H. Pelletier

Limits of using bilinear stress–strain curve for finite element modeling of nanoindentation response on bulk materials

Abstract For 10 years, finite element modeling (FEM) has been widely used to simulate elastic and plastic deformations beneath a pointed indenter. Hardness measurement with a sharp pyramidal indenter, such as Berkovich or Vickers, is one of the most complex mechanical problems. This is due to the 3D dimensional phenomenon associated with elastic and plastic deformations, with large strain field. Nowadays FEM is used to extract the true hardness of thin coatings, from the composite hardness measurements. The first step consists in assuming that the film and substrate behavior can be modeled with a bilinear law. Two parameters for the layer are extracted from the simulation fit of experimental load displacement curves: the yield strength, Y 0 , at the plastic strain of 0.2% and Youngs modulus, E . The aim of this work is to show on bulk pure metals the limits of the bilinear model. We show that the bilinear model does not define correctly the first step of strain hardening, which results in an overestimation of the yield stress or one need to adjust the target modulus at each indentation depth. Moreover we prove the strong influence of the indenter tip on the load–displacement curve. Finally, we propose a method to determine the tip radius, R , of the equivalent conical indenter used for FEM, to approximate the real Berkovich indenter geometry.

Surface roughness and morphology of three nanocomposites after two different polishing treatments by a multitechnique approach

OBJECTIVES The purpose of this study was to assess the surface roughness and morphology of three nanocomposites polished with two different polishing systems. METHODS Specimens made of hybrid composite (Tetric Ceram [TC] as control) and nanocomposites: nanofilled (Filtek Supreme [FS]), nanofilled hybrid (Grandio [Gr]), complex nanofilled hybrid (Synergy D6 [Syn]) were polished with CompoSystem [CS] or Sof-Lex [SL] polishing discs. The average surface roughness (Ra) before and after polishing was measured using optical profilometry. Both AFM and SEM techniques were additionally used to analyze the surface morphology after polishing with the aim of relating the surface morphology and the surface roughness. Statistical analysis was done by ANOVA using a general linear model (alpha=0.05) with an adjustment for multiple comparisons. RESULTS Within the same polishing system, FS exhibited the smoothest surface, followed by Syn, TC and Gr (p<0.0001). Sof-Lex polishing discs produced the smoothest surface compared to CompoSystem (p<0.0001). AFM and SEM observations confirmed that the surface roughness was related to the surface morphology and to the average filler size. SIGNIFICANCE Positive correlation between the average filler size and the surface roughness suggest that using nanoparticles in the formulation does not necessary improve the surface texture. The nanofilled composite FS, which contains only nanofillers, showed the best results when associated to Sof-Lex polishing discs.

Effect of high energy argon implantation into NiTi shape memory alloy

Abstract The super-elastic properties of NiTi alloy are used for several medical applications in dentistry, such as orthodontic wires (useful to correct the dental position), palatal expanders and endodontic instruments. Implantation process may be one solution to enhance the surface properties, corrosion, wear and fatigue resistance, of such instruments. A previous work, for which implantations were performed with N + and B + ions at 150 keV, has shown that the hardening mechanism is due to the formation of an amorphous layer whose thickness increased with dose in the near surface region. In this paper, in order to increase the treated thickness, especially the amorphous layer, argon implantations at high energy (1.5 MeV) have been performed. The structural modifications have been observed with Grazing Incidence X-ray Diffraction (GIXRD), as a function of Ar + dose. The processes involved in the crystalline to amorphous phase transformation of NiTi under ion beam irradiation have been particularly investigated. Characterisation of the near surface composition was made by Rutherford Backscattering Spectroscopy (RBS). Nanoindentation tests have been used to study the evolutions of hardness and elastic modulus through the treated thickness after implantation.

Correlation between hardness and structure of carbon-nitride thin films obtained by reactive pulsed laser deposition

Abstract Carbon-nitride thin films were synthesized by reactive pulsed laser deposition from graphite targets in low-pressure nitrogen. X-Ray photoelectron spectroscopy and nanoindentation measurements were performed in order to establish a connection between the composition, structure and hardness of the obtained thin films. We studied the variation of the sp 3 /sp 2 C bonded to N ratio with the increase of the N content of the thin layers. We found that the value of this ratio mainly determines the hardness of the carbon-nitride layers. The stability in time and/or under thermal heating of the CN bonds formed was also tested.

Pullout characteristics of percutaneous pedicle screws with different cement augmentation methods in elderly spines: An in vitro biomechanical study

BACKGROUND Vertebroplasty prefilling or fenestrated pedicle screw augmentation can be used to enhance pullout resistance in elderly patients. It is not clear which method offers the most reliable fixation strength if axial pullout and a bending moment is applied. The purpose of this study is to validate a new in vitro model aimed to reproduce a cut out mechanism of lumbar pedicle screws, to compare fixation strength in elderly spines with different cement augmentation techniques and to analyze factors that might influence the failure pattern. MATERIALS AND METHODS Six human specimens (82-100 years) were instrumented percutaneously at L2, L3 and L4 by non-augmented screws, vertebroplasty augmentation and fenestrated screws. Cement distribution (2 ml PMMA) was analyzed on CT. Vertebral endplates and the rod were oriented at 45° to the horizontal plane. The vertebral body was held by resin in a cylinder, linked to an unconstrained pivot, on which traction (10 N/s) was applied until rupture. Load-displacement curves were compared to simultaneous video recordings. RESULTS Median pullout forces were 488.5 N (195-500) for non-augmented screws, 643.5 N (270-1050) for vertebroplasty augmentation and 943.5 N (750-1084) for fenestrated screws. Cement augmentation through fenestrated screws led to significantly higher rupture forces compared to non-augmented screws (P=0.0039). The pullout force after vertebroplasty was variable and linked to cement distribution. A cement bolus around the distal screw tip led to pullout forces similar to non-augmented screws. A proximal cement bolus, as it was observed in fenestrated screws, led to higher pullout resistance. This cement distribution led to vertebral body fractures prior to screw pullout. CONCLUSION The experimental setup tended to reproduce a pullout mechanism observed on radiographs, combining axial pullout and a bending moment. Cement augmentation with fenestrated screws increased pullout resistance significantly, whereas the fixation strength with the vertebroplasty prefilling method was linked to the cement distribution.

Molecular dynamics simulations as a way to investigate the local physics of contact mechanics: a comparison between experimental data and numerical results

In this work, a mechanical analysis of normal contact using molecular dynamics (MD) simulations is presented. Conical indentation on amorphous polymer surfaces was simulated at various temperatures and indentation rates under displacement or load control. The results are qualitatively compared with experimental data from tests on epoxy materials with different glass transition temperatures (Tg), and show good agreement with experiments. Moreover, MD simulations of nano-indentation tests allow us to estimate the mechanical properties of the polymer films studied as in experimental nano-indentation tests, which demonstrates the relevance of this approach.

Creep of the contact with a spherical tip and recovery of the imprint on amorphous polymer surfaces

This paper is devoted to an analysis of the creep and recovery occurring on the surface of an amorphous polymer during indentation, a characteristic of the self-healing performance of the material. Creep and recovery were studied using a home made experimental device which allows one to record in situ the evolution of the imprint created by a probe. The influence of the temperature, the initial imposed strain and the creep duration were analysed. The evolution of the true contact area with time was followed during micro-indentation. We demonstrate the non-linear behaviour of the amorphous polymer studied during the creep and recovery phases. The results obtained allow calculation of the activation energy and activation volume of the polymer. These results are also compared with the predictions of the viscoelastic model of Lee and Radok and viscoplasticity is demonstrated for a reasonable initial mean contact strain as defined by Tabor. Nevertheless, it is still difficult to quantify the recovery and to define the conditions for the appearance of plasticity.

A Perspective of Pulsed Laser Deposition (PLD) in Surface Engineering: Alumina Coatings and Substrates

In this article, two original studies of the alumina as porous substrate and PLD (pulsed laser deposition) thin films in view of its biomedical and tribological applications are presented. The first biomedical study aimed to evaluate the role of Al2O3 on thin deposited nanostructures. For this purpose, cerium stabilized zirconia doped hydroxyapatite thin films were deposited by PLD onto high purity, high density alumina substrates with different low porosities. For deposition, an UV KrF* (λ=248 nm, τ ~ 25 ns) excimer laser was used for the multi-pulse irradiation of the targets. The nanostructured surface morphologies of the thin films with micro droplets were evidenced by atomic force microscopy and scanning electron microscopy and the compositions with a Ca/P ratio of 1.7 by energy dispersive spectroscopy. The films were seeded with mesenchymal stem cells for in vitro tests. The cells showed good attachment and spread and covered uniformly the surface of the samples. Different functions of substrate porosities are observed in the efficiency of developing long filopodia and of obtaining the optimal intracellular organization. The second study aimed to understand the influence of micro-structural and mechanical characteristics on the tribological behaviour of stainless steel samples with PLD alumina coatings produced using an UV KrF* (λ=248 nm, τ ~ 20 ns) excimer laser and a sintered alumina target. Various microscopic observation techniques were used in order to connect the tribological response to the amorphous microstructure of the coatings. The results correspond to the determination of the mechanical characteristics by nanoindentation tests, scratch tests, and a tribological behaviour analysis of the treated steel against 100Cr6. The films were stoichiometric, partially crystallized with an amorphous matrix and their surfaces had few particulates deposited on. The obtained values of hardness and elastic modulus of the films were in good agreements with literature data.

Pulsed Laser Deposition of Thin Coatings: Applications on Biomaterials

We report results on Pulsed Laser Deposition (PLD) of ceramic thin films for biomedical applica-tions. The coating of metallic implants with bioceramic thin films (e.g. calcium phosphates, in particular hydroxyapatite) has been proposed as a solution for combining the mechanical properties of the metallic material with the bioactive character of the ceramic layer, leading to a better integration of the entire implant with the newly remodelled bone. Other bioceramics (as e.g. alumina) exhibit a high degree of chemical inertness under physiological conditions, excellent wear resistance, ability to be polished to a high surface finish and excellent hardness as coating. Among the different methods to obtain ceramic coatings that have been widely used so far, PLD was focusing interest due to its versatility and controllability, the aptitude to synthesize and deposit uniform films, with an accurate control of the stoichiometry and crystallinity. We investigated the micro-structural and mechanical characteristics of PLD bioceramic coatings on metal substrate. Various microscopic observations and mechanical characterisations by nanoindentation and scratch tests were used in order to connect the mechanical response to the microstructure of the coatings. Our studies revealed that the pulsed-laser deposition technique appears to be a competitive candidate in biomedical applications as an extremely versatile technology

Residual Stress Gradient Study of Laser Shocked Aluminum Alloy by GIXRD analysis and FEM Simulation

Laser shock processing (LSP) is a new and competitive technology in comparison with classical mechanical surface treatments for alloys strengthening. The compressive residual stress could be induced by LSP, which further significantly affects materials surface properties. Aluminum based alloy 6056 is used for aeronautic components and their surface strengthening is very important to increase components’ durability. The main factors of LSP, which affect the surface properties of alloys, such as surface roughness and residual stress gradient, are studied and analyzed in present study. The sin2* method with pseudo-grazing incidence X-ray diffraction (GIXRD) is used to determine the residual stress gradient after LSP treatment. In addition, the FEM simulation of residual stress distribution induced by LSP is also carried out and the results are compared with experimental data.