Comparison of subcritical growth parameters of a Y-TZP obtained via cyclic or dynamic fatigue tests.

OBJECTIVE: The objective of this study was to 1) compare the stress corrosion coefficient (n) of a Y-TZP obtained by two fatigue tests: cyclic and dynamic and 2) evaluate the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and slow crack growth. METHODS: A total of 145 Y-TZP specimens were produced in accordance with the manufacturer's instructions. These specimens, measuring 4.0 × 3.0 × 25.0 mm, were used for dynamic (n = 70) and cyclic fatigue tests (n = 75). The specimens were obtained from CAD/CAM blocks, sectioned, and sintered in a furnace at 1530 °C with a heating rate of 25 °C/min. They were tested in their "as-sintered" form without any additional surface treatment. The fatigue tests were conducted using a four-point bending to obtain the slow crack growth parameters (n). The cyclic fatigue test was also conducted in two frequencies (2 and 10 Hz), using stress levels between 350 and 600 MPa. Data from these tests were analyzed using ASTM C 1368-00 formulas and Weibull statistics. Scanning electron microscope (SEM) was used for fracture surface analysis to identify the origin of the fracture. Critical defect size was measured and used, along with flexural strength values, to estimate fracture toughness. Dynamic fatigue test data were used to obtain subcritical crack growth (SCG) parameters and perform Weibull statistical analysis. The cyclic fatigue data were used in the General Log-linear Model equation using the ALTA PRO software. Data were analyzed using one-way ANOVA followed by Tukey post-hoc tests and Student's t-test at a significance level of p ≤ 0.05. RESULTS: In the dynamic fatigue test, the values obtained for σfo and n were 667 and 54, respectively. This parameter indicates how the strength of the material diminishes over time due to internal cracks. The Weibull parameters obtained from the same test results were m = 7.9, σ0 = 968, 9 and σ5% = 767, which indicates the reliability of the material. The Weibull parameters obtained by cyclic fatigue were statistically similar for the two frequencies used, the m* was 0.17 (2 Hz) and 0.21 (10 Hz); characteristic lifetimes (η) were 1.93 × 106 and 40,768, respectively. The n values obtained by cyclic fatigue were 48 and 40 at frequencies of 2 and 10 Hz, respectively. There was no effect of the frequency, the stress level or the interaction of the two in the Y-TZP lifetime, when analysed by General Log Linear Model. SIGNIFICANCE: the n values obtained by cyclic and dynamic fatigue tests showed no statistically significant difference and the effect of frequency in the characteristic lifetime and the existence of interaction between the cyclic fatigue and subcritical growth were not observed in the tested specimens.

Effect of Nd:YAG and CO2 Laser Irradiation on Prevention of Enamel Demineralization in Orthodontics: In Vitro Study.

OBJECTIVE: The aim of this study was to investigate Nd:YAG and CO2 laser effects in the prevention of demineralization in deeper layers of enamel via successive acid challenge cycles. BACKGROUND DATA: Lasers are promising in the prevention of enamel demineralization around the orthodontic brackets; however, there are very few studies that evaluate if the effects of treatment could be extended after successive acid challenge cycles due to permanent enamel structural alterations. MATERIALS AND METHODS: Human enamel samples were divided into five groups (n = 12): G1-application of 1.23% acidulated fluoride phosphate gel (AFP, control); G2-Nd:YAG laser irradiation (0.6 W, 84.9 J/cm2, 10 Hz, 110 µs, contact mode); G3-Nd:YAG laser irradiation associated with AFP; G4-CO2 laser irradiation (0.5 W, 28.6 J/cm2, 50 Hz, 5 µs, and 10 mm focal distance); and G5-CO2 laser irradiation associated with AFP. The samples were submitted to successive acid challenge cycles. Quantitative light-induced fluorescence and scanning electron microscopy were used to assess enamel demineralization. The data were statistically compared (α = 5%). RESULTS: G1: 50.87 ± 4.57; G2: 47.72 ± 2.87; G3: 50.96 ± 4.01; G4: 28.21 ± 2.19; and G5: 30.13 ± 6.38. The CO2 laser groups had significantly lower mineral losses than those observed in all other groups after successive acid challenge cycles. CONCLUSIONS: Only the CO2 laser (10.6 µm) irradiation prevents enamel demineralization around the orthodontic brackets even after exposure to successive acid challenges. The CO2 laser at 10.6 µm showed a deeper effect in enamel regarding caries prevention.

Comportamento de fadiga e perfis de tensões residuais de cerâmicas odontológicas / Fatigue behavior and residual stress profiles of dental ceramics

Capítulo 1. Objetivos: Medir e comparar o perfil de tensão residual da cerâmica de recobrimento aplicada sobre infraestruturas em zircônia tetragonal policristalina estabilizada por ítria (Y-TZP), em um compósito de alumina/zircônia (ZTA) e em uma alumina policristalina (AL). Material e Métodos: Os perfis de tensão residual de cada um dos materiais foi medido por meio do método "hole-drilling" em discos de 19 mm de diâmetro e 2,2 mm de espessura (0,7 mm de infraestrutura + 1,5 mm de porcelana de cobertura) .Resultados: Os espécimes de AL exibiram tensões de compressão que aumentaram numericamente com a profundidade, enquanto que as tensões compressivas mudaram para tensões de tração no interior das amostras de Y-TZP. As amostras de ZTA exibiram tensões de compressão na superfície, decrescendo com a profundidade até 0,6 mm, tornando-se compressivas novamente próximo à infraestrutura. Conclusão: O ZTA não apresentou tensões de tração em nenhum ponto ao longo da espessura da cerâmica de recobrimento. A AL apresentou um perfil de tensão residual mais favorável, gerado pelas tensões compressivas. A Y-TZP apresentou o perfil mais desfavorável, por apresentar tensões de tração próximo à infraestrutura. Capítulo 2. Objetivos: 1) Comparar os coeficientes de susceptibilidade ao crescimento subcrítico (n), de uma zircônia tetragonal policristalina estabilizada por ítria, obtidos por meio de dois ensaios de fadiga: cíclica e dinâmica. 2) Verificar o efeito da frequência usada no ensaio de fadiga cíclica no tempo de vida característico. Material e Métodos: Espécimes em forma de barras (25 mm x 4 mm x 3 mm) foram confeccionadas de acordo com as recomendações do fabricante. Os ensaios de fadiga dinâmica (n=70) e de fadiga cíclica (n=75) foram realizados por meio de resistência à flexão em quatro pontos para a obtenção dos parâmetros de crescimento de trincas subcrítico (SCG)

O ensaio de fadiga cíclica também foi realizado em duas frequências (2 e 10 Hz), utilizando valores de tensão máxima entre 350 e 600 MPa. Todos os espécimes fraturados foram inspecionados em microscópio eletrônico de varredura para que fosse identificada a origem da fratura. Os dados de fadiga dinâmica foram analisados por meio das fórmulas apresentadas na ASTM C 1368-00. Os dados de fadiga cíclica foram analisados por meio da estatística de Weibull e por meio de análise tipo "General Log Linear Model". Resultados: A Y-TZP apresentou valores dos parâmetros de SCG obtidos no ensaio de fadiga dinâmica de ?fo = 667 MPa e n = 54. Os parâmetros de Weibull obtidos a partir dos resultados do mesmo ensaio foram de m = 7,9, ?0 = 968,9 MPa e ?5% = 767 MPa. Os parâmetros de Weibull obtidos no ensaio de fadiga cíclica foram estatisticamente semelhantes para as duas frequências utilizadas, o m* foi de 0,17 para 2Hz e de 0,21 10Hz; os tempos de vida característicos (?, em número de ciclos) foram de 1,93 x 106 e 40.768, respectivamente para 2 e 10 Hz. O valor de n obtido na fadiga cíclica foi de 48 e 40, nas frequências de 2 e 10 Hz, respectivamente. Não foi observado o efeito da frequência da aplicação da tensão, do nível de tensão, nem da interação dos dois no tempo de vida da Y-TZP, quando analisados pelo General Log Linear Model. Conclusão: Os coeficientes de susceptibilidade ao crescimento subcrítico (n) obtidos por meio de fadiga cíclica e dinâmica foram semelhantes para a Y-TZP testada. Os extensos intervalos de confiança obtidos nos ensaios de fadiga cíclica indicaram não haver efeito da frequência usado nesse método de fadiga nos tempos de vida característicos da Y-TZP testada

Chapter 1. Objective: was: to measure and compare the residual stress profile in the veneering ceramic layered on three different polycrystalline ceramic frame-work materials: Y-TZP, alumina polycrystalline (AL) and zirconia toughened alumina (ZTA).Materials and Methods. The stress profile was measured with the hole-drilling method in bilayered disk samples of 19 mm diameter with a 0.7 mm thick Y-TZP, AL or ZTA framework and a 1.5 mm thick layer of the corresponding veneering ceramic. Results: The AL samples exhibited increasing compressive stresses with depth, while compressive stresses switching into interior tensile stresses were measured in Y-TZP samples. ZTA samples exhibited compressive stress at the ceramic surface, decreasing with depth up to 0.6 mm from the surface, and then becoming compressive again near the framework. Conclusions: Y-TZP samples exhibited a less favorable stress profile than those of AL and ZTA samples. Chapter 2. Objectives: 1) To compare the stress corrosion coefficient (n) of a Y-TZP, obtained by two fatigue tests: cyclic and dynamic and 2) Evaluate the effect of frequency used in the cyclic fatigue test in the characteristic lifetime. Methods: Bar shaped specimens (4 mm x 25 mm x 3 mm) were prepared according to the manufacturer's recommendations. The dynamic fatigue test (n = 70) and cyclic fatigue (n = 75) were performed by four-point bending to obtain the slow crack growth parameters (n) .The cyclic fatigue test was also conducted in two frequencies (2 and 10Hz), using stress levels between 350 and 600 MPa. All fractured specimens were inspected by SEM so that the origin of the fracture were identified. The dynamic fatigue data were analyzed using the formulas given in ASTM C 1368-00

Cyclic fatigue data were analyzed using Weibull statistics and by the Log General Linear Model. Results: The material showed values of parameters obtained in the SCG assay dynamic fatigue ?fo = 667 and n = 54. The Weibull parameters obtained from the same test results were m = 7.9, ?0 = 968, 9 and ?5% = 767. The Weibull parameters obtained by cyclic fatigue were statistically similar for the two frequencies used, the m* was 0.17 (2 Hz) and 0.21 (10 Hz); characteristic lifetimes (?) were 1.93 x 106 and 40768, respectively

Effect of water storage and surface treatment on the microshear bond strength of different luting cements to zirconia.

This study sought to determine the microshear bond strength of 4 luting cements to zirconia. Zirconia substrates were divided randomly into 16 groups (n = 12), according to the luting cement, surface treatment, and storage time. Matrices were placed onto each zirconia substrate, filled with the cements, and photocured for 40 seconds. Microshear bond strength was determined after storage periods of 48 hours and 75 days; the results were analyzed by ANOVA and Tukey's test (α = 0.05). Surface treatment increased the bond strength to zirconia for 3 of the 4 cements tested. Even after a relatively long period of water storage, the results indicated that surface treatment did not significantly decrease the bond strength values for the same 3 cements.

Efeito do envelhecimento por ciclagem mecânica sobre a resistência à flexão de cerâmicas odontológicas / Effect of aging by mechanical cycling on the flexural strength of dental ceramics

Os objetivos deste trabalho foram: 1) avaliar o efeito do envelhecimento por ciclagem mecânica (1 milhão de ciclos a uma freqüência de 2 Hz) sobre a resistência à flexão biaxial de três materiais cerâmicos utilizados para a construção infra-estruturas de próteses parciais fixas: a) uma zircônia tetragonal policristalina estabilizada por ítria (Y-TZP); b) uma alumina policristalina (AL) e c) um compósito de alumina e zircônia infiltrado por vidro (ICZ) e 2) determinar a influência da tensão gerada durante a ciclagem sobre a degradação da resistência das cerâmicas estudadas, já que para cada material a ciclagem foi realizada com dois níveis diferentes de tensão. Material e método: Espécimes em forma de disco (12 mm x 2 mm e 12 mm x 1 mm) foram confeccionados conforme as recomendações dos fabricantes. A resistência à flexão desses materiais foi determinada por meio do ensaio de flexão biaxial. Para o grupo controle de todos os materiais estudados (espécimes de 1 mm de espessura), foi realizada estatística de Weibull para determinação da resistência característica (0) e módulo de Weibull (m).

Após o envelhecimento por ciclagem mecânica, realizado para todos os materiais nas duas espessuras, os espécimes foram fraturados e os valores obtidos foram comparados com seus respectivos grupos controle. Resultados: O valor de m foi estatisticamente semelhante para todos os materiais, o ICZ (12,2) apresentou o maior valor comparado ao do Y-TZP (9,0) e do AL (8,4). Os valores de resistência característica, 0, apresentaram diferenças significantes para todos os materiais, 828 MPa para a Y-TZP, 405,8 MPa para a AL e 328 MPa para o ICZ. Não houve diferenças estatisticamente significantes entre as resistências medidas para os grupos controle e ciclado para nenhum dos materiais nas duas espessuras avaliadas. Conclusão: O envelhecimento por ciclagem mecânica não causou alterações significativas nos valores de resistência à flexão de nenhuma das cerâmicas testadas. O aumento no nível de tensão aplicada na ciclagem mecânica não gerou um aumento na degradação da resistência dos materiais estudados.

The objectives of this study were: 1) To evaluate the effect of aging by mechanical cycling (1 million cycles at a frequency of 2 Hz) on the biaxial flexural strength of three dental ceramics used as framework materials for fixed partial dentures (FPDs): a) yttria partially stabilized zirconia tetragonal polycrystals (Y-TZP), b) alumina polycrystals (AL) and c) alumina-based zirconia-reinforced glass infiltrated ceramic (ICZ) and 2) determine the influence of stress level generated during mechanical cycling on the flexural strength degradation of the studied ceramics. Materials and Methods: Disc-shaped specimens (12 mm x 2 mm and 12 mm x 1 mm) were prepared according to manufacturer's recommendations. The flexural strength of these materials was determined by biaxial flexure test for the control group of all materials. The Weibull statistics was performed to determine the characteristic strength (0) and Weibull modulus (m). After mechanical aging, the specimens were fractured and the values obtained were compared with their respective control groups. Results: No statistically significant differences were founded between the strength obtained for the control and cycled groups for any of the materials in the two thicknesses tested. The m value was similar for all materials, ICZ (12.2) which showed the highest value, followed by the Y-TZP (9) and AL (8.4). The values for characteristic strength (0) showed significant differences for all materials, 828 MPa for Y-TZP, 405.8 MPa for AL and 328 MPa for ICZ. Conclusion: Aging by mechanical cycling did not cause significant changes in the values of flexural strength for all the ceramics tested. The increase in the stress level during cyclic loading did not cause an increase in the strength degradation of the materials studied.