The influence of Zr layer thickness on contact deformation and fracture in a ZrN–Zr multilayer coating
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- Verma, N. & Jayaram, V. J Mater Sci (2012) 47: 1621. doi:10.1007/s10853-011-6001-y
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In order to understand the influence of ductile metal interlayer on the overall deformation behavior of metal/nitride multilayer, different configurations of metal and nitride layers were deposited and tested under indentation loading. To provide insight into the trends in deformation with multilayer spacings, an FEM model with elastic-perfect plastic metal layers alternate with an elastic nitride on top of an elastic–plastic substrate. The strong strain mismatch between the metal and nitride layers significantly alters the stress field under contact loading leading to micro-cracking in the nitride, large tensile stresses immediately below the contact, and a transition from columnar sliding in thin metal films to a more uniform bending and microcracking in thicker coatings.
Metal/nitride composite multilayer coatings can offer advantages over monolithic nitrides if they display higher toughness without compromising on hardness significantly [1, 2]. Metal interlayers are reported to enhance adhesion by reducing residual stress levels of the overall coating owing to shear deformation of metal, corrosion resistance by interrupting the corrosion path through the nitride at each interlayer and also possibly change the texture, in the case of TiN, to minimize the interfacial energy with the interlayer [3–9]. Different Ti through thickness distributions were checked for their influence on residual stress with constant, increasing, and decreasing metal content toward coating/substrate interface. Adhesion is not only dependent on total amount of metal but also dependent on the metal distribution and it is best for increasing metal amount near coating/substrate interface . These coating are deposited by many processes such as closed field unbalanced magnetron sputtering, dual ion beam sputtering and pulsed laser deposition. Many such systems have been studied, including metal/nitride systems such as TiN/Ti, ZrN/Zr, TiN/Ag, Al/SiC, ZrN/W, CrN/Cr, CrN/Cu, AlN/Al, TaN/Ta, and Al/Al2O3 [4–18]. The first metal/nitride coating was TiN/Ti . The microstructure of these coatings are mostly columnar with sharp interfaces and the nitride renucleates at each interface except in case of epitaxial ZrN/W. Nitrogen is believed to be present in the metal layer depending on its solubility and very thin layers degrade the multilayer mechanical properties due to the lack of sharp interfaces. In case of CrN/Cr multilayers, the microstructure coarsens as the layer becomes finer and less defective metal layers form due to the higher diffusivity of metal . AlN/Al shows that microstructure become coarser with larger layer thickness of AlN and better crystallization of layers which increases the surface roughness of film with increase in layer thickness. A preferred orientation (111) is seen in case of ZrN and TiN multilayers with their respective metals due to the large number of interfaces that relieve residual stresses. This orientation (111) is seen to show the best mechanical properties. Friction coefficient gets reduced in case of metal/nitride coatings and wear life increases due to metal ductility . ZrN/W show roughening of layers owing to kinetic effect of residual stress relaxation . Delamination of W from underlying ZrN layers is seen due to compressive stress in W. With decrease in layer spacing increase in residual stress is observed. Metal/nitride systems are believed to be more stable at high temperature as they can be non isostructural and immiscible compared to other nitride/nitride systems which degrade due to diffusional intermixing of layers .
Nanoindentation is being extensively used to extract the hardness and modulus of these coatings [24, 25] and correlating it with bilayer periodicity [26, 27] with the advantage of probing a small volume to extract only the properties of coating. Hardness and modulus always show dependence on volume fraction and bilayer periodicity . Hardness in all these systems are shown to increase with lower volume fraction of metal and with finer layer spacing it gets enhanced to above 40% more than the rule of mixture value . Modulus is mostly a function of volume fraction and shows no dependence on bilayer spacing . Different metals were also used to study the influence of different crystal structure and nitrogen reactivity on mechanical properties. Al(FCC), Ti(HCP) both having reactivity to nitrogen and Cu(FCC) with no reactivity to nitrogen were chosen for the study. All coatings were columnar and columns become continuous with finer layer spacing. One interlayer is sufficient to reduce the hardness and Ti interlayer did not reduce the hardness which might be due to N absorption during deposition. The residual stress in case of Al and Ti increases with number of interfaces but the opposite happens in case of Cu due to less reactivity of Cu with N. Hence Cu can deform and reduce compressive residual stress .
Tensile test on Al/Al2O3 showed that the strength was related to bilayer spacing and followed the Hall–Petch relation, with yield strengths ~μ/70 where μ is shear modulus of Al . The modulus is reported to be lower than the weighted average of layers and believed to be due to lower density and microcracking in the alumina layer. In case of indentations in Al/SiC multilayers, void formation in Al layers and shear banding was reported which was attributed to the constraint offered to deformation of soft phase by the harder SiC . It is shown in case of these coatings that the nitride grows with columnar grains and metal does not allow the columnar grains of nitride to grow throughout the whole coating. Columnar sliding is the common mode of deformation in these coatings which is greatly affected by the presence of metal interlayers as the columns are obstructed by the interlayer from sliding and so provide more resistance to deformation . There are reports on TiN/Al multilayers where the fraction of metal nitride was varied and coating of higher metal content was tested. The deformation was seen to be cooperative and the stiff nitride layer was also shown to undergo huge bending with equal volume fraction of nitride and metal and at very small length scales. In all other combinations of metal and nitride, the metal plastically deformed and caused the nitride to deform by cracking at higher layer spacing and shear banding at thinner layer spacing with unequal volume fractions of phases .
FEM has been used to evaluate the complex stress state under indentation in these composite coatings successfully. For example, it has been reported  that plastic flow in the metal during unloading can lead to changes in the calculated modulus using standard techniques. Metal plastically deforms and the modulus cannot be evaluated simply by unloading curve. Numerical simulations using HAFILM codes, which can simulate large plastic deformation and rotation, have reported the influence of interlayer properties and thickness on hardness, modulus and the ratio of H/E which is a measure of the propensity for cracking. The hardness and modulus decreases with increasing number of interlayers and H/E have no influence of number of interlayers for Ti interlayer [28, 29]. FEM calculations of nanoindentations on Al/SiC  revealed that the composite elastic properties may be obtained beyond an indentation depth of 8–10 bilayer spacings. It is reported that in case of a multilayer of alternate soft and hard layers the major shearing take place in soft layers and the hard layers merely slide over each other and so do not experience large bending stresses . Thus, the metal/nitride coating can show better performance over a single hard monolayer.
Despite extensive numerical analyses, the links with detailed microscopical observations are fewer [32, 33]. In the transition metal mononitride system, a further complicating factor is the tendency to form secondary nitrides, e.g., Ti2N, Cr2N, etc. One of the rare combinations that are stable is Zr–ZrN. Consequently, we have chosen this particular system to examine the role of multilayering on contact deformation supplemented with FEM calculations to understand the role of the stress field.
The ZrN is assumed to be elastic as the yield strength of these materials is very high compared to that of metal. The indenter is assumed to be perfectly rigid and all interfaces are perfectly bonded between all the layers. The yield stress for substrate used was taken to be 333 MPa and the strain hardening behavior was taken from literature . The FEM indentation was done under load control as done in the real situation with a maximum of 0.2 N in all cases.
Results and discussion
Nitride–metal multilayers display a transition with metal layer thickness in the mode of deformation beneath indentations which can be captured using FEM. Only columnar sliding is seen in case of monolayer and 30 nm metal layers while thicker layers display increasing amounts of microcracking in the nitride.
Plastic flow in the metal can generate very high tensile stresses in the nitride. These lead to tensile stresses even directly under the indentation where monolayers generally display high compressive stresses. The interface from which microcracks in the nitride initiate reverse as one moves toward the indentation periphery in agreement with the predictions from FEM.
The authors are grateful to Defense Research & Development Organization (Govt. of India) for financial support.