Horng-Hwa Lu

Effect of Y2O3 and Yb2O3 on the microstructure and mechanical properties of silicon nitride

The effect of Y2O3 and Yb2O3 on the microstructure and mechanical properties of silicon nitride was investigated. The effect of heat-treatment on the crystallization of grain boundary phases and on the high-temperature strength was also studied. Sinterability and α to β phase transformation of silicon nitride with Y2O3 were more pronounced than that with Yb2O3, for the same additive content. A wider grain size distribution and a larger average grain size were observed in the specimens with Y2O3. The specimens with Yb2O3 showed higher flexure strength, hardness and fracture toughness owing to a more frequent pullout of grains. Strength retention of silicon nitride at elevated temperatures was attributed to the crystallization and refractoriness of the grain boundary phases.

Microstructure in silicon nitride containing β-phase seeding: Part I

A proper powder preparation technique was used to develop elongated β–Si 3 N 4 particles as seeds from raw materials. The phase transformation and development of microstructure in Si 3 N 4 ceramics containing β–Si 3 N 4 seeds were investigated. The specimens of seeded silicon nitride had higher phase transformation rates than the specimens without seed. A core/shell structure was observed by transmission electron microscopy in seeded specimens owing to the difference in aluminum concentration in Si 3 N 4 grains. The misfit between the core and the shell was accommodated by interfacial dislocation that has a rotation character. The growth mode was epitaxial, although there was some compositional difference between the core and the shell. A relatively larger grain size and wider grain size distribution in seeded Si 3 N 4 specimens was due to the amount of β-phase seeds that could act as the nuclei, and the final modification of the microstructure was due to the coalescence process. The crack wake process characterized the mechanism of toughening.

Microstructure, fracture behavior and mechanical properties of TiN/Si3N4 composites

The grain morphology, microstructure, mechanical properties and fracture behavior of hot-pressed silicon nitride containing two different sizes of TiN particles were investigated in this study. No interfacial interactions were noticeable between TiN and Si3N4 up to a temperature of 1850 °C. The average aspect ratio and grain thickness of β-Si3N4 grains decreased slightly with the addition of TiN particles. The amplitude and frequency of the propagating path, and the crack deflection angles, increased with the content of large TiN particles. The toughening mechanisms in TiN/Si3N4 were attributed to the crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the β-Si3N4 matrix. The fracture toughness of Si3N4 was enhanced by 35% with the addition of 20% large TiN particles, hot-pressed at 1850 °C.

Reactive magnetron sputtering of indium tin oxide films on acrylics – morphology and bonding state

Abstract Indium tin oxide (ITO) films were deposited on acrylics by low temperature reactive magnetron sputtering. The effects of oxygen flow and bias voltage on the microstructure, surface morphology and bonding state of films were evaluated. In this investigation, X-ray photoelectron spectroscopy, X-ray diffraction, Atomic force microscope were used. It was found that the grain size of ITO films increased and surface roughness decreased with the increase of oxygen flow rate. The XPS spectra of In 3d and Sn 3d indicated that the oxygen flow had little effect on the binding energy of ITO films. The relative strength of O 2− II increased, while that of O 2− I decreased with increasing oxygen flow rate. The grain size increased with the bias voltage. However, at a maximum voltage of −90 V fine grains were detected due to the formation of numerous nuclei resulting from bombardment of high energy particles. The bias voltage had little effect on the bonding state of In, Sn and O ions.

Fabrication of multilaminated Si3N4–Si3N4/TiN composites and its anisotropic fracture behavior

Laminated composites containing alternate layers of Si 3 N 4 and TiN/Si 3 N 4 materials were used as model material for investigating the crack behaviors and mechanical properties. Results indicated that both strength and toughness in laminated composites were higher than that of monolithic silicon nitride. The failure profiles were affected by the stored strain energy prior to failure and the stress gradient in each layer. Cracks deviated successively from one layer to the other due to periodic stress distribution. Samples with better strength and toughness also had a longer crack propagation path and higher amplitude of crack deviation. The periodic stress distribution in laminated composites was confirmed by the measurements of indentation crack length. Results also suggested a tensile stress in the Si 3 N 4 layer and compressive stress in the TiN/Si 3 N 4 layer, in directions normal to the free sample interface.

The interfacial reaction in Cr 3 C 2 /Al 2 O 3 composites

This study investigates the effects of sintering atmosphere and temperature on the interfacial properties of Cr 3 C 2 /Al 2 O 3 composites. Thermodynamic considerations and calculations with computer-assisted methods for the equilibrium compositions in the Al–O–Cr–C system were used to simulate the interfacial reaction in Cr 3 C 2 /Al 2 O 3 composite during sintering. The results were in good agreement with the experimental analysis. Cr 3 C 2 is more stable during sintering in a system with carbon due to the lower equilibrium oxygen partial pressure. Controlling CO and O 2 gas concentration, Cr 3 C 2 first oxidized, decarbonized, and then transformed to Cr 7 C 3 before reacting with Al 2 O 3 . An interfacial reaction between Cr 3 C 2 and Al 2 O 3 was not observed.

Effects of two-step sintering on the microstructure of Si3N4

Abstract The microstructure of two-step sintered silicon nitride was studied. An investigation was undertaken concentrating on the effects of microstructure developed after the initial stage sintering on the grain size distribution and aspect ratio in β-grains during the final stage of sintering. Experimental results indicated that the grain size increased, grain size distribution became wider, and more volume of large grains was developed with increase of presintering temperature and time. The aspect ratio could increase or decrease with presintering temperature, depending on the duration of the initial stage sintering.

Role of pore structure in the nitridation of silicon/silicon nitride compacts

Abstract The effects of the pore-size distribution in silicon compacts on the nitridation kinetics were investigated. Various compositions of silicon nitride inert filler materials were added to silicon compacts to control pore structure. Samples having a higher pore volume and a larger pore diameter reacted faster at temperatures below 1200 °C, the reaction started at lower temperatures and the samples showed more weight gain and less residual silicon. The pore value was reduced and the strength was improved by decreasing nitridation rate.

INVESTIGATION OF SI3N4-TIN/SI3N4-SI3N4 TRILAYER COMPOSITES WITH RESIDUAL SURFACE COMPRESSION

The present study involved the fabrication of three-layered composites consisting of outer layers that contained Si 3 N 4 and an inner layer that contained TiN in a Si 3 N 4 matrix. Surface compressive stresses were developed upon cooling due to the relatively higher thermal expansion coefficient (CTE) in the inner layer. The flexural strength of layered Si 3 N 4 composites was substantially greater than that of monolithic Si 3 N 4 . This was attributed to the surface compressive stress. The effects of TiN composition and inner layer thickness on the mechanical properties were investigated. Layered samples containing 20 vol.% TiN had lower flexural strength than Si 3 N 4 –10% TiN/Si 3 N 4 –Si 3 N 4 due to the formation of microcracks in the inner layer. Crack behaviors in layered samples were affected by the residual stress, interface, and free sample surface. Both theoretical and experimental results indicated that the strength and toughness of layered composites were substantially greater than those of monolithic materials. The determination of fracture toughness in three-layered materials by the surface indentation technique should be done carefully due to the influence of residual stress.

The effects of atmosphere on the thermal debinding of injection moulded Si3N4 components

Abstract The thermal debinding of injection moulded silicon nitride components under nitrogen, air, and N 2 + 5% H 2 atmospheres at various temperatures was investigated. The softening points and bonding structure of binders, pore size distribution, density and strength were studied. Results indicated that a slow binder removing rate and control debinding atmosphere are essential to the sintered properties.