ICMCTF2005 Session B8-2: Hard and Multifunctional Nano-structured Coatings

After a brief overview of earlier work I shall show the differences in the properties of coatings where the superhardness results from energetic bombardment during their deposition, and superhard nanocomposites with high thermal and oxidation stability which were prepared according to our generic design principle. This principle will be explained with reference to the thermodynamic and kinetic constrains required for the successful reproduction of the results¹. The second part of my talk will be devoted to the presently ongoing discussion regarding the possible reason of the lack of reproducibility of our results, as claimed and published by other workers. On the basis of several recently published papers I shall show that the reason of the lack of the reproducibility was either an inappropriate choice of the deposition conditions, such as too low nitrogen pressure and/or deposition temperature in contradiction to our recipes described and justified in 1, or impurities, as also published some time ago². After clarifying these issues I shall concentrate on, a) the recent studies of the phase segregation during the deposition and, b) on the modelling of the mechanical properties of these materials by means of advanced finite element method (FEM). a) The thermally activated relaxation phenomena within the grain boundaries were studied by means of internal friction measurements. It was shown that superhard nanocomposites that were deposited according to our design principle have a stable nanostructure and, therefore, show no internal friction peak. In contrast, coatings in which the phase segregation was not complete during the deposition because of inappropriate choice of the conditions show internal friction peak associated with the relaxation of the nanostructure towards the stable state upon post-annealing. Recent thermodynamic calculations and experimental results confirmed the spinodal nature of the phase segregation in the TiN-Si₃N₄ as suggested earlier. b) An advanced FEM based on a new constitutional material model that accounts for the pressure dependence of elastic moduli and flow stress (as suggested recently³) allows us to model the non-linear behaviour of these materials the operate under extreme conditions. It is shown that the conventional linear mechanics that uses constant moduli and yield stress cannot describe such behaviour. This calls also for the development of new concepts for the evaluation of correct values of hardness from the load-depth-sensing indentation technique.

super 1@.Veprek and S.Reiprich, Thin Solid Films 268(1995)64

² S.Veprek et al. Electrochem.Soc.Proc.97/25(1997)317:Surf.Coat.Technol.108/109(1998)138

³ S.Veprek and A.S.Argon, J. Vac. Sci. Technol. B 20(2002)650

Si-Ti-N nanostructured coatings of 2-100 µm thickness were deposited using hypersonic plasma particle deposition [1]. The deposition rates varied from 2-60 µm/min, depending on reactant flow rates and composition of the plasma gas. During deposition, typically an Ar + H₂ plasma gas mixture was used. A plasma gas mixture of Ar + N₂ was also utilized to study the effect of using nitrogen as a plasma gas. Substrate temperature effects on coating properties were studied by varying the substrate temperature from 250-1000°C during deposition. Control of the substrate temperature allowed us to alter the crystalline fraction in the films and enabled us to analyze film hardness as a function of crystalline fraction of the coatings. Film post-processing was carried out using in-situ plasma sintering and the effects of post-processing on film properties were explored. Additionally, an oxygen gettering system was employed to remove trace amounts of oxygen present in the plasma gases.

In separate experiments with similar conditions, particle size distributions were measured by placing a sampling probe at the same location as the film substrate. Measured distributions from the sampling probe were compared to sizes obtained using the Scherrer and Warren-Averbach methods from X-ray diffraction patterns of the deposited films.

Microstructural characterization of the films was performed using scanning and transmission electron microscopy, Rutherford backscattering, X-ray photoelectron spectroscopy and X-ray diffraction. Focused ion beam milling was used to prepare TEM samples. Hardness of as-deposited films was evaluated by nanoindentation of polished film cross-sections. Measured hardness values, averaged over 10-15 locations for each film, equaled 17-28 GPa.

[1] N. P. Rao, N. Tymiak, J. Blum, A. Neuman, H. J. Lee, S. L. Girshick, P. H. McMurry and J. Heberlein, J. Aerosol Sci. 29, 707 (1998).

The family of Platit LARC^® nanocomposite coatings has recently been extended by the nACRo^® Al_xCr_1-xN/Si₃N₄ coatings. As in the related Ti system, a further improvement of hardness, wear- and temperature resistance has been achieved relative to conventional solid solution Al_xCr_1-xN coatings.

The properties and performance of the new coatings with different Si contents are compared to those of the latest members of the successful nACo^® Al_xTi_1-xN/Si₃N₄ coating family. Micro- and nanostructural similarities and differences to the Ti- and Cr-based systems are discussed. Laboratory oxidation tests, as well as machining field tests, illustrate the excellent resistance of both materials to extreme environments, thanks to their common intrinsic mechanism of structural and chemical stabilization. In the chromium-based nanocomposite system, a high level of stress control was achieved, allowing for the build-up of relatively thick coatings necessary for hobbing applications.

High-temperature oxidation resistance is one of key properties required for new advanced materials with unique physical and functional properties at temperatures exceeding 1000°C.

The article is devoted to a systematic investigation of the high-temperature oxidation resistance of ternary Zr-Si-N and W-Si-N films with a high (>20 at.%) Si content reactively sputtered from alloyed ZrSi₂ and WSi₂ targets, respectively. The films were deposited in a wide range of partial pressure of nitrogen. The oxidation resistance of the films was characterized by a high-temperature thermogravimetry, X-ray diffraction, microhardness measurements, scanning electron and optical microscopy.

The experiments revealed that there are two important factors influencing the high-temperature oxidation resistance of Zr-Si-N and W-Si-N films deposited on Si(100) substrate: (1) the phase composition of the films and (2) thermal stability of individual phases. The onset of the oxidation of the films shifts to higher temperatures with increasing content of nitrogen in the films and hence increasing content of Si₃N₄ phase. Amorphous films composed of phases based on nitrides of both elements, i.e. Si and Zr/W, exhibit the best oxidation resistance. The Zr-Si-N films exhibit higher oxidation resistance compared to that of the W-Si-N films with the same amount of a-Si₃N₄ phase. Zr-Si-N and W-Si-N films with a high (≥ 50 vol.%) amount of a-Si₃N₄ phase represent a new family of advanced materials with the high-temperature oxidation resistance exceeding 1000°C.

Recently it was suggested that a deeper understanding of the mechanical behaviour, and in this context, the mechanical properties of novel superhard nanocomposites with hardness of 40 to ≥ 100 GPa and a high elastic recovery requires consideration of the large-strain, non-linear behaviour of these materials and pressure enhancement of their elastic moduli¹ as well as associated changes in their plastic resistance. In this paper we present a new non-linear constitutive model based on the universal binding energy relation (UBER)². We use this model in a finite element calculation to model indentations into the superhard nanocomposite materials, accounting for the pressure enhancement of elastic moduli and of the plastic resistance. A comparison with a formulation based on classical elasticity, using constant, zero-pressure values of the moduli and pressure independent plastic resistance, shows that the enhancement of the elastic moduli and of the plastic resistance due to high pressure that develops in the material under the indenter has a significant effect on both overall indentation force/distance relations and details of the local stress and deformation fields. This enhancement has to be accounted for if correct modelling of the mechanical response of the material is to be achieved. The implications of these results for the evaluation of mechanical properties of such material from automated load-depth-sensing indentation techniques will be briefly discussed.

¹ S.Veprek and A.S.Argon, J. Vac. Sci. Technol. B 20(2002)650; S. Veprek et al., ibid A 21(2003)532

² J. H. Rose, J. R. Smith, F. Guinea and J. Ferrante, Phys. Rev. B 29 2963 (1984)