Abstract: In double glow plasma surface alloying(DGPSA), high vacancy concentration, high dislocation density and high concentration gradient of vacancies are produced at a definite depth under surface by ion bombardment.
This paper derives the expression of the diffusivity of molybdenum from diffusion flow. Based on vacancy mechanism, it also analyzes the effect of the high vacancy concentration and high concentration gradient of vacancies on diffusion of metal atoms. It points out that these are the main reason why the penetrating velocity of DGPSA is higher than that of conventional diffusion processes.

Key words: ion bombardment, vacancy concentration, concentration gradient of vacancies, concentration gradient of atoms