Hot Isostatic Pressing of Cubic Boron Nitride–Tungsten Carbide/Cobalt (cBN–WC/Co) Composites: Effect of cBN Particle Size and Some Processing Parameters on their Microstructure and Properties

Cubic boron nitride (cBN)-cemented carbide composites have gained attraction over the last few years because of their potential uses as wear parts. The densification behavior of cBN—hard metal composites by glass encapsulation hot isostatic pressing—has been investigated. Composites with different cBN grades (from 0/0.5 to 6/12 μm particle sizes) and cBN content (up to 50 vol%) were selected for this study. Near-full densification was obtained at temperatures between 1100° and 1200°C, and pressures between 150 and 200 MPa, respectively, while no phase transformation of cBN into the low-hardness hexagonal form has been detected by X-ray diffraction. The addition of cBN to the hard metal base material led to an increase of hardness, a significant increase of fracture toughness ( K _{I C} measured by Vickers indentation), and a moderate decrease of mechanical strength (determined by three-point bending).     

ZrO2增韧Al2O3—TiC系陶瓷复合材料的力学性能及其耐磨性能

本文通过对ＺｒＯ２增韧Ａｌ２Ｏ３－ＴｉＣ系复相陶瓷材料的制备工艺以及ＺｒＯ２含量的变化对材料断裂韧性、抗弯强度以及硬度的影响研究，采用Ｘ射线衍射法分析断口相变量随组成变化对多元系相陶瓷断裂韧性、抗弯强度的影响，同时分析在不同冲击工况下其耐冲蚀磨损特性与力学性能之间的关系。     

Mechanical and tribological properties of TiB2-SiC and TiB2-SiC-GNPs ceramic composites

TiB₂-SiC and TiB₂-SiC-graphene nanoplatelets (GNPs) composites were prepared using field-assisted sintering technology at 2100 °C in argon atmosphere, and the influence of the SiC and different GNPs addition on microstructure development, mechanical and tribological properties has been investigated. Instrumented hardness, bending strength, chevron-notched fracture toughness and ball-on-flat tribological tests were used for the testing and characterization of the composites. The addition of SiC significantly improved the bending strength and elastic modulus with values of 601 MPa and 474 GPa, respectively, but decreased the fracture toughness with a value of 4.8 MPa.m^1/2. The addition of GNPs has a positive effect on fracture toughness and flexural strength but a negative one on the hardness. The increasing amount of both GNPs has a positive influence on wear characteristics of the composites thanks to the described wear mechanisms.     

Microstructure and mechanical properties of Al2O3–Ti(C,N)–cBN composites prepared by a novel hot-forging process

Al₂O₃–cBN has received considerable attention in the field of ceramic cutting tools due to its high hardness, high wear resistance, and low cost, but poor interfacial bonding affects the performance of the composite. In this study, a novel hot-forging process was used to prepare high-performance Al₂O₃–cBN composites using Ti(C,N) as a binder. The evolution of the morphology, phase, and microstructure of the hot-forged Al₂O₃–Ti(C,N)–cBN composites was determined, and the mechanical properties were measured. The relative density of the composites increases significantly after hot forging, and the deformation of the composites increases with the hot-forging temperature. The highest performing Al₂O₃–Ti(C,N)–cBN composite was prepared by hot forging at 1600°C and has a hardness of 20 GPa, a bending strength of 647 MPa and a fracture toughness of 5.37 MPa m^1/2, which are superior to those of a directly hot-pressed sintered composite. However, at hot-forging temperatures higher than 1700°C, Al₅O₆N and TiB₂ are formed in the composite. In the composite hot forged at 1800°C, serrated grain boundaries promote the strength and toughness of the composite to 877 MPa and 6.76 MPa m^1/2, respectively. Therefore, the novel hot-forging process is expected to enhance material properties.     

Mechanical and thermal shock properties of Cf/SiBCN composite: Effect of sintering densification and fiber coating

In this work, resin-derived carbon coating was prepared on carbon fibers by polymer impregnation pyrolysis method, then silicoboron carbonitride powder was prepared by mechanical alloying, and finally carbon fiber-reinforced silicoboron carbonitride composites were prepared by hot-pressing process. The effects of sintering densification and fiber coating on microstructure, mechanical properties, thermal shock resistance, and failure mechanisms of the composites were studied. Fiber bridging hinders the sintering densification, causing more defects in fiber-dense area and lower strength. However, higher sintering temperature (1800–2000°C) can improve mechanical properties significantly, including bending strength, vickers hardness, and elastic module, because further sintering densification enhances matrix strength and fiber/matrix bonding strength, while the change of fracture toughness is not obvious (2.24–2.38 MPa·m^1/2) due to counteraction of higher debonding resistance and less pull-out length. However, fiber coating improves fracture toughness greatly via protecting carbon fibers from chemical corrosion and damage of thermal stress and external stress. Due to lower coefficient of thermal expansion, lower fiber loading ratio, less stress concentration at the fiber/matrix interface, and better defect healing effect, lower sintering temperature favors thermal shock resistance of composites, and thermal shock recession mechanisms are the damage of interface.     

High pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of thermodynamic condition and cBN volume fraction on their microstructure and properties

Tungsten carbide (WC) with different amounts of Cubic boron nitride (cBN) were synthesized by High Pressure-High Temperature (HPHT) method. The mapping correlation between thermodynamic condition, cBN addition, and microstructure, mechanical properties of WC–cBN composites was established and analyzed by response surface methodology. The main factors affecting the properties of composites were identified by ANOVA. The optimum thermodynamic condition was calculated. It was found that a minor phase transformation of cBN into the low-hardness hBN occurred at a temperature of 1300 °C and intensified at 1500 °C. The homogeneously dispersed cBN particles in the WC matrix promoted an improvement of hardness and fracture toughness, but the phase transition of cBN and its truss effect can dramatically reduce the mechanical properties. The Vickers hardness and fracture toughness of the well-sintered WC-cBN bulks reached a high value of 34 GPa and 13.6 MPa·m^1/2, which are improved by 17% and 52% respectively compared with the pure WC samples sintered under similar high-pressure level.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

热压法制备Al2O3/TiB2/AlN/TiN复合陶瓷

将金属Al,Al3Ti和TiB2以AlTiB中间合金的形式引入Al2O3基体材料中,利用热压法制备Al2O3/TiB2/AlN/TiN复合陶瓷.探讨了复合陶瓷致密化程度与AlTiB体积含量之间的关系.复合陶瓷在烧结过程中属过渡液相烧结.烧结过程中Al,Ti和N2(保护气氛)通过化学反应生成新相AIN和TiN.对热压烧结后材料的硬度、断裂韧性和抗弯强度进行了测试和分析.分析了复合陶瓷的力学性能随AlTiB体积含量变化的规律.比较了复合陶瓷1500℃和1600℃的相对密度及力学性能.探讨了复合陶瓷断面断裂方式的变化对其力学性能的影响,并分析了AlTiB中间合金的细化特性.     

Improved tribological properties of polyimide composites by micro–nano reinforcement

The hydroxylate carbon nanotubes (CNTs) were grafted by chemical method on the surface of the oxidized carbon fibers (CF) to improve the mechanical and tribological properties of polyimide (PI). The microstructure and fracture surface of the polyimide composites indicated that CF–CNTs hybrid as a multiscale reinforcement can distribute into the PI matrix homogeneously. Tribo-tests further showed that CF–CNTs hybrid had a better effect on hardness increment, impact strength enhancement, friction reduction, and wear resistance. Compared to the neat PI, the friction coefficient and wear rate of CF–CNTs/PI composite deceased by 23.2 and 55.9%, respectively. In particular, the loading capacity and high speed resistance of CF–CNTs/PI composite were greatly improved. The corresponding wear mechanisms were also discussed by observing the worn surface of the PI composites. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47900.     

Mechanical properties of SiCp/Al2O3 ceramic matrix composites prepared by directed oxidation of an aluminum alloy

Silicon carbide particulate reinforced alumina matrix composites were fabricated using DIrected Metal OXidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with appropriate dopants along with a preform of silicon carbide has led to the formation of alumina matrix surrounding silicon carbide particulates. SiC_p/Al₂O₃ ceramic matrix composites fabricated by the DIMOX process, possess enhanced mechanical properties such as flexural strength, fracture toughness and wear resistance, all at an affordable cost of fabrication. SiC_p/Al₂O₃ matrix composites were investigated for mechanical properties such as flexural strength, fracture toughness and hardness; the composite specimens were evaluated using standard procedures recommended by the ASTM. The SiC_p/Al₂O₃ ceramic matrix composites with SiC volume fractions from 0.35 to 0.43 were found to possess average bend strength in range 158-230 MPa and fracture toughness was found to be in range of 5.61-4.01 MPa√m. The specimen fractured under three-point loading as observed under scanning electron microscope was found to fail in brittle manner being the dominant mode. Further the composites were found to possess lower levels of porosity, among those prepared by DIMOX process.     

Effect of diopside addition on sintering and mechanical properties of alumina

In this paper, diopside was introduced in alumina as a sintering aid and fine structural alumina matrix ceramic materials were fabricated by pressureless sintering. The relative density, hardness, fracture toughness and bending strength of the new fabricated composites were measured. Tribological tests were carried out at a given rotation speed of 160 rpm and in a normal load ranged from 50 to 200 N. The experiment results show that the introduction of diopside can enhance densification rate, which may contribute to the improvement in mechanical properties and result in enhanced wear resistances. The effects of diopside on mechanical properties and microstructures of fine structural alumina matrix ceramic materials were analyzed and discussed.     

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

Alternating pyrolytic carbon/boron nitride (PyC/BN)_n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiC_f/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiC_f/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiC_f/SiC composites. The results indicated that the (PyC/BN)_n multilayer interface led to optimum flexural strength and fracture toughness of 566.0?MPa and 21.5?MPa?m^1/2, respectively, thus the fracture toughness of the composites was significantly improved.     

Mechanical and tribological properties of alumina-MWCNTs composites sintered by rapid hot-pressing

Alumina – MWCNTs composites were prepared using a novel approach. This process comprises functionalization of MWCNTs and stabilization of alumina-MWCNTs dispersion with subsequent freezing, which resulted in formation of granulated powders with homogeneous distribution of MWCNTs. The granulated powders were sintered by rapid hot pressing (RHP) at 1550 °C. Relative densities, microstructural analysis, tribological properties, fracture toughness and bending strength of prepared composites were investigated to reveal the effect of MWCNTs. Compared to pure alumina, bending strength and fracture toughness of dense alumina-5 vol.% MWCNTs composites decreased about 37% and 18%, respectively. At higher MWCNT contents, strength remained almost constant and fracture toughness slightly increased. Thus, the positive effect of CNTs on fracture toughness was demonstrated despite their counteracting effect on the refinement of the microstructure.     

Enhancing toughness and strength of SiC ceramics with reduced graphene oxide by HP sintering

In this paper, the silicon carbide-reduced graphene oxide (SiC/rGO) composites with different content of rGO are investigated. The hot pressing (HP) at 2100?°C for 60?min under a uniaxial pressure of 40?M?Pa resulted in a near fully-dense SiC/rGO composite. In addition, the influence of graphene reinforcement on the sintering process, microstructure, and mechanical properties (fracture toughness, bending strength, and Vickers hardness) of SiC/rGO composites is discussed. The fracture toughness of SiC/rGO composites (7.9MPam^1/2) was strongly enhanced by incorporating rGO into the SiC matrix, which was 97% higher than the solid-state sintering SiC ceramics (SSiC) by HP. Meanwhile, the bending strength of the composites reached 625?M?Pa, which was 17.3% higher than the reference materials (SSiC). The microstructure of the composites revealed that SiC grains were isolated by rGO platelets, which lead to the toughening of the composite through rGO pull out/debonding and crack bridging mechanisms.     

纤维氧化处理对SiCsf/SiC复合材料力学性能的影响

以Y2O3、Al2O3为烧结助剂,采用无压烧结法制备短碳化硅纤维(2～4mm)增强碳化硅(ShortSiCfiberreinforcedSiCcomposite,SiCsf/SiC)复合材料,研究了纤维氧化处理对SiCsf/SiC复合材料结构及力学性能的影响。采用X射线衍射(XRD)、扫描电镜(SEM)以及力学性能试验机对材料进行结构表征和力学性能测试。结果表明:纤维氧化处理后,复合材料的弯曲强度和断裂韧性均有大幅提高。当纤维含量达到5wt%时,复合材料断裂韧性为5.41MPa.m1/2,与原始纤维增强SiC样品相比,提高了6.5%;与无纤维增强SiC样品相比,提高了27%。扫描电镜显示纤维氧化处理后,纤维与基体结合紧密。     

改性碳纳米管增强热塑性聚氨酯复合材料的性能研究

以碳纳米管（CNTs）和热塑性聚氨酯（TPU）为原料,通过硫酸（H₂SO₄）/硝酸（HNO₃）混合溶液处理碳纳米管颗粒表面以达到改性的效果,使用改性过后的碳纳米管熔融共混制备出TPU/CNTs复合材料。研究了不同含量的CNTs对TPU基体的流变、力学、耐磨性以及热性能的影响。结果表明, 改性过后的CNTs在TPU基体中形成了良好的分散性和相容性;TPU/CNTs复合材料在高频剪切下保留了复合材料的加工流动性,并且复合材料的拉伸强度以及耐磨性相较于TPU有明显的增强,其中在改性碳纳米管含量较低时,复合材料的力学性能改善较为明显;改性CNTs的加入提高了TPU基体的熔融温度和结晶度;改性CNTs的加入提高了复合材料的热降解温度,提高了TPU基体的热稳定性。     

Tough Alumina/Polymer Layered Composites with High Ceramic Content

Ceramic composites found in nature, such as bone, nacre, and sponge spicule, often provide an effective resolution to a well‐known conflict between materials' strength and toughness. This arises, on the one hand, from their high ceramic content that ensures high strength of the material. On the other hand, various pathways are provided for stress dissipation, and thus toughness, due to their intricate hierarchical architectures. Such pathways include crack bridging, crack deflection, and delamination in the case of layered structures. On the basis of these inspiring ideas, we attempted here to create simultaneously strong and tough laminated alumina composite with high ceramic content. Composites were prepared from high‐grade commercial alumina with spin‐coated interlayers of ductile polymers (PMMA and PVA). The specimens' ultimate properties (strength, fracture toughness, and work of fracture) were measured by a four‐point bending method. In some cases, fracture toughness of the composites was increased by up to an order of magnitude, reminiscent of the natural layered composites. It is proposed that this increase may be attributed to an interlocking mechanism, often encountered in biological composites. The significance of sample architecture and the role of the interfacial and bulk properties of the interlayer material are discussed.     

Interfacial Microstructure and Chemistry of SiC/BN Dual-Coated Nicalon-Fiber-Reinforced Glass-Ceramic Matrix Composites

Glass-ceramic composites with improved high-temperature mechanical properties have been produced by incorporating continuous SiC fibers into a barium magnesium aluminosilicate matrix. Control of the fiber/matrix interface was achieved by a dual-layer coating of SiC/BN(C) applied to the fibers by CVD. The weakly bonded interface resulted in composites with high fracture toughness and strength up to 1100°C, and the composite system was oxidatively stable during long-term exposure to air at high temperatures. Composites with different thermal and mechanical histories were studied, and interfaces were characterized using transmission electron microscopy (TEM), Auger electron spectroscopy, and fiber pushout tests. Observations of interfacial microstructure were correlated with the mechanical properties of the composite and with interface properties determined from fiber push-out tests.     

High strength TiC matrix Fe28Al toughened composites prepared by spontaneous melt infiltration

Fe28Al bound TiC matrix composites with TiC content of 75–90% in volume (vol.%) were successfully fabricated by spontaneous melt infiltration. Amounts of Fe28Al in excess and below the pore volume of the TiC preform were used for optimization of fabrication techniques. Young's modulus, hardness, flexural strength and fracture toughness of the composites were measured. Four-point bending strength of Fe28Al/90–75 vol.% TiC ranges to 990–1260 MPa. The high strength is attributed to the good infiltration ability of molten Fe28Al in the porous TiC preform and to processing refinements. TiC preform pre-sintering and indirect infiltration all lead to fully dense and defect-free composites. The relationship between Vickers hardness and indentation fracture toughness and the dependence of mechanical properties on microstructure of the composites were also studied. Results of SEM and XRD analysis show TiC and Fe28Al as the only crystalline phases of the composite. Fe28Al ligaments have ductile behaviour and greatly toughen the composites. Crack front deviation during fracture also increased the fracture resistance of the composites.     

Preparation of Y2Si2O7/ZrO2 Composites and Their Composition – Mechanical Properties – Tribology Relationships

In this work, novel Y₂Si₂O₇/ZrO₂ composites were developed for structural and coating applications by taking advantage of their unique properties, such as good damage tolerance, tunable mechanical properties, and superior wear resistance. The γ‐Y₂Si₂O₇/ZrO₂ composites showed improved mechanical properties compared to the γ‐Y₂Si₂O₇ matrix material, that is, the Young's modulus was enhanced from 155 to 188 GPa (121%) and the flexural strength from 135 to 254 MPa (181%); when the amount of ZrO₂ was increased from 0 to 50 vol%, the γ‐Y₂Si₂O₇/ZrO₂ composites also presented relatively high facture toughness (>1.7 MPa·m^1/2), but this exhibited an inverse relationship with the ZrO₂ content. The composition–mechanical property–tribology relationships of the Y₂Si₂O₇/ZrO₂ composites were elucidated. The wear resistance of the composites is not only influenced by the applied load, hardness, strength, toughness, and rigidity but also effectively depends on micromechanical stability properties of the microstructures. The easy growth of subcritical microcracks in Y₂Si₂O₇ grains and at grain boundaries significantly contributes to the macroscopic fracture toughness, but promotes the pull‐out of individual grains, thus resulting in a lack of correlation between the wear rate and the macroscopic fracture toughness of the composites.