Necessary condition of diamond like carbon coated mechanical seals for nitrogen gas

Necessary conditions for usage of DLC coated mechanical seals were investigated by using ring on ring type sliding tests under various contact pressures and sliding speeds in dry and wet nitrogen. The wear mode of DLC coated SiC mechanical seal can be classified into two types by surface morphology of wear surface called ModeI with smooth surface that is useful as a seal and ModeII with relatively rough surface. The transition condition from ModeI to II is given by a critical μ2PV value, and show 0.025 under dry nitrogen and 0.070 under wet nitrogen. With the increase in PV value, arithmetic average roughness Ra decreases from 0.06 μm to 0.04 μm, and it leads to reduction of friction coefficient from 1.4 to 0.4. On the other hand, structural change of DLC films was not obtained within the condition of Mode I, and the friction was high. ModeI possibly has the desirable condition to DLC films for mechanical seal use. As a conclusion, the operation within critical μ2PV value range which depends on various conditions, and in addition, reduction of the friction coefficient in this range, achieve the increase of operating condition.