In the past few years, the Extreme High-Speed Laser Application (EHLA) process has been used as a coating technology alongside conventional processes due to its unique process characteristics and as an economical and sustainable alternative to traditional technologies. Compared to other LMD processes, the main energy input is into the powder material instead of into the substrate. This potentiates the achievement of to significantly higher surface and deposition rates as well as the coating of heat-sensitive substrates. Moreover, this increase in resource efficiency leads to a more sustainable and economically attractive process. To reduce component´s weight as well as secondary energy consumption, aluminum has become an essential base material in most industrial sectors. Aluminum is not simple to process its resistance is comparatively small due to its low hardness in relation to widely used steels. The low melting temperature of aluminum (approx. 750 °C) poses a great challenge when coating with, for example, iron-based alloys. Another challenge for laser-based systems is the reflectance of aluminum in the wavelength range of conventional laser beam sources (approx. between 1030 and 1070 nm). Therefore, for conventional laser-based processes, laser beam sources in other wavelength spectra, e.g., green or blue, are being developed to improve the processing of aluminum. Currently, commercially available multi-kW lasers in the visible light spectrum are still below the available power of IR-beam sources. In the context of this study, the feasibility of coating aluminum mase materials using EHLA is investigated. Besides the feasibility, the focus is to determine the maximum achievable surface and deposition rates up to the utilization of the available 8 kW infrared laser powder.

The coating that combines efficient microwave absorption performance and excellent tribological properties has important research value. Herein, firstly, the ceramic precursor was coated on expanded graphite (EG) or thermally expanded reduced graphene oxide (TERGO) sheets to prevent aggregation. Then, the treated EG and TERGO were homogeneously dispersed in the ceramic matrix to prepare two types of graphene-based/ceramic coatings by plasma spraying. The microwave absorption properties and tribological properties with different load of all coatings have been systematically investigated. Remarkably, the microwave absorption properties of the TERGO/Al2O3 coating achieve wide bandwidth (reflection loss ≤ − 10 dB coved 2.8 GHz), and strong absorption (reflection loss of −35.3 dB) as the thickness of 2.1 mm. Meanwhile, the TERGO/Al2O3 coating possesses excellent tribological properties, and the coefficient of friction and wear rate were decreased by 41 and 90% respectively compared with pure Al2O3 coating. This study paves the way for preparing microwave absorbing coating with tribological properties.

Biological hydroxyapatite (HA) coatings have been used to repair damaged bone exhibiting fluorosis. However, they are often inappropriate and have some undesirable effects. Consequently, modified HA coatings with functional elements have become popular. In the present work, selenium-doped HA (Se-HA) powders were first synthesized by chemical coprecipitation. A Se-HA nanostructure coating was then fabricated on the surface of a titanium (Ti) substrate by suspension plasma spraying. Energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, and x-ray powder diffraction demonstrated the homogeneous presence of Se within a Se-HA coating made from the appropriately doped powder. The Se was incorporated into the HA lattice by replacing some of the phosphate groups with selenite groups. The chemical stability of the coating was assessed by immersing samples in a citric acid-modified phosphate-buffered saline solution. The solubility of the Se-HA coating increased slightly owing to the introduction of Se. According to electrochemical tests, the dense Se-HA coating provided the Ti substrate with a reliable biocorrosion barrier. The in vitro bioactivity of the coating was characterized by immersing the samples in simulated body fluid for various times. The results suggested that the Se-HA coating had similar bone-like apatite formation capability to that of a conventional HA coating. Compared with the HA coating, the Se-HA coating effectively promoted the adhesion, proliferation, and differentiation of osteoblasts in the presence of fluoride ions. These results suggest that Se-HA coatings have some potential for improving bone regeneration in fluorosis patients.

Thermal spray coatings are material systems with unique structures and properties that have enabled the growth and evolution of key modern technologies (i.e., gas turbines, structurally integrated components, etc.). The inherent nature of these sprayed coatings, such as their distinctive thermal and mechanical properties, has been a driving force for maintaining industrial interest. Despite these benefits and proven success in several fields, the adoption of thermal spray technology in new applications (i.e., clean energy conversion, semiconductor thermally sprayed materials, biomedical applications, etc.) at times, however, has been hindered. One possible cause could be the difficulty in concurrently maintaining coating design considerations while overcoming the complexities of the coatings and their fabrication. For instance, a coating designer must consider inherent property anisotropy, in-flight decomposition of molten material (i.e., loss of stoichiometry), and occasionally the formation of amorphous materials during deposition. It is surmisable for these challenges to increase the risk of adoption of thermal spray technology in new fields. Nevertheless, industries other than those already mentioned have benefited from taking on the risk of implementing thermal spray coatings in their infrastructure. Benefits can be quantified, for example, based on reduced manufacturing cost or enhanced component performance. In this overview paper, a historical presentation of the technological development of thermal spray coatings in several of these industries is presented. Additionally, emerging industries that have not yet attained this level of thermal spray maturation will also be discussed. Finally, where applicable, the utility and benefits of multilayer functional thermal spray coating designs will be demonstrated.

In the present work, the coating surface subjected to heating in a vacuum drying oven without additional artificial modification exhibited a spontaneous change in its wettability from superhydrophilicity to superhydrophobicity. To clarify the mechanism of wettability transition and reveal the role of vacuum heating, plasma-sprayed metallic coatings with optimized biomimetic micro–nano-multiscale structure were taken as the research subjects. The influence of surface chemical composition on the wettability of coatings was emphatically studied via infrared spectroscopy and x-ray photoelectron spectroscopy. According to the results, the main reason for the wettability transition was the change in chemical state of the coating surface due to spontaneous adsorption of alkanes and alkyl chains with low surface free energy from the vacuum residual gas during vacuum heating. Heating promoted desorption of the initial physical adsorption layer and subsequent re-adsorption of residual gas, and vacuum system supplied the adsorbates, thereby providing the driving force for the spontaneous adsorption. The increase in heating temperature could promote re-adsorption of residual gas and improve the transition degree of wettability. The prepared superhydrophobic coatings had an excellent self-cleaning property. Therefore, this study not only opens up new prospects for rapidly achieving superhydrophobicity of surfaces, but also reveals the strong effect of vacuum drying on the surface wettability.

A novel method for preparing or repairing titanium-clad steel plates is proposed in this paper. Molten titanium powder is deposited onto the surface of Q235 steel using plasma transferred arc powder surfacing, creating metallurgical bonding of Ti/steel dissimilar metals. The influence of welding current and welding speed on the appearance, microstructure, and mechanical properties of the titanium-clad steel plate was studied. The results showed that welding parameters directly affect the heat input of the titanium surfacing layer during welding, changing the microstructure and strength of the titanium surfacing layer. In the low heat input mode, the interface microstructures consist of a TiFe layer and a TiFe2 layer. With increasing heat input, the phenomenon of intermetallic compound separation at the interface disappeared. Additionally, the surfacing was covered by a layer of eutectic structure instead of the ideal pure titanium. The brittle phase is the main factor limiting the shear resistance of titanium-steel welded joints. Under the shear load, the joint cracks mainly initiated in the intermetallic compound region and fractured with a brittle mode. The maximum shear strength was 142.05 MPa. Based on the analysis of interface microstructures, the bonding mechanism of titanium-clad steel plates made by PTA powder surfacing was revealed.

This paper aims to demonstrate the feasibility and effectiveness of using magnetic fields in the preparation of coatings. To achieve this, we have combined supersonic plasma spraying technology with a magnetic field to produce coatings that perform better. Specifically, we have prepared NiCrBSi-10%WC&Co coatings using supersonic plasma spraying under three conditions: no magnetic field, pulsed magnetic field, and constant magnetic field. In this study, we have compared and analyzed the droplet flight progression, coating morphology, coating micromorphology, and mechanical properties of the coatings under these three conditions. When both melted and unmelted particles spread over the matrix, broken edges appear. However, we found that the magnetic field can affect the magnitude and direction of the droplet velocity upon impact on the matrix. Additionally, the cross-sectional morphology of the coatings shows that the reinforcement aggregates due to the magnetic field’s promotion of internal flow during coating solidification. Further, our transmission electron microscopy analysis revealed that the magnetic field caused grain refinement. Moreover, we observed that the grain refinement effect was more significant under a constant magnetic field than under a pulsed magnetic field. Due to the growth of spreading morphology, enhanced phase aggregation, and fine-grain structure, the coatings produced in a magnetic field exhibited higher microhardness and adhesion properties. Overall, our results suggest that using magnetic fields in the preparation of coatings can significantly improve their performance.

In situ high-temperature x-ray diffraction (HT-XRD) was used in the present study to assess the coefficient of thermal expansion and recrystallization of Ni-5 wt.%Al coatings. Atmospheric plasma spray (APS) and detonation spray (DSC) techniques were used to deposit Ni-5 wt.%Al coatings on IN718 substrates. The coatings were examined using HT-XRD at ambient conditions (25 °C) up to high temperatures (1150 °C) under a vacuum pressure of around 10−4 mbar. Coefficients of thermal expansion (CTE), crystallite size (D) and lattice strain (ε) were determined by the Scherer and Williamson-Hall (W-H) method with a uniform strain model (UDM) using x-ray peak profile analysis (XPPA). The microstructure of the Ni-5 wt.%Al coatings was analyzed by field emission scanning electron microscopy (FESEM). No phase changes were observed in either coating, as the Ni-5 wt.%Al coatings consisted mainly of γ-Ni crystals with a face-centered cube (FCC) phase in both coating techniques. Lattice parameters as a function of temperature were used to calculate linear thermal expansion coefficients. The linear thermal expansion of Ni-5 wt.%Al coatings deposited by both thermal spray methods was discussed on the basis of process-induced microstructures.

The use of advanced thermal spray technology for surface modification of aluminum alloy parts for automobiles can greatly improve the surface properties and service life of the parts while saving resources and reducing environmental pollution. In this paper, a Ni60A coating was prepared on the surface of aluminum alloy 109 (ZL109) by supersonic plasma spraying (HEPJet). The spraying process parameters were optimized based on the response surface methodology (RSM). The effects of the main factors such as spraying power, spraying distance, and Ar flow rate on the porosity and microhardness of the coating were analyzed by the Box–Behnken method (BBD). The response surface of the porosity and microhardness of the coating was constructed by establishing the appropriate mathematical model. The response surface of porosity and microhardness of the coating was constructed by establishing a relevant mathematical model. The results show that the established mathematical model is reliable, and the spray power has the greatest influence on the porosity and microhardness of the coating. The optimal porosity and microhardness of the coating predicted by the model are 0.85% and 971.1HV0.1, respectively, which is close to the experimental results. The porosity and microhardness of the coating measured by the experiment are 0.87% and 959.8HV0.1, respectively. The coatings prepared with optimal parameters have good compactness, small porosity, and high hardness.

This study examines the effect of grit blasting on the surface and subsurface characteristics of low-carbon steel substrates under dry and wet environments. Three types of wet environments have been simulated: tap water, saline water, and ethyl alcohol. The variation in average surface roughness with grit size, standoff distance (SOD), and blasting time has been analyzed under all conditions. A mathematical model has been developed to estimate the increment in the contact area of each rough surface created by grit blasting. The optimal sets of process parameters for each environment have been suggested based on the maximum increment. The optimal conditions have been found to shift toward higher SODs and time periods under wet conditions. Vickers microhardness testing has been undertaken to study the hardness of the subsurface zone of the grit-blasted samples. Work hardening of the subsurface regions is higher under dry conditions compared to the same in wet conditions. The depth of the affected layer is higher in case of higher surface hardening. X-Ray Diffraction analysis has been carried out to identify the phases formed on the as-blasted substrate surfaces. Nascent surface metallurgy remains unchanged under dry and wet conditions, except in saline water environments where significant contamination has been observed. Subsurface microstructures of the blasted substrates have been studied using Field Emission Scanning Electron Microscopy and Energy Dispersive Spectroscopy. Subsurface damage is substantial under dry conditions. The deformation effect and formation of subsurface cracks are found to be localized in the vicinity of the grit-blasted surfaces. Among all the blasting environments, the alcohol medium offered reasonably high roughness with minimum hardening effect on the substrate.

Solution precursor plasma-sprayed (SPPS) yttrium aluminum garnet (YAG) thermal barrier coatings (TBCs) have previously been shown to have higher temperature capability and reduced thermal conductivity compared to state-of-the- art TBCs. This previous work was conducted using a relatively low enthalpy plasma gun (Metco 9 MB) and TBCs were deposited on laboratory specimens. The primary goal of this work was to advance the state of technology readiness of SPPS YAG TBC coatings by using a high enthalpy cascaded arc gun (Sinplex Pro) to produce varied microstructures optimized for specific engine components: a fuel nozzle tip, an annular combustor liner, and turbine ceramic outer air seals. The microstructure and properties of these TBCs have been characterized and shown to be superior to those obtained previously. Based on these favorable results, the processing technology was transferred to solar turbines incorporated. Their process optimization of coatings for the three engine components and the rig and engine testing of the coated components will be described in Part II of this paper.

Limited inter-splat bonding is a key problem in thermally sprayed coatings. The use of impinging particles with a high temperature is a crucial factor for obtaining superior bonding between splats. In this work, an attempt was made to raise the temperature of plasma-sprayed cast iron particles by using Mo-cladded gray cast iron (GCI) powder as the feedstock. The cladded powder was prepared by a ball-milling process. Heat transfer between the powder particles and plasma jet was numerically simulated by a finite element method. Cast iron coatings were prepared by atmospheric plasma spraying using both GCI and Mo-cladded GCI powders. The microstructure and wear resistance of both coatings were comparatively investigated. After ball-milling, the GCI powder was partially cladded by Mo, with a surface cladding ratio of 77.2%. The experimental results showed that Mo cladding inhibited particle oxidation by forming volatile molybdenum oxide. The simulated results showed that Mo cladding elevated the surface temperature of the impinging cast iron particles by 415 K. Consequently, the inter-splat bonding within the coating and wear resistance was significantly improved.

Arc-sprayed Fe-based coatings can provide a cheaper and more effective way of protecting cutting tools made of 65Mn steel against abrasive wear. Due to contact with hard and rocky soils, rotary tiller blades face extreme surface wear that reduces their service life. In this study, a new Fe-based coating was prepared on 65Mn steel substrates using arc spraying process. The influence of the process variables (spray distance, voltage, and current) on the microhardness and porosity was studied based on the response surface methodology (RSM). By maximizing microhardness and minimizing porosity, the optimum spraying parameters were voltage of 35 V, current of 150 A, and spray distance of 270 mm. Based on the process optimization, the desired coating was obtained with a microhardness and porosity of 783.62 ± 14.77 HV0.1 and 1.51 ± 0.49%, respectively. Validation tests of the coatings with the optimum conditions agree with the values predicted by the model. The microstructure, tensile bonding strength and wear tests of the optimized coatings were characterized. The Fe-based coating had a bonding strength of 40.8 ± 1.4 MPa. The wear rate of the coating was 12.26 × 10−6 mm3/Nm, which was about 2.4 times lower than that of the 65Mn steel substrate.

Air entrapment in drop impact onto solid surfaces has riveted the attention of the scientific community since it could drastically influence heat rate and splat porosity in additive manufacturing or thermal spraying processes. Due to the limitations of experimental methods, numerical methods have been gaining popularity in probing into air entrapment dynamics. To that end, this paper developed a paralleled phase field model. The model couples the Navier–Stokes equations with the Cahn–Hilliard equation to capture the liquid–gas interface, and adopts the liquid fraction, defined all over the computational domain, to distinguish between solid and liquid. The model is discretized using a finite difference method on a half-staggered grid. The model was first compared with experimental data, achieving reasonable agreement. Afterward, it was applied to a couple of cases. The air entrapment process was captured entirely and the interaction between solidification and air entrapment was also given. It shows that under the parameters studied here air entrapment may occur twice, one in the impacting center in the form of a bubble and the other in the form of a bubble ring. Besides, the effect of phase field mobility on air entrapment was examined as well.

Varied compositions of TiC (Titanium Carbide) and CuNi-Cr (Cupronickel-Chromium) have been examined as coating materials for hydro machinery steel (SS316) using HVOF spraying system. The slurry erosion experiments have been performed to evaluate these coatings’ effectiveness among themselves as well as against bare SS316 steel. In regard to this, the effect of different factors affecting the slurry wear of specimens, such as jet velocity, slurry concentration, and impingement angle have been examined. Further, porosity, microhardness, and surface roughness of the specimens have been measured, and the structure of the developed coatings was also analysed using XRD and SEM. Based on the outcomes of the experiments, it was converged that among the different coatings evaluated, 100% TiC based coating has shown significantly higher resistance to erosion in contrast to other candidate coatings which may be due to higher micro-hardness attribute availed by TiC granules. Moreover, as compared with bare SS316 steel, a substantial augmentation in the slurry wear resistance of SS316 steel was observed as contrasted with all the other developed coatings. Ductile mode of failure was noticed for specimen with CuNi-Cr as primary element, which was found to exhibit brittle mode with the increment in TiC percentage. Graphical Abstract

The impact resistance of coating/substrate systems is still lacking comprehensive understanding, especially for the thermal spray of wear-resistant coatings for coated components in potential applications with heavy-load impact service condition. The impact responses of a thermal spray WC-Ni coating/steel substrate were studied systematically by varying the coating thicknesses from 50 to 300 μm under an impact load of 1–10 kN. Besides conventionally employed crater depth/volume for impact evaluation, a critical impact load is identified to quantify the impact resistance with cracking. Considering the observed cracking modes, two critical loads of Lcc and Lcm have been defined corresponding to the cohesive failure of cracking in coatings and the mixed cohesive–adhesive failure including interface cracking, respectively. Increasing the coating thickness up to 200 μm raised the critical load Lcm from 7.4 to 11.1 kN, followed by a drop to 7.4 kN of Lcc at 300 μm. To reveal the influence of coating thickness, a major surface integrity parameter is proposed as the coating deformation in proportion to the total value of coating/substrate system in correlation to the impact resistance. The dependence of impact resistance on coating thickness is interpreted through the surface integrity changes by the thermodynamic mechanism of effective energy input and dissipation into the coating/substrate system from the external impact load.

The segregated effects of particle temperature and velocity on the morphology of plasma sprayed alumina splats and pore size distribution in the coatings were thoroughly studied. During spraying, the in-flight particle velocity was kept near constant while the particle temperature varied in a range, and vice versa. Relative proportions of different types of splats at various particle characteristics were investigated. Splat shape and size distributions were correlated to the pore size of the coatings. Fully flattened disk-shaped splats are predominant at the highest particle temperature. This led to a higher fraction of very fine pores (0-50 μm2) and, consequently, a lower overall porosity. The proportion of fully spread disk-shaped splat increased initially with an increase in particle velocity from 197 to 209 m/s. This effect resulted in a reduction in overall porosity by around 39%. A further increase in particle velocity from 209 to 247 m/s resulted in splat splashing and, hence, produced a slight reduction in the percentage of fully spread splats and concomitant increase in the fraction of splashed splats. This led to a slight increase in the overall porosity. No significant effect of particle velocity on the pore size distribution was identified in the considered velocity range.

Corrosion occurrence issue at high temperature has often come across in biomass boilers for the reason that burnt fuels mainly comprise of alkali, chlorine and other molten salts. Due to this, material depletion, leakages and unexpected shutdown of plants have been reported. Utilizing thermal spray for protective coatings is one of the striking solutions for obviating this issue. Commercial Inconel 625 and Inconel 718 powders were deposited on SA213-T22 boiler steel by means of a high velocity oxy-fuel (HVOF) spraying technique in the present investigation. In order to assess the performance of coating in actual environment, bare and coated samples were subjected to biomass-fired boiler for 15 cycles. The erosion-corrosion kinetics was determined using thickness loss data. The as-sprayed and eroded-corroded specimens were examined using different characterization techniques. XRD and SEM/EDS were utilized for examining the phases and surface morphologies of powder, coating and eroded-corroded samples. The HVOF sprayed coated steel outperformed than the bare steel in actual boiler environment.

To address the problem of wear and failure of the DZ125 superalloy turbine blades, a Tribaloy T-800 coating is fabricated on the DZ125 superalloy by laser cladding. The microstructure, phase composition, microhardness and wear resistance of the T-800 laser cladding coating are studied. The experimental results indicate that no cracks are detected within the coating, and a strong metallurgical bond between substrate and coating is observed. In addition, columnar crystals are formed in the bottom of the coating, while equiaxed crystals are formed in the middle and top of the coating. The T-800 laser cladding coating consists mainly of face-centered cubic α-Co solid solution and Co3Mo2Si hard phase. The shape of the Co3Mo2Si is granular and striped at the bottom of the coating, while the shape of the Co3Mo2Si in the middle as well as the top of the coating is striated and radiated. Moreover, the distribution density of Co3Mo2Si is highest in the middle of the coating compared to the top and bottom. The average microhardness of the T-800 laser cladding coating is approximately 535 HV0.5, which is 1.27 times that of the substrate. Furthermore, the average microhardness of the bottom, middle and top of the coating is 495 HV0.5, 569 HV0.5 and 537 HV0.5, respectively. The average coefficient of friction of the substrate is 0.6, and the wear mechanism of the substrate is abrasive wear, adhesive wear and oxidative wear. Compared to the substrate, the average coefficient of friction of the T-800 laser cladding coating is reduced by 42%. And, the wear mechanism of the T-800 laser cladding coating is slight abrasive wear, adhesive wear and oxidative wear.