Ultrathin graphene tribofilm formation during wear of Al₂O₃–graphene composites

Abstract

Graphene nanoplatelet (GNP)-reinforced aluminum oxide (Al 2 O 3 ) composites were sintered by spark plasma sintering in three different compositions (0, 0·5, 5 vol.% GNPs). To investigate the effects of graphene addition on the composites' wear resistance, ball-on-disk wear tests were conducted under very high normal load (40 N) by using a 3-mm-dia. ceramic counterface. Aluminum oxide-0·5 vol.% GNP exhibited 65% improvement in the wear resistance, while aluminum oxide-5 vol.% GNP displayed 53% poorer wear resistance as compared with aluminum oxide. The coefficient of friction was 0·45 for aluminum oxide-0·5 vol.% GNP, 0·40 for aluminum oxide-5 vol.% GNP and 0·60 for aluminum oxide. The highest wear resistance of aluminum oxide-0·5 vol.% GNP is attributed to formation of a continuous, protective and ultrathin graphene tribofilm on the wear surface. Tribofilm formation occurs due to the high shear forces induced by countersurface movement and localized heating, which causes GNP's delamination, overlap and welding together. In the case of aluminum oxide-5 vol.% GNP, poor dispersion and agglomeration of GNP results in a thick and discontinuous graphene tribofilm, which does not protect from the brittle fracture of aluminum oxide grains during wear.

From bulk to cellular structures: A review on ceramic/graphene filler composites

“…In those first works, Hvizdos et al [48] and Belmonte et al [49] showed the potential of graphene fillers for enhancing the tribological performance of Si 3 N 4 ceramics, especially increasing the wear resistance in about 60% under dry [48] and isooctane lubricating [49] testing conditions. Later on, new graphene containing ceramic and glass composites have been explored and, at present, reports on the tribological performance of Si 3 N 4 [48][49][50][51][52], Al 2 O 3 [53][54][55][56], SiC [57], SiO 2 [58], bioglass [59] bulk composites, as well as ZrO 2 [60], Y 2 O 3 -Al 2 O 3 -SiO 2 (YAS) [61] and CaSiO 3 [62] coating composites, can be found. Table 1 summarizes some relevant information (testing conditions and results) of all these works.…”

“…Focusing on Al 2 O 3 -and Si 3 N 4 -based composites, the 15 materials more extensively investigated, some interesting results can be observed. In the case of Al 2 O 3 /graphene composites, an excellent wear performance was reported for very low amounts (0.5-1.0 vol.%) of GNP [53][54][55][56]; while for works where higher fillers contents (up to ~ 10 vol.%) were introduced the wear resistance decreased due to the formation of GNP agglomerates during the processing of the composites [56] or to their poor mechanical properties [55]. However, to achieve the best results for Si 3 N 4 /graphene composites, larger GNP contents were required (~4.4 vol.%) [48,49,52].…”

Integration of graphene in poly(lactic) acid by 3D printing to develop creep and wear‐resistant hierarchical nanocomposites

“…Moreover, a larger amount of stress was encountered during the initial 10–15 min of the sliding test characterized by the decreased COF. It is proposed that during this period, the rise in temperature in the sliding interface along with the normal stress exerted by the counter material in the composite was high enough to dissociate graphene flakes from the polymer matrix , resulting in the formation of a graphene tribofilm, acting as a protective layer to the worn surface and as lubrication. After 15 min of the duration of the test, the stress exerted was not sufficient to continue with the lubrication mechanism causing the sudden increase in COF dominated by pure PLA wear.…”

Radial Injection in Suspension High Velocity Oxy-Fuel (S-HVOF) Thermal Spray of Graphene Nanoplatelets for Tribology

“…GNPs retain the good mechanical properties of graphene and also provide the lubricating characteristics of lamellar solids [11]. GNPs have been already used in form of nanocomposites with alumina [12,13] providing good wear response [14], thanks to the formation of a protective layer incorporating graphene.Considering the use of S-HVOF thermal spray, care must be taken in the way the feedstock is injected into the flame, since at this stage GNPs degradation can occur. There are two main ways of injection in thermal spray: the axial injection, where the feedstock is injected directly into the combustion chamber, and the radial injection, where it is injected into the flame from the side, outside the gun.…”

“…GNPs retain the good mechanical properties of graphene and also provide the lubricating characteristics of lamellar solids [11]. GNPs have been already used in form of nanocomposites with alumina [12,13] providing good wear response [14], thanks to the formation of a protective layer incorporating graphene.…”