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CrN coatings
SCT-21588; No of Pages 9 Surface & Coatings Technology xxx (2016) xxx–xxx
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Modeling intrinsic residual stresses built-up during growth of nanostructured multilayer NbN/CrN coatings Juliano Avelar Araujo a, Rafael Arthur Reghine Giorjão a,b, Jefferson Bettini b, Roberto Martins Souza c, André Paulo Tschiptschin a,b,⁎ a
Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, 05508-030 São Paulo, SP, Brazil LNNano - Brazilian Nanotechnology National Laboratory – CNPEM - Brazilian Center for Research in Energy and Materials, R. Giuseppe Máximo Scolfaro, 10000 - Polo II de Alta Tecnologia, 13083-970 Campinas, SP, Brazil c Mechanical Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2231, 05508-030 São Paulo, SP, Brazil b
a r t i c l e
i n f o
Article history: Received 14 April 2016 Revised 19 July 2016 Accepted in revised form 24 July 2016 Available online xxxx Keywords: Multilayer coatings Nanostructure Intrinsic residual stresses EELS chemical analysis Finite element modeling
a b s t r a c t Composition modulated NbN/CrN nanostructured multilayer coatings were deposited onto martensitic stainless steel. The coatings were obtained by cathodic arc process, in an industrial-size physical vapor deposition (CAPVD) chamber. The coatings reached 15 μm thick, consisting of multilayers of different periodicities (4 nm ≤ Λ ≤ 20 nm), which were achieved by varying the rotating speed of the samples in a 2-fold rotation table. The obtained coatings were characterized by High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Diffraction (SAD) in order to assess the crystal structure, epitaxy and degree of coherency of the NbN/CrN interface. The results showed that the coatings were formed by alternate individual layers of NbN and CrN separated by highly coherent interfaces, and revealed cross-contamination between the layers. Compositional variation measured across the multilayers, by Electron Energy Loss Spectroscopy (EELS) showed that the lower the coating periodicity (Ʌ), the greater is the cross-contamination in the NbN and CrN individual layers. A Finite Element Analysis (FEA) model was developed for assessing the intrinsic residual stresses due to differences of lattice parameters of the NbN and CrN structures, based on chemical composition gradients obtained from EELS measurements. The results show the build-up of stress gradients from the center of individual layers towards the interface, differently from other results published in literature. The smaller the periodicity the higher are the stress gradients developed near the NbN/CrN interfaces. The results are discussed based on the possible interaction of dislocations with stress fields developed near the NbN/CrN interfaces. The proposed model is consistent with former experimental results, which indicates that the smaller the periodicity, the greater is the coating hardness when periodicities range from 4 nm to 20 nm. © 2016 Elsevier B.V. All rights reserved.
1. Introduction CrN/NbN nanostructured multilayer coatings deposited by sputtering processes have been extensively investigated in the last two decades [1, 2]. However, the deposition of this type of coating by cathodic arc process has been understudied, especially regarding the variation of chemical composition of the mutually soluble layers (known as “Composition Modulation”), which are separated by diffused interfaces [3–6]. NbN/CrN nitride multilayers are isostructural and mutually miscible. Mixing of the constituents is likely to happen during deposition, leading to compositional gradients. The degree of mixing is a function of the
⁎ Corresponding author at: Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, 05508-030 São Paulo, SP, Brazil. E-mail address: [email protected] (A.P. Tschiptschin).
deposition conditions, particularly regarding the cross-contamination between the targets during the process [7,8]. Chu et al. [9] and Shinn et al. [10] describe superlattice structures for polycrystalline (TiN/NbN) coatings deposited by sputtering and using two opposite targets (Ti/Nb). According to the authors, “any intermixing of the layers due to ion irradiation was insufficient to eliminate the composition modulation and the associated hardness enhancement”. The authors observed an increasing on hardness when Ʌ (periodicity) decreased below 14 nm, in agreement with the trend observed for epitaxial nitrides [10]. Chawla et al. [11] studied the stress distribution generated in TiN/ AlN multilayer coating applied by sputtering process using a Finite Element Model (FEM). The authors considered an abrupt stress profile at the sub-layers interface. Yin et al. [12] built a FEM supposing cohesive bonds at the interfaces of TiN/CrN multilayer coating, in order to predict the formation and propagation of micro-cracks under indentation. In
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this model, the authors considered an abrupt interface between the sublayers, as well. Models that provide more accurate predictions have been developed by including the effects of several abrupt interfaces [12,13]. Some works have demonstrated, by dynamic diffraction (satellites peaks), that the composition modulation increases with increasing periodicity [12,14]. Moreover, Chu et al. [15] proposed a model for dislocation movement considering an arbitrary shaped composition modulation (sawtooth modulation) across and within the individual layers. Glow Discharge Optical Emission Spectroscopy (GDOES) and Atomic Emission Spectroscopy (AES) have been used for chemical composition modulation quantification. However, these techniques do not have the accuracy needed to detect compositional gradients in nanostructured multilayer coatings. Zhou et al. [16,17] analyzed the effect of the three-fold rotation movement on the composition modulation of TiAlN/VN deposited by sputtering. They obtained a significant chemical mixing between the layers when the periodicity was ~ 3 nm, as determined by EELS. Furthermore, the EELS technique has proved to be able to reveal the chemical composition profile through these nanostructured layers [18,19]. In this work, an assessment of the chemical modulation across NbN/ CrN multilayer coatings with different periodicities (Ʌ) was carried out. The cross-contamination during deposition resulted in chemical composition gradients, which were fed into a Finite Element Model to calculate intrinsic interfacial coherency stresses. Vegard's law was used to calculate the strain associated with corresponding lattice parameters variations. The effects of the compositional modulation on the intrinsic interfacial coherency stresses are discussed for coatings with four different periodicities. 2. Experimental procedure NbN/CrN nanostructured multilayer coatings were Cathodic Arc Physical Vapor (CAPVD) deposited onto gas nitrided martensitic stainless steel (AISI 440B) coupons in an industrial-size chamber. Three rectangular cathodes (two made of Cr and one of Nb) in alternate positions were fed with their own power supply. By varying the rotating speed of the table in the center of the chamber, the 15 μm thick coating was obtained with four different periodicities Λ, from 4 to 20 nm. The rotating speed controlled the time the substrate pass in front of each Nb or Cr targets in such a way that when it was multiplied by 2, 3 and 5, it lead to the formation of coatings with periodicities of 20 nm, 10 nm, 7.5 nm and 4 nm, respectively. The average deposition rate in all cases was 3 μm/h [20]. Cross sections of the samples were prepared by Focus Ion Beam (FIB) using a FEI Helios 660 dual beam microscope, in order to obtain thinfoils for TEM analysis. TEM analysis was carried out in a JEOL 2100F TEM operating at an accelerating voltage of 200 kV. The microscope was operated both in Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) modes. The TEM mode was primarily used for imaging and diffraction analysis, while STEM mode (STEM probe 0.7 nm – 150 pA) was adopted for image acquisitions alone and when coupled with EELS (Electron Energy Loss Spectroscopy – Gatan Tridiem 863) to generate a Spectrum Image [21]. TEM micrographs were recorded using ES500W (2 k × 2 k pixel) CCD camera and the camera of Gatan Tridiem 863. The EELS analysis was applied for measuring the composition modulation across the coating thickness. The high-resolution composition distribution was achieved by using probes of 0.2 and 0.7 nm with total inelastic mean free path t/ʎ (t = sample thickness, ʎ = total inelastic scattering electron mean free path in the sample) b 0.5 (typically between 0.2 and 0.4). Thus, the enlargement effect of probe size could be ignored in this work [22], since the maximum IMFP (Ineslatic mean free path) is 85 nm (CrN is 138 nm and NbN is 170 nm) and the beam convergence angle is 11 mrad in the case of the 0.7 nm probe. To
avoid a spatial drift during spectra image acquisition, a reference image was defined for drift correction every 4 s (Pixel time of 0.5 s and 2 s for 0.7 and 0.2 nm probes, respectively). 3. FEM modeling and assumptions A linear elastic model was designed using COMSOL finite element software. The model consists of 2D rectangles representing NbN and CrN individual layers. The width of the multilayer was set at 200 nm and the thickness of individual layers varied according to the dimensions measured in the transmission electron microscope. Triangular elements of 0.5 nm were selected for the multilayer region, as shown in Fig. 1. The bottom left corner of the model was pinned to restrict any movement and all other edges were stress free so that bending was permitted. For modeling strains in the NbN/CrN multilayer coating, d-spacings of pure NbN and CrN layers were considered to be those given by ICDD cards #38–1155 and #11–65, respectively. The lattice parameters were 0.439 nm for NbN and 0.415 nm for CrN, thus resulting in a difference around 5.5% between them. d-spacing of the coherent interface was assumed to be the average value between the lattice parameters of the individual sublayers, 0.425 nm. Young's modulus of pure NbN and CrN layers were taken as 348 GPa [23] and 245 GPa [24], respectively. The chemical composition along the coating thickness was provided by the EELS results and the intrinsic stresses were calculated based on a linear variation of lattice parameters with chemical composition, according to the rule of mixtures: ay ¼ aNbN C NbN þ aCrN C CrN y y
ð1Þ
Where, aNbN and aCrN are the lattice parameters of pure NbN and CrN layers respectively, CyNbN and CyCrN are the concentrations of the NbN and CrN layers in a specific region respectively and ay is the lattice parameter calculated in a particular region of the multilayer. Likewise, the elastic modulus in a particular region of the multilayer was calculated using the rule of mixtures: Ey ¼ ECrN C CrN þ ENbN C NbN y y
ð2Þ
Where, ENbN and ECrN are the elastic moduli of pure NbN and CrN layers, respectively, and Ey is the elastic modulus calculated in a particular region of the multilayer. To calculate the stresses, a material strain condition was applied, being calculated based on the local and interfacial lattice parameters. ey ¼
ay −aint aint
ð3Þ
Where aint is the lattice parameter of the NbN/CrN interface and ey is the resulting strain calculated for a particular region of the multilayer. 4. Results and discussion 4.1. TEM experimental results Fig. 2 shows high magnification images of the 7.5 nm periodicity coating, taken along the 15 μm NbN/CrN coating thickness. Image (a) was taken from a region close to the substrate and image (c) taken closer to the coating surface. Fig. 2 indicates that the NbN/CrN multilayer has an almost constant periodicity throughout the whole thickness. Variations on the sublayer thickness is smaller than 9%. Dark layers correspond to the NbN layers and bright ones to the CrN layers. The coating columnar microstructure (widths varying from 50 nm to 130 nm) overlaps the multilayer structure, maintaining the same orientation from layer to layer. The width of these columns may
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Fig. 1. Meshed geometry using triangular elements. (a) 2D rectangles model representing NbN and CrN individual layers. (b) Mesh geometry formed by triangular elements of 0.5 nm.
increase or decrease along the coating thickness, according to the simultaneous growth rule [25,26]. The inlets in Fig. 2 (b) and (d) are FFT (Fast Fourier Transform) images, which allow assessing the multilayer periodicity and the average atomic plane distances. FFT in the Fig. 2 (b) gives a periodicity of 7.5 nm. The detailed analysis of Fig. 2 indicates a 4.0 nm thick CrN phase and a 3.5 nm thick NbN phase. FFT in Fig. 2 (d) also shows that this coating is oriented in a [002] zone axis. The layers in the FFT are not completely flat, which indicates that orientations of the structure planes are changing along the coating thickness. This can be explained by the curvature of the multilayers observed in all images, which is probably due to substrate roughness and to the presence of macroparticles typical of the cathodic arc PVD process. 4.2. Coherent interfaces Fig. 3 shows HRTEM images of the nanostructured NbN/CrN multilayer coating with nominal 7.5 nm and 10 nm periodicities. Fig. 3a and b show 6.8 nm and 9.6 nm periodicities, respectively, thus providing b9% of variation. Measurements of individual layers thickness were carried out in Fig. 3 (a), giving CrN and NbN layers, 3.0 nm and 2.8 nm thick, respectively. The diffuse interface is 1.0 nm thick. In Fig. 3 (b), values for CrN, NbN and interface thicknesses are 3.4 nm, 5.0 nm, 1.2 nm, respectively, therefore indicating that coherent interfaces separate individual NbN and CrN layers. Fig. 4 shows nanodiffraction patterns taken from an area (50 nm diameter) of the 7.5 nm periodicity coating, depicted in Fig. 2 (d), enclosing both NbN and CrN layers. Both phases are face-centered
cubic (fcc) and textured in the [110] direction [27]. A double spot can be observed in the electron diffraction pattern in Fig. 4, implying a strong alignment. This result is an evidence of the presence of unperturbed atomic bonding between the NbN and CrN layers, which can be described as localized coherent growth. As a result, the multilayer coating presents a high degree of crystallinity and coherent interfaces. 4.3. Composition modulation by EELS EELS was applied to obtain high-resolution element distribution profiles for the NbN/CrN nanostructured multilayer with periodicities of 4, 7.5, 10 and 20 nm. In the case of the 7.5 nm periodicity coating, two resolutions of energy loss signals (~0.2 and ~0.7 nm) were applied in order to evaluate the resolution impact on the elemental map for this sort of coatings. The results (not shown here) indicate that probe size effects for 0.2 and 0.7 nm can be neglected. Spectroscopic images were obtained for all samples and quantification was carried out using Nb-M (205 eV) edge and Cr-L (575 eV) edge. Compositional quantification by EELS using L and M shell is more difficult and less reliable than Wavelength Dispersive Spectroscopy (WDS) [28]. Thus, WDS has been used to obtain the right k-factors [29]. The WDS analysis indicates that the coatings are stoichiometric, presenting the following chemical composition: Nb = 20 at.% (±1.0), Cr = 29 at.% (± 1.0) and N = 50 at.% (± 1.0) for all the periodicities studied. Nitrogen content as measured by EELS is around the same concentration 50 at.%, presenting variation of ± 2 at.% between NbN and CrN layers. Then, Cr and Nb compositions were normalized to 100% in the EELS chemical analysis.
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Fig. 2. High magnification images of the 7.5 nm periodicity coating, along the 15 μm thick coating. (a) region close to the substrate; (b) the inlet shows the FFT of the multilayer that allows assessing the periodicity; (c) region close to the coating surface; (d) the inlet shows the FFT that allows calculating average atomic planes distances.
Fig. 5 shows the STEM image of the coating with 7.5 nm periodicity , the Nb, Cr and composite maps, the spectral image obtained by EELS in the reference showed in the STEM image and EELS spectrums in the CrN and NbN rich layers. The Cr and Nb elemental distributions in the nanostructured CrN/NbN multilayer were evaluated by taking a line profile across the layers in the acquired images and further quantifying the concentrations. For all samples, two or three maps were obtained along the coating thickness.
The two point spectra were taken in positions 1 and 2 in the spectrum image. Position 1 was supposed to be a pure NbN layer as it was taken in the center of the bright layer. However, one can see a Cr L (edge) peak in the spectrum image for this position. Conversely, a Nb M (edge) peak is also observed in Position 2 spectrum image, which was supposed to be a pure CrN layer. These results clearly indicate Cr and Nb atoms intermixing in the layers, that can be related to cross-contamination during deposition.
Fig. 3. HRTEM images of the multilayer NbN/CrN coatings with (a) 7.5 periodicity and (b) 10.0 nm periodicity.
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Fig. 6 shows the relative concentrations of NbN and CrN along a line scan ~25 nm long. The chromium content tends to be dominant, since concentration levels reach up to 100 at.% in the CrN layers for the 10 nm and 20 nm periodicities coatings. This can be explained by the cathode configuration (two Cr cathodes and one Nb cathode) during NbN/CrN deposition. The results shown in Fig. 6 indicate that intermixing of chromium and niobium increases as periodicities decrease, most probably due to cross-contamination during deposition. The composition modulation persisted, even for the thinnest studied periodicity. The maximum composition amplitude increases with increasing periodicity; while for the 20 nm periodicity the average normalized compositional differences are as high as 100%, this value is ~ 65 at.% for both chromium and niobium in CrN and NbN individual layers, respectively, when the periodicity is 4 nm. Fig. 6 also shows that for decreasing periodicities, there is an increase of the concentration gradients at the interface. Fig. 4. Nano Beam Diffraction pattern of a 7.5 nm periodicity coating (Fig. 2 (d)), obtained from an area (50 nm diameter) enclosing both NbN and CrN layers.
Fig. 5. Elemental maps obtained by EELS in Spectrum mode. In order (top-to-bottom and left-to-right) the images are STEM micrograph, niobium rich map (green), chromium rich map (red), composite map of chromium + niobium, STEM Spectrum image and two point spectra obtained from niobium rich and chromium rich layers (7.5 nm periodicity). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Relative concentrations of Nb (blue) and Cr (brown) along 4 nm, 7.5 nm, 10 nm and 20 nm periodicities NbN/CrN multilayer coatings. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4.4. Residual stress FEM modeling A FE model was proposed to assess intrinsic elastic stresses, which were considered to be generated by compositional differences and corresponding variations of lattice parameters. To achieve this goal,
the concentration profiles obtained by EELS were fed into the model. The results of the simulations were analyzed by observing in the Xaxis the stresses generated, as shown in Fig. 7, for bilayer periodicities of 20, 10, 7.5 and 4.0 nm. Fig. 8 (a–d) shows the variation of the stress along the coatings for the 20, 10, 7.5 and 4 nm periodicities. The stress reaches maximum positive values (tensile residual stresses) at the center of CrN layers and maximum negative values (compressive residual stresses) at the center of NbN layers. In all cases, the stress equals zero at the NbN/ CrN interfaces. As periodicities decrease, a reduction of the maximum residual stress value is observed inside the CrN and NbN layers. Stress gradients at the coherent interfaces between NbN and CrN layers can be calculated by using Eq. (5), grad σ ¼
Fig. 7. Stress along a 7.5 nm periodicity coating, in the X direction.
σ max Λ=4
ð5Þ
where Λ is the periodicity and σmax is the maximum stress amplitude at the middle of NbN or CrN layers.
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Fig. 8. Stress variation along the coating calculated for the (a) 20 nm, (b) 10 nm, (c) 7.5 nm and (d) 4 nm bilayer thickness.
Table 1 shows the average maximum tensile and compressive stresses calculated by FEM in the middle of CrN and NbN layers for the four modeled periodicities. It also shows the stress gradients calculated at the coherent interfaces using Eq. (5). One can see that decreasing the periodicity leads to an increase in the maximum stresses inside individual layers. On the other hand, it leads to higher stress gradients at the coherent interfaces, which separate NbN and CrN individual layers. Fig. 9 shows the variation of the average stress gradients with the bilayer thickness. A continuous decrease of the average stress gradients at the NbN/CrN interfaces is observed. Fig. 10 shows the results obtained by Araujo et al. [20], indicating that Vickers microhardness of NbN/CrN coatings varies inversely with the bilayer thickness, in the same manner the variation of stress gradients does (Fig. 9). Coherency strengthening hinders dislocation motion, based on the interaction of interface coherency stresses with the elastic stress fields of dislocations [30,31]. This model can also be applied to multilayer structures as reported by Helmerson et al. [32]. Cahn [33] discussed how the mechanical properties of a cubic crystal should be affected by the long-range coherent composition fluctuations resulting from spinodal decomposition. Cahn states that for the same amplitude it matters whether the composition fluctuation is sinusoidal or approaching the square wave characteristic of longer aging. Using this reasoning, not only the maximum stresses found in the NbN and CrN layers play a role on coherency hardening of the coating, but also compositional gradients leading to stress gradients ahead of moving
dislocations. According to Eq. (5), for a given stress amplitude, the stress gradient may be greater or smaller depending on the periodicity of the multilayer coating. Both stress amplitudes and stress gradients can affect hardness, and both of them should be investigated to predict the variations of hardness with periodicity in the investigated coatings. Accordingly, steeper stress gradients may turn dislocation movement more difficult [15,31], when approaching NbN/CrN coherent interfaces. The smaller the periodicity, the larger are the stress gradients developed near the interface, imposing greater stress barriers to dislocation movement. Chawla et al. [11] investigated the interfacial coherency stresses generated in TiN/AlN multilayer films by FEM. In their study, they considered a constant lattice parameter inside each individual layer and did not take into account compositional gradients and lattice parameters variations inside the layers. As a result, they obtained square waves coherency stress profiles and do not mention any compositional gradient effects. Shinn et al. [10] studying TiN/NbN single crystals grown with periodicities varying from 1.6 nm to 450 nm, showed that hardness increased steadily as periodicity decreased in the range 450 nm to 4.6 nm, reaching a maximum hardness of 4600 kgf/mm2. Further decrease in periodicity, from 4.6 to 1.6 nm, led to a decrease in hardness down to 2000 kgf/mm2. The decrease is attributed to a continuous fall of the compositional modulation amplitude with decreasing periodicity, from 10 nm to 1.6 nm, up to very low values where both layers presented almost the same lattice parameter and no coherency stresses would
Table 1 Stress gradients calculated for the four modeled periodicities. Periodicity (nm)
Avg. tensile stress (GPa)
Avg. compressive stress (GPa)
Tensile gradient (GPa/nm)
Compressive gradient (GPa/nm)
Avg. stress gradient (GPa/nm)
20 10 7.5 4.0
4.88 7.15 5.40 5.38
−11.80 −8.90 −8.11 −6.26
0.97 2.86 2.88 5.38
−2.36 −3.56 −4.32 −5.85
1.66 3.21 3.60 5.61
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smaller the periodicity, the smaller are the maximum stresses at the center of NbN and CrN individual layers. On the other hand, the smaller the periodicity, the larger are the average stress gradients at the interface. This result is discussed considering the effect of stress barriers to movement of dislocations approaching NbN/CrN interfaces. The proposed model is consistent with former experimental results, which indicate that the smaller the periodicity, the greater the Vickers microhardness measured on coatings with periodicities ranging from 4 nm to 20 nm. Acknowledgments
Fig. 9. Variation of average stress gradient at the NbN/CrN interfaces with periodicity.
be expected. On the other hand, coherency strains obtained by measuring interplanar spacing by X-ray diffraction increased steadily with decreasing periodicity from 10 to 1.6 nm. The results obtained by EELS analysis and FEM suggest a separate effect of the concentration gradients exerted by coherent interfaces on the stress fields of dislocations, as discussed above. The results also suggest the possibility of varying the deposition rate and the rotation speed of the table to get a finer control of both the maximum compositional amplitudes and the stress gradients, allowing a better control of mechanical properties of this kind of multilayer coatings. 5. Conclusions The compositional modulation along four CAPVD multilayer NbN/ CrN coatings with different periodicities (20, 10, 7.5 and 4 nm) was investigated by HRTEM – High Resolution Transmission Electron Microscopy and EELS – Energy Loss Spectroscopy using STEM – Scanning Transmission Microscopy. HRTEM revealed CrN and NbN layers, grown with a high degree of crystallinity and a strong alignment, separated by coherent NbN/CrN interfaces. Chemical composition modulation measured by EELS showed that the higher the rotational speed of the CAPVD table, the lower was the periodicity, leading to greater cross-contamination during deposition. In the 4 nm periodicity coating, NbN layers contain as much as 15% Cr, while CrN layers contain as much as 15% Nb. A Finite Element Model – FEM fed with the results of compositional modulation allowed calculating the intrinsic residual stresses due to variations on lattice parameters. The results show that intrinsic residual stresses inside NbN and CrN layers vary with composition modulation, which is due to cross-contamination. Maximum tensile and compressive stresses were found in the center of NbN and CrN layers, respectively, while the residual stresses at the coherent interfaces are zero. The
Fig. 10. Variation of Vickers microhardness with periodicity. [Adapted from reference [20]].
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Modeling intrinsic residual stresses built-up during growth of nanostructured multilayer NbN/CrN coatings Juliano Avelar Araujo a, Rafael Arthur Reghine Giorjão a,b, Jefferson Bettini b, Roberto Martins Souza c, André Paulo Tschiptschin a,b,⁎ a
Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, 05508-030 São Paulo, SP, Brazil LNNano - Brazilian Nanotechnology National Laboratory – CNPEM - Brazilian Center for Research in Energy and Materials, R. Giuseppe Máximo Scolfaro, 10000 - Polo II de Alta Tecnologia, 13083-970 Campinas, SP, Brazil c Mechanical Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2231, 05508-030 São Paulo, SP, Brazil b
a r t i c l e
i n f o
Article history: Received 14 April 2016 Revised 19 July 2016 Accepted in revised form 24 July 2016 Available online xxxx Keywords: Multilayer coatings Nanostructure Intrinsic residual stresses EELS chemical analysis Finite element modeling
a b s t r a c t Composition modulated NbN/CrN nanostructured multilayer coatings were deposited onto martensitic stainless steel. The coatings were obtained by cathodic arc process, in an industrial-size physical vapor deposition (CAPVD) chamber. The coatings reached 15 μm thick, consisting of multilayers of different periodicities (4 nm ≤ Λ ≤ 20 nm), which were achieved by varying the rotating speed of the samples in a 2-fold rotation table. The obtained coatings were characterized by High Resolution Transmission Electron Microscopy (HRTEM) and Selected Area Diffraction (SAD) in order to assess the crystal structure, epitaxy and degree of coherency of the NbN/CrN interface. The results showed that the coatings were formed by alternate individual layers of NbN and CrN separated by highly coherent interfaces, and revealed cross-contamination between the layers. Compositional variation measured across the multilayers, by Electron Energy Loss Spectroscopy (EELS) showed that the lower the coating periodicity (Ʌ), the greater is the cross-contamination in the NbN and CrN individual layers. A Finite Element Analysis (FEA) model was developed for assessing the intrinsic residual stresses due to differences of lattice parameters of the NbN and CrN structures, based on chemical composition gradients obtained from EELS measurements. The results show the build-up of stress gradients from the center of individual layers towards the interface, differently from other results published in literature. The smaller the periodicity the higher are the stress gradients developed near the NbN/CrN interfaces. The results are discussed based on the possible interaction of dislocations with stress fields developed near the NbN/CrN interfaces. The proposed model is consistent with former experimental results, which indicates that the smaller the periodicity, the greater is the coating hardness when periodicities range from 4 nm to 20 nm. © 2016 Elsevier B.V. All rights reserved.
1. Introduction CrN/NbN nanostructured multilayer coatings deposited by sputtering processes have been extensively investigated in the last two decades [1, 2]. However, the deposition of this type of coating by cathodic arc process has been understudied, especially regarding the variation of chemical composition of the mutually soluble layers (known as “Composition Modulation”), which are separated by diffused interfaces [3–6]. NbN/CrN nitride multilayers are isostructural and mutually miscible. Mixing of the constituents is likely to happen during deposition, leading to compositional gradients. The degree of mixing is a function of the
⁎ Corresponding author at: Metallurgical and Materials Engineering Department, University of São Paulo, Av. Prof. Mello Moraes 2463, 05508-030 São Paulo, SP, Brazil. E-mail address: [email protected] (A.P. Tschiptschin).
deposition conditions, particularly regarding the cross-contamination between the targets during the process [7,8]. Chu et al. [9] and Shinn et al. [10] describe superlattice structures for polycrystalline (TiN/NbN) coatings deposited by sputtering and using two opposite targets (Ti/Nb). According to the authors, “any intermixing of the layers due to ion irradiation was insufficient to eliminate the composition modulation and the associated hardness enhancement”. The authors observed an increasing on hardness when Ʌ (periodicity) decreased below 14 nm, in agreement with the trend observed for epitaxial nitrides [10]. Chawla et al. [11] studied the stress distribution generated in TiN/ AlN multilayer coating applied by sputtering process using a Finite Element Model (FEM). The authors considered an abrupt stress profile at the sub-layers interface. Yin et al. [12] built a FEM supposing cohesive bonds at the interfaces of TiN/CrN multilayer coating, in order to predict the formation and propagation of micro-cracks under indentation. In
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this model, the authors considered an abrupt interface between the sublayers, as well. Models that provide more accurate predictions have been developed by including the effects of several abrupt interfaces [12,13]. Some works have demonstrated, by dynamic diffraction (satellites peaks), that the composition modulation increases with increasing periodicity [12,14]. Moreover, Chu et al. [15] proposed a model for dislocation movement considering an arbitrary shaped composition modulation (sawtooth modulation) across and within the individual layers. Glow Discharge Optical Emission Spectroscopy (GDOES) and Atomic Emission Spectroscopy (AES) have been used for chemical composition modulation quantification. However, these techniques do not have the accuracy needed to detect compositional gradients in nanostructured multilayer coatings. Zhou et al. [16,17] analyzed the effect of the three-fold rotation movement on the composition modulation of TiAlN/VN deposited by sputtering. They obtained a significant chemical mixing between the layers when the periodicity was ~ 3 nm, as determined by EELS. Furthermore, the EELS technique has proved to be able to reveal the chemical composition profile through these nanostructured layers [18,19]. In this work, an assessment of the chemical modulation across NbN/ CrN multilayer coatings with different periodicities (Ʌ) was carried out. The cross-contamination during deposition resulted in chemical composition gradients, which were fed into a Finite Element Model to calculate intrinsic interfacial coherency stresses. Vegard's law was used to calculate the strain associated with corresponding lattice parameters variations. The effects of the compositional modulation on the intrinsic interfacial coherency stresses are discussed for coatings with four different periodicities. 2. Experimental procedure NbN/CrN nanostructured multilayer coatings were Cathodic Arc Physical Vapor (CAPVD) deposited onto gas nitrided martensitic stainless steel (AISI 440B) coupons in an industrial-size chamber. Three rectangular cathodes (two made of Cr and one of Nb) in alternate positions were fed with their own power supply. By varying the rotating speed of the table in the center of the chamber, the 15 μm thick coating was obtained with four different periodicities Λ, from 4 to 20 nm. The rotating speed controlled the time the substrate pass in front of each Nb or Cr targets in such a way that when it was multiplied by 2, 3 and 5, it lead to the formation of coatings with periodicities of 20 nm, 10 nm, 7.5 nm and 4 nm, respectively. The average deposition rate in all cases was 3 μm/h [20]. Cross sections of the samples were prepared by Focus Ion Beam (FIB) using a FEI Helios 660 dual beam microscope, in order to obtain thinfoils for TEM analysis. TEM analysis was carried out in a JEOL 2100F TEM operating at an accelerating voltage of 200 kV. The microscope was operated both in Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) modes. The TEM mode was primarily used for imaging and diffraction analysis, while STEM mode (STEM probe 0.7 nm – 150 pA) was adopted for image acquisitions alone and when coupled with EELS (Electron Energy Loss Spectroscopy – Gatan Tridiem 863) to generate a Spectrum Image [21]. TEM micrographs were recorded using ES500W (2 k × 2 k pixel) CCD camera and the camera of Gatan Tridiem 863. The EELS analysis was applied for measuring the composition modulation across the coating thickness. The high-resolution composition distribution was achieved by using probes of 0.2 and 0.7 nm with total inelastic mean free path t/ʎ (t = sample thickness, ʎ = total inelastic scattering electron mean free path in the sample) b 0.5 (typically between 0.2 and 0.4). Thus, the enlargement effect of probe size could be ignored in this work [22], since the maximum IMFP (Ineslatic mean free path) is 85 nm (CrN is 138 nm and NbN is 170 nm) and the beam convergence angle is 11 mrad in the case of the 0.7 nm probe. To
avoid a spatial drift during spectra image acquisition, a reference image was defined for drift correction every 4 s (Pixel time of 0.5 s and 2 s for 0.7 and 0.2 nm probes, respectively). 3. FEM modeling and assumptions A linear elastic model was designed using COMSOL finite element software. The model consists of 2D rectangles representing NbN and CrN individual layers. The width of the multilayer was set at 200 nm and the thickness of individual layers varied according to the dimensions measured in the transmission electron microscope. Triangular elements of 0.5 nm were selected for the multilayer region, as shown in Fig. 1. The bottom left corner of the model was pinned to restrict any movement and all other edges were stress free so that bending was permitted. For modeling strains in the NbN/CrN multilayer coating, d-spacings of pure NbN and CrN layers were considered to be those given by ICDD cards #38–1155 and #11–65, respectively. The lattice parameters were 0.439 nm for NbN and 0.415 nm for CrN, thus resulting in a difference around 5.5% between them. d-spacing of the coherent interface was assumed to be the average value between the lattice parameters of the individual sublayers, 0.425 nm. Young's modulus of pure NbN and CrN layers were taken as 348 GPa [23] and 245 GPa [24], respectively. The chemical composition along the coating thickness was provided by the EELS results and the intrinsic stresses were calculated based on a linear variation of lattice parameters with chemical composition, according to the rule of mixtures: ay ¼ aNbN C NbN þ aCrN C CrN y y
ð1Þ
Where, aNbN and aCrN are the lattice parameters of pure NbN and CrN layers respectively, CyNbN and CyCrN are the concentrations of the NbN and CrN layers in a specific region respectively and ay is the lattice parameter calculated in a particular region of the multilayer. Likewise, the elastic modulus in a particular region of the multilayer was calculated using the rule of mixtures: Ey ¼ ECrN C CrN þ ENbN C NbN y y
ð2Þ
Where, ENbN and ECrN are the elastic moduli of pure NbN and CrN layers, respectively, and Ey is the elastic modulus calculated in a particular region of the multilayer. To calculate the stresses, a material strain condition was applied, being calculated based on the local and interfacial lattice parameters. ey ¼
ay −aint aint
ð3Þ
Where aint is the lattice parameter of the NbN/CrN interface and ey is the resulting strain calculated for a particular region of the multilayer. 4. Results and discussion 4.1. TEM experimental results Fig. 2 shows high magnification images of the 7.5 nm periodicity coating, taken along the 15 μm NbN/CrN coating thickness. Image (a) was taken from a region close to the substrate and image (c) taken closer to the coating surface. Fig. 2 indicates that the NbN/CrN multilayer has an almost constant periodicity throughout the whole thickness. Variations on the sublayer thickness is smaller than 9%. Dark layers correspond to the NbN layers and bright ones to the CrN layers. The coating columnar microstructure (widths varying from 50 nm to 130 nm) overlaps the multilayer structure, maintaining the same orientation from layer to layer. The width of these columns may
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Fig. 1. Meshed geometry using triangular elements. (a) 2D rectangles model representing NbN and CrN individual layers. (b) Mesh geometry formed by triangular elements of 0.5 nm.
increase or decrease along the coating thickness, according to the simultaneous growth rule [25,26]. The inlets in Fig. 2 (b) and (d) are FFT (Fast Fourier Transform) images, which allow assessing the multilayer periodicity and the average atomic plane distances. FFT in the Fig. 2 (b) gives a periodicity of 7.5 nm. The detailed analysis of Fig. 2 indicates a 4.0 nm thick CrN phase and a 3.5 nm thick NbN phase. FFT in Fig. 2 (d) also shows that this coating is oriented in a [002] zone axis. The layers in the FFT are not completely flat, which indicates that orientations of the structure planes are changing along the coating thickness. This can be explained by the curvature of the multilayers observed in all images, which is probably due to substrate roughness and to the presence of macroparticles typical of the cathodic arc PVD process. 4.2. Coherent interfaces Fig. 3 shows HRTEM images of the nanostructured NbN/CrN multilayer coating with nominal 7.5 nm and 10 nm periodicities. Fig. 3a and b show 6.8 nm and 9.6 nm periodicities, respectively, thus providing b9% of variation. Measurements of individual layers thickness were carried out in Fig. 3 (a), giving CrN and NbN layers, 3.0 nm and 2.8 nm thick, respectively. The diffuse interface is 1.0 nm thick. In Fig. 3 (b), values for CrN, NbN and interface thicknesses are 3.4 nm, 5.0 nm, 1.2 nm, respectively, therefore indicating that coherent interfaces separate individual NbN and CrN layers. Fig. 4 shows nanodiffraction patterns taken from an area (50 nm diameter) of the 7.5 nm periodicity coating, depicted in Fig. 2 (d), enclosing both NbN and CrN layers. Both phases are face-centered
cubic (fcc) and textured in the [110] direction [27]. A double spot can be observed in the electron diffraction pattern in Fig. 4, implying a strong alignment. This result is an evidence of the presence of unperturbed atomic bonding between the NbN and CrN layers, which can be described as localized coherent growth. As a result, the multilayer coating presents a high degree of crystallinity and coherent interfaces. 4.3. Composition modulation by EELS EELS was applied to obtain high-resolution element distribution profiles for the NbN/CrN nanostructured multilayer with periodicities of 4, 7.5, 10 and 20 nm. In the case of the 7.5 nm periodicity coating, two resolutions of energy loss signals (~0.2 and ~0.7 nm) were applied in order to evaluate the resolution impact on the elemental map for this sort of coatings. The results (not shown here) indicate that probe size effects for 0.2 and 0.7 nm can be neglected. Spectroscopic images were obtained for all samples and quantification was carried out using Nb-M (205 eV) edge and Cr-L (575 eV) edge. Compositional quantification by EELS using L and M shell is more difficult and less reliable than Wavelength Dispersive Spectroscopy (WDS) [28]. Thus, WDS has been used to obtain the right k-factors [29]. The WDS analysis indicates that the coatings are stoichiometric, presenting the following chemical composition: Nb = 20 at.% (±1.0), Cr = 29 at.% (± 1.0) and N = 50 at.% (± 1.0) for all the periodicities studied. Nitrogen content as measured by EELS is around the same concentration 50 at.%, presenting variation of ± 2 at.% between NbN and CrN layers. Then, Cr and Nb compositions were normalized to 100% in the EELS chemical analysis.
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Fig. 2. High magnification images of the 7.5 nm periodicity coating, along the 15 μm thick coating. (a) region close to the substrate; (b) the inlet shows the FFT of the multilayer that allows assessing the periodicity; (c) region close to the coating surface; (d) the inlet shows the FFT that allows calculating average atomic planes distances.
Fig. 5 shows the STEM image of the coating with 7.5 nm periodicity , the Nb, Cr and composite maps, the spectral image obtained by EELS in the reference showed in the STEM image and EELS spectrums in the CrN and NbN rich layers. The Cr and Nb elemental distributions in the nanostructured CrN/NbN multilayer were evaluated by taking a line profile across the layers in the acquired images and further quantifying the concentrations. For all samples, two or three maps were obtained along the coating thickness.
The two point spectra were taken in positions 1 and 2 in the spectrum image. Position 1 was supposed to be a pure NbN layer as it was taken in the center of the bright layer. However, one can see a Cr L (edge) peak in the spectrum image for this position. Conversely, a Nb M (edge) peak is also observed in Position 2 spectrum image, which was supposed to be a pure CrN layer. These results clearly indicate Cr and Nb atoms intermixing in the layers, that can be related to cross-contamination during deposition.
Fig. 3. HRTEM images of the multilayer NbN/CrN coatings with (a) 7.5 periodicity and (b) 10.0 nm periodicity.
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Fig. 6 shows the relative concentrations of NbN and CrN along a line scan ~25 nm long. The chromium content tends to be dominant, since concentration levels reach up to 100 at.% in the CrN layers for the 10 nm and 20 nm periodicities coatings. This can be explained by the cathode configuration (two Cr cathodes and one Nb cathode) during NbN/CrN deposition. The results shown in Fig. 6 indicate that intermixing of chromium and niobium increases as periodicities decrease, most probably due to cross-contamination during deposition. The composition modulation persisted, even for the thinnest studied periodicity. The maximum composition amplitude increases with increasing periodicity; while for the 20 nm periodicity the average normalized compositional differences are as high as 100%, this value is ~ 65 at.% for both chromium and niobium in CrN and NbN individual layers, respectively, when the periodicity is 4 nm. Fig. 6 also shows that for decreasing periodicities, there is an increase of the concentration gradients at the interface. Fig. 4. Nano Beam Diffraction pattern of a 7.5 nm periodicity coating (Fig. 2 (d)), obtained from an area (50 nm diameter) enclosing both NbN and CrN layers.
Fig. 5. Elemental maps obtained by EELS in Spectrum mode. In order (top-to-bottom and left-to-right) the images are STEM micrograph, niobium rich map (green), chromium rich map (red), composite map of chromium + niobium, STEM Spectrum image and two point spectra obtained from niobium rich and chromium rich layers (7.5 nm periodicity). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Relative concentrations of Nb (blue) and Cr (brown) along 4 nm, 7.5 nm, 10 nm and 20 nm periodicities NbN/CrN multilayer coatings. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4.4. Residual stress FEM modeling A FE model was proposed to assess intrinsic elastic stresses, which were considered to be generated by compositional differences and corresponding variations of lattice parameters. To achieve this goal,
the concentration profiles obtained by EELS were fed into the model. The results of the simulations were analyzed by observing in the Xaxis the stresses generated, as shown in Fig. 7, for bilayer periodicities of 20, 10, 7.5 and 4.0 nm. Fig. 8 (a–d) shows the variation of the stress along the coatings for the 20, 10, 7.5 and 4 nm periodicities. The stress reaches maximum positive values (tensile residual stresses) at the center of CrN layers and maximum negative values (compressive residual stresses) at the center of NbN layers. In all cases, the stress equals zero at the NbN/ CrN interfaces. As periodicities decrease, a reduction of the maximum residual stress value is observed inside the CrN and NbN layers. Stress gradients at the coherent interfaces between NbN and CrN layers can be calculated by using Eq. (5), grad σ ¼
Fig. 7. Stress along a 7.5 nm periodicity coating, in the X direction.
σ max Λ=4
ð5Þ
where Λ is the periodicity and σmax is the maximum stress amplitude at the middle of NbN or CrN layers.
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Fig. 8. Stress variation along the coating calculated for the (a) 20 nm, (b) 10 nm, (c) 7.5 nm and (d) 4 nm bilayer thickness.
Table 1 shows the average maximum tensile and compressive stresses calculated by FEM in the middle of CrN and NbN layers for the four modeled periodicities. It also shows the stress gradients calculated at the coherent interfaces using Eq. (5). One can see that decreasing the periodicity leads to an increase in the maximum stresses inside individual layers. On the other hand, it leads to higher stress gradients at the coherent interfaces, which separate NbN and CrN individual layers. Fig. 9 shows the variation of the average stress gradients with the bilayer thickness. A continuous decrease of the average stress gradients at the NbN/CrN interfaces is observed. Fig. 10 shows the results obtained by Araujo et al. [20], indicating that Vickers microhardness of NbN/CrN coatings varies inversely with the bilayer thickness, in the same manner the variation of stress gradients does (Fig. 9). Coherency strengthening hinders dislocation motion, based on the interaction of interface coherency stresses with the elastic stress fields of dislocations [30,31]. This model can also be applied to multilayer structures as reported by Helmerson et al. [32]. Cahn [33] discussed how the mechanical properties of a cubic crystal should be affected by the long-range coherent composition fluctuations resulting from spinodal decomposition. Cahn states that for the same amplitude it matters whether the composition fluctuation is sinusoidal or approaching the square wave characteristic of longer aging. Using this reasoning, not only the maximum stresses found in the NbN and CrN layers play a role on coherency hardening of the coating, but also compositional gradients leading to stress gradients ahead of moving
dislocations. According to Eq. (5), for a given stress amplitude, the stress gradient may be greater or smaller depending on the periodicity of the multilayer coating. Both stress amplitudes and stress gradients can affect hardness, and both of them should be investigated to predict the variations of hardness with periodicity in the investigated coatings. Accordingly, steeper stress gradients may turn dislocation movement more difficult [15,31], when approaching NbN/CrN coherent interfaces. The smaller the periodicity, the larger are the stress gradients developed near the interface, imposing greater stress barriers to dislocation movement. Chawla et al. [11] investigated the interfacial coherency stresses generated in TiN/AlN multilayer films by FEM. In their study, they considered a constant lattice parameter inside each individual layer and did not take into account compositional gradients and lattice parameters variations inside the layers. As a result, they obtained square waves coherency stress profiles and do not mention any compositional gradient effects. Shinn et al. [10] studying TiN/NbN single crystals grown with periodicities varying from 1.6 nm to 450 nm, showed that hardness increased steadily as periodicity decreased in the range 450 nm to 4.6 nm, reaching a maximum hardness of 4600 kgf/mm2. Further decrease in periodicity, from 4.6 to 1.6 nm, led to a decrease in hardness down to 2000 kgf/mm2. The decrease is attributed to a continuous fall of the compositional modulation amplitude with decreasing periodicity, from 10 nm to 1.6 nm, up to very low values where both layers presented almost the same lattice parameter and no coherency stresses would
Table 1 Stress gradients calculated for the four modeled periodicities. Periodicity (nm)
Avg. tensile stress (GPa)
Avg. compressive stress (GPa)
Tensile gradient (GPa/nm)
Compressive gradient (GPa/nm)
Avg. stress gradient (GPa/nm)
20 10 7.5 4.0
4.88 7.15 5.40 5.38
−11.80 −8.90 −8.11 −6.26
0.97 2.86 2.88 5.38
−2.36 −3.56 −4.32 −5.85
1.66 3.21 3.60 5.61
Please cite this article as: J.A. Araujo, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.07.108
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smaller the periodicity, the smaller are the maximum stresses at the center of NbN and CrN individual layers. On the other hand, the smaller the periodicity, the larger are the average stress gradients at the interface. This result is discussed considering the effect of stress barriers to movement of dislocations approaching NbN/CrN interfaces. The proposed model is consistent with former experimental results, which indicate that the smaller the periodicity, the greater the Vickers microhardness measured on coatings with periodicities ranging from 4 nm to 20 nm. Acknowledgments
Fig. 9. Variation of average stress gradient at the NbN/CrN interfaces with periodicity.
be expected. On the other hand, coherency strains obtained by measuring interplanar spacing by X-ray diffraction increased steadily with decreasing periodicity from 10 to 1.6 nm. The results obtained by EELS analysis and FEM suggest a separate effect of the concentration gradients exerted by coherent interfaces on the stress fields of dislocations, as discussed above. The results also suggest the possibility of varying the deposition rate and the rotation speed of the table to get a finer control of both the maximum compositional amplitudes and the stress gradients, allowing a better control of mechanical properties of this kind of multilayer coatings. 5. Conclusions The compositional modulation along four CAPVD multilayer NbN/ CrN coatings with different periodicities (20, 10, 7.5 and 4 nm) was investigated by HRTEM – High Resolution Transmission Electron Microscopy and EELS – Energy Loss Spectroscopy using STEM – Scanning Transmission Microscopy. HRTEM revealed CrN and NbN layers, grown with a high degree of crystallinity and a strong alignment, separated by coherent NbN/CrN interfaces. Chemical composition modulation measured by EELS showed that the higher the rotational speed of the CAPVD table, the lower was the periodicity, leading to greater cross-contamination during deposition. In the 4 nm periodicity coating, NbN layers contain as much as 15% Cr, while CrN layers contain as much as 15% Nb. A Finite Element Model – FEM fed with the results of compositional modulation allowed calculating the intrinsic residual stresses due to variations on lattice parameters. The results show that intrinsic residual stresses inside NbN and CrN layers vary with composition modulation, which is due to cross-contamination. Maximum tensile and compressive stresses were found in the center of NbN and CrN layers, respectively, while the residual stresses at the coherent interfaces are zero. The
Fig. 10. Variation of Vickers microhardness with periodicity. [Adapted from reference [20]].
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