Key Engineering Materials Vols. 602-603

Abstract: High hardness, high strength, high fracture toughness and low density are required for novel bulletproof materials. B₄C/SiC composite ceramic is one of the most potential candidates. In this study, B₄C/SiC composite ceramic was prepared by reaction sintering. The influence of B₄C content, species and content of carbon, sintering temperature on the mechanical properties of B₄C/SiC composite ceramic were studied. A high performance B₄C/SiC composite ceramic was sintered at 1750°C for 30 min. Phenolic resin and carbon black were both chosen as carbon sources, whose favorable contents were 10wt%, 5wt%, respectively. The density of sintered bodies reduces with B₄C content increases. To some extent, fracture toughness, bending strength improve initially and then deteriorate with the increase of B₄C content whose optimal amount is 30wt%. The optimal fracture toughness and bending strength of the B₄C/SiC composite ceramic are 5.07MPa·m^1/2 and 487MPa, respectively. Meanwhile, the Viker-hardness of the sintered body is 30.2GPa, the density is as low as 2.82g/cm³.

Abstract: Boron carbide is an attractive neutron absorbing material used both in Fast Breeder Reactors (FBR) and in Pressurised Water Reactors (PWR) owing to its very high absorption cross section for thermal neutrons, chemical stability and refractory character. In the present paper, ¹⁰B enriched B₄C ceramics are prepared by pressureless sintering at 19602160°C, under argon, using ¹⁰B boron carbide powder as raw material, 18 wt% phenolic resin as sintering aid. In the sintering temperature range, with the increasing of sintering temperature, both the relative density and flexural strength increase linearly, the average particle sizes increase from about 3μm at 1960°C to more than 30μm at 2160°C. The sample sintered at 1960°C has a 91.7% of relative density and 192 MPa of flexural strength and a homogeneous texture with 3-4μm particle size, which are enough for pellet application of reactors.

Abstract: In order to improve the sinterability of the cBN, surface oxidation treatment was conducted to form a B₂O₃ film on the surface of the cBN powders. The cBN powders (d₅₀ = 7.5 μm) were used as original powders. The heat treatment processes were 500, 800, 900, 950, 980 and 1000 °C holding for 30 minutes, respectively. The oxidation reactions which probably happened were calculated based on the thermodynamics. The experimental results showed that the oxidation starting temperature of cBN powders was higher than 800 °C. The higher the oxidation temperature, the greater the mass gain of the cBN powders.

Abstract: For engineering ceramics, cracks produced in preparation process, i.e. sintering and surface finishing, have a profound impact on the mechanical strength and wear. Under cavitation loading, the flaws on the surface and subsurface are preferentially eroded. In this paper, artificial cracks are introduced on WC ceramic by indentation of a Vickers indenter at a load of 10 kgf. Five positions on the cavitation eroded area are selected. The average length of the cracks is 71.6 mm. The cavitation erosion tests are terminated at 1, 3, and 5 min to reveal the evolution of cracks. A single pattern for the evolution of cracks is recognized. Two new cracks at the tip of indentation cracks first nucleate and then propagate (accompanied by pullout of grains). The angle between the two new cracks ranges from 30° to 45°. The evolution of adjunct two groups of cracks gives birth to local breakout finally. Steps which are provided limited resistance to breakout are observed. The nucleation and propagation of the four indentation cracks do not always occur simultaneously.

Abstract: In this work, atmospheric plasma spray (APS) technology was applied to fabricate ZrC-W composite coatings. The microstructure of the composite coatings was characterized. The influence of W content on the ablation-resistant and thermal shock properties of ZrC-W composite coatings was evaluated using a plasma flame. The results show that the ZrC-W composite coatings had typically lamellar microstructure, which was mainly made up of cubic ZrC, cubic W and a small amount of tetragonal ZrO₂. The ZrC-W coatings had improved ablation resistant and thermal shock properties compared with those of the pure ZrC coating. It was supposed that the improved density, thermal conductivity and toughness of the composite coatings contributed to this phenomenon.

Abstract: Binderless WC-based cemented carbides with different fractions (0-9 wt.%) of ZrC nanopowder were fabricated through spark plasma sintering at 1600 °C under a uniaxial pressure of 50 MPa. The addition effect of ZrC nanopowder on the phase composition and microstructure of the fabricated materials were explored with the help of X-ray diffraction and scanning electron microscope. The results indicated that W₂C phase was detected in the samples with 0-3 wt.% ZrC nanopowder, but with further increase in ZrC added fraction, ZrO₂ phase instead of W₂C phase was detected. The apparent density decreased gradually with the increase in added fraction of ZrC nanopowder, while the relative density increased initially and then decreased, reaching its maximum of about 98.2% when the added fraction of ZrC nanopowder was about 3 wt.%, indicating that appropriate added fraction of ZrC nanopowder can improve the densification of binderless WC cemented carbides. Without ZrC nanopowder, the coarsening and abnormal growth of WC grains were serious, resulting in many large prismatic WC grains in the samples. However, Such phenomena could be suppressed by adding ZrC nanopowder, resulting in much finer and more homogenous microstructure after 1-3 wt.% ZrC nanopowder was added. When the added fraction of ZrC nanopowder was higher than 3 wt.%, the agglomeration of ZrC nanopowder became more and more serious.

Abstract: The aim of this work was to determine the effect of composite additives on the thermal and mechanical properties of aluminum nitride (AlN) in detail. The composite system has not been studied in depth before. The hot-pressed AlN was prepared with Y₂O₃-Dy₂O₃-YF₃ and Y₂O₃-Dy₂O₃-CaO as the composite sintering additives. As the result, the thermal conductivities for the sintered body with two composite additives were 171 W/m.K and 152 W/m.K, respectively. The fracture toughness values calculated by the Evans & Clarkes’s equation for both of the samples were 2.34±0.09 MPa.m^1/2and 2.63±0.13 MPa.m^1/2 at 10 kg load. The toughness difference is the result of comprehensive effect of the grain size, the properties of the boundary phase, its distribution, and also the interactions between different phases.

Abstract: In this paper, the aluminum nitride (AlN) was fabricated by pressureless sintering with YF3 and various silicon compounds as the sintering aids. The phase, microstructure, density and thermal conductivity were characterized by XRD, SEM and laser thermal diffusivity method. The sample densities were detected varied from 3.17 to 3.30g/cm3 and room-temperature thermal conductivity varied from 196 to 233 W/m·K. Samples sintered with YF3 additives have the highest thermal conductivity. The sintering aids with SiO2, Si3N4 and SiC would decrease the density and the thermal conductivity obviously, and also change the fracture mode from the intergranular to transgranular , which is a key for the toughness of the AlN substrate.

Abstract: Highly dense AlN/CNT composite ceramics with 1-10% volume fractions of CNT were fabricated by spark plasma sintered (SPS) at 1400°C-1700°C. The results indicated that origination diameter of AlN had a great effect on microstructure and thermal conductivity. In details, for the system with AlN origination diameter of nanosized, the tubular structure of CNT has not been destructed, but when micro-sized AlN powder was adopted, the structure of CNT showed unstable at high temperature. Even though the degradation with incorporation of CNT into AlN, thermal conductivity of sintered AlN/CNT composites ceramics was evidently improved by adjusting content of additive Y₂O₃ and the sintering process. Both the real part and imaginary part of the composites of Ka-Band (26.540.0 GHz) increase with the increase of CNT content, in which the increase of imaginary part is more than that of real part, resulting in an increase of loss factor. The AlN/ CNT thermal conductivity composites with appropriate CNT content and sintering temperature possess good dielectric dissipation and thermal conductivity.

Abstract: Polycrystalline aluminum nitride (AlN) films were deposited on Si (111) substrates by radio frequency (RF) magnetron sputtering method in an nitrogen (N₂) + argon (Ar) gas mixture. The effect of the preparation conditions- sputtering pressure (p), sputtering power (w), gas mixture (Ar/N₂) and post-deposition annealing treatment -on the properties of AlN films were investigated by means of X-ray diffraction (XRD). Highly c-axis oriented AlN films were obtained with optimized growth parameters: p=0.3Pa, w=400w and Ar/N₂=2.