Bimodal Powder Research Articles

Microstructure and phase stability of EB-PVD alumina and alumina/zirconia for thermal barrier coating applications

This paper describes recent progress on research aimed at improving the performance of EB-PVD thermal barrier coatings (TBCs) for gas turbine applications by incorporating alumina as an oxygen diffusion barrier between the bond coat and zirconia. Two approaches are being investigated, either a single, discrete alumina layer, or a graded alumina/zirconia layer. This paper reports on preliminary experiments in which alumina was evaporated under different conditions, and the phase content and morphology of the coatings were characterized. Deposition rate and chamber pressure had a significant effect on the microstructure of the coatings; however, phase formation was essentially unaffected. In agreement with previously published results, α-Al 2O 3 was produced either by in situ deposition on substrates heated to temperatures near 1000 °C, or by heat treatment of metastable as-deposited alumina coatings at temperatures above 1100 °C. By continuously changing the vaporization ratio of alumina and zirconia as a function of deposition time, TBCs exhibiting composition gradients through the film thickness were produced. In related work, suitable source materials for alumina deposition were produced using a new sinterless powder processing route involving bimodal powder mixtures.

Improvements in conventional PM parts with bimodal powder mixtures: X. Chen et al. (University of Albberta, Edmonton, Canada.)

Improvements in conventional PM parts with bimodal powder mixtures: X. Chen et al. (University of Albberta, Edmonton, Canada.)

Densification behavior of dynamically shock compacted AI2O3/ZrO2 powders synthesized through rapid solidification

The rapidly solidified alumina-zirconia eutectic contains high volume fractions of nanocrystallinet- ZrO2, which makes the material a promising precursor for the manufacturer of fracture-resistant ceramic specimens. Unfortunately, conventional powder processing and sintering techniques are inadequate for the fabrication of dense specimens using this material. We have used dynamic shock compaction to facilitate the achievement of high density specimens which retain the unique microstructure of the precursor material. In an attempt to quantify the dynamics of the microstructural evolution which occurs during the compaction process, we have investigated the effect of various particle size distributions on the densification behavior of the material during the shock compaction and postcompaction sintering cycles. The shock compaction process produced high densities (∼73 to 78 pct of single-crystal theoretical) by inducing a highly efficient packing of the particles. A bimodal powder distribution was also compacted and this specimen exhibited a relative density of 86.2 pct, approximately 10 pct higher that those of the unimodal compositions. In this compact, the small particles efficiently filled the interstices between the larger particles. The high density of the bimodal compact did not translate to a high sintered density, however.

Effect of Additives on Packing and Sintering of Bimodal Size-Distributed Alumina Powder Mixtures

Effect of Additives on Packing and Sintering of Bimodal Size-Distributed Alumina Powder Mixtures

Processing of nanocomposite silicon nitride-mullite-alumina by reaction sintering

We describe a new method for processing Si 3N 4mulliteAl 2O 3 nanocomposites by reaction sintering of the green compacts after partial oxidation treatment. Bimodal powder compacts used in this process were prepared by a colloidal filtration method. Si 3N 4 and Al 2O 3 powders were dispersed in aqueous suspensions at pH=10 with the aid of an electrosteric stabilizer, polymethacrylic acid. A binary mixture of Al 2O 3 powder was used to attain high green densities. This process results in the following morphology: nanometer-sized Si 3N 4 particles that are distributed in the mullite phase which is embedded in an Al 2O 3 matrix.

Influence of coarse particle shape on packing and sintering of bimodal size-distributed alumina powder mixtures

Influence of coarse particle shape on packing and sintering of bimodal size-distributed alumina powder mixtures

Predication of packing and sintered density for bimodal powder mixtures: R.M. German (Pennsylvania State University, USA)

Predication of packing and sintered density for bimodal powder mixtures: R.M. German (Pennsylvania State University, USA)

Sintering densification for powder mixtures of varying distribution widths

A model for the sintering of bimodal powders is used to evaluate densification results for log-normal particle size distributions involving a constant median size on a weight basis. The model predicts a minimum in densification for distributions of intermediate width when the green density is held constant. This has been observed experimentally for sintered tungsten powders. As a consequence, wide particle size distributions are attractive for high packing and sintering densities, indicating possible new feedstock formulations for shaping technologies such as powder injection molding and slip casting.

Prediction of sintered density for bimodal powder mixtures

Bimodal mixtures improve the green density of powder systems and are used in processes such as slip casting and powder injection molding. The packing density can be predicted with reasonable accuracy, but there is great uncertainty in the sintered density of a bimodal mixture. The large/small composition effect on packing density and sintered density is treated using the specific volume. For a given composition, the packing density is accurately predicted when four parameters are known: particle size ratio, packing density of the small powder, packing density of the large powder, and mixture homogeneity. Prediction of the sintered density is possible from knowledge of the densification of the large and small powders and mixture homogeneity. The model is applied to bimodal mixtures of molybdenum, stainless steel, iron, and alumina. Certain criteria must be satisfied by the constituent powders for a bimodal mixture to exhibit the highest sintered density. In many situations, the highest sintered density occurs at the 100 pct small powder composition.

Relation between percolation and particle coordination in binary powder mixtures

Deformation processing of composite materials is often affected by the percolation of inclusions, which may form a connective network supporting a significant part of the applied pressure and to hinder the densification of the matrix powder. By numerical simulation it was found that the critical volume fraction of inclusions required to form the first percolative cluster strongly depends on the inclusion to matrix particle size ratio. Moreover the fraction of inclusions belonging to a connective cluster is a size-ratio-insensitive function of the inclusion-inclusion coordination number. Using this correlation and an analytical model for particle coordination, a simple scheme is proposed to predict the percolation within bimodal powder mixtures. Interest of these results for understanding pressure transmission during processing of particulate composites is also discussed.

Grain growth kinetics and microstructure in the high Tc YBa2Cu3O7−δ superconductor

The grain growth and microstructure development of YBa2Cu3O7−δ have been investigated utilizing two different starting particle size distributions (normal and bimodal). The grain growth exponent, n, was found to be about 0.21 for both normal and bimodal samples. An activation energy of 125 kJ/mole was calculated. The low value of n might be attributed to the high anisotropy of grain boundary energy in this system. Samples made from the bimodal powder were found to accelerate grain growth without introducing abnormal grain growth. Although most of the samples attained fractional densities greater than 0.95, the presence of various amounts of porosity (particularly in the case of the bimodal starting powder) did not affect the growth kinetics. The measured aspect ratio of grains did not significantly change during growth. A significant difference in aspect ratio was measured between samples made from the two different starting powders. Critical currents ranged from 10 to 120 A/cm2, but no concrete relationship with grain size was established. This implies that the grains produced by this experiment were in the size range where other factors, presumably microcracking, severely limited the current carrying capacity by the weak link effect.

HIP of Powder Mixtures

ABSTRACTModels of pressure-sintering and consolidation (such as that found in the HIP process) often assume a uniform spherical powder. However, it is known that use of mixtures, containing particles of differing composition and/or size, alters the initial powder packing and subseguent HIPing behavior. We have experimentally investigated these effects using the simplest form of mixture; a bi-modal powder with various size ratios and particle fractions. Packing experiments confirm the existence of “optimum” packing fractions for which the density is maximized over both monosized and the as-received (continuously distributed) powders. During HIPing these powder mixes remain relatively more dense than monosize packings. The consolidation of composite powder mixtures as well as microstructural evolution during HIP are also discussed. Results are compared with those predicted by modeling.

Analysis of Sintering of a Composite with a Glass or Ceramic Matrix

A recently developed theory for sintering of bimodal powders is applied to a composite where the matrix is either a glass or a ceramic. The filler phase is assumed to be rigid and nonsintering. It is shown that glass‐matrix composites should have predictable sintering behavior which follows the rule of mixtures, but ceramic‐matrix composites are expected to be more difficult to sinter, and their sintering behavior may deviate considerably from the rule of mixtures, even at low concentrations of the filler phase.

Sintering behavior of bi-modal powder compacts

Time dependent sintering of a bi-modal powder compact, consisting of two regions which sinter at different rates, is analyzed in detail. Complete solutions are obtained for the internal stress and for the densification rate. The important feature of the analysis is that it combines densification with deviatoric creep, since creep serves to relax the stress concentration produced by incompatible densification. The structure of the bi-modal compact is assumed to consist of a sphere of one type of material embedded in a matrix of the other material. Two cases are considered. In one case the matrix, and in the other the sphere, is assumed to sinter faster. The maximum tensile stress generated by incompatible sintering is found to be sensitive to a parameter, β, which is the ratio of the rate constant for creep, and the rate constant for densification. A large value of β reduces the magnitude of the stress. The generation of flaws as a result of this stress is considered. The influence of inhomogeneous sintering on the overall rate of shrinkage is calculated; we find that unless β is large the densification rate of the composite will deviate significantly from the simple rule-of-mixtures.

Sintering of Bimodally Distributed Alumina Powders

The densification of alumina powders prepared to have a bi‐modal particle‐size distribution with a coarse‐to‐fine particle‐size ratio of ≊ 10 can be predicted to a first approximation if the densification of the fine and coarse powders is known. Deviations from model prediction were attributed to compositional heterogeneity. Microstructural observations of bimodal powders show that only the fine powder undergoes grain growth. Thus, the grain‐size distribution becomes more uniform than the initial particle‐size distribution.