TiAl 3 -Al composite coatings are believed to hold promise for extending the service temperature range of titanium alloys used as structural materials. In this study, 0.6 x 40 mm Ti-6Al-4V specimens are coated with a 30 μm thick layer of TiAl 3 -Al by low-temperature HVOF spraying. Cross-sectional imaging shows that the as-sprayed coatings have a dense laminar microstructure and are well bonded to the substrate. Following the initial examination, the coating samples were placed in a muffle furnace, where they were held at 700 °C for up to 1000 h. Mass gain was detected starting at 200 h and remained nearly constant for the remainder of the test. This is an indication of excellent corrosion resistance, which is verified by SEM cross-sectioning and elemental EDS analysis. A brief explanation of the protective mechanism of the coating is provided.

Recently a Ti–TiAl 3 metal–intermetallic laminate (MIL) composite attracts growing attention because they have potential application in honeycomb or sandwich components of airplanes and as biomaterial with good bio-compatibility. Of the available processing techniques, diffusion bonding of elemental titanium and aluminum foils is an effective low-temperature method to synthesize the composite, allowing growth of the intermetallic layer. However, application of assembling and multi-pass cold rolling operations leads to fact that this technology is complex and expensive. The use of Cold Spray technology instead of aluminum foils utilization and multi-pass cold rolling to produce the Ti–TiAl 3 MIL composites is believed to be more effective. However, reaction diffusion kinetics of Ti-Al particulate composite differs from that of classical MIL composite and needs to be studied. The task of this paper is to define microstructural changes of Tl-TiAl 3 composite coating during cold spraying and reaction sintering. The optical microscopy, SEM, EDS, X-ray and microhardness examinations are presented and discussed.

In this paper, we reported our investigation on preparing inorganic phosphate ceramic coatings on the surface of γ-TiAl based alloy by air spray. The high temperature performance of the coatings was tested by thermal shock test, cyclic oxidation and isothermal oxidation at 950 °C. The results showed that the coatings exhibited significant improvement on high temperature anti-oxidation performance of the base alloy. No obvious cracks or spalling of the coatings occurred after oxidation experiment under isothermal oxidation condition at 950 °C for 1000 h. The characterization was carried out to analyze the microstructure of the coatings before and after oxidation test.

This work shows that with computer-controlled detonation spraying, the phase composition of coatings can be changed relative to that of the feedstock powders. New phases can appear in substantial quantities due to chemical reactions of reduction, oxidation, and nitridation as well as interfacial interactions between phases in composite powders. The key advantage of computer control is that it precisely regulates the quantity and stoichiometry of explosive gas mixtures. It has thereby been found that TiO 2 experiences partial reduction to titanium suboxides and that chemical reactions with nitrogen are also possible. It has also been found that when nitrogen is present, titanium aluminides, Ti 3 Al and TiAl, are likely to form nitrides in the sprayed coatings. Interfacial reactions between the phases of a composite have been studied, and in the case of the Ti 3 SiC 2 -Cu system, it has been found that deintercalation of Si can be prevented by maintaining relatively cold spraying conditions. At higher temperatures, coatings of an unusual phase composition form in which carbon-deficient TiCx inclusions are distributed in the Cu matrix as modified by the dissolution of silicon. The formation of new phases affects coating microstructure development and results in new microstructural features.

In this study, titanium and aluminum powders mixed in different ratios were deposited on stainless steel substrates by warm spraying. Microstructure and composition of as-sprayed and heat-treated samples were characterized and the effect of adding a third element was assessed. It was found that Al content has a major influence on the thickness and porosity of heat-treated Ti-Al coatings and that adding silicon to the powder mixtures reduces the melting point of Al, causing a loss of Al-Si particles during spraying.

Aluminum-titanium powder mixtures were deposited on γ-TiAl alloy substrates by cold spraying then heat treated for 5 h at 600, 650, and 700 °C. SEM and XRD examination showed that the treatment caused Al to diffuse into the substrate where it reacted with Ti, resulting in changes in microstructure. The diffusion of Al left pores in the fringes of the TiAl 3 phase, increasing the porosity of the coatings. A surplus of Al remained in the coatings after heat treatment at 600-650 °C, but at 700 °C, all Al was consumed, contributing to the formation of a continuous TiAl 3 layer.

Four powder blends of Al and Ti were cold sprayed on Ti-Al-Nb substrates at 300°C. Test samples were heat treated in Ar at 500 °C then exposed to 950 °C air for 100-500 h. It was found that oxidation rates were significantly reduced by the coatings, especially those with lower Ti content. However, four-point bending tests revealed that the deposition of the protective layer reduced the flexural strength of the coated substrate. The results indicate that oxidation is not the only factor influencing the mechanical properties of intermetallics at high temperatures.

Lightweight gamma titanium aluminide (γ-TiAl) intermetallic alloys have recently found application in low-pressure turbine blades in the aviation industry, but their use is currently limited to around 700 °C due to oxidation. This study evaluates the potential of various multilayer coating systems to increase the operating temperature range of γ-TiAl. The coating systems tested are based on a CoNiCrAlY topcoat for oxidation protection and a YSZ diffusion barrier, both applied by atmospheric plasma spraying using a three-cathode torch. Two bond coats, NiCrBSi and CoNiCrAlY, were also tested. Test specimens with bond coats withstood 1000 h of exposure at 900 °C without delamination and no detectable oxygen at the coating-substrate interface. Samples produced with varying feed rates showed that graded coatings can be achieved using the APS process.

Cold spraying was applied to deposit Ti2AlC on different substrate materials. The study of single impacts by scanning electron microscopy indicates that bonding of the first layer is mainly attributed to the deformation and shear instabilities of the substrates. Nevertheless, the irregularly shaped particles appear to flatten by the impact. This deformation seems to be attributed to local, internal shear, but also to internal fracture. By applying up to five passes under more sophisticated spray parameters, Ti2AlC - coatings with thicknesses of about 110 to 155 µm can be achieved. XRD analysis of the coating proves that the crystallographic structure of the feedstock can be retained during cold spraying. The coating microstructures show rather low porosity, but several cracks between spray layers. Successful build-up of more than one monolayer can probably be attributed to internal deformation and occurring shear instabilities within the highly anisotropic Ti2AlC - phase.

In previous work, a thermal spray multilayer system consisting of ZrO 2 and an MCrAlY top coat showed promising results regarding oxidation behavior of the γ-TiAl substrates tested, which encouraged further research activities. Diffusion of substrate material was successfully inhibited by a ceramic ZrO 2 coating. A building up of a dense and stable oxide layer could be achieved by additional application of an MCrAlY top coat, leading to improved oxidation resistance and thus showing feasibility. In this work the main focus for development was put on enhancing adhesion and lowering residual stresses of the coatings in order to allow long term and cyclic testing without delamination taking place. Being a very brittle material, Gamma Titanium Aluminides require special surface treatment to enable roughening which is crucial for a strong mechanical bond between substrate and coating. Alternatives to conventional grit blasting as a standard preparation method were investigated. These were micro-abrasive blasting and blasting at elevated temperature (≈300-550 °C) to allow a more ductile behavior. The paper will highlight the implications by means of these measures and will also show the present development status of the multilayer system.

Due to excellent mechanical properties and low density compared to super alloys (e.g. Ni-based alloys) Titanium Aluminide is often used as base material in the aerospace industry. But the thermodynamic conditions within turbines limit the capabilities of the material. At the moment γ-TiAl is used for parts, which have to withstand temperatures up to 700 °C. Above this temperature oxidation kinetics cause a thick oxide layer consisting of several oxides, which tend to fast chipping. Therefore the surface of the γ-TiAl is being destroyed and the material loses its excellent mechanical properties. To enable the use of this material at higher temperatures, the development of an oxidation protection coating is necessary. Several coating techniques e.g. EB-PVD were tried in the last years, but the oxidation behaviour of the γ-TiAl could not be significantly improved. Protective thermal spray coatings so far seem to be a promising technology in order to protect γ-TiAl components against oxidation. Therefore this technique was used within this work, which aims for the development of new oxidation protection coatings. A multilayer system was developed. The multilayer consists of a ceramic ZrO 2 -7Y 2 O 3 coating with a NiCoCrAlY top coat. In this case the ceramic coating avoids the diffusion of Ti or Al of the γ-TiAl into the MCrAlY coating or the other way around. The NiCoCrAlY coating improved the oxidation behaviour of the Titanium Aluminide by building a dense oxide layer on top of the multilayer. The paper will give an overview about the results of the oxidation tests with the new developed multilayer concept for protection of the γ-TiAl against oxidation.

Intermetallic TiAl coatings were applied to ferritic steels using plasma and HVOF spraying methods. The specimens were then placed in reducing sulfidizing atmospheres for high-temperature corrosion testing. This paper describes the experiments that were performed and presents and analyzes the results. In general, for the reasons given, the coatings performed better in an Ar-H 2 -H 2 S-atmosphere than in one containing CH 4 . Paper text in German.