Plasma-sprayed Ceramic Coatings Research Articles

Research and Application Status of Atmospheric Plasma Spray Torch in Internal Feedstock Mode

In comparison with typical external feedstock mode, powders internal injecting method can effectively enhance plasma sprayed ceramic coatings performance and improve powder deposition efficiency. In this feedstock mode, spray powders were injected into the hotter regions of a plasma jet, which had enough heat capacity to melt most of the particles instantly. Currently, several plasma spray guns in internal feedstock mode, such as Axial III and SG100, have been developed and commercialized. In this paper, characteristics of plasma spray guns in internal and external feedstock methods were summarized. And the research status of the plasma spray torch in internal feedstock mode was particularly presented. Finally, the development trend of different feedstock modes adopted by plasma spray gun was discussed.

Corrosion Resistance of Plasma Sprayed AT13 Composite Ceramic Coating on Mg

NiCoCrAlY/AT13 composite coating was prepared by plasma spraying technology on AZ91D substrate and sealed with three different methods. The corrosion resistances of the composite ceramic coatings with and without sealing were evaluated by immersion and electrochemical tests The results show that NiCoCrAlY/AT13 composite ceramic coating represents better corrosion property than single AT13 ceramic coating. The corrosion resistances of NiCoCrAlY/AT13 composite coatings with sealing are superior to that of unsealing coating. Among three sealed coatings, both coatings sealed with paraffin and varnish exhibit better corrosion resistance with corrosion rate of 0.130 g/(m2·h), 0.204 g/(m2·h), and Icorrof 1.754E-7 (A·cm-2), 9.493E-8 (A·cm-2) respectively, while the coating with epoxy resin sealing is relatively the worst one with corrosion rate of 0.744 g/(m2·h) and Icorrof 1.650E-7 (A·cm-2).

Study on Friction and Wear Performance of Plasma-Sprayed Metal Oxide Ceramic Coatings

Paper transferring gripper is a key part of printer. In this work, plasma spray technology was adopted for the coating of 5 different metal oxide ceramic layers on metallic substrate. The friction and wear performance of different plasma-sprayed metal oxide ceramic coating on the gripper surface were comparatively studied by friction and wear test. Results suggested that compared to 100% Al2O3 and 100% Cr2O3 ceramic coatings, Al2O3/Cr2O3 composite coatings had a higher friction coefficient and wear amount. Abrasive wear was the dominant wear mechanism for Al2O3/Cr2O3 composite coatings, while fatigue wear was dominant for 100% Al2O3 and 100%Cr2O3 ceramic coatings.

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.

Improvement of Adhesion and Cohesion in Plasma-Sprayed Ceramic Coatings by Heterogeneous Modification of Nonbonded Lamellar Interface Using High Strength Adhesive Infiltration

The mechanical properties and related performance of thermally sprayed ceramic coatings are degraded by their relatively low adhesion and cohesion resulting from the limited bonding at substrate/splat interface and splat/splat interface. In this study, the influence of high strength adhesive infiltration on the microstructure and erosion performance of plasma-sprayed Al2O3 coatings was investigated to understand the improving mechanism of adhesion and cohesion through heterogeneous modification of nonbonded interfaces. Element distribution maps proved that the adhesive can be infiltrated from the coating surface to the coating/substrate interface through the inter-connected open pores including in-plane nonbonded area and microcracks in splats. Both adhesion and cohesion can be significantly improved by the heterogeneous modification of nonbonded lamellar interfaces of both splat/splat and splat/substrate through adhesive infiltration. The adhesive strength of the coating was increased from several MPa to ~50 MPa after adhesive infiltration. The erosion resistance at a large particle jet angle was improved by a factor of 3 due to the significant improvement of the lamellar cohesion, although the erosion resistance at a small particle jet angle was not significantly influenced.

Chemical densification of plasma sprayed yttria stabilized zirconia (YSZ) coatings for high temperature wear and corrosion resistance

Plasma-sprayed yttria stabilized zirconia (YSZ) ceramic coatings have been widely used as wear- and corrosion-resistant coatings in high temperature applications and an aggressive environment due to their high hardness, wear resistance, heat and chemical resistance, and low thermal conductivity. The highly porous structure of plasma-sprayed ceramic coatings and their poor adhesion to the substrate usually lead to the coating degradation and failure. In this study, a two-layer system consisting of atmospheric plasma-sprayed 8wt.% yttria-stabilized zirconia (8YSZ) and Ni-based alloy coatings was post-treated by means of a novel chemical sealing process at moderate temperatures of 600–800°C. Microstructure characteristics of the YSZ coatings were studied using an electron probe micro-analyzer (EPMA). Results revealed that the ceramic top coat was densified by the precipitated zirconia in the open pores. Therefore, the sealed YSZ coatings exhibit reduced porosity, higher hardness and a better adhesion onto the bond coat. The mechanisms for the sealing process were also proposed.

Plasma‐Sprayed Ceramic Coatings for Osseointegration

Osseointegration of an orthopedic implant involves a cascade of cellular and extracellular biological events that take place at bone/implant interface. The process can be enhanced by coating implants with materials of various compositions, chemistry, topographies. The aim of this article was to review the literatures on development and application of numerous plasma‐sprayed ceramic coatings (including hydroxyapatite, wollastonite, dicalcium silicate, titanium oxide, and zirconia coatings) for establishing and maintaining the osseointegration based on some major biological requirements, such as good cytocompatibility, advantageous cell response, superior tissue response, effective anti‐infection activities.

Suspension and Air Plasma-Sprayed Ceramic Thermal Barrier Coatings with High Infrared Reflectance

Suspension and Air Plasma-Sprayed Ceramic Thermal Barrier Coatings with High Infrared Reflectance

Inelastic constitutive equation of plasma-sprayed ceramic thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) are a very important technology for protecting the hot parts of gas turbines (GTs) from a high-temperature environment. The coating stress generated in the operation of GTs brings cracking and peeling damage to the TBCs. Thus, it is necessary to evaluate precisely such coating stress in a TBC system. We have obtained a stress-strain curve for a freestanding ceramic coat specimen peeled from a TBC coated substrate by conducting the bending test. The test results have revealed that the ceramic coating deforms nonlinearly with the applied loading. In this study, an inelastic constitutive equation for the ceramic thermal barrier coatings deposited by APS is developed. The obtained results are as follows: (1) the micromechanics-based constitutive equation was formulated with micro crack density formed at splat boundary, and (2) it was shown that the numerical results for a nonlinearly deformed beam simulated by the developed constitutive equation agreed with the experimental results obtained by cantilever bending tests.

Process, Microstructure and Properties of Plasma-Sprayed Ceramic Coating on AZ91D Magnesium Alloy

Plasma spraying process was carried out to prepare Al2O3+13%TiO2 ceramic coating on AZ9Subscript text1D magnesium alloy by an orthogonal experiment. The microstructural characteristics and properties of coating were investigated. The results reveal that the bonding strength of optimized Al2O3+13%TiO2 coating reach 19.54MPa. The thermal shock failure of the coating is mainly due to fatigue under thermal cycling stress. The corrosion resistance of magnesium alloy is remarkably improved by plasma-sprayed Al2O3+13%TiO2 coating.

A novel test approach for plasma-sprayed coatings tested simultaneously under CMAS and thermal gradient cycling conditions

Plasma-sprayed ceramic coatings, used as thermal barrier or abradable coatings in high-pressure stages of gas turbines, are exposed to high thermo-mechanical loading due to harsh operating conditions. Under certain conditions they also have to withstand attack by calcium-magnesium-alumino-silicate (CMAS) deposits resulting from the ingestion of siliceous minerals with the intake air.Resistance to this kind of attack becomes more important at higher temperatures, when the melting temperature of the CMAS deposits is exceeded and a penetration into the coating microstructural features (cracks and pores) can take place. During cooling the CMAS solidifies and the coating loses its strain tolerance, which can lead to coating failure. Although the basic principles of failure seem to be understood, a detailed analysis of the mechanisms and the possibilities of avoiding delamination are still lacking, especially because there are as yet no adequate test beds.This paper investigates the possibility of testing such coatings in a burner rig test facility under thermal gradient cycling conditions and at the same time CMAS deposition. This novel and unique test approach promises a coating screening and characterization test under service conditions. The CMAS rig was established because the test conditions simulated here are closer to actual engine conditions, as compared to previous tests with primarily CMAS deposition and subsequent thermal furnace testing.The experimental setup of this new test approach is described and the applicability is confirmed. Furthermore, a first evaluation for plasma-sprayed coatings is presented. A significantly reduced lifetime was found for samples tested with CMAS attack in comparison to tests with water deposition only. The lifetime was also reduced compared to older results without any corrosive media. A decrease of nearly two orders of magnitude was found. A microstructural evaluation of the coatings is presented and the mechanisms and reasons for the very early failure are discussed.

Effect of Sealing Treatment on Corrosion Resistance of Plasma-Sprayed NiCrAl/Cr2O3-8 wt.%TiO2 Coating

Plasma-sprayed ceramic coatings inherently contain pores and micro-cracks which is deleterious when performed in aggressive environment. Various methods were applied to the as-sprayed coatings in order to improve the corrosion resistance. In the investigation of this study, plasma-sprayed NiCrAl/Cr2O3-8 wt.%TiO2 coatings were sealed by epoxy resin and silicone resin, respectively. Coatings were characterized by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), optical microscopy (OM) and x-ray diffraction (XRD). The possible corrosion mechanism was discussed. The results of salt spray test and electrochemical measurements indicated that after the sealing treatment, the porosity of coatings decreased obviously and a compact layer was formed to protect the coating from corrosion. The silicone resin proved to be more effective than epoxy resin in enhancing the corrosion resistance of the coatings used in this research.

Characterization and wear behavior of plasma-sprayed Al2O3 and ZrO25CaO coatings on cast iron substrate

Plasma spraying is one of the methods used for combating wear. Despite of its wide spread industrial use, little is known about the basic friction behavior and mechanism by which such coatings wear. In this work, the abrasive wear resistance of plasma-sprayed ceramic coatings on cast iron substrate has been investigated through pin-on-disc test. It was found that the coefficient of friction and wear affected mainly by splats and porosity, surface roughness, and coating thickness. The coefficient of friction is found to be more significantly affected by load than by other test parameters. This work also includes the characterization of coatings.

Comparison of corrosion resistance between Al2O3 and YSZ coatings against high temperature LiCl-Li2O molten salt

In this study, the high-temperature corrosion resistance of plasma-sprayed ceramic oxide coatings has been evaluated in a LiCl-Li2O molten salt under an oxidizing environment. Al2O3 and YSZ coatings were manufactured by atmospheric plasma spraying onto a Ni alloy substrate. Both the plasma-sprayed Al2O3 and YSZ coatings had a typical splat quenched microstructure which contained various types of defects, including incompletely filled pores, inter-splat pores and intra-splat microcracks. Corrosion resistance was evaluated by the thickness reduction of the coating as a function of the immersion time in the LiCl-Li2O molten salt at a temperature of 650 °C. A linear corrosion kinetic was found for the Al2O3 coating, while no thickness variation with time occurred for the YSZ coating. The ceramic oxide coatings were reacted with LiCl-Li2O molten salt to form a porous reaction layer of LiAl, Li5AlO4 and LiAl5O8 for the Al2O3 coating and a dense reaction layer of non-crystalline phase for the YSZ coating. The reaction products were also formed along the inside coating of the porous channel. The superior corrosion resistance of the YSZ coating was attributed to the formation of a dense protective oxide layer of non-crystalline reaction products on the surface and at the inter-splat pores of the coating.

Elastic and Conductive Properties of Plasma-Sprayed Ceramic Coatings in Relation to Their Microstructure: An Overview

This article focuses on micromechanics-based models that explicitly express the elastic and conductive properties of plasma-sprayed ceramic coatings in terms of relevant microstructural parameters. These parameters reflect, in an integral way, the density and the orientation distribution of microcracks; they apply to strongly oblate pores as well. On the other hand, the porosity parameter usually plays a secondary role. Partial contacts between crack faces—a factor of major importance—are reflected via appropriate reduction of crack densities. The effect of various “irregularities” of crack shapes is discussed. Case studies of YSZ coatings demonstrate how the micromechanics-based modeling can be used and directly interfaced with 2-D image data.

Nano-SiC Modified Plasma Sprayed Ceramic Coatings Prepared by Laser Sintering

This paper describes an investigation of nano-SiC reinforced ceramic coating, which has included NiCrAl and Al2O3+13wt%TiO2 coatings pre-produced by atmosphere plasma spraying, implemented by laser sintering. Commercial NiCrAl powders were plasma sprayed onto 45 steel substrates to produce a bond coating with thickness of ~100μm. The Al2O3-TiO2 based coating with ~500μm thickness was then plasma sprayed on top of the NiCrAl bond coating. With CO2 laser, nano-SiC powders were laser sintered on Al2O3-TiO2 based coatings. The microstructure and chemical composition of the modified based coatings were analyzed by such detection devices as scanning electronic microscope (SEM) and x-ray diffraction (XRD). The results show that the size of SiC grains has no obvious growth. In addition, due to the nanostructured SiC phase and laser remelting, the modified coatings exhibited better abrasion resistance than those unmodified samples.

Advanced thermal spray coatings for tribological applications

Thermal spray coating is one of the most common procedures to improve the tribological properties of materials. Ceramic and cermet coatings, especially those based on oxides (alumina, chromium oxide, etc.) and carbides, are widely used for wear protection. Improvements under investigations are related to the possible use of nanostructured coatings and to the potential application of hybrid techniques. As a possible alternative, solid lubrication is proposed as integration or replacement of the traditional liquid lubrication in the case of severe operative conditions when there is the possibility of a lack of lubricant or when environmental problems can arise from waste disposal. In the present paper, results from experimental activities on different types of abrasion-resistant thermal spray coatings are presented: graded coating systems, specifically designed for titanium-based alloys, obtained by reactive plasma spraying and physical vapour deposition of titanium nitrides; nanostructured WC-Co coatings deposited by high velocity oxy-fuel; plasma-sprayed ceramic coatings based on oxides containing nanophases either from nanostructured precursors or postprecipitated by purposely designed thermal treatments; self-lubricant coatings deposited by plasma spray with the inclusion of graphite.

Optical Coherence Tomography for the Inspection of Plasma-Sprayed Ceramic Coatings

Optical coherence tomography (OCT) is evaluated as a potential technique for microstructure characterization of plasma-sprayed ceramic coatings. OCT combines the principles of low-coherence interferometry and optical heterodyne detection to obtain both a high axial resolution and a high sensitivity to weakly backscattered light. It can be used to accurately locate interfaces where the refractive index changes abruptly within translucent materials. Therefore, OCT should be sensitive to interlamellar pores and splat interfaces within plasma-sprayed ceramic coatings. In the present work, OCT cross-sectional images of thin yttria-stabilized zirconia (YSZ) coatings are considered. The interferogram envelopes forming the collected images are analyzed individually to successfully gather information related to light attenuation inside the coatings. This light attenuation is shown to be related to the density of interfaces within the coatings as well as to the material oxidation state. The envelope analysis also allows the evaluation of the refractive index of the YSZ nontransformable tetragonal phase.

Nonlinear Behavior in Compression and Tension of Thermally Sprayed Ceramic Coatings

Mechanical properties of thermally sprayed coatings, especially of ceramics, are strongly influenced by a high density of mesoscopic defects, microcracks of dimensions between fractions of μm up to tens of μm. The anisotropic linear elastic stress–strain relations are valid only at very low deformations, e.g., |e| < 0.05%, with small values of Young’s moduli due to elastic openings and elastic partial closings of microcracks. At higher deformations, e.g., 0.05% < |e| < 0.4%, the stress–strain relations are strongly nonlinear. Under compressive stresses, elastic closing of microcracks leads to a gradual decrease of the microcrack density and to an increase of Young’s modulus in compression. Under tensile stresses, the microcracks slightly grow by inelastic processes; the microcrack density gradually increases and effective Young’s modulus in tension decreases. A two-parametric equation containing linear and quadratic terms is used to describe the nonlinear stress–strain curves of plasma-sprayed ceramic coatings. The effect of nonlinearity on the bending of beams with coatings and the nonlinear combination of external and residual stresses are discussed. The fracture of coatings at higher tensile stresses due to coalescence of the microcracks is mentioned.

Wear behaviour of thermally sprayed ceramic oxide coatings

The wear resistance of plasma-sprayed ceramic coatings (Al 2O 3, Al 2O 3–13%TiO 2, Cr 2O 3) has been investigated through pin-on-disk and dry sand-steel wheel tests, has been correlated to microstructural and micromechanical characteristics (microhardness, fracture toughness) and has been compared to well-known platings (such as Cr electroplating and electroless Ni) and HVOF-sprayed cermets (WC–17%Co, WC–10%Co–4%Cr). Plasma-sprayed ceramics are hard but brittle: dry particles abrasion occurs through splats detachment. The toughest coating (Al 2O 3) displays the highest wear resistance, which in fact overcomes HVOF-sprayed cermets and Cr electroplating, when a low number of wheel revolutions are considered. In pin-on-disk tests, no coating undergoes wear loss against the 100Cr6 ball, that possess lower hardness. Against the alumina ball, Al 2O 3 and Al 2O 3–TiO 2 coatings show high wear rates and friction coefficients (due to chemical affinity), while Cr 2O 3 possesses better wear resistance, lower friction coefficient and inflicts less wear on the counterpart. Cr 2O 3 wear scar consists in plastically deformed splats and debris forming a quite adherent protective tribofilm.