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Objective. The purpose of the study was to determine the hardness and fracture toughness of dental yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) manufactured by digital light processing (DLP) technology to study its clinical prospects. Methods. The experimental group was DLP-manufactured zirconia, and the control group was milled zirconia. The hardness was investigated under a range of test loads (0.49 N, 0.98 N, 1.96 N, 4.90 N, 9.81 N, 29.42 N, 49.03 N, 98.07 N, and 196.1 N). Meyer’s law was applied to describe the indentation size effect (ISE). Meanwhile, the PSR model and MPSR model were utilized to generate true hardness values. The cracks were observed to be induced by indentation under loads above 49.03 N, while the cracks showed the radial-median type under the load of 196.1 N, under which the fracture toughness was calculated. Results. The true hardness of DLP-manufactured zirconia was 1189 HV based on the PSR model and 1193 HV based on the MPSR model, a bit lower than that of milled zirconia. The fracture toughness was 3.43 ± 0.29   MPa √ m , which showed no statistical difference with the milled zirconia. Conclusion. The dental zirconia manufactured by the DLP 3D printing technique is similar to that manufactured by the conventional milling process in hardness and fracture toughness, thus having a promising future of clinical use.

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Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is currently a scientific and clinical research focus in the field of dental materials, owing to its excellent combination of mechanical features, aesthetics and biocompatibility [1]. Generally, zirconia-derived dental prostheses are fabricated using subtractive manufacturing (SM) technology, which applies computer aided design/computer-aided manufacturing (CAD/CAM) systems [2]. However, some problems with this technology have been recorded in recent years; for instance, it not only causes significant waste of materials and tools, but also has limitations in the fabrication of parts with complex geometries [3]. Consequently, additive manufacturing technologies have attracted increasing attention. Stereolithographic methods, including stereolithography and digital light processing (DLP), are promising for producing small and complex dental parts, which require both high accuracy and surface quality. In addition, these techniques significantly reduce the production steps and consumption of energy and raw materials [3,4].Generally, zirconia crystals are found in three structural entities, including the tetragonal and cubic phases at temperatures of >1170 °C and >2370 °C, respectively. The tetragonal phase is stabilized at ambient by yttria addition to generate a Y-TZP ceramic [1,5]. Currently, 3 mol% yttria stabilized Y-TZP (3Y-TZP) is the most commonly used zirconia ceramic in dentistry because of its excellent mechanical properties. However, this material is susceptible to low-temperature degradation (LTD) under moist conditions [1,6]. LTD involves a nucleation and growth process, which generally starts at the surface of polycrystalline ceramics and induces the tetragonal to monoclinic transformation over a temperature range of ~30 °C to 300 °C [6,7]. Furthermore, LTD may affect the biomechanical and surface physicochemical properties and the service life and quality of 3Y-TZPs, particularly for those applied in biomedical settings, which are exposed to complex environments in vivo, such as hip implants and dental restorations [8].