Stick with Ceramics

Ceramics are an incredibly diverse family of materials whose members span traditional ceramics (such as pottery and refractories) to the modern day engineering ceramics (such as alumina and silicon nitride) found in electronic devices, aerospace components and cutting tools.

Ceramics exhibit very strong ionic and/or covalent atomic bonding (stronger than the metallic bond) and this confers the properties commonly associated with ceramics; high hardness and chemical inertness. This strong bonding also accounts for the downside of ceramics, ie low tensile strength and low ductility.

Engineering ceramics give added functionality and/or enhanced performance. However, despite their many attractive properties, they cannot singly solve every application, and inevitably there is a need to join them to another component, frequently made from a different material. It is also difficult and expensive to produce large or complex shaped ceramic components.

Joining small and simple shaped parts together to form a complex component can overcome these limitations in size and geometry. Such a philosophy has been a major driving force in the study of ceramic/ceramic and ceramic/metal bonding. Joining has become widely acknowledged as being a key enabling technology for the utilisation of engineering ceramics. A key focus of joining techniques is to ensure that the joint is not the performance limiting, weak-link of the component.

There are many possible processes for joining ceramics, which can be broadly distinguished as mechanical attachment or chemical bonding. Chemical bonding is the more widely used and there are several potential processes, including adhesives, brazing, ultrasonic bonding, and diffusion bonding.

In general, the methods for mechanical attachment are simple in design. However, designs employing ceramic bolts or screw threads are both expensive and prone to cracking in the ceramic. Sharp changes in section such as the zigzag of a screw thread act as stress concentrators in the ceramic and should be avoided as a matter of best practice.

The most widely used method of mechanical attachment is shrink fitting, which utilises the difference in thermal expansion between ceramics and metals (metals usually expand more than ceramics on heating). The joint is formed at temperature; the metal expands sufficiently to surround the ceramic, then on cooling contracts to clamp it. Spark plugs are a good example of the use of shrink fitting, illustrated in Figure 2.

Organic adhesives, providing low temperature chemical bonding, can be readily used to bond ceramics for applications that do not require a hermetic seal. Their service temperature is limited to less than 150 C.

Ceramic adhesives have been developed that have a considerably higher service temperature (> 2000 C). Generally, they are based on alkali silicates or metal phosphates with different ceramic fillers, such as alumina, silica and zirconia. They are usually applied as pastes and cured at temperatures of around 1000 C. They are excellent for high temperatures, but can be friable and are not hermetic.

One of the most widely used processes for ceramic joining is brazing, which is a liquid phase process. The key element here is to ensure that the braze alloy wets and reacts with the ceramic. Most ceramics are inert to braze alloys; the braze simply balls up when molten, exhibiting similar behaviour to mercury on glass. Chemical reactivity is usually achieved by metallisation where the surface of the ceramic is pre-treated to encourage wetting during the brazing operation or by the use of active metal brazes which incorporate deliberate additions of elements that will react with the ceramic surface to render it wettable by the braze alloy. In either case, the brazing operation must be carried out in controlled atmosphere or vacuum.

One of the most common methods of metallisation is the ‘moly-manganese’ process, which has been found to be well suited to ceramics containing small amounts of intergranular glass, such as alumina. This route is used for producing ceramic to metal seals for high vacuum equipment. Once the ceramic is metallised, components can be brazed with a wide range of conventional metallic braze alloys. However, the moly-manganese process is only suited to ceramics that have an intergranular glassy phase. For ceramics without this glassy phase, other metallisation routes are required. Alternative metallisation routes include vapour deposition of metals such as titanium and zirconium.

A simpler brazing route is the use of active metal braze alloys. Figures 1 and 3 show a silicon nitride turbocharger rotor bonded to a steel shaft using an active Ag-Cu braze. This component is interesting because it also uses interlayers to overcome the mismatch in thermal expansion coefficient between silicon nitride and steel (~ 4 x 10-6/degree C and 12 x 10-6/degree C). These interlayers have an intermediate thermal expansion and lower modulus (stiffness) which can accommodate residual stresses generated on cooling. If the interlayers were not present and the ceramic and metal were brazed in direct contact, the ceramic would crack. This dome cracking is characteristic of thermal expansion mismatch.

The design and testing of ceramics is more critical than for any other class of material. The following principles should be kept in mind when designing with ceramics (and glasses):

* Define the working conditions and requirements as closely as possible, focusing on factors such as external loading, temperature regime and so on.

* The design must take into account the specific properties of the ceramic, such as high compressive strength; lower in tension bending or torsion.

* High stiffness (brittleness).

* Susceptibility to impact and point loading; high contact stresses (particularly at points of support or load transfer) are potentially detrimental. Thin, ductile, metal interlayers or inserts are sometimes used to distribute stress more evenly.

* Susceptibility to thermal shock; components subjected to temperature changes should have simple and symmetrical form.

* Avoid sharp changes in section and sharp corners. These increase the stress and should be avoided or rounded with a sufficient radius.

IF POSSIBLE:

* The ceramic should be thick walled in comparison with the metal.

* The ceramic should be in compression.

* Where possible, use soft ductile metals next to the ceramic.

Where joint dimensions are large, or there is a large difference in thermal expansion between the materials, design of the joint and joining process is critical.

TESTING

Mechanical testing provides information on the quality or integrity of components and can be used as a measure of the integrity of bonds.

Conventionally, bend, shear and tensile tests define static performance, where the load needed to cause failure is taken to characterise bond strength.

There is no simple answer to the question of what specimen geometry or test is best. Standards are lacking in the strength testing of ceramic joints and care should be taken when comparing the test results of one study to those of another. Best practice is to use whichever configuration most closely replicates the environment of the component in service.

Bond strength values depend upon the testing technique and the size of the specimen chosen. Bend test values tend to be 10 to 50% higher than tensile test values for joints in brittle materials. The presence of defects such as pores, cracks, no-bond regions and inclusions all act as stress concentrators and decrease the recorded strength.