FIELD

The present disclosure relates generally to composites and, more specifically, to oxidation protection systems for carbon-carbon composite structures.

BACKGROUND

Oxidation protection systems for carbon-carbon composites are typically designed to minimize loss of carbon material due to oxidation at operating conditions, which include temperatures of 900° C. (1652° F.) or higher. Phosphate-based oxidation protection systems may reduce infiltration of oxygen and oxidation catalysts into the composite structure. However, despite the use of such oxidation protection systems, significant oxidation of the carbon-carbon composites may still occur during operation of components such as, for example, aircraft braking systems. In addition, at such high operating temperatures, phosphate-based oxidation protection systems (OPS) applied to non-wear surfaces of brake disks may experience decreasing viscosity, which may cause the OPS to migrate away from non-wear surface edges proximate to a wear surface of the brake disk, leaving the composite material at or proximate to the non-wear surface edges vulnerable to oxidation. Even further, at the high operating temperatures, components in the OPS may oxidize, and in some cases, evaporate from the OPS, lessening the oxidation protection capabilities.

SUMMARY

A method for forming an oxidation protection system on a composite structure is provided. In various embodiments, the method may comprise forming a first slurry by combining a first pre-slurry composition with a first carrier fluid, wherein the first pre-slurry composition comprises a first phosphate glass composition; applying the first slurry to the composite structure; heating the composite structure to a temperature sufficient to form a base layer on the composite structure; forming a second slurry by combining a second pre-slurry composition with a second carrier fluid, wherein the second pre-slurry composition comprises a second phosphate glass composition and a strengthening compound comprising at least one of boron nitride, a metal oxide, or silicon carbide; applying the second slurry to the composite structure; and/or heating the composite structure to a temperature sufficient to form a sealing layer on the composite structure. In various embodiments, the strengthening compound may comprise boron nitride. In various embodiments, the metal oxide may comprise at least one of aluminum oxide, silicon dioxide, and titanium oxide.

In various embodiments, the second slurry may comprise between 0.1 and 6 weight percent boron nitride. In various embodiments, the second slurry may comprise between 2 and 5 weight percent boron nitride. In various embodiments, the second phosphate glass composition may comprise a glass frit, and wherein a weight ratio of boron nitride to the glass frit in the second slurry may be between 5:100 to 15:100. In various embodiments, the second phosphate glass composition may comprise a glass frit, and wherein a weight ratio of boron nitride to the glass frit in the second slurry may be between 10:100 to 12:100.

In various embodiments, the first pre-slurry composition may further comprise a silica compound comprising at least one of silica and a silica former. In various embodiments, the silica former may comprise at least one of a metal silicide, silicon, fumed silica, silicon carbide, and silicon carbonitride. In various embodiments, the first pre-slurry composition of the base layer may comprise between about 15 weight percent and about 30 weight percent boron nitride. In various embodiments, the first pre-slurry composition may comprise a first acid aluminum phosphate wherein a first molar ratio of aluminum to phosphate may be between 1 to 2 and 1 to 3.

In various embodiments, the method may further comprise applying a pretreating composition, wherein the applying comprises applying a first pretreating composition to an outer surface of the composite structure before the applying the first slurry, wherein the first pretreating composition comprises aluminum oxide and water; heating the pretreating composition; and/or applying a second pretreating composition comprising at least one of a phosphoric acid or an acid phosphate salt, and an aluminum salt on the first pretreating composition, wherein the composite structure is porous and the second pretreating composition penetrates at least a portion of a plurality of pores of the composite structure.

In various embodiments, at least one of the first phosphate glass composition or the second phosphate glass composition may be represented by the formula a(A′2O)x(P2O5)y1b(GfO)y2c(A″O)z:

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;

Gfis selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y1is a number in the range from about 0.100 to about 0.950;

y2is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y1+y2+z)=1; and

x<(y1+y2).

In various embodiments, an oxidation protection system disposed on an outer surface of a substrate may comprise a base layer comprising a first pre-slurry composition comprising a first phosphate glass composition; and/or a sealing layer comprising a second pre-slurry composition comprising a second phosphate glass composition and a strengthening compound comprising at least one of boron nitride, a metal oxide, or silicon carbide. The sealing layer may be disposed on the base layer. In various embodiments, the strengthening compound may comprise boron nitride. In various embodiments, the second phosphate glass composition may comprise a glass frit, and wherein a weight ratio of boron nitride to the glass frit in the sealing layer may be between 5:100 to 15:100. In various embodiments, the base layer may further comprise a silicon compound comprising at least one of silica and a silica former. In various embodiments, the silica former may be at least one of a metal silicide, silicon, fumed silica, silicon carbide, and silicon carbonitride.

In various embodiments, at least one of the first phosphate glass composition and the second phosphate glass composition is represented by the formula a(A′2O)x(P2O5)y1b(GfO)y2c(A″O)z:

A′ is selected from: lithium, sodium, potassium, rubidium, cesium, and mixtures thereof;

Gfis selected from: boron, silicon, sulfur, germanium, arsenic, antimony, and mixtures thereof;

A″ is selected from: vanadium, aluminum, tin, titanium, chromium, manganese, iron, cobalt, nickel, copper, mercury, zinc, thulium, lead, zirconium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, actinium, thorium, uranium, yttrium, gallium, magnesium, calcium, strontium, barium, tin, bismuth, cadmium, and mixtures thereof;

a is a number in the range from 1 to about 5;

b is a number in the range from 0 to about 10;

c is a number in the range from 0 to about 30;

x is a number in the range from about 0.050 to about 0.500;

y1is a number in the range from about 0.100 to about 0.950;

y2is a number in the range from 0 to about 0.20; and

z is a number in the range from about 0.01 to about 0.5;

(x+y1+y2+z)=1; and

x<(y1+y2).

In various embodiments, an aircraft brake disk may comprise a carbon-carbon composite structure comprising a non-wear surface; and/or an oxidation protection system disposed on the non-wear surface. The oxidation protection system may comprise a base layer comprising a first pre-slurry composition disposed on the non-wear surface, wherein the first pre-slurry composition comprises a first phosphate glass composition; and/or a sealing layer comprising a second pre-slurry composition comprising a second phosphate glass composition and boron nitride disposed on the base layer, wherein the sealing layer comprises between 5 and 15 percent weight boron nitride.

