Photographing impact of plasma-sprayed particles on metal substrates

Plasma-sprayed, molten molybdenum particles (∼40 μm diameter) were photographed during impact (with velocity ∼110 m/s) on Inconel surfaces that were maintained at either room temperature or at 400 °C. Some samples were also preheated at 400 °C for 3 h and then air-cooled to room temperature before spraying. A droplet approaching the surface was sensed using a photodetector, and after a known delay, a fast charge-coupled device camera was triggered to capture images of the spreading splat from the substrate front surface. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting the surfaces at room temperature disintegrated and splashed after achieving a maximum diameter >400 μm. Impact on preheated and heated Inconel produced splats with maximum diameters between 200 and 300 μm with less fragmentation. The cooling rate of splats on preheated Inconel was larger than that of splats on nonheated Inconel. Surface analysis showed that preheating Inconel reduced the surface skewness and kurtosis, resulting in improved splat-substrate contact. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

In-flight oxidation of aluminum in the twin-wire electric arc process

This paper examines the in-flight oxidation of aluminum sprayed in air using the twin-wire electric arc (TWEA) thermal spray process. Aerodynamic shear at the droplet surface increases the amount of in-flight oxidation by promoting entrainment of the surface oxides within the molten droplet and continually exposing fresh fluid available for oxidation. Mathematical predictions herein confirm experimental measurements that reveal an elevated, nearly constant surface temperature (∼2273 K) of the droplets during flight. The calculated oxide volume fraction of a “typical” droplet with internal circulation compares favorably to the experimentally determined oxide content (3.3–12.7%) for a typical TWEA-sprayed aluminum coating sprayed onto a room temperature substrate. It is concluded that internal circulation within the molten aluminum droplet is a significant source of oxidation. This effect produces an oxide content nearly two orders of magnitude larger than that of a droplet without continual oxidation.     

Use of thermal emission signals to characterize the impact of fully and partially molten plasma-sprayed zirconia particles on glass surfaces

Plasma-sprayed yttria-stabilized zirconia particles (∼ 40 µm diameter) were photographed during impact (velocity ∼ 200 m/s) on glass surfaces that were maintained at either room temperature or 400 °C. A droplet that approached the surface was sensed using a photodetector and after a known delay, a light source was triggered to illuminate the particle in order to photograph it with a charge-coupled device (CCD) camera. A rapid two-color pyrometer was used to collect the thermal radiation from the particles to follow the evolution of their temperature and size, in-flight and after impact. The fully molten particles spread into a thin liquid splat after impacting the surfaces. The partially molten particles disintegrated into small satellite fragments immediately upon impact. The surface area, as indicated by the pyrometric signals, of the partially molten particles during spreading was almost an order of magnitude smaller than that of the fully molten droplets. The pyrometric signals of partially molten zirconia showed significant fluctuations, lower maximum voltages, and were wider than those of fully molten zirconia. These signals, characteristic of the impact of partially molten zirconia, provide a novel method of identifying partially molten ceramic particles after impact on a flat surface.     

金属熔滴碰撞、铺张数值分析

基于金属熔滴沉积成形的3D打印技术,其成形质量与金属熔滴及基板参数有很大关系。本文对金属铝熔滴在不同参数下,比如：熔滴直径、滴落速度、比热、导热系数、潜热以及基板温度等,其凝固过程中的最大铺展因子关系进行了研究。采用VOF对自由表面进行追踪的方法,建立包括对流和相变的计算方程和能量方程的仿真模型,耦合水平集函数,跟踪熔融粒子与周围空气之间的接口,获得的最大传播因子之间的关系,与在文献中出现的实验数据吻合。     

In situ measurement system for deformation and solidification phenomena of yttria-stabilized zirconia droplets impinging on quartz glass substrate under plasma-spraying conditions

The authors have developed an in situ measurement system for precise one-to-one correlation between splat morphology and thermal history during particle impingement on a temperature-controlled substrate inside an airtight chamber under plasma-spraying conditions. The system has made it possible to collect about 10 single particles successively within a 10 s time frame, and to correlate exactly the relationship between the size, the temperature, and the impacting velocity of each droplet, and the morphology of the splats. The most striking finding is that a part of the yttria-stabilized zirconia (YSZ) droplets may be actually in supercooled condition before impinging, although a marked difference was not found in the splat morphology. In addition, as secondary results, we could evaluate the viscosity, μ, of YSZ, and the thermal contact resistance between YSZ splat and the quartz glass substrate as μ [Pa · s]=0.0037 exp (6110/T) and 3 × 10⁻⁶-4×10⁻⁵ m² K/W, respectively. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

等离子喷涂层片形成过程流体动力学模拟

建立了等离子喷涂层片形成过程的流动、传热和凝固耦合的三维数学模型,基于计算流体力学软件Fluent,运用有限体积法（FVM）离散控制方程,流体体积跟踪法（VOF）追踪熔滴自由表面,模拟了镍熔滴撞击基底表面形成层片的流体动力学过程,并对结果可视化输出.结果表明,撞击开始时刻,压力在撞击点出现最大值,熔滴内部压力场呈＂蘑菇云＂状,撞击压力分布从撞击点向熔滴上部递减;熔滴撞击后沿径向铺展,速度矢量场出现两个对称漩涡,在基底导热和熔体流动传热综合作用下,铺展熔滴内部温度场呈＂驼峰＂状分布;熔滴铺展速度最大值滞后最大撞击压力0.03μs出现.     

Laser sintering of ultradispersed iron-base powder materials

Experimental data on the formation of the structure and mechanical properties of layers produced from ultradispersed Fe-base powder mixtures under laser irradiation (λ ∼ 1 μm) with a radiation density q ∼ 10³ W/cm² and pulse duration 10⁻⁵ s are reported. The results of the study provide a background for the development of a method of producing complex metal alloys in a metastable condition with the help of high-concentration energy sources. The field of application of this method is the production of thin layers on the surface of highly loaded friction units.     

Laser-assisted replication of large-area nanostructures

We proposed and demonstrated a high-throughput fabrication method for large-area nanostructured polymers. The mold used consists of a quartz substrate and a nanostructured diamond-like carbon (DLC) thin film. A laser is irradiated from the back of the mold and only the DLC surface is directly heated. Then the surface of a polymethyl methacrylate (PMMA) film pressed by the mold is melted and the nanostructures are replicated. In this method, replication can be achieved with a low amount of heat and a short cycle time compared with conventional thermal replication. The effects of the laser power density, irradiation time, and environmental temperature on the replication area were experimentally investigated via the spot irradiation of a laser. Furthermore, the temperature distribution around the surfaces of the mold and polymer was investigated by performing numerical simulations. By scanning the laser, we successfully demonstrated the replication of a 500-nm-pitch pattern on a PMMA film with an area of 10 × 10 mm² in about 10 s. This technique is expected to lead to the high-throughput and low-energy fabrication of large-area nanostructured optical films.     

Kinetics of formation and growth of epitaxial SrTiO<Subscript>3</Subscript> films of single-crystal (001) SrTiO<Subscript>3</Subscript> supports

The aim of this study was to quantitatively estimate the kinetics of the formation and growth of oxide SrTiO₃ (STO) films using the method of the in situ reflection high-energy electron diffraction (RHEED) and compare the obtained results with the known growth models and theoretical estimates. The kinetics of the relaxation and crystallization of particles is studied under pulsed laser deposition (PLD) from oxide targets onto (001) STO supports or onto the surface of STO film growth at 650–800°C. Deposition frequencies of 0.1–10 Hz typical of PLD were used. The surface morphology and film structure was studied ex situ using the methods of AFM and X-ray-structural analysis. It was found that the time of relaxation of deposited particles is within the range of 2–20 s, which greatly exceeds or is comparable to the relative pulse duration. It was experimentally shown that structural distortions in epitaxial films for temperatures of ≤900°C are mainly due to the high rate of deposition and limited surface mobility of particles. The effect of structural relaxation in films is observed after the end of deposition; the time constant of bulk structural relaxation is ∼10 − ∼10² s or more. The obtained kinetic parameters of the formation of an oxide structure may be useful for the development of crystallization theory, as well as to optimize the conditions of epitaxial oxide film growth.     

等离子喷涂钼层片组织和晶体取向的EBSD分析

在抛光1Cr13不锈钢基底上采集了大气等离子喷涂的钼层片。利用台阶仪、扫描电子显微镜（SEM）、能谱仪（EDS）、电子背散射衍射（EBSD）等表征了层片的形貌、微观组织和晶体学取向分布。结果表明，钼层片呈“花瓣状”，并伴有铺展飞溅。基底在层片中心处因熔化/撞击形成“凹坑”，平均深度0.7μm。层片中心区域为细小的柱状晶，晶粒尺寸范围0.75~9.25μm，平均值为2.30μm；边缘区域为沿着径向拉长的晶粒，晶粒尺寸范围4.25~18.25μm，平均值8.50μm。层片边缘区域因翘曲，接触热阻较中心区域大，造成晶粒粗大。无论是层片中心区域还是边缘区域晶粒取向都随机分布，没有明显织构。     

Visible laser microfabrication of transparent plastic using Au nanoparticles-dispersed polymer film

A polymer film in which Au nanoparticles with average diameter of around 3 nm dispersed in ethylcellulose was applied to an absorber for laser microfabrication of a transparent plastic. Since the polymer film has a strong absorption at the wavelength of around 530 nm, it can be micromachined using focused low power Nd:YVO₄–SHG laser (CW, wavelength of 532 nm). When laser beam was irradiated on the polymer film coated on transparent substrate, the substrate under the polymer film which has no absorbance in the range of wavelength of laser beam was processed. A micropattern was clearly fabricated on transparent poly(methyl methacrylate) and polyethylene terephthalate using our polymer film. In poly(methyl methacrylate) substrate, the processed depth showed a maximum at the polymer film thickness of 15 μm under the condition of laser power of 23 mW. Finally, laser marking test on the transparent poly(methyl methacrylate) was demonstrated as an application of our system using the optimal polymer film thickness.     

Measuring depth-dependent dislocation densities and Elastic strains in an indented Ni-based Superalloy

The indentation-induced elastic-plastic zone in an IN 740 Ni-based superalloy was studied by three-dimensional (3-D) x-ray microdiffraction and electron back scattering diffraction (EBSD). Large lattice reorientations and the formation of geometrically necessary dislocations are observed in the area with a radius of ∼75 μm. A residual compression zone is found close to the indent edge. An elastic-plastic transition is observed at ∼20 μm from the indent edge. Depth dependent dislocation densities are determined at different distances from the indent edge.     

Simulation of cooling of liquid Al–33 wt.% Cu droplet impinging on a metallic substrate and its experimental validation

In the present work a model for heat transfer during collision of a falling liquid Al–33 wt.% Cu droplet on a 304 stainless steel substrate has been developed on a FLUENT 6.3.16 platform. The model simultaneously takes into account the fluid flow and heat transfer in the liquid droplet and the surrounding gas, and the heat transfer in the substrate. The liquid–gas interface was tracked using the volume of fluid method and the contact resistance between Al–33 wt.% Cu and the substrate was taken into account. The comprehensive model correctly predicted the total spread in the droplet. As per the predicted transient thermal field, the solidification front speed oscillated along the radius of the spread droplet. Based on the estimated front speeds at these locations and Jackson–Hunt plot for Al–33 wt.% Cu, the variation of interlamellar spacing along the radial direction was found. It matched well with the variation of the experimentally measured interlamellar spacing at different locations along the radius.     

Finite element analysis of effect of substrate surface roughness on liquid droplet impact and flattening process

A computational program using the finite element method has been developed to simulate the impact and flattening of a metal droplet impacting onto a solid surface with different surface roughness occurring in the plasma thermal spray. The model is based on Navier-Stokes equations combining with friction conditions on the substrate surface to simulate the effect of substrate surface roughness on the flattening process of the droplet. In this study, a moving free surface model based on the Lagrangian method with an automatic adaptive remeshing technique has been developed to handle the large deformation of droplets and to ensure the computational accuracy of the numerical results. The numerical results show that the substrate surface roughness has a significant influence on the spreading velocity, flattening ratio, flattening time, splat size, and shape. The spreading process of a droplet is governed not only by the inertia and viscous forces, but also by the frictional resistance of the substrate surface.     

等离子喷涂熔滴快速撞击基体非稳态凝固行为研究进展

涂层成形过程中的缺陷含量、残余应力、沉积效率、组织结构及力学性能等指标均会随着工艺参数与基体预处理状态的不同而发生显著变化,因而需要从更加微观的角度深入理解等离子喷涂涂层的微观构筑过程,即单个熔滴的铺展凝固现象。本研究分别从熔滴凝固的类型与机理、不同因素对熔滴凝固过程的影响及凝固斑点形态的定量表征方法 3个方面详细综述了等离子喷涂熔滴撞击基体快速凝固过程的研究现状。结果表明,熔滴的铺展形态主要可以分为5类,包括圆盘型、破碎型、放射型、花瓣型及气泡型,影响铺展过程的因素主要包括熔滴特性(速度、温度、粒径、材料属性、熔化状态等)与基体状态(表面粗糙形貌、表面化学状态、吸附物及冷凝物、界面润湿性及接触热阻等)2大类,综合采用一系列参数对熔滴铺展几何形态进行表征,可实现熔滴沉积质量的定量评价。     

Physical processes of the formation of structure and properties of films of transition-metal diborides

Physical processes are described that occur during the formation of the structure of film coatings of transition-metal diborides produced by the method of magnetron sputtering (in the DC and HF modes). The factors that affect the formation of the structure of the film coatings (energy of condensing atoms and substrate temperature) have been determined. The role of each factor in the formation of the film structure is shown. The optimum energy conditions for the formation of the transition-metal diboride films with the highest physicomechanical characteristics have been determined, i.e., the bias voltage equal to −50 V (DC regime) and ±50 V (HF regime); the substrate temperature ∼500°C. Under these conditions, there are formed hyperstoichiometric MB_2.4 films with a grain size of ∼20 nm and greater. If the energy supplied to the growing film is insufficient in order to stimulate its crystallization and the formation of a (00.1) growth texture, hypostoichiometric nanostructured or amorphous (clusterized) films are formed. A physical model of the formation of critical nuclei leading to growth of a columnar structure in the films of transition-metal diborides is suggested.     

激光加工超疏水表面抗结冰性能研究进展

在低温环境中，表面结冰会严重影响户外装备的运行效率和安全，基于疏水材料的新型被动式防除冰方法引起了广泛关注。超疏水表面凭借其优越的拒水、抑制冰核形成和降低冰黏附强度等能力，在防除冰技术领域表现出广阔的应用前景。激光加工技术具有高效率和灵活性，成为制备超疏水表面的有效方法，并被进一步用来研究表面的抗结冰性能。首先，概述了固体表面润湿理论和结冰机理。其次，综合评估了激光加工超疏水表面的抗结冰性能，包括静态水滴延迟结冰时间、动态水滴累积、冰黏附强度、延迟结霜与抗冻能力、表面积冰与除冰等方面。静态水滴延迟结冰时间受到水滴与表面接触界面的成核速率和传热速率的影响，动态水滴累积与表面润湿性密切相关，冰黏附强度反映了表面对冰的附着性和除冰的难易程度。超疏水表面具有显著的延迟结冰能力，但在低温高湿条件下，表面的超疏水性可能会减弱，甚至失效。除冰过程也可能破坏超疏水表面的微观结构，进而影响其持续的抗结冰性能。最后，对超疏水表面激光加工与抗结冰性能的未来研究方向进行了展望。     

Influence of Laser Power on the Hardening of Ti6Al4V Low-Pressure Steam Turbine Blade Material for Enhancing Water Droplet Erosion Resistance

To overcome water droplet erosion of Ti6Al4V alloy blade material used in low-pressure steam turbine (LPST) of high-rating nuclear and super critical thermal power plants, high-power diode laser (HPDL) surface treatment at two temperatures corresponding to two different power levels was carried out. During incubation as well as under prolonged erosion testing, the HPDL surface treatment of this alloy has enhanced its resistance significantly. This is due to the formation of fine-grained martensitic (ά) phase due to rapid heating and cooling associated with laser treatment. The droplet erosion test results after HPDL surface treatment on this alloy, SEM, XRD analysis, and residual stresses developed due to HPDL surface treatment are given in this paper.     

Epitaxial growth of tin ferrite thin films using pulsed laser deposition technique

Epitaxial thin films of tin ferrite (SnFe₂O₄) were deposited on (0 0 2) oriented strontium titanate (SrTiO₃) substrate using pulsed laser deposition method. The quality and epitaxial nature of the films were investigated by X-ray diffraction technique. The phi scan of the film and the substrate shows four folds symmetry indicating cube-on-cube epitaxial growth of the film on the substrate. The optical bandgap of the film was estimated to be 2.6 eV using optical transmittance data. Magnetic measurements indicate that the coercive field and remnant magnetization of the film decrease with increase in temperature. The presence of hysteresis loop in M vs. H plot at room temperature indicates the ferromagnetic nature of the film.     

Simultaneous independent measurement of splat diameter and cooling time during impact on a substrate of plasma-sprayed molybdenum particles

In thermal spray processes, the coating structure is the result of flattening and cooling of molten droplets on the substrate. The study of the cooling time and evolution of the splat size during impact is then of the highest importance to understand the influence of the spray parameters and substrate characteristics on the coating structure. Measurement of particle temperature during impact requires the use of a high-speed two-color pyrometer to collect the thermal emission of the particle during flattening. Simultaneous measurement of the splat size with this pyrometer is difficult since the size of the particle can change as it cools down. To measure the splat size independently, a new measurement technique has been developed. In this technique, the splat size is measured from the attenuation of the radiation of a laser beam illuminating the particle during impact. Results are presented for plasma-sprayed molybdenum particles impacting on a glass substrate at room temperature. It is shown that the molybdenum splat reaches its maximum extent about 2 μs after the impact. In this work, we show that this increase of the splat surface is followed by a phase during which the splat size decreases significantly during 2 to 3 μs.