Microstructure-property-process-quality relations for fabrication of monolithic zirconia restorations

Microstructures, mechanical properties and clinical processes significantly affect the surface quality of monolithic zirconia restorations. This study aims to understand the microstructure-property-processing-quality relations for the fabrication of monolithic zirconia restorations. Zirconia microstructures were characterized using scanning electron microscopy (SEM). Their mechanical properties associated with clinical processes were investigated using in-situ SEM nanoindentation techniques. Zirconia materials were subjected to CAD/CAM milling, sintering, polishing of exterior surfaces for oral function and sandblasting of intaglio surfaces for adhesion. Surface quality was measured using 3D optical profilometry. The two distinct porous (pre-sintered) and dense (sintered) microstructures yielded distinct maximum indentation depths, hardness H, the elastic moduli E, and indentation volumes. CAD/CAM milled pre-sintered zirconia had poor surface quality because its porous microstructure resulted in lower H/E and H3/E2 ratios, deeper indentation depths and volumes. Sintering diminished the average roughness of milled pre-sintered surfaces but did not heal the milling-induced peak and valley heights. Meanwhile, sintering-induced densification and grain growth increased H/E and H3/E2 ratios of sintered zirconia, making it difficult to polish. Polishing and subsequent sintering of the milled surfaces significantly improved the surface quality, achieving the lowest average roughness, peak and valley heights, and peak-to-valley heights, near zero skewness and reduced kurtosis. However, the lowest peak material portions and inverse areal material ratios obtained may compromize saliva retention and load-bearing capacity. Sandblasting of CAD/CAM milled-sintered surfaces with coarse abrasives achieved rougher surfaces than with finer abrasives. These microstructure-property-processing-surface quality relations provide technical insights into process selection and development for reliable monolithic zirconia restorations.