ICMCTF2010 Session EP: Symposium E Poster Session

In machining process, the tool-chip contact is characterized by intense thermo-mechanical loading. The coupling between thermal and mechanical loads may lead to tool failure, especially when machining refractory titanium alloys. Within such environments the efficiency of the coating material plays an important role in preserving the structural integrity of the cutting tool. The current study emphasizes the role of coating materials in enhancing wear resistance of the cutting tool and improving the tool-chip contact. The change in wear mechanisms with machining parameters such as cutting speed and coating material is discussed. The cutting tests were performed using an instrumented planer machine. Experiments were conducted on the Ti-6Al-4V titanium alloy (workpiece) and cemented carbide tool (tool substrate). Four coating materials were considered: (i)- diamond (thin layer, about 2-3µm), (ii)- diamond+TiB₂+CrN/DLC (Diamont Like Carbon) ( about 3,5µm), (iii)- diamond (thick layer), (iiii) Diameco⁺ (3µm). The performance of each coated tool was analyzed and compared to the uncoated one in terms of friction coefficient, contact length and tool wear. A study on the transient phenomenon of the tool degradation has been carried out according to the contact conditions which have been found strongly depended to the machining parameters.

Bearings are widely used in the automotive industry (gear box, wheel, steering power, crankshaft, etc) and play an essential role in the automotive reliability and performance. Extensive studied have been carried previously out on hard coatings and Me-CH coatings¹, this paper will report new results obtained with self-lubricated coatings that have shown exceptional results on other automotive parts. An experimental study was performed to assess the effect of self-lubricated coatings on the behaviour of radial and angular bearings that are designed to support thrust and/or radial loads. The bearings having large contact angles can support heavier thrust loads. They are suitable for a wide range of applications including high speed machine tool spindles or gear boxes. Self-lubricated coatings were deposited onto AISI 52100 bearing steel for laboratory tests and onto bearings for industrial tests. A laboratory pin on disc test rig was used and the tribological propertie s (friction-wear) were measured in various controlled environmental conditions (dry and lubricated). The surfaces were analysed by SEM-EDAX as well as STM/ AFM. The 4 best candidates were retained for the next industrial bearing test. Parameters such as vibration, friction/wear and temperature have been used to monitor bearing performance and the results are reported in the paper.

¹P. W. Gold and J. Loos Wear, 253( 2002)465-472.

[1] C. C. Chou, J. W. Lee, Y. I. Chen, Surf. Coat. Technol. 203 (2008) 704.

The paper reports on preparation of ~2000 to 3000 nm thick a-C coatings containing Mo, interrelationships between their mechanical properties, coefficient of friction (CoF) μ and wear k and the effect of Mo content in the a-C coating on these interrelationships. The Mo-C coatings were prepared by sputtering using an unbalanced magnetron (UM) equipped with a graphite targets (Ø=100 mm, 99.9% purity) fixed to the UM cathode with Mo ring of different inner diameter Ø_i. The content of Mo in the a-C coating was controlled by Ø_i. It is shown that (1) CoF μ and wear k of the coating strongly depend not only on its hardness H but also on its effective Young’s modulus E*=E/(1-ν²), the ratio H³/E*² characterizing the resistance of coating to plastic deformation and elastic recovery We and (2) the nc-Mo₂C/a-C coatings with low amount of Mo composed of Mo₂C nanograins dispersed in a-C matrix exhibit low values of μ≈0.07 and k≈10^-7mm³/Nm measured using CSM tribometer with WC ball at rotation speed v=0.05m/s, total sliding length l=1000 m and load L=2N; here E and ν is the Young’s modulus and the Poisson ratio, respectively. The CoF μ and wear k of a-C/Mo₂C coatings are compared with those of nc-TiC/a-C coatings with C/Ti>2 containing a low amount of nc-TiC nanograins.

Nanoindentation testing can become increasingly challenging at elevated temperature, particularly when the test temperature increases to 750°C. Effective thermal control and good experimental design are critical to obtaining reliable hardness and elastic modulus measurements. The importance of controlling the sample and indenter temperatures during the test and the experimental conditions to ensure that indentation creep does not influence elastic modulus measurements (particularly on metallic samples) are discussed. There are additional challenges in nanoindentation testing above the oxidation temperature of diamond, and the steps that can be taken to obtain reliable measurements in the temperature range 500-750°C are described.

Applications in thin film research, including cutting tool coatings (TiAlN, AlCrN, AlTiN, TiAlCrSiYN etc) and DLC will be presented. The optimum combination of hardness and toughness/plasticity varies with the severity and nature of the cutting conditions. For improved performance in interrupted cutting the plasticity index (plasticity index = plastic work/total work) is critical, with high values (i.e. not extremely high H/E) resulting in extended tool life. Nanoindentation at elevated temperature showed that the hardness of the coatings generally decreased and the plasticity index rose with increasing temperature. In high-speed turning hot hardness is the dominant factor whilst for interrupted cutting high hot hardness should be combined with improved plasticity for longer tool life.

Analytical modelling with Film Doctor software has shown that the temperature field modifies the von Mises stress distribution in contact applications. For example, optimisation of DLC coatings for applications involving friction requires 1) accurate experimental determination of the variation in coating and substrate mechanical properties with temperature 2) determination of the temperature field in the application 3) determination of the stress field in contact taking the temperature field into account.

Great emphases recently have been placed on such material characteristics as hardness and toughness in developing protective hard coatings. This study paper thus aims to investigate indentation behavior with a higher loading for the thick-layered TiCN coatings deposited by plasma enhanced magnetron sputtering (PEMS). Some literatures had been evidently proved that TiCN coatings can effectively prevent fractures from growing. The thick layer of TiCN exhibited its cracks after indentation. Deposited as thick as 20 μm, the TiCN coatings thus required a comparatively larger loading of 20 mN for an easy observation by SEM. Both wear and scratch experiments were employed to 20-μm thick layer of TiCN coatings. Besides, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were also used to investigate microstructures and morphologies of the TiCN coatings. The results indicated that fracture toughness of the TiCN coatings could be precisely predicted from merely observing how their cracks grew with varying loadings in the indentation.

Nowadays, ternary nanocomposite coatings are known for their high mechanical and oxidation resistance. Such films are composed of different phases, usually a nanocrystallised phase embedded into a thin amorphous one. This leads to the formation of an isotropic nanosized hard composite. The aim of this study is to understand the wear mechanisms of CrSiN nanocomposite coatings. Deposition was performed in an industrial arc evaporation PVD chamber, using specially designed Cr-Si targets. Single-layered CrN and CrSiN (2.8 at.% Si) coatings and the corresponding CrN/CrSiN multilayers (10 and 100 nm periods) were considered. Microstructure was investigated by XRD, SEM and TEM. Wear behaviour is investigated through tribological tests using a reciprocating ball-on-flat tribometer. Due to our specific configuration, using coated balls and uncoated flats, tests show a high severity, owing to increase cinematic length for the coating. Silicon addition leads to a detrimental decrease of the wear resistance in the case of singe-layered coatings, whereas the use of the multi-layered structure improves the durability. In order to better understand the mechanisms involved, mechanical properties were investigated by means of nanoindentation and SEM in situ tensile tests. Neither the hardness nor the strain to failure (H/E) could account for such wear results. On the contrary, cracking behaviour, discussed according to the Kelly-Tyson's model, is able to elucidate the high wear resistance of the multi-layered films.

The glass transition temperature determined by our technique is in close agreement with Dynamic Thermo Mechanical Analysis (DMTA) performed on free polymer films, mainly when taking into account time-temperature shift between both experiments. The great advantage of this nano-indentation technique lies in the ability to characterize the polymer films directly on their substrates, without any specific preparation.

Different studies have been carried out using nanoindentation as an investigation technique. In these studies, we have considered polyester thin films (20 μm thick) deposited on steel substrates. Films submitted to different UV weathering conditions (accelerated tests as well as natural exposure) were compared. The evolution of mechanical properties with aging conditions was determined.

In the present work we report the results of studies about the influence of Pt layers on the corrosion, wear and friction of TiALN coatings deposited on 316L stainless steel by magnetron sputtering. The coatings investigated were TiAlN//Pt, TiALN/TiAL multilayers and TiN. The thickness of the Pt layers was from 50 nm to 200 nm and the period of TiALN/TiAL and TiAlN/Pt multilayers were from 250 to 500 nm. The friction and wear tests were performed on a ball-on-flat tribometer and conducted in dry (unlubricated) conditions at room temperature. The loads used were 5N and 10N, the oscillating frequency was 5 Hz. The corrosion was studied using open circuit potential measurements and potentiodynamic polarization in ringer solutions. The structure of multilayers was studied by means of XRD. The surface topography and wore surface were studied by means of optical microscopy and profilometry. The results indicate that coefficient of friction (COF) of TIALN coatings decreased when metal layers are introduced and the corrosion resistance of TiAlN coatings is improved when Pt layers are deposited.

The scratch test is widely used in industry to test the adhesion and resilience of coatings. It is well known that the scratch test is highly sensitive to variations in the stylus geometry. Inconsistencies in scratch test results are a problem for quality control and acceptance testing and it is believed that the majority of test variability comes from variability in the geometry of the styli obtained from manufacturers and the change in geometry that occurs due to wear from contact with hard and/or rough surfaces. In 2002 the EC project REMAST (SMT4-CT98-2238) certified a European Certified Reference Material ref. BCR692 specifically to enable a rapid verification of stylus geometry (and instrument calibration). This has demonstrated that many Rockwell styli do not have a spherical tip and that the average radius of the tip can vary greatly. Part of this problem is that the measurement methods in ISO6508 are poorly defined.

Forging tools have a short lifetime compared to cold forming tools e.g. for sheet metal forming. The high local surface temperatures with alternating chilling conditions due to the spray cooling with water based cooling lubricants is provoking fatigue of the tool material. Crack initiation and crack growth due to thermal shock exposure often cause spalling of the tool steel material in the surface near regions. These are starting points for extensive wear. Hard coatings like CrN, TiAlN only have a minor influence on this behaviour and will spalling together with flakes of the surface near material. This is the reason why forging tools are mostly uncoated up to now.

Nitriding can increase the hardness and wear resistance at elevated temperatures and has become state of the art for hot forming tool steels in forging applications. Coincidental a decrease of the ductility can occur and will reduce the crack resistance of the tool surface especially under thermal shock conditions.

The main wear mechanism of hot forming tool steels will be discussed. Exemplary for plasma nitrided forging tools, the hot working steel 1.2367 which is used for the production of automotive components was analyzed. The influence of the nitriding parameters like temperature, nitrogen supply and the plasma excitation on the nitriding depth, the maximum hardness and the crack sensitivity will be presented.

It could be shown that the crack sensitivity of nitrided samples can be evaluated by the Rockwell indentation test as well as the scratch test, which both are normally used for the adhesion measurement of hard coatings. The application of these characterisation methods showed significant differences in the crack formation in dependency on the discussed nitriding processes.

Comparative application tests in the production of automotive components show the influence on the wear behaviour and lifetime of forging tools in an industrial environment.

All these damage phenomena (cracks or debonding) are strongly influenced by the presence of defects in the oxide layer. To take into account the probabilistic aspect, a statistical analysis of numerical results obtained for random sampling of limit stress distributions is carried out to characterize the inter-crack distance in the oxide layer.

We propose here a numerical approach capable of reproducing these fracture modes for Zr/ZrO₂ system.

The influence of every parameters: limit stress in the oxide layer, decohesion at the interface metal/oxide, substrate behavior and residual stress in the oxide layer are evaluated in terms of the inter-crack distance.

Finally, the parameters of the numerical models are identified from the results of experimental tests.

In this work the effect of surface wettability of nanopatterned diamond-like carbon (DLC) films on friction and wear behavior with sliding friction has investigated. Nanopatterned DLC films with surface roughness ranging from 5 to 9 nm were prepared by deposition of DLC film on the Si (100) substrate with nanoscale Cu dots. The following different surface wettability DLC films have been chosen: super-hydrophobicity (θ =164˚, RMS roughness = 9 nm), hydrophobicity (θ_water=96˚, RMS roughness = 5 nm) and hydrophilicity (θ_water=13˚, RMS roughness = 7 nm). Tests with these DLCs have been carried out with a ball-on-disk tribometer with hydrophilic Si₃N₄ ball coupling in air with 45~50 % relative humidity (R.H.) levels. The super-hydrophobic DLC film deposited using the 2.45 GHz surface wave-excited plasma CVD technique has been investigated with tetramethylilane (TMS: Si(CH₃)₄) gas. The hydrophobic/hydrophilic DLC film deposited using the r.f. magnetron sputtering and Ion implantation. The most important result achieved with the tribological tests and the FE-SEM examinations is that with couplings of a hydrophilic and a superhydrophobic DLC, self-lubrication has a greatly positive effect. The worn surface of the DLC film was smoothed significantly with decrease in surface energy, with sliding in air and water showing the lowest friction coefficient such as 0.065, 0.023. On the other hand, as wetting angle decreases, abrasive wear might have occurred when sliding in air due to the entrapment of large hard wear debris particle within the rubbing interface before being removed, leaving rough grooves on the worn surface. The role of air in reducing wear particle size of the super-hydrophobic DLC film has been attributed to the formation of low surface energy surfaces with self-cleaning effect.