The deformation and fracture of Ti-Si-x-N coatings of varying Si content were examined by cross-sectional transmission microscopy. Cross-sections were prepared through both undeformed coatings and coatings which had previously been indented using a 5 mu m radius spherically tipped diamond indenter. Samples with low silicon contents (0.8 and 1.5 at.%Si) had a columnar grain structure which deformed by inter-ranular cracking and shear sliding at the grain boundaries. Samples with higher silicon contents (9.5 and 12.5 at.%Si) consisted of TiN nanocrystals similar to 5 nm in size connected by an amorphous Si3N4 phase. These coatings contained a limited amount of intergranular cracking. The nanocrystalline coatings are thought to undergo quasi-plastic deformation behaviour via cracking oil a nanometer scale, where the crack size is of the order of the grain size. (c) 2004 Elsevier B.V. All rights reserved.