Investigation of wear mechanisms through in situ observation during microscratching inside the scanning electron microscope

Scratch experiments have been used in the past to investigate various contact phenomena such as abrasion mechanisms or asperity contacts. Depending on tip geometries, loading conditions and materials investigated, different crack propagation modes (radial or lateral cracks, etc.) or deformation modes (ploughing, chipping, etc.) may dominate the scratching process. The residual scratch path can yield some information about dominant deformation and fracture modes. It is, however, often not possible to uniquely correlate cracks and other phenomena with events on the recorded load-displacement curves. We have built a miniaturized microscratch device for use inside a scanning electron microscope (SEM) that allows the observation of the surface around the tip with sub-micrometer resolution during scratching. Using a conical indenter with spherical tip we demonstrate on different materials that the device is a powerful tool to observe initiation and propagation of cracks, to observe the flow of the material near the indenter (piling-up and sinking-in) and to study chip and particle formation mechanisms during microscratching. In GaAs, particles were observed to form in front and on the rear side of the tip via interaction of chevron cracks. In the case of a Fe-based bulk metallic glass, shear bands were observed to form in front of the tip leading to serrated chip formation. Discontinuities in the tip penetration during scratching of a polymer thin film were related to the onset of crack formation behind the tip and to the propagation of semi-circular cracks in front of the tip. The observed large elastic recovery of the polymer film at the rear side of the tip has to be taken into account for accurate contact area calculations

Investigation of wear mechanisms through in situ observation during microscratching inside the scanning electron microscope

Scratch experiments have been used in the past to investigate various contact phenomena such as abrasion mechanisms or asperity contacts. Depending on tip geometries, loading conditions and materials investigated, different crack propagation modes (radial or lateral cracks, etc.) or deformation modes (ploughing, chipping, etc.) may dominate the scratching process. The residual scratch path can yield some information about dominant deformation and fracture modes. It is, however, often not possible to uniquely correlate cracks and other phenomena with events on the recorded load–displacement curves. We have built a miniaturized microscratch device for use inside a scanning electron microscope (SEM) that allows the observation of the surface around the tip with sub-micrometer resolution during scratching. Using a conical indenter with spherical tip we demonstrate on different materials that the device is a powerful tool to observe initiation and propagation of cracks, to observe the flow of the material near the indenter (piling-up and sinking-in) and to study chip and particle formation mechanisms during microscratching. In GaAs, particles were observed to form in front and on the rear side of the tip via interaction of chevron cracks. In the case of a Fe-based bulk metallic glass, shear bands were observed to form in front of the tip leading to serrated chip formation. Discontinuities in the tip penetration during scratching of a polymer thin film were related to the onset of crack formation behind the tip and to the propagation of semi-circular cracks in front of the tip. The observed large elastic recovery of the polymer film at the rear side of the tip has to be taken into account for accurate contact area calculations

Correlating the rheological and mechanical response of polyurethane nanocomposites containing hyperbranched polymers

Cast polyurethane thermosets have been modified with randomly oriented exfoliated and intercalated montmorillonite clay usinghyperbranched polymers (HBP) as reactive additives to promote dispersion of the clay platelets. The influence of the clay content on thestiffness of the nanocomposites is discussed in terms of simplified classical micromechanical models that allow correlations to be establishedbetween the properties of the final polyurethane and the limiting high strain rate shear viscosity of the HBP or HBP/polyethylene glycolnanocomposite precursors. Such models imply certain restrictions on the potential for mechanical reinforcement with unoriented plateletsowing to crowding effects. Moreover, they appear unable to account simultaneously for the observed degrees of reinforcement in the glassyand the rubbery states, if currently accepted values for the stiffness of the MMT are assumed. Possible reasons for this are discussed

Irradiation control of the “SPIRAL1” target by measuring the ion beam intensity: “CICS” Project

International audienceIn order to obtain a more precise control on the irradiation of the targets of the “SPIRAL1” installation and to optimize the experiments schedule as well as the exploitation costs, a new criterion of safety is respected. This new safety criterion is the maximum dose (maximum number of ions stopped in the Spiral1 Target). To control this, an AQ system has been put in operation and more specifically a new device has been set up in order to measure the ion beam intensity and to calculate the number of particules per second. This value can then be integrated over time. This device mainly consists of two redundant instrumentations, which are acquired via a real time industrial controller. The accuracy of measurement is estimated taking into account the variation of beam, of the environment and of the installation. This system obtained the agreement of the French nuclear safety authorities and is operational since September 2007