Papers

ICMCTF2008 Session E2-2: Mechanical Properties and Adhesion

Thursday, May 1, 2008 1:30 PM in Room California

Thursday Afternoon

Start	Invited?	Item
1:30 PM		E2-2-1 Mechanical Characterization of Coatings Using Frequency Domain Photoacoustic Microscopy T.W. Murray (Boston University) A frequency domain photoacoustic microscopy system has been developed for the non-contact characterization of micro- and nanoscale materials. An amplified, intensity modulated continuous wave laser source is used to excite high frequency, narrow bandwidth acoustic waves and the displacement resulting from acoustic wave interaction with sample boundaries is detected using a path-stabilized Michelson interferometer coupled to a RF lock-in amplifier or vector network analyzer. The bandwidth reduction afforded by this technique allows for a significant improvement in signal-to-noise ratio over systems using pulsed laser excitation, and displacement sensitivity in the femtometer range is observed. Two methods for processing the measured frequency domain data are explored. In the first technique, the source is held at a fixed temporal frequency and scanned over the surface of the sample. The phase velocities of the generated acoustic waves are determined by analysis of the spatial frequencies in the wave field. In the second approach, the source modulation frequency is scanned over the bandwidth of interest and the transient response of the sample is reconstructed from the frequency domain data. The selections of frequency resolution and measurement bandwidth, and their effects on the resulting time domain signals, are detailed. Several applications of frequency domain photoacoustic microscopy are discussed including the mechanical characterization of thin films and coatings, resonant acoustic spectroscopy of membranes, and the photothermal operation of nanoelectromechanical systems.
2:10 PM		E2-2-4 Influence of Treatment Parameters on the Interfacial Shear Strength in a Peo Coated Aluminium Alloy D.T. Asquith, A.L. Yerokhin, Y.H. Tai, J.R. Yates, A. Matthews (University of Sheffield, United Kingdom) In recent years, the interest in oxide ceramic coated metals, in particular aluminium alloys has increased dramatically. Advanced coating processes, e.g. plasma electrolytic oxidation (PEO), are been used more frequently to enhance tribological and corrosion performances of treated surfaces. This has led to an interest in the failure of such coated systems and indeed the development of duplex treatments for increased fatigue life. The main failure mechanism in such systems is deemed to be due to brittle fracture of the ceramic layer; however this hypothesis remains so far untested. The influence of interfacial shear strength is crucial to the understanding of the failure mechanism in both simple treatments and duplex systems. In this work, effects of the coating characteristics on the interfacial shear strength of the PEO treated 2024 aluminium alloy are studied. Direct optical methods are used to measure surface strains and the shear lag method applied to infer the interfacial bond strength. Optical microscopy, SEM, EDX and XRD methods are used to reveal main characteristics of PEO coatings. Correlations between interfacial shear strength and coating parameters are discussed.
2:30 PM		E2-2-5 Evaluation of PVD Coatings and Surface Treated Dies Used in Aluminum Pressure Die Casting S. Myers, J.J. Moore, B. Mishra (Colorado School of Mines) The harsh and variable environment encountered in aluminum pressure die casting often results in premature failure, thereby creating a cost factor that includes large quantities of scrap and loss of productivity from down time. Aluminum pressure die casters are currently using a wide variety of PVD die coatings in conjunction with surface treatments in an effort to prolong die life and die casting campaigns. This paper examines the main die failure mechanisms, which include washout/erosion, soldering/corrosion, and thermal fatigue cracking of several commercial coatings and surface treated substrates, such as ferritic nitrocarburizing and ion nitriding. The controlling degradation mechanisms will be proposed and promising potential die coating systems and duplex systems will be proposed in addition to two unique laboratory tests known as the multi-mode tester and ease-of-release test.
2:50 PM		E2-2-6 Investigation of Creep in Electroplated Sn and Ni-Sn Alloy on Copper at Room Temperature by Nanoindentation J. Chen, S.J. Bull (Newcastle University, United Kingdom) Tin and tin alloy coatings have been deposited on a copper substrate as a potential electrode for a lithium battery. This electrode displays very good capacity but relatively poor lifetime in service which is due to a combination of its electrochemical and mechanical properties. Due to the large volume changes caused by new phase formation during the lithiation of tin during the charging cycle there is considerable stress generated in the coating and a low cycle fatigue component to failure during charge/discharge cycling. In this study, the creep of electroplated tin and tin alloy films at room temperature has been investigated by nanoindentation. It was found that at very low load (<1mN) significant creep is observed when holding at peak load. There is size-dependent creep behaviour in the Sn-film is caused by the tip rounding at low penetration but at larger penetration other mechanisms are also important. The relaxation of stress by creep is likely to be an important factor which affects coating failure during charge-discharge cycling.
3:10 PM		E2-2-7 Characterization of the Impact Response of Diamond-Like Carbon Coatings N.J. Uhure (University of Leicester, UK); M.E. Fitzpatrick (Open University, United Kingdom); S.V. Hainsworth (University of Leicester, United Kingdom) Diamond like carbon coatings are often used to decrease friction and wear in automotive applications. One of the difficulties of selecting appropriate coatings for automotive applications is in deciding which laboratory-based tests give an indication of performance in the engine. This study uses a non-lubricated repetitive inclined impact test following the approach of Zanoria and Blau¹ to evaluate a number of diamond-like-carbon coatings. The advantage of the inclined impact is that both normal and tangential loads are applied to the coating which more accurately mimics some of the contact conditions in the engine and also gives information on the fatigue behaviour of these thin coatings. Impact angles of 45, 60, 75 and 90° were used. The deformation modes during impact testing have been evaluated by scanning electron microscopy. The results from the impact testing are compared to the evaluation of the properties of the different coatings by a range of mechanical and tribological tests including nanoindentation, scratch testing, microabrasion wear testing and reciprocating sliding wear and the results are discussed in terms of the potential performance of the coatings tested in service. ¹ E.S. Zanoria and P.J. Blau, Wear 218 (1998) 66-77.
3:30 PM		E2-2-8 Damage Evolution and Fatigue Delamination of Diamond Films Under Repeated Loading S. Kamiya, H. Sekino (Nagoya Institute of Technology, Japan); H. Hanyu (OSG Corporation, Japan); J.C. Madaleno, J. Gracio (University of Aveiro, Portugal) It is well known that chemical vapor deposited (CVD) diamond thin films have excellent characteristics such as high hardness, high Young's modulus, high heat conductivity and chemical stability etc. In many applications, they are used under repeated loading; the case of wear resistant coatings on cutting tools is an extreme example. In such a case, sudden delamination of diamond film is frequently observed rather than gradual wear of diamond surface. From the tribological point of view, repeated scratch tests and impact tests were commonly applied to survey the possible fatigue damage of diamond coatings. However, due to the complicated dynamic stress field and additional surface interactions which are inevitable in such tribological tests, fundamental fatigue properties of diamond films deposited on substrates are not yet clear enough. In this study, it is aimed to investigate the details of fundamental fatigue damage of CVD diamond films deposited on silicon substrate. In order to eliminate complicated surface interaction under dynamic stress fields, fatigue experiment was performed with a quasi static cyclic loading applied by a spherical indenter at 10Hz. Since the strength of diamond films and their interface under monotonically increasing load is known to be significantly influenced by methane concentration in the source gas mixture, specimens were prepared with three different methane concentrations. The initial fatigue damage appeared on the substrate surface under the diamond film as small cracks observed on silicon surface (revealed by careful observation with microscopes after fatigue experiment), followed by partial delamination of the diamond film. Cracks in the film appeared after the initiation of delamination. Fatigue lifetime before delamination was successfully characterized as stress-number of cycles (S-N or Woehler) diagram for the three kinds of specimens deposited with different methane concentrations.
3:50 PM		E2-2-9 Scratch Mechanisms of Low Adhesion Multilayer Coatings on Glass X. Geng, D. Dalmas, E. Barthel (Laboratoire SVI, Unité mixte CNRS/Saint-Gobain, UMR 125, France); J.D. Kamminga (Saint-Gobain Recherche, France) Thin multilayer coatings deposited on glass are widely used for optical devices. Some of these coatings have low adhesion because of the silver layers they contain. One issue is the sensitivity to scratch damage. The aim of this study is to understand the scratch mechanisms for these brittle, low adhesion films. To refine the role of adhesion and friction coefficient in scratch mechanisms, we built a sphere/plane scratch-test which allows an in-situ microscopic observation of the scratch process. Different morphologies of scratches were characterized by SEM, EDS and FIB-TEM. As expected, the scratch mechanism strongly depends on these two parameters. Three steps were identified during scratching: scratch initiation, development and steady state. The scratch initiates in the center of the contact zone. SEM images show that it originates in curved cracks concave to the wake of the sphere. Then the cracked film remains stuck to the sphere due to confinement and pushes the crack to the front of contact. The friction coefficient at the sphere/coating interface has an important impact on these two stages. Finally the original contact area decreases due to the accumulation of debris until a steady state propagation mode. A stack of debris is pushed in front of the scratch. SEM/EDS analyses show that it consists of plastically accumulated silver under the contact and of small fragments of the brittle layers in front of the contact. FIB-TEM observations on the cross-sections of the scratches confirm that perfect interfacial rupture occurs at the interface beneath the silver. Finally, the impact of the friction coefficient and adhesion on these different steps will be discussed.
4:10 PM		E2-2-10 Effects of Substrate Bias Frequencies on the Characteristics of Chromium Nitride Coatings Deposited by Pulsed DC Reactive Magnetron Sputtering Y.-C. Kuo (National Taiwan University of Science and Technology, Taiwan); J.-W. Lee (Tung Nan University, Taiwan); L.-C. Chang (Huafan University, Taiwan); C.-J. Wang (National Taiwan University of Science and Technology, Taiwan) The chromium nitride coatings have been used widely in the molding industrial due to their good mechanical properties and corrosion resistance. In this work, the chromium nitride coatings were prepared by a bipolar symmetric pulsed DC reactive magnetron sputtering system at 2, 20, 50 and 80 kHz bias frequencies, respectively. The surface roughness and cross-sectional morphologies of coatings were evaluated via AFM, SEM and TEM, respectively. The crystalline structures of thin films were analyzed by an XRD. The chemical composition of coating was determined by an EPMA. The hardness and elastic modulus of coatings were measured by a nanoindenter. The Daimler-Benz Rockwell-C adhesion test, wear and scratch tests were further conducted to evaluate the tribological properties of coatings. The influences of substrate bias frequencies on the crystal structures and mechanical properties were discussed. It is observed that a phase transformation of CrN to Cr₂N occurred when the substrate bias frequencies increased. In addition, the mechanical properties of thin films were also strongly correlated with the substrate bias frequency.