Volume orientation: a practical solution to analyse the orientation of fibres in composite materials.

Mechanical properties of fibres reinforced composite materials depend on the type of fibres used, their percentage as well as their arrangement and orientation. As computer technology continues to improve, high-resolution computed tomography has proven to be an ideal instrument to analyse the structure of this kind of materials. In this context, various approaches have been proposed to detect the fibre orientation distribution and the relative degree of anisotropy of these composite materials. Some of these approaches are based on 'individual' measurements that isolate and reconstruct each single fibre and measure its properties. On the other hand, other approaches capture the characteristics of the fibre distribution by means of 'global' measurements computed on the entire set of tomographic data. The first methods are more precise but also more complex because they demand a procedure able to segment and separate each single fibre in the polymer, whereas the latter are easier to implement and can be applied even if fibre segmentation and separation is not effective or practicable. In this paper, a global method based on the technique called volume orientation - originally proposed several years ago to study the anisotropy of bone structures - is applied to fibre reinforced composite materials. This new approach does not require data acquired at very high resolution nor very complex procedures for individual segmentation of the fibres, but only binarised data through common thresholding procedures. The effectiveness of the proposed new approach is demonstrated by comparing it to the results obtained from a method based on individual measurements: when resolution and images quality are good enough, the volume orientation method gives results quite similar to the other approach. The analysis of three different case studies demonstrates its flexibility and its validity as an alternative to methods based on the separation of individual fibres, which are not always usable. The samples have been carefully selected in order to range between different attenuation contrast levels and also include a specimen subjected to mechanical testing which can be of great practical interest. LAY DESCRIPTION: Mechanical properties of fibres reinforced composite materials depend on the type of fibres used, their percentage as well as their arrangement and orientation. Today, both destructive and nondestructive techniques can be used in order to assess the fibre orientation. As computer technology continues to improve, high-resolution computed tomography has proven to be an ideal instrument to analyse the structure of this kind of materials, and then the fibre orientation distribution inside the material. In this context, various strategies have been proposed. Some of them require measurements that isolate and reconstruct each single fibre and measure its properties. On the other hand, other approaches capture the characteristics of the fibre distribution by means of 'global' measurements computed on the entire set of tomographic data. The first methods are more precise but also more complex because they demand a procedure able to detect and separate each single fibre in the polymer, whereas the latter are easier to implement and can be applied even if fibre segmentation and separation is not effective or practicable. In this paper, a global method based on the technique called volume orientation - originally proposed several years ago to study the microstructure of bone tissues - is applied to fibre reinforced composite materials. The aim of this work is to demonstrate that this new approach is easier to use. As a matter of fact, it does not require data acquired at very high resolution nor very complex procedures for individual segmentation of the fibres, but only binarised data through common thresholding procedures. The effectiveness of the proposed new approach is shown by comparing it to the results obtained from a method based on individual measurements: when spatial resolution and images quality are good enough, the volume orientation method gives results quite similar to the other already used approach. The analysis of three different case studies demonstrates its flexibility and its validity as an alternative to methods based on the separation of individual fibres, which are not always usable. The samples have been carefully selected in order to range between different attenuation contrast levels and different nature of the fibres (mineral, vegetable or synthetic). A specimen subjected to mechanical testing is also included, because of its great practical interest.

Relationship between the anisotropy tensor calculated through global and object measurements in high-resolution X-ray tomography on cellular and composite materials.

Structural anisotropy of two-phase materials can be evaluated through global measurements, as volume orientation or mean-intercept length methods do, or through statistics performed on a set of individual measurements. This last procedure is encouraged by recent improvements in the spatial resolution of conventional X-ray tomography. In this paper, the above-described approaches were compared in three case studies: a foam subjected to an in situ compression test, a second foam with a completely different cell morphology and a plastic material reinforced with short fibres. The approach based on the subdivision into distinguishable objects of the considered material phase has proved to be more sensitive in highlighting small deformations in the structure or small irregularities in an otherwise isotropic structure. On the other hand, the other approach is more general and is always usable. The two methods for calculating the fabric tensor tend to converge as the average anisotropy of individual objects in the statistical population increases. The use of Lambert's cylindrical equal-area projection of cell/fibre directions or local volume orientations is suggested, because the density of points is preserved from the sphere to the plane surface. Finally, a quick vector method to evaluate the anisotropy of the directions distribution has been presented, by defining a coherence index of the average direction.

New approach in Auger elemental relative sensitive factor calculation by using TEM-EDS analysis based on bi-layers of pure elements.

In fabrication of microelectronic devices two important steps are often recognized: i) all the processes performed on the wafer in order to build the active part of the devices and, ii) the assembly and packaging processes, typically performed on a chip, in order to fabricate interconnections between active part and exterior. The wafer back side is an active part of power devices and is normally coated with a stack of Ti-Ni-Au or Ti-Ni-Ag layers to ensure the best electrical contact with the frame on which the device is attached prior to the packaging. An important failure mechanism related to this particular process step is related to the diffusion of Ni to the surface of the stack that causes its oxidation on the back metal surface, inhibiting the correct connection to the metallic frame. Auger Electron Spectroscopy (AES) is a powerful analytical technique that can be used to detect this failure mechanism for its very high sensitivity in the characterization of surface layers. Unfortunately, its results are mainly qualitative. Quantitative extrapolations can be inaccurate using library Elemental Relative Sensitive Factor (ERSF) because they are mainly referred to a silicon substrate and could be not valid for a different matrix. A most accurate evaluation of the ERSF is based on the analysis, under identical experimental condition, of standard materials (with known concentration) that should be similar to the unknown sample and having the same matrix. However, the production of this kind of standard is not easy due to the mobility of Ni in Au and Ag. Another commonly used technique is the Energy Dispersive X-ray Spectrometry (EDS) which is less sensitive than the Auger and not sufficiently adequate for a quantitative analysis due to the limitation of the matrix correction methods. Recently, a new method to perform quantitative analysis by using Transmission Electron Microscopy (TEM) EDS was proposed, starting from bi-layers of pure elements. In this work we show how the use of TEM-EDS quantification of Ni in Ag could be a successful method for ERSF evaluation in order to overcome matrix effect in Auger quantification. For this purpose suitable foils of Ag/Al and Ni/Al were used. The validation of the method was performed on a sample with a tri-metal stack of Ti/Ni/Ag previously stimulated by means of a thermal budget to induce Ni migration on Ag surface. The quantitative analysis allowed us to use this characterized sample as AES standard for ERSF calculation.

Comparison of spatial resolutions obtained with different signal components in scanning electron microscopy.

Comparative studies on the ultimate spatial resolution of the Scanning Electron Microscope, using different components of the electron signal have been performed on specimens providing compositional contrast. By operating the microscope in conventional way as well as with a specifically designed set-up we have ascertained that the delocalized components of the signal provide a spatial resolution of the order of the beam size, even if the practical use can be limited by the noise. To amplify the contribution of the delocalized components of the signal, as backscattered electrons by a bulk specimen or forward scattered electrons by a thin specimen, we used a device consisting of a plate of a material with high secondary yield placed above or below the sample. An important practical implication arises from this study. A detecting system consisting of a standard Everhart-Thornley detector coupled with a converter of backscattered or transmitted electrons represents a high performance detecting device for low voltage observations.