STUDIES ON MECHANICAL, METALLURGICAL AND TRIBOLOGICAL CHARACTERISTICS OF RE MODIFIED THERMAL SPRAYED COATINGS

Abstract

The increasingly extreme conditions in which components are required to work have led to the use of thermal spray coatings in a lot of industrial applications. Thermal spraying is a process used to coat thin films of metallic or non metallic materials on to substrates. The study of the thermal sprayed coatings are useful, because they are used to restore desired dimensions; to improve resistance to wear, corrosion, oxidation, or a combination of these; and to provide specific electrical or thermal properties. In this dissertation, the microstructure, hardness and wear behavior of commercially available unmodified and RE and WC modified NiCrBSi powder (1004EN) coatings on mild steel substrate by flame spraying process have been presented. The effect of tungsten carbide (WC) particle (10 & 20wt.%) and rare earth oxides La203(1.2 wt.%)) & Ce02(0.8 wt.%)) addition to NiCrBSi on microstructure, hardness and abrasive wear behavior has been investigated. Further, the effect of re-melting of the unmodified and modified coatings by tungsten inert gas (TIG) process was also studied. All modified and unmodified Ni base alloy coatings were deposited using oxy-acetylene flame spraying process. The remelting of the all coating was done using TIG welding torch under controlled conditions. The microhardness test was carried out to get the hardness value. Wear behavior of the coatings was evaluated using pin-on-flat wear system against 120 & 600 abrasive grade SiC abrasive medium at different loads. SEM analysis of worn out samples was carried out to understand wear mechanism. Microstructure study of the coatings was done using optical microscopy and scanning electron microscopy. EDAX analysis of the micro-constituents of coating was done to study the distribution and composition of different elements in the coating. The XRD of the unmodified and modified coatings was also carried out to identify the various phases present in the coating. The results indicate that the addition of WC and RE oxides to NiCrBSi powder coating increased abrasive wear resistance and microhardness. Re-melting of the unmodified and modified powder coatings further enhanced microhardness and reduced wear rate. SEM study showed that wear of coatings resulted in formation of microgrooves, crater formation, side wise flow of material and plowing.