Indentation Fracture Behavior of Low Carbon Steel Thermal Spray Coatings: Role of Dry Sliding-Induced Tribolayer

摘要

Mechanical properties of a low carbon steel thermal spray coating, deposited from AISI 1010 feedstock, on Al-Si cylinder bores were studied using nano- and micro-indentation tests before and after the sliding tests. The coating's microstructure consisted of -Fe splats surrounded by FeO stringers and occasional oxide aggregates. Iron oxide aggregates fractured at the lowest indentation load (10gf). Vickers indentations performed on the ductile -Fe matrix of the coating caused separation of the oxide stringers within the indentation plastic zone. The probability of chipping-type fracture by separation of oxide stringers depended on the indentation load applied. Reciprocating dry sliding tests, performed on the coating against a CrN counterface, generated a 770 +/- 261-nm-thick tribolayer (hardness=0.36 +/- 0.1GPa; elastic modulus=38.4 +/- 5.3GPa). Raman spectroscopy revealed that the tribolayer consisted of Fe3O4, FeO and Fe2O3. The increase in the Weibull modulus, m, from 0.84 for the coating to 1.51 for the tribolayer-covered surface showed that the tribolayer decreased the probability of chipping fracture. A fracture mechanics argument was put forward to show that the tribolayer reduced the driving force that was available for chipping fracture and, in this way, protected the surface from damage during sliding contact.