Wilson Acchar

Enhancing the properties of ceramic products through mixture design and response surface analysis

In products consisting of several components, the physical and engineering properties are a function of the proportions of the components. In this work, statistical combinations of a three-component mixture were designed to obtain synergetic values of mechanical strength of a cement mixture, constituted by lime, fly ash and water. The response surface method using a mixture design of a constrained triangular surface was applied to analyse the data obtained. The results were very satisfactory for characterizing and predicting the fracture strength of hardened specimens as a function of the composition. It is concluded that the use of this mathematical procedure can be an important tool to help to understand the behaviour of these types of ceramic products.

Sintering behaviour of alumina–tungsten carbide composites

Abstract Alumina reinforced with tungsten carbide has been investigated as an alternative material for metalworking, combining resistance to high service temperatures and improved toughness. Pressureless sintered and hot-pressed Al 2 O 3 –WC composites were manufactured and characterised. The use of Y 2 O 3 as a sintering additive has also been evaluated. Additions of up to 30 wt.% WC resulted in limited grain boundary pinning and corresponding high densification. Although the addition of Y 2 O 3 improved sintering, the presence of a residual grain boundary phase (YAG) was harmful to the fracture toughness of the composites, as it affected the effectiveness of the crack deflection mechanism that takes place at the interfaces between Al 2 O 3 and WC grains. Hot-pressing resulted in hardness ∼17.5 GPa and fracture toughness ∼7 MPa m −1/2 , which is an improvement compared to alumina reinforced by other refractory carbides.

Thermal decomposition of illite

The effect of heat treatment on illite in air at temperatures ranging from 750 to 1150 °C was studied using the Mossbauer effect in 57Fe. The dependence of the Mossbauer parameters and relative percentage of the radiation absorption area was measured as a function of the firing temperature. The onset of thermal structural decomposition occurred at 800 °C. With rising temperature, the formation of hematite (Fe2O3) increased at the expense of the silicate mineral.

Incorporação de lama de mármore e granito em massas argilosas

A utilizacao de residuos industriais como aditivos na fabricacao de produtos cerâmicos vem despertando um crescente interesse dos pesquisadores nos ultimos anos e esta se tornando pratica comum. Este trabalho descreve a variacao do comportamento de uma argila utilizada numa industria de cerâmica vermelha, resultante de adicoes de uma lama de marmore e granito, tal como e produzida em uma industria de beneficiamento de pedras ornamentais do estado do Rio Grande do Norte. Misturas de argila e rejeito (10 - 50% em peso) foram compactadas uniaxialmente e sinterizadas a temperaturas entre 950 e 1150 °C. Os resultados obtidos por analise quimica e mineralogica (DRX e FRX), analises termicas (DTA, TG e dilatometria), medidas de distribuicao granulometrica, porosidade, absorcao de agua e tensao de ruptura a flexao, revelam que a lama de marmore e granito pode ser incorporada na massa de argila sem perda ou comprometimento das propriedades dos corpos sinterizados de cerâmica vermelha.

Laminated object manufacturing of LZSA glass‐ceramics

Purpose – This paper seeks to detail the fabrication of a glass‐ceramic substrate, based on the LiO2‐ZrO2‐SiO2‐Al2O3 (LZSA) system, by laminated object manufacturing (LOM) using water‐based cast tapes.Design/methodology/approach – Small amounts of ZrSiO4 were added to control the thermal expansion coefficient (TEC) of the original glass‐ceramic (LZSA5Zr: LZSA+5 wt% ZrSiO4). In order to verify the influence of the amount and nature of crystalline phases on the thermal and dielectric behavior of the material, LZSA and LZSA5Zr laminates were sintered at 700°C for 30 min and crystallized at either 800 or 850°C for 30 min.Findings – LZSA laminates (sintered and crystallized at 700 and 800°C, respectively) exhibited a relative density of ∼90 percent, a dielectric constant of 8.39, a dielectric loss tangent of 0.031 and TEC of 5.5×10−6 K−1 (25‐550°C). The addition of 5 wt% ZrSiO4 to original LZSA glass‐ceramics led to a nearly constant TEC value of 6×10−6 K−1 throughout the whole temperature interval (25‐800°C)....

Effect of Y2O3 addition on the densification and mechanical properties of alumina–niobium carbide composites

Alumina-based composites reinforced with refractory carbides are potential cutting tool materials. They exceed the capabilities of cemented carbides with respect to hot hardness and thermal stability, resulting in faster cutting speeds. Liquid-phase sintering of Al2O3–NbC composites was investigated as an alternative to pressure-assisted processes. Al2O3 reinforced by NbC (5–40 wt.%) was sintered with 3 wt.% Y2O3. In order to assess the effect of the formation of a liquid phase on the properties of the composites, sintering was carried out either below or above the Al2O3–Y2O3 eutectic temperature, at 1650 and 1800°C, respectively. Density, hardness, fracture toughness and wear resistance of the composite materials were evaluated. Liquid phase sintering did not affect the fracture toughness, but improved both the density and the hardness of the material, regardless of its NbC contents. Higher concentrations of NbC increased the wear resistance of the composite.

Reinforcing Al2O3 with W-Ti mixed carbide

Abstract Recent advances in high-speed cutting materials have focused on reinforcing alumina with different carbides and nitrides in order to improve hardness and fracture toughness. However, data on mixed carbides is still scarce. The potential use of this type of material for cutting tool applications has yet to be determined. The present study reports some preliminary results obtained reinforcing Al 2 O 3 with WTiC in the range of 5–30 wt.%. The material was hot-pressed at 1650°C for 30 min and then characterized. Vickers microhardness ( H V ) and fracture toughness ( K IC ) were evaluated by the indentation method. The addition of WTiC did not result in any significant change in the fracture toughness of alumina, however hardness values in excess of 22 GPa were obtained.

The increase of surface area of a Brazilian palygorskite clay activated with sulfuric acid solutions using a factorial design

Palygorskite is fibrous clay in which the structural tetrahedral and octahedral layers are organized in a way that structural channels are formed, leading to high surface area. However, impurities inside the channels and aggregated ones considerably reduce the available area. In order to increase the surface area, an activation treatment can be considered useful. The goal of this work is the activation of palygorskite from Guadalupe, Piaui, via sulfuric acid treatment using a two-level factorial design. The influence of three parameters (solution molarity, temperature and time) on BET surface area was determined. Moreover, samples were characterized via X-ray diffraction (XRD) and fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The largest surface area (282 m2/g) without considerable changes in clay structure and morphology was found in a sample treated with 5M H2SO4 at 70°C for 1h. The main parameters that favored the improvement of the surface area were the solutions molarity, temperature and their interaction.

Phase Transition behaviour of Tricalcium phosphate (TCP) doped with MgO and TiO2 as additives

In this study, the tricalcium phosphate [β-Ca3(PO4)2] behaviour using MgO and TiO2, respectively as additives has been investigated. The introduction of these additives is to control the phase transition of TCP during thermal process. The tricalcium phosphate (β,α) phases change when the temperature of sintering increase and/or during cooling down. To investigate the phase transition we examined tricalcium phosphate powder doped using 5 mol % of MgO or TiO2. The β-TCP and additives powders were mixture with acetone and dried using rota-vapour to eliminate all the solvent at 45°C for 3 h. After this procedure the powders were uniaxially pressed at 50 MPa and sintered in air-atmosphere at 1100°C to 1200°C for 5 h. All the sintered compacts were measurements by relative density, porosity, shrinkage, shrinkage rate, and the polished and fracture surfaces were investigated using a scanning electron microscope (SEM). To verify the transition phase the differential thermal analysis (DTA) and X-ray diffraction studies were carried out. The experimental results of relative density showed 92 % (MgO) and 90 % (TiO2) at 1200°C.

The influence of (Ti,W)C and NbC on the mechanical behavior of alumina

Recent studies published in the literature have focused on reinforcing alumina with different refractory carbides and nitrides in order to improve hardness, fracture toughness and wear resistance. The incorporation of hard particles as WC, (Ti,W)C and NbC on alumina matrix has shown to be a good alternative in improving the mechanical properties of the composite material. The present work reports some preliminary results obtained reinforcing alumina with 20 wt. (%) (Ti,W)C and 10 wt. (%) NbC. Alumina, (Ti,W)C and NbC powders were homogenized and mixed in a planetary ball mill and subsequently hot-pressed at 1650 °C under 20 MPa in flowing argon. Specimens were characterized by Vickers microhardness (HV), fracture toughness (KIC), X ray diffraction and scanning microscopy. The composite material showed hardness values of 19.5 GPa and fracture toughness values » 5.2 MPa.m1/2. The high fracture toughness encountered in this work is related to the crack deflection mechanism.