Shear testing and failure modelling of calcium phosphate coated AZ31 magnesium alloys for orthopaedic applications

Jonathan G. Acheson, E.A. Gallagher, J Ward, S McKillop, Brian FitzGibbon, A Boyd, BJ Meenan, P Lemoine, J.P. McGarry

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4 Citations (Scopus)
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Abstract

Magnesium orthopaedic fracture fixation devices can potentially provide significant clinical benefits, such as the elimination of secondary surgeries for device removal due to in-vivo resorption and reduced stress shielding due to reduced device stiffness. However, development, approval, and clinical adoption of magnesium devices has been hindered by the excessively high rates of in-vivo corrosion such that the structural integrity of the device can be catastrophically reduced before fracture healing occurs. Coating of devices with calcium phosphate coatings has been shown to significantly reduce corrosion rates, while enhancing osseointegration. However, the adhesion strength between the CaP coatings and magnesium substrates has not been previously investigated. Clinical insertion of fracture fixation devices such as intramedullary nails and k wires will impose significant shear loading on the coated surface of the implant. If the effective shear strength of the coating-device interface is not sufficiently high, the coating will be damaged and removed during device insertion. In the current study a bespoke experimental-computational approach is developed to provide a new understanding of the relationship between coating thickness, surface roughness, and effective shear strength of the CaP coating- Mg substrate interface. Nine test cases were created by adjusting either the deposition time (3 thickness values) or the surface treatment of the Mg alloy using SiC paper (3 roughness values) and double-lap shear testing was performed for these coating configurations. Strain development in the Mg substrates was monitored using strain gauges, and failure stress was determined for each configuration. Test results revealed that the effective shear strength of the coating-substrate interface is significantly higher for coatings on the rougher substrate surfaces when compared to those on smoother surfaces. Coating thickness was not found to significantly influence the effective shear strength over the range considered in this study (0.37–1.34 μm). Micro-scale finite element models of lap-shear tests were constructed using experimental profilometry data. Simulations of rough coating-substrate interfaces reveal that significant localised compression occurs at the coating-substrate interface in regions of large asperities. A novel cohesive zone formulation has been developed to simulate compression induced shear hardening, and the resultant simulations are found to accurately predict the significantly higher effective shear strength measured experimentally for rougher coatings compared to smoother Mg substrate surfaces.
Original languageEnglish
Article number127944
Number of pages14
JournalSurface and Coatings Technology
Volume429
Early online date25 Nov 2021
DOIs
Publication statusPublished (in print/issue) - 15 Jan 2022

Bibliographical note

Funding Information:
Ulster University acknowledges funding from the Department for the Economy (DfE), Northern Ireland (Grant USI 111). National University of Ireland Galway acknowledges funding from Science Foundation Ireland Centre-to-Centre grant 16/US-C2C/3291.Authors wish to acknowledge assistance from Dr. John Kelly with tensile testing and financial support for this work from a US-Ireland Centre-to-Centre R&D Partnership between Ulster University, the National University Ireland, Galway, North Carolina Agricultural and Technical State University, University of Pittsburgh, University of Cincinnati and Cincinnati Children's Hospital. The authors acknowledge access to computing facilities provided by the Irish Centre for High-End Computing (ICHEC).

Funding Information:
Ulster University acknowledges funding from the Department for the Economy (DfE), Northern Ireland (Grant USI 111 ). National University of Ireland Galway acknowledges funding from Science Foundation Ireland Centre-to-Centre grant 16/US-C2C/3291 .

Funding Information:
Authors wish to acknowledge assistance from Dr. John Kelly with tensile testing and financial support for this work from a US-Ireland Centre-to-Centre R&D Partnership between Ulster University , the National University Ireland, Galway , North Carolina Agricultural and Technical State University , University of Pittsburgh , University of Cincinnati and Cincinnati Children's Hospital . The authors acknowledge access to computing facilities provided by the Irish Centre for High-End Computing (ICHEC).

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • Calcium phosphate (CaP) coatings
  • Cohesive zone
  • Double lap shear
  • Finite element
  • Magnesium

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