Papers

ICMCTF1999 Session E4/F1: Mechanical Characterization - Micromechanical Testing and Modeing

Wednesday, April 14, 1999 8:30 AM in Room California

Wednesday Morning

Start	Invited?	Item
8:30 AM		E4/F1-1 Residual Stress Evolution During the Tensile Failure of a Coating on a Substrate E. Harry (CEREM, France); M. Ignat (CNRS-INPGrenoble, France); A. Rouzaud (CEREM, France) By X Ray diffraction determinations, we investigated the evolution of the residual stresses of a coating on a substrate system, when it was submitted to a pulling test. The manipulation consisted to deform progressively the coating on substrate system, determining at different levels of strains, the residual stresses in the coatings. Two sort of systems with W rich coatings were investigated. In both cases the substrate was on stainless steel. Under the pulling of the coating, then it’s progressive failure, the residual stresses showed a contineous evolution. The results are discussed with respect to the observed failure mechanisms and evolution of the cracks in the coating.
8:50 AM		E4/F1-2 On the Residual Stress of TiN and CrN PVD Coatings M.E. Silveira, J.R.T. Branco (CETEC, Brazil) Coating thickness and the corresponding mechanical properties, adhesion and wear behavior of coated parts depend on the residual stresses imposed by PVD processes. However, there is not enough information available about the interrelationship between residual stresses, substrate surface preparation and interlayer thickness, in special for CrN. To better understand this point and to improve control over the variability of PVD coatings behavior, this investigation was carried out. TiN and CrN coatings were deposited by electron beam evaporation ion plating on AISI M2 high-speed steel and AISI 304 stainless steel. The substrates were finished to 0,4 and 0,1 µm and placed at different positions in a BAI 640 R coating chamber. The effect of two levels of Cr interlayer thickness was also investigated. The residual stresses were determined by measuring the deflection of both strip and disc shape samples with a profilometer.
9:10 AM		E4/F1-3 Influence of Residual Stress Gradients on the Adhesion Strength of Sputtered Hard Coatings H.K. Toenshoff, B. Karpuschewski, A. Mohlfeld, H. Seegers (University of Hannover, Germany) Today's research activities covering the field of improving the properties of cutting tools are concentrated on optimizing manufacturing technologies and tool geometry as well as improved alloying of special cutting materials and coating of tools. Especially in the area of coated cutting inserts a high potential to enhance the wear resistance is still existing. As a result of the reduced machinability of new cutting tool materials high mechanical and thermal loads during grinding influence the subsurface properties. Hence, strong gradients of residual stresses are induced in the subsurface of the tools during manufacturing. Even with optimized coating parameters deposited PVD-coatings fail due to insufficient subsurface properties of the substrates. In this paper influences of residual stress distribution in subsurface layers on interface strength of PVD-coated carbides are presented. The investigations were carried out with WC-based cemented carbides coated by PVD-deposited (TiAl)N layers. Considered topics are the influence of grinding, micro blasting and water peening on the subsurface residual stress state of the coated carbide. Dependencies between stress distribution in subsurface layers and interface strength are highlighted. X-ray measurements were carried out by using a new method for evaluating residual stress gradients. This stress gradients in the subsurface layers of the substrate were determined using the non-linear distribution of the diffraction angle versus sin²Psi. Depth profiles of residual stresses before and after coating of the cemented carbide were determined. The results are compared with the well known sin²Psi-method by using different lattice planes and wavelengths which correspond to different penetration depths. The adhesion of the deposited coatings was analyzed by adhesion tests. In final cutting test wear behavior was observed during dry machining.
9:30 AM		E4/F1-4 The Role of Substrate Surface Roughness on The Adhesion of CVD Diamond Coatings to Cemented Carbide Tool Inserts M.A. Taher, W.F. Schmidt, A.P. Malshe (University of Arkansas) Since the discovery of diamond thin film synthesis using chemical vapor deposition (CVD), a wide range of applications have benefitted from this technology. In the machining industry, cemented carbide tool inserts were coated with thin CVD diamond films to extend their wear life beyond their nominal machining performance. Enhancing the adhesion of CVD diamond coatings to carbide tool inserts is a challenging difficulty that remains under investigation. Premature failure of CVD diamond coatings by flaking is caused by a combination of mechanical, chemical and thermal factors. One of the important mechanical factors that is known to exist is the interlocking action at the coating-substrate interface. Procedures such as mechanical scratching with diamond grit, sand blasting and grinding were shown to increase the surface roughness and improve adhesion. Yet a quantitative study relating adhesion to the state of roughness of the substrate has not been found. In this study, several cemented carbide cutting tools were treated by different methods to alter the roughness of the surface. Prior to CVD diamond deposition, Atomic Force Microscopy (AFM) was implemented to quantify the substrate surface roughness during the several stages of preparation. In addition, the change in residual stress of the substrate was monitored by X-ray diffraction. The adhesion of the diamond coatings was examined quantitatively by the use of a scraper tester and the results were correlated with respect to the initial surface roughness and residual stress of the substrate. Diamond coatings with a thickness of 20µm were deposited using a hot filament CVD reactor.
9:50 AM		E4/F1-5 Investigation of Triode Discharge Assisted Plasma Immersion Ion Implantation on The Galling Properties of Austenitic Stainless Steels M. Joseph, A.D. Wilson (University of Hull, United Kingdom); M. Coleman (Jessop Saville Limited, United Kingdom); A. Matthews (University of Hull, United Kingdom) The problem of galling with metallic materials used in certain high-load sliding contacts has been known for many years. Developments of surface treatments and alloying techniques can reduce (but not entirely eliminate) this problem. The aim of this investigation was to study the effect of plasma immersion ion implantation (PI³) of nitrogen and /or carbon in modifying the galling characteristics of austenitic stainless steel substrates. PI³ provides a non-directional method of implanting ions into materials using a low temperature plasma. A cathode sheath surrounds the workpiece and by superposition of high voltage pulses, the ions generated by the plasma are accelerated towards the workpiece surface and implanted to shallow depths. In this work a low pressure plasma (5-10mTorr) containing mixtures of argon with nitrogen and /or carbon was used, and pulsed negative bias voltages of between 5 and 15kV (with varying pulse-frequency between 0.1 and 1khz) were applied to the stainless steels. The treated surfaces were assessed using a modified scratch tester, optical microscopy, SEM and surface profiling to evaluate the critical load at which galling occurs. XRD and GDOES were used to evaluate the near surface region of the treated alloys. The near surface and bulk hardness were determined by Knoop microhardness and Rockwell C tests, respectively. The improvements in galling resistance which implantation and combined implantation /diffusion treatments can provide are examined; comparisons are made with other surface modification techniques such as PVD hard coating with TiN or CrN.
10:30 AM		E4/F1-7 Non-Destructive Evaluation of Mechanical Properties of Coatings and Surface Treatments B. Schultrich (Fraunhofer Institut f. Werkstoff und Strahltechnik, Germany); D. Schneider (Fraunhofer Institute f. Material and Beam Technology, Germany) Non-destructive testing has the task to evaluate the strength of technical components without damage. Different techniques of defectoscopy are applied to search flaws and defects which reduce the strength of the materials much below the theoretical values. High frequency ultrasonic microscopy has the potential to detect such small defects responsible for the failure of thin surface coatings used to improve the wear behaviour of the materials. Since this technique is still expensive and the test is time-consuming, additional methods are also of interest. Such technique should provide reliable results in short time with low costs. This is possible by measuring a physical property that may be expected to correlate with the strength of the material. The elastic constants are parameters that can be measured non-destructively also for very thin films. Young's modulus of the film appears in nearly all relations describing the mechanical behaviour of coated materials. Therefore, more and more efforts have been made to develop techniques for measuring the elastic parameters of thin films. The techniques of nano-indentation, Brillouin scattering, membrane deflection and line-focusing acoustic microscopy are described. The potential of the laser acoustic method is explained in special detail. This technique is based on surface acoustic waves which show frequency dispersion in coated materials. The dispersion is measured in a wide frequency range and Young's modulus, density or thickness of the film as well as the elastic modulus of the substrate can be deduced by fitting a theory. A robust technique was developed that allows quick and reliable measurements. Two versions were realised to measure on plane and cylindrical surfaces. It is demonstrated that the method can provide useful information about coatings and also enables to evaluate machining and damage layers. Thin multi-layer films consisting of two components can be tested. The method enables to determine the modulus of one component of the multi-layer film, if that of the other one is known. Examples are presented for hard and super-hard materials, machining layers on steel and damage layers on GaAs. Correlations of the elastic modulus of hard coatings with important properties as hardness, density, adhesion, bonding structure and stoichiometry reveal that the method is suitable to characterise the quality of the film materials. The effect of ion energy, ion bombardment intensity, angle of incidence, substrate temperature on the film modulus is shown that allows to optimise the deposition technology by means of the laser-acoustic technique.
11:10 AM		E4/F1-9 Scanning Electron Acousti Microscopy (SEAM) Imaging of Sub-surface Cracks in Coated Systems and Gears T.F. Page, B.A. Shaw, D.A. Hofmann (University of Newcastle, United Kingdom) In non-transparent solids, the presence of internal cracks or the sub-surface extent of surface-breaking cracks can be difficult to detect by conventional techniques, but can often successfully be imaged using acoustic techniques. A particular variant of the acoustic technique, scanning electron acoustic microscopy (SEAM), uses a pulsed electron beam in a conventional scanning electron microscope (SEM) to generate acoustic waves near the surface of the sample. The sample is bonded to a piezo-electric transducer whose output is fed back into the microscope imaging system via a phase-lock amplifier running synchronously with the beam-pulsing system. The depth below the surface at which acoustic waves are generated depends on the electron beam energy but is typically in the range 0-5 µm for beam energies of 0-30 kV - a figure attractively commensurate with the thickness of many thin hard coatings and the typical depths of fatigue-induced cracks in both gears and rolling element bearing systems. While the technique has been available for over a decade, and a number of useful applications demonstrated, it might be argued that the technique has found little applicability in the very research community to whom it may be most useful. In part, this maybe due to a complex theory needed to fully understand the contrast produced in SEAM images. Using examples from studies of thin hard coated systems and gears, we will demonstrate that the clear contrast arising from cracks oriented both parallel to and, sometimes, perpendicular to the surfaces of many samples, and that useful information can be provided regarding the debonding of coatings. We have also found it possible to delineate sub-surface contact and contact fatigue cracks allowing some information regarding crack orientation and extent to be deduced without the need for either serial or vertical sectioning of the sample.
11:30 AM		E4/F1-10 Mechanical Characterization and Tribological Evaluation of Ion Beam Assisted Carbon Nitride Coatings D.F. Wang, K. Kato, N. Umehara (Tohoku University, Japan) Sputter-coatings of carbon with nitrogen incorporation have been prepared by ion beam assisted depositing onto Si(111) substrates. The amorphous structure with tiny crystals in the coating has been observed and confirmed using a FE-TEM. The XPS and SIMS analyses have been revealed that the level of nitrogen incorporation is about 10%, and the composition distribution of both carbon and nitrogen atoms normal to the coating surface is homogeneous. A detailed Raman analysis however, has been further shown that higher nitrogen ion acceleration energy will incline to generating more sp2 carbon bonds. And the influences of different nitrogen incorporation parameters and different coating thicknesses on the mechanical properties of carbon nitride coatings have been then systematically and initially characterized from the view points of surface roughness, nano-indentation hardness and internal stress. Finally, the tribological properties of ion beam assisted carbon nitride coatings with a relation to the above mechanical characterization have also been evaluated in terms of nitrogen incorporation and coating thickness.
11:50 AM		E4/F1-11 Finite Element Simulation of the Development of Residual Stress in IAPVD Films D.J. Ward, A.F. Williams (University of Salford, United Kingdom) Ion Assisted Physical Vapour Deposited (IAPVD) Films typically have a high state of residual stress. This residual stress comprises two components: a thermal stress, which forms as the system cools to room temperature; and an intrinsic stress which is caused by the processes of deposition. Much work has been published on the tribology of surface coatings without consideration of the residual stress. It was therefore considered desirable to develop a Finite Element Simulation to be used either as a precursor to any realistic mechanical study of the behaviour of such surface coatings, or to be used as a tool to study the effects of varying the deposition parameters. IAPVD is a process of simultaneous ion bombardment and material condensation. Previous experimental work has shown that the residual stress is related to deposition parameters, such as incident ion and atom fluxes and energies, and recent Molecular Dynamics studies have indicated that trapped inert gas species play a major role in the mechanism for creation of the intrinsic stress. The F.E. simulation assumes that the processes of ion bombardment and material deposition are consecutive, but as the analysis time step tends to zero this assumption approximates the simultaneity of the processes. Suitable mathematical descriptions are employed in the bombarded region of the growing coating to simulate the macroscopic effects of the microscopic atomic collision phenomena and diffusion processes. The predicted trends of mean stress and its distribution are similar to those observed in published experimental work.