Densification Kinetics Research Articles

Sintering of 3D‐Printed Glass/HAp Composites

We report the sintering of 3D‐printed composites of 13‐93 bioactive glass and hydroxyapatite (HAp) powders. The sintering process is characterized on conventionally produced powder compacts with varying HAp content. A numeric approximation of the densification kinetics is then obtained on the basis of Frenkel, Mackenzie–Shuttleworth, and Einstein–Roscoe models, and optimized sintering conditions for 3D‐printed structures are derived. Fully isotropic sintering of complex cellular composites is obtained by continuous heating to 750°C at a rate of 2 K/min for a HAp content of 40 wt%. The approach can readily be generalized for printing and sintering of similar glass‐ceramic composites.

Densification Dynamics of Gadolinium-Doped Ceria upon Sintering

Densification Dynamics of Gadolinium-Doped Ceria upon Sintering

Densification Dynamics of Gadolinium-Doped Ceria upon Sintering

Densification behavior upon sintering is studied for gadolinium-doped ceria (GDC) by making use of X-ray diffraction, Archimedes method, high-resolution dilatometry (DLT), and element-specific positron annihilation spectroscopy. We found high concentration of vacancy-like nano-defects at GDC-crystallite interfaces participating in densification. Time-resolved length change and positron lifetime measurements enable to discuss the densification dynamics at the particle boundary relevant to a sintering neck and inside the particles. The particle boundary largely contributes to densification at the initial stage of sintering, whereas the crystallite interface gets to be responsible for prolonged densification. The densification inside the particle is developed by the growth of the crystallites followed by the transfer of Gd atoms from the interfaces to the crystallites.

Kinetic Analysis of Densification Behavior of Nano‐sized Tungsten Powder

The densification behavior of nano‐sized tungsten powder is examined using both nonisothermal and isothermal methods, and the kinetics of densification is evaluated using conventional sintering theories. The results show that nano‐sized powder, similar to micrometer‐sized powder, experiences three stages of sintering: initial, intermediate, and final. The initial densification of nano‐sized powder increased linearly with increasing time. The rate of densification accelerated after the compact reached above 50% relative density. The initial densification (when relative density <50%) was found to be the result of particle coarsening induced particle rearrangement. The apparent linear initial densification behavior is postulated to be due to the linear kinetic behavior of coarsening. Furthermore, surface diffusion can contribute to the initial densification of nano‐sized powder indirectly by being the primary mass transport mechanisms of the coarsening at the beginning of the sintering. The intermediate and final stages of nano sintering have similar densification behavior to those of micrometer‐sized powders, during which grain‐boundary diffusion is the main mechanism of densification.

Influence of particle size ratio on densification behaviour of AISI H13/AISI M3:2 powder mixture

Abstract The different densification kinetics of metal powders is one of the most important factors negatively affecting their possible co-sintering. This technological limit is further related to the relative fraction of the two powders, their shape and relative mean size. A decisive parameter, with this respect, is the so called particle size ratio defined as the ratio of the average particle size of the two powders. In this work the spark plasma co-sintering behaviour of an 80%vol AISI H13–20%vol AISI M3:2 blend is studied by considering nine different particle size ratios (dH13/dM3:2). The matrix of the relatively softer tool steel is strengthened by high speed steel. The highest values of density, hardness and fracture toughness are related to the smaller size of H13 particles (

Deformation and cracking during sintering of bimaterial components processed from ceramic and metal powder mixes. Part I: Experimental investigation

Parts composed of two ceramic–metal composite layers have been fabricated by co-sintering of two powder blends. The major constituent was the ceramic powder in one blend and the metal powder in the second one. This paper focuses on the mechanical analysis of the co-sintering process. This process has been observed thanks to an original optical dilatometry set up that provided images of the component in the course of sintering cycle. These images allowed following component shape changes throughout the thermal cycle and also evidenced the formation of cracks at the edges of a part at particular stages of the sintering cycle. These phenomena are interpreted from the mismatch between the densification kinetics of each powder blend sintered alone. In a companion paper the results of a finite element simulation of cosintering are compared to the experimental data displayed in the present paper.

Kinetics of nonisothermal hot pressing of boron carbide powder and its mixtures with alumina and additions of metallic aluminum

The paper proposes a method to determine the kinetic parameters of temperature-controlled hot pressing from the current temperature and height of specimens. This method substantially reduces the scope of experimental work in comparison with the conventional method for studying isothermal hot pressing. The kinetics of the densification of boron carbide powder and its composites with 20, 50, and 80 wt.% Al2O3 and 2 wt.% Al is studied during nonisothermal hot pressing. It is experimentally established that the kinetics of densification is controlled by the creep mechanisms in the matrix that forms a porous body. The creep activation energy for the matrix is estimated to be 8.1 eV for boron carbide, 7.6 eV for B4C + 20% Al2O3 + 2% Al, and 6.6 eV for B4C + 80% Al2O3 + 2% Al. The densification of the composite with 50 wt.% Al2O3 is controlled by the weekly temperature-dependent viscous flow of the matrix, for which the estimated activation energy is 1.7 eV. At higher temperatures, the viscous flow of the matrix is controlled by nonlinear creep the activation energy of 4.6 eV, which may correspond to superplasticity. The structure, phase constitution, and some mechanical properties of the composites are studied.

Influence of infiltration pressure on densification rate and microstructure of pyrocarbon during chemical vapor infiltration

Carbon/carbon composites were fabricated by chemical vapor infiltration of needled carbon fiber felts (70% porosity) with natural gas as precursor at different infiltration pressures. The thermodynamics of methane pyrolysis and the densification kinetics were used to investigate the influence of infiltration pressure on the infiltration rate and the microstructure of pyrocarbon. The microstructure of pyrocarbon was examined by polarized light microscopy. Results showed that the initial infiltration rate increased with pressure, whereas it decreased with pressure during the later stages of infiltration, which led to a decrease in the final density of the samples. The pyrocarbon microstructure depended markedly on the infiltration pressure at the given deposition temperature. Rough laminar (RL) carbon is formed as the matrix at the lowest pressure (approximately 1 kPa). At medium pressures (3, 5, and 10 kPa), isotropic (ISO) carbon is formed, followed by RL carbon, in the radial direction of the carbon fiber, with RL being the major matrix. Smooth laminar and ISO structures are the major phases at high pressures (15 kPa).

Low-temperature microwave sintering of TiN–SiC nanocomposites

Densification kinetics study during microwave sintering of titanium nitride-based nanocomposite has been conducted. A series of TiN–SiC compositions with 1, 3, 5 wt% of silicon carbide were microwave sintered at relatively low sintering temperatures (900–1,300 °C) for 0–30 min. The SiC content influenced on heating uniformity and final density and grain-size achieved. Densification process during microwave sintering obeyed the mechanism of grain-boundary diffusion with activation energy of 235 kJ mol−1. Microwave sintering resulted in fine microstructure (~300 nm) and hence high values of micro hardness (~20 GPa).

Spark Plasma co-Sintering of hot work and high speed steel powders for fabrication of a novel tool steel with composite microstructure

The capability to produce a hybrid tool steel with properties that can be modulated on the base of the specific application was investigated. Powder metallurgy (PM) offers the possibility to blend and co-sinter different powders to produce a hybrid material which combines the properties of the base materials. With the aim of producing a new steel with high hardness and good toughness, a hot work tool steel (HWTS) and a high speed steel (HSS) powder were selected. Four blends with different composition (HWTS-HSS: 20%–80%, 40%–60%, 60%–40%, 80%–20%) were produced by spark plasma sintering (SPS) and thermally treated. The influence of composition, particle size distribution and oxygen content was evaluated by the means of density, hardness and apparent fracture toughness of the blends. By selecting powders with a small size and narrow particles size distribution near full dense blends with good mechanical properties can be sintered. Large particles hinder the efficient co-sintering due to the different densification kinetics of the powders. High oxygen content in the base powders does not significantly influences the final density but negatively affects the consolidation process, strongly reducing toughness, particularly of the HWTS.

Synthesis of zirconium oxycarbide (ZrC xO y) powders: Influence of stoichiometry on densification kinetics during spark plasma sintering and on mechanical properties

Different stoichiometries of zirconium oxycarbide (ZrC x O y ) powders were synthesised by carboreduction to approach the role of the chemical composition on the densification behaviour during spark plasma sintering and on the mechanical properties. Chemical analyses were performed using complementary methods to determine the actual stoichiometry and homogeneity of powders. Evolution of relative density during SPS treatment and microstructure observations by SEM showed that oxygen and vacancy contents would enhance the densification kinetics by promoting the lattice diffusion of limiting species. A study of mechanical properties of SPS sintered specimens has highlighted their significant dependence on the oxycarbide stoichiometry, especially at high temperature.

The macroscopic model of the densification kinetics in the nonisothermic sintering of powders

The densification kinetics of a powder system has been described by a relatively simple analytical expression, which consists of only two dimensionless parameters. The first of them has been shown to be equal to the ratio between the maximum rate of the linear contraction and the total contraction of the sample multiplied by the time, for which the densification takes place; the second has been found to be equal to the ratio of the time required for the attainment of the maximum contraction velocity to the total densification time. The densification rate vs. time diagram is given by a curve having one or two maxima depending on values of the parameters entering the model. The study has been aimed at modeling the densification in sintering hard alloy mixtures.

Low temperature, transient liquid phase sintering of B2O3-SiO2-doped Nd:YAG transparent ceramics

Abstract

Blended elemental powder densification of Ti-6Al-4V by hot pressing

Abstract

Spark Plasma Sintering Kinetics of Pure α‐Alumina

The microstructure development of sintered alumina at different stages in the spark plasma sintering process has been investigated. Based on classical kinetics laws and adapted Coble creep models of hot pressing, the densification and grain growth kinetics were analyzed as a function of various parameters such as heating rate, sintering temperature, and dwell duration. It is found that sintering kinetics are greatly influenced by heating rates. The local temperature gradients at interparticle contacts during high heating rate can be from three to 10 times higher than in low heating rate cycles. Plastic yield might lead to instantaneous densification at an early stage of sintering. However, grain‐boundary diffusion probably dominates at low heating rate whereas grain coarsening, in respect of thermal equilibrium establishment, is unavoidable with high heating rate during the initial‐stage sintering at low temperatures. During the final‐stage sintering, fast grain growth mechanisms such as surface diffusion and pore‐controlled grain‐boundary migration may dominate over densification controlled by grain‐boundary sliding or lattice diffusion. The grain size–density trajectory corroborates that low heating rate is much favorable to achieve near full density with fine grain size at low sintering temperatures.

A Phenomenological Analysis of Sintering Mechanisms of W-Cu from the Effect of Copper Content on Densification Kinetics

The effect of addition of copper on the sintering of a W powder was systematically investigated by the analysis of dilatometric experiments on W and W-Cu compacts prepared with submicrometric powders. A pure W powder compact and a W-10 wt pct Cu powder compact with the same packing fraction of W particles were first studied, in order to analyze the effect of copper at fixed microstructure of the solid W particle packing. A more systematic set of experiments with different copper contents and W particle sizes was also qualitatively analyzed. A phenomenological model of sintering was developed and fitted in order to extrapolate the effect of copper content on sintering kinetics at fixed microstructure of the W particle skeleton. An interpretation of the sintering mechanisms was then proposed. Sintering of a W-Cu powder compact is the result of solid-state sintering of the W skeleton, enhanced by the capillary forces exerted by copper, with the superimposition of a particle rearrangement step after copper melting.

Effect of uniaxial load on the sintering behaviour of 45S5 Bioglass ® powder compacts

The effect of a uniaxial compressive load on the sintering behaviour of 45S5 Bioglass ® powder compacts was investigated by means of sinter-forging. In comparison to free sintering, densification kinetics was enhanced and the degree of crystallization was reduced. Significantly lower sintering temperatures, i.e. 610 °C instead of 1050 °C, can be employed to obtain dense Bioglass ® parts when sintering is performed under uniaxial load. The effect of mechanical loading on microstructure (pore density, shape and orientation) is discussed. The results of the investigation are relevant in connection with the development of sintered Bioglass ® substrates for bone replacement devices, where both porosity and crystallinity of the part require careful control and low densification temperatures are sought.

Densification and microstructural evolution during laser sintering of A356/SiC composite powders

This article reports experimental results on laser sintering of A356 aluminum alloy and A356/SiC composite powders. Effects of scan rate, sintering atmosphere, hatch spacing, and SiC volume fraction (up to 20%), and particle size (7 and 17 μm) on the densification were studied. The phase formation and microstructural development were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Laser sintering under argon atmosphere exhibited higher densification compared to nitrogen. A faster sintering kinetics was observed as the scan rate decreased. Except at a low SiC content (5 vol%), the composite powders exhibited lower densification kinetics. The densification was improved when finer SiC particles were utilized. Microstructural studies revealed directional solidification of aluminum melt to form columnar grains with inter-columnar silicon precipitates. In the presence of SiC particles, aluminum melt reacted with the ceramic particles to form Al4SiC4 plates.

Titanium foams produced by solid-state replication of NaCl powders

Open-celled titanium foams were fabricated by vacuum hot pressing of a blend of Ti and NaCl powders followed by NaCl removal in water. Densification kinetics of the Ti/NaCl blends are measured at 780 °C at various pressures (30–50 MPa), NaCl volume fractions (30–70%) and NaCl powder sizes (50–500 μm). As compared to pure Ti powders, densification kinetics of the blends is faster for relative densities below 92% due to rapid deformation of the NaCl powders. After dissolution, the flattened shape of the NaCl powders is replicated in the pores, resulting in an anisotropic porous structure. The foams exhibit good compressive strengths (e.g., 102 MPa for 50% porosity and 28 MPa for 67% porosity), low Young moduli (e.g., 29 GPa for 51% porosity) and ductile behavior up to compressive strain >60%.

Enhanced densification of Ti–6Al–4V powders by transformation-mismatch plasticity

The densification kinetics of Ti–6Al–4V powders with spherical or angular shapes are compared in uniaxial die pressing experiments between isothermal conditions (at 1020 °C, in the β-field, where deformation occurs by creep) and thermal cycling (between 860 and 1020 °C, within the range of the α–β phase transformation of the alloy, where transformation-mismatch plasticity is activated). Densification kinetics are only moderately affected by powder shape, but are markedly faster under thermal cycling than under isothermal conditions, as expected from the higher deformation rate achieved under transformation-mismatch plasticity conditions as compared to creep conditions. The densification curves for both creep and mismatch plasticity deformation mechanisms are successfully modeled for various applied stresses and for partial cycling, when transformation is incomplete. Tensile properties of specimens fully densified under thermal cycling conditions are similar to literature values from Ti–6Al–4V densified by isothermal hot isostatic pressing.