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Microstructures and tribological behaviour of oxynitrided austenitic stainless steel
Vacuum 146 (2017) 1e7
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Short communication
Microstructures and tribological behaviour of oxynitrided austenitic stainless steel Yang Li a, b, *, Yongyong He a, **, Shangzhou Zhang b, Xiaochun He b, Wei Wang a, Baoguo Hu a a b
State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, PR China Department of Materials Science and Engineering, Yantai University, Yantai, 264005, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2017 Received in revised form 11 September 2017 Accepted 13 September 2017 Available online 15 September 2017
This study investigates the microstructures and tribological behaviour in the modified layers of AISI 304 steel produced by active screen plasma oxynitriding. Compositional analysis of modified layers was carried out by field emission scanning electron microscopy, atomic force microscopy and high-resolution transmission electron microscopy. The results demonstrated that the modified composite duplex layer consisted of an amorphous and nanocrystalline oxynitrided layer (upper layer) and the nitrided layer (lower layer). Additionally, the wear resistance of the active screen plasma oxynitrided specimens was much higher than that of the untreated ones. The wear rate of the treated specimen decreased by approximately 95% compared with that of the untreated one. The analysis of the worn surface indicated that the treated specimen exhibited slight oxidation and abrasive wear, whereas the untreated one showed severe adhesive wear and plastic deformation. © 2017 Published by Elsevier Ltd.
Keywords: Oxy-nitrides Deposition Friction Iron alloys
1. Introduction As a widely used surface treatment technology to improve the hardness and wear resistance of austenitic stainless steels [1e8], plasma nitriding can be used for the formation of solid solution phases of high nitrogen content (20e35 at.%) on the surface of austenitic stainless steels, such as S phase [9e12]. In recent years, the active screen plasma nitriding (ASPN) method has been developed to address problems of conventional plasma nitriding [12e18]. With the deepening of nitrogen mass transfer and the deposition mechanism, ASPN technology has also been employed for deposition of the nanocrystalline surface layer on several substrates [19,20]. Partridge et al. [21,22] found that the Fe4N nanostructured particle layer was prepared from atoms and small clusters sputtered from an active screen. Dong et al. [23] suggested that the composite surface system containing hard S-phase and a nanocrystalline silver layer prepared by active screen plasma (ASP) alloying treatment led to significantly enhanced wear resistance of
* Corresponding author. State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. He). http://dx.doi.org/10.1016/j.vacuum.2017.09.026 0042-207X/© 2017 Published by Elsevier Ltd.
antimicrobial stainless steel. The wear and corrosion resistance can be further improved by the oxidation of the nitrided surface layer, especially using plasma oxidation [24,25]. Figueroa et al. [26] found that the friction coefficient for the post-oxidized specimen showed a decreasing trend when nitrogen atoms were substituted for oxygen atoms in the plasma nitrided and oxidized ferrous alloy. In addition, some oxynitride coatings are formed with the addition of oxygen, which provides better tribological resistance than pure nitride coatings in some fields [27,28]. Aiming at the preparation of an oxynitride modified layer through the ASP oxynitriding method, this work investigates the microstructure and tribological behaviour of the modified layer on AISI 304 austenitic stainless steel. 2. Experiments Test specimens cut from an AISI 304 austenitic stainless steel (0.06e0.10 C, 18.0e19.0 Cr, 8.5e9.0 Ni, 1.50e1.80 Mn, 0.36 Si, 0.005 S, 0.010 P, and the rest Fe (wt.%)) bar (f20 mm) with 6 mm thickness were adopted in the experiments. The cut specimens were grinded with several grades of emery papers and then mirror polished with diamond suspension (size 3.5 mm). Finally, acetone was used to thoroughly degrease the polished specimens. ASP oxynitriding experiments were carried out using an LDMC-
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20 pulsed plasma device. The ASP oxynitriding setups are shown in Fig. 1. The metallic screen was made of a low-carbon steel plate in which holes were uniformly distributed and some of the grooves were on the inner wall. The chemical composition (in wt.%) of the low-carbon steel plate was 0.18e0.24 C, 0.20e0.25 Cr, 0.35e0.65 Mn, 0.20e0.40 Si, and Fe in balance. The metallic cylinder of 36 cm diameter and 36 cm height was used to serve as part of the active screen. The specimen table was placed inside the metal screen and concurrently at the floating potential on an isolative material. The distance between the table surface and the top lid was 5 cm. The nearest distance from the mesh to the sample edge was approximately 4 cm. The grooves of 5 mm depth and 8 mm breadth had uniformly distributed round holes of 8 mm in diameter. The metallic mesh was connected to the cathode of the power supply. The hollow cathode discharge was formed by the grooves and cavities. The cathodic metal screen was used to heat the components and produce active species. The voltage and discharge current between the furnace well and metallic mesh were 780e820 V and 8e9 A, respectively. Treatments were performed at a pressure of 300 Pa using the NH3 with 2% oxygen. Under such conditions, processes were carried out at 440 C for 16 h. The XRD spectra of untreated and nitrided specimens were obtained using an X-ray diffractometer (Bruker D8 ADVANCE) with Cu-Ka radiation (l ¼ 1.5406 Å). Diffraction patterns were collected in BraggeBrentano configuration and with constant incident angles in the range of 20e70 . The specimens were etched using a chemical solution of (CuSO4$5H2O 5 g þ HCl 20 ml þ H2O 20 ml) to reveal the nitrided layer. The surface and cross-sectional topography of the treated specimens were studied using scanning electron microscopy (SEM, FEI Quanta 200 FEG). The evolution of surface morphology was investigated via atomic force microscopy (AFM, Nano Scope III) with ultralight tapping mode in atmosphere. Further microstructure characterization was performed by transmission electron microscopy using a JEOL-2100 microscope. Elemental analysis of treated samples was also performed by X-ray microanalysis using a QUANTAX 400 (Bruker, Germany) energy dispersive spectrometer. Surface microhardness measurements (load: 50 gf) were performed on the specimens using a Vickers' indenter. The surface roughness measurements were taken on the surface of the specimens before and after the treatments by using a stylus profilometer tester. The average surface roughness Ra (arithmetical mean deviation of the roughness profile from the mean line) was recorded. The reciprocating friction test was conducted using a universal micro-tribometer (UMT-3, CETR, USA) with a GCr15 bearing steel ball (d ¼ 4.0 mm, hardness 62 HRC) as counterpart. During the
reciprocating friction test, the linear speed was set as 15 cm/s, the applied load was set as 10 N, and the reciprocating length of the wear track was 10 mm. The topography and profiles of the wear tracks were measured by SEM and a 3D white light profilometer.
3. Results and discussion Fig. 2 shows the XRD patterns of untreated and treated specimens. The untreated specimen is an austenite phase in face centre cubic (fcc) structure, with diffraction angles of 43.8 and 51.0 . A very small ferrite diffraction peak is observable on the right of the 43.8 peak on the untreated specimen. This phase was formed during the preparation of the specimens. For the treated specimen, the S-phase diffraction peaks are primary, and there are a few CrN and ferrite diffraction peaks. The diffraction peaks of the CrN phase can be observed at approximately 37.5 , 43.8 and 64.5 , which suggests the presence of the chromium nitride precipitates. Sphase diffraction patterns (40.7 and 46.6 ) of the treated specimens exhibit a series of broader peaks that are shifted to lower 2q angles than those of the untreated specimens. This suggests a consequence of the expansion in the austenite lattice due to the nitrogen solubilization. The widening of the diffraction peaks is due to stacking faults, internal stress and nonuniformity of nitrogen atoms [29e31]. Fig. 3a shows the cross-sectional SEM images of the treated specimen. The formation of the modified layer with thickness of roughly 5.5 mm is clearly recognizable. The ASP oxynitrided specimen also shows higher surface hardness than those of the untreated specimen. The microhardness value measured from the treated surface is observed to be 760 HV0.05, which is approximately 3.2 times as hard as the untreated samples (240 HV0.05). The typical surface morphologies of treated specimens are shown in Fig. 3c and d. It can be seen that the austenitic grain boundaries are clear, but the slip bands cannot be observed on the modified surface (Fig. 3b). Observations under high magnification (Fig. 3d) reveal that the treated surface features particles deposition, which is related to the deposition effect during ASP oxynitriding. Fig. 3c shows the AFM surface micrographs of the treated specimens, from which nano-/micro-sized (80e160 nm) particles are clearly seen on the modified surface. According to the AFM images, the average surface roughness of the treated specimen is
Cavities
d=8 mm h=5mm Grooves φ=8 mm Fig. 1. Schematic diagram of the ASP oxynitriding equipment.
Fig. 2. XRD patterns of the untreated and treated samples.
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3
Fig. 3. SEM (a) cross-sectional image of the treated specimen; SEM (b, d) and AFM (c) images of surface morphology for the treated specimen.
approximately 23.2 nm, which is not consistent with other studies [1,32]. Gallo et al. [33] found that that the surface of austenitic stainless steels treated under the stated ASPN conditions exhibited two different contrasts: black areas and grey areas. The grey surfaces were covered with fine deposited particles, whereas no such particles could be found in the black areas. The total surface deposition of nano-/micro-sized particles is related to the hollow cathode discharge of the metal active screen. It is supposed that the steps present at the grain boundary and inside the upper grain are due to plastic deformation phenomena induced by ASP oxynitriding treatment. However, the slip bands are hardly observable owing to the presence of nanoparticles on the surface. The surface roughness of the untreated and treated specimens is evaluated by a stylus profiler. The surface roughness of the ASP oxynitrided specimen is more than 0.14 mm, which is approximately 7 times higher than that of the untreated substrate (0.02 mm). Several studies indicated the surface roughness of the ASPN nitrided sample in floating potential tended to be suppressed compared with DCPN nitrided sample in cathodic potential under micro-scale roughness [34]. More detailed TEM analysis of the treated specimen was carried out to clarify the composition of the modified layer (Fig. 4). The
thickness of the top deposition layer is approximately 120 nm. The SAD pattern in the inset of Fig. 4 exhibits a diffused background with hazy rings and various bright spots, which evidence the presence of amorphous and nanocrystalline phases. The results of energy dispersive spectroscopic analysis indicated that the amorphous region contained Fe, N, O, and a small amount of Cr. The deposition layer at area A was measured to contain 7.09 ± 0.69 wt% oxygen and 6.95 ± 0.89 wt% nitrogen, whereas the nitrided layer at area B was measured to contain 3.16 ± 0.39 wt% oxygen and 8.61 ± 0.85 wt% nitrogen. Moreover, no significant oxide phases are found in Fig. 2. This indicates that the oxygen content is highest in the deposited layer but rapidly decreases with increasing depth. The relatively high content of oxygen in the deposition layer can be ascribed to the amorphous structure. Wilhartitz et al. [35] found that oxygen in the nitrided layer could stabilize the fine crystalline structure. Under the deposition layer, the nitrided layer can be found. The phase composition of region B (Fig. 4) is mainly ferrite and CrN according to the SAD pattern. It can be concluded that the transformation of S/ ferrite þCrN is formed via a cellular transformation mechanism [36,37]. The carbon content of this region B is relatively high due to the carbon adsorption or contaminate. Fig. 5 shows the high-resolution transmission electron
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Y. Li et al. / Vacuum 146 (2017) 1e7
Fig. 4. Cross-sectional bright field TEM micrograph and selected area electron diffraction patterns and element compositions from areas A and B.
microscopy (HRTEM) images depicting the outer portion of the deposition layer. The bottom layer in the periodic lattice arrangement and top films in the slightly distorted atomic structure are clearly visible. The bottom layer is highly crystalline. The distance between the adjacent lattice fringes is approximately 0.191 nm, which agrees with the spacing (200) of the Fe4N phase. The formation of Fe4N can be further verified by the SAD pattern (the inset in Fig. 5a). The top film with thickness of 5 nm is formed on the outer surface. The d spacing of the core is 0.254 nm and indexed to the (311) crystal plane of the Fe3O4. It can be considered that the nitride layer is oxidized and the nitrogen atom is replaced by oxygen atoms. Liu et al. [38] proposed that a controlled method to prepare local Fe3O4 crystals in an Fe4N film using an electron beam. The HRTEM image in Fig. 5b suggests that the nanocrystalline particles are embedded in the amorphous phase. Fig. 6 shows the friction coefficients of the untreated and treated specimens. The friction coefficient of the untreated specimen shows several fluctuations during the whole testing period. For the treated specimen, the friction coefficient is approximately 0.15 for the first 20 s of sliding, after remaining very stable and smooth. The friction coefficient then rapidly increased and reached a steady
value after approximately 30s of sliding. The worn surfaces of the untreated specimen show severe plastic deformations (Fig. 7a). The depth of the wear track was characterized as 17.52 mm for the untreated specimen and 1.29 mm for the oxynitrided one. The wear rate of the treated specimen decreased by approximately 95% compared with the untreated one. This indicated that the wear resistance of AISI 304 stainless steel was improved significantly by the modified layer. According to the SEM wear track in Fig. 7c and the chemical compositions of Table 1, it can be found that the wear of the untreated specimen is typical adhesive and oxidation wear. However, the major wear mechanism in the centre of the wear track for the treated specimen is abrasion wear (Fig. 7d). This shows smooth wear tracks with practically no sign of material removal. Under higher magnification (Fig. 7f), the austenite grain boundaries remain after wear. The wear track is measured to contain 4.55 ± 0.63 wt% nitrogen and 6.02 ± 0.92 wt% oxygen, whereas the oxynitrided surface is measured to contain 4.74 ± 0.78 wt% nitrogen and 3.51 ± 0.45 wt% oxygen (Table 1). The wear track has higher oxygen content, and the wear surface involves oxidation wear. In the process of conventional plasma nitriding, the nitrogen ions directly affected the surface of stainless steel. The surface
Y. Li et al. / Vacuum 146 (2017) 1e7
5
1.2
Oxy-nitrided
Friction coefficients
1.0 0.8
Untreated
0.6 0.4 0.2 0.0
0
100
200
300
Time /s
400
500
600
Fig. 6. Friction coefficient of the untreated and treated samples.
Fig. 5. HRTEM image depicting the outer surface of the deposition layer (a). HRTEM image of the area between the deposition layer and nitrided layer (b).
roughness was increased, and slip and serious protrusion of the grain boundary occurred after nitriding, which were the unfavourable factors of friction. However, in this ASP oxynitriding experiment, the active screen with holes and grooves acted as a discharge cathode. Nitrogen atmosphere of high concentration and high activity was formed in the active screen owing to the hollow cathode discharge. In the initial stage of nitriding, the S phase layer was formed on the surface of austenitic stainless steel. With the extension of treatment time, the iron atoms were sputtered from the metal screen. The metal atoms were combined with nitrogen and oxygen to form nitrides and oxides on the substrate surface. The energetic positive ions cannot directly bombard the substrate surface, but the substrate surface was possibly bumped in the active screen space by many types of particles, such as that sputtered from the active screen. The active neutral nitrogen species in the plasma
flowed towards the surface of the specimens through the screen. The improved wear resistance is believed to be related to the formation of a duplex layer consisting of an amorphous and nanocrystalline oxynitride layer and the nitrided layer, as well as to the oxygen diffusion during the formation of the nitride phase layer. Stefaniszyn [39] analysed the structure and phase composition of AISI 316L steel after subjecting it to nitriding and oxynitriding under glow discharge conditions. The nitrided and oxynitrided layers increase the frictional wear resistance of the AISI 316L steel treated by glow-discharge-assisted nitriding and oxynitriding. Moreover, the effect of oxynitriding is better than that of nitriding. This initial value of the friction coefficient for the treated specimen was much lower, indicating probable the abrasion occurred between the oxy-nitrided layer and counterpart ball. The friction coefficient then rapidly increased and reached a steady value after approximately 30s of sliding. This suggested that the abrasion occurred between the nitrided layer and counterpart ball. The wear resistance of the oxynitrided stainless steel was improved because the higher hardness S phase. The friction mechanism of severe plastic deformation and adhesion wear produced during wearing of the untreated specimens could be converted into the friction mechanism of oxidation and slight abrasive wear owing to the formation of the duplex modified layer.
4. Conclusions Austenitic stainless steel was effectively treated using ASP oxynitriding. A duplex layer consisted of an amorphous/nanocrystalline oxynitrided layer and the nitrided layer. The deposition oxynitrided layer was composed of amorphous and nanocrystalline Fe3O4 and Fe4N phases, whereas the nitrided layer mainly contained the hard S phase and a small amount of CrN precipitation þ ferrite phases. It was believed that the improved wear properties were related to the presence of duplex modified layer formed by ASP oxynitrided on the surface. The friction mechanism was changed from the severe plastic deformation and adhesion wear into slight oxidation and abrasive wear.
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Y. Li et al. / Vacuum 146 (2017) 1e7
Fig. 7. SEM and 3D white light wear track of the untreated (a, c, e) and ASP treatment specimens (b, d, f).
Table 1 Chemical compositions (wt.%) of the surface and wear track of untreated and treated specimens from Fig. 7.
Surface (A) Track (B) Surface (C) Track (D)
N
O
Cr
e e 4.74 ± 0.78 4.55 ± 0.63
e 8.72 ± 0.71 3.51 ± 0.45 6.02 ± 0.92
16.96 16.12 12.74 15.14
Ni ± ± ± ±
1.01 0.87 0.92 0.73
Acknowledgments The project was supported by the National Key Basic Research Program of China (973) (2014CB046404), National Natural Science Foundation of China (51301149), and China Postdoctoral Science Foundation funded project (2015M570090).
7.76 6.99 5.61 9.45
Mn ± ± ± ±
0.32 0.54 0.43 0.82
1.53 1.14 1.44 1.50
Si ± ± ± ±
0.14 0.21 0.13 0.12
0.53 0.35 0.58 0.59
Fe ± ± ± ±
0.18 0.19 0.23 0.17
In In In In
balance balance balance balance
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Short communication
Microstructures and tribological behaviour of oxynitrided austenitic stainless steel Yang Li a, b, *, Yongyong He a, **, Shangzhou Zhang b, Xiaochun He b, Wei Wang a, Baoguo Hu a a b
State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, PR China Department of Materials Science and Engineering, Yantai University, Yantai, 264005, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2017 Received in revised form 11 September 2017 Accepted 13 September 2017 Available online 15 September 2017
This study investigates the microstructures and tribological behaviour in the modified layers of AISI 304 steel produced by active screen plasma oxynitriding. Compositional analysis of modified layers was carried out by field emission scanning electron microscopy, atomic force microscopy and high-resolution transmission electron microscopy. The results demonstrated that the modified composite duplex layer consisted of an amorphous and nanocrystalline oxynitrided layer (upper layer) and the nitrided layer (lower layer). Additionally, the wear resistance of the active screen plasma oxynitrided specimens was much higher than that of the untreated ones. The wear rate of the treated specimen decreased by approximately 95% compared with that of the untreated one. The analysis of the worn surface indicated that the treated specimen exhibited slight oxidation and abrasive wear, whereas the untreated one showed severe adhesive wear and plastic deformation. © 2017 Published by Elsevier Ltd.
Keywords: Oxy-nitrides Deposition Friction Iron alloys
1. Introduction As a widely used surface treatment technology to improve the hardness and wear resistance of austenitic stainless steels [1e8], plasma nitriding can be used for the formation of solid solution phases of high nitrogen content (20e35 at.%) on the surface of austenitic stainless steels, such as S phase [9e12]. In recent years, the active screen plasma nitriding (ASPN) method has been developed to address problems of conventional plasma nitriding [12e18]. With the deepening of nitrogen mass transfer and the deposition mechanism, ASPN technology has also been employed for deposition of the nanocrystalline surface layer on several substrates [19,20]. Partridge et al. [21,22] found that the Fe4N nanostructured particle layer was prepared from atoms and small clusters sputtered from an active screen. Dong et al. [23] suggested that the composite surface system containing hard S-phase and a nanocrystalline silver layer prepared by active screen plasma (ASP) alloying treatment led to significantly enhanced wear resistance of
* Corresponding author. State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. He). http://dx.doi.org/10.1016/j.vacuum.2017.09.026 0042-207X/© 2017 Published by Elsevier Ltd.
antimicrobial stainless steel. The wear and corrosion resistance can be further improved by the oxidation of the nitrided surface layer, especially using plasma oxidation [24,25]. Figueroa et al. [26] found that the friction coefficient for the post-oxidized specimen showed a decreasing trend when nitrogen atoms were substituted for oxygen atoms in the plasma nitrided and oxidized ferrous alloy. In addition, some oxynitride coatings are formed with the addition of oxygen, which provides better tribological resistance than pure nitride coatings in some fields [27,28]. Aiming at the preparation of an oxynitride modified layer through the ASP oxynitriding method, this work investigates the microstructure and tribological behaviour of the modified layer on AISI 304 austenitic stainless steel. 2. Experiments Test specimens cut from an AISI 304 austenitic stainless steel (0.06e0.10 C, 18.0e19.0 Cr, 8.5e9.0 Ni, 1.50e1.80 Mn, 0.36 Si, 0.005 S, 0.010 P, and the rest Fe (wt.%)) bar (f20 mm) with 6 mm thickness were adopted in the experiments. The cut specimens were grinded with several grades of emery papers and then mirror polished with diamond suspension (size 3.5 mm). Finally, acetone was used to thoroughly degrease the polished specimens. ASP oxynitriding experiments were carried out using an LDMC-
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20 pulsed plasma device. The ASP oxynitriding setups are shown in Fig. 1. The metallic screen was made of a low-carbon steel plate in which holes were uniformly distributed and some of the grooves were on the inner wall. The chemical composition (in wt.%) of the low-carbon steel plate was 0.18e0.24 C, 0.20e0.25 Cr, 0.35e0.65 Mn, 0.20e0.40 Si, and Fe in balance. The metallic cylinder of 36 cm diameter and 36 cm height was used to serve as part of the active screen. The specimen table was placed inside the metal screen and concurrently at the floating potential on an isolative material. The distance between the table surface and the top lid was 5 cm. The nearest distance from the mesh to the sample edge was approximately 4 cm. The grooves of 5 mm depth and 8 mm breadth had uniformly distributed round holes of 8 mm in diameter. The metallic mesh was connected to the cathode of the power supply. The hollow cathode discharge was formed by the grooves and cavities. The cathodic metal screen was used to heat the components and produce active species. The voltage and discharge current between the furnace well and metallic mesh were 780e820 V and 8e9 A, respectively. Treatments were performed at a pressure of 300 Pa using the NH3 with 2% oxygen. Under such conditions, processes were carried out at 440 C for 16 h. The XRD spectra of untreated and nitrided specimens were obtained using an X-ray diffractometer (Bruker D8 ADVANCE) with Cu-Ka radiation (l ¼ 1.5406 Å). Diffraction patterns were collected in BraggeBrentano configuration and with constant incident angles in the range of 20e70 . The specimens were etched using a chemical solution of (CuSO4$5H2O 5 g þ HCl 20 ml þ H2O 20 ml) to reveal the nitrided layer. The surface and cross-sectional topography of the treated specimens were studied using scanning electron microscopy (SEM, FEI Quanta 200 FEG). The evolution of surface morphology was investigated via atomic force microscopy (AFM, Nano Scope III) with ultralight tapping mode in atmosphere. Further microstructure characterization was performed by transmission electron microscopy using a JEOL-2100 microscope. Elemental analysis of treated samples was also performed by X-ray microanalysis using a QUANTAX 400 (Bruker, Germany) energy dispersive spectrometer. Surface microhardness measurements (load: 50 gf) were performed on the specimens using a Vickers' indenter. The surface roughness measurements were taken on the surface of the specimens before and after the treatments by using a stylus profilometer tester. The average surface roughness Ra (arithmetical mean deviation of the roughness profile from the mean line) was recorded. The reciprocating friction test was conducted using a universal micro-tribometer (UMT-3, CETR, USA) with a GCr15 bearing steel ball (d ¼ 4.0 mm, hardness 62 HRC) as counterpart. During the
reciprocating friction test, the linear speed was set as 15 cm/s, the applied load was set as 10 N, and the reciprocating length of the wear track was 10 mm. The topography and profiles of the wear tracks were measured by SEM and a 3D white light profilometer.
3. Results and discussion Fig. 2 shows the XRD patterns of untreated and treated specimens. The untreated specimen is an austenite phase in face centre cubic (fcc) structure, with diffraction angles of 43.8 and 51.0 . A very small ferrite diffraction peak is observable on the right of the 43.8 peak on the untreated specimen. This phase was formed during the preparation of the specimens. For the treated specimen, the S-phase diffraction peaks are primary, and there are a few CrN and ferrite diffraction peaks. The diffraction peaks of the CrN phase can be observed at approximately 37.5 , 43.8 and 64.5 , which suggests the presence of the chromium nitride precipitates. Sphase diffraction patterns (40.7 and 46.6 ) of the treated specimens exhibit a series of broader peaks that are shifted to lower 2q angles than those of the untreated specimens. This suggests a consequence of the expansion in the austenite lattice due to the nitrogen solubilization. The widening of the diffraction peaks is due to stacking faults, internal stress and nonuniformity of nitrogen atoms [29e31]. Fig. 3a shows the cross-sectional SEM images of the treated specimen. The formation of the modified layer with thickness of roughly 5.5 mm is clearly recognizable. The ASP oxynitrided specimen also shows higher surface hardness than those of the untreated specimen. The microhardness value measured from the treated surface is observed to be 760 HV0.05, which is approximately 3.2 times as hard as the untreated samples (240 HV0.05). The typical surface morphologies of treated specimens are shown in Fig. 3c and d. It can be seen that the austenitic grain boundaries are clear, but the slip bands cannot be observed on the modified surface (Fig. 3b). Observations under high magnification (Fig. 3d) reveal that the treated surface features particles deposition, which is related to the deposition effect during ASP oxynitriding. Fig. 3c shows the AFM surface micrographs of the treated specimens, from which nano-/micro-sized (80e160 nm) particles are clearly seen on the modified surface. According to the AFM images, the average surface roughness of the treated specimen is
Cavities
d=8 mm h=5mm Grooves φ=8 mm Fig. 1. Schematic diagram of the ASP oxynitriding equipment.
Fig. 2. XRD patterns of the untreated and treated samples.
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3
Fig. 3. SEM (a) cross-sectional image of the treated specimen; SEM (b, d) and AFM (c) images of surface morphology for the treated specimen.
approximately 23.2 nm, which is not consistent with other studies [1,32]. Gallo et al. [33] found that that the surface of austenitic stainless steels treated under the stated ASPN conditions exhibited two different contrasts: black areas and grey areas. The grey surfaces were covered with fine deposited particles, whereas no such particles could be found in the black areas. The total surface deposition of nano-/micro-sized particles is related to the hollow cathode discharge of the metal active screen. It is supposed that the steps present at the grain boundary and inside the upper grain are due to plastic deformation phenomena induced by ASP oxynitriding treatment. However, the slip bands are hardly observable owing to the presence of nanoparticles on the surface. The surface roughness of the untreated and treated specimens is evaluated by a stylus profiler. The surface roughness of the ASP oxynitrided specimen is more than 0.14 mm, which is approximately 7 times higher than that of the untreated substrate (0.02 mm). Several studies indicated the surface roughness of the ASPN nitrided sample in floating potential tended to be suppressed compared with DCPN nitrided sample in cathodic potential under micro-scale roughness [34]. More detailed TEM analysis of the treated specimen was carried out to clarify the composition of the modified layer (Fig. 4). The
thickness of the top deposition layer is approximately 120 nm. The SAD pattern in the inset of Fig. 4 exhibits a diffused background with hazy rings and various bright spots, which evidence the presence of amorphous and nanocrystalline phases. The results of energy dispersive spectroscopic analysis indicated that the amorphous region contained Fe, N, O, and a small amount of Cr. The deposition layer at area A was measured to contain 7.09 ± 0.69 wt% oxygen and 6.95 ± 0.89 wt% nitrogen, whereas the nitrided layer at area B was measured to contain 3.16 ± 0.39 wt% oxygen and 8.61 ± 0.85 wt% nitrogen. Moreover, no significant oxide phases are found in Fig. 2. This indicates that the oxygen content is highest in the deposited layer but rapidly decreases with increasing depth. The relatively high content of oxygen in the deposition layer can be ascribed to the amorphous structure. Wilhartitz et al. [35] found that oxygen in the nitrided layer could stabilize the fine crystalline structure. Under the deposition layer, the nitrided layer can be found. The phase composition of region B (Fig. 4) is mainly ferrite and CrN according to the SAD pattern. It can be concluded that the transformation of S/ ferrite þCrN is formed via a cellular transformation mechanism [36,37]. The carbon content of this region B is relatively high due to the carbon adsorption or contaminate. Fig. 5 shows the high-resolution transmission electron
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Fig. 4. Cross-sectional bright field TEM micrograph and selected area electron diffraction patterns and element compositions from areas A and B.
microscopy (HRTEM) images depicting the outer portion of the deposition layer. The bottom layer in the periodic lattice arrangement and top films in the slightly distorted atomic structure are clearly visible. The bottom layer is highly crystalline. The distance between the adjacent lattice fringes is approximately 0.191 nm, which agrees with the spacing (200) of the Fe4N phase. The formation of Fe4N can be further verified by the SAD pattern (the inset in Fig. 5a). The top film with thickness of 5 nm is formed on the outer surface. The d spacing of the core is 0.254 nm and indexed to the (311) crystal plane of the Fe3O4. It can be considered that the nitride layer is oxidized and the nitrogen atom is replaced by oxygen atoms. Liu et al. [38] proposed that a controlled method to prepare local Fe3O4 crystals in an Fe4N film using an electron beam. The HRTEM image in Fig. 5b suggests that the nanocrystalline particles are embedded in the amorphous phase. Fig. 6 shows the friction coefficients of the untreated and treated specimens. The friction coefficient of the untreated specimen shows several fluctuations during the whole testing period. For the treated specimen, the friction coefficient is approximately 0.15 for the first 20 s of sliding, after remaining very stable and smooth. The friction coefficient then rapidly increased and reached a steady
value after approximately 30s of sliding. The worn surfaces of the untreated specimen show severe plastic deformations (Fig. 7a). The depth of the wear track was characterized as 17.52 mm for the untreated specimen and 1.29 mm for the oxynitrided one. The wear rate of the treated specimen decreased by approximately 95% compared with the untreated one. This indicated that the wear resistance of AISI 304 stainless steel was improved significantly by the modified layer. According to the SEM wear track in Fig. 7c and the chemical compositions of Table 1, it can be found that the wear of the untreated specimen is typical adhesive and oxidation wear. However, the major wear mechanism in the centre of the wear track for the treated specimen is abrasion wear (Fig. 7d). This shows smooth wear tracks with practically no sign of material removal. Under higher magnification (Fig. 7f), the austenite grain boundaries remain after wear. The wear track is measured to contain 4.55 ± 0.63 wt% nitrogen and 6.02 ± 0.92 wt% oxygen, whereas the oxynitrided surface is measured to contain 4.74 ± 0.78 wt% nitrogen and 3.51 ± 0.45 wt% oxygen (Table 1). The wear track has higher oxygen content, and the wear surface involves oxidation wear. In the process of conventional plasma nitriding, the nitrogen ions directly affected the surface of stainless steel. The surface
Y. Li et al. / Vacuum 146 (2017) 1e7
5
1.2
Oxy-nitrided
Friction coefficients
1.0 0.8
Untreated
0.6 0.4 0.2 0.0
0
100
200
300
Time /s
400
500
600
Fig. 6. Friction coefficient of the untreated and treated samples.
Fig. 5. HRTEM image depicting the outer surface of the deposition layer (a). HRTEM image of the area between the deposition layer and nitrided layer (b).
roughness was increased, and slip and serious protrusion of the grain boundary occurred after nitriding, which were the unfavourable factors of friction. However, in this ASP oxynitriding experiment, the active screen with holes and grooves acted as a discharge cathode. Nitrogen atmosphere of high concentration and high activity was formed in the active screen owing to the hollow cathode discharge. In the initial stage of nitriding, the S phase layer was formed on the surface of austenitic stainless steel. With the extension of treatment time, the iron atoms were sputtered from the metal screen. The metal atoms were combined with nitrogen and oxygen to form nitrides and oxides on the substrate surface. The energetic positive ions cannot directly bombard the substrate surface, but the substrate surface was possibly bumped in the active screen space by many types of particles, such as that sputtered from the active screen. The active neutral nitrogen species in the plasma
flowed towards the surface of the specimens through the screen. The improved wear resistance is believed to be related to the formation of a duplex layer consisting of an amorphous and nanocrystalline oxynitride layer and the nitrided layer, as well as to the oxygen diffusion during the formation of the nitride phase layer. Stefaniszyn [39] analysed the structure and phase composition of AISI 316L steel after subjecting it to nitriding and oxynitriding under glow discharge conditions. The nitrided and oxynitrided layers increase the frictional wear resistance of the AISI 316L steel treated by glow-discharge-assisted nitriding and oxynitriding. Moreover, the effect of oxynitriding is better than that of nitriding. This initial value of the friction coefficient for the treated specimen was much lower, indicating probable the abrasion occurred between the oxy-nitrided layer and counterpart ball. The friction coefficient then rapidly increased and reached a steady value after approximately 30s of sliding. This suggested that the abrasion occurred between the nitrided layer and counterpart ball. The wear resistance of the oxynitrided stainless steel was improved because the higher hardness S phase. The friction mechanism of severe plastic deformation and adhesion wear produced during wearing of the untreated specimens could be converted into the friction mechanism of oxidation and slight abrasive wear owing to the formation of the duplex modified layer.
4. Conclusions Austenitic stainless steel was effectively treated using ASP oxynitriding. A duplex layer consisted of an amorphous/nanocrystalline oxynitrided layer and the nitrided layer. The deposition oxynitrided layer was composed of amorphous and nanocrystalline Fe3O4 and Fe4N phases, whereas the nitrided layer mainly contained the hard S phase and a small amount of CrN precipitation þ ferrite phases. It was believed that the improved wear properties were related to the presence of duplex modified layer formed by ASP oxynitrided on the surface. The friction mechanism was changed from the severe plastic deformation and adhesion wear into slight oxidation and abrasive wear.
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Fig. 7. SEM and 3D white light wear track of the untreated (a, c, e) and ASP treatment specimens (b, d, f).
Table 1 Chemical compositions (wt.%) of the surface and wear track of untreated and treated specimens from Fig. 7.
Surface (A) Track (B) Surface (C) Track (D)
N
O
Cr
e e 4.74 ± 0.78 4.55 ± 0.63
e 8.72 ± 0.71 3.51 ± 0.45 6.02 ± 0.92
16.96 16.12 12.74 15.14
Ni ± ± ± ±
1.01 0.87 0.92 0.73
Acknowledgments The project was supported by the National Key Basic Research Program of China (973) (2014CB046404), National Natural Science Foundation of China (51301149), and China Postdoctoral Science Foundation funded project (2015M570090).
7.76 6.99 5.61 9.45
Mn ± ± ± ±
0.32 0.54 0.43 0.82
1.53 1.14 1.44 1.50
Si ± ± ± ±
0.14 0.21 0.13 0.12
0.53 0.35 0.58 0.59
Fe ± ± ± ±
0.18 0.19 0.23 0.17
In In In In
balance balance balance balance
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