Characterization of surface mechanical properties of H13 steel implanted by plasma immersion ion implantation

Characterization of surface mechanical properties of H13 steel implanted by plasma immersion ion implantation

Journal of Materials Processing Technology 189 (2007) 367–373 Characterization of surface mechanical properties of H13 steel implanted by plasma imme...

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Journal of Materials Processing Technology 189 (2007) 367–373

Characterization of surface mechanical properties of H13 steel implanted by plasma immersion ion implantation Nurs¸en Saklako˘glu ∗ Celal Bayar University, Faculty of Engineering, Mechanical Engineering Department, 45140 Manisa, Turkey Received 7 June 2006; received in revised form 31 January 2007; accepted 5 February 2007

Abstract AISI H13 steel samples were implanted with nitrogen using plasma immersion ion implantation (PIII) at 320 or 380 ◦ C. In addition, PIII was used for simultaneous implantation of carbon and nitrogen into H13 steel at 380 ◦ C. Treated and untreated samples were studied by wear-friction testing, microhardness measurement, atomic force microscopy and glancing angle X-ray diffraction (XRD) diagnosis. Experimental results indicated that the hardness and wear resistance were improved by increasing the PIII treatment temperature, whereas the presence of C ions does the opposite. It is explained by a nitride formation and a lattice expansion between 1.7 and 2.23%. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; AISI H13 steel; Surface modification; Expanded ferrite; Friction; Wear

1. Introduction There has been an increasing demand for industrial surface treatments to enhance corrosion resistance, hardness, and to reduce friction coefficient and wear. Besides conventional coating technologies, research and development on surface engineering has focused on plasma nitriding and ion implantation [1]. One promising technology to improve the surface hardness without incurring additional problems as in poor adhesion is plasma immersion ion implantation (PIII), which allows fast and cost-effective modification of three-dimensional tools [2]. In this study, high negative voltage pulses are applied to workpieces immersed in a plasma for PIII. Positive ions from the plasma are accelerated towards the workpieces and bombarded the surface with energies equal to the applied voltage [3]. A typical PIII device is shown in Fig. 1 [4]. For practical purposes, the acceleration voltage is limited approximately to 40 kV (at higher voltages, X-ray emission through secondary electrons would be hazardous). In order to obtain the full ion energy at the target surface, the pressure must be kept sufficiently low (below 0.5 Pa). The high voltage has to be applied in 10 ␮s duration with repetition frequencies ranging from 10 Hz to 1 kHz to avoid excessive power loads into the target [5].



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One of the main reasons to have an interest in PIII is because it can be used as a hybrid treatment together with a diffusion process. When the temperature exceeds 250 ◦ C, diffusion of implanted species (such as nitrogen ions) plays a significant role in certain metals [1]. A nitrogen-strengthened diffusion zone is produced so that the zone extends well beyond the implantation range and gives rise to the increased surface hardness [6]. AISI H13 steel (DIN X40CrMoV51) is a high strength steel used commonly in industry as a raw material for tools for both hot and cold applications. The surfaces of these tools are usually strengthened by salt, gas or plasma nitriding to improve surface characteristics, such as friction or wear. In addition to the environmental concerns of these salt or gas nitriding processes, all of the cited methods result in a compound (or white) layer consisting of brittle iron nitrides. In plasma immersion ion implantation, it is possible to control this layer more easily [7]. Previous studies [1,7,8] were concentrated mainly on the study of phase composition, depth profiling, and hardness changing of AISI H13 steels and other steels after nitrogen PIII. In this paper, plasma immersion ion implantation in H13 steel was performed by nitrogen ions at 320 or 380 ◦ C. Besides hardness, phase composition and surface structures, wear resistance and friction coefficient were studied. It is known that carbon plasma treatments can also improve the surface properties. Previous efforts were also devoted to carry out a comparative study of PIII treatment with nitrogen, carbon, and nitrogen and carbon

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Fig. 1. A typical PIII device.

[9–13]. In this study, nitrogen and carbon plasma-immersion ion implantation (PIII) at 380 ◦ C are presented.

320 or 380 ◦ C, and the treatment time was chosen as 5 h, resulting in an incident nitrogen fluence of 2 × 1018 atoms/cm2 . Prior to the implantation, an Ar/H2 preclean at 15 kV/100 Hz pulsing during the heating process was performed. In addition, nitrogen plus carbon implantation was performed at 380 ◦ C to compare against pure nitrogen implantation. For nitrogen and carbon implantations, 90% N2 + 10% CH4 gases were used. The surfaces of the PIII treated and untread AISI H13 were investigated by atomic force microscopy (AFM). Nanohardness evaluations of the samples were conducted with a UMIS-2000 ultra microhardness indenter system. The hardness tests were performed directly onto the surface at loads of 50, 250 and 1000 mN. Five indents were made at each load on each sample from which average values were calculated. Wear measurements of the treated and untreated AISI H13 steel samples were performed on a pin-on-disc tribometer (CSEM) using a ruby ball (6 mm diameter) under dry conditions. The wear tests were continued up to 500 m (corresponding to 9969 turns) under a load of 5 N and a constant speed of 0.3 m/s. The coefficient of friction was recorded during wear testing by a transducer on the load arm of the tribometer. The mean width (W) of the wear tracks were measured by an optical microscope. The parameter W was used for evaluating the wear resistance of the implanted and unimplanted samples. The structural modifications of as-treated samples were investigated by XRD measurements using glancing angle X-ray diffraction (Siemens D500) with Co K␣ radiation.

2. Experimental details AISI H13 steel rod in as-received condition was cut into 4 mm thick, 30 mm diameter, discs. The composition of H13 steel is 5.27% Cr, 0.77% V, 0.83% Mn, 1.41% Mo, 0.36% C by weight. These samples were hardened at Gaws Heat Treatment Works in Australia under these following conditions: (1) Heated to 1020 ◦ C in a vacuum furnace. (2) Quenched in nitrogen gas. (3) Tempered for 2 h at 580 ◦ C and then polished to mirror-like finish. All samples were polished to a mirror finish and given a 3 min ultrasonic clean in ethanol to remove the residues from sample polishing. PIII treatment was performed at the Australian Nuclear Science and Technology Organisation (ANSTO), using the system which has been previously described elsewhere [14,15]. Fig. 2 shows a schematic illustration of the PIII system used in this study. The sample chamber was pumped to a base pressure of <2 × 10−17 mbar before each PIII treatment was begun. Nitrogen implantations were performed by applying negative high voltage pulses of 30 kV. The process temperature was

3. Results and discussion 3.1. Surface examination Surface examination was used to determine the change in sample surface roughness as a result of treatment, since such changes had been known to be an indirect indicator of the level of nitrided surface modification on samples. The surface appearance of the untreated and treated samples was observed by AFM scanning. Fig. 3 shows the 3-D surface appearance, and the profile of the samples. As it can be seen on the figure, PIII treatment increased the surface roughness. During the experimentation, PIII(380C) and the PIII(380C + CH4 ) processes agreed within the accuracy of the roughness measurements. Fig. 4 shows a comparison of the surface roughness of treated and untreated samples obtained from AFM diagnosis. 3.2. Hardness and wear

Fig. 2. A schematic illustration of the PIII system used in this study.

The hardness results for the PIII treated and untreated samples can be seen in Fig. 5. While a hardness enhancement of about ×2.2 was obtained for PIII(320C), it was ×2.5 for PIII(380C) during 50 mN loading. So as PIII process temperature increases, hardness improvement increases. For higher loading (1000 mN), while PIII(320C) led to about ×1.6 improvement in hardness, it was ×2.45 for PIII(380C). This shows that, as the PIII process temperature increases, the hardness improvement is continued up to rather deeper regions than for a lower PIII process temperature. The graph of hardness of PIII(380C + CH4 ) lies below that of PIII(380C). Therefore, the addition of carbon to the chamber during nitrogen implantation reduces the improvement in hardness. Uglov et al. [10] searched evolution of the microstructure of AISI M2 steel after plasma immersion nitrogen and carbon implantation. Nitrogen and carbon implantation led to decreased nitrogen concentration on the steel surface. A decrease in nitrogen concentration might be responsible for the lower

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Fig. 3. The surfaces of untreated and treated samples using AFM.

improvement in hardness. This result is reflected in the wear test results. Fig. 6 shows the wear tracks on the samples, which indicates the extent of the wear. Fig. 7 summarises the wear results extracted from Fig. 6. As the PIII process temperature increases, wear resistance increases. Nitrogen and carbon

implantation increased the wear rate of samples compared to that of the pure nitrogen PIII treatment. Fig. 7 also compares the wear width when Al2 O3 ball is used on treated and untreated samples. While H13 discs showed a wider wear track than that of the Al2 O3 ball for untreated and PIII(320C) treated condi-

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of nitrogen, and nitrogen + carbon implantation (the gas flow was 40 sccm N2 + 10 sccm CH4 ) [18]. They found no significant difference between the nitrogen, and the nitrogen and carbon implantations on friction coefficient. Saklakoglu et al. characterized austenitic stainless steel after plasma immersion nitrogen, and carbon implantation. Their results revealed no substantial changes on friction characteristics both treated and untreated samples [13]. The nitrogen and carbon implantation by PIII is a complex process and unfortunately, the detailed mechanism remains unknown. Fig. 4. The surface roughness of samples.

3.4. XRD-examination tions, the Al2 O3 ball showed a wider wear track than that of the H13 discs for PIII(380C) treatment because of a more hardened steel surface. And for the PIII(380C + CH4 ) condition, the H13 disc had a wider wear track because of a decreased hardness improvement. 3.3. Friction As it can be seen in Fig. 8, the friction curve for all PIII treated samples is slightly below than the untreated samples. Friction curve of PIII(380C) sample follows PIII(320C) up to approximately 70 m and then increases. While PIII(320C) treatment on the H13 surface creates purely abrasive wear on the Al2 O3 ball, PIII(380C) treatment is resulted in mostly adhesive wear. Abrasive and adhesive wear traces on both Al2 O3 ball and H13 disc can be shown in Fig. 6. It can be concluded that the adhesive wear increased the friction coefficient. Carbon films have attracted considerable attention due to their high hardness, low friction coefficient, and chemical inertness. Baba et al. states that carbon reduces the friction coefficient by forming solid lubricants [16]. In their study, nitrogen and carbon implantation by PIII showes no apparent improvement on friction. Uglov et al. searched AISI M2 high-speed steel modified by PIII feeded by N2 and CH4 mixture at the ratio of 1:1 [17]. For 7.5 m sliding distance nitrogen + carbon implantation reduced the friction, but after 20 m sliding distance nitrogen + carbon implantation exhibited more friction according to nitrogen implantation. Mandl et al. provides comparison

Fig. 5. The hardnesses plotted as a function of the applied load in mN.

Glancing angle X-ray diffraction patterns for untreated and PIII treated samples are shown in Fig. 9. The peaks in the PIII treated samples – below the positions of the standard ferrite peaks – indicate the formation of the expanded ferrite. The peak shifts are smaller for the carbon expanded ferrite. The shift of the peaks from their b.c.c. positions due to excessive carbon or nitrogen atoms causes to change the lattice constant. The interplanar spacing for cubic structures is given by a d= 2 (h + k2 + l2 )

(1)

where a is the lattice constant, and h, k and l are the Miller indices [19]. Using Eq. (1), the lattice constants can be calculated for untreated and treated samples. For the peak (1 1 0), the results are given in Table 1. As it is seen, nitrogen expanded ferrite causes an increase in the lattice constant. As the PIII process temperature increases, the lattice expansion also increases. In a recent publication [12], similar behavior was observed for AISI 316 L SS, producing an expanded austenite phase. While expanded austenite led to a lattice deformation 7% for AISI 316 L SS, nitrogen expanded ferrite resulted in a lattice deformation of 2.23% for the 380 ◦ C PIII process in this study. For nitrogen and carbon implantation, the lattice deformation value was 1.98%, which was lower than that of pure nitrogen. According to Blawert et al., nitrogen and carbon are randomly distributed in the middle of the cube edges, expanding the f.c.c. or b.c.c. unit cells [20]. It can be explained by assuming a high density of stacking faults. This shows that addition of C to the chamber during PIII treatment suppresses the formation of “nitrogen expanded ferrite” and decreases the nitriding effect. This is the reason for relatively smaller surface hardness and higher wear loss of the nitrogen and carbon implanted samples. In addition to the expanded ferrite formation, some FeN peaks on the glancing angle X-ray diffraction graph are also seen at 2-Theta = 44.65◦ ; 67.25◦ ; 82.55◦ . The peak of PIII(320C) lay slightly over the peaks of PIII(380C) and PIII(380C + CH4 ). It can be said that the lower temperature caused more nitride formation, but less lattice expansion for AISI H13 steel. Manova et al. studied martensitic stainless steel treated by PIII [21]. They reported that radiation induced effects and stress accumulation can be envisaged as a cause for lattice expansion after high fluence nitrogen implantation for both austenitic and martensitic steels.

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Fig. 6. The wear tracks on the samples.

Table 1 The lattice constants for untreated and treated samples on (1 1 0) peak Phase

Untreated α(110)

320C-5 h αN

380C-5 h αN

380C-5 h + CH4 αN

2θ d-spacing a-calculated Deformation ratio (%)

52,342 2,028 2,868 –

51,384 2,063 2,917 1,7

51,133 2,073 2,932 2,23

51,249 2,068 2,925 1,98

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Sienz et al. pointed out that the lattice expansion is anisotropic on a microscopic scale, i.e. a different expansion for different grain directions, with the largest expansion always observed for the (2 0 0) planes for f.c.c. lattices [22]. In this study, the anisotropic behaivour of PIII treated samples was not studied. But it can be concluded that b.c.c. lattices caused the largest expansion for the (1 1 0) planes in our study. 4. Conclusion

Fig. 7. The wear loss of samples.

Fig. 8. Friction coefficients of the samples against Al2 O3 under dry test conditions as a function of sliding distance.

In this paper, plasma immersion ion implantation in H13 steel was performed by nitrogen ions at 320 or 380 ◦ C. The effect of PIII process temperature on hardness, wear, friction and phase formation was accomplished by doing the experiment either at 320 or 380 ◦ C. Moreover, nitrogen and carbon implantations having 90% N2 + 10% CH4 gases were carried out at 380 ◦ C. The values for hardness, wear, friction and phase formation for this combination treatment were compared to those of pure nitrogen ions at 380 ◦ C. Nitrogen is a potent solid solution strengthener of ferrite. Although the solubility is low, implantation might allow concentration in excess of the solubility limit, and hence produce considerable hardening causing lattice expansion. Nitrogen has a relatively high diffusivity (characteristic of interstitial elements) and therefore formation of nitrides and carbonitrides may occur. Based on this work, several conclusions may be drawn, as follows: • Beside the different peak intensities, a variable lattice expansion was found. Similar to expanded austenite, high fluence nitrogen ion implantation leads to formation of expanded ferrite. No formation of carbon expanded ferrite was visible in the spectra.

Fig. 9. Glancing angle X-ray diffraction traces for untreated and PIII treated samples at various temperatures with and without C ions.

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• Nitrogen causes an expansion of the ferrite lattice, but carbon decreased the nitrogen expansion. So PIII treatment with nitrogen and carbon ions results in less hardness than obtained using pure nitrogen. • PIII treatments resulted in FeN formation. While the lower PIII process temperature led to slightly more FeN formation, it caused slightly less expanded ferrite formation. It can be said that the hardening effect results from both the nitride compound and expanded ferrite formation. • The microhardness for the near surface increased noticeably as a consequence of the PIII treatment as the process temperature increased. • The presence of C ions diminished the hardness improvement. • PIII treatment improved the wear resistance of H13 steel. While PIII(320C) process yielded an improvement of about 28%, it was 40% for PIII(380C), but only 24% for PIII(380C + CH4 ). • Although improvements of hardness and wear resistance, PIII treatment caused no obvious improvement for the friction coefficient. • It should be possible to reveal corresponding the anisotropic behaivour, activation energies and kinetics for these processes from further investigations. Acknowledgements In this study, samples were treated at the Australian Nuclear Science and Technology Organisation (ANSTO) laboratories using their plasma immersion ion implantation facilities. The author is grateful to Dr. George Collins and to Dr. Ken Short for their invaluable assistance. Special thanks are also due to Mr. Geoffrey Watt and Dr.˙I.Etem Saklako˘glu for their help and support. References [1] K.G. Kostov, M. Ueda, M. Lepiensky, P.C. Soares Jr., G.F. Gomes, M.M. Silva, H. Reuther, Surface modification of metal alloys by plasma immersion ion implantation and subsequent plasma nitriding, Surf. Coat. Technol. 186 (1–2) (2004) 204–208. [2] S. Mandl, E. Richter, R. Gunzel, W. Moller, Nitrogen plasma immersion ion implantation into high speed steel, Nucl. Instrum. Methods Phys. Res. B 148 (1999) 846–850. [3] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Ion-assisted surface modification by plasma immersion ion implantation, Surf. Coat. Technol. 103–104 (1998) 212–217. [4] A. Anders, Cathodic Arcs and High Power Pulsed Magnetron Sputtering: A Comparison of Plasma Formation and Thin Film Deposition, Seminars, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720.

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