In vitro mechanical integrity of hydroxyapatite coatings on Ti-6Al-4V implants under shear loading

Area of Science:

• Materials Science
• Mechanical Engineering
• Biomaterials Engineering

Background:

• Assessing the mechanical integrity of biomedical coatings is crucial for implant longevity.
• Conventional shear tests may not accurately represent in-vivo loading conditions for coatings.
• Plasma-sprayed hydroxyapatite (HA) coatings on titanium alloys are widely used in orthopedics.

Purpose of the Study:

• To develop and validate a new shear test method for evaluating coating mechanical behavior.
• To investigate the static and fatigue performance of plasma-sprayed HA coatings on Ti-6Al-4V substrates.
• To elucidate the failure mechanisms under shear loading.

Main Methods:

• Development of a new shear loading test method.
• Finite element simulations for test method validation.
• Static and cyclic shear testing of HA coatings on Ti-6Al-4V.
• Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) for interface analysis.

Main Results:

• The new test method provides shear load comparable to conventional methods.
• Static interface shear strength ranged from 25-40 MPa.
• A fatigue threshold for shear stress amplitude was identified, below which no fatigue damage occurred.
• AES/XPS revealed limited direct bonding (approx. 88% interface coverage) with mechanical interlocking dominating.

Conclusions:

• The developed shear test method is effective for characterizing coating mechanical behavior.
• HA coatings exhibit a fatigue threshold under cyclic shear loading.
• Failure initiates from interfacial microflaws, leading to coalescence and delamination.