Development and complex characterization of bio-tribological Cr/CrN + a-C:H (doped Cr) nano-multilayer protective coatings for carbon–fiber-composite materials

Abstract

Carbon fiber structures provide strength, stiffness, and fatigue resistance. Carbon-based materials show, however, significant oxidative degradation in air beginning at temperatures in the region of 400 °C. Therefore, a coating concept for carbon–carbon composites consists of an inner part, which serves as a structural link with stress compensation ability to the carbon substrate, and an outer part, which acts as a diffusion barrier. In the presented paper, chromium/chromium nitride (Cr/CrN) multilayer structure has been selected as the inner part. The outer part of the coating, in the presented paper, was hydrogenated amorphous carbon (a-C:H). Among doping metals, Cr, as one of the carbide formed elements, possesses an attractive combination of properties (corrosion resistance, wear resistance, etc.). Thus, in the presented paper, a-C:H part of the coating was implanted by Cr nanocrystals. Coatings were deposited by means of magnetron sputtering technique. They were subjected to complex investigations. Mechanisms of a mechanical wear of analyzed systems were presented, focusing on the cracking propagation in ball-on-disc tests using a 1 N and 5 N applied loads for 20 000 cycles. Complex microstructure analysis of presented nano-multilayer coatings, before and after mechanical tests, were performed by means of transmission electron microscopy (TEM). The microstructure characterization revealed that cracking, which was propagating in the outer part of the coating (in the carbon part) in the layer with lower nano-particle content, was stopped at the interface with the higher nano-particle content layer. In the case of the inner part of the coating (Cr/Cr₂N), ceramic layers showed brittle cracking, while metallic (Cr) ones deformed plastically.

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1. Introduction

Carbon–carbon (C/C) composites or carbon–fiber-composites (CFC) are increasingly being considered for aerospace application. In composite materials, the fibers impart strength, stiffness and fatigue resistance, while the carbon matrix holds the fibers together. A synergistic effect also exists between the fiber and the carbon matrix, which results in high fracture toughness and wear resistance. But, the most attractive properties of carbon–fiber-composites are their high specific strength and modulus. Based on their good mechanical performances, potential uses of structural carbon materials have been identified in future manned hypersonic vehicles and in other aircraft applications. However, most of these applications involve extended time periods in oxidizing environments. Unfortunately, carbon reacts rapidly with oxygen at temperatures around 500 °C and the composites are subject to oxidation degradation. Less is known about the effect of oxidation on the properties of CFC because they vary with the oxidation mechanisms and micro-structural features of the materials. But, it is well known that oxidation quickly degrades the properties of carbon materials. Therefore, numerous investigations have been conducted in an attempt to protect carbon based composites against oxidation.¹ Two ways are commonly used for the oxidation protection of CFC. The first one consists of the addition of some oxidation inhibitors into the matrix of the composite (such as phosphorus-, boron-, or silicon-containing additives).² These additives can effectively lower the oxidation rate of the CFC but they are not efficient for prolonged times at temperatures above 900–1000 °C. The second way is the deposition of external coatings on the composite surface; however, the biggest problem is thermal expansion mismatch between the coating material and the carbon substrates. Some attempts have been proposed in this field.^3–6

Applicable coating techniques are electrochemical/galvanic deposition or thermal and plasma spraying, which provide thick coatings of high load support.^7–9 However, they lack in adhesion during overloading, because their stiffness is too high and their plastic deformability is too low to follow substrate deflection. Alternatively, thick soft polymer coatings (pure and micro-/nanoparticle strengthened lacquers) possess high elasticity for bending to follow substrate deflection, but they fail in tribological resistance.¹⁰ Thin films of materials combining hardness and wear resistance with high compliance as well as high resistance to cohesive and adhesive crack propagation (high toughness) are future candidates for surface protection of high performance polymers.^11–13 Kääriäinen et al. proposed chromium nitride (Cr_xN) single layer coatings on CFC, but they were not effective in tribological protection.¹⁴ Although they achieved high adhesion of the magnetron-sputtered PVD coatings due to surface activation by high ion doses before film deposition or high substrate temperature, the realization of dense coatings was impossible. This was assigned to a very inhomogeneous and rough substrate surface, consisting of fibers with loose ends and polymer filler.

The paper deals with modern coatings development with appropriate adhesion to CFC substrate. In case of coatings for carbon composite substrates, proper coating materials selection is essential. Currently, in surface modification much attention is directed to multilayer coatings. The set of alternate layers of hard and soft phases as well as appropriate buffer layer application (the first layer from the substrate) can lead to coating quality improvement and increased adhesion to the substrate.^15–19 In case of protective coatings for CFC, the quality and type of the first buffer layer is the most essential, due to the differences in the thermal expansion coefficient and possibilities of high residual stress formation, especially at the coating/substrate interface. Multilayer coatings are composed of alternately stacked layers of hard and soft phases with thicknesses of a few to tens of nanometers. This combination of alternate layers may lead to considerable hardness, and high flexibility, good adhesion coatings to substrates as well as may lead to self-healing effect, which is important in substrate protection against oxidation. Possible cracks propagation through ceramic coating can be stopped at the ceramic/metallic interface by plastic flow. Energy of brittle cracking is compensated by plastic deformation. A coating concept for carbon–carbon composites should consist of an inner part, which serves as a structural link, with stress compensation ability, to the carbon substrate, and an outer part, which acts as a diffusion barrier. In the presented paper, as the inner part, chromium/chromium nitride (Cr/CrN) multilayer structure has been selected. The literature data indicate the particular meaning of Cr and CrN multilayer coatings. They are characterized by an appropriate crystallographic adjustment of subsequent constituent layers of Cr and CrN and by the creation of a transition layer between them with a thickness of several dozen of nanometers. This ensures a good connection between particular constituent layers and as a result also good maintenance properties: high adhesion, wear and corrosive resistance.²⁰ The outer part of the coating, in the presented paper, was hydrogenated amorphous carbon (a-C:H). It is well-known that a-C:H coatings have low friction coefficients and low specific wear rates. Thus, the amorphous carbon coatings are very promising tribo-materials. However, the poor adhesion strength to substrate, high residual stress and weak thermal stability would limit their application. Currently, many metallic elements (Ti, W, Ag, Cr etc.) have been utilized to modify their structure, and it has been proved that metal doping is an effective method to reduce the residual stress and enhance the adhesion strength of the film.²¹ In the presented paper, a-C:H part of the coating was implanted by Cr nanocrystals. The presented paper deals with microstructure description of the as deposited coatings as well as with description of microstructure changes of the coatings after mechanical wear. The elaborated materials are designed de novo for the elements of surgical tools. Thus, the coatings should be assembled to express biocompatible properties and not be recognized by the human enzymatic degradation system as well as they should be characterized by adequate mechanical properties, namely be wear resistant.

2. Materials and methods

2.1. Deposition technique

A hybrid PLD system (pulsed laser deposition + magnetron sputtering) equipped with high purity chromium (99.9% Cr) and carbon (graphite) targets was used for multilayer coatings deposition. The described coating contained two parts. Cr/Cr_xN (chromium/chromium nitride) multilayer was first deposited as an inner part from CFC substrate. The outer part was amorphous carbon (a-C:H) gradually implanted by chromium nano-crystals. Details of the deposition process had been described elsewhere.²² Coatings were deposited in four different deposition parameter sets, as it is presented in the Table 1. “A” variant and “B” variant differed only in deposition conditions. One third of the carbon part of the coating deposited in “A” variant was produced using 155 DC (direct current), while the last two thirds used 230 DC. Two thirds of the carbon part of the coating deposited in “B” variant was produced using 155 DC, while the last one third used 230 DC.

Table 1 Four different versions (deposition parameters set) of the deposition of the outer part of the coating

A variant
Step	Cr [W]	C [W]	C2H2 [sccm]	Ar [sccm]	N2 [sccm]	Time [min]
1	1400	50	5	45	0	14	1/3
2	1230	410	4.5	45.6	0	14
3	1060	780	3.8	46.2	0	14
4	890	1150	3.1	45.2	1.7	14	2/3
5	720	1520	2.5	44.2	3.3	14
6	550	1890	1.9	43.1	5	14
7	380	2260	1.3	42	6.7	14
8	210	2630	0.6	41.1	8.3	14
9	50	3000	0	40	10	14

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B variant
Step	Cr [W]	C [W]	C2H2 [sccm]	Ar [sccm]	N2 [sccm]	Time [min]
1	1400	50	5	45	0	14	2/3
2	1230	410	4.5	45.6	0	14
3	1060	780	3.8	46.2	0	14
4	890	1150	3.1	45.2	1.7	14
5	720	1520	2.5	44.2	3.3	14
6	550	1890	1.9	43.1	5	14
7	380	2260	1.3	42	6.7	14	1/3
8	210	2630	0.6	41.1	8.3	14
9	50	3000	0	40	10	14

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C variant
Step	Cr [W]	C [W]	C2H2 [sccm]	Ar50 [sccm]	N2 [sccm]	Time [min]
1	1400	50	20	30		14	2/3
2	1230	410	22.5	27.5		14
3	1060	780	25	25		14
4	890	1150	27.5	22.5		14
5	720	1520	30	20		14
6	550	1890	32.5	17.5		14
7	380	2260	35	15		14	1/3
8	210	2630	37.5	12.5		14
9	50	3000	40	10		14

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D variant
Step	Cr [W]	C [W]	C2H2 [sccm]	Ar50 [sccm]	N2 [sccm]	Time [min]
1	1400		20	30		14	2/3
2	1230		21.9	28.1		14
3	1060		23.8	26.2		14
4	890		25.6	24.4		14
5	720		27.5	22.5		14
6	550		29.4	20.6		14
7	380		31.2	18.8		14	1/3
8	210		33.1	16.9		14
9	50		35	15		14

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2.2. Mechanical experiments

The mechanical properties result from proper microstructure design. The mechanical properties of the coatings were investigated by means of nano-indentation (Berkovich indenter) and a ball-on-disc mechanical test using 1 N and 5 N applied loads for 20 000 cycles. All mechanical tests were performed in natural atmosphere (in air). An Al₂O₃ alumina ball with 6 mm diameter was used for the test. The linear speed of the ball, which was applied in the test was 0.06 m s⁻¹. Both tests' parameters are presented below:

(a) Low stress condition:

Load FN = 1 N; ball radius R = 3 mm; cycle number n = 20.000; friction radius r = 5 mm, linear speed v = 0.06 m s⁻¹; σ_H = 0.45 GPa.

(b) High stress condition:

Load FN = 5 N; ball radius R = 3 mm; cycle number n = 5.000; friction radius r = 4 mm, linear speed v = 0.05 m s⁻¹; σ_H = 0.8 GPa.

The parameters of the test, which were used in the test were the optimal ones for this kind of coatings deposited on such type of substrate. Before each test performance, the optimum test properties are usually looked for. Each type of coating composition and substrate type require new parameters test.

2.3. Techniques of microstructure investigation

The microstructure of the as-deposited coatings and after the mechanical tests, was studied using a transmission (TEM) (TECNAI G2 F20 FEG (200 kV)) electron microscope, which allows microstructure observation at the smallest scale.

Microstructure of all investigated coatings was similar. The difference between coatings was only in deposition conditions. It has been decided to present the microstructure of the coating, which according to mechanical test results, was characterized by the best properties (variant “A”).

Observations were carried out using TEM bright field mode (TEM BF), scanning TEM mode (STEM-observations in contrast depends on the atomic number Z) and especially the high resolution mode (HRTEM). Phase analyses were performed using selected area electron diffraction technique (SAEDF) and HRTEM. Chemical composition was characterized by energy dispersive X-ray spectroscopy technique (EDS). Thin foils for TEM observations were prepared using focused ion beam technique (FIB) using gallium ions. Only this technique allows preparation of thin foils directly from the place of interest (in this case from wear tracks). The QUANTA 200 3D DualBeam was used for thin foils preparation.

2.4. Cytotoxic analysis

Microstructure should influence the induction of inflammatory reactions that can start a rejection reaction or, in the case of medical tools, complications in surgery. Biological tests were performed as a screening tool for potential application for medical tools.

The cytotoxic effect was determined according to the ISO 10993-5:2009 standards.²³ Samples in size of 1.5 cm² were placed in confluent mouse fibroblast (L929; ATCC) cultures (about 5 × 10⁵ cells) and were incubated for 48 hours at 37 °C. Then, the cells were stained by propidium iodide (PI). Cultures incubated with the samples were analyzed in comparison with control cultures. Images were taken with the Exciter 5 confocal microscope equipped with a camera, and quantified using AxioVision 4.8 software (Carl Zeiss MicroImaging). A statistical analysis (two-way ANOVA and Tukey's post hoc test, P value smaller than 0.05 was considered as significant – Statistica 10.0 PL) was performed on three replicates from each treatment. The analysis revealed changes dependent on applied deposition parameter. The applied fluorescent marker (PI) is a colorant dye of the nuclei. It cannot penetrate the cell membrane of continuously living cells. Therefore, dead cells are stained. It penetrates into the interior of dead cells through cell membranes damaged in the process of necrosis and intercalates with DNA helix, emitting red light fluorescence (excitation 535 nm, emission 617 nm).

3. Results and discussion

3.1. Microstructure characterization of the as deposited Cr/Cr_xN + a-C:H + Cr nano multilayer coating deposited on carbon fiber composite material

Structure, microstructure, and nanostructure of surface treatments are critical aspects for surface engineering.²⁴

Microstructure characterization of the as deposited protective multilayer coating, described in the presented paper, was characterized mainly using the transmission electron microscopy technique (TEM).

Thin foils for TEM observation have been cut using focused ion beam technique (FIB) perpendicular to the carbon fibers, which is well seen in the image obtained by scanning electron microscope (Fig. 1a). Looking at the cross-section in STEM mode (scanning transmission electron microscopy), the quality of the deposited coating on such a complicated structure was very high (Fig. 1b). No delamination was noticed. The coating clearly reflects the surface roughness of the carbon–fiber-composite (CFC) substrate.

Fig. 1 Microstructure characterization of an as deposited coating; (a) image obtained using SEM technique; (b) image obtained by TEM technique in STEM mode.

The described coating contained two parts. The Cr/Cr_xN (chromium/chromium nitride) multilayer was deposited as a first part from CFC substrate (inner part). The second part was amorphous carbon (a-C:H) gradually implanted by chromium nano-crystals (outer part) (Fig. 2).

Fig. 2 Microstructure characterization of the coating at the cross-section, performed by TEM technique; (a) TEM BF image; (b) STEM image.

The purpose of the Cr/Cr_xN part was to reduce residual stress of total coating, especially at the substrate/coating interface, as well as to enhance coating adhesion to CFC substrate.

Because of the presence of a metallic phase (chromium – Cr), total coating had partially plastic properties. Such behavior has been described by the authors in their previous papers^15–17 as well as by other researchers.²⁵ The CrN and Cr lattice parameters allow a cube-on-cube close to epitaxial growth with a low mismatch (1.6%) (Fig. 3).²⁰ In the present case, the same diffraction contrast went through interfaces. It confirmed crystallographic dependence between Cr and Cr_xN phases. It is well seen on the TEM BF image (Fig. 2a).

Fig. 3 The scheme of the Cr and CrN cell growth assembly.¹²

High resolution analysis of the TEM technique (HRTEM), allowed us to established the mismatch between Cr and Cr_xN. It has been confirmed that the mismatch was at the level of 2.4 Å. Even such a good lattice fit caused formation of defects in the form of mixed dislocations in the structure of the metallic phase (Cr) (Fig. 4). It has been also confirmed that Cr_xN was presented as a Cr₂N.

Fig. 4 The HRTEM image of the Cr/Cr₂N interface.

The presence of defects in the form of dislocations can affect the strengthening of the structure of the metallic phase, which can impact positively on the mechanical properties of the whole coating.¹⁸

The second part of the coating, as it was mentioned at the beginning, formed a multilayer structure built of hydrogenated amorphous carbon (a-C:H) (other part of the total coating) (Fig. 5).

Fig. 5 Qualitative EDS chemical analysis; (a) STEM image; (b) diagram of selected elements distribution along the line marked at the STEM image; (c) maps of selected elements.

It is well known that a-C:H coatings have low friction coefficients and low specific wear rates. Thus, the a-C:H coatings are very promising bio-tribological materials for surgical tools application. However, poor adhesion strength to the substrate, high residual stress and weak thermal stability limit the application of a-C:H coatings.²¹ In the current work, Cr nano-grains have been utilized to modify the structure and properties of amorphous carbon. Cr nano-grains were gradually inserted into the carbon structure (Fig. 5).

In the STEM image, the layered structure of the a-C:H implanted by Cr nano-grains is clearly visible (Fig. 5a). Qualitative chemical analysis by EDS (energy dispersive X-ray spectroscopy) has been performed along the line marked on the STEM image (Fig. 5a). The results are presented in the form of a diagram of the selected elements distribution along the line (perpendicular to the substrate) (Fig. 5b). At the Cr line, steps were noticed. They are marked by arrows on the diagram. Each step corresponded to the individual a-C:H layer with different amounts of Cr nano-crystals content. The amount of Cr nano-grains was constantly decreasing towards the top of the coating, which has been presented in the maps of selected elements (Fig. 5c). Cr as one of the carbide formed elements possesses an attractive combination of properties (corrosion resistance, wear resistance, etc.). Thus, many researchers have paid more attention to the structures and tribological properties of Cr/a-C:H coatings.^26–33 In the case of the described coatings, the farther from the interface with a Cr/Cr₂N, the degree of a-C:H + Cr nano, the crystallinity was less. It has been well confirmed by selected area electron diffraction patterns (Fig. 6b).

Fig. 6 Microstructure characterization of the amorphous carbon part of the coating (outer part) using TEM technique; (a) TEM BF image; (b) selected electron diffraction patterns (phase analysis); (c) maps of selected elements obtained by EDS technique.

The further the distance from the interface with the Cr/Cr₂N part of the total coating, the more amorphous the character of the diffraction pattern.

3.2. Micromechanical properties of analyzed coatings

The wear rate was calculated for all variants of deposited coatings, but results should be treated only qualitatively due to the fact that the coatings were deposited on carbon–fiber composite substrates. Some of the carbon fibers were not sunk deeply into epoxy and had direct contact with the coating. This resulted in non-standard behavior of the coatings subjected to the wear test. Delamination appeared by chipping of the coating.

Coating that was deposited according to the “D” variant of the deposition condition was not taken under consideration. The residual stress formed during the deposition process, caused delamination of the amorphous carbon part of the coating. In the case of the coating deposited in the “A” variant of the deposition conditions, the cohesive cracks (first cracks) were formed under 8 N. The delamination of the coating (adhesive cracks) was found under 20 N of the applied load.

Coating, which was deposited in the “B” variant had very low adhesion. The cohesion cracks were formed under 3 N, while adhesive ones under 6 N. The last coating, which was taken under mechanical investigation, was deposited according to the “C” variant. The first cracks were formed under 10 N, while delamination occurred under 21 N.

The second micro-mechanical test was based on micro-indentation using a Berkovich indenter. The indenter was loaded up to the formation of first cracks. The crack formation was indicated by the appearance of the steps in the diagram (Fig. 7).

Fig. 7 Results of the indentation test of the coatings performed by Berkovich indenter.

The best fracture resistance properties were found for the coating deposited in the “A” variant.

The next micro-mechanical test was performed to characterize the wear resistant properties of analyzed coatings. The test was performed using the ball-on-disc mechanical test under 1 N and 5 N of the uploading (Fig. 8).

Fig. 8 Friction coefficient for the analyzed coatings under 1 N and 5 N wear tests.

Taking under consideration the 1 N test, for coating deposited under “A” variant of the deposition parameters, the friction coefficient was the lowest. This type of coating was the most wear resistant. From 0 to ∼6000 cycle number the friction coefficient raised the fastest, then it had a milder course.

In a first step of the sharper waveform the friction surfaces were adjusted, and some amount of tribo-film was formed. The second stage, where the course was milder, some amount of tribo-film (mainly graphite) occurred. Graphite as a good lubricant could reduce friction.

In case of the coating deposited under “B” variant of the deposition parameters, just at the beginning, the sudden increase, then decrease and again increase of the friction coefficient was observed. This sudden drop of the friction coefficient was also connected with the friction surfaces adjustment. However, it had another character that coating of the “A” variant did not have. Coating deposited under “B” variant had much lower adhesion than deposited in the “A” variant. This sudden jump of the friction coefficient could be connected with the removal of a much bigger part of the coating than in the “A” variant. Then, there was a stage of the tribo-film formation. After some time of the wear process, the graphite occurrence caused the friction coefficient courses of the coatings deposited under “A” as well as “B” variants to be very similar. In the case of the coating deposited under “C” variant, friction coefficient was the highest of all analyzed coatings. Its course increased very fast up to 5000 cycles. After this stage, the course was stable or even a little decreased. Probably, the formation of high amount of graphite caused this effect.

Looking at the graph of the 5 N test (Fig. 8), the friction coefficient course for coatings deposited under three different deposition parameters were very similar to those which were observed in the case of the 1 N test. In general, two stages were visible. The first stage connected with friction surfaces adjustment and the second stage was visible where graphite lubricant reduced the wear. Only the course of the friction coefficient of the coating deposited under “B” variant did not have this second stage. It may inform that in that case, graphite formation did not play a significant role in the reduction of the friction coefficient.

3.3. Microstructure characterization of the Cr/Cr₂N + a-C:H + Cr nano multilayer coating deposited on carbon fiber composite material, after mechanical tests

After the ball-on-disc mechanical test, changes of microstructure caused by mechanical wear were characterized. According to the micro-mechanical test, the best properties were found for the coating deposited in the “A” variant. Thus, the microstructure change analyses were performed for this type of coating. The topography of coating with wear tracks after 1 N and 20 000 cycles as well as after 5 N and 20 000 cycles has been presented in the image obtained by SEM technique (Fig. 9).

Fig. 9 Topography image of the coating after 1 N and 20 000 cycles wear test, and 5 N and 20 000 cycles wear test, performed by SEM technique.

Thin foils for TEM characterization were prepared directly from wear tracks using FIB technique. Only this technique allows preparation of thin foil for TEM and getting microstructure information in TEM precisely from the place of interest (in this case from the wear track).

At first, changes in microstructure caused by the higher uploading was characterized. The 5 N test is too much for such a sophisticated, very thin coating, but interesting was the fact how this type of coating acted under the pressure of such a large mechanical load.⁷ The cross-section of the coating after the 5 N test has been presented at the TEM bright field (TEM BF) image. The coating was seriously destroyed and fragmented, however it was still present on the substrate, fulfilling its protective role (Fig. 10).

Fig. 10 TEM BF image of the coating after 5 N and 20 000 cycles wear test.

Focusing on the upper part of the coating (a-C:H implated by Cr nano-crystals), the role of nano-crystals in amorphous carbon structure has been confirmed (Fig. 11).

Fig. 11 Microstructure characterization of the amorphous carbon part of the coating after 5 N and 20 000 cycles wear test; (a) TEM BF image; (b) HRTEM image.

It has been also noticed from the diffraction pattern (phase analysis) that Cr nano-crystals inserted into the a-C:H structure reacted with carbon forming chromium carbide nano-grains (Cr₂₃C₆). The HRTEM image showed that the crack propagating through a a-C:H layer with lower Cr₂₃C₆ nano-particles content has been stopped at the interface with another layer of a-C:H with higher amount of Cr₂₃C₆.

The Cr/Cr₂N part of the coating also played an important role in the cracking protection process.

Cr₂N ceramic layers brittle cracked, while metallic Cr deformed plastically, reducing the cracking energy (Fig. 12 and 13).

Fig. 12 Microstructure characterization of the Cr/Cr₂N part of the coating (first part from the substrate) after the 5 N and 20 000 cycles wear test; (a) TEM BF image; (b) TEM BF image-higher magnification; (c) STEM image.

Fig. 13 HRTEM image of the Cr/Cr₂N interface.

Plastic deformation in metallic Cr layers was realized at 45°, a typical angle for plastic deformation of polycrystalline, metallic materials.²⁴

During a wear process, the coating was cracked and fragmented, then parts of the coating were removed in the form of wear debris. In the next step of the wear, the removed material was crushed, mixed together and deposited again at the top of the coating forming so called tribo-film. The tribo-film material is normally formed at the top of the material, which is subjected to wear process, as a wear product (Fig. 14).

Fig. 14 Microstructure characterization of the tribo-film, performed using TEM technique.

The phase analysis of the tribo-film, which was performed using selected area electron diffraction technique, indicated the presence of chromium nitride (darker spots) and carbon in the form of graphite (bright matrix). The graphite is a very good lubricant.²⁵ Formation of this material during wear is an advantage, which may reduce the friction coefficient.

The 5 N and 20 000 cycles wear test (ball-on-disc) was too much for such a type of coating; however as it was proved, even under such high uploading conditions, the coating still fulfilled its role. The suitable uploading for such a type of coatings is ∼1 N. The 1 N and 20 000 cycles wear test has been performed. After the test, the changes in microstructure have been characterized using transmission electron microscopy technique (Fig. 15).

Fig. 15 Microstructure characterization of the coating at the cross-section, after the 1 N and 20 000 cycles wear test, performed by TEM technique in STEM mode.

Looking at the coating microstructure in the cross-section, it has been found that the coating was not so seriously destroyed as it was after the 5 N test. No cracks were found. Focusing on the surface topography, it has been noticed that at the top of the coating graphite was formed during the wear process (Fig. 16). The phase analysis performed by diffraction pattern, showed the presence of graphite and chromium carbides in the tribo-film. The results were confirmed by high resolution TEM technique (Fig. 17). The graphite material, as a very good lubricant, would influence the wear resistant properties of the coating during the wear process.

Fig. 16 Microstructure characterization of the tribo-film formed after 1 N and 20 000 cycles wear test, performed by TEM technique.

Fig. 17 The HRTEM analysis of the graphite in the tribo-film formed during the 1 N and 20 000 cycles wear test.

3.4. Biological investigations

Cytotoxicity was analyzed using eukaryotic cells (Fig. 18). Variants “A” and “B” of deposition differed only in current conditions; however, it influenced the bio-compatible properties of the material. The majority of the carbon part of the coating deposited in “A” variant was produced at 230 DC, while the majority of the carbon part of the coating deposited in the “B” variant was produced at 155 DC. It may be concluded that the higher the current of the deposition process, the better the biological properties. The “C” and “D” were deposited without nitrogen flow. It can be concluded that no nitrogen flow had a negative influence on the biological properties of the analyzed coatings. In the “D” variant, amorphous carbon was produced only because of the methane flow. There was no graphite target. It resulted in the worst biological properties.

Fig. 18 Cytotoxicity results depending on the deposition condition.

4. Conclusions

• A hybrid PLD system (pulsed laser deposition + magnetron sputtering) is a suitable technique for a complicated multilayered structure deposition on carbon–fiber-composite materials. This technique is also adequate for coating deposition on polymers.²³

• Quality of deposited Cr/CrN + a-C:H + Cr nano multilayer coatings was very high.

• Coatings clearly reflect the surface roughness of the carbon–fiber-composite (CFC) substrate.

• The same diffraction contrast went through interfaces in the Cr/Cr₂N part of the coating, which confirmed crystallographic dependence between Cr and Cr₂N phases.

• In the current work, Cr nano-grains have been utilized to modify the structure and properties of amorphous carbon-the second part of the coating; Cr nano-grains were gradually inserted into the carbon structure.

• After 5 N and 20 000 cycles wear test (ball-on-disc), coating was seriously destroyed and fragmented; however, it was still present at the substrate, fulfilling its protective role.

• Cr inserted into amorphous carbon reacted with carbon forming chromium carbides nano-particles (Cr₂₃C₆).

• The HRTEM image presented that a crack propagating through a a-C:H layer with lower Cr₂₃C₆ nanoparticles content has been stopped at the interface with another layer of a-C:H with higher amount of Cr₂₃C₆.

• The Cr/Cr₂N part of the coating also played an important role in cracking protection process. Cr₂N ceramic layers brittle cracked, while metallic Cr deformed plastically, reducing the cracking energy.

• The 1 N and 20 000 cycle wear test did not cause serious distortion of the coating. No cracks were found. At the surface of the graphite coating, a perfect lubricant was formed.

• The lowest cytotoxic effect was found for the coating deposited according to the “A” variant of deposition.

• Deposition current and adequate gas flow during coatings production may have an influence on their bio-compatible properties.

Acknowledgements

Research Project (National Science Centre): Multilayered, wear resistant, self-healing, protective coatings elaboration for carbon fiber composite materials. Number: 2012/06/M/ST8/00408. Research Project (National Science Centre): Bio-mechanical and microstructure analysis of multilayer nano-composite, protective coatings for metallic substrates for tissue interaction. Number: 2012/07/B/ST8/03396.

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