Michael Nöthe

Cooperative material transport during the early stage of sintering

The complex transport processes contributing to sintering are not yet fully understood, partially because in-situ observations of sintering in three dimensions (3D) are very difficult. Here we report a novel experiment in which monocrystalline copper spheres are first marked with microscopic boreholes drilled using a focused ion beam, after which high-resolution synchrotron X-ray tomography is carried out to measure translational, rolling and intrinsic rotation movements of some hundred spheres during sintering. Unlike in 1D and 2D systems, we show that, in 3D, intrinsic rotations are more pronounced than angular rolling rearrangements of the particle centres and become the dominant mechanism of particle movement. We conclude that in addition to the well-known neck growth and centre approach mechanisms, grain boundary sliding caused by the different crystallographic orientations of the individual spheres occurs.

Diffraction by image processing and its application in materials science

Abstract A spectral theory for constituents of macroscopically homogeneous random microstructures is developed and provided with a sound mathematical basis, where the spectrum obtained by Fourier methods corresponds to the angular intensity distribution of X-rays scattered by this constituent. It is shown that the fast Fourier transform applied to three-dimensional images of microstructures obtained by micro-tomography is a powerful tool of image processing. The applicability of this technique is demonstrated in the analysis of images of porous media.

Investigation of Sintering of Spherical Copper Powder by Micro Focus Computed Tomography (μCT) and Synchrotron Tomography

The two-particle model describes the approach of particle centres and the growth of the interparticle contacts during sintering of metal powders. Unfortunately the comprehensive description of processes inside of three dimensional specimens must consider the contribution of particle rearrangements. The recent developments of combined micro focus computed tomography (CT) and 3D photogrammetric image analyzing give the opportunity to obtain the experimental data required to overcome the shortcomings of sintering theories based on the two-particle model. The analysis of spherical poly and single crystalline copper powder was performed by CT. In addition a single crystal specimen was analyzed by high resolution synchrotron radiation tomography - a more sophisticated analysis method with very limited availability. The analysis of the 3D tomographic image by photogrammetric image analyzing yielded the positions and radii of all particles and their contact partners as well. A statistical analysis of the retrieved data was performed. The formation and breaking of necks during sintering could be observed. An in-depth analysis of the particle rotation with respect to the coordination number and local density will be presented.

A 1800 K furnace designed for in situ synchrotron microtomography.

A radiation furnace that covers the temperature range from room temperature up to 1800 K has been designed and constructed for in situ synchrotron microtomography. The furnace operates under a vacuum or under any inert gas atmosphere. The two 1000 W halogen heating lamps are water- and air-cooled. The samples are located at the focus of these lamp reflectors on a rotary feedthrough that is connected to a driving rotation stage below the furnace. The X-ray beam penetrates the furnace through two X-ray-transparent vacuum-sealed windows. Further windows can be used for temperature control, sample changing and gas inflow and outflow.

Analysis of Particle Rearrangement during Sintering by Micro Focus Computed Tomography (µCT)

The decrease of the distance between particle centers due to the growth of the sinter necks can be explained by the well known two-particle model. Unfortunately this model fails to provide a comprehensive description of the processes for 3D specimens. Furthermore, there is a significant discrepancy between the calculated and the measured shrinkage because particle rearrangements are not considered. Only the recently developed analysis of the particle movements inside of 3D specimens using micro focus computed tomography (μCT), combined with photogrammetric image analysis, can deliver the necessary experimental data to improve existing sintering theories. In this work, μCT analysis was applied to spherical copper powders. Based on photogrammetric image analysis, it is possible to determine the positions of all particle centers for tracking the particles over the entire sintering process and to follow the formation and breaking of the particle bonds. In this paper, we present an in-depth analysis of the obtained data. In the future, high resolution synchrotron radiation tomography will be utilized to obtain in-situ data and images of higher resolution.

Investigation of Sintering Processes by Tomography

Sintering especially of loose particle packings is accompanied by dimensional changes of the specimen. The growth of inter particle contacts under the influence of the Laplace pressure is well understood and described by the two particle model. On the other hand the understanding of other fundamental sintering phenomena i.e. cooperative material transport processes is rather vague. To overcome this limitation for near net-shape production of components by powder metallurgy an improved model of particle rearrangement processes is required. First efforts to obtain necessary experimental data were performed at 1D and 2D models. But recent improvements of high resolution synchrotron computer tomography (SCT) setups allow the acquisition of in-situ data of particle rearrangements. In-situ studies of particle rotations during sintering were conducted at the ESRF in Grenoble. The rotations during free sintering of monocrystalline particles were investigated during continuous heating up to 1050 °C or with frequent interruptions of the heating by 1 hr dwell times every 100 °C. In contrast to 1D specimens measured by Wieters the 3D specimens showed negligible rotations. This must be attributed to the constraints in 3D samples. To obtain a more detailed insight in the rotations the particles of one sample were marked. It is possible to show that the particles perform intrinsic rotations. Therefore a new rotation model is developed. The intrinsic rotations are confirmed by complementary EBSD analyses as well.

Volumetric analysis of a sinter process in time

We present the first fully three dimensional analysis of the sinter process of copper using particles of a realistic size. This has been made possible through the use of μCT. A 3D image processing chain, applied to each time step of this 4D dataset, followed by image registration and particle matching steps was used. This allows for the tracking of individual particle motions during the sintering process, which gives a large amount of information towards understanding this process.