ZrO2 Matrix Composites Research Articles

Micro/Nano Indentation Testing of Spark Plasma Sintered Al2O3 + ZrO2 + cBN Ceramics

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 °C and 1250 °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 μm 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.

Design and high-temperature tribological properties of CoCrW with rare earth fluoride composites

CoCrW matrix containing rare earth LaF3 self-lubricating composites were fabricated by using hot-pressing method. The microstructure, phases and high-temperature tribological properties of obtained composites were systematically studied. Friction and wear behaviors were evaluated by using ball-on-disc tribo-tester from RT (24 °C) to 1000 °C. Metal matrix consisted of two allotropes: ε (hcp) and γ (fcc). Mo had a solid solution strengthening effect and lubricating effect when it changed into compounds. Rare earth LaF3 and its compounds showed lubricating effect at elevated temperatures. The friction coefficients and wear resistance of reinforced composites were greatly enhanced with little hardness dropping. It was ascribed to the synergistic effect of in-situ formed and extra added solid lubricants (silver, LaCrO3, Ag2MoO4, AgF3 and metal oxides) as well as oxides film which changed the wear model of tribo-couples during sliding. The wear rate of CoCrW-10LaF3-8Mo-9Ag was about 10 times lower than those of reported Ni and ZrO2 matrix composites. The composites showed the slight abrasive wear and oxidative wear at elevated temperatures.

Nb2AlC-particle induced accelerated crack healing in ZrO2–matrix composites

The oxidation induced crack healing behavior of pre-cracked Nb2AlC particle loaded ZrO2-matrix composites was explored by annealing in air at 1200 °C for short periods of 10 and 20 min. Composites loaded with 0, 6.5, 13, and 19.5 vol% Nb2AlC powder dispersed in 3Y-TZP matrix powder were manufactured by spark plasma sintering (SPS) at 1300 °C. Semi-elliptical artificial surface cracks with a length exceeding 220 µm were produced by Vickers indentation. The modulus of rupture of virgin, indented and annealed samples was measured in three-point bending mode. Compared to single phase 3Y-TZP strength recovery of the Nb2AlC loaded composite upon annealing at 1200 °C in air is accelerated and reaches > 60% of the initial strength after a short healing period of 10 min only. A semi-empirical oxidation cohesive zone healing model was derived which describes the crack microstructure evolution as a combined effect of 3Y-TZP-matrix healing superimposed by Nb2AlC particle oxidation induced healing.

Thermochemical calculations of the oxidation behavior of Nb2AlC MAX phase in ZrO2–matrix composites

The oxidation behavior of Y-TZP/Nb2AlC composite was explored by thermo-chemical phase equilibrium calculations and experimental oxidation studies at 1200 °C. Growth of a compact surface reaction layer requires migration of oxygen from the surface to the reaction front and a gradient of oxygen content is likely to develop. The results of phase equilibria calculations show that at low oxygen concentration preferential oxidation of Al favors thermal degradation of Nb2AlC which yields Al2O3 and Nb-carbides. With increasing oxygen content various Nb-oxides are formed and finally a mixture of NbAlO4 and Nb2O5 is present after full oxidation of a Nb2AlC particle. Experimental measurements confirmed the formation of a compact oxidation reaction layer at 1200 °C composed mainly of NbAlO4 and a co-doped (Nb,Y)-TZP matrix.

REMOVED: Effect of ZrO2 particles dispersion on the microstructure and hardness of microwave sintering SiC containing ZrO2 matrix composites

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