Pulsed plasma-oxidation of nitrided steel samples

Pulsed plasma-oxidation of nitrided steel samples

Surface and Coatings Technology 174 – 175 (2003) 1220–1224 Pulsed plasma-oxidation of nitrided steel samples ˇ Bogdanovb, S. Zlatanovic´ c M. Zlatano...

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Surface and Coatings Technology 174 – 175 (2003) 1220–1224

Pulsed plasma-oxidation of nitrided steel samples ˇ Bogdanovb, S. Zlatanovic´ c M. Zlatanovic´ a,*, N. Popovic´ b, Z. a

Faculty of Electrical Engineering, Bulevar Kralja Aleksandra 73, 11120 Belgrade, Yugoslavia b ˇ 11000 Belgrade, Yugoslavia Nuclear Science Institute, Vinca, c University of California, San Diego, La Jolla, CA, USA

Abstract Plasma nitridedynitrocarburized samples made of steel grade DIN C15 were post oxidized in a mixtures of H2 and O2 gases using pulse power supply. Plasma nitrocarburizing post-oxidation were performed at 520 8C. The applied process is a combination of plasma nitriding, plasma nitrocarburizing and plasma-oxidation of the compound layer. Pulse plasma was used in all process steps at 5 kHz frequency and pauseypulse ratio 1:19. The XRD examination revealed the existence of the compound zone composed of of the g9 or g9q´ carbonitride phase with a thin overlayer of magnetite Fe3 O4 . The architecture of formed surface structure provides the unique mechanical and tribological properties with the diffusion zone responsible for load bearing capacity, wear and fatigue resistance and the double compound layer resistant to wear and corrosion. The results of Vickers microhardness measurements, optical microscopy, XRD and SEM analyses were presented. The obtained surface structures were compared with the samples salt bath nitrided in a conventional industrial unit. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma nitriding; Plasma nitrocarburizing; Salt bath nitrocarburizing; Post oxidation; Pulse plasma

1. Introduction Corrosion and wear resistant coatings play an important role in modern technology. Some coatings, such as galvanic and hard chromium, are very effective in suppressing the action of the aggressive surroundings, but suffer from the environment pollution. In order to enhance the performance of industrial components used under combined corrosive and mechanical wear, duplex technology w1x and several surface treatments which combine nitriding and oxidation were developed such as NITROTEC, TENIFER QPQ, SURSULFOXYNIT, NIOX NITROTEC and similar w2x. A post-oxidation process applied to salt bath nitrocarburized parts producing a single phase carbonitride overlayer ´-Fe2–3(N,C) was found to be very effective against both wear and corrosion, but due to the application of toxic components like cyanides, serious environmental problems were produced w3,4x. The efforts were also made to produce a monophase ´ carbonitride layer by plasma techniques. Plasma nitrocarburizing *Corresponding author. Tel.: q381-11-782-668; fax: q381-11324-86-81. ´ E-mail address: [email protected] (M. Zlatanovic).

usually results in formation of a mixed g9q´ compound zone inferior in tribological applications related especially to impact loads, but plasma nitrocarburized samples may be post-oxidized to obtain a magnetite superficial layer with enhanced wear and corrosion resistance w5–9x. The parameters of an oxidation process determine the oxygen concentration gradient at the oxide–nitride interface, the growth and structure of the superficial oxide layer, the micropore concentration and the oxide zone phase (Fe3O4, Fe2O3, FeO) and thickness. Based on high chemical potential of plasma state, the aim of the presented investigation was to develop an effective, environment clean, plasma post-oxidation process. 2. Experimental The experiments were performed in a 20 kVA pulse plasma nitriding unit JONPULS-MK with vacuum chamber ⭋300=700 mm3 w10x. The pulse plasma power supply generates the voltage pulses in the frequency range from 90 Hz to 16.7 kHz and with duty cycle from 5 to 95%. The samples made of DIN C15 plain carbon steel were plasma nitridedynitrocarburized in a hydrogen

0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00590-5

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Table 1 Parameters of plasma processing Sample no. Plasma nitriding All samples Plasma-oxidation 1 2 3 4

Time (h)

T (8C)

p (Pa)

qN2 (sccm)

qH2 (sccm)

qBP (sccm)

1 14

520 520

400 400

45 45

135 135

0 1.85

520 520 520 520

290 290 460 460

0 0 0 0

297 297 297 297

0 0 0 0

1.5 1.5 1.5 1.5

nitrogen atmosphere with low carbon content while one sample was salt bath nitrided in an industrial unit according to TENIFER process. Since the nitrocarburized samples were used as the substrates for postoxidation, the aim was to compare the initial surface structures of salt bath and plasma processed surfaces before the oxidation. Plasma nitridedynitrocarburized specimens were pulse plasma post-oxidized in a varied hydrogenyoxygen atmosphere at the temperature of Ts 520 8C. The process parameters of nitridingynitrocarburizing and plasma post-oxidation process are listed in Table 1. The flow of hydrogen, nitrogen, butane–propane and oxygen was denoted by qH2, qN2, qBP and qO2, respectively, p denotes the pressure and cO the atomic percent concentration of oxygen in the gas mixture. Plasma nitrocarburized specimens were post-oxidized using four different hydrogenyoxygen ratios as given in Table 1. The frequency of the unipolar pulse plasma supply was 5 kHz and duty cycle 95%, which corresponds to the pulse duration of 190 ms and pause duration of 10 ms. 3. Results and discussion

qO2 (sccm) 0 0

0 0

50 73.3 23.3 13.3

14.4 19.8 7.3 4.3

the high open pore concentration (Fig. 1b). The XRD examination (Fig. 2) revealed the existence of an ´ carbonitride layer with no indices of g9 phase, which is usual for the salt bath processes. No peaks of a-Fe reflection from the substrate are visible so that the phase structure close to the nitride–substrate interface cannot be discussed. During the ´ phase formation, nitrogen exceeding the saturation level recombines into molecular N2 thus creating voids that coalesce into columnar pores w11x. The g9 phase with lower content of nitrogen preferably segregates at the substrate–white layer interface with no pores formation. As revealed from Fig. 1a, the porosity of compound zone is more evident at the outer part of the layer, so that the existence of a thin g9 phase at the interface cannot be excluded. Salt bath nitriding followed by oxidation process proved to give a surface structure of enhanced corrosion resistance superior to corrosion properties of 20-mm thick hard chromium galvanic coating, but still there are some difficulties in reproducing a similar structure by plasma processes w3x. As compared to the salt bath treated sample, the aim of the plasma treatment experiments was to obtain a less porous, more compact nitridedynitrocarburized layer with a dense magnetite overlayer of good adhesion to the nitride.

The SEM micrographs of the cross-section of salt bath nitrided sample by TENIFER process are given in Fig. 1. Onto diffusion zone a compound layer was formed with the thickness over 30 mm (Fig. 1a). The topography of the surface of compound layer illustrates

Fig. 1. SEM micrographs of salt bath nitrided compound layer. (a) Cross section; (b) surface topography.

cO (at.%)

Fig. 2. XRD spectra of a salt bath nitrided sample.

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Fig. 3. Microhardness profile of plasma nitrocarburized sample.

All the samples were plasma nitridedynitrocarburized in the same batch according to the process parameters given in Table 1. The microhardness distribution over metallographic cross-section of a plasma nitrocarburized specimen is given in Fig. 3. Due to the long treatment time the nitrogen diffuses over 1 mm into the sample surface, which results in enhanced microhardness. The examination by the optical microscopy revealed the existence of a compound zone on nitrided and postoxidized samples. The thickness of ‘white layer’ on the

samples post-oxidized in H2 yO2 gas discharge containing only 4.3% of oxygen (Fig. 4a) is lower than on the sample treated in atmosphere with 19.8% of oxygen (Fig. 4b). Compared to the salt bath nitriding which produced an over 30-mm thick compound layer, the g9 carbonitride layer less than 8-mm thick was formed on plasma nitrocarburized samples. The XRD investigations revealed the existence of the g9 zone and an overlayer of magnetite Fe3O4 on the samples post-oxidized in a gas discharge containing 4.3% (Fig. 5a) and 19.8% (Fig. 5b) of oxygen. In the gas mixture with the maximum oxygen content used in experiments (19.8%) the layer richer in magnetite phase was produced. Fig. 6 illustrates the influence of post-oxidation process on nitrogen diffusion inside the surface zone. Beneath the magnetite layer, the g9 phase was transformed to the ´ phase after post-oxidation in 14.4% oxygen containing a hydrogenynitrogen mixture. It can be seen that the content of Fe3O4 phase is further increased as compared to the samples which XRD spectra are shown in Fig. 5. It stems from SEM analysis that a complex compound zone is formed on nitrocarburized and post-oxidized specimens. The structure of the surface zone of the sample treated in 4.3% oxygen-containing atmosphere

Fig. 4. Cross-section optical micrographs of nitrocarburized and oxidized samples: (a) 4.3%; (b) 19.8% oxygen in plasma (300=; nital 3%).

Fig. 5. XRD spectra of post-oxidized samples: (a) 4.3%; (b) 19.8% of oxygen in the gas discharge.

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ing atmosphere leads to the formation of the Fe2O3 oxide phase, which takes less Fe atoms than the magnetite. The concentration of released nitrogen atoms decreases and no precipitation of ´ phase was observed (Fig. 5b) after post-oxidation in 19.8% at. oxygen concentration in the gas mixture. It stems from SEM and XRD analysis that plasma post-oxidation processes offer flexibility in controlling the structure and properties of the oxide films and compound layers, which is, besides non-toxic characteristics, a clear advantage of plasma techniques. 4. Conclusions

Fig. 6. XRD spectra of the samples oxidized at 14.4% of oxygen in gas discharge.

is given in Fig. 7a and b. On g9 compound layer an oxide layer was formed (Fig. 7a) with a palisade structure clearly visible from Fig. 7b. In the case of increased oxygen content (19.8%) (Fig. 7c) the fine structured oxide layer was grown. The thickness of the white layer was close to 6 mm (4.3% oxygen) and 8 mm (19.8% oxygen). Compared to the ´ compound layer of salt bath nitrided specimen (Fig. 2), plasma nitridingynitrocarburizing and post-oxidation performed at the process conditions given in Table 1 produced only g9 or g9q´ phase white layers. The explanation of the observed transformation of the g9 into the nitrogen richer ´ phase during post-oxidation treatment (Fig. 6) requires some additional experiments. This effect may be due to the variation of released nitrogen atom concentration at the oxide–nitride interface. During the oxidation the iron atoms are incorporated in the oxide film and nitrogen atoms are released. Since a compact magnetite overlayer acts as the diffusion barrier, the nitrogen atoms diffuse into the compound g9 layer. By increasing the oxygen content in the gas mixture the concentration of nitrogen atoms at the oxide–nitride interface is also increased leading to the transformation of the g9 to ´ phase. As shown in Ref. w6x, a further increase of the oxygen content in process-

A combined surface treatment, which consists of the plasma nitridingynitrocarburizing and plasma post-oxidation, was successfully applied for surface treatment of plain carbon steel samples DIN C15. The surface layer was composed of the diffusion zone over 1-mm thick and the compound layer. The thickness and phase composition of the compound layer depends on the post-oxidation gas mixture. In low oxygen atmosphere an over 5-mm thick double compound layer was formed consisting of the g9 and Fe3O4 phase in which the g9 phase dominates. In oxygen rich processing plasma an over 7-mm thick white layer was formed with g9 phase and a magnetite Fe3O4 overlayer with increased content of the magnetite. The transformation of the g9 phase into the ´ phase was observed after the post-oxidation of plasma nitrocarburized sample in 14.4% oxygen containing H2 yO2 gas mixture. Compared to the conventional salt bath nitriding, which can produce an over 30-mm thick ´ carbonitride layer, plasma processing at the conditions used in experiments (Table 1) produced much thinner g9 carbonitride compound layer with small pore concentration and the magnetite Fe3O4 dense structure superficial layer with good adhesion to the nitride zone beneath. Acknowledgments This work was partially supported by the Serbian Ministry of Science and Technologies, Project MIS.3.02.0174.B.

Fig. 7. SEM micrographs of the samples treated in 4.3% oxygen gas: (a) cross-section; (b) surface, and in (c) 19.8% oxygen containing gas.

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