Date of Award
Doctor of Philosophy
Materials Science and Engineering
Hard, wear-resistant materials have potential utility as both coatings and bulk components in wear-prone applications. First-principle calculations performed by other investigators indicated that TaB2 and WB2 could be in the class of ultra-hard materials (hardness > 45 GPa), while MoB2 could be slightly less hard, but still in the "super-hard" range (hardness >30 GPa). However, relatively little empirical work has been done to confirm or refute such ultra-hard/super-hard expectations for the materials, and they have seen essentially no engineering use.
In this study, research was performed to determine whether:
MoB2, NbB2, TaB2, and WB2 have high hardness.
These borides have potential to be used as wear- and corrosion-resistant materials.
Their engineering properties could be improved by mixing two of the diborides together.
The study determined that only the mixed-phase diborides with no WB2 content showed an increase in hardness vis-à-vis the four pure compounds. However, all of the mixed-phase samples except the WB2-NbB2 specimens showed improved wear-resistance.
Ball-on-flat testing was conducted on the four pure diborides using a base 0W30 oil and also using 0W30 base oil containing 1% zinc dialkyldithiophosphate (ZDDP) additive, a wear-reducing agent that is sometimes added to lubricating oils. In these tests MoB2 showed the lowest steady-state coefficient of friction and the least wear of the steel counter-face. The ZDDP additive was found to actually degrade performance of the MoB2 sample, while having no effect on the other diborides.
Corrosion testing showed that none of the 28 samples reacted with deionized water or a 20% sodium hydroxide solution. All the samples showed reaction in the 5% sodium chloride aqueous solution, forming an adherent surface film that was not removed during a subsequent cleaning process. All three samples with 80% MoB2 content were severely attacked by a 50% nitric acid solution, and the pure MoB2 sample dissolved completely in nitric acid.
Ultimately, it was concluded that the sample with the best properties for engineering use was the Nb0.8Ta0.2B2 mixed phase diboride.
Austin Harold Shaw
Shaw, Austin Harold, "Physical properties of various conductive diborides and their binaries" (2015). Graduate Theses and Dissertations. 14496.