The boron oxide–boric acid system: Nanoscale mechanical and wear properties

Abstract

The film that forms spontaneously when boron oxide (B 2 O 3 ) is exposed to humid air is a solid lubricant. This film is usually assumed to be boric acid (H 3 BO 3 ), the stable bulk phase. We describe the nanometer-scale surface morphology, mechanical properties, and tribological properties of these films and compare them with crystals precipitated from saturated solutions of boric acid. Scanning force microscopy (SFM) and low-load indentation were the primary experimental tools. Mechanical properties and their variation with depth are reported. In all cases, the surfaces were covered with a layer that has different mechanical properties than the underlying bulk. The films formed on boron oxide showed no evidence of crystalline structure. A thin surface layer was rapidly removed, followed by slower wear of the underlying film. The thickness of this initial layer was sensitive to sample preparation conditions, including humidity. Friction on the worn surface was lower than on the as-formed surface in all cases. In contrast, the SFM tip was unable to cause any wear to the surface film on the precipitated crystals. Indentation pop-in features were common for precipitated crystals but did not occur on the films formed on boron oxide. The surface structures were more complex than assumed in models put forth previously to explain the mechanism of lubricity in the boron oxide-boric acid-water system.

Synthesis and mechanical properties of boron suboxide thin films

“…Ten single indent measurements were averaged. According to the suggestion in other studies, 3,7 the application of shear forces may transform the microstructure of H 3 BO 3 so as to exhibit a texture with bundles of basal planes. The x in BO x was plotted as a function of the O 2 partial pressure.…”

Atomic layer deposition of B2O3 thin films at room temperature

“…Source material pulsing was achieved by inert gas valving with pure nitrogen (N99.999%, Schmidlin UHPN 3000 N 2 generator) as a carrier and purging gas. Due to the hygroscopic nature of B 2 O 3 films, they were covered in the same pump-down with Al 2 O 3 deposited from (CH 3 ) 3 Al/H 2 O [37] (98%, Strem Chemicals, USA). All thin film depositions were carried out in a commercial F-120 ALD reactor manufactured by ASM Microchemistry Ltd. (Helsinki, Finland).…”

“…Nevertheless, chemical vapour deposition (CVD) or infrared heating of elemental B and B 2 O 3 have been employed for the preparation of B 2 O 3 nanowires [1,2]. In addition, boron oxide thin films react with air to form boric acid (H 3 BO 3 ) which finds uses as solid lubricant [3] or as self-assembled boric acid nanostructures [4]. Moreover, boron oxide is a constituent of borophosphosilicate [5,6] and glassy B 2 O 3 -Li 2 O-PbO [7] thin films as well as a dopant for diamond [8,9] and Si:H [10] thin films.…”

Origin of Ultra-Low Friction of Boric Acid: Role of Vapor Adsorption

“…Using literature values for boric acid mechanical properties [28] yields a Hertzian contact diameter of nearly 40 lm. Vapor environments were created by mixing a nitrogen stream saturated with the desired vapor with a dry nitrogen stream to reach a designated vapor partial pressure.…”

“…The initial dry sample spectrum shows a broad shoulder peak at 1260 cm -1 . This peak might be due to a small amount of boron oxide (B 2 O 3 ) formed locally upon extreme drying [28,42]. However, this peak disappears immediately as soon as the coating is exposed to humid environment, indicating its transformation into boric acid [42][43][44].…”