Optimal Densification Research Articles

Enhancing the sinterability and fracture toughness of Al2O3–TiN0.3 composites

We introduce a new and effective method for improving the fracture toughness of Al2O3-based composites through the addition of a nonstoichiometric material. Al2O3–TiN0.3 composites were sintered by spark plasma sintering with different TiN0.3 content at temperatures between 1300 and 1600°C for 10min and a micro-region diffusion phenomenon was observed at the Al2O3–TiN0.3 interface. Ti atoms from TiN0.3 diffused into Al2O3 to occupy Al sites, which led to the formation of Al vacancies that enabled the transport of aluminum by a vacancy mechanism. The optimal densification temperature of the Al2O3–30vol% TiN0.3 composite was approximately 1400°C. The maximum fracture toughness measured was 6.91MPam1/2, from the composite with 30vol% TiN0.3 sintered at 1500°C.

Fabrication and microstructure of CNTs activated sintered W–Nb alloys

Abstract In order to fabricate highly dense W-based alloys at low temperature, in the present work, high-energy ball milling and hot pressing were applied to fabricate W–Nb alloys (mass fraction of Nb varied from 0.5% to 5%), where CNTs were used as the activated sintering additives. The phase composition and microstructure were characterized by XRD and SEM equipped with EDS, respectively. The study found coupled effects of CNTs activated sintering and Nb addition on the enhanced sintering ability and refined microstructure of W at 1500 °C. The main results are: (i) XRD characterization revealed that the high-energy ball milling could significantly reduce the crystallite size of W particles and increase lattice distortion, which would enhance the sintering behavior of W alloys. (ii) The addition of CNTs to W (W–0.1CNTs) led to the formation of nanoscale interfacial layer between W grains during hot pressing, resulting in considerable densification and grain growth. Based on this result, it suggested that the activated sintering of W in the present work is due to an enhanced diffusion of W atoms along the GBs induced by CNTs. (iii) With the addition of CNTs to W–Nb alloys, the densification was improved again, but was not so obvious. The optimal densification was obtained for the W–0.1CNTs–1Nb specimen. Moreover, the microstructure characterization in CNTs activated sintered W–Nb alloys indicated that the distribution of sphere-like W(Nb) solid solution particles and decreased W grain sizes with increasing Nb content are the main microstructure features. (iv) The micro-hardness is varied from 4.84 to 6.07 GPa for the CNTs activated sintered W–Nb alloys, which is much higher than that of pure W specimen. The high micro-hardness could be attributed to the increased density and reduced grain size.

An Optimal Thermo-Hydro-Mechanical Densification (THM) Process for Densifying Balsam Fir Wood

To better utilize low-density softwood, a thermo-hydro-mechanical densification process performed in an open system was studied to enable the manufacture of densified wood with a hard surface, strong bonding, and good dimensional stability. This study was aimed at optimizing three densification parameters, i.e., compression ratio (CR), temperature, and time, for balsam fir (Abies balsamea (L.) Mill.). The Brinell surface hardness, bond strength, and thickness recovery ratio of densified fir were examined. It was found that the optimal densification parameters were a CR of 60%, a temperature of 230 ºC, and a time of 20 minutes. The surface hardness and bond strength of optimized densified fir were about 30 and 8 MPa, respectively. The thickness recovery ratio of the densified fir after a 2-hour cold water soaking and another 2-hour boiling treatment was about 10%. Because the densified fir in this study was used for indoor applications only, its thickness recovery ratio could be minimal under conditions of use.

X-Ray Pole Figure Analysis and Magnetic Properties of Microwave Sintered Sr-M-type Hexagonal Ferrites

Sr-M-type sintered hexagonal ferrites were prepared by the conventional and microwave sintering method. X-ray diffraction and pole figure analysis, scanning electron microscopy (SEM), and Robograph 2 magnetic properties test instrument were applied to character the structure and magnetic properties of sintered ferrites. The XRD results reveal all the sintered samples are composed of SrFe12O19 single phase with obvious c-axis alignment. The sample with MS treatment at 1050 ∘C exhibits the optimum magnetic alignment, showing the optimum (0 0 8) and (1 0 7) pole figures with almost all circular lines focused on the center and at about 30∘ of α, respectively. Few pores and optimal densification characters can be seen from the SEM image of strontium ferrites microwave sintered at 1050 ∘C, resulting in the optical magnetic properties as remanence (B r) of 410.5 mT, coercivity (H cj) of 372.2 kA/m, and maximum magnetic energy product ((BH)max) of 31.53 kJ/m3, respectively.

Effects of vacuum sintering temperature on mechanical properties and electric resistance for Cr50–Si50 optical target

In this study, two kinds of Cr and Si powders were mixed ingredients in set proportions (Cr/Si = 50∶50 in weight percentage), and different vacuum sintering temperatures were used to achieve the optimal densification process. The specimens used in this investigation have common microstructures of hardness, XRD, transverse rupture strength and resistivity of the Cr50–Si50 targets. The microstructure shows that the porosity tends to decrease as sintering temperature increases. Conversely, hardness and transverse rupture strength increase as sintering temperature increases. These results agree with the variation in porosity. Additionally, the resistivity tends to decrease with increasing sintering temperature. It should be correlated with the microstructure of Cr50–Si50 targets, increasing the density of CrSi2 could significantly decrease the resistance. Meanwhile, the resistivity decreases and conductivity increases as the neighbours are closer together, which also results from the decreasing porosity.

Shock synthesis and densification of tungsten based heavy alloys

Tungsten heavy alloys have been traditionally manufactured by thermal sintering were the compaction is achieved by heating under pressure the pre-compacted powders, to elevated temperatures, up to 3000 °C. Additives such as dopants are usually incorporated in the original powder mixtures in order to control the grain sizes, and mechanical properties of the sintered microstructures. This process is capable of producing high-density tungsten heavy alloys but has its own limitations. Shock induced synthesis (SIS) and shock compaction (SC) or dynamic compaction (DC), are processes used for the manufacture of advanced materials. In shock synthesis high detonation velocity explosives are used for the synthesis, whereas in shock compaction low detonation velocity explosives, elevated temperatures and prior coating of the powders are used to generate the shock waves required for optimal densification. Due to the high shock wave velocity, the temperatures within the capsule raise above the melting points of some or all the elements present in the powder mixtures. The liquid mixtures are then quenched in microseconds, which results in novel phases. Shock compaction offers the possibility of producing high temperatures necessary for strong metallurgical bonding within the particle surface regions where they are most required, while the internal powder mass, within the particles remain at relatively lower temperatures. A novel approach for producing dense and fine grain tungsten heavy alloys has been investigated. This approach combines reaction synthesis and dynamic consolidation. By combining these two processes tungsten heavy alloys with 98% of the theoretical density have been produced. In this work four tungsten–iron–nickel heavy alloys and one tungsten–copper–nickel heavy alloy were manufactured by shock synthesis and densification. The elemental powders were mixed and pre-compacted at 20 °C by packing them in cylindrical steel capsules of 25 mm diameter. Ammonite with a detonation velocity of 3.6 km/s was used as the main explosive. The samples were preheated to temperatures between 300 and 1000 °C before the application of the shock wave. The specimens were then characterized by optical microscopy, SEM analysis, EDS quantitative analysis, quantitative image analysis, X-ray diffraction, hardness and microhardness testing. This paper will discuss the effects of the shock parameters on the microstructure and strength of these five tungsten heavy alloys and compares the results obtained with those of alloys of similar compositions manufactured by thermal sintering.

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.

Pressureless sintering and elastic constants of Al 2O 3-SiC ‘nanocomposites’

A study is presented of how to obtain very near theoretical densities (≥99.6%) in Al 2O 3-SiC ‘nanocomposites’ by pressureless sintering. Several factors are considered in order to achieve the optimal densification. The most important factors are: good dispersion of SiC particles in the matrix, cold-isostatic pressing of green samples and use of a relatively coarse-sized bedding powder (which allows efficient effusion of carbon monoxide from the sample), correct choice of sintering temperature (which depends on the size of alumina particles), and the use of a nitrogen atmosphere during sintering. The elastic constants of the nanocomposites produced depend on the degree of densification. About 2% reduction from theoretical density leads to about a 6% reduction of the Young's modulus. The experimentally obtained moduli are correlated with three models that relate porosity to Young's modulus.

The influence of surface kinetics in modelling chemical vapour deposition processes in porous preforms

The isothermal chemical vapour infiltration (ICVI) process is a well known technique for the production of composites and the surface modification of porous preforms. Mathematical modelling of the process can provide a better understanding of the influence of individual process parameters on the deposition characteristics such as final porosity or deposition profiles in the pore network. The influence of different rate expressions for several binary compounds on the ICVI process is discussed. Experimental work is used to validate the importance of correct kinetic expressions in a continuous ICVI model for cylindrical pores. The predicted infiltration characteristics are compared with experimental results. The final densification and Thiele modulus, i.e. a number which is a measure for the diffusion limitations in a pore, are used for the evaluation of the presented model, and conditions are given for an optimal densification of a porous preform by the ICVI process for several binary compounds. The deposition profiles as predicted by the model calculations are in agreement with the experimentally determined deposition profiles of TiN and TiC in small tubes. Moreover, it can be concluded that the shape of the deposition profiles is determined by the heterogeneous reaction kinetics. There is only a qualitative agreement between the predicted densification and measured densification for the synthesis of TiN and TiB2 in sintered porous alumina. This mismatch can be explained in terms of a complexity of the pore network and differences in reaction kinetics. Model calculations reveal that there is a scattering for the predicted residual porosity as a function of the Thiele modulus for TiN. Moreover, this Thiele modulus can not fully account for the changes in densification at different temperatures. Given these uncertainties it is likely that a residual porosity of less than one percent can be obtained if the Thiele modulus is smaller than 1 × 10−4. However, a CVI process with such a small Thiele modulus will not be practical, because of the concomitant long process times. Therefore, more precise conditions for the individual process parameters, i.e. concentration, reactor pressure, and temperature are deduced from the model calculations.

Forced Chemical Vapor Infiltration of Tubular Geometries: Modeling, Design, and Scale-Up

ABSTRACTThe development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter.

Pressureless (liquid-phase) sintering of X-Phase and mullite/X-phase powders

Abstract X-Phase and mullite/X-phase composite powders, derived by carbothermal reduction and nitriding of kaolinitic clays, have been respectively sintered to 100 and 95% theoretical density by pressure-less sintering. Powders with lower proportions of mullite gave higher relative density values. Optimal densification could only be achieved by attention to a combination of factors such as powder composition, type of embedding powder, two-step sintering, temperature and time. With the aid of X-ray diffraction, scanning and transmission electron microscopy, it is shown that in mullite/X-phase powders, further transformation of mullite to X-phase occurs during sintering (in a N2 atmosphere), because of reduction in the sample by CO (in the sintering gas) formed by the oxidation of carbon from the graphite linings of the furnace. X-Phase nucleates directly from mullite. Kinetics of the process revealed that it is controlled by a liquid phase. X-Phase in both powder and sintered forms is triclinic (a = 9·68, b = 8·56, c = 11·21 A , α = 91·4, β = 124·4 and γ = 99·2) and (pseudo) orthorhombic (a = 9·66, b = 2·84, and c = 11·17 A ) when analysed by X-ray and electron diffraction, respectively.

Rapid densification of borophosphosilicate glass

The effect of densification, using a rapid thermal annealer, on properties of low pressure chemical vapor deposited borophosphosilicate glass films (4 wt.% B, 4 wt.% P) has been investigated. After densification, the wet etch rate of the films decreased by a factor of 12, while the infrared Si–O band peak height increased by 10%. Application of these findings to process monitoring and the impact of densification on subsequent processing have been examined. Results show that a single densification cycle at 950 °C/20 s induces glass flow behavior. Furthermore, for densification cycles from 500 to 1000 °C for 20 s followed by a fusion cycle of 1100 °C/20 s, densification temperature has no effect on the smoothing of glass over step structures. However, for contact sidewalls a densification cycle of at least 700 °C/20 s is required for sufficient tapering of the sharp corner. An optimal densification cycle for device processing was determined to be 800 °C/20 s.