The flow of a thin film of a yield-stress material on a rotating disc is considered. For the constitutive relation we use the biviscosity model, of which the usual Bingham model is a limiting case. We show that this limiting process conflicts with the usual thin film approximation, so that care is needed to avoid contradictions. It is found that the flow consists of three regions: (1) an inner region centered on the axis of rotation which is unyielded, (2) a shear flow region adjacent to the disc which is bounded above by the yield surface and (3) a region near to the free surface which is unyielded and has the yield surface as lower boundary. It is in the third region that difficulties occur in the straightforward ‘‘Bingham limit.’’ For on the one hand the material is rigid, and on the other it is supposed to move outwards radially. This conflict can be removed by using the biviscosity model and a suitable limiting process.
Insulated rail joint assemblies provide electrical insulation between two sections of rail
for signalling purposes. In this work, rail steel was successfully bonded to PSZ ceramic using an
active brazing technique. In order to increase the wettability of the PSZ ceramics, titanium coating
was deposited on the ceramic surface using a filtered arc deposition system. A filler metal called
BVAg-18 (60%Ag-30%Cu-10%Sn) was used and the joining was performed at a temperature of
750 °C.
Bonding between partially stabilised zirconia and rail steel with BVAg-18 filler metal was not
achieved using a standard brazing method. Bonding did occur with the BVAg-18 filler metal using
the advanced brazing technique of active metal brazing, with best results obtained using a brazing
temperature of 750oC and a dwell time of 10 minutes. The microstructure of the coating and joint
interface were characterised by XRD, SEM and EDS.
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