SiC/BN层状陶瓷耐损伤性能

采用压痕法在SiC/BN层状陶瓷试样的表面引入不同尺寸的表面裂纹,利用三点弯曲测量含裂纹试样的极限断裂应力,研究了不同尺寸的表面裂纹对层状陶瓷断裂强度的影响;根据压痕载荷-强度实验结果,测定层状陶瓷的阻力曲线,并与单相SiC陶瓷对比。结果表明,层状陶瓷的压痕强度对压痕裂纹深度的变化不敏感,阻力曲线呈上升型;而单相SiC陶瓷的压痕强度随压痕裂纹深度的增加急剧下降,阻力曲线呈平稳型,说明层状陶瓷具有优异的耐损伤性能和升值R-曲线行为。分析认为,裂纹在弱界面处发生偏折是层状陶瓷具有优良耐损伤性能和升值R-曲线行为的主要原因。这为陶瓷材料在含有一定的制备和加工缺陷以及承受冲击、磨损等接触损伤的条件下保持高强度提供了可能。     

在低合金钢基体表面沉积镶嵌结构界面金刚石膜

在低合金钢基体上磁控溅射镀Cr/Cu双层膜,电沉积铜-金刚石复合过渡层,在热丝CVD系统中沉积了连续的金刚石膜。用压痕实验研究了所沉积的金刚石膜/基结合性能,用扫描电镜(SEM)、X射线(XRD)、拉曼光谱表征了金刚石膜的表面形貌、相结构和内应力。结果表明,经CVD金刚石沉积后Cr层转化为铬铁碳化物层,阻止了Fe对界面金刚石的石墨化,并部分缓冲相变应力;较软的Cu层能有效缓解相变应力和热应力,降低了所沉积的金刚石膜内应力;用441N载荷对所沉积的金刚石膜进行压痕评估,压痕外缘只产生环状裂纹,表明膜基结合力较高。     

Si3N4/SiC层状结构陶瓷摩擦磨损性能研究

利用Falex多重样品球-三块式的摩擦磨损试验机,重点研究了单层Si3N4陶瓷,9层和5层Si3N4/SiC层状结构陶瓷在不同的外加载荷、不同的界面应力分布大小、不同的表面组成与层状结构陶瓷的摩擦磨损性能的关系.结果表明:随着外加载荷逐渐加大,材料的耐磨损性能降低,表现为达到磨损突变点的时间缩短,磨痕直径变大;界面残余压应力对层状结构陶瓷耐磨损性能的提高起着重要的作用,9层结构陶瓷的界面残余压应力高出5层结构的层状陶瓷20%左右,表现出更高的耐磨损性能;SiC Si3N4两相复合陶瓷极大地提高了材料表面的耐磨损性能,在同样外加载荷下,表现为达到磨损突变点的时间更长,磨痕直径更小.     

弱层成分对SiC/BN层状陶瓷阻力行为的影响

利用压痕2强度法测定了不同弱层成分的SiC/ BN 层状陶瓷的阻力曲线行为, 研究了BN 弱层成分对SiC/ BN 层状陶瓷的阻力曲线行为的影响, 并与SiC 块体陶瓷作对比。结果表明: 与SiC 块体陶瓷相比, SiC/ BN层状陶瓷具有更为明显的升值阻力曲线行为, 且其阻力曲线行为受弱层成分的影响。其中, BN 弱层中加入30 %Al₂O₃ 的SiC/ BN 层状陶瓷显示出更为优越的抗裂纹扩展能力, 其阻力曲线上升最陡, 上升幅度最大。分析认为,这与它们不同的增韧机制和界面的结合状态有关。原位增韧是SiC 块体陶瓷韧性提高的主要原因。裂纹遇到弱界面时发生偏转、分叉以及脱层等是层状陶瓷材料抗裂纹扩展能力提高的主要原因。而弱层成分又影响着裂纹的偏转, 适当结合强度的界面有利于裂纹的偏转, 结合强度太弱太强的界面都不利于裂纹的偏转。      

粗糙度对热障涂层冲蚀失效的影响及模型建立

采用气体喷砂试验机研究了大气等离子喷涂(APS) ZrO₂-7%Y₂O₃(7YSZ)热障涂层的冲蚀失效机理, 分析了陶瓷层表面粗糙度对热障涂层冲蚀磨损率及失效行为的影响, 研究了基于多孔层状的热障涂层在常温高速粒子90°攻角下的冲蚀失效特征, 探讨了多孔层状结构对涂层冲蚀失效演变机制的作用, 此外, 建立了基于粗糙度的冲蚀失效数学模型。研究结果表明: 冲蚀磨损率与陶瓷层表面粗糙度呈线性递增关系; 涂层在高速粒子冲蚀下多孔层状结构加剧了涂层的冲蚀失效; 涂层表面在粒子冲蚀下承受着非均匀、非连续的压-压脉动循环载荷, 在微凸粗糙表面承载着正应力和切应力的相互作用; 正应力迫使涂层出现凹坑, 切应力易导致涂层出现沟槽, 并同时以原喷涂态表面网状裂纹为策源地诱发裂纹在粒子晶界和层间界面萌生扩展导致涂层产生粒子和片状剥落, 最终涂层显示出典型的磨粒磨损和低周疲劳失效形式。     

应用纳米压痕法测量电沉积镍镀层残余应力的研究

应用纳米压痕法测量残余应力的2种理论模型对5种电沉积镍镀层中的残余应力在不同压痕深度处进行了测量,并与X射线衍射法的测量结果进行了比较.结果表明,压深位于薄膜/基底界面处的2种压痕法测量结果与X射线衍射法的测量结果相近,且Yun-Hee模型与其符合得更好.     

Ti3SiC2陶瓷的能量耗散机理

采用维氏和赫兹压痕法研究了Ti3SiC2接触损伤及其演变.结果表明,在维氏压痕接触损伤区从表面到纵深的不同损伤排序为:表面的晶粒粉碎,亚表面的晶粒分层或破碎,再远处的晶粒完好;在赫兹压痕接触损伤区剪切损伤带以内的晶粒破碎,剪切带以外的晶粒完好.因此,造成压痕处的局部能量耗散,使应力传递受限、应力集中下降,使这种三元层状陶瓷具有准塑性特征.用声发射(Acoustic Emission,简称AE)系统监测赫兹压痕加卸载过程中的局部损伤过程,发现在加载过程中声发射信号密集,卸载过程声发射信号稀疏,证明了损伤和局部能量耗散的不可逆性.Ti3SiC2陶瓷的能量局部耗散机理是弱晶界面开裂和晶粒分层导致的局部软化和破碎,在损伤区范围内吸收能量并使局部应力释放.     

Al2O3/ZrO2层状复合陶瓷的显微结构特征与强韧化的关系

利用自带能谱的扫描电镜(SEM)和透射电镜(TEM)仪,对Al2O3/ZrO2层状复合陶瓷的显微结构特征及断口形貌进行深入分析研究.研究结果表明:表面压应力的作用细化了晶粒,提高了可相变四方相的含量,增加了相变增韧的效果;沿界面两端表现出2种不同的断裂机制:界面以外靠近表面部分,其断裂主要是穿晶断裂;在基体层的断裂则表现为沿晶和穿晶混合型断裂方式.该断裂行为与ZrO2层状陶瓷的相变增韧机制密切相关.     

圆柱基体热障涂层热冲击下失效的瞬态应力分析

用等离子喷涂方法在合金圆柱体上制备了热障涂层,并用水冷的方法进行了热冲击实验,试样的失效现象为轴向的开裂和剥落.利用有限元分析工具对热障涂层的热冲击进行模拟,通过对涂层内部应力随时间分布的分析发现:热冲击过程中陶瓷层表面的周向应力随着时间由拉转变为压,冷却初期陶瓷层的周向拉应力值较大,超过了陶瓷层的抗拉强度,因而淬冷初期的周向应力是导致轴向裂纹萌发的主要原因;陶瓷层与粘结层接触面上的径向拉应力与粘结层的氧化生长引起界面开裂;轴向裂纹和界面开裂共同导致陶瓷层的剥落.     

热处理温度对热解炭及炭/炭复合材料力学性能的影响

以丙烷为气源,采用等温等压化学气相渗透技术制备了炭/炭复合材料,利用X射线衍射、偏光显微镜、扫描电镜、纳米压痕仪、三点弯曲法研究了热处理温度对热解炭以及炭/炭复合材料微观结构和力学性能的影响.微观结构观察显示随着热处理温度的升高,热解炭层间距减小,同时石墨化度提高;由于发生了局部应力石墨化,热解炭出现同心微裂纹,并且随热处理温度的升高裂纹的数量和宽度增加.纳米压痕测试表明,热解炭的纳米压痕行为是完全的弹性形变,完全卸载后热解炭表面没有残余压痕,但加载和卸载曲线没有重合而是存在一定的能量耗散,随着热处理温度的升高,热解炭的弹性模量增大.热处理后纤维强度降低,并且纤维与基体炭界面脱离,导致炭/炭复合材料的弯曲强度和模量下降.     

Mechanical analysis of indentation cracking in Al2O3/ZrO2 nanostructured laminated ceramics

Residual stress is not the only driving force for indentation cracking in Al2O3/ZrO2 laminated ceramics. An additional driving force is induced by martensitic transformation in the plastic zone beneath the indenter, whereas surface residual compressive stress controls the expansion of surface cracks. Contributions of the transformation driving force and surface residual stress are not considered in the traditional indentation and indentation-strength theory. Therefore, fracture toughness value measured by the traditional methods is usually lower than the practical one in Al2O3/ZrO2 laminated ceramics. When the improved SENB (Single edge notched beam method)-indentation-fracture method from Japanese Industrial Standard (No. JIS R 1607-1990) is used to measure fracture toughness of Al2O3/ZrO2 laminated ceramics, the value is more reliable.     

ZrO2 层状复合陶瓷的显微结构及晶体学位向关系

利用扫描和透射电子显微镜及X-rays 衍射仪等, 对ZrO₂ 层状复合陶瓷的显微形貌特征、断裂相变量及晶体学位向关系进行了深入研究。研究结果表明, ZrO₂ 层状陶瓷由于界面压应力的作用, 抑制了烧结过程中晶粒的生长速度及冷却后四方相向单斜相的转变, 提高了可相变四方相的含量, 提高了断裂相变量, 改善了材料的力学性能, 但却没有改变四方相和单斜相之间的晶体学位向关系, 在层状ZrO₂ 陶瓷中, (100) m/ / (010) t 的晶体学位向关系仍然存在。     

Calculation of crack tip phase transformation zones with the weight function method

Stabilized Zirconia ceramics can undergo a stress-induced tetragonal-to-monoclinic phase transformation. This way, a transformation zone with compressive stresses develops around crack tips, leading to an increase in fracture toughness. The increase in fracture toughness depends on the size of the transformation zone. Therefore, the ability to compute the phase transformation zone at a crack tip is crucial to determine the transformation toughening due to phase transformation. In the case of subcritical phase transformation, the crack tip phase transformation zone has been calculated using the finite element method. In some Zirconia ceramics, such as ceria-stabilized TZP Zirconia ceramics, an autocatalytic phase transformation takes place, leading to large, elongated transformation zones. As this supercritical phase transformation cannot be computed with finite elements, several methods for investigating supercritical phase transformation have been developed. In this paper, a method based on the weight function method will be described.     

R-curve behaviour of 9Ce-TZP zirconia ceramics

Stabilised zirconia ceramics may undergo a stress-induced tetragonal-to-monoclinic phase transformation. At crack tips, a transformation zone with compressive stresses develops, leading to an increase in fracture toughness, which depends on the size and geometry of the transformation zone. The influence of grain size on the R-curve behaviour and transformation zone size is investigated for five 9Ce-TZP zirconia ceramic materials of variable grain size.     

Surface strengthening and toughening of Si3N4/TiC layered composite by slip casting

Layered composite using monolithic Si₃N₄ as outer layers and Si₃N₄-15v/o TiC as inner core was fabricated by slip casting and pressureless sintering. As the composite is cooled from high sintering temperature, the difference in coefficients of thermal expansion between the constrained inner core and outer layer is expected to establish a compressive surface stress. The existence of this residual stress was verified by theoretical analysis and Vicker's indentation for the samples with various outer layer thickness. The layered composites exhibited greater strength, apparent fracture toughness and damage resistance due to the presence of compressive surface stresses in the layer.     

Calculation of Autocatalytic Phase Transformation Zones in Cracked and Uncracked Zirconia Ceramics

Stabilized zirconia ceramics can undergo a stress-induced tetragonal-to-monoclinic phase transformation. A transformation zone with compressive stresses develops at crack tips, leading to an increased fracture toughness which depends on the size and geometry of the transformation zone. In this paper, transformation zones in cracked and uncracked bodies for the case of autocatalytic phase transformation are computed using the weight function method and Eshelby's method. The results are compared with experimentally observed transformation zones.     

氧化锆增韧陶瓷（ZTC）的晶界结构

本文用 JEM200CX 超高分辨型透射电子显微镜对 ZTC 陶瓷的晶界结构进行了研宄。发现 Al_2O_3与ZrO_2之间的晶界存在共格点、共渗和无序结构形式,并发现晶界处常为 ZrO_2相变的成核区。ZrO_2(m)和ZrO_2(t)之间的晶界,既能成为 ZrO_2相变的成核区,也能阻止 ZrO_2相变的延续发展。只有当两个 ZrO_2晶粒的同类型晶面取向相近时,ZrO_2的相变才容易通过晶界而发展。ZrO_2(t)和 ZrO_2(t)之间的晶界具有较好的共格点结构,形成交错的锯齿状,这是 TZP 陶瓷具有高强和高韧的原因之一。     

Micron-sized fracture experiments on amorphous SiOx films and SiOx/SiNx multi-layers

In this study miniaturized monolithic cantilevers of thermally grown silicon oxide and multi-layer cantilevers of plasma enhanced chemical vapor deposited silicon oxide and nitride were mechanically characterized. In order to determine the fracture stress as well as the fracture toughness, un-notched and focused ion beam pre-notched cantilevers were tested. While the thickness of the monolithic cantilevers was varied from 280 nm to 2380 nm, the individual sub-layer thickness of the multi-layer cantilevers was adjusted to 50 nm. Bending experiments reveal a small increase of the fracture stresses with decreasing cantilever thicknesses. For the multi-layer stacks the tensile stress at fracture slightly exceeds the strength values of the corresponding monolithic materials. Furthermore, it is demonstrated that the specimens pre-notched by focused ion beam do not show significant changes in fracture toughness with varying pre-notch size. This makes the applied test a reproducible technique to determine fracture toughness of brittle films.