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A study on the abrasive and erosive wear behavior of arc-deposited Cr–N–O coatings on tool steel
Thin Solid Films 517 (2009) 1655–1661
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
A study on the abrasive and erosive wear behavior of arc-deposited Cr–N–O coatings on tool steel Cheng-Hsun Hsu ⁎, Yu-Dung Chen Department of Materials Engineering, Tatung University, 40 Chungshan North 3rd Sec., Taipei 104, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 21 December 2007 Received in revised form 10 September 2008 Accepted 18 September 2008 Available online 27 September 2008 Keywords: AISI M2 steel Cathodic arc evaporation Cr–N–O coatings Abrasion Erosion
a b s t r a c t In this study, the cathodic arc evaporation technique, by using the chromium target and controlling the flow rate of nitrogen/oxygen reactive gases, was utilized to deposit three different Cr–N–O coatings (CrN, CrN/Cr (N,O), CrN/Cr2O3) on AISI M2 tool steel. Two types of wear tests were applied to evaluate the abrasive and erosive wear behavior of the coated and uncoated specimens. One was the ball-on-disk abrasion test to measure the friction coefficient of these specimens. The other was the erosion test using Al2O3 particles (~ 177 µm in size and Mohr 7 scale) of about 5 g, and then the surface morphologies of the eroded specimens were observed. To further understand the coating effects on the two wear behaviors of M2 steel, coating structure, morphology, and adhesion were analyzed using XRD, SEM, and TEM, respectively. The results showed that surface roughness and adhesion of the double-layered coatings (CrN/Cr(N,O) and CrN/Cr2O3) were inferior to those of monolithic CrN, but their hardness and elastic modulus were superior to those of CrN. In the abrasive behavior, Cr–N–O coatings reduced the friction coefficient of M2 substrate. In particular, the CrN/Cr2O3 has the highest hardness/elastic (H/E) modulus ration, therefore the lowest friction coefficient, among all the coated-specimens tested. In the erosive behavior, the coated specimens exhibited better erosion resistance as compared to the uncoated ones, at the impingement angles of either 30o or 90o. Moreover, the erosion resistance of CrN/Cr(N,O) coatings was superior to that of CrN/Cr2O3 coatings due to its better adhesion. © 2008 Elsevier B.V. All rights reserved.
1. Introduction It is a well known fact that coatings of a wide variety of materials are commonly applied to substrate for different purposes. For instance, various PVD ceramic nitride films such as CrN, TiN, TiAlN, NbN, and their combination can be used to prolong the life of tool steels exposed to an abrasive or erosive environment in many engineering applications [1–5]. It was also reported that chromium nitride (CrN) deposited by PVD technology has been identified as one of the more promising protective coatings for the application of tool and mold materials in recent years [5–10]. In particular, CrN forms a dense and stable Cr2O3 thin film as a protective scale at a high working temperature. That is, the oxide layer could be quite effective as an oxygen diffusion barrier up to moderately high temperature (~ 900 °C) [10]. It is noted from some literature that Cr2O3 thin film can be synthesized using the unbalanced magnetron sputtering [11], ion implantation [12] and cathodic arc evaporation (CAE) methods [13,14]. The CAE method is particular noteworthy because it offers several advantages such as the high deposition rate, low processing
⁎ Corresponding author. Tel.: +886 2 2586 6410; fax: +886 2 2593 6897. E-mail address: [email protected] (C.-H. Hsu). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.083
temperature, and the ability to obtain dense structure with stoichiometric composition. Although previous studies [13,17,18] have confirmed certain properties of CrN and Cr2O3 hard coatings such as corrosion resistance and thermal stability, there is a lack of information on both the abrasive and erosive properties of tool steel treated with a combination of CrN and Cr2O3 coatings [19]. Thus, the present work aims to investigate the abrasive and erosive behaviors of AISI M2 tool steel coated with Cr–N–O double-layer films by the CAE process. That is, CrN film was first deposited on the steel as an interlayer, and then followed by the deposition of Cr2O3 as the superficial layer under different flow rates of reactive oxygen gas. Finally, abrasion and erosion tests were carried out in order to explore the effects of the hard coatings on the wear behaviors of the steel. 2. Experimental procedure 2.1. Substrate and CAE treatment The substrates were made of AISI M2 steel (0.8–0.9%C, 0.4%Si, 0.4% Mn, 3.8–4.5%Cr, 4.5–5.5%Mo, 5.5–6.7%W, 1.6–2.2%V, balance Fe), which were machined into circular and rectangular test coupons of 20 mm × 5 mm and 20 mm × 20 mm × 5 mm in size, respectively, for
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abrasive and erosive wear tests. Prior to the CAE treatment, the substrates were heat-treated to obtain the hardness of about 63 HRC to imitate the real tool and mold in applications [20]. Then, the substrates were mechanically ground and polished to an average surface roughness of approximately 0.06 µm (Ra value). After undergoing thorough wet cleaning in an ultrasonic bath, the specimens were then fixed in a chamber holder and subjected to Ar+ bombardment at the bias of −1 × 103 V for 10 min to ensure good adhesion of the deposited films. The pressure of argon gas during ion bombardment was kept at 0.1 Pa. The reactive gases of N2 and O2 were fed into the chamber to form CrN and Cr2O3 films, respectively. The chamber pressure during the deposition stage was kept at 3.3 Pa. The whole coating operation began with the formation of the CrN interlayer, and separately followed by either Cr(N,O) or Cr2O3 films as the outer layer in the same batch process. For comparison, another monolithic CrN film was also coated on steel for the same depositing time. A schematic diagram of the CAE system in this study is shown in Fig. 1 and the details of the processing parameters are given in Table 1. 2.2. Analysis of coating characteristics SEM (Hitachi-S4100, Japan) was utilized to observe cross-sectional morphology for the measurement of coating thickness. In addition, FEG-TEM (Philips Tecnai-F30, USA) was used to observe the dense interface of coatings at an incident beam voltage of 300 kV. The X-ray diffractometer (MAC Science-M21X, Japan) was employed to identify the coating structure by using Cu-target Kα radiation at 40 kV and 30 mA at a low incident angle of 2o and in the scanning angular (2θ) range from 20 to 120 degrees at 2 degree/minute. The hardness and elastic modulus of the films were measured by using a nanoindenter (MTS XP system, USA) under the applied load of 4 mN. Five readings were taken and averaged to represent the obtained data for each type of coating. A surface roughness analyzer (Mitutoyo Serftest SV-400, Japan) was applied to measure average surface roughness (Ra value) of the specimens. In addition, the adhesion strength of coatings was evaluated utilizing Rockwell C indentation testing with a load of 150 kg. 2.3. Abrasion and erosion tests In this study, the wear testing was divided into two types: the abrasion test and the dynamic erosion test. The abrasion tests of the coated and uncoated specimens were performed on a ball-on-disc tribometer (CSEM). The tests were conducted with no lubricant along a circular track of 30 mm diameter against a 6.0 mm diameter WC ball at 0.3 m/s under a normal load of 10 N in the ambient atmosphere. The
Fig. 1. Schematic diagram of the cathodic arc evaporation system.
Table 1 Processing parameters of the coatings in this experiment Target material Substrate temperature (°C) Chamber pressure (Pa) Substrate bias voltage (V) Evaporator current (A) Source to substrate distance (cm) Reactive gas (sccm)
Deposition time (min)
Cr (99.99%) 200–220 3.3 −150 70 15 Single-layer: N2(300); Double-layer: Interlayer N2(300), and then (1) outer layer N2(230)+ O2(70) as well as (2) outer layer only O2(180), respectively. 60
relationship between the friction coefficient and the travel distance was continuously recorded during the tests. Another wear-testing type, using a dry particle erosion tester as shown in Fig. 2, was adopted to evaluate the dynamic wear behaviors of all the specimens. Al2O3 angular particles (~ 177 µm in size and Mohr 7 scale) of about 5 g were used as the eroding carriers to impinge the specimen for the observation on its eroded surface. The erosion pressure was controlled at a constant value of 3 kg/cm2 by using an air compressor. The average particle velocity at a distance of 30 mm between the nozzle tip and specimen is 83.2 ms− 1. The internal diameter of the employed nozzle was 5 mm and the impact angles were fixed at 30o and 90o, respectively. 3. Results and discussion 3.1. Coating structure and morphology Fig. 3 shows the crystal structure of the deposited coatings in terms of the XRD patterns. For comparison, a single-layered specimen was identified as the CrN phase with the strong (200) plane. For one of the double-layered specimens, it was found that the XRD pattern mainly displayed the peaks of CrN and Cr2O3 crystal planes due to the simultaneous use of N2 (230 sccm) and O2 (70 sccm) reactive gases to deposit the outer layer. Thus the specimen had a superficial structure containing a mixture of CrN and Cr2O3 phases except for the interlayer of CrN, and here named CrN/Cr(N,O) coatings. For the other doublelayered specimen, its XRD pattern is similar to that of the above specimen in spite of the use of only O2 (180 sccm) reactive gas to deposit the outer layer. This is because the double-layered thin film comprises a CrN interlayer and a Cr2O3 outer layer, here also named CrN/Cr2O3 coatings in this study.
Fig. 2. Schematic arrangement of the erosion test. (θ: impingement angle).
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Fig. 3. XRD patterns of the resulting coatings.
Fig. 4. SEM cross-sectional view of coated specimens: (a) CrN, (b) CrN/Cr(N,O), and (c) CrN/Cr2O3.
Fig. 5. Surface morphology of coated specimens: (a) CrN, (b) CrN/Cr(N,O), and (c) CrN/ Cr2O3.
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Fig. 6. Comparison of surface roughness of the uncoated and coated specimens.
SEM was used to observed cross-sectional and surface morphologies of these coatings as shown in Figs. 4 and 5. The results clearly indicate that all the coatings have an approximate thickness of 1.5 µm
Fig. 8. Fracture morphology of the various coatings from the Rockwell ‘C’ adhesion test: (a) CrN, (b)CrN/Cr(N,O), and (c) CrN/Cr2O3.
under the same deposition time. Moreover, in terms of average surface roughness the CrN/Cr2O3 coating exhibited the highest Ra value (0.12 µm), followed by the CrN/Cr(N,O) coating (0.1 µm), and then the
Fig. 7. Comparison of coating mechanical properties for the various coated specimens: (a) film hardness, (b) elastic modulus, and (c) H/E ratio.
Fig. 9. Comparison of the friction coefficient of the coatings at a load of 10 N, sliding against a 6 mm WC ball counterface.
C.-H. Hsu, Y.-D. Chen / Thin Solid Films 517 (2009) 1655–1661
single-layer CrN (0.07 µm), compared with the substrate (0.06 µm), as shown in Fig. 6. The increase in the surface roughness of the coated specimens mainly results from the deposition of macro-particles during the CAE process [15,16]. It is noteworthy to show that, although different flow rates of the reactive gases were used to deposit the outer layer of the double-layered CrN/Cr(N,O) and CrN/Cr2O3 coatings (see Table 1), both the coatings had almost the same outer layer thickness of about 0.7 µm. According to a previous study [21], these results could be attributed to the more favorable reaction of chromium with oxygen as compared to that of chromium with nitrogen. The Cr2O3 layer is easily obtained by the CAE process because nitrogen is easily replaced by oxygen as a preferred reactive gas with chromium during the deposition process. As a result, this phenomenon also increases the surface roughness of the coatings (Fig. 6). It may be that the Cr2O3 coating has a less dense microstructure due to reduced sputter bombardment as it thickens and becomes insulating. 3.2. Coating hardness, elastic modulus and adhesion
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10 km travel distance, the wear pattern consisting of parallel straight lines and local exposed substrate is clearly seen in the CrN coated specimen (Fig. 10(a)). The wear pattern of double-layered coatings is shown in Fig. 10(b)–(c). Contrastingly, the straight lines become less evident when compared to the CrN coated specimen. The sequence of the H/E value of these coatings in this study is CrN/Cr2O3 N CrN/Cr(N, O) N CrN. Furthermore, CrN/Cr2O3 coatings exhibited the lowest friction coefficient (~ 0.3) against WC, followed by CrN/Cr(N,O) coatings (~ 0.45), and then by CrN coating (~ 0.6). On the other hand, the CrN/Cr2O3 coatings performed less well in the Rockwell C adhesion test – indicating that the Cr2O3 outer layer, although hard, may be more brittle (as possibly less well adhered) than the Cr(N,O) equivalent. 3.4. Analysis of erosive behavior Figs. 11 and 12 illustrate SEM micrographs of the uncoated and coated specimens after the single solid particle test at impingement
Fig. 7 shows a comparison of the hardness (H), elastic modulus (E), and hardness/elastic modulus (H/E) ratio among the double-layered coatings and CrN film. These data were obtained by using the nanoindenter with the indentation depth at approximately 100 nm under an applied load of 4 mN. It implies that all the values are mainly dominated by the superficial layer of the coatings without the substrate effect. The sequence in hardness is CrN/Cr2O3 (35.2 GPa) N CrN/Cr(N,O) (32.2 GPa) N CrN (23.0 GPa), whereas the sequence of elastic modulus is CrN/Cr(N,O) (352 GPa) N CrN/Cr2O3 (346 GPa) N CrN (294 GPa). Accordingly, CrN/Cr2O3 coatings has a highest H/E ratio, followed by CrN/Cr(N,O), and then by monolithic CrN film. Indentation tests were performed by using the Rockwell hardness tester (HRC) to evaluate the adhesion between film and substrate. Fractured morphologies of the coated specimens as shown in Fig. 8 were compared with a defined adhesion strength quality (ASQ) [22]. The results showed that the indentation of the CrN single-layered coating was cleaner and smoother, and belonged closely to the grade of HF1. For the double-layered coatings, CrN/Cr(N,O) coatings had a slight peeling failure around the indentation, approaching the grade of HF2. In addition, an obvious delamination failure was observed to occur in CrN/Cr2O3 double-layered coatings (~HF6). It implies that CrN/Cr(N,O) coatings possess better adhesion as compared to CrN/ Cr2O3 ones. 3.3. Analysis of abrasion behavior It is known that the wear-life of protective coatings mainly depends on a combined function of the friction coefficient, hardness and film adhesion. In this study, a CSEM tribometer was used to evaluate the wear performance of M2 steel with and without Cr–N–O coatings. A comparison of the coated and uncoated specimens regarding the relationship between the friction coefficient and the travel distance is shown in Fig. 9. It can be seen that all coated specimens have the stable and lower friction coefficient as compared to uncoated M2 substrate during the tribological test. In particular, the friction coefficient of the specimen with the double-layered coatings is superior to that with only the monolithic CrN coating. The results coincide well with the concepts described by Leyland and Matthews [23,24], who noticed that a high ratio of H/E can be indicative of good wear resistance in a disparate range of materials. Both high coating hardness and/or low coating modulus would reduce the ploughing effects and permit a reduced friction coefficient. That is, the higher coating hardness (H) infers less plastic yielding at a given contact pressure, and the lower coating modulus (E) allows the applied load to be distributed over a wider area thus reducing the maximum contact pressure at a given load. SEM micrographs of the worn craters corresponding to 10 km travel distance are shown in Fig. 10. After
Fig. 10. SEM micrograph of the coated specimens after 10 km travel distance in wear test: (a) CrN, (b) CrN/Cr(N,O), and (c) CrN/Cr2O3.
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Fig. 11. SEM micrograph of the specimens after erosion test at an impingement angle of 30°: (a) substrate, (b) CrN, (c) CrN/Cr(N,O), and (d) CrN/Cr2O3. Cutting groove and trace are indicated by arrows.
angles of 30o and 90o, respectively. For the low impingement angle of 30o, it was found that the uncoated M2 steel obviously had the cutting groove and failure damage on its eroded surface (Fig. 11(a)). In contrast, the surface of the three coated specimens showed only a relatively shallow and narrow cutting trace and no fracture (Fig. 11(b)– (d)). The results are attributed to the elastic modulus of the coatings which is high enough to resist the impinging normal force. It implies that the Cr–N–O coated specimens turn in a well-protected performance under the erosive-wear condition at a low impact angle. For the impingement at a high angle of 90o, however, all the specimens manifested the eroded surface morphology characterized by indented
craters, ridges, and cracking (Fig. 12(a)–(d)), even though they had protective coatings (Fig. 12(b)–(d)). This is because the thin films are penetrated by the more impinging force, resulting in the occurrence of the various damage behaviors on a specimen's surface. The CrN/Cr2O3 double-coated specimen was noticed to have a serious delamination to expose the CrN interlayer and substrate after the impingement of single solid particle. Its erosive damage area was more extensive than that of the CrN/Cr(N,O) specimen. The result can be attributed to the fact that the latter has a better adhesion than the former when they have similar hardness (32–35 GPa). The cross-sectional view of TEM high-resolution image for the CrN/Cr(N,O) coatings was further
Fig. 12. SEM micrograph of coated specimens after erosion test at an impingement angle of 90°: (a) substrate, (b) CrN, (c) CrN/Cr(N,O), and (d) CrN/Cr2O3.
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erosion resistance of the CrN/Cr(N,O) coated specimen is superior to the CrN/Cr2O3 coated one due to its better adhesion. Acknowledgement The authors are grateful to the Chinese National Science Council for its financial support (project No. NSC 93-2216-E-036-015). References
Fig. 13. TEM cross-sectional picture of the CrN/Cr(N,O) coating layer interface.
observed to manifest its dense interface of layered-structure as shown in Fig. 13, reflecting the superiority of CrN/Cr(N,O) coatings over CrN/ Cr2O3 and CrN coatings in erosion resistance. 4. Conclusions Due to the use of nitrogen and oxygen gases, CrN and/or Cr2O3 phases were sequentially formed on top of a CrN interlayer as doublelayered coatings by using the CAE system. Moreover, the affinity of chromium with oxygen is greater than that of chromium with nitrogen. For the Cr–N–O coatings applied in the abrasive performance of M2 steel, it showed an improvement on the friction coefficient in this study. Sliding against tungsten carbide, CrN/Cr2O3 coatings exhibit a low friction coefficient of about 0.3, followed by CrN/Cr(N,O) coatings (~0.45), and then by the CrN coating (~0.6). Concerning the erosion behavior at a low impingement angle of 30o, the Cr–N–O coatings seem to have the desirable erosion resistance due to a combination of high hardness and moderately low elastic modulus. However, increase the impinging angle up to 90o, all coatings are penetrated by the more normal force to generate erosion damage to different extents. In contrast to the abrasive behavior, the
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
A study on the abrasive and erosive wear behavior of arc-deposited Cr–N–O coatings on tool steel Cheng-Hsun Hsu ⁎, Yu-Dung Chen Department of Materials Engineering, Tatung University, 40 Chungshan North 3rd Sec., Taipei 104, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 21 December 2007 Received in revised form 10 September 2008 Accepted 18 September 2008 Available online 27 September 2008 Keywords: AISI M2 steel Cathodic arc evaporation Cr–N–O coatings Abrasion Erosion
a b s t r a c t In this study, the cathodic arc evaporation technique, by using the chromium target and controlling the flow rate of nitrogen/oxygen reactive gases, was utilized to deposit three different Cr–N–O coatings (CrN, CrN/Cr (N,O), CrN/Cr2O3) on AISI M2 tool steel. Two types of wear tests were applied to evaluate the abrasive and erosive wear behavior of the coated and uncoated specimens. One was the ball-on-disk abrasion test to measure the friction coefficient of these specimens. The other was the erosion test using Al2O3 particles (~ 177 µm in size and Mohr 7 scale) of about 5 g, and then the surface morphologies of the eroded specimens were observed. To further understand the coating effects on the two wear behaviors of M2 steel, coating structure, morphology, and adhesion were analyzed using XRD, SEM, and TEM, respectively. The results showed that surface roughness and adhesion of the double-layered coatings (CrN/Cr(N,O) and CrN/Cr2O3) were inferior to those of monolithic CrN, but their hardness and elastic modulus were superior to those of CrN. In the abrasive behavior, Cr–N–O coatings reduced the friction coefficient of M2 substrate. In particular, the CrN/Cr2O3 has the highest hardness/elastic (H/E) modulus ration, therefore the lowest friction coefficient, among all the coated-specimens tested. In the erosive behavior, the coated specimens exhibited better erosion resistance as compared to the uncoated ones, at the impingement angles of either 30o or 90o. Moreover, the erosion resistance of CrN/Cr(N,O) coatings was superior to that of CrN/Cr2O3 coatings due to its better adhesion. © 2008 Elsevier B.V. All rights reserved.
1. Introduction It is a well known fact that coatings of a wide variety of materials are commonly applied to substrate for different purposes. For instance, various PVD ceramic nitride films such as CrN, TiN, TiAlN, NbN, and their combination can be used to prolong the life of tool steels exposed to an abrasive or erosive environment in many engineering applications [1–5]. It was also reported that chromium nitride (CrN) deposited by PVD technology has been identified as one of the more promising protective coatings for the application of tool and mold materials in recent years [5–10]. In particular, CrN forms a dense and stable Cr2O3 thin film as a protective scale at a high working temperature. That is, the oxide layer could be quite effective as an oxygen diffusion barrier up to moderately high temperature (~ 900 °C) [10]. It is noted from some literature that Cr2O3 thin film can be synthesized using the unbalanced magnetron sputtering [11], ion implantation [12] and cathodic arc evaporation (CAE) methods [13,14]. The CAE method is particular noteworthy because it offers several advantages such as the high deposition rate, low processing
⁎ Corresponding author. Tel.: +886 2 2586 6410; fax: +886 2 2593 6897. E-mail address: [email protected] (C.-H. Hsu). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.083
temperature, and the ability to obtain dense structure with stoichiometric composition. Although previous studies [13,17,18] have confirmed certain properties of CrN and Cr2O3 hard coatings such as corrosion resistance and thermal stability, there is a lack of information on both the abrasive and erosive properties of tool steel treated with a combination of CrN and Cr2O3 coatings [19]. Thus, the present work aims to investigate the abrasive and erosive behaviors of AISI M2 tool steel coated with Cr–N–O double-layer films by the CAE process. That is, CrN film was first deposited on the steel as an interlayer, and then followed by the deposition of Cr2O3 as the superficial layer under different flow rates of reactive oxygen gas. Finally, abrasion and erosion tests were carried out in order to explore the effects of the hard coatings on the wear behaviors of the steel. 2. Experimental procedure 2.1. Substrate and CAE treatment The substrates were made of AISI M2 steel (0.8–0.9%C, 0.4%Si, 0.4% Mn, 3.8–4.5%Cr, 4.5–5.5%Mo, 5.5–6.7%W, 1.6–2.2%V, balance Fe), which were machined into circular and rectangular test coupons of 20 mm × 5 mm and 20 mm × 20 mm × 5 mm in size, respectively, for
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abrasive and erosive wear tests. Prior to the CAE treatment, the substrates were heat-treated to obtain the hardness of about 63 HRC to imitate the real tool and mold in applications [20]. Then, the substrates were mechanically ground and polished to an average surface roughness of approximately 0.06 µm (Ra value). After undergoing thorough wet cleaning in an ultrasonic bath, the specimens were then fixed in a chamber holder and subjected to Ar+ bombardment at the bias of −1 × 103 V for 10 min to ensure good adhesion of the deposited films. The pressure of argon gas during ion bombardment was kept at 0.1 Pa. The reactive gases of N2 and O2 were fed into the chamber to form CrN and Cr2O3 films, respectively. The chamber pressure during the deposition stage was kept at 3.3 Pa. The whole coating operation began with the formation of the CrN interlayer, and separately followed by either Cr(N,O) or Cr2O3 films as the outer layer in the same batch process. For comparison, another monolithic CrN film was also coated on steel for the same depositing time. A schematic diagram of the CAE system in this study is shown in Fig. 1 and the details of the processing parameters are given in Table 1. 2.2. Analysis of coating characteristics SEM (Hitachi-S4100, Japan) was utilized to observe cross-sectional morphology for the measurement of coating thickness. In addition, FEG-TEM (Philips Tecnai-F30, USA) was used to observe the dense interface of coatings at an incident beam voltage of 300 kV. The X-ray diffractometer (MAC Science-M21X, Japan) was employed to identify the coating structure by using Cu-target Kα radiation at 40 kV and 30 mA at a low incident angle of 2o and in the scanning angular (2θ) range from 20 to 120 degrees at 2 degree/minute. The hardness and elastic modulus of the films were measured by using a nanoindenter (MTS XP system, USA) under the applied load of 4 mN. Five readings were taken and averaged to represent the obtained data for each type of coating. A surface roughness analyzer (Mitutoyo Serftest SV-400, Japan) was applied to measure average surface roughness (Ra value) of the specimens. In addition, the adhesion strength of coatings was evaluated utilizing Rockwell C indentation testing with a load of 150 kg. 2.3. Abrasion and erosion tests In this study, the wear testing was divided into two types: the abrasion test and the dynamic erosion test. The abrasion tests of the coated and uncoated specimens were performed on a ball-on-disc tribometer (CSEM). The tests were conducted with no lubricant along a circular track of 30 mm diameter against a 6.0 mm diameter WC ball at 0.3 m/s under a normal load of 10 N in the ambient atmosphere. The
Fig. 1. Schematic diagram of the cathodic arc evaporation system.
Table 1 Processing parameters of the coatings in this experiment Target material Substrate temperature (°C) Chamber pressure (Pa) Substrate bias voltage (V) Evaporator current (A) Source to substrate distance (cm) Reactive gas (sccm)
Deposition time (min)
Cr (99.99%) 200–220 3.3 −150 70 15 Single-layer: N2(300); Double-layer: Interlayer N2(300), and then (1) outer layer N2(230)+ O2(70) as well as (2) outer layer only O2(180), respectively. 60
relationship between the friction coefficient and the travel distance was continuously recorded during the tests. Another wear-testing type, using a dry particle erosion tester as shown in Fig. 2, was adopted to evaluate the dynamic wear behaviors of all the specimens. Al2O3 angular particles (~ 177 µm in size and Mohr 7 scale) of about 5 g were used as the eroding carriers to impinge the specimen for the observation on its eroded surface. The erosion pressure was controlled at a constant value of 3 kg/cm2 by using an air compressor. The average particle velocity at a distance of 30 mm between the nozzle tip and specimen is 83.2 ms− 1. The internal diameter of the employed nozzle was 5 mm and the impact angles were fixed at 30o and 90o, respectively. 3. Results and discussion 3.1. Coating structure and morphology Fig. 3 shows the crystal structure of the deposited coatings in terms of the XRD patterns. For comparison, a single-layered specimen was identified as the CrN phase with the strong (200) plane. For one of the double-layered specimens, it was found that the XRD pattern mainly displayed the peaks of CrN and Cr2O3 crystal planes due to the simultaneous use of N2 (230 sccm) and O2 (70 sccm) reactive gases to deposit the outer layer. Thus the specimen had a superficial structure containing a mixture of CrN and Cr2O3 phases except for the interlayer of CrN, and here named CrN/Cr(N,O) coatings. For the other doublelayered specimen, its XRD pattern is similar to that of the above specimen in spite of the use of only O2 (180 sccm) reactive gas to deposit the outer layer. This is because the double-layered thin film comprises a CrN interlayer and a Cr2O3 outer layer, here also named CrN/Cr2O3 coatings in this study.
Fig. 2. Schematic arrangement of the erosion test. (θ: impingement angle).
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Fig. 3. XRD patterns of the resulting coatings.
Fig. 4. SEM cross-sectional view of coated specimens: (a) CrN, (b) CrN/Cr(N,O), and (c) CrN/Cr2O3.
Fig. 5. Surface morphology of coated specimens: (a) CrN, (b) CrN/Cr(N,O), and (c) CrN/ Cr2O3.
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Fig. 6. Comparison of surface roughness of the uncoated and coated specimens.
SEM was used to observed cross-sectional and surface morphologies of these coatings as shown in Figs. 4 and 5. The results clearly indicate that all the coatings have an approximate thickness of 1.5 µm
Fig. 8. Fracture morphology of the various coatings from the Rockwell ‘C’ adhesion test: (a) CrN, (b)CrN/Cr(N,O), and (c) CrN/Cr2O3.
under the same deposition time. Moreover, in terms of average surface roughness the CrN/Cr2O3 coating exhibited the highest Ra value (0.12 µm), followed by the CrN/Cr(N,O) coating (0.1 µm), and then the
Fig. 7. Comparison of coating mechanical properties for the various coated specimens: (a) film hardness, (b) elastic modulus, and (c) H/E ratio.
Fig. 9. Comparison of the friction coefficient of the coatings at a load of 10 N, sliding against a 6 mm WC ball counterface.
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single-layer CrN (0.07 µm), compared with the substrate (0.06 µm), as shown in Fig. 6. The increase in the surface roughness of the coated specimens mainly results from the deposition of macro-particles during the CAE process [15,16]. It is noteworthy to show that, although different flow rates of the reactive gases were used to deposit the outer layer of the double-layered CrN/Cr(N,O) and CrN/Cr2O3 coatings (see Table 1), both the coatings had almost the same outer layer thickness of about 0.7 µm. According to a previous study [21], these results could be attributed to the more favorable reaction of chromium with oxygen as compared to that of chromium with nitrogen. The Cr2O3 layer is easily obtained by the CAE process because nitrogen is easily replaced by oxygen as a preferred reactive gas with chromium during the deposition process. As a result, this phenomenon also increases the surface roughness of the coatings (Fig. 6). It may be that the Cr2O3 coating has a less dense microstructure due to reduced sputter bombardment as it thickens and becomes insulating. 3.2. Coating hardness, elastic modulus and adhesion
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10 km travel distance, the wear pattern consisting of parallel straight lines and local exposed substrate is clearly seen in the CrN coated specimen (Fig. 10(a)). The wear pattern of double-layered coatings is shown in Fig. 10(b)–(c). Contrastingly, the straight lines become less evident when compared to the CrN coated specimen. The sequence of the H/E value of these coatings in this study is CrN/Cr2O3 N CrN/Cr(N, O) N CrN. Furthermore, CrN/Cr2O3 coatings exhibited the lowest friction coefficient (~ 0.3) against WC, followed by CrN/Cr(N,O) coatings (~ 0.45), and then by CrN coating (~ 0.6). On the other hand, the CrN/Cr2O3 coatings performed less well in the Rockwell C adhesion test – indicating that the Cr2O3 outer layer, although hard, may be more brittle (as possibly less well adhered) than the Cr(N,O) equivalent. 3.4. Analysis of erosive behavior Figs. 11 and 12 illustrate SEM micrographs of the uncoated and coated specimens after the single solid particle test at impingement
Fig. 7 shows a comparison of the hardness (H), elastic modulus (E), and hardness/elastic modulus (H/E) ratio among the double-layered coatings and CrN film. These data were obtained by using the nanoindenter with the indentation depth at approximately 100 nm under an applied load of 4 mN. It implies that all the values are mainly dominated by the superficial layer of the coatings without the substrate effect. The sequence in hardness is CrN/Cr2O3 (35.2 GPa) N CrN/Cr(N,O) (32.2 GPa) N CrN (23.0 GPa), whereas the sequence of elastic modulus is CrN/Cr(N,O) (352 GPa) N CrN/Cr2O3 (346 GPa) N CrN (294 GPa). Accordingly, CrN/Cr2O3 coatings has a highest H/E ratio, followed by CrN/Cr(N,O), and then by monolithic CrN film. Indentation tests were performed by using the Rockwell hardness tester (HRC) to evaluate the adhesion between film and substrate. Fractured morphologies of the coated specimens as shown in Fig. 8 were compared with a defined adhesion strength quality (ASQ) [22]. The results showed that the indentation of the CrN single-layered coating was cleaner and smoother, and belonged closely to the grade of HF1. For the double-layered coatings, CrN/Cr(N,O) coatings had a slight peeling failure around the indentation, approaching the grade of HF2. In addition, an obvious delamination failure was observed to occur in CrN/Cr2O3 double-layered coatings (~HF6). It implies that CrN/Cr(N,O) coatings possess better adhesion as compared to CrN/ Cr2O3 ones. 3.3. Analysis of abrasion behavior It is known that the wear-life of protective coatings mainly depends on a combined function of the friction coefficient, hardness and film adhesion. In this study, a CSEM tribometer was used to evaluate the wear performance of M2 steel with and without Cr–N–O coatings. A comparison of the coated and uncoated specimens regarding the relationship between the friction coefficient and the travel distance is shown in Fig. 9. It can be seen that all coated specimens have the stable and lower friction coefficient as compared to uncoated M2 substrate during the tribological test. In particular, the friction coefficient of the specimen with the double-layered coatings is superior to that with only the monolithic CrN coating. The results coincide well with the concepts described by Leyland and Matthews [23,24], who noticed that a high ratio of H/E can be indicative of good wear resistance in a disparate range of materials. Both high coating hardness and/or low coating modulus would reduce the ploughing effects and permit a reduced friction coefficient. That is, the higher coating hardness (H) infers less plastic yielding at a given contact pressure, and the lower coating modulus (E) allows the applied load to be distributed over a wider area thus reducing the maximum contact pressure at a given load. SEM micrographs of the worn craters corresponding to 10 km travel distance are shown in Fig. 10. After
Fig. 10. SEM micrograph of the coated specimens after 10 km travel distance in wear test: (a) CrN, (b) CrN/Cr(N,O), and (c) CrN/Cr2O3.
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Fig. 11. SEM micrograph of the specimens after erosion test at an impingement angle of 30°: (a) substrate, (b) CrN, (c) CrN/Cr(N,O), and (d) CrN/Cr2O3. Cutting groove and trace are indicated by arrows.
angles of 30o and 90o, respectively. For the low impingement angle of 30o, it was found that the uncoated M2 steel obviously had the cutting groove and failure damage on its eroded surface (Fig. 11(a)). In contrast, the surface of the three coated specimens showed only a relatively shallow and narrow cutting trace and no fracture (Fig. 11(b)– (d)). The results are attributed to the elastic modulus of the coatings which is high enough to resist the impinging normal force. It implies that the Cr–N–O coated specimens turn in a well-protected performance under the erosive-wear condition at a low impact angle. For the impingement at a high angle of 90o, however, all the specimens manifested the eroded surface morphology characterized by indented
craters, ridges, and cracking (Fig. 12(a)–(d)), even though they had protective coatings (Fig. 12(b)–(d)). This is because the thin films are penetrated by the more impinging force, resulting in the occurrence of the various damage behaviors on a specimen's surface. The CrN/Cr2O3 double-coated specimen was noticed to have a serious delamination to expose the CrN interlayer and substrate after the impingement of single solid particle. Its erosive damage area was more extensive than that of the CrN/Cr(N,O) specimen. The result can be attributed to the fact that the latter has a better adhesion than the former when they have similar hardness (32–35 GPa). The cross-sectional view of TEM high-resolution image for the CrN/Cr(N,O) coatings was further
Fig. 12. SEM micrograph of coated specimens after erosion test at an impingement angle of 90°: (a) substrate, (b) CrN, (c) CrN/Cr(N,O), and (d) CrN/Cr2O3.
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erosion resistance of the CrN/Cr(N,O) coated specimen is superior to the CrN/Cr2O3 coated one due to its better adhesion. Acknowledgement The authors are grateful to the Chinese National Science Council for its financial support (project No. NSC 93-2216-E-036-015). References
Fig. 13. TEM cross-sectional picture of the CrN/Cr(N,O) coating layer interface.
observed to manifest its dense interface of layered-structure as shown in Fig. 13, reflecting the superiority of CrN/Cr(N,O) coatings over CrN/ Cr2O3 and CrN coatings in erosion resistance. 4. Conclusions Due to the use of nitrogen and oxygen gases, CrN and/or Cr2O3 phases were sequentially formed on top of a CrN interlayer as doublelayered coatings by using the CAE system. Moreover, the affinity of chromium with oxygen is greater than that of chromium with nitrogen. For the Cr–N–O coatings applied in the abrasive performance of M2 steel, it showed an improvement on the friction coefficient in this study. Sliding against tungsten carbide, CrN/Cr2O3 coatings exhibit a low friction coefficient of about 0.3, followed by CrN/Cr(N,O) coatings (~0.45), and then by the CrN coating (~0.6). Concerning the erosion behavior at a low impingement angle of 30o, the Cr–N–O coatings seem to have the desirable erosion resistance due to a combination of high hardness and moderately low elastic modulus. However, increase the impinging angle up to 90o, all coatings are penetrated by the more normal force to generate erosion damage to different extents. In contrast to the abrasive behavior, the
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