Free Sintering Research Articles

Sintering of mixtures of powders: Experiments and modelling

Powder metallurgy allows composite materials, notably metal–ceramic composites, to be produced by sintering a mixture of powders. This work analyses the sintering behaviour of metal powder mixed with ceramic inclusions. Experiments have been performed with mixtures of copper and alumina particles of about the same size and volume fractions of alumina between 5 and 30%. The densification kinetics of powder mixtures have been deduced from dilatometry measurement and the microstructure of sintered materials has been observed by scanning electron microscopy. The sintering process has also been modelled by using discrete element method. From both experimental and simulation results the effect of inert inclusions on the densification of metal-ceramic powder mixtures during free sintering is discussed. We found in particular that the densification was significantly delayed even for a volume fraction of inclusions as low as 5% and strongly hindered when this content got to 20%.

Influence of the processing conditions on the properties of the biferroic $\mbox{\sffamily\bfseries Pb(Fe}_{\hbox{\sffamily\bfseries\fontsize{12}{12}\selectfont 1/2}}{\mbox{\sffamily\bfseries Nb}}_{\hbox{\sffamily\bfseries\fontsize{12}{12}\selectfont 1/2}}{\mbox{\sffamily\bfseries )O}}_{\hbox{\sffamily\bfseries\fontsize{12}{12}\selectfont 3}}$ ceramics

The PbFe1 - xNbxO3 ferroelectromagnetic ceramic for x=0,5 (PFN) was obtained by one and two stage method and examined in the work. Synthesizing of ceramic powder was conducted as a result classic solid-state sintering methods (compacting method) and by powder calcinations method. Densification was carried out by free sintering and the hot uniaxial pressing method. The obtained specimens were subjected to microstructure and dielectric examinations and tests of direct current conductivity. The tests have shown that synthesizing a powder by a two stage columbite method through calcinations creates favorable conditions for obtaining the PbFe1 / 2Nb1 / 2O3 ceramic with the optimum applied parameters. PFN classified into smart materials can be used for building elements and components in electronic engineering and electrical engineering [1,2].

A further report on finite element analysis of sintering deformation using densification data—Error estimation and constrained sintering

In a previous paper, Kiani et al. [Kiani, S., Pan, J., Yeomans, J. A., Barriere, M. B. and Blanchart, P., Finite element analysis of sintering deformation using densification data instead of a constitutive law. J. Eur. Ceram. Soc., 2007, 27, 2377–2383] proposed an empirical numerical method to calculate the sintering deformation of ceramic powder compacts without knowing the viscosities and sintering potential. The method was validated by free sintering experiments using specimens with non-uniform initial densities. Two new developments are reported in this paper: (a) a method of error estimation is developed which can be used to check if the empirical analysis is valid after the analysis; (b) a range of case studies are presented showing that the empirical solutions provide very good approximations to the solutions obtained using full constitutive laws not only for free sintering but also for highly constrained sintering of single- or multi-layered films.

Experiments and Numerical Simulations on Sintering Process of Metal Injection Molded Components

To establish the constitutive model of sintering process,visco-plastic theory is adopted.The 316L stainless steel powder injection molded components are investigated as an example.The gravitational bending tests during sintering process and the free sintering experiments in dilatometer are realized to identify respectively the viscosity modulus and sintering stress in the constitutive model.The numerical simulations are realized via the finite element solver Abaqus by user subroutines for the prediction of shrinkage and deformation of product in the sintering process.The gravity,friction and powder-binder segregation resulted by injection process are taken into account for their effects on sintering process.The results obtained by simulations are consistent with the experimental ones.

Two-stage sintering inhibits abnormal grain growth during β to α transformation in SiC

Free sintering of SiC with Al, B, and C additions in two successive stages, first under nitrogen and then under argon, produced a near full-density ceramic with equiaxed grain structure. The β to α transformation proceeded to completion; however, the grain shape remained equiaxed due to the action of nitrogen present during the first stage of sintering. It is found that the β to α transformation is necessary but not sufficient for producing the microstructure of interlocking plates found in high-toughness SiC.

Improvement in the Temperature Stability of a BaTiO3‐Based Multilayer Ceramic Capacitor by Constrained Sintering

In this study, a self‐constrained BaTiO3 material system, composed of a low‐fire BaTiO3‐based X7R (ΔC/C±15% within −55° to 125°C) MLCC dielectric and a high‐fire, BaTiO3‐based X7R MLCC dielectrics eliminated sintering aid that are laminated on both sides of the BaTiO3‐based X7R MLCC, has been developed. The temperature dependence of capacitance of the BaTiO3‐based X7R MLCC is significantly improved to satisfy X8R requirements over the entire temperature range studied (ΔC/C±15% within –55° to 150°C) using the self‐constrained sintering. The curie temperature of BaTiO3‐based X7R MLCC increases on increasing the thickness of the constraining layers. Compared with the specimen fired by free sintering, a substantial reduction in grain size, leading to a decrease in dielectric constant, was observed in the specimen fired by constrained sintering with the thickness of the constraining layer being 170 μm. The increase in Curie temperature and decrease in grain size of the specimen fired by constrained sintering can be explained in terms of the presence of in‐plane tensile stress. The in‐plane tensile stress that introduces a friction force between the multilayer matrix and the nonshrinkage constraining layers to suppress the in‐plane shrinkage and to inhibit the rate of grain growth was formed during constrained sintering.

Effect of micro-structure on yield strength of porous solid: A comparative study of two simple cell models

An objective of this paper is to estimate an extent to which the yield strength of a powder compact is affected by its micro-structure. For this purpose, two opposite idealized structures of porous solid were investigated numerically. The first of them is a continuous matrix with isolated spherical pores simulating micro-geometry of the products obtained by the free sintering technology. The second one, being an array of spherical particles with isolated finite-sized contact spots, is typical for the powder compacts produced by cold pressing or intensive electro sintering. Their comparative study shows that the yield strength of porous solid is rather structure-sensitive and, therefore, the micro-structure parameters (and their evolution during consolidation) should be incorporated in the model in order to provide adequate description of their elastic–plastic behavior. It is found that behavior of both the considered materials can be described satisfactory by the Gurson–Tvergaard–Needleman (GTN) model. Numerical study shows that the coefficients entering the GTN-model are structure-dependent and, hence, can be effectively used as the internal state variables of porous solid. It opens a way of utilizing the available GTN-model based finite element software in computer simulation of the powder metallurgy technologies, with adequate account for micro-structure evolution of the consolidated powder compacts.

Nanostructured composite ceramic materials in the ZrO2-Al2O3 system

The composite ceramic materials based alumina and zirconia in the monoclinic form (10 wt %) or the tetragonal form stabilized by yttrium (7–45 wt %) are produced from metastable nanopowders through magnetic pulsed compaction followed by free sintering. The specific features revealed in the shrinkage are investigated at temperatures in the range 1400–1500°C. The development of the microstructure and polymorphic transformations are analyzed using X-ray diffraction and atomic-force microscopy. The fracture toughness and the microhardness of the samples are determined by the indentation technique.

Shrinkage Free Sintering of Low Temperature Cofired Ceramics by Glass Infilteration

The shrinkage variation of Low Temperature Cofired Ceramics(LTCC) limits the size of the substrates that impose limitations on embedded passive components. This paper focuses on the method of minimizing or controlling planar shrinkage and reducing distortion during firing. The laminated sheets of alumina and glass were sintered at varying temperature, and depending on the amount of the glass ceramics. When the sintered of multi-layer structure with Al2O3/Glass/Al2O3, the glass infiltrated entirely into Al2O3 layer at the temperature of about 950°C or higher.

Influence of Composition on Properties of BNT-BT Lead-Free Piezoceramics

Lead-free piezoelectric ceramics of (Bi 1/2Na 1/2)TiO 3-BaTiO 3(BNT-BT) were prepared by the conventional piezoelectric ceramic preparation technique (free air atmosphere sintering). The influence of BaTiO 3 additive amount and La 2O 3 additive amount on the properties of BNT-BT lead-free piezoceramics were investigated. The results show that the dielectric constant(ɛ) and piezoelectric strain constant ( d 33) of materials start increasing and then decreasing while BaTiO 3 additive amount increasing, the ɛ and d 33 of materials have maximum value (ɛ = 1650, d 33 = 120 PC · N −1) while x (BaTiO 3) = 0.06 mol. The ɛ and d 33 of materials start increasing and then decreasing while La 2O 3 additive amount increasing, the ɛ and d 33 of materials have maximum value (ɛ = 1684, d 33 = 153 PC · N −1) while w(La 2O 3) = 0.3%. The influence of La 2O 3 additive amount on the microstructure of BNT-BT piezoelectric ceramics was analysed by SEM (scanning electron microscope). The influence mechanism of La 2O 3 additive amount on the properties of BNT-BT piezoelectric ceramics was discussed. The BNT-BT ceramics with optimum comprehensive properties were obtained.

Simulations of the influence of rearrangement during sintering

A three-dimensional application of the discrete element method to free, constrained and pressure-assisted solid-state sintering of powders is presented. Special emphasis is laid on the generation of realistic random initial configuration as well as on the inertia effects. With this method, internal properties such as coordination number and pair correlation function are directly accessible and compare well with theoretical predictions and experimental data. The simulation scheme also enables one to investigate how grain rearrangements on a mesoscopic scale influence macroscopic properties such as densification rate and bulk and shear viscosity. Finally, crack formation during constrained sintering of low-density films is examined.

Rapid formation of α-sialon during spark plasma sintering: Its origin and implications

α-Sialons with varied chemical compositions and starting precursors were sintered in spark plasma sintering (SPS) furnace to verify the influence of electrical conduction, mechanical pressure and heat transfer efficiency. The internal temperature that the sample was exposed to and its evolution during the heating were experimentally estimated and discussed. XRD and SEM grain size analysis were carried out to evaluate the phase and microstructure evolution. Mechanical pressure was eliminated to create a free sintering condition under SPS. By free sintering and surrounding the sample with graphite wool the heat transfer was sufficiently retarded and a usual slow reaction was demonstrated. It is verified that the efficiency of heat transfer is more influential in the enhancement of α-sialon formation and grain growth by SPS than the efficiency of electric conduction.

Conventional Sintering Route for the Production of Alumina-Based Nanocomposites: A Microstructural Characterization

Two α-Al2O3/YAG composite powders have been prepared by reverse-strike precipitation, starting from chlorides aqueous solutions, the former containing 50 vol% of the two phases (labelled as AY50) and the latter made of 90 vol% of alumina and 10 vol% of YAG (AY90). The as-prepared powders were characterised by DTA/TG simultaneous analysis as well as by XRD analysis performed after calcination at different temperatures. A systematic TEM analysis was performed on AY50 powders pre-treated at different temperatures, in order to investigate the crystallites size evolution as a function of the temperature. After that, samples were compacted by uniaxial pressing and sintered at 1600°C for 3h. SEM observations revealed a homogeneous microstructure made of micronic alpha-alumina and YAG grains. For limiting grain growth through the decreasing of the maximum sintering temperature, an innovative activation procedure by coupling suitable thermal and mechanical treatments of the powders was performed. After that, high densification (>95% of the theoretical density) was easily achieved by performing a free sintering in the temperature range between 1320° and 1420°C, with different soaking times at the maximum temperature. The resulting sintered bodies showed an effective retention of the nano-size of the primary particles. By SEM, highly-homogeneous nanostructures, with an average grains size of about 200 and 300 nm for AY50 and AY90, respectively, were observed.

Discrete element modeling of metallic powder sintering

The free sintering of metallic powders is investigated by the discrete element method which allows the particulate nature of the material to be taken explicitly into account. A simple coarsening model is introduced and its use allows a favorable comparison of simulated densification rates with experimental data from the literature. The anisotropic shrinkage of numerical green compacts obtained from numerical close die compaction is also investigated.

Improvement in the accuracy of calculated interface morphologies within Monte Carlo simulations of sintering processes

Non-discrete energetic Monte Carlo methodology applied to two-dimensional constrained or free sintering of an infinite row of particles, showed the relevance of such a model for an accurate evaluation of the interface morphologies. Calculated interface morphologies for equilibrium and quasi-equilibrium states match perfectly those obtained using direct analytical methods.

Characterization of ZnO-Based Multilayer Varistor Sintered by Hot-Press Sintering

To attain the homogeneous microstructure of ZnO-based multilayer varistors (MLVs), hot-press sintering is conducted. To determine the effect of hot-press sintering on the microstructure, the dielectric and varistor properties of ZnO-based MLVs were investigated and compared with those of ZnO-based MLVs fabricated by free sintering. In hot-press sintering, the average grain size and the distribution of grain size of a ZnO-based MLVs are both reduced. The results show that the leakage current and nonlinear coefficient ( α) of ZnO-based MLVs can be improved. The increase in nonlinear coefficient ( α) is directly related to the inhibition of grain growth of ZnO grains when hot-press sintering is performed instead of free sintering. Note that the energy absorption capabilities in terms of peak current (PC) measurements of ZnO-based MLVs are remarkably improved due to the homogeneity of microstructure when hot-press sintering is conducted.

Accurate Sintering Morphologies FromMonte Carlo Modelling

Through the two-dimensional problems of the sintering of an infinite row and an infinite square arrangement of mono-sized particles, the relevance of the Monte Carlo methodology based on the non-discrete energetical model is pointed out to calculate equilibrium morphologies. In the absence of coarsening, and for the two cases of free and constrained sintering, the calculated stable morphologies are shown to precisely superimpose that obtained from analytical resolution. Therefore, an accurate drawing of surface curvatures and dihedral angles results from that methodology which is one of the required conditions to deal with more complex microstructures and to be able to compare the calculated microstructures with experiments.

Experimental and numerical analysis of the deformation of ferrite segments during sintering

The thermo-mechanical behaviour of hard ferrite powder compacts at high temperature is investigated with a view to simulating dimensional changes occurring during sintering of industrial components. Emphasis is laid on the anisotropy in both shrinkage and viscosity, which is induced by the prior pressing operation performed under magnetic field. Anisotropic shrinkage and viscosity are described according to an original approach based on an orthotropic thermo-elastic equation, which assumes an analogy between the thermal and elastic terms of this equation and the viscous and free sintering deformations. Finite element calculations using this constitutive equation are then carried out on two industrial parts. The comparison between the obtained numerical results and experimental measurements proves the relevance of the proposed model.

Constrained Sintering of Silver Circuit Paste

Densification kinetics and stress development during constrained sintering of a silver film on a rigid silicon substrate have been studied. Compared with free sintering, the sintering of constrained silver film exhibits a much lower densification and slower densification kinetics. The densification‐controlled mechanism changes from fast grain‐boundary diffusion kinetics for free sintering to slow lattice diffusion kinetics for constrained sintering. The in‐plane tensile stress developed during constrained sintering of silver film, measured using a noncontact laser‐scanning optical system, increases rapidly to a maximum level of 1.0–1.5 MPa initially, gradually decreases, and then becomes constant at 0.8–1.0 MPa. The maximum stress observed increases with increasing sintering temperature as a result of the faster densification rate. It is believed that the retardation of densification kinetics of constrained silver film is caused by a change in densification mechanism and the existence of in‐plane tensile stress.

Low-temperature sintering of BaZrO 3 and BaCeO 3 barrier materials

BaCe0.99Y0.01O3 – δ ceramics with a density over 95% was obtained by free sintering at 1000 °C using a combination of a fine precursor powder with a melt-forming sintering aid.