Hot-pressed Materials Research Articles

The microstructure and mechanical properties of ball-milled stainless steel powder: The effect of hot-pressing vs. laser sintering

The microstructure and mechanical properties of sintered stainless steel powder, of composition AISI 420, have been measured. Ball-milled powder comprising nanoscale grains was sintered to bulk specimens by two alternative routes: hot-pressing and microlaser sintering. The laser-sintered alloy has a porosity of 6% and comprises a mixture of delta ferrite and tempered martensite, and the relative volume fraction varies along the axis of the specimen due to a thermal cycle that evolves with progressive deposition. In contrast, the hot-pressed alloy has a porosity of 0.7% and exhibits a martensitic lath structure with carbide particles at the boundaries of the prior austenite grains. These differences in microstructure lead to significant differences in mechanical properties. For example, the uniaxial tensile strength of the hot-pressed material is one-half of its compressive strength, due to void initiation at the carbide particles at the prior austenite grain boundaries. Nanoindentation measurements reveal a size effect in hardness and also reveal the sensitivity of hardness to the presence of mechanical polishing and electropolishing.

Densification of hot-pressed composite materials for diamond tools

A set of compaction equations suitable for final stage of hot-press forming and sintering is deduced, all physical parameters are analyzed, the analysis results are consistent with a part of empirical formulas. Composites for diamond tools are taken as the subject of the investigation. Under the conditions of hot-forming sintering, the priority is given to the research of the relation between heat capacity Cp at constant pressure. By combining analysis of the set of compaction equations, we performed differential thermal analysis (DTA) in the differential scanning calorimetry (DSC) mode of heat capacity at constant pressure of pure Co powder, composite powder that consists of 663Cu and 35% Fe-based powder, analyzed the relationship between Cp and composite densification, and modified Cp of matrix composites containing 35% Fe-based composite powder for diamond tools in the mode of mixing a rare earth to make it as similar to Cp of pure Co as possible. Such modified powder is used in practical hot pressing of diamond tool composites and guides it with a set of compaction dynamical equations to radically improve the stability of practical performance (bending strength, hardness, impact ductility, porosity) of powder metallurgy composites and compaction of diamond tool composites.

Thermal Stress Analysis and Structure Parameter Selection for a Bi2Te3-Based Thermoelectric Module

The output power and conversion efficiency of thermoelectric modules (TEMs) are mainly determined by their material properties, i.e., Seebeck coefficient, electrical resistivity, and thermal conductivity. In practical applications, due to the influence of the harsh environment, the mechanical properties of TEMs should also be considered. Using the finite-element analysis (FEA) model in ANSYS software, we present the thermal stress distribution of a TEM based on the anisotropic mechanical properties and thermoelectric properties of hot-pressed materials. By analyzing the possibilities of damage along the cleavage plane of Bi2Te3-based thermoelectric materials and by optimizing the structure parameters, a TEM with better mechanical performance is obtained. Thus, a direction for improving the thermal stress resistance of TEMs is presented.

High-strength ceramic based on zirconium dioxide: preparation and properties

Strength properties are considered for ZrO2 hot-pressed ceramic materials stabilized with 4 – 6 wt.% Y2O3 in relation to original powder preparation method: solid phase synthesis, high-temperature salt thermolysis, combined hydroxide precipitation followed by freezing, drying and calcining, and the sol-gel method. Ceramic material with the highest strength in bending (up to 1350 MPa) are prepared from ZrO2 – 5 wt.% Y2O3 powders, synthesized by coprecipitation of hydroxides followed by freezing.

Compressive yield strength of the nanocrystalline Cu with Al 2O 3 dispersoid

Nanocrystalline (nc) Cu with Al 2O 3 dispersoid (∼4 vol.%) was successfully synthesized by simple cryo-milling at 210 K with a mixture of Cu 2O, Al, and Cu elemental powders. The milled powder was consolidated by hot pressing (HP) at 1123 K and 50 MPa for 2 h. TEM (Transmission Electron Microscopy) work revealed that both of the milled powder and the hot pressed (HPed) materials were comprised with a mixture of the nc-Cu and homogeneous distribution of Al 2O 3 dispersoids. The compressive and micro Vickers hardness tests were performed on the HPed materials (nanocrystalline Cu with 4 vol.% of Al 2O 3 dispersoid) at room temperature to characterize the mechanical properties of the materials. The compressive yield strength of the materials was as high as 863 MPa; the micro Vickers hardness 2600 MPa. The results of the mechanical tests apparently show that the relationship between the yield strength and the micro hardness of the HPed materials is in well agreement with Tabor's rule, H v = 3 σ y in MPa. The grain size of the nc-Cu was estimated by XRD using Scherrer's formula and TEM observation; the Al 2O 3 dispersoid size was measured from element mapping by STEM–EDS (Scanning Transmission Electron Microscopy–Energy Dispersive Spectroscopy) works. An attempt was made to quantify the possible strengthening effects of the nc-Cu materials with Al 2O 3 dispersoid. Two strengthening mechanisms were proposed for high hardness and yield strength of the materials, i.e., grain size and dispersion hardening effects.

Enhanced particle rearrangement during liquid phase spark plasma sintering of silicon nitride-based ceramics

Densification evolution has been analysed in liquid phase assisted spark plasma sintering (SPS) of silicon nitride (Si 3N 4)-based materials. Monolithic ceramics with variable amounts of Al 2O 3/Y 2O 3 sintering additives, and carbon nanotubes (CNTs) containing Si 3N 4 composites have been considered. The shrinkage behaviour of SPSed monolithic Si 3N 4 showed a noticeable enhancement of the particle rearrangement stage, exhibiting a complete wetting of the grain boundary phase even for the lowest additive content (1.70 vol.%), unlike the corresponding hot pressed (HPed) material. An improvement of the liquid phase wetting by the presence of electromechanical forces is proposed to explain the enhanced densification occurring in the particle rearrangement stage of the SPS process. Furthermore, the addition of CNTs seems to increase the efficiency of this mechanism, decreasing the particle rearrangement temperature.

A study of the nanostructure and hardness of electron beam evaporated TiAlBN Coatings

TiAlBN coatings have been deposited by electron beam (EB) evaporation from a single TiAlBN material source onto AISI 316 stainless steel substrates at a temperature of 450 °C and substrate bias of − 100 V. The stoichiometry and nanostructure have been studied by X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy. The hardness and elastic modulus were determined by nanoindentation. Five coatings have been deposited, three from hot-pressed TiAlBN material and two from hot isostatically pressed (HIPped) material. The coatings deposited from the hot-pressed material exhibited a nanocomposite nc-(Ti,Al)N/a-BN/a-(Ti,Al)B 2 structure, the relative phase fraction being consistent with that predicted by the equilibrium Ti–B–N phase diagram. Nanoindentation hardness values were in the range of 22 to 32 GPa. Using the HIPped material, coating (Ti,Al)B 0.29N 0.46 was found to have a phase composition of 72–79 mol.% nc-(Ti,Al)(N,B) 1 − x + 21–28 mol.% amorphous titanium boride and a hardness of 32 GPa. The second coating, (Ti,Al)B 0.66N 0.25, was X-ray amorphous with a nitride+boride multiphase composition and a hardness of 26 GPa. The nanostructure and structure–property relationships of all coatings are discussed in detail. Comparisons are made between the single-EB coatings deposited in this work and previously deposited twin-EB coatings. Twin-EB deposition gives rise to lower adatom mobilities, leading to (111) (Ti,Al)N preferential orientation, smaller grain sizes, less dense coatings and lower hardnesses.

Temperature-dependent thermal expansion of cast and hot-pressed LAST (Pb–Sb–Ag–Te) thermoelectric materials

The thermal expansion for two compositions of cast and hot-pressed LAST (Pb–Sb–Ag–Te) n-type thermoelectric materials has been measured between room temperature and 673 K via thermomechanical analysis (TMA). In addition, using high-temperature X-ray diffraction (HT-XRD), the thermal expansion for both cast and hot-pressed LAST materials was determined from the temperature-dependent lattice parameters measured between room temperature and 623 K. The TMA and HT-XRD determined values of the coefficient of thermal expansion (CTE) for the LAST compositions ranged between 20 × 10−6 K−1 and 24 × 10−6 K−1, which is comparable to the CTE values for other thermoelectric materials including PbTe and Bi2Te3. The CTE of the LAST specimens with a higher Ag content (Ag0.86Pb19Sb1.0Te20) exhibited a higher CTE value than that of the LAST material with a lower Ag content (Ag0.43Pb18Sb1.2Te20). In addition, a peak in the temperature-dependent CTE was observed between room temperature and approximately 450 K for both the cast and hot-pressed LAST with the Ag0.86Pb19Sb1.0Te20 composition, whereas the CTE of the Ag0.43Pb18Sb1.2Te20 specimen increased monotonically with temperature.

Electrical and mechanical properties of hot-pressed versus sintered LiTi 2(PO 4) 3

The electrical and mechanical properties of hot-pressed versus sintered LiTi 2(PO 4) 3 were investigated. The hot-pressed LiTi 2(PO 4) 3 had a higher density and larger average grain size than the sintered material. As a result of these microstructural differences the hot-pressed material exhibited a higher total ionic conductivity and lower hardness. The electronic conductivity of both materials was the same and increased by a factor of about 10 7 when the hot-pressed and sintered materials were heated under a reducing atmosphere.

Internal nitration in iron−chromium (manganese) nitride powder systems

The effect of the internal nitration of powder iron is examined by the introduction of nitrogen in the form of alloying additives such as chromium and manganese nitrides. It is established that the phase composition of these materials includes residual manganese and chromium nitrides and a solid solution of nitrogen in iron. To obtain a porousless or low-porosity hot-pressed material with evenly distributed fine inclusions of secondary nitrides and to maintain the composition, hot-pressed materials must be annealed in conditions that prevent nitrogen loss.

A Comparison of Friction Stir Processing of Mill Annealed and Investment Cast Ti-6Al-4V

Friction stir processing (FSP) of cast and mill annealed Ti-6Al-4V was studied to determine whether this thermomechanical processing technique is feasible for local microstructural modification to improve fatigue properties for aerospace engine applications. The ability to produce a friction stir processed surface layer is thought to provide a microstructure that is more resistant to fatigue crack initiation than the as-cast microstructure of the bulk. FSP resulted in a microstructure of either very fine equiaxed α or a colony microstructure with refined prior β grains when processing occurred below the β transus or above the β transus, respectively. The final microstructure observed in the stir zone was related to the processing conditions and was independent of the initial cast or wrought microstructures. Electron backscatter diffraction was used to assess the presence of microtexture in the stir zone. There was very little microtexture observed in sub-transus processed material. A moderate strength α phase texture was observed in the cast and wrought materials processed above the β transus. Fatigue tests performed on 4-point bend samples revealed a dramatic increase in fatigue life after FSP in both mill annealed (presented here) and cast + hot isostatic pressed materials (previously presented) although only a limited number of samples were available. Substantial tool wear is an issue and tungsten contamination is readily observed when FSP temperatures were below the β transus. This could be a concern in high temperature applications or for applications requiring postprocessing heat treatment.

Fabrication of toughened Cf/SiC whisker composites and their mechanical properties

In this paper, dense short carbon fiber reinforced silicon carbide matrix composites had been fabricated by hot-pressed (HP) sintering using Al 2O 3 and La 2O 3 as sintering additives. The results showed that the combination of Al 2O 3 and La 2O 3 system was effective to promote densification of short cut carbon fiber reinforced silicon carbide composites (Cf/SiC). The whisker structure of silicon carbide was formed during the annealed treatment at 2023 K for 1 h. However, it was noted that this structure was not observed in the as-received HP material. The mechanism of forming whisker structure was not clear, but this kind of whisker structure was helpful to improve mechanical properties. The combination of grain bridging, crack deflection and whisker debonding would improve the fracture toughness of the Cf/SiC composites.

Sintering and strength of hot-pressed ceramics based on titanium diboride

The paper examines how the grain-size composition of starting titan diboride powders and calcium silicon and calcium hexaboride additives influence the consolidation of titanium diboride in hot pressing and the structure and properties of hot-pressed materials. It is shown that 3 and 5 wt.% of calcium silicon decrease the pressing temperature by 200 °C, refine the structure, and improve the density and mechanical properties of titan diboride materials.

Microstructure and properties of an electroconductive SiC-based composite

In this work, an SiC-based electroconductive composite is obtained through simultaneous addition of MoSi2 and ZrB2 particles. The composite material is fully densified by hot pressing at 1860 °C and the microstructure is investigated by SEM-EDS analysis. Microstructural features and mechanical properties are compared to those of a monolithic hot-pressed SiC material. The MoSi2 and ZrB2 particles, besides increasing the electrical conductivity of the silicon carbide matrix, also act as reinforcement for the material. Room-temperature strength reaches the value of 850 MPa and the fracture toughness is 4.2 MPa m0.5. The composite electrical resistivity is of the order of 10−3 Ω cm.

Electrical Contact Fabrication and Measurements of Metals and Alloys to Thermoelectric Materials

AbstractLow electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators in order to convert heat to electricity. These contacts must be stable to high temperatures and through thermal cycling. A ratio of the contact resistance to the leg resistance below 0.1 is the goal for fabrication of a high efficiency thermoelectric power generation device. Here we present the fabrication procedures and characterization of contacts of metal alloys to Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds. Contacts were fabricated and measured for both ingot and hot pressed materials. Stainless steel 316 has shown a low resistance contact to these thermoelectric materials when the proper bonding conditions are used. Different time-temperature-pressure conditions for bonding to n-type and to p-type legs are presented. Contact resistances below 10μΩcm2 have been measured. In addition, break tests have shown bond strengths exceeding the semiconductor fracture strength. One of the considerations used in selecting iron alloys for electrical interconnects is the similarity in the coefficient of thermal expansion to the LAST and LASTT materials which is 18 ppm/°C and relatively temperature insensitive. Contacts to the thermoelectric materials were accomplished by diffusion bonding in a furnace developed in our lab at Michigan State University. The furnace is capable of reaching temperatures of up to 1000°C with a controlled atmosphere of a reducing gas. Fabrication procedures and contact data are presented in this paper.

Morphological, structural and optical properties of thin SiC layer growth onto silicon by pulsed laser deposition

Crystalline silicon carbide thin layers were grown on a p-type Si(1 0 0) substrate by pulsed laser deposition (PLD) using KrF excimer laser at λ=248 nm from a 6H–SiC hot-pressed target. The target “SiC” used to elaborate our SiC films is realized from a mixture of 1SiO 2 with 3C (carbon) “1SiO 2+3C” heated in an oven at 2500 °C (the target was a hot-pressed material and supplied by Goodfellow). The morphological, structural and optical properties of SiC layers were investigated by scanning electronic microscopy (SEM), high-resolution X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and UV–visible spectrophotometer. XRD analysis of the target showed that this latter is a hexagonal structure (6H–SiC). The XRD pattern shows that a 1.6 μm crystalline SiC layer was formed. In addition, a SIMS analysis gives a ratio Si/C of the thin SiC layer around 1.15 but the ratio Si/C of the target was found equal to 1.06, whereas one should have 1.0. This is due to the degree of the sensitivity of the SIMS technique and due to the higher ionization efficiency of Si compared to C atoms, all these which give different ratios. It is known that the PLD technique reproduces the same macroscopic property (optical, mechanical, structural, etc.) of the target. An optical gap ( E Gap) of the SiC layer of about 2.51 eV was obtained by reflectance measurement. Finally, a crystalline thin SiC layer of 1.6 μm was elaborated using PLD method at low-temperature deposition.

Microstructure and electrical properties of Si 3N 4–TiN composites sintered by hot pressing and spark plasma sintering

Electroconductive Si 3N 4–TiN composites were fabricated from micron-sized Si 3N 4 and nano-sized TiN powders by powder processing routes. The specimens were consolidated by hot pressing (HP) and spark plasma sintering (SPS). High densities (>98%) were obtained by sintering at temperatures ≥1500 °C using both sintering methods. The TiN phase grows rapidly during the liquid phase sintering, and its grain size increases with sintering temperature, leading to the increase in electrical resistivity. Dense materials produced by SPS possess significantly higher resistivity than the hot-pressed materials due to the larger TiN grain size and the presence of crystallized grain boundary phase.

Quality rating of a metal matrix-diamondcomposite from its thermal conductivity and electric resistance

The efficiency of hot-pressed diamond-containing composite materials (DCM) for various tool applications is greatly affected by microdefects, namely, the residual porosity of the metal matrix, damaged diamond grains, and imperfect diamond-matrix interfaces. An instrumental evaluation of these microdefects, predetermining the quality of a tool equipped with DCM, is rather difficult due to the small size, the nonstandard shape, and the strong heterogeneity of specimens. Proposed here is an alternative, nondestructive technique of DCM quality rating, which includes the measurement of electric resistance and thermal conductivity of diamond-containing composites and processing the obtained data by the methods of composite mechanics. It exploits the fact that diamond, being a dielectric, possesses an extremely high thermal conductivity, which allows estimating the residual porosity of a sintered metal matrix from the ratio of specific electric resistances, one being measured and another predicted by a theory. These data, in turn, are utilized to predict the thermal conductivity ofDCMwith an imperfect matrix. Matching with experiments, after solving the inverse problem gives the thermal resistance of diamond-matrix interface, which, within the frame work of the given model, simulates the damage of both the diamond grains and their bonds with the matrix. Thus, the numerical rating of quality is given in terms of two dimensionless parameters. The first one, 0 < K < 1, reflects the quality of the sintered metal matrix, whereas the second one, 0 < R <1, is an aggregate measure of the integrity of diamond grains and the perfection degree of composite interfaces. The quite satisfactory agreement observed between the theory and experiment confirms the efficiency of the technique and the reliability of the data obtained.

Evaluation of the quality of composite diamond-containing materials by their electrical and thermal conductivity

The serviceability of hot-pressed diamond-containing composite materials for application in tools depends substantially on the presence of microdefects, in particular, the residual porosity of the metal matrix, damaged diamond grains, and imperfect diamond-matrix interfaces. Experimental investigations of these defects are very difficult due to their small size and nonstandard shape as well as a strong inhomogeneity of the specimens. We propose an alternative technique of the nondestructive testing of the quality of diamond-containing composite materials, which includes measurement of the electrical resistance and thermal conductivity of a diamond-containing composite and analysis of the data obtained by the methods of composite mechanics. The quantitative estimate of quality is given in terms of two dimensionless parameters. The first of them, 0 < K < 1, reflects the quality of a sintered metal matrix, whereas the second, 0 < R < 1, is an aggregate measure of the integrity of diamond grains and the degree of perfection of the interfaces in a composite. The good agreement between theory and experiment corroborates the efficiency of this technique and the accuracy of the presented data.

Mechanochemical synthesis of Cu–Al 2O 3 nanocomposites

Nanocomposite powders of Cu–Al 2O 3 were produced by simple milling at room temperature in high-energy attritor mill using mixtures of Cu, Cu 2O and Al powder ingredients. The milled powders were then consolidated by hot pressing (HP) in high-density graphite die at 800, 850 and 900 °C under 50 MPa. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations showed that the consolidated materials were comprised of a mixture of nanocrystalline fcc-Cu and α-Al 2O 3 phase. The results of microstructural analysis also showed that Cu grains were of order of 20–30 nm with a bimodal size distribution of Al 2O 3 dispersoids: the coarse dispersoids are about 50–100 nm and fine ones about 10–20 nm. Average hardness number of the hot pressed materials consolidated at 850 °C increases from 70 to 103 HR B (Rockwell hardness number) with increasing vol.% of Al 2O 3.