Core-rim Microstructure Research Articles

Core-rim microstructure formation and mechanical properties of TiB2 ceramics with TiC, B4C, Co, and Mo sintering aids

This research investigates titanium diboride (TiB2) ceramics, renowned for their exceptional properties: high hardness, chemical inertness, and thermal stability. The challenge lies in achieving dense structures through pressureless sintering, given TiB2 ceramics' high melting point and low self-diffusion coefficient. To address this, a combined method of liquid phase and reactive sintering is employed, focusing on the influence of metallic cobalt (Co) and molybdenum (Mo) as sintering aids to enhance microstructural and mechanical properties. Vacuum sintering at 1900 °C facilitates the reaction between TiC and B4C, forming TiB2, with Co improving wettability and Mo inhibiting high-temperature grain growth. The matrix of 90TiB2+10(TiC + B4C + Co + Mo) recipe achieved 94.7 % relative density and a hardness of 31.97 GPa. Moreover, adding polyvinyl butyral (PVB) assists in creating gas evacuation channels during the sintering process. The research also reveals the reaction between TiC and B4C, forming (TiMoB)C with molybdenum, while the eutectic Co–Mo alloy dissolves into borides, promoting densification and forms dense core-rim structures.

Densification behavior of ZrB2–MoSi2–SiCw composite processed by multi stage spark plasma sintering

This study reports the densification behavior and novel design of core-rim microstructure for different ZrB2–MoSi2-SiCw (ZMSw) composites. Four ZrB2–MoSi2 composites containing various amounts of SiCw (0, 5, 10, and 20 vol%) were densified by multistage spark plasma sintering. Optimal combinations of sintering schedule, temperature, and composition are recognized for producing dense ZrB2-based ceramic-matrix composite with core-rim grain structure in the boride matrix, where ZrB2 core is surrounded by a solid solution of (Mo,W)Si2. Fragmentation, rearrangement, plastic deformation, and grain boundary diffusion mechanisms are responsible for densification of composites sintered by multistage SPS conditions at 1700 °C. Addition of SiCw significantly improves the density and mechanical properties of ZrB2–MoSi2–SiCw composites due to the formation of a core-rim structure.

Core-rim structured carbide MXene/SiO2 nanoplates as an ultrathin microwave absorber

Despite the high microwave attenuation ability of the titanium carbide MXene, exploiting the two-dimensional (2D) material as an efficient microwave absorber is a challenging task due to the strongly reflective feature caused by high electrical conductivity. Here, the plate-like MXene/SiO2 composites with a unique core-rim microstructure (MXene@SiO2) are rationally constructed by a simple Stöber method. The abundant functional groups on the edge of MXene leads to the formation of thick rim on MXene@SiO2 nanoplates, which remarkably increases the interfacial polarization loss. Meanwhile, the SiO2 coating with tunable thickness can balance the impedance at surface, which largely prevents the reflection of incident microwaves. As a result, very low specific reflection loss of −34.33 dB mm−1 in X band and −55.68 dB mm−1 in Ku band are realized in the composite with optimized coating thickness. Moreover, the reflection loss lower than −20 dB (MA efficiency more than 99%) can always be achieved with certain matching thickness lower than 2 mm in the whole X and Ku band. These findings imply that the 2D core-rim MXene@SiO2 nanoplates can be applied as ultrathin microwave absorber in various portable devices.

On the use of TiO2 in Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets

In this study, Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni cermets with varying [Mo]/([W] + [Mo]) mole ratios and constant [W] + [Mo] content were prepared by using TiO2 and carbon as substitutes for a part of Ti(C,N). In situ carbothermal reaction took place with subsequent liquid phase sintering. The thermal behavior and outgassing behavior were conducted using differential thermal analysis/thermal gravity analysis (DTA-TG) and mass-spectrometric evolving gas analysis (MS-EGA). The microstructure was investigated by scanning electron microscopy (SEM). After being sintered in N2 atmosphere, cermets without TiO2 additions generally showed rimless microstructures with black grains independently distributed from other phases, whereas the cermets with TiO2 addition showed complete core-rim microstructures. The average grain size generally decreased with increasing Mo content, and increased with increasing TiO2 substitution. Moreover, the average grain size increased with decreased N2 pressure from 50 mbar to 10 mbar at any [Mo]/([Mo] + [W]) ratio. The most interesting result was for the Mo-free cermets with 5 mol% TiO2 and 10 mol% TiO2 addition sintered in 50 mbar N2: both the hardness and fracture toughness increased compared with the cermets without TiO2 addition. The HV-KIC behavior of the majority of the cermets was superior to the industrial grades.

Influence of Microstructure Evolution on Mechanical Properties, Wear Resistance and Corrosion Resistance of Ti(C,N)-Based Cermet Tools with Various WC Additions

The effects of tungsten carbide addition on the microstructure, mechanical properties and corrosion resistance of Ti(C,N)-based cermets prepared by the sinter-HIP method are investigated through scanning electron microscope, X-ray diffraction, electron back scattered diffraction, mechanical test and electrochemical method. The results show that the appropriate WC addition has an important influence on promoting the formation of core-rim microstructure of cermet, increasing the wettability of ceramic phase and binder phase, and improving the mechanical properties. Ti(C,N)-based cermet with 5 wt% WC addition has the highest hardness of 92.4 HRA and excellent wear resistance. However, excessive WC addition causes the appearance of slight white core phase containing plenty of W and Ti element in microstructure, which increases the fracture toughness, and reduces the hardness and wear resistance of cermet. Additionally, the electrochemical result shows that the addition of 5 wt% WC in cermet has the optimal corrosion resistance in 1 mol/L hydrochloric acid solution.

Effects of TiB2 Content on the Microstructures and Mechanical Properties of Low-Cobalt Ti(C,N)-TiB2 Cermets Fabricated by Vacuum Hot Pressing

Ti(C,N)-TiB2 cermets were fabricated from Ti(C,N), TiB2, Co and WC powder mixtures via a vacuum hot pressing process. The influence of TiB2 content on their microstructures and mechanical properties was investigated. As a result of the elevated TiB2 contents, two types of core-rim microstructures were present in the Ti(C,N)-TiB2 cermets, and remarkably improved mechanical properties were achieved. With the increase of TiB2 content, the flexural strength, fracture toughness and hardness of the Ti(C,N)-TiB2 cermets first increased, and then decreased, while their relative density consistently decreased. Attributed to an integration of the intergranular and intrangranular fracture behaviors, the Ti(C,N)-TiB2 cermets with 20 wt% TiB2 content exhibited the best overall properties with the relative density, hardness, fracture toughness and flexural strength at 99.3%, 1 995 H V, 7.92 MPa·m1/2 and 1 114 MPa, respectively. The underlying mechanism for their enhanced properties was studied in detail.

Development of Ti(C,N)-based cermets with (Co,Fe,Ni)-based high entropy alloys as binder phase

High entropy alloys have been proposed as novel binder phases in cemented carbides and cermets. Many aspects related to the stability of these alloys during the liquid phase sintering process are still unclear and were addressed in this work. Consolidated Ti(C,N)-based cermets using four different (Co,Fe,Ni)-based high entropy alloys as the binder phase were obtained. The chosen alloys - CoCrCuFeNi, CoCrFeNiV, CoCrFeMnNi and CoFeMnNiV - were previously synthesized through mechanical alloying and a single alloyed solid solution phase with fcc structure and nanometric character was always obtained. The powdered alloys and the consolidated cermets were analyzed by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectrometry and transmission electron microscopy. Differential thermal analysis was employed to determine the melting point of the four high entropy alloys that ranged between 1310 °C and 1375 °C. Although a high temperature of 1575 °C was required to obtain the highest cermet densification by pressureless sintering, porosity still remained in most of the cermets. Best densification was achieved when CoCrFeNiV was used as the binder phase. During liquid phase sintering, different compositional changes were observed in the ceramic and binder phases. A core-rim microstructure was observed in cermets containing V in the alloys (CoCrFeNiV and CoFeMnNiV), since this element was incorporated to the carbonitride structure during sintering. A slight Cr segregation was detected in cermets containing Cr, leading to CrTi-rich alloys in small binder regions. However, a great Cu segregation was produced when CoCrCuFeNi was used, and the formation of two different fcc alloys -a Cu-rich and a Cu-depleted- was observed. Finally, a loss of Mn was also evidenced in CoCrFeMnNi and CoFeMnNiV, probably due to its sublimation at the sintering temperature.

Microstructural investigation and machining performance of NbC-Ti(C0.5N0.5) matrix cermets

In this study, Ni or Co + Ni bonded NbC matrix cermets with secondary carbides (Mo2C and WC) as well as Ti(C0.5N0.5) were prepared by liquid phase sintering at 1450 °C in vacuum. Detailed microstructural investigation was performed by SEM, EPMA and XRD analysis. The microstructure, mechanical properties as well as the C45 (HB140) steel turning performance of the cermets were investigated, and compared with a commercial Ti(C0.5N0.5) based cutting insert. The sintering results indicated that the partial substitution of NbC by Ti(C0.5N0.5) had a significant effect on the core-rim microstructure and refinement of NbC solution grains, as well as improved mechanical properties of the NbC-Ti(C0.5N0.5) cermets. The phase constitution and composition of the cermets were supported by thermodynamic equilibrium calculations. All sintered cermets were composed of a fcc solid solution metal binder and a cubic core-rim NbC based solution phase, as well as an independent N-rich Ti(C,N) phase. The NbC-Ti(C0.5N0.5) and Ti(C0.5N0.5) matrix cermets had a comparable HV30 of 1500–1600 kg/mm2 and a similar fracture toughness of 8.0 MPa m1/2.

Core-rim microstructure and properties of WC/Ni composites

WC/Ni55 composites were prepared by vacuum sintering. The dissolution of WC in Ni55 was analyzed by SEM and XRD, and the effect of WC dissolution on the microstructure and micro-hardness of the composites was researched. The results show that the edge of the WC particles in WC/Ni55 composites dissolves partially, and a core-rim structure of the WC particles was formed. A metallurgical interface with good bonding was formed between WC particles and the matrix. The stable size of rim has no obvious correspondence with the original size of WC particles. M6C and other hard phases were precipitated at the interface, which significantly improved the micro-hardness of the matrix. The new precipitate also improved the scratch resistance of the composites in some case.

Ostwald ripening and grain growth in Ti(C,N)-based cermets during liquid phase sintering

After liquid phase sintering, core-rim microstructure occurs in the grains of Ti(C,N)-based cermets. The mechanisms of the microstructural evolution during different sintering stages were investigated here. Our theoretical analyses show that the rim thickness of a grain is relatively independent of its initial radius when rim formation is dominated by Ostwald ripening during dissolution-reprecipitation stage, whereas it increases linearly with its initial radius when grain growth during subsequent cooling stage is the dominant mechanism. A geometric analysis via cross-sectioning technique is developed to identify dominant rim formation mechanism. Experiments with Ti(C,N)-based cermets show that rim formation is dominated by grain growth at low sintering temperatures and by Ostwald ripening at sufficiently high sintering temperatures.

Effect of mechanical vibration on the size and microstructure of titania granules produced by auto-granulation

Auto-granulation is the growth of particle clusters of fine, cohesive powders due to agitation of the bed, such as mechanical vibration. This clustering occurs without the addition of any binder to the system and the granules reach an equilibrium size due to the balance between disruptive and adhesive forces experienced by the clusters during process operations. For this reason, it is important in powder processing to be able to characterize this behavior. In this study, a sub-micron titania powder is mechanically vibrated under controlled conditions to induce clustering and promote auto-granulation. The amplitude and frequency of the vibration are varied to view their effect on the equilibrium granule size. Furthermore, imaging of cross-sections of the granules is conducted to provide insight into the internal microstructure and measure the packing fraction of the constituent particles. It is found that under all vibrational conditions investigated the particles exhibit a core–rim microstructure.

Mechanical properties and microstructure of Ti(C5N5)-TiB2-(W7Ti3)C composite cutting tool materials

Ti(C5,N5)-TiB2-(W7,Ti3)C composite ceramic cutting tool materials were prepared using the vacuum hot-pressed sintering technology. The effects of the sintering temperature and the heating rate on the mechanical properties and microstructure of different composite materials containing different contents of TiB2 were investigated. The sintering process of a higher temperature with a fast heating rate or a lower temperature with a slow heating rate was found to be beneficial to the mechanical properties of the composites. The better sintering temperature range for fabricating the composites was from 1585 to 1620 °C. The composite material with a content of 30 wt% TiB2 sintered at 1585 °C with either a heating rate of 90 or 50 °C/min had the prominent flexural strength. The composite with a content of 20 wt% TiB2 sintered at 1620 °C with a heating rate of 90 °C/min had the relatively better fracture toughness and hardness than that with a content of 30 wt% TiB2. The two core-rim microstructures were formed inside the TiB2 and Ti(C, N) grains, which further contributed to a compact microstructure and improved the mechanical properties of the composites. The optimum sintering temperature range for the formation of the core-rim microstructures was also from 1585 to 1620 °C.

Effect of heating rate on the mechanical properties and microstructure of Ti(C,N)-based cermets

An appropriate heating rate in the sintering process is crucial to obtain the Ti(C,N)-based cermets with superior properties. In this paper, Ti(C,N)-based cermets were sintered to investigate the influence of heating rate on the mechanical properties and microstructure of the cermet materials. The transverse rupture strength (TRS), Vickers hardness (HV) and fracture toughness (KIC) were tested. The microstructure, indention crack, fracture morphology and phase composition of the cermets were also studied by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results reveal that the heating rate has a great influence on the mechanical properties and microstructure of Ti(C,N)-based cermets. The cermets sintered at the heating rate of 3°C/min between 1300°C and 1430°C have the optimum comprehensive mechanical properties with a transverse rupture strength of 1605±107MPa, a hardness of 12.02±0.25GPa and a fracture toughness of 10.73±0.40MPam1/2. The heating rate can affect the reaction among the constituents of Ti(C,N)-based cermets and then influence the elements distribution in the core–rim microstructures and the lattice parameter of Ti(C,N) phase. When the heating rate is between 2°C/min and 5°C/min, the lower the heating rate is, the coarser the Ti(C,N) grains become. A higher heating rate is detrimental to the formation of core–rim microstructures, and a lower heating rate can result in grain coarsening and inhomogeneous microstructure. The observation of indention cracks and fracture surfaces show that the intergranular cracks and intergranular fractures mainly occur in the cermets with larger binder mean free path and medium grains. While the cleavage fractures appear more in the cermets with grain coarsening, and the transgranular fractures exist more in the cermets with non-fully developed fine grains.

Comparison of Ti(C,N)-based cermets processed by hot-pressing sintering and conventional pressureless sintering

A suitable sintering method is important to obtain the Ti(C,N)-based cermets with superior properties. In this paper, Ti(C,N)-based cermets were fabricated by hot-pressing sintering (HP) and conventional pressureless sintering (PLS) technology, respectively, to investigate the influence of different sintering methods on the microstructure and mechanical properties of cermets materials. The microstructure, fracture morphology, indention cracks and phase composition were observed and detected using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The transverse rupture strength (TRS), Vickers hardness (HV) and fracture toughness (KIC) were also measured. The results reveal that all of the Ti(C,N)-based cermets exhibit core–rim microstructures with black cores, white cores and grey rims embedded into metal binder phases. The grain size of the samples fabricated by HP is much finer and the structure is more compact than those fabricated by PLS, while there exist pores in the HP sintered samples. The sintering process has no influence on the phase composition of cermets, but affects the phase content and crystallinity. The samples fabricated by PLS present higher transverse rupture strength, fracture toughness and density than samples fabricated by HP. However, the HP sintered samples possess a higher hardness.

Influence of applied pressure during field-assisted sintering of Ti(C,N)–WC–FeAl based nanocomposite

Ti(C,N)–WC–FeAl based nanocomposites are processed by field-assisted sintering at 1500°C. The phase and microstructural evolution during the process under the influence of different applied pressures of 30, 50 and 100MPa are studied using x-ray diffraction and scanning electron microscopy. Lattice parameters of (Ti,W)(C,N) solid solution and binder phases after sintering are found to vary with applied pressure. The nanocomposite grains are observed to possess a core-rim microstructure. Microstructural variations in terms of type, size and fraction of “core-rim” structure as a function of applied pressure are investigated. The hardness and indentation fracture toughness values are in the range of 17.6–18.4GPa and 5.9–6.8MPa√m, respectively. These values are comparable with those reported for Ti(C,N)-based composites with metal binder.

Effect of TiN Addition on the Microstructure and Mechanical Properties of Ti(C, N)-Based Cermets

The effect of titanium nitride addition on the microstructure, porosity and mechanical properties of Ti(C, N)-based cermets were investigated in this work. Results show that cermets have typical core-rim microstructure, the grain size of hard phase refines and the porosity increases with the addition of titanium nitride content. The transverse rupture strength reaches maximum at about 15wt.% titanium nitride. The hardness declines with the incremental titanium nitride content.

Absence of the core–rim microstructure in TixTa1 − xCyN1 − y-based cermets developed from a pre-sintered carbonitride master alloy

(Ti,Ta)(C,N) solid solution-based cermets with cobalt as the binder phase were synthesised by a two-step milling process. The titanium–tantalum carbonitride solid solution (the ceramic phase) was obtained via a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental Ti, Ta, and graphite powder blends in a nitrogen atmosphere. Elemental Co (the binder phase) was added to the ceramic phase, and the mixture was homogenised by mechanical milling (MM). The powdered cermet was then sintered in a tubular furnace at temperatures ranging from 1400°C to 1600°C in an inert atmosphere. The chemical composition and microstructure of the sintered cermets were characterised as ceramic particles grown via a coalescence process and embedded in a complex (Ti,Ta)–Co intermetallic matrix. The absence of the typical core–rim microstructure was confirmed.

Fabrication and properties of Ti(C,N) based cermets reinforced by nano-CBN particles

Titanium carbontride (Ti(C,N)) based cermets with and without nano-cubic boron nitride (CBN) particles were prepared by microwave sintering in argon and nitrogen environment, respectively. Two kinds of core–rim microstructure, black core–grey rim and white core–grey rim, are shown in the cermets by scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). It is found that, for the cermet with 1.5% nano-CBN particles sintered at 1500°C for 30min in argon, its transverse rupture strength (TRS) and hardness are improved to about 25.9% and 1.4%, respectively. The SEM analysis shows that the inhibition effect of nano-CBN particles on the dissolution of Ti(C,N) is weakened with the increase of content of nano-CBN particles. Moreover, for the cermet sintered in argon reinforced by 1.5% nano-CBN particles, more fine black core–grey rims are found in the microstructure compared to the others. For the material sintered in nitrogen, its microstructure accompanied with many white core–grey rims in number and big black core and thin outer rim in size, results in high hardness and low TRS.

Inverse core–rim microstructure in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced self-sustaining reaction

Cermets with a nominal composition (Ti0.8Ta0.2C0.5N0.5— 20wt.% Co) were synthesised by a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental powder blends. The MSR allowed the production of a complex (Ti,Ta)(C,N) solid solution, which was the raw material used for the sintering process. The pressureless sintering process was performed at temperatures between 1400°C and 1600°C in an inert atmosphere. The microstructural characterisation showed a complex microstructure composed of a ceramic phase with an unusual inverse core–rim structure and a Ti–Ta–Co intermetallic phase that acted as the binder.

The addition of SiC particles into a MoSi 2-doped ZrB 2 matrix: Effects on densification, microstructure and thermo-physical properties

The densification, microstructure and properties were studied for MoSi 2-doped ZrB 2-containing SiC particles. MoSi 2 is added to ZrB 2 as sintering aid. The enhanced densification was driven by a pseudo-binary reaction involving ZrB 2 and transient Mo borides, which were formed as products of the reaction between MoSi 2 and the surface oxides on the ZrB 2 particles. The ZrB 2 grains had a core–rim microstructure. Room temperature strengths up to 800 MPa were measured. At 1500 °C (in air), strengths decreased to 240 MPa. The addition of SiC increased the fracture toughness (4.1 MPa m at RT), compared to ZrB 2 (3.4 MPa m at RT). The resistance to thermal shock of ZrB 2 also increased with the addition of SiC. The measured critical thermal shock temperature difference increased from 330 °C for ZrB 2 to 385 °C when SiC was added. The addition of SiC also decreased the oxidation rate of ZrB 2. The investigations gave evidence of the beneficial influence of MoSi 2 as sintering aid on densification and properties of ZrB 2-based materials.