Splat Microstructure Research Articles

Microstructure and adhesion of Cr 3C 2–NiCr vacuum plasma sprayed coatings

Vacuum plasma spraying (VPS) was used to spray a Cr 3C 2–NiCr coating of ∼ 150, 300 and 450 μm in thickness onto a plain carbon steel substrate, employing a commercially available Cr20Ni9.5C powder. The splat microstructures observed in the coating were found to consist of a NiCr matrix with a predominant Cr 3C 2 phase, besides Cr 7C 3 and Cr 2O 3. The adhesion of the coating to the substrate was evaluated by means of interfacial indentation techniques. It has been found that the interfacial toughness value changes from 7.6 to 10.1 MPa m 1/2 when the thickness increases from 150 to 450 μm. Also, it has been found that the parameter Kca o, determined by linear regression from the Kca versus 1 / e 2 curve by means of the interfacial indentation model advanced by Chicot et al., has a value of ∼ 9.8 MPa m 1/2.

Correlation between the Mechanical Properties and Splat Microstructures of an Air Plasma Sprayed Thermal Barrier Coating

Correlation between the Mechanical Properties and Splat Microstructures of an Air Plasma Sprayed Thermal Barrier Coating

Thermal Barrier Coatings Made by the Solution Precursor Plasma Spray Process

The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray process that can produce a wide variety of novel materials, including some with superior properties. The SPPS process involves injecting atomized droplets of a precursor solution into the plasma. The properties of resultant deposits depend on the time-temperature history of the droplets in the plasma, ranging from ultra-fine splats to unmelted crystalline particles to unpyrolized particles. By controlling the volume fraction of these three different constituents, a variety of coatings can be produced, all with a nanograin size. In this article, we will be reviewing research related to thermal barrier coatings, emphasizing the processing conditions necessary to obtain a range of microstructures and associated properties. The SPPS process produces a unique strain-tolerant, low-thermal conductivity microstructure consisting of (i) three-dimensional micrometer and nanometer pores, (ii) through-coating thickness (vertical) cracks, (iii) ultra-fine splats, and (iv) inter-pass boundaries. Both thin (0.12 mm) and thick (4 mm) coatings have been fabricated. The volume fraction of porosity can be varied from 10% to 40% while retaining the characteristic microstructure of vertical cracks and ultra-fine splats. The mechanism of vertical crack formation will be described.

Effect of substrate temperature and roughness on the solidification of copper plasma sprayed droplets

Flattening of an individual thermal sprayed particle on the substrate is a fundamental process for the coating formation. Coating properties such as porosity and adhesion depend strongly from the splats morphology. Also, the microstructure of the coating depends on the nucleation and the grain growth during splats solidification. The influence of the substrate temperature and roughness on the microstructure of copper splats was examined in the present study. Copper (−90 + 45 μm) was plasma sprayed under atmospheric conditions on mirror polished ( R a ∼ 0.02 μm) and on grit-blasted ( R a ∼ 1.33 μm) stainless steel substrates (AISI 304L). The substrates were preheated at three different temperatures: 200 °C, 250 °C and 300 °C. A Schottky field emission scanning electron microscope (SFE-SEM) was used in order to study the splats microstructure. In order to measure the grain size of the splats, image analysis software was used, while the chemical composition of the splats was studied by an electron probe microanalysis (EPMA). It was found that the splats cooling velocity is decreased with the increase of substrate temperature above the transition temperature. Also a further decrease of splats cooling velocity was provoked by increasing the mean substrate roughness.

Suppression of crystallization during high velocity impact quenching of alumina droplets: Observations and characterization

Droplets of alumina were deposited and quenched onto various smooth substrates using the high-velocity oxy-fuel (HVOF) thermal spray process. The microstructure of single splats was examined using optical microscopy, atomic force microscopy (AFM), focused ion beam assisted scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM). It was found that the high impact velocity inherent in HVOF formed splats that were in intimate contact with the substrate, leading to a high solidification rate up to 10 10 K/s. The solidified alumina splats exhibited a smooth surface devoid of “mud-crack” networks of surface micro-cracking, commonly observed in ceramic plasma sprayed systems. Microstructural examinations of the smooth areas of the splat revealed an amorphous microstructure as confirmed by selected area electron diffraction. In local areas of poor contact between the splat and substrate, a polycrystalline microstructure was discernible and a metastable γ-Al 2O 3 phase was detected. Second layer splats, that landed on top of a first layer splat and experienced slower cooling rate, were found to have surface cracking, and revealed principally a γ-Al 2O 3 nano-crystalline structure. The observation of amorphous alumina is explained by the suppression of a first order liquid-to-solid phase transformation, due to a high solidification rate that we estimate.

307 大気プラズマ溶射遮熱コーティングの機械的特性とスプラット組織構造との関連性(OS2-2 皮膜・薄膜の機械的特性,OS2 薄膜・皮膜とその膜構造の特性評価)

The correlation between the mechanical properties and the splat microstructure of an APSed TBC was studied by using the simple sintering model and the numerical simulation, i.e., the distinct element method. It was cleared from results of the numerical simulation that the mechanical properties of an APSed TBC were controlled with the change of the splat structure by the initial stage of the sintering. The change of mechanical properties in the APSed TBCs by the thermal exposure is predictable by using the proposed method in this work.

Effect of Steam Treatment During Plasma Spraying on the Microstructure of Hydroxyapatite Splats and Coatings

The major problems with plasma sprayed hydroxyapatite (HA) coatings for hard tissue replacement are severe HA decomposition and insufficient mechanical properties of the coatings. Loss of crystalline HA after the high-temperature spraying is due mainly to the loss of OH− in terms of water. The current study used steam to treat HA droplets and coatings during both in-flight and flattening stages during plasma spraying. The microstructure of the HA coatings and splats was characterized using scanning electron microscope, Raman spectroscopy, Fourier transform IR spectroscopy, and x-ray diffraction. Results showed that a significant increase in crystallinity of the HA coating was achieved through the steam treatment (e.g., from 58 to 79%). In addition, the effects were dependent on particle sizes of the HA feedstock, more increase in crystallinity of the coatings made from smaller powders was revealed. The Raman spectroscopy analyses on the individual splats and coatings indicate that the mechanism involves entrapping of water molecules by the individual HA droplets upon their impingement. It further suggests that the HA decomposition has already taken place before the impingement of the droplets on precoating or substrate. The improvement in crystallinity and phases, for example, from tricalcium phosphate and amorphous calcium phosphate to HA, was achieved by reversing the HA decomposition through providing extra OH−. Furthermore, the steam treatment during the spraying also accounts for remarkably increased adhesion strength from 9.09 to 23.13 MPa. The in vitro testing through immersing the HA coatings in simulated body fluid gives further evidence that the economic and simple steam treatment is promising in improving HA coating structure.

Effect of WC size on interface fracture toughness of WC–Co HVOF sprayed coatings

The interface fracture toughness of high velocity oxygen fuel (HVOF) sprayed coatings on carbon steel made of various types of WC–12 wt.% Co powders with different WC particle sizes of 0.2 to 7.0 μm was evaluated by the pre-notched four-point bending test to clarify the size effect of WC particles and to explore the superior adhesion mechanisms of this coating system. The correlation between the splat microstructure and the toughness variation was investigated by observing a cross-section of WC–Co splats around the interface using the focused-ion-beam (FIB) technique. The interface fracture toughness of WC–12 wt.% Co coating/carbon steel under a mixed Mode I/Mode II loading condition increases from 600 to 1800 J/m 2 as the WC particle size increases. Compaction of the underlying microstructure, the intrusion of WC particles into the substrate, and the volume fraction of the metallic binder phase in the coating are key factors in achieving excellent adhesion of this coating system.

Focused Ion Beam Study of Ni5Al Single Splat Microstructure

AbstractThermally sprayed (TS) coatings are widely used for surface engineering across a range of industries, including aerospace, infrastructure and biomedical. TS materials are formed via the successive impingement, rapid quenching and build-up of molten powder particles on a substrate. The impacted ‘splats’ are thus the fundamental microstructural constituents of the coatings, and their intrinsic properties, as well as intersplat bonding and morphology, dictate coating behavior. Beyond the obvious practical considerations, from a scientific standpoint, splats represent a fascinating template for study, due to the highly non-equilibrium processing conditions (rapid deceleration from sub-sonic velocities, million-degree/sec cooling rates). In the literature, many studies of isolated splats on substrates have been carried out, but these have focused on overall morphology (disc-shape vs fragmented). Direct observations of microstructure, in particular cross-section, are limited in the specimen preparation stage due to splat size (tens of microns in diameter, 1-2 microns in thickness). However, Focused Ion Beam (FIB) techniques have allowed this problem to be addressed in a robust manner; in this paper we will discuss such approaches to observe Ni5Al splats on stainless steel substrates. Cross-sections through the splat and the substrate were created by recourse to ion milling and the ion beam itself provided good channeling contrast for grain imaging. The typical splat microstructure with sub-micron Ni(Al) columnar grains, a chill zone at the bottom and a lift off area is observed in high detail. In addition, an amorphous aluminum oxide top layer of 100-200 nm is partially present on top of the Ni(Al) columnar grains. At the splat/substrate interface, defects such as micro- and nano-scale pores were characterized for the first time and will be discussed. These observations provide insights into splat and interface formation during the deposition process and may drastically improve our current understanding of Ni5Al splat properties.

Evaluation of fracture toughness of as plasma sprayed alumina–13 wt.% titania coatings by micro-indentation techniques

The fracture toughness of the as plasma sprayed alumina–13 wt.% titania (AT-13) coatings was evaluated using micro-indentation techniques. Indentations with smaller loads of 0.49 N show features such as partially melted particles to be extremely hard and brittle. However, the matrix made up of bi-modulus microstructure of splats was much tougher due to operative toughening mechanism for instance, crack bridging, which effectively retarded propagation of the cracks. Log plots of crack length ( C) versus load ( P) for loads varying from 1.96 to 9.8 N occurred with a slope of 0.65 ± 0.095 signifying sub-surface median or palmqvist type cracks to be rampant. The as sprayed microstructure made of pores; partially melted particles and splats were found to be anisotropic with regard to indentation toughness. Cracks were found to initiate and propagate easily in direction parallel to the coating growth directions than along the splat boundaries. Accordingly the bonding between the splats caused by plasma spray process could be deemed to be superior. The preferential directionality developed in the microstructure due to the imposed heat flow conditions essentially has weakened the microstructure.

Anisotropic resistivity in plasma-sprayed silicon thick films

Silicon thick films deposited by thermal plasma spray are of interest as inexpensive electronic materials for conformal meso-scale electronics applications. In addition they also serve as a model system for the investigation of electrical properties of coatings with layered anisotropy. In this study impedance spectroscopy was used to measure the complex resistivity of free-standing 64μm-thick polycrystalline silicon films deposited by thermal plasma spraying in an atmospheric ambient. Impedance spectroscopy measurements were taken in the through-thickness (across-splat) and edge-to-edge (in-splat) directions and revealed a resistivity difference of approximately 7.5±0.23 between the two directions. The complex resistivity results are explained on the basis of a brick-layer type model, associated with the layered splat microstructure obtained from cross-sectional transmission electron microscope imaging of the films. In addition a circuit-based model made up of parallel, resistor-capacitor elements in series, and Cole–Cole and Davidson–Cole impedance functions were used to fit the impedance data to extract material parameters and contributions from the grains and splat boundaries. Furthermore, thermal processing and phosphorus doping is shown to lead to higher and lower resistivity, respectively, in the films.

Effects of pores and interfaces on effective properties of plasma sprayed zirconia coatings

It is difficult to establish structure–property relationships in plasma sprayed coatings because of their unique and intermingled splat microstructures incorporating networks of various intrinsic process-dependent microdefects. In this paper, these coatings are characterized using two distinctly different approaches, both based on novel experimental techniques and computational modeling tools. In each approach, detailed finite element models are illustrated to represent the porous coatings fabricated with four different types of zirconia feed powder. One approach is based on small-angle neutron scattering (SANS) studies carried out to quantify microstructure. In this approach, the pore morphology is idealized by artificially rebuilding, based on the collective microstructural information obtained in terms of component porosities of three pore systems, their opening dimensions and orientation. The other method relies on image analysis of real microstructural images obtained using scanning electron microscopy (SEM). The finite element mesh for the actual cross-sectional model, generated by thresholding the SEM images, is constructed with the object oriented finite (OOF) element method. Through these two approaches, the effective thermal conductivity and elastic modulus along the spray, as well as the transverse directions are estimated for thermally sprayed yttria-stabilized zirconia (PSZ) coatings. Our results show the effectiveness of these computational approaches for estimating material properties with each approach having its strength and weakness. However, in comparison with experimentally measured properties, there exist some limitations in both approaches. To further probe the source of discrepancy within the measurements, the coatings are thermal cycled to reduce the effect of splat boundaries on properties. Additional models are constructed for these coatings and their analysis is carried out. For the first time, the influence of the splat interfaces on the effective properties of sprayed coatings is quantified.

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.

Microstructure of Plasma-Sprayed Cast Iron Splats with Different Particle Sizes

The superior wear-resistant property of cast irons is closely linked with their microstructure, in which graphite formation in plasmasprayed cast iron coatings causes distinct characteristics owing to its self-lubricating property. Since the solidification rate generally affects graphite formation, the optimum in spray parameters such as substrate temperature, ambient pressure, particle size and spray distance is required to slow down the solidification rate, as well as to improve the adhesive strength of splats. In this study, cast iron splats were induced on an aluminum alloy substrate by plasma spraying using alloyed cast iron powder high in silicon and aluminum in a low pressure argon atmosphere. Then, the effects of particle size on the microstructure and adhesive strength of splats were investigated by introducing the correlation between the solidification rate and the microstructure. Spraying with large particles leads to an increase in the number fraction of disk splats and a slight decrease in their adhesive strength. Cross-sectional observations reveal fine graphite growing in splats nearly perpendicular to the substrate surface.

Indentation fracture behavior of plasma-sprayed nanostructured Al2O3–13wt.%TiO2 coatings

Indentation crack growth resistance of nanostructured Al2O3–13wt.%TiO2 coatings plasma sprayed using nanosized powders was investigated. Comparisons were made between the nano-coatings and a commercial baseline coating of the same composition, Metco 130. In Metco 130 coatings that contain only the single-phase splat microstructure, long cracks initiate at the indent corners and propagate along splat boundaries. In contrast, the nano-coatings are composed of a bi-modal microstructure (a fully melted splat structure and a partially melted particulate structure), and the partially melted particulate region serves to trap and deflect the splat boundary cracks. The interface between the fully melted region and the partially melted region also provides additional crack arrest mechanisms. At optimized conditions, these toughening mechanisms can produce an approximately 100% improvement in the crack growth resistance. The optimized microstructure for the nano-coatings is the microstructure containing 15–20% of the partially melted particulate region, which can be systematically controlled by changing the plasma flame temperature.

Splat Morphology and Microstructure of Plasma Sprayed Cast Iron With Different Preheat Substrate Temperatures

A cast iron coating is a prime candidate for the surface modification of aluminum alloys for antiwear applications because cast iron is inexpensive and exhibits superior wear resistance arising from the self-lubricating properties of graphite. In the present study, fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate, including (1) the effects of preheat substrate temperature on the splat morphology, (2) the formation of a reaction layer and pores, and (3) the splat microstructure, were investigated in low-pressure plasma spraying. With an increasing substrate temperature, the splat morphology changes from a splash type to a disk and star shape. Deformed substrate ridges mainly resulting from the slight surface melting, are recognized adjacent to the splat periphery at high substrate temperatures. The flattening ratio of disk splats decreases with substrate temperature because the ridges act as an obstacle for splat expansion. A reaction layer composed of iron, aluminum, and oxygen is ready to form at high substrate temperatures, which, along with the deformed ridges, improves the adhesive strength of splats. However, the pores appear at the splat interface at low substrate temperatures, which hinder the formation of a reaction layer. The amount of graphitized carbon increases in cast iron splats with an increase in substrate temperature.

Microstructure development of Al 2O 3–13wt.%TiO 2 plasma sprayed coatings derived from nanocrystalline powders

The development of constituent phases and microstructure in plasma sprayed Al 2O 3–13wt.%TiO 2 coatings and reconstituted nanocrystalline feed powder was investigated as a function of processing conditions. The microstructure of the coatings was found to consist of two distinct regions; one of the regions was completely melted and quenched as splats, and the other was incompletely melted with a particulate microstructure retained from the starting agglomerates. The melted region predominantly consisted of nanometer-sized γ-Al 2O 3 with dissolved Ti 4+, whereas the partially melted region was primarily submicrometer-sized α-Al 2O 3 with small amounts of γ-Al 2O 3 with dissolved Ti 4+. The ratio of the splat microstructure to the particulate microstructure and thus the ratio of the γ-Al 2O 3 to α-Al 2O 3 can be controlled by a plasma spray parameter, defined as the critical plasma spray parameter (CPSP). This bimodal distribution of microstructure and grain size is expected to have favorable impact on mechanical properties of nanostructured coatings, as has been observed before.

Characterisation of electric arc spray formed Ni superalloy IN718

Nickel superalloy IN718 has been electric arc spray formed under a range of processing conditions as small diameter ring preforms. Top surface deposition temperature has been measured using an infrared thermal imaging video camera. The as-sprayed microstructure has been investigated by a combination of optical microscopy, image analysis and electron probe microanalysis, and characterised by the through thickness variation in porosity, grain morphology and microsegregation behaviour. Low top surface temperature preforms had an unfused splat microstructure with intersplat porosity. Intermediate temperature preforms had banded unfused and fused splat microstructures with some cross-splat columnar grains and low levels of porosity. High temperature preforms had banded unfused/fused splat and equiaxed grains. The extent of microsegregation depended upon the manufacturing temperature.

Mesoscopic Nonlinear Elastic Modulus of Thermal Barrier Coatings Determined by Cylindrical Punch Indentation

Cylindrical punch indentations are performed to determine the effective modulus of a plasma-sprayed ZrO2–8 wt% Y2O3 thermal barrier coating (TBC) as a function of coating depth. Cylindrical punch indentations offer significant advantages over pointed (Vickers, Berkovich, or Knoop) indentations for materials that do not exhibit linear elastic behavior. Cyclic loading with a cylindrical punch clearly shows the TBCs to exhibit nonlinear elastic behavior with significant hysteresis that is related to the compaction and internal sliding within the plasma-spray splat microstructure. Also, the effect of a high-heat-flux laser treatment is shown to produce a gradient both in the effective TBC modulus and degree of loading/unloading hysteresis with depth.

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.