Carbide Grades Research Articles

CVD Diamond: Erosion Resistant Hard Material

The present paper describes the erosion of 60 μ m thick diamond coatings on tungsten, 600 μ m thick free standing diamond brazed to tungsten carbide and two grades of cemented tungsten carbide and stainless steel type 316. The erosion rate of the diamond samples was between 12 and 30 times lower than that of tungsten carbide at a particle velocity of 268 m s-1. Moreover, the diamond coating displayed an erosion resistance approximately 2·4 times higher than the brazed diamond. However, pinholes were observed on the 60 μ m thick coating, which were not seen on the thicker diamond. Scanning acoustic microscopy showed these pinholes to be formed on regions of debonded coating and, therefore, they provide a visual indication of coating debonding, which eventually leads to the catastrophic failure of the coating. The mass loss associated with the formation of pinholes has been estimated to account for less than 5% of the total mass loss. Therefore, the coating life or time to failure tF rather than the volumetric erosion rate may be a more useful parameter by which the performance of different coatings can be compared.

Water‐Lubricated and Dry‐Running Properties of Ceramic Journal Bearings

AbstractJournal bearings and other well‐defined sliding surfaces are important for numerous moving mechanical assemblies. Quite often sliding surfaces are produced on particular components during fabrication, for which reason the manufacturer needs to know the properties of the materials in order to optimise production processes and the running‐in characteristics of the sliding surfaces. This paper describes application‐orientated tests to support the selection of ceramic materials for water‐lubricated and unlubricated journal bearings and similar sliding surfaces. The ceramics studied were different grades of alumina, sialon, silicon carbide, and zirconia. The most promising results were obtained with a dense, sintered silicon carbide.

Multifractal description of fracture morphology: investigation of the fractures of sintered carbides

In this paper, a new definition of probability is proposed, which should be applied in the case of a multifractal analysis of a fracture profile. This allows making the span of a multifractal spectrum dependent on the degree of the profile line development as well as on local changes of the distances between its points in places where overlaps and secondary cracks occur. The verification was performed for 28 specimens of seven sintered carbide grades, the fracture profiles of which overlaps had been removed. The new method enables one to apply a multifractal analysis for an evaluation of the fracture morphology by means of surface stereometry, reducing in this way the time consumption of the research methodology, and it also makes more objective owing to an analysis of a very large number of profiles.

Micro scale hardness distribution of rock types related to rock drill wear

All rock handling requires tool materials with special qualities. In crushing and screening, speciality steels are used to resist abrasion and other major wear mechanisms. In drilling and rotary crushing, WC/Co cemented carbides are used. On top of the extremely tough conditions for the material, one of the most difficult problems is the shifting character of the rock. Even within a single mineral ore, the character often shifts significantly. To optimise a cemented carbide grade for wear life in rock drilling, a suitable characterisation of the rock is required. To perform this characterisation of the rock, very low load hardness of calcite, magnetite, hematite, leptite, quartz, mica schist, granite and sandstone have been measured using nanoindentation. This fixed depth hardness was measured using 1 μm indentation. To get a high resolution mapping of the local hardness values, 3×160 closely spaced indentations were placed, covering an area of 3.2 mm×0.075 mm. This study elucidates the importance of scale in hardness measurements of rock, especially for rocks of multi-mineral character. The hardness distributions achieved at 1 μm fixed depth indentations also clearly indicated that micro hardness measurements at 500 g load is too blunt a tool for a representative rock characterisation. For minerals with wide distributions, such as hematite and mica schist, the micro hardness value could be as low as 1/5 of the top hardness values measured at 1 μm fixed depth.

Experiments and modelling of the compressive behaviour of two SiC ceramics

To analyse compressive damage generated during a high-energy impact on ceramic targets, quasi-static uniaxial compressive tests are performed on two silicon carbide grades. These two grades (S–SiC and SiC–B) are characterised by different processing routes, porosity levels and tensile strengths. A new specimen geometry is proposed. Post-mortem studies carried out on a specimen embedded before compression allow us to visualise the final damage pattern. A high-speed camera is also used. Photographs of cracking and debris ejection are obtained a few microseconds after the inception of failure. In addition, thin strips are cut out of S–SiC and SiC–B specimens loaded and unloaded before failure. An anisotropic damage model is introduced to explain the failure scenario. A first numerical simulation substantiates the hypothesis of failure caused by tensile stresses induced by a roughness defect. A second analysis using the mesh of a half specimen matches the different observations obtained from post-mortem studies and with the high-speed camera.

Thermal return reflection method for resolving emissivity and temperature in radiometric measurements

A radiometric method for resolving emissivity ε and temperature T in thermal emission measurements is presented. Thermal radiation from a viewed source is split by a beamsplitter between a radiometer and a mirror aligned to return a part of the thermal radiation back to the source. The ratio of the thermal signal with and without a return reflection provides a measurement of the emissivity without need of any other probing sources. The analytical expressions that establish this relationship are derived taking into account waveguide/optic losses and sources between the radiometer and viewed sample. The method is then applied to thermal measurements of several refractory materials at temperatures up to 1150 °C. A 137 GHz radiometer is used to measure the emissivity and temperature of an alumina brick, an Inconel 690 plate, and two grades of silicon carbide. Reasonable temperature agreement is achieved with an independent thermocouple measurement. However, when the emissivity approaches zero, as in the case of the Inconel plate, radiometric temperature determinations are inaccurate, though an emissivity near zero is correctly measured. This method is expected to be of considerable value to noncontact thermal analysis applications of materials.

All in an engineer’s life

All in an engineer’s life

Evaluation of Machining Performance of Martensitic Stainless Steel (JETHETE)

Two grades of coated carbide tools [CVD-Ti(C,N)/TiC/Al2O3 (T1) and PVD-TiN (T2)] were used in wet and dry turning of a martensitic stainless steel under various cutting conditions in order to assess the effect of the substrate and coating materials during machining. Microstructural alterations of the machined surfaces are predominantly plastic deformation of grain boundaries in addition to untempered and overtempered martensites. The use of an emulsified general-purpose oil as coolant increased tool life of the CVD multilayer coated insert grade and resulted in failure by fracture of the PVD-TiN-coated carbide grade due to its sharp cutting edge, reduced hardness caused by high cutting temperature, pitting corrosion of the TiN coating and the “hammering” effect of the flowing chip on the rake face.

Processing of Ta2O5 powders for electronic applications

The particle size, surface area, morphology, and purity of tantalum pentoxide are critical for some of its applications in the manufacture of several, electronic products. Although the purity of different grades of Ta2O5, such as standard technical grade, carbide grade, and optical grade, have been well documented, there is no report on the morphology and other surface characteristics of Ta2O5 powders. The objective of this paper is to review various methods and recent developments in the processing of tantalum oxide powders. The other objective of this paper is to report on the morphology, particle size, and surface area of tantalum oxide obtained from different methods of preparation. The work reported in this paper will be useful for researchers involved in the development of tantalum-related electronic materials.

Modelling the orthogonal machining process using coated cemented carbide cutting tools

Finite element methods were used to model the effect of coated and uncoated, cemented carbide cutting tools on the machinability of the nickel-based superalloy Inconel 718. Disposable coated and uncoated carbide inserts were used both experimentally and as finite element analysis (FEA) models to study how the stress distribution within different coatings and carbide grades compared to each other under a range of cutting conditions. The simulation of an orthogonal metal cutting process was performed using FORGE2, an elasto-plastic FEA package. All finite element models were assumed to be plane strain. The results include the stress and temperature distributions through the primary shear zone, the chip/tool contact region and the coating/substrate boundaries. The tool wear and stress distribution results from the finite element modelling agree favourably with those obtained from experimental work.

Cutting performance and wear characteristics of PVD coated and uncoated carbide tools in face milling Inconel 718 aerospace alloy

In this paper, cutting performance and failure characteristics of two PVD TiN coated and an uncoated tungsten carbide grades with identical geometry are presented. Face-milling tests of Inconel 718 superalloy were performed to investigate the effect of cutting speed and feed rate on tools performance under wet conditions. Tools were thoroughly examined under SEM at two stages in order to reveal the failure modes and wear mechanisms. These stages were after cutting for 5 s and when the tool failed. It was noted that the coating resulted in a marginal improvement, as it was delaminated by adhering workpiece material at the beginning of the cut, impeding the performance of the tool for the rest of the experiment. A combination of progressive chipping and flank wear was the general mode of tool failure, former being dominant at high speeds and the latter at the low speed region. Results showed that uncoated tool performed better than coated tools at low cutting speeds while coated tools gave slightly better performance as the speed was raised.

Effect of WC-Co substrates pre-treatment and microstructure on the adhesive toughness of CVD diamond

Well separated diamond particles were nucleated and grown by hot filament chemical vapor deposition (HFCVD) onto Co-cemented tungsten carbide (WC-Co). Two carbide grades were prepared. The former one was a ISO grade K10 carbide having a 1-μm average WC grain size, 5.8 wt.% Co and 0.2 wt.% VC. The latter one had a 6-μm average WC grain size, with 6 wt.% Co. Prior to deposition the substrates were submitted to two different pretreatments. The adhesive strength of deposited diamond crystallites was quantitatively determined in terms of interface toughness by directly applying an external load to the CVD diamond particles in the scanning electron microscope (SEM). The adhesive toughness was determined from the measurement of the maximum load required to scratch off the diamond crystallites. The variation of adhesive toughness was correlated to both the microstructure and the pretreatments of the substrate.

Production and applications of advanced grades of tungsten carbide and binder powders: B.Mende et al. (H.C.Starck GmbH, Goslar, Germany.)

An auxiliary power unit (APU) is presented that is fuelled with diesel, thermally self-sustaining, and based on a solid oxide fuel cell (SOFC). The APU is rated at 1 kW electrical, and can generate electrical power after a 3 h warm-up phase. System features include a “dry” catalytic partial oxidation (CPOX) diesel reformer, a 30 cell SOFC stack with an open cathode, and a porous-media afterburner. The APU does not require a supply of external water. The SOFC stack is an outcome of a development partnership with H.C. Starck GmbH and Fraunhofer IKTS, and is discussed in detail in an accompanying paper.

Automatic tool selection for milling operations Part 1: Cutting data generation

The selection of tools and cutting data is a central activity in process planning and is often liable to an element of subjectivity. It is further complicated by the wide range of choice presented by the various operation types and the huge portfolio of cutters and inserts available from many different tool manufacturers. This paper describes a procedure to select consistently and efficiently tools for rough and finish milling operations performed on a computer numerical controlled (CNC) machining centre. A wide range of milling operations is considered, including faces, square shoulders, slots, T-slots, pockets, holes and profiles. An initial set of feasible tools is generated that satisfy the constraints of the tool type, the operation geometry, the insert geometry and carbide grade, the workpiece material and the machine tool capacity. Each tool consists of a holder and one or more indexable carbide inserts. Aggressive cutting data are generated for each feasible tool using a rapid search procedure in the permissible depth/width/feed space for good chip control. The cutting data are further refined by a set of technological constraints, which include tool life, surface finish, machine power and available spindle speeds and feeds. The overall cutting data optimization criterion is selected by the user from minimum cost, maximum production rate or predefined tool life. A new optimization criterion, called ‘harshness’, allows the user to influence the chip thickness that is achieved for any given cutter. Any feasible tools that fail to satisfy all the constraints and optimization criteria are discarded.

Mechanical properties of titanium diboride based cermets

Mechanical properties of titanium diboride (TiB2) cermets critically depend on the composition of the binder phase. Both, fracture toughness and hardness are substantially increased by avoiding the formation of extremely brittle secondary borides which form during sintering by chemical reactions between TiB2 and the metallic additives. Fractographic observations of TiB2 cermets without secondary borides show the presence of ductile ligaments of the binder phase bridging the advancing crack tip. The powder metallurgy processing route applied to these materials allows modification of the binder phase structure from the ferritic iron-aluminium phase to Fe-Ni-Al austenite by changing the aluminium content of the powder mixtures. The highest toughness values have been obtained for the TiB2 cermets with an austenitic binder phase. X-ray diffraction analyses of the fracture surfaces of such samples show that the binder phase is metastable exhibiting stress induced martensitic transformation during fracture. This new family of materials presents an outstanding combination of hardness and toughness, comparable to those obtained with commercial grades of tungsten carbide (WC) hardmetals.

Modelling the orthogonal machining process using coated carbide cutting tools

In this paper finite element methods were used to determine the influence of various coated and uncoated tungsten carbide cutting tools on the machining of a nickel-based super alloy Inconel 718. Disposable coated and uncoated carbide inserts were used both experimentally and as FEA models to study how the stress distribution within different coatings and carbide grades compared to each other, under a range of cutting conditions. Simulation of an orthogonal metal cutting process was performed using FORGE2, an elasto-visco plastic FEA code. All FE models were assumed to be plane strain. The results include the stress and temperature distributions through the primary shear zone, the chip/tool contact region and the coating/substrate boundaries. The tool wear and stress results from the FE modelling agree favourably with those obtained from experimental work.

Studies of graded cemented carbides components

Graded structures obtained by coupling different WC grades, also containing cubic carbides type (Ta, Nb)C with Co as binder, were produced by vacuum sintering. Different batches of cylindrical samples were compacted by coupling two different grades and were sintered at 1450°C under vacuum. The hardness properties as well as the shrinkage due to sintering process are function of the employed grades and show gradual change between the single values of each of the coupled grades. The residual porosity was always lower than type A02, the observed microstructure shows very good link between the coupled grades, with the presence of a diffusion zone and without any influence of gravity on structure. The results show that it is possible to produce multilayer graded tools with a tough core, constituted by cheap carbide grades with high binder contents, covered by a very hard surface layer, achieving high economical and technical competitiveness for the production of tools for applications in high abrasive as well as in shock loaded environments.

Tool Life and Surface Integrity When Machining Inconel 718 With PVD- and CVD-Coated Tools

Test results show that when machining nickel-based Inconel 718 alloy under the cutting conditions investigated, the multilayer PVD-coated carbide grades gave the best performance in terms of tool life. This was due to their higher hardness, toughness, abrasion resistance, good heat transmission behavior of the multiple (TiN/TiCN/TiN) coatings as well as thicker coating layer (relative to the single-coated grade). The smaller grain size and higher density of their substrates also enhance the strength and hardness of the multi-coated PVD tools, thus providing higher resistance to attrition wear. Machining with the single-PVD TiN-coated grade generally produced a better surface finish due to the polishing action of the honed cutting edge and the generation of a uniform finishing edge with coating bottom on the trailing edge during machining. The absence of any significant tearing on the machined surfaces after cutting with the PVD- and CVD-coated tools at the cutting conditions investigated can be attributed to the low friction coefficient of the coating materials. Excessive plastic deformation, however, occurred on the machined surfaces. Increased hardness of the machined surface layer (up to 0.6 mm deep) also occurred as a result of high pressure and temperature generated during the machining operation. This tends to increase the hardening rate of Inconel 718. Presented at the 52nd Annual Meeting in Kansas City, Missouri, May 18–22, 1997

Experimental Observations on Flank Wear in Machining Spherodized Plain Carbon Steels

As a part of a comprehensive study on predicting tool wear, the authors' preliminary findings on flank wear are presented in this paper. To understand flank wear, commercially available plain carbon steels including AISI 1018, 1045, 1065 and 1095 were spherodized and turned using C6 grade carbide inserts coated with a single layer of TiN, Al2O3 or TiCN at various cutting speeds. The wear behavior of these inserts is studied in terms of the cementite content and the physical properties of coating materials at their respective cutting temperatures. An infrared pyrometer with fiber-optic cable was used to measure the temperature at a fixed point on the rake face of the inserts. An inverse estimation scheme using the 1-D ellipsoidal model of Yen and Wright (1) was employed to obtain the mean temperature of the tool-chip interface. The scheme suggested by Oxley (2) was then used to estimate the flank face temperature. The experimental results indicated that flank wear per sliding distance increases with cementite content and decreases with flank temperature. The general wear behavior of flank wear corresponds well with the prediction based on three-body wear model. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998

An investigation into microwave bonding mechanisms via a study of silicon carbide and zirconia

It is known that the glassy grain-boundary phase present in low-purity aluminas has two primary functions during direct microwave bonding. Firstly, it increases the dielectric loss of the host ceramic, allowing heating to occur; secondly, the bonding mechanism itself has been found to be based on viscous flow of the glassy grain-boundary phase. However, some evidence has also been found for the bonding of individual grains where they come into direct contact across the join line. To investigate the role of grain-boundary phases further, the microwave bonding of two different grades of silicon carbide and one grain of zirconia has been studied. A single-mode resonant cavity operating at 2450 MHz was used for both studies. The temperature and axial pressure were varied and the bonding time was kept to a minimum. Analysis of the resultant bonds indicated that both reaction-bonded silicon carbide and partially stabilized zirconia could be successfully joined using microwave energy with bonding times typically 10 min or less. For reaction-bonded silicon carbide ceramics, the silicon grain-boundary phase softened at the bonding temperature, allowing the butting faces to be "glued" together. Unlike the glassy grain-boundary phase for alumina ceramics, the silicon phase did not allow grain motion but always formed a discrete and continuous layer at the interface, even under optimum joining conditions. The work with zirconia confirmed that it is possible to join ceramics without the presence of a substantial grain-boundary phase. The mechanism is thought to be either solid-state diffusion and/or grain-boundary sliding. © 1998 Kluwer Academic Publishers