Investigation of a novel concept for the development of SiC nanophased reinforcement MoSi(2).

摘要

In this dissertation study a new processing method was developed to fabricate a {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite. The processing involved the pyrolysis and densification of {dollar}rm MoSisb2{dollar} powder particles coated with polycarbosilane. A uniform dispersion of nano-sized SiC particles in the {dollar}rm MoSisb2{dollar} matrix was observed. An optimum processing condition was determined through a statistical method (fractional factorial design). Limited mechanical property (strength, fracture toughness, and creep) measurements of the in-situ nanocomposites were carried out to study the efficacy of the chemical processing route for fabricating nanocomposites. The average room temperature flexural strength of the 14 v/o {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite was 750 MPa which was approximately a 500% improvement over that of the {dollar}rm MoSisb2{dollar} (160 MPa). The average flexural strengths of the 14 v/o {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite at 1100 and {dollar}rm 1250spcirc C{dollar} were 868 and 606 MPa, respectively. The improvement of the high temperature (at {dollar}rm 1250spcirc C){dollar} flexural strength of the 14 v/o {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite over that of the monolith (77 MPa at {dollar}rm 1250spcirc C){dollar} was approximately 800%. Room temperature fracture toughness {dollar}rm (Ksb{lcub}IC{rcub}){dollar} of the 14 v/o {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite was measured by Vicker's indentation method where a 45% increase over the monolithic {dollar}rm MoSisb2{dollar} was observed. The creep behavior of the 14 v/o {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite was studied and the creep rate at {dollar}rm 1200spcirc C{dollar} was 1.2 orders of magnitude lower than monolithic {dollar}rm MoSisb2{dollar} (based upon the calculation using the same grain size). SEM fractographic analysis revealed that the 14 v/o {dollar}rm SiCsb{lcub}pcs{rcub}/MoSisb2{dollar} in-situ nanocomposite exhibited a transgranular fracture mode, for both room and elevated temperatures, while monolithic {dollar}rm MoSisb2{dollar} exhibited a mixed mode at room temperature and mostly an intergranular fracture mode at {dollar}rm 1250spcirc C.{dollar} The in-situ nanocomposite processing method developed in this study could be applicable to many other nanocomposite systems (i.e., {dollar}rm SiCsb{lcub}pcs{rcub}/Sisb3Nsb4, SiCsb{lcub}pcs{rcub}/Alsb2Osb3, SiCsb{lcub}pcs{rcub}/SiC,{dollar} etc.).