Transformation-Toughened Silicon Nitride
Abstract
Composites, consisting of a silicon nitride (Si3N4) matrix containing dispersions of phase-stabilized zirconia (ZrO2), were prepared by colloidal processing and sintering to near full density. These materials exhibited improved strength and increased fracture toughness relative to the matrix alone. The effects of ZrO2 content, phase-stabilizer type and amount, and processing conditions on mechanical and thermophysical properties of the resultant composites were determined. The possibility of high-volume, low-cost production was demonstrated by successfully injection molding samples. Specific systems that appear promising for various alternative applications were developed. A composite containing 45 w/o ( approx. 30 v/o) ZrO2 phase-stabilized with Y2O3 exhibited a 50% improvement in fracture toughness, (6 MPa m1/2), high room temperature flexural strength (1000 MPa), low thermal expansion, and low thermal conductivity. Use of Y2O3 plus SiO2 as sintering aids with this composite was shown to maintain high strength to temperatures of 1400 C. A second system containing from 30 w/o ( approx. 20 v/o) to 45 w/o ( approx. 30 v/o) ZrO2 phase-stabilized with CaO also exhibited high fracture toughness and high strength. Further improvements (up to threefold) in fracture toughness were obtained by heat treatment. The CaO-containing system is limited to relatively low use temperatures of less than 700 C. Probable toughening due to the martensitic tetragonal to monoclinic transition of zirconia has been hypothesized. Potential applications for these toughened systems include cryogenic turbopump bearings, intermediate use temperature applications in diesel engines, and high-temperature turbine uses. Materials will have to be tailored and optimized for each of these areas.
Document Details
- Document Type
- Technical Report
- Publication Date
- Aug 01, 1988
- Accession Number
- ADA335147
Entities
People
- H. W. Carpenter
Organizations
- Oak Ridge National Laboratory