Indentation size effect of Y-TZP dental ceramics

Abstract

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: 0.49 N, 0.98 N, 1.96 N, 4.90, 9.81 N and 29.42 N. 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.

Introduction

Materials used in modern dentistry tend to be neutral, nanostructured, biocompatible and close to the tissue that is compensated. In addition, the tendency toward the production of single-phase materials should solve the problem of materials fracture and delamination. A closer understanding of the material dynamics with respect to design of the restoration and the intended use is required to enable these restorations to perform productively . Hardness is an important factor in the manufacture of prosthodontic crowns. This property provides crowns that can be thinner dimensions. This is especially important for oral rehabilitation where reduced occlusal relationship or where are extremely strong masticator forces. Example of such a material is zirconia or zirconium dioxide (ZrO 2 ), an inorganic metal oxide polycrystalline material. Zirconia has superior properties compared to other ceramics: high toughness, hardness, strength and Young’s modulus, biocompatibility, good wear resistance, good chemical and dimensional stability. Due its properties, zirconia based ceramic materials have received extensive interest in the past decades as important structural ceramic as well as biomedical materials like dental materials and implants . According to recommendation by the International Organization for Standardization (ISO, 1999), zirconia is the only dental ceramics that meets the requirements for flexural strength for fixed partial dentures .

Pure undoped zirconia is a polymorphic material that exists in three allotropes: monoclinic (m), tetragonal (t) and cubic (c). It can be transformed from one crystalline phase to another during firing. Pure zirconia at room temperature is monoclinic (m) and stable till 1170 °C. Between this temperature and 2370 °C, tetragonal (t) zirconia is formed, while cubic (c) zirconia is formed at temperatures above 2370 °C. Pure zirconia has a high melting point (2700 °C). After processing, and depending on the cooling process, the tetragonal phase becomes monoclinic at about 970 °C. This transformation is usually called a martensitic transformation and is affected by many factors and the volume expansion associated with this transformation is approximately 3–5%. This transformation is sufficient to exceed the elastic and fracture limits, resulting in cracks and flaws in ceramics. Finally, the result of this transformation is associated with an increase in volume of the grain . Generally, the increase in volume can be a problem of this material in practice, in connection with the zirconia and layer porcelain. This problem is trying to resolve with the production of monolithic zirconia crown in literature known like All-Y-TZP .

One of the important mechanical properties of ceramics materials is hardness. Hardness represents material resistance to plastic deformation. It is usually considered as non-destructive testing method, while it does not damage the specimen surface significantly. It is used very often, due to its general simplicity. When testing very hard materials, such as ceramics, it is common to use the Vickers ( HV ) or Knoop ( HK ) method, because their diamond pyramidal penetrators are able to leave visible indentations . In this paper, the Vickers method was used.

The Vickers hardness HV is calculated according to the next expression:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='HV=αFd2′>HV=αFd2HV=αFd2
H V = α F d 2

where F stands for applied load (N), d (mm) is the mean value of the indentation diagonals <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='(d=(d1+d2)/2)’>(d=(d1+d2)/2)(d=(d1+d2)/2)
( d = ( d 1 + d 2 ) / 2 )
, while α is the indenter’s geometrical constant, which equals 0.1891 for the Vickers diamond pyramid. In spite of its wide use, this method has few drawbacks, mainly related to the measurement uncertainty. Hardness values depend mostly on the surface quality of prepared sample, used hardness tester, as well as the influences of environmental and human factors. It is important to adequately prepare sample surface for hardness measuring by grinding and polishing, according to standard ceramographic procedure . It is known that surface preparation prior to hardness measurements (grinding and polishing) affects the hardness and fracture toughness values . Fracture toughness can be also determined by the indentation method, but the optimal load in the indentation fracture method is 196 and 294 N .

During the hardness testing, the applied load can influence the results. It means that the change in applied load causes changes in manifested (“apparent”) hardness, which is normally called indentation size effect (ISE). If material endures a decrease in hardness with the increasing load, this phenomenon is called normal ISE . It is also possible that material manifests an increase in hardness with the increasing indentation load, which is known as reverse ISE (RISE) . Both phenomena are illustrated schematically in Fig. 1 . This manifested, non-constant, hardness is often called “apparent” hardness, while the constant hardness, which usually occurs at higher indentation loads, is known as the “true” hardness or load-independent hardness.

Fig. 1
Schematic plot of hardness variation with test load, showing the indentation size effect (ISE) and reverse indentation size effect (RISE) .

The goal of this work is to analyze the observed ISE of two yttria-stabilized zirconia (Y-TZP) dental ceramics (without and with addition dental dye A3 by VITA Classical Shade Guide) using the traditional Meyer’s law, a proportional specimen resistance (PSR) model and a modified PSR model. We have expected that the A3 dental dye does not affect the Y-TZP ceramics “true” hardness (load independent hardness), but only gives a color. In order to confirm this hypothesis, we have conducted Vickers hardness measurements on Y-TZP samples with and without the addition of A3 dye, with the same surface finishing (polishing).

Materials and methods

All experiments were performed on two Y-TZP dental ceramics without and with addition of dental dye A3 by VITA Classical Shade Guide. Samples were provided by BruxZir, Glidewell Laboratories (Newport Beach, CA, USA). The chemical composition, according to the manufacturer’s declaration, of investigated Y-TZP dental ceramics is shown in Table 1 . The samples were made by the manufacturer, with dimensions of 10 mm × 10 mm × 2 mm. Sintering procedure was carried out by the usual standards of production of ceramic restorations by Glidewell Laboratories (Newport Beach, CA, USA). Samples were washed in distilled water in an ultrasonic bath and dried.

Table 1
Chemical composition of the Y-TZP dental ceramics expressed as weight percent (wt.%).
Sample wt.%
Y 2 O 3 HfO 2 Al 2 O 3 SiO 2 Fe 2 O 3 Na 2 O ZrO 2
Y-TZP ceramics 4.1 4.0 0.34 <0.01 <0.01 <0.01 Balance

The Vickers hardness was measured under the following loads: 0.49 N ( HV 0.05), 0.98 N ( HV 0.1), 1.96 N ( HV 0.2), 4.90 N ( HV 0.5), 9.81 N ( HV 1) and 29.42 N ( HV 3). The indentations were made under ambient laboratory conditions on the hardness tester Wilson-Wolpert Tukon 2100B (Instron, Grove City, PA, USA), while the diagonal was measured on an optical microscope Olympus BH (Olympus Imaging Corp., Tokyo, Japan), immediately after unloading. Thirty determinations for each load were performed on the both samples and the average value of diagonal lengths of the indentation for each load was used to calculate the hardness according to Eq. (1) .

Materials and methods

All experiments were performed on two Y-TZP dental ceramics without and with addition of dental dye A3 by VITA Classical Shade Guide. Samples were provided by BruxZir, Glidewell Laboratories (Newport Beach, CA, USA). The chemical composition, according to the manufacturer’s declaration, of investigated Y-TZP dental ceramics is shown in Table 1 . The samples were made by the manufacturer, with dimensions of 10 mm × 10 mm × 2 mm. Sintering procedure was carried out by the usual standards of production of ceramic restorations by Glidewell Laboratories (Newport Beach, CA, USA). Samples were washed in distilled water in an ultrasonic bath and dried.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Indentation size effect of Y-TZP dental ceramics
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