Abstract In this study, a novel self-healing concept is considered in order to increase the lifetime of thermal barrier coatings (TBCs) in modern gas turbines. For that purpose, SiC healing particles were introduced to conventional 8YSZ topcoats by using various plasma spray concepts, i.e., composite or multilayered coatings. All topcoats were sprayed by SG-100 plasma torch on previously deposited NiCrAlY bondcoats produced by conventional atmospheric plasma spraying. Coatings were subjected to thermal conductivity measurements by laser flash method up to 1000°C, isothermal oxidation testing up to 200h at 1100°C and finally thermal cyclic fatigue (TCF) lifetime testing at 1100°C. Microstructural coating evaluation was performed by scanning electronic microscope (SEM), in the as-produced and post high-temperature tested states. This was done to analyze the self-healing phenomena and its influence on the high-temperature performance of the newly developed TBCs.

Abstract The abradable coatings had significantly enhanced turbomachinery performance by acting as a sacrificial seal between rotating blades and stationary casing. Further improvement in seal design to meet the higher energy demand and increase the service time has been the key challenges to solve in the gas turbine industry. Honeycomb seals have become the industry standard clearance seal technique due to their unique design and high structural strength with minimum weight. The present study proposes a concept to form a thermal shock resistance structure to achieve higher temperature capability and improve the reliability of abradable seal structures. A cavity layer of honeycomb seal structure made of SS 321 alloy was coated with advanced high-temperature ZrO 2 +7.5%Y 2 O 3 +4% polyester seal material using TriplexPro-210 plasma spray system. The integrity of a seal structure was assessed by a cross-sectional analysis and evaluation of the coating microstructure. Additionally, the microhardness test was performed to estimate coating fracture toughness, and Object-Oriented Finite Element analysis was used to assess its thermo-mechanical performance. The concept proposed in this study should be further validated to develop the most capable innovative technology for advanced gas turbine abradable seal structures.

Abstract In an atmospheric plasma spray (APS) process, in-flight powder particle characteristics, such as the particle velocity and temperature, have significant influence on the coating formation. The nonlinear relationship between the input process parameters and in-flight particle characteristics is thus of paramount importance for coating properties design and quality control. It is also known that the ageing of torch electrodes affects this relationship. In recent years, machine learning algorithms have proven to be able to take into account such complex nonlinear interactions. This work illustrates the application of ensemble methods based on decision tree algorithms to evaluate and to predict in-flight particle temperature and velocity during an APS process considering torch electrodes ageing. Experiments were performed to record simultaneously the input process parameters, the in-flight powder particle characteristics and the electrodes usage time. Various spray durations were considered to emulate industrial coating spray production settings. Random forest and gradient boosting algorithms were used to rank and select the features for the APS process data recorded as the electrodes aged and the corresponding predictive models were compared. The time series aspect of the data will be examined.

Abstract Acquisition of a new LVPS and APS coating system at Delta Air Lines necessitated optimization of the coating parameters on both systems, especially for application of bond coat (LVPS) and top coat (APS) for a TBC coating system. To expedite the coating optimization, it was determined that a design of experiments (DOE) approach would best enable the establishment of the operating window for the two systems. Samples prepared were primarily evaluated for their performance while exposed to a cyclic oxidation cycle. Samples were also evaluated for the microstructure and composition using energy dispersive spectroscopy (EDS) analysis. Samples from the ceramic coating DOE were also evaluated for their erosion characteristics. Results indicate a low correlation between the individual bond coat parameters evaluated to the furnace cycle life. However, the top coat spray parameters were found to have a greater correlation to furnace cycle life and erosion performance.

Abstract As a critical technology, thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades, however, the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats, three new bond coat alloys have been designed and developed, and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys, such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase, absence of mixed / spinel oxides, and TGO self repair ability, which cannot be achieved by the existed MCrAlY alloys.

Abstract In this work, a novel liquid fuel HVOF process fueled with ethanol was used to prepare 75wt%Cr 3 C 2 –25wt%NiCr coatings on AISI304 stainless steel substrate. Taguchi method was employed to optimize the spray parameters (ethanol flow rate, oxygen flow rate, powder feed rate and standoff distance) to achieve better erosion resistance at 90° impact angle. The results indicated that ethanol flow rate and oxygen flow rate were identified as the highly contributing parameters on the erosion wear loss. The important sequence of the spray parameter is ethanol flow rate > oxygen flow rate > standoff distance > powder feed rate. The optimal spray parameter (OSP) for minimum erosion wear loss was obtained under ethanol flow rate of 28slph, oxygen flow rate of 420slpm, powder feed rate of 76.7 g/min and standoff distance of 300mm. The phase composition, microstructure, hardness, porosities, and the erosion wear behaviors of the coatings have been studied in detail. Besides, erosion wear testing of the optimized coating was conducted at 30°, 60° and 90° impact angle using air jet erosion testing machine. The SEM images of the erodent samples were taken to analyze the erosion mechanism.

Abstract Masking solutions for parts needing special treatments such as shot peening, grit blasting or thermal spray are now commonly used in industrial processes for the aerospace, industrial gas turbine, and medical industries. The ever-evolving special treatment methods require adapting the masking solutions so they retain their efficiency and quality level. This paper will describe the choices made in combining materials, geometry and surface finish of the masks to provide the necessary compliance with the thermal spraying processes or other special treatment requirements. We will demonstrate that successful masking designs are often the result of close collaboration between the operator’s engineering team and the masking solutions provider in order to comply with what is always a bespoke process. Innovations in materials and additive manufacturing enable the process towards tailored solutions which enable these industries to increase their productivity and level of manufacturing quality.

Abstract Tungsten heavy alloy (WHA) of W-Ni composition was deposited from a blend of standard thermal spray powders using a radio frequency inductively coupled plasma torch in a protective atmosphere. The coating contained a fully developed WHA structure, i.e., spherical W particles embedded in a Ni-rich matrix. Bending tensile strength R m , bending yield strength R p,0.2 , and elastic modulus were measured and compared with W-Ni-Co references fabricated by sintered and quenched (SQ) and forged and annealed (FA) powder metallurgy (PM) processes. The fatigue and fracture properties of the plasma spray deposits are comparable with those of the SQ-PM reference material, but inferior to those of the FA-PM reference. The results of various property tests are presented and analyzed in the paper.

Abstract Three different coatings were deposited using the Detonation Gun Spraying (DGS) technology from steel powders alone, and steel powers mixed with Fe3C and SiC particles, respectively. The microstructural characteristics of these coatings were examined and the hardness of each type of coating was studied. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. The unreinforced steel coating had the highest hardness among the three types of coatings, possibly due to a higher degree of amorphization in the coating compared to the other two samples. The microstructural observation confirmed the formation of dense coatings with a layered structure with good connectivity between layers with minimum defects and porosities in the interfacial regions.

Abstract The product quality of selective laser melting (SLM) is closely related to the alloy powder characteristics, including the size distribution and the oxygen content. In this work, the 316L stainless steel powder was prepared by a vacuum atomization furnace and sieved into a normal-sized distribution range from 15 to 53 μm with a median diameter of 37.4 μm, and a fine-sized distribution range from 10 to 38 μm with a median diameter of 18.9 μm. Then they were mixed with each other in different proportions. The results show that, under the condition of the same SLM parameters, the SLM part, with adding a large amount of fine-sized powder, has a lower density and strength, as well as more holes and spheroidized particles, compared with the SLM part with adding a small amount of finer-sized powder. Furthermore, the 316L stainless steel powder with a high oxygen content was prepared by a non-vacuum atomization furnace. Although the 316L stainless steel powder with a high oxygen content can be evenly spread in the SLM process, the surface layer of the powder is easy to form an oxide film during the cooling and solidification of powder inside the molten pool. Under the action of thermal stress, the small crack forms and expands along the oxide film, eventually leading to large cracks inside the melt channel.

Abstract Manufacturing of steel components is often done at high temperatures (HT) posing a serious challenge to components such as forming tools. Thermal spray coatings provide a cost-effective solution for surface protection under HT, corrosive environments and severe wear conditions. Thermally sprayed coatings based on cubic hard materials such as TiC and TiCN can provide an alternative to widely used Cr3C2-NiCr. While the latter possess a superb oxidation resistance and wear resistance at HT, they are prone to degradation in the presence of Mn, an element commonly alloyed in many modern steel grades such as TWIP (twinning-induced plasticity steel). In this study, a (Ti,Mo)(C,N)-29% Ni hardmetal feedstock powder was prepared by agglomeration and sintering. Coatings were deposited using a high velocity air-fuel (HVAF) spray process. The coating was benchmarked against a standard Cr3C2-NiCr coating obtained with the same spray process. Our work comprises analyses of the feedstock powder along with the resulting coating microstructure after deposition and heat treatment. Further, the HT sliding behavior against TWIP steel using a HT pin-on-disc tribometer at 700°C was investigated. The results showed a clear benefit of the TiCN-based coating, with almost no wear detected, while the Cr3C2-coating showed a significant wear loss. Based on these results, the TiCN-based coating is regarded as potential solution for prospective forming applications of modern high Mn steels, such as TWIP.

Abstract Internal diameter (ID) coating by means of thermal spraying for the wear and corrosion protection of components is currently experiencing growing interest in science and industry. While high-kinetic spray processes (such as HVOF, HVAF or warm spraying) in combination with cermet materials (e.g. WC-Co or Cr3C2-NiCr) are well established for this purpose in traditional coating of external OD (outer diameter) surfaces, they have hardly been used in the ID (internal diameter) area so far. Even though a few special ID spray guns with compact design and low combustion energy are by now available on the market, only little is known about the effects and interactions of the spray parameters on the particle behavior and the coating properties. Due to the mentioned gun specifications and the usually required short spray distances for ID coating, fine spray powders < 15 μm must be used to ensure sufficient melting and acceleration of the particles. In this study warm spraying of fine WC-12Co powders (-10 + 2 μm) using a novel spray gun “ID RED” (Thermico, Germany) was investigated. Statistical design of experiments (DoE) was employed to analyze and to model the influence of varying spray parameter settings on the in-flight particle behavior and the corresponding coating properties.

Abstract The effect of martensitic phase transformation on cavitation erosion resistance for a deposited layer prepared from a Fe-8Cr- C-1.5Al-Ti flux-cored wire of metastable steel was studied. A reference material of AISI 316L stainless steel was used as a substrate. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction was used to examine the phase transformation before and after cavitation tests. Also, the eroded surfaces of specimens were investigated by optical microscope (OM), scanning electron microscope (SEM), and 3D optical profilometer. The cavitation results revealed that the deposited layer exhibited a resistance to cavitation erosion approximately 10 times higher than the AISI 316L steel due to the martensitic phase transformation occurring during the cavitation process. The phase transformation plays a main role to minimize the cavitation damage of specimen. This is due to the fact that it contributes to obstructing movement of dislocations and increasing the hardness as a result of the increased hardening on the surface.

Abstract Pulsed plasma transferred arc surfacing is presently used in many industrial applications to make protective layers against corrosion, temperature exposition, and excessive wear. Increasing wear resistance is especially important in areas of industry where titanium alloys are used, such as aviation and cosmonautics, because the wear resistance of titanium alloys is often weak. One way to increase the wear resistance is to deposit or form a cermet with a titanium matrix (TMC) on the surface of the part. The present study deals with the fabrication and characterization of TMC based on B4C. TMC with B4C was formed by cofeeding Ti6Al4V and B4C powder into a melting pool. It has been found that the deposited, relatively thick layers have homogeneously dispersed B4C grains in the matrix. The deposits are metallurgically connected to the substrate - Ti6Al4V. The TMCs were investigated in terms of microstructure and chemical composition. Wear resistance was determined using the linear pin test.

Abstract Thermally sprayed Al 2 O 3 -TiO 2 ceramic coatings provide exceptional hardness and corrosion and wear resistance, but the high velocities at which they are applied result in an inherently porous structure that requires some type of remediation. This study evaluates the effectiveness of ultrasonic aluminum phosphate sealing treatments on plasma sprayed Al 2 O 3 -40TiO 2 ceramic coatings. The sealants were applied with and without ultrasonication (20-40 kHz) and were assessed using SEM/EDX analysis, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Test data indicate that optimum sealing, corresponding to the highest values of corrosion protection and erosion resistance, are achieved under ultrasonication at 30 kHz for 5 hours.

Abstract A typical structure of thermal spray coatings consisted of molten particles, semi-molten particles, oxides, pores and cracks. These factors caused the porosity of sprayed coatings, leading to a great influence on the coating properties, especially their wear-corrosion resistance. In this study, a post-spray sealing treatment of Cr3C2-NiCr/Al2O3-TiO2 plasma sprayed coatings was carried out, then their corrosion properties were evaluated, before and after the treatment. For sealing process, aluminum phosphate (APP) containing aluminum oxide (Al2O3) nanoparticles (~10 nm) was used. The permeability of APP into the sprayed coating was analyzed by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). The treatment efficiency for porosity and corrosion resistance of sprayed coatings were evaluated by electrochemical measurements, such as the potentiodynamic polarization and electrochemical impedance spectroscopy. In addition, the wear-corrosion resistance of the sealed coating was examined in 3.5 wt.% NaCl circulation solution containing 0.25 wt.% SiO2 particles. The obtained results showed that APP penetrated deeply through the sprayed coating. The incorporation of Al2O3 nanoparticles into APP sealant enhanced the treatment efficiency of porosity for sprayed coating. The effect of the post-treatment on corrosion protection of the sprayed coating has been discussed.

Abstract Anode erosion is a common concern in dc plasma spray torches. It depends largely on the heat flux brought by the arc and the dimensions, residence time, and mode of the arc attachment to a given location on the anode wall. This paper compares anode arc attachment modes predicted by LTE (local thermodynamic equilibrium) and 2-T (two-temperature) arc models that include the electrodes in the computational domain. The analysis is based on a commercial cascaded-anode plasma torch operated at high current (500 A) and low gas flow rate (60 NLPM of argon). It shows that the LTE model predicted a constricted anode arc attachment that moves on the anode ring while the 2-T model predicted a diffuse and steady arc attachment. The comparison between the predicted and measured arc voltage indicated that the 2-T prediction is closer to the actual voltage. A post-mortem observation of a new anode ring on a plasma torch operated under the same conditions confirmed the diffuse arc attachment predicted by the 2-T arc model.

Abstract Instabilities and fluctuations of the plasma jet can have a significant influence on the particle in-flight temperatures and velocities, thus affecting the properties of plasma sprayed coatings. Presented in this paper is a novel method for capturing the effects particles are exposed to in the plasma spraying process. High-speed camera images of a plasma jet generated by a cascaded three cathode plasma generator (TriplexPro-210) are recorded for varying operating conditions. The images are processed using the inverse Abel transform. This transformation accounts for the fact that the images represent a 2-D projection and generates correct intensity values of the plasma jet images. These images are then combined with particle tracks resulting from CFD simulations of the plasma jet to match the particles path with the recorded plasma jet. This new method allows a precise description of the plasma intensity experienced by individual particles with a high temporal resolution. The results show a high sensitivity of the method, it can even detect the influence of the plasma jet originating from the cascaded triple arc plasma generator, which is considered as rather stable, on the particles.

Abstract Additive manufacturing processes have been used to produce or repair components in different industry sectors like aerospace, automotive, and biomedical. In these processes, a part can be built by either melted particles as in selective laser melting (SLM) or solid-state particles as in the cold spray process. The cold spray has gained significant attention due to its potential for high deposition rate and nearly zero oxidation. However, the main concern associated with using the cold spray is the level of porosity in as-fabricated samples, altering their mechanical properties. These pores are primarily found in the regions where adiabatic shear instability does not occur. It is worth noting that the deformation of the impacted solid particle plays a vital role in reaching the shear instability. Therefore, for investigating the adiabatic shear instability region, an elastic-plastic simulation approach has been used. For this purpose, it is assumed that an elevated temperature solid Ti6Al4V particle impacts on a stainless-steel substrate surface at high velocity. The results show that increasing particle temperature will significantly enhance particle deformation because of thermal softening. Additionally, they illustrate that a material jet responsible for producing a bonding between particle and substrate by ejecting the broken oxide layer will be formed when the particle has a temperature above 1073 K and substrate remains at room temperature. In the end, it should be noted that increasing particle temperature up to 723 K will not have a significant effect on substrate deformation and final substrate temperature.

Abstract Thermal spray coatings are widely used to protect materials from corrosion, wear, and oxidation, but they have yet to reach their full potential because of porosity limitations and the detrimental effects of oxidation on interlamellar bonding. This paper investigates an atmospheric plasma spraying process that deposits oxide-free dense metallic coatings with well bonded lamellae. The process produces ultrahigh temperature metallic droplets, up to 2650 °C, using specially designed powders that are deoxidized in-flight through the evaporation or gasification of oxides. The impact of these oxide-free ultrahigh temperature droplets has a spreading-fusing, self-metallurgical bonding effect resulting in fully dense bulk-like metallic coatings. Various coating materials, including NiCrMo, 304SS-Mo, NiCrBSi, and Al, are investigated, demonstrating the versatility of the new technique.