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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. more...

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. more...

This paper presents the results of the investigation of NiCrRe coating deposited by plasma spray process at the atmospheric pressure on boiler steel substrate. These coatings were characterized by means of a scanning electron microscopy, and Energy-dispersive X-ray spectroscopy. The wear resistance of plasma sprayed NiCrRe coatings has been investigated under dry sliding conditions at applied load of 10 N in air. The continuous stiffness measurement (CSM) method was used for the investigation of nanohardness using Agilent G200 Nano-indenter in order to determine the mechanical properties of the coatings. Microstructural observations pointed out that the NiCr layer with white isles of rhenium possessed porosity, oxidized, un-melted and semi-melted particles, and inclusions. According to the results the thickness of the layer is 450 µm, the indentation modulus 158 ± 24.4 GPa, hardness 3.74 ± 0.76 GPa and the coefficient of friction is 0.45. more...

The high-velocity oxy-fuel technique (HVOF) was used to produce dense NiCrRe coating on boiler steel substrate with a minimal amount of oxide impurities and low porosity. Microstructure analysis, tribology and nano-hardness measurement have been realized with the aim to characterize the systems. The microstructure was studied using scanning electron microscopy and Energy-dispersive X-ray spectroscopy. Tribological characteristics have been studied under the dry sliding condition at applied loads of 5, 10 and 20 N using the ball-on-flat technique with SiC ball at room temperature. Nano-hardness was investigated in continuous stiffness measurement (CSM) mode, the indentation depth limit was 1500 nm. Microstructure analyses proved that the HVOF sprayed layer, with a thickness approximately 870 µm, contains a relatively low volume fraction of porosity with a chemical composition based on Nickel, Chromium, with white areas of Rhenium. The wear rate of the coating is significantly lower than the wear rate of 16Mo3 steel. The average values of indentation modulus and hardness were EIT = 237.6 GPa and HIT = 6.3 GPa, respectively. more...

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. more...

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. more...