Mechanical and biological properties of graded porous tantalum coatings deposited on titanium alloy implants by vacuum plasma spraying

Tantalum (Ta) has excellent mechanical properties, biocompatibility and chemical stability, and is more osteoconductive than titanium or cobalt-chromium alloys. However, it has a relatively high cost and is not easily fabricated. Accordingly, in the present study, a vacuum plasma spraying technique was used to deposit Ta powder in three successive layers on Ti6Al4V substrates. In the deposition process, the melting rate of the Ta powder was controlled by regulating the spray power and powder particle size in such a way as to form a coating with a porosity which increased from the inner layer to the outer layer. The Ta-coatings were processed by alkali treatment (AT), AT and heat treatment (AHT), and AHT with ultrasonic cleaning (AHT-UC), respectively. The average bond strength of the as-sprayed coating was approximately 54.5 +/- 2.3 MPa. Moreover, the average porosity and roughness R-a of the coating surface were around 13% and 22.2 mu m, respectively. The average porosity of the coating (with a thickness of 350-380 mu m) increased from 0.6% in the inner layer to 7.6% in the outer layer. The corresponding hardness and elastic modulus (E) values decreased from 240 +/- 20 to 167 +/- 62 HV0.1 and 148 +/- 5 to 123 +/- 4 GPa, respectively. Bone-like apatite inducement tests performed in simulated body fluid (SBF) showed a particularly rapid growth of apatite on the AHT-UC Ta-coating due to the formation of amorphous sodium tantalate during AT. Furthermore, the results obtained from initial in vitro biocompatibility tests showed that osteoblast-like osteosarcoma MG-63 cells had a significantly better attachment and spreading on the Ta-coating and AHT-UC coating following culturing for 3 h than on the untreated Ti6Al4V sample. In other words, both coatings provided a better initial biocompatibility than the native Ti6Al4V substrate.