Plasma-sprayed Coatings Research Articles

Effects of Laser-Remelting on the Microstructure, Hardness and Oscillating Wear Resistance of Atmospheric Plasma Sprayed Alumina-Rich Coatings

Thermally sprayed ceramic coatings such as alumina have a specific microstructure characterized by porosity and microcracks. In addition, a process-related phase transformation from α-Al2O3 to γ-Al2O3 typically occurs, which affects the properties of the coatings compared to sintered alumina. In a previous study, simultaneous additions of Cr2O3 and TiO2 have already extended and improved the property profile of pure alumina coating (i.e., sliding wear resistance and corrosion resistance against 1N H2SO4). Depending on the powder material used, the phase composition of the coatings differs considerably, influencing the property profile. Chemical integration through reactive bonding promises a previously untapped potential for improvement. In this study, these alumina-rich ternary oxide coatings are remelted by laser, and the effect of different parameters such as speed, laser power or distance on the macro- and microstructure of the coatings is investigated. For this purpose, both light microscopic and SEM examinations are used as well as the determination of the phase composition by XRD and element distribution by EDS. The created coating microstructures are studied with respect to hardness and oscillation wear resistance.

Crystallization behavior and thermal stability mechanism of plasma sprayed Al2O3-GAP amorphous ceramic coatings

A novel Al2O3-GdAlO3 (GAP) amorphous ceramic coating was in-situ deposited via atmospheric plasma spraying (APS). The crystallization behavior and thermal stability mechanism of Al2O3-GAP coating were studied via differential scanning calorimetry (DSC) under isochronal conditions. The nucleation process of Gd4Al2O9 (GAM) and α-Al2O3 phases were more sensitive than the growth process. On the contrary, the nucleation process of GAP phase was duller than the growth process. The activation energy of Al2O3-GAP coating was higher than those of almost all amorphous materials reported so far. GAM and α-Al2O3 phases had a semblable crystallization mechanism: the initial phase evolution was performed by diffusion-controlled growth with a decreasing nucleation rate. As the crystallization fraction increased, the nucleation rate continued to fall. In addition, it was demonstrated that Al2O3-GAP coating showed long failure time (te10%) of 3.0 × 107 min at 1000 K, which was conducive to the application of the coating under high temperature service conditions.

Preparation and CMAS Wettability Investigation of CMAS Corrosion Resistant Protective Layer with Micro-Nano Double Scale Structure

Solution precursor plasma spray (SPPS) can prepare thermal barrier coatings (TBCs) with nanostructures, which can modify the adhesion and wettability of molten silicate environmental deposits (CMAS) on the surface of TBCs, thereby improving the resistance of TBCs to CMAS corrosion. In this study, SPPS layers with micro-nano double scale structures were prepared on the surface of conventional atmospheric plasma spraying (APS) coatings. The effect of process parameters on the micro-nano double scale structures and the wetting and infiltration behavior of molten CMAS on the surface of coatings were investigated. The results show that micron structure is more sensitive to process parameters. Lower precursor viscosity, closer spraying distance, and smoother APS layer are favorable to form more typical and dense micron structures. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% and the steady-state contact angle increased up to three times. The reason is that the micro-nano double scale structures can effectively trap air and form an air layer between the coating surface and the molten CMAS. In addition, nano-particles play a more important role in the formation of the air layer, which in turn determines the steady-state wettability properties. While micron structures can influence the time needed to reach the steady state. However, the SPPS layers composed of nano-particles have a very loose structure and weak cohesion, and they degrade and fail rapidly after the infiltration of molten CMAS. Therefore, maintaining the excellent CMAS wetting resistance of the SPPS layers while taking into account their lifetime and reliability has become the focus of further research.

Wear behaviour of ceramic particle reinforced atmospheric plasma spray coatings on the cylinder running surface of internal combustion engines

Atmospheric plasma spray coatings can provide a solution for corrosion and wear resistant cylinder coating surfaces in hybrid powertrains. This article presents experimental results from a model study of metal matrix composite coating samples of chromium steel with varied ceramic content, in order to characterize the effect of hard particles and porous coating structure on friction and wear. Experiments were conducted on a high-frequency reciprocating rig with coated cast-iron cylinder segments and hard chromium coated piston ring segments. Samples were investigated under continuous and scarce lubrication conditions. A ceramic content of 35 wt% was found to be ideal in terms of friction and wear. Coatings with a higher ceramic content exhibited severe abrasive wear, whereas a ceramic content under 35 wt% allowed for increased adhesion between the ring and cylinder surfaces. A detailed investigation of focused ion beam milled sections of the coated cylinder wall segments revealed a stabilizing effect of the ceramic particles, which reduces the delamination of the coating structure.

Preparation, Microstructure and Thermal Conductivity of Plasma-Sprayed (Y0.8Gd0.2)3Al5O12 Coatings

Garnet-type rare earth aluminate compounds are one class of promising potential candidate materials for thermal barrier coatings (TBCs). In this paper, (Y0.8Gd0.2)3Al5O12 (GYAG) coatings are fabricated by air plasma spraying, with the microstructure, high-temperature phase stability, and thermal conductivity investigated. The results showed that the as-deposited GYAG coating was relatively dense, and contained garnet-type (Y0.8Gd0.2)3Al5O12 phase and a small amount of (Y,Gd)AlO3 and amorphous phases. The crystallized GYAG coating exhibited good phase stability from room temperature to 1450 °C. The coating had the lowest thermal conductivity of 1.17 W·m−1·K−1 at 800 °C, approximately 15% lower than that of the yttria-stabilized zirconia (YSZ) coating. After heat treatment at 1100 °C, the coating became denser with some fine grain precipitation, and formed a number of transverse and longitudinal cracks.

FEA Analysis and Experimental Investigation of the Ceramic Coating on Aluminum Piston Material By Plasma Spray Coating Technique

Abstract: Functionally graded materials are of widespread interest because of their superior properties such as corrosion, erosion and oxidation resistance, high hardness, chemical and thermal stability at cryogenic and high temperatures. These properties make them useful for many applications, including Thermal Barrier Coating (TBC) on metallic substrates used at high temperatures in the fields of aircraft and aerospace, especially for thermal protection of components in gas turbines and diesel engines. The application of TBC reduces the heat loss to the engine cooling-jacket through the surface exposed to the heat transfer such as the cylinder head, liner, piston crown and piston rings. The insulation of the combustion chamber with ceramic coating affects the combustion process and, hence, the performance and exhaust emissions characteristics of the engines improve. In this project, the main emphasis is placed on the study of thermal behavior of functionally graded coatings obtained by means of using a commercial code, ANSYS on aluminum and steel piston surfaces and the results are verified with numerical and experimental works. Keywords: Thermal Barrier Coating, Piston, Insulation, Corrosion, ANSYS, engine cooling-jacket.

A numerical investigation of the effect of thermal aging, processing and humidity on initiation and delayed cracking in plasma-sprayed coatings

The effect of aging by the relaxation of the initial thermal stresses related to the processing on the initiation and propagation of the inter-splat and the intra-spat cracks in plasma sprayed zirconia is analyzed using finite element model and a description of failure with cohesive surfaces. A multi-scale approach is adopted in which the inter-splat and intra-splat crack growth is described with a rate-temperature and humidity dependent cohesive zone model that mechanically represents the reaction-rupture mechanism underlying stress and environmentally assisted sub-critical failure. It is found that the relaxation of the initial thermal stresses generates a significant initial damage at the inter-splat scale by the nucleation of inter-splat cracks and a minor initial damage at the intra-splat scale. The results show that the rate of inter-splat cracks increases with the relative humidity and especially with the temperature at which the relaxation occurs. The effect of the initial damage generated by the thermal aging on the resistance of the polycrystal of plasma sprayed zirconia against intra-splat slow crack growth under static fatigue loading is investigated. The results show that the initial damage at the intra-splat scale does not affect its resistance against intra-splat slow crack growth. However, the initial damage at the inter-splat scale leads to an increase in the slow cracking rate for a loading level KI and a reduction in the threshold load K0 below which no slow crack growth occurs as the individual splat is embedded in a damaged equivalent continuum representing the overall splat structure. The aim of this work is to provide a reliable predictions and insight in long lasting applications of plasma sprayed ceramic materials.

Investigating Atmospheric Pressure Plasma Spray Coating of YPO4 and its Performance as a Corrosion Barrier Protective Layer Against Molten Uranium

Investigating Atmospheric Pressure Plasma Spray Coating of YPO4 and its Performance as a Corrosion Barrier Protective Layer Against Molten Uranium

Stability Study of Dielectric Properties of Plasma-Sprayed BaTiO3

Barium titanate coatings were, for the first time, sprayed by a high feed-rate plasma torch with water stabilization of the plasma. Two power levels of the torch were applied for spraying to cover steel substrates. Various substrate preheating levels from 125 °C to 377 °C were used to modify cooling conditions. Microstructure and phase composition including crystallinity quantification were observed. Dielectric measurements proved that the relative permittivity between 300 and 400 coatings is too temperature sensitive over 170 °C but fits the requirements of the EIA temperature coefficient between room temperature and 170 °C. Simultaneously, the loss tangent remains rather low, between 0.02 and 0.07, in a broad range of temperatures and frequencies. Annealing was performed in air to heal the oxygen deficiency, but only modified the microstructure insignificantly. The dielectric properties of as-sprayed and annealed samples were discussed, with the main finding that the temperature coefficient of permittivity was improved by annealing. This study contributes to the search for the suitability of plasma-sprayed BaTiO3 coatings for application in the electrical industry, namely by the optimization of conditions for high feed-rate spraying.

Ultrasonic characterization of elastic constants of plasma sprayed Al2O3 coatings based on simulated annealing algorithm

Macroscopic elasticity of plasma sprayed thermal barrier coatings (TBCs) is often anisotropic. Effective characterization of elastic constants is crucial for prediction and evaluation of TBC mechanical behaviors. However, elastic constants of TBCs sprayed on matrix is usually difficult to be determined. In this paper, an inversion method of TBC elastic constants is developed through ultrasonic back-reflection technique combined with Christoffel equation. An algorithm for calculating the coating ultrasonic velocities (longitudinal and transverse wave velocities) in different directions is proposed. A control device to adjust and measure the ultrasonic incident angle from water/coating/matrix/water multi-interface structure is designed. Five elastic constants of coating are inverted using the determined velocities combined with simulated annealing algorithm. Results show that the inverted elastic constants of plasma sprayed Al2O3 coating C11, C13, C33, C44, and C66 are 104.99 GPa, 22.09 GPa, 185.78 GPa, 28.43 GPa, and 26.64 GPa, respectively. The deviations between experiment and inversion velocities were less than 2.37%, and Young's modulus in three orthogonal directions show anisotropy. Then, statistical features of pore and micro-crack, including content, orientation, etc., were analyzed using laser scanning confocal microscope and scanning electron microscope, which primarily indicates that micro-cracks with preferred orientation (mainly parallel to x3 axis) are responsible for that elastic constant in the direction normal to the matrix is higher than that in the transverse direction.

Parametric Effect on Tribological Performance of Plasma-Sprayed Composite Coating on Bearing Steel

Parametric Effect on Tribological Performance of Plasma-Sprayed Composite Coating on Bearing Steel

Evaluation of solid particle erosion and electrochemical corrosion of plasma-sprayed WC/8YSZ coating on SS316

Plasma spray coating enhances the corrosion and wear resistance of the stainless steel structures installed in marine applications. Hence, it is imperative to study the effectiveness of various coating combinations on the material’s performance. The present work investigates the effects of three different plasma-sprayed coatings on the wear and corrosive resistance of austenitic stainless steel (SS316). The three compositions of the coating were prepared using (i) Tungsten carbide (WC), (ii) 8 wt%. Yttria Stabilized Zirconia (8YSZ), and (iii) 50 wt%. Tungsten carbide (WC) with 50 wt%. Yttria Stabilized Zirconia (8YSZ). Experiments were conducted as per ASTM G76 to determine the erosive wear with a mixture of high-velocity air and Al2O3 abrasive particles. The corrosive medium used in the electrochemical polarisation tests was 3.5 wt%. NaCl. A scanning electron microscope (SEM) was used to examine the surface morphology of the eroded and corroded coatings. Energy Dispersive x-ray Analysis (EDAX) and X Ray Diffractrometry (XRD) analysis were carried out to reveal the phase composition, elemental distribution, and lattice parameters of uncoated and coated samples. The study reveals that the composite coating (WC + 8YSZ) have superior wear resistance when exposed to a high-velocity erodent. Due to the robust particle adhesion and cohesiveness of the (WC + 8YSZ) composite coating, crack initiation and propagation are rarely found on the surface of the composite coating, as evidenced by the surface wear morphology analysis. Further investigation reveals that the 8YSZ coating has excellent corrosion resistance. The SEM-based corrosive wear topography analysis reveals that the 8YSZ phase on the coated surface acts as a diffusion barrier to the electrolytic medium and a passive protective layer over the coating. Consequently, the micro-chipping of particles during exposure to the corrosive medium is prevented.

Achieving high anti-sintering performance of plasma-sprayed YSZ thermal barrier coatings through pore structure design

Sintering is one of the critical factors leading to thermal and mechanical degradation of plasma-sprayed thermal barrier coatings (TBCs) in service under high temperature. The sintering resistance of the coating is expected to be improved through pore structure design. In this study, through a strategically improved powder feeding process, 8YSZ powders with fine-sized and coarse-sized pores were injected from the tail of plasma flame during plasma spraying to design porous coatings containing two differently flattened particles with different pore structures. The microstructural and property evolutions during sintering of the coatings were systematically studied compared with traditionally structured coatings. The effect of pore morphology on its sintering behavior was analyzed by quasi-in-situ observation of microstructure evolution. Results show that the anti-sintering performance of plasma-sprayed coating can be improved by adjusting the pore structure of coating with porous unmelted particles. In particular, the introduction of unmelted particles with coarse-sized pores can ensure that the coating has excellent thermal and mechanical properties for a long time. The results of this study provide guidance for structural design of anti-sintering plasma-sprayed coatings.

Microstructure and Corrosion Behavior of Atmospheric Plasma Sprayed NiCoCrAlFe High Entropy Alloy Coating.

High entropy alloys (HEAs) are multi-elemental alloy systems that exhibit a combination of exceptional mechanical and physical properties, and nowadays are validating their potential in the form of thermal sprayed coatings. In the present study, a novel synthesis method is presented to form high entropy alloy coatings. For this purpose, thermal sprayed coatings were deposited on Stainless Steel 316L substrates using atmospheric plasma spraying technique with subsequent annealing, at 1000 °C for 4 h, to assist alloy formation by thermal diffusion. The coatings in as-coated samples as well as in annealed forms were extensively studied by SEM for microstructure and cross-sectional analysis. Phase identification was performed by X-ray diffraction studies. The annealed coatings revealed a mixed BCC and FCC based HEA structure. Potentiodynamic corrosion behavior of SS316L sprayed as well as annealed coatings were also carried out in 3.5% NaCl solution and it was found that the HEA-based annealed coatings displayed the best corrosion resistance 0.83 (mpy), as compared to coated/non-annealed and SS 316 L that showed corrosion resistance of 7.60 (mpy) and 3.04 (mpy), respectively.

Effect of Melting Behavior of Al2O3 Powder on the Structure and Dielectric Properties of Plasma-Sprayed Coatings

Effect of Melting Behavior of Al2O3 Powder on the Structure and Dielectric Properties of Plasma-Sprayed Coatings

Plasma Spray Coating of Aluminum–Silicon-MWCNT Blends on Titanium Grade 5 Alloy Substrate for Enhanced Wear and Corrosion Resistance

Plasma Spray Coating of Aluminum–Silicon-MWCNT Blends on Titanium Grade 5 Alloy Substrate for Enhanced Wear and Corrosion Resistance

Comparative Study on Wear and Corrosion Behavior of Plasma Sprayed Fe73Cr2Si11B11C3 and Fe63Cr9P5B16C7 Metallic Glass Composite Coatings

Comparative Study on Wear and Corrosion Behavior of Plasma Sprayed Fe73Cr2Si11B11C3 and Fe63Cr9P5B16C7 Metallic Glass Composite Coatings

Effect of post coating processing on the morphological and mechanical properties of plasma Spray-reinforced hydroxyapatite coating

The biocompatible Hydroxyapatite (H.A.) coating helps titanium alloy recover and insulates implants from the body's environment. FDA recommends plasma spray to cover H.A. on body implants. H.A.'s poor mechanical properties limit its use in weight-bearing orthopaedic applications like hip and knee joints. This study examines the influence of H.A. reinforcement on plasma-sprayed coatings' microstructure and mechanical characteristics. The influence of thermal post-treatments on as-deposited (pure H.A. & reinforced H.A.) coatings was also examined. SEM images of as-deposited (pure H.A. & reinforced H.A.) coatings were taken before and after thermal processing (S.E.M.). The use of reinforcement improves as-sprayed coatings' microhardness and bond strength. After two hours at 700 °C in air, both substrates had ultrafine particles that may connect to bone and promote bone development.

Study on the morphological and mechanical properties of TaC reinforced plasma spray coating deposited on titanium alloy

The biocompatible Hydroxyapatite (H.A.) coating is often placed on titanium alloy to aid in the healing process and insulate the implant from the hostile bodily environment. Plasma spray is one of the best techniques to coat H.A. on body implants per the Food and Drug Association (F.D.A.), U.S.A. However, the poor mechanical qualities of H.A. limit its use in orthopaedic applications that must bear weight, such as hip and knee joints. The present investigation studies the effect of the addition of reinforcement with H.A. on the microstructural and mechanical properties of plasma-sprayed coatings. Pure H.A. and reinforced HA-coated samples' surface morphology was investigated using a scanning electron microscope (S.E.M/EDAX). The results indicate that the coating has a homogenous dense microstructure with some un-melted and melted particles and tiny cracks. The addition of reinforcement enhances the microhardness and bond strength of the coating.

Tribological Properties of Plasma Sprayed TiC-Graphite Composite Coatings

Tribological Properties of Plasma Sprayed TiC-Graphite Composite Coatings