Zirconia with strength gradients

Multilayered zirconium oxide with strength gradients is the focus of this article. A research group at LMU Munich investigated the influence of different sintering protocols on the mechanical and optical properties in different layers. In addition, the fracture load of three-unit bridges was analyzed in the initial state and after thermomechanical aging.

Zirconia is available in various versions today. For example, there are multilayered zirconium oxides with color and translucency gradients as well as a progression of flexural strength. The first zirconium oxide materials, consisting of 3 mol% yttrium-stabilized tetragonal zirconium oxide polycrystals (3Y-TZP), conquered the dental market due to their mechanical strength. A redistribution of the aluminum oxide particles to the grain boundaries, combined with a reduction in the total amount of aluminum oxide from 0,25 to 0,05 wt.%, led to the desired improvement in the optical properties. The further pursuit of this goal led to the development of zirconium oxides with an increased proportion of yttrium oxide (5Y-TZP). However, the high aesthetic properties of this group were accompanied by significantly reduced mechanical properties (flexural strength: ~ 500 MPa). This called into question the central competitive advantage of zirconium oxide materials over highly aesthetic silicate-based ceramics. As a result, 4Y-TZP zirconium oxides were developed – all-rounders with the aim of combining optical and mechanical properties.

Various zirconium oxide and insight into materials science

Based on X-ray diffraction analyses, the crystal phases of zirconium oxide can be classified as 

• cubic (c-phase), 
• tetragonal prime (t' phase), 
• tetragonal (t-phase) or 
• monoclinic (m-phase) 

While 3Y-TZP materials are characterized by their ability to undergo tetragonal-monoclinic phase transformation, which causes a volume expansion of 3-5% and is responsible for the high fracture toughness of the materials, an increase in the yttria content led to the development of fully stabilized zirconia. With the non-transformable t' and c phases, materials with an yttria content ≥5 mol% are insensitive to hydrothermal aging.

The latest development in zirconia is multi-layer blanks, such as KATANA Zirconia YML (Kuraray Noritake), which comprises four different vertical layers. These blanks mimic the different aesthetic appearance of natural teeth, from the translucent incisal edge to the more opaque neck of the tooth. This color gradient can be achieved either by gradual doping with pigments (color gradient blanks) or by combining different zirconia compositions in one blank (strength gradient blanks). 

Aim of the investigation of multilayered zirconia

In order to optimally integrate the production of zirconium oxide restorations into the digital chairside workflow, the time factor plays a crucial role. Therefore, high-speed and speed sintering protocols were developed. These protocols use a shortened holding time, which is compensated by an increased heating rate and firing temperature. However, variations in these sintering parameters affect the properties of the zirconium oxide. The aim of this study was to investigate the influence of different zirconium oxide materials (3/4Y-TZP, 4Y-TZP, 3Y-TZP) and sintering parameters on the grain size, crystal phases, translucency and biaxial flexural strength of the zirconium oxide in four different layers. In addition, the fracture load of three-unit bridges was analyzed both in the initial state and after thermomechanical aging.

material and methods 

Three zirconia materials (3/4Y-TZP, 4Y-TZP, 3Y-TZP) were sintered using high-speed, speed or conventional sintering protocols. Disc-shaped specimens embedded in four vertical layers of the blank were tested for their 

• Grain size, 
• Crystal phases (cubic (c), tetragonal-prime (t'), tetragonal (t), monoclinic (m)), 
• Translucency and 
• biaxial bending strength 

In addition, the fracture load of three-unit bridges was determined both in the initial state and after thermomechanical aging. The thermomechanical aging included 1.200.000 mechanical cycles, with a load of 50 N and a frequency of 1,5 Hz, as well as 6.000 thermal cycles at temperatures of 5°C/55°C and a dwell time of 60 seconds (Fig. 1). The types of fracture were classified and the collected data were subjected to statistical analysis.

Results

4Y-TZP showed a higher proportion of c- and t'-phase and a lower proportion of t-phase compared to 3Y-TZP (Fig. 2). In addition, 4Y-TZP showed better optical properties but worse mechanical properties compared to 3Y-TZP. The translucency of all materials decreased from layer 1 to layer 4. The highest breaking load values ​​were measured for 3/4Y-TZP, followed by 3Y-TZP, while 4Y-TZP had the lowest breaking load. For 4Y-TZP, the sintering parameters directly influenced the grain size (Fig. 3) and the translucency, while the mechanical properties remained largely unchanged. The sintering parameters influenced 3Y-TZP in different ways. After thermomechanical aging, comparable or even higher breaking load values ​​were observed.

Conclusions for multilayer zirconia

The highest fracture load values ​​of 3/4Y-TZP in this study underline the effectiveness of using strength gradients in multi-layer blanks. This enables high optical properties in the area of ​​the incisal edge and at the same time ensures robust mechanical properties in the deeper areas on which tensile forces act. Even after thermomechanical loading, all groups exceeded the maximum chewing forces, which indicates promising stability of the three-unit bridges examined.