Splat Microstructure Research Articles

Influence of spray parameters on particle deposition behaviour, microstructure, phase constitutions, and mechanical properties of high velocity oxy-fuel sprayed WC-Cr3C2-Co coatings

To optimise spray parameters for fabricating well-bonded and less decarburised coatings, the morphologies and cross-sectional microstructures of powders and splats, in addition to the microstructure, phase constitution, and mechanical properties of high-velocity oxy-fuel (HVOF)-sprayed WC–Cr3C2–Co coatings were systematically investigated. Significantly influenced by spray parameters, splat morphologies vary from ellipsoids to well-flattened pies with some ejectors or stripes on their periphery and a splat/substrate interface with good contact. Splat microstructures vary homogeneously from a minimal hard phase to non-uniform structures with some dense surface layers and smoothly outlined hard phases or to non-uniform structures with rough surfaces and angular hard phases. The coating microstructure evolves from a non-lamellar structure with many pores to a lamellar structure with a well-contacted interface and a smooth surface hard phase. The coatings mainly consist of the WC hard phase and a small fraction of decarburised and amorphous phases, in addition to Co, Cr3C2, Cr7C3, Cr2O3, and Cr2O5. Porosity and WC retention index vary from 0.89 ± 0.11 to 2.32 ± 1.75% and from 84.68 to 94.48%, respectively. Microhardness, elastic modulus, fracture toughness, adhesive strength, and wear rate of the coatings vary from 7.03 ± 0.64 to 11.96 ± 0.94 GPa, from 77.64 ± 13.77 to 244.73 ± 65.75 GPa, from 1.23 ± 0.1 to 2.66 ± 0.66 MPa m1/2, from 18.54 ± 1.21 to 63.84 ± 8.01 MPa, and from 1.38 ± 0.32 to 3.54 ± 0.79 × 10−2 mg/(N⋅m), respectively.

Microstructural study of Ni and Ni-20Cr particles plasma sprayed on stainless steel substrate at 300 ˚C

In thermal spray coatings, the bonding and adherence of the spray particles (called splats) to the substrate surface significantly influence the overall quality of the finished coating. In this study, both elemental Ni and Ni-20Cr powders were plasma sprayed onto polished stainless-steel substrates held at 300 ˚C. Splat formation for both feedstock materials was investigated and compared. Detailed studies of the microstructure of single splats as well as the splat-substrate interface were performed using scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The additions of Cr to nickel promoted improved adherence of the splat to the substrate and modified splat formation by improving wettability during spreading. More intriguingly, energy dispersive spectroscopy (EDS) mapping and line scans, performed on TEM cross-sections, revealed splat-substrate inter-diffusion and chemical inter-mixing. Of particular note, a ‘jet’ of iron/chromium-based material, arising presumably from the substrate, was noted to be intimately intermixed into the interior of the nickel splat.

High‐temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior

AbstractThe high‐temperature interaction between ~2.5 mg/cm2 of Na2SO4 and an atmospheric plasma sprayed (APS) Yb2Si2O7 topcoat–Si bond coat system on SiC CMC substrates was studied for times up to 240 h at 1000°C–1316°C in a 0.1% SO2–O2 gaseous environment. Yb2Si2O7 reacted with Na2SO4 to form Yb2SiO5 and an intergranular amorphous Na‐silicate phase. Below 1200°C, the reaction was sluggish, needing days to cause morphological changes to the “splat microstructure” associated with APS coatings. The reaction was rapid at 1200°C and above, needing only a few hours for the entire topcoat to transform into a granulated microstructure consisting of Yb2SiO5 and Yb2Si2O7 phases. Na2SO4 deposits infiltrated the Yb2Si2O7 topcoat and transformed into an amorphous Na‐silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb2SiO5 area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO2 thermally grown oxide (TGO) formed on the Si bond coat and rapid transport through the interpenetrating amorphous Na‐silicate grain boundary phase. It was predicted that nearly 2 weeks is needed for complete removal of Yb2SiO5 from the topcoat at 1316°C for a single applied loading of Na2SO4.

Effect of in-flight particle behavior on the morphology of flame sprayed Er2O3 splats

The morphology and microstructure of splats impact the comprehensive capability of a new coating methodology called chelate flame spraying (CFS). This study addresses the quantitative characterization of the spread morphologies of flame sprayed Er2O3 splats directly deposited under different spray conditions on aluminum alloy substrates with a mirror finish. The influence of the in-flight particle temperature and velocity, carrier gas type, and carrier gas ratio on the solidification mechanism of molten droplets was investigated. Image analysis methods were employed to identify single splats from the morphology observed with field-emission scanning electron microscopy (FE-SEM). In addition, Er2O3 films were synthesized on an Al–Mg alloy (A5052) substrate using N2 or O2 as the carrier gas. When O2 was used as the carrier gas, 109-μm-thick films were deposited on the A5052 substrate. The cross-sectional porosity of the films was 3.8%. In contrast, films with 101-μm thickness were synthesized on the A5052 substrate when N2 was used as the carrier gas. The cross-sectional porosity of these films was 13.8%. The results showed that the carrier gas type (N2) and carrier gas ratio had a significant effect on the flattening behavior of the molten droplets. A spraying method combined with multidimensional modes is proposed to control the morphology of the splats.

Mesoscale modeling of jet initiation behavior and microstructural evolution during cold spray single particle impact

Quasi-coarse-grained dynamics (QCGD) simulations are carried out to investigate the mesoscale deformation behavior during the impact of a 20 µm pure aluminum particle onto a substrate of pure aluminum at time and length scales relevant to cold spray deposition. A rigorous analysis of the evolution of pressure, temperature, strain, flow stress and microstructure is carried out to investigate the jetting mechanisms over a range of process parameters (impact velocity and particle temperature). The QCGD simulations identify a critical role of the pressure wave propagation in the initiation of a jet, i.e. outward flow of material at the particle/substrate interface periphery (edge). Jetting is observed to initiate when the shock wave interacts with the edge and results in localized softening of the metal in this region. This localized softening enables outward flow of the material and is accompanied by a release of the pressures in the particle and the substrate at the interface. Observations of final splat microstructures of systems that showed jetting revealed several new “small” grains in the range of 2–4 µm. These grains are mainly found at the interface, suggesting that recrystallization is favored in cold sprayed impacts of aluminum.

Acoustic Emission and Associated Damage Mechanism Analysis in 8YSZ Thermal Barrier Coatings Under Instrumented Indentation

Acoustic emission (AE) monitoring was used during indentation tests on the cross section of plasma-sprayed 8 wt.% yttria-stabilized zirconia (8YSZ) thermal barrier coatings to investigate the relationship between AE signals and the associated deformation or single cracking events. The damage evolution in 8YSZ was studied by examining signal characteristics with the aid of AE parameter analysis and wavelet packet decomposition. The results show that the coatings were firstly elasto-plastically deformed, and microcracks gradually developed around the indentation. Then, delamination occurred and the fracture was made up of some longer cracks and dozens of microcracks. AE signals originating from coating deformation, formation of microcracks and longer cracks show different amplitudes and frequencies. The results indicate that the features of AE parameters differ depending on the mechanical failure mechanism, and AE responses are closely related to the fracture behavior and are dependent on splat microstructure of the coatings.

Plasma-sprayed nickel splats on chromium substrates: The role of substrate preheating and thermal conductivity

The effect of chromium substrate preheating on the spreading behavior of plasma-sprayed nickel splats was characterized. The interfacial features along the splat-substrate interface were analyzed by focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In addition, the transient droplet spreading process was modelled in a simulation study. The results showed that the presence of a large fraction of surface moisture on the substrates induced splat fragmentation. However, splat fragmentation was completely constrained on chromium preheated to a low temperature (373 K) where a certain amount of surface moisture was still present. The high thermal conductivity of chromium substrates increased droplet solidification rates, decreasing the interaction time between the spreading droplet and the vaporized gas layer. Accordingly, splat fragmentation was suppressed and disk-shaped splats were formed. Fast solidification not only refined the final splat microstructure, but also promoted the formation of finger-splashed disk splats. In addition, the extent of elemental diffusion across the splat-substrate interface was decreased due to the low interfacial temperature.

The influence of powder properties on the adhesion strength and microstructural evolution of cold sprayed Ti6Al4V single splats

The adhesion strength and microstructure of single splats significantly influence the properties of cold sprayed coatings. Here, we compare the adhesion strength and microstructural changes of Ti6Al4V spherical powder (SP) particles with a martensitic microstructure and irregular powder (IP) particles with an equiaxed microstructure deposited by cold spray (CS). Splat adhesion tests were performed to determine the adhesion strength and electron channelling contrast imaging was done for microstructural analysis of splat cross-sections. IP splats formed a continuous bonded interface with the substrate resulting in a greater adhesion strength when compared to SP splats. IP splat cross-sections revealed ultrafine grains (UFG) near the interface followed by a highly deformed microstructure. SP splat cross-sections also showed UFG at the interface but largely retained the initial microstructure in the top portion of the splat due to poor deformability. The irregular morphology of the IP led to more adherent deposits while the equiaxed microstructure resulted in highly deformed post spray microstructures.

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.

Influence of microstructure on the mechanical integrity of plasma-sprayed TiO2 splat

The TiO2 splats with different microstructures were deposited by controlling the deposition temperature and the substrate crystalline structure, in order to investigate the effect of microstructures on the mechanical integrity of TiO2 splat. It was revealed that the cracking of TiO2 splat occurs by two different modes, i.e. intergranular cracking and intragranular cracking, which depend on splat microstructures. Both the morphologies and the spacing of cracks are significantly affected by the cracking mode. The area average crack spacing ranges from 5.0 to 10.4μm with the increase of splat grain size. In addition, it was found that the coarse grain splat and especially the monocrystalline one with (110) orientation spall off the substrate, while the fine grain splat is well bonded to the substrate. The cracking modes of TiO2 splat was discussed based on the stress-relieving mode, which is related to the accumulation and the relaxation of quenching stress.

Influence of microstructure on the optical property of plasma-sprayed Al, Cu, and Ag coatings

Metal materials have been used as thin films in many optical applications because of its excellent high reflectivity in the near-infrared range. However, studies on thick metal coatings deposited by plasma spraying have rarely been conducted. The current study investigates the relationship between optical property and microstructure for different metal plasma-sprayed coatings (Al, Cu, and Ag) and discusses the corresponding mechanical properties. The phase structure and surface morphology of as-sprayed powders and corresponding coatings were examined by X-ray diffraction and scanning electron microscopy, respectively. The optical and mechanical properties were characterized by UV–visible-near infrared spectroscopy, Multi-Specimen machine testing and Vickers microhardness testing. Results indicate that the optical behavior of metal plasma-sprayed coatings is related to their phase structure, splat microstructure, and surface roughness. Optimization of powders in molten state and the surface microstructure is proposed as an effective method to improve the optical property of these coatings. The molten state and low porosity also help to improve the mechanical properties of the coating materials.

Mechanism of Competitive Grain Growth in 8YSZ Splats Deposited by Plasma Spraying

The competitive growth mechanism of columnar grains generated from rapid solidification in plasma-sprayed yttria-stabilized zirconia (YSZ) splats collected on stainless steel at different temperatures was studied. The microstructure of YSZ splats was examined quantitatively by scanning electron microscope and x-ray diffraction. The crystalline orientation of columnar grains was investigated by electron backscattered diffraction. It was found that the competition between the columnar grains was governed by their relative growing rate. The columnar grains with growing face (001) had the slowest growing rate, while those with growing face {011} had the fastest growing rate. The degree of competition between the columnar grains was related to the nucleation rate for the splat at different substrate temperatures (T s). When the splat was at T s = 300 K or T s = 373 K, the relatively low nucleation rate resulted in less competition between the columnar grains; thus, no preferential orientation was observed. In contrast, for the splat at T s = 473 K, the higher nucleation rate resulted in more competition between the columnar grains, increasing the proportion of grains with the faster growing face {011}, and therefore, the columnar grains show 〈011〉 preferential orientation.

Microstructure of YSZ Coatings Deposited by PS-PVD Using 45 kW Shrouded Plasma Torch

In this study, a conventional 80 kW class plasma spraying system was used to produce yttria-stabilized-zirconia (YSZ) coatings by PS-PVD at a pressure of 100 Pa. A shroud was attached in the front of the plasma nozzle to restrain expansion of plasma jet. The torch was operated at an arc power of 45 kW and YSZ coatings were deposited at a powder feed rate of 0.2 g/min. Optical emission spectroscopy was used to diagnose the particle state in plasma jet. The surface morphology and cross-sectional morphology of coatings was characterized by field emission scanning electron microscope. It is found that the amount of YSZ evaporation is significantly enhanced through using a shroud. The coatings with a hybrid microstructure of splats and nanoclusters were deposited perpendicular to the coatings. The nanostructured clusters deposited out of the vapor are presented at splat interfaces. It is evident that using powders specially designed for PS-PVD and controlling heating of plasma jet to spray particles, PS-PVD deposition for a hybrid microstructure consisting of vapor phase deposit can be realized through conventional plasma spray system. Columnar grain structured YSZ can also be deposited by pure vapor phase at the side surface of the substrate.

Effect of intersplat interface bonding on the microstructure of plasma-sprayed Al2O3 coating

The interface bonding between lamellae dominates the properties and performance of plasma-sprayed ceramic coatings. In this study, the interlamellar interface bonding and its effect on the splat microstructure were examined using TEM analysis of the microstructure of a plasma-sprayed Al2O3 coating. The obtained results revealed that the intersplat interface microstructure depends significantly on the intersplat bonding. An amorphous phase was observed in the interface region in which the bonding was formed, while the γ-Al2O3 phase was observed at the interface where no bonding was formed. In addition, it was found that the interface bonding significantly influenced the interface microstructure of the coating. After heat treatment, the phase of the bonding between adjacent splats was transformed from amorphous to γ-Al2O3. In the interface region in which the amorphous phase recrystallization occurred, nanosized pores evolved owing to the volume shrinkage accompanying the transformation of alumina from the amorphous to the γ-Al2O3 phase.

Role of substrate temperature on microstructure formation in plasma-sprayed splats

The flattening nature of the individual splat in thermal spraying is the fundamental process for the coating fabrication process, thus numerous studies have been conducted to understand the splat formation process in thermal spraying. In the present investigation, commercially available pure Cu particles with diameter of several tens of micrometers were thermally sprayed onto mirror polished AISI304 substrates held at various temperatures during the spraying. The top surface, bottom surface, and cross section morphologies of splats collected under designated conditions were observed in detail. According to the observation, different splat microstructures could be found varied with the substrate temperatures. In particular, a typical layer composed of fine grains could be found at splat–substrate interface when the substrate was held at high temperature, which has a close relation with the favorable wetting and enhanced heat transfer from molten droplet to substrate. This layer could induce the rapid decrease of droplet viscosity at the bottom surface and prevent the further spreading of the splat, which promoted the formation of disk-shaped splat.

Relaxation scenarios in Fe–Pd and Fe–Pd–Cu ferromagnetic shape memory splats: Short range order and microstructure

Fe–Pd-based ferromagnetic shape memory alloys are based on metastable random solid solutions in the austenite and martensite phases, which require preparation from the equilibrium γ phase by rapid cooling. Employing splats, we explore the impact of annealing treatment on the disordered solid solution, phase formation and lattice defects using a combined conversion electron Mossbauer spectroscopy – X-ray diffraction study. We discuss indications of short range ordering tendencies as well as the impact of splat microstructure and defects on phase formation.

Study of the Splat Microstructure and the Effects of Substrate Heating on the Splat Formation for Ni-Cr Particles Plasma Sprayed onto Stainless Steel Substrates

The plasma spraying process is still poorly understood in term of the processes by which the coating is built up, especially coating interactions with the substrate. This present study enhances this understanding by studying, through a range of electron microscopy techniques, single NiCr splats plasma sprayed onto stainless steel substrates, which were first exposed to different heat treatments. The microstructure of the splats, particularly the splat-substrate interface, was characterized, and the formation of the observed features is discussed. Evidence of localized substrate melting and inter-mixing with the splat material was found, showing metallurgical bonding. The structures observed were also correlated to the treatment of the substrate, demonstrating how such treatments can influence the properties of the fully deposited coating by modifying the splat formation process. Most notably, heating the substrate during spraying was found to significantly modify splat formation by reducing splashing and increasing the extent of substrate melting.

Study of the Splat Microstructure, Splat-Substrate Interface, and the Effects of Substrate Heating on the Splat Formation for Ni-Cr Particles Plasma Sprayed on to Aluminum Substrates

This study investigates the mechanisms of formation of plasma-sprayed coatings, through the study of splat morphology and the splat-substrate interface of NiCr single splats sprayed onto Al substrates, at both micro- and nano-scale levels, using a range of electron microscopy techniques. This study provides direct observation of extensive substrate melting, together with chemical inter-mixing with the splat and the formation of metastable interfacial phases (including both non-equilibrium phases and metallic glasses). In addition, voids and a range of oxide phases were observed. The mechanisms of formation of these features are discussed. Two aluminum alloys (with and without Mg additions) and three different sets of substrate surface conditions (polished, and heat treated prior to and during spraying) were used to investigate the variations in splat morphologies induced by the different preparation conditions. It was found that the heating of substrate during spraying significantly reduced splashing.

Effects of Substrate Roughness on Splat Formation for Ni-Cr Particles Plasma Sprayed onto Aluminum Substrates

Roughening of the substrate, for instance by grit-blasting or etching, is often used before plasma spraying in order to provide a high degree of roughness that promotes mechanical interlocking of the sprayed coating and consequently improved adhesion. This study investigates the morphology and microstructure of NiCr splats formed on such rough Al substrates, where roughness was generated by a number of methods including grinding and etching. Cross sections of the splats and the splat-substrate interface were examined using a range of electron microscopy techniques. Localized substrate melting and chemical mixing with the splat material was observed, forming very particular structures. The formation of various oxides phases and voids was also noted and found to increase, along with the degree of the substrate melting, with increasing substrate roughness. The structures observed were related to the spray conditions and substrate morphology.

Microstructural Study of Splat Formation for HVOF Sprayed NiCr on Pre-Treated Aluminum Substrates

In this study, the mechanisms of splat formation and the influence of substrate surface conditions were studied for NiCr splats HVOF sprayed onto Al. The specimens, having undergone various substrate pre-treatments, were observed at high resolution using various electron microscopy techniques with emphasis on the splat microstructure and the splat-substrate interface. From the observations made here, a model for splat formation is proposed, whilst the effects of surface chemistry on this process were investigated. It was found that that due to the deformation of the substrate on impact of the partially molten particle, effective contact may be achieved between the substrate and the splat. However, in many cases, much of the initial projected particle did not adhere to the substrate after impact. The presence of oxide or hydroxide layers on the surface of the substrate was influential on the adhesion of the NiCr splat