Influences of hBN content and test mode on dry sliding tribological characteristics of B4C-hBN ceramics against bearing steel

The dry sliding tribological characteristics of B₄C-hBN ceramics with different contents of hBN against GCr15 bearing steel under two different test modes (upper disc on bottom pin test mode and upper pin on bottom disc test mode) have been evaluated using a pin-on-disc friction and wear tester. The experimental results show that, with increasing hBN content, the dry sliding tribological characteristics of B₄C-hBN/GCr15 bearing steel pairs have different variation rules under two different test modes. Under upper disc on bottom pin test mode, with increasing hBN content, the friction coefficients of B₄C-hBN/GCr15 bearing steel pairs decrease firstly and increase subsequently; however, under upper pin on bottom disc test mode, the friction coefficients of B₄C-hBN/GCr15 bearing steel pairs increase continuously with increasing hBN content. In this paper, the possible reasons for these interesting results are most deeply discussed.     

Controlled precipitation of W2B4 platelets and of β-WB nanolaminates for the in situ reinforcement of ternary TiB2-W2B4-CrB2 ceramics

Mechanical reinforcement of hard ultrahigh-temperature materials often requires complex processing techniques involving the mixing of platelet-shaped particles into the matrix - ultimately leading to inhomogeneities that compromise strength and fracture toughness. Thus, this paper investigates in situ reinforcement of a TiB₂ ceramic matrix by controlled precipitation of W₂B₄ platelets and β-WB nanolaminates. The results showed successful precipitation of: (i) W₂B₄ platelets according to the phase diagram, (ii) epitaxial β-WB lamellas, and (iii) spinodal, i.e. fully coherent W-rich lamellas which change to either W₂B₄ platelets or β-WB during ageing. The study of phase amount and microstructure as a function of homogenization temperature and annealing temperature/time regimes allows one to control the volume fraction, size as well as aspect ratio of precipitates. This enables the control of fracture toughness affected by crack deflection, particle pull-out, crack bridging and crack branching. Composition and milling treatment most affect microstructure. Successful precipitation is possible at 1650 °C.     

Tribological properties of SiC-B4C ceramics under dry sliding condition

SiC-B₄C ceramic composites with different ratios of SiC to B₄C were produced. The relative density, mechanical properties, initial surface characteristics, dry sliding tribological properties against SiC ball and worn surface characteristics of the SiC-B₄C ceramics were studied. Results of dry sliding tribological tests showed that, 40 wt. % SiC-60 wt. % B₄C ceramic composite had the best tribological properties in SiC-B₄C ceramic composites. A relief structure with height difference of 10−30 nm between B₄C grains and SiC grains is formed after dry sliding. This relief structure, on the one hand, can reduce real contact area on interface, decreasing adhesion effect, and on the other hand, can fix or trap the wear pieces formed on sliding interface during the dry sliding process, reducing the abrasive wear. However, there is a limit to the beneficial influence of decreased adhesion effect and reduced abrasive wear, and an optimum proportion of relief structure. Pores can also fix or trap some wear pieces, reducing the abrasive wear. Under the condition of strong bonding between SiC grains and B₄C grains, the SiC-B₄C ceramic composites with higher porosity can obtain better tribological properties. In addition, it is observed by AFM that the depth of scratch on B₄C grains is shallower than that on SiC grains. Hence, it is demonstrated by micro scale measurement that the wear rate of B₄C is lower than that of SiC in this study.     

Evaluation of tribological behavior of B4C–hBN ceramic composites under water-lubricated condition

The aim of this study is to evaluate the influence of hexagonal boron nitride (hBN) addition on the tribological behavior of B₄C-based ceramic composites under distilled water lubrication. Water-lubricated sliding tests of hot-pressed B₄C–hBN ceramic composites with different hBN amounts against pure B₄C ceramic were carried out on a pin-disc type wear apparatus. It was found that the addition of hBN into B₄C ceramic matrix resulted in a severe decrease of the friction coefficient from 0.373 for B₄C/B₄C sliding pair to 0.005 for B₄C–20 wt% hBN/B₄C sliding pair. A B₂O₃ tribochemical film formed on the worn surface of the B₄C–hBN specimen protected both B₄C–hBN and B₄C and facilitated the frictional surfaces to smooth. Therefore, the tribological behaviors of the pairs were significantly improved. The formation process of the film and its antifriction mechanism are discussed.     

Exploring processing,reactivity and performance of novel MAX phase/ultra-high temperature ceramic composites: The case study of Ti3SiC2

The reactivity behaviour between a MAX phase and ZrB₂ or WC was explored with the aim of developing novel composites by merging the benefits of the individual constituents. Hot pressing of Ti₃SiC₂ with 30 vol% ZrB₂ at 1450 °C led to notable microstructure re-assessment with formation of inter-locked sub-micrometric boride grains. This composite displayed enhanced hardness and showed a strength over 430 MPa up to 1200 °C, which is a great achievement considering the ductile behaviour of typical pure MAX compounds. However, addition of WC led to a highly porous composite with poor performance. These findings set a first basis for the progress of original light ceramics with combined hardness and failure tolerance over a broad temperature range.     

Fabrication and mechanisms of densification of ZrB2-based ultra high temperature ceramics by reactive hot pressing

Dense ZrB₂–SiC (25–30 vol%) composites have been produced by reactive hot pressing using stoichiometric Zr, B₄C, C and Si powder mixtures with and without Ni addition at 40 MPa, 1600 °C for 60 min. Nickel, a common additive to promote densification, is shown not to be essential; the presence of an ultra-fine microstructure containing a transient plastic ZrC phase is suggested to play a key role at low temperatures, while a transient liquid phase may be responsible at temperatures above 1350 °C. Hot Pressing of non-stoichiometric mixture of Zr, B₄C and Si at 40 MPa, 1600 °C for 30 min resulted in ZrB₂–ZrC_x–SiC (15 vol%) composites of 98% RD.     

Hot-pressing sintered BN-SiO2 composite ceramics with excellent thermal conductivity and dielectric properties for high frequency substrate

High thermal conductivity and low dielectric constant are the more and more important properties for high-frequency substrate materials to enhance their heat radiation and reduce signal delay. In this work, a series of BN-SiO₂ composite ceramics for high frequency application were successfully synthesized by hot-pressing sintering method. And their structures, thermal and dielectric properties were systematically studied. According to the results, the excellent thermal conductivity with low dielectric constant and low dielectric loss has been obtained in the BN-SiO₂ ceramic. Compared to the pure SiO₂, the sample with 50?wt% BN addition sintered at 1650?℃ exhibited excellent physical properties, including a high thermal conductivity of 6.75?W/m?K which is almost five times higher than that of pure SiO₂ and a low dielectric constant of 3.73. The achieved high thermal conductivity and appropriate dielectric property of the BN-SiO₂ composite ceramic make it a promising candidate for high-frequency substrate application.     

Self-propagating high-temperature synthesis of advanced ceramics MoSi2–HfB2–MoB

This study focuses on the investigation of the macrokinetic features of SHS (combustion synthesis) of elemental mixtures Mo–Hf–Si–B, in particular the mechanisms of structure and phase formation in the combustion front as well as the structure and properties of consolidated ceramics. Two routes for the fabrication of the composite SHS powder in system MoSi₂–HfB₂–MoB were used: (1) synthesis using Mo–Si–B and Hf–B mixtures followed by mixing of the combustion products and (2) synthesis using the four-component Mo–Hf–Si–B mixture. Dense ceramic samples with a homogeneous structure and low residual porosity (0.8–3.6%) were prepared by hot pressing of SHS powders. Although the particles size distribution and phase composition of SHS powders are similar for both synthesis routes, the structure and properties of both the composite SHS powders and hot-pressed ceramics differ considerably. Synthesis using the four-component Mo–Hf–Si–B mixture allows one to produce hierarchically ordered nanocomposite material with improved mechanical properties: hardness up to 17.6?GPa and fracture toughness up to 7.16?MPa?m^1/2.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Friction and wear behavior of Fe2AlB2 nanolaminates against GCr15 steel counterpart

The dependence of the friction and wear behavior of MAB-phase Fe₂AlB₂ against GCr15 steel counterpart on the sliding speed (0.8–6.4 m/s) and normal force (20–60 N) is investigated. The friction coefficient (0.3–0.7) decreases with increasing sliding speed and normal force, however, with some abnormalities for low sliding speeds of 0.8–1.6 m/s in the latter case. In contrast, overall increasing wear rates ((0.5–2.5) × 10^-5 mm³/(N·m)) are observed, with significantly high values at 0.8 m/s and 40–60 N. Surface oxidation is identified due to the high temperature. At 0.8–1.6 m/s, the surface tongue patterns and material migration are both indicative of adhesive wear, with the contribution of fatigue wear becoming more significant for sliding speeds of over 3.2 m/s. At 6.4 m/s and 40 N, the wear changes from deformation-controlled by shallow spalling to fracture-controlled by deep failure.     

A novel B2O3/B4C-modified composite adhesive with wide operative temperature range for alumina fiber fabric bonding

A heat resistance adhesive with wide operative temperature range for bonding alumina fiber fabric was developed by organic-inorganic modification. The polyvinyl alcohol modified by B₂O₃ generated a complex cross-linked network connected by B–O–C bonds, which can enhance the bonding strength of adhesive after heat-treatment from RT to 400 °C. Moreover, the addition of B₄C can enhance the bonding strength of adhesive after heat-treatment from 400 °C to 800 °C due to the formation of molten B₂O₃ and borosilicate glass. Significantly, the appropriate addition of B₄C can make the adhesive form a denser structure without transforming the fracture mode of the bonding joints, which is conducive to enhance the strength performance of bonding joints after sintered at 800 °C. On the contrary, the excessive addition of B₄C will transform the fracture mode of the bonding joints into brittle fracture, which will degrade the strength performance of bonding joints.     

Novel SiC/C composite targets for the production of radioisotopes for nuclear applications

A partially porous SiC ceramic, reinforced with 30 vol% short carbon fibers, was hot pressed and characterized as potential ISOL target for nuclear applications. Powder milling and hot pressing were effective for the realization of a ceramic with about 40% interconnected porosity in the 0.6–0.8 µm size range. A fiber-free porous SiC material was also synthesized for the sake of comparison. Compression strength of the fiber-rich SiC passed from about 200 MPa at room temperature to about 120 MPa upon testing at 1200 °C. The thermal conductivity was higher than the fiber-free SiC and other state-of-art ISOL target materials and was 48 W/m·K at 600 °C and decreased to 17 W/m·K at 1400 °C, owing to the porosity. Remarkably, this fiber-rich ceramic in form of thin disk, possessed suitable thermo-mechanical behavior to successfully withstand a 350 °C thermal gradient without failure.     

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.     

Two-step strategy for improving the tribological performance of Si3N4 ceramics: Controlled addition of SiC nanoparticles and graphene-based nanostructures

The tribological behaviour of silicon nitride (Si₃N₄) ceramics is investigated using a two-step strategy. A set of ceramic composites containing silicon carbide nanoparticles (SiC_n) is developed and, subsequently, graphene-based fillers are added to the Si₃N₄/SiC composite with the best tribological performance. The friction coefficient and the wear rate of Si₃N₄ are reduced up to 22 % and 40 %, respectively, when a 10 vol.% of SiC_n is incorporated into the ceramic matrix due to its improved mechanical response. Si₃N₄/SiC composites containing 11 vol.% of graphene nanoplatelets (GNPs) or reduced graphene oxide sheets (rGOs) are analysed under isooctane lubrication and dry testing. rGOs composite leads to an important decrease of the friction coefficient (50 %) under lubricated conditions, and an enhancement of the wear resistance (44 %) under dry sliding tests, as compared to the reference Si₃N₄/SiC. The best performance of rGOs composite is due to the nature of the lubricating tribofilm and its excellent toughness.     

Fabrication of Ti3SiC2 and Ti4SiC3 MAX phase ceramics through reduction of TiO2 with SiC

Ti₃SiC₂ and Ti₄SiC₃ MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO₂ using SiC as a reductant, followed by hot pressing of the products under 25 MPa of pressure at 1600 °C. It was found that both Ti₃SiC₂ and Ti₄SiC₃ may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti₃SiC₂ and Ti₄SiC₃ under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti₄SiC₃ compared to Ti₃SiC₂. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.     

Effect of nanometer SiC filler on the tribological behavior of PEEK under distilled water lubrication

The composites of polyetheretherketone (PEEK) filled with nanometer SiC of different proportions were prepared by compression molding. The tribological behaviors of the composites under lubrication of distilled water were investigated and compared with that under dry sliding, on an M‐200 friction and wear test rig, by running a plain carbon steel (AISI 1045 steel) ring against the composite block. The worn surfaces of nanometer SiC filled‐PEEK and the transfer film were observed by means of scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). As the results, nanometer SiC as the filler greatly improves the wear resistance of PEEK under dry sliding and distilled water lubrication, though the composites show different dependence of wear resistance on the filler content. Nanometer SiC‐filled PEEK showed signs of slight scuffing under distilled water lubrication, while a thin, uniform, and tenacious transfer film was formed on the surface of the counterpart steel ring. On the contrary, unfilled PEEK under lubrication of water showed signs of severe plowing and erosion, while the worn surface of the counterpart ring was very rough, and a discontinuous PEEK transfer film was formed. Thus, the different friction and wear behaviors of unfilled PEEK and nanometer SiC‐filled PEEK can be attributed to the different characteristics of the corresponding transfer films. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 609–614, 2000     

Influence of intergranular phases on edge integrity of Si3N4/SiC wood cutting tools

Si₃N₄/SiC composites used for industrial wood cutting were processed by a near net shape route involving gas pressure sintering with sintering additives such as Al₂O₃, La₂O₃, Y₂O₃ and MgO. The cutting edge integrity of these knives was tested in a cutting trial and compared to knives made by a hot pressing route. It was found that the intergranular phase has a crucial influence on the cutting edge integrity. The boundary phase was analysed by EFTEM and EDX mapping on TEM samples: in gas pressure sintered composites the crystallisation of the apatite Y₅Si₃O₁₂N phase was identified. In the hot pressed composite the boundary phase consisted only of silicates. These composites showed better edge stability than cutting tools with a Y-N-apatite phase. The formation of the type of intergranular phase was found to be determined by the amount of MgO sintering aid and the temperature of the post sintering heat treatment.     

Effect of MoS2/PTFE coatings on performance of Si3N4/TiC ceramics in dry sliding against WC/Co

To enhance the tribological performance of Si₃N₄/TiC ceramics, MoS₂/PTFE composite coatings were deposited on the ceramic substrate through spraying method. The micrographs and basic properties of the MoS₂/PTFE coated samples were investigated. Dry sliding friction experiments against WC/Co ball were performed with the coated ceramics and traditional ones. These results showed that the composite coatings could significantly reduce the friction coefficient of ceramics, and protect the substrate from adhesion wear. The primary tribological mechanisms of the coated ceramics were abrasive wear, coating spalling and delamination, and the tribological property was transited from slight wear to serious wear with the increase of load because of the lower surface hardness and shear strength. The possible mechanisms for the effects of MoS₂/PTFE composite coatings on the friction performance of ceramics were discussed.     

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.     

Tribological behaviors of FeCoNiCrAlx sliding against Si3N4 ceramics under high temperature condition

Tribological behaviors are system responses that not only depend on material properties but also hinge on external environmental factors. This work investigated the tribological behaviors of FeCoNiCrAl_x (x = 0.1, 0.5, 1) sliding against Si₃N₄ ceramics under high temperature conditions. According to experimental findings, the tribological properties of FeCoNiCrAl_x were enhanced as the Al element content increased, particularly FeCoNiCrAl₁ could resist the material softening under high-temperature conditions to enhance the wear resistance. Based on the friction coefficient changes, wear morphology, phase composition, and chemistry element, the high-temperature wear mechanisms of FeCoNiCrAl_x were discussed including adhesive, attrition, and oxidative wear. These new studies will lead to the further improvement of tribological data of high-entropy alloys.