ICMCTF2003 Session B8-2: Hard and Multifunctional Nano-structured Coatings

Monday, April 28, 2003 1:30 PM in Room Golden West

Monday Afternoon

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1:30 PM B8-2-1 The Role of Hardness and Elastic Modulus in Determining the Wear Resistance of Nanocomposite Coatings
A. Leyland, A. Matthews (The University of Sheffield, United Kingdom)

There is increasing scientific and commercial interest in the development of nanocomposite coatings, based particularly on coherent or semi-coherent mixtures of ceramic/amorphous or ceramic/metal phases deposited by PVD or CVD. In laboratory mechanical testing, extreme values of hardness (in some cases 70GPa, or higher) are often found for such nanocomposite coatings, similar to those claimed for intrinsically hard materials such as c-BN and diamond. High hardness is however often accompanied by an associated high elastic modulus (eg. 600GPa+ for c-BN and 850GPa+ for diamond) which, although desirable (in principle) for a cutting tool material and/or coating - where dimensional stability under load is obviously important - may in practice limit coating durability on low-modulus substrates and/or in wear applications other than machining. In reality, acceptable wear resistance can (depending on the contact conditions) often be achieved in a material whose hardness is little more than 30% higher than that of the counterface material with which it is in contact; thus high hardness (beyond a certain minimum requirement) is unlikely to provide further significant wear reduction. Conversely, differences in modulus between coating and substrate, if large, have the potential to generate substantial interfacial stresses under load, which may compromise coating adhesion and durability. Since the elastic moduli of many ceramic coating materials are in excess of 400GPa, whereas most engineering metals have moduli of ~200GPa (or substantially less), such circumstances are not uncommon. As well as elastic deflections under load, in some applications (eg. metal forming, or heavily-loaded gears) there may even be a risk of asperity - or near-surface - plastic deformation, which a coating may also have to accommodate.

In this paper therefore, we discuss the concept of H/E as an indicator of durability - since it provides a method to describe the 'elastic strain-to-failure' of a coating. However, rather than prescribing the H/E parameter as an arbitrary term, of universal applicability, we point to the importance of retaining sufficient coating 'H' (within the scope of trying to reduce coating 'E', to match that of the substrate). Furthermore, we consider the need (particularly in heavily-loaded contacts) for a coating to accommodate extensive substrate deformation; in this respect, film toughness (ie. 'engineering toughness' - in the sense of an ability to absorb deformation energy, whether elastic or plastic) needs to be considered. It may be that ceramic/metal and (particularly) metal/metal nanocomposites - the latter of which we are currently investigating - are able themselves (like ceramic/metal multilayers) to undergo some measure of plastic deformation in the 'soft' phase component, to accommodate large substrate strains, whilst continuing to provide wear protection. We discuss hardness, elastic modulus and wear test measurements (eg. pin-on-disc, reciprocating-sliding, impact or abrasion) on such nanocomposite coatings, with regard to the implications of the ratio of H to E for practical tribological coating applications, where close matching of the coating/substrate interfacial elastic properties (and/or an ability to withstand plastic strain, if necessary) may be of more importance than extreme hardness.

2:10 PM B8-2-3 Mechanical Behavior and Structure-Property Relationships in Nanocomposite TiN/SiN1.3 Coatings Prepared by Low Temperature PECVD
P. Jedrzejowski, J.E. Klemberg-Sapieha, L. Martinu (École Polytechique de Montréal, Canada)
Nanocomposite hard coatings were fabricated by PECVD from TiCl4/SiH4/N2/H2/Ar gas mixtures at substrate temperatures between 300°C and 500°C. The mechanical characteristics such as micro- and nanohardness, toughness, Young's modulus, stress, and friction were determined by classical and depth-sensing indentations, by tribometry and by curvature method. Optical properties such as color, refractive index and extinction coefficient were evaluated using combined spectrophotometry and spectroscopic ellipsometry. Film microstructure was studied by XRD, SEM, ERD-TOF, XPS, and AFM. For films consisting of about 10 nm size TiN grains incorporated in an amorphous SiN1.3 matrix, we found a Young's modulus of >250 GPa and >350 GPa, a hardness of >30 GPa and >45 GPa, a compressive stresses of about 1 GPa and about 2.5 GPa for low and high deposition temperatures, respectively. Throughout this work we particularly address the methodological approaches in the evaluation of the film mechanical properties, while comparing different models proposed in recent literature.
2:30 PM B8-2-4 Characterization of High Temperature Deposited Ti-Si-N Coatings
X.D. Zhang, W.J. Meng (Louisiana State University); L.E. Rehn, P.M. Baldo (Argonne National Laboratory)
Characterization of structure and mechanical properties of Ti-Si-N coatings deposited by a high-density plasma assisted hybrid CVD/PVD technique at high temperatures is carried out and compared to Ti-Si-N coatings deposited at lower temperatures. The extent of phase separation within Ti-Si-N coatings is probed by combining X-ray spectroscopy, transmission electron microscopy, and X-ray diffraction. The influence of the extent of phase separation on mechanical properties is probed by instrumented nanoindentation. The present results aim to illustrate the dependence of mechanical properties of ceramic nanocomposite coatings on the detailed nm scale structure.
2:50 PM B8-2-5 Structure and Hardness Enhancement of Multilayered a-BON / nc-TiN and a-BON / nc-SiC Thin Films Prepared by Combination Methods of Plasma Assisted and Thermal MOCVD
D.-C. Lim, S.-B. Lee, J.-H. Boo (Sungkyunkwan University, South Korea)
Multilayered a-BON / nc-TiN thin films have deposited on Si(100) substrates in the deposition temperature range of 300 ¡­ 500 °C by low frequency RF derived plasma assisted MOCVD and multilayered a-BON/nc-SiC thin films were grown by combination methods of PACVD and thermal MOCVD in the deposition temperature range of 600 ­ 900 °C to analysis the mechanism of obtaining new materials with high hardness by combining soft amorphous material with hard crystalline material. Trimethylborate (TMB), tetrakisdimethylaminotitanium (TDMAT), and Diethylmethylsilane (DEMS) precursors were used to grow BON, TiN, and SiC thin films, respectively Ar and N2 gases were used as a plasma source and a reactive gas as well as additional nitrogen source. In this study, we have mainly investigated the relationship between structure and hardness enhancement of the coating layers on the effects of layer thickness and the number of layers. The results show that surface structure of top layer and interface structure of layer by layer affect the hardness enhancement in multilayered a-BON / nc-TiN thin films. Also the hardness of the film was dependent on the number of layer and thickness of each layer. Hardness obtained from a-BON / nc-TiN bilayer was 30 Gpa. In addition, we could obtain hardness enhancement in mutilayered a-BON / nc-SiC thin films.
3:10 PM B8-2-6 Microstructures and Mechanical Behaviour of TiAlCrN Multilayer Thin Films
A.E. Santana, A. Karimi (Swiss Federal Institute of Technology (EPFL), Switzerland); V.H. Derflinger, A. Schütze (Balzers Ltd., Liechtenstein)
The chromium content and multilayering effects on mechanical properties and microstructures of TiAlCrN thin films were studied using nanoindentation measurements. The films were deposited onto the WC-Co substrate using arc plasma PVD method. Film microstructure was characterized by X-Ray diffraction, transmission electron microscopy and the chemical composition by Rutherford back scattering spectroscopy. Addition of chromium favors the formation of columnar structures in TiAlCrN. In the case of 400 nm bilayer thickness of TiAlCrN/TiAlN the TiAlN layer interrupt the formation of columns, while for 10 nm bilayer thickness a perfectly columnar growth is obtained. Incorporation of chromium into the film as well as the multilayer structure contributes to hardness enhancement. The effect of Cr is explained by favoring the formation of hard cubic TiAlCrN phase instead of softer hexagonal TiAlN present in single layer films. The hardness enhancement due to multilayering is associated with a Hall-Petch like effect and its consequent structure refinement. Indentation induced crack morphology was observed to be influenced by the film structures. The appearance of the coarse columns in Cr rich film modifies crack morphologies from well-organized straight patterns in TiAlN into a network of irregular cracks in TiAlCrN due to greater contribution of column boundary sliding.
3:30 PM B8-2-7 Mechanical and Structural Properties of Various Alloyed TiAlN-Based Hard Coatings
V.H. Derflinger, A. Schütze, M. Ante, W. Kalss (Balzers Ltd., Liechtenstein)
Most of the modern tools used in cutting operations are coated with PVD hard coatings based on TiAlN. Machining of hard materials above 60 HRC still remains one of the most difficult tasks. Especially mechanical properties such as thermal stability, hot hardness, and toughness are of interest when designing a coating especially used in this area of applications. Here, the cathodic arc technology was used to deposit various TiAlN-based hard films. The influence of different alloy elements with respect to mechanical, physical, and structural properties will be discussed.
3:50 PM B8-2-8 Thermal Stability and Tribological Properties of TiAlN/VN Superlattice Coatings
P.H. Mayrhofer (University of Leoben, Austria); P.Eh. Hovsepian, W.-D. Münz (Sheffield Hallam University, United Kingdom); C. Mitterer (University of Leoben, Austria)
TiAlN/VN superlattices are potential candidates for dry cutting due to their high hardness and excellent tribological properties. It has been reported that VN easily oxidizes at relatively low temperatures where the V2O5 phase formed show lubricious properties. In addition, bulk V2O5 melts at 685°C which may further reduce friction at high temperatures. The aim of this work is to investigate the effects of these mechanisms on the tribological properties of TiAlN/VN superlattices. The coatings were prepared in an industrial-sized PVD coater using the combined cathodic arc/unbalanced magnetron deposition technique. Oxide formation of the coatings and melting of V2O5 were investigated by dynamical thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) up to 1450°C in an argon/oxygen atmosphere. On heating the coating samples which were chemically removed from their low-alloyed steel substrates, an exothermal peak appeared during the DSC measurements indicating oxidation. The oxidation threshold was found to be at approximately 450°C during the DSC as well as the TGA measurements. The DSC investigations also indicate the presence of a melting peak at about 650°C. As indicated by X-ray diffraction and DSC the melting phase is the previously formed V2O5. Dry sliding experiments against an alumina ball were performed using a ball-on-disc tribometer in the temperature range between 25°C and 700°C. Up to the melting point of V2O5 the friction coefficient increases from 0.58 to 0.95. It drops to about 0.18 at 700°C as a liquid surface oxide phase is formed. Scanning electron microscope investigations of the wear track after the ball-on-disc test at 700°C verified the presence of a liquid phase. The results obtained show that addition of VN shows high potential to reduce friction due to the formation and melting of V2O5.
4:10 PM B8-2-9 Structure and Mechanical Properies of Zirconium-tin-boron Metallic Nanocomposite Coatings Deposited by Magnetron Sputtering
C. Tsotsos, M. Joseph (University of Hull, United Kingdom); A. Leyland, A. Matthews (The University of Sheffield, United Kingdom)
In this work we examine the microstructural and tribological properties of coatings based on the low miscibility Zr-Sn binary metal system with interstitial additions of B. Boron doped Zr(B) phases within the coating provide an increase in hardness whilst maintaining a low elastic modulus; a further increase in the strain tolerance of the coatings is provided by the secondary immiscible Sn phase. Therefore coatings with significantly high hardness, low elastic modulus and superior elastic recovery can effectively be used to treat soft steels and light alloys for abrasion and impact applications. It is generally claimed that the higher the hardness of the coating the more resistant it is to wear, which is not always the case. We believe that low modulus metallic coatings may be more beneficial in treating light alloys and steels since the respective elastic moduli of coating and substrate can be more closely matched. Coatings were deposited on AISI M2 tool steel and AISI 316 austenitic stainless steel coupons by reactive magnetron sputtering in a filament-enhanced dc unbalanced magnetron system. Coating structure is characterised using scanning electron microscopy (SEM) and glancing angle X-ray diffraction (XRD) analysis; composition analyses are carried out by means of glow-discharge optical emission spectroscopy (GDOES). Surface hardness and elastic moduli of the films are determined by Knoop microhardness and Vickers depth-sensing ultra-microhardness measurements. Micro-abrasion, high frequency ball-on-plate impact and reciprocating sliding wear tests are used to correlate the microstructure and composition to coating mechanical properties.
4:50 PM B8-2-11 Dry Machining: Nanostructured Coatings for Automotive, Aerospace and Dies Industries
M. Rostagno (Centro Ricerche FIAT, Italy); S. Veprek (Technical University Munich, Germany); S. Durante (Centro Ricerche FIAT, Italy); D. Franchi (Ferioli & Gianotti div. Genta-Platit, Italy); F. Rabezzana (Metec Technologies, Italy)
In the last decades, research for materials innovation in manufacturing fields pushed towards the development of machining technologies able to sustain continuous challenges. Nanotechnologies will have a key-role in the machining of difficult to cut materials, especially as ultra hard coatings due to their high hardness, toughness and oxidation resistance. This paper gives an overview about applications of hard nanostructured coatings in industrial cutting test. These last ones were carried out in automotive, aerospace and dies machining shops and were focused on three classes of difficult to cut materials: -steels and cast iron (hardened steels, high speed steels, innovative cast iron) -light alloys (aluminum alloys, titanium alloys) -high temperature resistant super-alloys (Inconel 718, Rene 80, IN 100) Different coatings chemical compositions, coating thickness, cutting parameters and lubrication conditions were tested in order to build a know-how of metals machineability with the innovative nanostructured coatings. Cutting tools wear analysis together with coatings characterization provided with data useful to improve further performance in tool life and reliability. The main target has been to extend the dry machining as far as possible. Replacement of coolant with dry machining will bring an improvement of the ecological impact with strong benefits on process efficiency and cost reduction.
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