The synthesis (PVD and CVD) and bulk properties of the M_nN-SiN_x (M = transition metal) family of nanocrystalline superhard composites have been extensively studied over the past three decades. Among this large group of nanocomposites, TiN-SiN_x has received special attention as a model system due to its exceptionally high Vicker's hardness (from 50 to 100 GPa, depending upon the presence of TiSi₂ phases). The commonly accepted model for the phase structure of TiN-SiN_x is that it consists of a set of nanometer-sized TiN crystallites surrounded by an amorphous SiN_x tissue-phase.

In an attempt to gain better understanding of the origin of TiN-SiN_x superhardness, we use in-situ STM and LEED to investigate the atomic-scale structure of the SiN_x/TiN interface, of which very little is known. Samples were prepared in a multi-chamber UHV (base pressure < 10^-10 Torr) system by reactive magnetron sputtering of epitaxial TiN layers onto single-crystal TiN(001) or TiN(111) substrates at temperatures between 700 and 900°C followed by formation of SiN_x overlayers at temperatures ranging from 600 to 800°C.

We show both topographic (STM) and diffraction (LEED) evidence that (a) SiN_x overlayers on TiN are crystalline with reconstructions including 2x2, 3x3, and 1x5, depending upon SiN_x coverage, surface orientation, and annealing temperature; and (b) TiN grows epitaxially on top of the SiN_x layers. Specifically, our results show that for SiN_x coverages near 1 ML, where maximum TiN-SiN_x hardness is achieved, the SiN_x layer is "not amorphous" as deposited.