Date of Award

Fall 12-2021

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor

Yingchao Yang

Second Committee Member

Zhihe Jin

Third Committee Member

Vincent Caccese

Additional Committee Members

Ling Li

Bo Li

Abstract

Composites reinforced by aramid fibers may suffer poor interface interactions during mechanical loading due to the smooth and chemically inert surface of the fibers. Three main strategies have been used to improve the interface shear strength (IFSS) including increasing surface roughness, enriching fiber surface with functional groups, and surface coating with ceramic particles. Plasma treatment is usually employed to functionalize and roughen surface of the fibers, leading to strengthening of the interface between the fibers and polymer matrix. A concern for fibers reinforced polymers is that the IFSS would significantly decrease once the composite is exposed to elevated temperatures. To maintain the mechanical performance of the composite at elevated temperatures, a polymer-derived-ceramics (PDC) precursor additive is used to stabilize the interface of the fabricated composite. Testing has shown that the IFSS of composites reinforced by untreated fibers is 29.32 ± 1.00 MPa and 26.10 ± 5.23 MPa at 50 °C and 100 °C, respectively, which surpasses 23.46 ± 2.32 MPa, the IFSS without PDC stabilization at room temperature. Regarding the functional groups, they have been created on the surface of the fibers through plasma or chemical treatments to improve the mechanical performance of aramid fiber reinforced composites. Unfortunately, these functional groups may reorient toward the fiber and be passivated by polar molecules in air over time and then become inactive and unable to bond to the matrix. The active period of the functional groups after a plasma treatment has been carefully evaluated. It is found that the fiber surface can be reactivated by drying with and without water molecule pretreatment. With drying at 100 °C for 10 hours, the IFSS can be restored from ~23 MPa to ~30 MPa. Regarding the last method to enhance the IFSS, chemical vaper deposition (CVD) and physical vaper deposition (PVD) have been used as an alternative methods to improve the IFSS by synthesizing ceramic nanoparticles MoO3 on the aramid fibers. A few advantages include no damage to aramid fibers which will help preserve mechanical properties of the aramid fibers and no concern on passivation as ceramic nanoparticles are stable compared to functional groups. Contrast to the bare improvement on IFSS from the MoO3 grown with PVD, the IFSS reaches 31 MPa from the nanoparticles fabricated with CVD, which suggests CVD would deliver a better interface interaction between MoO3 and fibers. These projects will benefit the engineering of aramid fibers into composites by largely taking advantage of their superior mechanical properties. Further research into interactions between heat treatment and strengthening additives would help us fundamentally understand the interface of advanced composite materials, as well as provide theoretical guidance for designing the interface between reinforcement and matrix.

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