Si3N4 Phase Research Articles

The effect of oxygen vacancies on the optical and thermophysical properties of (1-x)Si3N4 – xAl2O3 ceramics

Interest in composite ceramics based on silicon nitride (Si3N4) and aluminum oxide (Al2O3) is due to the possibility of using them as semiconductor applications associated with their operation under conditions of external influences, including heating. The paper presents the results of assessing the influence of variations in the phase composition of (1-x)Si3N4 – xAl2O3 ceramics on alterations in optical and thermophysical parameters, as well as their relationships. Analysis of the optical characteristics of the ceramics under study showed that changes in the phase composition associated with the dominance of the Al2O3 phase in the composition of the ceramics lead to a change in covalent bonds and an increase in the number of ionic bonds, which also affects the change in the semiconductor nature of the ceramics, and therefore, a change in the thermophysical properties. According to the assessment of changes in the thermal conductivity coefficient of the studied ceramics, with the dominance of the Al2O3 phase in the composition, the observed growth in the concentration of structural defects, as well as oxygen vacancies, results in the thermal conductivity coefficient decrease, as well as an elevation in the thermal expansion of ceramics during thermal heating. It has been determined that the presence of a combination of Al2(SiO4)O and Si3N4 phases in the ceramic composition leads to an increase in the thermal conductivity coefficient, as well as maintaining its stability over a wide temperature range.

Effect of Phase Composition Variation of Oxy–Nitride Composite Ceramics on Heat Resistance and Preservation of Strength Parameters

The aim of this study is to determine the effect of changes in the phase composition of Al2O3–Si3N4 ceramics that were obtained using the method of mechanochemical solid-phase grinding on their resistance to the process of long-term thermal exposure, accompanied by the processes of oxidation and softening. The relevance of this research consists of determining the influence of the phase composition of ceramics on the change in their strength and thermophysical parameters, on the basis of which, we can draw a conclusion about the optimal composition of composite ceramics that have great prospects in the field of fire-resistant, heat-resistant, or radiation-resistant structural materials. During this study, the dynamics of the changes in the phase transformations of the xAl2O3–(1−x)Si3N4 ceramics, with variations in the ratio of the components, initiated by the thermal annealing of the samples, was established. According to the assessment of the phase transformations with variations in the ratio of the components, it was found that thermal annealing in an air environment at an Al2O3 concentration in the order of 0.3–0.5 M leads to the formation of an orthorhombic Al2(SiO4)O phase and an elevation in its contribution at concentrations above 0.5 M, which causes a rise in the thermophysical parameters and resistance to high-temperature degradation. During the heat resistance tests, it was found that the formation of the composite ceramics with the Si3N4(SiO2)/Al2(SiO4)O/Al2O3 phase composition results in an increase in the stability of their strength properties when exposed to thermally induced oxidation, which has a negative impact on their resistance to softening and a decrease in hardness. Moreover, the presence of the Al2(SiO4)O phase in the composition of the ceramics causes a slowdown in the processes of thermal oxidation of the Si3N4 phase under prolonged temperature exposure, alongside an increase in the degradation resistance of strength properties by more than 4–7 times, in comparison with the softening data established for single-component ceramics.

Tackling residual tensile stress in AlN-on-Si nucleation layers via the controlled Si(111) surface nitridation

This work is devoted to the study of the influence of controlled Si(111) surface nitridation on the epitaxial growth of AlN-on-Si nucleation layers with reduced tensile stress on ordered crystalline silicon nitride phase. The Si(111) surface nitridation process was performed at low ammonia flux and substrate temperatures in the range of 700–900 °C and was studied using RHEED and STM techniques. A universal criterion, namely the stage of the nitridation process completion is introduced, taking into account the influence of substrate temperature, ammonia flux and nitridation time. The 100 nm AlN nucleation layer on silicon substrates grown by ammonia molecular beam epitaxy is studied using AFM, XRD, HR-TEM and Raman spectroscopy techniques. The Raman data show that reducing the nitridation temperature from 900 °C to 700 °C not only deteriorates the crystalline quality of the subsequent AlN nucleation layers, but also reduces the residual tensile stress by almost 30 %. In the present contribution, micro-Raman spectroscopy is used to determine the nature of the defects formed during the high temperature growth of the AlN nucleation layers and confirms them to be inversion domains. The HR-TEM technique was used to study the AlN/Si interface in AlN-on-Si nucleation layers grown on a nitridated silicon surface at 700 °C and 900 °C at the optimum stage of the nitridation process completion. HR-TEM images of AlN nucleation layers revealed regions with different AlN/Si(111) interfaces: 1) AlN/amorph-Si3N4/Si, 2) AlN/SiN(8 × 8)/Si, and 3) AlN/Si with a sharp interface boundary. Using fast Fourier transform image analysis, it is shown that the presence of amorphous Si3N4 phase inclusions in the AlN/Si interface boundary introduces tensile stresses in the AlN nucleation layer which can be reduced by lowering the nitridation temperature. The results obtained clearly show that one of the causes of cracks in III-nitride layers grown on silicon substrates is the formation of tensile AlN layers with a high content of the amorphous Si3N4 phase at the AlN/Si interface, which is characteristic of silicon nitridation at elevated temperatures (> 700 °C).

Synthesis of Si3N4 powder with a controllable alpha/beta ratio by sol-gel assisted carbothermal reduction and nitridation

The controllable α/β ratio of Si3N4 powder was synthesized using tetraethyl orthosilicate (TEOS), glucose, MgCl2 and YCl3·6H2O by sol-gel assisted carbothermal reduction and nitridation (CRN). This study aimed to investigate the impact of various parameters, including a H2O/TEOS mol ratio of the sol, a C/Si mol ratio, sintering aids, and the temperature of CRN on the phase transformation of α-Si3N4. The results demonstrated that the H2O/TEOS ratio significantly influenced the relative content of α and β phase in Si3N4 by changing the morphological characteristics of SiO2. When the H2O/TEOS ratio was 100, β-Si3N4 powder with an approximate β content over 88 wt% was synthesized at 1500 °C. This remarkable phase transformation was likely facilitated by the eutectic liquid phase consisting of MgSiO3/Mg2SiO4. The SEM image revealed rod-like whiskers with a length around 1.4 μm for the β-Si3N4 grains. Due to the presence of liquid phase, the synthesis of spherical-like α-Si3N4 nano-powder with an α-phase content of 91 wt% was achieved at a lower temperature of 1450 °C, with an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The size of the particles was about 70 nm due to the reduced reaction temperature. Furthermore, without the addition of any sintering aids or Si3N4 seeds, high purity α-Si3N4 with an α phase content approaching 96 wt% could be synthesized at 1500 °C, using an H2O/TEOS ratio of 15 and a C/Si ratio of 4. The resulting α-Si3N4 powder exhibited an extremely regular hexagonal prism shape with a length of approximately 2 μm.

Structure and corrosion properties of multilayer metal/nitride/oxide ceramic coatings formed on austenitic-martensitic steel by magnetron deposition

The design of protective coatings preventing the degradation and failure of austenitic steels in humid climate is a relevant issue. In this work, a conventional magnetron sputtering method was utilized in order to fabricate a four-layer Ni/Al-Si-N/Ni/Al-Si-O coating onto austenitic-martensitic steel. At the first stage of deposition, a three-layer coating was formed in an Ar+N2 atmosphere using a mosaic target (Ni or Al-Si), while the second stage included the deposition of the outer ceramic Al-Si-O layer in an Ar+O2 gas mixture. The studies of the structure, hardness, adhesion strength and electrochemical behavior (in 3.5 wt.% NaCl) of the «coating/steel» system were performed by means of X-ray Diffraction, electron microscopy, mechanical (indentation and scratch) tests and electrochemical impedance spectroscopy. The ceramic Al-Si-O layer exhibits an amorphous structure. The adjacent nickel layers have submicrocrystalline columnar grains. The ceramic Al-Si-N layer possesses a nanocrystalline columnar structure based on AlN + Si3N4 phases. Hardness behavior of the multilayer coating varies nonmonotonically due to a contribution of the soft amorphous phase in the upper surface layer and high-strength nanocrystalline phases in the underlying sublayers. It has been revealed that as-deposited multilayer coating decreases a corrosion rate of the substrate. The layered structure of the coating hinders the transport and diffusion of chloride ions to the substrate and decreases the corrosion rate of the substrate. However, a transport of chloride ions into the inner layers of the coating may occur through the microstructural defects preserved in the amorphous Al-Si-O layer. The scheme describing an initiation of corrosion damage and formation of corrosion products under critical corrosion conditions was proposed.

The effect of h-BN content on mechanical properties, microstructure and machinability of hot-pressed h-BN/Si3N4 composites

Si3N4 ceramics now face new applications and challenges in the semiconductor field. The preparation of h-BN/Si3N4 machinable ceramics incorporating 5–30 vol% h-BN via hot-pressing was carried out. This study systematically explored how the varying h-BN content influenced the composite's microstructure, mechanical attributes, and machinability. The results showed that some of the h-BN was encapsulated by growing Si3N4 grains to form an intragranular structure during the process of hot-pressing, while the remaining h-BN exhibited uniform distribution along grain boundaries. The Si3N4 phase transition in composites was inhibited by excessive h-BN, accompanied by a higher decrease in densification. The sample containing 20 vol% h-BN exhibited the best comprehensive performance, with high mechanical strength and excellent machinability. The bending strength and fracture toughness remained high at 862 MPa and 10.3 MPa m1/2, and could be effortlessly machined with cemented carbide drills. Additionally, the machinability of the composites was systematically characterized through observations of crack propagation paths, fitting of R-curves, and mechanical drilling tests.

Influence of silicon content on high temperature tribology performance of TiAlCrSiN coatings

A series of TiAlCrSiXN coatings with various silicon contents were fabricated on Si(100) and 304 stainless steel substrates using a radiofrequency magnetron sputtering system, aiming to examine the effects of various Si content on wear behaviors at room temperature and 700 °C. The addition of silicon notably increased the coatings' hardness from 21 GPa to 29 GPa, attributed to significant compressive residual stress (−3.4 GPa) that inhibited dislocation movement. Room temperature wear tests revealed that wear rates varied between 0.6 and 3.7 (10−6mm3N−1 m−1) with increasing silicon content. The lowest wear rate is observed at a silicon content of 3.1 at.%, which is associated with an ideal residual stress of −1.2 GPa. This stress level played a crucial role in preventing crack propagation and delamination, thus enhancing wear protection. At 700 °C, the coatings' wear resistance was mainly governed by their oxidation resistance, with the most resistant coatings exhibiting wear rates from 1.6 to 4.4 (10−6mm3N−1 m−1). The silicon addition promoted the formation of an amorphous Si3N4 phase, serving as an oxygen diffusion barrier, thus enhancing resistance to oxidation at elevated temperatures. The findings suggest that TiAlCrSiN coatings offer a promising solution for advanced tribological applications, demonstrating improved performance at temperatures up to 700 °C.

Fabrication of silicon nitride with high thermal conductivity and flexural strength by hot‐pressing flowing sintering

AbstractThe silicon nitride (Si3N4) possessing enhanced thermal conductivity and flexural strength was sintered by a hot‐pressing flowing sintering (HPFS) method. The effects of extrusion deformation and sintering additives on the density, phase, microstructure, and performances of Si3N4 fabricated by HPFS were investigated. As the strain increased from 67% to 167%, the mean grain diameter of Si3N4 with MgSiN2 as an additive increased by 49%, reaching 1.36 ± .6 µm. The addition of MgSiN2–Y2O3 resulted in a significantly stronger degree of texturing and smaller grain diameter in Si3N4, compared to the incorporation of MgSiN2 alone. The thermal conductivity of Si3N4 with added MgSiN2–Y2O3 increased to 109.1 W m−1 K−1 in the flow direction, and the flexural strength reached the maximum value of 1250.8 ± 39 MPa in the hot‐pressing direction. The results showed that the thermal conductivity and flexure strength of Si3N4 were enhanced due to the formation of texture and the reduction of flaws induced by HPFS. Moreover, HPFS method exhibited outstanding controllability over the microstructures and properties of materials.

Functionally graded structure of a nitride-strengthened Mg2Si-based hybrid composite

The ex-situ incorporation of the secondary SiC reinforcement, along with the in-situ incorporation of the tertiary and quaternary Mg3N2 and Si3N4 phases, in the primary matrix of Mg2Si is employed in order to provide ultimate wear resistance based on the laser-irradiation-induced inclusion of N2 gas during laser powder bed fusion. This is substantialized based on both the thermal diffusion- and chemical reaction-based metallurgy of the Mg2Si–SiC/nitride hybrid composite. This study also proposes a functional platform for systematically modulating a functionally graded structure and modeling build-direction-dependent architectonics during additive manufacturing. This strategy enables the development of a compositional gradient from the center to the edge of each melt pool of the Mg2Si–SiC/nitride hybrid composite. Consequently, the coefficient of friction of the hybrid composite exhibits a 309.3% decrease to –1.67 compared to –0.54 for the conventional nonreinforced Mg2Si structure, while the tensile strength exhibits a 171.3% increase to 831.5 MPa compared to 485.3 MPa for the conventional structure. This outstanding mechanical behavior is due to the (1) the complementary and synergistic reinforcement effects of the SiC and nitride compounds, each of which possesses an intrinsically high hardness, and (2) the strong adhesion of these compounds to the Mg2Si matrix despite their small sizes and low concentrations.

Atomic scale formation mechanism of the Amorphous-Nanocrystalline biphase structure in TiSiN Coating: Phase field crystal simulation and experimental characterization

TiSiN nanocomposite coatings have exceptional strength, toughness, and corrosion resistance due to their unique biphase amorphous/nanocrystalline structure with tiny TiN grains embedded in an amorphous Si3N4 phase. However, the unclear formation process makes microstructural control challenging. In this study the phase field crystal method has been employed to analyze the microstructure evolution in TiSiN coatings, uncovering a four-step formation mechanism for the amorphous-nanocrystalline structure. The initial stage results in appearance of concentration heterogeneities and the development of the triangular-like short-range order (Stage I). This stage is followed by the nucleation of nanocrystals in Ti-rich zones and the transformation of triangular short-range to the long-range order with a square symmetry (Stage II). Then, during coarsening stage, Ti-rich polycrystalline nanocrystals are formed (Stage Ⅲ) followed by the vacancy migration from the polycrystalline nanocrystals to amorphous phase and formation of monocrystalline Ti nanocrystals embedded in amorphous phase (Stage Ⅳ). The simulation results are compared qualitatively with aberration-corrected High-Resolution Scanning Transmission Electron Microscopy and High-Resolution Transmission Electron Microscopy images from our study.

Effect of electric field on the structure of SiCN pyrolyzed at 1600 °C

Application of electric field is known to influence the phase transformation and sintering conditions of ceramics. In this work, the effect of electric field on the phase structure evolution of SiCN pyrolyzed at 1600 °C was studied in comparison with conventional pressureless pyrolyzed SiCN. The structural information was characterized via X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and Raman spectroscopy. It was found that SiCN ceramic prepared via conventional pressureless pyrolysis was composed of SiC, Si3N4 and free carbon phases, while that produced via electric field-assisted pyrolysis consisted of Si3N4 and free carbon phases. No crystalline SiC was detected in SiCN ceramic pyrolyzed via the field-assisted method, indicating that the electric field affected the carbothermal reaction between Si3N4 and C phases and changed the formation mechanism of SiC. Moreover, the electric field promoted structural ordering rearrangement of the free carbon phase and increased the defect content therein.

Effect of Si contents on microstructure and mechanical characteristics of TiAlSiN thin film deposited by HiPIMS using different Ti[sbnd]Si target compositions

High Power Impulse Magnetron Sputtering (HiPIMS) PVD deposition technology offers smoother coating enhanced mechanical properties and improved substrate-coating adhesion and has become a superior alternative to direct current magnetron sputtering (DCMS). The technology utilizes high peak power with a small pulse duration, leading to a much denser plasma in the order of 1018 m−3. This results in dense, defect-free, and high-quality smoother coatings. In this study, TiAlSiN coatings were deposited on WC-10Co (wt%) cermet using HiPIMS technology with two different TiSi contents. A pair of TiSi targets, either of compositions Ti77Si23 or of Ti66Si34, were used during the depositions along with another pair of Ti40Al60, in successive cathode-stages. The impact of Ti and Si atomic percentages on the microstructure and mechanical properties of the coatings was investigated using various techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), 3D-surface profiling, nano-indentation, Rockwell indentation-adhesion, and scratch test. The coating's thickness and deposition rate were estimated from the cross-section morphology. The results suggested that the TiAlN phase likely existed in an amorphous phase of Si3N4 in TiAlSiN coatings. As the Ti atomic percentage in the layer increased, the hardness improved, reaching up to 42 GPa (max). Additionally, the coating surface roughness decreased from 14.3 nm to 10.9 nm, and the grain size decreased from 10 nm to 9 nm. The adhesion strength observed under the Rockwell-indentation test was rated as HF1 in both cases. Furthermore, the indentation test revealed similar crack patterns, but an increase in Ti content significantly improved the coating-substrate adhesion, as confirmed by the Scratch test. The scratch test demonstrated a maximum load of 173 N during the complete delamination (Lc3) of TiAlSiN from the carbide substrate.

The new nanocapsule structure and cyclic tribological properties of Mo2N/Ag/Si3N4 nanocomposite film

Nanostructured films present versatile properties in the area of friction. This paper introduced a adaptive nanostructured film with multiple cycle service capability. Based on the concept of capsule sustained-release, a series of Mo2N/Ag/Si3N4 composite films with different Si contents were prepared. The results show that the films were mainly composed of fcc-Mo2N, fcc-Ag and Si3N4, presenting a “capsule structure” with the encapsulation of Mo2N/Ag by Si3N4. The crystalline Si3N4 wrap the Mo2N to form a co-extended growth structure, when the Si content is lower than 6.3 at. %. However, the coherent interface could be destroyed for the film with higher content of Si (>6.3 at. %), owing to the transforming to an amorphous phase of Si3N4. When annealed at 500 °C, the growth of Ag content on the surface is higher for film with 6.3 at. % Si than that of film with 11.4 at. % Si, indicating that the capsule structure with amorphous Si3N4 can hinder the diffusion of Ag. With the increase of Si content, the hardness of the film increases firstly and then decreases gradually. The maximum hardness value of 34.3 GPa can be obtained for the film with 6.3 at. % Si. The average friction coefficient of Mo2N/Ag/Si3N4 composite films with 6.3 and 11.4 at.% Si contents decreased from 0.48 to 0.33 and from 0.42 to 0.31 respectively with the increase of friction temperature from 200 °C to 700 °C under friction test. At the same temperature, the friction coefficient for film with 6.3 at.% Si is always lower than that of film with 11.4 at.% Si, but the wear rate is always slightly higher. The temperature-cycling friction times of nanocomposite films consisting of amorphous Si3N4 capsule structure with11.4 at.% Si is twice that of crystalline Si3N4 with 6.3 at % Si. The amorphous “capsule” structure can improve the multi-cycle serviceability of the film.

Interaction of Si Atom with the (001) Surface of TiN, AlN and TaN Compounds

Nowadays, the application of multicomponent coatings with multiphase nanocrystalline structure is the most promising direction in the search for wear-resistant protective coatings with a full set of necessary operational properties. Nanocrystalline multicomponent coatings based on the Ti-Al-Ta-Si-N system have a high hardness combined with thermal stability and oxidation resistance. Silicon atoms are weakly soluble in the TiN, Ti1−xAlxN, and TaN crystalline phases of the Ti-Al-Ta-Si-N system and interact preferentially with N atoms, forming the amorphous Si3N4 phase. In this context, it is important to first study the peculiarities of the interaction of Si atoms with the simplest structural units of the Ti-Al-Ta-Si-N system, such as TiN, AlN, and TaN compounds with the NaCl structure. This work is devoted to the study of the interaction of a Si atom with the (001) surface of AlN, TiN, and TaN compounds with the NaCl structure using ab initio calculations. This provides information for a deep understanding of the initial stages of the formation of different crystallites of the considered composite. It was established that the adsorption of silicon on the (001) surface of AlN, TiN, and TaN significantly increases the relaxation of the surface layers and leads to an increase in the corrugation observed on the clean surfaces. The largest corrugation is observed on the surface of the TaN compound. The most energetically favorable adsorption positions of Si atoms were found to be the position of Si above the N atom on the TiN and TaN surfaces and the quadruple coordinated position on the AlN surface. The valence electron density distribution and the crystal orbital Hamiltonian population were studied to identify the type of Si atom bonding with the (001) surface of AlN, TiN, and TaN compounds. It was found that silicon forms predominantly covalent bonds with the nearest metal and nitrogen atoms, except for the quadruple coordinated position on the surface of TiN and TaN, where there is a high degree of ionic bonding of silicon with surface atoms.

The effect of ZnO:Si3N4 nanocomposite interlayer on electrical properties and interface-states between metal-semiconductor in Al/p-Si (MS) structures

In the study, the fabrication processes of the Al/p-Si (MS), Al/ZnO/p-Si (MIS1), and Al/(ZnO:Si3N4)/p-Si (MIS2) structures were described and the effect of the ZnO:Si3N4 interface layer in MS was investigated in detail. The ZnO and (ZnO:Si3N4) thin films were coated on p-Si substrates and annealed at 500 °C for 1 h. Si3N4 and ZnO phases were identified via X-ray diffraction (XRD) and Raman spectra. FESEM images demonstrated that ZnO nanorods grew on Si3N4 particles with a heterogeneous nucleation mechanism. The I–V characterization was performed in the dark (±4 V). The basic diode parameters (barrier height (BH), series/shunt (Rs/Rsh) resistances, rectifying rate (RR=If/Ir), and ideality factor (n)) were calculated from various methods. The RR, and Rsh values increased, but n, Rs and BH decreased by insulating layer in the MIS2 The energy-dependent profiles of interface-states (Nss) were also extracted from the I–V plots according to voltage-dependent n and BH. The MIS2 provided the best passivation effect with decreasing interface traps.

Silicon nanoparticles encapsulated in Si3N4/carbon sheaths as an anode material for lithium-ion batteries

Novel composite materials comprising of silicon nanoparticles (SiNPs) encapsulated with thin layers of silicon nitride and reduced graphene oxide shells (Si@Si3N4@rGO) are prepared using a simple and scalable method. The composite exhibits significantly improved cycling stability and rate capability compared to bare SiNPs. The presence of inactive α and β phases of Si3N4 increases the mechanical endurance of SiNPs. Amorphous SiN x , which is possibly present with Si3N4, also contributes to high capacity and Li-ion migration. The rGO sheath enhances the electronic conduction and improves the rate capability. 15-Si@Si3N4@rGO, which is prepared by sintering SiNPs for 15 min at 1300 °C, spontaneous-coating GO on Si@Si3N4, and reducing GO to rGO, delivers the highest specific capacity of 1396 mAh g−1 after 100 cycles at a current density of 0.5 A g−1. The improved electrochemical performance of 15-Si@Si3N4@rGO is attributed to the unique combination of positive effects by Si3N4 and rGO shells, in which Si3N4 mitigates the issue of large volume changes of Si during charge/discharge, and rGO provides efficient electron conduction pathways. Si@Si3N4@rGO composites are likely to have great potential for a high-performance anode in lithium-ion batteries.

Effect of Different Si Target Currents on Performance of Magnetron Sputtering Wear Resistant Coatings

In order to ameliorate the properties of the coating material on the surface of the key moving parts of artillery, the influence of different Si target currents on microstructure and mechanical performances of magnetron sputtering TiAlMoSiN coating was studied. The phase structure of the coating, microstructure, coating adhesion, friction and wear properties were characterized, respectively. The deposited TiAlMoSiN coatings were mainly composed of TiN phase, Mo2N phase, amorphous Si3N4 phase and amorphous AlN phase. A typical columnar nano-multilayer structure was formed on the TiAlMoSiN coating after magnetron sputtering process. When the Si target current reached 1.0A, the coating hardness was supreme, valued 32.6 GPa. Along with the addition of the Si target current, the hardness and the bonding strength between the coatings and 45 steel substrates declined. As the Si target current was 1.5 A, TiAlMoSiN coating has the lowest friction coefficient and wear rate. Moreover, it was found out the coating with Si target current at 1.0A had shown the optimal surface properties.

Prediction of four Si3N4 compounds by first-principles calculations

Four Si3N4 crystal structures were predicted using an ab initio evolutionary methodology. The mechanical and dynamic stabilities were confirmed by the density functional theory assuming zero-pressure conditions. Energetic stability calculations indicated that the structures are metastable phases at ambient pressure, but their formation is more favorable at high pressures. At zero pressure, the densities of the hp-Si3N4, cp-Si3N4, oc-Si3N4, and ti-Si3N4 phases were 3.21, 3.28, 3.70, and 3.24 g/cm3, respectively. The calculated band structures and densities of states indicated that they have semiconductive properties, with gaps ranging from 0.754 to 3.968 eV. Mechanical property calculations revealed that the hardness of the Si3N4 compounds ranged between 11.2 and 23.3 GPa, which were higher than the corresponding values for the synthesized Si3N4 phases. These four Si3N4 structures are potentially valuable candidates for the synthesis of Si3N4 compounds.

Study of Structural, Mechanical, and Corrosion Resistance of a Nanocomposite CrSiN/CrN/Cr Coating Deposited on AZ31: Effects of Deposition Time

To improve the surface properties of Mg alloys and expand the applications of CrN-based materials, composite CrSiN coatings consisting of amorphous Si3N4 and nano CrN phases have been prepared on AZ31 based on the theory of fine grain strengthening and multigrain boundaries. The effect of the thickness of the coating on the structure and properties was investigated. The microstructure was studied by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The mechanical properties, adhesion properties, and corrosion resistance were investigated using a nanoindentater, scratch testers, and electrochemical workstations. The results show that the coating consists of a face-centered cubic CrN phase, that Si3N4 is not found in the diffraction pattern, and that the HRTEM images show a composite structure of amorphous and nanocrystalline phases. With the increase in deposition time (thickness), the surface roughness decreases, the defects disappear, and the interface has no visible defects. Moreover, the hardness and elastic modulus of the coating increase, corrosion resistance improves, adhesion performance first increases and then decreases. The adhesion between coating and substrate reaches the maximum when sputtering time is 50 min, which corresponds to the CrSiN thickness of 0.79 μm.

Analysis of Connecting Rod Made by Using Micro Si<sub>3</sub>N<sub>4</sub> Particulates Reinforced with Al2024 Alloy

In the present work, an attempt has been made to synthesize metal matrix composite using Al2024 as matrix material with Si3N4 particulates and K2TiF6 reinforcement using liquid metallurgy route in particular stir casting technique. The addition level of reinforcement is being varied from 4-8% in steps of 4 wt%. For each composite, reinforcement particles were preheated to a temperature of 500ºC and then dispersed in steps of three into the vortex of molten Al2024 alloy rather than introducing all at once, there by trying to improve wettability and distribution. Microstructural characterization was carried out for the above prepared composites by taking specimens from central portion of the casting by microstructural studies and SEM analysis. Tensile, Impact, and Fatigue properties of the prepared composite were studied before and after addition of Al2024 particulates to note the extent of improvement. Microstructural characterization of the composites has revealed fairly uniform distribution of Si3N4 particulates and some amount of grain refinement in the specimens. SEM analysis revealed the presence of Si3N4 and other phases. Further, the Tensile and Impact strength of the composite found increased with increased filler content.