Developing fully mixed oxidation-resistant hafnium aluminum diboride hard coatings deposited by low temperature CVD

Hard protective coatings are often made from oxides, carbides, nitrides and borides. However, an all-rounder compound with ubiquitous applications usually does not exist. Alloying with other elements is often needed to improve properties such as oxidation resistance or fracture toughness. Thus, this dissertation explores the microstructure-property space of novel HfB2-AlB2 alloy coating system. Hf1-xAlxBy alloy thin films are deposited at ≤ 300°C by chemical vapor deposition from two volatile precursors, Hf(BH4)4 and AlH3N(CH3)3. This method allows for control over a wide range of Al content x, here 0.41 to 0.78, which controls the as-deposited microstructure. The as-deposited state is nanocrystalline with equiaxed grains of diameter ~ 5 nm, for 0.40 < x < 0.66. Al contents between 0.41-0.66 afford a solid solution with no resolvable segregation into regions of HfB2, AlB2, or Al. Excess Al (x > 0.66) generates an amorphous phase, and Al-rich domains are present in a film with x = 0.78. Different as-deposited microstructures exhibit different oxidation behaviors and mechanical response. After annealing at 600°C for 8 hours in air, unalloyed HfBy is consumed to form a porous oxide; by contrast, the Hf1-xAlxBy alloy forms a uniform and much thinner oxide. Upon annealing at 700°C for 1 hour in air, the grain size and phase are stable, and the alloy film does not crack or delaminate from the substrate. An AlOx layer, ~ 22-50 nm thick depending on the composition x, forms on the surface and slows the rate of further oxidation. Annealing coatings with excess aluminum (x > 0.66) at 700 °C results in the surface passivation being breached by the formation of aluminum borate, consistent with literature reports of borate formation at ≥ 700 °C. Hardness, Young’s modulus, plasticity index, and fracture toughness for films with x = 0 to 0.78 are determined using cube corner nanoindentation. A hardness of ~ 20 GPa and plasticity index of ~ 0.33 are maintained for aluminum contents x = 0 to 0.66; for larger values the hardness declines and the plasticity increases. The fracture toughness increases as a function of aluminum content, with no cracks detectable for x ≥ 0.66. For engineering applications that require a good combination of hardness, toughness, and high temperature oxidation resistance, coatings with 0.41 ≤ x ≤ 0.66 are expected to perform well. Therefore, Hf1-xAlxBy films are excellent candidates for applications that require a hard, chemically resistant coating at temperatures below 700°C.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Shrivastav, Samyukta

Contributors dc:contributor

• Abelson, John R
• Krogstad, Jessica A
• Girolami, Gregory S
• Bellon, Pascal
• Shoemaker, Daniel P

• Hafnium aluminum diboride
• Ceramics thin films
• Materials Characterization

Statement dc:rights

• Copyright 2025 Samyukta Shrivastav

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129494

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129494