Fracture toughness of coated TiCN–WC–Co cermets with graded composition

Mixed TiCN–WC–Co cermets are developed to improve at the same time toughness and resistance to deformation of materials for cutting tool applications. Moreover, graded materials joining optimum properties according to the functional part of the tool are elaborated. To this end, TiCN–WC–Co cermets are interesting because they develop a WC–Co layer at the surface during the sintering. This tough layer at the surface limits the crack propagation that can lead to the rupture of the tool. Such materials show a good resistance to the deformation in the bulk and a good toughness at the surface, where the cracks are initiated upon machining. Cutting tools are often coated by CVD to improve the wear resistance. This paper proposes a method to measure the toughness K_IC at high temperature by using this CVD coating for initial crack formation. The coating thickness is the precrack length of traditional K_IC measurements. Samples are fractured by three point bend tests. The rupture stress is measured by Weibull statistics. This method is particularly interesting for graded structure materials where the influence of surface layers on toughness must be estimated. The comparison between cermets with and without WC–Co layer shows an improvement of 28% of the toughness when the layer is present. The possible bias of internal stresses on the results is discussed.     

Mechanical properties of WC–10Co cemented carbides sintered from nanocrystalline spray conversion processed powders

Mechanical properties and microstructures of nanocrystalline WC–10Co cemented carbides were investigated. The nanocrystalline WC–10Co cemented carbide powders were manufactured by reduction and carbonization of the nanocrystalline precursor powders which were prepared by spray drying process of solution containing ammonia meta-tungstate (AMT) and cobalt nitrate. The WC powders were about 100 nm in diameter mixed homogeneously with Co binder phase and were sintered at 1375 °C under a pressure of 1 mTorr. In order to compare the microstructures and mechanical properties with those of nanocrystalline WC–10Co, commercial WC powders in a diameter range of 0.57–4 μm were mixed with Co powders, and were sintered at the same conditions as those of nanocrystalline powders. TaC, Cr₃C₂ and VC of varying amount were added into nanocrystalline WC–10Co cemented carbides as grain growth inhibitors. To investigate the microstructure of Co binder phase in the WC–10Co cemented carbides, Co–W–C alloy was fabricated at the temperature of sintering process for the WC–10Co cemented carbides. The hardness of WC–10Co cemented carbides increased with decreasing WC grain size following a Hall–Petch-type relationship. The fracture toughness of WC–10Co cemented carbides increases with increasing HCP/FCC ratio of Co binder phase by HCP/FCC phase transformation.     

Study of the mechanical properties of TiCN–WC–CO hardmetals by the interpretation of internal friction spectra

TiCN–WC–Mo–Co mixed carbide hardmetals have an interesting application potential for cutting tool fabrication combining the high toughness of WC–Co with the resistance to plastic deformation of TiCN–Co cermets. Mechanical spectroscopy (MS) is used in order to separate the effects of the constituents on the mechanical properties. Internal friction (IF) spectra are measured in a torsion pendulum on WC–TiCN–Mo–Co samples where TiCN/WC ratio is varied as well as the Co content. Six components of the characteristic IF spectrum of WC–TiCN–Mo–Co have been identified and interpreted. Two peaks are located in the cobalt, two peaks in the TiCN phase and two peaks in the ceramic grain boundaries. Four temperature domains are defined depending on the mechanical behaviour: brittle (I), anelastic (IIa), limited plasticity (IIb) and extended plasticity (III). The anelastic domain is characterized by the bulk deformation of cobalt. In the limited plasticity domain, both cobalt and TiCN are deformed by dislocation movement. The high temperature extended plasticity should be attributed to grain boundary sliding in the ceramic phase (mainly WC) enhanced by cobalt diffusion in the grain boundaries.     

An experimental study of the sintering of nanocrystalline WC–Co powders

Since nanocrystalline WC–Co powder was produced over a decade ago, the sintering of nanocrystalline powders remains a technological challenge. The goal of sintering nanocrystalline powders is not only to achieve full densification but also to retain nanocrystalline grain sizes. This is difficult because of rapid grain growth at high temperatures. Previous studies on the sintering of nanocrystalline WC–Co have shown that grains grow rapidly during the early stage of sintering. But there are few studies on the mechanisms of grain growth and densification during this stage. This paper presents the results of an experimental investigation on grain growth and densification of nanocrystalline WC–Co powders during heat-up at equilibrium solid-state temperatures. The results have shown that nanocrystalline WC grains grow rapidly during this period concurrently with rapid densification. The rapid densification and grain growth are partially attributed to the surface energy anisotropy of tungsten carbide. The effects of vanadium carbide on grain growth at solid state during heat-up are also discussed.     

On the influence of binder content and binder composition on the mechanical properties of hardmetals

In order to achieve an understanding of damage mechanisms and the role of the binder phase in the fatigue behaviour of hardmetals, two series of hardmetals with 7, 15 and 27 wt% of Co and CoNiFe binder phases were investigated in a reversed bending stress apparatus. Microstructural investigations were carried out using SEM and TEM. Finite element simulations based on real microstructure cut-outs were used to analyse the stress and strain states. While both series of hardmetals exhibit inert strengths which depend strongly on the binder content, their fatigue behaviour is totally different. For the WC–Co grades, no significant influence of the binder content on the fatigue behaviour was found. In contrast to this, the fatigue behaviour of the WC–CoNiFe grades showed a clear dependence on the binder content. The microstructural investigations revealed that the mechanism of stacking fault formation and phase transformation in the Co binder phase, together with a stress concentration in the binder pools are responsible for the fatigue behaviour of the WC–Co grades. In WC–CoNiFe grades, no stacking faults or transformed zones were observed. In addition, the CoNiFe binder phase reveals a very ductile flow nature, which results in large plastically deformed zones in the binder phase.     

Semi-empirical relationship between the hardness, grain size and mean free path of WC–Co

The Vickers hardness of several well-characterized grades of WC–Co (23 in total) was measured. The mean grain size of these samples ranged from 0.6 to 5.0 μm and the cobalt content from 6 to 50 wt%. An empirical formula between hardness of WC–Co, grain size of WC and mean free path in Co was obtained. It was found that the empirical formula fitted our measured hardness well. However, when used against results of other researchers, it did not reproduce them satisfactorily at values higher than 1500 HV. A theoretical model, based on the assumption that plastic deformation of WC–Co begins at the WC–WC necks and that the mean length of the cross-section of the WC–Co necks is proportional to the square root of the mean WC grain size, was subsequently derived. The results obtained from this model were in good agreement with those of the empirical formula and like the empirical formula, did not reproduce high hardness values of other researchers. Thus, the model was modified by introducing semi-empirical terms which led to a satisfactory fitting of the data.     

Fundamentals of liquid phase sintering for modern cermets and functionally graded cemented carbonitrides (FGCC)

Metallurgical reactions and microstructure developments during sintering of modern cermets and functionally graded cemented carbonitrides (FGCC) were investigated by modern analytical methods such as mass spectrometer (MS), differential thermal analysis (DTA), differential scanning calorimeter (DSC), dilatometer (DIL), microscopy and analytical electronic microscopy with energy dispersive spectrometer (EDS). The complex phase reactions and phase equilibria in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni were studied. The melting behavior models in the systems of TiC–WC/MoC–Ni/Co, TiC–TiN–WC–Co and TiCN–TaC–WC–Co have been established. By an in-depth understanding of the mechanisms that govern the sintering processing and metallurgical reactions, new cermets and different types of FGCC with desired microstructures and properties were developed.     

Determination of the composition range suitable to the formation of WC–(V,W)Cx–Co materials

The phase equilibria of the system W–V–C–Co are determined experimentally in the composition range corresponding to small additions of VC_x in the cemented carbides WC–Co and to larger VC_x contents in the hard materials WC–(V,W)C_x–Co. The results are obtained from the microanalysis of the phase compositions in samples heat-treated at 1450 and 1200 °C. Two four-phase fields, {Cosolution+WC+(V,W)C_x+C} and {Cosolution+WC+(V, W)C_x+η}, are determined. The narrow domain between the two four-phase fields defines the composition range suitable to the formation of the hard materials WC–(V,W)C_x–Co. Some interfacial features important for the microstructure of the materials are underlined.     

Grain size measurement methods and models for nanograined WC–Co

Development of nanograined WC–Co presents challenges for grain size measurement. In this study, standard and nanocrystalline WC–Co powders are processed by conventional and spark sintering. The resulting microstructures are characterized by image analysis of scanning electron microscopy micrographs, X-ray line broadening, and magnetic coercivity. The results are analyzed and a property map relating coercivity to WC grain size is developed. Equations for interface-controlled grain growth are transformed into the master sintering curve form and are used to analyze the grain size data from the three measurement techniques.     

Evaluation of WC–17Co and WC–10Co–4Cr thermal spray coatings by HVOF on the fatigue and corrosion strength of AISI 4340 steel

It is known that chromium electroplating is related to the reduction in the fatigue strength of base metal. However, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. Environmental requirements are an important point to be considered in the search for possible alternatives to hard chrome plating. Aircraft landing gear manufactures are considering WC thermal spray coating applied by the high-velocity oxygen-fuel (HVOF) process an alternative candidate, which shows performance at least comparable to results, obtained for hard chrome plating. The aim of this study is to compare the influence of WC–17Co and WC–10Co–4Cr coatings applied by HVOF process and hard chromium electroplating on the fatigue strength of AISI 4340 steel, with and without shot peening. S–N curves were obtained in axial fatigue test for base material, chromium plated and tungsten carbide coated specimens. Tungsten carbide thermal spray coating results in higher fatigue strength when compared to hard chromium electroplated. Shot peening prior to thermal spraying showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel. Experimental data showed higher axial fatigue and corrosion resistance in salt fog exposure for samples WC–10Co–4Cr HVOF coated when compared with WC–17Co. Fracture surface analysis by scanning electron microscopy (SEM) indicated the existence of a uniform coverage of nearly all substrates.     

The temperature ranges for maximum effectiveness of grain growth inhibitors in WC–Co alloys

In order to more effectively control the WC grain growth during liquid phase sintering of submicron WC–Co alloys, the temperature ranges where common grain growth inhibitors are most active have been determined. In the study, small additions (0.5% by weight) of TiC, TaC, NbC, VC, and Cr₃C₂ were added to WC–6Co and WC–10Co alloys. In some cases, a small amount of coarse WC powder was added to simulate the effect of discontinuous grain growth. The powder compacts were vacuum sintered at temperatures ranging from 1300 to 1575 °C. The magnetic coercivity and microstructure results indicated that VC is equally effective in limiting continuous grain growth for all temperatures within the temperature range while the other inhibitors become more effective with temperature. VC was also found to be the most effective inhibitor of discontinuous grain growth. The results of the investigation are expressed in terms of the effectiveness of grain growth inhibition as determined by microstructural analysis, magnetic coercivity and hardness.     

Rapid sintering of ultrafine WC–Ni cermets

Using high-frequency induction-heated sintering (HFIHS), cermets of WC–xNi (x = 8, 10, and 12 wt.%) were consolidated to a density of about 98%. The sintered samples had WC grain size of 300 nm. The hardness was significantly higher than that obtained with conventionally sintered WC–Co and WC–Ni cermets without a reduction in fracture toughness.     

Effect of WC particle size on the microstructure, mechanical properties and fracture behavior of WC–(W, Ti, Ta) C–6 wt% Co cemented carbides

This study deals with the microstructure and mechanical properties of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides fabricated by conventional sintering. The conventional WC particles of 4 μm size and ultrafine particles of 0.2 μm were introduced in the system with varying ratio. The ratios of conventional WC particles to ultrafine WC particles were 2:1, 1:1, and 1:2. The microstructures of sintered WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides were sensitively dependent on the ratio of conventional WC particles to ultrafine WC particles. The rim phase increased with the increase in the amount of ultrafine particles. Hardness of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbide increased with increase in the amount of rim phase and decrease in the average grain size of WC particles. The bending strength showed the similar trend of the hardness. The fracture morphologies are reported. The fracture behavior changed from mixed mode to transgranular fracture mode, when the ratio of conventional WC particles to ultrafine WC particles was changed from 2:1 to 1:2.     

A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing sintering

Novel cemented carbides (W_0.4Al_0.6)C_0.5–Co with different cobalt contents were prepared by mechanical alloying and hot-pressing technique. Hot-pressing technique as a common technique was performed to fabricate the bulk bodies of the hard alloys. The novel cemented carbides have good mechanical properties compared with WC–Co. The density and operation cost of the novel material were much lower than the WC–Co system. It was easy to process submicroscale sintering with the novel materials and obtain the rounded particles in the bulk materials. There is almost no η-phase in the (W_0.4Al_0.6)C_0.5–Co cemented carbides system although the carbon deficient obtains the astonishing value of 50%.     

The influence of carbon content on formation of carbo-nitride free surface layers in cemented carbides

To increase crack propagation resistance in cemented carbide cutting tools, it is sometimes of interest to create tough surface zones in the substrates. A way to do this is to use so-called gradient sintering in the manufacturing of the cutting tool. In this sintering process a nitrogen and titanium containing cemented carbide is sintered in a nitrogen free atmosphere. The difference in nitrogen activity between atmosphere and cutting tool during sintering will create an outward nitrogen diffusion. Due to thermodynamical coupling between nitrogen and titanium, this gives rise to an inward titanium diffusion, which creates a surface zone depleted of hard cubic carbo-nitrides, and enriched in ductile binder phase. By varying the carbon content of the material, the nitrogen activity is affected, and this in turn affects the surface zone formation.

In this report, Ti(C,N)–(Ti,W)C–WC–Co, Ti(C,N)–NbC–WC–Co, and Ti(C,N)–TaC–WC–Co cemented carbides were studied. All three materials were produced in series with varied carbon content, in order to study the effect of carbon on gradient surface zone formation.     

A novel route to prepare ultrafine-grained WC–Co cemented carbides

A new way of synthesizing pure WC–Co composite powder by in situ reduction and carbonization reactions of WO_2.9, Co₃O₄ and carbon black powders is presented. The ultrafine-grained cemented carbide bulk has been prepared by spark plasma sintering (SPS) the composite powder. The preparation process, microstructure, and properties of the composite powder and the bulk were investigated and characterized. Both the temperature and time in the present method of combing reduction and carbonization reactions and SPS technique are apparently reduced as compared with the conventional methods. The resultant WC–Co bulk shows a homogeneous fine-grain microstructure and good combined mechanical properties.     

Picosecond laser machining of tungsten carbide

Hard materials such as tungsten carbide (WC) are extensively used in cutting tools in high-value manufacturing, and the machining of these materials with sufficient speed and quality is essential to exploit their full potential. Over the last two decades, short (nanosecond (ns)) and ultra-short (picosecond (ps); femtosecond (fs)) pulse laser machining has been evaluated by various researchers and proposed as an alternative to the current state-of-the-art machining techniques for advanced materials like WC, which include mechanical grinding and electrical discharge machining. However, most of the established/existing research on this topic is based on low power lasers, which may not be adopted in industrial production environments due to its low material removal rate. This paper presents the results of a fundamental study, on using a 300 W picosecond laser for the deep machining of tungsten carbide. The influence of various laser parameters on the geometric precision and quality (surface and sub-surface) of the ablated area was analysed, and the ablation mechanism is discussed in detail. Laser pulse frequency and scanning speed have minimal effect on ablation rate at high power levels. The surface roughness of the ablated area increases with the ablation depth. At optimal conditions, no significant thermal defects such as a recast layer, micro crack or heat affected zone were observed, even at a high average power of 300 W. The material removal rate (MRR) seems to be proportional to the average power of the laser, and a removal rate of around 45 mm³ per minute can be achieved at 300 W power level. Edge wall taper appears to be a significant issue that needs to be resolved to enable industrial exploitation of high power ultra-short pulse lasers.     

WC–Co substrate surface pretreatments with aluminum compounds prior to polycrystalline CVD diamond deposition

Without pretreatment, the adherence of polycrystalline chemical vapour deposition (CVD) diamond films on WC–Co substrates is rather poor due to Co presence in the binder phase. Therefore several attempts to immobilize the Co on the substrate surface have already been investigated. However, results are not fully satisfying. In this paper, solutions or suspensions of various Al compounds have been applied to the WC–Co surface to form stable Co/Al compounds before diamond deposition. By means of SIMS depth profile analysis it is shown that surface pretreatment with Al(OC₂H₅)₃/conc. CH₃COOH or Al(NO₃)₃ leads to the formation of Co/Al compounds which suppress the Co mobility resulting in improved adherence of the diamond layer. Evidence for Co/Al compounds has also been found after pretreatment with Al(OH)₂–OOCCH₃ in neutral solution, although the quality of the diamond crystals was changed to ballas. The use of Al(OH)₂–OOCCH₃ in a basic as well as an acid medium produced no Co/Al reaction.     

Thermal stress analysis of HVOF sprayed WC–Co/NiAl multilayer coatings on stainless steel substrate using finite element methods

The scope of this study is to find out the effects of thermal cycling on the coating-substrate system of WC–Co coatings by finite element modeling. With this regard, WC–Co/NiAl coating layers were successfully deposited on 316 L stainless steel substrates by using a HVOF technique and microstructural observations were carried out using SEM. The SEM study revealed that the coating was very dense with very low oxide content and had a very good contact with the substrate, indicating a very good bonding to the substrate. Thermal cycling tests were performed at the temperature range of 373 and 873 °K without external load. In finite element modeling (FEM), thermal residual stresses, developed during and after thermal cycling, were determined by using ANSYS software package. It was found that the stress distributions were obtained in the WC–Co/NiAl architectured coating systems during heating and cooling steps because of the different thermal and mechanical properties of the coating layers and substrates. According to thermal analysis results, the calculated tensile stresses were higher than the compressive stresses and also thermal stress components for x-direction were bigger than for y-direction.