Synthesis and tribological characterization of cold-sprayed Ni-based composite coatings containing Ag, MoS2 and h-BN

Abstract

The tribo components often fail to perform in a desired way due to absence of adequate lubrication in severe conditions and may cause loss of material. Even under room temperature (RT), it is desirable to maintain the proper lubrication for efficient functioning. In recent years, many efforts have been made to develop a coating material which can facilitate low friction and wear properties in diverse working conditions. The proper use of the solid lubricants can deliver utmost lubricity and, therefore, minimize the loss of materials. Solid lubricating coatings have found various industrial applications in harsh environmental conditions, such as in internal combustion engine, gas turbine and aircraft. As far as development of coating is concerned, cold spray (CS) technology has been observed to develop the coatings at relatively low temperature (avoids decomposition) as compared to conventional thermal spray technologies (plasma spray & high velocity oxy fuel). In the current investigation, synergistic response of the participating solid lubricants viz. silver (Ag), molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN) on the tribological properties of the developed coatings were studied in various working regime of loads. The tribological characteristics of different composite coatings, such as NiAl-MoS2-Ag (NB0), NiAl-MoS2-Ag-5 wt. % hBN (NB5), NiAl-MoS2-Ag-7.5 wt % hBN (NB7.5) and NiAl-MoS2-Ag-10 wt % hBN (NB10) against alumina ball were explored at various loads (6, 11, 16 and 21 N) and at a fixed speed of 0.3 m/s under room temperature (RT). The fixed concentration of Ag and MoS2 with varying weight percentage of h-BN (0, 5, 7.5 and 10 wt %) were introduced as solid lubricants for evaluating the lubricating potential of h-BN. During the tests, coefficient of friction (COF) and wear rate were observed to lessen as the load increased from 6 to 16 N. However, increased COF and wear rate were noticed for all the participating coatings at higher load (21 N). The ideal content of hBN in the deposited coatings was ascertained in order to get the utmost favourable lubricating conditions. It was observed that NB7.5 coating exhibited the enhanced tribological characteristics under the aforesaid operating conditions. Coating NB7.5 shows the best friction and wear characteristics during each testing load and reached a COF (0.19) and wear rate (2.1 × 10−5 mm3/Nm) at 16 N load. The observed wear mechanisms were analysed on the basis of the synergy shown by Ag, MoS2, and hBN as well as the optimal hBN level in the coating (NB7.5) which helped in attaining the improved tribological properties.