Microstructure,mechanical properties and toughening mechanisms of graphene reinforced Al2O3-WC-TiC composite ceramic tool material

The low fracture toughness of Al₂O₃-based ceramics limited their practical application in cutting tools. In this work, graphene was chosen to reinforce Al₂O₃-WC-TiC composite ceramic tool materials by hot pressing. Microstructure, mechanical properties and toughening mechanisms of the composite ceramic tool materials were investigated. The results indicated that the more refined and denser composite microstructures were obtained with the introduction of graphene. The optimal flexural strength, Vickers hardness, indentation fracture toughness were 646.31?±?20.78?MPa, 24.64?±?0.42?GPa, 9.42?±?0.40?MPa?m^1/2, respectively, at 0.5?vol% of graphene content, which were significantly improved compared to ceramic tool material without graphene. The main toughening mechanisms originated from weak interfaces induced by graphene, and rugged fractured surface, grain refinement, graphene pull-out, crack deflection, crack bridging, micro-crack and surface peeling were responsible for the increase of fracture toughness values.     

Microstructure evolution,mechanical and tribological properties of Ti3(Al,Sn)C2/Al2O3 composites

In this paper, in situ formed Ti₃(Al,Sn)C₂/Al₂O₃ composites were fabricated by sintering the mixture of Ti₃AlC₂ and SnO₂. The Al atoms could diffuse out of the Ti₃AlC₂ layered structure to react with SnO₂, resulting in the formation of Ti₃(Al,Sn)C₂ solid solution and Al₂O₃. When the SnO₂ content was 20?wt.%, the sintered Ti₃(Al,Sn)C₂/Al₂O₃ composite exhibited the best overall mechanical properties, because of the optimized cooperative strengthening effect of solution strengthening and Al₂O₃ enhancement. When the SnO₂ content increased up to 30?wt.%, the flexural strength and fracture toughness of Ti₃(Al,Sn)C₂/Al₂O₃ composite dramatically decreased on account of the large accumulation of generated Al₂O₃. Moreover, according to the SiC ball-on-flat wear tests, it was found that the wear resistance of Ti₃(Al,Sn)C₂/Al₂O₃ composites was significantly improved as the SnO₂ content increased.     

Fabrication of in-situ Ti(C,N) phase toughened Al2O3 based ceramics from natural bauxite

Al₂O₃–Ti(C,N) ceramics were fabricated via carbothermal reduction nitridation method with high-titania special-grade bauxite as the raw material. The formation mechanism of in-situ Ti(C,N) phase and its effect on the properties of materials are discussed. After nitrided at 1700 °C, Ti(C,N) phase could be formed in-situ with appropriate C/TiO₂ molar ratio. Due to the residual stress field formed by Ti(C,N) particles, the path of crack propagation is changed, leading to the crack deflection and pinning. Therefore, the mechanical properties of the materials are improved by forming in-situ Ti(C,N) phase. With a C/TiO₂ molar ratio of 2.2 and nitridation temperature of 1700 °C, Al₂O₃–Ti(C,N) ceramic with a hardness of 13.9 GPa, a fracture toughness of 8.28 MPa m^1/2 and a flexural strength of 387 MPa could be fabricated.     

Influence of hBN content on mechanical and tribological properties of Si3N4/BN ceramic composites

Silicon nitride materials containing 1–5 wt% of hexagonal boron nitride (micro-sized or nano-sized) were prepared by hot-isostatic pressing at 1700 °C for 3 h. Effect of hBN content on microstructure, mechanical and tribological properties has been investigated. As expected, the increase of hBN content resulted in a sharp decrease of hardness, elastic modulus and bending strength of Si₃N₄/BN composites. In addition, the fracture toughness of Si₃N₄/micro BN composites was enhanced comparing to monolithic Si₃N₄ because of toughening mechanisms in the form of crack deflection, crack branching and pullout of large BN platelets. The friction coefficient was not influenced by BN addition to Si₃N₄/BN ceramics. An improvement of wear resistance (one order of magnitude) was observed when the micro hBN powder was added to Si₃N₄ matrix. Mechanical wear (micro-failure) and humidity-driven tribochemical reaction were found as main wear mechanisms in all studied materials.     

Cutting performance,failure mechanisms and tribological properties of GNPs reinforced Al2O3/Ti(C,N) ceramic tool in high speed turning of Inconel 718

Cutting performance and failure mechanisms of graphene nano-platelets (GNPs) reinforced ceramic cutting tool ATG (short for Al₂O₃/Ti(C,N)/GNPs) in continuous dry turning of Inconel 718 up to a cutting speed of 500 m/min were investigated in comparison with those of commercial Sialon tool KY1540. The results show that ATG tool shows similar cutting performance with KY1540 tool at the speed range of 150–300 m/min, but greatly outperforms KY1540 when the cutting speed range of 400–500 m/min for higher hardness, wear resistance, chemical inertness and fracture toughness. Flank wear, notch wear, chipping and flaking are the reasons for tool failure of ATG. The wear modes are related to cutting speed, and adhesion wear is found to be the dominant failure mechanism of ATG. It is believed that GNPs play a significant role in improving mechanical properties and tribological properties which contributed to excellent resistance to abrasive wear and fracture. Turning Inconel 718 in dry and high speed via using ATG ceramic tool is an efficient and eco-friendly method.     

Microstructure,mechanical properties,and fracture behaviour of Ti(C,N)-based cermets with a composite structure

Ti(C,N)-based cermets with a composite structure were designed to maintain the balance between strength and toughness. The cermets with the composite structure comprised coarse particles and the matrix, and the coarse particles included fine hard phases compounded in the matrix. A new hard phase grain with a four-layered structure was found. The composite structure of the cermet can contribute to high toughness, and the grain with the four-layered structure in the composite structure imparts high strength and toughness. As the granule size increases, the fracture toughness of the cermets increased, but the hardness and transverse rupture strength (TRS) showed the opposite trend. The toughening mechanisms of the cermet were crack branching, crack bridging, crack deflection, and formation of tear ridges.     

Effect of Ge on microstructure and mechanical properties of Ti3SiC2/Al2O3 composites

Owing to the good physicochemical compatibility and complementary mechanical properties of Ti₃SiC₂ and Al₂O₃, Ti₃SiC₂/Al₂O₃ composites are considered as ideal structural materials. However, TiC and TiSi₂ typically coexist during the synthesis of Ti₃SiC₂/Al₂O₃ composites through an in-situ reaction, which adversely affects the mechanical properties of the resulting composites. In this study, Ti₃SiC₂/Al₂O₃ composites were prepared via in-situ hot pressing sintering at 1450 °C. Ge, which was used as a sintering aid, improved the purity and mechanical properties of the Ti₃SiC₂/Al₂O₃ composites. This is because Ge replaced some of the Si atoms to compensate the evaporation loss of Si to form Ti₃(Si_1-xGe_x)C₂, which showed a crystal structure similar to that of Ti₃SiC₂. Furthermore, the molten Ge accelerated the diffusion reaction of the raw materials, increasing the overall density of the Ti₃SiC₂/Al₂O₃ composites. The optimum Ge amount for improving the mechanical properties of the composites was found to be 0.3 mol. The flexural strength, fracture toughness, and microhardness of the composite with the optimum Ge amount were 640.2 MPa, 6.57 MPa m^1/2, and 16.21 GPa, respectively. The formation of Ti₃(Si_1-xGe_x)C₂ was confirmed by carrying out X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy analyses. A model crystal structure of Ti₃(Si_1-xGe_x)C₂ doped with 0.3 mol Ge was established by calculating the solid solubility of Ge.     

Effect of Y2O3-stabilized ZrO2 whiskers on the microstructure,mechanical and wear resistance properties of Al2O3 based ceramic composites

The aim of this study was to improve the mechanical properties of Al₂O₃ ceramics by the addition of Y₂O₃-stabilized ZrO₂ whiskers (designated as Al₂O₃/YSZ_W composite) through the flux method and hot-pressing technology. The effect of YSZ_W content on their microstructure, phase composition and transformability, mechanical properties, and wear resistance was systematically investigated. The Al₂O₃/YSZ_W composites containing 10 wt% YSZ_W exhibited the best mechanical performance, including the highest content of YSZ_W tetragonal phase and transformability as well as the largest values in their relative density (99.5%), hardness (1969 HV), fracture toughness (9.57 MPa m^1/2) and flexural strength (855 MPa). The strengthening and toughening of the Al₂O₃/YSZ_W composites were attributed to the YSZ_W tetragonal-monoclinic phase transformation as well as the whiskers reinforcing effect. Furthermore, the Al₂O₃/YSZ_W composites also showed the highest friction and wearing properties.     

Effects of Pr6O11 on the microstructure and mechanical properties of Ti/Al2O3 composites prepared by pressureless sintering

Ti/Al₂O₃ composites with different volume percentages of Pr₆O₁₁ added (0–12.0  vol.%) were prepared by pressureless sintering at 1600 °C for 1.5 h. The influences of Pr₆O₁₁ on the composition, microstructure and mechanical properties of the composites were characterized and investigated. The results showed that Pr₆O₁₁ could promote the sintering of the composites by generating some new interfacial reaction products, such as AlTiO₂, Pr₂Ti₂O₇ and PrAlO₃. Pr₆O₁₁ could also inhibit the production of TiAl and Ti₃Al by the same mechanism. Additionally, Pr₆O₁₁ changed hexagonal alumina to tetragonal alumina. The latter could improve the mechanical properties of the composites by the effects of crack deflection and particle pullout when it was present in proper amounts. Composites showed satisfactory comprehensive properties when the content of Pr₆O₁₁ was no more than 3.0 vol.%.     

Synthesis,microstructure and mechanical properties of Cr2AlC/Al2O3 in situ composites by reactive hot pressing

Al₂O₃ particle-reinforced Cr₂AlC in situ composites were successfully fabricated from powder mixtures of Cr₃C₂, Cr, Al, and Cr₂O₃ by a reactive hot-pressing method at 1400 °C. A possible synthesis mechanism was proposed to explain the formation of the composites in which Al₂O₃ was formed by the aluminothermic reaction between Al and Cr₂O₃, meanwhile, Cr₃C₂, Al, together with Cr reacted to form Cr₂AlC in a shortened reaction route. The effect of Al₂O₃ addition on the microstructure and mechanical properties of Cr₂AlC/Al₂O₃ composites was investigated. The results indicated that the as-sintered products consisted of Cr₂AlC matrix and Al₂O₃ reinforcement, and the in situ formed fine Al₂O₃ particles dispersed at the matrix grain boundaries. The flexural strength and Vickers hardness of the composites increased gradually with increasing Al₂O₃ content. But the fracture toughness peaked at 6.0 MPa m^1/2 when the Al₂O₃ content reached 11 vol.%. The strengthening and toughening mechanism was also discussed.     

Effects of non-stoichiometric defects on the dielectric properties of (Ba0.5Sr0.5)xTiO3-ZnGa2O4 composite ceramics

In order to explore the effects of non-stoichiometric defects on the dielectric properties of composite ceramics, 70 wt% (Ba_0.5Sr_0.5)_xTiO₃-30 wt%ZnGa₂O₄ ((BS)_xT50-ZG, x = (Ba + Sr)/Ti = 0.99, 1.00, 1.01 and 1.05) composite ceramics were fabricated by the traditional sintering technique. The association between structure and dielectric properties has been studied. The results show that the distortion of the crystal lattice brought by the partial Schottky defects, namely [V″_Ba,Sr–V^˙˙_O]^× and [V′′′′_Ti–2V^˙˙_O]^×, induces a decrease in the Curie temperature of (BS)_xT50-ZG composite ceramics. The orientation of the elastic dipoles brought by oxygen vacancies causes the pinning of the domain walls, which reduces the dielectric loss at low frequencies. Tunability is related to the dipole polarization caused by [V″_Ba,Sr–V^˙˙_O]^× and [V′′′′_Ti–2V^˙˙_O]^× defect complexes. In addition, compared with the composite ceramic with x = 1, the Q values of the composite ceramics with x < 1 and x > 1 decreases due to the deterioration of the microstructure homogeneity and the enhancement of the disorder of the B-site cations in (BS)_xT50.     

Effects of hafnium content on microstructures and properties of newly developed (Ti,Hf)(C,N) ceramics

In this study, novel (Ti,Hf)(C,N) ceramics with varying hafnium contents were fabricated via carbothermal reduction–nitridation and subsequent spark plasma sintering. The influence of Hf addition on the mechanical properties, wear properties, and corrosion resistance of the (Ti,Hf)(C,N) ceramics was systematically studied. The introduction of Hf promoted the sintering densification of the ceramics in the sintering process. The prepared (Ti,Hf)(C,N) ceramics exhibited excellent mechanical and wear properties owing to refinement and solution-strengthening mechanisms. The (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic demonstrated higher Vickers hardness and fracture toughness, measuring 1997 HV5 and 4.28 MPa m^1/2, respectively, compared to the pure Ti(C_0.5,N_0.5) ceramic which exhibited values of 1635 HV5 and 3.94 MPa MPa m^1/2. The wear scar depth of the (Ti_0.9,Hf_0.1)(C_0.5,N_0.5) ceramic sample was 57.36% to that of the Ti(C_0.5,N_0.5) ceramic. Additionally, the addition of Hf improved the corrosion resistance of (Ti,Hf)(C,N) ceramics in a 0.5 M NaOH solution. The potential applications of (Ti,Hf)(C,N) ceramics include machining tools and wear-resistant parts.     

Effect of YAG content on creep resistance and mechanical properties of Al2O3-YAG composite

Comprehensive study on effect of YAG amount on densification, creep resistance and room-temperature mechanical properties of Al₂O₃-YAG composite pressureless sintered at 1600 °C was conducted. The main goal was to optimize the amount of YAG in order to fabricate a composite with improved creep resistance and sufficiently good room-temperature mechanical properties. The composite was made by mixing a commercially available Al₂O₃ powder with fine YAG powder obtained by glycine-nitrate combustion synthesis starting from aluminum nitrate and yttrium nitrate. Increased driving force for sintering of fine YAG powder allowed fabrication of dense Al₂O₃-YAG composite with up to 30 vol% YAG. The presence of YAG was found to be very effective in improving creep resistance of Al₂O₃-YAG composite. Large Y³⁺ ions blocked diffusion along Al₂O₃ grain boundaries, reduced diffusivity and therefore enhanced creep resistance of Al₂O₃-YAG composite which continuously increased as the YAG amount increased. Тhe presence of YAG was also found to improve mechanical properties such as hardness and elastic modulus. The improvement of these properties was ascribed to increased density of Al₂O₃-YAG composites owing to high sintering activity of YAG powder. While fracture strength of the composite can be as high as that of monolithic Al₂O₃, fracture toughness of composite decreased continuously as the YAG content increased. The decrease was ascribed to transgranular fracture of both YAG and Al₂O₃ grains in samples containing larger amounts of YAG. The proper balance between fracture toughness and creep resistance was found in composite containing 18 vol% YAG which had considerably improved creep resistance accompanied by a relatively small decrease in fracture toughness.     

Effects of Al2O3 addition on sintering behaviour,microstructure, and physical properties of Al2O3/vitrified bond CBN composite

The effects of Al₂O₃ content on the sintering behaviour, microstructure, and physical properties of Al₂O₃/vitrified bonds (SiO₂–Al₂O₃–B₂O₃–BaO–Na₂O–Li₂O–ZnO–MgO) and Al₂O₃/vitrified bond cubic boron nitride (CBN) composites were systematically investigated using X-ray diffraction, differential scanning calorimetry, dilatometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Various amounts of Al₂O₃ promoted the formation of BaAl₂Si₂O₈ and γ-LiAlSi₂O₆, increasing the relative crystallinity of the Al₂O₃/vitrified composite from 85.0 to 93.2%, resulting in residual compressive stress on BaAl₂Si₂O₈, thereby influencing the thermal behaviour and mechanical properties of the Al₂O₃/vitrified composite. The bulk density, porosity, flexural strength, hardness, and thermal conductivity of 57.5 wt% Al₂O₃ sintered at 950 °C were 3.12 g/cm³, 6.1%, 169 MPa, 90.5 HRC, and 4.17 W/(m·K), respectively. The coefficient of thermal expansion of the bonding material was 3.83 × 10^?6 °C^?1, which was comparable to that of CBN, and the number of N–Al bonds were increased, which boosted the flexural strength of the Al₂O₃/vitrified CBN composite to 81 MPa. The excellent mechanical properties, compact structure, and suitable interfacial bonding state with the CBN grains of the Al₂O₃/vitrified composite make it a promising high-performance bonding material for superhard abrasive tools.     

Influence of hexagonal boron nitride nanosheets on phase transformation,microstructure evolution and mechanical properties of Si3N4 ceramics

Fully dense Si₃N₄ materials with 1 wt.% (～ 1.5 vol.%) and 2 wt.% (～ 3.0 vol.%) h-BN nanosheets were prepared by spark plasma sintering at 1750 °C with the dwell of 7 min under a pressure of 50 MPa in a vacuum. BN nanosheets with different dimensions were prepared by ultrasound-assisted liquid phase exfoliation of h-BN powder, followed by centrifugation at different speeds (1000 rpm and 3000 rpm). The addition of BN nanosheets hindered the particle rearrangement stage of sintering, which resulted in the delayed α→β phase transformation of Si₃N₄. To study a direct effect of BN nanosheets on the mechanical properties of Si₃N₄, the results were compared to the monolithic Si₃N₄ with similar grain size and α/β-Si₃N₄ ratio. The addition of 2 wt.% h-BN nanosheets (1000 rpm) increased both the fracture toughness (～ 26 %) and the strength (～ 45 %) of Si₃N₄, when compared to the monolithic Si₃N₄ with similar microstructure.     

Improvement of microstructure,mechanical properties and cutting performance of Ti(C,N)-based cermets by ultrafine La2O3 additions

Ti(C,N)-WC-Mo₂C-TaC-Co-Ni cermets with various content of La₂O₃ were prepared by gas-pressure sintering at 1450 °C. The effects of ultrafine La₂O₃ additions (0, 0.05, 0.1 and 0.2 wt%) on the microstructure, mechanical properties, wear resistance and cutting performance of cermets were explored. In the microstructure of cermets, the La₂O₃ particles and dissolved La element in binder phases were observed, which could inhibit the dissolution-precipitation process of ceramics phases during liquid-sintering. Furthermore, the La₂O₃ could absorb and react with the impurity Al element with low melting point from raw powders, avoiding the appearance of liquid phase at the low temperature and partial overheating during sintering process. These mechanisms could inhibit the abnormal growth of Ti(C,N) core-(Ti,W,Mo,Ta)(C,N) rim structures effectively, leading to the thinning of brittle rim phases and coarsening of wear-proof Ti(C,N) particles. The decrease of proportion of brittle rim phase and ultrafine Ti(C,N) particles promoted the fracture toughness. The increase of proportion and grain size of Ti(C,N) improved the hardness, wear resistance and cutting performance significantly. However, the excessive addition of La₂O₃ would result in the agglomeration of La₂O₃, causing the sharp decline of mechanical properties and cutting performance. The cermet with 0.1 wt% La₂O₃ addition possessed the optimal mechanical properties with Vickers hardness, transverse rupture strength and fracture toughness of 1710 (HV30) Kgf/mm², 2480 MPa and 11.7 MPa m^1/2, respectively.     

热压烧结Al2O3-ZrB2-SiC复相陶瓷的高温力学性能研究

为提高Al2O3陶瓷的高温力学性能,采用热压烧结工艺(烧结温度1 800℃,烧结压力20 MPa,保温1 h)制备了Al2O3-ZrB2-SiC复相陶瓷(简称AZS),并研究了ZrB2含量对Al2O3基陶瓷高温抗折强度和抗热震性的影响。结果表明:1)在Al2O3基陶瓷中加入第二相ZrB2能有效改善材料的高温抗折强度和高温强度保持率,在1 000和1 200℃时,加入20%体积分数ZrB2的AZS陶瓷试样具有最高的高温抗折强度,而加入24%体积分数ZrB2的AZS陶瓷试样具有最高的高温强度保持率。2)AZS陶瓷的抗热震性能优于纯Al2O3陶瓷。经100℃温差急冷后,加入20%体积分数ZrB2的AZS陶瓷具有最高的残余强度,比纯Al2O3陶瓷提高了17.2%;经300和500℃温差急冷后,加入24%体积分数ZrB2的AZS陶瓷都具有最高的残余强度,比Al2O3陶瓷分别提高了35.3%和20.9%。     

Effects of TiC/TiN addition on the microstructure and mechanical properties of ultra-fine grade Ti (C, N)–Ni cermets

Two series of Ti (C, N)-based cermets, one with TiC addition and the other with TiN addition, were fabricated by conventional powder metallurgy technique. The initial powder particle size of the main hard phase components (Ti (C, N), TiC and TiN) was nano/submicron-sized, in order to achieve an ultra-fine grade final microstructure. The TiC and TiN addition can improve the mechanical properties of Ti (C, N)-based cermets to some degree. Ultra-fine grade Ti (C, N)-based cermets present a typical core/rim (black core and grayish rim) as well as a new kind of bright core and grayish rim structure. The average metallic constituent of this bright core is determined to be 62 at% Ti, 25 at% Mo, and 13 at% W by SEM–EDX. The bright core structure is believed to be formed during the solid state sintering stage, as extremely small Ti (C, N)/TiC/TiN particles are completely consumed by surrounding large WC and Mo₂C particles. Low carbon activity in the binder phase will result in the formation (Ni₂Mo₂W)C_x intermetallic phase, and the presence of this phase plays a very important role in determining the mechanical properties of TiN addition cermets.     

Regulation of Y2O3 addition on structure and properties of Al2O3/ZrO2(Y2O3) directionally solidified eutectic ceramic

Al₂O₃/ZrO₂(Y₂O₃) directionally solidified eutectic ceramics (DSECs) with different Y₂O₃ addition were prepared. Three polymorphs of ZrO₂: m-ZrO₂, t-ZrO₂ and c-ZrO₂ appeared and disappeared in order with the increase of Y₂O₃ addition, the effects of Y₂O₃ addition amount on the mass fraction of ZrO₂ polymorphs were quantitatively analyzed. The gradual decrease of area fraction and average size of colony was attributed to the increase of constitutional undercooling region, the variation of interphase spacing in the colonies was explained via selection mechanism of dendrite tip. The maximum hardness and fracture toughness were 18.35 GPa and 8.55 MPa m^1/2 obtained at 9 mol% and 3 mol% Y₂O₃ addition, respectively. The hardness was determined by the mass fraction of m-ZrO₂ phase, the area fraction of intercolony and the residual compressive stress, and the toughening mechanisms were composed of microcrack toughening, t-m transformation toughening and residual stress toughening.     

Synergistic effects of TiB2 and graphene nanoplatelets on the mechanical and electrical properties of B4C ceramic

Monolithic B₄C, B₄C–TiB₂, and B₄C–TiB₂–graphene nanoplatelets (GNPs) were fabricated by hot pressing (HP) at 1900 °C for 1 h under an axial pressure of 30 MPa. The microstructures and mechanical and electrical properties of the B₄C composites were investigated. The results show that the GNPs are distributed homogeneously in B₄C-based ceramic composites. Compared with monolithic B₄C, the TiB₂–GNPs-containing B₄C composite exhibits an approximately 68 % increase in flexural strength and a 169 % increase in fracture toughness due to the synergistic effects of TiB₂ particles and GNPs. The toughening mechanisms mainly include TiB₂ crack deflection, crack branching, transgranular fracture and GNPs crack deflection, crack bridging, and GNPs pull-out. Additionally, the electrical conductivity of the B₄C composite reinforced with dual fillers is three orders of magnitude higher than that of monolithic B₄C due to the establishment of a conductive network. The addition of GNPs can efficiently connect the isolated conductive TiB₂ particles in the B₄C matrix and provides an additional channel for electron migration.