The article presents a discussion on the use of hexagonal boron nitride as an additive to lubricants. Based on the analysis of the research, factors determining its application were identified. These include particle size distribution, their morphology, specific surface area, and porosity. Next, the research identifying these properties for the four types of h-BN was described. Based on the results, the possible mechanisms of the influence of individual h-BN types were described. It was also found that the use of h-BN nanoparticles as lubricants seems to be promising.

This study investigates the influence of hexagonal boron nitride (h-BN) particle size and concentration on the tribological performance of lithium and calcium greases. Formulations containing h-BN nanoparticles and microparticles at 1%, 3%, 5%, and 10% by weight were evaluated in ball-on-flat reciprocating tests under three load conditions. The tests were conducted using a steel ball and a steel plate. The most favorable results were obtained for greases with 3% h-BN, characterized by an average particle size of 130 nm and the highest nanoparticle content. In lithium grease, this formulation reduced friction by up to 9.7% and wear by up to 69.2% compared to the base grease. In calcium grease, the same additive concentration led to reductions of up to 18.2% in friction and 70.2% in wear. Tribological performance was significantly influenced by the type of base grease, which affected the dispersion of the additive and its ability to form protective surface layers. SEM/EDS analysis of the surfaces after testing revealed that the dominant lubrication mechanisms included shearing-sliding and surface-mending effects. This study confirms that h-BN—especially in nanoparticle form—is an effective additive for improving the performance of greases.

Hexagonal boron nitride is being considered as an additive for greases due to its structure and physical and chemical properties. In the context of the application of such lubricants in real tribological systems, it is important to recognise the effect of hexagonal boron nitride not only on tribological properties, but also on other functional properties of this group of lubricants. In the present study, tests including dropping point, penetration and mechanical stability were carried out. Additionally, particular focus was placed on the properties of the additive itself, including particle size distribution and adsorption properties, as determined by scanning electron microscopy and low-temperature adsorption isotherms. The introduction of hexagonal boron nitride particles into lithium and calcium greases resulted in enhanced resistance to high temperature and prolonged mechanical stress. This phenomenon was attributed to the type of base grease and the modifications in the configuration of the grease's spatial network that ensued as a result of the incorporation of solid particles. It was found that an additive with a smaller particle size and a significant proportion of nanoparticle fractions, and a more developed porous structure, was more effective. Microscopic observations of the structure of the greases confirmed that the solid particles were deposited in the spatial network of the greases. The distribution of hexagonal boron nitride in the grease structure was found to be contingent upon the physical and chemical properties of the additive. Furthermore, the type of base grease, including the arrangement of the soap fibre network, was identified as a contributing factor. Graphical Abstract

The paper presents the test results of lubricity properties of Hydrol L-HL 32 hydraulic oil, Mobilube 1 SHC 75w-90 gear oil, and two base oils: SN-150 and SN-650, containing hexagonal boron nitride. To conduct the required tests, a four ball machine was used. This device is commonly used when assessing lubricity properties of oils and greases. The following normative lubricity parameters were defined: the value of welding point Pz [daN], the value of last non-seizing load Pn [daN], the value of seizing at smoothy growing load Pt [daN], as well as the wear limiting capacity load Goz [daN/mm2]. Furthermore, several non-normative tests were completed that covered indication of two parameters making it possible to complete an assessment of lubricity properties of oils. Based on the tests, it was determined that hexagonal boron nitride is a good additive that improves lubricity properties. Improvement in the above properties was recorded both in the case of hydraulic oil and gear oil, as well as oil bases. Relatively better improvement results were obtained in the case of oil bases, where the improvement in the results after using boron nitride, was more visible than in the case of hydraulic and gear oil. The best improvement results for hydraulic oil were obtained after adding 4% h-BN, whereas in the case of the SN-150 oil base, this value was 2% h-BN. The best improvement results for gear oil and SN-650 oil base were also obtained after adding 4% h-BN. The possibility of using the values of h-BN bigger than 4% was excluded. It was determined that bigger content of h-BN in oil generates negative effects (under four ball conditions). Keywords: tribology, lubricity, boron nitride.

The paper presents the tests results of lubricity properties of base greases, containing hexagonal boron nitride (h-BN). To conduct the required tests, a four ball machine was used. This device is commonly used when assessing lubricity properties of oils and greases. The following normative lubricity parameters were defined: welding point Pz [daN], the value of last non-seizing load Pn [daN], seizing at smoothy growing load Pt [daN], as well as the wear limiting capacity load Goz [daN/mm2]. Furthermore, several non-normative tests were completed that covered indication of two parameters making it possible to complete an assessment of lubricity properties of oils. Based on the conducted research, it was found that hexagonal boron nitride is an additive that improves the lubricating properties of the base grease. The best effects of the improvement, taking into account the adopted tribological criteria, were noted for a sample containing 10% (m/m) hexagonal boron nitride with an average grain diameter of 65÷75 nm. It was found that the lower granulation of the additive (h-BN) positively influences the efficiency of the grease lubrication of the base, under the conditions of tests on a four-ball model apparatus. Keywords: tribology, lubricity, boron nitride, greases