Splat Thickness Research Articles

Non-equilibrium solidification of the molten metal droplets impacting on a solid surface

A measuring method for the kinetic coefficient and activation energy of molten metals has been developed. This method is based on a splat thickness measurement of a molten metal droplet deposited on a polished metal substrate. An analytical solution of a non-equilibrium crystallization of a molten metal droplet impacting on a solid substrate relates the thickness of the splat to the kinetic coefficient. The dimensionless number showing the departure of the equilibrium phase transition from the non-equilibrium transition follows from the theory. The predicted values of the kinetic coefficient and activation energy agree well with the existing literature data.

Predicting splat morphology in a thermal spray process

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 particle diameter and velocity, 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 solid layer growth is very slow ( Θ ≪ 1), the splat 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 ∼ 0.1–0.3), the splat is restricted from spreading too far and does not become thin enough to rupture, resulting in disk splats. When solid layer growth is rapid ( Θ > 0.3), it obstructs liquid from flowing outward during droplet impact, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data and found to agree well.

Adhesive and bending failure of thermal sprayed hydroxyapatite coatings: Effect of nanostructures at interface and crack propagation phenomenon during bending

Hydroxyapatite (HA) coatings have shown promising effects on rapid bone remodeling and suitable functional life in orthopedic and dental applications. However, the major problem encountered by the HA-coated implants is the failure of the coating due to its insufficient mechanical properties. The present study investigated the influence of the microstructure near to the coating/substrate interface on the adhesion of the coatings. In addition, the crack propagation behavior within the coatings was studied through 4-point bend test. Results showed that nanostructures (30–110nm) within the HA coatings were achieved by high velocity oxy-fuel (HVOF) spraying. Comparison among HVOF HA coatings, which were deposited using different starting feedstock, suggests detrimental effect of the perpendicular-to-substrate nano-cuboids on adhesion of the coatings. The presence of the grains with hexagonal shape (<250nm in length and <50nm in diameter) triggered a deteriorated adhesion. Granular nanosized grains at the interface give rise to enhanced adhesion through improved mechanical interlocking. Formation mechanism of the nanosized grains was discussed in this paper. Furthermore, the 4-point bend test revealed consistent crack propagation path that the cracks actually grow within the coating with a direction parallel to the interface, and approximately several to 20 microns thick coatings were remained on the substrate. The critical strain energy release rate exhibited a value of ∼1.15kJm−2. During the crack propagation, kinking and trapping of the bending cracks were decided by the flaws within the coating, which were mainly located at splats’ interface. The interface between the first layer (with one splat thickness) and the second is believed to be the weakest zone in the nanostructured coating.

RAPID SOLIDIFICATION STUDY OF STABILIZED ZIRCONIA UNDER PLASMA SPRAYING

This paper presents a numerical study of transient heat transfer and phase change of a thermal spray coating problem. The early solidification of yttria stabilized zirconia splat deposited on a cooled substrate is considered. These phenomena are quantified in terms of some working parameters such that: the splat thickness, the splat/substrate contact quality, the latent heat of solidification and the initial temperatures. A two-dimensional heat transfer model, extended phase change, is developed with an enthalpy formulation in order to predict the splat/substrate thermal story and its effects on plasma surface interaction. The results are in good agreement with an experimental one for the case of a nickel splatting on an alumina substrate.

Effect of Surface Roughness on Remelting and Stresses During Splat Solidification [PowerPoint Submission

AbstractIn order to understand the microstructural evolution in plasma sprayed coatings, the solidification process was modeled using a 2-D FEM model based on enthalpy formation. Studies of the surface of the coatings showed surface roughnesses across multiple length scales. The model was used to examine the effects of the substrate and splat temperatures and the surface roughness features on the onset of remelting of the underlying surface on which the splat solidifies. The surface roughness was found to promote remelting, indicating that it was an important parameter that determines splat solidification. The temperatures of the splat and substrate were consolidated into one non-dimensional parameter that captured the onset of remelting with a non-dimensional remelting point.A fully coupled thermo-mechanical finite element model was also run for a single splat case, to provide more insight stress buildup during solidification. An important result was that the relative size of the surface roughness features, as compared to the splat thickness, is very important. Very large wavelengths compared to splat thickness lead to smaller stresses, since the solidification and the interface are essentially 1-D. Very small wavelengths compared to splat thickness also leads to reduced stresses, since the solidification front quickly becomes 1-D. Only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration, which are locations of microcrack formation.

Microcrack formation in plasma sprayed thermal barrier coatings

Air plasma sprayed ZrO 2–8wt%Y 2O 3 thermal barrier coatings were deposited under tightly controlled conditions. The lengths and orientations of the horizontal cracks and vertical cracks in these coatings were characterized in detail, and process/structure maps of the crack distribution as a function of particle and substrate states were constructed. A fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. The model also showed that surface roughness plays a key role in determining the magnitude of maximum stresses, and that only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration.

Impact of molten metal droplets on the tip of a pin projecting from a flat surface

We photographed impact of molten zinc and tin droplets on a flat steel surface from which the tip of a small pin was projecting. The height of the pin and its distance from the droplet center was varied. A three-dimensional model of droplet impact and solidification was used to predict splat shapes formed by droplets after they had spread and frozen. The model calculated velocity and pressure distributions in droplets and the growth of solid layers. When the offset distance of the pin was less than the droplet radius liquid flowed over the pin so that it had relatively little impact on the final splat shape. At larger offset distances a liquid sheet jetted from under the droplet after impact and impinged on the vertical surface of the pin. If the pin height was sufficiently large, approximately the same as the final splat thickness, it obstructed flow of liquid so that the solidified splat had a V-shaped notch in it. If pin height was made less than the average splat thickness liquid flowed over it and the final splat was circular, but the pin reduced flow velocity and suppressed growth of fingers along the edges of the splat that passed over the pin. Reduced velocities also resulted in faster growth of the solidified layer around the pin.

Effect of Surface Geometry on Stress Generation in Thermal Barrier Coatings During Plasma Spray Deposition

ABSTRACTA fully coupled thermo-mechanical finite element model was used to study the buildup of stresses during splat solidification, and to understand the effect of deposition conditions on crack formation during plasma spray deposition. Through the simulation, the locations and magnitudes of maximum stresses were identified, where crack formation would presumably initiate. The model showed that the stresses scaled with the temperature difference between the superheated splat and the substrate. The simulation further showed that the stresses scale with the three geometric parameters, and two independent geometric ratios were defined; ζ (defined as t/λ) and ψ (defined as A/λ). 2D maps of maximum S11 and S22 under different combinations of ζ and ψ were constructed. The mappings showed that only roughness features on the scale of splat thickness were important in providing locations of maximum stress concentration.

Optimization for plasma spraying processes by numerical simulation

Numerical simulation of plasma spraying processes from the injection of metallic particles to formation of the coating was conducted by integrating plasma thermofluid, and splat and coating formation models. The first model clarifies the electromagnetic thermofluid fields of a particle-laden plasma flow impinging onto the substrate, as well as the characteristics of the fine particles injected in the plasma flow. The second model clarifies the splat thickness and diameter of a particle using the melted particle impact velocity and temperature obtained by the first model. Furthermore, the optimal particle impact velocity and temperature were derived from the stable splat criterion given by the integrated model.

Influence of Plasma Spray Parameters on Formation and Morphology of ZrO2–8 wt% Y2O3 Deposits

Spray prints of thermal spray coatings were created on glass slides for air‐plasma‐sprayed 8‐wt%‐yttria‐partially‐stabilized zirconia (YSZ) deposits. The spray parameters such as carrier gas flow rate, standoff distance, and torch power were systematically changed to investigate the influence of these parameters on the YSZ deposit characteristics. The deposit properties such as deposition efficiency (DE), substrate coverage, deposit thickness, and roughness were measured. The deposits sprayed with a 3.5–4.0 L/min carrier gas flow rate at an 80 mm standoff distance exhibited higher values of DE within the range of studied process parameters. The DE increased as much as 25% by varying the carrier gas flow rate from 2.0 to 4.0 L/min. The deposits sprayed at a higher standoff distance and low torch power gave poor deposit characteristics. The deposit characteristics were compared with the in‐flight particle parameters and revealed that the deposit characteristics strongly depended on the in‐flight particle temperature. Using the in‐flight particle properties, the flattening ratio and the splat thickness were calculated. The average size of particles adhering to the substrate was found to drastically change with a change of process conditions, being much less than the average size of the starting powder.

A study of the microstructure of yttria-stabilized zirconia deposited by inductively coupled plasma spraying

Zirconia stabilized with 20 wt.% yttria was deposited to a thick free-standing type, ∼7 mm, by (inductively coupled plasma spraying) (ICPS). The spheroidization of particles and the microstructure of deposits were analyzed. Spheroidization fraction dependence on spray parameters such as particle size, H2 gas mixing quantity, and probe position was studied. Effects of parameters on the spheroidization of particles were analyzed by ANOVA (analysis of variance) (ANOVA). ANOVA results showed that the spheroidization fraction largely depend on H2 gas mixing quantity and particle size, and there are also some dependence on probe position and H2 gas mixing quantity. After melting, particles kept their chemical composition homogeneously from the center to their surface without segregation or evaporation. The degree of deformation of the diameter of the splat over the diameter of the spheroidized particle was approximately 320%, and splat thickness in the deposit varies between 2 µm and 3 µm depending on the deposition condition. The yttrium concentration gradient of the interlayer boundary appeared linear in the range of 0.5 to 1 µm. X-ray diffraction analysis and a transmission electron microscope (TEM) micrograph showed that low yttrium content particles resulted in tetragonal phase in deposit. The major characteristics of the microstructure of the thick free-standing deposit and solidification mode were studied. Microstructure of the bottom part of the deposit represented equiaxed or cellular structure. Equiaxed small grains prevailed when the droplets were quenched rapidly on substrate. The middle part of the deposit showed large columnar grains, of about 100 µm thick and 300 µm long. This may be due to high substrate or deposit temperatures and results in recrystallization and grain growth. The effects of the parameters, such as H2 gas mixing quantity, particle size, spraying distance, and probe position, on the microstructure of the deposits were evaluated. The H2 gas mixing quantity of Ar/H2=120/20 L/min compared to Ar/H2=120/10 L/min resulted in larger grain size and thicker cellular in chill. Grain shapes were affected by the heat removal rate from the deposit to its surrounding. Deposition with larger particle size showed heterogeneous grain size, insufficient particle melting, and incomplete recrystallization. The effect of probe position was less than the others.

Interfaces in Rapidly Solidified Zirconia-Yttria

Rapidly spread and solidified molten droplets (splats) are basic building units of thermal-sprayed coatings. Various interfaces of zirconia-yttria (7%wt. yttria) single splats produced by atmospheric plasma spraying have been investigated using cross-sectional high-resolution transmission electron microscopy. The interface between the substrate and the rapidly solidified splats is of utmost importance for the adhesion of coatings formed by this method though other interfaces also have important effects. Melting and resolidification of the substrate are commonly observed as a result of rapid heat transfer from molten droplets. Possible formation of a thin interfacial layer between the substrate and the splat depends on the substrate material and its temperature. The interfacial layer may be either nanocrystalline or amorphous and is composed of both the substrate and splat material. Bond coats of NiCrAlY prevent the formation of this interfacial layer. Rapid solidification, together with good contact at the splat/substrate interface favors copious nucleation followed by growth of narrow columnar grains through the splat thickness, starting at the substrate interface. The zirconia grain width ranges from 30 to 100 nm, while their length is determined by the thickness of the solidified splat and is typically on the order of microns. Boundaries between the columnar grains are clean, without any second phases or amorphous regions, unlike the boundaries in conventionally prepared ceramic materials, although they are occasionally cracked. Growth of narrow columnar grains in splats solidifying on previous splats is very often epitaxial.

Effect of substrate deformation on droplet flattening in thermal spraying

The analytical formulae describing the time evolution of the splat thickness and radius and the rates of their variation in the process of flattening of droplets during thermal spraying taking into account the splat curvature induced by the droplet impact are obtained. The time of finishing of the flattening process is established and is compared with those for the smooth and rough substrate surfaces. The role of the surface effects is discussed. Splat–substrate mechanical interaction is considered and the analytical formulae for the pressure developed upon the droplet impact taking into account the surface curvature are obtained. The theoretical results obtained agree with the experimentally observed tendencies of the droplet flattening.

Reactive spraying of nickel-aluminide coatings

Reactive spraying of nickel aluminides was accomplished via reaction synthesis techniques in which nickel and aluminum powders were fed through a direct- current plasma torch onto carbon steel substrates. The as- sprayed coatings obtained by reactive spraying were characterized by x- ray diffraction and microscopic techniques. Reactive spraying of nickel and aluminum resulted in coatings consisting of Ni, Al, Ni3Al, NiAl3, Ni5Al3, NiAl, and Al2O3, depending on the experimental conditions. Nickel aluminide phases observed in plasma spray depositions were compared with the phases obtained by combustion synthesis techniques, and the formation of phases in reactive spraying was attributed to the exothermic reaction between splats of aluminum and nickel. Primary and secondary reactions leading to the formation of nickel aluminides were also examined. The splat thickness and the reaction layer suppressed the formation of desired equilibrium phases such as Ni3Al and NiAl. As- sprayed coatings were annealed to enhance the diffusional reactions between the product phases and aluminum and nickel. Coatings obtained by reactive spraying of elemental powders were compared with as- sprayed and annealed coatings obtained with a bond coat material in which nickel was deposited onto aluminum particles.

Flattening of composite powder particles during thermal spraying

An investigation is conducted of the time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of composites (agglomerated) powder particles at smooth and rough surfaces during thermal spraying. Considerations include the splat solidification, droplet mass loss, solid volume fraction, and variations of splat- surface friction and splat solidification velocity due to the presence of the solid phase. Effective values of the droplet viscosity, impact velocity, and Reynolds number taking into account characteristic features of the flattening process are introduced, and analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are established. Results can be used to predict the splat flattening parameters.

Effect of droplet impact angle on flattening of splat in thermal spraying

Analytical correlations between the flattening characteristics and the Reynolds number and the spraying angle are obtained. The final splat thickness is shown to decrease and the final splat radius is found to vary nonuniformly with a decrease in the spraying angle. The theoretical results obtained agree with those of the experimentally observed tendencies of thermal spraying at off-normal angles.

Microstructural investigation of plasma-sprayed ceramic splats

Alumina and yttria stabilized zirconia (YSZ) (8 wt.% Y 2O 3) were plasma sprayed on smooth ( R a∼0.1–0.4 μm) substrates preheated to 300°C in order to study the splat formation. This preheating temperature was chosen to obtain very regular disk shaped splats quite adherent to the substrate made either of stainless steel 304 L, plasma sprayed alumina, YSZ polished coatings or pyrex. Thermal contact resistance evaluations suggested that the true contact area between the lamella and the substrate was less than the apparent area covered by the splat because of entrapped gas, surface contamination or oxide film. This resulted in good contact area through which the heat flux was released during crystal growth and poor ones for which the heat flux had to flow through the already solidified parts. In this paper, atomic force microscopy (AFM) of the splats, linked to results obtained by fast pyrometry (50 ns), SEM and XRD, enabled us to show different types of crystallisation fields related to the true splat contacts with the substrate. Different spraying angles and substrates have shown that solidification started before the flattening process was completed which explained the lenticular shaped splats obtained when preheating the substrates to 300°C. A microcolumnar structure (column diameters between 125 and 400 nm depending on the solidification time) orthogonal to the substrate was observed in good contact area. This solidification time could be varied for example for YSZ splats between 0.7 and 7.6 μs depending on substrate diffusivity and splat thickness linked to particle impact velocity. Much bigger crystals (one to two orders of magnitude) were observed in poor contact area. These poor contact areas were either distributed all over the surface of the splats for alumina or in the splat rim for alumina and YSZ. For the splat rim this poor contact is owing to the curling of the splat because of stress accommodation. For the big crystals formation within alumina splats its seems that it depends on substrate roughness and, for example, they can be observed along a slight scratch at the surface of a 304 L stainless steel substrate. The influence of surface roughness is drastically increased when the liquid material of the splat is still flowing at the end of the flattening stage, i.e. when spraying on an inclined substrate. In this case very small irregularities promote the formation of big crystals for both alumina and YSZ. When spraying on cold substrates the AFM observation is very difficult owing to a very distorted topography of the splat and its poor adhesion to the substrate. However, when possible, AFM shows in the splat central part “islands” of good contact surrounded by rims and in the fringes of the splat many “fingers”, the structure of which is similar to that of the rims.

Investigation of droplet flattening during thermal spraying

Formulae describing the evolution with time, and the final values of splat thickness, splat radius and the rates of their variation in the process of droplet flattening during thermal spraying are obtained. They take into account surface roughness, surface-splat friction, splat solidification, and droplet mass loss. On the basis of these formulae the flattening process was investigated. The theoretical results obtained agree well with the experimental data and can be used for predictive purposes.

Analysis of splat formation during flattening of thermally sprayed droplets

Formulae describing the evolution with time, and the final values of the splat thickness, the splat radius and the rates of their variation in the process of the droplet flattening during thermal spraying are obtained. They take into account the surface roughness, surface-splat friction, splat solidification and droplet mass loss. On the basis of these formulae the flattening process was investigated. The theoretical results obtained agree well with the experimental data and can be used for predictive purposes.

Influence of solidification on the flattening of droplets during thermal spraying

The analytical formulae describing the time evolution as well as the final values of the splat thickness and radius and the rates of their variation in the process of flattening of droplets during thermal spraying are obtained. They take into account the solidification of the splat and mass loss of an impinging droplet. The theoretical results obtained agree well with the experimental data and can be used for the prediction of splat flattening parameters.