This revolutionary development provides an excellent protective oxide coating for titanium aluminide with very strong substrate bonds. Presently, titanium aluminide compounds do not form a good protective oxide coating that prevents oxygen from diffusing into the titanium aluminide and forming oxides that degrade the compound.
This is because in air or oxygen, alpha-Al2O3 and TiO2 will form simultaneously, but they are not compatible oxides. They do not form a solid solution together. If these are the only oxides formed, they will form weak oxides that do not bond together and will form only a weak bond to the titanium aluminide substrate. As a result, the oxides are likely to peel from the substrate.
Forming a Protective Coating on Titanium Aluminide
These issues are overcome and the protective coating is accomplished by using a two-step process that begins by causing aluminum to be the dominant oxide former. In the first-step of this process, a very strong alpha-Al2O3/titanium aluminide substrate interface bond is formed. This is followed by a growth of only alpha-Al2O3 oxide above this interface until an alpha-Al2O3 oxide layer, at least 5000 angstroms thick, is formed above the interface. An attempt is made during this growth period to form a fully oxidized alpha-Al2O3 oxide, with the oxygen and aluminum atoms in their preferred sites, and to form an ordered structure within the alpha-Al2O3 molecules. Meanwhile, titanium atoms diffusing from the titanium aluminide substrate may momentarily form an oxide, but aluminum atoms in the area will immediately reduce the oxide to titanium and oxygen. However, since titanium atoms can diffuse faster than aluminum atoms in alpha-Al2O3 oxide, these titanium atoms will diffuse outward through the alpha-Al2O3 oxide and form Ti2O3 molecules above the alpha-Al2O3 oxide coating. Alpha-Al2O3 and Ti2O3 molecules are fully compatible because they form the same crystalline structure and have very similar lattice parameters. At this point, the original alpha-Al2O3 oxide layer will continue to grow slowly, but the mixed alpha-Al2O3 and Ti2O3 oxides will grow much more quickly.
During the second-step process, TiO2 will begin to form in a very accommodating way, as fiber-like, micrometer-diameter columns embedded in the alpha-Al2O3/Ti2O3 matrix and growing approximately perpendicular to the plane of the titanium aluminide surface. The overall structure consists of a strongly adherent alpha-Al2O3/titanium aluminide interface with at least a 5000 angstrom thick alpha-Al2O3 oxide growing above it, TiO2 fiber-like, micrometer-diameter columns embedded in the mixed alpha-Al2O3/Ti2O3 matrix and strengthening it with the final surface showing the TiO2 fibers protruding upward from the top of the mixed oxide matrix in which the fibers are embedded. This type of structure has been grown to thicknesses as great as 50 micrometers. Stylometer measurements have shown that the cohesive strength of the overall mixed oxide is a function of the degree of oxidation that has taken place. As the mixed oxide approaches a state of nearly full oxidation, the cohesive strength approaches the very high cohesive strength of the original alpha-Al2O3 oxide coating measured at the end of the first-step process.
Benefits of Using The Protective Coating on Titanium Aluminide
- Creates an absolute oxygen barrier coating for the titanium aluminide substrate.
- Acts as a self-healing coating with surface titanium oxides, in the event some surface destruction by an impinging particle, by diffusing titanium atoms forward to the surface to react with oxygen to affect a repair.
- The ability of the overall alpha-Al2O3/Ti2O3/TiO2 mixed oxide system to protect the surface of titanium aluminide better than any known mix of oxides. At the same time, each of the oxides in the overall coating has a very high cohesive strength and each is joined to each other and to the substrate with very high strength adhesive bonds.
- Provides an atmospherically exposed surface with fiber-like, micrometer-diameter crystalline columns, embedded in the mixed alpha-Al2O3 and Ti2O3 matrix, that have grown in a nearly perpendicular direction relative to the plane of the titanium aluminide substrate surface. The outer surface may attach well to an applied Thermal Barrier Coat (TBC).
Applications for the Coating
- Aircraft manufacturing
- Aircraft engine manufacturing
- Automobile manufacturing
- Superalloy products
- Manufacturing of superalloys.
- Steel and stainless steel products
- Manufacturing of iron, nickel, chromium, and cobalt based alloys
- Products made from iron, nickel, chromium, and cobalt based alloys
- Products that must be protected from oxidation over a wide range of temperatures
- Products from alloys that must be protected from atmospheric oxygen at all operable temperatures
Source: NextTechs Technologies
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