Korrosion thermisch gespritzter Schichten auf der Basis von Aluminiumoxid

Corrosion of thermally sprayed coatings based on aluminium oxide In this paper, the results of corrosion investigations performed on thermally sprayed coatings with different compositions in the Al₂O₃‐TiO₂ system (Al₂O₃, Al₂O₃‐3 %TiO₂, Al₂O₃‐40 %TiO₂, Al₂TiO₅) are presented. The coatings were deposited on corrosion‐resistant stainless steel substrates using APS and HVOF processes. The coatings were characterized by means of optical microscopy and SEM of metallographically prepared cross sections as well as surfaces before and after corrosion testing. The changes in phase composition occurring during spraying were studied by X‐ray diffraction. The corrosion experiments were performed with 1 N solutions of NaOH and H₂SO₄ at room temperature, 60 °C, and 85 °C. In contrast to expectations, APS‐sprayed coatings were found to be more corrosion‐resistant than the denser HVOF‐sprayed coatings were.     

Corrosion resistance of the Al2O3+ZrO2 thermal barrier coatings on stainless steel substrates

Al₂O₃ + ZrO₂ composite thermal barrier coatings (TBCs) were deposited on SUS304 substrates using the gas tunnel type plasma spraying method. Groups of coating samples with different mixing ratios of Al₂O₃ and different thicknesses were respectively obtained. The graded microstructure of the coatings was examined using optical microscopy (OM) and a scanning electron microscope (SEM). It is well known that the thermal oxidation of interfaces is the main reason for the spallation of TBC coatings, because sprayed TBCs contain micropores and microcracks. The anodic polarization characterization of sprayed TBCs provides a way, to some extent, to investigate the mechanism of the interface oxidation. In this research, anodic polarization was performed to investigate the effect of coatings on the corrosion resistance. The results showed that a higher alumina mixing ratio and thicker coatings lead to higher corrosion resistance. The corrosion potential and deactivated corrosion current of the samples were correlated and analyzed according to the coating porosity, because the through pores in the coatings provided the way for the oxidants of the ambient solution to access the interface. The analysis indicated that lowering the porosity and increasing the gradient of coating porosity help lowering the oxidation.     

A comparative study on thermally sprayed alumina based ceramic coatings

Aluminium oxide is relatively cheap material, abundant almosteverywhere and therefore it is widely used for thermal sprayapplications. Various alumina based powder containing 13 wt. Titania, two different 40 wt.% Zirconia and three differentcompositions of alumina-zirconia-chromia were deposited byatmospheric plasma spraying (APS) and high power plasma spraying(HPPS). The coatings obtained were evaluated by optical microscopy,microhardness measurements, X-ray diffraction and porositymeasurements. Moreover, abrasion and friction wear resistance wereevaluated by using Pin-on-Disc machine. Microhardness values of APScoatings are relatively high as compared to HPPS coatings except inalumina-zirconia-chromia coatings. HPPS have higher hardness values.APS coatings are much coarser and show higher porosity values thanHPPS coatings. The best wear/friction behaviour exhibited coatingAl₂O₃-40 wt.% ZrO₂ that deposited from agglomerated andsintered powder type.     

Thermisches Spritzen vanadiumkarbidverstärkter Eisenbasisschichten

Thermal spraying of vanadium carbide reinforced iron based HVOF sprayed cermet coatings for wear protection like WC‐Co(Cr) and Cr₃C₂‐NiCr have found a broad range of applications in the past. By using chromium containing matrices, they exhibit good corrosion resistance along with outstanding wear resistance. In present research in the area of powder metallurgy and PTA welding iron based alloys with high content of chromium and vanadium have been developed, revealing similar properties and therefore being a cost efficient alternative to established cermet coatings. HVOF sprayed coatings of these iron based alloys are investigated regarding their economic applicability and their corrosion and wear properties.     

Ti-Al球磨粉反应等离子喷涂涂层的组织与性能

为制备Ti-Al金属间化合物复合涂层并研究其性能,以机械球磨的Ti-Al混合粉在Q235钢表面进行反应等离子喷涂实验,分别采用X射线衍射、扫描电子显微镜对涂层的成分、显微组织进行了分析,并测试了涂层的结合强度、显微硬度和耐腐蚀性能.结果表明:涂层由Al3Ti、TiN、Al2O3、少量TiAl与Ti3Al、以及残留的Al和Ti组成;球磨可促进喷涂时的反应,但喷涂时Al和Ti仍未完全反应,且在空气环境中喷涂容易氧化和氮化;涂层与基体之间是镶嵌式的机械结合,结合强度平均为30.24 MPa;涂层表面的显微硬度平均为206.1 HV,涂层的耐腐蚀性优于基体.总体上看,当球磨时间较长、电流较大、喷涂距离较大、气流量较小时,喷涂时的反应较充分,且涂层比较均匀、致密,其强度、硬度以及耐腐蚀性能较高.     

Microstructure and wear mechanisms of thermal-sprayed alumina coatings

Alumina coatings, deposited on metallic substrates by two thermal spraying techniques, were tested against sintered alumina in a pin-on-disc tribometer. The wear mechanics involved were investigated with respect to the microstructural characteristics of the coatings. The denser Al₂O₃ coatings presented higher wear resistance than the more porous ones.     

Surface roughness characterization of thermally sprayed and precision machined WC–Co and Alloy-625 coatings

Surface roughness characterization of thermally sprayed and precision machined WC–Co and Alloy-625 coatings was carried out. The objectives were to demonstrate that such difficult-to-machine coating surfaces could be machined with commercial machines and tools, and to characterize the surface finish of the machined coatings. The coatings were obtained using two thermal spraying processes: arc spraying and high velocity oxy-fuel spraying. Different machining techniques were tried to optimize the surface finishing of the coatings based on surface finish and time required. Machined samples were then examined using stylus roughness testers. Surface roughness parameters R_a and R_q were measured using various cut-off lengths, sampling lengths, and numbers of sampling and cut-off lengths to characterize the scaling behavior of the surfaces. Diamond turning of WC–Co demonstrated the advantage of high material removal rates, and diamond grinding of WC–Co produced good surface finish with relatively high material removal rates. Precision-machined WC–Co and Alloy-625 surfaces could be identified as self-affine fractals in a statistical sense within the correlation length, i.e. within the length scales studied from 0.08 to 8 mm. The surface roughness heights of the machined surfaces were found to be dependent on the scale of cut-off length as a power law.     

Effects of hydroxyapatite/Zr and bioglass/Zr coatings on morphology and corrosion behaviour of Rex-734 alloy

To improve corrosion resistance of metallic implant surfaces, Rex-734 alloy was coated with two different bio-ceramics; single-Hydroxyapatite (HA), double-HA/Zirconia(Zr) and double-Bioglass (BG)/Zr by using sol–gel method. Porous surface morphologies at low crack density were obtained after coating and sintering processes. Corrosion characteristics of coatings were determined by Open circuit potential and Potentiodynamic polarization measurements during corrosion tests. Hardness and adhesion strength of coating layers were measured and their surface morphologies before and after corrosion were characterized by scanning electron microscope (SEM), XRD and EDX. Through the SEM analysis, it was observed that corrosion caused degradation and sphere-like formations appeared with dimples on the coated surfaces. The coated substrates that exhibit high crack density, the corrosion was more effective by disturbing and transmitting through the coating layer, produced CrO₃ and Cr₃O₈ oxide formation. It was found that the addition of Zr provided an increase in adhesion strength and corrosion resistance of the coatings. However, BG/Zr coatings had lower adhesion strength than the HA/Zr coatings, but showed higher corrosion resistance.     

Thermisch gespritzte Schichten im System Al2O3–Cr2O3–TiO2 – ein Update

With exception of ZrO₂, the individual oxides and binary compositions in the system Al₂O₃–Cr₂O₃–TiO₂ are the most important oxide materials for the preparation of thermally sprayed coatings. In this contribution selected results of recent own research activities are summarized. This includes the comparison of microstructures, phase compositions, and properties of coatings, deposited by atmospheric plasma spraying (APS) and high velocity oxy‐fuel (HVOF) spraying. The possibilities arriving from the use of suspensions as feedstock are reviewed. Special attention is paid to the advantage of use of binary compositions in this system. Tribological, electrical and corrosion properties of the coatings are discussed.     

Self-fluxing coatings for stationary gas turbines

Self-fluxing alloys are becoming more and more interesting for hardware applications in stationary gas turbines. This is due to their ease of application which involves plasma spraying and sintering. These coatings are basically Cr₂O₃ formers which provide protection against sulphidation.As a result of the demand for greater efficiency in gas turbines and the consequent increase in operating temperature oxidation has become the basic form of corrosion attack and therefore oxides other than Cr₂O₃, i.e.Al₂O₃, are required.In this paper we shall describe an attempt that was made to modify a self-fluxing alloy (Ni-Cr-Fe-Si-B) with aluminium in order to produce protective Al₂O₃ scales. The coating is applied using air plasma spraying. The variation in the sinter temperature and the oxidation and corrosion resistance properties due to aluminium are given. The effect of aluminium on the tensile properties is illustrated using data from a ductile-brittle transition temperature test.     

Untersuchung der Haftmechanismen kaltgasgespritzter Al‐Schichten auf Al2O3

The metallization of ceramics by means of cold gas spraying has been in the focus of numerous publications in the recent past. However, the bonding mechanisms of metallic coatings on non‐ductile substrates are still not fully understood. Former investigations of titanium coatings on corundum revealed that a combination of recrystallisation induced by adiabatic shear processes and heteroepitaxial growth might be responsible for the high adhesions strengths of coatings applied on smooth ceramic surfaces. In the present work, it is intended to examine the interface area of cold gas sprayed aluminum on alumina substrates. Besides a variation of powder fraction and substrate temperature, it is investigated if a downstream heat treatment has an influence on tensile strength and hardness of the coatings. The splat formation of single particles is investigated by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to examine the Al/Al₂O₃ interface. First results suggest that mechanical clamping is the primary bonding mechanism on polycrystalline coatings with a sub‐micrometer‐scaled surface roughness, while heteroepitaxial growth is the main bonding mechanism for Al coatings on single‐crystalline, atomically smooth sapphire (α‐Al₂O₃) substrates. Heteroepitaxy is promoted by deformation‐induced recrystallisation of the cold gas‐sprayed aluminum.     

Corrosion behavior of plasma sprayed ceramic and metallic coatings on carbon steel in simulated seawater

Al₂O₃, ZrO₂ and Ni60 coatings were produced on carbon steels by plasma spray. Ni60 was used as the bond coat in all the cases. The microstructure of these coatings was analyzed by scanning electron microscopy (SEM). The corrosion behavior of the plasma spray coated samples as well as uncoated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in simulated seawater. The results showed that Ni60 coating protected carbon steels against the corrosion and plasma spraying ceramic powders on metallic coating improved the corrosion resistance of the coatings further. The corrosion resistance of the Al₂O₃ coating was superior to that of the ZrO₂ coating due to the relatively few defects in Al₂O₃ coating.     

Surface modification of magnesium alloy AZ31 by hydrofluoric acid treatment and its effect on the corrosion behaviour

In the present study, the effect of hydrofluoric acid (HF) treatment on the surface composition and corrosion behaviour of the magnesium alloy AZ 31 was investigated. The HF treatment of the samples was performed with various concentrations and at different treatment times. The samples surfaces were analysed by Fourier transform infrared spectroscopy, optical emission spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The results showed the formation of hydroxides, oxides and compounds of the general formula Mg(OH)_xF_2 − x on the samples surfaces, as well as variations on impurities concentrations. The process led to distinct surfaces, each having its specific corrosion resistance, which was evaluated by electrochemical impedance spectroscopy and potentio-dynamic polarization. The most improved corrosion protection was achieved using the concentrations of 14 and 20 mol L^− 1 and 24 h of treatment time, resulting in corrosion rates 20 times lower than those of untreated samples. These two solutions also resulted in an improved corrosion protection for further polymeric coatings, showing that this treatment is an excellent pre-treatment for corrosion protective layers on magnesium alloys.     

Hard transparent dielectric coatings

Hard transparent corrosion-resistant dielectric coatings were investigated (a) for application to the direct protection of glass surfaces, (b) for application to the protection of thin film metallic window coatings used for reflection or transmission of solar radiation and (c) for use in codeposited metal-ceramic coatings. The dielectric coatings were Al₂O₃, SiO₂ and SiC in the thickness range 100–100 000 Å. The coatings were deposited under varying conditions of substrate temperature, gas composition, r.f. power and substrate preparation. The results of the following measurements on the dielectric coatings are presented: diamond pyramid hardness tests, optical reflectance and transmittance in the region 0.6–4.0 eV and corrosion resistance tests under cyclic conditions. The results showed a considerable improvement in the scratch resistance and hardness of glass by layering of hard dielectric coatings 2–5 μm thick.     

Nanotechnology-associated coatings for aircrafts

Polymeric epoxy-based composites are modified with nanopowders of silicon oxide (∼ 100 nm). By the method of spraying, these composites are applied to specimens of 2024-T3 aluminum alloy preliminary treated with molybdate solutions to get conversion layers. Three types of polymeric coatings are considered: reference, treated by silica, and with additional polyurethane coatings. The aim of modification of polymeric coatings is to absorb and/or block unwanted ions/molecules (Cl⁻, O₂, OH⁻, H₂O, etc.) and improve the protective properties of the films. The tests carried out by the method of electrochemical impedance spectroscopy, in a salt-fog chamber, and by immersion in a 0.5 M NaCl solution reveal high anticorrosion characteristics of the coating. New coatings are promising for the corrosion protection in the aircraft industry. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 103–108, May–June, 2007.     

Corrosion protection and formation mechanism of anodic coating on SiCp/Al metal matrix composite

The corrosion protection from sulfuric acid anodized coatings on 2024 aluminum and SiC particle reinforced 2024 aluminum metal matrix composite (SiC_p/2024Al MMC) in 3.5 wt.% NaCl aqueous solution was investigated using electrochemical methods. The results show that the anodized coating on 2024Al provides good corrosion protection to 3.5 wt.% NaCl, and the anodized coating on the SiC_p/2024Al MMC provides some corrosion protection, but it is not as effective as for 2024Al because non-uniformity in thickness and cavities present are associated with the SiC particulates. Cavities above SiC particles are the reason that the anodized coating on the MMC cannot be completely sealed by hot water as with anodic Al alloy. SiC particle anodizes at a significantly reduced rate compared with the adjacent Al matrix. This gives rise to alumina film encroachment beneath the particle and occlusion of the partly anodized particle in the coating. It was found that the barrier layer of anodized Al MMC is not continuous, and it is composed primarily of the barrier layer of anodized Al matrix and a barrier-type SiO₂ film on occluded SiC particles in the coating. A new formation mechanism of coating growth during anodizing of a SiC_p/2024Al MMC was proposed.     

Formation and Study of Porous Alumina and Catalytic Coatings by the Use of Cold Gas Dynamic Spraying Method

Cold gas dynamic spraying was used to obtain coatings based on γ-Al₂O₃ and Pd-CeO₂/γ-Al₂O₃ catalyst on a stainless steel foil. The coatings were studied by thermal cycling, BET, SEM, TEM, and XPS methods. The textural and adhesion characteristics of the coatings were found to depend on the composition of initial alumina powders with metallic aluminum (weight ratios of the components were varied from 2:8 to 8:2). It was shown that a mixture containing 25% γ-Al₂O₃ and 75% Al provides the optimal properties of the coatings. The effect produced by a method of the active component introduction into the coating on the catalytic properties of samples was examined. A considerable advantage of Pd introduction by impregnation in the preliminarily sprayed γ-Al₂O₃ layer over the direct spraying of the finished catalytic composition on the foil was demonstrated.     

Studies on the properties of high-velocity oxy-fuel thermal spray coatings for higher temperature applications

Materials operating at high temperatures in corrosive media suffer erosion-corrosion wear, oxidation, and hot corrosion. Among various methods used for the protection of the surfaces against degradation, we can especially mention the technology of application of coatings by high-velocity oxy-fuel spraying, which gives coatings which high strength and hardness, low (less than 1%) porosity, and high erosion-corrosion and wear resistances. The characteristics of the coatings and their protective properties are presented. The role of some high-velocity oxy-fuel coatings in the protection of metals and alloys against degradation at high temperatures in various media is demonstrated. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 6, pp. 80–95, November–December, 2005.     

Corrosion behaviour of steel with PVD CrxN coatings

The application of PVD coatings for wear protection of tools is well known. Since many years, TiN coated cutting and forming tools are state of the art. In contrast, the application of PVD coatings on machine parts is not standard today. This is caused by the problems of coating deposition on components as well as the fact that wear protection and corrosion protection is demanded for many parts with longer lifetime. TiN produced by means of PVD technique is good for wear protection, but with respect to corrosion there are problems. On the other hand electropolated chromium is a reliable coating to resist corrosion, but wear resistance is limited. PVD Cr_xN coatings promise to combine the advantages of hard coatings and electropolated chromium. The present study focuses on the corrosion properties of magnetron sputtered Cr_xN coatings. Different types of coatings on steel substrates with various amounts of nitrogen were investigated in order to take into account aspects of coating deposition resp. coating material, coating structure and coating morphology. Additionally several graded and multilayer coatings were studied to show influences of coating system design. Electroplated hard chromium was used as reference material for corrosion resistance. To explain the corrosion behaviour, crystallographic phases and structure of coatings were analysed by X‐ray diffraction and morphology by SEM. It could be shown that the corrosion behaviour depends on all these parameters and that 8 μm chromium nitride provides the same corrosion protection as 48 μm electroplated chromium.     

Characterisations of HVOF sprayed NiCrBSi coatings on Ni- and Fe-based superalloys and evaluation of cyclic oxidation behaviour of some Ni-based superalloys in molten salt environment

Microstructure plays a predominant role in determining material behaviour. Increasing microstructure uniformity has long been considered a fruitful means of improving thermal, chemical and mechanical properties of the materials. High velocity oxy-fuel (HVOF) is one of the emerging technologies among the thermal spraying techniques, for producing uniform and dense coatings, having high hardness and good adhesion values. In this study, HVOF technique was used to deposit NiCrBSi coatings, approximately 250-300 μm thick, on the Ni- and Fe-based superalloys for hot corrosion applications. The coatings were characterised in relation to coating thickness, porosity, microhardness and microstructure. The hot corrosion behaviour of the coatings deposited on nickel-based superalloys after exposure to molten salt (Na₂SO₄-60% V₂O₅) at 900 °C under cyclic conditions was also studied. The techniques used in the present investigation include X-ray diffraction, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and electron probe microanalysis (EPMA). The thermogravimetric technique was used to establish kinetics of corrosion. The structure of the as sprayed NiCrBSi coating mainly consisted of γ-nickel solid solution containing small fraction of Cr₇C₃ and Ni₃B phases. Very weak peaks of NiCr₂O₄ spinel oxides were also formed during spraying of the coatings. Some porosity (less than 1.4%) and inclusions were observed in the structure of the coatings. Coating microhardness values were found to be in the range of 750-930 Hv (Vickers Hardness) on different substrates. The NiCrBSi coating was found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. The hot corrosion resistance imparted by NiCrBSi coatings may be attributed to the formation of oxides of silicon, chromium, nickel and spinels of nickel and chromium.