Microscopic study on the interfacial strength of hydrogenated amorphous carbon coating systems

Microscopic study on the interfacial strength of hydrogenated amorphous carbon coating systems

Surface & Coatings Technology 205 (2011) 3429–3433 Contents lists available at ScienceDirect Surface & Coatings Technology j o u r n a l h o m e p a...

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Surface & Coatings Technology 205 (2011) 3429–3433

Contents lists available at ScienceDirect

Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Microscopic study on the interfacial strength of hydrogenated amorphous carbon coating systems Jens Schaufler a,⁎, Guang Yang b, Karsten Durst a, Erdmann Spiecker b, Mathias Göken a a b

Department of Materials Science and Engineering, University Erlangen-Nürnberg, Germany Center for Nanoanalysis and Electron Microscopy (CENEM), University Erlangen-Nürnberg, Germany

a r t i c l e

i n f o

Article history: Received 21 October 2010 Accepted in revised form 1 December 2010 Available online 9 December 2010 Keywords: Diamond-like carbon Interface Adhesion strength Cr adhesion layer

a b s t r a c t The microstructure and the adhesion strength of two hydrogenated amorphous carbon (a-C:H) coatings on steel substrates with a Cr adhesion layer were investigated by Rockwell C testing, Focused Ion Beam and High Resolution Transmission Electron Microscopy (HRTEM). Slight variation of the coatings in the ramp layer between Cr and a-C:H resulted in a good or poor adhesion behaviour. Both coatings exhibited in the ramp layer a quasi amorphous matrix structure with short-range ordered substructures. No crystalline chromium carbides could be detected in these regions. Focused investigations of the chemical gradients with Energy Filtered TEM (EFTEM) and Energy Dispersive X-Ray (EDX) spectrometry revealed a clear difference in the chemical composition of the ramp layers. Smooth gradients together with a nanocomposite structure seem to provide a high stability of the ramp layer against delamination. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Amorphous carbon coatings typically show a number of beneficial properties such as high hardness [1], low friction coefficients [2] and chemical inertness [3]. These coatings can be processed with various deposition methods [4–6]. On an industrial scale the PlasmaEnhanced-Chemical-Vapor-Deposition (PECVD) method is a frequent technique to deposit hard and wear resistant hydrogenated amorphous carbon coatings (a-C:H) for high load applications [7–9]. Coating systems produced by PECVD often exhibit low adhesion strength to the substrate because of high internal stresses [10]. Therefore additional adhesion interlayers between the substrate and the coating are crucial for a good and reliable performance in the application. Various forms of sputtered Ti-, Cr- and Si-based adhesion layers have been investigated [11]. Sputtered Cr adhesion layers have been found being a well performing adhesion layer for a-C:H on steel. First Cr is deposited in a sputter process and subsequently a process gas like acetylen is introduced. Cr forms stable carbides, which form a gradient structure between the pure Cr layer and the carbon rich a-C:H coating. Cr has a thermal expansion coefficient between that of steel and a-C:H (Cr: 4.9 x 10− 6 K− 1, a-C:H: 2.3 x 10− 6 K− 1, Fe: 11.8 x 10− 6 K− 1 [12]), which leads to low stresses at the steel/Cr interface as wells as at the Cr/a-C:H interface [13]. The adhesion strength of hard coatings can be determined by using a Rockwell C ⁎ Corresponding author. Department of Materials Science and Engineering, University Erlangen-Nürnberg, Institute1: General Materials Properties, Martensstrasse 5, 91058 Erlangen, Germany. Tel.: +49 9131 8525449; fax: +49 9131 8527504. E-mail address: jens.schaufl[email protected] (J. Schaufler). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.005

indentation test. According to the VDI 3198 standard [14] the adhesion of the coating system is divided into six classifications, starting with HF1 (very good adhesion) to HF6 (poor adhesion). The Rockwell C test induces a radial tensile stress field, which leads to radial crack formation around the indent. Furthermore, this deformation causes shear stresses at the interface between the steel substrate, the adhesion layer and the a-C:H coating, which can lead to delamination. In previous work the failure behaviour and the damage formation of hard carbon coatings on ductile substrates under uniaxial tensile loading conditions were examined [15,16]. It has been found, that brittle cracks which are found under tensile strain in the a-C:H coating are progressing directly through the adhesion layer to the substrate surface. Furthermore, dislocation deformation bands were found on both sides of the sputtered Cr adhesion layer, indicating a good adhesion between the substrate and the adhesion layer. Delamination should thus appear in the upper region of the adhesion layer, namely near the boundary between the adhesion layer and the a-C:H coating. In this work the microstructure as well as the chemical composition of two a-C:H coating systems with Cr adhesion layers, were investigated with Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM). The coatings were deposited in a similar process, but with a small variation in the deposition parameters of the ramp layer, which leads to different adhesion properties ranging from good to poor behaviour. The crystal structure of the two different ramp layers was analysed with selected area diffraction (SAD) and High Resolution TEM (HRTEM) techniques. Chemical gradients were investigated with Energy Filtered TEM (EFTEM) and Energy Dispersive X-Ray (EDX) spectrometry. Using FIB cuts, the damaged areas

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around the Rockwell C indents were closely examined and the weak areas in the adhesion layers leading to failure were characterised. 2. Experimental 2.1. Deposition Piston pins of 16MnCr5 were coated in a Balzers RS90 PVD/PECVD equipment. The specimens were mounted on a carousel-holder and exposed to a permanent axial rotation while processing. As an initial step, the specimen surfaces were cleaned and activated with Ar-ion bombardment for 30 min, using a low-voltage DC as plasma source and a voltage of −150 V to accelerate the ions on the substrate. The metallic Cr adhesion layer was deposited via a PVD process with four Cr targets. The sputter process then progressed to a PECVD process and a transitional Cr–C layer (from now on known as ramp layer), with a gradual change in the Cr content to final a-C:H coating composition, was applied to the metallic Cr adhesion layer. Therefore the power of the Cr targets was slowly reduced, while the acetylene gas (purity 2.6) flow content was continuously increased. The ratio of these two parameters, the gas flow of acetylene and the power on the Cr targets effectively defines the growing process and the microstructure of the ramp layer which is crucial for a good adhesion of the coating. The coating was deposited via a MF-PECVD process at a temperature of 190 °C and contains residual compressive stress in a range of − 2 GPa determined elsewhere using beam curvature tests. The variation of the ramp layer parameters in the deposition process of the two coating systems leads to a distinct adhesion behaviour determined with a Gnehm Swiss Max 300 Rockwell C indentation test at a load of 150 kg. A slow increase of the gas flow/late shut down of the Cr targets (variation 1) leads to a good adhesion strength. Only radial cracking with no or very limited delamination (Fig. 1a) can be found after the Rockwell C test. In contrast, a strong increase of the gas flow/early shut down of the Cr targets (variation 2) results in a poor adhesion. The SEM micrographs of the Rockwell C indent reveal just a small number of radial cracks combined with strong delamination in the vicinity of the indent (Fig. 1b). The two coating systems will be denominated as HF1 (variation 1) and HF6 (variation 2) according to their different delamination behaviour (Fig. 1). 2.2. Microstructural characterisation The FIB cross sections and the TEM samples for the failure analysis were prepared with a Zeiss crossbeam 1540 in the vicinity of the Rockwell C indents (see Fig. 1), where cracking and delamination of

the a-C:H coating occur. All TEM samples were prepared using an insitu lift out technique in the FIB with a last fine milling step of 50 pA at 30 kV to achieve electron transparency. Finally, the amorphization of the lamellae was reduced with a polishing step at a low current at 5 kV. The basic microstructure and the failure modes were investigated with a TEM CM 30 equipped with a Tietz 1 K 1 K CCD camera to collect selected area diffraction (SAD) patterns of defined regions. The EDX investigations were done with an Oxford PentaFET detector. High Resolution TEM investigations, Energy Filtered TEM (EFTEM) studies as well as the nanodiffraction studies of the columnar Cr structure were performed at 300 kV using a FEI Titan 300 80–300 microscope equipped with an image spherical aberration corrector. The EFTEM images were recorded with a Gatan image filter (GIF) Model 863 Tridiem (Gatan, Pleasanton, USA). The High Resolution TEM investigations and the diffraction analysis were focused on four characteristic zones of the adhesion layers: I) sputtered chromium structure (only diffraction analysis), II) first structural part of the ramp layer, III) upper part of the ramp layer and VI) a-C:H. Since detailed investigations of the chromium layer (I) revealed no structural variations, its structure was exemplarily discussed for system HF6. 3. Results 3.1. Microstructural characterisation In Figs. 2 and 3, TEM images of FIBed lamellae of the two coatings together with the corresponding EFTEM images and EDX measurements are shown. For both coatings, a sputtered columnar Cr layer with a width of 260 nm and a chromium content of around 90 at.% is found. For the HF1 coating, the columnar Cr [I] layer is followed by a 300 nm thick sublayer exhibiting a small Cr grain size [II]. Towards the a-C:H layer the Cr amount decreases from 85 at.% to 70 at.% and the number of Cr grains gradually decreases (Fig. 2). This sublayer is followed by a 240 nm thick, dense, solid area with no recognizable features where the Cr content decreases from 70 at.% to 40 at.%. The last 150 nm of the adhesion layer [III] are characterised by a slight coral-like microstructure and a decrease to nearly 0 at.% Cr at the interface to the a-C:H coating. For the HF6 coating a relatively sharp transition between Cr and the ramp layer is found, without clear indication of grain structure (Fig. 3). The region is subdivided into two coral-like sublayers with different densities. In the first dense, coral-like zone [II] with a width of 140 nm the Cr content decreases strongly from ~85 at.% to 60 at.%. It is followed by the second coral-like sublayer ([III], width 230 nm)

Fig. 1. a-C:H coatings on steel with different adhesion qualities, as indicated by the damage behaviour after Rockwell C indentation: a) HF1: good adhesion—periodical radial cracking around the indent can be observed; b) HF6: poor adhesion—large fields of delamination with just a small number of radial cracks around the indent can be found. The marked areas show the regions of interest, where the damage mechanisms were closely investigated by TEM and FIB cross sections.

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Fig. 2. Bright field (BF) TEM images with the corresponding EFTEM RGB (red: carbon, green: iron and blue: chromium) elemental maps and EDX measurements of the Cr content for HF1. Coating system HF1 shows a small gradient in Cr/C, no large steps in the chemical composition can be found.

with a drop in the Cr amount from 60 at.% to nearly 0 at.% at the interface to the a-C:H coating (Fig. 3). For both coating systems, a change in the gradient of the Cr content is related to a change in the microstructure. Clear differences in the gradients of the ramp layer can be observed. The EFTEM elemental maps depicted as RGB images in Figs. 2 and 3 (red: C, green: Fe and blue: Cr) qualitatively support these results. It is found, that the adhesion layer of coating system HF1 shows a continuous decrease of the Cr content towards the ramp layer/a-C:H interface. The HF6 image shows that similar to the EDX measurements a different gradient of Cr and C as well as an abrupt jump in the chemical composition appears between areas II and III. A wave-like distribution of the chemical elements in the ramp layer can be found in both coating systems. This characteristic structure can be assigned to the permanent axial rotation of the sample in the deposition process. The Cr layer [I] consists out of a highly defective columnar structure with a preferred growing orientation of b110N (Fig. 4). The SAD investigations of zone II close to the interface to zone I show evidence for nanocrystalline chromium structures for both HF1 and HF6. However the diffraction signal is influenced by the nanocrystalline Cr layer of area I. The analysis of the SAD patterns of area III of system HF1 (Fig. 5b, c) and HF6 (Fig. 6b, c) shows no indication for periodical long-range ordered Cr-structures. More structural details become visible in the corresponding HRTEM micrographs of the different regions. There crystalline chromium precipitates located in a short-range ordered amorphous matrix can be found in area II of HF1, indicating a nanocomposite structure (Fig. 5a). In contrast, no crystalline chromi-

um precipitates are found in area II for system HF6. The high resolution images of area II only indicate a short-range ordered matrix (Fig. 6a). For the interface between ramp layer and a-C:H, no longrange ordered crystalline structures are found (Figs. 5c, 6c) for HF1 and HF6. The a-C:H is fully amorphous in the HRTEM images. 3.2. Failure analysis after Rockwell C indentation—FIB/SEM/TEM An investigation of the cross section (Fig. 7a) of the well adhered coating system shows, that delamination takes place at the ramp layer/a-C:H boundary which results in a smooth interface with no internal damage of the ramp layer. The radial cracks in the a-C:H coating around the indent propagate perpendicular to the interfaces. These cracks sometimes propagate along the steel/adhesion layer interface. A high magnification TEM picture of such a delamination of the whole adhesion layer is shown in Fig. 7b. At high strains the crack propagates through the adhesion layer along the steel–chromium interface. The adhesion layer itself shows a good resistance against internal cracking. No branching of the crack has been observed. In the FIB cross section (Fig. 7c) of the delaminated areas of HF6, it is found that the ramp layer is strongly damaged in the upper part close to the former boundary of the ramp layer and the a-C:H coating. In contrast to the coating system HF1, no periodical radial cracks have been found in this cross section. At higher magnification the TEM image shows a crack passing through the ramp layer along the interface of the two characteristic structural zones (Fig. 7d). This weak area of the ramp layer obviously shows low internal stability against crack formation.

Fig. 3. Bright field (BF) TEM images with the corresponding EFTEM RGB (red: carbon, green: iron and blue: chromium) elemental maps and EDX measurements of the Cr content for HF6. The coating system HF6 exhibits an abrupt decrease of the Cr amount in the ramp layer. The microstructural features can be correlated with the gradient of the chemical elements.

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Fig. 4. BFTEM of the sputtered columnar chromium structure (HF6, region I) showing a sharp interface to the steel substrate. The exemplary nanodiffraction patterns of one columnar Cr structure give evidence for the preferred b110N orientation.

4. Discussion While the overall microstructure of both coatings is similar there are important differences in the structural and chemical gradients of the ramp layers. For both coatings, no evidence of crystalline chromium carbide structures can be identified. The HRTEM investigations of the characteristic regions II and III just show some crystalline chromium for HF1 and indication of shortrange ordered structures for HF1 and HF6, which also can be verified with the SAD diffraction patterns. This shows a good agreement with the work of Romero et al. [17], Detroye et al. [18] and Schuster et al. [19]. These investigations reveal the formation of crystalline chromium carbide structures with various deposition methods (PVD and CVD) at process temperatures much higher than those used for the PECVD process for a-C:H deposition in this work. By Rockwell C testing of the HF1 coating a periodical crack structure is found in the vicinity of the indent. Similar to uniaxial tensile tests, the crack

spacing is an indicator for the locally applied strain. A small distance between the cracks in the coating thus indicates following an Agrawal-Raj [20] type of model a good interfacial strength. In contrast the HF6 coating does not exhibit any periodical radial cracking, and rather large patches of coating delaminated from the substrate were found. The failure behaviour of the two coatings is clearly related to the chemical composition and microstructure. The abrupt change in the Cr-gradient in HF6 seems to lead to a weak bonding strength inside the ramp layer, which causes the observed structural instability in the upper part of the ramp layer. In contrast, the HF1 coating does not show any evidence of delamination inside the ramp layer. Obviously, the smooth Cr-gradient results in a sufficiently strong internal bonding under the applied loading conditions. However, some limited delamination occurs between the ramp layer and the aC:H coating. It should be mentioned here, that the distribution of the residual stresses plays also an important role in regard to the structural stability of the ramp layer. Since the hydrogenated amorphous carbon coating contains high compressive stresses [21,22] a complex residual stress profile is expected in the ramp layer. Close investigation on the residual stresses using the FIB-based ILR method published by Massl et al. [23] could help to understand the influence of residual stress on the failure behaviour. However, the current work on this topic shows certain challenges applying the described method on highly stressed a-C:H coatings on ductile steel substrates. 5. Conclusions In this work two a-C:H coatings with good and poor adhesions in a Rockwell C test have been investigated in terms of microstructure, chemical composition and failure behaviour. It is found that the chemical gradients and the microstructure of the ramp layer between the Cr and a-C:H is of crucial importance for good adhesion. For both coatings, no formation of crystalline chromium carbide was found. In the coating showing good adhesion a smooth gradient in the chemical composition of the ramp layer resulted in the formation of a nanocomposite structure, consisting of an amorphous C-rich matrix with Cr precipitates. In contrast, the ramp layer of the coating showing

Fig. 5. The SAD patterns of the ramp layer for coating system HF1 indicate nanocrystalline Cr for zone II. For zone III only evidence for short-range ordered structures can be found. The HRTEM images of zone II show a number of Cr precipitates embedded in a short-range ordered amorphous matrix. Zone III reveals no indications for long-range ordered crystalline structures. The HRTEM micrograph gives evidence for the fully amorphous structure of the a-C:H coating.

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Fig. 6. The SAD analysis of zone II for HF6 show indications for nanocrystalline Cr. The SAD patterns of zone III give no evidence for long-range ordered structures. The HRTEM micrographs reveal more structural details of zone II. In contrast to the SAD investigations, no crystalline structures can be found for zone II. The HRTEM of zone III shows no longrange ordered crystalline structures. The a-C:H is fully amorphous in the HRTEM image.

Acknowledgements The authors gratefully acknowledge the funding of the German Research Council (DFG), which, within the framework of its ‘Excellence Initiative’ supports the cluster of Excellence ‘Engineering of Advanced Materials’ at the University of Erlangen-Nürnberg.

References

Fig. 7. Cross sections of the a-C:H coating system showing the typical failure modes after Rockwell C indentation: a) HF1 FIB cross section: periodical cracks run through the coating and the adhesion layer; b) HF1 high magnification TEM image of the adhesion layer: a crack propagates through the adhesion layer and along the steel/adhesion layer interface; c) HF6 FIB cross section: the ramp layer is partially damaged—wide fields of the coating are delaminated; d) HF6 TEM image of the adhesion layer: crack branching parallel to the interface (see dotted markers) can be observed.

bad adhesion was characterised by a steep chemical gradient and no Cr precipitates. The smooth gradient together with the nanocomposite structure are therefore believed to be of key importance for a good adhesion behaviour.

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