Indentation size effect of Y-TZP dental ceramics

Investigation of indentation size effect and R-curve behaviour of Li2O-SiO2 and Li2O-2SiO2 glass ceramics

Indentation size effect (ISE) and R-curve behaviour of Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics are investigated using micro-indentation and indentation-strength (IS) techniques, respectively. Vickers micro-indentations were applied on both materials at the load of 0.10-19.6 N to determine the load influence on the measured hardness. For the IS-measured fracture toughness, the load ranged from 1.96 to 19.6 N. The hardness decreased with increasing load by 20% and 18% on Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics, respectively, indicating the ISE behaviour on both materials. The fracture toughness increased with the load by 27% and 59% on Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics, respectively, signifying the R-curve behaviour. The ISE behaviour of both materials was analysed using the Meyer's, Hays-Kendall (HK), proportional specimen resistance (PSR), Nix-Gao (NG), modified PSR (MPSR) and elastic plastic deformation (EPD) models while the R-curve behaviour was analysed by the fractional power law. The Meyer's index of both materials was less than 2, strongly confirming the ISE existence. The HK, PSR and NG models were only suitable to determine intrinsic Vickers hardness for Li₂O-2SiO₂ glass ceramic while the MPSR and EPD models were successful for both materials. The fractional power law gave higher R-curve steepness for Li₂O-2SiO₂ than Li₂O-SiO₂ glass ceramics. Also, material and brittleness indices predicted, respectively, higher quasi-plasticity and better machinability for Li₂O-2SiO₂ than Li₂O-SiO₂ glass ceramics indicating superior performance in the former to the latter. Finally, this study presents a new significant insight into the micro-mechanisms of fracture tolerance behaviour of these glass ceramics which is critical to their functional performance as structural ceramics.

Determination of Hardness and Fracture Toughness of Y-TZP Manufactured by Digital Light Processing through the Indentation Technique

Objective

The purpose of the study was to determine the hardness and fracture toughness of dental yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) manufactured by digital light processing (DLP) technology to study its clinical prospects.

Methods

The experimental group was DLP-manufactured zirconia, and the control group was milled zirconia. The hardness was investigated under a range of test loads (0.49 N, 0.98 N, 1.96 N, 4.90 N, 9.81 N, 29.42 N, 49.03 N, 98.07 N, and 196.1 N). Meyer's law was applied to describe the indentation size effect (ISE). Meanwhile, the PSR model and MPSR model were utilized to generate true hardness values. The cracks were observed to be induced by indentation under loads above 49.03 N, while the cracks showed the radial-median type under the load of 196.1 N, under which the fracture toughness was calculated.

Results

The true hardness of DLP-manufactured zirconia was 1189 HV based on the PSR model and 1193 HV based on the MPSR model, a bit lower than that of milled zirconia. The fracture toughness was 3.43 ± 0.29 MPa√m, which showed no statistical difference with the milled zirconia.

Conclusion

The dental zirconia manufactured by the DLP 3D printing technique is similar to that manufactured by the conventional milling process in hardness and fracture toughness, thus having a promising future of clinical use.

The effect of different aging protocols on the flexural strength and phase transformations of two monolithic zirconia ceramics

Introduction:

The aim of the present study was to investigate how different aging protocols can affect the flexural strength and phase transformations of yttrium-stabilized zirconia ceramics (Y-TZP) for monolithic restorations.

Materials and methods:

Bar-shaped specimens from two zirconia ceramics bars were divided into three groups: a. no treatment (c), b. aging in an autoclave (a), and c. thermal cycling (t). The flexural strength was determined by the 3-point bending test and statistical analysis was performed to determine significant differences ( p< 0.05). Weibull statistics was used to analyze the dispersion of strength values while surface microstructural analysis was performed through X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM).

Results:

Aging did not significantly affect the flexural strength but differences were recorded between the two groups, with group A presenting higher strength values and m-phase percentages.

Conclusions:

The observed differences between the two ceramics could be attributed to variations in composition and processing.

Mechanical and Surface Properties of Monolithic Zirconia

Purpose:

This study compared monolithic zirconia with conventional ones based on mean roughness (Ra), Vickers hardness (VHN), topography, transmittance, grain size, flexural strength (FS), Weibull modulus, and fractographic behavior.

Methods and Materials:

One monolithic (Prettau Zircon [PR group]) and two conventional (ICE Zirkon Transluzent [IZ group] and BloomZir [BL group]) zirconias were used. Specimens were tested using a profilometer, a microhardness tester, a scanning electron microscope, a spectrophotometer, and a Universal Testing Machine (EMIC DL 2000). Ra, VHN, grain size, and transmittance were analyzed using the Kruskal-Wallis test associated with Dunn test (α=0.05). FS was analyzed using one-way analysis of variance with the Tukey honestly significant difference test (α=0.05).

Results:

Means and standard deviations of roughness, after sintering (Ra, in μm) and VHN, were, respectively, 0.11 ± 0.01, 1452.16 ± 79.49, for the PR group; 0.12 ± 0.02, 1466.72 ± 91.76, for the IZ group; and 0.21 ± 0.08, 1516.06 ± 104.02, for the BL group. BL was statistically rougher (p&lt;0.01) than PR and IZ. Hardness was statistically similar (p=0.30) for all groups. Means and standard deviations of FS (in MPa) were 846.65 ± 81.97 for the PR group, 808.88 ± 117.99 for the IZ group, and 771.81 ± 114.43 for the BL group, with no statistical difference (p&gt;0.05). Weibull moduli were 12.47 for the PR group, 7.24 for the IZ group, and 6.31 for the BL group, with no statistical differences. The PR and BL groups had higher transmittance values and grain sizes than the IZ group (p&lt;0.05). Although the BL group had some fractures that originated in the center of the tensile surface, fractographic analyses showed the same fracture pattern.

Conclusions:

All tested zirconia showed similar VHN, and the monolithic zirconia had similar roughness compared to one of the conventional zirconias. In addition, the monolithic zirconia showed similar flexural strength and Weibull modulus compared to the others even though its mean grain size was larger. The total transmittance of monolithic zirconia was higher than only one of the conventional zirconias tested.