Pilar Miranzo

TOWARDS DURABLE THERMAL BARRIER COATINGS WITH NOVEL MICROSTRUCTURES DEPOSITED BY SOLUTION- PRECURSOR PLASMA SPRAY

The feasibility of a new processing method—solution precursor plasma spray (SPPS)—for the deposition of ZrO2-based thermal barrier coatings (TBCs) with novel structures has been demonstrated. These desirable structures in the new TBCs appear to be responsible for their improved thermal cycling life relative to conventional plasma-sprayed TBCs. Preliminary results from experiments aimed at understanding the SPPS deposition mechanisms suggest that nanometer-scale particles form in the plasma flame, followed by their deposition by sintering onto the substrate in the intense heat of the plasma flame. The SPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

Thermal conductivity of ceramics in the ZrO 2 -GdO 1.5 system

Low thermal conductivity ceramics in the ZrO 2 -GdO 1 . 5 system have potential in structural (refractories, thermal barrier coatings, thermal protection) and nuclear applications. To that end, the thermal conductivities of hot-pressed x GdO 1 . 5 . (1 - x)ZrO 2 (where x = 0.05, 0.15, 0.31, 0.50, 0.62, 0.75, 0.89, and 1.00) solid solutions were measured, for the first time, as a function of temperature in the range 25 to 700 °C. On the ZrO 2 -rich side, the thermal conductivity first decreased rapidly with increasing concentration of GdO 1 . 5 and then reached a plateau. On the GdO 1 . 5 -rich side, the decrease in the thermal conductivity with increasing concentration of ZrO 2 was less pronounced. The thermal conductivity was less sensitive to the composition with increasing temperature. The thermal conductivity of pyrochlore Gd 2 Zr 2 O 7 (x = 0.5) was higher than that of surrounding compositions at all temperatures. A semiempirical phonon-scattering theory was used to analyze the experimental thermal conductivity data. In the case of pure ZrO 2 and GdO 1 . 5 , the dependence of the thermal conductivity to the absolute temperature (T) was less than 1/T. Therefore, the minimum thermal conductivity theory was applied, which better described the temperature dependence of the thermal conductivity of pure ZrO 2 and GdO 1 . 5 . In the case of solid solutions, phonon scattering by cation mass fluctuations and additional scattering by oxygen vacancies on the ZrO 2 -rich side and by gadolinium vacancies on the GdO 1 . 5 -rich side seemed to account for the composition and temperature dependence of the thermal conductivity.

Elastic/plastic indentation in ceramics: a fracture toughness determination method

An extensive literature survey of the evolution of the indentation technique is made. The elastic/plastic solution for the spherical cavity expansion is used to obtain a physical explanation for the quantitative measurement of the radial cracks that occur at the indentation impression corners. Finally an expression based on the mentioned theory is given. Experimental data from a serie of brittle materials are well fitted by this expression.

Multicomponent toughened ceramic materials obtained by reaction sintering: Part 1 ZrO2-Al2O3-SiO2-CaO system

Fully dense zirconia toughened ceramics with a mullite matrix from the basis of information on the quaternary system ZrO2-Al2O3-SiO2-CaO, in a temperature range as low as 1425 to 1450° C, have been obtained by reaction sintering of zircon/alumina/calcium carbonate mixtures. The shrinkage, advance of reaction, microstructure and mechanical properties of different compositions are reported. The results are explained in terms of a transitory liquid phase that appears at temperatures lower than 1400° C.

Multicomponent toughened ceramic materials obtained by reaction sintering

Very dense zirconia-toughened ceramics with a mullite matrix based on the quaternary system ZrO2-Al2O3-SiO2-MgO in the temperature range as low as 1450 to 1500° C have been obtained by reaction sintering of zircon/alumina/magnesia mixtures. The shrinkage, advance of reaction, microstructure, densification mechanism and mechanical properties are reported. The results are explained in terms of transitory and permanent liquids that appear at ≤1425° C and ~ 1450° C respectively.

Thermal conductivity of Al2O3/SiC platelet composites

The thermal conductivity of hot-pressed Al2O3/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al2O3/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselmans approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites.

Joining mechanism in Si3N4 bonded with a Ni–Cr–B interlayer

Abstract A commercially available Ni–Cr brazing alloy foil with B infused at its surface layer was used to join silicon nitride ceramics. The joining region showed a diffusion zone at the middle of the interlayer and a reaction layer at the silicon nitride/brazing alloy interface. The reaction layer mainly consisted of CrN and BN while the diffusion layer was formed by Ni [Cr, Si] solid solution with some CrN precipitates. A reaction mechanism is proposed to explain the microstructure and phases observed in the bonding interlayer.

Microstructure and mechanical strength of Si3N4/Ni solid state bonded interfaces

Joining of hot pressed silicon nitride using Ni interlayers was done by diffusion bonding. Results were compared for two bonding temperatures: 1050 and 1150°C. For the lower joining temperature, no reaction was found but a limited diffusion of atomic Si into the Ni lattice was observed while for the higher temperature a thin reaction layer was found. Some nickel silicides and precipitates of a Y-rich phase were detected within this layer and an extensive diffusion zone of Si into Ni was measured. The mechanical properties of the joints were evaluated by shear and bend tests varying the thickness of the Ni interlayer. Bending strengths of 235 MPa were achieved at room temperature.

Effect of α-/β Si3N4-phase ratio and microstructure on the tribological behaviour up to 700°C

Abstract The present work analyses the effect of mechanical and microstructural characteristics in the tribological behaviour of Si3N4 materials. Special attention was given to the α/β phase ratio and the coarseness of the microstructure. Wear experiments were performed in a pin-on-disc tribometer using self-mated Si3N4 pairs varying the sliding speed and the test temperature. In the case of fine and medium microstructures, fracture and delamination was the main wear mechanism while for heterogeneous coarse material deformation related to intergranular microfracture was prevalent. The latter present higher wear rates at room temperature and low sliding speed but showed the best wear resistance when either sliding speed or temperature were increased.

Tribological characteristics of self-mated couples of Si3N4–SiC composites in the range 22–700°C

Abstract The tribological behaviour of self-mated pairs of Si 3 N 4 , Si 3 N 4 /SiC P (platelets) and Si 3 N 4 /SiC N (nanosized) hot pressed materials was studied in a pin-on-disc configuration. Tests were performed without lubrication, at a fixed load of 5 N, using different temperatures (room temperature −700°C) and sliding speeds (0.5–2 m s −1 ). Friction coefficients were usually above 0.5, being almost independent of sliding speed and test temperature. For all test conditions, the Si 3 N 4 /SiC P composite had the lowest friction coefficient, due to the presence of SiC platelets with smooth cleavage planes oriented parallel to the sliding surface. Wear coefficients were above 10 −6 mm 3 N −1 m −1 denoting a generalised severe wear mode, controlled by surface microcracking. No net differences were found between the wear behaviour of monolithic and composite materials under this wear regime. Wear decreased with sliding speed due to the protective action of a coherent tribofilm resulting from debris aggregation. Conversely, wear coefficients showed a steep increase above room temperature, to values around 10 −4 mm 3 N −1 m −1 at 700°C.