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Etching

Although certain information may be obtained from as-polished specimens, the microstructure is usually visible only after etching. Only features which exhibit a significant difference in reflectivity (10 % or greater) can be viewed without etching. This is true of microstructural features with strong color differences or with large differences in hardness causing relief formation. Cracks, pores, pits, and nonmetallic inclusions may be observed in the as-polished condition. In most cases, a polished specimen will not exhibit its microstructure because incident light is uniformly reflected. Since small differences in reflectivity cannot be recognized by the human eye, some means of producing image contrast must be employed. Although this has become known as "etching" in metallography, it does not alway refer to selective chemical dissolution of various structural features. There are numerous ways of achieving contrast. These methods may clasified as optical, electrochemical (chemical), or physical, depending on whether the process alters the surface or leaves if intact.

The purpose of etching is to optically enhance microstructural features such as grain size and phase features. Etching selectively alters these microstructural features based on composition, stress, or crystal structure. The most common technique for etching is selective chemical etching and numerous formulations have been used over the years. Other techniques such as molten salt, electrolytic, thermal and plasma etching have also found specialized applications.

Chemical Etching

Chemical etching is based on the aplication of certain illumination methods, all of which use the Kohler illumination principle. This principle also underlies common bright-filed illumination. These illumination modes are dark field, polarized light, phase contrast and interference contrast. They are available in many commercially produced microscopes, and in most cases, the mode may be put into operation with few simple manipulations. There is distinct advantage in employing optical etching rather than those technique which aleter the specimen surafce. Chemical and physical etching require considerable time and effort and there is always a danger of producing artifacts which lead to misinterpretations.

Chemical etching - slectively attacks specific microstructural features. It generally consists of a mixture of acids or bases with oxidizing or reducing agents. For more technical information on selective chemical etching consult corrosion books which discuss the relationship between pH and Eh (oxidation/reduction potentials), often known as Eh-pH diagrams or Pourbaix diagrams. Over the years numerous chemical etchants have been formulated.

Electrochemical Etching

During the process of electrochemical etching of metallic specimen, reduction and oxidation process (redox process) take place. All metals in contact with the solution have a prononounced tendency to become ionized by releasing (losing) electrons. The degree to which this reaction takes place may be recorded by measuring the electrochemical potential. This is done by comparing the potential of metal versus the standard potential of a reference electrode. The tabulation of various metals results in the electromotive series of elements:

Li+ , Na+ , K+, Ca++ , Ba++ , Be++ , Mg++ , Al+++ , Mn++ , Zn++ ,
Cr++ , Cd++ , Ti+ , Co++ , Ni++ , Pb++ , Fe+++ , H+ , Sn++++ ,
Sb+++ , Bi+++ , As+++ , Cu++ , Ag+ , Hg++, Au+++ , Pt+++.

The elements are listed in order of decreasing electroaffinity. All elements preceding hydrogen are attacked by acids with the evolution of hydrogen (H2). All elements following hydrogen cannot be attacked without the addition of an oxidizing agent. Thus, microstructural elements of different electrochemical potential are attacked at different rates. This produces differential etching, resulting in microstructural contrast. Electrochemical etching may be considered as "forced corrosion". The differences in potential of the microstructural elements cause a subdivison into a network of very small anodic or cathodic regions. These miniature cells cannot originate from differences in phase composition only, but also have to come from irregularities in the crystal structure as they are present - for example, at grain boundaries and from other inhomogeneities such as:

Inhomogeneities resulting from deformation, which are less reistant to attack than undeformed material.

Unevenness in the formation of oxidation layers

Concentration fluctuation in the electrolyte

Differences in electrolyte velocity

Differences in the oxygen content of the electrolyte

Differences in the illumination intensity, which can initiate diferences inpotential

Because of differences in potential between microstructural features, dissolution of the surface proceeds at different rates, producing contrast. Contrast can also originate from layers formed simultaneously with material dissolution. This is true in precipitation etching and heat tinting where surface reactions are involved. In precipitation etching the material is first dissolved at the surface; it then reacts with certain components of the etchant to form insoluble compounds. These compounds precepitate selectively on the surface, causing interference colors or heavy layers of a specific color. During heat tinting, coloration of the surface takes place at different rates according to the reaction charcteristics of different microstructural elements under the given conditions of atmosphere and temperature.

A wide variety of etchants is available, including acid, bases, neutral solutions, mixtures of solutions, molten salts and gases. The stability of many etching solutions is limited; redox potentials change with time. Changes may also occur while the etchant isin use, such that it must be discarded after a limited time.

Etching times range from several seconds to some hours. When no instractions are given, progress is judged by the appearance of the surface during etching. Usually, the surface will become less reflective as etching proceeds. Etching time and temperature are closely related; by increasing the temperature, the time can usually be decreased. Most etching is performed at room temperature.

Conventional chemical etching is the oldest and most commonly applied technique for production microstructural contrast. In this technique, the etchant reacts with the specimen surface without the use of an external current supply. Etching proceeds by selective dissolution according to the electrochemical characteristic of the componenet areas.

In electrolytic or anodic etching, an electrical potential is applied to the specimen by means of an external circuit. Typical setup consist, the specimen (anode) and its counterelectrode (cathode) immersed in an elctrolyte.

Another type of electrolytic etching requires more sophisticated electrochemical equipment (potentiostats). This equipment uses a three electrode configuration (anode, cathode, reference) which allows the current and voltage to be adjusted independent of each other. In comparison to the two electrode system, voltage is altered with the electrolyte dictating the current conditions. On the other hand, the electrolyte for the three electrode configuration only requires a conductive solution such as KCl, instead of highly corrosive acids or bases.

On completion of any chemical or electrochemical etching process, the specimen should be rinsed in clean water to remove the chemicals and stop any reactions from proceeding futher. After specimens are water rinsed, they should be rinsed in alcohol and dried in a stream of warm air. The use of alcohol speeds up the drying action and prevents the formation of water spots.

Physical Etching

Basic physical phenomena are also often used to develop strucural contrast, mainly when conventional chemical or electrolytic techniques fail. They have the advantage of leaving surfaces free from chemical residues and also offer adavantages where electrochemical etching is difficult - for example, when there is an extremely large difference in electrochemical potential between microstructural elements, or when chemical etchants produce ruinous stains or residues. Some probable applications of these methods are plated layers, welds joining highly dissimular materials, porous materials, and ceramics.

Cathodic Vacuum Etching

Cathodic vacuum etching, also referred to as ion etching, produces structural contrast by selective removal of atoms from the sample surface. This is accomplished by using high-energy ions (such as argon) accelerated by voltages of 1 to 10 kV. Individual atoms are removed at various rates, depending on the microstructural details such as crystal orientation of the individual grains, grain boundaries, etc.

Plasma etching is a lesser known technique that has been used to enhance the phase structure of high strength ceramics such as silicon nitride. For silicon nitride, the plasma is a high temperature flouride gas, which reacts with the silicon nitride surface producing a silicon flouride gas. This etching technique reveals the intragrain microstructure of silicon nitride.

Molten Saltetching

Molten Saltetching is a combination of thermal and chemical etching techniques. Molten salt etching is useful for grain size analysis for hard to etch materials such as ceramics. The technique takes advantage of the higher internal energy associated at a materials grain boundaries. At the elevated temperature of molten salts, the higher energy at the grain boundaries is relieved, producing a rounded grain boundary edge which can be observed by optical or electron microscope techniques. Some common molten salts are listed in the following table.

Thermal Etching

Thermal etchingis a useful etching technique for ceramic materials. Thermal etching is technique that relieves the higher energy associated at the grain boundaries of a material. By heating the ceramic material to temperatures below the sintering temperature of the material and holding for a period of minutes to hours the grain boudaries will seek a level of lower energy. The result is that the grain boundary edge will become rounded so that it can be evaluated by optical or electron microscope techniques.

Depending upon the ceramic material, the atmospheric condition of the furnace may need to be controlled. For example, etching silicon nitride will require either a vacuum or an inert atmosphere of nitrogen or argon to prevent oxidation of the surface to silicon dioxide.