Determining thermal properties of thin coatings using the “thermal mirror” method

We suggest a photothermal method for measuring thermal properties of opaque coatings. The coating–substrate assembly is irradiated by the repetitive pulse power laser which causes a nonstationary buckling of the coating. Thermal properties of the coating are determined by comparing the measured phase shift in the wave component of the temporal dependence of beam deflection angle of the low power laser with theoretical predictions obtained by solving equations of thermal elasticity.     

Spall Characterization in Epoxy Via Laser Spallation

Background: The strength of materials under extreme dynamic loading conditions, such as in the case of shock wave loading, is assessed from their spallation characteristics. Under laboratory conditions, flyer plate impact, or sometimes laser-induced stress waves, is employed to instigate spall in a material. These methods are often combined with velocity interferometer system for any reflector (VISAR) technique for performing transient measurements. Although the VISAR can record the velocity of extremely fast-moving surfaces, it requires a complex optical setup and a specialized data reduction technique. Objective: In this study, a simpler approach is adopted by extending laser spallation method to determine the spall strength of epoxy, while performing in situ interferometric measurements, directly on top of thick epoxy films. Methods: The glass/epoxy test samples are prepared by transferring an aluminum coating on top of epoxy layers with different thicknesses. Laser-induced stress waves transmit across the substrate/film interface and induce subsurface failure in the epoxy at sufficiently high incident laser energy. The nature and magnitude of the waves are deciphered from the out-of-plane displacement histories of the top reflective sample surfaces, which are recorded by using a Michelson interferometer. Results: The interferometric data reveal the development of two (temporally) well-separated stress waves: an ablation-induced high-amplitude short-duration longitudinal pulse, which is referred to as the primary wave, and a secondary wave, which travels at a comparatively slower speed. The complex constructive interaction of the two waves develops a high-magnitude tensile stress region in the epoxy layer. The spall strength is quantified by superimposing the two stress wave histories associated with the critical energy fluence. Conclusions: The spall depths predicted from spatiotemporal wave travel analyses are in excellent agreement with the experimental observations. The newly adopted methodology estimates the spall strength of epoxy as 260?±?20 MPa.

     

改进SPH方法在陶瓷材料层裂数值模拟中的应用

为了探讨铝飞片撞击陶瓷材料时的层裂现象,采用改进SPH方法模拟应力波在陶瓷材料中的传播。结果表明,当离散粒子分布不均匀时,数值模拟计算的自由面速度时程曲线与实测曲线吻合良好。对比CSPM方法,改进SPH方法的精度更高。提出适用于数值模拟的陶瓷材料损伤演化方程,对脉冲载荷下陶瓷/钢层合板层裂的破坏过程进行数值模拟,结果表明,由于陶瓷的波阻抗高于钢的,且抗压强度远高于抗拉强度,因此拉应力引起的层裂破坏是主要的。即使在材料内部传播的只是弹性压缩波,当弹性波到达材料界面时,由界面反射引起的卸载波也能导致陶瓷发生层裂破坏。     

Dynamic torsion testing of nanocrystalline coatings using high-speed photography and digital image correlation

The strength and ductility of microcrystalline and nanocrystalline tungstsen carbide-cobalt (WC-Co) cermets have been evaluated by employing a stored energy Kolsky bar apparatus, high-speed photography and digital image correlation. The test specimens were thin-walled tubular AI7075-T6 substrates 250 μm thick, coated with a 300 μm thick microcrystalline or nanocrystalline WC-Co layer with an average grain size of about 3 μm and 100 nm, respectively. Dynamic torsion experiments reported in this paper reveal a shear modulus of 50 GPa and a shear strength of about 50 MPa for both microcrystalline and nanocrystalline WC-Co coatings. The use of high-speed photography along with digital image correlation has shown that damage to the coating coincides with a significant softening on the stress-strain curve. The coating failed in mode III, and strong interactions between the crack faces were probably responsible for the increase in load after failure of the coating. The overall failure of the coating-substrate system was not brittle but rather progressive and controlled by the ductility of the aluminum substrate. A methodology for investigating damage kinetics and failure has been established. This methodology can be applied to examine the behavior of other advanced materials that can be manufactured as coatings on ductile substrates. Manufacturing coatings free of initial microcracks remains a significant challenge. Research on optimization of the spray deposition parameters should be pursued to produce high-quality nanostructured coatings that can fully exploit the benefits of nano-size grains.     

Convective heat transfer in laser-irradiated graphite

The gas dynamic and thermal processes developing near the surface of graphite after exposure to a 20-nsec laser pulse with an energy E- 0.1–1 J and a wavelength of 0.6943 m are investigated experimentally and by mathematical modeling. The times required for the shock wave to degenerate into an acoustic wave are also considered. Typical density profiles over the axial section of the inhomogeneity are presented for various moments of time. It is noted that the rate of ascent of the thermal inhomogeneity is much higher than the free convection velocity. The convective heat-transfer processes are studied in detail through numerical solution of the system of two-dimensional Navier-Stokes equations. The results of the calculations are in satisfactory agreement with the experimental data.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 180–182, May–June, 1989.     

Impact damage of laminated composite with energy dissipation: Part II — Damage evolution

Having developed the methodology for analyzing the failure of a ceramic/rubber/steel composite laminate impacted by a tungsten rod in Part I, Part II of the work is concerned with the progressive damage process where material continuity would be interrupted at different locations and time intervals. Depending on the time rate dependent threshold values of the surface and volume energy density, the degree and extent of damage by fragmentation, mass loss, etc. are determined by finite element calculations for time steps of 0.15, 5.0, 7.5, 10, 20, 21 and 21.5 μs. Stresses and strains possess an oscillatory character in time; they alternate in sign as the impact waves bounce back and forth in the three-layered dissimilar materials.Local strain rates of approximately 10⁵, 10³ and 10⁴ s⁻¹ are formed in the ceramic, rubber and steel layer respectively at locations underneath the tungsten rod after 16 μs of impact. A more wide range of strain ratio would have prevailed for a homogeneous layer of the same thickness. The tungsten rod is now badly fragmented while cracking near the surface of the ceramic is also predicted. Local temperature and dissipation energy density rise rapidly as time approached 20 μs. The maximum surface and volume.energy density in the ceramic near the impact region reached 260 MPa · m and 6.39 MPa, respectively. Complete disintegration of the tungsten rods occurred at 21.5 μs. At this time, the ceramic layer is perforated and the rubber layer is partially cracked. The back-up steel plate, however, remained in tack. These predictions agree qualitatively with past observations.     

Thermally generated stress waves in a dispersive elastic rod

The propagation of thermally generated stress waves in a dispersive elastic rod was investigated both experimentally and analytically. In the experimental investigation, the end of a circular colored-glass rod was heated very rapidly by the deposition of luminous energy from a Q-switched ruby laser. The light from the laser was directed parallel to the axis of the rod and deposited on the polished end of the rod. The depth of deposition was of the same order as the radius of the rod. The length of the energy pulse from the laser was 20 nsec. This results in heating at such a rate that it can be considered as instantaneous when compared to the mechanical response of the material used. The resulting stress wave was measured using a thin quartz crystal in a Hopkinson pressure-bar arrangement. Radial inertia precluded the use of the simple wave equation; Love's modified wave equation was used to describe the motion. The thermoelastic problem was reduced to a homogeneous partial differential equation with appropriate initial and boundary conditions which is solved by the separation of variables technique. The experimental results are in good agreement with Love's theory. The amplitude of the stress waves was found to be directly proportional to the total energy deposited. The very short stress pulses generated by Q-switched laser deposition on the end of the thin rod gave rise to the higher modes of longitudinal wave propagation. The existence of wave propagation in a thin rod at near dilatational velocities was experimentally confirmed. It is concluded that the experimental techniques developed can be used to model stress-wave generation due to electromagnetic-energy depositions. Also, laser deposition provides an efficient means for generating the higher modes of longitudinal wave propagation in thin rods. Paper was presented at 1968 SESA Spring Meeting held in Albany, N. Y., on May 7–10. This work was supported by the U. S. Atomic Energy Commission at University of California, Lawrence Radiation Laboratory, Livermore, Calif.     

等离子喷涂镍基合金涂层内聚强度的截面划痕表征

制备了3类不同成分的等离子喷涂Ni Cr BSi涂层,利用截面大载荷划痕测试方法表征了涂层内聚结合强度,评价了涂层在油润滑条件下的滑动磨损行为,讨论了涂层内部残余应力、显微硬度、内聚强度与涂层耐磨性之间的内在关联.结果表明:在本文所用涂层沉积与划痕测试条件下,Ni Cr BSi涂层内聚强度的高低次序为Ni25Ni35Ni45,涂层显微硬度的高低次序与喷涂原始粉末硬度次序同为Ni25Ni35Ni45;涂层耐磨性与显微硬度间无对应关系,而与内聚结合强度关系密切,三种涂层耐磨性高低次序为Ni25Ni35Ni45;Ni Cr BSi涂层在低载荷条件下的磨损失效形式主要为磨粒磨损,随着载荷增加,涂层内部微小孔隙、裂纹等缺陷在残余拉应力和摩擦剪切力作用下诱发裂纹扩展并引发材料剥落,使疲劳剥落主导了涂层的磨损失效.     

Mixed-mode interfacial adhesive strength of a thin film on an anisotropic substrate

The mixed-mode interfacial adhesion strength between a gold (Au) thin film and an anisotropic passivated silicon (Si) substrate is measured using laser-induced stress wave loading. Test specimens are prepared by bonding a fused silica (FS) prism to the back side of a 〈1 0 0〉 Si substrate with a thin silicon nitride (Si_xN_y) passivation layer deposited on the top surface. A high-amplitude stress wave is developed by pulsed laser ablation of a sacrificial absorbing layer on one of the lateral surfaces of the FS prism. Due to the negative non-linear elastic properties of the FS, the compressive stress wave evolves into a decompression shock with fast fall time. Careful selection of the incident angle between the pulse and the FS/Si interface generates a mode-converted shear wave in refraction, subjecting the Si_xN_y/Au thin film interface to dynamic mixed-mode loading, sufficient to cause interfacial fracture. A detailed analysis of the anisotropic wave propagation combined with interferometric measurements of surface displacements enables calculation of the interfacial stresses developed under mixed-mode loading. The mixed-mode interfacial strength is compared to the interfacial strength measured under purely tensile loading.     

Analysis methods and size effects in the indentation fracture toughness assessment of very thin oxide coatings on glass

Very thin oxide coatings (<100 nm) which are used as anti-reflection and barrier layers in low emissivity architectural glass have been studied by nanoindentation methods to determine the effect of coating thickness on fracture toughness. Traditional microindentation-derived methods to determine the fracture toughness are unsuitable for assessing very thin coatings (<500 nm) and alternative energy-based models are required depending on what features are visible in indentation load–displacement curves. In cases where radial cracks are formed and grow in a discontinuous manner there are excursions in the load–displacement curve which can be the basis for analysis. In cases where picture frame cracks are observed there are no such features and an alternative approach based on assessment of irreversible work of indentation is required. This paper reviews the methods for obtaining fracture toughness data for very thin coatings and assesses the existence of size effects in the mechanical response of oxide coatings with different thickness on a glass substrate. For oxide coatings in the thickness range 100 to 400 nm no size effects in fracture toughness were observed.     

Mixed-mode failure of thin films using laser-generated shear waves

A new test method is developed for studying mixed-mode interfacial failure of thin films using laser generated stress waves. Guided by recent parametric studies of laser-induced tensile spallation, we successfully extend this technique to achieve mixed-mode loading conditions. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin-film surface. A shear wave is obtained with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions reveals significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading.     

Blast wave propagation in air from a gaseous charge

In the point explosion problem it is assumed that an instantaneous release of finite energy causing shock wave propagation in the ambient gas occurs at a space point. The results of the solution of the problem of such blasts are contained in [1–4]. This point model is applied for the determination of shock wave parameters when the initial pressure in a sphere of finite radius exceeds the ambient air pressure by 2–3 orders of magnitude. The possibility of such a flow simulation at a certain distance from the charge is shown in papers [4, 5] as applied to the blast of a charge of condensed explosive and in [6, 8] as applied to the expansion of a finite volume of strongly compressed hot gas. In certain practical problems the initial pressure in a volume of finite dimensions exceeds atmospheric pressure by a factor 10–15 only. Such cases arise, for example, in the detonation of gaseous fuel-air mixtures. The present paper considers the problem of shock wave propagation in air, caused by explosion of gaseous charge of spherical or cylindrical shape. A numerical solution is obtained in a range of values of the specific energy of the charge characteristic for fuel-air detonation mixtures by means of the method of characteristics without secondary shock wave separation. The influence of the initial conditions of the gas charge explosion (specific energy, nature of initiation, and others) is investigated and compared with the point case with respect to the pressure difference across the shock wave and the positive overpressure pulse.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 110–118, May–June, 1986.     

涂层/基体材料界面结合强度测量方法的现状与展望

界面结合强度是涂层/基体材料体系中的一项重要力学性能指标.而表征与评价涂层/基体材料的界面结合强度又得依靠实验 方法的测定.由于涂层/基体材料体系的多样性与复杂性, 至今还没有形成适合于测量这类材料的界面结合强度的标准方法. 目前, 常用来测量涂层/基体材料的界面结合强度的方法有：拉伸法、剪切法、弯曲法、划痕法、压入法等.本文就目前表征 与评价涂层/基体材料界面结合强度的测量方法做了综述, 讨论了它们的适用范围, 比较了它们的优势与不足.     

Measurement of coatings' elastic properties by mechanical methods: Part 2. Application to thermal barrier coatings

Elastic properties of a thermal barrier ceramic coating composed of an NiCoCrAIY bond coat and a ZrO₂(Y₂O₃) top coat were measured by a four-point bending rig in the temperature range 20°C–900°C. Different types of specimens (i.e., with bond coat only or with bond coat and top coat, on one side or on both sides) were employed. Test procedures were based on the theory discussed in Part 1 to enhance accuracy and to estimate confidence intervals. In particular, the method employed at high temperature was calibrated at room temperature by comparing the results with those obtained by methods with low sensitivity to layer thicknesses. For the bond coat, Young's modulus was found to be temperature independent up to about 500°C; a decreasing trend was observed above this temperature. For the top coat, a slightly temperature range examined. A possible explanation is given on the basis of phase transformation and the microstructure of the two layers. At room temperature, Poisson's ratio for the bond coat was found to be near 0.3, whereas a near zero value was measured for the top coat.     

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.     

Absorption spectroscopy of mid and neighboring Z plasmas: Iron, nickel,copper and germanium

Opacities of four medium Z element plasmas (iron, nickel, copper and germanium) have been measured at the LULI-2000 facility in similar conditions: temperatures between 15 and 25 eV and densities between 2 and 10 mg/cm³, in a wavelength range (8–18 Å) including the strong 2p–3d structures.Two laser beams from the LULI facility were used in the nanosecond-picosecond configuration. The NANO-2000 beam (at λ = 0.53 μm) heated a gold hohlraum with an energy between 30 and 150 J with a duration of 0.6 ns. Samples covering half a hohlraum hole were thus radiatively heated. The picosecond pulse PICO-2000 beam (at λ = 1.053 μm) has been used to produce a short (about 10 ps) X-ray backlighter in order to reduce time variations of temperatures and densities during the measurement. A crystal high-resolution spectrometer was used as the main diagnostic to record at the same time the non-absorbed and the absorbed backlighter spectra. Radiation temperatures were measured using a broadband spectrometer. 1D and 2D simulations have been performed in order to estimate hydrodynamic plasmas parameters.The measured spectra have been compared with theoretical ones obtained using either the superconfiguration code SCO or the detailed term accounting code HULLAC. These comparisons allow us to check the modeling of the statistical broadening and of the spin-orbit splitting of the 2p–3d transitions and related effects such as the interaction between relativistic subconfigurations belonging to the same non-relativistic configuration.     

Warm dense matter at the bench-top: Fs-laser-induced confined micro-explosion

We report the experimental evidence for creation of Warm Dense Matter (WDM) in ultrafast laser-induced micro-explosion inside a sapphire (Al₂O₃) crystal. We show that the WDM can be formed by a 100 nJ fs-pulse if the following conditions are satisfied: (1) the laser pulse is tightly focused to inside of the bulk of transparent material so the intensity at focus is two orders of magnitude higher than the optical breakdown threshold; (2) the pulse duration is shorter than the electron-ion energy exchange time; and, (3) the absorbed energy density is above the Young’s modulus for the material studied. The empty void created inside a sapphire crystal surrounded by a shell of compressed material provides the direct evidence of the maximum pressure above the Young’s modulus of sapphire (∼400 GPa). Synchrotron X-ray diffraction (XRD) analysis of the shell revealed the presence of novel super-dense bcc-Al crystalline phase predicted at pressures above ∼380 GPa theoretically, which has never been observed experimentally before neither in nature in laboratory experiments. These results show that confined micro-explosion induced by tightly focussed fs-laser inside a transparent solid opens new routes for synthesis of new materials and study of WDM at a laboratory bench-top.     

含WC陶瓷相电弧喷涂层耐磨粒磨损性能的研究

采用电弧喷涂含WC-CoNi金属陶瓷粉末的粉芯丝材,在低碳钢基体上制备铁基复合涂层,采用MLS-225型湿砂橡胶轮磨损试验机评价铁基复合涂层的耐磨粒磨损性能,利用光学显微镜、扫描电子显微镜和X射线衍射仪对涂层的显微组织结构、磨损表面及其相组成进行分析.结果表明:含WC陶瓷相涂层的耐磨粒磨损性能较好,相对Q235钢提高约9倍;当粉芯中WC质量分数低于25%时,随着WC含量增加,涂层的硬度和耐磨性增加;当粉芯中WC质量分数超过25%后,涂层的耐磨性有所下降;电弧喷涂含WC陶瓷相涂层的磨损机制主要为硬质相的脆性剥离和轻微的塑性切削,在磨粒磨损条件下硬度较低的金属基体先磨损,硬度较高的WC和Fe3B硬质相起到阻止石英砂磨损的作用,从而降低了涂层的磨损.     

MeV proton generation and efficiency from an intense laser irradiated foil

The proton energy distribution generated from the interaction of an intense (Iλ² ≈ 10²⁰ W/cm² μm²) short-pulse (100 fs) laser with a thin foil is investigated using energy resolved measurements and 2D collisional PIC-hybrid simulations. The measured absolute proton spectrum is well matched by a 1.7 MeV exponential function for energies <11 MeV. The proton conversion efficiency from hot electrons ≈6%. Simulations predict a strong radial dependence on the maximum proton energy and on the radial extent of 12 Å hydrocarbon depletion region. C and O ions in the hydrocarbon layer gain significant energies, limiting the efficiency to the protons. The efficiency scaling for ion mixtures is derived using a simple model, and is shown to strongly depend on the cooling rate of the hot electrons. Simulations using hydrogen-rich, layered targets predict much higher efficiencies.     

Effect of a laser beam on plexiglas

An analysis is given of the optical effects during the focusing of a laser beam inside a transparent dielectric (plexiglas). Two types of damage were established. One is connected with the appearance of microcracks and the other with large plane cracks. Transition from one type of damage to the other is observed when the focal length of the lenses (energy density in the specimen) is varied and the pulse length is changed from 10^–3 to 10^–8 sec. The effect of a laser beam on metals and ionic crystals was investigated in [1–3]. A number of effects associated with structural changes and a specific material damage were found. There is also some interest in the effects of laser light on transparent dielectrics (glasses and polymers). In the present paper we report results of an investigation of the effect of laser beams on plexiglas.The authors are grateful to G. I. Barenblatt and B. Ya. Zel'dovich for valuable advice in formulating the problem and discussions of the results and to V. V. Kireev, G. F. Kuzmin, and O. E. Marin for assistance in the experiments.