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Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding
Surface and Coatings Technology 133᎐1134 Ž2000. 259᎐263
Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding E. Menthe a,U , A. Bulak a , J. Olfe b, A. Zimmermanna , K.-T. Rie a a
Institut fur und Plasmatechnische Werkstoffentwicklung, TU Braunschweig, Germany ¨ Oberflachentechnik ¨ b Fraunhofer Institut fur Braunschweig, Germany ¨ Schicht- und Oberflachentechnik, ¨
Abstract In this paper, we report on a series of experiments designed to study the influence of plasma nitriding on the mechanical properties of austenitic stainless steel. Plasma nitriding experiments were conducted on AISI 304L steel in a temperature range of 375᎐475⬚C using pulsed-DC plasma with different N2 ᎐H 2 gas mixtures and treatment times. Firstly the formation and the microstructure of the modified layer will be highlighted followed by the results of hardness measurement, adhesion testing, wear resistance and fatigue life tests. The modified surface was analyzed directly after plasma nitriding as well as using a depth profiling method. The microhardness after plasma nitriding is increased up to 19 GPa, that is a factor of five higher compared to the untreated material Ž3.3 GPa.. The adhesion is examined by Rockwell indentation and scratch test. No delamination of the treated layer could be observed. The wear rate after plasma nitriding is significantly reduced compared to the untreated material. Plasma nitriding produces compressive stress inside the modified layer, which can be easily derived from the bending of thin metal foil, which was treated only on one side. The treatment influences the fatigue life, which can be raised by a factor of 10 at a low stress level Ž230 MPa.. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Austenitic stainless steel; Plasma nitriding; S-Phase; Mechanical properties
1. Introduction Nitriding at temperatures of 450⬚C or below offers the opportunity to enhance the mechanical properties of austenitic stainless steels without affecting the excellent corrosion resistance of this material. In this case a metastable phase named ‘S-phase’ or ‘␥ N-Phase’ with different properties is formed. Nitrogen remains in solid solution inside this new phase without removing chromium from the austenitic structure by precipitation of CrN. This type of layer can be produced by different surface treatments, i.e. plasma nitriding w1᎐5,11,12x, plasma immersion ion implantation w9,10x, ion beam nitriding or ion implantation w6᎐8x. Hardness U
Corresponding author. DaimlerChrysler AG, Research and Technology ŽFT4rST., Wilhelm-Runge-Strasse 11, D-89081 Ulm, Germany. Tel.: q49-731505-2242; fax: q49-731505-4106. E-mail address: [email protected] ŽE. Menthe..
and corrosion resistance of this type of layer has been described in many papers but less information exists on the mechanical or technological properties. The wear and the friction behavior as well as the adhesion is of great importance for industrial applications, in case of tubes and fittings the fatigue strength plays an important role.
2. Experimental details We have conducted a series of experiments on AISI 304L austenitic stainless steel at temperatures in the range of 375᎐475⬚C using pulsed-DC plasma nitriding with different N2 ᎐H 2 gas mixtures and treatment times. The structure of the nitrided layers was analyzed by means of Bragg Brentano- and glancing-angle X-ray diffraction, optical microscopy and EPMA. The surface hardness was measured with a nanoindenter with final
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 3 0 - 0
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E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
loads of 0.05, 0.25 and 1 mN. Typically, five indentations were made up to each final load and the results averaged. Wear and friction tests were performed on a pin-on-disk machine, the wear volume was determined by a profilometer. Fatigue tests were carried out on the base material before and after plasma nitriding at two different temperatures. The fatigue specimens are cylindrically shaped with a gauge section of 10 mm in diameter and 14 mm in length. After nitriding for 10 h at 425⬚C a uniform thickness of 10 m of the S-phase was found over the gauge length. Nearly the same thickness of the S-phase was achieved after nitriding at 475⬚C for 5 h. The fatigue tests were triangular load controlled push᎐pull tests with the same maximum load in tension and compression Ž R s y1.. The loading frequency was 1 Hz. The fatigue life Nf is defined by the number of cycles to fracture. The resistance against uniform corrosion was studied in 5 N H 2 SO4 , the pitting corrosion in 2% NaCl solution. These analyses were applied on as-treated samples as well as after removing the layer stepwise by grinding and polishing. Samples to be treated were cut from a heat-treated bar of AISI 304L, ground and mirror polished. A second heat treatment was conducted in order to retransform the stress-induced martensite that was formed during sample preparation. Sputter cleaning was performed in an Ar᎐H 2 atmosphere prior to plasma nitriding with a pulsed-DC glow discharge under the conditions listed in Table 1. All samples were cooled after plasma nitriding in vacuum.
Fig. 1. Bragg᎐Brentano XRD pattern obtained for plasma nitrided sample treated for 5 h at 450⬚C.
Two main peaks representing the 1114 and the 2004 planes of the S-phase and a weak peak of CrN- 2004 are visible. The d-spacing of the S-phase is approximately 9% larger when compared to the untreated stainless steel indicating the formation of a new phase. The formation of this type of layer strongly depends on the treatment parameters. This can be seen by the layer thickness w11x or by the microhardness after nanoindentation ŽTable 2.. The typical hardness of the S-phase layer is approximately 19 GPa. The hardness and the thickness of the modified layer decreases on reduction of the nitrogen partial pressure of the treatment gas, the treatment time, the treatment temperature or the duty cycle.
Table 2 Results of nanoindentation Žmicrohardness 1 mN load. as a function of Ža. treatment time and gas composition and Žb. as a function of treatment temperature and duty cycle
3. Results and discussion 3.1. Microstructure after plasma nitriding
Ža. Time Žmin.
Fig. 1 shows the typical Bragg᎐Brentano XRD of plasma nitrided stainless steel treated for 5 h at 450⬚C. Table 1 Parameters for plasma nitriding treatments Parameter
Sputter cleaning
Plasma nitriding
Temperature T Ž⬚C. Pulse duration Pd Žs. Pulse pause Pp Žs. Voltage U ŽV. Total pressure P ŽPa. Time t Žh. Treatment gas Ar Žvol.%. N2 Žvol.%. H2 Žvol.%.
100᎐350 50 330 410 100 1
350᎐500 50᎐300 200᎐1000 400᎐600 600 0.5᎐16
80 ᎐ 20
᎐ 20᎐80 80᎐20
Microhardness ŽGPa. Nitrogenrhydrogen ratio 80r20
50r50
20r80
30 60 120 180 240 300 540 960
17.9 16.1 18.1 19.6 19.1 19.7 18.4 19.3
6.7 16.6 17.4 18.9 19.6 18.8 19.0
10.9 8.2 18.5 19.0 19.9
Žb. Temperature Ž⬚C.
Microhardness ŽGPa. Pulse durationrpulse pause ratio 50:1000 50:500 50:330 3.5 3.4 5.6 3.3 4.2 16.4 3.3 4.1 20.2 3.6 3.6 20.7 3.7 4.3 19.7
375 400 425 450 475
50:200 16.9 17.8 18.7 18.5
E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
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Fig. 2. SEM-micrograph of a plasma nitrided Ž5 h, 450⬚C. disc after Rockwell indentation test.
Fig. 4. SEM-micrograph of a fracture section of a plasma nitrided Ž5 h, 450⬚C. sample.
3.2. Mechanical properties
layer are still in good contact. Fig. 5 shows first results on the influence of plasma nitriding on the SrN-curves. In the Low Cycle Fatigue ŽLCF. range Ž Nf - 50 000. no influence of nitriding was apparent. Even if the number of tests was relatively low in the High Cycle Fatigue ŽHCF. range at lower cyclic loads, a trend towards increased fatigue life due to plasma nitriding could be observed. Increased fatigue life after nitriding is a well-known phenomenon for different kinds of steels and is attributed to the formation of internal compressive stresses during the diffusion treatment. The formation of compressive stress inside the modified layer after plasma nitriding can be easily derived by the bending of a thin metal foil treated only on one side. The internal stress is superimposed to the external load and leads to a reduction of the effective stress in tension. Since only tensile stress produces fatigue cracks and contributes to crack progress a reduction of the effective stress in tension increases the time to fatigue failure. In the case of HCF most of the fatigue life is spent
Fig. 2 shows the surface of a plasma nitrided sample after Rockwell indentation. Some small cracks are visible around the edge of the indentation but no delamination of the layer takes place. Therefore, the adhesion strength can be classified to the highest group ŽHF1. according to Rockwell indentation test standard w13x. A scratch test was also performed in order to measure the adhesion strength ŽFig. 3.. Even at 100 N load no delamination was observed, only some cracks in the scratch track appear. According to Burnett and Rickerby w14x these cracks were caused due to bending of the layer or due to exceeding the tensile strength. The very good contact of the modified layer to the substrate material is also visible in a simple bending test. In order to produce a brittle cracking of the layer the test has been performed at low temperatures Žy192⬚C.. It is obvious from the SEM micrograph ŽFig. 4. that the substrate material is still ductile, the layer itself seems to be very compact with good adhesion to the substrate material. Even very small parts of the
Fig. 3. SEM-micrograph at the end of a scratch track Ž100 N. carried out on a plasma nitrided Ž5 h, 450⬚C. sample.
Fig. 5. Number of cycles to failure for different stress amplitudes ŽS᎐N curves. of untreated and plasma nitrided austenitic stainless stee l.
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E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
Fig. 6. Wear track cross-section area of untreated and plasma nitrided ŽPN, 5 h, 450⬚C. austenitic stainless steel discs obtained with various loads and pin materials Žload: 5 N, ¨ : 0.05 mrs, rel. H: 40᎐60%, RT..
during the crack initiation whereas in the LCF-range crack growth in the bulk material controls fatigue life. Since the bulk material is not influenced by plasma nitriding no effect on the fatigue life during LCF could be expected. 3.3. Wear and friction The wear rate of untreated austenitic stainless steel is compared to the plasma nitrided material in Fig. 6. The wear resistance after plasma nitriding is significantly increased ᎏ practically no wear track is visible on the treated disc for a load of 5 N. Even after an increase of the test load by a factor of four, the wear rate on the treated disc is still significantly lower than that obtained on the untreated material. The main wear regime observed for the untreated discs are grain pull-out, whereas abrasive wear dominates for the plasma nitrided discs. This is also reflected in the friction coefficient ŽFig. 7.. Untreated and plasma nitrided pins made of austenitic stainless steel were tested against different materials. In case of untreated stainless steel a high friction coefficient in a range of 0.7 was measured which is typical for this material. After
Fig. 7. Friction coefficient of plasma nitrided ŽPN, 5 h, 450⬚C. pins against different substrate materials Žload: 5N, ¨ : 0.05 mrs, rel. H: 40᎐60%, RT.. Materials: Poly Tetra Fluor Ethylene ŽPTFE.; polyethylene ŽPE.; aluminum ŽAl.; stainless steel ŽSS.
Fig. 8. Anodic polarisation curves of plasma nitrided Ž5 h, 450⬚C. samples measured after 10 cycles in 5 N H 2 SO4 . The depth profile was obtained due to careful removing part of the surface layer by grinding.
plasma nitriding of only one partner the friction coefficient is strongly reduced. 3.4. Corrosion resistance Fig. 8 shows the influence of plasma nitriding on the uniform corrosion resistance measured in 5 N H 2 SO4 . The initial critical current density is slightly more than one decade higher compared to an untreated sample measured to have an average current density of 100 Arcm2 . After removing parts of the surface layer the critical current density is reduced and reaches nearly the level of the untreated material. The behavior against pitting corrosion shows an opposite tendency. In this case the resistance of the plasma nitrided material is strongly increased, especially nitriding with high nitrogen partial pressure leads to this increased resistant against pitting corrosion. After removing the surface layer stepwise the pitting corrosion resistance decreases ŽFig. 9. but is still significantly higher compared to the untreated material.
Fig. 9. Anodic polarisation curves of plasma nitrided Ž5 h, 450⬚C. samples measured after 10 cycles in 2% NaCl. The depth profile was obtained due to careful removing part of the surface layer by grinding.
E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
4. Conclusions A new phase with outstanding mechanical and technological properties is formed after plasma nitriding. The microhardness of this layer is increased by a factor of five compared to the untreated material and the wear is strongly reduced even at higher loads. The friction coefficient of plasma nitrided austenitic steel against ferritic or austenitic steel is lower compared to the untreated material. The fatigue strength in the high cycle fatigue range at lower cyclic loads is also increased due to plasma nitriding. The increased fatigue life is a well-known phenomenon for different kinds of steels and is attributed to the formation of internal compressive stresses during the plasma nitriding treatment. The results show that plasma nitriding of austenitic stainless steel is a suitable process for improving the mechanical and the technological properties without significantly effecting the corrosion resistance of this material.
Acknowledgements The authors gratefully acknowledge the financial support from the ‘Deutsche Forschungsgemeinschaft’ for part of this research work. They would also like to
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thank Prof. G.K. Wolf and Dr. S. Schell ŽUniversity of Heidelberg. for corrosion measurement. References w1x K. Ichii, K. Fujimura, T. Takase, Technology reports of Kansai University, No. 27, March 1986. w2x P.A. Dearnley, A. Namvar, G.G.A. Hibberd, T. Bell, Proceedings of the 1st International Conference on plasma surface engineering, Garmisch-Partenkirchen, 1988, DGM Informationsgesellschaft, Oberursel, 1989, p. 219. w3x A. Saker, C.h. Leroy, H. Michel, C. Frantz, Mater. Sci. Eng. A140 Ž1990. 702. w4x E. Menthe, K.-T. Rie, J.W. Schultze, S. Simson, Surf. Coat. Technol. 74᎐75 Ž1995. 412. w5x K. Marchev, M. Landis, R. Vallerio, B.C. Giessen, Surf. Coat. Technol. 116᎐119 Ž1998. 184. w6x R. Leutenecker, G. Wagner, T. Louis, U. Gonser, L. Guzman, A. Molinari, Mater. Sci. Eng. A115 Ž1989. 229. ¨ ¨ D.L. Williamson, J. Appl. Phys. 77 Ž1995. 3839. w7x O. Ozturk, w8x D.L. Williamson, J.A. Davis, P.J. Wilbur, Surf. Coat. Technol. 103᎐104 Ž1998. 178. w9x M. Samandi, B.A. Shedden, D.I. Smith, Surf. Coat. Technol. 59 Ž1993. 261. w10x G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, X. Li, M. Samandi, Surf. Coat. Technol. 74᎐75 Ž1995. 417. w11x E. Menthe, K.-T. Rie, Surf. Coat. Technol. 99 Ž1998. 198. w12x B. Larisch, U. Brusky, H.-J. Spies, Surf. Coat. Technol. 99 Ž1998. 205. w13x H. Jehn, G. Reiners, DIN Fachbericht 39; Haftfestigkeit, Beuth Verlag, Berlin, 1993, p. 210. w14x P.J. Burnett, D.S. Rickerby, Thin Solid Films 154 Ž1987. 403.
Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding E. Menthe a,U , A. Bulak a , J. Olfe b, A. Zimmermanna , K.-T. Rie a a
Institut fur und Plasmatechnische Werkstoffentwicklung, TU Braunschweig, Germany ¨ Oberflachentechnik ¨ b Fraunhofer Institut fur Braunschweig, Germany ¨ Schicht- und Oberflachentechnik, ¨
Abstract In this paper, we report on a series of experiments designed to study the influence of plasma nitriding on the mechanical properties of austenitic stainless steel. Plasma nitriding experiments were conducted on AISI 304L steel in a temperature range of 375᎐475⬚C using pulsed-DC plasma with different N2 ᎐H 2 gas mixtures and treatment times. Firstly the formation and the microstructure of the modified layer will be highlighted followed by the results of hardness measurement, adhesion testing, wear resistance and fatigue life tests. The modified surface was analyzed directly after plasma nitriding as well as using a depth profiling method. The microhardness after plasma nitriding is increased up to 19 GPa, that is a factor of five higher compared to the untreated material Ž3.3 GPa.. The adhesion is examined by Rockwell indentation and scratch test. No delamination of the treated layer could be observed. The wear rate after plasma nitriding is significantly reduced compared to the untreated material. Plasma nitriding produces compressive stress inside the modified layer, which can be easily derived from the bending of thin metal foil, which was treated only on one side. The treatment influences the fatigue life, which can be raised by a factor of 10 at a low stress level Ž230 MPa.. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Austenitic stainless steel; Plasma nitriding; S-Phase; Mechanical properties
1. Introduction Nitriding at temperatures of 450⬚C or below offers the opportunity to enhance the mechanical properties of austenitic stainless steels without affecting the excellent corrosion resistance of this material. In this case a metastable phase named ‘S-phase’ or ‘␥ N-Phase’ with different properties is formed. Nitrogen remains in solid solution inside this new phase without removing chromium from the austenitic structure by precipitation of CrN. This type of layer can be produced by different surface treatments, i.e. plasma nitriding w1᎐5,11,12x, plasma immersion ion implantation w9,10x, ion beam nitriding or ion implantation w6᎐8x. Hardness U
Corresponding author. DaimlerChrysler AG, Research and Technology ŽFT4rST., Wilhelm-Runge-Strasse 11, D-89081 Ulm, Germany. Tel.: q49-731505-2242; fax: q49-731505-4106. E-mail address: [email protected] ŽE. Menthe..
and corrosion resistance of this type of layer has been described in many papers but less information exists on the mechanical or technological properties. The wear and the friction behavior as well as the adhesion is of great importance for industrial applications, in case of tubes and fittings the fatigue strength plays an important role.
2. Experimental details We have conducted a series of experiments on AISI 304L austenitic stainless steel at temperatures in the range of 375᎐475⬚C using pulsed-DC plasma nitriding with different N2 ᎐H 2 gas mixtures and treatment times. The structure of the nitrided layers was analyzed by means of Bragg Brentano- and glancing-angle X-ray diffraction, optical microscopy and EPMA. The surface hardness was measured with a nanoindenter with final
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 3 0 - 0
260
E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
loads of 0.05, 0.25 and 1 mN. Typically, five indentations were made up to each final load and the results averaged. Wear and friction tests were performed on a pin-on-disk machine, the wear volume was determined by a profilometer. Fatigue tests were carried out on the base material before and after plasma nitriding at two different temperatures. The fatigue specimens are cylindrically shaped with a gauge section of 10 mm in diameter and 14 mm in length. After nitriding for 10 h at 425⬚C a uniform thickness of 10 m of the S-phase was found over the gauge length. Nearly the same thickness of the S-phase was achieved after nitriding at 475⬚C for 5 h. The fatigue tests were triangular load controlled push᎐pull tests with the same maximum load in tension and compression Ž R s y1.. The loading frequency was 1 Hz. The fatigue life Nf is defined by the number of cycles to fracture. The resistance against uniform corrosion was studied in 5 N H 2 SO4 , the pitting corrosion in 2% NaCl solution. These analyses were applied on as-treated samples as well as after removing the layer stepwise by grinding and polishing. Samples to be treated were cut from a heat-treated bar of AISI 304L, ground and mirror polished. A second heat treatment was conducted in order to retransform the stress-induced martensite that was formed during sample preparation. Sputter cleaning was performed in an Ar᎐H 2 atmosphere prior to plasma nitriding with a pulsed-DC glow discharge under the conditions listed in Table 1. All samples were cooled after plasma nitriding in vacuum.
Fig. 1. Bragg᎐Brentano XRD pattern obtained for plasma nitrided sample treated for 5 h at 450⬚C.
Two main peaks representing the 1114 and the 2004 planes of the S-phase and a weak peak of CrN- 2004 are visible. The d-spacing of the S-phase is approximately 9% larger when compared to the untreated stainless steel indicating the formation of a new phase. The formation of this type of layer strongly depends on the treatment parameters. This can be seen by the layer thickness w11x or by the microhardness after nanoindentation ŽTable 2.. The typical hardness of the S-phase layer is approximately 19 GPa. The hardness and the thickness of the modified layer decreases on reduction of the nitrogen partial pressure of the treatment gas, the treatment time, the treatment temperature or the duty cycle.
Table 2 Results of nanoindentation Žmicrohardness 1 mN load. as a function of Ža. treatment time and gas composition and Žb. as a function of treatment temperature and duty cycle
3. Results and discussion 3.1. Microstructure after plasma nitriding
Ža. Time Žmin.
Fig. 1 shows the typical Bragg᎐Brentano XRD of plasma nitrided stainless steel treated for 5 h at 450⬚C. Table 1 Parameters for plasma nitriding treatments Parameter
Sputter cleaning
Plasma nitriding
Temperature T Ž⬚C. Pulse duration Pd Žs. Pulse pause Pp Žs. Voltage U ŽV. Total pressure P ŽPa. Time t Žh. Treatment gas Ar Žvol.%. N2 Žvol.%. H2 Žvol.%.
100᎐350 50 330 410 100 1
350᎐500 50᎐300 200᎐1000 400᎐600 600 0.5᎐16
80 ᎐ 20
᎐ 20᎐80 80᎐20
Microhardness ŽGPa. Nitrogenrhydrogen ratio 80r20
50r50
20r80
30 60 120 180 240 300 540 960
17.9 16.1 18.1 19.6 19.1 19.7 18.4 19.3
6.7 16.6 17.4 18.9 19.6 18.8 19.0
10.9 8.2 18.5 19.0 19.9
Žb. Temperature Ž⬚C.
Microhardness ŽGPa. Pulse durationrpulse pause ratio 50:1000 50:500 50:330 3.5 3.4 5.6 3.3 4.2 16.4 3.3 4.1 20.2 3.6 3.6 20.7 3.7 4.3 19.7
375 400 425 450 475
50:200 16.9 17.8 18.7 18.5
E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
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Fig. 2. SEM-micrograph of a plasma nitrided Ž5 h, 450⬚C. disc after Rockwell indentation test.
Fig. 4. SEM-micrograph of a fracture section of a plasma nitrided Ž5 h, 450⬚C. sample.
3.2. Mechanical properties
layer are still in good contact. Fig. 5 shows first results on the influence of plasma nitriding on the SrN-curves. In the Low Cycle Fatigue ŽLCF. range Ž Nf - 50 000. no influence of nitriding was apparent. Even if the number of tests was relatively low in the High Cycle Fatigue ŽHCF. range at lower cyclic loads, a trend towards increased fatigue life due to plasma nitriding could be observed. Increased fatigue life after nitriding is a well-known phenomenon for different kinds of steels and is attributed to the formation of internal compressive stresses during the diffusion treatment. The formation of compressive stress inside the modified layer after plasma nitriding can be easily derived by the bending of a thin metal foil treated only on one side. The internal stress is superimposed to the external load and leads to a reduction of the effective stress in tension. Since only tensile stress produces fatigue cracks and contributes to crack progress a reduction of the effective stress in tension increases the time to fatigue failure. In the case of HCF most of the fatigue life is spent
Fig. 2 shows the surface of a plasma nitrided sample after Rockwell indentation. Some small cracks are visible around the edge of the indentation but no delamination of the layer takes place. Therefore, the adhesion strength can be classified to the highest group ŽHF1. according to Rockwell indentation test standard w13x. A scratch test was also performed in order to measure the adhesion strength ŽFig. 3.. Even at 100 N load no delamination was observed, only some cracks in the scratch track appear. According to Burnett and Rickerby w14x these cracks were caused due to bending of the layer or due to exceeding the tensile strength. The very good contact of the modified layer to the substrate material is also visible in a simple bending test. In order to produce a brittle cracking of the layer the test has been performed at low temperatures Žy192⬚C.. It is obvious from the SEM micrograph ŽFig. 4. that the substrate material is still ductile, the layer itself seems to be very compact with good adhesion to the substrate material. Even very small parts of the
Fig. 3. SEM-micrograph at the end of a scratch track Ž100 N. carried out on a plasma nitrided Ž5 h, 450⬚C. sample.
Fig. 5. Number of cycles to failure for different stress amplitudes ŽS᎐N curves. of untreated and plasma nitrided austenitic stainless stee l.
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E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
Fig. 6. Wear track cross-section area of untreated and plasma nitrided ŽPN, 5 h, 450⬚C. austenitic stainless steel discs obtained with various loads and pin materials Žload: 5 N, ¨ : 0.05 mrs, rel. H: 40᎐60%, RT..
during the crack initiation whereas in the LCF-range crack growth in the bulk material controls fatigue life. Since the bulk material is not influenced by plasma nitriding no effect on the fatigue life during LCF could be expected. 3.3. Wear and friction The wear rate of untreated austenitic stainless steel is compared to the plasma nitrided material in Fig. 6. The wear resistance after plasma nitriding is significantly increased ᎏ practically no wear track is visible on the treated disc for a load of 5 N. Even after an increase of the test load by a factor of four, the wear rate on the treated disc is still significantly lower than that obtained on the untreated material. The main wear regime observed for the untreated discs are grain pull-out, whereas abrasive wear dominates for the plasma nitrided discs. This is also reflected in the friction coefficient ŽFig. 7.. Untreated and plasma nitrided pins made of austenitic stainless steel were tested against different materials. In case of untreated stainless steel a high friction coefficient in a range of 0.7 was measured which is typical for this material. After
Fig. 7. Friction coefficient of plasma nitrided ŽPN, 5 h, 450⬚C. pins against different substrate materials Žload: 5N, ¨ : 0.05 mrs, rel. H: 40᎐60%, RT.. Materials: Poly Tetra Fluor Ethylene ŽPTFE.; polyethylene ŽPE.; aluminum ŽAl.; stainless steel ŽSS.
Fig. 8. Anodic polarisation curves of plasma nitrided Ž5 h, 450⬚C. samples measured after 10 cycles in 5 N H 2 SO4 . The depth profile was obtained due to careful removing part of the surface layer by grinding.
plasma nitriding of only one partner the friction coefficient is strongly reduced. 3.4. Corrosion resistance Fig. 8 shows the influence of plasma nitriding on the uniform corrosion resistance measured in 5 N H 2 SO4 . The initial critical current density is slightly more than one decade higher compared to an untreated sample measured to have an average current density of 100 Arcm2 . After removing parts of the surface layer the critical current density is reduced and reaches nearly the level of the untreated material. The behavior against pitting corrosion shows an opposite tendency. In this case the resistance of the plasma nitrided material is strongly increased, especially nitriding with high nitrogen partial pressure leads to this increased resistant against pitting corrosion. After removing the surface layer stepwise the pitting corrosion resistance decreases ŽFig. 9. but is still significantly higher compared to the untreated material.
Fig. 9. Anodic polarisation curves of plasma nitrided Ž5 h, 450⬚C. samples measured after 10 cycles in 2% NaCl. The depth profile was obtained due to careful removing part of the surface layer by grinding.
E. Menthe et al. r Surface and Coatings Technology 133᎐134 (2000) 259᎐263
4. Conclusions A new phase with outstanding mechanical and technological properties is formed after plasma nitriding. The microhardness of this layer is increased by a factor of five compared to the untreated material and the wear is strongly reduced even at higher loads. The friction coefficient of plasma nitrided austenitic steel against ferritic or austenitic steel is lower compared to the untreated material. The fatigue strength in the high cycle fatigue range at lower cyclic loads is also increased due to plasma nitriding. The increased fatigue life is a well-known phenomenon for different kinds of steels and is attributed to the formation of internal compressive stresses during the plasma nitriding treatment. The results show that plasma nitriding of austenitic stainless steel is a suitable process for improving the mechanical and the technological properties without significantly effecting the corrosion resistance of this material.
Acknowledgements The authors gratefully acknowledge the financial support from the ‘Deutsche Forschungsgemeinschaft’ for part of this research work. They would also like to
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