Plasma-sprayed Coatings Research Articles

Development and characterization of corrosion-resistant plasma-sprayed Ni-Al composite coatings via graphene oxide fillers

In this study, the authors propose a procedure to enhance pure Ni-Al powders through graphene oxide dispersion in water for plasma spraying. The enhanced powders were utilized to coat A36 steel via plasma spraying. Next, the powders and the coatings were characterized by energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, micro-Raman spectroscopy, and field emission scanning electron microscopy. Given the capability of the generated X-ray to characterize near surfaces to a depth of a few micrometers, the performed X-ray microanalyses convey that the carbon species incorporated within the powders through the graphene oxide dispersion can act as nanofillers reducing the number of holes/pores in the plasma-sprayed coatings. Potentiodynamic polarization testing and electrochemical impedance spectroscopy were used to investigate the corrosion behavior of the samples at two intervals: after 1 h and after 30 days of exposure to a 3.5 % NaCl solution. According to the results of the corrosion testing, the enhanced coatings can display about 2.35 times lower corrosion current density compared to pure Ni-Al coatings, with the corrosion potentials of the enhanced coatings being higher than that of pure Ni-Al coatings (i.e., improved corrosion resistance). This improvement in corrosion resistance is also evident when the enhanced coatings are compared to the A36 substrate after 30 days of exposure to the solution. It appears that the incorporated carbon nanofillers, by reducing deep holes and pores that communicate with the surface of the substrate, ultimately determine the highest corrosion resistance. Thus, the outcome offers enhanced Ni-Al coatings possessing superior anticorrosion properties that typical plasma-sprayed Ni-Al bond coats may not exhibit.

Microstructure and properties of plasma sprayed NbC-Al2O3 composite coatings

Niobium carbide has received widespread attention due to its high melting point, high strength, high chemical stability and high wear resistance, but the current preparation of niobium carbide coatings using plasma spraying technology suffers from low deposition efficiency, poor fusibility of the composite powders, high porosity, and high defects in the coating microstructure. Therefore, the microstructure and properties of NbC-Al2O3 composite coatings prepared by plasma spraying NbC-Al2O3-SiC composite powder and reactive plasma spraying Nb2O5-SiC-Al composite powder were comparatively investigated in this paper. The reaction mechanism of Nb2O5-SiC-Al composite powder during plasma spraying as well as the mechanism of the formation of coatings were analyzed in detail. The results showed that compared with the ex-situ NbC-Al2O3-SiC coating fabricated by plasma spraying, the in-situ Nb2O5-SiC-Al composite coating prepared by reactive plasma spraying had an obvious layer structure, low porosity (5.8%), high hardness (1141 HV0.1), high toughness (22.94 J/m2), and good scratch resistance properties.

Microstructure evolution and oxidation resistance of plasma sprayed AlSi-doped AlCoCrFeNi coatings with post-annealing

High entropy alloy (HEA) coatings of equimolar AlCoCrFeNi typically exhibit a lower oxidation rate at high temperatures by forming a protective passivation film. However, the metal elements consumption during long-term oxidation limitted the application. In this work, AlCoCrFeNi HEA coatings doped by AlSi as a supplement to passivation elements were prepared by atmospheric plasma spraying (APS), and AlSi capsules were diffused uniformly into the coating through annealing treatment to offset the element consumption during high-temperature oxidation. Results showed that annealing promoted Al and Si atoms diffusing into the solid solution, which stabilized BCC and inhibited FCC formation. During the oxidation at 900 °C, a protective Al2O3 film was formed on the coating surface, and AlSi capsules continuously transported Al ions to the consumption zone and reduced oxidation rate to 0.0015 g/cm2. The HEA coating doped by passivation element capsules provided a new approach for the design of novel antioxidant coatings.

Effect of powders recycling on microstructure evolution and wear mechanism of plasma sprayed Ni625-WC composite coating

The utilization rate of metal powders during the preparation of plasma-sprayed coatings is typically below 70 %, which has raised significant concerns regarding efficiency. The microstructure evolution and wear mechanism of plasma-sprayed Ni625-WC composite coatings with received powders (C-p1) and recovered powders (C-p2) were studied to investigate the feasibility of powder recycling. The results showed that the boundary layer at WC particles in C-p2 was 2.84 μm thicker than that in C-p1. Decarburized WC products were dispersed nucleation to form block M23C6 at the boundary of WC particles and within Inconel 625 matrix in C-p1. In contrast, the larger contact area of block M23C6 after initial heating promoted the nucleation and growth of acicular M23C6 in C-p2. In addition, the wear rate of C-p2 is 9.1 % lower than that of C-p1. Although the higher elastic modulus (E) of C-p1 caused the Inconel 625 matrix to adhere more strongly to WC particles, resulting in higher degree adhesive wear rather than exfoliations and less adhesion in C-p2, the presence of harder and more uniformly distributed acicular M23C6, and higher microhardness (H)/E ratio in C-p2, improved the wear resistance.

The Influence of Injection Mode and Spray Distance on Wear Resistance of Al2O3+13 wt.% TiO2 coatings

Abstract Atmospheric plasma spraying (APS) allows deposition of ceramic coatings on metallic substrate, significantly increasing the wear resistance of the component. Coating’s microstructure and consequently its properties, depend on heat treatment of the feedstock particles. This property can be controlled by process parameters, especially electric power and spray distance. On the other hand, some plasma torches allow introducing another variable, an injection mode, which could be realized in external or in internal way. In the presented research, wear resistance of Al2O3+13 wt.% TiO2 coatings deposited under various values of spray distance and different injection modes were examined using ASTM G-99 procedure. The impact of spraying parameters on the coating’s microstructure was established and connected to the functional properties of the manufactured deposits. Observed differences indicated influence of injection mode and spray distance value. Coatings deposited with internal feedstock injection exhibit lower porosity and slightly higher hardness than these made with external injection mode. In terms of wear resistance for both spray distance the wear factor was lower for internal injection system. Obtained results confirmed assumed thesis, that internal injection mode improves heat treatment and consequently ameliorate wear performance of plasma sprayed alumina-titania coatings.

Influence of added La2O3 on the microstructure, mechanical properties, and tribocorrosion resistance of TiB2–Ni plasma-sprayed coatings

To enhance the tribocorrosion resistance of 7075-T6 aluminum alloy in deep sea environments, TiB2–Ni coatings with varying La2O3 content were deposited on 7075-T6 aluminum alloy substrates using plasma spray technology. The effects of different La2O3 contents on the organization, structure, and properties of the coatings were analyzed. The results indicated that La2O3 incorporation suppressed brittle phase formation such as Ni3B; 1.0 wt% La2O3 addition reduced Ni3B by 16.9%. The coating's corrosion resistance significantly improved with La2O3 doping - the self-corrosion current density of the 1.0 wt% La2O3-doped coating decreased from 71.2 nA/cm2 to 30.2 nA/cm2.

Lunar regolith for plasma spray coatings

On the threshold of the first human residences on the Moon, a key requirement for early colonists will be to become rapidly self-sufficient, as the transportation of supplies and materials will be limited. ISRU (In Situ Resources Utilisation) focuses on this particular aspect and proposes the use of lunar regolith as a possible solution. In this study, we consider using this prime resource to manufacture thick coatings using the plasma spray technique. A lunar regolith simulant was successfully deposited on steel substrates and was found to form very adhering coatings, exhibiting similar properties and chemistry to the raw material, that is, mare basalts. This kind of newly formed coating could play various shielding roles, such as those of a thermal barrier or radiation shield.

Wear-Resistant Low-friction Atmospheric Pressure Plasma Spray Coatings for Sustainable (Bio-based Recyclable) Materials

Coatings from polyetheretherketone (PEEK), polyamide 12 (PA12), molybdenumdisulfide (MoS2), zinc (Zn), and graphite (C) powder mixtures were deposited on PA6, PA12, and PEEK substrates by an atmospheric pressure plasma (APP) spray jet system. Several tenth of µm thick coatings on PA6 and PA12 substrates result in an almost halved surface roughness Ra ~8 µm, Rq ~10 µm and Rz ~60 µm, whereas a significant increase of all surface roughness parameters is observed for PEEK substrates (Ra < 1 µm → 4 µm, Rq < 1 µm → 5 µm, Rz < 5 µm → 20 µm). The surface roughness, powder composition, and selected APP process parameter strongly influence the coefficient of friction (COF) and specific wear rate ki of the APP coatings in rotational ball-on-disc tribological testing. The COF of PA12/MoS2/C coatings on PA6 substrates manufactured by selective laser sintering (SLS) is ~0.2 after 628 m sliding distance, resulting in a very low calculated ki of 6.3 × 10−7 mm3/Nm. A similarly low COF and ki was observed for PEEK coatings deposited at a current of 75 A and 60 mm jet–substrate distance on SLS PA12 substrate. Although the COF of Zn/C/MoS2 coatings on PEEK drops down below 0.1 after 1884 m sliding distance under nitrogen atmosphere the corresponding ki of 5.6 × 10−5 mm3/Nm is higher. Still all calculated specific wear rates are significantly lower than the reported values of polyamide-polytetrafluorethylene (PTFE)-polyethylene composites (1.9–8.0 × 10−2 mm3/Nm) and partly even outperform PEEK-PTFE composites (1.0 × 10−7–2.5 × 10−6), currently applied in demanding wear regimes.

Plasma-Sprayed Osseoconductive Hydroxylapatite Coatings for Endoprosthetic Hip Implants: Phase Composition, Microstructure, Properties, and Biomedical Functions

This contribution attempts to provide a state-of-the-art account of the physicochemical and biomedical properties of the plasma-sprayed hydroxylapatite (HAp) coatings that are routinely applied to the surfaces of metallic endoprosthetic and dental root implants designed to replace or restore the lost functions of diseased or damaged tissues of the human body. Even though the residence time of powder particles of HAp in the plasma jet is extremely short, the high temperature applied induces compositional and structural changes in the precursor HAp that severely affect its chemical and physical properties and in turn its biomedical performance. These changes are based on the incongruent melting behavior of HAp and can be traced, among many other analytical techniques, by high resolution synchrotron X-ray diffraction, vibrational (Raman) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. In vivo reactions of the plasma-sprayed coatings to extracellular fluid (ECF) can be assessed and predicted by in vitro testing using simulated body fluids (SBFs) as proxy agents. Ways to safeguard the appropriate biological performance of HAp coatings in long-term service by controlling their phase content, porosity, surface roughness, residual stress distribution, and adhesion to the implant surface are being discussed.

Influence of Yttria content in YSZ: An evaluation of water-based SPS coatings and spark plasma sintered pellets

The inherent phase instability of the state-of-the-art 7–8 mol% partially stabilised Yttria-Stabilised Zirconia (YO1.5, 8 YSZ, tetragonal) under a molten CMAS attack lacks the technological readiness needed to increase the gas inlet temperatures for more thermally efficient gas turbine engines. In this study, the concentration of Yttria in YSZ was systemically increased to impart CMAS resistance and their applicability via emerging Suspension Plasma Sprayed (SPS) coatings and Spark Plasma Sintered (SpPS) pellets under simulated conditions was evaluated. The fully stabilised higher Yttria YSZ compositions (21.4 mol% and 50.2 mol%, cubic) severely restricted the CMAS infiltration and interaction areas, with the later composition forming an apatite layer and the trans-granular cracking and chipping of YSZ into layers reduced with an increase in Yttria content. However, their application into water-based SPS coatings resulted in a complete coating failure following the high-temperature CMAS tests. During Furnace Cycling Test (FCT) tests, the 8 YSZ coating survived 34 thermal cycles compared to just one on the 50.2 mol% YSZ coatings. The premature delamination of the higher Yttria coating seems to have been caused by the spallation associated with horizontal cracks within the coating. The YSZ composition could be modified into CMAS-resistant chemistry; however, their applicability as a functioning TBC with inherent microstructural features, such as the Dense Vertically Cracked (DVC) coating strategy examined in this study, requires new coating design strategies that impart sustainable thermal cycling performance.

On reinforcing the friction stir weld joints of AA5086-H116 using the plasma spray coatings

The fatigue properties of friction stir weld joints of AA5086-H116 alloy, reinforced with Al2O3 particles using the plasma spray coating method, were investigated after single and double weld passes. A relatively uniform distribution of reinforced particles was achieved with superior properties, even higher than that of the parent alloy. The reinforced joints had enhanced hardness as well as joint efficiency, the double pass being superior to the single pass, but at the cost of reduced total elongation. Furthermore, the reinforced double-pass joints exhibited a remarkable ∼ 55%, ∼ 14%, and ∼ 89% increase in fatigue strength compared to the as-weld, the base material, and single-pass joints, respectively. The fractographic examination mainly revealed a defect-free crack initiation mechanism with a few cases of the ones induced by defects. The superior fatigue performance was mainly attributed to the uniform spatial and size distribution of fine Al2O3 particles within the weld nugget zone, thanks to the novel weld reinforcement method used in this study, which provides a promising avenue for improving the fatigue properties of weld joints.

Microstructural, mechanical, room, and high temperature tribological properties of graphene oxide reinforced plasma sprayed alumina nanocomposite coatings

Microstructural, mechanical, room, and high temperature tribological properties of graphene oxide reinforced plasma sprayed alumina nanocomposite coatings

Role of powder morphology on α-phase content in plasma sprayed alumina coatings

Despite various studies addressing the retention of α phase in thermal sprayed alumina coatings, a noticeable research gap persists regarding the impact of feedstock morphology on phase retention and properties. In this work, we explored this simple way to enhance the retention of the α phase and studied its impact on mechanical and dielectric properties. To achieve this, we deposited alumina powders with two different morphologies (such as angular and spherical) with similar particle size using plasma spraying. After deposition, we quantified the retention of α phase present in the coatings using the Rietveld refinement method. From this analysis, we noticed that spherical morphology (SM) powder exhibited the highest percentage of α phase retention (82.81 %) as compared to angular morphology (AM) powder (48.8 %). In turn, SM-coating demonstrated superior mechanical properties, with improvements in hardness, elastic modulus and fracture toughness of 21.9 % 13.7 % and 25 % respectively, compared to AM-coating. This improved mechanical properties in SM-coating is attributed to high α phase retention, synergistic effect of both fully melted (FM) and partial melted (PM) regions of coating microstructure which hindered the crack propagation. Furthermore, the SM-coating exhibited superior dielectric properties, with a dielectric strength 9.9 % higher than the AM-coating. This dielectric performance is ascribed to high retained α phase and generation of discontinuous crack due to presence of PM regions in the SM-coating. These findings suggest the impact of feedstock powder morphology on coating performance and offer avenues for optimizing coating processes in diverse technological fields.

Hot Corrosion Behavior of La2Ce2O7-Based Plasma-Sprayed Coating

La2Ce2O7 (LC) has been identified as a promising thermal barrier coating (TBC) for use up to 1250 °C. In this study, a TBC system was deposited on grit-blasted Inconel 738 using atmospheric plasma spraying (APS) with NiCrAlY as the bond coat, followed by YSZ, and then LC as the top layer. The coatings were exposed to a mixture of molten Na2SO4 (45 wt.%) and V2O5 (55 wt.%) at 950 °C for hot corrosion. On the top LC layer, LaVO4, CeVO4 and CeO(1.66–2.00) formed as hot corrosion products after 4 h exposure. A reaction between YSZ and the corrosion products could not be observed due to the absence of YVO4. The hot corrosion mechanism of the LC-based TBC is also discussed in this study.

Material system and tribological mechanism of plasma sprayed wear resistant coatings: Overview

Challenging environments in aviation, automotive, electric power, and various industrial sectors are increasing industry and technological demands, resulting in machine damage and energy loss from friction and wear between interacting surfaces. Addressing this issue requires the development of new and advanced materials, processing techniques, and surface modification methods to enhance performance and reduce friction and wear. Plasma spraying technology is essential for material surface reinforcement and modification because of its low cost and stability. This paper reviews and summarizes the research progress on plasma spraying wear-resistant coating material systems and tribological mechanisms, covering the working process and principles of plasma spraying equipment. It also details wear-resistant coating material systems, including iron-based, nickel-based, copper-based, other metal materials, oxides, and monoxide ceramic materials.Furthermore, it addresses the wear-resistant and friction-reduction mechanisms of wear-resistant coatings, aiming to summarize the tribological strengthening mechanism concerning wear resistance and friction reduction. Moreover, this paper covers aspects such as powder design, sealing treatment, laser remelting, and other methods for regulating the wear resistance of coatings, examining their influence on the wear resistance of coatings in material design, spraying process, and post-spraying treatment. Plasma spraying wear-resistant coatings should be used along with doping modification and other methods to study the new spraying materials; furthermore, the optimization of the spraying process and post-treatment methods should be investigated to prepare a wear-resistant coating that meets the performance of the actual working conditions. At the same time, ultrahigh temperature wear-resistant coatings and super lubricating coatings are also future research priorities. In addition, when comparing plasma transfer arc spraying technology and plasma enhanced high-speed arc spraying technology with plasma non-transfer arc spraying technology, the jet temperature is higher, and the cost is lower; however, since the process is still in its infancy, the follow-up should be strengthened in this area of technical research. This paper provides a theoretical basis and reference for researching plasma spraying wear-resistant coatings in various fields.

Study on pin temperature and wear behaviour of plasma sprayed Ni-Cr coatings with nano Al2O3 particle

ABSTRACT In this study, the dry sliding wear behaviour of plasma sprayed composite Ni-Cr coatings with nano Al2O3 particle concentrations of 0, 5, 10, 15, 20, and 100 wt.% on Al6061 substrate material was analysed. The coatings were characterised by scanning electron microscopy. The results showed higher porosity and coating thickness values for 100% and 20% nano Al2O3 particle coatings. The microhardness was found to increase with an increase in nano Al2O3 particle concentration in the Ni-Cr coating matrix. The porosity values were found to decrease with an increase in coating thickness. It is found that the specific wear rate of the Ni-Cr/15% Al2O3 particle coating is reduced by 54.02% compared to the Al2O3 particle coating.

Enhanced CMAS and hot corrosion degradation of YSZ thermal barrier coating with nano powders

Thermal barrier coatings are used to protect hot section parts of gas turbine engines. These coatings are subject to corrosion due to impurity elements in the fuel and dust absorbed from the atmosphere into the engine. The rough and porous surface nature of plasma-sprayed coatings facilitates the penetration of these factors into the coating. Coatings are damaged prematurely as a result of corrosion. In this study, nano-structure YSZ, YSZ + Al2O3, YSZ + TiO2, and YSZ + Al2O3 + TiO2 coatings were deposited on plasma sprayed YSZ coating by electrophoretic deposition method. Hot corrosion and CMAS resistance were investigated. The coatings were characterized by SEM, XRD, AFM, and microhardness before corrosion tests. The changes in the coatings after corrosion tests were characterized by SEM and XRD. The penetration behavior of corrosive factors was examined by elemental mapping with EDS. Nanostructure coatings deposited on APS YSZ made the surfaces of as-sprayed coatings more stable. The ability of these nanolayers to prevent pure penetration into coatings during hot corrosion and CMAS is limited. However, the Al2O3 + TiO2-doped nanostructure YSZ reduced the bond-layer oxidation. In addition, the addition of Al2O3 + TiO2 to the EPD deposited layer also reduced the tetragonal-monoclinic phase transformation that occurred in YSZ as a result of corrosion. Al2O3 and TiO2 additives alone are not effective in improving the corrosion behavior of coatings. Nanostructured coatings deposited with the EPD technique can help protect plasma-sprayed thermal barrier coatings against corrosion.

Cavitation Erosion Investigations of Hard Carbide and Nitride Based Novel Coatings Manufactured by Plasma Spraying

Cavitation erosion (CE) poses a significant challenge in the operation and maintenance of hydropower turbines, leading to substantial economic and operational losses. This study addresses this issue by investigating the effectiveness of plasma-sprayed coatings in mitigating CE on SS316 steel. Si3N4+TiC + VC and Cr-Ni based coatings were applied using a plasma spraying process. The research encompasses a comprehensive analysis of the coatings’ characteristics, including microhardness, surface roughness, porosity percentage, and coating thickness. Furthermore, CE tests were conducted under various conditions, considering factors such as impingement angle, stand-off distance, and jet velocity. The eroded surfaces were analyzed using scanning electron microscopy (SEM) to gain insights into the failure modes. The findings demonstrate that the Si3N4+TiC + VC and Cr-Ni based coatings significantly enhance the resistance of SS316 steel to CE.

Effect of Different Post Processing Techniques on Microstructure and Mechanical Properties of Atmospheric Plasma Sprayed WC-17Co Coatings

Effect of Different Post Processing Techniques on Microstructure and Mechanical Properties of Atmospheric Plasma Sprayed WC-17Co Coatings

Influence of Ni Content on Microstructure and Hardness of Nickel-Graphite Abradable Seal Coatings Produced by Plasma Spraying

The influence of the initial shape of graphite powder and Ni content on the structure and properties of the nickel-graphite powders and plasma-sprayed coatings is investigated. The irregularly and spherically shaped graphite powders were produced by mechanical crushing and mechanical crushing with rolling, respectively. It is shown, that spherically shaped graphite powder has advantages in flowability and better deposition of Ni layer. The nickel-graphite core-shell powders with 50 and 75 wt.% Ni content were used to deposit coatings by plasma spraying. The composition, structure, and hardness of the nickel-graphite coatings were studied.