Low-cost surface modification of a biomedical Zr-2.5Nb alloy fabricated by electron beam melting

Abstract

• Thermal oxidation is an effective way to prepare oxide layer on a Zr-2.5Nb alloy fabricated by electron beam melting (EBM). • The morphology, phase constituents, and surface color of oxide layer change in the oxidation process. • Grain refinement significantly delays the breakaway oxidation. • A dense and black oxide layer with a hardness of 902 HV is prepared on a EBM- built Zr-2.5Nb alloy by oxidation at 600°C for 3 hours. The Zr-2.5Nb alloy with a fine microstructure consisting of α laths was successfully prepared by electron beam melting (EBM). The thermal oxidation behaviors and kinetics of the as-built, and the EBM-built and hot isostatically pressed (HIPed) Zr-2.5Nb materials in a temperature range of 450–600°C were investigated and compared with those of the alloy prepared by conventional casting and forging. It was found that the oxidation kinetics of the as-built and the forged materials followed the parabolic rate law during isothermal oxidation at 550°C, but the HIPed materials exhibited a parabolic-to-linear kinetic transition, suggesting that the larger grain sizes enhanced the oxidation. The oxide layers of all materials were composed of a large fraction of monoclinic zirconia phase ( m -ZrO 2 ) and a small fraction of tetragonal zirconia phase ( t -ZrO 2 ), and transformed from t -ZrO 2 to m -ZrO 2 with increasing oxidation time. The surface hardness of the as-built, the forged and the HIPed materials increased from 215, 204, and 188 HV before oxidation to 902, 1070, and 1137 HV after oxidation, respectively. The cross-sections of the materials showed the presence of micropores and microcracks inside the oxide layers with thicknesses ranging from 4 to 8 µm. With the oxidation temperature of 600°C and oxidation time duration of 3 h, a dense black m -ZrO 2 oxide layer with smooth surface and 902 HV hardness was obtained on the EBM as-built Zr-2.5Nb materials.