where a significant decrease, close to 50%, was observed compared to the TiAlN and TaN monolayers. The increase in the number of interfaces facilitates the relaxation of the structure for both intrinsic and extrinsic stresses. Through FIB, it was possible to study the cross-sections of the nanoindentation footprints and identify the main deformation mechanisms present in the coatings studied. For monolayer coatings, inter-columnar shear was observed as the main mechanism. Additionally, the shear in the TaN coatings progressively evolved until the substrate was reached, evidenced in the formation of steps in the coating-substrate interface. On the other hand, in multilayer coatings a significant decrease in inter-columnar shear was observed as the period of the coatings decreased. The adhesion of the coatings was one of the most significant results obtained in the present work. It was possible to observe increase two times the critical loads of TiAlN/TaN multilayer coatings compared to individual TiAlN and TaN coatings (individual critical loads of 9 N and 18 N), reaching critical loads of 37 N for coatings with a period of 10 nm. It is important to emphasize that all multilayer coatings exhibited superior adhesions to the constituent monolayers. Finally, it was possible to clearly observe the influence of multilayer architecture and the bi-layer period on mechanical and tribological properties. Values for both friction coefficients and wear rates showed a significant decrease. Acknowledgments The authors are grateful to the Comité para el Desarrollo de la Investigación (CODI) for financing this project (no. PRG-2014-934), to the University of Antioquia and Colciencias for the Ph.D. sponsorship agreement (647). Additionally, the authors want to thank in a very special way the Centro para la Investigación en Materiales Avanzados (CIMAV) for all the support and disposition of the equipment used for the coatings characterization. Finally, the authors want to thank Roberto Talamantes for his invaluable technical work and assistance in nanoindentation test and Wilber Antunez for his invaluable technical work and assistance in FE-SEM analyses. References [1] S. PalDey, S.C. Deevi, Properties of single layer and gradient (Ti,Al)N coatings, Mater. Sci. Eng. A 361 (2003) 1–8, https://doi.org/10.1016/S0921-5093(03) 00473-8. [2] S. Veprek, R.F. Zhang, M.G.J. Veprek-Heijman, S.H. Sheng, A.S. Argon, Superhard nanocomposites: origin of hardness enhancement, properties and applications, Surf. Coatings Technol. 204 (2010) 1898–1906, https://doi.org/10.1016/j.surfcoat. 2009.09.033. [3] R. Rodríguez-Baracaldo, J.A. Benito, E.S. Puchi-Cabrera, M.H. Staia, High temperature wear resistance of (TiAl)N PVD coating on untreated and gas nitrided AISI H13 steel with different heat treatments, Wear. 262 (2007) 380–389, https://doi. org/10.1016/j.wear.2006.06.010.
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hardness and toughness, J. Alloys Compd. 725 (2017) 1188–1198, https://doi.org/ 10.1016/j.jallcom.2017.07.010. S.J. Bull, Failure mode maps in the thin film scratch adhesion test, Tribol. Int. 30 (1997) 491–498, https://doi.org/10.1016/S0301-679X(97)00012-1. L. hui Zhu, C. Song, W. yang N, Y. xiong Liu, Effect of 10% Si addition on cathodic arc evaporated TiAlSiN coatings, Trans. Nonferrous Met. Soc. China (English Ed. 26 (2016) 1638–1646. doi:https://doi.org/10.1016/S1003-6326(16)64273-5. W.Y.H. Liew, J.L.L. Jie, L.Y. Yan, J. Dayou, C.S. Sipaut, M.F. Bin Madlan, Frictional and wear behaviour of AlCrN, TiN, TiAlN single-layer coatings, and TiAlN/AlCrN, AlN/TiN nano-multilayer coatings in dry sliding, Procedia Eng. 68 (2013) 512–517, https://doi.org/10.1016/j.proeng.2013.12.214. A.J. Gant, M.G. Gee, L.P. Orkney, The wear and friction behaviour of engineering coatings in ambient air and dry nitrogen, Wear. 271 (2011) 2164–2175, https://doi. org/10.1016/j.wear.2011.02.032. P. Wiecinski, J. Smolik, H. Garbacz, J. Bonarski, A. Mazurkiewicz, K.J. Kurzyd, Microstructure and properties of metal/ceramic and ceramic/ceramic multilayer coatings on titanium alloy Ti6Al4V, Surf. Eng. (2016), https://doi.org/10.1016/j.
surfcoat.2016.11.003. [35] M.Z. Huq, J.P. Celis, Expressing wear rate in sliding contacts based on dissipated energy, Wear. 252 (2002) 375–383, https://doi.org/10.1016/S0043-1648(01) 00867-5. [36] T.Y. Tsui, G.M. Pharr, W.C. Oliver, C.S. Bhatia, R.L. White, S. Anders, A. Anders, I.G. Brown, Nanoindentation and nanoscratching of hard carbon coatings for magnetic disks, MRS Proc. 383 (1995) 447, https://doi.org/10.1557/PROC-383447. [37] M. Pfeiler, K. Kutschej, M. Penoy, C. Michotte, C. Mitterer, M. Kathrein, The influence of bias voltage on structure and mechanical/tribological properties of arc evaporated Ti-Al-V-N coatings, Surf. Coatings Technol. 202 (2007) 1050–1054, https://doi.org/10.1016/j.surfcoat.2007.07.045. [38] M. Pfeiler, G.A. Fontalvo, J. Wagner, K. Kutschej, M. Penoy, C. Michotte, C. Mitterer, M. Kathrein, Arc evaporation of Ti-Al-Ta-N coatings: the effect of bias voltage and ta on high-temperature tribological properties, Tribol. Lett. 30 (2008) 91–97, https://doi.org/10.1007/s11249-008-9313-6.
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Mechanical and tribological properties of nanostructured TiAlN/TaN coatings deposited by DC magnetron sputtering
T
Elbert Contreras Romeroa, , Abel Hurtado Macíasb, Juan Méndez Nonellb,c, Oscar Solís Cantob, Maryory Gómez Boteroa ⁎
a
Centro de Investigación, Innovación y Desarrollo de Materiales -CIDEMAT-, Universidad de Antioquia UdeA, Calle 70 N° 52-21, Medellín, Colombia Centro de Investigación en Materiales Avanzados -CIMAV-, Miguel de Cervantes 120, Complejo Industrial Chihuahua, C.P. 31136 Chihuahua, Mexico c Cinvestav Saltillo, Av. Industria Metalúrgica 1062, Ramos Arizpe, Coahuila 25900, Mexico b
ARTICLE INFO
ABSTRACT
Keywords: Nanoscale multilayer coatings Tribological performance Mechanical performance FIB analysis
In this research, nanoscale TiAlN/TaN multilayer coatings was deposited by DC magnetron sputtering, controlling the substrate rotation speed in order to obtain different periods and to evaluate their effect on mechanical and tribological properties. FE-SEM images showed multilayer architecture of coatings with larger bilayer period, TEM analysis were necessary to observe the architecture for coatings with periods below 25 nm. Mechanical properties exhibited a parallel behavior. Hardness, Young's modulus and recovery percentage increase progressively as the bilayer period of multilayer coatings decreases. Regarding to the residual stresses, a significant reduction of residual stress was observed for multilayer coatings compare to monolayer constituent monolayers, decreasing the residual stress from −8 GPa up to −1 GPa. By FIB, it was possible to study the deformation mechanisms in nanoindentation tests; inter-columnar shear was the main mechanisms observed. The increase in critical load of coatings was one of the most relevant results obtained in the investigation, it was possible to increase two times the critical load of multilayer coatings with ʌ = 10 nm, compare to TiAlN and TaN monolayer coatings. With Regard to tribological properties, it was clearly observe the influence of multilayer architecture and the bilayer period. TiAlN/TaN coatings with ʌ = 5 mn exhibited friction coefficients lower than 0.2 and wear rates 2 time lower than the bare substrate. The optical images suggest the presence of tribooxidation reactions and with EDS it was possible to confirm the oxidation of coatings.
1. Introduction In recent decades, TiAlN coatings have become an indispensable alternative in the protection of machining tools and engineering components due to their mechanical and tribological properties, and good oxidation resistance, especially at high temperatures [1–4]. However, the main disadvantage associated with hard monolayer coatings has been their poor toughness [5,6]. This is the reason why scientific work and materials research on coatings for industrial applications has focused on the search for strategies to allow the continuous improvement of the mechanical and tribological performance of coatings. In this regard, the combination of two or more materials in multilayer architecture to develop coatings with higher performance than conventional monolayers has become one of the most highly-studied strategies in recent years. Significantly increased hardness, decreased friction coefficients and wear rates, increased adhesion (a product of the decrease in residual stresses), and resistance to high temperatures
⁎
and corrosion, have been the main advantages of coatings with multilayer architecture [7–9]. The interfaces act as barriers against the movement of dislocations and the propagation of cracks, and as thermal barriers, as well as promoting stress relaxation, these being the main reasons for the improvement in the relevant properties. Based on this, it could be assumed that the number of interfaces should be increased as much as possible. However, several studies have shown an optimal number of interfaces where there is a significant increase in the relevant properties, and this optimum is intrinsic to the materials and the deposition conditions. Therefore, research and development regarding multilayers has focused on the systematic study of new combinations of materials. Many combinations have been used to improve the performance of TiAlN coatings. TiAlN/CrN coatings have shown increased thermal stability even at 800 °C [7]. TiAlN/TiAl coatings exhibit hardness of up to 30 GPa, an increase of up to 60% compared to their constituent monolayers, as well as a decrease in friction coefficients of up to 0.3 and
Corresponding author. E-mail address: [email protected] (E. Contreras Romero).
https://doi.org/10.1016/j.surfcoat.2019.124941 Received 9 April 2019; Received in revised form 16 August 2019; Accepted 23 August 2019 Available online 31 August 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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an increase in adhesion of up to 50% [10]. Many other combinations have, like those previously mentioned, shown significant improvements in mechanical and tribological performance. TaN coatings, due to their multifunctional properties, are subject to increasing interest as an alternative to conventional MexN (Me = Ti, Cr, Mo, etc.) coatings in different fields of application. Ta–N is a widely used material for producing hard coatings, wear resistant layers, thin film resistors, and diffusion barriers in integrated circuits, among other applications [11]. The main reason for the improvement in the properties of multilayer coatings is the interruption of the columnar growth of the coatings, grain growth and the formation of an interface that acts as a barrier to movement in the dislocations, the propagation of cracks and the diffusion of elements along the coating. Therefore, the main investigations have focused on the study that has the number of interfaces (i.e. bilayer period) in the mechanical, tribological, thermal, corrosion properties, among others. Shi et al. report the increase in hardness and adhesion of NbN-NbB2/NbN coatings as a function of the bi-layer period [12]. Kim et al. report an increase in hardness with values higher than 45 GPa, friction coefficients lower than 0.2 and a significant decrease in the wear rate of CrN/TiAlN coatings as a function of the bilayer period [13], it is important to note that the investigations have focused on obtaining coatings at a nanoscale (below 100 nm). Given the above mentioned properties of (Ti,Al)N and TaN coatings, the combination of these materials in a TiAlN/TaN multilayer architecture could result in coatings with enhanced mechanical, tribological, and thermal properties with multifunctional applications. Furthermore, in the literature there is a lack of research of the mechanical and tribological properties of these two materials in a multilayer architecture. For these reasons, the measurements, evaluations, and discussions in this work constitute an important contribution for future research on the development of nanostructured materials of this type.
Ti-Al and Ta targets, respectively. Prior to the deposition of these monolayer coatings, a tantalum adhesion layer (~80 nm) was deposited with a power of 1450 W, an argon flow of 30 sccm and substrate rotation speed of 12 rpm, for 5 min. Once the adhesion layer had been deposited, a 7 sccm nitrogen flow was applied for 3 h for the formation of the respective nitrides. Five (Ti,Al)N/TaN multilayer coatings were deposited for 2 h, obtaining an average thickness of 1500 nm and bilayer numbers n = 23, 31, 60, 150 and 300, using substrate rotation speeds of 0.2, 0.5, 1.0, 3.0 and 4.0 rpm, respectively. A simple one-fold rotation system was used to rotate the samples, and a frequency converter was used to control the substrate speed rotation. All other process parameters were the same as those used for the deposition of the monolayer coatings. The influence of the bilayer period of the nanolayered coatings on the mechanical and tribological properties was studied. 2.2. Coatings characterization In order to observe the coating morphology, transverse images were taken using a JEOL JSM-7401F field emission scanning electron microscope (FE- SEM). Transmission electron microscopy (TEM) was also performed using JEOL JEM-2200FS equipment in order to identify the microstructure and interfaces of the coatings at a nanometric scale. The cross-sectional sample preparation was performed using JEOL JEM9320 FIB focused ion beam (FIB) equipment, both for TEM analysis and for the cross-sectional nanoindentation analysis. Mechanical properties such as hardness, Young modulus, and recovery percentages of the TiAlN/TaN coatings were measured using an Agilent G200 nanoindenter with an XP Berkovich diamond tip and a radius of curvature of 20 ± 5 nm. The nanoindenter was calibrated using a standard fused silica sample (13 GPa). The test parameters of the area function were: C0 = 24.02, C1 = −179.21, C2 = 6703.01, C3 = −25,333.40 and C5 = 18,810.3. The mechanical properties were measured using the Oliver and Pharr method [14], maximum load of 2 mN, Poisson ratio of υ = 0.22 and holding time of 2 s. In order to evaluate the deformation mechanisms present during the nanoindentation tests, it was necessary to increase the indentation load to 10 mN, in order to obtain a penetration depth close to 500 nm. The adhesion of the coating to the substrates was evaluated by progressive load scratch tests. The scratch test was measured according to ASTM C1624 [15]. Scratch tracks with a length of 10 mm were generated with a scratch tester (Revetest CSM, Switzerland) utilizing a Rockwell C diamond indenter with 200 μm radius. The normal force was linearly increased from 0 to 100 N, the load rate was 1 N/mm and the scratch time was 60 s. The tribological properties of the films were evaluated by a CSM Instruments tribometer with a rotating ball-on-disc setup. A WC-6wt %Co bearing was used as a ball counterpart with a diameter of 6 mm. Relative velocity of 0.04 m/s, normal load of 1 N, wear track radius of 0.2 cm and 1200 cycles were set as testing conditions. The tests were carried out at 298 ± 2 K, in ambient air, at a relative humidity ranging from 40 to 50%. In order to calculate the average friction coefficient, the first step was to determine the limit between the initial regime and the steady regime, selecting the 400 cycles as the inflection point between both regimes. Then, the friction coefficient data was selected only in steady regime, i.e. from 400 to 1200 cycles. The wear rate was calculated by volume loss (eq. 1), using a Bruker DektakXT profilometer to calculate the area worn in the wear tracks. The worn area was always measured at the end of each test, i.e. after 1200 cycles, which in meters is equivalent to 15 m. In addition, the profilometer was used to measure the radius of curvature of silicon substrates with dimensions of 3 cm × 1 cm, before and after the coating process, in order to calculate the residual stresses of the coatings using Stoney's equation.
2. Experimental procedure 2.1. Coatings deposition Five nanolayered TiAlN/TaN coatings were deposited onto AISI M2 substrates with dimensions of 1.6 cm diameter and 0.3 cm thickness, using a direct current magnetron sputtering system. Additionally, TiAlN and TaN monolayer coatings were deposited for comparison purposes. Heat treatment was carried out in order to increase the hardness of the substrates. This consisted of preheating of the substrates at 1100 K, followed by an austenization at 1470 K and then oil quenching at 823 K. A tempering process with slow heating to 823 K was carried out immediately after quenching. The final hardness of the AISI M2 substrates was 64 ± 2 HRC (~7 GPa). The substrate was polished using SiC emery paper with different grit sizes, and then polished to a mirror finish with 1 μm diamond paste. Finally, an ultrasonic cleaning process was carried out on the substrate using a 3:4 solution of ethanol:acetone for 0.5 h, in order to remove any contamination from the polish. A magnetron sputtering system with dimensions of 550 × 600 × 800 mm3, designed and built by the research team, was used for the coating deposition process. Planar targets of Ta (99.9 wt%) and Ti-Al (50–50 wt%) with dimensions of 500 × 100 × 6 mm3 were used, and were placed facing one another. The chamber was evacuated to a background pressure below 10−3 Pa. Once the background pressure was reached, the targets were subjected to a plasma cleaning process using an argon flow of 40 sccm and chamber pressure of 3 Pa. The cleaning power was 500 W for both the Ti-Al and Ta targets, and the temperature was 523 K. Finally, the same plasma cleaning process was carried out on the substrates, using a bias voltage of −700 V and a duty cycle of 80 μs/10 μs. For the deposition of the TaN and TiAlN monolayer coatings, a mixture of gases (Ar/N2) with 19% nitrogen was used. Deposition conditions were as follows: temperature of 523 K, working pressure of 0.45 Pa, bias voltage of −70 V, and power of 1700 W and 1450 W for 2
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3. Results
Table 1 Setup parameters of TiAlN, TaN and TiAlN/TaN coatings: substrate speed rotation, bilayer number and bilayer period. Deposited coatings TiAlN TaN TiAlN/TaN TiAlN/TaN TiAlN/TaN TiAlN/TaN TiAlN/TaN
#1 #2 #3 #4 #5
Substrate speed rotation (rpm) 12 12 0.2 0.5 1.0 3.0 4.0
Bilayer number 1 1 23 31 60 150 300
Table 1 presents information on the TiAlN and TaN monolayer coatings and the five TiAlN/TaN multilayer coatings, together with the substrate speed rotation used to deposit the coatings, the bilayer number, and the experimental bilayer period. It should be noted that, as expected, as the substrate rotation speed increases, the number of times that the substrates passes in front of each of the targets increases (in less time), and therefore the bilayer number increases and the thickness of the bilayer period decreases. Cross-sectional FIB cuts of TiAlN, TaN monolayer coatings and representative multilayer TiAlN/TaN coatings with bilayer period ʌ = 65 nm (0.2 rpm) and ʌ = 48 nm (0.5 rpm) are shown in Fig. 1. It is possible to observe homogenous and compact coatings without delamination between the coating and substrate. Fig. 1 shows each of the constituent layers of the coatings studied (adhesion layer, coating, gold and carbon). The AISI M2 steel substrate is observed at the base, followed in turn by a pure tantalum adhesion layer (~100 nm). Next, the respective coating (TaN, TiAlN, TiAlN/TaN) and a gold layer in order to improve the conductivity of the samples. Finally, a carbon layer acting
Bilayer period (nm) 800⁎ 1500⁎ 65 48 25 10 5
* Thickness of the monolayer coatings.
k=
A. 2. . r l. d
(1)
where A is the calculated worn area in the wear track by profilometry, r is the radius of the wear track, l is the load applied in the tribological test and d is the total distance in the tribological test.
Fig. 1. Cross sectional FE-SEM image of monolayer a) TaN, b) TiAlN and TiAlN/TaN multilayer coatings with c) 0.2 rpm and d) 0.5 rpm.
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Fig. 2. TEM Analysis of representative TiAlN/TaN multilayer coating deposited with 1.0 rpm, a) Low magnified image STEM mode with HAADF coatings, b) and c) EDS line scan in TiAlN/TaN in STEM-HAADF mode, d) calculated bilayer period of TiAlN/TaN, e) HRTEM image in TiAlN/TaN coatings, f) and g) Fast Fourier Transform analysis in TaN and TiAlN coatings respectively.
as a protective layer that prevents unwanted erosion in FIB cutting. The cross-sectional images show TaN coatings (Fig. 1a) with thickness around 1500 nm, while the TiAlN coatings (Fig. 1b) exhibit thickness around 800 nm. It is important to remember that both monolayer coatings was deposited during 3 h. The difference in the thickness of the TaN and TiAlN coatings has been attributed in previous works to the low sputtering rate of the Ti-Al target resulting from the formation of intermetallic TiAl3 and to the difference in formation energies between TiAlN and TaN [16]. In Fig. 1c, TiAlN/TaN multilayer architecture of the coating deposited at 0.2 rpm is observed, with thickness ~1500 nm, number of bilayers n = 23 and ʌ = 65 nm. Meanwhile, Fig. 1d shows the coating deposited at 0.5 rpm where, similarly to the coatings deposited at 0.2 rpm, TiAlN/TaN multilayer architecture was identified, thickness of ~1500 nm, bilayer number n = 31 and ʌ = 48 nm. In these coatings, it was not possible to observe the multilayer architecture, reason why TEM analysis was used to calculate the period of these coatings. The representative study of the TiAlN/TaN deposited at 1.0 rpm is presented in Fig. 2. Fig. 2 shows the structure, morphology and multilayer architecture of TiAlN/TaN coatings deposited with 1.0 rpm. Fig. 2a shows the cross
section and at low-magnified image which was prepared by FIB. The calculated bilayer period of TiAlN/TaN coatings was approximately ʌ = 25 nm as shown in Fig. 2d. In order to corroborate the chemical composition and distinguish if there is an interphase between TiAlN and TaN coatings, it was analyzed in a region with more magnification in mode STEM High-Angle Annular Dark Field (HAADF) as shown in the Fig. 2b. In this figure it can be clearly observed how the TaN layer is the brightest due to in STEM mode the heaviest elements such as Ta are highlighted. Additionally, a linear EDS scan analysis was performed in order to see the modulation of the chemical composition of each coating as shown in Fig. 2c. According to the EDS analysis, it is clearly observed the modulation of the Ta (blue spectrum line), Ti (green spectrum line) and Al (yellow spectrum line) elements. Whereas the Ta increase the Al and Ti decrease. It validate the chemical composition and the formation of a multilayer architecture. Moreover, in the Fig. 2e it is observed the identification of the TiAlN and TaN layer, a small inter-diffusion zone can be clearly observed, suggesting a non-zero interface between TaN and TiAlN constituent coatings in HRTEM mode. Fig. 4f and g show the analysis of the Fast Furrier Transform (FFT) in a region marked with a square on each coating in HRTEM mode. 4
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Fig. 3. Nanomechanical properties of nanolayered TiAlN/TaN coatings measured by nanoindentation: a) load-unload curves, b) recovery percentage of coatings after nanoindentation tests, c) hardness and young modulus, d) H/⁎E and H3/*E2 ratios.
Fig. 4. Cross-sectional FE-SEM image of nanoindentation footprints of TiAlN and TaN monolayer coatings.
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Fig. 5. Representative cross-sectional FE-SEM image of nanoindentation footprints of TiAlN/TaN nanolayered coatings with ʌ = 65 nm and ʌ = 48 nm.
Performing an analysis of the interatomic distances of the FFT, the planes corresponding to each of the phases were determined. These figures clearly show how the crystals are oriented. TaN layer has face centered cubic (fcc) with (2,2,0) and (2,-2,0) planes with the interatomic distances of 1.5312 Å agree PDF reference code 01-089-5196. While the TiAlN layer has face centered cubic (fcc) too with (1,1,1) and (1,-1,1) planes with the interatomic distances of 2.450 Å agree PDF reference code 00-031-1403. This interatomic distance is slightly greater than TiN, due to the interstitial aluminum incorporation within the TiN lattice, forming a (Ti,Al)N solid solution. Nanomechanical properties of the TiAlN/TaN multilayer coatings were studied by nanoindentation, obtained results are presented in Fig. 3. Fig. 3a shows the loading-unloading curves of multilayer coatings deposited with different bilayer periods. The curves obtained are characteristic of elastoplastic behavior, with a progressive increase in the load to 2 mN and a maximum depth of 73 nm for the coatings with ʌ = 65 nm. It is possible to observe a progressive reduction in penetration depth to h = 59 nm as the bilayer period of the coatings reduces to 10 nm. Finally, a slight increase in penetration depth is observed for the coatings deposited with ʌ = 5 nm. As previously mentioned the curves indicate that the coatings were subjected to an elastoplastic deformation. This means that part of the total energy involved in the nanoindentation test is recovered as elastic energy, and another part is dissipated as plastic energy (associated with the plastic deformation of the material during the indentation). Calculating the area below each of the loading curves (total energy applied) and unloading curves (elastic energy) it is possible to determine the percentage of plastic energy dissipated during the test for each of the coatings [17,18]. Recovery percentages of TiAlN/TaN are shown in Fig. 3b, it is possible to observe a progressive increase in the recovery percentages of the TiAlN/TaN multilayer coatings as the coating period decreases. TiAlN/TaN coatings with ʌ = 65 nm showed a recovery percentage of 42%, while the recovery percentage of coatings with ʌ = 10 nm and ʌ = 25 nm it is around 56%. In addition, it is important to note that coatings deposited with 10, 25 and 48 nm exhibited higher recovery percentages than the TiAlN and TaN monolayer coatings. Fig. 3c shows the hardness and Young's modulus values calculated from the loading-unloading curves using the Oliver and Pharr
method. A progressive increase in hardness and Young's modulus of the TiAlN/TaN multilayer coatings is observed as the coating period decreases, reaching values of H = 38 GPa and E = 472 GPa for TiAlN/ TAN coatings deposited with ʌ = 10 nm. However, multilayer coatings with ʌ = 5 nm, a sudden fall in both properties up to 35% is observed. The increase in hardness and Young's modulus of multilayer coatings can be attributed to the increase in the number of interfaces, which act as barriers for the propagation of dislocations. However, as the bilayer period continued to decrease, a sudden decrease in hardness and young modulus was observed. According to Chu and Barnett, the main explanation for this phenomenon is that the decrease of the bilayer period below 5 nm leads an interface mixture which leads to lower modulation amplitude [19]. Considering the solubility of tantalum within the TiAlN matrix, it is possible to affirm that this phenomenon may be the main reason for the decrease in the hardness and Young's modulus of coatings with a bilayer period below 5 nm. While mechanical and tribological performance improves as the
Fig. 6. Residual stresses of TiAlN/TaN multilayer coatings, deposited with different bilayer period, measured by the curvature method and Stoney equation. 6
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period is reduced (increasing the number of interfaces), Holleck reported that the number of interfaces (known as interface volume) plays a fundamental role in the properties of multilayer coatings. Moreover, Holleck reported that all materials have an optimum number of interfaces where their performance is improved significantly, this optimum in ceramic multilayer coatings is generally between 100 and 500 [20]. TiBCN/CNx [8], TiN/VN [21], TiAlN/CrAlN [22] and Ti0.33Al0.67N/CrN [23] are some examples that have been studied the influence of bilayer period on mechanical and tribological performance. Additionally, optimum number of layers for mechanical performance of the coatings was identified. In the present work, optimal hardness and Young's modulus are observed for the coatings between 10 and 20 nm, considering the coatings thickness of 1500 nm, the optimal interface volume is between 75 and 150. The results of the H/⁎E and H3/⁎E2 ratios, associated with elastic strain to failure and resistance to plastic deformation respectively, are presented in Fig. 3d. Different investigations have identified a limit of H/⁎E = 0.1 for coatings with high resistance to plastic deformation and low elastic strain to failure [18,24–26]. In Fig. 3d, it can be observed that the H/⁎E ratios of the TiAlN/TaN multilayer coatings progressively increase as the period reduces. The coatings with ʌ = 25 nm and ʌ = 10 nm exhibited values above 0.1, suggesting high elastic strain to failure. Parallel behavior presents the relationship H3/⁎E2, authors report that this relationship can be a parameter to predict the tribological behavior of coatings. In the tribology results obtained (Fig. 8), the
correlation between both properties will be deepened. Fig. 4a shows the lamella of the monolayer TiAlN and TaN coatings, extracted and milled by FIB, observed transversally in FE-SEM. The cross section of two indentations can clearly be observed at 5000×, with a separation of 15 μm to avoid influence between the indentations. In Fig. 4b (20,000×) the cross section of one indentation is shown. It is possible to identify the adhesion layer, the TiAlN coating, gold layer and protective carbon layer. The elastoplastic deformation induced in both the TiAlN and M2 substrate can be observed, with evidence of inter-columnar shearing across both the coating and the adhesion layer, dissipating once the ductile substrate is reached. In Fig. 4c at (50,000×) the inter-columnar shearing can be observed more clearly, with no evidence of lateral, inclined or curved cracks. With regard to the indentations made to the TaN coatings, in Fig. 4d (5000×) two indentations with separation of 15 μm is observed once again. In Fig. 4e (20,000×) one of the indentations is observed, with characteristic pyramid shape in the top of the coating. Inter-columnar shearing is observed, with greater intensity compared to the TiAlN coatings. Finally, in Fig. 4f (50,000×) it can be observed in greater detail that the only deformation mechanism present is inter-columnar shearing, with evidence of characteristic slip in the coating-substrate interface, which is progressive until reaching the center of the indentation. Representative images of the indentations made to the multilayer coatings are presented in Fig. 5. In Fig. 5a (5000×) two indentations made to the coatings deposited with period ʌ = 65 nm are observed.
Fig. 7. Optical and FE-SEM image of the scratch up to Lc3 of nanolayered TiAlN/TaN coatings. 7
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Fig. 8. a) Lc3, H/⁎E and H3/⁎E2 in function of the bilayer period of multilayer coatings and b) Lc3 vs H/⁎E and H3/⁎E2 and its respective cubic fitting.
Each of the constituent layers and the pyramid-shaped indentations are clearly observed in the surface of the coatings. In Fig. 5b (20,000×), only inter-columnar dislocations can be observed as a failure mechanism, as in the case of the TaN coatings. However, a reduction in the number of dislocations present is observed. In Fig. 5c (50,000×), it can be observed that the inter-columnar dislocation leads to transverse shearing of the TiAlN/TaN multilayer, increasing the number of dislocated layers as this reaches the center of the indentation, which is consistent with the force concentration during the nanoindentation test. Fig. 5d, e and f present the indentations of the coatings with ʌ = 48 nm. Similarly to the TiAlN/TaN coatings with ʌ = 65 nm, the multilayer structure is observed, although there are significant differences in the deformation mechanism. In Fig. 5e (20,000×) no evidence of any type of inter-columnar shearing or lateral, inclined or curved cracks is observed. In the top of the surface in Fig. 5f the pyramid shape of the Berkovich indentation is clearly observed, while in the coatingsubstrate interface the indentation footprint shape changes from pyramid to semi-circular. Based on what is observed, it can be suggested that the energy associated to the nanoindentation test is dissipated progressively between interfaces as it advances across the coating. The curvature method together with the Stoney's equation has been widely used to measure the residual stresses of coatings deposited by sputtering; Fig. 6 shows the residual stresses of TiAlN, TaN monolayer coatings and TiAlN/TaN multilayer coatings. Both monolayer coatings exhibited high residual stress, −8,69 GPa for TaN coatings and −7,01 GPa for TiAlN. The negative sign by convention has been adapted to indicate that the stresses are compressive. Regarding to TiAlN/TaN multilayer coating it is possible to observe a significant reduction in residual stress, the reduction is close to 50% for TiAlN/TaN deposited with ʌ = 65 nm (−3.98 GPa). Additionally, there is a clearly trend towards a reduction in residual stress as the bilayer period decreases. Each of the interfaces acts as a stress dissipater, relaxing the structure and decreasing the residual stresses, therefore, as the number of interfaces increases (decrease in the bilayer period) the decrease in residual stresses becomes evident [27–29]. Fig. 7 shows optical and scanning electron microscopy of the tracks obtained in the scratch tests of the monolayer TiAlN and TaN coatings and multilayer TiAlN/TaN coatings. It can be observed that all the multilayer coatings exhibit higher critical loads compare to the monolayer coatings. Additionally, it can be seen that TaN coatings showed a critical load twice as high as the TiAlN monolayer, probably due to this coating having greater chemical affinity with the Ta adhesion film than TiAlN. The TiAlN and TaN monolayer coatings exhibited Lc3 critical loads of 8.9 N and 18.1 N, respectively. With regard to the multilayer TiAlN/ TaN coatings, a progressive increase in the adherence of the coatings as the period decreases can be observed, with Lc3 values of 37.1 N for the coatings deposited with ʌ = 10 nm, falling to 31 N when the period is
reduced to ʌ = 5 nm. At low loads, plastic deformation of the substratecoating system occurs. The most important aspects to highlight are as follows: i) the significant increase in adherence of the multilayer coatings by up to 90% in comparison with the TaN monolayer and, ii) the clearly correlation between the adherence of the TiAlN/TaN coatings and their mechanical properties (recovery percentage, H/E ratio and H3/⁎E2). In all scratches, sudden failure of the coatings is observed as the Lc3 critical load is reached. Propagation of compressive, semicircular spallation is observed from the center of the scratch line to the outside, leaving the substrate completely bare even in areas outside the edges of the scratch. Based on the optical evidence and on the ASTM C 1424-05 standard, the main failure mechanism in the coatings appears to be compressive wedging-spallation. This is typical of the brittle fracture generally present in hard coatings deposited on hard substrates such as HSS, as a result of the compressive force generated ahead of the indenter as it advances [30,31]. The Lc3 values has been plotted together with H/⁎E and H3/⁎E2 (Fig. 8a), in addition has been plotted the Lc3 vs H/⁎E and H3/⁎E2, respectively (Fig. 8b), in order to find a mathematical correlation between the properties. As expected, the Lc3 values showed the same trend obtained for H/⁎E and H3/⁎E2 (Fig. 8a). This correlation is congruent if it is considered that, during the scratch test, the coatings are subjected to a shear stresses, these stresses induce a plastic deformation in the material, therefore, as the ratio of the values H/⁎E and H3/⁎E2 increases, coatings with greater resistance to shear stresses are expected and consequently an increase in adhesion is observed. Additionally, the critical load Lc3 vs H/⁎E and H3/⁎E2 was plotted in order to find a
Fig. 9. Friction records of monolayer TiAlN, TaN and TiAlN/TaN multilayer coatings deposited with different bilayer period. 8
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steady regime around 300 cycles is reached. Other reports also show friction coefficients between 0.6 and 0.8 for TiAlN, depending on the counterbody used and the speed at which the test is performed [32,33]. The exception is the coating with ʌ = 5 nm, which retains a low friction coefficient throughout the test. The monolayer coatings show greater fluctuation in friction values than the multilayer coatings, suggesting that during these tests, contact between the tribological surfaces was more chemically and mechanically stable. The average value of the friction coefficients and wear rates in steady state are presented in Fig. 10. All the multilayer coatings showed friction coefficients lower than the monolayer coatings, showing a reduction in friction coefficient as the period of the multilayers decreased, with the lowest friction coefficient of 0.22 obtained in the multilayer with ʌ = 5 nm. Considering that tribological tests consist mainly in the measurement of the tangent force, which, when divided the normal load, is obtained the friction coefficient. The decrease in friction coefficients is mainly associated with the increase in the number of interfaces, which help dissipate the energy associated with the tribological test [34,35]. The wear rates exhibit parallel behavior to the friction coefficients, exhibiting lower values as the multilayer period decreases, with the lowest value of 1.8 × 10−4 mm3/Nm obtained in the coating with ʌ = 5 nm, which corresponds to a value 10 times lower compared to the monolayer coatings. All the multilayer coatings presented wear rates one order of magnitude lower than those of the monolayer coatings do. Although the best mechanical properties were presented by the coating with a ʌ = 10 nm, the coating showed the best tribological properties with ʌ = 5 nm. In the 80's Tsui et al. introduce the Jhonson's equation suggesting that the H3/⁎E2 ratio can be a useful parameter for predicting the plastic deformation resistance of coatings [36]. However, in recent years multiple investigations have been reported that differ from the one proposed by Tsui, arguing that the decrease of Young's modulus goes against the search of the increase in toughness. Additionally, Young's modulus and hardness are directly correlated properties and it is not trivial to modify one without affecting the other. In the TiAlN/ TaN multilayer coatings studied in this research, there is no direct or
Fig. 10. Friction coefficients in steady regime and wear rates of TiAlN/TaN multilayer coatings deposited with different bilayer periods.
mathematical expression that adjusts the correlation between the respective properties. A cubic correlation was found between each of the properties, the correlation between the critical load Lc3 and the H/⁎E ratio can be expressed using the cubic function y = − 3.7105 + 0.4611x − 0.0179x2 + 2.2933x10−4x3, with an R2 adjustment of 0.9730, suggesting a high fitting between experimental and theoretical values. In the other hand, the correlation between Lc3 and H3/⁎E2 ratio can be expressed using the cubic function y = − 0.2654 + 0.0408x − 0.0016x2 + 1.9766x10−5x3, with an R2 adjustment of 0.9851. Fig. 9 shows the friction records for the multilayer and monolayer systems. Excluding TiAlN coatings, initially, in the first 130 cycles, all coatings show low friction coefficients (0.2) similar to TaN coatings, undoubtedly because the last layer of the multilayer systems is TaN, while the TiAlN monolayer starts with a higher value (0.4). These values increase suddenly and quickly (after 50 cycles) up to 0.8 once the
Fig. 11. Optical images of wear tracks of TiAlN/TaN multilayer coatings deposited with a) ʌ = 65 nm, b) ʌ = 25 nm, c) ʌ = 10 nm y d) ʌ = 5 nm. 9
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indirect correlation between the H3/⁎E2 ratio and the tribological behavior of the coatings. Authors such as Abad et al. have reported the same phenomenon in TiBC coatings [25]. Therefore, it is important to note that the concept associated with H3/*E2 ratio is not universal, should be used with caution and should be studied further. The optical microscopy images of the multilayer coatings reveal the presence of remaining products adhering on the wear tracks of the coatings (Fig. 11). Fig. 11a shows the wear track of TiAlN/TaN coatings with ʌ = 65 nm. It is possible to observe the presence of debris on each external side of the wear track, additionally, the adhesion of debris formed within the wear track was observed. The presence of litmus colors suggests tribo-oxidative reactions between TiAlN/TaN coatings and WC-Co counterbody. Similar behavior can be seen in Fig. 11b (ʌ = 48 nm), however, there is a decrease in debris adhesion at the external wear track, while the size and number of debris adhered within the wear track increases. In Fig. 11c, it is possible to identify the formation of grooves in the TiAlN/TaN coatings with ʌ = 10 nm, suggesting abrasive wear as the main wear mechanism. In the SEM images of the wear tracks shown in Fig. 12, wear debris adhering to the tribological wear track is observed. In Fig. 12a and b, it is possible to identify flake-like debris, this kind of debris are generated during the delamination process due to plastic shear strain accumulation at the wearing surfaces. Two selected EDS in different zones and a mapping was carried out to study the debris adhered on the tribological wear tracks. Fig. 12d shown the selected areas analyzed by EDS, area 1
onto the debris and area 2 outside them. In Fig. 12e, the oxygen distribution obtained in the EDX mapping is shown. Two zones are clearly observed, one with high oxygen concentration corresponding to area 1 associated with the adhered debris and second one with a low oxygen concentration corresponding to area 2, free of debris. Fig. 12f shows the EDS spectra corresponding to each of the studied areas. Additionally, the element atomic percentages is shown. Congruent with the mapping, area 1 shows high oxygen content, with percentages higher than 50 at.%, with which it is possible to affirm that the debris formed during the tribological test correspond to oxides. On the other hand, in area 2, it can be seen that the oxygen concentration is very low (5%) so it is possible to affirm that this zone corresponds to the coating and that the tribological test has not yet exposed the substrate. Based on the SEM and optical microscopy images, it can be said that the increase in the period caused a parallel increase in the formation of tribofilms on the tribological wear track, resulting in higher friction coefficients as the contact with the tribological surface began to be governed by oxides formed with oxygen and humidity in the atmosphere. Other authors have found similar behavior in monolayer and multilayer AlCrN, TiAlN, TiN, TiAlVN, TiAlTaN and AlN system [37]. The tribological behavior of these nitride coatings could be strongly influenced by the presence of humidity at room temperature, as the continuous formation of oxides in sliding contact can result the removal of material by abrasive mechanisms, as occurred in the multilayer coating with ʌ = 10 nm, showed in Fig. 11c [32–38].
Fig. 12. SEM images of wear tracks of TiAlN/TaN multilayer coatings deposited with a) ʌ = 65 nm, b) ʌ = 10 nm, c) ʌ = 5 nm, d) zoom of selected area in the wear track of coatings with ʌ = 10 nm, e) EDS mapping of oxygen in the selected zone and f) the atomic composition of selected zone. 10
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4. Conclusions
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It was possible to modify the deposition of the TiAlN/TaN coatings using only the rotation speed of the substrates, by progressively decreasing the bilayer period as the rotation speed of the substrates increased. As expected, in coatings with larger bilayer periods it was possible to identify the multilayer architecture using FIB for transversal cuts and FE-SEM for the cross-section. For coatings below ʌ = 25 nm, TEM analysis was necessary to calculate the period of the coatings. All multilayer coatings exhibited a defined structure between layers, however, through TEM it was possible to observe that the coatings presented a non-zero interface. This means that there was an interdiffusion zone between each of the layers. With regard to the mechanical properties of the coatings, the recovery percentages, hardness, and Young's modulus exhibit parallel behavior. All properties increase progressively as the period of the coatings decreases. However, there is an optimum point between 10 and 25 nm after which the properties decrease suddenly. Similar behavior was shown for residual stresses, where a significant decrease, close to 50%, was observed compared to the TiAlN and TaN monolayers. The increase in the number of interfaces facilitates the relaxation of the structure for both intrinsic and extrinsic stresses. Through FIB, it was possible to study the cross-sections of the nanoindentation footprints and identify the main deformation mechanisms present in the coatings studied. For monolayer coatings, inter-columnar shear was observed as the main mechanism. Additionally, the shear in the TaN coatings progressively evolved until the substrate was reached, evidenced in the formation of steps in the coating-substrate interface. On the other hand, in multilayer coatings a significant decrease in inter-columnar shear was observed as the period of the coatings decreased. The adhesion of the coatings was one of the most significant results obtained in the present work. It was possible to observe increase two times the critical loads of TiAlN/TaN multilayer coatings compared to individual TiAlN and TaN coatings (individual critical loads of 9 N and 18 N), reaching critical loads of 37 N for coatings with a period of 10 nm. It is important to emphasize that all multilayer coatings exhibited superior adhesions to the constituent monolayers. Finally, it was possible to clearly observe the influence of multilayer architecture and the bi-layer period on mechanical and tribological properties. Values for both friction coefficients and wear rates showed a significant decrease. Acknowledgments The authors are grateful to the Comité para el Desarrollo de la Investigación (CODI) for financing this project (no. PRG-2014-934), to the University of Antioquia and Colciencias for the Ph.D. sponsorship agreement (647). Additionally, the authors want to thank in a very special way the Centro para la Investigación en Materiales Avanzados (CIMAV) for all the support and disposition of the equipment used for the coatings characterization. Finally, the authors want to thank Roberto Talamantes for his invaluable technical work and assistance in nanoindentation test and Wilber Antunez for his invaluable technical work and assistance in FE-SEM analyses. References [1] S. PalDey, S.C. Deevi, Properties of single layer and gradient (Ti,Al)N coatings, Mater. Sci. Eng. A 361 (2003) 1–8, https://doi.org/10.1016/S0921-5093(03) 00473-8. [2] S. Veprek, R.F. Zhang, M.G.J. Veprek-Heijman, S.H. Sheng, A.S. Argon, Superhard nanocomposites: origin of hardness enhancement, properties and applications, Surf. Coatings Technol. 204 (2010) 1898–1906, https://doi.org/10.1016/j.surfcoat. 2009.09.033. [3] R. Rodríguez-Baracaldo, J.A. Benito, E.S. Puchi-Cabrera, M.H. Staia, High temperature wear resistance of (TiAl)N PVD coating on untreated and gas nitrided AISI H13 steel with different heat treatments, Wear. 262 (2007) 380–389, https://doi. org/10.1016/j.wear.2006.06.010.
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