Quantitative estimation of closed cell porosity in low density ceramic composites using X-ray microtomography.

X-ray Microtomography is a proven tool for phase fraction analysis of multi-phase systems, provided that each phase is adequately partitioned by some means of data processing. For porosity in materials containing low-density ceramic phases, differentiation between pores and the low-density phase(s) can be intractable due to low scattering in the low-density phase, particularly if small pores necessitate low binning. We present a novel, combined methodology for accurate porosity analysis-despite these shortcomings. A 3-stage process is proposed, consisting of (1) Signal/noise enhancement using non-local means denoising, (2) Phase segmentation using a convolutional neural network, and (3) Quantitative analysis of the resulting 3D pore metrics. This particular combination of denoising and segmentation is robust against the fragmentation of common segmentation algorithms, while avoiding the volitional aspects of model selection associated with histogram fitting. We discuss the procedure applied to ternary phase SiC-TiC-diamond composites produced by reactive spark plasma sintering with porosity spanning 2-9 vol%.

Synthesis of Au-MoS(2) nanocomposites: thermal and friction-induced changes to the structure.

The synthesis of Au-MoS2 nanocomposite thin films and the evolution of their structures during film growth, in situ transmission electron microscopy (TEM) heating, and sliding contact were investigated. TEM revealed that the films deposited at ambient (room) temperature (RT) consisted of 2-4 nm size Au particles in a matrix of MoS2. With increasing growth temperatures, the nanocomposite film exhibited structural changes: the Au nanoparticles coarsened by diffusion-driven Ostwald ripening to 5-10 nm size and the MoS2 basal planes encapsulated the Au nanoparticles thereby forming a solid Au-core MoS2 structure. However, when the RT deposited film was heated inside the TEM, the highly ordered MoS2 basal planes did not encapsulate the Au, suggesting that MoS2 surface diffusivity during film growth is different than MoS2 bulk diffusion. Increases in MoS2 crystallinity and coarsening of Au nanoparticles (up to 10 nm at 600 °C) were observed during in situ TEM heating of the RT deposited film. Sliding contact during friction and wear tests resulted in a pressure-induced reorientation of MoS2 basal planes parallel to the sliding direction. The subsurface coarsened Au nanoparticles also provide an underlying load support allowing shear of surface MoS2 basal planes.