Growth Of Elongated Grains Research Articles

Effect of Ultra-High Pressure Sintering and Spark Plasma Sintering and Subsequent Heat Treatment on the Properties of Si3N4 Ceramics.

In this study, coarse Beta silicon nitride (β-Si3N4) powder was used as the raw material to fabricate dense Si3N4 ceramics using two different methods of ultra-high pressure sintering and spark plasma sintering at 1550 °C, followed by heat treatment at 1750 °C. The densification, microstructure, mechanical properties, and thermal conductivity of samples were investigated comparatively. The results indicate that spark plasma sintering can fabricate dense Si3N4 ceramics with a relative density of 99.2% in a shorter time and promote α-to-β phase transition. Coarse β-Si3N4 grains were partially fragmented during ultra-high pressure sintering under high pressure of 5 GPa, thereby reducing the number of the nucleus, which is conducive to the growth of elongated grains. The UHP sample with no fine α-Si3N4 powder addition achieved the highest fracture strength (822 MPa) and fracture toughness (6.6 MPa·m1/2). The addition of partial fine α-Si3N4 powder facilitated the densification of the SPS samples and promoted the growth of elongated grains. The β-Si3N4 ceramics SPS sintered with fine α-Si3N4 powder addition obtained the best comprehensive performance, including the highest density of 99.8%, hardness of 1890 HV, fracture strength of 817 MPa, fracture toughness of 6.2 MPa·m1/2, and thermal conductivity of 71 W·m−1·K−1.

Growth kinetics of ceramic grains during liquid phase sintering in Mo2FeB2-based cermets

Mo2FeB2-based cermets with different hard phases were fabricated by reaction boronizing sintering. SEM, EDS, and TEM were utilized to evaluate the growth kinetics of Mo2FeB2 hard-phase grains with different morphologies during liquid phase sintering. During the sintering stages of L1 and L2, the growth of elongated grains along the long-axis conformed to the cubic law. Combined with the observed rough interface between the hard and binder phases, the growth mechanism along the long-axis could be identified as diffusion-controlled since atoms did not have to overcome large energy barriers to nucleate. Meanwhile, the grain growth along the short-axis was primarily accomplished via 2-D nucleation on the smooth interface, which was mainly controlled by interface reaction during the liquid phase sintering since the driving force in the short-axis direction failed to meet the requirements of critical driving force (∆Gc). In addition, the principal control mechanism of nearly equiaxed grains growing in L1 and L2 was identical to the long-axis direction of elongated grains. Nevertheless, the third phase (Fe23B6) at the interface of nearly equiaxed grains affected grain growth, resulting in variations in grain size and growth activation energy during the L1 and L2 sintering stages.

Effect of LiYO2 addition on sintering behavior and indentation properties of silicon nitride ceramics

Abstract The influence of the sintering additive LiYO2 (5– 15 wt.%) on sintering behavior, microstructure and mechanical properties of Si3N4 ceramics was investigated. Since LiYO2 enables densification of Si3N4 at extraordinarily low temperatures, sintering was carried out in the range from 1200– 1700 °C. Densification was found to be enhanced with increasing additive content due to an increasing volume fraction of the liquid. The phase transformation and grain growth occurred through a solution-reprecipitation mechanism, where the precipitation took place preferentially on pre-existing β-Si3N4 nuclei (of which the starting powder already contained 20 wt.%). The indentation fracture toughness increased with both sintering time and additive content as a result of the growth of elongated grains.

Inconel-steel multilayers by liquid dispersed metal powder bed fusion: Microstructure, residual stress and property gradients

Synthesis of multi-metal hybrid structures with narrow heat affected zones, limited residual stresses and secondary phase occurrence represents a serious scientific and technological challenge. In this work, liquid dispersed metal powder bed fusion was used to additively manufacture a multilayered structure based on alternating Inconel 625 alloy (IN625) and 316L stainless steel (316L) layers on a 316L base plate. Analytical scanning and transmission electron microscopies, high-energy synchrotron X-ray diffraction and nanoindentation analysis reveal sharp compositional, structural and microstructural boundaries between alternating 60 μm thick alloys’ sub-regions and unique microstructures at macro-, micro- and nano-scales. The periodic occurrence of IN625 and 316L sub-regions is correlated with a cross-sectional hardness increase and decrease and compressive stress decrease and increase, respectively. The laser scanning strategy induced a growth of elongated grains separated by zig-zag low-angle grain boundaries, which correlate with the occurrence of zig-zag cracks propagating in the growth direction. A sharp <110> fiber texture within the 316L regions turns gradually into a <100> fiber texture in the IN625 regions. The occurrence of the C-like stress gradient with a pronounced surface tensile stress of about 500 MPa is interpreted by the temperature gradient mechanism model. Chemical analysis indicates a formation of reinforcing spherical chromium-metal-oxide nano-dispersoids and demonstrates a possibility for reactive additive manufacturing and microstructural design at the nanoscale, as a remarkable attribute of the deposition process. Finally, the study shows that the novel approach represents an effective tool to combine dissimilar metallic alloys into unique bionic hierarchical microstructures with possible synergetic properties.

Al-B-C 조제 β-SiC의 스파크 플라즈마 소결에 미치는 α-SiC seed 첨가 영향: 미세 구조 변화

The unique features of spark plasma sintering process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. <TEX>$\beta$</TEX>-SiC powder with 0, 2, 6, 10 wt% of <TEX>$\alpha$</TEX>-SiC particles (seeds) and 4 wt% of Al-B-C (sintering aids) were spark plasma sintered at <TEX>$1700-1850^{\circ}C$</TEX> for 10 min. The heating rate, applied pressure and sintering atmosphere were kept at <TEX>$100^{\circ}C/min$</TEX>, 40 MPa and a flowing Ar gas (500 CC/min). Microstructural development of SiC as function of seed content and temperature during spark plasma sintering was investigated quantitatively and statistically using image analysis. Quantitative image analyses on the sintered SiC ceramics were conducted on the grain size, aspect ratio and grain size distribution of SiC. The microstructure of SiC sintered up to <TEX>$1700^{\circ}C$</TEX> consisted of equiaxed grains. In contrast, the growth of large elongated SiC grains in small matrix grains was shown in sintered bodies at <TEX>$1750^{\circ}C$</TEX> and the plate-like grains interlocking microstructure had been developed by increasing sintering temperature. The introduction of <TEX>$\alpha$</TEX>-SiC seeds into <TEX>$\beta$</TEX>-SiC accelerated the grain growth of elongated grains during sintering, resulting in the plate-like grains interlocking microstructure. In the <TEX>$\alpha$</TEX>-SiC seeds added in <TEX>$\beta$</TEX>-SiC, the rate of grain growth decreased with <TEX>$\alpha$</TEX>-SiC seed content, however, bulk density and aspect ratio of grains in sintered body increased.

In situ growth of elongated Al 2O 3 grains induced by Al nanoparticles

We have reported the in situ growth of elongated grains induced by Al nanoparticles for the fabrication of high-pure and high toughness Al 2O 3 ceramics. It is found that the introduced Al nanoparticles have a profound effect on the anisotropic growth of Al 2O 3 grains. As compared to the conventional techniques by adding impurities to induce the anisotropic grain growth, Al particles could be transformed into Al 2O 3 after sintering in air, leading to the formation of high-pure Al 2O 3 ceramics without contamination. The elongated grains induced by the Al nanoparticles make noticeable improvements of bending strength and fracture toughness of Al 2O 3 ceramics. Current work might benefit the fabrication of high-pure and high toughness Al 2O 3 ceramics with in situ grown elongated grains.

Effects of Technical Parameters on the Phase Composition and Microstructure of Yb α-SiAlON Prepared by Combustion Synthesis

In this paper, Yb a-sialon powders with elongated grains have been prepared by combustion synthesis. The effects of diluents, nitrogen pressure, adding a-sialon seeds and NH4F on the phase transformation and microstructure development of the samples are discussed systematically. The experimental results show that the addition of diluents, proper nitrogen pressure and additive NH4F can facilitate the nitridation of Si. It was also found that the added a-sialon seeds and NH4F are beneficial to the nucleation and growth of elongated grains.

Effect of LiYO2 addition on sintering behavior and indentation properties of silicon nitride ceramics

Abstract The influence of the sintering additive LiYO2 (5–15wt.%) on sintering behavior, microstructure and mechanical properties of Si3N4 ceramics was investigated. Since LiYO2 enables densification of Si3N4 at extraordinarily low temperatures, sintering was carried out in the range from 1200–1700°C. Densification was found to be enhanced with increasing additive content due to an increasing volume fraction of the liquid. The phase transformation and grain growth occurred through a solution-reprecipitation mechanism, where the precipitation took place preferentially on pre-existing -Si3N4 nuclei (of which the starting powder already contained 20wt.%). The indentation fracture toughness increased with both sintering time and additive content as a result of the growth of elongated grains.

Pressureless sintering of in-situ toughened Yb α-SiAlON ceramics by adding seed crystals prepared by combustion synthesis

Single-phase Yb α-SiAlON ceramics in-situ toughened by elongated grains were prepared by pressureless sintering, with the crystalline powder prepared by combustion synthesis used as seed crystals. The effects of seed addition on the densification, phase transformation, microstructure development, and mechanical properties of the samples were investigated in detail. From the experimental results, it was found that the added seed crystals could promote the phase transformation and facilitate the growth of elongated grains. By adding seed crystals the fracture toughness of the sintered α-SiAlON ceramics was considerably improved, with no obvious degradation in the bulk density and hardness at the same time.

Development of sialon ceramics for lubricated sliding applications

AbstractMicrostructural design and characterization of mechanical as well as tribological properties of sialon ceramics are the aim of this work. A potential application for these materials are fuel injection pumps for gasoline engines. The currently used metal parts face severe tribological damage with increasing pumping pressures and thus a replacement is needed. Sialon ceramics offer a wide variety of compositional parameters which allow to tailor microstructure and mechanical properties in order to meet the tribological requirements. Since phase boundaries in the sialon system shift depending on the additive elements used, investigations of the phase relations have been performed for two different sintering additives in order to gain informations about reasonable starting compositions. Then, the ratio of the sialon modifications, the amount of grain boundary phase and the grain morphologies have been correlated with the initial composition and the amount of sintering additives. For high hardness values an increase of the fraction of the α‐sialon modification in mixed α/β‐sialon materials is beneficial. The fracture toughness can be raised by promoting growth of elongated grains. It can be done either by achieving higher β‐sialon fractions in mixed α/β‐sialon ceramics or by sintering with an excess of additive in pure α‐sialon materials. Under lubricated sliding with isooctane, sialon ceramics showed a more advantageous behaviour with regard to friction coefficients and wear than commercial silicon nitride and coarse‐grained alumina.

Microstructural control of b-silicon nitride ceramics to improve thermal conductivity

An advanced silicon nitride material with high isotropic thermal conductivity (149 W m-1 K-1) has been developed. This high thermal conductivity was achieved with a process that combines high-quality seed crystals with a suitable additive system to promote grain growth. In this process, the addition of β-Si3N4 seed crystals was found to be effective in improving thermal conductivity due to their low defect and impurity concentrations. The seed crystals seem to work as nuclei for controlling grain growth during the sintering process. Controlling the growth of elongated grains so that they do not interact with each other seems important for suppressing the generation of new defects inside the grains.

Growth of elongated grains in α-SiAlON ceramics

The microstructural evolution of pressureless sintered α-SiAlON, which was made from different particle sizes of α-Si 3N 4 as starting powders have been investigated. The microstructure of the specimens prepared from as-received powder of 0.55 μm in average size, which was sintered at 1850°C, consisted of equiaxed α-SiAlON grains. However, by decreasing the initial α-powder size to 0.27 μm, the growth of elongated grains was enhanced, resulting in a microstructure with elongated grains of a high aspect ratio. In addition, when the specimen prepared from as-received α-Si 3N 4 powder of 0.55 μm in average size was heat treated at 1600°C for 10 h, and then sintered at 1950°C for 1 h, elongated grains were also formed, probably due to the nuclei size reduction by the dissolution of remaining α-Si 3N 4 during intermediate heat treatment. The specimen prepared from finer powders showed increased values of hardness and fracture toughness as a result of the microstructure containing elongated grains.

Effect of β-Si 3N 4 starting powder size on elongated grain growth in β-Si 3N 4 ceramics

The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si 3N 4 as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si 3N 4 grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.

Interfaces of polymeric precursors with ceramics/metals

Preceramic polymers that yield ceramic materials on heating have been investigated as precursors to fibers, binders, powders, and coatings. These materials have also been used as processing additives for ceramics, rheological control agents, and aids for controlling the microstructure of ceramics and coatings. The reactivity of the precursors with ceramic and metal powders or with substrate surfaces opens up many possibilities for improved processing of materials, fabrication of novel materials, and the preparation of materials and coatings with improved properties. Although reactions at interfaces between polymeric precursors and ceramics or metals are not yet well understood, the beneficial effects of using precursors have been demonstrated. Interfacial reactions between polymers and other materials depend on the polymer composition and rheological properties, the ceramic or metallic material composition and surface states, and the heat treatments and environments. Interfaces range from relatively simple interfaces between a pure polymer-derived material and a substrate, as reflected by the formation of coatings or the use of polymers to infiltrate porous materials, to complex interfaces between polymers, various powders, and occasionally substrates. Examples of complex interactions include the use of precursors as binders or microstructural control agents and the direct formation of bulk materials or matrix materialsmore » from polymer/powder compositions. This paper gives specific examples of the interactions of polymeric precursors with ceramics to provide controlled microstructure development during sintering of Si[sub 3]N[sub 4] (in-situ growth of elongated grains) and to form near-net-shape mullite and mullite-based coatings with unique properties.« less

Effect of Grain Growth of β‐Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

β‐Si3N4 powder containing 1 mol% of equimolar Y2O3–Nd2O3 was gas‐pressure sintered at 2000°C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30‐MPa nitrogen gas. These materials had a microstructure of “in‐situ composites” as a result of exaggerated grain growth of some β Si3N4 grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa‐m1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas‐pressure sintering of β‐Si3N4 powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.