Sintering Behavior of Nano-Ceramic Coatings
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ABSTRACT This paper concentrates on the densification of sol gel derived thin nano-ceramic coatings by laser radiation and by furnace treatment. Deposition of the film is done by means of spin coating of Zirconia. The sintering of the layer is observed using a number of different measuring techniques. Especially, attention is paid to the densification behavior and the grain growth in the layer, investigated by scanning electron microscopy and ellipsometry. Also the different crystal phases observed are addressed. During sintering at a constant temperature a change in the sintering behavior with time is observed. This change is observed for most temperatures. Both the initial fast sintering of the process (for all sintering temperatures), and the final slow sintering (only at the higher sintering temperatures) are explained. INTRODUCTION A decade ago the emergence of the field of coatings was greatly driven by the realization that the surface is often the most important part of any engineering component. Structural components fail by wear, corrosion, high cycle fatigue etc., that is to say affected by the surface conditions. Consequently, an appropriate approach would be to modify the surface layer of a base-material or to apply a coating, so as to provide enhanced performance [1, 21. A number of techniques were developed for coating fabrication. But the fabrication of coatings is often a difficult and laborious process. The conventional method of slipcasting to produce ceramic foils suffers from thickness limitations. An industrially applicable method to obtain coatings is wet-chemical processing, also called sol-gel processing [3,4]. The sol-gel method allows films to be made with almost any composition and degree of porosity. The problem of homogeneity, often encountered in the processing of powders, is absent in the sol-gel preparation technique because no comminution is required. Moreover the processing temperatures can be significantly reduced, and combining different coating liquids (hybrid systems) is easy. This latter aspect offers a great deal of freedom to fabricate coatings with different properties. In our work the sol-gel concept is combined with inkjet technology and laser treatment of surfaces. The entire combination is called 'Stereostiction' [5]. A solution containing nano-sized ceramic particles is fed to an inkjet nozzle that generates a softwarecontrolled pattern on a surface. Afterwards the drops are exposed to an intense laser beam that dries and densifies the drops, thereby forming a sintered ceramic layer. The heating damage to the product by the intense laser beam is minimized by using short laser pulses resulting in very small heat affected zones. It is possible to apply different coating compositions using multicolor techniques, which nowadays are in a mature stage of development. 3-D products up to several millimeters can be manufactured by repetitive passing of the inkjet nozzle, drying and sintering. However, sintering behavior of the sol-gel coating is a crucial aspect and very little
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