Corrosion and wear performances of hydroxyapatite and boron-containing TiO2 composite coatings on Ti6Al7Nb alloy

Plasma electrolytic oxidation coupled with hydrothermal treatment is a relatively new technique to form a hydroxyapatite/TiO2 layer on titanium alloys for biomedical applications. Hence the process allows achieving a bioactive and bactericidal surface by using electrolytes that contain ions (such as calcium-phosphorus and boron) necessary for desired properties. The coating properties are controllable by adjusting the parameters in the PEO process. In the present study, an electrolyte that contains both calcium, phosphorus, and boron ions was used to form a rough and porous oxide layer on Ti6Al7Nb which is known to be less toxic than the most widely used Ti alloy for biomedical applications, Ti6Al4V. A hydroxyapatite and boron-containing oxide layer was obtained after plasma electrolytic oxidation and hydrothermal treatment. Coatings were examined by XRD, XPS, SEM, contact angle measurement system, micro-hardness tester, wear tester, and corrosion measurement system. The results showed that the wear and the corrosion properties of all coated samples increased. Especially boron doping enhanced both the wear and corrosion resistance. Relatively the best corrosion resistance was achieved from CaP-B and the best wear resistance was from HA-B samples. The hardness values and mean surface roughness of all coated samples also increased while the average friction coefficients decreased. The hardness increased from 323 +/- 5 HV0.1 to 1084 +/- 16 HV0.1 where the coefficient of friction decreased from 0.5672 +/- 0.01 to 0.4697 +/- 0.03.