Calcium Phosphate Coatings
Porous hydroxyapatite has been accepted that due to its unfavourable mechanical properties it cannot be used under load bearing purposes. For this reason hydroxyapatite has been used as thin film coatings on metallic alloys. Of the metallic alloys investigated titanium based alloys have shown to be the material of preference for thin film coatings. Titanium alloys possesses good mechanical strength and fatigue resistance under load bearing conditions. They are lightweight, with high strength to weight ratios.
Calcium Phosphate Coating Deposition Processes
Of the coating techniques utilized, thermal spraying tends to be the most commonly used and analysed. This technique has been faced with challenges of producing a controllable resorption response in clinical situations. Besides the set backs, thermally sprayed coatings are continually being improved by using different compositions and post heat treatments which converts amorphous phases to crystalline calcium phosphates. Other techniques are being investigated. Techniques that are capable of producing thin coatings include pulsed-laser deposition and sputtering which, like thermal spraying involves high - temperature processing. Other techniques such as electrodeposition, and sol-gel utilise lower temperatures and avoid the challenge associated with the structural instability of hydroxyapatite at elevated temperatures.
The Advantages of the Sol-Gel Process
The advantages of sol-gel technique are numerous; it results in a stoichiometric, homogeneous and pure coating due to mixing on the molecular scale; reduced firing temperatures due to small particles sizes with high surface areas; it has the ability to produce uniform fine-grained structures (Figure 1); the use of different chemical routes (alkoxide or aqueous based); and their ease of application to complex shapes with a range of coating techniques those being dip, spin, and spray coating. The lower processing temperature has another advantage; it avoids the phase transition (~1156 K) observed in titanium based alloys used for biomedical devices.
Figure 1. (a) SEM and (b) AFM image of a sol-gel (alkoxide) derived hydroxyapatite coating.