ICMCTF2003 Session B1: Sputtering Coatings and Technologies

Nanocrystal-(Ti,Al)N_x / amorphous-SiN_ycomposite films were prepared by co-deposition processes. The effects of deposition parameters on composition, texture, lattice parameter, preferred orientation, grain size, grain morphology, fractured cross-sections, surface morphology, and roughness of (Ti,Al)N_x films were analyzed by x-ray photoelectron spectroscope, x-ray diffraction, transmission electron microscopy, scanning electron microscope, and atomic force microscope. The mechanical properties were also investigated.

The results indicated that the composition of nanocomposite films was uniform and could be controlled through the co-deposition process. Nanocrystal (Ti,Al)N_x embedded in amorphous SiN_y matrix with (200) preferred orientation was observed. The content of amorphous SiN_y has substantial influence on the stresses release, columnar structure, surface morphology, hardness, nanograin size and dome-shaped.

Alumina as top-coat of hard coatings is considered as the suitable material for oxidation-resistance and heat-resistance layer of cutting tools. Particularly, α-alumina with corundum structure is considered as the best because of its higher thermal stability than any other crystal structure of alumina. However, such film can be commercially deposited only by CVD at temperature above 1000°C.

In this work, the deposition of crystalline PVD-Alumina at lower temperature on cemented carbide substrates by reactive magnetron sputtering was reported. The films were deposited from the metallic aluminum targets in Ar and O₂ mixture at the substrate temperature from 600°C to 780°C in the production scale PVD system. By combining discharge voltage control and optical emission feedback, the deposition rate as high as 0.8µm/hr was achieved. Under the optimized conditions, the formations of single phase α-Al₂O₃ with good crystallinity was confirmed by low angle XRD analysis. The deposited films were characterized by hardness and adhesion, and the film compositions were measured by XPS. The structure and morphology of the films were observed by SEM.

Metallic tin and its alloys have been utilized since the Bronze Age, and their properties have been well-known as well as tin oxides, which have also been extensively studied and already put to practical use to gas-sensing and transparent-conductive devices. On the other hand, the characteristics of any binary tin-nitrogen compound have not been clarified. In this study, we report on the influences of negative bias on structural properties of tin-nitride thin films prepared by reactive sputtering.

Tin-nitride films were prepared by using an rf magnetron sputtering system. The nitrogen pressure was kept at 1 Pa. We varied a substrate bias in the ranges of 0 ~ -200 V. The crystal structure of the deposited films was characterized by X-ray diffraction (XRD). The chemical bonding states and the chemical compositions were investigated by X-ray photoelectron spectroscopy (XPS). Cross-sectional structure of the films was observed by using both a scanning electron microscope (SEM) and a transmission electron microscope (TEM).

Some sharp diffraction peaks were observed in the XRD patterns. From the peak positions, we confirmed the crystal structure of the tin-nitride film is spinel. The crystallinity of the films was strongly influenced by the substrate bias. Preffered orientation of the deposited films varied from 111 to 311 with increase in the negative bias. XPS analyses revealed that the Sn-N bonds in the films are more covalent than Sn-O in tin oxide. In the cross-sectional SEM images, we observed that the films consist of two layers: the surface-side layer shows clear columnar structure while the substrate-side one has no texture, which means the polycrystalline tin-nitride grow on amorphous tin-nitride buffer layers.

Sliding and abrasive wear tests were performed on chromium nitride (CrN) coatings deposited on mirror-polished M2 high speed steel substrates by the novel high power impulse magnetron sputter (HIPIMS) deposition technique utilising peak cathode powers of 3000 Wcm^-2, applied at a repetition frequency of 50 Hz and a duty cycle of <1%. The samples were metal ion etched using non-reactive HIPIMS at a substrate bias voltage of -1200 V. Subsequently a 2 µm thick CrN was deposited by reactive HIPIMS at a deposition rate of 0.5 µmh^-1 on the samples positioned at a distance of 10 cm from the target. No bias was used during the deposition. The ion saturation current measured by a flat electrostatic probe reached peak values of 50 mAcm^-2 corresponding to a peak ion-to-neutral ratio of Ji/Jn = 700 during the maximum of the target power pulse. Secondary electron microscope observations of the surface showed a roughening of the samples, which exposed the grain structure and was caused by intensive ion etching during the pretreatment stage. The microstructure of the CrN coatings was highly dense and columnar as observed by transmission electron microscopy.

The CrN films were evaluated in pin-on-disk tests under a normal load of 5 N and using an Al₂O₃ ball as a counterpart. Abrasive wear tests were carried out using SiC slurry and normal loads of 300 mN. Sliding wear coefficients of 2.3x10^-16 m³N^-1m^-1 measured for these films were lower by factor of 4 and the roughness of the wear track was significantly reduced compared to conventional unbalanced magnetron (UBM) sputtered CrN films. The abrasive wear coefficient, K_C, was 2.2x10^-13 m³N^-1m^-1 representing an improvement by a factor of 3 over CrN deposited by conventional UBM sputtering and a wear resistance comparable to that of CrN based superlattice structured coatings such as CrN/NbN and CrN/TiAlN.

Nanoscale multilayer C/Cr coatings (bilayer thickness, Δ =1.6 nm) were produced by the combined steered cathodic arc/ unbalanced magnetron deposition technique. The mirror-polished M2 substrates were treated by Cr⁺ ion etching/ implantation followed by the deposition of a 0.2 µm thick CrN base layer. A 1.6 µm thick C/Cr multilayer coating was then deposited by non-reactive unbalanced magnetron sputtering while rotating the substrates in front of three graphite and one Cr target. During the multilayer deposition, a bias potential of -75 V was applied to the substrates. The ion flux measured by a flat electrostatic probe was 1.2 mAcm ^-2 and the deposition rate was 0.4 µmh^-1, which resulted in ion-to-neutral ratio of Ji/Jn = 11.

XTEM diffraction- and Z-contrast imaging investigations revealed a novel nanostructure in which the basic nano-lamellae obtained as a result of substrate rotation in front of the C and Cr targets were modified by ion-irradiation induced nanoclumnar structure. The intense ion-irradiation of the immiscible film components caused local enrichment of Cr and C that propagated in the growth direction resulting in Cr-rich nanocolunms separated by C-rich boundaries.

Tribological studies of the composite Cr/C coatings were conducted using a pin on disk apparatus under a load of 5 N, at velocities of 0.13 to 0.15 and for distances of ~ 500 m in dry nitrogen (~ 0% humidity) and open air (30% relative humidity). Results indicated that the Cr-C composite coatings provided friction coefficients of 0.7-0.8 in dry nitrogen while the values were significantly lower in open air, 0.21 to 0.24, during sliding against both the coated and uncoated balls. In air a transfer film was present on the sliding ball surfaces and the wear of coated and uncoated surfaces was low. Critical load values exceeding Lc = 75 N were measured in scratch test, showing excellent adhesion of the coating.

The M_n+1AX_n-materials are newly discovered ceramics with unique properties, as reported for the bulk materials¹. An example is Ti₃SiC₂, which exhibits typical metallic properties (e.g., low resistivity) in combination with ceramic attributes such as high hardness, oxidation resistance and self-lubrication. Calculations for the electronic and mechanical properties of this phase^2,3 have further promoted the interest for surface engineering applications. Recently, we showed that single crystal Ti₃SiC₂ films can be deposited on MgO(111) substrates at growth temperatures below 1000°C, using three different d.c. magnetron sputtering processes^4,5.

We present a study on the growth of Ti-Si-C as a function of substrate temperature and substrate bias on different substrate materials including single crystals and engineering components. We evaluate the phase composition, phase stability and microstructure evolution in films grown by magnetron sputtering from individual Ti, Si and C targets as well as from a Ti₃SiC₂ compound target. Analysis with SEM, TEM, XRD, XPS and nanoindentation show that well-defined films of different microstructure ranging from epitaxial (at T>700°C) to nanocrystalline/amorphous (below 600°C) can be deposited with a smooth, nodular surface morphology. Electrical measurements yield contact resistances below 100µΩ.

¹M.W. Barsoum, Prog. Solid St. Chem. 28 (2000) 201-281.

²R. Ahuja et al., Appl. Phys. Lett. 76 (2000) 2226

³B. Holm et al., Appl. Phys. Lett. 79 (2001) 1450

⁴J.-P. Palmquist et al., Appl. Phys. Lett. 81 (2002) 835

⁵T. Seppänen et al., Proc. 53rd Ann. Meet. Scand. Soc. Electron Microscopy (Ed. J. Keränen and K. Sillanpää, Tampere University, Finland, ISSN 1455-4518, 2002), p.142.