An attempt is made to explain the anisotropy of indentation-induced deformation and fracture of sapphire for different crystallographic planes indented by a variously oriented triangular diamond indenter. The anisotropic surface deformation and fracture of the indented crystal are discussed using a new effective resolved shear stress (ERSS) model. The patterns of the surface traces of twins and cracks are predicted on the basis of the ERSS-calculations performed for all the slip and twinning systems which are potentially activated during room temperature indentation. The theoretical prediction of the ERSS model agrees very well with the observed hardness anisotropy. The present work is the first application of the continuous indentation test to the anisotropic surface deformation of single crystals.

The anisotropy of surface deformation of sapphire : continuous indentation of triangular indenter

The in situ synthesis/consolidation of B4C–TaB2 eutectic composites by spark plasma sintering (SPS) is reported. Samples for the evaluation of bending strength were cut from specimens with diameters of 30 mm. The sample prepared for the three-point flexural strength test had fibers of tantalum diboride with diameter of 1.3 ± 0.4 μm distributed in the B4C matrix, thereby reducing composites brittleness and yielding an indentation fracture toughness of up to 4.5 MPa·m1/2. Furthermore, the Vickers hardness of B4C–TaB2 eutectics formed by SPS was as high as 26 GPa at an indentation load of 9.8 N. The flexural strength of the B4C–TaB2 system has been reported for the first time. Some steps were identified in the load–displacement curve, suggesting that micro- and macrocracking occurred during the flexural test. Ceramic composites with a eutectic structure exhibited a room-temperature strength of 430 ± 25 MPa. Compared with other eutectic composites of boron carbide with transition-metal diborides, room-temperature strength the B4C–TaB2 was 40% higher than that of B4C–TiB2 ceramics, demonstrating advantage of the in situ synthesis/consolidation of eutectic composites by SPS.

High‐Strength B4C–TaB2 Eutectic Composites Obtained via In Situ by Spark Plasma Sintering

Bulk niobium diboride ceramics were consolidated by spark plasma sintering (SPS) at 1900 °C. SPS resulted in dense specimens with a density of 98% of the theoretical density and a mean grain size of 6 μm. During the SPS consolidation, the hexagonal boron nitride (h-BN) was formed from B2O3 on the powder particle surface and residual adsorbed nitrogen in the raw diboride powder. The room-temperature strength of these NbB2 bulks was 420 MPa. The flexural strength of the NbB2 ceramics remained unchanged up to 1600 °C. At 1700 °C an increase in strength to 450 MPa was observed, which was accompanied by the disappearance of the secondary h-BN phase. Finally, at 1800 °C signs of plastic deformation were observed. Fractographic analysis revealed a number of etching pits and steplike surfaces suggestive of high-temperature deformation. The temperature dependence of the flexural strength of NbB2 bulks prepared by SPS was compared with data for monolithic TiB2, HfB2 and ZrB2. Our analysis suggested that the thermal stresses accumulated during SPS consolidation may lead to additional strengthening at elevated temperatures.

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High-temperature strength and plastic deformation behavior of niobium diboride consolidated by spark plasma sintering

https://dr.ntu.edu.sg/bitstream/10356/83645/1/Consolidation%20of%20B4C-TaB2%20eutectic%20composites%20by%20spark%20plasma%20sintering.pdf

Consolidation of B4C-TaB2 eutectic composites by spark plasma sintering

Fracture and property relationships in the double diboride ceramic composites by spark plasma sintering of TiB 2 and NbB 2

The high temperature Knoop hardness of NbB2 and TaB2 single crystals was measured in the temperature range 20–1100°C on the (0001) plane in the 〈1010〉 and 〈1120〉 indentation directions and on the {1010} plane in the 〈1210〉 and [0001] directions. In every case the hardness value H decreased monotonically with increasing temperature, but the H-T curves all showed a step at around 0.3T/Tm where Tm is the melting temperature, suggesting a change in the deformation mechanism at this temperature. At all temperatures the inequalities H 〈10 1 0〉(0001) > H 〈11 2 0〉(0001) and H 〈1 2 10〉{10 1 0} > H [0001]{10 1 0} were obtained for NbB2 and TaB2. This hardness anisotropy indicates that slip was mainly on the prismatic system and this was confirmed by slip line observations.

T. Sugimura

Papers

High temperature hardness and slip system of NbB2 and TaB2 single crystals