Date of Award

6-29-2018

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Fracture mechanics, Lithium Battery, PVD Coatings, Thermal spray coatings, Tribology

Supervisor

A. Alpas

Rights

info:eu-repo/semantics/openAccess

Abstract

The objective of this research is to develop advanced material characterization and mechanical testing methods for C-, Mo- based and ferrous tribological coatings to be used in lightweight engines, and manufacturing of lightweight materials. These new characterization methods are also applied to the investigation of damage processes in energy materials, namely to investigate degradation in C- based electrodes used in Li-ion batteries. Evaluations of the damage mechanisms in these materials under static and sliding contact stress conditions, as well as under different environments and temperatures encountered under the actual service conditions create several challenges. Accordingly, an in-situ mechanical testing set-up was developed to study the fracture behaviour of coating materials under different strain gradients simulating sliding contact and voltage gradients stimulating lithium batteries’ operation conditions. Thermal spray low carbon steel coatings were used for mass reduction in powertrains to replace iron liners. The crack initiation and propagation processes were captured and fracture mechanisms were delineated. Cracks were formed at oxide layers, whose propagation behaviour were described by a mixed mode I and mode II with stress intensity factors KI and KII used to determine the crack propagation direction. According to the experimental results, new guidelines for the coating design consisting of distribution, angle and length of oxide stringers were proposed for optimization of the coatings’ fracture resistance. Dry wear tests were performed on the coatings to simulate the oil starvation condition in combustion engines. The tribolayer formed acted as an energy-absorbing entity and reduced the driving force necessary for FeO stringer separation in the low carbon coating. Wear maps for these Ti-MoS2 and B4C coatings along with the DLC were constructed as a function of temperature and for different environments, and these maps provided engineering guidelines for optimization of the working conditions of these coatings. MoS2 coatings exhibited low coefficient of friction (COF) when sliding against aluminum, however, their COF showed high sensitivity to moisture. A MoS2 transfer layer was formed on the counterface and maintained low wear rate and low COF (<0.1) throughout the temperature range from 25 to 400 °C in an oxygen free environment (dry N2). C- based coatings namely B4C were subjected to sliding tests under different environments. In high humidity atmospheres where sliding induced graphitization occurred --as observed by Raman, FTIR and XPS--the passivation of graphitized tribolayers maintained a low COF for B4C. An increase in the humidity from 25% to 85% RH resulted in reduction of COF to 0.18. The lowest COF values of 0.07 were achieved in iso-propyl alcohol as the OH- radicals passivated the surfaces. Lithiation/de-lithiation cycles induced cracks in isostatically pressed graphite electrode and reduced the capacity of lithium battery. The crack-growth rate, da/dt, depended on the stress intensity factor at the crack tip and could be expressed as da/dt = AΔKIn. A two-stage crack-growth behaviour was determined with n=51.3 in the first stage and n=9.9 in the second stage. The reduction of ΔK to ΔKeff and reduced the crack-growth rates in the second stage are caused by a rough crack face morphology and generation of thick solid electrolyte reduction products on the crack flanks resulted in premature closing of cracks, indicating the battery performance degradation could be slowed down by this phenomenon. From a practical point of view, by exploring the failure mechanisms and tribological properties of thermal spray coatings, the work presented in this dissertation has provided guidelines for designing stable and wear resistant microstructures. For C- and Mo- based coatings, operating temperatures and environment conditions were determined to help with establishing processing windows for manufacturing applications. Battery electrodes for energy applications, selections of electrolytes and electrode materials for a longer life time were suggested.

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