E L S E V I E R Surface and Coatings Technology 71 (1995) 233-238

81111FACE COATIN68

HEII#OLO6Y

Pores in hydrogenated amorphous carbon films on stainless steel

U. M U l l e r a, R. H a u e r t a, B. O r a l a, M . T o b l e r b

a Swiss Federal Laboratories for Materials Testing and Research (EMPA), CH-8600 D~ibendolf Switzerland b Berna AG, Industriestrasse 36, CH-4600 Olten, Switzerland

Received 16 May 1994

Abstract

The aim of this work is to investigate the influence of surface precleaning procedures prior to deposition of hydrogenated amorphous carbon films onto stainless steel. In industrial coating processes the substrates are first degreased and then Ar-ion sputter-cleaned in the deposition chamber just prior to film deposition. This sputter cleaning, which removes the native surface oxide layer as well as unknown contaminations, is carried out rather thoroughly to assure a good adhesion of the hard carbon film. In these films holes or pores are found when inspected with the scanning electron microscope. We demonstrate that these are not pores in the coating but holes in the stainless steel substrate at locations of manganese sulphur inclusions. They are produced during the sputter-cleaning procedure owing to a higher sputter yield of the manganese-sulphur inclusions with respect to the stainless steel substrate. By carefully selecting the surface preparation procedures, the depth of these holes may be reduced below the surface roughness and can then be neglected. We also show that the Ar-ion sputter etching increases the surface roughness by less than 2% of the thickness of the removed layer and that the amorphous carbon film growth is extremely uniform, the increase in surface roughness being less than 1% of the film thickness for 100 nm thick coatings.

Keywords: Amorphous carbon; Holes; Adhesion; Pores; Coatings

1. Introduction

Hydrogenated amorphous carbon (a-C: H) or dia- mond-like carbon (DLC) films as an ultra-hard and low-friction coating have invoked widespread interest in research and application. Considerable efforts are being made to understand their basic properties and also to control and improve them [1] . Special properties of particular interest are chemical inertness [2] , biocom- patibility [3,4], high electrical resistivity and infrared transparency [5]. A variety of processes to deposit carbon films are employed, of which r.f. plasma depos- ition is the most widely used because of its ability to coat workpieces uniformly almost independent of their shape.

To date, many papers have been published investigat- ing the microstructure and atomic structure of amor- phous carbon films [1] . Also, many papers have been written exploring their industrial applications [6] , focus- ing on the macrostructural behaviour of these films, such as adhesion, friction or film thickness uniformity. To our knowledge no paper has been published investi- gating the microstructure of the films over macroscopic areas.

0257-8972/95/$09.50 ~:) 1995 Elsevier Science S.A. All rights reserved SSD1 0257-8972(94)02319-L

Holes have been observed with scanning electron microscopy on hard amorphous carbon films deposited onto a variety of substrate materials. Since they were correlated with failures of the films as corrosion protec- tives they were thought of as being pores in the DLC films. Such pores would not only make the coating useless as a corrosion-protective layer but would also not be acceptable in low-friction coatings or in medical implants. Similar features have been observed on a polished multiple arc evaporation deposited titanium nitride coating on a Ti-6A1-4V substrate by Davidson and Mishra [7]. There, they were explained by the removal of micro-size droplets during polishing, which were deposited inadvertently during the coating process.

In this article we investigate the surface of stainless steel substrates after mechanical polishing, after plasma cleaning and after film deposition, with atomic force microscopy (AFM), scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). We thus show the development of different surface features in the process of cleaning procedures, e.g. the effects of preferen- tial sputtering and the evolution of the surface roughness. Sj6str6m et al. [8] mentioned such effects but did not elaborate on them. They recommended using only low

234 U. Mallet" et al. / Stoface and Coatings Teehnology 71 (1995) 233 238

ion energies and high ion fluxes as well as aiming for a low background pressure. Whereas the last point is absolutely important to preserve the cleanliness of an in-situ-cleaned surface at least until deposition starts, the first two cannot be controlled independently in r.f. iI ~ plasma deposition. Wright and Page [9] noted further- / % t more that the substrate topography is replicated on the °~ coating surface, o i~

l 9

2. Experimental

Q

z

II*

The a - C : H films were prepared by r.f. plasma (13.56MHz) deposition from acetylene in an all- stainless-steel high-vacuum system with a system base pressure better than 2 x 10 6 Pa. The films were depos- ited onto standard stainless steel substrates (D! N 1.4301, AISI 304), which were mounted on the r.f. powered electrode. The r.f. generator was regulated to yield a constant self-bias. The substrates were first mechanically polished using diamond powder down to 0.25 lam size just prior to introduction into the vacuum system. Then, the substrate was Ar plasma cleaned at 3.4 Pa and - 6 0 0 V self-bias for a certain time. Using a Talysurf profilometer a sputter rate of 5 nm min 1 was measured under these working conditions. Just prior to switching to acetylene the self-bias was increased to 700 V. The acetylene was then introduced to a pressure of 2.0 Pa and simultaneously the argon was turned off so that a smooth transition from sputter cleaning to film depos- ition was achieved without interrupting the plasma. The power dissipated in the plasma was 1 W cm -z and the growth rate of the a - C : H film under the conditions described above was 40 nm min 1.

The conductivity of these films is around 2 × 104 f~ in. The films which were grown to a thickness of ~ 10 ~tm showed a Vickers hardness of 35 GPa for test loads of 200 and 500 raN. Auger electron spectroscopy (AES) was performed on a Perkin Elmer PHI 4300 scanning Auger microscope system. AES measurements on the a - C : H films showed no other elements than carbon, proving the cleanliness of the deposition system. Atomic force measurements were performed in air in the contact force mode with constant force to obtain real space topographical information of the surfaces. The quality of the surface finish is determined by the root mean square (rms) roughness calculated by the AFM software Nanoscope III with softwear, Version 2.51 (1993), Digital instruments Inc., Santa Barbara, CA. These rms rough- ness values are obtained from images with scan sizes of around 5-10 lain with a resolution of 512 x 512 pixels. Adhesion of the coatings is examined according to VDI 3198 [10] performing an optical analysis of the film at the border of Rockwell C indentations made into the substrate.

Fig. 1, A region of high pore density on a stainless steel substrate covered with a 100nm thick a - C : H film. Pore sizes are mostly between 0.5 and I gm with one larger pore of 2 ~tm size.

3. Results and discussion

Fig. 1 shows an SEM image (7kV accelerating voltage) of a 100nm thick a - C : H film on a 30min Ar-sputter-cleaned stainless steel substrate. Polishing lines as well as pores with diameters 0.5-1 ~tm and one larger pore of 2 ~tm size can be seen. Generally these pores have diameters between 0.5 gm and 3 ~tm, are mostly round, and are found for film thicknesses less than approximately 1 ~tm. Auger electron spectroscopy measurements showed that the entire surface consisted of carbon only, in the holes as well as in the surroundings.

In Fig. 2(a) a typical Auger spectrum of the coating is shown, containing only the carbon peak and no impurity peaks. Removing the a - C : H film by Ar ion sputtering revealed that the thickness of the carbon coating was the same, within 10%, in the surroundings of the holes as well as in the holes. The Auger spectra shown in Figs. 2(b) and 2(c) were taken after the complete removal of the hydrogenated amorphous carbon film. Fig. 2(b) shows the spectrum acquired in the hole, which consists mainly of sulphur and manganese as well as of some chromium. Fig. 2(c) shows the spectrum from the sur- roundings, corresponding to the one expected for stain- less steel. We therefore conclude that the holes found in the SEM images are not pores in the film but holes in the substrate, presumably due to different sputter effi- ciencies of the stainless steel and the manganese sulphur (MnS) inclusions, which are common in this type of steel.

To verify the above assumption and to measure the depth of these holes we performed atomic force micro- scopy measurements at all steps of the surface prepara- tion and film deposition. Table 1 contains a summary of the AFM results and Figs. 3 5 show typical topographi- cal AFM images of the differently prepared surfaces.

U. Mallet" et al. ,/Su@tce and Coatings Technology 71 (1995) 233 238 235

.z_ C

W

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v "1o

- - 7 - 7 "f ]" r r r

c

a) a-C:H film

b) hole

steel

Fe Fe

.L 1 k J - _ _ ~ J

200 400 600 800

kinetic energy [eV]

Fig. 2. (a) A typical Auger spectrum of the hydrogenated amorphous carbon film as found everywhere on the film. (b) The Auger spectrum in a hole and (c) in the surroundings of the hole, both after complete removal of the coating with Ar ion sputtering.

Fig. 3 shows an AFM image of a mechanically pol- ished stainless steel surface right after the last polishing step. The polishing lines are clearly visible and the surface rms roughness is 0.4 0 .7nm parallel and 1.0 1.5 nm normal to these lines. Line distances vary between 0.25 and 1 lam. Overall rms roughness values of around 1 nm indicate a nearly atomically flat surface.

Fig. 3. An AFM image of a mechanically polished stainless steel surface right after the last polishing step. The polishing lines are the most prominent feature and the surface roughness is 0.4 0.7 nm parallel and 1.0 1.5 nm normal to these lines.

Figs. 4(a) and 4(b) show two different areas of the same substrate as in Fig. 3 after 15 min of Ar plasma sputtering at - 6 0 0 V self bias. Some of the polishing lines are still visible but now regions with different corrugations are visible. Flat regions as in the lower part of Fig. 4(a) have an rms roughness of approximately 1.3 nm, whereas porous regions as in the upper part of Fig. 4(a) have ~2.5 nm rms roughness. These regions correspond to different grains. A few holes with depths from 10 to 35 nm and diameters between 400 and 700 nm are found. A typical hole is shown in Fig. 4(b), with a depth of 35 nm and a diameter of 500 nm.

Fig. 5 shows two different areas of a substrate surface after 30 min of Ar plasma sputtering. Fig. 5(a) two grains are visible. The height difference between the two is

13 nm and they are due to different Ar sputter yields on different crystallographic orientations. Measured roughnesses lie in the range 2.5 4.0 nm on either grain. Some deeper polishing lines are still visible. Holes found

Table 1 Comparison of different precleaning procedures with and without a-C : H fihn. All results are obtained from AFM measurements

Surface Roughness (rms) Prominent feature Holes

Mechanically polished Parallel to polishing: 0.4 0.7 nm Polishing lines None found Normal to polishing: 1.0 1.5 nm

Mechanically polished and Flat region: 1.3 nm No grains, but regions of different additionally 15 min Ar sputtered Porous region: 2.5 nm roughness can be distinguished

Mechanically polished and Some grains are clearly visible, additionally 30 min Ar sputtered height differences: 10 15 nm

100 nm a-C : H fihn on 1 min Polishing lines clearly visible Ar sputtered substrate

100 nm a-C : H film on 30 min Granules Ar sputtered substrate

2.5 4.0 nm

Parallel to polishing: 0.7 1.5 nm Normal: 1.0- 2.0 nm 1.5 3.0 nm

12 holes investigated: depth: 10 35 nm diameter: 400 700 nm 20 holes investigated: depth: 35 55 nm diameter: 400 nm 1 pm None found

l l holes investigated: depth: 55 105 nm diameter: 250 800 nm

236 [2 Miiller et aL /Sur/iu:e and Coatings Technology 71 (1995) 233 238

(a) (a)

(b)

Fig. 4. (a), (b) AFM images of two different arcas of the same substrate as in Fig. 2 after 15 min of Ar plasma sputtering at 600 V self bias. Flat regions as in the lower part of (a) have an rms roughness of approximately 1.3 ran, whereas porous regions as in the upper part have ~2.5 nm rms roughness. The hole in (b) has a depth of 35 nm and a diameter of 500 ran.

(b)

Fig. 5. (a, b) AFM images of the stainless steel substrate surface after 30 rain of Ar plasma sputtering. In (a) different grains are visible. The height differences are ~ 13 nm. Measured rms roughnesses lie in the range 2.5 4.0 nm on either grain. In (b) a typical hole ion-sputter- etched into the substrate with 55 nm depth and a diameter of 750 nm is shown.

are between 45 and 55 nm deep and have diameters ranging from 400 to 900 nm. On average the depth of these holes is approximately twice the depth of the holes found on the surface with 15 min Ar sputtering, in accordance with the doubled sputter time. Fig. 5(b) shows a typical hole ion-sputter-etched into the substrate with 55 nm depth and a diameter of 750 nm.

The large variation of the hole depths as shown in the summary in Table 1 may be explained by two reasons. First, manganese is added to these steels to bind the sulphur and to prevent grain boundary embrit- tlement and, therefore, the stoichiometry of the manga- nese sulphur may vary for different inclusions, resulting in a varying sputter yield. Second, the electric field created by the self-bias which defines the Ar ion flux and energy is distorted owing to the forming hole therefore changing the sputter efficiency.

Fig. 6 shows 100nm thick a - C : H films grown on stainless steel. In Fig. 6(a) the substrate was Ar sputter cleaned for I rain whereas in Fig. 6(b) it was cleaned for 30min. In Fig. 6(b) a hole is shown with a depth of 73 nm and a diameter of 450 nm. As can be seen from comparing Fig. 6(a) with Fig. 3 and Fig. 6(b) with Fig. 5(b) the surface morphology has not changed as a result of the film deposition.

These AFM results show that the rms roughness increases slightly from 0.9 to 3.3 nm when the surface has been sputter cleaned for 30 rain, corresponding to the removal of a 150 nm layer of steel. Therefore the surface roughness increases by less than 2% of the removed layer thickness. Owing to a higher sputter yield of the manganese-sulphur inclusions - - approximately 7 nm min 1 compared with the 5 nm rain 1 measured for stainless steel - - holes are etched into the surface at

U. MiUler et al. / Smface and Coatings Technology 71 (1995) 233 238 237

(a)

(b)

Fig. 6. AFM images of 100 nm thick a-C:H films grown on stainless steel. In (a) the substrate was Ar sputter cleaned for 1 min whereas in (b) it was cleaned for 30 rain. In (b) a hole is shown with a depth of 73 nm and a diameter of 450 nm.

the positions of MnS inclusions. These holes are not visible as long as their depth is less than the surface roughness. After 100 nm a - C : H deposition the rough- ness has not changed, demonstrating that the film growth is extremely uniform. The detection limit is approxi- mately 1 nm, corresponding to 1% of the film thickness. This is in accordance with the finding of Yamamoto that a - C : H films grow in a layer-by-layer mode on CoCrTa substrates [ 11 ]. This result also reiterates AFM observations by Vandentop et al. [ 12], who studied the initial stages of growth of a-C : H films, e.g. after 10 s of deposition time, on silicon (100) and found a very smooth topology with less than 0.3 nm rms roughness for an initial rms roughness of <0.3 nm, both for scan sizes of 180 nm.

As said in the introduction the Ar-ion sputter-cleaning (responsible for the formation of holes) is mandatory to assure good adhesion of the coating to the substrate. We therefore have to investigate if lowering the Ar sputter time to as low as 1 min to reduce the hole depth below the surface roughness is still enough to enable good adhesion. To test the influence of the Ar cleaning

procedure on the film adhesion, Rockwell C indentations have been performed. In Fig. 7 Rockwell C indentations on (a) 1 min Ar sputtered and (c) unsputtered substrates are shown, each coated with 2 lain a -C :H. Fig. 7(a) reveals excellent adhesion, since only a few crack lines are going outwards from the border of the indentation crater, whereas in Fig. 7(c) large flakes are broken off around the indentation. Figs. 7(b) and (d) show the identical indentations as Figs. 7(a) and (c) respectively after one week of storage under ambient conditions. In (b) no difference is seen compared with (a), proving again the excellent adhesion, whereas in (d) self-ablation (no mechanical load had been applied) going out from the indentation crater of the film occurs, probably owing to the diffusion of oxygen and water along the interface.

On well-polished surfaces the natural surface oxide- contamination layer as measured immediately after the last polishing step with XPS depth profiling showed a thickness of approximately 3 nm. This oxide-contamina- tion layer is therefore completely removed by 1 min of Ar sputtering removing 5 nm material from the surface. Lankford et al. studied the adherence of DLC films on various substrates including stainless steel and a titanium alloy (Ti-A16-V4), which were chemically or thermally precleaned [13]. They found that the adhesion on substrates which were heated in vacuum to 225-250 °C as a pretreatment had improved but was still not good. This result may be explained by the removal of water and volatile hydrocarbons from the surface by heating but especially not the oxide layer, whereas in our prepa- ration scheme the whole oxide-contamination layer is removed.

4. Conclusions

Using Ar pre-sputtering for surface cleaning prior to a - C : H deposition, holes may be ion-sputter-etched into stainless steel surfaces at manganese-sulphur inclusions. The depth of these holes depends on the sputter time. The increase in overall rms roughness due to this clean- ing is less than 2% of the removed layer thickness. The a-C : H film growth on stainless steel is extremely uniform for film thicknesses at least up to 100 nm and adapts perfectly to the surface shape. For rough surfaces, which have to be Ar sputter cleaned for a long time to yield good adhesion of the a - C : H film, hole formation is of no concern because of their high initial roughness. In other words, the hole formation may be neglected as long as the hole depth is smaller than the surface roughness.

We have thus demonstrated that perfectly flat hydro- genated amorphous carbon films can be deposited on stainless steel if the substrate is carefully prepared; i.e. the surface is well polished to allow reduction of the sputter duration to time just short enough to remove the surface oxide layer. These films also have excellent adherence and are from this point of view very well

238 U. Miiller et al. / Surface and Coatings Technology 71 (1995) 233--238

Fig. 7. Adhesion of an a-C:H film on stainless steel. The dark hole is a Rockwell C indentation. (b), (d) The identical indentations as in (a) and (c) respectively after one week of storage under ambient conditions. (a), (b) The excellent adhesion on the 1 rain Ar sputter cleaned surface, (b) showing no alteration. (c), (d) Insufficient adhesion on the unsputtered surface, (d) showing the ongoing self-ablation of the film.

suited to be used as coatings on medical implants and other applications where extremely flat surfaces are required.

Acknowledgements

We thank G. Hobi for his technical assistance. Financial support by the Swiss Priority Program on Materials Research (PPM) is gratefully acknowledged.

References

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