CoMoCrSi coatings prepared by high-velocity oxygen fuel spraying: microstructure and mechanical properties at elevated temperatures up to 800 °C

The microstructures, mechanical properties, and tribological behaviors from room temperature (RT) to 800 degrees C of HOVF-sprayed CoMoCrSi coatings were investigated in detail. The as-sprayed CoMoCrSi coatings were found to be predominantly composed of intermetallic Laves phases, i.e., Co7Mo6, Co3Mo2Si, Cr3Si, and some amorphous phases. The as-sprayed coatings possessed a compact and typical lamellar microstructure and balanced mechanical property; their Vickers hardness decreased from 855.9 +/- 16 HV5.0 at RT to 583.9 +/- 10 HV5.0 at 800 degrees C due to a normal soft phenomenon of material in hot environment. Further, between room temperature and 400 degrees C, the as-sprayed coatings suffered serious mechanical wear without any lubricant tribolayer forming on the worn surface, indicating that they would not function as good anti-wear materials at low temperatures. In particular, the coatings exhibited a brittle fracture coupled with abrasive wear at RT, obvious abrasive wear at 200 degrees C, and severe adhesive wear at 400 degrees C that with the highest friction coefficient of 0.65 and wear rate of 35.79 x 10(-6) mm(3)/(N.m). As the test temperature increased to 600 and 800 degrees C, the friction coefficient of the coating decreased to 0.45 and 0.26, respectively, and the corresponding wear rates reached 0.135 x 10(-6) mm(3)/(N.m) and 0.288 x 10(-6) mm(3)/(N.m), with a difference of approximately two orders of magnitude between the low- and high-temperature wear rate. This result further confirmed that the as-sprayed coatings are a better choice of abrasion-resistant materials for high-temperature applications. After sliding tests at 800 degrees C, numerous metallic oxides i.e., Co3O4, MoO3, and bimetallic oxides such as CoMoO4 and Co2CrO4 of nanometer size (50-100 nm) were identified in the continuous protective layer formed on the worn surface. These oxides played an important role of lubrication and reduced direct contact between the coating and its counterpart during the sliding process, leading to a decrease in the friction coefficient and material loss. The main wear mechanisms of the coatings at this temperature range are slight adhesive wear coupled with abrasive wear.