Fabrication And Properties Of ZrO₂/SUS316L Gradient Composite Pipes

In recent years, functionally graded materials (FGM) have been widely adopted in a great many areas, and most of its applications are practical. However, most of the researches concerning FGM are preparation of tabular specimens so far, and little attention has been given to the preparation of gradient composite pipes. In this dissertation, ZrO2/SUS316L gradient composite pipes were preparated by a new method tape casting-roll forming process, at the same time, Symmetrical and nonsymmetrical type Zirconia/Stainless steel (ZrO2/SUS316L) gradient composite plates were fabricated。According to the curvature of deformation of sintered plate, gradient composite pipes were designed. The process (gradient optimization,pressure between rollers and sintering process) on fabrication was analyzed; Macrophotograph and microphotograph of composite pipes were observed; element and microhardness distribution of pipe wall were analysed; Thermal Shock Test was designed and compressive strength was tested. The results showed that:ZrO2/SUS316L symmetrical gradent compoisite plate without significant bending phenomenon, and better bonding at interlayers. when ZrO2/SUS316L asymmetric gradient composite plate size is larger than 2cm, and in the case of pressureless sintering, plates were bended with a certain curvature. Moreover, the curvature was calculated by derived equation, the results of calculation and of test agree fairly closely.As a result of gradient optatimization, the residual stress would be died down. When the pressure P<5MPa, green gradient composite pipes cannot be obtained, and layers combined not closely; when P(?)15Mpa, Surfaces of pipes are rough, there are corrugation phenomenon. According to the third impact factor, the average cooling rate of l0vol.%,15vol.%,20vol.% gradient composite pipes can be estimated 0.75℃/min,0.8℃/min,0.8℃/min respectively.The pipes have no defects such as crack, warpage and flatted. The pipes wall distributed continuous at interface due to diffusion and concentration gradients, and was transitive because no mutation of components and elements. Holes in the wall arise from ZrO2-rich region, if powders were pre-mixed, defects can be reduced.After thermal shock test, microstructure of transitional boundaries varies inappreciably, which also shows good interfacial bonding between adjacent layers and beter thermal microstructure stability. but those among diferent layers cannot be compared directly. In order to improve properties of sintered composite pipes, both compsitional diference and characteristic diference between adjacent layers should be slight.Compression test shows that: with the transition rate decreased, the compressive strength decreased, but increased the fracture energy consumption and enhanced the bonding strength between layers; With the diameter increased, the compressive strength reduced; With the wall thickness increased, the compressive strength enhanced; Crack propagation show that the anisotropy of the gradient composite pipe wall would lead to crack deflection or bifurcation. In which mixed-phase and matrix phase would also affect the crack propagation path and crack would extend along the weaker interface.