Effects of surface treatments on high cycle corrosion fatigue of metallic implant materials

Long term corrosion fatigue properties of two materials which are candidates for skeletal implants — cold worked c.p. Niobium and c.p. Tantalum — have been investigated. Fatigue properties have been compared to two implant materials in clinical use — c.p. Titanium and Ti–6Al–7Nb alloy. Constant amplitude fatigue experiments (S–N curves) were performed at ultrasonic frequency (20 kHz) with two different surface structures (ground surface and blasted and shot peened surface) in ambient air and in a corrosive fluid similar to the body fluid in the oral cavity. The endurance limit at 2×108 cycles of all materials decreased by 5–20% if they were cycled in corrosive fluid instead of ambient air. The loss of fatigue strength is more pronounced for ground Ti–6Al–7Nb alloy and c.p. Ti than for ground c.p. Nb cw and c.p. Ta cw. Fracture surfaces show a more pronounced embrittlement of ground Ti–6Al–7Nb alloy and c.p. Titanium after cycling in corrosive fluid than ground c.p. Tantalum and c.p. Niobium. A beneficial influence of surface structuring by blasting and shot peening on the fatigue properties was found for all materials in both environments. Fatigue loading using ultrasonic frequency allows one to select appropriate implant materials and to determine their very-high cycle corrosion-fatigue behaviour within reasonable testing times. Though the obtained high-frequency values may not be fully representative of actual in vivo behaviour, they are regarded as useful material characterizing values.