Spark Plasma Sintering of Hybrid Nanocomposites of Hydroxyapatite Reinforced with CNTs and SS316L for Biomedical Applications

Background: The development of new bioimplants with enhanced mechanical and biomedical properties have great impetus for researchers in the field of biomaterials. Metallic materials such as stainless steel 316L (SS316L), applied for bioimplants are compatible to the human osteoblast cells and bear good toughness. However, they suffer by corrosion and their elastic moduli are very high than the application where they need to be used. On the other hand, ceramics such as hydroxyapatite (HAP), is biocompatible as well as bioactive material and helps in bone grafting during the course of bone recovery, it has the inherent brittle nature and low fracture toughness. Therefore, to overcome these issues, a hybrid combination of HAP, SS316L and carbon nanotubes (CNTs) has been synthesized and characterized in the present investigation. Methods: CNTs were acid treated to functionalize their surface and cleaned prior their addition to the composites. The mixing of nano-hydroxyapatite (HAP(n)), SS316L and CNTs was carried out by nitrogen gas purging followed by the ball milling to insure the homogeneous mixing of the powders. In three compositions, monolithic HAP(n), nanocomposites of CNTs reinforced HAP(n), and hybrid nano- composites of CNTs and SS316L reinforced HAP(n) has been fabricated by spark plasma sintering (SPS) technique. Results: SEM analysis of SPS samples showed enhanced sintering of HAP-CNT nanocomposites, which also showed significant sintering behavior when combined with SS316L. Good densification was achieved in the nanocomposites. No phase change was observed for HAP at relatively higher sintering temperatures (1100 degrees C) of SPS and tricalcium phosphate phase was not detected by XRD analysis. This represents the characteristic advantage with enhanced sintering behavior by SPS technique. Fracture toughness was found to increase with the addition of CNTs and SS316L in HAP(n), while hardness initially enhanced with the addition of nonreinforcement (CNTs) in HAP(n) and then decrease for HAP(n)-CNT-SS316L hybrid nanocomposites due to presence of SS316L. Conclusions: A homogeneous distribution of CNTs and SPS technique resulted in the improved mechanical properties for HAP(n)-CNT-SS316L hybrid nanocomposites than other composites and suggested their application as bioimplant materials.