F. Carassiti

A novel pillar indentation splitting test for measuring fracture toughness of thin ceramic coatings

The fracture toughness of thin ceramic films is an important material property that plays a role in determining the in-service mechanical performance and adhesion of this important class of engineering materials. Unfortunately, measurement of thin film fracture toughness is affected by influences from the substrate and the large residual stresses that can exist in the films. In this paper, we explore a promising new technique that potentially overcomes these issues based on nanoindentation testing of micro-pillars produced by focused ion beam milling of the films. By making the pillar diameter approximately equal to its length, the residual stress in the upper portion of the pillar is almost fully relaxed, and when indented with a sharp Berkovich indenter, the pillars fracture by splitting at reproducible loads that are readily quantified by a sudden displacement excursion in the load displacement behaviour. Cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius and fracture toughness for a given material. The main novel aspect of this work is that neither crack geometries nor crack sizes need to be measured post test. In addition, the residual stress can be measured at the same time with toughness, by comparison of the indentation results obtained on the stress-free pillars and the as-deposited film. The method is tested on three different hard coatings created by physical vapour deposition, namely titanium nitride, chromium nitride and a CrAlN/Si3N4 nanocomposite. Results compare well to independently measured values of fracture toughness for the three brittle films. The technique offers several benefits over existing methods.

Characterization and hardness modelling of alternate TIN/TICN multilayer cathodic arc PVD coating on tool steel

Abstract TiCN coatings on the market today are in general multi-layer TiN/ Ti(C x N 1− x ) coatings. Such multilayered film configuration enables optimisation of the film/substrate as well as the film/worked material interactions, by controlling the internal stress state, fatigue toughness, hardness and superficial composition of these Ti(C x N 1− x ) coatings. This paper presents the results of the investigations on a wear resistant coating made by alternate layers of TiN and Ti(C x N 1− x ) (nominal 0.5 μm each), deposited on S600 tool steel by reactive cathodic arc evaporation using a reactive gaseous mixture of methane and nitrogen. Microstructural and compositional characterisation were carried out using ball crater tests, Optical Microscopy, Scanning Electron Microscopy associated with Energy Dispersive Microanalysis and Image Analysis. Micro hardness measurement were evaluated by means of the Chicot and Lesage volume law of mixture model to cope with the problem of the multiple influence of the different layers and the substrate on the real multilayer surface hardness. In order to predict the surface hardness, the model needs the knowledge of the properties of each type of layers and of the substrate (Young modulus, H 0 =hardness at infinitely small load and n =strain hardening coefficient, or ISE index). These properties were measured using microindentation tests from ad hoc samples of single TiN 0.5 μm layer film, single Ti(C x N 1− x ) 0.5 μm layer film and the uncoated substrate. Young modulus for the TiN and TiCN were evaluated with load–displacement nanoindentation tests; Young modulus for the substrate is from manufacturer. Thickness, composition profiles and microstructure of each film were used to qualify the data input for the model. Experimental measurements on the composite surface hardness allowed then the verification of the predictions.

Evaluation of chloride content in concrete by X-ray fluorescence

Determination of chloride content in concrete structures can be carried out by the X-ray fluorescence technique. However some problems affect measurement validity and limit the convenience of using XRF analysis when a limited number of determinations have to be performed. Measurements carried out on chloride contaminated concrete and mixtures of NaCl and concrete show the influence of distribution and grain size of chloride containing phase on analytical results. It is shown that chloride fluorescence intensity, at the same concentration level and at very low concentrations, is greatly reduced when chloride is present in the concrete matrix as NaCl salt instead of being dissolved in the concrete pore solution.

Residual stress measurement at the micrometer scale: focused ion beam (FIB) milling and nanoindentation testing

We present a new procedure for the determination of surface elastic residual stress by instrumented sharp indentation, based on nanoindentation testing on focused ion beam (FIB) milled micro-pillars. Finite element modeling (FEM) of strain relief after FIB milling of annular trenches demonstrates that full relaxation of pre-existing residual stress state occurs when the depth of the trench approaches the diameter of the remaining pillar. Considering this, the average residual stress present in the sample material can be calculated by the comparison of two different sets of load–depth curves, the first one obtained at the center of stress relieved pillars, the second on the undisturbed (residually stressed) surface. Analytical modeling of the contact stress distribution in non-halfspace conditions was adopted to take into consideration the additional boundary conditions given by the edges of pillars and the elastic properties of the substrate (in case of coatings). The results are presented for residual stress evaluation of a 3.8-µm TiN coating on WC–Co substrate obtained by cathodic arc evaporation-physical vapor deposition (CAE-PVD) techniques, showing an average compressive stress state of −5.63 GPa. This result is in close agreement with the estimation obtained by XRD (sin2 ψ method) analysis of −5.84 GPa of the same sample, adopting the same elastic constants.

Superconducting and microstructural studies on sputtered niobium thin films for accelerating cavity applications

The aim of the present research activity was to verify the influence of the applied bias voltage on the microstructural and functional properties of magnetron sputtering physical vapour deposition (MS-PVD) niobium thin films for use in superconducting resonant cavities for particle accelerators. Four different sets of samples were produced, by varying both the applied bias voltage and the nature of the substrate (copper or quartz). The morphological, microstructural, and mechanical properties of the coatings were experimentally determined by focused ion beam scanning electron microscopy (FIB-SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and nanoindentation techniques, and then correlated to the applied bias and nature of the substrate. The superconducting properties (critical temperature Tc and residual resistivity) were determined by a calibrated four-contact probe and a cryogenic apparatus. The microstructures and surface properties of biased films grown on copper and quartz were compared. The observed differences are likely connected to the low conductivity of quartz that induces a re-sputtering effect and a consequent modification of the superconducting performances.

Low-temperature oxidation of silicon nitride by water in supercritical condition

Abstract Oxidation tests carried out in supercritical water (400–500 °C) revealed a noticeable corrosion attack on the silicon nitride surface in spite of the low temperatures. Increasing water pressure generally caused an increase in the oxidation phenomena. Oxidation scale evolution depended strictly on the silicon nitride sintering process, sintering aids, porosity and impurities, which influenced oxidation kinetics and surface morphology. In such test conditions the solubility of silica in water seemed to have a great influence on the stability of the oxidation scale, mainly in reaction-bonded silicon nitride samples.

An Innovative Non-contact Method to Determine Surface Free Energy on Micro-areas

Surface free energy (SFE) is a property which depends on the chemical state and roughness of the surface and it is necessary to develop a reliable method to evaluate SFE value on a small area, taking into account these two different contributions. Today contact angle methods are the most used and they allow to evaluate the global mean value of SFE on areas of mm2 size. With these methods, it is not possible to evaluate the effects of roughness, surface defects, chemical contamination on SFE value. In addition, it is difficult to determine the surface free energy value on small components which have dimensions smaller than drop diameter. Nanoindentation and atomic force microscopy techniques provide alternative direct measurement methods to evaluate the SFE on small areas (on the order of μm2 or nm2) through a contact mechanism triggered by the contact of two bodies. In order to evaluate the adhesion properties, currently three models, Johnson– Kendall–Roberts, Maugis–Dugdale and Derjaguin–Muller–Toporov, use the value of pull-off force (force required to separate the indenter tip from the sample). All influences of surface morphology on SFE values are lost using these methods. In fact the adhesion value obtained refers to the energy balance between two conformal surfaces, which depends mainly on the morphology of the harder material (i.e., diamond tip). In this work we describe a new methodology for the SFE determination consisting in the modeling and quantitative evaluation of the interaction between the tip and sample surface during the approach phase in a nanoindentation test. During the test, the nanoindenter tip is attracted to the sample surface until the sample reaction forces become significant (in this case physical contact between two bodies is achieved). The SFE value is evaluated using experimental force of attraction and displacement of the nanoindenter spherical tip when it approaches the sample surface. In this method the sample surface is not altered by the tip, therefore unlike pull-off force method, it could be very useful to evaluate the actual SFE considering the effect of sample morphology (controlled roughness or pattern).Surface free energy (SFE) is a property which depends on the chemical state and roughness of the surface and it is necessary to develop a reliable method to evaluate SFE value on a small area, taking into account these two different contributions. Today contact angle methods are the most used and they allow to evaluate the global mean value of SFE on areas of mm2 size. With these methods, it is not possible to evaluate the effects of roughness, surface defects, chemical contamination on SFE value. In addition, it is difficult to determine the surface free energy value on small components which have dimensions smaller than drop diameter. Nanoindentation and atomic force microscopy techniques provide alternative direct measurement methods to evaluate the SFE on small areas (on the order of μm2 or nm2) through a contact mechanism triggered by the contact of two bodies. In order to evaluate the adhesion properties, currently three models, Johnson– Kendall–Roberts, Maugis–Dugdale and Derjaguin–Muller–Toporov, ...

Elastic anisotropy of coatings by AFM analysis of microindentations

Abstract Atomic force microscope analysis of low load Vickers and Knoop indentation marks was performed to obtain a statistically consistent method for the mechanical characterisation of physical vapour deposition (PVD) coatings and the evaluation of elastic anisotropy. Indents shape analysis were elaborated with three different models for the evaluation of Youngs modulus. Subsequently, roughness, elastic/plastic behaviour and intrinsic hardness have been numerically evaluated. Results showed that the intrinsic hardness evaluation of the films for applied loads as low as 50 mN is statistically supported by atomic force microscopy analysis of the indents, especially when diagonal length is smaller than 2/3 m. Moreover, some of magnetron sputtering PVDPlease confirm definition of MS. coatings showed mechanical anisotropy: normal to surface Youngs modulus was higher than in plane Youngs modulus. Such information would not have been obtained by nanoindentation testing alone and could be extremely important for the process development and production of high performance coatings for specific tribological applications

Focused ion beam and transmission electron microscopy as a powerful tool to understand localized corrosion phenomena

Abstract The present study deals with focused ion beam (FIB) coupled with transmission electron microscopy (TEM) analysis of corrosion mechanisms in cathodic arc evaporation (CAE)-PVD CrN/NbN multilayer coatings on aluminum alloy substrate. Samples were exposed to a 500-h ASTM B-117 salt spray test. Surface morphology of tested specimens was preliminary analyzed by SEM-EDS techniques. The corrosion rate in CAE-PVD coated components (ASTM B-117 standard) was significantly lower than uncoated substrates. Crevice corrosion in relation to surface defects (microdroplets) was observed after FIB and TEM cross-section analysis. Nevertheless, results of TEM analysis also showed a more complex crevice/galvanic corrosion mechanism in relation to the bottom droplet/coating voids, close to the coating/substrate interface, which could be concurrent with usually proposed corrosion mechanisms. Moreover, modification of coating microstructure and composition is observed close to the peripheral area of droplets. These results give further and deeper insight into the corrosion mechanisms for PVD thin coatings, and could only be obtained by FIB sectioning and TEM analysis. Obtained results also allow the correct selection of the most suitable coating material and process parameters as a function of the nature of the substrate and the specific environmental conditions.

Interfacial reactions in Al/SiC composites produced by low pressure plasma spray

Metal matrix composite tapes, reinforced by long silicon carbide fibres produced by low pressure plasma spray are characterised by the scarce presence of reaction compounds at the fibre/matrix interface. However, a complete and comprehensive investigation of the dissolution, diffusion and chemical reaction phenomena taking place in the interfacial area during the deposition is most important for the evaluation of the adhesion strength between the reinforcement and the base material and, consequently, of the mechanical behaviour of the material. In the case of interest for the present paper, the characterisation of the interface has been carried out by means of scanning electron microscopy, energy dispersive spectroscopy and X ray difraction analysis. Moreover, the quality of the fibre/matrix interface of the Al/SiC composites was evaluated by means of push-out tests, aimed to the identification of the transmission mechanism of the load from the matrix to the fibre, and to quantify the adhesion strength.