Technological Real Surfaces

Technological surfaces or engineering surfaces are terms that are used to describe the real surfaces of engineering materials. These layers, along with the underlying bulk material, are the real substrate that must be altered to produce the desired surface properties. Invariably the real surface differs chemically from the bulk material by having surface layers of reacted and adsorbed material such as oxides and hydrocarbons. The surface chemistry, morphology, and mechanical properties of the real surface can be very important to the adhesion and film formation process. The underlying bulk material can be important to the performance of the surface. For example, a wear coating on a soft substrate will not function well if, under load, it is fractured by the deformation of the underlying substrate. Also, good film adhesion cannot be obtained when the substrate surface is mechanically weak, because failure can occur in the near-surface material. The bulk material can influence the surface preparation and the deposition process by continual outgassing and outdiffusion of internal constituents.

Some of the surface properties that affect the formation and properties of the deposited film are:

• Surface chemistry--affects the adatom-surface reaction and nucleation density. Chemistry can affect the stability of the interface formed by the deposition.

• Contamination (particulate and film, local or uniform)--affects surface chemistry and nucleation of the adatoms on the surface. Particulate contamination generates pinholes in the deposited film.

• Surface morphology--affects the angle-of-incidence of the depositing atoms and thus the film growth. Geometrical shadowing of the surface from the depositing adatom flux reduces surface coverage. Surface morphology can affect the film properties and stability.

• Mechanical properties—affects film adhesion and deformation under load

• Outgassing and outdiffusion--affects nucleation and film contamination

• Homogeneity of the surface--affects uniformity of film properties over the surface

In particular, the surface morphology can have an important effect on the film properties. Figure 1 shows the effect of surface morphology and particulate contamination on surface coverage and pinhole formation. Also, the surface morphology can affect the average angle-of-incidence of the adatom flux, which has a large effect on the development of the columnar morphology in atomistically deposited films.

Fig. 1 Surface morphology effects on pinhole formation

The nature of the real surface depends on its formation, handling, and storage history. In order to have reproducible film properties, the substrate surface must be reproducible. This reproducibility is attained by careful specification of the substrate material, careful incoming inspection procedures, careful surface preparation, and appropriate handling and storage of the material.

Surface preparation is the process of preparing a surface for the film/coating deposition process (Ref 1). Surface preparation may mean cleaning (removal of contaminants), but it can also include surface treatments to change the properties of the surface in a desirable way, such as roughening or smoothing the surface, making a harder surface by plasma treatment (i.e., plasma nitriding) or shot peening, or "activating" the surface, such as the oxygen plasma treatment of a polymer surface. Often surface preparation consists of two distinct stages. The first is "external cleaning," which takes place outside the deposition system in a controlled environment. This processing environment is designed to control recontamination after cleaning. For example, to control recontamination by particulates, a filtered air "cleanroom" is used. External cleaning can consist of both "gross cleaning," which removes a portion of the substrate surface material, and "specific cleaning," which removes specific contaminants such as hydrocarbons or salts. The second stage of surface preparation is "in situ cleaning," which is performed in the deposition system. For example, hydrocarbon contamination can be removed from some surfaces by exposing them to an oxygen plasma in the deposition system.

Care must be taken to ensure that the surface preparation process does not change the surface in an undesirable or uncontrolled manner, such as selective leaching of one phase of a two-phase surface. One objective of any surface preparation procedure is to produce as homogeneous a surface as possible. Reproducible surface preparation, as well as associated handling and storage techniques, are obtained by having appropriate specifications for the process, handling, and storage procedures used. In addition, recontamination of the prepared surface in the deposition chamber and by the deposition process is a major consideration.

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