Flattening and solidification of thermally sprayed particles

In this article, molybdenum particles were plasma sprayed on copper, zirconia, and glass substrates. The impact of the molten particles was monitored using a fast two-color optical fiber pyrometer focused on a small spot on the substrate surface. The apparent duration of the flattening process and the cooling speed, both determined from the pyrometer signals, were found to depend on the substrate conditions and to vary with coating thickness. The substrate material and its roughness were also found to influence the texture in the sprayed coatings. Furthermore, a transient thermal flow numerical model was used to compute reliable thermal histories of the impinging particles and the underlying lamellae, the interfacial thermal resistance being determined by comparison of experimental thermograms with computed ones.     

In-flight particle surface temperature measurement: Influence of the plasma light scattered by the particles

The application of optical pyrometry to low- melting- point plasma- sprayed particles can be limited by the plasma light scattered by the particles themselves. From spectroscopic measurements of the plasma between 650 and 1050 nm and using the Mie scattering theory, the intensity of scattered light has been determined in the case of nickel particles sprayed using an Ar/He plasma. The results show that, even in spectral regions between the atomic lines of the plasma gas, the scattered light can be important compared to the thermal emission of the particles. This scattered light leads to values of measured temperatures, which are all the more overestimated because the particle temperature is low and the particle/torch distance short. For a 50- Μm nickel particle at 1550 ‡C, located 10 cm from the torch, the measurement error made with a double wavelength pyrometer is estimated at 100 ‡C. This work is based on a presentation made at the 1993 National Thermal Spray Conference, Anaheim, California, USA.     

The deformation and cooling of ceramic particles sprayed with a thermal radio-frequency plasma under atmospheric conditions

Common thermal-spray techniques use the strong acceleration of powder particles to produce dense ceramic coatings with high bond strength. The residence time of the powder particles within the plasma jet is correspondingly low, and only relatively small particles can be molten. In this work, on the contrary, an inductively coupled radio-frequency (RF) inductively coupled plasma (ICP) torch was used to spray large oxide-ceramic powder particles under atmospheric conditions. The slow plasma flow of a RF plasma leads to large residence times of the powder particles, so that the powder size of the feedstock can be 100 μm and more. It was observed that these particles will not be strongly accelerated in the plasma and that their velocity at the moment of impact is in the range of 10 to 20 m/s. Ceramic coatings were ICP sprayed with a low porosity and a high bond strength, similar to direct current (DC) or high-velocity-oxygen-fuel (HVOF) sprayed coatings. The morphology of ICP-sprayed particles on smooth steel surfaces, as a function of the surface temperature, is described and compared with DC plasma-sprayed splats. Furthermore, the degree of deformation was measured and determined by different models, and the pronounced contact zones formed between the pancake and the substrate were investigated. The ICP-sprayed ceramic coatings show some special properties, such as the absence of metastable crystalline phases, which are common in other spray technologies.     

Comparison of plasma-sprayed alumina coatings by RF and DC plasma spraying

Splat size and shape- factor distributions for plasma- sprayed alumina particles on various substrates were studied using a setup derived from the line- scan test. Direct- current (dc) and radiofrequency (rf) plasma torches were used to study the influence of particle velocity at impact. The influence of substrate temperature prior to spraying also was studied. Splats collected on smooth substrates kept below 100 °C were extensively fingered and had poor substrate contact. When the substrates were heated to 300 °C before spraying, the splats became disk- shaped and their substrate contact was very good. Similar results were obtained for rough substrates. Coating adhesion decreased with particle velocity and was lower for the dc plasma torch when using larger particles, which did not melt as well as smaller ones. Melting and adhesion were much improved with the rf torch.     

Structure of plasma-sprayed zirconia coatings tailored by controlling the temperature and velocity of the sprayed particles

The correlation between particle temperature and velocity with the structure of plasma-sprayed zirconia coatings is studied to determine which parameter most strongly influences the coating structure. The particle temperature and velocity are measured using an integrated optical monitoring system positioned normal to the spraying axis. The total porosity, angular crack distribution, and thermal diffusivity are correlated with the particle temperature and velocity. Results show that the temperature of the sprayed particles has a larger effect on the coating properties than the velocity in the conditions investigated.     

Experimental study of particle deposition characteristics of alumina using plasma spraying

Plasma spraying is one of the most versatile techniques used to form coatings for protection against oxidation, corrosion, and wear. The plasma spraying is ideally suited for refractory materials, but there are a number of variables that need to be controlled to obtain dense coatings. In spite of considerable progress made in the theoretical understanding of this complex process, there is a need for a simple method to evaluate the interaction between the plasma flame and powder particles that form the coatings. As reported in the literature, this involves metallographic observation of the powders collected from the plasma. In the present study, the structure and morphology of plasma-sprayed splats are experimentally investigated using different power levels and spray distances for alumina powder. The results show that the splashing occurs during splatting of a completely molten droplet. It is found that at higher power levels and shorter spray distances, spreading of molten droplets improves considerably.     

氧化铝种分过程理论生长模型

根据理想生长过程粒度分布变化的特性,通过粒数衡算方程建立了氧化铝种分理论生长过程粒度分布变化的数学模型。根据实验数据求解了不同温度条件下理论生长过程的模型参数,并讨论了模型参数与种分反应温度的关系,比较了模型预测粒度分布与实测粒度分布的偏差。该模型在55~200μm的粒径段内,能近似地对种分过程粒度分布变化进行预测。     

In-flight oxidation of stainless steel particles in plasma spraying

Air engulfment by the plasma jet in air plasma spraying (APS) causes in-flight oxidation of metallic particles. This oxidation, often complex and difficult to explain by classic diffusion-controlled oxidation, is governed by several mechanisms. This paper highlights the possible in-flight oxidation mechanisms in metallic particles with a focus on the convective oxidation. Two different types of austenitic stainless steel particles were air plasma sprayed using a direct current plasma gun and were collected in an argon atmosphere. Preliminary experiments indicated that different mechanisms are likely to occur during the in-flight oxidation of particles. The mass transfer from surface to interior of particle occurred, forming oxide nodules within particles. The mass transfer is governed by convective movements inside liquid particles within the plasma jet core due to the plasma-particle kinematic viscosity ratio greater than 50 and particle Reynolds number (Re) higher than 20. The nodules were composed of metastable phases consisting of mixed oxide of Fe and Cr. Convective movements within particles ceased roughly outside of the plasma jet core, and classic surface oxidation was found to be the dominant phenomenon forming the surface oxide layer. Moreover, the molten surface oxide outside the jet core may become entrained toward the tail of the particle if plasma conditions promote a higher particle Re number. The major oxide phase in collected particles was FeCr₂O₄, in a nonstoichiometric form of Fe_3−x Cr _x O₄. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

Improved CFD modeling of full dissolution of alumina particles in aluminum electrolysis cells considering agglomerate formation

The full alumina dissolution process in aluminum electrolysis cells was investigated using an improved computational fluid dynamics (CFD) model based on the previous researches by consideration of agglomerate formation. The results show that the total mass of alumina agglomerate and its maximum size are mainly dependent on the feeding amount and increase with increasing it. Higher superheat can effectively inhibit the agglomerate formation and thus promote the full alumina dissolution behavior. The full alumina dissolution process mainly includes a fast stage and a slow stage, with an average dissolution rate of 17.24 kg/min and 1.53 kg/min, respectively. About 50% (mass percentage) of the total alumina particles, almost all of which are the well-dispersed alumina fine grains, dissolve within the fast dissolution stage of about 10 s. The maximum values of the average dissolution rate and final percentage of the cumulative dissolved alumina mass are obtained with a feeding amount of 1.8 kg for a superheat of 12 °C. The formation of the alumina agglomerates and slow dissolution characteristics play a dominant role in the full dissolution of alumina particles.     

冷喷涂Cu粒子参量对其碰撞变形行为的影响

采用有限元数值计算方法研究了冷喷涂过程中Cu粒子与Cu基体的碰撞变形行为,探讨了粒子速度、温度对其碰撞基体后的变形行为、界面温度变化与粒子和基体的接触面积的影响．结果表明,随粒子碰撞速度的增加,粒子扁平率与碰撞界面温度增加、接触面积增大．证实了存在使碰撞界面发生绝热剪切失稳变形的临界速度,该速度与粒子沉积的临界速度一致．当粒子速度大于产生绝热剪切失稳变形的临界速度时,粒子的变形扁平率显著增加,且界面温度与有效接触界面面积也显著增加;随碰撞前粒子温度的增加,碰撞界面的温度也显著增加．高达粒子材料熔点的界面温度与有效接触面积的显著增加,将有助于粒子与基体之间冶金结合的形成．     

Isothermal oxidation resistance comparison between air plasma sprayed, vacuum plasma sprayed and high velocity oxygen fuel sprayed CoNiCrAlY bond coats

Commercial CoNiCrAlY powders with the same chemical composition were sprayed by vacuum plasma spraying (VPS), air plasma spraying (APS) and high velocity oxygen fuel (HVOF) onto Hastelloy X superalloy substrates obtaining coatings of comparable thickness. After coating, samples were maintained at 1273 K in air for different periods up to 3000 h. Morphological, microstructural and compositional analyses were performed in order to assess the high temperature oxidation resistance provided by the different spraying systems. HVOF technique provided bond coats with higher oxidation resistance compared to APS and VPS.     

Effect of direct-current plasma fluctuations on in-flight particle parameters: Part II

This paper is the continuation of previous work,^[1] in which plasma fluctuations were shown to produce significant time-dependent variations in the in-flight particle temperature and velocity, as well as in the number of detected particles. In this paper, the impact of the plasma fluctuations on the coating microstructure and deposition efficiency is demonstrated. Alumina coatings and deposition efficiencies, obtained with two sets of spray conditions showing similar in-flight particle conditions (velocity and temperature) with the DPV-2000 but displaying very different voltage fluctuations, are compared. The coating produced in the less stable plasma condition (C-I) is found to be more porous and contains a larger number of unmelted particles than the other coating produced in more steady plasma conditions (C-II). Moreover, condition C-I yields a significantly lower deposition efficiency. Such large discrepancies must be traced back to the physical characteristics of the particle jet. Laser illumination of the particle jet is used to probe particles too cold to be detected by pyrometric means. Cold particles are found in a much larger proportion in C-I than in C-II. They are ascribed to particles that are injected when the plasma is in a low enthalpy state. Periodic time-dependent variations in the in-flight characteristics of cold and hot particles, synchronous with the voltage fluctuations, are revealed.     

In-Flight particle measurements of twin wire electric arc sprayed aluminum

A real-time, nonintrusive measurement technique was successfully applied to a Tafa Model 9000 (TAFA Incorporated, Concord, NH) twin wire electric arc thermal spray system to simultaneously measure particle size, velocity, and temperature within the spray plume. Aluminum wire was sprayed with the current varied from 100 to 300 amp, and the gun pressure (air flowrate) varied from 40 to 75 psia. For all cases, the average sizes of the molten aluminum particles along the spray centerline range from 33 to 53 μm. The particles accelerate to peak velocities between 130 and 190 m/s, then decelerate slightly as they travel downstream. The average centerline particle temperature ranges from 2004 to 2056 °C, and the temperature profile remains fairly flat throughout transport to the substrate. A stagnation pressure probe was used to quantify the gas flow regime in the unladen jet. The wires were found to have a pronounced effect on the flow, resulting in a complex three-dimensional flowfield with mixed regions of subsonic and supersonic flow.     

Study of phase changes in plasma sprayed deposits

The formation of a plasma-sprayed coating that exhibits predictable properties requires the control of many process variables. The phase changes that take place during plasma spraying are significant material variables that should be controlled. Several different materials were deposited in air with a water-stabilized plasma torch (model PAL 160). Usually, air was used as a carrier gas for the powder; however, argon was also used for some coatings. The injected powders (NiAl, Ni, ZrSiO₄-based, Al₂O₃-based, etc.) as well as the coatings were studied for, among other properties, their structure, particle size, microhardness, and chemical and phase composition. Phase changes induced by the different cooling rates of molten particles after their impact on a substrate are illustrated for ZrSiO₄. It has also been found that the oxidizing power of the water-stabilized torch is less than previously believed. For example, coatings produced with nickel powder injected with argon as the carrier gas exhibited almost no oxides. Significant element redistribution during plasma spraying was demonstrated with a two-phase NiAl feedstock powder. The coating exhibited almost all the phases that are present in the binary NiAl alloy as well as envelopes of oxides and traces of amorphous phase.     

In situ characterization of small-particle plasma sprayed powders

The effect of various small-particle plasma spray powder injection parameters on the in situ particle position, velocity, and temperature is measured for yttria-stabilized zirconia and yttrium-aluminum-garnet powder. Using full-factorial experiments and multiple regression analysis, carrier gas flow, injector angle, and powder feeder disc speed were found to significantly affect the particle properties. Temperature and velocity were inversely related; on average, the cooler particles traveled faster. These properties also correlated to the particle position in the flame, where particles above the centerline of the flame traveled faster. The trends are discussed on the basis of residence time in the flame, as well as in terms of particle size segregation effects. Coating density and splat geometry reflect the temperature and velocity differences between the runs. Slower, hotter particles possessed more intrasplat and intersplat porosity and less splat-substrate contact area, leading to lower overall coating density.     

Computational study and experimental comparison of the in-flight particle behavior for an external injection plasma spray process

A three-dimensional computational fluid dynamic (CFD) analysis using Fluent V5.4 was conducted on the in-flight particle behavior during the plasma spraying process with external injection. The spray process was modeled as a steady jet issuing from the torch nozzle via the heating of the are gas by an electric are within the nozzle. The stochastic discrete model was used for the particle distribution. The particle temperature, velocity, and size inside the plasma plume at a specified standoff distance have been investigated. The results show that carrier gas flow rate variation from 2 standard liters per minute (slm) to 4.0 slm can increase the centerline particle mean temperature and mean velocity by 10% and 16%, respectively, at the specified standoff distance. A further increase of the carrier gas flow rate to 6 slm did not change the particle temperature, but the particle velocity was decreased by 20%. It was also found that an increase in the total arc gas flow rate from 52 slm to 61 slm, with all other process parameters unchanged, resulted in a 17% higher particle velocity, but 6% lower particle temperature. Some of these computational findings were experimentally confirmed by Kucuk et al. For a given process parameter setting, the kinetic and thermal energy extracted by the particles reached a maximum for carrier gas flow rate of about 3.5–4.0 slm.     

Enhanced corrosion resistance of porous NiTi with plasma sprayed alumina coating

In this study, corrosion behaviour of porous NiTi modified by plasma sprayed alumina coating has been investigated. Scanning electron microscopy and X-ray diffraction techniques were applied for the morphology and microstructure characterisation, while linear sweep voltammetry and electrochemical impedance spectroscopy were used for investigation of corrosion behaviour of coated and uncoated NiTi specimens. Induced couple plasma was conducted to measure ion release of the specimens in simulated body fluid at 37°C. The plasma sprayed Al₂O₃ coating on the porous NiTi improved the surface characteristics for biomedical applications. The alumina coating significantly hampered Ni ion release from the surface. In spite of slight decrease in corrosion resistance of the coated specimens, the corrosion mechanism changed from pitting to general corrosion. The breakdown phenomenon was not detected in the coated specimens, as well. Overall, it can be concluded the longevity of the coated specimen in the simulated biological system was enhanced, comparing to bare NiTi specimens.     

Structure and mechanical properties of plasma sprayed nanostructured alumina and FeCuAl-alumina cermet coatings

Nanostructured alumina (Al₂O₃) and nanostructured cermet coatings containing alumina dispersed in a FeCu or FeCuAl matrix, were deposited by atmospheric plasma spraying (APS) from nanostructured powders. These coatings were characterized by SEM, EDAX, TEM, XRD and nanoindentation. Friction and wear behaviour were investigated by sliding and abrasion tests. TEM and XRD revealed that a nanostructuring was retained in the APS deposited coatings.The nanostructured ceramic and cermet coatings were compared in terms of coefficient of friction and wear resistance. Nanostructured cermet coatings appeared to offer a better wear resistance under sliding and abrasion tests than nanostructured Al₂O₃ coatings. The role of Fe, Cu, and Al additions to the Al₂O₃ coatings on friction and wear behaviour, was investigated.In the case of FeCu- and FeCuAl-based cermet coatings containing alumina, though the starting material consist of only two compounds, the coatings contain up to four different phases after plasma spraying. The mechanical properties of these different phases namely crack sensitivity and elasto-plastic deformation was determined by nanoindentation. The failure mechanisms were investigated and an attempt was made to establish a ‘structure-property’ relationship. It was shown that an appropriate balance between hard and soft phases results in optimum tribological properties of the nanostructured cermet coatings.     

Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement

During plasma spraying of alumina with the stable α phase in the starting powder, metastable phases tend to form in the final coating. This is attributed to the rapid quenching associated with the process. In this paper the weight fraction of metastable phase formed, i.e., stable phase retained, and has been estimated using Rietveld refinement of X-ray diffraction data. This weight fraction depends on the process parameters like standoff distance, primary and secondary gas flow rate, nozzle size, etc., which in turn control particle melting. Under favourable melting conditions the weight fraction of the metastable phases approaches 1.     

Behavior of Ni-Al particles in argon: Helium plasma jets

To better understand the plasma spray coating process, an experimental study of the interaction between a subsonic thermal plasma jet and injected nickel- aluminum particles was performed. The velocity, temperature, and composition of the argon/helium gas flow field was mapped using an enthalpy probe/mass spectrometer system. The sprayed particle flow field was examined by simultaneously measuring the size, velocity, and temperature of individual particles. Particle and gas temperatures were compared at the nominal substrate stand- off distance and axially along the median particle trajectory. Temperature and velocity differences between the particle and the gas surrounding it are shown to vary substantially depending on the trajectory of the particles. On the median trajectory, the average particle is transferring heat and momentum back to the plasma by the time it reaches the substrate. Because the exchange of heat and momentum is highly dependent on the particle residence time in the core of the plasma, the condition of particles at the substrate can be optimized by controlling the particle trajectory through the plasma.