Fracture toughness enhancement

Oxides such as CaO, MgO, and Y2O2 are added to Zr02 to stabili2e the tetragonal phase at temperatures below the tetragonal to monoclinic phase-transition temperature. Without stabili2et, the phase transition occurs spontaneously at temperatures below 850—1000°C, and no fracture toughness enhancement occurs (25). 

Recently siloxane-imide copolymers have received specific attention due to various unique properties displayed by these materials which include fracture toughness, enhanced adhesion, improved dielectric properties, increased solubility, and excellent atomic oxygen resistance 1S3). The first report on the synthesis of poly(siloxane-imides) appeared in 1966, where PMDA (pyromellitic dianhydride) was reacted with an amine-terminated siloxane dimer and subsequently imidized 166>. Two years later, Greber 167) reported the synthesis of a series of poly(siloxane-imide) and poly(siloxane-ester-imide) copolymers using different siloxane backbones. However no physical characterization data were reported. 

These conclusions imply that fundamental studies of the phase separation process in polysulfone/thermoset nuxtures are needed to better understand the kinetics of the situation. General conclusions about the range of variables that allow for the development of a desirable morphology compared to others are needed to be able to take full advantage of the fracture toughness enhancements... 

A number of examples exist that indicate the potential for increasing the toughness of brittle matrix by dispersing a ductile phase. The basics of the mechanism of fracture toughness enhancement were amply analyzed in the literature. Several models were developed which commonly predict the increase of toughness AGc as... 

T. Kobayashi, H. Toda. Toughness enhancement based on fracture mechanical simulation of Al-SiC composite. Mater Sci Forum 242 193, 1997. 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

Intermetallic alloys (compounds) are becoming of increasing interest as materials which possess significantly enhanced high temperature capabilities compared to many conventional metallic alloys. However, they suffer intrinsic problems associated with low-temperature ductility and fracture toughness. Two types of... 

Research Focus Enhancing the fracture toughness of poly(L-lactide-co-glycolide) by introducing phase separation using y-caprolactone. 

Two terpolymers of poly(L-lactide-co-glycolide) were prepared in this application that had enhanced fracture toughness by incorporating -y-caprolactone by ring-opening polymerization using either stannous octoate or stannous trifluoromethane sulfonate as the catalyst. The products synthesized by this method were ... 

Siaions consist of three-dimensional arrays of (Si—Al) (O.N)4 tetrahe-dra. These oxynitrides are traditionally fabricated with silicon nitride An example is beta-sialon, where the O and Si are partially replaced by N and Al, respectively. Advanced sialons are now being researched to enhance fracture toughness and iinpioved cieep properties,... 

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