Pump-probe optical methods such as transient grating spectroscopy (TGS) enable rapid, nondestructive thermoelastic property measurements. But, in phonon-dominated ceramics, they can underpredict bulk thermal diffusivity when long mean free path phonons do not equilibrate over experimental length scales. We combine in situ TGS with ${\mathrm{Si}}^{4+}$ ion irradiation of chemical vapor deposited 3C-SiC (300 and $550{}^{\circ }\mathrm{C}$, 0.5–1 dpa) and density functional theory informed Boltzmann transport equation solutions to understand the origins of this offset. We show how the discrepancy between laser flash analysis (LFA) and TGS-measured thermal diffusivity varies with grain-boundary density, temperature, and defect concentration. We introduce a dimensionless suppression factor that accounts for this discrepancy, and we demonstrate its utility by using it to show an agreement between the thermal defect resistance of neutron irradiated 3C-SiC (measured using LFA) and ion irradiated 3C-SiC (measured using TGS). This theory-informed experimental framework enables quantitative, in situ tracking of ion irradiation damage induced thermal transport degradation in ceramics.

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• Ballistic transport
• Lattice thermal conductivity
• Phonons
• Radiation damage
• Thermal properties