1 - Erich Schmid Institute

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1 Introduction
Figure 1.4: Typical evolution of average film stress σav as a function of film thickness tf during the
deposition procedure. The curve describes the three stages of film growth (compressive
- tensile - compressive stress) for vapor deposited polycrystalline films at deposition temperature.
In-situ monitoring of the average stresses during deposition shows that the film
thickness at which the change from tensile to compressive stresses occurs (between stages
2 and 3) depends strongly on the materials involved and can lie between a few nanometers
and a few hundred nanometers. 1
It has to be emphasised that this curve describes the average film stresses at deposition
temperature and that the stresses at room temperature are superimposed by the thermal
stresses that arise owing to the difference of the coefficients of thermal expansion of the
substrate and the coating.
1.3.2 Thermal Stresses
Thermal stress is an important issue in coated and uncoated components and can arise
during fabrication and in service. Components or semi-finished parts are usually fabricated
at elevated temperatures to obtain the desired material properties or simply because
it is required by the fabrication process. In terms of massive parts, residual stresses can
be introduced when cooling the material with a high cooling rate, which can produce
stresses as high as the yield stress between the shell and the core, especially if the material
exhibits a low yield strength and a low thermal conductivity. This effect can be
amplified or weakened when the material undergoes one or more phase transformations
during this treatment. 10
Regarding coated components, it does not require high heating or cooling rates to
produce thermal residual stresses because often the coating and the substrate material
exhibit different coefficients of thermal expansion (CTE). Depending on the relative magnitude
of the CTEs and the corresponding Young’s moduli, such stresses can reach the
yield stress of the film, which can lead to fracture in case of tensile stresses in the coating
or delamination of the coating from the substrate material in case of high compressive
stresses and insufficient adhesion. An estimation of the order of magnitude of thermal
stresses is demonstrated by means of a simple example. We consider a cutting tool consisting
of a 2µm thick CrN film deposited on a steel substrate by means of a conventional
CVD process at a typical deposition temperature of 800 ◦ C. The thermal stresses in the
coating induced by cooling down the system to room temperature reach about 3GPa
compression according to the difference in the CTEs of steel (≈ 13 · 10 −6 K −1 ) 16 and CrN
(6 · 10 −6 K −1 ). 17
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