Influence of sandblasting on zirconia in restorative dentistry

Abstract

La utilización de circona tetragonal policristalina estabilizada con 3 % mol de itria (3Y-TZP) para fabricar coronas e implantes ha sufrido una fuerte expansión recientemente debido a las buenas propiedades mecánicas, estéticas y de biocompatibilidad que posee este material. La microestructura y composición exacta de 3Y-TZP son específicamente diseñadas por los fabricantes para adecuarse a las normativas existentes. Durante su procesamiento, las prótesis cerámicas son tratadas superficialmente por métodos como el arenado para mejorar su adhesión al cemento protésico y a la porcelana que cubre la pieza dental. Ahora bien, no todos los fabricantes dentales recomiendan el arenado de las coronas previamente a su implantación, ya que el arenado puede introducir defectos superficiales que pueden afectar la integridad estructural de la prótesis así como producir cambios cristalográficos en la superficie. A pesar de que el efecto de arenado en circona ya ha sido parcialmente estudiado, no se han considerado exhaustivamente muchos aspectos como la severidad de las condiciones de arenado, el efecto en las propiedades superficiales, el daño subsuperficial y los cambios de fase. Una comprensión detallada de estos aspectos es necesaria para escoger correctamente las condiciones de trabajo del arenado y para mejorar el diseño microestructural de estos materiales. Esto mejoraría la vida a largo plazo de los implantes cerámicos evitando o retrasando posibles fallos de la pieza. En este trabajo, se ha estudiado el efecto del arenado en 3Y-TZP con diferentes tamaños de grano bajo diferentes condiciones de arenado. También se ha estudiado el efecto de la adición de de nanotubos de carbono multicapa (MWCNT, 0.5-2 vol. %) a una matriz de 3Y-TZP. Una parte del trabajo ha consistido en el estudio de las propiedades mecánicas y de resistencia a la degradación hidrotérmica de 3Y-TZP nanométrica y de los nanocompuestos 3YTZPMWCNT con tamaño de grano nanométrico (90-150 nm) producidos por “spark plasma sintering”, los cuales se ha encontrado que poseen una menor tenacidad de fractura por indentación que 3Y-TZP con tamaño de grano de 300 nm. La adición de un 2% en volumen de MWCNT aumenta la tenacidad de fractura por indentación en alrededor de un 15% con respecto a la matriz del mismo tamaño de grano. El módulo de elasticidad apenas cambia mientras que la dureza disminuye ligeramente.

The use of tetragonal zirconia polycrystals (3Y-TZP) in dental restorations such as crowns and implants has recently increased attention due to their very good aesthetic appearance and mechanical properties in addition to biocompatibility. The restorations undergo several surface treatments such as sandblasting for better adhesion to luting cements and veneering porcelain. However, there is some controversy about using sandblasted crowns, as sandblasting introduces surface flaws and defects that can compromise the strength of the crown as well as crystallographic changes at the surface. Though the effect of sandblasting in zirconia has been previously studied to some extent, many issues like severity of the conditions, effect on surface mechanical properties, subsurface damage and phase transformation zone size have not been still fully addressed. Comprehensive understanding of these aspects will help in choosing better sandblasting conditions and also to improve the microstructural design of the materials for long term performance of the restorations so that clinical failures can be avoided or delayed.

In this thesis, the effect of sandblasting on 3 mol% yttria stabilized zirconia (3Y-TZP) with different grain sizes has been studied under different sandblasting conditions. Additionally, nanocomposites formed by adding multiwall carbon nanotubes (0.5-2 vol. %) to 3Y-TZP matrix have been also studied. Initially, the study has been focused in the mechanical properties and hydrothermal degradation resistance of nanometric grain size 3Y-TZP and zirconia multiwall carbon nanotubes nanocomposites (3YTZP-MWCNT). Nanometric grain size 3Y-TZP (90-150 nm) produced by spark plasma sintering have slightly lower toughness compared to standard zirconia with grain size 300 nm. Adding multiwall carbon nanotubes improve the indentation fracture toughness nearly 15% compared to monolithic materials. Elastic modulus hardly changes while hardness decreases slightly for 2 vol.% nanotubes.

The materials were subjected to sandblasting using two particle sizes, two pressures and two impact angles, After sandblasting the materials were analyzed looking for roughness, phase transformation and damage. In addition the change in mechanical properties and in hydrothermal degradation resistance induced by sandblasting was evaluated.

It has been found that increasing particle size and pressure increases surface roughness. The bi-axial strength of zirconia has been studied only in standard 300 nm grain size 3Y-TZP. The main result has been to show that at impact angle of 90º the biaxial strength increases when sandblasted with 110 um particles while it decreases with 250 um particles. On the other hand, the strength slightly increases when sandblasted under an impact angle of 30° irrespective of the particle size. By using nanoindentation it is shown that mild sandblasting conditions (110 um particle size, 2 bars pressure) have no effect on the surface mechanical properties such as, elastic modulus and contact hardness. A model based on the formation of residual compressive stresses is presented in order to explain the indentation the shorter length of the indentation cracks in sandblasted material as well as to rationalize the increase in strength of sandblasted material under mild sandblasted conditions.

The microstructural change induced by sandblasting near the surface consists of: i) a thin layer of plastically deformed grains; ii) phase transformation; and iii) occasional microcracking. The fraction of monoclinic volume fraction induced after sandblasting under the studied conditions is of about 10-15%, and with transformation up to a depth of about 10-13 um. Finally it is shown that in sandblasted conventional 3Y-TZP, the kinetics of hydrothermal degradation are slower than in the starting material.