Mechanical properties and corrosion resistance of laser cladding iron-based coatings with two types of NbC reinforcement

In this work, iron-based alloy coating without NbC (No-NbC), directly added NbC (A-NbC) and in-situ synthesized NbC (I-NbC) reinforced composite coatings were fabricated on the SS304 substrate surface to compare the strengthening effects of two types of NbC introduction mechanisms on the iron-based alloy. All as-clad coatings presented a complete and dense structure, and compared to A-NbC composite coating, I-NbC composite coating achieved a fine dispersion distribution of the reinforcement. Unlike A-NbC coating with α-Fe and NbC, it was revealed that α-Fe, γ-Fe, NbC and Cr23C6 existed in I-NbC composite coating, and the possibility of in-situ synthesized NbC has been confirmed through thermodynamic theory. Additionally, the introduction of NbC notably refined the grain size of iron-based alloy systems and restrained the generation of high angle grain boundaries (HAGBs). Compared to substrate and No-NbC coating, A-NbC and I-NbC composite coating achieved the higher average microhardness value of 524.6 ± 12 HV and 534.9 ± 4 HV, respectively, mainly relying on the precipitation of NbC in the coating. Meanwhile, the results in the dry sliding friction test found that due to the dispersion and uniformity of in situ synthesized reinforcements, I-NbC composite coating presented a satisfactory wear resistance with the lowest specific wear rate of 6.55 × 10−6 mm3/N m and the minimum friction coefficient of 0.627. NbC particles were considered to be detrimental to the corrosion resistance of coatings in electrochemical corrosion tests due to galvanic corrosion between them and the matrix. However, I-NbC composite coating provided a favorable condition for the formation of passivation film by introducing Cr and Nb atoms, with the minimum Icorr of only 7.55 × 10−8 A cm−2, which was much less than that of other as-clad coatings.