Analysis of gas flow evolution and shock wave decay in detonation thermal spraying systems

The reactive Euler equations with variable gas properties are solved in both axisymmetric and plane two-dimensional flows to analyze the gas flow evolution, shock wave decay, and shock reflections in pulsed detonation thermal spraying (PDTS) systems. The gas phase governing equations are numerically solved using a high-resolution shock capturing numerical method. Expansion-compression waves are formed upon external gas expansion and persist for a long time (on the time scale of a PDTS cycle) with wide fluctuations in the gas velocity and temperature. The results show that the reflected shock wave from the substrate dies out extremely fast that micron-sized particles used in PDTS do not encounter these transients. The external shock wave decay is also analyzed for different reactive mixtures and flow geometries and is related to the truncation of the computational domain and the implementation of numerical boundary conditions at the open end boundaries.     

Gas-particle interaction in detonation spraying systems

A model is developed to describe dynamic interaction of particles with the carrier gas during detonation spraying. Equations of mass, energy, and momentum conservation are integrated numerically for the two-phase particle-gas flow with the Hugoniot boundary conditions at the detonation wave front. Velocity and temperature of the sprayed powder and the gas parameters are calculated self-consistently, taking into account effects of friction and cooling of the gas in the vicinity of the gun barrel and effects of particle-gas interaction on the parameters of the gas phase. Calculations are performed for tungsten carbide particles of 30 μm diam and a 1.8 m long detonation gun using a stoichiometric mixture of oxygen and propane. Distributions of gas and particle parameters along the barrel are calculated for various moments of time. Tungsten carbide particles of 30 μm reach an exit velocity of 1278 m/s and a temperature of 1950 K. Exit particle velocity is a nonmonotonic function of the loading distance,L, with a distinct maximum atL = 75 cm. The proposed model can be applied to a broad range of problems related to detonation coating technology and allows evaluation of the effectiveness of various designs and optimization of operational parameters of detonation spraying systems.     

Gas dynamical parameters of detonation powder spraying

An investigation is conducted of the gas dynamics of a gas detonation coating process and the mechanism of particle acceleration by the shock wave inside the coating apparatus. Velocities of gas detonation in different gas mixtures are analyzed by applying the conventional hydrodynamic theory of detonation, and the effect of addition gases on the velocity of detonation in oxygen/hydrogen and oxygen/acetylene mixtures is studied. The authors propose a model that allows calculation of particle acceleration and final velocity. This model utilizes the Chapman-Jouquet picture of detonation and assumptions about the linear distribution of the velocity of detonation products behind the front of the detonation wave. The kinetics of particle acceleration by a detonation wave exhibits several novel features and is distinctly different from particle acceleration in other methods of spraying, such as plasma and high-velocity oxyfuel. There is a nonmonotonic dependence of particle velocity upon its coordinate and change in the direction of particle acceleration. Loading distance and total barrel length are important parameters that affect final particle velocity. Results indicate that final particle velocity and, as a consequence, the quality of detonation coatings can be significantly affected by changing the gas mixture composition and the powder loading distance while keeping the remaining operational parameters constant.     

Control of structural and phase processes in detonation spraying of coatings

Issues of obtaining coatings with optinum structure and properties from nickel-clad powdered aluminium oxide are studied. A production method of metal–ceramic composition is offered, the ceramic component gradually increasing from the lower layer to the upper one, ensuring optimal properties of the coating.     

爆炸喷涂Al-Cu-Cr准晶涂层的组织及硬度研究

为了研究热喷涂工艺对爆炸喷涂Al-Cu-Cr准晶涂层组织和硬度的影响规律，采用三种爆炸喷涂工艺参数在Q235A低碳钢基体上制备涂层，借助XRD、SEM和OM等技术手段对粉末和涂层的组织结构进行分析，并检测涂层横截面的孔隙率和显微硬度。结果表明，用于喷涂的A165Cu20Cr15准晶粉末中含有二十面体准晶相i-A165Cu24Cr11和极少量具有单斜结构的晶体相θ-Al13Cr2（即Al83Cu4Cr13）；而爆炸喷涂涂层中除i和θ两相外，生成了新相——体心立方结构的α-Al69Cu18Cr13（准晶i的晶体类似相）和Al2O3相。涂层呈典型层状结构，其它条件不变的情况下，涂层中各晶体相与准晶相i最强峰衍射强度的比值α／i、θ/i和Al2O3／i随爆炸喷涂工作气体流速的成比例提高而增加，同时涂层截面的孔隙率下降而显微硬度HV0.1升高。     

等离子喷涂与脉冲等离子爆炸复合涂层的氧化组织分析

采用等离子喷涂在镍基高温合金GH4049表面制备了CoNiCrAlY涂层,随后对涂层进行了脉冲等离子爆炸和真空预氧化处理,并对涂层进行了1050℃保温10 h和50 h的恒温静态氧化实验。结果表明:经过脉冲等离子爆炸处理后的涂层表面形貌更加平坦,脉冲等离子爆炸处理使得涂层表面的片层结构转变为致密结构,减少了氧气渗入涂层的通道,有效延缓了涂层的氧化过程,且没有改变涂层的化学成分;在1050℃空气中氧化50 h后,涂层的物相组成与未氧化处理的基本一致。     

纳米粒子堆垛模型及其热喷涂传热数值分析

将纳米粒子团聚大颗粒进行抽象处理,提出以理想堆垛结构为假设的纳米团聚颗粒模型。以ANSYS有限元软件为平台,模拟Al2O3-13%TiO2(质量分数)团聚纳米颗粒在热喷涂环境下的传热过程,分析传热时间、团聚颗粒直径和孔隙率等对传热的影响,并分析团聚颗粒在热喷涂传热后的组织结构差异。结果表明:在一定传热条件下,纳米团聚颗粒可以保持部分纳米粒子或长大为亚微米晶形态,且团聚颗粒直径越大,这类组织就越容易形成,而团聚颗粒孔隙率在0.48以下时,对组织形态的影响较弱。     

WC-12Ni/NiCrBSi爆炸喷涂涂层制备及其滑动磨损特性

利用爆炸喷涂方法制备了不同比例的WC-12Ni/NiCrBSi涂层,研究了NiCrBSi含量对涂层的组织形貌、硬度与结合强度的影响,用扫描电镜和能谱仪分析了涂层的滑动磨损行为.结果表明:由于NiCrBSi的加入,改善了WC颗粒的熔融状态,提高了涂层的结合力,涂层中含15%的NiCrBSi时,可以明显的提高涂层的磨损抗力.     

激波对电场辅助冷喷涂纳米粒子速度的影响

通过建立二维数值分析模型,分析了不同进气温度及压力时,带电纳米粒子在激波和电场力耦合作用下的加速特性以及对粒子撞击基板速度的影响。结果表明:弓形激波气体压缩层是造成纳米粒子减速的主要因素,弓形激波气体压缩层越薄粒子撞击基板时速度越大;外加电场时,带电纳米粒子撞击基板的速度得到较大提升。     

Scaling analysis and prediction of thermal aspects of the plasma spraying process using a discrete particle approach

On the basis of a discrete particle approach, a scaling analysis was used to predict features of the thermal plasma spraying process. Correlations were obtained using the analysis and they were subsequently used to predict two important features: the state of the particle at the moment of impact on the substrate, and the nature of solidification process. Limitations and restrictions were also identified in the development of the analysis that can be used to infer the resulting structure of coating. The correlations that were developed might be utilized in optimizing the thermal plasma spraying process, as well as in producing new types of coatings.     

Plasma spraying of Wc-Co part I: Theoretical investigation of particle heating and acceleration during spraying

Plasma-sprayed WC-Co coatings are used extensively in a variety of wear-resistant applications. The quality of these sprayed coatings depends greatly on the temperature and velocity of the powder particles impacting the substrate. Because it is both expensive and difficult to experimentally determine these particle parameters, the present study deals with a theoretical investigation of particle heatup and acceleration during plasma spraying of WC-Co based on a recently developed model. The effect of WC-Co particle size on the evolution of particle temperature and velocity is examined through calculations performed under typical spraying conditions. The implications of the powder particles, assuming an off-axis trajectory during their traverse through the plasma flame, are also discussed.     

Measurement and numerical simulation of particle velocity in cold spraying

The velocity of cold spray particles was measured by a diagnostic system designed for thermal spray particles that is based on thermal radiation. A laser beam was used to illuminate the cold spray particles in cold spraying to obtain a sufficient radiant energy intensity for detection. The measurement was carried out for copper particles of different mean particle sizes. The particle velocity was also estimated using a two-dimensional axisymmetric model developed previously. The simulated velocity agreed well with the measured result. This fact indicates that particle velocity in cold spraying can be predicted reasonably by simulation. Therefore, it is possible to optimize the cold spray process with the aid of the simulation results. 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.     

爆炸喷涂WC-Co/CoMoCrSi复合涂层的组织与性能

为了提高CoMoCrSi涂层的结合强度和力学性能,采用爆炸喷涂技术制备了WC-12Co涂层作为过渡层的WC-Co/CoMoCrSi复合涂层,借助SEM和EDS等手段分析了涂层截面组织形貌和化学元素组成,采用显微硬度计、万能拉伸机及销盘式摩擦磨损试验机等研究了涂层的力学及摩擦磨损性能。结果表明：在氧燃充枪比为60%的喷涂参数下,制备WC-Co/CoMoCrSi复合涂层平均结合强度高达66 MPa,涂层截面组织致密、均匀,孔隙率小于0.6%,平均显微硬度为667 HV0.1,复合涂层摩擦因数0.53~0.56,具有优异的耐磨性能。     

爆炸喷涂CoNiCrAlY-Al2O3涂层的组织及高温耐磨性研究

采用三种爆炸喷涂工艺在SCH19耐热钢基体上制备CoNiCrAlY-Al2O3高温耐磨涂层,借助XRD、SEM和EDS等技术对涂层的组织结构进行分析,并检测涂层的表面粗糙度、显微硬度和高温磨损失重。结果表明,涂层由γ-Ni（Co,Cr）、γ’-Ni3Al和α-Al2O3组成,涂层呈典型的层状结构,适当提高乙炔氧气流量比,可改善涂层的组织结构,并且使其在900℃的高温下具有更优异的耐磨性能。     

Interaction of particles with carrier gas in HVOF spraying systems

Several designs of high-velocity oxygen fuel (HVOF) thermal spray systems have been created during the last decade. The most advanced systems are now producing coatings comparable in quality to detonation (D-gun) coatings. This paper presents numerical analysis of the interaction of dispersive particles with the carrying gas flow for three different HVOF systems, along with a method to calculate the parameters of sprayed particles that highlights the advantages and limitations of each design. The method includes gas dynamical calculations of the gas flow in an accelerating channel and calculations of the injected par-motion and thermal state (temperature and melted mass fraction). The calculations were performed for particles of tungsten carbide, aluminum oxide, and zirconium oxide with size distributions of 10 to 80 μm. Two conventional types of HVOF systems were considered: those with a supersonic accelerating channel and those with a subsonic accelerating channel (without a de Laval nozzle). A novel design is pro-posed that contains a combined gas dynamical path with functionally separated regions of heating and acceleration. The regularities and distinctions in the behavior of the metallic and ceramic oxide particles are discussed for different jet configurations. The results obtained indicate that it is possible to signifi-cantly affect particle parameters by using the new configuration solutions without creating construction complications.     

A computational fluid dynamic analysis of gas and particle flow in flame spraying

The flame spraying process, which is a common industrial thermal spraying application, has been analyzed by means of three-dimensional computational fluid dynamics (CFD) simulations. The process used at the Volvo Aero Corporation for the coating of fan and compressor housings has been modeled. The process uses the Metco 6P torch (Metco, Westbury, NY), which ejects a mixture of acetylene and oxygen at high speed through a ring of 16 orifices to form the flame. A stream of argon gas flowing through an orifice in the center of the ring carries a powder of nickel-covered bentonite through the flame to the spray substrate. The torch is cooled by a flow of air through an outer ring of 9 orifices. The simulation emulated reality closely by including the individual inlets for fuel, cooling air, and injected particles. The gas combustion was simulated as a turbulent, multicomponent chemically reacting flow. The standard, two-equation k-ε turbulence model was used. The chemical reaction rates appeared as source terms in the species transport equations. They were computed from the contributions of the Arrhenius rate expressions and the Magnussen and Hjertager eddy dissipation model. The first simulations included several intermediate chemical substances whose predicted concentration agreed favorably with measurements. Later, more simplified simulations incorporated only the global chemical reaction involving the initial and the final products, with corrections to the thermal properties being made to account for the missing intermediaries. The gas velocity and temperature fields predicted by the later simulations compared satisfactorily to those predicted by the earlier, more elaborate, ones. Therefore, the final simulations, which incorporated injected particles, were conducted employing the simplified model with only the global reaction. An in-house finite difference code was developed to calculate particle properties. Allowance was made for elliptical shapes, phase changes, and internal heat transfer with regard to the composite material. The particle velocities and temperatures predicted by the final simulations compared fairly well with experimental results obtained with the optical DPV2000 system.     

Effect of oxidation on droplet flattening and splat-substrate interaction in thermal spraying

The processes of oxidation that occur during particle inflight motion and during splat solidification in an oxygen-rich atmosphere were considered for the thermal spray process. The effect of oxidation on droplet flattening, splat-substrate mechanical and thermal interaction, splat morphology, and development of coating porosity and adhesion was studied. The influence of wetting and oxygen dissolution on flattening and splat-substrate adhesion was also investigated. The results from theoretical treatment agree with experimental observations.     

Optimization of spray conditions in cold spraying based on numerical analysis of particle velocity

The effects of the parameters involved in cold spray on the acceleration of particles are systematically investigated by a CFD code in order to reveal the main factors influencing significantly particle velocity. The parameters involved include nozzle geometry parameters, processing parameters and properties of spray particles. It is found that driving gas type, operating pressure and temperature are main processing parameters which influence particle velocity. As for nozzle geometry, the expansion ratio and divergent section length of spray gun nozzle show significant effects. Moreover, the density, size and morphology of powder also have significant effects on particle velocity. The effects of those main parameters are summarized in a comprehensive equation obtained through nonlinear regression of the simulated results for the estimation of particle velocity. The interactions of the parameters on particle acceleration can be examined through the equation. Moreover, the optimization of the dimensions of spray gun nozzle and spray parameters can be realized based on the obtained results.     

等离子喷涂YPSZ陶瓷梯度涂层的组织与抗热震性能

利用等离子喷涂工艺在汽车铝活塞表面制备了氧化钇部分稳定的氧化锆(YPSZ)陶瓷梯度涂层,用电子扫描显微镜、金相显微镜等手段对涂层从组织形貌、物相、显微硬度和热震性能进行了分析.结果表明:基体与涂层结合紧密;涂层在冷却过程中发生了t-ZrO2→m-ZrO2的相变过程;涂层的热震性能良好,可满足活塞的使用要求.     

CoCrAlY表面改性后热障涂层高温氧化及热震性能

采用大气等离子喷涂技术（APS）在镍基高温合金表面制备了CoCrAlY粘结层,利用电子束蒸发镀膜在CoCrAlY表面蒸镀纳米铝膜,并使用强流脉冲电子束熔敷纳米铝膜进行表面改性,最后使用APS在表面改性后的CoCrAlY表面沉积陶瓷层制备了热障涂层.在空气环境中对热障涂层进行高温氧化试验和热震试验.结果表明,CoCrAlY表面改性后热障涂层经1 050℃静态空气氧化后,界面处生成的热生长氧化物（TGO）具有较高的连续性和致密性,有效阻碍了氧化的进一步发展且避免尖角型氧化物的形成,提高了热障涂层的抗氧化能力;在1 050℃高温加热后10℃水淬热震条件下,脱落率仅为2%左右.