Indentation toughness of Al2O3-CNT nanocomposites

In the last few years new ceramic/carbon nanotube composites have been developed and a number of authors have reported improved mechanical and functional properties in the case of ceramic/CNT composites compared to the monolithic materials. according to the results reinforcing by CNTs in many cases improved the fracture toughness of Al2O3, however, this toughening is not evident, and some of the variations may purely arise from using different testing techniques. The improvement in fracture toughness is due to bridging of the crack surfaces by CNTs during the crack propagation by CNT pullout mechanism, which strongly depends on the interfacial bonding between CNTs and the matrix. The aim of the present work is to investigate the effect of addition of carbon nanotubes and carbon black on the indentation toughness of alumina-CNT and alumina-carbon black nanocomposites

Highly wear-resistant and low-friction Si3N4 composites by addition of graphene nanoplatelets approaching the 2D limit

Abstract Graphene nanoplatelets (GNPs) have emerged as one of the most promising filler materials for improving the tribological performance of ceramic composites due to their outstanding solid lubricant properties as well as mechanical and thermal stability. Yet, the addition of GNPs has so far enabled only a very limited improvement in the tribological properties of ceramics, particularly concerning the reduction of their friction coefficient. This is most likely due to the challenges of achieving a continuous lubricating and protecting tribo-film through a high GNP coverage of the exposed surfaces. Here we demonstrate that this can be achieved by efficiently increasing the exfoliation degree of GNPs down to the few-layer (FL) range. By employing FL-GNPs as filler material, the wear resistance of Si3N4 composites can be increased by more than twenty times, the friction coefficient reduced to nearly its half, while the other mechanical properties are also preserved or improved. Confocal Raman spectroscopy measurements revealed that at the origin of the spectacular improvement of the tribological properties is the formation of a continuous FL- GNP tribo-film, already at 5 wt% FL-GNP content

Mechanical and electrical properties of Al2O3-CNT nanocomposites

This work describes the microstructure, indentation toughness and electrical conductivity of alumina-carbon black and alumina-carbon nanotubes nanocomposites prepared by spark plasma sintering. Materials have been studied by SEM, Vickers indentation technique and two-point electrical conductivity measurement. The addition of 5 % CNTs increased the indentation toughness from 3.24 MPa m(1/2) to 4.14 MPa m(1/2). The electrical conductivity of alumina-CNT nanocomposites is approximately ten times higher in comparison to the alumina-carbon black nanocomposites due to the fibrous nature and high aspect ratio of CNTs

Wear resistance of Al2O3–CNT ceramic nanocomposites at room and high temperatures

This work describes the microstructure, indentation toughness, room and high temperature tribological properties of alumina–carbon nanotubes nanocomposites with various contents of carbon nanotubes prepared by spark plasma sintering. Materials have been studied by SEM, TEM, Vickers indentation technique on the microhardness and nanohardness testers and, by high temperature ball-on-disk tribometer. The microstructure, CNT dispersion, and fracture surface were studied and wearing mechanisms: fiber pull-out, CNT crushing and formation of transferred film were identified. The addition of 5% CNTs increased the indentation toughness from 3.24 MPa m1/2 to 4.14 MPa m1/2. The coefficient of friction of alumina–CNT nanocomposites is approximately three times lower in comparison to the alumina monolithic material due to the lubricating effect of carbon nanotubes during sliding

Microstructure and properties of zirconia/carbon nanofiber composites

The effect of the addition of carbon nanofibers (CNFs) on the microstructure, fracture/mechanical and electrical properties of the CNF/zirconia composite has been investigated. The microstructure of both ZrO2 and ZrO2–CNF composites consists of a very low grain sized matrix (approximately 160 nm) with relatively well dispersed carbon nanofibers in the composite. The mechanical properties slightly decreased after the addition of CNFs to the ZrO2 but the electrical resistivity decreased significantly, exhibiting approximately 0.1 Ω cm

Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO2 and ZrO2–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10−10 S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO2/ZrO2 boundaries

Abrasive Wear of High-Carbon Low-Alloyed Austenite Steel: Microhardness, Microstructure and X-ray Characteristics of Worn Surface

A high-carbon, high-silicon steel (1.21 wt% C, 2.56 wt% Mn, 1.59 wt% Si) was subjected to quenching from 900 and 1000 °C, resulting in microstructures containing 60 and 94% of retained austenite, respectively. Subsequent abrasive wear tests of quenched samples were performed using two-body abrasion and three-body abrasion testing machines. Investigations on worn surface and subsurface were carried out using SEM, XRD, and microhardness measurement. It was found that the highest microhardness of worn surface (about 1400 HV0.05) was achieved on samples quenched from 900 °C after three-body abrasion. Microhardness of samples after two-body abrasion was noticeably smaller. with a maximum of about 1200 HV0.05. This difference correlates with microstructure investigations along with XRD results. Three-body abrasion has produced a significantly deeper deformed layer; corresponding diffractograms show bigger values of the full width at half maximum parameter (FWHM) for both α and γ alone standing peaks. The obtained results are discussed in the light of possible differences in abrasive wear conditions and differing stability of retained austenite after quenching from different temperatures. It is shown that a structure of metastable austenite may be used as a detector for wear conditions, as the sensitivity of such austenite to phase transformation strongly depends on wear conditions, and even small changes in the latter lead to significant differences in the properties of the worn surface