Analysis of Corrosion Behavior and Surface Properties of Plasma-Sprayed HA/Ta Coating on CoCr Alloy
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The cobalt–chromium (CoCr) alloys have been extensively used as implants, especially in total joint replacements and in odontology, due to their superior mechanical properties and wear resistance in vivo. However, the excessive release of the ‘Co’ and ‘Cr’ ions from CoCr implants can lead to adverse health issues, such as hypersensitivity and inflammatory reactions. The present study aimed to improve the corrosion resistance of a medical grade CoCr alloy (ASTM F-1537) plasma-sprayed with tantalum (Ta)-reinforced hydroxyapatite (HA) coating. The weight percent (wt.%) of Ta content in HA coating was varied at three levels, i.e., 10, 20, and 30%. In vitro corrosion behavior was investigated by electrochemical measurements in Ringer’s solution along with surface properties analysis. The results revealed an increase in surface hardness value with an incremental increase in Ta content in the HA coating. The surface of HA as well as Ta-reinforced HA coatings possessed adequate roughness and demonstrated hydrophilic nature. With the Ta reinforcement in HA coating, the Ecorr values shifted toward nobler potentials and Icorr values declined noticeably which indicated an increase in corrosion resistance of the surface. The results of the study indicate that the proposed Ta reinforcement in HA is potentially important for CoCr bio-implant applications.
Keywordsimplants cobalt–chromium corrosion hydroxyapatite plasma spray tantalum
Medical and surgical therapies have been progressed immensely due to the extensive research. However, despite the advancements, such as advances in suppressive antimicrobial therapy (Ref 1), development antibiotic-loaded bone cement (Ref 2), progress in biomedical imaging and monitoring (Ref 3, 4), evolution of debridement and implant retention procedure (Ref 5), and the surge in the use of antibiotic-impregnated articulating spacers (Ref 6), the infection and osteomyelitis rates are still noteworthy with 4-64% in open long-bone fractures and about 1% in total joint replacements (TJR) (Ref 7). The by-products and debris produced due to the corrosion of implants lead to aseptic osteolysis, which accounts for almost two-thirds of TJR failures (Ref 8). CoCr alloys have been widely used as orthopedic implants, especially in TJR and in odontology, due to robust mechanical properties and lower wear in vivo compared to titanium alloys and stainless steels (Ref 9). However, the excessive release of ions, such as Co2+, Cr3+, and Cr6+, from CoCr implants results in hypersensitivity and inflammatory reactions as well as chromosome breakage and DNA damage, cell apoptosis, and later necrosis (Ref 10, 11). HA (Ca10(PO4)6(OH)2) coatings are considered as a promising method to prevent the excessive release of these ions (Ref 12, 13). HA has chemical and biological similarities to the human hard tissue (bone) (Ref 14, 15), and its coating can boost bone cell growth (i.e., osteoblast) activity which enhances the interaction between implant and the surrounding tissue (termed as “osseointegration”) (Ref 16). To improve the performance of CoCr alloys for biomedical applications, HA coating had been investigated in the earlier studies, in which HA coatings were mainly deposited by using electrochemical deposition, electrophoretic deposition, and sol–gel method (Ref 17-20). It is worthwhile to mention that plasma spraying is the only clinically accepted coating method for HA coating on a metallic substrate as well as it is commercially viable and a repeatable technique with high deposition efficiency (Ref 21). Various reinforcements such as silicon oxide (SiO2), calcium phosphate (CaP), titanium oxide (TiO2), zirconium oxide (ZrO2), polycaprolactide (PCL), and zinc had been tried in HA coating to further enhance its performance for the bio-implant applications (Ref 22-25).
The clinical practices of the use of Ta as an implant started about half a century ago in a wide range of diagnostic and implant applications such as cranioplasty plates, orthopedic and dental implants, vascular clips and endovascular stent (Ref 26-29). However, relatively high cost and casting ability issues due to the complex shapes required were major obstacles in the acceptance of Ta as a bulk material for implants manufacturing (Ref 30, 31). This issue leads to a new approach of using Ta as a coating constituent for the surface modification of traditional biomaterials. Moreover, Ta in combination with HA is rarely studied compared to other materials formulations. By using Ta as a reinforcement in HA, the prominent cause of implant failure, i.e., corrosion (Ref 32), can be addressed at relatively low expense compared to a bulk Ta implant.
In addition to corrosion properties, the analysis of surface properties, such as surface hardness, roughness, and wettability, is also essential because these properties influence the corrosion resistance and biocompatibility. Surface hardness is a critical property, which is frequently associated with the surface degradation rate of implants (Ref 33). Surface roughness is often related to biocompatibility because attachment of the biomolecules and cell tissues depend upon the surface area provided by the implant (Ref 34). Generally, the rougher the surface, then the better will be the attachment of biomolecules and cell tissues. Surface wettability has attracted the attention of researchers due to its correlation with the biologic response of implants. The rate at which a surface is being hydrated decides its wettability. The rate and ease of attachment of biomolecules to the surface of an implant depend on the rate of surface hydration (Ref 35).
The results of corrosion behavior and surface property analyses of plasma-sprayed Ta-reinforced HA coating on CoCr alloy (ASTM F-1537) have not been reported. In this study, plasma-sprayed coatings of HA, Ta, HA reinforced with 10, 20, and 30 wt.% of Ta (HA-10Ta, HA-20Ta, and HA-30Ta) were developed on CoCr alloy. The corrosion behavior of uncoated and coated samples was investigated using electrochemical corrosion testing along with surface property analyses.
Materials and Methods
Deposition of Coatings
Coatings of HA, Ta, HA-10Ta, HA-20Ta, and HA-30Ta were produced using plasma spray processing at MEC Pvt. Ltd., Jodhpur, India. Plasma spray gun MF4 (MEC Pvt. Ltd., Jodhpur, India) was used for plasma spraying. The spraying parameters were: an arc voltage of 63 volts, arc current of 600 A, traverse speed of 10 mm/s, rotating speed 30 rpm, and six number of passes. The flow rate of the primary gas (argon) and secondary gas (hydrogen) was 40 SLPM and 5 SLPM, respectively. The standoff distance and distance between spray beads was maintained at 75 and 125 mm, respectively.
Characterization of Coatings
The microhardness of the surface was evaluated from the polished cross section of the coatings using a microhardness testing machine (Wolpert Wilson 402MVD, Aachen, Germany). For microhardness testing, 50 gf load was used for 15-s dwell time. The roughness of the surfaces was analyzed by using a surface roughness tester (Mitutoyo SJ-210, Kamasaki, Japan). Wettability of surface was evaluated from the contact angle measurement between Ringer’s solution droplet and the surface using a goniometer (First Ten Angstroms FTA2000, Portsmouth, USA). Fifteen readings of each surface property were taken and the average value is reported.
Electrochemical Corrosion Analysis
The corrosion behavior of the coated and uncoated samples was evaluated by using the potentiodynamic polarization technique. Electrochemical corrosion tests were conducted using a Potentiostat/Galvanostat/ZRA (Gamry G-750, Warminster, USA) with the standard three-electrode arrangement. Ag/AgCl (saturated KCl) and graphite rod served as reference and counter electrodes, respectively, while the uncoated/coated sample formed working electrode. For simulating human body fluid conditions, Ringer’s solution (Nice Chemicals Pvt. Ltd., Kochi, India) was used as an electrolyte with the chemical composition (in gram/L) as 9 NaCl, 0.24 CaCl2, 0.43 KCl, and 0.2 NaHCO3 at pH 7.2. The potentiodynamic scan was performed from an initial potential of -0.25 V to the final potential of 0.25 V with respect to open-circuit potential at a scan rate of 0.5 mV/Sec. The potentiodynamic curve was then analyzed with Echem Analyst Software (Gamry Instruments, Warminster, USA) to acquire the values of electrochemical parameters by using Tafel extrapolation. Each sample was immersed in Ringer’s solution for 24 h before conducting the experiments to obtain a relatively stable state. Only a surface area of 1 cm2 was exposed to the Ringer’s solution for each sample by covering all the other sides with epoxy. Testing of each sample was performed at least thrice to establish a good agreement between the curves.
Results and Discussion
In case of Ta and Ta-reinforced HA coatings, peaks corresponding to Ta oxide (JCPDS card no. 19-1300) were also observed. The earlier studies on Ta-based coatings, in which the coatings were deposited by using sol–gel method (Ref 38), electron beam melting (Ref 39), and anodization (Ref 40), suggested that the Ta oxide content in the coating was advantageous for the improved corrosion resistance and biological properties of the surface. When the Ta reinforcement increased in HA coating, the amorphous hump corresponding to β-TCP reduced and the peaks corresponding to TTCP lowered as well. The overall intensity of HA peaks slightly increases with increase in Ta reinforcement in HA. HA coating represented crystallinity of 74.6%, whereas HA-10Ta, HA-20Ta, and HA-30Ta coatings had a crystallinity of 78.7, 82.2 and 84.1%, respectively.
The difference in the thermal conductivities of HA and Ta could explain the change in crystallinity for the coatings. The thermal conductivity of Ta (57 W/m K) is about 81 times higher than HA (0.7 W/m K) (Ref 41, 42), which can create a thermal gradient across HA/Ta boundary. It is reported in the literature that when the thermal conductivity of the second constituent is significantly higher than HA, it allows the lower cooling rate to adjacent HA region/splats (Ref 43). Reportedly, rapid cooling rate is one of the main reasons due to which crystallinity of HA phase decreases after plasma spraying (Ref 44). Due to the presence of Ta, the slower cooling rate of the coating could be the reason for the higher crystallinity of HA phase for Ta-reinforced coatings.
Surface Properties Analysis
The value of average surface roughness (Ra) of the uncoated CoCr alloy sample after air blasting was 1.16 ± 0.2 µm. The values of Ra for coated samples are depicted in Fig. 6(b). Ta-coated sample possessed the lowest surface roughness. The Ra values of HA coating and HA-10Ta coating were very close. But in the case of HA-20 Ta and HA-30Ta samples, there was a noticeable decrease in the surface roughness as compared to HA coating. It can also be seen in the SEM micrographs (Fig. 5) that the surface seemed relatively flattened with lesser melt-resolidified particles as the Ta content increased in HA. The previous studies which analyzed the effect of surface finish on biocompatibility suggest that an adequately rough surface of the bio-implant improves protein absorption at the surface, which results in better cell tissue/bone adherence with the bio-implant (Ref 34, 51). The obtained values of surface roughness for HA and reinforced HA coating were within the range of surface roughness produced by plasmas-sprayed coatings (Ra = 2-6 µm) as reported by Gross and Babovic (Ref 52).
Corrosion parameters determined by Tafel Extrapolation method
βa, e−3 V/decade
βc, e−3 V/decade
The Ecorr value of uncoated CoCr alloy was the lowest and for coated samples, the order for Ecorr value was: Ta < HA < HA-10Ta < HA-20Ta < HA-30 Ta. The Ta coating and HA coating improved the Ecorr value of the uncoated CoCr and this value further shifted to nobler direction with Ta-reinforced HA coatings. The Icorr value of uncoated CoCr was the highest among all the samples. The Ta-coated samples revealed lower Icorr values than the uncoated sample but higher than HA coating. The results of electrochemical corrosion testing are in agreement with the earlier studies on HA-coated CoCr alloys which revealed that the corrosion resistance of uncoated specimens was improved by HA coating (Ref 18-20). The surface microcracks (Fig. 5b) present at the Ta coating could be the reason for its lower Ecorr and higher Icorr values than the HA coating. Any structural imperfection in the coating such as a microcrack facilitates corrosion under the coating or on the surface of the substrate, which leads to leaching of ions to the corrosive media and thus resulting in a decrease in protection ability/corrosion resistance of the surface (Ref 60). With the Ta reinforcement in HA, the Icorr value further decreased and HA-30Ta-coated sample exhibited lowest Icorr which is about 11.5 times lower than the uncoated CoCr alloy. Fathi and Azam (Ref 61) also reported that HA/Ta-coated specimen revealed to be significantly higher corrosion resistant than the bare stainless steel 316L. In the present study, the difference between the crystallinity of HA and Ta-reinforced HA coatings could explain the decrease in Icorr value, i.e., enhancement of corrosion resistance. Mohajernia et al. (Ref 62) reported that the higher phase purity of HA coating improves corrosion resistance as well as the biological performance of the bio-implants. The crystallinity of HA phase was increased with the incremental increase of Ta reinforcement in HA coating and consequently Icorr decreased. In addition, the surface roughness is also an important factor which influences the corrosion behavior of a surface. Dudin et al. (Ref 63) suggested that pit formation on the rougher surface can happen easier as compared to a smoother surface. The pit formation leads to pitting corrosion, which results in lower protection ability of the surface. The earlier studies also demonstrated that corrosion resistance decreases with increased surface roughness (Ref 22, 63, 64). It has been proved that the ions resulting from the corrosion of metallic implants may affect cell metabolism, i.e., cell behavior may be affected by corrosion current (Ref 61). In the present study, the Ta reinforcement enhanced the corrosion resistance as well as the crystallinity of HA coating which could improve the long-term survivability and osseointegration (Ref 65).
To detect the changes in the composition of the coating, if any, EDX analysis was carried out after electrochemical corrosion testing. EDX data revealed that the Ca:P decreased for HA as well as Ta-reinforced HA-coated samples after exposure to Ringer’s solution. The value of Ca/P for HA, HA-10Ta, HA-20Ta, and HA-30Ta coatings were 1.72, 1.70, 1.69, and 1.68, respectively. The oxygen content for HA-, HA-10Ta-, HA-20Ta-, and HA-30Ta-coated samples was observed to be increased from 69.9, 67.4, 64.2, and 63.1% to 72.4, 69.6, 66.7, and 64.3%, respectively. The increase in oxygen content and a decrease in Ca:P after immersion in Ringer’s solution/simulated body fluid as observed in the present investigation was also reported in the earlier studies conducted on HA (Ref 15, 22, 66). The increased oxygen content of the coating can also enhance the attachment of biomolecules to the surface of bio-implant (Ref 59).
The Ta-reinforced HA coatings demonstrated better crystallinity than pure HA coating and fraction of amorphous phases decreases with the increase in Ta content in the coating.
The surface of as-sprayed HA coating and Ta-reinforced HA coatings found to be free from any microcrack and a sound and adherent interface between substrate and coating was observed. However, microcracks were present on the surface as well as at the substrate-coating interface of the pure Ta coating.
With the increase in Ta reinforcement in HA coating, microhardness of the surface increased significantly. However, lower surface roughness was observed in case of Ta-reinforced HA coatings as compared to the pure HA coating.
The surfaces of the CoCr alloy, as well as Ta coating, were hydrophobic. HA-coated surface nature revealed hydrophilic nature. With the Ta reinforcement, the contact angles between Ringer’s solution droplet and the surface increased slightly, but the nature of the surface remained hydrophilic.
The corrosion resistance uncoated CoCr alloy was enhanced by pure HA coating as well as Ta-reinforced HA coatings, whereas Ta-coated sample demonstrated relatively lower corrosion resistance as compared to other coated samples. A decrease in the Icorr for the Ta-reinforced HA coatings was observed (~ 7, 24 and 26% for HA-10Ta, HA-20Ta, and HA-30Ta, respectively) as compared to the HA coating.
After exposure to Ringer’s solution, the microcracks which were present in the as-sprayed Ta coating became more prominent and some microcracks were observed on the surface of HA coating. Ta-reinforced HA coatings effectively maintained the morphological integrity confirming their superior protection ability.
The authors would like to express gratitude to Medicoat, France and Zapp Precision Metals, Germany for sponsoring HA power and CoCr alloy substrates, respectively. The authors would also like to express gratitude to Mechanical Engineering Department, IIT Ropar, India, for providing research facilities for corrosion and surface analysis for this research work.
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