ICMCTF2011 Session B4-2: Properties and Characterization of Hard Coatings and Surfaces

Multicomponent coatings based on refractory transition metal carbides and nitrides attract a great attention as protective layers on the surface of cutting and stamping tools, as well as various mechanical components working under load-bearing conditions at evaluated temperatures. Important coating properties for such applications are high hardness, stiffness, wear-, corrosion-, and oxidation resistance, as well as high thermal stability and low friction. The desired properties can be achieved by complex alloying with other elements such as Al, Cr, Si, etc. The aim of this work was a comparative investigation of the structure and properties of Al- and Cr-doped TiSiCN coatings deposited by sputtering of composite TiAlSiCN and TiCrSiCN targets produced by self-propagating high-temperature synthesis method. Particular attention was paid to the investigation of thermal stability, high-temperature oxidation resistance and tribological characteristics. In addition, the comparison of lifetimes of coated and uncoated WC-Co end mills in dry milling of chromium steel (X12BF brand mark, 52-53 HRC) was fulfilled. The results obtained show that the Ti(Al,Si)SiCN coatings with hardness of 30 GPa showed thermal stability up to 1200^oC. The TiAlSiCN coating was more oxidation resistant than the TiCrSiCN coating. After 1 h exposure at 1000^oC the TiCrSiCN and TiAlSiCN coatings, 2.7 mm thick, were only partly oxidized with oxidation depth of 0.7 (TiAlSiCN) and 1.1 mm (TiCrSiCN). The end mills with TCrSiCN coatings showed superior dry cutting performance against high-chromium steel compared to the TiAlCN, Cr- and Si-doped TiAlCN coatings. This can be attributed to its better mechanical (high hardness and elasticity) and tribological (low friction and wear) properties.

Due to their outstanding mechanical properties at temperatures < 1000°C, ternary Ti_1-xAl_xN thin films attract a huge industrial interest. At temperatures around and above 900°C the thermally induced spinodal decomposition of metastable cubic Ti_1-xAl_xN results in an age-hardening effect during the first stages of decomposition. The advent of the stable wurtzite AlN phase, after forming c-AlN-rich domains, goes along with reduced mechanical properties.

In the present study, we highlight the influence of two refractory elements, Nb and Ta, on the thermal stability of magnetron sputtered Ti_1-xAl_xN thin films. We thereby investigate the development of structure and mechanical properties from a single phase cubic coating after deposition towards their stable constituents as a function of annealing temperature. It is shown, that apart from a solid solution hardening effect in the as-deposited state, vacuum annealing to 900°C initiates a decomposition of the c-(Ti_1-xAl_x)_1-yNb_yN coatings, akin to Ti_1-xAl_xN, to form a dual phase structure of w-AlN and c-Ti_1-yNb_yN. However, the formation of w-AlN and c-Ti_1-yTa_yN during the thermally induced decomposition of quaternary c-(Ti_1-xAl_x)_1-yTa_yN thin films is effectively retarded to higher temperatures with increasing amount of Ta.

This combined approach, implementing structural information, mechanical properties and comparison to ab initio calculations emphasizes the underlying structure-property relationships in Nb and Ta alloyed quaternary Ti_1-xAl_xN based thin films.

Thermal stability of protective tool coatings is a crucial factor for high speed and dry cutting during which the temperature can exceed 1000°C. Coatings retaining their mechanical properties at elevated temperatures is therefore of interest. This study reports an improved thermal stability of TiAlN when Cr is added. Cubic Ti_(1-x-y)Al_yCr_xN (x<0.4, 0.45<y<0.6) PVD layers were deposited by cathodic arc evaporation using compound TiAlCr cathodes in a N₂ atmosphere onto WC-Co substrates. After post deposition annealing at temperatures up to 1400°C the phase evolution and the mechanical properties of the coatings were investigated using nanoindentation, X-ray diffraction and analytical transmission electron microscopy. In-situ investigations of the thermal response of the coatings were investigated with differential scanning calorimetry.

Additionally, first principle calculations are reported. The disorder problem is, on the basis of previous work [1], treated as a coherent potential approximation with local relaxation implemented by means of the independent sublattice model, yielding the mixing free energy of the cubic TiAlCrN system. The mixing enthalpies are positive, giving a driving force for decomposition, except for systems with high contents of Cr.

The crystal structure of the as deposited coatings is cubic (B1). The hardness of the TiAlCrN coatings is maintained or even increased up to annealing temperatures of 1000°C, indicating an age hardening process, which is affected by the Cr-content. This is supported by transmission electron microscopy observations showing compositional modulations similar to what have previously been reported for TiAlN [2], however strongly affected by the Cr-content in accordance with our theoretical predictions. Results from differential scanning calorimetry demonstrate that the thermal stability of TiAlN can be increased by several hundreds degrees through Cr alloying.

[1] B. Alling, et al., Phys. Rev. B 75, 045123 (2007)

[2] A. Knutsson, et al., Appl. Phys.Lett. 93 (2008) 143110

DLC (diamond-like carbon)-coatings stand for excellent wear and friction properties. Due to their good tribological properties these coatings increase the tool life, reduce the power consumption and improve the surface finish of the work piece. However, under different humid conditions DLC-coatings react very differently. Hydrogen-free a-C-coatings show excellent frictions properties under wet conditions because of the thin water film which is formed between the coating surface and the counter part. Under dry conditions the free C-atoms at the surface bond to the counter body material which deteriorates the frictions coefficient. The hydrogenated a-C:H films act contrary to this and have extraordinary tribological properties under dry conditions since the C-atoms at the surface are hydratized and not available for any bonding with the oppositional material. Under wet conditions water molecules are weakly absorbed by the a-C:H-coatings so the interaction between the coating surface and the tribological counter part changes to a dipole-like interaction which is disadvantageous or the tribological performance. According to this, the hydrogen-content plays an important role for the wear and friction behavior of DLC coatings under different humid conditions. This work focuses on the quantification of the hydrogen content of differently bias a-C and a-C:H top layered coating systems and their influence on the tribological behavior under dry and wet conditions.

By means of a magnetron sputter device two different DLC-coating systems, one with an a-C-top layer and the other one with an a-C:H-top layer have been deposited . In order to quantify the hydrogen content within the layers GDOES (Glow Discharge Optical Emission Spectroscopy) was used. In combination with the results of the tribological tests under different humid conditions using a Ball-on-disc-tester, correlations between the hydrogen content, the bias voltage and the wear and friction performance were made.