ICMCTF2012 Session B6-2: Coating Design and Architectures

Physical properties of nanocrystalline thin films are strongly related to their structure. It, in turn, depends on the deposition conditions, which affect atomistic processes acting during film growth. An understanding of the growth-structure-property relations in thin solid films is thus crucial in optimizing their performance. The attempt of this study is to reveal the origin of depth-profile variations in film texture, morphology, stress state and thermal properties based on the evolutionary nature of the film structure due to competitive growth. We will demonstrate how the variation in the stress state and thermal properties is related to the volume fraction of grain boundaries, which typically develops in nanocrystalline films to a different extent depending on the actual growth stage. The role of weakly bonded atoms of the interfacial area will be discussed with respect to the development of intrinsic and thermal stresses, which are related to the efficiency of defect generation by incident particles from the deposition flux, the incorporation of an excess of weakly bonded surface adatoms in grain boundaries, the degree of interaction between adjacent grains during film growth and thermal expansion. Based on these findings, a guideline how to tailor the stress state and stress gradients in nanocrystalline films by their controlled growth will be given. Moreover, a concept for enhancement of mechanical properties and wear resistance by structural design of single and multilayered coatings based on nanoindentation experiments and analytical modeling will be discussed.

A new design of decorative coatings will be presented. The coatings have a multilayer design consisting of alternating metallic W and W-O layers. The coatings were deposited by magnetron sputtering from a tungsten target and pulsing the oxygen as reactive gas. The controlled injection of the reactive gas can produce a concentration profile gradient from pure tungsten to tungsten trioxide, determining the final apparent colour of the coating. To this gradient layer corresponds a graded refractive index.

A dynamic sputtering model was built to simulate the growth of the coating during the reactive gas pulsing which was validated by direct measurement of the gradient of the oxygen content in the deposited coatings. By depositing monolitic coatings with increasing oxygen coatings it was possible to determine the optical properties for each individual coating and, thus, the gradient of the optical properties in the multilayer film. These results were used for an optical model allowing the optical properties of the deposited tungsten oxide layer to be described, again validated by experimental analysis. This procedure allows the deposition of coatings with the desired colour by using the models to finding the optimal oxygen pulse parameters. In this way, a suitable gradient chemical composition layer will be deposited to which corresponds a specific refractive index gradient which will give rise to a specific colour. This proposed method can be easily applied to almost any metal/metal oxide system.

Sarakinos et al. has recently highlighted the importance of defects for the phase formation in high power pulsed sputtered HfNO thin films.[1]

Using ab initio calculations, the role of defects in TiAlN as well as oxygen incorporation in TiAlN were studied. Vacancies, substitutions, interstitials and combinations thereof in different configurations have been investigated in terms of crystal energies, enthalpies of formation and bulk moduli.

The energy of mixing of TiAlN and hypothetical isostructural TiAlO is negative which may imply the possibility to form TiAlNO in NaCl structure. The influence on enthalpy of formation of metal vacancies is calculated as well as on enthalpy of formation of interstitial oxygen. It is shown that oxygen on the nitrogen sublattice leads to spontaneous incorporation of interstitial oxygen. Possible reasons are discussed.

Thin films of TiAlNO are prepared using high power pulsed magnetron sputtering of a TiAl target in mixed nitrogen and oxygen atmosphere. It is shown that a high oxygen flux leads to the formation of amorphous films. The influence of temperature on structure, and elastic propertiesis determined. Furthermore, thermal stability and thermogravimetric data are presented.

[1] K. Sarakinos, D. Music, S. Mráz, M. to Baben, K. Jiang, F. Nahif, A. Braun, C. Zilkens, S. Konstantinidis, F. Renaux, D. Cossement, F. Munnik, and J.M. Schneider: On the phase formation of sputtered hafnium oxide and oxynitride films, [http://jap.aip.org/japiau] 108 (1) (2010) 014909-1.

In previous ICMCTFs, we have presented the formation and structure-property relation in Al-Cr-O-N oxynitride system. In this paper we report on the latest results on this system which correspond to the growth of fcc-(Al,Cr)₂O₃ at early stage of deposition which gives rise to the corundum structure a-(Al,Cr)₂O₃ subsequently. This transition has a significant effect on film properties. Although the formation of a-(Al,Cr)₂O₃ is frequently observed under our deposition conditions, the occurrence of fcc-(Al,Cr)₂O₃ is rather unexpected. First a cubic interlayer which was used for adhesion of coating was suspected for epitaxial growth of fcc-(Al,Cr)₂O₃. To remove this effect an interlayer with hexagonal structure was applied instead of cubic one and same growth behaviour was observed. XPS studies and simulation of XRD diffractions showed that the formation of fcc-(Al,Cr)₂O₃ with the (200) preferred orientation can be originated from the presence of a metastable CrO films with a B1 structure. The replacement of Cr²⁺ by Al³⁺ results in the development of vacancies in the Cr positions that leads to the stabilization of fcc-(Al,Cr)₂O₃. However, as the thickness of coating increases, the occupation of the antibonding orbitals leads to a loss of structural stability and, thus, the system will transform into the thermodynamically stable a-(Al,Cr)₂O₃. In this paper the formation mechanism of the fcc and corundum (Al,Cr)₂O₃ will be discussed with respect to structural properties.

Many components need a protection against superimposed corrosion and wear (abrasion, erosion) loading, e.q. in off-shore applications. The goal of the research has been to develop PVD multilayer coating by systematically altering the layer architecture in order to protect components against corrosive environments and erosive loadings. The development of the PVD multilayer coatings was staged in four phases. These are investigations of corrosion-, wear-, combined erosion corrosion-resistance and verification of the development, for example by field testing.

This paper deals with first phase of the development, i.e. the investigation of the corrosion resistance of the PVD multilayer coating. Polarisation tests have been identified to be an adequate tool to investigate the different multilayer architectures.

The investigated coatings were applied on a plasma nitrided mild steel and consist of a CrN/CrCN interlayer, different numbers of graded (g) CrCg/aC layers and an a-C:H top layer. The investigations were focused on the necessary number of CrCg/aC layers and the thickness of the a-C:H top layer to achieve excellent corrosion protection. In addition to the influence of the architecture various substrate pre-treatments like substrate nitriding and polishing have been investigated regarding their potential to improve the corrosion resistance as well.

As a result of the PVD multilayer development no corrosion could be detected when immersing in artificial seawater after more than 500 hours.