The effect of interface diffusion between tungsten and graphite on embrittlement has not been examined over the tungsten ductile-brittle transition temperature. To analyze interface embrittlement with tungsten carbide (WC) formation and hardness, a reactive diffusion barrier model was adapted to clarify the roles of leak rate, lag time, and impurity. Plasma-sprayed tungsten (PS-W) on graphite with molybdenum interlayer (diffusion barrier) was fabricated using plasma-spray. The carbon concentration and hardness were measured using energy-dispersive X-ray spectroscopy and micro-indentation after furnace experiments relevant to plasma-facing component upper limit temperature (1470 K). The lag time and the leak rate were determined by the model with different impurity amounts (10-30 at. %) and barrier thicknesses (1-40 μm). It is worth noting that the lag time determines embrittlement threshold time because it delays the onset of diffusion, and it is expanded with thicker barrier and impurity (0.07-21000 ms). The leak rate represents the embrittlement rate since it limits the diffusion flux, and it does not depend on impurity but on barrier thickness. Diffusion-induced interface embrittlement was measured and estimated based on WC fraction. The embrittlement can be spatially expanded with time, suggesting that interface embrittlement can be severe for KSTAR long-term operation.