Development of novel carbon nanotubes (CNT) reinforced plasma sprayed baghdadite coatings

Abstract

Biomaterials are natural or synthetic materials to substitute the damaged biological structure to restore its form and function. The synovial joints like hip, knee and shoulder suffers from degenerative diseases like osteoarthritis that results in the cartilage breakdown. According to published data, orthopedic implant market is experiencing significant growth. The implant life depends upon multiple factors like age, gender, and patient body mass index and it ranges from 10 to 15 years. Therefore, it is important to increase the lifespan of orthopedic implants, which will delay the need for replacement surgeries and improving the comfort of implant users. The main requirements of load-bearing implants are mechanical properties like tensile strength, hardness, modulus, and elongation, and should be non-toxic and non-inflammatory. Apart from that, high corrosion and wear resistance are also required to enhance the life of implants which usually depends on the surface properties of the implants. The surface modification techniques can help in improving the biocompatibility of materials like titanium by coating it with bioactive material and hence, improving the osseointegration. Amongst different coating methods, plasma-spray is the most widely used because of its high deposition rate, desired coating thickness, cost-effectiveness and reliability. Hydroxyapatite (HA) coatings by plasma spray techniques are commercially accepted and approved by the FDA (Food and Drug Administration) and used in knee, hip, shoulder, ankle, and dental implants. However, its intrinsic brittle nature and inadequate mechanical properties restrict its usage as a coating material, declaring it unreliable for load-bearing bioimplants. Another bioceramic, Baghdadite (BAG, Ca3ZrSi2O9), has emerged as a highly promising biomaterial for facilitating bone and tissue regeneration. As reported in the literature, BAG exhibits superior mechanical properties compared to hydroxyapatite (HA). Carbon nanotubes (CNT) have high tensile strength, high thermal conductivity, and excellent chemical stability. Over the past 15-20 years, CNT has emerged as a promising reinforcement material for bioimplant coating applications, contributing significantly to advancements in both mechanical and biological properties. In the present work, bioceramic coatings reinforced with CNT using the plasma-spray technique were developed. Four different coatings were prepared which include HA, BAG, BAG reinforced with 1wt%CNT and BAG reinforced with 2wt%CNT on titanium substrate. The developed coatings were characterized using SEM (Scanning Electron Microscopy), EDS (Energy Dispersive Spectroscopy), XRD (X-ray Diffraction), XPS (X-ray Photoelectron Microscopy), and Raman spectroscopy. The surface morphology of the coatings exhibits typical plasma spray characteristics of pores, microcracks, and irregular melting. The XRD analysis confirmed the presence of hydroxyapatite and baghdadite phases in the HA and BAG coatings respectively. Raman spectroscopy validated the presence of CNT in the coatings after the plasma spray deposition. The coatings were firmly bonded to the substrate and thickness in the range of 350-450 μm. The porosity and surface roughness values of the coatings were also measured. The BAG coatings have higher surface roughness values than HA coatings and it increased further with CNT reinforcement. The porosity values were decreased with CNT addition due to the ability of CNT to infiltrate the inter-lamellar spaces and fill the gaps at the intersplat region. The microhardness values increased with the reinforcement of CNT in BAG coatings by 38.7%. It can be ascribed to the reduction in porosity and effective anchoring of CNT within the BAG ceramic matrix. The scratch testing of the coatings indicated the reduction in wear volume loss and scratch rate due to the decrease in penetration depth and scratch width by the addition of 2wt% CNT in BAG coatings. The increase in critical load values in BAG-2CNT coatings demonstrated the enhancement of the cohesion strength of the coatings. The CNT provided improved interlocking and mechanical anchorage among the splats. The electrochemical corrosion analysis of the coatings was performed by linear polarization test and Electrochemical Impedance Spectroscopy (EIS) in SBF at 37ºC. The corrosion current density value was reduced by 80% for BAG-2CNT coatings as compared to the BAG coatings. Further, the EIS results showed the increase in the polarization resistance values in CNT-reinforced coatings. The post corrosion analysis indicated the reduction of defects like pores and microcracks in the CNT reinforced coatings which leads to the enhancement of the corrosion resistance. The CNT having higher thermal conductivity and acting as an inert barrier to the SBF can be the other major reasons. The in-vitro biocompatibility studies revealed the good cell attachment, growth and spreading of the MC3T3-E1 cells on the surface of the coatings. The filopodia development indicated the efficient colonization and spreading of the cells on the coating surface. The cell viability and proliferation analysis indicated no cytotoxicity is caused by the CNT reinforcement in the coatings. The BAG-2CNT coatings have the highest cell viability % at the end of day 1, 3 and 7. In summary, this study demonstrates how the plasma spray BAG coatings with CNT reinforcement exhibits favorable mechanical, electrochemical characteristics as well as biocompatibility.