Elastic–plastic analysis of ultrafine-grained Si2N2O–Si3N4 composites by nanoindentation and finite element simulation

Ultrafine-grained Si₂N₂O–Si₃N₄ composites are fabricated by hot-press sintering of amorphous nano-sized silicon nitride powders at 1600, 1650, and 1700 °C with nano-sized Al₂O₃ and Y₂O₃ as additives. Sintered materials of increasing average grain sizes of 280, 360, and 480 nm were obtained with increasing sintering temperature. Hardness and elastic modulus are tested by nanoindentation. Finite element simulations of the nanoindentation are performed to study the elastic and plastic mechanical properties based on the elastic modulus and P–h curve that were obtained through the nanoindentation tests. A theoretical method is proposed for calculating the stress–strain relationship of brittle ceramic materials based on the experimental nanoindentation data. Several relevant coefficients in the theoretical calculation formula are determined by comparing the calculation and simulation results.     

B4C对Si3N4/Si2N2O结合SiC材料抗热震性能的影响

首先以Si粉、SiO₂微粉为原料，先在700℃空气气氛处理，然后在1 400℃氮气气氛下合成Si₂N₂O,研究了B₄C添加量(外加质量分数分别为0、1.0%、2.0%、3.0%、4.0%)对Si₂N₂O合成效果的影响。然后根据B₄C最优加入量，先在700℃空气气氛保温5 h,然后在1 400℃氮气气氛保温5 h制备了Si₃N₄/Si₂N₂O结合SiC试样。采用1 300℃风冷5次后试样的抗折强度保持率评价其抗热震性，分析了热震前后试样的物相组成和显微结构。结果表明：1)合成Si₂N₂O的B₄C最优添加量为3%(w);在700℃空气处理时，B₄C优先和气氛中O₂反应生成液相B₂O₃,为1 400℃氮...     

Si2N2O标准生成吉布斯自由能的测定

用两种方法测量了Si2N2O的标准生成吉布斯自由能ΔfGθSi2N2O:其一是在0.1 MPa氮气压力条件下,在1 073～1 869 K范围内,利用Y2O3稳定ZrO2固体电解质,以Cr和Cr2O3为参比电极,用极化法测试;其二是在1 673和1 773 K下通过Ni-NiO平衡控制氧分压法测试。结果表明,两者吻合很好,其表达式可写为:ΔfGθSi2N2O=-960.627+0.292T±6.2(kJ·mol-1),其中1 073 K≤T≤1 869 K。     

Si3N4粉粒度对振荡压力烧结Si3N4陶瓷的影响

为了提高Si₃N₄陶瓷的烧结致密度,采用振荡压力烧结工艺分别在1 745和1 775℃制备了Si₃N₄陶瓷,主要研究了Si₃N₄粉的粒度（平均粒径分别为0.4、2.0、2.3μm）对Si₃N₄陶瓷的显微结构和性能的影响。结果显示：1)在两种温度的振荡压力烧结工艺下,由三种不同粒度的Si₃N₄粉制备的Si₃N₄陶瓷的相对密度都很大,为99.65%～99.86%,彼此相差很小。2)由平均粒径为0.2μm的Si₃N₄粉在1 745℃烧结制备的试样的微观结构最均匀,其β-Si₃N₄晶粒平均长径比、抗弯强度和维氏硬度均最大,分别达到5.0、（1 364±65） MPa和（15.72±0.8） GPa;由平均粒径为2.3μm的Si₃     

Sn/Si3N4复合镀层中纳米Si3N4含量测定方法的比较

采用电熔解法溶解Sn/Si3N4复合镀层,以紫外可见分光光度法测定了镀层中纳米Si3N4的含量,并与传统酸溶解及重量法对比。结果表明,电解法在10 min内使镀层完全溶解且在测定波长448 nm处纳米Si3N4溶液的吸光度值与其浓度在10～200 mg/L范围保持良好的线性关系,该方法简单,省时,误差小,回收率98.5%～100%。     

Si3N4-TiN陶瓷氧化模型

氮化硅-氮化钛陶瓷的氧化行为是建立在对其微观结构的观察,对这种氧化模型现象进行了描述,并对其氧化动力学模型进行研究。当温度<1 000℃,只有TiN相被氧化。此氧化过程是由氧气通过TiO2扩散来控制的,由氧化动力学抛物线方程来描述。超过1 000℃的过程是非常复杂的,那是因为同时发生Si3N4和TiN相的两种氧化反应。三种氧化模型都被清晰的扩散机理所控制,是接连发生的。第1步,Si3N4和TiN相被独立的氧化,它们分别被氧化为SiO2和TiO2相。Si3N4的氧化是由氧通过SiO2扩散控制的,而TiN的氧化由钛通过TiO2扩散所控制。第2步,TiN的氧化被氧通过TiO2扩散控制,而通过SiO2被氧化形成Si3N4。第3步,TiN和Si3N4的氧化由氧通过二氧化硅层扩散控制。动力学方程可以由这三种氧化模型的任意一种决定。     

Microstructure and reaction phases in Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was self-jointed using a filler alloy of Cu–Pd–Ti, and the microstructure of the joint was analyzed. By using a filler alloy of Cu76.5Pd8.5Ti15 (at.%), a high quality Si₃N₄/Si₃N₄ joint was obtained by brazing at 1100–1200 °C for 30 min under a pressure of 2 × 10⁻³ MPa. The microstructure of the Si₃N₄/Si₃N₄ joint which was observed by EPMA, XRD and TEM, and the results indicated that a reaction layer of TiN existed at the interface between Si₃N₄ ceramic and filler alloy. The center of the joint was Cu base solid solution containing Pd, and some reaction phases of TiN, PdTiSi and Pd₂Si found in the Cu [Pd] solid solution.     

Joining of Si3N4 to Si3N4 using a AuPd(Co,Ni)–V filler alloy and the interfacial reactions

Au38.0–Pd28.0–Co18.0–Ni7.0–V9.0 (in wt%) alloy was designed as a filler for joining Si₃N₄. The filler alloy showed a contact angle of 77.2° on Si₃N₄ ceramic at 1473 K. The Si₃N₄/Si₃N₄ joint brazed with the rapidly-solidified filler foils at 1443 K for 10 min exhibits an average three-point bend strength of 320.7 MPa at room temperature and the strength values are 217.9 MPa and 102.9 MPa at 1073 K and 1173 K respectively. The interfacial reaction products were composed of V₂N and Pd₂Si, and the elements Co and Ni in the brazing alloy did not participate in the interfacial reactions. The coarse-network-like distribution of refractory Pd₂Si compound within the Au–Pd–Co–Ni alloy matrix throughout the joint contributes to the stable high-temperature joint strengths.     

凝胶注模成型制备Si3N4-SiC材料

以SiC颗粒(3.36～0.15 mm)、SiC细粉(≤19μm)、改性Si粉(≤26μm)、改性Al粉、α-Si3N4粉为原料制成固相体积分数为75%的悬浮体,采用凝胶注模成型工艺和氮化烧成工艺制备Si3N4-SiC材料,并研究了SiC系悬浮体的流变特性以及氮化烧成后Si3N4-SiC材料的性能和显微结构。结果表明:1)悬浮体在切变速率10～160 s-1范围内表现出剪切变稀的特征,满足凝胶注模成型的要求;其流变特征符合Sisko模型,流变方程为η=4.625 4+2 172.9γ-0.450 3,拟合流变曲线与试验流变曲线基本吻合。2)脱模后湿坯的抗折强度为16 MPa,体积密度为2.57 g.cm-3;干燥后坯体结构致密,SiC大颗粒被Si粉和SiC粉均匀包裹。3)氮化烧成后试样的Si3N4含量、常温抗折强度和高温抗折强度均高于非凝胶注模成型的工业产品;Si3N4晶体发育完整,形成了Si3N4晶体紧密环绕SiC颗粒的均匀结构。     

Si3N4粉体表面化学分析及表面改性

Si3N4陶瓷是一种重要的结构陶瓷,本文着重阐述了Si3N4粉体的表面分析方法,以及为了提高固相体积分数而进行的Si3N4粉体表面改性方面的进展.     

Si粉氮化法合成Si3N4-SiC材料

以SiC、Si粉和Al2O3微粉为主要原料,羧甲基纤维素(CMC)为临时结合剂,采用氮化反应烧结法合成了Si3N4-SiC材料,主要研究了Si粉的粒度(≤0.074、≤0.044 mm)和加入量(质量分数分别为15%、17%、19%、21%)、烧成温度(分别为1 380、1 400、1 420、1 430、1 440、1 460和1 480℃)、Al2O3微粉添加量(质量分数分别为0、1%、2%、3%、4%,取代相应量的SiC粉)对Si3N4-SiC材料的显气孔率、体积密度、常温耐压强度、常温抗折强度、高温抗折强度及Si3N4含量的影响。结果表明:1)采用粒度较细Si粉的试样具有较高的致密度、常温强度、高温抗折强度和Si3N4含量;随着Si粉加入量的增加,试样的致密度略有增大但变化不大,常温强度和Si3N4含量逐渐增大,而高温抗折强度先增大后减小;2)适当提高烧成温度会明显改善Si3N4-SiC材料的高温抗折强度,但当温度超过1 440℃反而略有下降;3)添加Al2O3微粉对烧后试样的致密度、常温强度和高温抗折强度有益。综合来看,Si粉的适宜添加量(质量分数)为17%,较适宜的烧成温度为1 420～1 440℃,Al2O3微粉的适宜添加质量分数为2%。     

导电 Si3N4 基复相陶瓷研究进展

Si_3N_4陶瓷具有优异的力学性能和导热性能,然而其固有的高硬度和脆性极大地限制了其加工性能。通过添加导电相改善Si3N4陶瓷的导电性能可实现对Si_3N_4陶瓷的电火花加工。添加的导电相主要包括钛基化合物(TiN、TiC、TiC N、TiB_2)、锆基化合物(Zr B_2、Zr N)和MoSi_2等导电陶瓷以及碳纳米管(CNT)、碳纳米纤维(CNF)、石墨烯纳米片(GNP)等导电碳基纳米材料。本论文详细回顾了Si_3N_4基导电陶瓷的研究进展,并对今后Si_3N_4基导电陶瓷的发展趋势进行了展望。     

Simultaneous synthesis and consolidation of nanostructured TaSi2–Si3N4 composite by pulsed current activated combustion

Dense nanostructured 4TaSi₂–Si₃N₄ composite was synthesized by pulsed current activated combustion synthesis (PCACS) method within 3 min in one step from mechanically activated powders of TaN and Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense 4TaSi₂–Si₃N₄ composite with relative density of up to 98% was produced under simultaneous application of a 60 MPa pressure and the pulsed current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

基于激光切割的Si3N4陶瓷断裂韧性测试方法

提出一种采用激光切割技术在Si3N4陶瓷表面预制微小切口,并结合SENB法测定陶瓷材料断裂韧性的新方法。利用连续激光束在陶瓷表面加工出切口,在三点弯曲实验前后分别运用激光共聚焦显微镜(LSCM)和扫描电镜(SEM)测量切口宽度和深度,而后计算陶瓷材料断裂韧性。在此基础上分析激光输出功率P、激光辐照光斑直径D和激光切割速率Vw与材料断裂韧性值的内在联系。结果表明:输出的激光能量密度达到陶瓷切割加工阈值后,光束在试件表面制得对应切口;切口深宽比为4.3～4.8时测得的Si3N4陶瓷断裂韧性值具有较高精度。     

Microstructure evolution of Si3N4/Si3N4 joint brazed with Ag–Cu–Ti + SiCp composite filler

The Si₃N₄ ceramic was brazed by Ag–Cu–Ti + SiCp composite filler (p = particle) prepared by mechanical mixing. Effects of the content of Ti and SiC particles on microstructure of the joint were investigated. A reliable Si₃N₄/Si₃N₄ joint was achieved by using Ag–Cu–Ti + SiCp composite filler at 1173 K for 10 min. A continuous and compact reaction layer, with a suitable thickness, forms at the Si₃N₄/braze interface. The SiC particles react with Ti in the brazing layers, forming Ti₃SiC₂ thin layers around the SiC particles themselves and Ti₅Si₃ small particles in the Ag[Cu] and Cu[Ag] based solid solution. The higher content of SiC particles in the filler (≥10 vol%) depresses interfacial bonding strength between the Si₃N₄ substrate and composite brazing layer due to the thinner reaction layer and the bad fluidity of the filler. The Ti₃SiC₂ → TiC + Ti₅Si₃ reaction occurs when Ti concentration around SiC particles in the filler increases.     

Synthesizing high α-phase Si3N4 powders containing sintering additives

Fine grain α-phase silicon nitride (Si₃N₄) ceramic powders were produced via carbothermic reduction of colloidal SiO₂, which contained pre-mixed additives of sintering aids primarily consisting of oxides such as MgO and Y₂O₃. The powders that were pre-mixed in the starting reactants were chosen based on the final powder composition and on type and amount of the secondary phases desired for sintering. After synthesizing, powder properties were examined using standard characterization techniques (XRD, SEM, BET, etc). This technique of ceramic synthesis has advantages in providing nitride-based ceramic powders, which contain secondary in situ phases that are distributed as sintering additives. Silicon nitride ceramic powders synthesized using this method might therefore be readily sintered because the homogeneously distributed sintering additives were present in the starting materials. In this work, the processing parameters are described in terms of the synthesis conditions.     

TiN/Si3N4复合材料的磁控溅射制备及其电性能研究

利用直流反应磁控溅射法在Si3N4陶瓷基体上制备了TiN导电薄膜。采用X射线衍射仪(XRD)、扫描电镜(SEM)和电子能谱(EDS)对薄膜的物相组成以及表面形貌进行分析,表明TiN薄膜均匀,且与基体有较强的附着力。采用SZ82型四探针测试仪对薄膜进行了方阻随厚度变化的分析,表明薄膜的厚度对薄膜的电性能有很大的影响。     

用硅灰合成无碳Si3N4/SiC纳米粉末

用机械与同步热激活法首次从废硅灰中合成无碳Si3N4/SiC纳米粉末。通过这种新方法可在1 465℃形成晶粒尺寸小至45nm的Si3N4/SiC纳米粉末。为了合成无碳Si3N4/SiC纳米粉末,研究了两种不同方法移除纳米粉末中游离碳的有效性。这两种方法分别使用氢气和空气。研究发现,虽然使用氢气时Si3N4和SiC是稳定的,但是这种方法不是很有效。相比之下,使用空气移除游离碳是有效的。游离碳除去后立即停止使用空气,Si3N4/SiC纳米粉末也会有少量被氧化。本文提供了明确的途径将硅灰有效合成无碳Si3N4/SiC纳米粉末。     

Effects of whisker-like β-Si3N4 seeds on phase transformation and mechanical properties of α/β Si3N4 composites using MgSiN2 as additives

α/β Si₃N₄ composites with β-Si₃N₄ content ranging from 26% to 100% were hot-pressed with or without β-Si₃N₄ seeds, using MgSiN₂ as additives, and their mechanical properties were investigated. When the α-Si₃N₄ content was over 58%, the microhardness of α/β Si₃N₄ composites was in the range of 23–24 GPa, and then the indentation hardness decreases with decreasing the content of α-Si₃N₄, whether with β-Si₃N₄ seeds or not. The toughness increased with increasing elongated β-Si₃N₄ grains, which improved fracture resistance by crack bridging, pull out or the crack deflection mechanism, and reached the maximum value of 7.0 MPa m^1/2 with 1 wt% β-seeds. In comparison with α/β Si₃N₄ composite with a similar phase composition, the fracture strength was improved by adding β-Si₃N₄ seeds because of the relatively smaller grain sizes and higher toughness. The α/β Si₃N₄ composite with 5 wt% β-seeds showed a high strength of 1253 MPa, a high hardness of 20.9 GPa and a toughness of 6.9 MPa m^1/2.