Multi-stage spark plasma sintering to study the densification mechanisms of boron carbide

The densification kinetics of boron carbide (B₄C) during multi-stage spark plasma sintering was studied. The densification mechanisms were analyzed according to the stress exponent n and the apparent activation energy Q_d using a creep deformation model. The results showed that the densification mechanisms were controlled by viscous flow and grain boundary diffusion at the low effective stress with initial temperature range of 1600–2000 °C, while the dominant mechanism is the dislocation climb at the effective stress regime with final temperature of 2100 °C and the multi-stage sintering can reduce the apparent activation energy. Meanwhile, the scheme of multi-stage sintering can obtain nearly theoretical dense B₄C and avoid grain growth. Therefore, the basic mechanical properties suggesting a good combination of high hardness (37.63GPa) and bending strength (539.86 MPa) was obtained by the multi-stage sintering.     

Spark plasma sintering of pure TiCN: Densification mechanism,grain growth and mechanical properties

This study aims to disclose the densification mechanism and grain growth behaviors during the spark plasma sintering (SPS) of undoped TiCN powder. The SPS experiments were performed under temperatures ranging from 1600 °C to 2200 °C and a fixed pressure of 50 MPa. The sintering mechanisms were described in different models according to two grain growth behaviors: densification without grain growth at low temperatures (1600–1700 °C) and grain growth without apparent densification at higher temperatures (1800–2200 °C). At the constant grain stage, a creep model is applied to describe the densification process. In addition, the effective stress exponents, n, are calculated, indicating that the densification can be attributed to both grain boundary sliding (n = 1.5) and dislocation climbing (n = 3.13 or n = 4.29). During the second stage of sintering, the grain growth model reveals that the grain-growth is controlled by grain boundary diffusion. In addition, the Vickers hardness varies from 4326 Hv to 6762 Hv when the density ranges from 90% to 96.3%.     

Microstructure evolution and densification kinetics of Al2O3/Ti(C,N) ceramic tool material by microwave sintering

The microstructure evolution and densification kinetics of Al₂O₃/Ti(C,N) ceramic tool material during microwave sintering were studied. The density and grain growth significantly increases at the temperatures higher than 1400 °C. The calculated kinetics parameter n indicates that volume diffusion is the main densification mechanism when the sintering temperature is below 1300 °C, while grain boundary diffusion plays a leading role in the densification process when the sintering temperature is higher than 1300 °C. The grain growth activation energy of Al₂O₃/Ti(C,N) composite is 48.82 KJ/mol, which is much lower than those of monolithic Al₂O₃ in the microwave sintering and conventional sintering. The results suggested that the Al₂O₃/Ti(C,N) ceramic tool material with nearly full densification and fine grains can be prepared by two-step microwave sintering.     

Densification Kinetics of CeO<Subscript>2</Subscript> Reinforced 8 Mol.% Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Stabilized ZrO<Subscript>2</Subscript> Ceramics

The grain growth kinetics of 8YSZ ceramics processed using spark plasma sintering (SPS) has been investigated in the temperature ranging from 1100°C to 1500°C. The activation energy during SPS densification was obtained as 332 kJ/mol with grain boundary diffusion as a dominant mechanism. Further, the effect of CeO₂ on the densification kinetics of 8YSZ ceramic processed via SPS and conventional sintering (CS) has been delineated. The lower grain boundary mobility of CS-processed composites (an order of magnitude lower than SPS) is attributed to the solute drag and lattice distortion mechanism. However, no significant change in the grain boundary mobility was observed with CeO₂ addition (~?14.7–43.9?×?10^?18 m³/N/s for CS and 107.2–116.7?×?10^?18 m³/N/s for SPS) revealing that the defect concentration is nearly constant in 8YSZ. The study highlights the effect of sintering techniques (SPS and CS) and reinforcement (CeO₂) on engineering the desired microstructure of 8YSZ ceramic.     

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.     

Progress on grain growth dynamics in sintering of nano-powders

Nanostructured materials, characterized by an ultrafine grain size, have stimulated much research interest by virtue of their unusual mechanical, electrical, optical, and magnetic properties. In this paper, the sintering process of nano-powders were reviewed, to which sintering of the traditional materials compared. The microstructural development, i.e., grain growth and densification during sintering as well as the mechanism of crystal surface diffusion and boundary migration were analyzed, and the dynamic models on sintering process were summarized by the relationship of grain growth and pores size, interface diffusion, densification rate, and sintering temperature. Finally, the research tendency of this major on the basis of above models was discussed.     

Titanium sintering science: A review of atomic events during densification

The sintering densification trajectory for titanium powder is identified in terms of the interaction between mass transport processes and microstructure evolution. During initial heating, as surface oxides dissolve, surface diffusion forms bonds between contacting particles without densification. Grain boundaries form in the bonds due to random crystal orientations at the contacts. Except for mixed powder Kirkendall swelling, subsequent diffusion in these interparticle grain boundaries leads to densification. Most importantly, the alpha-beta transformation provides strain, defects, and interfaces that accelerate densification in the 800–1100 °C temperature range. This is below a typical peak sintering temperature. Final densification involves beta phase volume diffusion and grain boundary diffusion. Densification slows due to grain growth and the loss of grain boundary area. Pores close near 92% density to trap impurities and reaction products inside the closed pores, often limiting sintered density to about 95% of theoretical. High final density requires slow heating or long holds at intermediate temperatures to evaporate impurities prior to pore closure. The master sintering curve is a means to link densification to process parameters without concern over detailing this cascade of transport mechanisms and microstructure changes.     

Densification and grain growth during isothermal sintering of Mo and mechanically alloyed Mo–TZM

Isothermal sintering behavior of pure molybdenum (Mo) and mechanically alloyed Mo–TZM (Mo–0.6Ti–0.2Zr–0.02C) has been investigated in the temperature range 1000–1800 °C. A linear relationship has been found to exist between logarithms of increment in density and time. Although the volume diffusion has been found to be the dominant sintering mechanism, a significant contribution from grain boundary diffusion is also identified. Both the diffusion coefficients (D_v) obtained from shrinkage data and the grain boundary mobility (M_b) during grain growth are found to be lower for Mo–TZM due to the presence of carbides in the microstructure. The grain boundary migration is restricted due to the presence of carbides and porosities in the microstructure.     

Temperature dependence of microstructure evolution during hot pressing of ZrB2–30 vol.% SiC composites

Microstructure evolutions of ZrB₂–30 vol.% SiC composites, prepared by hot pressing at different processing temperatures (1700, 1850 and 2000 °C) for 30 min under 10 MPa, were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM). The microstructures of the fabricated composites were compared with and the effects of the processing temperature on the sintering process and densification behavior during the hot pressing were found. The amount and the orientation of dislocations which were indicated by TEM analysis in the sample hot pressed at 1700 °C showed that no plastic deformation and atomic diffusion occurred. But the presence of amorphous phases and rearrangement of particles are signs of the fact that liquid phase sintering and particle fragmentation/rearrangement is the main densification mechanism. On the other hand, in the sample hot pressed at 1850 °C, aggregation of dislocations behind the grain boundaries and the presence of twinnings addressed wide plastic deformations which were introduced as the main densification mechanism at 1850 °C. Finally in the sample hot pressed at 2000 °C, lower amounts of un-oriented dislocations and also some annealing twinnings were observed in TEM micrographs together with fractographical SEM analysis and showed that the atomic diffusion is the dominant densification mechanism of hot pressed ZrB₂–30 vol.% SiC composite.     

The effect of yttrium on densification and grain growth in α-alumina

The grain growth and densification have been investigated in very high-purity α-alumina doped with varying amounts of yttrium (0 to 3000 wt ppm of yttria) and sintered in air at 1450, 1550 and 1650 °C. Yttrium doping inhibited densification and coarsening at 1450 °C, but had very little effect at 1550 °C and no effect at 1650 °C. The change in densification behaviour is suggested to be related to the transition with increasing temperature from grain boundary diffusion to lattice diffusion controlled densification. The coarsening rate increases faster with temperature than the densification rate. This was correlated with a higher measured activation energy for grain growth than for the diffusion processes, which control the densification.     

Spark plasma sintering of B4C ceramics: The effects of milling medium and TiB2 addition

Boron carbide (B₄C) ceramics, with a relative density up to 98.4% and limited grain growth, were prepared at 1600-1800 °C by spark plasma sintering (SPS) technique. The effects of powder milling medium (water and 2-propanol) on the powders' surface characteristics and TiB₂ addition on the sintering densification were investigated. The ball milling processing of B₄C powders in water can promote the sintering of B₄C ceramics. A B₂O₃ layer on B₄C particle surface is concluded to promote the densification of the B₄C ceramics at an early sintering stage. This B₂O₃ layer, which normally inhibits the densification process at the final stage of the sintering, can be reduced through reaction with TiB₂ particles, resulting in further densification of the B₄C ceramics.     

Spark plasma sintering of W-10Ti high-purity sputtering target: Densification mechanism and microstructure evolution

The densification mechanism and microstructure evolution of W-10Ti sputtering target prepared by spark plasma sintering (SPS) method at a temperature ranges from 900 to 1600 °C, with dwelling time of 6 min and fixed pressure of 30 MPa were investigated. Densification occurs mainly at low temperatures (900 to 1300 °C), while grain growth occurs at high temperatures (1400 to 1600 °C). The creep model has been used to reveal the densification process. The effective stress exponent n is calculated systematically, which indicates that the densification process is mainly due to the particle rearrangement (n < 1), grain boundary diffusion (n = 1–2), and dislocation climbing (n = 3.77 or 4.14). In addition, the apparent activation energy Q_d is calculated to be 119.30 and 271.79 kJ/mol when the effective stress exponent n is equal to 1 and 2, respectively. It is also found that the microstructure of W-10Ti alloys is greatly affected by the sintering temperatures. The solution between W and Ti significantly improves with the increase of the sintering temperature. The solubility of W in βTi(W) exceeded the eutectoid point (28.97 wt% W) and the eutectoid structure (βW(Ti) + αTi) forms in cooling process when the temperature is up to 1300 °C. With the temperature increasing to 1500 °C, the composition of the βTi(W) phase is located in the miscibility gap of the (βTi(W), βW(Ti)) system, which tends to decompose in to βTi(W) and βW(Ti) phases.     

Oxidation kinetics of Y‐doped FeCrAl‐alloys in low and high pO2 gases

A model Fe‐20Cr‐5Al‐0.05Y alloy was oxidized in Ar‐20%O₂ and Ar‐4%H₂‐7%H₂O at 1200–1300 °C. Two‐stage oxidation experiments using oxygen isotope tracers showed that inward oxygen diffusion was predominant in both gases, but more isotope exchange was observed in the H₂/H₂O gas reaction. The alumina scales formed in both gases were composed of columnar grains, the lateral size of which increased linearly with depth beneath the scale surface. Thermogravimetric measurement of oxygen uptake revealed kinetics which were intermediate to parabolic and cubic kinetic rate laws. A model based on grain boundary diffusion control coupled with competitive oxide grain growth accounts satisfactorily for the results when the requirement for a divergence‐free flux within the scale is imposed. This treatment shows that the oxide grain boundary diffusion coefficient is lower when H₂O is the oxidant. It is concluded that hydrogen slows the grain boundary diffusion process by altering the nature of the diffusing species.     

Sintering of binderless cubic boron nitride and its modification by β-Si3N4 additive for hard machining applications

This study present the results of HP-HT sintering, microstructure, properties, and performance of binderless cBN tool material. Within the investigated sintering temperature range of 1900–2600 °C the optimum was found to be 2200–2300 °C. Lower temperature results in incomplete diffusion bonding between cBN grains, while higher temperature results in high degree of recrystallization of initial structure, grain growth, and even formation of hexagonal boron nitride in triple joints. Introduction of stress-inducing β-Si₃N₄ minor inclusions resulted in high overall mechanical and thermal properties: HK = 41 GPa; K_IC = 12.6 MPa·m^1/2; λ = 180 W/(m·K). Machining experiments in roughing of hardened tool steels show that binderless cBN material provides high performance in terms of resistance to tool cratering, chipping, and tool fracture.     

Low temperature processing of dense samarium-doped CeO2 ceramics: sintering and grain growth behaviors

Samarium-doped CeO₂ is a leading electrolyte for applications in solid oxide fuel cells (SOFCs), which requires a typical sintering temperature of 1400–1600 °C. By synthesizing reactive powders via carbonate precipitation, fully dense CeO₂ ceramics doped with 0–20 at.% of samarium have been fabricated in this work via pressureless sintering at a significantly lowered temperature of 1000 °C. The resultant ceramics show ultrafine grain sizes of ∼0.15–0.75 μm, depending upon the dopant concentration. Sintering studies indicated that samarium doping retards both densification and grain growth but increases the rate ratios of the two in the intermediate stage of sintering. Subsequent investigations on the grain growth in the fully densified ceramics also showed the suppressing effects of dopant, which tend to saturate at 10 at.% of samarium. The activation energy for grain growth increased from ∼186 to ∼254 kJ/mol by raising the samarium concentration from 5 to 20 at.%.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Effect of the addition of silicon nitride on sintering behaviour and microstructure of zirconium diboride

Using Si₃N₄ as a sintering aid in ZrB₂ greatly improved densification and microstructure compared to additive-free zirconium diboride. Nearly fully dense material was obtained by hot pressing at 1700 °C. The microstructure consists of fine ZrB₂ grains and of grain boundary phases (BN, ZrO₂, ZrSi₂, B–N–O–Si–Zr glassy phase) mainly located at triple points.     

Monte-carlo modeling of grain growth in Zr equal channel angular pressed and recrystallized

Grain boundary character distribution in equal-channel-angular pressed Zr was studied. Using a die design of 90°/20° and an operation temperature of 350°C. The initial grain size of 20 μm was reduced to about 270 nm with 4 passes via route B_c. The grain growth kinetics of the recrystallized state was obtained by experiment and Monte-Carlo computer simulation, respectively, which showed good agreement. Based on kinetics and morphological characteristics, it was concluded that the grain coarsening mechanism was governed by normal grain growth. No sign of abnormal grain growth was detected either in the experiment or in simulation despite taking into consideration anisotropy in grain boundary energy as well as its mobility. This indicates that grain boundaries produced by severely deformed Zr are stable against explosive coarsening. The evolution characteristics of the microstructure in the present ECA pressed and recrystallized Zr differed from those of cold rolled Ti in that the grain boundary misorientation distribution and texture were rather stable during grain growth. Jointly Appointed by the Center for Advanced Aerospace Materials This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Microstructure and optical properties of transparent alumina

The microstructure and optical properties are evaluated for alumina sintered by spark plasma sintering at temperatures between 1100 and 1550 °C. With increasing sintering temperature, grain growth and densification occur up to 1250 °C, and above 1300 °C, rapid grain growth and pore growth occur. Light transmission increases with the densification and decreases with the grain/pore growth. It is found that the total forward transmission and the reflection of light are related to the porosity and the pore growth, whereas the in-line transmission and the light absorption are related to the grain size and the defects, respectively. The relationships are explained by using the Mie scattering theory, model prediction and observed microstructural characteristics.     

Influence of B4C coating on graphitization for diamond/WC-Fe-Ni composite

Diamond/WC-Fe-Ni composite is a potential composition for impregnated diamond drill bits. It is necessary to avoid the graphitization of the diamond from Fe and Ni under the powder metallurgy process. Boron carbide (B₄C) was coated on diamond, and diamond/WC-Fe-Ni composites were consolidated by hot pressing at different temperatures. The influences of sintering temperature and interfacial structure on bending strength and wear behavior were investigated. The bending strength for diamond/WC-Fe-Ni composite was dependent on matrix densification and interfacial graphitization. Un-coated diamond was eroded by Fe-Ni matrix and partially converted to graphite during the sintering process at all sintering temperatures. In opposite, B₄C coating was beneficial to matrix densification at a lower sintering temperature, and delayed the appearance of graphitization to around 1300 °C. Therefore, the diamond/WC-Fe-Ni composites with B₄C coating exhibited larger bending strength and better wear behavior at a relative low sintering temperature.