Development and Characterization of Biomedical Porous Ti - Nb - Ag Alloy through Powder Metallurgy Method

Abstract

One of the major concerns in biomedical implants is the mismatch in the elastic modulus of the implant material and the bones leading to stress shield effect. The present investigation focuses on the development of low elastic modulus, porous Ti−Nb−Ag alloy through powder metallurgy (PM) space holder method. Elemental powders of Ti, Nb and Ag with varying amounts were mixed with the powders of space holder (NH4HCO3). These powders are blended using ball milling for 1h, 5h, 10 h, 15 h, and 20 h. The powders were compacted by applying a load of 500 MPa. These compacts were initially calcinated at 200ºC for 2 h to remove the space holder and then finally sintered at 1200ºC for 3 h under ultrahigh vacuum sintering furnace. Microstructure of the porous alloys exhibited micropores, macropores and interconnected pore structures. It was found that with increasing ball milling time, the porosity and pore size decreased while the mechanical properties and electrochemical corrosion properties [in simulated body fluid (SBF)] were improved. XRD results indicated formation of small amount  martensite phase and intermetallic compound of Ti2Ag along with the α and β phases. Role of Nb was studied with various Nb content (x = 25, 30 and 35 wt%) in Porous Ti−xNb−5Ag alloys. Increase in Nb content led to decrease in porosity, reduction in both the elastic modulus and compression strength but improved corrosion resistance in SBF. Samples with different porosity levels (22% to 68%) with pore size ranging from 98 μm to 130 μm were fabricated by varying the amount of space holder. Increase in porosity further leads to the reduction in the compression strength, elastic modulus and also corrosion resistance in SBF. Tribocorrosion behaviour of porous Ti−20Nb−5Ag alloys were evaluated in SBF solution by applying various loads (0 N, 1N, 5N, 10N). The results indicate that increasing the applied loads lead to a material degradation and corrosion. The porous Ti−20Nb−5Ag samples are alkali-heat treated using 5 M NaOH, to aid the hydroxyapatite formation in SBF. Alkali treated samples were immersed in SBF for 7, 14 and 21 days at 37 ºC to examine the hydroxyapatite formation. The Ca/P ratio confirmed the formation of adequate hydroxyapatite coating. Further, the electrochemical corrosion test was conducted on hydroxyapatite coated porous alloy in SBF. The hydroxyapatite coated porous alloy after 21 days of immersion in SBF shows excellent corrosion resistance. The cytotoxicity test was conducted on the porous Ti−20Nb−5Ag alloy using MG-63 human osteoblast cells by incubating for 1, 4, and 7 days. The results indicated excellent cell growth and proliferation on porous alloy surface. Cytotoxicity test confirms that developed porous sample has non-toxic in nature and highly suitable for implant application.