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Corrosion resistance of chromium nitride and oxynitride layers produced under glow discharge conditions
Surface and Coatings Technology 130 Ž2000. 274᎐279
Corrosion resistance of chromium nitride and oxynitride layers produced under glow discharge conditions T. Wierzchon ´U , I. Ulbin-Pokorska, K. Sikorski Faculty of Materials Science and Engineering, Warsaw Uni¨ ersity of Technology, 02-507 Warsaw, Woloska 141, Poland Received 14 December 1999; accepted in revised form 5 April 2000
Abstract The paper presents the results of investigations of the structure and corrosion resistance of chromium nitride, oxynitride and oxide layers produced on steels by electrochemical chromium deposition combined with glow discharge assisted nitriding, oxynitriding and oxydizing processes. The layers obtained were of the types: CrNq Cr2 Nq Cr q ŽCr,Fe.; Cr2 Nq Cr q ŽCr,Fe.; CrNq Cr2 Nq Cr q ŽCr,Fe. 7 C 3; Cr2 Nq Cr q ŽCr,Fe. 7 C 3; CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 and Cr2 O 3. The corrosion resistance of the layers is high and can be modified by changing the process parameters, such as the temperature and chemical composition of the gas atmosphere. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Chromium nitride; Chromium oxynitride; Chemical composition; Plasma nitriding; Corrosion resistance
1. Introduction The requirements for modern structural materials of high performance properties have prompted the development of various material treatment techniques and their combination. Such combined methods facilitate controlling the chemical composition, phase composition, microstructure and residual stresses state of the treated layers or coatings, thereby controlling their performance properties and, on the other hand, permit the elimination of drawbacks, e.g. high porosity of monolayer coatings, or improving their adhesion to the substrate due to the diffusion involved in the process. Literature reports indicate an increased interest of industry in chromium nitride layers due to their higher hardness and erosion resistance compared to those of
U
Corresponding author. Fax: q7-4822-8484947. E-mail address: [email protected] ŽT. Wierzchon ´..
chromium coatings, and a good frictional wear and corrosion resistance at elevated temperatures as well w1᎐13x. Therefore, apart from using various PVD techniques w1᎐10x, relatively new methods of producing these layers, such as electrochemical chromium deposition combined with glow discharge assisted nitriding w11᎐13x or with ion implantation w14᎐16x are being investigated. This paper presents the results of examinations of the surface morphology and corrosion resistance of the composite layers, produced by glow discharge assisted nitriding, oxynitriding w17x and oxydizing of the chromium preplated steel.
2. Experimental methods The substrate materials were Armco iron and AISI 1045 steel which were covered electrochemically Žusing a 250 grdm3 CrO 3 , 2.5 grdm3 H 2 SO4 , 5᎐10 grdm3
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 9 6 - 4
T. Wierzchon ´ et al. r Surface and Coatings Technology 130 (2000) 274᎐279
275
Fig. 1. Microstructures of the composite layers: Cr2 Nq Cr q ŽCr,Fe. on Armco iron Ža. and Cr2 Nq Cr q ŽCr,Fe. 7 C 3 on 45 steel Žb., produced by glow discharge assisted nitriding at a temperature of 850⬚C.
Cr2 O 3 solution. with chromium coatings 10᎐50 m thick, and then were subjected to: 1. glow discharge assisted nitriding at temperatures between 500 and 850⬚C in a nitrogenrhydrogen atmosphere at various process durations Ž1᎐10 h.; 2. oxynitriding at a temperature of 560⬚C in a nitrogenrhydrogen atmosphere with an addition of approximately 2 vol.% of air for 10 h; and 3. oxydizing at a temperature of 400⬚C in an airrargon atmosphere for 4 h. The layers, thus produced, were examined metallographically, their phase compositions were determined using a Philips 1830 X-ray diffractometer with a CoK ␣ ˚ .. The quantitative distribution radiation Ž s 1.7902 A of the elements in the layers was examined with a Cameca Semprobe SU-30 X-ray microanalyser at an accelerating voltage of 15 kV and a current intensity of 20 nA, using the Cameca correction program based on the Pouchou᎐Pichoir method w18x. The surfaces of the samples were observed in a Tesla BS 300 SEM. The corrosion resistance was measured by the potentiodynamic method in which the samples were immersed in a 0.5 M NaCl solution at a temperature of 25⬚C and polarized from a potential of y1000 mV in the anodic
Fig. 2. Distributions of chromium, iron and nitrogen in the Cr2 Nq Cr q ŽCr,Fe. layer on Armco iron Ža., and the distributions of chromium, nitrogen and carbon in the composite Cr2 Nq Cr q ŽCr,Fe. 7 C 3 layer on 45 steel precoated with Cr to a thickness of 15 m Žb..
direction with a potential varying rate of 50 mVrmin and 10 mVrmin within the corrosion potential region. The potential was measured with respect to that of a calomel electrode ŽSCE.. The potentiodynamic measurements were performed using a fully computerized Atlas᎐Sollich system. Prior to the measurements, the samples were maintained in the test solution for 24 h so as to let the corrosion potential stabilize.
3. Results and discussion The microphotographs of the composite layers produced on chromium preplated by glow discharge assisted nitriding are shown in Fig. 1. The layers consist
Fig. 3. Polarization curves of the CrNq Cr2 N chromium nitride layers produced on Armco iron by glow discharge assisted nitriding at temperatures of 600, 700 and 800⬚C and a process duration of 1 h compared with the polarization curve of a galvanic chromium layer.
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Fig. 4. Appearance of the surface of chromium nitrides of the CrNq Cr2 N type after nitriding at a temperature of 560⬚C for 1 h Ža. and for 10 h Žb..
of Cr2 Nq Cr q ŽCr,Fe. on Armco iron and Cr2 Nq Cr q ŽCr,Fe. 7 C 3 on AISI 1045 steel. The distributions of the elements in the layers are shown in Fig. 2. The oxynitriding process gave the following layers: CrŽN,O. q Cr q ŽCr,Fe. on Armco iron and CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 on AISI 1045 steel. The surface hardness of the top chromium nitride layers ŽCrNq Cr2 N. was approximately 2200 HV 0.02 and that of the chromium oxynitride layers CrŽN,O. with approximately 30 at.% of oxygen ᎏ approximately 1300 HV 0.02. The ŽCr,Fe. 7 C 3 zone formed on AISI 1045 steel as a result of the mutual diffusion of chromium and iron and the diffusion of carbon from the substrate into the
chromium layer, has a hardness of the order of 2600 HV 0.05 w11x. The formation and properties of chromium nitride layers strongly depend on the treatment temperature. This effect is well illustrated by the short duration glow discharge assisted nitriding of Cr-coated Armco iron with a coating thickness of approximately 15 m. Fig. 3 shows the corrosion resistance of the nitrided layers formed by nitriding at various temperatures for 1 h. We can clearly see that, as the nitriding temperature decreases, the corrosion resistance of the chromium nitrides increases, i.e. the corrosion potential becomes more positive whereas the corrosion current density and the current density in the passive state decrease. This is due to the formation of a homogeneous, finegrained chromium nitride structure within the nearsurface zone of the layer ŽFig. 4.. These relationships still hold when the treatment time is increased ŽFigs. 3 and 5.. This suggests that the corrosion resistance of these layers is chiefly settled during the initial stages of the formation of chromium nitride layers. Fig. 6 shows the appearance of the surface of the CrNq Cr2 Nq Cr q ŽCr,Fe. layers on Armco iron before and after the corrosion measurements. In galvanic chromium coatings Žapprox. 15 m thick., we observe first a very active underfilm crevice corrosion and then a uniform corrosion of the substrate ŽFig. 6a., which can even result in the layer spalling. After the glow discharge assisted nitriding, the crevices in the chromium coating appear to be filled, which causes the significant increase in the corrosion resistance of the layers. Similar advantageous effects occur during glow discharge assisted oxynitriding. The surface zone of chromium oxynitrides of the CrŽN,O. type formed during this process increases the corrosion resistance even more than the chromium nitride layer formed by glow discharge nitriding alone ŽFigs. 7 and 5., making it comparable with the highest corrosion resistance of the Cr2 O 3 chromium oxide layer produced under glow discharge conditions at a temperature of 400⬚C ŽFig. 7..
Fig. 5. Polarization curves of the CrNq Cr2 Nq Cr q ŽCr,Fe. surface layers produced on Armco iron by glow discharge assisted nitriding at various process durations.
T. Wierzchon ´ et al. r Surface and Coatings Technology 130 (2000) 274᎐279
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Fig. 6. Appearance of the surface of a chromium layer produced by the electrochemical method Ža. and by glow discharge assisted nitriding at 560⬚C Žb. before and after corrosion measurements.
Surface examinations of the composite CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer on AISI 1045 steel have shown that initially it is built of regular ‘islets’ 5᎐6 m in diameter and oblong bands that represent the crevices in the chromium coating ŽFig. 8a.. As the treatment time increases, the number of the ‘islets’ increases so that they join with one another to form larger agglomerates and finally a solid layer. However, as the treatment time is prolonged, the layer spalls in the regions above the filled crevices, especially at the edges of the samples ŽFig. 8b.. This is most probably due to the increased residual stresses induced in the CrŽN,O. layer. Fig. 9 shows the surface distributions of chromium, nitrogen and oxygen in the central region of the sample. We can see that the signal of nitrogen and oxygen within the band positioned along the line of analysis Žmarked in Fig. 9a. is stronger than that outside this band. Fig. 10a,b shows the chromium, nitrogen and oxygen distributions at the edges of the sample, along a line of analysis perpendicular to a crevice where the CrŽN,O. layer is spallingx. We can see from these figures that the crevices present in the chromium coating have been filled with chromium oxides and nitrides. Quantitative examinations of the element concentrations in the CrŽN,O. layer Ž50 m chromium precoated sample oxynitrided for 10 h. made in various regions of the sample have shown that the layer is quite homogeneous and contains 32.36 at.% of nitrogen, 27.44 at.%
of oxygen and 37.40 at.% of chromium on average. The fact that the corrosion resistance of the oxynitrided CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layers is better than that of solely nitrided layers and can be compared with the corrosion resistance of the Cr2 O 3 chromium oxides may result from the presence of oxygen in the nearsurface zone of this layer. 4. Conclusions Glow discharge assisted nitriding of chromium pre-
Fig. 7. Polarization curves of the CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer produced by glow discharge assisted oxynitriding of 1045 steel precoated with a 50-m chromium film and Cr2 O 3 layer produced by glow discharge assisted oxydizing of Armco iron precoated with a 15-m chromium film compared with the polarization curves of the starting materials.
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Fig. 8. An SE Žsecondary electrons. image of the surface of a CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer.
coated steels gives surface layers of the types: CrNq Cr2 Nq Cr q ŽCr,Fe., Cr2 Nq Cr q ŽCr,Fe.; or CrNq Cr2 Nq Cr q ŽCr,Fe. 7 C 3 , Cr2 Nq Cr q ŽCr,Fe. 7 C 3 , depending on the steel grade used as the substrate and the temperature of the process. When nitriding is carried out above 800⬚C, we obtain the composite layers with the top layer composed of chromium nitride ŽCr2 N., whereas below this temperature, within the 530᎐700⬚C range, the top layer contains ŽCrNq Cr2 N. nitrides. The corrosion resistance of these layers depends on the composition and the microstructure of the chromium nitride zone Ž1᎐5 m thick.. A higher corrosion resistance ᎏ comparable with that of Cr2 O 3 chromium oxide ᎏ can be achieved by using a glow
Fig. 9. The surface of a CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer produced at T s 560⬚C, Ps 2.4 hPa, t s 10 h, an SE image and the distributions of nitrogen ŽN., chromium ŽCr. and oxygen ŽO. Ža., and the distribution of nitrogen, chromium and oxygen along the line indicated in Ža. and Žb. in the central region of the sample.
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discharge assisted oxynitriding process which gives CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 type layers. The composite layers thus produced have a diffusion character and show a good adhesion and a uniform surface structure. Therefore, they can essentially widen the application range of chromium coatings formed on steel. References w1x R. Aharonov, B. Coll, R. Fontana, Surf. Coat. Technol. 61 Ž1993. 223. w2x J.P. Terrat, A. Gaucher, H. Hadj-Rabah, R.Y. Fillit, Surf. Coat. Technol. 45 Ž1991. 59. w3x O. Knotek, F. Loffler, H.J. Scholl, Surf. Coat. Technol. 45 ¨ Ž1991. 53. w4x S. De Rossi, S. Luridiana, Proceedings of the 11th Congress of the International Federation For Heat Treatment And Surface Engineering, Florence, Oct. 1998, vol. II, 39. w5x G. Berg, C.H. Friedrich, E. Broszeit, C.H. Berger, Surf. Coat. Technol. 86᎐87 Ž1996. 184. w6x G. Bertrand, C. Savall, C. Meunier, Surf. Coat. Technol. 96 Ž1997. 323. w7x M. Sonobe, K. Shiozawa, K. Motobayashi, JSME Int. J. A 40 Ž4. Ž1997. 436. w8x C. Gautier, J. Machet, Thin Solid Films 295 Ž1997. 43. w9x Y. Fu, X. Zhu, B. Tang et al., Wear 217 Ž1998. 159. w10x P. Engel, G. Schwarz, G.K. Wolf, Surf. Coat. Technol. 98 Ž1998. 1002. w11x T. Wierzchon, ´ I. Ulbin-Pokorska, K. Sikorski, J. Trojanowski, Vacuum 53 Ž1999. 473. w12x J.P. Ge, Plat. Surf. Finish. 83 Ž5. Ž1996. 146. w13x E. Menthe, K.T. Rie, Surf. Coat. Technol. 112 Ž1999. 217. w14x J.I. Onate, Trans. Inst. Met. Finish. 65 Ž3,s. Ž1987. 99. w15x W.C. Oliver, R. Hutchings, J.B. Pethica, Metall. Trans. 15A Ž12. Ž1984. 2221. w16x R. Hutchings, Mat. Sci. Eng. 69 Ž1. Ž1985. 129. w17x K. Sikorski, T. Wierzchon, ´ Proceedings of the X Conference on Electron Microscopy of Solids. E. Jezierska, J.A. Kozubowski ŽEds.., Warsaw-Serock, Sept. 1999, 343. w18x J.L. Pouchou, F. Pichoir, Rech. Aerosp. 3 Ž1984. 121.
Fig. 10. The surface of a CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer produced at T s 560⬚C, Ps 2.4 hPa, t s 10 h, an SE image and the distributions of nitrogen ŽN., chromium ŽCr. and oxygen Ž0. Ža., and the distribution of nitrogen, chromium and oxygen along the line indicated in Ža. and Žb. at the edge of the sample.
Corrosion resistance of chromium nitride and oxynitride layers produced under glow discharge conditions T. Wierzchon ´U , I. Ulbin-Pokorska, K. Sikorski Faculty of Materials Science and Engineering, Warsaw Uni¨ ersity of Technology, 02-507 Warsaw, Woloska 141, Poland Received 14 December 1999; accepted in revised form 5 April 2000
Abstract The paper presents the results of investigations of the structure and corrosion resistance of chromium nitride, oxynitride and oxide layers produced on steels by electrochemical chromium deposition combined with glow discharge assisted nitriding, oxynitriding and oxydizing processes. The layers obtained were of the types: CrNq Cr2 Nq Cr q ŽCr,Fe.; Cr2 Nq Cr q ŽCr,Fe.; CrNq Cr2 Nq Cr q ŽCr,Fe. 7 C 3; Cr2 Nq Cr q ŽCr,Fe. 7 C 3; CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 and Cr2 O 3. The corrosion resistance of the layers is high and can be modified by changing the process parameters, such as the temperature and chemical composition of the gas atmosphere. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Chromium nitride; Chromium oxynitride; Chemical composition; Plasma nitriding; Corrosion resistance
1. Introduction The requirements for modern structural materials of high performance properties have prompted the development of various material treatment techniques and their combination. Such combined methods facilitate controlling the chemical composition, phase composition, microstructure and residual stresses state of the treated layers or coatings, thereby controlling their performance properties and, on the other hand, permit the elimination of drawbacks, e.g. high porosity of monolayer coatings, or improving their adhesion to the substrate due to the diffusion involved in the process. Literature reports indicate an increased interest of industry in chromium nitride layers due to their higher hardness and erosion resistance compared to those of
U
Corresponding author. Fax: q7-4822-8484947. E-mail address: [email protected] ŽT. Wierzchon ´..
chromium coatings, and a good frictional wear and corrosion resistance at elevated temperatures as well w1᎐13x. Therefore, apart from using various PVD techniques w1᎐10x, relatively new methods of producing these layers, such as electrochemical chromium deposition combined with glow discharge assisted nitriding w11᎐13x or with ion implantation w14᎐16x are being investigated. This paper presents the results of examinations of the surface morphology and corrosion resistance of the composite layers, produced by glow discharge assisted nitriding, oxynitriding w17x and oxydizing of the chromium preplated steel.
2. Experimental methods The substrate materials were Armco iron and AISI 1045 steel which were covered electrochemically Žusing a 250 grdm3 CrO 3 , 2.5 grdm3 H 2 SO4 , 5᎐10 grdm3
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 9 6 - 4
T. Wierzchon ´ et al. r Surface and Coatings Technology 130 (2000) 274᎐279
275
Fig. 1. Microstructures of the composite layers: Cr2 Nq Cr q ŽCr,Fe. on Armco iron Ža. and Cr2 Nq Cr q ŽCr,Fe. 7 C 3 on 45 steel Žb., produced by glow discharge assisted nitriding at a temperature of 850⬚C.
Cr2 O 3 solution. with chromium coatings 10᎐50 m thick, and then were subjected to: 1. glow discharge assisted nitriding at temperatures between 500 and 850⬚C in a nitrogenrhydrogen atmosphere at various process durations Ž1᎐10 h.; 2. oxynitriding at a temperature of 560⬚C in a nitrogenrhydrogen atmosphere with an addition of approximately 2 vol.% of air for 10 h; and 3. oxydizing at a temperature of 400⬚C in an airrargon atmosphere for 4 h. The layers, thus produced, were examined metallographically, their phase compositions were determined using a Philips 1830 X-ray diffractometer with a CoK ␣ ˚ .. The quantitative distribution radiation Ž s 1.7902 A of the elements in the layers was examined with a Cameca Semprobe SU-30 X-ray microanalyser at an accelerating voltage of 15 kV and a current intensity of 20 nA, using the Cameca correction program based on the Pouchou᎐Pichoir method w18x. The surfaces of the samples were observed in a Tesla BS 300 SEM. The corrosion resistance was measured by the potentiodynamic method in which the samples were immersed in a 0.5 M NaCl solution at a temperature of 25⬚C and polarized from a potential of y1000 mV in the anodic
Fig. 2. Distributions of chromium, iron and nitrogen in the Cr2 Nq Cr q ŽCr,Fe. layer on Armco iron Ža., and the distributions of chromium, nitrogen and carbon in the composite Cr2 Nq Cr q ŽCr,Fe. 7 C 3 layer on 45 steel precoated with Cr to a thickness of 15 m Žb..
direction with a potential varying rate of 50 mVrmin and 10 mVrmin within the corrosion potential region. The potential was measured with respect to that of a calomel electrode ŽSCE.. The potentiodynamic measurements were performed using a fully computerized Atlas᎐Sollich system. Prior to the measurements, the samples were maintained in the test solution for 24 h so as to let the corrosion potential stabilize.
3. Results and discussion The microphotographs of the composite layers produced on chromium preplated by glow discharge assisted nitriding are shown in Fig. 1. The layers consist
Fig. 3. Polarization curves of the CrNq Cr2 N chromium nitride layers produced on Armco iron by glow discharge assisted nitriding at temperatures of 600, 700 and 800⬚C and a process duration of 1 h compared with the polarization curve of a galvanic chromium layer.
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Fig. 4. Appearance of the surface of chromium nitrides of the CrNq Cr2 N type after nitriding at a temperature of 560⬚C for 1 h Ža. and for 10 h Žb..
of Cr2 Nq Cr q ŽCr,Fe. on Armco iron and Cr2 Nq Cr q ŽCr,Fe. 7 C 3 on AISI 1045 steel. The distributions of the elements in the layers are shown in Fig. 2. The oxynitriding process gave the following layers: CrŽN,O. q Cr q ŽCr,Fe. on Armco iron and CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 on AISI 1045 steel. The surface hardness of the top chromium nitride layers ŽCrNq Cr2 N. was approximately 2200 HV 0.02 and that of the chromium oxynitride layers CrŽN,O. with approximately 30 at.% of oxygen ᎏ approximately 1300 HV 0.02. The ŽCr,Fe. 7 C 3 zone formed on AISI 1045 steel as a result of the mutual diffusion of chromium and iron and the diffusion of carbon from the substrate into the
chromium layer, has a hardness of the order of 2600 HV 0.05 w11x. The formation and properties of chromium nitride layers strongly depend on the treatment temperature. This effect is well illustrated by the short duration glow discharge assisted nitriding of Cr-coated Armco iron with a coating thickness of approximately 15 m. Fig. 3 shows the corrosion resistance of the nitrided layers formed by nitriding at various temperatures for 1 h. We can clearly see that, as the nitriding temperature decreases, the corrosion resistance of the chromium nitrides increases, i.e. the corrosion potential becomes more positive whereas the corrosion current density and the current density in the passive state decrease. This is due to the formation of a homogeneous, finegrained chromium nitride structure within the nearsurface zone of the layer ŽFig. 4.. These relationships still hold when the treatment time is increased ŽFigs. 3 and 5.. This suggests that the corrosion resistance of these layers is chiefly settled during the initial stages of the formation of chromium nitride layers. Fig. 6 shows the appearance of the surface of the CrNq Cr2 Nq Cr q ŽCr,Fe. layers on Armco iron before and after the corrosion measurements. In galvanic chromium coatings Žapprox. 15 m thick., we observe first a very active underfilm crevice corrosion and then a uniform corrosion of the substrate ŽFig. 6a., which can even result in the layer spalling. After the glow discharge assisted nitriding, the crevices in the chromium coating appear to be filled, which causes the significant increase in the corrosion resistance of the layers. Similar advantageous effects occur during glow discharge assisted oxynitriding. The surface zone of chromium oxynitrides of the CrŽN,O. type formed during this process increases the corrosion resistance even more than the chromium nitride layer formed by glow discharge nitriding alone ŽFigs. 7 and 5., making it comparable with the highest corrosion resistance of the Cr2 O 3 chromium oxide layer produced under glow discharge conditions at a temperature of 400⬚C ŽFig. 7..
Fig. 5. Polarization curves of the CrNq Cr2 Nq Cr q ŽCr,Fe. surface layers produced on Armco iron by glow discharge assisted nitriding at various process durations.
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Fig. 6. Appearance of the surface of a chromium layer produced by the electrochemical method Ža. and by glow discharge assisted nitriding at 560⬚C Žb. before and after corrosion measurements.
Surface examinations of the composite CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer on AISI 1045 steel have shown that initially it is built of regular ‘islets’ 5᎐6 m in diameter and oblong bands that represent the crevices in the chromium coating ŽFig. 8a.. As the treatment time increases, the number of the ‘islets’ increases so that they join with one another to form larger agglomerates and finally a solid layer. However, as the treatment time is prolonged, the layer spalls in the regions above the filled crevices, especially at the edges of the samples ŽFig. 8b.. This is most probably due to the increased residual stresses induced in the CrŽN,O. layer. Fig. 9 shows the surface distributions of chromium, nitrogen and oxygen in the central region of the sample. We can see that the signal of nitrogen and oxygen within the band positioned along the line of analysis Žmarked in Fig. 9a. is stronger than that outside this band. Fig. 10a,b shows the chromium, nitrogen and oxygen distributions at the edges of the sample, along a line of analysis perpendicular to a crevice where the CrŽN,O. layer is spallingx. We can see from these figures that the crevices present in the chromium coating have been filled with chromium oxides and nitrides. Quantitative examinations of the element concentrations in the CrŽN,O. layer Ž50 m chromium precoated sample oxynitrided for 10 h. made in various regions of the sample have shown that the layer is quite homogeneous and contains 32.36 at.% of nitrogen, 27.44 at.%
of oxygen and 37.40 at.% of chromium on average. The fact that the corrosion resistance of the oxynitrided CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layers is better than that of solely nitrided layers and can be compared with the corrosion resistance of the Cr2 O 3 chromium oxides may result from the presence of oxygen in the nearsurface zone of this layer. 4. Conclusions Glow discharge assisted nitriding of chromium pre-
Fig. 7. Polarization curves of the CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer produced by glow discharge assisted oxynitriding of 1045 steel precoated with a 50-m chromium film and Cr2 O 3 layer produced by glow discharge assisted oxydizing of Armco iron precoated with a 15-m chromium film compared with the polarization curves of the starting materials.
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Fig. 8. An SE Žsecondary electrons. image of the surface of a CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer.
coated steels gives surface layers of the types: CrNq Cr2 Nq Cr q ŽCr,Fe., Cr2 Nq Cr q ŽCr,Fe.; or CrNq Cr2 Nq Cr q ŽCr,Fe. 7 C 3 , Cr2 Nq Cr q ŽCr,Fe. 7 C 3 , depending on the steel grade used as the substrate and the temperature of the process. When nitriding is carried out above 800⬚C, we obtain the composite layers with the top layer composed of chromium nitride ŽCr2 N., whereas below this temperature, within the 530᎐700⬚C range, the top layer contains ŽCrNq Cr2 N. nitrides. The corrosion resistance of these layers depends on the composition and the microstructure of the chromium nitride zone Ž1᎐5 m thick.. A higher corrosion resistance ᎏ comparable with that of Cr2 O 3 chromium oxide ᎏ can be achieved by using a glow
Fig. 9. The surface of a CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer produced at T s 560⬚C, Ps 2.4 hPa, t s 10 h, an SE image and the distributions of nitrogen ŽN., chromium ŽCr. and oxygen ŽO. Ža., and the distribution of nitrogen, chromium and oxygen along the line indicated in Ža. and Žb. in the central region of the sample.
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discharge assisted oxynitriding process which gives CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 type layers. The composite layers thus produced have a diffusion character and show a good adhesion and a uniform surface structure. Therefore, they can essentially widen the application range of chromium coatings formed on steel. References w1x R. Aharonov, B. Coll, R. Fontana, Surf. Coat. Technol. 61 Ž1993. 223. w2x J.P. Terrat, A. Gaucher, H. Hadj-Rabah, R.Y. Fillit, Surf. Coat. Technol. 45 Ž1991. 59. w3x O. Knotek, F. Loffler, H.J. Scholl, Surf. Coat. Technol. 45 ¨ Ž1991. 53. w4x S. De Rossi, S. Luridiana, Proceedings of the 11th Congress of the International Federation For Heat Treatment And Surface Engineering, Florence, Oct. 1998, vol. II, 39. w5x G. Berg, C.H. Friedrich, E. Broszeit, C.H. Berger, Surf. Coat. Technol. 86᎐87 Ž1996. 184. w6x G. Bertrand, C. Savall, C. Meunier, Surf. Coat. Technol. 96 Ž1997. 323. w7x M. Sonobe, K. Shiozawa, K. Motobayashi, JSME Int. J. A 40 Ž4. Ž1997. 436. w8x C. Gautier, J. Machet, Thin Solid Films 295 Ž1997. 43. w9x Y. Fu, X. Zhu, B. Tang et al., Wear 217 Ž1998. 159. w10x P. Engel, G. Schwarz, G.K. Wolf, Surf. Coat. Technol. 98 Ž1998. 1002. w11x T. Wierzchon, ´ I. Ulbin-Pokorska, K. Sikorski, J. Trojanowski, Vacuum 53 Ž1999. 473. w12x J.P. Ge, Plat. Surf. Finish. 83 Ž5. Ž1996. 146. w13x E. Menthe, K.T. Rie, Surf. Coat. Technol. 112 Ž1999. 217. w14x J.I. Onate, Trans. Inst. Met. Finish. 65 Ž3,s. Ž1987. 99. w15x W.C. Oliver, R. Hutchings, J.B. Pethica, Metall. Trans. 15A Ž12. Ž1984. 2221. w16x R. Hutchings, Mat. Sci. Eng. 69 Ž1. Ž1985. 129. w17x K. Sikorski, T. Wierzchon, ´ Proceedings of the X Conference on Electron Microscopy of Solids. E. Jezierska, J.A. Kozubowski ŽEds.., Warsaw-Serock, Sept. 1999, 343. w18x J.L. Pouchou, F. Pichoir, Rech. Aerosp. 3 Ž1984. 121.
Fig. 10. The surface of a CrŽN,O. q Cr q ŽCr,Fe. 7 C 3 layer produced at T s 560⬚C, Ps 2.4 hPa, t s 10 h, an SE image and the distributions of nitrogen ŽN., chromium ŽCr. and oxygen Ž0. Ža., and the distribution of nitrogen, chromium and oxygen along the line indicated in Ža. and Žb. at the edge of the sample.