Jorge M. Antunes

Ultra-microhardness testing procedure with Vickers indenter

Abstract Depth-sensing indentation equipment is widely used for evaluation of the hardness and Youngs modulus of materials. The depth resolution of this technique allows the use of ultra-low loads. However, aspects related to the determination of the contact area under indentation should be cautiously considered when using this equipment. These are related to the geometrical imperfections of the tip, the diamond pyramidal punch and the formation of pileup or the presence of sink-in, which alter the shape and size of the indent. These and other aspects, such as the thermal drift of the equipment and the scattering at the zero indentation depth position related to surface finishing, are discussed in this work. A study concerning the hardness and the Youngs modulus results determined by Vickers indentation on different materials was performed. Samples of fused silica, BK7 glass, aluminium, copper and mild steel (for which the values of Youngs modulus were previously known) were tested using indentation loads in the range 10–1000 mN. Moreover, two methods are proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP). The present methods are based on: (i) analysis of the punch profile using atomic force microscopy (AFM); and (ii) a linear penetration-depth function correction (LM), based on knowledge of the values of the Youngs modulus of several materials. By applying these methods to the indentation load/indentation depth results, it was possible to draw some conclusions about the benefit of the AFM and LM methods now under proposal.

Young's modulus of thin films using depth-sensing indentation

A new methodology for the determination of Youngs modulus of thin films, using a single hardness test measurement of film/substrate composites, has been developed. It is based on a recently proposed weight function, the reciprocal of the Gao function. Firstly, results of three-dimensional numerical simulation of the Vickers hardness tests on several fictitious composites are considered. Then, the methodology is checked experimentally by using depth-sensing indentation results on real composite materials.

Mechanical Behaviour of FSW Aluminium Tailored Blanks

The mechanical behaviour of homogeneous and inhomogeneous FSW aluminium tailored blanks is analysed in this paper. The heterogeneity in mechanical properties across the different weld zones is discussed based on hardness testing results. Tensile and formability test results are also shown and the mechanical behaviour of the welds is discussed in relation to the base materials. Despite the hardness tests have indicated very small differences in hardness, between the welds and the base materials, and the tensile test results also showed similarities in mechanical behaviour, the formability tests revealed additional difficulties in forming the welded sheets.

On the evaluation of the ductility of thin films

A new tensile test procedure has been developed to evaluate the ductility of thin films deposited on a substrate. The tensile sample has a continuously variable cross-sectional area, resulting in a continuous strain gradient along the sample after deformation. The films present cracks where the strain imposed exceeds their ductility. The strain attained at the boundary of the region where cracks appear characterises the films ductility. This tensile test procedure has been used to evaluate the ductility of TiAl films deposited on an AISI 304 steel tensile sample. Cracks were observed by optical microscopy. The results of the evolution of the mean distance between cracks as a function of the deformation value are presented and discussed. A three-dimensional finite element code was used to simulate the deformation of this tensile sample. Special attention is devoted to the analysis of the state of stress and strain in the composite film/substrate. A similar study was made of a conventional tensile test sample to confirm and validate the results obtained from the modified sample. Moreover, the influence of the presence of cracks on the stress and strain distributions was also studied by numerical simulation. The experimental tests on conventional samples need to be interrupted at several strain values in order to follow the evolution of the cracks during the deformation. The ductility results obtained from conventional samples do not differ from those obtained with the modified sample. These conclusions and the simplicity of the method demonstrate the advantage of using the continuously variable cross-sectional area sample to study the ductility of thin films.

Numerical Simulation of the Mechanical Behaviour of the Multi-Walled Carbon Nanotubes

The mechanical behaviour of non-chiral multi-walled carbon nanotubes under tensile and bending loading conditions was investigated. For this purpose, a simplified finite element model of armchair and zigzag multi-walled carbon nanotubes, which does not take into account the van der Waals forces acting between layers, was tested in order to evaluate their tensile and bending rigidities, as well as the Young’s modulus. The current numerical simulation results are compared with data reported in the literature. The robustness of the simplified model for evaluation of the Young’s modulus of multi-walled carbon nanotubes is discussed.

Numerical Simulation of the Mechanical Behaviour of Single-Walled Carbon Nanotube Heterojunctions

The study of the mechanical behaviour of single-walled carbon nanotube heterojunctions has been carried out, implementing nanoscale continuum approach. A three-dimensional finite element model is used in order to evaluate the elastic behaviour of cone heterojunctions. It is shown that the bending rigidity of heterojunctions is sensitive to bending conditions. The torsional rigidity does not depend on torsion conditions. Both rigidities of the heterojunction are compared with those of the thinner and thicker constituent nanotubes.

Mechanical characterization of thin films by depth-sensing indentation

The increasing use of thin films in various fields of industry have created the necessity to develop methodologies for evaluating their mechanical properties. The hardness test is a widely used technique for determining the mechanical properties of bulk materials and thin films. The test equipment for depth-sensing indentation (DSI) extended the application of the hardness test to scales close to few interatomic distances. Moreover, the DSI technique allows evaluation of not only the hardness but also other mechanical properties, such as Young’s modulus. In this context, the aim of this chapter is to review the main issues associated with the use of DSI tests in the mechanical characterization of thin films. It focuses on the methods and procedures which can be used in experimental DSI tests in order to evaluate the hardness and Young’s modulus of thin films.

The Influence of the Substrate on the Mechanical Properties of Si-Doped DLC Thin Films

In this work, Si-doped DLC films were deposited on stainless steel (316SS) and polycarbonate (PC) substrates by RF-PACVD in gas mixtures of SiH4+CH4, with 2, 5 and 10 vol.% SiH4. The increase of the Si content in the films led to a progressive drop in the hardness from 30 GPa down to 23 GPa whereas the elastic modulus increased from 124 GPa up to 146 GPa, as measured in the SS coated substrates. In the case of coated PC samples pop-in was observed in the loading curve which was interpreted by finite element simulation and nanoscratching techniques.

Numerical Simulation of Ultramicrohardness Tests in Thin Films

The main problem for characterization of thin coatings, using the ultramicrohardness tests, is related to the determination of the contributions of the substrate and the film for the measured film hardness. Numerical simulation of the ultramicrohardness test can be a helpful tool for a better understanding of the influence of different parameters that are involved in the ultramicrohardness tests of thin films. In this work, specific three-dimensional numerical simulation software, HAFILM, was used to simulate ultramicrohardness tests in three different pars of materials. To perform this study two Vickers indenters and three different film thicknesses were used, in the simulation. Introduction In experimental ultramicrohardness tests, some difficulties emerge for determination of the mechanical properties, such as the Hardness and Young’s modulus [1]. They are related to the geometrical imperfections of the indenter tip and to the different contributions of the substrate and the film for the hardness results. Experimental tests, in coatings, show that Hardness and Young’s modulus results are strongly dependent on the ratio between the film thickness and indentation depth used in the tests, mainly in the case of very thin films [2]. In this work, three-dimensional numerical simulations of the ultramicrohardness test were performed in order to understand the influence of the parameters involved in the experimental tests [3,4]. On the numerical simulation tests, two Vickers indenters with different offsets and three composites with three different thicknesses were used. The finite element home code HAFILM [5], specifically developed to simulate the hardness tests, was used in the simulations Finally, to test the numerical simulations accuracy, results from the simulations are compared with experimental ones. The Finite Element Code HAFILM The mechanical model of the finite element code HAFILM considers the ultramicrohardness test as a process of large deformations and rotations. The plastic behaviour of the material can be described by the anisotropic Hill’s criterion with isotropic and kinematic hardening. The elastic behaviour is considered isotropic. It is assumed that contact with friction exists between the sample and a rigid indenter and is modelled using a classical Coulomb law. To associate the static equilibrium problem with the contact with friction, an augmented multiplier is applied in the mechanical formulation. This leads to a mixed formulation, where the kinematic (material displacements) and static (contact forces) variables are the final unknowns of the problem. For its resolution, the HAFILM code uses a fully implicit algorithm of the Newton-Raphson type. All nonlinearities, induced by the elastoplastic behaviour of the material and by the contact with friction, are treated within a single iterative loop [6]. Materials Science Forum Online: 2004-05-15 ISSN: 1662-9752, Vols. 455-456, pp 694-698 doi:10.4028/www.scientific.net/MSF.455-456.694

The Effect of Vacancy Defects on the Evaluation of the Mechanical Properties of Single-Wall Carbon Nanotubes: Numerical Simulation Study

A three-dimensional finite element model is used in order to evaluate the tensile and bending rigidities and, subsequently, Young’s modulus of non-chiral and chiral single-walled carbon nanotubes containing vacancy defects. It is shown that the Young’s modulus of single-walled carbon nanotubes with vacancies is sensitive to the presence of vacancy defects in nanotube: it decreases with increasing of the density of vacancy defects.