Titanium nitride is emerging as an alternative plasmonic nanomaterial to costly (e.g., Au) or potentially toxic (e.g., Ag) noble metals. So TiN is most promising as replacement for Au-based nanostructures in photothermal cancer therapy due to its localized surface plasmon resonance (LSPR) in the desired near-infrared region (750-900 nm). In such biological applications, a SiO2 surface is most desirable as it enables easy functionalization with specific moieties. Here, TiN nanoparticles coated with SiO2 films of precisely controlled thickness are prepared by nitridation of flame-made monocrystalline SiO2-coated TiO2 particles (similar to 40 nm in crystal size). The average SiO2 thickness and polydispersity are obtained by microscopy and are in good agreement with those from fluid-particle dynamic simulations. The coating quality is assessed by photocatalysis of methylene blue by SiO2-coated and -mixed (co-oxidized) TiO2 particles. Nitridation results in polycrystalline TiN (similar to 20 nm) and proceeds from the particle surface inward even through the most hermetically coated (similar to 7 nm in SiO2 thickness) particles as determined by microscopy and elemental mapping. A photothermally optimal SiO2 shell thickness of a couple of nanometers, however, is found and attributed to plasmonic decoupling and stoichiometry of the constituent TiN particles. The biocompatibility of these particles was excellent as assessed with THP-1 monocytes.