SLM printing of cermet powders: Inhomogeneity from atomic scale to microstructure

It is very challenging for 3D printing based on the selective laser melting (SLM) technology to obtain cermet bulk materials with high density and homogeneous microstructures. In this work, the SLM process of the cermet powders was studied by both simulations and experiments using the WC-Co cemented carbides as an example. The results indicated that the evolution of the ceramic and metallic phases in the cermet particle during the heating, melting and solidification processes were all significantly inhomogeneous from atomic scale to mesoscale microstructures. As a consequence, the microstructural defects were caused intrinsically in the printed bulk material. The formation and growth of the bonding necks between the particles were mainly completed at the later stage of laser heating and the early stage of solidification. Both simulations and experiments demonstrated that thin amorphous layers formed at the ceramics/metal interfaces. This work disclosed the mechanisms for the evolution from the atomic scale to microstructure during the SLM printing of cermet powders, and discovered the origin of the defects in the printed cermet bulk materials.     

Coking-resistant NbOx-Ni-Ce0.8Sm0.2O1.9 anode material for methanol-fueled solid oxide fuel cells

NbO_x is added in Ni-Ce_0.8Sm_0.2O_1.9 by impregnation as an anode material for solid oxide fuel cells fed with methanol. Nb (IV) and Nb (V) exist in the reduced anode. The addition of Nb reduces the binding energy of Ni. The catalytic activity of the anode and the performance of the single cell both increase with the increase of Nb. At 700 °C, the cell with 5NbO_x-Ni-Ce_0.8Sm_0.2O_1.9 anode and Ce_0.8Sm_0.2O_1.9-carbonate electrolyte shows a output power density of 687 mW cm^?2. Meanwhile, water produced in the anode is absorbed by NbO_x and forms surface hydroxyl groups, which facilitates the removal of carbon. The addition of NbO_x decreases the amount of deposited carbon in the humidified methanol atmosphere significantly, and an improved stability of the single cell is achieved.     

Influence of Ni/Co binders and Mo2C on the microstructure evolution and mechanical properties of (Ti0.93W0.07)C–based cermets

In this research, solid–solution powder of (Ti_0.93W_0.07)C was synthesized by high–energy ball mill method followed by carbothermal reduction process. Subsequently, the acquired powder was blended with Ni/Co and Mo₂C secondary carbide, and sintered under the optimized temperature (1510?°C) for 1?h to produce the modulated cermets. A typical core–rim structure formation with solid–solution phases was confirmed by backscattered electrons studies using a Field Emission electron scanning microscope. The hardness of the synthesized cermets was enhanced by increasing the specific amount of Mo₂C. The acquired results demonstrate that the binder type has a prominent influence on the microstructure and hardness of the prepared cermets. The hardness of (Ti_0.93W_0.07)C–xMo₂C–Ni cermet increased ~ 9%, when nickel was partially substituted by cobalt.     

Effects of TiN addition on the properties of hot pressed TiN–Ti/Al2O3 composites

TiN–Ti/Al₂O₃ composites of varying TiN content (0–20?vol%) were prepared by vacuum hot-pressing sintering at different temperatures (1400?°C and 1500?°C) to investigate how TiN affected the mechanical properties and electrical conductivity of the composites. Sintered samples with added TiN exhibited better performance than those without it. The sample with 20?vol% TiN sintered 1500?°C had an optimal relative density of 99.49, Vickers hardness of 14.94?GPa, flexural strength of 321.55?MPa, and electrical resistivity of 1474.7?μΩ?cm. However, this increased temperature did not improve the best sample resistivity of 930.3?μΩ?cm, which was obtained at 1400?°C. Form SEM images and XRD patterns, the positive effect of TiN on composite mechanical properties may be ascribed to its good performance of high hardness and strength, a decrease of the brittle intermetallic phase, the form of AlTi₃N, and the impact of the fine-grained strength of the TiN phase.     

Tribological behaviour of a 3Y-TZP/Ta ceramic-metal biocomposite against ultrahigh molecular weight polyethylene (UHMWPE)

This study aims to evaluate the tribological behaviour of 3Y-TZP/Ta (20 vol%) ceramic-metal composites and 3Y-TZP monolithic ceramic prepared by spark plasma sintering (SPS) against ultrahigh molecular weight polyethylene (UHMWPE). According to the results of pin (UHMWPE)-on-flat wear test under dry conditions, the UHMWPE – 3Y-TZP/Ta system exhibited lower volume loss and friction coefficient than the UHMWPE – monolithic ceramic combination due to the presence of an autolubricating layer that provides sufficient lubrication for reducing the friction. Owing to the lubrication of the liquid media, under wet conditions obtained using simulated body fluid (SBF), similar behaviour is observed in both cases. Additionally, the ceramic and biocomposite materials were subjected to a low temperature degradation (LTD) process (often referred to as “ageing”) to evaluate the changes in the tribological behaviour after this treatment. In this particular case, the wear properties of the UHMWPE-biocomposite system were found to be less influenced by ageing in contrast to the case of the UHMWPE-zirconia monolithic material. In addition to their exceptional mechanical performance, 3Y-TZP/Ta composites also showed high resistance to low temperature degradation and good tribological properties, making them promising candidates for biomedical applications, especially for orthopaedic implants.     

The impact of porosity,pH2 and pH2O on the polarisation resistance of Ni–BaZr0.85Y0.15O3−δ cermet anodes for Protonic Ceramic Fuel Cells (PCFCs)

The effect of porosity and atmosphere on the polarisation resistance of Ni-BZY cermet anodes for protonic ceramic fuel cells (PCFCs) was investigated using electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The level of porosity was adjusted by the use of differing amounts of starch porogen. The total polarisation resistance of the cermet anode (Rp) generally showed an increase with increasing porosity. The high frequency polarisation resistance (R2) was shown to have low atmosphere dependence but presented a strong correlation to microstructural variations. In contrast, the low frequency polarisation resistance (R3) was shown to have a low dependence on humidity but demonstrated a strong negative dependence on pH₂ with a value of unity. Subsequent discussion relates the lower frequency response to the dissociative adsorption of H₂ on the anode surface. The present study highlights that porogens are not required for peak performance in PCFC anodes under standard operating conditions, a result contrary to that of their oxide-ion conducting cermet counterparts.     

Fast preparation of ZTA-TiC-FeCrNi cermets by high-gravity combustion synthesis

ZTA-TiC-FeCrNi cermets are prepared by a fast and furnace-free way called high-gravity combustion synthesis. The synthesized cermet samples show the maximum relative density of 97.6% and a hierarchical microstructure with grain sizes from submicron to >50 µm. The content of TiC has a strong influence on the microstructure and mechanical properties of the cermet samples. A higher TiC content results in refined microstructure, improved hardness, and reduced coefficient of friction. With increasing TiC content, the strength and toughness of the samples first increase and then drops, and reach the maximum of 469±26 MPa and 11.3±0.2 MPa m^1/2 at 20% TiC. Compared with commercial polycrystalline Al₂O₃ ceramics, the ZTA-TiC-FeCrNi cermets exhibit better wear resistance, and the volume loss is lower by one magnitude than Al₂O₃ under the same condition.     

Creep behaviour of porous SOFC electrodes: Measurement and application to Ni-8YSZ cermets

A methodology is proposed in this study to investigate the creep properties of porous Ni-8YSZ cermet. Creep experiments have been conducted under reducing atmosphere at the typical SOFC operating temperatures. Specimens have been loaded in a four-point bending test bench. A special attention has been paid in this work to the analytical and numerical modelling of the mechanical test. It has been highlighted that Ni-8YSZ exhibits substantial creep strain rates even at relative low temperatures (700 °C < T < 850 °C). The creep exponent has been found to be just slightly higher than unity (1 < n < 2) while the activation energy has been determined equal to Q = 115 kJ mol⁻¹.High-temperature plastic strains of both Ni and 8YSZ phases have been estimated through the local stress acting on the cermet particles. This analysis indicates that creep behaviour of the Ni-8YSZ composite is not influenced by the metallic phase, but is controlled by the deformation of the 8YSZ matrix. It is also proposed that cermet creep mechanism involves Zr⁴⁺cations diffusion at the surface rather than in the bulk of the 8YSZ material.Impact of the Ni-8YSZ cermet creep on the internal stresses distribution in SOFC is discussed considering the anode supported cell (ASC) design. It is shown that cermet creep strain can induce a substantial stress decrease in the thin electrolyte.     

Effects of Cr3C2 addition on the erosion–corrosion resistance of Ti(C,N)-based cermets in alkaline conditions

Effects of Cr₃C₂ on the erosion–corrosion behavior of Ti(C,N)-based cermets are studied in alkaline conditions. The results indicate that the erosion–corrosion resistance of cermets is improved with proper Cr₃C₂ content. Corrosion performance of cermets is deteriorated by Cr₃C₂ addition in NaOH solution. With the increase of Cr₃C₂, the erosion–corrosion behavior of Ti(C,N)-based cermets is classified to be erosion regime, erosion–corrosion regime, corrosion–erosion regime and corrosion regime. Materials degradation is determined by particles erosion for cermets with low Cr₃C₂ content, while for materials containing more Cr₃C₂ addition, binder corrosion and subsequent erosion are responsible for materials deterioration.     

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.