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c-BN multilayer system
Applied Surface Science 257 (2010) 1098–1104
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Enhancement of surface mechanical properties by using TiN[BCN/BN]n /c-BN multilayer system ˜ ˜ c , L. Yate d , J. Esteve d , P. Prieto b,e H. Moreno a , J.C. Caicedo b,∗ , C. Amaya b , J. Munoz-Salda na a
Laboratorio de Recubrimientos Duros, CDT-ASTIN SENA, Cali, Colombia Grupo de Películas Delgadas, Universidad del Valle, Cali, Colombia Centro de Investigación y de Estudios Avanzados del IPN, Unidad Querétaro, México, Mexico d Department de Física Aplicada i Óptica, Universitat de Barcelona, Catalunya, Spain e Centro de Excelencia en Nuevos Materiales, CENM, Cali, Colombia b c
a r t i c l e
i n f o
Article history: Received 22 June 2010 Received in revised form 3 August 2010 Accepted 4 August 2010 Available online 11 August 2010 PACS: 61.05.c 62.20.Qp 68.65.Ac Keywords: Multilayer coatings Magnetron sputtering Mechanical properties
a b s t r a c t The aim of this work is to improve the mechanical properties of AISI 4140 steel substrates by using a TiN[BCN/BN]n /c-BN multilayer system as a protective coating. TiN[BCN/BN]n /c-BN multilayered coatings via reactive r.f. magnetron sputtering technique were grown, systematically varying the length period () and the number of bilayers (n) because one bilayer (n = 1) represents two different layers (tBCN + tBN ), thus the total thickness of the coating and all other growth parameters were maintained constant. The coatings were characterized by Fourier transform infrared spectroscopy showing bands associated with h-BN bonds and c-BN stretching vibrations centered at 1400 cm−1 and 1100 cm−1 , respectively. Coating composition and multilayer modulation were studied via secondary ion mass spectroscopy. Atomic force microscopy analysis revealed a reduction in grain size and roughness when the bilayer number (n) increased and the bilayer period decreased. Finally, enhancement of mechanical properties was determined via nanoindentation measurements. The best behavior was obtained when the bilayer period () was 80 nm (n = 25), yielding the relative highest hardness (∼30 GPa) and elastic modulus (230 GPa). The values for the hardness and elastic modulus are 1.5 and 1.7 times greater than the coating with n = 1, respectively. The enhancement effects in multilayered coatings could be attributed to different mechanisms for layer formation with nanometric thickness due to the Hall–Petch effect; because this effect, originally used to explain increased hardness with decreasing grain size in bulk polycrystalline metals, has also been used to explain hardness enhancements in multilayered coatings taking into account the thickness reduction at individual single layers that make up the multilayered system. The Hall–Petch model based on dislocation motion within layered and across layer interfaces has been successfully applied to multilayered coatings to explain this hardness enhancement. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The PVD method via magnetron sputtering processes has been used for coating cutting tools since the 1980s. Thus, among the processes used for this purpose, magnetron sputtering and its reactive variants have proven to be very successful. Many studies reported in the literature, coating materials such as TiCN [1], AlCN [2], YSZ [3], CrAlN [4], and BiMnO3 [5] have been synthesized with high stability, confirming the advantages of the PVD process. Surface engineering of metallic substrates with protective coatings like single-layered hard coatings such as TiCN, BCN, and BN, has
∗ Corresponding author at: Universidad del Valle, Grupo de Películas Delgadas, Calle 13 # 100 - 00 Melemdez, Cali, Valle, Colombia. Tel.: +57 2 339 46 10; fax: +57 2 339 32 37. E-mail address: [email protected] (J.C. Caicedo). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.024
played an important role a few years ago, due to the mechanical and tribological properties of metallic substrates reflected in higher hardness and toughness, as well as in high corrosion resistance compared to those of steel substrates used in industrial applications [6]. On the other hand, the isostructural multilayered systems, composed by metal/ceramic and ceramic/ceramic superlattices, have received much attention because these combinations can exhibit high hardness values, often increasing by more than 100% over the rule-of-mixture values, while retaining good ductility and high anticorrosive properties [7]. Several metal/ceramic and ceramic/ceramic superlattices have been studied during the past 10 years, including Ti/TiN [8], TiCN/TiNbCN [9], BCN/CN/BN [10], or the B4 C/BCN/c-BN [11]. The nitride coatings, characterized by high hardness, and wear resistance can be used for strengthening and protection in industrial steels. A significant disadvantage of such coatings is their thickness, which in many cases cannot be exceeding 5 m. The use of the physical vapor deposition (PVD)
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techniques such as magnetron sputtering has been allowed with the development of multilayered coatings having a higher combination of properties to enhance the strength of machine parts. One of the most important parameters determining the service properties of nitride coatings is the size and quantity of the grains and pores, which as a result of the thinness of the protective layer pierce it all the way through to the base and serve as mechanical damage origins. It is known that PVD coatings have significantly less grain size, roughness and porosity in comparison to coatings obtained via other methods of vacuum technology and also through gas thermal spraying methods. In some cases, magnetron sputtering makes it possible to obtain pore-free coatings mainly in multilayered with a thickness in the order of 1 m, which is the result of the high packing density of the atoms being concentrated. However, according to data from some works, the porosity of nitride coatings may reach 5%, differing from the multilayer nitrides where the porosity decreases to 1% [12]. Therefore, the mechanical resistance of multilayer nitride coatings is higher than that of single-layered coatings, which may be explained by their greater thickness and features of their microstructure, including the absence of circular crystal characteristic of single-layered coatings; moreover, failure of nitride coatings under the action of a mechanical effect occurs at structural defects, which determine their mechanical resistance and tribological properties [12]. In this work, the BCN/BCN multilayered coatings have been deposited on TiN buffer-layer films to increase adhesion to steel substrates; also, the c-BN over-layer (top layer) coatings have been deposited on BCN/BCN multilayered coatings because the c-BN coatings show relevant mechanical properties. The BCN/BN multilayered coatings are used as a coupling system between TiN and c-BN coating. With this in mind, TiN[BCN/BN]n /cBN multilayered coatings with total thickness of 3 m have been designed and deposited on Si (1 0 0) and AISI 4140 steel substrates with bilayer periods in a broad range, from nanometers to hundreds of nanometers, to study the microstructural evolution with decreasing bilayer thickness () and their related mechanical properties for possible industrial applications in processes where high mechanical performance is required.
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assembly was analyzed via scanning electron microscope (SEM), by using a Leika 360 Cambridge Instrument equipped with a height sensitivity back-scattered electron detector. Grain size and roughness were determined via atomic force microscopy (AFM-Asylum Research MFP-3D® ) and calculated by scanning probe image processor (SPIP® ) that is the standard program for processing and presenting AFM data and this software has become the de facto standard for image processing at nanoscale. In this work the SPIP was used in the grain analysis for a quantitative study of grain size and moreover the SPIP was used for an advanced measurement of the surface roughness [9]. Hardness and elastic modulus measurements were performed by using a nanoindenter (UBI1-Hysitron) under load and unload modes with a matrix measurement of 25 points and maximum load of 10 mN. 3. Results and discussion 3.1. FTIR analysis Infrared spectrum of [BCN/BN]1 /c-BN coating deposited on Si(1 0 0) is shown in Fig. 1. The spectrum presents an intense absorption feature located at approximately 1400 cm−1 . This band originates from lattice stretching vibrations of the h-BN structure [13]. The peak around 1100 cm−1 can be related to Sp3 B–N (cBN) [14,15]. Peaks centered at 849 cm−1 , 747 cm−1 , and 1000 cm−1 can be associated with C–H, wurtzite hexagonal boron nitride (wh-BN), and boron carbide (B4 C) (B–C) bonds with vibrations of non-conjugated alkenes and azometinic groups, respectively [16,17]. C–H line comes possible from hydrocarbon contamination due to environmental effect. Cubic and hexagonal microdomains were obtained in the nitride coatings at 300 ◦ C, and nitrogen flux of 3.7 sccm, though no IR materials of Sp3 -like (cubic-BN-like) B–C bond could be available for reference to specify their presence in B–C–N and c-BN. Because three-dimensional bonds among B, C, and N atoms were unambiguously determined, it may be concluded that the amorphous B–C–N coatings are composed mainly of cubic c-BN-like and hexagonal-BN-like plain microdomains. The results agree well with other FTIR spectra obtained for B–C–N, wh-BN, and h-BN materials deposited via physical processes [16–18].
2. Experimental details 3.2. SIMS analysis TiN[BCN/BN]n /c-BN multilayered coatings were deposited onto silicon (1 0 0) and AISI 4140 steel substrates by using a multitarget magnetron reactive sputtering technique with an r.f. source (13.56 MHz) and stoichiometric Ti, B4 C, and BN targets with purity at 99.9% for all targets. The deposition parameters to obtain highquality coatings were sputtering power of 300 W for Ti, 350 W for B4 C, and 500 W for the BN target; substrate temperature of 300 ◦ C. The sputtering gas was a mixture of Ar 93% (50 sccm) and N2 7% (3.7 sccm) and a total working pressure of 6 × 10−3 mbar was used for all films (TiN, BCN and BN). An unbalanced r.f. bias voltage was applied, which generates a negative signal fixed at −100 V. Also, our magnetron sputtering device has a positioning substrate system in relationship to target spot; this parameter permits varying the bilayer number among 1, 5, 10, 20, and 25 bilayers, changing the bilayer period, therefore, the deposition times were the same for all multilayered coatings but the individual layers within the different bilayer numbers were deposited with different holding times in front of the targets. The Fourier transform infrared (FTIR) analysis of the coatings was carried out with a Shimatzu 8000 spectrometer, which uses a ceramic-type Nerst source in the range of 550–1500 cm−1 in transmission mode. Secondary ion mass spectroscopy (SIMS, Atomika 488) was carried out using oxygen ions at 9 keV and 400 nA, for the multilayer with thicker periods, and at 4.5 keV and 300 nA for the thinner periods in order to improve depth resolution and to avoid mixing drawbacks. The multilayer
Secondary ion mass spectroscopy (SIMS) analysis showed a clear modulation in all the multilayer coating compositions, confirming the previously designed multilayer structures. Fig. 2 depicts the
Fig. 1. FTIR transmittance spectrum for [BCN/BN]1 /c-BN coating grown at r.f. negative bias voltage of −100 V.
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Fig. 3. SEM micrograph of TiN[BCN/BN]10 /c-BN coatings n = 10, = 200 nm.
Fig. 2. SIMS depth-profile analysis for a TiN[BCN/BN]n /c-BN multilayered coating grown with bilayer number (n = 25) and bilayer period ( = 80 nm).
SIMS compositional depth-profile analysis of the TiN[BCN/BN]n /cBN multilayered coatings grown with n = 25 bilayered and bilayer period of = 80 nm. The modulation of carbon signal (12 C+ ) is clearly observed as SIMS analysis progresses in depth, confirming the number of bilayers (25) in this coating from the number of maxima in SIMS profile (B–C–N layers). The modulation in 12 C+ signal is 5 times lower than the 14 N+ signal in this case; but it reaches down to 10 times in multilayered with thicker bilayer periods, where SIMS sputtering mixing has less appreciable effects in depth resolution. Moreover, the 11 B+ signal from boron carbon nitride
multilayer, 48 Ti+ signal from titanium nitride deposited as single layer, and 56 Fe+ from steel substrate are recorded. The 12 C+ , 11 B+ and 14 N+ signals reach the same values measured in the B–C–N and B–N single-layered coatings; suggesting that boron carbon nitride and boron nitride have the same composition, both within all multilayered and in single-layered coatings. 3.3. Scanning electron microscopy analysis A first glimpse on BCN/BN multilayer modulation and microstructure was accomplished by SEM micrographs. Fig. 3 presents the cross-sectional image of a TiN[BCN/BN]10 /c-BN coating with n = 10 ( = 200 nm) (Fig. 3). The darkest contrast of B–C–N layers with respect to B–N layers allowed a clear determination of
Fig. 4. AFM images for a TiN[BCN/BN]n /c-BN multilayer with n = 1, 5, 10, 15, 20, and 25 (a–f) grown at r.f. negative bias voltage of −100 V.
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Fig. 5. Surface measurements obtained via AFM and SPIP analysis for TiN[BCN/BN]n /c-BN multilayered with a total thickness of 3 m and bilayer period from = 2 m to 80 nm: (a) correlation between roughness and the bilayer number, and (b) correlation between grain size and the bilayer number.
the layer structure. These TiN[BCN/BN]10 /c-BN coatings presented a single layer of TiN with thickness around 100 nm, the BCN/BN multilayer with thickness around 2 m, and c-BN with thickness around 800 nm with well-defined and uniform periodicity. All the multilayer stacks were resolved by SEM and confirmed quite precisely the previously designed values of bilayer thickness, as well as the total thicknesses. The only slight deviation observed by SEM imaging was on relative thicknesses. In Fig. 3, the BCN layers seem a bit thicker than the B–N ones. These deviations are difficult to evaluate because of the poor resolution of the BCN/BN interfaces in this technique. 3.4. Atomic force microscopy analysis AFM was carried out to quantitatively study the surface morphology of our samples in relation to a decrease in bilayer periods or an increase of bilayer numbers in TiN[BCN/BN]n /c-BN multilayered coatings deposited onto Si (1 0 0). Fig. 4 shows AFM images for multilayered coatings with statistical distribution of grain size that was analyzed by 15 m × 15 m AFM images for the (a) n = 1, (b) n = 5, (c) n = 10, (d) n = 15, (e) n = 20, and (f) n = 25. The correlation between roughness part (a) and grain size part (b) with the bilayer number is shown in Fig. 5a and b, respectively. The quantitative values were extracted from the AFM images by means of statistical analysis scanning probe image processor (SPIP® ). The TiN[BCN/BN]10 /c-BN multilayer coatings has the same total thickness, around 3.0 m. The roughness was greater than for a coating with n = 1 or high bilayer period = 2 m, and lower for multilayered coatings with a higher bilayer number (n = 25) or lower bilayer period = 80 nm, indicating that coatings with (n = 1) grow more disordered than do multilayered with (n = 25). The grains of the multilayered coatings are smaller than those for the coatings with a low bilayer period, because the Ar+ ion bombardment on coatings stimulates a greater number of nucleation places, because of the reduction of individual thicknesses for each layer when the bilayer period is reduced and bilayer numbers are increased; this implicates a decrease in the entire surface roughness [9]. In this work was take in account that grain size values comes from c-BN top layer, but c-BN grain size is affected by changes in grain size of inner layers (BCN/BN) as function of reduction in the bilayer period () or increase in the bilayer number (n), because the BCN/BN multilayered were used for improve the adhesion of c-BN (hardness top layer) to TiN single layer. On the other hand, many authors reported a correlation between AFM and XRD results, because the AFM analysis delivers surface information (with lat-
eral resolution), XRD collects information from the space around the normal vector on the surface (from a cross-section); and if this is taken into account it is possible to consider that PVD coating columns can grow “vertically” from the substrate to the “top” and can change their cross-section when bilayer thickness is modified. 3.5. Mechanical properties Typical load–displacement indentation curves of multilayered coatings are shown in Fig. 6, using a standard Berkovich indenter. The indentation matrix images via AFM are shown in Fig. 4. The values of elasticity modulus, Er , and hardness, H, were obtained by using Oliver and Pharr’s method in multilayered coatings deposited onto AISI 4140 steel substrates [19]. Hardness values of the TiN[BCN/BN]n /c-multilayer measured by nanoindentation are presented in Fig. 7a, as functions of period thickness, , and/or in function with bilayer number, n. Elastic modulus values of multilayered coatings are also presented in Fig. 7b, showing relevant differences in their values. The hardness and elastic modulus in these multilayered coatings varied from 19 to 30 GPa and from 130 to 225 GPa, respectively. The highest hardness of the multilayer set, 30 GPa, was obtained by the thinnest bilayer period ( = 80 nm). This increase in mechanical properties is related to the remarked
Fig. 6. Nanoindentation measurements showing load–displacement indentation curves of TiN[BCN/BN]10 /c-BN multilayered coatings.
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Fig. 7. Mechanical properties for TiN[BCN/BN]n /c-BN multilayered coatings with bilayer period from = 2 m to 80 nm: (a) hardness as a function of bilayer number (n),and (b) elastic modulus as a function of bilayer number (n).
heterostructure effect that is present when a perfect assembly occurs for BCN and BN coatings with c-BN single layer deposited over BCN/BN system and, moreover, the BCN/BN periodicity that remained constant for lowest bilayer period analyzed by SIMS and SEM results. The enhancement in the mechanical properties can be associated with hardness improvement by using multilayered materials. This improvement has led to lower grain size, as observed from AFM images in Fig. 4, generating an increase of the density for the hard coating [9,20,21]. The high interface density of nanoscale multilayered structures contributes to impeding dislocation motion and the dislocation glide across the interfaces, which would require a critical yield stress being related to the difference in the elastic shear modulus of the single-layered materials [22–24]. Many authors have used the Hall–Petch effect to explain the material hardening, therefore, it is possible to apply the Hall–Petch effect when the ceramic materials like hard multilayer coatings with a bilayer period () higher than that of 5.2 nm are obtained, because the multilayered with lower are within nanoscale regime, and thus, the dislocations should not occur in nanoscale structures below a certain value of grain sizes [25]. Fig. 7a shows that the hardness for TiN[BCN/BN]n /c-BN coatings is in relation to the reduction of the modulation period. Therefore from AFM results are possible observed that indentation track exhibit low deformation size when the mechanical properties are improved as function of a increasing in the bilayer number. It is argued that there are some correlations between the hardness of a multilayered coating and the modulation period [22,23]. Hardness enhancement for TiN[BCN/BN]n /c-BN was attributed to many interfaces that blocked the dislocation movement across the interface between B–C–N layer and B–N layer due to differences in the shear module of the individual layer materials, and by coherency strain causing periodical strain–stress fields in the case of latticemismatched multilayered coatings. Each interface also functions like a grain boundary in a Hall–Petch [26] related mechanism such as that of the dislocation pile-up that offers a strong interaction again with interfaces in general; therefore, the interfaces and strain harden layer presents a serve crack tip deflectors which strengthens the mechanical properties into the coatings [22–24]. According to Kim et al. [24] another condition for enhancing the hardness of multilayered coatings with respect to the single-layer coating is that the layer in the multilayered system has to be discrete, which was found in this study via SIMS and SEM analyses (Figs. 2 and 3) respectively. Also, the enhancement of mechanical properties for a TiN[BCN/BN]n /c-BN multilayer is according to the increase in hard-
ness such as what has been shown in multilayered-type coatings for diverse material systems (e.g. TiCN/TiNbCN, TiN/VN, TiN/NbN, TiN/CrN, TiN/TiAlN, and TiC/TiB2 ) [9,16,25,27]. Yashar et al. [26] used a Hall–Petch approach to model the mechanical behavior of multilayered materials with layer thicknesses as low as 1–100 nm, transforming the Hall–Petch approach into the next equation presented by Caicedo et al. [9]: Hm = H(f1+f2) + kIM Dt −1/2
(1)
where Hm is the multilayer hardness, H(f1+f2) is the hardness from layer 1 and layer 2, kIM is a constant measuring the relative hardening contribution of the interface between layer 1 and layer 2, and Dt is the bilayer period () [9]. The model predicts the overall behavior of the hardness on that is observed in most multilayered systems. Previous work has shown that a maximum hardness would be expected when the individual components of the multilayer have a relative equal thickness, as presented in this paper [9,28]. Furthermore, a dependence of the elasticity with the bilayer number (n) or bilayer period () was observed; the higher bilayer number corresponds to the higher elastic modulus. Therefore, it is possible to conclude that the elasticity and elastic recovery (R) of the multilayer coatings are improved by increasing the interface number. So, from the nanoindentation measurement, the typical values of elasticity modulus, Er , and hardness, H were obtained by using the Oliver and Pharr’s method [19]. Thus the elastic recovery for all TiN[BCN/BN]n /c-BN multilayered coatings was calculated by using the following equation [9]: R=
ımax − ıp ımax
(2)
where ımax is the maximum displacement and ıp is the residual or plastic displacement [9,29]. The equation data were taken from the load–penetration depth curves of indentations for each coating according to Fig. 6. Fig. 8a shows the increase in the elastic recovery as function of increase of bilayer number when ımax and ıp values from load and displacements results (Fig. 6), are introduced in Eq. (2), thus it is possible to observe the interface effect on multilayer system, because when the bilayer number (n) increases, in this way the interface number is increased, therefore, can generate a hardening on multilayer hard coating, which is evidenced in the improvement of elastic recovery (R). In another way according to Kim et al. [30] there is a relationship between elasticity modulus and hardness named plastic deforma-
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Fig. 8. Elasto-plastic properties with plastic deformation for TiN[BCN/BN]n /c-BN multilayered coatings with bilayer period from = 2 m to 80 nm: (a) elastic recovery, R, and (b) plastic deformation resistance.
tion resistance (H3 /E2 ratio), this relation was calculated for all multilayered coatings in function of the bilayer number or bilayer period. Fig. 8b shows a considerable increase in the resistance to the plastic deformation (H3 /E2 ) as a function of the increasing of the bilayer number (n), this fact is due to the hardness and the elasticity modulus also increasing as the interface number increased for all multilayer coatings. This enhancement in plastic deformation resistance occurs when bilayer period () decreases, increasing the interface number for coatings with total thickness, producing, thus the crystallite refinement, point defect formations and the increase of interface number which improving the hardness of the TiN[BCN/BN]n /c-BN multilayer coatings. As shown in Fig. 8, the TiN[BCN/BN]n /c-BN multilayer coatings increased the plastic deformation resistance and elastic recovery with respect to coatings deposited with lower bilayer number. The maximum value was reached for the bilayer number (n = 25) and the bilayer period ( = 80 nm), i.e. the plastic deformation due to the applied load is more markedly reduced than that of other multilayered systems with fewer bilayer numbers. This effect is clearly correlated to the reduction of grain size, increasing coating density, hardness, and elastic recovery [9]. In general the bilayer number (n) or bilayer period () has some effects on multilayer that has been shown to exhibit very high hardness in multilayer composed of a nitride (e.g. BN) and a carbonitride (e.g. BCN). Nitride is used to provide significant ductility while the carbonitride gives the high hardness. In this nitride/carbonitride multilayer system, dislocation motion across layer interfaces should be difficult since the layers typically have different crystal structures showing the increase in the mechanical properties. The first theoretical explanation of the mechanical properties of multilayered materials as function of bilayer number (n) or bilayer period () was considered by Koehler [12]. He showed that dislocation motion is inhibited in a multilayered structure due to the image force on dislocations created by the different dislocation line energies in each layer when the bilayer number is increased. By creating a multilayer structure in which the two layers have a low (layer A) and a high (layer B) shear modulus, a shear stress of the order of G/100 (where G is the shear modulus) would be required to drive dislocations through the structure. This shear stress is an extremely high value, which has the same order of magnitude as the theoretical strength of a solid [26]. Koehler also noted that each layer should be thin enough so that dislocation generation cannot occur within the layers. Therefore, in multilayered with thin layer, the bilayer number (n) or bilayer period () produces
a stress field (− BN + BCN ) which increase the hardness. On the other hands the effect of (single) c-BN top layer is very important in the industrial application because the c-BN material exhibit high hardness which can be show relevant tribological properties, but this material present high residual stress, low adherence to steel substrate, so in this sense in necessary to applied the multilayer system (e.g. TiN[BCN/BN]n ) that help to increase the adherence of cBN top layer and relieving the residual stress within c-BN top layer. Then, the c-BN hardness and multilayered structure contribute in the improving of general mechanical properties which has been observed in TiN[BCN/BN]n /c-BN multilayer coatings. On the other hand with an indentation depth of the indenter of around 200 nm, the hardness and especially the Young’s modulus of the c-BN coatings (top layer) on tools were determined to be influenced by the films underneath. This influence is responsible for the slightly lower hardness values for c-BN films on tool substrates compared to c-BN films on silicon substrates and for the considerable variation of the Young’s modulus. A more detailed investigation of the elastic constants leads to higher values for the Young’s modulus. The results of these investigations will be published elsewhere [31,32]. However, the results of mechanical characterization of the c-BN-layer system on tools and on silicon are nearly the same and confirm the outstanding properties of c-BN coatings. Specifically, the hardness is twice as high as for typical TiN or TiAlN coatings [31]. In combination with the very low abrasive wear rates and the relatively low friction coefficient, the results emphasize the high potential of c-BN as a super hard and wear resistant tool coating. In comparison to the already previously published results [33–35], it was possible to increase the c-BN layer thickness on industrial AISI 4140 steel substrates with a TiN[BCN/BN]n multilayered precoating slightly over 1 m for c-BN top layer and simultaneously increase the elastic recovery, R, and plastic deformation resistance. So, with these properties of the c-BN coated cutting tools, application tests in turning and milling with high performance can be feasible. 4. Summary TiN[BCN/BN]n /c-BN multilayered coatings were successfully deposited by r.f. magnetron sputtering with bilayer periods ranging from 2 m to 80 nm. The FTIR results show the BN vibrations with the presence of c-BN. The SIMS and SEM analyses exhibit the multilayer evidence with a well-defined and uniform periodicity. AFM and SEM analyses confirmed well-defined multilayered structures with grain size and roughness lower than expected
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for coatings deposited with (n = 1) and, moreover, this work showed important variations in layer thickness ratio. The highest hardness and elastic modulus, 30 GPa and 225 GPa, respectively, were observed for TiN[BCN/BN]n /c-BN deposited with n = 25 and = 80 nm. The enhancement in hardness of the multilayered coating was attributed to the many interfaces that blocked the dislocation movement across the interface between B–C–N layers and B–N layers due to differences in the shear module of the individual layer materials, and by coherency strain causing periodical strain–stress fields in the case of lattice-mismatched multilayered coatings. Therefore, the highest hardness was found for the nanometric coatings (n = 25, = 80 nm), reaching a maximum value of 30 GPa. The explanations for this hardness enhancement in TiN[BCN/BN]n /c-BN multilayer are based on interfaces acting on dislocations and Hall–Petch models, which give a good overall picture of the hardness enhancements observed. Furthermore an increase in the elastic recovery resistance and plastic deformation resistance for TiN[BCN/BN]25 /c-BN ( = 80 nm) was observed with respect to the multilayer coating deposited with lower bilayer number (n = 1). So, the maximum value was reached for the coating deposited with (n = 25), therefore, an increase of the elastic recovery (R) and plastic deformation resistance around 54% and 26%, respectively was observed. So, this study revealed that bilayer period () and bilayer number (n) have a marked influence on the mechanical properties of all the coatings. Acknowledgement This work was supported by the Center of Excellence for Novel Materials (CENM) under Colciencias/CENM contract # RC-0432005. Colciencias-Univalle. References [1] J.C. Caicedo, C. Amaya, L. Yate, W. Aperador, G. Zambrano, M.E. Gómez, ˜ ˜ P. Prieto, Appl. Surf. Sci. 256 (2010) J. Alvarado-Rivera, J. Munoz-Salda na, 2876–2883. [2] L. Yate, J.C. Caicedo, A. Hurtado Macias, F.J. Espinoza-Beltrán, G. Zambrano, J. ˜ ˜ P. Prieto, Surf. Coat. Technol. 203 (2009) 1904–1907. Munoz-Salda na, ˜ ˜ G. [3] C. Amaya, W. Aperador, J.C. Caicedo, F.J. Espinoza-Beltrán, J. Munoz-Salda na, Zambrano, P. Prieto, Corros. Sci. 51 (2009) 2994–2999.
[4] J.E. Sanchéz, O.M. Sanchéz, L. Ipaz, W. Aperador, J.C. Caicedo, C. Amaya, M.A. ˜ ˜ G. Zambrano, Hernández Landaverde, F. Espinoza Beltran, J. Munoz-Salda na, Appl. Surf. Sci. 256 (2010) 2380–2387. [5] M. Grizalez, E. Martínez, J.C. Caicedo, J. Heiras, P. Prieto, Microelectron. J. 39 (2008) 1308–1310. [6] B.S. Kim, G.S. Kim, S.Y. Lee, B.Y. Lee, Surf. Coat. Technol. 202 (2008) 5526. [7] J. Romero, J. Esteve, A. Lousa, Surf. Coat. Technol. 188 (2004) 338–343. [8] J.C. Caicedo, C. Amaya, L. Yate, O. Nos, M.E. Gómez, P. Prieto, Mater. Sci. Eng. B 171 (2010) 56–61. [9] J.C. Caicedo, C. Amaya, L. Yate, G. Zambrano, M.E. Gómez, J. Alvarado-Rivera, J. ˜ ˜ P. Prieto, Appl. Surf. Sci. 256 (2010) 5898–5904. Munoz-Salda na, [10] C. Morant, D. Cáceres, J.M. Sanz, E. Elizalde, Diamond Relat. Mater. 16 (2007) 1441–1444. [11] G. Bejarano, J.M. Caicedo, E. Baca, P. Prieto, A.G. Balogh, S. Enders, Thin Solid Films 494 (2006) 53–57. [12] J.S. Koehler, Phys. Rev. B 2 (1970) 547. [13] X. Chu, S.A. Barnett, J. Appl. Phys. 77 (1995) 4403. [14] M. Murakawa, S. Watanabe, S. Miyake, Mater. Sci. Eng. A 140 (1991) 753–758. [15] N. Tanabe, T. Hayashi, M. Iwaki, Diamond Relat. Mater. 1 (1992) 151–156. [16] A. Weber, U. Bringmann, R. Nikulski, C.-P. Klages, Diamond Relat. Mater. 2 (1993) 201–206. [17] S. Noyan Dilek, H.Ö. Özbelge, N.A. Sezgi, T. Dou, Ind. Eng. Chem. Res. 40 (3) (2001) 751–755. [18] M.P. Johansson, L. Hultman, S. Daaud, K. Bewilogua, H. Lüthje, A. Schütze, S. Kouptsidis, G.S.A.M. Theunissen, Thin Solid Films 287 (1996) 193–201. [19] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. ˜ [20] J.C. Caicedo, G. Bejarano, M.E. Gómez, P. Prieto, C. Cortéz, J. Munoz, Phys. Stat. Sol. C 4 (11) (2007) 4127–4133. [21] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996) 173–184. [22] M. Stueber, H. Holleck, H. Leiste, K. Seemann, S. Ulrich, C. Ziebert, J. Alloy Compd. 483 (2009) 321–333. [23] S.-K. Tien, J.-G. Duh, Thin Solid Films 494 (2006) 173–178. [24] G.S. Kim, S.Y. Lee, J.H. Hahn, Surf. Coat. Technol. 171 (2003) 91–95. [25] J.-K. Park, Y.-J. Baik, Surf. Coat. Technol. 200 (2005) 1519–1523. [26] P.C. Yashar, W.D. Sproul, Vacuum 55 (1999) 179–190. [27] H. Holleck, M. Lahres, P. Woll, Surf. Coat. Technol. 41 (1990) 179–190. [28] D.B. Lewis, I. Wadsworth, W.D. Münz, R. Kuzel Jr., V. Valvoda, Surf. Coat. Technol. 116–119 (1999) 285. [29] V. Hajek, K. Rusnak, J. Vlcek, L. Martinu, H.M. Hawthorne, Wear 213 (1997) 80. [30] G.S. Kim, S.Y. Lee, J.H. Hahn, Surf. Coat. Technol. 171 (2003) 91. [31] M. Keunecke, E. Wiemann, K. Weigel, S.T. Park, K. Bewilogu, Thin Solid Films 515 (2006) 967–972. [32] F. Richter, M. Herrmann, F. Molnar, T. Chudoba, N. Schwarzer, M. Keunecke, K. Bewilogua, X.W. Zhang, H.-G. Boyen, P. Ziemann, Surf. Coat. Technol. 201 (2006) 3577. [33] K. Yamamoto, M. Keunecke, K. Bewilogua, Thin Solid Films 377–378 (2000) 331. [34] M. Keunecke, K. Bewilogua, E. Wiemann, K. Weigel, R. Wittorf, H. Thomsen, Thin Solid Films 494 (2006) 58. [35] K. Yamamoto, M. Keunecke, K. Bewilogua, Z. Czigany, L. Hultmann, Surf. Coat. Technol. 142–144 (2001) 881.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhancement of surface mechanical properties by using TiN[BCN/BN]n /c-BN multilayer system ˜ ˜ c , L. Yate d , J. Esteve d , P. Prieto b,e H. Moreno a , J.C. Caicedo b,∗ , C. Amaya b , J. Munoz-Salda na a
Laboratorio de Recubrimientos Duros, CDT-ASTIN SENA, Cali, Colombia Grupo de Películas Delgadas, Universidad del Valle, Cali, Colombia Centro de Investigación y de Estudios Avanzados del IPN, Unidad Querétaro, México, Mexico d Department de Física Aplicada i Óptica, Universitat de Barcelona, Catalunya, Spain e Centro de Excelencia en Nuevos Materiales, CENM, Cali, Colombia b c
a r t i c l e
i n f o
Article history: Received 22 June 2010 Received in revised form 3 August 2010 Accepted 4 August 2010 Available online 11 August 2010 PACS: 61.05.c 62.20.Qp 68.65.Ac Keywords: Multilayer coatings Magnetron sputtering Mechanical properties
a b s t r a c t The aim of this work is to improve the mechanical properties of AISI 4140 steel substrates by using a TiN[BCN/BN]n /c-BN multilayer system as a protective coating. TiN[BCN/BN]n /c-BN multilayered coatings via reactive r.f. magnetron sputtering technique were grown, systematically varying the length period () and the number of bilayers (n) because one bilayer (n = 1) represents two different layers (tBCN + tBN ), thus the total thickness of the coating and all other growth parameters were maintained constant. The coatings were characterized by Fourier transform infrared spectroscopy showing bands associated with h-BN bonds and c-BN stretching vibrations centered at 1400 cm−1 and 1100 cm−1 , respectively. Coating composition and multilayer modulation were studied via secondary ion mass spectroscopy. Atomic force microscopy analysis revealed a reduction in grain size and roughness when the bilayer number (n) increased and the bilayer period decreased. Finally, enhancement of mechanical properties was determined via nanoindentation measurements. The best behavior was obtained when the bilayer period () was 80 nm (n = 25), yielding the relative highest hardness (∼30 GPa) and elastic modulus (230 GPa). The values for the hardness and elastic modulus are 1.5 and 1.7 times greater than the coating with n = 1, respectively. The enhancement effects in multilayered coatings could be attributed to different mechanisms for layer formation with nanometric thickness due to the Hall–Petch effect; because this effect, originally used to explain increased hardness with decreasing grain size in bulk polycrystalline metals, has also been used to explain hardness enhancements in multilayered coatings taking into account the thickness reduction at individual single layers that make up the multilayered system. The Hall–Petch model based on dislocation motion within layered and across layer interfaces has been successfully applied to multilayered coatings to explain this hardness enhancement. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The PVD method via magnetron sputtering processes has been used for coating cutting tools since the 1980s. Thus, among the processes used for this purpose, magnetron sputtering and its reactive variants have proven to be very successful. Many studies reported in the literature, coating materials such as TiCN [1], AlCN [2], YSZ [3], CrAlN [4], and BiMnO3 [5] have been synthesized with high stability, confirming the advantages of the PVD process. Surface engineering of metallic substrates with protective coatings like single-layered hard coatings such as TiCN, BCN, and BN, has
∗ Corresponding author at: Universidad del Valle, Grupo de Películas Delgadas, Calle 13 # 100 - 00 Melemdez, Cali, Valle, Colombia. Tel.: +57 2 339 46 10; fax: +57 2 339 32 37. E-mail address: [email protected] (J.C. Caicedo). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.024
played an important role a few years ago, due to the mechanical and tribological properties of metallic substrates reflected in higher hardness and toughness, as well as in high corrosion resistance compared to those of steel substrates used in industrial applications [6]. On the other hand, the isostructural multilayered systems, composed by metal/ceramic and ceramic/ceramic superlattices, have received much attention because these combinations can exhibit high hardness values, often increasing by more than 100% over the rule-of-mixture values, while retaining good ductility and high anticorrosive properties [7]. Several metal/ceramic and ceramic/ceramic superlattices have been studied during the past 10 years, including Ti/TiN [8], TiCN/TiNbCN [9], BCN/CN/BN [10], or the B4 C/BCN/c-BN [11]. The nitride coatings, characterized by high hardness, and wear resistance can be used for strengthening and protection in industrial steels. A significant disadvantage of such coatings is their thickness, which in many cases cannot be exceeding 5 m. The use of the physical vapor deposition (PVD)
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techniques such as magnetron sputtering has been allowed with the development of multilayered coatings having a higher combination of properties to enhance the strength of machine parts. One of the most important parameters determining the service properties of nitride coatings is the size and quantity of the grains and pores, which as a result of the thinness of the protective layer pierce it all the way through to the base and serve as mechanical damage origins. It is known that PVD coatings have significantly less grain size, roughness and porosity in comparison to coatings obtained via other methods of vacuum technology and also through gas thermal spraying methods. In some cases, magnetron sputtering makes it possible to obtain pore-free coatings mainly in multilayered with a thickness in the order of 1 m, which is the result of the high packing density of the atoms being concentrated. However, according to data from some works, the porosity of nitride coatings may reach 5%, differing from the multilayer nitrides where the porosity decreases to 1% [12]. Therefore, the mechanical resistance of multilayer nitride coatings is higher than that of single-layered coatings, which may be explained by their greater thickness and features of their microstructure, including the absence of circular crystal characteristic of single-layered coatings; moreover, failure of nitride coatings under the action of a mechanical effect occurs at structural defects, which determine their mechanical resistance and tribological properties [12]. In this work, the BCN/BCN multilayered coatings have been deposited on TiN buffer-layer films to increase adhesion to steel substrates; also, the c-BN over-layer (top layer) coatings have been deposited on BCN/BCN multilayered coatings because the c-BN coatings show relevant mechanical properties. The BCN/BN multilayered coatings are used as a coupling system between TiN and c-BN coating. With this in mind, TiN[BCN/BN]n /cBN multilayered coatings with total thickness of 3 m have been designed and deposited on Si (1 0 0) and AISI 4140 steel substrates with bilayer periods in a broad range, from nanometers to hundreds of nanometers, to study the microstructural evolution with decreasing bilayer thickness () and their related mechanical properties for possible industrial applications in processes where high mechanical performance is required.
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assembly was analyzed via scanning electron microscope (SEM), by using a Leika 360 Cambridge Instrument equipped with a height sensitivity back-scattered electron detector. Grain size and roughness were determined via atomic force microscopy (AFM-Asylum Research MFP-3D® ) and calculated by scanning probe image processor (SPIP® ) that is the standard program for processing and presenting AFM data and this software has become the de facto standard for image processing at nanoscale. In this work the SPIP was used in the grain analysis for a quantitative study of grain size and moreover the SPIP was used for an advanced measurement of the surface roughness [9]. Hardness and elastic modulus measurements were performed by using a nanoindenter (UBI1-Hysitron) under load and unload modes with a matrix measurement of 25 points and maximum load of 10 mN. 3. Results and discussion 3.1. FTIR analysis Infrared spectrum of [BCN/BN]1 /c-BN coating deposited on Si(1 0 0) is shown in Fig. 1. The spectrum presents an intense absorption feature located at approximately 1400 cm−1 . This band originates from lattice stretching vibrations of the h-BN structure [13]. The peak around 1100 cm−1 can be related to Sp3 B–N (cBN) [14,15]. Peaks centered at 849 cm−1 , 747 cm−1 , and 1000 cm−1 can be associated with C–H, wurtzite hexagonal boron nitride (wh-BN), and boron carbide (B4 C) (B–C) bonds with vibrations of non-conjugated alkenes and azometinic groups, respectively [16,17]. C–H line comes possible from hydrocarbon contamination due to environmental effect. Cubic and hexagonal microdomains were obtained in the nitride coatings at 300 ◦ C, and nitrogen flux of 3.7 sccm, though no IR materials of Sp3 -like (cubic-BN-like) B–C bond could be available for reference to specify their presence in B–C–N and c-BN. Because three-dimensional bonds among B, C, and N atoms were unambiguously determined, it may be concluded that the amorphous B–C–N coatings are composed mainly of cubic c-BN-like and hexagonal-BN-like plain microdomains. The results agree well with other FTIR spectra obtained for B–C–N, wh-BN, and h-BN materials deposited via physical processes [16–18].
2. Experimental details 3.2. SIMS analysis TiN[BCN/BN]n /c-BN multilayered coatings were deposited onto silicon (1 0 0) and AISI 4140 steel substrates by using a multitarget magnetron reactive sputtering technique with an r.f. source (13.56 MHz) and stoichiometric Ti, B4 C, and BN targets with purity at 99.9% for all targets. The deposition parameters to obtain highquality coatings were sputtering power of 300 W for Ti, 350 W for B4 C, and 500 W for the BN target; substrate temperature of 300 ◦ C. The sputtering gas was a mixture of Ar 93% (50 sccm) and N2 7% (3.7 sccm) and a total working pressure of 6 × 10−3 mbar was used for all films (TiN, BCN and BN). An unbalanced r.f. bias voltage was applied, which generates a negative signal fixed at −100 V. Also, our magnetron sputtering device has a positioning substrate system in relationship to target spot; this parameter permits varying the bilayer number among 1, 5, 10, 20, and 25 bilayers, changing the bilayer period, therefore, the deposition times were the same for all multilayered coatings but the individual layers within the different bilayer numbers were deposited with different holding times in front of the targets. The Fourier transform infrared (FTIR) analysis of the coatings was carried out with a Shimatzu 8000 spectrometer, which uses a ceramic-type Nerst source in the range of 550–1500 cm−1 in transmission mode. Secondary ion mass spectroscopy (SIMS, Atomika 488) was carried out using oxygen ions at 9 keV and 400 nA, for the multilayer with thicker periods, and at 4.5 keV and 300 nA for the thinner periods in order to improve depth resolution and to avoid mixing drawbacks. The multilayer
Secondary ion mass spectroscopy (SIMS) analysis showed a clear modulation in all the multilayer coating compositions, confirming the previously designed multilayer structures. Fig. 2 depicts the
Fig. 1. FTIR transmittance spectrum for [BCN/BN]1 /c-BN coating grown at r.f. negative bias voltage of −100 V.
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Fig. 3. SEM micrograph of TiN[BCN/BN]10 /c-BN coatings n = 10, = 200 nm.
Fig. 2. SIMS depth-profile analysis for a TiN[BCN/BN]n /c-BN multilayered coating grown with bilayer number (n = 25) and bilayer period ( = 80 nm).
SIMS compositional depth-profile analysis of the TiN[BCN/BN]n /cBN multilayered coatings grown with n = 25 bilayered and bilayer period of = 80 nm. The modulation of carbon signal (12 C+ ) is clearly observed as SIMS analysis progresses in depth, confirming the number of bilayers (25) in this coating from the number of maxima in SIMS profile (B–C–N layers). The modulation in 12 C+ signal is 5 times lower than the 14 N+ signal in this case; but it reaches down to 10 times in multilayered with thicker bilayer periods, where SIMS sputtering mixing has less appreciable effects in depth resolution. Moreover, the 11 B+ signal from boron carbon nitride
multilayer, 48 Ti+ signal from titanium nitride deposited as single layer, and 56 Fe+ from steel substrate are recorded. The 12 C+ , 11 B+ and 14 N+ signals reach the same values measured in the B–C–N and B–N single-layered coatings; suggesting that boron carbon nitride and boron nitride have the same composition, both within all multilayered and in single-layered coatings. 3.3. Scanning electron microscopy analysis A first glimpse on BCN/BN multilayer modulation and microstructure was accomplished by SEM micrographs. Fig. 3 presents the cross-sectional image of a TiN[BCN/BN]10 /c-BN coating with n = 10 ( = 200 nm) (Fig. 3). The darkest contrast of B–C–N layers with respect to B–N layers allowed a clear determination of
Fig. 4. AFM images for a TiN[BCN/BN]n /c-BN multilayer with n = 1, 5, 10, 15, 20, and 25 (a–f) grown at r.f. negative bias voltage of −100 V.
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Fig. 5. Surface measurements obtained via AFM and SPIP analysis for TiN[BCN/BN]n /c-BN multilayered with a total thickness of 3 m and bilayer period from = 2 m to 80 nm: (a) correlation between roughness and the bilayer number, and (b) correlation between grain size and the bilayer number.
the layer structure. These TiN[BCN/BN]10 /c-BN coatings presented a single layer of TiN with thickness around 100 nm, the BCN/BN multilayer with thickness around 2 m, and c-BN with thickness around 800 nm with well-defined and uniform periodicity. All the multilayer stacks were resolved by SEM and confirmed quite precisely the previously designed values of bilayer thickness, as well as the total thicknesses. The only slight deviation observed by SEM imaging was on relative thicknesses. In Fig. 3, the BCN layers seem a bit thicker than the B–N ones. These deviations are difficult to evaluate because of the poor resolution of the BCN/BN interfaces in this technique. 3.4. Atomic force microscopy analysis AFM was carried out to quantitatively study the surface morphology of our samples in relation to a decrease in bilayer periods or an increase of bilayer numbers in TiN[BCN/BN]n /c-BN multilayered coatings deposited onto Si (1 0 0). Fig. 4 shows AFM images for multilayered coatings with statistical distribution of grain size that was analyzed by 15 m × 15 m AFM images for the (a) n = 1, (b) n = 5, (c) n = 10, (d) n = 15, (e) n = 20, and (f) n = 25. The correlation between roughness part (a) and grain size part (b) with the bilayer number is shown in Fig. 5a and b, respectively. The quantitative values were extracted from the AFM images by means of statistical analysis scanning probe image processor (SPIP® ). The TiN[BCN/BN]10 /c-BN multilayer coatings has the same total thickness, around 3.0 m. The roughness was greater than for a coating with n = 1 or high bilayer period = 2 m, and lower for multilayered coatings with a higher bilayer number (n = 25) or lower bilayer period = 80 nm, indicating that coatings with (n = 1) grow more disordered than do multilayered with (n = 25). The grains of the multilayered coatings are smaller than those for the coatings with a low bilayer period, because the Ar+ ion bombardment on coatings stimulates a greater number of nucleation places, because of the reduction of individual thicknesses for each layer when the bilayer period is reduced and bilayer numbers are increased; this implicates a decrease in the entire surface roughness [9]. In this work was take in account that grain size values comes from c-BN top layer, but c-BN grain size is affected by changes in grain size of inner layers (BCN/BN) as function of reduction in the bilayer period () or increase in the bilayer number (n), because the BCN/BN multilayered were used for improve the adhesion of c-BN (hardness top layer) to TiN single layer. On the other hand, many authors reported a correlation between AFM and XRD results, because the AFM analysis delivers surface information (with lat-
eral resolution), XRD collects information from the space around the normal vector on the surface (from a cross-section); and if this is taken into account it is possible to consider that PVD coating columns can grow “vertically” from the substrate to the “top” and can change their cross-section when bilayer thickness is modified. 3.5. Mechanical properties Typical load–displacement indentation curves of multilayered coatings are shown in Fig. 6, using a standard Berkovich indenter. The indentation matrix images via AFM are shown in Fig. 4. The values of elasticity modulus, Er , and hardness, H, were obtained by using Oliver and Pharr’s method in multilayered coatings deposited onto AISI 4140 steel substrates [19]. Hardness values of the TiN[BCN/BN]n /c-multilayer measured by nanoindentation are presented in Fig. 7a, as functions of period thickness, , and/or in function with bilayer number, n. Elastic modulus values of multilayered coatings are also presented in Fig. 7b, showing relevant differences in their values. The hardness and elastic modulus in these multilayered coatings varied from 19 to 30 GPa and from 130 to 225 GPa, respectively. The highest hardness of the multilayer set, 30 GPa, was obtained by the thinnest bilayer period ( = 80 nm). This increase in mechanical properties is related to the remarked
Fig. 6. Nanoindentation measurements showing load–displacement indentation curves of TiN[BCN/BN]10 /c-BN multilayered coatings.
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Fig. 7. Mechanical properties for TiN[BCN/BN]n /c-BN multilayered coatings with bilayer period from = 2 m to 80 nm: (a) hardness as a function of bilayer number (n),and (b) elastic modulus as a function of bilayer number (n).
heterostructure effect that is present when a perfect assembly occurs for BCN and BN coatings with c-BN single layer deposited over BCN/BN system and, moreover, the BCN/BN periodicity that remained constant for lowest bilayer period analyzed by SIMS and SEM results. The enhancement in the mechanical properties can be associated with hardness improvement by using multilayered materials. This improvement has led to lower grain size, as observed from AFM images in Fig. 4, generating an increase of the density for the hard coating [9,20,21]. The high interface density of nanoscale multilayered structures contributes to impeding dislocation motion and the dislocation glide across the interfaces, which would require a critical yield stress being related to the difference in the elastic shear modulus of the single-layered materials [22–24]. Many authors have used the Hall–Petch effect to explain the material hardening, therefore, it is possible to apply the Hall–Petch effect when the ceramic materials like hard multilayer coatings with a bilayer period () higher than that of 5.2 nm are obtained, because the multilayered with lower are within nanoscale regime, and thus, the dislocations should not occur in nanoscale structures below a certain value of grain sizes [25]. Fig. 7a shows that the hardness for TiN[BCN/BN]n /c-BN coatings is in relation to the reduction of the modulation period. Therefore from AFM results are possible observed that indentation track exhibit low deformation size when the mechanical properties are improved as function of a increasing in the bilayer number. It is argued that there are some correlations between the hardness of a multilayered coating and the modulation period [22,23]. Hardness enhancement for TiN[BCN/BN]n /c-BN was attributed to many interfaces that blocked the dislocation movement across the interface between B–C–N layer and B–N layer due to differences in the shear module of the individual layer materials, and by coherency strain causing periodical strain–stress fields in the case of latticemismatched multilayered coatings. Each interface also functions like a grain boundary in a Hall–Petch [26] related mechanism such as that of the dislocation pile-up that offers a strong interaction again with interfaces in general; therefore, the interfaces and strain harden layer presents a serve crack tip deflectors which strengthens the mechanical properties into the coatings [22–24]. According to Kim et al. [24] another condition for enhancing the hardness of multilayered coatings with respect to the single-layer coating is that the layer in the multilayered system has to be discrete, which was found in this study via SIMS and SEM analyses (Figs. 2 and 3) respectively. Also, the enhancement of mechanical properties for a TiN[BCN/BN]n /c-BN multilayer is according to the increase in hard-
ness such as what has been shown in multilayered-type coatings for diverse material systems (e.g. TiCN/TiNbCN, TiN/VN, TiN/NbN, TiN/CrN, TiN/TiAlN, and TiC/TiB2 ) [9,16,25,27]. Yashar et al. [26] used a Hall–Petch approach to model the mechanical behavior of multilayered materials with layer thicknesses as low as 1–100 nm, transforming the Hall–Petch approach into the next equation presented by Caicedo et al. [9]: Hm = H(f1+f2) + kIM Dt −1/2
(1)
where Hm is the multilayer hardness, H(f1+f2) is the hardness from layer 1 and layer 2, kIM is a constant measuring the relative hardening contribution of the interface between layer 1 and layer 2, and Dt is the bilayer period () [9]. The model predicts the overall behavior of the hardness on that is observed in most multilayered systems. Previous work has shown that a maximum hardness would be expected when the individual components of the multilayer have a relative equal thickness, as presented in this paper [9,28]. Furthermore, a dependence of the elasticity with the bilayer number (n) or bilayer period () was observed; the higher bilayer number corresponds to the higher elastic modulus. Therefore, it is possible to conclude that the elasticity and elastic recovery (R) of the multilayer coatings are improved by increasing the interface number. So, from the nanoindentation measurement, the typical values of elasticity modulus, Er , and hardness, H were obtained by using the Oliver and Pharr’s method [19]. Thus the elastic recovery for all TiN[BCN/BN]n /c-BN multilayered coatings was calculated by using the following equation [9]: R=
ımax − ıp ımax
(2)
where ımax is the maximum displacement and ıp is the residual or plastic displacement [9,29]. The equation data were taken from the load–penetration depth curves of indentations for each coating according to Fig. 6. Fig. 8a shows the increase in the elastic recovery as function of increase of bilayer number when ımax and ıp values from load and displacements results (Fig. 6), are introduced in Eq. (2), thus it is possible to observe the interface effect on multilayer system, because when the bilayer number (n) increases, in this way the interface number is increased, therefore, can generate a hardening on multilayer hard coating, which is evidenced in the improvement of elastic recovery (R). In another way according to Kim et al. [30] there is a relationship between elasticity modulus and hardness named plastic deforma-
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Fig. 8. Elasto-plastic properties with plastic deformation for TiN[BCN/BN]n /c-BN multilayered coatings with bilayer period from = 2 m to 80 nm: (a) elastic recovery, R, and (b) plastic deformation resistance.
tion resistance (H3 /E2 ratio), this relation was calculated for all multilayered coatings in function of the bilayer number or bilayer period. Fig. 8b shows a considerable increase in the resistance to the plastic deformation (H3 /E2 ) as a function of the increasing of the bilayer number (n), this fact is due to the hardness and the elasticity modulus also increasing as the interface number increased for all multilayer coatings. This enhancement in plastic deformation resistance occurs when bilayer period () decreases, increasing the interface number for coatings with total thickness, producing, thus the crystallite refinement, point defect formations and the increase of interface number which improving the hardness of the TiN[BCN/BN]n /c-BN multilayer coatings. As shown in Fig. 8, the TiN[BCN/BN]n /c-BN multilayer coatings increased the plastic deformation resistance and elastic recovery with respect to coatings deposited with lower bilayer number. The maximum value was reached for the bilayer number (n = 25) and the bilayer period ( = 80 nm), i.e. the plastic deformation due to the applied load is more markedly reduced than that of other multilayered systems with fewer bilayer numbers. This effect is clearly correlated to the reduction of grain size, increasing coating density, hardness, and elastic recovery [9]. In general the bilayer number (n) or bilayer period () has some effects on multilayer that has been shown to exhibit very high hardness in multilayer composed of a nitride (e.g. BN) and a carbonitride (e.g. BCN). Nitride is used to provide significant ductility while the carbonitride gives the high hardness. In this nitride/carbonitride multilayer system, dislocation motion across layer interfaces should be difficult since the layers typically have different crystal structures showing the increase in the mechanical properties. The first theoretical explanation of the mechanical properties of multilayered materials as function of bilayer number (n) or bilayer period () was considered by Koehler [12]. He showed that dislocation motion is inhibited in a multilayered structure due to the image force on dislocations created by the different dislocation line energies in each layer when the bilayer number is increased. By creating a multilayer structure in which the two layers have a low (layer A) and a high (layer B) shear modulus, a shear stress of the order of G/100 (where G is the shear modulus) would be required to drive dislocations through the structure. This shear stress is an extremely high value, which has the same order of magnitude as the theoretical strength of a solid [26]. Koehler also noted that each layer should be thin enough so that dislocation generation cannot occur within the layers. Therefore, in multilayered with thin layer, the bilayer number (n) or bilayer period () produces
a stress field (− BN + BCN ) which increase the hardness. On the other hands the effect of (single) c-BN top layer is very important in the industrial application because the c-BN material exhibit high hardness which can be show relevant tribological properties, but this material present high residual stress, low adherence to steel substrate, so in this sense in necessary to applied the multilayer system (e.g. TiN[BCN/BN]n ) that help to increase the adherence of cBN top layer and relieving the residual stress within c-BN top layer. Then, the c-BN hardness and multilayered structure contribute in the improving of general mechanical properties which has been observed in TiN[BCN/BN]n /c-BN multilayer coatings. On the other hand with an indentation depth of the indenter of around 200 nm, the hardness and especially the Young’s modulus of the c-BN coatings (top layer) on tools were determined to be influenced by the films underneath. This influence is responsible for the slightly lower hardness values for c-BN films on tool substrates compared to c-BN films on silicon substrates and for the considerable variation of the Young’s modulus. A more detailed investigation of the elastic constants leads to higher values for the Young’s modulus. The results of these investigations will be published elsewhere [31,32]. However, the results of mechanical characterization of the c-BN-layer system on tools and on silicon are nearly the same and confirm the outstanding properties of c-BN coatings. Specifically, the hardness is twice as high as for typical TiN or TiAlN coatings [31]. In combination with the very low abrasive wear rates and the relatively low friction coefficient, the results emphasize the high potential of c-BN as a super hard and wear resistant tool coating. In comparison to the already previously published results [33–35], it was possible to increase the c-BN layer thickness on industrial AISI 4140 steel substrates with a TiN[BCN/BN]n multilayered precoating slightly over 1 m for c-BN top layer and simultaneously increase the elastic recovery, R, and plastic deformation resistance. So, with these properties of the c-BN coated cutting tools, application tests in turning and milling with high performance can be feasible. 4. Summary TiN[BCN/BN]n /c-BN multilayered coatings were successfully deposited by r.f. magnetron sputtering with bilayer periods ranging from 2 m to 80 nm. The FTIR results show the BN vibrations with the presence of c-BN. The SIMS and SEM analyses exhibit the multilayer evidence with a well-defined and uniform periodicity. AFM and SEM analyses confirmed well-defined multilayered structures with grain size and roughness lower than expected
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for coatings deposited with (n = 1) and, moreover, this work showed important variations in layer thickness ratio. The highest hardness and elastic modulus, 30 GPa and 225 GPa, respectively, were observed for TiN[BCN/BN]n /c-BN deposited with n = 25 and = 80 nm. The enhancement in hardness of the multilayered coating was attributed to the many interfaces that blocked the dislocation movement across the interface between B–C–N layers and B–N layers due to differences in the shear module of the individual layer materials, and by coherency strain causing periodical strain–stress fields in the case of lattice-mismatched multilayered coatings. Therefore, the highest hardness was found for the nanometric coatings (n = 25, = 80 nm), reaching a maximum value of 30 GPa. The explanations for this hardness enhancement in TiN[BCN/BN]n /c-BN multilayer are based on interfaces acting on dislocations and Hall–Petch models, which give a good overall picture of the hardness enhancements observed. Furthermore an increase in the elastic recovery resistance and plastic deformation resistance for TiN[BCN/BN]25 /c-BN ( = 80 nm) was observed with respect to the multilayer coating deposited with lower bilayer number (n = 1). So, the maximum value was reached for the coating deposited with (n = 25), therefore, an increase of the elastic recovery (R) and plastic deformation resistance around 54% and 26%, respectively was observed. So, this study revealed that bilayer period () and bilayer number (n) have a marked influence on the mechanical properties of all the coatings. Acknowledgement This work was supported by the Center of Excellence for Novel Materials (CENM) under Colciencias/CENM contract # RC-0432005. Colciencias-Univalle. References [1] J.C. Caicedo, C. Amaya, L. Yate, W. Aperador, G. Zambrano, M.E. Gómez, ˜ ˜ P. Prieto, Appl. Surf. Sci. 256 (2010) J. Alvarado-Rivera, J. Munoz-Salda na, 2876–2883. [2] L. Yate, J.C. Caicedo, A. Hurtado Macias, F.J. Espinoza-Beltrán, G. Zambrano, J. ˜ ˜ P. Prieto, Surf. Coat. Technol. 203 (2009) 1904–1907. Munoz-Salda na, ˜ ˜ G. [3] C. Amaya, W. Aperador, J.C. Caicedo, F.J. Espinoza-Beltrán, J. Munoz-Salda na, Zambrano, P. Prieto, Corros. Sci. 51 (2009) 2994–2999.
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