Two-step Hot Pressing Research Articles

Thermal Conductivity and Flexural Strength of Two-Step Hot-Pressed SiC Ceramics

Effects of two-step hot-pressing on microstructure, thermal conductivity and flexural strength of silicon carbide (SiC) ceramics were investigated. A novel combination of aluminium nitride (AlN), La2O3 and LaF3 was employed as sinter additives in this study. SiC ceramics prepared through both one-step (OS samples) and two-step (TS samples) hot-pressing method exhibited high relative densities. Compared with OS samples, the TS samples yielded finer microstructures with both higher thermal conductivities and flexural strengths. Impedance spectroscopy analysis was employed to reinforce the investigation on thermal conductivity variations in different samples. According to the analysis results, TS samples exhibited higher fitting grain and grain boundary resistances, which stood for a lower concentration of vacancies, indicating that two-step hot-pressing is more beneficial to the elimination of defects and improvement of thermal conductivity in SiC ceramics. On the other hand, TS samples were strengthened by a combined effect of finer microstructure and formation of SiC-AlN solid solution, thereby obtained higher flexural strengths. Hence, the present study suggests that two-step hot-pressing is more favorable to the improvement of both thermal conductivity and flexural strength of SiC ceramics with AlN-La2O3-LaF3 additive combinations.

Use of the fiberglass reinforcement method in thermoplastic mouthguard materials to improve flexural properties for enhancement of functionality.

The purpose of this study was to evaluate the application of fiberglass reinforcement method in thermoplastic mouthguard materials to improve flexural properties and adhesive strength. Commonly used two types of commercial mouth guard materials (ethylene-vinyl acetate copolymer-based and polyolefin-based) were reinforced with glass fiber clothes by two-step hot press. Flexural strength and adhesive strength with each base material were examine via three-point bending test and delamination test, respectively. Ethylene-vinyl acetate copolymer-based fiberglass-reinforced material has significantly greater adhesive strength with base material and improvement of flexural properties compared with polyolefin-based material. These results suggest that flexural properties of both conventional commercial mouthguard materials were improved when the glass-fiber-reinforced method was applied to reinforce mouthguard materials, and more, ethylene-vinyl acetate copolymer was more desirable for the base material.

Transparent and strong polymer nanocomposites generated from Pickering emulsion gels stabilized by cellulose nanofibrils

We report here the development of transparent and strong polymer composites reinforced by unmodified cellulose nanofibrils (CNFs) with a Pickering emulsion gelation strategy. The CNFs entangle and firmly stabilize on the surface of emulsion droplets containing polymethyl methacrylate (PMMA) solution, leading to the gelation of the emulsions. CNFs/PMMA composites were generated via vacuum filtration and solvent washing of the gel and a subsequent two-step hot pressing. The composites contained a unique self-assembled two-tier hierarchy of CNFs networks and demonstrate promising transparency, tensile strength, flexibility, and an extremely low thermal expansion. Remarkably, these properties are highly tunable with varying the concentration of CNFs and the volume ratio of the water to oil phase. This work offers a facile route to realize the well dispersion of unmodified CNFs in hydrophobic polymer matrix and achieve high performance of polymeric materials reinforced by CNFs.

Study on Hot Press Forming Process of Large Curvilinear Generatrix Workpiece of Ti55 High-Temperature Titanium Alloy

In order to manufacture complex curvilinear generatrix workpieces of high-temperature titanium alloy, the hot tensile behavior of Ti55 alloy sheet was tested and the hot press forming process was investigated using Finite Element Method (FEM) simulation and experiment. The hot tensile experiments of Ti55 rolled sheet were conducted at the temperatures of 800–900 °C with the strain rates of 0.001–0.1 s−1. According to the results of hot tensile tests and microstructure evolution, the proper hot press forming parameters were determined as the temperature of 850 °C and the strain rates of 0.001–0.01 s−1. The wrinkling mechanism in the transition region was analyzed and the initial blank sheet geometry was optimized by FE simulation of hot press forming. The two-step hot press forming process was better to produce the complex sheet workpiece of Ti55 alloy than the one-step hot forming scheme, which could restrain the wrinkling trend and ensure the microstructure and mechanical properties of the hot formed workpieces.

A Thermoplastic Multilayered Carbon-Fabric/Polycarbonate Laminate Prepared by a Two-Step Hot-Press Technique.

Carbon fiber (CF) reinforced thermoplastic composites have gradually become increasingly popular in composite production owing to their lower hazard level, good structural flexibility and recyclability. In this work, a multilayered carbon–fabric/polycarbonate laminate (multi-CFPL) was fabricated by a two-step hot-press process, mainly based on the thermoplastic properties of its polycarbonate (PC) matrix. Different from the conventional one-step method, the two-step hot-press process was composed of two separate procedures. First, a unit-hot-press operation was introduced to prepare a single-layered carbon–fabric/PC laminate (simplified as unit-CFPL). Subsequently, a laminating-hot-press was employed to compress several as-prepared unit-CFPLs bonded together. This combined process aims to reduce the hot-press temperature and pressure, as well as facilitate the structure designability of this new composite. Several mechanical investigations were conducted to analyze the effect of the hot-press parameters and unit-CFPL numbers on the performance of this multi-CFPL material, including flexural, uniaxial tensile and impact tests. The results reveal that the multi-CFPL exhibits a good stability of flexural and tensile properties in terms of strength and modulus. Furthermore, during impact tests, the multi-CFPL presents an accelerated growth of peak force and energy absorption capability with increasing unit-CFPL layers.

Graded Si3N4 ceramics with hard surface and tough core by two-step hot pressing

Graded Si3N4 ceramics with hard surface and tough core were prepared by two-step hot pressing with the homogenous starting composition. The inner Si3N4 layer was firstly hot-pressed at 1800°C, subsequently covered with Si3N4 powders on both sides, and finally hot-pressed at 1600°C. After two-step hot pressing, the resulting ceramics exhibited a zoned microstructure, differentiated by the phase assemblage of Si3N4 and grain size. The outer layers were well bonded to the inner layer. The outer layer exhibited bimodal and fine-grained microstructure, whereas the inner layer exhibited bimodal and coarse-grained microstructure. Vickers hardness of outer and inner layers were 18.1±0.2GPa and 16.0±0.2GPa, respectively, and fracture toughness were 4.2±0.1MPam1/2 and 5.5±0.2MPam1/2, respectively.

Preparation of dense amorphous Al2O3–ZrO2–Y2O3 by two-step hot pressing

Two-step hot pressing was reported for the first time in the densification of amorphous oxides. Amorphous AZY powder was synthesized by sol–gel method, and densified by conventional hot pressing (CHP) and two-step hot pressing (TSHP). In CHP, the sample was easily crystallized and the maximum realizable density was 93.5% under 350°C for 1min before crystallization. In TSHP, uniform amorphous AZY with relative density up to 98% was achieved under T1=350°C, T2=200°C for 15min. Based on the key problem in preparation of dense bulk amorphous oxides, TSHP solved the competition between densification with crystallization, and provided new methods to densification of amorphous oxides.

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB2 particles were investigated. The densification on-set temperature (Td-micron) and grain growth on-set temperature (Tg-micron) for micron-sized ZrB2 particles were about 1500°C and 1800°C, respectively. And the densification on-set temperature (Td-nano) and grain growth on-set temperature (Tg-nano) for nano-sized ZrB2 particles were about 1300°C and 1500°C, respectively. A bimodal micron/nano-ZrB2 ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2±35.8MPa and an improved fracture toughness of 7.2±0.4MPa·m1/2 were obtained. The measured critical thermal shock temperature difference (ΔTc) for this bimodal micron/nano-ZrB2 ceramic was as high as 433°C.

Microstructure and properties of self-reinforced α-sialon ceramics doped with Lu 2O 3

Lu-α-sialon ceramics containing elongated grains embedded into fine α-sialon matrix were obtained by applying a two-step hot-press procedure. Incorporation of excess Lu 2O 3 could promote anisotropic growth of α-sialon grains, and the quantity and aspect ratio of elongated α-sialon grains increase with the amount of Lu 2O 3 addition. The elongated α-sialon grains nucleate and grow epitaxially on the α-Si 3N 4 core, and the initial precipitation on the core is richer in Lu and Al content than the subsequent precipitation. The obtained Lu-α-sialons possess improved mechanical properties, the Vickers hardness, flexural strength and fracture toughness of the materials with 4 wt%Lu 2O 3 could reach with value of 22 GPa, 620.4 MPa and 4.8 MPa m 1/2, respectively. The Lu-α-sialons also demonstrate excellent high-temperature strength due to the presence of the refractory intergranular phase. The strength of the material with 4 wt%Lu 2O 3 at 1400 °C could retain 87% of its room-temperature strength.

In Situ Toughened α-Sialon Ceramics Doped with Lutecia and Ytterbia

Ytterbia and Lutecia were adopted to stabilize α-sialon ceramics during a two-step hot press sintering, respectively. Although Yb3+ and Lu3+ possess the similar small ion size (0.86 Å and 0.85 Å) as two adjacent rare-earth elements, Yb- and Lu-α-sialon ceramics present very different microstructure and properties. Yb-α-sialon ceramic is fully composed of α-sialon phase without any secondary phase. The ceramic incorporated with Lu2O3 also consists of α-sialon phase, but a trace amount of intergranular phase J′ (Lu4Si2-xAlxO7+xN2-x) still remains at the boundary. Microstructure observation indicates the fully equiaxial grain morphology of Yb-α-sialon ceramic. However, elongated grains appear in the Lu-α-sialon ceramic, coexisting with small equiaxed grains. Owing to the almost full α-sialon phase assemblage, both ceramics present high hardness with the value of over 21 GPa. Lu-α-sialon ceramic possesses good toughness, which is attributed to the elongated grains assisting the facilitation of toughening mechanism. The equiaxied grain morphology results in a comparatively lower toughness for Yb-α-sialon ceramic.

Phase transformation and microstructure development of in situ toughened Yb α-SiAlON ceramics prepared by two-step hot-pressing

In this paper in situ toughened Yb α-SiAlON ceramics with elongated grains were prepared by two-step hot-pressing. The phase transformation and microstructure development of the samples during the two-step sintering were discussed in detail. It was found that at low temperatures, the formation of intermediate phases reduced the amount of liquid phase and thus slowed down the dissolution and mass transportation processes. In this way, the grain growth was depressed and fine grains were obtained. At the elevated temperature, the intermediate phases would be dissolved, resulting in abundant liquid phase and fast anisotropic grain growth. By the two-step sintering procedure, the nucleation and grain growth could be well manipulated and coarse elongated grains could be produced. These elongated grains improved the fracture toughness of the sintered ceramics via the mechanisms such as grain pull-out and debonding, which were clearly observed by SEM. In addition, the preferred orientation of the elongated α-SiAlON grains was also studied in the light of their growth habit.

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Abstract Self-reinforced in situ Si3N4 composite material was prepared with high amount of La2O3 and Y2O3 additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si3N4 grains.