CERAMICS

Prepared by:

F. M. HoskingSandia National Laboratories

J. J. StephensSandia National Laboratories

Contents

Introduction 458

Ceramic Materials 458

Brazing Processes 462

Joint Design 464

Joint Functionalityand ProcessDevelopment 467

Processes andEquipment 467

Surface Preparationand Cleaning 469

Bibliography 469

SuggestedReading List 469

CHAPTER 24

Photograph courtesy of Altair Technologies, Incorporated

AWS BRAZING HANDBOOK 457

INTRODUCTION

458 CHAPTER 24—CERAMICS AWS BRAZING HANDBOOK

Ceramics1,2 are inorganic nonmetallic materialsthat can be separated into two broad categories—traditional ceramics and structural ceramics. A com-mon characteristic of ceramic materials is that theyare manufactured from powders that are formed to adesired shape with a glassy binder and then heated toa high temperature with or without the applicationof external pressure to achieve a final densifiedbrazement.

Traditional ceramics include clay products andrefractories. These materials typically have low den-sities (because of relatively high porosity content).They are normally used in high-temperature applica-tions for which brazing is not practical.

Structural ceramics include monolithic materialssuch as aluminum oxide (Al2O3), zirconium oxide(ZrO2), silicon carbide (SiC), aluminum nitride(AlN), silicon nitride (Si3N4), and silicon-aluminumoxynitrides (SiAlONs) as well as composites madeentirely of ceramics like Al2O3 containing SiC whis-kers or SiC containing titanium diboride particles.Care is generally taken during the manufacture ofstructural ceramics to ensure that the chemical com-position is controlled and high densities (or relativelylow porosity contents) are achieved. The brazing ofstructural ceramics is possible and widely practiced.

The technological interest in structural ceramics isdirectly related to their unique properties as com-pared to metals. Many ceramics are characterized byhigh strength, not only at room temperature but alsoat elevated temperatures. Silicon carbide, for exam-

1. Originally sponsored by the U.S. Department of Energy, Assis-tant Secretary for Conservation and Renewable Energy, Office ofTransportation Technologies, as part of the Ceramic Technologyfor Advanced Heat Engines Project of the Advanced MaterialsDevelopment Program, under contract DE-AC05-840R21400with Martin Marietta Energy Systems, Inc.2. The submitted manuscript was authored by a contractor of theU.S. Government under contract No. DE-AC05-840R21400.Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this con-tribution, or allow others to do so, for U.S. Government purposes.

ple, can maintain a tensile strength in excess of 29 ×103 pounds per square inch (psi) (200 megapascals[MPa]) at 2786°F (1530°C), the melting point ofiron. Other ceramics like Si3N4 and certain ceramiccomposites also maintain similar strengths at hightemperatures.

In addition to high strength, other properties thatmake ceramics attractive candidates for applicationsthat have usually been reserved for metallic alloysinclude excellent wear resistance, high hardness,excellent corrosion and oxidation resistance, lowthermal expansion, and high electrical resistivity.

Structural ceramics are being used or consideredfor use as cutting tools, bearings, machine tool com-ponents, dies, pump seals, high-temperature heatexchangers, and a variety of internal combustion andturbine engine components. The typical properties ofseveral ceramic materials and metallic alloys are pre-sented in Table 24.1.

Even though there is keen interest in the develop-ment of structural ceramics and their use in new andunusual engineering applications, it is in the electron-ics industry where the largest fraction of ceramics isactually being used. Likewise, while the developmentof brazing technologies for materials like ZrO2,Si3N4, and SiC has been pursued vigorously in recentyears, Al2O3 is still the most widely used structuralceramic with a sizeable commercial market. For thisreason, the brazing of Al2O3 using commerciallyavailable practices is emphasized throughout theremainder of this chapter.

CERAMIC MATERIALS

Ceramics are generally defined as inorganic, non-metallic materials. Typical examples include alu-mina, silicon nitride, silicon carbide, aluminumnitride, and zirconia. Most ceramics are based onoxide, carbide, and nitride compounds. What differ-

CERAMICSCHAPTER 24