Mechanics of Solid Interfaces

Chapter 9

Damage in Thin Films on Substrates

9.1. Overview

The necessity to constantly improve the various functional properties of surfaces and bulks has led to the widespread use of thin films and coatings, with very diverse objectives: resistance to corrosion, to wear, to erosion; friction reduction; thermal, optical, electrical and chemical barriers; etc. The materials used are themselves very diverse, (metallic alloys, ceramics and polymers) depending on the desired application, as are the microstructures of the films and their deposit processes.

Whatever their function, coatings by their very nature raise mechanical difficulties: their elastic and plastic mechanical behaviors are different from those of the substrate supporting them. Any mechanical loading of the part therefore causes the appearance of additional strains – and the associated stresses – due to the necessity of ensuring the compatibility deformation of the coating connected to its substrate. For the same reasons, any variation in temperature of the coating–substrate structure generates thermomechanical stresses, since the thermal expansion coefficients of the two materials are usually different. This second type of stress often results from the process of depositing the coating, which is frequently accompanied by an increase in temperature, while the covered part is then manipulated and used at a different temperature.

Whatever their physical/chemical origin (see Chapter 8), these stresses present two common ...