Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Sintering,microstructural development,and electrical properties of gadolinia-doped ceria electrolyte with bismuth oxide as a sintering aid

A considerable reduction (≥250 °C) in the sintering temperature, enhancement of the sintering density, and a slight improvement of the electrical properties, can be achieved by using bismuth oxide in the range of 0.2 to 2 wt.%, as a sintering aid for gadolinia-doped ceria (GDC) ceramic electrolytes. Dilatometric experiment (CHR) and SEM observations indicate that a liquid phase-assisting sintering mechanism contributes to the improvement in sintering density for bismuth oxide concentrations exceeding 0.5 wt.%. The addition of small amount of Bi₂O₃ ≤0.5 wt.% also results in the achievement of highly dense ceramic bodies (≥99% of theoretical density) after sintering at 1200 °C for 4 h, which indicates that the addition of Bi₂O₃ to gadolinia-doped ceria promoted the sintering process by a cooperating volume diffusion-liquid phase-assisting mechanism. Based on the lattice constant data, the solid solubility limit of Bi₂O₃ in gadolinia-ceria is, probably, lower than 1.0 wt.%. Grain size also increased with increasing Bi₂O₃ content up to 0.5 wt.% and then it decreased with further addition of Bi₂O₃. The addition of the smaller amounts of bismuth oxide, i.e., ≤1.0 wt.% Bi₂O₃ slightly enhanced the total ionic electrical conductivity of the gadolinia-doped ceria electrolyte. The sintering temperature strongly influenced the electrical conductivity of the doped-GDC ceramics. The best sample was that containing 1.0 wt.% Bi₂O₃ sintered at 1400 °C for 2 h which had an ionic electrical conductivity of 4 S m⁻¹ at 700 °C, and an activation energy of 0.58 eV for the oxide-ion conduction process in air.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

Interaction of titanium diboride/boron nitride evaporation boats with aluminium

Hexagonal boron nitride/titanium diboride composites are widely used as evaporation boats for aluminium deposition to produce functional and decorative layers on different target materials. The lifetime of such a material is limited mainly by the interaction of the metal with the ceramic substrate, but the corrosion mechanism has still not yet been thoroughly investigated and understood. In this article the corrosion mechanism for the evaporation boats used was investigated using thermodynamic calculations, FESEM, EDX and XRD phase analysis. The analysis showed that hexagonal boron nitride (hBN), which is thermodynamically less stable than TiB₂, is passivated during the application process through the formation of AlN surface layers, whereas the thermodynamically more stable TiB₂ phase dissolves and Ti-rich components precipitate in cooler regions of the evaporation boats.     

Sintering and conductivity of Sc-doped CaZrO3 with Fe2O3 as a sintering aid

This paper reports the effect of 0.1–0.5 wt% Fe₂O₃ addition on sintering and electrical properties of CaZr_0.95Sc_0.05O_3-δ ceramics synthesized by combustion method. Addition of the sintering aid was shown to enhance ceramic densification and grain coarsening at a reduced sintering temperature and a shorter holding time (1430 °C, 2 h). Effect of the sintering aid on electrical conductivity of the ceramics was investigated using impedance spectroscopy. The highest total conductivity was achieved for the composition with 0.5 wt% Fe₂O₃; it was about an order of magnitude higher than that of the composition without Fe₂O₃. The effect of Fe₂O₃ addition on the conductivity of the grain interior and grain boundaries has been discussed. It was concluded that ceramic densification, grain coarsening and formation of small amounts of calcium ferrite at the grain boundaries upon Fe₂O₃ addition were responsible for the conductivity enhancement.     

Room-temperature mechanical properties of a high-entropy diboride

The mechanical properties of a (Hf,Mo,Nb,Ta,W,Zr)B₂ high-entropy ceramic were measured at room temperature. A two-step synthesis process was utilized to produce the (Hf,Mo,Nb,Ta,W,Zr)B₂ ceramics. The process consisted of a boro/carbothermal reduction reaction followed by solid solution formation and densification through spark plasma sintering. Nominally, phase pure (Hf,Mo,Nb,Ta,W,Zr)B₂ was sintered to near full density (8.98 g/cm³) at 2000°C. The mean grain size was 6 ± 2 µm with a maximum grain size of 17 µm. Flexural strength was 528 ± 53 MPa, Young's modulus was 520 ± 12 GPa, fracture toughness was 3.9 ± 1.2 MPa·m^1/2, and hardness (HV_0.2) was 33.1 ± 1.1 GPa. A Griffith-type analysis determined the strength limiting flaw to be the largest grains in the microstructure. This is one of the first reports of a variety of mechanical properties of a six-component high-entropy diboride.     

Sintering,oxidation and mechanical properties of hot pressed aluminium nitride

Aluminium nitride samples were prepared by hot pressing of fine powder without additives. The sintering process is described. Different porosities were obtained and the ratio total porosity/open porosity was measured. Tests of corrosion in air were performed. The main mechanical properties (hardness, strength, toughness, thermal shock resistance...) were measured and the effects of porosity and temperature studied. The results obtained are compared with the mechanical properties of other structural ceramics.     

Microstructure and mechanical properties of B4C based ceramics with Fe3Al as sintering aid by spark plasma sintering

B₄C based ceramics were fabricated with different Fe₃Al contents as sintering aids by spark plasma sintering at relatively low temperature (1700 °C) in vacuum by applying 50 MPa pressure and held at 1700 °C for 5 min. The effect of Fe₃Al additions (from 0 to 9 wt%) on the microstructure and mechanical properties of B₄C has been studied. The composition and microstructure of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA) equipped with WDS (wavelength dispersive spectrometry) and EDS. The mixtures of B₄C and Fe₃Al underwent a major reaction in which the metal borides and B₄C were encountered as major crystallographic phases. The sample with 7 wt% of Fe₃Al as a sintering aid was found to have 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa m^1/2 fracture toughness which is higher than that of pure B₄C.     

Reticulated open-cellular aluminum nitride ceramic foams: Effect of sintering aids on microstructural,thermal, and mechanical properties

Open-celled aluminum nitride ceramic foams were prepared by the polymer sponge replication technique involving aqueous dispersions of passivated AlN. The amount of the Y₂O₃ and Dy₂O₃ as sintering aid was varied, and the effects on the densification, microstructure formation, phase composition, and finally, the thermal conductivity were investigated. A typical thermal conductivity of 1.1 W m⁻¹ K⁻¹ was determined for foams at a porosity level of 94.3 vol.%, on average. This measured foam thermal conductivity was subsequently modeled using different porosity ↔ thermal conductivity relations considering the different hierarchical levels of porosity in these foams. From these models, the thermal conductivity of the bulk AlN strut material was determined, correlated with the strut microstructure and the phase composition, and compared to literature data.     

Dynamic mechanical properties of polystyrene–aluminum nitride composite

A composite of aluminum nitride (AlN) particles dispersed around polystyrene matrix particles was synthesized in this study. The purpose of using this microstructure is to improve the thermal properties of a polymer at a low filler content with a minimal increase in the dielectric constant of the polymer composite to meet the material requirements for electronic packaging. The dynamic mechanical properties of this type of polystyrene–AlN composite were investigated here. The experimental results indicate that the dynamic mechanical property of the polystyrene–AlN composite is a function of the polystyrene particle size, AlN filler concentration, and temperature under this dispersion state. The addition of an AlN concentration into polystyrene increases both the storage modulus and the α‐transition temperature. The smaller polystyrene particle size gives a higher storage modulus and damping peak. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1348–1353, 2000     

Microstructure and mechanical properties of TaC ceramics with 1–7.5 mol% Si as sintering aid

Metallic Si as sintering aid was effective in densifying tantalum carbide ceramic (TaC) by spark plasma sintering (SPS) at 1700°C. Full density was reached at 5.0 mol% Si addition (equivalent to 1.088% Si in weight) and above. Enhanced densification of TaC ceramic with Si was associated with decrease in oxygen content from ~0.24 wt% in TaC powder to ~0.03 wt% in consolidated specimen. Rest of the oxygen species was collected at multigrain conjunctions to form SiO₂‐based liquid at high temperatures. Upon cooling, Ta, Si, O, and C dissolving in the liquid precipitated minor phases of TaSi_x and SiC of low concentrations. Microstructure of TaC ceramics was refined by the Si addition, with average grain size decreasing from 11±8 μm at 1.0 mol% Si to 3±2 μm at 7.5 mol% Si addition. Ta solute in SiC and Si solute in TaC were evidenced. TaC ceramic containing 7.5 mol% Si had a relatively good flexural strength and fracture toughness of 646±51 MPa and 5.0 MPa·m^1/2, respectively.     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

Hydrolysis protection and sintering of aluminum nitride powders with yttria nanofilms

Aluminum nitride (AlN) is a promising material for electronic substrates and heat sinks. However, AlN powders react with water that adversely affects final part properties and necessitates processing in organic solvents, increasing the cost of AlN parts. Small quantities of yttrium oxide (Y₂O₃) are commonly added to AlN particles to enable liquid phase sintering. To mitigate the reaction of AlN particles with water, particle atomic layer deposition (ALD) was used to coat AlN powders with conformal films of Y₂O₃ prior to densification and powder processing. When AlN particles were coated with 6 nm thick films of amorphous Y₂O₃, the hydrolysis reaction was significantly suppressed over 48 h, demonstrating that Y₂O₃ nanofilms on AlN powders act as a barrier coating in an aqueous solution. AlN powders with Y₂O₃ addition by particle ALD sintered to high relative densities (≥90% theoretical) after sintering at 1800°C for 50 min.     

Processing and mechanical properties of Si3N4 composites employing polymer-derived SiAlOC as sintering aid

Silicon nitride ceramics have been densified with polymer-derived SiAlOC sintering aid. Dense samples were prepared at relatively mild temperatures (1600 °C) from blends with 30 wt.% of pyrolysed SiAlOC additives. Decreasing the SiAlOC aid content to 15 wt.% resulted in porous Si₃N₄ samples (～85% rel. density). The properties of dense samples were influenced by the remaining SiAlOC glass (HV = 15.5 GPa, K_IC = 4 MPa m^1/2). Increasing the sintering temperature to 1780 °C for 5 min significantly changed the phase composition and properties of the composites. The major phase was O′-sialon in the sintered samples. Additional annealing of the samples at 1530 °C for 16 h further decreased the amount of the residual glassy phase and consequently affected the mechanical properties. The Vickers hardness of dense samples was 18.5 GPa and the fracture resistance ranged between 4.0 and 4.5 MPa m^1/2. The compressive creep test (1400 °C/100 MPa/24 h) of the SNA30-A sample sintered at 1600 °C for 30 min without an additional crystallisation step showed a promising low creep rate of 8.6 × 10^?8 s^?1. Further improvement of creep resistance is expected for the crystallised samples.     

Pressureless densification and mechanical properties of hafnium diboride doped with B4C: From solid state sintering to liquid phase sintering

A pressureless sintering process, using a small amount of boron carbide (≤2 wt%) as sintering aid, was developed for the densification of hafnium diboride. Hafnium diboride ceramics with high relative density were obtained when the sintering temperature changed from 2100 °C to 2350 °C. However, the sintering mechanism was varied from solid state sintering (SSS, below 2300 °C) to liquid phase sintering (LPS, above 2300 °C). Boron carbide addition improved densification by removing the oxide impurities during solid state sintering and by forming a liquid phase which was well wetting hafnium diboride grains during liquid phase sintering process. The different roles of B₄C on the microstructure development and mechanical properties of the sintered ceramics were investigated.     

Crystal structure and mechanical properties of titanium nitride films synthesized by magnetron sputtering with a hot target

The effect of nitrogen consumption and the discharge current density on the crystal structure of titanium nitride films synthesized by the method of reactive magnetron sputtering of a hot target at a constant current is studied by X-ray phase analysis. It is found that the phase of titanium nitride of a cubic crystal system is formed in the films. An increase in the current density leads to the texturing of the crystals parallel to the (111) plane in the films. In this case, the hardness and the modulus of normal elasticity are increased significantly.     

The effect of boron nitride nanosheets on the mechanical and thermal properties of aluminum nitride ceramics

The boron nitride nanosheets (BNNSs)/aluminum nitride (AlN) composites were prepared by hot press sintering at 1600°C. The microstructure, mechanical properties, and thermal conductivity of the samples were measured, and the effect of adding BNNSs to AlN ceramics on the properties was studied. It is found that the addition of BNNSs can effectively improve the mechanical properties of AlN. When the additional amount is 1 wt%, the bending strength of the sample reaches the maximum value of 456.6 MPa, which is 23.1% higher than that of the AlN sample without BNNSs. The fracture toughness of the sample is 4.47 MPa m^1/2, a 68.7% improvement over the sample without BNNSs. The composites obtained in the experiment have brilliant mechanical properties.     

Pressureless sintering of highly transparent aluminum oxynitride ceramic by precipitation-coating with sintering aid

AlON transparent ceramic is an ideal material for photoelectric windows, domes, and transparent armor. This ceramic is commonly fabricated by ball-mill mixing of AlON powder with appropriate sintering aids; however, it is difficult to homogeneously disperse trace sintering aids using this method and contamination is easily introduced, which may generate pores and flaws in the ceramic, resulting in degraded properties. To address this problem, we adopted a wet-chemical method to precipitation-coat sintering aids on the surface of AlON powder to achieve homogeneous distribution and enable nano-size sintering aids. Pressureless sintered AlON with 0.5 mass% Y₂O₃-coating gave in-line transmittance of 83.5% at a wavelength of 1064 nm and flexural strength of 326.3 MPa; the corresponding values for the ball-milled product were 79.3% and 304.6 MPa. Our results demonstrate that this method of sintering-aid coating has potential to fabricate AlON ceramic with excellent performance.     

Sintering effect on mechanical properties of composites of natural hydroxyapatites and titanium

This study presents the fabrication and characterization of composite materials of hydroxyapatite and Ti. Hydroxyapatite (HA) powder was obtained from bovine bones (BHA) and human enamel (EHA) via calcination technique. Fine powders of HA were admixed with 5 and 10 wt.% fine powder of metallic Ti. Powder-compacts were sintered at different temperatures between 1000 and 1300 °C. Compression strength, Vickers microhardness and elastic modulus as well as density were measured. SEM and X-ray diffraction studies were also conducted. The experimental results showed that addition of Ti to EHA and BHA decreases the elastic modulus, comparing to samples of pure BHA. The best mechanical properties for BHA–Ti composites were obtained after sintering in the range of 1200–1300 °C and for EHA–Ti composites in the range of 1100–1300 °C.     

Sintering behavior and mechanical properties of spark plasma sintering SiO2-MgO ceramics

SiO₂-MgO ceramics containing different weight fractions (0, 0.5, 1, 2, and 4 wt%) of SiO₂ powder were prepared by mixing nano MgO powder, and the powder mixtures were densified by spark plasma sintering (SPS). The effect of SiO₂ addition and SPS method on the sintering behavior, microstructure and mechanical properties were investigated. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The highest relative density, flexural strength and hardness of 2 wt% SiO₂-MgO ceramics reached 99.98%, 253.99 ± 7.47 MPa and 7.56 ± 0.21 GPa when sintered at 1400 °C by SPS, respectively. The observed improvement in the sintering behavior and mechanical properties are mainly attributed to grain boundary "strengthening" and intragranular "weakening" of the MgO matrix. Furthermore, the spark plasma sintering temperature could be decreased by more than 100 °C as compared with the HP method, SPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.