ICMCTF2008 Session B5-2: Properties and Characterization of Hard Coatings and Surfaces

Wednesday, April 30, 2008 1:30 PM in Room Golden West

Wednesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	B5-2-1 Nano-Layered Hard Coatings: from Fundamentals to Applications S.A. Barnett (Northwestern University) This talk will cover the current understanding of the deposition, structure, and properties of nano-layered hard coatings, with emphasis on mechanical properties including hardness, stress, and adhesion. Stability of nano-layers under the high temperature conditions relevant to tool coatings will be discussed. Applications of hard nano-layered coatings will be described.
2:10 PM		B5-2-3 Hardness of and Stress in Tungsten Carbide-Diamond Like Carbon Multilayer Coatings B.R. Pujada, F.D. Tichelaar, G.C.A.M. Janssen (Delft University of Technology, Netherlands) Hardness of and stress in tungsten carbide-diamond like carbon (WC-DLC) multilayer coatings sputtered in reactive argon/acetylene plasma have been studied as a function of the bilayer thickness. The multilayer structure has been deposited by a periodic modulation of the reactive acetylene gas flow from 0 to 5 sccm, producing a series of pure WC layer and W-C:H amorphous layer. Internal stress and microstructure have been investigated by wafer curvature and transmission electron microscopy whereas hardness by nanoindentation. In a previous paper on WC-DLC multilayers [Appl. Phys. Letts. 90, 021913 (2007)] we reported an interface stress of 11 N/m. This huge interface stress was partially attributed to the shrinkage of the very thin carbon (C) and tungsten (W) layers of ~ 2 nm present between the pure WC layer and the W-C:H amorphous layer. We also found that by reducing the bilayer thickness from 30 to 5 nm, the hardness decrease from 21 to 17 GPa. By changing the layout of the gas lines in the deposition equipment, we now are able to deposit multilayers without the very thin C and W layers. Our results show that even without these layers, the internal compressive stress exhibits the same dependence with the bilayer thickness as reported before and a huge interface stress of 8 N/m is still present in the multilayer. In this new series of multilayers, we are reporting a hardness of 17 GPa, and contrarily to the observed before, it does not dependence on the bilayer thickness. These results are discussed in terms of the internal microstructure of the individual layers and the structure of the interfaces.
2:30 PM		B5-2-4 Heat Treatment Induced Phase Separation and Phase Transformation of Zr(N,O) Thin Films by Ion Plating J.-H. Huang, T.H. Wu (National Tsing Hua University at Hsinchu, Taiwan); G.-P. Yu (National Tsing Hua University, Taiwan) Recently, transition metal oxynitrides, such as TiN_xO_y and ZrN_xO_y, have been emerging as a new class of decorative material owing to their adjustable colorations. In this study, nanocrystalline Zr(N,O) thin films, consisting of ZrN and monoclinic ZrO₂ were deposited on p-type Si (100) substrates using a hollow cathode discharge ion-plating system. With increasing oxygen flow rate ranging from 0 to 10 sccm, the primary phase of the as-deposited film evolved from pure ZrN phase to nearly amorphous structure and further to a single m-ZrO₂ phase. The Zr(N,O) films were then annealed at 700 and 900°C for 2 hours under controlled atmosphere. After annealing, phase transformation was found to occur in the samples deposited at 8 and 10 sccm oxygen flow rates, where Zr₂ON₂ was derived from the m-ZrO₂ phase. The purpose of this research is to study the phase separation and phase transformation of the Zr(N,O) films induced by heat treatment. A hypothesis based on change of crystal structure was proposed to explain the phenomenon. Phase transformation could significantly relieve the residual stress, and the residual stress of whole thin films was almost equal to that of each constituent phase for all samples after annealing at 900^oC. In addition, the packing density of thin films mostly increased after heat treatment, especially for the samples prepared at 8 and 10 sccm oxygen flow rates, which undergone phase transformation induced by annealing. The coloration for the as-deposited films varied from light gold to slate gray as the oxygen content increased, and it remained generally the same after heat treatment.
2:50 PM		B5-2-5 Investigation of Atomic Layer Deposited Lubricous Nanocrystalline ZrO₂ Thin Films and Their Interfaces by Spectroscopic Ellipsometry, X-ray Reflectivity, Grazing Incidence XRD and High Resolution TEM M.C. Romanes, T.W. Scharf (The University of North Texas) Atomic layer deposition (ALD) was used to grow low friction nanocrystalline zirconia thin films using precursors of tetrakis(dimethylamido) zirconium(IV) and water at various temperatures and thicknesses (up to ~200nm). The physical and microstructural properties of these films were studied using variable angle spectroscopic ellipsometry (VASE), x-ray reflectivity (XRR), grazing incidence x-ray diffraction (GIXRD) and high resolution transmission electron microscopy (HRTEM). These techniques were used to evaluate subtle variations in the films’ surface roughness, thickness, density, crystal structure and interface with the substrate. It was determined that the films grown at lower temperatures (250°C) contained mosaic-type nanocolumnar grains with mixed tetragonal (110) and (101) orientations through thickness. The higher temperature films (350°C) have smaller grains near the substrate and similar but less dense mosaic-type nanocolumnar grains on top of them. This resulted in density gradients across the film thickness which is consistent with XRR results. The lower sliding friction coefficients, ~0.2, were exhibited by the films with higher proportion of (101)_T to (110)_T orientated mosaic-type grains. Friction mechanisms will be discussed and recommendations will be given regarding high temperature solid lubricant applications.
3:10 PM		B5-2-6 Tribological Studies on PVD/HVOF Duplex Coatings on Ti₆Al₄V Substrate E. Bemporad (University of Rome “ROMA TRE”, Italy); M. Sebastiani (University of Rome “ROMA TRE”); E.S. Puchi, M.H. Staia (Universidad Central de Venezuela) Thin hard coatings are widely used to improve tribological performances of mechanical components, even if endurance and reliability problems do still exist when high contact stress or contact fatigue resistance are needed. Differences in hardness, stiffness and H/E ratio between coating and substrate often require a more complex coating design procedure, and several problems still remain unsolved. In the present paper, tribological performances of duplex coatings (Ti/TiN PVD and WC-Co HVOF) on Ti₆Al₄V substrate were investigated by analytical modelling and experimental assessment of mechanical/tribological properties. Analytical study of the contact stress distribution under spherical indenter was performed, varying thickness and position of the Ti buffer layer. Mechanical and tribological properties of produced coatings were measured by micro- and nano-indentation testing, scratch test, HRC adhesion test, ball-on-disk sliding wear test under 5% NaCl wet environment and salt spray test (500h). Morphology and composition of the worn surface, as well as failure modes under scratch testing (i.e. sub-surface layer delamination, crack propagation modes) were investigated by Focussed Ion Beam techniques. Results showed that the use of a WC-Co interlayer with intermediate hardness and stiffness provides a significant increase of load carrying capacity and wear resistance of samples, as predicted by analytical modelling. Furthermore, the presence of a Ti buffer layer also gives a noteworthy improvement in adhesion of the PVD top layer, likely due to residual stress relaxation and improved crack propagation mechanisms due to the multilayered structure.
3:30 PM		B5-2-7 Simulation of Molecular Dynamics Associated With Hardness of Metal/Metal Superlattice Thin Films Y.Y. Cheng, C.C. Lee (National Central University, Taiwan) Maximum hardness of metallic multilayer films is strongly dependent on the modulation period .In this work, the molecular dynamics simulations of an argon cluster impact a thin films surface are performed to investigate the correlation between the hardness and the modulation period in the metal/metal multilayer films. The argon cluster with high acceleration energy 3 keV impacts the surface of metal multilayer thin film consisting of approximately 350,000 metal atoms. The modulation period of multilayered metal films are prepared ranging from 5nm~10nm. A typical hemispherical crater forms on a surface approximately several pico-second following the impact. The hardness of thin films B, the acceleration energy E of the cluster, and the depth of the crater h, are correlated using the formula h~(E/B)^1/3. The size of the crater varied with surface hardness and the cluster parameters (such as acceleration energy E and the number of cluster atoms). In this work,the simulation are performed with same cluster and the crater depth is measured to determine the hardness of the thin films. The simulation results present hardness of metal/metal multilayer film increases with decreasing modulation period of multilayer film. The simulation results indicate that restricted dislocation movement within and between layers in multilayer film enhances the hardness of the multilayer film during the modulation period in the nanometer scale. Via simulation results, the suitable modulation period can be determined to enhance thin film hardness. The simulation results are also compared with the results in experiment.
3:50 PM		B5-2-8 Tribological and Corrosion Behaviour of NbN, a-Nb₂O₅ and Nb(O,N) Multilayer Coatings Deposited by Reactive Magnetron Sputtering M. Fenker, H. Kappl (FEM, Germany) Monolayered and multilayered films from the system Nb(O,N) have been deposited onto high speed steel (HSS) by reactive magnetron sputtering. For the multilayered films the reactive gas pulsing process (RGPP) has been used, by pulsing the oxygen flow. A mounting triangle was used as pulse shape, resulting in multilayered coatings with bilayer periods of 10 and 100 nm. The tribological behaviour of the coatings was studied by using a pin-on-disk tribometer (counterparts: 100Cr6, Al₂O₃ and WC-Co balls). The wear tracks were studied by scanning electron microscopy, energy dispersive X-ray spectroscopy and profilometry. For 100Cr₆ a friction coefficient of 0.6 was found for the multilayered coatings in comparison to 0.8-1.1 for the NbN and a-Nb₂O₅ coatings. For Al₂O₃ as counterpart material abrasive wear was found as wear mechanism. The harder the coatings the lower was the wear track depth after the tribotest, resulting in the lowest wear track depth for the NbN and the highest wear track depth for the a-Nb₂O₅ coating. Against WC-Co balls the friction coefficient of all films was in the range of 0.5-0.8. The corrosion behaviour of the Nb(O,N) films was examined by potentiondynamic corrosion tests in NaCl solution and by salt spray tests. For some of the multilayered coatings the passivation current density was as low as 10^-8 A/cm² in comparison to about 10^-5 A/cm² for the NbN-coated HSS. However, in salt spray tests only the a-Nb2O5 coating revealed an excellent corrosion behaviour with < 1 pit/cm². Apparently, the multilayered nanostructure of the Nb(O,N) coatings do not improve their corrosion behaviour.
4:10 PM		B5-2-9 Origin of Transformation-Induced Lateral Defects in Sputter Deposited Yttria-Stabilized Zirconia J.R. Piascik (RTI International); T. Guda (University of Texas at San Antonio); J.Y. Thompson (NOVA Southeastern University); B.R. Stoner (RTI International) There has been much interest in yttria-stabilized zirconia (YSZ) thin films as strengthening coatings for single and multi-layer composites. Unlike bulk YSZ with micron sized equiaxed grains, YSZ thin films have grain sizes in the nanometer range that are highly anisotropic. Intrinsic and thermal stresses, generated during deposition, may provide sufficient activation to initiate an in situ transformation that significantly alters the thin film microstructure and mechanical properties. We will present evidence of in situ martensitic, tetragonal to monoclinic phase transformation (T?M) for YSZ sputter-deposited thin films and a model that describes the subsequent defect formation during film growth. Earlier work demonstrated that substrate bias-assisted deposition resulted in higher film stress, increased monoclinic phase, and a series of lateral defects throughout the film structure. It is proposed that stresses generated during film growth are sufficient to induce a T?M phase transformation. Lateral defects form as a result of stress-relief along the columnar grains which undergo rotational strain and volumetric expansion associated with the martensitic transformation. Larger defects, spanning an entire grain are formed, separating transformed sections of the columnar structure. Similarly, smaller microcracks form, within a partially transformed zone, producing regions of T?M phase transitions. Both defect structures are presented in this study with supporting analysis from high resolution TEM and Fourier Transform (FFT) diffractograms. Finite Element Analysis (FEA) allowed for the system to be simulated to better understand the defect generation and growth model.