Apparent fracture toughness of the CoB-CO₂B interface

In this study, new results for the interfacial fracture toughness of the CoB-Co₂B layer formed at the surface of the ASTM F1537, Standard Specification for Wrought Cobalt-28Chromium6Molybdenum Alloys for Surgical Implants (UNS R31537, UNS R31538, and UNS R31539), alloy were estimated. Initially, the cobalt boride layers were developed by means of the powder-pack boriding process at 1,173 K with 6 h of exposure, and 1,223 K with 8 h of exposure. The depth-sensing Vickers microindentation tests were performed using applied loads ranging between 1 and 2.3 N to generate a crack along the CoB-Co₂B interface. The apparent fracture toughness ðK_caÞ of the CoB-Co₂B interface was estimated using a half-penny cracking model and considering the interfacial values of the Young's modulus, hardness, and the critical point (P_c, a_c), in which P_c was considered as a criterion of the adhesion between CoB and Co₂B. To verify the influence of the cobalt boride layer thickness and the magnitude of the residual stresses developed on CoB-Co₂B interface, the cracking model was extended to estimate the K_ca for the boriding condition at 1,273 K with 6 h of exposure. In addition, the magnitude of the shear stresses on the CoB-Co₂B interface was analyzed by the finite element method as a function of the indentation loads of 1 and 2.3 N using the boriding conditions of 1,173 K with 6 h of exposure and 1,223 K with 8 h of exposure. The results showed that the interfacial fracture toughness of the CoB-Co₂B increased as a function of the CoB layer thickness, whereas the magnitude of the compressive residual stresses decreased for the thicker cobalt boride layer formed at 1,273 K with 6 h of exposure. Finally, the distribution of the maximum shear stresses located on the CoB-Co₂B interface oscillated from 3.9 to 4.4 GPa according to the indentation loads.