Search

Fully dense ZrB 2 –TiC composites were produced by reactive pressureless sintering using ZrB 2 and TiC as the starting reactants. The effect of TiC content on microstructure and mechanical properties of ZrB 2 –TiC composites was investigated. The densification and grain refinement were achieved due to the reaction and solid solution coupling effect. Besides, elongated (Ti, Zr)B 2 plate-like grains with a high aspect ratio were obtained for the first time by the reaction of ZrB 2 and TiC. The ZrB 2 -70 mol%TiC composite exhibited good comprehensive mechanical properties of high hardness of 29.0 GPa, flexural strength of 555 MPa, and fracture toughness of 5.4 MPa m1/2 due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects. [ABSTRACT FROM AUTHOR]

Boron nitride-barium aluminosilicate (BN-BAS) composite ceramics were fabricated by the sol-gel method and hot-pressed sintering. The effects of the introduction method of BAS and sintering temperature on the phase assemblage, density, apparent porosity, mechanical properties and microstructures of the h-BN composite ceramics were investigated. The following two introduction methods were adopted: (A) BAS gel powder was prepared by the sol-gel method and then mixed with h-BN for hot pressing sintering, and (B) the BN-BAS composite powder was first prepared by the sol-gel method and then hot-pressed and sintered. The results indicate that the density, bending strength and fracture toughness of the latter are higher than those of the former when other conditions are the same. In the gel step of Group B, BAS has been coated on the surface of BN particles in dots, chemical bonding has existed, the distribution of BAS is more uniform, and it can better serve as a sintering aid. The BN-BAS composites prepared by both methods achieved the maximum mechanical properties at a sintering temperature of 1700 °C, and the density, apparent porosity, bending strength and fracture toughness of Group A were 2.40 g/cm3, 1.82%, 132.8 MPa and 2.3 MPa m1/2, while those of Group B were 2.42 g/cm3, 1.45%, 162.9 MPa and 2.7 MP m1/2. [ABSTRACT FROM AUTHOR]

• The adding vanadium to (TiZrHfVNbTa)C favors eliminating Nb segregation and enhancing densification. • Excess adding vanadium reduces single-phase-forming ability of (TiZrHfVNbTa)C, due to the lattice distortion increment. • Mechanical properties of (TiZrHfVNbTa)C is optimized via adjusting vanadium content and sintering temperature. A series of (TiZrHfVNbTa)C high-entropy ceramics with different vanadium contents have been fabricated by pressureless sintering at 2300 °C–2500 °C for 1 h, utilizing self-synthesized carbide powders obtained by carbothermal reduction. The addition of vanadium is beneficial to promote densification process and refine grain, as well as facilitate the homogeneous distribution of metal elements. The distribution of pores is also modified, almost entirely existing at grain boundary, and the integral mechanical properties achieve optimization. However, excess adding vanadium does not favor forming a single-phase (TiZrHfVNbTa)C high-entropy ceramic. The optimal (TiZrHfVNbTa)C high-entropy ceramic sintered at 2300 °C possesses a high relative density of 97.5 % and homogeneous microstructure with small grain size of 1.2 μm. The flexural strength and Vickers hardness reach 473 MPa and 24.9 GPa, respectively. This work has established a cost-effective and convenient preparation of novel (TiZrHfVNbTa)C high-entropy carbide ceramics. [ABSTRACT FROM AUTHOR]

Monolithic α-SiC particles and in situ β-Si3N4-reinforced barium aluminosilicate (BAS) glass ceramic matrix composites with good mechanical properties were obtained by spark plasma sintering. However, attempts at the synthesis of ternary phase composites in order to maximize the reinforcing effects by co-adding α-SiC and β-Si3N4 were only partly successful. β-Si3N4 grain growth was remarkably hindered by the coexistent α-SiC, thus the resultant composite suffered a loss of reinforcement efficiency. [Copyright &y& Elsevier]

Abstract: A 20vol.% ZrCp/W composite is fabricated by hot-pressing, and its mechanical properties can be significantly improved by the addition of ZrC particles. On the other side, the residual pores within ZrC particles clustering regions which are some latent microcrack sources are mainly caused by the relative low sinterability of ZrC particles. In order to eliminate the pores within the particle clusters, the as-sintered composite is annealed at 2300°C for 1h. The microstructure of the annealed composite becomes more uniform and densified. Components of the as-sintered and annealed composites are composed of a dilute solid solution tungsten-matrix, (Zr, W)C solid solution particles and a new W2C phase, but the content of W2C phase in the annealed composite increases. After annealing, the strength properties of the composite decrease, but its plasticity increases. [Copyright &y& Elsevier]

Abstract: A detailed microstructural and mechanical characterization of a PM processed 20vol.% ZrCp/W composite disk indicates that the composite from the edge to the center of the disk has different phase compositions and mechanical properties. The difference is caused by a temperature gradient in the disk during heating and cooling. [Copyright &y& Elsevier]

Abstract: SiB0.5C1.5N0.5 nanopowders were synthesized by mechanical alloying method, and a predominantly amorphous with a small volume fraction of nanocrystalline phase was formed in the powders. The results showed that the crystallites were SiC, and the polytype of SiC were 3C and 6H. SiB0.5C1.5N0.5 powders were consolidated by spark plasma sintering (SPS) technique at 1800°C under different sintering pressure. The grain boundaries of SiC and BCN phase could not be clearly distinguished for the ceramics sintered under 25MPa. When the pressure increased to 40MPa, SiC phase almost completely crystallized, and its grain size was less than 1μm. BCN phase were mainly distributed at boundary of SiC grain. Some amorphous structures were found in the sintered ceramics. Relative densities of the ceramics increased from 76.6% to 92.4%, and mechanical property values increased two or three times with the increase of sintering pressure. For the ceramics sintered under a pressure of 40MPa, flexural strength, fracture toughness, elastic modulus and Vickers’ hardness attained to 311.5MPa, 3.45MPam1/2, 133.0GPa and 3.68GPa, respectively. The mass of the SiBCN ceramics lose by several percent below 930°C in air, and then keep unchanged above 1100°C. [Copyright &y& Elsevier]

MgO-Al 2 O 3 -SiO 2 (MAS) glass ceramic was used as liquid sintering aid to densify h -BN ceramics, and effects of sintering temperature on the phase composition of MAS, texture behavior of h -BN grains and properties of the composite ceramics were studied. h -BN grains will react with MAS by forming N-M (Al3+ and Si4+) coordinate-covalent bonds and inhibit the crystallization of glass phase. The h -BN grains will rotate parallel to the pressure direction due to the joint action of the liquid sintering aid and the pressure. The sintering temperature will significantly affect the properties of the composite ceramics by effecting the texture behavior of h -BN grains. And higher sintering temperature of 1900 °C will result in the decline of mechanical properties and increase of thermal conductivity due to the decrease of texture degree caused by the grain growth of h -BN. [ABSTRACT FROM AUTHOR]

In this study, amorphous SiBCN powders were mechanical milled and consolidated by low-temperature sintering at different pressure to provide an insight into the densification and mechanical behavior of dense amorphous SiBCN monoliths. Results show that the mechanical properties undergo a gradual variation closely related to the microstructural evolution. The increased degree of densification with few defects and free volume results in significantly improving Young’s modulus and nano hardness in case of high pressure application. The consolidation at relatively low pressure (3 GPa) occurs by heavy deformation with the support of limited short-range diffusion, whereas the densification at higher pressure (5 GPa) is mostly diffusion controlled showing a classic sintering mechanism. At 1000 °C, higher pressure (5 GPa) suppresses crystallization leading to an effective solid-state reaction, while SiBCN monoliths consolidated by lower pressure (3–4 GPa) show a hybrid structure of dominant amorphous phases and few nanocrystals. [ABSTRACT FROM AUTHOR]

A novel pressure-assisted transient viscous phase sintering process has been used to fabricate dense BN-ZrO 2 -SiC composites at relatively low sintering temperatures. The sintering behavior and densification mechanism of BN-ZrO 2 -SiC composites incorporated with B 2 O 3 as transient viscous phase have been investigated. A rapid densification process occurs at about 1300 °C, which is attributed to the in-situ formation of SiO 2 by chemical reactions among ZrO 2 , SiC and B 2 O 3 . BN-ZrO 2 -SiC composites sintered at 1300 °C under 100 MPa exhibit a relative density of 95.9%, a flexural strength of 242 MPa and a fracture toughness of 4.1 MPa m 1/2 . [ABSTRACT FROM AUTHOR]

Pure h-BN ceramics were processed at different sintering temperatures and hold times using ball milled nanocrystalline or amorphous h-BN powders. Increases in sintering temperature induced greater grain growth but reduced relative densities and mechanical properties. Changes in sintering hold times caused further increases in grain sizes and relative density, but had little impact on mechanical properties. The grain sizes play an important aspect of strength in h-BN ceramics. The relationship of flexural strength and grain sizes in h-BN ceramics conforms to Hall-Petch behavior. When the grain size of h-BN ceramics increased from 0.3 μm to 8 μm, the flexural strength declined from 163 ± 6 to 84 ± 4 MPa. The mechanical properties of h-BN ceramics depend both on relative densities and grain sizes. [ABSTRACT FROM AUTHOR]

Abstract: Porous titanium was prepared by pressureless sintering of titanium beads with diameters of 100, 200, 400 and 600μm. The results indicated that the mechanical properties of porous titanium changed significantly with different bead diameters. Plastic deformations such as necking phenomenon and dimple structure were observed on the fracture surface of porous titanium sintered by beads with diameter of 100μm. However, it was difficult to find this phenomenon on the porous titanium with a titanium bead diameter of 600μm. The microarc oxidized coatings were deposited on its surface to improve the bioactivity of porous titanium. Furthermore, the cefazolin sodium/chitosan composited films were fabricated on the microarc oxidized coatings for overcoming the inflammation due to implantation, showing good slow-release ability by addition of chitosan. And the release kinetic process of cefazolin sodium in composited films could be possibly fitted by a polynomial model. [Copyright &y& Elsevier]

In the present paper, aluminum matrix composites (AMCs) reinforced by nanodiamond (ND) were fabricated by hot-press sintering. The morphology and microstructure of ND/Al composite powders for different milling hours were observed. The effect of ND content on microstructure, mechanical properties and tribological behavior of AMCs was investigated. The relative density, hardness, compressive strength and wear resistance of AMCs with different ND contents were compared. The flake-shape aluminum particles with uniformly distributed ND particles were formed by ball milling for 15 h. A laminated structure of AMCs was formed by hot-press sintering at 550 °C for 5 min. ND was distributed at the grain boundary of Al matrix. At the interface between Al and ND, a layer of Al with high dislocation density was formed, which was closely connected with a layer of amorphous carbon. With increasing of ND content, more and larger pores were presented in AMCs, which caused a decrease in relative density. The hardness, compressive strength and wear resistance of AMCs were significantly improved in comparison with sintered aluminum. The AMC with 2.5% ND obtained the highest hardness and compressive strength and the lowest coefficient of friction and wear rate. [Display omitted] • A laminated structure is formed in the sintered ND-Al composite material. • At the interface between Al and ND a layer of Al with high dislocation density is formed. • The composite with 2.5% ND exhibits the highest hardness and compressive strength and the lowest COF and wear rate. [ABSTRACT FROM AUTHOR]

Abstract: TaC/SiC composites with 20vol.% SiC addition were densified by spark plasma sintering at 1600–1900°C for 5min under 40MPa. Effects of sintering temperatures on the densification, microstructures and mechanical properties of composites were investigated. The results showed the materials achieved >98% of theoretical density at a temperature as low as 1600°C. While the TaC grains grew slightly with the sintering temperature increasing, the SiC particles in materials decreased in size. Equiaxed to elongated grain morphology transformation was observed in the SiC phase in the 1900°C material to obtain a higher flexural strength and fracture toughness of 715MPa and 6.7MPam1/2, respectively. Lattice enlargement of the TaC phase in the 1900°C material suggested possible Si diffusion into TaC grains. Ta was also detected in SiC grains by energy dispersive spectroscopy. Glassy pockets present at multi-grain junctions explained the enhanced densification. [Copyright &y& Elsevier]

Abstract: TaC/SiC composites with 5–40vol.% 6H–SiC were fabricated by spark plasma sintering at 1800°C for 5min. Effects of SiC concentrations on the densification, microstructure, and mechanical properties of the materials were investigated. The results showed the densification process of the TaC/SiC composites was composed of six sequent stages. SiC addition was effective in eliminating the closed porosities in the final stage to yield fully dense TaC/SiC composites at 20vol.% SiC concentrations and above. TaC grain sizes decreased with SiC concentration increase. The 40vol.% SiC composition had the highest flexure strength and fracture toughness up to 703MPa and 6.8MPam1/2 due to the reinforcement of the SiC agent. The TaC/SiC composites with 20 and 40vol.% SiC showed good flexural strength at 1400°C. [Copyright &y& Elsevier]

Abstract: A TaC composite with ∼11wt% in situ TaB2 was fabricated by spark plasma sintering at 1600–1900°C. The densification process was studied by analysis of the densifying shrinkage of the powder compacts. Three distinct stages for densification were determined. The starting powder mixture of TaC, B4C and Ta completed reaction to form the desired TaC and TaB2 at temperature <1583°C. At around ∼1750°C, the TaC/TaB2 material significantly improved relative density to ∼95% with rapid grain growth. The final densification took place very rapidly at 1900°C by releasing a high pressure vapor. [Copyright &y& Elsevier]

Abstract: Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si3N4 green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si2N2O was formed through reaction between Si3N4 and silica sol at a temperature above 1550°C. Si3N4/Si2N2O composite ceramics with a low linear shrinkage of 1.3–5.7%, a superior strength of 95–180MPa and a moderate dielectric constant of 4.0–5.0 (at 21–39GHz) were obtained by varying infiltration cycle and sintering temperature. [Copyright &y& Elsevier]

Abstract: cBN/β-SiAlON composites with different cBN contents were fabricated by spark plasma sintering (SPS) process. The densification, thermal stability of cBN, microstructure and mechanical properties of the resultant composites were investigated. The addition of cBN particles to β-SiAlON could increase the Vickers hardness, flexural strength and fracture toughness of β-SiAlON. It is attributed to the high density, no obvious cBN to hBN transformation and good interface bonding between cBN grains and β-SiAlON matrix. [Copyright &y& Elsevier]

Abstract: Y-α-SiAlON (Y1/3Si10Al2ON15) ceramics with 5wt.%BaAl2Si2O8 (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600°C without holding and using a heating rate of 100°Cmin−1, but the transformation from α-Si3N4 to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800°C for 5min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2GPa and fracture toughness of 6.8MPam1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300°C. [Copyright &y& Elsevier]