Hot-pressed Materials Research Articles

Kinetics of compaction of hot-pressed composite materials in the TiN - AlN system

The kinetics of compaction of hot-pressed materials in the TiN - AlN system has been studied. The shrinkage of TiN - AlN composites (for an AlN concentration varying within 10 – 90 vol.%) occurs within 10 – 15 min in the temperature range 1750 – 1850°C at a pressure of 65 MPa. The calculated activation energy for hot-pressed ceramic [50% TiN + 50% AlN] was found to be 5.0 eV, intermediate between the activation energies for neat TiN and AlN species. This implies that the sintering of hot-pressed heterophase mixtures characterized by a decrease of temperature under isothermal conditions is an activated process.

Lossy AlN–SiC composites fabricated by spark plasma sintering

Dense lossy AlN–SiC composites doped with Y2O3 were fabricated by spark plasma sintering (SPS). Both the microstructure and the dielectric properties are different from those of hot-pressed samples. Microstructure analysis reveals little solid solution (AlN)x(SiC)1-x is formed. Scanning evanescent microwave microscopy images show that the materials by SPS exhibit large contrast in dielectric permittivity, whereas the hot-pressed materials show very mild fluctuation in dielectric permittivity over the samples. The results indicate that AlN–SiC composites fabricated by SPS can be treated approximately as a mechanical mixture of AlN and SiC when estimating complex permittivity of the composite in the microwave range. The complex permittivity of the composites with different SiC contents can be phenomenologically predicted by effective medium approximation.

Structure and Certain Properties of Hot-Pressed Boron Carbide-Based Ceramic with Calcium-Silicon Additive

The influence which the composition of powder mixtures, the treatment conditions which the mixtures are subjected to, and the conditions under which the hot-pressed composite materials B4C – (5-10 mass%) calcium-silicon are fabricated exert on the structure, nature of failure, and mechanical properties of these materials is investigated. Optimum properties are possessed by material containing 10 mass% of addition. It is shown that the structure, morphology, and dispersivity, as well as the nature of the distribution of the components that are added to the composite material (secondary phase) vary as the temperature of hot pressing changes. Maximal mechanical characteristics of the composite material (σbend = 560 MPa, K 1c = 4.7 MPa·m1/2, HV = 37 GPa) are attained at hot-pressing temperatures in the range 2000-2100°C.

Conditions of Development of an Activated Contact Surface in the Formation of Hot-Pressed Powder Materials

Conditions of Development of an Activated Contact Surface in the Formation of Hot-Pressed Powder Materials

Reaction sintering of fluorine-doped MgSiN 2

Reaction bonded MgSiN 2 (RBMSN) with CaF 2 and YbF 3, as sintering additives was prepared by direct nitridation of Si/Mg 2Si/Mg/Si 3N 4 powder compact in a temperature range 1350–1550 °C. The influence of the ratio of constituents on the final phase composition, the amount of sintering additives and the influence of sintering temperature are discussed. It was shown that the starting powders should be in a ratio Mg 2Si/Mg⩾3 and Si 3N 4/Si tot⩾0.5 respectively, to obtain single-phase MgSiN 2. The oxygen content of MgSiN 2 was in the range of 0.4–0.7 wt.%. The final density of pressureless sintered RBMSN with 4 vol.% fluorine additives was 85% of theoretical density. Higher density (∼92%) was obtained for the reaction-bonded composite consisting of 48 vol.% MgSiN 2 and 52 vol.% Si 3N 4, due to the volume increase after the nitridation of silicon. The mechanical properties of fluorine-doped RBMSN were further improved by hot-pressing at 1600 °C. The 4-point bending strength and Vickers hardness (HV1) of hot-pressed materials are 427 MPa and 20.8 GPa, respectively. The indentation fracture toughness is 5.3 MPa m 1/2, due to the presence of elongated β-Si 3N 4 grains. The thermal conductivity of the dense reaction bonded MgSiN 2 is 22 W m −1K −1.

Effects of heat treatment and sintering additives on thermal conductivity and electrical resistivity in fine-grained SiC ceramics

The effects of heat treatment and sintering additives on the thermal conductivity and electrical resistivity of fine-grained SiC materials were investigated. The thermal conductivity and the electrical resistivity of dense SiC materials were measured at room temperature by a laser flash technique and a current-voltage method, respectively. The results indicated that the thermal conductivity and electrical resistivity of the SiC materials were dependent on the sintering additives and the resultant microstructure. Annealed materials with oxide additives developed microstructures consisting of elongated grains of various α/β–SiC polytypes. In contrast, annealed materials with oxynitride additives had microstructures consisting of fine equiaxed grains entirely of β–SiC phase. For the annealed materials with oxide additives the observed thermal conductivity was over 110 W/mK. For the annealed materials with oxynitride additives the observed value was 47 W/mK. The electrical resistivity of a hot-pressed material with oxide sintering additives decreased after annealing. For annealed materials with oxynitride additives, the electrical resistivity was even lower. High-resolution electron microscopy revealed a thin amorphous phase along the grain boundaries. Energy dispersive x-ray spectroscopy results showed that there was segregation of both Al and O atoms and a very small amount of Y atoms at grain boundaries. The results indicated that the chemistry and structure of the grain boundary has significant influence on thermal and electrical properties in SiC.

Synthesis of (Ca,Mg)-α-sialon from slag by self-propagating high-temperature synthesis

The synthesis of (Ca,Mg)-α-sialon powders using slag as a starting material by self-propagating high-temperature synthesis (SHS) is reported for the first time. The only crystalline phase detected in the synthesized products was α-sialon. The characters of the synthesized powders were observed and compared with that of conventionally hot-pressed materials. More metal cations were absorbed into the α-sialon structure of the SHS synthesized powders. Fully densified sialon ceramics can be obtained using the SHS powders by hot-pressing. The hardness and fracture toughness of the hot-pressed α-sialon ceramics reached 16 GPa and 5.1 MPa m−1/2 respectively.

Tailoring the mechanical properties of silicon carbide ceramics by modification of the intergranular phase chemistry and microstructure

Abstract Liquid-phase-sintered SiC has attracted increasing interest for its ability to form an in-situ toughened material and its potentially superior mechanical properties relative to the solid-state-sintered SiC. In the present work, a submicron-size β-SiC powder was densified with additives of various combinations of rare-earth oxides (RE 2 O 3 ; RE=La, Nd, Y and Yb) and alumina by hot-pressing, and the hot-pressed materials were further annealed at higher temperatures. The phase compositions, microstructures and mechanical properties of the hot-pressed and the annealed materials were characterized. It was found that the mechanical properties were strongly influenced by the type of the sintering additives. The additives also affected the microstructural development during annealing. While the fracture toughness of all the annealed materials continuously increased with increasing annealing temperatures, however, the flexural strength either increased or decreased with annealing temperatures, depending on the kinds of the RE 2 O 3 additives. By using appropriate rare-earth oxides, e.g. La 2 O 3 or Nd 2 O 3 , fracture toughness and flexural strength synergetically improved after an annealing treatment.

Thermoelectric Properties of n-type Bismuth Telluride Based Alloys Prepared by Hot Pressing and Zone Melting Method

Alloys of Bi2Te3 rich side of Bi2Te3-Bi2Se3 were prepared by the zone melting method and the hot pressing method in order to compare their thermoelectric properties. When specimens were hot pressed thermoelectric properties changed as a function of particle size, pressing time and hot pressing temperature. The reasons for the variation of the thermoelectric properties were investigated by examining influences of following parameters; oxidation, mechanical deformation during pulverization and the hot pressing temperature. Thermoelectric properties of zone melted ingot were largely affected by dopants when composition is fixed, whereas thermoelectric properties of hot pressed material were mainly related with the variation of the carrier concentrations caused by the generation of electrically active defects from many sources. Defects induced by the mechanical deformation and oxygen causes generation of donors. Defect concentration is also altered with different hot pressing temperatures depending on the amount of previously received mechanical deformation.

Investigation of the Structure and Properties of Materials in the SiC ― TiB2 System

The structure and properties of sintered and hot-pressed materials of the SiC ― TiB2 system were investigated. The optimal conditions for obtaining dense ceramics with minimum grain size of the phase components was determined. It was established that composites containing from 25 to 50 vol.@percnt; TiB2 have a bend strength equal to 450 MPa at 20°C; this increases to ∼500 MPa at 1600°C for ceramics with a pseudoeutectic structure. The material is highly heat resistant in the temperature range 900-1200°C.

Influence of annealing treatments on microstructure and toughness of liquid-phase-sintered silicon carbide

The possibility of anin situtoughening through the β → α phase transition was evaluated on liquid-phase-sintered SiC (with Al2O3and Y2O3as additives). Dense hot-pressed materials with an equiaxed morphology were annealed at 1850, 1900 and 1950 °C for 1 to 4 h so the effect of time, temperature, composition, and amount of second phase could be investigated. Microstructure features revealed that 1900 °C was the best temperature in producing a high percentage of elongated grains with limited grain coarsening and reduction of second-phase pockets. Mean grain size and aspect ratios increased from 0.5 to 1.5 μm and to 5, respectively, during the first 2 h of treatment at 1900 °C and then maintained a constant value. The amount and composition of second phase influenced the rate of transformation from equiaxed to elongated grain morphology. Toughness increased from 3.3 to 5.5 Mpa · m1/2in 1900 °C annealed samples due to a crack-deflection mechanism.

Borided Hot-Pressed Powder Materials Based on Iron. Part 2. Effects of Hot Pressing Conditions on Surface Quality and Properties of Borided Powder Materials

Hot pressing is considered for borided and unborided powder blanks. In the hot additional pressing of unborided steel compacts, protection from decarburization is required. The hot pressing of borided blanks should be conducted at 1200°C in the presence of liquid eutectic phase. This produces good quality in the borided layer. Cracks do not arise at the ends of the blanks. The cracks that arise on the side surfaces at the start of deformation are filled by liquid phase in the final stage. The best conditions have been established for heating the blanks made by solid-phase and liquid-phase boriding.

Influence of powder characteristics and powder processing routes on microstructure and properties of hot pressed silicon nitride materials

Four different commercial Si3N4 powders were hot pressed with the addition of La2O3 and Y2O3 as sintering aids. Two powder processing routes were set up: addition of sintering aid powders by ultrasonic dispersion, addition of nanodispersed amorphous additive species by chemical coprecipitation. The following aspects were analyzed: characteristics of starting powders and powder mixtures, with reference to surface modification (electrokinetic behaviour and surface properties) induced by the powder treatment; sintering behaviour of the powder mixtures; influence of raw powders characteristics and processing route on microstructure and properties of dense materials. The microstructural characteristics of hot pressed materials (grain size, aspect ratio, grain boundary phases) were found to be dependent on powder characteristics and its process history. Significant variation of the mechanical properties (Young modulus, hardness, toughness and strength) were related to microstructural features. Strength, for example, ranges from 600 to 1200 MPa at room temperature and from 400 to 1000 MPa at 1200°C; toughness ranges from about 4 to about 6 MPam1/2.

Copper alloy–stainless steel bonded laminates for fusion reactor applications: tensile strength and microstructure

The tensile strength of copper alloy–stainless steel bi-layer panels joined by hot isostatic pressing or explosive bonding was tested over a temperature range 25–350°C. The bonding processes caused changes in interfacial microstructures which affected the interfacial mechanical properties. Hot isostatic pressed materials studied included: precipitation strengthened CuNiBe or dispersion strengthened Cu–Al 2O 3 bonded to 316L stainless steel plates and Cu–Al 2O 3 bonded to Cu–Al 2O 3. An explosive bonded panel of Cu–Al 2O 3–316L stainless steel was also examined. Hot isostatic pressed panels showed similar decreasing shear strengths with increasing temperature, while the explosive bonded panel displayed substantially higher strength at 25°C but declined rapidly as the test temperature increased. Tensile strengths of all panels were nominally lower than the constituent bulk material strengths and delamination near the copper alloy–stainless steel interface was the dominate failure mode in the bi-metallic panels. The joining processes were found to alter the microstructure of the bond interfacial regions in the form of microporosity and Fe–Cr–B precipitate formation, NiBe precipitate morphology changes and interdiffusion of constituent elements.

Effects of Microstructure on Superplastic Behavior and Deformation Mechanisms in β‐Silicon Nitride Ceramics

The influence of grain shape and size on superplastic behavior and deformation mechanisms was investigated in annealed β‐silicon nitride materials and compared with the results for hot‐pressed material. The microstructure of the annealed materials consisted of fine equiaxed β‐grains together with some elongated ones. Similar to the deformation behavior in the hot‐pressed material, strain hardening did not occur in these annealed materials. Moreover, in contrast to the deformation behavior under tension, grain alignment under compression resulting from the development of a mild texture did not give rise to strain hardening. An annealed material with small elongated grains had a flow‐stress dependency of n= 1, whereas other annealed materials with large elongated grains exhibited a flow‐stress dependency of n= 1.6. In terms of texture development and the effect of grain shape on the creep rate when diffusion was the rate‐controlling mechanism, a single curve with a stress exponent of ∼1 and a grain‐size exponent of 3 were obtained for all materials. This suggests that the deformation mechanism in these annealed materials was the same as that of fine equiaxed β‐silicon nitride.

Zirconia–spinel composites. Part II: mechanical properties

The mechanical properties (fracture strength, fracture toughness, and Vickers microhardness) of MgAl 2O 4 and x wt% ZrO 2–MgAl 2O 4 (1 ≤ x ≤ 30) hot-pressed materials were measured at room temperature. Two kinds of materials, which were prepared from either ball-milled or attrited powders, were investigated. Compared with an unreinforced spinel, a twofold (or greater) increase in both strengthening and toughening was measured for materials with the highest zirconia content (30 wt%). It was found that dispersing only 1 wt% of zirconia in the spinel produced a marked increase in the Vickers microhardness. The total zirconia content appears to be a key parameter.

熱電変換材料 高速フレーム溶射によるＦｅＳｉ２熱電材料の溶射成形

Large sized n-and p-type FeSi2 thermoelectric devices were spray-formed by high-velocity oxy-fuel thermal spraying (HVOF) method. One is a plate 100mm square with the thickness of 6mm and another is a tube-type device which was formed on a stainless tube (25 mm in diameter, 500 mm long) with the thickness of 5 mm. Their microstructure and physical properties were compared to those of hot-pressed material. The bulk density of HVOF-formed FeSi2 was lower than that of hot-pressed materials due to higher amount of pore. For n-type FeSi2, Seebeck coefficient and thermal conductivity of HVOF-formed material were comparable to those of hot-pressed material. While electrical conductivities of both n-and p-type HVOF-FeSi2 were quite lower than that of hot-pressed n-type material, resulted in low performance as thermo electric device.

Tribological characteristics of self-mated couples of Si 3N 4–SiC composites in the range 22–700°C

The tribological behaviour of self-mated pairs of Si 3N 4, Si 3N 4/SiC P (platelets) and Si 3N 4/SiC N (nanosized) hot pressed materials was studied in a pin-on-disc configuration. Tests were performed without lubrication, at a fixed load of 5 N, using different temperatures (room temperature −700°C) and sliding speeds (0.5–2 m s −1). Friction coefficients were usually above 0.5, being almost independent of sliding speed and test temperature. For all test conditions, the Si 3N 4/SiC P composite had the lowest friction coefficient, due to the presence of SiC platelets with smooth cleavage planes oriented parallel to the sliding surface. Wear coefficients were above 10 −6 mm 3 N −1 m −1 denoting a generalised severe wear mode, controlled by surface microcracking. No net differences were found between the wear behaviour of monolithic and composite materials under this wear regime. Wear decreased with sliding speed due to the protective action of a coherent tribofilm resulting from debris aggregation. Conversely, wear coefficients showed a steep increase above room temperature, to values around 10 −4 mm 3 N −1 m −1 at 700°C.

Thermodynamics of the Al–C–O system and properties of SiC–AlN–Al 2OC composites

Based on a recent thermodynamic evaluation of the Al–C–O system, the standard Gibbs free energies of formation of both aluminium oxycarbides Al 4O 4C and Al 2OC are given, and a classical stability diagram is shown at 2100 K. Because Al 2OC is unstable below 1715 °C, the stable würtzite compound 2AlN.Al 2OC has been preferred, and formed in-situ as the second phase in SiC-based composites. Starting with commercial powders of α-SiC, AlN, Al 2O 3 and Al 4C 3, dense materials are obtained by pressureless sintering (up to 2020 °C) or hot-pressing (up to 1950 °C), owing to the liquid phase from the Al 2O 3–Al 4C 3 system. The existence of a miscibility gap is shown, and the microstructures are fine grained and equiaxed. Compared with SiC–Al 2OC alloys, the hot-pressed materials with 90 wt% SiC exhibit slightly higher mechanical properties and a good retention nearly up to 1500 °C.

Effect of the method of fabricating SiC, Si3N4, and a1n specimens on their resistance to hightemperature oxidation

The methods of high-pressure sintering (5 GPa) and, for comparison, conventional hot pressing are used for fabricating high-density specimens from SiC, Si3N4, and A1N with the use of activating additives and without them. The high-temperature oxidation is studied by the method of thermogravimetry at 1400°C with a 150-min hold. The introduction of additives in order to activate the sintering increases the resistance of hot-pressed specimens to high-temperature oxidation and does not influence the stability of materials obtained in a high-pressure apparatus. The corrosion resistance of materials sintered under a high pressure turns out to be much higher than that of hot-pressed materials, which is connected with the formation of superfine-grained densely fit oxide films