Wear behavior of plasma nitrided AISI 420 stainless steel

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S. P. Brühl et al.: Wear behavior of plasma nitrided AISI 420 stainless steel

Sonia P. Brühla, Raúl Charadiaa, Carlos Sanchezb, Mariana H. Staiab

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W 2008 Carl Hanser Verlag, Munich, Germany

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Surface Engineering Group, Universidad Tecnológica Nacional, FRCU, Concepción del Uruguay, Argentina CENMACOR, Facultad de Ingeniería, Universidad Central de Venezuela, Caracas, Venezuela

Wear behavior of plasma nitrided AISI 420 stainless steel Martensitic AISI 420 stainless steel was plasma nitrided using a DC N2 – H2 pulsed discharge, after thermal treatment at two different tempering temperatures. Sliding wear behavior was determined by means of a ball-on-disk tribometer at room temperature and without lubrication. Wear volume was calculated from the information provided by an optical profilometer and wear scars were analyzed by Scanning Electron Microscopy. The results indicated that the nitrided samples exhibited improved wear resistance when compared to the untreated specimens, but corrosion resistance was diminished due to the presence of the outer nitrided layer. However, when this layer was removed mechanically considerable improvement in the corrosion resistance was achieved without impairing the wear resistance of the whole system, which was 200 % higher compared to the wear resistance of the non-nitrided specimens. Keywords: Plasma nitriding; Adhesive wear; Martensitic stainless steel; Ball-on-disk

1. Introduction Martensitic stainless steels are materials of choice for application where good wear resistance is a key performance indicator in addition to the superior corrosion resistance shown by stainless steel. Such applications include turbine blades, roller bearings and, in medicine, for example, operation needles and bone saws employed for orthopedic surgery. For all of these applications, improved surface hardness, as can be produced in other alloys by nitriding, is desirable [1 – 3]. Plasma nitriding is a useful method of enhancing the metallurgical, mechanical and tribological properties of engineering materials. It is a thermochemical case hardening technique where atomic nitrogen is diffused into the surface of the specimen from the surrounding plasma. During plasma nitriding, the sputtering excites the energetic positive ions, which effectively remove contaminants such as oxides from the component surface, accelerate nitrogen mass transfer from the plasma to the component. Accordingly, plasma nitriding is particularly suitable for surface modification of stainless steel, where the presence of oxide (i. e. Cr2O3) on the surface may prevent nitrogen mass transfer during conventional gaseous nitriding. This treatment has economic, environmental, and metallurgical advantages over other methods [3 – 7]. There have been many reports on the wear characteristics of nitrided austenitic stainless steel [7 – 10]. However, the Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 7

wear behavior of nitrided martensitic stainless steel has not been studied systematically so far [11 – 15]. In this work, a structural investigation of martensitic stainless steel is presented, nitrided after two different heat treatments, together with results and discussion of wear behavior in a ball-ondisk experiment.

2. Experimental Two series of AISI 420 martensitic stainless steel samples were ion nitrided, but with different thermal treatments prior to the nitriding process. The chemical composition in mass percent of AISI 420 is 0.38 % C, 13 % Cr, 0.44 % Mn, 0.42 % Si, 0.07 % Mo, 0.02 % P and balance Fe. The specimens were submitted to heat treatment because the material was received in the annealed condition (a ferritic matrix with M23C6 carbide dispersion). The specimens were heated up to 1050 8C, and held at this austenitizing temperature for 20 min in a non-controlled atmosphere. After this treatment the samples were quenched in oil and tempered at 400 8C and 580 8C for 2 h. Under these conditions a martensitic structure was obtained without loss of toughness and related mechanical properties, such as ductility and fracture toughness. Table 1 presents a summary of the different specimens, treatments and tests referred in this work. The nitriding facility is essentially a discharge chamber of stainless steel, 100 mm in diameter and 300 mm in height. The cathode is a negatively biased stainless steel plate isolated from the chamber. The counter electrode is a concentric cylinder and between them a 500 Hz DC pulsed discharge is sustained. Heat is supplied by a resistance array surrounding the cathode, allowing separate control of sample temperature and joule heating by current density. For this purpose, a thermocouple is attached to the cathode and connected to a proportional – integral – derivative controller Table 1. Specimens with thermal treatment prior to nitriding. Experiment

Tempering Specimen temperature

Nitrided

Analysis

100

400 8C

100-P 100-1 100-2

· ·

wear wear sectioned

101

580 8C

101-P 101-1 101-2 101-3

· · ·

wear sectioned wear wear

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Fig. 1. Schematic drawing of the plasma nitriding reactor.

(PID controller). A schematic drawing of the facility is depicted in Fig. 1. Disk-like specimens were machined from an AISI 420 stainless steel bar, 25 mm in diameter and 6 mm in height. The samples were manually ground using SiC paper down to 1200 grade and then placed in cavities situated on the cathode exposing only one face to the plasma. For AISI 420 stainless steel, plasma nitriding is carried out ten degrees lower than for austenitic steels, namely 400 8C, because this steel contains less chromium (13 % compared to 18 % in AISI 304) and corrosion sensitization is easily achieved when nitrides are formed [15]. The process initiates with two hours sputtering in an argon – hydrogen glow discharge. The aim of the sputtering is not only to clean the surface but also to remove the passive oxide and allow the penetration and diffusion of nitrogen. Once sputtering is finished, argon is replaced by nitrogen and the gas flows are adjusted to have an N2-25 % and H2-75 % vol. flowing plasma. The nitriding process starts when the temperature reaches 400 8C. Current density is maintained constant, with pressure and bias voltage varying to achieve that. As the nitriding takes place at a relative high pressure at 12 mbar, an oxygen getter is placed in the chamber. Table 2 summarizes the sputtering and nitriding parameters. After nitriding is finished, samples are slowly cooled down in the same working atmosphere. Surface hardness was measured with a Shimadzu HV-2 Microhardness Tester equipped with Vickers indenter. An applied load of 0.49 N was chosen for the surface. After that, the specimens were transversely sectioned, mounted 780

in resin and polished in order to study the metallographic features. The recorded hardness – depth profiles ranged from 5 microns to 60 microns below the surface using 0.245 N load. A preliminary study of corrosion was carried out by placing a drop of 10 % CuSO4 aqueous solution on the nitrided surface. The presence of copper as a deposit on the surface indicates chromium precipitation and, consequently, sensitization of the material has taken place, implying the need for grinding and polishing away this layer. However, if the surface was corrosion resistant then the ball-on-disk wear tests could be carried out. The ball-on-disk test was carried out in air at room temperature, with a 3 mm radius Al2O3 ball as counterpart. A 5 N normal load was employed leading to a Hertzian contact pressure of 900 MPa. The disk speed was fixed to 10 cm s – 1 around a track 7 mm in diameter, without lubrication and without debris removal. The total path was chosen as 1000 m. Table 2. Process parameters. Sputtering

Nitriding

Gases and proportion (%vol.) Ar 60 % – H2 40 % N2 25 % – H2 75 % Process Time 2h 10 h Bias – Mean Value 400 – 500 V 500 – 600 V DC Pulsed Frequency 500 Hz – 50 % on 500 Hz – 50 % on Current Density 0.13 mA · cm – 2 0.13 mA · cm – 2 Temperature 100 to 200 8C 400 8C

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After the wear test, the surface was examined by means of scanning electron microscopy (SEM). Wear scars were analyzed with a Zygo 3D Surface Profilometer (based on scanning white-light interferometry) and the volume wear loss was calculated from the wear track plain profiles. The transferred material remnant on the balls was also analyzed by means of SEM and energy-dispersive X-ray analysis (EDAX).

3. Results 3.1. Microstructure and hardness Martensitic stainless steel, as annealed, has a low hardness of 200 – 220 HV. After the thermal treatments, the specimens which had been tempered at 400 8C had a surface hardness of 405 HV, whilst those tempered at 580 8C presented a hardness of only 285 HV (see Table 3). After ion nitriding the surface hardness increased to values which varied between 800 and 1000 HV. In order to characterize the nitrided layer, specimen 100-2 was sectioned and prepared for metallographic examination. The sample was etched with Vilella reagent which allowed the examination of a modified layer of 17 – 18 microns width, as shown in Fig. 2. Based on a previous work published by the authors [16] it can be asserted, taking into account the XRD (X-ray diffraction) analysis, that this layer corresponds to a diffusion zone, where the nitrogen is present on interstitial sites of the tetragonally distorted bcc ferrite lattice, called “expanded martensite” [11, 14]. However, this nomenclature is not very suitable since martensite is a stressed structure itself, therefore “expanded ferrite” could be a more appropriate name [15]. Unlike the results obtained when nitriding medium alloy steels such as AISI 4140, this is a nitrogen rich layer without nitride precipitation. Specimen 101-2 showed a similar layer, but 23 – 24 microns in width. Another specimen, 101-3, corresponding to Experiment 101, was tested for corrosion with CuSO4 and a copper deposit was observed. Subsequently, it was slightly polished and put again in contact with CuSO4. Copper deposition recurred, indicating that a top layer was formed during ion nitriding in these experimental conditions which is not suitable for corrosion applications. Even though the deposit was not easily visible in many zones, it could be seen when using optical microscopy indicating that the nitrided layer suffers from corrosion attack, mainly pitting (Fig. 3). Also, the presence of a thin “black layer” on top of the white layer was observed. This corresponds to a narrow zone characterized by precipitation of nitrides, which is hard and wear resistant, but which is not corrosion resistant due to the chromium depletion which takes place during nitriding. In previous works published in the literature [16, 17], XRD results have indicated that a-Fe is formed toTable 3. Surface Hardness – HV0.05 – 5 % Std dev. Specimen

Before nitriding

After nitriding

100-1 100-2 101-1 101-2 101-3

405 407 290 285 285

1160 1045 1010 915 360 (ground)


Fig. 2. Optical micrograph of the cross-section of ion nitrided AISI 420, showing the nitrided layer in specimen 100-2.

Fig. 3. Micrograph showing the nitrided layer and the microstructure of the base material (quenched and tempered at 400 8C) of the specimen 101-1 with corrosion marks.

gether with iron and chromium nitrides. Below this layer, a white layer forms, indicating that nitrogen remains in solid solution exhibiting better corrosion resistance than the non-nitrided bulk material. With the aim of having this white layer on the top, the 101-3 specimen was ground until the corrosion test was negative. The surface hardness was measured again and, of course, presented a hardness of 360 HV, which was lower than the previous one but still higher than the hardness corresponding to the base material whose value was determined as 285 HV (see Table 3). After the wear test, the sample was analyzed with optical microscopy and it could be seen than more than 20 microns had been removed and only a narrow 3 – 4 microns white layer remained, without a well-defined interface with the bulk material. Specimens 100-1, 101-2 and ground 101-3 were subjected to a sliding wear test carried out on a ball-on-disk tribometer. For comparison, two of the non-nitrided, 100-P and 101-P, samples were also tested. Hardness – depth profiles were also analyzed for the 100-2 and 101-1 sectioned specimens. Sample 100-2 had a 781

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Fig. 4. Hardness – depth profiles across the cross-section of nitrided specimens 100-2 (tempered at 400 8C, core hardness 405 HV) and 101-1 (tempered at 580 8C, core hardness 285 HV).

higher hardness than that of the 101-1 specimen irrespective of the depth value, as shown in Fig. 4. Therefore, it would be expected that in the former case, the sample would exhibit higher yield strength in tension, and also higher shear yield strength.

(b)

3.2. Ball-on-disk wear test and wear profile

Fig. 5. Plain profiles across the wear track in Exp. 101, (a) nitrided, (b) non-nitrided. Note that the two figures have a different height scale.

After the wear test, the surface was cleaned ultrasonically with acetone and isopropyl alcohol and it was ready for optical profilometry studies, which were carried out by means of a Zygo 3D Surface Profiler. The study of the wear tracks included measuring their depth. Subsequently, the mean transversal area and the volume loss were calculated. A preliminary surface examination shows clearly the difference between the wear scars, that of the nitrided specimen and the non-nitrided one. Besides the difference in magnitude, it was observed that the shape of the scar was also slightly different, being more irregular for the non nitrided specimen. This indicates that for the latter higher plastic deformation took place at the edge of the scar. Table 4 shows the results of the wear rate, calculated as mass loss divided by load and total path. A relative value was calculated compared to the reference specimen, i. e. the non nitrided sample. The product of wear rate and hardness, on an arbitrary scale, is presented in the last column. Table 4 and Fig. 5 show the difference in the values corresponding to the wear rate as well as depth of the wear scar, which are almost one order of magnitude smaller for the nitrided specimen in comparison with the non-

nitrided one, thus indicating the beneficial effect of this treatment. A graphical representation of the relative volume loss is shown in Fig. 6. The vertical axis is broken in order to com-

Fig. 6. Graphical representation revealing the volume loss in all specimens. A break was introduced in the volume loss scale (vertical axis) because of the big difference in magnitudes.

Table 4. Comparative wear rates.

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Specimen

Wear scar depth (lm)

Wear Rate (mm3(N · m) – 1)

Wear Rate relative to non-nitrided

Wear Rate · Hardness

100-P 100-1 101-P 101-2 101-3

12.2 1.1 15.8 2.2 10.6

4.65 · 10 – 5 2.30 · 10 – 6 7.12 · 10 – 5 5.42 · 10 – 6 3.77 · 10 – 5

1 0.049 1 0.076 0.53

1883 267 2029 496 1357


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pare the magnitudes and the difference between the nitrided and non nitrided specimens.


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3.3. Friction coefficient In general, the friction coefficient revealed no significant difference between non-nitrided and nitrided specimens during the ball-on-disk test. The non-nitrided 100-P and 101-P specimens had a mean friction coefficient, ld, corresponding to the steady state of 0.71 ± 0.05, meanwhile for the nitrided samples its value varied between 0.77 and 0.80. Nevertheless, in the plot corresponding to specimen 101-2, interesting behavior can be observed such as the change in the mean value of ld from 0.66 to 0.88, as shown in Fig. 7, indicating that the tribological pair has undergone an important change. It is considered that this sudden change could be due to the material transference via “cold welding” from the nitrided surface to the alumina ball without wearing off the nitrided layer completely, since it was found that the wear scar of this specimen possessed a depth of only 2.2 lm, which is considerably smaller than the thickness of the nitrided layer of about 20 lm (see Fig. 2).

(a)

3.4. SEM analysis of balls and worn surfaces SEM examination of the alumina balls after the wear test revealed “cold welded” material, which was transferred to the ball. Figure 8 shows the ball contact surfaces which were the counterparts of samples 100-1 (a) and 101-2 (b). EDAX analysis was performed in two different zones of the transferred material area: the smoother and light gray surface denoted with a white “x” in Fig. 8 and the contiguous zone, darker and inhomogeneous. The spectra shown in Fig. 9, where the black trace with the unfilled area corresponds to the light gray zones labeled “x” in Fig. 8 and it is superimposed with a red and filled trace, taken in the contiguous zone. It can be seen that the only differences between the two zones are in the Fe and Al peaks, which are circled with dotted lines. Therefore, it can be concluded from the EDAX analysis that the light gray zones (“x”) are composed mainly of Fe, showing that this is the material transferred

(b) Fig. 8. SEM images (with Back-Scattered Electrons) of the alumina ball surfaces after the wear test, (a) Exp. 101-2 and (b) Exp. 100-1. Transferred material is marked with a white “x”.

Fig. 9. EDAX analysis of the “x” zones from Fig. 8, the curve in black, not filled, superimposed with the EDAX spectrum of the other zone in red. Note the differences between the two zones in the Fe and Al peaks, circled with dotted lines.

Fig. 7. Variation of the friction coefficient with the sliding distance during wear testing of the nitrided specimen 101-2.


from the nitrided steel to the ball via “cold welding”, and the rest is mainly Al, indicating that this is the eroded zone of the alumina ball. It can also be seen that the material transfer is more extensive for the ball which was in contact with the 101-2 specimen compared with that for the 100-1 specimen. The latter exhibited the best wear resistance. 783

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(b) Fig. 10. Morphology of wear tracks in Experiment 100: (a) nitrided and (b) non-nitrided, (c) and (d) higher magnification.

(c)

(d)

In order to analyze the wear mechanisms the scars were also studied by means of SEM. The SEM micrographs of the worn specimens are shown in Fig. 10. As can be seen from Fig. 10a with low magnification, the surface of the non-nitrided specimen was severely deformed and scored. From that it is deduced that the main wear mechanism was adhesion. At higher magnification, as shown in Fig. 10b, it is obvious that the surface deformed plastically and the presence of oxides is indicated by the dark zones in this micrograph. On the other hand, in the nitrided sample 100-1, only microwear was observed and no adhesion occurred; only a few micro-cracks can be seen.

4. Discussion The plasma nitriding treatment introduces nitrogen into the steel and it has been proved to be a very suitable process for increasing the hardness of the treated material. Therefore, the role of the nitrided layer is to present to the counterpart a harder surface, which can resist plastic deformation and avoid adhesion [18]. The wear scar appearance studied using SEM provides information about contact conditions, and this information is useful for the determination of the wear modes [19, 20]. In this case, comparison of both worn surfaces showed that the non-nitrided specimen underwent severe wear, adhesion, abrasion and plastic deformation, as can be deduced from the morphology of the wear track. In contrast, little damage was observed for the nitrided specimens. The significant increase in surface hardness of the nitrided specimens minimizes both adhesive friction and deformation of the asperities between the two surfaces in contact. The nitrided layer has limited ductility, thus decreasing the mutual solid solubility of the surfaces in contact, which results in good resistance to sliding wear. Specimen 101-2 shows a higher wear rate that was twice that of specimen 100-1. The difference between the re784

sponses to the wear test of the two samples does not depend on surface hardness (Table 4), but it can be understood considering the change in the friction coefficient caused by an appreciable amount of “cold welded” material transferred between the mating surfaces that occurred in experiment 101-2, implying a change in the initial characteristics of the tribological pair. The material transfer depends on the mechanical properties of the nitrided case and interface, which can be analyzed by means of the hardness depth profile in Fig. 4. The hardness decrease between surface and case is far more pronounced in specimen 101-2, indicating a stronger reduction in the shear yield strength than for 100-1 sample. Considering similar problems found in the literature [21 – 23], it was verified that the shear yield strength in this experiment should be of the same order of magnitude as the applied stresses. It may thus be concluded that the higher wear rate in specimen 101-2 compared with 100-1 was caused by major plastic deformation beneath the surface, which could cause mechanical failure and the subsequent detachment of surface material. More work should be done to evaluate the local stresses and strength, and to relate these to the wear mechanisms involved. Hardness can be easily achieved in stainless steel using plasma nitriding, but corrosion resistance is not always preserved. When the temperature or the current density is high enough, Fe and Cr nitrides precipitate out and corrosion response is decreased because of the chromium depletion. In austenitic stainless steel, lowering the temperature easily controls this nitride formation and preserves corrosion resistance, indicated as a white layer. In martensitic stainless steel, although not observed in the micrograph, it seems that a thin “black layer” could not be avoided, and the specimen did not exhibit good corrosion resistance after the nitriding treatment. However, it was shown that under this layer remains another “white” layer with nitrogen in solution, hard enough and suitable Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 7

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from the corrosion resistance point of view. The results of the ball-on-disk test and the SEM micrographs revealed an improvement in the adhesive wear behavior of about 50 %, even though the hardest layer was mechanically removed.

5. Conclusion Ion nitriding is an efficient procedure for improving the tribological response of stainless steel. Hardness can be strongly increased and a compact surface layer is generated that protects the material against wear. However, much care should be taken in establishing the plasma processing parameters, such as current density and temperature, in order to obtain a layer with high nitrogen concentration but without the formation of nitrides, i. e. without impairing their corrosion resistance. The nitrided layer possesses outstanding tribological properties, evidenced by the very superior performance during the ball-on-disk test compared to the non-nitrided material. The wear resistance of this martensitic stainless steel was 20 times improved for specimen 100 and 13 times for 101, showing that it is better to have a harder surface prior to nitriding (specimen 101 was tempered at 400 8C and was harder than 100, tempered at 580 8C, prior to the nitriding treatment). On the other hand, corrosion resistance was affected, because the nitriding temperature of 410 8C was not effective in avoiding compound layer formation. After grinding away the outer layer, the corrosion behavior was similar to that of the original material although its high hardness was lost. However, the remnant layer contributed to an increase in the wear resistance of nearly 200 %. These results do not imply that the plasma nitriding process is not a promising process for hardening martensitic steels. However, great care must be taken when establishing the plasma process parameters with the aim of avoiding chromium nitride precipitation, i. e. decreasing their corrosion resistance. This could be achieved with a lower process temperature and a smaller nitrogen concentration in the gas mixture, which imply the use of a lower current density for a longer duration of the process. The hardness – depth profile of the modified region is of primary importance; the surface stress applied should not penetrate deeper than this layer or reach the local shear strength, because this could produce plastic deformation and detachment of material, as occurred in experiments 101-2 and 101-3, the latter possessed a thinner layer. Thus, this hardening technique is suitable for improving the wear resistance of small precision engineering components, such as milling cutters, twist drills and press dies for example, where the maximum extent of wear is small and where the stress imposed in service decays rapidly along the depth into the surface [23]. Calculations or simulations regarding local stress and local yield shear strength should be carried out to establish precisely the load-bearing capacity of a nitrided system and so generate a guide for the engineering design of mechanical components.

This work is part of a Cooperative Program between the National University of Technology, Argentina and the Central University of Venezuela in Caracas, supported by Ademat Network, ALFA II Program of the EU, Project Nr. II-0240-B1-AT-RT-CT. The work was also supported in part from the Venezuelan FONACIT (National Foundation for Science and Technology) and the Regional Faculty of Concepción del Uruguay, UTN, Argentina. Some of the authors would like to acknowledge the Venezuelan National Council for Science, Technology and Innovation (FONACIT) through the projects S1-2001000759 and UCV-F 2001000600’’ and to CDCH-UCV. Other authors acknowledge the financial support of UTN and Regional Faculty of Concepción del Uruguay, through the project PID 25/D018 and D021. Also special thanks for the student of UTN Argentina and research assistant Mr. S. Suárez Vallejo for the collaboration in the nitriding experiments and the SEM observations. References [1] W. Fender, R. Brown: Adv. Mater. Processes 163 (2005) 36. [2] C.E. Pinedo, W.A. Monteiro: J. Mater. Sci. Lett. 20 (2001) 147. doi:10.1023/A:1006723225515 [3] K.-T. Rie, E. Menthe, A. Matthews, K. Legg, J. Chin: MRS Bulletin, August 1996. [4] H. Michel, T. Czerwiec, M. Gantois, D. Ablitzer, A. Ricard: Surf. Coat. Technol. 72 (1995) 103. doi:10.1016/0257-8972(94)02339-5 [5] T. Bell, C.X. Li: Adv. Mater. Processes 160, 6 (2002) 49. [6] T. Czerwiec, N. Renevier, H. Michel: Surf. Coat. Technol. 131 (2000) 267. doi:10.1016/S0257-8972(00)00792-1 [7] E. Menthe, A. Bulak, J. Olfe, A. Zimmermann, K.-T. Rie: Surf. Coat. Technol. 133 – 134 (2000) 259. doi:10.1016/S0257-8972(00)00930-0 [8] M.P. Fewell, D.R.G. Mitchell, J.M. Priest; K.T. Short, G.A. Collins: Surf. Coat. Technol. 131 (2000) 300. doi:10.1016/S02578972(00)00804-5 [9] D.L. Williamson, P.J. Wilbur, F.R. Fickett, S. Parascandola: in T. Bell, K. Akamatsu (Eds.), Stainless Steel 2000; 2001, Maney Publ., 333 – 352. [10] C.X. Li, T. Bell: Wear 256 (2004) 1144. doi:10.1016/j.wear.2003.07.006 [11] K. Marchev, C.V.: Cooper, B.C. Giessen: Surf. Coat. Technol. 99 (1998) 229. doi:10.1016/S0257-8972(97)00533-1 [12] Y. Fu, A. Batchelor, N.L. Loh, K.W. Tan: Wear 219 (1998) 169. doi:10.1016/S0043-1648(98)00184-7 [13] I. Alphonsa, A. Chainani, P.M. Raole, B. Ganguli, P.I. John: Surf. Coat. Technol. 150 (2002) 263. doi:10.1016/S0257-8972(01)01536-5 [14] S.K. Kim, J.S. Yoo, J.M. Priest, M.P. Fewell: Surf. Coat. Technol. 163 – 164 (2003) 380. doi:10.1016/S0257-8972(02)00631-X [15] P. Corengia, G. Ybarra, C. Moina, A. Cabo, E. Broitman: Surf. Coat. Technol. 187 (2004) 63. doi:10.1016/j.surfcoat.2004.01.031 [16] S.P. Brühl, R. Charadia, N. Mingolo, J. Cimetta, M.A. Guitar, S. Suárez, M. Duarte, in: Soc. Chilena de Metalurgia y Mat, Universidad de La Serena (Eds.) Proc. of Congress CONAMET/ SAM, 2004, Santiago de Chile, 917 – 922. http://www.materiales-sam.org.ar/sitio/biblioteca/chile/INDEX01.HTM [17] S.P. Brühl, N. Mingolo, V. Vanzulli, A. Cabo, E. Forlerer, D. Peix, M.A. Guitar, in: Asociación Argentina de Materiales (Eds.) Proc. Jornadas SAM/Congreso CONAMET/Simposio Materia 2003, ISBN 987-20975-0-X, pp. 734 – 737. http://www.materialessam.org.ar/sitio/biblioteca/bariloche/Trabajos/A08/0815.PDF [18] M.H. Staia, A. Fragiel, S.P. Brühl, J.N. Feugeas, B.J. Gómez: Thin Solid Films 377 – 378 (2000) 650. doi:10.1016/S0040-6090(00)01446-2 [19] R.G. Bayer: Wear analysis for engineers, HNB Publishing, New York (2002). [20] P. Blau (Ed.): ASM Handbook Vol.18 “Friction, Lubrication & Wear Technology”, 1992, ASM international. [21] D. Manova, D. Hirsch, E. Richter, S. Mändl, H. Neumann, B. Rauschenbach: Surf. Coat. Technol. 201 (2007) 8329 – 8333. doi:10.1016/j.surfcoat.2006.10.060 [22] T. Bell, Y. Sun: Surf. Eng. 6 (1990) 133. [23] I.M. Hutchings: Tribology – Friction and wear of engineering materials, Butterworth-Heinemann, Oxford (2001) 208.

(Received July 12, 2007; accepted April 22, 2008) Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 7

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Correspondence Address: Prof. Dr. Sonia P. Brühl Universidad Tecnológica Nacional Facultad Reg. Concepción del Uruguay Ingeniero Pereira 676 E3264 BTD C. del Uruguay República Argentina Tel.: +54 3442 425541 Fax: +54 3442 423803 E-mail: [email protected] [email protected]

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DOI 10.3139/146.101692 Int. J. Mat. Res. (formerly Z. Metallkd.) 99 (2008) 7; page 779 – 786 # Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282

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