Hardness of alumina/silicon carbide nanocomposites at various silicon carbide volume percentages

Vickers indentation was employed to measure the microhardness of monolithic alumina and six alumina-based nanocomposites consisting of variable silicon carbide nanoparticle volume percentages of 0.3% to 20%. Indentation tests were performed over a broad range of loads from 0.5N to 40N. The resultant hardness-load curves exhibit cumulative increases in the apparent hardness based on the silicon carbide content and reveal each sample suffers from a prominent indentation size effect (ISE). Herein, we present a comprehensive analysis of this data using Meyer’s Law, the proportional specimen resistance model (PSR) and the modified proportional specimen resistance model (MPSR) and employ TEM imagery to detail potential mechanisms by which silicon carbide nano-reinforcements influence the “true hardness” and the ISE

Friction surface structure of a Cf/C-SiC composite brake disc after bedding testing on a full-scale dynamometer

We have examined friction surface structure of a carbon ceramic brake disc tested on a full-scale dynamometer with microscopy techniques. The bedded friction surface is composed of two types of regions: transferred materials (TM) and SiC. The TM regions were formed through the deposition of wear debris into surface voids, followed by compaction and crystallite refinement during braking. A thin friction layer (FL) was developed on top of TM and SiC regions with nano-sized copper/iron oxide crystallites as the primary constituent. Analysis shows that debris generated from pad is the main source of TM and FL. No evidence shows chemical diffusion bonding between TM and composite constituent. On silicon carbide surface, dislocations were activated as the sources of surface fracture

Friction and surface fracture of a silicon carbide ceramic brake disc tested against a steel pad

Friction coefficients of a SiC ceramic disc were measured on a laboratory-scale dynamometer by testing against a mild steel pad under different initial braking speeds, and its friction surface was investigated with microscopy techniques. At bedding, averaged friction coefficient for a braking stop varied significantly with the initial braking speed; after bedding, it converged to ~0.6, regardless of braking speed. Surface fracture on the SiC disc was responsible for the transformation from a flat surface into a rough one, making ploughing a dominant friction mechanism at bedded stage. It was found that fracture surface and non-contact regions directly contributed friction coefficient variation at bedding stage. Friction layer composed of iron oxides and plastic deformation with partial dislocations activated appeared on SiC surface, but were unsustainable owing to surface fracture. A quantitative analysis is provided to understand friction coefficient variation and SiC surface fracture during braking

Ductile deformation in alumina ceramics under quasi-static to dynamic contact impact

Florescence spectroscopy and TEM has been used to study the ductile deformation of alumina ceramics underneath an impact contact. The contact was generated by a spherical tungsten carbide indenter under quasi-static, drop weight and ballistic loading conditions. In all circumstances, a ductile deformation region containing dislocations developed below each contact impression. The dislocation density distribution complies to the shear stress distribution predicted by the Hertzian contact model. Ballistic loading resulted in secondary material flow, giving a maximum dislocation density 5-10 times higher than that dictated by the Hertzian contact model. Quantification of dislocation density distribution allowed a critical shear stress for dislocation generation to be estimated. In this alumina ceramic, the critical shear stress is estimated as 2.55 ± 0.10 GPa. Cold work hardening and comminution under dynamic loading are discussed as possible mechanisms for the enhanced dislocation activity under dynamic impact

Plastic deformation and cracking resistance of SiC ceramics measured by indentation

Hardness is deemed as a measurement of resistance to plastic deformation of materials, but cracking, accompanying the deformation of ceramics, has recently been reasonably interpreted as one of the essential contributors to the widely known size effect of hardness, also called the indentation size effect. On the basis of the physical activities behind the indentation size effect, we intend to have the indentation size effect measurement adapted as a means to quantitatively measure the plastic deformation and the cracking damage of ceramics. We studied three types silicon carbide ceramics that were manufactured from same chemical formulation and processing route. Two of the three were given a post-sintering annealing at different dwell times, giving a distinctive microstructure from homogeneous equiaxial grains to highly heterogeneous elongated grains. By fitting the indentation size effect data with proportional specimen resistance model, the a1 and a2 parameters are extracted with good confidence. The a2 value is interpreted as the “true hardness”, the determined values of which were found to be similar for all samples. However, the a1, relatable to the indentation size effect, increased dramatically together with the indentation-induced cracking around Vickers indents observed under SEM. Considering that the cracking resistances of all three samples showed limited variance, we discuss, using a simple illustrative model, the possible microstructural factors which may contribute to the cracking damage exhibited under a defined loading condition

Chemically bonded phosphate ceramics reinforced with carbon nanotubes

We report herein, a scalable method for the preparation of alumina (Al2O3)-phosphate ceramics reinforced with carbon nanotubes (CNTs). All composites were manufactured by direct on-site growth of CNTs on ceramic particles via catalytic chemical vapour deposition. Introduction of catalyst metals to the substrate was achieved through two simple approaches, drip-coating and vacuum filtration, both of which have been reviewed. Transmission electron microscopy was utilised to investigate the interface between the Al2O3 surface and the in-situ CNTs. Resultant ceramics were produced by impregnating phosphoric acid into the Al2O3+CNT nanocomposite powder followed by die-pressing. In order to maintain the integrity of the CNTs, dehydration/curing was performed at 130-150○C. Scanning electron microscopy was elected to comparatively characterise the microstructure of this type of ceramic nanocomposite against its monolithic equivalent. Possible mechanisms by which specific features have formed are discussed

Plastic deformation of polycrystalline aluina introduced by scaled-down drop-weight impacts

We present our findings after scaled-down drop-weight tests, performed under relatively low loading conditions and employing a small-scale spherical indenter as a projectile, to boost the strain rate and energy density of the impact, resulted in the generation of a cavity of measurable depth on the surface of a pure, fully dense, alumina ceramic. We demonstrate that activated dislocations are a main contributor in the formation of the residual impression with an estimated maximum density of ~4.02×1014 m−2

Near-surface structure and residual stress in as-machined synthetic graphite

We have used optical and electron microscopy and Raman spectroscopy to study the structural changes and residual stress induced by typical industrial machining and laboratory polishing of a synthetic graphite. An abrasion layer of up to 35 nm in thickness formed on both machined and polished surfaces, giving the same ID/IG ratios evidencing graphite crystal refinement from an La of ~110 nm down to an average of 21 nm, but with different residual compression levels. For the as-polished sample, structural change was limited to the near surface region. Underneath the as-machined surface, large pores were filled with crushed material; graphite crystals were split into multi-layered graphene units that were rearranged through kinking. Graphite crystal refinement in the sub-surface region, measured by La, showed an exponential relationship with depth (z) to a depth of 35–40 μm. The positive shift of the G band in the Raman spectrum indicates a residual compression accompanied by refinement with the highest average of ~2.5 GPa on top, followed by an exponential decay inside the refined region; beyond that depth, the compression decreased linearly down to a depth of ~200 μm. Mechanisms for the refinement and residual compression are discussed with the support of atomistic modelling

In-situ micro bend testing of SiC and the effects of Ga+ ion damage

The Young’s modulus of 6H single crystal silicon carbide (SiC) was tested with micro cantilevers that had a range of cross-sectional dimensions with surfaces cleaned under different accelerating voltages of Ga+ beam. A clear size effect is seen with Young’s modulus decreasing as the cross-sectional area reduces. One of the possible reasons for such size effect is the Ga+ induced damage on all surfaces of the cantilever. Transmission electron microscopy (TEM) was used to analyse the degree of damage, and the measurements of damage is compared to predictions by SRIM irradiation simulation

Contact damage of silicon carbide ceramics with different grain structures measured by Hertzian and Vickers indentation

We have used Hertzian and Vickers indentation to investigate contact damage in sintered SiC ceramics, one consisting of uniform, fine-grains and the other a coarse, elongated grain structure. Cracking-resistance measured by Hertzian indentation, showed no discernible difference, nor did the Vickers hardness. However, numerical analysis of the Vickers indentation size effect, performed using the proportional specimen resistance model, indicates 77.3% greater surface energy, mostly realised through cracking, is experienced by heterogeneous SiC per unit area of indentation impression. This is typified by an observable increase in the number of radial cracks generated around Vickers impressions, which has been found to artificially increase the K IC determined by Vickers indentation fracture. Quantitative measurements of pre-existing flaws by Hertzian indentation show that heterogeneous SiC retains a higher density of larger flaws. Relationships between the differences in cracking around Vickers indents and the pre-existing flaw populations of these two SiC ceramics are discussed