# Additive Manufacturing of Ceramic Reference Spheres by Stereolithography (SLA)
## 1 Introduction  
The benefits of additive manufacturing (AM) compared to traditional manufacturing methods are widely acknowledged for its capacity to produce parts from various materials, intricate shapes, with great speed, enough precision, cost-effectiveness, and eliminating the need for fixtures and jigs [1, 2, 3]. These advantages are particularly significant when considering ceramic components, where their properties such as high melting temperatures, low ductility, and susceptibility to thermal shock, make conventional forming methodsexpensive and difficult, often resulting in faulty parts, with defects and surface microcracks. But these issues, that have persisted for years in conventional methods, continue nowadays in additively manufactured ceramic parts due to the lack of on-site inspection and systematic procedures throughout the design, processing, and post-processing stages. Among the various AM technologies for advanced ceramics, those based on photopolymerization stand out for producing parts with superior mechanical properties, high precision, and excellent surface finish [4]. Within this category, Vat Photopolymerization by Ultraviolet Laser for Ceramics technique (VPP-UVL/C according to ISO/ASTM 52900 [5]), commonly known as stereolithography (SLA), has shown promising results, particularly with materials like alumina, zirconia, and hydroxyapatite [6]. The SLA process for ceramics involves four critical stages in two phases: printing (production phase), cleaning, debinding, and sintering (post-processing phase). The raw material consists of a paste containing a suspension of ceramic particles in a photosensitive resin, along with diluents and binders. In the initial printing stage, a green (oversized) part is fabricated via photopolymerization, layer by layer, facilitated by ultraviolet light acting upon the photosensitive resins [7]. The paste is evenly distributed onto the printing bed using a blade that adjusts the layer height during each pass. Throughout this phase, layer displacements may occur, and microbubbles can form due to air entrapment during paste application [8]. The strategic use of supports is crucial to prevent structural failure resulting from the forces exerted during the process, which could lead to deformation, breakage of thin sections, or detachment of the part from the bed [9]. The stresses induced are contingent upon the rheological properties of the paste and the velocity and thickness of the layers, all relative to a given blade geometry. The cleaning stage is critical, as the printed parts do not have sufficient consistency and are susceptible to breakage when the surrounding paste is removed. This involves removing all the ceramic paste that has not been photopolymerized and that not only surrounds the piece but is also stored in the hollows and interstices of the piece's geometry. In the debinding stage, which constitutes the second phase, the removal of the organics of the photopolymerized resin is achieved through high temperature-time cycles. This process involves a combination of polymer evaporation, oxidation-induced decomposition, and thermal degradation. It stands as the most crucial stage, as any cracks or deformations occurring during debinding due to a significant reduction in the volume and, therefore, cannot be rectified in subsequent sintering phases. Initially, the evaporation of diluents creates an open porosity, facilitating the diffusion and subsequent evaporation of polymeric components undergoing pyrolysis. At the end of this stage, parts exhibit significant porosity, reaching up to 40%, resulting in a low mechanical strength of the parts [10]. In the fourth stage, known as sintering, the ceramic component devoid of resins undergoes exposure to high temperatures. During this phase, the porous structure is densified, fostering a robust bond among the ceramic particles. This densification invariably leads to volumetric contraction, ultimately resulting in the part attaining its intended design dimensions.