Abstract
Objectives
The aim of this study was to evaluate the mechanical properties and the subcritical crack growth behavior of a presintered dental Y-TZP (Kavo Everest ZS) and a hot isostatic pressed Y-TZP (Kavo Everest ZH) and to perform life data analysis.
Methods
For each material 150 bending bars were produced. The initial fracture strength was determined in a four-point bending test. The subcritical crack growth parameters n and A were determined in a dynamic fatigue method at four decreasing loading rates from 110 MPa/s to 0.11 MPa/s in distilled water at 38 °C. For each loading rate Weibull statistics were performed and the Weibull moduli m and characteristic strengths σ 0 were calculated. Using these data, strength–fracture probability–life time (SPT) predictions were derived for 1 day, 1 year, 5 years and 10 years, based on a static crack growth mechanism.
Results
The “hipped” Y-TZP ceramic exhibited a higher initial strength ( σ c = 1618.18), characteristic strength ( σ 0 = 837.15) and fracture toughness ( K IC = 4.52 MPa/m 1/2 ) than the pre-sintered ceramic ( σ c = 1431, σ 0 = 745.46 and K IC = 3.17 MPa/m 1/2 , respectively). Fatigue parameters, n and A , were 28.5 and 7.97 × 10 −24 for Everest ZH and 30.15 and 5.47 × 10 −20 for Everest ZS. The predicted fracture stress at 5% failure probability for a lifetime of 10 years was 259.34 MPa for Everest ZH and 263.2 MPa for Everest ZS.
Conclusions
Although the “hipped” Y-TZP showed favorable initial mechanical properties, no significant difference could be found in the susceptibility of both ceramics to subcritical crack growth and their long-term strength.
1
Introduction
All-ceramic materials have established themselves as a competitive alternative to common metal–ceramic crowns and fixed partial dentures (FPD), due to their excellent esthetic properties and biocompatibility. The use of toughened oxide ceramics such as Y ttrium-oxide T etragonal Z irconia P olycrystals (Y-TZP) has widened the indication of ceramic materials, enabling their use for FPD supported by teeth or implants in the posterior region where high stress rates are expected.
The main reasons for the wide application field of Y-TZP as a core material for dental restorations are their exceptionally high mechanical reliability, including superior fracture toughness compared with conventional brittle ceramics. The improved toughness of yttrium-oxide partially stabilized zirconia ceramics results from the stress-induced transformation of metastable tetragonal zirconia particles . This transformation leads to the development of localized compressive stresses being generated around and at the crack tip preventing further crack propagation .
Different processing methods of Y-TZP have been proposed in order to affect the microstructure of the material and its aging resistance . Firstly hot isostatic pressing (HIP) of Y-TZP has been introduced as a method for reducing porosities and consists of the simultaneous application of external pressure and high temperature. During post-HIP, the density is increased up to an excess of 99% of the theoretical density, major processing flaws can be healed, and an improved microstructure is developed . Different studies have shown that hot isostatic pressing improves fracture strength, toughness and reliability of Y-TZP as well as the susceptibility to low temperature degradation than the unhipeded material .
Densely sintered Y-TZP is processed for dental purposes using CAD/CAM (computer-aided design/computer-aided manufacturing) system by means of milling of a Y-TZP block. During milling of densely sintered ceramic blanks, there also is the danger of surface and structural defects on the ceramic. Caused by diamond burs, these can also negatively impact the permanent strength of the ceramics . The hard machining of densely sintered Y-TZP with diamond burs is more time and labor intensive and also involves increased wear on the milling instruments. Mainly for this reasons dental restorations are mostly machined substractively by CAD/CAM from partially sintered ceramic blocks in an enlarged form and then are subsequently post-sintered in appropriate furnaces to reach the fully sintered stage.
The long-term stability of Y-TZP in a wet environment is limited by the subcritical crack growth (SCCG) under stress and the low temperature degradation (LTD), caused by the progressive spontaneous transformation of the tetragonal phase into monoclinic. Subcritical crack growth in dental ceramics is attributed to stress corrosion cracking in the corrosive oral environment . This phenomenon is caused by the combined effect of high stresses at the crack tip and the presence of water or body fluid molecules, reducing the surface energy at the crack tip and allowing the crack propagation at a load below K I = K IC (where K I is a stress intensity factor and K IC is the fracture toughness). That means that under appropriate conditions, cracks keep on growing for some time in a slow manner until the critical stress intensity factor is exceeded and catastrophic failure results without warning.
The stress intensity factor at the tip of a surface crack subjected to flexure is given by:
K = σ ⋅ a ⋅ Y
where σ is applied flexure stress, a is the depth of the critical flow and Y is a function of crack shape ( Y = π 1/2 for small crack lengths) .
Under such a driving force, the slow crack growth rate may be assumed to follow a power law relation :
v = d a d t = A ⋅ K I n
The crack growth parameters A and n are depending on the material, the temperature and the environment and can be determined by measuring the crack growth of macrocracks and relating it to the stress intensity factor (direct methods) or by indirect methods such as static and constant stress-rate tests .
The subcritical crack growth parameters n and A characterize the growth rate of flaws in ceramics. For constant stress the induced stress is constant, σ ( t ) = σ a and Eqs. (1) and (2) can be integrated to obtain time to failure:
t f = 2 ( n − 2 ) ⋅ A ⋅ Y 2 ⋅ K I C n − 2 ⋅ ( σ c n − 2 − σ a n − 2 )
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