Development and evaluation of TiB2–AlN ceramic composites sintered by spark plasma

Titanium diboride (TiB₂) is a ceramic material with high mechanical resistance, chemical stability, and hardness at high temperatures. Sintering this material requires high temperatures and long sintering times. Non-conventional sintering techniques such as spark plasma sintering (SPS) can densify materials considered difficult to sinter. In this study, TiB₂–AIN (aluminum nitride) composites were sintered by using the SPS technique at different sintering temperatures (1500 °C, 1600 °C, 1700 °C, 1800 °C, and 1900 °C). x-ray diffraction was used to identify the phases in the composites. mechanical properties such as hardness and indentation fracture toughness was obtained using a vickers indenter. Different toughening mechanisms were identified, and good densification results were obtained using shorter times and lower temperatures than those previously reported.     

Temperature dependent hardness and strength properties of TiB2 with TiSi2 sinter-aid

From the perspective of high temperature structural applications, it is important to evaluate temperature dependent mechanical properties of titanium diboride (TiB₂) ceramics. The present study reports the effect of TiSi₂ content (up to 10 wt.%) and temperature on hardness and strength of TiB₂. The hardness properties were measured from room temperature (RT)—900 °C in vacuum; the four-point flexural strength properties were evaluated at selected temperatures in air up to 1000 °C. An attempt has been made to discuss the difference in hardness and strength properties with sinter-aid amount and microstructure. Our experimental results clearly indicated that the addition of 2.5 wt.% TiSi₂ to TiB₂ resulted almost full densification at a lower hot pressing temperature of 1650 °C without compromising on the high temperature strength and hardness properties. The hot pressed TiB₂–2.5 wt.% TiSi₂ ceramic could retain moderate strength of more than 400 MPa and hardness of 9 GPa at 1000 °C and 900 °C, respectively.     

Influence of sintering temperature on microstructure and mechanical properties of WC-8Ni cemented carbide produced by vacuum sintering

The WC-8Ni powder was prepared by the ball milling method, then consolidated via a vacuum sintering technique. The influence of sintering temperature varying from 1375 °C up to 1500 °C on microstructure and mechanical properties of WC-8Ni cemented carbide was investigated. The best mechanical properties of the samples have been achieved at sintering temperature of 1450 °C. At which the relative density, hardness and fracture toughness (K_IC) of the samples are 99.81%, 13.23 GPa and 24.22 MPa m^1/2, respectively. The effect of η-phase identified by Murakami etching method and XRD technique on the mechanical properties was also discussed.     

Study of processing routes for WC-MgO composites with varying MgO contents consolidated by FAST/SPS

Two different preparation routes were applied to process WC-MgO composites with varying MgO contents (4.1 wt.% and 5.9 wt.% MgO). WC-MgO powder mixtures were synthesized by a milling process at 600 rpm for 6 h of partially oxidized WC (WC + WO₃), Mg₃N₂ and C. Alternatively, WC and MgO as initial powders were used. For consolidation of the powder mixtures the field-assisted sintering technology (FAST) was used. X-ray diffraction shows that samples out of different powder mixtures and sintered between 1600 °C and 1750 °C exhibited WC, MgO and the W₂C phase independent of the preparation route of the powder mixtures. A higher density and better mechanical properties (hardness and indentation fracture toughness) of WC-MgO were achieved of pure WC and MgO as initial powders were consolidated by FAST. It was found that a lower MgO content results in higher hardness values and in a slightly decreased indentation fracture toughness.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Effect of pressure and temperature on densification,microstructure and mechanical properties of spark plasma sintered silicon carbide processed with β-silicon carbide nanopowder and sintering additives

The effects of applied pressure and temperature during spark plasma sintering (SPS) of additive-containing nanocrystalline silicon carbide on its densification, microstructure, and mechanical properties have been investigated. Both relative density and grain size are found to increase with temperature. Furthermore, with increase in pressure at constant temperature, the relative density improves significantly, whereas the grain size decreases. Reasonably high relative density (~96%) is achieved on carrying out SPS at 1300 °C under applied pressure of 75 MPa for 5 min, with a maximum of ~97.7% at 1500 °C under 50 MPa for 5 min. TEM studies have shown the presence of an amorphous phase at grain boundaries and triple points, which confirms the formation of liquid phase during sintering and its significant contribution to densification of SiC at relatively lower temperatures (≤1400 °C). The relative density decreases on raising the SPS temperature beyond 1500 °C, probably due to pores caused by vaporization of the liquid phase. Whereas β-SiC is observed in the microstructures for SPS carried out at temperatures ≤1500 °C, α-SiC evolves and its volume fraction increases with further increase in SPS temperatures. Both hardness and Young׳s modulus increase with increase in relative density, whereas indentation fracture toughness appears to be higher in case of two-phase microstructure containing α and β-SiC.     

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.     

Thermal stability and mechanical properties of HVOF/PVD duplex ceramic coatings produced by HVOF and cathodic vacuum arc

Duplex ceramic coatings, consisting of an inner NiCr-Cr₃C₂-based coating and an outmost AlCrN film, were produced on the steel substrate in succession by velocity oxygen-fuel spraying (HVOF) and cathodic vacuum arc methods, and then isochronally annealed at annealing temperatures below 900 °C for 2 h. The thermal stability and mechanical properties of the annealed samples were systematically studied by means of X-ray diffraction, Optical microscope and transmission electron microscope, in association with mechanical property measurements. The results show that the microstructure, phase evolution and mechanical properties of duplex ceramic coatings are significantly dependent on the annealing temperature. Metastable fcc-AlCrN solid solution in AlCrN film first decomposes to rich-Al and rich-Cr domains by spinodal decomposition at 700 °C, leading to a notable increase in hardness due to its smaller grain size and high elastic strain field, and then to equiaxed hcp-AlN and Cr₂N by the nucleation and growth at 900 °C, leading to a notable decrease in hardness due to the recrystallization and the formation of hcp-AlN. Meanwhile, the both decarburization of Cr₃C₂ to Cr₇C₃ occurs at 800 °C, but becomes more intensive at 900 °C, leading to a notable loss in hardness. In addition, the dissolution of Cr₃C₂ produces high density of porosity, which also reduces the hardness. The hardness tests show the following ordering of load-bearing capacity for the duplex ceramic coatings: 700 °C>As-deposited >800 °C>900 °C. Tribological property measurements demonstrate that the wear resistance of the tested duplex ceramic coatings obeys the following ordering: 700 °C>As-deposited >800 °C>900 °C. The improved wear resistance is due to high surface hardness, load-bearing capacity and thermal stability. In addition, the wear mechanisms are shown.     

Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.     

Environment-dependent indentation recovery of select soda-lime silicate glasses

Indentations made on silicate glasses can easily be affected by the environment. In the present work, indentations were made on select commercial float glasses as well as on experimental soda-lime silicate glasses using a 1 mm diameter spherical tungsten carbide ball-mounted Brinell indenter. Recovery of indentations made on the glass samples was measured in different environments, namely, 100 °C, room temperature/room humidity and 100% relative humidity, as a function of time by using a Zygo laser non-contact profiliometer. Elastic (Young's modulus, bulk modulus, shear modulus and Poisson's ratio) and indentation (Vickers hardness, fracture toughness, brittleness and fracture surface energy) properties of the glasses were also determined by a pulse-echo and Vickers indentation methods, respectively, to correlate with the recovery of indentations. The elastic properties and Vickers hardness are directly proportional to the packing ions present in the glass structure and the strength of an individual bond, whereas the brittleness and fracture toughness more likely depend on molar volume of the glasses. According to the applied environment, a recovery rate of indentations follows the order: room temperature/room humidity <100% relative humidity <100 °C, regardless of glass composition. The reason for higher recovery rate of indentations is attributed to the structural relaxation, which is promoted by a thermodynamic driving force at 100 °C, and stored strain energy in deformation zone, allowing the indentations to regain their original configurations at certain points.     

Nanoindentation study of nickel manganite ceramics obtained by a complex polymerization method

The chemical synthesis of nickel manganite powder was performed by a complex polymerization method (CPM). The obtained fine nanoscaled powders were uniaxially pressed and sintered at different temperatures: 1000–1200 °C for 2 h, and different atmospheres: air and oxygen. The highest density was obtained for the sample sintered at 1200 °C in oxygen atmosphere. The energy for direct band gap transition (Eg) calculated from the Tauc plot decreases from 1.51 to 1.40 eV with the increase of the sintering temperature. Indentation experiments were carried out using a three-sided pyramidal (Berkovich) diamond tip, and Young's modulus of elasticity and hardness of NTC (negative temperature coefficient) ceramics at various indentation depths were calculated. The highest hardness (0.754 GPa) and elastic modulus (16.888 GPa) are exhibited by the ceramics sintered at highest temperature in oxygen atmosphere.     

Fluidized bed chemical vapor deposition of pyrolytic carbon-III. Relationship between microstructure and mechanical properties

Pyrolytic carbon (PyC) coatings with different densities were produced by fluidized bed chemical vapor deposition under different deposition conditions. Their Young’s modulus and hardness were measured by nano-indentation, whereas the deformation behavior was studied through analyzing the force–displacement curves of the indentations. The deformation mechanism of PyC under indentation is attributed to the slip of the graphene planes, and its reversibility is discussed in terms of the defects of the microstructure. We observed a linear relationship between the density of PyC’s and their Young’s modulus and hardness, for densities lower than 1.9 g/cm³. Above this value, the mechanical properties were controlled by the amount of interstitial defects. Samples were also heat treated at 1800 °C and 2000 °C, and their changes in microstructure, hardness and Young’s modulus are discussed as a function of density.     

Reactive sintering cBN-Ti-Al composites by spark plasma sintering

When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB₂, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m^0.5, respectively.     

Understanding the mechanical properties of hot pressed Ba-doped S-phase SiAlON ceramics

In this contribution, we report the analysis and interpretation of the mechanical property measurements for a new class of SiAlON ceramic. The hardness and indentation fracture toughness were measured on the hot pressed Ba-doped S-SiAlON ceramic using Vickers indentation at varying loads (up to 300 N). An important observation was that all the investigated S-SiAlON exhibited the characteristic rising R-curve behavior with a maximum toughness of up to 10–12 MPa m^1/2 for ceramics, hot pressed both at 1700 and 1750 °C. Crack deflection by large elongated S-phase grains and crack bridging by β-Si₃N₄ needles has been found to be the major toughening mechanisms for the observed high toughness. Theoretical estimates, using a toughening model based on crack bridging and deflection by platelet shaped ‘S’-phase grains and β-Si₃N₄ needles, reveal the interfacial friction of around 200 MPa. Careful analysis of the indentation data reveals the average (apparent) hardness modestly increases with indent load in all S-SiAlON samples, with more significant effect for S-SiAlON, hot pressed at 1600 °C. This effect has been analyzed in the light of the established model of ‘indentation-induced cracking’ phenomenon. Our experimental results suggest that a modest combination of average hardness of 15 GPa and indentation toughness of around 12 MPa m^1/2 could be achieved in Ba-S-SiAlON ceramic and further improvement requires microstructural tailoring.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Mechanical,morphological and dielectric properties of sintered mullite ceramics at two different heating rates prepared from alkaline monophasic salts

The sintering behavior of nanocrystalline orthorhombic mullite powders was investigated. The changes in microstructure, mechanical and dielectric properties with two different heating rates were explained. Microstructural characteristics depending on heating rate were explained at different sintering temperatures. Dielectric properties of prepared mullite nanocomposites were studied to examine the synthesized mullite ceramics as high permittivity materials in the microwave range. It was indicated that a sharp decrease in bulk density was observed at 1600 °C due to the exaggerated growth of mullite grains. Moreover, a maximum hardness value of 4.97 GPa was obtained at 1600 °C with slow heating rate (5 °min^?1). The DC electrical resistivity with a slow heating rate at 1300 °C was approximately three times the value of the mullite sample sintered with a fast heating rate (30 °min^?1). The minimum dielectric loss of about 0.017 at 1.5 GHz was achieved at a sintering temperature of 1500 °C with a slow heating rate.     

Dry sliding wear behavior of β-Sialon ceramics at wide range temperature from 25 to 800 °C

The β-Sialons are potential candidates for high temperature application because of their excellent comprehensive performances. However, there is little research about dry sliding wear behavior of β-Sialons at wide range temperature. This study aims at revealing the mechanisms of how temperature, microstructure and mechanical properties affect the tribological properties of such composites. Four kinds of β-Sialons are prepared and their wear properties are characterized from 25 to 800 °C. Results show that β-Sialons have a preferable tribological property at 25 °C, which is ascribed to its excellent mechanical properties. Whereas, with temperature increasing, the wear rate increases two orders of magnitudes compared to 25 °C, owing to the reduced hardness and increased thermal stress of the sample. At 800 °C, the wear rate of composites decreases with z values increasing, which is attributed to the tribo-chemical reaction and generate more Al₂O₃ in β-Sialons with higher z values during sliding process.     

Microstructure and mechanical properties at room and elevated temperatures of reactively hot pressed TiB2–TiC–SiC composite ceramic tool materials

TiB₂-based composite ceramic tool materials with different amounts of TiC and SiC were fabricated via a reactive hot pressing process. The mechanical properties at room temperature and flexural strength at 800–1300 °C were tested in ambient air. The composition and microstructure before and after the high-temperature strength tests were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectrometer (EDS). The flexural strength increment/degradation mechanisms at elevated temperatures were investigated. In-situ synthesized TiC improved the sinterability and mechanical properties of the materials at both room and elevated temperatures. Comparing with TTS (TiB₂–15.9 wt%TiC–10.6 wt%SiC) and TS (TiB₂–22.4 wt%SiC), TTS3 (TiB₂–8.1 wt%TiC–16.4 wt%SiC) had the optimum room temperature mechanical properties, i.e., flexural strength of 862 MPa, fracture toughness of 6.4 MPa m^1/2, hardness of 22.8 GPa, and relative density of 99.3%. The improved mechanical properties were ascribed to the fine grain size. The flexural strength of the TTS composite at 800 °C was higher than that at room temperature. The improvement of the flexural strength was attributed to the healing of preexisting flaws and the relief of residual stress. Substantial strength degradation took place when the temperature exceeded 1000 °C, due to softening of the grain boundaries, surface oxidation and elastic modulus degradation.     

Densification,microstructure and mechanical properties of hot pressed tantalum carbide

Monolithic tantalum carbide (TaC) ceramics were prepared by hot pressing in order to investigate the effect of hot pressing temperature on the densification behavior, microstructure and mechanical properties of TaC. Monolithic TaC sample hot pressed at 2000 °C for 45 min under 40 MPa, with relative density value above 97%, Vickers hardness of 15.7 GPa and fracture toughness of 4.1 MPa m^1/2 was obtained. Fracture surfaces investigations of the samples, which were carried out using the SEM analysis, showed a significant grain growth by increasing the hot pressing temperature from 1700 to 2000 °C. Also, based on the X-ray diffraction pattern, a decrease in the lattice parameter of hot pressed TaC sample was observed.     

Production of nepheline/quartz ceramics from geopolymer mortars

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C. Nepheline (NaAlSiO₄) and carnegieite (NaAlSiO₄) form at 900 °C in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (～275 MPa) and high Vickers hardness (～350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.