**3.2. Wear track morphologies of gray iron rings**

The materials transfer from the bamboo specimens to the iron surface occurred due to adhesion. In the initial stage, transferred material formed some patches on the gray iron ring surface, as shown in **Figure 4a**. As the sliding distance was increased, transferred material patches were extended along the sliding direction because of crushing action and further adhesion. When the interfacial contact reached a steady state, the adhesion transfer film was in a relatively steady state as shown in **Figure 4b**. The transferred material film did not cover the entire friction surface of the iron ring, as the material transferred to the ring surfaces could be detached. This transferring-detaching process resulted in the adhesive wear of bamboo. Features of adhesive wear were also found on worn surfaces of bamboo specimens and will be discussed in the following sections.

**Figure 3.** The wear volume of the three types of bamboo specimens versus the normal load at sliding velocity of (a) 0.42 m s−1 and (b) 0.84 m s−1.

**Figure 4.** Stereographs of wear track of the gray iron ring.
