Microstructural and in vitro characterization of 45S5 bioactive glass coatings deposited by solution precursor plasma spraying (SPPS)
Introduction
Bioactive materials were developed as an improvement with regard to the bioinert materials typically used in the field of medicine [1,2]. The most studied bioactive material, which has been used for long time, is hydroxyapatite. This biomaterial is employed in a wide range of medical applications [[3], [4], [5]].
Not long ago, Prof. Hench discovered by accident a glass material composed of a silicate network incorporating sodium, calcium and phosphorus (45% SiO2, 24.5% CaO, 24.5% Na2O and 6% P2O5, in wt%) [6,7]. This glass, known as Bioglass® or 45S5 bioactive glass, has proved to be more bioactive and to possess better osseointegration than hydroxyapatite [8,9], so that it can be used in different clinical treatments such as periodontal disease, bone regeneration or in middle ear surgery [7,[10], [11], [12]]. In addition, the US Food and Drug Administration (FDA) approves its employment in medical applications [7].
The main disadvantage of this material is its brittleness, which limits its utilisation in load–bearing applications [13,14]. To solve this problem, researchers started to develop the deposition of bioactive glass onto metallic substrates developing a composite layer which combines good mechanical properties with high bioactivity. Different techniques have been studied to deposit this type of coatings, i.e. enamelling, glazing, magnetron sputtering and pulsed laser deposition [[15], [16], [17], [18], [19]]. Among all these techniques, plasma spraying is the most employed method due to the high deposition rate, the good control of the substrate degradation (compared to the other deposition techniques) and the possibility of controlling the morphology, thickness and structure of the coating, and therefore its properties [15]. In addition, plasma spraying gives the chance of easily producing dense coatings which are suitable to be scaled–up for implantable devices.
Typically, plasma sprayed coatings have been deposited using glass powder. These powders can be obtained either by the melting and quenching method or by the sol–gel method accompanied by subsequent thermal treatment [20,21]. Literature shows that bioactive glass coatings from powder feedstocks with good bioactivity can be deposited by plasma spraying. However, the coatings exhibited a cracked, highly porous microstructure with poor adhesion to the substrate [20,22].
Recently, the employment of liquid feedstocks such as suspensions and solutions instead of powder feedstocks has received great interest in the thermal spray community for different materials, due to the unique coating properties achieved, whereas thinner (40–50 μm) submicron– to nano–sized coatings could be easily produced [[23], [24], [25], [26], [27]].
For the case of bioactive glass coatings, its deposition from suspension feedstocks has been recently addressed. Thus, reported findings showed that final coatings displayed similar microstructures to those obtained from powder feedstocks [8,28,29]. On the contrary, the utilisation of solution feedstocks in the development of glass coatings has been hardly investigated while significant advantages can be obtained. On the one hand, thinner and nanostructured coatings with a more homogeneous microstructure can be engineered. On the other hand, using solutions allows one to work with pure feedstocks comparing to powders and suspensions, since the preparation of both feedstocks implies a series of laborious steps which can often introduce some contaminants in the working material.
In a previous work, the authors presented a preliminary study on the possibility of obtaining bioactive coatings using solution precursor plasma spraying (SPPS) [30]. Therefore, this current work aims to complete the previous research by determining the stability of the solution feedstocks as well as analysing their thermal behaviour inside the plasma torch. In addition, the effect of the employed spraying parameters on the morphology of the coatings was investigated. A mechanical characterization by scratch technique was carried out on bioactive glass coatings obtained by plasma spray for the first time. Finally, a complete study of the coatings bioactivity was developed for long soaking times.
Section snippets
Preparation of the solution feedstock
Following the previous work [30], bioactive glass 45S5 was selected as the working composition. Solutions were prepared with a precursor concentration of 4 M (4 mol of precursors per litre of solution), using water as a solvent. The reactants listed below were used to synthesise the solution feedstock:
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Tetraethyl orthosilicate or TEOS (C8H20O4Si synthesis grade, Merck, Germany) as a source of SiO2.
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Triethyl phosphate or TEP (C6H15O4P synthesis grade, Merck, Germany) as a source of P2O5.
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Calcium
Bioactive glass solutions characterization
The obtained solution feedstocks showed a composition very close to that of the 45S5 bioactive glass (Table 2). No significant differences between the solutions containing different amount of catalyst were found since all of them were made from the same amount of each precursor.
Concerning the DTA–TG analysis, the results are represented in Fig. 1. It can be seen that the three feedstocks displayed a similar behaviour, since the same amount of precursors was used in the preparation of each sol
Conclusions
Bioactive glass solution feedstocks were developed with different amounts of catalyst to hydrolyse the alkoxides. As–prepared, all feedstock solutions were suitable to be injected into the plasma torch as they had low viscosity at higher shear rates. In addition, their thermal behaviour made them suitable to obtain bioactive glass coatings. However, higher amount of catalyst resulted in a non–stable feedstock over time because the solution gelation quickly occurred.
The sol containing 0.2 M of
Acknowledgements
The authors of the present work thank Universitat Jaume I of Castellón the support provided in funding action 3.1. of the Research Promotion Plan (PREDOC/2015/50) and The European Virtual Institute on Knowledge–based Multifunctional Materials ASBL (KMM–VIN) for the KMM–VIN Research Fellowship (call 2016).
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