Guide to Surface Engineering

Surface Engineering embraces those processes which modify the surfaces of engineering components to improve their in-service performance, useful working lifetimes, aesthetic appearance or economics of production. It means 'modifying the surface' of a material or component to confer surface properties which are different from the bulk properties. The purpose may be to minimise corrosion, reduce frictional energy losses, reduce wear, act as a diffusion barrier, provide thermal insulation, exclude certain wavelengths of radiation, promote radiation electronic interactions, electrically insulate or simply improve the aesthetic appearance for the surface.

This guide concentrates on engineering applications, where the main concern is with the potential of surface damage; i.e. damage that might occur by corrosion, wear or any other aspect of the environment.

There are several possibile degradation processes:

• Chemical: acids, alkalis, salts, solvents, etc
• Thermal: high temperatures, direct flames, oxidising gases, etc
• Atmospheric: humidity, rain, snow, sunlight, sea water, erosion, etc
• Mechanical: abrasion, vibration, adhesion, galling, etc

The general protective process consists of four steps:

• Preparation and conditioning of the metal surface
• Determination of the best coating needed for the material and its use (environment)
• Control of the coating process
• Maintenance of the protective coating

The surface engineering process can be divided into two main groups;

1. Surface Preparation Processes,which clean and prepare component surfaces.

2. Surface Treatments, which give the material the desired properties. There are many ways of treating metal surfaces to enhance the corrosion resistance and/or their tribological properties. These may be grouped into three broad categories:

• Category 1 -  Modifying the surface without altering the substrate's chemical constitution. In this case, the existing metallurgy of the component is changed within the surface regions, either by thermal or mechanical means, to increase its hardness.
• Category 2 - Changing the surface layers by altering the alloy chemistry. New elements are diffused into the surface, usually at elevated temperatures, so that the outer layers are changed in composition and properties compared to those of the bulk.
• Category 3 Adding layers of material to the surface. This group incorporates a wide group of coating processes, where a material different from the bulk is laid upon the surface. Unlike the first two categories, there will be a clear boundary at the coating/substrate interface and the adhesion of the coating is a primary issue.