ICMCTF1998 Session B1-2: Sputter Techniques & Deposition of Multi- & Nano-scaled Layered Coatings

TiAlN coatings have been known since the mid 1980's to exhibit high resistance against oxidation and abrasive wear [1]. However, only recently an industrial breakthroguh has been achieved promoted by the progress in dry, high speed machining using speed machine tools equipped with PVD coated cemented carbide cutting tools including solid carbide end mills and drills. Today TiAlN coatings are routinely produced by magnetron sputtering and cathodic arc evaporation.

A two fold improvement of TiAlN coatings has been developed by the incorporation of typically 2 At % Y into the coating. Ti_0.43Al_0.52Cr_0.03Y_0.02N coatings deposited by the combined steered arc/unbalanced magnetron deposition technique are characterised by an extremely fine grained equi-axed micorstructure [2] and both an enhanced compressive stress (7 Gpa) and hardness (HK 2700). The incorporated Y enhances [1] the high temperature oxidation resistance still further and [2] stabilises coating/substrate interface by suppressing the diffusion e.g. of iron and chromium through the film. In fact At % Y containing TiAlN coatings are thermally stable up to approx. 900 °C.

Cemented carbide tools coated with Ti_o.43Al_0.52Cr_0.03Y_0.02N may be operated in dry cutting of die steels with up to Hr_c65, cutting speeds up to 25000 rpm and linear feeds up to 10 m/min.

[1] W.-D./ Münz, J. Vac. Sci. Technol., A4 (1986) 2717

{2] L.A. Donohue, I.J. Smith, W.-D. Münz, I. Petrov. J.E. Greene, to be published in Surf. Coat. Technol.

TiAl_xB_yN_z coatings (with x = 0.1-0.8, y = 0.2-0.5, z = 0-0.8)have been synthesised by reactive magnetron sputtering in an Ar/N₂ plasma at temperatures between 150 and 400 degrees C. Previous studies of Ti-B-N coatings ¹, consisting of the TiN, BN and TiB₂ phases, showed a progressive increase in hardness from 18 to 40 GPa in the same temperature range. Previously hardness values of up to 20 GPa were reported for TiAl_xB_yN_z coatings deposited at 200 degrees C ². The goal of this investigation was to study in detail the influence of the temperature on the structural, mechanical and tribological properties of these coatings.

The composition of the films was determined by Auger Electron Spectroscopy and Glow Discharge Optical Emission Spectroscopy; chemical state information was gathered by X-ray Photoelectron Spectroscopy. Glancing-angle X-ray Diffraction was used to evaluate the lattice parameter and peak shape parameters, which determine coating micro- and macro-stresses, whilst Scanning macro-stresses, whilst Scanning Electron Microscopy techniques were used to evaluate coating morphology and topography. Knoop microhardness (HK), elastic modulus by Ultrasonic Surface Acoustic Waves, Rockwell ‘C’ indentation and scratch adhesion, dry sliding pin-on-disc and reciprocating wear, ball-on-plate impact and abrasive wear wheel tests were performed to evaluate the mechanical properties of the coatings.

Previous results ² showed that high hardness values can be obtained from a number of ternary/quaternary compositions, and that coating performance can be optimised in terms of a hardness/toughness compromise if the content and distribution of ‘soft’ phases such as Ti or h-BN can be controlled; these effects are further examined with the modified set of coatings reported here.

¹ L.Chalex-Combadiere and J.Machet, Study of the deposition process of TiBN films obtained by d.c. magnetron sputtering, TATF’96 Proceedings, supplement ‘Le Vide science, technique et applications’ no 279 Janvier-Fevrier-Mars 1996, 116-119. ² C.Rebholz, H.Ziegele, J.M.Schneider, A.Leyland, S.L.Rohde and A.Matthews, Structure, mechanical and tribological properties of sputtered and elecron-beam evaporated PVD hard coatings within the TiB, TiBN and TiAlBN systems, AVS’97 Proceedings, 20-24 October 1997, San Jose, USA.

Triode ion plating is a well known physical vapor deposition technique for the production of hard coatings like TiN, Ti(C,N), (Ti,Al)N, CrN, etc. ... In triode ion plating the four most important process parameters are: (1) low voltage arc current, (2) high voltage electron gun current, (3) total pressure and (4) substrate voltage. To investigate the influence of these process parameters on the plasma parameters and the energy spectra of ions and neutrals, a flat Langmuir probe was positioned in the main argon plasma body. Furthermore, an energy- and mass-analyzer was oriented to the crucible during the evaporation of titanium without reactive gases.

The substrate bias had no significant influence on the plasma parameters and the energy spectra. The HV electron gun, used to melt the titanium, changed the plasma parameters only at high current values, but the energy spectra of the Ti ions changed drastically. The LV arc current and the total pressure were the main process parameters that did effect both the probe characteristics and the energy spectra of ions and neutrals.

Under well controlled reactive unbalanced magnetron sputtering conditions CrN_x coatings can be reproducbly deposited as Cr, Cr+N Cr₂N and CrN phases and mixtures of Cr+N and Cr₂N as a function of the nitrogen content [1]. The structures and properties can be further influenced by variation of bias voltage and bias current density. At floating potential (U_B = -25 v) the lattice parameter coincides with that of the bulk value for CrN and the compressive stress approaches almost zero Gpa. Increasing the negative bias voltage to -200 V a steep increase of the lattice parameter together with an increase in the stress to only 1 Gpa um^-1 at U_B = -200 V was obsrved. In parallel a change in texture was observed. At bias voltages the {200} becomes the major texture. Increasing the bias current density leads also to an increase in the lattice parameter and internal stress from approximately zero Gpa to 1.13 Gpa um^-1 at a bias current density of 6 mA cm^-2. Increasing ion bombardment densifies the coating as can be observed from the increase of HV 2120 at 1 mA cm^-3 to HV 2440 at 6 mA cm^-3 at a bias voltage U_B = -100 V. The ion energy (bias voltage) obviously influenced the hardness more than the ion flux (bias current density).

At U_B = 0 a hardness value of HV 1290 was observed in contrast to HV 2200 at U_B = -200 V. SNMS depth profile analysis revealed an extremely uniform homogenous nitrogen composition over the total coating thickness. Whereas the variation of the bias current density hardly influenced the amount of nitrogen incorporated, a slight minimum in the nitrogen concentration was observed at Ub = -50V to U_B = -100 V.