GB2252981A

GB2252981A - Diffusion barrier coating for titanium alloys involving alloying

Info

Publication number: GB2252981A
Application number: GB9203439A
Authority: GB
Inventors: Albert G Tobin
Original assignee: Grumman Aerospace Corp
Current assignee: Grumman Corp
Priority date: 1991-02-19
Filing date: 1992-02-18
Publication date: 1992-08-26
Also published as: FR2672906A1; DE4204447A1; GB9203439D0

Abstract

In order to produce an oxidation-resistant surface for titanium aluminides and alloys, for use in aerospace structures, a refractory metal is deposited on a substrate of the titanium material to form a diffusion barrier. The surface of the barrier is then alloyed with a metal to form a protective oxide scale on exposure to a high-temperature oxidizing medium. In a preferred embodiment niobium or tantalum is used as the refractory barrier layer. Alloying of the barrier layer may be effected by applying a surface alloy consisting of alloy compositions of Cr, Ni, Fe, Co and Al is either alloyed to the surface alloy layer either by aluminization or by co-sputtering Al with the other elements which alloy then alloys with the barrier layer surface, suitably with heating. Alternatively a film of one of Ni, Fe, Cr or Co is applied onto a Nb film and then a layer of Al is evaporated thereon.

Description

DIFFUSION BARRIER COATING FOR TITANIUM ALLOYS The present invention

relates to oxidation protection for metals, and more particularly to titanium alloys and titanium aluminides operating at high temperatures.

High temperature, lightweight aerospace structural composites are required for high Mach number airframes and next generation propulsion systems requiring high strengthto-weight and stiffness-to-weight ratios. Titanium alloys and titanium aluminides are attractive materials for this application. Although considerable effort ha5gone into alloy and metal matrix composite development, the applicability of these advanced structural materials at elevated temperatures will ultimately depend on their resistance to the oxidizing gases that are an integral part of their operating environment. This is due to the sensitivity of the mechanical properties of titanium alloys to oxygen embrittlement. Because of the high solubility (i.e., 30 at. %) and diffusivity of oxygen in titanium at elevated temperatures, exposure to an oxidizing environment leads to the growth of a non-protective oxide film that becomes a source for oxygen dissolution into the alloy substrate.

Significant reductions in ductility, fatigue and creep properties of titanium alloys and titanium aluminides may occur since oxygen concentrations exceeding 1 at. % in the alloy matrix are sufficient to cause significant loss in ductility and fracture toughness. It is thus expected that under operating conditions cracks will nucleate and propagate from such an embrittled surface layer. The application of overlay coatings to protect against oxidation may also lead to localized embrittlement of the substrate and subsequent loss of mechanical properties due to either interdiffusion and/or brittle intermetallic compound formation at the coat ing-substrate interface. In addition, thermal and/or mechanical strains (i.e., fatigue) that may occur during usage can lead to cracking or spallation of the coating with subsequent loss of protectiveness. Surface modifications that provide oxidation protection over a wide range of operating conditions and which do not degrade substrate mechanical properties are needed to fulfill the high temperature structural potential of titanium alloys and titanium aluminides.

In the prior art successful surface modifications were developed for vanadium-base alloys for use in oxidizing gases in fusion reactor blanket applications. It was shown that surface alloying of vanadium base-alloys with chromium was effective in preventing both oxygen absorption and oxygen diffusion into the alloy substrate in moist He environments. This principle was applied to surface protection of several commercial titanium alloys and a2 titanium aluminides in which aluminum was diffused into the alloy via a vapor transport process and led to a significant improvement in cyclic air oxidation resistance up to 8150C. However, it was also shown that this process may also lead to some reduction in tensile properties of the substrate.

According to the present invention, there is provided a coating method for protecting a substrate of titanium material comprising the steps: depositing a diffusion barrier layer of refractory metal on the substrate; and depositing an oxidation-resistant metal layer on the barrier layer for alloying with the surface of the barrier layer.

A novel method is provided for surface modification that not only improves high temperature oxidation resistance but also preserves the baseline fatigue and fracture properties of titanium alloys in high temperature oxidizing environments.

In order to provide an oxidation-resistant surface on titanium alloys that does not degrade mechanical properties of the baseline material, it is necessary to eliminate the potentially damaging effects of the coating-substrate interdiffusion via the introduction of a ductile diffusion barrier layer on the surface. The unique feature of this disclosure is that the barrier layer will not lead to the formation of brittle intermetallic compounds at the substrate-barrier interface. This is needed to guarantee that the titanium alloy substrate will not be embrittled by the oxidation- resistant coating and to prevent the formation of a non-protective titanium oxide film on exposure to the oxidizing environment. The barrier layer is subsequently alloyed with specific alloying elements that will provide a protective oxide scale (i.e., A1203) on exposure to an oxidizing environment. The choice of materials and processing steps to apply the barrier layer and to create an oxidation-resistant alloyed layer on the surface of the barrier layer constitute the main features of this disclosure.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. I is a diagrammatic illustration of the layers deposited on a substrate of titanium material in accordance with the present invention; FIG. 2 is a plot of oxidation test results of the present method.

Either of the pure refractory metals Nb and Ta have been selected to act as the diffusion barrier layer 14 for the titanium material substrate 10 shown in FIG. 1. This choice of is based upon the low diffusivity of Ti in these materials and their high solubility and lack of brittle intermetallic compound formation with the titanium. The oxidationresistant surface alloy layers consists of selected ternary or quartenary alloy compositions of Cr, Ni, Fe, Co, and Al is either alloyed to the surface alloy layers either by aluminization or by co-sputtering Al with the other elements. The choice of these particular elements are based upon a careful survey of the literature of oxidationresistant Nb and Ta alloys in which very low oxidation rates have been observed at temperatures up to 1200"C in air environments. The diffusion barrier layer may be created by a variety of conventional coating processes including but not exclusively limited to sputtering, electroplating, chemical vapor deposition, physical vapor deposition, ion beam techniques and/or combinations of these techniques. It is desirable (but not necessary) to co-deposit (i.e., via co-sputtering or co- evaporation) the oxidation-resistant alloy layer on to the barrier layer to guarantee the proper alloying of the barrier layer surface for maximum oxidation protection. However, vacuum heat treatments of surface alloy coated parts in the range 700-1100C for periods of up to 24 hours may be required to achieve the desired alloy composition on the barrier layer surface. The choice of heat treat time and temperature is dictated by the particular surface alloy composition of interest.

The following example illustrates this invention.

EXAMPLE

Titanium alloy substrates (1.27cmxl.27cmxO.0762cm) (0.5"xO.5"xO.030") consisting of titanium aluminide (Ti-25Al-1ONb3V-1Md) were prepared by mechanical polishing to 600 grit SiC abrasive and rinsed in acetone and methanol. The specimens were sputtered on both sides with an Nb f ilm for 2 hours. This produced 2-59M thick sputtered Nb coating on each side of the specimen.

A coating of Ni was subsequently sputtered on the Nb sputtered substrate. similar experiments were performed using Fe, Cr, and Co sputtered films. Sputter-coated specimens were then placed into a vacuum chamber containing a charge of pure Al in a graphite mold and vacuum heated to 1000C for 1-4 hours. During this process the Al vaporized and reacted with the surface to form an Nb-Ni-Al alloy on the barrier layer surface. Similar surface reactions occurred with the Fe, Cr, and Co surface modifications.

Coated specimens were placed in an air oxidizing furnace (at 1 atm) pre-heated to 815-9000C and oxidized for various times. After each exposure the specimens were weighed and compared with a corresponding uncoated specimen. FIG. 2 illustrates oxidation typical test results for Ti25Al-lONb-3V-lMo alloy sputter coated with Nb barrier, Ni and Al vapor aluminization at 10000C for 4 hours. These results clearly show that the surface modified layers exhibited a significant improvement in oxidation resistance over the uncoated specimen. Scanning electron microscopy studies indicated that the barrier layer of Nb had been preserved although some titanium had diffused into it.

In summary, the present method for producing an oxidation-resistant surface f or titanium aluminides and alloys involves the introduction of a refractory metal diffusion barrier that is metallurgically compatible with the alloy substrate. The surface of the diffusion barrier is then alloyed to form a protective oxide scale on exposure to a high temperature oxidizing medium. The advantage of this approach is that it provides oxidation protection for the substrate by preventing titanium diffusion to the surface and prevents embrittlement of the alloy substrate due to coating-substrate interactions.

Claims

CLAIMS:

1. A coating method for protecting a substrate of titanium material comprising the steps:

depositing a diffusion barrier layer of refractory metal on the substrate; and depositing an oxidation-resistant metal layer on the barrier layer for alloying with the surface of the barrier layer.

2. The method as claimed in claim 1, wherein the barrier layer refractory metal is selected from niobium and tantalum.

3. The method as claimed in claim 1 or 2, wherein the intermediate oxidation-resistant layer is selected from nickel, iron, chromium, cobalt and aluminum.

4. A multi-layer oxidation coating for a substrate of titanium material comprising:

a barrier layer of refractory metal which minimizes diffusion thereacross between the substrate and the coating external of the barrier layer; and an oxidation-resistant layer alloyed with the surface of the barrier layer; wherein the barrier layer refractory metal is selected from niobium and tantalum; and further wherein the intermediate oxidation-resistant layer is selected from nickel, iron, chromium, cobalt and aluminum.

5. A coating method substantially as hereinbefore described with reference to the foregoing Example.

6. A multi-layer oxidation coating produced on a substrate of titanium material, wherever produced by the method of any one of claims 1 to 3 and 5.