In medical applications, especially for hip and knee implants, a CoCr alloy is mostly used, due to good mechanical, anticorrosive and tribological characteristics [1
]. CoCr alloys have also been used as screws in trauma plating systems. In this case, a low osseointegration, compared to Ti alloys, could facilitate an easier removal after the healing of the bone fracture. Due to its higher strength, a CoCr alloy was also used for idiopathic scoliosis applications, where the results proved to be better in the case of the CoCr alloy compared to stainless steel (SS) and a Ti alloy [4
]. Another application was the use of a CoCr alloy in implants used for the correction of spine deformities, due to the high rigidity of the CoCr rod compared to SS- and Ti-based ones [5
]. CoCr was also found to be well adapted in dentistry for its good castability, especially the wrought alloys. Guide wires, clips, orthodontic arch wires and catheters are among the main applications in this field [7
]. Thus, the decreased corrosion resistance of SS and the low wear resistance of Ti alloys make it difficult to replace CoCr alloys in a wide range of applications.
Despite these good properties, in clinical practice the implants made of a CoCr alloy exhibited a high rate of failure due to various complications after implantation: (i) high toxicity as a result of the migration of toxic metal ions; (ii) a high amount of wear debris surrounding the peri-implant tissues and body organs, mainly due to the low wear resistance in biological fluids; (iii) low bioactivity abilities; (iv) a non-hydrophilic surface [1
]. During the friction process, wear debris of CoCr alloys were generated in different size and shapes [9
] and migrated to the periprosthetic tissues, leading to a failure of the implants. The wear of hip or knee joints is a complex process that involves many factors, such as the material and geometry of the implant, synovial fluid properties (various protein levels) and the patients’ lifestyles and body weight. Thus, the wear particles larger than 0.5–10 µm (round to oval to irregular shapes) play a dramatic role as third-body wear, leading to an intense wear process [9
]. The main problem with these debris is their high size, as cells (i.e., macrophages, fibroblasts, giant cells, neutrophils, lymphocytes, osteoclasts) will interact with these debris, leading to a chronic inflammatory response [9
]. Another problem can be the release of Co and Cr ions into the synovial fluid and their correlated increased concentration in the blood. Although considered essential elements for the body, their increase in concentration can be detrimental for certain functions [15
]. Thus, their long-term exposure can lead to cellular effects in the adjacent tissue and even to necrosis [16
]. Nevertheless, in order to eliminate the mentioned disadvantages found in CoCr alloy medical implants, several modifications of alloy surfaces were carried out over recent decades by: (i) coating, using various types of techniques (PVD, CVD, ion implantation, plasma spray); (ii) surface structuring (laser processing, sand blasting, acid etching, anodization); (iii) micro arc oxidation; (iv) electrochemical oxidation [1
]. The PVD method chosen for this study, namely the cathodic arc method, combines both a high degree of ionization of the ejected particles and a high efficiency of the evaporation process. Even though the initial energies are about 20 eV for light elements and around 200 eV for heavy elements, the final ion velocities (in the range of 1–2 × 104
m/s) were found to be independent of the cathode material and ion charge state, due to electron–ion coupling [18
]. Thus, with an enhanced atom mobility and surface diffusion, due to the higher energy of the ions, the obtained materials have favorable conditions in order to obtain different coating properties [20
The present study aimed to analyze TiSiCN as a possible coating solution to improve the corrosion and wear resistance of CoCr alloys used for orthopedic implants. For comparison, TiCN and uncoated CoCr were used as control groups. The TiCN coating was selected as a reference because it has good mechanical properties and good corrosion resistance, and an acceptable wear resistance in dry environments [21
]. By the addition of Si into TiCN, it was expected to significantly reduce the friction and wear process, as well as to improve the corrosion resistance of the CoCr alloy. It was reported that the addition of Si governs grain refinement and Si-containing coatings present superior friction and wear performance [24
]. Furthermore, it was demonstrated that the addition of Si to various materials with biological applications enhances the proliferation and differentiation of human osteoblasts, accelerating the osseointegration process [25
]. Additionally, Si–N thin films proved to have remarkable properties, which included high thermal stability and chemical inertness, in addition to those already mentioned [26
]. A survey of the literature shows that TiSiCN coatings have a superior tribological performance, but tests were performed mainly in conditions used in industrial applications, such as cutting tools and the automotive industry. Their main advantages are low friction, high wear resistance, good mechanical properties such as toughness and high resistance on plastic deformation [27
]. For medical applications, however, they have not yet been tested. Nevertheless, an alternative solution was the addition of Zr and Cr to the Si–N and Si–C–N matrix for severe wear and corrosive applications [33
In the present study, the coatings were obtained by the cathodic arc evaporation method on the CoCr substrates under a mixture of CH4 and N2 gases. The investigation included the examination of elemental and phase composition, texture, structure, morphology and mechanical properties (Young modulus, hardness, roughness, stress). Special attention was devoted to the corrosion resistance performed in a 90% DMEM + 10% FBS solution, at 37 ± 0.5 °C. In order to understand the damaging effect of the corrosion test, the morphology and roughness after corrosion were evaluated. Electrochemical impedance spectroscopy (EIS) was also performed in order to investigate the behavior of the proposed systems. Thus, this method gave an insight into the electrochemical processes which occurred at the material–electrolyte interfaces. The research was conducted in order to find new and improved structures as a better solution to optimize the safety and efficacy of biomaterials.
2. Materials and Methods
2.1. Coatings Preparation
Both coatings were prepared by the cathodic arc deposition process. For TiCN, one Ti cathode (99.99% purity) was used, while for TiSiCN, a Ti+Si cathode (85 at.% Ti and 15 at.% Si; 99.9% purity) was used. The cathodes were supplied from Cathay Advanced Materials Limited, Guangdong, China. The coatings were prepared using both CH4
as reactive gases (99.999% purity, Linde). The position of each cathode inside the deposition chamber was the same as those described in [33
]. In order to guarantee the uniform thickness of the coatings, the substrate holder was rotated by 15 rot/min. The CoCr substrates (ASTM F75 CoCr alloy) were cut into 12 mm discs and polished up to Ra roughness of 46.9 ± 5.9 nm. Each substrate was ultrasonically cleaned in trichloroethylene and flushed with dry nitrogen, and then introduced in the deposition chamber. To eliminate any impurity, the substrates were sputter etched with Ar+
ions (1 keV) for 15 min. The deposition parameters are listed in Table 1
, and were maintained constant during all deposition runs. The same negative substrate bias voltage was applied on both cathodes and the substrate temperature was around 320 °C. The thickness of the coatings was around 2.5 µm.
2.2. Coatings Characterization
Energy dispersive spectrometry (EDS) was used for the analysis of the elemental composition (EDS, Quantax70, Bruker, Tokyo, Japan). The morphology was examined by scanning electron microscopy (SEM, Hitachi TM3030Plus, Tokyo, Japan). Phase composition was studied by the X-ray diffraction method (XRD, SmartLab diffractometer, Rigaku, Tokyo, Japan), using Cu Kα radiation, from 10° to 100° with a step size of 0.02°/min. For the thickness and surface roughness determination, a surface profilometer (Dektak 150, Bruker, Billerica, MA, USA) was used. Surface roughness was measured for each investigated sample on five line-scans, each on a distance of 4000 μm.
The mechanical properties of the coatings were determined using a Hysitron Premier TI nanoindentation unit equipped with a Berkovich indenter tip of 100 nm radius and a total included angle of 142.3°, respectively. Prior to any sample testing, the Z-axis calibration was performed in the air and the machine compliance was assured using a fused quartz standard calibration sample with known hardness (H = 9.25 GPa ± 10%) and elastic modulus (E = 69.6 GPa ± 10%). In order to perform the nanoindentation experiments, a 15 × 15 µm2 area was previously scanned using the same Berkovich diamond tip at a normal force of 2 µN to investigate the surface roughness for the subsequent indents. Additionally, the indents were intentionally located at least 5 µm apart from each other, whilst an applied force of 10 mN was employed for every nanoindentation test. The force–displacement curves were recorded using a gradual force increase up to 10 mN in a 7-s time interval, followed by a 2-s dwell time at the maximum force of 10 mN and a gradual force decrease within the next 7 s, until the complete tip retraction from the coatings surface.
The electrochemical behavior of the investigated specimens was analyzed by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), using a VersaSTAT 3 Potentiostat/Galvanostat system. The measurements were performed in 90% DMEM + 10% FBS, at 37 ± 0.5 °C, using a typical three electrode setup with a Pt grid counter electrode (CE) and a saturated calomel (saturated KCl) (SCE) as the reference electrode (RE), while the working electrode consisted of uncoated CoCr and TiCN or TiSiCN coated CoCr substrates, respectively.
The open circuit potential (EOC) was monitored for 1 h, starting after the sample’s immersion. Linear polarization, Tafel and potentiodynamic curves were performed by applying a potential of −20 to 20 mV vs. EOC, −50 to 250 mV vs. EOC and −1 V vs. EOC to 2 V vs. RE, respectively, with a scanning rate of 0.167 mV/s. For the linear polarization measurements, the testing conditions were selected based on preliminary results, in such a way that the applied potential gave a linear behavior. The selected value was used to accommodate all the investigated systems. The EIS measurements were performed over a range of frequencies (0.1 ÷ 104 Hz), by applying a sinusoidal signal of 10 mV RMS vs. EOC.
The nature of the electrolyte, scanning rate, temperature, impurities, anode material and surface state of the samples are a part of the parameters, which can influence the electrochemical reactions. For example, the surface texture of the working electrode (investigated sample) is one of the most important parameters, which has an influence on the Tafel slopes, and consequently on the corrosion rate. Surface roughness has an important effect on general or pitting corrosion and the nucleation of metastable pitting. Skewness and kurtosis were used to identify the corrosion mechanism. Reid et al. have patented a method and apparatus for the identification of corrosion in metal objects and defining typical values of skewness and kurtosis for the identification of corrosion mechanism [59
]. The pitting mechanism appears when the Sk
< −2. In all our cases, the Sk
had values greater than −2, indicating that a general corrosion mechanism can be found for all investigated surfaces. Regarding the kurtosis, if the value is < 3, general corrosion can be observed, while for kurtosis > 3, pitting corrosion can be found. Considering this, it can be seen that for all investigated surfaces, a general corrosion mechanism has been identified. If the surface is rough, then a larger area could contribute to the increase in the corrosion current or to the corrosion rate. Therefore, a decreased surface roughness will lead to a better corrosion resistance [60
]. According to this statement, the TiCN-coated surface was rougher than TiSiCN, and this is probably the reason for its better corrosion resistance. For the uncoated CoCr, the roughness is not a factor which has a major influence on the corrosion resistance. Compared to the coated samples, the CoCr uncoated substrate had a smaller roughness, while its corrosion resistance was worse than for both coatings. Clearly, the surface roughness affects the corrosive behavior of materials (i.e., metals, alloys, coatings) and the nature of its effects (increase or decrease in the degree of corrosion) depends on the nature of the material.
To the best of our knowledge, there is no direct relation between hardness and corrosion resistance. Hard coatings can be subjected to surface microcracks, and then a localized penetration of corrosive solution will take place, leading to a galvanic cell, which accelerates the corrosive process. Hardness is important for load-bearing implants because a hardened material can have the ability to withstand wear. Taking into account the results of the present study, TiSiCN was harder than CoCr and the TiCN coatings and was more adequate for the proposed application.
For load-bearing implants, resistance to plastic deformation is an important factor and it can be described by the H3
ratio. Moreover, a material with a high H3
ratio resists plastic deformation during low load contact events and exhibits a higher yield strength [61
]. It is also generally accepted that the H/E ratio can be considered an important indicator of a good wear resistance of the surface [63
]. Thus, the improvement of the H/E ratio and, consequently, of the resistance to plastic deformation (H3
ratio) of the load-bearing implant may offer advantages, such as less surface damage and increased durability. In this study, the TiSiCN coatings have an H3
ratio equal to 1.1, which is higher than the one for TiCN (0.63), indicating that TiSiCN has a superior toughness and it can offer a better resistance to plastic deformation and good wear resistance.
The addition of Si to TiCN coatings leads to a grain refinement, and the crystallite size (d) was decreased to 16.4 nm in the case TiCN and to 14.6 nm in the case of TiSiCN. The formation of new defects, especially dislocations, is also responsible for the reduction in the crystallite size. The strain in the TiSiCN coatings (ε = 0.012) was lower compared to the TiCN coatings (ε = 0.053). The reason for this decrease could be attributed to different factors. One reason could be due to the addition of Si, which has atomic radii (0.111) smaller than that of Ti (0.146 nm), leading to a disorder of the crystal lattice, which is also evident by XRD diffraction (peaks were shifted when compared with the TiCN standard). The second reason could be attributed to amorphous phases in which Si can be found (Si, SiNx, SiCN) or C=C phases at grain boundaries, which are detrimental for crystallite development. TiSiCN has a higher C content than TiSiC. It is difficult to separate these factors and to know their contribution. However, the crystalline disorder becomes more pronounced by an increase in carbon content, which is also suggested by the decrease in the crystallite size and by the decrease in microstrain. Moreover, Franceschini et al. reported a strong dependence of stress on the nitrogen content in a-C:H films; at a low N content, the stress is high [66
]. This effect is difficult to see in our coatings, because the N content is reduced after the addition of Si, but it is a minor reduction. This result can also have a major influence on the corrosion resistance of TiSiCN. This coating probably presents fewer defects and it is more compact than TiCN.
When the crystallite size decreases, the corrosion current density decreases and polarization resistance increases, which means that the corrosion resistance of the coatings increases with decreasing grain size. Thus, the TiSiCN coatings, which have the smallest crystalline size, were more resistant to corrosive attack. The dependence of corrosion resistance on crystallite size can be ascribed to the BOLS mechanism [67
]. In the grain boundaries, there are undercoordinated atoms with lowered residual cohesive energy which possess high energy, these atoms exist in unstable states and an increase in their percent will lead to an increase of corrosion resistance [68
]. This finding is also sustained by the strain ε
value. In both cases, the strain was low, but after the addition of Si, the strain was significantly reduced. When the strain decreases, the corrosion resistance of the coatings increases. Thus, the correlation between high corrosion resistance and low strain and small crystallites can be explained in terms of the “bond-order-length-strength correlation mechanism”, meaning that the undercoordinated atoms found on the surface or in grain boundaries take the responsibility of the good corrosion resistance. In the current paper, along with the addition of Si, the Qdl
parameter was also decreasing, and this result could be due to a smaller crystallite size obtained by the TiSiCN coating. Thus, the decreased generation of defects, in the case of this coating, had a beneficial effect on the protective properties. It was shown that defects within a structure can cause localized corrosion at the coating–substrate interface, due to the electrolyte ingress [69
]. In addition, Rpore
indicated that the resistivity of the electrolyte in the pores had the highest value in this case, which can be also correlated with the lack of defects. The α values ranged from about 0.87 to 0.90, and the deviation from an ideal capacitor was ascribed to differences in roughness, as was shown. The dependence between roughness, capacitance and associated α values was demonstrated [69
], although there are also some other factors which can be influences, such as thickness and the dielectric constant of the material.
The present study aimed to find a new and improved possible solution for load-bearing implants. For this purpose, titanium-based carbonitrides with and without the addition of Si were investigated and compared. Both coatings obtained by the cathodic arc deposition method had an almost stoichiometric structure, being solid solution, which consisted of a mixture of TiC and TiN, with a face-centered cubic (FCC) structure. The crystallite size decreased with the addition of Si into the TiCN matrix, the crystallite size of TiCN was 16.4 nm, while for TiSiCN it was 14.6 nm. Both coated surfaces exhibited a uniform coverage, with some microparticles from the ion sputtering and ejection of the particles during the deposition process. After the addition of Si into TiCN, the Ra roughness values decreased, indicating a beneficial effect of Si.
All investigated surfaces had positive skewness, which is adequate for load-bearing implants, which work in corrosive environments. The hardness of the TiCN coating was 36.6 ± 2.9 GPa and it was significantly increased to 47.4 ± 1 GPa when small amounts of Si were added into the TiCN layer structure, while the elastic modulus was increased from 277 ± 8 GPa to 310 ± 2 GPa. A significant increase in resistance to plastic deformation (H3/E2 ratio) from 0.63 to 1.1 was found after the addition of Si into the TiCN matrix.
The most electropositive value of corrosion potential was found for the TiSiCN coating (-14 mV), as well as the smallest value of corrosion current density (49.6 nA cm2), indicating good corrosion resistance. The TiSiCN coating exhibited the best protection after immersion in 90% DMEM + 10% FBS, the best capacitive character, indicated by the low value of Qdl, and the highest resistance through the pores generated by the defects of the coatings and the electrolyte ingress, indicated by Rpore and Rct.
According to the conducted research, TiSiCN coatings have shown good mechanical properties and high corrosion resistance and are a good alternative for the coating of load-bearing implants.