Tomography of indentation cracks in feldspathic dental porcelain on zirconia

Comparison between different fracture toughness techniques in zirconia dental ceramics

Vickers indentation (IF) and single-edge-V-notched beam (SEVNB), to measure the fracture toughness (K_IC ) of zirconia-based dental ceramics and mathematical models were proposed to establish a correlation between both. Zirconia (ZrO₂ ) stabilized with 3 mol. % of Y₂ O₃ (3Y-TZP) and 5 mol% of Y₂ O₃ (5Y-PSZ) were compacted (n = 42) and sintered for 2 h at different temperatures (1475°C, 1500°C, 1550°C, or 1600°C). After sintering, they were characterized by relative density using the ASTM C373-88 standard, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The average grain size was measured according to the ASTM E1382-97 standard. The fracture toughness (K_IC ) was determined using two methods: Vickers indentation fracture toughness (K_IC-IF ): method based on mathematical modeling that considers the parameters used for the Vickers hardness test and Fracture toughness by the single-edge-V-notch-beam (K_IC-SEVNB ): method proposed by ISO 23146:08. The main phases of the 3Y-TZP and 5Y-PSZ ceramics were ZrO₂ -tetragonal and ZrO₂ -cubic, respectively. The 3Y-TZP specimens showed equiaxed grains with average grain sizes ranging from 0.55 to 0.79 μm. The grain sizes of 5Y-PSZ of specimens sintered at 1475°C and 1600°C were 0.62 and 2.32 μm, respectively. For all ceramics the crack size ratio was c/a < 2.5, suggesting a Palmqvist-type crack system. The fracture toughness measured by the Vickers indentation method (K_IC-IF ) and by the SEVNB method (K_IC-SEVNB ) was the same when the experimental data were fit to a mathematical model.

Residual stress estimated by nanoindentation in pontics and abutments of veneered zirconia fixed dental prostheses

Glass ceramics’ fractures in zirconia fixed dental prosthesis (FDP) remains a clinical challenge since it has higher fracture rates than the gold standard, metal ceramic FDP. Nanoindentation has been shown a reliable tool to determine residual stress of ceramic systems, which can ultimately correlate to failure-proneness.

Effect of handling material on mechanical and optical properties of feldspathic porcelain

OBJECTIVE

To evaluate the optical, chemical, and mechanical properties of feldspathic porcelain after handling with different instrument materials.

MATERIAL AND METHODS

Feldspathic porcelain was manipulated with different spatula compositions: metal spatula (MS), plastic spatula (PS), and glass spatula (GS) for the fabrication of 30 (n = 10) disks. Contrast ratio (CR), translucency parameter (TP), and surface Knoop microhardness (KHN) were measured. The color change (ΔE₀₀ ) was evaluated using the CIEDE2000 system. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used for surface analysis. Data were analyzed using one-way ANOVA and Tukey test (P < 0.05).

RESULTS

All groups presented different morphological surfaces with higher presence of Al on the MS. PS group presented lower Al, Si, K than MS, and GS. Higher CR was observed for PS (0.734; P < 0.043), followed by MS (0.696; P < 0.043) and GS (0.65; P < 0.011). The highest TP (13.06) and KHN (386.27) were presented by GS (P < 0.001). MS and PS presented similar KHN results. The higher ΔE₀₀ were found for plastic/metal comparison. Also, the L^* values for the MS group (67.49) were lower than the other groups.

CONCLUSION

The use of metal spatula promoted higher color alteration during feldspathic porcelain manipulation than did the other materials. Handling with glass instrument promoted higher microhardness than other spatula materials.

CLINICAL SIGNIFICANCE

The effect of the material used for ceramic handling on feldspathic porcelain properties is often ignored. This study shows that the handling spatula material must be carefully chosen to avoid inadvertent changes to the feldspathic porcelain restoration.

Effects of Manufacturing and Finishing Techniques of Feldspathic Ceramics on Surface Topography, Biofilm Formation, and Cell Viability for Human Gingival Fibroblasts

Purpose:

Feldspathic ceramic restorations can be obtained by different techniques (stratification or computer-aided design/computer-aided manufacturing [CAD/CAM] blocks) and finishing procedures (polishing or glaze application). This study evaluated the effects of techniques and finishing procedures on surface properties, biofilm formation, and viability of human gingival fibroblasts (FMM-1) in contact with these materials.

Methods and Materials:

Ceramic specimens were obtained through a stratification technique (Vita VM9) and from CAD/CAM blocks (Vita Blocs Mark II; both Vita Zahnfabrik) and their surfaces were finished by polishing (ceramisté diamond rubbers + polishing paste; “p” subgroups) or glaze spray application + sintering (“g” subgroups). Roughness (Ra and RSm parameters) and surface free energy (SFE) were measured. Early biofilm formation of Streptococcus mutans, Streptococcus sanguinis, and Candida albicans was evaluated by counting colony-forming units (CFU). MTT (3-[4.5-dimethyl-thiazol-2-yl-]-2.5-diphenyl tetrazolium bromide) cytotoxicity test evaluated cellular viability for the growth of FMM-1 after 24 hours and seven days of contact. Scanning electron microscopy (SEM) and three-dimensional optical profilometry were performed to qualitatively analyze the surface. Data were analyzed by analysis of variance, Tukey test, and t-test (all α=0.05).

Results:

Polished samples presented lower roughness (Ra, p=0.015; RSm, p=0.049) and higher SFE (p=0.00). Streptococci had higher CFU in all groups, but the CFU of C albicans was lower for polished samples. Biofilm formation was influenced by the interaction of all factors (p=0.018), and the materials showed no cytotoxicity to FMM-1 growth.

Conclusions:

Polishing resulted in the lowest values for surface roughness and higher SFE values. Polished ceramics showed less C albicans adherence while the adherence of Streptococci was greater than C albicans in all conditions.

Effects of porcelain thickness on the flexural strength and crack propagation in a bilayered zirconia system

OBJECTIVE

This study evaluated the influence of porcelain (VM9, VITA Zahnfabrik, Germany) thickness on the flexural strength and crack propagation in bilayered zirconia systems (YZ, VITA Zahnfabrik, Germany).

MATERIAL AND METHODS

Thirty zirconia bars (20.0x4.0x1.0 mm) and six zirconia blocks (12.0x7.5x1.2 mm) were prepared and veneered with porcelain with different thickness: 1 mm, 2 mm, or 3 mm. The bars of each experimental group (n=10) were subjected to four-point flexural strength testing. In each ceramic block, a Vickers indentation was created under a load of 10 kgf for 10 seconds, for the propagation of cracks.

RESULTS

The results of flexural strength were evaluated by One-way ANOVA and Tukey's test, with a significance level of 5%. The factor "thickness of the porcelain" was statistically significant (p=0.001) and the l-mm group presented the highest values of flexural strength. The cracks were predominant among the bending specimens with 1 and 2 mm of porcelain, and catastrophic failures were found in 50% of 3-mm-thick porcelain. After the indentation of blocks, the most severe defects were observed in blocks with 3-mm-thick porcelain.

CONCLUSION

The smallest (1 mm) thickness of porcelain on the zirconia infrastructure presented higher values of flexural strength. Better resistance to defect propagation was observed near the porcelain/ zirconia interface for all groups. Higher flexural strength was found for a thinner porcelain layer in a bilayered zirconia system. The damage caused by a Vickers indentation near and far the interface with the zirconia shows that the stress profiles are different.

Indentation size effect of Y-TZP dental ceramics

OBJECTIVE

The purpose of this study was to investigate and analyze the indentation size effect (ISE) in Vickers hardness of monolithic yttria partially stabilized zirconia (Y-TZP) dental ceramics without and with the addition of dental dye A3. The ISE is analyzed using the Mayer law, a proportional specimen resistance (PSR) model and a modified proportional specimen resistance (MPSR) model.

METHODS

Two samples of Y-TZP dental ceramics, trade names BruxZir (provided by Glidewell Laboratories, CA, USA), were investigated. The first sample was polished Y-TZP and the second sample was polished Y-TZP with the addition of dental dye A3, by VITA Classical Shade Guide. The Vickers hardness was measured under the following loads: 0.49N, 0.98N, 1.96N, 4.90, 9.81N and 29.42N. Thirty indentations were made on each sample, under each load. Relationships between the applied load, F, and the resulting indentation size, d, have been analyzed by the Mayer law, the PSR model and the MPSR model.

RESULTS

The Meyer index (n) for both Y-TZP dental ceramics is less than 2, which indicates that hardness is dependent on test loads. The PSR model and the MPSR model were used to calculate "true" Vickers hardness or load-independent hardness.

SIGNIFICANCE

All applied mathematical models are suitable for the data analysis, which is confirmed with high correlation coefficients, but the best correlation between measured values and mathematical models was achieved with the MPSR model with a correlation coefficient of 0.9999.

Hertzian contact response and damage tolerance of dental ceramics

OBJECTIVES

To determine the contact response and damage tolerance or strength degradation of a range of dental CAD/CAM ceramic materials including novel polymer-infiltrated-ceramic-network (PICN) materials by means of spherical indentations at various loads and indenter radii.

METHODS

The seven tested materials included Mark II, PICN test materials 1 and 2, In-Ceram Alumina, VM 9, In-Ceram YZ (Vita Zahnfabrik, Bad Saeckingen, Germany) and IPS e.max CAD, (Ivoclar Vivadent, Schaan, Liechtenstein). To evaluate the damage tolerance and role of indenter size, indentations with tungsten carbide spheres (0.5mm and 1.25mm radius) were placed on bending bars with varying loads (1.96-1000N). The indented bending bars were subsequently loaded to fracture in three-point bending. The contact induced damage was analyzed by light microscopy (LM) and SEM. The spherical contact response was measured on polished surfaces.

RESULTS

The initial strengths for the individual materials were found to reduce above specific indentation loads, which were a function of the indenter radius. Employing a 0.5mm radius sphere resulted in the following strength degrading loads and ordering of materials: VM9 (98N)<MarkII-PICN1 (147N)<ICAlumina-e.maxCAD (300N)<PICN2-YTZP (500N). For the materials indented with the 1.25mm sphere, higher loads were required for the onset of strength degradation: VM9 (190N)<MarkII (300N)<PICN1 (400N)<e.maxCAD (500N)<ICAlumina (700N)<PICN2 (1000N)<YTZP (above 1000N). Two different damage modes were observed by SEM and LM - brittle cone cracking and plastic deformation. The PICN materials exhibited elastic-plastic behavior with creep. In contrast YTZP showed entirely elastic behavior upon loading with both spheres.

SIGNIFICANCE

This study aims to emulate the likely clinical behavior of contact loading by opposing cusps to dental restorative ceramic materials by utilizing spherical indentations at various loads and sphere diameters.