Microstructure Of Silicon Nitride Research Articles

Effect of nano-sized sintering additives on microstructure and mechanical properties of Si3N4 ceramics

Nano-sized sintering additives such as MgO, Al2O3, Y2O3 and La2O3 are added to process Si3N4 based ceramics. The effects of such sintering additives on the phase transformation, mechanical properties and microstructure of silicon nitride were investigated. XRD analysis of sintered silicon nitride with these sintering additives have shown the complete transformation of α-Si3N4 into β-Si3N4. The silicon nitride ceramics sintered with nano-sized additives (4MgO-4Al2O3–5La2O3) have exhibited the highest fracture toughness. This is not only due to the weak glass|grain interface but also due to the formation of high aspect ratio grains (β-Si3N4). Superior mechanical properties of Si3N4 can be obtained by way of adding small and large RE3+ radii RE2O3 (Y2O3 and La2O3) together than the addition of single RE2O3.

Hypothetic impact of chemical bonding on the moisture resistance of amorphous SixNyHz by plasma-enhanced chemical vapor deposition

The relationship between the microstructure of silicon nitride and its sensitivity to moisture was studied. The effectiveness of Si-H rich and N-H rich silicon nitride layers was measured under attack from water in vapor and liquid states. For water vapor attack, samples are exposed to vapor at 85 °C with a relative humidity of 85% during 1600 hours; for liquid water attack, samples are dipped in water at 60, 85 and 100 °C for 200 hours. The water resistance of the Si-H rich and N-H rich silicon nitride layers was evaluated by measuring: (i) the thickness of the silicon dioxide formed after their oxidation with water vapor, (ii) the rate of dissolution of the silicon nitride in liquid water and (iii) the corresponding activation of energy. This evaluation was performed by coupling spectroscopic ellipsometry, infra-red and X-ray photoelectron spectrometry analyses. The results revealed that for Si-H rich layer, 10 nm of silicon dioxide was formed during the water vapor attack; for liquid water attack, a high activation energy (0.88 eV) and a low dissolution rate were observed regardless of the water temperature. For N-H rich layers, approximatively 6–8 nm of silicon dioxide was formed and a low activation energy (0.64 eV) with a high dissolution rate were observed. All of these observations lead to the conclusion that the N-H rich layers could be less resistant to moisture because the isoelectronic relationship between Si2N-H and −H2O+ facilitated their deterioration in water. Moreover, a higher rate of nanoporosity for N-H rich layers than Si-H rich layer could complete this hypothesis.

On the influence of transgranular and intergranular failure mechanisms during dynamic loading of silicon nitride

Plane-strain tensile loading numerical simulations of dynamic crack propagation in silicon nitride microstructures are conducted. The strength and toughness are evaluated as a function of strain rate and microstructural parameters, including grain size and density of needle-shaped grains. The silicon nitride microstructures are built using Voronoi tessellation for constructing regular grains and a merging procedure to generate elongated grains. Dynamic insertion of cohesive elements representing transgranular and intergranular cracking is a key feature of the modeling. The results show that inertia and elongated grains both contribute to the rate hardening of the specimen. The simulations reveal the existence of a threshold opening rate for intergranular cracks to transform into transgranular ones. Moreover, a higher percentage of transgranular fracture causes higher toughness.

Variation of microstructure on the fracture surface of dense silicon nitride by laser power in LAM

Ceramics have attractive properties to metals and polymers, and they are consequently useful for specific applications. Silicon nitride has been studied extensively and has been found to possess promising thermal and mechanical properties at high temperatures. However, it has drawbacks such as brittleness and large scatter in its mechanical properties. As a result, it is difficult to fabricate complex shapes using traditional methods because of high cost and difficulties to machine the components. Today, ceramic parts have limited use for production of simple shaped parts and low quantities. This study explores the possibility of employing silicon nitride for more diverse applications using laser-assisted machining (LAM). Because the surface of the workpiece is locally heated by an intense laser source prior to material removal, softening and damage of the workpiece surface make machining of ceramics easy. Among the parameters of LAM, laser power is one of the decisive factors during the process. In this study, fractured cross sections were observed to examine how the microstructure of silicon nitride was changed by laser power. The deformation of microstructure near the surface of the workpiece increases when the laser power increases. It is found that increasing the laser power facilitates cutting of silicon nitride but results in detrimental heat effects on the surface of the workpiece.

Mechanisms of toughening in silicon nitrides: The roles of crack bridging and microstructure

Crack-bridging mechanisms can provide substantial increases in toughness coupled with strength in ceramics. Herein, we describe the various bridging mechanisms, their toughening contributions and how they are affected by microstructure in silicon nitride ceramics, which are a classic example where both high strength and toughness are achieved. Crack growth resistance curves ( R-curves) for seven different silicon nitrides doped with various metal oxides and with different microstructures were measured, and bridging stress distributions were calculated for each. Based on an analysis of those results combined with results of the relative interfacial toughness of two of the ceramics, a new mechanistic theory for the evolution of the R-curve is proposed. The initial steep rise in toughness is attributed to the formation of elastic bridges that experience no debonding. This mechanism has not previously been recognized in the literature and the high strength of these materials (up to 1 GPa) is here attributed primarily to that mechanism. As those bridges begin to fracture and the mechanism becomes saturated, a change in the R-curve slope is observed and the more traditional mechanisms of partially debonded elastic and fully debonded frictional bridges dominate the continuing rise in toughness. Furthermore, the saturation of each mechanism is associated with a change in slope of the R-curve. The results of this study provide a fundamental insight into how to optimize silicon nitride microstructures for high strength and toughness.

Measurement of Indentation Fracture Toughness of Silicon Nitride Ceramics: I, Effect of Microstructure of Materials

Effect of microstructure of silicon nitride on the fracture toughness, KIc evaluated by the IF method was studied with various indentation loads ranging from 49 N to 490 N, since practical assessment of fracture toughness of small Si3N4 parts is needed in the ceramic ball bearing market. The plot of KIc against the as-indented crack length revealed the rising R-curve behavior for the coarse Si3N4 and slight R-curve for the fine Si3N4. By comparing KIc estimated from the SEPB and IF methods using 4 different equations, it was revealed that the IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. These results were discussed in conjunction with their R-curve behavior and the effective crack length in the SEPB specimens.

Effect of the microstructure of Si 3N 4 on the adhesion strength of TiN film on Si 3N 4

The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si 3N 4 was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase (α or β) of starting Si 3N 4 powder, and sintering temperature. The microstructure of Si 3N 4 was characterized in terms of grain size, aspect ratio of the elongated grain, and β-to-α phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si 3N 4 were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si 3N 4, such as critical loads for yield ( P y) and for ring cracking ( P c). The effect of the microstructure of Si 3N 4 on adhesion strength evaluated by scratch test is discussed. TiN films on Si 3N 4 showed high adhesion strengths in the range of 80–140 N. Hardness and the P y of Si 3N 4 substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si 3N 4, substrates with finer grain sizes and higher α phase ratios, which show high hardness and high P y, were suitable for higher adhesion strength of TiN film.

Preparation of Submicron Silicon Nitride Powders via Self-propagating High-Temperature Synthesis

Nitridation of commercial ferrosilicon via self-propagating high-temperature synthesis is described. The nitrogen pressure is shown to influence the conversion and the combustion rate of the alloy. Dilution of the starting alloy with the final product or an ammonium salt influences the nitrogen content of the nitrided material. The high nitrogen content of the reaction products made it possible to isolate a sufficient amount of silicon nitride with a particle size of 0.43 µm by acid enrichment in an HCl solution. The microstructure of silicon nitride prepared under different conditions is studied by electron microscopy. Electron diffraction results indicate that the final product contains, in addition to the well-known α and β phases (hexagonal structure), tetragonal and orthorhombic Si3N4 polymorphs.

Imaging and microanalysis of liquid phase sintered silicon-based ceramic microstructures

This review paper is focussed on the characterization of the microstructural development during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the Si3N4 or SiC structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the TEM, are discussed and related to the fabrication process and the overall composition of the ceramic material. It is demonstrated that combined high resolution analytical and spatial information from chemically and structurally distinct fine scale features, such as grain boundary films of residual glass, is obtained by electron spectroscopic imaging and subsequent computation of elemental distribution images. These images reveal that residual glassy grain boundary films are rich in oxygen and cations originating from the metal oxide/nitride additives, consistent with fine probe EDX analysis in the FEGTEM. Elemental analysis with high spatial resolution has also shown that grain growth into pockets of residual liquid/glass is associated with diffusion profiles in the glass in front of the growing grain. High resolution imaging in the TEM and elemental maps computed from electron energy filtered images show that the intergranular film thickness, in general, varies within a particular silicon nitride or sialon microstructure. Furthermore, grain boundaries, apparently free from residual glass may co-exist with glass-containing grain boundaries in some silicon nitride microstructures. In addition to the choice and weight fraction of sintering additives, factors such as the ionic radius of the cations originating from the additives, the local nano-scale chemistry and the relative grain orientation have an effect on the volume fraction and morphology of the intergranular microstructure.

Strength and Microstructure of Silicon Nitride Fabricated by Post-Sintering Process Using Low-Purity Silicon Powder as Raw Materials

Influence of sintering temperature on the strength and micro-structure were investigated on silicon nitride specimens fabricated by pressureless post-sintering process using low-purity silicon powder as raw materials. Densification of reaction-bonded silicon nitride with sintering aid did not proceed further upon setting the post-sintering temperature at 1600 or 1650°C due to the volatilization of species and poor sinterbility. Sintering at 1850°C provided almost fully densified silicon nitride having an average strength of 521 MPa, the grains of this materials were relatively fine but did not develop into a rod-like shape.

Effects of rare-earth oxide additions and heat-treatment temperature on the transformation and microstructure of silicon nitride

In this paper, mixtures of Si3N4 powder with different added amounts of single rare-earth oxide Re2O3 (Re = Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) were heat treated at 1600–1800 °C and the effects of Re2O3 on the transformation and the microstructure of Si3N4 were investigated. Compared with the original Si3N4 powder, the transformation ratio of the Si3N4 powders obviously increased and the phase transformation finishing temperatures decreased by about 100 °C when different rare-earth oxides were added. When the heat-treatment temperature was 1700 °C, the transformation ratio of the Si3N4 powder was about 100%. When the heat-treatment temperature was 1600 °C and the amount of Re2O3 addition was 3 mol%, an anomalous transformation ratio occurred. In addition, it was found that the transformation ratio of Si3N4 changed periodically with the increase of the atomic number of the lanthanide when the heat-treatment temperature was 1650 °C or less. The SEM image of the Si3N4 powders obtained at 1700 °C indicated that the heavy lanthanide oxides such as Er2O3 and Yb2O3 were very helpful to develop rod-like β-Si3N4 particles.

Study on the thermal conductivity and microstructure of silicon nitride used for power electronic substrate

The thermal conductivity and microstructure of Si 3N 4 ceramics used for power electronic substrate were studied in present paper. Thermal conductivity of Si 3N 4 ceramics sintered at different temperatures varied greatly. The microstructure of the specimens was studied with X-ray diffraction, EDS and transmission electronic microscope to reveal the difference in thermal conductivity. The experimental results showed that microstructure of Si 3N 4 ceramics sintered at different temperatures had different components in their grain phase and grain boundary phase, while electrical properties showed not much difference with different sintering temperatures.

Microstructure of Silicon Nitride Containing β‐Phase Seeds: II

The microstructure of silicon nitride containing different percentages of β‐seeds was investigated. The average grain size and volume fraction of large grains increased with the incorporation of β‐seeds. The length and aspect ratio of large grains in sintered Si3N4 initially increased by incorporating β‐seeds, and then decreased. Similar trends were observed in the fracture toughness. The toughening mechanisms and fracture behaviors were correlated with the grain morphology measured from image analysis. Plots of measured toughness versus volume fraction of large grains fit very well with the equation derived by Becher and Budiansky, which strongly suggested the essential role of debonding length in the toughness of silicon nitride. Further improvement of fracture toughness in silicon nitride may be possible by tailoring the amount of incorporated β‐phase seeds and leading to optimize the volume fraction, grain size, and aspect ratio of the elongated grains.

Two cores in one grain in the microstructure of silicon nitride prepared with aligned whisker seeds

Abstract The microstructure of silicon nitride with aligned silicon nitride whisker seeds was examined by electron microscopy. Silicon nitride sintered with yttria and alumina showed a “core-rim” structure. A few grains of the sample sintered with yttria and alumina had more than one core within one grain. This was explained in terms of coalescence of the grains growing from separate cores. A boundary with small misfit was observed by TEM, also supporting the possibility of grain coalescence.

Sintering and microstructure of silicon nitride with magnesia and cerium additives

This paper deals with the pressureless sintering and microstructure of Si 3N 4–MgO–CeO 2 ceramics. The sintered Si 3N 4–MgO–CeO 2 ceramics achieved a relative density of 98.5% and a bending strength of 950 MPa. XRD, TEM and EDAX analysis indicated that the main composition of the glassy phase in the sintered Si 3N 4–MgO–CeO 2 ceramics was cerium silicate and that it hardly contained any MgO: during sintering, MgO would crystallize. Abnormal grain growth occurred at 1850°C, which led to microcracks and dislocation. Some silicon whiskers were growing in fractal patterns during TEM observation, which provided a good experimental example for further study of fractal growth.

Microstructural characterisation of metal ion implanted silicon nitride

The microstructure of silicon nitride after metal ion implantation at different dosages has been studied. A metal vapour vacuum arc (MEVVA) ion source was employed to implant Ni, Ti and Ni+Ti ions into silicon nitride. Characterisation of the implanted surfaces was carried out by Rutherford backscattering spectrometry (RBS) and cross-sectional transmission electron microscopy (XTEM), in conjunction with nano-beam electron diffraction (NBED) analysis. It was established that the thickness of the implanted layer was in the range of 210–280 nm, depending on implantation species and energy. Titanium or nickel implantation under present conditions resulted in the formation of an amorphous layer and precipitation of titanium or nickel nitrides. However, when Ti and Ni were both implanted successively at half the total dose, the modified layer was fully amorphised with no evidence of precipitation.

Heterogeneous Formation of Oriented Silicon Oxynitride on α-Si3N4 Seed Crystals : Habits And Radiation Stability

Lath-like silicon oxynitride crystals have often been observed in the microstructure of silicon nitride based ceramics after processing. They are usually located in glassy regions which are siliceous solidified sintering aid liquid, and usually contain a small (∼100nm) a-Si3N4 crystal. These nitride crystals are considered to be seeds, incompletely dissolved in the melt, that are heterogeneous nucleation sites for the oxynitride crystals. We present here the first observations of morphological and crystallographic habits between the seed nanocrystals and the host oxynitride laths.Fig. 1 shows a typical oxynitride lath containing a nitride seed crystal. The lath is surrounded by glass and ß-Si3N4 particles, and a small cristobalite particle (a minor constituent). This microstructure is from an Si02-Si3N4 ceramic processed with Al2O3 sintering aid. The same oxynitride lath/seed structures were observed when other sintering aids (eg. Y2O3, MgO, ZrO2) were used, so they are independent of sintering aid.

Contact Fatigue in Silicon Nitride

A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine (F)‐almost exclusively fully‐developed cone cracks; (ii) medium (M)‐well developed but smaller cone cracks, plus modest subsurface quasi‐plastic damage; (iii) coarse (C)‐intense quasi‐plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on inert strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture during contact, although the M material shows signs of enhanced failure from quasi‐plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single‐cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi‐plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long‐lifetime applications, specifically in concentrated loading, are considered.

A Study of Grain Growth and Microstructure Control in Silicon Nitride by Computer Simulation

AbstractA grain growth simulation technique is developed for microstructure design of ceramic materials by extending the Potts model. The simulation model has successfully reproduced the self-reinforced microstructure which is peculiar to silicon nitride, consists of large elongated grains and small equiaxed grains.To control the microstructure of silicon nitride further, a seeding technique is effective and often employed. Grain growth behavior of seed particles in the simulation is studied. It is found that the seed grains grow with a relatively high growth rate during the early stage of grain growth. In contrast in the later stage, the difference between microstructures with and without seed particles decreases. This behavior agrees well with that of actual silicon nitride. These results demonstrate the consistency of the simulation.

Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: I, Mechanical Characterization

In this Part I of a two‐part study of Hertzian indentation in silicon nitride we characterize irreversible contact damage as a function of microstructure. Three controlled silicon nitride microstructures are examined, representing a progression toward greater long‐crack toughness: fine (F), bimodal with predominantly equiaxed α grains; medium (M), bimodal with mostly β grains of intermediate size; and coarse (C), with almost exclusively elongated β grains. An effect of increasing the microstructural heterogeneity in this sequence is to suppress ring cracking around the indenter, ultimately to a degree beyond that expected from increased toughness alone. Along with the crack suppression is a parallel tendency to enhanced damage accumulation beneath the indenter, such that the contact in the coarsest microstructure is predominantly quasi‐plastic. The characterization of damage includes the following: determination of indentation stress‐strain curves, to measure the level of quasi‐plasticity; measurement of threshold loads for the initiation of ring cracking and subsurface yield, to quantify the competing damage processes; and measurement of characteristic dimensions of the ensuing cracks and deformation zones in their well‐developed stages. These quantitative results are considered in terms of formal contact mechanics, along with finite element modeling to generate the essentially elastic‐plastic fields in the different silicon nitride structures. This contact mechanics description serves also as the basis for subsequent analysis of strength degradation in Part II. Implications concerning microstructural design of silicon nitride ceramics for specific applications, notably bearings, are considered.