Microstructural evolution in calcium phosphate thin films

Abstract

Hydroxyapatite (HA) is an important class of biomaterial which has been extensively researched as a coating on metallic implant devices and/or as a model for the study of cells on bioactive surfaces. The composition of such coatings, when compared with the raw HA target powder, has led to them being termed as calcium phosphate (CaP) thin films with varying “HA-like” properties. The ability of the coating to deliver some or all of the properties of the sources material is limited by the unit cell composition that originates from the crystallinity of the coating during its growth. Although structural models exist for HA as a powder, the unit cell composition of CaP thin films has not yet been determined at the level necessary to understand the influences that effects from substrate topography and thermal annealing have on these properties.

This work describes a detailed investigation of the evolution of the microstructure that occurs via grain growth in sputter deposited CaP thin films. The approach taken here utilises a multi-source RF magnetron sputter deposition system to create the relevant CaP surfaces and the application of advanced characterisation techniques and data analysis methodologies to investigate the structural properties. The effects of the key operational aspect of the process, most particularly the sputtering parameters and subsequent thermal processing on the extent of stress incorporation determined from crystallographic data and quantitative phase analysis have indicated that the choice of deposition parameters is the most critical consideration in terms of coating reproducibility and quality.

The relationship between quality of the precursor HA (target) powder, thickness of the deposited layer and the unit cell composition of coating have all been addressed. It has been shown that the composition of the coating and its evolving microstructure are determined through preferred crystallographic orientation in the plane of lowest surface energy which limits the size and angle of curvature of the grains present in the coating. The size and shape of the grains produced influences the distribution of residual stress levels, i.e. the mechanical stability of the coating and also the concentration of the various atoms that are distributed across and within the thin film.

An experimentally derived structural model has been created for CaP sputter deposited thin films which are in advance of any produced heretofore. The resulting monoclinic P21/b structure is calcium deficient and represents a dehydroxylated apatite more like dahllite or bone mineral with carbonate substitution occurring at the A and B sites in Ca_10-x (HPO⁻³₄) (PO₄ ,CO⁻²₃ ) (OH⁻,CO⁻²₃ )_2-x  or Ca_10-x  (HPO⁻³₄) (PO₄ ,CO⁻²₃) (OH⁻)_2-x ·ηH₂Owhere 0 < x < 1 and η = 0 − 2.5 for the case where charge balance is achieved by interstitial water. The approach developed here has also been applied successfully to the study of TiO₂/HA films created by diffusion of the TiO₂ through a sputter CaP thin film at temperatures up to 700 ºC.

Since surface structure and function are intimately related properties this detailed characterisation of CaP thin film crystallography with microstructural properties is critical to the provision of reproducible surfaces that have a highly qualified and predictable biological response.

Date of Award	Apr 2011
Original language	English
Sponsors	VCRS
Supervisor	Brian Meenan (Supervisor) & Jim McLaughlin (Supervisor)