Mo-Si 3 N 4 functionally graded composites with 6 layers (each layer being app. 100-130μm thick) were processed using powder mixtures and hot-pressing at 1750°C. The Mo/Si 3 N 4 -ratios of the layers were chosen as 100/0, 80/20, 60/40, 40/60, 20/80, 0/100. After hot-pressing, the FGM displayed channel cracks (normal to layer direction) at the second layer. However, no crack propagation into the 100Mo/0Si 3 N 4 -layer and into the Si 3 N 4 -rich layers was observed. Comprehensive microstructure and phase analysis showed, that except for the 100Mo/0Si 3 N 4 -layer, no metallic Mo was present in the other layers, indicating extensive reaction between Mo and Si 3 N 4 to form various compounds and phases of Mo x Si y . Above approximately 30% Si 3 N 4 content, part of Si 3 N 4 could be maintained. Below this content, Si 3 N 4 disintegrated completely, yielding N 2 -evaporation and producing a porous microstructure. Although Mo x Si y -phases alone display brittle fracture behaviour and reduced crack resistance if compared with Si 3 N 4 , they exhibited appropriate crack deflection mechanisms in the layers where they coexisted with Si 3 N 4 Cracks initiated by Vickers indenture were deflected and/or arrested at the phase boundaries. Also, crack energy was consumed through microcrack formation and opening in the Mo x Si y -phases.