Mechanical Properties and Adhesion

Thursday, May 2, 2013 8:00 AM in Room Golden West

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8:00 AM E2-4-1 3D Micro Scratch Tests in Combination with a Comprehensive Stress Analysis – a New Tool for the Understanding of Surface Failures
Thomas Chudoba (ASMEC Advanced Surface Mechanics GmbH, Radeberg, Germany); Norbert Schwarzer (Saxonian Institute of Surface Mechancis, Germany); Astrid Gies (OC Oerlikon Balzers AG, Liechtenstein)

Beside the Rockwell adhesion test, the scratch test is the standard tool for the investigation of adhesion properties of thin hard coatings. Typically such tests are carried out using a Rockwell C indenter with a tip radius of 200 µm and relatively high forces in the range between 10N - 100 N. However, the results are difficult to compare since they depend on substrate material, film thickness, scratch speed, perfectness of the tip and other parameters (see [1]). Generally, such tests allow mostly a ranking of different coating systems but no analysis of failure reasons.

In the last years modifications of the scratch test have been developed which are mainly designed for the micro range with forces up to 2N. Tips with a radius of typically 10 µm or smaller and precise displacement measurements with nanometer resolution allow a scan of the surface before and after the test with the scratch tip itself. These scans can give valuable information about the surface profile which influences the local stresses and about the remaining surface failures, especially the elastic – plastic transition. The measurement of the depth under load allows an estimation of the elastic and plastic deformation energy, introduced into the material, and allows often detecting the point of abrupt coating failure.

However, all these data are not sufficient to understand the reason of coating failures and their localization and to use this knowledge for the design of better coating systems. This requires a comprehensive analysis of the local stresses [2]. The elastic parameters of all layers and the substrate, the area function of the tip, the accurate lateral position of the tip in relation to the rough surface and the tilting moment of the shaft, holding the tip, have to be known for the calculation. It is further an improvement when not only the surface profile within the scratch track is considered but the full 3D surface topography in a certain range around the track.

Using a variety of coatings on different substrates such 3D stress analysis will be presented and it will be shown how it can be used for failure analysis.

[1] REMAST, A certified reference material for the scratch test, European project SMT3-CT98-2238, Final technical report, February 2002

[2] 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, Surface and Coatings Technology, volume 206, issue 6, year 2011, pp. 1327 - 1335

8:20 AM E2-4-2 A New Dynamic Impact and Sliding Wear Testing Method for the Tribological Evaluation of Treated Surfaces
Panos Epaminonda, Claus Rebholz (University of Cyprus, Cyprus)

Several well established testing methods (e.g. pin-on-disk, fretting and impact tests) have been widely used to study treated surfaces and coatings on various substrates. However, many of these existing techniques have limitations in their ability to characterize materials, since they mainly focus on a single mode of loading and wear (e.g. only impact or sliding). In this study, the design of a new Dynamic Impact and Sliding Test (DIST) for the tribo-mechanical evaluation of surfaces under complex loading conditions is presented, where the surfaces are simultaneously subjected to sliding and impact loading. Such modes exist in many critical applications, from biomedical (e.g. hip/knee implants) to automotive applications (e.g. diesel injectors, engine valves, cam shafts), in cutting tools, general machine parts and systems, etc. Instruments and techniques for combined loading situations (such as the proposed DIST) offer a feasible way for fast, economical and reliable evaluation of complex tribo-systems with high practical and industrial interest. Expected benefits include the time and cost effective evaluation of various surfaces and the better understanding of their peculiarities under such multi mode loading conditions. Some of the unique characteristics of the DIST (e.g. combined impact and sliding testing; wear area in a single point; pre-setting of desired maximum wear depth possible; evaluation of materials’ properties and behavior in a single run) are described and discussed, and also the evaluation method of the expected results and possible limitations and difficulties.

8:40 AM E2-4-3 Laser Shock Adhesion Test (LASAT) of EB-PVD TBCs: Towards an Industrial Application
Geoffrey Bégué, Vincent Guipont, Michel Jeandin (Mines-ParisTech, France); Pascal Bilhe, Jean-Yves Guédou (Snecma, SAFRAN Group, France)
The assessment of the adhesion strength of yttria (7%wt.)-stabilized (7YSZ) EB-PVD TBC is a key issue to better understand the life duration before spallation of coated turbine blades. Laser Shock Adhesion Test (LASAT) consists in focussing a pulsed nanosecond laser on the metallic side (MS) of a coated plate. Using different laser power densities permits to achieve different levels of tensile stress at the interface that could lead to the coating debonding. Two methods were previously developed to measure the interface strength on TBCs. First, adhesion threshold could be determined by LASAT-1D through the searching of a “LASAT threshold” that is the lowest laser energy to debond the coating. The LASAT-2D method consists in measuring the evolution of the interfacial crack diameter when increasing the laser power density. On TBCs the crack diameter is easily revealed by the presence of a white spot due to change of optical properties of the debonded ceramic. The conventional configuration to test TBC coupons by LASAT involves a laser shock applied on the metallic side (MS) with water as a confinement media. Unfortunately, in case of industrial turbine blades, the substrate cannot be reached by the laser beam. In this paper we introduce a new and fully original configuration (Snecma/Armines patent FR 1157284) by implementing the laser shock onto the ceramic side (CS). MS and CS LASAT-2D configurations are compared experimentally through the observation of the interfacial damaging and numerically by implementing the calculation of shock wave propagation for different substrate thicknesses. TBC as-coated plates (50×30×2 and 50×30×1 mm3) of a nickel-based superalloy (AM1) with a (Ni,Pt)Al bond-coat and EB-PVD 7YSZ top-coat were used as samples. The CS configuration required a laser absorbing black tape and a transparent adhesive as confinement media to replace water (Fig. 1). The resulting shock wave that propagated through the TBC was monitored using photonic Doppler velocimetry (PDV) to calibrate the pressure profile. A similar change of optical properties was also obtained with CS-LASAT (Fig. 2). Laser-shocked interfaces were thoroughly observed and evolution of the interfacial crack diameter was studied (Fig. 3). This allowed introducing successfully the LASAT-2D applied to TBC involving the CS configuration. A numerical FEM Abaqus simulation considering the 2D propagation of the shock wave was implemented to calculate the temporal profile pressure and the maximum stress at the interface. In the near future, the LASAT-2D with CS configuration could be applied on turbine blades with TBCs to control the adhesion in a non destructive manner.
9:00 AM E2-4-4 Self-organized Thin Film Buckling Patterns
Sergey Grachev, Jean-Yvon Faou (Saint-Gobain Recherche, France); Guillame Parry (SIMaP, France); Etienne Barthel (Saint-Gobain Recherche, France)
Thin films may spontaneously buckle under compression forming blisters of various shapes. When attached to a substrate, the buckling phenomenon is restricted by adhesion to the substrate. The interplay between the driving force to buckle and the counter-force of adhesion often results in formation of periodic patterns, as telephone-cord-like blisters. On one hand, the film buckling is described by the non-linear Föppl–von Kármán equations and has been well reproduced by finite element method (FEM) simulations. On the other hand, the delamination process involves the fracture of the interface under a mixed mode loading (both with normal and shear forces), which influences the energy needed to crack the interface, that is the interfacial toughness. Recently we have shown the importance of taking into account both these phenomena [Faou et.al., Phys.Rev.Lett. v.108, 2012]. In the current work we make a step further and study the critical conditions for the blisters to branch and to form periodic two-dimensional patterns. The parameters space was thoroughly studied by coupled buckling and mixed mode adhesion model FEM simulations. We show that the period of the blisters rather than their width is defined by the parameters of the system (stress, thickness, materials properties). The branching and the formation of patterns can occur in a certain area of the parameter space, and their period can be predicted. The results of simulations are confirmed by experimental observations of buckling of the sputter-deposited films. In particular, the self-organized hexagonal network was both simulated and observed experimentally with the period of the features of ~10 μm. We anticipate that such patterns can be scaled down to the nano-scale.
9:20 AM E2-4-5 Determination of the Young´s Modulus of Hard Coatings on Soft Polymer Substrates
Thomas Sander, Stephan Tremmel, Sandro Wartzack (Friedrich-Alexander-University Erlangen-Nuremberg, Germany)
The knowledge of the Young´s modulus for both, coating and substrate material is essential for an accurate numerical simulation or the calculation of stresses from measured displacements. While measuring the modulus of elasticity of the substrate material is state of the art, the determination for the coating is often difficult. Although several measurement methods for the elastic properties of coatings exist, the application on a compound of a polymer substrate and a hard coating is still a major challenge. The influence of the very soft polymeric substrate material on a nanoindentation of a thin hard layer is significant. Other measurement methods like tensile testing, resonance method or cantilever method require highly accurate sample geometries or lengthy preparations. The presented method uses both, experimental and numerical results. The indentation depth is measured by spherical indenters. Compared to a Berkovich or Vickers indenter the spherical indenters are less sensitive to the surface roughness and allow higher indentation loads without plastification or even coating failure. In addition to the experimental results, the test is simulated with FEM models and varying parameters such as coating thickness, Young´s modulus of substrate and coating, radius of the indenter or applied load. Knowing the coating thickness and the Young´s modulus of the substrate, the Young´s modulus of the coating can be determined by matching the indentation depth of the experimental measurements with the numerical results. The influence of the substrate is considered. In order to save time during the characterization of the coated samples, the parametric FEM simulations are carried out previously according to a statistically based sampling plan. After the experimental measurement, the results can be directly received from the data pool using a meta model (e.g. the response surface methodology). Time consuming subsequent simulations can be avoided. The results are shown for different polymeric substrate materials and thin hard coatings. However, the method can also be used for other substrate materials.
9:40 AM E2-4-6 Nanoscale Mechanical Mapping at a Wide Range of Deformation Rates with AFM
Bede Pittenger, Steve Minne, Chanmin Su (Bruker Nano Surfaces Division, US)

Atomic force microscopes (AFM) can measure and map mechanical properties of materials with very high resolution. Over the years, the methods of mechanical mapping have evolved from slow force volume to multiple-frequency based dynamic measurements using TappingMode and contact resonance.

Recently, real-time control of the peak force of the tip-sample interaction has led to a fundamental change in AFM imaging, providing quantitative mapping of mechanical properties at unprecedented resolution. During material property mapping, the time scale of tip-sample interaction now spans from microseconds to seconds, tip sample forces can be controlled from piconewtons to micronewtons, and spatial resolution can reach sub-nanometer. AFM has become a unique mechanical measurement tool having large dynamic range (1kPa to 100GPa in modulus) with the flexibility to integrate with other physical property characterization techniques in versatile environments.

This presentation will review this recent progress, providing examples from a wide range of fields that demonstrate the dynamic range of the measurements, and the speed and resolution with which they were obtained. Additionally, the effect of time dependent material properties on the measurements will be explored.

10:00 AM E2-4-7 The Effective Indenter Concept and its Extension into the Time Domain
Nick Bierwisch, Norbert Schwarzer (Saxonian Institute of Surface Mechanics, Germany)

It's well known that the theoretical basis of the standard analysis method for nanoindentation measurements, the Oliver & Pharr method [1], is the concept of the effective indenter. This concept was introduced by Bolshakov et al in 1995 [2]. Since then many papers have appeared demonstrating that it is not used only in analysing nanoindenation curves. It's applicable to more complex mechanical contact experiments on layered materials for normal and multi-axial loading conditions [3,4] too.

Within this work the concept will be applied to materials with time dependent behaviour, coated and microstructured systems. In addition its possible to determine correct generic material parameters like Young's modulus (time dependent if neccessary) and yield strength.

[1] W. C. Oliver, G. M. Pharr, J. Mat. Res. 7 (1992) 1564-1583.

[2] A. Bolshakov, W. C. Oliver, G. M. Pharr, MRS Symp. Proc 356, p 675 (1995)

[3] N. Schwarzer, Phil. Mag. 86 (33-35) 21 Nov – 11 Dec 2006, 5153 – 5767

[4] N. Schwarzer, J. Mater. Res., Vol. 24, No. 3, March 2009, 1032 - 1036

10:20 AM E2-4-8 Determining Average Effective X-ray Elastic Constant (AEXEC) of Hard Coatings by Combining cos2α sin2ψ X-ray Diffraction and Laser Curvature Methods
Anni Wang, Ge-Ping Yu, Jia-Hong Huang (National Tsing Hua University, Taiwan, Republic of China)

The X-ray elastic constants (XECs) are the major sources of experimental error in stress measurement for thin films and coatings. The selection of XEC is usually problematic and often leads to an undesirable error. To improve the accuracy in stress measurement, it is necessary to acquire reliable elastic constants. In this study, we proposed a new X-ray elastic constant for hard coatings named AEXEC (average effective X-ray elastic constant) that can be determined by combining cos2αsin2ψ X-ray diffraction and laser curvature methods. TiN hard coating on Si (100) substrate was selected as our model system, where the residual stress was determined by two techniques, namely, laser curvature technique using Stoney’s equation and sin2ψ XRD methods accompanying with AEXEC. Since the residual stress obtained by laser curvature technique (ORS) covers the entire thickness, ORS can be regard as the bulk average stress of the film. The elastic constant (E/1+ν) can be calculated by substituting the ORS into the slope of strain vs. cos2αsin2ψ plot acquiring from grazing incident X-ray diffraction, where the corresponding penetration depth of X-ray must encompass the coating thickness. The effective elastic constant (EXEC), E/1+ν, can be used for X-ray stress measurement without knowing the individual values of E and ν. However, the XECs are usually sampled from one specific diffraction direction with a few groups of grains, which may have statistical fluctuation at different rotational angles (Φ). The AEXEC (average effective X-ray elastic constant) is the average value of EXEC at several rotational angles, which can increase the sampling volume and reduce the statistical fluctuation. By comparing the AEXEC of TiN coating with different elastic constants, the AEXEC is comparable to the elastic constants determined from nanoindentation and sin2ψ XRD method. Furthermore, applying AEXEC in the conventional sin2ψ XRD stress measurement gave an acceptable deviation from ORS, ranging from 0% to 17%. The results indicated that AEXEC effectively reduced the statistical fluctuation in the stress measurements. Therefore, the proposed method provides a nondestructive technique to acquire reliable XECs in hard coatings, and the values are comparable to those determined by nanoindentation.

10:40 AM E2-4-9 Fatigue Property Enhancements of Crystalline Metallic Substrates by Coating Thin Film Metallic Glasses
Chia-Hao Chang, Jinn.P Chu, Cheng-Min Lee (National Taiwan Univ. of Sci. and Tech., Taiwan, Republic of China)
Zr-based thin film metallic glasses (TFMGs) are deposited on the crystalline metallic substrates by magnetron sputtering. Four-point-bending fatigue tests are conducted on those coated materials. It has been found that fatigue properties of materials can be considerably improved, and the enhancements varied with the maximum applied stresses. The reduction of the surface roughness after film deposition, good adhesion between the film and the substrate, and the high hardness, strength, and good ductility of TFMGs, are the major factors for the fatigue properties enhancements. The effects of TFMGs on fatigue properties of alloy substrates will be discussed.
11:00 AM E2-4-10 Bending Ductility Enhancement of Bulk Metallic Glass by Surface Treatment s
Jinn.P Chu, Chia-Chi Yu (National Taiwan Univ. of Sci. and Tech., Taiwan, Republic of China)

Bulk metallic glasses (BMGs) are normally fractured with very limited plastic strain at room temperature, severely restricting their application as engineering materials. Since deformation in BMGs is accommodated by shear bands and surface offsets, it is important to promote homogenous formation and propagation of shear bands in order to obtain measureable plastic strains.

In this study, artificial scratches are generated on tensile surface and then annealed by rapid thermal annealing below Tg, followed by magnetron sputter coating of a layer of thin-film metallic glass. The surface modifications result in increasing of bending plastic strain and more homogeneous shear band formed. The effects of surface treatments on the mechanism of shear band formation and multiplication as well as the bending plasticity will be discussed.
11:20 AM E2-4-11 Crystal Orientation Effect on the Mechanical Behaviour of Al2O3 Coatings at Ambient Temperature
Vineet Bhakhri (Imperial College London - South Kensington Campus, UK); Rachid Msaoubi (Seco tools AB); Finn Giuliani (Imperial College London - South Kensington Campus, UK); Emmanouil Bouzakis (Fraunhofer Project Center for Coatings in Manufacturing (PCCM), Greece)
Micro-pillar uni-axial compression testing provides a way to study the mechanical behaviour of materials in a much simpler stress state and plastic flow in individual slip system can be studied. This technique is employed to investigate in to the room-temperature plasticity of otherwise brittle 10mm thick Al2O3 coatings. Two different surface orientation coatings, (001) and (012), preferably oriented for Prismatic and Basal slips respectively, were deposited by chemical vapour deposition (CVD) technique on WC-Co cemented carbide substrates. Micro-pillars with approximate dimensions of 0.4mm in diameter and 1.5mm in height were fabricated with-in a single grain on these coating surfaces. These structures were then subjected to compression using a flat-punch indenter tip at a constant loading rate of 0.15mN/s. The estimated critical resolved shear-stress (CRSS) was found to be of the order of 11.5GPa for Basal slip and 4.0GPa for prismatic slip. These results are consistent with the data obtained by Lagerlof & Heuer [1] for these two slip orientations at elevated temperatures. Higher CRSS for Basal slip in Al2O3 is attributed to the splitting of a dislocation into two non-colinear partials separated by high-energy stacking faults. While, the dislocation structures in Prism slip were shown to dissociate into three collinear partials separated by low-energy stacking faults resulting in lower CRSS. (1) K.P.D. Lagerlog & A.H. Heuer; J. Amer. Ceram. Soc. 77[2]: 385-97 (1994).