The effect of systematic modifications in the chemistry of the phase-boundary film on the macroscopic mechanical properties of Si3N4-matrix composites was investigated. Model composite materials, containing SiC or WC platelets, were prepared and only the bulk anion composition of the glassy-SiO2 intergranular phase was varied by adding increasing amounts of fluorine to the material. Detailed material characterizations by high-resolution electron microscopy (HREM) and Raman spectroscopy on both undoped and F-doped composites allowed to derive a structural model of the phase-boundary film as well as to evaluate the average microscopic stresses acting on it. In addition, high-temperature internal friction measurements provided an estimate of the grain-boundary relaxation temperature as a function of the F content. Noticeable variations of both elastic modulus and fracture energy of the composite were detected upon F addition, which were related to a spontaneous process of phase-boundary microcracking upon cooling. A threshold of the F-content was found for microcrack formation and its existence is theoretically explained according to a percolation process of non-bridged SiO4-tetrahedra, which arises from the incorporation of F into the intergranular film network.