Laser micro-patterning of dental zirconia: effects on microstructure and reliability
- Autores: Erica Roitero
- Directores de la Tesis: Emilio Jiménez Piqué (dir. tes.), Frank T. Mücklich (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Diego Gómez García (presid.), Luis Llanes Pitarch (secret.), Rosa I. Merino (voc.)
- Programa de doctorado: Programa de Doctorado Erasmus Mundus en Ciencia e Ingeniería de Materiales Avanzados / Advanced Materials Science and Engineering por la Universidad Politécnica de Catalunya; Linköpings Universitet(Suecia); Luleå Tekniska Universitet(Suecia); Universität des Saarlandes(Alemania) y Université de Lorraine(Francia)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Tetragonal polycrystalline zirconia stabilized with 3% mol of yttria (3Y-TZP) is a popular bioceramic, increasingly used for dental applications thanks to its good mechanical properties, biocompatibility and aesthetic outcome. The addition of stabilizers oxides retains the metastable tetragonal structure, which can transform to the stable monoclinic form upon mechanical or chemical solicitation with its associated volume increment. If the phase transformation is induced by high stresses at the tip of an advancing crack, transformation toughening mechanisms are activated hindering further propagation of the crack and making this material more tough and damage tolerant. On the other hand, when 3Y-TZP is exposed to humid environment at moderate temperatures water species diffuse inside the tetragonal lattice triggering progressive phase transformation and the consequent degradation of the surface. This is known as low temperature degradation (LTD), hydrothermal degradation or aging and is one of the main drawbacks of this family of materials, especially for biomedical applications.
There has been a growing interest in modifying the surface topography in order to influence the biological response to these biomaterials. Among the available methods, laser patterning is one of the most promising because it is a non-contact and therefore not-contaminating technique and it allows great precision. In particular, Direct Laser Interference Patterning (DLIP) allows patterning at the micrometric- and nanometric-scale in a single-step process. Laser beams interference creates a periodical distribution of intensity that produces the desired pattern on the surface. It has been successfully applied on metals, polymers and ceramics for different applications spanning from antimicrobial coatings to tribological applications. Its application to 3Y-TZP seems therefore promising but needs a thorough characterization in order to ensure the short and long term stability.
This work contributes to understand how nanosecond laser interference patterning modifies the topography and microstructure of 3Y-TZP and how these changes influence the integrity and reliability of the material after the laser treatment. DLIP in the nanosecond regime is a suitable technique to introduce periodical topography at the micrometric scale on the surface of 3Y-TZP. Laser-material interaction mainly results in thermal effects, producing the desired topography alteration thanks to material melting and liquid flow. Pattern geometry and overall surface roughness can be modified independently varying laser fluence and number of pulses. The surface treatment is not free of collateral damage due to thermal shock. Microcracking, recrystallization, residual stresses, phase transformation and texturization are produced on the treated surface. These microstructural modifications are concentrated in a thin layer of material (1 µm thick) and are not a concern for mechanical integrity of the treated material. However, pre-existing defects on the surface can interact with the laser beam, becoming larger critical defects that lower the overall mechanical resistance. Therefore, a good surface finish, ideally free of defects, would ensure an optimal outcome of the surface treatment. Finally, the LTD resistance is reduced by the laser treatment because of the monoclinic phase and residual stresses induced by thermal shock. An annealing treatment is capable of restoring the LTD resistance, even more than before the laser treatment thanks to the texture induced in the tetragonal phase.
Therefore, to ensure the good outcome of such laser patterning on 3Y-TZP a thermal treatment (at 1200°C for 1 hour) is recommended to ensure the long term reliability and a defect-free surface is recommendrecommended to ensure the long term reliability and a defect-free surface is advisable to reduce the detrimental effect of laser on mechanical properties.
