Surface/interface phenomena in nano‐multilayer coating under severing tribological conditions

An extensive study of surface/interface phenomena during wear of an adaptive TiAlCrSiYN/TiAlCrN nano‐multilayer coating deposited using physical vapor deposition was undertaken under increasingly severe tribological conditions associated with dry end milling of H13 hardened tool steel. The results of FEM modeling on the temperature/stress distribution at different cutting speeds outline actual cutting conditions on the both rake and flank frictional surfaces of the coated tool. Studies of the surface/interface phenomena were made by means of SEM/high‐resolution transmission electron microscopy/XPS analyses. Results demonstrate that intensifying tribological conditions facilitates improved wear performance of the adaptive coating layer. In extreme tribological conditions of ultra‐performance machining (cutting speed of 500 m/min), the self‐organization process establishes entirely through the formation of a nano‐scale layer of dynamically re‐generating tribo‐ceramic films. The formation of these surface nano‐films results in exceptionally efficient protection of the underlying coating layers. In response to the extreme external environment, the coating layer remained almost undamaged during a long run, demonstrating the capacity to efficiently replenish necessary tribo‐ceramic films. In this way, interconnection of various surface and undersurface processes is established in the hierarchically structured tribo‐films/coating layer. This integral performance is responsible for exceptional wear resistance under intensifying and extreme tribological conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

Fast and slow modes of the propagation of the combustion front in heterogeneous systems

Experimental results are presented in favor of the existence of fast and slow modes of the propagation of the combustion front in diluted heterogeneous mixtures of reactive particles. A theoretical combustion model is proposed to explain the existence of these modes. The transition from the fast to the slow mode, which occurs in a narrow range of the degree of dilution of the mixture by inert powder, is associated with the break of a percolation cluster formed by reagent particles that are in direct contact with each other. After such a break of the cluster, the thermal energy of combusting particles is still insufficient to maintain the combustion wave. A sharp decrease in the front velocity in this case is associated with the necessity of heating inert regions inevitably appearing in its path.