Influence of spraying variables on structure and properties of plasma sprayed alumina coatings
TL;DR: An experimental statistical design study on the plasma spraying of alumina powder has been carried out Coating experiments were conducted, using a Taguchi full factorial L 16 design parametric approach, to study the effect of four key plasma processing variables on the coating quality as mentioned in this paper.
Abstract: An experimental statistical design study on the plasma spraying of alumina powder has been carried out Coating experiments were conducted, using a Taguchi full factorial L 16 design parametric approach, to study the effect of four key plasma processing variables on the coating quality, namely, primary gas flow rate, arc current, powder feed rate, and spray distance Optical microscopy, scanning electron microscopy, XRD, image analysis, and hardness testing were used for characterisation The resulting as sprayed coating characteristics were quantified with respect to roughness, microhardness, porosity, and microstructure Through statistical calculation (analysis of variance), the parameters that have significant influence on the structure and properties of the coatings were identified and their relative importance and contribution ratios to overall variance were studied The Taguchi evaluation employed in the present investigation showed that an improvement in the coating properties could be ach
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TL;DR: In this article, X-ray powder diffraction and Rietveld refinements were explored as potential tools for the determination of the amorphous content in plasma sprayed alumina coatings.
Abstract: Plasma-sprayed alumina coatings mainly consist of γ-alumina with minor amounts of α-alumina due to incorporation of incompletely fused powder. The presence of amorphous materials has also been mentioned in the literature, but not quantified. In this work, X-ray powder diffraction and Rietveld refinements were explored as potential tools for the determination of the amorphous content in plasma sprayed alumina coatings. To cross-check the accuracy of the Rietveld analysis, standard additions of amorphous alumina were performed. Both approaches provided consistent results supporting the validity of the Rietveld method for routine quantification of the amorphous phase in plasma-sprayed alumina. For the as-sprayed coatings studied in the present work, the amount of amorphous alumina was found to be 12.0 ± 0.7 wt.%.
37 citations
TL;DR: In this paper, an attempt is made to produce high-quality alumina (Al2O3) coatings by optimizing the detonation spray process parameters following a (L16-24) factorial design approach.
Abstract: The increasing demands for high-quality coatings has made it inevitable that the surface coating industry would put more effort into precisely controlling the coating process. Statistical design of experiments is an effective method for finding the optimum spray parameters to enhance thermal spray coating properties. In the present investigation, an attempt is made to produce high-quality alumina (Al2O3) coatings by optimizing the detonation spray process parameters following a (L16-24) factorial design approach. The process parameters that were varied include the fuel ratio, carrier gas flow rate, frequency of detonations and spray distance. The coating characteristics were quantified with respect to roughness, hardness and porosity. The performance of the coatings was quantitively evaluated using erosion, abrasion and sliding wear testing. Through statistical analysis of the experimental results, performed by the ANOVA method, the significance of each process parameter together with an optimal variable combination was obtained for the desired coating attributes. Confirmation experiments were conducted to verify the optimal spray parameter combination, which clearly showed the possibility of producing high-quality Al2O3 coatings.
34 citations
TL;DR: In this paper, a 20kW hollow cathode plasma spray torch was manufactured and its performances were examined by spraying alumina coatings on mild steel substrates with different critical plasma spray parameters.
Abstract: A 20 kW hollow cathode plasma spray torch was manufactured and its performances were examined by spraying alumina coatings on mild steel substrates with different critical plasma spray parameters (CPSP: 0.12, 0.24, 0.36, 0.48). CPSP is defined as the ratio between the torch input power and primary plasma forming gas flow rate. The effects of CPSP on the microstructure, porosity, microhardness, surface roughness, sliding wear and erosive wear properties of the coatings were investigated. The electrothermal efficiency and the excitation temperature of the plasma jet were also investigated for different CPSP. The microstructure and phase composition of the alumina coatings were examined using scanning electron microscope and X-ray diffraction (XRD) analysis respectively. XRD analysis revealed that alumina particles during plasma spraying reach sufficient melting state prior to the impact on the substrate with a velocity comparable to that of conventional plasma spraying. The experimental results have shown that the CPSP has significant influence on the microstructure and other properties of the deposited alumina coating. It also showed that the hollow cathode plasma spray torch is a suitable tool to coat the dense ceramic materials with low input power levels.
26 citations
TL;DR: In this article, it was shown that in all cases the titanium present in the solute appears to be merely trapped in the metastable γ phase and a newer type of metastable X phase which has high solubility for titanium, as reported in the literature, did not form in any of the processes studied.
Abstract: AT-13 wt.% type powders, depending on the process used for spraying has shown variable type of resultant phases in the as sprayed conditions. In a detonation gun process it appears that freezing of the molten plume with the same composition as the liquid was possible, in contrast to this in a typical plasma spraying process preferential evaporation and phase separation of the solute from the molten plume was prevalent. These variations to the solidification paths has resulted in a totally metastable γ phase in the former process and in the latter process heterogeneous nucleation of α phase in addition to the normal nucleation of metastable γ phase was observed. In all cases the titanium present in the solute appears to be merely trapped in the metastable γ phase and a newer type of metastable “X” phase which has high solubility for titanium, as reported in the literature, did not form in any of the processes studied. The imposed rapid solidification condition had caused all the phases stabilized to be of nanocrystalline sizes which could be verified by Hall–Williamson analysis. Owing to the solute trapping of titanium the lattice of metastable γ phase was severely micro strained and titanium cation was nearly insoluble in α phase.
22 citations
TL;DR: In this paper, the effect of substrate surface temperature on the crystalline structure of plasma-sprayed Al2O3 splats was studied to clarify the phase selection process during the splat quenching for the development of crystalline structures.
Abstract: The effect of substrate surface temperature on the crystalline structure of plasma-sprayed Al2O3 splats was studied to clarify the phase selection process during the splat quenching for the development of crystalline structure. Al2O3 splats were deposited by atmospheric plasma spraying on the polished alumina substrate preheated to different temperatures from 573 K to 1173 K. The TEM samples of cross-sectional alumina splats were prepared by focused ion beam assisted with scanning electron microscopy (FIB-SEM). The crystalline structure of splats was examined using high resolution transmission electron microscopy (HR-TEM). Results showed that the alumina splats deposited at the substrate surface temperature higher than 573 K were well bonded to the alumina substrate. Moreover, the crystalline structure changed significantly with substrate temperature. The crystalline structure of alumina splats changed from amorphous phase through meta-stable γ-Al2O3 crystallites to α-Al2O3 crystallites with the increase of the substrate temperature from 573 K to 1173 K. The present results clearly indicate that the crystalline structure formation depends on the surface temperature of the substrate on which droplet impacts. Thus, it is possible to control the splat crystalline structure by controlling substrate surface temperature prior molten droplet impact.
22 citations
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TL;DR: In this paper, the mechanism of formation of plasma-sprayed coatings was examined and related to the microstructure produced, and it was shown that the real area of contact between individual lamellae within the coating and between lamella and substrate is much less than the apparent area because of adsorbed and entrapped gas, oxide films or other contamination.
Abstract: The mechanism of formation of plasma-sprayed coatings was examined and related to the microstructure produced. The evidence suggests that the real area of contact between individual lamellae within the coating and between lamellae and substrate is much less than the apparent area because of adsorbed and entrapped gas, oxide films or other contamination. The measured fracture toughness parameters for cohesive failure of coatings are generally much lower than would be expected for complete wetting of previously solidified material by impinging droplets, reflecting the imperfect contact between lamellae. Similar considerations apply to the lamellae-substrate interface at which the contact angle would generally be greater than for lamellae-lamellae interfaces. The difference between the fracture toughness values for ceramic and metallic coatings and the role of a metallic subcoat under ceramic coatings can be explained in terms of plastic deformation of metallic lamellae. The very high adhesive fracture toughness of Ni Al coatings on steel implies more effective contact rather than inherently stronger bonding between contact points. This may be due to aluminothermic reduction of the oxide film on steel. Improvement of the mechanical properties of plasma-sprayed coatings requires methods for increasing the real area of contact between lamellae and between lamellae and substrate.
367 citations
TL;DR: In this paper, the formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of the metastable to stable forms, and the thermal history of the particles during solidification.
Abstract: The formation of metastable phases in plasma- and flame-prepared alumina particles is examined in terms of the classical nucleation theory, rate of transformation of metastable to stable forms, and the thermal history of the particles during solidification. It is suggested that homogeneous nucleation of the solidification of liquid droplets at considerable undercooling results in the formation ofγ-Al2O3 rather thanα-Al2O3 because of its lower critical free energy for nucleation. The phase finally observed depends upon the thermal history of the particles during evolution of the heat of fusion and upon the kinetics of the transformation of the nucleating phase to the stable phase. This means that the cooling rate of the particles is relatively unimportant and under the conditions existing in flames and plasmas, metastable alumina will be formed on solidification. The metastable form will be retained on cooling particles less than approximately 10 μm diameter, but particles larger than this may transform toα-Al2O3 during the solidification exotherm
278 citations
TL;DR: In this paper, the main characteristics of the plasma spraying process of alumina deposits are described, i.e., the temperature and flow field of the plasmas obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation.
Abstract: In this paper are described the main characteristics of the plasma spraying process of alumina deposits, i.e., the temperature and flow field of the plasma jets obtained with the classical spraying torches, the injection of the particles into the plasma jet, the particle surface temperature and velocities in the plasma (measured for calibrated alumina particles), and the coating generation. The measurements on the alumina particles are compared with the predictions of a mathematical model. The experimental and computed particle velocities are in rather good agreement. However, this is not the case for the particle surface temperature. Possible reasons for the discrepancy are proposed (influence of the carrier gas, thermophoretic forces, and poor penetration of the particles into the plasma core even for an injection velocity twice that of the optimal calculated one, as shown by recent measurements). Finally the correlations between the particle velocities and surface temperature, and the properties of the alumina coating (porosity, crystal structure, mechanical properties) are studied.
69 citations
TL;DR: In this article, the wear and friction behaviours of detonation-gun and plasma-sprayed hard coatings were investigated under dry sliding conditions, and the results indicated that D-gun-sprays had higher hardnesses, densities and wear resistances than the corresponding plasma-Sprayed coatings.
Abstract: The wear and friction behaviours of detonation-gun- (D-gun) and plasma-sprayed hard coatings were investigated under dry sliding conditions The coating materials studied included Cr 2 O 3 , WC-12%Co, WC-20%Co, Al 2 O 3 , TiO 2 , Al 2 O 3 -40%TiO 2 , TiC-20%Ni and Cr 3 C 2 -25%NiCr Sintered WC-6%Co and steel MoCN315M were also investigated as references Scanning electron microscopy analysis was carried out to study the wear mechanisms The results indicated that D-gun-sprayed hard coatings had higher hardnesses, densities and wear resistances than the corresponding plasma-sprayed coatings D-gun-sprayed Cr 2 O 3 showed the highest wear resistance among the materials studied The wear resistance of D-gun-sprayed Cr 2 O 3 was even higher than that of sintered WC-6%Co As a result of this study, the wear-resistant order of D-gun- and plasma-sprayed hard coatings as well as sintered WC-6%Co was given
60 citations
TL;DR: In this paper, a statistical design of experimental study of the D-gun spraying of Al 2 O 3 powder is presented, where the attributes of the coatings are correlated with the changes in operating parameters and their relative importance and contribution ratios to overall variance are calculated.
Abstract: Detonation gun (D-gun) spraying is one of the most promising thermal spray variants for depositing high quality wear resistant coatings. Of all the ceramic materials that can be D-gun sprayed, alumina (Al 2 O 3 ) is the most widely established and these coatings have already gained industrial acceptance for diverse applications. The present study deals with a statistical design of experimental study of the D-gun spraying of Al 2 O 3 powder. Coating experiments were conducted, using a Taguchi-full factorial (L 16 ) design parametric study, to optimize the D-gun spray process parameters. Four selected important spraying parameters were considered in their upper and lower levels of the predefined range according to the test matrix, in order to display the range of processing conditions and their effect on the coating quality. Optical microscopy, scanning electron microscopy, X-ray diffraction, image analysis and hardness testing was used for characterization. Coating qualities are discussed with respect to surface roughness, hardness, porosity and microstructure. The attributes of the coatings are correlated with the changes in operating parameters and their relative importance and contribution ratios to overall variance are calculated.
55 citations