PACVD

Accepted Manuscript Mechanical and wear characteristics of a-C:H/Cr/AlN/Ti multilayer films deposited by PVD/PACVD

M. Pătru, C. Gabor, D. Cristea, G. Oncioiu, D. Munteanu PII: DOI: Reference:

S0257-8972(16)31406-2 doi: 10.1016/j.surfcoat.2016.12.109 SCT 21966

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

30 August 2016 9 December 2016 27 December 2016

Please cite this article as: M. Pătru, C. Gabor, D. Cristea, G. Oncioiu, D. Munteanu , Mechanical and wear characteristics of a-C:H/Cr/AlN/Ti multilayer films deposited by PVD/PACVD. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2016), doi: 10.1016/j.surfcoat.2016.12.109

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ACCEPTED MANUSCRIPT MECHANICAL AND WEAR CHARACTERISTICS OF a-C:H/Cr/AlN/Ti MULTILAYER FILMS DEPOSITED BY PVD/PACVD

M. Pătru1, C. Gabor1, D. Cristea1, G. Oncioiu2, D. Munteanu1 Department of Materials Science, Transylvania University, 500036 Brasov, Romania Institute for Nuclear Research Pitesti, 1 Campului Str., 115400 Mioveni, Romania

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Corresponding author: [email protected]

Abstract

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In the present work, the feasibility from the mechanical behavior point of view of embedding

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piezoelectric AlN/Ti interlayer sensors between wear resistant a:C-H coatings and steel substrates was evaluated. The wear sensor composed by a bottom electrode of Ti and a piezoelectric AlN layer, along

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with the adhesion/top electrode Cr interlayer were deposited by magnetron sputtering (MS), while

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plasma assisted chemical vapor deposition (PACVD) was used to build up the wear resistant diamondlike carbon coating.

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The effects of the AlN/Ti interlayer on the properties of this system of coatings were investigated by

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several means. X-Ray photoelectron spectroscopy was used in order to assess the type of chemical bonds present in the coatings. The surface and section morphologies were observed by scanning electron microscopy, while the mechanical characteristics (hardness, adherence to the substrate and wear behavior) were analyzed by nanoindentation, scratch tests and ball-on-disc friction tests. The results showed that the presence of AlN/Ti interlayers is reducing the carbon sp3 content and is increasing the bonded oxygen at the surface of the coatings, which, in turn, leads to an increased chemical activity, giving rise to abrasive wear and a higher friction coefficient. The adhesion to the steel substrate of these multilayer coatings, compared to an a:C-H/Cr coating, used as reference, was

ACCEPTED MANUSCRIPT greatly improved. However, the hardness was gradually degraded, compared to the same a:C-H/Cr film. This indicates that the AlN/Ti interlayers might decrease the mechanical strength of the top DLC films, but can bring benefits in terms of adhesion to the steel substrate. The mechanical and wear characteristics of the a:C-H/Cr/AlN/Ti/steel system are dependent on, and can be varied due to the deposition process parameters.

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Keywords: DLC coating; wear resistance; Cr/AlN/Ti sensor.

1. Introduction.

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Aluminum nitride AlN thin films, among other piezoelectric thin film materials, are of high interest for sensor and actuator applications, mainly due to their excellent thermal, mechanical and chemical

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stability, coupled with good piezoelectric properties (piezoelectric coefficients, permittivity, etc). The

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influence of certain parameters and characteristics on AlN films and their mechanical and piezoelectric properties has already been reported: bottom electrode material type and AlN thin film thickness [1],

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substrate crystal orientation [2], usage temperature [3], the influence of c-axis orientation [4],

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sputtering atmosphere and doping elements [5,6], sputtering parameters and thermal treatments [7], interlayer material type [8], to name a few. Diamond-like-carbon (DLC) materials, which possess a

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combination of diamond (sp3) bonds and graphite (sp2) bonds, show an excellent friction behavior and

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are good candidates for surface coatings in a wide range of tribological applications. However, certain drawbacks have been reported: delamination and abrupt wear acceleration when deposited on Ti6Al4V alloy [9], high wear rate when deposited on titanium substrates [10], and increased substrate corrosion [11]. The addition of chromium or titanium as buffer layers or interlayers was reported to improve the adhesion to the substrate, to increase the hardness and to allow better flexibility of the system [12]. In this paper we are presenting our results concerning the mechanical and wear behavior of a aC:H/Cr/AlN/Ti/steel multilayer system, which could potentially be used as a wear resistant coating, as well as a wear monitoring sensor, due to the addition of a piezoelectric AlN interlayer. The proposed

ACCEPTED MANUSCRIPT sensor element, which could allow reporting the wear rate, is a piezoelectric thin film, known to produce an electrical charge in response to an applied mechanical stress, caused by the delamination of the wear resistant coating. Furthermore, using multilayered coatings could provide a few other added benefits: i) the multilayered coating can minimize the thermal fatigue failure; ii) it can significantly increase the toughness and lifetime of the coating (the fatigue cracks can change their propagation

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directions at layer interfaces); iii) it can minimize the mismatch of CTE (coefficients of thermal

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expansion) and stresses inherent in the coating system; iv) the fracture toughness can be improved by varying the elastic modulus throughout the coating thickness with certain compensation of the coating

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hardness.

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2. Experimental procedure.

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A deposition chamber from Oerlikon-Balzers was used to deposit the samples studied for this paper. The chamber is equipped with both PACVD, to deposit the a:C-H films, as well as MS, to deposit the

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remaining layers: Ti, AlN, Cr. For the deposition of the a:C-H layers we used a mixture of Ar (plasma

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gas) and C2H2 (precursory), while for the remaining layers we used Ar (plasma gas) and N2 (reactive gas), if needed. The final configuration of the samples was obtained without removing the samples

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from the deposition chamber: first the magnetron sputtered films were obtained, the samples were

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moved towards the PACVD system and the a:C-H films were deposited. Five sets of samples were obtained, as follows: one set of reference samples, used as comparison (a:C-H/Cr), and four sets of a:CH/Cr/AlN/Ti deposited with varying deposition parameters only for the AlN interlayers (deposition time and reactive gas partial pressure). The remaining layers (Cr, Ti, a-C:H) were obtained with identical parameters for each sample. The deposition conditions for the layers used for this application are presented in greater detail in Table 1 (bottom Ti and top Cr electrodes), Table 2 (a-C:H layers), and Table 3 (AlN interlayers). Mirror polished 100Cr6 steel was used as substrate for all samples.

ACCEPTED MANUSCRIPT X-ray photoelectron spectroscopy (XPS – ESCALAB 250 spectrometer with monochromatic Al Kα radiation and 0.1 eV passing energy) was used to determine the chemical bonding of the a:C-H films. The XPS analysis was performed in ultra-high vacuum conditions with a sputter cleaning source to remove any undesired contaminants. The surface morphology and roughness of the samples were studied using a scanning electron microscope (SEM), equipped with energy dispersive X-Ray (EDX)

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spectroscopy (SEM model FEI Inspect S).

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The hardness and elastic modulus of the a:C-H layers were obtained by nanoidentation, using a Nano Indenter G200 (Agilent Technologies), in dynamic mode, by measuring the contact stiffness as a

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function of the penetration depth. A Berkovich geometry indenter with a nominal tip radius of 20 nm was used. In order to avoid both the Indentation Size Effect (ISE), which might appear for lower

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penetrations depth, and the substrate influence (in this case, the influence of the intermediary layers)

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which could appear for higher penetration depths, the hardness and elastic modulus values that were considered accurate were obtained for penetration depths equal to or slightly larger than 10% of the

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a:C-H film thickness. At least 10 indentations were performed on each sample, in order to minimize the

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measuring errors.

The adhesion of the films to the steel substrate was assessed using a Micro Scratch Tester from CSM

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Instruments. A steel tipped stylus (100Cr6 steel) with a Rockwell geometry (tip radius = 100 µm) is

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drawn on the sample surface, while the load applied on the stylus is increased linearly, up to 30 N. The critical load values of interest, which signify the destructive events observed on the samples, were obtained after optical analysis of the wear tracks, and these are defined as follow: Lc1– the load necessary for the emergence of the first cracks in the film; Lc2– the load corresponding to the first delamination of the film; Lc3– the load responsible for the delamination of more than 50% of the film from the wear track. The length of the tests was set at 3 mm, being confined to the samples’ area of relatively uniform thickness. Three tracks were made on each measured sample, with a displacement on the Y axis of 0.3 mm between each track, and the critical load values were averaged.

ACCEPTED MANUSCRIPT The wear behavior was assessed using a rotation tribometer from CSM Instruments (Switzerland), using 6 mm diameter 100Cr6 steel balls as friction couples, and a constant applied load of 15 N. The dynamic friction coefficient is measured by a LVDT transducer (Linear Variable Differential Transformer), while the wear coefficient was calculated using the following formula:

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(1)

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where V represents the volume of material which is dislodged from the sample, L represents the applied

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load (15 N) and d represents the distance of the wear test. The volume of the dislodged material is calculated using the area of the wear track profile. The wear track profile area was measured using a

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Taylor-Hobson Surtronic 25 profiler.

3. Results and discussion.

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3.1. X-ray photoelectron spectroscopy (XPS).

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Films composition analysis using ESCA techniques (Electron Spectroscopy for Chemical Analysis) namely XPS - X-rays photoelectron spectroscopy and XAES - X-ray excited Auger electron

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spectroscopy, enables the study of chemical bonds in carbon films and deceleration between sp2 and

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sp3 bonds. The obtained results (XPS spectra for maximum C1s and XAES spectra) are presented in Figure 1. According to the literature [13, 14], the XPS spectra peaks corresponding to the C1s line from

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the 284 eV, 285 eV, and 287 eV energy values are assigned to sp2 bonds, sp3 bonds and C-O bonds respectively. Higher binding energy components of the C1s peak reveal the presence of oxygen bonds. Oxygen induces a peak displacement to higher values with ~ 1.5 eV for each C-O bond (for instance =C=O bond induces a 3 eV chemical displacement, while the –O–C=O bond a 4.5 eV displacement to higher binding energy) [15]. The energy values that include the main Auger C KLL transition structures for DLC type carbon films are located in the 250 eV – 280 eV domain. According to the literature, the main characteristic structure for graphite appears at 269 eV, for diamond at 262 eV,

ACCEPTED MANUSCRIPT while for hydrogenated DLC films at 265 eV [13]. The chemical bond type within the studied carbon films, as well as their atomic percentage obtained from the maximum C1s and KLL Auger lines analysis are shown in tables 5.3 and 5.4 respectively. The deconvolution of the C1s maximum corresponding to the reference sample (a:C-H/Cr) (Figure 1.e)) indicates two maxima associated to the sp2 and sp3 carbide hybridization states, while in the case

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of a:C-H/Cr/AlN/Ti samples (Figure 1.a), 1.b), 1.c), 1.d)) a new component, at a higher binding energy

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value appears, which indicates C-O type chemical bounds, determining a slowly decrease of the C-sp2 and C-sp3 content.

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In the case of a:C-H/Cr/AlN/Ti samples the physico-chemical processes between substrate and the gas mixtures used in PACVD processes enhances the formation of C-O chemical bonds in the upper DLC

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film due to an increased chemical reactivity through ionization of the oxygen found in the AlN/Ti

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intermediate layer. The presence of oxygen is explained mainly due to the residual atmospheric gas not evacuated in its entirety from the deposition chamber.

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Carbon films properties were reported to depend significantly on the chemical bonds type and

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percentage. Therefore, the differences which were observed between samples can influence the film hardness and wear behavior especially. In certain wear conditions the C-O bonds could potentially

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generate tribo-chemical reactions resulting in fretting wear failures like: increased surface reactivity,

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protection film removal, roughness alteration, acceleration of fatigue processes on upper layers.

3.2. Scanning electron microscopy (SEM). The micrographs obtained by scanning electron microscopy on the sample surface at a 1500× magnification, illustrated in figures 2.a) to 2.e) (left side), exhibit increased surface defects in the case of a:C-H/Cr/AlN/Ti samples compared with those from the reference a:C-H/Cr sample. These features can be explained through the incorporation in the carbon layer of the adsorbed oxygen from the intermediate AlN/Ti layers during the PACVD process.

ACCEPTED MANUSCRIPT Figures 2.a) - 2.e) (right side) show the cross-section micrographs of the samples, obtained at a 20000× magnification. On these cross-section micrographs, both the component layer thickness and the overall multilayer system thickness can be observed. The values presented on the SEM micrographs represent average thicknesses, resulted from at least five measurements per component, in different locations. The standard deviation did not exceed 10%. The images highlight the multilayer structure without

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noticeable cracks or interlayer discontinuities. The Ti, AlN and Cr intermediate layers obtained by MS

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present a columnar growth, while the a:C-H upper layer is, as expected, featureless. The Ti and AlN intermediate layers influence the Cr film growth by decreasing its horizontal growth tendency and by

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forming grains that germinate at the existing grain limits leading to a compact structure. The compactness of the multilayered a:C-H/Cr/AlN/Ti samples can be explained by the interlayers

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morphology and columnar structure, which can vary as a function of the deposition parameters [7].

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More exactly, using adequate deposition parameters for the AlN/Ti interlayers can lead to a dense and compact multilayer structure, ensuring better adherence and cohesion to the substrate and between

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layers.

3.3. Mechanical properties.

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Figure 3 represents the variation of the hardness and elastic modulus, as function of the penetration

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depth, for the a:C-H/Cr/AlN/Ti - 4 sample. The remaining samples, a:C-H/Cr/AlN/Ti – 1, a:CH/Cr/AlN/Ti – 2, a:C-H/Cr/AlN/Ti – 3, as well as the reference sample, exhibit an almost identical type of variation concerning the hardness and elastic modulus (higher initial values, a stagnation region, followed by a steady decrease, all as function of the penetration depth). One can notice that the hardness variation starts from relatively high values, followed by a stagnation region (stabilization), in the 200 ÷ 450 nm domain (starting from ~13% of the film thickness), which we consider the accurate representation of the film hardness. Furthermore, the softer Cr interlayer leads to the steady decrease of the a:C-H/Cr/AlN/Ti and a:C-H/Cr system hardness. The average hardness recorded for the a:C-H/Cr

ACCEPTED MANUSCRIPT reference sample was 30.70 GPa, in good agreement with the values reported in the literature for these types of films [16, 17]. The remaining a:C-H/Cr/AlN/Ti samples exhibit slightly lower hardness values, which can be attributed to the lower hardness of the intermediary layers. The hardness and elastic modulus values can be observed in table 6. According to the XPS results, after the deconvolution of the C1s peaks, we observed the occurrence of a component which was attributed to C-O bonds, resulting in

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a lower C – sp3 content. The mechanical properties of diamond-like carbon films are highly dependent

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on the carbon hybrid states, more specifically on the sp3 / (sp3 + sp2) ratio. Furthermore, a significant decrease of the elastic modulus values was observed, compared to the reference sample. This

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phenomenon could be attributed to the possible occurrence of creep and/or plastic deformation during indentation, especially considering the highly anisotropic nature of the samples in question. The

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influence of these types of occurrences is much more noticeable on the evolution of the elastic

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modulus, when compared to the evolution of the hardness, mainly due to the fact that the elastic modulus values are calculated using the unloading curves [18]. The higher homogeneity of the a:C-

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H/Cr coatings could be inferred from the significant differences related to the standard deviation: the

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a:C-H/Cr reference coating exhibits lower measuring errors, when compared to the a:C-H/Cr/AlN/Ti coatings. In addition to adequate piezoelectric properties, the AlN/Ti interlayers should provide the

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a:C-H/Cr/AlN/Ti/steel tribological system with good mechanical support, especially for the a:C-H wear

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resistant layer. As it was shown elsewhere by the authors [7], the deposition parameters (ex: the reactive gas partial pressure and the deposition time) can greatly influence the chemical, structural and, consequently, the mechanical properties of the AlN/Ti interlayers. The a:C-H/Cr/AlN/Ti tribological system should exhibit firstly good adhesion to the steel substrate, and secondly increased cohesion between the interlayers, in order to avoid complete or partial delamination. Some of the contributing factors for improved adhesion are: chemical interactions at the interfaces, reduced discrepancies between the mechanical properties of the interlayers (hardness and elastic modulus), interlayer thicknesses, to name a few. The critical load values measured as a result of scratch testing can be

ACCEPTED MANUSCRIPT observed in Table 6. With the exception of the a:C-H reference sample and a:C-H/Cr/AlN/Ti – 4 sample, which clearly exhibited the Lc2 critical load (10.29 N for the reference sample and 17.26 N for the a:C-H/Cr/AlN/Ti – 4 sample), there was no noticeable distinction between the Lc2 and Lc3 loads, severe delamination occurring immediately after the first cracks in the coatings. The SEM micrographs for samples a:C-H/Cr/AlN/Ti – 1, a:C-H/Cr/AlN/Ti – 4 and a:C-H/Cr can be observed in Figure 4.

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Samples a:C-H/Cr/AlN/Ti – 2 and a:C-H/Cr/AlN/Ti – 3 exhibited an almost identical behavior as

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sample a:C-H/Cr/AlN/Ti – 1 during scratch testing. The morphological analysis clearly shows two types of behavior: samples a:C-H/Cr/AlN/Ti - 1, a:C-H/Cr/AlN/Ti - 2, and a:C-H/Cr/AlN/Ti - 3 exhibit

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gross spallation, severe delamination, and chipping at a significant distance from the scratch track, while the remaining samples exhibit buckling/wedging and plastic deformation prior to delamination.

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This behavior could be explained when one observes the difference in coating thickness for the AlN

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interlayers: sample a:C-H/Cr/AlN/Ti – 1 = 1876 nm, a:C-H/Cr/AlN/Ti – 2 = 1486 nm, a:CH/Cr/AlN/Ti – 3 = 2225 nm, while sample a:C-H/Cr/AlN/Ti – 4 = 4120 nm, due to the increased

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deposition time for the AlN interlayer. The internal stresses due to the deposition process, combined

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with the significantly lower thickness for the first three samples, create the optimum conditions for gross spallation and severe delamination [19]. A higher thickness for the AlN interlayer might better

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accommodate the internal residual stresses in the system, therefore changing the delamination behavior.

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To better understand the adhesion behavior, nanoindentation tests were performed on the individual layers, using the SACS method (small-angle cross-section), with a 5mN load. The SACS hardness results for the AlN interlayers and the steel substrate are presented in Table 6. The magnitude of the mismatch between hardness values, in conjunction with the layer thickness, seem to play a role on the adhesion behavior: i) the samples which exhibit the lowest Lc1 critical loads (a:C-H/Cr/AlN/Ti – 1 and a:C-H/Cr/AlN/Ti – 3) are almost as hard as each other (the AlN interlayer); ii) sample a:C-H/Cr/AlN/Ti – 2 exhibits a slightly better Lc1 value, which could be attributed to the much lower mismatch between the hardness values (27.27 GPa for the a:C-H layer, compared to 17.07GPa for the AlN interlayer),

ACCEPTED MANUSCRIPT however the thickness of the AlN interlayer is the lowest out of all samples, which does not lead to a significant adhesion improvement; iii) sample a:C-H/Cr/AlN/Ti – 4 exhibits increased adhesion, mainly due to the almost negligible difference between the hardness values of the AlN interlayer and the steel substrate, followed by the much thicker AlN layer, compared to the remaining samples. Furthermore, as it was shown in section 3.2, the type of growth of the AlN interlayer might also play a

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role in these types of delamination behaviors. Samples a:C-H/Cr/AlN/Ti - 1, a:C-H/Cr/AlN/Ti - 2, and

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a:C-H/Cr/AlN/Ti - 3 exhibit columnar growth. As the thickness of the film is increased, the structure becomes finer, more compact.

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Further still, the appearance of the cross-section images (SEM, figure 2) indicates differences in bonding between the reference a:C-H coating and the substrate, compared to the multi-layer systems

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and the substrate. In case of the reference sample, a substance-to-substance bonding seems to be

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present, while in case of the multi-layer systems no metallurgical connection between the layers and the substrates can be seen.

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The dry wear behavior was assessed using 100Cr6 steel balls as friction counterparts, with an applied

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load of 15 N. Some of the parameters (sliding speed, sliding distance) and average values for the dynamic friction coefficient and wear coefficient can be observed in Table 7. One can observe that the

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average dynamic friction coefficient values, for each set of tests (the same sliding distance), are rather

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similar, albeit the lowest values being always exhibited by the reference samples. Regardless of sliding distance, the Cr intermediary layer was not reached. Furthermore, the wear test parameters have a significant influence on the wear behavior: for higher sliding speeds and sliding distances the wear coefficient exhibits lower values. This behavior can be explained by the formation of a reaction film at the sample/counterpart interface, with dry lubrication properties. The relative motion and friction between the sample surface and the friction counterpart generates heat, the point of contact could easily reach the 300-400 °C temperature range. According to the literature [16], in this domain of temperatures the diffusion of hydrogen out of the a:C-H film takes place, and the sp3 bonds are

ACCEPTED MANUSCRIPT replaced by sp2 bonds. At the surface of the a:C-H film there is a tendency of graphite formation, which, with increasing sliding distances, leads to the formation of a dry lubricant layer. These observations show that the wear behavior of a:C-H films is highly dependent on the testing parameters. A similar phenomenon was reported elsewhere [20]. As expected, the addition of AlN/Ti interlayers does not influence significantly the wear behavior. However, the addition of some surface defects for

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each deposited layer in the a:C-H/Cr/AlN/Ti system leads to surface features (presented in section 3.2)

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which can cause abrasive wear marks and dislodgement of material from the surface. To conclude this section, finding the optimum deposition parameters for all the interlayers is of paramount importance

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due to their major effect on the mechanical behavior.

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4. Conclusions.

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Wear resistant a:C-H/Cr/AlN/Ti and a:C-H/Cr thin films were obtained using a PVD – PACVD combined deposition system. X-Ray photoelectron spectroscopy, scanning electron microscopy,

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nanoindentation tests, scratch tests and ball-on-disc friction tests were applied in order to highlight and

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compare the sample properties. The results showed that the presence of AlN/Ti interlayers reduces the sp3/sp2 carbon ratio and favors the appearance of C-O bonds in the upper DLC layer determining a

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growth of the friction coefficient value. The adhesion to the steel substrate of these multilayer coatings,

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compared to an a:C-H/Cr coating, used as reference, was improved by up to 68%. Instead, the hardness was gradually degraded, compared to the same a:C-H/Cr film. This indicates that the AlN/Ti interlayers might decrease the mechanical strength of the top a:C-H films, but can bring benefits in terms of adhesion to the steel substrate. The results have shown that the mechanical and wear characteristics of the a:C-H/Cr/AlN/Ti/steel system are surface and additive dependent, and can vary depending on the deposition process parameters.

ACCEPTED MANUSCRIPT Acknowledgement. We hereby acknowledge the structural founds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS

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2637, ctr. No 11/2009) for providing the infrastructure used in this work.

ACCEPTED MANUSCRIPT References. [1] S. Marauska, et al. Procedia Eng. 25 (2011) 1341 – 1344 [2] Nathan Jackson. Vacuum 132 (2016) 47-52 [3] C. Tu, J.E.-Y Lee. Sens. Actuators, A 244 (2016) 15–23 [4] M. Schneider, et al. Procedia Eng. 87 (2014) 1493 – 1496

[7] M. Patru et al. Appl. Surf. Sci. 354 (2015) 267–278

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[6] J.S. Cherng, T.Y. Chen. Curr. Appl. Phys. 11 (2011) S371-S375

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[8] T. Kamohara et al. Thin Solid Films 515 (2007) 4565–4569

[9] L.-y. Huang, et al, Diam. Relat. Mater. 10 (2001) 1448–1456.

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[10] T. Manhabosco, I. Müller. Tribol. Lett. 33 (2009) 193–197.

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[11] A.M.M. dos Santos, et al. Corros. Sci. 82 (2014) 297–303 [12] S. Sahoo, et al. J. Non-Cryst. Solids 386 (2014) 14–18

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[13] C. Niu, et al., Science 261 (1993) 334-337.

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[14] P. Merel, et al, Appl. Surf. Sci. 136 (1998) 105 – 110. [15] V. Andrei, G. Vlaicu. Romanian J. of Phys. 48 (2003) 427-438.

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[16] M. Yutaka, et al. Trib. Int. 62 (2013) 130 – 140.

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[17] J.C.A Batista, et al. Surf. Coat. Technol. 174 (2003) 891-898. [18] M. Kot. Arch. Civ. Mech. Eng. 12 (2012) 464 – 470. [19] S. Suzuki, et al, J. Adhes. Sci. Technol. 8 (1994) 261–271. [20] C. Charitidis, et al, Lubricants 1 (2013) 22-47.

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[5] Genshui Ke, et al. Surf. Coat. Technol. 290 (2016) 87–93

ACCEPTED MANUSCRIPT Tables

Table 1. Deposition parameters for the bottom Ti electrode and top Cr films. Parameter

Bottom Ti film

Top Cr film

Ti

Cr

Sputtering power:

1.2 kW

1.2 kW

Working pressure:

5×10-4 mbar

5×10-4 mbar

Ar partial pressure:

Ar – 0.25 mbar

Deposition rate:

0.8 µm/h

Deposition time:

1h

Target-substrate distance:

250 mm

Ar – 0.25 mbar

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Target (high purity):

0.4 µm/h 1h 250 mm

ACCEPTED MANUSCRIPT Table 2. Deposition parameters for the a-C:H layers. Deposition parameters

a-C:H thin films

Precursory gas:

C2H2 5×10-4 mbar

Precursory gas flow:

400 ml/min

Substrate temperature:

180 ºC

Bias voltage:

500V

Deposition rate:

1.5 µm/h

Deposition time:

1h

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Working pressure:

ACCEPTED MANUSCRIPT Table 3. Deposition parameters for the AlN interlayers. a:C-H/Cr/AlN/Ti - 1

a:C-H/Cr/AlN/Ti - 2

a:C-H/Cr/AlN/Ti - 3

a:C-H/Cr/AlN/Ti - 4

Target (high purity):

Al

Al

Al

Al

Sputtering power:

1.5 kW

1.5 kW

1.5 kW

1.5 kW

Working pressure:

5×10-4 mbar

5×10-4 mbar

5×10-4 mbar

5×10-4 mbar

Reactive gas partial pressure:

N2 – 0.05mbar

N2 – 0.05mbar

N2 – 0.1 mbar

N2 – 0.1 mbar

Deposition rate:

1.2 µm/h

1.2 µm/h

1.2 µm/h

Deposition time:

2h

3 h*

Target-substrate distance:

250 mm

250 mm

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Parameter

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2h

250 mm

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* - 6 cycles of 20 minutes of deposition, followed by 10 minutes break.

1.2 µm/h 4h 250 mm

ACCEPTED MANUSCRIPT Table 4. Peak positions for the C-sp2, C-sp3 and C – O bonds, obtained from the deconvolution of the XPS curves (C1s maximum).

C - sp2

C - sp3

C-O

At. (%)

Peak (eV)

At. (%)

Peak (eV)

At. (%)

a:C-H/Cr/AlN/Ti - 1

284.38

24.06

285.00

56.97

286.60

18.97

a:C-H/Cr/AlN/Ti - 2

284.56

37.54

285.43

44.02

287.10

18.44

a:C-H/Cr/AlN/Ti - 3

284.45

24.25

285.11

42.74

286.16

33.01

a:C-H/Cr/AlN/Ti - 4

284.56

35.22

285.37

40.56

286.94

24.22

a:C-H/Cr - reference

284.54

26.43

285.28

73.57

-

-

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Peak (eV)

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C - sp3

C-O

At. (%)

Peak (eV)

At. (%)

Peak (eV)

At. (%)

a:C-H/Cr/AlN/Ti - 1

274.31

15.79

267.50

60.86

256.64

23.35

a:C-H/Cr/AlN/Ti - 2

273.10

24.43

266.23

52.33

256.47

22.24

a:C-H/Cr/AlN/Ti - 3

273.29

29.92

266.67

41.11

a:C-H/Cr/AlN/Ti - 4

269.20

27.80

262.82

45.97

a:C-H/Cr - reference

271.66

32.36

262.99

IP

CR

US AN M ED PT CE AC

T

Peak (eV)

67.64

257.47

28.97

256.45

26.03

-

-

ACCEPTED MANUSCRIPT Table 6. Mechanical characteristics: critical adhesion loads (Lc1, Lc3), hardness (Hit), elastic modulus (Eit). Critical adhesion loads

Nanoindentation

Sample

Hit (GPa) Lc1 (N)

Lc3 (N)

Hit (GPa)

Type of delamination

Eit (GPa)

4.30±0.97

14.46±1.2

28.76±2.23

234.2±21.4

11.65±1.05

Gross spallation

a:C-H/Cr/AlN/Ti -2

4.52±0.11

14.24±0.4

27.27±1.99

229.7±20.2

17.07±0.96

Gross spallation

a:C-H/Cr/AlN/Ti -3

4.24±0.56

12.07±1.5

26.93±2.02

230.1±21.0

11.67±1.11

Gross spallation

a:C-H/Cr/AlN/Ti -4

5.81±0.25

27.38±0.5

25.77±2.24

228.3±25.2

11.39±0.80

Plastic deformation

a:C-H/Cr - reference

4.79±0.24

20.48±1.1

30.70±1.35

301.70±34.5

-

Plastic deformation

-

-

-

-

10.22±0.51

-

AC

CE

PT

ED

M

CR

US

AN

Substrate

T

a:C-H/Cr/AlN/Ti -1

IP

AlN (SACS)

ACCEPTED MANUSCRIPT Table 7. Wear characteristics: dynamic friction coefficient and wear coefficient. distance = 100m;

distance = 500m;

distance = 1000m; linear

linear speed = 24.5cm/s

linear speed = 31.42 cm/s

speed = 36.70cm/s

Q

µ

(mm3/(N×m))

Q

µ

(mm3/(N×m))

a:C-H/Cr/Ti/AlN -1

0.215

5.50 × 10-7

0.141

1.84 × 10-7

a:C-H/Cr/Ti/AlN -2

0.204

6.17 × 10-7

0.135

2.20 × 10-7

a:C-H/Cr/Ti/AlN -3

0.187

7.07 × 10-7

0.139

1.88 × 10-7

a:C-H/Cr/Ti/AlN -4

0.173

6.57 × 10-7

0.119

a:C-H/Cr - reference

0.177

5.97 × 10-7

0.102

0.121

8.61 × 10-8 8.27 × 10-8

0.157

6.14 × 10-8

2.12 × 10-7

0.128

6.06 × 10-8

1.96 × 10-7

0.099

7.83 × 10-8

CR

IP

0.149

US AN M ED PT CE AC

Q (mm3/(N×m))

T

µ

ACCEPTED MANUSCRIPT Figure captions:

Fig.1. Deconvolution of the C1s maximum XPS spectra. a) a:C-H/Cr/AlN/Ti - 1, b) a:C-H/Cr/AlN/Ti 2, c) a:C-H/Cr/AlN/Ti - 3, d) a:C-H/Cr/AlN/Ti - 4, e) a:C-H/Cr reference.

Fig. 2. SEM analysis. a) a:C-H/Cr/AlN/Ti - 1, b) a:C-H/Cr/AlN/Ti - 2, c) a:C-H/Cr/AlN/Ti - 3, d) a:C-

IP

T

H/Cr/AlN/Ti - 4, e) a:C-H/Cr reference.

CR

Fig. 3. Hardness and elastic modulus as function of the penetration depth, for sample a:C-H/Cr/AlN/Ti

US

- 4.

Fig. 4. Scratch track SEM analysis for sample: a) a:C-H/Cr/AlN/Ti – 1, b) a:C-H/Cr/AlN/Ti – 4, c)

AC

CE

PT

ED

M

AN

a:C-H/Cr reference.

ACCEPTED MANUSCRIPT Fig. 1a

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig.1b

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig.1c

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig.1d

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig.1e

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig. 2a

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig. 2b

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig. 2c

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig. 2d

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig. 2e

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Fig. 3

35

T P

I R

a:C-H/Cr/AlN/Ti - 4

30

C S

Eit

U N

25

Hardness (GPa)

Hit

15

D E

M

300 250 200 150

C A

0 -5 0

200

500

350

E C

5

550

400

T P

10

600

450

A

20

650

100

400

600

800

1000

1200

Penetration depth (nm)

1400

1600

50 1800

Elastic modulus (GPa)

40

ACCEPTED MANUSCRIPT Fig. 4

T P

I R

C S

A

U N

D E

T P

C A

E C

M

ACCEPTED MANUSCRIPT Highlights



a:C-H/Cr/AlN/Ti wear resistant multilayer coatings were deposited by PVD/PACVD;



the AlN/Ti layers decrease the carbon sp3 content and increase the bonded oxygen;



the adhesion to the steel substrate, compared to an a:C-H/Cr coating, was improved;



the a:C-H/Cr/AlN/Ti/steel system properties vary with the deposition parameters.

C S

I R

A

U N

D E

T P

C A

E C

M

T P