Evolution of friction and wear in amorphous carbon thin films

The main goal of this study is to acquire a deeper understanding of the friction and wear evolution during contact sliding on amorphous carbon overcoats. Therefore, hydrogenated and nitrogenated amorphous carbon films were produced using magnetron sputtering. The film roughness, density and thickness vary with dopant concentration in the sputtering atmosphere. A friction tester was developed and built to evaluate tribological properties of a-C films during continuous contact sliding tests. The durability of the a-C:H film with the higher density and roughness is the greatest and it exhibits the lowest friction force. The friction forces for a-C:N films are significantly higher than for a-C:H. Wear in the a-C films is evaluated using atomic and lateral force microscopy (AFM and LFM). An optimal Fourier (Wiener) filter is developed and used to quantify the wear induced anisotropy in surfaces. LFM reveals the wear tracks more readily than AFM, i.e. while topographical changes are modest, significant surface alterations may be present. A film prepared by magnetron sputtering in a 0.5% H\sb2 in Ar atmosphere is subjected to a detailed friction and wear study. The friction increases monotonically until debris is formed in the interface. The topographical changes before debris formation are limited to gradual smoothing preferentially in the wear direction at the top of the surface. The sliding contact has significant effect on the lateral force images. Even after only 7,500 head the surface in the wear track is partitioned into two sections: an unaltered section with unchanged lateral force, and an altered section where the local coefficient of friction has a different value. The magnitude of the local coefficient of friction does not change with wear time. FE SEM with low energy incident electrons monitors changes in surface conductivity in the wear track indicating a chemical alteration of the surface. LFM corroborates that the macroscopic friction force is proportional to the real area of contact. We develop a wear model that assumes proportionality between the real area of contact and the macroscopic friction force. The model describes the friction force in a-C:H films until debris is formed in the interface

Fretting of titanium nitride and diamond-like carbon coatings at high frequencies and low amplitude

Physically vapour-deposited TiN and chemically vapour-deposited diamond-like carbon (DLC) coatings were tested against corundum under conditions of fretting at frequencies of 750 and 1500 Hz and at a displacement amplitude of 1 mu m. A ballon-flat configuration without external lubrication was used. The energy dissipated in the contact was calculated from the evaluation of experimental tangential force-displacement hysteresis loops. Fretting damage and material modifications in the contact zone were evaluated with light optical microscopy, scanning electron microscopy, laser profilometry and micro-Raman spectroscopy. Information from on-line mechanical analysis and from off-line material behaviour analysis is used to present an explanation of the damage processes. The degradation of the TiN coating is dominated by the development of an oxidation reaction. Under certain conditions the amorphous structure of the DLC will undergo partial graphitization. A comparison with fretting tests at lower testing frequency and higher displacement amplitude shows that the mechanisms of degradation are very different.status: publishe