Abstract: Sn-Zn alloy is employed to conduct wetting tests over Si₃N₄/2024Al composite (45% Si₃N₄) in vacuum under different temperatures.On surface of the composite formed a thick Sn diffusion layer,and then the layer grows thicker as temperature increases.At 530℃,Sn diffusion layer thickness reached 1300 μm,while diffusion is not observed when temperature is below 330℃.Sn diffusion mechanism was studied based on Young's equation and cavity theory by five groups contrast tests.The results indicated that the volatilization of Zn in vacuum and second phase strengthening particles are necessary to accomplish Sn layer.