纳米Si3N4 对TC4钛合金微弧氧化层微观结构和性能的影响

在基电解液中加入氮化硅纳米颗粒,对TC4钛合金进行微弧氧化（MAO）处理,研究了Si₃N₄浓度对微弧氧化层表面形貌、耐蚀性和耐磨性的影响。添加Si₃N₄的MAO层呈现多孔结构,当Si₃N₄浓度为1 g/L时,涂层厚度最大,且经过7 d的酸腐蚀试验,该涂层的耐蚀性良好,腐蚀速率最低,约为0.057 mg·cm^-2·d^-1。随着Si₃N₄的加入,MAO涂层的抗菌性能先升高后降低。当Si₃N₄的添加量为1 g/L时,该MAO层的抗菌性能最好。Si₃N₄的加入能明显提高涂层在模拟海水中的耐磨性。当Si₃N₄的添加量为3和4 g/L时,所得涂层的摩擦系数低且稳定,且添加3 g/L Si₃N₄制备来的MAO涂层表现出优异的耐磨性。     

Properties and fracture processes of Si3N4/ Si3N4 joints brazed using Cu–Zn–Ti filler alloy

Abstract

Si₃N₄ ceramic was jointed to itself using a filler alloy of Cu-Zn-Ti at 1123-1323 K for 0.3-2.7 ks. Ti content in the Cu-Zn-Ti filler alloy was varied from 5 to 20 at.-%. The effect of brazing parameters, such as brazing temperature, holding time and Ti content, on the mechanical properties and facture processes of the Si₃N₄/Si₃N₄ joint were investigated. The results indicated that the increased brazing temperature, holding time and Ti content increase the thickness of the interfacial reaction zone in the Si₃N₄/filler alloy, and the size and amount of the reaction phases in the filler alloy. Their increases lead to increasing shear strength of the joint. The fracture behaviour of the Si₃N₄/Si₃N₄ joint greatly depends on the microstructure of the joint. A suitable thick reaction zone with reaction phases yields the high strength of the Si₃N₄/ Si₃N₄ joint.     

Brazing Si3N4 ceramic to AISI 5140 steel under pressure

Pressures (0 to 40 MPa) were applied to the joints of Si₃N₄ ceramic to 5140 steel during vacuum brazing with Ag-Cu-Ti active filler metal. Pressurization started at various temperatures (873,973, and 1073 K) and ended at room temperature during cooling. Results show that there is an optimum starting temperature to pressurize, at which the maximum room temperature shear strength of the joint is obtained.     

Study of the mechanical properties,toughening and strengthening mechanisms of Si3N4/Si3N4w/TiN nanocomposite ceramic tool materials

Si₃N₄/Si₃N_4w/TiN nanocomposites were fabricated by a hot-pressing technology with different sintering processes. The effect of nanoscale TiN and Si₃N_4w on the mechanical properties was investigated. The microstructure and indention cracks were observed by scanning electron microscopy, transmission electron microscopy and energy-dispersive spectrometry investigations. The research results showed that Si₃N₄/Si₃N_4w20/TiN5 nanocomposites containing 5 vol.% of nanoscale TiN and 20 vol.% of nanoscale Si₃N_4w, which were sintered under a pressure of 30 MPa at a temperature of 1650 °C for 40 min, had optimum mechanical properties. The addition of both nanoscale TiN and nanoscale Si₃N_4w contributed to the microstructural evolution and an improvement of the mechanical properties. The toughening and strengthening mechanisms are discussed for Si₃N₄/Si₃N_4w20/TiN5 nanocomposites.     

Thermal shock behavior of Si3N4-TiN nano-composites

Si₃N₄-TiN nano-composites were fabricated by hot press sintering nano-sized Si₃N₄ and TiN powders. The microstructure, mechanical properties and thermal shock behavior of Si₃N₄-TiN nano-composites were investigated. The addition of proper amount TiN particles can significantly increase the flexural strength and the fracture toughness. Si₃N₄-TiN nano-composites showed both higher critical temperature difference and higher residual strength compared with those of monolithic silicon nitride nano-ceramic when the amount of TiN is less than 15 wt.%. But a further increase in the amount of TiN leaded to a decrease in the thermal shock resistance.     

Corrosion behavior of nanostructured Ni‐Si3N4 composite films: A study of electrochemical impedance spectroscopy

Ni‐Si₃N₄ nanocomposite films with both the consecutive Ni crystallites and dispersed Si₃N₄ particles in the nanometer range have been fabricated using DC electroplating technique, and characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), and X‐ray diffraction (XRD). The corrosion resistance of the Ni‐Si₃N₄ nanocomposite film has been compared to that of pure Ni coating through polarization. Meanwhile, the corrosion process of Ni‐Si₃N₄ nanocomposite film in neutral 3.5% NaCl solution has been investigated using electrochemical impedance spectroscopy (EIS). The results show that the Ni‐Si₃N₄ nanocomposite film is more resistant to corrosion than the pure Ni coating. The corrosion of Ni‐Si₃N₄ nanocomposite film is controlled by electrochemical step, and the whole corrosion process is divided into two sequential stages. The main corrosion type of Ni‐Si₃N₄ nanocomposite films in neutral 3.5% NaCl solution is pitting.     

采用Ag-Cu-Ti+Mo复合钎料钎焊Si3N4陶瓷

采用Ag-Cu-Ti+Mo复合钎料连接Si3N4陶瓷,利用SEM,TEM,Nanoindentation研究了钎料内钼颗粒含量对接头组织和力学性能的影响.结果表明,在Si3N4/钎料界面处形成了一层致密的反应层,该反应层由TiN和Ti5Si3组成.接头的中间部分由银基固溶体、铜基固溶体、钼颗粒和Ti-Cu金属间化合物组成.借助于纳米压痕技术测定了接头内Ti-Cu化合物以及钎料金属的弹性模量和硬度值.随着钎料内钼颗粒含量的提高,母材/钎料界面反应层厚度逐渐降低;钎料金属中Ti-Cu化合物数量增多;此外,银和铜基固溶体组织逐渐变得细小.当添加5%Mo时,得到最高的接头强度429.4 MPa,该强度相比合金钎料提高了114.7%.     

Densification, microstructure, and fracture behavior of TiC/Si3N4 composites by spark plasma sintering

TiC/Si₃N₄ composites were prepared using the β-Si₃N₄ powder synthesized by self-propagating high-temperature synthesis (SHS) and 35 wt.% TiC by spark plasma sintering. Y₂O₃ and Al₂O₃ were added as sintering additives. The almost full sintered density and the highest fracture toughness (8.48 MPa·m^½) values of Si₃N₄-based ceramics could be achieved at 1550°C. No interfacial interactions were noticeable between TiC and Si₃N₄. The toughening mechanisms in TiC/Si₃N₄ composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si₃N₄ matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.     

Preparation and characterization of Si3N4/TiN nanocomposites ceramic tool materials

Si₃N₄/TiN nanocomposites ceramic tool materials were sintered under 30 MPa at 1650 °C for 40 min. The effects of nano-scale TiN on the mechanical properties and microstructure were investigated. The strengthening and toughening mechanisms of Si₃N₄/TiN nanocomposites were studied by observing the fracture surfaces, samples for TEM and cracks. The oxidation resistance of Si₃N₄/TiN nanocomposites was also discussed based on the observation of microstructure. The results showed that the elongated Si₃N₄ grains which were pulled out from the fracture surfaces were reduced with the increase of the addition of nano-scale TiN, and many intragranular TiN grains were observed in Si₃N₄/TiN nanocomposites. The greater crack deflection and microcrack provided the contributions to the strengthening and toughening mechanisms for Si₃N₄/1 vol%TiN nanocomposite. However, Si₃N₄/TiN nanocomposites were oxidized strongly at higher temperature.     

Enhanced tribological behaviors of sintered polycrystalline diamond by annealing treatment under humid condition

The tribological behaviors of the 700 °C annealed sintered polycrystalline diamond (PCD) at various relative humidity (RH) levels were systematically investigated. The comparison of tribological behaviors between the 700 °C annealed PCD and the pristine PCD was made to further understand the tribological mechanisms. The results reveal that the friction inducing carbonaceous transfer film and oxidation and hydrolysis induced tribochemistry reaction dominant the tribological behaviors of the annealed PCD at various RH levels. The low coefficient of friction (COF) obtained in dry environments is attributed to carbonaceous transfer film on the worn Si₃N₄ surface, which was formed by the layers shearing action of massive tiny diamond grains exfoliated from the annealed PCD surface. The graphitization, oxidation and stress relaxation of the PCD induced by the 700 °C annealing treatment make the tiny diamond grains more easily to exfoliate and be grinded on the Si₃N₄ interface. It facilitates the formation of friction reducing carbonaceous transfer film, and finally results in the 30% lower COFs than those of pristine PCD at low RH levels (5%–50% RH). Meanwhile, an enhanced wear resistance of PCD can be achieved after 700 °C annealing treatment. The tribochemistry reaction induced by the oxidation and hydrolysis of Si₃N₄ governs the tribological behaviors of the annealed PCD at high RH levels (60%–99.9% RH). It reveals higher COFs accompanied with serious wear of Si₃N₄ ball and nearly no wear loss of annealed PCD. The produced SiO₂ and silicic acid embeds into massive spalling pits on the annealed PCD surface, resulting in slighter wear of the PCD and Si₃N₄ than that of the pristine PCD/Si₃N₄. These results propose that the tribological behaviors of PCD under humid environment can be significantly improved by the 700 °C annealing treatment.     

Effects of tribochemical reaction on tribological behaviors of Si3N4/polycrystalline diamond in hydrochloric acid

The interfacial tribo-chemistry significantly affects the tribological behaviors of polycrystalline diamond (PCD)/Si₃N₄, especially introducing acid solution. The results indicate that the friction coefficient rises with increased PH while Si₃N₄ wears severer and then slighter. Besides, when the hydrogen ions were introducing in the testing environment, the hydrolysis reaction of Si₃N₄ and oxidation reaction of PCD were promoted on the surfaces with the formation of Si-OH and C-OH. The tribo-pair surfaces absorded hydrogen ions by protonation reaction to form the double electron layer in this process, during which significantly affect the tribological behavior of Si₃N₄. This work elucidates that the protonation effect of hydrogen ions and bridge effect of hydroxyls, leading to the friction and wear mechanism of PCD/Si₃N₄.     

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

Gradients in elastic modulus for improved contact-damage resistance. part ii: the silicon nitride–silicon carbide system

In an effort to create elastic-modulus (E) graded materials for contact-damage resistance—free of substantial amounts of glass—silicon nitride (Si₃N₄)-silicon carbide (SiC) graded materials were processed. The structure of these graded materials is such that Si₃N₄ (E=300 GPa) is at the contact surface and SiC (E=400 GPa) is in the interior, with a stepwise gradient in composition existing between the two over a depth of 1.6 mm. A pressureless, liquid-phase co-sintering method, in conjunction with a powder-layering technique, was used to achieve this structure. The liquid phase used was yttrium aluminum garnet (YAG). Under spherical indentation, cone-cracks did not form in the graded material, but some inelastic shear deformation was observed. Cone cracks formed in both the monolithic Si₃N₄ and the monolithic SiC end member materials under identical indentation conditions. Finite element analysis (FEA) of the stresses associated with indentation revealed that the maximum principal tensile stresses outside the Hertzian contact circle, which drive the classical cone-cracks, are reduced by approximately 12% in the graded material relative to the monolithic silicon nitride case. This reduction is significantly lower than what was calculated for the Si₃N₄-glass case (Part I), owing to the shallower, linear E-gradient over a 1.6 mm depth in Si₃N₄-SiC, as compared with the power-law, steeper E-gradient over 0.4 mm depth in the Si₃N₄-glass. It appears that, in addition to the E-gradient, the inelastic deformation contributes to the suppression of cone cracks in the Si₃N₄-SiC graded material. It is suggested that compressive residual stresses may be present in the Si₃N₄-SiC graded material, which are also likely to aid in the suppression of cone-cracks.     

Morphology and ablation mechanism of Cf/Si3N4 composites ablated by oxyacetylene torch at 2200°C

Abstract

Si₃N₄ ceramic matrix composites reinforced by carbon fibres (C_f/Si₃N₄) were prepared by low pressure chemical vapour infiltration at 1250°C using SiCl₄ and NH₃ as precursor. The as prepared C_f/Si₃N₄ composites were ablated to determine the mechanism of the ablation resistance and oxidisation resistance by oxyacetylene torch at 2200°C. The morphology and microstructure of the composites were examined by scanning electron microscopy. The phase compositions of the composites were confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction. The results indicated that the matrix of the C_f/Si₃N₄ composites was composed of the amorphous Si₃N₄ and nanometre α-Si₃N₄. A central ablation region and a ring oxidisation region appeared on the surface of the as ablated C_f/Si₃N₄ composites. Sublimation of the Si₃N₄ matrix and oxidation of the carbon fibres are the main ablation behaviours in the central region. Oxidation of the Si₃N₄ matrix and deposition of SiO₂ particles are the main ablation behaviour in the ring region. A large number of SiO₂ liquid droplets produced during ablation were retained and formed spherical solid particles on the surface of the ring region after ablation. For the mismatch of the coefficient of thermal expansion of the carbon fibres and the Si₃N₄ matrix, Si₃N₄ matrix was cracked under the thermal impact of the oxyacetylene flame. With the passive oxidation of the as cracked surface, the continuous SiO₂ liquid was formed in the ring region. Subsequently, some residual Si₃N₄ particles were covered by transparent SiO₂ layer to form an amber-like microstructure.     

Fabrication of platinum coating on continuous porous SiC–Si3N4 composites by the electroless deposition process

Continuous porous SiC–Si₃N₄ composite fabricated by a multi-pass extrusion process was investigated using SEM, XRD and TEM techniques. In the continuous pore regions, many single crystalline Si₃N₄ whiskers with an average diameter of about 1 μm, which could increase the surface area of the porous composites, were observed. However, to make a platinum coating on the composite, the electroless deposition method was adapted. The spot-type platinum particles, with a diameter of about 50–500 nm, attached to the SiC–Si₃N₄ matrix as well as the surface of the Si₃N₄ whiskers, which can enhance the catalytic activity.     

The Effect of Si3N4 Additions on the Oxidation Resistance of TiAl Alloys

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si₃N₄ particles were studied at 1173 and1273 K in 1 atm of air. The Si₃N₄ dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti₅Si₃ and dissolvednitrogen. The oxide scale formed on TiAl–Si₃N₄alloys consisted of an outer TiO₂, an intermediate(Al₂O₃+TiO₂), and an inner(TiO₂+Al₂O₃) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO₂ particlesbelow the outer TiO₂ layer, and the improved scale adhesion bySi₃N₄ dispersions were attributable mainly to theincreased oxidation resistance compared to the Si₃N₄-freeTiAl alloys. Marker experiments showed that, for TiAl–Si₃N₄ alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.     

Development of Si3Na/Al composite by pressureless melt infiltration

Pressureless infiltration process to synthesize Si3N4/Al composite was investigated. Al-2%Mg alloy was infiltrated into Si3N4 and Si3N4 containing 10% Al2O3 preforms in the atmosphere of nitrogen. It is possible to infiltrate Al-2%Mg alloy in Si3N4 and Si3N4 containing 10% Al2O3 preforms. The growth of the dense composite of useful thickness was facilitated by the presence of magnesium powder at the interface and by flowing nitrogen. During infiltration Si3N4 reacted with aluminium to form Si and AIN, the growth of composite was found to proceed in two ways, depending on the Al2O3 content in the initial preform. Firstly, preform without Al2O3 content gives rise to AIN, Al3.27Si0.47 and Al type phases after infiltration. Secondly, perform with 10% Al2O3 content gives rise to AIN-Al2O3 solid solution phase （AION）, MgAl2O4, Al and Si type phases. AlON phase was only present in composite, containing 10% Al2O3 in the Si3N4 preforms before infiltration.     

Growth Kinetics and Microstructure Evolution of Intermediate Phases in MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-WSi<Subscript>2</Subscript>/Mo Diffusion Couples

The growth kinetics and silicon diffusion coefficients of intermediate silicide phases in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple prepared by spark plasma sintering were investigated in temperatures ranging from 1200 to 1500 °C. The intermediate silicide phases were characterized by x-ray diffraction. The microstructures and components of the MoSi₂-Si₃N₄-WSi₂/Mo composites were investigated using scanning electron microscope with energy-dispersive spectroscopy. A special microstructure with MoSi₂ core surrounded by a thin layer of (Mo,W)Si₂ ring was found in the MoSi₂-Si₃N₄-WSi₂ composites. The intermediate layers of Mo₅Si₃ and (Mo,W)₅Si₃ in the MoSi₂-Si₃N₄-WSi₂/Mo diffusion couples were formed at different diffusion stages, which grew parabolically. Activation energy of the growth of intermediate layers in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple was calculated to be 316 ± 23 kJ/mol. Besides, the hindering effect of WSi₂ addition on the growth of intermediate layers was illustrated by comparing the silicon diffusion coefficients in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo and MoSi₂-3.5 vol.% Si₃N₄/Mo diffusion couples. MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂ coating on Mo substrate exhibited a better high-temperature oxidation resistance in air than that of MoSi₂-3.5 vol.% Si₃N₄ coating.     

Vitreous bond silicon carbide wheel for grinding of silicon nitride

This study investigates the grinding of sintered silicon nitride using a SiC wheel with a fine abrasive grit size and dense vitreous bond. The difference of hardness between the green SiC abrasive and sintered Si₃N₄ workpiece (25.5 vs. 13.7 GPa) is small. Large grinding forces, particularly the specific tangential grinding forces, are observed in SiC grinding of Si₃N₄. The measured specific grinding energy is high, 400–6000 J/mm³, and follows an inverse relationship relative to the maximum uncut chip thickness as observed in other grinding studies. The SiC wheel wears fast in grinding Si₃N₄. The G-ratio varies from 2 to 12. Two unique features in SiC grinding of Si₃N₄ are the trend of increasing G-ratio at higher material removal rate and the excellent surface integrity, with 0.04–0.1 μm R_a and no visible surface damage. For a specific material removal rate, surface cracks along the grinding direction are generated on the ground surface. The problem of chatter vibration was identified at high material removal rates. Periodic and uneven wheel loading marks and clusters of workpiece surface cracks across the grinding direction could be observed at high material removal rates. This study demonstrates that the SiC grinding wheel can be utilized for precision form grinding of Si₃N₄ to achieve good surface integrity under a limited material removal rate.     

Preparation of pure nano-grained Si2N2O ceramic

Pure Si₂N₂O ceramic is fabricated by nitridizing a powder mixture of Si and SiO₂. Results show that the prepared Si₂N₂O is composed of nano-particles with a size of about 50 nm. Analysis and discussion are focused on the formation mechanism of the nano-grained Si₂N₂O ceramic based on thermodynamics calculation, micro-morphologies and phase composition analysis. Preferential formation of Si₃N₄ on the surface of nano-SiO₂ particles at below melting point of Si and then successive in-situ transformation of SiO₂ and Si₃N₄ into Si₂N₂O at above melting point of Si are considered as the key reasons to form the pure nano-grained Si₂N₂O ceramic.