Modeling intrinsic residual stresses built-up during growth of nanostructured multilayer NbN/CrN coatings
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], [4], [5], [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 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 this model, the authors considered an abrupt interface between the sub-layers, 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.
Section snippets
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
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
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
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
Acknowledgments
The authors acknowledge the supports of CNPq processes n. 151653/2010-0, 481918/2010-8 and 486104/2012-5, FAPESP process n. 2012/50890-0 and the University of São Paulo for supporting the Center for Research on Tribology and Surface Engineering – NAP – TRIBES.
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