Ž . Diamond and Related Materials 10 2001 10301035 Low friction ta-C films with hydrogen reservoirs J. Koskinen a,  , H. Ronkainen a , S. Varjus a , T. Muukkonen a , K. Holmberg a , T. Sajavaara b a VTT Manufacturing Technology, P.O. Box 1703, FIN-02044 VTT Espoo, Finland b Accelerator laboratory, Uni  ersity of Helsinki, Helsinki, Finland Abstract Ž . 3 Diamond-like tetrahedral amorphous carbon ta-C films, which contain no hydrogen and high content of sp bonds have a Ž . very high hardness and elastic modulus up to 600 GPa . They have also excellent tribological properties in atmospheres that Ž . contain humidity 50% RH or also when submerged in water. However, in dry atmospheres the co-efficient of sliding friction against bearing steel, for instance, increases to values of over 0.7. In this paper ta-C films were modified by adding hydrogen to the films. The ta-C films were deposited by using non-filtered pulsed vacuum arc deposition in the ambient pressure of hydrogen and hydrocarbon gases. A novel layered structure containing titanium layers with hydrogen and carbon have also been grown by using filtered metal arc in combination with the pulsed carbon arc. The hydrogen content and the composition of the films were Ž . measured by using time-of-flight elastic recoil detection analysis TOF-ERDA . The hardness of the films was measured by nanoindentation. The tribological performance of the coatings were determined by using pin-on-disk tests. The counter part Ž . material was steel AISI 52100 , the normal load applied 5 N and the sliding velocity 0.02 ms. In the modified films the hydrogen content was up to 16 at.% in ta-C and up to approximately 50 at.% in the titanium layers. A co-efficient of friction of approximately 0.2 at dry atmosphere was measured for hydrogen-containing ta-C and also for layered structures where a hydrogen-containing Ti layer was buried under a hydrogen free ta-C layer. The results indicate that the lowered co-efficient of friction is a result of hydrogen transport to the contact surfaces with the aid of surface diffusion through the pinholes of the ta-C layer.  2001 Elsevier Science B.V. All rights reserved. Keywords: ta-C; Hydrogen content; Dry sliding; Tribology 1. Introduction Carbon films deposited by arc evaporation process possess a high fraction of sp 3 bonds and are nearly free of hydrogen. Such amorphous films are called tetrahe- Ž .  dral amorphous carbon films ta-C 1 and they reveal properties close to those of diamond. The typical properties of the ta-C films are high  Corresponding author. Tel.: 358-9-456-5413; fax: 358-9-463- 118. Ž . E-mail address: jari.koskinen@vtt.fi J. Koskinen . hardness and elastic modulus. Thus, they provide excel- lent wear resistance and low coefficient of friction against most other materials such as metals and cer- amics when the tests are performed in normal air Ž . relative humidity RH approx. 50% or even in water   2,3 . However, the co-efficient of friction has been reported to be high when the films are tested in vac-   uum or in dry conditions 4,5 . This is opposite to what has been reported of the tribological properties of Ž .   hydrogenated amorphous carbon films a-C:H 6,7 . The a-C:H films have a significantly lower friction in dry conditions compared to, e.g. normal air RH 50%. There are several industrial applications which operate 0925-963501$ - see front matter  2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 9 2 5 - 9 6 3 5 00 00488-X