Structure and mechanical properties of arc evaporated Ti–Al–O–N thin films J. Sjölén a, ⁎ , L. Karlsson a , S. Braun b , R. Murdey b , A. Hörling b , L. Hultman b a Seco Tools AB, SE-737 82, Fagersta, Sweden b Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden Received 18 October 2006; accepted in revised form 11 December 2006 Available online 22 January 2007 Abstract The structure, mechanical properties, and machining performance of arc evaporated Ti–Al–O–N coatings have been investigated for an Al 0.66 Ti 0.34 target composition and O 2 /(O 2 +N 2 ) gas flow-ratio varied between 0 to 24%. The coating structure was analysed using SEM, EDX, XRD, XPS, TEM, and STEM. Mechanical properties were analysed using nanoindentation and the deformation behaviour was analysed by probing the nanoindentation craters. The coatings performances in cutting tests were evaluated in a turning application in low carbon steel (DIN Ck45). It is shown that the addition of oxygen into the arc deposition process leads to the formation of a dual layer structure. It consists of an initial cubic NaCl-structure solid solution phase formed closest to the substrate, containing up to 35 at.% oxygen (O/O+N), followed by steady-state growth of a nanocomposite compound layer comprised of Al 2 O 3 , AlN, TiN, and Ti(O,N). The addition of oxygen increases the ductility of the coatings, which improves the performances in cutting tests. At high levels of oxygen, (N 13 at.%), however, the performance is dramatically reduced as a result of increased crater wear. © 2006 Elsevier B.V. All rights reserved. Keywords: TiAlON; Arc-evaporation; Nanostructure; Mechanical properties 1. Introduction The development of wear resistant coatings for cutting applications is rapidly progressing. Novel materials and structural design concepts are being introduced to offer further means for optimisation. The technological driving forces are to enhance wear resistant properties, e.g., mechanical strength and thermal and chemical stability for the cutting tool operating conditions. There is also a need to increase the fundamental understanding of vapour phase growth and reactions in thin films on atomistic and nanostructured levels and to explore the unique properties and structures of thin films. For both activities, ceramics are the materials of choice. The Ti–Al–N–O phase diagram show a wealth of different structures with, e.g., Ti x O y , Ti x Ny, Al x N y , Al x O y , Al x Ti y , and Al x Ti y N z , [1], most of which exist in several polytypes. To our knowledge, no thermodynam- ically stable quaternary phase has been reported. Several of the above phases have useful wear resistant properties where TiN, Al 2 O 3 , and (Ti,Al)N serve as the prime examples[2–5]. There are several motives for choosing (Ti,Al)(O,N) as a wear resistant coating material for metal cutting applications: 1) improved oxidation resistance, 2) improved chemical wear resistance, 3) alloy hardening, and 4) grain size hardening. First, for the oxidation and chemical wear resistance, we note that Al 2 O 3 coatings produced by CVD exhibit excellent thermal and chemical wear resistance. A desire to synthesize these coatings at lower temperature using PVD techniques in a productive way has, however, been found cumbersome [6,7]. Some of the difficulties encountered are associated with the low deposition rates and the control of the phase formed. In the endeavours of producing a PVD-coating with similar properties as for CVD, metastable solid solution (Ti,Al)N coatings, however, have been successfully applied. Explanations for their good high-temperature performance are claimed to be in the ability of (Ti,Al)N to form an outer protective Al 2 O 3 layer by oxidation of the surface in tool operation [8]. While the oxidation mechanism is well worked out in annealing experi- ments, the actual conditions for oxygen accessing the interface between the tool and work piece during metal cutting operation Surface & Coatings Technology 201 (2007) 6392 – 6403 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. E-mail address: Jacob.Sjolen@secotools.com (J. Sjölén). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.12.006