Effect of Photopolymerization Printing Parameters on Zirconia Green Bodies

A research team led by Gurminder Singh from the Indian Institute of Technology published a paper in Ceramics International titled "Additive manufacturing of yttria-stabilized zirconia using digital light processing: Green density and surface roughness analysis." The study investigates the influence of DLP printing parameters on the green density and surface roughness of zirconia during the printing process.



Original article link: https://doi.org/10.1016/j.ceramint.2024.04.021
 

Research Background

The combination of Digital Light Processing (DLP) technology with pressureless sintering has emerged as a promising approach for fabricating precise and complex yttria-stabilized zirconia (YSZ) components. This process involves several key steps, including the fabrication of green bodies via DLP, debinding to remove the binder, and sintering to achieve densification. However, the optimization and understanding of parameters at each stage remain insufficient for the production of defect-free, fully densified components.

Figure Analysis
Figure 1(a) shows the SEM image of the component printed with the parameters corresponding to specimen No. 16 in Table 3. Image analysis reveals two distinct types of void regions. The first type, referred to as intralayer voids, is observed on the surface within the printed layers, while the second type, known as interlayer voids, appears between adjacent printed layers (Figure 1(b)). The formation of these voids can be mainly attributed to two factors: (1) air entrapment within the slurry during the recoating process, and (2) nonuniform distribution of ceramic particles while spreading the slurry using the doctor blade system.

Figure 1. SEM images of DLP printed specimens

In the 20 sets of experiments, the green density was found to vary from 2.89 gm/cm³ to 3.32 gm/cm³. Figure 2(a) shows the effects of three parameters (layer thickness, orientation angle, and light intensity) on the green density. Figure 2(b) describes the percentage contribution of these parameters to the green density, indicating that the contributions of layer thickness and light intensity are significantly greater than those of other parameters.

Figure 2(a). Effects of the parameters on green body density; (b) Percentage contribution of each parameter to green body density.

Figure 3.presents 3D surface plots and line graphs showing the combined effects of (a) layer thickness and orientation angle, and (b) light intensity and orientation angle on the green body density.

Figure 3(a) shows the interaction between layer thickness and orientation angle through line and surface plots. At higher orientation angles, the effect of layer thickness is more pronounced than at lower orientation angles. When the layer thickness is large, the scattering effect within the green body increases, resulting in a reduced density. In addition to high layer thickness, using a higher orientation angle leads to large voids in both intralayer and interlayer regions, further decreasing the green body density. Moreover, at small layer thickness combined with low orientation angles, the green body density is also relatively low. Therefore, the highest green body density is achieved with an intermediate combination of layer thickness and orientation angle. Figure 3(b) illustrates the interaction effect of orientation angle and light intensity on green body density. At higher orientation angles, the influence of light intensity is very significant, whereas at lower orientation angles, no single parameter shows a dominant effect.

Figure 4. SEM images of the intra-layer region, inter-layer region, and optical surface profile, where (a–c) correspond to samples fabricated with optimized parameters and (d–e) show the respective images of samples fabricated with unoptimized parameters.

Figure 4 shows SEM images of the “intra-layer” region, “inter-layer” region, and surface topography of specimens printed using the optimized parameters (layer thickness: 75 μm, orientation angle: 50°, UV light intensity: 15 mW/cm²) and arbitrary parameters. In specimens printed with non-optimized parameters, improper layer bonding due to high layer thickness, large orientation angle, and low light intensity during printing resulted in numerous pores (highlighted by yellow circles). Figure 4(e) further illustrates that, compared with specimens printed under optimized conditions, those fabricated with non-optimized parameters exhibited substantial deposition of uncured slurry at the interfaces. Figures 4(c) and 4(f) compare the surface profiles of specimens printed with optimized and non-optimized parameters, respectively, showing that the surfaces of specimens produced under non-optimized conditions exhibited higher irregularity.

Conclusions
The primary objective of this study was to investigate the effects of various processing parameters on the properties of green bodies fabricated using yttria-stabilized zirconia (YSZ) slurry via digital light processing (DLP). A central composite design (CCD) approach was employed to establish the experimental design and develop a quadratic statistical model, elucidating the relationships between key process parameters—namely, layer thickness, part orientation angle, and light intensity—and two response variables: green body density and surface roughness. The results revealed that the density of the green body decreased as the layer thickness increased from 10 μm to 70 μm, while an increase in orientation angle from 0° to 90° and light intensity from 10 mW/cm² to 50 mW/cm² led to higher density. Variations in layer thickness and part orientation angle significantly influenced surface roughness, whereas the effect of light intensity on surface roughness was comparatively minor.