Effect of post-annealing on the thermal stability and residual stresses in CVD (Al,Ti)N coatings investigated by in situ synchrotron diffraction

The stress in thin wear resistant coatings is of great importance for the performance and service life of tools for metal cutting. In this work we have performed detailed investigations of the phase stability and temperature- dependent residual stresses in Al-rich AlxTi1-xN x Ti 1-x N ((Al,Ti)N) coatings deposited by chemical vapor deposition (CVD) on cemented carbide substrates. One as-deposited (Al,Ti)N coating and one coating post-annealed at 850 degrees C for 3 h were heated to 1200 degrees C while the structure and residual stresses were monitored by in situ high energy synchrotron X-ray diffraction. In the as-deposited state, the coating is in tensile stress at room temperature, but post-annealing resulted in a reduction of the room temperature residual stress. This lowering can be explained by growth of hexagonal AlN (hAlN) at the (Al,Ti)N grain boundaries during the isothermal hold time. Upon heating, the temperature-dependence of the residual stresses in both coatings are initially controlled by the mismatch in coefficients of thermal expansion (CTE) with the substrate, which leads to compressive stresses at typical service temperatures. Decomposition starts gradually at around 850-900 degrees C, resulting in an accelerated development of large compressive stresses with increasing temperatures, until the entire coating is transformed at temperatures just below 1100 degrees C. The growth of hAlN initiates slightly higher in temperature after post- annealing, whereas the upper limit for complete transformation remains unaffected. The lowered room temperature tensile stress after post-annealing leads to higher compressive stress at service temperatures, which is expected to improve the performance and service time of the coated tool.