Surface properties after ageing of dispersion ceramic and its influence on strength

The primary purpose of this work was the examination of the surface and the strength of ATZ ceramic after hydrothermal treatment. Hydrothermal treatment of ATZ ceramic leads to changes of roughness and phase composition on the surface. The phase composition and the microstructure on the surface and within the peripheral zone were determined after applying a stepwise broad ion beam polishing technique and quantitative phase analysis. With this technique a penetration depth of the phase transformation of less than 14 µm was observed. Investigations of the cross sectional area by means of FESEM have shown a transformation zone of about 9 µm. Both methods lead to comparable results with a very small, morphologically changed peripheral layer. The study of the strength after ageing with different thickness of the samples showed that for a thickness of 2.0 mm there is no negative influence on mechanical behaviour. In consideration of this fact no significant change of tribological properties is expected

Inspection of microstructure and phase composition of a dispersion ceramic after hydrothermal treatment

The primary purpose of this work was the examination of the surface of ATZ ceramic after hydrothermal treatment. The surface was inspected concerning its roughness. The phase composition and the microstructure on the surface and within the peripheral zone were determined after applying a stepwise broad ion beam polishing technique. After each preparation step, the surface was analysed with X-ray diffractometry (XRD) and the phase composition was calculated. Additionally, the cross sectional area of the sample was observed with a field emission scanning electron microscope technique (FESEM). Hydrothermal treatment of ATZ ceramic leads to changes of roughness and phase composition on the surface. But the penetration depth of the phase transformation is only 14 micron. The result of this last step of only 0.5 wt% monoclinic zirconia corresponds to the initial content after fabrication. That means that the tetragonal/monoclinic transformation only takes place in this small surface layer of a thickness of less than 14 micron. This is in good agreement with the FESEM analysis where a defect peripheral zone of a thickness of about 9 micron was observed

Thermophysical and microstructural studies on thermally sprayed tungsten carbide-cobalt coatings

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components, and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes, the knowledge of thermophysical, mechanical, and other material data is required. However, this information is still missing today. In this study, the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700 °C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity, and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50 °C to 35.4 W/(mK) at 700 °C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) to take into account the stru ctural and compositional changes. Extensive XRD and FESEM studies were performed to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison

Electrochemical corrosion of liquid phase sintered silicon carbide ceramics

Liquid-phase sintered silicon carbide (LPS-SiC) is silicon carbide ceramic which contains sintering additives forming a liquid phase during sintering. These additives segregate in the grain boundary phase during cooling. The usually used Al2O3 dissolves partially in the SiC-grains and therefore changing the conductivity of the SiC (core rim structure). This study is focused on the electrochemical properties of LPS-SiC with yttria and alumina as sintering additives. Electrochemical corrosion behaviour of LPS-SiC has been determined by linear and cyclic voltammetry in acidic and alkaline solution. The effect of anodic oxidation on the material has been monitored by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX) as well as by atomic force microscopy (AFM). The core-rim structure of the investigated materials plays a decisive role in the vulnerability towards corrosion. If oxidative attack was found to occur under anodic polarization, it happened preferentially in the rim region of the SiC-grains, while the core of the SiC-grains remained basically unaffected

Field-Assisted Densification of Superhard B6O Materials with Y2O3/Al2O3 Addition

B6O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that Al2O3 can be utilized as an effective sintering additive, similar to the addition of Y2O3/Al2O3 that was used in this work. The densification behavior of the material as a function of applied pressure, its microstructure evolution, and the resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B2O3, Al2O3, and Y2O3, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average a hardness of 33 GPa, a fracture toughness of 4 MPa. m(1/2), and a strength value of 520 MPa

Field-assisted densification of superhard B6O materials with Y2O3/Al2O3 addition

B6O is a possible candidate of superhard materials with a hardness of 45 GPa measured on single crystals. Up to now, densification of these materials was only possible at high pressure. However, recently it was found that different oxides can be utilized as effective sintering additives. In this work the effect of addition of Y2O3/Al2O3 on the densification behaviour as a function of applied pressure, its microstructure evolution, and resulting mechanical properties were investigated. A strong dependence of the densification with increasing pressure was found. The material revealed characteristic triple junctions filled with amorphous residue composed of B2O3, Al2O3 and Y2O3, while no amorphous grain-boundary films were observed along internal interfaces. Mechanical testing revealed on average hardness of 33 GPa, a fracture toughness of 4 MPam1/2, and a strength value of 500 MPa