Spark plasma sintering of alumina reinforced with tungsten carbide-cobalt systems

Alumina-based ceramic composites have attracted many research interests for several decades due to their good mechanical properties and the abundance of alumina (Al2O3) raw materials. This study was aimed at investigating the mechanical properties, sliding wear properties and microstructure of alumina tungsten carbide-cobalt (Al2O3/WC-Co). In this study, Al2O3/WC-Co admixed powders were consolidated and densified via spark plasma sintering (SPS) technique at a holding time of 5 to 10 minutes. The sintering temperatures were varied between 1600 and 1800 oC. The heating rates were varied from 75 to 150 oC/min and the cooling rates were 200 oC/min and free cooling. Al2O3 powders were reinforced with 5, 10 and 15 vol% WC-12wt%Co. The admixed powders were consolidated to relative densities ranging from 96.5 to 99.98 % of their theoretical density. XRD was utilized for phase identification of both the sintered samples and the admixed powders. Al2O3 phase was present in all the samples, while WC phase was present in the composite materials, the Co phase was not identified in both the powder and sample phase match. The XRD results revealed no new phases formed either during powder milling or sintering of the samples. The microstructures of the samples were evaluated using a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS). The microstructure of the pure Al2O3 had an average grain size of 40 μm while the average grain size of the sintered Al2O3/5vol%(WC-12wt%Co) was 36 μm. It was clear that the addition of WC-12wt%Co inhibited grain growth of the Al2O3 matrix during the sintering process.

The hardness of the samples was measured using the Vickers hardness with a diamond indenter. Hardness of the Al2O3 samples was found to be lower than that of the reinforced counterparts. The increase in the composition of the additives led to an increase in the hardness of the samples. Fracture toughness was computed using the Palmqvist crack method. It was found that increasing the amount of additives significantly increases the fracture toughness. Grain refinement was postulated to have a pronounced impact on the toughness properties of the samples. Al2O3/WC-Co systems had better wear resistance to dry sliding test as compared to pure Al2O3 ceramic. WC-12wt%Co has proven to be an ideal reinforcement material for Al2O3 with improved mechanical properties and wear properties.