Microstructure Of Silicon Nitride Ceramics Research Articles

Investigation on mechanical properties and microstructure of silicon nitride ceramics fabricated by spark plasma sintering

In order to prepare high-performance and stable performance silicon nitride-based ceramic cutting tools, orthogonal experiment was carried out firstly to study the influence of sintering temperature and sintering aids (MgSiN2, Y2O3 and CeO2) on the densification and mechanical properties of the Si3N4 materials by spark plasma sintering. The results showed that Si3N4 ceramics with 5 wt%MgSiN2, 3 wt%Y2O3 and 1 wt%CeO2 sintering aids had the best relative density and mechanical properties at 1650 ℃. Secondly, the effects of sintering temperature on the densification, microstructure and mechanical properties of Si3N4 ceramics with 5 wt%MgSiN2, 3 wt%Y2O3 and 1 wt%CeO2 sintering aids were studied in detail. The results showed that the relative density of Si3N4 ceramics reached the maximum of 99.40 ± 0.14% at 1650 ℃, the Vickers hardness and fracture toughness were 16.53 ± 0.12 GPa and 6.89 ± 0.20 MPa m1/2, respectively. The grain shapes were composed of equiaxed α-Si3N4 and long columnar β-Si3N4, and the transformation rate of α-Si3N4 to β-Si3N4 was 42.5%. Finally, the statistical distribution of the relative density, phase transformation rate and mechanical properties of Si3N4 ceramics with 5 wt%MgSiN2, 3 wt%Y2O3 and 1 wt%CeO2 sintering aids at 1650 ℃ was investigated. It was found that the mechanical properties of silicon nitride ceramics sintered by spark plasma sintering were less dispersive, and the hardness and fracture toughness obeyed the two parameters Weibull distribution well.

種結晶を用いた窒化ケイ素セラミックスの微構造制御と機能発現

It has been reported that the size and distribution of β-Si3N4 particles existing in silicon nitride raw powder are crucial factors governing the microstructure of silicon nitride ceramics. Considering such the grain growth behavior, a processing strategy for controlling the microstructure of silicon nitride ceramics has been developed. The key point of this process is to control the size, distribution and orientation of large elongated β-Si3N4 grains via seeding of morphologically regulated β-Si3N4 single crystal particles. Silicon nitrides with textured microstructure where elongated β-Si3N4 grains were unidirectionally or two-dimensionally orientated could be fabricated by combining the seeding with forming methods of extrusion or tape casting. The textured silicon nitride exhibited superior mechanical properties such as high strength, high fracture toughness, high creep resistance, and had a high Weibull modulus of about 50. Moreover it has been revealed that the material exhibits unique tribological and thermal properties.

Effect of Microstructure of Silicon Nitride Ceramics on Wear Mechanism

A silicon nitride sintered body with uni-directionally aligned β-Si3N4 elongated grains (UA-SN) was fabricated and the effect of microstructural anisotropy on wear properties was investigated. Block-on-Ring tests without lubricants were carried out using the UA-SN as block specimens and commercially supplied Si3N4 as ring specimens. In addition, scratch tests were also carried out using a diamond stylus having 25μm of tip radius with the normal load increasing continuously up to 1.47N. For UA-SN, tribological properties were evaluated in three directions with respect to the grain alignment; in the direction normal to the grain alignment on the plane normal to the grain alignment (N), and in the direction parallel (L) to and perpendicular (T) to the grain alignment in the side plane. The N direction exhibited the highest wear resistance, and the worn surface of this plane was quite smooth, in contrast to the other specimens whose surfaces were irregular owing to grain removing. For the scratch test, there was an inflexion point in friction force during increasing load for each specimen. The starting states of wear, such as grain removing, were observed at this point and the loads at this point among specimens correlated with the result of wear resistance obtained from wear tests. Microfracture of the silicon nitride induced by scratching started at the grain boundary as microcracks. Wear was caused by subsequent grain removing following propagation and combination of these microcracks. It is suggested that the high wear resistance achieved in the plane normal to the grain alignment is attributable to the inhibition of crack propagation along the sliding surface by the stacked elongated grains normal to the sliding surface.

Wear behaviour of α-Si 3N 4 ceramics reinforced by rod-like β-Si 3N 4 grains

The microstructure of silicon nitride ceramics has a significant affect on their mechanical and tribological properties. The grain morphology, in particular, is a significant factor for controlling wear resistance. In this study, α-Si 3N 4 with various amounts of in-plane aligned rod-like β-Si 3N 4 grains were fabricated by hot pressing the tape cast and stacked green bodies with a small amount of rod-like β-Si 3N 4 seeds. The effect of anisotropy in microstructure and β-Si 3N 4 content on tribological properties was investigated by means of block-on-ring tests without lubricant using commercially supplied Si 3N 4 as ring specimens. Three sliding directions were set for the texured Si 3N 4 used as the block specimens in the friction test; directions perpendicular to and parallel to the casting direction on the casting plane (referred to as C-perp and C-para, respectively), and a direction parallel to the casting plane on the stacking plane (referred to as S). The effect of sliding direction on wear rate was small for the textured Si 3N 4 with less than 20% β-Si 3N 4 content, but it became larger with the increasing β-Si 3N 4 content. The C-para specimen exhibited larger wear rate than the other specimens of textured Si 3N 4 with more than 30% β content. The worn surface of the C-para specimen was irregular owing to grain pull-out, in contrast to the other specimens whose surfaces were quite smooth. In addition, for the textured Si 3N 4 with more than 30% β content the wear resistance of the S specimen was higher than of the C-perp specimen. The wear behaviour of the textured Si 3N 4 with high β content can be explained by the toughness anisotropy, assuming that wear proceeds through generation of microcracking, followed by their propagation mainly parallel to the sliding surface.

Effects of α/β ratio in starting powder on microstructure and mechanical properties of silicon nitride ceramics

The effects of α/β ratio in the starting powder on microstructure development and mechanical properties of sintered silicon nitride ceramics were investigated. With increasing β-phase content, the microstructure became finer, the finest-grained microstructure was developed from the powder with pure β-phase, whereas a duplex microstructure was developed from high α-phase content powder. The results show that the microstructure of silicon nitride ceramics can be tailored by changing the α/β ratio in the starting powder. The mechanical properties have an optimum region where the α/β ratio equals 1. Both flexural strength and fracture toughness can be improved by controlling size, number, aspect ratio and size distribution of the rod-like β-Si 3N 4 grains

Influence of Yttria–Alumina Content on Sintering Behavior and Microstructure of Silicon Nitride Ceramics

The present study investigates the influence of the content of Y2O3–Al2O3 sintering additive on the sintering behavior and microstructure of Si3N4 ceramics. The Y2O3:Al2O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering‐additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β‐Si3N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction.

Effect of α‐Si3N4Particle Size on the Microstructural Evolution of Si3N4Ceramics

The effects of initial particle size on the microstructure of silicon nitride ceramics produced by pressureless sintering have been investigated. The microstructures of the silicon nitride ceramics varied considerably with the size of the initial powder. With decreasing powder size, abnormal grain growth was enhanced, which resulted in significant bimodal distribution of grain size. The observed results are discussed in relation to the two‐dimensional nucleation and growth theory for faceted crystals.

Densification of Silicon Nitride Ceramics by Gas Pressure Sintering

The densification and the microstructure of silicon nitride ceramics depends on processing parameters, mainly the sintering conditions and the amount and type of sintering aids necessary to promote liquid phase sintering. In this work, selected compositions of silicon nitride containing different amounts and type of sintering additives, were normal sintered at 1700°C and 1750°C, and gas pressure sintered under 25, 60 and 100 bar of nitrogen pressure, varying, also, the soaking time and final sintering temperature. Densification rates ware determined using a dilatometer coupled to the gas pressure sintering furnace. Gas pressure sintering has shown to be an effective route for densification of silicon nitride ceramics with low quantities of sintering aids. Densities greater than 99.5% of theoretical density were obtained for compositions containing only 5wt.% of sintering aids by using multiple steps of temperatures up to 1950°C and 100 bar N 2 pressure.

Elastic Modulus of Oxidation-formed Silicon Nitride Composite

Silicon nitride ceramics are generally used in high temperature oxidizing environments. The phase composition and microstructure of silicon nitride ceramics change during oxidation resulting in changes in the mechanical properties of materials. The oxidation behaviour of silicon nitride is dealt with in numerous studies [e.g. 1±7]. In these papers the effects of the oxidation on the room-, and the high-temperature strength and fracture toughness were studied [1±5]. The phase composition and the thickness of the surface oxide layer also studied by several authors [6, 7]. The published studies in this ®eld concentrated on the mechanical properties relevant for utilization but little attention was paid to the elastic properties. It is known that the Young's modulus gives valuable information about the structural changes of ceramics during sintering [8], therefore it was assumed that the measurement of the modulus of oxidized samples would contribute to the understanding of the complex process of oxidation. In this paper, the Young's modulus of oxidized silicon nitride ceramics is studied. The sample oxidized at elevated temperatures is considered as a composite material that consists of an internal part and a surface oxide layer. The elastic modulus of the whole sample was measured by the standard four point bending method, that of the internal region and the surface oxide layer was determined by the depth sensing indentation (DSI) method. A formula was derived for the calculation of the modulus of the composite from the moduli of the parts. Mixtures of Si3N4, AlN, Al2O3 and Y2O3 powders were milled in ethanol in a planetary type alumina ball mill. The composition of materials is given in Table I. Samples were compacted by dry pressing at 220 MPa. The materials were sintered in high purity nitrogen by a two-step sinter-HIP method using BN embedding powder. The dimensions of as-sintered specimens were approximately 3:5 mm 3 5 mm 3 50 mm. Oxidation took place in static air at 800 8C, 1000 8C, 1150 8C, 1300 8C and 1400 8C for up to 48 h. The Young's modulus of the samples was determined before and after oxidation by a room temperature four point bending test with spans of 40 and 20 mm. The elastic moduli for different compositions and temperatures of oxidation are shown in Fig. 1, the one standard deviation is less than 5 GPa. The Young's modulus of the samples began to decrease at an oxidation temperature of 1300 8C for materials X, A and F and a drastic reduction occurred at 1400 8C for all the compositions investigated. On the surface of the samples oxidized at 1300 8C and 1400 8C a white, rough oxide layer appeared. The role of this oxide layer in the change of the elastic modulus will be analysed in the following. The oxidized rectangular specimen can be considered as a composite material that consists of an internal part and an oxide layer (Fig. 2a). The Young's modulus of the internal region and the surface oxide layer of the materials oxidized at 1300 8C and 1400 8C was measured by the depth sensing indentation (DSI) method. The surface layer was removed by grinding before the measurement of the modulus of the internal part of the material. The DSI measurements were carried out by a Shimadzu dynamic ultra-micro hardness tester (DUH 202). Each DSI test consisted of a loading±unloading cycle. During the loading period a Vickers indenter penetrated into the surface of the sample at constant loading rate (70 mN sy1) and the same rate was applied in the unloading period when the pyramid moved backwards. The maximum load in each test was 500 mN. The Young's modulus of the samples was calculated from the indentation curves using the method of Oliver and Pharr [9]. The average modulus values of the internal region (Ei) and the surface oxide layer (El) are shown in Table II (the one standard deviation is about 10 GPa). The Young's modulus of the oxide layer of material B oxidized at 1400 8C is much lower than that of other studied materials. The reason for this deviation is under investigation. A simple relationship between the modulus of the

Microstructure of silicon nitride ceramics sintered with rare-earth oxides

Microstructures of Si 3N 4 with crystallized grain boundaries of rare-earth silicates were investigated. β-Si 3N 4 grains in samples sintered with Yb 2O 3 were more elongated than those sintered with Dy 2O 3. Amorphous thin layers between crystallized grain boundaries of silicate and Si 3N 4 grains were recognized for both specimens sintered with Dy 2O 3 and with Yb 2O 3. In the sample sintered with Dy 2O 3, very clean grain boundaries between Si 3N 4 grains which did not contain the heavy elements were frequently recognized, while samples with Yb 2O 3 had an amorphous grain boundary phase containing Yb. Unusual structures with distorted lattice images in regions rich in Si and O were discovered in crystalline Dy-silicate phases.

Relationship between microstructure and mechanical properties of silicon nitride ceramics

Abstract

Microstructure of Silicon Nitride Ceramics and Morphology of Surface Damage Induced by Round Indenter

Microstructure of Silicon Nitride Ceramics and Morphology of Surface Damage Induced by Round Indenter