The wear and corrosion properties of stainless steel nitrided by low-pressure plasma-arc source ion nitriding at low temperatures

The wear and corrosion properties of stainless steel nitrided by low-pressure plasma-arc source ion nitriding at low temperatures

Surface and Coatings Technology 130 Ž2000. 304᎐308 The wear and corrosion properties of stainless steel nitrided by low-pressure plasma-arc source io...

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Surface and Coatings Technology 130 Ž2000. 304᎐308

The wear and corrosion properties of stainless steel nitrided by low-pressure plasma-arc source ion nitriding at low temperatures Wang LiangU , Xu Bin, Yu Zhiwei, Shi Yaqin Institute of Metals and Technology, Dalian Maritime Uni¨ ersity, Dalian 116024, PR China Received 2 February 2000; accepted in revised form 29 April 2000

Abstract Low pressure plasma arc discharge-assisted nitriding of AISI 304 austenitic stainless steel is a process that produces surface layers with useful properties such as a high surface hardness of approximately 1500 Hv0.1 and a high resistance to frictional wear and corrosion. The phase composition, the thickness, the microstructure and the surface topography of the nitrided layer, as well as its properties, depend essentially on the process parameters. Among them, the processing temperature is the most important factor for forming a hard layer with good wear and corrosion resistance. Nitriding austenitic stainless steel at approximately 420⬚C for 70 min can produce a thin layer of 7᎐8 ␮m with very high hardness and good corrosion resistance on the surface. The microstructure was studied by optical microscopy and both glancing angle and conventional Bragg᎐Brentano Ž ␪᎐2␪. symmetric geometry X-ray diffraction ŽXRD.. The formation of expanded austenite was observed. Measurements of the wear depths indicated that the wear resistance of austenitic stainless steel can be improved greatly by nitriding at approximately 420⬚C using low-pressure plasma-arc source ion nitriding. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Low pressure plasma arc source; Nitriding; Stainless steel; Wear and pitting corrosion

1. Introduction Austenitic stainless steels are known for their excellent corrosion resistance based on a native surface oxide layer, but low hardness leads to a short lifetime in industrial applications with intensive wear. In addition, several austenitic stainless steels such as AISI 304 suffer extensive pitting or localized corrosion in the presence of halide ions. Except for surface treatments, these materials can only be hardened by cold forming. Nitriding, which is an effective technique for strength-

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Corresponding author. E-mail address: [email protected] ŽW. Liang..

ening the surface of materials to improve their surface hardness, tribological and corrosion properties, has been developed for many years. However, for austenitic stainless steels at a temperature above 500⬚C, the nitrided layer formed is accompanied by CrN precipitation, resulting in a significant decrease in corrosion resistance. In recent years, efforts have been made to obtain a metastable nitrogen super-saturated austenite, called ␥N or S-phase, on the surface of the austenitic stainless steel by various nitriding processes at a low temperature of approximately 400⬚C, where the mobility of the chromium is low enough to avoid precipitation of chromium nitride in the nitrided layer w1x. The purpose of this paper was to obtain a thicker nitrided layer Ž; 10 ␮m. consisting of expanded

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W. Liang et al. r Surface and Coatings Technology 130 (2000) 304᎐308

Fig. 1. Schematic diagram of the low-pressure plasma-arc source ion nitriding furnace.

austenite by low-pressure plasma-arc source ion nitriding at temperature ; 420⬚C for 70 min, and to investigate the wear and corrosion properties of the layer.

2. Experiments The material used in this work is a variant of AISI 304L austenitic stainless steel with the following chemical composition Žwt.%.: C, 0.03; Si, 0.70; Ti, 0.14; Cr, 18.9; Ni, 9.2; and Fe in balance. Disk-type wear testing specimens of 35 mm in diameter and 2 mm in thickness were cut from a bar. Before nitriding, the specimens were surface ground, followed by mechanical grinding. Corrosion testing samples of 15 mm in diameter and 8 mm in thickness were prepared in same way. The original structure of the samples was austenite with a trace of ferrite created by the mechanical grinding process in preparation. Low-pressure plasma-arc source ion nitriding was carried out using a 15 kW ion nitriding and ion plating unit Žsee Fig. 1.. The hollow cathode generated an arc discharge plasma, which contained a high portion of directed electrons with an enhanced mean energy, the so-called low voltage electron beam, resulting in very effective ionization of the gas. Consequently, very high plasma densities could be achieved. Another advantage was the uncoupling of the plasma generation from the substrate. Independent substrate biasing provided a means for independent ion flux and ion energy control. The processing time was kept constant at 70 min, and ammonia was used as the working gas. The working pressure was approximately 0.4 Pa. The arc current was 40 A with a voltage of 50 V. A negative bias of 1000 V was applied on the substrates. A heater was used to maintain the substrate temperature at approximately 420⬚C. The current density on the substrate surface was 0.6᎐0.8 mArcm2. The nitriding process was started immediately as soon as the temperature was reached without pre-cleaning by ion sputtering. NH3 was used instead of N2 or N2 q H2 as the nitrogen source in order to maximize the energy of

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species, since NHq has an energyrŽ N nucleus . of 3 approximately 1.65 times greater than that of N2q for the same applied voltage w2x. The composition, structure and microstructure of the resulting nitrided layers formed on the AISI 304 austenitic stainless steel at a temperature of 420⬚C were determined by using optical microscopy, X-ray diffraction, Auger electron spectroscopy and electron probe microstructure analyses ŽEPMA.. XRD was done on the sample that was nitrided at 420⬚C for 70 min with Cu Ka radiation in the conventional Bragg᎐Brentano Ž ␪᎐2␪. symmetric geometry and glancing angle geometry at an incidence angle of 10⬚. The microhardness of the nitrided layer was measured by a Vicker’s microhardness tester with a load of 1 N. Sliding wear tests were carried out using a pin-on-disc wear tester without lubrication. During testing, the stainless steel disc rotated against a stationary ball of AISI 52100 high carbon᎐chromium bearing steel with a diameter of 12.8 mm. This steel had been quenched and tempered to give a hardness of HRC 63᎐64. A sliding speed of 0.3 mrs was used for all the tests with different loads. The wear properties and mechanisms were evaluated by measuring the depths of the sliding tracks left on the surface and observing the worn surface by SEM. The corrosion measurements were made by potentiadynamic polarization. An EG and G M342 potentiostat was used. The electrolyte was 3.5% NaCl solution. The potential sweep was 10 mVrs. All measurements were conducted for an area of 10-mm diameter and at room temperature. The morphology of the corroded samples was analyzed by SEM.

3. Results and discussion Fig. 2 shows the microstructure of a cross-section of the AISI 304 stainless steel surface nitrided at 420⬚C for 70 min. A white layer of 7᎐8-␮m thickness, with no evident diffusion layer, was produced. No other microstructural feature was revealed in the cross-section of the layer produced at this temperature by optical met-

Fig. 2. Micrograph of the cross-section of an AISI 304 austenitic stainless steel sample nitrided at 420⬚C for 70 min.

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Fig. 3. Grazing incidence XRD and conventional Bragg᎐Brentano XRD patterns obtained after nitriding AISI 304 austenitic stainless steel at 420⬚C for 70 min.

allographic technique. X-Ray diffraction analysis showed that the white layer consisted of the expanded austenite Ž ␥N . phase with a small amount of ␧M martensite created by the induced stress. Many researchers consider it to be a supersaturated nitrogen solid solution in austenite. Fig. 3 shows both a typical glancing angle XRD pattern obtained with an incidence angle of 10⬚ and a Bragg᎐Brentano Ž ␪᎐2␪. pattern. The lattice planes corresponding to the peaks are labeled in the spectra. The main diffraction peaks were attributed to the austenite expanded by nitrogen i.e. ␥N , which could be obtained with all nitriding processes including conventional d.c. plasma nitriding w3,4x, plasma source ion implantation w5x, plasma immersion ion implantation w6x, low-energy nitrogen ion implantation, and other methods w7᎐10x. There was no evident difference in peak position and shape between the two diffraction patterns, except for a small peak of ␧M martensite, indicating that the nitrogen concentration in the zone of 3᎐5 ␮m near the surface was almost homogenous. Such modified surface layers had a nitrogen concentration, as measured by EPMA and AES, of ; 40 at.%. The diffraction peaks from the substrate were not visible in these two types of XRD patterns, indicating that the nitrided layer is relatively thick. The microhardness measured for treated sample was Hv

1500 in comparison to Hv 320 for the untreated sample. The sliding wear behavior of the untreated and treated samples was assessed using a pin-on-disc test. Fig. 4 shows the depth of the wear scar on specimens nitrided in NH3 for 70 min at 420⬚C with different loads compared with that of unnitrided specimens. The untreated steels suffered severe wear. A mixture of adhesion, abrasion and plastic deformation was observed in and around the wear track. Soon after the start of the test, metallic debris was observed. Large variations in the friction force suggested that the steel and ball were sticking together leading to adhesive wear, whereas for the nitrided samples, there was only a trace of very fine oxide particles generated by friction. These tribological tests with loads of 20-N indicated that the coefficient of friction was essentially reduced from 0.4᎐0.5 to 0.16᎐0.2 by ion-nitriding. With the load above 20 N, the friction coefficient of the treated sample was essentially similar to that of the untreated sample, remaining at a value of 0.5᎐0.6. However, the wear rate was reduced by a factor of approximately 4 after treatment. Micrographs of the wear tracks for both the untreated and treated surfaces after testing at a load of 10 N for 60 min are shown in Fig. 5. After testing for 60 min, the worn surface of a nitrided disk was very smooth, contrasting with the rough worn surface of an unnitrided disk. On unnitrided austenitic stainless steel specimens, severe adhesive and abrasive wear was observed, whereas the nitrided specimens revealed only very mild abrasive wear, a small amount of tribo-chemical reaction and some plastic deformation as the layer was pressed in to the substrate at higher loads. Comparison of the worn surfaces revealed very different wear behavior for unnitrided and nitrided specimens. The super-saturated nitrogen in austenite caused by nitriding increased the strength of the AISI 304 austenitic stainless steel and could introduce extremely high compressive residual

Fig. 4. Depth of the wear scar on AISI 304 austenitic stainless steel specimen nitrided at 420⬚C for 70 min compared with that of unnitrided sample.

W. Liang et al. r Surface and Coatings Technology 130 (2000) 304᎐308

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Fig. 5. SEM micrographs of disc wear tracks after testing for 60 min at a load of 10 N.

stresses in the nitrided layer. These compressive stresses would tend to close existing microcracks or impede their formation during wear, hence retarding the formation of wear particles by the tensile stresses produced during sliding. The unnitrided materials suffered severe wear, with abrasion and a combined formation of metallic and oxidized wear particles generated by severe adhesion and plowing. On the contrary, the nitrided material showed polishing of the grinding marks, producing a smooth oxidized surface with a rapid transition towards a mild oxidative wear mechanism. This adherent oxide layer formed on the surface of nitrided samples acts as a lubricating layer, preventing metal-to-metal contact, decreasing the friction coefficient, reducing adhesive wear and the stresses transmitted to the rubbing surfaces, and, therefore, reducing their wear. Wear proceeds by an oxidative-type mechanism. After the nitriding treatment the wear regime changed completely. The severe metallic wear regime was replaced by a much milder oxide wear behavior. As long as the nitrided layer remained intact, the measured wear track depth seemed to be a result of plastic deformation in the substrate with the modified layer being pressed into the substrate. Fig. 6 presents the potentiodynamic curves of anodic polarization of nitrided and unnitrided AISI 304 stain-

less steel in aerated 3.5% NaCl. The pitting potential for the samples is defined as the potential where there is an abrupt rise in the current. The polarization curves in Fig. 6 indicate that the unnitrided sample did not passivate and underwent a continuous dissolution while the nitrided sample has an evident passive region between y80 and 500 mV. The anodic current density was reduced significantly after nitriding, which indicated that the nitrogen incorporated in the surface layer impeded the anodic dissolution, resulting in a very low anodic current. Consequently, the passivability and pitting resistance of AISI 304 austenitic steel was evidently improved by low-pressure plasma-arc source ion nitriding. Fig. 7 compares the surface microstructures of unnitrided and plasma nitrided samples at

Fig. 6. Potentiodynamic polarization curves for untreated and treated AISI 304 stainless steel in 3.5% NaCl solution.

Fig. 7. SEM micrographs of unnitrided and nitrided sample surface after corrosion in 3.5% NaCl solution.

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420⬚C. The formation of corrosion pits could be confirmed by the naked eye from its small size and round shape. Pitting corrosion was observed only for the unnitrided sample. The corrosion pits, with an average size of 250᎐300 ␮m on the surface, were clearly visible. For the nitrided sample, pitting corrosion did not occur, so corrosion pits were not found on the surface. The corrosion resistance is much enhanced by lowpressure plasma-arc source ion nitriding at the temperature of 420⬚C. It has been well established that the resistance against corrosion of stainless steel is maintained or improved by the presence of the ␥N layer at the surface by ion-nitriding treatment at a temperature below 450⬚C w11᎐13x. The corrosion resistance of nitrided layer in aqueous solution of NaCl is due to the formation of an adhering anodic passivation layer. It is clear from this study that the nitriding treatment enhances the passive film formation of austenitic stainless steel.

4. Conclusions 1. Low-pressure plasma-arc source ion nitriding at low temperatures has been a successful way of obtaining a hard nitrogen rich layer with a thickness of 7᎐8 ␮m for a short processing time, without the formation of chromium nitrides, and enhancing the passivation of the stainless steel substrate in NaCl solution.

2. A reduction in wear and coefficient of friction was obtained after nitriding. The higher hardness and large compressive stress of sample nitrided is also responsible for its better wear resistance. 3. The wear mechanism of AISI 304 austenitic stainless steel is completely changed after nitriding. The severe metallic wear regime is replaced by a much milder wear behavior. References w1x R. Wei, Surf. Coat. Technol. 83 Ž1996. 218. w2x C.P. Munson, R.J. Faehl, I. Henins et al., Surf. Coat. Technol. 84 Ž1996. 528. w3x S.P. Hannula, P. Nenonen, J.P. Hirvonen, Thin Solid Films 181 Ž1989. 343. w4x E. Menthe, K.-T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74᎐75 Ž1995. 412. w5x R. Gunzel, M. Betzl, I. Alphonsa, B. Ganguly, P.I. John, S. Mukherjee, Surf. Coat. Technol. 112 Ž1999. 307. w6x M. Samandi, B.A. Shedden, D.I. Smith, G.A. Collins, R. Hutchings, J. Tendys, Surf. Coat. Technol. 59 Ž1992. 261. w7x D.L. Williamson, L. Wang, R. Wei, P.J. Wilbur, Mater. Lett. 9 Ž1990. 302. w8x O. Ozturk, D.L. Williamson, J. Appl. Phys. 77 Ž1995. 3839. w9x M.J. Baldwin, G.A. Collins, M.P. Fewell et al., Jpn. J. Appl. Phys. 26 Ž1997. 4941. w10x N. Renevier, P. Collignon, H. Michel, T. Czerwiec, Surf. Coat. Technol. 86᎐87 Ž1996. 285. w11x C. Blawert, A. Weisheit, B.L. Mordike, F.M. Knoop, Surf. Coat. Technol. 85 Ž1996. 15. w12x Z.L. Zhang, T. Bell, Surf. Eng. 1 Ž1985. 131. w13x M.K. Lei, Z.L. Zhang, J. Vac. Sci. Technol. A 13 Ž1995. 2986.