Microwave plasma steel nitriding using different H2-N2 gas mixtures

We report the plasma parameters of a microwave discharge immersed in a magnetic field (ECR type) studied as a function of the type of gas used to induce the discharge. Plasma diagnostics included a Langmuir probe, an ion energy analyzer and an optical emission spectroscopy system. Results of the diagnostics are discussed in connection with the influence of the plasma parameters on the nitriding of steels. Different H 2 – N2 mixtures were used to form the plasma and carry out the nitriding of AISI-316 stainless steel, with samples at the plasma floating potential, so that the energy of the ions hitting the sample surface depends on the difference between the plasma potential and the floating potential. Treatments were done in the low temperature regime (~ 450 C), where the steel keeps its anticorrosive properties and its structure corresponds to the so-called expanded austenite according to our X-ray diffraction results. The highest surface hardness was obtained when a 60-40 (H2-N2) gas mixture was used, corresponding to a 380% hardness increase for a 50-min. treatment.