Residual stresses in coatings comprise the contribution of the thermal expansion mismatch between coating and substrate (thermal stress) and the intrinsic stress, the latter related to the structural evolution of the coatings during its growth and influenced by deposition conditions [1]. Residual stress is a crucial concern in coated cutting tool technology, as the properties, performance, and durability of the coatings are directly affected by the level of residual stress [2].

In this work we compared the residual stresses of TiAl(Si)N monolayers and multilayers with different methods: i) measurement of Si wafer curvature with a contact profilometer and calculation using the Stoney’s equation; ii) application of the sin²ψ method (XRD) while evaluating the influence of substrate (Si and WC Co) and hkl reflection; iii) calculation of the thermal stress contribution from the mismatch between the thermal expansion coefficients of the substrates and the constituent phases of the coatings along the deposition temperature. Three Ti-Al-(Si)-N types of monolayers were deposited on Si (100) and WC-Co by High Power Impulse Magnetron Sputtering at 450 °C using three compound targets with different Ti:Al ratios (50:50 and 40:60) and Si contents (0 and 6 at. %): Ti₅₀Al₅₀, Ti₄₇Al₄₇Si₆, and Ti₃₇Al₅₇Si₆. Films deposited using Ti₅₀Al₅₀ and Ti₄₇Al₄₇Si₆ were formed by cubic nitrides (labelled as c and c_Si), while films deposited with Ti₃₇Al₅₇Si₆ showed hexagonal phases (labelled as h_Si). Several c_Si+c and c_Si+h_Si multilayers with different bilayer thickness (7-14 nm) were investigated in addition to monolayers.

The h_Si monolayer presented the lowest residual stress (-0.75 GPa), in agreement with the calculation of thermal stress considering its main constituent phase (hexagonal nitride). A good agreement between curvature and sin²ψ method was observed for the residual stress of c_Si monolayer on Si substrate using (220) as reflection peak, -1.07 and -1.13 GPa, respectively (Fig. 1). However, higher values (-1.45 GPa) were found when the (200) reflection peak was used for both substrates. The multilayer systems showed an increase of at least 30% in the residual stress compared to their component monolayers, regardless the bilayer thickness. The differences in chemical composition and mechanical properties of each sublayer raised the disagreement between wafer curvature and sin²ψ methods to 65%. The validity of the expressions used for calculations and the influence of the material parameters will be discussed.

[1] R. Daniel et al. Acta Materialia, 2011, 59, 6631-6645.
[2] C. A. Johnson; J.A. Ruud; R. Bruce; D. Wortman Surface and coatings technology, 1998, 108-109, 80-85.