The Effect of Stress Relaxation on the Microstructure and Hardness Evolution of Pure Amorphous‐Carbon and C/Ti Multilayer Films

Abstract

Research on amorphous-carbon (a-C) films prepared by pulsed-laser deposition (PLD) has reported varying percentages of the sp 3 -hybridized C bond from low to as high as 85% by changing the process parameters, [1][2][3] due to the effect of PLD on energetic species. When a high-energy laser impinges on the graphite target, carbon ions, atoms or clusters with high energy are ejected in a plume, which subsequently ''condenses'' on a low-temperature substrate (often less than 100 8C), to deposit in sp 3 bonds. [4,5] The sp 3 -rich a-C films have attracted great interest. [4,[6][7][8] However, the high internal stress of the sp 3 -bonded disorder phase reduces the adhesion performance of a-C films to substrates. The thicknesses of the sp 3 -rich, pure a-C films reported by many studies are mostly less than 200 nm. [9,10] For films to be deposited with a larger thickness, incorporation of elements or multilayer modulation is introduced, [11][12][13][14][15][16][17] otherwise the films will crumble or delaminate due to over-accumulation of the stress.The stress-driven structure instability is one major specialty of a-C films containing a high sp 3 bond ratio. Many studies have reported the relaxation of a-C films through an sp 3 -to-sp 2 bond-conversion process, which generally reduces the stress and density, and results in hardness evolution. [10,18] The thermal relaxation of films by heat treatment or an elevated deposition temperature has been well studied. [18][19][20][21] There is an optimal energy for species, above which, possibly exceeding 300 eV or at a fluence larger than 80 J cm À2 , [4,[22][23][24] structural relaxation of the as-deposited films will occur. It is an important issue, when the stress-containing a-C film strips off, how the microstructure of the film evolves without substrate binding. Until now, there have been few studies on adhesion-failure-induced relaxation of the stress of a-C films. In this work, the effects of adhesion failure on the microstructure and hardness evolution of pure a-C films were studied by comparing them with adherent films. Furthermore, the influence of multilayer design on the properties of the films was also investigated. To minimize the possible effects of energy-induced relaxation, laser fluences much lower than the above-stated upper threshold were applied to deposit the a-C films.Pure amorphous-carbon (a-C) and C/Ti multilayer films are prepared by pulsed-laser deposition. For each fluence (7 or 10 J cm À2 ), a-C films are deposited with two durations (35 and 120 min) to vary the thickness. The thin films have an sp 3 bond ratio and hardness dependant on the fluence. The as-deposited, thick a-C films delaminate and provide a more-nanosized graphitic microstructure, and a lower sp 3 bond ratio and hardness compared to thin a-C films, correlating with stress relaxation of the films. The C/Ti multilayer films can be deposited at large thicknesses due to the low internal stress, while the sp 3 bond ratio and the hardness increase with decreasing thickness of the Ti bil...

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