Use of silicon or carbonitriding interface in the adhesion of Ag-DLC film on titanium alloy: a comparative study

Abstract

A Hollow Cathode Plasma Enhanced Chemical Vapor Deposition (HC-PECVD) reactor was used to deposit silver doped Diamond-Like Carbon (Ag-DLC) films on Ti6Al4V alloy employing two methodologies: i) producing a silicon interlayer, using tetramethylsilane (TMS) as silicon precursor, varying the argon flow of the hollow cathode; and ii) carbonitriding the substrate. Profilometry, Raman, and Secondary Ion Mass Spectrometry (SIMS), as well as nanohardness, micro-scratch, scratch, and VDI 3198 indentation tests were used to evaluate the characteristics of the films and their adhesion on the substrates. The results demonstrated that the argon flow can be used for tuning the Ag-DLC film’s hardness, toughness, and adherence on silicon interlayers. The carbonitriding process, in turn, provided an improvement in the film toughness compared with non-carbonitrided samples. Considering the lower cost and easier handling of N₂ compared to the silicon precursors commonly available (TMS, HDMSO, SiH₄, etc.), the carbonitriding process proved more appropriate to improve the adhesion of the Ag-DLC films on the Ti6Al4V alloy.

Keywords:
diamond-like carbon; PECVD; hollow cathode; carbonitriding; titanium; scratch

1. Introduction

Hydrogenated amorphous carbon (a-C:H) thin films are largely used as functional coating in biomedical applications where the wear strength is a requirement (Cemin et al., 2014; Meškinis & Tamulevičienė, 2011; Bonetti et al., 2006; Hauert et al., 2013). However, when the substrate is a metallic alloy, the film/substrate adherence becomes impaired due to the high compressive stress generated during the ion bombardment on the surface and the difference between the thermal expansion coefficients of both materials (Cemin et al., 2014; Bonetti et al., 2006; Chehung & Chien-Hung, 2008). Such different properties often result in delamination or peeling of the film and the consequent ion release from the interlayer or substrate to the organic environment. In the long term, this situation can result in toxic effects and allergic reactions (Singh et al., 2009; Gajski et al., 2014; De Morais et al., 2009; Oliver et al., 2019; Balachandran et al., 2020), requiring use of interlayers, which withstand the stress field generated by ion bombardment and increase the film toughness (Cemin et al., 2014; Bonetti et al., 2006).

Nanometric metallic interlayers containing Al, Cr, or Si are commonly used in this way with the metal precursors in liquid or gaseous phases (Cemin et al., 2014; Meškinis & Tamulevičienė, 2011; Liu et al., 2004). For substrates composed of titanium alloys, the silicon interlayers are the most widely employed, presenting excellent performance (Cemin et al., 2014; Bonetti et al., 2006). Although DLC films employed in environments where they are subject to chemical attacks by body fluids or friction during the use, the film can be damaged, exposing the silicon interlayer. Often, this exposition can result in a severe risk of stress corrosion cracking, due to the silicon/oxygen affinity and the high tribological loads that the interlayer is subjected to (Meškinis & Tamulevičienė, 2011; Hauert et al., 2013). Furthermore, according to Batory et al. (2015), when in contact with blood, the incorporation of silicon in the DLC film suppresses the formation of sp² graphitic clusters and promotes the hybridization of sp³, which decreases the hemocompatibility of the film and can lead to the thrombosis. This result was also confirmed by Choi et al. (2008), who found that decreasing the sp²/sp³ ratio reduced the hemocompatibility and increased the hydrophilicity of DLC films. According to some studies (Sayes et al., 2007; Chen & Von, 2005; Valko et al., 2006; Kaewamatawong et al., 2006; Lin et al., 2006), the release of SiO₂ to the organism can also induce inflammatory and oxidative stress response and lead to cell damage. Furthermore, most of the liquid and gaseous silicon precursors are highly flammable and hazardous, requiring additional care during the handling.

To overcome the difficulties presented by use of silicon interlayers and its precursors, other alternatives are proposed to improve the film adherence, among them the use of a multilayer system (Bewilogua et al., 2011; Hainsworth & Uhure, 2013; Campoccia et al., 2013) with different functionalities, to grant compositelike characteristics to the surface. However, multilayer deposition is a complex process, sometimes demanding the use of several precursors and with a high risk of adhesive failure between the layers (Bewilogua et al., 2011, Campoccia et al., 2013). The use of thermochemical processes, such as nitriding and carbonitriding of the titanium surface for biomedical use, has been studied, but generally for direct contact with the organic tissues (Khandaker et al., 2016; Da Silva et al., 2011; Lin et al., 2011; Çaha et al., 2019; Ali et al., 2019). According to Khandaker et al. (2016) and Van Hove et al. (2015), the titanium surface nitriding promotes the surface bombardment with nitrogen ions and the consequent formation of Ti₂N and TiN_x (0.43 < x < 1.08) precipitates in the α-Ti matrix. Titanium nitrides are marked by their high biocompatibility and blood tolerability allied with high wear and fracture strength (Khandaker et al., 2016; Van Hove et al., 2015; She et al., 2015). On the other hand, thermochemical treatment of the titanium surface replacing the silicon interlayer has not gained attention.

In summary, DLC films for coating biomaterials have been studied ostensibly through various deposition techniques (Zia, A. W. & Birkett, M., 2021), however, the technique of PECVD operated with a hollow cathode, as a way of doping DLC films with Ag for use in biomaterials, is still little explored. Therefore, this study aims to evaluate the influence of a silicon interlayer compared to carbonitriding on the adhesion of Ag-doped DLC films on titanium alloys using a hollow cathode PECVD reactor. The study was conducted using two methodologies: i) using a silicon interlayer and ii) carbonitriding the titanium substrates. The characteristics of the films were evaluated by means of profilometry, Raman spectroscopy and secondary ion mass spectrometry (SIMS), as well as nano-hardness, micro-scratches, scratch tests and VDI 3198 (1991) standard indentation tests.

2. Experimental procedures

Titanium alloy disks (Ti6Al4V), with 10 mm diameter and 3 mm thickness, were used as substrates, which were prepared using 280 to 2,000 grit sandpaper and polished with 0.6 µm colloidal silica. After that, the deposition of DLC films and the carbonitriding processes were performed in a cylindrical hollow cathode PECVD reactor with 300 mm diameter and 450 mm height, using an EM18 Edwards vacuum pump. The main plasma in the substrate holder was generated by a pulsed DC Pinacle Plus 5 kW + 5 kW power supply from Advanced Energy. The sample temperature was monitored by a thermocouple installed at the bottom of the substrate holder. All the samples were doped with silver nanoparticles for trials to be performed in a later study. The hollow cathode plasma used to dope the DLC film with Ag was placed inside the reactor, near the substrate holder. An electric power of 60W was applied on a silver hollow cathode. An argon flow (99.99% purity) inside the hollow cathode carried the silver to the samples The dimensions of the hollow cathode are illustrated in Figure 1.

Figure 1
Schematic representation of the hollow cathode showing its dimensions and materials that compose it.

On an AISI 300 stainless steel planar cathode (diameter: 100mm; thickness: 5mm) an electric voltage was applied (varied from 300V to 450V). This plasma system was used to deposit Ag-DLC films on Ti6Al4V using two kinds of interfaces: i) a silicon interlayer, using tetramethylsilane (TMS) as the silicon precursor, varying the argon flow of the hollow cathode and ii) carbonitriding the substrate.

A “ring shower” placed on the top of the chamber supplied methane (CH₄ - 99.99% purity) and/or N₂, used as carbon and nitrogen precursors, respectively. For a better evaluation of Ag-DLC film adherences, the samples were divided into two groups (Table 1), both containing carbonitrided and non-carbonitrided samples. In the first one (Group 1), in which the process times were smaller, the aim was to analyze the behavior of the DLC films subjected to low loads in the scratch test. Hence, the DLC film adherence was characterized by nanohardness and micro-scratch test. For the second group (Group 2), the process times were increased to obtain thicker films and analyze the behavior of the DLC films under greater loads in the scratch test.

Table 1
PECVD process parameters for Groups 1 and 2.

Group	Sample	Treatment	Interlayer/Carbonitriding			DLC
			Pressure (Torr)	Temperature (ºC)	Time (min)	Pressure (Torr)	Power (W)	Time (min)	Argon flow
1	Si20	Silicon Interlayer	3	530	10	0.4	15	15	20
	Si80	Silicon Interlayer			10				80
	CN20-1	Carbonitriding			60				20
2	Si60	Silicon Interlayer	3	530	30	0.4	15	60	60
	Si100	Silicon Interlayer			30				100
	CN20-2	Carbonitrided			90				100
	CN20-3	Carbonitrided			90				100

The schematic diagram of the plasma system is in Figure 2.

Figure 2
Schematic diagram showing the main components of the hollow cathode PECVD reactor.

For both groups, the deposition processes followed the steps:

a) First it was heated to 530 °C and pressure of 3 Torr for carbonitriding (samples CN20-1, CN20-2, and CN20-3) and for silicon interlayer (samples Si20, Si60, Si80, and Si100), at different process times, see Table 1.

In the carbonitriding process, N₂ (80 sccm) and CH₄ (5 sccm) were used as nitrogen and carbon precursors, respectively. The silicon interlayers were grown using tetramethylsilane (TMS) as the silicon precursor.

b) The substrate was cooled from 530 °C to the Ag-DLC work temperature (approximately 150 °C);

c) Ag-DLC film were deposited at pressure of 0.4 Torr, using 5 sccm of methane as carbon precursor. For Group 1, the DLC deposition time was 15 min and for Group 2, 60 min. The argon flow was changed during the DLC film deposition for evaluation of its relationship with the film adherence to the substrate.

During the substrate cooling, the cooling rate of non-carbonitrided samples have always been higher than the cooling rate of that carbonitrided, the latter always resulting in thicker Ag-DLC films. This occurs because the thermal conductivity of the silicon interlayers (250 – 270 W.m^-1. K^-1) (Román-Manso et al., 2016) is higher than that of the carbonitrided layers (15 – 30 W.m^-1. K^-1) (Zhang, 1993).

For the first group of samples (Group 1), the following characterization methods were performed:

- Profilometry: to measure film thickness using a KLA Tencor P-7, with scan speed of 50 µm/s, acquisition rate of 200 Hz, load of 2 mg and resolution of 13 µm/0.0078Å. Si (100) wafers were used as witness samples.

- Raman Spectroscopy: Horiba Evolution, with a 600 lines/mm diffraction grid. The adopted procedure used three accumulations with an acquisition time of 30 s.

- Secondary Ion Mass Spectrometry: to determine the chemical composition depth profile using a SIMS Workstation from Hyden Analytical with a MAXIM Quadrupole for mass detection, working pressure of 10^-8 Torr, resolution of 1/300 amu, argon ion gun, and removal capacity of 1 nm. The chemical analysis of the coating was performed using an Ar⁺ ion beam with energy of 3 kV incident on the film surface.

- Nanohardness test: using an Anton Paar Micro-Scratch Tester (MST3) with a Berkovich diamond indenter. The acquisition rate was 10 Hz, approach distance of 3,000 nm, linear loading with an approach speed and retract speed of 16,000 nm/min, maximum load of 3 mN, loading rate of 6 mN/min, unloading rate of 6 mN/min, pause of 10 s, and stiffness threshold of 500 µN/µm. This characterization provided the film hardness in MPa. The room temperature was 25 °C and the relative humidity 53%.

- Micro-Scratch test: An Anton Paar Micro-Scratch Tester (MST3) with an HRC conic diamond indenter was used. The advance speed was 1 mm/min. The start load was 30 mN and the end load was of 10,000 mN, with a speed of 1 mm/min applied over 2 mm of distance. The ASTM C1624-05 (2015) Norm was adopted as a test procedure, with the following three stages of critical loads: Lc1 for crack starting, Lc2 for starting of substrate exposition, and Lc3 for full substrate exposition.

- Rockwell C hardness for Group 1 test: using a Wilson Durometer model 1JR with load of 150 kgf.

- Rockwell C hardness for Group 2 test: CETR Tribometer, model UMT with a Rockwell C diamond tip with radius of 200 µm, speed of 166 µm/s, acquisition time of 30 s, over 5 mm of distance. The load used was 20 N. Table 1 shows the process parameters of both groups.

3. Results and discussion

3.1 Analyses of Group 1

3.1.1 Profilometry

Figure 3 shows DLC film thicknesses of the Group 1 samples. The increase in the argon flow reduced the film thickness from around 0.8 µm to 0.25 µm. This result is related to the combined effect of argon and CH₄ flows of the DLC film deposition process, which involves not only reduction in the deposition rate, but also an increase in the etching rate promoted by argon atoms (Jones & Nelson, 2016; Kim & Lee, 2003).

Figure 3
Thickness profiles of DLC films of the Group 1 samples.

The effect of the etching process can also be seen by the profile curve obtained in the sample Si80, where the DLC film surface is rougher than the surface of the Si20 sample.

The carbonitrided sample CN20-1 in turn, presented the thickest DLC film among the samples, of around 1 µm, caused by the slow cooling rate of carbonitriding process in comparison with non-carbonitriding process. This is related to the higher thermal conductivity of the silicon interlayers compared with that of the carbonitrided layers, resulting in greater time with the plasma turned on during the cooling of the sample until the DLC film deposition temperature.

3.1.2 Raman spectroscopy

Raman spectra of the samples Si20 and Si80 presented D and G bands characteristic of DLC films, located between the wavelengths 1,350 cm^-1 and 1,580 cm^-1 (Figure 4). While the G band of the sample Si20 was located at 1,575 cm^-1, for sample Si80, in which the DLC film was grown with the higher argon flow, this band was located at 1,558 cm^-1, moving away from the crystalline graphite wavelength (1,580 cm^-1), showing an increase in the film amorphization (Robertson, 2002; Grenadyorov, 2018) due to the increased argon ions bombardment (Hauert, 2004).

Figure 4
Raman spectra of the samples Si20, Si80, and CN20-1.

The comparison of the FWHMG values of 122 cm^-1 for sample Si20 and 140 cm^-1 for sample Si80 also evidence this tendency. The D band of both samples did not present substantial changes.

The sample CN20-1 also presented D and G bands characteristic of DLC films, with D and G values located at 1,336 cm^-1 and 1,545 cm^-1, respectively. The FWHMG of this sample was 157 cm^-1, which characterizes an amorphization degree larger than for both other samples, suggesting a DLC film with higher density and elevated hardness (Lima-Oliveira et al., 2012). This result is related to the diffusion-barrier formed by carbonitriding process on the titanium surface, hampering the rapid diffusion of hydrogen and carbon into the substrate. This barrier promotes the formation of carbides and saturation of carbon on the titanium surface (Fu et al., 2000).

3.1.3 Secondary ion mass spectrometry - SIMS

Figure 5 and Figure 6 show the chemical composition gradient of the main chemical elements in the Ag-DLC film of the CN20-1 sample. The goals of SIMS characterization were to evaluate the effectiveness of the carbonitriding process, by means of the nitrogen presence in the DLC/substrate interface, and also estimate the Ag-DLC film thickness. In both figures, the chemical analysis from Ag-DLC surface up to Ti substrate is plotted from left to right, showing a reduction of both nitrogen content (from 8,000 to 2,000 counts) and carbon (from 100,000 counts to very low values). These results indicated that the thickness of the film is around 1 µm, and this is the interface between the substrate and the DLC film. Although the N counts decreased near the substrate, its presence was still detected below. the Ti surface, confirming the carbonitriding of the sample. The low count of nitrogen in the substrate could be related to the higher ionization energy of this element and the stability of its formed compounds. Furthermore, Ti was detected throughout the film due the surface sputtering process during the deposition. From 1 µm below the DLC film surface, its count increased, confirming that the SIMS electron beam reached the titanium alloy surface. The increase of H and Ag on the film surface is related to the interruption of the argon flow in the last minute of the process, maintaining only the N₂ and CH₄ flowing through the silver hollow cathode. The aim of this procedure was to enrich the DLC film surface with silver, to improve further biological response. The high hydrogen count on the DLC film surface could also be related to the active role of titanium in the hydride formation, which may explain the quick increase of both elements near this region (Lima-Oliveira et al., 2012; Fu et al., 2003).

Figure 5
SIMS curves showing N, Ti, and Ag gradients throughout the DLC film for sample CN20-1.

Figure 6
SIMS curves indicating C and H gradients throughout the DLC film for sample CN20-1.

3.1.4 Nanohardness and micro-scratch tests

As can be seen in Figure 7, for samples with silicon interlayer (Si20 and Si80), the increase in argon flow (from 20 to 80 sccm) allowed a hardness of 10,440 MPa to be reached, an average increment of more than 2,000 MPa in relation to Si20 hardness.

Figure 7
Nanohardness for samples of Group 1.

If the substrates were a very hard material, at least some part of this increase could be because sample Si80 has an indentation depth (± 0.18 nm for all samples) greater than 20% of the film thickness and the substrate hardness would affect the measurement (Lin et al., 2004). However, the Ti6Al4V alloy used in this study, has a low hardness of around 505 MPa. An increased FWHMG indicates the rise in the sp³ hybridization, which characterizes films with best adherence and mechanical strength that are suitable for biomedical applications where the material is subject to load bearing and wear (Hellmich et al., 1998; Koike et al., 1992). Several studies found that the increase in the argon flow promotes the rise in the ion bombardment leading to the preferential removal of sp² sites due to its lower bonding energy (Bigerelle et al., 2005; Lackner et al., 2012; Chau-Chang et al., 2013; Vetter, 2014). Robertson (1994; 2002) found that the carbon atoms implanted during the plasma deposition promotes a metastable increase of the film density and supports sp³ bonding. Khettache et al. (2002) and Peng et al. (2001) also confirmed that a-C:H films grown with higher argon flows presented greater sp³ bonding due to the diffusion barrier formed by ions embedded in the surface.

In the carbonitrided sample, the film hardness was even greater, reaching 12,495 MPa. Although Vandevelde et al. (1998) and Lin et al. (2004) associated the presence of nitrogen in the DLC film to a decrease in its hardness and quality, Bootkul et al.(2014) and Derradji et al.(2005) found that increasing the N content in the DLC film, induced changes in the arrangement of C sp³ sites from graphitic to pyridinelike structures and nitrile bonds, which breaks the interconnection of the graphitic structure of the film and can induce some increase in the hardness. Despite the great thickness of the DLC film in this sample, to a certain extent, its high hardness can also be affected by increased substrate tensile strength, due to the precipitation of very hard compounds, such as TiC_xN_y, Ti₂N, and TiN_x, on the alloy surface and by solid solution hardening of α-Ti matrix by nitrogen (Perry, 1981; Tamura et al., 2002).

The micro-scratch test provides a quantitative engineering measurement of the practical adhesion strength and damage resistance of the coating-substrate system as a function of the applied normal force (ASTM C1624-05). The friction coefficients are presented in Figure 8 from the micro-scratch tests performed on the three samples of Group 1. While for sample Si20, the friction coefficient started at values lower than 0.05 and after 0.1 mm increased quickly to values over 0.4, for sample Si80, they were under 0.15 up to 1.8 mm, denoting upper adhesion strength. On the other hand, sample CN20-1 presented a different behavior according to the tip advance. The friction coefficient started in 0.15 and presented some peaks with values between 0.25 and 0.30 along the track, returning to 0.15 again. For a better understanding of the failure mechanisms, a detailed analysis was conducted comparing the critical loads (Lc), failure modes, and acoustic emission (AE) signals.

Figure 8
Friction coefficient for samples Si20, Si80, and CN20-1.

Figure 9 shows the result of the micro-scratch test for sample Si20, and in the upper section, exhibits the image of the track generated by the indenter during the test. Right below, the friction coefficient along the track is presented, then the applied normal force (diagonal line) and the acoustic emission during the path. The bottom section illustrates the magnification of the areas where the critical loads occurred, indicated by dashed arrows. The horizontal black arrows indicate the vertical axis related to that curve. Using the average value of AE signal from sample Si20, it was possible to determine the place where the cracks started. The first Chevron cracks, cohesive failures with a “V” shape (Perry, 1981; ASTM C1624- 05), occurred between 0.1 mm and 0.2 mm from the track beginning, with Lc1 = 900 mN. The AE signal was primal for the definition of Lc2. From 0.6 mm to 1.2 mm, this signal presented an average gain of 20%, increasing from 1.28% to 1.61%. This denotes that around 0.5 mm, the progressive load reached the critical value Lc2 = 2,900 mN and the film cracking is not sufficient to relieve the stress promoted by the indenter, signaling a region where the delamination (adhesive failure) was the main failure mode (Perry, 1981; ASTM C1624-05). After 1.2 mm, AE presented a new gain of 55%, increasing from 1.61% to 2.5%, denoting the full exposition of the film.

Figure 9
Friction coefficient, acoustic emission and start point of critical loads of sample Si20.

Figure 10 shows the result of the micro-scratch test for sample Si80 and exhibits the evolution of the same parameters presented in Figure 9. In this case, all the critical loads measured for sample Si80 are higher than those of the sample Si20, reaching the maximum value of Lc3 = 9,100 mN, while for Si20 the Lc 3 remained at 6,100 mN (Figure 11). Another relevant result was the different amplitude of the AE signal for both samples. While the signal amplitude for Si20 was limited to 5%, for Si80, the AE signal exceeded 40% (Figure 10). According to Sekler et al. (1988) and Tomastik et al. (2018), the signal amplitude is proportional to the energy released by microcracks formed during the indenter pathway. Considering this result and the fact that the nanohardness of the sample Si80 DLC film presented a Young’s Modulus of 10.4 GPa, while for the sample Si20, this value was 9.1 GPa, it is possible to state that the failure formation energy for sample Si80 was higher than for the Si20 sample. This could explain the amplitude difference of the AE signals and suggests an improvement in the cohesive strength of the Si80 DLC film, confirming the important role of the increased argon flow in the sp³ bonding formation of this sample.

Figure 10
Friction coefficient, acoustic emission, and start point for critical loads of sample Si80.

Figure 11
Critical loads for samples Si20, Si80, and CN20-1.

The borders of the scratch tracks of both samples also presented some outstanding differences in the failure modes (Figure 12). This can be seen in the areas bounded by dashed lines for samples Si20 and Si80, in the track borders after 1 mm of the scratch track starting. While in the sample Si20, the failures were characterized by film detachment in this region, in sample Si80, the film buckling was the prevalent failure mode. This result suggests that although the increased argon flow promoted a decreased sp²/sp³ ratio, it also promoted the increased Young’s Modulus and enabled the deposition of a more ductile film, which was able to withstand the buckling promoted by the indenter without breaks and resulted in best adherence (Bull & Berasetegui, 2006; Burnett & Rickerby, 1988). For the toughness of the DLC films, some researchers have used only the lower critical load (Lc1) to indicate the “crack toughness” of the films (Pei et al., 2005; Shum et al., 2004; Ligot et al., 2000), while others (Yonghui & Xiang, 2015; Zhang & Zhang, 2012; Zhang et al., 2005) have pointed out that this parameter should be proportional to both, the lower critical load (Lc1) and the difference between the load where the film starts to peel (Lc2) and the lower (Lc1) critical load. Obviously, a film can have an early crack, but if it fractures or peels off at very high load, the film has a very high “toughness” because, during the measurement, the coating successfully resisted the propagation of the crack, so it is very reasonable that Lc2 should be considered in the film toughness measurement. This quantitative calculus of the film toughness suggested by Zhang & Zhang (2012) was performed by using the following equation:

Figure 12
Failure modes in the track borders after 1 mm of the scratch track starting: sample Si20 (delamination), Si80 (buckling), and CN20-1 (buckling).

$CPR=Lc1*\left\{K*\left(Lc2-Lc1\right)\right\}$

where: - CPR is the Crack Propagation Resistance; - K is a correction coefficient related to the film properties.

Applying this formula to both DLC films of samples Si20 and Si80, the following results were obtained:

${\text{CPR Si}}_{20}=1.80\times 106.K$

${\text{CPR Si}}_{80}=2.55\times 106.K$

The ratio between CPR Si₈₀ and CPR Si₂₀ results in a 41% increase in the CPR, evidencing the high effectiveness of the increased argon flow on the toughness of the DLC film. Figure 11 shows a comparison of the critical loads of the three Group 1 samples.

In the micro-scratch test of sample CN20-1 (Figure 13), the values of Lc1 and Lc2 remained near those obtained for sample Si20 (Figure 11). Even with the absence of a silicon interlayer, the friction coefficient of CN20-1 was 0.15 in the beginning of the test remained under 0.2 up to 0.6 mm. Between 0.6 mm and 1.3 mm, the friction coefficient presented oscillations with high peaks that returned to the lower level, which according to Perry (1981) characterize carbonitrided surfaces. Considering the purposed biomedical use of this film, for many applications a very low friction coefficient is not interesting or necessary because in general, it means a hydrophilic surface, which can thwart the cellular adhesion (Campoccia et al., 2013; Caravaca, 2016; Liu et al., 2004; Vasudev et al., 2013). On the other hand, sample CN20-1 presented substantial adhesion strength along the micro-scratch track, with greater Lc2 and Lc3 values than the Si20 sample (Figure 11). The improvement in the adhesion strength of sample CN20-1 compared with the sample Si20 is also characterized by the failure mode in the track border at the 1 mm position of the micro-scratch test (Figure 12), where the film withstood the buckling promoted by the indenter without breaks resulting in high adherence (Bull & Berasetegui, 2006; Burnett & Rickerby, 1988). The choice of the position at 0.9 mm for Lc2 critical load occurrence, and not some point between 0.5 mm and 0.7 mm, was based on the procedure defined in the ASTM C1624-05, which establishes that any critical load should be associated with the occurrence of the same failure in both sides of the track. Quantitatively, the CPR value for sample CN20-1 was 1.92 × 106.K, achieving a 6.6% increase in the crack toughness for the Si20 sample.

Figure 13
Friction coefficient, acoustic emission, and start point of critical loads for sample CN20-1.

3.1.5 VDI 3198 indentation test

The VDI 3198 indentation test is a fast characterization method for film adherence (Lugo et al., 2017; Capote et al., 2018; Standard VDI 3198, 1991). The test uses a conical indenter to penetrate a diamond indenter into a coating surface, causing extensive plastic deformation of the substrate and fracture of the coating. Figure 14 is the SEM images of the indentation areas at 45° slope for Group 1 samples. Table 2 provides the EDX chemical analysis of both the DLC films (Area 1) and the delaminated area (Area 2), after VDI 3198 indentation test for each sample. As can be seen, sample Si20 presented extensive delamination of the DLC film and silicon interlayer, fully exposing the titanium substrate (Figure 14), which can be confirmed by the high amount of Ti (73.8 wt%) and Al (9 wt%) detected in Area 2. For Sample Si80, the DLC film delaminated, while silicon interlayer remaining adhered to the substrate. In this case, Area 2 presented Ti and Al amounts of 9.9 wt% and 2.2 wt%, respectively. The amount of Si and O were 13 wt% and 5.4 wt%, respectively, denoting characteristics of the silicon interlayer.

Figure 14
Comparison of film bending around HRC indenter imprint for samples Si20, Si80, and CN20-1 (SEM - 45° slope - 150x).

Table 2
SEM chemical analysis of DLC film and delaminated area for Group 1 samples.

Sample	Area 1 (%wt.					Area 2 (%wt.
	C	Si	Ti	O	Al	C	Si	Ti	O	Al
Si20	77.8	7.9	7.3	5.6	0.9	73.8	0.2	73.8	2.7	9.0
Si80	77.4	10.4	6.7	3.8	0.8	69.4	13.0	9.9	5.4	1.2
CN20-1	75.0	0.2	21.5	-	2.5	10.5	0.4	67.0	11.0	8.0

Unlike sample Si20, at the borders of the delaminated area of sample Si80, little parts of the film (white dashed lines) are visible, which bended without breaking, demonstrating a slight improvement in the film toughness. In the case of sample CN20-1, the delaminated region (Area 2) presented elevated amounts of Ti and Al, but not as high as sample Si20. Considering that the carbon amount in this area was 10.5 wt%, this result suggests the presence of compounds with high stability like carbonitrides (Ti₂N and TiN_x), as mentioned by Khandaker et al. (2016) and Van Hove et al. (2015). This sample also presented a peculiar aspect around the imprint border, where it is possible to see areas (white dashed lines) in which the film bent without breaking, evidencing a pronounced improvement in the film toughness.

3.2 Group 2

3.2.1 Profilometry

Figure 15 shows the profilometry characterization of the Group 2 samples. Both Si60 and Si100 presented a thickness of around 5 µm, although the last one had a rougher surface owing to the increase in the ion bombardment caused by higher argon flow in this process deposition. Samples CN20-2 and CN20-3, in turn, presented DLC film thicknesses of 8.8 µm and 8.3 µm, respectively, the highest values among all the DLC films. This result is related to the greater time needed to cool the substrate from 530 to 150 °C, in which the plasma is turned on. In general, these films are smoother than those without the carbonitrided layer. According to Luo (2009), for hard films deposited by plasma, increasing the thickness trends to generate a smoother film surface due the low influence of the substrate roughness and the extended time that the indenter remains sliding on the film.

Figure 15
DLC film thicknesses for Group 2 samples.

3.2.2 Scratch-test

Figure 16 shows the scratch tests performed on Group 2. The upper and central regions of this figure present the friction coefficients of both non-carbonitrided (Si60 and Si100) and carbonitrided (CN20-2 and CN20-3) samples, while the lower region presents the normal force used in the tests.

Figure 16
Friction coefficients and normal loads of theSi60, Si100, CN20-2, and CN20-3 samples.

The non-carbonitrided samples (Si60 and Si100), maintained their friction coefficients under 0.15 up to 3.2 mm, even with a load of 20 N, which is higher than that used for non-carbonitrided samples in Group 1 (Si20 and Si80). This behavior is related to the promotion of the sp3 hybridization due to the increased argon flow, which characterizes films with best mechanical strength (Hellmich et al., 1998; Steffen et al., 1992; Koike et al., 1992).

However, beyond the influence of the argon flow, the increase of the film thickness from Group 1 to Group 2 also seems to have influenced the holding of the friction coefficient at low values when comparing samples Si60 (Group 2) with Si80 (Group 1), for example. Ma et al. (2003) observed the dependence of tribological properties of DLC films related to its thickness. Sheeja et al. (2002), Qi et al. (2001), and Luo (2009) observed that for hard films, the friction coefficient decreases with the increasing of its thickness, due the low influence of the substrate roughness and the extended time that the indenter keeps sliding on the film. Despite the better performance of the Group 2 non-carbonitrided, samples compared to the Group 1 samples, their performance did not surpass those obtained for Group 2 carbonitrided samples (CN20-2 and CN20-3). This result can be seen at least up to 4.5 mm, where the DLC film from sample CN20-2 presented a failure and its friction coefficient grew abruptly. This failure can be related to some film imperfection or surface contamination, because the DLC film of sample CN20-3, grown with the same conditions, maintained its friction coefficient under 0.15 up to the end of the test, withstanding a normal force up to 15 N, which is 50% above the maximum load (10 N) employed in the CN20-1 sample.

4. Conclusions

In this study, experiments were performed in a hollow cathode PECVD reactor aiming to compare the adhesion of DLC films grown on titanium substrates using a silicon interlayer and DLC films grown directly on carbonitrided titanium substrates. The argon flow was also changed to analyze its influence on the properties and adhesion of the DLC films. Increasing the argon flow from 20 sccm to 80 sccm in the samples with silicon interlayer increased the film hardness by more than 2,000 MPa in. The rising of the argon flow also improved the silicon interlayer adherence, observed in the VDI 3198 indentation test and promoted an increase of 41% in the film toughness. The samples where the DLC films were grown with higher process times, presented a higher adherence, due to the role of the film thickness in decreasing the influence of substrate roughness. These results confirm that the argon flow in the PECVD process can be used for tuning the hardness, adherence, and toughness of DLC films. The carbonitriding process, in turn, promoted an increase of 6.6% in the DLC film toughness and was able to maintain the friction coefficient at lower values than those obtained with the use of silicon interlayers. Considering this result and the reduced cost and easier handling of the nitrogen precursors, the carbonitriding process is more appropriate to improve the adhesion of the Ag-DLC films on Ti6Al4V alloy.

Acknowledgement

The authors acknowledge the financial support of the Brazilian Research Foundations CNPq (grant no. 560054/2010-6), CAPES, FINEP, and FAPESP (grant no. 2011/14038-4). They also thank the following institutions for use of their characterization equipments: Universidade Federal de São Paulo – UNIFESP, Instituto Tecnológico de Aeronáutica - ITA/ DCTA, and Instituto de Estudos Avançados - IEAv/DCTA.

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Publication Dates

• Publication in this collection
  06 Sept 2024
• Date of issue
  Oct-Dec 2024

History

• Received
  24 Jan 2024
• Accepted
  27 Mar 2024