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.     

Three‐Layered Hollow Nanospheres Based Coatings with Ultrahigh‐Performance of Energy‐Saving,Antireflection, and Self‐Cleaning for Smart Windows

In this study, a well‐controlled interfacial engineering method for the synthesis of SiO₂/TiO₂/VO₂ three‐layered hollow nanospheres (TLHNs) and TLHNs‐based multifunctional coatings is reported. The as‐prepared coatings allow for an outstanding integration of thermochromism from the outer VO₂(M) layer, photocatalytic self‐cleaning capability from the middle TiO₂(A) layer, and antireflective property from internal SiO₂ HNs. The TLHNs coatings exhibit excellent optical performance with ultrahigh luminous transmittance (T_lum‐l = 74%) and an improved solar modulation ability (ΔT_sol = 12%). To the best knowledge, this integrated optical performance is the highest ever reported for TiO₂/VO₂‐based thermochromic coatings. An ingenious computation model is proposed, which allows the n_eff of nanostructured coatings to be rapidly obtained. The experimental and calculated results reveal that the unique three‐layered structure significantly reduces the refractive index (from 2.25 to 1.33 at 600 nm) and reflectance (Rave, from 22.3 to 5.3%) in the visible region as compared with dense coatings. Infrared thermal imaging characterization and self‐cleaning tests provide valid evidence of SiO₂/TiO₂/VO₂ TLHNs coatings' potential for energy‐saving and self‐cleaning smart windows. The exciting inexpensive and universal fabrication process for well‐defined structures may inspire various developments in processable and multifunctional devices.     

Lubrication mechanisms of C‐MoS2‐Fe2O3 (Fe3O4) nano‐composite lubricants at the rubbing interfaces of non‐copper coated solid wires against the contact tube

Graphite‐MoS₂‐Fe₂O₃ (Fe₃O₄) nano‐composite lubricating coatings were prepared on the surfaces of non‐copper coated solid wires by a mechanical coating technique. The tribological behaviours of graphite‐MoS₂‐Fe₂O₃ (Fe₃O₄) coatings at the rubbing interfaces of welding wires against the contact tube were investigated. The results demonstrate that the lubricating properties of graphite‐Fe₃O₄ coatings outperform the lubricating properties of graphite‐Fe₂O₃ coatings. The anti‐wear performance of the contact tube is strengthened with increasing nano‐MoS₂ contents. Layers of protective tribofilms are formed at the rubbing interfaces of welding wires against a contact tube by tribochemical reaction among lubricants. The tribofilms are composed of FeO, MoO₃ and FeMoO₄ with excellent lubricating properties. They can avoid direct contact of welding wires against the contact tube, thus decreasing contact tube wear. With the transition of the contact tube wear from mild to severe, the dominant wear mechanisms of contact tube change from fatigue peeling and oxidative wear to abrasive wear and arc ablation.     

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.     

High Temperature Behavior of Si3N4 and Yb2SiO5 Coated Carbon Fibers for Silicon‐Nitride CMC

For oxide‐free ceramic matrix composites (CMC), with Si₃N₄ matrix and carbon fiber reinforcement, for extreme high temperature applications, protective coatings of the C‐fibers are investigated. Two different coatings are compared: reactive CVD‐derived pure Si₃N₄ coatings to investigate C‐fiber‐matrix reactions and powder based Yb‐silicate coatings to reveal potential reactions with the Yb‐silicate additive serving as sintering aid for Si₃N₄. The reactivity toward carbon in nitrogen atmosphere is studied in the temperature interval from 20 °C up to 1700 °C. A new ceramic phase – an Yb‐carbido‐nitiridosilicate, Yb₂Si₄CN₆–is found as product of carbothermal reduction of the Yb‐silicate. The carbothermal reduction occurs also with other RE‐silicates, RE = Yb, Er, Y, Gd, and Sm while SiC is found as reaction product on carbon fibers coated with pure Si₃N₄. The oxidation resistance of the coated fibers in air was investigated in the temperature interval up to 1000 °C, and the apparent activation energy of oxidation was analyzed based on DTA‐EGA results. The oxidation kinetic reveals a significant increase of onset point of oxidation temperature by up to 150 K for Si₃N₄ coated short carbon fibers obtained from the reactive CVD coating process. Such fibers have a high application potential for carbon‐fiber reinforced Si₃N₄‐CMC. The role of Yb₂Si₄CN₆ as reinforcement for Si₃N₄‐CMC is discussed based on bond strength comparison of carbides (SiC), nitride silicates (SiAlON), and nitrides (Si₃N₄).     

MoS2/ta‐C‐Kombinationsschichten hergestellt durch Laser‐Arc‐Technologie

MoS₂/ta‐C coatings produced by laserarc‐technology A series of MoS₂ and combined MoS₂/ta‐C coatings were prepared by lasercontrolled arc evaporation (Laser‐Arc) in order to study the tribological coating behaviour under vacuum and atmospheric conditions. Very low friction coefficients down to 0.005 were measured under high vacuum. By using a ta‐C underlayer beneath the MoS₂ a increased lifetime up to 5×10⁵ load cycles could be obtained. Also under atmospheric conditions the underlayer had a beneficial effect on coating performance.     

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.     

Synthesis and characterization of functionally graded nickel‐nickel coated ZrO2 composite coating

Electroless‐nickel plated ZrO₂ (NCZ) particles have been used to produce a functionally graded nickel‐electroless‐nickel plated ZrO₂ composite coating. So, electroless‐nickel plated ZrO₂ particles concentration was continuously increased from 0 to an optimum value in the electroplating bath (Watt's bath). The substrate was ST37 steel and the thickness of the coating was approximately 50 μm. Also a uniformly distributed nickel‐electroless‐nickel plated ZrO₂ composite coating has been manufactured as comparison. The composite coatings were characterized by scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. Structure and phase composition were identified by X‐ray diffraction analysis. Microhardness of the coatings was evaluated by employing a Vickers instrument. Three‐point bend test was carried out to compare the adhesion strength of the coatings. Dry sliding wear tests were performed using a pin‐on‐disk wear apparatus. The electrochemical behavior of the coatings was studied by electrochemical impedance spectroscopy. The microhardness measurements showed that, with increasing the co‐electrodeposited electroless‐nickel plated ZrO₂ particle content in the nickel matrix, the microhardness increases from interface towards the surface of the functionally graded composite coating. Bend, wear and electrochemical test results confirmed that the functionally graded composite coating has higher adhesion, wear resistance and corrosion resistance as compared with the uniformly distributed coating. This has been attributed to lower mechanical mismatch between coating and substrate in functionally graded composite coating with respect to the uniformly distributed one.     

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.     

Effect of cobalt content on microstructure and property of electroplated nickel‐cobalt alloy coatings

Nickel‐cobalt alloys were electrodeposited on copper sheets in sulfate bath containing 288.5 g/l NiSO₄·6H₂O, 30 g/l CoSO₄·7H₂O, 40 g/l HBO₃, 15 g/l NaCl and 0.08 g/l lauryl sodium sulfate. The effects of cobalt content on microstructure, microhardness, and wear resistance of electroplating nickel‐cobalt alloys were studied by using SEM and XRD techniques, and microhardness tester and wear tester. The relationship between the microhardness of nickel‐cobalt alloy coatings and heat treatment procedures was also investigated. The experimental results show that cobalt content (W_t) in coating increases with Co²⁺/(Co²⁺ + Ni²⁺)% (X) in plating solution. Fitted regression equation is as following: W_t = –0.7399 + 2.2847X – 0.0133X². The increase of cobalt content leads to that the longitudinal section morphology of coating transforms from the cone into sphericity and at last into the shape of willow leaf, and its structure transforms from face centered cubic (fcc) nickel solid solution into fcc cobalt solid solution and at last into hcp cobalt solid solution. The increase of cobalt content results in the increase of microhardness of nickel‐cobalt alloy coatings, and the hardness reaches a maximum value (363 HV) when cobalt content is 54.9%. After heat treatment at 400°C and 600°C, the microhardness of coatings begins to decrease except the coating containing 79.2% Co. Moreover, the wear resistance of electroplated coatings increases with the increase of cobalt content.     

PECVDTM Coatings in Industrial Applications

The adoption of GPI's Linear PECVD^TM process has accelerated over the last year. Several R&D and production reactor modules were installed at major companies and a turnkey in‐line system was completed. Through these installations, customers are confirming the superior deposition rate, uniformity and stability of Linear PECVD^TM compared to reactive sputtering. Today Linear PECVD^TM is being used to deposit oxide and nitride films on a variety of substrates. The films include SiO₂, TiO₂, Al2₂O₃, SiN, ZnO, and SnO and the applications include multi‐layer AR coatings, single layer oxides and nitrides in combination with sputtered films, thin‐film solar coatings, barrier films, anti‐smudge coatings and TCO's. This progress demonstrates that GPI's Linear PECVD^TM technology may soon displace reactive sputtering of oxides and nitrides for large area substrates in architectural glass, flat panel display and flexible web applications.     

Neue Aspekte bei der physikalischen Abscheidung kompakter Dünnschichten aus der Gasphase am Beispiel einer praxisrelevanten Festschmierstoffschicht

New aspects regarding sputter‐depositing dense coatings, in particular a solid lubricant of practical interest Using MoS₂‐based solid lubricant films as an example, this paper focuses on approaches to preventing sputter‐deposited films from attaining columnar microstructures. This is how it becomes possible to deposit tribological coatings that are characterised by a high durability. As a result of scanning electron microscopy on microstructures, texture measurements by means of XRD, and surface analysis, focusing on sputter‐deposited MoS₂ films, a concept of how to develop thick and dense MoS₂‐based solid lubricant films was generated. At an early stage of film growth a dense and flawless microstructure forms. In order to make the favourable properties of these underlayers available for tribological applications, metallic intralayers were introduced, by means of which the MoS₂ film growth was to be repeatedly interrupted.     

Elaboration of Monophasic and Biphasic Calcium Phosphate Coatings on Ti6Al4V Substrate by Pulsed Electrodeposition Current

Calcium phosphate coatings on Ti6Al4V substrates are elaborated by pulsed electrodeposition. The surface morphology and chemical composition of the coatings are characterized by SEM–EDS. The obtained results are systematically confirmed at the nanometre scale using TEM. Moreover, XRD is performed in order to identify the coatings phases. The results show that pulsed electrodeposition allows uniform coatings to be obtained without the holes and craters usually observed with classical electrodeposition. After appropriate heat treatment, these coatings have a biphasic composition of stoichiometric hydroxyapatite and β‐tricalcium phosphate. Moreover, the addition of 9% H₂O₂ to the electrolyte leads to monophasic coatings made of stoichiometric hydroxyapatite. As an indication of the passive nature of the electrodeposited coating, electrochemical potentiodynamic tests are performed in physiological solution in order to determine the corrosion behaviour of these coatings.     

Self‐Organized Nanostructures in Hard Ceramic Coatings

Nanostructures have attracted increasing interest in modern development of hard coatings for wear‐resistant applications. In plasma‐assisted vapor deposited thin films, nanostructures can evolve during growth or a post‐deposition annealing treatment. In this review we demonstrate, using TiB_2.4, TiN–TiB₂, Ti_0.34Al_0.66N, and Ti(N,B) as model‐coatings, the development of nanostructures and its influence on the mechanical properties of ceramic thin films. For TiB_2.4 and TiN–TiB₂ a two‐dimensional and three‐dimensional nanostructure, respectively, organizes itself during growth by segregation driven processes. Growth of Ti_0.34Al_0.66N and Ti(N,B) results in the formation of a supersaturated TiN based phase, which tends to decompose into its stable constituents during post‐deposition annealing via the formation of nm‐sized domains. As the hardness of a material is determined by resistance to bond distortion and dislocation formation and motion, which depend on the amount and constitution of obstacles provided, there is a direct relation between hardness and nanostructure.     

Effects of Conformal Nanoscale Coatings on Thermal Performance of Vertically Aligned Carbon Nanotubes

The high aspect ratio and the porous nature of spatially oriented forest‐like carbon nanotube (CNT) structures represent a unique opportunity to engineer a novel class of nanoscale assemblies. By combining CNTs and conformal coatings, a 3D lightweight scaffold with tailored behavior can be achieved. The effect of nanoscale coatings, aluminum oxide (Al₂O₃) and nonstoichiometric amorphous silicon carbide (a‐SiC), on the thermal transport efficiency of high aspect ratio vertically aligned CNTs, is reported herein. The thermal performance of the CNT‐based nanostructure strongly depends on the achieved porosity, the coating material and its infiltration within the nanotube network. An unprecedented enhancement in terms of effective thermal conductivity in a‐SiC coated CNTs has been obtained: 181% compared to the as‐grown CNTs and Al₂O₃ coated CNTs. Furthermore, the integration of coated high aspect ratio CNTs in an epoxy molding compound demonstrates that, next to the required thermal conductivity, the mechanical compliance for thermal interface applications can also be achieved through coating infiltration into foam‐like CNT forests.     

Verarbeitung von feinen Spritzwerkstoffen zur Verbesserung von Korrosions‐ und Verschleißschutzeigenschaften von thermisch gespritzten Schichten

About the application of fine spray materials for improvements in terms of corrosion and wear protection of thermal spray coatings Within a research project the spray materials Cr₂O₃, Cr₃C₂ NiCr and WCCoCr were considered. Process parameters were determined, optimized and validated allowing the application of very fine spray material fractions (< 25 μm) with plasma and high velocity oxy‐fuel spray systems. In addition to improved coating properties like low porosity and improved corrosion resistance the application of near net shape coatings was enabled. The subsequent reduced effort for after‐treatment of the coatings can contribute to further rationalization of thermal spray processes.     

The preparation of hard and super‐hydrophilic MgB2 coating by spray pyrolysis deposition

In this study, we report spray pyrolysis deposition using an alternative precursor solution for the fabrication of MgB₂ films. Polycrystalline MgB₂ films were prepared by spray pyrolysis, a precursor solution of magnesium diboride nanoparticles, sodium hypophosphite, sodium succinate, sodium acetate and dimethyl sulfoxide on AZ91 magnesium alloys. The spray was carried out using argon as carrier gas at a temperature of 150 °C and a spray rate of 5 ml/min for 60 min. After spraying, the deposited samples were annealed at 300 °C for 15, 30 and 45 min in order to investigate morphological changes and crystallization behaviour. The microstructure, hardness and wettability properties of approximately 30 μm coatings were investigated by X‐ray diffraction, scanning electron microscopy, microhardness tester and contact angle meter. Produced coatings showed dense and homogenous structural formation with strong grain connections. As‐deposited MgB₂ films showed the most pronounced preferred orientation with the (101) reflection and the highest hardness value compared to other annealed coatings at different times. Besides, all the synthesized coatings had a super‐hydrophilic surface.     

Laser‐Raman and Nuclear Magnetic Resonance (NMR) studies on plasma‐sprayed hydroxylapatite coatings: Influence of bioinert bond coats on phase composition and resorption kinetics in simulated body fluid

The structure and phase composition of HAp coatings deposited onto Ti6Al4V coupons (50x20x2mm) by atmospheric plasma spraying (APS) were studied by laser‐Raman spectroscopy, ³¹P‐ and ¹H‐MAS‐NMR and 2D‐³¹P/¹H HETCOR‐CP‐NMR spectroscopy, and XRD with Rietveld refinement. The samples investigated comprised APS HAp coatings with and without a TiO₂ bond coat as well as coatings incubated for different lengths of time (up to 12 weeks) in simulated body fluid (SBF) under physiological conditions. In APS coatings the presence of a bond coat increased the proportion of well‐ordered crystalline HAp at the expense of distorted apatite‐like structures such as oxyHAp and oxyapatite, and thermal decomposition products such as tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP), and also decreased the amount of amorphous calcium phosphate (ACP). Incubation in SBF further advanced the proportion of crystalline HAp since the disordered structures, the thermal decomposition products, and ACP exhibit substantially higher solubility.     

From Alloying to Nanocomposites—Improved Performance of Hard Coatings

Nowadays a variety of different hard coatings are commercially available, the most widely used ones are TiN, TiC, TiCN, TiAlN, CrN, Al₂O₃, and combinations thereof, as well as some coatings with lubricating properties such as diamond‐like carbon (DLC), WC/C or MoS₂. To fulfil the industrial demands for improved coatings, a lower friction, a longer lifetime, a desired biological behavior or a better thermal stability in different environments, improved and application adapted coatings are developed. The different properties of a coating can be tuned to a desired value by alloying with suitable elements. Composite materials such as multilayer coatings and isotropic nanocomposite coatings, having structures in the nanometer range, can even show properties which can not be obtained by a single coating material alone. The authors review research and development work on the improvement of the overall coating performance. It mainly addresses alloying, the development of multilayer systems and the recently emerged field of nanocomposite coatings.     

Langzeitverhalten und Einsatzgrenzen von plasmagespritzten CeO2‐ und Y2O3‐ stabilisierten ZrO2‐Wärmedämmschichten

Long‐term behaviour and application limits of plasma‐sprayed CeO₂‐ and Y₂O₃‐stabilized ZrO₂ thermal barrier coatings Investigations of changes in phase composition and mechanical properties of CeO₂ – and Y₂O₃ – stabilized plasma‐sprayed ZrO₂‐based thermal barrier coatings (TBC) after long‐term heat treatments at typical service temperatures were performed. Experimental studies include X‐ray diffraction and mechanical testing. TBCs with a high amount of the tetragonal equilibrium phase t (8 mol‐% CeO₂) show strong degradation due to the high amount of transformation to the monoclinic phase and the related decrease in strength. TBCs with a high amount of tetragonal or cubic non‐equilibrium phases t′ or c′ are more suitable. Among these, TBCs with higher CeO₂‐stabilizer contents (19.5 mol‐% CeO₂/1.5 mol‐% Y₂O₃ and 35 mol‐% CeO₂) show less amounts of monoclinic phase with respect to Y₂O₃‐stabilized TBCs (4.5 mol‐% Y₂O₃), as commercially used. Therefore they seem to have a high potential for long‐term applications.