Microstructure and mechanical properties of fine-grained boron carbide ceramics fabricated by high-pressure hot pressing combined with high-energy ball milling

Dense and fine-grained boron carbide (B₄C) ceramics were fabricated via high-pressure hot pressing (100?MPa) using powders, which are prepared by high-energy ball milling. These powders were sintered at a low temperature (1800?°C) without any sintering aid. The dense and fine-grained B₄C ceramics demonstrate super high hardness, outstanding fracture toughness and modern flexure strength. The milled powders were characterised by disordered crystal structure and ultrafine particle size that ranges from a few nanometres to a few hundred nanometres. The combined contributions of high pressure and the characteristic of the milled powders guaranteed that the dense fine-grained microstructure was achieved at only 1800?°C. The grain size distribution of the ceramics was inhomogeneous and ranged from 70?nm to 1.6?µm. However, the average grain size was fine at only 430?nm, which partially contributed to the super high hardness of the B₄C ceramics. The locally concentrated areas of the small grains changed the fracture mode of the B₄C ceramics from the complete transgranular fracture to a mixture of transgranular and intergranular fractures, thereby enhancing the toughness of the B₄C ceramics. The relative density, Vickers hardness, flexure strength and fracture toughness of the obtained B₄C ceramics reached up to 99.5%, 41.3?GPa, 564?MPa and 4.41?MPa?m^1/2, respectively.     

Microstructure and mechanical properties of boron carbide/graphene nanoplatelets composites fabricated by hot pressing

In this study, boron carbide (B₄C)-graphene nanoplatelets (GNPs) composites, with enhanced strength and toughness, were fabricated by hot pressing at 1950 °C under a pressure of 30 MPa for 1 h. Microstructure analysis revealed that the GNPs are homogenously dispersed within the B₄C matrix. Raman spectroscopy and electron microscopy showed the orientation of the GNPs in the composites. The effects of the amount of GNPs on the microstructure and mechanical properties of the composites were also investigated. The optimal mechanical properties were achieved using 1 wt% GNPs. The relative density, Vickers hardness, flexure strength, and fracture toughness of the B₄C-GNPs composite ceramic were found to be 99.12%, 32.8 GPa, 508 MPa, and 4.66 MPa m^1/2, respectively. The main toughening mechanisms included crack deflection in three dimensions, GNPs pull-out, and crack bridging. The curled and semi-wrapped GNPs encapsulated individual B₄C grains to resist GNPs pull-out and to deflect propagating cracks.     

The microstructure and mechanical properties of Ni/Al2O3 composites by in-situ generated CaAl12O19 and ZrO2 via hot pressing sintering

Ni/Al₂O₃ composites with a varying mass fraction of CaZrO₃ (0–12 wt%) were prepared by vacuum hot pressing sintering at 1650 °C under a pressure of 30 MPa for 30 min to investigate how CaZrO₃ affect the mechanical properties and morphology of the composites. The results show that CaZrO₃ can react with Al₂O₃ and form new strengthening and reinforcing phases of CaAl₁₂O₁₉ and ZrO₂, which can promote complete densification and solve the problem of uneven distribution due to the poor wettability between Al₂O₃ and Ni. Additionally, composites showed satisfactory mechanical properties when 6.0–9.0 wt% CaZrO3 was added and the major toughening mechanism involved the typical fracture of delamination and the transgranular mode.     

Microstructure and anisotropic mechanical properties of B6.5C-TiB2-SiC-BN composites fabricated by reactive hot pressing

B_6.5C-TiB₂-SiC-BN composite ceramics were prepared by a novel solid-state reaction using TiCN, B, and Si as raw materials. The final products obtained by hot pressing at 1950 °C possessed a fine microstructure, homogeneous distribution, and excellent mechanical properties. The obtained bulk B_6.5C-TiB₂-SiC-BN composite ceramic shows a high relative density (98.8 %). The mechanical properties of the composites are anisotropic because of the orientation growth and structural characteristics of TiB₂ and h-BN grains. The values of hardness, bending strength, and fracture toughness measured along the hot-pressing direction were 19.6 GPa, 801 MPa, and 4.30 MPa m^1/2, respectively, which were higher than those measured perpendicular to the hot-pressing direction. The formation of twin structures in B_6.5C and SiC grains and the crack deflections induced by h-BN and TiB₂ grains are beneficial for improving the mechanical properties of these composites.     

Effects of pressure on densification behaviour,microstructures and mechanical properties of boron carbide ceramics fabricated by hot pressing

Effects of pressure, from ordinary (30 MPa) to high pressure (110 MPa), on densification behaviour, microstructures and mechanical properties of boron carbide ceramics sintered by hot pressing are investigated. With increasing pressure, the relative density sharply increases within 30–75 MPa, slowly increases within 75–100 MPa and finally stagnates. For samples within 75–100 MPa, densification begins at approximately 1000 °C, and the dominant densification process ends before the soaking stage. High relative densities of 98.49% and 99.76% are achieved. For samples within 30–50 MPa, densification begins at approximately 1500 °C, and the soaking stage (initial 20 min) is still important for the dominant densification process. The final relative densities are only 87.90% and 92.32%. The above-mentioned differences are derived from contributions of pressure, and the dominant densification mechanism under high pressure is plastic deformation. The average grain size of the samples slightly increases with increasing soaking time. The grain size under higher pressure is larger than that under lower pressure at corresponding periods because grains grow easily with reduced pores. Vickers hardness and fracture toughness increase as grain size decreases in fully dense samples. However, when the samples do not achieve full density, relative density becomes more influential than grain size in hardness and toughness. A soaking time of 30 min is enough for samples under 100 MPa. Prolonging the soaking time has deleterious effects on mechanical properties. The relative density, grain size, hardness and fracture toughness of the samples under 100 MPa for 30 min are 99.73%, 1.96 µm, 37.85 GPa and 3.94 MPa m^1/2, respectively.     

Preparation and mechanical properties of SiCw-Al2O3-YAG ceramic composite by hot oscillatory pressing

SiC_w-Al₂O₃-YAG ceramic composites were prepared by hot oscillatory pressing (HOP) and traditional hot pressing (HP). The results showed that compared with static pressure, the oscillatory pressure could effectively promote densi?cation and mechanical properties of the composites. The sample prepared by HOP exhibited higher hardness (15.72 ± 0.20 GPa) and fracture toughness (7.13 ± 0.19 MPa m^1/2). The current work suggests that HOP could be an effective technique for the preparation of whisker reinforced ceramic composites.     

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.     

Effect of ball milling treatment on the microstructures and properties of Cr2AlC powders and hot pressed bulk ceramics

Ball milling was used on synthesized Cr₂AlC powders, and dense Cr₂AlC bulk ceramics with almost pure phase were fabricated by hot pressing these ball milled Cr₂AlC powders at 1300 °C for 2 h under 30 MPa. The phase compositions and the microstructures of powders and bulk ceramics were characterized. The mechanical properties and dislocation analysis of bulk ceramics were also investigated. Our results indicated that the grain size of powders became uniform and smaller after ball milling, which was also inherited in the bulk ceramics. Moreover, the mechanical properties of hot pressed Cr₂AlC ceramics, including flexural strength, fracture toughness, and Vickers hardness, significantly increased by using ball milled powders treated at speed 200 rpm. The organization of hexagonal dislocation networks during hot pressing and reduction of grain size both had a positive effect on improving mechanical properties.     

Effects of Ta2O5 on mechanical properties and elements diffusion of Ti/Al2O3 composites prepared via hot pressing sintering

Homogeneous Ti/Al₂O₃ composites with different volume percentages of Ta₂O₅ addition were prepared at different temperatures via hot pressing sintering. Laminated Ti/Al₂O₃ composites with different volume percentages of Ta₂O₅ added were prepared. The effects of Ta₂O₅ on the composition, microstructure, mechanical properties and elements diffusion of the composites were characterized and investigated. Ta₂O₅ inhibited the production of TiAl and Ti₃Al by forming solid solution with Ti or new reaction product of Al. This solid solution melted and filled the void of Al₂O₃ phase to increase the density of Ti/Al₂O₃ composites at high temperature. Mechanical properties had also been improved by this phenomenon. Because Al element couldn’t diffuse in Ta or react with it, Al couldn’t diffuse through the Ta-enriched area at the interface of Ti and Al₂O₃.     

Study on mechanical properties of hot pressing sintered mullite-ZrO2 composites with finite element method

In this study, mullite–zirconia (ZrO₂) composites were fabricated by hot pressing sintering method. The effects of sintering temperature and holding time on the microstructures, phase compositions and mechanical properties of the composites were investigated. The results indicated that the size of t-ZrO₂ grain varies with sintering temperature and holding time, and the maximum flexural strength of 674.05?MPa and fracture toughness of 12.08?MPam^1/2 are obtained when the sintering temperature is 1500?°C with holding times of 20 and 60?min, respectively. Finite element method was employed to analyze the relationship between grain size and mechanical properties of mullite–ZrO₂ composites for the first time. The results showed that the maximum stress on mullite–ZrO₂ interface increases with the growth of t-ZrO₂ grain size, which enhances the generation and propagation of cracks on grain boundaries significantly and degrades the flexural strength and fracture toughness of the mullite–ZrO₂ composite ceramics.     

Effect of Al2O3 particle size on the microstructure,phase transformation and mechanical properties of hot pressed Al2O3-CA6-ZrO2/Ni multi-phase composites

Al₂O₃-CA6-ZrO₂/Ni multi-phase composites were fabricated by vacuum hot pressing sintering at 1650 °C under the pressure of 30 MPa for 30 min. The microstructural evolution rule of the composites was investigated as a function of Al₂O₃ particle size. Upon increasing the Al₂O₃ particle size to 30 μm, the generated CA6 underwent a transformation from unfixed type to a plate-like pattern and to a combined CA6-Al₂O₃ matrix, whereas the fracture mode of m-ZrO₂ changed from an intergranular fracture to an intergranular and transgranular mixed type due to the improved interface binding energy. Additionally, satisfactory mechanical properties of the composites were achieved when the Al₂O₃ particle size was 30 μm. Under the synergistic effect of different strengthening and reinforcing phases, the inhomogeneous distribution caused by poor wettability between Al₂O₃ and Ni was effectively solved by the distributions of “intercrystalline type” and “intracrystalline type” for the Ni phase. The mechanisms of the microstructural evolution, phase transformation and improved mechanical properties are discussed in detail.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

Comparative investigation of microstructure,mechanical properties and strengthening mechanisms of Al-12Si/TiB2 fabricated by selective laser melting and hot pressing

Near-fully dense Al-12Si matrix composites reinforced with TiB₂ ceramic particles (2?wt%) were successfully fabricated by selective laser melting (SLM) and hot pressing (HP) of powder mixtures. TiB₂ ceramic particles are homogeneously distributed in the Al-12Si matrix at the micrometer-scale owing to a very good wetting between molten Al-12Si alloy and TiB₂ ceramic. The microstructural analysis of the as-fabricated SLM samples show the formation of a supersaturated α-Al phase and the decrease of free residual Si with respect to the hot-pressed ones. Both composites exhibit a fine microstructure with a grain size of ~?5.1?µm and ~?5.8?µm for SLM- and HP-fabricated samples with addition of TiB₂ ceramic particles. The SLM Al-12Si/TiB₂ composite exhibits significantly improved microhardness (~?142?±?6.0 HV_0.05) and yield strength (~247?±?4.0?MPa) compared to the corresponding HP one. Fine cell morphology and nanostructured dispersion strengthening are responsible for the improved mechanical strength of the Al-12Si/TiB₂ composite processed by SLM.     

O''-Sialon-ZrO2-SiC(nm)的制备及力学性能

采用热压烧结工艺合成了O’ Sialon -ZrO2 -SiC(nm) 复合材料 ,研究了该复合材料的力学性能及显微结构。结果表明 ,材料抗折强度随温度的变化属Ⅰ类曲线变化规律 ,Sialon、SiC多晶以及晶粒粗大、层状结构SiC的存在可能是导致材料强度不高的主要原因。断口的显微结构研究表明 ,复合材料在室温时的断裂以穿晶断裂为主 ,中、高温时则为沿晶断裂     

Si3N4/graphene nanocomposites for tribological application in aqueous environments prepared by attritor milling and hot pressing

Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si₃N₄ structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si₃N₄/MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si₃N₄ ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.     

Microstructure and mechanical property of (TiB2-SiC) agglomerate-toughened B4C-TiB2-SiC composites

B₄C-TiB₂-SiC composites toughened by (TiB₂-SiC) agglomerates were prepared via reactive hot pressing with B₄C and TiSi₂ as raw materials. Phase composition, microstructure, and mechanical properties of the fabricated composites were investigated. The function of (TiB₂-SiC) agglomerates was analyzed, and the strengthening and toughening mechanism were also discussed. Results indicated that some of the in situ formed TiB₂ and SiC were interlocked to form special (TiB₂-SiC) agglomerates in the matrix. The good comprehensive performances of 510 MPa flexural strength, 5.84 MPa·m^1/2 fracture toughness, and 31.93 GPa hardness were obtained in the composites fabricated with 30 wt% TiSi₂. The in situ introduced fine TiB₂ and SiC grains refined the grains of B₄C due to the pinning effect, which enhanced the strength. The special (TiB₂-SiC) agglomerates and the existing toughening phenomena such as crack deflection, branching, and microcrack regions induced by the mismatch of thermal expansion coefficients, had cumulative effects on improving the fracture toughness.     

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.     

Microtopography and mechanical properties of vacuum hot pressing Al/B4C composites

Al/B₄C composites with various volume contents of B₄C (5%, 10%, 15%, 20%, and 25%) reinforcing the Al matrix, have been fabricated by vacuum hot press sintering at 680 °C, with a soaking time of 90 min and external pressure of 30 MPa. Mechanical properties, phase composition, and microstructure of the Al/B₄C composites are discussed to reveal the physical properties of the composites. Field emission transmission electron microscopy and selected area electron diffraction have been employed to verify the interior structure and crystal growth direction, respectively. The Vickers hardness, fracture strength, tensile strength, and maximum force attained the optimal values of 108.45 ± 4.02 HV, 585.70 ± 23.26 MPa, 196.18 ± 2.48 MPa, and 4.44 ± 0.17 kN, respectively, for 25 vol% B₄C/Al composites. The static compression strength increased before the 15 vol% B₄C addition and then decreased, acquiring the highest value of 292.15 ± 2.09 MPa for 15 vol% B₄C/Al composites. In general, the relative density and ductility of these composites consistently increased, with an increase in the volume content of Al, achieving a maximum of 99.22% and 54.63 ± 7.34%, respectively, for 5 vol% B₄C/Al composites.     

Mechanical,tribological and electrical properties of ZrB2 reinforced Cu processed via milling and high-pressure hot pressing

The present work investigates the effect of (0–10 wt%) ZrB₂ reinforcement on densification, mechanical, tribological and electrical properties of Cu. The consolidation of Cu–ZrB₂ samples was carried out using a hot press (temperature: 500 °C, pressure: 500 MPa, time: 30 min, vacuum pressure: 1.3 × 10^-2 mbar). The bulk density of the hot-pressed Cu composites decreased from 8.84 g/cc to 8.16 g/cc and the relative density of samples lowered from 98.6% to 92.1% with the addition of ZrB₂. The incorporation of hard ZrB₂ (up to 10 wt%) improved the hardness of Cu (1.32–2.55 GPa). However, the yield strength and compressive strength of Cu composites increased up to 5 wt% ZrB₂, and further addition of ZrB₂ lowered its strength. The yield strength of Cu samples varied from 602 to 672 MPa and the compressive strength between ~834 and 971 MPa. On the other hand, the coefficient of friction (COF) (from 0.49 to 0.18) and wear rate (from 49.3 × 10^-3 mm³/Nm to 9.1 × 10^-3 mm³/Nm) of Cu–ZrB₂ samples considerably decreased with the addition of ZrB₂. Significantly low wear was observed with Cu-10 wt% ZrB₂ (Cu-10Z) samples, which is 5.41 times less than pure Cu. As far as the wear mechanisms are concerned, in pure Cu, continuous chips (wear debris) were formed during sliding wear by plowing. Whereas the major amount of material loss was occurred due to the plowing mechanism with discontinuous and short chip formation for Cu–ZrB₂ composites. The electrical conductivity of Cu–ZrB₂ samples decreased from 75.7% IACS to 44.1% IACS. In particular, Cu with ZrB₂ (up to 3 wt%) could retain the conductivity of 66.8% IACS. This study reveals that the addition of ZrB₂ (up to 3 wt%) is advantageous to have a good combination of properties for Cu.     

Influence of B4C nanoparticles on mechanical behaviour of Silicon brass nanocomposite through mechanical alloying and hot pressing

This research article has concentrated to develop a novel silicon brass of [82Cu4Si14Zn]_100-x – x wt.% B₄C (x = 0, 3, 6, 9, and 12) nanocomposites which were synthesized by mechanical alloying followed by vacuum hot pressing for consolidation of powders into bulk samples. Single vial planetary ball mill was used to synthesize the nanocomposite powders in which the ball-to-powder ratio of 10:1 with the milling time of 20 h was used. The milled powders were compacted and sintered simultaneously using vacuum hot pressing equipment for 1 h at 900 °C. The structural, mechanical and tribological properties were characterized and investigated by x-ray line profile analysis (XRD), scanning electron microscopy (SEM), electron backscattered diffraction images (EBSD), energy dispersive x-ray spectroscopy (EDS), Vickers microhardness, compression test, and dry sliding wear behaviour analysis. It has been found that B₄C nanoparticles had homogeneously distributed and embedded in the nanocrystallite matrix. As a result, the fabricated nanocomposites were exhibited superior properties than the conventional alloy. Here, 12 wt% B₄C reinforced silicon brass of bulk nanocomposite was produced higher hardness and compressive strength than the unreinforcement matrix. Further, the worn morphologies were evidenced the mild wear occurred at higher reinforced nanocomposites owing to decohesion and lower wear rate with considerable wear resistance.