Atmospheric Plasma Spray Process Research Articles

Microstructural and profilometric analysis of thermally sprayed cylinder wall coatings

AbstractCylinder bore coatings are widely used in automotive internal combustion engines to replace cylinder liners. During the atmospheric plasma spraying process, the coatings are oxidized and controlled by the Si content of the steel powder used as the coating raw material. This phenomenon affects the technological process, the microstructure, and the properties of the formed coatings. The research aims to investigate how the Si content of two commercial grades of steel powder commonly used in industry affects the undesirable consequences observed in practice, such as clogging of powder nozzles or large variations in coating hardness. The analyses and industrial experiments show that increasing the Si content can contribute significantly to these undesirable phenomena.

Microstructure and wear behaviour of AlCoCrFeNi-coated SS316L by atmospheric plasma spray process

ABSTRACT The capacity to endure harsh wear in demanding conditions in stainless steel drops under extreme nature and applications. Protecting the surface by providing a coating layer supports the usage in harsh conditions. In this work, SS316L is coated with AlCoCrFeNi high-entropy alloy (HEA) by atmospheric plasma spray process and annealed at 600°C for 2 hours. The AlCoCrFeNi HEA exhibited spherical particles with bcc phase and 20 µm particle size. The coating morphology revealed a uniform coating with a homogeneous distribution of HEA particles over a thickness of 150 µm. The coating post-annealing offered improved microhardness by 12% than the coated sample before annealing. The wear test was executed by varying load, sliding distance, and sliding velocity at normal temperature, 400°C and 600°C and the corresponding worn surface was analysed. The coated samples after annealing showed 57.6%, 87.5%, and 65.4% improved wear resistance at normal temperature than the coated sample before annealing at minimum levels of load, sliding velocity and distance. The wear rate of coated and annealed samples revealed 5.2%, 4.5%, and 4.4% better wear resistance at 400°C than the coated samples before annealing. The worn surface morphology showcased wear mechanisms to be delamination, abrasive wear, and oxide layer formation under all conditions.

Wear and Corrosion Properties of Pure Mo Coating Layer Manufactured by Atmospheric Plasma Spray Process

A pure molybdenum (Mo) coating layer was manufactured by using the atmospheric plasma spray (APS) process and its wear and corrosion characteristics were investigated in this study. A Mo coating layer was prepared to a thickness of approximately 480 μm, and it had sound physical properties with a porosity of 2.9% and hardness of 434 Hv. Room temperature dry wear characteristics were measured through a ball-on-disk test under load conditions of 5 N, 10 N and 15 N. Based on the coefficient of friction graph at 5 N and 10 N, the oxides formed during wear functioned as a wear lubricant, thereby confirming an increase in wear resistance. However, at 15 N, wear behavior changed, and wear occurred due to splat pulling out. A potentiodynamic polarization test was conducted under an artificial seawater atmosphere, and Ecorr and Icorr measured 0.717 V and 7.2E-5 A/cm2, respectively. Corrosion mainly occurred at the splat boundary and pores that were present in the initial state. Based on the findings above, the potential application of APS Mo coating material was also discussed.

Hierarchical microporous Ni-based electrodes enable “Two Birds with One Stone” in highly efficient and robust anion exchange membrane water electrolysis (AEMWE)

Anion exchange membrane water electrolysis (AEMWE) is currently the most promising technology to produce green hydrogen. However, the lack of cost-effective and scalable methods for fabricating robust and highly active non-noble metal electrodes primarily inhibits its large-scale industrialization. This study unveils an effective strategy for tackling this challenge by creating a novel hierarchical conical-microporous nickel-based electrode, through a synergistic implementation of both rapid and scalable atmospheric plasma spraying (APS) and laser texturing (LT) processes. The resultant NA-LT-CA electrodes exhibits remarkable catalytic performances for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Notably, the AEMWE cell with NA-LT-CA electrodes yields a remarkable enhancement of cell efficiency, toward a reduction in cell voltage of 244 mV at 0.8 A cm−2, compared to the NA-CA electrode (without LT) cell. The notable achievements stem from the improved bubble dynamic contributed by the introduced hierarchical micropores into NA-LT-CA electrodes by the LT process. Moreover, the cell equipped with the NA-LT-CA electrodes demonstrates an outstanding durability, maintaining its performance for 1000 h without visible degradation under 0.8 A cm−2, which can be ascribed to the distinctive “pinning effect” produced by the transition layer during the LT process, adeptly preventing the catalytic layer peeling off even under industrial-scale current densities. Notably, introducing the LT process delivers a dual benefit, akin to achieving “Two Birds with One Stone.”. This work supports the effectiveness of combining APS and LT processes as a potent strategy for fabricating high-efficiency and enduring electrodes, thus advancing AEMWE for practical industrial applications.

Resistance of ceramic / metal Functionally Graded Materials in the flame of a combustion chamber under harsh thermal and environmental conditions

Ceramic/metal Functionally Graded Materials (FGM) are efficient systems to increase the performance of materials used in thrusters, which operate under harsh thermal and environmental conditions. In this context, an experimental study has been carried out to firstly develop the FGM using atmospheric plasma spraying process and secondly investigate the resistance of the materials in the flame of a combustion chamber under oxidising atmosphere. Several configurations of FGM have been tested, in order to analyse the influence of the different layer thickness and composition, on their resistance. The link between the different FGM configurations and their endurance in the combustion flame has been established. The best performance was demonstrated with a thin ceramic layer of rich yttria stabilised hafnia deposited on the top surface. All the configurations showed an excellent resistance with limited degradation signs.

Internal Diameter Atmospheric-Plasma-Sprayed High-Performance YSZ-Based Thermal Barrier Coatings

Thermal barrier coatings (TBCs) play a crucial role in enhancing the operating temperature of key equipment. However, when TBCs are sprayed on the internal surface of these components, there are issues related to the imperfect internal diameter atmospheric plasma spray (ID-APS) process and the unsatisfactory performance of TBCs in internal holes. Yttria-stabilized zirconia (YSZ), YSZ + Gadolinium zirconate (GZ) and YSZ + tantalate (RETaO4) TBCs were prepared using the SM F210 ID-APS gun. The thermal shock and particle erosion resistance of the coatings were compared, and the relationship between the structure and properties was investigated. The results show that the hardness, fracture toughness and erosion resistance of the three coatings are the largest in the YSZ coating and the smallest in the GZ coating. YSZ coating also has the best thermal shock resistance. These research findings have important guiding significance for the improvement of the ID-APS process.

Reduced amount of contamination particle generated by CF4/Ar/O2 plasma corrosion of Y2O3 materials: Influence of defluorination process

Y2O3 coatings during atmospheric plasma spray (APS) processes are used to suppress the corrosion of internal process chamber parts for semiconductor plasma etching. Highly corrosive fluorine-based plasma corrodes the surface, causing contamination particle and process drift, which are the main causes of yield reduction. In this paper, defluorination is proposed to reduce the amount of contamination particle generation by removing the YOxFy layer on the surface of APS-Y2O3 material after plasma etching. After exposure to CF4/Ar/O2 plasma, chemical–mechanical polishing occurred using KOH, surfactant, and piranha solutions, respectively. The highest and lowest defluorination efficiencies were obtained with the piranha and surfactant solutions, respectively. The results were validated by field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. In particular, the piranha solution generated the lowest amount of contamination particles and lowest amount of etching. In contrast, the thickness reduction due to the defluorination process resulted in the highest breakdown voltage among the surfactant solutions, thereby prompting optimization studies. Finally, the mechanisms of the plasma etching and defluorination processes were derived, and a new method for controlling the amount of contamination particle generation was presented.

Improved corrosion properties of novel Fe-Cr-B based metamorphic alloy designed for thermal spray process by adding Nb and Mo

ABSTRACT In this study, new metamorphic alloy powder (M+) with an optimised composition of Fe-Cr-B-Mo-Nb appropriate for thermal spray process was developed by adding Mo and Nb, and the novel metamorphic alloy coating was fabricated by atmospheric plasma spray (APS) process. Thereafter, the corrosion resistances of the coatings were investigated and compared with those of commercial Fe-Cr-B based metamorphic alloy (M). The high-temperature oxidation resistance of the developed M+ powder feedstock was 2.18 times better than that of the commercial M. The in-flight oxidation occurring during APS process in M+ alloy decreased by 39.5% compared to that in M alloy. The passivation behaviour of M+ alloy was more stable than that of M alloy in a 3.5 wt% NaCl solution. The effect of adding Nb and Mo on the corrosion resistance of M+ coatings was also discussed in connection with microstructural changes.

Systematic Approach to Optimize Technological and Economical Aspects of Atmospheric Plasma Sprayed Thermal Barrier Coatings

Plasma‐sprayed yttria‐stabilized zirconia coatings have been used in gas turbines for decades. They are applied for thermal insulation to increase operating temperature and hence efficiency and component's lifetime. To keep manufacturing costs low, especially deposition efficiency is important. However, increasing it is also related to a reduction in porosity, affecting the insulating properties of the layer. To find an optimal combination of efficiency and technological performance, a systematic study of the most affecting parameters of the atmospheric plasma spraying process is conducted, using response surface methodology. In detail, the influence of current, spraying distance, and hydrogen gas flow is investigated with respect to the deposition efficiency, porosity, microstructure, and mechanical properties of the coatings. Characterization is carried out by scanning electron microscopy, microindentation tests, and three‐point bending tests. The models generated based on these measured properties allow predictions of the system responses for any parameter variation in the investigated design space. In addition, a numerical model is developed for targeted optimization of the coating properties. This can be used to produce optimized coatings for load‐flexible gas turbines with high deposition efficiency, high porosity, and at the same time advantageous mechanical properties.

Determination of wear resistance of nickel-carbide alloy coating by atmospheric plasma spray technique on 30MnB5 alloy steel used in cultivator blades

The wear that occurs during tillage in agricultural machinery shortens the material’s life and increases costs. This study investigated the microstructure and morphology of cultivator blades coated with atmospheric plasma spray (APS) process and tested under real field conditions using OM, SEM, EDX, and XRD techniques. Abrasion tests were conducted in an area of 110 hectares and under two different soil moisture conditions. Material properties were investigated using hardness and surface roughness tests. The coating process was successful both mechanically and metallurgically by applying Ni-WC ceramic-metal (cermet) powder on the surface of 30MnB5 steel substrate with APS. The oxide and carbide structures detected on the coating surface by XRD studies resulted in a 1.5 times higher coating hardness. Due to the hard carbide phases in the coating matrix, the grooves on the wear surfaces were shallow and narrow compared to the substrate sample. The surface roughness values of the coated sample were approximately two times higher than the substrate sample. Due to the microstructure and high hardness, the coated specimen experienced three times less abrasion in both humidity conditions. The APS method can be successfully applied to the machined parts of tillage machines. In the near future, the cost of agricultural production can be reduced, and the environment can benefit from this method. In conclusion, this study presents a feasible approach to improving the wear resistance of Ni-WC ceramic-metal alloy by plasma spray coating under real field conditions.

Effect of Cr2O3 on the microstructure and wear resistance of coatings prepared from Cr2O3-SiC-Al composite powders

Introducing the in-situ reaction synthesis into the thermal spraying process, the content of each phase in the raw material will directly affect the phase and microstructure of the coating obtained by spraying. Therefore, composite powders with different Cr2O3–SiC–Al mass ratios were used as coating materials, and composite coatings were prepared using an atmospheric plasma spraying process. By analyzing the coatings' phase composition, microstructure, wear resistance, and other properties, the effect of the variation of Cr2O3 content in the composite powders on the performance of the coating was investigated. The results showed that with the decrease of Cr2O3 content in the composite powder, the coatings’ compactness, microstructure uniformity, and wear resistance decreased. The coating prepared by Cr2O3–SiC–Al composite powder with a mass ratio of 65:12:23 had good compactness and microstructure uniformity, the microhardness and toughness were also the best among the three groups of coatings, and it presented higher wear resistance.

Comparison of tribological performances of plasma sprayed YSZ, YSZ/Ag, YSZ/MoO3 and YSZ/Ag/MoO3 coatings from 25 to 800 °C

YSZ, YSZ/Ag, YSZ/MoO3 and YSZ/Ag/MoO3 coatings were prepared on Inconel 718 alloys substrates by the atmospheric plasma spraying process. Their microstructures, mechanical and tribological properties were compared. The results show that the presence of soft Ag and dual addition of Ag and MoO3 exhibit enhanced impact resistance of YSZ coating, nevertheless the inclusion of MoO3 conversely deteriorates this property of YSZ coating. As referenced with YSZ coating, all composite coatings exhibit improved tribological properties from 25 to 800 °C. The presence of Ag in YSZ matrix provides effective function on reducing friction and wear at ambient temperatures, surprisingly YSZ/MoO3 composite coating demonstrate worse wear due to its poor mechanical strength. In contrast, the dual dopants of Ag and MoO3 are in favor of outstanding tribological performance. Characterization of the worn surface of YSZ/Ag coating reveals the enriched Ag island exists being responsible for its better frictional performance at ambient temperature. A distinct tribo-layer covers on the wear surface of YSZ/Ag/MoO3 coating at 800 °C, primarily consisting of Ag2MoO4, which ultimately contributes to reduced friction and improved wear resistance. The formation of Ag2MoO4 and its working mechanism are proposed for the tribological characteristics of YSZ/Ag/MoO3 coating over 500 °C.

Characterisation and property evaluation of High Entropy Alloy coating on 316L steel via thermal spray synthesis

The advancement in the alloys led the development of High Entropy Alloys (HEA) that possess improved performance and characteristics. In the current study, AlCoCrFeNi HEA powder was synthesised using gas atomisation and coated onto SS316L stainless steel substrate using the atmospheric plasma spraying process. The microstructural characterisation of the synthesised HEA powder exhibited spherical morphology with a mean particle size of 20 µm. The crystal structure is observed to be BCC phase. The coated samples displayed uniform coating with homogenous distribution of HEA over the coating thickness of 150 µm. The microhardness of the coated samples had an increment of 2.46 times against the uncoated samples due to the excellent bonding and fine dispersion of HEA. The subsequent wear test performed by varying the process parameters revealed improved wear resistance on the coated samples than the uncoated samples. The lower range of load, sliding velocity and sliding distance of coated samples offered 83.92 %, 76.56 % and 74.59 % increments in wear resistance respectively and the characterisation of the worn surface revealed the mechanism to be plastic delamination and adhesive wear. The corrosion test conducted in 3.5 wt% NaCl electrolytic solution also exhibited better corrosion resistance for the coated samples due to the formation of an oxide layer along the coating acting as a protective layer for the enhanced resistance to corrosion.

Simulation of 1500 °C Thermal Shock for Novel Structural Thermal/Environmental Barrier Coatings System

In recent years, with the development of SiC composites in aero-engine hot-end components, environmental barrier coatings (EBCs) have received extensive attention. Moreover, in order to elevate the service temperature, it is a developing trend to apply thermal barrier coatings (TBCs) with low thermal conductivity on EBCs coating system to form thermal/environmental barrier coatings (T/EBCs). However, the combination of high coefficient of thermal expansion (CTE) of TBCs with low CTE of EBCs often leads to premature failure due to excessive thermal expansion mismatch stress. However, a novel structural thermal barrier coating with embedded micro-agglomerated particles (EMAP TBC) by using atmospheric plasma spraying (APS) process has brought hope to solve this problem due to its low elastic modulus. Therefore, in this study, an innovative EMAP Gd2Zr2O7 T/EBCs coating system (EMAP Gd2Zr2O7/Yb2Si2O7/Si) under 1500 °C flame thermal shock was simulated and systematically studied on the SiC substrate. The results showed that the EMAP Gd2Zr2O7 T/EBCs coating system has much lower thermal stress than the conventional Gd2Zr2O7/Yb2Si2O7/Si T/EBCs coating system. Furthermore, when the thickness of each layer of the EMAP Gd2Zr2O7 T/EBCs coating system varies, to meet the thermal insulation requirements of Yb2Si2O7 layer and reduce the thermal shock stress, the thickness of the EMAP Gd2Zr2O7 layer is recommend being about 100 μm. Meanwhile, the thicknesses of Yb2Si2O7 and Si layers can be set as large as needed. In addition, with the increase in Yb2SiO5 doping content in the Yb2Si2O7 intermediate layer, the EMAP Gd2Zr2O7 T/EBCs coating system suffers a greater risk of spalling failure due to the increase in thermal stress.

Karakterizacija prevlake cirkonijum oksida stabilizovanog cerijum i itrijum oksidom deponovane na veznoj prevlaci nikl hrom aluminijum kobalt itrijum oksid

Zirconium dioxide ZrO2 is used as a basic material for thermal barriers and as a biomaterial for manufacturing parts for hip implants in orthopedic surgery and in dentistry for making crowns. Use of ZrO2 ceramics as an insulating material and biomaterial is due to its good chemical and dimensional stability, mechanical strength, toughness and elastic modulus. The aim of this study was to analyze the characteristics of zirconium oxide coating of stabilized cerium and yttrium oxide deposited by atmospheric plasma spraying (APS) process on the bond coating of nickel chromium cobalt yttrium oxide. Coatings were deposited on cold substrates and on substrates preheated to 180°C. Composite powder nickel chromium/aluminum/cobalt/yttrium oxide was used for the production of bond layers and for the ceramic layers zirconium oxide stabilized with cerium and yttrium oxide powder. Testing the quality of coatings was done by measuring microhardness of layers using the HV method and for bond tensile strength of the coating system tensile testing was applied. Metallographic evaluation of the share of pores in the bond and ceramic layers was performed with image analysis - processing of images from a light microscope. Powder particle morphology and EDS analysis were performed on the SEM. The obtained results confirm that the preheating temperature of the substrate has a significant impact on structural mechanical properties of the tested coatings system.

Adaptive Control of the Atmospheric Plasma Spray Process for Functionally Graded Thermal Barrier Coatings

Functionally graded coatings (FGCs) have a material composition continuously varying through the thickness but uniform in the surface parallel to the coated substrate. When used as a thermal barrier on a metallic substrate, the coating composition varies from an almost pure metal near the substrate to a pure ceramic adjacent to the outer surface exposed to a hot environment. Challenging issues in producing high quality FGCs in the presence of external disturbances with an atmospheric plasma spray process (APSP) include controlling the mean temperature, the mean axial velocity, and the positions of the constituent material particles when they arrive at the substrate to be coated. The unavoidable disturbances include fluctuations in the arc voltage and clogging of the powder in the delivery system. For a two-constituent coating, this work proposes using three modified robust model reference adaptive controllers based on the σ-modified laws and low frequency learning. One controller adjusts the current and flow rates of argon and hydrogen into the torch. The other two controllers adjust the distance of the two powder injector ports from the plasma jet axis and the average injection velocity of each powder. It is shown through numerical experiments that the three controllers implemented in an APSP consistently produce high-quality FGCs.

Characterization and tribological behaviour of Indian clam seashell-derived hydroxyapatite coating applied on titanium alloy by plasma spray technique

Various hydroxyapatite (HA) powders synthesized at different temperatures are deposited on titanium alloy by using an atmospheric plasma spray process. These different HA powders were synthesized from Indian clam seashells through the hydrothermal technique at varying temperatures from 700 to 1000 °C for a 2 h time duration in our previous study. The synthesized HA powders are spray-dried to obtain agglomerated powders suitable for spraying during the coating application. Crystallite size, Ca/P ratio, and crystallinity of agglomerated HA powders and their respective coatings are estimated by standard methods. The microstructure and phases of the feedstock and coating materials are investigated by using a field-emission scanning electron microscope (FESEM) and X-ray diffractometer (XRD), respectively. Further, the HA coatings are characterized in terms of surface roughness, microhardness, porosity, adhesion strength, and wear resistance through the stylus profilometer, Vickers micro-hardness tester, image analysis technique, scratch tester, and ball-on-disc tribometer, respectively. The average surface roughness (Ra) and porosity of the coating are decreased with an increase in the synthesis temperature. The minimum Ra and porosity obtained for the 1000 °C coating sample suggest a high degree of melting of such powder particles. However, the highest adhesion strength noticed in the case of the 900 °C coating sample is due to the high compatibility of such coating material with Ti-alloy substrate in terms of thermal properties. The 900 °C coating sample has also shown the highest microhardness and wear-resistance properties due to its maximum crystallinity among all the HA coatings.

Reactive synthesis of ZrC-ZrSi2 composite coating by atmospheric plasma spraying

In-situ reaction between zirconium and silicon carbide was introduced to atmospheric plasma spraying process to prepare ZrC-ZrSi2 composite coating. The in-situ fabricated ZrC-ZrSi2 composite coating was featured with dense microstructure and promising mechanical properties. Well-arranged whiskers and nanosized spheric grains were observed in the as-prepared composite coating. Water-quenching experiment was conducted to investigate the formation mechanism of the target phase (ZrC and ZrSi2) and the final coating. This work validated the feasibility of preparing ZrC composite coating using Zr-SiC reactive system and furthermore, gave references for preparing ultra-high temperature ceramic coatings by reactive synthesis method.

TEM exploration of in-situ nanostructured composite coating fabricated by plasma spraying 8YSZ-Al-SiC composite powder

A novel in-situ synthesis method, which introduced 8YSZ-Al-SiC reaction system to atmospheric plasma spraying process, was put forward to prepare ZrC composite coating. Phase transformation between 8YSZ and 15YSZ caused by in-situ reactions was observed. Efforts were focused on TEM investigation in order to explore in-situ generated microstructure containing nanosized ZrC, ZrSi 2 and 15YSZ grains in the composite coating. Moreover, the relationship between the mechanical properties and microstructure of the ZrC composite coating was preliminarily established based on the micron-indentation test. Results confirmed the feasibility of preparing ZrC composite coating by plasma spraying 8YSZ-Al-SiC composite powder and further proved the positive effect of in-situ generated microstructure of the ZrC composite coating on its mechanical properties.

Tribological and corrosion performance of an atmospheric plasma sprayed AlCoCr0.5Ni high-entropy alloy coating

High entropy alloys (HEAs) are an established new class of materials exhibiting excellent mechanical and functional properties. The well-studied AlCoCrFeNi HEA has been modified to AlCoCr0.5Ni HEA in view of the latter's better oxidation resistance. The elimination of Fe and reduction in Cr content is expected to improve the HEA's corrosion and tribological performance. Thus, an AlCoCr0.5Ni HEA coating was fabricated via the atmospheric plasma spray (APS) process using mechanically alloyed (MA) feedstock. The microstructural characteristics of both the MA feedstock and coating have been investigated. The sliding wear behaviour of the HEA coating was evaluated at both room temperature and 500 °C. In addition, the electrochemical compatibility of the coating was analyzed in seawater. The microstructural results revealed that the MA feedstock was composed of BCC/B2 and FCC phases, which were retained in the HEA coating along with minor oxides. The wear resistance of the AlCoCr0.5Ni HEA coating was superior to the AlCoCrFeNi HEA coating both at room temperature and 500 °C. The high wear resistance of the coating was attributed to the formation of equal concentrations of the BCC/B2 and FCC phases. The coating exhibited a greater tendency of being cathodic (passive) than both an AlCoCrFeNi HEA coating and SS316L with identical polarization behaviour.