CHAPTER 21

GRAPHITE AND CARBON-CARBON COMPOSITES

Prepared by:A. E. ShapiroTitanium Brazing, Incorporated

ContentsIntroduction 382

Applications 382

Base Materials 383

Brazing Characteristics 388

Brazing Filler MetalCompositions andBrazing Methods 397

Prebrazing Treatmentand the BrazingProcedure 400

Diffusion Brazing ofGraphite to Steelwithout Adding aBrazing Filler Metal 402

Diffusion Brazing withCarbide Synthesis forthe Production ofHeat-Resistant Joints 402

Strength ofBrazed Joints 403

Troubleshooting 405

Bibliography 406

Photograph courtesy of Scarrott Metallurgical Company

AWS BRAZING HANDBOOK 381

INTRODUCTION

382 CHAPTER 21—GRAPHITE AND CARBON-CARBON COMPOSITES AWS BRAZING HANDBOOK

With respect to brazing, graphite and carbon-carbon (C/C) composites are somewhat similar tostructural ceramics and ceramic-matrix composites.However, although both types of material have highmelting points and relatively low coefficients of ther-mal expansion, structural ceramics are compatiblewith oxidizing environments, while carbon-basedmaterials typically begin oxidizing at temperaturesranging from 842°F to 1200°F (450°C to 650°C),depending on the perfection of the carbon structureand its purity.

In addition, ceramics generally have little or noporosity, while many carbon materials have signifi-cant amounts of open porosity. This porosity compli-cates the brazing process by wicking molten brazingfiller materials away from the joint. On the otherhand, carbon materials have higher thermal conduc-tivity, and coupled with their low coefficients of ther-mal expansion and high strength, they are much lesssensitive to thermal shock. The hot strength ofgraphite increases with temperatures from up to3630°F to 4530°F (2000°C to 2500°C). Therefore,at such high temperatures, the graphite and C/Ccomposites are the only materials that can maintainhigh strength and toughness. Finally, carbon materi-als are also more creep resistant than ceramics.

Solid scientific and industrial knowledge accumu-lated during the past three decades confirms thatbrazing and diffusion welding are the only joiningtechniques that provide sufficient strength and reli-ability of joints between carbon-based materials,both in similar and dissimilar combinations withmetals and ceramics.

Carbon materials vary widely in their degree ofcrystallinity; crystal orientation; and in the size,quantity, and distribution of porosity in the micro-structure. These factors, which are strongly depen-dent on the precursor materials and on processing,govern the physical and mechanical properties ofthese products. This potential for variation in prop-erties [such as the coefficient of thermal expansion(CTE)] can be advantageous because properties thatare significant from a brazing standpoint can bereadily modified by processing. For example, co-efficients of thermal expansion can be varied from0.8 × 10–6°F–1 (1.4 × 10–6°C–1) in the fiber directionin fiber-reinforced composites to 4.6 × 10–6°F–1 (8 ×10–6°C–1) for isostatically molded isotropic graphites.

APPLICATIONS

Graphite and C/C composites find widespread usein nuclear, aerospace, and electronic applications.They are also used as electrodes in metallurgical pro-cesses, fixtures and heating elements in vacuum fur-naces, chemical equipment and instruments, filters inbiochemistry, extractive metallurgy, and wastewatertreatment. Specialized uses include guide vanes, electricmotor brushes and switches, bushings and bearings,high-temperature heat exchangers, and plumbing, aswell as heart valves, and high-performance brakelinings.

GRAPHITE AND CARBON-CARBON COMPOSITES

CHAPTER 21