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.

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. [

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.

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.

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.

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.

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.

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.

Joining dense Si3N4 to porous Si3N4 by using an anorthite based glass-ceramic

This study focuses on the design,characterization and testing of a novel anorthite based glass-ceramic solder for joining dense Si3N4 and porous Si3N4 ceramic.It was found that the glass solder possesses a favorable wetting ability on two ceramic surfaces.Additionally,the influence of crystallization treatment on the microstructure and shear strength of joints was investigated.The results showed that CaAl2 Si2O8 was the main product in the seam.After crystallization treatment,the crystallinitiy of the seam and shear strength of joints were improved obviously.The shear strength of joints after crystallization was 52 MPa at room temperature,and it increased to 59 MPa at 850℃.Finally,the joining mechanism of the dense Si3N4/glass-ceramic/porous Si3N4 bonded joints was proposed.Intergranular diffusion of elements was identified as the main mechanism of bonding in the interface of dense Si3N4/glass-ceramic.In the porous Si3N4 side,the bonding was achieved by infiltration that was controlled by capillarity and resistance of the viscous forces.

ROLLING CONTACT FATIGUE BEHAVIOR OF CERAMIC BALLS LUBRICATED BY LUBRICANTS WITH EXTREME PRESSURE ADDITIVES

An experiment is conducted to investigate the effects of lubricant10#, which contains extreme pressure additives T304 and T305, on the rolling contact fatigue （RCF） life of the contact pairs of a Si3N4 ceramic ball and a steel rod. The experimental investigation is carried out using a ball-rod RCF test rig. The results show that the extreme pressure additives increase the anti-contact-fatigue performance of ceramic balls; When the content of the additives varies from 1% to 5%, the increasing gradient of the RCF life curve decreases; And the oil sample with 1% T305 additive corresponds to the maximal gradient of the RCF life curve, with the RCF life being increased by about 10.77 times. The fatigue surface of the ceramic ball is analyzed with scanning electron microscope （SEM） and X-ray electron dispersion analysis（EDAX）, and the physical model of extreme pressure additives＇ increasing the RCF life of the ceramic ball is proposed. It is found that the extreme pressure additives form a corrosive film and a transfer film on the surface of the ceramic ball, which decrease the surface tangential stress, and to increase the surface energy is the most effective means for increasing the RCF life.

Effect of geometric parameters at open turbine combined structure on flow field distribution of dry granulation for Si_(3)N_(4) powder

To improve the uniformity of the flow field and the poor axial velocity in the chamber of Si3N4 dry granulation, the influence of geometric parameters at open turbinecombined structure on the flow field distribution is studied. The Euler–Euler gas-solidtwo-phase flow model is established and the physical model of dry granulation chamberunder the combined structure is simplified. Under the same radial structure, the volumedistribution and velocity field of Si3N4 particles in the granulation chamber with a different number and angle of the axial structure at the open turbine are analyzed by theCFD method. The influence of the axial structure at the open turbine on the flow fielddistribution of Si3N4 particles under different geometric parameters is compared. Theresults show that the axial structure of the open turbine in the granulation chamber isthe most uniform when the number of blades is 6 and the inclination angle is 45◦, andthe circulating flow of the upper and lower parts of Si3N4 powder is strong.