Cemented carbide is a composite material of hard WC and soft Co, and has been widely used as a tool material because of its excellent balance of hardness and toughness. Among them, superfine cemented carbide with WC grain size of less than 1 μm is widely used as a base material for cutting tools and wear-resistant tools because it can achieve both high hardness and high strength. When using superfine cemented carbide as a cutting tool, it is essential to adopt the finer WC particles with higher Co dispersibility, in addition to the appropriate addition of grain growth inhibitors. If the WC particles can be finer, the hardness and transverse rupture strength of the alloy can be expected to improve. In addition, if Co particles, which are softer than WC particles, can be distributed more finely and homogeneously in the mixed powder, the mechanical properties of the alloy may be further improved. In this paper, we report on the improvement of Co dispersibility and the development of new ultra-fine WC, and introduce the cutting performance of the developed practical tools using above the WC powder.

Inhibition mechanism of grain growth by metal carbide addition for WC-Co based ultrafine-grained cemented carbide was studied to develop 100 nm-grained cemented carbide. Two hypotheses of inhibition mechanism, a) inhibition by adsorption atoms at steps/kinks on WC crystal surface and b) inhibition by segregation layers on WC basal plane, were proposed. To judge which hypothesis is based on the inhibition mechanism, observation of TEM microstructure and analysis of segregation amount on WC/Co and WC/WC interfaces in WC-Co based cemented carbides including metal carbide additives were performed.

Determining the presence or absence of segregation layer on WC crystal surface during liquid phase sintering was the judgement point of inhibition mechanism. The judgement was considered comparison between predictive phenomena on each hypothesis and experiment results. The experimental results supported that grain growth inhibited by adsorption atoms at steps/kinks on WC crystal surface during liquid phase sintering and segregation layer formed by precipitation during cooling after liquid phase sintering. Segregation mechanism of grain growth inhibitor was proposed based on the composition, solubility limit in liquid Co and that in solid Co of the inhibitor.

Developments of 100 nm-grained WC-Co based cemented carbides with cobalt content of 4 and 10% were succeeded based on the resolution of inhibition mechanism. The hardness and transvers rupture strength of developed cemented carbides were 2300 HV and 4.7 GPa for 10%Co, 2600 HV and 3.0 GPa for 4%Co, respectively. Using the technique, 0.2 µm-grained cemented carbides were commercialized.

This study was intended to investigate the mechanical property at high temperature and cutting performance of cemented carbide for machining of exotic materials, such as nickel alloys, cobalt alloys, and titanium alloys. Exotic materials are widely used for equipment and parts in the aerospace and automotive industries due to their superior heat and corrosion resistance. When machining exotic materials, its low thermal conductivity causes the cutting edge temperature to be higher than when cutting steel, and the hardness of the tool material decreases, resulting in extremely low tool life. However, because cemented carbide in particular contains metals in its composition, plastic deformation of the cemented carbide cannot be prevented even when the coating has sufficient heat resistance. The WC-Co-NiCr alloy, in which NiCr partially replaces Co as the binder phase of cemented carbide, has improved high-temperature properties while maintaining room temperature properties compared with conventional WC-Co alloys, and tool life has been improved in machining exotic alloys.

This study was intended to investigate the mechanical property and cutting performance of AlTiBN coating for machining of exotic materials such as heat-resistant alloys and Titanium alloys, which has been increasing for aerospace, energy and medical industries. In conventional PVD coatings for cutting tools, Cr and Si has often been added to AlTiN in order to increase hardness and heat resistance, achieving high wear resistance. However, these methods increase hardness as well as Young’s modulus, which reduces toughness of coating. In exotic materials milling, coatings with a high Young’s modulus tends to develop internal cracks, leading to early destruction of the coatings. In this research, optimizing B content of AlTiBN coating, we realized low Young’s modulus while maintaining high hardness. This is probably because the pure AlTiN changed to a mixed crystal structure of cubic and hexagonal by the addition of B. When the cutting performance of this AlTiBN coating was evaluated in milling for Inconel 718, it was found that internal cracks were suppressed at the initial stage of processing. As a result, cemented carbide tool with the AlTiBN coating achieved approximately 1.6 times longer tool life than that with B-free AlTiN coating.

A nanosecond pulse electric field is defined as the application of an electric field with a width of approximately 10 nanoseconds to 1 microsecond. Recent developments in power semiconductors such as SiC and GaN have made it possible to design power supplies with higher voltage and single pulse width. In such nanosecond pulsed electric fields, there are four major characteristics. The most important of these features is that dielectric breakdown does not occur even when an electric field higher than the breakdown voltage is applied because the pulse application time is shorter than the time during which breakdown occurs. In other words, the nanosecond pulsed electric field can be used to realize a material creation process with a higher electric field than before. In addition, when used in conjunction with optical switches, an electric field can be applied at a targeted timing. Short-time electrochemical reactions can be generated. Since the repetition frequency can be varied, the resonance phenomenon by frequency matching can be utilized as in the case of a high-frequency power supply. In this paper, we will explain the material process that makes use of these features.

A non-firing solidification method using surface-activated ceramic powders with mechanochemical treatment and solidified without a high-temperature sintering process has been studied in our group. In this method, the surfaces of ceramic particles are activated by grinding, and this activated state is used as a driving force to bond the particles together. It is essential to computationally understand the mechanical activation and solidification processes in the non-firing process at various molecular and bulk scales. This paper first outlines the differences between the non-firing solidification process and the conventional sintering process. Next, we introduce our simulation results of the non-firing solidification process and discuss future prospects.

Powder metallurgy is a process for compacting and sintering powders such as metals and ceramics, and has been attracting new attention in recent years. This paper presents research examples in which simulation methods such as molecular dynamics (MD) method, Monte Carlo (MC) method, finite element method (FEM), and discrete element method (DEM) are applied to powder metallurgy. The MD method is used to calculate physical property values such as interfacial energy and diffusion coefficient that change depending on the type of grain boundaries. The MC method is used to calculate the microstructural development due to the sintering and grain growth. The microstructure can be predicted when the sintering process is changed. The FEM is used to calculate deformation and strain. The sintering simulation combined with the MC method can calculate the microstructural changes and deformation simultaneously. In the compaction simulation, density distribution due to the friction with the mold can be analyzed. The DEM is used to calculate the compaction structure. The interaction of the friction and stiffness between particles and particle/mold can be analyzed. As powder metallurgy technology develops, process design that considers the whole process will be important. Supporting technology based on simulation will become increasingly important.

Ground round bars made of WC-20 mass%Co cemented carbides were used to accurately determine the changes in shape. The bars were heated at 1673 K and cooled under different conditions to investigate the changes in Co content and diameter (d) in the longitudinal direction. When the bars were heated in a conventional sintering furnace, rapid cooling increased the Co content and d at both ends, while slow cooling increased those at the center. In the latter case, the amount of change was smaller. When the bar was heated using a tubular furnace and then cooled under 30 K of temperature gradient in the longitudinal direction, the Co content and d on the higher temperature side decreased and those on lower temperature side increased. The relationship between the change in Co content and the change in d could be explained quantitatively by considering the migration of the liquid phase in the temperature range of solid-liquid Co coexistence region. This study revealed that when WC-Co cemented carbides were fabricated using the conventional sintering furnace, an inevitable temperature gradient in the specimen during cooing caused the liquid phase migration and the shape distortion, regardless of the cooling rate.

In order to investigate the cause of the liquid phase migration (LPM) and the shape distortion when WC-Co cemented carbides (alloys) undergo a temperature gradient during cooling, round bars of WC-10 mass%Co alloys are inserted halfway under the coil of the induction heating furnace and partially heated at 1673 K. The results showed that Co migrated from the higher temperature side to the lower one and increase of Co content and bar diameter occurred in a zone at the outside of the coil. The alloy containing free carbon had a significantly narrow Co-increased zone and the zone located at a lower temperature. The volume transfer (V) of Co during heating changed in proportional to the square root of the heating time (t). It was considered that the phenomenon was the same as the LPM between a couple of alloys having different Co content in our previous report. We have concluded that the Co-increased zone in this study corresponds to the Co solid-liquid coexistence zone during partial heating and the Co increase is caused by the migration pressure occurred in the zone, which we named Π_SL. Π_SL should be generated because F_WC/Co(S) derived from γ_WC/Co(S) and γ_Co(L)/Co(S) is higher than F_WC/Co(L) derived from γ_WC/Co(L).

WC-Ti(C,N)-Cr₃C₂-Co ultrafine-grained cemented carbides with different contents of C, N and Co binder phases were fabricated. Their microstructures and mechanical properties were examined, and the relationship between the existing form of Ti(C,N) and various conditions was mainly discussed. The hardness of the samples increased and their fracture toughness decreased with decreasing C content. The average T.R.S. peaked at 4.8 GPa on the low-C side. Their microstructures significantly changed when the N₂ partial pressure changed. At a N₂ partial pressure of 2.6 kPa, the microstructure became finer and the average T.R.S. was the highest. Meanwhile, at N₂ partial pressures of 0 and 11.7 kPa, the microstructure became coarser and the average T.R.S. decreased. As the content of Co binder phase decreased, the number of WC/WC interfaces increased. The average T.R.S. was maximum at 16.4 vol% Co, and decreases with the Co content, but was higher than those of conventional cemented carbides. The existing form of Ti(C,N) changed depending on the N content. Herein, a N₂ partial pressure of 2.6 kPa and low C contents were deemed optimal.

Using ultrafine WC powders with different impurities contents, WC-10%Co ultrafine-grained cemented carbides with 5-9 mass% VC addition to the Co content were sintered and then HIPed or HIP-RQ (re-sintered with quenching after HIP). Their transverse-rupture strengths were measured, and the relationship between the stress σ_d acting on the fracture source and its source size 2a was evaluated using σ_d^-1 - a^1/2 and σ_d - A^-1/4 (= σ_d - (2a)^-1/2) correlation diagrams. Each cemented carbide had limiting strength which was constant σ_d when 2a ≦ 10 μm, regardless of 2a, in both diagrams. The limiting strength of σ_d was varied from 3.20 to 4.04 GPa, depending on the WC powder, VC addition amount, and HIP and HIP-RQ treatment. While the grinding residual stress proposal was unlikely to be the main cause for ultrafine-grained cemented carbide, the defect expansion proposal due to plastic deformation around the defect was able to explain a part of the phenomenon. It was revealed that if the stress applied to the defect varies due to interactions between defects, this could be the cause of the limiting strength phenomenon. By considering this interaction, it is possible to explain phenomena that could not be explained by the previous proposals.

The bending strength of ultrafine-grained cemented carbides exceeds that of medium-grained cemented carbides. This was thought to be due to the smaller size of defects within the ultrafine-grained cemented carbides. However, this attribution had not been verified because the specimens broke into numerous small pieces during bending tests. In addition, previous studies on the bending strength of medium-grained cemented carbides reported that there is limiting strength which is a region of not increasing bending strength by reducing the defect size. Therefore, in this study, we conducted the tensile test, which is unlikely to produce many small pieces, and the bending test to determine the cause of the high strength in ultrafine-grained cemented carbides. The ratio of bending strength to tensile strength was comparable between the medium- and ultrafine-grained cemented carbides. The number of fragments produced by fractures were almost same between the medium- and ultrafine-grained cemented carbides in case of the almost same strength. It was clear that the tensile strength of both medium- and ultrafine-grained cemented carbides increased as the defect size decreased with no sign of limiting strength. These results suggest that further strength improvements can be achieved by controlling defect size in ultrafine-grained cemented carbides.

Ultrafine-grained binderless cemented carbides using 0.4 µm WC particles were prepared by adding Cr₃C₂, VC, TaC, βt and various sizes of Ti(C,N) particles. SEM observations of the polished and fractured surfaces revealed that the WC grains became finer after the addition of the carbides or carbonitrides. In the Ti(C,N)-added binderless cemented carbides, the WC grains became finer with decreasing Ti(C,N) particle size and increasing Ti(C,N) content. The grain size of WC in the binderless cemented carbide containing 0.1 μm Ti(C,N) was comparable to that of the Cr₃C₂-containing binderless cemented carbide, although not as fine as that of the VC-containing binderless cemented carbide. The binderless cemented carbides containing fine Ti(C,N) grains were harder and had higher strengths than the other carbides. The combined addition of fine Ti(C,N) and Cr₃C₂ resulted in more homogeneous WC grains. These results indicate that the grain growth inhibition of binderless cemented carbides varies depending on the type and grain size of the added carbonitrides. These findings will contribute to the future development of binderless cemented carbides.

Composite of Ca-α-SiAlON and β-SiAlON was obtained by hot pressing of mixture of commercially available Ca-α-SiAlON and SiO₂ powders under 40 MPa at 1800°C for 2hours. The Ca-α-SiAlON powder used in this study was manufactured with combustion synthesis technique. Effect of SiO₂ additive on the mechanical properties of the SiAlON composite was investigated by changing the amount of the additives; 1, 3, 5 and 7 wt%. The highest strength, 736 MPa for the 4 points bending strength, was obtained in the SiAlON composite sintered with the addition of 5 wt% SiO₂ powder. From the results of XRD, SEM, and EPMA analysis, we found the formation of columnar β-SiAlON grains facilitated by the addition of SiO₂ powder led to the high strength.

In shis study, yield stress equation, flow rules and physical properties of barium titanate (BaTiO₃) are experimentally obtained. These basic equations and coefficient of flow stress are introduced to finite element method and sintering behavior of barium titanate bulk is simulated. From the uni-axis compression test, yield stress equation of BaTiO₃ is expressed by Shima’s equation and flow rule is written by plastic equiation. The coefficient of flow stress of BaTiO₃ is significantly smaller than literature Alumina and the value is different when the grain size is different. The experimental sintering deformation is quantitatively reproduced by numerical simulation, where basic equation and physical properties are exprerimentally introduced. In addition, the coefficient of flow stress could be determined by simulation sensitivily analysis. Through this study, experimentally obtained yield stress equation, flow rules and coefficient of flow stress are numerically validated.

The simulation for sintering of co-firing double layer compacts and substrate-constrained compacts were obtained by coupled Monte Carlo method and finite element method. In the coupled simulation of a co-firing compacts with different sintering shrinkage curves, the amount of warpage during and after sintering changed significantly. As a result of detailed confirmation of the results, it was found that there was a difference between the porosity obtained by the Monte Carlo method and by finite element method, so the coupled simulation was modified to minimize the porosity difference. As a result of the coupled simulation of constraint sintering, it was not contracted in the surface direction, but only in the thickness direction, and the results were represented in the previous experimental results. We found that coupled simulation can be applied as a tool to design and analyze the sintering behavior of products obtained by co-firing and constrained sintering.

Monte Carlo simulation was performed to investigate the effect of sintering on the changes in microstructure, porosity, and pore distribution of thermal barrier coating layers synthesized by electron beam physical vapor deposition. First, the simulation well expressed the characteristic porous structures such as columnar and feather-like structures, which were fabricated under the condition of substrate rotation. Secondly, the sintering of such a porous structure by high-temperature annealing could be demonstrated using the simulation. The microstructure of the layer close to the substrate surface shows fine grains and pores, suggesting ease of sintering and grain growth, which is clearly demonstrated from the experimental results of the yttria-stabilized zirconia layer deposited by electron beam physical vapor deposition. The simulation also reveals the sintering behavior of the porous including heterogeneous structure and pore distribution. By introducing a small sintering step to the deposition process during the simulation, the experimental values of the porosity observed in the layers could be successfully explained. The doubled effects of sintering on porosity during both deposition and exposure to high temperature environment enhanced the heterogeneity of the microstructures of the layers.

The effects of the addition of platinum group metals Ru, Rh and Ir on the microstructure and mechanical properties of cemented carbide at room and high temperatures were investigated. It was found that the increase in hardness by the addition of platinum group elements was due to the grain growth inhibition of WC and solid solution hardening of the binder phase. Furthermore, when creep deformation resistance was investigated as a mechanical property at high temperature, it was found that the strain rate of the WC-Co with Ru, Rh and Ir were lower than that of the WC-Co alloy. As a practical test, cutting test was conducted using martensitic stainless steel, which is used as a material for turbine blades. It was found that cemented carbides containing Ru had superior plastic deformation resistance compared to alloy without Ru.