Sol-gel processing of lithium disilicate - Part II Crystallization and microstructure development of coatings

Lithium disilicate (Li₂Si₂O₅) coatings were prepared by spin-coating alkoxide solutions on to substrates [Si, SiO₂, polycrystalline (poly) Si, sapphire] and heating isothermally at 500-600 °C. The effects of solution chemistry, coating thickness and substrate type on crystallization behaviour and microstructure development were investigated using atomic force microscopy, X-ray diffraction and transmission electron microscopy. Amorphous dried coatings began to crystallize into Li₂Si₂O₅ at 500-550 °C. Coatings prepared on Si substrates (with a thin native oxide) using Li-Si methoxyethoxide solution crystallized into microstructures with large grain sizes (ca. 2-5 μm diameter) as compared with the coating thickness (<0.3 μm). Nucleation rate in these coatings could be increased (and hence transformation rate increased and grain size decreased) by: (1) adding H₂PtCl₆to the solution to act as nucleation agent; (2) increasing the thickness of the coating; or (3) using a crystalline substrate (sapphire or poly Si). Coatings prepared using Li-Si ethoxide solution had fine-grained microstructures (≤0.5 μm diameter) for all substrates. Chemical heterogeneity in the ethoxide system may have increased nucleation rate. Nucleation rate in this system could be decreased by using partially hydrolysed tetraethylorthosilicate as the Si precursor. The relationship between solution chemistry and microstructure was used to tailor microstructures in multilayer coatings.