Fabrication and flexural strength of porous Si3N4 ceramics with Li2CO3 and Y2O3 as sintering additives

By employing sintering additives of Li2CO3 and Y2O3,porous Si3N4 ceramics are prepared after experiencing the processes of sintering and post-vacuum heat treatment at 1680 and 1550°C,respectively.The experimental results demonstrate the completed phase transformation fromαtoβ-Si3N4 in Si3N4 ceramic samples with a amount of 1.60 wt%Li2CO3(0.65 wt%Li2O)and 0.33 wt%Y2O3 additives.The as-synthesized porous Si3N4 ceramics exhibit high flexural strength((126.7±2.7)MPa)and high open porosity of 50.4%at elevated temperature(1200°C).These results are attributed to the significant role of added Li2CO3 as sintering additive,where the volatilization of intergranular glassy phase occurs during sintering process.Therefore,porous Si3N4 ceramics with desired mechanical property prepared by altering the addition of sintering additives demonstrate their great potential as a promising candidate for high temperature applications.

Study on Microstructures of Micron Si_3N_4 Composite Ceramic with Addition of Nano SiC and SiC Whisker

The microstructures and crack propagating characteristic of Si 3N 4 (μ)/SiC (n, w) composite ceramic were studied with AEM. The Si 3N 4 (μ)/SiC (n, w) composite ceramic consists of β Si 3N 4, β SiC, a small amount of α Si 3N 4 and α SiC, and amorphous phase. Most of Si 3N 4 grains were equiaxed crystal and also there were some bulky columnar ones. Most of SiC particles and SiC whiskers distributed at the Si 3N 4 grain boundaries and a few of smaller SiC particles in the Si 3N 4 grains. Most of amorphous structure was in the junction of several Si 3N 4 grains and thin amorphous layer was observed only at a few of Si 3N 4 boundaries. Failured cracks propagated mainly along the boundaries of the Si 3N 4 grains and partially passed through Si 3N 4 grains. The path of crack propagating might change, branching and twisting of the cracks might occur when the expanding crack meet the SiC particle and/or SiC whisker. Effect of the microstructure on strength and toughness of the composite ceramic was briefly discussed.

Effect of Y_2O_3, CeO_2 on Sintering Properties of Si_3N_4 Ceramics

The effect of rare earth oxides Y 2O 3 or CeO 2 on sintering properties of Si 3N 4 ceramics was studied and the mechanism of assisting action during sintering was analyzed. The results indicate that the best sintering properties appear in Si 3N 4 ceramics with 5% Y 2O 3 or 8% CeO 2. Secondary crystallites are formed at grain boundaries after heat treatment, which decreases the amount of glass phase and contributes to the improvement of high temperature mechanical properties of silicon nitride.

Effect of Metallurgical Behaviour at the Interface between Ceramic and Interlayer on the Si_3N_4/1.25Cr-0.5Mo Steel Joint Strength

By using newly developed CuNi5~25Ti16~28 B rapldly solidifled brazing filler the joining of Si3 N4/1.25Cr-0.5Mo steel has been carried out with interlayer method. If employing the interlayer structure of steel (0.2 mm)/W (2.0 mm)/Ni(0.2 mm), the joint strength can be increased greatly compared with employing that of Ni/W/Ni, and the three point bend strength of the Joint shows the value of 261 MPa. The metallurgical behaviour at the interface between Si3N4 and the interlayer has been studied. It is found that Fe participated in the interfacial reactions between Si3N4 and the brazing filler at the Si3N4/steel (0.2 mm) interface and the compound Fe5Si3 was produced. However, since the reactions of Fe with the active Ti are weaker than those of Ni with Ti, the normal inter facial reactions were still assured at the interface of Si3N4/steel (0.2 mm) instead of Si3N4/Ni (0.2 mm), resulting in the improvement of the joint strength. The mechanism of the formation of Fe5Si3 is also discussed. Finally, some ideas to further ameliorate and simplify the interlayer structure are put forward.

Effect of the Fourth Element on Bonding of Silicon Nitride Ceramics with Y_2O_3-Al_2O_3-SiO_2 Glass Solders

Bonding of Si 3N 4 ceramic was performed with Y 2O 3 Al 2O 3 SiO 2(YAS) X glass solders,which were mixed with TiO 2 (YT) and Si 3N 4 (YN), respectively. The effects of bonding conditions and interfacial reaction on the joint strength were studied. The joint strength in different bonding conditions was measured by four point bending tests. The interfacial microstructures were observed and analyzed by SEM, EPMA and XRD. It is shown that with the increase of bonding temperature and holding time, the joint strength increases reaching a peak, and then decreases. When TiO 2 is put into YAS solder,the bonding interface with Si 3N 4/(Y Sialon glass+TiN)/TiN/Y Sialon glass is formed. When YAS solder is mixed with Si 3N 4 powder, the interfacial residual thermal stress may be decreased, and then the joint strength is enhanced. According to microanalyses, the bonding strength is related to interfacial reaction.

Moisture-proof and Enhanced Effect of Inorganic Coating on Porous Si_3N_4 Ceramic

Inorganic coating was fabricated on the surface of the porous Si3N4 ceramic by polymer derived（PD） and spraying technology, via using vinyl-polysilazane（PSN-1） as a preceramic polymer and Si3N4 and lithium aluminosilicate（LAS） powders as fillers. The phase and microstructure of the coatings were analyzed by X-ray diffraction（XRD） analysis and scanning electron microscopy（SEM）, respectively. The effect of the coatings on mechanical property and humidity resistance of the porous Si3N4 ceramic was investigated. The experimental results showed that we successfully fabricated the uniform and dense coating which preferably combined with the substrate upon the addition of fillers. The bending strength of the porous Si3N4 ceramic sprayed the coating increased by more than 18%, and the surface hardness increased by 1.7 times. The apparent porosity of the materials reduced by an average of 97.7%, and water absorption was below 0.5%. Therefore, the prepared coating with preferable density had an obviously moisture-proof and enhanced effect on the porous Si3N4 ceramic.

Evaluation of the Properties of Si3N4/Si3N4 Joint Brazed Using a Filler Alloy Containing Pd

Si3N4 ceramic was jointed to itself using a filler alloy of Cu76.5Pd8.5Ti15, and the mechanical properties of the jointwere measured and analyzed. By using a filler alloy of Cu76.5Pd8.5Ti15, the SisN4/SisN4 joints were obtained bybrazing at 1373～1473 K for 0.9～5.4 ks under a pressure of 2×10-3 MPa. The effect of brazing parameters on theshear strength of the joint was investigated. When the brazing temperature and holding time is 1423 K and 5.4 ksrespectively, the maximum shear strength of the Si3N4/Si3N4 joint is obtained to be 198 MPa.

Bonding of Silicon Nitride Ceramic Composite with RE_2O_3Al_2O_3SiO_2 Glass Solders

Liquid bonding of Si 3N 4 ceramic composite was carried out with RE 2O 3 Al 2O 3 SiO 2 glass solders. The effect of bonding conditions and interfacial reaction on the joint strength was studied. The joint strength under different bonding conditions was measured by four point bending tests. The interfacial microstructures were observed and analyzed by SEM, EPMA and XRD. It is shown that the liquid glass solders react with Si 3N 4 at interface, forming the Si 3N 4/Si 2N 2O/Y(La) sialon glass/Y(La) sialon glass gradient interface. With the increase of bonding temperature and holding time, the joint strength first increased reaching a peak, and then decreased. According to microanalyses, LaYO 3 precipitated from joint glass improves joint strength at room and high temperature.

OBSERVATION ON THE MICROSTRUCTURE OF SINTERED Si_3N_MgO-CeO_2 CERAMICS

The microstructure of the pressureless sin-tered Si3N4 ceramics with MgO-CeO2 has been studied by TEM. The glassy phase is observed and confirmed directly by microdiffraction. EDAX analysis results show that the main function of the CeO2 lies in the glass phase which hardly contains any MgO. The cerium silicate galssy phase is good to wet Si3N4 and MgO-CeO2 is a most effective sintering aid for Si3N4. Excessive grain growth occurs at above 1850℃, which is harmful to the mechanical properties. Mi-crocracks and dislocations are observed in the excessive large grains.

Tribological performances of Si_3N_4 ceramic in vacuum

The tribologieal performances between Si3N4 ceramic balls and GCr15 steel disks without lubrication both in air and in vacuum （6 × 10^-3Pa） are investigated. The results show that the friction coefficient and wear in vacuum are smaller than those in air. The friction mainly occurs between steel and steel in air because the steel is obvious to adhere on the ceramic surface. The ceramic and steel are hard to adhere in vacuum. The function of mechanical plough and the rotation of small wear particles in the contact region reduce the friction and wear between the ceramic and steel in vacuum.

Research Progress of Interfacial Chemism of High Efficiency Organic Grinding Fluid for Si_3N_4 Ceramics

Grinding is a most important machining method for Si3N4 ceramics. Utilizing interracial chemistry reaction membrane between grinding fluid and Si3N4 ceramics can reduce friction factor, soften surface layer and meanwhile improve the grinding efficiency, which is a bran-new research direction. This article, based on high efficiency grinding of Si3N4 ceramics by the way of half plasticity removal, descanted on the assistant function of interface chemistry reaction to improve the removal rate of Si3N4 ceramics in the application of organic grinding fluids represented by alcohols grinding fluid. To target action mechanism research, it applies the methods of classification, comparison and induction, and advanced test equipments to explore the effects of long, short carbochain alcohol and their water solutions acting as grinding fluids. In addition, it also discusses the effective function of three groups of organic matters such as perhalogeno hydrocarbon, cationic surfactant and tetra ethoxysilane acting as grinding fluid components from different angles, reveals their mechanisms of action, and supplies theoretical basis for the development of machining ceramic grinding fluid of high efficiency, low cost.

Strength and interfacial microstructure of Si3N4 joint brazed with amorphous Ti-Zr-Ni-Cu filler metal

In this paper, the vacuum brazing of Si3N4 ceramic was carried out with Ti40Zr25Ni15Cu20 amorphous filler metal. The interfacial microstructure was investigated by scanning electron microscopy （ SEM ）, energy dispersive spectroscopy （EDS） etc. According to the analysis, the interface reaction layer was mode up of TiN abut on the ceramic and the Ti-Si, Zr-Si compounds. The influence of brazing temperature and holding time on the joint strength was also studied. The results shows that the joint strength first increased and then decreased with the increasing of holding time and brazing temperature. The joint strength was significantly affected by the thickness of the reaction layer. Under the same experimental conditions, the joint brazed with amorphous filler metal exhibits much higher strength compared with the one brazed with crystalline filler metal with the same composition. To achieve higher joint strength at relatively low temperature, it is favorable to use the amorphous filler metal than the crystalline filler metal.

Solid-liquid state pressure bonding of Si_3N_4 ceramics with aluminum based alloys and its mechanism

Solid liquid state pressure bonding of Si 3N 4 ceramics with aluminum based alloys, which contain a small amount of intermetallic compounds Al 3Ti or Al 3Zr, was investigated. With this new method, the heat resistant properties of the bonding zone metal are improved, and the joints’ strengths at high temperature is increased. The joints’ shear strength at room temperature and at 600 ℃ reach 126～133 MPa and 32～34 MPa, respectively, with suitable bonding pressure. The reaction between aluminum and Si 3N 4 ceramics, which produces Al Si N O type compounds is the dominant interfacial reaction, while the reactions between the second active element Ti or Zr in the aluminum based alloys and Si 3N 4 ceramics also occur to some extend. [

压力和温度对等离子活化烧结Si3N4陶瓷致密化及相变的影响

以Mg2Si为烧结助剂,采用等离子活化烧结制备了Si3N4陶瓷,研究了烧结过程中压力、烧结温度对Si3N4陶瓷致密化以及相变过程的影响。结果表明:适当地提高烧结压力有助于致密化以及相转变,其原因在于外加压力有助于粉体颗粒的重新排列,以及液相在空隙的填充,明显提升相转变程度以及缩短致密化温度。而压力超过了50MPa之后由于致密化温度提前使得液相与粉体颗粒的接触面积减小,导致相转变过程受到了一定程度的阻碍。另外温度对于Si3N4陶瓷的致密化以及相转变也有着重要的影响,30MPa压力下至少需要1400℃以上温度才能使得Si3N4陶瓷烧结致密以及相转变发生。

Si-C-N_(np)/Si_3N_4复合材料的室温和高温显微结构与力学性能

以 Si- C- N纳米微粉为增强相 ,Si3N4 为基相 ,采用热压的方法制备了 Si Cp/ Si3N4 纳米复相陶瓷。应用扫描电镜 (SEM)、高分辨透射电镜 (HRTEM)对其结构进行了观察 ,并讨论了结构与性能之间的关系。结果表明 ,所得的 Si Cp/ Si3N4 复相陶瓷的室温力学性能比氮化硅单相陶瓷有较大的提高 ,而 135 0℃断裂韧性达 14 .6 6 MPa· m1 / 2 。Si C微晶在晶粒内和在晶界玻璃相内的钉扎作用是材料高温性能提高的主要因素。

新型Si_3N_4/TiC_(nano)复合陶瓷材料的制备及性能

本文利用热压烧结方式制备了新型Si3N4/TiCnano复合陶瓷材料,对材料的配方及烧结工艺进行了优化,采用XRD、SEM等手段分析了材料的微观结构,研究了烧结条件对材料性能的影响机理。结果表明,所研究的材料具有良好的力学性能。

INTERFACIAL REACTION BEHAVIOUR AND JOINT STRENGTH OF TiC_p/Si_3N_4 COMPOSITE BONDED WITH Al FOIL

１ＩＮＴＲＯＤＵＣＴＩＯＮＴｉＣｐ／Ｓｉ３Ｎ４ｃｏｍｐｏｓｉｔｅｃｅｒａｍｉｃｓｈａｖｅｅｘｃｅｌｌｅｎｔｐｈｙｓｉｃａｌ，ｃｈｅｍｉｃａｌａｎｄｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｉｅｓ，ａｎｄｔｈｅｙａｒｅａｐｐｌｉｅｄｗｉｄｅｌｙｉｎｓｅｖｅ...

Mechanical and dielectric properties of porous and wave-transparent Si3N4–Si3N4 composite ceramics fabricated by 3D printing combined with chemical vapor infiltration

Porous Si3N4–Si3N4 composite ceramics were fabricated by 3D printing combined with low-pressure chemical vapor infiltration(CVI).This technique could effectively improve the designability of porous Si3N4 ceramics and optimize the mechanical and dielectric properties.The effects of process parameters including the deposition time and heat treatment on the microstructure and properties of porous Si3N4–Si3N4 composite ceramics were studied.The study highlights following:When CVI processing time was increased from 0 to 12 h,the porosity decreased from68.65%to 26.07%and the density increased from 0.99 to 2.02 g/cm3.At the same time,the dielectric constant gradually increased from 1.72 to 3.60;however,the dielectric loss always remained less than0.01,indicating the excellent electromagnetic(EM)wave-transparent performance of porous Si3N4–Si3N4 composite ceramics.The maximum flexural strength of 47±2 MPa was achieved when the deposition time attained 6 h.After heat treatment,the porosity increased from 26.07%to 36.02%and the dielectric constant got a slight increase from 3.60 to 3.70 with the dielectric loss still maintaining lower than 0.01.It has been demonstrated that the porous Si3N4–Si3N4 composite ceramics are a promising structural and EM wave-transparent material suitable for high temperature service.

用Y_2O_3Al_2O_3SiO_2Si_3N_4钎料连接氮化硅复相陶瓷

研究了用Ｙ２Ｏ３Ａｌ２Ｏ３ＳｉＯ２Ｓｉ３Ｎ４ 钎料对氮化硅复相陶瓷的连接． 对连接界面进行ＳＥＭ、ＥＰＭＡ和ＸＲＤ分析．接头强度随着保温时间、连接温度的增加而逐渐增加． 在达到峰值后，连接强度逐渐降低．在ＹＡＳ钎料中添加氮化硅， 可以降低接头界面的热应力，改善接头强度． 微观分析表明：接头强度的变化主要与界面反应和界面孔洞损伤有关．

Solid-Liquid State Bonding of Si3N4 Ceramics with Ceramic-Modified Brazing Alloy

Solid-liquid state bonding of Si3N4 ceramics with TiN-modified Ag-Cu-Ti brazing alloy was used'- to enhance joint strength. The effects of the TiN particles on the microstructures, interfacial reactions, and room-temperature properties of the joints were investigated. The results show that the TiN particles are gen- erally well dispersed in the Ag-Cu eutectic base and the interface between them is both clean and com-pact. Changes in the TiN volume fractions from 0 to 20% exert no noticeable effect on the interfacial reac-tion between Ag-Cu-Ti and the substrates. Other bonding parameters being constant, the TiN volume frac-tion in the filler material plays a key role in the joint properties. For TiN volume fractions below 20%, the joints are reinforced, especially joints with 5% and 20% TiN. The average shearing strength of joints with 5% TiN is 200.8 MPa, 30% higher than that of joints with no TiN (154.1 MPa). However, for TiN volumes frac- tions above 20%, the joint strengths decrease.