ICMCTF2013 Session B6-2: Coating Design and Architectures

Aluminization of steels using Pack Cementaion entails the risk of crack formation within the coating. This is due to a detrimental combination of the brittleness of the aluminium rich intermetallic phases that form, the coating thickness and the mismatch of the coefficients of thermal expansion (CTE) between the substrate and the coating. By means of a new coating design the desired coating properties can be achieved by predicting the required coating manufacturing parameters such as the process temperature, process time and powder composition. Thus, coatings consisting of intermetallic phases (iron and nickel aluminides), which are less brittle and have a CTE which is closer to that of the substrate can be designed with a controlled coating thickness that guarantees the well known good oxidation behaviour of aluminides. Furthermore, a modification using co-deposition of aluminium and another element such as silicon is considered in this coating design.

Nanocrystalline and nanostructured thin films with grain size below 100nm exhibit typically inhomogeneous depth gradients of microstructure, strain and physical properties varying at the nano-scale. Currently, however, it is not trivial to reveal how these gradients relate to the macroscopic film behaviour. One of the main reasons is the lack of reliable experimental techniques which can provide thickness-dependent data with sub-micron resolution.

In this contribution, our recent results from position-resolved synchrotron X-ray diffraction (XRD) studies of microstructure and strain in nanocrystalline films will be presented. The experiments are based on position-resolved XRD performed using monochromatic beams with diameters down to 100 nm. On the examples of CrN, TiN and TiAlN coatings, it will be demonstrated that the newly developed approaches can be used to analyse lateral- and thickness-dependent gradients of strain, crystallographic texture, phases and grain size with sub-micron resolution.

Additionally, results from mechanical tests obtained from bending experiments on micro-cantilevers and indentation will be used to illustrate variability and anisotropy of mechanical properties in nanocrystalline coatings.

Finally, it will be demonstrated that the scanning X-ray diffraction studies and local mechanical characterization can be used as an effective tool to reveal structure-property relationship in inhomogeneous thin films and coatings at sub-micron scale.

In the field of hard and wear-resistant coatings, thin films based on the system aluminium and chromium represent the state of the art. With the addition of nitrogen and/or oxygen ceramic coatings covering a wide compositional range can be synthesised. In the present study, a mass-to-charge analyser was used to investigate the charge state resolved ion energy distribution functions of Al and Cr as well as the surrounding gases Ar, N₂ and O₂. The number of Al⁺⁺ ions increased by changing the background gas following the sequence from Ar, N₂ to O₂ leading to a higher mean charge state of Al. In contrast, the Cr charge states were reduced. Cr⁺⁺⁺ ions were only found in Ar atmosphere while the number of Cr⁺⁺ ions strongly reduced in O₂ as compared to Ar and N₂ atmosphere. In terms of cathode composition, both Al ions, Al⁺ and Al⁺⁺, showed an increase in energy as the Al content in the cathodes was increased. The highest energies of up to 120 eV were recorded in O₂ atmosphere. The Cr ions showed a similar but less pronounced energy dependence on the cathode composition as the Al ions. The observed ion energy distributions were interpreted in the established framework of plasma generation at cathode spots and ion-gas interactions.