GB2239617A

GB2239617A - Coating for preventing oxygen embrittlement

Info

Publication number: GB2239617A
Application number: GB9027463A
Authority: GB
Inventors: Martin Thoma; Monika Bindl
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1989-12-20
Filing date: 1990-12-19
Publication date: 1991-07-10
Anticipated expiration: 2010-12-19
Also published as: FR2655996B1; GB9027463D0; DE3942025C1; GB2239617B; FR2655996A1

Abstract

Coating for preventing oxygen embrittlement on titanium components at elevated temperatures consists of sodium silicate (water glass) paint and silicon powder, in a silicon powder to sodium silicate paint ratio of 1:08 to 1:1.4 by weight, which effectively prevents the formation of oxygen diffusion layers. The protective coating exhibits a strong bond with the component and resists detachment also under gross alterations in temperature.

Description

Method for preventing oxygen embrittlement This invention relates to a

protective coating and to a method for preventing oxygen embrittlement of titanium components at elevated temperatures in an oxygenous atmosphere.

These elevated temperatures run in the 400 C to 900 C range. depending on the application.

For instance, when titanium alloy components are formed, these are heated to temperatures around 600 C to 900 C to take advantage of the improved formability at these temperatures.

it is primarily in engine aircraft construction that components are increasingly being manufactured from titanium, which is characterized by high strength at low weight. The objective here is to make thermally stressed components, such as blades in the aft compressor stages, from this material. although in advanced engine designs, these components are already undergoing temperatures of about 600 C.

When titanium components are heated, howeveri oxidation and oxygen diffusion occur on the component surface at temperatures above 500 C. Diffusion typically produces brittle surface layers of increased hardness whose thickness varies with the duration and temperature of exposure.

- 2 Such brittle diffusion layers are mostly undesirablei for the reason that they cause cracking in the component and so impair its strength and fatigue life.

To prevent the Impairment of component properties by the heat, resort is made to coatings which firmly adhere to the component surface.

Attempts are accordingly being made to prevent the formation of oxygen diffusion zones and the attending impairment of the component properties by coating the titanium component. Typical applications in this respect are nickel or aluminum diffusion coatings (aluminizing), although these are disadvantaged by brittle phases forming between the titanium and the coating material. While this indeed prevents formation of the one brittle phase, i.e. that with oxygen, it regrettably causes another brittle phase instead.

US Patent Specification 2.959,503, e.g., relates to a protective coating designed to prevent oxygen diffusion on titanium components, where a vitreous layer is formed on the component to remain there temporarily while the component undergoes heat treatment. After heat treatment the vitreous layer is caused to detach by dipping the component in water. This protective vitreous coating does not form until at temperatures exceeding 700 C. These elevated temperatures involve the hazard of embrittlement and incipient injury of the substrate already at the time of coating.

This surface layer is little suitable for permanently coating titanium components, considering that it adheres poorly to the component and detaches immediately it is subjected to alternating thermal loads.

Another method, disclosed in DE-OS 22 38 5921 is to brush ferrous components with a layer of siliceous water g 1 lass -1 1 glue to prevent scaling. The protective action derives from the fact that by diffusion. the silicon enters into chemical combination with the iron of the substrate to form ferrosilicon.

1 This method is disadvantageous in that, again, a diffusion layer is formed that exhibits hardness and brittleness properties much like those of the oxygen diffusion layers. Also. said method assumes the presence of iron, whereas the method of the present invention relates to titanium alloys.

Protective coatings with borides. carbides, nitrides. etc. have also been tested, which in fact may prevent embrittlement of the substrate. but involve a marked deterioration in strength.

in a broad aspect of the present invention a protective coating and a method for protectively coating titanium components is provided which permanently prevents the formation of oxygen diffusion compounds but does not impair the strength of the base material.

It Is a particular object of the present invention to provide a protective coating consisting of sodium silicate paint and silicon powder. where the Si powder-to-sodium silicate paint ratio amounts to 1:08 to 1:1.4 by weight.

The essential advantages afforded by this invention are that the inventive method provides a protective coating for titanium components which effectively prevents the formation of oxygen diffusion layers at elevated temperatures. Said protective coating strongly adheres to the component and resists detachment also in the presence of widely alternating temperatures. It is suited for effectively preventing oxygen embrittlement and corrosion at elevated temperatures not only on titanium components. but also on other substrates.

Another essential advantage is that the protective coating of the present invention permanently adheres to the Ti substrate and can therefore find upe. e.g., in a turbine engine for permanent protection from oxidation.

s Still another advantage is that when the method of the present invention is used. the strength is not appreciably degraded and virtually no negative effects on the components are indicated otherwise. Depositing the paint by spraying, atomizing or brushing poses little problem technically. and air drying again simplifies the process and reduces cost over conventional methods.

In a further advantageous aspect of the present invention the sodium silicate paint consists of 36% to 42% sodium silicate, which makes for excellent bond strength.

To obtain a good surface finish the particle size of the Si powder, which is added in an approximate 1:1 ratio. should be under 20 /un, where 60% to 80% should even have a particle size under 5 pm. This prevents the surface from being rough, and the paint can be processed and applied for better results.

In a further aspect of the present invention the place of pure silicon powder as an addition is taken by an Si Al alloy powder. While Al powder alone will not give the positive effects of the Si powder. it was learned with some surprise that the Si Al alloy powder serves well as an addition. In this application the alloy powder should contain aluminum and silicon in a ratio of 3:1 to 1:10 by weight,-with the particle size preferably matching that of pure silicon powder.

EXAMPLE:

A paint is prepared from 40% by weight sodium silicate and 60% water. As a pigment, Si powder of a particle size of 80% under 5 /ura, the remainder under 20 pm, was added to the paint in a 1:1 ratio by weight.

The surface of a component in IMI 834 (5.8% Ali 4.5% Sn, 4% Zr, 0.7% No. 0.5% Mo. 0.4 Sil 0.06% Ci. remainder titanium) is thoroughly cleaned and dry blasted. It is then brushed with a layer of sodium silicate paint about 35 /um thick.

The paint is air dried for about 30 minutes. Subsequently the component is heat treated at 600 C for 1000 hours.

No oxidation, oxygen embrittlement or metal reaction occurred. The paint coat adhered well also after heat treatment.

Claims

Claims:

1. Protective coating to prevent oxygen embrittlement on titanium components at elevated temperatures in an oxygenous atmosphere, characterized in that the coating consists of sodium silicate paint and silicon powder in a Si powder-to-sodium silicate paint ratio of 1:0.8 to 1:1A by weight.

2. Protective coating of Claim 1, characterized in that the sodium silicate paint consists of 30% to 50 % by weight sodium silicate and the remainder water.

3. Protective coating of Claim 2, characterized in that the sodium silicate paint contains 36% to 42% sodium silicate.

4. Protective coating of Claim 1, characterized in that the coating thickness of the paint coating is 5 /un to So /un, preferably 20 pm to 40 pm.

5. Protective coating of Claim li characterized in that the Si powder has a particle size of under 20 pm.

i- 1 1

6. Protective coating of Claim 5, characterized in that at least 60% and preferably 80% of the Si powder has a particle size under 5 pm.

Protective coating of Claim 1, characterized in that the Si powder is alloyed with aluminum.

8. Protective coating of Claim 7. charhcterized in that the Si Al alloy contains silicon and aluminum in a ratio of 1: 3 to 10: 1. preferably 1: li by weight.

9. Method for coating a titanium component with a protective coating of claim 1, characterized in that the sodium silicate paint is mixed with Si powder in a Si powder-to-sodium silicate paint ratio of 1:0.8 to 1:1A by weight and then deposited on the component and dried at room temperature.

10. Method of Claim 9, characterized in that the sodium silicate paint coating is sprayed or brushed onto the component surface.

11. Method of Claim 9. characterized in that prior to coating, the component surface is cleaned and dry blasted.

12. Method of Claim 9. characterized in that the paint is allowed to dry on the component surface at room temperature for a period of 0.5 to 2 hours.

13. Use of the protective coating of one of the Claims 1 to 8 on compressor blades, compressor casings and.connectIng parts of a gas turbine.

14. A protective coating substantially as herein described with reference to the example given.

15. A method for coating a titanium component with a protective coating substantially as herein described with reference to the example given.

Published 1991 at'Me Patent Office, State House. 66/71 High Holborn, London WC1R 477. Further copies may he obtained from Sales Branch. Unit 6. N.'ne Mile Point Cwmfelinfach. Cross Keys. Newport. NP) 7HZ. Printed by Multiplex techniques ltd, St MarY Cray, Kent.