Edoardo Bemporad

Surface analysis of biocompatible coatings on titanium

The coatings of hydroxyapatite, which is widely used for orthopaedic and dental prothesis, were deposited by using the dip-coating method. The layers of hydroxyapatite were grown on commercial Ti substrates. In order to improve the adhesion of hydroxyapatite, the substrate was a priori covered with titania or calcium titanate by using the sol-gel technique. For comparison, commercial samples of hydroxyapatite coating (manufactured by means of plasma-spray apparatus) were analysed. The chemical composition and the structure of the coatings (TiO2, CaTiO3 and hydroxyapatite) were studied by using X-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), X-ray diffraction (XRD) and secondary electron microscopy (SEM) techniques. The data of quantitative XPS analysis and the surface images (SAM and SEM) displayed the superior quality (cleanness, homogeneity, etc.) of hydroxyapatite deposited by sol-gel in comparison with commercial samples investigated.

Parametric study of an HVOF process for the deposition of nanostructured WC-Co coatings

Nanocrystalline WC-Co coatings were deposited by high velocity oxyfuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermochemical and gas-dynamical properties of gas and particle flows within the combustion flame were identified in various operating conditions by computational fluid-dynamics (CFD) simulation. Significant improvements of the mechanical properties of the coatings were obtained: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness.

Interaction of mercury vapour with thin films of gold

Thin films of gold deposited on a silicon substrate were used as electrical sensors of mercury vapour. The samples were exposed to mercury vapour at room temperature, changing the Hg concentration and the exposure time. The chemical composition and morphology of the films exposed to mercury vapour were studied by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), scanning Auger microscopy (SAM) and secondary electron microscopy (SEM). In the samples exposed for a short time, the mercury was adsorbed by thin surface sublayer of Au film. In the case of long exposures, the transformation of the uniform Au film to a dendritic-like coalesced AuHg amalgam occurred, i.e. the morphology of the film was modified irreversibly. This transformation is caused by insufficient adhesion of gold to the substrate.

Tensile experiments and SEM fractography on bovine subchondral bone

Subchondral bone undecalcified samples, extracted from bovine femoral heads, are subjected to a direct tensile load. The Youngs modulus of each sample is determined from repeated tests within the elastic limit. In a last test, the tensile load is increased up to the specimen failure, determining the ultimate tensile strength. The investigation is performed on both dry and wet specimens. The measured Youngs modulus for dry samples is 10.3+/-2.5GPa, while that of wet samples is 3.5+/-1.2GPa. The ultimate tensile strengths are 36+/-10 and 30+/-7.5MPa for dry and wet specimens, respectively. SEM micrographs of failure surfaces show characteristic lamellar bone structures, with lamellae composed of calcified collagen fibers. Rudimentary osteon-like structures are also observed. Failure surfaces of wet samples show a marked fiber pull-out, while delamination predominates in dry samples. The obtained results are interpreted on the basis of the deformation mechanisms typical of fiber-reinforced laminated composite materials.

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.

Influence of mechanical properties of tungsten carbide–cobalt thermal spray coatings on their solid particle erosion behaviour

Abstract The present investigation has been carried out in order to study the erosion wear behaviour of WC–Co base thermal spray coatings. WC–12Co and WC–10Co–4Cr coatings were deposited by means of high velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90° using SiC particles of ∼50 μm in diameter as erodent, at a velocity of 83·4 m s−1. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC–10Co–4Cr coating exhibited a higher erosive wear resistance as compared to the WC–12Co coating.

Laser-induced crystallization of amorphous silicon-carbon alloys studied by Raman microspectroscopy

Abstract CW-laser crystallization of amorphous silicon carbon alloys has been investigated as a function of both laser power density and alloy composition. Irradiation of such alloys generally yields phase segregation into crystalline silicon and crystalline graphite, depending on the laser power density, while crystalline SiC phase can be obtained only when nearly stoichiometric amorphous silicon–carbon alloy are irradiated.

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.

Residual micro-stress distributions in heat-pressed ceramic on zirconia and porcelain-fused to metal systems: Analysis by FIB–DIC ring-core method and correlation with fracture toughness

OBJECTIVES The production of fixed partial dentures (FPDs) induces complex residual stress profiles, due to both the thermal expansion coefficient mismatch between the veneering ceramic and the framework and to the thermal gradients occurring during the final cooling. Detailed knowledge of residual stress distributions in the veneering ceramics is important to understand the interface phenomena with the framework and the consequences of the different firing systems. The first objective of this study was to analyse the residual stress distribution in heat-pressed ceramic on zirconia core with micrometer spatial resolution, with also a focus on the stress at the interface versus porcelain-fused-to-metal samples. The second purpose was to correlate the residual stress with the fracture toughness. METHODS The micron-scale focused ion beam (FIB) ring-core method was used to map the residual stress over the cross-sections of the veneering ceramics. The methodology is based on FIB micro-milling of annular trenches, combined with high-resolution in situ scanning electron microscope (SEM) imaging, a full field strain analysis by digital image correlation (DIC) and numerical models for residual stress calculation. Fracture toughness was evaluated by using high load Vickers indentation and hardness/modulus were measured by nanoindentation testing also across the interfaces. RESULTS Both prosthetic systems showed a compressive stress at the ceramic surface on a micron-scale. The stress profile for porcelain fused to metal (PFM) showed a transition to tensile stress at the half of the layer, whilst the stress in proximity of the interface was more compressive in both the cases. Residual stress on a micron scale are higher in magnitude than the corresponding macro-scale values reported in the literature, due to the stress relaxation given, at larger scales, by micro-voids and cracks. The stress field was directly correlated with the indentation fracture toughness, which was higher in those areas where the compressive stress is greater. Stress analysis in correspondence of interfacial porosity for the zirconia sample also showed that micro-defects could induce local modifications of the residual stress field, which may even locally generate a tensile stress state. SIGNIFICANCE The interfacial stress in dental systems was analysed on a micron scale and can give further insights into the process/property/performance correlation for this class of materials. In particular, interfacial and/or local modifications of the residual stress are expected to have a significant influence on crack nucleation mechanism in correspondence of micro-defects. A direct correlation between residual stress distribution and fracture toughness was proposed. It is noteworthy that the method can be used to study real crowns and bridges. In fact, complex geometries can be easily analysed by this procedure.