Non-intrusive measurement of electron, vibrational, rotational temperatures and active species concentration in N2-H2 cathodic cage plasma

The cathodic cage plasma nitriding (CCPN) is an innovative surface engineering method, which overcomes the shortcomings associated with conventional plasma nitriding (CPN). In current study, optical emission spectroscopy (OES) and optical actinometry are used to diagnose the electron, vibrational and rotational temperature, relative active species density (N, N2+) and nitrogen dissociation fraction in hydrogen admixed nitrogen plasma (with 4% argon as actinometer) in CCPN reactor. The electron temperature is estimated using modified Boltzmann plot method, vibrational temperature using N2(C, v′) state distribution, while rotational temperature is estimated using Boltzmann plot method. To estimate the nitrogen dissociation fraction, actinometery as well as intensity ratio method is employed. The electron and vibrational temperature depicts an increasing trend, while rotational temperature decreases with hydrogen admixture. The relative density of [N] is found to be increasing up to 40% hydrogen admixture, however, relative density of [N2+] is decreasing. This study presents clear picture of hydrogen admixing on the surface characteristics: it depicts that with hydrogen admixture, in addition to oxide layer removal and increased ionization, the generation of active species is also influenced. It suggests that with regard to both N and N2+ density, 40% hydrogen is beneficial.