When tribological contacts operate under boundary lubrication, lubricant films are not thick enough to fully separate the two surfaces. In these cases, surface to surface contacts occur, while asperity interactions and surface adhesion dominate the friction. In this thesis, a boundary lubrication friction model will be described with these two mechanisms considered. Both numerical analysis and experimental works were employed to build and validate the model. To characterize the tribofilms formed during boundary lubrication, the thickness and chemical composition of the tribofilms were firstly quantified by AFM, FIB-SAM and XPS. Afterwards, nano-indentations were performed on the tribofilms to determine their mechanical properties. A numerical model based on the semi-analytical method was developed to calculate the tangential forces caused by asperity interactions. Elasto-plastic deformations of both tribofilm and substrate were considered. The effects of tribofilm thickness, interference depth and mechanical properties of tribofilm on friction were investigated. The sensitivity of SAM calculating real contact areas is dealt with in this thesis. The real contact areas of a sinusoidal wavy surface on a flat were calculated by both SAM and an analytical method. The results were compared and an extrapolation was proposed to acquire a better estimation of the real contact area calculated by SAM. Finally, the effect of adhesive forces and asperity interactions on friction were combined and the total frictions were calculated. The results were validated by pinon-disc test results. The thesis consists of two parts. The first part (part A) summarizes the work in six chapters. The second part (part B) lists the journal articles in which the details are described.