Plastic deformation mechanisms in tungsten carbide

Transmission electron microscopy studies of dislocations formed by plastic deformation in tungsten carbide have confirmed a slip deformation mechanism involving the {1 0 ¯1 0} 0 0 0 1 system. Direct visual evidence also confirmed the existence of extended dislocations formed by a suggested dissociation 1/31 1 ¯2 3=1/62 0 ¯2 3+1/60 2 ¯2 3 which can further interact with other extended dislocations on pairs of intersecting {1 1 ¯2 2} pyramidal planes. The proposed interaction is suggested as a means of reducing both dislocation strain energy and atomic misfit strains due to antiphase boundary formation and leads to a form of sessile dislocation arrangement similar to the Lomer Cottrell lock.     

Coarsening of hafnium carbide particles in tungsten

The coarsening behaviour of finely dispersed HfC particles in a W-HfC alloy was investigated by monitoring the growth rate of the particles. An activation energy of 480 kJ mol^–1 was obtained for the process. Diffusion experiments of hafnium in tungsten were conducted at temperatures between 1773 and 2573 K using a secondary ion mass spectroscopy technique to determine the diffusion contribution to the coarsening process. The diffusion process at high temperature is controlled by lattice diffusion with an activation energy of 335 kJ mol^–1 whereas that at low temperature is governed by grain-boundary diffusion with an activation energy of 170 kJ mol^–1. It appears that the coarsening process is controlled by two energy barriers: one dictated by the diffusivity of hafnium and the other by the solubility limit as a function of temperature. The strain energy required to dissociate the carbide particles into individual species was also considered. The effects of the coarsening of HfC particles in a dispersion-strengthened W-0.4 mol% HfC alloy on recrystallization and creep deformation were illustrated using a concerted experimental modelling analysis. Results show that the strengthening effect of the HfC particles is significantly reduced at temperatures above 1800 K, due to particle coarsening.     

Stress induced cavitation in cobalt bonded tungsten carbide

Tensile stress-strain curves of three grades of Co-bonded WC composite have been obtained near 1200° C. Extensive void formation is observed in specimens extended to failure. A qualitative model for deformation based on atomic migration of Co in the bonding Co phase is proposed to account for the observations.     

Coatings made of tungsten carbide and tantalum carbide for machining tools

Continuous evolution of cutting parameters has been a constant requirement for the manufacture of machining tools. This has promoted the development of new technologies to reduce the production time as well as cost without affecting the competitiveness. In the present work, high speed tools have been coated with WC and TaC by means of chemical vapour deposition and tested in a high speed milling machine. Damages on the tips of these cutting tools were analysed by scanning electron microscope. These observations show that the coating is heterogeneous and affected by the type and composition of various phases formed. Results indicate that the coating made from WC is more highly resistant to attrition than that made of TaC. Surfaces that have been machined by the coated tools are evaluated by means of the average roughness parameter     

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

Fracture analysis of cobalt-bonded tungsten carbide composites

Specimens of WC-Co were indented to measure the resulting crack size and unindented samples were fractured in three-point flexure to obtain strength and to measure characteristic features on the fracture surface. Fracture toughness was determined using indentation techniques and compared to those determined using fractography. We show that principles of fracture mechanics can be applied to WC-Co composites and can be used to analyse the fracture process.The fracture surfaces were examined by scanning electron microscopy and optical microscopy. Characteristic features observed in glasses, single crystals and polycrystalline materials known as mirror, mist, hackle and crack branching were identified for these composites. We discuss the importance of fracture surface analysis in determining the fracture origins and in the failure analysis of WC-Co composites.     

Crack branching and fracture mirrors in cemented tungsten carbide

Fracture mirrors are investigated in several WC-Co grades with the aim of determining crack branching criteria. The size of the mirrors is found to increase with increasing cobalt content and with decreasing carbide grain size. The results are explained in terms of the dependence on cobalt content and grain size of the free surface energy and the elastic strain at fracture.     

Microstructure and properties of CVD tungsten carbide from tungsten hexafluoride and dimethyl ether

Tungsten carbide was deposited from tungsten hexafluoride, dimethyl ether, and hydrogen using a horizontal, cold-wall reactor. The effects of substrate temperature, reactor pressure, and reagent ratio on the coating growth rate, morphology, composition, and microhardness were studied. Under most conditions, the solid deposit was primarily W₃C with minor amounts of W. The tungsten carbide growth rate data fit an Arrhenius rate expression for temperatures from 425 to 550°C and had an activation energy of 24kcal/mol at 70mmHg total pressure and a WF₆/DME ratio of 6.3. A variety of surface morphologies and microstructures were observed. The microhardness of the coated substrates increased with coating thickness to a maximum value of 2400kg/mm².     

Enhancement in the microhardness of arc plasma melted tungsten carbide

Microhardness is found to increase significantly in arc plasma melted tungsten carbide. To understand the mechanism for such increase, tungsten carbide powder was mixed with tungsten metal powder to prepare mixtures of seven different compositions. The mixtures with varying WC/W ratio were pelletized and melted in an arc plasma followed by cooling in the furnace. It is observed that microhardness value enhances in the product when WC/W₂C ratio becomes high. Based on our microstructural finding of <100> WC hard faces and lamellar/acicular structures (due to martensite transformation) carried out by XRD, optical microscope and SEM, an attempt has been made to understand the reason behind the enhancement in microhardness.     

The formation of tungsten carbide in cobalt-base wear-resistant coating alloys

Tungsten carbide-cobalt alloys cannot be produced by melting because of a peritectic reaction in the W-C system; they are produced by sintering. Tungsten carbide-cobalt powders (mixed, agglomerated, sintered) can be plasma sprayed or deposited by other techniques but they cannot be fused afterwards without decomposition of the tungsten monocarbide that provides the best mechanical properties for many applications.Wear-resistant cobalt alloys were developed 60 years ago and are based on cobalt-chromium-tungsten-carbon. During studies of the CoCrWC system with various carbon concentrations and at various temperatures we identified MC, M₂C, M₇C₃, M₂₃C₆, M₆C, M₁₂C and M₂₈C carbides. The solidifying M₆C carbide is unstable over a certain concentration range of chromium and decomposes to form tungsten carbide (WC). On heat treatment the tungsten-containing M₆C forms WC in a cobalt-chromium matrix if the chromium content is less than 5 wt.%. It is therefore possible to produce a sprayed and fused or welded layer of WC-cobalt alloy. The rate of WC formation depends on temperature and time. Time-temperature-decomposition diagrams have been established for four alloys. The structures of the heat-treated alloys are similar to those of sintered tungsten carbide-cobalt alloys.     

Dissimilar laser brazing of boron nitride and tungsten carbide

Using laser brazing process, we made the dissimilar joint of the boron nitride and tungsten carbide with an excellent functionality. In order to investigate the characteristic of joint, observation and structural analysis of its interface by the electron probe micro-analysis (EPMA) and the scanning acoustic microscopy were performed. The wetting property between h-BN and Ag–Cu–Ti braze was excellent, therefore no gaps were seen between them. Moreover, it was suggested that the Ti element, which is the active ingredient in Ag–Cu–Ti braze, reacted with N in h-BN to generate Ti–N composite phase as an interfacial precipitation and the generation of Ti–N composite phase was affected by the concentration of Ti in Ag–Cu–Ti braze. All fracture was formed in h-BN body near the interface and it seemed that the distribution of shear strength of 1.25%Ti to 1.68%Ti was within the margin of variation of bulk strength of h-BN.     

纳米碳化钨粉体的热稳定性研究

以工业化生产设备条件制备纳米碳化钨粉末,对样品在不同温度下退火,采用X射线衍射法(XRD)、比表面积法(BET)、冷场发射扫描电子显微镜(FESEM)、透射电子显微镜(TEM)、激光光散射法对退火前后样品的物相、粒度及其分布进行表征.研究结果表明,温度是影响纳米WC颗粒长大的一个重要因素,随着退火温度升高,纳米WC颗粒尺寸随之增大.同时在不同升温速率下测的DSC吸收峰值,计算了纳米WC颗粒的长大激活能.在此基础上,对实验现象产生的原因进行了分析讨论.     

Effects of lower cobalt binder concentrations in sintering of tungsten carbide

Cemented tungsten carbides have received much attention because of their superior characteristics. Traditional cemented tungsten carbides usually contain 3–30 wt% binder phase. In this paper, WC with low Co concentration less than 3 wt% is studied using traditional powder metallurgy. The binder phase has tremendous effect on sinterability of WC. High sinterability and high hardness can be achieved for the WC (0.7 μm) with 0.5 wt% Co. Abnormal grain growth (AGG) is often observed in sintering WC with small amount of Co. It seems that AGG is affected by the concentration of Co and a range of Co concentrations may exist for the large amount of AGG. To control the grain size, VC is added to inhibit the grain growth of WC. It is observed that the hardness is affected by the amount of addition of VC. Controlling the ratio of C/W less than unity at low Co concentrations will result in the production of W₂C phase. The hardness of WC–Co is affected by the amount of W₂C phase in the sample and W₂C is stable during the normal cooling process.     

Carbon,carbide and silicide coatings

The elements carbon and silicon are the significant constituents of the refractory coating materials discussed in this paper, namely B₄C, pyrocarbon, TiC, SiC, Si₃N₄ and MoSi₂. Because of their hardness and their chemical and oxidation resistance, these materials are of increasing interest as coatings on various substrates. However, their physical and chemical properties limit the number of suitable plating methods. Whilst the CVD technique is the most versatile, the sintering technique is a good alternative, especially if thicker coatings are required.The different coating materials are discussed in detail. The ways in which different coatings can be modified for special applications, e.g. isotropic pyrocarbon coatings for bioengineering purposes, are discussed. Pure silicon carbide and silicon nitride coatings become important as protection against internal oxidation and as barriers in carbon fibre reinforced composites.The problems arising from the simultaneous deposition of two elements to form a specific compound are demonstrated with the examples of SiC, Si₃N₄ and B₄C. The necessity of avoiding chemical reactions with the substrate during deposition is exemplified by the case of TiC coatings on carbon fibres.Finally the technique of preparing metal silicide coatings on refractory metals and carbon by plasma spraying and isostatic hot pressing as protection against oxidation is explained, and the modification of the coatings, in particular to improve thermal cycling behaviour, is discussed.     

Carbon fiber reinforced hafnium carbide composite

Hafnium carbide is proposed as a structural material for aerospace applications at ultra high temperatures. The chemical vapor deposition technique was used as a method to produce monolithic hafnium carbide (HfC) and tantalum carbide (TaC). The microstructure of HfC and TaC were studied using analytical techniques. The addition of tantalum carbide (TaC) in the HfC matrix was studied to improve the microstructure. The microstructure of HfC, TaC and co-deposited hafnium carbide-tantalum carbide (HfC/TaC) were comparable and consisted of large columnar grains. Two major problems associated with HfC, TaC, and HfC/TaC as a monolithic are lack of damage tolerance (toughness) and insufficient strength at very high temperatures. A carbon fiber reinforced HfC matrix composite has been developed to promote graceful failure using a pyrolytic graphite interface between the reinforcement and the matrix. The advantages of using carbon fiber reinforcement with a pyrolytic graphite interface are reflected in superior strain capability reaching up to 2%. The tensile strength of the composite was 26 MPa and needs further improvement. Heat treatment of the composite showed that HfC did not undergo any phase transformations and that the phases comprising composite were are thermochemically compatible.     

Diamond chemical vapour deposition on seeded cemented tungsten carbide substrates

Diamond particles were deposited onto seeded cemented tungsten carbide (WC-Co) substrates using conventional hot-filament chemical vapour deposition (HFCVD) and time-modulated CVD (TMCVD) processes. The substrates were pre-seeded ultrasonically with diamond particles of different grit sizes. In this investigation, we employ timed methane (CH₄) gas modulations, which are an integral part of our TMCVD process in order to enhance diamond nucleation density. During diamond deposition using the conventional HFCVD process, methane gas flow was maintained constant. The total hydrogen flow into the reactor during TMCVD process was higher than in the HFCVD process. Hydrogen etching can be expectedly more prominent in the TMCVD process than in HFCVD of diamond particles. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results showed that a proper selection of the diamond grit size for seeding using ultrasounds can lead to enhancement in the nucleation density values of about two orders of magnitude (10⁷ to 10⁹ cm^− 2). The TMCVD process using the different seeded substrates can result in high nucleation density values of up to 10¹⁰ cm^− 2.     

Machining and morphological evaluation of diamond coated tungsten carbide drills

This paper reports the results of drilling tests on various workpiece materials using diamond coated tungsten carbide drills. The performance of the coated drills were compared with uncoated control drills. Hot filament chemical vapor deposition was used for coating the pre-treated drills and film coating morphology and stress characteristics were studied prior to drilling. Forces and torques were measured during drilling and the results indicate catastrophic failure and short tool-life of the coated drills for all workpiece materials. Drill failure reasons are attributed to crystal clustering and point loading of the cutting edges. Further research issues are also identified.     

R.F. magnetron sputtered tungsten carbide thin films

Thin films of tungsten carbide have been deposited on stainless steel substrates held at 500°C by r.f. reactive magnetron sputtering in two different modes of introducing argon and acetylene gases called normal and high rate mode. A single phase fcc-WC is formed in the normal mode whereas a mixture of A-15-W₃C, hexagonal-WC and graphitic- and diamond-carbon is found in the high rate mode. A microhardness value as high as 3200 kgf/mm² (as compared to the bulk value of 1800 kgf/mm²) is obtained in the film deposited by normal mode.