Stress measurements in thermal loaded (Ti,Al) N hard coatings

Extreme thermal load of (Ti,AI)N coated WC-Co tools during the dry high-spe ed cutting process results in large thermal stresses superimposing the resi dual stresses in the films. This can cause layer damage by cracking and del amination. Therefore a stress determination in (Ti,AI)N coatings can help t o understand these damaging processes. Fine crystalline (Ti,ADN hard coatin gs with an Al/Ti ratio of 1.33 and a thickness of 1, 3, 6 and 14 mum, respe ctively, were deposited onto WC-6% Co hard-metal substrates or Si(100) wafe rs, respectively, from TiCl4/AlCl3/N-2/Ar/H-2 gas atmosphere using the plas ma-assisted CVD (PACVD) process. Thermal load was applied either by anneali ng or by thermal laser shock. Stress state of the coatings before and after thermal load was analysed using X-ray diffraction methods or the substrate curvature method. In the as-deposited state of the (Ti,Al)N films compress ive stress values between 300 MPa and some GPa at the film surface were mea sured caused by deposition mechanism and different thermal expansion coeffi cients of the film and the substrate. Stress values decrease towards the su bstrate and change to tensile stress up to 2.5 GPa near the film-substrate interface. At temperatures above 450 degreesC stress relaxation mechanisms occur during substrate curvature experiments, which result in tensile stres ses up to approximately 200 MPa after cooling. In laser shock experiments w ith homogenised laser beam profiles (DeltaE ' /E < 10% across the laser spo t) typical crack networks were generated inside the laser spot region when the energy density reached a critical value, corresponding to a surface tem perature of approximately 750 degreesC for irradiation times between 1 and 10 ms. However, there are remaining questions for the explanation of the la tter fact by means of the stress measurements. (C) 2001 Elsevier Science B. V. All rights reserved.