Diamond-Like Carbon Films with Low Internal Stress by a Simple Bilayer Approach

Koumei Baba,Wolfgang Ensinger,Stefan Flege,Ruriko Hatada

doi:10.3390/coatings10070696

Koumei Baba, Wolfgang Ensinger + Show 2 more

Open Access

https://doi.org/10.3390/coatings10070696

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Journal: Coatings	Publication Date: Jul 19, 2020
Citations: 4	License type: CC BY 4.0

Affiliation: Sanno Medical Center, Technical University of Darmstadt

Abstract

Amorphous carbon films with a high hardness usually suffer from high internal stress. To deposit films with a hard top surface but reduced internal stress, a simple bilayer approach was used. Films were prepared by plasma source ion implantation, using only hydrocarbon precursors. The single layer with the highest hardness (deposited by a low direct current (DC) voltage and radio frequency (RF) generation of the plasma) has the highest internal stress with more than 3.5 GPa. By adding an interlayer with a lower hardness, the resulting stress of the bilayer film can be reduced to below 1.4 GPa while maintaining the high hardness of the top layer. By avoiding metallic interlayers or dopants within the films, the deposition process can be kept simple and cost-effective, and it is also suitable for three-dimensional samples.

Highlights

One of the main applications of amorphous carbon films is as protective coating
A high sp3 content of the film is equivalent to a high hardness [2,3,4] and to high internal stress of the film [5], which might lead to adhesion failure and delamination
The coefficient of thermal expansion (CTE) of amorphous hydrogenated films falls into the range of the average CTE of graphite and the CTE of diamond; it depends on the sp3 content of the film [7]

Summary

Introduction

One of the main applications of amorphous carbon films is as protective coating (e.g., for tribological applications). Since internal stress is inherent in hard diamond-like carbon (DLC) films, some countermeasures have been developed, namely (a) thermal annealing, (b) the addition of dopants to the DLC film and (c) the addition of one or more interlayers between substrate and DLC film. The interlayer here is a DLC layer, namely a softer one with a higher sp content and lower internal stress. We selected the simplest approach possible to investigate its effectiveness: a DLC bilayer was deposited (i.e., one softer interlayer and one harder film on top; 1 period) This bilayer approach should result in a lower internal stress of the whole DLC film while keeping the higher hardness of the outer layer. By avoiding the use of metals and limiting the precursors to hydrocarbon gases, the coating of three-dimensional samples by low stress layers would be feasible

Materials and Methods

Results and about

Deposition

Hardness andare internal stress the samples prepared with

Findings

Discussion