Houzheng Wu

The strength of Al2O3/SiC nanocomposites after grinding and annealing

Alumina matrix nanocomposites containing about 5 vol.% SiC of < 100 nm mean particle size show a substantial increase in strength after machining and annealing. The final strength is controlled by the annealing process and achieves the same level after a coarse machining using 150-grit diamond as is achieved using a complex lapping and polishing sequence. In all cases the final anneal leads to an increase in strength The nanocomposite retains a significantly higher residual surface-compression stress level than an equivalent grai size alumina after machining. A remnant stress, of about 20% of the initial level, is retained even after 10 h annealing at 1250 C. Hertzian indentation and measurements of Rayleigh ave velocity show that the nanocomposite surfaces contain defects of smaller size and density than are found in equivalent aluminas. Annealing appears to result in healing of existing surface defects thus increasing the nanocomposite strength while leaving its toughness unchanged. The crack-healing mechanism is associated with a chemical process on the nanocomposite surface which has been tentatively identified as an oxidation leading to amorphous mullite formation.

Subsurface damage analysis by TEM and 3D FIB crack mapping in alumina and alumina/5vol.%SiC nanocomposites

Abstract TEM and 3D crack analysis by focused ion beam (FIB) cross sectioning have been used to quantify the subsurface damage beneath scratches made by a 120° cone indenter loaded to 1 N in monolithic polycrystalline alumina and alumina/5vol.%SiC nanocomposites. In the nanocomposite, an extensive plastic deformation zone was found under the scratch grooves, extending beyond the first layer of grains to a maximum plastic deformation depth of ~7 μm below the surface of the track. In the alumina, however, the plastically deformed region only extends to a maximum depth of ~4 μm and is contained within the first layer of grains adjacent to the groove surface. The 3D morphologies of the cracks under the scratches have been determined by FIB sectioning, showing that a high density of microcracks exists under the scratches in both ceramics. Differences between the plastic deformation and subsurface facture modes of the alumina and the nanocomposite are discussed.

Residual stress and subsurface damage in machined alumina and alumina/silicon carbide nanocomposite ceramics

We have used TEM and Hertzian indentation to study the interrelation between subsurface damage and residual stress introduced by grinding and diamond polishing surfaces of polycrystalline alumina and 5%SiC/alumina nanocomposites. In all cases a layer of high dislocation density was found near the surface. This varied in thickness from about 300 nm for alumina polished with 1 μm diamond grit to greater than 6 μm for a nanocomposite surface wheel-ground with 150 μm diamond grit. For a given finishing process the nanocomposites showed a greater depth of dislocation activity than alumina. In alumina, extensive basal twinning was found beneath the ground surfaces. Hertzian indentation data indicates a residual compressive stress of about 1500 MPa confined to the dislocation-containing region. Mechanisms for the enhanced dislocation activity in the nanocomposites are discussed.

Local ordering of oxygen vacancies in cubic zirconia (ZrO2) stabilized with yttria (Y2O2) and magnesia (MgO) I. Electron diffuse scattering study

Abstract Diffuse scattering, being highly intensive and confined in reciprocal space to a regular geometric surface exhibiting the translation symmetry of the bcc reciprocal lattice, is observed by electron diffraction from partially stabilized zirconia with Y2 O3 and MgO. This material is a structural ceramic having potential applications at high temperatures (> 1000°C). A three-dimensional geometric model of the diffuse scattering distributed in reciprocal space is constructed and an analytical equation corresponding to this model is established. In the reciprocal-space unit cell, the diffuse scattering distributes on the surfaces of eight rotational ellipsoids whose rotational axes are along the (111) directions of the bcc reciprocal lattice. Possible sources regarding the formation of the diffuse scattering are discussed. It is concluded that local ordering of oxygen vacancies in the cubic phase matrix, corresponding possibly to a transition state changing from the disordered cubic fluorite structure ...

Plasma augmented laser welding of 6 mm steel plate

Abstract A hybrid laser arc welding system incorporating a plasma arc in a triangulated configuration (plasma augmented laser welding – PALW) was used to produce bead on plate welds in 6 mm low carbon steel. Laser powers up to 5 kW and arc currents up to 150 A were investigated. Plasma augmented laser welding allowed an increase in full penetration welding speed of 100% or an increase in penetration of between 25 and 100%, compared with the laser alone. At speeds of 2 m min−1 and above, PALW and laser alone were found to be equal with respect to fusion zone area per kW (mm2 kW−1). Plasma augmented laser welding was equivalent in terms of welding energy utilisation (mm3 J−1 of fusion zone) at 2 m min−1 and superior above 2 m min−1. Full penetrating PALW allows significant energy losses via the keyhole root exit. Only when the keyhole root closes can the full benefits of the hybrid process be manifested in the workpiece.

Subsurface damage in alumina and alumina–silicon carbide nanocomposites

The subsurface plastic deformation below alumina (Al2O3) and Al2O3–silicon carbide (SiC) nanocomposite surfaces subjected to grinding, polishing and annealing has been measured by high-resolution grazing-incidence parallel-beam X-ray powder diffraction and transmission electron microscopy. The variation with angle in the full width at half-height maximum (FWHM) of the X-ray Bragg peaks was successfully modelled by a FWHM distribution that fell exponentially with increasing depth. Consistent parameters were extracted from data taken using both prism and pyramidal reflecting planes. Correlation was found between the depth at which the FWHM fell to 1/e of the surface value and the depth of damage observed by transmission electron microscopy. The associated surface strain in the nanocomposite was found to increase linearly with increasing diameter of the diamond polishing particles. In ground 5 vol.% SiC nanocomposite, these random surface strains fell by a factor of 7 and the depth of damage increased by a factor of 3 after annealing at 1250°C for 2 h. No differences were observed in the Bragg peak FWHM as a function of angle for material polished with 1 µm diamond grit before and after annealing.

Fracture toughness of a zirconia engineering ceramic and the effects thereon of surface processing with fibre laser radiation

Vickers hardness indentation tests were employed to investigate the near-surface changes in the hardness of a fibre laser-treated and an as-received ZrO2 engineering ceramic. Indents were created using 5, 20, and 30 kg loads to obtain the hardness. Optical microscopy, white-light interferometry, and a coordinate measuring machine were then used to observe the crack lengths and crack geometry. Palmqvist and half-penny median crack profiles were found, which dictated the selection of the group of equations used herein. Computational and analytical approaches were then adapted to determine the K1c of ZrO2. It was found that the best applicable equation was: K1c = 0.016 (E/H)1/2 (P/c3/2), which was confirmed to be 42 per cent accurate in producing K1c values within the range of 8 to 12 MPa m1/2 for ZrO2. Fibre laser surface treatment reduced the surface hardness and produced smaller crack lengths in comparison with the as-received surface. The surface crack lengths, hardness, and indentation loads were found to be important, particularly the crack length, which significantly influenced the end K1c value when K1c = 0.016 (E/H)1/2 (P/c3/2) was used. This is because, the longer the crack lengths, the lower the ceramics resistance to indentation. This, in turn, increased the end K1c value. Also, the hardness influences the K1c, and a softer surface was produced by the fibre laser treatment; this resulted in higher resistance to crack propagation and enhanced the ceramics K1c. Increasing the indentation load also varied the end K1c value, as higher indentation loads resulted in a bigger diamond footprint, and the ceramic exhibited longer crack lengths.

Sub-micron thin film intrinsic Josephson junctions

We have fabricated sub-micron intrinsic Josephson junctions in thin films of Tl-Ba-Ca-Cu-O using two differing techniques suited to different applications. By using lateral focussed ion-beam milling we have created arrays of intrinsic junctions in c-axis oriented films. Such arrays, with areas as low as 0.25 /spl mu/m/sup 2/, display large hysteresis comparable to that observed in single-crystal intrinsic junctions. By using normal focussed ion-beam milling we have created arrays in mis-aligned films grown on vicinal substrates. In arrays of area less than 0.4 /spl mu/m/sup 2/ we observe Josephson phase diffusion and a suppressed critical current, showing that charging effects may be significant in these junctions.

Buffer layers for Tl-2212 thin films on MgO and sapphire substrates

High quality Tl-2212 films can be readily grown on LaAlO substrates, but the design of complex microwave devices in these films is made more difficult by the twinning of the substrate. CeO buffer layers on sapphire have been demonstrated to offer excellent substrates for Tl-2212 films (1). We have been exploring alternative architectures for buffer layers on MgO and sapphire. Epitaxial Tl-2212 superconducting thin films have been success- fully grown on Gd O -buffered MgO substrates by sputter deposi- tion of a Tl-free precursor followed by an ex-situ thalliation anneal. The Tl-2212 films were aligned with the -axis normal to the film surface, and also had excellent in-plane orientation with Tl-2212 aligned with the Gd O and MgO . The critical tem- peratures of the films ranged from 100-103 K and transport and inductive critical current densities of 7.5 10 A/cm at 77 K have been achieved. We have also fabricated epitaxial GdAlO buffer layers on sapphire substrates which support epitaxial growth of Tl-2212 but with relatively poor microstructure. High quality films have been prepared on the new La,Sr Al,Ta O (LSAT) sub- strates.

Determination of the depth distribution of subsurface damage during polishing of alumina

High-resolution grazing-incidence parallel-beam powder diffraction has been used to detect the subsurface damage below alumina surfaces subjected to polishing with cerium oxide or diamond. Despite very significant evolution of the surface morphology, no changes in the subsurface strains were observed over a 20min period of polishing with ceria. For both polishing materials, the variation with angle in the full-width at half-height maximum was successfully modelled by a strain distribution that fell exponentially with increasing depth. Although the surface amplitude and depth dependence parameter are coupled, we have been able to place upper limits on the depth to which the damage extends. Under realistic assumptions, the depth of damage induced by 1µm diamond paste is comparable with that from 3µm ceria polish.