Effect of Energy and Temperature on Tetrahedral Amorphous Carbon Coatings Deposited by Filtered Laser-Arc

Abstract

In this study, both the plasma process of filtered laser-arc evaporation and the resulting properties of tetrahedral amorphous carbon coatings are investigated. The energy distribution of the plasma species and the arc spot dynamics during the arc evaporation are described. Different ta-C coatings are synthesized by varying the bias pulse time and temperature during deposition. An increase in hardness was observed with the increased overlapping of the bias and arc pulse times. External heating resulted in a significant loss of hardness. A strong discrepancy between the in-plane properties and the properties in the film normal direction was detected specifically for a medium temperature of 120 °C during deposition. Investigations using electron microscopy revealed that this strong anisotropy can be explained by the formation of nanocrystalline graphite areas and their orientation toward the film's normal direction. This novel coating type differs from standard amorphous a-C and ta-C coatings and offers new possibilities for superior mechanical behavior due to its combination of a high hardness and low in-plane Young's Modulus.

Keywords: coating; diamond-like carbon (DLC); friction; laser-arc; superhardness; tetrahedral amorphous carbon (ta-C); wear.

Figure 1

(a) Scheme of the coating systems with (1) graphite cathode, (2) laser scanner, (3) anode, (4) filter, (5) chamber/ground, (6) radiation heater, (7) magnetron, (8) substrate rotation, and (9) carbon plasma. (b) View of the coating system.

Figure 2

Energy distribution of single- and double-charged carbon ions at 1100–1450 A.

Figure 3

Time-resolved arc current and different bias pulse overlaps and arc-spot images spreading at 1450 A at (a) 0 µs (laser ignition), (b) 100 µs, (c) 150 µs (maximum current), and (d) 300 µs.

Figure 4

Dependence of the Young’s modulus (measured using two different techniques, nanoindentation and LAwave) and hardness on bias pulse length at 100 V, series A/B/C (a), and temperature, series B/D/E (b).

Figure 5

Young’s modulus and hardness values measured by nanoindentation in coating D. The values were obtained in different remaining coating thicknesses using a wedge-shaped area which was created by grinding a calotte into the layer. A microscope image of a calotte area is included.

Figure 6

Raman plots (a) and results of the fitted Raman spectra are shown as I_D/I_G peak height ratio over G peak position (b), visualizing independent influence of temperature and bias voltage on structural parameters. The blue arrow indicates direct correlation with an increased sp³ ratio, hardness, and Young’s modulus, where I_D/I_G ≈ 0. The red arrow indicates an increase in graphitic nanoclusters with an emerging D peak.

Figure 7

(a): in-plane Young’s modulus and hardness values measured by nanoindentation in coating D on a prepared cross-section. The values obtained by conventional, depth-resolved hardness measurement are included for illustration (grey symbols). The Young’s modulus value as measured by LAwave is added for comparison (dashed green line). (b): schematic illustration of the measuring directions of the different measuring techniques.

Figure 8

(a)TEM cross-section images of coating D. In particular, the higher magnification (b) shows a partition of the C-coating into a relatively structureless zone I and a nanostructured zone II for the main part of the coating.

Figure 9

TEM cross-section images of coating D in high resolution and embedded SAD images. The assignment of zones I (a) and II (b) is shown in the image to the right in Figure 8.