Nanocomposite Ceramic Tool Materials Research Articles

Simulation of Microscopic Fracture Behavior in Nanocomposite Ceramic Tool Materials

In this paper, the microstructures of nanocomposite ceramic tool materials are represented through Voronoi tessellation. A cohesive element model is established to perform the crack propagation simulation by introducing cohesive elements with fracture criteria into microstructure models. Both intergranular and transgranular cracking are considered in this work. The influences of nanoparticle size, microstructure type, nanoparticle volume content and interface fracture energy are analyzed, respectively. The simulation results show that the nanoparticles have changed the fracture pattern from intergranular mode in single-phase materials to intergranular–transgranular–mixed mode. It is mainly the nanoparticles along grain boundaries that have an impact on the fracture pattern change in nanocomposite ceramic tool materials. Microstructures with smaller nanoparticles, in which there are more nanoparticles dispersed along matrix grain boundaries, have higher fracture toughness. Microstructures with a nanoparticle volume content of 15% have the most obvious transgranular fracture phenomenon and the highest critical fracture energy release rate. A strong interface is useful for enhancing the fracture toughness of nanocomposite ceramic tool materials.

Effects of graphene pull‐out on the interfacial mechanical properties of alumina/graphene nanocomposite ceramic tool materials: Molecular dynamics analysis and material preparation

AbstractThe interfacial mechanical properties of the alumina/graphene (Al2O3/GR) nanocomposite ceramic tool materials were studied by molecular dynamic simulations. The effects of pull‐out velocity of graphene, system temperature, vacancy defect, and wrinkled graphene on the interfacial mechanical properties were investigated. Results show that a large interfacial shear stress (382 MPa) exists between the Al2O3/GR interface. The maximum pull‐out force and interfacial shear stress increase with increasing graphene pull‐out velocity. The increase of temperature reduces the maximum pull‐out force and interfacial shear stress. Moreover, the load transfer capacity is reduced and the creep of the matrix occurs. These factors reduce the fracture toughness of the Al2O3/GR nanocomposites. A moderate amount of vacancy defects in graphene increases the interfacial pull‐out force and shear stress of Al2O3/GR nanocomposite ceramic tool. Compared with Al2O3/GR nanocomposite ceramic tools, the interfacial interaction energy and shear stress of pleated graphene and Al2O3 were increased by 13.87% and 41.08% respectively, with better toughening and reinforcement. The results are of significance in further understanding of the interfacial mechanical properties at the nanoscale and then beneficial to improve the mechanical properties and engineering performance of Al2O3/GR composite ceramic materials.

Phase field simulation of grain growth in Al2O3-based composite ceramic cutting tool materials containing second phase nanoparticles and pores

A phase field model was established to simulate the grain growth of Al2O3-based composite ceramic tool materials containing second phase nanoparticles and pores. The free energy parameters of the model are directly related to the surface and grain boundary energies of Al2O3 such that the grain growth process can be quantitatively analysed. The model was used to investigate the effects of the volume fraction and size of second phase particles on grain growth in Al2O3 with a certain initial pore volume fraction. Findings show that pores and second phase particles jointly hinder grain growth. When the radii of second phase nanoparticles are the same, as the number of particles increase, most of the grain boundaries occur as straight lines in the microstructure, and grain size decreases. It is beneficial to obtain fine and uniform microstructure. However, when the particles content reaches a certain level, the second phase particles tend to agglomerate at the grain boundary of matrix grains, which will result in a decrease in the properties of nanocomposite ceramic tool materials. The study also found that, at a constant volume fraction of particles, with the decrease of particle radius, the grain size decreases. When the second phase particle size is reduced to 50 nm, relatively more intragranular microstructure was observed, which is considered to be the main reason for increasing the toughness of nano composite ceramic tool materials. Through analyzing the simulation results, the optimal combination of the content and size of the second phase particles can provide theoretical guidance for Al2O3 nano composite ceramic tool materials design and preparation.

Thermal and mechanical shock resistances of Si3N4/(W, Ti)C graded nano-composite ceramic tool material

Thermal and mechanical shock resistances of a newly developed Si3N4/(W, Ti)C graded ceramic tool material were investigated using the water quenching methods and intermittent cutting test, respectively. One homogeneous material was used as a competitor. In the quench-strength tests, the retained flexural strength slightly decreased at first and then dramatically decreased, and after that it decreased gradually once again. The critical temperature difference was 600 °C for the graded material and 500 °C for the homogeneous material. The initiation, propagation and merge of microcracks were the main reasons that leaded to the sharp decrease of bending strength. The graded material had a smaller crack propagation length value than the homogeneous material after the same thermal shock cycles in the indentation-quench tests. In the intermittent cutting test, the tools failed mainly due to ladder-like fracture around the cutting nose. Tool failure mechanisms included mechanical shock microcracks, adhesion, and chipping. After the same number of impacts, the graded tool showed much smaller flank wear. Reasonable distribution of composition resulted in smaller thermal shock tensile stress in the surface layer of the graded material. The residual compressive stress introduced by the graded structure could alleviate some external thermal and mechanical stress. These effects inhibited the growth of microcracks and made the graded material higher thermal and mechanical shock resistances.

High temperature mechanical properties of Si3N4/(W, Ti)C graded nano-composite ceramic tool material

A Si3N4/(W, Ti)C graded nano-composite ceramic tool material was developed and its high temperature flexural strength and fracture toughness were studied. With increased testing temperature, the flexural strength demonstrates a tendency to first decrease gradually and then decrease sharply, and the transition temperature is 1000 °C. The softening effect of glassy phases, the oxidation effect and the surface cracks result in the gradual decrease of flexural strength while the growth and merge of microcracks and cavities results in the sharp decrease. The fracture toughness under elevated temperatures increases gradually and then decreases dramatically, and the transition temperature is 1000 °C as well. The plastic deformation behavior, microcrack toughening effect and the healing effect of SiO2 layer result in the gradual increase of fracture toughness while liquefaction of the grain-boundary phase results in the sharp decrease. Due to reasonable composition distribution and induced compressive residual stress in the surface layers, the graded material shows a significantly better high temperature properties than the homogenous one.

Simulation of microcrack initiation in nanocomposite ceramic tool materials

In this work, the microcrack initiation behavior of nanocomposite ceramic tool materials caused by the residual stress initiated during the cooling stage is simulated by means of a micromechanical model based on the Voronoi tessellation and the cohesive element theory. Influences of microstructure morphologies such as nanoparticle size, nanoparticle volume content and microstructure types on the microcrack distribution and material properties are analyzed respectively. The conclusions are useful for providing theoretical basis for the development of nanocomposite ceramic tool materials.

Microscopic cracking simulation of nanocomposite ceramic tool materials under the consideration of residual stress

In this paper, the numerical simulation of crack propagation in microstructures of Al2O3/SiCn nanocomposite ceramic tool materials is carried out by means of a micromechanical model based on the Voronoi tessellation and the cohesive element theory in order to explore the relationship between microstructure morphologies and mechanical properties. The residual stress initiated due to the mismatch of thermal expansion coefficients and elastic modulus between matrix grains and nanoparticles is considered in the simulation. The effects of microstructure types and nanoparticle volume content on the residual stress field and the fracture behavior are analyzed, respectively.

Design and fabrication of Si3N4/(W, Ti)C graded nano-composite ceramic tool materials

Si3N4/(W, Ti)C graded nano-composite ceramic tool materials with different thickness ratios and number of layers were fabricated by hot pressing technology. The flexural strength, fracture toughness and hardness of the sintered composites were tested and the microstructure and indention cracks were observed. The experiment results showed that the five-layer graded nano-composites with a thickness ratio of 0.2, which were sintered under a pressure of 30MPa at 1700°C in vacuum condition for 45min, had the optimum comprehensive mechanical properties with a flexural strength of 1080.3MPa, a hardness of 17.64GPa, and a fracture toughness of 10.87MPa·m1/2. The formation of elongated β-Si3N4 grains contributes to the favorable mechanical properties. The graded structure can induce residual compressive stress in the surface layer and enhance the mechanical properties. The strengthening and toughening mechanisms are a synergistic effect of intergranular and transgranular fracture, crack bridging and deflection.

Fabrication and mechanical properties of Si3N4/(W, Ti)C/Co graded nano-composite ceramic tool materials

Si3N4/(W, Ti)C/Co nano-composite ceramic tool materials with multilayered structure were fabricated by hot pressing technology. The composites without Co were used as the surface layers and the composites containing Co were used as the inner layers. The flexural strength, fracture toughness, and hardness of the composites were tested, and the fracture surfaces and indention cracks were observed. The experimental results showed that the flexural strength and fracture toughness were largely increased by the graded structure and the high hardness was still maintained. The five-layer graded nano-composites, with a thickness ratio of 0.2, which were sintered under a pressure of 30MPa at 1700°C in vacuum condition for 45min, had the optimum comprehensive mechanical properties with a flexural strength of 992MPa, a hardness of 17.83GPa, and a fracture toughness of 10.5MPa·m1/2. The formation of elongated β-Si3N4 grains and the residual compressive stress in the surface layer induced by the graded structure contributed to the improved properties. A synergistic effect of intergranular and transgranular fracture, crack bridging, and deflection contributed to the strengthening and toughening mechanisms.

Fabrication and Mechanical Properties of Homogenous and Graded Si<sub>3</sub>N<sub>4</sub>/(W, Ti)C Nano-Composite Ceramic Tool Materials

Close attention has been paid to Functional graded materials (FGMs) worldwide for their novel design ideas and outstanding properties. To verify the advantage of FGMS in the design of ceramic tool materials, Si3N4/(W, Ti)C nanocomposite ceramic tool materials with homogenous and graded structure were fabricated by hot pressing and sintering technology. The flexural strength, fracture toughness and hardness of the sintered composites were tested and compared. The experimental results showed that the graded structure improved mechanical properties of the ceramic tool materials, especially the flexural strength and fracture toughness. The introduction of residual compressive stress in the surface layer contributes to the improvement of the properties .

Preparation and characterization of Si3N4/(W, Ti)C nano-composite ceramic tool materials

Si3N4-based composite ceramic tool materials reinforced with nanoscale Si3N4 and microscale (W, Ti)C were fabricated by hot pressing and sintering technology. The flexural strength, fracture toughness and hardness of the sintered composites were tested and the microstructure and indention cracks were observed. The experimental results showed that the Si3N4/(W, Ti)C nanocomposites containing 18vol% of nanoscale Si3N4 and 20vol% of microscale (W, Ti)C, which were sintered under a pressure of 30MPa at 1700°C in vacuum condition for 45min, had the optimum comprehensive mechanical properties. The appropriate addition of microscale (W, Ti)C and nanoscale Si3N4 accelerated the formation of elongated β-Si3N4 grains and contributed to the improvement of mechanical properties. The strengthening and toughening mechanisms for the Si3N4/(W, Ti)C composite ceramic tool materials was a synergistic effect of intergranular and transgranular fracture, crack bridging, deflection and branching.

Dynamic fatigue behavior of Al2O3/TiC micro–nano-composite ceramic tool materials at ambient and high temperatures

Dynamic fatigue behavior of Al2O3/TiC micro–nano-composite ceramic tool materials with/without metallic phase was investigated using three-point bending technique at different loading rates at ambient and high temperatures in air. When the loading rate was low, the flexural strength of two kinds of materials increased with the increase of loading rate at 23°C and 900°C, but they were more susceptible to slow crack growth at 900°C. The composite with cobalt exhibited better slow crack growth resistance than that of the composite without cobalt at ambient and high temperatures. The slow crack growth zone of specimens all originated from the tensile surface. Slow crack growth was generally a result of a stress corrosion process involving the stable growth of pre-existing defects (machining cracks on the tensile surface and residual pores in the bulk material). Crack initiation zone became larger at higher temperature, which should be attributed to the oxidation-assisted defects and the low dynamic viscosity of Co-based grain boundary phases. At 1200°C, the composite with cobalt exhibited larger plastic deformation at the loading rates of 0.08 and 0.8MPa/s, and the plastic deformation was carried out by dislocation slip and twinning. The dominated failure mechanism at the lower loading rate was ascribed to creep rather than stress-corrosion cracking.

Simulation of the Effect of Sintering Temperature on Microstructure Evolution in Nanocomposite Ceramic Tool Materials

A Monte Carlo Potts model coupled with sintering temperature for the sintering process of nanocomposite ceramic tool materials is proposed, the relation between grain growth and sintering temperature is presented. The grain growth process at different sintering temperature is investigated in this model, and the effect of sintering temperature on microstructure evolution is discussed, it is found that the mean grain size increases with the increase of sintering temperature during simulation, and nanoparticles are easier to enter into matrix grains to form intragranular-type microstructure at higher temperature. The simulation results are in accordance with the experimental observations.

Simulation of the Effect of Nano Phase Size on Microstructure Evolution in Nanocomposite Ceramic Tool Materials

Monte Carlo simulation method of microstructure evolution in nanocomposite ceramic tool materials is set up. Two-phased material systems with the same area fractions but various sizes of nanoparticles are designed, and the processes of microstructure evolution are simulated. The influences of nanoparticle size on microstructure and grain growth are observed and discussed quantitatively. It is found that the mean size of matrix grains increases with the increase of nanophase size during simulation, and intragranular-type microstructure is more difficult to form. The material Al2O3/nanoSiC is developed and the simulated results are verified.

Simulation of the Effect of Sintering Pressure on Microstructure Evolution in Nanocomposite Ceramic Tool Materials

A Monte Carlo Potts model coupled with sintering pressure for the sintering process of nanocomposite ceramic tool materials is proposed, the relation between grain growth and sintering pressure is presented. The grain growth process at different sintering pressure is investigated in this model, and the effect of sintering pressure on microstructure evolution is discussed, it is found that the mean grain size increases with the increase of sintering pressure during simulation. The results from this simulation are shown to correlate well with the experimental observations.

Preparation and characterization of Al2O3/TiC micro–nano-composite ceramic tool materials

Abstract An Al 2 O 3 -based composite ceramic tool material reinforced with micro-scale and nano-scale TiC particles was fabricated by a hot-pressing technology with cobalt additive in different sintering processes. The microstructure, indention cracks and phase composition of composites were characterized with scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The experimental results showed that Al 2 O 3 /TiC μ /TiC n micro–nano-composite containing 6 vol% nano-scale TiC and 35 vol% micro-scale TiC, which were sintered under a pressure of 32 MPa at a temperature of 1650 °C in vacuum for 20 min, had optimum mechanical properties. The addition of both nano-scale TiC and Co contributed to the microstructure evolution and the improvement of mechanical properties. Effects of nano-scale TiC on mechanical properties were investigated. The toughening and strengthening mechanisms of micro–nano-composites were discussed.

Three-dimensional simulation of microstructure evolution for three-phase nano-composite ceramic tool materials

It is believed that adding particles can improve mechanical properties of ceramic tool materials. So far, two-phase composites materials have been widely investigated. However, materials with two-kinds of particles are seldom researched. In this paper, microstructure evolution of two-phase and three-phase nano-ceramic materials are simulated and compared. The effects of particle volume fraction, radius, distributions and grain boundary energy ratios on microstructure evolution are analyzed respectively. It shows that adding two kinds of particles can receive better grain refinement than just adding one kind of particles. The higher volume fraction and smaller-sized nano-particles have stronger pinning force on matrix grain growth. The nano-particles located around grain boundaries have better inhibition effect than those distributed randomly on matrix grain growth. Grain boundary energy ratio has significant influence on pinning effect of particles. The nano-particles not only restrain grain boundary migration, but also prevent the elimination of small grains and refine matrix grains.

Numerical Simulation on the Sintering Process of Nanocomposite Ceramic Tool Materials

A Monte Carlo Potts model for the sintering process of two-phase nanocomposite ceramic tool materials is proposed. The grain growth, pore diffusion and vacancy annihilation at grain boundaries are simulated in the model. Simulation results are presented and discussed. It is found that pore and nanophase can pin the ceramic matrix and decrease the growth rate of matrix grain during the microstructure evolution. The addition of nanoparticles is not only beneficial to the refinement of ceramic matrix grain and the formation of intra/intergranular-type microstructure but also advantageous to the densification of ceramic materials.

Three Dimensional Monte Carlo Simulation of Microstructure Evolution in Presence of Pores and Impurities for Three-Phase Nanocomposite Ceramic Tool Materials

A modified three-dimensional Monte Carlo (MC) model in presence of pores and impurities for three-phase nanocomposite ceramic tool materials is successfully established in this paper. Pore migration by surface diffusion is incorporated into the MC model and it is applied to observe and scientific quantitative characterization of three dimensional microstructure evolution and densification process. Some modifications are applied to the simulation algorithm to improve the computing efficiency. The influence of pores on the particle and impurity loaded grain boundaries is simulated and investigated for the study of grain growth kinetics. The relationship between porosity and density is also analyzed. The results indicate that the higher the porosity is, the lower the density will be.

Three Dimensional Monte Carlo Simulation of Microstructure Evolution in Presence of Pores for Three-Phase Nano-Composite Ceramic Tool Materials

A new three-dimensional Monte Carlo (MC) model in presence of pores of microstructure evolution for three-phase nano-composite ceramic tool materials is successfully established to simulate the grain growth during sintering process in this paper. The defect-free microstructure evolution and microstructure evolution in presence of pore are simulated and investigated. The results show that the new MC model can well simulate the grain growth and pores shrinkage during densification process. Compared with defect-free system, the grain growth velocity can be slow down obviously owning to the existence of pores.