WO1997042135A1

WO1997042135A1 - Improved oxidation protection for carbon/carbon composites and graphites

Info

Publication number: WO1997042135A1
Application number: PCT/US1997/007189
Authority: WO
Inventors: Terence B. Walker; Richard Donaldson; Philip Whalen
Original assignee: Alliedsignal Inc.
Priority date: 1996-05-02
Filing date: 1997-05-01
Publication date: 1997-11-13

Abstract

The invention provides a method for surface and subsurface deposition of very fine particle size silicon carbide (SiC) on exposed surfaces of carbon/carbon composites and graphites. Additionally, the invention provides a method for combining this process with the application of various phosphoric acid based penetrants to yield a binary oxidation protection system which provides outstanding oxidation protection, even at temperatures exceeding 850 °C.

Description

IMPROVED OXIDATION PROTECTION FOR CARBON/CARBON COMPOSITES AND GRAPHITES

This patent application is a continuation that claims the priority and benefit of provisional application 60/01 6,740, filed May 2, 1 996.

The present invention relates to oxidation protection for carbon/carbon composites, and in particular to oxidation protection for carbon/carbon composite aircraft brake discs.

Substantial evidence exists that SiC incorporated into the structure of carbon/carbon composites or graphites will inhibit oxidation of the finished carbon article, and numerous methods have been developed to incorporate SiC into various structures. Additionally, surface coatings of SiC have been applied via CVD (using silane and other gaseous precursors) to carbon/carbon articles for the purpose of oxidation protection. Both techniques involve complicated processes and/or equipment. It is highly desirable to provide a simple technique for development of both surface and subsurface SiC using economical precursors and simple application techniques. Additionally, phosphoric acid based penetrants have been used extensively to inhibit oxidation of carbon/carbon composite articles, such as in McAllister et al. U.S. Patent 4,837,073. The applicability of these systems has typically been limited to temperatures below 700°C. It is highly desirable to combine an SiC coating with various penetrants to provide improved oxidation resistance for a carbon/carbon composite. The present invention describes a simple technique to combine the previously applied SiC coating with various penetrants to form a binary surface treatment which provides outstanding oxidation resistance. The overall objective of the invention is to protect carbon/carbon composites or graphites at elevated temperatures up to and exceeding 850°C. The present invention provides solutions to the above by providing a carbon/carbon composite or graphite article adapted to resist oxidation which comprises: a. a substrate of fiber-reinforced carbon/carbon composite or graphite, and b. a surface and subsurface silicon carbide treatment which is achieved via the application of colloidal silica to the surface of the substrate and conversion of the treatment to silicon carbide via reaction with the substrate during heat treatment.

The invention is described in detail below, and the sole Figure illustrates the improved oxidation protection of the present invention. The invention may be applied to various carbon/carbon materials. Testing and evaluation, as described below, was done on CARBENIX® 4000 (AlliedSignal, Inc.), an aircraft brake carbon/carbon composite friction material, consisting of PAN precursor carbon fibers, densified with carbon utilizing CVD.

A solution of colloidal silica, such as LUDOX® AS-40 (Dupont Chemicals), is applied to the surfaces of the carbon/carbon article, either by brushing or spraying, or using pressure impregnation. Since substantial porosity exists in these materials, substantial subsurface penetration occurs. Drying of the article at approximately 80°C leaves a fine deposit of silica on all treated areas. Conversion of the SiO₂ deposit to SiC is accomplished by heat treatment of the treated article to 1600-1 800°C using standard inert gas (e.g. N₂ or argon) heat treatment equipment, such as is used in the heat treatment of carbon/carbon friction materials for aircraft brakes.

Treated articles (cylindrical coupons, 1 .9" diameter by .1 20" thick) were shown to have superior oxidation resistance compared with untreated articles when tested in flowing air (3scfh) at 1 500°F, for four hours. Weight loss for treated articles averaged 1 1 .5% while untreated articles showed an average weight loss of 1 7.5%. (See Chart I below.)

A variety of phosphoric acid based anti-oxidation penetrants may be used in combination with the above described SiC pretreatment to obtain outstanding oxidation resistance. Typical penetrant formulations are shown in Chart II below taken from McAllister et al. U.S. Patent No. 4,837,073 which is incorporated by reference herein. Additionally, penetrants A through E described therein may be modified with the addition of .1 %-50 wt. % sub-micron sized particles of Si₃N₄ (silicon nitride) or .1 -50 wt. % silica as colloidal silica suspension. Penetrants are applied to the SiC treated surface and then cured at 700-900°C in an inert gas atmosphere. It has been theorized that the SiC surface treatment enhances absorption and binding of the applied penetrants to the carbon/carbon surface.

Test specimens ( 1 .9" diameter, .120" thick cylindrical coupons, CARBENIX® 4000) were exposed to flowing air (3 scfh) at a temperature of 1 600°F (871 °C) for 4 hours. Weight loss under these conditions was measured to evaluate effectiveness of oxidation protection. Oxidative weight loss directly measures loss of heat sink mass as would be seen in field oxidation of carbon/carbon composite friction materials. Additionally, strong correlations have been shown between weight loss due to oxidation and loss of structural strength. Weight loss for each set of specimens was calculated and ranged from .5 to 10%. Specimens treated with the binary systems showed reduced weight loss (50 to 95% less) than specimens treated with the penetrants alone. Bare carbon specimens showed weight losses in excess of 50%. Comparison of the effects of the SiC pretreatment on oxidation resistance with various penetrants is shown below in the chart entitled "Oxidation Testing of Binary Systems" . Comparisons are shown for articles utilizing no pretreatment (" Plain Substrate"), an anti-oxidative carbon CVD pretreatment ("AO CVD Substrate"), and the described SiC pretreatment ("SiC Substrate"). "AO CVD Substrate" is a high-density surface coating of graphitic carbon used previously to enhance the anti-oxidation properties of phosphoric acid based penetrants. (Penetrant C is described in Chart II above, Penetrant Q is Penetrant C modified with 31 .87 parts of LUDOX®, while Penetrant Y is Penetrant C modified with sub-micron Si₃N₄ at a ratio of 31.9 g Si₃N₄ to 90 cc Penetrant C). From the sole Figure, it is clear from oxidation testing that the binary system comprising penetrant treated "SiC Substrate" exhibits superior oxidation resistance.

Claims

WHAT IS CLAIMED IS:

1 . A carbon/carbon composite or graphite article adapted to resist oxidation which comprises: a. a substrate of fiber-reinforced carbon/carbon composite or graphite, and b. a surface and subsurface silicon carbide treatment which is achieved via the application of colloidal silica to the surface of the substrate and conversion of the treatment to silicon carbide via reaction with the substrate during heat treatment.

2. The article in accordance with claim 1 , wherein the article has subsequently been treated with phosphoric acid based penetrant comprising 1 0-50% H₂O/ 20-70% H₃PO₄/ 0-25% MnP0₄ ^'1 .6H₂0/ 0-30% AIH₂P0₄/0-2% B₂O₃/ 0-10% Zn₃P0₄.

3. The article in accordance with claim 2, wherein the described penetrant has been modified with the addition of .1 -50% by weight of one of sub-micron sized Si₃N₄ and colloidal silica suspension.

4. The article in accordance with claim 1 , wherein the heat treatment is at a temperature greater than 1 600°C in an inert gas.

5. The article in accordance with claim 1 , wherein the heat treatment includes drying of the colloidal silica.

6. A method for protecting a substrate from environmental degradation, comprising the steps of:

(a) applying colloidal silica to the substrate, and

(b) heating the substrate.

7. The method in accordance with claim 6, further comprising the step of drying the applied colloidal silica.

8. The method in accordance with claim 6, further comprising treating the substrate with a penetrant, and curing the penetrant.