ICMCTF2013 Session TS2-2: Advanced Characterization of Coatings and Thin Films

In recent years it has become current practice to employ a wide range of test methods to assess the adhesive properties of thin films and coatings, ranging from the simple tape-peel test to the more advanced pull-off and 3-point bending tests. Many such methods have high operator subjectivity, high data variability and in many cases are unsuitable for industrial Quality Control (QC) use where assessment is done regularly and often by several operators. The Rockwell indentation test (ISO 26443 and VDI 3198) has become common for evaluating the adhesion of hard coatings deposited on various substrates, but the applied load (1472 N) is often too high causing the 200 µm radius diamond indenter to plunge too deeply into the substrate making it difficult to classify the resultant coating damage around the edges. The progressive load scratch test (ASTM C1624, ISO EN 1071-3, ISO 20502) is now widely used to characterize the adhesion of thin hard coatings with high repeatability. However, a direct correlation between these two methods has never been attempted.

It has been proposed that a smaller radius diamond indenter (possibly 50 µm radius) could be employed with the Rockwell indentation test, using a lower applied load, in order to concentrate the maximum stress nearer to the sample surface causing more focused failure of the coating-substrate interface. The objective of this study is to fabricate samples representative of the 6 failure categories of the standard Rockwell test, to then evaluate the same coatings with lower load and smaller indenter radius, and thirdly to test the same samples with the progressive load scratch test. It is hoped that this could lead to better correlation of data obtained with all such methods and aid industry in better QC control of their coatings.

Caused by the many effects taking place during a general tribological test it is a relatively ambiguous goal to create not just a tribo-model or simulation tool, but to make it fit for inversion and thus, analysis of such tribological tests. The talk features an approach where tribo parameters, like Archard's wear depth parameter kd, are extracted from effective interaction potentials [1], which themselves are built up and fed from more physical-oriented measurements like Nanoindentation and PHYSICAL scratch. By using such effective material potentials one can derive critical loading situations leading to failure (decomposition strength). A subsequent connection of these decomposition or failure states with the corresponding stress or strain distributions allows the development of rather comprehensive tribological parameter models applicable in wear and fatigue simulations as demonstrated in this work.

From this a new global increment wear model has been developed on the basis of the effective indenter concept [1, 2] by using the extended Hertzian approach [3]. The models do not only allow to analyze certain tribological experiments like the well known pin-on disk test or the more recently developed nano-fretting tests, but also to forward simulate such tests and even give hints for better component life-time predictions. Within the talk a few examples will be shown.

[1] N. Schwarzer, Short note on the effect of pressure induced

increase of Young's modulus, Phil. Mag., submitted July 2011

[2] N. Schwarzer, G. M. Pharr: „On the evaluation of stresses during

nanoindentation with sharp indenters”, Thin Solid Films, Vol. 469-470C pp.

194-200

[3] N. Schwarzer, T. Chudoba, G. M. Pharr: „On the evaluation of

stresses for coated materials during nanoindentation with sharp indenters”, Surf. Coat. Technol, Vol 200/14-15 pp 4220-4226

[4] N. Schwarzer: "The extended Hertzian theory and its uses in

analysing indentation experiments", Phil. Mag. 86(33-35) 21 Nov - 11 Dec

2006 5153 – 5767, Special Issue: “Instrumented Indentation Testing in Materials Research and Development”

Radio Frequency (RF) glow discharge optical emission spectroscopy (GD-OES) has been developed in our laboratory to characterize various coatings and thin films. Thick dielectrics (insulators) such as anodized coatings, typically > 40 µm, are used in semiconductor, coating equipment, defense, aerospace, automotive, architectural, medical, marine, sporting goods, home appliances, and recreation. These thick coatings can be analyzed by RF GD-OES for elemental composition, spatial elemental distribution, and trace impurities. The use of RF plasma for material sputtering, excitation and ionization avoids many intrinsic limitations associated with traditional electron beam, ion beam or x-ray techniques. Deep depth profiling a > 40 µm anodized coating (e.g. Type III hard coatings) throughout its entire thickness down to its substrate at nm depth resolution can only be accomplished by RF GD-OES.

With simultaneous multi-element (40 or more including H, C, N, O, Na, Mg, Al, P, S, Cl, Fe…) profiling capability (real-time profiling), RF GD-OES has been successfully used to examine coating uniformity, interfacial contamination, and surface stoichiometry (<10 nm). The RF GD-OES results demonstrate the usefulness of this technique in optimizing anodization processes, facilitating base material (aluminum alloys) selection, controlling surface and interfacial contamination, and preventing premature failure of an anodized coating or subsequent layer. The signal intensity produced by RF GD-OES has a simple and well-defined mathematical (linear) relationship with elemental concentration in a material. A wide linear dynamic range (over six orders of magnitude) possessed by RF GD-OES coupled with various NIST traceable material standards developed in our laboratory have made accurate surface, interfacial and bulk analyses possible.

Thin nanocrystalline coatings featuring outstanding mechanical properties have gained much interest in recent years. The application of deposition techniques operating far from the thermodynamic equilibrium results in the formation of pronounced depth inhomogeneities in microstructure and strain over coating thickness compared to their bulk counterparts. Thus, instead of surface sensitive parameters obtained using indentation techniques, knowledge about their mechanical behaviour is required in order to assess the relationship between process conditions, thickness-dependent coating properties and functional behaviour. The presented approach is a step forward in the characterization of thickness-averaged mechanical properties of crystalline coatings at the micrometer scale. It consists of an etching step of the substrate to isolate the coating without affecting its interface to surface inhomogeneity, thus preserving its nucleation zone. Afterwards, a rectangular cantilever is fabricated by cutting the free-standing coating with a focused gallium ion beam. The cantilever beam is loaded by an indenter, where load-displacement curves are recorded until fracture. Then, the fracture surfaces are evaluated by scanning electron microscopy. Using this approach, the fracture mechanism can be identified and linked to coating microstructure and residual stress state. The influence of deposition parameters, annealing treatments and coating architectures on the fracture properties of representative CrN and TiAlN coatings will be demonstrated. The approach allows the identification of mechanisms, which are responsible for the deterioration of the coating performance and which are often not well understood or remain unrevealed by other characterization techniques.

Titanium and Chromium based nitrides find widespread applications as wear resistant coatings on cutting and forming tools. There have been continuous efforts to enhance their performance through various means, including designing their architecture in the form of nanocrystalline, nanocomposite or multilayer coatings. The mechanical properties of these coatings are significantly influenced by the crystal structure, microstructure and composition of the coating. Consequently, there exists an opportunity to control the coating performance by either tailoring the composition, through addition of alloying elements to achieve different stoichiometry, or the microstructure, through introduction of multilayers and nanosized grains. In the case of multilayer coatings, deposition of alternate layers with varying modulus is known to enhance crack resistance and, in turn, the toughness of the coating.

In the present study, Ti-Al-Cr-N based coatings were deposited by cathodic arc evaporation in monolayer and multilayer configurations on tool material substrates. The Al and Cr percentage in the coatings was varied in the range 0-65% by suitably manipulating the process parameters and four different compositions were comprehensively investigated. XRD analysis was carried out to ascertain the crystal structure while microstructural characterization was carried out by FIB-SEM and TEM to observe the variation in grain structure on the surface and cross section of the coatings. Stresses were measured employing an X-ray residual stress analyzer. Mechanical characterization was done using nanoindentation to determine the hardness and modulus, and scratch testing to assess adhesion. In situ scanning probe microscopy was also used to map the mechanical properties of the multilayer coatings. The hardness and modulus of the coatings were found to vary with Al and Cr percentage and correlated with the plasticity, toughness and residual stresses present in the coatings. Results suggest that there exists a critical Al content at which a maximum in hardness is obtained. Furthermore, the variation in sub-surface deformation during scratch testing and toughness with changing composition, and due to the multilayer configuration in the coating, was studied. Mapping of modulus variation between different layers of the multilayer coatings revealed that a substantial modulus mismatch existed between alternate layers that could lead to enhanced toughness. The implications of this study in enabling identification of a suitable coating composition and architecture to yield properties desirable for industrial applications will also be discussed.