Characterization of mechanical properties of thin films using nanoindentation test

Using finite element modeling (FEM), this work investigates using finite element modeling (FEM) the mechanical behavior of film on substrate composites during the penetration of a rigid tip. In order to understand the magnitude of the substrate effect, the difference of strain gradient through the thickness of a given layer, deposited first on a softer substrate and then on an harder substrate can be observed. In this specific case, up to a critical ratio (h/t) = 0.35 (with h the indentation depth and t the film thickness), the mechanical behavior of the layer is quite similar. But, for h/t > 0.35, two different behaviors may be observed: (i) in the first case H_f/H_S  1 (with H_f and H_S, respectively, the film and substrate hardness values), the total strain remains contained within the film thickness up to a ratio h/t close to 1 and (ii) in the second case H_f/H_S  1, the total strain extends deeply into the substrate. These results show that the empirical 10% rule is not valid, even for a hard film on a softer substrate. The main error is caused by a wrong estimation of the contact depth between the indenter tip and the film surface. Indeed, the simulation runs exhibit the formation of pile-up depending on the ratios (h/t) and Y_f/Y_S (with Y_f and Y_S, respectively, the film and substrate yield stress values). As a function of the used model for calculating the contact depth, at least three variation of hardness may be found from load–displacement curves obtained by FEM. In these conditions, it seems ambiguous to try to determine a weighting function to extract meaningful mechanical properties of the thin film. Another way to determine film properties consists in using the loading phase. A relationship between the applied load (P) and the indentation depth (h) is studied during the loading phase. For the case of a soft film on harder substrate (H_f/H_S  1), it is possible to determine the yield stress of the film, from the previous relationship. This approach is applied to experimental amorphous Al₂O₃ films formed by electron beam evaporation on silicon substrate.     

Characterisation of nanolayered aluminium/palladium thin films using nanoindentation

Structure, hardness, and elastic modulus of nanolayered aluminium/palladium thin films, with individual layer thickness varying from 1 nm to 40 nm, were investigated using transmission electron microscopy (TEM) and nanoindentation. TEM micrographs indicated a sharp but not flat Al-Pd interface. With just 6.5% (v/v) Pd a hardness enhancement of ~ 200% was observed for nanolayered Al/Pd compared to the hardness of pure Al film. A maximum hardness enhancement of up to 350% was observed for nanolayered Al/Pd samples compared to the hardness of pure Al film when bilayer thickness was 2 nm and Pd was 50% (v/v). Modulus enhancement was also observed for the nanolayered thin films.     

Assessment of the toughness of fibre-reinforced concrete using the R-curve approach

The fracture response of brittle materials like concrete can be characterised using modified linear elastic fracture mechanics models, such as the rising resistance curve (R-curve). In this study, the R-curve, determined by utilising the experimental response in the notched beam test with mode I fracture, is proposed as a measure of the toughness of fibre-reinforced concretes (FRCs). The unambiguity of the R-curve obtained is assessed by comparing the predicted flexural response of specimens of different geometries with experimental data. The variation in the R-curves with the dosage of steel and polymer fibres is also discussed.     

Fracture mechanics analysis of thin coatings under spherical indentation

Spherical indentation of a thin, hard coating bonded to a thick substrate is investigated. The bending of the coating over the softer substrate induces concentrated tensile stresses on the lower and upper coating surfaces, from which transverse cracks may ensue. This work is primarily concerned with ring cracks originating from the top surface of the coating. In-situ indentation tests are carried out on a model glass/polycarbonate bi-layer, with the coating thickness and the indenter radius being the main test variables. As the coating thickness is decreased, the critical load to initiate ring cracks progressively departs from that associated with a critical surface stress, the effect that increases with increasing the indenter radius. A fracture mechanics approach in conjunction with the FEM technique is used to elucidate the onset of cylindrical ring cracks in thin-film bi-layer structures due to spherical indentation. The analysis, conducted as a function of the coating thickness and the indenter radius, reveals the existence of bending-induced compression stress regions ahead of the crack tip, which tend to shield the crack or increase the fracture resistance. The specific behavior is dictated by a complex interplay between the contact radius, a, the coating thickness, d, and the crack length, c. An interesting manifestation of this shielding mechanism is that when the coating surface contains flaws of various sizes, small flaws in this population may be more detrimental than large ones. Incorporation of this aspect into the analysis led to a good correlation with the experimental results. In the limit case of point-load, a closed-form, approximate solution for the stress intensity factors and the critical loads is obtained. This solution constitutes a lower bound for the critical loads, and is furthermore directly applicable to finite size indenters provided da. In the limit c/d/to0, a failure stress criterion may be used irrespective of the ball radius, r. The analysis in this case reveals that decreasing either d/r or the coating/substrate modulus ratio tend to favor ring cracking over radial type cracking. The transition between these two failure modes is identified explicitly as a function of the system parameters.     

Assessment of bioengineering ceramic coatings using the wetting method

The wettability of coatings, including ceramic ones, which show considerable promise for the use on bioengineering products, with physical solution (0.9% NaCl) have been studied. It has been found that the use of coatings of all types under study increases the wetting angles on the surface as compared with the initial metal materials (stainless steel of the 12X18H10T grade, titanium alloy of the BT6-grade, Co-Cr-Mo alloy), which serves as a prerequisite for an improvement in the biocompatibility of implants. The degree of the coating bioinertness increases in the following order: titanium nitride → diamond carbon films → aluminum nitride → titanium oxide (anodic oxide film) → nitride (oxidized) → oxide → titanium oxide (anodic-spark coating).     

Analysis of coatings and thin films using energetic ions

Energetic ion analysis techniques provide non-destructive information on the depth distribution of atomic composition in the near-surface (1–10 μm) region of a solid sample. The techniques are quantitative and are not complicated by the presence of chemical or matrix effects. Generalized nuclear reaction analysis is described and its application to the measurement of the stoichiometry of Ta₂O₅ films and BeO coatings on Cu-Be is briefly discussed.     

Measurement of mechanical properties of multilayer coatings by nanoindentation

Abstract

Multilayer protective coatings of alternate aluminium and titanium diboride TiB₂ layers have been tested by nanoindentation to measure both hardness and Young's modulus values. The initial results show that the values obtained depend upon the depth of indentation. An alternative view is presented to show that by considering the percentage of each coating in contact with the indenter a single relationship between either hardness or Young's modulus and the amount of aluminium layer penetrated can be produced. This technique allows the influence of the percentage ceramic on the results obtained to be identified. Comparison of the nanoindentation results with three point bending tests show how the coating structure influences the results obtained.     

Measuring mechanical properties of plasma-sprayed alumina coatings by nanoindentation technique

AISI 1020 steel substrate is coated with alumina as feedstock material using plasma spraying process in order to correlate the microstructural features with mechanical properties of coating. The present work focuses on the effects of microstructural inhomogeneity on mechanical properties of alumina coating through nanoindentation technique. Young’s modulus and hardness of the alumina coating are analytically evaluated. Indentation stress–strain curves are generated from the experimentally obtained load–displacement curves to characterise the mechanical properties of the coating. The results have shown large variation in hardness and Young’s modulus of alumina due to microstructural inhomogeneity of the coating.     

Characterisation of the mechanical properties of plasma-polymerised coatings by nanoindentation and nanotribology

Nanoindentation and SFM tip-induced wear (nanotribology) have shown clear differences in the mechanical properties of plasma-polymerised coatings when compared to conventional thermoplastics. Plasma polymers deposited in-plasma are much harder, stiffer and wear resistant than conventional polymers. Plasma polymers deposited downstream at low plasma power exhibit viscous behaviour. They are susceptible to nanowear and the resulting morphology of the worn region is completely different to what is observed on conventional polymers such as poly(ethylene terephthalate) and polystyrene.     

Efficient charge carrier control on single walled carbon nanotube thin film transistors using water soluble polymer coatings

SWCNT-based thin-film transistors (TFTs) typically display unipolar p-type electrical characteristics in ambient condition due to the O₂/H₂O redox couple. However, complementary circuits that combine both p and n channels are preferred due to lower power requirements. Typical approaches with small molecule or polymeric dopants often yield ambipolar devices, or unstable n-type devices while concomitantly suppressing the on-current and mobility. Herein, we demonstrate a charge carrier control strategy using aqueous-based polymeric coatings that enable n-type devices with comparable performance to p-type devices. Specifically, we used a polyvinyl alcohol (PVA) coating layer containing a minority fraction of polyethyleneimine (PEI) (0.06–1.1?% w/w) to effectively switch the transfer characteristics from p-type to n-type, while maintaining decent electrical characteristics. Moreover, we demonstrate the ability to fine-tune the n branch threshold voltage via the annealing temperature. A similar strategy provides a balanced p branch on-current by incorporating PVA as a minor component (0.1-6?% w/w) into a polyacrylic acid (PAA) matrix. Through effective n-type conversion and p-type balancing, we demonstrate a simple SWCNT-based inverter. Considering the low-cost, environmentally friendly compositions and aqueous processability, this approach is attractive for large scale complementary printable circuits.

     

Mechanical properties evaluation of fluor-doped diamond-like carbon coatings by nanoindentation

Fluor-doped diamond-like carbon (DLC) films were grown by RF sputtering, varying the CF₄ partial pressure and the total gas pressure. Nanoindentation is required to evaluate the mechanical properties of such very thin films. The Vickers nanohardness and the Young's modulus values fall in the 55-70 GPa and 235-326 GPa ranges, respectively, the lowest values being observed for the highest degree of fluorine content in the film (80% CF₄ in the gas mixture) and for the highest processing pressure (1.5 kPa).     

Simulations of nanoindentation in a thin amorphous metal film

Nanoindentation was simulated in a two dimensional model metallic glass thin film using molecular dynamics. Strain localization was observed in simulations where the system was sufficiently structurally relaxed prior to deformation. Indentation simulations utilized an atomic indenter that adhered to the surface or a frictionless indenter. Boundary conditions were varied to constrain the film or allow the film to relax in-plane to examine the effect on shear band formation. The most constrained system, i.e. that with the atomic indenter and the constrained boundaries exhibited the highest hardness.     

Fracture toughness measurement of thin films on compliant substrate using controlled buckling test

Thin films and multilayered structures are increasingly used in the industry. One of the important mechanical properties of these thin layers is the fracture toughness, which may be quite different from the known value of the bulk sample due to microstructural difference. In the design towards device flexibility and scratch resistance, for example, fracture toughness is an important parameter of consideration. This work presents a testing scheme using controlled buckling experiment to determine the fracture toughness of brittle thin films prepared on compliant substrates. When the film is under tension, steady-state channelling cracks form in parallel to each other. Critical fracture strain can be calculated by the measuring the displacement of the buckled plate. The fracture toughness can then be obtained with the help of finite element calculation. When the substrate experiences plastic deformation, the energy release rate is increased by the degree of plasticity. Fracture toughness measurement of two types of thin film Cu-Sn intermetallic compounds has been given to illustrate the merits of such a test scheme.     

Measurement of fracture toughness by nanoindentation methods: Recent advances and future challenges

In this paper, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever, beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials. For pillar splitting, cohesive zone finite element simulations are used to validate a simple relationship between the critical load at failure, the pillar radius, and the fracture toughness for a range of material properties and coating/substrate combinations. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ∼25% is found between the pillar splitting technique and double-cantilever testing. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred.     

Indentation modulus extrapolation and thickness estimation of ta-C coatings from nanoindentation

Coatings used in tribological applications often exhibit high hardness and stiffness to achieve high wear resistance. One coating characterization method frequently used is nanoindentation which allows the determination of indentation hardness and indentation modulus among other material properties. The indentation modulus describes the elastic surface behavior during indentation and is, among hardness, a direct indicator for wear resistance. To obtain the true indentation modulus of a coating, it must be measured with varying loads and then extrapolated to zero load. Current recommendation of the standard ISO 14577-4:2016 is a linear extrapolation which fits poorly for nonlinear curves. Such nonlinear curves are commonly found for high hardness mismatches between coating and substrate, for example, superhard tetrahedral amorphous carbon coatings (ta-C) on a steel substrate. In this study, we present a new empirical fit model, henceforth named sigmoid. This fit model is compared to several existing fit models described in the literature using a large number of nanoindentation measurements on ta-C coatings with wide ranges of indentation modulus and coating thickness. This is done by employing a user-independent and model agnostic fitting methodology. It is shown that the sigmoid model outperforms all other models in the combination of goodness of fit and stability of fit. Furthermore, we demonstrate that the sigmoid model’s fit parameter directly correlates with coating thickness and thus allows for a new approach of determining ta-C coating thickness from nanoindentation.