Effect of particle size and sintering processes on mechanical properties of cemented carbides

Abstract

Cemented carbide has been widely used as hard materials in industrial machining as cutting tools and as moulds in metal shaping. Prior research suggests that many processing techniques have been applied to improve its manufacturing conditions, microstructural characteristics and mechanical properties, namely hardness, fracture toughness and wear resistance. Although it is considered to be one of the most stable composites in terms of industrial requirements, challenges have been faced in the areas and routes of manufacturing, and consolidation with a desired set of mechanical properties via different processing techniques. In the field of cemented carbides, the understanding of the processes such as powder refinement and consolidation or sintering, contributing to the mechanical properties is critical. The aim of this project was to examine three different groups of powder size of Tungsten Carbide-Cobalt (WC-Co), which is the dominant compositional elements among all cemented carbides, and four consolidation techniques including plasma, microwave and conventional sintering, in order to understand the relationship between these processes and mechanical properties. This study started with analysing three different particle size for both Tungsten Carbide (WC) and Cobalt (Co), which then resulted into three different types of mixed powder samples based on their size. Once the compositions were made through either high or low energy ball milling, experiments proceeded to compaction and sintering stage. The chosen composition for the study was WC – 7.5 wt. % Co, which was kept constant, while particle size, sintering process, sintering temperature, pressure and other parameters were varied. The final and overall objective was to establish a three way relationship among particle size, processing routes and mechanical properties of WC – 7.5 wt. % Co. It was found that Pulse Plasma Sintering (PPS) method is the most successful in achieving excellent physical and mechanical properties including density (fully dense), hardness (2000 HV) and fracture toughness (15.3 MPa√m), and displays significantly improved microstructural behaviour in cemented carbides sintered at lower than conventional temperature, ensuring time and energy efficiency. Spark Plasma and Microwave Sintering were found to be the most efficient and effective after PPS in terms of mechanical properties, considering the entire particle size range and the other variable parameters. This thesis first outlines the basic understanding of cemented carbides, their fields of application, types, and processes that are involved during their manufacture. It then presents an overview article which is a chapter as well that presents an understanding of what has been done specifically in the areas of processing techniques, through powder refinement and consolidation highlighting the areas where challenges are faced. The mechanical properties along with certain microstructural aspects like grain growth and phases are also elaborated as part of this paper. The second paper discusses the processing of particles before sintering processes are applied and how a homogeneous and optimized mixture can be achieved. Four more chapters come from four papers that are either published or under review focusing on the different consolidation processes and their effects on mechanical properties and microstructural behaviour of WC- 7.5 wt. % Co. The thesis finally brings together a conclusion from all the separate segments of the project, highlighting the key findings and comparisons and provides information on future work. Future work will involve further investigation on sintering mechanisms and to resolve one of the most critical question, whether the presence of plasma can be confirmed during the use of some of the modern sintering methods.