Master Sintering Curve Concept Research Articles

Sintering activation energies of anisotropic layered and particle alumina/zirconia-based composites and their mechanical response

Information on the sintering activation energy is currently focused on evaluation of single-phase ceramic systems. This work shows the results of high-temperature dilatometry measurements of layered and particle composites based on alumina and zirconia. Layered composites with different layer thickness ratios and particle composites with variable composition in the entire concentration range were prepared by electrophoretic deposition allowing manufacturing composites with precious design and strongly bonded interfaces. The phenomena observed during the high-temperature dilatometry measurements are discussed, and the data were used to calculate the sintering activation energies of composites using the modified Master Sintering Curve concept. By covering a wide range of composite designs, it was possible to determine differences in activation energies and to show their dependence on the direction in the case of laminate composites given by the directionally dependent sintering behaviour. Sintering activation energies of layered composites were always higher than for monoliths due to constrained sintering showing maximum sintering activation energies at lower volumes of zirconia in the layers for longitudinal and transversal orientation of the samples. A similar trend was identified in particle composites due to slowed down alumina densification by the pinning effect. Additionally, mechanical properties represented by Vickers hardness and indentation elastic modulus were related to the microstructure developed during sintering. The effects of interconnectivity of phases present in the composites together with other parameters of the microstructure were described.

Impact of high energy ball milling on densification behaviour of magnesium aluminate spinel evaluated by master sintering curve and constant rate of heating approach

The present study investigates the influences of high energy ball-milling on densification of a commercial sub-micron magnesium aluminate spinel powder. High energy milling decreases the crystallite size and modifies the particle size distribution of powder resulting in better compaction of green bodies. Milling, also, increases the number of structural defects.The concept of master sintering curve (MSC) and the constant rate of heating (CRH) approach were employed to evaluate the activation energy of sintering. It was demonstrated that, according to MSC, the milled samples exhibit lower densification activation energy compared to the as-received powder (530 kJ mol−1 vs 750 kJ mol−1). Analysing the densification using CRH approach reveals that the sintering activation energy of milled samples remains constant during the sintering process, while that of as-received samples increases with densification. The estimated activation energy using MSC method is thus an average value of the values measured over the whole process of sintering.

Activated liquid phase sintering of W–Cu and Mo–Cu

A model for densification based on the master sintering curve concept is applied to solid-state sintering of W and Mo and to liquid-phase sintering of W–10Cu and Mo–18Cu. The effects of small additions of Ni, Co, Fe, and Pd to W–10Cu and Mo–18Cu are investigated. Separate grain growth models for solid-state and liquid-phase sintering are also applied to these systems. The model predictions are compared to experimental results to determine the effects of the activators on densification and grain growth mechanisms. Ni and Pd additions only slightly reduce the activation energy for densification, while Co and Fe have a much larger effect. The effects of Co and Fe additions on grain growth rate constants are not adequately described by consideration of their effects on the solubility of W and Mo in the liquid phase. The experimental results with Co and Fe additions are better described by a solid-state grain growth model. The effects of Co and Fe additions on densification and grain growth are consistent with enhanced diffusion through a segregated solid phase during liquid-phase sintering.

Processing Route to Fully Dense Nanostructured HAp Bioceramics: From Powder Synthesis to Sintering

The study is devoted to the processing of hydroxyapatite (HAp) nanopowder to develop fully dense nanostructured bioceramics by pressureless sintering. The sintering behavior of stoichiometric HAp prepared by hydrothermal processing was investigated by nonisothermal, two‐step, and conventional sintering. By low‐temperature two‐step sintering (TSS), at 900°C and 850°C, with appropriate dwell time, dense bioceramics without final‐stage grain growth and average grain size of 75 nm was obtained. A concept of master sintering curve was applied, enabling control of sintering process, estimation of effective activation energy for sintering of HAp nanopowder, and qualitative understanding of sintering mechanisms. According to estimated activation energy of 412.6 kJ/mol, low sintering temperature and particles' microstructure as the dominant sintering mechanism we proposed diffuse‐viscous flow controlled by grain boundary diffusion. HAp nanoparticles comprising of different interior and boundary regions as ordered/disordered microstructure are found to be of an advantage for low‐temperature sintering.

Master debinding curves for solvent extraction of binders in powder injection molding

This study successfully extends the master sintering curve concept to model removal of polymeric binders in powder injection molded bodies. In this demonstration the focus is solvent debinding. Master debinding curves (MDC) were used to estimate the activation energy for debinding. Binder removal curves were developed for solvent extraction of polyethylene glycol (PEG) from injection molded shapes made from silicon nitride doped with yttria and spinel (Si3N4 5% Y2O3 5% MgAl2O4). The extraction for different shapes incorporated a shape parameter in the standard master curve equation, giving activation energy of 12.4kJ/mol extraction of PEG. Master debinding curves were also developed for wicking debinding. The analysis showed lower activation energies for solvent extraction and wicking compared to thermal debinding. The lower activation energy suggests easier removal by solvent or wicking versus pyrolysis. The study provides a unifying quantitative framework for comparing and predicting the effects of material, process and geometry on binder removal. The master debinding curves can be utilized in design and optimization of binder removal.

Master sintering curve concepts applied to full-density supersolidus liquid phase sintering of 316L stainless steel powder

Supersolidus liquid phase sintering (SLPS) is an important liquid phase sintering variant that is used to densify prealloyed powders. The densification behaviour of a boron doped 316L stainless steel during SLPS is treated using a master sintering curve (MSC) concept based on the integral time–temperature work of sintering. With this approach, the MSC analysis is able to identify a fundamental change in the densification rate that is indicative of a mechanistic shift during SLPS.

The Pressure-Assisted Master Sintering Surface of Metallic Powder Mixture

The use of master sintering curve (MSC) is an effective method to predict densification during sintering or hot pressing. Empirical models of MSC in early studies were developed using a single kind of powder or a major powder with a little amount of sintering additives. For the present study, the powder mixture consisting of 6 different metallic materials was used to see whether the concept of MSC can be applicable to the hot pressing of metallic powder mixture. Three data sets having different heating rates merged onto a single curve when the density was plotted as a logarithmic function of Θ as well as the best estimation of the activation energy was found, where Θ is the integral of temperature function over time. It was found that the density versus Θ profile can be used to predict the final density of metallic powder mixture at a given pressure regardless of heating history. The study was also extended to a range of pressures from 31.4 to 88.3 MPa to generate the pressure-assisted master sintering surface (PMSS).

Master Sintering Curve Formulated from Constitutive Models

The classical master sintering curve (MSC) is derived from empirical sintering model and is applicable over a range of heating rates and temperatures. For simplicity, the MSC approach was modified by assuming one dominant densification mechanism to evaluate and predict densification response. However, the concept of MSC can be extended well beyond the original formulation or the subsequent simplifications. To this effect, generalized formulations are proposed based on several constitutive equations including both grain growth and densification. These formulations can be used very effectively to obtain material properties that in turn can be used in finite‐element method to improve the accuracy of the simulations.

An Analysis of Four Different Approaches to Predict and Control Sintering

Understanding and predicting sintering, which have been goals since the first attempts to mathematically describe the sintering process in the 1950s, are necessary to eliminate machining after sintering and to reliably predict and control the sintered microstructure and the resultant mechanical and other desired properties. In this study, four different sintering models are evaluated relative to one another and the experimental data, revealing their attributes, deficiencies, and modifications/improvements in order to facilitate their application, including the following: (i) a microstructure‐based model for solid state sintering, mainly developed by Riedel and Svoboda (RS); (ii) a viscous sintering (SOVS) model developed by Skorohod and advanced by Olevsky; (iii) a Kinetic Monte Carlo (KMC) model provided by Tikare; and (iv) the master sintering curve (MSC) approach introduced by Johnson et al. For different reasons, all four models have deficiencies that preclude achieving the most challenging goal of being able to comprehensively understand and predict sintering behavior: (i) the RS and the KMC models are complicated and difficult to use; (ii) the SOVS model cannot predict microstructure evolution; and (iii) the KMC model and the MSC have no stresses in their mathematical description, so they cannot simulate the effects of external forces. Each model also has attributes: (i) the KMC model allows one to follow the evolution of mesostructure; (ii) the MSC concept and the RS model are suitable for predicting densification curves for a wide variety of temperature–time profiles; and (iii) the SOVS and the RS models, which are implemented into finite element codes, can be used to predict density gradients and the warping of complex shape parts. Individually and together, the MSC, KMC, SOVS, and RS models can be useful tools to advance the fundamental understanding and improve the control of sintering.

Master sintering curves of two different alumina powder compacts

Concept of Master Sintering Curve is a strong tool for optimizing sintering schedule. The sintering behaviour can be predicted, and sintering activation energy can be calculated with the help of few dilatometric measurements. In this paper an automatic procedure was used to calculate Master Sintering Curves of two different alumina compacts. The sintering activation energies were determined as 640 kJ/mol for alumina with particle size of 240 nm, respective 770 kJ/mol for alumina with particle size of 110 nm. The possibility to predict sintering behaviour with the help of Master Sintering Curve was verified. .

DENSIFICATION AND GRAIN GROWTH OF NANOCRYSTALLINE NIAL POWDER DURING PRESSURELESS SINTERING

Nanocrystalline NiAl powder was prepared by mechanical alloying. The prepared powder was cold compacted and sintered at different temperatures and times. The crystallite size of green and sintered compacts was estimated from X-Ray Diffraction peak profile analysis. Thereafter, the Generalized Parabolic Grain Growth model and Master Sintering Curve concept based on Combined Stage Sintering model were employed to investigate the grain growth and densification behavior of nanocrystalline NiAl powder during sintering, respectively. The results of modeling were in a very good agreement to that of experiment. Finally, by comparing the modeling results the sintering parameters were optimized.

Grain growth behavior of tungsten heavy alloys based on the master sintering curve concept

Time and temperature relationships for grain growth by Ostwald ripening are transformed into the master sintering curve (MSC) form. The resulting MSC equations are used to analyze grain size data obtained from W-Ni-Fe heavy alloys that were quenched from sintering temperatures ranging from 1200 °C to 1500 °C. Activation energies for grain growth during both solid-state and liquid-phase sintering are calculated and compared to experimental values for diffusion. The MSC equations are further modified to include the effect of the solid volume fraction on the grain growth constant. A sensitivity analysis of material properties and process variables shows that the grain growth rate constant is most affected by changes in the sintering temperature and activation energy.

Application of Work-of-sintering concepts in powder metals

One of the current challenges facing the particulate materials industry is developing simple, accurate models to predict sintered properties. Work-of-sintering concepts, where time-temperature integrals are used in such models, offer a solution to this problem. In this study, the master sintering curve concept is applied to several powder metal systems: 17 to 4PH stainless steel, 316L stainless steel, nickel, niobium, tungsten heavy alloys with two different compositions, and molybdenum. A detailed explanation of the construction of these curves is given, including methods used to calculate the apparent activation energy for sintering and to curve-fit experimental data to a sigmoid function describing the master sintering curve. Discussion of the results shows that the master sintering curve can be applied to powder metal systems, even those that use liquid phase during sintering.

Densification behavior of tungsten heavy alloy based on master sintering curve concept

The master sintering curve (MSC) theory is modified by substituting the densification ratio (φ) for the densification parameter (ψ) to identify regions where shrinkage occurs by a similar combination of sintering mechanisms. The modified MSC theory is used to analyze the results of dilatometry experiments conducted with W-Ni-Fe heavy alloys, in which a phase change occurs during sintering. Apparent activation energies for sintering in three regions (solid state, transition, and liquid phase) are calculated. These activation energies are compared with experimental values for diffusion and other mass-transport phenomena to identify the dominant mechanisms in each region. A series of master sinter curves for varying W contents are developed into a master sintering surface that includes tungsten content and integral work.

Master sintering curve concepts as applied to the sintering of molybdenum

Sintering experiments with various molybdenum powders are rationalized using a master sintering curve concept that collapses density, grain size, or other parameters onto a single curve. In this case, the integral work of sintering is developed and customized for different green densities. Construction of the master sintering curve is described to show how the curve can be normalized with respect to green density effects. Various grades of molybdenum for metal injection molding and die compaction are included in the analysis, processed over a range of heating cycles. Die-compacted samples of varying green densities are used to illustrate the efficacy of the new, normalized master sintering curve concept. Sintering cycle optimization is one possible outcome from this analysis.

Application of the Master Sintering Curve Theory to Non-Isothermal Sintering of BaTiO<sub>3</sub> Ceramics

In this paper a practical approach to the analysis of sintering of BaTiO3 using the Master Sintering Curve concept has been presented. Non-isothermal sintering of high-purity non-doped BaTiO3 ceramics was monitored using a sensitive dilatometer at three different heating rates (10, 20 and 30 oC/min) up to 1380oC. Densification of BaTiO3 during sintering was analyzed using the Master Curve Sintering Theory. A MSC was defined characterizing the sintering behavior of barium-titanate regardless of the heating rate. Construction of the MSC enabled estimation of the process activation energy. Using defined MSC, densification behavior of BaTiO3 ceramics during sintering can be predicted for arbitrary temperature-time excursions and these predictions can be used in controlling and planning the sintering process of this material.

The pressure-assisted master sintering surface

It has been reported that the master sintering curve (MSC), in which the sintered density is a unique function of the integral of a temperature function over time, is insensitive to the heating path. The present research was undertaken to determine whether the MSC concept is applicable to hot pressing, and to develop the pressure-assisted master sintering surface for alumina. Densification of Sumitomo AKP30 alumina was continuously recorded during heating at 10°C/min at fixed pressures from 7 to 34.5 MPa. Final densities computed from the dilatometer traces were in excellent agreement with values determined by the Archimedes method. The thermocouple was calibrated using the melting point of the Ni/C eutectic. An accuracy of ±2°C was established. The pressure-assisted master sintering surface was successfully constructed. Using this surface, the final density can be predicted to about 1% accuracy for a fixed pressure and an arbitrary temperature-time path.