1

Introduction

Yttria-partially-stabilized tetragonal zirconia polycrystalline ceramic (3Y-TZP) represents an important prosthodontic therapy option in aesthetically pleasing, all-ceramic dental restorations. Its main advantages are its chemical inertness, bio-compatibility and exceptionally high strength and fracture toughness. The last of these are attributable to its ability to undergo a martensitic phase transformation ( t-m ) of the thermodynamically metastable tetragonal grains into the stable monoclinic form as a result of mechanical stress, thereby developing transformation toughening .

The phase instability of 3Y-TZP ceramics, however, also has a disadvantageous side, known as low-temperature degradation (LTD), i.e., ageing. When the material is exposed to moisture at slightly elevated temperatures the tetragonal grains on the surface will start transforming spontaneously to the monoclinic phase . Due to the volume expansion associated with the t-m transformation, the process is accompanied by surface roughening, grain pull-outs and extensive micro-cracking, which may ultimately lead to strength degradation unless a small amount of alumina (0.25%) is added to decelerate the LTD , notably by strengthening the grain boundaries .

The moisture-induced t – m phase transformation progresses from the surface to the interior by a mechanism similar to the corrosion of metals, forming a degraded surface layer with a distinct boundary to the underlying, unaffected material .

The strong reduction of the elastic modulus and the hardness of the degraded surface were confirmed with a nano-indentation test . At higher indentation loads extensive micro-cracking appears along the impression edges, leading to chipping of the damaged area around the indentation .

LTD is temperature-dependent and proceeds most rapidly at temperatures between 200 and 300 °C . However, it has also been shown to occur at human body temperature at a much lower rate . The t – m phase transformation is also sensitive to the chemical and phase composition, microstructure, surface topography and testing conditions .

Nowadays, most forms of dental zirconia are of the same chemical composition (containing about 3 mol% of yttria in the solid solution), but they differ in their sintering temperature, which in turn has a direct influence on the grain size and thereby on the related stress-and/or moisture-induced transformability, ageing resistance and mechanical properties. In general, fine-grained 3Y-TZPs sintered at lower temperatures exhibit a superior ageing resistance and a higher strength, but a lower fracture toughness and damage tolerance, as compared to their coarser-grained counterparts, achieved by sintering at higher temperatures . The sintering conditions, in particular the firing temperature of dental 3Y-TZP ceramics, are starting-powder-specific and are generally recommended by the producer. Depending on the specific surface area, crystallite and aggregate sizes, the presence of dopants and the compaction ability of the starting powder used, the final sintering temperatures are typically between 1350 and 1550 °C . In restorative dentistry, finer grains are advocated not only for their higher initial strength and an enhanced ageing resistance, but also for the increased light transmittance achieved with grain sizes well below the wavelength of visible light , resulting in higher translucency and improved aesthetics. Conversely, in the case that superior damage tolerance is targeted, a higher-opacity, coarser-grained ceramic might be chosen.

After sintering, grinding and polishing are commonly required to adjust the occlusion, whereas airborne-particle abrasion is used to clean the cementation surface and to increase the surface area and improve the bonding with luting cements . Because 3Y-TZP ceramic exhibits a stress-induced t – m transformation (i.e., toughening), the ground or abraded surfaces will be constrained and also damaged. While rough ground surfaces are commonly fine ground and/or polished to remove the most heavily damaged upper layer, the airborne-particle-abraded surfaces are only cleaned in terms of dust and adhered debris. This process does not alter the characteristic topography of the abraded surfaces, showing extensive plastic deformation, sharp scars, (sub)surface cracks and welded airborne-particle debris . Regardless of whether it introduces surface flaws, airborne-particle abrasion generally increases the strength of 3Y-TZP ceramics, because, it appears that, the surface strengthening prevails over the mechanical damage . Furthermore, residual compressive stress and grain refinement were found to suppress LTD , which in turn should have relevance for extending the lifetime of zirconia-based dental restorations that are exposed to humidity and cyclic stressing under clinical conditions.

It has recently been demonstrated that the initial monotonic strength of mechanically damaged and/or hydrothermally treated dental 3Y-TZP ceramics provides a very good basis for identifying whether failure would occur during subsequent cyclic stressing . Since the initial strength is likely to be the major factor determining the survival of dental restorations, our study was designed to investigate the influence of airborne-particle abrasion and accelerated ageing on the strength of two sets of specimens of 3Y-TZP ceramics with the same nominal chemical composition, but differing in their grain size and transformability achieved by sintering at 1400 °C and 1500 °C. These two sintering temperatures were selected on the basis of the reported difference in the grain size, and the properties of the materials produced from the same powder grade .

2

Materials and methods

Biomedical grade 3Y-TZP powder (TZ-3YB-|E, Tosoh, Tokyo, Japan) was used to fabricate the materials used in this study. This powder grade is a commonly used raw material for the preparation of commercially available, pre-sintered zirconia blanks, containing 3 mol% of yttria in the solid solution and 0.25 wt.% of alumina addition intended to decelerate ageing. The powder is granulated and contains about 3 wt.% of an organic binder.

Disc-shaped specimens of 20 mm in diameter and 2 mm thick were uniaxially dry pressed at 150 MPa in a floating die and randomly divided into two groups of 90. One group was sintered at 1400 °C for 2 h and the other group was sintered at 1500 °C for 2 h. After sintering, the specimens were 15.80 ± 0.02 mm in diameter and 1.48 ± 0.04 mm thick. The grain-size evaluations were made on FE-SEM (Carl Zeiss, Supra 35LV, Oberkochen, Germany) micrographs using the linear-intercept method, based on the ASTM E112-13 standard without introducing any correction factors .

The relative density of both materials was determined with Archimedes’ method using distilled water as the immersion liquid. Even though the specimens were sintered in the two-phase ( t + c ) region, the relative densities were calculated by adopting a theoretical density of ρ _T = 6.08 g/cm ³ for the tetragonal phase. The sintered discs were divided into 18 groups of 10 samples (labelled with the letters A to S) and subjected to the fully crossed experimental protocol of airborne-particle abrasion and accelerated ageing, as presented in Table 1 .

The airborne-particle abrasion was performed with 50-μm or 110-μm Al ₂ O ₃ in an air-abrasion unit (Basic IS, Renfert Dental, Hilzingen, Germany). During the procedure, the nozzle tip was moved evenly and perpendicularly across the disc surface for 20 s at a pre-determined distance of 20 mm and pressure of 2.5 bar. Only one side of the discs was abraded and the treated specimens were then ultrasonically cleaned in acetone for 10 min.

The specimens were aged in deionized water under isothermal conditions at 134 °C. Short-term (12 h) and long-term (48 h) ageing were performed.

Three randomly selected samples from each group were subjected to crystallographic phase analysis by X-ray diffraction (XRD) using Cu-Kα radiation over the range 25–40° 2 θ (X’Pert PRO X-ray diffractometer, PANalytical, Almelo, The Netherlands). The amount of transformed monoclinic zirconia ( Xm ) was estimated from the diffractograms using the Garvie and Nicholson formula .

The biaxial flexural strength was measured in a universal material-testing machine (Quasar 100, Galdabini, Cardano al Campo, Italy) using the piston-on-3-balls method. The crosshead speed was 1 mm/min. The airborne-particle-abraded specimens were loaded with the treated side under tension. The load (N) at fracture was recorded and the biaxial flexure strength was calculated in accordance with Wachtman et al. .

Statistical analyses were conducted with the statistical software package R 3.1.2 . Levene’s test was used to assess the homogeneity of variances and the three-way ANOVA was performed. As presented in Table 1 , the fixed factors were the sintering temperature (2 levels: 1400 °C or 1500 °C), the airborne-particle abrasion (3 levels: no abrasion, 50-μm particles, 110-μm particles) and the accelerated ageing regime (3 levels: no ageing, 12 h of accelerated ageing, 48 h of accelerated ageing). Statistically significant interactions were further interpreted with the function glht from the multcomp package by defining custom contrasts between the treatments. The p -values were adjusted with the Bonferroni correction method and the significance level was set to α = 0.05. Multiple comparisons with Tukey’s HSD test were used to further assess the differences among the mean values in flexural strength and monoclinic content across the treatment groups.

2

Materials and methods

Biomedical grade 3Y-TZP powder (TZ-3YB-|E, Tosoh, Tokyo, Japan) was used to fabricate the materials used in this study. This powder grade is a commonly used raw material for the preparation of commercially available, pre-sintered zirconia blanks, containing 3 mol% of yttria in the solid solution and 0.25 wt.% of alumina addition intended to decelerate ageing. The powder is granulated and contains about 3 wt.% of an organic binder.

Disc-shaped specimens of 20 mm in diameter and 2 mm thick were uniaxially dry pressed at 150 MPa in a floating die and randomly divided into two groups of 90. One group was sintered at 1400 °C for 2 h and the other group was sintered at 1500 °C for 2 h. After sintering, the specimens were 15.80 ± 0.02 mm in diameter and 1.48 ± 0.04 mm thick. The grain-size evaluations were made on FE-SEM (Carl Zeiss, Supra 35LV, Oberkochen, Germany) micrographs using the linear-intercept method, based on the ASTM E112-13 standard without introducing any correction factors .

The relative density of both materials was determined with Archimedes’ method using distilled water as the immersion liquid. Even though the specimens were sintered in the two-phase ( t + c ) region, the relative densities were calculated by adopting a theoretical density of ρ _T = 6.08 g/cm ³ for the tetragonal phase. The sintered discs were divided into 18 groups of 10 samples (labelled with the letters A to S) and subjected to the fully crossed experimental protocol of airborne-particle abrasion and accelerated ageing, as presented in Table 1 .

Table 1

Mean values and standard deviations for the flexural strength and the monoclinic content ( Xm ) in the treatment groups. Values marked with the same characters are not significantly different from each other (Tukey’s HSD test, α = 0.05). Lower-case letters are used for the strength data and upper-case letters are used for the Xm data.

Sintering temperature	Group	Airborne-particle abrasion	Accelerated ageing (134 °C)	Flexural strength in MPa (SD)	Xm in % (SD)
1400 °C	A (control)	–	–	1093.5 gh (129.26)	<1.0 F
	B	50 μm	–	1384.95 abcd (119.19)	3.0 EF (0.2)
	C	110 μm	–	1311.89 cdef (139.81)	3.9 EF (0.7)
	D	–	12 h	1118.68 fgh (179.91)	1.3 F (0.1)
	E	50 μm	12 h	1340.37 bcde (135.01)	7.1 EF (0.2)
	F	110 μm	12 h	1304.87 cdef (139.81)	8.2 EF (0.4)
	G	–	48 h	925.12 h (70.99)	31.0 BC (11.6)
	H	50 μm	48 h	1064.99 gh (80.44)	28.1 BC (0.6)
	I	110 μm	48 h	1022.3 gh (126.02)	22.2 CD (0.5)
1500 °C	J (control)	–	–	1019.0 gh (99.80)	<1.0 F
	K	50 μm	–	1492.97 abc (120.23)	4.5 EF (0.2)
	L	110 μm	–	1558.07 a (150.57)	7.0 EF (0.2)
	M	–	12 h	1163.56 efg (100.69)	24.7 BCD (0.9)
	N	50 μm	12 h	1448.62 abc (205.86)	12.8 DEF (1.0)
	O	110 μm	12 h	1538.13 ab (119.18)	14.9 DE (1.0)
	P	–	48 h	1167.13 efg (55.69)	67.8 A (0.2)
	R	50 μm	48 h	1215.31d defg (115.40)	36.6 B (3.3)
	S	110 μm	48 h	1351.17 bcde (97.87)	36.9 B (0.3)

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