ICMCTF2012 Session EP: Symposium E Poster Session

To obtain tribological parameters like Archards wear depth parameter k_d usually requires some severe effort in performing and analyzing complex tribological experiments. The poster features an approach where such parameters are extracted from effective interaction potentials [1], which themselves are built up and fed from more physical measurements like nanoindentation and physical scratch test [2]. By using such effective material potentials one can derive critical loading situations leading to failure (decomposition strength). A subsequent connection of these decomposition or failure states with the corresponding stress or strain distributions allows the development of rather comprehensive tribological parameter models applicable in wear and fatigue simulations as demonstrated in this work.

[1] N. Schwarzer: "Short note on the effect of pressure induced increase of Young's modulus". Phil. Mag., submitted July 2011

[2] N. Schwarzer et al.: "Optimization of the scratch test for specific coating designs". Surface & Coatings Technology, accepted 2011.

The diamond-like carbon (DLC) films have been widely applied to various areas for as protective coating for its superior properties of high hardness, low friction coefficient, high wear resistance, and chemical inertness.

In this study, the tetrahedral amorphous carbon (ta-C) films were synthesized by using a self-design filtered cathodic vacuum arc (FCVA) system. The effect of substrate bias( from -50V to -250V) on the surface morphology of the films were observed by scanning electron microscopy (SEM). The ta-C film shows a uniform and smooth surface morphology and a dense cross-section texture. The electron microscopy, atomic force microscopy and Raman spectroscopy were employed to characterize the microstructure and carbon bond properties of DLC coatings.

According to the results of Raman and X-ray photoelectron spectroscopy (XPS) we found that the sp³ fractions between 70% ~ 80%. When substrate bias was -100V, the maximum hardness of the film was 32 GPa. The surface particle density was decreased 60% than unfiltered process and the particle size was less than 2nm.

Keyword : Filtered Cathodic-Arc Deposition, Diamond-Like Carbon, surface flatness

The paper reports on preparation of Mo-N-O thin films deposited by reactive magnetron sputtering in an argon-oxygen-nitrogen atmosphere. The effect of oxygen and nitrogen content in gas mixture on mechanical properties (hardness H, effective Young´s modulus E^*, elastic recovery W_e) and tribological properties (coefficient of friction μ, wear rate k) of Mo-N-O films were investigated in detail. Correlations between the mechanical and the tribological properties of the Mo-N-O film were also investigated. It was found that (i) there is no signifficant difference between the tribological behaviour of the δ-MoN and γ-Mo₂N films (ii) the addition of oxygen in discharge results in increase of the coefficient of friction (from 0.3 to 0.5) (iii) hardness (H), effective Young´s modulus (E^*) E^*=E/(1-ν²) and the ratio H³/E^*2, characterizing the resistance of film to plastic deformation, increases with increasing amount of nitrogen in film (iv) the deposition rate of the MoN film decreases with increasing partial pressure of N₂ from 283 to 140 nm/min.

A new sophisticated wear model has been developed on the basis of the effective indenter concept [1, 2] by using the extended Hertzian approach [3]. Fed properly from physically performed experiments like ordinary Nanoindentation and lateral Nanoindentation via effective interaction potentials the models do not only allow to analyze certain tribological experiments like the well known pin-on disk test, but also to forward simulate such tests and even give hints for better component life-time predictions. The work will show a few examples.

[1] N. Schwarzer, G. M. Pharr: „On the evaluation of stresses during nanoindentation with sharp indenters ”, Thin Solid Films, Vol.469-470C pp. 194-200

[2] N. Schwarzer, T. Chudoba, G. M. Pharr: „On the evaluation of stresses for coated materials during nanoindentation with sharp indenters ”, Surf. Coat. Technol, Vol 200/14-15 pp 4220-4226

[3] N. Schwarzer: "The extended Hertzian theory and its uses in analysing indentation experiments", Phil. Mag. 86(33-35) 21 Nov - 11 Dec 2006 5153 – 5767, Special Issue: “Instrumented Indentation Testing in Materials Research and Development”

A microtensile testing for studying the cyclic fatigue mechanical properties of freestanding nanocrystalline copper thin film with thickness of sub-micrometer was performed to observe its mechanical response under tension-tension fatigue experiments with a variety of stress amplitude and mean stress conditions at cyclic loading frequencies up to 20 Hz. Tensile sample loading was applied using a piezoelectric actuator. Loads were measured using a capacitance gap sensor with a mechanical coupling to the sample. The experiments were carried out with feedback to give load control on sputter deposited 300, 500 and 700 nm Cu thin films. Loading cycles to failure reached over 10^6 at low mean load with a trend of decreasing cycles to failure with increasing mean load and load amplitude as anticipated. The cyclic results provided clear evidence for a cyclic creep rate dependent and change in failure mechanism from crack formation to extended plasticity as the mean load and load amplitude are decreased. Moreover, the length scale dependence on fatigue mechanism and cyclic creep of tested films were observed.

The ceramic coatings have fulfilled a wide range of functions such as wear resistance, energy storage, and optical properties. Due to their low fracture toughness, however, they usually show complex crack characteristics when these ceramic coatings are in contact with foreign objects. Such local cracks can affect the integration of the entire component as well as the function of the coatings. In order to optimise the material selection, design and fabrication, it is essential to determine the fracture toughness. Nanoindentation tests have been widely used to evaluate fracture toughness of brittle materials. With decrease of the coating thickness and the introduction of complex composition and structure, the fracture morphology becomes more complicated. This increases the difficulty in determining fracture toughness. For the thin coatings, energy based models have been shown to be effective.

Based on determination of unloading curve at the start and end points of the crack induced pop-in, one can assess fracture toughness. However, the existing models give a loose bound of fracture toughness limits.

Therefore, an improved method has been proposed based on a refined approach to determine the unloading curve at the start and end points of the crack induced pop-in. The semi-analytical generalized expressions have been presented to determine the fracture toughness of coatings for nanoindentation tests performed under both load and displacement control. This has provided a valuable theoretical guideline to determine fracture toughness from the energy point of view. Furthermore, this approach has been validated by nanoindentation tests on various coatings.

An optimized magnetron sputtered TiSiN coating on a high speed steel substrate was fabricated using a combination of direct current (DC) and radio frequency (RF) power in a physical vapor deposition (PVD) technique. Controlled samples were also developed for a comparison, whereby the DC power was varied at 300 and 400 W. The optimized sample was coated at 500 W. A scratch test was performed to investigate wear resistant of the coating. The test was done with at least nine constant load wear cycles on the coating. The coating deposited with DC power of 300 and 400 W failed after 4^th and 6^th cycles, but the coating of 500 W resisted wear even after 9^th cycle. This result was supported with the optical image of scratch track, whereby no evidence of chipping, delamination and total exposure of substrate were observed. Mechanical property of optimized coating was also investigated for tribological performance. The value of effective ratio of hardness to relative modulus (H/E_r*) was discussed to relate with the wear resistance. Furthermore, failure mode of 3 distinguished stages of deposited coating was explored.

In micro-injection moulding of small, precise and complex-shaped polymer parts, adhesion between the mould and the polymer needs to be minimized to achieve the requested tolerances of the parts and sufficient lifetime of the mould. Therefore, friction and wear behavior of the coating materials TiN, CrN, Al₂O₃, CrAlN, TiAlN and TiCN as well as an uncoated high-speed steel were studied against the polymers of interest polyoxymethylene (POM), polyamide 6.6 (PA 6.6) and polyether ether ketone (PEEK). Ball-on-disk tests in ambient air were performed at half of the melting temperature for each polymer. For POM and PEEK, the lowest friction (~0.15) and counterpart wear volume (7.8 x 10^-4 mm³) were observed for CrN, whereas for PA 6.6 TiCN and TiAlN showed the lowest friction (~0.55) and CrN the lowest counterpart wear volume (1.2 x 10^-3 mm³). Ex-situ Raman analyses carried out on the wear tracks of both parties revealed that chemical changes in the contact were only observed for the high-speed steel, giving evidence of the presence of hematite and magnetite and most importantly transfer material of the polymer counterparts. Correlations between the Raman results as well as roughness and surface energy to the obtained friction and wear properties provided new insights regarding the understanding of the tribological behavior.

The effect of surface defects on the residual stress distribution in thin coatings has been investigated by an innovative high resolution methodology, which mainly consists of incremental focused ion beam (FIB) ring-core milling (IµRCM), combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained with sub-micron spatial resolution (both lateral and in-depth), by comparison of the experimentally measured surface strain with finite element modeling using Schajer’s integral method.

Commercial Titanium Nitride (TiN) and Chromium Nitride (CrN) coatings were produced by Cathodic Arc Evaporation Physical Vapor Deposition (CAE-PVD) on tool steel substrate, which were prepared at three different roughness levels.

Preliminary characterization of coatings consisted on nanoindentation testing, FIB cross section microstructural analysis and XRD (sin2ψ) average residual stress measurement and micr-scratch testing on each coating.

Local residual stress were measured by the IµRCM method nearby to droplets of different size, with the main aim of investigating the effect of such defect on the residual stress in-depth distribution. FIB cross section were also prepared in correspondence of each test, in order to investigate the changes of microstructure and growth mechanism due to the defect. Residual stress were also measured in correspondence of surface roughness defects (polishing scratches, wrinkles, asperities). A series of stress measurements were finally performed on the homogeneous defect-free coating

Significant differences were found in terms of residual stress and stress gradient in proximity of surface defects, in comparison with the residual stress measured in defect-free areas.

In case of micro-droplets, such differences were attributed to significant modification of growth mechanisms and microstructure of the coating. This assumption was supported by the microstructural FIB-SEM observation, which showed anomalous grain growth and changes in crystal orientation in correspondence of a droplet.

Finally, the observed surface residual stress local variation where correlated to the failure modes during scratch test, which could be affected by local residual stress peaks.

Research for alternatives to electroplated hard chromium coatings is continued because of their tremendous environmental and health hazard concern. Among the various possibilities, recently, electroplated Co-W alloys show promising results like abrasive wear and corrosion resistance close to and even better than electroplated hard chromium. In the present study, the Co-W alloy matrix is further strengthened by incorporating nano-size WC particle ( 500 nm) via electrochemical codeposition technique. The matrix cobalt helps in binding the WC particles and in return particles strengthen the matrix along with host tungsten as alloy element. A sulphate-tungstate based electrolyte was used to electrodeposit the Co-W alloys and Co-W-WC nanocomposites. In order to increase the particle-content within the alloy matrix, effect of current density and particle loading within the electrolyte were investigated in detail. Nanoindentation was used to evaluate the nanohardness and elastic modulus of the CoW alloys and CoW-WC nanocomposites deposited under identical current density conditions in order to study the strengthening performances. No drastic change in hardness was observed in case of nanocomposite as compared to alloy counterpart. On the other hand, a significant improvement in wear behaviour was noticed for CoW-WC nanocomposites in comparison to CoW alloy coatings. The fretting tests were performed at a normal load of 2-10N, keeping the total displacement amplitude at 200 μm, the frequency at 5 Hz for 10000-100000 cycles in humid air (50% RH) at 23 °C. A corundum (Al₂O₃) ball of 10 mm diameter was used as counter body. Typical wear volume measured in case of CoW-WC (deposited at 50mA.cm^-2) under 3D optical profilometer was almost half of CoW alloy (deposited at 50mA.cm^-2), after fretting test under identical conditions. Post deposition annealing of these coatings was done to found influence on hardness and wear rate. The observed wear rate data of coatings will be discussed in the light of microstructure, crystal structure and surface morphology of the as deposited coating and wear scar analysis. A special attention was given to correlate nanohardness, elastic modulus with the measured wear rate of these coatings.

Aluminum is one of light metals and thermal conductivity of aluminum and its alloy is excellent. However, tribological properties of aluminum and aluminum alloys are worse compared with iron based material.

On the other hand, “Duplex coating” is a one of hardening processing such as thin nitride film formation on the surface of nitrided layer. The adhesion strength was improved by introducing of the chemical bonding between the film and the substrate using this method.

In this study, duplex coating of Diamond Like Carbon coating on the nitride layer were studied to improve the tribological properties of aluminum and aluminum alloys.

AC8A and ADC12 were used as the substrates. The mixture of NH₃ and H₂ was used as a reaction gas of radical nitriding. DLC films were prepared on the substrates directly or after radical nitriding. Evaluation of adhesion strength was performed by observation of peeling areas using SEM after Rockwell indentation test. Tribologcal properties were evaluated by a friction tester.

DLC peaks were observed in the Raman spectra of samples after DLC coating. The hardness of the sample of duplex coating was higher than that of direct coating. Surface SEM image of duplex coating sample showed no-peeling off and the adhesion had been improved.

From the result of tribological estimation, friction coefficient of the DLC duplex coating samples was lower than that of Al alloys.

As a result, improvement of the hardness, adhesion strength, friction properties were confirmed for duplex coating.

Grooved surface texture (30 μm in depth, 150 μm in width and 500 μm in pitch) was manufactured on the surface of compacted graphite iron samples (brake disc material) by electromachining . The difference of friction-induced vibration and noise properties between groove-texture d and smooth surfaces was studied in a ball-on-flat configuration under different normal loads of 3, 5, 10, 20, 40 N. A Si₃N₄ 10 mm diameter sphere was used as counterface ball. The dominant frequencies of the interface forces and noise were analyzed and t he influence of grooved surface texture on friction-induced vibration and noise behaviors was investigated. The test results showed that the normal load had significant influence on the friction noise level of the smooth surface but little influence on that of the groove-textured surface. For the grooved surface texture adopted in this work, friction noise was prone to occur even under low normal load of 3 N. No friction noise occurred at the smooth surface until the normal load increased from 5 N to 10 N, but its level increased rapidly approaching to that of the groove-textured surface. The groove-textured surface adopted in this work exhibited higher coefficient of friction and wear resistance as compared to the smooth surface, but it made the friction system easier to generate multi-frequency friction vibration and friction noise with more complex frequency components. The existence of grooved surface texture changed the fiction and wear behaviors as well as friction-induced vibration and noise properties of the frictional surfaces, further study will be performed to gain understanding for their corresponding relationship.

Drilling is one of main machining procedures in industry and the quality of the drill plays a fundamental role in the productivity and efficiency the process. The quality determines the useable tool life, the characteristics of the machined holes, the machining time and the electrical power needed. However, ascertaining this quality from a surface and tribological characterization of the drill bit is difficult. There are many different kinds of drills commercially available with variations in their cutting edge, the tool material, the surface treatment, etc. and these result in drill bits of a great variety of prices.

Though tribological property of cast iron is excellent and used as sliding parts and rotating parts, its hardness is not so high. To improve wear resistance and fatigue strength of cast iron, induction hardening and nitriding are usually used. However, induction hardening is rapidly heated and quenched over the transformation point of the steel. So, it needs to be considered as stress and dimensional changes. In contrast, nitriding is treated by under the transformation point of the steel. So, stress and dimensional changes after nitriding are smaller than those of induction hardening. However, formation of compound layer and holes formed on the surface are problems for conventional nitriding of cast iron. On the other hand, radical nitriding is one of the surface hardening methods and it is possible to keep surface roughness smooth like as initial surface to control the plasma state precisely. Investigation was carried out on the surface modification of cast iron substrates using radical nitriding.

The surface of substrates was cleaned by H₂ etching before radical nitriding. The condition of H₂ etching was follows: pressure; 133Pa, applied voltage; -380V and -600V, H₂ flow rate; 50SCCM, external heater temperature; 843K and processing time; 10min, respectively. The condition of radical nitriding was follows: pressure; 133Pa, applied voltage; -380V and -600V, NH₃ flow rate; 100SCCM, H₂ flow rate; 100SCCM, external heater temperature; 843K and processing time; 60min, respectively. The evaluations of the nitrided layer were carried out by surface profiler, micro vickers hardness tester and ball-on-disk friction tester. Measurements of friction coefficient were conducted by using of load; 0.5N, turning radius; 2.4mm, speed of sliding; 6.2mm/s and counterpart materials SUJ2 4.76mm in diameter, respectively.

Surface roughness of the substrate after nitriding was worse than that of the no-treated cast iron substrate, and it was depended on the radical nitriding conditions. The surface states were changed rough by higher applied voltage. Vickers hardness of the substrate after nitriding was depended on the conditions. Effects of the H₂ etching conditions were smaller than the radical nitriding conditions. As a result of estimation on the tribological properties, the harder specimens showed lower friction coefficient.

As a result of surface modification of cast iron substrates using radical nitriding, improvements of mechanical properties were recognized by treating at appropriate conditions.

Duplex Stainless Steels (DSSs) that are formed by a approximately equal content of ferritic and austenitic phases are employed in the petrochemical industry. In the present work we submitted SAF2101 and SAF2205 to ion nitriding process by plasma immersion ion implantation (PI3) at 300 ºC and 380 ºC during 1h and 4h. Nitriding at surfaces in longitudinal and transversal directions to austenitic and ferritic grains orientation were compared. X-ray diffraction allowed identifying the surface microstructure after the nitriding. Nitriding at 300ºC promotes only N in solid solution (expanded austenite and ferrite). Treatments at 380ºC form iron nitrides at different stoichiometries in addition to N in solid solution. The thickness of N modified layers varies from 1 to 8 μm depending working PI3 parameters. Mechanical properties were investigated by instrumented indentation at nanoscale, following the Oliver and Pharr method. Tribological behaviour was investigated by reciprocating sliding process at severe dry condition using WC(Co) and SiC balls as counter bodies. Nanoscratch tests were employed to investigate the surface brittle response under increasing loadings employing pyramidal stylus. The hardness at shallow depths is about 15 GPa and decreases to the bulk value (3-3.5 GPa) for both duplex steels. The highest hardness values are not modified at the surface by increasing the nitriding time as observed at shallow depths by nanoindentation tests. In addition, nitriding at surfaces oriented longitudinally to grains orientation or transversally to them did not cause modification in hardness measured profiles. The behaviors after nanoscratch tests for the steels nitrided at 300ºC are compared to pristine surface to evaluate the effect of occurrence of plastic deformation and/or fractures on grain boundaries on austenitic and ferritic phases. The results of conventional tribological tests for the both steels were compared considering the nitriding conditions (300ºC and 380ºC). Even in the presence of thin N modified layers, as in the case of nitriding at 300°C by 1h, the wear resistance can improve by 10 times in respect to untreated sample. The effect of grain orientation in respect to nitriding and wear sliding test is discussed.

It is well known that coating deposited on a cutting tool can improves the wear resistance of the tool, and hence prolongs the tool life. The performance of a coating is strongly depends on its mechanical and chemical properties. In machining process, the type of chosen coating depends on the cutting condition, due to properties of the applied coating material. In addition, there are also many factors that influence the performance of a coating, such as the coating thickness, the composition ratio, the sequences of layer in a multilayer coatings and the deposition method. In this paper, the properties of TiCN and TiCN/ZrN were characterised by using thermal shock and wear tests. The substrate material made from carbide-based cutting tool was also developed in house. From the characterisation analysis, it was found that the performance of TiCN and TiCN/ZrN coatings were comparable, and in some cases better than the commercial TiN coated carbide cutting tool.

This work aims to investigate the effects of coating type on the cutting induced damage and surface integrity when dry machining glass/epoxy composites at intermediate fibers’ orientations. The wear rate mechanisms and the material removal process affecting the surface finish were analyzed on both the uncoated tungsten carbide (WC) insert and CVD and PCD multi-layers (ML) coated inserts using the Atomic Force Microscope (AFM). The developed cutting forces and the generated mechanisms of chip formation were also investigated and correlated with the AFM measurements. The experimental findings showed good performance of coating layers to alter tool wear with comparison to PCD tool. It was found also that the improved adhesive properties of coating layers of carbide coated tool are capable to dissipate the sequential shock due to alternation of phases within the material. The aspect of cut surface of fiber was also discussed in details in order to explain the effects of fiber orientation on accelerating the tool damage.

Keywords: Dry cutting; Glass/epoxy; ML Coatings; Wear; AFM.