The Effect of Phase Composition on the Mechanical Properties of LTCC Material

For the fabrication of complex, micro‐electromechanical systems (MEMS) devices based on low‐temperature co‐fired ceramic (LTCC), higher firing temperatures and longer times than those proposed by the LTCC producer are needed. These changes to the thermal budget may influence the material properties and consequently its functional properties. The effect of the firing conditions on the LTCC DuPont 951 and thus on the phase composition, that is, the alumina/anorthite ratio and porosity, on the mechanical properties is presented. The samples fired at low temperatures (800°C) had a high porosity (7%), which significantly contributed to the low elastic modulus (100 GPa) and the low mechanical strength of the LTCC (140 MPa). The samples fired at 850°C, which had only 1% of porosity, resulted in an elastic modulus of 122 GPa and a flexural strength of 224 MPa. A further increase in the temperature contributed to a slight decrease in the elastic modulus, while no significant difference in the flexural strength could be observed. The enhancement of the flexural strength with an increasing firing temperature was mainly related to a decrease in the porosity and to a lesser extent to the different ratio of the alumina/anorthite phases. The effect of firing time on the phase composition at selected temperatures (i.e., 100 h at 700 and 800°C) is also discussed.     

The microstructure,coefficient of thermal expansion and flexural strength of cordierite ceramics prepared from alumina with different particle sizes

Cordierite ceramics were produced from alumina with 5 and 0.65 μm particle sizes or AlOOH and talc, clays and feldspar, to determine the influence of the alumina particle size on the microstructure, coefficient of thermal expansion (CTE) and flexural strength (FS) of the ceramics. After sintering at 1300 °C the ceramics made from 5-μm-sized alumina consisted of cordierite, glass, quartz, mullite and alumina, and had the highest density, FS and CTE. The alumina grains act as inclusions, from which the trajectories of the cracks were deflected or terminated, which increases the FS and CTE. The ceramics from sub-micrometre-sized alumina or AlOOH contained a negligable amount and no alumina, respectively, together with other phases. This is reflected in the low CTE and FS. The cordierite ceramic with the lowest CTE of ∼2.0 × 10⁻⁶ K⁻¹ and a high FS of 100 MPa was prepared from the 0.65-μm-sized alumina particles.     

Temperature induced gelation of non–aqueous alumina suspension using oleic acid as dispersant

A novel temperature induced gelation method for alumina suspension using oleic acid as dispersant is reported. Non–aqueous suspension with high solid loading and low viscosity is prepared using normal octane as solvent. Influence of oleic acid on the dispersion of suspension was investigated. There was a well disperse alumina suspension with 1.3 wt% oleic acid. Influence of gelation temperature on the coagulation process and properties of green body was investigated. The sufficiently high viscosity to coagulate the suspension was achieved at −20 °C. The gelation temperature was controlled between the melting point of dispersant and solvent. The gelation mechanism is proposed that alumina suspension is destabilized by dispersant separating out from the solvent and removing from the alumina particles surface. The alumina green body with wet compressive strength of 1.07 MPa can be demolded without deformation by treating 53 vol% alumina suspension at −20 °C for 12 h. After being sintered at 1550 °C for 3 h, dense alumina ceramics with relative density of 98.62% and flexural strength of 371±25 MPa have been obtained by this method.     

Effect of YSZ-incorporated glass-based composite coating on oxidation behavior of K438G superalloy at 1000 °C

A glass-based composite coating incorporating YSZ particles was prepared by sintering on K438G superalloy substrates. The YSZ additions increased the cyclic oxidation resistance at 1000 °C, while the formation of zircon resulting from interfacial reactions between YSZ and the glass matrix worked reversely. Besides, the YSZ inclusions changed the crystallization behavior of the glass matrix, and only anorthite precipitated during cyclic oxidation. Due to the synergy of sand-blasting and sealing effect of the glass-based coating, the oxidation behavior of K438G was changed and a layer of alumina instead of chromia formed at the substrate/coating interface. Furthermore, a gahnite layer formed at the alumina/gahnite interface because of interfacial reactions between alumina and the glass matrix, leading to the formation of a bi-layered thermally grown oxide. Thus, the alumina layer was protected from the attack of the active glass matrix. Accordingly, the coated K438G superalloy exhibited satisfactory oxidation resistance at 1000 °C.     

Fabrication and characterization of anorthite-based ceramic using mineral raw materials

The purpose of this study is to design a novel single crystalline phase ceramic based on anorthite whose properties fulfill the tableware market requirements such as high appearance quality, strength and thermal shock resistance. To obtain the single phase anorthite ceramic, ball clay, quartz, calcite, feldspar and alumina were used as raw materials. The single phase anorthite ceramic was fabricated by slip casting and sintering at 1230 °C for 1 h. It has a high flexural strength of 103 MPa, which is higher than that of the conventional porcelain. The single phase anorthite ceramic had relatively low (4.9 × 10^?6 K^?1) thermal expansion coefficient which can be matched with applicable glaze easily. Furthermore, the single phase anorthite ceramic had high degree of whiteness (L* = 94) and excellent translucency behavior which could achieve a high-quality decorative effect.     

Sodalite zeolite as an alternative all-ceramic infiltrating material for alumina and zirconia toughened alumina frameworks

The purpose of this study was to determine the effects of synthesized sodalite zeolite infiltration achieved by a direct in-situ hydrothermal reaction followed by sintering process on the flexural strength and hardness of alumina and zirconia-toughened alumina (ZTA) frameworks. Ceramic core materials were prepared as disk-shaped specimens with 16 mm diameter and 1.2±0.2 mm thickness. The case-study group was synthesized sodalite zeolite-infiltrated alumina (IA-SOD) and synthesized sodalite zeolite-infiltrated ZTA (IZ-SOD); and the control group was glass-infiltrated alumina (IA-glass) and glass-infiltrated ZTA (IZ-glass). The biaxial flexural strength (piston-on-three-balls test) and Vickers microhardness were compared among groups (n=10 specimens in each group). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to investigate the structural characteristics of specimens at the fracture and cross-sectional surfaces. For both IA-SOD and IZ-SOD, the biaxial flexural strength exceeded the required value of 100–150 MPa as specified by ISO 6872(2015), indicating their potential as all-ceramic core materials. The flexural strengths and Vickers microhardness of IZ-SOD were respectively 324.7 MPa and 1162 VHN, while these values were measured 233.6 MPa and 1013 VHN for IA-SOD. The mechanical properties and microstructure of core materials have been advocated as crucial to the clinical performance of all-ceramic dental restorations. This investigation provides data regarding the flexural strength, hardness and microstructure of partially sintered alumina and ZTA frameworks with synthesized sodalite zeolite infiltration.     

Influence of graphitization treatment on microstructure and flexural strength of C/C-ZrC-SiC composites fabricated via reactive melt infiltration

C/C-ZrC-SiC composites were prepared by chemical vapor infiltration (CVI) and molten salt assisted reactive melt infiltration (RMI). The microstructure of low density and high density C/C composites without graphitization (LC/HC) and graphitization at 2000 °C (LCG/HCG) were compared. Moreover, the effects of graphitization of LC and HC on the microstructure and flexural strength of C/C-ZrC-SiC composites were investigated in detail. The composites prepared by infiltration of LC and LCG had lower flexural strength, 220.01 ± 21.18 MPa and 197.94 ± 19.05 MPa, respectively. However, the composites prepared by HC and HCG presented higher flexural strength, 308.76 ± 12.35 MPa and 289.62 ± 8.70 MPa, respectively. This was due to the phenomenon of fiber erosion in both LC and LCG during the RMI process. After graphitization, the flexural strength of C/C-ZrC-SiC composites prepared by RMI decreased, but the fracture behavior of the composites tends to be more mild. The decreased strength of the composites were caused by the increased matrix cracks, fiber damage in high temperature and the weak interfacial bonding. The improve of failure behavior of the composites was due to interface debonding between the fiber and matrix, and composites can consume the fracture energy through fiber pull-out.     

Microstructure and mechanical properties of hot pressed Al2O3/SiC nanocomposites

Al₂O₃/SiC micro/nano composites containing different volume fractions (5, 10, 15, and 20 vol.%) of SiC were prepared by mixing a sub-micron alumina powder with respective amounts of either micro- or nano-sized silicon carbide powders. The powder mixtures were hot pressed 1 h at 1740 °C and 30 MPa in the atmosphere of Ar. The effect of SiC addition on the microstructure and mechanical properties, i.e. hardness, fracture toughness, and room temperature flexural strength were investigated. The flexural strength increased with increasing volume fraction of silicon carbide particles. The maximum flexural strength (655 ± 90 MPa) was achieved for the composite containing 20 vol.% of coarse-grained SiC, which is more than twice as high as in the Al₂O₃ reference. Hardness and fracture toughness were also moderately improved. The observed improvement of mechanical properties is mainly attributed to alumina matrix grain refinement and grain boundary reinforcement.     

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.     

Influence of debinding holding time on mechanical properties of 3D-printed alumina ceramic cores

Herein, alumina green bodies are fabricated by three dimensional (3D) printing technology, then, the influence of debinding holding time under vacuum and argon on mechanical properties is systematically investigated by comparing the changes in microstructure, bulk density, open porosity, grain connection situation and flexural strength of ceramics. The flexural strength of alumina ceramics acquired the maximum values of 26.4 ± 0.7 MPa and 25.1 ± 0.5 MPa after debinding under vacuum and argon for 120 min and 180 min, respectively. However, the alumina ceramics rendered the flexural strength of 19.4 ± 0.6 MPa and 9.5 ± 0.4 MPa under vacuum and argon without extended holding time, respectively. The relatively low mechanical properties can be mainly attributed to the weak interlayer binding force, which is caused by layer-by-layer forming mode during 3D printing process and anisotropic shrinkage during the sintering process. Moreover, the alumina ceramics exhibited moderate bulk density and open porosity of 2.4 g/cm³ and 42% after the sintering process, respectively, which are mainly influenced by the microstructural evolution of alumina ceramics during thermal treatment. Also, the diffusion of gases is achieved by curing of photosensitive resin and influenced by different holding times during debinding, affecting the mechanical properties of sintered ceramics. The mechanical properties of as-sintered ceramics are suitable for the utilization of ceramic cores in the manufacturing of hollow blades.     

Wet Green‐State Joining of Alumina Ceramics Without Paste

A new technique for the green‐state joining of alumina ceramics without paste has been developed. Wet alumina green body, gelcast by PIBM (a copolymer of isobutylene and maleic anhydride) gel system, is used as a parent green body. The joining process involves integrating two wet green bodies in a gelling state, followed by drying and sintering. For example, wet green bodies after aging 10 h were joined, and the joint region of the dried and sintered sample exhibited a homogeneous microstructure without defects or an interlayer. The flexural strength of the joined sample (468.5 ± 62.9 MPa) was nearly equivalent to that of a monolithic sample (470.6 ± 27.4 MPa). The joining mechanism was discussed on the basis of interaction among PIBM molecular chains. The proposed joining technique is simple and promising for producing advanced ceramics with large sizes and/or complicated shapes.     

Microstructure and mechanical properties improvement of the Nextel™ 610 fiber reinforced alumina composite

The alumina slurry with high solid content was prepared, and a rapid lamination route for fabricate the Nextel? 610 fiber reinforced alumina composite was proposed in this work. The microstructure and mechanical properties of the as-received all-oxide composite were investigated by a series of techniques. The shrinkage cracks in matrix were reduced, while porous structure in composite was maintained. The N610/alumina composite has weak matrix and weak interface, as the Young’s modulus of the alumina matrix and the interfacial shear strength of the composite are 140.8±2.5GPa and 129.1±14.6MPa. The mechanical properties of the composite are much higher than lots of oxide/oxide composites, given its flexural strength, interlaminar shear strength and the fracture toughness are 398.4±5.7MPa、27.0±0.5MPa and 14.1±0.9MPa·m^1/2, respectively. The flexural strength of the virgin composite keep stable at 25–1050 °C, while dramatically decrease at 1100–1200 °C.     

Processing and properties of Nextel™ 720 fiber reinforced alumina-zirconia ceramic matrix composites

The continuous Nextel? 720 fiber-reinforced zirconia/alumina ceramic matrix composites (CMCs) were prepared by slurry infiltration process and precursor infiltration pyrolysis (PIP) process. The introduction of submicron zirconia powders into the aqueous slurry was optimized to offer comprehensively good sintering activity, high thermal resistance and good mechanical properties for the CMCs. Meanwhile, the zirconia and alumina preceramic polymers were used to strengthen the porous ceramic matrix through the PIP process. The final CMC sample achieved a high flexural strength of 200 MPa after one infiltration cycle of alumina preceramic polymer and thermal treatment at 1150 °C for 2 h. The flexural strength retention of the improved CMC sample was 104% and 89% respectively after thermal exposure at 1100 °C and 1200 °C for 24 h.     

Effect of zirconia content and particle size on the properties of 3D-printed alumina-based ceramic cores

Alumina-based ceramic cores are used to manufacture the internal structures of hollow alloy blades, requiring both high precision and moderate properties. In this work, zirconia is regarded as a promoter to improve the mechanical properties of sintered ceramic. The effect of zirconia content and particle size on the microstructure and mechanical properties of ceramics was evaluated. The results indicate that the flexural strength of sintered ceramics reached the maximum of 14.5 ± 0.5 MPa when 20 wt% micron-sized (10 μm) zirconia (agglomerate size, consistent with the alumina particle size) was added, and 26.5±2.5 MPa when 15 wt% 0.3 μm zirconia was added. Zirconia with submicron-sized (0.3 μm) particles effectively filled the pores between alumina particles, thus leading to the maximum flexural strength with a relatively low content. The corresponding sintered ceramics had a bulk density of 2.0 g/cm³ and open porosity of 59.6%.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

Coating optimization of yield pseudoplastic paste-based stereolithography 3D printing of alumina ceramic core

Yield pseudoplastic paste is prepared for self-supported stereolithography (SLA) 3D printing with critical shear stress of 290.7 Pa and viscosity of 28810 mPa·s under strain rate of 30 s^?1. Simulation analysis of paste coating is carried out based on established constitutive equation. Furthermore, corresponding stress field is analyzed under different layer thicknesses (25–100 μm) and blade speeds (1.5–4.5 mm/s). Then, flexural strength, porosity, surface roughness and microstructure of components, printed with different layer thicknesses and sintered at different temperatures, are systematically investigated. Finally, complex-shaped cores are fabricated with flexural strength of 38.56±1.45 MPa, porosity of 21.57±0.8% and surface roughness of R_a < 3 μm. The optimized parameters include layer thickness of 25–50 μm, blade speed of 3.5 mm/s, and sintering temperature of 1300 ^oC. Moreover, step-like undulating fracture morphology of the core is observed along printing layer, which shows that the coating process has an important influence on the mechanical properties of ceramic parts.     

High strength and high light transmittance sapphire/sapphire joints bonded using a La2O3-Al2O3-SiO2 glass filler

A novel La₂O₃-Al₂O₃-SiO₂ (LAS) glass was used as filler to join transparent sapphire for obtaining high strength and high light transmittance joints. The results show that the LAS glass filler had compatible coefficient of thermal expansion (CTE) with sapphire and excellent wetting ability on sapphire. During the joining process, no interfacial reaction occurred and the brazing seams were in a completely amorphous state under fast cooling conditions (~50 °C/min). With increased joining temperature, the mutual dissolution and diffusion between sapphire and the LAS filler were enhanced. The flexural strength of joints first increased and then decreased with an increase in the joining temperature from 1400 °C to 1550 °C. The optimal flexural strength of joint reached 325 MPa, which almost was the same as the strength of sapphire substrate. At 500 nm, the in-line transmittance of this joint was 80.5%, which was close to that of sapphire (84.2%).     

Ultra-high strength medium-entropy (Ti,Zr,Ta)C ceramics at 1800°C by consolidating a core-shell structured powder

We report for the first time the synthesis of a core-shell structured composite powder with a core of Zr(Ti,Ta)C and a shell of Ti,Ta(Zr)C at 1700°C and investigate the formation mechanism for the core-shell structure. The medium-entropy (Ti,Zr,Ta)C ceramics with fine grains (1.1 ± 0.4 μm) and relative density of 94.8% was prepared by hot-pressing at 2100°C. The flexural strength of (Ti,Zr,Ta)C at 1000°C (493 ± 21 MPa) was close to the room temperature (511 ± 52 MPa). As the temperature increased from 1600°C to 1800°C, the flexural strength was increased significantly, with an ultra-high flexural strength of 725 ± 32 MPa at 1800°C. The existence of the core-shell structure in the powder suppressed the grain growth due to the sluggish diffusion effect. The ultra-high strength of (Ti,Zr,Ta)C ceramics was attributed to its fine microstructures, high fracture toughness, and the reinforced the grain boundary strength.     

Continuous carbon fiber reinforced ZrB2-SiC composites fabricated by direct ink writing combined with low-temperature hot-pressing

As one of additive manufacturing techniques, direct ink writing has significant advantages in the manufacture of ceramic matrix composites, nevertheless, the poor impregnability of ceramic slurry makes it difficult to fill the interior of fiber bundles, causing poor mechanical properties. Here, ultrasound-assisted fiber separation technique was introduced to impregnate ceramic slurry with a continuous carbon fiber bundle during direct ink writing of continuous carbon fiber/ceramic green body and subsequent low temperature hot-pressing was combined to improve its robustness. Suitable thickness of carbon coating could bring to high fracture resistance, whereas excessively thick carbon coating will adversely affect the mechanical properties. A carbon interface with thickness around 110 nm was incorporated, the flexural strength, fracture toughness and work of fracture of C_f/ZrB₂-SiC composite reached 388.3 MPa, 10.04 MPa·m^1/2 and 2380 J/m², respectively. Therefore, direct ink writing combined with low temperature hot-pressing, was effective to fabricate high-performance ceramic matrix composites.     

Development of SiC–SiC joint by reaction bonding method using SiC/C tapes as the interlayer

SiC/C tapes with different compositions and thicknesses were used to join pressureless sintered silicon carbide ceramics by reaction bonding method. The microstructure of the joints and the influences of joint thickness and residual silicon content in joint layer on the 4-point flexural strength of as joined SiC ceramics have been investigated. Specimens with high flexural strength can be achieved through the control of the composition and the thickness of the joint layer. The highest flexural strength of the joined specimens with the joint thickness of 13 μm can reach 346 ± 35 MPa and 439 ± 31 MPa at room temperature and 1250 °C, respectively. The microstructure development and the reaction bonding mechanism were also studied.