Degradation of TiAlN coatings by the accelerated life test using pulsed laser ablation

Abstract

The degradation behaviors of TiAlN coatings were evaluated by an accelerated life test under thermo-mechanical loading using a Nd:YAG pulsed laser ablation system. Pulse laser levels were chosen based on finite element analysis, and failure was determined when the specimen hardness decreased below 50% from its initial value. The accelerated life test was conducted on both TiAlN and TiN coatings deposited on an Inconel 617 substrate. The predicted lifetimes were obtained through statistical analysis of the failure time data. Surface and cross-sectional microstructural observations of the ablated spots were made to identify the cause for the difference in lifetimes since the TiAlN coating lasted longer than the TiN coatings by a factor of ~1.7. Pores formed on the TiAlN coating surface by the repeated laser ablation, and surface cracking and delamination of the coating layers were observed on the ablated spots in the failed specimens. The TiN coating also showed surface cracks with fewer surface pores than the TiAlN coating, but spalling of the coating layer occurred due to bursting of the substrate. Secondary ion mass spectrometry (SIMS) depth profiling revealed that surface oxidation increased in the TiN coating as the number of laser pulses increased, while the TiAlN coating showed less surface oxidation. The surface oxidation and spalling of the coating layer is considered to be the dominant influence on the decrease in hardness of the TiN coating. The TiAlN coating retains its hardness longer due to its lower thermal conductivity and higher oxidation resistance, as compared to the TiN coating.