ICMCTF2012 Session E2-1: Mechanical Properties and Adhesion

Thin film multilayers, where the layer thickness is between 5 and 25 nm, have been shown to exhibit significant strength enhancements over the constituent components, appealing for wear resistant coatings. The vast majority of research in this area has been focused on bi-layer systems (e.g. Cu-Ni, Cu-Nb). The particular strengthening mechanisms depend on the interface structure; FCC-FCC interfaces tend to strengthen due to elastic modulus mismatch while FCC-BCC interfaces do not transmit dislocations and additionally can provide the ability to shear to accommodate the presence of dislocations. Additionally, the ability to have locally disordered interfaces provides a sink for defects due to radiation damage. However, these high strength materials often do not have substantial ability to sustain high strains, and their ductility decreases with decreasing layer thickness. Recently we have demonstrated that tri-layer films, Cu-Ni-Nb, exhibit additional strain hardening due to the ability to have dislocations in the FCC layers cross slip because of the presence of the FCC-BCC interface adding strength to the system. This presentation will demonstrate the use of nanoindentation techniques to extract strain hardening behavior from thin films, and compare the results from microtensile behavior of free standing films with those of the films on oxidized silicon substrates. The hardness behavior is tracked as a function of included angle of the indenter to generated different effective strains. The pile up around the indentation is also tracked to correlate to the strain hardening coefficient. These complementary techniques are then compared to tensile testing of free standing, sub-micron thick films using digital image correlation for strain measurements. A combination of molecular dynamics and dislocation dynamics is used to demonstrate the likely mechanism which causes this additional strain hardening behavior.

The performance of cutting and forming tools can be significantly improved by ta-C coatings. Different applications of such tools implicate coating deposition on different materials. Moreover, the pre-treatment of the tools to be coated becomes complicated due to the necessity to perform the deposition at low temperature. Therefore it follows that a procedure to predict coating adhesion on different substrate materials would be of great benefit in order to design straightforwardly the coating-substrate system.

In this work, the mechanical properties of ta-C coatings deposited on 1.2842 (90MnCrV8) steel and tungsten carbide (6wt.% Co) have been investigated. The specific Young's moduli and yield strengths were derived from nano-indentation measurements, and multi-axial load stress profiles were simulated accordingly to Ref. [1]. The von Mises and normal stress profiles obtained from simulations are utilized to predict the locations within the coating-substrate systems where either the yield strengths or the critical tensile stresses are exceeded. This prediction is confirmed by scratch tests for which the load range and indenter geometry is optimized for the depth of interest by simulation (test dimensioning as elaborated in [1]). On the basis of these results, it was assumed that stress relaxation could be significantly dependent of the substrate material (i.e. steel or tungsten carbide). Therefore, non-destructive X-ray diffraction stress-depth profiling [2] was used to investigate the near-surface regions of both substrate materials. Thus, on this basis a straightforward interface design suitable for particular applications becomes possible.

[1] N. Schwarzer, Q.-H. Duong, N. Bierwisch, G. Favaro, M. Fuchs, P. Kempe, B. Widrig & J. Ramm: Optimization of the Scratch Test for Specific Coating Designs, submitted to SCT, accepted August 2011

[2] A. Kumar, U. Welzel & E.J. Mittemeijer: A method for the non-destructive analysis of gradients of mechanical stresses by X-ray diffraction measurements at fixed penetration/information depths, J. Appl. Crystal. 39, 633, 2006

The interface between a coating and a substrate is often crucial for the performance of coating systems. During deformation of the substrate, shear stresses are transferred at the interface into the coating, leading there eventually to cracking and delamination. In this work, the properties of carbon coatings on ductile metallic substrates (diamond on Ti and a:C-H on steel) are studied, using new in-situ methods for analyzing the stress transfer as well as the interfacial strength of the coating in dependency of the microstructure and the local chemical composition.

The first part of the talk is concerned with the analysis of the stress transfer from a ductile Ti-substrate to a brittle diamond coating under tensile straining using micro-Raman spectroscopy and analytical modeling. The coating contains initially compressive residual stresses of ~-5.4 GPa, which turn into the tensile regime during plastic straining of the substrate. Once the fracture strength of the coating of ~1.5 GPa is reached, normal cracks appear in the coating followed by a reduction in crack spacing and finally delamination. The stress measurements across different cracked coating segments using Raman spectroscopy, indicated tensile stresses at the middle and compression near the edges of the segment under tensile load. Coating fragmentation leads to a relaxation of the stress within the cracked coating segment. Further cracking of the smaller segments requires larger strains. The classical shear lag model is extended to derive the stress distribution in the coating bonded to the substrate, considering both residual stress and cracking using a fracture criterion. The model captures nicely the failure behavior of the coating as well as the stress profiles in cracked coating segments.

In the second part of the talk two a-C:H-coating systems on steel with the same microstructure, but different adhesion layers and qualitative different adhesion behaviour were investigated. The coatings were characterized in terms of their mechanical properties, microstructure and the chemical composition using nanoindentation tests and Auger electron spectroscopy on small angle cross sections of the a-C:H-coatings. There strong gradients in the mechanical properties indicate bad adhesive properties for one coating. For quantitative measurement of the interfacial fracture strength and fracture toughness, micro-cantilevers with a length of about 1.5 µm and a thickness of about 0.5 µm were prepared by focused ion beam so that the highest bending stress or stress intensity occurs right at the interface. Using a force measurement systems, the micro-cantilevers were loaded inside the SEM and the fracture properties were evaluated. The interfacial fracture strength of the different interfaces is discussed in terms of the microstructure, the local mechanical properties and the chemical gradients in the adhesion layer. It is found that a reduction in the bending strength of the interface of ~40% results on a macroscopic scale in a change from good to bad adhesion properties. Furthermore the results of fracture toughness and bending strength of the interface are compared to the properties of pure a-c:H.

Using these different approaches, a better understanding on the damage behavior of thin brittle coatings on ductile substrates is achieved and the local adhesion strength is correlated to the coating microstructure and chemical composition.

Funding by DFG Cluster of Excellence Engineering of Advanced Materials (EAM)

[1] Ö. Özkanat, B. Salgin, M. Rohwerder, J. M. C. Mol, J. H. W. de Wit , H. Terryn, Journal of Physical Chemistry C 2012, 116, 1805.

[2] Ö. Özkanat, B. Salgin, M. Rohwerder, J. H. W. de Wit , J. M. C. Mol, H. Terryn, Surface and Interface Analysis 2012, Accepted.

In this work, TiB₂-MoS₂ composite coatings deposited by closed-field unbalanced magnetron sputtering (CFUBMS) technique using Taguchi L₉(3⁴) experimental method.The structural properties of TiB₂-MoS₂ composite coatings were analyzed SEM and XRD.The hardness of coatings were measured using microhardness tester.Adhesion and fatique properties of coatings have been scratch tested in two modes. A multi-mode operation was used as sliding-fatigue, like multi-pass scratching in the same track at different fractions of critical load (unidirectional sliding) and a standard mode using progressive load operation. Failure mechanisms were discussed according to SEM examinations of the scratch tracks.

Key Words:TiB₂-MoS₂ ,Taguchi method, Adhesion, Multi-pass scratch

In this study, diamond coating adhesion on cobalt-cemented tungsten-carbide (WC-Co) substrates was investigated using scratch testing. In particular, the methodology was applied to evaluate the effects of the coating thickness and interlayer on coating delaminations. In the coating thickness effect study, substrate surface preparations, to remove the surface cobalt, prior to diamond depositions was common chemical etching using Murakami solutions . On the other hand, to study the interlayer effect, by halting the catalytic effect of the cobalt binder, two different interlayers, Cr/CrN/Cr and Ti/TiN/Ti, were deposited to WC-Co substrate surfaces (no chemical etching) by using a commercial physical vapor deposition (PVD) system in a thickness architecture of 200nm/1.5µ m/1.5µ m, respectively. Diamond films were synthesized by using a hot-filament chemical vapor deposition (HFCVD) reactor at a gas mixture of 6 sccm of CH₄ and 60 sccm of H₂, with varied deposition times.

Scratch testing was conducted on the fabricated specimens using a commercial machine, at a maximum normal load of 20 N and a speed of 2 mm/min. It is noted that the onset of coating delamination can be clearly identified by high-intensity acoustic emission (AE) signals when such events occur, which can be used to determine the critical load. Scratched track geometry was also characterized by white-light interferometry and scanning electron microscopy.

The results show that the adhesion of the diamond coating increases with the increased coating thickness, with a nearly linear relation, in the range tested. In a previous investigation, finite element (FE) simulations of scratching on a diamond-coated carbide were developed, using a cohesive zone model, to evaluate coating delaminations related to interface characteristics. The FE model was applied in this study to investigate the coating thickness effect on delamination critical loads and the results suggest a linear relation too. For the two types of interlayer materials tested, either of them seems to be effective and the diamond coating with Ti-interlayer shows poorer adhesion comparing to the Cr-interlayer coating.

It will be shown how relatively simple models simulating the bond interaction in solids applying effective potentials like Lennard-Jones and Morse can be used to investigate the effect of pressure induced changes of Young's modulus and yield strength of these solids. Relatively simple dependencies of the bulk-modulus B on the pressure P being completely free of microscopic material parameters are derived wherever the solid bond interaction can be described or at least partially described by Lennard-Jones potentials. Instead of bond energies and length only specific integral constants like Young's modulus and Poisson's ratio are of need. The influence of the pressure induced Young's modulus change B(P) is discussed especially with respect to mechanical contact experiments. Thereby it is also shown how the extraction of critical decomposition loading situations could help to obtain somewhat more generic parameters for tribological simulations and life-time predictions.

In a second part of the presentation and based on the results obtained for B(P) a theoretical feasibility study will tackle the question whether or whether not it is possible to obtain hardness values higher than 100GPa (so called "ultra hardness") for nano-composite TiN/a-Si₃N₄ and nano-composite TiN/a-Si₃N₄/TiSi₂ coating materials. For this an effective indenter concept is used which also takes into account the pressure induced increase of the Young's modulus and yield strength during indentation. It will be shown that these hardnesses could IN PRINCIPLE be obtained.