ReviewThe effect of process conditions on the properties of bioactive films prepared by magnetron sputtering
Introduction
Hydroxyapatite (HA), Ca10(PO4)6(OH)2, which is the main chemical constituent of bone, is nowadays considered a useful biocompatible material. Unfortunately, HA is brittle and the prepared bone implants can provide poor mechanical performance. This drawback is overcome by the development of HA coatings on metals. This takes advantage of the bioactive behavior of the HA ceramic and preserves the mechanical characteristics of the metallic substrate. Presently, the commercial method applied for growing HA coatings with interest in bone implantology is plasma spraying [1], [2], [3]. However, this method presents a few drawbacks, including thermal decomposition of HA from the high-temperature plasma (up to 30,000 °C) and fractures in thick-film coatings of >50 μm in thickness [4], [5], [6]. To resolve these problems, other deposition methods such as biomimetic process [7], sol–gel [8], [9], dipping [10], electrophoretic deposition [11], [12], and radio frequent (RF) magnetron sputtering [13] have been investigated. Although these experimental processes have successfully overcome some of the problems associated with plasma spraying, there are unique limitations with each [2], [14], [15], [16]. For example, dip methods and electrophoresis techniques showed weak bonding strength between the coating and substrate [4], [17]; the biomimetic coatings made with a supersaturated calcifying solutions are difficult to prepare on an industrial scale [18]; the sol–gel deposited coatings are easy to crack in drying processing [16]. In comparison, RF magnetron sputtering exhibits more advantages for producing bioactive films: (1) the sputtered films have good adherence properties [2], [17], [19]; (2) the films are more uniform and more dense, even on complex implant designs [20], [21]; (3) the substrate temperature is low during deposition [22]; and (4) this technology can be applied for large-scale industrial production [23].
RF magnetron sputtering is an improved ion-sputtering method and has been applied to deposit thin bioactive HA films on titanium (Ti) oral implants [24]. In RF magnetron-sputtering deposition, an RF field generates plasma between a sputtering target and the substrate holder. In magnetron sputtering, a magnetic configuration is present below the target, which traps electrons nearby the target. There, the electrons cause ionizations in the sputtering gas. Part of the ions is accelerated towards the target by the voltage drop, the so-called plasma sheath, which is present between the plasma and the target. The bombarding ions cause the ejection of target species and secondary electrons. The secondary electrons maintain the discharge [22]. The atoms from that target travel to and bond with the substrate, becoming the part of thin films.
The structure and properties of magnetron-sputtered films largely depend upon the rate at which energy is delivered to the growing film by concurrent ion bombardment. In other words, film properties are governed by internal parameters like ion density and ion current to the substrate. The application of external parameters in a magnetron-sputtering system such as discharge power, gas composition, process pressure, base pressure, gas flow rates, substrate temperature, bias and targets-substrate distance, significantly affects these internal parameters and the resulting film properties [25], [26]. By adjusting the deposition parameters, thin, uniform, and dense bioactive films can be obtained.
Despite these excellent properties, the as-sputtered films have a low HA crystalline which in turn affects the stability or dissolution behavior of the implants [27], [28], [29]. This problem can be solved by post heat treatments, however, which may be result in weakened bond strength due to thermal expansion mismatching between substrate and film. The differences in thermal expansion can be reduced by controlling the film composition, which is greatly influenced by initial target material. Consequently, the properties of the bioactive films are also determined by post-deposition treatments and initial target materials [30]. The relationships between processing conditions and structure, elemental composition and properties of the bioactive films are summarized in this paper.
Section snippets
Deposition conditions of magnetron sputtering
The sputter deposited films had a uniform and dense structure, especially on complex implant designs, like threaded implants [24]. In cell culture experiments, magnetron-sputtered HA films can indeed stimulate extracellular matrix and induce apatite formation [31]. However, the main limitation of RF sputtering of HA is the possible alteration of the HA structure and composition. The calcium–phosphate (Ca/P) ratios measured for sputtered HA films are often different from the stoichiometric ratio
Annealing
As far as we know, the as-sputtered Ca–P films are amorphous or only weakly crystalline. The dissolution/precipitation behavior of the various Ca–P ceramics is influenced by their chemical structure and crystallinity. This low degree of crystallinity accelerates the dissolution of HA films in a living body, and this high rate of dissolution leads to the disappearance of the films before bone tissue can bond to the film, at an early stage after implantation [63], [64]. High-crystal quality films
Conclusion
RF magnetron sputtering is a promising technique for forming bioactive films (HA and its composite films). The adhesion of the film to its substrate and the growth of crystalline structures of the films are predominant factors in determining the performance and reliability of dental and orthopedic implant applications. The structure and properties of films are particularly depended on the process conditions, which include deposition parameters, heat treatments and initial target materials.
Acknowledgments
This work was financially supported by Department of Science and Technology of Jinan, Shandong Province, P. R. China (Grant No. 051070).
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