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On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel
Author’s Accepted Manuscript On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel F. Cemin, C.D. Boeira, C.A. Figueroa
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To appear in: Tribiology International Received date: 28 May 2015 Revised date: 8 September 2015 Accepted date: 27 September 2015 Cite this article as: F. Cemin, C.D. Boeira and C.A. Figueroa, On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2015.09.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel
F. Cemina, C. D. Boeiraa, and C. A. Figueroaa,b,*
a
CCET, Universidade de Caxias do Sul, Caxias do Sul, 95070-560, Brazil b
Plasmar Tecnologia Ltda., Caxias do Sul, 95030-775, Brazil
Keywords: Interface; adhesion; DLC; steel; Hertz contact theory; shear stress.
*
Corresponding author: [email protected]
Abstract DLC is a promising material to be used in technological issues. However, its poor adhesion on steels keeps away from a wide range of applications. Thus, an understanding of the physicochemical structure of the DLC/interlayer/steel system in correlation with a mechanical model is mandatory in order to explain the tribological behavior as a whole. In this work, DLC was deposited on AISI 4140 low-alloy steel through a silicon-containing interlayer. Two different delamination critical loads were determined and they were associated with different failure mechanisms that took place at the outermost and at the innermost interfaces of the silicon-containing interlayer. The Hertz contact theory was used to evaluate the shear stress distribution at the applied normal loads during the scratch tests. 1
1. Introduction: Diamond-like carbon (DLC) thin films are state-of-the-art materials based on hydrogenated amorphous carbon (a-C:H) that may provide both reduced wear and ultra-low friction [1,2,3]. In particular, steel surfaces of mechanical pieces in relative movement of engines and gear boxes can be tailored with such a type of thin film for energy efficiency issues [4,5]. However, the poor adhesion of DLC films on ferrous alloys reduces a wide range of industrial applications [6,7,8,9,10,11,12]. Interlayers containing Al, Cr, W, Ti or Si are commonly employed in the endeavor to improve the DLC adhesion on metallic alloys. Furthermore, the adhesion layer between the steel substrate and the DLC thin film, usually called as interlayer, intermediate layer or bond layer, is understood as a bi-dimensional layer without further details. After a careful analysis of the state-of-the-art works about the nature of the adhesion interlayer, some works showed the cross-section images of DLC/interlayer/substrate systems, but no evidence about the existence and/or function of the two interfaces that constitute the interlayer was found [10,13]. Most of works carried out in this area generally uses PVD technologies where the adhesion layer is composed by a buffer layer of metal and/or metallic nitride/carbide interlayers without further descriptions of the interlayer physicochemical structure that promotes adhesion and even to analyze the tribological behavior as a whole [14]. A previous work has shown that the propagation and growth of a crack took places at the interface of a single thin silver film (10 nm) embedded in a brittle multilayer under sliding contact by a mechanism that requires a combination of tensile in-plane stresses and shear [15]. Moreover, in the case of DLC coatings deposited on ductile substrate, an elastic-plastic behavior was detected as the load increased due to plastic deformation in the substrate and
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cracking in the coating [16]. For hard coatings like DLC, the cracks are initiated not only at the tail of the contact zone on the DLC surface but also at the interface [17]. An atomistic model to the understanding that will be introduced in this work was already proposed. However, such a model of the different types of atomic interactions and chemical bonding is introducing the practical adhesion and the interfacial stress between a coating and a substrate, but only one interface was invoked [18]. Indeed, a single interlayer is composed, at least, by two interfaces with different physicochemical structures. Consequently, both interfaces might act in different ways due to different mechanical strengths imparted by different chemical environments. Moreover, if a second body is in mechanical contact, an induced shear stress will be developed and will be different at each interface. Indeed, when a normal force is applied, the induced shear stress depends on depth according to the Hertz contact model [19] and may cause the delamination of the thin film in a region where weak chemical bonds and more defects are concentrated, i.e., at one of the interfaces. The aim of this work is to show, for the very first time, that the silicon-containing adhesion interlayer is constituted by two different interfaces and both can control the delamination of DLC thin films on steels. In particular, the correlation between the shear stress distribution and the interfacial adhesion forces at each interface will be discussed.
2. Material and Methods: In our experiments, the material system is constituted by an outermost DLC film 1.8 μm thick deposited on the AISI 4140 steel intermediated by a bond layer of hydrogenated silicon carbide (SiCx:H), obtained by plasma enhanced chemical vapor deposition (PECVD) assisted by electrostatic confinement [20,21]. 3
The DLC films were deposited on discs with 13 mm of diameter and 2 mm of thickness from the same AISI 4140 plain steel bar. The samples were mirror polished using standard metallographic techniques and cleaned ultrasonically in acetone bath for 30 min before being loaded into a laboratory scale chamber for deposition. The plasma electrostatic confining is obtained by a multi-electrode array, which enhances the density of active species, as it has been previously published [22]. The background deposition chamber pressure is 1.5 Pa by using mechanical pumps. For all the reported processes, the glow discharge was sustained by a negative pulsed direct current power supply (10 kHz), duty cycle of 40% and +30 V constant bias positive between the negative pulses. Prior to deposition, the substrates were Ar+ ion cleaned plasma during 30 min at a chamber pressure of 10 Pa and an applied voltage of -500 V. The silicon-containing interlayer was then deposited from a vapor mixture formed by Si(CH3)4 (TMS) and argon (~80%–20% proportion in partial pressure, respectively). The working pressure used for the interlayer depositions was ~60 Pa at a voltage of -500 V for 10 min. The substrate temperature for the different studied samples expanded from 100oC to 550°C by using a resistive heating. After growing of the siliconcontaining interlayers, the DLC films were deposited when the substrate temperature reached 80°C in a gaseous mixture of acetylene (C2H2) and argon (75%-25% proportion in sccm, respectively). The total gas pressure was 10 Pa and the glow discharge was maintained at an applied voltage of -800 V during 60 min. The microstructure, morphology and thickness of the DLC films were determined by scanning electron microscopy (SEM - Shimadzu SSX-550) by analyzing the plain view and cross-section of the samples. The qualitative chemical composition of the films was studied on the same SEM apparatus by energy dispersive X-ray spectroscopy (EDS). In depth
4
chemical profiles were obtained by glow discharge optical emission spectroscopy (GDOES Horiba GD-Profiler 2). The scratch tests were performed by using a nano-tribometer from Micro Materials Ltd. (NanoTest-600 model) with a conical diamond tip (25 m of final radius). The scratch tests were carried out in order to analyze the failure mechanism to delaminate the DLC and quantify the adhesion of DLC on steel through the delamination critical load. A normal load of 0.01 mN was applied in the firsts 100 μm of the sliding and then the load was linearly increased at a rate of 0.064 mN.s-1 up to reach a final normal load of 500 mN, as the tip was dragged through a distance of 7,813 μm over the sample surface. The sliding speed was 1 m.s-1 and each scratch test was repeated 5 times. The applied normal load to induce failure (delamination) in the DLC was considered as the critical load and this value was determined by using both the measured tangential forces and direct microscopic observation.
3. Hertz contact theory and calculations: The experimental results of the critical loads for delamination of DLC and DLC + interlayer were correlated with calculations about the depth for maximum shear stress and the shear stress distribution at different applied normal loads by using the Hertz contact theory. Figure 1 shows a schematic of the contact condition which was considered in the present work. The conical indenter tip finalizes in a half sphere which is in mechanical contact with the coating surface (DLC) in sliding motion. We assumed that the mechanical system can be approximated by the interaction of a rigid sphere (diamond indenter tip) on a flat surface (DLC) in static condition (without sliding). One must remark that our tribological system shows a very low coefficient of friction (lower than 0.07 at the first DLC delamination). Therefore, we also assumed a frictionless contact, which is one of the original 5
boundary condition for Hertz's original analysis, i. e., the static condition (see page 203 in Ref. 19 and page 110 in Ref 23]. In such a case, the Hertz pressure on a circular region renders the following Hertizian stress equations along the z-axis (depth in our case) [19]:
rp0 = p0 = - (1 + ).{1 - (z/a)tan-1(a/z)} + (1/2).(1 + z2/a2)-1
zp0 = -(1 + z2/a2)-1
Where the principal shear stress is given by [19]:
= (1/2).ǀ Z - ǀ
We calculated the maximum shear stress at different depths and the shear stress distributions by using the MESYS Hertzian Stress Calculation - Version 04-2015 [24]. Table 1 introduces the mechanical properties [25,26] that were used in the calculations. Also, the mechanical properties of the SiCx:H interlayer were considered similar to those of DLC.
4. Results and Discussion: Figure 2 a shows the images of two samples obtained with different SiCx:H interlayers deposited at temperatures of 200oC and 300oC followed by a posterior deposition of the same the DLC thin film on top. The surface of the sample with the SiCx:H interlayer deposited at 300oC shows the typical black color of DLC films and no signal of delamination is apparent, while spontaneous delamination (no external forces applied) of the DLC thin film occurred for the sample with the SiCx:H interlayer deposited at 200oC. However, one can 6
notice that the SiCx:H interlayer still remains on the sample obtained at 200oC due to the color gradient at the center of the sample Figures 2 b and c show the SEM images in cross-section for both samples. For the DLC film with the silicon-containing interlayer deposited at 200oC, one can see that the adhesion failure takes place at the outermost interface (DLC/SiCx:H) instead of at the innermost interface (SiCx:H/steel) (please, see Fig. 2 b). Indeed, this type of adhesion failure is determining a failure mechanism at the outermost interface (DLC/SiCx:H), as observed in Figure 2 a. It is important to note that a critical load of (313.8 ± 6.6) mN must be applied to delaminate the DLC thin film when the interlayer was deposited at 300oC. By considering the material system above-described, both interfaces of the adhesion interlayer would have different chemical structures that lead to change the tribological behavior as a whole. Much care should be taken into account in the understanding of both the outermost and innermost interfaces if the tribology of DLC on steel through adhesion interlayers must be controlled. Moreover, most of works published up to now does not discuss this important point of view of the adhesion interlayer, i.e, neither specific considerations are given in respect to the existence of two interfaces nor their chemical structures and roles in the adhesion interlayer are described. Figure 3 shows the chemical composition profile of the sample where the interlayer was deposited at 200oC. For clarity, the composition of each region is stressed and they are different from a chemical point of view. One can see that the interlayer is composed by silicon, carbon, hydrogen, and residual oxygen and it is in the middle of a thin film containing carbon, hydrogen and residual oxygen and the substrate containing, basically, iron atoms. Thus, two different interfaces should be developed in such a material system: the outermost aC:H(O)/SiCx:H(O) and the innermost SiCx:H(O)/Fe(C).
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Under the light of these different chemical environments, an interlayer has two different possible points of failure, i.e., the outermost and the innermost interfaces. In our studied material system, the outermost interface is constituted by carbon-carbon and carbonsilicon bonds and the innermost interface is constituted by carbon-iron and silicon-iron bonds, both acting as a nanoglue [27]. These different chemical structures, i.e., different energies involving different chemical bonds at both interfaces must change the adhesion energy of each interface, which could induce different critical loads for thin film delamination depending on the type and density of chemical bonding at each interface. Figures 4 a to f show the SEM images and EDS analysis of the worn surfaces after sliding experiments for two samples where the interlayers were deposited at 300oC and 550oC. At the top, Fig. 4 b shows the SEM image in a region where the DLC was removed and Fig. 4 a and c show the EDS analysis for iron and silicon, respectively, for the sample with the interlayer deposited at 300oC. At the bottom, Fig. 4 e shows the SEM image in a region where the DLC was removed and Fig. 4 d and f show the EDS analysis for iron and silicon, respectively, for the sample with the interlayer deposited at 550oC. In both cases, the sliding direction was the same and the SEM images show the surface microstructure of a wear track where the delamination of the DLC film is evident (see Fig. 4 b and e). However, the chemical analysis provides different information. On one hand, in the sample with the interlayer deposited at 300oC, both iron and silicon were detected on the surface of the region where the DLC film was removed. On the other hand, in the sample with the interlayer deposited at 550oC, only iron is apparent in the region where DLC delamination was removed. Moreover, in the sample with the interlayer deposited at 300oC the silicon signal is brighter than the iron signal, which suggests that the silicon-containing interlayer still remains on the substrate (steel). Therefore, the silicon-containing interlayer was removed together 8
with the DLC film in the sample with the interlayer deposited at 550oC, indicating a different adhesion failure mechanism, in this new case, at the innermost interface (SiCx:H/steel). Figure 5 a and b show the SEM images of the sample with the interlayer deposited at 300oC. The SEM images were taken in a tilt of 3 degrees in order to analyze both the surface and cross-section of sample. These images are giving more evidence about two different failure mechanisms for the DLC delamination. Whereas the silicon-containing interlayer remains adhered on the steel substrate after DLC delamination in the proposed “failure mechanism I” (please, see the wear track in Fig. 5 a), the same interlayer was removed together with the DLC film in the proposed “failure mechanism II” (please, see the wear track in Fig. 5 b). Moreover, the failure mechanisms to delaminate the DLC films rendered two different critical loads, being Lc2 related to the failure mechanism I and Lc3 related to the failure mechanism II. Lc1 is defined as the critical load to start the plastic deformation of DLC thin films and it is not the focus of the present work (no delamination). Table 2 introduces the critical loads for both types of DLC delamination (with and without the removal of the interlayer) for the sample with the interlayer deposited at 300oC. One can see that Lc3 is greater than Lc2. According to the Hertz contact model [19], the shear stress distribution shows its maximum value underneath the surface and, also, depends on depth with the applied normal load. The shear stress induces mismatching in the DLC/interlayer/substrate sandwich structure where there are two different interfaces: DLC/interlayer (the outermost interface) and interlayer/substrate (the innermost interlayer). Chemically speaking, the interface is the weakest point of the sandwich structure due to the high concentration of dangling bonds and defects that decrease the mechanical strength of the material system as a whole. Taking into account that chemical point of view, the delamination failure is to be initiated with more 9
probability at any interface (the outermost and/or the innermost interfaces) than in the coating, silicon-containing interlayer or substrate bulks. A previous work has shown that the maximum shear stress is moved away from the coating/substrate interface with the increasing thickness of the coating [28]. Moreover, a multilayer architecture decreases the maximum shear stress and can avoid the coating delamination [29,30]. However, to the best of our knowledge, no evidence was found about the correlation among the shear stress distribution, the applied normal load and the interface position in depth concerning the delamination mechanism of a DLC/interlayer/substrate sandwich structure. It is important to note again that although we performed unidirectional sliding tests (dynamic condition), the Hertzian stress equations that were used represent a static condition (normal load without sliding) due to we considered a frictionless tribological system (coefficient of friction 0.07) [19,23]. Thus, we are estimating the real dynamic situation by considering a frictionless sliding motion. In addition, previous results have shown that the critical shear stress does not change (within the experimental error) at different DLC coating thicknesses by using the Herzt theory in a dynamic situation [31]. Indeed, this work was performed in a similar experimental setup with a diamond tip sliding on a flat DLC coating, which is a tribological system of very low friction [31]. Figure 6 shows the evolution of the depth where the shear stress is a maximum as a function of the normal load by applying the Hertz contact model in a mechanical system constituted by a diamond sphere on a flat DLC surface (please, see Fig. 1). One can see that the depth for maximum shear stress increases monotonously with the increasing of the applied normal load. Moreover, in the range of the applied normal loads in the present work, the depth for maximum shear stress goes from approximately 0.5 m to 2.3 m. For the sample with the interlayer deposited at 300oC, the thicknesses of the DLC thin film and the silicon-containing 10
interlayer are, approximately, 1.80 m and 0.3 m, respectively, and they are also shown in Fig. 6. It is important to note that these thicknesses (1.8 m for DLC only and 2.1 m for DLC plus SiCx:H interlayer) are in agreement with the depths for maximum shear stress calculated with the measured critical loads Lc2 and Lc3 (please, see both Table 2 and Fig. 6). Figure 7 a and b show a schematic representation of both failure mechanisms (I and II) in combination with the shear stress distribution at the average applied normal load for Lc 2 and Lc3, respectively. In both cases, the sample with the interlayer deposited at 300oC is considered. According to our calculations, the maximum shear stresses when normal forces of 313.8 mN and 461.4 mN are applied (please, see Lc2 and Lc3 in Table 2) take place at a depth of approximately 1.80 m and 2.10 m, i.e., the thickness where the outermost interface DLC/SiCx:H is located and the thickness where the innermost interface SiCx:H/steel is located, respectively. By analyzing the shear stress distributions, the innermost interface seems to be more strength than the outermost interface in terms of adhesion. Indeed, the removal of both the DLC and the silicon-containing interlayer occurs at a higher shear stress than the removal of DLC only. Thus, the adhesion energy due to Si-Fe chemical bonds plus defects (innermost interface) is greater than C-Si and C-C chemical bonds plus defects (outermost interface) [20,26]. In the light of these findings, four different tribological scenarios may be described: 1. when the shear stress is higher than both adhesion forces at the outermost and innermost interfaces, either the DLC or DLC + interlayer would be removed; 2. when the shear stress is higher than the adhesion force at the innermost interlayer but lower than the adhesion force at the outermost interlayer, the DLC + interlayer would be removed only; 3. when the shear stress is higher than the adhesion force at the outermost interlayer but lower than the adhesion force at the innermost interlayer, the DLC would be removed only; 4. when the shear stress is 11
lower than both adhesion forces at the outermost and innermost interfaces, neither the DLC nor interlayer would be removed. Finally, these shear stress distributions could be used to analyze the chemical strength in terms of the adhesion energy at each interface, focusing in atomistic models that deal with the quantity and force of the chemical bonding at the interfaces.
5. Conclusions: This understanding of the relevance of considering the outermost and innermost interfaces of a SiCx:H interlayer could open new pathways not only in the improvement of the tribological behavior for DLC deposited on steel, but also to tailor the interface chemistry that controls the DLC adhesion on steels by using intermediate layers. The Hertz contact theory can be used to calculate the shear stress distribution and the depth for maximum shear stress in order to correlate and evaluate the adhesion strength at the interfaces of the adhesion interlayer. Consequently, plasma processes that are used to obtain DLC could be optimized in order to increase the adhesion of such thin films in parts of mechanical devices that aim to reduce friction and save energy.
Acknowledgements: The authors are grateful to UCS, INCT-INES-CNPq (Grant # 554336/2010-3), CAPES (Grant # Brafitec 087/11), PETROBRAS, and SUMA2 Network Project, 7th Framework Program of the European Commission (IRSES Project # 318903) for financial support. FC and CAF are CNPq fellows. FC is in part supported by PETROBRAS.
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[21] Dufrène SMM, Cemin F, Soares MRF, Aguzzoli C, Maia da Costa MEH, Baumvol IJR, Figueroa CA. Hydrogenated amorphous carbon thin films deposited by plasma-assisted chemical vapour deposition enhanced by electrostatic confinement: structure, properties and modelling. Appl Phys A: Mater Sci Process 2014;117:1217-25. [22] Corujeira Gallo S, Crespi AE, Cemin F, Figueroa CA, Baumvol IJR. Electrostatically Confined Plasma in Segmented Hollow Cathode Geometries for Surface Engineering. IEEE Trans Plasma Sci 2011;39:3028–32. [23] Fischer-Cripps, A. C. Introduction to contact mechanics. 2nd Edition. Springer; 2007. [24] http://www.mesys.ch/?page_id=157&lang=en. [25] Cho S-J, Lee K-R, Eun, KY, Hahn JH, Ko D-H. Determination of elastic modulus and Poisson's ratio of diamond-likecarbon flms. Thin Solid Films 1999;341:207-10. [26] Spear KE, Dismukes JP. Synthetic Diamond – Emerging CVD Science and Technology. 1st ed. New York: Wiley; 1994 (p. 315). [27] Cemin F, Bim LT, Leidens LM, Morales M, Baumvol IJR, Alvarez F, Figueroa CA. Identification of the chemical bonding prompting adhesion of a-C:H thin films on ferrous alloys intermediated by a SiCx:H buffer layer. ACS Appl. Mat. & Interfaces. In Press. [28] Xiaohui D, Hongwei Z, Lei W. Hertzian Contact Analysis of Ceramic/Metal Functionally Graded Coating. Advanced Materials Research 2013; 787:582-587. [29] Lackner JM, Major L, Kot M. Microscale interpretation of tribological phenomena in Ti/TiN soft-hard multilayer coatings on soft austenite steel substrates. Bull. Pol. Ac.: Tech. 2011;59:343-355 [30] Zhang XC, Xu BS, Wang HD, Wu YX, Jiang Y. Hertzian contact response of singlelayer, functionally graded and sandwich coatings. Mat. Design 2007;28:47-54.
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[31] Choi J, Ishii K, Kumagai T, Kato T; Hikita Y. Evaluation of Adhesion Strength of DLC Films Using Critical Shear Stress. J. Japanese Soc. of Tribologists 2009;54:138-44.
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Figure Captions
Figure 1: Schematic of the contact condition where the conical indenter, which finalizes in a half sphere, is in mechanical contact with the coating surface (DLC).
Figure 2: (a) Surface images of two samples obtained at different SiCx:H interlayer deposition temperatures (200oC and 300oC); (b) and (c) show the SEM images (in crosssection) of both samples where the DLC/interlayer/steel sandwich structure is apparent.
Figure 3: Chemical composition profile of the sample where the interlayer was deposited at 200oC. The DLC/interlayer/steel system is chemically described.
Figure 4: The SEM images and EDS analysis of the worn surfaces for two samples where the interlayers were deposited at 300oC and 550oC. Fig. 3 b/e show the SEM image of surfaces and Fig. 3 a/d and c/f show the EDS analysis for iron and silicon, respectively (interlayer deposited at 300oC/550oC).
Figure 5 a and b: Failure mechanisms for Lc2 and Lc3, respectively, in the sample where the interlayer was deposited at 300oC.
Figure 6: Depth for maximum shear stress as a function of the applied normal load calculated by using the Hertz contact theory. The thicknesses of the DLC and the DLC + interlayer are also shown (interlayer deposited at 300oC).
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Figure 7 a and b: Schematic representation of the failure mechanisms (I and II) of DLC delamination at the outermost and the innermost interfaces, respectively, of for the same interlayer deposited at 300oC. The shear stress distributions at the measured critical loads (L2 and Lc3) and the depths for maximum shear stress are also shown.
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Figure 1
Figure 1 Cemin et al.
Figure 2
No external forces applied for
Critical load for DLC delamination
DLC delamination
298 ± 7 mN
Figure 2 abc Cemin et al.
Figure 3
Figure 3 Cemin et al.
Figure 4
Figure 4 a-f Cemin et al.
Figure 5
Figure 5 a and b Cemin et al.
Depth for maximum shear stress (µm)
Figure 6
3.0
Thickness of DLC film plus SiCx:H interlayer
2.5 2.0 1.5
Thickness of DLC film only
1.0 0.5 0.0
200
400
600
800
Applied normal load (mN) Figure 6 Cemin et al.
Figure 7a
Figure 7a Cemin et al.
Figure 7b
Figure 7b Cemin et al.
Table 1
Table 1: Mechanical properties for diamond and DLC used in the calculations involving the Hertz contact theory and MESYS (software for surface engineering).
Elastic modulus (GPa) Poisson's ratio
Diamond 1200 0.2
DLC 90 0.22
Table 2
Table 2: Different critical loads determined by performing scratch tests and direct microscopic observation for the sample with the interlayer deposited at 300oC.
Lc1 Lc2 Lc3
Normal Load (mN) Standard Deviation 70.5 6.1 313.8 6.6 461.4 6.0
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S0301-679X(15)00446-6 http://dx.doi.org/10.1016/j.triboint.2015.09.044 JTRI3871
To appear in: Tribiology International Received date: 28 May 2015 Revised date: 8 September 2015 Accepted date: 27 September 2015 Cite this article as: F. Cemin, C.D. Boeira and C.A. Figueroa, On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2015.09.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
On the understanding of the silicon-containing adhesion interlayer in DLC deposited on steel
F. Cemina, C. D. Boeiraa, and C. A. Figueroaa,b,*
a
CCET, Universidade de Caxias do Sul, Caxias do Sul, 95070-560, Brazil b
Plasmar Tecnologia Ltda., Caxias do Sul, 95030-775, Brazil
Keywords: Interface; adhesion; DLC; steel; Hertz contact theory; shear stress.
*
Corresponding author: [email protected]
Abstract DLC is a promising material to be used in technological issues. However, its poor adhesion on steels keeps away from a wide range of applications. Thus, an understanding of the physicochemical structure of the DLC/interlayer/steel system in correlation with a mechanical model is mandatory in order to explain the tribological behavior as a whole. In this work, DLC was deposited on AISI 4140 low-alloy steel through a silicon-containing interlayer. Two different delamination critical loads were determined and they were associated with different failure mechanisms that took place at the outermost and at the innermost interfaces of the silicon-containing interlayer. The Hertz contact theory was used to evaluate the shear stress distribution at the applied normal loads during the scratch tests. 1
1. Introduction: Diamond-like carbon (DLC) thin films are state-of-the-art materials based on hydrogenated amorphous carbon (a-C:H) that may provide both reduced wear and ultra-low friction [1,2,3]. In particular, steel surfaces of mechanical pieces in relative movement of engines and gear boxes can be tailored with such a type of thin film for energy efficiency issues [4,5]. However, the poor adhesion of DLC films on ferrous alloys reduces a wide range of industrial applications [6,7,8,9,10,11,12]. Interlayers containing Al, Cr, W, Ti or Si are commonly employed in the endeavor to improve the DLC adhesion on metallic alloys. Furthermore, the adhesion layer between the steel substrate and the DLC thin film, usually called as interlayer, intermediate layer or bond layer, is understood as a bi-dimensional layer without further details. After a careful analysis of the state-of-the-art works about the nature of the adhesion interlayer, some works showed the cross-section images of DLC/interlayer/substrate systems, but no evidence about the existence and/or function of the two interfaces that constitute the interlayer was found [10,13]. Most of works carried out in this area generally uses PVD technologies where the adhesion layer is composed by a buffer layer of metal and/or metallic nitride/carbide interlayers without further descriptions of the interlayer physicochemical structure that promotes adhesion and even to analyze the tribological behavior as a whole [14]. A previous work has shown that the propagation and growth of a crack took places at the interface of a single thin silver film (10 nm) embedded in a brittle multilayer under sliding contact by a mechanism that requires a combination of tensile in-plane stresses and shear [15]. Moreover, in the case of DLC coatings deposited on ductile substrate, an elastic-plastic behavior was detected as the load increased due to plastic deformation in the substrate and
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cracking in the coating [16]. For hard coatings like DLC, the cracks are initiated not only at the tail of the contact zone on the DLC surface but also at the interface [17]. An atomistic model to the understanding that will be introduced in this work was already proposed. However, such a model of the different types of atomic interactions and chemical bonding is introducing the practical adhesion and the interfacial stress between a coating and a substrate, but only one interface was invoked [18]. Indeed, a single interlayer is composed, at least, by two interfaces with different physicochemical structures. Consequently, both interfaces might act in different ways due to different mechanical strengths imparted by different chemical environments. Moreover, if a second body is in mechanical contact, an induced shear stress will be developed and will be different at each interface. Indeed, when a normal force is applied, the induced shear stress depends on depth according to the Hertz contact model [19] and may cause the delamination of the thin film in a region where weak chemical bonds and more defects are concentrated, i.e., at one of the interfaces. The aim of this work is to show, for the very first time, that the silicon-containing adhesion interlayer is constituted by two different interfaces and both can control the delamination of DLC thin films on steels. In particular, the correlation between the shear stress distribution and the interfacial adhesion forces at each interface will be discussed.
2. Material and Methods: In our experiments, the material system is constituted by an outermost DLC film 1.8 μm thick deposited on the AISI 4140 steel intermediated by a bond layer of hydrogenated silicon carbide (SiCx:H), obtained by plasma enhanced chemical vapor deposition (PECVD) assisted by electrostatic confinement [20,21]. 3
The DLC films were deposited on discs with 13 mm of diameter and 2 mm of thickness from the same AISI 4140 plain steel bar. The samples were mirror polished using standard metallographic techniques and cleaned ultrasonically in acetone bath for 30 min before being loaded into a laboratory scale chamber for deposition. The plasma electrostatic confining is obtained by a multi-electrode array, which enhances the density of active species, as it has been previously published [22]. The background deposition chamber pressure is 1.5 Pa by using mechanical pumps. For all the reported processes, the glow discharge was sustained by a negative pulsed direct current power supply (10 kHz), duty cycle of 40% and +30 V constant bias positive between the negative pulses. Prior to deposition, the substrates were Ar+ ion cleaned plasma during 30 min at a chamber pressure of 10 Pa and an applied voltage of -500 V. The silicon-containing interlayer was then deposited from a vapor mixture formed by Si(CH3)4 (TMS) and argon (~80%–20% proportion in partial pressure, respectively). The working pressure used for the interlayer depositions was ~60 Pa at a voltage of -500 V for 10 min. The substrate temperature for the different studied samples expanded from 100oC to 550°C by using a resistive heating. After growing of the siliconcontaining interlayers, the DLC films were deposited when the substrate temperature reached 80°C in a gaseous mixture of acetylene (C2H2) and argon (75%-25% proportion in sccm, respectively). The total gas pressure was 10 Pa and the glow discharge was maintained at an applied voltage of -800 V during 60 min. The microstructure, morphology and thickness of the DLC films were determined by scanning electron microscopy (SEM - Shimadzu SSX-550) by analyzing the plain view and cross-section of the samples. The qualitative chemical composition of the films was studied on the same SEM apparatus by energy dispersive X-ray spectroscopy (EDS). In depth
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chemical profiles were obtained by glow discharge optical emission spectroscopy (GDOES Horiba GD-Profiler 2). The scratch tests were performed by using a nano-tribometer from Micro Materials Ltd. (NanoTest-600 model) with a conical diamond tip (25 m of final radius). The scratch tests were carried out in order to analyze the failure mechanism to delaminate the DLC and quantify the adhesion of DLC on steel through the delamination critical load. A normal load of 0.01 mN was applied in the firsts 100 μm of the sliding and then the load was linearly increased at a rate of 0.064 mN.s-1 up to reach a final normal load of 500 mN, as the tip was dragged through a distance of 7,813 μm over the sample surface. The sliding speed was 1 m.s-1 and each scratch test was repeated 5 times. The applied normal load to induce failure (delamination) in the DLC was considered as the critical load and this value was determined by using both the measured tangential forces and direct microscopic observation.
3. Hertz contact theory and calculations: The experimental results of the critical loads for delamination of DLC and DLC + interlayer were correlated with calculations about the depth for maximum shear stress and the shear stress distribution at different applied normal loads by using the Hertz contact theory. Figure 1 shows a schematic of the contact condition which was considered in the present work. The conical indenter tip finalizes in a half sphere which is in mechanical contact with the coating surface (DLC) in sliding motion. We assumed that the mechanical system can be approximated by the interaction of a rigid sphere (diamond indenter tip) on a flat surface (DLC) in static condition (without sliding). One must remark that our tribological system shows a very low coefficient of friction (lower than 0.07 at the first DLC delamination). Therefore, we also assumed a frictionless contact, which is one of the original 5
boundary condition for Hertz's original analysis, i. e., the static condition (see page 203 in Ref. 19 and page 110 in Ref 23]. In such a case, the Hertz pressure on a circular region renders the following Hertizian stress equations along the z-axis (depth in our case) [19]:
rp0 = p0 = - (1 + ).{1 - (z/a)tan-1(a/z)} + (1/2).(1 + z2/a2)-1
zp0 = -(1 + z2/a2)-1
Where the principal shear stress is given by [19]:
= (1/2).ǀ Z - ǀ
We calculated the maximum shear stress at different depths and the shear stress distributions by using the MESYS Hertzian Stress Calculation - Version 04-2015 [24]. Table 1 introduces the mechanical properties [25,26] that were used in the calculations. Also, the mechanical properties of the SiCx:H interlayer were considered similar to those of DLC.
4. Results and Discussion: Figure 2 a shows the images of two samples obtained with different SiCx:H interlayers deposited at temperatures of 200oC and 300oC followed by a posterior deposition of the same the DLC thin film on top. The surface of the sample with the SiCx:H interlayer deposited at 300oC shows the typical black color of DLC films and no signal of delamination is apparent, while spontaneous delamination (no external forces applied) of the DLC thin film occurred for the sample with the SiCx:H interlayer deposited at 200oC. However, one can 6
notice that the SiCx:H interlayer still remains on the sample obtained at 200oC due to the color gradient at the center of the sample Figures 2 b and c show the SEM images in cross-section for both samples. For the DLC film with the silicon-containing interlayer deposited at 200oC, one can see that the adhesion failure takes place at the outermost interface (DLC/SiCx:H) instead of at the innermost interface (SiCx:H/steel) (please, see Fig. 2 b). Indeed, this type of adhesion failure is determining a failure mechanism at the outermost interface (DLC/SiCx:H), as observed in Figure 2 a. It is important to note that a critical load of (313.8 ± 6.6) mN must be applied to delaminate the DLC thin film when the interlayer was deposited at 300oC. By considering the material system above-described, both interfaces of the adhesion interlayer would have different chemical structures that lead to change the tribological behavior as a whole. Much care should be taken into account in the understanding of both the outermost and innermost interfaces if the tribology of DLC on steel through adhesion interlayers must be controlled. Moreover, most of works published up to now does not discuss this important point of view of the adhesion interlayer, i.e, neither specific considerations are given in respect to the existence of two interfaces nor their chemical structures and roles in the adhesion interlayer are described. Figure 3 shows the chemical composition profile of the sample where the interlayer was deposited at 200oC. For clarity, the composition of each region is stressed and they are different from a chemical point of view. One can see that the interlayer is composed by silicon, carbon, hydrogen, and residual oxygen and it is in the middle of a thin film containing carbon, hydrogen and residual oxygen and the substrate containing, basically, iron atoms. Thus, two different interfaces should be developed in such a material system: the outermost aC:H(O)/SiCx:H(O) and the innermost SiCx:H(O)/Fe(C).
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Under the light of these different chemical environments, an interlayer has two different possible points of failure, i.e., the outermost and the innermost interfaces. In our studied material system, the outermost interface is constituted by carbon-carbon and carbonsilicon bonds and the innermost interface is constituted by carbon-iron and silicon-iron bonds, both acting as a nanoglue [27]. These different chemical structures, i.e., different energies involving different chemical bonds at both interfaces must change the adhesion energy of each interface, which could induce different critical loads for thin film delamination depending on the type and density of chemical bonding at each interface. Figures 4 a to f show the SEM images and EDS analysis of the worn surfaces after sliding experiments for two samples where the interlayers were deposited at 300oC and 550oC. At the top, Fig. 4 b shows the SEM image in a region where the DLC was removed and Fig. 4 a and c show the EDS analysis for iron and silicon, respectively, for the sample with the interlayer deposited at 300oC. At the bottom, Fig. 4 e shows the SEM image in a region where the DLC was removed and Fig. 4 d and f show the EDS analysis for iron and silicon, respectively, for the sample with the interlayer deposited at 550oC. In both cases, the sliding direction was the same and the SEM images show the surface microstructure of a wear track where the delamination of the DLC film is evident (see Fig. 4 b and e). However, the chemical analysis provides different information. On one hand, in the sample with the interlayer deposited at 300oC, both iron and silicon were detected on the surface of the region where the DLC film was removed. On the other hand, in the sample with the interlayer deposited at 550oC, only iron is apparent in the region where DLC delamination was removed. Moreover, in the sample with the interlayer deposited at 300oC the silicon signal is brighter than the iron signal, which suggests that the silicon-containing interlayer still remains on the substrate (steel). Therefore, the silicon-containing interlayer was removed together 8
with the DLC film in the sample with the interlayer deposited at 550oC, indicating a different adhesion failure mechanism, in this new case, at the innermost interface (SiCx:H/steel). Figure 5 a and b show the SEM images of the sample with the interlayer deposited at 300oC. The SEM images were taken in a tilt of 3 degrees in order to analyze both the surface and cross-section of sample. These images are giving more evidence about two different failure mechanisms for the DLC delamination. Whereas the silicon-containing interlayer remains adhered on the steel substrate after DLC delamination in the proposed “failure mechanism I” (please, see the wear track in Fig. 5 a), the same interlayer was removed together with the DLC film in the proposed “failure mechanism II” (please, see the wear track in Fig. 5 b). Moreover, the failure mechanisms to delaminate the DLC films rendered two different critical loads, being Lc2 related to the failure mechanism I and Lc3 related to the failure mechanism II. Lc1 is defined as the critical load to start the plastic deformation of DLC thin films and it is not the focus of the present work (no delamination). Table 2 introduces the critical loads for both types of DLC delamination (with and without the removal of the interlayer) for the sample with the interlayer deposited at 300oC. One can see that Lc3 is greater than Lc2. According to the Hertz contact model [19], the shear stress distribution shows its maximum value underneath the surface and, also, depends on depth with the applied normal load. The shear stress induces mismatching in the DLC/interlayer/substrate sandwich structure where there are two different interfaces: DLC/interlayer (the outermost interface) and interlayer/substrate (the innermost interlayer). Chemically speaking, the interface is the weakest point of the sandwich structure due to the high concentration of dangling bonds and defects that decrease the mechanical strength of the material system as a whole. Taking into account that chemical point of view, the delamination failure is to be initiated with more 9
probability at any interface (the outermost and/or the innermost interfaces) than in the coating, silicon-containing interlayer or substrate bulks. A previous work has shown that the maximum shear stress is moved away from the coating/substrate interface with the increasing thickness of the coating [28]. Moreover, a multilayer architecture decreases the maximum shear stress and can avoid the coating delamination [29,30]. However, to the best of our knowledge, no evidence was found about the correlation among the shear stress distribution, the applied normal load and the interface position in depth concerning the delamination mechanism of a DLC/interlayer/substrate sandwich structure. It is important to note again that although we performed unidirectional sliding tests (dynamic condition), the Hertzian stress equations that were used represent a static condition (normal load without sliding) due to we considered a frictionless tribological system (coefficient of friction 0.07) [19,23]. Thus, we are estimating the real dynamic situation by considering a frictionless sliding motion. In addition, previous results have shown that the critical shear stress does not change (within the experimental error) at different DLC coating thicknesses by using the Herzt theory in a dynamic situation [31]. Indeed, this work was performed in a similar experimental setup with a diamond tip sliding on a flat DLC coating, which is a tribological system of very low friction [31]. Figure 6 shows the evolution of the depth where the shear stress is a maximum as a function of the normal load by applying the Hertz contact model in a mechanical system constituted by a diamond sphere on a flat DLC surface (please, see Fig. 1). One can see that the depth for maximum shear stress increases monotonously with the increasing of the applied normal load. Moreover, in the range of the applied normal loads in the present work, the depth for maximum shear stress goes from approximately 0.5 m to 2.3 m. For the sample with the interlayer deposited at 300oC, the thicknesses of the DLC thin film and the silicon-containing 10
interlayer are, approximately, 1.80 m and 0.3 m, respectively, and they are also shown in Fig. 6. It is important to note that these thicknesses (1.8 m for DLC only and 2.1 m for DLC plus SiCx:H interlayer) are in agreement with the depths for maximum shear stress calculated with the measured critical loads Lc2 and Lc3 (please, see both Table 2 and Fig. 6). Figure 7 a and b show a schematic representation of both failure mechanisms (I and II) in combination with the shear stress distribution at the average applied normal load for Lc 2 and Lc3, respectively. In both cases, the sample with the interlayer deposited at 300oC is considered. According to our calculations, the maximum shear stresses when normal forces of 313.8 mN and 461.4 mN are applied (please, see Lc2 and Lc3 in Table 2) take place at a depth of approximately 1.80 m and 2.10 m, i.e., the thickness where the outermost interface DLC/SiCx:H is located and the thickness where the innermost interface SiCx:H/steel is located, respectively. By analyzing the shear stress distributions, the innermost interface seems to be more strength than the outermost interface in terms of adhesion. Indeed, the removal of both the DLC and the silicon-containing interlayer occurs at a higher shear stress than the removal of DLC only. Thus, the adhesion energy due to Si-Fe chemical bonds plus defects (innermost interface) is greater than C-Si and C-C chemical bonds plus defects (outermost interface) [20,26]. In the light of these findings, four different tribological scenarios may be described: 1. when the shear stress is higher than both adhesion forces at the outermost and innermost interfaces, either the DLC or DLC + interlayer would be removed; 2. when the shear stress is higher than the adhesion force at the innermost interlayer but lower than the adhesion force at the outermost interlayer, the DLC + interlayer would be removed only; 3. when the shear stress is higher than the adhesion force at the outermost interlayer but lower than the adhesion force at the innermost interlayer, the DLC would be removed only; 4. when the shear stress is 11
lower than both adhesion forces at the outermost and innermost interfaces, neither the DLC nor interlayer would be removed. Finally, these shear stress distributions could be used to analyze the chemical strength in terms of the adhesion energy at each interface, focusing in atomistic models that deal with the quantity and force of the chemical bonding at the interfaces.
5. Conclusions: This understanding of the relevance of considering the outermost and innermost interfaces of a SiCx:H interlayer could open new pathways not only in the improvement of the tribological behavior for DLC deposited on steel, but also to tailor the interface chemistry that controls the DLC adhesion on steels by using intermediate layers. The Hertz contact theory can be used to calculate the shear stress distribution and the depth for maximum shear stress in order to correlate and evaluate the adhesion strength at the interfaces of the adhesion interlayer. Consequently, plasma processes that are used to obtain DLC could be optimized in order to increase the adhesion of such thin films in parts of mechanical devices that aim to reduce friction and save energy.
Acknowledgements: The authors are grateful to UCS, INCT-INES-CNPq (Grant # 554336/2010-3), CAPES (Grant # Brafitec 087/11), PETROBRAS, and SUMA2 Network Project, 7th Framework Program of the European Commission (IRSES Project # 318903) for financial support. FC and CAF are CNPq fellows. FC is in part supported by PETROBRAS.
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References
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[21] Dufrène SMM, Cemin F, Soares MRF, Aguzzoli C, Maia da Costa MEH, Baumvol IJR, Figueroa CA. Hydrogenated amorphous carbon thin films deposited by plasma-assisted chemical vapour deposition enhanced by electrostatic confinement: structure, properties and modelling. Appl Phys A: Mater Sci Process 2014;117:1217-25. [22] Corujeira Gallo S, Crespi AE, Cemin F, Figueroa CA, Baumvol IJR. Electrostatically Confined Plasma in Segmented Hollow Cathode Geometries for Surface Engineering. IEEE Trans Plasma Sci 2011;39:3028–32. [23] Fischer-Cripps, A. C. Introduction to contact mechanics. 2nd Edition. Springer; 2007. [24] http://www.mesys.ch/?page_id=157&lang=en. [25] Cho S-J, Lee K-R, Eun, KY, Hahn JH, Ko D-H. Determination of elastic modulus and Poisson's ratio of diamond-likecarbon flms. Thin Solid Films 1999;341:207-10. [26] Spear KE, Dismukes JP. Synthetic Diamond – Emerging CVD Science and Technology. 1st ed. New York: Wiley; 1994 (p. 315). [27] Cemin F, Bim LT, Leidens LM, Morales M, Baumvol IJR, Alvarez F, Figueroa CA. Identification of the chemical bonding prompting adhesion of a-C:H thin films on ferrous alloys intermediated by a SiCx:H buffer layer. ACS Appl. Mat. & Interfaces. In Press. [28] Xiaohui D, Hongwei Z, Lei W. Hertzian Contact Analysis of Ceramic/Metal Functionally Graded Coating. Advanced Materials Research 2013; 787:582-587. [29] Lackner JM, Major L, Kot M. Microscale interpretation of tribological phenomena in Ti/TiN soft-hard multilayer coatings on soft austenite steel substrates. Bull. Pol. Ac.: Tech. 2011;59:343-355 [30] Zhang XC, Xu BS, Wang HD, Wu YX, Jiang Y. Hertzian contact response of singlelayer, functionally graded and sandwich coatings. Mat. Design 2007;28:47-54.
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Figure Captions
Figure 1: Schematic of the contact condition where the conical indenter, which finalizes in a half sphere, is in mechanical contact with the coating surface (DLC).
Figure 2: (a) Surface images of two samples obtained at different SiCx:H interlayer deposition temperatures (200oC and 300oC); (b) and (c) show the SEM images (in crosssection) of both samples where the DLC/interlayer/steel sandwich structure is apparent.
Figure 3: Chemical composition profile of the sample where the interlayer was deposited at 200oC. The DLC/interlayer/steel system is chemically described.
Figure 4: The SEM images and EDS analysis of the worn surfaces for two samples where the interlayers were deposited at 300oC and 550oC. Fig. 3 b/e show the SEM image of surfaces and Fig. 3 a/d and c/f show the EDS analysis for iron and silicon, respectively (interlayer deposited at 300oC/550oC).
Figure 5 a and b: Failure mechanisms for Lc2 and Lc3, respectively, in the sample where the interlayer was deposited at 300oC.
Figure 6: Depth for maximum shear stress as a function of the applied normal load calculated by using the Hertz contact theory. The thicknesses of the DLC and the DLC + interlayer are also shown (interlayer deposited at 300oC).
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Figure 7 a and b: Schematic representation of the failure mechanisms (I and II) of DLC delamination at the outermost and the innermost interfaces, respectively, of for the same interlayer deposited at 300oC. The shear stress distributions at the measured critical loads (L2 and Lc3) and the depths for maximum shear stress are also shown.
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Figure 1
Figure 1 Cemin et al.
Figure 2
No external forces applied for
Critical load for DLC delamination
DLC delamination
298 ± 7 mN
Figure 2 abc Cemin et al.
Figure 3
Figure 3 Cemin et al.
Figure 4
Figure 4 a-f Cemin et al.
Figure 5
Figure 5 a and b Cemin et al.
Depth for maximum shear stress (µm)
Figure 6
3.0
Thickness of DLC film plus SiCx:H interlayer
2.5 2.0 1.5
Thickness of DLC film only
1.0 0.5 0.0
200
400
600
800
Applied normal load (mN) Figure 6 Cemin et al.
Figure 7a
Figure 7a Cemin et al.
Figure 7b
Figure 7b Cemin et al.
Table 1
Table 1: Mechanical properties for diamond and DLC used in the calculations involving the Hertz contact theory and MESYS (software for surface engineering).
Elastic modulus (GPa) Poisson's ratio
Diamond 1200 0.2
DLC 90 0.22
Table 2
Table 2: Different critical loads determined by performing scratch tests and direct microscopic observation for the sample with the interlayer deposited at 300oC.
Lc1 Lc2 Lc3
Normal Load (mN) Standard Deviation 70.5 6.1 313.8 6.6 461.4 6.0