Plasma Sprayed Alumina Coatings Research Articles

Influence of Graphite and Zirconia Addition on the Tribological Properties of Plasma-Sprayed Alumina Coatings

Al2O3, Al2O3-graphite and Al2O3-ZrO2 coatings were formed on the C45 steel rolls using atmospheric plasma spraying. The influence of graphite and zirconia addition on the surface morphology, phase composition and tribological properties under dry sliding conditions using 30 N load were analyzed. It was found that the addition of graphite or ZrO2 slightly affected the fraction of the α-Al2O3 and γ-Al2O3 phases in the alumina coatings. The highest mass loss rate (~8.84 × 10−4 g/s) was obtained for the friction pair of C45 steel roll and steel plate. The friction coefficient of the Al2O3-graphite coating was slightly lower (up to 7%) compared to the coating of Al2O3-ZrO2. However, the friction pair of Al2O3-ZrO2 coating and steel plate demonstrated the highest wear resistance under dry sliding conditions. The increase in the wear resistance of the Al2O3-graphite and Al2O3-ZrO2 coatings is due to the formation of tribofilm in the sliding contact zone.

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.

Splat Shape and Pore Size Distribution in Plasma Sprayed Alumina Coating at Various In-flight Particle Conditions

Splat Shape and Pore Size Distribution in Plasma Sprayed Alumina Coating at Various In-flight Particle Conditions

Evolution of alumina phase structure in thermal plasma processing

Alumina (Al2O3) remains one the most important engineering ceramic for industrial applications. In addition to the α phase, transition alumina phases have interesting characteristics. Controlling the obtained phase structure from alumina melt requires processes with extreme cooling rates and therefore limits the tailoring capabilities. This study investigates how the cooling rate of pure alumina affects its microstructural properties and phase structure in plasma-based processing. The paper reports phase changes in micron sized granulated alumina particles in high-temperature plasma spheroidization and compares the results to plasma sprayed alumina coatings. Both plasma processes involve melting of the material followed by subsequent rapid cooling. Direct comparison on the alumina phase transitions is obtained for the two methodically distinct processing routes, creating unique microstructures due to difference in their cooling rates.

Study of the Mechanical Properties and Thermal Control Performance of Plasma-Sprayed Alumina Coating on Aluminum Alloy Surface

Thermal control coating is an important means of ensuring that a spacecraft remains operational at high temperatures. Due to limitations regarding preparation technology and material properties, the mechanical properties of the conventional thermal control coatings still need to be improved. To solve this problem, nanostructured alumina coatings (NCs) and conventional alumina coatings (CCs) were prepared using plasma-spraying technology. The microscopic morphology, phase structure, hardness, and thermal control properties (solar absorptance (αs) and emissivity (ε)) of the nanostructured alumina coatings were investigated and compared with those of conventional alumina coatings. The results show that the NC has a higher hardness value (1168.8 HV) and that its reflectivity exceeds 75% in the wavelength range of 446–1586 nm, while a high degree of emissivity of 0.863–0.87 is still maintained at 300–393 K. Furthermore, the results show that these highly reflective properties are related to the phase composition and internal micromorphology of the NC, whereby the solar absorption of the coating is reduced due to the increase in the alpha phase content (21.4%), the high porosity (5.21%) and the nanoparticles favoring the internal scattering. All these properties can improve the performance of this CC coating with low solar absorptance (αs) and high emissivity (ε).

Metastable Phase Formation, Microstructure, and Dielectric Properties in Plasma-Sprayed Alumina Ceramic Coatings

The need for new solutions for electrical insulation is growing due to the increased electrification in numerous industrial sectors, opening the door for innovation. Plasma spraying is a fast and efficient way to deposit various ceramics as electrical insulators, which are used in conditions where polymers are not suitable. Alumina (Al2O3) is among the most employed ceramics in the coating industry since it exhibits good dielectric properties, high hardness, and high melting point, while still being cost-effective. Various parameters (e.g., feedstock type, spray distance, plasma power) significantly influence the resulting coating in terms of microstructure, porosity, and metastable phase formation. Consequently, these parameters need to be investigated to estimate the impact on the dielectric properties of plasma-sprayed alumina coatings. In this work, alumina coatings with different spray distances have been prepared via atmospheric plasma spray (APS) on copper substrates. The microstructure, porosity, and corresponding phase formation have been analyzed with optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Moreover, we present an in-depth analysis of the fundamental dielectric properties e.g., direct current (DC) resistance, breakdown strength, dielectric loss tangent, and permittivity. Our results show that decreasing spray distance reduces the resistivity from 6.31 × 109Ωm (130 mm) to 6.33 × 108Ωm (70 mm), while at the same time enhances the formation of the metastable δ-Al2O3 phase. Furthermore, space charge polarization is determined as the main polarization mechanism at low frequencies.

Effect of Sealing on the Corrosion Resistance of Plasma-Sprayed Alumina Coatings

Effect of Sealing on the Corrosion Resistance of Plasma-Sprayed Alumina Coatings

Evaluation of Thermal Cycling Performance of Plasma Sprayed Alumina Coating on Inconel 600 with Different Bond Coats

Evaluation of Thermal Cycling Performance of Plasma Sprayed Alumina Coating on Inconel 600 with Different Bond Coats

Incorporation of graphene nano platelets in suspension plasma sprayed alumina coatings for improved tribological properties

Graphene possesses high fracture toughness and excellent lubrication properties, which can be exploited to enhance tribological performance of coating systems utilized to combat wear. In this work, suspension plasma spray (SPS) process was employed to deposit a composite, graphene nano-platelets (GNP) incorporated alumina coating. For comparison, monolithic alumina was also deposited utilizing identical spray conditions. The as-deposited coatings were characterized in detail for their microstructure, porosity content, hardness, fracture toughness and phase composition. Raman analysis of the as-deposited composite coating confirmed retention of GNP. The composite coating also showed good microstructural integrity, comparable porosity, higher fracture toughness and similar alumina phase composition as the monolithic alumina coating. The as-deposited coatings were subjected to dry sliding wear tests. The GNP incorporated composite coating showed lower CoF and lower specific wear rate than the pure alumina coating. Additionally, the counter surface also showed a lower wear rate in case of the composite coating. Post-wear analysis performed by SEM/EDS showed differences in the coating wear track and in the ball wear track of monolithic and composite coatings. Furthermore, Raman analysis in the wear track of composite coating confirmed the presence of GNP. The micro-indentation and wear test results indicate that the presence of GNP in the composite coating aided in improving fracture toughness, lowering CoF and specific wear rate compared to the monolithic coating. Results from this work demonstrated retention of GNP in an SPS processed coating, which can be further exploited to design superior wear-resistant coatings.

Elaboration and characterisation of plasma sprayed alumina coatings on nickel with nickel oxide interlayer

A process implying the pre-oxidation of nickel substrates was proposed alternatively to the usual Atmospheric Plasma Spraying (APS) of alumina on sandblasted substrates. This process comprised two steps, the substrate pre-oxidation (instead of the sandblasting of the substrate) followed by APS. The pre-oxidation formed a thin layer of the NiO oxide, 0.7–3.5 μm thick, which entirely covered the nickel substrate. During APS, the deposit did not react with NiO due to the very high cooling rate of the alumina splats at the surface of the substrates. The characterisation of the interfacial zone showed the crystallographic continuity of the different lattices, from the nickel substrate (f.c.c.) to the NiO interlayer (f.c.c.) and to the monoclinic γ-Al2O3 coating. The NiO interlayer played the role of a buffer zone that adapted the crystal lattices of the three phases. The adhesion of the coatings was determined according to the ASTM C633 test modified by reducing the contact area substrate/coating, in order to quantify the adhesion of strongly-linked coatings. The coating adhesion reached 105 MPa for the pre-oxidized samples when NiO was 1.8 μm thick, instead of 48.5 MPa for the sandblasted samples and only 8 MPa for the simply polished nickel.

Improvement in Tribological Properties of Plasma-Sprayed Alumina Coating upon Carbon Nanotube Reinforcement

Crushed alumina powder heterocoagulated with 1 or 2 wt.% carbon nanotube (CNT) was used to produce plasma-sprayed coatings with improved tribological properties. Heterocoagulation is a colloidal processing technique, and it was used to produce a homogenous mixture of CNT in alumina. These coatings were compared with a pristine alumina coating using a pin-on-disk setup under various speed–load conditions. A 6-mm-diameter WC–Co ball was used as counterbody. The wear resistance of the coatings increased in the following order: pristine alumina, alumina with 1 wt.% CNT, and alumina with 2 wt.% CNT. Wear progressed by low cycle fatigue, microfracture, and finally pulverization of the already dislodged wear debris on repeated rubbing by the counterbody. The higher wear resistance of the reinforced coatings is attributed to their higher hardness and fracture toughness. The pristine alumina coating was found to crumble owing to diffuse intergranular fracture beneath the surface during rubbing under higher speed and load. Upon crumbling, the pulverized debris assumed a paste-like form and was ejected at the top under the compressive action exerted by the counterbody at its leading edge. The high wear of the pristine alumina coating is attributed to this effect.

Porosity Characterization and Its Effect on Thermal Properties of APS-Sprayed Alumina Coatings

In the thermal spraying process, the porosity of ceramic coatings contributes directly to the efficiency of the thermal insulation. The size, shape, and distribution of the pores determine the level of both thermal and sintering resistance. In this work, three different atmospheric plasma sprayed (APS) alumina coatings were fabricated with the same spraying parameters using alumina powders with fine, medium, and coarse particle size. The microstructure of the obtained coatings was analyzed regarding the obtained total porosity, pore size, and pore shape. It was found that it is expedient to divide the pore size range into fine, medium, and large sizes. The shape was characterized with regard to the circularity aspect. In this way, all types of cracks can be considered as oblate pores and were included in the calculation of the total porosity. In the case of using fine feedstock powder, the densest coatings were produced among all coatings, and the fraction of fine pores and cracks are thereby substantially higher. However, the total porosity increases with increasing feedstock powder size. A connection was also made between thermal insulation and porosity fraction which includes fine pores and cracks.

Efficacy of graphene nanoplatelets on splat morphology and microstructure of plasma sprayed alumina coatings

Splat morphology and its microstructure are of significance to affect the comprehensive properties of plasma sprayed ceramic coatings. This paper studies the effect of graphene nanoplatelets (GNPs) on the splat morphology and microstructure evolution of plasma sprayed alumina coatings. With an increasing GNP content, splat morphology became more disk-shaped and the average diameter of the splat decreased from 120 ± 28 μm for pure Al2O3 to 83 ± 17 μm for 1.0 wt% GNP/Al2O3. Simulated splat morphology presented a good agreement with the experimental observations. GNP agglomeration at the periphery of a droplet accelerate the formation of solid ring resulting in a decrease in splat diameter, and homogeneously distributed GNPs inside a droplet is expected to simultaneously promote rapid solidification of inward melt following the formation of a solid ring, which suppresses splat fragment. Meanwhile, columnar grain refinement and GNP pull-out are responsible for the composite splats free of microcracks.

DEM model for simulation of crack propagation in plasma-sprayed alumina coatings

In this work, a Discrete Element Method (DEM)-based numerical model has been developed in C++ program, which enables to simulate the crack propagation of plasma-sprayed Al2O3 coatings thus enabling to predict their failure. DEM-based approach is combined with the image analysis procedure applied on two-dimensional (2D) cross-section images of as-sprayed coatings. This procedure allows to build up the model of the coating microstructure taking into account defects which can be regarded as pores and cracks. Pores are then discriminated from cracks and these latter are in turn categorized into two types intra- and inter-splat cracks which have been considered in the simulations of crack propagation. Results indicate that the Young's modulus of as-sprayed coatings depends strongly on the geometric features of the porosity, namely the shape and the distribution. The simulation results on crack propagation exhibit that the generation of cracks which begin inside the splat due to the intra-splat cracks yield large cracks which propagate vertically until to reach the surface of the coating, while inter-splat cracks located at the interface between splats evolve by creating a bifurcation through an interlinking mechanism with other cracks.

Effect of temperature and humidity on dielectric properties of thermally sprayed alumina coatings

Breakdown strength, DC resistivity, permittivity and loss of thermally sprayed alumina coatings were studied at various temperatures and relative humidities. The studied coatings were sprayed by utilizing three different spray techniques: flame, high-velocity oxygen fuel (HVOF) and plasma spraying. Breakdown behavior of HVOF sprayed alumina were studied up to very high temperatures (800 °C). At 20–180°C, no significant trend could be seen in the breakdown strength of HVOF and plasma sprayed alumina coatings. The breakdown strength of alumina coatings decreased gradually from 300 to 800 °C reaching a value which was only 14% of the breakdown strength measured at 20 °C/RH 20%. Increasing humidity (from 20 to 90%) decreased the DC resistivity of the alumina coatings five orders of magnitude. Correspondingly, permittivity and losses increased with the humidity; in most cases with a notable contribution due to DC conduction. The material behavior may be linked to the microstructure of coatings consisting of amorphous and crystalline regions with interfaces in between. Moreover, the alumina coatings exhibited notable amount of highly hygroscopic γ-phase which also affected the moisture sensitivity of the coatings.

Grinding of ceramics – sintered ceramics versus ceramic coatings

This paper reports the study of grinding characteristics of sintered alumina ceramic and plasma sprayed alumina coatings with respect to grinding forces, specific grinding energy, force ratio and surface integrity at high wheel speed and moderate downfeed. The ground surface of sintered ceramic showed some signatures of plastic deformation along with micro-brittle fracture whereas, in plasma sprayed ceramic coatings, micro-brittle fracture was the predominant mode of material removal under the same grinding condition. This is attributed to the comparable value of critical depth (dc) in ceramics and very low value of dc in ceramic coatings as compared to maximum uncut chip thickness. In sintered alumina, the specific tangential and normal forces are 0.10 and 0.30 N/mm respectively. On the other hand, the forces observed in plasma sprayed alumina coatings were low, and the maximum forces recorded were 0.05 and 0.23 N/mm respectively. SEM images of chips showed micro-brittle fracture as the predominant mode of material removal. Average surface roughness was found to be low in sintered alumina as compared to that in plasma sprayed coatings. Sintered ceramic was free from residual stress whereas in ceramic coatings, tensile residual stresses were observed.

Predicting the Deposition Efficiency of Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy by Response Surface Methodology

Plasma sprayed ceramic coatings are successfully employed in many industrial applications, where high wear and corrosion resistance with thermal insulation are needed. Plasma spray parameters such as power, stand-off distance and powder feed rate have significant influence on coating characteristics like deposition efficiency. This paper presents the use of statistical techniques specially response surface methodology (RSM), analysis of variance, and regression analysis to develop empirical relationships to predict deposition efficiency of plasma sprayed alumina coatings on AZ31B magnesium alloy. The developed empirical relationships can be efficiently used to predict deposition efficiency of plasma sprayed alumina coatings at 95% confidence level. Response graphs and contour plots were constructed to identify the optimum plasma spray parameters to attain maximum deposition efficiency in alumina coatings. Further, correlating the spray parameters with coating properties permits the identification of characteristics regime to achieve desired quality of coatings.

Structure and electrical insulation characteristics of plasma-sprayed alumina coatings under pressure

In this work, alumina coatings were fabricated on 316LN austenitic stainless steel by a plasma spray technique. The pressure dependence of the surface electrical resistivity of alumina coatings was investigated in detail. A combination of scanning electron microscopy and X-ray diffraction was employed to understand the microstructure and properties of the as-sprayed alumina coatings. The coatings can endure high pressures under a practical working environment. The surface electrical resistivity of the alumina coatings decreases continuously with an increase in pressure to 250MPa. Interestingly, the surface resistivity is still greater than 107Ω·mm for 250MPa, demonstrating that the coatings have good electrical insulation properties and can be fully utilized in the magnet support of ITER.

Development of suspension plasma sprayed alumina coatings with high enthalpy plasma torch

Deposition of aluminium oxide (Al2O3) coatings from liquid feedstocks using a high enthalpy hybrid water-stabilized plasma torch was investigated. The entire process included the optimization of spraying parameters using a commercial ready-to-spray suspension, optimization and deposition of custom-made suspensions and one solution and evaluation of mechanical properties of the most durable coatings. Acquired coating microstructures varied from porous with typical columnar morphology to well-sintered dense structures with low porosity. The microstructural features were clearly linked to the feedstock formulation. The most durable coating with the highest density microstructure prepared from ethanol-based suspension showed outstanding mechanical properties, e.g. high values of hardness (up to 1211HV0.3), tensile adhesion strength (up to 51MPa) and wear resistance (material removal rate as low as 1.94·10−5mm3·N−1·m−1 as measured by pin-on-disc test). Moreover, deposition from a water-based solution of aluminium nitrate resulted in deposition of highly porous coating composed predominantly of stable α-alumina phase.

Aluminum phosphate sealing to improve insulation resistance of plasma-sprayed alumina coating

ABSTRACTThe insulation resistance of conventional atmospheric plasma-sprayed alumina coatings with 10–15% porosity is ∼1011 Ω. The presence of pores, lamellae boundaries, and other non-fillings dampens the insulation resistance of the coating. In the present study, aluminum phosphate was used to seal the surface of plasma-sprayed alumina coating and evaluate the effect of sealing on the insulation resistance and its thermal cycling response. Sealing was carried out with three concentrations of sealant (P/Al molar ratio of 3, 10, and 15). Characterization by X-ray diffraction and scanning electron microscopy revealed the primary sealing phase as aluminum metaphosphate and effective sealing of the pores by the aluminum phosphate phases. Insulation resistance is improved by two orders of magnitude after sealing the coated samples. Sealing with P/Al molar ratio 3 exhibited maximum insulation resistance of ∼1013 Ω at room temperature. Thermal cycling studies between 650°C and 200°C on the sealed samples showed deterioration in thermal cycling life after sealing.