Densification Curves Research Articles

Evaluation of the mechanical stability of asphalt mixture using the gyratory compactor

In this study, four asphalt mixtures were produced using two different aggregate sources, three aggregate gradations, and six asphalt contents. Super Gyratory Compactor SGC was used to compact all mixtures. The SGC produced densification curves were used to determine volumetric properties of the mixture. It is necessary to evaluate the construction and the densification caused due to the traffic loading. Considering the Construction Densification Index CDI, (the area that is under the densification curve ranges from density at eight gyrations to a density of 92 % of the theoretical maximum specific gravity), that stands for work validation through the period of construction to achieve the percentage of air void of about 8 %. Since all the mixtures used in this study are Superpave designed mixtures, it is important to determine whether the Traffic Force Index (TFI) value can be identified with the stability of mixture as measured by the Marshall stability. The Superpave Gyratory results show very interesting indications that the SGC can be adopted as a tool for the estimation of the mechanical stability of mixtures. According to the presented results, the traffic index TFI, estimated from the SGC, is associated with the Marshall stability.

Densification behavior of dry spent coffee ground powders: Experimental analysis and predictive methods

The knowledge of bulk properties and densification behavior of Spent Coffee Ground (SCG) powders can be useful in handling such powders through the typical processing units of the soluble coffee industries. In this paper we measured the bulk densities of SCG mixtures with mean sizes from 225 to 550 μm as a function of tapping and tested a number of selected equations to predict the densification behavior of these mixtures. Densification kinetics of SCG powders was found to be overwhelmingly driven by the mass fraction of fines in the mixture. The equations proposed by Kawakita and Ludde (1971), Knight et al. (1995) and Mallol et al. (2008) predicted well the variation of bulk density of SCG powders throughout tapping. Nevertheless, the Kawakita and Ludde's equation is recommended as the most suitable because the fitted parameters in this equation could be correlated to the powders' mean size and Hausner ratio. Besides, three methods to predict the densification curves were proposed. In the first method, the powders' mean diameter is used to predict the densification kinetics and in the second the angle of repose and the loose bulk density of the samples are used. Finally, the third method is based on coupling the densification equation to the Modified Linear-Mixture Packing Model. The three approaches led to predictions of bulk density with deviations <10% in comparison to the experimental data and can be recommended to estimate the densification kinetic behavior of industrial SCG powders.

Fabrication and microstructure development of Yb:YAG transparent ceramics from co‐precipitated powders without additives

AbstractSintering additives are generally considered to be important for improving densification in fabrication of transparent ceramics. However, the sintering aids as impurities doped in the laser materials would decrease the laser output power and produce additional heat during laser operation. In this work, Yb:YAG ceramics were vacuum‐sintered without additives at different temperatures for various soaking time through using ball‐milled powders synthesized by co‐precipitation route. The densification behavior and grain growth kinetics of Yb:YAG ceramics were systematically investigated through densification curves and microstructural characterizations. It was determined that the densification in the 1500°C‐1600°C temperature range was controlled by a grain‐boundary diffusion. It is revealed that the volume diffusion is the main mechanism controlling the grain growth between 1600°C and 1750°C. Although SiO2 additives can promote densification during low‐temperature sintering, the optical transmittance of Yb:YAG ceramic with no additives, sintered at 1800°C for 15 hours, reaches a maximum of 83.4% at 1064 nm, very close to the measured transmittance value of Yb:YAG single crystal. The optical attenuation loss was measured at 1064 nm in Yb:YAG transparent ceramic, to be 0.0035 cm−1, a value close to that observed for single crystals.

Influence of aggregates angularity on the locking point of asphalt mixtures

Aggregate angularity plays a critical role in the performance of asphalt mixtures. Current asphalt mix design methods do not specify compaction effort according to aggregate angularity or locking points. This study utilised two methods of compaction, gyratory (Superpave Gyratory Compactor) and impact (Marshall Hammer), to investigate the influence of aggregate angularity on the locking point of asphalt mixtures. For gyratory compaction, densification curves were used to determine the gyratory locking point. For impact compaction, an accelerometer was connected to the Marshall Hammer to register asphalt response, by which the impact locking point was determined. Five mixtures were designed and produced in the laboratory with different aggregate angularities. The mixtures were tested for their locking points and then related to aggregate angularities. Results show that locking point could be determined for most of the mixtures in the study by the gyration and impact methods, except for the mixture composed of uncrushed, round aggregates compacted with the Marshall Hammer. The locking point of asphalt mixtures was highly related to aggregate angularity. A higher aggregate angularity resulted in a lower locking point. Mixtures produced with only crushed aggregates showed locking points much lower than mixtures made with both crushed and uncrushed aggregates, while mixtures made with only uncrushed aggregates had much higher locking points than those made with both aggregates. The findings from this study indicate that compaction effort in asphalt mix design should be specified according to the locking point of the mixtures.

Effect of aggregate contact characteristics on densification properties of asphalt mixture

The densification properties of asphalt mixture are an external manifestation of aggregate contact action, including the friction, interlocking and dislocation of aggregates as well as the cohesion of asphalt binder during compaction. The asphalt binder also has a lubrication effect on the contact action at the high temperature. To investigate the effect of aggregate contact characteristics on the densification properties of asphalt mixture, an aggregate contact device was developed and aggregate contact tests were conducted on different conditions. Two parameters, F that refers to the maximum contact force composed of friction and interlocking force and E that refers to the required energy during the whole test, were gained according to the contact test curve. Considering the lubrication of asphalt binder at the high temperature, the MF and ME refer to the dry mixture, while the AMF and AME refer to the asphalt mixture. The densification parameters were obtained by means of analyzing the densification curves. Then the relationships between the parameters of aggregate contact and those of densification of asphalt mixture were discussed. The results indicate that the aggregate contact action composed of friction and interlocking force increases with the enlargement of nominal maximum aggregate size (NMAS) of the asphalt mixture, and the values of MF and ME increase with the increase of NMAS. The asphalt binder has a significant lubrication effect on the aggregate contact at the high temperature. The contact action of asphalt mixture with a dense gradation is lubricated enough is prone to be compacted. Comparing to test temperature and asphalt content, the gradation of mixture and its NMAS are more easily to affect the contact action, which has the significant effect on the densification properties of the asphalt mixture.

Sintering behavior of NbC based cemented carbides bonded with M2 high speed steel

Densification behavior of NbC-12 wt.% M2 high speed steel (HSS) cemented carbide was studied using a thermo-mechanical analyzer. To be able to survey the complete sintering process, model-free and model-based kinetic analysis were combined. According to this approach, the sintering process was divided into two main parts. The first part is associated with high shrinkage and according to the model-free analysis, three steps can be identified in this part. In the second part, the presence of solution-reprecipitation can be identified according to the developed models for liquid phase sintering. By dividing the sintering behavior into two main parts, it is possible to overcome the effect of limited NbC solubility in Fe during the heating process. This effect can be observed in the densification curves as a slight expansion at the end of the first sintering part. It is assumed that poor wetting performance at low temperatures results in the outflow of the liquid phase to the surface of the compacts which results in a fake densification. It was evident that wetting performance improves significantly at elevated temperatures (above 1310 °C) inducing a re-infiltration of liquid drops on the surface inside the inner pores and the expansion in the shrinkage curve.

Densification and microstructure evolution of W-TiC-Y2O3 during spark plasma sintering

Spark plasma sintering (SPS) is one of the methods used to achieve the low-temperature densification of refractory metal materials. In this study, powder prepared through a wet chemical method was consolidated via SPS at 1100 °C, 1200 °C, 1350 °C, 1600 °C, and 1800 °C to obtain a high-performance W-TiC-Y2O3 composite material. Densification was studied by analyzing the densification curve and changes in the microstructure of the samples. This process could be divided into three stages: the bonding stage, the sintering neck growth stage, and the shrinkage and spherification stage of closed pores. Surface diffusion and grain boundary diffusion played different roles in densification. The density, grain size, and Vickers hardness of the tungsten material increased significantly as temperature increased. This study evaluated the sintering process and provided a basis for obtaining high-performance tungsten materials through SPS.

2D-simulation of Nanopowder High-Speed Compaction

The paper concerns the nanopowder high-speed, $10^4$ - $10^9$ s${}^{-1}$, compaction processes modeling by a two-dimensional granular dynamics method. Nanoparticles interaction, in addition to known contact laws, included dispersive attraction, formation of a strong interparticle bonding (powder agglomeration) as well as the forces caused by viscous stresses in the contact region. For different densification rates, the "pressure vs. density" curves (densification curves) were calculated. Relaxation of the stresses after the compression stage was analyzed as well. The densification curves analysis allowed us to suggest the dependence of compaction pressure as a function of strain rate. It was found that in contrast to the plastic flow of metals, where the yield strength is proportional to the logarithm of the strain rate, the power-law dependence of applied pressure on the strain rate as $p\propto v^{1/4}$ was established for the modeled nanosized powders.

Activation Energy and Mechanism of the Molybdenum Disilicide Sintering Process

This paper presents dilatometric analysis data for the sintering of off-the-shelf molybdenum disilicide (MoSi2) powder prepared by magnesiothermic synthesis. We have obtained continuous shrinkage curves for MoSi2 powder compacts with an initial relative density of 70% at different heating rates: 5, 10, 20, and 30°C/min. From a quantitative analysis of densification curves for the compacts, the activation energy for the initial stage of sintering has been determined to be Q = 695 kJ/mol. It has been shown that the dominant process in the initial stage of MoSi2 powder sintering is volume diffusion from grain boundaries and surfaces.

Research on the Densification Process of the Mechanically-Alloyed Amorphous 2Si-B-3C-N Powder

A mixture of the commercially available cubic silicon powder, hexagonal boron nitride powder and graphite powder was mechanically alloyed to prepare amorphous 2Si-B-3C-N composite powder. The amorphous powder was heated up to 1900°C in nitrogen, with a heating rate of 20°C/min and under a pressure of 80 MPa. Careful investigation was carried out on the densification curve, the microstructure and the mechanical properties of the prepared ceramics. Results show that the amorphous 2Si-B-3C-N powder mainly consists of near-spherical agglomerates, with an average size of 3.5±2.4 micrometers. When the amorphous powder was hot pressed, the densification process mainly included three stages, the denser packing of powder particles with the help of axial pressure, the initial sintering at about 1500-1800°C, and the rapid sintering at temperatures approximately higher than 1830°C. When the 2Si-B-3C-N ceramic was hot pressed at 1900°C for 10-30 mins, it exhibited large volume shrinkage, noticeable reduction of pores, and significantly improvement of density and mechanical properties. The applied high temperature and large pressure may give rise to severe plastic deformation, viscous flow and creep of powder particles, which greatly contribute to the rapid densification of the amorphous 2Si-B-3C-N powder.

Compaction behavior of bimodal iron nanopowder agglomerate

The compaction behavior of structure-modified Fe nanopowders having agglomerates of spherical shape and a bimodal particle size distribution was investigated in terms of the particle packing and microstructure during compaction process. To fabricate the structure-modified Fe nanopowders, the spray-dried Fe2O3 nanopowder was hydrogen reduced at 400, 450 and 550 °C to control the degree of particle growth in agglomerates. The compaction behavior of fabricated Fe nanopowders was investigated by two different ways. In die compaction experiments, it was found that the bimodal nanopowder reduced at 550 °C showed higher green density compared to nanopowders reduced at 400 and 450 °C over the entire range of compacting pressure. Furthermore, the compact of the bimodal nanopowder achieved a remarkable green density over 80% of theoretical density (TD) at 1000 MPa pressure without any defects due to the effective packing by filling the interstices between the large nanoparticles with small nanoparticles. The obtained density-pressure data was fitted by the Panelli and Ambrozio Filho equation to compare the densification capacity between the fabricated Fe nanopowders. To investigate the agglomerate deformation occurrence at the low pressure range, a continuous compaction experiment was conducted; the observations were explained in terms of microstructural development on the basis of the apparent yield pressure obtained from continuous densification curves.

Inert atmosphere processing of hydroxyapatite in the presence of lithium iron phosphate

The present study describes sintering behaviour of hydroxyapatite (HAp) upon addition of lithium iron phosphate (LFP) (1–10 wt.%) system in inert (Ar) atmosphere. The interaction between materials and melting of LFP influenced early and intermediate stages of HAp sintering, shifting the densification curves towards low-temperature side. Analysis of densification process indicated significant differences upon LFP addition. The reaction mechanism that assumes the initial interaction between phosphates from LFP and calcium from HAp was proposed, generating calcium vacancies and contributing to HAp densification. Cross-sections of sintered samples showed changes in microstructural properties, with uniform atomic distribution and presence of Li2Fe3O4 spherical inclusions (200 nm) located at grain boundaries of calcium phosphate matrix. The Rietveld refinement analysis indicated changes in structural and microstructural parameters like crystallite size, anisotropy and microstructural strain of HAp upon LFP addition. Mechanical characterisation indicated improvements in fracture behaviour upon LFP addition.

Preparation of silicon carbide/carbon fiber composites through high-temperature spark plasma sintering

This study discusses the potentials of spark plasma sintering (SPS) integrated with high temperature process that can enable sintering of SiC/Cf composites without any sintering aids. The random distribution of carbon fibers was obtained through mixing composite components in ethanol by using a shaker mill for 10 min. The corresponding sintering process was carried out at 1900 and 2200 °C with 50 MPa pressure applied at maximum temperature. The results showed that 89 ± 0.9 and 97 ± 0.8% of the theoretical density can be obtained for sintering temperatures of 1900 and 2200 °C, respectively. The densification curves were plotted to monitor sintering behavior with punch displacement changes. The appropriate bonding between SiC particles and carbon fibers was detected using FE-SEM for sample which was sintered at 2200 °C. The clear maximum in hardness (2992 ± 33 Vickers), bending strength (427 ± 26 MPa) and fracture toughness (4.2 ± 0.3 MPa m1/2) were identified for sample sintered at 2200 °C. XRD investigations supposed that SiC and carbon were the only crystalline phases in both sintered samples.

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

AbstractIn this study, calcium lanthanum sulfide (CaLa2S4, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.

Reactive sintering and microstructure development of tungsten carbide-AISI 304 stainless steel cemented carbides

Sintering of WC-stainless steel (SS) composites within a typical binder range from 6 up to 15 wt% SS was investigated through constant heating rate dilatometry, in vacuum conditions, complemented by differential thermal analysis and by the study of the high temperature wetting behavior of SS on WC. The densification starts ∼900 °C with a typical densification curve for all compositions, where three distinct regions are discernible: the first one with a slow densification rate, followed by a second region where a sharp increase in the densification rate up to a maximum value dependent on the binder amount is observed and, finally, a third one with a slowdown of the densification rate until the end of the thermal cycle. The attained final density at 1450 °C is dependent on the binder amount, increasing proportionally to its initial content. The final microstructure presents a normal grain size distribution and appreciable amounts of eta-phase, besides the major WC phase and residual iron rich phase. The reactive densification behavior and the role of the liquid phase are interpreted accordingly with structural and kinetic data.

NMR Pore Structure and Dynamic Characteristics of Sandstone Caused by Ambient Freeze-Thaw Action

For a deeper understanding of the freeze-thaw weathering effects on the microstructure evolution and deterioration of dynamic mechanical properties of rock, the present paper conducted the nuclear magnetic resonance (NMR) tests and impact loading experiments on sandstone under different freeze-thaw cycles. The results of NMR test show that, with the increase of freeze-thaw cycles, the pores expand and pores size tends to be uniform. The experimental results show that the stress-strain curves all go through four stages, namely, densification, elasticity, yielding, and failure. The densification curve is shorter, and the slope of elasticity curve decreases as the freeze-thaw cycles increase. With increasing freeze-thaw cycles, the dynamic peak stress decreases and energy absorption of sandstone increases. The dynamic failure form is an axial splitting failure, and the fragments increase and the size diminishes with increasing freeze-thaw cycles. The higher the porosity is, the more severe the degradation of dynamic characteristics is. An increase model for the relationships between the porosity or energy absorption and freeze-thaw cycles number was built to reveal the increasing trend with the freeze-thaw cycles increase; meanwhile, a decay model was built to predict the dynamic compressive strength degradation of rock after repeated freeze-thaw cycles.

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B4C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B4C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040°C and 1600–1700°C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B4C–SiC nanocomposites were obtained at 1700°C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B4C grains and some nanosized B4C and SiC particles were embedded into B4C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800°C reached 99.2±0.4%, 35.8±0.9GPa and 6.8±0.2MPam1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B4C–SiC composites.

Cold sintering process for ZrO2‐based ceramics: significantly enhanced densification evolution in yttria‐doped ZrO2

AbstractWe recently developed a novel technique of cold sintering process (CSP) to obtain dense ceramics at extraordinarily low temperatures. In this communication, we demonstrate the feasibility of applying CSP to zirconia‐based ceramics. As exemplified by 3Y‐TZP ceramics, a significantly enhanced densification evolution is observed. Water is simply utilized as a sintering aid to assist the ceramic densification under an applied external pressure. The low‐temperature advantage of CSP outstands in contrast to the densification curves compiled from other sintering techniques. A gradual monoclinic‐to‐tetragonal phase transformation is revealed in correspondence to the densification development, as well as contributes to the mechanical hardness evolution. A Vickers Hardness reaches ~10.5 GPa after annealing the cold‐sintered ceramics at 1100°C, which is comparable to those values reported in the previous studies at higher sintering temperatures. Such a sintering methodology is of significant importance as it provides a roadmap for cost‐effective processing of zirconia‐based ceramics and composites that enable broad practical applications.

The influence of SPS heating rates on the synthesis reaction of tantalum diboride

TaB2 is a material from the Ultra High Temperature Ceramics group and is rather unexplored because it is difficult to procure the raw materials and to densify TaB2. Using SPS technique to realize reactive sintering processes of powders mixture according to the reaction Ta+2B→TaB2 makes it possible to achieve TaB2 in one technological step. The aim of the study was to determine the influence of heating rates on the synthesis reaction and on the multistage densification mechanisms during SPS processes. The mixture was sintered at constant parameters of 2200°C, 48MPa for 5min with the usage of heating rates from 50°C/min up to 400°C/min. The densification processes were studied through analyzing the shrinkage of powder compacts during SPS (Spark Plasma Sintering) processes. The comparison of the densification curves indicates that the reactions do not proceed completely at slow heating rates. Namely, too low heating rates contribute to the sintering of tantalum before the synthesis reaction and demonstrate the presence of boron in liquid state. The best material obtained in this study has Young's modulus 571GPa, Vickers hardness 20.7GPa (HV1) and indentation fracture toughness KIC 4.7MPam1/2.

Assessment of solid state and liquid phase sintering models by comparison of isothermal densification kinetics in W and W-Cu systems

The role of liquid phase and grain growth during the intermediate stage of sintering is investigated by comparing the isothermal densification rate of the tungsten skeleton in two systems: solid-state sintered W and liquid-phase sintered W-10wt%Cu. Densification curves at 1380 °C, i.e. well after completion of capillary rearrangement in W-Cu, are confronted to the predictions of a model that considers a single representative grain size with quasi-equilibrium interfaces and a monotonic increase in grain size and coordination with density. In both systems, the densification mechanism is taken to be governed by W diffusion along grain boundaries. For system W-Cu, a phenomenological law is proposed to allow accounting for the decrease in liquid surface curvature during funicular filling of the pores by the liquid-phase. The kinetics of grain growth is described via current literature model. Good agreement is demonstrated between experimental measurements and model predictions when relying on material parameter values given in literature. In system W-Cu, the detrimental effect of the decrease in dihedral angle is, at the beginning of the funicular stage, compensated by the contribution to sintering stress brought about by the concave curvature of liquid/vapour interface. The sensitivity of computational results to model hypotheses and material parameters is assessed. The most influential parameters are diffusivities along W grain boundaries and along W/vapour and W/Cu interfaces.