This impervious layer or boundary will retard the free diffusion of moisture. This condition is referred to as case hardening.
这一不透气的壳层(或边界层)会阻滞水分自由扩散，这种情形称为表面硬化。
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The paper also briefs the nozzle or moving blade surface hardening treatment technology, including plasma spraying and diffusion layer for preventing the nozzle and moving blade from SPE.