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

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.

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 HK_25g =580 at -75V to HK_25g = 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 U_b = -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 U_b = -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 β_Nb 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 U_b = -150 V. The appearance of the β phase may explain earlier lower reported adhesion values in these types of coatings.

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.

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.

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.