Contact Nanofatigue Shows Crack Growth in Amorphous Calcium Phosphate on Ti, Co-Cr and Stainless Steel

Abstract

Fatigue testing of load-bearing coated implants is usually very time-consuming and so a new contact fatigue test using a nano-indenter has been evaluated. A cube corner indenter provided the fastest indication of failure, through crack formation, compared to a spherical indenter. Contact fatigue was performed on sintered hydroxyapatite and then on splats produced on titanium, stainless steel and cobalt-chrome surfaces, either made on room temperature or 250 oC preheated surfaces. Sintered hydroxyapatite showed continual plastic deformation, but this is not that apparent for splats on metal surfaces. Substrate preheating was found to induce cracking in splats, explained by greater thermal residual stresses. Endurance during contact fatigue, measured as time to crack formation, was the lowest for splats on titanium followed by Co-Cr and stainless steel. The splat on titanium showed both cracking and plastic deformation during testing. Good agreement has been reached with previous studies with cracking directed to the substrate without splat delamination. Contact fatigue with the nano-indenter easily and quickly identifies cracking events that previously required detection with acoustic emission, and shows good feasibility for mechanical testing of discs and splats produced by thermal spraying, spray forming, laser-ablation, aerosol jet and ink jet printing