Mechanical Properties and Adhesion

Wednesday, May 2, 2001 1:30 PM in Room San Diego

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1:30 PM F1/E4-2-1 High Cycle Fatigue of Thermal Spray Coatings and Substrates
R.L. Holtz, K Sadananda (Naval Research Laboratory); T. Tsakalakos, M. Croft (Rutgers University)
General fatigue damage to a coated substrate can occur to the coating, to the substrate, or to the interface between the coating and substrate. Undesirable results of fatigue damage can be cracks in the coating, separation of the coating from the substrate, or in some cases premature failure of the substrate. Residual stresses at the coating-substrate interface can accelerate fatigue crack initiation in a substrate. Substrate crack initiation also can be accelerated by the presence of defects or cracks in the coating under some instances. These effects can cause a coated part to fail much earlier than an uncoated part, even if the coating otherwise has beneficial effects such as wear resistance or corrosion resistance. We report experimental bending fatigue results of tungsten-carbide/cobalt and alumina-titania ceramic thermal spray coatings deposited onto stainless steel, carbon steel, alloy steel and Monel. We describe an analysis of the trends in general fatigue tolerance for various coating-substrate combinations, and describe mechanisms of premature fatigue failures of coated substrates. Nanostructured coatings are expected to have less residual stress effects due to enhanced creep rates. We compare the fatigue tolerance of nanostructured tungsten-carbide/cobalt and alumina-titania ceramic coatings versus conventional microstructured coatings on the various substrates and correlate with experimental residual stress profiles.
1:50 PM F1/E4-2-2 Interfacial Adhesion to Study the Role of Residual Stresses Upon Adhesion of Thermal Sprayed Coatings
J. Lesage (UST Lille, IUT A, France)
The interfacial indentation test was developed these last years as an alternative method to the use of standard tests in order to study adhesion of thermal sprayed coatings. In this method the application of increasing loads on a Vickers indentor used as a wedge between the coating and the substrate, allows to determine the critical load for which cracking is initiated at the interface between the coating and the substrate. It was observed that the crack formed at the interface has a semi-circular (half penny) shape. Since the global behavior of the bi-material can be considered as brittle, the authors have proposed earlier to associate an apparent interface toughness to the critical load. For that purpose, they consider that Linear Fracture Mechanics concepts can applied and that the properties of a unique material could be derived from that of the two materials coating and substrate. This methodology has already proved is usefulness for the study of adhesive properties of various thermal sprayed materials on metallic substrates. Using literature data as well as experimental results, the authors propose this time a new phenomenological model which allows to calculate the stresses by comparing the interfacial indentation behavior after or before an annealing treatment that decreases the stresses in a great extent. Applying this model to NiCr coatings, it was found that the residual stresses are inversely proportional to the thickness of the coatings. In addition, a good correlation between the calculated residual stresses and that determined by X-Ray diffraction was observed.
2:10 PM F1/E4-2-3 An Evaluation Software of the Impact Test Results to Quantify the Creep Behaviour of Monolayer Thick Plasma Sprayed Coatings
K.-D. Bouzakis, A. Lontos, K. David, G. Maliaris ( Aristoteles University of Thessaloniki, Greece)
The mechanical and creep properties of various plasma sprayed coatings on machine elements are required in order to ensure their safe operation. In former investigation plasma sprayed coatings were examined by means of the impact tester to determine their creep behaviour. The deposition strategy as well as the applied conditions during the deposition process is responsible for the multiform mechanical and creep properties of coating materials. In the present paper an evaluation software of impact test results to determine mechanical and creep properties of thick monolayer plasma sprayed coating is exhibited. The stress strain rates of various coatings versus the implied stress are predicted by means of the developed procedure. The determined properties are used to define the operational safety factor against creep of plasma sprayed coatings on machine elements as well as for the optimum design of multilayer coatings without creep.
2:30 PM F1/E4-2-4 Interpretation of Coated Tools Performance in Milling and in Impact Testing Based on Thin Hard Coatings Stress-Strain Curves
K.-D. Bouzakis, N. Michailidis ( Aristoteles University of Thessaloniki, Greece); G. Erkens (CemeCon GmbH, Germany)
Thin hard Physical Vapour Deposited (PVD) coatings wide use in various technical applications necessitates the precise knowledge of their mechanical properties. In the Laboratory for Machine Tools and Manufacturing Engineering of the Aristoteles University of Thessaloniki a Finite Elements Method (FEM) continuous simulation of the nanoindentation hardness test was developed and is applied in order to extract the coating stress strain curves. @paragraph@ In the present paper coated specimen on high speed steel (HSS) and on cemented carbide (HM) were examined to extract coatings stress strain curves and the results shown that these curves were independent of the substrate material as well as of the applied during the nanohardness measurement indentation force. @paragraph@ The aforementioned procedure was applied to determine the coatings constitutive laws, as for example of the same coating deposited at various temperatures, of coatings differently heat treated, manufactured through Physical Vapour Deposition or low temperature Chemical Vapor Deposition etc. @paragraph@ The obtained coating elastic and plastic properties were used to explain the fracture progress during the impact test of a certain coating, deposited on various substrates as for instance on a high speed steel or on a cemented carbide specimen. The extracted stress-strain curves were also considered in order to predict the fatigue wear of coated inserts in milling. Herewith Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) microspectral investigations of the used cutting edges were conducted to check the tool failure. Furthermore the FEM simulation of the contact between the tool and the workpiece enables a quantitative description of the occurring during the cutting process mechanical stresses leading to coating failure.
2:50 PM F1/E4-2-5 Mar Resistance of Automotive Clearcoats
P. Bertrand-Lambotte, J.L. Loubet (Ecole Centrale de Lyon, LTDS, France); C. Verpy (PSA Peugeot Citroen, France); S. Pavan (Ecole Centrale de Lyon, LTDS, France)
Automotive clearcoats are subjected to different kinds of environmental damage : UV radiation, acid rain, trees sap, birds droppings, stone chipping, marring... Marring, which is mainly due to car wash brushes, decreases the gloss of automotive paint and alter the colour. Optical and atomic force microscope imaging of clearcoats samples brushed by an automated car wash reveal both plastic scratches and fractures. We have drawn the same type of scratches in a controlled manner : the applied load is controlled, the deformation and the strain rate are known. First the Young's modulus and the hardness of the clearcoats were calculated from indentation tests at different strain rate values. Next, scratching tests providing greater strain rates allowed us to extend the hardness results from indentation tests. These experiments carried out at room temperature have been supplemented by indentation tests performed at different temperatures (from -10°C to 45°C). Thus, the evolution of the hardness versus strain rate at different temperatures has been measured and a time-temperature superposition curve has then been plotted. After building this curve, we have determined the activation energy of each clearcoat. The energies are closed to the values found in literature for a @beta@ transition. And it turns out that mar resistance would largely be dependent of the temperature position of the @beta@ transition in comparison with the temperature of the @alpha@ transition (both determined at 1 Hz).
3:10 PM F1/E4-2-6 Surface Engineering With Graded Multilayer Coatings: A Numerical Study Based on the Boundary Element Method
C. Saizonou (INPL-Ecole des Mines, France); R. Kouitat-Njiwa (UHP Nancy I Ecole des Mines, France); J. von Stebut (INPL-Ecole des Mines, France)
In a preceding numerical study we have shown the contact stress field in a bi-layer composite to strongly depend on the coating/substrate ratio of the corresponding elastic moduli as well as on the generalised thickness t/a@subH@ of the coatings (t being the absolute coating thickness and a@subH@ the Hertzian (elastic!) contact radius. To a first order approximation the position of the Tresca stress maximum at - 0.67 a@subH@ cannot be changed. For coatings more compliant than the substrate, the coating-induced resolved in-depth Tresca contact stress field enhancements (as compared to the uncoated substrate!) are confined in the coating, with an important step discontinuity at the interface, while in the substrate the stress field varies as for the uncoated material. For coatings stiffer than their substrate, with medium generalised thickness(t/a@subH@>0.5), the corresponding overall Tresca stress is lowered in the coating with a major bending stress gradient in the very near surface region. For very thin coatings (t/a@subH@ < 0.2) this bending stress gradient dominates within the entire coating thickness. In the present paper we consider graded coatings with moduli progressively increasing from the coating/substrate interface towards the extreme coating surface. We show that a stack of 5 - 10 individual layers is in general sufficient to optimise the overall coating stress field. Thus major stress field modifications can be restricted to the coating alone, as for coatings more compliant than the substrate, while the stress discontinuity at the coating/substrate interface can be considerably reduced. In exchange, a major bending stress gradient close to the very surface has to be accepted as for monolithic, high stiffness coatings. For very thin graded coatings (equivalent to high contact pressures), the position of the Tresca stress maximum in the substrate remains (as for the uncoated material) at z - 0.67 a@subH@, well below the coating stack/substrate interface.
3:30 PM F1/E4-2-7 A Study of Fatigue Behavior of DLC Films Using Acoustic Emission Monitoring
Y. Gachon, F. Vaux, Ch. Heau (HEF, France)
In a previous paper @footnote 1@ it was shown that tribological tests performed with acoustic emission (AE)monitoring alllowed to draw fatigue maps of DLC films. But until now, no correlation had been established with deposition parameters. During a long mechanical test AE detection provides information on the time of damage appearance. It also enables to interrupt a test with less empirism to observe all the stages of degradation process. The difficulty of AE is to draw a link between the signals and the type of occurring damage. For this purpose, a preliminary study was conducted using a scratch test apparatus. This mechanical test requires very few time so it is a good way to choose the right operating parameters to detect AE signals (they radically differ from one coating to another). When examining the scratch a correspondence may be easily established between several damage mechanisms (cracking, spalling, coating delamination) and the characteristics of resulting AE signals. Using these results a study was performed using a ball on disc tribometer. Tests were performed with several load values. In a first phase, the tribometer was switched off as soon as a given amount of AE events (corresponding to film removal) were detected. Afterwards tests of shorter duration were performed to observe degradation process of the films. Surface topography and EDAX analysis were used to get further information about wear and transfer mechanisms. Some effects of elaboration parameters on fatigue strength of the films have been identified.
3:50 PM F1/E4-2-8 Evaluation of the Fracture Toughness of Silicon Carbide Thin Films through Two Different Indentation Techniques
R.M. Souza, M.G.V. Cuppari, G. Pintaude G. Pintaude Pintaude (Polytechnic School of the University of Sao Paulo, Brazil); J. Vilcarromero (UNICAMP, Brazil); M.N.P. Carreno, A. Sinatora (Polytechnic School of the University of Sao Paulo, Brazil)
In this work, silicon carbide (SiC) coated specimens were prepared in a plasma enhanced chemical vapor deposition (PECVD) chamber. Deposition runs with different process parameters, such as substrate temperature, film stoichiometry and deposition pressure, were conducted to provide specimens with different mechanical properties. In each run, both brittle silicon substrates and soft aluminum and stainless steel substrates were coated. Two different indentation techniques were then conducted to evaluate the fracture toughness of the films. The specimens with brittle substrates were indented using a nanoindentation equipment and the fracture toughness of the SiC films was evaluated following the procedures described in the literature. The specimens with soft substrates were indented by a 1.59 mm (1/16 in.) diameter sphere, which applied a normal load of 50 N. In this case, the film fracture toughness was evaluated based on the spacing of the circular cracks observed close to the contact edge of the indentations. A good correlation was obtained when the results from both indentation techniques were compared.
4:10 PM F1/E4-2-9 Measurement of Mechanical Properties of Single and Multi-layered Nitride Thin Films by Depth Sensing Nanoindentation and SEM/AFM Techniques.
A.K. Sikder, I.M. Irfan, P.B. Zantye, A. Kumar, J.M. Anthony (University of South Florida)
Indentation testing at the nanometer scale has become one of the most widely used techniques for the investigation of localized mechanical behavior of materials. Mechanical properties of thin films differ significantly from those of bulk materials due to the effects of interfaces, microstructure and thick underlying substrates. Starting from the microelectronic devices to wear resistant hard coatings nanoindentation has become a very fast and useful technique to evaluate their mechanical properties. It is interesting to find the influences of layer thickness and the number of layers on the mechanical properties of multi-layered system. There is evidence that fine layered structures show a greatly enhanced resistance to plastic flow. @paragraph@ In this study we will present the results of nanoindentation tests to evaluate mechanical properties of nitride (TiN, CrN, TiCN and TiAlN) single and multi-layered thin films. Films were coated on steel substrates using cathodic arc deposition technique. We have extended the measurement to the multilayer coating with indenter displacements increasing to different layer thicknesses. Nanoindentation was performed by MTS NanoIndenter® XP with a Berkovich indenter. @paragraph@ We have also observed the indentations and defects created by nanoindentation using scanning electron microscopy (SEM) and atomic force microscopy (AFM). It is found that large majority of indents show material pileup, and the projected areas of these indents, as measured with the SEM/AFM, are significantly different than those calculated from the unloading curves. Also using SEM/AFM we have observed elastic/plastic deformation, cracking, phase transformation etc. Experimental results obtained in this studies are discussed and compared with the results obtained by finite-element modeling.
4:30 PM F1/E4-2-10 Properties of Al@sub 2@O@sub 3@ Films Deposited Using PVD Twin-Mag Sputtering.
A. Schuete (Balzers Inc., Liechtenstein)
Because of their hardness and chemical inertness under elevated temperatures crystalline Al@sub 2@O@sub 3@ films are used in machining applications as wear resistant coatings and also serve as a thermal and diffusion barrier. So far only commercialized CVD processes are available to deposit these layers at about 1000 °C. In this work PVD Twin-Mag sputtering has been used to deposit crystaline Al2O3 films on cemented carbide and HSS inserts at 500°C. The films have been analyzed using REM, TEM, and XRD and show a glassy fine grained crystalline morphology. Mechanical Properties will be shown and first results of milling tests comparing this films to TiAlN and different multilayers will be discussed.
4:50 PM F1/E4-2-11 On the Evaluation of the Absolute Hardness of TiN@sub x@ and TiC Deposits
E.S. Puchi-Cabrera, J.A. Berríos (Universidad Central de Venezuela); D.G. Teer (Teer Coatings Ltd., United Kingdom)
The present investigation has been conducted in order to evaluate the absolute hardness of three TiN@sub x@ coatings deposited on 316L stainless steel and one TiC coating formed on a high carbon steel, by means of the models earlier advanced by Jönsson and Hogmark (1984), Chicot and Lessage (1995) and Korsunsky and co-workers (1998). The TiN@sub x@ deposition was carried out at Teer Coatings Ltd., Hartlebury, U. K., by means of closed field unbalanced magnetron sputtering (PVD). Microhardness measurements were carried out both on the TiN@sub x@ coated samples and on the uncoated substrate employing loads of 5, 10, 25, 50, 100, 200 and 400 g. At least twelve indentations were conducted for each load. The indentation diagonal values were input into the three models considered, in order to evaluate their capability of describing the experimental data. The analysis conducted on the samples coated with a TiC deposit was carried out employing the same data reported by Chicot et al. (2000). It has been concluded that the absolute hardness of a film deposited onto a substrate can be evaluated from hardness measurements conducted on the composite system (substrate + coating) employing different loads and most of the models available for this purpose. However, it is believed that, taking into consideration the number of parameters involved, the model advanced by Korsunsky and co-workers (1998) provides the best description of the data.