Al-based Coatings Research Articles

Influence of ternary elements (X = Si, B, Cr) on TiAlN coating deposited by magnetron sputtering process with single alloying targets

Ti–Al based single alloying targets were prepared by mechanical alloying and spark plasma sintering. Also, Si, B, and Cr added targets were prepared to improve wear and corrosion properties. The investigation on the alloying targets showed that their microstructures were about 20–30nm in size and all the elements were homogeneously distributed.Ternary Ti–Al-based coatings were deposited by magnetron sputtering method with various alloying targets, such as TiAlSi, TiAlB and TiAlCr. The composition of the coating was almost the same with that of the target. Their microstructures and mechanical properties were investigated by X-ray diffractometer, field emission scanning electron microscope, nano-indenter, tribometer and etc. Moreover, effects of 3rd element on the wear and corrosion properties of Ti–Al–N based coatings were studied here. From the results of TiAlN-X coating properties, the sample coated with TiAlN-B showed the highest hardness. Moreover, TiAlN-Cr coatings showed the best friction coefficient in oil condition and best corrosion resistance with the lowest corrosion current density. Therefore, easy method of coating TiAlN with ternary element using single target, was demonstrated as well as its improved structural properties.

Wear Evaluation of SiCp Reinforced Al Matrix Composites Coatings

Thermal spray technology provides wear-resistant coating on the surface of mechanical components. In this study, wear characteristics of SiCp reinforced Al matrix composite coatings were evaluated. SiCp reinforced Al matrix composite coatings were fabricated using the mixed powders which have different fraction of the SiC reinforcement, 0, 20, 40 and 60 vol% on Al 6061 substrate by thermal spray process. Dry sliding wear tests were performed on these coatings using varied sliding speeds and applied loads. Wear behavior of these Al-based composite coatings were investigated using scanning electron microscope(SEM), energy dispersive X-ray spectroscopy(EDX) and X-ray diffraction(XRD). The composites coatings reinforced with 40 vol% of SiC particles showed the most excellent wear resistance. Although the wear rate increased according to the increase in the applied load, it decreased according to the increase in the sliding speed. The major wear behavior of the SiCp reinforced Al matrix composite coatings is transferred from the adhesive wear to the abrasive wear according to the increase in the sliding speed and the applied load. These results were significantly influenced by the formation of mechanical mixed layer(MML).

Aluminum-Based Barrier Development for Nuclear Fusion Applications

Abstract In the helium-cooled lead-lithium (HCLL) design concept for blanket structures of a future fusion power plant, the eutectic Pb-15.7Li alloy is used as a liquid tritium breeder and is in direct contact with the structural material, e.g., ferritic-martensitic steels. Compatibility tests showed that a high corrosion attack appears and that the dissolved steel elements like Fe and Cr form precipitates, with a high risk of system blockages. Therefore, coatings as corrosion barriers are required for a reliable plant operation. Because of safety reasons, a distinct permeation reduction of the tritium, produced in the Pb-Li alloy by nuclear reactions, into the environment has to be limited, too. Previous developments already showed that Al-based coatings can perform very well as anti-corrosion and tritium permeation (T-permeation) reduction barriers in a PbLi environment. However, industrially relevant coating technologies for fusion application are still missing and/or cannot fulfil low activation criteria. Electrochemical deposition processes are highly flexible and used widely for the production of functional coatings, e.g., for corrosion protection. Nevertheless, because of its unique electrochemistry, Al cannot be deposited from water-based systems. Therefore, the development of two different Al coating processes from nonaqueous systems was initiated and proofed by the successful generation of protective and reproducible scales after heat treatment.

Development of advanced Al coating processes for future application as anti-corrosion and T-permeation barriers

Reduced ferritic-martensitic steels (e.g. Eurofer 97) are foreseen in future fusion technology as structural material in the HCLL concept, however, they show strong dissolution attack in Pb–15.7Li. Corrosion testing in Picolo loop revealed dramatically values of about 400 μm/year at flow rates of 0.22 m/s at 550 °C. This large amount of corrosion products is transported and will be deposited as precipitates at cooler positions with the risk of line blockages as found in Picolo testing. Thus, reliable TBM function claims anti-corrosion barriers. Previous studies (e.g. Hot-Dip Aluminization) showed that Al-based coatings have such anti-corrosion and also T-permeation reduction behavior. However, industrially relevant coating technologies are missing and Hot-Dip cannot fulfill low activation criteria. Electro-chemical deposition promises such features, however water-based systems are unqualified. The investigation of Al coating methods from non-aqueous systems succeeded in deposition of Al-scales in high reproducible and controllable thickness from two different types of electrolytes. The first one is based on organic aromates as electrolyte and the second variant uses organic salt melts (ionic liquids). The specific characteristics and advantages will be discussed as well as the formation of the protective scales after heat treatment and their behavior in Pb–15.7Li.

A Study on Cavitation Erosion and Corrosion Behavior of Al-, Zn-, Cu-, and Fe-Based Coatings Prepared by Arc Spraying

Investigation to find a suitable coating material for a rudder application has been carried out in this study. Ten different coatings were prepared by arc spraying with Al-, Zn-, Cu-, and Fe-based wire feedstock. Both the cavitation erosion and marine corrosion behavior of the arc-sprayed coatings were evaluated, and compared with the conventional anti-corrosion paint. In terms of marine corrosion resistance, aluminum coating was the best among the tested coating systems while stainless steel coating showed the highest resistance against cavitation erosion. In addition, the effects of both the Si composition in Al-based coatings and the Ni composition in Cu- and Fe- based coatings were discussed in this study.

Possible Explanation for the Efficiency of Al-Based Coatings on LiCoO2: Surface Properties of LiCo1−xAlxO2 Solid Solution

Aluminum-based coatings are commonly used in lithium-ion batteries to modify the surface of LiCoO2 particles, to limit cobalt dissolution in the electrolyte at high voltage. It was shown that the formation of a LiCo1−xAlxO2 solid solution occurs at the interface between the coating and the core material. In this paper, we investigated the surface properties of LiCo1−xAlxO2 materials by X-ray photoelectron spectroscopy. We explored the surface acid−base properties of these materials by adsorption of gaseous probe molecules (NH3 and SO2) followed by XPS analyses. We showed that the basic character of the LiCo1−xAlxO2 surface strongly decreases when x increases, which makes these materials less reactive than LiCoO2 toward acidic species (such as HF) that are present in LiPF6-based electrolytes. This is a possible explanation for the efficiency of Al-based coatings to protect LiCoO2 against cobalt dissolution in the electrolyte.

Mechanical and corrosion properties of dc magnetron sputtered Al/Cr multilayers

Mechanical and corrosion properties of Al/Cr multilayer coatings deposited by dc magnetron sputtering are investigated and compared to pure Al and Cr coatings. The objective of this study is to increase the mechanical properties of sacrificial Al-based coatings. The results indicate that mechanical and corrosion properties depend on the architecture organisation. The reduction of the layer thickness allows an increase of the coating hardness. The nanolayer architecture leads to a “hard and tough” material compared to the other architectures. Moreover, the intrinsic corrosion resistance is enhanced, as the corrosion and the pitting corrosion potentials are progressively shifted towards positive values. The nanolayer coating presents the highest intrinsic corrosion resistance but multilayer and nanolayer Al/Cr coatings become nobler than the steel substrate. The corrosion resistance of the coated steel is then strongly dependent on the microscopic coating defects which act as preferential pathways for the corrosive solution. So the nanolayer architecture organisation improves the mechanical properties but does not permit to conserve the sacrificial behaviour of the aluminium based coating, which is harmful for the corrosion protection of coated steel.

Porosity-dependent cyclic-oxidation resistance at 850 °C of annealed Ni–Al-based coatings via electroplating

This study was focused on the cyclic-oxidation behaviors of porous Ni–Al-based coatings, and some alternative methods such as slow annealing and alloying with oxide additives to achieve better cyclic-oxidation resistance at intermediate temperatures. It was found that excellent cyclic-oxidation resistance at 850 °C could be obtained for the coatings even with low Al content when the initial porosity was less than 7%. When porosity was in the range of 7% to 11%, the coatings exhibited inferior resistance to cyclic oxidation in the initial stage, but improved resistance at the latter stage as pores are healed by diffusion. The formation of thin and close-packed oxide film on coating surfaces reduced the pore-healing rate in the coatings. Adding reactive oxides had little effect on oxidation resistance and pore healing under the current intermediated temperature of 850 °C and the relative short testing time (1000 h).

Microstructural, thermal and mechanical properties of HVOF sprayed Ni–Al-based bond coatings on stainless steel substrate

This work focuses on the microstructural, thermal and mechanical properties of NiAl coatings fabricated on 316 L stainless steel substrates by using the high-velocity oxygen fuel (HVOF) method. With this regard, NiAl-based coatings were fabricated on 316 L stainless steel substrates by using HVOF technique. The produced coatings were extensively analyzed with respect to X-ray diffraction (XRD), optical microscope (OM), image analyzer, scanning electron microscope (SEM), microhardness and surface roughness testers. Adhesion strength of the coatings was also measured by using a scratch tester. The effect of thermal cycling on the failure behaviour of the Ni–Al-based coating has been carried out without an external load at temperatures between 47 °C and 600 °C for the period of 303 s cycles. XRD study revealed that Ni 3Al phase is present in the coatings. The metallographic observations showed that all coatings had a similar coating microstructure and consisted of high-quality contact to the substrate, highly dense structure, low oxide and porosity contents. In addition to microstructural results, microhardness values of Ni–Al-based coatings and stainless steel were measured to be 450 HV and 220 HV before thermal cycling. These microhardness values drastically decreased after thermal cyclic tests. The adhesion strength value of Ni–Al-based coating was found that 85.3 MPa. In order to determine wear loss, friction coefficient and wear mechanism, wear tests were performed the Ni–Al-coated stainless steel substrates at 49 N and 89 N under dry sliding conditions through a pin-on-plate arrangement against AISI 303 L steel counter body. Regarding as wear mechanism of the coatings, the microstructural analysis of worn surfaces was examined by SEM. It was found that the friction coefficients of NiAl coating for 49 N and 89 N loads were in the range of 0.289 and 0.329 and the extent of the plastic deformation at 49 N was less than that at 89 N under dry sliding conditions. Mechanical properties of coating were examined by Shimadzu Dynamic Ultra-microhardness test machine for estimating Young's modulus due to load–unload sensing analysis and in addition to mechanical investigation of hardness–depth curves of Ni–Al-based coatings was obtained.

Microstructure and mechanical properties of sputtered intermetallic Al–Au coatings

Intermetallic Al-based coatings such as Al–Ni, Al–Cr, and Al–Pt are widely used as bond coats and/or as oxidation protection of turbine blades. Investigations during earlier work indicated promising thermal stability and oxidation resistance of intermetallic Al2Au-based coatings. Hence, here we study in detail the microstructure and mechanical properties of unbalanced magnetron sputtered Al–Au films with Al∕Au at. % ratios of 1.85, 2.00, and 4.32. The film with an Al∕Au ratio of 2.00 has a single-phase Al2Au dense columnar structure in the as-deposited state and a hardness of ∼8GPa. With increasing Al and Au contents the morphology changes into a more equiaxed structure with additional Al, AlAu, and Au phases, respectively. During growth, excess Al of the film with an Al∕Au ratio of 4.32 forms lamellar nanosegregations (∼3nm thick) in the Al2Au domains. The simultaneous nucleation and growth of Al2Au, AlAu, and Au phases in the coatings with an Al∕Au ratio of 1.85, with their different growth mechanisms, leads to the formation of pores having an average size of ∼150nm. These Al- and Au-rich films, compared to Al2Au, have reduced hardnesses of ∼4 and ∼2GPa, respectively, due to the additional soft metallic phases and pores present. Our results show that within the Al–Au system the single-phase intermetallic Al2Au films exhibit the best structural and mechanical properties for protecting oxidation sensitive materials.

Modern coatings in high performance cutting applications

Modern, state-of-the-art, PVD coatings are required to fulfill a variety of different applications. Each metal cutting operation requires an optimal combination of various film parameters to achieve a high end cutting performance. Especially, Al-based coatings such as AlTiN- and AlCrN-coatings show very good results in high performance metal cutting applications. Wear resistance, thermal stability such as oxidation resistance and hardness at elevated temperatures are key issues within these cutting operations. In this paper the influence of these properties on Al-based nitride coatings in relation to metal cutting tests such as milling and drilling will be discussed.

Magnetron Sputtered Intermetallic Al2Au and Al-Zr-Y Coatings for the Oxidation Protection of γ-TiAl

AbstractAl-based intermetallic coatings are widely used as bond coats and for oxidation protection of turbine blades and engine components. Here we present and discuss the structural and thermal properties of novel unbalanced-magnetron sputtered Al-based coatings (single-phase intermetallic Al2Au, binary Al-Zr, and nano-structured Al-Zr-Y) developed to protect gamma-TiAl from environmental attack at elevated temperatures. Al-Zr films exhibit a coarse-grained dual-phase Al3Zr2-Al2Zr structure and are nano-structured by alloying with ~5, 10, and 14 at% Y.Combined dynamic differential-scanning calorimetry and thermogravimetric analyses up to a temperature of 1150 °C reveal that the Al2Au film is very stable with only marginal mass gain from oxidation found between 800 and 1000 °C. High temperature X-ray diffraction shows that this coating retains its (311) texture up to 900 °C where Al2O3 formation leads to the depletion of Al in Al2Au and subsequently the precipitation of intermetallic AlAu. When gamma-TiAl is coated with Al2Au and exposed to cyclic oxidation tests at 750 and 850 °C good oxidation resistance is obtained as a protective oxide layer is formed.Dual-phase Al3Zr2-Al2Zr coatings form ZrO2 and Al2O3 in oxidizing atmosphere. However, the phase transition from monoclinic (m-)ZrO2 to tetragonal (t-)ZrO2 with the accompanying volume change causes flaking of the oxide. Yttrium addition to the Al-Zr films stabilizes the cubic (c-) and t-ZrO2 and hence avoids the fatal tetragonal-monocline transformation. The thermally grown c-ZrO2 based oxides allow good adhesion to thermal barrier coatings which are themselves based on c-ZrO2.

Duplex Al-based thermal spray coatings for corrosion protection in high temperature refinery applications

The application of thermal spray coatings has been effective in preventing corrosion of steel and iron products. It has been used in a wide range of applications spreading from the petroleum to the food industry. In this work, the performance and effectiveness of a two-layered aluminum-based thermal spray coating applied to an ASTM A387 G11 steel was evaluated. The coating structure was comprised of an inner Al-Fe-Cr layer and an outer layer of aluminum. Coated samples were tested in the reactor zone of a fluid catalytic cracking unit (FCCU) of a petrochemical plant for 2.5 years. The reactor zone temperature was about 793 K (520 °C) and the environment was a mixed gas containing sulfur, oxygen and carbon. Laboratory-scale tests were also conducted on the coated samples in order to gain a better understanding of the corrosive effect of the gaseous species present in the FCCU atmosphere. Porosity present in the thermal spray coatings allowed the penetration of the atmosphere corrodents, which instigated intergranular corrosion of the steel substrate. The presence of an inner Al-Fe-Cr layer did not prevent coating spallation, which further contributed to the internal corrosion process.

Vapour phase alloy design of corrosion-resistant overlay coatings

This paper examines the design of hot corrosion-resistant NiCrAl-based coatings by co-deposition from multiple sources. Initial studies examined the deposition of a wide range of ternary alloy compositions by magnetron sputtering or co-evaporation from multiple EB sources. In the magnetron sputtering work a segmented target, consisting of pure Ni, Cr and Al, was used. For the co-evaporation studies, binary and pure element source materials were used. In both deposition procedures new coating compositions were produced by mixing these source materials in the vapour phase. The new coatings were deposited onto pure Al 2O 3 substrates to ensure that no interaction between the coatings and substrate would occur during the corrosion tests. Low temperature hot corrosion of the novel coating compositions was assessed using an ‘ash recoat’ test procedure. Samples were coated in sodium chloride (at 0.75 mg cm −2) every 20 h and exposed in a flowing air-1 % SO 2 equilibrated to an air-SO 2-SO 3 environment, at 750°C for times up to 100 h. Pack aluminised IN738 and uncoated IN738 were included in the test as reference materials and these exhibited classic type II hot corrosion morphologies, as did some of the new coatings tested. Other new coatings resulted in similar deposit chemistries but showed a much lower tendency to form corrosion pits. By modelling the corrosion behaviour along quasibinary sections drawn through the NiCrAl ternary diagram, it has been possible to construct an iso-corrosion contour map at 750°C. These studies have highlighted a new coating composition capable of resisting type II hot corrosion, which on the basis of short time tests offers a six-fold improvement in hot corrosion resistance over conventional NiCrAlY overlay coating compositions as measured using this ‘ash recoat’ test procedure.

Formation of various coatings and their behaviour under particle bombardment

The behaviour of Al-based coatings was tested under H +, D + and He + irradiation in the energy regime from 100 eV to 8 keV. The sputtering yields of Al−Si, SAP and Al 2O 3 were measured both for solid materials and for coatings on stainless steel. The hydrogen yield data for SAP, Al−Si and Al 2O 3 are similar and correspond to the values on pure Al. This is probably due to a reduction of A1 2O 3 by the implanted hydrogen. Sputtering yields for He on Al and Al 2O 3 differ more than an order of magnitude below 1 keV. Coatings show the same sputtering yield as the corresponding solid material. But the structure and the mechanical properties strongly depend on the coating method. It was found that coatings obtained by reactive sputtering give the best properties as far as density, adsorption of residual gas and sputtering are concerned. The coating method also seems to be most appropriate for application in fusion devices. However, additional properties have to be tested in order to assess application in reactor systems.