ELECTROSPRAYING OF GLASSES—PREPARATION OF GLASS COATINGS ON GLASS

Abstract

Glass coatings on ceramic or metallic substrates are prepared by two different methods. In a two-step process the glass powder is deposited on the glass substrate by methods such as airless spraying of enamel or glaze suspensions, dipping in enamel suspensions, electrospraying or electrophoretic deposition of enamel or glaze suspensions, and electrostatic powder spraying. The particle size of the glass powders which are generally prepared by milling, ranges from 500 nm to 100 μm. Therefore, these layers of glass powder have to be densified by melting in a second step. As for glass substrates this processing temperature should not exceed 500–700°C to prevent bending of the substrate, low melting glass powders have to be applied in this case. Alternatively, the glass powders can be heated to melting temperatures while spraying (e.g. flame or plasma spraying) to reduce the thermal load of the substrate (one-step process). Unfortunately, the molten glass particles freeze very quickly when they hit on the cold substrate and the density of the deposited glass layer is lower compared to the two-step process due to trapped residual pores. It is the objective of our work, to improve the coating methods of glass by combining these different methods and to reduce the processing temperatures by applying nanosized glass powders. Due to the high surface activity, these powders can be sintered to transparent glass below the critical crystallization temperature. Therefore, the processing temperature can be significantly reduced compared to the melting technique. In this paper the eletrospraying of glass melts and glass suspensions is presented which are new methods for preparing glass coatings on glass substrates. The process of electro-melt spraying is similar to the LMIS process, but instead of molten metal a glass melt is atomized under vacuum. The electrical conductivity and surface tension of these glass melts are higher than that of water but lower than that of molten metal. The droplet diameter of atomized glass melts lies between 100 nm and 10 μm. The molten droplet build up a dense coating on a hot substrate and are sintered in situ. In our case a flat glass substrate is mounted on a metal counter electrode. Due to the high electric fields which are needed for forming a Taylor cone of a glass melt, a vacuum of about 10 -2 mbar is necessary to prevent electrical break-down. At this pressure most technical glasses show foaming. Thus, a new type of crucible was developed, where the glass melt is under normal pressure and the capillary tip is under vacuum. Additionally, this crucible enables a continuous production of glass powders. For the electrospraying of glass suspensions a new process was developed, the so-called electro-flame spraying. In contrast to the classical electrospraying, a hydrogen–oxygen flame is used as a counter electrode to build up the electric field for creating the Taylor cone at the tip of a capillary tube. In this flame the dispersing liquid of the suspension is vaporized and the glass particles are molten. Due to the high surface charge, the molten glass drops split up in several smaller droplets which are deposited on a substrate forming a dense glass layer. Finally, the synthesized glass powders and deposited glass coatings are characterized.