共价晶体硬度的微观理论

基于共价固体硬度正比于单位面积上每根化学键对压头的阻抗之和的假设,提出了估计极性共价固体硬度的微观理论.计算了新近合成的立方尖晶石结构Si3N4的理论硬度,计算值与最近报道的实验值吻合.最后,预测了高密度C3N4异构体的理论硬度.     

氮化碳晶体的合成

1989年A．Y．Liu和M．L．Cohen根据β-Si3N4的晶体结构，用C替换Si，在局域态密度近似下采用第一性赝势能带法从理论上预言了β-C3N4这种硬度可以和金刚石相媲美而在自然界中尚未发现的新的共价化合物。1996年，Teter和Hemley通过计算认为C3N4可能具有5种结构，即α相，β相，立方相、准立方相以及类石墨相。除了类石墨相外，其他4种结构物质的硬度都可以与金刚石相比拟。     

β-C_3N_4的研究进展

自从1989年Liu等人预言β-C3N4可能具有超高硬度等特殊性能以来，这种材料得到了科技界和工业界的广泛重视，然而至今尚未获得与理论预测完全相符的实验结果。鉴于这种情况，一方面采用更新的材料制备与检测手段进行合成以获得真正的β-C3N4晶体；另一方面对碳、氮反应所形成的固体材料进行性能研究以寻找可供使用的特殊性能。为此对近年来β-C3N4的研究进行了综合评述。     

铌酸锂晶体化学键性质研究

利用平均能带模型研究了ＬiNbO3晶体化学键性质,结果表明,铌氧键的共价性为０．２,锂氧键的共价性为０．４２,利用由共价性和极化率定义的化学环境因子h计算LiNbO3中Ｃr^3 的Ｒacah参数和ＬiNbO3中Ｆe^2 的穆斯堡尔同质异能位移均与实验值一致。     

Ni-Ti系金属间化合物的电子结构与结合能计算

基于固体与分子经验电子理论(EET),计算了Ni-Ti合金系金属间化合物的价电子结构与理论结合能.计算结果表明,NiTi、NiTi2与Ni3Ti中,分占不同晶位的Ni、Ti原子对应不同的杂阶.构成Ni-Ti系金属间化合物化学键的成分非常复杂,既有占主导作用的共价成分,也有金属和离子成分.NiTi、NiTi2与Ni3Ti的理论结合能分别为-458.83kJ/mol、-447.10kJ/mol与-437.37kJ/mol,理论值与实验值在一级近似下相吻合.3种化合物中,NiTi的结合能数值最大,可预见其结构稳定性最强.     

通过键共价性增加策略获得高性能Cu3SbSe4基热电材料(英文)

通过Cu₃SbSe₄-MTe (M=Ge/Sn)固溶体系的对比研究,本文报道了一种增加键共价性来弱化电声耦合进而获得高迁移率的策略,也证实了该策略能提升Cu₃SbSe₄化合物的热电性能.研究发现,相比于SnTe固溶,GeTe固溶赋予Cu₃SbSe₄化合物在相似载流子浓度情况下更高的迁移率,进而获得更优越的电输运性能.密度泛函理论计算表明,与SnTe固溶相比,GeTe固溶能增加Cu₃SbSe₄化学键共价性因而具有优越的电输运性能.最终,在Cu₃SbSe₄-1%GeTe样品中取得了0.8的最大热电优值(@648 K)和0.41的平均热电优值(300–648 K).此研究表明,通过增加键共价性来弱化电声耦合进而获得高迁移率是提升热电性能的有效途径.     

熔盐电解法和固体粉末法在纯镍表面渗硼的对比研究

本工作分别采用熔盐电解法和固体粉末法对纯镍表面进行渗硼处理,以提高镍的表面硬度。熔盐电解法以硼砂(Na₂O₄B₇·10H₂O)作硼源,碳酸钠(Na₂CO₃)作支持电解质,电流密度为750 A/m²;固体粉末法采用碳化硼(B₄C)和氟硼酸钾(KBF₄)作为渗硼剂。利用扫描电镜(SEM)和能谱仪(EDS)分析渗硼试样的断面形貌和元素含量,利用X射线衍射仪(XRD)分析试样的物相。结果表明：采用熔盐电解法和固体粉末法在纯镍表面渗硼,均得到由Ni₂B和Ni₃B组成的渗层;熔盐电解法在电解温度为950℃、电解时间为4 h时渗层厚度约为184.35μm,渗层表面硬度值为1 755HK;固体粉末法在渗硼温度为950℃、渗硼时间为10 h时所得渗层厚度约为176.35μm,渗层的表面硬度值是1 526HK;镍经过渗硼后其表面硬度值有明显的增加,渗层厚度越大,表面硬...     

类金刚石碳膜的结构及其微动磨损行为

采用非平衡磁控溅射与等离子源离子注入(PSII)的混合技术，在1Crl8Ni9Ti不锈钢上制备N／Ti，N／TiN／C／DLC多层膜，研究其结构和微动磨损性能，并与N注入层比较。结果表明，N注入层内形成了CrN和Fe3N等氮化物相；多层膜内形成了TiN，Ti2N和CrN等化舍物相。PSII技术能够提高1Crl8Ni9Ti不锈钢的表面硬度，N注入层的硬度约为基体硬度的2．5倍，DLC多层膜的硬度约为基体硬度的4倍。N注入层和DLC多层膜都能够提高1Crl8Ni9Ti的抗微动磨损性，虽然DLC多层膜比N注入层薄，但其抗微动磨损性能更好。     

TiN/Si3N4纳米多层膜的生长结构与超硬效应

采用磁控溅射方法制备了一系列不同Si3N4和TiN层厚的TiN/Si3N4纳米多层膜,采用X射线衍射、高分辨电子显微分析和微力学探针表征了薄膜的微结构和力学性能,研究了Si3N4和TiN层厚对多层膜生长结构和力学性能的影响.结果表明:当Si3N4层厚小于0.7 nm时,原为非晶的Si3N4在TiN的模板作用下晶化并与之形成共格外延生长的柱状晶,使TiN/Si3N4多层膜产生硬度和弹性模量异常升高的超硬效应.最高硬度和弹性模量分别为34.0 GPa和353.5 GPa.当其厚度大于1.3 nm时,Si3N4呈现非晶态,阻断了TiN的外延生长,多层膜的力学性能明显降低.此外,TiN层厚的增加也会对TiN/Si3N4多层膜的生长结构和力学性能造成影响,随着TiN层厚的增加,多层膜的硬度和弹性模量缓慢下降.     

蓝宝石和硅衬底上氮化硅薄膜的制备和性能研究

采用射频磁控反应溅射法,以高纯Si为靶材,高纯N2气为反应气体,在蓝宝石和硅衬底上制备了氮化硅薄膜.并对Si3N4薄膜的沉积速率、成分、结构及红外光学性能等进行了研究.实验结果表明,沉积薄膜中Si和N的比接近3∶4,形成了Si3N4化合物,呈非晶态结构.制备的Si3N4薄膜的硬度明显高于蓝宝石衬底的硬度,且与蓝宝石衬底结合牢固,可提供良好的保护性能.     

TiN/Si3N4纳米晶复合膜的微结构和强化机制

采用高分辨透射电子显微镜对高硬度的TiN/Si3N4纳米晶复合膜的观察发现,这类薄膜的微结构与Veprek提出的nc-TiN/a-Si3N4模型有很大不同：复合膜中的TiN晶粒为平均直径约10nm的柱状晶,存在于柱晶之间的Si3N4界面相厚度为0.5～0.7nm,呈现晶体态,并与TiN形成共格界面.进一步采用二维结构的TiN/Si3N4纳米多层膜的模拟研究表明,Si3N4层在厚度约＜0.7nm时因TiN层晶体结构的模板作用而晶化,并与TiN层形成共格外延生长结构,多层膜相应产生硬度升高的超硬效应.由于TiN晶体层模板效应的短程性,Si3N4层随厚度微小增加到1.0nm后即转变为非晶态,其与TiN的共格界面因而遭到破坏,多层膜的硬度也随之迅速降低.基于以上结果,本文对TiN/Si3N4纳米晶复合膜的强化机制提出了一种不同于nc-TiN/a-Si3N4模型的新解释.     

纳米Si3N4及其与石墨、MoS2混合填充PTFE摩擦磨损性能研究

用M-2000型摩擦磨损试验机对纳米Si3N4及其与石墨、MoS2混合填充聚四氟乙烯(PTFE)复合材料在干摩擦条件下与45#钢对磨时摩擦磨损性能进行了研究,用洛氏硬度仪对其进行了测量,用扫描电子显微镜对磨损表面进行了观察.结果表明:纳米Si3N4的加入能提高PTFE复合材料的硬度和耐磨性,纳米Si3N4与MoS2混合填充会使PTFE复合材料的耐磨性能提高更多,特别是在载荷增大时其耐磨效果更好.纳米Si3N4能阻止PTFE复合材料中磨损微裂纹的产生,在纳米Si3N4的富聚区,磨损微裂纹较少,在纳米Si3N4的贫聚区,磨损的微裂纹较多.纳米Si3N4填充PTFE复合材料的摩擦系数比纯PTFE大,且随着载荷增加有所减小,石墨的加入可降低PTFE的摩擦系数.     

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.     

Synthesis of cubic zirconium and hafnium nitride having Th3P4 structure

High-pressure synthesis is a powerful method for the preparation of novel materials with high elastic moduli and hardness. Additionally, such materials may exhibit interesting thermal, optoelectronic, semiconductuing, magnetic or superconducting properties. Here, we report on the high-pressure synthesis of zirconium and hafnium nitrides with the stoichiometry M3N4, where M = Zr, Hf. Synthesis experiments were performed in a laser-heated diamond anvil cell at pressures up to 18 GPa and temperatures up to 3,000 K. We observed formation of cubic Zr3N4 and Hf3N4 (c-M3N4) with a Th3P4-structure, where M-cations are eightfold coordinated by N anions. The c-M3N4 phases are the first binary nitrides with such a high coordination number. Both compounds exhibit high bulk moduli around 250 GPa, which indicates high hardness. Moreover, the new nitrides, c-Zr3N4 and c-Hf3N4, may be the first members of a larger group of transition metal and/or lanthanide nitrides with interesting ferromagnetic or superconducting behaviour.     

Interracial Bonding Strength of TiN Film Coated on Si3N4 Ceramic Substrate

The fraction of TiN/Si3N4 in the cross section was observed with scanning electric microscope （SEM）, and residual stresses of TiN coated on the surface of Si3N4 ceramic were measured with X-ray diffraction （XRD）.The hardness of TiN film was measured, and bonding strength of TiN film coated on Si3N4 substrate was measured by scratching method. The formed mechanism of residual stress and the failure mechanism of the bonding interface in the film were analyzed, and the adhesion mechanism of TiN film was investigated preliminarily. The results show that residual stresses of TiN film are all behaved as compressive stress, and TiN film is represented smoothly with brittle fracture, which is closely bonded with Si3N4 substrate. TiN film has high hardness and bonding strength of about 500 MPa, which could satisfy usage requests of the surface of cutting Si3N4 ceramic.     

Interfacial Bonding Strength of TiN Film Coated on Si3N4 Ceramic Substrate

The fraction of TiN/Si3N4 in the cross section was observed with scanning electric microscope (SEM), and residual stresses of TiN coated on the surface of Si3N4 ceramic were measured with X-ray diffraction (XRD).The hardness of TiN film was measured, and bonding strength of TiN film coated on Si3N4 substrate was measured by scratching method. The formed mechanism of residual stress and the failure mechanism of the bonding interface in the film were analyzed, and the adhesion mechanism of TiN film was investigated preliminarily. The results show that residual stresses of TiN film are all behaved as compressive stress, and TiN film is represented smoothly with brittle fracture, which is closely bonded with Si3N4 substrate. TiN film has high hardness and bonding strength of about 500 MPa, which could satisfy usage requests of the surface of cutting Si3N4 ceramic.     

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。     

Fabrication and evaluation of SixNy coatings for total joint replacements

Wear particles from the bearing surfaces of joint implants are one of the main limiting factors for total implant longevity. Si(3)N(4) is a potential wear resistant alternative for total joint replacements. In this study, Si(x)N(y)-coatings were deposited on cobalt chromium-discs and Si-wafers by a physical vapour deposition process. The tribological properties, as well as surface appearance, chemical composition, phase composition, structure and hardness of these coatings were analysed. The coatings were found to be amorphous or nanocrystalline, with a hardness and coefficient of friction against Si(3)N(4) similar to that found for bulk Si(3)N(4). The low wear rate of the coatings indicates that they have a potential as bearing surfaces of joint replacements. The adhesion to the substrates remains to be improved.     

AlN-Si_3N_4膜

用直流反应溅射沉积AlN－Si3N4膜。通过红外吸收光谱和X射线衍射来验证薄膜的结构。通过各种测量，给出了吸收系数、电阻率、应力、硬度和结合力。实验表明，AlN－Si3N4膜具有良好的光、电和机械特性。     

反应溅射Ti-Si-N纳米晶复合薄膜的微结构与力学性能

采用Ar、N2 和SiH4混合气体反应溅射制备了一系列不同Si含量的Ti Si N复合膜 ,用EDS、XRD、TEM和微力学探针研究了复合膜的微结构和力学性能。结果表明 ,通过控制混合气体中SiH4分压可以方便地获得不同Si含量的Ti Si N复合膜。当Si含量为 (4～ 9)at%时 ,复合膜得到强化 ,最高硬度和弹性模量分别为 34 2GPa和 398GPa。进一步增加Si含量 ,复合膜的力学性能逐步降低。微结构研究发现 ,高硬度的Ti Si N复合膜呈现Si3 N4界面相分隔TiN纳米晶的微结构特征 ,其中TiN纳米晶的直径约为 2 0nm ,Si3 N4界面相的厚度小于 1nm。