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A (Hf-Ta-Zr-Nb-Ti)C high-entropy carbide was prepared by ball milling and a two-step Spark Plasma Sintering process, achieving a single-phase ceramic sample with a high relative density of 99.4 %. The wear resistance of the sample was measured by tribology and micro-scale mechanical behaviour was studied by nanoindentation on both the non-deformed and worn surfaces. Grains and the vicinity of grain boundaries exhibited high hardness values of 38.5 ± 0.5 GPa and 35.5 ± 1.0 GPa with similar Young’s moduli of 562 ± 11 GPa and 547 ± 16 GPa, respectively. The dominant wear mechanism was mechanical wear with limited grain pull-out and fracture, and with a localized and thin tribo-layer formation. The specific wear rate exhibited an increase with the increasing load from 2.53·10−6 mm3/Nm at 5 N to 9.03·10−6 mm3/Nm at 50 N. This was correlated to the decrease of nanohardness of the worn surfaces with increasing wear load, which is attributed to the increased number of microcracks.

Al2O3 + 30 vol% ZrO2 matrix composites with 20 and 30 vol% cBN have been prepared with the optimized processing route, using spark plasma sintering (SPS) at temperatures of 1400 &deg, C and 1250 &deg, C. The influence of cBN addition on the microstructure characteristics, micro/nanohardness, elastic modulus, and crack-extension resistance of the composites and their constitutions have been investigated using scanning electron microscopy (SEM), statistical analyses of the individual grain size and micro/nanoindentation methods. The matrix consists of alumina and zirconia grains with grain sizes/diameter of approximately 220 and 160 nm with approximately 1.9 &mu, m cBN grains in the Al2O3 + ZrO2 + cBN composites. The microhardness is slightly increasing with cBN addition from 16.2 to 17.1 GPa and the crack-extension resistance from 3.72 to 4.29 MPa.m1/2. The toughening mechanisms are in the form of crack deflection, crack branching, and crack bridging. The nanohardness and indentation modulus of the matrix are approximately 30 and 420 GPa, and the cBN grains 70 and 777 GPa, respectively.

Nano-mechanical and wear characteristics of ZrB2 + Al2O3:Cr3+,Nd3+ (Al2O3 co-doped with Cr3+ and Nd3+ ions) luminescent ceramic composites have been investigated using nanoindentation and tribological tests. Nanoindentation was carried out using a Berkovich diamond tip applying continuous stiffness measuring (CSM) mode with a maximum depth of 200 nm. The wear behaviour was studied during dry sliding in the air with a SiC counter body at applied loads of 5 and 50 N, and sliding speed of 0.1 m/s. The nanohardness of the ZrB2 and Al2O3 are very similar with mean values of 34.2 GPa and 32.6 GPa, respectively The Young modulus for ZrB2 is higher with a mean value of 555.6 GPa in comparison to the Young modulus of alumina with mean value of 473.8 GPa. Pop – in effect was found during the indentation of ZrB2 grains at indentation loads from 0.2–8.0 mN. The lowest coefficient of friction was measured for ZrB2 + 8%Al2O3:Cr3+,Nd3+ at 5 N load with value of approximately 0.5. The wear rate of the system ZrB2 + 32 % Al2O3:Cr3+,Nd3+ at 5 N is significantly lower in comparison to the wear rate of ZrB2 + 8% Al2O3:Cr3+,Nd3+ but at a load of 50 N the wear of the systems is very similar approximately 1. 10−5 mm3/N.m. Tribolayer formation connected with debris origin, oxidation and tribochemical reactions were characteristic for both composites with similar chemical composition but different size and thickness of tribolayers.

Silver and graphene nanoplatelets (Ag-GNPs) have been employed as reinforcements to prepare the self-lubricating silicon nitride matrix composites via 3D ball milling (Turbula) and spark plasma sintering. The prepared composites were characterized by scanning electron microscope with energy dispersive spectroscopy, Vickers hardness tester and reciprocating ball tribometer. Fracture surface morphology of the sintered composites indicated the potential reinforcement by the ductile silver phase. The mechanical property testing revealed that Si3N4 composites with Ag and GNPs incorporation exhibited lower hardness and slightly lower toughness compared with Si3N4 monolithic material. However, the coefficient of friction and wear in composites exhibited the lower values in 1 N friction force testing range.

Si3N4/3wt.% multi-walled carbon nanotubes (MWCNTs) composites were prepared from oxidized ?-Si3N4 powders and sintering additives by hot isostatic pressing (HIP). The Si3N4 powders were oxidized at 1000?C in ambient air environment for 10 and 20 h and the powder mixture was sintered at 1700?C for 3 h under the pressure of 20MPa in nitrogen. The relationship between oxidation, microstructure, tribology and mechanical properties was studied. The dispersion of MWCNTs was not optimal, as they were found mainly in the form of bundles between the ?-Si3N4 grains. Neither Si2N2O nor other oxide phase were observed after the sintering. This was probably caused by the presence of MWCNTs at the grain boundaries of the Si3N4 grains causing reduction of oxide phases. The density, hardness, bending strengths and friction coefficient of the composites increased with the oxidation time of the Si3N4 powder.