Plasma Sprayed Alumina Coatings Research Articles

Prediction and optimization of process variables to maximize the Young's modulus of plasma sprayed alumina coatings on AZ31B magnesium alloy

Abstract Like other manufacturing techniques, plasma spraying has also a non-linear behavior because of the contribution of many coating variables. This characteristic results in finding optimal factor combination difficult. Subsequently, the issue can be solved through effective and strategic statistical procedures integrated with systematic experimental data. Plasma spray parameters such as power, stand-off distance and powder feed rate have significant influence on coating characteristics like Young's modulus. This paper presents the use of statistical techniques in specifically response surface methodology (RSM), analysis of variance, and regression analysis to develop empirical relationship to predict Young's modulus of plasma-sprayed alumina coatings. The developed empirical relationships can be effectively used to predict Young's modulus 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 Young's modulus in alumina coatings. A linear regression relationship was established between porosity and Young's modulus of the alumina coatings.

Comparison of Artificial Neural Networks (ANN) and Response Surface Methodology (RSM) Modeling Approaches in Predicting the Deposition Efficiency of Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy

Comparison of Artificial Neural Networks (ANN) and Response Surface Methodology (RSM) Modeling Approaches in Predicting the Deposition Efficiency of Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy

Phase stabilization of plasma-sprayed alumina coatings by spraying mechanically blended alumina–chromia powders

ABSTRACTA study of phase stabilization is performed on atmospheric plasma-sprayed alumina–chromia coating applied on low-carbon steel substrates. Alumina–chromia mixture with varying amounts of chromia content (1 wt.% to 6 wt.%) is used for this purpose. X-ray diffraction (XRD) analysis is carried out to investigate the different phase compositions of the coatings. Quantification of the different phases present in the coatings is performed by subsequent use of Rietveld refinement method. Surface morphology, microhardness, and wear behavior of the different coatings are also observed. Rietveld analyses performed on coatings ensure the stabilization of metastable phases present in the alumina coatings by chromia addition. Significant increase of the α -alumina and (AlxCr1-x)2O3 content is observed in alumina coatings with 4 wt.% chromia content. This is supported by the improved hardness and wear-resistant properties of the alumina coatings containing 4 wt.% chromia. The minimum surface roughness of the coating is also observed for alumina coatings with 4 wt.% chromia content among all the alumina coatings with different chromia content. The formation of alumina–chromia solid solution and solidification of α-alumina are found to be the reasons behind the enhancement in mechanical properties of the coating.

Measuring mechanical properties of plasma-sprayed alumina coatings by nanoindentation technique

AISI 1020 steel substrate is coated with alumina as feedstock material using plasma spraying process in order to correlate the microstructural features with mechanical properties of coating. The present work focuses on the effects of microstructural inhomogeneity on mechanical properties of alumina coating through nanoindentation technique. Young’s modulus and hardness of the alumina coating are analytically evaluated. Indentation stress–strain curves are generated from the experimentally obtained load–displacement curves to characterise the mechanical properties of the coating. The results have shown large variation in hardness and Young’s modulus of alumina due to microstructural inhomogeneity of the coating.

Microstructural and Corrosion Properties of Atmospheric Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy Under Sodium Chloride Environment

AbstractThe corrosion deterioration process of plasma sprayed alumina coatings on AZ31B magnesium alloy was investigated using immersion corrosion test in NaCl solution at different chloride ion concentrations, pH value and immersion time. The experiments were conducted based on a three factor, five level, central composite rotatable design matrix with full replications technique. Response surface methodology was used to develop the relationship. The developed relationship can be effectively used to predict the corrosion rate of plasma sprayed alumina coatings on AZ31B magnesium alloy at 95 % confidence level. The results showed that the corrosion deterioration of alumina coated magnesium alloy in NaCl solutions was significantly influenced by chloride ion concentration and pH value. The alumina coatings were found to be highly susceptible to localized damage, and could not provide an effective corrosion protection to Mg alloy substrate in solutions containing acidic environments (pH3), higher chloride concentrations and immersion time.

Effect of pH, Chloride Ion Concentration and Immersion Time on the Microstructural and Corrosion Properties of Atmospheric Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy Under Immersion Environment

Effect of pH, Chloride Ion Concentration and Immersion Time on the Microstructural and Corrosion Properties of Atmospheric Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy Under Immersion Environment

Developing Empirical Relationships to Predict Porosity and Microhardness of Atmospheric Plasma-Sprayed Alumina Coatings on AZ31B Magnesium Alloy

AbstractPlasma-sprayed alumina coatings find wide-ranging applications by virtue of their inherent wear and corrosion resistance and attractive dielectric and diffusion barrier properties. In this work, empirical relationships were developed to predict the porosity and microhardness of alumina coatings by incorporating independently controllable atmospheric plasma-sprayed operational parameters (input power, stand-off distance and powder feed rate) using response surface methodology (RSM). A central composite rotatable design with three factors and five levels was chosen to minimize the number of experimental conditions. Within the scope of the design space, the input power and the stand-off distance appeared to be the most significant two parameters affecting the responses among the three investigated process parameters. A linear regression relationship was also established between porosity and microhardness of the alumina coatings.

Predicting the immersion corrosion behaviour in NaCl solution of atmospheric plasma sprayed alumina coatings on AZ31B magnesium alloy

The corrosion deterioration process of uncoated and plasma sprayed alumina coated AZ31B magnesium alloy, was investigated using immersion technique. The aim of the present work is to compare the corrosion behaviour of uncoated substrate and alumina plasma spray coatings on AZ31B magnesium alloy in NaCl environment at different pH values, chloride ion concentration and immersion time. Empirical relationship was established to predict the effect of immersion test parameters on corrosion rate. The corrosion products were examined by XRD, SEM/EDAX and X-ray mapping techniques. The results of immersion test in NaCl solution showed that the alumina coating exhibited a much better corrosion resistance than that of the bare Mg alloy.

Corrosion performance of atmospheric plasma sprayed alumina coatings on AZ31B magnesium alloy under immersion environment

Plasma sprayed ceramic coatings are successfully used in many industrial applications, where high wear and corrosion resistance with thermal insulation are required. The alumina powders were plasma sprayed on AZ31B magnesium alloy with three different plasma spraying parameters. In the present work, the influence of plasma spray parameters on the corrosion behavior of the coatings was investigated. The corrosion behavior of the coated samples was evaluated by immersion corrosion test in 3.5 wt% NaCl solution. Empirical relationship was established to predict the corrosion rate of plasma sprayed alumina coatings by incorporating process parameters. The experiments were conducted based on a three factor, five-level, central composite rotatable design matrix. The developed relationship can be effectively used to predict the corrosion rate of alumina coatings at 95% confidence level. The results indicate that the input power has the greatest influence on corrosion rate, followed by stand-off distance and powder feed rate.

Influence of chloride ion concentration on immersion corrosion behaviour of plasma sprayed alumina coatings on AZ31B magnesium alloy

Abstract Corrosion attack of aluminium and magnesium based alloys is a major issue worldwide. The corrosion degradation of an uncoated and atmospheric plasma sprayed alumina (APS) coatings on AZ31B magnesium alloy was investigated using immersion corrosion test in NaCl solutions of different chloride ion concentrations viz., 0.01 M, 0.2 M, 0.6 M and 1 M. The corroded surface was characterized by an optical microscope and X-ray diffraction. The results showed that the corrosion deterioration of uncoated and coated samples were significantly influenced by chloride ion concentration. The uncoated magnesium and alumina coatings were found to offer a superior corrosion resistance in lower chloride ion concentration NaCl solutions (0.01 M and 0.2 M NaCl). On the other hand the coatings and Mg alloy substrate were found to be highly susceptible to localized damage, and could not provide an effective corrosion protection in solutions containing higher chloride concentrations (0.6 M and 1 M). It was found that the corrosion resistance of the ceramic coatings and base metal gets deteriorated with the increase in the chloride concentrations.

Influence of Critical Parameters on the Properties of Plasma Sprayed Coatings Prepared from Spray Dried Alumina Powder

Plasma sprayable α-alumina powder was prepared by spray drying process. The powder was characterized by field emission scanning electron microscopy (FESEM), powder X-ray diffractometry, particle size analysis and flowability measurement. In the present work, the influences of three different critical plasma spray parameter (CPSP) values (675, 825 and 937.5 W/nl.pm) on the crystallographic forms, microstructure, microhardness, surface roughness, wear and corrosion resistance of the plasma sprayed alumina coatings were investigated. The coatings were characterized by X-ray diffractometry and the analysis revealed the presence of some traces of γ-alumina along with α-alumina for the coating obtained at higher CPSP. The surface roughness, microhardness, corrosion and wear resistance of the coatings were evaluated. Plasma sprayed alumina coating obtained at the highest CPSP exhibited improved wear and corrosion resistance compared to the other two coatings which is attributed to the dense nature of the coating.

Establishing empirical relationships to predict porosity level and corrosion rate of atmospheric plasma-sprayed alumina coatings on AZ31B magnesium alloy

Plasma sprayed ceramic coatings are successfully used in many industrial applications, where high wear and corrosion resistance with thermal insulation are required. In this work, empirical relationships were developed to predict the porosity and corrosion rate of alumina coatings by incorporating independently controllable atmospheric plasma spray operational parameters (input power, stand-off distance and powder feed rate) using response surface methodology (RSM). A central composite rotatable design with three factors and five levels was chosen to minimize the number of experimental conditions. Within the scope of the design space, the input power and the stand-off distance appeared to be the most significant two parameters affecting the responses among the three investigated process parameters. A linear regression relationship was also established between porosity and corrosion rate of the alumina coatings. Further, sensitivity analysis was carried out and compared with the relative impact of three process parameters on porosity level and corrosion rate to verify the measurement errors on the values of the uncertainty in estimated parameters.

Influence of pH, chloride ion concentration and spraying time on the corrosion behaviour of atmospheric plasma sprayed alumina coatings

The corrosion behaviour of the plasma sprayed alumina coatings was evaluated by conducting salt fog test in NaCl solution at different chloride ion concentrations, pH value and spraying time. Empirical relationship was developed to predict the corrosion rate of alumina coatings by incorporating the corrosion process parameters. The corroded surface was characterised by scanning electron microscopy (SEM) and X-ray diffraction. The results showed that the corrosion deterioration of alumina coating was significantly influenced by chloride ion concentration and pH value. The alumina coatings were found to be highly susceptible to localised damage. It could not provide an effective corrosion protection to Mg alloy substrate in solutions containing acidic environments (pH 3), higher chloride concentrations and spraying time.

Prediction of Porosity Level and Corrosion Rate of Atmospheric Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy

Prediction of Porosity Level and Corrosion Rate of Atmospheric Plasma Sprayed Alumina Coatings on AZ31B Magnesium Alloy

Characterization of Nonmelted Particles and Molten Splats in Plasma-Sprayed Al2O3 Coatings by a Combination of Scanning Electron Microscopy, X-ray Diffraction Analysis, and Confocal Raman Analysis

Plasma-sprayed ceramic coatings are deposited by flattened splats together with some nonmelted particles. In this study, both nonmelted particles and molten splats in plasma-sprayed alumina coatings were examined by a combination of scanning electron microscopy, x-ray diffraction analysis, and confocal Raman analysis to aim at the comprehensive understanding and quantitative evaluation of coating microstructure. Results showed that the nonmelted particles can be clearly identified from the polished cross section due to the raised pattern morphology (with convexes) resulting from the higher hardness of the nonmelted particles than molten splats. Raman analysis revealed that there was little α-Al2O3 phase in the flattened splat region, which confirmed that α-Al2O3 phase in the coating originated only from nonmelted particles. Moreover, it is proposed that attention should be paid to the weak bonding of the nonmelted particles to surrounding flattened splats during the preparation of samples for the quantitative characterization of coating microstructure.

Effect of critical plasma spray parameters on the microstructure, microhardness and wear and corrosion resistance of plasma sprayed alumina coatings

Plasma sprayed ceramic coatings are successfully used in many industrial applications, where high wear and corrosion resistance with thermal insulation are required. Plasma sprayable alumina powders with γ- and α-forms have been prepared by chemical precipitation route by varying the calcination temperatures of the powders and without the spray drying step. The powders are characterized by scanning electron microscopy, powder X-ray diffractometry, particle size analysis and flowability. The alumina powders are plasma sprayed at three different critical plasma spray parameters (CPSP; 675, 825 and 937.5W/nLpm). In the present work, the influence of CPSP values on the crystallographic forms of alumina, microstructure, microhardness, surface roughness and wear behavior of the coatings is investigated. Plasma sprayed alumina coating with a CPSP of 825 shows higher wear and corrosion resistance compared to other coatings. The coatings are subjected to thermal cycling at 1100°C and coatings plasma sprayed at lower and moderate CPSPs are intact even after 700 thermal cycles. Also, the effect of starting phases (α and γ-alumina) on the preferential orientation of [113] of the plasma sprayed coating is studied as a function of CPSP.

Mechanical properties of conventional and nanostructured plasma sprayed alumina coatings

The hardness and the elastic modulus measured by microindentation of three different types of plasma sprayed alumina coatings have been compared. Usually, such coatings present porosity and heterogeneity which affect the measurement of the mechanical properties. To take such effects into account along with the indentation size effect which is relevant to all hardness studies, the Proportional Specimen Resistance model is applied. The three alumina coatings show closely similar mechanical properties at indentation loads exceeding 1 N, i.e., macrohardness around 5.7GPa, indentation size effect parameter around 5.5MPamm and elastic modulus around 160GPa. For loads below 1N, the extrapolated values of the macrohardness of crushed and agglomerated alumina coatings increased to 8.5GPa, while the indentation size effect parameter has the same value, and the elastic modulus increased to 320GPa. However, no significant change in the measured values of hardness and the elastic modulus of the nanostructured alumina coating has been observed. This result is attributed to porosity and the bimodal microstructure of the nanostructured coating where a semimolten phase coexists along with the fully molten phases.

Enhancement of scattering and reflectance properties of plasma-sprayed alumina coatings by controlling the porosity

The plasma-spraying process generates materials with typical, porous and complex, microstructures. Inspired by dielectric multilayer mirrors (DMMs), thermal sprayed media may be used in the field of optics, particularly for making scattering and reflecting coatings suitable for a large range of wavelengths. In fact, pores inside plasma sprayed matrix create numerous optical index discontinuities, similarly to the gaps created in DMMs, in order to obtain high reflectivity.The porosity of coatings microstructure can be customized by selection of plasma sprayed process parameters. This study aimed to optimize scattering and reflectance properties in porous alumina by the control of spray parameters resulting in the optimized porosity. A self-supporting bi-layer with a diffuse reflectance over 90% over a large band of wavelengths was obtained.The first layer (micro-structured), which is thick enough to support the free standing, was prepared by atmospheric plasma spraying (APS). The second layer (nanostructured) was manufactured by suspension plasma spraying (SPS) over the first layer in order to enhance the reflectance at short wavelengths.

Behaviour of alumina-coated 304L steel in a Waste-to-Energy plant

Pieces of 304L stainless steel, alumina-coated by plasma spraying, were placed during one week at about 1170K in the furnace of the Waste-to-Energy plant of Limoges, France. The resistance against corrosion was only slightly improved compared to non-coated alloy. The origin of this unsatisfactory result lies in the cracking of the coating that occurred when the pieces were introduced in the furnace, due to the great difference in the Coefficient of Thermal Expansion (CTE) of alumina and 304L steel. However, the plasma-sprayed alumina coatings themselves appear as impervious enough to provide an efficient protection of this alloy against the very corrosive atmosphere of the incinerator.

Electrochemical Characterization of Plasma Sprayed Alumina Coatings

Open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) were employed to characterize the corrosion behavior of plasma-sprayed alumina-coated mild steel in 3.5 wt% NaCl solu-tion. Alumina-coated steel showed higher OCP and lower corrosion current (icorr.) compared with the steel substrate. However, localized corrosion probably occurs at the coat/steel interface when immersed in the corrosive media. The reason for that is the penetration of corrosive solution into the steel surface through the pores of accumulated alumina layers. The corrosion products (mainly iron oxides) accumulate inside the pores and on the coating surface. The presence of iron oxide slightly improved the corrosion resistance.