原子层沉积氧化铝薄膜摩擦学性能研究

不同温度条件下,采用原子层沉积(ALD)技术在单晶硅基底表面制备了Al2O3薄膜.利用原子力显微镜观察了Al2O3薄膜的表面形貌和粗糙度,利用纳米压痕仪测定了薄膜的硬度,并通过UMT-2型往复摩擦磨损试验机(球-盘接触方式)考察了制备温度、载荷和对偶球对Al2O3薄膜的摩擦学性能的影响.结果表明:不同温度条件下制备得到的Al2O3薄膜的粗糙度不同;制备温度为100和200℃的Al2O3薄膜的摩擦性能较优;在所用载荷范围内,摩擦系数存在最低值;与不同对偶球对摩时,由于对偶球硬度不同,Al2O3薄膜呈现不同的摩擦磨损现象.     

利用 AFM 机械刻蚀技术在OTS自组装单分子膜表面制备纳米图案及其力学性能研究

在羟基化Si(111)表面制备十八烷基三氯硅烷(OTS)自组装单分子膜,利用AFM机械刻蚀技术,通过自编程序设计针尖走向,以金刚石针尖作为加工工具,在OTS自组装单分子膜的Si(111)表面刻蚀不同结构的纳米图案,通过预设针尖移动距离和扫描头Z向位移等参数控制纳米结构的大小和深度.结果表明:利用原子力显微镜(AFM)机械刻蚀技术在OTS自组装单分子膜的Si(111)表面加工出大小、深度可控的纳米图案,其中载荷与加工深度基本呈线性关系;所获得的纳米图案结构清晰,刻蚀试验的重复性较好;经过刻蚀后的图案化区域为裸露的SiO2/Si表面,与OTS覆盖区域相比其摩擦力和粘附力较大,其表面性质具有明显的可分辨性.     

用侧向力显微镜对不同非晶态GeSbTe薄膜的摩擦性能研究

采用侧向力显微镜研究了磁控溅射方法制备的GeSbTe薄膜在大气环境中的纳米级摩擦性能,考虑了相对湿度、扫描速度及表面粗糙度对其摩擦性能的影响,对比不同成分的GeSbTe薄膜的摩擦特性.结果表明:在相对湿度较大时,扫描速度对针尖和GeSbTe薄膜之间的摩擦力影响很大;在其它条件相同、外加载荷较大时,同一载荷下的摩擦力与表面粗糙度呈线性关系,但在外加载荷较小的情况下,二者呈现非线性变化规律;相对湿度对Ge2Sb2Te5薄膜和针尖的粘附力影响较GeSb2Te4薄膜弱,且粘附力使得摩擦系数减小;在同一相对湿度下,由于薄膜成分的变化导致硬度不同,其对薄膜的摩擦性能也有一定影响.     

表面粗糙度对UHMWPE微动摩擦磨损性能的影响

采用SRV-4微动摩擦磨损试验机,研究了在干摩擦和水润滑条件下,表面粗糙度对UHMWPE微动摩擦磨损性能的影响.结果表明:干摩擦时,随着UHMWPE表面粗糙度的增加,摩擦系数先降低后升高,比磨损率则单调递增.利用光学显微镜和扫描电子显微镜对磨损表面形貌进行分析观测,发现干摩擦时表面粗糙度较小的UHMWPE磨损表面有少量犁沟,并伴随轻微的塑性变形,随着表面粗糙度的增加,摩擦副接触表面间的黏合点增多,黏着磨损加剧,且在对偶钢球的表面形成转移膜.而在水润滑条件下,摩擦系数和比磨损率显著降低,随着表面粗糙度的增加,摩擦系数和比磨损率同干摩擦时的变化趋势一致,磨损以磨粒磨损为主.     

基于表面粗糙度影响的摩擦声发射特性研究

接触表面粗糙度是影响摩擦的重要因素之一,而摩擦为复杂的动态过程,在线测量非常困难.为了研究表面粗糙度对摩擦的影响,利用声发射技术对不同粗糙度表面的摩擦过程进行了检测.研究结果表明：摩擦系数随着表面粗糙度的增大而增大,但当粗糙度增至一定程度时,摩擦系数随着表面粗糙度的增大而减小;声发射信号各参数（振铃计数、振幅、能量）随表面粗糙度的变化规律与摩擦系数随粗糙度的变化规律一致;声发射信号各参数与表面粗糙度存在密切的对应关系,因此,用声发射技术实时监测摩擦过程是可行的.     

45~#钢表面织构上硬脂酸薄膜减摩性能的研究

本文中提出了一种在45#钢表面构筑具备优异减摩耐磨性能的薄膜的简易方法.首先采用高浓度氢氧化钠溶液在钢表面制备沟槽状表面织构,然后沉积硬脂酸分子得到减摩耐磨薄膜.用扫描电子显微镜、原子力显微镜、接触角测量仪、X射线光电子能谱仪以及X射线衍射仪等手段表征了薄膜的形成机制、表面形貌和化学组分,并利用微纳米摩擦磨损试验机研究薄膜在干摩擦条件下的减摩耐磨特性.研究结果发现,在经化学刻蚀形成织构的钢表面所沉积的硬脂酸薄膜具有优异的减摩耐磨性能.     

摩擦热对UHMWPE/钢摩擦副摩擦性能的影响

通过控制销盘试验中的散热条件测定了摩擦热对UHMWPE/钢摩擦副摩擦性能的影响;利用统计学相关系数理论对摩擦系数和表层温度的相关性进行定量比较,发现不良散热条件使其相关系数显著增大;根据Taylor Hobson轮廓仪测量试样表面形貌三维初始表面高度,尝试用元胞自动机方法进行了摩擦表面温度分布模拟计算,并结合磨损表面形貌分析探讨摩擦热对UHMWPE磨损机理的影响.结果表明:摩擦系统的热传导条件严重影响UHMWPE的摩擦性能;摩擦热效应使得局部接触表面温度超过熔点,摩擦热的积累效应使UHMWPE的磨损机理由粘着磨损变为熔融剪切磨损;在聚合物材料的摩擦试验及应用研究中必须考虑摩擦热的影响.     

两种螺旋角干气密封环端面摩擦特性试验研究

干气密封环端面的接触摩擦不容忽视,在槽型设计与优化时也应该考虑端面的摩擦特性.利用端面摩擦磨损试验机,分别对16°和18°两种螺旋角的螺旋槽干气密封环进行测试,通过对比分析磨损量、摩擦温度、摩擦系数以及表面粗糙度来讨论密封环的摩擦特性和两种螺旋角对端面摩擦性能的影响.结果表明:18°螺旋角试验的磨损量、摩擦温升和摩擦系数均稍大于16°螺旋角试验,说明18°螺旋角试件的磨损比16°螺旋角的更加剧烈;石墨环被螺旋槽覆盖的外圈粗糙度的变化有较大的差异,内圈粗糙度变化相似,说明槽型参数的改变对石墨环磨损的影响主要体现在外圈.试验结果为今后端面摩擦学性能的研究及槽型优化提供了依据.     

非晶含氢碳薄膜摩擦与抗擦伤性能的实验研究

用射频等离子体增强化学气相沉积法在钛合金表面制备了非晶含氢碳（a-C:H)薄膜,利用原小孩子显微镜、纳米力学探针、划痕仪以及滑动摩擦和微动摩擦试验仪器等研究了其表面形貌、粗糙度、硬度和摩擦性能,分析了薄膜的表面粗糙度、硬度及载荷对摩擦系数的影响。结果表明：随着薄膜厚度的增加,硬度略有提高,划痕临界载荷明显提高,而表面粗糙度先增加后降低,最后趋于稳定;室温空气中的滑动摩擦系数随硬度的增大以及表面粗糙度和载荷的减小而降低;在微动摩擦试验中,随相对湿度增加,摩擦系数降低,这有利于其在体液环境中的应用;薄膜具有良好的耐磨性能,经5000次摩擦后磨损很小。     

纳米CeO2颗粒的制备及其化学机械抛光性能研究

以硝酸铈和六亚甲基四胺为原料制备出不同粒径的纳米CeO2粉体颗粒,将纳米CeO2粉体配制成抛光液并用于砷化镓晶片的化学机械抛光.结果表明,不同尺寸的纳米磨料具有不同的抛光效果,采用粒度8 nm的CeO2磨料抛光后微观表面粗糙度最低(0.740 nm),采用粒度小于或大于8 nm的CeO2磨料抛光后其表面粗糙度值均较高.通过简化的固-固接触模型分析,认为当粒度过小时,磨料难以穿透软质层,表现为化学抛光为主,表面凹坑较多,表面粗糙度较高;当粒度大于一定值时,随着磨料粒度增加,嵌入基体部分的深度加大,使得粗糙度出现上升趋势.提出当磨料嵌入晶片表面的最大深度等于或接近于软质层厚度时,在理论上应具有最佳的抛光效果.     

The role of enhanced coated surface in pool boiling CHF in FC-72

 The purpose of Critical heat flux (CHF) experiments was to determine the role of various types and thickness of enhanced coated surface on a horizontal, vertically oriented ribbon heaters made of Ti and Steel 1010 of different thickness. Saturated pool boiling in FC-72 at atmospheric pressure was used in the experiment. The microstructure and surface topography are important factors in pool boiling CHF. In conditions of highly wetting FC-72 and increased roughness, CHF increased by 6 to 12%. However, CHF increased by 29% with greater topographic unevenness of the surface and lower roughness, which was obtained by etching Steel 1010 in H₂SO₄ acid. CHF also increased when the content of metals and metal oxides particles in the coating were increased. The CHF ratio of enhanced coated surface to a ribbon heater featuring a standard surface finish is up to 2.3. In addition, the asymptotic CHF of these uncoated heaters was considerably exceeded. Received on 17 January 2000     

Experimental study of the characteristics and mechanism of pool boiling CHF enhancement using nanofluids

The pool boiling critical heat flux (CHF) performance of aqueous nanofluids was investigated using various nanoparticles of TiO₂, Al₂O₃, and SiO₂. The usage of a nanofluid as a working fluid can significantly enhance the pool boiling CHF, which was found to be strongly dependent on the kind of nanoparticle, as well as its concentration. A nanoparticle surface coating was observed on the heating surface after the experiment. The CHF of pure water on the nanoparticle-coated surface was higher than that of nanofluids for all cases. This revealed that the cause of the CHF enhancement using nanofluids is due to the fact that the heater surface is modified by the nanoparticle deposition. The mechanism of CHF enhancement due to the nanoparticle coating was discussed, relating it to surface wettability, surface roughness, and maximum capillary wicking height of the nanoparticle-coated surface.     

Heat transfer and bubble formation on horizontal copper tubes with different diameters and roughness structures

Heat transfer in flooded evaporators of the refrigeration, air conditioning or process industries is mainly enhanced by modifying the surface structure of evaporator tubes in the micro and/or macro range. To quantify the effect of such modifications, however, the influence of the basic roughness structure on the heated surface has to be separated. Starting from recent publications, experimental results of heat transfer and bubble formation from horizontal copper tubes with different outer diameters (8 or 25 mm) and roughness structures to various boiling liquids are analyzed in this paper to improve our knowledge of the specific events connected with the formation of bubbles at active nucleation sites and their effect on local heat transfer. It is shown that a single, standardized roughness parameter like the (integral) mean roughness height P _a is not sufficient to explain the effect of the heating surface structure on nucleate boiling heat transfer. Instead, detailed information on characteristic roughness parameters of the heated surfaces is necessary for the analysis, making it possible to define the size and form of cavities included in the roughness structure and their positions on the surface. An analysis that aims in this direction is given in a separate contribution to this special issue by A. Luke, who prepared the surfaces and provided the basic data on the set of standardized roughness parameters, the probability distributions of which are used in this paper.     

Z切石英在氟化氢铵溶液中的腐蚀特性

对Z切石英在氟化氢铵溶液中的腐蚀特性进行了研究。首先研究了Z向腐蚀速率随腐蚀温度和腐蚀液浓度的变化关系,然后研究了Z向表面腐蚀粗糙度随腐蚀深度、腐蚀液体浓度、腐蚀温度的变化规律,最后将石英在氟化氢铵溶液中的腐蚀特性跟在BHF溶液中的腐蚀特性进行了简单对比。试验结果表明:腐蚀速率随腐蚀温度和腐蚀液浓度增加而增加;Z向腐蚀表面粗糙度随腐蚀时间的增加而增加;提高腐蚀液体浓度有利于减小腐蚀表面粗糙度;提高腐蚀液体温度有利于减小表面粗糙度;石英在氟化氢铵溶液腐蚀具有更高的腐蚀效率和更小的腐蚀表面粗糙度。本研究结果能够为石英MEMS器件的设计和工艺提供有益帮助。     

Lattice–Boltzmann simulations for analysing the detachment of micron-sized spherical particles from surfaces with large-scale roughness structures

Fully resolved numerical simulations of a micron-sized spherical particle residing on a surface with large-scale roughness are performed by using the Lattice–Boltzmann method. The aim is to investigate the influence of surface roughness on the detachment of fine drug particles from larger carrier particles for transporting fine drug particles in a DPI (dry powder inhaler). Often the carrier surface is modified by mechanical treatments for modifying the surface roughness in order to reduce the adhesion force of drug particles. Therefore, drug particle removal from the carrier surface is equivalent to the detachment of a sphere from a rough plane surface. Here a sphere with a diameter of 5 μm at a particle Reynolds number of 1.0, 3.5 and 10 are considered. The surface roughness is described as regularly spaced semi-cylindrical asperities (with the axes oriented normal to the flow direction) on a smooth surface. The influence of asperity distance and size ratio (i.e. the radius of the semi-cylinder to the particle radius, R_c/R_d) on particle adhesion and detachment are studied. The asperity distance is varied in the range 1.2 < L/R_d < 2 and the semi-cylinder radius between 0.5 < R_c/R_d < 0.75. The required particle resolution and domain size are appropriately selected based on numerical studies, and a parametric analysis is performed to investigate the relationship between the contact distance (i.e. half the distance between the particle contact points on two neighbouring semi-cylinders), the asperity distance, the size ratio, and the height of the particle centroid from the plane wall. The drag, lift and torque acting on the spherical particle are measured for different particle Reynolds numbers, asperity distances and sizes or diameters. The detachment of particles from rough surfaces can occur through lift-off, sliding and rolling, and the corresponding detachment models are constructed for the case of rough surfaces. These studies will be the basis for developing Lagrangian detachment models that eventually should allow the optimisation of dry powder inhaler performance through computational fluid dynamics.     

齿向修形对滤波减速器润滑性能的影响分析

综合考虑了滤波减速器齿向修形参数、真实齿面粗糙度和瞬态效应等因素,建立了轮齿混合润滑数学模型,数值计算了不同修形参数值对应不同啮合点的最大压力和中心膜厚,分析了齿面粗糙度和转速对润滑性能的影响.结果表明:修形参数r和Ry均存在一个优化范围,使得轮齿表面最大油膜压力显著降低,边缘效应弱化,而中心膜厚则随着r和Ry的增大而逐渐增大;未修形轮齿边缘油膜压力受粗糙度的影响而急剧增大,边缘效应更加显著,修形后轮齿的边缘效应得到了明显改善,因此,轮齿修形也因粗糙表面的存在而显得更加重要;随着转速逐渐降低,轮齿表面的平均油膜厚度逐渐变小,接触比逐渐增大,轮齿表面由弹流润滑逐渐转为混合润滑,最后演变为边界润滑.     

Large-scale spanwise periodicity in a turbulent boundary layer induced by highly ordered and directional surface roughness

The effect of converging–diverging riblet-type surface roughness (riblets arranged in a ‘herringbone’ pattern) are investigated experimentally in a zero pressure gradient turbulent boundary layer. For this initial parametric investigation three different parameters of the surface roughness are analysed in detail; the converging–diverging riblet yaw angle α, the streamwise fetch or development length over the rough surface F_x and the viscous-scaled riblet height h⁺. It is observed that this highly directional surface roughness pattern induces a large-scale spanwise periodicity onto the boundary layer, resulting in a pronounced spanwise modification of the boundary layer thickness. Hot-wire measurements reveal that above the diverging region, the local mean velocity increases while the turbulent intensity decreases, resulting in a thinner overall boundary layer thickness in these locations. The opposite situation occurs over the converging region, where the local mean velocity is decreased and the turbulent intensity increases, producing a locally thicker boundary layer. Increasing the converging–diverging angle or the viscous-scaled riblet height results in stronger spanwise perturbations. For the strongest convergent–divergent angle, the spanwise variation of the boundary layer thickness between the diverging and converging region is almost a factor of two. Such a large variation is remarkable considering that the riblet height is only 1% of the unperturbed boundary layer thickness. Increasing the fetch seems to cause the perturbations to grow further from the surface, while the overall strength of the induced high and low speed regions remain relatively unaltered. Further analysis of the pre-multiplied energy spectra suggests that the surface roughness has modified or redistributed the largest scale energetic structures.     

接触表面不平度对摩擦尖叫噪声的影响

针对摩擦尖叫的间歇性特点,建立了考虑摩擦接触表面不平度的3自由度摩擦自激振动机理模型.应用数值方法分析了接触表面不平度倾角对系统稳定性及瞬态动力学特性的影响,并进行了基于销-盘装置的摩擦尖叫噪声试验研究.结果表明:系统的稳定性主要取决于系统自身的耦合特性,但也与摩擦特性和接触表面不平度倾角的综合作用有关.当接触表面不平度倾角增大时,系统趋向不稳定,会激发尖叫噪声.     

The effect of a systematic change in surface roughness skewness on turbulence and drag

Previous work by the authors (Flack and Schultz, 2010) has identified the root-mean-square roughness height, k_rms, and the skewness, Sk, of the surface elevation distribution as important parameters in scaling the skin-friction drag on rough surfaces. In this study, three surfaces are tested in turbulent boundary layer flow at a friction Reynolds number, Re_τ = 1600–2200. All the surfaces have similar root-mean-square roughness height, while the skewness is varied. Measurements are presented using both two-component LDV and PIV. The results show the anticipated trend of increasing skin-friction drag with increasing skewness. The largest increase in drag occurs going from negative skewness to zero skewness with a more modest increase going from zero to positive skewness. Some differences in the mean velocity and Reynolds stress profiles are observed for the three surfaces. However, these differences are confined to a region close to the rough surface, and the mean velocity and Reynolds stress profiles collapse away from the wall when scaled in outer variables. The turbulence structure as documented through two-point spatial correlations of velocity is also observed to be very similar over the three surfaces. These results support Townsend’s (1976) concept of outer-layer similarity that the wall boundary condition exerts no direct influence on the turbulence structure away from the wall except in setting the velocity and length scales for the outer layer.     

Outer layer similarity in fully rough turbulent boundary layers

Turbulent boundary layer measurements were made on a flat plate covered with uniform spheres and also on the same surface with the addition of a finer-scale grit roughness. The measurements were carried out in a closed return water tunnel, over a momentum thickness Reynolds number (Re) range of 3,000–15,000, using a two-component, laser Doppler velocimeter (LDV). The results show that the mean profiles for all the surfaces collapse well in velocity defect form. Using the maximum peak to trough height (R_t) as the roughness length scale (k), the roughness functions (U⁺) for both surfaces collapse, indicating that roughness texture has no effect on U⁺. The Reynolds stresses for the two rough surfaces also show good agreement throughout the entire boundary layer and collapse with smooth wall results outside of the roughness sublayer. Quadrant analysis and the velocity triple products show changes in the rough wall boundary layers that are confined to y<8k_s, where k_s is the equivalent sand roughness height. The present results provide support for Townsends wall similarity hypothesis for uniform three-dimensional roughness. However, departures from wall similarity may be observed for rough surfaces where 5k_s is large compared to the thickness of the inner layer.