Evolution of microstructure,mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y₂O₃–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.     

Fabrication of Gd2O3‐MgO nanocomposite optical ceramics with varied crystallographic modifications of Gd2O3 constituent

The fabrication of Gd₂O₃‐MgO nanocomposite optical ceramics via hot‐pressing using sol‐gel derived cubic‐Gd₂O₃ and MgO nanopowders was investigated. The precursor powder calcined at 600°C had an average particle size of 12 nm. The effects of hot‐pressing temperature on constituent phases, microstructure, mid‐infrared transmittance, and microhardness were studied. The crystallographic modifications of Gd₂O₃ phase varied with the increase in sintering temperature from 1250 to 1350°C. The monoclinic‐Gd₂O₃ phase was retained for the composite sintered at 1350°C and the sample had an average grain size of 90 nm, excellent transmission (80.4%‐84.8%) over 3‐6 μm wavelength range, and enhanced hardness value of 14.1 GPa.     

High-pressure work hardening of alumina

We report a method for strengthening microcrystalline alumina compaction by work hardening under conditions of high pressure and temperature, in which the hardness significantly improved. Micro-sized spherical alumina powders were treated without additives under 5.5 GPa–14 GPa and 600 °C–1200 °C. A polycrystalline sample sintered at 12 GPa and 900 °C yielded a Vickers hardness of 30.0 GPa, which was 1.5 times that of single-crystal alumina and the highest value reported to date for alumina. By analysing the density, hardness, and microstructure of the sintered samples, the results demonstrated that under ultra-high pressure and below the recrystallisation temperature of alumina, plastic deformation of the grains produced many substructures such as stacking faults and lattice distortions showing obvious work hardening effects. Combined with well-bonded crystalline and amorphous grain boundaries, the hardness of the samples was unexpectedly enhanced.     

Visible Light Transparency for Polycrystalline Ceramics of MgO·2Al2O3 and MgO·2.5Al2O3 Spinel Solid Solutions

Magnesium aluminate spinel solid solutions with the alumina‐rich compositions MgO·2Al₂O₃ and MgO·2.5Al₂O₃ have been prepared as polycrystalline ceramics with average in‐line transmissions at 550 nm of 85.5 ± 0.3% and 80.9 ± 0.4%, respectively. Starting powders are prepared from combinations of high purity Mg(OH)₂ and γ‐Al₂O₃ thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The optimum amount of LiF added varies with the alumina composition of the spinel solid solution. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa in Ar. Both compositions exhibit exaggerated grain growth with average sizes well over 500 μm. Knoop hardness measurements are 11.2 ± 0.3 GPa for MgO·2Al₂O₃ and 11.0 ± 0.4 GPa for MgO·2.5Al₂O₃.     

Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Oxidation resistance and thermal stability of the SiC-ZrB2 composite ceramic fibers

In this study, continuous SiC-ZrB₂ composite ceramic fibers were synthesized from a novel pre-ceramic polymer of polyzirconocenecarbosilane (PZCS) via melt spinning, electron beam cross-linking, pyrolysis, and finally sintering at 1800°C under argon. The ZrB₂ particles with an average grain size of 30.7 nm were found to be uniformly dispersed in the SiC with a mean size of 59.7 nm, as calculated using the Scherrer equation. The polycrystalline fibers exhibit dense morphologies without any obvious holes or cracks. The tensile strength of the fibers was greater than 2.0 GPa, and their elastic modulus was ~380 GPa. After oxidation at 1200°C for 1 hour, the strength of the fibers did not decrease despite a small loss of elastic modulus. Compared to the advanced commercial SiC fibers of Tyranno SA, the fibers exhibited improved high-temperature creep resistance in the temperature range 1300-1500°C.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.     

Fabrication of Highly Textured Fine‐Grained α‐Alumina by Templated Grain Growth of Nanoscale Precursors

[0001] textured alumina ceramics with a fine grain size were fabricated between 1400°C and 1600°C via templated grain growth (TGG) using fine alumina platelets (~0.6 and ~3 μm diameter) aligned by tape casting in either a 50 nm α‐Al₂O₃ matrix powder, or in a seeded boehmite sol. The 3 μm templates could be readily aligned by tape casting in both matrices (orientation parameters r = 0.27 and 0.18, respectively), whereas 0.6 μm diameter templates were well aligned in the seeded boehmite sol only (r = 0.29). Improved alignment in boehmite sols is attributed to inorganic gelation, resulting in a strongly pseudo‐plastic rheology that preserves template alignment against the influence of Brownian motion. The in situ formation of fine α‐Al₂O₃ matrix after transformation in the seeded boehmite system results in a higher driving force for TGG and improves texture development. The combination of 3 μm templates with a seeded boehmite matrix results in extremely high texture qualities (texture fraction f = 0.97–0.99, r = 0.17) while maintaining a relatively fine grain size (5–10 μm in diameter and 1.5–3 μm in thickness). Although undoped samples can be fully textured at 1600°C, adding as little as ~0.25 wt% CaO/SiO₂ dopant improves TGG kinetics and yields full texture at 1400°C.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

Real indentation hardness of nano-polycrystalline cBN synthesized by direct conversion sintering under HPHT

The hardness characteristic of nano-polycrystalline cBN synthesized by direct conversion sintering was thoroughly investigated using Vickers and Knoop indenters. It was found that nano-polycrystals consisting of smaller cBN grains increase the elastic recovery of indentations during unloading of the indenters and the diagonal of Vickers indentations and the minor diagonal of Knoop indentations significantly decrease in length. Thus, if a Vickers indenter is used, the apparent hardness value increases, making it impossible to perform an accurate evaluation, e.g. incorrect Vickers hardness values in excess of 80 GPa were obtained from nano-polycrystalline cBN with a grain size of 50 nm or less. On the other hand, it was verified that a Knoop indenter ensures an accurate hardness evaluation even if the constituent grains are fine because its major diagonal length which is used for measurement is less susceptible to elastic recovery. In an accurate evaluation of the hardness of different types of nano-polycrystalline cBN using a Knoop indenter, the hardness of each type of cBN was around 45 GPa, and there was no clear Hall–Petch relationship between hardness and grain size without a slight bell-like correlation. These results suggest that reported hardness values higher than 80 GPa of similar nano-polycrystalline cBN evaluated using a Vickers indenter are incorrect values caused by elastic recovery occurring at the indentation.     

Infrared‐Transparent Y2O3–MgO Nanocomposites Fabricated by the Glucose Sol–Gel Combustion and Hot‐Pressing Technique

A glucose sol–gel combustion method has been developed to synthesize composite nanopowders with equal volume fractions of Y₂O₃ and MgO. The synthesis involves the generation of precursor foam containing Y³⁺ and Mg²⁺ cations via the chemical and thermal degradation of glucose molecules in aqueous solutions. Subsequent calcination of the foam gave the composite nanopowders uniform composition and surface areas of 44–62 m²/g depending on the relative amount of glucose. Then the nanopowder with an average particle size of 19 nm was consolidated by the hot‐pressing technique with different sintering temperatures. The fabricated nanocomposite is mid‐infrared transparent as the result of fine grains, narrow grain size distribution, and uniform phase domains. The transmittance increases with increase in the sintering temperature and reaches 83.5% at 3–5 μm mid‐infrared wave range once the temperature reaches 1350°C, which is close to the theoretical value of 85%. And it is noteworthy that the cutoff wavelength reaches 9.6 μm, which is superior to those of spinel, AlON, and sapphire. And the Vickers hardness of the sample reaches 10.0 ± 0.1 GPa, which is significantly higher than those of the coarse grained single‐phase MgO and Y₂O₃. The results indicate that the glucose sol–gel combustion and hot‐pressing technique is an effective method to fabricate infrared transparent Y₂O₃–MgO nanocomposites.     

Effect of nano CuO addition on the tribo-mechanical behavior of alumina ceramics in non-conformal contact

Sintering of alumina from 1500°C to 1650°C and tribo-mechanical properties at room temperature had been investigated using nano CuO as a sintering aid. Bulk density gradually increases with sintering temperature from 1500°C to 1600°C and is optimized at 1600°C, beyond this, bulk density does not significantly increase at 1650°C. The addition of 2 wt% CuO showed the best result on densification. Densification of about 97.74% was attained at 1600°C with the incorporation of 2 wt% CuO. Nano CuO at grain boundaries forms CuAl₂O₄ liquid which modifies the morphology of the grain and improves mechanical properties. The formation of self-lubricating tribo-film on the wear track results in a low coefficient of friction <0.2 and reduces specific wear rate. 4 wt% CuO addition increases contact tensile stress (σ_max) by 51.2% and high Hertzian contact pressure (P_max≈1.51 GPa) causes plastic deformation of wear track. The re-solidified strengthening bond phase on the wear track simultaneously increases in friction coefficient and wear resistance with CuO addition. The optimizing effect of CuO addition shows that 2 wt% significantly decreases wear rate, and increases hardness and fracture toughness.     

Effect of manganese addition on sintering behavior and mechanical properties of alumina toughened zirconia

The effect of MnO₂ additives on the sintering behavior and mechanical properties of alumina-toughened zirconia (ATZ, with 10 vol% alumina) composites was investigated by incorporating different amounts of MnO₂ (0, 0.5, 1.0, and 1.5 wt%) and sintering at various temperatures ranging from 1300 to 1450 °C. The addition of MnO₂ up to 1.0 wt% improved the sintered density, hardness, flexural strength, and fracture toughness of the composite. However, the addition of 1.5 wt% MnO₂ degraded the relative density, hardness, and flexural strength of the composite due to the transformation of the ZrO₂ phase from tetragonal to monoclinic and grain coarsening. Optimal results were obtained with 1.0 wt% MnO₂ and sintering at 1450 °C, which improved the mechanical properties (hardness: 13.5 GPa, flexural strength: 1.2 GPa, fracture toughness: 8.5 MPa m^1/2) and lowered the sintering temperature compared to the conventional sintering temperature of ATZ composites (1550 °C). Thus, the ATZ composite doped with MnO₂ is a promising material for structural engineering ceramics owing to its improved mechanical properties and lower sintering temperature.     

Low‐Temperature Sintering of HfC/SiC Nanocomposites Using HfSi2‐C Additives

HfC/SiC nanocomposites were fabricated via the reactive spark plasma sintering (R‐SPS) of a nano‐HfC powder and HfSi₂‐C sintering additives. The densification temperature decreased to 1750°C as the additive content increased. XRD analysis indicated the formation of pure HfC–(19.3–33.8 vol%) SiC within the sintered composites without residual silicide or oxide phases or secondary nonoxide phases. Ultrafine and homogeneously distributed HfC (470 nm) and SiC (300 nm) grains were obtained in the dense composites using nano‐HfC powder through the high‐energy ball‐milling of the raw powders and R‐SPS. Grain growth was further suppressed by the low‐temperature sintering using R‐SPS. No amorphous phase was identified at the grain boundary. The maximum Vickers hardness, Young's modulus, and fracture toughness values of the HfC/SiC nanocomposites were 22 GPa, 292 GPa, and 2.44 MPa·m^1/2, respectively.     

Mechanical properties of nano-polycrystalline cBN synthesized by direct conversion sintering under HPHT

Using hBN and pBN as starting materials, various types of binderless polycrystalline cBN (BL-PcBN) were synthesized in the pressure range of 8–20 GPa and temperature range of 1300–2400 °C, and their mechanical properties were evaluated. In the synthesis pressure range of 10 GPa and higher, the hardness of BL-PcBN showed a correlation not with the synthesis pressure, but with the synthesis temperature. Binderless polycrystalline cBN synthesized at about 2200 °C exhibited the highest mechanical properties, for both starting materials. Specifically, BL-PcBN(h) (100–300 nm grain size) synthesized from hBN at 10 GPa and 2200 °C showed a hardness of 45 GPa, transverse rupture strength of 1.6 GPa. In contrast, BL-PcBN(p) synthesized from pBN at the same temperature had finer grain size (50–100 nm) and exhibited the same level of hardness but lower strength properties (transverse rupture strength of approx. 1.3 GPa) than BL-PcBN(h). Consequently, the material that exhibited the best mechanical properties was BL-PcBN(h) synthesized at 10 GPa and 2200 °C. A prototype micro ball end mill made of this material was examined in a mirror-like (polished-like) finishing test using high-strength hardened steel. This ball end mill achieved a fine finishing surface with a surface roughness (Ra) of 20 nm or better. The test revealed the high potential of this material for use as a high-precision cutting tool for high strength ferrous materials.     

Properties of an Infrared‐Transparent MgO:Y2O3 Nanocomposite

A 50:50 vol% MgO–Y₂O₃ nanocomposite with ~150 nm grain size was prepared in an attempt to make 3–5 μm infrared‐transmitting windows with increased durability and thermal shock resistance. Flexure strength of the composite at 21°C is 679 MPa for 0.88 cm² under load. Hardness is consistent with that of the constituents with similar grain size. For 3‐mm‐thick material at 4.85 μm, the total scatter loss is 1.5%, forward scatter is 0.2%, and absorptance is 1.8%. Optical scatter below 2 μm is 100%. Variable intensity OH absorption (~6% absorptance) is observed near 3 μm. The refractive index is ~0.4% below the volume‐fraction‐weighted average of those of the constituents. Thermal expansion is equal to the volume‐fraction‐weighted average of expansion of the constituents. Specific heat capacity is equal to the mass‐fraction‐weighted average of heat capacities of the constituents. Thermal conductivity lies between those of the constituents up to 1200 K. Elastic constants lie between those of the constituents. The Hasselman mild thermal shock resistance parameter for the composite is twice as great as that of common 3–5 μm window materials, but half as great as that of c‐plane sapphire.     

Investigation of the structural and mechanical properties of micro-/nano-sized Al2O3 and cBN composites prepared by spark plasma sintering

Alumina-cubic boron nitride (cBN) composites were prepared using the spark plasma sintering (SPS) technique. Alpha-alumina powders with particle sizes of ∼15 µm and ∼150 nm were used as the matrix while cBN particles with and without nickel coating were used as reinforcement agents. The amount of both coated and uncoated cBN reinforcements for each type of matrix was varied between 10 to 30 wt%. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. We studied the effect of the size of the starting alumina powder particles, as well as the effect of the nickel coating, on the phase transformation from cBN to hBN (hexagonal boron nitride) and on the thermo-mechanical properties of the composites. In contrast to micro-sized alumina, utilization of nano-sized alumina as the starting powder was observed to have played a pivotal role in preventing the cBN-to-hBN transformation. The composites prepared using nano-sized alumina reinforced with nickel-coated 30 wt% cBN showed the highest relative density of 99% along with the highest Vickers hardness (H_v2) value of 29 GPa. Because the compositions made with micro-sized alumina underwent the phase transformation from cBN to hBN, their relative densification as well as hardness values were relatively low (20.9–22.8 GPa). However, the nickel coating on the cBN reinforcement particles hindered the cBN-to-hBN transformation in the micro-sized alumina matrix, resulting in improved hardness values of up to 24.64 GPa.     

Mechanical and high temperature oxidation behavior of HfB2-ZrB2-SiC-B4C-CNT composites joined with and without Ni interlayer

Difficulties associated with the manufacturing of diboride based large and complex shapes mandate them to be joined for extreme applications. In the present work, spark plasma sintering is utilized to join HfB₂-ZrB₂-SiC-B₄C-CNT (HZSBC) based composites with Ni-interlayer at 1100 °C and without interlayer at 1800 °C. Microstructural analysis has elicited the formation of Ni diffused reaction zone (150–200 μm), and unaffected HZSBC composite for HZSBC-Ni-HZSBC joined composite (at 1100 °C), which gets merged into a homogeneous microstructure (without any distinguishable interface) in the HZSBC-HZSBC joint (without Ni-interlayer). An overall reduction of 18% and 14% is elicited in hardness and elastic modulus, respectively, in the HZSBC-Ni-HZSBC joint, whereas in HZSBC-HZSBC joined composite, uniform hardness and elastic modulus of ～22 GPa and ~398 GPa, respectively, is observed. Further, the bending and shear strength of the joined composites was obtained to be ～209 MPa and ～41 MPa, respectively, for HZSBC-Ni-HZSBC, which increased to ~342 MPa and ～81 MPa, respectively, for HZSBC-HZSBC. In conjunction, though superior high-temperature (1500 °C) oxidation protection is witnessed in HZSBC-HZSBC joint, with a thinner oxide layer, both display SiO₂-rich protective glassy layer, making these composites potential candidates for use in atmospheric re-entry conditions.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.     

Nano–Nano Composite Powders of Lanthanum Zirconate and Yttria‐Stabilized Zirconia by Spray Pyrolysis

Powders composing of La₂Zr₂O₇ (LZ) and (Zr_0.8Y_0.2)O_1.9 (10YSZ) phases (volume ratio = 1:1) were synthesized by using a sol‐spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ–10YSZ powders were investigated. XRD results showed that the grain size of LZ and 10YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single‐phase LZ and 10YSZ powders were in the range 9%–36% and 37%–86%. The activation energy for grain growth of LZ and 10YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single‐phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10YSZ phases in the composite.