Master of Engineering Science
Mechanical and Materials Engineering
Yttria stabilized Tetragonal Zirconia Polycrystal (3Y-TZP) materials veneered with porcelain are preferred for clinical practice due to their bio-functionality, biocompatibility, and affordability. However, these materials are often subjected to premature clinical failures due to combination of subsurface flaws and tensile stresses within porcelains. This work focuses on performing a critical literature review and evaluating veneer characteristics, i.e. structural relaxation, to form a suitable explanation for the development of subsurface stresses. User material subroutines, UEXPAN and UTRS are developed, validated and integrated in ABAQUS finite element solver. With the use of validated models, it is shown that faster cooling rates and high veneer thickness results in higher subsurface tensile stresses due to lack of structural relaxation. Slow cooling rates and lower veneer thickness result in desired compressive stresses in the subsurface. This work shows that structural relaxation in veneer can be used for tailoring the desired stress for the extending lifespan of veneered 3Y-TZP dental restorations.
Summary for Lay Audience
Zirconia-based dental materials are widely used for dental restorations due to their excellent aesthetics, bio-functionality, biocompatibility, and affordability compared to other dental materials. Zirconia material is veneered with glassy porcelain to achieve a suitable aesthetic appearance. Unfortunately, clinically such veneered glassy porcelain suffers premature failures due to subsurface manufacturing defects and undesired stresses. These undesired stresses are induced during the last heat treatment conducted for glazing of the veneering dental porcelains.
Extensive work is performed in understanding and identifying various parameters that can potentially result in such premature failures. Nevertheless, the problem remains unresolved. One objective of this research is to conduct a critical literature review to identify all possible parameters and their effects on this material’s performance.
Dental porcelains exhibit glass-like behavior. Evaluation of glass characteristics is important for understanding how glass behaves when subjected to high temperature. Structural relaxation is one of the glass characteristics that is never explored in dental porcelains to evaluate its effect on the stress state. Understanding the effects of structural relaxation on the state of the stress is another objective of this research.
Finite element numerical method is used for analysis in this project. This type of analysis is a technique that uses a software to predict the behavior of varied type of physical problems in defined conditions. Basically, finite element method is a mathematical procedure used to calculate approximate solutions for complicated engineering problems by solving equations with the use of commercial softwares. This finite element analysis have shown that faster cooling rates and higher veneer thicknesses result in undesired subsurface tensile stresses due to lack of structural relaxation. Such undesired tensile stresses may lead to premature failure during clinical applications. It is shown that lower cooling rates and lower veneer thicknesses result in desired compressive stresses, which increases the resistance to surface defect.
By modelling the development of stress state with respect to structural relaxation, this research focuses on a promising way forward to improve the lifespan of dental porcelains veneered on zirconia.
Shelar, Prashant S., "A critical review and finite element study of structural relaxation in veneered 3Y-TZP dental structures" (2021). Electronic Thesis and Dissertation Repository. 7644.