Helen Reveron

Towards long lasting zirconia-based composites for dental implants: Transformation induced plasticity and its consequence on ceramic reliability

Zirconia-based composites were developed through an innovative processing route able to tune compositional and microstructural features very precisely. Fully-dense ceria-stabilized zirconia ceramics (84vol% Ce-TZP) containing equiaxed alumina (8vol%Al2O3) and elongated strontium hexa-aluminate (8vol% SrAl12O19) second phases were obtained by conventional sintering. This work deals with the effect of the zirconia stabilization degree (CeO2 in the range 10.0-11.5mol%) on the transformability and mechanical properties of Ce-TZP-Al2O3-SrAl12O19 materials. Vickers hardness, biaxial flexural strength and Single-edge V-notched beam tests revealed a strong influence of ceria content on the mechanical properties. Composites with 11.0mol% CeO2 or above exhibited the classical behaviour of brittle ceramics, with no apparent plasticity and very low strain to failure. On the contrary, composites with 10.5mol% CeO2 or less showed large transformation-induced plasticity and almost no dispersion in strength data. Materials with 10.5mol% of ceria showed the highest values in terms of biaxial bending strength (up to 1.1GPa) and fracture toughness (>10MPa√m). In these ceramics, as zirconia transformation precedes failure, the Weibull modulus was exceptionally high and reached a value of 60, which is in the range typically reported for metals. The results achieved demonstrate the high potential of using these new strong, tough and stable zirconia-based composites in structural biomedical applications. STATEMENT OF SIGNIFICANCE Yttria-stabilized (Y-TZP) zirconia ceramics are increasingly used for developing metal-free restorations and dental implants. Despite their success related to their excellent mechanical resistance, Y-TZP can undergo Low Temperature Degradation which could be responsible for restoration damage or even worst the failure of the implant. Current research is focusing on strategies to improve the LTD resistance of Y-TZP or to develop alternative composites with better stability in vivo. In this work the mechanical characterization of a new type of very-stable zirconia-based composites is presented. These materials are composed of ceria-stabilized zirconia (84vol%Ce-TZP) containing two second phases (α-alumina and strontium hexa-aluminate) and exhibit exceptional strength, toughness and ductility, which may allow the processing of dental implants with a perfect reliability and longer lifetime.

Surface Coating of Oxide Powders: A New Synthesis Method to Process Biomedical Grade Nano-Composites

Composite and nanocomposite ceramics have achieved special interest in recent years when used for biomedical applications. They have demonstrated, in some cases, increased performance, reliability, and stability in vivo, with respect to pure monolithic ceramics. Current research aims at developing new compositions and architectures to further increase their properties. However, the ability to tailor the microstructure requires the careful control of all steps of manufacturing, from the synthesis of composite nanopowders, to their processing and sintering. This review aims at deepening understanding of the critical issues associated with the manufacturing of nanocomposite ceramics, focusing on the key role of the synthesis methods to develop homogeneous and tailored microstructures. In this frame, the authors have developed an innovative method, named “surface-coating process”, in which matrix oxide powders are coated with inorganic precursors of the second phase. The method is illustrated into two case studies; the former, on Zirconia Toughened Alumina (ZTA) materials for orthopedic applications, and the latter, on Zirconia-based composites for dental implants, discussing the advances and the potential of the method, which can become a valuable alternative to the current synthesis process already used at a clinical and industrial scale.

Effect of loading configuration on strength values in a highly transformable zirconia-based composite.

OBJECTIVES The aim of this work was to determine mechanical properties of a highly transformable 10Ce-TZP/Al2O3/La2AlO3 composite, currently developed as a biomaterial for dental application, and to investigate the effect of loading configuration on its flexural strength. METHODS Fracture toughness is determined by the single-edge-V-notched beam (SEVNB) method. Strength measurements were conducted by four-point bending and biaxial bending tests (piston-on-three balls) according to ISO 6872, dedicated to ceramic materials in dentistry. RESULTS Strength obtained by either four-point or biaxial bending are very different, and take the values of 596MPa and 1470MPa respectively. It is demonstrated that the difference in measured strength cannot be attributed to the effect of volume on strength, generally predicted by the standard Weibull analysis, but to different transformation behaviors for the two bending configurations. More extensive transformation occurs in the biaxial configuration, with a lower autocatalytic transformation stress threshold, resulting to substantial compressive residual stresses. SIGNIFICANCE The significant influence of the loading configuration on the strength should be integrated when designing a component from a highly transformable ceramic.

Design and development of dental ceramics: Examples of current innovations and future concepts

Abstract Ceramics for restorative dentistry should satisfy some major requirements: biocompatibility, esthetic properties, and mechanical performance, to ensure proper reliability and lifetime to the dental devices. By keeping in mind such requirements and related applications, this chapter aims to provide some guidelines for the design and development of innovative ceramics for the next generation of dental products and to indicate the direction for future developments. Three main strategies are illustrated here concerning the design of new dental restorative materials. The first strategy focuses on strong, tough, and stable zirconia-based ceramics, needed for dental parts and implants subjected to the highest stresses. The second is the biomimetic approach, in which the design of dental ceramics is inspired by the human tooth architecture and functions. Functionally graded ceramics as well as mineralized tissue-based hierarchical structures have recently been designed and developed to this aim. The third ambitious strategy relates to tissue engineering in dentistry, showing the most recent advances for regenerating the whole-tooth complex in a biological manner.

Design and Processing of Novel Ceramic Composite Structures for Use in Medical Surgery

. In order to fulfill the clinical requirements for strong, tough and stable ceramics for dental applications, we have designed and developed innovative Ceria-stabilized zirconia (Ce-TZP)-based composites. In particular, we have added two kinds of second phases to the Ce-TZP matrix: equiaxed a-Al2O3 grains, for increasing the hardness and the fracture strength, and elongated hexa-aluminates (both SrAl12O19 and CeMgAl11O19), to provide an additional toughening effect by crack deflection/bridging mechanisms. In order to carefully control the composition and the microstructure in those complex composite systems, we have used a novel surface-coating approach for the preparation of the nanostructured composite powders, which allows a perfect tailoring of the microstructural, morphological and compositional features of the composites. Once optimized the sintering cycle for each composite material, both composites reached full densification. Mechanical properties (Vickers hardness, flexural strength and fracture toughness) were evaluated, while the zirconia transformability was followed by means of an optical microscope during load-unload bending tests. The sensitivity to ageing was estimated by autoclave treatments. In spite of a remarkable different behavior – mainly in terms of stress-induced tetragonal to monoclinic zirconia transformability - both materials showed excellent mechanical properties as well as a negligible sensitivity to ageing, thus demonstrating their high potential for new reliable and safe devices for structural biomedical applications.