Mapping of mechanical properties of WC–Co using nanoindentation

High-resolution measurements of mechanical properties are of immense importance in metallurgy. Measuring the intrinsic properties of each phase separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. In this work, nanoindentation has been used to reveal mechanical properties of different phases in WC–Co with very high resolution. The resolution limits of the equipment and this material as well as various techniques to accomplish the measurements were evaluated. By making indents only 0.1 μm apart and using very low loads (1 mN) it was possible to distinguish between the two different phases, WC and Co. Maps created from the measured properties, hardness and Young’s modulus, were in excellent agreement with SEM images of the same area. Furthermore, it was also possible to detect an increased hardness of the Co binder phase by a factor of four as compared to the bulk hardness of Co. This work verifies experimentally what several authors have proposed earlier based on modelling. This revised version was published online in August 2006 with corrections to the Cover Date.     

Metallurgical principles of cryogenically treated tool steels��a review on the current state of science

The cryogenic treatment of tool steels has transformed over centuries from black art to science, but the metallurgical principles responsible for increase in wear resistance, tensile strength, toughness, and stability are still disputed. Metallurgists comprehend how tool steels respond to cryogenic treatment, but they also understand that for many years, the cryogenic treatment of tool steels had the reputation of being a quick fix for poor heat treatment practice. During the cryogenic treatment of tool steels, the process modifies the carbon present in the tool steels. However, cryogenic treatment has not been widely adopted by the cutting tools industry due to lack of understanding of the fundamental metallurgical mechanisms and due to the wide variation in reported research findings. In the present paper, an attempt has been made to review the literature on metallurgical changes that occurred during the cryogenic treatment of tool steels to benefit the cutting tools industry. The prominent reasons found to be responsible for improving the mechanical properties of tool steels are transformation of retained austenite to martensite and precipitation of fine carbides.     

Surface finishing: Impact on tribological characteristics of WC–Co hardmetals

Flat samples of WC–Co hardmetals with 6–12 wt%Co were surface finished by grinding, polishing and wire-electro-discharge machining (EDM). Comparative dry reciprocating sliding experiments against WC–6 wt%Co pins were performed using a Plint TE77 tribometer. Tribological characteristics were recorded online. Wear surfaces were characterized by surface scanning topography and scanning electron microscopy. Wire-EDM’ed samples exhibited higher friction and wear compared to ground and polished equivalents. This trend was correlated to X-ray diffraction measurements revealing tensile residual surface stresses in WC after wire-EDM contrary to compressive surface stresses after grinding and polishing. However, finer executed EDM reduces friction and wear significantly.     

Wear characteristics of Cr3C2–NiCr and WC–Co coatings deposited by LPG fueled HVOF

Wear behavior of the HVOF deposited Cr₃C₂–NiCr and WC–Co coatings on Fe-base steels were evaluated by the pin-on-disc mechanism. The constant normal load applied to the pin was 49 N and sliding distance was 4500 m with velocity of 1 m/s, at ambient temperature and humidity. The specific wear rate of WC–Co coating was 3 mm³/N m and Cr₃C₂–NiCr coating was 5.3 mm³/N m. SEM/EDAX and XRD techniques were used to analyze the worn out surface and wear debris. The Fe₂O₃ was identified as the major phase in the wear debris. The wear mechanism is mild adhesive wear in nature.     

Some study on electrical discharge machining of ({WC+TiC+TaC/NbC}–Co) cemented carbide

Electrical discharge machining process is a potential method of shaping ({WC+TiC+TaC/NbC}–Co) cemented carbide known for its superior hardness and compressive strength at high temperature and resistance to diffusion wear. Yet, detailed study on electrical discharge machining of this material is lacking in the literature. In the present investigation, therefore, mathematical models are developed for material removal rate, wear ratio, and surface roughness in electrical discharge machining of this cemented carbide using the procedure of statistical design of experiments. Current setting, pulse on time, and pulse off time are chosen as input parameters. Based on available machine settings, a face-centered central composite design is selected for meaningful experimentations. The procedure may be extended to develop a data bank for such type of materials. Further, to reveal the attributes behind the removal of material from the work-piece surface, scanning electron micrographs are studied. It appears that sufficient superheating of work-piece material and subsurface boiling is essential for efficient material removal and that formation of pock marks due to burst of blisters and associated crack formation may be controlled by choosing a proper dielectric.     

Wear test of cemented tungsten carbide at high atmospheric temperature (400°C)

Cemented tungsten carbide–cobalt (WC–Co) alloy has good mechanical properties, so it is widely used for cutting tools, wear-resistant dies and rolls, and shock-resistant punching dies. In this study, a wear test between cemented WC–Co alloy and carbon steel is carried out using a pin-on-disc wear-testing machine. The characteristics as to wear rate, coefficient of friction and surface roughness are investigated. Pin specimen and disc specimen surfaces are analyzed by a fluorescent X-ray analyzer. In addition, the wear characteristics of WC–Co alloy are confirmed by SEM observation of the microstructure of the cemented WC–Co alloy specimen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Microabrasion of WC–Co hardmetals in corrosive media

In microabrasion, the influences of corrosive carrier media on the mechanisms and severity of material removal are still poorly understood. This means that with a few notable exceptions, good models for the prediction of materials behaviour are still not readily available. Ball cratering is still a technique which is in its infancy, but is yet sufficiently developed for a test programme to be undertaken using corrosive media.In this preliminary study, the technique has been applied to the microabrasion wear of a selection WC-based hardmetals. For the alkaline and neutral carrier media it was found that volume loss was inversely proportional to hardness. This was not in the case for the acidic media. Scanning electron microscopy of the wear surfaces indicated that the breakdown of the hardness–volume loss relationship in the acidic media was due to the coarser carbide grains (i.e. in the hardmetals of lower bulk hardness) remaining in situ rather than being removed by abrasive processes. Longer-term tests indicated that there were transients in the material removal process, giving a higher wear rate at the start of a test.     

Dry sliding wear characteristics of glass–epoxy composite filled with silicon carbide and graphite particles

The dry sliding wear characteristics of a glass–epoxy (G–E) composite, filled with both silicon carbide (SiC_p) and graphite (Gr), were studied using a pin-on-disc test apparatus. The specific wear rate was determined as a function of sliding velocity, applied load and sliding distance. The laminates were fabricated by the hand lay-up technique. The volume percentage of filler materials in the composite was varied, silicon carbide was varied from 5 to 10% whereas graphite was kept constant at 5%. The excellent wear resistance was obtained with glass–epoxy containing fillers. The transfer film formed on the counter surface was confirmed to be effective in improving the wear characteristics of filled G–E composites. The influence of applied load is more on specific wear rate compared to the other two wear parameters. The worn surfaces of composites were examined with scanning electron microscopy (SEM) to investigate the probable wear mechanisms. It was found that in the early stage of wear, the fillers contribution is significant. The process of transfer film, debris formation and fiber breakage accounts for the wear at much later stages.     

Effect of Tribo-Oxidation on Friction and Wear Behaviour of HVOF Sprayed WC–10Co–4Cr Coating

Tribology Letters - In current investigation, tribological properties of TiAl matrix composite reinforced with 15 vol%Ti2AlN and TiAl alloy prepared through in situ reactive method of...     

Wear behaviour of laser cladded and plasma sprayed WCCo coatings

The usefulness of WCCo cermets as wear resistant material for coatings is determined by the cladding technique employed. This paper compares the features of an 83% WCCo coating on an AISI 1043 steel substrate using two different application techniques: plasma spraying and laser cladding. Results show significantly less porosity, improved coating hardness and better layer-substrate adherence in laser cladded than in plasma sprayed coatings. This causes them to have different wear behaviour which was determined using a method developed on the basis of the PV factor theory using sliding linear contact of flat-cylinder type. The method proved that wear rate (V_d′) is directly proportional to the product of coefficient of friction (μ), load (C) and applied speed (V), V_d′ = KμCV, where proportionality constant, K, is different for every material and depends on conditions such as lubrication, temperature, etc. To study wear behaviour, laser cladded and plasma sprayed 83% WC-Co coatings, under extreme lubrication, were placed against a hardened and tempered AISI 1043 steel, at different load and sliding speed rates. As a result constant K was estimated for each coating. The tests also showed that wear rate in laser deposited coatings is approximately 34% lower than in plasma sprayed coatings.     

I–V curve characteristics of solar cells on composite substrate under mechanical loading

This research is aimed to conduct a characterization of I–V (current-voltage) curve changes in order to interpret the effects of physical defects such as microcracks of solar cells installed over structures to which mechanical load is applied on the power generation performance of solar cells, as well as to conduct fractography to interpret causes for reducing the power generation performance of the cells. To accomplish this, tensile specimens to which a mechanical load would be applied were produced using composite materials, which are representative light materials and solar cells that were attached to the specimens using the adhesive property of EVA film. In the experiment, mono-crystalline silicon solar cells were used, which are breakable and whose efficiency is approximately 24.2% (lab.). Also, in order to evaluate the power generation performance of solar cells under mechanical loading in real-time, a measuring device was designed to compare and evaluate the time point and property of fracture under mechanical loading. Moreover, fractography was conducted to analyze and consider causes for reducing the efficiency of solar cells. As a result of the experiment, it was found that the mechanical load applied to solar cells caused cracks to the surface and interior of the cells, which in turn reduced the I_sc value and the V_oc value of the I–V curve. The present study provides an initial set of valuation parameters in the maintenance and management of installed solar cells when applying solar cell technology to dynamic structures.     

Experimental investigation and optimization of EDM process parameters for machining of aluminum boron carbide (Al–B4C) composite

This study investigates the effect of electric discharge machining (EDM) process parameters [current, pulse-on time (T_on), pulse-off time (T_off) and electrode material] on material removal rate (MRR), electrode wear rate (EWR) and surface roughness (SR) during machining of aluminum boron carbide (Al–B₄C) composite. This article also summarizes a brief literature review related to aluminum metal matrix composites (Al-MMCs) based on different process and response parameters, work and tool material along with their sizes, dielectric fluid and different optimization techniques used. The MMC used in the present work is stir casted using 5% (wt) B₄C particles of 50 micron size in Al 6061 metal matrix. Taguchi technique is used for the design of experiments (L₉-orthogonal array), while the experimental results are analyzed using analysis of variance (ANOVA). Response table for average value of MRR, EWR and SR shows that current is the most significant factor for MRR and SR, while electrode material is most important for EWR. ANOVA also confirms similar results. It is also observed that the optimum level of process parameters for maximum MRR is A₃B₁C₃D₃, for minimum EWR is A₁B₂C₃D₁, and for SR is A₁B₃C₃D₃.     

Effect of 10 wt% VC on the Friction and Sliding Wear of Spark Plasma–Sintered WC–12 wt% Co Cemented Carbides

The effect of 10 wt% VC addition on the friction and sliding wear response of WC–12 wt% Co cemented carbides produced by spark plasma sintering (SPS) was studied. The SPS of WC–12 wt% Co alloys with and without 10 wt% VC, at 1100 and 1130°C, respectively, yielded dense materials with minimal porosity. No eta phase was found in any of the alloys. The WC–12 wt% Co–10 wt% VC alloy showed the formation of a hard WV₄C₅ phase, which improved the alloy's hardness. Friction and dry sliding wear tests were done using a ball-on-disk configuration under an applied load of 10 N and sliding speed of 0.26 m.s^?1, and a 100Cr-steel ball was used as the counterface. A significant improvement in the sliding wear response of the harder and more fracture tough WC–12 wt% Co–10 wt% VC alloy compared to the WC–12 wt% Co alloy was found. Analysis of the worn surfaces by scanning electron microscopy showed that the wear mechanisms included plastic deformation, preferential binder removal, adhesion, and carbide grain cracking and fragmentation.     

Microstructural and abrasive characteristics of high carbon Fe–Cr–C hardfacing alloy

A series of high carbon Fe–Cr–C hardfacing alloys were produced by gas tungsten arc welding (GTAW). Chromium and graphite alloy fillers were used to deposit hardfacing alloys on ASTM A36 steel substrates. Depending on the four different graphite additions in these alloy fillers, this research produced hypereutectic microstructures of Fe–Cr phase and (Cr,Fe)₇C₃ carbides on hard-facing alloys. The microstructural results indicated that primary (Cr,Fe)₇C₃ carbides and eutectic colonies of [Cr–Fe+(Cr,Fe)₇C₃] existed in hardfacing alloys. With increasing the C contents of the hardfacing alloys, the fraction of primary (Cr,Fe)₇C₃ carbides increased and their size decreased. The hardness of hardfacing alloys increased with fraction of primary (Cr.Fe)₇C₃ carbides. Regarding the abrasive characteristics, the wear resistance of hardfacing alloys were related to the fraction of primary (Cr,Fe)₇C₃ carbides. The wear mechanism was also dominated by the fraction of primary (Cr,Fe)₇C₃ carbides. Fewer primary carbides resulted in continuous scratches worn on the surface of hardfacing alloy. In addition, the formation of craters resulted from the fracture of carbides. However, the scratches became discontinuous with increasing fraction of the carbides. More primary carbides can effectively prevent the eutectic colonies from the damage of abrasive particles.     

Friction and wear behaviour of plasma sprayed WC–12Co coating sliding against Si3N4 under lubrication of ionic liquids

Abstract

The friction and wear behaviour of a WC–12Co coating prepared by plasma spraying sliding against a Si₃N₄ ceramic ball, under the lubrication of liquid paraffin and ionic liquids 1-methyl-3-butylimidazolium hexafluorophosphate and 1-methyl-3-hexylimidazolium hexafluorophosphate at room temperature, was investigated using an SRV tester. The morphology and elemental distribution of the worn coating surfaces were characterised by means of scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analyser (EDXA) attachment, and the chemical state of typical elements in the boundary lubricating film on the worn coating surface was analysed by means of X-ray photoelectron spectroscopy (XPS). The SEM/EDXA analysis shows that phosphorus is uniformly distributed on the worn coating surface lubricated by ionic liquids. The XPS results also indicate that the boundary lubricating film is mainly composed of CoF₂ and PF_x and the tribochemical reaction products contribute to reducing the friction and wear of the plasma sprayed WC–12Co coating.     

Dry Reciprocating Sliding Friction and Wear Response of WC–Ni Cemented Carbides

A number of WC–Ni based cemented carbide grades with distinctive binder contents were tested with the goal to evaluate their dry reciprocating sliding friction and wear behaviour against WC–6 wt.%Co cemented carbide using a Plint TE77 tribometer and distinctive normal contact loads. The generated wear tracks were analysed by scanning electron microscopy and quantified volumetrically using surface scanning topography. The experimental results revealed one WC–Ni grade with superior wear performance.