ICMCTF2003 Session F1/E3-2: Mechanical Properties and Adhesion

Tuesday, April 29, 2003 1:30 PM in Room Royal Palm 4-6

Tuesday Afternoon

Time Period TuA Sessions | Abstract Timeline | Topic F Sessions | Time Periods | Topics | ICMCTF2003 Schedule

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1:30 PM F1/E3-2-1 Measurement of Residual Stress by Nanoindentation*
G.M. Pharr, Y.J. Park (University of Tennessee and Oak Ridge National Laboratory); C.M. Lepienski (Oak Ridge National Laboratory and Universidade Federal do Parana, Brazil)

It has long been recognized that residual stresses in monolithic and thin film materials influence their measured hardness in a manner that may be useful in measuring and characterizing the stress. However, making such measurements from nanoindentation load-displacement data has proven difficult for a number of reasons. New experimental results are presented that examine the influence of residual stress on nanoindentation data obtained with spherical indenters. Although significant effects are observed in the elastic-plastic transition regime, accurate measurement of the stress is difficult to achieve due to practical considerations arising from factors such as surface roughness, substrate effects, and the lack of a good mechanics model that can be used to quantify the stress. Insights from finite element simulations which help to elucidate the problems are presented and discussed.

* Research at the Oak Ridge National Laboratory SHaRE Collaborative Research Center was sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.

2:10 PM F1/E3-2-3 Use of Synchrotron Radiation on the Evaluation of the Residual Stresses Along the Surface of Titanium Nitride Films Deposited onto Cemented Carbide (WC-Co) Substrates
R.M. Souza, G. Katsuragi (University of Sao Paulo, Brazil); J.T.N. Medeiros (Federal University of Rio Grande do Norte, Brazil); L.G. Martinez (Energetic and Nuclear Research Institute, Brazil); C. Giles (University of Campinas, Brazil)
In this work, thin films of titanium nitride were deposited using commercial physical vapor deposition (PVD) techniques with different process parameters. The substrates were triangular cemented carbide (WC-Co) cutting inserts with edges 16 mm long. An x-ray diffraction beamline (XD2) at the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas, Brazil, was used to measure the film residual stresses. The sin2ψ method was used with ψ angles varying from - 45 to 45 degrees and measurements were conducted at two perpendicular directions. The high intensity of a monochromatic beam, with selected wavelength, allowed the reduction of the size of the incident beam at the specimen to approximately 1 mm x 3 mm. This incident beam size was located at different regions of the surface of each specimen, which permitted the evaluation of the variation of the film residual stresses along the surface. Results indicated that with this technique it was possible to identify differences in residual stresses not only from one deposition run to the other, but also from different regions of the surface of each specimen.
2:30 PM F1/E3-2-4 Development of Stress in DLC Based Coating Deposited by Filtered Cathodic Vacuum Arc
M.P. Delplancke-Ogletree (Universite Libre de Bruxelles, Belgium); O.R. Monteiro (Lawrence Berkeley National Laboratory)
In the literature there is a running debate on the importance of stress in DLC films for the development of hardness. Two opposite views are presented. On one side authors are considering that high hardness can not be obtained without the development of high compressive stresses in the growing layers. On the other side, it is said that the important observed stresses are an artefact of the deposition process and are not linked to the development of hardness. To try to clarify the debate and to bring new experimental evidences, we built an inexpensive technique for in-situ stress measurement during PIIID deposition of DLC based layers. The depositions were carried out in different conditions: bias voltage and duty cycle, pulse length. The stress in the growing layer was monitored continuously. The hardness of the films was then determined by nanoindentation. In this paper, we will present the stress monitoring technique as well as the observed correlations between the hardness, the stress and the deposition conditions.
2:50 PM F1/E3-2-5 Influence of Ion Bombardment on the Properties and Microstructure of UBM Deposited Nb Coatings.
T. Savisalo, D.B. Lewis, P.Eh. Hovsepian, W.-D. Münz (Sheffield Hallam University, United Kingdom)

Niobium PVD coating have been shown to have high corrosion resistance. Thin Nb coatings have also show good potential for the corrosion protection of low alloy steels as a stand alone coating or as a part of multilayer corrosion and wear resistant coating. To establish full potential for these coatings the effects of the increase in bias voltage during the deposition of Nb coatings were investigated polished mild steel and stainless steel samples.

The changes were investigated in two sets of samples with either approximately 1 μm or 3 μm thick Nb coatings. The bias voltage was varied from -75V to -150V. Increasing bias voltage increased the hardness of the coating from HK25g =580 at -75V to HK25g = 820 at -150V.

The effects to the microstructure were examined using XTEM and XRD. XRD measurement developed strong {110} texture in all samples. However, the specimens deposited at low bias voltage Ub = -75 V showed more clearly defined grains and thus higher peak maximum for the {110} reflection. Increased ion bombardment led to increasing residual compressive stresses. In parallel to the increasing residual stress the amount of Ar incorporated in the lattice increased with increasing bias voltage. Ar contents of the order of 2.5 at % were measured by EDX in the coating deposited at Ub = -150 V. Furthermore, in the XRD patterns the {220} reflection became increasingly asymmetric with increasing bias voltage, which leads one to the conclusion that the tetragonal βNb is forming at the higher bias with a lattice parameter in the 'a' axis almost identical to that of the bcc Nb phase. The incorporation of Ar the existence of the β phase and higher residual stresses explains the higher hardness at Ub = -150 V. The appearance of the β phase may explain earlier lower reported adhesion values in these types of coatings.

3:30 PM F1/E3-2-7 Mechanisms of Stress Generation in Thin Ta Coatings at Elevated Temperatures
B.L. French, J.C. Bilello (University of Michigan, Ann Arbor)

Sputtered Ta thin films on Si (100), subjected to a 1 hour anneal in air at 585°C, have been observed to generate large compressive stresses due to thermal expansion mismatch, formation of less-dense oxide phases, differential thermal stress caused by uneven heating, and entrapped gases. Films deposited at Ar pressures below 0.93 Pa develop significantly larger stresses during annealing than films grown at higher pressures. This disparity results in delamination and adherence of low and high-pressure films, respectively. This study utilizes atomic force microscopy, x-ray diffraction, and x-ray specular reflectivity to examine the mechanisms by which the morphology, phase, and density affect the stress-generation in these sputtered Ta films. A model is developed to explain the differences in stress-generation between films grown at different pressures, and is used to predict the stress for a given film as a function of time and annealing temperature.

This work is sponsored, in part, by the ARO under GRANT# DAAG 55-98-1-0382 and DAAD 19-02-1-0335. The authors would like to thank USDoE for support of work performed at the Stanford Synchrotron Radiation Laboratory.

3:50 PM F1/E3-2-8 Mechanical Strength Improvement of Electrical Discharge Machined Cemented Carbides Through PVD (TiN, TiAlN) Coatings
B. Casas (ETSEIB-Universitat Politècnica de Catalunya, Spain); A. Lousa (Universitat de Barcelona, Spain); M. Anglada (ETSEIB-Universitat Politècnica de Catalunya, Spain); J. Esteve (Universitat de Barcelona, Spain); L. Llanes (ETSEIB-Universitat Politècnica de Catalunya, Spain)
Electrical discharge machining (EDM) is an alternative shaping route for manufacturing complex component shapes of hard and brittle materials as cemented carbides. However, this results in a heat affected surface layer with poor surface integrity including unfavorable residual stresses and cracks. The EDM-induced surface damage of these materials often lead to their mechanical degradation. In this study, the improvement of mechanical strength of a EDMed fine-grained WC-10%wt Co hardmetal by physical vapor deposition (PVD) coatings is investigated. Titanium nitride (TiN) and nanoestructured titanium aluminum nitride (TiAlN) films have been deposited on WC-Co substrates with different surface finish variants: sequential EDM involving several surface finish steps, and conventional grinding and polishing employing diamond as abrasive. Fracture resistance of coated samples was measured under four-point bending tests and compared to that of uncoated samples for all conditions studied. A detailed fractographic examination has been conducted through scanning electron microscopy in order to identify and characterize strength-controlling defects. The residual stresses in the PVD films have been measured by X-ray diffractometer technique. Experimental results confirm the detrimental effect of the EDM treatment on the fracture strength of cemented carbides. Furthermore, they show that deposition of TiN and TiAlN on the different EDM-related surface variants evaluated yield a significant fracture resistance increase. On the other hand, changes of mechanical properties of the reference polished condition and subsequently coated have not been observed. These findings are discussed in terms of the type and location of the strength-limiting flaws and the residual stresses in the PVD films.
4:10 PM F1/E3-2-9 Determination of Residual Stress Alterations in pvd Coatings and their Substrates Induced by Mechanical and Heat Treatments, Based on a FEM Supported Continuous Simulation of the Nanoindentation
K.-D. Bouzakis, G. Skordaris, I. Mirisidis, S. Hadjiyiannis, I. Anastopoulos, N. Michailidis (Aristoteles University of Thessaloniki, Greece); G. Erkens, R. Cremer (CemeCon AG, Germany)

The high potentials to increase the cutting performance of coated tools through appropriate mechanical or heat treatments have led to a wide use of related techniques in manufacturing of coated tools. Through such treatments residual stress alterations in coating and substrate materials are induced, resulting to strength properties modifications. The quantitative determination of coatings residual stresses and their alterations is usually conducted by means of X-ray measurements, however with restricted accuracy in the case of thin hard coatings.

In the frame of the present paper, a procedure based on nanoindentations and a FEM supported evaluation of the corresponding measurement results has been developed, enabling an accurate definition of coatings residual stress alterations. Hereupon the yield strength differences are correlated to internal stress ones in the coating and in their substrates after various treatments. The developed method is based on a Finite Elements Method (FEM) supported calculation of the principal and equivalent stresses occurring at various positions in the coating and substrate material, at the moment of the material plastic deformation beginning in these locations during the nanoindentation.

By means of the developed method, the occurring stress alterations in cemented carbide inserts as well as within their coatings after micro-blasting procedures can be detected. Moreover the superficial stress modifications in cemented carbide inserts and in their coatings induced by heat treatments can be quantitatively determined.

4:30 PM F1/E3-2-10 Deformation Processes in TiN Thin Films on Ductile Substrates
M.J. Hoffman (The Universtiy of New South Wales, Australia); Z.-H. Xie, J. Cairney, R. Moon, P.R. Munroe (The University of New South Wales, Australia)

The tribological properties of TiN coatings are particularly important for their use as cutting tools and bearings where they are applied to ductile, usually steel, substrates. Considerable research has been undertaken into developing means of assessing the quality these coatings and the means in which they deform. Much of this research has been based upon indentation behaviour.

In this study, nano-indentation was undertaking on 1-2 µm thick TiN coatings on stainless steel substrates using peak loads ranging from 50 mN to 500 mN. Spherical-tipped conical diamond indenters were used. Coatings were applied using physical vapour deposition. Following indentation, a focused-ion-beam (FIB) was used to mill a cross-section of the indentation and subsequently to prepare transmission electron microscope specimens of the indented region, including the TiN/steel interface.

It was revealed that extensive shear cracking occurred between the TiN grains which was also observed on the nano-indentation load-displacement plots. With subsequent loading, shear sliding occurred between the grains resulting in a step-like interface at the steel. The cracks blunted at the TiN/steel interface and no interfacial cracking was observed. Additionally, Hertzian cone cracks were observed in the coating at the edge of the indent but not in the region below the indent, indicating that they formed during unloading. A large number of dislocations were observed in the TiN coating.

An energy-based model has been developed to assess the quality of the coatings. It is based upon the concept of assuming that the total energy associated with nano-indentation of the system is the superposition of film cracking and deformation and plastic deformation of the substrate. By undertaking indentations of the substrate material at comparable strains it is possible to ascertain the plastic deformation energy of the coatings.

Time Period TuA Sessions | Abstract Timeline | Topic F Sessions | Time Periods | Topics | ICMCTF2003 Schedule