Coating of Surfaces

Coating of glass can be done in different ways. A differentiation between optical or mechanical reasons has to be done. Optical coatings are applied for antireflection, heat insulation or UV protection. These coatings are relevant to increase the pressure resistance of thin hollow glass fibers.

The protection of the surface from defects and lubrication are the primary purposes of coatings. The application of a cover on the surface also can be used to heal existing defects and to change the surface properties [134]. An effective way of coating is to spray a dispersion of organic components in water directly after drawing process on the surface of the fibers, whereby the organic component can be e.g. polyacrylate. The heat of the glass leads to vaporization of the water and an organic coat forms on the surface.

Especially in the case of glass fibers and yarns the process of applying a coating has to be done almost immediately after the formation because of their abrasiveness [135].

The deposition of inorganic non-metallic layers on the glass surface improves mechanical characteristics as well as optical and chemical characteristics, which is called the sol-gel process. A dispersion of organic and inorganic materials is deposited on the glass surface to create a homogeneous phase. Afterward the hardening by heating up to temperatures between TH = 400 °C and TH = 550 °C is initiated. This forms a thin layer with a thickness of up to 1 micrometer by using inorganic brine as dispersion [94]. A usage of hybrid polymer as dispersion features the advantages of lower hardening temperatures of only about T_H = 150 °C or UV-light but leads to thicker layer up to 20 micrometer [94]. The adaption of CTE of substrate material (glass) and layer has to be considered. Thickness of layer affects the time of hardening and can lead to thermal stresses in material. Experimental tests showed that even a very thin layer of 0.2 micrometer increases the mechanical resistance of glass [95].

Another feasible method of glass coating is the chemical vapor deposition process (CVD-process). Selected coatings with highest pureness are manageable. So called precursors in gaseous phase flow over the surface of a substrate by dint of inert carrier

gas; near or directly on the surface, chemical reactions at specific temperatures take place. As a result of that reaction, the final product is formed and creates a thin layer on the surface of substrate. The creation of gaseous phase requires a heating of the precursors, thus the whole process setup has to be heated up to avoid condensation of them on colder parts.

The process can be divided into several single steps [95]:

1. Creating a gaseous phase of precursors;

2. Transport to substrate surface by carrier gas;

3. Adsorption;

4. Chemical reaction and decomposition of adsorbed components;

5. Formation of layer and desorption of unwanted reaction products;

6. Cooling.

Coating with CVD-process is deployed e.g. in hot end coating of containment glass, the formed layer fills up micro cracks and increases the resistance and solidity of glass components.

The atomic layer deposition process (ALD) as modifies CVD-method can be used to form coatings on glass surfaces as well. Contrary to the conventional CVD, in this connection precursors are lead separately into the heated reaction chamber. Between the single precursors, either a purging by inert gas or evacuating of reaction chamber ensures the outlet of non-reacted gases. Thus, the single reaction steps are separated from each other. A two-component process can be summarized as followed:

1. Discharge of the first precursor and limited chemical reaction;

2. Purging with inert gas or evacuation to remove non reacted components from step 1;

3. Discharge of second precursor with limited chemical reaction;

4. Purging with inert gas or evacuation to remove not reacted components from step 3.

The result of these steps is one process cycle which can be repeated several times.

Dependent on the number of process cycles, the thickness of coating layer varies. That method produces accurate and conformal layer even at nanoscale [136]. The thin layer film covers the surface of the treated glass and cracks are partially or completely filled up. A schematic illustration is given in the next figure.

Figure 25: Schematic illustration of ALD coated glass, the surface of glass (bright region) is covered by nanoscale defects, ALD coating (dark region) fills the cracks completely or partially [136]

Since optical perfect surface features invisible micro defects, which decreases the mechanical resistance, with the coating jacket the crack tip will be smoothed and the radius is decreased. Additionally, the potential of formation of local stress peaks is decreased. Experimental tests were carried out and showed increase of tensile strength between 45.7 % up to 89.3 % [136], [137] by coating with ALD before cutting. If glass is cut first before coating, then the tensile strength can be increased again about 9 % [137].

In that case the ALD film not only heals the surface flaws but also heals the cracks caused by cutting in a conventional manner.

The described methods and processes of coating surfaces are feasible to form coating layers on different shapes and with different consequences to the material characteristics. These methods are feasible not only for flat glass but also for round or complex shapes. In view of pressure resistance of hollow glass fibers, the increase of mechanical resistance caused by reducing or preventing surface defects is the main advantage. Also layers with sun protective or heat insulating effects are feasible, which are not described as precisely.

Further coating processes are classified as physical vapor deposition. The layer material is also vaporized and is comparable to CVD, but on the surface no reaction takes place.

The vaporized material forms a homogeneous layer caused by condensation. That process is adaptable to flat glass; in contrast the coating of complex or round shapes is not possible. That is why the PVD is not adaptive to glass fibers.