Indentation size effect of Y-TZP dental ceramics
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 [1]. 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 (ZrO2), 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 [2], [3], [4], [5]. 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 [6].
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 [7]. 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 [8].
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 [9]. In this paper, the Vickers method was used.
The Vickers hardness HV is calculated according to the next expression:where F stands for applied load (N), d (mm) is the mean value of the indentation diagonals , 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 [9], [10]. It is known that surface preparation prior to hardness measurements (grinding and polishing) affects the hardness and fracture toughness values [11], [12]. Fracture toughness can be also determined by the indentation method, but the optimal load in the indentation fracture method is 196 and 294 N [13].
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 [9], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. It is also possible that material manifests an increase in hardness with the increasing indentation load, which is known as reverse ISE (RISE) [16]. Both phenomena are illustrated schematically in Fig. 1 [9]. 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.
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).
Section snippets
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
Results
The Vickers hardness was calculated according the Eq. (1). Obtained data show variations of calculated hardness with the applied load. Fig. 2 represents the change in calculated hardness values of investigated Y-TZP dental ceramics with the applied indentation load, all data are averages of thirty values; therefore error bars are plotted. Error bars characterize the accuracy. Both Y-TZP dental ceramics showed the decrease in hardness with the applied load increase, which is known as normal
Discussion
The Meyer's number n represents a measure of the ISE. Specifically, n < 2 indicates the normal ISE, e.g. the hardness decreases with the applied load increase, while the reverse ISE, or the increase in hardness with the applied load increase, is characterized by n > 2. If n = 2, the applied load has no influence on the hardness values. In other words, the measured values represent the “true” hardness values. Meyer's index values obtained by linear regression analysis showed that both investigated
Conclusion
Results presented in this paper show that investigated Y-TZP dental ceramics samples (with and without the addition of dental dye) undergo the normal indentation size effect, when measuring Vickers hardness. This means that measured hardness decreases with an increase in applied indentation load. Three different mathematical models were used for analysis of the normal indentation size effect: the Meyer's law, the proportional specimen resistance (PSR) and modified proportional specimen
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