Our results suggest that staining of printed zirconia did not affect flexural strength (rejection of hypothesis 1 while the factors nesting orientation (only varied with ZD) and material did affect flexural strength (acceptance of hypotheses 2 and 3).
The flexural strength of LC samples observed in this study were consistent with those reported in a previous study [
16]. However, flaws, voids or delaminations can occur between the layers of printed zirconia [
10,
11,
16], and these problems may be enhanced by post-processing of the material [
11,
17,
20,
21]. To standardize the workflow, all materials in this study were used in accordance with the manufacturer’s instructions. For LC samples, prefabricated cartridges were used directly in the 3D printer, whereas for ZD samples, the slurry had to prepared and mixed before use. Furthermore, the LC printer prepares a thin film of zirconia during each step, onto which the object is lowered until a 25-μm gap (printing layer thickness) is formed with the platform. In contrast, the ZD printer lowers the object into the slurry vat so that slurry flows over the object; after this, the slurry is wiped off with a squeegee to create a film thickness of 50 μm. Our results suggest that this technique might favor small air pockets in the ZD printing slurry, creating voids and increasing porosity in the sintered zirconia. When comparing the facture modes of vertically nested specimens, almost all ZD specimens fractured along the layer interfaces, whereas no favored fracture directions could be observed for LC specimens. This might be caused due to more air inclusions at the interfaces of ZD specimens (Fig.
7). Since fracture surfaces of horizontally nested specimens did not show such porosity, we assumed that this problem was restricted to the layers’ seams. With LC, each cured layer sticks to the bottom of the vat and has to be pulled off before the next layer can be generated. This is not necessary with the bottom-to-top building direction of the ZD printer. Thus, in use of ZD, only the free slurry surface is light-cured, theoretically leading to less object torsions. In this study, the default light curing parameters were used. A possible impact of varied lighting duration/intensity in terms of improving layers’ interfaces should be the topic of future studies. For the sake of completeness, it should be kept in mind that slurry provision is more extensive in use of ZD. In order to minimize slurry consumption, slurry remaining in the vat has to be prepared again prior to the next printing job making the approach time consuming and technique sensitive. Back to facture strengths, cleaning and debinding were more conservative for LC than for ZD, with LC using a low percentage isopropanol while ZD uses nearly pure isopropanol. Liebermann et al. showed that cleaning procedures can degrade the material [
17]. Furthermore, the debinding and firing time for ZD was roughly 70% of that for LC. SEM images (Figs.
6,
7 and
8) revealed voids and/or flaws in both LC and ZD samples, in agreement with previous literature [
11,
16,
21], but porosity and flaws were more prominent in the printing layer interfaces of ZD samples as described above. These flaws can initiate cracks, which might cause failure at rather low (nominal) tensile stress values [
11]. In our study, ZD samples with tensile stresses acting perpendicular to printing layer interfaces (x- and y-orientation) had only 70% of the strength of samples with layers parallel to the tensile stress direction (z-orientation). This finding, together with the fact that x- and y-oriented samples typically fractured in two halves along the printing layer interfaces, indicates that the layer interfaces are the weak spot of ZD-printed zirconia. Internal pilot studies have shown that the strength of LC samples is not affected by nesting orientation, which is why LC samples were only tested with one nesting orientation (x-orientation). Both LC and ZD samples showed a mean biaxial flexural strength of > 500 MPa, making them at least class 4 materials according to ISO 6872 standards. For clinical success, small failure rates are needed. Our results indicate similar critical stress values between 500 and 600 MPa correlating with a failure rate of 5% for LC and ZD samples. Since mean strength in all test groups was > 800 MPa, LC and ZD also fulfill the prerequisites of class 5 materials. Due to the high variability, such a recommendation should be handled with care. Importantly, the cylindrical discs used in this study have a very favorable shape for debinding. In dental restorations with sharp edges (such as fissures on the occlusal surfaces) and thick objects (such as pontics), debinding might create more flaws compared with the discs. Further studies are needed to show the performance of real 3D-printed restorations.
With real crowns, high tensile stresses occur at the occlusal surface next to contact points and at the inner crown surface beneath contact points if the wall is not thick. The tensile stress is highest parallel to the crown surface; therefore, a horizontal orientation of the occlusal surface is favorable. However, high tensile stress will never be limited to one location because of complex mechanical factors, such as the geometry and loading of dental restorations. Therefore, material anisotropy cannot be completely solved by simply choosing an appropriate nesting orientation. Further investigations should find ways to improve ZD printing as well as debinding and sintering protocols.