André Luiz Molisani

Porosity dependence of elastic constants in aluminum nitride ceramics

Aluminum nitride is a relatively new engineering ceramic and its mechanical properties have not been extensively studied. In this work, the effects of porosity (~0 to 38%) on sonic velocities, Poissons ratio, and elastic moduli of partially sintered A1N, both pure and with additives, were investigated. The elastic constants, determined by the ultrasonic pulse-echo method, were significantly influenced by the porosity, but not by the different types of additives used (2 wt. (%) of CaCO3, CaO, and Y2O3). All elastic constants evaluated decreased with increasing porosity, but the rates of decrease were higher for elastic moduli compared do sonic velocities and Poissons ratio. The results were analyzed in the light of stress concentration and loading bearing area models proposed in the literature.

Mechanical Properties and Microstructure of Zinc Oxide Varistor Ceramics

ZnO varistors are nonlinear resistors used as surge arresters in power transmission and distribution for the protection of electronic devices. Electrical characteristics of these materials have been extensively studied, but their mechanical behavior is not completely understood. It has been suggested that the breakdown of ZnO varistors is related to microstructure heterogeneities and processing defects. These defects are the same that usually control the mechanical strength of ceramic materials. In this work, mechanical properties (flexural strength, fracture toughness, elastic constants, and hardness) of five commercial blocks of ZnO varistors (class I) from different producers were measured and correlated to their microstructure. Pore fraction and size significantly affected the flexural strength.

Development of Y-TZP Pre-Sintered Blocks for CAD-CAM Machining of Dental Prostheses

The aim of this work was to establish a route to produce pre-sintered blocks of Y-TZP (yttria stabilized tetragonal zirconia polycrystal) suitable to be machined in a commercial CADCAM system, used to manufacture crowns and bridge frameworks for dental applications. Two commercial Y-TZP powders were investigated. The powders were pressed with different compaction pressures (40 to 500 MPa) and pre-sintered at temperature ranging from 900 to 1100°C. Vickers hardness, biaxial flexural strength, and linear shrinkage could be correlated to the relative density of pre-sintered samples. Using an empirical equation, pre-sintered blocks with a pre-defined density could be prepared. The blocks withstood the machining in a CAD-CAM system, and the machined and sintered crowns presented good adaptation.

Synthesis of Potassium Titanate Fibers for Friction Materials

Potassium hexa and octatitanate fibers have been proposed as reinforcement for friction materials. The aim of this work was to establish a calcination route to produce these fibers, using commercial anatase and potassium carbonate powders. These powders were dry mixed with TiO2/K2O molar ratio, n, of 3.0, 3.5, and 4.0, and then calcined at 950, 1050, and 1150°C for 3 h. Calcined powders were milled, washed in warm water with different pHs, and heat treated to crystallize the fibers. The best conditions to growth long fibers were n=3.0 and 1050°C, in the twofase field (liquid + K2Ti4O9). Controlled ion-exchange with water removed K+ ions from K2Ti4O9 fibers resulting in potassium hexa or octatitanate fibers after the second heat-treatment. Fibers with sub-micrometer thickness (~0.6 μm) and average length of ~20 μm could be prepared.

Variables that Affect the Indentation Fracture Testing (IF) of a Dental Porcelain

The aim of this study was to investigate the influence of the indentation load, the period waste between indentation and measurement, and the gold coating on the fracture toughness (K Ic ) of a feldspatic porcelain (d.Sign, Ivoclar) measured by the Indentation Fracture Technique (IF). Porcelain beams (∼3x4x20mm 3 ) were fabricated by sintering green bodies at 870 C under vacuum with rapid heating and cooling rates. The samples presented microstructure composed of a glassy matrix, second phase particles and residual pores. Vickers hardness tester was used to produce indentation patterns on mirror polished surfaces. The diagonal of indentation (2a) and the length of crack (2c) were measured in an optical microscopy with better resolution than the hardness tester. The increase of indentation load from 0,2 to 5kg caused: i) increase of c/a ratio from 1,8 to 3,3; ii) almost no variation of Vickers hardness number; and iii) constancy of K Ic value from 0,3kg load. It was observed no influence of a gold coating on K Ic measurement. K Ic suffered significant reduction with time, mainly in the first hour.

Intermediate Oxide Layers for Direct Bonding of Copper (DBC) to Aluminum Nitride Ceramic Substrates

DBC is a process where copper foils are bonded to ceramic substrates for manufacturing hybrid electronic circuits and packages with high power-handling capabilities. For aluminum nitride (AlN) ceramics, a heat-treatment is required to grow an oxide layer to promote the bonding with copper. The oxidation treatment, however, must be conducted in special conditions to avoid the occurrence of severe cracking. In this work, an alternative method is proposed to form an intermediate oxide layers for DBC to AlN substrates. By this method, eutectic powder mixtures (CuO-CaO and CuO-Al2O3 systems) were applied to dense AlN substrates and then heat-treated at 1200 °C for 1 h in air. Different types of AlN ceramics sintered between 1650 and 1700 °C for 4 h in nitrogen atmosphere with additives of the system Y2O3-CaO-SrO-Li2O were investigated. The prepared oxide layers (thickness of ~25 m) presented good microstructural joining with the AlN substrates (characterized by SEM and EDS analysis), and did not affect significantly the thermal conductivity in the working temperature range of electronic devices (~100 to 50 W/m.K determined by laser flash method between 100 and 200 °C) compared to the AlN substrates.

Sintering of Commercial Leucite-Based Dental Porcelains

The aim of this work was to study the effect of temperature and heating rate on the densification of two leucite-based dental porcelains: one low-fusion and one high-fusion commercial leucite porcelains (Dentsply-Ceramco). Porcelain powders were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), particle size distribution, and helium picnometry. Bar samples were sintered from 650 to 1050oC, using heating rate of 55oC and 10oC/min. Sintered samples were characterized in terms of bulk density, measured by the Archimedes method in water, and fractured surface microstructure by scanning electron microscopy (SEM). The results show that densification increases with increasing temperature and the increase in heating rate has no effect on the densification of the porcelains studied.

Sintering Behavior of Aluminum Nitride Doped with Y2O3

The aim of this study was to investigate the sinterability of a commercial AIN powder doped with Y 2 O 3 . It was done by sintering several samples with different compositions in a graphite furnace using a nitrogen atmosphere. The effect of the sintering temperature (between 1500 and 1700 °C) was studied by isochronal heat treatments whilst its kinetics was studied by isothermal sintering treatments at 1600 °C for different times. Sintered samples were characterised by density measurements, secondary electron image analysis and energy dispersive spectroscopy in SEM, to investigate their microstructural evolution. Pure AIN samples showed low sinterability in both series of experiments, reaching maximum relative densities of approximately 60%, when they were sintered at 1600 °C for 8 h and 1700 °C for 1 h. Additions of Y 2 O 3 promoted the densification of AlN. However, the range of sintering temperatures utilised in this work was not enough to produce high density ceramics. The samples doped with Y 2 O 3 presented lower weight loss than pure AIN samples.