Plasma-sprayed Chromium Oxide Research Articles

다구찌 기법에 의한 Cr2O3 플라즈마 코팅의 공정최적화

In the present study, process optimization for plasma-sprayed chromium oxide coating was performed using Taguchi method and analysis of variance (ANOVA). Chromium oxide coating specimens were fabricated by plasma-spray process according to L9 orthogonal array table. Hardness and porosity for each specimen was measured. The approach of Taguchi design for multi-response optimization problem is based on loss function of the quality characteristics. Multi-response signal-to-noise (MRSN) ratio was calculated for the response variables and the optimum combination level of variables, that is, the spray distance, the argon flow rate, the powder feed rate, and the surface velocity were obtained.

Porosity and Its Significance in Plasma-Sprayed Coatings

Porosity in plasma-sprayed coatings is vital for most engineering applications. Porosity has its merits and demerits depending on the functionality of the coating and the immediate working environment. Consequently, the formation mechanisms and development of porosity have been extensively explored to find out modes of controlling porosity in plasma-sprayed coatings. In this work, a comprehensive review of porosity on plasma-sprayed coatings is established. The formation and development of porosity on plasma-sprayed coatings are governed by set spraying parameters. Optimized set spraying parameters have been used to achieve the most favorable coatings with minimum defects. Even with the optimized set spraying parameters, defects like porosity still occur. Here, we discuss other ways that can be used to control porosity in plasma-sprayed coating with emphasis to atmospheric plasma-sprayed chromium oxide coatings. Techniques like multilayer coatings, nanostructured coatings, doping with rare earth elements, laser surface re-melting and a combination of the above methods have been suggested in adjusting porosity. The influences of porosity on microstructure, properties of plasma-sprayed coatings and the measurement methods of porosity have also been reviewed.

Effect of arc\u2013current and spray distance on elastic modulus and fracture toughness of plasma-sprayed chromium oxide coatings

In this experimental study, chromium oxide powder was sprayed on a low-carbon steel substrate using the atmospheric plasma spray process. The current and standoff distances (SODs) were varied to study their effect on the fracture toughness of the coatings. Theoretically, as the arc current increases, the melting of the ceramic oxide should increase and this in turn should lead to the formation of a dense coating. However, it was observed that if the arc power is too high and because the particle size of the powder is small (approximately 30 μm), the particles tend to fly away from the plasma core. Similarly, an appropriate SOD should provide the particles with more melting time, thus resulting in a dense coating. On the other hand, a larger SOD leads to the solidification of the molten particles before the droplets can reach the substrate. All these effects may lead to substantial variation in the fracture toughness of the coating. The present paper attempts to correlate the plasma spraying parameters and microstructure of the coating with the fracture toughness and other primary coating properties.

Hot Corrosion Studies of Plasma-Sprayed Chromium Oxide Coatings on Boiler Tube Steel at 850 °C in Simulated Boiler Environment

High-temperature corrosion behavior of plasma-sprayed aluminum oxide coatings on ASME-SA213-T-22 boiler tube steel at 850° for 25 cycles has been studied. The Cr2O3 plasma coatings were sprayed at M/S Metallizing Equipment Co. Pvt. Ltd., Jodhpur. The experiments were performed in salt mixture of 60% NaSO4–40% V2O5 in silicon-carbide tube furnace. The experiments were done for 25 cycles, and each cycle consisted of 1 h of exposing the specimen at 850° followed by cooling at room temperature for 20 min. The thermogravimetric technique was used to establish the kinetics of corrosion. Corrosion products were analyzed, and it was found that coated samples show better corrosion resistance behavior than uncoated samples at 850°.

Electrochemical investigation of chromium oxide‐coated Ti‐6Al‐4V and Co‐Cr‐Mo alloy substrates

Hard coatings for articulating surfaces of total joint replacements may improve the overall wear resistance. However, any coating approach must take account of changes in corrosion behavior. This preliminary assessment analyzes the corrosion kinetics, impedance and mechanical-electrochemical stability of 100 μm thick plasma sprayed chromium oxide (Cr₂O₃) coatings on bearing surfaces in comparison to the native alloy oxide films on Co-Cr-Mo and Ti-6Al-6V. Cyclic potentiodynamic polarization, electrochemical impedance spectroscopy, and mechanical abrasion under potentiostatic conditions were performed on coated and substrate surfaces in physiological saline. SEM analysis characterized the coating morphology. The results showed that the corrosion current density values of chromium oxide coatings (0.4-1.2 μA/cm²) were of the same order of magnitude as Ti-6Al-4V alloy. Mechanical abrasion did not increase corrosion rates of chromium oxide coatings but did for uncoated Co-Cr-Mo and Ti-6Al-4V. The impedance response of chromium oxide coatings was very different than Co-Cr-Mo and Ti-6Al-4V native oxides characterized by a defected coating model. More of a frequency-independent purely resistive response was seen in mid-frequency range for the coatings (CPE(coat) : 40-280 nF/cm² (rad/s)(1-α) , α: 0.67-0.83) whereas a more capacitive character is seen for Co-Cr-Mo and Ti-6Al-4V (CPE(ox) around 20 μF/cm² (rad/s)(1-α) , α around 0.9). Pores, interparticle gaps and incomplete fusion typical for thermal spray coatings were present in these oxides which could have influenced corrosion resistance. The coating microstructure could have allowed some fluid penetration. Overall, these coatings appear to have suitable corrosion properties for wear surfaces.

On the Self-Affine Fractal Geometry of Plasma-Sprayed Surfaces

Surface roughness strongly controls essential properties of thermally sprayed wear- and corrosion-resistant coatings including their mechanical adhesion to the substrate, tribological performance, efficient retention of lubricating materials, and also the presence of sufficient carrying surface able to support the wear couple along the line of contact expressed by the Abbott-Firestone curve. The determination of the surface fractal geometry may yield useful information on the topography of plasma-sprayed coatings beyond that provided by a single roughness parameter such as R a or R z. The fractal geometry of atmospheric plasma-sprayed chromium oxide coatings, deposited according to two different statistical experimental design protocols, was assessed through determination of the Hurst exponent H of fractal Brownian motion (fBM), as well as the area-scaled fractal complexity (ASFC) obtained by triangular tessellation (“patchwork” method). Attempts were made to correlate fractality with coating adhesion strength.

Wear resistance of chromium oxide nanostructured coatings

Nanostructured plasma-sprayed chromium oxide ceramic coatings, with a higher wear resistance than coatings obtained using micrometric powder as starting material have been developed. The influence of the starting powder, i.e. Cr 2O 3 fused crushed powders and Cr 2O 3 nanopowders having grains size of 100 nm, has been investigated. The nanopowders are reconstituted into spherical micrometer sized granules by the spray-drying process before plasma spraying. It is shown that due to their specific microstructure the wear resistance of the nanostructured coatings is more than 20 times higher compared to the coatings obtained using micrometric powder.

Correlation between surface roughness of plasma-sprayed chromium oxide coatings and powder grain size distribution: a fractal approach

Attempts were made to correlate the arithmetic mean surface roughness R a and the 10-point height values R z of atmospheric plasma-sprayed (APS) chromium oxide coatings with fractal properties such as the relative area for 2 μm 2 patch size (RAPS), the smooth-rough crossover (SRC) and the area-scaled fractal complexity (ASFC). In particular, the measured surface roughness of the coatings correlates with high statistical probability with the grain size distribution of the selected spray powders. It was found that the SRC value is useful to characterize the low and high cutoff values of the grain size distribution of thermal spray powders. Hence fractal analysis is a potential analytical tool to quantify the geometric complexity of the surface structure of plasma-sprayed coatings as a function of scale of observation.

Microstructure and tribological properties of plasma-sprayed chromium oxide–molybdenum oxide composite coatings

Plasma-sprayed composite coatings of Cr2O3–MoO3 were studied to identify their microstructure and to understand the influence of molybdenum oxide composition in the coatings on their tribological behavior. Cr2O3–MoO3 composite powders and Cr2O3 powders were fabricated using a spray-drying method and plasma-sprayed coatings of these powders were produced to evaluate their tribological performance. Wear tests were conducted with a reciprocating motion at room temperature and 450 °C under dry sliding condition. Measurement of friction coefficient was made in association with the sliding cycle. The physical characteristics of surfaces were investigated by scanning electron microscopy and chemical composition of the coating surfaces was analyzed using a X-ray photoelectron spectrometer and a transmission electron microscope (TEM). The study showed that the micro-hardness of coatings containing MoO3 were higher than that composed of Cr2O3 alone. The result of TEM analysis indicated that chromium/molybdenum was precipitated in the grain and grain boundary. The friction coefficients of the MoO3-added coatings were lower than that without MoO3 addition at both test temperatures. However, the addition of MoO3 in the Cr2O3 coatings did not significantly improve the wear performance of the coatings at both test temperatures. Dispersed smooth films were formed in the worn surface for all coatings. These wear protecting layers, formed by plastic deformation of adhered and compacted debris particles to the surface, strongly influence the friction of the coatings as already observed by the authors with different plasma-sprayed coatings. The chemical composition of these films varied depending on the test temperature.

Microstructural Study of Aluminum Phosphate-Sealed, Plasma-Sprayed Chromium Oxide Coating

Microstructural characterization of aluminum phosphate-sealed, plasma-sprayed chromium oxide coating was carried out in order to study the strengthening mechanisms of the aluminum phosphate sealant in the coating. Characterization was performed using x-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The structure of the sealed coating was lamellar with columnar α-Cr2O3 grains extending through the lamella thickness. Amorphous aluminum phosphate sealant had penetrated into the structural defects of the coating such as cracks, gaps, and pores between the lamellae. The relative composition was 25 at.% aluminum and 75 at.% phosphorus for the sealant in the coating, giving the molar ratio P/Al of 3, which corresponds to that of metaphosphates Al(PO3)3. There is no indication of reaction products from the chemical reactions between the sealant and the coating. Thus, the aluminum phosphate sealing in the chromium oxide coatings can be explained mainly by adhesive binding resulting from the formation of the condensed phosphates with the appropriate adhesive properties to the coating, and not by chemical bonding resulting from the chemical reactions between the sealant and the coating.

Bond Strength of Plasma Sprayed Chromium Oxide Coatings

Plasma spraying is an attractive method to protect the components from severe conditions. Chromium oxide coatings were used to develop the wear resistance because of their good wear properties. One of the problems of plasma sprayed coating is the bond strength. As an effort to improve the bond strength, the Cr 2 O 3 coatings formed on mild steel substrates by the plasma spraying were subjected to annealing and hot press treatment. After each treatment, the bond strengths were compared with the as-sprayed coatings. The bond strength, which was not improved by only annealing, was improved by incorporating the composite layer. Annealed coating after forming the composite layer showed about 14 MPa higher bond strength than that of without the composite layer.

Tribological behaviour of plasma-sprayed chromium oxide coating

A plasma-sprayed chromium oxide coating was studied to gain a better understanding of its tribological behaviour. Both dry and lubricated wear tests were conducted with a reciprocating motion at room temperature and at 450°C for the dry wear tests, and at room temperature and 200°C for the lubricated wear tests. Under dry sliding conditions, dispersed smooth surface films were formed by plastic deformation of compacted debris particles that adhered to the surface and these films strongly influence the friction coefficient and wear rate of the coating. Considerable quantity of CrO 3 was detected at room temperature whereas CrO 2 was detected at 450°C. CrO 2 in the smooth film seems more favorable than CrO 3 in reducing friction. Under lubricated sliding conditions, tribochemical reaction films of different species of carbon–oxygen bond units were formed depending on the test temperature. The surface films appear to be effective in reducing friction and preventing wear. The dominant species with carbon-associated bonds in the film was identified as graphite at both room temperature and high temperature. Among the compounds formed in the film, graphite appears to be most important in determining the tribological performance of the film.

Friction and wear of bearing alloy with plasma sprayed chromium oxide in sliding contact : J.E. Fernandex et al (University of Oviedo, Asturias, Spain)

Friction and wear of bearing alloy with plasma sprayed chromium oxide in sliding contact : J.E. Fernandex et al (University of Oviedo, Asturias, Spain)

Piston ring coatings for high horsepower diesel engines

Thermal-spray-deposited face coatings were developed for a top compression piston ring operating under the high pressures and temperatures of high horsepower diesel engines. Coatings were deposited by plasma spray and high velocity oxygen fuel (HVOF) techniques onto the piston ring faces. The coatings were evaluated by wear and engine tests. Three coatings were deposited by a plasma spray technique: molybdenum/chromium carbide, molybdenum/molybdenum carbide, and chromium oxide. Nickel chromium/chromium carbide powder was deposited by a HVOF technique. On the basis of wear and engine testing, HVOF nickel chromium/chromium carbide, plasma-sprayed chromium oxide and plasma-sprayed molybdenum/chromium carbide were identified to have wear resistances superior to that of today's electroplated chromium and molybdenum carbide coatings. Cylinder liner wear was found to be generally equivalent or lower for the thermal spray coatings when compared with the electroplated chromium.

Evaluation of thermally sprayed coatings under reciprocating lubricated wear conditions

To find an alternate coating to hard electroplated chrome in internal combustion engines, wear tests and metallurgical characterization have been performed on plasma-sprayed chromium oxide, metal-arc-sprayed martensitic stainless steel, and electroplated chromium coatings applied to steel base material. A wear test rig was fabricated that simulated the reciprocating sliding wear under lubrication encountered in internal combustion engines. The chromium oxide coating was found to perform equally well compared to the hard chrome coating that is conventionally used.

Developments in the specular reflection spectroscopy of surfaces using FTIR: Application to plasma-sprayed chromium oxide deposits

The multiple advantages of FTIR spectroscopy result in considerable improvement in surface analysis using specular reflection. This method is applied in the present study of partially-oriented chromium oxide surfaces prepared by the plasma-spray technique. The results indicate that IR reflection spectroscopy can be used to evaluate the effective orientation of polycrystalline surfaces and, hence, their quality. Furthermore, it is suggested that this method can be used to optimize the various parameters involved in the plasma-spray process.

Plasma-sprayed chromium oxide: control of mechanical properties using infrared specular reflection

The infrared reflection spectra of Cr 2O 3 samples prepared by the plasma-spray technique are analyzed using the Kramers-Kronig method. As the degree of orientation of the crystalline surface, and hence its mechanical properties, depends on the deposition conditions, infrared reflection spectroscopy provides a convenient method of controlling the quality of plasma-sprayed deposits.

Wear prevention in molds used to mold boron filled elastomers

An investigation was conducted to identify surface coatings which would improve the wear characteristics of compression molds used to mold contoured parts from elastomers filled with crystalline boron particles. An analysis was made of the mode of wear present on the surfaces of a production mold, followed by selection and modification of a capillary rheometer as the test device. Test specimens were coated by electroplating, electroless plating, plasma spraying, chemical vapor deposition, sputtering and a fused salt process. Testing was conducted under conditions simulating those encountered in the production molding operation. Techniques used in evaluating the results included optical microscopy, scanning electron microscopy, transmission electron microscopy and surface profilometry. The results are expressed as a ratio of volumetric wear of the tested coating to that of unprotected mold steel. Results indicate that titanium diboride (TiB 2) applied by chemical vapor deposition provides wear resistance superior to plasma sprayed aluminum oxide mixed with titanium dioxide, plasma sprayed chromium oxide, electrodeposited chromium, electroless nickel containing synthetic diamond powder, chemical vapor deposited tungsten carbide (W/W 2C), aluminum oxide and aluminum oxide over chromium carbide. Sputtered titanium diboride was also superior to these coatings and to sputtered titanium carbide and boron carbide.