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WCrTiAlN multilayer coatings deposited on nitrided AISI 4140 steel
Materials Characterization 147 (2019) 353–364
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Microstructure, mechanical and adhesive properties of CrN/CrTiAlSiN/ WCrTiAlN multilayer coatings deposited on nitrided AISI 4140 steel ⁎
Yang Lia,b, Zhongli Liua, Jianbin Luob, Shangzhou Zhanga, , Jianxun Qiua, Yongyong Heb, a b
T
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School of Nuclear Equipment and Nuclear Engineering, Yantai University, Yantai, 264005, PR China State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coating Multilayers Nitriding Nanoindentation Scratch test
A duplex treatment of plasma nitriding (PN), followed by the deposition of a CrN/CrTiAlSiN/WCrTiAlN multilayer coating, was performed on AISI 4140 steel. Untreated, plasma-nitrided, only-coated (just the coating) and duplex-treated (PN + coating) AISI 4140 steel were microstructurally and mechanically characterized. The nitrided compound layer, consisting of mainly ε-Fe2-3N and γ′-Fe4N phases, was approximately 8.0 μm thick. The total thickness of the multilayer coating, which consisted of a top W-Cr-Ti-Al-N multilayer, a periodic Cr-Ti-Al-SiN interlayer, and a CrN bottom layer, was 2.3 μm. The CrN/CrTiAlSiN/WCrTiAlN multilayered coatings possessed a super-hardness of 47.9 GPa and the highest elastic modulus of 424.8 GPa in the all samples. The microhardness (2200 HV0.05) of the duplex-treated sample was significantly higher than that of the only-coated (740 HV0.05) and nitrided (900 HV0.05) samples. The scratch tests revealed that the adhesion strength of the duplex-treated sample increased significantly due to a gradient-hardness support of the coating-nitrided layer interface.
1. Introduction Transition metal nitride coatings, such as CrN and TiN, have excellent mechanical, tribological and corrosion resistance, which can improve the surface properties of the workpiece and prolong the service life of the workpiece [1–7]. The working conditions such as heavy loads, high temperature, wear and corrosion are complicated and harsh and require a workpiece surface with high hardness, high-strength and excellent wear-resistant and anti-fatigue properties. However, with an increase in the hardness and strength, the toughness and adhesion will inevitably decrease, resulting in the deterioration of the wear resistance, corrosion resistance and fatigue resistance [8,9]. Recently, compared with single-layer coatings, nanoscale multilayer coatings have been widely studied because of their stronger performance [10–16]. It is a specific form of multilayered coating materials that alternately grow in thin layers of several nanometers to several tens of nanometers and maintains strictly periodicity [17–21]. It suggested that the multilayer coatings have lower residual stresses mainly because of stress relaxation at the interfaces [22]. Specifically, the strength and ductility of transition nitride multilayers are attributed to the modulation periods and choice of the constituent layers [23–25]. Nanocrystalline (Ni) thin film exhibits poor thermo-mechanical properties due to its unstable microstructure at elevated temperature.
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Feng and Li et al. [26] endowed a new approach to solve above issue via addition of nano-multilayers and incorporation of W for nanocrystalline Ni-based films, to provide novel Ni/Ni3AlW nano-composite multilayered structure with high hardness and good thermal stability. In terms of Ni/Ni3Al multilayer, the hardness reaches the small value of 4.76 GPa at 800 °C. On the contrary, the outstanding hardness (15.6 GPa) of the 12.5 at.% W doped Ni/Ni3Al-W multilayer can be attributed to combination of precipitation and residual layer interface strengthening via the Orowan mechanisms. The introduction of W into the CrN coatings increased their hardness values, which was attributed to the Hall–Petch effect from the refinement of the crystal size and the formation of a highly covalently bonded W2N phase in the CrN coatings [27,28]. Yan-Zuo et al. [29] have reported that the improvement in tribomechanical properties of a CrN/WN multilayer coating could be attributed to the existence of its superlattice structure. In addition, some research has indicated the benefits of W2N as an incorporated sublayer in a multilayer architecture because of its unique characteristic, such as super-hardness and chemical inertness [30,31]. As to the hardness, the hard coatings are usually divided into two groups: (1) hard coatings having hardness < 40 GPa, and (2) superhard coatings having hardness > 40 GPa [32]. However, these coatings have a higher hardness and stiffness than the soft substrates, and support of the coating is a concern in high stress contact [33]. The
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Y. He).
https://doi.org/10.1016/j.matchar.2018.11.017 Received 13 October 2018; Received in revised form 14 November 2018; Accepted 14 November 2018 Available online 15 November 2018 1044-5803/ © 2018 Elsevier Inc. All rights reserved.
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Fig. 1. (a) Schematic representation of the PN sample; (b) Schematic representation of the only-coating sample; (c) Schematic representation of the duplex PN + coating sample.
Fig. 2. X-ray diffraction patterns of the different samples. (a) Untreated; (b) PN; (c) only-coating; (d) PN + Coating.
provides a hard and brittle coating with strong support but also reduces the hardness gradient at the interface between the coating and substrate [35,39,40]. A smoother hardness gradient reduces the stress in the coating during loading, and the stress distribution at the interface is more even, and thus, the possibility of damaging or cracking the coating is reduced. A smoother hardness gradient reduces the stress in the coating during loading, and the stress distribution at the interface is more even, and thus, the possibility of damaging or cracking the coating is reduced [41].
tribological performance of these coatings is limited, in many cases by plastic deformation of the substrate, which can result in the failure of the coating [33,34]. The duplex-treated system, which combines plasma nitriding (PN) with a hard coating, is effective in improving the adhesion of the coating-substrate interface and the wear resistance [35–38]. The introduction of a nitrided layer not only significantly improves the load ability of the substrate surface and the coating–substrate bonding force but also improves the surface fatigue strength, thermal shock and chemical resistance. This layer not only
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Fig. 3. Cross-sectional SEM micrographs and EDS mapping of the nitrided layer on AISI 4140 steel by PN treatment.
Fig. 4. Cross-sectional SEM micrographs and EDS mapping of the only coating deposited on AISI 4140 steel.
acetone. The nitriding process was carried out in an LDMC-20 nitriding furnace (LDMC-20F, WHRCLS, China) [42,43]. The nitriding was performed at 525 °C for 6 h. The working pressure was maintained at 400 Pa using NH3 gas. Depositions were conducted in an industrial cathodic arc evaporation system (HCCE-280, Aomet Nano Technology Ltd., China). The CrN/CrTiAlSiN/WCrTiAlN coatings were deposited onto the polished and nitrided AISI 4140 steel samples. To create the multilayers, 3 tungsten, 3 chromium,1 Al-Ti, and 1 Al-Ti-Si targets were used. Each target had a power source supply with electric currents, depending on the desired deposition rate. The samples were sputter cleaned in an argon plasma with an overall pressure of 2 Pa, and the temperature reached 350 °C. After achieving a base pressure of 10−4 Pa, the deposition pressure was fixed at 3 × 10−1 Pa. First, a very thin Cr interlayer was deposited onto the substrate. Cr was used as an intermediate adhesion layer to improve the adhesion of the coating on the substrate. Then, N2 with a flow speed of 100 sccm was added as a reactive gas. The CrN layer was deposited as the second interlayer. Afterwards, the selection of the sputtering targets (Cr, Al-Ti, and Al-Ti-
In this study, AISI 4140 steel samples were plasma nitrided, and subsequently, a CrN/CrTiAlSiN/WCrTiAlN multilayer coating was deposited on both the untreated and nitrided surfaces. The structure and composition of the different samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM). The mechanical properties were also measured by microhardness and nanoindentation testing. The scratch adhesion strength of the multilayer coating on diffusion-treated substrates was discussed.
2. Experimental The material used in this study is commercial AISI 4140 steel (composition: 0.040 wt% C, ≤0.05 wt% Si, 0.129 wt% Mn, 0.006 wt% P, 0.007 wt% S). Disc samples approximately 6.0 mm thick were cut from a 20 mm diameter rod and wet ground with emery paper from a 240 grit down to a 2000 grit. Then, the samples were mirror polished with a diamond suspension (size 3.5 μm) to produce a fine surface finish. The surfaces of the samples were cleaned ultrasonically in 355
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Fig. 5. Cross-sectional SEM micrographs and EDS mapping of the duplex layers on the nitrided AISI 4140 steel by the duplex PN + coating treatment.
Fig. 6. (a) Cross-sectional TEM bright-field micrograph of the multilayer coating; (b) TEM image and (c) SAED pattern of the W-Cr-Al-Ti-N coating; (d) TEM image and (e) SAED pattern of the Cr-Al-Ti-Si-N coating.
involves two sequential procedures (nitriding and coating). Fig. 1c describes the composition and structures of the duplex layers, where the graded coatings are deposited on the nitrided AISI 4140 steel surface. The fundamental concepts of surface composite design relate to constructing the composite layer in a specified order so that the periodic pairs of the nitrided layer and multilayer coating function effectively [17]. The strengthening mechanisms in adherence can be related to the diffusion layer between the coating and the substrate [44]. On the basis of this concept, a composite structure, consisting of a gradienthardness support nitrided layer and a hard and high-strength multilayer coating, is designed. The crystalline condition of the nitrided layer and the coating was characterized by XRD with Cu Ka radiation (λ = 0.15406 nm) at 40 kV and 40 mA (D8-Advance, Bruker, Germany). The analysis of each
Si) depended on the category of the CrTiAlSiN layers. The thickness of the individual layers was controlled by adjusting the deposition time by power supplies of the different targets. Here, during the deposition, the bias current was also monitored at the rotation table and the substrate samples. The individual layer thickness of WCrTiAlN and CrTiAlN layer was controlled by alternately switching the time for shutters with the aid of a programmable logic control to form the top WCrTiAlN multilayers. Fig. 1 provides the schematic showing the detailed structures of the individual layers. PN involves the introduction of nitrogen atoms into the component surface to produce a compound layer and a diffusion layer on the surface of AISI 4140 steel, as schematically shown in Fig. 1a. Fig. 1b provides the CrN/CrTiAlSiN/WCrTiAlN-graded coating compositions and the schematic. Furthermore, duplex surface treatment 356
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Fig. 7. (a) Cross-sectional TEM image for the upper part of coating; (b) EDS mapping results on the purple lined area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sample was conducted within the range of 30° < 2θ < 90° with an increment of 0.05°/step and a time per step of 2 s. The morphology was observed by field emission scanning electron microscopy (FESEM, Hitachi-S4800, Japan). The elemental composition of the coatings was detected by energy dispersive X-ray analysis (EDX). The morphologies and microstructures of the different samples were investigated using TEM (Tecnai G20, FEI, USA). The characterization of the different precipitates was performed by means of bright-field TEM, HRTEM, selected-area electron diffraction (SAED) patterns, and EDX. The Vickers microhardness was measured using a hardness tester (Struers Duramin-A300, Denmark) under a load of 50 g for 15 s.
Table 1 Chemical compositions (at.%) of the representative elements of the analysis points (1 to 6) from the Fig. 6. Point
N
W
Cr
Al
Ti
1 2 3 4 5 6
43.45 44.08 44.38 44.58 46.46 47.67
22.64 19.41 21.16 18.36 14.36 –
16.68 17.98 17.34 18.80 20.24 34.18
11.70 12.28 11.34 11.95 12.52 10.53
5.53 6.25 5.78 6.31 6.42 5.94
Si – – – – 1.68
Fig. 8. (a) EDX line scan of the W-Cr-Al-Ti-N coating from the lines I in the Fig. 6a. (b) EDX line scan of the Cr-Al-Ti-Si-N coating from the lines II in the Fig. 6a. 357
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Fig. 9. HRTEM micrograph of the W-Cr-Al-Ti-N and Cr-W-Al-Ti-N nanolayers.
Fig. 10. HRTEM micrograph of the Cr-Al-Ti-Si-N nanolayers.
and α(211) are found at 45°, 66° and 82°, respectively (Fig. 2a). Fig. 3 shows the SEM cross-sectional images of the nitrided layers of the PN sample. An 8.0–9.0 μm compound layer was formed on the surface, and the EDS results show a N-rich surface. The compound layers consisted of hexagonal close packed (hcp) ε-Fe2–3N and face centered cubic (fcc) γ′-Fe4N nitride phases (Fig. 2b). Fig. 4 shows the cross-sectional morphology of the only-coating sample. The total thickness of the multilayer coating was 2.3 μm, which consisted of a top W-Cr-Ti-Al-N multilayer (thickness = 0.7 μm) and a periodic Cr-Ti-Al-Si-N interlayer (thickness = 1.6 μm). From Fig. 2c, the XRD spectra of the only-coating sample indicates that the multilayer coating mainly contains the W2N, CrN, TiN and AlN phases. The shift of the CrN and TiN diffraction patterns toward higher 2θ values (compared with the Joint Committee on Powder Diffraction Standards (JCPDS) data) is in relation to the compressive residual stress characteristics for those multilayer systems. In addition, the peaks of the Fe substrate are detected due to the thin thickness of the coating. The
Nanoindentation tests were performed with an instrumented indenter (Nano-indenter Tester NHT2, CSM, Switzerland) to determine the indentation hardness and the Young's modulus of the different samples. The adhesion behaviors were evaluated using an automatic scratch instrument (WS-2005, LICP, China) equipped with acoustic emission. The only-coating and duplex samples were scratched at the initial load of 0.05 N, which was gradually increased to a final load of 60 N with the increasing rate of 60 N/min. The scratch length was 4 mm. The normal load at which the failure occurs, the critical load, was determined by SEM after each scratch test. 3. Results and Discussion 3.1. Structure of the Nitrided Layer and Multilayer Coatings Fig. 2 shows the XRD patterns of the different samples. For untreated AISI 4140 steel, three diffraction ferrite peaks of α(110), α(200) 358
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Fig. 11. TEM image of the cross-sectional duplex layers and its SAED patterns at different positions.
further indicate that the Cr, Al and Ti atoms are uniformly distributed in the inner layer of the coating. The presence of N in all the coatings is also observed. The duplex sample shows that a multilayer coating is deposited on the surface of the nitrided compound layer (Fig. 5). The mapping covered the entire coated layer and the partially nitrided layer. The duplex-treated sample shows the distribution of the N, W, Cr, Ni, Al and Fe elements from the surface to the nitride layer. For the duplex sample, the W2N, CrN, TiN and AlN phases are predominant, and small peaks of ε-Fe2–3N and γ′-Fe4N are also detected (Fig. 2d). The TEM microstructure on a cross-section of the multilayered coating is shown in Fig. 6a. A nanolayered architecture with sharp interfaces is displayed. This image shows the typical microstructure of the sample with only a coating, which is a multilayer coating formed by a 0.7 μm thick WCrTiAlN layer and 1.6 μm thick CrTiAlSiN layer. The multilayered coating can be divided into two distinct regions. The first deposited bottom layers are columnar structure, while the latter grown top layers are flat region. The ring pattern of the top WCrTiAlN layer (as shown in Fig. 6c) is also identified as the (111), (200), (220) and (311) directions of the NaCl type structure, which is consistent with the XRD results of the multilayer coating (Fig. 2c). The micrograph in Fig. 6b and c shows that the WCrTiAlN layer is continuous and dense with nanocrystalline grains, which is indicating that this layer exhibits a nanocrystalline structure. Fig. 6b is the cross-section TEM image of the nano-multilayers, which shows a dark strip and a light strip. The EDS mapping analyses (Fig. 7) were performed over the WCrTiAlN multilayer coating (shown as red rectangular area in Fig. 6a), and the results are also presented in Table 1. According to the EDS mapping results shown in Fig. 7b, the outer layer is composed of W, Cr, Al, Ti and N, while the inner layer
Table 2 Chemical compositions (at.%) of representative elements of the analysis points (7 to 10) from the Fig. 11. Point
N
W
Cr
Al
Ti
Si
Fe
7 8 9 10
47.98 51.32 11.93 11.89
– – – –
42.95 46.19 2.22 2.68
2.46 – – –
1.42 – – –
0.25 – – 0.77
– 0.38 75.12 77.78
diffraction pattern of the coating presents four diffraction peaks centered at approximately 37.1°, 42.8°, 62.2° and 74.3°, which can be assigned to the fcc W2N (JCPDS card, no. 25-1257) (111), (200), (220) and (311) planes, respectively. These peaks shift to toward lower angle in W2N, which may be related to the increase in the in-plane residual stress [30]. The W2N phase has the lowest formation enthalpy with nitrogen compared to the other W2N phases. The enthalpy of formation of WN = −15 kJ/mol, where W2N = −22 kJ/mol [45]. Additionally, the AlN phase is represented by peaks at approximately 43.3° and 74.0°. The diffraction peaks located at 2θ of 37.0°, 43.0° and 74.5° are identified as the (111), (200) and (311) planes of the fcc CrN/TiN structure (JCPDS card no. 11-0065 and 38-1420). Compared with the database of the CrN phase (JCPDS card no. 11-0065), these peaks are slightly shifted to a higher diffraction angle, indicating a decrease in the lattice parameters as the Cr atoms were replaced by the smaller Al atom [46]. The intensity of the (200) diffraction peak mainly depends on the magnitude of the composition modulation in the superlattice structure [30,47]. To confirm the distributed elemental composition, SEM-EDS mapping examinations were carried out, as shown in Fig. 4. It found that the W is mainly distributed in the outermost layer. The results 359
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Fig. 12. HRTEM micrograph showing the interfaces between the Cr coating and nitrided layer.
layer (Fig. 10) shows that the lattice fringes continuously cross the layer interfaces, and the spacing of the fringes is maintained at 0.208 nm. These fringes are attributed to the (200) plane of the CrN, AlN and TiN crystal lattices. These results reveal the epitaxial and coordinated growth of these nano-multilayers. The larger column size in the CrTiAlSiN layer corresponds to the discrete diffraction spots in the SAED patterns, thus confirming the coarser grain size in this layer. The increased number of continuous diffraction rings in the SAED pattern of CrTiAlSiN compared with that of the WCrTiAlN coatings imply the finer grain size of the CrTiAlSiN coatings. The TEM images and corresponding SAED pattern of the nitrided layer and the coatings are demonstrated in Fig. 11. The results show both a bright-field TEM image of a cross-section of the coating and SAED patterns along the growth direction. Clearly, there are three kinds of layers: the nitrided layer, the Cr layer and the CrN layer. The EDS point analyses of the different regions are presented in Table 2. Point 7 differs from other points (6,8–10) by revealing relatively high concentrations of N and Cr and relatively low concentrations of Al, Ti and Si. In contrast, point 8 is similar to the initial thin Cr layer, where only high concentrations of Cr and N are found, which indicate the presence of a CrN layer. The SAED pattern shown in Fig. 11b is indexed according to the fcc CrN structure with (111), (200), (220) and (311) reflections. The presence of a significant amount of Fe and N and a low concentration of Cr at point 9 indicates that a nitrided layer remains on the AISI 4140 steel. An analysis of the SAED pattern (Fig. 11c) of the nitrided layer reveals the hcp ε-Fe2–3N phase, which is consistent with the d-spacing from the XRD pattern of the PN sample (Fig. 2b). The coatings were deposited as an initial thin Cr layer (∼150 nm), followed by a transition zone around the Cr-N monolithic layer. Three
does not contain W. Fig. 8a further shows the EDS intensities of W, Cr, Al, Ti and N along a scan line, and the results indicate that an increase in the W content corresponds to a decrease in the Cr, Al, Ti and N values. The EDS line I scan results show a higher content of W in the bright layer and the lower content in the dark layer. The EDS point analysis also proves that W is different in the dark and light strips. The chemical composition of the dark strip (point 1) identified by EDS (at.%) is 43.45% N, 22.64% W, 16.68%Cr, Al 11.70%, and 5.53% Ti. The chemical composition of the light strip (point 2) identified by EDS (at.%) is 44.08% N, 19.41% W, 17.98% Cr, 12.28% Al, and 6.25% Ti. In Fig. 9, a lattice-resolution HRTEM image of the top coating is marked by a red square in Fig. 6a. It can be seen that the dark strip in the WCrTiAlN coating is approximately 12.0 nm. Three different lattice fringes with a lattice spacing of approximately 0.232 nm, 0.208 nm, and 0.142 nm are detected. Those lattice fringes correspond to the (111), (200), (220) and (311) orientations of W2N (JCPDS card no. 25-1257), CrN (111) (JCPDS card no. 11–0065), TiN (JCPDS card no. 38-1420), and AlN (JCPDS card no. 46-1200), respectively. These results are in good agreement with the results obtained from its corresponding XRD and SAED patterns. The SAED patterns (Fig. 6c) show continuous and sharpening diffraction rings, which indicated a very fine grain structure. Fig. 6d and e show the cross-sectional TEM micrographs and SAED patterns of the CrTiAlSiN layer. The fcc (111), (200), (220) and (311) reflections of the CrN, AlN and TiN phases are identified, which indicates that the CrTiAlSiN layer has a fcc crystal structure. The distribution of the elements in the CrTiAlSiN layer is shown by EDS elemental line (II) in Fig. 8b. The HRTEM micrograph of the CrTiAlSiN
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lattice spacings. The fringe with a lattice spacing of approximately 0.204 nm corresponds to the (200) plane of the Cr layer, and the other fringe with a lattice spacing of approximately 0.219 nm is ascribed to the (111) plane of the hcp Fe2−3N phase. 3.2. Mechanical Properties Typical examples of the load-displacement curves corresponding to different samples, i.e., the AISI 4140 steel matrix, the nitrided layer and the only-coated sample, are depicted in Fig. 13a. These curves correspond to the tests with a maximum penetration depth of approximately 238 nm, then of ~197 nm and finally those with the depth of ~129 nm. The residual depths of these curves are approximately 190, 127 and 53 nm. The maximum contact depth of the multilayer coating of the only-coated sample is ~0.13 μm, which is much smaller than 2.3 μm (10% of the coating thickness, in the Fig. 4), indicating that the contribution to the deformation behavior from the underlying substrate is negligible [48,49]. Therefore, the indentation data accurately reflect the mechanical behavior of the multilayer coatings. Nano-hardness and elastic modulus values of the different samples are shown in Fig. 13b. The values of the nano-hardness and elastic modulus values for AISI 4140 steel matrix are 6.8 GPa and 227 GPa, respectively. Compared with the matrix, the nitrided layer has both a higher hardness (12.5 GPa) and elastic modulus (230 GPa). The increase in the hardness from 6.8 GPa to 12.5 GPa is due to the interstitial diffusion of nitrogen in α-Fe [50,51]. The multilayer coating has a maximum nano-hardness and elastic modulus of 47.9 GPa, and 424.8 GPa, respectively. According to previous studies [9,52], the hardness distribution of various CrN-based coatings is within the range of 20-35GPa. The coating with hardness above 40 GPa is classified as superhard. It is widely accepted that a greater hardness to elastic modulus value, and thus a greater plastic deformation resistance ration, H3/E*2, indicates a better resistance to plastic deformation and to abrasion damage by localized plastic deformation against a blunt and rigid contact [53–55]. The H/E* and H3/E*2 values of the different samples are shown in Fig. 13c. The H/E* values are 0.03, 0.06, and 0.124 for the untreated, nitrided, and multilayers samples, respectively. J. Musil [55] concluded that a new task in the development of advanced hard nanocomposite coatings with enhanced toughness is to produce coatings with a value that satisfies high H/E* ≥ 0.1. The nitrided layer exhibits an H3/E*2 value higher than 0.046, whereas the untreated matrix demonstrates a low H3/E*2 value of 0.006. The H3/E*2 ratio of approximately 0.609 obtained for the CrN/CrTiAlSiN/WCrTiAlN multilayer is substantially higher than those reported for common hard coatings such as CrN (~0.18) [56], WN (~0.32) [56], CrAlN (0.25) [57], CrTiAlN (~0.31) [58], and W-Si–N (~0.51) [59]. This finding means that the CrN/CrTiAlSiN/WCrTiAlN multilayer coating is likely to exhibit a higher threshold load for the initiation of plastic deformation than the CrN-based coatings without W. The improved toughness in the superlattice CrN/CrTiAlSiN/WCrTiAlN multilayer coatings can be explained as crack deflection at the interfaces and the dissipation of crack energy by plastic deformation in the crack tip by introducing a large number of interfaces with different elastic moduli [8,60]. Microhardness measurements were carried out on the surface of the different samples; the surface microhardness values are reported in Fig. 14a. The nitriding treatments facilitate an increase in the surface hardness (900 HV0.05), which is approximately 2.6 times greater than that of the untreated matrix (350 HV0.05). The high hardness of the nitrided layer is attributed to changes in the composition of the εFe2−3N and γ′-Fe4N nitrides [61]. The hardness level of only the coating is nearly 2.1 times higher than that of the untreated matrix (740 HV0.05). The highest microhardness value (approximately 2200 HV0.05) was obtained in the case of the duplex treatment. The microhardness profiles of the coating deposited on polished and nitrided AISI 4140 steels are shown in Fig. 14b. The nitrided samples
Fig. 13. (a) Load-displacement curves of the different samples under nanoindentation; (b) Nanohardness and elastic modulus of the different samples; (c) H/E and H3/E⁎2 ratios derived from the nanohardness and elastic moduli of the different samples.
distinct layers can be found in the Fig. 11, the bottom nitrided layer, followed by the Cr bonding layer, and then the outer CrN layer. The Cr adhesive layer consists of coarse columnar grains and is well-bonded to the nitrided layer. The CrN layer begins with a transition layer with fine columnar grains that become coarser toward the outer surface of the coating. The magnified HRTEM image in Fig. 12 displays two kinds of 361
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Fig. 14. Surface microhardness (a) and hardness depth profile (b) of the different samples.
Fig. 15. The scratch track morphologies and acoustic emission of the scratch tests for the sample with only the coating.
34 and 51 N, respectively. For the sample with only the coating, an abrupt amplitude increase in the acoustic emission signal exists at a normal load of approximately 34 N, followed by a series of continuous acoustic emission peaks. As the applied load increases further, the scratch marks continue to widen, and the spalling of the coating becomes more serious accordingly (Fig. 15b). Fig. 15c shows large chipped pieces and severely deformed substrate in the exposed area at the maximum load of 60 N. An analysis of the scratch track on the duplex sample (Fig. 16) shows that the width of the scratch is smaller than that of the untreated substrate. It found that the degree of plastic deformation on the ends of the scratch is less than that of the only-coating sample without
show high hardness values, which remain approximately constant through the diffusion layer and then gently decrease to the matrix values. Obviously, the thickness of the diffusion layer was approximately 160 μm for the nitrided samples. The hardness of the duplex-treated sample decreases gradually to the value of the substrate through the diffusion layer, while the hardness rapidly drops to the substrate hardness for the only-coating sample. Figs. 15 and 16 show a scratch track and the relationship between the scratch distance and the scratch force obtained for the multilayer coating on substrates without and with nitriding treatment, respectively. The critical loads for the multilayer coating on the substrates without and with nitriding treatment are estimated to be approximately 362
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Fig. 16. The scratch track morphologies and acoustic emission of scratch tests for the duplex sample (PN + coating).
1. After PN treatment, the α-ferrite peaks disappear, while the ε-Fe2–3N and γ′-Fe4N nitrides form in the nitride layer. It was shown that PN gives rise to a 160 μm-modified strengthening layer due to the diffusion of nitrogen, leading to a surface hardness of 900 HV0.05 compared to 350 HV0.05 for the untreated one. 2. The XRD and TEM results reveal that the CrN/CrTiAlSiN/WCrTiAlN multilayer coatings consist a mixture of W2N, CrN, TiN and AlN phases. An ultrahigh hardness of 47.9 GPa and the highest H3/E⁎2 ratio of 0.609 are obtained. These findings are mainly attributed to the formation of W2N phases and a good gradient distribution in the multilayer coating. 3. The duplex-treated sample is more effective in improving the adhesion than the only-coating sample. This is possibly the nitrided layer with a gradient-hardness support as an interlayer formed between the multilayer coating and the substrate.
nitriding. A smaller width of the scratch demonstrates a potentially higher mechanical load bearing capacity of the system [39]. Some chipping occurred on the edges of the scratch tracks, and tensile macrocracks were built on the scratch track (Fig. 16b). While the final load is 60 N (Fig. 16c), the nitrided layer is not exposed, and the coating is not completely detached from the nitrided layer for the duplex sample. An adhesion enhancement in the multilayer coatings on nitrided AISI 4140 steel is owing to a gradual change in the hardness from the multilayer coating to the substrate. It should be noted that the relatively low critical loads for the onlycoating sample is related to the soft substrate. The coating was too stiff to buckle leading to the formation of compressive shear cracks at the weak interface of the coating and substrate. Besides, there are multitudinous cracks perpendicular to the scratch direction due to the plastic deformation of the substrate. However, the duplex sample will exhibit better adhesion, due to the increased deformation resistance and load bearing capacity of the substrates during nitriding process. This conclusion is consistent with those reported in the literatures [62–64], in which the plasma nitriding pre-treatment was used to increase the adhesion of the coatings on the different substrates.
Acknowledgements The project was supported by the National Key Basic Research Program of China (973) (2014CB046404), the Shandong Provincial Natural Science Foundation, China (ZR2018MEE016), the Shandong Provincial Key Research and Development Plan, China (2017GGX20140), and the National Natural Science Foundation of China (51301149).
4. Conclusions A superlattice structured CrN/CrTiAlSiN/WCrTiAlN multilayer coating was successfully prepared on un-nitrided and nitrided AISI 4140 steel by a vacuum cathodic arc evaporation system. The experimental results show that formation of W2N phases can further improve the structural integrity as well as the mechanical properties of the CrNbased multilayer coatings. The conclusions of this study can be summarized as follows:
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Microstructure, mechanical and adhesive properties of CrN/CrTiAlSiN/ WCrTiAlN multilayer coatings deposited on nitrided AISI 4140 steel ⁎
Yang Lia,b, Zhongli Liua, Jianbin Luob, Shangzhou Zhanga, , Jianxun Qiua, Yongyong Heb, a b
T
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School of Nuclear Equipment and Nuclear Engineering, Yantai University, Yantai, 264005, PR China State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coating Multilayers Nitriding Nanoindentation Scratch test
A duplex treatment of plasma nitriding (PN), followed by the deposition of a CrN/CrTiAlSiN/WCrTiAlN multilayer coating, was performed on AISI 4140 steel. Untreated, plasma-nitrided, only-coated (just the coating) and duplex-treated (PN + coating) AISI 4140 steel were microstructurally and mechanically characterized. The nitrided compound layer, consisting of mainly ε-Fe2-3N and γ′-Fe4N phases, was approximately 8.0 μm thick. The total thickness of the multilayer coating, which consisted of a top W-Cr-Ti-Al-N multilayer, a periodic Cr-Ti-Al-SiN interlayer, and a CrN bottom layer, was 2.3 μm. The CrN/CrTiAlSiN/WCrTiAlN multilayered coatings possessed a super-hardness of 47.9 GPa and the highest elastic modulus of 424.8 GPa in the all samples. The microhardness (2200 HV0.05) of the duplex-treated sample was significantly higher than that of the only-coated (740 HV0.05) and nitrided (900 HV0.05) samples. The scratch tests revealed that the adhesion strength of the duplex-treated sample increased significantly due to a gradient-hardness support of the coating-nitrided layer interface.
1. Introduction Transition metal nitride coatings, such as CrN and TiN, have excellent mechanical, tribological and corrosion resistance, which can improve the surface properties of the workpiece and prolong the service life of the workpiece [1–7]. The working conditions such as heavy loads, high temperature, wear and corrosion are complicated and harsh and require a workpiece surface with high hardness, high-strength and excellent wear-resistant and anti-fatigue properties. However, with an increase in the hardness and strength, the toughness and adhesion will inevitably decrease, resulting in the deterioration of the wear resistance, corrosion resistance and fatigue resistance [8,9]. Recently, compared with single-layer coatings, nanoscale multilayer coatings have been widely studied because of their stronger performance [10–16]. It is a specific form of multilayered coating materials that alternately grow in thin layers of several nanometers to several tens of nanometers and maintains strictly periodicity [17–21]. It suggested that the multilayer coatings have lower residual stresses mainly because of stress relaxation at the interfaces [22]. Specifically, the strength and ductility of transition nitride multilayers are attributed to the modulation periods and choice of the constituent layers [23–25]. Nanocrystalline (Ni) thin film exhibits poor thermo-mechanical properties due to its unstable microstructure at elevated temperature.
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Feng and Li et al. [26] endowed a new approach to solve above issue via addition of nano-multilayers and incorporation of W for nanocrystalline Ni-based films, to provide novel Ni/Ni3AlW nano-composite multilayered structure with high hardness and good thermal stability. In terms of Ni/Ni3Al multilayer, the hardness reaches the small value of 4.76 GPa at 800 °C. On the contrary, the outstanding hardness (15.6 GPa) of the 12.5 at.% W doped Ni/Ni3Al-W multilayer can be attributed to combination of precipitation and residual layer interface strengthening via the Orowan mechanisms. The introduction of W into the CrN coatings increased their hardness values, which was attributed to the Hall–Petch effect from the refinement of the crystal size and the formation of a highly covalently bonded W2N phase in the CrN coatings [27,28]. Yan-Zuo et al. [29] have reported that the improvement in tribomechanical properties of a CrN/WN multilayer coating could be attributed to the existence of its superlattice structure. In addition, some research has indicated the benefits of W2N as an incorporated sublayer in a multilayer architecture because of its unique characteristic, such as super-hardness and chemical inertness [30,31]. As to the hardness, the hard coatings are usually divided into two groups: (1) hard coatings having hardness < 40 GPa, and (2) superhard coatings having hardness > 40 GPa [32]. However, these coatings have a higher hardness and stiffness than the soft substrates, and support of the coating is a concern in high stress contact [33]. The
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Y. He).
https://doi.org/10.1016/j.matchar.2018.11.017 Received 13 October 2018; Received in revised form 14 November 2018; Accepted 14 November 2018 Available online 15 November 2018 1044-5803/ © 2018 Elsevier Inc. All rights reserved.
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Fig. 1. (a) Schematic representation of the PN sample; (b) Schematic representation of the only-coating sample; (c) Schematic representation of the duplex PN + coating sample.
Fig. 2. X-ray diffraction patterns of the different samples. (a) Untreated; (b) PN; (c) only-coating; (d) PN + Coating.
provides a hard and brittle coating with strong support but also reduces the hardness gradient at the interface between the coating and substrate [35,39,40]. A smoother hardness gradient reduces the stress in the coating during loading, and the stress distribution at the interface is more even, and thus, the possibility of damaging or cracking the coating is reduced. A smoother hardness gradient reduces the stress in the coating during loading, and the stress distribution at the interface is more even, and thus, the possibility of damaging or cracking the coating is reduced [41].
tribological performance of these coatings is limited, in many cases by plastic deformation of the substrate, which can result in the failure of the coating [33,34]. The duplex-treated system, which combines plasma nitriding (PN) with a hard coating, is effective in improving the adhesion of the coating-substrate interface and the wear resistance [35–38]. The introduction of a nitrided layer not only significantly improves the load ability of the substrate surface and the coating–substrate bonding force but also improves the surface fatigue strength, thermal shock and chemical resistance. This layer not only
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Fig. 3. Cross-sectional SEM micrographs and EDS mapping of the nitrided layer on AISI 4140 steel by PN treatment.
Fig. 4. Cross-sectional SEM micrographs and EDS mapping of the only coating deposited on AISI 4140 steel.
acetone. The nitriding process was carried out in an LDMC-20 nitriding furnace (LDMC-20F, WHRCLS, China) [42,43]. The nitriding was performed at 525 °C for 6 h. The working pressure was maintained at 400 Pa using NH3 gas. Depositions were conducted in an industrial cathodic arc evaporation system (HCCE-280, Aomet Nano Technology Ltd., China). The CrN/CrTiAlSiN/WCrTiAlN coatings were deposited onto the polished and nitrided AISI 4140 steel samples. To create the multilayers, 3 tungsten, 3 chromium,1 Al-Ti, and 1 Al-Ti-Si targets were used. Each target had a power source supply with electric currents, depending on the desired deposition rate. The samples were sputter cleaned in an argon plasma with an overall pressure of 2 Pa, and the temperature reached 350 °C. After achieving a base pressure of 10−4 Pa, the deposition pressure was fixed at 3 × 10−1 Pa. First, a very thin Cr interlayer was deposited onto the substrate. Cr was used as an intermediate adhesion layer to improve the adhesion of the coating on the substrate. Then, N2 with a flow speed of 100 sccm was added as a reactive gas. The CrN layer was deposited as the second interlayer. Afterwards, the selection of the sputtering targets (Cr, Al-Ti, and Al-Ti-
In this study, AISI 4140 steel samples were plasma nitrided, and subsequently, a CrN/CrTiAlSiN/WCrTiAlN multilayer coating was deposited on both the untreated and nitrided surfaces. The structure and composition of the different samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM). The mechanical properties were also measured by microhardness and nanoindentation testing. The scratch adhesion strength of the multilayer coating on diffusion-treated substrates was discussed.
2. Experimental The material used in this study is commercial AISI 4140 steel (composition: 0.040 wt% C, ≤0.05 wt% Si, 0.129 wt% Mn, 0.006 wt% P, 0.007 wt% S). Disc samples approximately 6.0 mm thick were cut from a 20 mm diameter rod and wet ground with emery paper from a 240 grit down to a 2000 grit. Then, the samples were mirror polished with a diamond suspension (size 3.5 μm) to produce a fine surface finish. The surfaces of the samples were cleaned ultrasonically in 355
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Fig. 5. Cross-sectional SEM micrographs and EDS mapping of the duplex layers on the nitrided AISI 4140 steel by the duplex PN + coating treatment.
Fig. 6. (a) Cross-sectional TEM bright-field micrograph of the multilayer coating; (b) TEM image and (c) SAED pattern of the W-Cr-Al-Ti-N coating; (d) TEM image and (e) SAED pattern of the Cr-Al-Ti-Si-N coating.
involves two sequential procedures (nitriding and coating). Fig. 1c describes the composition and structures of the duplex layers, where the graded coatings are deposited on the nitrided AISI 4140 steel surface. The fundamental concepts of surface composite design relate to constructing the composite layer in a specified order so that the periodic pairs of the nitrided layer and multilayer coating function effectively [17]. The strengthening mechanisms in adherence can be related to the diffusion layer between the coating and the substrate [44]. On the basis of this concept, a composite structure, consisting of a gradienthardness support nitrided layer and a hard and high-strength multilayer coating, is designed. The crystalline condition of the nitrided layer and the coating was characterized by XRD with Cu Ka radiation (λ = 0.15406 nm) at 40 kV and 40 mA (D8-Advance, Bruker, Germany). The analysis of each
Si) depended on the category of the CrTiAlSiN layers. The thickness of the individual layers was controlled by adjusting the deposition time by power supplies of the different targets. Here, during the deposition, the bias current was also monitored at the rotation table and the substrate samples. The individual layer thickness of WCrTiAlN and CrTiAlN layer was controlled by alternately switching the time for shutters with the aid of a programmable logic control to form the top WCrTiAlN multilayers. Fig. 1 provides the schematic showing the detailed structures of the individual layers. PN involves the introduction of nitrogen atoms into the component surface to produce a compound layer and a diffusion layer on the surface of AISI 4140 steel, as schematically shown in Fig. 1a. Fig. 1b provides the CrN/CrTiAlSiN/WCrTiAlN-graded coating compositions and the schematic. Furthermore, duplex surface treatment 356
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Fig. 7. (a) Cross-sectional TEM image for the upper part of coating; (b) EDS mapping results on the purple lined area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sample was conducted within the range of 30° < 2θ < 90° with an increment of 0.05°/step and a time per step of 2 s. The morphology was observed by field emission scanning electron microscopy (FESEM, Hitachi-S4800, Japan). The elemental composition of the coatings was detected by energy dispersive X-ray analysis (EDX). The morphologies and microstructures of the different samples were investigated using TEM (Tecnai G20, FEI, USA). The characterization of the different precipitates was performed by means of bright-field TEM, HRTEM, selected-area electron diffraction (SAED) patterns, and EDX. The Vickers microhardness was measured using a hardness tester (Struers Duramin-A300, Denmark) under a load of 50 g for 15 s.
Table 1 Chemical compositions (at.%) of the representative elements of the analysis points (1 to 6) from the Fig. 6. Point
N
W
Cr
Al
Ti
1 2 3 4 5 6
43.45 44.08 44.38 44.58 46.46 47.67
22.64 19.41 21.16 18.36 14.36 –
16.68 17.98 17.34 18.80 20.24 34.18
11.70 12.28 11.34 11.95 12.52 10.53
5.53 6.25 5.78 6.31 6.42 5.94
Si – – – – 1.68
Fig. 8. (a) EDX line scan of the W-Cr-Al-Ti-N coating from the lines I in the Fig. 6a. (b) EDX line scan of the Cr-Al-Ti-Si-N coating from the lines II in the Fig. 6a. 357
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Fig. 9. HRTEM micrograph of the W-Cr-Al-Ti-N and Cr-W-Al-Ti-N nanolayers.
Fig. 10. HRTEM micrograph of the Cr-Al-Ti-Si-N nanolayers.
and α(211) are found at 45°, 66° and 82°, respectively (Fig. 2a). Fig. 3 shows the SEM cross-sectional images of the nitrided layers of the PN sample. An 8.0–9.0 μm compound layer was formed on the surface, and the EDS results show a N-rich surface. The compound layers consisted of hexagonal close packed (hcp) ε-Fe2–3N and face centered cubic (fcc) γ′-Fe4N nitride phases (Fig. 2b). Fig. 4 shows the cross-sectional morphology of the only-coating sample. The total thickness of the multilayer coating was 2.3 μm, which consisted of a top W-Cr-Ti-Al-N multilayer (thickness = 0.7 μm) and a periodic Cr-Ti-Al-Si-N interlayer (thickness = 1.6 μm). From Fig. 2c, the XRD spectra of the only-coating sample indicates that the multilayer coating mainly contains the W2N, CrN, TiN and AlN phases. The shift of the CrN and TiN diffraction patterns toward higher 2θ values (compared with the Joint Committee on Powder Diffraction Standards (JCPDS) data) is in relation to the compressive residual stress characteristics for those multilayer systems. In addition, the peaks of the Fe substrate are detected due to the thin thickness of the coating. The
Nanoindentation tests were performed with an instrumented indenter (Nano-indenter Tester NHT2, CSM, Switzerland) to determine the indentation hardness and the Young's modulus of the different samples. The adhesion behaviors were evaluated using an automatic scratch instrument (WS-2005, LICP, China) equipped with acoustic emission. The only-coating and duplex samples were scratched at the initial load of 0.05 N, which was gradually increased to a final load of 60 N with the increasing rate of 60 N/min. The scratch length was 4 mm. The normal load at which the failure occurs, the critical load, was determined by SEM after each scratch test. 3. Results and Discussion 3.1. Structure of the Nitrided Layer and Multilayer Coatings Fig. 2 shows the XRD patterns of the different samples. For untreated AISI 4140 steel, three diffraction ferrite peaks of α(110), α(200) 358
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Fig. 11. TEM image of the cross-sectional duplex layers and its SAED patterns at different positions.
further indicate that the Cr, Al and Ti atoms are uniformly distributed in the inner layer of the coating. The presence of N in all the coatings is also observed. The duplex sample shows that a multilayer coating is deposited on the surface of the nitrided compound layer (Fig. 5). The mapping covered the entire coated layer and the partially nitrided layer. The duplex-treated sample shows the distribution of the N, W, Cr, Ni, Al and Fe elements from the surface to the nitride layer. For the duplex sample, the W2N, CrN, TiN and AlN phases are predominant, and small peaks of ε-Fe2–3N and γ′-Fe4N are also detected (Fig. 2d). The TEM microstructure on a cross-section of the multilayered coating is shown in Fig. 6a. A nanolayered architecture with sharp interfaces is displayed. This image shows the typical microstructure of the sample with only a coating, which is a multilayer coating formed by a 0.7 μm thick WCrTiAlN layer and 1.6 μm thick CrTiAlSiN layer. The multilayered coating can be divided into two distinct regions. The first deposited bottom layers are columnar structure, while the latter grown top layers are flat region. The ring pattern of the top WCrTiAlN layer (as shown in Fig. 6c) is also identified as the (111), (200), (220) and (311) directions of the NaCl type structure, which is consistent with the XRD results of the multilayer coating (Fig. 2c). The micrograph in Fig. 6b and c shows that the WCrTiAlN layer is continuous and dense with nanocrystalline grains, which is indicating that this layer exhibits a nanocrystalline structure. Fig. 6b is the cross-section TEM image of the nano-multilayers, which shows a dark strip and a light strip. The EDS mapping analyses (Fig. 7) were performed over the WCrTiAlN multilayer coating (shown as red rectangular area in Fig. 6a), and the results are also presented in Table 1. According to the EDS mapping results shown in Fig. 7b, the outer layer is composed of W, Cr, Al, Ti and N, while the inner layer
Table 2 Chemical compositions (at.%) of representative elements of the analysis points (7 to 10) from the Fig. 11. Point
N
W
Cr
Al
Ti
Si
Fe
7 8 9 10
47.98 51.32 11.93 11.89
– – – –
42.95 46.19 2.22 2.68
2.46 – – –
1.42 – – –
0.25 – – 0.77
– 0.38 75.12 77.78
diffraction pattern of the coating presents four diffraction peaks centered at approximately 37.1°, 42.8°, 62.2° and 74.3°, which can be assigned to the fcc W2N (JCPDS card, no. 25-1257) (111), (200), (220) and (311) planes, respectively. These peaks shift to toward lower angle in W2N, which may be related to the increase in the in-plane residual stress [30]. The W2N phase has the lowest formation enthalpy with nitrogen compared to the other W2N phases. The enthalpy of formation of WN = −15 kJ/mol, where W2N = −22 kJ/mol [45]. Additionally, the AlN phase is represented by peaks at approximately 43.3° and 74.0°. The diffraction peaks located at 2θ of 37.0°, 43.0° and 74.5° are identified as the (111), (200) and (311) planes of the fcc CrN/TiN structure (JCPDS card no. 11-0065 and 38-1420). Compared with the database of the CrN phase (JCPDS card no. 11-0065), these peaks are slightly shifted to a higher diffraction angle, indicating a decrease in the lattice parameters as the Cr atoms were replaced by the smaller Al atom [46]. The intensity of the (200) diffraction peak mainly depends on the magnitude of the composition modulation in the superlattice structure [30,47]. To confirm the distributed elemental composition, SEM-EDS mapping examinations were carried out, as shown in Fig. 4. It found that the W is mainly distributed in the outermost layer. The results 359
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Fig. 12. HRTEM micrograph showing the interfaces between the Cr coating and nitrided layer.
layer (Fig. 10) shows that the lattice fringes continuously cross the layer interfaces, and the spacing of the fringes is maintained at 0.208 nm. These fringes are attributed to the (200) plane of the CrN, AlN and TiN crystal lattices. These results reveal the epitaxial and coordinated growth of these nano-multilayers. The larger column size in the CrTiAlSiN layer corresponds to the discrete diffraction spots in the SAED patterns, thus confirming the coarser grain size in this layer. The increased number of continuous diffraction rings in the SAED pattern of CrTiAlSiN compared with that of the WCrTiAlN coatings imply the finer grain size of the CrTiAlSiN coatings. The TEM images and corresponding SAED pattern of the nitrided layer and the coatings are demonstrated in Fig. 11. The results show both a bright-field TEM image of a cross-section of the coating and SAED patterns along the growth direction. Clearly, there are three kinds of layers: the nitrided layer, the Cr layer and the CrN layer. The EDS point analyses of the different regions are presented in Table 2. Point 7 differs from other points (6,8–10) by revealing relatively high concentrations of N and Cr and relatively low concentrations of Al, Ti and Si. In contrast, point 8 is similar to the initial thin Cr layer, where only high concentrations of Cr and N are found, which indicate the presence of a CrN layer. The SAED pattern shown in Fig. 11b is indexed according to the fcc CrN structure with (111), (200), (220) and (311) reflections. The presence of a significant amount of Fe and N and a low concentration of Cr at point 9 indicates that a nitrided layer remains on the AISI 4140 steel. An analysis of the SAED pattern (Fig. 11c) of the nitrided layer reveals the hcp ε-Fe2–3N phase, which is consistent with the d-spacing from the XRD pattern of the PN sample (Fig. 2b). The coatings were deposited as an initial thin Cr layer (∼150 nm), followed by a transition zone around the Cr-N monolithic layer. Three
does not contain W. Fig. 8a further shows the EDS intensities of W, Cr, Al, Ti and N along a scan line, and the results indicate that an increase in the W content corresponds to a decrease in the Cr, Al, Ti and N values. The EDS line I scan results show a higher content of W in the bright layer and the lower content in the dark layer. The EDS point analysis also proves that W is different in the dark and light strips. The chemical composition of the dark strip (point 1) identified by EDS (at.%) is 43.45% N, 22.64% W, 16.68%Cr, Al 11.70%, and 5.53% Ti. The chemical composition of the light strip (point 2) identified by EDS (at.%) is 44.08% N, 19.41% W, 17.98% Cr, 12.28% Al, and 6.25% Ti. In Fig. 9, a lattice-resolution HRTEM image of the top coating is marked by a red square in Fig. 6a. It can be seen that the dark strip in the WCrTiAlN coating is approximately 12.0 nm. Three different lattice fringes with a lattice spacing of approximately 0.232 nm, 0.208 nm, and 0.142 nm are detected. Those lattice fringes correspond to the (111), (200), (220) and (311) orientations of W2N (JCPDS card no. 25-1257), CrN (111) (JCPDS card no. 11–0065), TiN (JCPDS card no. 38-1420), and AlN (JCPDS card no. 46-1200), respectively. These results are in good agreement with the results obtained from its corresponding XRD and SAED patterns. The SAED patterns (Fig. 6c) show continuous and sharpening diffraction rings, which indicated a very fine grain structure. Fig. 6d and e show the cross-sectional TEM micrographs and SAED patterns of the CrTiAlSiN layer. The fcc (111), (200), (220) and (311) reflections of the CrN, AlN and TiN phases are identified, which indicates that the CrTiAlSiN layer has a fcc crystal structure. The distribution of the elements in the CrTiAlSiN layer is shown by EDS elemental line (II) in Fig. 8b. The HRTEM micrograph of the CrTiAlSiN
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lattice spacings. The fringe with a lattice spacing of approximately 0.204 nm corresponds to the (200) plane of the Cr layer, and the other fringe with a lattice spacing of approximately 0.219 nm is ascribed to the (111) plane of the hcp Fe2−3N phase. 3.2. Mechanical Properties Typical examples of the load-displacement curves corresponding to different samples, i.e., the AISI 4140 steel matrix, the nitrided layer and the only-coated sample, are depicted in Fig. 13a. These curves correspond to the tests with a maximum penetration depth of approximately 238 nm, then of ~197 nm and finally those with the depth of ~129 nm. The residual depths of these curves are approximately 190, 127 and 53 nm. The maximum contact depth of the multilayer coating of the only-coated sample is ~0.13 μm, which is much smaller than 2.3 μm (10% of the coating thickness, in the Fig. 4), indicating that the contribution to the deformation behavior from the underlying substrate is negligible [48,49]. Therefore, the indentation data accurately reflect the mechanical behavior of the multilayer coatings. Nano-hardness and elastic modulus values of the different samples are shown in Fig. 13b. The values of the nano-hardness and elastic modulus values for AISI 4140 steel matrix are 6.8 GPa and 227 GPa, respectively. Compared with the matrix, the nitrided layer has both a higher hardness (12.5 GPa) and elastic modulus (230 GPa). The increase in the hardness from 6.8 GPa to 12.5 GPa is due to the interstitial diffusion of nitrogen in α-Fe [50,51]. The multilayer coating has a maximum nano-hardness and elastic modulus of 47.9 GPa, and 424.8 GPa, respectively. According to previous studies [9,52], the hardness distribution of various CrN-based coatings is within the range of 20-35GPa. The coating with hardness above 40 GPa is classified as superhard. It is widely accepted that a greater hardness to elastic modulus value, and thus a greater plastic deformation resistance ration, H3/E*2, indicates a better resistance to plastic deformation and to abrasion damage by localized plastic deformation against a blunt and rigid contact [53–55]. The H/E* and H3/E*2 values of the different samples are shown in Fig. 13c. The H/E* values are 0.03, 0.06, and 0.124 for the untreated, nitrided, and multilayers samples, respectively. J. Musil [55] concluded that a new task in the development of advanced hard nanocomposite coatings with enhanced toughness is to produce coatings with a value that satisfies high H/E* ≥ 0.1. The nitrided layer exhibits an H3/E*2 value higher than 0.046, whereas the untreated matrix demonstrates a low H3/E*2 value of 0.006. The H3/E*2 ratio of approximately 0.609 obtained for the CrN/CrTiAlSiN/WCrTiAlN multilayer is substantially higher than those reported for common hard coatings such as CrN (~0.18) [56], WN (~0.32) [56], CrAlN (0.25) [57], CrTiAlN (~0.31) [58], and W-Si–N (~0.51) [59]. This finding means that the CrN/CrTiAlSiN/WCrTiAlN multilayer coating is likely to exhibit a higher threshold load for the initiation of plastic deformation than the CrN-based coatings without W. The improved toughness in the superlattice CrN/CrTiAlSiN/WCrTiAlN multilayer coatings can be explained as crack deflection at the interfaces and the dissipation of crack energy by plastic deformation in the crack tip by introducing a large number of interfaces with different elastic moduli [8,60]. Microhardness measurements were carried out on the surface of the different samples; the surface microhardness values are reported in Fig. 14a. The nitriding treatments facilitate an increase in the surface hardness (900 HV0.05), which is approximately 2.6 times greater than that of the untreated matrix (350 HV0.05). The high hardness of the nitrided layer is attributed to changes in the composition of the εFe2−3N and γ′-Fe4N nitrides [61]. The hardness level of only the coating is nearly 2.1 times higher than that of the untreated matrix (740 HV0.05). The highest microhardness value (approximately 2200 HV0.05) was obtained in the case of the duplex treatment. The microhardness profiles of the coating deposited on polished and nitrided AISI 4140 steels are shown in Fig. 14b. The nitrided samples
Fig. 13. (a) Load-displacement curves of the different samples under nanoindentation; (b) Nanohardness and elastic modulus of the different samples; (c) H/E and H3/E⁎2 ratios derived from the nanohardness and elastic moduli of the different samples.
distinct layers can be found in the Fig. 11, the bottom nitrided layer, followed by the Cr bonding layer, and then the outer CrN layer. The Cr adhesive layer consists of coarse columnar grains and is well-bonded to the nitrided layer. The CrN layer begins with a transition layer with fine columnar grains that become coarser toward the outer surface of the coating. The magnified HRTEM image in Fig. 12 displays two kinds of 361
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Fig. 14. Surface microhardness (a) and hardness depth profile (b) of the different samples.
Fig. 15. The scratch track morphologies and acoustic emission of the scratch tests for the sample with only the coating.
34 and 51 N, respectively. For the sample with only the coating, an abrupt amplitude increase in the acoustic emission signal exists at a normal load of approximately 34 N, followed by a series of continuous acoustic emission peaks. As the applied load increases further, the scratch marks continue to widen, and the spalling of the coating becomes more serious accordingly (Fig. 15b). Fig. 15c shows large chipped pieces and severely deformed substrate in the exposed area at the maximum load of 60 N. An analysis of the scratch track on the duplex sample (Fig. 16) shows that the width of the scratch is smaller than that of the untreated substrate. It found that the degree of plastic deformation on the ends of the scratch is less than that of the only-coating sample without
show high hardness values, which remain approximately constant through the diffusion layer and then gently decrease to the matrix values. Obviously, the thickness of the diffusion layer was approximately 160 μm for the nitrided samples. The hardness of the duplex-treated sample decreases gradually to the value of the substrate through the diffusion layer, while the hardness rapidly drops to the substrate hardness for the only-coating sample. Figs. 15 and 16 show a scratch track and the relationship between the scratch distance and the scratch force obtained for the multilayer coating on substrates without and with nitriding treatment, respectively. The critical loads for the multilayer coating on the substrates without and with nitriding treatment are estimated to be approximately 362
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Fig. 16. The scratch track morphologies and acoustic emission of scratch tests for the duplex sample (PN + coating).
1. After PN treatment, the α-ferrite peaks disappear, while the ε-Fe2–3N and γ′-Fe4N nitrides form in the nitride layer. It was shown that PN gives rise to a 160 μm-modified strengthening layer due to the diffusion of nitrogen, leading to a surface hardness of 900 HV0.05 compared to 350 HV0.05 for the untreated one. 2. The XRD and TEM results reveal that the CrN/CrTiAlSiN/WCrTiAlN multilayer coatings consist a mixture of W2N, CrN, TiN and AlN phases. An ultrahigh hardness of 47.9 GPa and the highest H3/E⁎2 ratio of 0.609 are obtained. These findings are mainly attributed to the formation of W2N phases and a good gradient distribution in the multilayer coating. 3. The duplex-treated sample is more effective in improving the adhesion than the only-coating sample. This is possibly the nitrided layer with a gradient-hardness support as an interlayer formed between the multilayer coating and the substrate.
nitriding. A smaller width of the scratch demonstrates a potentially higher mechanical load bearing capacity of the system [39]. Some chipping occurred on the edges of the scratch tracks, and tensile macrocracks were built on the scratch track (Fig. 16b). While the final load is 60 N (Fig. 16c), the nitrided layer is not exposed, and the coating is not completely detached from the nitrided layer for the duplex sample. An adhesion enhancement in the multilayer coatings on nitrided AISI 4140 steel is owing to a gradual change in the hardness from the multilayer coating to the substrate. It should be noted that the relatively low critical loads for the onlycoating sample is related to the soft substrate. The coating was too stiff to buckle leading to the formation of compressive shear cracks at the weak interface of the coating and substrate. Besides, there are multitudinous cracks perpendicular to the scratch direction due to the plastic deformation of the substrate. However, the duplex sample will exhibit better adhesion, due to the increased deformation resistance and load bearing capacity of the substrates during nitriding process. This conclusion is consistent with those reported in the literatures [62–64], in which the plasma nitriding pre-treatment was used to increase the adhesion of the coatings on the different substrates.
Acknowledgements The project was supported by the National Key Basic Research Program of China (973) (2014CB046404), the Shandong Provincial Natural Science Foundation, China (ZR2018MEE016), the Shandong Provincial Key Research and Development Plan, China (2017GGX20140), and the National Natural Science Foundation of China (51301149).
4. Conclusions A superlattice structured CrN/CrTiAlSiN/WCrTiAlN multilayer coating was successfully prepared on un-nitrided and nitrided AISI 4140 steel by a vacuum cathodic arc evaporation system. The experimental results show that formation of W2N phases can further improve the structural integrity as well as the mechanical properties of the CrNbased multilayer coatings. The conclusions of this study can be summarized as follows:
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