Kazuhiro Akiyama

Atomic structures of a liquid-phase bonded metal/nitride heterointerface

Liquid-phase bonding is a technologically important method to fabricate high-performance metal/ceramic heterostructures used for power electronic devices. However, the atomic-scale mechanisms of how these two dissimilar crystals specifically bond at the interfaces are still not well understood. Here we analyse the atomically-resolved structure of a liquid-phase bonded heterointerface between Al alloy and AlN single crystal using aberration corrected scanning transmission electron microscopy (STEM). In addition, energy-dispersive X-ray microanalysis, using dual silicon drift X-ray detectors in STEM, was performed to analyze the local chemistry of the interface. We find that a monolayer of MgO is spontaneously formed on the AlN substrate surface and that a polarity-inverted monolayer of AlN is grown on top of it. Thus, the Al alloy is bonded with the polarity-inverted AlN monolayer, creating a complex atomic-scale layered structure, facilitating the bonding between the two dissimilar crystals during liquid-phase bonding processes. Density-functional-theory calculations confirm that the bonding stability is strongly dependent on the polarity and stacking of AlN and MgO monolayers. Understanding the spontaneous formation of layered transition structures at the heterointerface will be key in fabricating very stable Al alloy/AlN heterointerface required for high reliability power electronic devices.

A study of the adhesion between CVD layers and a cemented carbide substrate by AEM analysis

Abstract The interface between chemical vapor deposition (CVD) coated layers and cemented carbide substrate of coated cemented carbide inserts was analyzed by analytical electron microscopy (AEM). This analysis was carried out from the viewpoint of the adhesion of the coated layer. From the results of elemental analysis, it became clear that the amounts of W and Co diffused from the substrate into the coated layer were different among three coating conditions. A columnar TiCN grain produced in relatively low temperature scarcely contained these elements, and they were detected only at the grain boundary. On the other hand, these elements diffused inside the grains when the shape of the grain was granular as TiN. Strong adhesion of coated layer was obtained when a proper amount of these elements was diffused. Coated inserts containing an interfacial layer with suitable diffused materials and main columnar TiCN layer showed good cutting performance.