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.     

Surface modification of Cr3C2–NiCr cermet coatings by direct diode laser

Thermal spraying has emerged as an important tool of increasingly sophisticated surface engineering technology, and it is being used widely to repair and surface modification in metallic parts. The Cr₃C₂–NiCr sprayed coatings are frequently used as wear resistant coatings against abrasion and erosion at high temperature up to 1173 K, and in corrosive environments. Hardness and microstructure of Cr₃C₂–NiCr cermet coatings fused by direct diode laser process was compared with that formed by high-velocity oxygen fuel spraying (HVOF) process. The effect of beam characteristics (power density, power, scanning speed, etc.) was examined on the surface modification of sprayed coatings. In this study, we treated Cr₃C₂–25%NiCr cermet coatings by laser irradiation process and examined its hardness compared with that formed by HVOF process. Consequently, the average hardness of laser-treated Cr₃C₂–25%NiCr cermet coatings has been found out to be higher than that of HVOF coatings. Laser remelting improved markedly the wear resistance of HVOF sprayed Cr₃C₂–25%NiCr cermet coatings.     

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.     

Thermal spraying of a nitrogen alloyed austenitic steel

Stainless steel coatings provide an alternative to protect steel surfaces against corrosive attack. The 316 L stainless steel coatings have been conventionally produced by different spraying processes for such applications. Because the nitrogen alloyed stainless steels exhibit not only superior mechanical properties, but also better corrosion behaviour than conventional stainless steels, in this study the coatings of a nitrogen alloyed austenitic steel were produced using a high velocity oxy-fuel (HVOF) spraying process and an atmospheric plasma spraying (APS) process. Due to much stronger deformation strengthening, the coatings deposited by the HVOF spraying process presented a much higher microhardness than the coatings deposited by the APS process. Moreover, the coatings deposited by the HVOF spraying process were also more corrosion resistant than the coatings deposited by the APS process, because the oxidation of the powder material during HVOF spraying was much lower than that during APS. Compared with the coatings of the conventional stainless steel 316 L, the nitrogen alloyed steel coating deposited by the HVOF spraying process showed a much better corrosion performance.     

MAX-phase coatings produced by thermal spraying

This paper presents a comparative study on the Ti₂AlC coatings produced by different thermal spray methods, as Ti₂AlC is one of the most studied materials from the MAX-phase family. Microstructural analysis of coatings produced by High Velocity Air Fuel (HVAF), Cold Spray and High Velocity Oxygen Fuel (HVOF) has been carried out by means of the scanning electron microscopy equipped with an energy dispersive spectrometer (EDS). The volume fraction of porosity was determined using the ASTM standard E562. The phase characterization of the as-received powder and as-sprayed coatings was conducted using the X-ray diffraction with CrKα radiation. Impact of the spray parameters on the porosity and the mechanical properties of the coatings are discussed. The results show that the spraying temperature and velocity play a crucial role in coatings characteristics.     

Microstructural evolutions of nickel-aluminide nanocomposites during powder synthesis and thermal spray processes

The synthesis and microstructural evolutions of the NiAl-15 wt% (Al₂O₃–13% TiO₂) nanocomposite powders were studied. These nanocomposite powders are used as feedstock materials for thermal spray applications. These powders were prepared using high and low-energy mechanical milling of the Ni, Al powders and Al₂O₃–13% TiO₂ nanoparticle mixtures. High and low-energy ball-milled nanocomposite powders were also sprayed by means of high-velocity oxy fuel (HVOF) and air plasma spraying (APS) techniques respectively. The results showed that the formation of the NiAl intermetallic phase was noticed after 8 h of high-energy ball milling with nanometric grain sizes but in a low-energy ball mill, the powder particles contained only α-Ni solid solution with no trace of the intermetallic phase after 25 h of milling. The crystallite sizes in HVOF coating were in the nanometric range and the coating and feedstock powders showed the same phases. However, under the APS conditions, the coating was composed of the NiAl intermetallic phase in the α-Ni solid solution matrix. In both of the nanocomposite coatings, reinforcing nanoparticles (Al₂O₃–13% TiO₂) were located at the grain boundaries of the coatings and pinned the boundaries, therefore, the grain growth was prohibited during the thermal spraying processes.     

Microstructural characterization of high velocity oxy-fuel sprayed coatings of Inconel 625

High velocity oxy-fuel (HVOF) thermal spraying is being increasingly used to deposit high quality surface coatings. In the present study HVOF spraying was used to deposit coatings of the Ni-based alloy Inconel 625 onto mild steel substrates and the structure of the sprayed coatings were related to the processing conditions employed. The microstructural characteristics of the deposits were investigated using X-ray diffraction together with optical, scanning electron and transmission electron microscopy. The as-sprayed microstructure was found to consist of Ni-based metallic regions together with oxides exhibiting the Cr2O3 and NiCr2O4 crystal structures. Transmission electron microscopy revealed that although the metallic regions were predominantly highly alloyed, Ni-rich grains (depleted in solute) were also present in all coatings. Three processing variables were examined: oxygen to fuel gas ratio, total gas flow rate in the gun and combustion chamber length. All were found to significantly influence the oxide contents of coatings as measured by X-ray diffraction methods. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of high velocity oxy-fuel parameters on properties of nanostructured TiO<Subscript>2</Subscript> coatings

A liquid fuel high velocity oxy-fuel (HVOF) thermal spray process has been used to deposit TiO₂ nanostructured coatings utilizing a commercially available nanopowder as the feedstock. The coatings were characterized by means of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. Photocatalytic activity was evaluated as a rate constant of decomposition reaction of methylene blue (MB) determined from the changes of relative concentration of MB with UV irradiation time. The results indicate that the sprayed TiO₂ coatings were composed of both TiO₂ phases viz. anatase and rutile, with different phase contents and crystallite sizes. A high anatase content of 80% by volume was achieved at 0·00015, fuel-to-oxygen ratio with nanostructure coating by grain size smaller than feedstock powder. Photocatalytic activity evaluation results indicated that all the TiO₂ coatings are effective to degradation MB under UV radiation and their activities differ in different spray conditions. It is found that fuel flow rate strongly influenced on phase transformation of anatase to rutile and by optimizing the rate which can promote structural transformation and grain coarsening in coating and improving photocatalytic activity.     

Influence of gas flow during vacuum cold spraying of nano-porous TiO2 film by using strengthened nanostructured powder on performance of dye-sensitized solar cell

Nano-porous TiO₂ films, which can be applied to the flexible dye-sensitized solar cell (DSC), were deposited by vacuum cold spraying at room temperature with the strengthened nanostructured TiO₂ powder as feedstock. The spraying was conducted under different accelerating gas flows resulting in various particle velocities. Results show that the short-circuit photocurrent density of the cell (N719 dye) increases from 8.3 to 9.8 mA/cm² with the increase in gas flow from 3 to 7.5 L/min. A maximum overall energy conversion efficiency of 4.2% was obtained for the DSC with the TiO₂ film deposited at the gas flow of 7.5 L/min. The influence of particle velocity on the electron transport parameters and cell performance was discussed to reveal the important role of particle velocity in the formation of particle connection through high impact pressure during vacuum cold spraying.     

Failure morphologies of cyclically oxidized ZrO2-based thermal barrier coatings

Abstract

Advanced and baseline thermal barrier coatings (TBCs) were thermal cycle tested in air at 1163°C until delamination or spallation of the ceramic top coat. The top coat of the advanced TBC’s consisted of ZrO₂ with various amounts of Y₂O₃, Yb₂O₃, Gd₂O₃, or Nd₂O₃ dopants. The composition of the top coat of the baseline TBC was ZrO₂-8wt.%Y₂O₃. All top coats were deposited by air plasma spraying. A NiCrAlY or NiCoCrAlY bond coat was deposited by low pressure plasma spraying onto a single-crystal, Ni-base superalloy. The TBC lifetime for the baseline coatings was approximately 190 cycles (45 minutes at 1163°C per cycle) while the lifetime for the advanced coatings was as high as 425 cycles. The fracture surfaces and sample cross sections were examined after TBC failure by SEM and optical microscopy, and the top coats were further examined by X-ray diffraction. These post-test studies revealed that the fracture path largely followed splat boundaries with some trans-splat fracture. However, there were no obvious distinguishing features which explained the difference in TBC lifetimes between some of the advanced and baseline coatings.     

等离子体喷涂氧化钛涂层的生物活性研究

以纳米TiO₂粉末为喷涂原料, 采用大气等离子体喷涂技术在医用钛合金上制备氧化钛涂层. 利用酸和碱溶液对氧化钛涂层表面进行生物活化处理, 体外模拟体液浸泡实验考察涂层的生物活性. 采用XRD、SEM、FTIR、EDS等测试技术对改性前后氧化钛涂层的生物活性进行表征. 结果表明: 氧化钛涂层和钛合金基体的结合强度较高, 其值高达40MPa, 涂层的耐模拟体液腐蚀性优于钛合金. 酸和碱溶液表面改性后的氧化钛涂层经模拟体液浸泡可在其表面生成含有碳酸根的羟基磷灰石(类骨磷灰石), 显示良好的生物活性.     

Effect of laser glazing on the thermo-mechanical properties of plasma-sprayed LaTi2Al9O19 thermal barrier coatings

Conventional duplex (DL) and functionally graded (FG) LaTi₂Al₉O₁₉ (LTA) coatings were deposited over C263 nickel alloy by air plasma spray (APS) and compared with subsequent laser glazing processes. The effect of laser glazing on adhesion strength and thermal barrier performance was investigated. The thermal barrier effect was measured using the temperature difference technique involving infrared (IR) rapid heater and the adhesion strength was measured using the scratch tester. The surface morphology and microstructure were analyzed by optical microscopy (OM), Scanning Electron Microscope (SEM) and 3D profilometer. Based on the experimental results, the laser glazing showed a remarkable temperature drop after IR rapid heating. The changes in porosity and grain refinement make more contributions to the temperature drop of the laser-glazed coatings than that of as-sprayed coatings. The temperature drop is about 110°C for laser-glazed LTA FG coating after 100?s of IR flash, while the drop in DL as-sprayed coating is 60°C compared to the base material.     

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.     

Untersuchungen zur Beständigkeit thermisch gespritzter Schichten im Vergleich zu Hartchromschichten bei durch Abrasion dominierter tribologischer Beanspruchung

Investigations on thermal spray coatings resistance against abrasion dominated tribological load in comparison to hard chromium coatings HVOF iron and nickel based hard alloy as well as WC/Co(Cr) and Cr₃C₂/Ni20Cr coatings are compared to APS Al₂O₃/TiO₂ and Cr₂O₃, powder flame sprayed and fused composite coatings consisting of NiCrBSi and WC/Co and electrolytically deposited hard chromium coatings concerning their wear behavior for tribological load by lose abrasive particles (ASTM G65 and ASTM G75). Thereby the influence of newly developed HVOF torch combustion chambers with reduced critical diameter and divergent expansion nozzles that both permit increased combustion gas and therefore also particle velocities on microstructure and wear resistance of the produced coatings is studied. While there is no improvement of wear resistance for hard alloy coatings compared to mild steel substrates for the specific tribological boundary conditions of these tests, especially the carbide reinforced coatings permit improvement by more than one order of magnitude in ASTM G65 tests and even more than two orders of magnitude in ASTM G75 tests. Also, for both types of tribological load HVOF coatings with WC as reinforcing phase are clearly superior to electrolytically deposited hard chromium coatings. Both use of the combustion chamber with reduced critical diameter and the expansion nozzles with divergent contour result in improved wear resistance of the thereby produced coatings. The specific wear mechanisms are deduced based on SEM examination of worn specimen surfaces.     

Untersuchung der Einflüsse von Substratrauheit und Spritzpartikelgröße auf die Haftung thermisch gespritzter Schichten

About the influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings The influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings was researched systematically. In addition to established spray materials (Cr₂O₃, WCCo, NiCr) and spraying processes (atmospheric plasma spraying (APS), high velocity flame spraying (HVOF)) different substrate materials (steel, stainless steel, aluminum) were included in the research work as well.     

Photo-catalytic film deposition by atmospheric TPCVD

TiO₂ film were deposited by atmospheric thermal plasma chemical vapor deposition (TPCVD) method to investigate the rapid process for functional film deposition. The experiment was conducted under the condition where working gas was Ar, working gas flow rate was 20 l/min, deposition distance was varied from 30 to 200 mm and spraying time was 10 min. Ethanol diluted titanium tetra buthoxide was used as starting material. Consequently, even in this process TiO₂ films including anatase could be deposited, and the results of wettability and methylene blue decoloration testings suggest that the TiO₂ films have good photo-catalytic property. Besides, by using extension nozzle, columnar structure film could be deposited due to a perfect starting material vaporization. From these results, the proposed process seemed to be highly promising for the rapid formation of functional thin films.     

Improvement of adhesion of plasma-sprayed Al2O3 coatings by using dry-ice blasting

Dry-ice blasting, as an environment-friendly method, was introduced for the first time into atmospheric plasma spraying for improving properties of Al₂O₃ coatings. The tensile adhesion of the coating was examined. The microstructure of the coating was characterized using scanning electron microscopy. The temperature evolutions during the spraying were measured using an infrared pyrometer measurement system. The adhesive strength of Al₂O₃ coating deposited with dry-ice blasting exceeded 60 MPa, which was nearly increased by 30% compared with that of the coating deposited with conventional air cooling. The comparison of adhesions and microstructures of Al₂O₃ coatings plasma-sprayed with dry-ice blasting and with air cooling revealed that dry-ice blasting can optimize the coated substrate besides a cooling effect, and consequently resulted in the improved adhesion of plasma-sprayed Al₂O₃ coatings.     

Effects of heat treatment on adhesion strength of thermal barrier coating systems

Thermal barrier coatings (TBCs) are widely used as protective and insulative coatings on hot section components of gas turbines and their applications, like blades and combustion chambers. The quality and performance properties of TBCs are of great importance in terms of their resistance to service conditions. In a TBC system, there is a close relationship between the adhesion properties of coating layers. The adhesion strength of TBCs varies depending on the coating technique used and the surface treatments. In this study, CoNiCrAlY and YSZ (ZrO₂ + Y₂O₃) powders were deposited on stainless steel substrate. High Velocity Oxy-Fuel (HVOF) and Atmospheric Plasma Spraying (APS) techniques were used to produce the bond coats. The ceramic top layers on CoNiCrAlY bond coats were produced by the APS technique. The TBC specimens were subjected to heat-treatment tests. Adhesion strength for top coat/bond coat interface of as-sprayed and heat-treated samples was investigated. The results showed that the heat treatment of the coatings in different temperatures led to an increase in the adhesion strength of TBCs.     

Mikrostruktur und Eigenschaften HVOF‐gespritzter Schichten im System TiO2 – Cr2O3

Microstructure and properties of HVOF‐sprayed coatings of the TiO₂ – Cr₂O₃ system Thermally sprayed titanium oxide coatings are known for their good tribological properties and their electrical conductivity. The latter is due to oxygen deficiency from the stoichiometric composition TiO₂. These lattice defects can be ordered and are called crystallograhic shear planes. These structures are known as Magnéli phases. At high temperature in oxygen‐containing atmospheres the material forms isolating TiO₂, therefore the application under such conditions is restricted. At the titania‐rich side of the system TiO₂‐Cr₂O₃ also compounds with the structure of Magnéli‐phases are formed. According to information from the literature, these phases are stable in oxygen‐containing atmospheres and are therefore promising for corresponding coating applications at elevated temperatures. In this paper first results of systematic studies of microstructure and properties of HVOF‐sprayed coatings are presented.     

Thermisch gespritzte Beschichtungen für den Armaturenbau

This study discusses the development of various thermally sprayed coatings for the use in the valve industry. Based on established coating systems for wear protection, different solid lubricants were integrated into these coatings in order to optimize the wear and friction behavior at application oriented loads. Wear protection coatings based on WC/CoCr and Cr₃C₂/NiCr were applied. As solid lubricants, nickel-encapsulated graphite and hexagonal boron nitride were investigated. The thermal spraying processes high velocity oxygen fuel flame spraying (HVOF) and the novel ultra high velocity flame spraying (UHVOF) were investigated. The results show that through an appropriate coating composition a wear reduction of 53 % and a friction reduction of 31 % are possible.