Significance of quenching stress in the cohesion and adhesion of thermally sprayed coatings

In thermal spraying, molten particles strike a solid surface, where they are flattened and quenched within a very short time. Considerable in-plane tensile stress on the order of 100 MPa can develop within each splat during quenching after solidification because thermal contraction of the particle is constrained by the underlying solid. Ni-20Cr alloy and alumina powders have been plasma sprayed in air onto steel substrates that were maintained at about 473 K. The influence of spraying conditions such as spray distance on the magnitude of the quenching stress have been studied by measuring the curvature of the substrate during spraying. Mechanical properties such as Young’s modulus and bend strength of the deposited coatings have also been measured. A strong correlation was found between the quenching stress and the strength of Ni-20Cr coatings, which suggests that the strength of interlamellar bonding limits the quenching stress at such temperature. Presented at ITSC ’92, June, 1992, Orlando, Florida.     

Predicting Quenching and Cooling Stresses within HVOF Deposits

Due to the nature of the HVOF and other thermal spray processes, residual stress build up in thick deposits is a significant and limiting problem since it impedes the coating behavior in service. The residual stress-state that evolves in a deposit is largely dependent on the thermal conditions to which the substrate/coating system has been subjected, and is a combination of quenching stresses, peening stresses that develop in some cases in HVOF, both of which arise during deposition, and cooling stresses, postdeposition. It follows that precise control of these phenomena is essential, if a thick deposit or one with low levels of residual stress are to be thermally sprayed. This paper applies looks at analytical and finite element techniques used to predict quenching and cooling stresses within tungsten carbide-cobalt thermally sprayed deposits. The analysis investigates and predicts the quenching and cooling stresses using improved analytical and finite element analysis techniques by validating the models with experimental results such as X-ray diffraction and the hole drilling method. The result of this paper is a thermo-mechanical equation for quenching stress which includes the effects of misfit strain, the Poisson’s effect, variation of coating and substrate thicknesses, thermal expansion, and process temperature effects.     

Understanding the Formation of Limited Interlamellar Bonding in Plasma Sprayed Ceramic Coatings Based on the Concept of Intrinsic Bonding Temperature

Interlamellar bonding is an important factor controlling the mechanical, thermal and electrical properties of plasma sprayed ceramic coatings. In order to understand the formation of limited interlamellar bonding, a theoretical model is proposed based on the concept of the intrinsic bonding temperature. The numerical simulation of the interface temperature between a molten splat and underlying splats was performed for splats with uniform and non-uniform thickness, in order to reveal the conditions for the interlamellar bonding formation. The interlamellar bonding ratio was theoretically estimated based on the bonding forming conditions. The features of interlamellar bonding revealed by the simulation agree well with the experimental observations. The bonding ratio of plasma sprayed coatings is significantly influenced by the distribution of splat thickness. According to the distribution of Al₂O₃ splat thickness in the coating, the theoretical estimation of bonding ratio yielded a value of 0.41 for the plasma sprayed Al₂O₃ coating at the ambient atmosphere conditions, which is reasonably consistent with the observation value. Therefore, the limited interlamellar bonding can be reasonably explained based on the sufficient condition that the maximum interface temperature between a molten splat and underlying splats is larger than the intrinsic bonding temperature.     

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。     

Intrinsic residual stresses in single splats produced by thermal spray processes

Changes in processing parameters strongly affect the structure and properties of thermally sprayed coatings and, consequently, their performance. Residual stress in the deposits is a factor that needs consideration, since it has direct influence on the processability and integrity of the sprayed material. In order to enhance the understanding of this phenomenon, a study of measurements of residual stresses on a single particle level was undertaken. The deposit is built-up with the successive impingement of micron-sized droplet and therefore an understanding of the single splat microstructure and properties will provide a fundamental understanding of the underlying mechanisms. Residual stresses in thin coatings, as well as isolated particles—splats—deposited on stainless steel substrates were investigated using X-ray microdiffraction. Plasma sprayed molybdenum and cold sprayed copper were studied. The key process parameters considered were: in-flight particle energy and substrate temperature in the first case, and particle velocity in the latter. The results will be discussed with respect to the influence of each of these parameters, contribution of quenching and thermal stress component and splat formation. Further, the coating build-up from individual particles and the associated factors influencing residual stress will be discussed.     

基底对等离子喷涂Mo层片形成的影响

运用扫描电镜和台阶仪研究了在不同状态下的1Cr18Ni9Ti不锈钢和纯Cu基底上大气等离子喷涂Mo层片的形成机制。结果表明:室温光滑基底上沉积的Mo层片均呈不规则飞溅花瓣状,随预热温度升高,形成的层片逐渐变得较完整;熔滴撞击铺展过程中在基底上可能会形成凹坑;基底导热率越高,层片铺展度越小;粗糙基底表面阻碍熔滴铺展,促进熔体飞溅。理论分析表明,"花瓣状"层片的形成是熔体表面张力、熔体与基底接触热阻的不均匀性和基底凹坑相互作用的结果;基底凹坑的形成主要由基底熔化所控制。     

Epitaxial Growth and Cracking Mechanisms of Thermally Sprayed Ceramic Splats

In the present study, the epitaxial growth and cracking mechanisms of thermally sprayed ceramic splats were explored. We report, for the first time, the epitaxial growth of various splat/substrate combinations at low substrate temperatures (100 °C) and large lattice mismatch (? 11.26%). Our results suggest that thermal spray deposition was essentially a liquid-phase epitaxy, readily forming chemical bonding. The interface temperature was also estimated. The results convincingly demonstrated that atoms only need to diffuse and rearrange over a sufficiently short range during extremely rapid solidification. Concurrently, severe cracking occurred in the epitaxial splat/substrate systems, which indicated high tensile stress was produced during splat deposition. The origin of the tensile stress was attributed to the strong constraint of the locally heated substrate by its cold surroundings.     

Relationships between the microstructure and properties of thermally sprayed deposits

Thermally sprayed deposits have layered structure composed of individual splats. The individual splats have quenching microstructure of quasi-stable preferred fine grains. However, this fine-grained microstructure of the deposits is usually not reflected by improved performance of the deposits because a layered structure with two-dimensional voids occurs between lamellar interfaces. The microstructure of the thermal spray deposits with the emphasis on the layer structural parameters is reviewed. Conventionally, one of the most common quantitative parameters used to characterize the microstructure of the thermally sprayed deposits is the porosity, measured by different methods. However, it is illustrated that the relationships between properties and porosity for bulk porous materials processed by conventional processes cannot be applied to thermally sprayed deposits owing to the two-dimensional characteristics of voids. The total porosity in the deposits is not meaningful from the viewpoint of prediction of the deposit properties. An idealized structural model and related parameters, instead of porosity, are proposed to characterize quantitatively the microstructure of the thermally sprayed deposit. The relationships between the properties and the structural parameters are presented for the plasma-sprayed ceramic deposits based on the proposed microstructure model. The properties include the Young’s modulus, fracture toughness, erosion resistance, and thermal conductivity of the plasma sprayed ceramic deposits. The correlations of theoretical relationships with reported experimental data are discussed. An agreement of theoretical with observed values suggests that the lamellar structure of the deposit with limited interface bonding is the dominant factor controlling the performance of the deposit. An erratum to this article is available at .     

On-Line Measurement of Plasma-Sprayed Ni-Particles during Impact on a Ti-Surface: Influence of Surface Oxidation

The objective of this study was to analyze the impact of plasma-sprayed Ni5%Al particles on polished and grit-blasted Ti6Al4V samples under oxidized and nonoxidized conditions. For this purpose, measurements of thermal radiation and velocity of individual plasma-sprayed particles were carried out. From the thermal radiation at impact, splat diameter during flattening and temperature evolution during cooling were evaluated. Characteristic parameters related to the quality of contact between the splat and the substrate were retrieved. The flattening speed was introduced to characterize wetting, while the cooling rate was used to characterize solidification. The idea was to get a signature of particle impact for a given surface roughness and oxidation state by identifying parameters which strongly affect the splat behavior. Sieved Ni5%Al powder in a narrow range (+65 −75 μm) was sprayed on four sets of titanium alloy surfaces, consisting of polished and grit-blasted samples, one set had a nonoxidized surface and the other one was oxidized in an oven at 600 °C for two hours. Resulting splats after impact were characterized by scanning electron microscopy, the splats on oxidized surface showed pores in their core and detached fingers at the periphery. The cooling rate and flattening degree significantly increased on the oxidized smooth surface compared to the nonoxidized one. This trend was not found in grit-blasted surfaces, which implies that impact phenomena are different on grit-blasted surfaces than on smooth surfaces thus further work is needed.     

Splat shapes in a thermal spray coating process: Simulations and experiments

We studied the deposition of nickel particles in a plasma spray on a stainless steel surface using both experiments and numerical simulations. We developed a three-dimensional computational model of free-surface fluid flow that includes heat transfer and solidification and used it to simulate the impact of nickel partcles. In our experiments, particles landing on a polished stainless steel surface at a temperature below 300 °C splashed and formed irregular splats, whereas those deposited on substrates heated above 400 °C formed round disk splats. Simulations showed that formation of fingers around the periphery of a spreading drop is caused by the presence of a solid layer. Droplets that spread completely before the onset of solidification will not splash. To sufficiently delay the instant at which solidification started in our simulations to obtain disk splats, we had to increase the thermal contact resistance between the droplet and the substrate by an order of magnitude. We measured the thickness of the oxide layer on the test surfaces used in our experiments and confirmed that heating them creates an oxide layer on the surface that increases the thermal contact resistance. We demonstrated that the numerical model could be used to simulate the deposition of multiple droplets on a surface to build up a coating.     

Residual stress in thermal spray coatings measured by curvature based on 3D digital image correlation technique

Residual stress is an important factor in thermally sprayed deposits which affects both processing and performance. High stress can influence the structural integrity of sprayed parts and impair their function. Therefore, it is important to know the stress state, understand its origin and be able to control it. In this paper, three-dimensional digital image correlation as a non-destructive full-field optical measurement method is used to measure the strip curvature on thermal spray coatings given a NiCrCoAlY bonding layer and an yttria-stabilized zirconia (YSZ) layer on 304 stainless steel substrate as an example. The stress profile through thickness can be determined in each step of deposition by finite element method based on an inverse analysis and the stress interpretation of curvature. The individual contribution of quenching and thermal stresses to residual stress is analyzed from temperature evolution during post-deposition cooling. In addition, the multilayer progressing deposition and elastic-plastic model are used for the accurate predictions in the simulations.     

热喷涂NiCoCrAlYTa+7YSZ热障涂层颗粒沉积行为

分别采用低压等离子喷涂和大气等离子喷涂在K4169基体上收集了NiCoCrAlYTa颗粒沉积物及涂层,并对颗粒沉积物的形貌及涂层性能进行了观察分析。结果表明:低压等离子喷涂收集到的单个NiCoCrAlYTa扁平颗粒主要呈圆盘状,涂层致密且氧含量低。而大气等离子喷涂收集到的扁平颗粒主要呈溅射状,涂层孔隙率和氧含量均较高。又在经镜面抛光的NiCoCrAlYTa涂层和K4169基体上分别收集了7YSZ颗粒沉积物,并对其沉积形貌进行了观察分析,结果表明:在K4169基体上收集到的7YSZ颗粒沉积物主要呈圆盘状,表面存在大量的网状微裂纹及宏观环状贯通裂纹。在镜面抛光的NiCoCrAlYTa涂层表面收集的7YSZ颗粒沉积物,周围有少量的指状溅射物,中心部存在一定数量的网状微裂纹,但宏观环状裂纹消失。     

Formation of Solid Splats During Thermal Spray Deposition

This paper reviews the findings of recent research on the formation of solid splats by the impact of thermal spray particles on solid substrates. It discusses methods of describing the substrate, by characterizing both chemical (oxide layers) and physical (surface topography, adsorbed and condensed contaminants) aspects. Recent experiments done to observe impact of thermal spray particle are surveyed and techniques used to photograph particle impact and measure cooling rates described. The use of numerical modeling to simulate impact and deformation of impacting particles is appraised. Two different break-up mechanisms are identified: solidification around the edges of splats; and perforations in the interior of thin liquid films created by droplet spreading without solidification. These two modes can be reproduced in numerical models by varying the value of thermal contact resistance between the splat and substrate. A simple criterion to predict the final splat shape is presented.     

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 1

Properties of the coatings developed during thermal spraying are essentially determined by rapid solidification of splats formed as a result of impingement of the melted powder particles onto a substrate surface. The processes of flattening droplets and formation of splats in thermal spraying have been studied intensively during the last two decades. The last review on this topic was published at the end of 1994. Since then many papers have been dedicated to investigating splat formation, taking into account such important issues as roughness of the substrate surface, wetting phenomena, and splashing. This review, consisting of two parts, includes the main results obtained since 1994 and examines the influence of solidification of the lower part of the splat, substrate roughness, wetting at the substrate-coating interface, substrate deformation, oxidation, and splashing on the dynamics of flattening of droplets and the formation of splats. Flattening of composite powder particles, splat-substrate interaction, and development of splat-substrate adhesion and splat porosity are discussed. Part 1 of the review covers the following issues, which significantly influence the droplet flattening and splat formation: droplet solidification during flattening and roughness of the substrate surface, composite morphology of the powder particles, and oxidation processes. The results provide a better understanding of the thermal spray processes to increase their efficiency.     

Quenching,thermal and residual stress in plasma sprayed deposits: NiCrAlY and YSZ coatings

Residual stress is an important factor in thermally sprayed deposits which affects both processing and performance. High stress magnitudes and/or concentrations can undermine the structural integrity of sprayed parts and impair their functionality. Therefore, it is important to know the stress state, understand its generation and be able to control it. Results of experimental stress determination in plasma sprayed deposits are presented. Neutron diffraction as a non-destructive and phase-distinctive measurement method was used to determine residual stress profiles in thick NiCrAlY and yttria-stabilized zirconia (YSZ) deposits. Measurements were complemented with calculations based on experimentally determined material properties, which allowed for separation of quenching and thermal stress contributions to final residual stress. Since the application of neutron diffraction to plasma sprayed deposits is relatively novel, certain verification measurements were performed. Specimens were prepared at two different deposition temperatures, to determine the effect of temperature on the stresses and relevant deposit properties.     

Mathematical Modeling and Numerical Simulation of Splat Cooling in Plasma Spray Coatings

Particle deformation and cooling significantly affect the characteristics of thermally sprayed coatings, such as the adhesion and cohesion strength between a splat and a substrate and between splats, as well as the internal stresses of deposits. It is essential to understand these processes for the successful industrial application of thermal spray technology. However, to date, the microstructure of the boundary of a splat and the substrate has not been clarified, although much research has been conducted on splat formation and the cooling process. We have developed a microstructure model of the boundary between the splat and the substrate, based on splat morphology obtained from experiments. In the model, it is assumed that gaps, or voids, and contact areas are arranged on the splat boundary with the substrate in an orderly fashion. The model includes phase changes and heat resistance simulating the function of the microstructure during splat cooling. Assumptions in the model are that ambient gas trapped in the gaps, or voids, transfers heat only by conduction and not by convection or radiation. The results of the simulation indicated that the extent of gaps, or voids, significantly affects the rate of decrease of the average temperature of the splat surface, as well as the temperature distribution inside the splat.     

Effects of Surface Chemistry on Splat Formation During Plasma Spraying

Ni-Cr single splats were plasma-sprayed at room temperature onto aluminum and stainless steel substrates, which were modified by thermal and hydrothermal treatments to control the oxide surface chemistry. The proportions of the different splat types were found to vary as a function of substrate pretreatment, especially when the pretreatment involved heating. It was observed that surface roughness did not correlate with changes in splat morphology. Substrate surfaces were characterized by X-ray photoelectron spectroscopy using in situ heating in vacuum to determine the effect of thermal pretreatment on substrate surface chemistry. It was found that the surface layers were composed primarily of oxyhydroxides. When the substrates were heated to 350 °C, water vapor was released from the dehydration of oxyhydroxide. Preheating the substrate can remove the water prior to spraying: preheated substrates had improved the physical contact between the splat and substrate, which enhanced the formation of disk splats and increased the number of splats.     

Splat microstructure of plasma sprayed cast iron with different chamber pressures

Achieving a plasma sprayed cast iron coating containing graphite requires stringent control on spray parameters that synergistically influence the coating properties and thus the performance. The microstructure of cast iron splats greatly depends on spray parameters such as substrate temperature, chamber pressure, and spray distance. This paper presents the effect of chamber pressure on the splat microstructure, including oxides and graphite. At low chamber pressures, most splats exhibit a disk shape with high flattening ratios, whereas star-shaped splats extensively appear at high chamber pressures. Spraying at high chamber pressures causes the formation of pores and thick oxide zones at the splat/substrate interface, mainly due to the atmospheric gases, which are responsible for a decrease in splat adhesion. Spraying in Ar atmosphere reduces the splat oxidation due to a decrease in the oxygen partial pressure. Small deformed substrate ridges are observed adjacent to the periphery of splats sprayed at low chamber pressures whereas no ridges are detected at high chamber pressures. Ridge formation generates a kind of mechanical bond, which increases the adhesive strength. Since the molten droplets impinge with high velocity and thus high flattening ratio at low chamber pressures, the solidification rate becomes faster, and graphite formation is resultantly hindered.     

Modeling of a substrate thermomechanical behavior during plasma spraying

Plasma coating performances and lifetimes may be ruined during service conditions because of uncontrolled residual stress development within the coating. This study presents the results of a CAST3M^® thermomechanical numerical model which purpose is to simulate the different residual stresses development within the duplex coating–substrate during the coating built-up and its comparison with the experimental results. To achieve the thermal spray process understanding all the thermal fluxes transferred to a metallic beam and surrounding temperatures were measured so as to provide the CAST3M^® model with precise boundary conditions, corresponding to a specific geometry. The residual stresses were experimentally determined by the in situ curvature measurement and, afterwards, by the hole drilling method. The plasma torch stand-off distance, the relative torch/substrate velocity and the substrate material were considered as the parameters of this study. The main results concern the substrate temperature and deflection during the preheating stage, the thermal energy transferred by the molten splats to the substrate together with the quenching stress and the development of thermal stress during the final cooling.     

Effect of Ambient Pressure on Flattening Behavior of Thermal Sprayed Particles

Cu splats were thermally sprayed onto the mirror polished SUS304 substrate surface at various ambient pressures ranging from 6.66 to 101.33 kPa. The effect of ambient pressure on the flattening behavior of the particle was systematically investigated. It was observed that only around 10% or less of disk-shaped splats deposited at atmospheric pressure. The splat shape on the flat substrate had a transitional changing tendency from a splash splat to a disk one with a decrease of the ambient pressure. The tendency of splash splat change with increasing the ambient pressure agreed with the BET curve, which indicates that adsorption/desorption of the adsorbed gas/condensation plays an important role on the flattening behavior of thermal sprayed particles. Moreover, a part of substrates were polished to a mirror finish and heated to 573 K for 10 min, then elapsed to air atmosphere for different duration of up to 1 h. The fundamental static wetting behavior of the once heated substrate surface by a water droplet was investigated. The contact angle measurement results agreed well with the splat morphologies. No chemical change and surface topography change took place with the elapse time increasing. Hence, the occurrence of desorption caused by reducing the ambient pressure or by substrate preheating provided good wetting. Wetting of substrate surface by molten particles may dominate the flattening behavior of thermal sprayed particles.