L’indentation instrumentée est largement utilisée pour la détermination des propriétés mécaniques des matériaux, principalement la dureté et le module d’élasticité. Pour obtenir des données fiables, plusieurs corrections comme la prise en compte du défaut de pointe et la complaisance de l’instrument doivent être apportées dans la méthodologie d’analyse des données de l’essai. Malgré tout, ces corrections ne suffisent plus pour la caractérisation de matériaux fragiles et poreux, qu’ils soient sous une forme massive ou de revêtement. En effet, d’autres sources d’erreurs peuvent provenir de la fragilité de ces matériaux, mais aussi de la représentativité des mesures locales dans le cas des matériaux hétérogènes, ou de l'influence du substrat dans le cas des revêtements. Pour limiter ces effets, l’indenteur Knoop semble être un bon candidat car, à charge équivalente, sa surface de contact est plus grande et la profondeur d’indentation plus faible. Toutefois, par rapport aux indenteurs habituels, le retour élastique du matériau au voisinage de l'empreinte Knoop, qui doit être pris en compte dans les calculs des propriétés, n’est pas bien connu. Ceci constitue donc un frein à son utilisation. C’est pourquoi, nous proposons d’adapter la méthodologie conventionnelle proposée par Oliver et Pharr pour obtenir des résultats fiables par indentation Knoop. Pour atteindre cet objectif, nous analysons d’abord des résultats obtenus par indentation de différents matériaux céramiques denses en comparant les indentations Knoop et Vickers, ce dernier étant utilisé comme référence. Tout d’abord, nous analysons le retour élastique en indentation Knoop et nous montrons qu’il est possible de retrouver les mêmes valeurs qu’en indentation Vickers en intégrant un facteur correctif lié simplement à des grandeurs géométriques de l’empreinte. Cette approche est valable qu’il s’agisse de la détermination du module d’élasticité ou de la dureté. Toutefois, pour cette dernière propriété, il est nécessaire de bien préciser la définition utilisée pour son calcul. Nous en justifions le choix. Ensuite nous appliquons la méthodologie d’analyse ainsi établie pour l’étude de la réponse mécanique des matériaux poreux sous forme massive et de revêtement où non seulement le paramètre de porosité joue un rôle important sur la réponse du matériau, mais aussi son hétérogénéité, sa rugosité et son épaisseur. De manière générale, nous montrons que l’indenteur Knoop présente un intérêt certain par rapport aux indenteurs habituels, tout au moins dans le cas des céramiques. / Instrumented indentation is widely used for the assessment of mechanical properties, mainly hardness and elastic modulus. In order to obtain reliable results, analysis methodology has to refer several corrections, such as tip defect and frame compliance. However, these corrections are insufficient for the characterization of brittle and porous materials, whether bulk or coatings. Not only brittleness but also local surrounding in the case of heterogeneous materials and substrate influence in the case of coatings raise error concerns. To limit these unwanted effects, the Knoop indenter appears to be a favorable candidate because of the larger contact area and lower indentation depth at equivalent loads. However, compared to other commonly used indenters, radial elastic recovery near Knoop indent that has to be taken into account in the analysis methodology, is not well known. This constitutes therefore an obstacle for its use. Thereof, we propose to adapt the conventional methodology proposed by Oliver and Pharr to obtain reliable results with Knoop indentation. To achieve this purpose, we analyze the indentation data of different dense ceramic materials by comparing Knoop and Vickers indentations, this latter being used as a reference. First, we analyze the elastic recovery using Knoop indenter. We show that correlation to Vickers indentation results is possible using a geometric correction factor. This approach is valid for the determination of elastic modulus and hardness. However, for the latter property, it is necessary to specify the definition used for its calculation. The established analysis methodology was applied for studying the mechanical response of porous bulk materials and coatings, with respect to material porosity, roughness and thickness. It was confirmed that Knoop indenter may be favorable compared to the commonly used indenters, at least in the case of ceramics.

Na indústria automobilística há uma grande preocupação em relação ao custo e ao peso das peças utilizadas em projetos e os adesivos vem sendo utilizados para substituir a solda ponto com esta finalidade. Neste contexto o adesivo epóxi estrutural Betamate 1496 vem sendo empregado pela Volkswagen do Brasil S.A para substituição de solda a ponto na fabricação da porta dianteira do veículo Pólo Hatch. O adesivo Betamate 1496 é utilizado na fabricação da carroçaria do veículo Pólo Hatch da Volkswagen do Brasil SA com essa finalidade. O objetivo do presente trabalho é propor e testar uma metodologia para avaliar o estado de cura do adesivo epóxi Betamate 1496 por meio da determinação do seu módulo de elasticidade e de sua dureza, quando este é empregado na fabricação das portas dianteiras do veículo Pólo Hatch. Estas propriedades foram determinadas pela medição das propriedades mecânicas do adesivo em amostras retiradas em seis posições da porta em duas fases de processamento: estado pré-curado, durante a montagem da porta e estado curado, após a cura final na estufa de pintura. O estudo foi realizado através do ensaio de indentação instrumentada, utilizando os procedimentos experimentais propostos por Franco Jr. e colaboradores e Pintaúde e colaboradores na avaliação das propriedades do polímero. O estado de cura do polímero foi avaliado de modo qualitativo também pela técnica de calorimetria diferencial exploratória (Differential Scanning Calormetry). Os resultados obtidos pela metodologia proposta por Franco Jr. e colaboradores apresentaram grande dispersão de valores, provavelmente pelo fato do método propagar erros de arredondamento na aplicação do algoritmo. Conforme observado nos resultados experimentais obtidos pela metodologia proposta por Pintaúde e colaboradores, no estado pré-curado todas as amostras apresentam valores de módulo de elasticidade muito próximos, em torno de 1,0 + 0,1. GPa, independentemente da posição na porta ou do tempo de descarregamento, já no estado curado observa-se uma diferença significativa dos valores de módulo de elasticidade, E, e dureza H dependendo da posição original da amostra na porta, com as amostras retiradas das posições inferiores apresentando E=0,8 + 0,1 GPa, enquanto que as demais apresentam E=1,0+0,1 GPa, os valores de dureza, H, para todas as amostras curadas foi H=0,03 GPa. A analise qualitativa do ensaio térmico pelas as curvas obtidas no ensaio térmico de DSC mostra que as curvas da amostra no estado pré-curado e curado são idênticas, indicando o ciclo de cura do polímero já se completa na fase de montagem da porta, antes mesmo da cura final na estufa de pintura. / Cost and weight reduction are major concerns of the automobilist industry, this leads to the increasing substitution of spot weld by adhesives in the assembling of automobile parts. In this context Volkswagen do Brazil S.A uses the Betamate 1496 structural epoxy adhesive in the manufacture of the chassis of the Polo v. Hatch. The aim of the present work is to suggest and to test a methodology to evaluate the state of cure of this adhesive by means of the determination of its hardness and elastic modulus when used in the manufacture of the front doors of the Polo v. Hatch vehicle. These properties have been determined by the measurement in six samples removed from different positions at the two phases of processing: after assembling of the chassis and cured state, after final cure at painting. The study was carried through instrumented indentation testing, which used the experimental procedures suggested by Franco Jr. and collaborators and Pintaúde and collaborators. The state of cure of polymer was evaluated also in qualitative way by Differential Scanning Calorimetry. The results obtained by the Franco Jr. methodology show great dispersion of values, probable due to the fact that the method propagates rounding errors in the application of the algorithm. The experimental results obtained by the Pintaúde methodology show that, in the state after assembling of the chassis all samples present elastic modulus, E, around 1,0 + 0.1 GPa, independently of the position in the door. In the cured state is observed depending on the sample’s position at the door: samples taken from the bottom of the door shaved E=0,8 + 0,1 GPa, while the remaining showed E= 1,0 + 0,8 GPa. The hardness, H, of the adhesive is constant (H=0,03 GPa) for all samples. The comparison of DSC curves obtained from the incurred and cured samples, showed that they are very similar, suggesting that the cure cycle of the polymer is already completed at assembling stage of the door.

Na indústria automobilística há uma grande preocupação em relação ao custo e ao peso das peças utilizadas em projetos e os adesivos vem sendo utilizados para substituir a solda ponto com esta finalidade. Neste contexto o adesivo epóxi estrutural Betamate 1496 vem sendo empregado pela Volkswagen do Brasil S.A para substituição de solda a ponto na fabricação da porta dianteira do veículo Pólo Hatch. O adesivo Betamate 1496 é utilizado na fabricação da carroçaria do veículo Pólo Hatch da Volkswagen do Brasil SA com essa finalidade. O objetivo do presente trabalho é propor e testar uma metodologia para avaliar o estado de cura do adesivo epóxi Betamate 1496 por meio da determinação do seu módulo de elasticidade e de sua dureza, quando este é empregado na fabricação das portas dianteiras do veículo Pólo Hatch. Estas propriedades foram determinadas pela medição das propriedades mecânicas do adesivo em amostras retiradas em seis posições da porta em duas fases de processamento: estado pré-curado, durante a montagem da porta e estado curado, após a cura final na estufa de pintura. O estudo foi realizado através do ensaio de indentação instrumentada, utilizando os procedimentos experimentais propostos por Franco Jr. e colaboradores e Pintaúde e colaboradores na avaliação das propriedades do polímero. O estado de cura do polímero foi avaliado de modo qualitativo também pela técnica de calorimetria diferencial exploratória (Differential Scanning Calormetry). Os resultados obtidos pela metodologia proposta por Franco Jr. e colaboradores apresentaram grande dispersão de valores, provavelmente pelo fato do método propagar erros de arredondamento na aplicação do algoritmo. Conforme observado nos resultados experimentais obtidos pela metodologia proposta por Pintaúde e colaboradores, no estado pré-curado todas as amostras apresentam valores de módulo de elasticidade muito próximos, em torno de 1,0 + 0,1. GPa, independentemente da posição na porta ou do tempo de descarregamento, já no estado curado observa-se uma diferença significativa dos valores de módulo de elasticidade, E, e dureza H dependendo da posição original da amostra na porta, com as amostras retiradas das posições inferiores apresentando E=0,8 + 0,1 GPa, enquanto que as demais apresentam E=1,0+0,1 GPa, os valores de dureza, H, para todas as amostras curadas foi H=0,03 GPa. A analise qualitativa do ensaio térmico pelas as curvas obtidas no ensaio térmico de DSC mostra que as curvas da amostra no estado pré-curado e curado são idênticas, indicando o ciclo de cura do polímero já se completa na fase de montagem da porta, antes mesmo da cura final na estufa de pintura. / Cost and weight reduction are major concerns of the automobilist industry, this leads to the increasing substitution of spot weld by adhesives in the assembling of automobile parts. In this context Volkswagen do Brazil S.A uses the Betamate 1496 structural epoxy adhesive in the manufacture of the chassis of the Polo v. Hatch. The aim of the present work is to suggest and to test a methodology to evaluate the state of cure of this adhesive by means of the determination of its hardness and elastic modulus when used in the manufacture of the front doors of the Polo v. Hatch vehicle. These properties have been determined by the measurement in six samples removed from different positions at the two phases of processing: after assembling of the chassis and cured state, after final cure at painting. The study was carried through instrumented indentation testing, which used the experimental procedures suggested by Franco Jr. and collaborators and Pintaúde and collaborators. The state of cure of polymer was evaluated also in qualitative way by Differential Scanning Calorimetry. The results obtained by the Franco Jr. methodology show great dispersion of values, probable due to the fact that the method propagates rounding errors in the application of the algorithm. The experimental results obtained by the Pintaúde methodology show that, in the state after assembling of the chassis all samples present elastic modulus, E, around 1,0 + 0.1 GPa, independently of the position in the door. In the cured state is observed depending on the sample’s position at the door: samples taken from the bottom of the door shaved E=0,8 + 0,1 GPa, while the remaining showed E= 1,0 + 0,8 GPa. The hardness, H, of the adhesive is constant (H=0,03 GPa) for all samples. The comparison of DSC curves obtained from the incurred and cured samples, showed that they are very similar, suggesting that the cure cycle of the polymer is already completed at assembling stage of the door.

Instrumented indentation testing was used to determine viscoelastic properties of bitumen, i.e. shear relaxation modulus. Review of previous studied reveals that indentation testing technique has been drawing increasing attempts in investigations of binder material mechanical properties. Various properties of bitumen, e.g. elastic, viscoelastic, have been successfully determined by indentation testing at different scales and test conditions. The response of bitumen to indentation was studied extensively under a wide range of test parameters such as temperature, loading rate, indenter geometry, etc. This experimental tool was also applied to asphalt cements grading and microstructure study. However, there have been limited numbers of studies at macro levels with the use of spherical indenters to characterize bitumen properties. This motivated the present study. Spherical indentations have been performed with balls of different curvature radii at -5 oC. Load – displacement curves have been measured and used to determine the relaxation moduli of the studied bitumen. Repeatability of the measurements has been evaluated. Obtained results were compared with the observations from DSR relaxation test. The influence of the measurement scale and load level on the results of instrumented indentation testing has been investigated. It has been found that instrumented indentation was able to characterize accurately the viscoelastic behavior of bitumen which can be described by Prony series and agrees well with the results from DSR tests. Nevertheless, the elastic solution failed to produce a proper description of bitumen response during loading phase.

Bulk Metallic Glasses (BMGs), also known as amorphous metals, are of considerable scientific and commercial interest due to their random or chaotic structure. Given their potential use as engineering materials, there is a concomitant need to establish their mechanical properties. However, BMGs are not conveniently available in sufficient volumes (especially experimental and combinatorial compositions), making property determination via conventional tensile or compression testing problematic. Instrumented indentation is ideally suited for this purpose because the testing requires only small sampling volumes and can probe multiaxial deformation characteristics at various length scales. In this technique, conducted generally on a sub-micron regime, the depth of penetration of an indenter, usually a diamond, is measured as a function of the applied load and expressed graphically as load (P) - displacement (h) curves from which a host of mechanical properties can be extracted and studied. In this work, a methodology for using instrumented indentation at nano- and micro- scales to determine the mechanical response of BMGs was developed and implemented. The implementation primarily focused on deformation in the elastic regime but included preliminary results related to the onset of inelastic deformation. The methodology developed included calibration techniques, formulations to extract the machine compliances, verifications using standards and verification for uniqueness of instrument deformation under a spherical indenter. The methodology was different for the two platforms used based on the load-depth response characteristics of the instrument. In the case of the Micro Test platform, the load-depth response of the instrument was linear. In the case of the Nano Test platform, the instrument load-depth response followed a 3/2 power law, representative of Hertzian behavior. The load-depth response of the instrument was determined by subtracting the theoretical response from the corresponding raw load-depth response obtained by elastically indenting a standard steel specimen of known modulus. The true response of the sample was then obtained by subtracting the instrument's response from the corresponding uncorrected load-depth response (raw data). An analytical model to describe the load-train compliance was developed. The methodology was verified using quartz and tungsten standards. Indentation experiments were conducted on Zr41.25Ti13.75Cu12.5Ni10Be22.5 (Vitreloy 1), Cu60Hf25Ti15, Cu60Zr30Ti10 and Fe60Co7Zr10Mo5W2B16 bulk metallic glasses using spherical indenters with diameters 2.8 mm and 100 [micro]m. The spherical geometry results in a simpler stress distribution under the indenter (when compared to a sharp geometry) and furthermore by recourse to spherical indenters the onset of plastic deformation was delayed. In the case of the Zr-based BMG, the experiments showed that the elastic response did not depend on the diameter of the indenter used indicative of the absence of residual stresses in the sample. Large scale plastic deformation was observed when the sample was indented using a smaller diameter indenter. Log scale analysis (i.e., examining the results on a log load vs. log depth response to check for deviation from Hertzian behavior) showed a deviation from a 3/2 fit indicating a deviation from elastic behavior. The onset implied a yield strength value of ~ 4 GPa, higher than the value reported in the literature (~ 2 GPa). Hence, it is believed that the first signs of plastic deformation occurred at lower loads than the predicted loads from the log scale analysis procedure and is expected to occur as discrete bursts. Discrete plastic events or "pop-ins" were observed in the load-depth indentation responses under quasistatic loading conditions, which were believed to be associated with shear band activity. An attempt was made to formulate a mathematical model based on three yield criteria (Drucker-Prager, Mohr-Coulomb and von Mises). Based on the von Mises predictions and comparable experiments on a quartz standard, it was established that the pop-ins observed were real and not an instrument artifact. Multiple load cycles following partial unload experiments showed that the pop-ins affected the subsequent indentation response. The moduli and the yield strength values obtained for the Cu-based BMGs were comparable to the values reported in the literature. There was significant scatter in the indentation data from the Fe-based BMG. Porosity and lack of 100 % compaction were believed to be the reasons for scatter in the data. The financial support of NSF through grant DMR 0314212 is gratefully acknowledged.

Semiconductor die attach materials for high voltage, high reliability analog devices require high thermal conductivity and retention of dielectric strength. A comparative study of effective thermal conductivity and dielectric strength of selected thermoset/ceramic composites was conducted to determine the effect of ceramic particle size and ceramic particle loading on thermoset polymers. The polymer chosen for this study is bismaleimide, a common aerospace material chosen for its strength and thermal stability. The reinforcing material chosen for this study is a ceramic, hexagonal boron nitride. Thermal conductivity and dielectric breakdown strength are measured in low and high concentrations of hexagonal boron nitride. Adhesive fracture toughness of the composite is evaluated on copper to determine the composite’s adhesive qualities. SEM imaging of composite cross-sections is used to visualize particle orientation within the matrix. Micro-indentation is used to measure mechanical properties of the composites which display increased mechanical performance in loading beyond the percolation threshold of the material. Thermal conductivity of the base polymer increases by a factor of 50 in 80%wt loading of 50µm hBN accompanied by a 10% increase in composite dielectric strength. A relationship between particle size and effective thermal conductivity is established through comparison of experimental data with an empirical model of effective thermal conductivity of composite materials.

<p>Understanding the behavior of materials with nanoscale features is important because of both the  size  of  engineering  devices  and  the  internal  microstructure  of  more  bulk  materials.  Many electronic  components  have  been  miniaturized  in  recent  years  to  attend  the  high  demand  of technology development. Similarly, new stronger bulk metallic materials use nm-scale grain sizes or  precipitates  to  increase  their  strength  over  more  conventionally  processed  alloys.  Nanoscale testing   also   offers   a   route   for   mechanical   behavior   understanding   at   the   microscale. Nanoindentation  has  been  used  to  find  structure-properties  relationships  of  nanostructured materials due to its high load-depth resolution and versatility of the test. Nanoindentation can be used  to  find  hardness  and  modulus  of  the  materials,  important  characteristics  to  evaluate mechanical  performance.  An  introduction  to  strengthening  mechanism  and  generalities  of nanoindentation is shown in Chapter 1.</p> <p>This thesis explores how traditional strengthening mechanisms for bulk materials, can be  applied  to  nanomaterials  and  how  the  microstructure  could  be  tailored  to  achieve  the  desired outcomes on the specific materials studied. The first one is the study of mechanical properties of Nanometallic  Foams  (NMF)  and  its  relationship  with  the  nanostructure.  NMFs  of  pure  copper, CuNi and CuZn alloys were fabricated and tested to find the predominant structural and chemical parameters  of  the  mechanical  properties.  Research  on  how  to  control  and  tailor  the  structural parameters of NMF with viscosity of the precursors is shown in Chapter 2. The relative density was  the  most  predominant  parameter  among  the  structural  parameters  studied.  However,  when relative density parameter is isolated, NMF  are more susceptible to strengthen by second phase precipitation instead of solid solution. The solid solution strengthening mechanism was validated with  MD  simulation  and  agrees  with  the  experimental  findings  that  showed  the  addition  of  Ni atoms to Cu have a moderate effect on the mechanical properties. Chapter 3 presents these findings The  second  example  presented  shows  the  strengthening  effect  of  precipitates  in  nanometallic multilayer. The precipitation was achieved by aging treatment. High temperature nanomechanical testing is also presented in Chapter 4. The third and final example, presented in chapter 5, shows how  the  second  phase  precipitation  and  dispersion  strengthening  of  lead-free  solder  SAC  305 compares  between  samples  aged  for  nine  years  at  body  temperature  and  an  accelerated  aging treatments.</p>

A caracterização de materiais utilizando a técnica de indentação instrumentada difundiu-se significativamente na última década, devido às melhorias dos instrumentos que permitem ensaios por esta técnica e à necessidade de se fazer ensaios em pequenos volumes de materiais, como, por exemplo, em filmes finos e materiais com superfícies modificadas por tratamentos superficiais. Neste texto, abordou-se a elaboração de um algoritmo que permita o estudo da resposta de indentação de superfícies de diferentes materiais metálicos, durante e após o contato com um indentador agudo, empregando simulações por elementos finitos e análise dimensional. Na obtenção do algoritmo analisaram-se os efeitos da formação de borda (pile-up) ou da retração (sink-in) do material indentado, o coeficiente de atrito indentador-amostra, as deformações elásticas do indentador, a geometria do indentador e a variação experimental. As relações obtidas permitiram identificar uma falta fundamental de relação única entre as propriedades mecânicas e a forma da curva de indentação para curvas com razão d r/dmax>0,9, onde d r é a profundidade residual da indentação e dmax é o deslocamento máximo do indentador durante o ensaio. Da mesma forma, concluiu-se que a relação de Sneddon tem que ser corrigida tanto pela geometria da área de contato indentador-amostra quanto pela razão entre os módulos elásticos do material e do indentador (E/Ei). Como a área de contato indentador-amostra é afetada não só pela geometria do indentador mas também pelo nível de pile-up ou sink-in nos indentadores piramidais, uma relação foi identificada entre o nível de pile-up/sink-in e o fator de correção b da equação de Sneddon para os indentadores Vickers e Berkovich. Adicionalmente, pequenas diferenças foram observadas entre as curvas P-h (onde h é a profundidade de indentação abaixo da superfície original da amostra) e as curvas P-d (onde d é o valor da aproximação mútua entre indentador e amostra durante a indentação) para um mesmo valor de módulo reduzido Er quando a razão E/Ei é grande. Assim, o módulo reduzido pode sobreestimar ou subestimar a rigidez do indentador, dependendo das propriedades do material indentado. As análises neste trabalho permitiram igualmente observar que as principais limitações na obtenção das propriedades elasto-plásticas usando a curva de indentação instrumentada vêm da falta de unicidade, seguida pela geometria do indentador, isto é, diferenças entre o cone equivalente e os piramidais Vickers e Berkovich, assim como desvios em relação à geometria ideal do indentador, o que inclui arredondamento da sua ponta. Quando não há unicidade, sabendo-se informações adicionais à curva P-d, por exemplo, o valor da área residual da indentação ou o módulo elástico, uma solução única das propriedades mecânicas pode ser obtida. Em paralelo, a variação experimental pode limitar de forma significativa a precisão no cálculo das propriedades, enquanto o atrito indentador-amostra e as deformações do indentador têm efeitos menos significativos. Em termos gerais, observa-se que as funções que compõem o algoritmo desenvolvido neste trabalho permitem: (i) predizer as curvas carga-deslocamento (curvas P-d), produto do ensaio de indentação instrumentada, para um conjunto de propriedades elasto-plásticas conhecidas; (ii) identificar quando uma mesma curva P-d pode ser obtida de mais de um conjunto de propriedades do material indentado (iii) calcular as propriedades mecânicas (dureza, módulo elástico, coeficiente de encruamento e limite de escoamento) de um material usando a curva P-d quando d r/dmax<0,8 ou faixas de propriedades de materiais quando d r/dmax>0,8 e (iv) calcular as propriedades mecânicas (dureza, módulo elástico, coeficiente de encruamento e limite de escoamento) de um material usando a curva P-d e a área residual da indentação. / The interest in material characterization using instrumented indentation techniques has significantly increased in the last decade, due to improvements in testing instruments and the need to carry out tests on small volumes of materials, such as thin films or materials with surfaces modified by other surface treatments. This work addresses the development of an algorithm to analyze the indentation response of a group of metallic materials, during and after the contact with a sharp indenter, using finite element simulations and dimensional analysis. The formulation of the algorithm considered the effects of pile-up or sink-in of the indented material around the indentation, the friction coefficient between the indenter and the sample, the elastic deformation of the indenter, and the defects of the indenter tip. An analysis considering algorithm output and experimental variation was also conducted. The results allowed identifying a fundamental lack of unique relationship between the mechanical properties and the shape of the indentation curve for indentation curves with ratio d r/dmax>0,9, where d r is the residual indentation depth and dmax is the maximum indenter displacement in the test. Similarly, results allowed concluding that Sneddons equation requires a correction by both the geometry of the contact area and the ratio between the elastic moduli of the material and the indenter (E/Ei). As the shape of contact area is affected not only by the geometry of the indenter but also by the level of pile-up or sink-in in pyramidal indenters, a relationship was observed between the level of pile-up/sink-in and the correction factor b in the Sneddons equation for Vickers and Berkovich indenters. Additionally, it was found that the deformation of the indenter is not fully incorporated into indentation data analysis by the consideration of a reduced modulus (Er). Small differences between P-h curves (where h is the indentation depth below the original surface) and P-d curves (where d is the indenter/sample mutual approach) were observed for the same Er when the ratio E/Ei is large. Thus, the reduced modulus can overestimate or underestimate the indenter stiffness, depending on the mechanical properties of the indented material. Additionally, the analysis in this work has identified that the most important limitations in mechanical properties estimation using the indentation curve arise from the lack of uniqueness, followed by deviations in indenter geometry, such as differences between equivalent cone and pyramidal Vickers or Berkovich and tip defects. When non-uniqueness is present, unique solution may be obtained with the knowledge of additional information, in conjunction with P-d data, such as the residual indentation area or the elastic modulus. Furthermore, even when a unique solution is available the experimental variation may significantly decrease the accuracy in mechanical properties estimation, whereas friction and indenter deformation have less significant effects. In general, it was observed that the proposed algorithm allows: (i) predicting the load-displacement curves P-d of instrumented indentation tests for a set of known elastic-plastic mechanical properties, (ii) identifying when the same P-d curve can be obtained from more than one set of mechanical properties of the indented material, (iii) calculating the mechanical properties (hardness, elastic modulus, yield stress and strain hardening coefficient) from P-d curves when d r/dmax<0,8 or possible ranges for each mechanical property when d r/dmax>0,8 and (iv) calculating the mechanical properties (hardness, elastic modulus, yield stress and strain hardening coefficient) from P-d curves and the knowledge of the residual indentation area.

Cette thèse est un travail prospectif sur la caractérisation multi-échelle de matériaux à gradient de propriétés générés par des traitements de surface de type mécanique (grenaillage à air comprimé ou par ultrasons) ou thermochimique (nitruration, implantation ionique, cémentation basse température). Les apports de plusieurs techniques de caractérisation (microscopie électronique à balayage, spectrométrie, indentation instrumentée, microscopie interférométrique), à différentes échelles, et l’existence possible d’une signature des traitements de surface étudiés sur le matériau ont été examinés. Une analyse multi-échelle des échantillons grenaillés par ultrasons a permis d’établir un lien entre les paramètres procédé et la rugosité du matériau. Une approche originale statistique a été proposée pour déterminer la dureté d’un matériau modifié par un traitement de surface donné sans altérer la surface par une rectification. Elle a permis d’établir un lien entre la rugosité des échantillons grenaillés par ultrasons et leur dureté. Une recherche bibliographique détaillée a été réalisée sur la simulation de l’essai d’indentation instrumentée par éléments finis en étudiant une centaine d’articles afin d’évaluer l’influence des hypothèses des modèles sur leurs résultats. A l’aide d’un modèle éléments finis, la sensibilité des courbes d’indentation à une variation des paramètres matériau a été examinée. Cela a permis de mettre en place une réflexion sur l’identification des propriétés d’un matériau à gradient à l’aide de l’essai d’indentation. / This thesis is a prospective work on the multiscale characterization of plastically graded materials obtained with mechanical (shot peening using air pressure or ultrasounds) or thermochemical (nitriding, ion implantation, low temperature carburizing) surface treatments. The benefits of several characterization techniques (scanning electron microscopy, spectrometry, instrumented indentation, interferometric microscopy), at different scales, and the possible existence of a signature of the studied surface treatments on the material were examined. A multiscale analysis of the ultrasonically shot-peened specimens linked the process conditions with the material roughness. A novel statistical approach was proposed to determine the treated material roughness without deteriorating the surface through resurfacing. The latter enabled to establish a link between the shot-peened specimen roughness and their hardness. A detailed literature review of a hundred articles examined the IV simulation of the instrumented indentation test with finite elements in order to assess the effect of different hypotheses on the simulation results. Using a finite element model, the sensitivity of indentation curves to a variation of material parameters was examined. The latter enabled to reflect on the identification of the properties of plastically grade materials using the instrumented indentation test.

L'indentation instrumentée est un outil pratique et puissant pour sonder les propriétés mécaniques des matériaux à petite échelle. Cependant, plusieurs erreurs (rugosité de surface, effet de taille d’indentation, la détermination de premier point de contact, etc.) affectent l'essai d’indentation instrumentée (e.g. non reproductibilité de la courbe d’indentation) et conduisent à des imprécisions dans la détermination des propriétés mécaniques des matériaux analysés. Une approche originale est développée dans cette thèse pour la caractérisation précise des propriétés mécaniques des matériaux. Cette approche fondée sur une analyse statistique des courbes d’indentation avec la prise en compte d’erreur dans la détermination du premier point de contact et des effets de la rugosité de surface. L’approche est basée sur une minimisation de la distance (défini comme l'erreur de la profondeur de contact initiale) entre l’ensemble des courbes expérimentales et celles simulées par le modèle de Bernhard de manière à générer une courbe maitresse « unique » représentative du faisceau de courbes expérimentales. La méthode proposée permet de calculer à partir de cette courbe maitresse la macro-dureté et le module d’Young du matériau en tenant compte des erreurs dues à la rugosité de surface et à l'effet de taille en indentation pour les faibles profondeurs de pénétration. La robustesse de la méthode est prouvée par son application à différents groupes d’échantillons, i.e. panels de matériaux à propriétés mécaniques diverses, différents traitements de surface (polissage, sablage) et différentes pointes d’indentation permettant de générer différents états de contraintes locaux. Une liaison quantitative entre la rugosité de surface et l'écart type de l'erreur de la profondeur de contact initiale est établie grâce à une analyse multi- échelle de la rugosité de la surface. La méthode proposée permet de caractériser les propriétés mécaniques des matériaux sans avoir recours à la préparation de surface pouvant potentiellement altérer ses propriétés (e.g. génération de contraintes résiduelles, contamination de surface…). / Instrumented indentation is a practical and powerful tool for probing the mechanical properties of materials at small scales. However, several errors (surface roughness, indentation size effect, determination of first contact point, etc…) affect the instrumented indentation testing (e.g. the low reproducibility of the indentation curves) and lead to inaccuracies in the determination of mechanical properties of materials analyzed. An original approach is developed in this thesis for the accurate characterization of the mechanical properties of materials. This approach is established by a statistical analysis of the indentation curves with taking account of error in determining the first contact point and effects of the surface roughness. This approach is basing on a minimization of the distance (defined as the initial contact depth error) between the experimental indentation curves and the ones simulated with Bernhard’s model in order to generate a “unique” representative curve which enables to represent all the experimental curves. The proposed method permits to calculate the macro-hardness and the Young’s modulus of materials from this representative curve with the consideration of the errors due to the surface roughness and the indentation size effect for shallow penetration. The robustness of the method is proved by its application to different groups of specimens, i.e. different materials with various mechanical properties, different surface preparation methods (polishing, sandblasting) and different indenter tips to generate different states of local stresses. A quantitative link between the surface roughness and the standard deviation of initial contact depth error is established by a multi-scale surface roughness analyzing. The proposed method enables to characterize the mechanical properties of materials without resorting to the surface preparation which may potentially alter its properties (e.g. generation of residual stresses, surface contamination ...).