Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Fabrication of mullite ceramics by rotary forging and pressureless sintering

Mullite ceramics were fabricated from boehmite/silica diphasic gels using a rotary forging compaction technique and pressureless sintering. Almost fully dense mullite samples were obtained after sintering at 1350°C for 2 h. The microstructure of sintered samples comprised fine (average size <1 μm), equiaxed grains. The samples showed superior sintering behaviour in comparison to those fabricated using conventional uniaxial and isostatic pressing. Powder compaction by rotary forging is thought to generate viscous deformation of the contact points increasing the interparticle contact area as well as increasing the packing density by breaking down hard agglomerates more effectively with concomitant rearrangement of the primary powder particles, thus promoting greater densification at relatively low temperatures. ©     

Densification of copper oxide doped alumina toughened zirconia by conventional sintering

The effect of copper oxide doping (0.05–1 wt%) on the densification, microstructure evolution and mechanical characteristics of alumina toughened zirconia (ATZ: 80 wt% Y-TZP + 20 wt% Al₂O₃) ceramic composites was investigated. Green samples were pressureless sintered using a short hold time of 12 min at temperatures varying from 1250 °C to 1500 °C. The incorporation of up to 0.2 wt% copper oxide was beneficial in promoting densification at low sintering temperature and improving the mechanical properties of ATZ without affecting the tetragonal phase stability. It was found that 0.2 wt% copper oxide addition was most efficacious, and the samples could attain a relative density of approximately 92% at 1250 °C, approximately 97% dense at 1350 °C and above 99% dense at 1450–1500 °C. This approach was also accompanied by an improvement in the Vickers hardness (12.7 GPa) and fracture toughness (6.94 MPam^1/2) when consolidated at 1450 °C/12 min. In comparison, the undoped composite exhibited relative densities of approximately 80% at 1250 °C, 87% at 1350 °C and approximately 97%–98% at 1450 °C-1500 °C. However, the study also found that higher dopant levels (0.5 wt% and 1 wt%) was not beneficial because the tetragonal zirconia phase was disrupted upon cooling from sintering, resulting in the monoclinic phase formation. In addition, low densification and poor mechanical properties were obtained.     

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.     

Non-stoichiometry of (TiZrHfVNbTa)Cx and its significance to the microstructure and mechanical properties

(TiZrHfVNbTa)C_x with variable stoichiometry are fabricated by pressureless sintering utilizing self-synthesized carbide powders via carbothermal reduction reaction. The densification behavior and microstructure evolution coupled with corresponding adjustable mechanical properties are investigated. The single-phase rock-salt crystal structure is retained despite the carbon stoichiometry approaching 0.6, indicating (TiZrHfVNbTa)C_x can maintain structural stability even containing high carbon vacancy. The carbon vacancy is beneficial for promoting densification procedure. The relative density of (TiZrHfVNbTa)C_0.6 sintered at 2150 °C can reach 97.9 %, while the similar value for (TiZrHfVNbTa)C_1.0 is obtained even at 2400 °C. While, remarkable grain growth accompanied by decline in relative density also occurs for lower carbon stoichiometry. With the variation of carbon content, the concentration of carbon-metal bonds changes gradually, leading to the adjustable mechanical properties. This work provides a potential approach to synthesize non-stoichiometric high-entropy carbides with high carbon vacancy via low-temperature pressureless sintering.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Effects of CaCO3 content on the densification of aluminum nitride

The effect of adding up to 13.4 wt.% CaCO₃ on the densification behavior of aluminium nitride (AlN) was investigated during pressureless sintering between 1100 and 2000 °C. The presence of second-phases, weight losses, Ca contents, and microstructures of sintered samples were correlated with the densification curves. Two microstructural aspects determined the densification of aluminum nitride with CaCO₃: second-phase evolution path and formation of large pores. Additions of small amounts of CaCO₃ caused the formation of higher melting point calcium aluminates (mainly CA₂) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed. For larger CaCO₃ amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C₁₂A₇ related eutectic temperature. Irrespective of the initial CaCO₃ content, all second-phase evolution paths converged to CA phase above 1600 °C, suggesting that during sintering of AlN with CaO at high temperatures, a liquid phase with composition of CA phase is more stable than others compositions. The effect of this composition changing on densification is discussed. Large pores were formed in the sites originally occupied by large particles of CaCO₃ and retarded the bulk densification in samples with high additive contents.     

Slow sintering in garnet-containing Y and Gd zirconate–aluminate mixtures for thermal barrier coatings

Mixtures of rare-earth zirconates and aluminates containing Y or Y + Gd that form a two-phase garnet–fluorite mixture exhibit much slower sintering than pure fluorite at 1400°C. An equivalent Y-free, Gd-containing composition that forms a perovskite aluminate instead of garnet showed faster densification after the metastable garnet decomposes. At 1500°C, the Y-free sample also showed the fastest initial sintering rate, whereas there was more divergence in the sintering rate for the samples containing Y + Gd. The zirconate–aluminate with equimolar Y + Gd shows the slowest densification at 1500°C and retains ∼25% porosity after 250 h. The results highlight possibilities for designing compliant thermal barrier coatings that can retain significant porosity at 1400°C or higher.     

Wear resistant boron carbide compacts produced by pressureless sintering

Boron carbide compacts were produced by pressureless sintering at 2200 °C/2 h and 2250 °C/2 h in Ar atmosphere, using a starting powder with a particle size smaller than 3 µm. Effects of carbon addition (3.5 wt%) and methanol washing of the starting powder were investigated on the densification, Vickers hardness, and micro-abrasive wear resistance of the samples. The removal of oxide phases by methanol washing allowed the production, with no sintering additive, of highly densified (93.6% of theoretical density), hard (25.4 GPa), and highly wear resistant (wear coefficient =2.9×10^–14 m³/N.m) boron carbide compacts sintered at 2250 °C. This optimized combination of properties was a consequence of a reduced grain growth without the deleterious effects associated to the carbon addition. Methanol washing of the starting powder is a simple and general approach to produce, without additives, high quality, wear resistant boron carbide compacts by pressureless sintering.     

Microstructure evolution during spark plasma sintering of FJS-1 lunar soil simulant

A spark plasma sintering (SPS) process has been explored to densify FJS-lunar soil simulants for structural applications in space explorations. The effect of SPS conditions, such as temperature and pressure, on the densification behavior, phase transformation, microstructural evolution, and mechanical properties of FJS-1 have been examined by conducting the X-ray diffraction analysis, electron microscopy imaging, and nano/micro indentation testing. Test analysis results were also compared to results from the FJS-1 powder and sintered samples without pressure. The FJS-1 powder was composed of sodian anorthite, augite, pigeonite, and iron titanium oxide. When FJS-lunar soil simulants were sintered without pressure, the main phase evolved from sodian anorthite to the intermediate sodian anorthite, jadeite and glass, and iron titanium oxide at 1000°C, which were further transformed into filiform and feather-shaped augite and schorlomite at 1100°C. Most densification processes in pressureless sintering occurred at 1050°C-1100°C. During the SPS process, the main phases were sodian anorthite, pigeonite, and iron titanium oxide at 900°C. These phases were transformed to sodian anorthite, glass, and feather-shaped augite at 1000°C and 1050°C, with the nucleation of dendritic schorlomite at 1050°C. Significant densification by SPS can be observed as low as 900°C, which indicates that the application of pressure can substantially lower the sintering temperature. The SPSed samples showed higher Vickers microhardness than the pressureless sintered samples. The mechanical properties of the local phases were represented by the contour maps of elastic modulus and nanohardness. Multiscale mechanical test results along with the microstructural characteristics further imply that the SPS can be considered a promising in-situ resource utilization (ISRU) method to densify lunar soils.     

Pressureless sintering of beta silicon carbide nanoparticles

This study reports the pressureless sintering of cubic phase silicon carbide nanoparticles (β-SiC). Green blended compounds made of SiC nano-sized powder, a fugitive binder and a sintering agent (boron carbide, B₄C), have been prepared. The binder is removed at low temperature (e.g. 800 °C) and the pressureless sintering studied between 1900 and 2100 °C. The nearly theoretical density (98% relative density) was obtained after 30 min at 2100 °C.The structural and microstructural evolutions during the heat treatment were characterised. The high temperatures needed for the sintering result in the β-SiC to α-SiC transformation which is revealed by the change of the composite microstructure. From 1900 °C, dense samples are composed of β-SiC grains surrounding α-SiC platelets in a well-defined orientation.TEM investigations and calculation of the activation energy of the sintering provided insight to the densification mechanism.     

Properties and ballistic tests of strong B4C-TiB2 composites densified by gas pressure sintering

B₄C-TiB₂ composites with classical 75/25 vol ratio were sintered by pressureless sintering with and without gas pressure application in the final stage of densification, using a novel prototypal furnace. A small fraction of WC was introduced through high energy milling of the starting powders with WC-Co spheres. High energy milling facilitated the densification thanks to incorporation of WC impurities acting as sintering aid, and size reduction of the starting powders. Strength, stiffness and toughness of the ceramic densified at 2050 °C via gas pressure sintering were even better than hot pressed composites at 1900 °C. Depth of Penetration tests on plates with 3–5 mm thickness demonstrated that the gas pressure sintered material had a superior performance compared to the hot pressed one. This work also revealed that hardness was not the property spotting the best ballistic performance.     

Densification kinetics and sintering behavior of UO2 and 0.5 wt.%MnO-doped UO2

In this work, the sintering kinetics of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by a high-temperature dilatometer heated up to 1500°C. In addition, the sintering behavior of pure UO₂ and 0.5 wt.%MnO-doped UO₂ was studied by pressureless sintering technique. The results showed that MnO doping enhanced the grain boundary diffusion of UO₂, which can effectively decrease the densification temperature and promote grain growth. The sintering temperature of UO₂ was significantly reduced by about 200°C with the addition of 0.5 wt.%MnO. The microscopic morphology studies showed that there were still fine particles agglomerated, forming sintered spheres in the matrix even if no severe agglomeration and bimodal size distribution were observed in raw UO₂ powder. The microstructure evolution of the sintered sphere and UO₂ matrix during the densification process was studied by isothermal sintering. Finally, the present analyses indicated that the densification of UO₂ matrix can be accelerated by adding MnO or increasing the sintering temperature, thus improving the densification inhomogeneity of UO₂ matrix.     

Solid-state pressureless sintering of silicon carbide below 2000 °C

To date, solid-state pressureless sintering of silicon carbide powder requires sintering aids and high sintering temperature (>2100 °C) in order to achieve high sintered density (>95% T.D.). Two-step sintering (TSS) method can allow to set sintering temperature lower than that conventionally required. So, pressureless two-step sintering process was successfully applied for solid-state sintering (boron carbide and carbon as sintering additives) of commercial SiC powder at 1980 °C. Microstructure and mechanical properties of TSS-SiC were evaluated and compared to those obtained with the conventional sintering (SSiC) process performed at 2130 °C. TSS-SiC showed finer microstructure and higher flexural strength than SSiC with very similar density (98.4% T.D. for TSS-SiC and 98.6% T.D. for SSiC).     

Preparation of machinable cordierite/mica composite by low-temperature sintering

In order to fabricate machinable cordierite/mica composite at low temperatures, the mica-composition glass powder was mixed with the conventional magnesia, alumina and silica powders which are raw materials of cordierite, compacted and fired in a sealed platinum container. By the addition of the 40 mass% mica-composition glass powder, machinable cordierite/mica composite was obtained. The machinability was caused by the interlocking microstructure of mica developed in the composite. In the firing process, mica crystallized at about 730 °C, cordierite was suddenly formed at 1050–1100 °C and the densification progressed markedly at 1000–1100 °C. The formation and sintering of cordierite were strongly promoted by a small amount of gaseous fluorine and/or fluorides. It was considered that fluorine and fluorides such as AlF₃ evaporated from the mica-composition glass at >800 °C and gaseous HF was formed in the sealed platinum container by the reaction of fluorine with water evaporated from the glass.     

Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Second-phase evolution and densification behavior of AlN with CaO–Y2O3–C multicomponent additive system

AlN compacts with different CaO–Y₂O₃–C mixtures were sintered between 1100 and 1850 °C to understand the effects of the in situ formed reducing atmosphere on the densification behavior and evolution of the second-phases. AlN with Y₂O₃ densified at 1750 °C, but the addition of C changed the second-phases evolution towards Y-rich phases that delayed the densification. For AlN containing CaO, the second-phases were little influenced by the reducing atmosphere, but the addition of C increased the evaporation of the second-phase compounds during sintering, limiting the densification due to the reduction of the liquid-phase fraction and the gas trapping inside the pores. AlN with CaO–Y₂O₃ mixtures could be completely densified at 1650 °C, but the addition of C inhibited the densification below this sintering temperature because liquid-phase had poor wetting and spreading characteristics and the second-phase a high melting point (>1800 °C).     

Heating parameter optimization and optical properties of Nd:YAG transparent ceramics prepared by microwave sintering

Nd-doped YAG transparent ceramics were prepared by microwave sintering. In this paper, the green bodies from high-purity commercial powders were sintered from 900 °C to 1750 °C for different lengths of time (0.5–2 h) by microwave heating. By optimizing the microwave heating parameters (the heating rate at different stages of microwave sintering, sintering temperature and holding time), the microstructures and optical properties of transparent ceramics can be effectively improved. The phase transformation, densification process and optical properties of Nd:YAG transparent ceramics were discussed. The liquid phases strongly absorb microwave radiation and affect the sintering results of samples during microwave sintering. The highest in-line transmittances of Nd:YAG transparent ceramic fabricated at 1750 °C for 2 h were 76.5% at 400 nm and 80.6% at 1064 nm. The fluorescence emission spectra and lifetime depending on different heating conditions were also discussed.     

Sintering behaviour and microstructure of 3Y-TZP + 8 mol% CuO nano-powder composite

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate complexation–precipitation, respectively. A significant enhancement in sintering activity of 3Y-TZP nano-powders, without presence of liquid phase, was achieved by addition of 8 mol% CuO nano-powder, resulting in an extremely fast densification between 750 and 900 °C. This enhancement in sintering activity was explained by an increase in grain-boundary mobility as caused by dissolution of CuO in the 3Y-TZP matrix. The nano-powder composite was densified to 96% by pressureless sintering at 1130 °C for 1 h. Considerable tetragonal to monoclinic phase transformation of the zirconia phase was observed by high temperature XRD analysis. This zirconia phase transformation is discussed in terms of reactions between CuO and yttria as segregated to the 3Y-TZP grain boundaries.     

Phase and microstructural evolution during the heat treatment of Sm–Ca–α-sialon ceramics

The phase and microstructural evolution of multi-cation Sm–Ca–α-sialon ceramics was investigated. Six samples were prepared, ranging from a pure Sm-sialon to a pure Ca-sialon, with calcium replacing samarium in 20 eq% increments, thus maintaining an equivalent design composition in all samples. After pressureless sintering at 1820 °C for 2 h, all samples were subsequently heat treated up to 192 h at 1450 and 1300 °C. The amount of grain boundary glass in the samples after sintering was observed to decrease with increasing calcium levels. A M′_ss or M′_ss-gehlenite solid solution was observed to form during the 1450 °C heat treatment of all Sm-containing samples, and this phase forms in clusters in the high-Sm samples. The thermal stability of the α-sialon phase was improved in the multi-cation systems. Heat treatment at 1300 °C produces SmAlO₃ in the high-Sm samples, a M′_ss-gehlenite solid solution in the high-Ca samples, and a Sm–Ca–apatite phase in some intermediate samples.