Microstructure and tribological performance of NbC-Ni cermets modified by VC and Mo2C

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo₂C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo₂C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents. The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV₃₀) of 1077 ± 22 kg/mm² and an indentation toughness of 9.1 ± 0.5 MPa·m^1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm², whereas the toughness increased to 11.3 ± 0.1 MPa·m^1/2. Addition of 5 and 10 vol% Mo₂C into a NbC-12 vol% Ni mixtures generated the same values in HV₃₀ and K_IC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC + 4 vol% Mo₂C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m^1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.     

Microstructure and properties of copper composite containing in situ NbC reinforcement: Effects of milling speed

This paper presents a study on the effects of milling speed on the properties of in situ copper-based composite produced by mechanical alloying followed by cold pressing and sintering. A powdered mixture of copper, niobium and graphite with the composition of Cu–30%NbC was milled at various speeds (100, 200, 300 and 400 rpm). The NbC phase started to precipitate in the as-milled powder after 30 h milling at 400 rpm and the formation was completed after sintering at 950 °C. Enhancements of NbC phase formation with a reduction in Cu crystallite size were observed with the increase of milling speed. Density, hardness and electrical conductivity of the sintered composite were evaluated. An increase in milling speed resulted in an increase in sintered density and hardness but a reduction of electrical conductivity. The changes in the properties were correlated to the formation of NbC phase and refinement of copper and niobium carbide crystallite size since higher milling speed is associated with higher kinetic energy per hit.     

Microstructure instability in TiC-316L stainless steel cermets

Titanium carbide (TiC) based cermets are commonly used in wear and corrosion resistance applications. The microstructural evolution, and related compositional instability, of TiC-based cermets prepared with a 316-L stainless steel binder is described in the present work. Samples were fabricated using a simple vacuum melt-infiltration procedure, with 5 to 30 vol.% binder. Infiltration temperatures ranged from 1475 °C to 1550 °C, held for up to 240 min, typically resulting in sintered samples with densities in excess of 99% of theoretical. It is demonstrated that irregularly shaped grains (concave/hollow) can arise after sintering, especially at 1475 °C, which is discussed in terms of the ‘instability of the solid-liquid interface’ theory. It is demonstrated that a complex, multi-layer core-rim structure arose for the cermets, with accommodation of selected steel constituents into the rim of the TiC grains. In particular, it is shown that the Mo in the original 316-L stainless steel is essentially fully depleted from the metallic binder phase, forming a Mo-rich inner-rim layer on the TiC grain cores.     

Mechanical properties and rapid sintering of nanostructured WC and WC–TiAl3 hard materials by the pulsed current activated heating

Tungsten carbides are primarily used as cutting tools and abrasive materials in the form of composites with a binder metal, such as Co or Ni. However, these binder phases have inferior chemical characteristics compared to the carbide phase and the high cost of Ni or Co. Therefore, low corrosion resistance of the WC–Ni and WC–Co cermets has generated interest in recent years for alternative binder phases. In this study, TiAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC–TiAl₃ with a relative density of up to 99% was obtained within 2 min by PCAS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered WC and WC–TiAl₃ were lower than 100 nm. The addition of TiAl₃ to WC enhanced the toughness without great decrease of hardness due to crack deflection and decrease of grain size.     

Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

The effects of microstructure on Vickers indentation damage in TiC-316L stainless steel cermets

Titanium carbide (TiC) based cermets are commonly used as wear resistance and corrosion resistance components. In the present work, the effects of microstructure of TiC-316L stainless steel cermets are assessed in terms of Vickers indentation damage, with both the steel binder content and TiC grain size varied. Binder contents from 5 to 30 vol.% were examined, with samples fabricated using a simple vacuum melt-infiltration procedure at temperatures between 1475 °C and 1550 °C (held for up to 240 minutes). Two primary Vickers indentation-cracking patterns arise in these materials, namely median or Palmqvist cracks, and this response relates to both the volume fraction of ductile metal binder present and the binder ligament dimension. Focused ion beam microscopy has been utilised for sub-surface evaluation of the cracks, to confirm the anticipated crack patterns.     

Effect of Mo addition on microstructure and mechanical properties of (Ti,W)C solid solution based cermets

(Ti,W)C solid solution was synthesized by milling a mixture of C and oxides and then reducing it at 1350 °C for 2 h. The microstructure and mechanical properties of (Ti,W)C solid solution based cermets with various Mo additions were systemically studied. The dark core–gray rim carbide grains and/or gray carbide grains embedded in black Ni based binder phase were observed. The grain size of the cermets decreased with increasing Mo addition, while excessive Mo addition would result in agglomeration and inhomogeneity of the grains. The dark core was transformed from (Ti,W)C into (Ti,W,Mo)C and Mo concentration in the dark core increased with increasing Mo content. The fracture toughness of the cermets decreased with the increase of Mo content, while the hardness and TRS reached a peak value at 10 wt.% and 15 wt.% Mo additions respectively, and declined with the further increase of Mo content.     

Erosion-corrosion behavior of Ti(C,N)-based cermets containing different secondary carbides

The present work investigated the effects of secondary carbides (Mo₂C\WC\TaC\NbC) on the erosion-corrosion behavior of Ti(C,N)-based cermets. The results indicate that the erosion-corrosion resistance of Ti(C,N)-based cermets is enhanced in the order of NbC, TaC, WC and Mo2C addition. The contribution of erosion to the erosion-corrosion of Ti(C,N)-based cermets is much more significant than that of corrosion, and it increases with the decreased mechanical properties. The synergistic effect plays a dominant role in the degradation of Ti(C,N)-based cermets in erosion-corrosion conditions. There are two modes to ceramic phase degradation in erosion conditions: large ceramic grains are prone to deterioration through crack initiation and propagation → grain fracture → fragment removal; finer ceramic grains trend to be pulled out after the deterioration of binder and interface. The binder loss is determined by the corrosion resistance of binder, the erosion resistance of binder and the erosion resistance of ceramic phase.     

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.     

The fabrication of multi-core structure cermets based on (Ti,W,Ta)CN and TiCN solid-solution powders

A novel multi-core structure cermets consisted of both black-core/rim structure and grey-core/rim structure were obtained by partially replacing TiCN powder with (Ti,20W,15Ta)CN powder via low-pressure sintering process. The toughness and strength of TiCN-based cermets were optimized and its feature of high hardness was maintained simultaneously. Systematically, it was investigated that the influences of various weight ratios of both (Ti,20W,15Ta)CN/TiCN and Co/Ni on the microstructure and mechanical properties of the multi-core cermets. The results showed that the addition of (Ti,20W,15Ta)CN powder could cause the refinement of the core size and the occurrence of the secondary phase (W,Mo,Ti)_3 + x(Co,Ni)_3 − xC (0 < x ≤ 1), both of which are responsible for the significant improvement of the mechanical properties. The appearance of the secondary phase was found under two circumstances, one was when the weight ratio of (Ti,20W,15Ta)CN/TiCN was 6:4 while that of Co/Ni was 5:5(cermet M60) and the other was when that of (Ti,20W,15Ta)CN/TiCN was 5:5 with pure Co binder (cermet C50). And there is a monotonous escalation of the fracture toughness (K_IC) of the cermets while increasing the (Ti,20W,15Ta)CN content. The optimal comprehensive mechanical performance was found in cermet M60 with transverse rupture strength (TRS) of 1903.32 MPa, Vickers hardness (HV₃₀) of 16.33 GPa and fracture toughness of 12.19 MPa·m^1/2.     

Effects of sintering processes on the mechanical properties and microstructure of Ti(C,N)-based cermet cutting tool materials

In this study, two types of Ti(C_0.7,N_0.3)-based cermet cutting tool materials (Ti(C,N)–Mo–Ni–Co, named as TMNC, and Ti(C,N)–WC–Mo–Ni–Co–TaC–HfC, named as TWMNCTH) were fabricated by the hot pressed sintering process at different temperatures (from 1380 °C to 1500 °C) for different holding times (from 30 min to 60 min) in a vacuum atmosphere and at a compressive stress of 32 MPa. The polished surface and the fracture surface of the two types of cermets were observed by a scanning electron microscope (BSE/SEM) and energy dispersive spectrometry (EDS), and the relationships among sintering processes, mechanical properties and microstructure were discussed. The experimental results showed that the sintering temperature and holding time both had a great influence on the flexural strength and a small effect on the hardness and the fracture toughness of the two types of cermets. The two cermets both had the optimal comprehensive mechanical properties when they were sintered at 1400 °C for 30 min. The sintering temperature and holding time also had a great influence on the microstructure of the two cermets, and the grain sizes increased when the sintering temperature varied from 1400 °C to 1500 °C and the holding time varied from 30 min to 60 min. The properties and microstructure of the two cermets were also compared. The results indicated that the cermet TWMNCTH had a lower flexural strength, a similar value of fracture toughness, a higher hardness and a thicker rim in the microstructure.     

Preparation of self-lubricating Ti(C,N)-based cermets by solid carburization and wear behavior

In this work, self-lubricating Ti(C,N)-based cermets were prepared by solid carburization. The sintered cermets were wrapped by carburizing agent and sintered again at 1440 °C with different time. The microstructure and composition of cermets were studied. The wear behavior of cermets containing graphite phase was also evaluated using a block-on-ring tribometer. The results showed that the carbon content increased gradually in binder phase with carburizing time. When the carburizing time was 3 h, the carbon got saturated in binder phase. When the carbon content exceeded the solubility in the binder, excessive carbon precipitated and formed graphite phase. Uniformly distributed graphite clusters formed in cermets after carburization for 4 h. The graphite clusters consisted of flocked graphite particles. With the carburizing time extended to 5 h, the graphite clusters became large and some of them interlaced together. Besides, the wear results indicated that the volume loss of cermets containing graphite phase was half of that without graphite due to the formation of smooth tribofilm on the worn surface of cermets.     

Effect of carbon content on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with different carbon contents were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The transverse rupture strength (TRS), hardness (HRA) and fracture toughness (K_IC) were also measured. The free carbon present in the green compact significantly decreased the grain size; however, a high carbon content resulted in the formation of graphite phase and Fe₃C phase. An increasing carbon content promoted the dissolution of Mo in the binder phase. In addition, the binder phase varied from ferrite to martensite with increasing carbon content. The highest hardness was found for the cermets with 0.5 wt.% carbon addition, whereas the cermets without carbon addition exhibited the maximum TRS and fracture toughness.     

Rapid consolidation of nanostuctured WC-FeAl3 by pulsed current activated heating and its mechanical properties

In the case of cemented WC, Ni or Co is added as a binder for the formation of composite structures. However, the high cost of Ni or Co, the low hardness and the low corrosion resistance of the WC-Ni and WC-Co cermets have generated interest in recent years for alternative binder phases. In this study, FeAl₃ was used as a novel binder and consolidated by the pulsed current activated sintering (PCAS) method. Highly dense WC-FeAl₃ with a relative density of up to 97% was obtained within 2 min by PCAS under a pressure of 80 MPa. The average grain sizes of the sintered WC and WC-FeAl₃ were lower than 100 nm. The addition of FeAl₃ to WC enhanced the mechanical properties and increase of relative density.     

Sintering of tungsten powder with and without tungsten carbide additive by field assisted sintering technology

Tungsten powder (0.6–0.9 μm) was sintered by field assisted sintering technology (FAST) at various processing conditions. The sample sintered with in-situ hydrogen reduction pretreatment and pulsed electric current during heating showed the lowest amount of oxygen. The maximum relative density achieved was 98.5%, which is from the sample sintered at 2000 °C, 85 MPa for 30 min. However, the corresponding sintered grain size was 22.2 μm. To minimize grain growth, nano tungsten carbide powder (0.1–0.2 μm) was used as sintering additive. By mixing 5 and 10 vol.% WC with W powder, densification was enhanced and finer grain size was obtained. Relative density above 99% with grain size around 3 μm was achieved in W–10 vol.% WC sintered at 1700 °C, 85 MPa, for 5 min.     

Preparation of polycrystalline cubic boron nitride compact by high-pressure infiltration using cemented carbide

Polycrystalline cubic boron nitride (PcBN) compacts, using the infiltrating method in situ by cemented carbide (WC–Co) substrate, were sintered under high temperature and high pressure (HPHT, 5.2 GPa, 1450 °C for 6 min). The microstructure morphology, phase composition and hardness of PcBN compacts were investigated by using scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The experimental results show that the WC and Co from WC–Co substrate spread into cubic boron nitride (cBN) layer through melting permeability under HPHT. The binder phases of WC, MoCoB and Co₃W₃C realized the interface compound of PcBN compact, and the PcBN layer formed a dense concrete microstructure. Additionally the Vickers hardness of 29.3 GPa and cutting test were performed when sintered by using cBN grain size of 10–14 μm.     

Sub-micron binderless tungsten carbide sintering behavior under high pressure and high temperature

Pure tungsten carbide (WC) compacts of about 200 nm grain size were prepared by high pressure and high temperature (HPHT) method. The best property sample with high relative density (99.2%), high Vickers hardness (2925 kg·mm^− 2) and high fracture toughness (8.9 MPa·m^1/2) was obtained in the condition of 1500 °C temperature and 5 GPa pressure. By means of scanning electron microscopy (SEM) and transmission electron microscope (TEM) observations, a large number of twins and stacking faults appeared in sintered samples, and the grain size of sintered samples maintained in the initial range. The XRD patterns of bulk samples reveal that there is a phase transition from WC to W₂C with the increasing of temperature. Moreover, the effect of HPHT condition for sintering kinetics, microstructure evolutions, and mechanical properties of the sintered samples were also discussed.     

The effect of volume fraction of WC particles on wear behavior of in-situ WC/Fe composites by spark plasma sintering

Tungsten carbide (WC) particles have been in-situ synthesized through the reaction between tungsten particles and carbide particles by spark plasma sintering (SPS). The composites with different WC content were comparatively observed by the techniques of scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM), X-ray diffraction, hardness and pin-to-disc abrasive wear test. The results showed that the formed WC particles were homogenously distributed in the iron matrix with the size of smaller than 25 μm. Additionally, with the increasing of the WC content, the hardness of composites, the microhardness of matrix and the wear resistance increased, but there was no change significantly between 32 vol% WC/Fe composites and 42 vol% WC/Fe composites. The composites possessed excellent wear resistance comparing the specific wear rate determined in the present work to the martensitic wear-resistant steel under the load of 80 N after a sliding distance of ~ 950 m. The specific wear rate of the martensitic wear-resistant steel was a factor of 24 and 48 times higher than WC/Fe composites, when the content of WC was 32 vol% and 42 vol% in WC/Fe composites, respectively. The main wear mechanism was synthetic of abrasion wear and oxidation wear. The wear performance of 32 vol% WC/Fe composites didn't appear to be much different from 42 vol% WC/Fe composites, due to the WC particles in the 42 vol% composites produced stress concentration easily, which could ultimately induce the creak initiation around WC particles in the subsurface (near wear surface) and propagation to wear surface promoting the breakup of surface film.     

Microstructure characterization of large TiC-Mo-Ni cermet tiles

A detailed characterization of large (150 mm × 150 mm), 6 to 12 mm thick, commercially produced tiles of a TiC-Mo-Ni cermet with ~ 13 vol% Ni binder and a microstructure consistent with processing via self-propagating high temperature synthesis (SHS) has been conducted. Many mechanical property defining attributes of the materials were highly reproducible, including the composition, phase content, TiC particle size distribution (with an average particle diameter of 8–10 μm), and density of 5.52 × 10³ kgm^− 3. However, sufficient variability in the distribution of metal elements within the carbide particles, interparticle contiguity, the distribution of porosity, and residual stress were discovered that the mechanical behavior is expected to exhibit significant variability. The spheroidal shaped TiC particles had a multilayered (onion ring like) composition with rings of locally higher Mo concentration, rather than the more usual Ti-rich core and a single Mo-rich rim that enhances wetting with the Ni-binder. The TiC particles also had a high contiguity factor of 0.30–0.47. Recent assessments of liquid phase sintered cermets indicate significant loss of fracture resistance as the contiguity increases above 0.25. Hot isostatic pressing (HIP) at 1250 °C and 100 MPa was unable to reduce the porosity, which remained as large pockets of insufficient metal binder material (a form of shrinkage porosity) between the spheroidal carbide particles. X-ray diffraction measurements indicated the presence of significant residual stress in the as-received and the HIP condition materials. A stress relief heat treatment at 900 °C succeeded in eliminating this residual stress consistent with its origination from thermal gradients associated with rapid cooling.     

Study of cemented carbonitrides with nickel as binder: Experimental investigations and computer calculations

Cobalt is the most common binder in cemented carbides industry. However, there are some interests in use of alternatives. The similarity in properties has made nickel the first choice. In the present work, the effect of initial composition on modern hardmetals containing transition metal carbides/carbonitrides that are called “cemented carbonitrides” with nickel as binder was investigated. Change in quantity of additive carbides and tungsten to carbon (C/W) weight ratio through applying metallic tungsten powder in primary powder mixture had some effects on final hardness, transverse rupture strength, and microstructure of studied alloys. Addition of vanadium carbide not more than 0.2 wt.%, increased the final hardness. Application of (Ta,Nb)C solid solution carbide cancelled the grain refinement effect of VC. Formation of eta (η) phase was observed in SEM micrographs as a result of increase in W/C weight ratio. Surface modified layers with thicknesses ranging from 55 to 65 μm called Cubic Free Layer (CFL) was observed in vacuum sintered specimens. DICTRA® module from Thermo-Calc® software package was applied for simulation of CFL formation process in studied alloys. Formation of eta phase and consumption of metallic binder was predicted using calculations of Thermo-Calc® ver. P software. A state of the art technique was developed to prove linear relationship between Labyrinth factor (λ) and binder phase volume fraction (f).