Dependence between friction of laser interference patterned carbon and the thin ﬁlm morphology Teja Roch a,b, ⁎, Dimitri Benke b , Stephan Milles a , Aljoscha Roch b , Tim Kunze b , Andrés Lasagni a,b a Institute of Manufacturing Technology, Technische Universität Dresden, Dresden 01062, Germany b Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstr. 28, Dresden 01277, Germany abstract article info Article history: Received 25 November 2014 Received in revised form 7 January 2015 Accepted 8 February 2015 Available online 10 February 2015 Keywords: Diamond-like carbon Tribology Radiation induced effects Laser modiﬁcation Laser materials Tetrahedral amorphous carbon coatings exhibit excellent tribological properties in terms of friction (~0.1) and wear (~10 -9 mm 3 /N/m). In this work, the dependence between micro structural changes due to laser structuring and the tribological properties of ta-C are discussed. The laser structuring is made by using a holographic tech- nique called direct laser interference patterning (DLIP). Within this technique an 8 ns pulsed UV-laser (wave- length 355 nm) is used, to produce cross-like patterns with structural periods ranging from 2 μm to 10 μm. The inﬂuence of the patterns on the frictional behavior is investigated under linear reciprocating sliding condi- tions with ball on disk method and non-lubricated conditions. It is found that depending on the pattern period the friction is either increased or reduced compared to an unpatterned reference sample. The decrease of the fric- tion coefﬁcient is explained by a reduction of surface contact area and a high hardness of the non-ablated ta-C ﬁlms. However, the increased friction results from thermally induced changes in the morphology of the ta-C ﬁlm. This assumption is substantiated by thermal simulation of the DLIP process. Additionally the frictional prop- erties of DLIP processed ta-C- and steel surfaces vs. steel probes with and without a ta-C coating are compared. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Today's industry spends considerable efforts in optimization of pro- cesses to reduce energy losses in mechanical parts. An improved perfor- mance of mechanical parts is typically accompanied by an increased life cycle (reduced wear) and a reduced energy consumption (reduced friction). One route to achieve these requirements is to separate the basic material properties from the surface properties. Thus, the main function of a part remains intact but new advantageous surface proper- ties can be generated. One major ﬁeld of functionalized surfaces is tribo- logical applications in order to reduce friction, wear and abrasion. By comparing different tribological coatings, diamond like carbon (DLC) ﬁlms combine some of the most advantageous properties such as chem- ical inertness, low wear and low friction. Basically DLC-coatings can be divided in coatings with minor or major hydrogen contents [1]. Addi- tionally, these coatings can be classiﬁed according to their sp 3 -content. Within these deﬁnitions, coatings with a low hydrogen content (around 0,1–5%) and high sp 3 -content (N 70%) are called tetrahedral amorphous carbon (ta-C). These coatings offer an extreme high hardness and low friction even under non-lubricated conditions [2]. Ta-C coatings pre- pared with pulsed arc technology, exhibit empirical relations between the density (ρ), sp 3 -content (F), elastic modulus (E) and hardness (h). The relation between sp 3 -content and thin ﬁlm density is given by [3]: F ¼ ρ-1:92 1:37 : ð1Þ Furthermore, the density can be correlated with the elastic modulus [4] as follows: ρ ¼ 1:79 g  cm -3 h i  1 þ E 780 GPa ½  - E 1620 GPa ½    2   : ð2Þ Thus, a higher elastic modulus corresponds to a higher density and concomitantly higher sp 3 -content. Additionally, it has been observed that the thin ﬁlm hardness is directly proportional to the elastic modulus [4]: h ¼ 0:1  E: ð3Þ It has to be noted, that these relations are based on the variation of the sp 3 -content in the ta-C thin ﬁlms. A limiting factor of these empirical relations is the crystallinity of the sp 2 -bonded carbon network. This property might inﬂuence the elastic modulus or the optical properties independent from the sp 3 /sp 2 -ratio [5,6]. The tribological properties of ta-C, especially the coefﬁcient of fric- tion (COF), correlate linearly to the sp 3 -content of the thin ﬁlm [7]. Diamond & Related Materials 55 (2015) 16–21 ⁎ Corresponding author at: Institute of Manufacturing Technology, Technische Universität Dresden, Dresden 01062, Germany. E-mail address: teja.roch@iws.fraunhofer.de (T. Roch). http://dx.doi.org/10.1016/j.diamond.2015.02.002 0925-9635/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond