A new technique to measure the true contact area using nanoindentation testing

Nanoindentation technique requires the determination of projected contact area under load for calculation of modulus and hardness of materials. This projected contact area is usually calculated by models which take into account the pile-up or sink-in phenomena around the tip. The most commonly used model was developed by Oliver and Pharr [1] which can precisely model the sink-in around the tip, but cannot account for pile-up. Another model developed by Loubet et al can be used [2]. It can take into account the pile-up and the sink-in phenomena and can precisely measure the projected contact area for a large range of materials, except for materials with high strain hardening exponent. Other techniques, like post mortem measurements, can be used. However these measurements do not take into account the elastic recovery during unloading. A new technique to estimate the true projected contact area will be presented. It consists of combining two models (The Dao et al. model and the Kermouche et al. model) that are used normally to calculate the representative stress and the representative strain in indentation. Consequently, the projected contact area calculation does not depend on any contact area model. Moreover, it can account for the pile-up or sink-in phenomenon and the strain hardening of the material, which is not possible with the actual models used. This new technique requires measuring indentations parameters like the maximum load, the contact stiffness and the loading curvature. It requires also the use of two tetrahedral indenters: a Berkovich tip and a tetrahedral tip where the included semi-angle is 50°. The method was tested on three different samples: glass, PMMA and 100C6 steel. For indentations on glass and PMMA samples, the projected contact area was precisely measured. For indentations on 100C6 steel sample, the method was adapted to take into account the Indentation Size Effect observed at small indentation depths. The projected contact area values measured with this new technique will be presented and compared to the values calculated with classical literature models. Also, the limits of the technique will be discusse

Investigation of contact-induced near-surface materials transformations using nanomechanical testing.

Mechanical surface treatments, such as shot peening – burnishing – deep rolling, are known for their efficiency to improve resistance to abrasive wear and local fatigue crack propagation. They are based on repeated contact loadings that create large plastic strains in the near-surface leading to compressive residual stress field and local grain refinement (Tribologically Tranformed Surfaces, Fig1). A significant gradient of mechanical properties over 100 µm is usually observed. This paper aims to present a methodology based on nanomechanical testing –i.e. micropillar compression, nanoindentation - and EBSD measurements to explain microstructure changes induced by such treatments. This methodology is applied to various cases ranging from severe shot peening (Fig1) to sliding friction contacts (Fig2). Please click Additional Files below to see the full abstract

Microshear mechanical properties measurements on tribolayers

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High strain rates micromechanical behavior of materials: A coupled experimental and numerical approach

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Mechanical behavior and size effects of polymer/amorphous NiB composites with 3D micro‐ architectures

Micro-architectured materials are a new class of hierarchical cellular material with outstanding properties. By designing advantageous cellular geometries and combining the material size effects at the nanometer scale, lightweight hybrid micro-architectured materials with hierarchical cellular structures and tailored structural properties are achieved. Previous papers have reported the mechanical properties of ceramic/polymer composites but few studies have examined the properties of similar structures with metal coatings instead of ceramic. To estimate the mechanical performance of polymer cellular structure reinforced with a metal coating, we combined 3D laser lithography and electroless deposition of an amorphous layer of NiB to produce metal/polymer hybrid structures. In this poster, the fabrication of 3D hybrid structures by electroless deposition aiming at achieving high and yet low density material will be presented. We also studied the mechanical response of micro-architectured structures as a function of the architecture design and the thickness of the amorphous NiB layer on their deformation mechanisms. In situ SEM microcompression experiments revealed a change in the deformation behavior with the NiB layer thickness, suggesting that the deformation mechanism and the buckling behavior are controlled by the size induced brittle-to-ductile transition in the NiB layer. In addition, the energy absorption properties demonstrate the possibility of tuning the energy absorption efficiency with adequate designs. Please click Additional Files below to see the full abstract

Indentation relaxation test: Opportunities and limitations

Small scale characterization of material’s mechanical behavior has been performed for fifty years using indentation tests. Many developments have been made in order to improve the reliability of both measurements and interpretations. However, determination of material’s time dependent mechanical properties by means of nanoindentation techniques is still to be enhanced 1. It is proposed to investigate the indentation relaxation – i.e. constant displacement – test as an alternative to the commonly used indentation creep – i.e. constant load – test. Effects of loading strain rate on the measured relaxation behavior are studied, analytically, from a linear viscoelastic model. It is shown that constant strain rate loading guarantees a depth-independent measure of the relaxation behavior. Moreover, indentation strain rate (ISR) affects the relaxation spectrum 2 up to a critical time constant 3 (see figure 1). These effects, highlighted analytically, are confirmed experimentally on PMMA. Limitations of the indentation relaxation test are also discussed. Two main difficulties arise from this kind of experiment. Acquisition of reliable measurements is limited, for long time characterization, by the system drift and, for short time, by the displacement control loop. A particular care has been taken in tuning the control feedback gains to limit displacement overshoot. Very low drift rate has been attained – under 0.015 nm.s-1 – This allowed for measurements at constant displacement up to 600 s. Please click Additional Files below to see the full abstract

High-temperature nano-impact testing of a hard-coating system

Forging and cutting tools for high-temperature applications are often protected using hard nanostructured ceramic coatings. While a moderate amount of knowledge exists for material properties at room temperatures, significantly less is known about the system constituents at the elevated temperatures generated during service. For rational engineering design of such systems, it is therefore important to have methodologies for testing these materials to understand their properties under such conditions (i.e. high strain rate, temperature, or impact). In this work, we present our first results using a newly developed Alemnis piezo actuated nanoindenter device which utilizes dynamic indentation testing at frequencies approaching 10 kHz. A sinusoidal displacement amplitude input is provided, while a stage heater allows for sample temperatures exceeding 500 °C. Thermal drift can be minimized through high frequency, and therefore low contact time, impacts. We investigated a thin (4.65 μm) physical vapor deposited chromium nitride (CrN) ceramic coating, which had been deposited onto plasma nitrided tool steel. Forces of approximately 400 mN were applied sinusoidally at 500 Hz using a 5 μm diameter diamond flat-punch at room temperature, 200°C, 300°C, 400°C and 500°C. It was found that increasing the number of impacts led to plastic deformation and fatiguing of the hard ceramic coating. At 300°C a transition to increased material flow and consequently larger crater size, and crack initiation and propagation in the ceramic, was observed. These ceramic deformation results are understood using high-resolution scanning electron microscopy (HR-SEM), elastic simulations, and large scale batch processing of force-deformation data which are generated during high-frequency measurement and collected at a sampling rate of 50 kHz. The results are further put into context by understanding recently measured small-scale high-temperature fracture toughness and yield strength properties of thin CrN films. The presented results are the first for in situ high-temperature nano-impact testing, and will be useful for hard coatings industries involving high service temperatures and high impact strain rates, such as for forging processes

Constant contact stiffness indentation relaxation test

Nanoindentation test is of great interest to characterize small scale mechanical behavior, thus a large literature exists on the field. Nevertheless, measurements of time dependent mechanical properties by this technique is still to be improved 1. It is proposed to investigate the indentation relaxation from a different point of view. Indentation relaxation tests are usually performed keeping a constant displacement over a prescribed time duration 2. This experimental procedure is consequently very sensitive to the system drift. Hence, determination of relaxation behavior is limited to few hundreds of seconds in the best cases. Weihs and Pethica 3 and Maier et al. 4, proposed to use the continuous contact stiffness measurement as a robust measure of the contact area. Based on these studies, a novel experimental procedure has been developed. Contact stiffness is kept constant after loading to a prescribed depth, for a define period, while displacement and load are monitored. As the contact stiffness measurement is not sensitive to drift, this method allowed to perform relaxation experiments with very long hold segment. Experiments on fused silica and polymers - i.e. PMMA, PC and PS - at room temperature have been performed with a constant contact stiffness maintained up to 10 hours. It has been shown that the dispersion on the force, F, was greatly reduced (see Figure 1). This could be understood as constant contact stiffness experiments were much less affected by the system drift than constant displacement ones. This new method opens the way to time dependent mechanical characterization in a wider range of conditions, especially long time experiments and high temperature indentation tests. Please click Additional Files below to see the full abstract

Quantification of mechanical properties gradient by nano-indentation and microcompression testing on mechanically-induced transformed surfaces

In the industry, there are several techniques which improve the service lifetime of materials by increasing the local mechanical properties in the near-surface. In the case of mechanical surface treatments (such as impact-based), the material is exposed to repeated mechanical loadings, producing a severe plastic deformation in the surface, and then leading to a local refinement of the microstructure into the affected zone (Tribologically Transformed Surfaces - TTS). The microstructure’s transformation is characterized by a progressive increment of the grain size from the surface until the bulk material. Consequently, very interesting physical properties such as high hardness and better tribological properties are exhibit in these mechanically-induced transformed surfaces. Nowadays, it is well-known that the grain size gradient generated provokes an evolution on the mechanical properties in the impacted zone over a few tens of microns. However, a simple micro-hardness test is not quite enough to quantify precisely the engendered variation of mechanical properties due to the heterogeneity of the transformed surface. The main issue of this work is to assess and describe precisely the elastic-plastic behavior and the distribution of mechanical properties on deformed zones of a model material (pure iron). In our project, a characterization of the transformed microstructure, as well as a statistics measurement of the grain size distribution on the cross-section of the sample is presented firstly. Afterwards a methodology based on nano-indentation tests (Fig.1) and in-situ micro-pillars compression tests (Fig.2) is implemented to quantify the evolution of mechanical properties starting from the near-surface. A relation between the hardness gradient and the microstructure evolution is established, as well as a comparison between the properties measured by both techniques is discussed