YOUR AIMS

On completion of this lesson you should be able to:

• appreciate how modification of the surface properties of materials can significantly affect the performance of components in service
• recognise the importance of suitably preparing a surface prior to the application of a coating and the ways in which this can be achieved
• discuss the nature of some of the processes involved in applying surface coatings
• discuss the characteristic engineering properties of a particular coating and the reasons for its use in specific applications.

SELF-ASSESSMENT QUESTIONS

1. One of the reasons metallic surfaces are usually subject to pretreatment prior to the application of a coating is to remove any superficial corrosion which may be on the surface. If they were not removed, what types of problems may result?

2. The nature of the electroplating process requires the design of components to be electroplated to be given due consideration. What are the three major factors and features which need to be borne in mind?

3. The protection provided by a coating of zinc on steel components is not solely due to the barrier that the coating provides. What other effects enhance the protective nature of the coating?

4. The use of the reducing agent sodium hypophosphite results in the incorporation of phosphorus into electroless deposited nickel coatings. Is this an advantage or a disadvantage?

5. Galvanising of steel by hot dipping leads to the formation of Zn-Fe alloy layers on the surface which, although brittle, have better corrosion resistance than the zinc metal itself. The coating is often subsequently annealed. Why is this done?

6. The life of sprayed coatings may be increased significantly by sealing the coating following its application. What advantages do sealers provide? Provide three examples of typical sealers.

7. Outline the essential similarities and differences between the processes of chemical vapour deposition (CVD) and physical vapour deposition (PVD).