Formation and high-temperature oxidation of thermal-barrier coatings with Ti-Al-Cr binding layer

Abstract

The behavior of TiAlCr detonation coatings with an outside thermal barrier layer in high-temperature oxidation in air is examined. It is established that the efficiency of thermal-barrier coatings with a zirconia external layer on substrates of Ti-rich alloys and γ-TiAl depends on the oxidation of the binding TiAlCr layer and the diffusion at the coating-substrate interface. It is shown that the presence of a ceramic layer has no fundamental effect on the oxidation of γ-TiAl-based coatings. The behavior of TiAlCr coatings in high-temperature oxidation depends on the substrate structure, which determines the nature of diffusion at the coating-substrate interface. If the substrate is made of titanium-rich alloys, there is active diffusion of Al and Cr from and of Ti into the coating. Since the Al/Ti activity ratio changes, a mixed layer of TiO 2 and Al 2 O 3 forms on the surface. In case of the γ-TiAl substrate, a layer on the TiAlCr surface consists of Al 2 O 3 alone.Protective high-temperature coatings of chromium-containing titanium aluminides, which can act both as a surface layer and as a binding layer in a thermal-barrier coating, hold much promise for increasing oxidation resistance of alloys based on titanium and γ-titanium aluminide to 900-1000°C.Thermal-barrier coatings are used in various industries to protect gas-turbine engine parts, diesels, different space devices against the environment at high temperatures. Thermal-barrier coatings usually consist of a binding metal layer and an outer yttria-stabilized zirconia layer. The service life of a coating greatly depends on the capability of the binding layer to form a solid Al 2 O 3 layer at the interface with the outer ceramic layer to protect the base from oxidation.In spite of numerous research efforts [1-6], the phase formation in the Ti-Al-O system in metastable conditions, which ensure the maximum high-temperature oxidation resistance, is still a pressing issue.It has been experimentally established that the temperature range over which γ-titanium aluminide remains stable at 49 to 66 at.% Ti, according to the phase diagram, is limited to 750-800°C [7]. Sale consisting of a Al 2 O 3 and TiO 2 mixture is usually formed in the surface materials layer at these temperatures. The presence of rutile is undesirable as this loose oxide cannot protect the metal sublayer against oxygen; moreover, it degrades very quickly.Replacing some part of aluminum with chromium is a promising method of increasing the oxidation resistance of alloys based on titanium aluminide to 900-1100°C, fracture roughness being adequate. An analysis of research efforts on the development of oxidation-resistant Ti-Al-Cr alloy with acceptable properties has identified two areas. The first area deals with attempts to use γ-titanium aluminide, which has good mechanical properties, as a base and then increase the oxidation resistance of the alloy by doping from 800 to 1100°C. It is established that the excessive amount of chromium makes such materials brittle. For example, the paper [1] rep...

Thermal spraying of transition metal aluminides: An overview

“…Detonation Gun: Oliker and workers [149e151] have compared the coating structure obtained by spraying a prealloyed g-TiAl powder and a mechanical alloyed Ti þ Al mixture, obtaining the exact powder structure in the resulting deposit for the first case and a ceramic compound based on Al 2 TiO 5 with TiN inclusions. Regarding Cr-doping, D-gun TiAlCr coatings (AL153 -TiAl 3 -based alloys group-and TiAlCrSc alloys -g-TiAl-based group-on substrates of g-TiAl and VT-16) have also been used for binding in thermal barrier systems [152]; after being tested by thermal cycling in air at 900 C for 56 h, concluding that not the ceramic but the composition of the substrates greatly influences the protection behaviour (Fig. 10).…”

Thermal and Cold Spray: Recent Developments

“…For NiAl intermetallic and eutectic (β-NiAl + γ-Re) the resistance at oxidation up to 1100°C was controlled by the ratio between rates at which Al and Ni oxides are formed [192]. For thermal barrier bond coats Ti-Al-Cr [193] were tested. The behavior of TiAlCr depends on the substrate structure which determines the nature of diffusion.…”