ICMCTF1999 Session D4/E5: Mechanical Properties and Applications of Diamond and Related Materials Coatings

Various deposition conditions of diamond on Ti-6Al-4V alloys at 600°C in a microwave plasma were used to obtain different film characteristics. Both the nucleation and growth steps were varied to change the diamond films structure, their surface roughness and their composition, that is the sp³/(sp³+sp²) ratio. Coatings with overlayer of different composition were also deposited. Moreover some films were polished after diamond deposition to obtain a final roughness of about 8 nm (r.m.s.).

The diamond film characteristics were studied by visible and UV Raman spectroscopy, X-ray diffraction and AFM. Surface roughness of columnar polycrystalline and polynucleated nanocrystalline diamond films were determined in the 20-100 nm range. The sp³/(sp³+sp²) ratio was also varied from nearly one to about 50%.

These films were deposited on Ti-6Al-4V disks and pins. Sliding experiments were conducted in ambient air on a rotating pin-on-disk tribometer with uncoated and coated Ti-6Al-4V pins. The friction coefficient of uncoated pins on polycrystalline diamond films is in the 0.05-0.1 range after a running in period which is as short as the surface roughness is low. The nanocrystalline diamond films provide especially low friction coefficient and wear rate in comparison to polycrystalline films. The whole results including those obtained with the polished diamond films show the influence of the surface roughness. The influence of the sp³/(sp³+sp²) ratio however cannot be neglected. No coating failure or crack, and no stress relaxation are observed on the disk track even at high load. Sliding tests between two diamond-coated counterfaces evidence a self-polishing mechanism that leads to ultra smooth wear tracks and a low final wear rate. No more damage is produced by titanium arising in the contact when the diamond pin tip is partly worn during a long test.

Diamond-like carbon (DLC) films exhibit poor microstructural stability and properties at elevated temperatures. In this study, the effect of annealing on the stability of DLC films alloyed with silicon and deposited on steel is investigated. A comprehensive study of the mechanical properties, including adhesion and fracture strength, is carried out by a novel method combining normal indentations using micro- and macroindentors with finite element calculations of the indentation. The mechanical properties of the layers are correlated to structural changes in the film and to interface reactions.

While it has become a common practice to determine hardness and the Young's modulus of thin films by nanoindentation and to calculate residual stresses from the bending of the film/substrate system, evaluation of the interface toughness, which is a measure of adhesion, and of the film rupture strength is less straightforward. Here, Hertzian-type ring cracks are generated in the film by nanoindentation of the film/substrate system with spherical diamond tips. From the critical load for crack generation the film rupture strength is deduced using FEM. Similarly, Rockwell C hardness tests in combination with calculations are performed to measure the interface toughness.

Applying these methods to DLC films on steel, it has been found that the Young's modulus decreases with increasing silicon content and the residual stress drops below 1GPa. The rupture strength is close to its theoretical limit of E/10. Annealing up to 500°C reduces the adhesion energy significantly. The variation of mechanical properties can be attributed to structural changes in the film as investigated by Raman spectroscopy, X-ray photoelectron spectroscopy.

Crystalline ß-C₃N₄ was predicted to exhibit extreme properties, such as hardness, comparable to that of diamond. Although the synthesis of this crystalline metastable phase has not been fully confirmed yet, the already prepared amorphous CN_x films possess excellent mechanical properties, including high hardness, elastic recovery (up to 90 %), and interesting tribological characteristics. Films having such properties, and prepared at the temperatures above 200 °C, using magnetron sputtering, are predicted to possess fullerene-like microstructure.

In the present work we deposited amorphous CN_x films on Si substrates by reactive DC magnetron sputtering of graphite target in nitrogen plasma at a substrate temperature of 600 °C. The films are substoichiometric in nitrogen, with its concentration ranging from 12 to 24 at. %. Elastic recoil detection (ERD) analysis revealed a concentration of hydrogen between 1 and 5 at. % in the bulk of the films. The nitrogen concentration increased while the hydrogen concentration decreased with the substrate bias voltage ranging from -300 to -700 V. Increased hydrogen content was accompanied by decreased quality of mechanical properties (hardness from 23 to 2 GPa, elastic recovery from 74 to 40 %, lower adhesion), by higher film electrical resistance (from 20 to 970 Ωcm), and by the formation of C-H and N-H bonds (confirmed by FTIR). We suggest that excessive amount (> 1 at. %) of hydrogen in the films inhibits crosslinking between graphite-like planes containing carbon and nitrogen, and it thus hampers formation of the fullerene-like microstructure.