Purpose: This study investigated the effects of grinding, polishing and aging on physical properties using self-made zirconia and commonly used ultraclear ceramic materials and glass ceramics in the clinic. Methods: The samples were prepared using 3% yttria-stabilized zirconia ceramic (3Y-TZP) powder containing alumina, which was granulated by ball milling. Then, it is pressed into a circular sheet together with Upcera and Wieland materials. The glass ceramic materials are molded. Finally, all materials were subjected to hydrothermal aging. Results: The self-made zirconia had better permeability than the commonly used ultratransparent ceramic materials and glass ceramics. The polishing after grinding improved the surface morphology and roughness of tooth transparent ceramic materials; Polishing after grinding improved the aging resistance of zirconia materials. The bending strength of self-made samples was less than that of two kinds of ceramics commonly used in clinic, but greater than that of glass ceramics. Aging improved the bending strength of zirconia. Conclusions: The self-made zirconia had better permeability than ultratransparent all-ceramic materials and glass ceramics, and its bending strength was better than that of glass ceramics. Grinding and polishing could improve not only the surface morphology and roughness but also the strength and aging resistance.

Keywords:

1. Introduction

Zirconia material is the main material used in odontology to produce all-ceramic restorations and implant abutments such as crowns and bridges. It is usually white and has the important advantages of being metal free in color appearance and reliable in mechanical strength.^1,2 Zirconia has three typical polymorphisms: monoclinic, tetragonal and cubic phases. In addition, an orthorhombic phase of zirconia exists under high pressure, and a rhombic phase forms on the ground zirconia surface.^3,4 At room temperature, pure zirconia exists in a monoclinic phase. However, with increasing temperature, the monoclinic phase transforms into the tetragonal phase. Moreover, the tetragonal phase transformation becomes stable at temperatures > 1,170°C. After high-temperature sintering, when zirconia is cooled to room temperature, the tetragonal phase changes again to a monoclinic phase with 3–5% volume increase. Because of the presence of significant cracks in zirconia, the resulting strain exceeds the elasticity and fracture strength of zirconia.

Further, 3% yttrium oxide–stabilized zirconia (3Y-TZP) is a major component of dentate zirconia materials, and Y-TZP has superior mechanical properties compared with other dentate materials⁵ (flexural strength between 750MPa and 1300MPa^6,7). This material is biologically inert⁷ and has a high X-ray opacity. Its yttrium oxide content and optical properties, such as translucency, are similar to those of enamel. Y-TZP has several crystal forms: as a mineral, under standard conditions, ZrO₂ is monoclinic; under high-temperature conditions, three tetragonal phases and one cubic phase exist.^8,9,10,11,12 The cubic phase has a fluorite structure type¹³ and converts into a tetragonal phase with decreasing yttrium oxide content and temperature.^8,9,10 These three tetragonal phase volumes are different, resulting from phase separation due to quenching after sintering of Y-TZP. According to Lipkin et al., the yttrium oxide poor-yttrium phase component initially transforms into the tetragonal phase t and subsequently into the monoclinic phase M. Yttrium-rich phase components transform into the tetragonal phase $t$ and cubic phase C.¹⁰ The tetragonal phase is slightly distorted, with an obvious group subunit chain connection to the cubic phase. Therefore, the transition from the cubic phase to the tetragonal phase is not destructive.^8,9 However, this situation does not hold for phase transition from the tetragonal phase $t$ to the monoclinic phase M, which is accompanied by a large volume change and a large shear strain from 8% to 9%.¹⁴ This may lead to microcracking and an increased number of surface irregularities.¹⁵ The results include an increasing buckling strength loss²⁰ and, in a worst-case scenario, failure associated with a proportional increase in the monoclinic phase.^{10,16,17,18,19}

Previous studies reported that zirconia surfaces, when ground or polished, formed a surface stress layer, and if this adjustment process lasted for a longer time, the depth stress increased, leading to a decrease in flexural strength. This reduction was related to the degree of phase transition from the tetragonal to monoclinic phases, which was dependent on the amount of grinding or polishing, the type of instrument used, the particle size, and the heat generated during adjustment.²¹ 3Y-TZP nanohardness was greater than 20 GPA, which was comparable to that of ceramics and enamel. Drills with higher grinding efficiency are required to grind or polish this hard material. Therefore, a diamond sand high-speed grinding polishing bur is recommended, but minimal research has been conducted on its effect on the physical properties of zirconia. The grain size of the ground material, whether it is polished after use, does not currently have a clear standard for use. Moreover, the control of pressure and temperature by the operator can also have an impact. Therefore, the pressure, temperature, and time was controlled in this study using an automatic grinding polishing instrument.

The application of zirconia ceramics in the oral field has increased in recent years. At present, commonly used zirconium dioxide restorations in clinical practice mostly use imported zirconium dioxide powder; however, it is relatively expensive. Domestic zirconium dioxide restorations are inferior to imported products in terms of their physiochemical characteristics and aesthetic properties, although they are less expensive than imported zirconium dioxide restorations. Also, their clinical popularity is not high. These reasons limit the choice and application of zirconium dioxide restorations in clinical work. Our group has been working on the development of a homemade autonomous-brand zirconium dioxide powder and restorations with excellent physicochemical characteristics and aesthetic properties. The aim is to develop a product more competitive with the same products or technologies at home and abroad, not only significantly improving the performance of the currently used dental zirconia but also reducing the price of the clinical end restorations. Less cost is required for patients to obtain the same treatment, which greatly increases the rate of clinical selection of zirconia restorations and improves the level of oral health care of the population.

In this study, we investigated the effects of grinding, polishing, and aging on the physical properties of the four materials by measuring the surface morphologies, surface roughness, crystal-phase composition, nanohardness, nanomodulus and flexural strength of homemade ultratransparent zirconia materials; two kinds of clinically commonly used ultratransparent zirconia materials; and glass ceramics. The purpose was to provide theoretical support for the choice of all-ceramic materials by clinical practitioners and the change in all-ceramic materials during use after grinding and polishing, and to promote the development of our dentate zirconia materials in the medical field.

2. Materials and Methods

2.1. Specimen preparation

The samples were prepared using 3Y-TZP powder (tz-3y-e, TOSOH, Japan) with 0.25wt% alumina content that had been pelleted by ball milling (SP3, Nanjing University Instrument Factory, China). Weighing 2 g each, the fully automated powder tablet press (ZY-40TD, Engjian, China) pressed the samples into a circular shape of 2mm height. Parafilm membrane-wrapped samples were put into an electrically cold isostatic press (CIP-50M) at 20MPa and cold isostatically pressed for 1 min, requiring a step-up to 20MPa in 10min. Sample specification: after sintering according to the standard of iso6872-2015, 20 round-shaped samples were obtained, which were 20mm in diameter and 2mm in height. They were polished with 320-grit silicon carbide sandpaper to eliminate any initial error.

Twenty disk-shaped zirconia specimens, each 20mm in diameter and 2mm in height, were prepared from Upcera zirconia material (ST, Upcera, China) and Wieland zirconia material (Zenostar T1, Wieland, Germany) and polished with 320-grit silicon carbide sandpaper to eliminate any initial error.

The sample specifications were checked using Vernier calipers, and samples that met the criteria were enrolled in this trial. After ethanol light scrubbing, the samples were dried at room temperature for further use.

The selected specimens with intact surfaces (diameter 20mm, height 2mm) were heated in a high-temperature box resistance furnace (ksl-1750x, Kejing, China) at a rate of 3°C/min, held at 600°C for 1h, heated continuously up to 1450°C for 2h, and finally cooled down with the furnace. According to the ISO 6872-2015 standard, specimens with a diameter of 12mm and a height of 1mm were selected in this experiment.

Glass-ceramic materials (IPS E-MAX HT, Ivoclar, Liechtenstein), 20 specimens, 12mm in diameter and 1mm in height, which had been die cast according to the specification requirements, were purchased directly.

2.2. Grinding and polishing

The samples were randomly grouped as shown in Table 1.

**Table 1. Surface treatment received by each group.**
Group	Number	Surface treatment method
Blank control group	A1, A2, A3, and A4	Not processed
Clinical negative control group	B1, B2, and B3	Grinding: 600 rpm, 1min; polishing: 1000 rpm, 2min
Experimental grinding and polishing group	C	Grinding: 600 rpm, 1min; polishing: 1000 rpm, 2min

Group A was the blank control group without any treatment: 10 self-made samples in group A1 and 10 Upcera zirconia materials in group A2 (ST, Upcera, China), 10 Wieland zirconia materials in group A3 (Zenostar T1, Wieland, Germany) and 10 glass ceramics in group A4 (IPS E-MAX HT, Ivolar, Liechtenstein).

Group B was a clinical negative control group, with clinical grinding and polishing: 10 pieces of Upcera zirconia in group B1 (ST, Upcera, China), 10 pieces of Wieland zirconia in group B2 (Zenostar T1, Wieland, Germany) and 10 pieces of glass ceramics in group B3 (IPS E-MAX HT, Ivolar, Liechtenstein).

Group C was the self-made sample polishing group.

Group A was treated as a blank control group without grinding. The samples in the clinical negative control group (group B) and the experimental grinding and polishing group (Group C) were ground with a 400-mesh silicon carbide waterproof sandpaper (Suisun, Korea) and semi-automatic grinder (Unipol-820, China). The top and bottom surfaces were ground at a speed of 600 rpm for 1min. After grinding, 2000-mesh silicon carbide waterproof sandpaper (Suisun, Korea) and an automatic metallographic sample polishing machine [ympz-1-300 (250), China] were used. The top and bottom surfaces were polished at 1000 rpm for 2min.

2.3. Hydrothermal aging

The samples were subjected to hydrothermal aging in an autoclave (xsf-z, Weihai Xingyu Chemical Machinery Co., Ltd., China) at 134°C and 200kPa for 5h to simulate the low-temperature aging resistance in zirconia, which met the standard of ISO1335-2008. The amount of phase-change zirconia after the aforementioned treatment was roughly equivalent to the amount of zirconia implants used for 15–20 years.²² The groups without thermal aging were called A0 (A10, A20, A30, and A40), B0 (B10, B20, and B30), and C0, and the groups with thermal aging for 5 h were called A5 (A15, A25, A35, and A45) and B5 (B15, B25, and B35), and C5, and the number of each group was 5.

2.4. Translucency

The translucency in A1, A2, A3 and A4 was measured using a UV spectrophotometer (uv-3600plus, Shimadzu, Japan).

2.5. Surface morphology and roughness

The surface morphology and roughness were measured using a scanning electron microscope (SEM; Phenom XL, Feiner, the Netherlands) and a laser scanning confocal microscope (LSCM; RTC, mft-5000, USA).

2.6. Crystal-phase analysis

The phase types and contents were identified using an X-ray diffractometer (Ultima IV, Rigaku, Japan). The analysis range in each group ( $n$ = 5) was 20–100°, the step was 0.02°, and the counting time of each step was 0.02 s. The content of monoclinic zirconia was calculated using the formula proposed by Tyagi B:²³ $Xm = [Im (11 \bar{1}) + Im (111)] ∕ [Im (11 \bar{1}) + Im (111) + It (111)]$ , where Im (11 $\bar{1}$ ) represents the intensity of the monoclinic peak and It (111) represents the intensity of the tetragonal peak. The X-ray diffraction spectrum was analyzed using pdxl2 software.

2.7. Nanoindentation

The polished surface was tested using nanoindentation (G200, Keysight, USA) to evaluate the hardness and elastic modulus of the phase transition zone.

2.8. Biaxial flexural test

According to the standard ISO 6872-2008, biaxial flexural tests were carried out on each group of specimens ( $n = 5$ ) using a universal testing machine (Ag-X, Shimadzu, Japan). The disk-shaped specimen was placed at the center of the die (Fig. 1) and supported with 300 steel balls with a diameter of 4.5mm, with a distance of 120° on the support circle with a diameter of 10mm. The glaze connecting the support ball was removed and polished. A flat punch with a diameter of 1.4mm was used to apply an increased load (0.52mm/min) to the center of the specimen until the specimen was completely broken. The flexural strength was calculated according to ISO 6872-2008 :

where $σ$ , $P$ and $b$ represent the maximum tensile stress (MPa), the total load at the fracture (N), and the thickness at the starting point of fracture (mm), respectively. In addition, $v = 0.32$ (zirconia), $r_{1}$ is the radius of the support circle (5mm), $r_{2}$ is the radius of the loading area (0.7mm) and $r_{3}$ is the radius of the sample (6mm).

2.9. Fracture analysis test

The specimen fragments were collected after the biaxial flexural test. The fracture morphology, fracture source, and crack propagation direction were observed and analyzed using an SEM (Phenom XL, Feiner, Netherlands).

2.10. Data analysis

Origin was used to plot the surface roughness, nanohardness, nanomodulus and flexural strength, and pdxl2 software was used to calculate the content of monoclinic zirconia. Statistical analysis was performed using IBM SPSS 20.0 software. The data of surface roughness value, nanohardness and nanomodulus, and flexural strength were obtained using $x \pm s$ for statistical description. The statistical method adopted the independent-samples $t$ test; $α = 0.05$ was the inspection level.

3. Results

3.1. Translucency

The transmittance of A1, A2, A3 and A4 materials was tested. The transmittance characteristics of the four materials under continuous visible light with a wavelength range of 200–800nm are shown in Fig. 2. A1 had good transmittance under this visible light. Conclusion: A1 > A4 > A3 > A2.

3.2. Surface morphology and roughness

3.2.1. SEM results of polishing after grinding

SEM images are shown in Fig. 3. Grinding eliminated the initial scratches caused before sintering and flattened the surface, but new scratches were generated. The scratches in the experimental grinding groups (C0 and C5 groups) were shallower than those in the blank groups (A10 and A15 groups). The scratches in the experimental grinding group (B10 and B15 groups) were shallower than those in the blank groups (A20 and A25 groups). The scratches in the experimental grinding group (B20 and B25 groups) were shallower than those in the blank groups (A30 and A35 groups). The scratches in the experimental grinding groups (B30 and B35 groups) were shallower than those in the blank groups (A40 and A45 groups). In addition, the grinding group showed more consistent scratches in one direction than the blank group.

3.2.2. Surface morphology results of polishing after grinding

The LSCM image of three-dimensional topography/topology is shown in Fig. 4. Compared with the grinding group (C0 group), the grooves in the blank control group (A10 group) were more obvious. After grinding (C0 group), the sample showed a smoother surface, and the three-dimensional morphology of the surface had fewer “gullies” and “protrusions.” This clearly showed that the area between staggered scratches was smoothed by grinding and polishing.

LSCM image also provided the statistics of surface roughness value. As shown in Fig. 5, the surface roughness in the blank control group (A10 group) was 1.3148 $μ$ m. The surface roughness after grinding (C0 group) was 0.9656 $μ$ m. The images showed the following trend of surface roughness: A10 group $>$ C0 group, and A20 group $>$ B10 group, A30 group $>$ B20 group and A40 group $>$ B30 group. The difference was statistically significant ( $P < 0.05$ ), indicating that grinding and polishing made the surface smoother. The images also depicted the following trend: A15 group $>$ C5 group, A25 group $>$ B15 group, A35 group $>$ B25 group and A45 group $>$ B35 group, and the difference was statistically significant (P < 0.05), indicating that aging after grinding and polishing also made the surface smoother.

3.3. Crystal-phase analysis

In crystal-phase analysis, the content of the monoclinic phase (Xm) and tetragonal phase in the polishing experiment after grinding was expressed by the peak intensity in X-ray diffraction data (Table 2 and Fig. 6). Before aging, monoclinic zirconia was not detected in the blank control groups (A10, A20, A30 and A40 groups), clinical negative control groups (B10, B20 and B30 groups), and experimental grinding and polishing group (C0 group). After aging, the monoclinic zirconia generated on the surface in the grinding and polishing groups (C5, B15 and B25 groups) was less than that on the nongrinding surface (A15, A25 and A35 groups). Monoclinic zirconia was not detected in the glass ceramics (B35 and A45 groups) mainly composed of silica. Therefore, polishing did not cause phase transformation, but it improved the aging resistance of zirconia materials.

**Table 2. Monoclinic phase content in grinding and polishing experiment (Xm).**
Group	Xm (%)	Group	Xm(%)
A10	No detected	A15	26.97
A20	No detected	A25	29.32
A30	No detected	A35	3.61
A40	No detected	A45	No detected
B10	No detected	B15	6.92
B20	No detected	B25	1.40
B30	No detected	B35	No detected
C0	No detected	C5	21.45

Fig. 6. Monoclinic phase content (Xm).
*Notes*: m (11 $\bar{1}$ ) and m (111) represent the intensity of the monoclinic peak, and t (111) represents the intensity of the tetragonal peak.

3.4. Nanoindentation

Table 3 and Fig. 7 list the nanohardness and nanomodulus of the surface of four ceramic materials. Grinding, polishing and aging had no effect on the nanohardness and nanomodulus of the three zirconia materials and glass-ceramic surfaces, and the difference was not statistically significant. The nanohardness and nanomodulus of self-made zirconia materials were not different from those of super-transparent all-ceramic materials commonly used in clinic, but better than that of glass ceramics, and the difference was statistically significant (P < 0.05).

**Table 3. Nanohardness (GPa) and nanomodulus (GPa).**
Group	Nanohardness	Nanomodulus	Group	Nanohardness	Nanomodulus
A10	24.772±1.212	302.849±25.814	A15	27.134±0.814	303.488±14.767
A20	20.491±0.546	267.491±46.954	A25	21.457±2.126	269.844±32.582
A30	16.489±0.223	245.769±22.341	A35	16.786±0.145	250.146±32.564
A40	8.425±0.245	215.489±13.254	A45	8.892±0.144	219.487±21.775
B10	22.491±0.455	277.493±23.897	B15	23.462±0.961	278.635±18.705
B20	18.131±0.196	251.41±18.465	B25	18.344±0.282	255.634±16.429
B30	8.631±0.368	218.533±12.084	B35	8.954±0.189	220.456±39.551
C0	27.892±0.523	303.674±31.025	C5	28.993±1.421	305.706±23.092

3.5. Biaxial flexural test

The flexural strength in each group was significantly different. Based on Fig. 8, the following conclusions were drawn:

A30 and A20 $>$ A10 $>$ A40, and the difference was statistically significant (P < 0.05). That is, the flexural strength of the self-made sample was less than that of the two common clinical ceramics, but greater than that of glass ceramics.

A10 $<$ C0, and the difference was statistically significant (P < 0.05); A20 $<$ B10, A30 $<$ B20, and A4 $<$ B30, and the difference was not statistically significant (P > 0.05). That is, only one material increased the flexural strength after grinding and polishing. Or, the self-made ultratransparent dental zirconia increased the flexural strength after grinding and polishing, while the other three materials caused no significant change.

3.6. Fracture analysis

Zirconia fragments used for fracture analysis were obtained from the flexural strength test. The starting point of fracture (Figs. 9 and 10) was close to the position where the flat punch was applied. The number, size and direction of cracks in each group were different and complex. The samples in the blank control group showed a relatively uniform uneven appearance with staggered small cracks (A10, A20, A30 and A40 in Fig. 9). At the magnification of 500 times, the fracture morphology of the samples in the clinical negative control group (B10, B20 and B30 in Fig. 9) and the samples in the experimental grinding group (C0) was more consistent than that of the samples in the blank control group. Therefore, polishing after grinding affected the cracks near or far from the fracture. The cracks were regular, and the direction was more consistent.

Fig. 9. SEM of the fracture surface of unaged sample (×500 times): blank control group samples (A10, A20, A30 and A40), clinical negative control group samples (B10, B20 and B30), and experimental grinding group samples (C0). The scale is 50 $μ$ m.

Fig. 10. Scanning electron micrographs of fractured specimens after aging (×500 fold): blank control samples after aging (A15, A25, A35 and A45), clinical negative control samples after aging (B15, B25 and B35), and an experimental grinding group samples after aging (C5). Scale bar is 50 $μ$ m.

Aging greatly changed the fracture morphology of the samples, the surface was uneven, and even small hills and fractures appeared. The appearance of small ridges and long cracks replaced the relatively uniform uneven appearance (A10, A20, A30 and A40 in Fig. 9 and A15, A25, A35 and A45 in Fig. 10). The small ridge was mainly located near the starting point of the crack, and the long crack was far away from the starting point of the crack (A15, A25, A35 and A45 in Fig. 10). The cracks near the crack starting point were short and staggered, while the cracks far away from the crack starting point were large and aligned.

4. Discussion

This study aimed to elucidate the effects of grinding, polishing, and aging on the physical properties, such as surface morphology, surface roughness, crystal-phase composition, nanohardness, nanomodulus and flexural strength, of homemade ultraclear dentate zirconia materials, commonly used ultraclear all-ceramic materials and glass ceramics in clinical practice. The purposes were to compare the merits of homemade-like ultratransparent dentate zirconia materials with those of commonly used ultratransparent all-ceramic materials and glass ceramics in the clinic, and to explore the necessity of grinding polishing for dentate transparent ceramics, so as to provide a theoretical basis for the clinical selection of materials and grinding polishing operations.

In this study, self-made high-light transmission dentate zirconia materials were developed. Various detection indicators and comparisons with commonly used materials in the clinic were used to prepare for later stepwise achievement transformation and industrialization. The objective was to not only significantly improve the performance of the currently used dental zirconia but also reduce the price of the clinical end restorations. Less cost would be required for patients to obtain the same treatment, thus greatly increasing the rate of clinical selection of zirconia restorations and improving the level of oral health care of the population.

Untreated zirconia and glass-ceramic specimens with staggered small cracks in this study (A20, A30 and A40 in Fig. 9) had relatively uniform fracture morphologies, implying that the zirconia and glass-ceramic materials used in this study were reliable.²⁴ The sample preparation was done by the same physician according to the ISO6872-2015 standard, thus minimizing errors.

Different from the transparent ceramic materials used in other studies, this study used the self-made samples, that is, 3% yttrium-stabilized zirconia ceramic (3Y-TZP) powder with 0.25wt% alumina content and 3% yttria content. Other ultratransparent zirconia materials used in this study, Upcera zirconia material (ST, Upcera, China) and Wieland zirconia material (zenostar T1, Wieland, Germany), had a yttrium content of 4.5–6.0% and zirconia content of more than 99%, which were comparable with self-made samples. Glass-ceramic materials (IPS E-MAX HT, Ivolar, principality of Liechtenstein) were mainly composed of silica and zirconia. The main components were different from the aforementioned materials, but they are widely used in clinic. They can also be used to study the effects of grinding, polishing and aging.

Yttrium oxide-containing zirconia has high strength, but the aging resistance and transparency decrease with the increase in sintering temperature after 1400°C due to the segregation of yttrium oxide to zirconia grain boundaries and sintering temperature-related grain size.^25,26,27,28

A large number of studies at home and abroad have proved how grinding and polishing affect the mechanical behavior of zirconia: Generally speaking, grinding changes the surface stress state, induces phase transformation, and introduces defects, so as to increase or reduce the strength of zirconia.^{29,30,31,32,33,34} Different grinding tools, pressure, temperature, and time have important effects on grinding and polishing. Intensive grinding, such as grinding with a high-speed head and a large-size dental bit, is more likely to trigger wider phase transformation and introduce larger defects. Grinding with an automatic grinder, while controlling the pressure and temperature, triggers a small phase transition and produces small or no obvious defects.^35,36,37

The results of this study showed that, under the continuous visible light with the wavelength range of 200–800nm, the transmittance characteristics of the four materials, from good to bad, were self-made sample $>$ glass ceramics $>$ Wieland zirconia material $>$ Upcera zirconia material, which proved that the self-made sample material in this study had the best translucency.

The polishing experiment after grinding found that the initial scratches induced before sintering were eliminated and the surface flattened, but new scratches were generated. For the finely ground surface, the old scratches were covered by new scratches to achieve a neat arrangement. Hence, the old and new scratches were staggered. Therefore, in clinical operation, the requirements for the operator improved. It was necessary to select a fine grinding head to avoid more severe inappropriate grinding and polishing due to excessive force.

According to the surface roughness value, it was found that aging after grinding and polishing made the surface smoother. Tuncer et al.³⁸ studied the surface roughness of seven materials after aging. Their report showed that more than 10,000 aging times did not significantly affect the surface roughness value of composites, which was consistent with the experimental results. That is, after dental materials were used in the mouth for more than 20 years, the surface was not significantly roughened.

The crystal-phase analysis showed that monoclinic zirconia was not detected in the blank control, clinical negative control, and experimental grinding and polishing groups before aging. After aging, the monoclinic zirconia generated on the surface in the grinding and polishing groups (C5, B15 and B25 groups) was less than that on the nongrinding surface (A15, A25 and A35 groups). Monoclinic zirconia was not detected in the glass ceramics (B35 and A45 groups) mainly composed of silica. Therefore, after grinding, polishing did not cause phase transformation. The stress layer produced by grinding impeded the penetration of water into materials and effectively improved the aging resistance of zirconia materials, which was consistent with the results of the study by Pereira and Cattani-Lorente et al.^39,40,41 The homemade samples obtained after grinding and polishing had the same anti-aging property.

The results of nanonaindentation showed that grinding, polishing, and aging had no effect on the nanohardness and nanomodulus of the three zirconia materials and glass-ceramic surfaces. The nanohardness and nanomodulus were mainly related to the type of material. Hence, although Lai et al.⁴² found that the nanohardness and nanomodulus were also related to the stress state of surface zirconia, not all groups had changed nanohardness and nanomodulus. Evidence to show that grinding, polishing, and aging were enough to affect nanohardness and nanomodulus was insufficient. The modulus of self-made zirconia ceramics was no better than that of self-made nanoceramics, but no difference was found between self-made zirconia ceramics and nanoceramics. It showed that the self-made material had reached the clinical average hardness level.

The biaxial flexural test results showed that the flexural strength of the self-made sample was less than that of the two common clinical ceramics but greater than that of glass ceramics, which needed to be improved. The flexural strength of self-made ultratransparent dental zirconia increased after grinding, with no significant change in the other three materials. Due to the phase transformation caused by grinding and polishing, the compressive stress layer was generated through grain expansion, which hindered the propagation of microcracks and delayed failure. As described by Pittayachawa and Pereira et al.,^40,43 the phase transformation toughening (PTT) of zirconia material increased the flexural strength of self-made ultratransparent dental zirconia after grinding. However, several reports showed that PTT could not eliminate the adverse effects of all defects caused by grinding and polishing. Therefore, it might even lead to the reduction of flexural strength,^{33,37,44,45,46} which was consistent with the results that no obvious change occurred in the other three materials in this study.

The fracture analysis test results showed that polishing after grinding affected the cracks near or far from the fracture. The cracks were regular, and the direction was more consistent. Aging greatly changed the fracture morphology of the sample, the surface was uneven, and even small hills and fractures appeared. The appearance of small ridges and long cracks replaced the relatively uniform uneven appearance. The small ridge was mainly located near the starting point of the crack, and the long crack was far away from the starting point of the crack. The cracks near the crack starting point were short and staggered, while the cracks far away from the crack starting point were large and aligned. These changes in fracture morphology might reflect the stress distribution in the material. When the surface was flattened and scratched by grinding and polishing, the stress state of the treated surface changed, resulting in a compressive stress field with increased strength.^40,47

5. Conclusions

This study investigated the effects of grinding polishing and aging on the physical properties, including surface morphology, surface roughness, crystal-phase composition, nanohardness, nanomodulus and flexural strength of two clinically commonly used types of self-made ultratransparent zirconia materials and glass ceramics. It also evaluated the excellent properties of the self-made materials by measuring the effects of grinding polishing and aging on the physical properties of the four materials. The objectives were to provide theoretical support for the choice of all-ceramic materials by clinical practitioners and the change in all-ceramic materials during use after grinding polishing, and also to promote the development and promotion of our dentate zirconia materials in the medical field. The analysis of the experimental results led to the following conclusions:

(1)	Self-made ultratransparent zirconia was more permeable than the ultratransparent zirconia materials and glass ceramics in the clinical negative control group.
(2)	The flexural strength of self-made ultratransparent zirconia was better than that of the ultratransparent zirconia materials in the clinical control group and also better than that of glass ceramics.
(3)	The nanohardness and nanomodulus of the homemade zirconia material were not different from those of the clinically commonly used ultratransparent zirconia materials but were better than those of glass ceramics.
(4)	Grinding after polishing improved the surface topography and roughness of the four dentate transparent zirconia materials. Grinding and polishing were beneficial for improving the strength of homemade ultratransparent zirconia and the aging resistance of three zirconia materials.

Due to a large number of clinically available all-ceramic materials, only two clinically commonly used ultratransparent zirconia materials and one glass-ceramic were selected for this study. This study had certain limitations in the choice of the types of all-ceramic materials. However, the results of this study on three kinds of all-ceramic materials commonly used in the clinic might inspire the investigators to conduct further research on the effects of grinding and polishing on homemade zirconia materials and other commonly used materials in the clinic. The findings laid the foundation for the next study on material biosafety.

This study provided theoretical support for the choice of all-ceramic materials by clinical practitioners and the changes during the use of all-ceramic materials after grinding and polishing, the excellent translucency exhibited by the homemade materials, and the comparable nanohardness and nanomodulus to those of the commonly used ultratransparent zirconia materials in the clinic. It might promote the continued research in our subject group to develop homemade autonomous-brand zirconia powders and restorations with excellent physicochemical characteristics and esthetic properties. The research development of products translated into reducing the material costs required by patients to obtain the same treatment would greatly increase the rate of clinical selection for zirconia restorations and improve the oral health care of the population.

5.1. Ethical compliance

The research experiments conducted in this study with animals or humans were approved by the ethics committee and responsible authorities of our research organization(s), following all guidelines, regulations, and legal and ethical standards as required for humans or animals.

Acknowledgment

This study was supported by grants from the science and technology project of Xinjiang Uygur Autonomous Region, China (Grant/Award Number 2019E0279).

Conflicts of Interest

The authors declare no conflicts of interest.

References

1. Kim DJ, Transformation of tetragonal zirconia, Sci Technol Ceram Mater 88 :157–165, 1993. Google Scholar
2. Miyazaki T, Nakamura T, Matsumura H et al., Current status of zirconia restoration, J Prosthodont Res 57 :236–261, 2013. Crossref, Web of Science, Google Scholar
3. Garvie RC, Hannink RH, Pascoe RT, Ceramic steel, Nature Cell Biol 4 :703–704, 1975. Google Scholar
4. Sakuma T, Yoshizawa YI, Suto H, The rhombohedral phase produced in partially-stabilized zirconia, J Mater Sci Lett 4 :29–30, 1985. Crossref, Google Scholar
5. Conrad HJ, Seong WJ, Pesun IJ, Current ceramic materials and systems with clinical recommendations: A systematic review, J Prosthet Dent 98 :389–404, 2007. Crossref, Web of Science, Google Scholar
6. Zhang Y, Lawn BR, Novel zirconia materials in dentistry, J Dent Res 97 :140–147, 2018. Crossref, Web of Science, Google Scholar
7. Kelly JR, Denry I, Stabilized zirconia as a structural ceramic: An overview, Dent Mater 24 :289–298, 2008. Crossref, Web of Science, Google Scholar
8. Kisi EH, Howard CJ, Crystal structures of zirconia phases and their inter-relation, Key Eng Mater 36 :153–154, 1998. Google Scholar
9. Krogstad JA, Gao Y, Bai J et al., In situ diffraction study of the high-temperature decomposition of t0 -Zirconia, J Am Ceram Soc 98 :247–254, 2015. Crossref, Web of Science, Google Scholar
10. Lipkin DM, Krogstad JA, Gao Y et al., Phase Evolution upon aging of air-plasma sprayed t0-zirconia coatings: I-synchrotron X-ray diffraction, J Am Ceram Soc 96 :290–298, 2013. Crossref, Web of Science, Google Scholar
11. Yashima M, Sasaki S, Kakihana M et al., Oxygen-induced structural change of the tetragonal phase around the tetragonal–cubic phase boundary in ZrO2–YO1.5 solid solutions, Acta Crystallogr B 51 :381, 1995. Crossref, Google Scholar
12. Duwez P, Brown FH, The zirconia-yttria system, J Electrochem Soc 98 :356–362, 1951. Crossref, Web of Science, Google Scholar
13. Scott HG, Phase relationships in the zirconia-yttria system, J Mater Sci 10 :1527–1535, 1975. Crossref, Web of Science, Google Scholar
14. Hashim D, Cionca N, Courvoisier DS et al., A systematic review of the clinical survival of zirconia implants, Clin Oral Investig 20 :1403–1417, 2016. Crossref, Web of Science, Google Scholar
15. Chevalier J, Cales B, Drouin JM. Low-temperature aging of Y-TZP ceramics, J Am Ceram Soc 82 :2150–2154, 2004. Crossref, Google Scholar
16. Masonis JL, Bourne RB, Ries MD et al., Zirconia femoral head fractures: A clinical and retrieval analysis, J Arthroplast 19 :898–905, 2004. Crossref, Web of Science, Google Scholar
17. Gahlert M, Burtscher D, Grunert I et al., Failure analysis of fractured dental zirconia implants, Clin Oral Implants Res 23 :287–293, 2012. Crossref, Web of Science, Google Scholar
18. Keuper M, Berthold C, Nickel KG, Long-time aging in 3 mol % yttria-stabilized tetragonal zirconia polycrystals at human body temperature, Acta Biomater 10 :951–959, 2014. Crossref, Web of Science, Google Scholar
19. Tholey MJ, Berthold C, Swain MV et al., XRD2 Micro-diffraction analysis of the interface between Y-TZP and veneering porcelain: Role of application methods, Dent Mater 26 :545–552, 2010. Crossref, Web of Science, Google Scholar
20. Botelho MG, Dangay S, Shih K et al., The effect of surface treatments on dental zirconia: An analysis of biaxialflexural strength, surface roughness and phase transformation, J Dent 75 :65–73, 2018. Crossref, Web of Science, Google Scholar
21. Işeri U, Ozkurt Z, Yalnız A et al., Comparison of different grinding procedures on the flexural strength of zirconia, J Prosthet Dent 107 :309–315, 2012. Crossref, Web of Science, Google Scholar
22. Chevalier J, Gremillard L, Deville S, Low-Temperature degradation of zirconia and implications for biomedical implants, Mater Res, 37 :1–32, 2007. Crossref, Web of Science, Google Scholar
23. Tyagi B, Sidhpuria K, Shaik B et al., Synthesis of nanocrystalline zirconia using sol-gel and precipitation techniques, Indus Eng Chem Res 45 :8643–8650, 2006. Crossref, Web of Science, Google Scholar
24. Mccabe JF, Carrick TE, A statistical approach to the mechanical testing of dental materials, Dent Mater 2 :139–142, 1986. Crossref, Web of Science, Google Scholar
25. Cotič J, Jevnikar P, Kocjan A et al., Complexity of the relationships between the sintering-temperature-dependent grain size airborne-particle abrasion, ageing and strength of 3Y-TZP ceramics, Dent Mater, 32 :510–518, 2016. Crossref, Web of Science, Google Scholar
26. Zhang F, Vanmeensel K, Batuk M et al., Highly-translucent strong and aging-resistant 3Y-TZP ceramics for dental restoration by grain boundary segregation, Acta Biomater 16 :215–222, 2015. Crossref, Web of Science, Google Scholar
27. Zhang Y, Making yttria-stabilized tetragonal zirconia translucent, Dent Mater, 30 :1195–1203, 2014. Crossref, Web of Science, Google Scholar
28. Inokoshi M, Zhang F, De Munck J et al., Influence of sintering conditions on low-temperature degradation of dental zirconia, Dent Mater 30 :669–678, 2014. Crossref, Web of Science, Google Scholar
29. Zhang KM, Shen Z, Effect of multistep processing technique on the formation of micro-defects and residual stresses in zirconia dental restorations, J Prosthodont 23 :206–212, 2013. Google Scholar
30. Pereira GKR, Amaral M, Simoneti R, Effect of grinding with diamond-disc and -bur on the mechanical behavior of a Y-TZP ceramic, J Mech Behav Biomed 37 :133–140, 2014. Crossref, Web of Science, Google Scholar
31. Işerı U, Ozkurt Z, Kazazoğlu E et al., Influence of grinding procedures on the flexural strength of zirconia ceramics, Braz Dent J 21 :528–532, 2010. Crossref, Google Scholar
32. Paula M, Ranulfo B, Leite TP et al., Effect of titania addition and sintering temperature on the microstructure, optical, mechanical and biological properties of the Y-TZP/TiO2 composite, Dent Mater 36 :1418–1429, 2020. Crossref, Web of Science, Google Scholar
33. Schmitter M, Lotze G, Bömicke W et al., Influence of surface treatment on the in-vitro fracture resistance of zirconia-based all-ceramic anterior crowns, Dent Mater 31 :1552–1560, 2015. Crossref, Web of Science, Google Scholar
34. Lee K, Choe H, Heo Y et al., Effect of different grinding burs on the physical properties of zirconia, J Adv Prosthodont 80 :137, 2016. Crossref, Web of Science, Google Scholar
35. Kosma T, Oblak C, Jevnikar P et al., The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic, Dent Mater 15 :426–433, 1999. Crossref, Web of Science, Google Scholar
36. Canneto J, Cattani-Lorente M, Durual S et al., Grinding damage assessment on four high-strength ceramics, Dent Mater 32 :171–182, 2016. Crossref, Web of Science, Google Scholar
37. Hjerppe J, Narhi TO, Vallittu PK et al., Surface roughness and the flexural and bend strength of zirconia after different surface treatments, J Prosthet Dent 116 :577–583, 2016. Crossref, Web of Science, Google Scholar
38. Tuncer S, Demirci M, Tiryaki M et al., The effect of a modeling resin and thermocycling on the surface hardness, roughness, and color of different resin composites, J Esthetic Restorative Dentis 25 :404–419, 2013. Crossref, Web of Science, Google Scholar
39. Pereira GKR, Silvestri T, Amaral M et al., Fatigue limit of polycrystalline zirconium oxide ceramics: Effect of grinding and low-temperature aging, J Mech Behav Biomed 61 :45–54, 2016. Crossref, Web of Science, Google Scholar
40. Pereira G, Amaral M, Cesar PF et al., Effect of low-temperature aging on the mechanical behavior of ground Y-TZP, J Mech Behav Biomed 45 :183–192, 2015. Crossref, Web of Science, Google Scholar
41. Cattani-Lorente M, Scherrer SS, Durual S et al., Effect of different surface treatments on the hydrothermal degradation of a 3Y-TZP ceramic for dental implants, Dent Mater 30 :1136–1146, 2014. Crossref, Web of Science, Google Scholar
42. Lai X, Si WJ, Jiang DY et al., Effects of small-grit grinding and glazing on mechanical behaviors and ageing resistance of a super-translucent dental zirconia, J Dentist 66 :23–31, 2017. Crossref, Web of Science, Google Scholar
43. Pittayachawan P, McDonald A, Young A et al., Knowles, flexural strength, fatigue life, and stress-induced phase transformation study of Y-TZP dental ceramic, J Biomed Mater Res B Appl Biomater 88 :366–377, 2009. Crossref, Web of Science, Google Scholar
44. Kosmač T, Dakskobler A, Oblak Č et al., The strength and hydrothermal stability of Y-TZP ceramics for dental applications, Int J Appl Ceram Technol 40 :164–174, 2007. Crossref, Web of Science, Google Scholar
45. Kosmac T, Oblak C, Jevnikar P et al., Marion, Strength and reliability of surface treated Y-TZP dental ceramics, J Biomed Mater Res, 53 :304–313, 2000. Crossref, Web of Science, Google Scholar
46. Aboushelib HM, Effect of surface treatment on flexural strength of zirconia bars, J Prosthet Dent 104 :98–104, 2010. Crossref, Web of Science, Google Scholar
47. Pereira GKR, Silvestri T, Camargo R et al., Mechanical behavior of a Y-TZP ceramic for monolithic restorations: Effect of grinding and low-temperature aging, Mater Sci Eng 63 :70–77, 2016. Crossref, Web of Science, Google Scholar

Vol. 22, No. 08

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History

Received 10 February 2022

Accepted 28 May 2022

Published: 13 October 2022

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This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC BY) License which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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