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%.     

Influence of the pulse shape on the EDM performance of Si3N4-TiN ceramic composite

This paper describes the influence of the EDM discharge pulse shape on the machining performances and material removal mechanisms of Si₃N₄-TiN. Dramatic differences of material removal, ranging from classical melting to chemical decomposition, are observed by applying different pulse shapes such as the iso-energetic or relaxation type discharge pulses. It not just leads to the change of surface texture and machining performances, but also has influences on the ceramic properties. An EDM strategy is developed for the production of ceramic components in Si₃N₄-TiN and validated through the fabrication of a high temperature mesoscopic gas turbine.     

采用Ti/Ag-Cu/Cu中间层钎焊连接Si3N4陶瓷和TiAl合金的界面结构及性能

采用Ti/Ag-Cu/Cu中间层实现了Si₃N₄陶瓷与TiAl合金的钎焊连接,获得了良好的接头.利用SEM,EDS等微观手段,分析了接头界面结构和元素分布情况.结果表明,Si₃N₄陶瓷/Ti/Ag-Cu/Cu/TiAl典型界面微观结构可能为:Si₃N₄/TiN/Ti-Si/Cu-Ti+Ag（s,s）+Cu（s,s）/AlCuTi/TiAl.在连接温度1 133 K、保温时间30 min、接头压力0.040 MPa时,接头四点弯曲强度达到最大值170 MPa.     

Deposition and characterization of CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings prepared using reactive DC unbalanced magnetron sputtering

Nanocomposite coatings of CrN/Si₃N₄ and CrAlN/Si₃N₄ with varying silicon contents were synthesized using a reactive direct current (DC) unbalanced magnetron sputtering system. The Cr and CrAl targets were sputtered using a DC power supply and the Si target was sputtered using an asymmetric bipolar-pulsed DC power supply, in Ar + N₂ plasma. The coatings were approximately 1.5 μm thick and were characterized using X-ray diffraction (XRD), nanoindentation, X-ray photoelectron spectroscopy and atomic force microscopy. Both the CrN/Si₃N₄ and CrAlN/Si₃N₄ nanocomposite coatings exhibited cubic B1 NaCl structure in the XRD data, at low silicon contents (< 9 at.%). A maximum hardness and elastic modulus of 29 and 305 GPa, respectively were obtained from the nanoindentation data for CrN/Si₃N₄ nanocomposite coatings, at a silicon content of 7.5 at.%. (cf., 24 and 285 GPa, respectively for CrN). The hardness and elastic modulus decreased significantly with further increase in silicon content. CrAlN/Si₃N₄ nanocomposite coatings exhibited a hardness and elastic modulus of 32 and 305 GPa, respectively at a silicon content of 7.5 at.% (cf., 31 and 298 GPa, respectively for CrAlN). The thermal stability of the coatings was studied by heating the coatings in air for 30 min in the temperature range of 400-900 °C. The microstructural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data of the heat-treated coatings in air indicated that CrN/Si₃N₄ and CrAlN/Si₃N₄ nanocomposite coatings, with a silicon content of approximately 7.5 at.% were thermally stable up to 700 and 900 °C, respectively.     

Structure and mechanical properties of TiAlSiN/Si3N4 multilayer coatings

TiAlSiN/Si₃N₄ multilayer coatings which have different separate layer thicknesses of TiAlSiN or Si₃N₄ were deposited onto glass sheets, single-crystal silicon wafers and polished WC-Co substrates by reactive magnetron co-sputtering. The morphology, crystalline structure and thickness of the as-prepared multilayer coatings were characterized by TEM, SEM, XRD and film thickness measuring instrument. The mechanical properties of the coatings were evaluated by a nanoindenter. The effects of monolayer thickness on the microstructure and properties of TiAlSiN/Si₃N₄ multilayer coatings were explored. The coatings showed the highest hardness when the thickness of Si₃N₄ and TiAlSiN monolayers was 0.33 nm and 5.8 nm, respectively. The oxidation characteristics of the coatings were studied at temperatures ranging from 700 °C to 900 °C for oxidation time up to 20 h in air. It was found that the coatings displayed good oxidation resistance.     

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。     

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.     

Effects of pelletization of reactants and diluents on the combustion synthesis of Si3N4 powder

Reactants were pelletized, and then combustion synthesis of Si₃N₄ powder was achieved under the N₂ pressure of 3.0 MPa. Effects of the pelletization of reactants and Si₃N₄ diluents on the combustion process parameters and the characteristics of products were studied. The combustion mode of single reactant pellet was preliminarily discussed. The results indicated that the combustion reaction of single pellet was layer-by-layer from the surface to the core, which led to two peaks on the combustion temperature variation waves. With increasing the diluents content, the morphologies of Si₃N₄ particles changed from short rods into equiaxed grains, and residual Si in the final products obviously decreased. Single phase β-Si₃N₄ powder mainly contained equiaxed grains was prepared when 58 wt.% diluents was added.     

Influence of normal load and sliding speed on the tribological property of amorphous carbon nitride coatings sliding against Si3N4 balls in water

Amorphous carbon nitride (a-CN_x) coatings were deposited on Si₃N₄ disks by an ion beam assisted deposition system. The composition, structure and hardness of the a-CN_x coatings were characterized by Auger electronic spectroscopy, Raman spectroscopy and nano-indentation tester, respectively. The influences of normal load and sliding speed on the friction coefficients and the specific wear rates for the a-CN_x/Si₃N₄ tribo-pairs were investigated and analyzed synthetically by ball-on-disk tribometer. The worn surfaces were observed by optical microscope. The results showed that the a-CN_x coatings contained 12 at.% nitrogen, and their structure was a mixture of sp²and sp³ bonds. The a-CN_x coatings’ nanohardness was 29 GPa. The influence of sliding speed on the friction coefficients and the specific wear rate of the CN_x coatings was more obvious than that of normal load. The friction coefficients and the specific wear rate of the CN_x coatings decreased as the sliding speed increased. At a sliding speed higher than 0.1 m/s, the friction coefficients were less than 0.04. The specific wear rates of the a-CN_x coatings were higher than those of Si₃N₄ balls at a sliding speed below 0.1 m/s, while the specific wear rates of the a-CN_x coatings and the Si₃N₄ balls all fluctuated around a lower level of 10^− 8 mm³/Nm as the sliding speed increased beyond 0.2 m/s. To describe the wear behavior of a-CN_x coatings sliding against Si₃N₄ balls in water with normal loads of 3-15 N and sliding speeds of 0.05-0.5 m/s, the wear-mechanism map for the a-CN_x/Si₃N₄ tribo-pairs in water was developed.     

Preparation and properties of sintered molybdenum doped with La2O3/MoSi2

Sintered Mo with the addition of La₂O₃/MoSi₂ was prepared via the process of solid–solid doping + powder metallurgy. X-ray diffraction experiment, hardness test, three-point bending test and high-temperature tensile test were carried out to characterize the samples. The XRD pattern of a typical sample shows that the sintered Mo was mainly composed of Mo, La₂O₃ and Mo₅Si₃. Mo₅Si₃ was probably formed through the reaction between MoSi₂ and the Mo matrix. Densities and fracture toughnesses of both doped Mo and pure Mo were measured and contrasted. Sintered Mo with the addition of 0.2 wt% La₂O₃/MoSi₂ has the highest toughness, while more addition of La₂O₃/MoSi₂ has smaller effect on improving toughness or even embrittles Mo. The results of three-point bending test and high-temperature tensile test show that the bending strength and high-temperature tensile strength of doped Mo are both higher than those of pure Mo. The formation of Mo₅Si₃ improves the high-temperature strength. The La₂O₃/Mo₅Si₃ dispersed in the Mo matrix refined the grains, and thus strengthened the Mo matrix by dispersion strengthening and grain refinement.     

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.     

Preparation of Sr2Si5N8:Eu for white light-emitting diodes by multi-step heat treatment

In order to enhance luminescence properties of Eu²⁺-doped ternary nitride phosphor for white light-emitting diodes (LEDs), Sr₂Si₅N₈:Eu²⁺ phosphors with different Eu²⁺ concentrations were synthesized using a multi-step heat treatment. Impurities and luminescence properties of prepared Sr₂Si₅N₈:Eu²⁺ phosphors were investigated using X-ray diffraction (XRD) and photoluminescence spectroscopy. Excitation spectra of Sr₂Si₅N₈:Eu²⁺ phosphor showed broad excitation bands by both UV and blue light. Emission peak positions in spectra were red-shifted from 613 to 671 nm as Eu²⁺ ion concentrations increased. Phosphors following a multi-step heat treatment showed excellent luminescence properties. These included high emission intensity and very low thermal quenching compared to measurements using a commercially available YAG:Ce³⁺ phosphor.     

Corrosion of hot pressed Si3N4-TiN composite in sulphuric acid aqueous solution

The corrosion behaviour of an electroconductive Si₃N₄-35 vol% TiN composite, hot pressed with the addition of Al₂O₃ and Y₂O₃ as sintering aids, was studied in 1.8 M sulphuric acid aqueous solution at RT, 40 and 70 °C up to 400 h. The corrosion follows linear kinetics at RT and at 40 °C, involving only the progressive chemical dissolution of grain boundary phases, in the system Al-Y-Si-Ti-O-N. Chemical attack of TiN occurs at 70 °C, while Si₃N₄ is not affected by the selected corrosive environment.The effect of the corrosion on flexural strength, fracture toughness and electrical resistivity were investigated. Very high strength levels are maintained after corrosion for 400 h at room temperature, while the strength decreases of about 10% and 16% after the treatment at 40 and 70 °C, respectively. The electrical resistivity rises after corrosion at 40 and 70 °C, in line with the progressive chemical dissolution of the conductive TiN particles.     

Corrosion behavior of Si3N4-TiN composite in sulphuric acid

The corrosion behavior of a hot pressed Si₃N₄-TiN electroconductive composite, with Al₂O₃ and Y₂O₃ as sintering aids, was studied in 1.8 M H₂SO₄ aqueous solution at 70 and 90 °C up to 200 h in an open system. At 70 °C an outer corrosion zone with strong removal of the glassy grain boundary phase is developing on top of a zone of leaching. The outer zone exposes the formerly protected TiN-grains and results in an increasing level of Ti removal with time and overall nearly linear kinetics. At 90 °C the leaching of the network modifiers from the grain boundary phase dominates. A growing silica-rich residue inside the corrosion zone develops, while TiN exposure is developing very slowly: the overall kinetics is nearly parabolic. Thus simple extrapolation from lower to higher temperature behavior is not correct and at long times damage can even be lower at higher temperature.     

The oxidation of cobalt based alloys containing partially dissolved particles of Si3N4 at 1000°C

The isothermal oxidation behavior of Co-Cr and Co-Cr-Si alloys with and without 5, 10, and 15 vol.% dispersions of unstable Si₃N₄ particles was studied in 1 atm of oxygen at 1000°C. The dispersion of Si₃N₄ which dissolved partially in the matrix, greatly reduced the oxidation rate of Co-Cr alloys. Silicon nitride was found to promote the formation of continuous Cr₂O₃ layers at low chromium concentrations. Furthermore, the unstable Si₃N₄ was more effective in reducing the oxidation rate than an alloy containing an equivalent amount of silicon. Additions of 15vol.% Si₃N₄ tended to increase the oxidation rate by forming nonprotective SiO₂ particles which disrupted the protective Cr₂O₃ scale. The mechanism of oxidation was altered due to the Si₃N₄ additions. Marker experiments indicated oxygen diffusion inward via the CoO lattice, rather than a combination of both oxygen and metal ion motion which is encountered in Co-Cr alloys.     

TEM and DFT investigation of CVD TiN/κ-Al2O3 multilayer coatings

This paper investigates the interfacial structure in hot-wall CVD TiN/κ-Al₂O₃ multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the κ-Al₂O₃ layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/κ-Al₂O₃ coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/κ-Al₂O₃ coatings, are not present. However, the orientation relationships at the TiN/κ-Al₂O₃ interfaces are the same in both multilayers. The HREM investigations show that κ-Al₂O₃ (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, γ-Al₂O₃ can be grown, at least partly stabilized by the cube-on-cube orientation relationship between γ-Al₂O₃ and the underlying TiN. The DFT calculations show very similar adsorption strengths for an O monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fcc site. O adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for κ-Al₂O₃ on TiN(111) than on TiC(111) and a higher stability for κ-Al₂O₃ than for α-Al₂O₃ on both TiC and TiN. This indicates that the observed higher stability of κ-Al₂O₃ on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of κ-Al₂O₃ over α-Al₂O₃ can be partly attributed to the mismatch.     

Thermal plasma synthesis of SiC- Si3N4 composite powders from SiCH3CI3- NH3- H2

SiC-Si₃N₄ composite powders were synthesized by introducing trichloromethylsilane, ammonia, and hydrogen into a high-temperature radiofrequency (RF) thermal plasma argon gas. Powders were characterized by XRD, TEM, and FT-IR. Silicon carbide and silicon nitride were formed independently into separate powders. Silicon carbide was formed as β-SiC crystalline powder and silicon nitride was in an amorphous state. The crystalline SiC powders were in the size range of 75 to 200 nm and amorphous Si₃N₄ powders were 5 to 60 nm. When the mole ratio of ammonia to trichloromethylsilane was between 1 and 2, SiC-Si₃N₄ composite powders were formed, and when it was higher than 4, Si₃N₄ powders were formed.     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al₂O₃ composites were studied. The effect of TiC and Al₂O₃ starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δa). The mechanical properties and thermal fatigue resistance of TiC/Al₂O₃ composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al₂O₃ composites is due to a “true” cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

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.     

自反应电弧喷涂原位合成Ti(C,N)-TiB2-Al2O3复相陶瓷涂层

以Ti+B4C为反应药芯、Al为外皮材料制备反应型喷涂丝材,探讨利用自反应电弧喷涂技术在45钢基体表面制备复相陶瓷涂层的可行性。以X射线衍射仪(XRD)、扫描电子显微镜(SEM)和能谱仪(EDS)等方法分析、观察了涂层的组织与结构,测试了涂层的主要力学性能。结果表明:利用制备的药芯丝材进行喷涂试验,可获得由TiB2、TiB、TiC0.3N0.7、TiN、Al2O3、AlN等多相组成的复相陶瓷涂层。涂层呈典型的层状结构,其连续的基体相内弥散分布着离散的第二、第三相。涂层与基体间的结合强度为18.9MPa,涂层的平均显微硬度与弹性模量分别为735.4HV0.2和461.4GPa,摩擦因数在0.45～0.50之间,耐磨性能较基体材料提高3倍以上。