ICMCTF2015 Session B6-2: Coating Design and Architectures

Design and development of new thin-film materials with multiple functionalities and unique properties in a single material system are presently of great scientific and technological interest. Multifunctional thin-film materials have an enormous potential to enable new device and system capabilities, and to improve current technology efficiency, reliability and flexibility while reducing size, weight, cost and power consumption. Such materials can find applications in different scientific and industrial areas including intelligent electronic devices and sensors, energy systems, aerospace vehicles, medicine instruments, etc.

In our laboratories, we focus our research on the design, preparation and characterization of new multifunctional ceramic coatings based on multielement nitrides, borides, carbides or oxides exhibiting a combination of several beneficial properties, e.g., high hardness, high optical transparency, high or low electrical and thermal conductivity, anisotropic thermal conductivity, high thermal stability and oxidation resistance, low stress, high resistance to cracking, antibacterial activity, etc. The ternary, quaternary and/or quinary coatings allow us to control their elemental composition, structure and consequently their physical and functional properties. These coatings are deposited by reactive dc or dc pulsed magnetron sputtering from one or two targets in argon-nitrogen or argon-oxygen gas mixtures. Afterwards, they are characterized by several analytical techniques to have a detailed insight into various mechanisms responsible for their properties.

My talk will be aimed at several coating systems including (1) amorphous Si-B-C-N coatings with an exceptionally high thermal stability and oxidation resistance, very high optical transparency and low thermal conductivity and expansion, (2) Zr-B-C-N coatings with very high hardness, high electrical conductivity and good oxidation resistance, (3) Hf-B-Si-C(-N) coatings with very high thermal stability of high electrical conductivity in air, and (4) ternary oxide-based coatings with high optical transparency and resistance to cracking. The correlation between composition, as-deposited or thermally-activated structure and properties will be discussed.

In a previous study we presented results of AlCrN coating deposition experiments applying AlCr powder metallurgical targets in an industrial magnetron sputtering coating machine. In these experiments we investigated the influence of the Al/Cr ratio in the targets and of the deposition process parameters on the coating’s mechanical properties.

In this following study we present the influence of the bias voltage variation on the hardness of AlCrN coatings deposited from targets with different Al/Cr metal ratios (Al/Cr= 60/40, 70/30 and 66/34) as well as introducing a third element to the sputtering target by alloying 10 at% Si to the initial materials. The influence of Si on the properties of deposited coatings have been investigated by estimating the physical properties of the coatings as hardness, elastic modulus, adhesion, crystallographic orientation and oxidation resistance by annealing experiments.

Another goal of this work was to investigate the impact of different aluminium and chromium ratios in the Si-alloyed target on the above mentioned coating properties. For this reason different AlCr targets with Al/Cr ratios of 1:1, 2:1 and 3:2 with remaining constant Si-content have been used for sputtering tests.

The increase of energy efficiency and useful life of materials in their application environment is the top priority in the development of resource-friendly materials. The greatest technical importance is the optimization of the friction and wear behavior (tribological properties) of the material surface. In order to minimize friction energy losses and wear, new research and development activities focus on material concepts which enable a self-adaptation of the surface to the occurring load and stress situation (e.g. change in the surface pressure, the surface roughness of the sliding counter body, the size of the abrasive particles, and specifically occurring overloads ).

Over millions of years various biomaterials have been developed through evolution in animals and plants, which are optimally adapted to the environment and the life. In this work human skin and pearly (nacre) have been selected as the bionic (biomimetic) models to develop new wear-resistant coating surfaces for plastics and composites.

The goal of the first example of the presented work is the biomimetic comparison of wrinkling effects on the nano- and micrometer scale in thin films and on the micro- and millimeter scale of human skin. Ti, Ag, Au, W, C, and TiN thin films with 10 to 500 nm thickness were deposited by pulsed laser deposition (PLD) on smooth polyurethane, polycarbonate, polyamide 6, and polyimide polymer substrates. Wrinkling is obviously the dominating surface topography formation mechanism for compounds with hard surface on soft bulk material. Wrinkling occurs on the micro to millimeter scale for skin and on the nano to micrometer scale for polymers with inorganic thin coatings. Mechanically, the wrinkles were found to have high impact on the elongation. In the elastic region of the stress–strain curve, they are smoothing out with increasing strain.

The second example will biomimetically compare the deformation in multi-layered magnetron sputtered coatings of nano-scaled hard TiN and DLC in combination with soft Ti layers to the deformation of nacre. In hydrated nacre shearing of hard aragonite tablets occurs on deformable polysaccharide/protein interlayers similar to hard coating segments on soft metal layers. TEM investigations around deformations from spherical tip indents revealed a step-by-step fragmentation of nano- scaled hard layers and shearing of Ti in between. Deformation curves revealed for the multilayer deformation highly tough behavior, but lower ultimate maximum stress as found for TiN single layers.

This contribution deals with hard nanocrystalline conductive thermally stable materials MBCN (M = Ti, Zr, Hf). The materials were prepared in the form of thin films by pulsed reactive dc magnetron sputtering of M₄₅(B₄C)₅₅ targets in N₂+Ar plasma. All films exhibit very smooth defect-free surfaces and low compressive stress (due to the adaptively varied discharge pressure). The materials exhibit oxidation resistance (mass change <0.01 mg/cm²) of around 600°C at low N content and 750°C at enhanced N content (and well above 1000°C upon Si incorporation).

We focus on the complex relationships between the metal element choice (at fixed contents of the non-metal elements and fixed deposition parameters), materials structure and materials properties. The experimental results are compared with and explained by ab-initio calculations. Most importantly, we show that the transition from Ti through Zr to Hf leads to an increasing preference to form stable MB_xC_yN_1-x-y solid solutions.

At low N contents (compositions around M₄₁B₃₀C₈N₂₀) the aforementioned trend leads to a transition from x-ray amorphous TiBCN through nc-ZrBCN (fcc-ZrB_xC_yN_1-x-y + fcc-ZrN or ZrC_yN_1-y + amorphous phase) to nc-HfBCN (fcc-HfB_xC_yN_1-x-y + amorphous phase). This transition from almost amorphous to nanocomposite structures significantly improves material hardness, H (from 21 to 33-37 GPa), H/E^* ratio (from 0.098 to 0.132-0.133) and elastic recovery (from 67 to 82-85%). In parallel, the materials combine low electrical resistivity (on the order of 10^-6 Ωm) with optical transparency (extinction coefficient at 550 nm 0.06 and 0.014 for M=Zr and M=Hf, respectively).

At enhanced N contents (approximately from 20 to 50 at.%) the transition from TiBCN (which is homogenous) to ZrBCN and especially HfBCN (where small conductive nanocrystals are predicted to be separated by an insulating amorphous phase) dramatically increases the electrical resistivity (from the order of 10^-6 [M=Ti] to 10³ [M=Zr] - 10⁶ [M=Hf] Ωm).

Collectively, the consistent theoretical and experimental results provide guidelines for the design of future hard, oxidation resistant, electrically conductive and/or transparent MBCN materials for different technological applications, including the identification of the most thermally stable structures or structures which are prone to form at high temperatures.

[1] J. Houska, J. Kohout, J. Vlcek, Thin Solid Films 542, 225 (2013)

[2] M. Zhang, J. Jiang, J. Vlcek, J. Houska, J. Kohout, E.I. Meletis, Acta Materialia 77, 212 (2014)

[3] J. Kohout, J. Vlcek, J. Houska, P. Mares, R. Cerstvy, P. Zeman, M. Zhang, J. Jiang, E.I. Meletis, S. Zuzjakova, Surf. Coat. Technol., in print (2014)

The replacement of expensive stainless steel in various socio-economic sectors such as mechanical or alimentary is an issue that would be possible to solve by developing an architectured coating on low-cost carbon steel. This innovative system is constituted of a coating whose properties will depend on the aimed application. This latter is about complex pieces in contact with different kinds of fluids (hot and/or containing particles) when functioning. Consequently, the expected coating function is to protect effectively the equipment from corrosion,abrasion and erosion.

Only few studies deal with coatings obtained by sol-gel process on low carbon steel. It has been reported in the literature that silica-epoxy based sol-gel have been developed as barrier layers which when densified, would improve the hardness of the coatings.^1,2 Coatings developed for anticorrosion performances are not enough efficient to ensure a good protection of carbon steels from their constant contact in aggressive media.³

The mechanical properties of sol-gel coatings are also enhanced by the introduction of inorganic oxides nanoparticles such as silica¹, alumina⁴, zirconia⁵ and ceria⁵. Taking into account the specificity of the DC04 steel, we developed architectured coatings displaying its extreme surface densified in order to give steel corrosion and wear protection.

The morphology and microstructure of the coatings were characterized by 3D optical microscopy, scanning electron microscopy and interferometry. Analysis by ¹³C and ²⁹Si solid state NMR were used to assess the level of polymerization of the organic and inorganic networks. The validation of work was established by the implementation of immersion tests in corrosive solutions and characterizations by electrochemical impedance spectroscopy, nanoscratch and nanoindentation. Standard test methods were achieved to evaluate wear resistance of such coatings.

This work was performed in the framework of a joint laboratory, called CETIMAT, where CIRIMAT and CETIM collaborate for some aspects of their research."

¹ J.-M. Yeh et al., J. Applied Polym. Sci. 112 (2009) 1933-1942.

² M. Grundwürmer et al., Wear 263 (2007) 318-329.

³ I.B. Singh et al., Corrosion Science 50 (2008) 639-649.

⁴ S. Turri et al., J. Applied Polym. Sci. 118 (2010) 1720-1727.

⁵ G. Tsaneva et al.,J. Univ. Chem. Technol. Metall. 43 (2) (2008) 231-238.

Most of the literature has dealt with stationary heat sinks and sources in connection with Moving Boundary Problems. The classic work of Carslaw and Jaeger (1), claims only certain solutions known for certain geometries, for moving heat sources. The moving heat source used in laser surface melting is treated by various transformations together with a decoupling for the heat and mass transfer terms. It is known from recent work by Sands(2), that the scale for heat diffusion is different from conduction, and when small times are involved as in rapid solidification the effect may become pronounced. Some of the earlier works on the interface stability are by Mullins Sekerka (3) and Pedroso Domoto (4).

Apart from the classic work of Mullins Sekerka (3) in the 60's very little published work has appeared on the stability of the solid liquid interface. It is essential to understand this aspect because instabilities in solidification processes will affect the finish of the surface. Recent work by Hao and Lawrence (5) has appeared, specific to laser melting. The classic problem was known as the Stefan problem and formulated over 100 years ago, yet people are still struggling to give theoretical solutions.

References

1. Carslaw HS, Jaeger JC, "Conduction of Heat in Solids", 2nd Ed, Clarendon Oxford, 1959

2. Sands, D, Appl. Phys A., 88, 179-189, 2007

3. Mullins WW and Sekerka RF, JAP, 35, 444-451, 1964

4. Pedroso RI, and Domoto GA, IJHMT, 1816-19, 1973

5. Hao L and Lawrence J, Proc Roy Soc. (A), 462, 43-47 , 2006