Study of cellular femoral stem for stress shielding and interface stability

Implant failure is due to stress shielding and interface micromotion. The application of porous structures in the femoral implant has a great effect on reducing stress shielding and improving the stability of the bone-implant interface. The performance of femoral stems with triply periodic minimal surface (TPMS) structures, IWP, and Gyroid structures was evaluated using finite element analysis. We studied the stress shielding phenomenon of the porous femoral stem based on the ability of stress transfer to the femur. The micromotion at the bone-implant interface was explored for different porous femoral stems. The effect of gradient structure design was investigated in the axial direction of the stem. These gradient designs involved a stem with an increasing volume fraction in the axial direction (IAGS) and a decreasing volume fraction along the stem (DAGS). The results showed that the axial stiffness of the stem has a direct effect on stress shielding and an inverse relation to bone-implant micromotion. The finite element analysis results inferred that bone resorption is higher in the stems with IWP structure than in Gyroid at the same volume fraction. Axially graded stems transfer higher stress to the femur than homogenous porous stems. DAGS design of IWP and Gyroid and IAGS Gyroid increased the stress on the proximal-medial of the femur. Homogeneous porous stems with high porosity (80% porosity for IWP and 70% porosity for Gyroid) and DAGS design exhibited low stress shielding and controlled bone-implant interface micromotion within an acceptable range for bone ingrowth.

Fabrication of AlCoCrFeNi High-Entropy Alloy Coating on an AISI 304 Substrate via a CoFe2Ni Intermediate Layer

Through laser metal deposition, attempts were made to coat AlCoCrFeNi, a high-entropy alloy (HEA), on an AISI 304 stainless steel substrate to integrate their properties. However, the direct coating of the AlCoCrFeNi HEA on the AISI 304 substrate was found to be unviable due to cracks at the interface between these two materials. The difference in compositional change was suspected to be the source of the cracks. Therefore, a new transition route was performed by coating an intermediate layer of CoFe₂Ni on the AISI 304 substrate. Investigations into the microstructure, phase composition, elemental composition and Vickers hardness were carried out in this study. Consistent metallurgical bonding was observed along both of the interfaces. It was found that the AlCoCrFeNi alloy solidified into a dendritic microstructure. The X-ray diffraction pattern revealed a transition of the crystal structure of the AISI 304 substrate to the AlCoCrFeNi HEA. An intermediate step in hardness was observed between the AISI 304 substrate and the AlCoCrFeNi HEA. The AlCoCrFeNi alloy fabricated was found to have an average hardness of 418 HV, while the CoFe₂Ni intermediate layer had an average hardness of 275 HV.