TiN coatings in C/Mg-Al composites: Microstructure, nanochemistry and function

The characteristic features of TiN interlayers were investigated in three different materials: firstly TiN-coated C-fibres of the high-tensile-strength type in pure Mg; secondly TiN-coated fibres in a Mg alloy containing 5wt% Al (AM50); thirdly, for comparison, the latter alloy reinforced with the uncoated fibres. The results were obtained using high-voltage electron microscopy, high-resolution electron microscopy, energy-filtered transmission electron microscopy and scanning transmission electron microscopy in combination with energy-dispersive X-ray spectroscopy and electron-energy-loss spectroscopy, including analyses of near-edge fine structures. The chemically vapour-deposited coating was proven to consist partly of a nanocrystalline titanium carbonitride (TiC x N y ) rather than of titanium nitride (TiN). It varies in thickness mainly between 10 and 30 nm. In the composite with the pure Mg matrix, the interfacial structure is characterized by an almost stringent separation of matrix and fibre by the coating. A graphitic ribbon 10–15 nm thick was only rarely observed at the coating/matrix interface. Thus, the coating almost fully prevents the C transport across the fibre/matrix interface. Accordingly, changing the matrix to the Mg—Al alloy did not initiate a distinct formation of aluminium carbides. On the other hand, combining the same matrix with the uncoated fibre resulted in the substantial formation of many Al2MgC2 carbides of different sizes at the fibre/matrix interface, which embrittle the composite as described in our former work. Therefore, an appropriate titanium nitride interlayer enables the applicability of Mg matrices with high contents of Al, which is of particular interest for the manufacturing of hybrid components.