WANG Dong, JIANG Tao, ZHAO Bo, DING Zhengwei, SHI Shurui
(Xi'an Technological University, School of Mechatronic Engineering, Xi'an 710021, Shaanxi, China)
Extended abstract:[Background and purposes] High-speed cutting is widely used in industry, because of its high efficiency and superior machining quality. Silicon nitride (Si3N4) ceramic cutting tools, owing to their high hardness, excellent corrosion and oxidation resistance, are considered ideal candidates for high-speed machining tools. Because Si3N4 ceramics have relatively low thermal conductivity, self-lubricating phases should be introduced into the ceramic tools to improve cutting wear behavior and extend tool life. Among current self-lubrication strategies, incorporation of solid lubricants is widely adopted, due to its simple and environmentally friendly fabrication process. Carbon nanotubes (CNTs), as a common solid lubricant, possess strong C–C bonds that give them excellent mechanical properties, while their tubular/lamellar structure provides favorable friction-reducing performance, so CNTs-containing composites exhibit markedly improved tribological behavior. However, introducing CNTs into tool materials would degrade the mechanical strength of the matrix. Therefore, it is necessary to achieve a synergistic optimization of mechanical and tribological properties after the addition of CNTs. In this study, Si3N4 was chosen as the matrix, with CNTs as the solid lubricant and Al2O3/Y2O3 as sintering aids. Self-lubricating Si3N4 ceramic tool materials were fabricated by using spark plasma sintering, while their mechanical properties and friction wear behavior were analyzed to develop Si3N4-based self-lubricating ceramic tools with excellent comprehensive performance.[Methods] To mitigate the agglomeration of CNTs, ultrasonic agitation was employed to achieve uniform dispersion of CNTs, while controlling the sonication time to prevent CNTs breakage during processing and thereby preserve a high aspect ratio. The components except CNTs were weighed according to the designed compositions and mixed with an appropriate amount of anhydrous ethanol, followed by sonication and stirring for 40 min to achieve preliminary powder homogenization. The weighed WC grinding balls and the slurry were placed into a milling jar and ball-milled for 24 h. After ball milling, the pre-dispersed CNTs were added to the mixture and ball-milled for an additional 30 min to further promote uniform dispersion of CNTs within the matrix powder. The slurry was then dried at vacuum at 140 ℃ for 3 h, followed by sieving through a 200-mesh screen to obtain homogeneous composite powder. The measured composite powder was loaded into a graphite die and cold-pressed with a hydraulic press to form green bodies, which were subsequently densified by using SPS. The sintered compacts were finally processed by cutting, rough grinding, fine grinding and polishing to produce self-lubricating ceramic cutting tool samples. Mechanical characterization was performed using a three-point bending tester and a Vickers hardness tester to measure flexural strength, Vickers hardness and fracture toughness. To evaluate friction and wear behavior, pin-on-disk tests were conducted with Si3N4 balls at an 8 N load and rotational speed of 100 r·min−1, for test duration of 30 min, with wear radius of 5 mm, while the wear rates were determined according to mass loss.[Results] After sintering at 1650 ℃, grains are fully developed, porosity is significantly reduced, and the surface element distribution is uniform with no obvious defects, thus giving rise to superior mechanical properties. After sintering at 1700 ℃, a large number of interconnected pores are present, together with excessive grain growth, causing a marked decline in flexural strength and fracture toughness. At 1750 ℃ and 1800 ℃, continuous pores no longer appear, but a considerable number of fine pores remain, so mechanical properties partially recover. With increasing sintering temperature, Vickers hardness and flexural strength first increase, then decrease and eventually slightly recover, while fracture toughness increases initially, then decreases and eventually levels off. The sample sintered for 7 min has flatter fracture surface than the others, which accounts for its higher flexural strength. By contrast, samples sintered for 9 min, 11 min and 13 min exhibit various degrees of grain pull‑out. Vickers hardness and fracture toughness both increase first and then decrease, while fracture toughness reaches nearly equal maxima for the 9 min and 11 min samples. Flexural strength increases, then decreases and finally recovers slightly. With the addition of CNTs, both the hardness and flexural strength decrease. The friction coefficient shows an increase followed by a decrease, with the highest value to be present for the sample with 1.0 wt.% CNTs. The 1.5 wt.% and 2.0 wt.% samples have lower friction coefficients than the sample without CNTs, with the lowest value at 2.0 wt.%. At 1.5 wt.% CNTs, the wear rate is reduced by 66.7% relative to the CNT‑free sample, demonstrating excellent wear resistance. With 2.0 wt.% CNTs, due to the reduced hardness, abundant wear debris are produced, which rapidly destroy the protective oxide films, so that they cannot adhere to the wear surface. Nevertheless, the oxide particles formed still provide effective lubrication, resulting in the lowest friction coefficient.[Conclusions] The optimal sintering conditions were determined to be 1650 ℃ for 7 min. It is found that, the sample with 1.5 wt.% CNTs exhibited optimal performance, with Vickers hardness of 13.73 GPa, fracture toughness of 8.53 MPa·m1/2, flexural strength of 529.25 MPa, friction coefficient of 0.6334 and wear rate of 1.327×10−6 g·N−1·m−1. Sintering temperature has strong influence on mechanical properties by affecting pore formation, which is the lowest in the sample sintered at 1650 ℃. Furthermore, the dwell time significantly affects fracture morphology, while the A 7 min dwell time yields a relatively smooth fracture surface, whereas 9 min, 11 min and 13 min dwell times led to grain pull‑out and hence reduction in flexural strength. With increasing content of CNTs, Vickers hardness and flexural strength are decreased, while formation of oxide films in the contact/tribological region is promoted.
Key words: silicon nitride ceramics; carbon nanotubes; self-lubrication; friction and wear