High-rate deposition of thick (Cr,Al)ON coatings by high speed physical vapor deposition

Abstract

Inspired by diffusion barriers, (Cr,Al)ON coatings are developed as oxidation protection for gamma titanium aluminide alloys (γ-TiAl), offering oxidation resistance for applications at elevated temperatures, such as turbines. In this study, (Cr,Al)N and (Cr,Al)ON coatings were deposited by means of high speed physical vapor deposition (HS-PVD) technology, basing on hollow cathode discharge (HCD) and gas flow sputtering. Using argon as plasma gas and transport medium, the HS-PVD combines the advantages of thermal spraying and physical vapor deposition, which provides very high deposition rates. The objectives of this study were to synthesize thick PVD (Cr,Al)ON coatings and to understand how various process parameters influence coating morphology, deposition rate and phase composition. Keeping other parameters constant, the argon gas flow was varied between QAr=6000sccm and QAr=12,000sccm, different N2:O2 gas flow mixtures from 1:2 to 4:1 were introduced into the coating chamber and the bias voltage was adjusted between UB=−25V and UB=−150V. A special target configuration was applied for synthesizing coatings with different Cr:Al ratios within one coating cycle. The atomic Cr:Al ratio was varied between 25/75 and 75/25. The results showed that thick s>35μm (Cr,Al)ON coatings can be synthesized very efficiently at a deposition rate ds/dt≈20μm/h. Strong influences of argon gas flow and bias voltage on the deposition rate were revealed. Furthermore, stable plasma processes were observed, even at high oxygen flows QO2=600sccm, which would lead to target poisoning in conventional PVD processes. In this way, the HS-PVD technology reveals a high potential for the deposition of high temperature oxidation protective coatings and extends the application range of conventional PVD technology significantly.