S.Y. Chen

Growth kinetics of sintered NiO/ZrO2 (5 mol.% Y2O3) composites

Abstract This work uses X-ray diffraction and scanning electron microscopy to investigate the solution of NiO in ZrO 2 (with 5 mol% Y 2 O 3 , mainly in a metastable t′ phase as plasma sprayed powders) and the resultant growth kinetics of the NiO/ZrO 2 composite at 1600 °C. The tt-ZrO 2 transforms into cubic (c-) ZrO 2 within 1 h upon firing, and room temperature X-ray lattice parameter results indicate that the solubility of NiO in c-ZrO 2 is about 3 mol%. Within the solubility limit, both sintering and growth of c-ZrO 2 grains were enhanced by NiO addition. A further increase in the NiO content results in the formation of NiO particles which hinder the grain growth of c-ZrO 2 . For specimens with 10 mol% NiO the c-ZrO 2 grains were found to follow the t 1 3 growth kinetics, i.e. bulk diffusion controlled, while the intergranular and intragranular NiO particles coarsen according to t 1 4 and t 1 5 laws respectively. The interfacial energy of NiO/c-ZrO 2 is about 0.48 ± 0.1 J m 2 , inferred from the geometry of the NiO particles at the grain boundary.

Effect of NiO dissolution on the transformation of plasma-sprayed Y-PSZ

Abstract Plasma-sprayed Y-PSZ (5 mol.% Y 2 O 3 ) with and without 10 mol.% additions of NiO-source precursor was fired at temperatures between 1200 and 1500 °C to study the dissolution effect of NiO on the transformation behaviour of zirconia. Postsintering heat treatment at 1200 and 1300 °C caused significant dissolution of NiO in Y-PSZ, as indicated by the increasing c/a ratio of tetragonal (t-) or non-transformable t′-zirconia and the contraction of the cubic (c) and monoclinic (m) lattices. The NiO dissolution enhanced the decomposition of t′ phase to c + t zirconia phases, but suppressed the subsequent t → m transformation. The effect of NiO dissolution on the properties of the plasma-sprayed coating of Y-PSZ is discussed.

Phase equilibria of zirconia-dispersed ceramic in NiOAl2O3ZrO2 system

Abstract Powders of NiO, Al 2 O 3 and ZrO 2 mixed in various ratios were sintered between 1200 and 1600°C and their phase equilibria studied by X-ray diffraction and electron microscopy. Partially stabilized ZrO 2 (PSZ) consisting of tetragonal ZrO 2 (t-ZrO 2 ) precipitated in the cubic ZrO 2 (c-ZrO 2 ) matrix was not formed because the solubility of NiO and/or Al 2 O 3 in ZrO 2 was not enough to stabilize the c-ZrO 2 phase. Instead, t-ZrO 2 particles were constrained in the matrix phase of NiO and/or spinel which was formed by reacting NiO with Al 2 O 3 . At 1600°C, t-ZrO 2 equilibrated with the stoichiometric NiAl 2 O 4 spinel when free NiO was present, with spinel of varied nickel-to-aluminium stoichiometry when Al 2 O 3 content exceeded NiO, and with Al 2 O 3 -rich spinel and free Al 2 O 3 (corundum) when Al 2 O 3 was saturated in spinel. The implications of these results to TBCs are discussed.

Annealing-induced defect clustering at Ni1−xOY-partially-stabilized ZrO2 interfaces

Abstract Annealing-induced grain boundary relaxation of Ni1−xO (bunsenite) particles were studied in a simplified composite system consisting of dispersed bunsenite in yttria-partially-stabilized zirconia (Y-PSZ) (5 mol.% Y2O3) which have been sintered (1573 K for 5 h) with added Ni1−xO (10 mol.%) followed by annealing at 1573 K for up to 300 h. Transmission electron microscopy observations indicated that the incoherent Ni1−xO particles occurred within or at the boundaries of the ZrO2 grains which were saturated with the Ni1−xO component. At grain boundaries of these Ni1−xO particles, defect clusters and spinel oxide were produced to form a shell by prolonged aging (e.g. for 300 h), but not for 100 h or less. Commensurate side-band diffraction spots with a unit about 1/2.5 times the lattice parameter of bunsenite in the reciprocal space were diffracted from these defect clusters, indicating that they were periodically spaced. The results of scanning transmission electron microscopy coupled with energy-dispersive X-ray analysis and the consideration of defect chemistry suggested tentatively that defect clusters contain extrinsic nickel vacancies (VNi″ and/or VNi′ and charge-compensating defects, e.g. predominantly the dopant ZrNi·· and/or Zri····. The roles of dopants in the defect clustering at Ni1−xO-Y-PSZ interfaces are discussed.

Polymorphic transformation of t′ phase in Yttria partially stabilized zirconia

Abstract Plasma-sprayed yttria partially stabilized zirconia (5 mol.% Y 2 O 3 ) with and without 10 mol.% additions of NiO source precursor were fired at temperatures between 1200 and 1600°C, and studied by X-ray diffraction and electron microscopy. The samples free of NiO underwent metastable tetragonal to metastable cubic (t′−c′) transformation above a critical aging temperature T 0 between 1400 and 1500°C, and then decomposed to equilibrium cubic (c) and tetragonal (t) assemblages. At 1600 °C, the t′−c′ transformation of NiO-free specimens was complete within 1 h and the subsequent decomposition to c+t assemblages took about 16 h. The dissolution of NiO in zirconia enhanced the formation of equilibrium c phase above 1400°C.

Effects of grain boundary carbide on the stress-strain curves of type 316 stainless steel

Abstract The presence of dense grain boundary carbide can result in a decrease in the uniform elongation and an increase in the tensile strength and strain hardening rate of type 316 stainless steel at room temperature. The stress-strain curves are analyzed using the Ludwigson equation σ = k 1 ϵ n 1 + exp ( k 2 + n 2 ϵ ). It is found than n 2 and n 2 decrease, k 1 increases slightly while k 2 is not affected by the presence of grain boundary carbide. The strain at which the Ludwigson equation reduces to the Ludwik equation (i.e. σ = k 1 ϵ n 1 ) decreases with increasing grain size and by the presence of grain boundary carbide. The significance of the results are discussed.

Microstructural development of zirconia-dispersed ceramic in the NiOAl2O3ZrO2 system

Abstract Zirconia-dispersed ceramic (ZDC) prepared by mixing powders of NiOZrO 2 adesignated as NZ, 1 : 1 and 5 : 1 in molar ratios) and NiOAl 2 O 3 (designated as NAZ, 3:5:1 and 5:3:1 in molar ratios) followed by sintering between 1200 and 1600°C for 1–40 h were studied by electron microscopy. The vapour phase of Ni contributed significantly to the reaction of NiO and Al 2 O 3 to form spinel oxide of varied NiAl stoichiometry in the NAZ specimens and facilitated densification in both the NAZ and the NZ specimens. In both systems interfacial diffusion controlled the growth of the dragged ZrO 2 particles, resulting in the formation of larger intergranular ZrO 2 par ticles which were faceted and generally too large to retain the tetragonal symmetry. Exaggerated grain growth occured for NAZ specimens when sintered (1600 °C) in a two-phase compositional region (namely ZrO 2 and Al 2 O 3 -rich spinel in the N 3 A 5 Z 1 specimen) but not in a three-phase compositional region (namely NiO, ZrO 2 and stoichiometric spinel in the N 5 A 3 Z 1 specimen). On cooling, the spinel phase confined the intragranular tetragonal ZrO 2 particles up to about 0.2 μm in size. The formation mechanisms of ZDCs in NZ and NAZ systems are discussed.

Confined ZrO2 particles in NiO · nAl2O3 spinel

Abstract Microstructures of confined ZrO 2 particles in NiO · n Al 2 O 3 ( n =1.67) spinel and residual strain of the spinel matrix were studied by transmission electron microscopy. The intragranular ZrO 2 particles in NiO · n Al 2 O 3 spinel are sometimes slightly faceted following the crystallographic planes {111} of the latter. Similar to that in the non-cubic matrices ( e.g. Al 2 O 3 and mullite) the transformation of the tetragonal-ZrO 2 (t-ZrO 2 ) particles either intragranular or intergranular resulted in the formation of monoclinic ZrO 2 (m-ZrO 2 ) twins which appeared mosaic like within the particle, but secondary deformation twins occurred near the grain boundary. In the vicinity of the intragranular ZrO 2 particles the spinel matrix was anisotropically strained, both dilatational and compressional, by the t-ZrO 2 and m-ZrO 2 phases respectively.

Study of selective laser melting process for cubic models with various porosities

Additive manufacturing (AM) has the advantages of fabricating complex models for application. In this study, cube structure with various porosity is manufactured by AM of selective laser melting (SLM). CAD software is used to create the micro porosity of sample models from 60 %~80 %. The models are printed by Ti-6Al-4V powders. The morphology of the printed porous structures are examined by OM and SEM. Results show that only minor geometric deviation between the designed CAD pattern and the printed samples.