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Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering
Journal Pre-proofs Full Length Article Hard yet tough CrN/Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering Y.X. Ou, X.P. Ouyang, B. Liao, X. Zhang, S. Zhang PII: DOI: Reference:
S0169-4332(19)32984-8 https://doi.org/10.1016/j.apsusc.2019.144168 APSUSC 144168
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
4 July 2019 18 September 2019 23 September 2019
Please cite this article as: Y.X. Ou, X.P. Ouyang, B. Liao, X. Zhang, S. Zhang, Hard yet tough CrN/Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144168
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Hard yet tough CrN/Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering Y.X. Ou a, X.P. Ouyang b*, B. Liao c, X. Zhang c, S. Zhang d* Beijing Radiation Center, Beijing Academy of Science and Technology, Beijing 100875, China Shaanxi Engineering Research Center of Controllable Neutron Source, School of Science, Xijing University, Xi'an, 710123, China c Key Laboratory of Beam technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China d Faculty of Materials and Energy, Southwest University, Chongqing 400715, China a
b
Abstract Hard yet tough coatings with simultaneously high hardness and toughness represent a new class of high-performance coatings, which are vital for tribological applications due to the enhanced surface/interface properties. In this work, CrN/Si3N4 multilayer coatings with various Si content were reactively deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering. The coatings exhibit smooth surface morphology, well-defined nc-CrN/a-Si3N4 interfaces, uniform thickness and highly dense architecture. Microstructure evolution is induced by Si content changing from scaly nanocolumnar to densely packed fibrous grains to tapered crystallites. These features allow the achievement of the increased properties in terms of high hardness of 30.6 GPa, H/E* of 0.099, H3/E*2 of 0.387, elastic recover We of 58.5% and fracture toughness KIC of 3.69 MPa∙m1/2. Hard yet tough CrN/Si3N4 multilayer coatings show excellent tribological properties by enhanced cooperative deformability through energy dissipation and stress relaxation at well-arranged nc-CrN/a-Si3N4 interfaces to reduce crack initiation and propagation, and adhesion failure, thanks to the enhanced H, H/E*, H3/E*2, We and KIC. Meanwhile, under limited normal load conditions, uniform tribolayers (Cr2O3, FeO(OH), Fe2O3 and Fe3O4) caused by tribochemical reaction on the worn surface also act to make contributions towards reducing friction and wear as a lubricant. Keywords: CrN/Si3N4 multilayer coatings; Deep oscillation magnetron sputtering; Hard yet tough; Tribological behaviors
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* e-mail: [email protected] (X.P. Ouyang), [email protected] (S. Zhang)
1. Introduction All along, a combination of hardness and toughness is a highly desired feature for ceramic coatings [1-4]. Much has been done over the years in one way or another. A potential route to achieve this aim is by the optimized coating design and advanced deposition techniques to obtain nanoscale, multilayer and nanocomposite coatings [5-8]. It is found that nanostructured multilayer coatings composed by transition-metal nitrides have attracted enormous research interests in scientific research and industrial applications due to their high hardness and wear and corrosion resistance [9,10]. In many cases, remarkably enhanced performance is always achieved by superlattice coatings, thanks to the stability of coherent interface structure [11-13]. Hence, heterostructured multilayer or nanocomposite coatings with a strong thermodynamic stability are expected to develop by the combination of nanocrystalline and amorphous phases [14,15]. As suggested by Veprek et al[16], nc-TiN/a-Si3N4 nanocomposite coatings have a potential to achieve superhardness up to 100 GPa. However, hardness-toughness ratio is indeed an open problem to handle. Because stable nanocomposite structure is formed by selforganization resulted from a strong, thermodynamically driven, and spinodal phase segregation. As for heterostructured multilayer coatings, i.e., nc-CrN/a-Si3N4 multilayer coatings, their desired performance is always achieved by the precise control of modulated bilayer period (Λ) [17,18]. In addition, CrN/Si3N4 multilayer coatings combine the advantages of single CrN and Si3N4 coatings in terms of low internal stress, and good wear and corrosion resistance. The enhanced damping capacity of hardness and toughness of multilayer coatings may be associated with interface structure, grain boundary sliding and energy dissipation mechanism localized at interfaces [17-19]. The earlier studies are mainly devoted to explore the origin of hardening mechanism of hard coatings in order to achieve superhardness (>40 GPa) as much as possible because hardness is considered as a unique
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criterion according to classical wear theory [1,6,20,21]. However, friction and wear response of hard ceramic coatings are usually complex, depending on many factors, atmosphere, counterpart materials, sample conditions, wear parameters, etc [22,23]. Therefore, hardness does not always correlate with tribological properties of ceramic coatings. Besides, superhard coatings are prone to brittle fracture in friction and wear process due to low toughness. The improved toughness of hard coatings is beneficial for the increased resistance against crack initiation and propagation deriving from stress accumulation in the vicinity of structure imperfections [2,3]. Hence, coatings with high hardness and high toughness are effectively resistant to plastic deformation, chipping, flaking, and even catastrophic failure in friction and wear applications. The characterization of hardness of hard coatings is easily carried out using micro/nanoindentation[24], but as for the measurement of toughness, it is still an open problem due to the sensitive effect of substrates [25,26]. It is known that friction and wear properties of hard coatings are influenced by hardness and toughness as well as hardness and effective Young’s modulus ratios (H/E* and H3/E*2) [6,27,28]. But beyond those, structure evolution of the coatings, chemical reactions on sliding contact surface, grain boundary sliding and interface behavior during dry sliding wear process are still lack of systematic and comprehensive studies. It is also lack of advanced knowledge of correlations of crack initiation and propagation at heterostructured interfaces under shear stress and normal loads, which may be closely associated with wear response and durability of hard coatings. Development of advanced preparation technology of coatings requires delicate control over surface integrity, dense microstructure, grain size, residual stress state, hardness and toughness by a virtually arc-free plasma with high density and low ion energy [3-5,29,30]. High-power impulse magnetron sputtering exhibits a great advantage in high-performance coatings over conventional magnetron sputtering [31,32]. As a novel high-power impulse magnetron sputtering, deep oscillation magnetron sputtering (DOMS) can generate a virtually arc-free plasma using a series of voltage oscillation pulse for depositions of high-quality coatings [6,7]. Therefore, CrN/Si3N4 multilayer coatings with high hardness and toughness may be accomplished through precise control of heterostructured interfaces and microstructure.
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AISI 304L stainless steels have been widely used in nuclear power plants to fabricate mechanical components, such as pumps and valves. However, their poor friction and wear properties result in severe deterioration of working performance and lifetime, especially in harsh working environment. In this work, CrN/Si3N4 multilayer coatings with various Si content were deposited on AISI 304L stainless steel and Si(100) substrates using the combined deep oscillation magnetron sputtering with pulsed dc magnetron sputtering. The effect of Si content on microstructure, and mechanical and tribological properties were investigated to explore the correlations of surface morphology and interface structure with hardness and fracture toughness. Moreover, tribological behaviors of hard yet tough CrN/Si3N4 multilayer coatings under various normal loads were studied to explore the response of surface/interface with cracking to further understand the related mechanism.
2. Experimental details 2.1 Depositions of CrN/Si3N4 multilayer coatings CrN/Si3N4 multilayer coatings with various Si content were deposited on mirror polished AISI 304L stainless steel and Si(100) substrates using the combined deep oscillation magnetron sputtering (DOMS) and pulsed dc magnetron sputtering (PDCMS) by sputtering pure Cr and Si targets (292mm× 102mm× 6.4mm) with the purity of 99.95%. The two targets were oppositely installed in the chamber (Φ 470 mm) with an unbalanced magnetron sputtering system. Fig.1A shows schematic diagram of deposition system for CrN/Si3N4 multilayer coatings. The substrates were carefully cleaned by acetone and alcohol in turn, and then were fixed at the distance of 120 mm away from target surface before coating depositions. The basic pressure in chamber was below 10-4 Pa. The substrates were etched by glow discharge Ar+ plasma for 30 min at -500 V bias pulsed at 100 kHz and 90% duty cycle. And then, about 200 nm-thick Cr adhesion layer was deposited at a -60 V dc substrate bias. CrN/Si3N4 multilayer coatings with the thickness of about 5.6-7.4 m were deposited by the combined DOMS+PDCMS in an Ar/N2 mixture with a work pressure of 0.67 Pa and N2 flow rate of 23 sccm. The substrate holder stops by the front of Cr and Si target for 3 s and 8 s, respectively, using a rotation system controlled by a settled computer
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program. The Cr target was sputtered by PDCMS power supply using Advance Energy Pinnacle Pluspower sources at 1.0 kW (100 kHz and 90% duty cycle). Si target was sputtered by DOMS power supply using HIPIMS CypriumTM plasma generator at various powers of 0.15-1.0 kW. The voltage oscillation-on (on) and -off (off) times were 2 μs and 40 μs, respectively. Fig.1B shows discharge voltage and current oscillation pulse packages in one DOMS pulse. Average and peak values of power, voltage and current of Si target are 0.15-0.8 kW, 5.6-6.1 kW, 289-307 V, 754-832 V, 21-24 A and 58-63 A, respectively. The frequency ranges from 53 to 263 Hz. Substrate bias voltage of -60 V (AXIS, Zpulser LLC) were used for all coating depositions. 2.2 Characterization of composition, structure and surface morphology The chemical composition of CrN/Si3N4 multilayer coatings deposited at various Si target power were identified using electron probe microanalysis (EPMA, SHIMADZU EPMA-1600). The chemical bonding state of the coatings with 15.2 at.% Si were detected using X-ray photoelectron spectroscopy (XPS, PHI-5300 ESCA). The crystal structure and phase composition were characterized using X-ray diffraction (XRD, Siemens KRISTALLOFLEX-810) in -2 configuration with Cu K-alpha radiation. The Λ of CrN/Si3N4 multilayer coatings was calculated using the modified Bragg law according to low-angle XRD patterns. The microstructure and surface morphology were examined using a field emission scanning electron microscope (FESEM, ZEISS SUPRA55 VP). Residual stress state in as-deposited coatings on double polished AISI 304L stainless steel wafers (50 mm×10 mm×0.8 mm) was measured by FST1000 film stress tester, which is developed according to Stoney equation. Hence, the measured residual stress ranged from -0.6 to -2.3 GPa with increasing Si content from 1.1 to 26.1 at.%. At 15.2 at.% Si, residual stress is -1.2 GPa. 2.3 Mechanical properties The hardness (H), Young’s modulus (E) and elastic recovery (We) of CrN/Si3N4 multilayer coatings were measured using a nanoindenter (Nanoindenter XPTM, MTS Systems Corporation) equipped with a Berkovich diamond indenter using Oliver-Pharr method [33] according to the load-displacement curve. The applied load was 10 mN and the indentation depth was set below 10% of the film thickness to minimize the substrate
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effect. H/E* and H3/E*2 ratios are calculated by hardness and effect Young’s modulus, where E*=E/(1−ν2) is the effective Young's modulus and ν is the poisson's ratio. The fracture toughness was calculated by the length of radial cracks of Vickers indentations made by microhardness tester (MMT-X, Matsuzawa) under the applied load of 0.1 N. The following equations are used to calculate fracture toughness KIC of the coatings [34]. 1/2
K IC
E P 3/2 H c
(1)
where is the empirical constant (=0.016 for Vickers indenter). H, E, P and c are hardness, Young’s modulus, applied max load and the length of radial cracks, respectively. The depth of Vickers indentations is less than 10% of coating thickness. The applied load must carefully choose in order to eliminate the substrate contribution. 2.4 Tribological properties Tribological properties of CrN/Si3N4 multilayer coatings were studied using Universal Micro-Tribotester (UMT-2, Center for Tribology) in a reciprocating configuration against AISI 304L stainless steel ball (6 mm in diameter) under a normal load of 1-4 N at 1 Hz for reciprocating sliding time of 1 h. The cross-section profiles of wear tracks are examined using Taylor Hobson surface profilometer to calculate specific wear rate using the integration function in Origin software. Worn surface morphologies were analyzed using FESEM equipped with energy dispersive spectroscopy (EDS) to study wear mechanism. Also, oxide tribolayers formed on worn surface were characterized using micro-Raman spectroscopy (HR800, HORIBA) with the wavelength range of 100-1500 nm. In addition, the microstructure evolution during dry sliding tests was investigated using a transmission electron microscope (TEM, Philips/FEI CM200). Cross-sectional TEM samples of worn surface was prepared by focus ion beams, and the detailed HRTEM investigations with EDS results were carried out to further explore wear response and mechanism.
3. Results and discussion 3.1 Coating depositions
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Nc-CrN/a-Si3N4 multilayer coatings with various Si content were deposited by the combined DOMS (DOMS pulse is shown in Fig.1B) and pulsed dc magnetron sputtering in a closed field unbalanced magnetron sputtering system (Fig.1A). The correlations of crack initiation and propagation at heterostructured interfaces with wear response and durability are investigated in details to further explore the effect of nanostructure mechanism of CrN/Si3N4 multilayer coatings on friction and wear behavior. It is found that hardness and toughness of coatings can be controlled through heterostructured interfaces of the nanostructured multilayer or nanocomposite coatings of closely related chemical composition [4-8,19,29]. Thus, CrN/Si3N4 multilayer coatings with various Si content are expected to synthesize hard yet tough coatings by the combined DOMS+PDCMS. The Si target is sputtered by various DOMS powers to create the increase of Si content and Si/(Si+Cr) ratio as well as Si3N4 layer thickness corresponding to Λ. The Λ of CrN/Si3N4 multilayer coatings ranged from 7.6 to 16.7 nm with increasing Si content from 1.1 to 25.1 at.%. With an increase in Si target power from 0.15 to 1.0 kW, Si content in the coatings accordingly increases from 1.1 to 25.1 at.%, while Cr decrease from 47.5 to 22.5 at.%, indicating that Si element has a stronger competitive advantage in binding capacity with N element over Cr element. N content shows a slight changes from 51.4 to 52.7 at.% due to the fixed N2 gas flow rate during the coating depositions. The Si/(Si+Cr) ratio ranges from 2.26% to 52.7%. 3.2 Chemical composition and structure Fig.2A shows XRD patterns of CrN/Si3N4 multilayer coatings with various Si content deposited by the combined DOMS+PDCMS. Apart from diffraction peaks of 304L stainless steel labeled as SS, a strong (220) peak and two weak (111) and (200) peaks from cubic CrN phase are detected in the coatings with low Si content of 1.1 at.%. (220) peak gradually disappears, and the intensity of (200) increases with increasing Si content. Correspondingly, the intensity factor of diffraction peak (f(hkl)), i.e. f(220) = I(220)/(I(220)+I(111)+I(200)), increases from 47.3% for (220) peak at 1.1 at.% Si to 83.8% for (200) peak at 25.1 at.% Si. It is noted that a broadening and strong (200) peak is clearly obtained in the coatings with 15.2 at.% Si, attributed to the refinement of grain size and the overlap of the peak from Cr adhesion layer (43.4o). Besides, the strong intensity of diffraction peaks of the coatings with Λ of 8.9 and 15.2 nm is observed from low-angle 7
XRD patterns, which reveals the formation of sharp heterostructured interfaces in the coatings, as shown in Fig.2B. In order to determine chemical bonding state of the elements in CrN/Si3N4 multilayer coatings with 15.2 at.% Si content, we conducted XPS core level spectra of Cr 2p, N1s and Si2p (Fig.2C). The asymmetric Cr 2p peak composes of two peaks at binding energy (BE) of 575.5 and 576.7 eV originated from CrN and CrOx phases, respectively. According to XRD results, the Cr-N BE at 575.5 eV is related to crystalline cubic CrN phase. The Cr-O BE of 576.7 eV probably resulted from residual surface oxide layer after sputter etching before XPS analysis. A strong peak at BE of 396.7 eV and a weak peak at BE of 399.8 eV in asymmetric N1s spectra are assigned to CrN and Si3N4 phases, respectively. A single peak at BE of 101.5 eV is observed in Si2p spectra, which reveals the presence of Si3N4 amorphous phase in the coatings due to no evidence of crystalline Si3N4 phase identified in XRD patterns. Weak Si2p signal is detected because of CrN layer on top surface of CrN/Si3N4 multilayer coatings and low XPS analysis depth. 3.3 Mechanical properties The assessment of hard yet tough coatings is carried out using H, H/E* and H3/E*2 ratios (related to the resistance of elastic strain to failure and plastic deformation, respectively), elastic recovery We and fracture toughness KIC. It is reported that the enhanced cooperative deformation of coating/substrate system by increasing toughness of coatings, cohesion/adhesion and substrate hardness[13], which has close correlations with friction and wear properties [6-8]. Fig.3(A-C) illustrates hardness H, Young’s modulus E, H/E*, H3/E*2, elastic recovery We and fracture toughness KIC of CrN/Si3N4 multilayer coatings with various Si content. H, E, H/E*, H3/E*,2 We and KIC exhibit an initial increase from 17.3 GPa, 195 GPa, 0.083, 0.12, 38.9%, 1.29 MPa∙m1/2 to 30.6 GPa, 284 GPa, 0.096, 0.29, 58.5% and 3.69 MPa∙m1/2, followed by a decrease to 21.3 GPa, 267 GPa, 0.075, 0.12, 43.4%, 1.60 MPa∙m1/2. It is noted that the coatings with 15.2 at.% Si reach highest value of H, E, We, KIC, H/E* and H3/E*2. When the Si content in the range of 8.9-15.2 at.%, hard yet tough CrN/Si3N4 multilayer coatings are obtained. The Si content is below 8.9 at.% or above 15.2 at.%, the brittleness will gradually increase. Fig.3(D-G) presents the cross-sectional FESEM images inserted with surface morphologies of CrN/Si3N4 multilayer coatings with various Si content. According to the 8
extended structure zone diagram (SZD)[35], the cross-sectional morphology of the coatings with low 1.1 at.% Si exhibits rough columnar structure (Zone 2), while surface morphology shows scaly feature probably due to low substrate temperature caused by ion bombardment effect at low Si target power. With an increase of Si content, the coatings possess dense microstructure and smooth surface morphology together with the refinement of columnar grain. At 15.2 at.% Si content, the refined packet of nano-grains (Zone T) are obtained with disappearance of columnar structure. The coating with 25.1 at.% Si shows porous microstructure with a large amount of voids observed at the grain boundaries (Zone 1). Fig.3(H-L) shows the optical micrographs of Vickers microindentations of CrN/Si3N4 multilayer coatings with various Si content. The radial cracks are observed on the surface, and the length of cracks decrease, indicating the increase of fracture toughness KIC (Eq 1)[34]. Fig.4 shows cross-sectional morphology of CrN/Si3N4 multilayer coatings with 15.2 at.% Si after cracking. At first, the pre-cracks were made at one edge side of Si substrate, and then carefully pressed using tweezers to make cracking, as shown in Fig.4A. It is clearly seen from FESEM image inserted (Fig.4B) that the coatings exhibit “flexible feature” without the formation of cracks along with the irregular fracture of Si substrate. Fig.4C presents the detailed information at the edge of coating deformation from white square from Fig.4B. There are no cracks observed on the severe deformation zone. It is inferred that CrN/Si3N4 multilayer coatings with 15.2 at.% Si possess good hard yet tough properties in irregular cracking of Si substrate. Hence, Si content promotes the preferred orientation of nanocrystalline CrN changing from (220) to (200), together with the refinement of nc-CrN due to the broadening diffraction peaks. The microstructure evolution induced by increasing Si content changes from rough columnar structure Zone 2 (1.1 at.% Si) to densely packed fibrous grains Zone T (15.2 at.% Si) to porous tapered crystallites separated by voids Zone 1 (25.1 at.% Si). The ion-enhanced depositions by virtually arc-free DOMS plasma with high density contribute for the formation of well-defined interfaces between nc-CrN and a-Si3N4 layers. In addition, the strong XRD peaks also reveal high crystallinity of ncCrN on a-Si3N4 plane to form the nanocrystalline/amorphous multilayer structure. It is noted that CrN/Si3N4 multilayer coatings with 8.9-15.2 at.% Si exhibit high surface
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integrity including smooth and uniform surface morphology without the formation of micro-particles in the coatings. The ion-enhanced depositions by DOMS plasma contribute for the remarkable improvements in surface and interface morphologies, as well as microstructure of nanostructured coatings, as compared to those of the coatings deposited by conventional magnetron sputtering [7,6,17,27]. Therefore, it is inferred that high surface integrity, dense microstructure and refined nano-grains promote the enhancement of hardness and toughness to form hard yet tough CrN/Si3N4 multilayer coatings due to the dislocation motions hindered at sharp nanocrystalline/amorphous interfaces and dense grain boundaries. 3.4 Tribological behaviors Fig.5A plots coefficient of friction (COF) of CrN/Si3N4 multilayer coatings with 15.2 at.% Si against AISI 304L stainless steel (SS) balls under the load of 1-4 N. When the applied load is 1 N, COF sharply increases to a stable value of 0.45, compared with a long run-in period. As shown in the FESEM images of worn surface and EDX patterns in selected area (Fig.5(C-E)), the coatings suffered from mild oxidative wear due to the formation of uniform oxide films consisting of Cr2O3, FeO(OH) and Fe3O4 (Fig.5B). COF has large fluctuations at the beginning of 15 min, and gradually reaches a stable value of 0.63 in the next 15 min under the applied load of 3 N. Accordingly, a large amount of wear debris composing of Cr2O3, FeO(OH), Fe2O3 and Fe3O4 is accumulated within wear track resulted from severe adhesive and oxidative wear (Fig.5(F-H)). At 4 N, the COF also shows the oscillation feature during the run-in period of 0-13 min and reaches 0.74. The coatings suffered from severe adhesive and oxidative wear, resulting in thick oxide films (Fig.5B) formed on worn surface (Fig.5(I-K)). The measured specific wear rate is 1.3273×10-7mm-3N-1m-1 with increasing normal load from 1 to 4 N. It is known that the nature of coatings, substrates and counterpart materials, as well as sliding wear conditions (normal loads, sliding speed, etc.) result in complex wear response due to considerable differences in contact mechanism [13,23,28]. Therefore, the evaluation of durability and wear resistance of coatings must refer to microstructure evolution of the sublayer in coating/substrate system except COF, wear rate and worn morphology. Fig.6 presents the schematic of investigations on friction and wear behavior
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for CrN/Si3N4 multilayer coatings using ball-on-disc tribometer under different normal loads. At a lower load (F1), mass transfer would occur on contact surface. Meanwhile, the high-performance coating/substrate system is expected to promote cooperative deformation. However, at a higher load (F2), severe plastic deformation of substrate and total coating failure would happen. Thus, we focus on the interactions of crack initiation propagation in nc-CrN/a-Si3N4 heterostructured interfaces in the hard yet tough coatings with wear response on sliding contact surface. Fig.7 demonstrates TEM investigations of worn surface of CrN/Si3N4 multilayer coatings with 15.2 at.% Si under the applied load of 1 N. As shown in Fig.7A, uniform oxide films covered on worn surface is observed without the severe coating delamination. Also, no cracks are formed through coating thickness. The inserted cross-sectional morphology of worn surface by FIB cutting presents the whole multilayered structure, and selected area electron diffraction (SAED) patterns reveal typical B1 cubic structure of the coatings with the (111), (200) and (220) plane reflections, which are assigned to nc-CrN phase consistent with XRD and XPS results. Fig.7B is the elements distribution in the selected area by the EDS lateral scanning. It is confirmed that CrN and Si3N4 nanolayer are well arranged to form the multilayer structure. In addition, Fe, Cr and O elements are assembled in the top surface, agreed with Raman results in Fig.5B. Under the shear force during dry sliding tests, CrN/Si3N4 multilayer coatings are carefully peeled off layer by layer just like peeling the onion along the sliding direction. Meanwhile, cooperative deformation is observed in the sublayer area of the coatings (Fig.7C). Moreover, the flexible deformation of interfaces between nanocrystalline CrN and amorphous Si3N4 layers (Fig.7D). It is inferred that crack initiation and propagation on the sliding contact surface occurred in only several nanolayers of CrN/Si3N4 multilayer coatings with 15.2 at.% Si under normal and shear force. Moreover, the ncCrN layer is prone to crack initiation and propagation, while the a-Si3N4 amorphous layer is a function in preventing further cracking until the wear loss of amorphous layer. Afterwards, nc-CrN and a-Si3N4 layers continually repeat cracking behavior above again during sliding wear tests. Fig.8 reveals TEM investigations of worn surface of CrN/Si3N4 multilayer coatings with 15.2 at.% Si under the applied load of 4 N. Wear track is covered by thick oxide
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films with small cracks (Fig.8A). The debris of broken coatings is embedded into the AISI 304L substrate resulting in severe plastic deformation, together with the formation of oxides films assigned to Cr2O3, FeO(OH), Fe2O3 and Fe3O4 (Fig. 8B) on sliding contact surface according to Raman results (Fig.5B). The hardening zone resulted from severe plastic deformation and coating debris embedding into substrate, which promotes the loading capacity (Fig. 8C). The SAED patterns inserted in Fig.8C still show cubic (111), (200), and (220) diffraction rings assigned to nc-CrN phase, indicating a stable ncCrN/a-Si3N4 multilayer structure. As shown in HRTEM image in Fig.8D, however, the interfaces between nc-CrN and a-Si3N4 layers exhibit a flexible deformation, probably related to strain energy dissipation localized at interfaces. At a normal load of 1 N, CrN/Si3N4 multilayer coatings with 15.2 at.% Si are subjected to mild oxidative wear with low and stable COF of 0.45 against AISI 304L SS ball. The smooth worn surface reveals the formation of uniform thin oxides films without the accumulated wear debris due to mass transfer from 304L SS counterpart, which has a function in the decrease of COF. It is well known that ceramic coatings are always prone to brittle fracture, resulting in a higher wear rate than metals under the same friction and wear conditions [36,37]. However, in this case, hard yet tough CrN/Si3N4 multilayer coatings deposited on soft substrate (304L SS) show a good cooperative deformation of densely well-defined heterostructured nc-CrN/a-Si3N4 interfaces. It is clearly seen that nc-CrN layer is peeled off layer by layer just like peeling the onion along the sliding direction. In addition, nc-CrN layers are fractured and plastically deformed, and heterostructured interfaces are irregularly deformed. It is indicated that microcracking including crack initiation and propagation only occurred in sublayers due to energy dissipation and stress relaxation at interfaces via interfacial deformation. Moreover, there are no cracks formed in the nc-CrN layers, even though their severe plastic deformation, probably owing to low internal stress of -1.2 GPa. Hence, cooperative deformation attributes to high H, H/E*, and We, beneficial to reducing cracks initiation. High H3/E*2 and KIC contribute for the block of cracks propagation up to fracture. Si target are sputtered by DOMS instead of RF magnetron sputtering in order to generate high quality amorphous Si3N4 layers to avoid much higher penetration of depositing particles[29]. In
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addition, the enhanced interface strength and dense microstructure eliminate the risk of interface crack propagation and adhesion failure [13,38]. With an increase in applied normal load to 4 N, the COF of the coatings increases to 0.74 within running-in period of 15 min. The coatings suffered from severe adhesive and oxidative wear with the formation of thick oxide films covered on wear tracks, resulting in the increase and fluctuation of COF. In general, oxides films formed at sliding contact surface will reduce friction and wear apart from hard wear debris ascribed to coating failure. The changes in COF are related to the coating failure under large loading in excess of the capacity of coatings, as well as the severe plastic deformation of 304L SS substrate. Highly applied normal load results in a larger plastic deformation zone in subsurface of substrate together with the increased bearing capacity enhanced by hard coating debris. Dense nc-CrN/a-Si3N4 interfaces with severe deformation still keep tightly bonding multilayer structure without crack propagation at interface, while sudden failure through whole thickness occurred during sliding test. However, it is reported that lateral and edge cracks in nc-TiAlN/a-Si3N4 nanocomposite coating (NC) and lateral and intercolumnar cracks at interfaces in TiN/NC multilayer coatings deposited by cathodic arc PVD are observed under the normal load[39]. It is also well demonstrated that virtually arc-free DOMS plasma with high density contributes for the formation of dense microstructure and sharp interfaces in CrN/Si3N4 multilayer coatings. Due to different selections of friction and wear conditions including coatings, substrates, etc., crack initiation and propagation at sliding contact surface, sub-surface and interfaces give raise to considerable difference in wear response depending on hardness and toughness of hard coatings [13,28]. Therefore, hard yet tough CrN/Si3N4 multilayer coatings with high H, H/E*, H3/E*2, We and KIC are enhanced by densely well-arranged heterostructured interfaces to promote excellent cooperative deformation to reduce the cracking through coating thickness. Meanwhile, uniform oxide films in the upper part of the tribolayer on worn surface also made contribution towards reducing friction as a lubricant.
4. Conclusion CrN/Si3N4 multilayer coatings with 1.1-25.1 at.% Si were deposited using the combined deep oscillation magnetron sputtering (DOMS) with pulsed dc magnetron
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sputtering. DOMS plasma with high ionization and density achieved by a series of optimized voltage oscillation packets contributes for tailoring chemical composition, microstructure and properties. With an increase of Si content in the coatings, hardness, H/E*, H3/E*2, We and KIC show the same trend of initial increase, followed by a decrease. CrN/Si3N4 multilayer coatings with 15.2 at.% Si possess high hardness of 30.6 GPa, H/E* of 0.099, H3/E*2 of 0.387, elastic recover We of 58.5% and fracture toughness KIC of 3.69 MPa∙m1/2 due to the formation of smooth surface morphology, highly dense architecture, well-defined nc-CrN/a-Si3N4 interfaces. Moreover, at normal load of 1 N, the low COF of 0.45 and specific wear rate of 1.3×10-7mm-3N-1m-1 are obtained respectively. Hard yet tough CrN/Si3N4 multilayer coatings exhibit good cooperative deformability in terms of energy dissipation and stress relaxation at well-arranged ncCrN/a-Si3N4 heterostructured interfaces to reduce cracks initiation and propagation and adhesion failure, thanks to H, H/E*, H3/E*2, We and KIC. Meanwhile, uniform oxide films consisting of Cr2O3, FeO(OH), Fe2O3 and Fe3O4 on worn surface make contributions towards reducing friction as a lubricant due to tribochemical reaction. However, the applied normal load in excess of the capacity of hard yet tough coatings results in the increased COF of 0.74 and specific wear rate of 2.73×10-5mm-3N-1m-1, and the formation of plastic deformation hardening zone caused by hard coating debris. Dominated wear mechanism changes from mild oxidative wear to the mixture of severe adhesive and oxidative wear with increasingly applied normal load.
Acknowledgement This work is partly supported by National Natural Science Foundation Joint Fund Key Project (Grant No. U1865206), Guangdong Province Key Area Research and Development Program (Grant No. 2019B090909002) and Fundamental Research Funds (Grant No. SWU118105) for the Central Universities.
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Figure captions: Fig.1 Hybrid depositions for CrN/Si3N4 multilayer coatings using the combination of deep oscillation magnetron sputtering (DOMS) and pulsed dc magnetron sputtering (PDCMS): A, The schematic of DOMS+PDCMS in a closed field unbalanced magnetron sputtering system. B, Typical pulse shape of voltage and current oscillation with micro pulse-on (2 s) and -off time (40 s). Fig.2 Structure and chemical bonding state of CrN/Si3N4 multilayer coatings: A, High-angle XRD patterns for the coatings with 1.1-25.1 at.% Si. B, low-angle XRD patterns of the coatings with 8.9 and 15.2 at.% Si. C, XPS core level spectra of Cr 2p, N1s Si2p, and O1s for the coatings with 15.2 at.% Si. Fig.3 Microstructure and mechanical properties of CrN/Si3N4 multilayer coatings: A, Hardness and Young’s modulus. B, H/E* and H3/E*2 ratios. C, We and KIC. D-G, field emission SEM morphologies of the coatings with various Si content. H-L, optical morphologies of micro-indentations. Fig.4 Cross-sectional morphology of CrN/Si3N4 multilayer coatings with 15.2 at.% Si after cracking: A, cooperative deformation of the coating along the Si substrate cracking. B, Crack preparation. C, detailed information at the edge of coating deformation from white square from B. Fig.5 Friction and wear properties of CrN/Si3N4 multilayer coatings with 15.2 at.% Si: A, Coefficient of friction. B, Raman spectra of worn surface. C-E, Surface morphologies and EDS of worn surface under normal load of at 1 N, 3 N (F-H), and 4 N (I-K) at 1 Hz. Fig.6 The schematic of investigations on friction and wear behavior for CrN/Si3N4 multilayer coatings using ball-on-disc tribometer under different normal loads. Fig.7 Cross-sectional TEM morphologies of worn surface of CrN/Si3N4 multilayer coatings under the normal load of 1 N: A, worn surface morphology inserted with cross-sectional images by FIB cutting and SAED patterns. B, elements distribution in the selected area by EDS lateral scanning. C, deformation and delamination of sublayers. D, cooperative deformation of heterostructured interfaces and detachment mechanism. Fig.8 Cross-sectional TEM morphologies of worn surface of CrN/Si3N4 multilayer coatings under the normal load of 4 N: A, worn surface morphology. B, elements distribution in the selected area. C, adjacent severe plastic deformation area inserted with SAED patterns. D, HRTEM images of coating debris.
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Combined Deep Oscillation Magnetron Sputtering Hard yet tough CrN/Si3N4 multilayer coatings Tribological behaviors of hard yet tough coatings PDCMS power supply
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Highlights 1. CrN/Si3N4 coatings are deposited by combined deep oscillation magnetron sputtering. 2. Hard yet tough coatings have smooth surface morphology and well-defined interfaces. 3. High hardness, H/E*, H3/E*2, We and KIC characterized hard yet tough coatings. 4. Hard yet tough coatings have high cracking resistance to improve friction and wear.
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S0169-4332(19)32984-8 https://doi.org/10.1016/j.apsusc.2019.144168 APSUSC 144168
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
4 July 2019 18 September 2019 23 September 2019
Please cite this article as: Y.X. Ou, X.P. Ouyang, B. Liao, X. Zhang, S. Zhang, Hard yet tough CrN/Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144168
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Hard yet tough CrN/Si3N4 multilayer coatings deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering Y.X. Ou a, X.P. Ouyang b*, B. Liao c, X. Zhang c, S. Zhang d* Beijing Radiation Center, Beijing Academy of Science and Technology, Beijing 100875, China Shaanxi Engineering Research Center of Controllable Neutron Source, School of Science, Xijing University, Xi'an, 710123, China c Key Laboratory of Beam technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China d Faculty of Materials and Energy, Southwest University, Chongqing 400715, China a
b
Abstract Hard yet tough coatings with simultaneously high hardness and toughness represent a new class of high-performance coatings, which are vital for tribological applications due to the enhanced surface/interface properties. In this work, CrN/Si3N4 multilayer coatings with various Si content were reactively deposited by the combined deep oscillation magnetron sputtering and pulsed dc magnetron sputtering. The coatings exhibit smooth surface morphology, well-defined nc-CrN/a-Si3N4 interfaces, uniform thickness and highly dense architecture. Microstructure evolution is induced by Si content changing from scaly nanocolumnar to densely packed fibrous grains to tapered crystallites. These features allow the achievement of the increased properties in terms of high hardness of 30.6 GPa, H/E* of 0.099, H3/E*2 of 0.387, elastic recover We of 58.5% and fracture toughness KIC of 3.69 MPa∙m1/2. Hard yet tough CrN/Si3N4 multilayer coatings show excellent tribological properties by enhanced cooperative deformability through energy dissipation and stress relaxation at well-arranged nc-CrN/a-Si3N4 interfaces to reduce crack initiation and propagation, and adhesion failure, thanks to the enhanced H, H/E*, H3/E*2, We and KIC. Meanwhile, under limited normal load conditions, uniform tribolayers (Cr2O3, FeO(OH), Fe2O3 and Fe3O4) caused by tribochemical reaction on the worn surface also act to make contributions towards reducing friction and wear as a lubricant. Keywords: CrN/Si3N4 multilayer coatings; Deep oscillation magnetron sputtering; Hard yet tough; Tribological behaviors
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* e-mail: [email protected] (X.P. Ouyang), [email protected] (S. Zhang)
1. Introduction All along, a combination of hardness and toughness is a highly desired feature for ceramic coatings [1-4]. Much has been done over the years in one way or another. A potential route to achieve this aim is by the optimized coating design and advanced deposition techniques to obtain nanoscale, multilayer and nanocomposite coatings [5-8]. It is found that nanostructured multilayer coatings composed by transition-metal nitrides have attracted enormous research interests in scientific research and industrial applications due to their high hardness and wear and corrosion resistance [9,10]. In many cases, remarkably enhanced performance is always achieved by superlattice coatings, thanks to the stability of coherent interface structure [11-13]. Hence, heterostructured multilayer or nanocomposite coatings with a strong thermodynamic stability are expected to develop by the combination of nanocrystalline and amorphous phases [14,15]. As suggested by Veprek et al[16], nc-TiN/a-Si3N4 nanocomposite coatings have a potential to achieve superhardness up to 100 GPa. However, hardness-toughness ratio is indeed an open problem to handle. Because stable nanocomposite structure is formed by selforganization resulted from a strong, thermodynamically driven, and spinodal phase segregation. As for heterostructured multilayer coatings, i.e., nc-CrN/a-Si3N4 multilayer coatings, their desired performance is always achieved by the precise control of modulated bilayer period (Λ) [17,18]. In addition, CrN/Si3N4 multilayer coatings combine the advantages of single CrN and Si3N4 coatings in terms of low internal stress, and good wear and corrosion resistance. The enhanced damping capacity of hardness and toughness of multilayer coatings may be associated with interface structure, grain boundary sliding and energy dissipation mechanism localized at interfaces [17-19]. The earlier studies are mainly devoted to explore the origin of hardening mechanism of hard coatings in order to achieve superhardness (>40 GPa) as much as possible because hardness is considered as a unique
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criterion according to classical wear theory [1,6,20,21]. However, friction and wear response of hard ceramic coatings are usually complex, depending on many factors, atmosphere, counterpart materials, sample conditions, wear parameters, etc [22,23]. Therefore, hardness does not always correlate with tribological properties of ceramic coatings. Besides, superhard coatings are prone to brittle fracture in friction and wear process due to low toughness. The improved toughness of hard coatings is beneficial for the increased resistance against crack initiation and propagation deriving from stress accumulation in the vicinity of structure imperfections [2,3]. Hence, coatings with high hardness and high toughness are effectively resistant to plastic deformation, chipping, flaking, and even catastrophic failure in friction and wear applications. The characterization of hardness of hard coatings is easily carried out using micro/nanoindentation[24], but as for the measurement of toughness, it is still an open problem due to the sensitive effect of substrates [25,26]. It is known that friction and wear properties of hard coatings are influenced by hardness and toughness as well as hardness and effective Young’s modulus ratios (H/E* and H3/E*2) [6,27,28]. But beyond those, structure evolution of the coatings, chemical reactions on sliding contact surface, grain boundary sliding and interface behavior during dry sliding wear process are still lack of systematic and comprehensive studies. It is also lack of advanced knowledge of correlations of crack initiation and propagation at heterostructured interfaces under shear stress and normal loads, which may be closely associated with wear response and durability of hard coatings. Development of advanced preparation technology of coatings requires delicate control over surface integrity, dense microstructure, grain size, residual stress state, hardness and toughness by a virtually arc-free plasma with high density and low ion energy [3-5,29,30]. High-power impulse magnetron sputtering exhibits a great advantage in high-performance coatings over conventional magnetron sputtering [31,32]. As a novel high-power impulse magnetron sputtering, deep oscillation magnetron sputtering (DOMS) can generate a virtually arc-free plasma using a series of voltage oscillation pulse for depositions of high-quality coatings [6,7]. Therefore, CrN/Si3N4 multilayer coatings with high hardness and toughness may be accomplished through precise control of heterostructured interfaces and microstructure.
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AISI 304L stainless steels have been widely used in nuclear power plants to fabricate mechanical components, such as pumps and valves. However, their poor friction and wear properties result in severe deterioration of working performance and lifetime, especially in harsh working environment. In this work, CrN/Si3N4 multilayer coatings with various Si content were deposited on AISI 304L stainless steel and Si(100) substrates using the combined deep oscillation magnetron sputtering with pulsed dc magnetron sputtering. The effect of Si content on microstructure, and mechanical and tribological properties were investigated to explore the correlations of surface morphology and interface structure with hardness and fracture toughness. Moreover, tribological behaviors of hard yet tough CrN/Si3N4 multilayer coatings under various normal loads were studied to explore the response of surface/interface with cracking to further understand the related mechanism.
2. Experimental details 2.1 Depositions of CrN/Si3N4 multilayer coatings CrN/Si3N4 multilayer coatings with various Si content were deposited on mirror polished AISI 304L stainless steel and Si(100) substrates using the combined deep oscillation magnetron sputtering (DOMS) and pulsed dc magnetron sputtering (PDCMS) by sputtering pure Cr and Si targets (292mm× 102mm× 6.4mm) with the purity of 99.95%. The two targets were oppositely installed in the chamber (Φ 470 mm) with an unbalanced magnetron sputtering system. Fig.1A shows schematic diagram of deposition system for CrN/Si3N4 multilayer coatings. The substrates were carefully cleaned by acetone and alcohol in turn, and then were fixed at the distance of 120 mm away from target surface before coating depositions. The basic pressure in chamber was below 10-4 Pa. The substrates were etched by glow discharge Ar+ plasma for 30 min at -500 V bias pulsed at 100 kHz and 90% duty cycle. And then, about 200 nm-thick Cr adhesion layer was deposited at a -60 V dc substrate bias. CrN/Si3N4 multilayer coatings with the thickness of about 5.6-7.4 m were deposited by the combined DOMS+PDCMS in an Ar/N2 mixture with a work pressure of 0.67 Pa and N2 flow rate of 23 sccm. The substrate holder stops by the front of Cr and Si target for 3 s and 8 s, respectively, using a rotation system controlled by a settled computer
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program. The Cr target was sputtered by PDCMS power supply using Advance Energy Pinnacle Pluspower sources at 1.0 kW (100 kHz and 90% duty cycle). Si target was sputtered by DOMS power supply using HIPIMS CypriumTM plasma generator at various powers of 0.15-1.0 kW. The voltage oscillation-on (on) and -off (off) times were 2 μs and 40 μs, respectively. Fig.1B shows discharge voltage and current oscillation pulse packages in one DOMS pulse. Average and peak values of power, voltage and current of Si target are 0.15-0.8 kW, 5.6-6.1 kW, 289-307 V, 754-832 V, 21-24 A and 58-63 A, respectively. The frequency ranges from 53 to 263 Hz. Substrate bias voltage of -60 V (AXIS, Zpulser LLC) were used for all coating depositions. 2.2 Characterization of composition, structure and surface morphology The chemical composition of CrN/Si3N4 multilayer coatings deposited at various Si target power were identified using electron probe microanalysis (EPMA, SHIMADZU EPMA-1600). The chemical bonding state of the coatings with 15.2 at.% Si were detected using X-ray photoelectron spectroscopy (XPS, PHI-5300 ESCA). The crystal structure and phase composition were characterized using X-ray diffraction (XRD, Siemens KRISTALLOFLEX-810) in -2 configuration with Cu K-alpha radiation. The Λ of CrN/Si3N4 multilayer coatings was calculated using the modified Bragg law according to low-angle XRD patterns. The microstructure and surface morphology were examined using a field emission scanning electron microscope (FESEM, ZEISS SUPRA55 VP). Residual stress state in as-deposited coatings on double polished AISI 304L stainless steel wafers (50 mm×10 mm×0.8 mm) was measured by FST1000 film stress tester, which is developed according to Stoney equation. Hence, the measured residual stress ranged from -0.6 to -2.3 GPa with increasing Si content from 1.1 to 26.1 at.%. At 15.2 at.% Si, residual stress is -1.2 GPa. 2.3 Mechanical properties The hardness (H), Young’s modulus (E) and elastic recovery (We) of CrN/Si3N4 multilayer coatings were measured using a nanoindenter (Nanoindenter XPTM, MTS Systems Corporation) equipped with a Berkovich diamond indenter using Oliver-Pharr method [33] according to the load-displacement curve. The applied load was 10 mN and the indentation depth was set below 10% of the film thickness to minimize the substrate
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effect. H/E* and H3/E*2 ratios are calculated by hardness and effect Young’s modulus, where E*=E/(1−ν2) is the effective Young's modulus and ν is the poisson's ratio. The fracture toughness was calculated by the length of radial cracks of Vickers indentations made by microhardness tester (MMT-X, Matsuzawa) under the applied load of 0.1 N. The following equations are used to calculate fracture toughness KIC of the coatings [34]. 1/2
K IC
E P 3/2 H c
(1)
where is the empirical constant (=0.016 for Vickers indenter). H, E, P and c are hardness, Young’s modulus, applied max load and the length of radial cracks, respectively. The depth of Vickers indentations is less than 10% of coating thickness. The applied load must carefully choose in order to eliminate the substrate contribution. 2.4 Tribological properties Tribological properties of CrN/Si3N4 multilayer coatings were studied using Universal Micro-Tribotester (UMT-2, Center for Tribology) in a reciprocating configuration against AISI 304L stainless steel ball (6 mm in diameter) under a normal load of 1-4 N at 1 Hz for reciprocating sliding time of 1 h. The cross-section profiles of wear tracks are examined using Taylor Hobson surface profilometer to calculate specific wear rate using the integration function in Origin software. Worn surface morphologies were analyzed using FESEM equipped with energy dispersive spectroscopy (EDS) to study wear mechanism. Also, oxide tribolayers formed on worn surface were characterized using micro-Raman spectroscopy (HR800, HORIBA) with the wavelength range of 100-1500 nm. In addition, the microstructure evolution during dry sliding tests was investigated using a transmission electron microscope (TEM, Philips/FEI CM200). Cross-sectional TEM samples of worn surface was prepared by focus ion beams, and the detailed HRTEM investigations with EDS results were carried out to further explore wear response and mechanism.
3. Results and discussion 3.1 Coating depositions
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Nc-CrN/a-Si3N4 multilayer coatings with various Si content were deposited by the combined DOMS (DOMS pulse is shown in Fig.1B) and pulsed dc magnetron sputtering in a closed field unbalanced magnetron sputtering system (Fig.1A). The correlations of crack initiation and propagation at heterostructured interfaces with wear response and durability are investigated in details to further explore the effect of nanostructure mechanism of CrN/Si3N4 multilayer coatings on friction and wear behavior. It is found that hardness and toughness of coatings can be controlled through heterostructured interfaces of the nanostructured multilayer or nanocomposite coatings of closely related chemical composition [4-8,19,29]. Thus, CrN/Si3N4 multilayer coatings with various Si content are expected to synthesize hard yet tough coatings by the combined DOMS+PDCMS. The Si target is sputtered by various DOMS powers to create the increase of Si content and Si/(Si+Cr) ratio as well as Si3N4 layer thickness corresponding to Λ. The Λ of CrN/Si3N4 multilayer coatings ranged from 7.6 to 16.7 nm with increasing Si content from 1.1 to 25.1 at.%. With an increase in Si target power from 0.15 to 1.0 kW, Si content in the coatings accordingly increases from 1.1 to 25.1 at.%, while Cr decrease from 47.5 to 22.5 at.%, indicating that Si element has a stronger competitive advantage in binding capacity with N element over Cr element. N content shows a slight changes from 51.4 to 52.7 at.% due to the fixed N2 gas flow rate during the coating depositions. The Si/(Si+Cr) ratio ranges from 2.26% to 52.7%. 3.2 Chemical composition and structure Fig.2A shows XRD patterns of CrN/Si3N4 multilayer coatings with various Si content deposited by the combined DOMS+PDCMS. Apart from diffraction peaks of 304L stainless steel labeled as SS, a strong (220) peak and two weak (111) and (200) peaks from cubic CrN phase are detected in the coatings with low Si content of 1.1 at.%. (220) peak gradually disappears, and the intensity of (200) increases with increasing Si content. Correspondingly, the intensity factor of diffraction peak (f(hkl)), i.e. f(220) = I(220)/(I(220)+I(111)+I(200)), increases from 47.3% for (220) peak at 1.1 at.% Si to 83.8% for (200) peak at 25.1 at.% Si. It is noted that a broadening and strong (200) peak is clearly obtained in the coatings with 15.2 at.% Si, attributed to the refinement of grain size and the overlap of the peak from Cr adhesion layer (43.4o). Besides, the strong intensity of diffraction peaks of the coatings with Λ of 8.9 and 15.2 nm is observed from low-angle 7
XRD patterns, which reveals the formation of sharp heterostructured interfaces in the coatings, as shown in Fig.2B. In order to determine chemical bonding state of the elements in CrN/Si3N4 multilayer coatings with 15.2 at.% Si content, we conducted XPS core level spectra of Cr 2p, N1s and Si2p (Fig.2C). The asymmetric Cr 2p peak composes of two peaks at binding energy (BE) of 575.5 and 576.7 eV originated from CrN and CrOx phases, respectively. According to XRD results, the Cr-N BE at 575.5 eV is related to crystalline cubic CrN phase. The Cr-O BE of 576.7 eV probably resulted from residual surface oxide layer after sputter etching before XPS analysis. A strong peak at BE of 396.7 eV and a weak peak at BE of 399.8 eV in asymmetric N1s spectra are assigned to CrN and Si3N4 phases, respectively. A single peak at BE of 101.5 eV is observed in Si2p spectra, which reveals the presence of Si3N4 amorphous phase in the coatings due to no evidence of crystalline Si3N4 phase identified in XRD patterns. Weak Si2p signal is detected because of CrN layer on top surface of CrN/Si3N4 multilayer coatings and low XPS analysis depth. 3.3 Mechanical properties The assessment of hard yet tough coatings is carried out using H, H/E* and H3/E*2 ratios (related to the resistance of elastic strain to failure and plastic deformation, respectively), elastic recovery We and fracture toughness KIC. It is reported that the enhanced cooperative deformation of coating/substrate system by increasing toughness of coatings, cohesion/adhesion and substrate hardness[13], which has close correlations with friction and wear properties [6-8]. Fig.3(A-C) illustrates hardness H, Young’s modulus E, H/E*, H3/E*2, elastic recovery We and fracture toughness KIC of CrN/Si3N4 multilayer coatings with various Si content. H, E, H/E*, H3/E*,2 We and KIC exhibit an initial increase from 17.3 GPa, 195 GPa, 0.083, 0.12, 38.9%, 1.29 MPa∙m1/2 to 30.6 GPa, 284 GPa, 0.096, 0.29, 58.5% and 3.69 MPa∙m1/2, followed by a decrease to 21.3 GPa, 267 GPa, 0.075, 0.12, 43.4%, 1.60 MPa∙m1/2. It is noted that the coatings with 15.2 at.% Si reach highest value of H, E, We, KIC, H/E* and H3/E*2. When the Si content in the range of 8.9-15.2 at.%, hard yet tough CrN/Si3N4 multilayer coatings are obtained. The Si content is below 8.9 at.% or above 15.2 at.%, the brittleness will gradually increase. Fig.3(D-G) presents the cross-sectional FESEM images inserted with surface morphologies of CrN/Si3N4 multilayer coatings with various Si content. According to the 8
extended structure zone diagram (SZD)[35], the cross-sectional morphology of the coatings with low 1.1 at.% Si exhibits rough columnar structure (Zone 2), while surface morphology shows scaly feature probably due to low substrate temperature caused by ion bombardment effect at low Si target power. With an increase of Si content, the coatings possess dense microstructure and smooth surface morphology together with the refinement of columnar grain. At 15.2 at.% Si content, the refined packet of nano-grains (Zone T) are obtained with disappearance of columnar structure. The coating with 25.1 at.% Si shows porous microstructure with a large amount of voids observed at the grain boundaries (Zone 1). Fig.3(H-L) shows the optical micrographs of Vickers microindentations of CrN/Si3N4 multilayer coatings with various Si content. The radial cracks are observed on the surface, and the length of cracks decrease, indicating the increase of fracture toughness KIC (Eq 1)[34]. Fig.4 shows cross-sectional morphology of CrN/Si3N4 multilayer coatings with 15.2 at.% Si after cracking. At first, the pre-cracks were made at one edge side of Si substrate, and then carefully pressed using tweezers to make cracking, as shown in Fig.4A. It is clearly seen from FESEM image inserted (Fig.4B) that the coatings exhibit “flexible feature” without the formation of cracks along with the irregular fracture of Si substrate. Fig.4C presents the detailed information at the edge of coating deformation from white square from Fig.4B. There are no cracks observed on the severe deformation zone. It is inferred that CrN/Si3N4 multilayer coatings with 15.2 at.% Si possess good hard yet tough properties in irregular cracking of Si substrate. Hence, Si content promotes the preferred orientation of nanocrystalline CrN changing from (220) to (200), together with the refinement of nc-CrN due to the broadening diffraction peaks. The microstructure evolution induced by increasing Si content changes from rough columnar structure Zone 2 (1.1 at.% Si) to densely packed fibrous grains Zone T (15.2 at.% Si) to porous tapered crystallites separated by voids Zone 1 (25.1 at.% Si). The ion-enhanced depositions by virtually arc-free DOMS plasma with high density contribute for the formation of well-defined interfaces between nc-CrN and a-Si3N4 layers. In addition, the strong XRD peaks also reveal high crystallinity of ncCrN on a-Si3N4 plane to form the nanocrystalline/amorphous multilayer structure. It is noted that CrN/Si3N4 multilayer coatings with 8.9-15.2 at.% Si exhibit high surface
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integrity including smooth and uniform surface morphology without the formation of micro-particles in the coatings. The ion-enhanced depositions by DOMS plasma contribute for the remarkable improvements in surface and interface morphologies, as well as microstructure of nanostructured coatings, as compared to those of the coatings deposited by conventional magnetron sputtering [7,6,17,27]. Therefore, it is inferred that high surface integrity, dense microstructure and refined nano-grains promote the enhancement of hardness and toughness to form hard yet tough CrN/Si3N4 multilayer coatings due to the dislocation motions hindered at sharp nanocrystalline/amorphous interfaces and dense grain boundaries. 3.4 Tribological behaviors Fig.5A plots coefficient of friction (COF) of CrN/Si3N4 multilayer coatings with 15.2 at.% Si against AISI 304L stainless steel (SS) balls under the load of 1-4 N. When the applied load is 1 N, COF sharply increases to a stable value of 0.45, compared with a long run-in period. As shown in the FESEM images of worn surface and EDX patterns in selected area (Fig.5(C-E)), the coatings suffered from mild oxidative wear due to the formation of uniform oxide films consisting of Cr2O3, FeO(OH) and Fe3O4 (Fig.5B). COF has large fluctuations at the beginning of 15 min, and gradually reaches a stable value of 0.63 in the next 15 min under the applied load of 3 N. Accordingly, a large amount of wear debris composing of Cr2O3, FeO(OH), Fe2O3 and Fe3O4 is accumulated within wear track resulted from severe adhesive and oxidative wear (Fig.5(F-H)). At 4 N, the COF also shows the oscillation feature during the run-in period of 0-13 min and reaches 0.74. The coatings suffered from severe adhesive and oxidative wear, resulting in thick oxide films (Fig.5B) formed on worn surface (Fig.5(I-K)). The measured specific wear rate is 1.3273×10-7mm-3N-1m-1 with increasing normal load from 1 to 4 N. It is known that the nature of coatings, substrates and counterpart materials, as well as sliding wear conditions (normal loads, sliding speed, etc.) result in complex wear response due to considerable differences in contact mechanism [13,23,28]. Therefore, the evaluation of durability and wear resistance of coatings must refer to microstructure evolution of the sublayer in coating/substrate system except COF, wear rate and worn morphology. Fig.6 presents the schematic of investigations on friction and wear behavior
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for CrN/Si3N4 multilayer coatings using ball-on-disc tribometer under different normal loads. At a lower load (F1), mass transfer would occur on contact surface. Meanwhile, the high-performance coating/substrate system is expected to promote cooperative deformation. However, at a higher load (F2), severe plastic deformation of substrate and total coating failure would happen. Thus, we focus on the interactions of crack initiation propagation in nc-CrN/a-Si3N4 heterostructured interfaces in the hard yet tough coatings with wear response on sliding contact surface. Fig.7 demonstrates TEM investigations of worn surface of CrN/Si3N4 multilayer coatings with 15.2 at.% Si under the applied load of 1 N. As shown in Fig.7A, uniform oxide films covered on worn surface is observed without the severe coating delamination. Also, no cracks are formed through coating thickness. The inserted cross-sectional morphology of worn surface by FIB cutting presents the whole multilayered structure, and selected area electron diffraction (SAED) patterns reveal typical B1 cubic structure of the coatings with the (111), (200) and (220) plane reflections, which are assigned to nc-CrN phase consistent with XRD and XPS results. Fig.7B is the elements distribution in the selected area by the EDS lateral scanning. It is confirmed that CrN and Si3N4 nanolayer are well arranged to form the multilayer structure. In addition, Fe, Cr and O elements are assembled in the top surface, agreed with Raman results in Fig.5B. Under the shear force during dry sliding tests, CrN/Si3N4 multilayer coatings are carefully peeled off layer by layer just like peeling the onion along the sliding direction. Meanwhile, cooperative deformation is observed in the sublayer area of the coatings (Fig.7C). Moreover, the flexible deformation of interfaces between nanocrystalline CrN and amorphous Si3N4 layers (Fig.7D). It is inferred that crack initiation and propagation on the sliding contact surface occurred in only several nanolayers of CrN/Si3N4 multilayer coatings with 15.2 at.% Si under normal and shear force. Moreover, the ncCrN layer is prone to crack initiation and propagation, while the a-Si3N4 amorphous layer is a function in preventing further cracking until the wear loss of amorphous layer. Afterwards, nc-CrN and a-Si3N4 layers continually repeat cracking behavior above again during sliding wear tests. Fig.8 reveals TEM investigations of worn surface of CrN/Si3N4 multilayer coatings with 15.2 at.% Si under the applied load of 4 N. Wear track is covered by thick oxide
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films with small cracks (Fig.8A). The debris of broken coatings is embedded into the AISI 304L substrate resulting in severe plastic deformation, together with the formation of oxides films assigned to Cr2O3, FeO(OH), Fe2O3 and Fe3O4 (Fig. 8B) on sliding contact surface according to Raman results (Fig.5B). The hardening zone resulted from severe plastic deformation and coating debris embedding into substrate, which promotes the loading capacity (Fig. 8C). The SAED patterns inserted in Fig.8C still show cubic (111), (200), and (220) diffraction rings assigned to nc-CrN phase, indicating a stable ncCrN/a-Si3N4 multilayer structure. As shown in HRTEM image in Fig.8D, however, the interfaces between nc-CrN and a-Si3N4 layers exhibit a flexible deformation, probably related to strain energy dissipation localized at interfaces. At a normal load of 1 N, CrN/Si3N4 multilayer coatings with 15.2 at.% Si are subjected to mild oxidative wear with low and stable COF of 0.45 against AISI 304L SS ball. The smooth worn surface reveals the formation of uniform thin oxides films without the accumulated wear debris due to mass transfer from 304L SS counterpart, which has a function in the decrease of COF. It is well known that ceramic coatings are always prone to brittle fracture, resulting in a higher wear rate than metals under the same friction and wear conditions [36,37]. However, in this case, hard yet tough CrN/Si3N4 multilayer coatings deposited on soft substrate (304L SS) show a good cooperative deformation of densely well-defined heterostructured nc-CrN/a-Si3N4 interfaces. It is clearly seen that nc-CrN layer is peeled off layer by layer just like peeling the onion along the sliding direction. In addition, nc-CrN layers are fractured and plastically deformed, and heterostructured interfaces are irregularly deformed. It is indicated that microcracking including crack initiation and propagation only occurred in sublayers due to energy dissipation and stress relaxation at interfaces via interfacial deformation. Moreover, there are no cracks formed in the nc-CrN layers, even though their severe plastic deformation, probably owing to low internal stress of -1.2 GPa. Hence, cooperative deformation attributes to high H, H/E*, and We, beneficial to reducing cracks initiation. High H3/E*2 and KIC contribute for the block of cracks propagation up to fracture. Si target are sputtered by DOMS instead of RF magnetron sputtering in order to generate high quality amorphous Si3N4 layers to avoid much higher penetration of depositing particles[29]. In
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addition, the enhanced interface strength and dense microstructure eliminate the risk of interface crack propagation and adhesion failure [13,38]. With an increase in applied normal load to 4 N, the COF of the coatings increases to 0.74 within running-in period of 15 min. The coatings suffered from severe adhesive and oxidative wear with the formation of thick oxide films covered on wear tracks, resulting in the increase and fluctuation of COF. In general, oxides films formed at sliding contact surface will reduce friction and wear apart from hard wear debris ascribed to coating failure. The changes in COF are related to the coating failure under large loading in excess of the capacity of coatings, as well as the severe plastic deformation of 304L SS substrate. Highly applied normal load results in a larger plastic deformation zone in subsurface of substrate together with the increased bearing capacity enhanced by hard coating debris. Dense nc-CrN/a-Si3N4 interfaces with severe deformation still keep tightly bonding multilayer structure without crack propagation at interface, while sudden failure through whole thickness occurred during sliding test. However, it is reported that lateral and edge cracks in nc-TiAlN/a-Si3N4 nanocomposite coating (NC) and lateral and intercolumnar cracks at interfaces in TiN/NC multilayer coatings deposited by cathodic arc PVD are observed under the normal load[39]. It is also well demonstrated that virtually arc-free DOMS plasma with high density contributes for the formation of dense microstructure and sharp interfaces in CrN/Si3N4 multilayer coatings. Due to different selections of friction and wear conditions including coatings, substrates, etc., crack initiation and propagation at sliding contact surface, sub-surface and interfaces give raise to considerable difference in wear response depending on hardness and toughness of hard coatings [13,28]. Therefore, hard yet tough CrN/Si3N4 multilayer coatings with high H, H/E*, H3/E*2, We and KIC are enhanced by densely well-arranged heterostructured interfaces to promote excellent cooperative deformation to reduce the cracking through coating thickness. Meanwhile, uniform oxide films in the upper part of the tribolayer on worn surface also made contribution towards reducing friction as a lubricant.
4. Conclusion CrN/Si3N4 multilayer coatings with 1.1-25.1 at.% Si were deposited using the combined deep oscillation magnetron sputtering (DOMS) with pulsed dc magnetron
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sputtering. DOMS plasma with high ionization and density achieved by a series of optimized voltage oscillation packets contributes for tailoring chemical composition, microstructure and properties. With an increase of Si content in the coatings, hardness, H/E*, H3/E*2, We and KIC show the same trend of initial increase, followed by a decrease. CrN/Si3N4 multilayer coatings with 15.2 at.% Si possess high hardness of 30.6 GPa, H/E* of 0.099, H3/E*2 of 0.387, elastic recover We of 58.5% and fracture toughness KIC of 3.69 MPa∙m1/2 due to the formation of smooth surface morphology, highly dense architecture, well-defined nc-CrN/a-Si3N4 interfaces. Moreover, at normal load of 1 N, the low COF of 0.45 and specific wear rate of 1.3×10-7mm-3N-1m-1 are obtained respectively. Hard yet tough CrN/Si3N4 multilayer coatings exhibit good cooperative deformability in terms of energy dissipation and stress relaxation at well-arranged ncCrN/a-Si3N4 heterostructured interfaces to reduce cracks initiation and propagation and adhesion failure, thanks to H, H/E*, H3/E*2, We and KIC. Meanwhile, uniform oxide films consisting of Cr2O3, FeO(OH), Fe2O3 and Fe3O4 on worn surface make contributions towards reducing friction as a lubricant due to tribochemical reaction. However, the applied normal load in excess of the capacity of hard yet tough coatings results in the increased COF of 0.74 and specific wear rate of 2.73×10-5mm-3N-1m-1, and the formation of plastic deformation hardening zone caused by hard coating debris. Dominated wear mechanism changes from mild oxidative wear to the mixture of severe adhesive and oxidative wear with increasingly applied normal load.
Acknowledgement This work is partly supported by National Natural Science Foundation Joint Fund Key Project (Grant No. U1865206), Guangdong Province Key Area Research and Development Program (Grant No. 2019B090909002) and Fundamental Research Funds (Grant No. SWU118105) for the Central Universities.
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Figure captions: Fig.1 Hybrid depositions for CrN/Si3N4 multilayer coatings using the combination of deep oscillation magnetron sputtering (DOMS) and pulsed dc magnetron sputtering (PDCMS): A, The schematic of DOMS+PDCMS in a closed field unbalanced magnetron sputtering system. B, Typical pulse shape of voltage and current oscillation with micro pulse-on (2 s) and -off time (40 s). Fig.2 Structure and chemical bonding state of CrN/Si3N4 multilayer coatings: A, High-angle XRD patterns for the coatings with 1.1-25.1 at.% Si. B, low-angle XRD patterns of the coatings with 8.9 and 15.2 at.% Si. C, XPS core level spectra of Cr 2p, N1s Si2p, and O1s for the coatings with 15.2 at.% Si. Fig.3 Microstructure and mechanical properties of CrN/Si3N4 multilayer coatings: A, Hardness and Young’s modulus. B, H/E* and H3/E*2 ratios. C, We and KIC. D-G, field emission SEM morphologies of the coatings with various Si content. H-L, optical morphologies of micro-indentations. Fig.4 Cross-sectional morphology of CrN/Si3N4 multilayer coatings with 15.2 at.% Si after cracking: A, cooperative deformation of the coating along the Si substrate cracking. B, Crack preparation. C, detailed information at the edge of coating deformation from white square from B. Fig.5 Friction and wear properties of CrN/Si3N4 multilayer coatings with 15.2 at.% Si: A, Coefficient of friction. B, Raman spectra of worn surface. C-E, Surface morphologies and EDS of worn surface under normal load of at 1 N, 3 N (F-H), and 4 N (I-K) at 1 Hz. Fig.6 The schematic of investigations on friction and wear behavior for CrN/Si3N4 multilayer coatings using ball-on-disc tribometer under different normal loads. Fig.7 Cross-sectional TEM morphologies of worn surface of CrN/Si3N4 multilayer coatings under the normal load of 1 N: A, worn surface morphology inserted with cross-sectional images by FIB cutting and SAED patterns. B, elements distribution in the selected area by EDS lateral scanning. C, deformation and delamination of sublayers. D, cooperative deformation of heterostructured interfaces and detachment mechanism. Fig.8 Cross-sectional TEM morphologies of worn surface of CrN/Si3N4 multilayer coatings under the normal load of 4 N: A, worn surface morphology. B, elements distribution in the selected area. C, adjacent severe plastic deformation area inserted with SAED patterns. D, HRTEM images of coating debris.
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Highlights 1. CrN/Si3N4 coatings are deposited by combined deep oscillation magnetron sputtering. 2. Hard yet tough coatings have smooth surface morphology and well-defined interfaces. 3. High hardness, H/E*, H3/E*2, We and KIC characterized hard yet tough coatings. 4. Hard yet tough coatings have high cracking resistance to improve friction and wear.
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