Tribology Online, 11, 3 (2016) 460-465. ISSN 1881-2198 DOI 10.2474/trol.11.460
Article
Surface Modification of Tool Steel by Atmospheric-Pressure Plasma Nitriding Using Dielectric Barrier Discharge Junji Miyamoto1)*, Takashi Inoue1), Kazushige Tokuno1), Hideo Tsutamori1) and Petros Abraha2) 1)
Department of Mechanical Engineering, Daido University 10-3 Takiharu-cho, Minami-ku, Nagoya, Aichi 457-8530, Japan 2) Department of Mechanical Engineering, Meijo University 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya, Aichi 468-8502, Japan * Corresponding author:
[email protected] ( Manuscript received 06 July 2015; accepted 05 May 2016; published 31 May 2016 )
The plasma nitriding of tool steel under atmospheric-pressure was performed using a dielectric barrier discharge method, resulting in the formation of a uniform nitrided layer. In this study, the tribology properties of the nitrided layer generated by atmospheric-pressure plasma nitriding were investigated. The results showed that the surface hardness of the tool steel nitrided by the atmospheric-pressure plasma method were increased by more than twofold compared with that of the core material. The surface hardness and the thickness of the nitrided layer were uniform, with values of 1300 HV and 30 m, respectively. In addition, the wear rate of the sample nitrided by the atmospheric-pressure plasma method was decreased by more than 25 times compared with that of the untreated sample. Only the emission of the N2 second positive system and Ar were detected by the optical emission spectroscopic observation of the generated plasma. For this reason, we consider that the nitriding of this research caused by the dissociation of nonexcited N2, NH3, NH2, and NH etc. on the sample like a gas nitriding. Keywords: plasma nitriding, atmospheric pressure, dielectric barrier discharge, tool steel, tribology properties
1. Introduction Plasma nitriding is a unique prospective method for treating various engineering materials to achieve higher surface hardness, while maintaining the material's core properties. Nitriding results in a diffusion layer and a compound layer. These layers also increase the wear, fatigue, and corrosion resistance as well as the surface hardness of the material. At present, glow discharge plasma nitriding is the most common plasma nitriding method [1-3]. However, glow discharge plasma nitriding requires a vacuum system to generate plasma under vacuum. Therefore, most traditional plasma nitriding can be performed only as a batch process and capital costs become very high [4]. For this reason, we are developing a new plasma nitriding method that is conducted under atmospheric pressure. In recent years, Ichiki et al. have succeeded in hardening tool steel using a pulse-arc plasma jet under atmospheric pressure [5]. However, the thickness of the resulting nitride layer was not uniform. As a result, they observed that nitriding was possible only for a limited area. It is suggested that nonuniform thickness of this nitride layer resulted in an insufficient
Copyright © 2016 Japanese Society of Tribologists
improvement of the steel’s tribological properties, such as its wear resistance. Thus, the nitride layer formed by atmospheric-pressure plasma nitriding must be uniform to improve the steels tribological properties to the same extent as conventional plasma nitriding. Uniform plasma density and a wider plasma than that generated in the plasma jet method are required to form a uniform nitrided layer. To attain a wide plasma, we have adopted the method of atmospheric-pressure plasma nitriding by dielectric barrier discharge for two reasons. First, plasma can be generated stably because of a dielectric barrier between the electrode and the sample. Second, the uniformity of plasma can be easily achieved through the use of a wide electrode. Recently, the technique of plasma generated using dielectric barrier discharge at atmospheric barrier discharge has been rapidly developed [6]. New applications of the plasma using dielectric barrier discharge were investigated considering the surface treatment such as nitriding and coating deposition [6-8]. This previous work shows that it is possible to realize plasma nitriding at atmospheric pressure by using dielectric barrier discharge. However, the uniformity of treated surface, properties of nitrided layer, and nitriding process mechanism has not been 460
Surface Modification of Tool Steel by Atmospheric-Pressure Plasma Nitriding Using Dielectric Barrier Discharge
clarified. In this paper, we propose a plasma nitriding method involving dielectric barrier discharge under atmospheric pressure and investigate the state of plasma and the tribological properties of the resulting nitrided layer. We expected the thickness of the nitrided layer to be uniform because of the wide plasma-generated dielectric-barrier discharge. Moreover, the excited species were investigated using a spectroscopic observation of the generated plasma to discuss the mechanism of the nitriding by dielectric barrier discharge. 2. Experimental methods 2.1. Samples and experimental apparatus The sample material was SKD61 tool steel. Discs with a diameter of 20 mm and thickness of 2 mm were quenched at 1030°C and then tempered to 540°C. The hardness of the heat-treated samples was 630 HV. Their chemical composition is shown in Table 1. The surfaces of the samples were ground and polished successively using 6-, 3-, and 1- m diamond slurry. The average surface roughness of the polished samples was Ra = 5 nm. In this research, plasma nitriding of tool steels was performed by the dielectric barrier discharge method and the plasma jet method, which is a conventional method, to allow a comparison of the uniformity of hardness of the resulting nitrided surfaces. The apparatus of the plasma jet method and that of dielectlic barrier discharge method are shown in Fig. 1(a,b), respectively. In the plasma jet method, plasma was generated via a pulsed voltage biased between two electrodes (one outside the glass tube and the other inside it). In the dielectric barrier discharge, plasma was generated by a 0-V potential difference between the pulsed voltage biased electrode and the sample. The sample must be heated during plasma nitriding. Therefore, a heater is required at the
Fig. 1
Schematics apparatus
Material SKD61
of
C 0.36
the
plasma
base of the sample to heat the sample while it is being irradiated with plasma. 2.2. Experimental methods and conditions To generate plasma, the electrodes must be negatively or positively biased. In our setup, the sample bias voltage was a bipolar pulse of Vp-p = 7 kV. In these experiments, the samples were subjected to the plasma jet method and dielectric barrier discharge method for 3 h. The mass flow rates of argon and nitrogen were 10 slm and 1 slm, respectively. Surface hardening cannot be achieved without hydrogen addition; in the absence of hydrogen, only surface oxidation occurs. Thus, the mass flow rate of hydrogen was 0.5 slm. The distance between the glass tube and sample for the plasma jet method was 10 mm. The distance between the insulator and sample for the dielectric barrier discharge method was 1 mm. The sample temperature was maintained at 500°C by a heater positioned at the base of the sample. Prior to the generation of plasma, residual oxygen was purged with nitrogen gas introduced through the nozzle at 10 slm for 10 min to avoid sample oxidation. The surface and cross-sectional hardness of nitrided samples was investigated using a micro-Vickers hardness tester. The load of hardness test was performed on 0.01 kgf to measure limited range. Surface analysis was performed by X-ray diffraction on an instrument equipped with a Cu-Kα radiation source. Schematic diagram of the wear test is shown in Fig. 2. The wear rate of nitrided samples was investigated using 8 mm aluminum oxide (hardness 1400 HV) balls in a dry-atmosphere ball-on-disc tribometer. The applied load was 10 N; the samples were rubbed for 5000 cycles at 100 rpm (the diameter of the wear track was 5 mm and sliding speed was 8.3 mm/s). Average contact pressure Pmean before wear was 884 MPa from calculations. The surface roughness of the nitrided samples was investigated using a stylus touch-type roughness meter. The measurement distance for investigation of the
nitriding
Fig. 2
Schematic diagram of the wear test
Table 1
Chemical composition of the sample (mass%)
Si 0.92
Mn 0.43
Japanese Society of Tribologists (http://www.tribology.jp/)
P 0.008
S
