Influence of Preaging Temperature on the Indentation Strength of 3Y-TZP Aged in Ambient Atmosphere

Yttria-stabilized zirconia (3Y-TZP) containing 0.25% Al2O3, which is resistant to low temperature degradation (LTD), was aged for 10 h at 130–220 °C in air. The aged specimens were subsequently indented at loads ranging from 9.8 to 490 N using a Vickers indenter. The influence of preaging temperature on the biaxial strength of the specimens was investigated to elucidate the relationship between the extent of LTD and the strength of zirconia restorations that underwent LTD. The indented strength of the specimens increased as the preaging temperature was increased higher than 160 °C, which was accompanied by extensive t-ZrO2 (t) to m-ZrO2 (m) and c-ZrO2 (c) to r-ZrO2 (r) phase transformations. The influence of preaging temperature on the indented strength was rationalized by the residual stresses raised by the t→m transformation and the reversal of tensile residual stress on the aged specimen surface due to the indentation. The results suggested that the longevity of restorations would not be deteriorated if the aged restorations retain compressive residual stress on the surface, which corresponds to the extent of t→m phase transformation less than 52% in ambient environment.


Introduction
Zirconia is currently one of the most commonly used ceramic materials in restorative dentistry. The application of the first generation of 3 mol.% Y 2 O 3 -stabilized tetragonal zirconia polycrystal (3Y-TZP) was limited to frameworks for all-ceramic restorations, on which feldspathic porcelain was layered, owing to its insufficient translucency [1]. However, veneering is a technique sensitive and time-consuming procedure, accompanied by significant tooth reduction and a high risk of chipping, which is a major cause of failure in zirconia veneering ceramics [2,3]. To alleviate such drawbacks, the monolithic zirconia restorations are adapted [4]. However, there are concerns regarding low temperature degradation (LTD) because monolithic zirconia restorations are directly exposed to the humid oral environment and masticatory force. The extent of LTD is determined by the fraction of monoclinic phase of ZrO 2 (m-ZrO 2 ) transformed from tetragonal crystal structure of ZrO 2 (t-ZrO 2 ) during aging. Indeed, in vivo aging of 3Y-TZP in the oral environment led to~12% of m-ZrO 2 formation after exposing for 24 months [5]. Even 60 days of intra-oral aging of flat 3Y-TZP specimens, exposed to the oral cavity, resulted in 4.7-7.7% of m-ZrO 2 and deteriorated the strength by 32-39.4% [6]. LTD is a phenomenon in which Y 2 O 3 -containing t-ZrO 2 transforms to m-ZrO 2 to deteriorate mechanical properties during aging at temperatures of 100-300 • C and is accelerated in the presence of moisture, such as in the oral environment [7].
LTD is originated from the fact that at ambient temperature t-ZrO 2 is internally strained by the adaptation of Zr cation in a distorted eightfold coordination environment because the size of Zr 4+ is too small to be coordinated to eight oxygen ions in the fluorite property is not affected by aging because the aging conditions, such as aging temperature or time, are not sufficient to influence the strength [28][29][30]. Thus, the influence of LTD on the flexural strength of 3Y-TZP has not been clearly elucidated.
Ceramics become fractured when the applied stress intensity exceeds the critical stress intensity factor, that is, the fracture toughness. Intrinsic flaws that control the strength can be introduced during the fabrication process such as powder compaction, forming, drying, firing, and cooling [31]. Conversely, extrinsic flaws can be introduced into dental ceramics by dental clinicians or technicians during grinding or polishing ceramic restorations [31], as well as because of masticatory force or parafunctional habits such as bruxism and clenching [32]. Such extrinsic damages may cause reduced strength, resulting in premature failure of dental ceramics [33]. Although dental 3Y-TZPs are strong enough to withstand the extrinsic damages [34,35], the damage coupled with LTD may become a strength limiting flaw. Nevertheless, there has been no systematic attempt to evaluate the influence of extrinsic stimuli on the strength of monolithic zirconia restorations encountered by LTD. To simulate the extrinsic flaws, Vickers indentation was applied to the 3Y-TZP that was aged in ambient atmosphere prior to the indentation. Although the indentation loads used in this study are slightly higher than the mastication load, a comparable damage may occur in procedure of grinding or polishing by dental clinicians or technicians during dental practices [33]. In this study, the extent of LTD was evaluated using the Rietveld refinement and the influence of preaging temperature on the strength of 3Y-TZP, aged in air and then indented, was investigated to elucidate the aging dependence of strength.

Materials and Methods
3Y-TZP powder was used for the specimens and the material information is listed in Table 1. The powder was pressed uniaxially at 125 MPa in a steel die, then pressed isostatically at 186 MPa to produce pellet-shaped green compacts (diameter: 20 mm, height: 1.5 mm). The pressed specimens were debinded for 55 h up to 985 • C, and then sintered for 2-20 h at 1550 • C on an alumina crucible in air. To prevent potential interaction between the specimens and the crucible, the top surface of the sintered specimens was used exclusively for XRD and indentation. After sintering, the diameter and thickness of the specimens were 15.7 ± 0.05 mm and 1.22 ± 0.04 mm, respectively. No surface treatment was performed prior to the experiments. Initially, 20 specimens of each were sintered at 1550 • C for 2, 8, 14, and 20 h, and half of each group was indented at 9.8 N. After the initial trials, 350 pellets were fabricated by sintering at 1550 • C for 20 h and randomly divided into five groups (n = 70) based on aging temperatures of 130, 160, 190, and 220 • C, including as-sintered specimens for the control. Each group of specimens was subdivided again by 10 specimens for indentations at loads of 0, 9.8, 49, 98, 196, 294, and 490 N. All specimens were aged for 10 h at each temperature in air prior to the indentations. The microstructures of the sintered specimens, thermally etched for 15 min at 1500 • C, were observed using a scanning electron microscope (SEM, S4700, Hitachi, Tokyo, Japan) at ×30,000 magnifications with an accelerating voltage of 20 kV.
The bulk density of the fully sintered blocks was determined in accordance with ISO/DIS 18754 [36], using distilled water as the immersion liquid. The m-ZrO 2 fraction was determined using Rietveld refinements from the XRD data for the specimens, which were collected on a Bragg-Brentano diffractometer (PANalytical, Almelo, The Netherlands) with a CuKα X-ray tube, a Bragg-BrentanoHD optical module, a position-sensitive PIXcel3D detector, and Soller slits (0.02 rad). The scan range covered 2θ = 20-90 • at a step size of ∆2θ = 0.0217 • . The data reduction and structure determination from the XRD data were performed using the HighScore Plus suite.
For the Vickers indentations, predetermined loads ranging from 9.8 to 490 N were indented for 10 s at the center of each specimen using a digital Vickers hardness tester (Highwood HWDV-7, TTS Unlimited Inc., Osaka, Japan). Immediately after unloading, a drop of silicon oil was applied on the indented surface to prevent possible humidity assisted-crack growth. The biaxial flexural strength was determined in accordance with the ISO 6872:2015 standard for dental ceramics [37]. The indented surface of each specimen was positioned to face the three supporting steel balls (diameter: 2.7 mm), positioned 120 • apart on a support circle, then loaded at a crosshead speed of 1.0 mm/min using a universal testing machine (Instron 3365, Canton, MA, USA). To avoid contact damage, an ethylene-vinyl acetate foil was inserted between a flat punch (diameter: 1.4 mm) and specimen to ascertain an even contact.
Statistical analyses were performed using R software (version 3.6.2, R Foundation for Statistical Computing, Vienna, Austria). As the data showed non-normal distribution, they were analyzed using non-parametric statistical test, aligned rank transform analysis of variance (ART ANOVA), and multiple comparison using Bonfferoni post hoc test. The effect of independent variables such as the aging temperature and indentation load on biaxial strength, and the interaction of the two independent variables were determined. For all statistical analyses, a significance level of 5% was used.

Results
The SEM micrographs of 3Y-TZP sintered at 1550 • C for 2-20 h are shown in Figure 1. After sintering for 2 h, the grain boundaries were well defined, and the size was determined to be 0.53 µm (Figure 1a) and increased to 0.72 µm after sintering for 8 h ( Figure 1b). As time further increased, the grain boundaries became obscure because of pulverization of the grains into pristine grains. The unclear boundaries and pulverization made it difficult to determine the grain sizes after sintering longer than 14 h (Figure 1c,d). The XRD patterns of the specimens revealed that the t→m phase transformation extended with the sintering time as demonstrated by the increase in the peak heights for m-ZrO 2 and the decrease in the peak intensity of t-ZrO 2 in Figure 2. This suggests that the uneven microstructures were mainly due to the volume expansion accompanied by the t→m phase transformation and boosted because of thermal etching after polishing for SEM observations, which may ease the restriction that retained the pristine grains inside the grain boundaries.
In Figure 2, the increase in the phase transformation is manifested by the increased peak intensities of m-ZrO 2 along with an asymmetric broadening of the (101) peak of t-ZrO 2 with increasing sintering time. After sintering for 14 h the broadening of the peak at 30 • becomes visible. The peak for c(111) is imbedded in the peak of t(101) and the peak for c(020) is shown as a separate peak at 35.2 • between two t-ZrO 2 peaks. The markers for r-ZrO 2 related to the broadening are supposed to appear at 2θ of~30 • and~35 • but are not apparent because the extent of c→r phase transformation is limited. The peak intensity of m(111) at 31.5 • for m-ZrO 2 is so weak that the peak intensity ratio of m(111)/m(111) is anomalously high compared to the ratio of 1.47 that is found in the ICDD # 00-037-1484 for m-ZrO 2 . In the XRD pattern for 20 h it is interesting to note that the peak intensity of m(002) at 34.2 • is higher than that of m(111) even though the intensities are listed as 21 and 68, respectively. No peak overlapping for m-ZrO 2 was observed in the entire scan range. deconvolution of the compound peak at ~35° showed the r(102 _ ) peak of r-ZrO2 at 34.8° embedded near the c(020) peak of c-ZrO2 at 35.1°. In this study the extent of t→m transformation was determined by the volume fraction ratio of m-ZrO2/(m-ZrO2 + t-ZrO2) based on the analysis by the Rietveld refinement procedure. The m-ZrO2 volume fractions in the specimens sintered at 1550 °C for 2, 8, 14, and 20 h ( Figure 2) were calculated to be 15.4, 27.7, 39.6, and 44.6%, respectively.   In Figure 3 the t(101) peak at 30.2 • from the 3Y-TZP sintered for 20 h at 1550 • C was deconvoluted into t-, r-, and c-ZrO 2 phases by the Rietveld refinement. All peaks including the asymmetric peak were well identified with the given structure models including r-and c-ZrO 2 phases. The Rietveld refinement revealed that the asymmetric peak is composed of r(003) at 29.8 • and r(101) at 30.0 • of r-ZrO 2 and c(111) at 30.2 • of c-ZrO 2 besides t(101) of t-ZrO 2 . Thus, c-ZrO 2 completely overlaps t-ZrO 2 in the asymmetric peak. The deconvolution of the compound peak at~35 • showed the r(102) peak of r-ZrO 2 at 34.8 • embedded near the c(020) peak of c-ZrO 2 at 35.1 • . In this study the extent of t→m transformation was determined by the volume fraction ratio of m-ZrO 2 /(m-ZrO 2 + t-ZrO 2 ) based on the analysis by the Rietveld refinement procedure. The m-ZrO 2 volume fractions in the specimens sintered at 1550 • C for 2, 8, 14, and 20 h ( Figure 2) were calculated to be 15.4, 27.7, 39.6, and 44.6%, respectively.  The influence of the m-ZrO2 fraction, originated from the different sintering time (Figure 1), on the biaxial strength is depicted in Figure 4. The strength of the as-sintered The influence of the m-ZrO 2 fraction, originated from the different sintering time (Figure 1), on the biaxial strength is depicted in Figure 4. The strength of the as-sintered specimens tended to decrease steadily upon the fraction. Conversely, the strength increased asymptotically with the fraction when 9.8 N of indentation was applied to the specimens. Although the strength of the indented specimen sintered for 2 h was lower than that of the as-sintered specimen by 334 MPa, the difference became nullified for the specimen sintered for 20 h. The decrease and increase in strength of the as-sintered and indented specimens, respectively, with the sintering time are likely influenced by both the enlargement of the grain size ( Figure 1) and the increase in the extent of t→m phase transformation ( Figure 2). than that of the as-sintered specimen by 334 MPa, the difference became nullified for the specimen sintered for 20 h. The decrease and increase in strength of the as-sintered and indented specimens, respectively, with the sintering time are likely influenced by both the enlargement of the grain size ( Figure 1) and the increase in the extent of t→m phase transformation ( Figure 2). The influence of aging temperature on the phase change was shown in Figure 5. The asymmetric broadening of the (101) peak of t-ZrO2 was also observed and increased in proportion to LTD. After aging at 190 °C the peak shape of t(101) disrupted to result in the transformation of 77.7 vol.% of t-ZrO2 into m-ZrO2. Concurrently, the r(102 _ ) became prominent at 34.8° and a marked preference of the m(002) orientation at 34.2° was observed. After aging at 220 °C the asymmetricity disappeared and 96.8 vol.% t-ZrO2 was transformed to m-ZrO2. The shift of the peak at 30.2° toward a lower 2θ and the disappearance of c(020) at 35.1° indicate that the t→m and c→r phase transformations were nearly completed. The m-ZrO2 volume fractions in the as-sintered and the specimens aged at 130, 160, 190, and 220 °C for 10 h were 35.6, 40.9, 52.2, 77.7, and 96.8%, respectively.
The Rietveld refinement revealed no significant changes in the lattice parameters of m-ZrO2 as the aging temperatures increased. However, the lattice constant of β, the angle between a and c axes, varied with LTD as depicted in Figure 6. The β of 99.03° for the assintered 3Y-TZP remained nearly unchanged up to 130 °C, then expanded upon further increasing the temperature suggesting that LTD is closely related to the variation of β. The lattice constants of the m-ZrO2 were calculated by the Rietveld refinement through all possible Bragg reflections in the corresponding crystal structures in the XRD pattern. Since it underwent a process of defining the correct peak position through correction of the zero- The influence of aging temperature on the phase change was shown in Figure 5. The asymmetric broadening of the (101) peak of t-ZrO 2 was also observed and increased in proportion to LTD. After aging at 190 • C the peak shape of t(101) disrupted to result in the transformation of 77.7 vol.% of t-ZrO 2 into m-ZrO 2 . Concurrently, the r(102) became prominent at 34.8 • and a marked preference of the m(002) orientation at 34.2 • was observed. After aging at 220 • C the asymmetricity disappeared and 96.8 vol.% t-ZrO 2 was transformed to m-ZrO 2 . The shift of the peak at 30.2 • toward a lower 2θ and the disappearance of c(020) at 35.1 • indicate that the t→m and c→r phase transformations were nearly completed. The m-ZrO 2 volume fractions in the as-sintered and the specimens aged at 130, 160, 190, and 220 • C for 10 h were 35.6, 40.9, 52.2, 77.7, and 96.8%, respectively.
The Rietveld refinement revealed no significant changes in the lattice parameters of m-ZrO 2 as the aging temperatures increased. However, the lattice constant of β, the angle between a and c axes, varied with LTD as depicted in Figure 6. The β of 99.03 • for the as-sintered 3Y-TZP remained nearly unchanged up to 130 • C, then expanded upon further increasing the temperature suggesting that LTD is closely related to the variation of β. The lattice constants of the m-ZrO 2 were calculated by the Rietveld refinement through all possible Bragg reflections in the corresponding crystal structures in the XRD pattern. Since it underwent a process of defining the correct peak position through correction of the zero-shift error and/or sample displacement error, the estimated standard deviation involved in β ranges from 0.0015 to 0.0036 • .
The bulk density of 3Y-TZP sintered at 1550 • C for 20 h was 6.01 g/cm 3 , corresponding to 98.5% of the theoretical density (6.10 g/cm 3 ) of 3Y-TZP. The density decreased slightly as the aging temperature increased from 130 to 220 • C in Table 2, where all specimens were aged for 10 h at each temperature. The decrease in density was concurrent with the increase in the extent of t→m phase transformation ( Figure 5) because the theoretical density of m-ZrO 2 (5.83 g/cm 3 ) is lower than that of t-ZrO 2 .

°C.
Aging Temperature, °C As-sintered 130 160 190 220 Bulk density, g/cm 3 6.01 ± 0.02 6.01 ± 0.01 5.97 ± 0.02 5.97 ± 0.02 5.96 ± 0.01 Figure 5. X-ray diffraction patterns of 3Y-TZP aged for 10 h at temperatures ranging from 130 to 22 °C after sintering at 1550 °C for 20 h. The pattern for the as-sintered specimen was added for com parison. The asymmetric broadening, associated with r-ZrO2 and c-ZrO2, increases with aging tem perature and corresponds to r-ZrO2 at 220 °C suggesting that aging results in both t→m and c→ phase transformations.

Aging Temperature, • C
As-sintered 130 160 190 220 Bulk density, g/cm 3 6.01 ± 0.02 6.01 ± 0.01 5.97 ± 0.02 5.97 ± 0.02 5.96 ± 0.01 In Figure 7 the fractions of t→m transformation for the specimens in Figure 5 were plotted along with the corresponding biaxial strength as a function of the aging temperature. As the aging temperatures increased, the m-ZrO 2 content increased and the biaxial strength decreased, except for aging at 130 • C, showing a slightly higher strength than the assintered specimen even though the m-ZrO 2 fraction was extended moderately from 35.6% to 40.9%. As the aging temperature increased further, the strength decreased abruptly in the same manner as the increase in the extent of t→m transformation.  In Figure 8 the involvement of the preaging temperature in the indented strength was demonstrated by the plot of the strength as a function of the temperature. The 3Y-TZP specimens were sintered at 1550 °C for 20 h, then aged for 10 h at 130-220 °C; the assintered specimen was included as a control. The specimens were indented at loads ranging from 9.8 to 490 N prior to the strength measurement and a set of specimens without In Figure 8 the involvement of the preaging temperature in the indented strength was demonstrated by the plot of the strength as a function of the temperature. The 3Y-TZP specimens were sintered at 1550 • C for 20 h, then aged for 10 h at 130-220 • C; the as-sintered specimen was included as a control. The specimens were indented at loads ranging from 9.8 to 490 N prior to the strength measurement and a set of specimens without indentation was added to facilitate comparison. The strength of the indented specimens tended to increase with the preaging temperature, and the increasing trend diminished as the indentation load increased. Similar results were also observed in Figure 4, where the biaxial strength of the specimens indented at 9.8 N increased with m-ZrO 2 content, which was varied by sintering times at 1550 • C. Notably, the strength of the specimens, aged at 160-220 • C and then indented with a load of 9.8 N, was higher than the identical specimens without indentation. The contribution of indentation on the increase in strength became progressively prominent up to a load of 98 N, then became less noticeable upon further increasing the loads. Based on the statistical analysis, the aging temperature and indentation load significantly impacted the biaxial flexural strength 3Y-TZP (p < 0.05, Bonferroni post hoc test). Furthermore, there was significant interaction between two independent variables of preaging temperature and indentation load (p < 0.05, Bonferroni post hoc test), as listed in Table 3.
further increasing the loads. Based on the statistical analysis, the aging temperature and indentation load significantly impacted the biaxial flexural strength 3Y-TZP (p < 0.05, Bonferroni post hoc test). Furthermore, there was significant interaction between two independent variables of preaging temperature and indentation load (p < 0.05, Bonferroni post hoc test), as listed in Table 3.

Discussion
This study was initially designed to examine the extent of the t→m phase transformation of 3Y-TZP for up to 100 h at a temperature range of 130-220 °C at 30 °C intervals in air. Primary tests showed that the LTD resistant 3Y-TZP containing 0.25 wt.% Al2O3, prepared by sintering at 1550 °C for 2 h, underwent an insignificant amount of phase transformation after aging for 100 h at the lowest intended temperature of 130 °C. Since it

Discussion
This study was initially designed to examine the extent of the t→m phase transformation of 3Y-TZP for up to 100 h at a temperature range of 130-220 • C at 30 • C intervals in air. Primary tests showed that the LTD resistant 3Y-TZP containing 0.25 wt.% Al 2 O 3 , prepared by sintering at 1550 • C for 2 h, underwent an insignificant amount of phase transformation after aging for 100 h at the lowest intended temperature of 130 • C. Since it caused difficulty in determining the extent for aging times shorter than 100 h, the 3Y-TZP was sintered at 1550 • C for 20 h to expedite the transformation and was used for the remaining experiments. The 3Y-TZP, employed in this investigation, is known to be less vulnerable to LTD because the Al 2 O 3 content of 0.25 wt.% decreases the grain boundary energy of t-ZrO 2 to suppress LTD as a result of the grain boundary segregation of Al 3+ ions [38]. Furthermore, this aging study was performed in ambient atmosphere to minimize the involvement of water species, which has been reported as the cause of LTD in numerous reports on this subject. This study demonstrates that LTD tolerant 3Y-TZP encounters t→m phase transformation in an ambient atmosphere, where no intentional water species are involved like autoclave treatments, as 3Y-TZP is stressed internally by the enlargement of grain size.
The t→m transformation governing LTD proceeds in air as long as the grain size is sufficiently large to cause sufficient residual tensile stress at the grain boundaries and corners [39]. The grain size dependence of the transformation in Figures 1 and 2 stems from the fact that the local tensile stress concentration, due to the thermal expansion anisotropy (TEA) of the a and c axes of tetragonal zirconia lattice, scales with the grain size [39,40]. The dependency is manifested by the increased t→m phase transformation temperature and fracture toughness with the increase in grain size. The TEA in Y-TZP decreases with increasing Y 2 O 3 content in ZrO 2 [41] leading to the phase stability against LTD and consequently to a low fracture toughness of tetragonal zirconia. Conversely, TEA increases with the addition of pentavalent cation oxides, such as Nb 2 O 5 and Ta 2 O 5 , to Y-TZP [42], resulting in phase instability favorable to LTD [7,8,43] and enhanced fracture toughness [8,44]. In this context, the stabilizer for t-ZrO 2 , which consequently prevents LTD, can be defined as oxides that reduce TEA when added to ZrO 2 to form solid solutions. Due to the dominant contribution of residual stress to LTD, it occurs in environments where no water is involved, such as in vacuum and dried air [43] and moisture-free silicon oil [8], once the tetragonal solid solutions are stressed internally by high TEA, which is commonly accompanied by high c/a axial ratios of the tetragonal lattice [44]. Thus, in this study, the 3Y-TZP containing Al 2 O 3 , which prohibits LTD, experienced extensive LTD at ambient atmosphere because the residual stress was sufficiently large to trigger the phase transformation. These observations confirm that environments containing water or water vapor are not prerequisites for LTD but accelerate LTD [8,43].
Additionally, the residual stress influences the strength of 3Y-TZP in the t→m phase transformation, which can be envisaged from the strength of the specimens indented with 9.8 N in Figure 4. Without the indentation the strength decreased gradually with the increasing m-ZrO 2 fraction due to the increase in sintering time. The decrease is likely resulted from a combined contribution of the increase in the grain size ( Figure 1) in accordance with the Hall-Petch relationship and the t→m transformation ( Figure 2). However, the increase in the indented strength with the grain sizes suggests that the main reason behind the decrease in the strength is the transformation related residual stress. An analogous trend was also observed when the specimen was aged at 130, 160, 190, and 220 • C for 10 h in Figure 7, where the influence of grain size is not involved. These observations consist with the reports that the strength decreases with LTD [25][26][27]. The decrease in the strength has been rationalized based on the argument that when the depth of the transformed layer is greater than the critical crack size of the specimen, determined by the linear fracture mechanics, the strength decreases in proportion to the m-ZrO 2 fraction [45]. If this were the case, it would be hard to interpret the opposite trend that the strength of the identical but indented specimens increased with the fraction. This is because impressions by the indentation should behave as an additional defect besides the layer to aggravate the strength. Thus, an alternate rationale is required for the interpretation of the decrease in strength with the increase in the t→m phase transformation.
The possible involvement of the residual internal stress on the strength of the aged t-ZrO 2 was further examined using Vickers indentation in Figure 8. After conducting Vickers indentation on the surface aged at various temperatures, the strength increased with the aging temperatures for all the specimens indented at loads ranging from 9.8 to 490 N. This result is analogues to the reports on the strengthening by LTD [22][23][24]. The increasing rate of strength with the aging temperature was highest at 98 N and reduced as the load increased. The strength of the specimens indented at 9.8 N was even higher than that of non-indented specimens at aging temperatures of 160-220 • C. The unexpected result was related to the residual stress around the indentation. The impression formed by the Vickers indent is generally described by a hemispherical compressively plasticized zone beneath the indent, surrounded by the elastic matrix. The elastic/plastic contact generates residual stresses around the indentation, which play a significant role in the fracture process of ceramics [46]. Particularly, the compressive residual stress originates from the constraint on the expanded indentation region from the unyielded material surrounding it [47]. Thus, the fracture toughness of indented brittle ceramics is given by the applied stress intensity factor and indentation residual stress intensity factor [48]. Herein, the influence of indentation on the strength of the aged 3Y-TZP is possibly related to the compressive residual stresses from both the indentation and the revelation of the compressive layer beneath the tensile surface.
The t-ZrO 2 and m-ZrO 2 grains on the surface of the aged specimens experience tensile and compressive stress, respectively [15,16]. The compressive residual stress can be manifested by the β lattice constant in Figure 6, which is significantly smaller than 99.22 • of the constant of m-ZrO 2 compiled in the ICDD # 00-037-1484. When an indenter is placed on the surface of the aged specimens, the residual compressive zone beneath the indenter and partially exposed compressive layer by the indentation contribute to retarded crack growth in the form of indentation residual stress intensity [48]. Furthermore, the m-ZrO 2 on the aged surface may transform back to t-ZrO 2 as the indentation expands and envelops the aged material in the highly compressive region beneath the indenter [47]. The decreased elastic modulus by LTD [49] may facilitate the envelopment because materials with low ratios of the modulus/strength easily accommodate the indentation volume into the plastically deformed zone beneath the indenter [50]. The freshly created compressive surface by indentation combined with the residual stress intensity factor may cause increased strength of the indented specimens in Figures 4 and 8. The residual stress originates from hydrostatic compressive stress accompanying the t→m phase transformation in the plastic zone [51]. As the plastic zone size increases with the indent load, the residual tensile stress located on the edge of the impression, which is produced by mismatch tractions on the surrounding matrix by the plastically deformed zone [46], may play a role in the distraction of the compressive field. Consequently, the constructive influence of indentation on the strength of the aged specimens reduces at loads exceeding 196 N and nearly disappears at a load of 490 N. Additionally, the complete disappearance at a load of 490 N may be associated with the decrease in residual stress due to lateral cracking [48].
The stress-induced martensitic t→m phase transformation is characterized by an athermal and diffusionless process; however, the kinetics of LTD involve isothermal and diffusion-controlled phase transformation [8]. When the initial t→m transformation occurred, the shape strain comprising shear and dilational strains exerts opposing stresses in the surrounding matrix to place a local internal stress that could cause further stressinduced t→m transformation leading to the autocatalytic transformation [40]. This can occur immediately at the moment of the crack propagation to contribute to the phase transformation toughening. In LTD, however, the opposing stresses are accumulated by the isothermal t→m transformation through the diffusion of oxygen vacancies and may be accommodated to trigger the c→r transformation. This is supported by the fact that the c→r transformation is a stress induced phase transformation [52] and the c-ZrO 2 content in 3Y-TZP decreases with prolonged aging time [20]. When the opposing stresses become large enough by aging at temperatures higher than 160 • C, the remnant shear stresses expand abruptly the β lattice constant in m-ZrO 2 from 99.03 to 99.11 • (Figure 6) to relieve the compressive stress in m-ZrO 2 resulting in additional tensile residual stress on the aged surface. The sudden increase in β is likely related to the collapse of t(101) peak and the preferred orientation of m(002) as observed in Figure 5. This is analogous to the autocatalytic effect in a diffusion-controlled manner. As LTD proceeds, thus, the fractions of t-ZrO 2 and c-ZrO 2 decrease and those of m-ZrO 2 and r-ZrO 2 increase.
The m-ZrO 2 fraction on the surface aged at 160 • C was 52.2% in Figure 7. The relaxation of the compressive stress is supported by the fact that the steady increase in the compressive stress in m-ZrO 2 with aging was halted but the tensile stress in t-ZrO 2 increased continuously when the m-ZrO 2 fraction was higher than approximately 50% [15]. In this case the aged specimens lose their strength with an increase in LTD and the strength becomes subject to the influence of the indentation as shown in Figures 7 and 8. Conversely, when the opposing stress is consumed only for the c→r transformation, the β lattice parameter remains nearly constant at about 99.03 • and the aged strength increases with LTD possibly because the compressive stress in m-ZrO 2 overwhelms the tensile stress in the untransformed t-ZrO 2 . This is the case where the indentation strength of specimens preaged at temperatures lower than 130 • C increases slightly with LTD in Figures 7 and 8. The transition of LTD dependence of the strength was observed from 3Y-TZP subjected to accelerated aging at 134 • C, where the transition occurred after 14.7 h of aging [53]. It should be noted that the m-ZrO 2 fraction that determines the dominancy of the residual stress state on the aged surface may not be fixed for 3Y-TZP but rather varies with the magnitude of the intrinsic or extrinsic residual stresses in specimens.
As previously mentioned, the influence of LTD on the flexural strength of Y-TZP is controversial [22][23][24][25][26][27][28][29][30]. Based on observations of the indentation dependence of the strength of aged 3Y-TZP, controversies surrounding the effect of aging on the strength of 3Y-TZP might be rationalized by considering the residual stress state on the surface of the aged specimens. The argument of residual stress can facilitate to envision the mechanism for LTD in such that the oxygen vacancies in t-ZrO 2 under the residual tensile stress on the surface migrate into the bulk overcoming the activation barrier of~83 kJ/mol along the residual stress gradient to relieve the internal stress in t-ZrO 2 lattice [7,8,43]. This study demonstrates that the stress state can be envisaged by the measurements of the indentation strength of 3Y-TZP after aging. The surface residual stresses can be encountered, not only by the t→m transformation, but during the preparation of strength specimens such as the severity of grinding for dimensioning sample size, heat treatment after machining, abnormal thermal expansion mismatch between 3Y-TZP specimens and a ceramic substrate where the specimens are placed during sintering, and thickness of the bending specimens. In clinical situations, the occlusal adjustment of the restoration may be a cause of surface residual stress, which may influence the longevity of the restorations.

Conclusions
Even LTD resistant 3Y-TZP containing 0.25% Al 2 O 3 underwent LTD in air, where no intentional water species were involved, once the internal residual stress is sufficiently large to induce t→m phase transformation. The transformation was accompanied by c→r transformation, which is responsible for the asymmetric broadening of the (101) peak of t-ZrO 2 lattice. The β lattice constant of m-ZrO 2 lattice was highly contracted by LTD demonstrating compressive residual stress state in m-ZrO 2 . The magnitude of β depended on the build-up of opposing stress in the surrounding matrix imposed by the t→m transformation. The influence of preaging temperature on the β value and the indented strength of 3Y-TZP suggested that as the m-ZrO 2 is under a heavy compressive stress at relatively early stage of LTD, the aged strength increases with LTD, and as the compressive stress is relaxed after an extensive LTD, the strength deteriorates with LTD. The transition occurred at the m-ZrO 2 fraction of 52% as determined by the Rietveld refinements.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.