Showing papers in "International Journal of Refractory Metals & Hard Materials in 1998"

Hardness to toughness relationship of fine-grained WC-Co hardmetals

Abstract: The hardness to toughness relationship of fine-grained WC-Co hardmetals was studied based on Palmqvist indentation toughness measurements. Sixty-five commercial and lab-sintered hardmetals of different composition, microstructure and manufacturing history were investigated to build up a representative hardness/toughness measurement band. This band is then used to discuss the influence of the various alloy- and process-related parameters on the hardness to toughness relationship of WC-Co composites. Beyond that, optimal hardness/toughness combinations can be assessed for the hardness range of 1400–2200 HV30. In general, the higher the hardness of the alloys, the longer were the indentation cracks, indicating a decrease in fracture toughness with increasing hardness. However, at a certain hardness, the toughness of individual alloys varied significantly. For example, at HV30:1670, the sum of crack lengths varied between 287 μm (high toughness) and 449 μm (low toughness), which corresponds to fracture toughness values of 11.5 and 9.2 MNm−32, respectively. Very fine-grained hardmetals (ultrafine grades) were shown to be not necessarily tougher than coarser grained alloys (submicron grades), in particular in the hardness range of 1450–2000 HV30, although they exhibit significantly more binder at a given hardness. Only in the high hardness range of > 2000 HV30 might they be of advantage. Samples, exclusively doped with Cr3C2 as growth inhibitor exhibit more favorable hardness/toughness combinations than comparable VC-doped alloys. However, other parameters, such as sintering temperature, sintering time, or the gross carbon content of the respective alloys must be taken into consideration for obtaining optimal hardness/toughness combinations.

Microwave sintering of hardmetals

Abstract: Application of microwave radiation as a heat source for sintering of hardmetal is described. Sintering of hardmetal with microwaves leads to a finer microstructure because of lower sintering temperatures and shorter processing times. A further variant is the microwave reaction sintering of a powder mixture of metallic tungsten, carbon and cobalt to obtain finer microstructures than by the conventional route. Moreover, this process offers a great potential for simplifying and shortening the process sequence in hardmetal production. The in-situ formation of WC-platelets during microwave reaction sintering was observed.

High temperature mechanical behaviour of Ti(C,N)-Mo-Co cermets

Abstract: The high temperature behaviour and the microstructure of Ti(C,N)-Mo 2 C-Co cermets were investigated in order to describe the deformation mechanisms that can appear during their use in cutting tool applications. Different grades were considered in order to take into account the effects of the molybdenum and of the cobalt content. The results of three point bend tests, consisting in constant strain rate tests and creep tests performed between 1173 and 1553 K, suggest a description of the mechanical behaviour in three temperature domains: elastic and brittle from room temperature to about 1150 K, tough with limited plasticity between 1150 and 1350 K and creep above 1350 K. Internal friction (IF) measurements were performed at high temperature using forced oscillations. Five thermally activated relaxation peaks and one thermally activated high temperature background were observed. Each peak is related to the motion of a structural defect (dislocation, grain boundary) in one of the two cermet phases. The results obtained from the IF measurements and from the three point bend tests were confirmed by transmission and scanning electron microscope observations, as well as by in situ electron microscope observations performed at high temperature.

WC-Co based cemented carbides with large Cr3C2 additions

Abstract: Five model alloys based on WC-16.6 vol% Co with different amounts of Cr3C2 (0–12 vol%) substituting WC were studied. Their microstructures were characterised with X-ray diffraction, scanning electron microscopy and transmission electron microscopy in combination with energy dispersive X-ray analysis. Thermodynamic calculations on the C-Co-Cr-W system were carried out using the Thermo-Calc software. The microstructures were related to previously published results on properties (magnetic coercivity, hardness, transverse rupture strength and indentation fracture toughness) of these materials. The aim of this work was to investigate whether the positive effect of small Cr3C2 additions on grain refinement and various properties remains also for large Cr3C2 additions. For Cr3C2 additions larger than 2 vol%, the most obvious effect on the microstructure was the formation of a chromium- and cobalt-rich M7C3 carbide. The Cr/Co ratio of this phase depends on the amount of Cr3C2 substituting WC. Some large WC grains were present in the WC-Co material, but not in the materials with Cr3C2 additions. Apart from this, no major changes of the WC grain size with Cr3C2 content were observed. The partial replacement of tough binder by presumably brittle M7C3 carbide might explain the lower toughness of these materials.

A new look at the influences of load, grain size and grain boundaries on the room temperature hardness of ceramics

Abstract: The measured load (size) effect on the hardness is modelled assuming that for the extension of the plastically deformed zone, growth and multiplication of pre-existing elements of plasticity are more effective than the generation of new dislocations and twins in the virgin material around. This idea also explains the decreasing load effect at smaller grain sizes. Therefore, microhardness approaches must not be used to investigate grain size effects in ceramic microstructures. The comparison of the real grain size effect in sintered Al 2 O 3 with the indentation size (load) effect in sapphire single crystals (shown to simulate the grain size effect in polycrystals but avoiding the influence of grain boundaries) reveals an important contribution of the grain boundaries to the permanent deformation at the indentation site at room temperature even for coarser microstructures and rules out the chances for a strong hardness increase in oxide ceramics at grain sizes <100 nm. However, first results indicate a possibly different behavior in binder-free carbides where the covalent character of interface bonding may reduce the degree of grain boundary deformation at room temperature as it is known to reduce the microplastic deformability of the crystal lattice.

A study of grain boundaries in a binderless cemented carbide

Abstract: The microstructure of a binderless cemented carbide with < 0.5 wt% Co has been analysed using transmission electron microscopy (TEM) in combination with an electron energy loss spectroscopy (EELS) detector and an imaging filter. In particular, microstructural properties of the grain boundaries were studied. The analysis identified different kinds of grain boundaries in the material: boundaries between WC grains containing nanometre-thick layers of Co; WC-WC grain boundaries with perfect lattice matching and no or very small amounts of Co; and boundaries between WC and TiC grains containing high amounts of C, but no Co. The influence of the grain boundaries on the mechanical properties is discussed.

Activation parameters for dislocation glide in α-SiC

Abstract: The critical resolved shear stress for activating the 〈211¯0〉(0001) slip system of monocrystalline α-SiC ( 6H and 4H polytypes) has been determined as a function of test temperature and strain rate via constant-displacement compression tests. Tests were conducted at temperatures between 550 and 1300 °C and strain rates between 3.1 × 10 −5 s −1 and 6.5 × 10 −4 s −1 . The current study shows that α-SiC crystals can be plastically deformed via relatively modest resolved shear stresses on the basal plane at temperatures as low as 550 °C. Two different methods to determine the activation parameters for dislocation glide have been examined. Transmission electron microscopy (TEM) was used to rationalize some of the results.

Wear characteristics of TiC, Ti(C,N), TiN and Al2O3 coatings in the turning of conventional and Ca-treated steels

Abstract: The wear characteristics of single layers of TiC, Ti(C,N), TiN and Al2O3 were investigated during turning of conventional and Ca-treated quenched and tempered Al-killed steels. The experimental coatings were deposited using chemical vapour deposition (CVD) or moderate temperature CVD (MTCVD) on cemented carbide substrates of a single composition and the coatings were of similar thicknesses (7 ± 1 μm). The wear mechanisms and layer formation were studied using scanning electron microscopy, optical microscopy and X-ray diffraction. Inclusion modification appeared to be an effective means of enhancing machinability and all experimental coatings exhibited about 20% better performance as a result of Ca-treatment. In particular, the crater wear of the experimental coatings — excluding Al2O3 — was clearly reduced. Comparative cutting tests revealed important differences between the coating materials. Wear mechanisms of the experimental coatings are discussed in detail.

Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition

Abstract: Dense, uniform, mullite coatings have been deposited by chemical vapor deposition on SiC substrates, using a AlCl3-SiCl4-CO2-H2 system The typical coating microstructure consisted of a thin layer of nanocrystallites of γ-Al2O3 in vitreous silica at the coating-substrate interface, with columnar mullite grains over this interfacial layer The composition of the coating was graded such that the outer surface of the coating was highly alumina rich The changes in the coating microstructure with processing parameters are discussed The ability of mullite to incorporate such large composition variations is discussed in the light of vacancy formation as theAl/Si ratio is increased and the ordering of these vacancies leads to changes in lattice parameters The formation of domains was studied by measuring the spacing of superlattice spots in electron diffraction patterns and the relationship between domain size andAl/Si ratio is discussed

Diamond coatings and cBN coatings for tools

Abstract: The introduction of diamond-coated tools for industrial machining of materials, formerly hampered mainly by coating adhesion difficulties, has, in the recent years, found considerable success, thanks to solutions of this main problem worked out in several R&D laboratories. Compared with the state of the art as to the diamond coating of hard metal and ceramic tools, coating of steel with diamond or cubic boron nitride (cBN) still needs a considerable amount of R&D work to be done, before field testing of tools can actually be started.

Diamond growth with boron addition

Abstract: Diamond coatings were produced on Si substrates by the hot-filament method, with B(C2H5)3 added to the gas phase. Ratios of B(C2H5)3: CH4 up to 0.01 (10000 ppm) were used which gave boron concentrations up to 3% in the layer according to secondary ion mass spectrometry (SIMS) and elastic recoil detection (ERD) measurements. The characteristic Raman peak of diamond at 1332 cm−1 decreases with increasing boron incorporation. Studying this effect in detail shows that on (100) facets the Raman peak still can be observed while on (111) it is already severely deteriorated. TEM and localized EELS spectra show high boron incorporation in the (111) growth sectors and low boron concentration in the (100) sectors. With cathodoluminescence spectroscopy measurements electronic properties were determined. The Mott-transition from semiconductor to metal-like conduction was found to occur at 0.11% B, which is in agreement with published Hall-measurements.

Sintering of alumina-niobium carbide composite

Abstract: Several studies have been focused on particulate-dispersed Al 2 O 3 composites in order to improve both room and high temperature mechanical properties and wear resistance. In the present work Al 2 O 3 -NbC composites have been pressureless sintered and their microstructures analysed as a function of NbC and Y 2 O 3 concentration, the latter added as sintering aid. The compositions used in this study were Al 2 O 3 -xNbC and (Al 2 O 3 3%Y 2 O 3 )-xNbC, (x = 10, 20 and 40 wt%) and the sintering was performed at 1650 °C/30 min and 1750 °C/15 min. A density greater than 96% of the theoretical density was reached even for those materials sintered at 1650 °C. The observed microstructure was more homogeneous for the samples with Y 2 O 3 addition and the Y 3 Al 5 O 12 phase was detected. The Al 2 O 3 grain growth restraining due to the NbC concentration was more pronouncedly in samples sintered at 1750 °C.

Nanophase and superfine cemented carbides processed by powder injection molding

Abstract: Two commercial superfine and one innovative nanophase cemented carbide powders were used in this study. They cover straight and complex grades of cemented carbides. The nanophase cemented carbides are characterized by a combination of high hardness and strength. However, the nanophase cemented carbides do not have a favorable moldability. The as-received nanophase particles are agglomerated hollow balls. Therefore, milling was used on the as-received nanophase powder to form a particle suitable for injection molding. Even so, manufacture of nanophase cemented carbides via powder injection molding (PIM) is highly challenging. Several processing parameters affect the success, including powder moldability, binder chemistry, mixture homogeneity, molding parameters, debinding cycle, and sintering densification level. Binder-related factors were the focus of this study, including binder chemistry, binder-powder compatibility, feedstock preparation, molding conditions, and binder removal after molding.

Surface Brillouin scattering studies on vanadium carbide

Abstract: Vanadium carbides in a range of carbon to metal ratio were investigated by surface Brillouin scattering (SBS) on the (100) and (110) surfaces. The surface acoustic wave (SAW) velocity varies with the crystal surface and direction and displays an approximately linear increase with the carbon to metal ratio. The tensor elastic constants of the samples were extracted and, while they vary significantly from the different carbon to metal ratio, the value of anisotropy ratio remains close to 0.8 for all samples. The directional dependence of the Young's modulus and shear modulus are obtained from the tensor elastic constants. In the directions chosen, both Young's modulus and the shear modulus reduce with decreasing carbon to metal ratio, with this behaviour being directionally dependent.

Potassium bubble growth in doped tungsten

Abstract: Potassium filled submicroscopic bubbles play a decisive role in determining the metallurgical properties of doped tungsten wires. Besides the influence of bubbles on the recrystallization behaviour and the high temperature creep strength, the bubbles may also form the nuclei for the generation of large cavities or voids which influence the fracture behaviour and lamp failure. The most accepted mechanism of bubble coarsening is stress assisted bubble growth; however, this needs as prerequisite a critical bubble size dependence on the stress system of the tungsten coil. It will be shown that a possible mechanism for the coarsening until the critical size is the diffusion controlled transport of potassium provided there are corresponding diffusion paths, as for example grain boundaries or dislocations. In addition, the new model also explains the formation of voids in doped tungsten wire and gives the basis for a material limited failure mechanism in halogen lamps.

CVD diamond films as protective coatings on titanium alloys

Abstract: The solid particle erosion resistance of CVD diamond coated T1-6A1-4V samples has been investigated in comparison with PVD hard coatings. In particular, diamond films grown with 4% methane gas concentration show excellent erosion resistance under both a 30 ° and a 90 ° incidence angle. Diamond coatings deposited applying 1% methane possess a somewhat lower particle impact resistance. The PVD coatings show almost no positive effect on the erosion behaviour of Ti-6A1-4V. Different failure mechanisms of diamond films grown with 1 and 4% methane gas concentration were identified: for the first an intercrystalline and for the latter a transgranular crack propagation was observed. This indicates relatively weak crystal boundaries for the diamond coatings grown with 1% methane concentration. Additional evidence for lower boundary strength was found when applying Rockwell C indents onto the films.

Molybdenum alloys for glass-to-metal seals

Abstract: The requirements for Mo-wire for hard glass sealing (matched seal) and Mo ESS-ribbon for quartz glass sealing (ductile-metal seal) will be discussed. Comparing the effect of various oxide additions to molybdenum, a correlation between the percentage of ionic bonding character of the oxide and the recrystallization temperature of the ODS-Mo could be found, owing to particle refining during deformation and heat treatment. For hard glass sealing the particle multiplication factor should be as high as possible (e.g. Mo-La2O3) in order to achieve ductility after the pinch-sealing operation. Quartz glass sealing demands an ESS-ribbon with a recrystallization temperature in the range of 1250 °C. A higher recrystallization temperature leads to higher stresses in the quartz glass (higher risk of quartz cracks), a lower recrystallization temperature results in a reduced yield owing to cracks in the ESS-ribbon. A good adherence between the ESS-ribbon and the quartz glass, which is a prerequisite for a long-term vacuum tight seal, is determined by mechanical bonding, chemical bonding, wettability and surface purity of the ESS-ribbon. The Y2O3-Ce2O3 particles in MY (Mo-0.47 wt% Y2O3-0.08 wt% Ce2O3) which are surrounded by etching grooves lead to a “zip-fastener effect” (improved mechanical bonding), a reduced wetting angle (pure Mo: 97.2 ± 6.5 °, MY: 79.7 ± 8.5 °; T = 2000 dg°C, Ar) and the formation of Y2Si2O7. Using MY instead of pure Mo ESS-ribbon, the life time of halogen lamps at base temperatures of 430–540 °C could be improved by a factor of 2–3.

Nitriding of dilute Mo-Ti alloys at a low temperature of 1373 K

Abstract: For Mo-0.5 mass% Ti and pure Mo alloy nitrided in a NH 3 gas at a relatively low temperature of 1373 K, microstructural observations through optical and transmission electron microscopes, X-ray diffraction analysis and hardness measurements were carried out. A surface nitriding layer with very high hardness of approximately Hv ~ 1800 consisted of two Mo-nitride regions: an outer one of γ-Mo 2 N and an inner one of β-Mo 2 N. The inward diffusion of nitrogen is a rate-controlling process in the growth of the surface nitriding layer. In Mo-Ti alloy additionally an internal nitriding layer with relatively high hardness of Hv ~ 800 was formed beneath the surface nitriding layer. Such high hardness in the internal nitriding layer was found to result from the uniform dispersion of extremely fine plate-like particles of titanium nitride. The particles are approximately 0.4 nm thick and have coherent strain field in the matrix.

Fracture in CVD diamond

Abstract: The fracture behaviour of thick, textured films of chemical-vapour-deposited diamond is discussed with particular emphasis on the influences asserted by the polycrystalline microstructure. Cracking is investigated on two different scales, firstly where it is large in that it traverses many grains and pertains to the fracture of the bulk. The second is where the fractures are localised, resulting from repeated small particle impacts and the mechanisms of material removal at the grain-size-scale are elucidated. The behaviour at this smaller level can be rationalised in terms intermediate to bulk fracture and that observed in single crystal diamond. The effects of grain size, grain boundaries, crystallographic orientation, twinning, internal stresses and pre-existing flaws are discussed. A new value for the fracture toughness is calculated and the gravimetric erosion rate for different surface orientations measured.

Microstructure of the surface zone in a heat-treated cermet material

Abstract: The microstructure of a Ti(C, N)-(Ti, W)(C, N)-(Ti, Ta)(C, N)-WC-Mo 2 C-Co cermet with a heat-treatment induced surface gradient has been studied. The post-sintering heat-treatment was performed at 1200 °C in a nitrogen atmosphere and the resulting microstructure was compared to that of the as-sintered material. The microstructures were characterised with optical microscopy, X-ray diffraction, electron microprobe analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, electron energy-loss spectroscopy and energy-filtered transmission electron microscopy. It was found that a 50-μm deep nitrogen-rich surface zone was introduced by the heat treatment. η-phase was observed in the surface zone. The nitrogen gradient caused diffusion of titanium towards the surface, thereby forcing the cobalt-rich binder to be transported inwards. The composition of the binder phase and outer rim of the carbonitride grains showed only minor changes with depth. The variations in binder composition may be a direct consequence of the heat treatment, but are also likely to be influenced by the formation of η-phase. A nitrogen-rich phase was present in the surface zone of the heat-treated material both as thin layers (≈ 70 nm) surrounding the carbonitride grains and as small parts of some carbonitride grains. In addition, the Ti(C, N) cores in the surface region were found to have different N/(C + N) ratios.

Interactions of hard metal substrates during diamond deposition

Abstract: Deposition and adhesion of diamond coatings on hard metal substrates depend sensitively on the substrate composition. In particular during deposition on low-carbon hard metal substrates carbon is picked up by the substrate from the carbon-containing environment and the formation of a diamond layer onto the hard metal is retarded. The process runs over a long period of deposition and can lead to a partial dissolution of the diamond coating at the diamond/hard metal interface, thereby weakening the interface adhesion. To investigate this interaction during deposition, experiments were carried out on carbon deficient substrates, containing small amounts of η-carbides (Co3W3C). Using these model alloys it is easy to demonstrate the carbon diffusion into the substrate by observing the migration of the Co3W3C+C⇒Co(C, W)+WC reaction front from the surface into the bulk material. It is shown, that at 900 °C both the C and W diffusion rate as well as the reaction rate are high enough to transform all η-carbides into Co(C, W) and WC. At 20 h deposition time the reaction front was progressed already about 0.9 mm into the bulk of the substrate. The carbon saturation of the cobalt binder during deposition and the partial dissolution of the diamond layer can be furthermore demonstrated by lattice parameter measurements performed in different depths segments of the substrate and SIMS measurements across the interface.

Some properties and cutting performance of polycrystalline cubic boron nitride with no additives

Abstract: Hexagonal BN disks made by chemical vapor deposition were treated under ultra high pressure of 6.8 GPa and at temperatures from 1800 to 2500 °C for 1.8 ks. When treated at temperatures above 2100 °C, only a c-BN phase was detected. The sample treated at 2100 °C had a homogeneous microstructure having fine grain size less than 1 μm, whereas that treated at 2300 °C had a heterogeneous one with some large grains. The sample treated at 2500 °C showed remarkable grain growth. Some physical properties and cutting performance of these compacts were investigated. Then hardness clearly decreased at 2500 °C and the thermal conductivity was higher when synthesized at 2500 °C than at 2100 °C. The fine grained compact of 2100 °C exhibited better wear resistance than the coarse grained one of 2500 °C in cutting of Co base super alloy and cemented carbide.

Solid-state1H NMR studies of different tungsten blue oxides and related substances

Abstract: The doping of 'tungsten blue oxide' (TBO) with K, Si and Al leads to the outstanding high-temperature creep resistance of the so-called non-sag (NS) tungsten wire. The composition of the blue-colored TBO depends on the reduction conditions of ammonium paratungstate, (NH 4 ) 10 [H 2 W 12 O 42 ].4H 2 O, such as temperature, atmosphere, type of rotary kiln or pusher-type furnace used and rate of feed through the furnace. Besides crystalline compounds (WO 3 , hexagonal tungsten bronze phase, W 20 O 58 , W 18 O 49 , WO 2 ) the industrially produced TBO powders may contain up to 50% of amorphous phases. The wide-line and especially the high-resolution solid-state 1 H NMR investigation of five industrially manufactured TBOs and seven reference substances show a distinction between five proton-containing species. Now, together with chemical, quantitative X-ray powder diffraction and NH + 4 /K + exchange analyses, we are able to give a complete and consistent characterization of different TBOs. Only TBOs with a certain content of hexagonal ammonium tungsten bronze, h-(NH 4 ) η WO 3 (η≤0.33), reveal the incorporation of K + via an ion exchange mechanism during the doping process.

The 700 °C tensile behavior of Mo-0.5Ti-0.08Zr-0.025C (TZM) extruded bar measured transverse and parallel to the billet extrusion axis

Abstract: The 700 °C deformation and fracture behavior of tensile samples extracted from both hot-worked and recrystallized Mo-0.5Ti-0. 08Zr-0.025C (TZM) extrudate has been analyzed. The investigation considers the orientation dependence (longitudinal, transverse) of the tensile strain axis to the extrusion axis of the billet and how this dependence effects the strain to failure. The influence of the normal plastic anisotropy parameter, r, and its effect on ductility has been established. The strain-rate sensitivity of each temper has been estimated based on post-uniform elongation behavior. The study has revealed that limited ductility is observed in hot-worked samples tension tested transverse to the extrusion axis of the billet. The combined effects of multiple crack initiation sites, revealed by SEM fractography, and an r-value of 0.49 are thought responsible. The origin of the cracks and the role extrusion plays in deformation anisotropy are discussed. The nearly isotropic deformation behavior of the recrystallized alloy is also investigated.

Nucleation and growth of combustion flame deposited diamond on silicon nitride

Abstract: The effect of substrate temperature ( T s ) on the nucleation and growth of diamond on silicon nitride (Si 3 N 4 ) based substrates deposited via the oxy-acetylene combustion flame technique was investigated. The diamond deposits were characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The nucleation density of the resulting deposits, which was of the order 10 5 nuclei/cm 2 , was used to approximate the activation energy for heterogeneous nucleation of diamond as32 – 40 kcal/mol. An Arrhenius plot of particle growth rate was used to calculate the activation energy for diamond growth as9.4 and 8.3 kcal/mol in the center and outside annulus of the deposit, respectively. These results suggest that the heterogeneous nucleation of diamond is a highly energetic process and may in fact be responsible for the observed low nucleation density of diamond on Si 3 N 4 . Thermodynamic analysis of gas/substrate reactions under conventional process conditions predicted that SiC formation, which is known to be a necessary precursor to diamond nucleation on Si, is energetically forbidden. Via kinetic and thermodynamic considerations, a patented in situ multistage deposition technique was developed which yielded continuous diamond coatings on Si 3 N 4 substrates without extensive substrate preparation.

Apparent reversibility of the β-w → α-w transformation

Abstract: On ground of experiments and thermodynamic data we make an attempt to modelize the building and transformation of beta tungsten in binary W-O and ternary W-O-K systems. We have not been able to confirm some indications in the literature about reversibility of the β-W → α-W transformation, but we have found an oxidation-reduction cycle by which alpha tungsten can be retransformed into beta tungsten in a ternary system. The loop of the cycle can be made quite narrow by choosing small temperature and/orH 2 O/H 2 partial pressure differences which leads to an apparent reversibility.

Deposition of BCN coatings from trisdimethylaminoboran by hot-filament and microwave plasma activation

Abstract: This investigation is a preliminary trial to grow cubic BN or cubic ternary BCN phases without application of high pressure. In contrast to diamond superhard coatings based on boron and nitrogen can so far not be made by low-pressure CVD methods. Single source precursors containing boron/carbon/nitrogen, have the potential to be decomposed to BN, BC and BNC phases, which are of technological interest for wear-resistant tool coatings if they are superhard, especially c-BN can also be used for machining of ferrous alloys. In this work by pyrolitic decomposition of tris(dimethylamino)borane B(NMe 2 ) 3 [B:N:C = 1:3:6], thin films were deposited on silicon wafers or on diamond coated silicon wafers. Microwave plasma CVD lead to coating qualities from powder-like to smooth morphology and varying C contents, which can be influenced by the argon/nitrogen ratio in the gas phase. The layer growth rate ranged from 0.5 to 1.8 μm/h. Elastic recoil detection analysis showed a homogeneous distribution of boron, carbon and nitrogen in the coating. Hot-filament CVD yields layer qualities from smooth to more crystalline morphology, which can be influenced by the reaction pressure. The layer growth rate ranged from 0.15 to 1 μm/h. IR-spectroscopy confirmed the formation of h-BN and X-ray diffraction of the deposited layers revealed a low grade of crystallinity. Hardness and mechanical properties of the formed B x C y N z coatings were not measured now.

Indentation fracture and soft impresser fatigue in sapphire and polycrystalline alumina

Abstract: The soft impresser technique has many advantages compared with conventional diamond pyramid indentation for studying the deformation and fracture of materials that would normally be considered ‘brittle’. In particular, as utilised in this study, it can be used to study the evolution of deformation and crack initiation and propagation under contact fatigue conditions. Alumina, both in its sintered polycrystalline form and as single crystals (sapphire), has been investigated because of its many uses in engineering load-bearing applications. Intially conical tool steel impressers were used to subject the flat ceramic surfaces to mean pressures of 5.2 ± 2.4 GPa for up to 7.5 × 10 6 cycles. Plastic deformation in the polycrystalline alumina was observed after only 10 3 loadings, followed by cracking at the edge of the contact zone and subsequent crack interlinking and radial extension. The sapphire showed no signs of deformation or cracking under these conditions, but did deform and crack after only one cycle when a harder WC/Co impresser was used.

Refractory metals: crucial components for light sources

Abstract: More than 40 elements of the periodic table are used in the lighting industry as auxiliary materials and as light emitting compounds in the solid as well as in the gaseous state. The so-called ‘refractory metals’ are of crucial importance for all existing types of industrial light sources. They comprise the elements tungsten, molybdenum, niobium, tantalum, rhenium, zirconium and hafnium. Wire and coils made out of potassium-doped non-sag tungsten became an indispensable part of all commercial incandescent lamps. Coils in halogen incandescent lamps represent parts with the highest energy load to a solid-state material in technology. The demands are still rising to increase efficacy and lamp life in many respects. Thoriated tungsten became the preferred material for electrodes in nearly all makes of discharge lamps which contribute immensely to energy saving in lighting. The replacement of thoria in these applications is not a trivial task. Electrical power supply into the silica vessels of halogen incandescent lamps or metal halide discharge lamps would be impossible without molybdenum feather-edged ribbon of some 10 μm in thickness; niobium is used in form of small cups in high-pressure sodium lamps. Tungsten metallurgy initiated a tremendous progress in vacuum technology and powder metallurgy. The introduction of gas-filled lamps made it necessary to develop separation methods to make rare gases available and, by adding halogen compounds to avoid wall blackening, ppm-technologies were first applied for highly reactive gases on industrial scale. In addition, a new understanding about lamp quality requires other features, like surface finish, to be taken into account. It is beyond any doubt that the investigations on refractory metals in the lighting industry will be a pacemaker for other technologies in the future.

New adhesion measurement technique for coated cutting tool materials

Abstract: Adhesion properties of wear resistant coatings on cutting tool materials are essential to their performance in technical applications It is therefore necessary to characterize the coating adhesion by an appropriate measurement technique which reveals both critical adhesion values as well as information on time dependent debonding In particular, a quantitative and reproducible technique is required rather than largely qualitative methods such as indent or scratch tests In addition, for multilayer coatings, the location of the weakest interface is required to facilitate improvement of interfacial integrity Accordingly a sandwich double cantilever beam (DCB) test was developed and used to measure the adhesion properties of coatings on cermets Additionally, the structure of the coatings in the as fabricated state and after mechanical testings were characterized by optical microscopy, SEM and TEM With quantitative adhesion measurements and investigations of the interface microstructure, a comprehensive characterization of coating adhesion on cutting toolmaterials was achieved