Engineering Ceramic Microstructures and Machining

Azom.com
June 2, 2011

Engineering Ceramics

The common engineering ceramic materials can be identified as Aluminium Oxide (Alumina), Silicon Carbide, Silicon Nitride, Sialon and Zirconia. These materials are classified as “Engineering” or sometimes “Advanced Ceramics” as they have mechanical and physical properties which lead to their application in many “engineering” environments.

Engineering ceramics can service applications which previously would have been considered unsuitable for a “ceramic solution” due to their inherent brittleness, low strength or other properties such as the low thermal shock resistance displayed by “traditional ceramics”.

To provide one simple example, a traditional ceramic material such as porcelain, displays a measured bending strength in the region of 150 MPa, whereas a high quality Silicon Nitride or Zirconia display bending strength values 5-10 times higher.

Microstructure

One of the key material factors behind such property improvements is the quality of the materials microstructure.

What do we mean by microstructure?

If we take one of the engineering ceramic materials we have mentioned above in its fully fired or sintered condition i.e. the strongest and hardest form of the material and machine it using a diamond grinding wheel and diamond lapping techniques to produce a highly polished a flat surface, that surface can then be examined with a microscope to reveal the “microstructure”. Or, in other words, the structure of the material at a magnification that allows us to see features at the scale of 1 µm.

In addition to the machining, it may also be a pre-requisite that before the structure can be viewed microscopically it is necessary to “etch” the surface to reveal the underling features. This may be achieved by a heat treatment to reveal the key surface structures or chemical etching with acids to reveal grain boundary features by preferentially removing grain boundary glassy phases.

Key Microstructural Features of Engineering Ceramics

The key elements of the microstructure are the individual grains, grain boundaries, pores or defects as show in Figure 1. below.

Figure 1. Microstructure of a typical dense ceramic

The grains are formed as the compacted grains of ceramic powder try to reduce their surface area by coalescing together during sintering.

The grain boundaries are formed at the intersection of the grains and often contain impurities that form “glassy” phases. The glassy phases aiding in the sliding process that occurs as the grains coalesce.

Pores are formed due to the inability of the powder compact to sinter to full density i.e. the theoretical density that can be obtained by the individual grains eradicating all the pre-existing porosity of the as-pressed powder compact.

Figure 2: From SEM Micrograph of Tape Cast Silicon Nitride illustrating the elongated grains of Beta Silicon Nitride

How Does the Microstructure of a Ceramic Affect Its Performance?

A major field of academic study in the field of engineering ceramics relates to what is known as “structure-property” relationships. Confirming the important relationship between the microstructure of an engineering ceramic and measured properties such as strength, toughness or hardness.

It is worth examining each of these properties to allow us to subsequently analyse the effect of microstructure.

Strength

The strength of an engineering ceramic is typically measured by machining a bar of material into a solid bar 3x4x45 mm which is the subject to a 3 point bending strength test.

Toughness

The toughness of engineering ceramics in simple terms describes the ability of the material to withstand the propagation or movement of a crack throughout the body of the material. It is often measure by using the type of diamond indenter used in conventional hardness tests.

Hardness

Hardness is perhaps the simplest of all property measurements as it is generally measured by the depth of penetration of an indenter into the surface of the material, either a pyramidal indentation in Vickers hardness or a conical indenter in Rockwell hardness testing.

The Relationship between Properties and Performance

When the strength of an engineering ceramic is measure it is predominantly the case that the measured failure stress is determined by the fundamental load carrying ability of the structure but also the critical surface defect at the point of maximum stress.

The fundamental load carrying ability of the structure is determined by the prior processing and the ability of the ceramic structure to sinter to full density. The surface defect that will cause failure is determined by the quality of the machining operation.

A typical fracture surface of an alumina ceramic subject to a strength test is shown in Figure 3. It is easily discernible that the fracture has proceeded along the weakest points namely the grain boundaries.

Figure 3. Fracture surface of a monolithic alumina.

The Effect of Machining and Microstructure

During the machining or grinding process employed with engineering ceramics, the graphic below (Figure 4.) illustrates the potentially deleterious effects of the grinding process being conducted with excessive or inappropriate force.

Figure 4. Graphical example of machining defects.

The grinding stresses leads to both surface and subsurface defects, both of which may generate the critical defect at failure.

Equally, such defects can also reduce the measured values of toughness and hardness.


Summary

It is therefore obvious that when selecting an engineering ceramic to be used in any engineering component, it is important to select the material on the basis of its mechanical and physical properties but also it is important to ensure that the component is machined by experts who appreciate the microstructure property relationships of engineering ceramics and the potentially catastrophic effects of inappropriate machining techniques.

All of these processes required highly skilled operators and high precision equipment to achieve the highest levels of precision, surface finish and low levels of surface and sub-surface damage.

For further details visit Insaco website.