Microstructural control of green bodies prepared from Si-based multi-component non-aqueous slurries and their effects on fabrication of Si3N4 ceramics through post-reaction sintering

A Si-based slurry containing Si particles covered with Y₂O₃ and MgO nanoparticles (NPs) has been successfully prepared and then applied to shape Si-based green compacts for the fabrication of silicon nitride (Si₃N₄) ceramics via post-reaction sintering. It was found that Y₂O₃ and MgO NPs modified with polyethyleneimine-oleic acid complex (PEI-OA) could be effectively attached to Si particles by simple mixing in dense toluene slurry. Field emission scanning electron microscopy observations confirmed the attachment of PEI-OA-modified sintering aids to Si particles without forming large NP agglomerates. The adsorption of the PEI-OA-modified sintering aids and PEI-OA on the surface of Si particles drastically improved the stability of the Si-based toluene slurry, which was subsequently molded through wet vacuum casting and dewaxed to fabricate a Si-based green body. The obtained green body was nitrided at 1375?°C for 4?h at a N₂ pressure of 0.15?MPa and further sintered at 1850?°C for 2?h at a N₂ pressure of 0.9?MPa. The adsorption of sintering aid particles on the Si surface reduced the number of contact points between Si particles in the green body, which effectively suppressed the Si melting process during nitriding and improved the characteristics of the produced nitride body such as the degree of nitriding and α/(α+β) ratio of Si₃N₄, leading to the successful fabrication of high-density Si₃N₄ ceramics during the subsequent densification step.     

Si3N4-SiC-TiN陶瓷燃烧合成工艺研究

以TiSi2和SiC为原料,利用SHS方法合成Si3N4-SiC-TiN复相陶瓷.在不同稀释剂含量及氮气压力下进行燃烧合成,计算了毛坯转化率和产物相对密度,并对产物进行了XRD分析.结果表明,氮气压力增高有利于提高转化率及产物相对密度.反应物转化率随稀释剂含量增加而增大.孔隙率为53%(体积分数)毛坯,稀释剂SiC含量为35%(质量分数)压坯相对密度达到最大值,且当稀释剂含量高于35%(质量分数)时,SiC发生氮化反应,生成Si3N4和C.     

Effect of Si3N4 Addition on the Mechanical Properties, Microstructures, and Wear Resistance of Ti-6Al-4V Alloy

In order to improve the wear resistance of Ti-6Al-4V, different amounts of Si3N4 powder were added into the alloy powder and sintered at 1250℃. Porous titanium alloy with higher wear resistance was successfully fabricated. At sintering temperature, reaction took place and a new hard phase of Ti5Si3 formed. The mechanical properties of the fabricated alloys with different amounts of Si3N4 addition were investigated. The hardness of Ti-6Al-4V, which is the index of wear resistance, was increased by the addition of Si3N4. Amounts of Si3N4 addition have very significant influences on hardness and compressive strength. In present study,titanium alloy with 5 wt pct Si3N4 addition has 62% microhardness and 45% overall bulk hardness increase,respectively. In contrast, it has a 16.4% strength reduction. Wear resistance was evaluated by the weight loss during wear test. A new phase of Ti5Si3 was detected by electron probe microanalyzer （EPMA） and X-ray diffraction （XRD） method. The original Si3N4 decomposed during sintering and transformed into titanium silicide. Porous structure was achieved due to the sintering reaction.     

Investigation on tool wear process of milling wave-transmitting Si3N4 ceramics

Due to the low fracture toughness of wave-transmitting Si₃N₄ ceramics, the special material removal mechanism causes the tool wear to be different. The paper presents the tool wear forms and mechanism under different milling depth. The effect of tool wear on cutting force and machined surface morphology is discussed. Tests have been performed under typical conditions of cutting depth of 0.3 mm (in plastic-domain processing) and 0.4 mm (in brittle-domain processing). The results show that the abrasive wear caused by the chips is the main mechanism of the cutting edge wear and the flank wear, the increase of the side edge rear angle with tool wear is the main cause of the chipping phenomenon. The cutting depth is a significant influence parameter to the wear characteristics, and two types have been distinguished. As the material removal volume ascending, the cutting edge wear and the flank face wear has a stable period, and the root-mean-square deviation of processing surface increases to 1.6 μm, while that increase with the material removal volume continuously, and the processing surface decreases to 1.4 μm. It has been proved that the cutting force tends to increase first and then decrease as the material removal volume is about 4320 mm³.     

Influence of TiC additions on the oxidation behaviour of Si3N4-based ceramics

The oxidation behaviour in air of Si3N4 ceramics containing Y2O3 and Al2O3 as sintering aids, with and without a dispersion of TiC particles, has been studied between 1200 and 1400 °C. The influence of TiC additions on the oxidation kinetics is discussed in comparison with the results obtained for the Si3N4–Y2O3–Al2O3 reference material. However, in all cases and within the experimental temperature range, the oxidation kinetics were observed to be of a parabolic type. The microstructure of the oxide scales formed, which has been characterized by scanning and transmission electron microscopy, was observed to depend on the oxidation temperature and the initial composition of the material. © 1998 Chapman & Hall     

The effect of a small amount of impurity on the oxidation of Si3N4 ceramics

To investigate the effect of a small amount of impurity, especially Ca(CaO), on the oxidation of Si3N4 ceramics at high temperatures, two kinds of Si3N4 ceramics with different amounts of impurities were fabricated. The decrease in the strength of Si3N4 ceramics containing calcium as an impurity became severe when oxidation time was longer and temperature was higher. The decrease in the strength was caused by the formation of pits. It is estimated that pits are formed due to the high concentration of calcium in the oxidized layer near to the pits. This revised version was published online in November 2006 with corrections to the Cover Date.     

SiC_W含量对SiC_W/Si_3N_4复合陶瓷力学及高温微波性能影响

以α-Si_3N_4粉、β-SiC_W为原料,Al_2O_3、Y_2O_3为烧结助剂,采用凝胶注模工艺制备了SiC_W/Si_3N_4复合陶瓷材料,烧结温度为1 650℃,保温1.5h。研究了SiC_W加入含量对SiC_W/Si_3N_4复合陶瓷的微观结构、力学及常温/高温微波吸收性能的影响。结果表明:随着SiC_W含量的增加,SiC_W/Si_3N_4复合陶瓷的抗弯强度和断裂韧性都有先増后减的趋势,当含量为10wt%时,抗弯强度达到最大值505MPa,断裂韧性达9.515MPa·m1/2。常温介电常数在SiC_W含量为10wt%时,实部达最大值12,在12GHz最大吸收值为-21dB。高温介电常数随着SiC_W含量的增加有先增后减的趋势,在含量为10wt%时,实部达到最大值12.5。相比于纯Si_3N_4陶瓷,当SiC_W含量为10wt%时,SiC_W/Si_3N_4复合陶瓷在11.7GHz左右最大吸收可达-27dB,有效吸收频带(小于-5dB)为11.2~12.3GHz。     

Effect of grinding on the texture and reactivity of Si3N4

It has been concluded that the preliminary dry grinding of α-Si₃N₄ and β-Si₃N₄ in a planetary ball mill dramatically increases their reactivity towards the corrosion by oxygen and water at high temperature (1480°C). It has been shown that the reactivity of both phases with water vapour is considerably higher than that with oxygen. Moreover, α-Si₃N₄ has a corrosion resistance considerably higher than β-Si₃N₄ in spite of the fact that α-Si₃N₄ ground samples have a smaller crystal size and a higher concentration of microstrains than the ground β-Si₃N₄ samples. The different corrosion resistance of both silicon nitrides has been attributed to the difference in their crystalline structures, what would lead to different oxidation mechanisms. Amorphous silica is the product obtained from the oxidation of α-Si₃N₄ by oxygen or water, while α-cristobalite is the oxidation product obtained from the corrosion of β-Si₃N₄.     

Fabrication of 6061 aluminum alloy reinforced with Si3N4/n-Gr and its wear performance optimization using integrated RSM-GA

In this research work the dry sliding wear behavior of a hybrid aluminum metal matrix composite is evaluated. Al 6061 is used as a matrix material while Si₃N₄ and nanographite powder (3–15 wt%) are used as reinforcements. These two reinforcements (50 wt% of each) were blended in a high-energy ball mill for homogeneous mixing to derive the sound aluminum matrix composite (AMC). The hybrid composite is made by the stir casting route and its wear rate was investigated against an EN32 steel disc surface, using a pin-on-disc tribometer. Integrated response surface methodology (RSM) and genetic algorithm (GA) are used to optimize the pin-on-disc process parameters. Analysis of variance (ANOVA) shows that sliding distance plays a major role on the dry sliding wear rate followed by load, sliding speed and reinforcements. Two-factor interactions and quadratic terms have also significant contributions. GA suggested a minimum wear rate value of 0.827 mg at optimized setting. Microstructural analysis by scanning electron microscopy (SEM) reveals that very fine grooves are obtained at optimized settings while at other settings severe ploughing is observed. Transition of wear mechanism takes place with the increase of speed (i.e., temperature between the two rubbing surfaces) from abrasive to adhesive.     

Effect of sintering additives on the behaviour of SiC whisker-reinforced Si3N4 composites

The effects of sintering additives on the microstructural development, whisker stability, oxidation resistance and room-temperature mechanical properties of the SiC whisker-reinforced Si₃N₄ matrix composites were investigated. Seven different combinations of Y₂O₃ and Al₂O₃ were used as sintering additives. The composites containing 20 vol % SiC whiskers were densified by hot pressing. The microstructure of the resulting composites was characterized using X-ray diffraction, scanning and transmission electron microscopy. Oxidation testing of the composite at 1400 °C was conducted to investigate the relationship between matrix compositions and oxidation resistance. The flexural strength, fracture toughness and crack propagation patterns were also characterized and correlated with the microstructural features.     

Effect of interface chemistry on the mechanical properties of Si3N4-matrix composites

The effect of systematic modifications in the chemistry of the phase-boundary film on the macroscopic mechanical properties of Si₃N₄-matrix composites was investigated. Model composite materials, containing SiC or WC platelets, were prepared and only the bulk anion composition of the glassy-SiO₂ intergranular phase was varied by adding increasing amounts of fluorine to the material. Detailed material characterizations by high-resolution electron microscopy (HREM) and Raman spectroscopy on both undoped and F-doped composites allowed to derive a structural model of the phase-boundary film as well as to evaluate the average microscopic stresses acting on it. In addition, high-temperature internal friction measurements provided an estimate of the grain-boundary relaxation temperature as a function of the F content. Noticeable variations of both elastic modulus and fracture energy of the composite were detected upon F addition, which were related to a spontaneous process of phase-boundary microcracking upon cooling. A threshold of the F-content was found for microcrack formation and its existence is theoretically explained according to a percolation process of non-bridged SiO₄-tetrahedra, which arises from the incorporation of F into the intergranular film network.     

Effect of particulate additions on the contact damage resistance of hot-pressed Si3N4

Silicon nitride matrix composites containing particles of SiC, TiC, and BN were fabricated and tested for improved contact damage resistance at 900° C in an oxidizing atmosphere. Contact damage resistance was characterized with profilometry, scanning electron microscopy, bend tests, and coefficient of friction measurements. The results of these tests indicated that composites containing TiC particles showed dramatically improved friction and wear behaviour compared to the SiC- and BN-containing composites, as well as the monolithic Si₃N₄. Auger spectroscopy indicated that the improved behaviour was due to the formation of a lubricious oxide containing both titanium and silicon on the surface of the composite and the transfer of some of the oxide to the rider.     

Effect of starting PMMA content on microstructure and properties of gel casting BN/Si3N4 ceramics with spherical-shaped pore structures

By gel casting with polymethylmethacrylate microbeads (PMMA) as pore-forming agent, porous boron nitride/silicon nitride (BN/Si₃N₄) composite ceramics were successfully prepared. The obtained ceramic shows bimodal hierarchical structures that composed of spherical-shaped micro pores depending on PMMA content and irregular sub-micro pores formed by the stacking of ceramic particles. Porosity of the porous BN/Si₃N₄ ceramics can be well controlled from 53.0 to 60.6 % by the PMMA content from 10 to 40 wt%, as well as the mechanical and dielectric properties. Effect of PMMA content on phase composition and the relationship between microstructure and the basic properties of the porous BN/Si₃N₄ ceramics was discussed in detail. Microstructure analysis reveals that the sub-micro pores acted as channels between micro pores. BN particles have a relatively denser distribution on the wall of spherical-shaped micro pores with a window between micro and sub-micro pores, and resulting in a half-closed micro pore structure, which is meaningful for material design with concentration of BN particles on the wall of pore structure.     

氮化硅针晶的分离研

结合氮化硅浆料的悬浮性能,提出了液相分离针晶的新方法。测出了粉体的zeta电位。探讨了pH值和分散剂的含量对浆料的悬浮性能的影响,并对浆料做了沉降分析。结果表明,当DarvanC的含量为0.15wt%,pH值为10时,浆料的性能满足分离要求。SEM显示,分离后,针晶中已经基本没有球状颗粒。     

粉料粒度对氮化硅陶瓷性能的影响

用球磨的方式将平均粒径为6.46μm的β-氮化硅原料粉细化为不同粒度的亚微米级起始粉料,加入适宜的添加剂,以无压液相烧结的方法研究起始粉料粒度对氮化硅陶瓷的强度、硬度等机械性能的影响。用扫描电镜、X射线衍射等方法测试分析试样的显微结构、相组成等。结果表明,试样密度、强度、硬度等均随起始粉料粒度的减小而增大;试样显微结构显示了烧结体晶粒尺寸随起始粉料粒度的减小而减小,且粒度小的试样烧结的晶粒均匀、致密;烧结后试样的物相主要是β-氮化硅和少量黄长石,粉料的粒度对烧结后样品的物相没有影响。     

稀释剂量对反应烧结Si_3N_4陶瓷结构和性能的影响

以工业Si粉为原料,α-Si3N4粉为稀释剂,聚乙烯醇为粘结剂,采用反应烧结工艺制备了Si3N4陶瓷。研究了稀释剂量对反应烧结Si3N4陶瓷的体积密度、开气孔率、相组成、微结构、弯曲强度和抗热震性的影响。结果表明,随稀释剂量的增加,Si3N4陶瓷的体积密度从2.27g/cm3降至2.04g/cm3,开气孔率从23%升至33.8%。Si3N4陶瓷由α-Si3N4、β-Si3N4和少量单质Si组成。Si3N4主要以针状晶形式存在,残留Si呈不规则块体。随着稀释剂量的增加,4组Si3N4陶瓷的三点抗弯强度分别为119MPa、112MPa、146MPa和113MPa;经50次800℃至室温空冷热震后,其强度保持率分别为81.5%、90.2%、87%和88.5%,表现出较好的抗热震性。     

hBN含量对Si3N4-hBN复相陶瓷性能和微观结构的影响

利用纳米级粉体经热压烧结制备了Si₃N₄-hBN复相陶瓷, 研究了hBN含量对Si₃N₄-hBN复相陶瓷致密度、力学性能、摩擦学性能、微观结构的影响。用阿基米德排水法、三点弯曲法和维氏压痕法测量材料的致密度、力学性能; 用摩擦磨损试验机测试材料的摩擦学性能; 用XRD、EDAX和SEM、LSCM分析观察材料的物相组成和微观结构。研究结果表明, 随着hBN含量的增加, 复相陶瓷的密度将会持续下降, 气孔率先是急剧上升, 然后趋于平缓, 力学性能持续下降, 干摩擦条件下复相陶瓷与GCr15配副的摩擦学性能呈现先提高后降低的趋势, 当hBN含量低于20wt%时, 随着hBN含量的增加, 摩擦系数和磨损率逐渐减小; 当hBN含量大于20wt%时, 摩擦系数和磨损率急剧增大; hBN含量为20wt%时, 获得最低的摩擦系数为0.31。hBN的引入直接影响Si₃N₄-hBN复相陶瓷的微观组织结构, 进而影响复合陶瓷的力学性能和摩擦学性能。     

钎缝厚度对陶瓷连接接头强度的影响

采用不同厚度的Ag-Cu-Ti钎料连接Si3N4陶瓷材料,研究了钎缝厚度对连接强度的影响规律,利用扫描电镜观察了接头微观结构.试验发现,在一定钎缝厚度范围内,随着钎缝厚度的增加,接头强度随之提高.研究表明:钎缝厚度影响连接强度的主要原因在于对残余应力的缓解,但钎缝厚度过厚时,对应力的缓解作用变化不大,而钎缝的拘束作用减弱,接头强度有下降的趋势;在相同连接规范下,钎缝厚度与反应层厚度之间也存在一定的关系.