Rhombohedral Phase Formation in Yttria-Stabilized Zirconia Induced by Dental Technical Tools and Its Impact on Dental Applications

In the study the influence of different dental technical tools on the surface temperature and phase composition of fixed dental prostheses (FDPs) made of yttria-partially stabilized zirconia polycrystals (3Y-/4Y-/5Y-PSZ) was investigated. FDPs were fabricated by using computer-aided manufacturing (CAM). The FDPs were treated with a contra-angle handpiece equipped with different burs and polishers. The resulting surface temperatures were measured with a thermographic camera, and the resulting phase transformations were investigated by X-ray diffraction and quantified by Rietveld refinement. Processing with burs resulted in no phase transformation, but a preferred orientation shift. Using coarse polisher induced a phase transformation to the rhombohedral phase, while fine polishers produced no relevant phase transformations and no preferred orientation shift. Compared to the monoclinic phase (ca. 9% theoretical volume increase), which is associated with low-temperature degradation (LTD), the rhombohedral phase is much more voluminous (ca. 15% theoretical volume increase) and distorted and, therefore, has a greater degradation potential.


Introduction
In recent years, tooth-coloured materials such as ceramics and resin composites have become very popular for application in monolithic aesthetic dental restorations [1][2][3][4][5]. As yttria-partly stabilized zirconia (Y-PSZ), compared to other dental materials, features outstanding mechanical performance [6] (flexural strength between 750 and 1300 MPa [7,8]) and its optical properties such as translucency [9] are almost similar to enamel. The material is biologically inert [8], has a high X-ray opacity, AND can be used for a broad variety of dental restorations, including fixed dental prostheses (FDPs), primary telescopic crowns, dental implants, and implant superstructures [4,5]. The survival rates are very high and similar to metal alloys [10]. They ranged for zirconia implants from about 76% (first generation [11]) to 95% after one to seven years [12], for fixed complete dentures (FCDs) 100% after one to seven years [5]), and for frameworks 100% after ten years [13].
The translucency of Y-PSZ increases with a reduction in the grain size of the pressed powder, the porosity, and the aluminium content, and with an increase in the yttria content [14]. For the question of indication, the yttrium oxide content in particular has proven to be a decisive material parameter, as it can be used to control the phase composition and thus, above all the optical and mechanical properties.
Because of the resulting local volume expansion and the optical biaxiality, this aging phenomena (low temperature degradation; LTD) degrades the physical and optical properties of zirconia severely [15,18,19,21,28,29].
During cooling after sintering, the initially present tetragonal phase T separates into yttria-rich (C, T ) and yttria-lean (M, T) phases [16,17,30]. Because of the low yttria solubility of the monoclinic phase, only the yttria-lean tetragonal phase T transforms in the monoclinic phase M, while the yttria-rich tetragonal phase T and the cubic phase C are not transformable and, therefore, not subjected to low-temperature degradation [18,29].
Prosthetic treatments include the examination of aesthetic and functional parameters such as the marginal fit between crown and tooth, proximal contact, and static and dynamic occlusion [37]. In many cases, occlusal adjustments are necessary, as interferences in the occlusion may cause problems with teeth and the temporomandibular joint, which may ultimately impair quality of life [38]. Occlusal adjustments are commonly performed with fine diamond burs, which are followed by a polishing regime. Adequate polishing is particularly relevant in restorations fabricated from zirconia, as insufficiently polished zirconia surfaces may cause increased abrasion in antagonistic natural tooth tissues [39,40].
The study examined the influence of commonly used diamond burs and polishing equipment on the phase composition of fixed dental prostheses (FDPs) fabricated from 3Y-, 4Y-, and 5Y-PSZs. The boundary conditions necessary for the phase transformation (temperature distribution, mechanical effects) were determined experimentally. The results were compared to know phase transformation driven phenomena such as LTD.  To simulate dental adjustments in occlusion, coarse (842KR) and fine (8837KR) diamond burs (both from Komet Dental, Lemgo, Germany) were used with an EXPERTtorque Mini LUX E677 L contra-angle handpiece (KaVo, Biberach an der Riß, Deutschland). Subsequently, polishing was simulated using typical coarse (Cera Glaze P3032A) and fine (Cera Glaze P30032A) polishers (both by NTI, Khala, Germany) ( Table 3). The manual mechanical vertical load, which was applied to the bur/polisher while processing was analysed using a universal testing machine (Retro line, ZwickRoell, Ulm, Germany). The force was applied directly on the measurement spot (Table 3).

Thermographic Analysis
Processing with the burs and polishers was filmed using an MWIR A6700-InSb thermal imaging camera (Teledyne FLIR LLC, Wilsonville, Oregon, USA) with a thermal sensitivity of <18 mK. The dataset was analysed using ResearchIR 4.40.11 software (Teledyne FLIR LLC, Wilsonville, Oregon). The temperatures on the crowns were measured directly after removing the bur from the surface. For each measurement, two temperature ranges (77.4-216 and 146.6-323.6 • C for the burs and 20-98.1 • C and 146.6-216 • C for the polishers) were applied.

X-ray Diffraction (XRD)
The phase composition ( Figure 1) was analysed using a D8 Discover (Bruker AXS Advanced X-ray Solutions GmbH, Karlsruhe, Germany) X-ray diffractometer with a VÅNTEC-500 (Vantec Thermal Technologies, Fremont, CA, USA) area detector. CuKα radiation (λ = 1.5418 Å) and X-ray settings of 40 kV and 40 mA were used. For gathering data, the measurement setup described by Wertz et al. [47] was applied. The integration was carried out with the software DIFFRAC.EVA (Version 3.1; Bruker AXS Advanced X-ray Solutions GmbH, Karlsruhe, Germany).
Important reflections at~35 • ,~60 • , and~74 • in 2Θ (Figure 2a-c) were used for differentiating the tetragonal phase T from the tetragonal phase T and the cubic phase C.   The increasing surface roughness induced by treatment with dental technical tools was compensated with a surface roughness correction [51]. Texture effects were refined using a preferred orientation approach according to March-Dollase [52,53]. The resulting refinement was improved using optical and numerical parameters in an iterative process. Diamond 4 software (Version 4.6.5, Crystal Impact GbR, Bonn, Germany) was used to display the phases and to determine parameters such as the number of atoms per unit cell.

Results
In general, identical phase compositions were found between the manufacturers for the processing steps and the different yttria contents. Therefore, only changes which were detected for both manufactures are presented in detail in the sections. The phase composition of all measurements after Rietveld refinement are deposited in the Appendix A (Tables A1-A6).

Coarse and Fine Diamond Burs
The usage of all coarse and fine diamond burs induced no phase transformation, but a preferred orientation shift from 1 1 0 T/T to 0 0 1 T/T .          Comparing the temperature profile induced by treatment with the coarse and fine (Table 4) diamond burs, it is obvious that both treatments produced similarly high temperatures on the surfaces. Table 4. Maximum temperatures measured on the diamond bur and FDP surfaces.

Coarse and Fine Polisher
When coarse polishers ( Figure 6) were applied on all samples listed in Table 1, substantial formation of a rhombohedral (trigonal R) phase was induced. The fine polisher induced no relevant phase transformation (c.f. Tables A1-A6).       (highlighted parts of Figure 6).
The thermograms (Figure 8) gathered during treatments with coarse and fine polishers showed large temperature differences between the two treatments.    The temperatures observed on the surface of the FDPs and polishers were much higher during treatment with the coarse polisher than with the fine polisher (Table 5).

Discussion
Following the arguments of Wertz et al. [47], we have used a phase model for Rietveld refinement, which includes the two tetragonal phases T and T as well as cubic phase C, monoclinic phase M, and trigonal/rhombohedral phase R. In regard to the tetragonal phases, the authors showed that including both phases substantially improves the refinement.

Coarse and Fine Diamond Burs
The usage of diamond burs leads to a preferred orientation shift, but no phase transformation. In the current study, the fine polisher induced no such preferred orientation shift, probably because of its finer surface structure.
Since one stability mechanism in tetragonal zirconia includes a layered structure with strong (2.09-2.10 Å) Zr-O 1 bonds within and weak (2.33-2.35 Å) Zr-O 2 bonds between the layers [26,54], a preferred orientation shift (Figures 3 and 4) through coarse or fine diamond burs may change this layered structure (Figure 9).

Figure 7.
Comparison of the observed curves of the FDPs fabricated from 3Y_DD_4 (sintered, fine polisher; red) and 3Y_DD_5 (sintered, fine polisher; black) around 30° (a) and 50° (b) in 2Θ (highlighted parts of Figure 6).   . Schematic idealized illustration of grains with a preferred orientation of crystal structure and their impact on a diffractogram. The initially randomly oriented grains (left) are oriented towards 0 0 1 T/T (right). Since parts of the grains still have a preferred orientation towards 1 1 0 T/T , a 1 1 0 T/T reflection remains. It should be noted that reflections 1 1 0 T/T and 0 0 1 T/T are located in two different spatial planes, and therefore, cannot be represented in a 2D drawing.

Coarse and Fine Polisher
The coarse polisher induced the formation of a rhombohedral phase (R). Because of its metastability, the rhombohedral phase of zirconia has only rarely been reported. While it has been discussed [31,32,47,55] in other studies [20,56], typical reflections for the rhombohedral phase (R) were measured but not discussed.
The elementary cell (Table 6 and Figure 10) of the rhombohedral phase (R) features a 15% higher volume per atom ratio than the cubic (C) and tetragonal phases (T, T ) and a shear of 30 • (cf. monoclinic elementary cell: 4-5% [15] to 9% higher; 9 • shear [15]). Additionally, in the rhombohedral phase, every zirconium atom is bonded with only six oxygen atoms instead of eight atoms, as in the cubic and tetragonal phases (monoclinic phase: seven O-bondings per Zr-atom).  Cubic  8  12  133  11  Tetragonal  8  6  67  11  Monoclinic  7  12  144  12  Rhombohedral  6  57  748  13 Materials 2022, 15, x FOR PEER REVIEW 5 of 5 1 1 0T/T'' reflection remains. It should be noted that reflections 1 1 0T/T'' and 0 0 1T/T'' are located in two different spatial planes, and therefore, cannot be represented in a 2D drawing. Figure 10. Schematic and idealized illustration of the formation of the rhombohedral phase and its consequences at the grain and unit cell levels. Polishing with coarse polishers induced a phase transformation to the rhombohedral phase (above). In comparison to the transformation to the monoclinic phase, a higher volume expansion (values based on atoms per volume ratio; cf. Table 6) and shear occur (below).  Table 6) and shear occur (below).
When the FDP is heated after polishing in dental practice (e.g., for glace firing), a retransformation from the rhombohedral to the tetragonal phase would be possible, which may cause shrinkage and microstrain formation.
These effects may be stronger than the effects of the LTD-related monoclinic phase transformation because of the stronger distortion (Table 6), the higher volume expansion, and the larger shear of the rhombohedral phase.
Kitano et al. [31] indicated that a rhombohedral to monoclinic phase transformation may also be operative. Therefore, the rhombohedral phase may also be subjected to processes such as low-temperature degradation (LTD) and potentially promote them.
Only treatment with the coarse polisher resulted in the formation of the rhombohedral phase. Subsequent firing (e.g., glace firing or regeneration firing) could retransform the rhombohedral phase into a tetragonal or cubic phase, which may cause additional stresses by shrinkage phenomena. The different results can be explained by the higher temperatures induced by treatment with the coarse polisher (Table 5) and the entry of shear forces [46,57].
The current study is limited by the low thickness of the surface, which can be analysed by X-ray diffraction (XRD), the harsh loading conditions, and the limited number of dental technical tools (Table 3) we used. Therefore, the practical consequences on this the laboratory study must determine in further investigations.

Conclusions
Our experiments within the limitations of the study show some conditions of the formation of a rhombohedral phase fraction (15-30%) of 3/4/5Y-PSZ through polishing with dental handpieces.

1.
Polishing with coarse polishers can induce a partial rhombohedral phase transformation.

2.
All diamond burs used induce a break up of the layer structure of Y-PSZ and a subsequent preferred orientation shift to 0 0 1 T/T at the expense of 1 1 0 T/T , but no phase transformation.

3.
Treatment with fine polishers did not induce any relevant phase transformation.
Similar to the tetragonal to monoclinic phase transformation, the rhombohedral phase transformation induced a volume expansion (15% higher volume per atom ratio than the cubic or tetragonal phases) and shear (30 • ). Since only the coarse polisher induced this phase transformation, this indicates a metastable phase stability field, in which this phase transformation occurs.
Further studies should address the conditions and prevention of the formation of the rhombohedral phase, especially the combination of different polishers in practically used polishing sets. It is necessary to clarify the practical impact of the above observations on the material properties (mechanical and optical) and clinical performance of FDPs fabricated from Y-PSZs. Additionally, the stability of the rhombohedral phase against subsequent firings and low-temperature degradation should be investigated.