Internal tension

Internal tension, also known as residual stresses, are mechanical stresses that remain inside the material even in the absence of external forces or loads. These stresses can be generated due to various processes, such as plastic deformation, heat treatment, chemical or phase changes, deposition of thin films or welding. In thin PVD layers compressive stresses reach values around 2 GPa. Experimentally, however, it is possible to prepare stress-free layers or with stresses of 7-9 GPa. Such high residual stresses can fundamentally affect the mechanical properties of the substrate, especially its strength characteristics and resistance to fatigue processes during cyclic loading. The presence of internal compressive stresses can lead to an increase in fatigue limits and thereby prolong the life of components subjected to periodic mechanical stress.

The origin of residual stresses in PVD layers is predominantly the deformation of crystal lattices arising under non-equilibrium conditions of deposition and, for higher deposition temperatures, then different coefficients of thermal expansion of layers and underlying materials.

Measurement

A typical method of measuring the residual stresses of PVD layers is the use of X-ray diffraction in the Bragg-Brentano configuration GAXRD (glancing angle XRD). The size of the lattice parameters is determined from the obtained diffraction profiles, and by comparing them with the values ​​of the theoretical stress-free lattice parameters, the nature and value of the residual stresses are calculated. The method is instrumentally quite demanding. The world’s preeminent expert in the field is prof. David Rafaja from TU Freiberg.

An example of the diffraction profiles of different variants of the TiAlN layer is shown in Fig. 1 taken from the article by Prof. Rafaja.

A less sophisticated and operationally undemanding macro tension measurement is the determination of the deformation of thin, one-sided, coated tapes. The residual stress is compensated by its bending. Fig. 2 shows an example of measurement.

Values

• substoichiometric CrNx, WCx 0-1 GPa
• common PVD layers TiN, CrN, AlTiN, AlCrN 1-2 GPa
• TiCN, TiSiN, TiB2 2-3 GPa
• where C > 4GPa
• experimental layers 7-9 GPa

Reference

• one of the wonderful articles by the team of prof. Rafaja: Ch.Wustefeld, D,Rafaja, Effect of the aluminum content and the bias voltage on the microstructure formation in Ti1-xAlxN protective coatings grown by cathodic arc evaporation, Surface & Coatings Technology 205 (2010) 1345-1349.

Application

A technically interesting application is the coating of 3D printed structural parts in order to improve the fatigue behavior. The PVD coating creates a surface barrier against the formation and propagation of fatigue cracks associated with the inhomogeneous structure of 3D printing.