Like the Olympics, the Plansee Seminar comes around just once every four years. And like the Olympics, everyone concerned wants to be present, either actively or in the audience. Unusually for such an occasion, the Seminar is organised by a commercial international company at its headquarters in Reutte, in the Austrian Tyrol close to the Plansee, the lake that gives it its name. Metallwerk Plansee was one of the pioneers of powder metallurgy, and its seminars encouraged the interchange of ideas at the highest levels of science and technology.
The subjects of this event represent the peak of achievement in powder metallurgy; the production, characteristics and application of sintered refractory metals like tungsten and tantalum, often requiring exceptionally high temperatures, and of hardmetals based on ceramics like tungsten and titanium carbides, where microporosity that would be unnoticed in sintered steel or copper alloys would cause immediate rejection of such critical tool materials.
If the theme is exciting, the programme is formidable. In five hectic days, well over a hundred papers will be crammed into nineteen regular oral, ‘special interest’ oral and poster sessions, cunningly devised so that those with an interest only in refractory metals OR hard materials will miss nothing, whilst those with an interest in both (like myself) will constantly be faced with problems of selection and a need to be in two places at the same time. Away from technical presentations, networking and social events will fill all available time.
This preview feature is based on advance abstracts and therefore subject to change. Only the key”author is identified here, also expected to be the presenter. The headings are those employed by the Seminar organisers for the relevant sessions and the papers are reviewed in order of scheduled presentation. This is both a “taster” for those attending and, for those interested but unable to be present, encouragement to obtain the full papers when published by Plansee.
The Plansee Seminar takes place from 3–7 June, 2013 in Reutte, Austria.
Oral presentations: Materials
Processing and properties of functionally graded hardmetals, Z.Z. Fang, University of Utah, USA
The author’s process for making functionally graded cemented tungsten carbide (FG WC-Co) is based on the well-known carburising heat treatment of standard WC-Co materials. During carburising, the carbon gradient induces a gradient in the volume fraction of cobalt phase which then migrates, giving a final microstructure with cobalt composition gradient from surface to the interior. The process can either be applied to fully sintered off-the-shelf WC-Co products or be part of the sintering process. As a result of the gradient microstructure, wear resistance of the material is improved significantly without sacrificing bulk fracture toughness. Impact and fatigue resistance are also improved, attributed to residual surface compressive stress.
Design of graded cemented carbides with Fe-Ni-Co binders assisted by thermodynamic and kinetic modelling, J. García, Sandvik Coromant R&D, Sweden
In many industrial applications, functionally graded outer surfaces in cemented carbide substrates are advantageous prior to applying wear-resistant CVD or PVD coatings. Examples are given of combinations of experimental processes and thermodynamic modelling methods for the design and production of graded microstructures. The roles of the binder phase (Fe-Ni-Co), carbide content and processing variables on the formation of gradients as well as the thermodynamic simulation of kinetics are examined. Computational thermodynamics is a powerful tool to assist alloy development, predict microstructure formation and evolution and shorten developmental lead-times. Simulation tools are the commercial programs ThermoCalc (thermodynamics) and Dictra (kinetics), combined with special thermodynamic databases.
Understanding cobalt layer formation: in situ observation and new insights on the mechanism, E.Sachet, North Carolina State University, USA
Cobalt surface layer formation during sintering has been known since the beginnings of hardmetal production, a layer of cobalt binder sometimes being formed on the surface of cemented carbides during cooling. The morphology of these layers ranged from discrete thin surface layers without any measurable effect on the bulk tungsten carbide to stratified layers of cobalt parallel to the surface, that disintegrated the carbide network. The effects of grain size, binder content, cooling conditions and alloy composition on cobalt layer formation have been thoroughly discussed in literature, and various models proposed. In this work, in situ observation of layer formation was used to formulate a new mechanism that explained all known morphologies and made predictions for various alloys and sintering conditions. The authors discuss the need for detailed knowledge of these phase relations to further understand the layer formation.
Cobalt capping: a technique for improving the transverse rupture strength, fracture toughness and wettability by braze alloys of WC-Co hardmetals, I. Konyashin, Element Six GmbH, Germany
The formation of cobalt layers on a surface of hardmetal articles during sintering is described in literature as ‘cobalt capping.’ According to this author, the mechanism is not well understood and there is no information on its technological implementations. He will propose a mechanism explaining the presence or absence of cobalt layers on hardmetals and report results on the fabrication of Co-coated hardmetals based on the cobalt capping phenomenon. The proposed mechanism is based on capillarity acting on liquid Co in narrow channels between WC grains in the hardmetal near-surface layer. The formation of continuous, ductile and defect-free Co coatings on the surface of different hardmetal grades can increase the TRS of unground hardmetals by up to 50%, due to ‘healing’ surface defects by the Co coatings. Fracture toughness and wettability by braze alloys of the surface of WC-Co articles are also dramatically improved by the presence of the Co coatings (which is how we used them in the 1950s – KJAB).
Miraculous long-range migration behaviour of rare earth additives in WC–Co cemented carbides, L. Zhang, State Key Laboratory of Powder Metallurgy, Central South University, China
Research on cemented carbides doped with the rare earth lanthanum Ln started in the 1960s. Since then, apart from economic factors, instability in quality control has troubled the industry. This research concludes that long-range migration of Ln during the sintering process is behind the instability. Because of the huge difference (more than 29%) in atomic radius between La/Ce/Pr/Nd and W, or La/Ce/Pr/Nd and Co, the author describes as an amazing phenomenon long-range migration of Ln in WC–Co alloys. This paper reports the long-range migration of Ln towards surfaces during the sintering process, the in situ formation of a layer-structured Ln2O2S phase with self-lubricating properties on the working surfaces of inserts, the formation mechanism of Ln-containing dispersed phases and how to stimulate and control long-range migration behaviour.
Effect of carbon activity on the shape and size distribution of WC, I. Borgh, KTH Royal Institute of Technology, Sweden
The properties of cemented carbides depend strongly on WC grain size and it is thus crucial to control coarsening of WC during processing. This work aimed to study carbon activity, just one parameter affecting coarsening. Five WC-Co alloys with differing carbon activity, from graphite equilibrium to η-phase equilibrium, were sintered at 1410°C for 1h. Grain size distribution was experimentally measured using EBSD (electron backscatter diffraction) and the basal plane shape of WC grains evaluated for the each alloy. No clear basal plane shape change was seen with change in carbon activity. Average WC grain size increased and grain size distribution was degraded with increasing carbon activity.
A study of the decomposition of the mixed (Ti,Zr)C phase, P. Hedström, KTH Royal Institute of Technology, Sweden
Hardness is one of the key parameters for high cutting performance of hardmetal cutting tools (though there is no simple relationship between hardness and performance – KJAB). Additions of TiC or ZrC to the WC-Co matrix form an extra cubic phase and increase the hardness. However, the mixed (Ti,Zr)C was believed by the authors to generate even higher hardness increases, connected to the thermodynamic stability of the mixed (Ti,Zr)C phase. At high temperatures a homogeneous solution of (Ti,Zr)C was stable, but when the temperature was decreased the (Ti,Zr)C decomposed into TiC and ZrC through phase separation. In this work, the decomposition was investigated by studying phase separation kinetics. After the homogeneous carbide solution was first formed by synthesis at 2200°C, phase separation was investigated by X-ray diffraction and electron microscopy after ageing treatments at temperatures from 2000-1200°C. Experimental work was supplemented by calculations of phase stability.
Optimisation of WC particle size, Ni binder content and Mo2C addition for improved SPS WC-TiC-Ni cemented carbides, R. Genga, University of the Witwatersrand, South Africa
The effects of spark plasma sintering (SPS), WC starting particle size (0.1-0.8µm) and Ni binder content on WC-Ni cemented carbides, on attaining mechanical properties comparable to WC-Co cemented carbides, were investigated. The influence of Mo2C on densification behaviour and microstructure was also studied. There was slight grain growth of the submicron (0.8µm) WC powders after SPS, although large angular WC grains of up to 1µm occurred in the nano (0.1µm) and ultrafine (0.4 µm) WC powders (almost certainly due to much poorer grain size distribution in the finest powders – KJAB). Additions of 1-5wt%Mo2C to WC-6.25TiC-9.3Ni (wt%) improved the wetting of WC by Ni, leading to improved microstructure, with reduced WC grain growth, particularly in the nano and ultrafine compositions, which increased the hardness. Optimum binder content was 7wt%. This gave hardness values of >21 GPa, while retaining fracture toughness of >11 MPa.m1/2 and modulus of elasticity of >550 MPa, giving good mechanical properties comparable to conventionally sintered WC-Co cemented carbides with hardnesses between 12.50 and 13.63 GPa, fracture toughnesses between 12.40 and 13.90 MPa.m1/2 and modulus of elasticity ~580 GPa.
Hot press sintering of hard material master alloys synthesised in situ by mechanically induced self-sustaining reaction, J.M. Córdoba, Materials Science Institute of Seville, Spain
Ti0.9Ta0.1C0.5N0.5/Co powdered cermets synthesised by mechanically induced self-sustaining reaction was sintered by hot press. The effects of the microstructural parameters studied by image analysis (contiguity, mean free path, ceramic particle size and distribution) on the microhardness, and fracture toughness were related to the sintering conditions. Different sintering temperatures joined to different sintering times and sintering atmospheres drove to the best combination of parameters to obtain the more homogeneous microstructure and the greater mechanical properties.
Microstructure evolution of nanocrystalline WC-10Co hardmetals during sintering, C. Lin, General Research Institute for Non-ferrous Metals, China
Microstructural features of nanocrystalline WC-10Co hardmetals sintered at 1250-1350? were investigated by FESEM and quantitative picture analysis. Changes of residual porosity, morphology and grain-size distribution of WC hard phase with temperature were quantitatively analysed. The forms of VC, Cr3C2 and rare-earth additives and their influences on the cobalt binder were observed and evaluated by TEM and HRTEM. It was found that RE dopant kept more Cr3C2 in the Co binder, which helped to obtain WC-10Co hardmetal with 110 nm average WC grain size and 2120 HV10 hardness.
Nanoscaled hardmetals - fiction or reality?, V. Richter, Fraunhofer IKTS, Germany
‘Nanoscale’ or ‘nanosize’ as a description of fine powders was an attempt by the PM industry in general, and the hardmetals industry in particular, to attach a ‘nano’ label to their products and thus attract more research funds. Particles up to 400nm – or even 200nm - in diameter are no more ‘nanoscale’ than a 200µm (0.2mm) particle is ‘micron-scale.’ Nevertheless, the writer of this paper describes these so-called ‘nanoscaled’ materials as being seen as an important breakthrough, boosting productivity and encouraging new processes and applications.
In the field of hardmetals, numerous scientific papers have been published in recent years but, according to this paper, appropriate commercial products are still missing.
After an overview of the history of grain-size refinement and the classification of hardmetals, the paper describes the manufacture of hardmetals with binder contents of 0-15 wt%, possessing extraordinarily high hardness, good toughness and nominal grain size of the hard phase in the sintered material below 200 nm. The effects of raw materials, particularly the influence of the grain size of the starting powders of tungsten carbide, binder and grain-growth inhibitors, and of composition, on sintering behaviour and properties, are discussed. Alternative binders, grain-growth inhibitors and milling technologies are also considered. A manufacturing route based on laboratory technique can be scaled up to production level. Several applications are outlined and first results reported.
Applications
Precipitate of M7C3 carbides during sintering of TiCN-WC-Ni-Co-Cr alloys used in hot rolling applications, I. Iparraguirre, CEIT-Centro de Estudios e Investigaciones, Spain
Densification of TiCN-WC-Cr-Ni cermets was analysed by means of dilatometry and calorimetry. Shrinkage phenomena in both solid and liquid phase were enhanced by Cr additions. Carbothermal reduction of oxides occurred at much lower temperatures for high Cr contents. Dissolution of additives used in powder mixes and the rim formation also depended on the amount of Cr added. Thus TiC and WC dissolution took place at lower temperatures as Cr content increased, and in all cases rim formation required the presence of a liquid phase.
New applications for polymer matrix materials based on hard material fillers, A. Kayser, ESK Ceramics GmbH & Co.KG, German
Advanced polymer materials frequently replace metals, graphite, hard materials or ceramics in new applications. The development and marketing of polymer/ceramic and polymer/hard-material composites opens new business opportunities for hard material producers.
Two examples of successful product development and market introduction are described. Polymer/SiC composite bearings for fluid handling have friction coefficients <0.01 and extended lifetimes compared with graphite bearings. Polymer/BN composite electronic components achieve thermal conductivities >10 W/m K, replacing metal heat sinks and housings in high-volume applications like LED lighting.
Investigations on adhesive wear mechanism in turning of stainless steels, M. Collin, Sandvik Coromant, Sweden
(Back in the 1950s, I showed that the adhesive wear then commonplace when hardmetals machined stainless steels at ‘HSS’ speeds could be avoided by machining at much higher speeds with heat-resistant alloy carbides. There were no heat-resistant coatings in those days. The work described here faced a similar problem but no possibility of the same solution – KJAB).
This report covers investigations of adhesive wear mechanisms in parting-off and face-turning operations on machinability-improved stainless steel with coated carbide tools. Both operations are characterised by cutting speeds being reduced to zero when the centre of the workpiece is reached. Unsurprisingly, the results showed shorter tool life when machining to the centre compared with machining to a hole.
At low cutting speed, BUE (built-up edge) and BUL (built-up layer) were observed, the materials in which had been strongly strain-hardened. The compositions were similar to that of the workpiece material, indicating that the adhesion process primarily involved mechanical sticking and smearing rather than chemical reactions.
Another effect of using lower cutting speeds was that the coatings were removed from regions on the rake faces. The lower the cutting speed, the larger the wear. Coatings were removed piece by piece in small fragments, fractures often being found in the coatings with cracks propagating from the coating into the substrate. Adhesive wear of the cemented carbide on a larger scale resulted in rough-surfaced pits. When the adhesive wear occured on a smaller scale the worn surface was semi-rough. A third type of surface morphology was smooth but not primarily seen as a result of adhesive wear.
The investigation showed that lower tool life when parting off or face turning to the centre compared to when parting off or face turning to a hole could be attributed to low cutting speed and formation of adhesive layers, after which the layers took part in adhesive wear mechanisms. The abstract suggests that no solution to the problem was found, other than replacement of the worn tools or inserts.
Effect of gradation by reactive imbibition on commercial WC-Co drilling tools used in oil and gas industries, O. Ther, Mines Paristech, France
Hardmetals used in rock drilling, mining tools and wear resistant parts possess an excellent compromise between hardness and toughness. Graded structure improve these two properties simultaneously and thus increase the service life of drilling tools.
A continuous composition gradient of several millimetres thickness is generated in commercial WC-Co drilling tools by a reactive imbibition method. The effects of this process were studied in terms of microhardness, cobalt concentration and WC grain size. A continuous gradient of about 300HV on 8mm substrate thickness for PDC cutters is obtained by a one-step imbibition process in combination with a boron-rich coating deposited on the free surface.
This gradient is preserved and sometimes improved after the HPHT (high pressure/high temperature) process used for diamond table deposition on the WC-Co substrate. The service life in abrasion of a graded PDC cutter can apparently reach an improvement of 160% in comparison with a standard PDC cutter tested in the same conditions. An improvement of about 50% is obtained for shock resistance.
Chromium-containing TiAlN coatings for metal cutting, J. Kohlscheen, Kennametal Shared Services GmbH, Germany
TiAlN coatings are widely used for protecting cutting tools. Either arc or sputter PVD is used for deposition of hard nitride coatings. To enhance coating properties like oxidation resistance further chemical elements can be added. Chromium can form wear-resistant nitrides and is known to have a beneficial effect on oxidation resistance. Cutting test results in steel and cast iron evaluate a chromium addition to sputtered TiAlN coatings.
A tribological approach towards engineering the wear behaviour of PVD coatings, M. Morstein, PLATIT AG, Switzerland
Wear-resistant coatings on cutting and forming tools are challenged by abrasion, intermittent high shear forces and chemical attack by the working environment. Given the complexity of wear phenomena observed for coated cemented carbide or powder-metallurgical steel tools, the author questions the extent to which individual wear contributions can be analysed by tribological methods.
Novel conventional and silicon-containing nanostructured PVD coatings, deposited by means of the cylindrical rotating arc cathodes method, were investigated by friction and wear tests at a range of temperatures. Wear rates and failure mechanisms on PM steels and different types of cemented carbide were determined by profilometry, FIB-SEM and EDX mapping, then related to coating physical properties such as nanohardness, toughness and, for the high-temperature tests, oxidation resistance.
Coating performance in applications such as tapping, milling or fine blanking can be correlated not only to mere wear resistance, but also to the different set of requirements for the substrate/coating interface. Adhesive phenomena were monitored by a new method for workpiece material build-up values at low to moderate sliding speeds.
Modelling and Simulation, Strength of WC/WC grain boundaries in WC-Co from atomistic calculations, M. Petisme, Chalmers University of Technology, Sweden
First-principle calculations and interatomic potentials were used to assess grain-boundary strength in WC-Co hardmetals. With density functional theory (DFT), the author investigated the impact on the work of separation (with and without local rearrangement of atoms) of segregated transition metal atoms. A strengthening effect of Co atoms (as well as of Ni, Fe, and Mn) was found in sub-monolayers, but the effect of carbide-forming atoms (Ti, V, Cr, Zr, Nb, and Ta) was small. DFT calculations were used for simulations at high temperature and experimental data employed to construct an interatomic potential of the Tersoff form for the W-C-Co system. Results indicated limits in temperature and shear stress for the onset of grain-boundary sliding.
Numerical simulation of microcrack initiation and propagation in WC/Co hardmetal under fatigue loading using a hybrid FEM-XFEM damage model, U.A. Özden, IWM RWTH Aachen University, Germany
Fatigue is a common type of failure in long term applications of WC/Co hardmetals. In mechanical response the hard and brittle carbides are elastic, whereas the tough Co binder is elastoplastic. It has often been observed that the microcracks leading into larger fractures in WC/Co originate from individual carbide grains or clusters located at the component surface, followed by crack propagation in the binder phase. Based on those observations, a hybrid model was generated for this study, evaluating the initiation of microcracks in the carbide phase based on a linear elastic fracture mechanics (LEFM) approach in XFEM and a ductile damage model for stable microcrack propagation in the binder phase with conventional FEM. A two dimensional plane stress model was prepared based on images taken in a scanning electron microscope (SEM) in order to imitate the surface crack evaluation. The proposed model successfully generates a microcrack pattern similar to the experimental observations.
Nanometer-thick equilibrium films in doped cemented carbides, S. Johansson, Sandvik Coromant R&D, Sweden
The last two decades have witnessed tremendous progress in the calculation methods for materials properties, the cornerstone of which is the density-functional theory (DFT) mentioned in an earlier paper.
In this paper an interfacial phase diagram was derived using DFT calculations for interface energetics. Cluster expansions based on these were used in simulations to obtain configurational free energies. The method was applied to study the segregation of different transition metal atoms to WC/Co interfaces and to construct the corresponding ‘complexion’ phase diagram. A connection with real materials was made by CALPHAD-type analysis for the adjoining bulk phases.
It was found that nanometre-thick equilibrium films of cubic carbides existed in V-doped material for a wide range of temperatures and chemical potentials of V corresponding to V additions below the (V,W)Cx solubility limit. These equilibrium nanostructures did not break up at liquid-phase sintering conditions and could be used to tailor materials with specific macroscopic properties.
Applying computational thermodynamic and kinetics to analyse the effect of N in hardmetals, K. Frisk, Swerea KIMAB, Sweden
Coupling of thermodynamic and kinetic calculations with experimental investigations is well-established for alloy and process development. Cubic carbonitrides are important constituents in hardmetals. The stability and composition of these phases in a hardmetal depends on several factors, including alloy composition, solubility of W in the carbonitrides, carbon and N activities and carbonitride intermixing. To understand these effects application of computational techniques is valuable.
In this paper work the background information needed to describe the phase equilibria and kinetics is described, and it is shown how experimental validation is performed. Examples of applications are given, and the strengths and limitations of the methods described.
Mechanical properties
Toughening and fatigue micromechanisms in hardmetals: FESEM/FIB tomography characterisation, J.M. Tarragó, Universitat Politècnica de Catalunya, Spain
Though hardmetals possess extraordinary combinations of hardness and toughness, they are prone to strength degradation under cyclic mechanical loads. As the toughening and fatigue-susceptible agent in cemented carbides, the metallic binder phase plays a key role, since its effective ductility is critical for defining crack-shielding effects and cyclic induced degradation. However, experimental data on toughening and fatigue micromechanisms have usually been presented in the literature on the basis of post-failure fractographic examination. In this work, detailed characterisation of crack/microstructure interaction during stable crack growth in hardmetals was attempted. FESEM/FIB tomography was combined with systematic mechanical and indentation testing, under monotonic and cyclic loads, to assess crack extension behaviour of hardmetals. The study highlights differences regarding failure micromechanisms under monotonic and cyclic loads.
High resolution observation and comparison of crack propagation in hardmetals during monotonic and fatigue fracture, K. Mingard, NPL, United Kingdom
Understanding the effects of microstructure on the propagation of different modes of fracture through a hardmetal is key to the modelling of hardmetal performance and ultimately the development of new grades with improved strength and toughness. Post-failure fracture surface observations contributed to this objective in previously published research. However, in this study a unique ultra-stiff rig developed at NPL for use in the scanning electron microscope has enabled direct, in situ, comparative observation of crack propagation during monotonic and fatigue loading. A range of hardmetal compositions with binder contents 6 to 25% have been studied and key differences in the metrology of local strain and phase morphologies at the crack tip in different fracture modes have been related to underlying microstructures.
Fatigue testing of hardmetals in the gigacycle range, A. Betzwar-Kotas, University of Vienna, Austria
Fatigue testing of hard and brittle materials is experimentally difficult, particularly with axial loading and very high loading cycle values. In the work described, ultrasonic resonance testing was employed to characterise the gigacycle fatigue behaviour of hardmetals, an adaptation of a testing routine developed for tool steels. Fixing the specimens to the resonance system was crucial, as was bar length tuning to optimise resonating behaviour.
A further relevant parameter was surface state. High compressive residual stresses were introduced by grinding and polishing, relieved by a suitable annealing treatment. Specimen cooling employed an aqueous solution containing a corrosion inhibitor.
Using the optimised testing routine, well-defined S-N curves with fairly low scatter could be obtained, with no levelling out up to 1010 cycles. The existence of a fatigue limit could therefore be excluded, even at these high loading cycle values.
Damage induced under contact fatigue on ceramic-coated hardmetals, L. Llanes, Universitat Politècnica de Catalunya, Spain
The tribological and mechanical response of ceramic coatings has mainly been investigated on the basis of indentation, scratch or tribological testing techniques. Knowledge of their behaviour under repetitive contact loading is scarce. Information on contact fatigue of coated systems was required, if they were to be used effectively.
In this investigation the approach was to assess the contact fatigue behaviour of a fine-grained WC-Co cemented carbide with two distinct ceramic films, TiN and WC, either as monolayers or bilayers, by means of spherical indentation tests. Results indicated that circular cracking of the coating was a more appropriate choice than interfacial delamination for defining critical damage in the materials studied. From this perspective, coated cemented carbides were found to be fatigue susceptible under contact loading, though effective fatigue sensitivity and failure are apparently dependent on ceramic and coating characteristicss.
Evolution of the microstructure during creep testing of WC-Co based cemented carbides, M.A. Yousfi, Chalmers University of Technology, Sweden
This study follows the evolution of WC grain size during the high temperature creep deformation of a cemented carbide. The material had a Co content of 8.37 wt.% and a smaller additions of Cr (0.84 wt.%). A hot compressive creep test was performed at 1000°C under an applied load of 900 MPa for 700 seconds. WC grain size was determined before and after deformation by two different methods, linear intercept measurement from SEM images and electron backscatter diffraction. Results indicated that WC grain growth, preferentially perpendicular to the load axis, took place during creep deformation. The author suggests that plastic deformation of WC grains leads to dislocations at WC grain surfaces, and that these sites facilitate nucleation of atomic layers of W and C, and thus grain growth, at the comparatively low temperature. In addition, formation of binder-phase lamellae was observed at some WC/WC grain boundaries, suggesting that grain boundary sliding also took place during deformation.
Abrasion simulation for WC/Co hardmetals, M. Gee, National Physical Laboratory, United Kingdom
This paper describes a computer-controlled micro-tribometer system modified to carry out simulated abrasion experiments, where areas on polished samples of WC/Co were subjected to sets of randomly distributed scratches (both spatially and in terms of load). The time progression of damage was examined in simulations with different numbers of scratches up to 10,000 in a millimetre-wide area. A number of different hardmetal grades were examined.
Microstructural damage mechanisms were characterised by 3D optical microscopy and high resolution SEM, combined with selected 3D FIB-SEM analysis. The examination was combined with knowledge of the scratching history, including friction results, that had been recorded for any specific position on the sample.
In line with previous observations, it was found that the damage that occurred involved removal of cobalt from the binder-phase region of the sample, cracking of WC grains and re-embedment of WC fragments in the remaining binder phase. The results obtained were compared with single-point multiple scratches and with the results of macroscopic wear tests.
CVD coating
CVD Ti1-xAlxN coatings for large scale production, H. Holzschuh, SuCoTec AG, Switzerland
Over the last 20 years Ti1-xAlxN coatings were the domain of PVD but were limited to x <0.65. Developed in a lab-scale CVD system, details of the first cubic Ti1-xAlxN coatings containing high AlN (x>0.8) were published in 2008. Wear behaviour and other properties were remarkable. In 2011 a tool manufacturer developed an in-house process* to produce CVD Ti1-xAlxN coated inserts. Features were high Al content (>70%), high hardness, a phase mixture of cubic and hexagonal and excellent cutting results.
The author reports Ti1-xAlxN results achieved with the first commercially available CVD systems SCT600 and SCT400. The newly developed NH3 module has demonstrated its ability to coat homogenously single-phased cubic Ti1-xAlxN in medium temperatures with Al contents >70%. By varying the gas parameters, phases of the Ti1-xAlxN coatings can be tailored (pure cubic, or mixed cubic and hexagonal) while keeping AlN content high. The coated samples are characterised in terms of structure, composition, physical and mechanical properties.
*see following paper
Novel TiAlN coating by medium temperature low pressure CVD, R. Pitonak, Boehlerit GmbH & Co. KG, Austria
Until recently, the only way to produce Ti1-xAlxN hard coatings with NaCl structure was by a PVD process, with maximum x = 0.65. This work describes how Ti1-xAlxN coatings were prepared with x > 0.9 by medium-temperature CVD (MT-CVD).
Significant differences were found when the structure and properties of coatings deposited in lab-scale and production were compared. The lab-scale CVD system as well as the modified production-scale MT-CVD plant used TiCl4, AlCl3, NH3, HCl, N2 and H2 as process gases. If the lab-scale system were applied, nearly pure fcc- Ti1-xAlxN with x=0.91 was obtained. The production-scale Ti1-xAlxN coatings contained the phases fcc-Ti(Al)N, fcc-Al(Ti)N and w-AlN. Specific lamellar structures were observed with HR-TEM, confined to minute areas of a few hundred nm and consisting of alternating w-AlN and fcc-Ti(Al)N. The lab-scale coatings with the highest fraction of fcc-Ti1-xAlxN exhibited the highest hardness and Young’s modulus of 40 GPa and 537 GPa, respectively. The production-scale coatings also exhibited high hardness, between 32 and 36 GPa, regardless of their high fraction of w-AlN. Both lab-scale and production-scale Ti1-xAlxN coatings retained moderate compressive stress. The industrial coatings exhibited extraordinary high oxidation resistance up to 1100°C (air/1h), considerably higher than known from TiAlN- and even AlCrN PVD coatings. Milling tests showed remarkable results in GJS-600 cast iron and also in 1.2311 steel, but in the latter case only when lubricant was used.
Structure and properties of Al2O3 Coatings Prepared by LPCVD with AlCl3 and N2O, M. Höhn, Fraunhofer Institut IKTS Dresden, Germany
CVD alumina is one of the most important components of modern coating systems for cutting tools. Hardness and wear resistance can be strongly affected by controlled adjustments to the process. The aim of this work was to examine the structure and properties of CVD alumina coatings prepared with the precursor N2O.
Layer deposition was carried out in a low-pressure CVD process using a gas mixture of AlCl3, N2O, Ar, N2 and H2 at a pressure of 0.5 to 1.0 kPa and deposition temperature between 920 and 1030°C. The coatings were characterised with respect to phase composition, crystal structure, hardness and wear behaviour.
The layers exhibited a smooth surface and fine-grained crystal structure with crystal size between 30 and 100nm. The coatings were predominantly a phase mixture of ?- and ?-Al2O3. The layers were surprisingly hard, up to 30.3 GPa.
Thermal stability and cutting performance of titanium or zirconium doped κ-Al2O3 coatings by chemical vapour deposition, M. Okude, Mitsubishi Materials Corporation, Japan
The doping effects of metastable κ-Al2O3 coatings on thermal stability and cutting performance were investigated. κ-Al2O3 coatings were deposited on TiCN-coated cemented carbide substrates by conventional hot-wall CVD with AlCl3-HCl-CO2-H2 and TiCl4 or ZrCl4 gas atmospheres. Compared with non-doped κ-Al2O3, titanium or zirconium doping had a remarkably inhibiting effect on the thermal transformation from κ-Al2O3 to α-Al2O3. Although zirconium-doped κ- Al2O3 coatings were stable after heat treatment at 1040ºC for 5 hours, non-doped and titanium-doped κ- Al2O3 coatings almost transformed to α- Al2O3.
In addition, zirconium oxide overlays were deposited on the surfaces of κ- Al2O3 coatings to control thermal transformation. It was found that zirconium oxide overlays enhanced the thermal transformation for non-doped κ- Al2O3 coating, but a zirconium oxide overlay had little effect on the thermal stability of doped κ- Al2O3 coatings. After alloy steel turning, the abrasion-resistance values of transformed α- Al2O3 coatings were rather better than non-transformed κ- Al2O3 coatings.
Thermal stress relaxation of dry-blasted α- and κ-Al2O3 CVD hard coatings, N. Schalk, Materials Centre Leoben Forschung GmbH, Austria
Post-deposition blasting treatments of CVD coated cemented carbide cutting tools are gaining in interest. For this paper, α- and κ-Al2O3 coatings were dry-blasted using different pressures (0.8, 1.2 and 1.5 bar) as well as globular and edged blast media. Higher pressure increased compressive residual stresses, as measured by X-ray diffraction. Both blasting media exerted minor influence on the roughness of α-Al2O3, but the edged material caused significant increase in roughness for κ-Al2O3. The dry-blasted samples were vacuum annealed, particular attention being paid to the relaxation of introduced residual compressive stress. In addition, the effects of blasting and thermal stress degradation on the tribological behaviour were studied by ball-on-disc tests. For α-Al2O3 dry-blasted at high pressures, stresses start to relax at temperatures of about 500°C, but higher onset temperatures have been obtained for those blasted at lower pressures as well as for κ-Al2O3.
In-situ residual stress analysis of coated cutting tools at thermal cycling conditions, E. Ramos Moore, Universitaet des Saarlandes, Germany
This work investigated thermal residual stress behaviour during thermal cycling of CVD Ti(C,N) coatings on WC-Co cemented carbides, the carbonitride coatings featuring C/N composition gradients. Residual stresses were measured in-situ, as-coated and after shot-blasting. The thermal cycling treatment consisted of heating the samples from room temperature to 800ºC and then cooling back to room temperature. Both initial residual stress state and residual stress behaviour under thermal cycling were influenced by the pre-treatment process. As-coated Ti(C,N) CVD layers presented tensile residual stresses at room temperature, compressive residual stresses at high temperatures and tensile residual stresses when the system was cooled again to room temperature. Top-blasting induced compressive stress on the initial state of the coatings, which eventually changed to tensile stress after thermal cycling.
PVD coating
Combinatorial materials science: from materials discovery and optimisation to materials design approaches for thin films and bulk materials, J.M. Schneider, RWTH Aachen University, Germany
This paper provides an overview of modern materials development, from discovery and optimisation toward materials design, based on combinatorial thin-film materials science. Implications for bulk materials are discussed. The combinatorial approach, combining materials synthesis of thin-film compositions with high-throughput property characterisation, helps to delineate composition/structure/property relationships and thus identify composition windows with enhanced properties. These results are relevant for the design of coating and bulk materials. Most importantly for materials design, theoretical models and hypotheses can be critically appraised.
The author claims the combination of modern electronic structure calculations with highly efficient combinatorial thin-film composition-based techniques as an effective tool for knowledge-based materials design of hard coatings, nanolaminates, thermoelectrics, metallic glasses and steel. Quantum-mechanically guided design strategies for Mo2BC as well as FeMn alloys are examples of data obtained for thin films also proving relevant for bulk materials.
Impact of Al on structure and mechanical properties of NbN and TaN, Z. Zehua, Montanuniversität Leoben, Austria
Transition metal nitrides (TMN) are widely used as wear-resistant hard coatings for such applications as machining, casting and hot-forming. However, TMN’s rapidly oxidise and often form porous oxides, permitting rapid scale growth. Thus ternary TM(1-x)AlxN coatings are the focus of many research activities, since Al promotes the formation of dense oxides. However, the superior thermal and mechanical properties of TM(1-x)AlxN variants are mainly obtained for face-centred-cubic (fcc) structures, requiring detailed information on phase stability ranges. NbNy and TaNy coatings in particular are highly complex, due to their variety of crystallographic phases. Stoichiometric NbN and TaN can crystallise in either hexagonal or cubic structures. Through the incorporation of Al, the fcc structure is rapidly stabilised for x=0.1?0.44 (Nb(1-x)AlxN) and 0.1?0.36 (Ta(1-x)AlxN). Highest hardnesses were obtained for Nb0.56Al0.44N with about 32 GPa and Ta0.64Al0.36N with about 34 GPa. Single-phase hexagonal Nb(1-x)AlxN and Ta(1-x)Alx coatings with Al contents above x=0.61 and 0.65, exhibited hardnesses around 23 GPa. Results were in excellent agreement with computations.
About the synthesis of next generation high oxidation resistant hard coatings by means of novel high ionisation hybrid PVD processing, G. Erkens, Sulzer Metaplas GmbH, Germany
Technology and process experts meet current and future challenges with a variety of coatings and surface treatments. One novel approach is the use of hybrid PVD technology, which applies when several processes for the generation of layer-forming particles are combined in a single system. HIPAC (High-Ionisation Plasma Assisted Coating) sputtering technology, in combination with the APA-Arc (Advanced Plasma Assisted) technique represents a hybrid approach to coated high performance precision tooling.
The work described highlights novel microalloyed hybrid coatings such as (Al,Ti)N/SiBNC(O) and vanadium-alloyed films. The test coatings were evaluated by SEM, EDX and nano-indentation hardness measurements. Oxidation tests were carried out at elevated temperatures. The significant oxidation resistance of SiBNC(O) and the low friction at elevated temperatures of VXN illustrated the technical potential for applications where high thermal stability, high oxidation resistance and low friction were predominant requirements. Cutting-test results confirmed this.
Phase formation at the surface of Al-Hf targets in reactive cathodic arc evaporation and the correlation with the synthesised coatings, X. Maeder, Centre Suisse d’Electronique et de Microtechnique CSEM SA, Switzerland
Surfaces of Al/Hf composite targets eo be used as cathodes in arc evaporation were investigated for atomic compositions Al75Hf25, Al70Hf30 and Al67Hf33. The narrow range of compositions was selected to study the sensitivity of the surface modifications with respect to each target composition. The targets were operated with oxygen as the only reactive gas. The phase composition at the target surface was determined by XRD analysis and compared for the different process conditions. Layer compositions were determined by Rutherford backscattering spectrometry and X-ray diffraction analysis for the metallic and oxide phases in the layers. Phase compositions of layer and target surface were compared and the possible processes during target surface modifications discussed.
Influence of Si on the arc evaporation behaviour of Al-Cr targets and structural evolution of (Al0.7Cr0.3)2O3 oxide coatings, J. Paulitsch, Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, Vienna University of Technology, Austria.
Cathodic arc evaporation is a standard deposition technique to synthesise high quality coatings, though the technique is very sensitive to the reactive gas pressure used, especially when preparing Al/Cr oxides. Recent studies showed that increasing the oxygen partial pressure encouraged the formation of crystalline (Al0.7Cr0.3)2O3 coatings. However, higher oxygen partial pressure also promotes the formation of Al-rich oxide islands at the target surface which, in turn, influence arc behaviour as well as film quality. Therefore, powder metallurgical AlxCr1-x-ySiy targets with Si contents y of 1,2,5 and 10 at% were studied for arc behaviour and suitability to prepare corundum-type Al/Cr/O oxides. The investigation showed that (Al0.7Cr0.3)2O3 targets with 5at% Si minimised the unfavourable oxide island formation and promoted the formation of dense (Al0.7Cr0.3)2O3 coatings with preferred crystallographic orientations, even at low substrate temperatures of 500°C. The Si content of these coatings was below the detection limit of elastic recoil detection analysis.
The results clearly indicated that sophisticated target design was essential for the preparation of high-quality coatings.
Spatial correlation of tensile residual stress and thermal fatigue damage in coated cemented carbide milling inserts, T. Teppernegg, Materials Centre Leoben Forschung GmbH, Austria
Thermomechanical fatigue induced by interrupted tool/workpiece contact and wear are the main damage mechanisms for coated hardmetal milling inserts. This study examined the damage mechanism by local correlation of the emergence of tensile residual stress and damage by cracking. The milling inserts, cutting normalised 42CrMo4 steel, were made of fine-grained WC/8wt%Co coated with arc-evaporated TiAlN. To focus on early signs of damage, the inserts carried out a predefined number of contacts with the workpiece and the development of residual stresses in the WC phase was followed as a function of the contact number. Their position along the cutting edge was examined via synchrotron X-ray diffraction with high lateral resolution.
In order to document the degree of damage present at various stages of tool life, selected milling inserts were prepared by focused ion beam milling and investigated by scanning electron microscopy. The location of the first observed significant tensile residual stress was compared with the position of the most pronounced thermal fatigue crack at the end of service life. The position of tensile residual stresses in an insert with 1000 tool/workpiece contacts correlated well with the position of thermal fatigue cracks in milling inserts at the end of service life.
Means to improve thermal stability of TiAlN hard coatings, M. Oden, Linköping University, Sweden
TiAlN decomposes isostructurally at elevated temperatures by spinodal decomposition into coherent nanometre-sized domains rich in Ti and Al, resulting in improved hardness. Further heating causes transformation to hexagonal AlN, which decreases hardness. The paper describes ways to delay this transformation to higher temperatures, using a combination of simulations and experimental techniques.
The first method investigated was based on multilayer stacking. 20nm TiN interlayers were introduced to promote spinodal decomposition and suppress the hexagonal transformation, improving the thermal stability and wear resistance of the cutting-tool inserts. The physics governing this improvement are discussed in terms of surface-directed spinodal decomposition and pressure stabilisation of cubic AlN.
The second concept was alloying. The approach here was to alter the enthalphy of mixing to modify the miscibility gap. The author demonstrates his approach with the model system Cr-TiAlN and shows that in terms of thermal stability large beneficial effects can be achieved.
Microstructure of cathodic arc evaporated (Al,Ti)N hard coatings deposited at different orientations to the target, D. Rafaja, TU Bergakademie Freiberg, Germany
This contribution investigated the effect of deposition geometry on the microstructure of aluminium-rich (Al,Ti)N coatings deposited by cathodic arc evaporation from a single vertical cathode. Detailed examination of the microstructure of coatings located either vertically or horizontally in the deposition apparatus was a first step in the quantitative characterisation of cutting tools with complex geometry. Three sample series were deposited at bias voltages of -20, -40 and -80V, each series consisting of vertical and horizontal samples. Coating microstructures were described in terms of phase composition, lattice parameters of face-centred cubic and wurtzitic (Al,Ti)N, and crystallite size. It was found that a higher bias voltage stabilised the cubic phase of (Al,Ti)N and reduced the size of cubic crystallites, predominantly in vertical coatings.
Two possible reasons for the stabilisation of the cubic phase of aluminium-rich (Al,Ti)N were considered; microstructural defects and lattice strains induced by ion impact. In the samples under study local fluctuations of lattice strains were thought to be responsible for the concurrent formation of cubic and wurtzitic (Al,Ti)N rather than fluctuations of the Ti and Al concentrations.
Advances in deposition equipment and process technology for HiPIMS coatings for cutting tools, L. Hultman, Linköping University, Swede
HiPIMS (high-power impulse magnetron sputtering) is characterised by short power pulses and extremely short signal rise time. A completely new HiPIMS power supply design is the subject of this presentation, claimed to feed electrical energy into the plasma more efficiently.
Commercial coatings for cutting tools are frequently of the Ti1-xAlxN type. Such a system needs an understanding of the HiPIMS effect on different target materials, including elemental and sintered compound types. The paper describes the result of intensive plasma diagnostics and their correlation with film structure, composition and properties for TiAlN HiPIMS coatings.
Case studies showed that HiPIMS benefits for cutting inserts included “hitherto unachieved evenness of coating distribution around flank and rake face as well as perfect film formation at the cutting edge.”
SEM images revealed the “dense morphology” of HiPIMS coatings, which therefore combine high hardness with relatively low Young’s modulus, indicating high coating toughness. The effective bombardment of the growing film with highly ionised material further improves HiPIMS coating surfaces.
Application of (Ti,Cr,Al,Si)N PVD coatings in hard machining of PM high speed steel, K. Bobzin, RWTH Aachen University, Germany
PM high speed steels are increasingly employed as tool materials for cold work applications due to their high abrasion resistance and toughness. Nonetheless, machining of these materials is very challenging due to the vanadium, chromium and iron carbides which lead to high abrasive wear on cutting edges. In addition, the high toughness of these materials leads to adhesive wear.
To meet these challenges, nitride nanocomposite coatings seemed to be promising. In the work described, (Ti,Cr,Al,Si)N coatings were deposited on cemented carbides via MSIP (Magnetron Sputter Ion Plating) and investigated using XRD, SEM and scratch tests. The developed coating was deposited on ball-nose endmills and compared in milling S790PM, 62HRC (1.3345, M3 class 2) with two different commercial coatings, (Ti,Al)N deposited via MSIP and (Ti,Cr,Al,Si)N deposited via arc process. Flank wear width of 100µm was chosen as tool life criterion. Tool life was increased by the newly developed coating.
Effect of grain size of targets on arc behaviour and coating characteristics for AlTiN and AlCrN coatings deposited by CARC+ technology from powder metallurgical targets, F. Papa, Hauzer Techno Coating BV, The Netherlands
Several types of materials, such as AlTi and AlCr, applied for a variety of advanced hard coating applications are produced by powder metallurgical techniques due to advantages such as homogeneous elemental distribution for a wide range of compositions. Powder metallurgical targets have been produced from Aluminum and Titanium/Chromium powders with grain sizes of 20, 100 and 500 microns. Due to the various grain sizes in these targets, it can be expected that arc movement, the conditions on target surface and consequently the coating characteristics such as droplet size and density may be affected. AlTiN and AlCrN coatings deposited by CARC+ technology under various conditions from such targets have been analysed with high resolution 2D/3D imaging in order to characterise the surface state of the coatings.
Development of new cathodic arc source and its application, K. Yamamoto, Kobe Steel Ltd., Japan
Cathodic arc source with a new magnetic field design was developed. This new evaporation source (SFC, super fine cathode) is designed to cover the drawbacks of conventional arc source; droplets and generation of excessive residual stress. Due to the new magnetic field configuration, number of the cathode spots as well as the movement velocity was increased. This results in significant decrease of droplets emission and better surface roughness. Surface roughness of 3 um TiAlN by conventional arc source is approximately 0.1 um of Ra and it is less than 0.05 um by SFC. Compared with same level of substrate bias, residual stress of coatings deposited by SFC is 1/2 to 1/3 the one by conventional cathode. SFC deposited thick and optimised TiAlN coating (up to 20 um) was applied to heavy turning cutting inserts for high strength cast iron (CGI) which thick CVD oxide coating (TiN-TICN-Al2O3) is dominating. Result indicates that SFC TiAlN can compete with CVD coated inserts in roughing cutting condition.
Special interest sessions: Hardmetal powder production and recycling
Review of the hardmetal recycling market and the role of the zinc process as a recycling option, T. Karhumaa, Tikomet Oy, Finland
Recycling in the hardmetal industry has greatly gained in popularity since 2005 due to the increase in tungsten prices. The recent developments in hardmetal recycling, the various recycling options and their market shares in Europe are reviewed and the future outlook is discussed.
A significant new development in recent years has been the emergence of the zinc process as a new recycling option. The basics of the zinc process are reviewed and the current position of the zinc process in the recycling market is discussed.
Both lab and production scale experiments were carried out where WC-Co zinc reclaim powders were blended with virgin raw materials and the effect of such additions on the processing of powders and properties of sintered hardmetal were analysed. Results of both laboratory and production scale tests are shown and discussed.
The results show that the WC-Co zinc reclaim powders when added in the right amounts and processed properly have no negative effect on the properties of sintered hardmetal.
Based on the results various applications for zinc reclaim powders are discussed
Numerical simulation of organic binder decomposition during thermal debinding, T. Kraft, Fraunhofer Institute for Mechanics of Materials IWM, Germany
In several powder technological processing routes debinding of green parts can be critical due to possible damage formation. Especially large parts or very dense compacts in combination with higher thermal heating rates are vulnerable. In this paper we present a model which describes the chemical decomposition of organic binders and the consequential development of an internal gas pressure. It includes the combined Maxwell-Stefan and Knudsen diffusion and the seepage flow of multiple gaseous reaction products through a porous body. The model is implemented into the finite element program ABAQUS. The internal stresses developing due to this gaseous overpressure during decomposition are shown for some simple examples. Predicting these stresses can indicate critical temperature regions and help to optimise debinding conditions.
Effect of WC material properties and milling parameters on the grain size and sintering behaviour of coarse WC powder, A. Johansson, Seco Tools AB, Sweden
The wet-milling and sintering behaviour of two coarse WC powders was studied experimentally. Microscopy, laser light scattering, BET adsorption isotherms, X-ray powder diffraction, and magnetic coercivity measurements were used to quantify the effect of material and process parameters on the evolution of the WC grain size distribution throughout the process chain. Analysis of the results showed that the WC grain size reduction with increasing milling time is characterised by a power-law behaviour. The evaluation of the effect of several process parameters on milling efficiency could be facilitated by the use of simple scaling factors. In addition, the correlation between WC grain size after ball-milling and after sintering was investigated.
Property changes induced in submicron WC through various final milling techniques, R. Cook, Global Tungsten & Powders Corp., USA
Milling techniques vary in both the energy and mechanism applied to powder samples. Due to these variations, powder properties and the resulting sintered properties change accordingly. This study looks at these changes by comparing a wide variety of commonly employed milling operations including ball milling, attritor milling, pin milling and rod milling as applied to sub-micron WC. A wide range of physical, chemical and sintered properties are examined to see changes due to milling in sub-micron WC and WC+Co. From these studies certain attributes such as densities, particle size distributions and sintered shrinkage changes can be directly modeled by the milling time and type, however each mill style (combination of impact and shear) has a different phenomenological response.
Rim-nitrided WC powder: preparation, properties and use for hardmetals, M. Wetzel, TU Bergakademie Freiberg, Germany
Nitrogen uptake of commercial WC powders of different grain size was studied by high-pressure nitridation (≤ 200bar N2). The nitrided powders were characterised by XRD, chemical analysis, SEM, TEM and for their grain-size distribution. Interestingly, it was found that a perceptible amount of nitrogen dissolves in WC. As the diffusion rate of N in WC is extremely slow, nitridation occurs in the outermost rim of the WC particles. Upon appropriate nitridation conditions the powders are even purified with respect to the oxygen level and do not show particle growth.
Sintering experiments were performed with these powders for a possible use in the preparation of hardmetals. It turned out that nitrogen can retard low temperature grain growth and thus hardmetals of reduced grain size can be obtained. Mass-spectrometric delubing experiments showed that the powders loose nitrogen upon sintering. If the employed powder particles have a steep N gradient (essentially a nitrogen-rich fringe area) no porosity remains in the hardmetals, contrary that of powders with uniform nitrogen distribution with a similar overall nitrogen content.
Analysis of WC with increased Ta doping, J. Weidow, Chalmers University of Technology, Sweden
Tungsten and tantalum metal powders were co-carburised to yield a mixture of cubic and hexagonal carbide. The carburisation was made through a two-step carburisation process with (W,Ta)2C powder as an intermediate product. X-ray diffraction analysis showed that the lattice parameters of the hexagonal phase in the fully carburised powder were larger than those of pure WC indicating the formation of a mixed crystal carbide, (W,Ta)C. The powder with the largest lattice parameters was investigated in detail. A method to produce atom probe tomography specimens of this powder was developed. The atom probe tomography measurement showed the Ta solubility expressed as Ta/(Ta+W) to be as high as 0.086 (i.e. about 8.6 at%). In addition, it was found with electron backscatter diffraction that the (W,Ta)C grains had a large fraction of Σ2 grain boundaries as well as a small fraction of what was suggested as Σ4 grain boundaries.
Special interest: Hardmetal trends in processing
Thermal spraying – a technology for hardmetal coating solutions, L.-M. Berger, Fraunhofer Institut IWS, Germany
Thermal spray processes represent an important and rapidly growing group of surface modification technologies, it is the only surface technology allowing to produce hardmetal coating solutions. It it thus complementary to sintering technology and applicable to large parts.
The contribution reviews the development of these coatings, including the different processes and the feedstock materials, and presents the technical possibilities of state-of-the-art coating solutions. Hardmetal coatings are based also mostly on WC, but also on Cr3C2. Cubic hard phases based on TiC represent an alternative. Specifics of the hardmetal compositions for coatings (large Cr3C2 addition, high binder contents) are considered. The need of fundamental research is outlined for understanding the specifics of the metallurgical reactions during the spray processes, e.g.the formation and properties of (W,Cr)2C. The microstructure-property relationships are discussed in dependence of the feedstock powder properties and the specifics of the spray processes applied. Besides the traditional use as wear resistant coatings, new applications, such as service under contact loading is envisaged.
Development of a HVOF WC based thick coating, J. He, Global Tungsten & Powders Corp., USA
To protect from severe wear, gas/oil and other mining industries need very thick wear-resistant layers, currently produced by PTA techniques. It is known that HVOF thermal spray coatings have advantages over PTA and plasma coatings in wear resistance, based on its microstructure formed at lower temperatures and higher particle velocities. This work will present the microstructure and mechanical property of WC-10%Ni HVOF thick coatings of up to 1,000 μm. The coating were produced using both densified and non-densified thermal spray powders. The thick coatings were sprayed on both rotating and still substrates. The resulting microstructure is comparable to the typical HVOF coatings with no cracks or large voids either in the coatings or near interface between coating and substrate. Thick coatings have the same level of porosity (less than 0.5%), microhardness (~1030HV300) and bond strength (~ 10 ksi) as the corresponding normal HVOF coatings in the 100 to 300 µm range.
Rapid synthesis of ultrafine WC-Co cemented carbides, X. Song, Beijing University of Technology, China
Recently we have developed novel routes to prepare in a rapid way the ultrafine WC-Co cermets, which have potential applications in the industrial fields. One is the rapid synthesis of the WC-Co composite powder with controllable particle size by in situ reduction-carbonisation reactions using metal oxides and carbon black as the raw powders. The powder products, which have advantages in the pure phase constitution, uniform particle size distribution and homogeneous distribution of the binder phase, are available as the high-performance coating materials, as well as the sintering powders for the preparation of the cermet bulks. The second is a one-step route for synthesising the cermet bulks through reaction sintering in a spark plasma sintering system. This route is applicable to prepare the dense nanocrystalline cermet bulks and is feasible to be developed as a net-forming fabrication technique for certain cermet products. Along with the development of the preparation methods, the mechanisms of the rapid synthesis have been studied, and the prospects concerning the applications of the techniques are proposed.
Characterisation of gas reactions during ceramic and hardmetal production, T. Gestrich, Fraunhofer Institute IKTS Dresden, Germany
Gas reactions are critical steps during the thermal treatment for the production of hard materials like hardmetals (e.g. WC-Co) or ceramics (e.g. Liquid Phase Sintered SiC). Thermogravimetry (mass changes), Differential Scanning Calorimetry (caloric effects) and Mass Spectrometry (gas analysis) are suitable methods to study gas reactions. Thermodilatometry (length changes) can give valuable additional information. It is shown how gas reactions are influenced by grain size, grain growth inhibitors, and amount or composition of binder phase respectively. Through kinetic analysis of thermoanalytical results it is possible to predict the thermal behaviour for any temperature-time profile or special mass change behaviour. This knowledge can be used to avoid problems like por osity which influence important properties of the sintered material, such as the strength and / or the carbon balance. The created process knowledge may lead to a better cost efficiency, to stabilisation of properties and improvement of technology.
On the interaction of alumina during sintering of cemented carbides, A. Bicherl, Wolfram Bergbau und Hütten AG, Austria
Even today sintering of certain cemented carbide parts is carried out in push-type furnaces under hydrogen atmosphere. In this case the parts are embedded in a coarse-grained alumina/carbon fill. Other grades are sintered in vacuum on a graphite tray coated by a fine suspension of alumina.
During the sintering sequence the oxygen partial pressure within the heating zone is significantly reduced through the presence of carbon, and a partial carbothermic decomposition of alumina takes place, resulting in an up-take of aluminium in the binder phase during sintering.
It is demonstrated, that both temperature and atmosphere determine the amount of aluminium in solution. Under extreme sintering conditions even the formation of intermetallic phases can be observed. The amount of aluminium is measured as a function of temperature and respective alloy system (Co-, Ni-, or Fe-base), and the influence of dissolved aluminium on subsequent cooling and the properties of the parts will be discussed.
Selective laser sintering of tungsten carbide inlays for local wear protection of injection moulding tools, H. Koehler, BIAS - Bremer Institut für angewandte Strahltechnik, Germany
Materials of tools for injection moulding like steel 1.2343 need to withstand different types of wear. Typically tool wear varies locally, i.e. it is increased dramatically on spots of injection. Hence, inserts of wear resistant material are used in such areas. The selective laser sintering/melting process was chosen to produce near net shape parts out of a variety of materials. The highly wear-resistant alloys WC-Co 83/17 and WC-Co 88/12 were chosen to be generated on steel substrates. Major challenge was the determination of process parameters to meet desired densities and avoid cracking. The influence of process parameters and weld trajectories on the density was investigated. It was found that the key to achieve densities of >95% through the complete specimens was the combination of comparatively low scanning speed, large beam diameter and checker board shaped weld trajectories where the single boards were to be made with overlap. Inserts fulfilled the requirements for brazing to the tool steel 1.2343. The complete process chain of generating and joining wear protective inlays could be realised.