<p>Engineering amorphous dielectric films with tunable thermal conductivity is advantageous for the thermal management of semiconductor devices and thermal insulation of aerospace applications. Here, we demonstrate that incorporating dense dispersed amorphous Al(Ti)N (~1 nm or above) nanoparticles having phase volume fractions from 6 to 70 %, has a negligible effect on the intrinsic thermal conductivity of the amorphous Si₃N₄ matrix (~2 W m⁻¹K⁻¹), in which the wave-like ‘propagons’ in Allen-Feldmann theory are believed to be unsupressed and non-tuned. By contrast, incorporating (5–15 nm) crystalline TiN phases significantly increases the thermal conductivity (up to 15 W m⁻¹K⁻¹). Critically, the micrometre-thick Si₃N₄/AlN and Si₃N₄/TiN amorphous matrix dual-phase nanocomposite coatings exhibit excellent thermal stability upon exposure to ambient air at 1000 °C for 50 h. These findings shed light on the phonon transport mechanism regarding the effects of the second phase and pave a design pathway for engineering amorphous coatings displaying unprecedented high thermal conductivity and excellent thermal stability.</p>