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

The characteristic features of TIN interlayers were investigated in three d ifferent materials firstly TiN-coated C-fibres of the high-tensile-strength type in pure Mg; secondly TiN-coated fibres in, a Mg alloy containing 5 wt % Al (AM50); thirdly, for comparison, the latter alloy reinforced with the uncoated fibres. The results were obtained using high-voltage electron micr oscopy, high-resolution electron microscopy, energy-filtered transmission e lectron microscopy and scanning transmission electron microscopy in combina tion with energy-dispersive X-ray spectroscopy and electron energy-loss spe ctroscopy, including analyses of near-edge fine structures. The chemically vapour-deposited coating was proven to consist partly of a nanocrystalline titanium carbonitride (TiCxNy) rather than of titanium nitride (TiN). It va ries in thickness mainly between 10 and 30nm. In the composite with the pur e Mg matrix, the interfacial structure is characterized by an almost string ent 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, th e coating almost fully prevents the C transport across the fibre/matrix int erface. Accordingly, changing the matrix to the Mg-Al alloy did not initiat e a distinct formation of aluminium carbides. On the other hand, combining the same matrix with the uncoated fibre resulted in the substantial formati on of many Al2MgC2 carbides of different sizes at the fibre/matrix interfac e, which embrittle the composite as described in our former work. Therefore , an appropriate titanium nitride interlayer enables the applicability of M g matrices with high contents of Al, which is of particular interest for th e manufacturing of hybrid components.