Organic and inorganic sols

In the 80’s, H. Schmidt reported successful preparation of a new family of sol-gel based materials named as “Ormocers”, organically modified ceramics [64]. It is obvious that the

constitution of the coating solution is great importance. For example, containing of fluorine in Ormocers coatings act as anti-sticking constituent for glass containers application [65].

“Sol” – a colloidal dispersion of practices in a liquid; on the other hand, “sols” – are typically multi component systems consisting of an inorganic phase dispersed in a solvent mixture. “Gel” – is a giant aggregate or molecule that extends throughout the sols [66, 67]. Sol-gel materials are peculiar because they often contain more than one solvent, each solvent differing in volatility and surface tension [66]. The precursors could be classified as inorganic or metal organic precursors which participate in a polymerization (gelation) process.

The organic and inorganic components can interpenetrate each other on a nanometer scale. Depending on the interaction between organic and inorganic components, hybrids are divided into two classes: (1) hybrids consisting of organic molecules, oligomers or low molecular weight polymers embedded in an inorganic matrix to which they are held by weak hydrogen bond or van der Waals force and (2) in those, the organic and inorganic components are bonded to each other by strong covalent or partially covalent chemical bonds [68-71]. The physical properties of coatings varied from brittle and hard to rubbery and soft depending on the ratio of the organic to the inorganic constituents [65].

2.7.2 Method of Depositions

In practically, there are several methods available for applying liquid coatings to substrates; the best choice depends on several factors including solution viscosity, coating speed and desired coating thickness. Most commonly used methods for sol-gel deposition are dip coating and spin coatings [68-71]. The film microstructure depends on the size and extent of branching (or aggregation) of the solution species prior to film deposition and relative rates of condensation and evaporation during film deposition. Physics of film formation examines the dipping an spinning processes with respect to such parameters as withdrawal rate, spin speed, viscosity, surface tension, and evaporation rate. The reactions which occur during this sol-gel process can be classified in two categories: hydrolysis and condensation reactions.

¾ Dip Coating:

In the dip coating process, sol-gel materials involves more than one competition between viscous, capillary and gravitational forces; the mechanisms which control final film thickness and microstructure very complex as shown in figure below :

Film thinning by gravitational draining is assisted by vigorous evaporation. Differential evaporation may trigger several events at and beneath the liquid/gas interface. First it may lead to concentration variations along the gas-liquid interface; theses variations leads to surface tension gradients, which contribute to the surface stress and alter the flow. Second, differential evaporation leads to diffusion of the volatile species towards the surface and non-volatile ones

As the substrate is withdrawn upwards, a layer of solution is deposited, and a combination of viscous drag and gravitational forces determine the film thickness, H [68, 69]:

H = c1 (ηUo/ρg) ^0.5 (1)

Where - is the viscosity, the withdrawal speed and is a constant. The thickness of a dip coated film commonly in the range of 50-500nm [68, 69]. For sol-gel coatings, the formation of critical coating thickness, has been defined

Tc = EGc/ Aσ² (2)

Where E is young’s modulus of the film, A is a dimensionless proportionality constant, and the energy required to form two new crack surfaces. The mechanism of sol gel thin film deposition is shown Figure 2-11.

Figure: 2-10 Representation of dip coating process. [68]

Figure: 2-11 Representation of thin film deposition mechanism [66].

2.7.3 Role of Solvents in In-situ Solution

Solvent acted as the coating carrier. The removal of solvent or drying of the coating proceeds simultaneously with continues condensation and solidification of the gel network. The origin of stress developed during drying of a solidified coating is due to the constrained shrinkage and low rate of solvent loss after solidification [68-71]. The solvent content at solidification should be minimized in order to lower the stress in the coating [69]. It is very

important to limit the condensation reaction rate during the removal of solvent upon drying, so that the volume fraction of solvent at solidification is kept small.

The drying rate plays a very important role in the development of stress and formation of cracks particularly in the late stages and depends on the rate at which solvent or volatile components diffuse to the free surface of the coating and the rate at which the vapor is transported away in the gas[68, 69].

2.7.4 Advantages of Sol-Gel Coating Process

Through the sol-gel method well control microstructural (e.g. high surface areas and small pore size) films obtained directly from the gel state. Porous structures created in solution are preserved, which lead to the application in filtration, insulation, separations, sensors and antireflective surfaces. The advantages of sol-gel process are summarized as following.

ƒ Better homogeneity and purity from raw materials

ƒ Lower temperature of preparation:

• Save energy;

• Minimize evaporation losses;

• Minimize air pollution;

• No reactions with containers, thus purity;

• Bypass phase separation;

• Bypass crystallization.

However, the disadvantages of sol-gel processing include the cost of raw materials, shrinkage that accompanies drying and sintering, and processing times.

2.7.5 Physical Properties of Sol-Gel Thin Film

The physical properties of a coating include hardness, residual stresses, tensile strength and Poisson’s ratio, expansion coefficient and elastic module etc. These properties can be manipulated by the porosity, residual OH, chemistry, structure, unreacted organics, thickness and the uppermost temperature and duration of the heat-treatment [65, 72]. The expansion coefficient and elastic module of the substrate also have an influence on the properties of the coatings. The complexity of sol-gel film is shown in Figure 2-12. The relationship between physical properties of coatings and other interdependent variables such as sol gel chemistry, process parameters and thickness of coating are still unclear.

Figure: 2-12 Complexity of sol-gel coating [65].

2.6.6 Importance of Sol-Gel Al2O3 Coating

The Al2O3 coatings prepared by sol-gel process have been used for mechanical, optical, semi-conductor and microelectronic applications, because of its excellent properties such as good mechanical strength, high hardness, high resistance to radiation, corrosion resistance, excellent antiwear ability, high abrasive nature, chemical inertness, insulating and optical properties

achieved through low temperature sol-gel process. Selection of metaloxide and solvent is very important for a desired high quality of nano scale film in sol-gel processing. The tribological behavior of water based (aqueous sols) sol-gel Al2O3 coatings was evaluated by Zhang et al [74].

The results show sol-gel Al2O3 coatings exhibit better wear resistance, toughness and long life with low coefficient of friction [74, 77].

Kim et al, reported that sol-gel derived Al2O3 buffer layer (< 10nm) acted as diffusion barrier between the substrate and Pt film which improved the microstructural and electrical properties of PZT ferroelectric films for nonvolatile memory devices [75]. The Inter-diffusion of reactive elements was effectively prevented from the substrate by the Al2O3 diffusion barrier. In practice, aluminum oxide film can be obtained by different techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), Atomic layer deposition (ALD) and electroplating methods [74, 81]. However, the above mentioned methods are difficult to deposit a film with good surface coverage and thickness uniformity for the samples with complex profile or microstructured surface.

The optical and mechanical property of amorphous Al2O3 film is shown in Table 2-2.

The Transmittance range is in the range of 0.2~7 μm which is very suitable utilized as the transparent protective coating. The high melting temperature and hardness Al₂O₃ film can significantly improve thermal stability and wear resistance for both of metallic molds and glass performs.