The influence of air plasma sprayed alumina coating geometry, microstructure, interface roughness on its delamination and crack propagation resistance during low temperature thermal cycling, i.e. thermal mismatch stress, is investigated both numerically and experimentally. Previous studies on thermal cycling loading concentrate on flat, numerically designed locally curved specimens and/or mathematically modeled roughness without extension towards real coating morphology, which renders the conclusions less practically driven. Results show that arbitrarily oriented cracks originate predominantly near the coating/substrate interface and propagate along zones of high tensile and shear residual stress. The crack path deflection was attributed to the complex stress concentration structure resultant from the intricate microstructural porosity and coating general convex geometry. Microstructural features such as porosity increase the interfacial and coating tensile stress, which may lead to important delamination processes even during low temperature thermal cycling.

Thermal-sprayed coatings have been extensively used in aerospace with the main purpose to overcome critical challenges such as abrasive wear, corrosion, and erosion under high temperatures and pressures. Such protective coatings can also play a crucial role in optimizing the efficiency of gas turbine engines and therefore in reducing fuel consumption and CO 2 emissions. CuAl-based thermal sprayed coatings are commonly employed in tribological interfaces within gas turbine engines to improve the fretting wear resistance. These coatings are typically deposited by more traditional thermal spray techniques such as Air Plasma Spray (APS), which can result in high amounts of oxidation within the coating. The main purpose of this study is to critically evaluate lower temperature deposition techniques such as High Velocity Oxygen Fuel (HVOF). More specifically, commercially available Cu-10Al powders were deposited by APS and HVOF and compared in terms of their microstructural, mechanical properties, and tribological behavior at various temperatures. The results showed that the friction coefficient for both coatings was equivalent at room temperature while it was lower for the APS coating at high temperature. Similarly, the specific wear rates showed little difference between the different deposition processes at room temperature while the APS coating had a lower wear rate at elevated temperature when compared to the HVOF coating. The differences in the friction and wear behavior were attributed to differences in the interfacial processes.

A new challenge in the transport systems concerns with improving efficiency. Thermal swing coatings are interesting candidates for internal combustion engines due to their potential to reduce cooling requirements and increase efficiency. K 2 Ti 6 O 13 (KTO) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying through powder structure design and optimization of deposition conditions. The thermophysical properties of plasma-sprayed KTO deposits and their effect on the thermal swing have been investigated. Their thermal conductivities were tested by a laser flash method and the thermal performance of the coatings was further examined by thermal swing test. The phases, nominal chemical compositions and microstructure of KTO deposits were characterized by X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS). The results indicated that the chemical composition change occurs to the coatings resulting in a deviation from nominal stoichiometry due to chemical reactions between the plasma gas and particles. The thermal conductivity of the coating is very sensitive to the coating compositions, and the coating prepared using porous powder under pure argon presents a single K 2 Ti 6 O 13 phase and high porosity, and the lowest thermal conductivity of 0.85 W/m·K.

Plasma Transferred Wire Arc (PTWA) is a well-established thermal spray process that is used in high-volume production by multiple automotive OEMs. Benefits of these PTWA thermal spray coatings include closer bore spacing, improved thermal transfer, lower bore distortion, increased resistance to corrosion and abrasion, reductions in weight and friction, enhanced durability, and product cost savings. For automobiles, this leads to increased fuel economy and lower emissions. Millions of engine cylinder bores per year are coated using the PTWA thermal spray process. To ensure optimal surface coatings, it is vital to monitor the process variables. Although some process monitoring already exists in current production, new technological advancements allow for additional variables to be monitored. Arc voltage is of particular importance as it can be viewed real-time in situ to the PTWA process to determine the curvature of the feedstock wire. Straight wire is ideal for achieving peak system performance. If the wire has excessive curvature, it can lead to out-of-tolerance conditions that detrimentally affect the quality of the surface coating. Therefore, in-situ monitoring of wire curvature is both desirable and necessary for producing the highest quality PTWA thermal spray coatings possible.

Temperature sensors are critical components in many industrial and research applications, particularly in harsh environments where high temperatures, corrosion and mechanical stress are prevalent. In this paper, we investigate the use of plasma spray technique as a versatile and simple method to print thermocouples and Resistance Temperature Detectors (RTDs) on metallic and ceramic substrates. The thermocouples based on NiCr-NiAl coatings were directly printed using thick metallic masks, while the RTD’s were structured using laser ablation. The manufacturing methods and the preliminary characterization of these temperature sensors are presented and discussed.

The application of thermally sprayed coatings on CFRPs has gained great interest to enhance thermal and tribological properties and several processes have been optimized. However, for the coating of internal surfaces of tubes there is no sufficient technical solution. This paper introduces a novel and unique process technique for coating the internal surfaces of CFRP tubes using the transplantation of thermally sprayed coatings. A negative shape tube with defined surface and material properties was used as a mandrel and coated using atmospheric plasma spraying (APS). The CFRP was then produced using filament winding onto the coating, and after curing, the specimen was separated from the mandrel. With this process innovation, CFRP tubes with internal ceramic or metallic coatings can be produced without any thermal degradation of the polymeric matrix or damage to the carbon fibers. Compared to conventional coating methods, this novel process technique has several advantages. It allows for the production of internal coatings with low roughness of R z = 10 μm as sprayed without post-processing. The specimens also have a significantly lower tendency to corrode compared to conventional coated CFRPs. A high adhesion strength of the coatings of 15.9 MPa was achieved and the hardness of the internal ceramic coating is 918 HV0.1

With an increasing demand for lower fuel consumption of different means of transportation, the demand for lightweight construction materials is rising. In this frame, usually metallic parts can be replaced by components consisting of fiberreinforced plastics. On the other hand, the components lose their electromagnetic field (EMF) shielding properties, which are required for many applications such as housings for electrical components. This issue can be solved by applying electrically conductive foils or meshes, often by a manual process that increases the time of production and process. In this publication, the application and parameter influence of thermally sprayed electrically conductive coatings for EMFshielding applications is discussed. Laser structuring is used as a novel surface preparation process, for the subsequent thermal spray process. The influence of the used laser-parameters is discussed accordingly. The coatings are applied by the wire-arc spray with Zinc feedstock as well as the atmospheric plasma spray (APS) process with Copper feedstock. It was found that coating properties such as adhesion strength, EMF-shield strength as well as electrical properties are provided by the proposed technology.

The metallic bond coat is generally utilized to increase the coating adhesion and the adhesion of thermal spray bond coat is of essential importance to applications. However, it usually depends on mechanical bonding with a low adhesive strength. In this study, a novel metal bond coat with high cohesion strength is proposed by plasma-spraying Mo-clad Ni-based or Fe-based spherical powder particles. Mo-cladding ensures the heating of spray particles to a high temperature higher than the melting point of Mo and prevents metal core from oxidation during spraying. Theoretical analysis on the splatsubstrate/ splat interface temperature and experimental examination into coating-substrate interface microstructure were performed to reveal the metallurgical bonding formation mechanism. The local melting of substrate surface and resultant bond coating by impacting high temperature droplets creates metallurgical bonding throughout the interfaces between substrate and bond coat, and within bond coat. The experiments were conducted with different substrates in different surface processing conditions including Ni-based alloy, stainless steel and low carbon steel. All pull-off tests yielded strong adhesion higher than the adhesives strength of 80 MPa. The present results revealed that Mo-clad metal powders can be used as new bond coat materials and high performance bond coat can be deposited by atmospheric plasma spraying.

Hybrid plasma spraying has been proved to provide novel coating microstructures as a result of the simultaneous injection of a dry coarse powder and a liquid feedstock into the plasma jet. Such microstructure contains both large splats originating from the conventional dry powder and finely dispersed miniature splats deposited from the liquid. This approach enables preparation of coatings from virtually all materials which are conventionally processed using plasma spraying. However, incorporation of materials susceptible to decomposition at high temperatures is still challenging even using this concept due to the high thermal energy provided to all feedstocks to be deposited. Hereby, we propose an innovative approach of incorporation of thermally-sensitive materials into a coating sprayed using a high-enthalpy plasma torch. As a case study, Al 2 O 3 was sprayed from dry coarse powder and MoS 2 was sprayed from the suspension which was deposited directly onto the substrates, i.e., by-passing the hot plasma jet. The retention of the added material in the coating was evaluated using scanning electron microscopy and X-ray diffraction.

Ni/Co-based alloys have been widely employed as bond coats (BCs) in thermal barrier coatings (TBCs) to provide oxidation resistance through the formation of a dense thermally grown oxide (TGO) layer. TGO thickening is a major contributor to TBC failure. Conventional approaches to minimize its growth have included refinement/optimization of the BC composition, deposition techniques, and post-treatments. However, these approaches have only led to incremental improvements in TBC performance and do not directly address the effect of the thin interfacial oxide layer on the TBC lifetime. In a shift from conventional thinking, the development of an Al 4 C 3 -Ni alloy composite BC aims to overcome the challenges generated by current TGOs. Post-deposition heat treatment tailors the coating microstructure to form a continuous internal carbide network. At elevated temperatures, the Al 4 C 3 preferentially oxidizes to form an interlacing protective Al 2 O 3 “root” that provides better TGO anchoring and reduces TBC thermal mismatch with the substrate. In this paper, the coatings were manufactured through gas-shrouded plasma spraying using various parameters to optimize the degree of inflight carbide dissolution and minimize the extent of coating porosity and cracking. XRD and carbon analysis were performed on the coatings and the microstructure was observed using SEM. Differences between coatings are discussed in relation to the spraying parameters.

Conventionally, bulk WC and Cr 3 C 2 -based carbide compositions have been used independently of each other. However, recent investigations have begun to explore combining these carbides together within the same composite/hardmetal coating system. This research builds on earlier work characterising 42%wt% WC-42%wt% Cr 3 C 2 - 16%wt% Ni coatings sprayed under “low”, “medium” and “high” thermal input conditions, to assess their compositions and microstructures after heat treatment in air at 900°C for up to 30 days. Coatings were deposited by HVOF, Ar-He and Ar- H 2 shrouded plasmas respectively, onto Alloy 625 substrates with Ni20Cr bond-coats and top-coats. The coating compositions and lattice parameters were quantified by Rietveld peak fitting of XRD patterns. The microstructures were analysed from cross sectional backscatter electron micrographs. Rapid phase development occurred within the first five days, beyond which the compositions and microstructures remained stable. The microstructures retained extremely fine, sub-micron grain sizes, while the carbide phases exhibited high degrees of metastable alloying, even after 30 days at 900°C. The coating compositions are discussed, and a mechanism proposed to account for the rate of development and overall metastable microstructure.

In this study, Al 2 O 3 -based coatings with varying TiO 2 contents (0, 3, 13, and 40%) were fabricated using atmospheric plasma spraying technique. To compare the superiority of the samples, their thermal properties (thermal conductivity and thermal shock resistance) were characterized. As observed, Al 2 O 3 - 40%TiO 2 (A-40T) coating exhibited relatively superior thermal insulation and thermal shock resistance at 600°C. According to the microstructure and phase analysis, this finding can be attributed to the special phase, Al 2 TiO 5 , and the pre-existing microcracks inside the coating. Thus, A-40T manifested excellent characteristics for thermal insulation application compared with pure Al 2 O 3 and low-TiO 2 content coatings.

Composite coatings using mixed alloy matrices reinforced with carbon-based solid lubricants as feedstock materials were prepared by atmospheric plasma spraying. The aim of the present study was to investigate the tribological characteristics of such coatings exploring potential benefits of CNTs as nano-additive to reduce friction and wear, improving lubrication conditions during operation in tribosystems, such as piston ring – cylinder liner systems. The chemical composition of feedstock materials and the thermal spray parameters during coatings deposition are correlated to friction coefficient and wear rate using pin-on-disk measurements. The developed coatings hybrid behaviour is studied. Co-based cermet as well as metal alloy anti-wear performance along with the promoted lubrication conditions during operation is revealed. The dependence of the developed coatings quality and performance on the characteristics of the feedstock powder is thoroughly discussed.

Hybrid plasma spraying combines plasma spraying of dry powders and liquids (suspensions and solutions). Combination of these two approaches allows deposition of microstructures consisting of both conventional coarse and ultrafine splats. Moreover, splats with dissimilar size may have also different chemistry. Such combination is potentially interesting for many fields of thermal spraying, including thermal barrier coatings (TBCs), as novel microstructures may be economically and relatively easily obtained. The technology has recently reached a level, where coatings with interesting hybrid microstructures may be reliably deposited, so that their potential for practical applications may be evaluated. In this study, first experimental TBCs with YSZ-based hybrid topcoat were deposited by hybrid water/argon stabilized plasma (WSP-H) technology. Al 2 O 3 and YAG were selected as secondary phase deposited from suspension as both provide strong materials contrast in scanning electron microscope (SEM) so they can be used as “markers” in the coating microstructure. Samples were exposed to thermal cycling simulating in-service TBC conditions in order to test their thermal shock resistance. Changes of the coating microstructure were studied by SEM analysis and X-ray diffraction.

Thermal spray processes benefit from workpiece cooling to prevent overheating of the substrate and to retain metallurgical properties (e.g., temper). Cold-gas “plume quenching” is a plume-targeting cooling technique, where an argon curtain is directed laterally above the substrate surface to re-direct high temperature gases without impacting particle motion. However, there has been little investigation of its effect on the molten particles and the resulting coating properties. This study examined high- and medium- density tantalum and nickel coatings, fabricated by Controlled Atmosphere Plasma Spray with and without plume quenching on aluminum and titanium substrates. To compare the effect of plume quenching, the deposition efficiency was calculated through coating mass gain, and the coating density, stiffness, and adhesion were measured. The tantalum and nickel coatings were largely unaffected by plume quenching with respect to deposition efficiencies, coating density, adhesion, and stiffness. These results indicate that a plume quench could be used without affecting the coating properties for high- and medium-density metals while providing the benefit of substrate cooling that increases with higher plume quench gas flow rates.

Atmospheric plasma spraying (APS) is characterized by complex interactions between input, process and output variables. Process control relies heavily on human expert knowledge and experience. Process diagnostics can provide additional information to the operator and support cognitive processes in task execution. When using non-cascaded torch systems, significant plasma fluctuations occur, affecting the coating quality. High-frequency fluctuations can only be detected by suitable diagnostic systems and interpreted by experienced APS operators. In this study, the state of the plasma jet (area, fluctuation) is investigated depending on total plasma gas flow rate (50 vs. 65 l/min) and the H 2 content of plasma gases (17, 20 and 23 vol. %) using high-speed camera pictures. To evaluate plasma fluctuation effects, particle temperature and velocity as well as resulting coating properties (thickness and porosity) are determined for two ceramic systems. The results show that fluctuations of the plasma jet have a significant effect on the particle state and coating quality. The use of a high-speed camera to evaluate the stability of the plasma jet is an attractive method that, when properly integrated, has the potential to provide the human operator with important information to allow rapid assessment of input parameters or the condition of the plasma torch.

NiAl coating can be used as bond coats for thermal barrier coatings (TBCs) with good ductility and excellent resistance against high temperature oxidation. In this study, nickel-coated aluminum composite powders were used to prepare NiAl intermetallic compound coatings on nickel-based superalloys using an air plasma spray (APS), high-velocity oxygen-fuel (HVOF) and cold spray (CS) processes. Different spraying parameters in the HVOF and CS processes were used to make different coating microstructures, and the coating prepared by the APS technique served as a control for the HVOF and CS processes. The microstructure and phase constitution of the coatings were studied using XRD, SEM and EDS. The results indicate that the deformation behavior of the NiAl powder was different under the different spraying parameters. Less defects of oxides and inclusions were observed in the CS coatings compared with the HVOF coatings.

The enhancement of the surface characteristics and corrosion resistance of cobalt alloys is under continuous examination for its biomedical applications. In this work, the investigation of corrosion performance of cobalt alloy coated with HA and HA/ZnO reinforced powders using plasma spray technique revealed that on the continuous increase of ZnO reinforcement the corrosion resistance improved progressively. The increment in surface hardness and drop in surface roughness was examined with the rise in ZnO content. Each coated sample exhibits a hydrophilic property. According to SEM and EDX investigations, homogeneous distribution of HA/ZnO coatings and intact reinforcement of ZnO in pure HA powder was noticed. All of the coated specimens maintain their morphological integrity, ensuring excellent protection of the prepared samples. The obtained outcomes denote HA/ZnO reinforced coatings on CoCr alloy as a suitable combination of enhanced surface properties and excellent corrosion resistance for future bone implant practices.

The formation of Nickel coatings on stainless steel substrates and YSZ (Yttria-Stabilized Zirconia) on NiCrAlY in the Atmospheric Plasma Spray (APS) process is investigated. Coating formation over a substrate with an arbitrary shape (an inclined step in this paper) is considered. The topography of the coatings, as well as their microstructure, e.g., porosity, average thickness, and average roughness, are evaluated. An algorithm, which is based on the Monte-Carlo stochastic model, is employed. The significant difference between the coating temperature and that of the substrate leads to the formation of residual thermal stresses. These stresses are analyzed using Object Oriented Finite-element software (OOF) developed by the National Institute of Standards and Technology (NIST). An image of the cross-section of the coating is imported into the code, which utilizes an adaptive meshing technique and Finite- Element Method to calculate residual thermal stresses. The maximum stress in the coatings occurs at the interface between the coating and the substrate. The coatings' topography and microstructure are compared with those of the experiments.

The study of self-lubricant coatings is a rapidly developing research field that is expected to address major issues arising from operation under high loads and thermal stresses of machine parts. It is of utmost importance that various critical tribological pairs such as crankshaft bearings, piston rings, etc. maintain their integrity, thus contributing to the engines efficient and reliable operation for long periods of time. Therefore, a choice of high-strength metals is required, with special treatments and/or coatings in combination with sufficient lubrication. In the present study, liquid lubricants encapsulated in poly(urea-formaldehyde) were incorporated in ceramic-metal matrices for the production of composite coatings by Atmospheric Plasma Spraying. Aspects concerning the survival of the capsules during their flight through the high temperature plasma flame, the maintenance of their integrity at impact with the substrate with high kinetic energy, their homogeneous dispersion within the coating mass, thermal spray deposition parameters adjustment and optimization are thoroughly discussed. Coating properties such as roughness, friction, adhesion strength, and wear behaviour were also investigated. Microstructural characteristics and friction-wear behaviour were found to be critical to the durability of coating.