Fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate are investigated in the present study: focusing on the effects of preheat substrate temperature (T S ) and chamber pressure (P C ) on the splat morphology, the adhesive strength of splats, the formation of a reaction layer and graphite. Splash-type splats appear at low T S but disk and star-shaped splats arise at high T S . Deformed substrate ridges, mainly due to the slight surface melting, are formed adjacent to the splat periphery at high T S . At low T S , pores are observed at the splat/substrate interface, which cause a decrease in the adhesion of splats. In contrast, a reaction layer composed of iron, aluminum and oxygen is ready to form at high T S . The amount of graphitized carbon increases in cast iron splats with T S . At a low P C of 26.3 kPa, disk-type splats are in the majority at a constant T S of 473 K. As P C increases, star-shaped splats appear along with disk splats. The flattening ratio of disk splats decreases with the increase of P C , because of a decrease in the kinetic energy and temperature of molten droplets. An interfacial oxide layer composed of iron, aluminum and oxygen is ready to form at high P C . The number of pores intensively increases with P C , which leads to a decrease in the adhesive strength of splats. The amount of formed graphite in cast iron splats slightly increases with P C , however, that of a rapidly solidified phase of Fe-Si-C decreases because of lowering of the solidification rate.

It has been said that plasma-sprayed ceramics particles are often supercooled before the impact on substrate. Some numerical models of the droplet impact actually included the supercooling effects. However, there is no report that has experimentally confirmed the effects on splat morphology. Therefore, in this research, we have mainly investigated the supercooling effects on splat morphology as well as splat microstructure. To achieve this, we developed an in-situ measurement technique utilizing radiation from a melt particle to monitor the impact of single particle successively under plasma spraying. The system was able to identify each single particle, which enabled us to correlate the splat morphology with impact velocity and thermal history of each particle during the impact. Yttria-stabilized zirconia powders were sprayed onto quartz glass substrate by the argon-hydrogen dc-rf hybrid plasma under atmospheric pressure. Waveforms of emissions and thermal history obtained during the impact were precisely analyzed. Especially, we closely examined thermal history during particle spreading to find the recalescence. In addition, splat morphologies were examined statistically in relation to their thermal histories. Based on the measurement, we also evaluated the viscosity of zirconia, cooling rate, and thermal contact resistance experimentally.

A study was performed to examine the effects of starting powder composition, substrate thermal conductivity, and substrate temperature on the composition and structure of individual Al-Cu-Fe splats formed during thermal spraying. The fraction of quasicrystalline phase which formed was found to depend on the chemistry and solidification history of the splats. Due to evaporative loss of Al during spraying, an initial powder composition higher in Al produced splats closer to the desired composition, which yielded more of the quasicrystalline phase. Deposition onto lower thermal conductivity surfaces resulted in an increase in the quasicrystalline phase, as did solidification onto higher temperature substrates.

Splats formed during a thermal spray process may be either highly fragmented or intact and disk-like. To predict this change in splat morphology, a dimensionless solidification parameter (Θ), which takes into account factors such as the substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, has been defined. Θ is the ratio of the thickness of the solid layer formed in the splat while it is spreading, to the splat thickness. The value of Θ can be calculated from simple analytical models of splat solidification and spreading. If the solid layer growth is very slow (Θ << 1), the droplet spreads out to a large extent. Once it reaches maximum spread it becomes so thin that it ruptures, producing fragmented splats. If, however, the solid layer thickness is significant (Θ ~ 0.1 – 0.4), the droplet is restricted from spreading too far and does not become thin enough to rupture. Under such circumstances, disk-type splats are expected. When the solid layer growth is rapid (Θ~1), spreading of the droplet is significantly obstructed by the solid layer, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data.

Ni20Cr splats were sprayed onto polished substrates at different preheating temperatures in an argon atmosphere by Low Pressure Plasma Spray to reveal the dominating factor on the effective interface bonding formation. The splat morphology, microstructure and splat-substrate interface bonding were characterized by SEM and EBSD. The interface for examination of typical splats was prepared by FIB. Disk splats were obtained on AISI 304 stainless steel substrates preheated to temperatures of 100 °C (cooling from 350 °C), 350 and 550 °C. Moreover, typical distinct two-zone microstructure feature was observed on the splat surface by SEM and EBSD, including central coarse grain and marginal fine grain. When the preheating temperature was higher than 350 °C, effective bonding formed only in the entire central coarse zone, whereas no effective bonding was observed in the fine grain zone. By using glass, copper, nickel and 304 SS as substrates, it was found that increasing thermal conductivity of metallic substrates has little effect on splat diameter and morphology and however decreased the area fraction of central coarse grain zone. It was revealed that the melt/substrate interface temperature plays a crucial role on the interface bonding formation.

Disk splats are usually observed when the deposition temperature exceeds the transition temperature, whereas thick oxide layer will reduce the adhesion resulting from high deposition temperature. In present study, single molybdenum splats were sprayed onto polished molybdenum substrates with different preheating processes to clarify the effect of surface oxidation on the splat formation. Three preheating processes included heating the substrate to 350 °C, 550 °C, and cooling the substrate from 550 °C to 350 °C, which were performed in argon atmosphere. The chemistry and compositions of substrate surface was examined by XPS. The cross sections of splats were prepared by focus-ion-beam (FIB), and then characterized by SEM. Nearly disc-shaped splat with small fingers in the periphery was observed on the substrate preheated to 350 °C. Perfect disc-shape splat was deposited at 550 °C. Flower-shaped splat exhibited a central core and discrete periphery detached by some voids on the substrate preheated to 350 °C (cooling down from 550 °C). The results of peeling off splats by carbon tape and morphology of FIB sampled cross-sections indicated that no effective bonding formed in the splat-substrate interface for the substrate ever heated to 550 °C, due to the increasing content of MoO 3 on preheated molybdenum surface.

Copper splats are deposited on the flat stainless steel surface at the ambient and preheated conditions. The splashing occurs as the splats are deposited at an ambient atmosphere. The characteristics of the splashing occurring at different splat regions during spreading of the droplet are examined. The splashing can be classified into two types according to the splashing mechanisms. At the surrounding region of the splat larger than flattening ratio about 1.5 to 2, the radial splashing takes place by jetting-away of splat materials, which leads to the formation of a splat with a reduced diameter. At the central area of the reduced splat, the upward splashing occurs through the blowing up of the top surface layer which results from the high pressure of gas bubbles. At the preheated condition which can remove surface adsorbates, no evident splashing occurs under the normal spray conditions. Two types of splashing can be explained by the gases evolved through evaporation of the adsorbates resulting from the heating of the high temperature droplet. The spreading of the droplet involved in the wave urging flow is presented.

A thermal spray coating is formed through successive impact, flattening, rapid cooling and solidification processes of a stream of spray droplets. Splashing may occur during droplet flattening process. Recent studies suggested that splashing can be suppressed when a molten droplet impacts on a preheated flat substrate. In this study, the splatting behavior in plasma spray is examined using molten spray droplets of different Reynolds number. Splats are deposited on preheated flat stainless steel surface. The morphology of splats is examined using optical microscopy and scanning electron microscopy. To adjust Reynolds number of spray droplets, copper droplets are produced using both Ar-H 2 and Ar-He-H 2 plasma jets under different operating conditions. As a result, the Reynolds number of spray droplets have been varied from about 18,000 to 90,000. It has been found that Reynolds number will influence splashing phenomena during splatting and consequent splat morphology. At low Reynolds number, splats present a regular disc morphology. However, when Reynolds number was increased up to about 5x104, the severe splashing around periphery of splat droplet was clearly observed despite the preheating of substrate. Based on the morphology of splats, a model for the spreading of molten droplet is proposed to explain the effect of Reynolds number on the flattening behavior of molten spray droplet.

This paper presents the results of a study of the morphology of alumina splats deposited on stainless steel and alumina substrates. The substrates were either plasma sprayed or coated via plasma enhanced CVD. Substrates that were plasma sprayed were annealed if necessary to get specific phase structures, then polished to around 0.4 μm (Ra). CVD-coated substrates with an Ra ~6 nm and a columnar amorphous structure were sprayed as deposited. Splat studies show that the crystal structure of alumina substrates and the release of entrapped gas have a major influence on splat formation. For plasma sprayed coatings, disk-shaped splats with excellent adhesion properties were obtained on hot γ alumina, while on α alumina, splat shape and morphology were irregular and adhesion very poor. The effect of entrapped gas, on the other hand, can be seen in the splats that formed on the CVD-coated substrates. These splats were very porous and, in many, most of alumina flowed out to rim. As the paper explains, this is the result of gas release upon impact of molten particles, which reduces wettability and thermal contact between the splat and substrate.

Many properties (thermal, electrical, mechanical) of thermal sprayed coatings are strongly linked to the real contacts between the “piled-up” splats. The quality of this contact depends on droplet parameters at impact (size, temperature, velocity) and substrate parameters (temperature, topography). Two different techniques have been developed in order to study the plasma sprayed particle behaviour at impact. The first one allows direct observation under direct current (dc) plasma spray conditions, while the latter one, based on the millimetre sized free falling drop, enables the visualization of flattening phenomena, but at larger scale. These two techniques bring complementary approaches and results. The latter show that flattening time and cooling rate of the lamellae (metallic and ceramic) are improved with the stainless steel substrate surface modification at the nanoscale when corresponding to a positive skewness parameter obtained by preheating it over the transition temperature. Experiments of wettability show that the presence of nanopeaks increases the contact angle of the liquid on the substrates and reduces thermal contact resistance at interface. It has also been shown that, when adsorbates and condensates are not eliminated from the surface, even with a positive skewness, the thermal contact resistance is increased.

In this study, stainless steel splats were deposited on preheated stainless steel substrates with oxide scales of different thickness in inert low-pressure plasma spay (LPPS) conditions to examine the effect of in-situ oxidation of prior splats on the morphology and bonding of subsequently formed splats. Splat-substrate interface cross-sections were prepared by focus-ion-beam milling. Splat morphology and bonding state with the substrate were characterized by SEM. The results show that with oxide films up to 35 nm thick, disk-type splats are deposited that bond well to the substrate except in the periphery region. As oxide films become thicker (100 nm) and present a surface with micro-scale roughness, splats take on a finger-like shape with poor bonding at the interface.

The various thermal spraying methods available include the plasma process, which uses a plasma flame to melt a fine powder before it is sprayed onto a substrate, and the High Velocity Oxy-Fuel (HVOF) spray process, in which the flame is made from the combustion of oxygen. These methods differ both in the temperature and velocity with which the molten particles impact the substrate, leading to different coating characteristics. This includes differences in splat morphology and the nature of microstructural interactions at the splat-substrate interface. That is, features such as local melting of the substrate, the existence of porosity and the presence of oxides. For this study a nickel-chromium powder was sprayed onto mirror-polished stainless steel substrates using both plasma spray and HVOF to form single splats. These splats, and their interface with the substrate, were characterized using a range of microstructural characterization techniques and the observed differences were correlated to the spray conditions used.

Impacting of a molten droplet with melting point much higher than substrate results in melting of substrate around the impact area. The melting of the substrate surface to certain depth alters the flow direction of droplet fluid. The significant change of fluid flow direction leads to detaching of fluid from contact with the substrate. Consequently, splashing occurs during droplet spreading process. In the present study, Mo splats were formed on stainless steel substrate under different plasma spraying conditions. For comparison, Mo splats were also deposited on Mo surface. The substrate surface was polished prior to deposition. The powders used have a narrow particle size distribution. The results show that the morphology of splats depends significantly on the thermal interaction between the molten particle and the substrate. The splat observed was only a central part of an ideal disk-like complete splat. The typical pattern of Mo splats was the split type presenting a small split structure on stainless steel substrate surface. With Mo particles, the preheating of steel substrate has no effect on splat morphology. On the other hand, disk-like type Mo splat with a reduced diameter of a dimple-like structure at the central area of the splat was formed on Mo substrate and splashing can be suppressed through substrate preheating. Based on the experimental results, a surface-melting- induced splashing model was proposed to explain the formation mechanism of Mo splat on steel surface.

Polypropylene (PP) was flame sprayed onto rough mild steel substrates at room temperature (RT) that was preheated at 70 °C, 120 °C, and 170 °C. Single solidified droplets (splats) were collected and analysed to understand how processing variables influenced the thermal spray coating characteristics. The splat morphology was characterized in detail using optical and scanning electron microscopy (SEM). The splats exhibited a disk-like shape with a large central viscous core and a fully melted wide rim with a thin edge. The splat size increased with increasing substrate temperature. A unique flat microstructure was observed on the surface of the splat deposited onto the RT substrate, whereas a flowing pattern appeared on the splat surfaces deposited onto the preheated substrates and the pattern increased by increasing the substrate temperature. The results of this study revealed improved splat-substrate adhesion by heating the substrate from RT to 170 °C. On the basis of the result, the influence of substrate parameters on splat morphologies was employed to establish a relationship between the microstructural characteristics and processing variables of flame sprayed polymeric coatings.

Aiming at clarifying the individual splat formation mechanism in thermal spray process, commercially available metallic powders were thermally sprayed onto AISI304 substrate surface. The splats changed from a distorted shape with splash to a disk-shaped splat in flattening after collision onto substrate surface, through substrate preheating and/or reducing the ambient pressure. Accordingly, both substrate temperature and ambient pressure have an equivalent effect on the shape transition. The observation on the bottom surface morphology of single splat indicated that the ring-shaped initial solidification might play an important role during splat formation process. As a simulation of the real thermal spray process, free falling experiment has been conducted. The thermal history of the free falling metal droplet onto AISI304 substrate indicated that the flattening pattern is decided so quicky just after collision onto solid surface, which is enough earlier to the finalization of the flattening.

Understanding the impacting phenomena of yttria-stabilized zirconia (YSZ) particles and following coating formation in plasma spraying process is of importance to control and design the microstructure of coatings such as thermal barrier coatings. To this aim, recently, the authors have developed a novel in situ monitoring system for particle impacts under atmospheric dc plasma spraying conditions. This system utilized a high-speed video camera coupled with a long-distance microscope and was capable of capturing the particle-impinging phenomena at one million frames per second. To understand the coating formation mechanism, two approaches were attempted, that is, observation of the single splat formation and the following coating formation as the integration of splats. In the former case, the deformation and cooling processes of YSZ droplets impinging on substrates were captured successfully. In the latter case, multiple-droplet-impacting phenomena were observed as an ensemble treatment. Representing coating process, the tower formation (1- dimensional) and bead formation (2-dimentional) were observed under typical plasma spray conditions for thermal barrier coatings. By using a triggering system coupled with the motion of a robot, impact events were recorded for every pass. The obtained images clearly showed the coating formation resulted by the integration of single splats.

Dry-ice blasting, as an environmental-friendly method, was used to pretreat the substrate to be coated. In the present paper plasma-sprayed CoNiCrAlY splats were examined on the dry-ice blasted substrate. The cleaning effect of dry-ice blasting was demonstrated accompanying the condensation phenomenon, which is also harmful for the formation of ideal disk-like splat. A solution of ensuring the substrate temperature over dew point temperature was proposed for the proper application of dry-ice blasting during droplet flattening.

Joint research work between the University of Limoges and the State University of New York, Stony Brook, has been carried out on the impact and solidification of plasma sprayed zirconia particles. A measurement device, consisting of a phase doppler particle analyser and a pyrometer, was used to correlate the characteristic parameters of splats to those of the substrate and to the size, velocity and temperature of the impacting particles.

This is the second paper of a two part series based on an interdisciplinary research investigation between the University of Limoges, France, and the State University of New York, Stony Brook, USA, aimed at fundamental understanding of the plasma-particle interaction, deposit formation dynamics and microstructure development. In this paper, the microstructure development during plasma spraying of zirconia is investigated from the point of view of deposition parameters and splat formation (part I). Splats and deposits have been produced at Limoges and Stony Brook under controlled conditions of particle parameters and substrate temperatures. The zirconia splat microstructures thus obtained are examined for their shape factors, grain size, crystallographic texture and defects. Further the deposits were analyzed for phases, porosity and mechanical properties in an effort to develop a process-microstructure property relationship. The results suggest a strong role played by the deposition temperature on the microstructure and properties of the deposit.

It is well accepted that the morphology and microstructure of the splats have a strong influence on the characteristics and properties of thermally sprayed coatings. McPherson has made pioneering and outstanding contributions in the above area, especially for plasma sprayed coatings. Recently, splat morphology - microstructure - properties correlation has also been attempted in the case of HVOF thermal spray coatings. However, only limited data is available in the case of detonation sprayed coatings inspite of the fact that DS coatings have been available commercially for a long time. In the present work, the influence of particle velocity and temperature on the splat morphology and also area coverage of the splat has been studied for detonation sprayed Al 2 O 3 particles on a mild steel substrate. Further, the effect of two detonation spray process variables namely, oxy fuel ratio and shot frequency on splat morphology and splat area coverage has been evaluated. The above correlation has then been utilized to understand the variation of deposition efficiency of detonation sprayed Al 2 O 3 coatings on mild steel as a function of spray process parameters.