The effect of argon admixing on nitriding of plain carbon steel in N2 and N2-H2 plasma
Introduction
Nitriding is a cost-effective thermochemical surface engineering technique to enhance the tribological and mechanical properties of various materials [1, 2]. Around a century ago, liquid and gas nitriding techniques were introduced, which exhibits certain limitations like environmentally hazardous, poor nitriding excellence and expensive [3]. Alternatively, those techniques were replaced by direct current plasma nitriding (DCPN), which is based upon gasses ionization due to the potential difference among cathodic substrate holder and the counter electrode. Due to environmental friendly, its demand grows rapidly at earlier stages, but on commercial-scale applications, it was not warmly adapted. It happens mainly due to non-uniform distribution of temperature, non-uniform nitriding at edges and hollow cathode effect [4].
These drawbacks lead to the development of cathodic cage plasma nitriding CCPN (originally it was termed as through cage plasma nitriding) which involves the isolation of discharge from the sample surface to a metallic cathodic screen [5]. In this configuration, samples are at the floating potential instead of cathodic potential, and hence the problems related with DCPN can be escaped. Furthermore, this system has additional advantages including reduced sensitivity to grease or rust, no needs of the conductivity of samples and also valuable for aesthetic perspective [[6], [7], [8]].
The nitrogen-containing discharges are important in diverse applications like surface processing of polymers, sterilization and surface hardening of various materials including steels [[9], [10], [11]]. In these applications, nitrogen atomic and ionic species are of significant importance. However, the production of these species by dissociation and ionization process is limited because of the higher bond strength of nitrogen (9.67 eV) over other molecules [12]. These processes in nitrogen gas can be boosted by admixing hydrogen, and therefore nitriding process is usually conducted in hydrogen admixed nitrogen discharges. It is due to the fact that hydrogen has a small collisional cross section over nitrogen and additionally it can remove the native oxides barrier to nitrogen diffusion. Various authors [[13], [14], [15], [16], [17], [18]] reported that addition of hydrogen up to certain fraction can enhance the surface hardness, as well as nitride layer thickness in various systems including RF-discharges, diode plasma nitriding reactor and DC, glow discharges. In CCPN reactor, Sousa et al. [19] found that for AISI-316, surface hardness, corrosion resistance and layer thickness can be improved for a small quantity of hydrogen. Recently, some authors [[20], [21], [22], [23], [24]] implemented various diagnostics such as optical emission spectroscopy, Langmuir probe, quadrupole mass spectrometry and tunable diode laser infrared absorption spectroscopy, and concluded that hydrogen concentration influences the active precursors in processing. In our previous reports [25, 26], we found that in CCPN reactor, nitrogen active species concentration is highest when the nitrogen/hydrogen are mixed in 3/2 ratio.
In last two decades, besides hydrogen admixed nitrogen discharges, various authors reported that addition of argon can also enhance the production of nitrogen active species through Penning ionization and dissociation processes. Reyes et al. [27] found that argon addition up to 70% enhances the production of nitrogen active species and electron temperature, and suggested that such admixture is probably suitable for nitriding. Rehman et al. [28] reported increased dissociation of nitrogen by addition of argon higher than 60% in RF-CCP. Itagaki et al. [29] described an increase in dissociation rate of nitrogen by addition of argon higher than 80% in electron cyclotron resonance plasma. In inductively coupled discharges, Song et al. [30] found an enhancement in nitrogen atomic species density by adding 30% argon in nitrogen. Saloum et al. [12] diagnosed radio-frequency hollow cathode discharge and showed that nitrogen dissociation fraction is maximum at 50% argon admixture. Recently, we have reported [31] that admixing of argon gas in nitrogen discharge in CCPN reactor enhances the production of nitrogen active species, electron temperature and electron number density.
Although the diagnostics results show that addition of argon gas in nitrogen discharge increases the active species responsible for nitriding. However, it is questionable that only the addition of argon without hydrogen is enough to attain the better surface properties of steels. To the best of authors' knowledge, the comparison of argon addition with and without the presence of hydrogen in nitrogen discharge is not studied so far, and it is expected to be valuable for better surface improvement.
In this study, the effect of argon gas admixing (10–30%) in nitrogen and nitrogen-hydrogen mixture (having ratio 3/2) discharge on surface properties of plain carbon steels are compared.
Section snippets
Plasma reactor
The plasma reactor used in the current experiment is drawn schematically in Fig. 1. The body of the chamber is of cylindrical geometry made up of austenite stainless steel AISI-304 with dimension (height ~ 335 mm and internal radius 155 mm). It is at a grounded potential (acting as an anode), whereas a metallic screen is installed at mid of reactor which is biased by a negative pulse of the power supply (acting as a cathode, so it is usually termed as a cathodic cage). The cathodic cage is also
Surface hardness observation
The hardness of samples nitrided at various conditions as a function of indentation loads is plotted in Fig. 2. It clearly illustrates that hardness of nitrided samples is improved irrespective of gasses composition over the base material (~154 HV). Even the sample nitrided in pure nitrogen (labelled as A) also shows an increase in hardness, which contradicts the results of DCPN in which no increase in hardness is reported [35]. This fact is probably attributed to the mechanism of CCPN, which
Conclusions and final remarks
This work mainly based on practical analysis of previous literature reports that addition of argon and hydrogen can enhance the nitrogen reactive species, which are responsible for nitriding. However, in this study, in the first step, argon is added in nitrogen plasma, and secondly, argon is added to nitrogen-hydrogen plasma. Main achievements from this analysis are as follows:
- 1.
The hardness of plain carbon steel cannot be significantly improved in nitriding processing by addition of argon in
Acknowledgement
The author Dr. J.C. Díaz-Guillén acknowledges partial support from Mexican National Council of Science and Technology CONACYT under Grant Project No. CB-2015-257705. Dr. M. Shafiq acknowledges partial financial support from Quaid-i-Azam University Research Fund (URF 2016-17) for Plasma Physics Laboratory.
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