**2. Production of nanocomposites**

incident energetic particles, and/or resultant recoil atoms, with surface atoms. A measure of the removal rate of surface atoms is the sputter yield Y, defined as the ratio between the number of sputter ejected species and the number of incident particles. Sputtering only occurs if the incident particles have enough energy to overcome the binding energy of the elements that constitute the material. In a deposition chamber under reduced pressure, a discharge is established between the cathode (target material from where the particles are ejected) and the anode (substrate onto which the coating will be formed). Usually there are three distinct phases in the sputtering process: (i) ejection of the target material; (ii) transport of the ejected material and (iii) nucleation and growth of the film. The control of several deposition parameters in each one of these three phases allow to a perfect control over the deposited thin film and obtaining tailor made materials, which are not predicted by conventional thermodynamics. The ejection of the target material is achieved by simply momentum transfer between the ionic rare gas positive ions with the constituent material of the target. Argon is one of the most commonly used rare gases. It is an inert gas and therefore has the advantage of not change the chemistry of the target material by reacting with the ejected species. It has an atomic weight that guarantees the adequate sputtering of most of the chemical elements, has a low cost and high availability in the market, and can be purchased with high purity. In the course of the ionization inside the chamber, a plasma is formed which needs to be stable, in order for the process to proceed. In order to induce the plasma positive ions collision with the target, they must be accelerated by a negative potential. For this reason, the applied deposition power must be high, so that the ions can acquire sufficient energy to promote the ejection of secondary electrons from the target, contributing to the maintenance of plasma. The pressure in the deposition chamber needs to be well-controlled in order to promote a high number of collisions between the secondary electrons and the gas atoms, allowing ionization to proceed and ensure the maintenance of the plasma. However, it must not be too high, as in this case, argon ions will suffer too many collisions in their path, diminishing the free mean path, leading to loss of energy and arriving at the target with lower energy that the one needed to promote sputtering. If they have enough energy, the ejected particles from the target are transported through the plasma to the substrate, where they eventually deposit and form the coating. The trajectory of the ejected particles is random, in their direction, following a cosine law. The process of the formation of the coating can also be considered in three phases (**Figure 1**). In the first phase, the atoms ejected from the target transfer their kinetic energy to the substrate, becoming "adatoms," that is, nonbonded atoms. In the second phase, the adatoms diffuse superficially over the substrate where they either suffer desorption or are accommodated in a low surface energy location onto the substrate. Finally, the third phase takes place when the atoms read-

66 Nanocomposites - Recent Evolutions

just their position in the network by bulk diffusion processes.

The properties of the films deposited by this technique depend on the material of the target, the gas used for the discharge and deposition parameters such as pressure, target-distance, polarization of the substrate and the chemical composition of the discharge gas. In fact, in addition to the nonreactive noble gas, the discharge gas, others can be added such as oxygen, nitrogen or methane. In these cases, the sputtering is said to occur in a reactive mode. The use of a magnetron associated with the cathode creates a magnetic field that imposes compulsory

trajectories to the electrons ejected by the target, increasing the bombardment density.

The use of composites is usually required when the materials or its surface must present a combination of properties/characteristics that are not possible to obtain from a single material. The materials for the matrix can be either organic, polymeric materials, or inorganic, such as ceramics and metals or metal alloys. In addition, the fillers can belong to these two classes of materials. When one of the dimensions of the filler is of nanometric scale, lower than 100 nm, it is called nanofiller and the resulting material a nanocomposite. The most commonly used inorganic nanofillers are carbides, nitrides, borides, oxides, metallic particles, clay, carbon nanotubes, nanodiamond and, more recently, graphene. Regarding the organic nanofillers, the most common, depending on the application, are polytetrafluoroethylene, nanocellulose fibers or cellulose nanocrystals. Nevertheless, some combinations of matrix and reinforcement are not able to be produced by conventional methods. In fact, the reinforcement of a metallic matrix with an organic nanofiller is forbidden, as the processing temperature of the metal material occurs at a temperature that completely degrades the organic filler. There are an extensive number of excellent reviews on the production of nanocomposites (see Refs. [3, 4] as examples) and some brief examples, based in these two references, are given.

The sol–gel method is suitable for obtaining nanocomposites coatings with a thickness up to 1 μm. However, the method presents several drawbacks, namely its application onto metallic substrates, crackability and not appropriate if a thermal treatment is required. This method can be used in combination with sputtering or electrodeposition.

Chemical vapor deposition (CVD) method is usually used for the fabrication of inorganic/ inorganic nanocomposites coatings and, sometimes, in order to improve the quality of the coatings, the aerosol-assisted CVD can be used. Some organic (matrix)/inorganic nanocomposites have also been produced by this method.

Physical vapor deposition (PVD) technology includes evaporation and sputtering, among other less used technologies. It can be used for the production of hybrid nanocomposites and, especially sputtering, as described in the next point of this chapter, is one of the most versatile for the production of a great combination of inorganic/organic nanocomposites.

The thermal spray method is often used for making nanocomposites with a metallic matrix. The spray material is a nanosized metallic or alloyed powder that is dispersed into a solution in order to conduct plasma thermal spraying.

was higher in the co-deposited samples compared to the control samples without Mg [9]. Another element that is known to cause the increase of the number of osteoblasts and reduce the activity of osteoclasts is strontium (Sr). This is the reason why it is recommended in several countries as treatment of osteoporosis [10]. Its integration into hydroxyapatite coatings demonstrated a 46% increase in the area of contact between bone and implant when compared to the HA monolithic coatings [11]. This study also demonstrated that the osteoconductivity of doped coatings was not only faster but also provided a better quality of bone-implant integration.

Hybrid Nanocomposites Produced by Sputtering: Interaction with Eukaryotic and Prokaryotic Cells

Moreover, other metallic elements such as cobalt (Co), chromium (Cr) and nickel (Ni) were used in hydroxyapatite composites [12] to study their effect on the *in vitro* growth of an apatite layer. This study demonstrated that metal ions can be incorporated during the mineralization process affecting its structure and size and, consequently, the quality of the mineral coating.

decreased the porosity and surface area of the coating and inhibited the proliferation of microorganisms, implying their possible use as biomaterials that may reduce the inflammatory process, according to a published study [13]. The same type of nanocomposites was also obtained

ment, adhesion and growth of mesenchymal cells, as well as their ability to differentiate into

with highly osteoinductive capacity, allowing their possible use as coatings for implants that support high loads. Moreover, also doping HA films with magnesium oxide (MgO) allowed to obtain better corrosion resistance and lower porosity of the coatings both when in contact with simulated human fluids and under conditions of osteoclastic resorption *in vivo* [14].

Silicon (Si) is a chemical element that is found in active zones of calcification. Its absence is associated with a poor production of collagen that reduces bone proliferation leading to the appearance of deformations and lesions. For this reason, it is a very important chemical element in the early stages of bone mineralization and soft tissue development [15]. Some of the invoked benefits are related to the release of small amounts of silicon ions, which stimulate the activity of seven families of genes, increasing osteoblasts proliferation and differentiation. In addition, silicon increases the solubility of the coatings by generating a more electronegative surface resulting in a surface biologically equivalent to apatite [16]. The development, in our laboratory, of hybrid sputtered nanocomposite HA/Si coatings confirmed these claims

The coatings were deposited, onto 316L stainless steel, by sputtering r.f. magnetron from HA target doped with silicon foils. The choice of steel is related to economic factors (especially in the context of economic crisis) given the price of the materials used in this type of implant titanium and its alloys. The use of 2 and 4 10 × 10 mm Si foils gave rise to coatings with atomic

The thin films, with thickness of approximately 700 nm, demonstrated a nanocomposite structure (**Figure 2**) with an average surface roughness (Sa) of 15 , 63 and 29 nm for HA, HA/Si2 and HA/Si4, respectively. All the surfaces presented moderate overall hydrophilic characteristics, although the surfaces of HA doped with elemental Si reveal a heterogeneous distribution of

(10 wt%) to HA to determine the potential of the composite in the develop-


http://dx.doi.org/10.5772/intechopen.79048

into HA,

69

Also, all ceramic composites, such as those obtained from the incorporation of ZrO<sup>2</sup>

by adding TiO<sup>2</sup>

osteoblasts. The results indicated that TiO<sup>2</sup>

and the results presented here have never been published before.

percentages of Si of 2 (HA/Si2) and 4 (HA/Si4), respectively.

Electroless deposition method is often used for producing nanocomposite coatings with polymeric matrix where the nanofillers are carbides, nitrides, borides or other polymers. In order to improve the mechanical and degradation properties of the coatings, a thermal treatment is made.

Electrodeposition method is used for the production of nanocomposites which contain organic nanofillers dispersed in organic or inorganic matrix. By changing the duty cycle and frequency during pulsed electrodeposition can also produce nanocomposite coatings.

Spray coating and spin coating methods are widely used for the preparation of polymericbased nanocomposites. However, the latter can only be used for coating flat substrates and, therefore, is mostly used for the preparation of thin-film nanocomposite coatings.

Dip coating is a method widely used in the industry. It consists of soaking the substrate in a solution with the nanocomposite and pulled up at a constant and controlled speed. Some of the advantages of this method are that the solution can be reused until evaporation or depletion. The disadvantages are mainly related with the poor adhesion between the substrate and the coating, which makes this method unsuitable for a number of applications.
