表面溶胶-凝胶法制备TiO2薄膜的微观结构及其摩擦磨损性能研究

采用表面溶胶-凝胶法在普通载玻片上制备TiO2纳米结构薄膜,用X射线光电子能谱仪(XPS)、X射线衍射仪(XRD)和原子力显微镜(AFM)分析薄膜组成结构及其表面形貌,用UMT-2型摩擦磨损试验机考察表面溶胶-凝胶法和传统体溶胶-凝胶法制备的TiO2薄膜的摩擦磨损性能,用扫描电子显微镜(SEM)观察分析薄膜表面及其磨痕表面形貌.结果表明,与传统体溶胶-凝胶法相比,表面溶胶-凝胶法制备的薄膜摩擦磨损性能得到明显改善.这是由于多次干燥过程在一定程度上避免了传统体溶胶-凝胶工艺的后续干燥和热处理过程中大量残留溶剂瞬时溢出而导致的结构缺陷,从而使得薄膜具有更高的致密性,降低了薄膜表面粗糙度,提高了抗磨性.     

A parametric study of laser induced thin film spallation

We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.     

Micron-scale channel formation by the release and bond-back of pre-stressed thin films: A finite element analysis

Buckling of thin films on a rigid substrate during use or fabrication is a well-known but unwanted phenomenon. However, this phenomenon can also be exploited to generate well-controlled patterns at the micro and nano-scale. These patterned surfaces find various technological applications such as optical gratings or micro/nano-fluidic channels. In this article, we present a numerical model that accounts for the buckling-up of pre-strained thin films by a reduction of the interface toughness and the subsequent bond-back. Channels are formed whose dimensions can be controlled by tuning the film dimensions, film thickness and stiffness, the eigenstrain in the film and the cohesive interface energy between the film and the substrate. We will show how the buckling-up and draping back processes can be captured in terms of a limited set of dimensionless parameters, providing quantitative insight on how these parameters should be tuned to generate a specified channel geometry.     

Thermo-mechanical solution of film/substrate systems under local thermal load and application to laser lift-off of GaN/sapphire structures

Film/substrate structures may undergo a localized thermal load, which can induce stresses, deformation and defects. In this paper, we present the solutions of temperature and stresses in a film/substrate structure under a local thermal load on the film surface. Then, the generalized Stoney formula, which connects the curvature of deformation and the stress field is obtained. The present solution takes into account the non-uniformity of the temperature field both in the width and thickness directions of the film. The thermo-mechanical solution is applied to the analysis of the temperature distribution, stresses, and damage of a GaN/sapphire system during the laser lift-off (LLO) process. It is shown that the laser with the Gaussian distribution of energy density causes much smaller tensile stresses at the edge of the heated area in the film than the laser with the uniform distribution of energy density, and thus can avoid damage to the GaN films separated from the substrate.     

Heat transfer mechanisms during short-pulse laser heating of two-layer composite thin films

 Using a simple perturbation technique, an analytical investigation is presented for the heat transfer mechanisms during ultrafast laser heating of two-layer composite thin slabs from a microscopic point of view. The composite slab consists of two thin metal films which are in perfect thermal contact. The microscopic parabolic two-step model is adopted to describe the behavior of the composite slab. In the microscopic two-step model, the heating process is modeled by the deposition of radiation energy on electrons, the transport of energy by electrons, and the heating of the material lattice through electron–phonon interactions. The proposed perturbation technique is used when the normalized temperature difference between the solid lattice and the electron gas is relatively a small perturbed quantity. Received on 20 September 2000 / Published online: 29 November 2001     

Multi-layer thin films/substrate system subjected to non-uniform misfit strains

Current methodologies used for the inference of thin film stress through curvature measurement are strictly restricted to stress and curvature states that are assumed to remain uniform over the entire film/substrate system. These methodologies have recently been extended to a single layer of thin film deposited on a substrate subjected to the non-uniform misfit strain in the thin film. Such methodologies are further extended to multi-layer thin films deposited on a substrate in the present study. Each thin film may have its own non-uniform misfit strain. We derive relations between the stresses in each thin film and the change of system curvatures due to the deposition of each thin film. The interface shear stresses between the adjacent films and between the thin film and the substrate are also obtained from the system curvatures. This provides the basis for the experimental determination of thin film stresses in multi-layer thin films on a substrate.     

Thin film/substrate systems featuring arbitrary film thickness and misfit strain distributions. Part I: Analysis for obtaining film stress from non-local curvature information

Current methodologies used for the inference of thin film stress through curvature measurements are strictly restricted to stress and curvature states which are assumed to remain uniform over the entire film/substrate system. Recently Huang, Rosakis and co-workers [Huang, Y., Ngo, D., Rosakis, A.J., 2005. Non-uniform, axisymmetric misfit strain: in thin films bonded on plate substrates/substrate systems: the relation between non-uniform film stresses and system curvatures. Acta Mech. Sin. 21, 362–370; Huang, Y., Rosakis A.J., 2005. Extension of Stoney’s Formula to non-uniform temperature distributions in thin film/substrate systems. The case of radial symmetry. J. Mech. Phys. Solids 53, 2483–2500; Ngo, D., Huang, Y., Rosakis, A. J., Feng, X. 2006. Spatially non-uniform, isotropic misfit strain in thin films bonded on plate substrates: the relation between non-uniform film stresses and system curvatures. Thin Solid Films (in press)] established methods for film/substrate system subject to non-uniform misfit strain and temperature changes. The film stresses were found to depend non-locally on system curvatures (i.e., depend on the full-field curvatures). The existing methods, however, all assume uniform film thickness which is often violated in the thin film/substrate system. We extend these methods to arbitrarily non-uniform film thickness for the thin film/substrate system subject to non-uniform misfit strain. Remarkably the stress-curvature relation for uniform film thickness still holds if the film thickness is replaced by its local value at the point where the stress is evaluated. This result has been experimentally validated in Part II of this paper.     

Spontaneous instability of soft thin films on curved substrates due to van der Waals interaction

The linear bifurcation theory is used to investigate the stability of soft thin films bonded to curved substrates. It is found that such a film can spontaneously lose its stability due to van der Waals or electrostatic interaction when its thickness reduces to the order of microns or nanometers. We first present the generic method for analyzing the surface stability of a thin film interacting with the substrate and then discuss several important geometric configurations with either a positive or negative mean curvature. The critical conditions for the onset of spontaneous instability in these representative examples are established analytically. Besides the surface energy and Poisson's ratio of the thin film, the curvature of the substrate is demonstrated to have a significant influence on the wrinkling behavior of the film. The results suggest that one may fabricate nanopatterns or enhance the surface stability of soft thin films on curved solid surfaces by modulating the mechanical properties of the films and/or such geometrical properties as film thickness and substrate curvature. This study can also help to understand various phenomena associated with surface instability.     

Experimental and theoretical/numerical investigations of thin films bonding strength

A laser spallation facility has been developed to measure the strength of planar interfaces between a substrate and a thin coating. This quantity is a central requirement in contemporary thin film and protective coatings technology and its successful measurement should improve the scientific/technological potential for the design of advanced composites, protective coatings of composites that operate in hostile environments, and in joining of dissimilar materials. The technique involves impinging a laser pulse of ultra short duration on the rear surface of the substrate, which is coated by a thin layer of energy absorbing metal such as Sn and Pb. The explosive evaporation of the metallic layer, confined between a fused quartz crystal and the substrate, induces a compressive shock wave, which propagates through the substrate toward the material interface. Upon reflection from the free surface of the coating, the pressure pulse is converted into a tensile wave which, under certain conditions, can lead to spallation at the interface. It is shown by mathematical simulation that atomic bond rupture is the mechanism of separation in this experiment. Since the interaction of laser energy with matter is a complicated, highly non-linear process, our investigations, at first, were based on measurement of the pressure pulse generated by the threshold flux level that leads to spallation, by using a micro-electronics device with a piezo-electric crystal, and on computation of the tensile stress experienced at the material interface, by numerical simulation of the induced stress wave propagation. Several substrate/coating (ceramic/ceramic and ceramic/metal) systems have been investigated such as, 1–15 μm SiC by CVD, 1–4 μm TiC and TiN by PVD coatings on sapphire substrates, as well as 1–2 μm Au, Sn and Ag coatings by sputtering on sapphire, fused quartz and glass substrates. For identically prepared specimens, the measured threshold energy levels are reproducible, thus leading to reproducible bond strength values, while the spall size, as expected, is dependent on the laser pulse energy level. Finally, the bond strength values obtained are in very good agreement with similar data derived by direct experimental techniques based on Laser-Doppler-Interferometry.     

Exact solution of heat transfer from a stretching surface with variable heat flux

An exact expression of the temperature distribution is constructed for the heat transfer from a stretching surface with prescribed power law heat flux. The stretching velocity is inversely proportional to the one third power of the distance measured along the surface from a thin slit. The final result is expressed in terms of hypergeometric functions. Although the exact solution is accomplished, some physically unrealistic phenomena are encounters for specific conditions. The temperature parameter which prescribe the surface heat flux, strongly affects those situations. Two types of temperature distribution are discussed: dimensionless temperatures with and without scaling to the dimensionless surface temperature. The expression of the temperature distribution without scaling is lucid to understand the heat transfer characteristics. Received on 23 July 1997     

微构件自由弯曲回弹的数值模拟

回弹是影响弯曲成形精度的一个重要因素,由于尺度效应的存在,微构件的弯曲回弹问题更复杂.利用动态显式与静态隐式算法相结合的方法数值模拟了C1200黄铜V形微构件的自由弯曲,与其超薄板三点弯曲的实验结果吻合度很好,验证了该方法模拟微构件弯曲回弹的可行性.分析了板厚t、晶粒大小d、t/d(板厚与晶粒大小比值)和残余应力对回弹的影响,结果显示t/d可以作为表征回弹的重要参数,回弹量随着t/d减小而增大;随着t和d的增大,工件的残余应力也增大,会影响工件的质量.可以采用适当热处理工艺增大材料的晶粒尺寸来减少回弹,当然也要兼顾残余应力对工件质量的影响.     

Buckling of a stiff thin film on a pre-strained bi-layer substrate

Controlled buckling can impart stretchable mechanics to brittle materials when integrated as thin films on soft, elastomeric substrates. Typical elastomers are permeable to fluids, however, and therefor unable to provide robust barriers to entry of water, for instance, into devices built with the supported thin films. In addition, the mechanical strength of a system dominated by a soft substrate is often unsatisfactory for realistic applications. We show that introduction of a bi-layer substrate yields a robust, high strength system that maintains stretchable characteristics, with a soft layer on top of a relatively stiff layer in the substrate. As a mechanical protection, a soft encapsulation layer can be used on top of the device and the stretchability of the encapsulated system is smaller than that of the system without encapsulation. A simple, analytic model, validated by numerical analysis and FEA, is established for stiff thin films on a bi-layer substrate, and is useful to the design of stretchable systems.     

Determining the Elastic Modulus of Compliant Thin Films Supported on Substrates from Flat Punch Indentation Measurements

Instrumented indentation is a technique that can be used to measure the elastic properties of soft thin films supported on stiffer substrates, including polymer films, cellulosic sheets, and thin layers of biological materials. When measuring thin film properties using indentation, the effect of the substrate must be considered. Most existing models for determining the properties of thin films from indentation measurements were developed for metal and dielectric films bonded to semiconductor substrates and have been applied to systems with film-substrate modulus ratios between 0.1 and 10. In the present work, flat punch indentation of a thin film either bonded to or in contact with a substrate is examined using finite element modeling. A broad range of film-substrate modulus ratios from 0.0001 to 1 are investigated. As the substrate is effectively rigid compared to the film when the film-substrate modulus ratio is less than 0.0001, the results are also useful for understanding systems with lower film-substrate modulus ratios. The effects of the contact radius, film thickness, elastic properties, and friction between the film and the substrate on the measured stiffness were quantified using finite element modeling in order to understand how the elastic properties of the film can be extracted from indentation measurements. A semi-analytical model was developed to describe the finite element modeling results and facilitate the use of the results to analyze experimental measurements. The model was validated through analysis of indentation measurements of thin polyethylene sheets that were supported on substrates of various stiffness.     

Exact harmonic solutions to Guyer–Krumhansl-type equation and application to heat transport in thin films

A system of hyperbolic-type inhomogeneous differential equations (DE) is considered for non-Fourier heat transfer in thin films. Exact harmonic solutions to Guyer–Krumhansl-type heat equation and to the system of inhomogeneous DE are obtained in Cauchy- and Dirichlet-type conditions. The contribution of the ballistic-type heat transport, of the Cattaneo heat waves and of the Fourier heat diffusion is discussed and compared with each other in various conditions. The application of the study to the ballistic heat transport in thin films is performed. Rapid evolution of the ballistic quasi-temperature component in low-dimensional systems is elucidated and compared with slow evolution of its diffusive counterpart. The effect of the ballistic quasi-temperature component on the evolution of the complete quasi-temperature is explored. In this context, the influence of the Knudsen number and of Cauchy- and Dirichlet-type conditions on the evolution of the temperature distribution is explored. The comparative analysis of the obtained solutions is performed.     

A suppression method for thermomechanical stresses during the laser processing of structural elements

A method to reduce thermomechanical stresses during the laser processing of a beam-like specimen along its side surface and a thin disk along its central circular hole is considered. Some analytical temperature expressions are obtained in both these cases. The problems of determining thermal stresses during the heating of specimens with consideration of heat transfer on their surfaces are solved. The solutions obtained with heat transfer and without it are compared. It is shown that the side surface blowing can be used to suppress the fracture of heated specimens.     

Strain tunability of dielectric and ferroelectric properties in epitaxial lead titanate thin films

Many distinguished properties of epitaxial ferroelectric thin films can be tunable through the misfit strain. The strain tunability of ferroelectric and dielectric properties in epitaxial lead titanate ultrathin films is numerically investigated by using a phase field model, in which the surface effect of polarization is taken into account. The response of polarization to the applied electric field in the thickness direction is examined with different misfit strains at room temperature. It is found that a compressive misfit strain increases the coercive field and the remanent polarization while a tensile misfit strain decreases both of them. The nonlinear dielectric constants of the thin films with tensile misfit strains are much larger than those of the thin films without misfit strains, which are attributed to the existence of the a/c/a/c multiple domains in the thin films under tensile misfit strains.     

纳米压痕法测磁控溅射铝薄膜屈服应力

为了在考虑残余应力下测量出磁控溅射铝薄膜的屈服应力,提出了一种实验测量方法,通过曲率测试法和球形压头纳米压痕法测出磁控溅射铝薄膜的屈服应力．建立球形压痕力学模型,并用ANSYS对球形压痕进行力学有限元仿真,利用直流磁控溅射技术在硅基上淀积一层1 μm厚的铝薄膜,首先通过曲率测试法测量膜内等双轴残余应力,再利用最小二乘曲线拟合法从薄膜/基底系统的球形压头纳米压痕实验数据中提取出铝薄膜的屈服应力,测得磁控溅射铝薄膜的屈服应力为371 MPa．该方法也可以用来研究其他材料的薄膜和小体积材料的力学特性．     

Pure-shear Failure of Thin Films by Laser-induced Shear Waves

The measurement of mode-dependent thin film interfacial properties is important in evaluating the quality of the interfaces between thin films and substrates. Previous work has proved that tensile and mixed-mode strength of a thin film/substrate interface can be evaluated using a laser-induced thin film spallation technique. To further examine the application regime of this technique and identify the individual roles of the tensile and shear stress in the resulting interfacial failure, a special sample design is adopted in the current work to realize pure-shear loading at the thin film/substrate interface. Our result demonstrates that for sufficiently high stress amplitude, interfacial failure can be induced solely by the in-plane shear stress and the stress can be quantitatively determined from optical interferometric measurements. Together with the previous tensile and mixed-mode studies, a complete picture of the mode-dependent thin film interfacial strength can now be reliably determined using the laser-induced thin film spallation techniques.     

Simulation of glass cutting with an impinging hot air jet

Thermal cutting of glass sheet due to an impinging hot air jet is simulated and analyzed. Induced thermal stresses due to the moving heat source can be used to stably initiate and attract a crack toward the jet axis. Relative motion between the jet and glass sheet then can be used to cut the glass sheet. This paper presents a theoretical study of this process for straight cuts. Process simulation is accomplished by analyzing the coupled temperature and stress fields together with the fracture mechanics criteria for the crack growth.A finite element remeshing technique is employed for the analysis and singular elements are used around the crack tip for a more precise computing of the stress intensity factor. It is shown that a certain minimum air jet temperature for a given nozzle velocity and a certain maximum air jet velocity for a given temperature are required for continuous cutting. The results of the simulation show a good agreement with the published results in the literature. However a variating nature is detected for the distance between the crack tip and the air jet nozzle from a starting value to the steady-state one.     

Effect of wall emissivities on radiation heat transfer in glass tank forehearths

In an earlier work, we had proposed a two-band, nongrey radiative transfer model for heat transfer in forehearths with simultaneous optically thick and thin approximations for molten glass interiors and at boundaries. Here using the same model, the radiative interaction of the top-crown and bottomrefratory walls with interior layers of shallow molton glass is studied by varying the wall emissivities. The forehearth exit temperature profiles for higher wall emissivities (0.9) show better conditioning of the glass for white flint glasses (optically thin).