Thermal Spray Using a High-Frequency Pulse Detonation Combustor Operated in the Liquid-Purge Mode

Experiments on thermal spray by pulsed detonations at 150 Hz were conducted. Two types of pulse detonation combustors were used, one operated in the inert gas purge (GAP) mode and the other in the liquid-purge (LIP) mode. In both modes, all gases were supplied in the valveless mode. The GAP mode is free of moving components, although the explosive mixture is unavoidably diluted with the inert gas used for the purge of the hot burned gas. In the LIP mode, pure fuel-oxygen combustion can be realized, although a liquid-droplet injector must be actuated cyclically. The objective of this work was to demonstrate a higher spraying temperature in the LIP mode. First, the temperature of CoNiCrAlY particles heated by pulsed detonations was measured. As a result, the spraying temperature in the LIP mode was higher than that in the GAP mode by about 1000 K. Second, the temperature of yttria-stabilized zirconia (YSZ) particles, whose melting point was almost 2800 °C, heated by pulsed detonations in the LIP mode was measured. As a result, the YSZ particles were heated up to about 2500 °C. Finally, a thermal spray experiment using YSZ particles was conducted, and a coating with low porosity was successfully deposited.     

In Situ Monitoring of Particle Consolidation During Low Pressure Cold Spray by Ultrasonic Techniques

This study attempts to test the viability of the examination of the cold spray process using acoustic methods, specifically in situ testing during the actual spray process itself. Multiple composites studied by flat and multi-channel transducers as well as the results of actual online measurements are presented. It is shown that the final thickness as well as the dynamics of buildup can be evaluated (including plotting rates of buildup). Cross sections of the coating thickness are also easy to obtain and show true profiles of the coating. The data can also be used to generate real estimates for nozzle speed and spray diameter. Finally, comparisons of real thickness and acoustically estimated thickness show a close linear relationship. The data clearly show that online acoustic measurement is a viable method for estimating thickness buildup.     

Determination of Thermal Spray Coating Property with Curvature Measurements

Real-time curvature measurement of a coating-substrate system during deposition has facilitated the monitoring of coating stresses and provided additional insights into thermal spray deposition mechanisms. However, the non-equilibrium state of coating formation along with harsh spray booth environment introduces complexity not only in data interpretation but also in the coating properties estimation. In this paper, a new procedure is proposed to estimate the elastic modulus of thermal sprayed ceramic coatings using in situ curvature and temperature measurements. In order to correlate the measurable parameters to coating elastic modulus, a systematic study is conducted to develop a suitable methodology. First, various finite element model analyses are carried out to formulate suitable relations between the measurements and elastic modulus. Subsequently, experiments are conducted to validate the procedure to estimate coating moduli. The results are compared with more accurate measurements obtained from post-deposition characterization technique under low temperature thermal cycles. The comparison suggests that the moduli estimated using the proposed procedure are in good agreements with those obtained from the post-deposition technique. Further, the nonlinear response of coatings are evaluated from the estimated moduli during deposition and cool down, which offer additional information on the characteristics of thermal spray coatings.     

Low Thermal Conductivity Yttria-Stabilized Zirconia Thermal Barrier Coatings Using the Solution Precursor Plasma Spray Process

The primary function of thermal barrier coatings (TBCs) is to insulate the underlying metal from high temperature gases in gas turbine engines. As a consequence, low thermal conductivity and high durability are the primary properties of interest. In this work, the solution precursor plasma spray (SPPS) process was used to create layered porosity, called inter-pass boundaries, in yttria-stabilized zirconia (YSZ) TBCs. IPBs have been shown to be effective in reducing thermal conductivity. Optimization of the IPB microstructure by the SPPS process produced YSZ TBCs with a thermal conductivity of 0.6 W/mK, an approximately 50% reduction compared to standard air plasma sprayed (APS) coatings. In preliminary tests, SPPS YSZ with IPBs exhibited equal or greater furnace thermal cycles and erosion resistance compared to regular SPPS and commercially made APS YSZ TBCs.     

Microstructures of Metallic NiCrBSi Coatings Manufactured via Hybrid Plasma Spray and In Situ Laser Remelting Process

This paper deals with coating alternatives to hard chromium plating. Thermal spraying is already used in industry, but results are not always satisfactory for reasons of porosity and microstructures. In this study, atmospheric plasma spraying (APS) and in situ laser irradiation by diode laser processes were combined to modify the structural characteristics of thick NiCrBSi alloy layers. The microstructure evolution was studied, and results show that in situ laser remelting induced the growth of a dendritic structure, which strongly decreased the porosity of as-sprayed coatings and increased the adhesion on the substrate. Moreover, no phase transition after laser treatment was observed. Lastly, a mechanical investigation demonstrated that the combination between plasma spray and in situ melting with a diode laser could result in very good mechanical properties. The increase of the laser incident power involved an increase of the mean contact pressure, along with coating hardness. The hybrid process appears to be a possible alternative to hard chromium plating, in order to protect mechanical parts, because of the improved mechanical properties of the NiCrBSi layer.     

Peening Effect of Thermal Spray Coating Process

The present study investigated the influence of grit blasting, feedstock powder, and thermal spraying technology on performance near the surface on the substrate’s side. The experimental results show that both the grit-blasting process and thermal spraying process harden the substrate, and microhardness on or near the surface was noticeably increased. Grit blasting created deformed regions next to the surface of the substrate and interface between entrapped grits and substrate. Initial equiaxed grains in the deformed regions were elongated and spirally oriented surrounding impact spots. There were no visible changes in microstructure caused by thermal spraying, and the elongated grain regions remained in the coated substrate. Substrate hardening was attributed to grit blasting and associated heating due to flame rather than powder particle impacting during thermal spraying, thus feedstock powder and individual thermal spray technology had no influence on the hardening.     

Heat Flux Estimation of a Flame Thermal Spray Process Using a Thermally Thin Composite Calorimeter

Temperature measurements take on prime importance in the field of the thermal spray coating since the temperature variation greatly affects the formation of splat morphology and also the coating properties and qualities. The evaluation of the heat flux is therefore essential since temperature variation comes from the energy transfer and conduction of the thermal system. The aim of this study is to estimate the heat flux of a flame thermal spray by solving an inverse heat conduction problem. Firstly, the substrate material and geometry are well designed so that the Biot number is small enough to conform to the lumped capacitance conditions. A lumped capacitance model of a substrate with its coating subjected to a uniform echelon heat flux is evaluated by solving a heat balance equation in the Laplace domain. Then, a thermally thin calorimeter is designed and the experimental thermogram is obtained by embedding a thin-wire micro-thermocouple onto the front and rear faces of the substrate. The forced convective heat transfer coefficient as well as the net incident heat flux density brought to the substrate during the thermal spray process are estimated. The theoretical composite surface temperature is compared to the experimental recording, the result showing a good agreement.     

Passive and Pulse-Echo Ultrasonic Monitoring of Cold Spray Process

To fully understand particle interactions with both substrate and neighboring particles in cold spray, ultrasound wave generation and ultrasonic monitoring of particle impacts were studied. The multi-channel ultrasonic system works in pulse-echo and passive modes. In pauses between pulse-echo data acquisition frames the system is in the passive mode receiving the signals generated by the particle impacts. The particles being deposited generate the ultrasonic signals in a very wideband frequency range at more than 40?dB signal-to-noise ratio. The particle impact signals are considered as a sum of the background wideband noise and the sparse strong pulses observed when the nozzle passes over the location of the transducer. It was shown that these components of the particle impact signal can be separated by threshold processing. The results of the passive monitoring are confirmed by the ultrasonic pulse-echo and direct measurements of a deposited coating geometry.     

Optimization of Plasma Spray Process Using Statistical Methods

The microstructure features of coatings produced by a plasma spray process are affected significantly by the process parameters such as powder size, spray gun nozzle size, total plasma gas flow, ratio of H₂ + N₂ over total gas flow, and so on. This article presents a study of the effects of these parameters on the microstructure (porosity, formation of crack, unmelted particle and oxide phase) of NiCrAlY coatings deposited by the Mettech Axial III™ System. A Taguchi array is used to design the spraying process parameters. The results of the microstructure evaluation are used to generate regression equations for the prediction of coating microstructure based on process parameters. The results predicted from the regression equations are in good agreement with the experimental results according to a confidence level of 0.95. Among the parameters examined, the powder size and the ratio of H₂ + N₂ over total gas flow rate are the most significant parameters affecting the occurrence of crack, porosity, unmelted particle and oxide. Within the range of the designed process parameters, lower powder size and higher ratio of H₂ + N₂ over total gas flow rate lead to less cracks, pores, unmelted particles but more oxides. Nozzle size has marginal influence on oxides which increase with nozzle size. Gas flow rate has no direct influence on any coating feature evaluated with the range of variation.     

Plasma and Particle Temperature Measurements in Thermal Spray: Approaches and Applications

Growing demands on the quality of thermally sprayed coatings require reliable methods to monitor and optimize the spraying processes. Thus, the importance of diagnostic methods is increasing. A critical requirement of diagnostic methods in thermal spray is the accurate measurement of temperatures. This refers to the hot working gases as well as to the in-flight temperature of the particles. This article gives a review of plasma and particle temperature measurements in thermal spray. The enthalpy probe, optical emission spectroscopy, and computer tomography are introduced for plasma measurements. To determine the in-flight particle temperatures mainly multicolor pyrometry is applied and is hence described in detail. The theoretical background, operation principles and setups are given for each technique. Special interest is attached to calibration methods, application limits, and sources of errors. Furthermore, examples of fields of application are given in the form of results of current research work.     

Using Thermal Spray and Laser Micromachining to Fabricate Sensors

The use of thermal spray to fabricate sensors directly onto engineering components is an emerging technology. The capabilities for sensor fabrication are considerably enhanced with the use of laser micromachining, in which feature sizes as small as 15-20 μm can be achieved. Such feature sizes are required for a variety of sensors, including strain and heat flux sensors, thermopiles and microheaters. Ultrafast lasers—lasers with pulse durations ≲1 ps—are particularly well suited for the multimaterial/multilayer processing required to fabricate sensors from materials deposited using thermal spray. In this work, the key issues associated with laser micromachining of thermal spray coatings for sensor applications are presented. Both resistive strain gages and microheaters are discussed in detail as representative sensor designs that require the fine feature and linewidth capability that laser micromachining provides, including details of their fabrication and practical design restrictions. This article summarizes with recommendations for future work.     

变速箱拨叉热喷涂工艺和质量控制

菲亚特系列C539变速箱的3种拨叉制造技术要求较高,其性能好坏直接影响变速箱的换档操作性能、可靠性及寿命.由于热喷涂处理可使工件具有较好的耐磨性和润滑性能,以及能提高工件的疲劳强度,而且工艺灵活、投资少、见效快,所以在产品设计中,要求对拨叉的工作面进行Cu-Al和Ni-Al热喷涂处理.目前拨叉喷涂是南亚变速箱厂的关键工序,已连续生产5年,技术不断完善,产品质量已相当可靠和稳定.     

Thermoelectric Device Fabrication Using Thermal Spray and Laser Micromachining

Thermoelectric generators (TEGs) are solid-state devices that convert heat directly into electricity. They are used in many engineering applications such as vehicle and industrial waste-heat recovery systems to provide electrical power, improve operating efficiency and reduce costs. State-of-art TEG manufacturing is based on prefabricated materials and a labor-intensive process involving soldering, epoxy bonding, and mechanical clamping for assembly. This reduces their durability and raises costs. Additive manufacturing technologies, such as thermal spray, present opportunities to overcome these challenges. In this work, TEGs have been fabricated for the first time using thermal spray technology and laser micromachining. The TEGs are fabricated directly onto engineering component surfaces. First, current fabrication techniques of TEGs are presented. Next, the steps required to fabricate a thermal spray-based TEG module, including the formation of the metallic interconnect layers and the thermoelectric legs are presented. A technique for bridging the air gap between two adjacent thermoelectric elements for the top layer using a sacrificial filler material is also demonstrated. A flat 50.8 mm × 50.8 mm TEG module is fabricated using this method and its performance is experimentally characterized and found to be in agreement with expected values of open-circuit voltage based on the materials used.     

热喷涂基体表面前处理技术的研究进展

热喷涂涂层与基体机械咬合的结合机理决定了基体表面前处理是热喷涂涂层中非常重要的处理工艺。文中概述了当前广泛应用的喷砂处理的工艺特点,指出砂粒易在基体表面镶嵌和对基体造成损伤是喷砂工艺的主要缺点,讨论了喷砂对高温合金单晶材料和超高强钢疲劳性能的影响,研究了软质基体表面超音速火焰喷涂WC涂层的免喷砂工艺。同时介绍了近年来其他热喷涂基体表面前处理方面的研究热点,包括高压水射流处理技术、机械粗化技术以及激光表面前处理,并重点阐述了其基本原理、特点及应用情况。     

Listening to Radiation Damage In Situ: Passive and Active Acoustic Techniques

JOM - Knowing when, why, and how materials evolve, degrade, or fail in radiation environments is pivotal to a wide range of fields from semiconductor processing to advanced nuclear reactor design....     

Worker Exposure Monitoring of Suspended Particles in a Thermal Spray Industry

The purpose of the present work was the investigation and characterization of the quality of air in a thermal spray industry, in Greece. The activities that take place in the specific facility, as well as in most other similar industries, include thermal spraying and several mechanical and metallurgical tasks that generate airborne particles, such as grit-blasting, cutting and grinding of metallic components. Since the main focus of this work was the workers exposure to airborne particles and heavy metals, portable air samplers with quartz fiber filters, were used daily for 8 h. Three samplers, carried from different employees, were used for a period of 1 month. Results showed that both particles and heavy metals concentrations were low, even in the production site, which was the most susceptible area. The only exceptions were observed in the case of cleaning and maintenance activities in the thermal spray booth and in the case of spraying outside the booth. The main reason for the low concentrations is the fact that most of the activities that could produce high-particle concentrations are conducted in closed, well-ventilated systems. Statistical elaboration of results showed that particles are correlated with Ni, Cu, Co. The same conclusion is extracted for Fe, Mn. These correlations indicate possible common sources.     

Slicing Process Monitoring of Quartz Glass Ferrules using Acoustic Emission

Inappropriate conditions and blade wear in the glass ferrule slicing process are likely to cause chipping, scratching, and inaccuracies in ferrules. To limit such abnormal occurrences, in-process monitoring of the slicing process is necessary. The AE (Acoustic Emission) signal is known to be a useful tool to detect cracks and chippings in cutting brittle materials such as ceramics and glass, because high-frequency acoustic waves are emitted in material fracture. In this study, the glass ferrule slicing process was analyzed and modeled in two stages - cutting and polishing - according to the relative position of the blade and the glass rod. The AErms (root mean square of AE) for various slicing cases, was acquired and investigated both qualitatively and quantitatively based on the process model, the AE feature was extracted for each abnormal state: the high-frequency component of AE for chipping and the low-frequency component for wear. In this paper, a monitoring algorithm using two index parameters - kurtosis and level ratio - of AErms is proposed to discriminate abnormal states - chipping and wear - in the glass ferrule slicing operation.     

Identifying Indicators of Progress in Thermal Spray Research Using Bibliometrics Analysis

We investigated the research publications on thermal spray in the period of 1985-2015 using the data from Web of Science, Scopus and SciVal^®. Bibliometrics analysis was employed to elucidate the country and institution distribution in various thermal spray research areas and to characterize the trends of topic change and technology progress. Results show that China, USA, Japan, Germany, India and France were the top countries in thermal spray research, and Xi’an Jiaotong University, Universite de Technologie Belfort–Montbeliard, Shanghai Institute of Ceramics, ETH Zurich, National Research Council of Canada, University of Limoges were among the top institutions that had high scholarly research output during 2005-2015. The terms of the titles, keywords and abstracts of the publications were analyzed by the Latent Dirichlet Allocation model and visually mapped using the VOSviewer software to reveal the progress of thermal spray technology. It is found that thermal barrier coating was consistently the main research area in thermal spray, and high-velocity oxy-fuel spray and cold spray developed rapidly in the last 10 years.     

Thermal Barrier Coatings Made by the Solution Precursor Plasma Spray Process

The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray process that can produce a wide variety of novel materials, including some with superior properties. The SPPS process involves injecting atomized droplets of a precursor solution into the plasma. The properties of resultant deposits depend on the time-temperature history of the droplets in the plasma, ranging from ultra-fine splats to unmelted crystalline particles to unpyrolized particles. By controlling the volume fraction of these three different constituents, a variety of coatings can be produced, all with a nanograin size. In this article, we will be reviewing research related to thermal barrier coatings, emphasizing the processing conditions necessary to obtain a range of microstructures and associated properties. The SPPS process produces a unique strain-tolerant, low-thermal conductivity microstructure consisting of (i) three-dimensional micrometer and nanometer pores, (ii) through-coating thickness (vertical) cracks, (iii) ultra-fine splats, and (iv) inter-pass boundaries. Both thin (0.12 mm) and thick (4 mm) coatings have been fabricated. The volume fraction of porosity can be varied from 10% to 40% while retaining the characteristic microstructure of vertical cracks and ultra-fine splats. The mechanism of vertical crack formation will be described.     

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H₂), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H₂ as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.