L.B. Varela-Jimenez¹, V. Simova¹, A. Miletic¹, P.R. Avila¹, O. Zabeida¹, P. Immich², G. Negrea², R. Schäfer³, D. Schorn⁴, Th. Schuette⁵, S. Williams⁶, J.E. Klemberg-Sapieha¹, L. Martinu^1
1Polytechnique Montreal, Quebec, Canada
²IHI Hauzer Techno Coating BV, Venlo, The Netherlands
³robeko GmbH & Co. KG, Mehlingen, Germany
⁴MAGPULS GmbH, Sinzheim, Germany
⁵PLASUS GmbH, Mering, Germany
⁶Sputtering Components Inc, Owatonna, MN
In the present work, we systematically study the effect of the coating growth conditions on the development of residual stress (RS). This investigation is meant to develop approaches to mitigate RS in general, and to take it one step further: to tailor the stress to better oppose the stress imposed by different levels of loading during tribological conditions applied in real-life applications. TiAlN on WC-Co substrates was used as a model system.
The coatings were prepared using Ti_0.5Al_0.5N compound targets in two setups, one equipped with a planar magnetron (PM), and another one with a 500 mm long cylindrical magnetron (CM). In situ, real-time monitoring by optical emission spectroscopy (OES) was applied for the study of the process and film growth conditions.
Structural characterization and RS measurements were performed using multi-hkl grazing incidence X-ray diffraction (MGIXRD) in a standard laboratory XRD system as well as in the Canadian Light Source X-ray radiation synchrotron facility. Tribo-mechanical properties were measured by depth-sensing indentation, pin-on-disc and ball-on-flat tribometers, and micro scratch testing, while further analyses were performed by optical profilometry, SEM and EDS.
In the first part, we studied a single-layer system while controlling the substrate bias (V_B). Hardness increased from 20 GPa (no bias) to 30 GPa for V_B = -60 V without any heating. This correlates with the increase in compressive RS from -0.9 GPa to -5.5 GPa and the corresponding decrease in the grain size (from 16 nm to 9 nm). Substrate heating enhanced the mechanical properties, accompanied by a lower compressive RS. Combined substrate temperature (350 °C) and biasing gave rise to even lower stress (-2.3 GPa).
In the second part, two film architectures were investigated, namely a single TiAlN layer with a graded interface due to V_B variation, and a 14-layer architecture when individual layers were deposited at different V_B values between -60 V and -100 V. RS varied from -2.0 to -7.6 GPa that strongly correlated with the tribological performance. Specifically, high stress reduced the abrasive wear by hindering crack initiation and propagation and contributed to low wear rates for short sliding distances. In contrast, lower stress led to high wear rates for longer sliding distances. The TiAlN coatings outperform the substrate at high temperatures (700 °C) due to its severe oxidation. Wear rate of the coating increased with a longer sliding distance, possibly due to a reduction of the RS.
The results clearly show that tuning the residual stress can effectively be used to tailor the mechanical and tribological properties of the TiAlN (and other) protective coatings for enhanced performance at specific application-related loading conditions.