The use of lasers in the formation of Vitreous layers on surfaces
The action of a laser beam on a cement-based material, such as concrete, can provide the necessary energy to convert the cement into a glass. The concrete surface is non-uniform, owing to the presence of aggregate pieces. Consequently, a non-uniform glass results. If an additional, uniform cement layer is applied to the surface prior to laser processing, a uniform vitreous layer can be formed on the concrete surface.
This technique can be applied to the “fixing” of hazardous materials, such as radioactive dust, to stable substrates. The material is immobilised, reducing the danger of inhalation or ingestion. A similar cement coating can be applied to other building materials, such as steel and brick, resulting again in a uniform, vitreous sealing layer.
The heating effect of the laser in forming this glass can have a detrimental effect on the mechanical integrity of the coating and substrate. Direct glazing of concrete or the application of a thin coating layer shows that dehydration can occur, which destroys bonds and thus reduces mechanical strength. To avoid this, an insulating layer can be applied beneath the vitrifiable layer, in order to thermally insulate the substrate from excessive temperature rises. Several interfaces are therefore formed within the system, dependent both upon the temperature reached, and the specific material. The heat-affected zone (HAZ) provides the “weak link” in the coated material, and generally mechanical failure occurs within it.
The substrate/coating system can be characterised in terms of these interfaces and the temperatures attained. The subsequent performance of the system is dictated by the nature of the materials and processing applied.
The effect of various processing parameters (e.g. laser type, power density, sample traverse speed) has been investigated. The effect of incorrect processing is mechanical failure of the coating or substrate, resulting in cracking and spalling of the material. Microscopic inspection and mechanical pull-off tests have been used to determine the optimum processing conditions.
KeywordsHeat Affect Zone Traverse Speed Concrete Surface Cement Matrix Thermal Interface
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