Development of a methodology for fabrication and characterization of nanophased reinforced composites from the silicon carbide-aluminum nitride system.

摘要

The fabrication and characterization of nanophased reinforced composites from SiC-AlN compositions, by heat treatment within the spinodal decomposition zone, were investigated. A series of SiC-AlN compositions were hot pressed at 2100{dollar}spcirc{dollar}C with a one hour soak time under a pressure of 35 MPa in vacuum. For the fabrication of nanophased reinforced composites, hot-pressed 2H-wurtzite solid solution specimens of 25, 50, and 75 mol% AlN were chosen and heat treated at 1750{dollar}spcirc{dollar}C for 150 hours in a flowing N{dollar}sb2{dollar} atmosphere. Analyses of the microstructure, crystallographic phase, lattice parameters, residual stress, texture, peak-breadth, and composition were conducted. Properties regarding the hot-pressed and the heat-treated specimens were measured.; SiC-AlN 2H-wurtzite solid solutions for 25 to 100 mol% AlN were formed via hot pressing. Microstructures and morphologies of the hot-pressed specimens varied throughout the composition with a decreasing needle-like structure up to 25 mol% AlN and with increasingly equiaxed grains from 25 to 100 mol% AlN.; Nano precipitates with typical modulated tweed-type structures were found through the (2110) zone axis on the three heat-treated compositions. Compositions associated with the modulated tweed-type structure were also successfully found to modulate around average compositions within nano-intervals. These two significant results strongly confirmed the formation of in-situ nanophased reinforced composites. Two significant findings associated with the characterization of the modulated structures were established. First a method for the determination of modulation orientation was developed by adapting the Khachaturyan-Mayo-Tsakalakos approach and combining it with a TEM image and its corresponding diffraction pattern, as well as lattice parameter data. The modulation orientations are orthogonal to the {dollar}{lcub}01=12{rcub}{dollar} planes that make angles of 46.70{dollar}spcirc{dollar}, 46.90{dollar}spcirc{dollar}, and 47.11{dollar}spcirc{dollar} to the (0001) for the three compositions. Second, HRTEM combined with a very fine EDS probe of 1nm resolution resolved the presence of nano-interval compositional modulation within the modulated tweed-type structure.; The expansion of lattice parameters and the presence of a modulated structure did not cause significant residual stresses in the heat-treated specimens. The residual stresses and the textures between the hot-pressed and heat-treated specimens were not significantly different. These results indicate the stability of the macro grain structure after the heat treatment which is consistent with the similar lattice parameters and structures of the two materials.; For the case of room-temperature properties of the hot-pressed specimens, density and coefficient of thermal expansion increase linearly with increasing AlN content while Young's modulus decreases linearly. Knoop-microhardness decreases linearly with increasing AlN content within the 2H-wurtzite solid solution region. Flexural strengths and fracture toughness increase up to 25 mol% AlN and then decrease from 25 to 100 mol% AlN. This is believed to be due to the microstructures and intrinsic bonding qualities of the SiC-AlN specimens. The results of coefficient of thermal expansion and Young's moduli at elevated temperatures, and flexural strengths at 1250{dollar}spcirc{dollar}C suggest the possible use of SiC-AlN compositions for moderate temperatures.; A comparison of the heat-treated to the hot-pressed specimens showed that the Knoop-microhardness values are not significantly different. Flexural strengths are consistently higher for 75 mol% AlN. No conclusions can be drawn for 25 and 50 mol% AlN due to data inconsistencies. However, the flexural strengths have lower standard deviations suggesting that the heat treatment might cause homogenization. Micro-indentation toughness values are higher but not significa