Tribological properties of HVOF as-sprayed and heat treated Co–Mo–Cr–Si coatings

HVOF-sprayed Co–28%Mo–17%Cr–3%Si alloy tribological performance was tested in the as-sprayed condition and after thermal treatments at 200, 400, 600 °C for 1 h. As-sprayed coating possesses low hardness, undergoes adhesive wear against 100Cr6 steel and displays an high-friction coefficient causing relevant thermal effects. The 600 °C-heat treatment increases microhardness, thus preventing adhesive wear and reducing friction.     

An elastic–plastic contact model of ellipsoid bodies

A theoretical model for the elastic–plastic contact of ellipsoid bodies is presented in this paper. Relation of the contact parameters, such as the mean contact pressure, the contact area and the contact load as a function of the contact interference are modeled in the three different contact regimes: elastic, elastic–plastic and fully plastic. The model is verified by experimental results and is compared with published theoretical models. Very good agreement between the present model and the experimental results are found compared to the prediction of the other contact models.     

Hardness, Toughness, and Scratchability of Germanium–Selenium Chalcogenide Glasses

Ge–Se chalcogenide glasses are characterized by relatively low hardness (0.39–2.35 GPa) and low fracture toughness (0.1–0.28 MPa·m^1/2). Actually, the hardness of chalcogen-rich glasses is low enough so that the brittleness parameter, B = H / K _c , is lower than that of silicate glasses. Whereas hardness and Young's modulus increase with increasing germanium contents, fracture toughness follows a trend similar to that of the density and exhibits a maximum for the Ge₂₀Se₈₀ composition, which corresponds to the rigidity percolation threshold. Optical microscopy and atomic force microscopy observations suggest that the indentation deformation proceeds by a localized shear deformation phenomenon. Glasses in the chalcogen-rich region behave viscoelastically at room temperature. As a consequence, an increase of the loading time results in a decrease of hardness and toughness.     

Temperature-Dependent Indentation Behavior of Transformation-Toughened Zirconia-Based Ceramics

Indentation behavior of Ce-TZP, Y-TZP, and Mg-PSZ between room temperature and 1300°C was investigated. Hardness decreased with increasing temperature for all three materials, but indentation cracking increased with increasing temperature. The opposing temperature dependences are discussed in terms of dislocation and transformation plasticity.     

Application of instrumented indentation technique for enhanced fitness-for-service assessment of pipeline crack

While most in-field non-destructive technologies for structural integrity diagnosis focus on precise crack detection, a novel instrumented indentation technique for non-destructively determining tensile properties in fields is introduced here. The goal of this work is to apply the newly-developed indentation technique to in-field fitness-for-service (FFS) assessment of linepipe cracks. As one step to verify its applicability, tensile properties in base metal and girth weldement of API-X65-graded linepipe were evaluated by this indentation technique and provided for construction of material-specific failure assessment diagrams (FADs). Results are discussed in terms of the accuracy of the indentation data and how the FAD assessment results are affected by the variation in local tensile properties measured by indenting small target regions such as heat-affected zones (HAZs). Based on the results, we suggest that the indentation technique may be useful for reducing possible difficulties in flaw assessment (arising from the use of incorrect tensile properties) by providing reliably practical data for FAD construction.     

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.     

冷加工过程中316L奥氏体不锈钢硬度压痕尺寸效应分析

通过理论结合试验的方法对冷加工过程中316L奥氏体不锈钢硬度压痕尺寸效应（ISE）进行了分析,并将根据理论模型得到的真实硬度与测试硬度进行了对比。结果表明：不同伸长率下316L奥氏体不锈钢呈现正ISE现象。通过对三种常见的理论模型分析发现,修正比例试样阻力(MPSR)模型能更准确地描述冷加工下316L奥氏体不锈钢的ISE现象,并通过此模型得到了伸长率为0、15%、30%、40%下真实硬度值分别为159.16 MPa、228.07 MPa、259.72 MPa、282.54 MPa,且验证得出各伸长率下的真实硬度与屈服强度之间存在线性关系,随后结合材料的真实硬度值发现,在9.8 N压头载荷下测出经不同静态单轴拉伸后的316 L不锈钢维氏硬度几乎不存在ISE现象。     

A quantitative analysis of the indentation fracture of fused silica

It is unclear how the densification of fused silica influences the damage of its precision optics subjected to machining. This paper presents a quantitative analysis of the indentation fracture of fused silica involving densification with the embedded center of dilation (ECD) model. The Hertzian stress field and the ECD-induced stress field were superposed to provide the overall stress distribution in the loading stage. A new method was established to accurately determine the strength of the ECD-induced stress field with densification effects. With the aid of the ECD model, the starting locations, initiation stages and initiation sequence of crack morphologies were predicted by analyzing the stress fields. To quantitatively study the initiation of conical cracks in fused silica, the strain energy release rate was calculated by linear elastic fracture mechanics (LEFM). The predicted minimum threshold load leading to conical cracking was consistent with the measured values.     

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si₃N₄/Si₂N₂O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si₂N₂O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si₂N₂O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si₃N4/Si₂N₂O and fine grained β- Si₃N₄ ceramics.     

单晶硅表层机械力学特性的各向异性分析

为探究挠性筋结构单晶硅材料的各向异性特性以及KOH腐蚀工艺对其力学性能的影响规律,进行纳米压痕实验,并结合原子力显微镜观察单晶硅表层3个主晶面上压痕裂纹形貌随晶向的变化规律,分析单晶硅材料表层弹性模量、硬度、断裂韧性等机械力学特性参数在(001)、(110)及(111)3个主要晶面上沿各个晶向的变化规律;分析挠性筋结构单晶硅材料(001)晶面的KOH腐蚀工艺对其材料表面机械特性的影响规律.结果表明:挠性筋单晶硅在(001)晶面上弹性模量的各向异性变化幅度明显,硬度及断裂韧性各向异性的变化幅度不大;挠性筋单晶硅在(110)晶面弹性模量和断裂韧性的各向异性变化幅度明显,硬度各向异性变化幅度不大;挠性筋单晶硅在(111)晶面硬度值、弹性模量及断裂韧性参数的变化幅度幅值均较小;确定了单晶硅表层3个晶面裂纹最易扩展的晶向方向,KOH腐蚀工艺使得单晶硅表面质量降低,腐蚀后暴露的表面微裂纹、缺陷等会使得单晶硅(001)晶面表层硬度、断裂韧性降低,从而降低了挠性筋结构的实际断裂强度.