The sintering behavior microstructure and materials properties of tungsten carbide - alumina nanocomposites fabricated by spark plasma sintering

Abstract

In this study the densification behavior of WC-Al2O3 composites prepared via spark plasma sintering (SPS) was investigated The initial materials were fabricated using a metal-organic chemical vapor deposition process in which tungsten hexacarbonyl W(CO)6 was used as a precursor and WC-Al2O3 powders were used as the matrix in a spouted chamber The decomposition of W(CO)6 produced W species that coated the WC-Al2O3 powder and carbonized in CH4-H2 mixing gas to form tungsten carbide-alumina composite powder The tungsten carbide-alumina composite powder was sintered via SPS in the temperature range of 1200 to 1400 °C In the above temperature range secondary phases of W including WC and W2C were found to co-exist in the SPS-treated samples and WC decomposed to form W2C at 1350 °C The density hardness and electrical resistivity of the SPS-treated samples were investigated Before synthesis of tungsten carbide-alumina composite powder nanostructured tungsten carbide particles were successfully synthesized by metal–organic chemical vapor deposition (MOCVD) in a spouted bed followed by carburization in CH4-H2 atmosphere in the temperature range 700–900°C The carburization process was little bit of complex which involved the coating of carbon on the outer surface of the decomposed W(CO)6 precursor particles and then followed by carbon diffusion into the particles leading to formation of nanostructured WC via an intermediate metastable phase W2C The carbon deficient phase W2C was formed initially at lower carburization temperature and then transformed to stable WC phase by increasing the temperature and holding time The materials characteristics of tungsten carbide-alumina composite powder are summarized as follows The addition of WC improved the sintering process and mechanical properties of WC-Al2O3 matrix by hindering its grain growth Due to the refined microstructure of composites the hardness and fracture toughness value were found to be increased with the decrease of the WC-Al2O3 the grain size The WC-Al2O3 composites show maximum toughness of 6 1 MPa‧m1/2 and hardness value of 24 GPa which are higher than those of monolithic alumina Microstructure observations indicate that WC nanoparticles are dispersed within the alumina matrix which limits the grain growth of alumina matrix The fracture mode changes from intergranular in the case of monolithic Al2O3 to transgranular mode for nanocomposites to reinforce their mechanical properties Various alumina/tungsten carbide based nanocomposites have been fabricated by spark plasma sintering and their wear properties have been investigated by performing ball-on-disk type wear test at room temperature under ambient environment The results reveal that the main fracture behavior of the Al2O3/tungsten carbide composites sliding against Al2O3 balls is the plastic deformation Crack formation and grain pull-out in the wear processes are responsible for enhancing the wear rate

Date of Award	2015 Aug 11
Original language	English
Supervisor	Jow-Lay Huang (Supervisor)