MicroCT was performed using a 3D imaging system (GE phoenix v|tome|x, Germany) with a scanning resolution of 32 μm to evaluate the green sample and sintered samples made of 3YSZ. X-ray source voltage of 80 kV with a beam current of 70 mA for green samples and 150 mA for sintered samples were used. Beam current was optimized to match the density difference between green and sintered bodies. The size and morphology of the voids were analyzed using VGStudio MAX (Volume Graphics, Germany).

Three-dimensional precision analysis/part comparison analysis was performed using software VG Studio Max (Volume Graphics, Germany), 3-Matic research 11.0, and CATIA V5 R19 to assess material homogeneity, shrinkage, and deviations during sintering that arise from warping or bending. The overall accuracy of this process in replicating the clinically acceptable marginal accuracy of the designed model without chipping of fine edges was assessed to analyze applicability in dental ceramics. The process essentially involves four steps (Figure 3).

For optimizing the green machining parameters and analyzing the effect of path interval on final surface roughness, profilometry was performed for samples machined with different path intervals ranging from 0.05 mm to 0.4 mm with 0.05 mm increment. A contact profilometer of Taylor Hobson precision, Form Talysurf Series 2 with a conical tip, was used for the purpose. Measurement direction was maintained perpendicular to the direction of the tool path. Also, a 3D optical surface profilometer-Bruker, Model-Contour GT, with Vision 64 control and analysis software was used to generate a 3D surface mapping of the machined sample with optimized path interval to assess roughness before and after sintering.

Phase analysis of the alumina and 3YSZ samples in green and sintered state was performed using an XRD pattern of a high-resolution X-ray diffractometer (Panalytica High-Resolution XRD-I, PW 3040/60) measured at 40 kV and 30 mA using Ni-filtered Cu K_α radiation (2θ = 10–80°). Intensity peaks of green and sintered samples were matched with the JCPDS file using the Xpert® analysis software.

A scanning electron microscope (Zeiss, Jena, Germany) was used to evaluate crystal size, morphology, and distribution on the machined surfaces of alumina and 3YSZ after sintering. In the bulk of the sample, defects and grain boundary distribution were evaluated on the fracture surface. The macrostructure of the sintered samples was examined with a stereo zoom microscope (Zeiss, Jena, Germany) to evaluate the machined features. The dust leftover after machining of the green samples was examined using a stereo zoom microscope in both reflectance and transmittance modes and compared with the dust from the machining of a partially sintered alumina blank, as shown in Figure 9.

The density of the green and sintered samples of alumina and 3YSZ in a water medium was evaluated using the Archimedes principle. Ten samples were randomly selected from extruded samples as well as machined samples after drying and sintering. Dry weight (w1) and wet suspended weight (w2) after 24 h soaking in vacuum were measured with a sartorius digital balance. Density was measured with the following equation:

Density (ρ)=w1w1-w2 … (1)

These values were converted to % theoretical density by assuming the theoretical density of alumina to be 3.95 ×10³ kg/m³ and that of 3 Mol% YSZ to be 6.05 ×10³ kg/m³.

Vickers hardness of the sintered alumina and 3YSZ was measured on the polished cross-sections of the samples using a diamond indenter with 200 gF.

Micro Computer Tomography (MicroCT) analysis of the alumina incisor bridge and 3YSZ molar bridge is shown in Figure 4. Both the images show a random distribution of pores with negligible pore volume (0.06 and 0.08% of the total volume). Tavares et al. reported a similar void analysis of commercially available sintered glass-reinforced ceramic blanks of IPS e.max and Rosetta SM CAD-CAM systems. They reported a porosity value of 0.05–0.1% for IPS e. max and 0.07–0.13% for Rosetta SM, with uniform pore distribution by MicroCT evaluation (17). The voids in this study could have formed during kneading or sintering and indicated the regions where powder distribution and packing were not homogeneous. The concentration of these voids would deteriorate the strength of the ceramic, especially when they are present in strategic areas such as thin contacts and edges between the teeth; however, in both the bridges, they are haphazardly distributed with two to three larger voids constituting 90% of the pore volume. In addition, they are aligned parallel to the occlusal plane, where the effect on the fracture toughness would be minimal. Since the blanks were prepared by molding the dough with low stress, the pores were remnant in the final body; however, while extruding the dough at higher stress, these voids could be minimized, and the green body could be void-free as discussed in a previous study (15).

Precision/tolerance analysis is performed after machining the bridge with respect to the reference surface file, as shown in Figure 5A, where the overall deviation is ranging between +0.52 and −0.79 mm. Slight deviation near the undercuts of buccal and lingual cuspal regions was inevitable because of the limitations with the three-axes machining and angular face cutting edges of the tools used; however, using the advanced five-axis CNC and automatic tool changing machines, which are specifically designed for use in clinical machining of pre-sintered or sintered ceramic blanks, these deviations could be further minimized. The shrinkage of 3YSZ blanks was studied separately, and the original model was scaled up before machining for compensating the shrinkage. The 3YSZ had shown anisotropic shrinkage of 16.5% in all directions. Overall dimensional tolerance between the original model and extracted surface file of the sintered model is shown in Figure 5B, where it can be observed that the deviation is in the range of +0.56–0.79 mm. In both tolerance studies, the cementation surface has shown minimal deviation in the range of −0.2–+0.2 mm, which is comparable with the clinically acceptable marginal accuracy of 0.1–0.2 mm with regard to longevity (18, 19). Warpage on edges and cusps during binder burnout and sintering of machined body, arising from non-homogeneous powder packing of the green body, could affect the marginal fitting on the prepared abutments. Hence, the tolerance between extracted surface files of the machined model and the scaled-up sintered model assuming 16.5% shrinkage is analyzed, as shown in Figure 5C. The precision was superior, with an overall deviation in the range of −0.25–+0.25 mm, while the deviation on the cementation surface and edges were in the range of −0.08–+0.08 mm. In addition, the deviation was uniform, representing homogeneous powder packing and minimal warpage. Hence, this technology can be safely adopted for the precise machining of the ceramic prosthesis in a dental laboratory. Rapid prototyping by stereolithography of zirconia slurry resulted in a zirconia core bridge with deviation in the range of −0.4–+0.4 mm (8). A good fitting of the cementation surface on the prepared tooth is crucial for minimizing micromotion, plaque accumulation, and increasing the longevity of the fixed restoration (20).

Figure 6 shows the contact roughness of sintered samples machined in a green state with different path intervals where it is evident that the average roughness (Ra) and root mean squared roughness (Rq) values were similar for path intervals from 0.05 to 0.2 mm. Upon further increasing the path interval, the roughness increased gradually from 0.25 to 0.35 mm interval. Beyond 0.35 mm, the roughness increased rapidly, which would result in an unacceptable surface finish both esthetically and functionally. Hence, a 0.2 mm path interval could be considered as the optimized parameter, which produces the best profile in the least possible time frame. Also, the conical tool used for roughing and finishing has an edge width of 0.2 mm, as shown in Table 1, which could be the cause of similar roughness until the 0.2 mm interval.

The roughness for the green machined surface of alumina and 3YSZ with optimized path interval, pre and post-sintering, is evaluated with an optical profilometer, as shown in Figure 7. The surface roughness of both materials is compared, and the result has indicated that before sintering, the Ra values of the green machined surface for alumina and 3YSZ were 1.77 and 1.24 μm, respectively. After sintering, they increased to 2.46 and 2.24 μm, respectively. Correspondingly, the Rq values before sintering were 2.35 and 1.6 μm and increased to 3.33 and 3.07 μm, respectively, after sintering for alumina and 3YSZ. The Rq values represent the SD in the distribution of surface height measurements (21). The lower surface roughness of the green machined surface before sintering could be attributed to the presence of a binder between the particles and the smaller size of the un-sintered particles. Upon sintering the same surface, the roughness increased slightly because of the grain growth and binder burnout. Furthermore, these values are slightly higher than the roughness values measured with the contact profilometer, as shown in Figure 6. This could have arisen from the difference in resolution of probes, optical properties of the sintered material, and the area under measurement; however, the roughness values achieved after sintering with a 0.2 mm path interval using both methods are in good agreement with the literature by pre-sintered machining (22). They further suggest that a better shear bonding of the restoration with the luting resin cement would require a roughness value <3 μm. As the roughness increases, there is a tendency for void formation with decreased wetting of luting cement while flowing due to air inclusions (22).

Occlusal surface with greater roughness would result in wear of antagonistic tooth. Zirconia restorations with roughness ~2.5–5 μm have resulted in a clinically simulated grinding and polishing study by Hmaidouch et al. (23). However, in this study, similar roughness was observed without any surface alteration after sintering. In this technique, the finishing parameters of green machining would also influence the sintered surface roughness. Hence, machining parameters were optimized with a focus on clinically acceptable surface finish in the least possible time frame. Roughness was also slightly higher on the alumina surface than the YSZ surface even though the machining parameters were the same in both, probably because of the difference in the particle size of starting powders. Along with roughness, the wettability of luting cement is also influenced by the presence of functional groups arising from etching treatment. Furthermore, physicochemical characteristics could clarify the influence of green machining on the shear bonding of bridges with luting cement.

The x-ray diffraction pattern of both alumina and 3YSZ samples in green and sintered states is shown in Figure 8. The alumina samples have α phase, and corresponding reflections on h k l lattice planes were mentioned. The peaks of the green sample were matching with that of the sintered samples, indicating that proper sintering and phase crystallization occurred without the formation of any unwanted phases; however, in the green 3YSZ sample, peaks corresponding to the monoclinic phase were prominent along with the tetragonal phase, whereas, after sintering, only tetragonal peaks were visible. This reveals that a beneficial tetragonal phase transformation has occurred during sintering, which is stable after cooling owing to yttria doping in the starting powder. Hence, it can be said that this novel dough processing did not affect or hinder the appropriate phase formation during processing.

Electron microscopy images of the sintered surfaces of alumina and 3YSZ produced by green machining were examined for topography and the presence of voids and defects, and evaluation of crystallite size, grain morphology, and grain size distribution (Figure 9). It is evident that the prominent types of brittle surface damages associated with sintered body machining, such as grain chipping and inter- and intra-granular defects, were absent in the green machined surface. During machining of sintered surface, preventing the formation of these cracks with a ductile removal mechanism requires highly controlled machining with a very low critical cutting depth and removal rate using ultra-speed tools in the presence of a coolant (24). All these factors invariably increase the cost of the machined component; however, in the present process, the parameters and the tools used, as shown in Table 1, were not very demanding yet successfully produced a high-quality surface finish after sintering.

The alumina surface demonstrates elongated coarse grains, which are crack-stable. In addition, it has a combination of fine as well as elongated grains that might add to the strength and contribute toward stable crack-free alumina (25). The absence of voids, as observed in the bulk of the sintered 3YSZ, could be attributed to superior densification that occurred during the sintering step, and the average grain size of ~ 826 nm was achieved. The zirconia fracture pattern from the crack initiation point (9.c white arrow) reveals that the crack propagated because of forced rupture from bending where both grains pullout and trans-granular fractures were observed.

Stereo zoom microscopy of the machined dust in the green and partially sintered state, as shown in Figure 10, reveals that green machining resulted in very fine dust compared with partially sintered machining. Hence, less force would be required by tools for particle removal without compromising feed rate during machining. In general, green samples by slurry casting or hot isostatic pressing tend to be hard with local heating and chipping during machining. However, the green body in this study has a soft vinyl polymer adhesive and a lubricant coating the particles, resulting in reduced hardness without compromising the green strength. Consequently, the abrasiveness of ceramic particles would also be reduced, increasing tool life significantly. Also, it was observed that the green debris can be reused without post-processing to break down agglomerates. Addition of solvent and repeating the dough process would be sufficient to obtain similar dough quality, suggesting that the binder and lubricant coating were unaltered during machining. This would further reduce wastage and make the product affordable, especially in dental ceramics, where rear earth materials, such as zirconia, are preferred. It also explains the finding that the force required for particle removal was minimal. When greater force is applied during tooling, it results in a localized rise in temperature, degrading the polymer binders.

Furthermore, the tools used for machining are examined with the microscope (Figure 11) along with an unused one. The roughing and finishing tool used for machining green samples for ~4 h retained its edge sharpness and diamond coating, whereas the bur used for roughing a partially sintered alumina blank for just ~ 30 min lost its tip and diamond coating. A good quality tip is crucial for proper edge retention and successful green machining, as it is found that failure during green machining (Figure 1D) mostly occurs when the tool has lost its cutting edge (Figure 11). Blunting of cutting edge and loss of diamond coating result in increased surface roughness, high-frequency vibrations, and greater dynamic forces with localized heating in the blank, failing machining (26, 27). It is to be noted that the tools used in this study may not be intended for a ceramic machining purpose and that many advanced tools with long life and efficiency are available. However, we intended to emphasize the ease and minimal force required for machining the dried green blanks produced by this novel dough technique.

The apparent green body density of the alumina and YSZ achieved was 68.8 ± 1 and 57.4 ± 0.9, respectively. After sintering, the apparent density attained was 97 ± 1.2 and 97 ± 2.1 for alumina and YSZ, respectively. In both cases, the green body apparent density was considerably higher than that of the previous reports (13, 28, 29). A high green density results in uniform shrinkage and minimal defects in the components after sintering. Machining of the dense green body produces a smooth surface contour with good resolution and edge retention. This can be observed in the MicroCT analysis discussed above where the calculated pore volume was negligible. The hardness of the sintered alumina and 3YSZ measured by the Vickers indenter, using 200 gF is shown in Table 2. These results also match with those reported for dental ceramics made of alumina, 3YSZ and their composites (8, 30, 31).

Flexural strength and Weibull analysis was performed in the previous study, where the three-point flexural strength of the extruded alumina and YSZ achieved was 408.3 ± 61.47 MPa and 690.5 ± 51.5 MPa, respectively (15). The Weibull analysis on the flexural strength of sintered alumina resulted in modulus 5.4 with the scale parameter of 404.6 MPa, whereas for Zirconia, the modulus is 10.02 with the scale parameter of 665.17 Mpa (15). The green body hardness of both dried blanks, as measured by nanoindentation, was 0.3 ± 0.1 Mpa, and the three-point flexural strength was 5.3 ± 0.8 Mpa. In the case of YSZ, the strength was inferior to the commercially available dental zirconia, which usually employs additives to control grain growth and improve strength (32, 33). In addition, final strength depends on the interplay of different processing parameters, such as sintering time, temperature, doping, and grain size distribution. Further, both the alumina and YSZ processed by this dough process as presented in the previous study showed strength greater than those reported for ceramics used in dental restorations (34, 35). The longevity of a dental restoration under constant cyclic loading and a complex set of masticatory forces is influenced by fatigue in the prosthesis. In addition, the presence of moisture and an acidic oral environment degrades the strength in zirconia ceramics because of the low-temperature degradation effect (36, 37). Hence, cyclic loading in simulated oral fluids would give a clear understanding of the stability of these monolith bridges.