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To follow the interaction with a heavy alkane, the hexatriacontane, and a remote argon–oxygen plasma, the O2 (b1Σ g+, v = 0 − X3Σ g−, v′ = 0) transition at 762 nm can be used. A strong correlation with the material transformation is observed and attributed to atomic oxygen whose recombination releases a quantity of heat that affects the treatment temperature.

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Interactions between a late Ar-O 2 post-discharge and the hexatriacontane (HTC), a long-chain alkane, are shown to depend on the thermal flux released by surface reactions that makes the temperatures of the sample and the gas phase drift in an uncontrolled manner as a function of time. Since the transformations of the hexatriacontane depend on these temperatures, the initial value of the temperature and that of the oxygen concentration are key parameters that control the whole transformation process. A thorough description of the different steps of the transformation undergone by the hexatriacontane is given, explaining the origins of the limitation of the material etching. Pulsing the plasma shows that optimizing the etching process requires to work at low temperature, a too strong heating of the sample leading to functionalization and reticulation that limit the etching of the HTC.

This work reports experimental results on sintered PIM 316L stainless steel low-temperature plasma nitriding. The effect of treatment temperature and time on process kinetics, microstructure and surface characteristics of the nitrided samples were investigated. Nitriding was carried out at temperatures of 350, 380, 410 and 440 °C , and times of 4, 8 and 16 h, using a gas mixture composed by 60% N 2 + 20% H 2 + 20% Ar, at a gas flow rate of 5.00 × 10 -6 Nm 3 s −1, and a pressure of 800 Pa. The treated samples were characterized by scanning electron microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma nitriding is a diffusion controlled process. The calculated activation energy for nitrided layer growth was 111.4 kJmol -1 . Apparently precipitation-free layers were produced in this study. It was also observed that the higher the treatment temperature and time the higher is the obtained surface hardness. Hardness up to 1343 HV 0.025 was verified for samples nitrided at 440 °C. Finally, the characterization of the treated surface indicates the formation of cracks, which were observed in regions adjacent to the original pores after the treatment.

Characterization of the interaction between an argon-oxygen post-discharge and hexatriacontane (C36H74) is carried out. Optical emission spectroscopy using the atmospheric band O2 (b1Σg +, v=0 X3Σg −, v’=0) at 760 nm gives simultaneously the evolution of the O(3P) concentration above the surface and the gas temperature by simulation of the rotational spectrum of the transition. Surface reactions contribute to the heating in the sample and to a substantial increase in the gas temperature. Finally, a strong correlation between the time evolutions of the transition intensity and the sample temperature is observed, suggesting that O(3P) is the main reactive species that produces the heating and the chemical changes in the HTC.

Plasma technology used to perform thermochemical treatments is well established for the majority of steels, but it is not the case for the different stainless steel classes. Thus, important scientific and technological achievements can be expected in the coming years regarding plasma-assisted thermochemical treatment of such steels. The metallurgical aspects as well as the application cost-efficiency of stainless steels impose specific requirements for the thermochemical treatment, such as easy native chromium-rich oxide layer removal and surface activation at low temperature, which do not appear for other steel classes plain, low-alloy, and tool steels . Thus, due to the highly reactive physicochemical environment created by the plasma, plasma-assisted technology presents advantages over other conventional technologies like those performed in gas or liquid environments. Low temperature is needed to avoid the reduction of corrosion resistance of stainless steels, by suppressing chromium carbide/nitride precipitation, and, in this case, good surface properties are achieved by the formation of treated layers containing metastable phases. Such attributes make the low-temperature plasma thermochemical treatments of stainless steels an important R&D field in the domain of plasma technology and surface treatments, and the goal of this chapter is to introduce the reader to this important topic.

Stearic acid (CH 3 (CH 2 ) 16 COOH) was treated with Ar and Ar-O 2 (10%) pulsed DC discharges created by a cathode-anode confined system to elucidate the role of oxygen in plasma cleaning. The treatment time (5 to 120 minutes) and plasma gas mixture (Ar and Ar-O 2 ) were varied, and the results showed that the mass variation of stearic acid after Ar-O 2 plasma exposure was greater than that of pure Ar plasma treatment. Thus, compared to Ar*, active oxygen species (O and O 2 , in all states) enhance the etching process, regardless of their concentration. During the treatments, a liquid phase developed at the melting temperature of stearic acid, and differential thermal analyses showed that the formation of a liquid phase was associated with the breakage of bonds due to treatment with an Ar or Ar-O 2 plasma. After treatment with Ar and Ar-O 2 plasmas, the sample surface was significantly modified, especially when Ar-O 2 was utilized. The role of oxygen in the treatment process is to break carbonaceous chains by forming oxidized products and/or to act as a barrier again ramification, which accelerates the etching of stearic acid.