Splat Morphology Research Articles

Raman spectroscopy assisted residual stress measurement of plasma sprayed and laser remelted zirconia splats and coatings

This investigation deals with Raman and X-ray assisted stress measurement of plasma sprayed and laser remelted zirconia splats and coating. A Raman peak of tetragonal zirconia phase at 635.38 cm−1 was used as a reference peak. The tensile residual stress in the as-sprayed and laser remelted single splat lied in the range of 1–2 GPa and 0.2–0.9 GPa, respectively. Surface residual stress of the as-sprayed coating is tensile and its magnitude increases upon laser remelting. Raman spectroscopy results are in agreement with those obtained using X-ray diffraction. It appears that Raman spectroscopy is suitable for quick and accurate measurement of residual stress for small sized Raman active objects and stress depth profiling of the bulk coating. Splat morphology was correlated with particle velocity and temperature. Microstructural aspects and mechanical properties of as-sprayed and laser remelted coating were also investigated.

A computational approach for predicting microstructure and mechanical properties of plasma sprayed ceramic coatings from powder to bulk

In this study, a multi-pronged computational approach is developed to predict the effect of spray parameters on aluminum oxide splat formation and mechanical properties of the coatings. Effect of plasma power and substrate preheat temperature on the splat formation and coating's elastic modulus are studied. The splat morphology is investigated using computational fluid dynamics approach. Simulated splat morphologies show a good agreement with the experimentally obtained splats. Three-dimensional coating structure is constructed using the stochastic approach with different simulated splat morphologies, such as the disk, fragmented and fingered shapes, and their volume fractions. Microstructure-based Finite Element Analysis is used to compute the elastic modulus of the coating, which is found to be higher (~12%) than the experimentally obtained value. A correction factor (fis) is introduced, which takes into account inter-splat cracks, interface bonding, and other effects like curling of splats and splat sliding. After the correction factor, the computed elastic modulus for simulated coating was comparable to the experimental values. This study shows that the proposed computational approach can predict the mechanical properties of the coating and is promising for developing plasma-sprayed coatings with predictable properties. This methodology can be extended to other materials systems resulting in a desktop manufacturing approach of plasma sprayed coatings with a reduced number of experiments.

Efficacy of graphene nanoplatelets on splat morphology and microstructure of plasma sprayed alumina coatings

Splat morphology and its microstructure are of significance to affect the comprehensive properties of plasma sprayed ceramic coatings. This paper studies the effect of graphene nanoplatelets (GNPs) on the splat morphology and microstructure evolution of plasma sprayed alumina coatings. With an increasing GNP content, splat morphology became more disk-shaped and the average diameter of the splat decreased from 120 ± 28 μm for pure Al2O3 to 83 ± 17 μm for 1.0 wt% GNP/Al2O3. Simulated splat morphology presented a good agreement with the experimental observations. GNP agglomeration at the periphery of a droplet accelerate the formation of solid ring resulting in a decrease in splat diameter, and homogeneously distributed GNPs inside a droplet is expected to simultaneously promote rapid solidification of inward melt following the formation of a solid ring, which suppresses splat fragment. Meanwhile, columnar grain refinement and GNP pull-out are responsible for the composite splats free of microcracks.

The Morphology Analysis of Plasma-Sprayed Cast Iron Splats at Different Substrate Temperatures via Fractal Dimension and Circularity Methods

Abstract Plasma-sprayed cast iron splats were deposited onto polished aluminum substrates preheated to different temperatures ranging from 25°C to 250°C. The morphology of single splat was observed by a field emission scanning electron microscope. Quantitative characterization methods, including fractal dimension (FD) and circularity analyses of the splat profile, were employed to identify the difference in morphology of the splats with the change of the substrate temperature. The results showed that the substrate temperature has a significant effect on the spreading of molten droplets and the morphology of resultant splats through changing the solidification rate of the droplets. With the increment of substrate temperature, the homogeneous and sufficient spreading of the droplets resulted from low solidification rate reduces the splashing of the droplets. In addition, the evaporation of adsorbed moisture on the substrate improves the wettability between the spreading droplet and the substrate, then benefits the homogeneous spreading of the molten droplet. As a result, a distinct decline in the FD value was observed. It was also suggested that the FD analysis could be used to characterize the morphology of the splat more effectively while the circularity method was heavily dependent on the area of the splat.

Characterization Studies of Slurry-Sprayed Mullite–Nickel Coatings on ASTM 1018 Steel

Slurry spray technique (SST) is a simplistic deposition technique which could easily be contrived to the choice of material for appropriate functionality. With the continual technological progress and manufacturing efficiency, the appropriacy of SST needs to be expanded to meet the growing demand for surface engineering. Functionally graded mullite–nickel $$(\hbox {3Al}_{2}\hbox {O}_{3}\cdot \hbox {2SiO}_{2}\hbox {-Ni})$$ -based environmental barrier coatings were slurry sprayed on ASTM 1018 steel. This study reports the findings pertaining to the fundamental mechanical, thermal, corrosion and microstructural investigations performed to evaluate and ascertain the utility of the coatings produced by SST. Sound and resilient coatings with density in the range 1238– $$5320\hbox { kg/m}^{3}$$ and coating thickness in the range of 99– $$380\,\upmu \hbox {m}$$ were attained at distinct processing parameters. Within the experimental design space, a maximum adhesion strength value of 18.15 MPa was measured during tensile adhesion test. The developed coatings with microhardness up to $$204.2\hbox { HV}_{0.1}$$ , exhibit the surface roughness value ranged from 3.27 to $$7.98\,\upmu \hbox {m}$$ (Ra) and the porosity with an overall mean value of 9.9% by volume. During the thermal cycling testing, the majority of the specimens did survive up to 30 thermal cycles, which encapsulate acceptable thermal shock resistance of the coatings. The corrosion studies of the mullite–nickel coatings performed by immersion test in $$0.5\hbox {M Na}_{2}\hbox {SO}_{4}$$ proclaimed a decent corrosion rate of 5.217 mpy. The microstructural studies supported by SEM/EDS and XRD demonstrate as-sprayed splat morphology and the continuous interface and suggest a good adherence of the coatings fabricated wielding SST.

Effect of substrate temperature on the microstructure and interface bonding formation of plasma sprayed Ni20Cr splat

Ni20Cr splats were sprayed onto polished substrates at different preheating temperatures in an Ar atmosphere by low pressure plasma spray to reveal the dominating factor driving the bonding formation of the splats. The splat morphology, microstructure and splat-substrate interface were characterized by SEM, EBSD and TEM. The interface for examination of typical splats was prepared by focus ion beam (FIB). Disk-like splats were obtained on AISI 304 stainless steel substrate preheated to temperatures of 100 °C (cooling down from 350 °C) and 350 °C, whereas few short projections in the periphery were observed on the splat deposited at 550 °C. For all splats, three typical distinct zones with different microstructure features were observed on the splat surface by SEM and EBSD, including central coarse grain zone, transitional medium grain zone and peripheral fine grain zone. The interface bonding formation is closely related to the microstructure distribution. TEM observations demonstrated that the bonded interface only formed in the central coarse grain zone which provides a facile approach to access the bonding of plasma sprayed splat. The present work also reveals that increasing the substrate temperature cannot significantly improve the interface bonding due to the existence of oxide layer. The mechanism dominating the formation of those microstructures was also discussed.

Understanding the formation of plasma-sprayed Ni20Cr splats through interface observation

In the formation of plasma-sprayed metallic splats, substrate preheating plays a critical role in determining the splat morphology as well as the splat-substrate and the splat-splat interfacial interactions. In the present work, the splat formation mechanism of regular-shaped Ni20Cr splats on stainless steel substrates preheated at 423 K, 573 K and 673 K was studied. Focused ion beam (FIB) microscopy and transmission electron microscope (TEM) were used to characterize the splat solidification microstructure, pore formation, splat delamination, and the elemental diffusion along the splat-substrate interface. The substrate surface roughness was consistent up to 573 K preheating temperature, while the great degree of surface oxide formation increased the roughness at 673 K. For disk splats, good interfacial contact with a long distance of elemental diffusion was observed near the center. With increased substrate preheating temperatures, the degree of curling-up of the splat edge decreased. At high substrate preheating temperatures (673 K) where splats slowly solidified, Cr segregated to the top surface forming a thick layer of NiCr2O4 spinel. During FIB milling, a dense layer consisting of Fe and Cr was formed at the delaminated splat edge due to substrate material redeposition.

Oxidation behavior of nanostructured and conventional MoSi2 plasma-sprayed coatings

The present study was conducted to evaluate and compare oxidation behavior of nanostructured and conventional MoSi2 plasma-sprayed coatings. For this purpose, the synthesized and conventional MoSi2 powders were deposited by argon-shrouded plasma spraying (ASPS). The results showed that the nanostructured coat consisted of nanoparticles of solid or semisolid state entrapped in a typical splat morphology. The nanostructured coat showed a high concentration of non-transformed α-MoSi2 phase. The formation of a continuous SiO2 layer revealed a protective effect of plasma-sprayed coatings at a high temperature of 1000 ℃. The adhesion strength, hardness, and roughness of nanostructured and conventional MoSi2 coatings were also determined in this work.

Influence of Lamellar Interface Morphology on Cracking Resistance of Plasma-Sprayed YSZ Coatings

Splat morphology is an important factor that influences the mechanical properties and durability of thermal barrier coatings (TBCs). In this study, yttria-stabilized zirconia (YSZ) coatings with different lamellar interface morphologies were deposited by atmospheric plasma spraying (APS) using feedstocks with different particle sizes. The influence of lamellar interface roughness on the cracking resistance of the coatings was investigated. Furthermore, the thermal shock and erosion resistance of coatings deposited by two different powders was evaluated. It was found that the particle size of the feedstock powder affects the stacking morphology of the splat that forms the coating. Coatings fabricated from coarse YSZ powders (45–60 μm) show a relatively rough inter-lamellar surface, with a roughness about 3 times greater than those faricated from fine powders (15–25 μm). Coatings prepared with fine powders tend to form large cracks parallel to the substrate direction under indentation, while no cracking phenomena were found in coatings prepared with coarse powders. Due to the higher cracking resistance, coatings prepared with coarse powders show better thermal shock and erosion resistances than those with fine powders. The results of this study provide a reference for the design and optimization of the microstructure of TBCs.

Detonation spraying behaviour of refractory metals: Case studies for Mo and Ta-based powders

In thermal spraying of refractory metal powders, two major issues need to be solved: particles of materials having high melting temperatures should be heated to reach a semi-molten/molten state or temperatures close to the melting point, while oxidation of the metals should be prevented. It has long been believed that it is rather difficult, if not impossible, to produce high-quality refractory metal coatings by detonation spraying. In this work, we demonstrated the capability of the detonation spraying method to produce tantalum-based and molybdenum coatings of low porosity. Using a computer-controlled detonation spray (CCDS2000) facility, the detonation spraying behaviour of a molybdenum powder and a partially oxidized tantalum powder was studied. Spraying was conducted onto steel substrates using an acetylene-oxygen mixture with O2/C2H2 = 1.1. The spraying process was studied by means of analyzing the splat morphology and calculating the particle temperatures and velocities. According to the X-ray diffraction phase analysis, the metals did not experience oxidation during the deposition. Rather, partial reduction of the oxide phase contained in the Ta-based powder occurred during spraying.

Influence of Impact Conditions on Feedstock Deposition Behavior of Cold-Sprayed Fe-Based Metallic Glass

Cold spray is a promising method by which to deposit dense Fe-based metallic glass coatings on conventional metal substrates. Relatively low process temperatures offer the potential to prevent the crystallization of amorphous feedstock powders while still providing adequate particle softening for bonding and coating formation. In this study, Fe48Mo14Cr15Y2C15B6 powder was sprayed onto a mild steel substrate, using a variety of process conditions, to investigate the feasibility of forming well-bonded amorphous Fe-based coatings. Particle splat adhesion was examined relative to impact conditions, and the limiting values of temperature and velocity associated with successful softening and adhesion were empirically established. Variability of particle sizes, impact temperatures, and impact velocities resulted in splat morphologies ranging from well-adhered deformed particles to substrate craters formed by rebounded particles and a variety of particle/substrate interface conditions. Transmission electron microscopy studies revealed the presence of a thin oxide layer between well-adhered particles and the substrate, suggesting that bonding is feasible even with an increased oxygen content at the interface. Results indicate that the proper optimization of cold spray process parameters supports the formation of Fe-based metallic glass coatings that successfully retain their amorphous structure, as well as the superior corrosion and wear-resistant properties of the feedstock powder.

A highly strain and damage‐tolerant thermal barrier coating fabricated by electro‐sprayed zirconia hollow spheres

AbstractAn approach to make air plasma sprayed (APS) thermal barrier coatings (TBCs) with the enhanced strain and damage tolerance was reported, using a novel hollow spheres produced by electro‐spraying (ESP) technique. Compared with agglomerated & sintered (A&S) and hollow spherical (HOSP) yttria‐stabilized zirconia (YSZ) powders, the ESP powder showed a unique network microstructure and the TBCs exhibited a 2‐3 times longer thermal cycling lifetime. The splat morphology and the top coats microstructure were investigated. Some semi‐melted ESP particles were observed in the as‐sprayed top coat. The indentation coupled with the Raman mapping technique was employed to evaluate the strain and damage tolerance of the TBCs. The coatings deposited by the ESP powder show a lower in‐plane stiffness determined by three‐point bending tests. It is proposed that the superior performance is attributed to the lower amount of the short microcracks (0.5‐4 μm) with low angle (<45°) and the semi‐melted ESP particles remained in the YSZ top coat.

A novel method to predict the highest hardness of plasma sprayed coating without micro-defects

The plasma sprayed coatings are stacked by splats, which are regarded generally as the elementary units of coating. Many researchers have focused on the morphology and formation mechanism of splat. However, a novel method to predict the highest hardness of plasma sprayed coating without micro-defects is proposed according to the nanohardness of splat in this paper. The effectiveness of this novel method was examined by experiments. Firstly, the microstructure of splats and coating, meanwhile the 3D topography of the splats were observed by SEM (SU1510) and video microscope (VHX-2000). Secondly, the nanohardness of splats was evaluated by nanoindentation (NHT) in order to be compared with microhardness of coating measured by microhardness tester (HV-1000A). The results show that the nanohardness of splats with diameter of 70 μm, 100 μm and 140 μm were in the scope of 11∼12 GPa while the microhardness of coating were in the range of 8∼9 GPa. Because the splats had not micro-defects such as pores and cracks in the nanohardness evaluated nano-zone, the nanohardness of the splats can be utilized to predict the highest hardness of coating without micro-defects. This method indicates the maximum of sprayed coating hardness and will reduce the test number to get high hardness coating for better wear resistance.

The Influence of Process Parameters on the Structure, Phase Composition, and Texture of Micro-Plasma Sprayed Hydroxyapatite Coatings

In this study, hydroxyapatite (HA) coatings were deposited on Ti-6Al-4V by micro-plasma spraying (MPS). The influence of the process parameters on the microstructure of HA coatings was investigated. The splat morphology and spreading behavior were examined to understand the influence of process parameters on the coating. The texture strength of HA coatings was characterized by X-ray diffraction (XRD). The texture coefficients were all applied to characterize the variation in texture. The morphology of splats and coatings were characterized by scanning electron microscopy (SEM). XRD pattern shows that the texture intensity of the c-axis of HA was greatly influenced by spraying distance and spraying current. SEM reveals the different texture strength of HA coatings with different ratios of columnar grains. The strongest c-axis texture was found in the coating by 60 mm spraying distance with a spraying current of 40 A. In the cross-section SEM images of the coating with the strongest c-axis texture, uniform distribution columnar grains were observed in the upper part (~100 μm). The investigation of splats indicates that columnar grain growth occurs after fully melted particles impact the heated substrate. By controlling the melting state prior to in-flight particle impacts, columnar grain growth can be achieved during slow solidification of the disk shape splat during MPS.

Metallization of polyether ether ketone (PEEK) by copper coating via cold spray

The metallization of polymer-based materials via cold spray has been attractive to improve its thermal and electrical properties, while the direct deposition of thick Cu coating was rarely reported. In this work, relatively low process parameters were used to metallize PEEK substrate via cold spray to enhance the electrical properties of PEEK. The coating microstructure and properties such as microhardness and electrical conductivity were investigated at different propelling gas pressures. A thick and dense Cu coating was produced on PEEK substrate with only three passes of deposition. Based on Raman spectra, the polymer structures of PEEK substrate were well retained after high-velocity impact in cold spray. Meanwhile, the electrical conductivity and microhardness of the cold sprayed coating on PEEK increase at a higher propelling gas pressure. At last, the splat morphology of single Cu particle deposition on PEEK accompanying the finite element analysis were used to discuss the deposition mechanism of metallic particle on polymer surface. More prominent material jet was formed at the substrate at a higher propelling gas pressure as the adiabatic shear instability is more evident at higher particle impact velocity, which facilitates the deposition of metallic coating on PEEK.

Effect of spraying parameter and injector angle on the properties of in-flight particles and alumina coatings on Al alloy with PA-HT

Bonding strength is one of the most important properties of plasma sprayed coatings, especially ceramic coatings on complexly shaped light alloys, which is mainly determined by the properties of in-flight particles and their flattening behaviors on substrate surface. Consequently, the influence of current, voltage, primary gas flow rate and injector angle on temperature and velocity of in-flight alumina particles and morphology of splats on Al alloy with plasma arc-heat treatment (PA-HT) were investigated in detail by DPV Evolution, optical microscope and 3D non-contact surface mapping profile. The bonding strength, hardness and wear behavior of corresponding alumina coatings were measured by universal testing machine, nano-indentation test and wear test. Results showed that particle temperature and velocity increased with increase of input current and voltage, while their trends were reverse with increasing primary gas flow rate. The droplets could melt Al alloy surface and penetrate into its interior that greatly affected the morphology and corresponding shape factor (SF) of splats. Moreover, reducing injector angle caused droplets to glide on substrate surface and then to form “sole-like” splats. The morphology change of these splats obviously influenced the interfacial bonding strength, compactness, hardness and wear resistance of alumina coatings.

Deposition mechanism of plasma sprayed droplets on textured surfaces with different diameter-to-distance ratios

The laser method was used to process dimples with different diameters and distances on ASTM 1045 steel substrates. Ni60 alloy powders were deposited on the textured substrates using atmosphere plasma spray technology. Single sprayed particles were collected to study the spreading behavior. Scanning electron microscopy was applied to characterize the cross-section morphology of splats and coatings, investigating the deposition behavior of droplets. Image analysis was used to calculate the open angle of first particle, thus also providing information on the subsequent particles. Transmission electron microscopy was used to investigate the crystal structure of coatings near the interface. The bonding strength of coatings was evaluated by pull-off tests. The impact of various texture parameters on the adhesion strength of coatings was investigated. The results showed that bonding strength improved with increasing texture diameter and decreasing texture distance. Wetting behavior between two particles as well as bond style between particles and substrates have great influence on bond strength of coatings with the variant texture diameter and distance. The diameter-to-distance ratio was found to be the factor directly determining the adhesion strength between substrates and coatings.

Influence of Feedstock Feed Rate on the Properties of SPPS-Deposited Cobalt Ferrite Splats

Cobalt ferrite sol-gel solution was deposited onto mild steel substrates to form single splats using a solution precursor plasma spray (SPPS) process. It was revealed that the thermal oxidation of cobalt and iron salts occurred mainly into six different formations of deposited splats. These formulations were cobalt monoxide (CoO), maghemite (γ-Fe2O3), cobalt oxide (Co3O4), cobalt oxyhydroxide (CoOOH), goethite (α-FeOOH), hematite (α-Fe2O3), magnetite (Fe3O4), and cobalt ferrite (CoFe2O4). The surface topographies of the splats exhibited burned, unmolten, partially molten, and completely molten splats. The feed rates of SPPS process played the important role on several properties of the splat such as splat morphologies and percentage of the obtained composite splat phases.

Microsecond-scale formation of Ni[sbnd]Ti intermetallics in thermal spray coatings

Intermetallic phase formation due to diffusion reactions between nickel and titanium in a fast dynamic system, such as the thermal spray coating process is reported here for the first time. In the present work, we examined the possibility of the ultra-fast (in the order of microseconds) reaction of nickel single molten droplets deposited by plasma spray on a titanium substrate and the role of this interfacial reaction on the droplet spreading process. The splat morphology and splat-substrate interfaces were characterised using FIB and TEM. Titanium was found to diffuse into the nickel splat, but not vice versa. Excellent splat-substrate contact was observed with the co-existence of three interfacial layers of TiNi3, TiNi, and Ti2Ni having a total thickness from 0.4 to 1μm. In contrast, poor contact and delamination was observed at the splat periphery without the formation of interfacial layers. This indicated that the microsecond chemical reactions occurring between the nickel droplet and titanium substrate can enhance the interface bonding. We also found that the diffusion reaction to form intermetallics occurred after the splat solidified, thus it seems unlikely to influence the spreading of the splat on the solid surface.

Impact-disrupted gunshot residue: A sub-micron analysis using a novel collection protocol

Abstract The analysis of gunshot residue (GSR) has played an integral role within the legal system in relation to shooting cases. With a characteristic elemental composition of lead, antimony, barium, and a typically discriminative spheroidal morphology, the presence and distribution of GSR can aid in firearm investigations. In this experiment, three shots of low velocity rim-fire ammunition were fired over polished silicon collection substrates placed at six intervals over a 100 cm range. The samples were analysed using a Field Emission Gun Scanning Electron Microscope (FEG-SEM) in conjunction with an X-flash Energy Dispersive X-ray (EDX) detector, allowing for GSR particle analyses of composition and structure at the sub-micron level. The results of this experiment indicate that although classic spheroidal particles are present consistently throughout the entire range of samples their sizes vary significantly, and at certain distances from the firearm particles with an irregular morphology were discerned, forming “impact-disrupted” GSR particles, henceforth colloquially referred to as “splats”. Upon further analysis, trends with regards to the formation of these splat particles were distinguished. An increase in splat frequency was observed starting at 10 cm from the firearm, with 147 mm −2 splat density, reaching a maximal flux at 40 cm (451 mm −2 ), followed by a gradual decrease to the maximum range sampled. Moreover, the structural morphology of the splats changes throughout the sampling range. At the distances closest to the firearm, molten-looking particles were formed, demonstrating the metallic residues were in a liquid state when their flight path was disrupted. However, at increased distances-primarily where the discharge plume was at maximum dispersion and moving away from the firearm, the residues have had time to cool in-fight resulting in semi-congealed and solid particles that subsequently disrupted upon impact, forming more structured as well as disaggregated splats. The relative compositions of the characteristic elements that are present in GSR also change in the different splat morphologies sampled, which may contribute to the particles' physical structures. Two distinct populations of splats were also observed: circular and elongated, which suggest the residues hit the substrate at different angles. The difference in the splat impact angle can be ascribed to the position of the residues within the firearm discharge plume; particles get caught up in the vortex that is created by the discharge gases behind the projectile as it leaves the barrel, thereby affecting their directionality and flight time. This reasoning could also justify the existence of both spheroidal and splat particles at certain distances. The novel sampling and analytical techniques used in this experiment have provided previously unknown information in relation to GSR structure and formation which could have greater implications to its current analysis amongst laboratories and law enforcement agencies worldwide.