Ti(CN) BASED CERMET SYSTEMS FOR THE IMPROVED WEAR AND MACHINING PERFORMANCE

Abstract

Ti(CN)-based cermets are reported to possess a unique combination of properties like low density, high hardness and reasonable toughness, ability to undergo plastic deformation, superior wear and corrosion resistance, high strength, high thermal and electrical conductivity. They are proved to be more promising for high speed machining purposes. They possess light weight, superior mechanical properties, resistant to wear, provide improved surface finishing and offer superior geometrical work-piece accuracy than conventionally used tools. The composition of Ti(CN)-based cermets significantly influences properties and the performance. Addition of WC enhances densification during sintering. WC gets readily wetted by both molten nickel and cobalt, thus improving sinterability. Nickel is the basic binder having the highest wettability effect with hard phase. The grain size of the hard phase becomes finer with addition of cobalt. The cobalt addition also improves machinability and decreases solubility of TiN in metallic melt providing the stability of carbonitrides. The literature indicates that TiCN-Ni with WC is the preferred composition for superior wear resistance. Further improvement in the performance of TiCN-based cermets in wear or machining conditions can be achieved with design of new composition. Addition of TaC cermet is expected to improve the high temperature wear resistance because of its thermal shock resistance and the deformable characteristics. The major aim of the present study is to develop TiCN-Ni/Co-WC-TaC cermets for the improvement of performance in wear and machining conditions. The work presented in the present thesis can be broadly divided into three parts: (i) preparing newly designed Ti(CN)-based cermets and their characterization; (ii) understanding tribological behavior of the sintered cermets in sliding wear conditions and; (iii) estimating performance of the selected cermets in dry machining conditions. The major mechanisms of material removal in wear and machining conditions are particularly elucidated. Ti(CN) based cermets with compositions Ti(CN)-5WC-20Ni, Ti(CN)-5WC-20Ni-5TaC, Ti(CN)-5WC-10Ni-10Co, and Ti(CN)-5WC-10Ni-10Co-5TaC were processed via conventional sintering and spark plasma sintering (SPS) techniques. The microstructure of the sintered cermets was characterized in terms of carbides (core + rim) size, ceramic contiguity and mean free path of the binder. Hardness and indentation toughness were ii estimated for the sintered cermets. High densities are obtained for cermets prepared via SPS than conventional sintering than conventional sintering. Addition of TaC in Ti(CN)-WC-Ni/Co cermets resulted in high hardness and indentation toughness. Refined size and least fraction of adjacent ceramic phase are attributed for improved properties of spark plasma sintered Ti(CN)-5WC-10Ni-10Co-5TaC. To understand the friction and wear behavior of sintered Ti(CN) based cermets, unlubricated sliding wear behavior at different loads was studied. Three different commercially available counterbody balls (steel, cemented carbide and silicon carbide) were selected. The dominant mechanisms of material removal on the worn cermet discs as well as counter bodies in the selected sliding conditions were elucidated as function of cermet composition and sintering technique. It was found that among the investigated cermets, the Ti(CN)-5WC-10Ni-10Co-5TaC cermet exhibited stabilized friction and reduced wear due to formation of a strongly adherent tribochemical layer and is found to be more promising for superior performance in sliding wear conditions against any counterbody and load. Debris particles were collected after wear tests and their shape and size also studied to obtain a better understanding of friction and wear behavior of the investigated cermets. Continuing, performance of the sintered Ti(CN) based cermets was studied in machining conditions against 304 stainless steel rod and compared with commercially available cemented carbide tool. Turning operations were performed at different cutting speeds and time intervals for a given feed rate, depth of cut and cutting force was recorded. Dominant crater wear mechanisms were studied on the Ti(CN) based cermets tools and carbide tip tool. Increased intensity of crack, grain pull-out and fracture is observed in conventional sintered Ti(CN)-5WC-20Ni cermet tool, whereas increased resistance against crack or fracture is observed in conventionally sintered and Spark plasma sintered Ti(CN)-5WC-10Ni-10Co-5TaC cermet tools. Further damage during turning is restricted in spark plasma sintered Ti(CN)-5WC-10Ni-10Co-5TaC cermet tool due to formation of adhered layer beneath the tool face. Summarizing, the present study essentially demonstrates the potential of newly designed cermet composition of Ti(CN)-WC-Ni-Co-TaC for superior performance in wear and machining conditions. Particularly, the design of new composition and degradation mechanisms in sliding wear and turning conditions are highlighted. In other words, an attempt is made to understand the relation of sintering conditions, composition, iii microstructure, mechanical properties, wear behavior and tool performance of newly designed TaC added Ti(CN)-WC-Ni/Co cermets.