Annealing Effect After RF (Radio Frequency) Sintering of Dental Zirconia Block with Dramatically Reduced Sintering Time: Experimental Study

Abstract

Objectives: Radio frequency (RF) induction sintering has demonstrated superior performance compared to conventional sintering methods in previous studies. Furthermore, the annealing process is expected to further enhance the mechanical properties of sintered zirconia. This study aimed to investigate the effects of annealing on RF-sintered zirconia and provide empirical evidence supporting its role in optimizing sintering outcomes. Methods: A custom-built RF induction sintering furnace was utilized to process zirconia specimens under various annealing conditions (temperature range, annealing time). The sintered specimens underwent three-point flexural strength testing, followed by microstructural analysis using scanning electron microscopy (SEM). Statistical analysis was performed using one-way ANOVA and Tukey’s post hoc tests to assess the significance of differences between groups. Results: The optimal sintering temperature for RF induction sintering was determined to be 1350 °C, with a minimum annealing duration of 20 min at 1220 °C. Notably, even in the absence of annealing, RF sintering at 1350 °C for 20 min produced specimens with higher flexural strength than those obtained through conventional sintering methods. However, due to variability in mechanical properties, the incorporation of annealing is recommended for clinical applications to ensure consistency and reliability. Conclusions: RF induction sintering significantly reduced both energy consumption and processing time compared to conventional sintering techniques, particularly when combined with annealing. While full densification could be achieved within 20 min without annealing, a total processing time of 30 min, including annealing, was found to enhance process stability and ensure reliable mechanical properties. These findings suggest that both sintering and annealing are critical for achieving optimal densification in zirconia, with annealing playing a key role in improving consistency and reproducibility.

1. Introduction

Zirconia sintering plays a critical role in the fabrication of dental prosthetics, particularly for crowns and implants [1]. Conventionally, this process relies on electric furnaces, which require prolonged heating cycles of up to eight hours to achieve full densification [2]. Despite significant advancements in digital dentistry—such as computer aided design/computer-aided manufacturing (CAD/CAM) systems and intraoral 3D scanning technologies that have streamlined prosthetic design and fabrication [3,4]—the sintering step remains a major bottleneck, impeding the realization of efficient same-day prosthetic workflows [5,6,7].

To address this time-consuming process, researchers have explored various methods to accelerate sintering, including the use of higher power delivery in electric furnaces. However, such approaches often result in tradeoffs in microstructural uniformity, grain growth control, and mechanical reliability. Consequently, alternative sintering techniques have been investigated—such as microwave sintering, arc discharge sintering, and ultrasonic sintering—all of which aim to reduce processing time while preserving material integrity [8,9,10,11].

Among these methods, radio frequency (RF) induction heating has received relatively little attention, despite offering inherent advantages such as rapid, localized heating, high energy efficiency, and compatibility with controlled thermal profiles. Previous studies involving RF heating have primarily focused on its ability to reduce sintering time, yet the complementary role of annealing in enhancing material properties has not been thoroughly examined. Our study addresses this gap by integrating RF induction sintering with a post-sintering annealing step, aiming to improve both efficiency and mechanical performance.

This approach is particularly novel in that it combines fast, energy-efficient RF heating with targeted annealing, resulting in enhanced phase stability, densification uniformity, and flexural strength. In performing so, the proposed method offers a compelling pathway toward the clinical implementation of same-day zirconia prosthetics without compromising material quality.

In our preliminary investigations, RF (radio frequency) induction sintering was found to reduce the zirconia sintering time to approximately 40 min, which is a substantial improvement over the conventional 8 h duration [12]. Moreover, it was hypothesized that incorporating an annealing step during RF sintering could further lower the required sintering temperature, thereby minimizing total energy consumption while enhancing mechanical properties.

This study aimed to determine the optimal sintering and annealing conditions for dental zirconia using RF induction sintering. Specifically, the effects of sintering temperature, sintering time, and annealing duration on the flexural strength and microstructure of zirconia were systematically investigated to improve processing efficiency and mechanical reliability.

2. Materials and Methods

Prior to the main experiment, a preliminary study was conducted to determine the optimal temperature parameters for RF induction sintering. Both the preliminary and main experiments were performed using a custom-designed RF induction sintering apparatus developed specifically for this study. Zirconia specimens were fabricated and prepared in accordance with ISO 9693-2 [13] to ensure consistency and reproducibility of the results.

2.1. Experimental Setup and Characterization

A custom-built 3 kW RF (radio frequency) induction sintering apparatus was developed for this study (Figure 1).

The apparatus was enclosed in a 3D-printed carbon filament casing for enhanced durability and thermal resistance and included a liquid cooling system. Accurate temperature calibration was achieved using an infrared laser thermometer, and the system’s operating frequency was tuned to approximately 500 kHz by adjusting the inductance (L = 101 μH) and capacitance (C = 0.001 μF), as shown in Equation (1):

$$f_0=1/\left(2\pi \sqrt{LC}\right)$$

(1)

• f₀ is the frequency generated by the inverter in Hertz (Hz).
• L is the inductance of the RF coil in Henrys (H).
• C is the value of the capacitor connected in parallel with the RF coil in Farads (F).

A schematic diagram of the apparatus design and its core components is shown in Figure 2.

The system functions by converting AC to DC and then back to high-frequency AC to generate alternating magnetic flux, inducing eddy currents within conductive materials. Zirconia specimens (Acucera, Pocheon, Republic of Korea), stabilized with 3 mol% yttria, were prepared following ISO 9693-2 guidelines. The specimen dimensions were (25 ± 1) mm × (3 ± 0.1) mm × (0.5 ± 0.05) mm. Due to the electrically insulating nature of zirconia at room temperature, specimens were placed in graphite crucibles to promote initial heating via induction.

The sintering temperature increased nonlinearly with time due to the rapid RF induction effect. A detailed temperature profile during the sintering process is provided in

Table 1. The temperature profile of the RF induction furnace chamber measured using an infrared thermometer.

Time (minute)	Temperature (°C)
0	20
1	703
2	1161
3	1352
4	1371
5	1386
6	1413
7	1440
8	1455
9	1470
10	1486
11	1496
12	1513
13	1520
20	1531

.

Flexural strength optimization was first explored in a preliminary study. Forty zirconia specimens were divided into four groups (n = 10 each) and sintered at 1200 °C, 1300 °C, 1400 °C, and 1500 °C for 20 min. The sample size was determined using G*Power 3.1 (Heinrich-Heine-Universität Düsseldorf, Germany), with parameters set to α = 0.05, effect size = 0.7, number of groups = 4, and statistical power = 0.95 [14]. Based on the results, 1350 °C was identified as the approximate optimal temperature.

Zirconia behaves as an electrical insulator at room temperature but transitions into a conductive state once it exceeds a critical temperature. At this elevated temperature, the RF coil induces voltage within the zirconia due to the alternating magnetic flux generated by the applied high-frequency AC power. This phenomenon, governed by Faraday’s Law of Electromagnetic Induction, is mathematically expressed in Equation (2).

$$\mathsf{\epsilon }=-d\mathsf{\Phi }/d\mathrm{t}$$

(2)

• ε is the induced voltage.
• Φ is the induced magnetic flux inside the zirconia.

Equation (3) describes the interdependence of eddy currents, the primary source of heat generation and the electrical resistance of zirconia. As the zirconia transitions into a conductive state at elevated temperatures, eddy currents are induced within the material, leading to Joule heating, which further accelerates the sintering process.

$$P=I^2\mathrm{R}$$

(3)

• I is the induced current (eddy current) inside the zirconia.
• R is the electrical resistance of the zirconia.
• P is the heat generated inside the zirconia by the eddy current.

Alternatively, by taking into account the volume of the material and its intrinsic properties, the power dissipation per unit volume can be expressed as Equation (4):

$$P_v=B^2{d}^2{f}^2/6\rho$$

(4)

• $P_v$is the power loss per unit volume.
• B is the magnetic flux density.
• d is the thickness of the material.
• f is the frequency of the AC.
• ρ is the electrical resistivity of the material.

The equipment used in this experiment was designed to output 3 kW of power. The operating frequency was determined by the capacity of the components used, where C = 0.001 µF and L = 101 µH. Based on the calculation using Equation (1), the resulting frequency was 500 kHz.

Zirconia is an electrical insulator at room temperature, which necessitates additional measures to enable heating via RF induction. To overcome this limitation, graphite, a highly conductive material with a melting point well above the sintering temperature of zirconia, was used as the crucible.

The main experiment was then conducted with four groups (n = 10 per group), processed under varied sintering and annealing durations:

• Control: 1440 °C for 20 min ⟶ 1220 °C for 20 min.
• RF20/20: 1350 °C for 20 min ⟶ 1220 °C for 20 min.
• RF10/20: 1350 °C for 10 min ⟶ 1220 °C for 20 min.
• RF20/10: 1350 °C for 20 min ⟶ 1220 °C for 10 min.

Flexural strength testing was conducted using a universal testing machine (Instron 3345, Instron, Norwood, OH, USA) under three-point bending conditions (span: 10 mm; crosshead speed: 0.5 mm/min).

Microstructural evaluation was performed via FE-SEM (S4800, Hitachi, Tokyo, Japan) at 5 kV and ×30,000 magnification. Crystallographic analysis was conducted using XRD (Dmax2500/PC, Rigaku, Tokyo, Japan) to confirm tetragonal phase formation. These methods enabled analysis of the effects of processing parameters on both mechanical and microstructural outcomes.

2.2. Statistical Analysis

Statistical analysis was performed to evaluate whether the differences observed between the experimental and control groups were statistically significant. A simple numerical comparison, such as reporting percentage increases in flexural strength, does not inherently imply statistical relevance. To ensure the robustness of the findings, non-parametric tests, including one-way analysis of variance (ANOVA) and post hoc Tukey’s test, were employed. These methods provide a rigorous evaluation of whether the observed differences reflected true effects rather than random variation. By incorporating statistical validation, this study enhanced the scientific credibility and quantitative reliability of the experimental results, which are presented in the following sections.

3. Results

3.1. Flexural Strength Test

3.1.1. Results Graph for Preliminary Study

Figure 3 presents the average flexural strength of the specimens for each experimental group. The samples sintered at 1350 °C exhibited the highest average flexural strength among the experimental groups. However, this group also showed a relatively large standard deviation.

This variability can be attributed to two main factors. First, the RF induction heating generates a non-uniform eddy current distribution inside the graphite crucible—strongest at the center and weaker toward the edges. As a result, samples located centrally reached the target temperature faster and achieved more uniform densification, whereas edge-positioned samples experienced delayed heating and incomplete sintering. Second, during the 1350 °C sintering process, some zirconia specimens lost their upright position despite being supported by beads intended to keep them centered. These specimens fell over during heating, which likely led to inconsistent heat exposure and contributed to the increased variation in mechanical properties.

3.1.2. Results Table for Main Test

Table 2. RF sintering and annealing results.

	Control	RF20/20	RF10/20	RF20/10
Sintering condition	1440 °C 20 min 1220 °C 20 min	1350 °C 20 min 1220 °C 20 min	1350 °C 10 min 1220 °C 20 min	1350 °C 20 min 1220 °C 10 min
Flexural strength	190.4 MPa	282.0 MPa	266.4 MPa	224.0 MPa
Standard deviation	11.744	52.482	39.714	24.144

summarizes the flexural strength results corresponding to different annealing durations at the optimal sintering temperature of 1350 °C, as determined in the preliminary study. The data indicate a positive correlation between longer annealing times and improved mechanical properties. Notably, all recorded values exceeded the 167.5 MPa flexural strength reported for conventional sintering in previous studies [12].

3.2. Scanning Electron Microscope Images

Main Test Results

Figure 4b shows the control group, which was sintered at 1440 °C for 20 min and annealed at 1220 °C for 20 min. Figure 4c illustrates an experimental group sintered at 1350 °C for 20 min, followed by annealing at 1220 °C for 20 min. Figure 4d presents a group sintered at 1350 °C for 10 min with the same annealing duration. Figure 4e depicts a group sintered at 1350 °C for 20 min, followed by a shorter annealing period of 10 min at 1220 °C. Finally, Figure 4f captures the early stage of phase transformation, taken during the first 2 min of sintering at 1350 °C.

Figure 5 shows an SEM image of unsintered zirconia, where the particle sizes range from 1 μm to 2 μm. The particles are well separated, exhibiting clear and well-defined boundaries.

Figure 4a (reference) presents an SEM image of zirconia sintered using a conventional electric furnace, a widely utilized method in the field. Unlike Figure 5, where distinct particle boundaries are visible, the outlines of individual particles are largely indistinguishable. Most of the material underwent extensive fusion, forming a large, consolidated mass, with only a few particles remaining intact.

Figure 4b (control) displays an SEM image of zirconia sintered using an RF furnace. Compared to those in Figure 4a, the crystal sizes range between 3 μm and 4 μm. The degree of clustering is less pronounced and the crystals exhibit a more even distribution.

Figure 4c (RF20/20) presents an SEM image of zirconia sintered under different RF sintering conditions in terms of temperature and duration. The crystals are approximately 2 μm in size, displaying a relatively uniform distribution. While the particles are in close contact, their individual outlines remain distinguishable.

Figure 4d (RF10/20) illustrates an SEM image of zirconia sintered under the same conditions as that in Figure 4c (RF20/20) but with a modified sintering time. Although the crystal size remains comparable to that in Figure 4c, the degree of fusion is greater, making individual outlines more difficult to distinguish.

Figure 4e (RF20/10) shows an SEM image of zirconia sintered under identical conditions to those in Figure 4c, except for an adjusted annealing time. While the crystal size distribution remains around 2 μm, the degree of fusion is lower compared to Figure 4c, resulting in more clearly defined particle outlines.

Figure 4f (RF2/0) presents an SEM image of zirconia in the initial stage of sintering. Compared to Figure 5, which depicts unsintered zirconia, there is an increase in intergranular spacing, indicative of the early stages of densification.

A simple visual comparison of SEM images is insufficient to reliably assess the differences in physical properties of zirconia sintered under varying conditions. Therefore, to enable a more objective and quantitative evaluation, porosity was analyzed using the image processing software ImageJ (Ver. 1.54p, NIH, Bethesda, MD, USA). The acquired SEM images were converted to 8-bit grayscale, and appropriate thresholding was applied to distinguish pores from solid regions. The porosity was calculated as the ratio of the total pore area to the entire image area. For this analysis, two representative specimens were selected: the specimen fabricated using conventional electric sintering (Figure 4a, conventional, reference) and the specimen that exhibited the highest flexural strength in this study using RF induction sintering (Figure 4c, RF20/20). The porosity of the conventional sintered specimen (Figure 4a, conventional, reference) was approximately 74.3%, whereas the RF-sintered specimen (Figure 4c, RF20/20) exhibited a porosity of 59.8%. The flexural strength of the specimen shown in Figure 4c (RF20/20) was higher than that of the specimen in Figure 4a (conventional, reference) and this result is further supported by the porosity analysis. As expected, as seen in Figure 4c, RF20/20 exhibited a lower porosity than the conventional reference (Figure 4a), which helps explain the superior mechanical performance of the RF-sintered specimen.

3.3. X-Ray Diffraction Results

Figure 6a (1580 °C, 8 h) shows zirconia specimens fabricated using the conventional sintering method currently employed in clinical practice. This group was included to enable cross-validation with the RF sintering approach. Figure 6b (1440 °C, 20 min ⟶ 1220°C, 20 min) presents data from a previous study in which annealing was incorporated into the RF sintering process. This group served as a comparative reference to evaluate the impact of modified sintering conditions. Figure 6c (1350 °C, 20 min ⟶ 1220 °C, 20 min) represents the optimal RF sintering conditions identified in this study, which yielded the highest flexural strength. This group was included to allow direct comparison with previously reported techniques. Figure 6d (1220 °C, 20 min) illustrates the annealing-only condition, serving as a baseline to assess the influence of sintering on the mechanical properties of zirconia.

In Figure 6, the XRD data are further analyzed by integrating the peak areas around the major diffraction angles corresponding to tetragonal (t-ZrO₂) and monoclinic (m-ZrO₂) phases. Based on this semi-quantitative analysis, the t-ZrO₂ content is estimated to be approximately 80.9%, while the m-ZrO₂ content is about 19.1% in Figure 6a. Figure 6b displays a similar pattern but the t-ZrO₂ content is estimated to be approximately 76.2%, while the m-ZrO₂ content is about 23.8%. In Figure 6c, the t-ZrO₂ content is estimated to be approximately 92.7%, while the m-ZrO₂ content is about 7.3%. Figure 6d displays that the t-ZrO₂ content is estimated to be approximately 89.8%, while the m-ZrO₂ content is about 10.2%.

3.4. Statistical Analysis

The standard deviations and 95% confidence intervals for each group are presented in

Table 3. ANOVA results, SPSS 27. These results indicate std. deviation and confidence interval.

					95% CI
(I) Group	N	Mean	Std. Deviation	Std. Error.	Lower Bound	Upper Bound
control	10	190.3913	11.74436	3.71389	181.9899	198.7927
RF20/20	10	282.0364	52.48262	16.59646	244.4926	319.5802
RF10/20	10	266.3627	39.71490	12.55896	237.9524	294.7730
RF20/10	10	223.9538	24.14455	7.63518	206.6818	241.2258

. The control showed the lowest average flexural strength (190.4 MPa), whereas the experimental groups RF20/20, RF10/20, and RF20/10) showed values of 282.0 MPa, 266.4 MPa, and 224.0 MPa, respectively. The corresponding SEM images for these groups are shown in Figure 4b–e.

The ANOVA results (

Table 4. ANOVA results, SPSS 27. These results indicate a difference between the groups in the main test.

	Sum of Squares	df	Mean Square	F	Sig.
Between groups	51,786.719	3	17,262.240	13.666	<0.000
Within groups	45,473.293	36	1263.147
Total	97,260.012	39

) indicate a statistically significant difference among the groups (p < 0.001), confirming that the variation in sintering and annealing conditions meaningfully influenced the flexural strength. However, this analysis alone does not reveal which groups differed from one another, so a post hoc Tukey test was conducted to assess pairwise differences (

Table 5. Tukey’s post hoc test result, SPSS 27.

					95% CI
(I) Group	(J) Group	Mean Difference (I-J)	Std. Error	Sig.	Lower Bound	Upper Bound
control	2.00	−91.64510*	15.89432	<0.001	−134.4521	−48.8381
	3.00	−75.97140*	15.89432	<0.001	−118.7784	−33.1644
	4.00	−33.56250	15.89432	0.169	−76.3695	9.2445
RF20/20	1.00	91.64510*	15.89432	<0.001	48.8381	134.4521
	3.00	15.67370	15.89432	0.758	−27.1333	58.4807
	4.00	58.08260*	15.89432	0.004	15.2756	100.8896
RF10/20	1.00	75.97140*	15.89432	<0.001	33.1644	118.7784
	2.00	−15.67370	15.89432	0.758	−58.4807	27.1333
	4.00	42.40890	15.89432	0.053	−0.3981	85.2159
RF20/10	1.00	33.56250	15.89432	0.169	−9.2445	76.3695
	2.00	−58.08260*	15.89432	0.004	−100.8896	−15.2756
	3.00	−42.40890	15.89432	0.053	−85.2159	0.3981

).

The post hoc analysis revealed that both RF20/20 and RF10/20 exhibited significantly higher flexural strength than the control group (p < 0.001), highlighting the advantage of RF sintering over conventional methods. Interestingly, no significant difference was found between RF20/20 and RF10/20 (p = 0.758), suggesting that a reduction in initial sintering duration (from 20 to 10 min) does not compromise mechanical performance, provided that sufficient annealing is applied. This implies that total energy input could be reduced without a loss of material quality.

In contrast, the RF20/10 group showed a significant decrease in strength compared to RF20/20 (p = 0.004), despite sharing the same sintering time. This indicates that reducing annealing time negatively impacts densification and final strength. Furthermore, RF20/10 showed no significant difference from the control group (p = 0.169), suggesting that short annealing negates the benefits of the RF sintering process.

Table 6. Categorization by homogeneity, SPSS 27.

Strength
Tukey HSDa		Subset for Significance Level of 0.05
Group	N	1	2	3
Control	10	190.3913
RF20/10	10	223.9538	223.9538
RF10/20	10		266.3627	266.3627
RF20/20	10			282.0364
Sig.		0.169	0.053	0.758

and Figure 7 present the homogeneity subsets. RF20/20 and RF10/20 were classified in the highest performance subset, confirming that both conditions produced similarly high flexural strength. Meanwhile, RF20/10 and the control group were grouped together in a lower subset, reflecting their statistically and functionally inferior performance. Notably, the fact that RF10/20 achieved comparable strength to RF20/20 despite having a shorter sintering time demonstrates that a well-optimized annealing phase can compensate for shorter sintering, enabling faster processing without sacrificing mechanical integrity.

These results collectively support the conclusion that in RF induction sintering, annealing duration plays a more dominant role than sintering time in determining flexural strength. Therefore, optimizing the annealing phase is essential for achieving high-strength zirconia in shorter processing times.

4. Discussion

Previous studies have demonstrated that RF induction sintering yields superior results compared to conventional sintering methods [12], highlighting its potential for clinical applications. In particular, RF induction sintering significantly reduced the total sintering time from 8 h to 40 min while achieving a flexural strength of 282.0 MPa (RF20/20), which was substantially higher than that obtained through conventional sintering (167.483 MPa, reference). This suggests that RF induction sintering enhances diffusion-driven densification, facilitating rapid particle bonding while maintaining structural integrity. These characteristics can enhance the feasibility of one-day prosthetics.

This study aimed to further optimize the sintering process by incorporating an annealing step to reduce the total processing time while ensuring that the flexural strength remained comparable to or exceeded the results of previous experiments. Based on the preliminary test, an optimal sintering temperature of 1350 °C was identified, which is significantly lower than that reported in prior studies. For example, Choi (2024) found that conventional sintering requires an optimal temperature of 1550 °C [12], whereas RF induction sintering in this study achieved comparable mechanical properties at 1350 °C. This discrepancy is attributed to the rapid localized heating effect of RF induction, which promotes uniform densification at reduced temperatures.

The initial experiments explored sintering temperatures ranging from 1100 °C to 1500 °C, given that previous studies have shown increased flexural strength with rising temperatures up to 1500 °C [15,16]. As shown in Figure 5, SEM images of unsintered zirconia were obtained to evaluate initial particle characteristics. The results indicate that particle size and clustering significantly influence the bending strength of zirconia. A direct comparison of the SEM images of unsintered zirconia (Figure 5) and those subjected to RF induction sintering (Figure 4f, RF2/0) revealed that as sintering progressed, particle boundaries began to blur, indicating partial fusion. Notably, the highest flexural strength was achieved when particles were well-separated, bonded without excessive melting, and exhibited minimal void formation.

These observations confirm that as sintering progressed, particles moved closer together, leading to crystallization and bond formation between grains. The size, uniformity, and degree of particle clustering directly influenced the mechanical properties of the final product. This correlation is further supported by SEM images (Figure 4a–f), which demonstrate a clear relationship between microstructural characteristics and flexural strength.

To further investigate the effects of sintering temperature, samples were processed at 1440 °C and 1350 °C while maintaining a constant sintering time [17]. As shown in Figure 4, the SEM image of zirconia sintered at 1350 °C (Figure 4c, RF20/20) exhibited a higher density with minimal voids compared to that of zirconia sintered at 1440 °C (Figure 4b, control). This finding aligns with conventional sintering studies, which report that increased sintering temperatures result in grain coarsening and reduced material density [18,19,20].

XRD analysis (Figure 6) further supports these findings. Samples sintered at 1350 °C primarily exhibited the tetragonal phase with minimal monoclinic content, whereas samples sintered at higher temperatures (>1400 °C) showed grain coarsening and increased monoclinic phase formation, leading to reduced flexural strength. The microstructural differences observed in SEM images (Figure 4c–e) are consistent with this phase transformation behavior. Specifically, Figure 4c (RF20/20) demonstrates a well-sintered structure with minimal voids, whereas Figure 4e (RF20/10) shows numerous voids and Figure 4d (RF10/20) exhibits partial melting and grain bonding.

To examine the influence of annealing time, the sintering duration was kept constant while the annealing duration was varied. As shown in Figure 4c (RF20/20) and Figure 4e (RF20/10), both samples were sintered for 20 min; however, the annealing time in Figure 4e was limited to 10 min. The flexural strength of Figure 4c (282.0 MPa) was significantly higher than that of Figure 4e (224.0 MPa) and statistical analysis (Tukey’s post hoc test, Table 5) confirmed a significant difference (p < 0.05). This suggests that prolonged annealing facilitated phase stabilization and grain boundary refinement, ultimately improving mechanical strength.

Annealing is a post-sintering thermal process that plays a crucial role in stabilizing the microstructure of ceramics such as zirconia. During RF induction sintering, rapid heating may lead to non-uniform grain growth, internal stresses, and incomplete phase transformation. The annealing process allows for thermal relaxation, enabling atoms to diffuse and reorient into lower-energy configurations. This promotes grain boundary healing, a reduction in microstructural defects, and the stabilization of the metastable tetragonal phase, which is responsible for transformation toughening and enhanced mechanical strength. Furthermore, annealing contributes to the improved reproducibility of material properties by minimizing residual stresses and facilitating phase homogeneity.

Next, the annealing time was held constant while varying the sintering time. As shown in Figure 4c (RF20/20) and Figure 4d (RF10/20), RF10/20 underwent a shorter sintering time of 10 min while maintaining a 20 min annealing step. The flexural strength of RF10/20 was slightly lower at 266.4 MPa compared to that of RF20/20. However, statistical analysis indicated that the differences between these two groups were not significant, suggesting that a 10 min sintering time may be sufficient under RF induction conditions.

A more direct comparison of annealing effects was conducted by comparing Figure 4d,e (RF20/10). Although both samples underwent a total processing time of 30 min, the process for the sample shown in Figure 4d included 20 min of annealing, whereas the process for the sample shown in Figure 4e involved 20 min of sintering. The flexural strengths were 266.4 MPa and 224.0 MPa, respectively, with statistical analysis confirming significant differences. This further emphasizes the importance of adequate annealing time in achieving superior mechanical properties.

Overall, these findings suggest that while a 10 min sintering time may be sufficient, securing a minimum annealing duration of 20 min yields optimal results. Compared to conventional sintering, RF sintering consistently demonstrated superior mechanical properties when performed at 1350 °C for at least 20 min, regardless of whether annealing was included. However, to ensure stable clinical outcomes, annealing is recommended as it enhances phase stabilization and mechanical integrity.

Despite these promising results, this study has several limitations. The relatively small sample size, due to funding constraints, restricted the implementation of random allocation and blind assessment, potentially introducing bias in data interpretation. Future studies should aim to increase the sample size and incorporate a double-blind experimental design to enhance the reliability of the findings. Additionally, long-term stability and fatigue resistance evaluations are necessary to validate the clinical applicability of RF induction sintering. Nevertheless, this study provides strong evidence that RF sintering at 1350 °C with optimized sintering and annealing times can significantly enhance the efficiency and mechanical performance of zirconia, paving the way for its broader clinical implementation.

5. Conclusions

This study demonstrates that radio frequency (RF) induction sintering can significantly reduce the total processing time for dental zirconia while achieving superior mechanical performance. The optimal sintering temperature was identified as 1350 °C, and a minimum annealing time of 20 min at 1220 °C was found to be essential for phase stabilization and flexural strength enhancement. Notably, even with a shortened sintering duration of 10 min, comparable strength was achieved when annealing was sufficient. Statistical analysis confirmed the significance of these improvements. Overall, RF induction sintering combined with proper annealing offers a promising alternative to conventional sintering, enabling rapid and reliable processing for clinical applications.