ICMCTF2010 Session E2-3: Mechanical Properties and Adhesion

The increasing use of polymeric materials as thin coatings requires appropriate mechanical characterization methods at micro and nano-scale taking into account time-dependent and temperature effects. A dynamic nanoindentation technique has been developed for viscoelastic analysis of polymer thin films.

This study is focused on the case of polymer layer thickness larger than 1 µm. For these polymer layers three scales has to be considered as a function of the ratio penetration depth (h) to the thickness layer (t).

For very small h, lower than 100 nm, and very small h/t ratio some changes from the bulk behaviour can be obtained. Experiments have demonstrated that the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. Using dynamic nanoindentation technique some changes from the bulk behaviour are here related.

For the middle range regime small h/t ratio and h bigger than 100 nm it can be possible to consider material as bulk material. Dynamic nanoindentation technique had shown agreement between bulk measurements and those on polymer thin films. In this talk it is shown how a quantitative thermorheological analysis can be done.

For larger h and h/t ratio, this study discusses the interaction between mechanical properties of thin layer and those of their substrate. The underlined questions are: how the substrate's mechanical properties affect those of the film; how the layer's thickness affects its mechanical properties. Model was used to estimate the elastic modulus of the film from the global measured value. For thin and "compliant" polymer layers deposited on "hard" substrates, an increase of the film elastic modulus was observed along indentation. This increase is attributed to the "anvil effect": the bulk elastic modulus increases with the hydrostatic pressure which results from the compression of the film.

In this work, a series of two-dimensional plane-strain finite element analyses was conducted to further understand the stress distribution during tensile tests on coated systems. Besides the film and the substrate, the finite element mesh also considered a number of cracks perpendicular to the film/substrate interface. Different from analyses commonly found in the literature, the mechanical behavior of both film and substrate was considered elastic-perfectly plastic. The variables in the analysis were the number of cracks in the film, the distance between two consecutive cracks and the film yield stress. Results were analyzed based on the normal stresses parallel to the loading axis (σ_x), which are responsible for cohesive failures that are observed in the film during this type of test. Results indicated that the presence of plasticity in the film has significantly reduced the value of σ_x at the film/substrate interface and close to the pre-defined crack tips. Thus, thin film plasticity favors new crack nucleation at film surface, similar to what is commonly seen in practice, and not at the interface.

This paper presents an experimental investigation on thermal fatigue of ion nitrided and duplex-treated (ion nitriding followed by PAPVD coatings) AISI H13 hot-work tool steel. One ion nitrided system and five duplex-treated systems [TiN, TiAlN, (CrN/TiN)x3, CrN, (Cr/CrN)x8] were categorised based on the intensity of thermal fatigue cracks observed after testing. An apparatus based on high frequency induction heating and water spray cooling was used for thermal fatigue testing under the following conditions: maximum temperature at specimen surface 600°C, minimum temperature at specimen surface 80°C, heating time per cycle 15 seconds, cooling time per cycle 10 seconds and total number of thermal cycles for test termination: 500, 1000 and 2000. The behavior of the diffusion layer (ion nitriding), mono- and multi-layered coatings upon thermal fatigue intensity was discussed according to surface crack density and crack propagation depth at different test intervals. Finally, based on the results of the investigation, the applicability of surface treatments in hot metalworking tools subjected to thermal fatigue is discussed in this paper.

Semiconducting nanowires, e.g. from silicon (Si NWs) can be grown either epitaxially on crystalline substrates or e.g. on glass so that no epitaxy is involved. This so called /bottom up /growth/ /approach is explored now next to the well established /top down /technology which is based on materials removeal using masks. Both methods for nanowire realization are studied and 1D structures are realized between 10 nm and 1µm in diameter, i.e. in structure dimensions that permit new applications in sensors photonic and electronic devices. In order to be able to incorporate these SiNWs in new device designs, their physical properties have to be known. This presentation shows structural and mechanical characterization of SiNWs and already integrated SiNWs for example embedded in transparent conductive or insulating oxide layers.

The SiNW characterization is based on a combination of microscopy techniques. Transmission electron microscopy to reveal the structural properties down to the atomic scale and scanning electron microscopy

(SEM) to reveal SiNW morphology. In addition, a nanomanipulator built into an SEM is used to contact the nanowires and characterize them electrically (e.g. /p/-/n /doping profiles can be detected by electron beam induced current imaging). This nanomanipulator inside the SEM is used to manipulate the SiNWs so that bending as well as tensile testing are possible with real time visual feedback. In particular, tensile experiments can be performed in which the specimen is strained uniformly. This is important to reduce the influence of surface effects, for example when measuring Young's modulus of an unknown material or a composite material such as a SiNW with a conformally wrapped oxide layer. In order to precisely and automatically extract data from the experiments, an image analysis tool is used that can track objects with subpixel resolution. Finite element calculations support measurements.

The example of SiNWs as the absorber in a novel thin film solar cell concept will be discussed in terms of mechanical integrity and electrical capabilities.

In the frame of an ANR Pnano project (Cmonano), we develop a biaxial tensile module at the French synchrotron facility SOLEIL in order to study and model the elastic behavior of nanostructured thin films. The mechanical characterization of such structures and the relationship with the microstructure required for further development of technological applications, are still poorly explored. The project encompasses the elaboration of thin films of controlled microstructure, the experimental characterization of their mechanical response under biaxial loading and synchrotron X-ray diffraction, and the modeling of the observed mechanical behavior in view of the nanostructure. Metallic thin films are produced using PVD methods onto polymeric substrates, i.e. polyimide cruciform-shaped substrates. The film-substrate composites are then deformed in situ in an X-ray goniometer. The morphology and texture of the coatings are characterized using TEM and XRD respectively. The modeling using homogenization methods, is developed and takes into account both the crystallographic texture and morphology of the metallic films. The first results concerning W thin films in situ deformed with this new biaxial tensile modulus will be presented.

Brush-plating (selective plating) of gold and silver is mainly used for decorative or electric applications. This process is up to 60 times faster than bath plating and has made reductions in amount of deposited materials possible. The laying of the coating, however, is accompanied by generation of residual stresses in it. Residual stresses are in many cases high and may cause significant shape changes or contribute to damage development (cracking, delamination due to poor bonding between the coating and substrate). Thus, accurate measurement and prediction of residual stresses is important for an understanding and ultimate control of delamination and cracking of coatings.

In this study residual stresses were determined in gold and silver coatings deposited from a commercial SIFCO Dalic Solution (Gold (Hard Alloy), Code SPS 5370, and Silver Hard Heavy Build, Code SPS 3080) on brass unclosed thin-walled ring substrates.

The calculation formula is extended Brenner and Senderoff´s formula which takes into consideration the real shape of the substrate, and the difference between the elasticity moduli of coating and substrate materials [ H. Lille, et al., Materials Science, Lithuania, 3(2008), 226]. Residual stresses in the coating were investigated depending on current density. D eposition took place at the current densities 0.05 to 0.50 amp/cm² and the cathode velocity 23.4 m/min. The sensitivity of the method was studied and the expanded uncertainties of the computed mean values of the residual stresses are presented. Residual stresses represented tensile stresses and their values ranged from 191±29 to 402±61 N/mm² and 43.6±7.1 to 102.0±16.6 N/mm² for gold and silver, respectively. These values are higher than the respective values of stresses in coatings obtained from bath solution.

Relaxation of residual stresses was observed. The values of residual stresses in the gold (deposited at the current densities 0.15 to 0.35 A/cm²) and silver coatings decreased markedly. After one week they were about two times lower and after nineteen months more than four times lower than the respective values obtained immediately after deposition. However, residual stresses in the gold coatings deposited at the current densities 0.05; 0.10; 0.40 and 0.50 A/cm² decreased somewhat less: for example, after nineteen months their values were more than twice lower than the respective initial values.

The microstructure of the studied coatings was investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM).

Diamond Like Carbon (DLC) thin films are being developed for use in a wide range of applications (biomedical, semiconductor, etc). In order to simulate (and if possible extend) the service life of a particular coating system and to improve efficiency, it is important to characterize the true coating material properties. This can be particularly difficult for thin films whose thickness is often less than 50 nm (common in the magnetic hard disk industry, for example). This paper will focus on the particular challenges which are encountered when attempting to measure a wide range of properties, including hardness, elastic modulus, coating adhesion, strain-hardening exponent, fracture toughness, viscoelasticity and creep. A specific focus will be placed on the ability of a thermal-drift-free nanoindentation system to accurately and reproducibly measure the elastic/plastic transition in such thin films and why this can be of importance in obtaining stress-strain information about the coating. The advantages of measuring creep decay over significant time periods (> 5 minutes) and guaranteeing that such creep is entirely material-dependent (and not having a component originating from the measurement equipment) will also be covered.