Research On Friction Materials For TBM Driving Clutch Plate

Tunnel boring machine (TBM) is the most advanced system for great tunnel grubbing, and TBM clutch friction plate is one of the most important parts for power transferring. When boring, TBM suffered heavy impact and librating loading, the main parts, such as scraper, scratch plates and friction plates, etc., are easy damaged and these accessories have to depend on imports at present. TBM clutch plates are one of the most important driving transfer parts. TBM clutch plates are worn heavily due to the complicated geologic condition. With the development of long and great tunnels in railways, highways and undergrounds, it is a key problem to develop friction materials with high properties. The purpose of this dissertation is to develop a new kind of friction composites suitable for TBM clutch plates.The work state and failure form of TBM clutch friction plates,as well as the present friction materials, were analyzed in this paper. The uniform design method of 5 factors and 27 levels was used to design and optimize the match composition (wt%) : bonder Polyether-ether-ketone (PEEK) 16.1%, 304 stainless steel fiber 4.7%, carbon fiber 12.5%, cashew nut powder 13.6%, chromite powder FeCr2O4 11.1%, barite powder BaSO4 5.1%, graphite and sulfide stibium Sb2S3 11.5%, aluminum powder 6.2%, kaolin powder and fluorite CaF2 10.7%.According to the orthogonal test, the parameters of hot-press and curing process were determined by testing density, hardness and impact strength. Specimens were manufactured by the conventional dry-processing procedure for friction linings, comprising mixing, pre-heating, hot pressing and heat treatment. The specimens were hot-pressed at 310-340°C and 25-40 MPa, holding 3-5 min/mm, then the molded specimens were post-cured at 80°C, 150°C, 270°C for 30min respectively and 310°C for 12 hours.For samples of the optimized match composition, the friction coefficient was stable between 0.394-0.476 at 100-350°C, and the heat-fading ratio was 4.8%. The wear loss was 0.31×10-7 cm3(N·m)-1 at temperature of 350°C. meaning the perfect properties of stability and recovery with higher bending strength and impact toughness. The friction and wear mechanism was analyzed through observing the worn surface morphology by scanning electron microscope (SEM), Field-emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared spectrometry (IR). The results showed that the wear mechanism is that the particle abrasion occurs at low temperature, the adherence abrasion and particle abrasion take place at the temperature between 200～350°C. The transferred layer is formed, and big debris particles of lath-shape and sheet-shape are found on the worn surface. The thermal degradation of the PEEK does not occur until its temperature rises to higher than 350°C.The process of abrasion is the repeating cycles that the friction materials undergo continuous furrowing, mechanical abrasion, adhesion, formation of transfer film, deformation, propagating and growing of microcrack, delamination, fatigue and wear. The friction film exerts a great impact on the tribological properties of the friction couple. In sliding process, plate shaped transfer films are a compositional mix of components including barite (BaSO4), sulfide stibium (Sb2S3), chromite powder(FeCr2O4), steel fibres and grey cast disc of the mate materials. The composite materials undergo a series of mechanical, thermal and chemical changes that compounds of barium, iron and sulfur formed on the contact interface. Two kinds of surface film structure are formed: one is cystiform microstructure with PEEK matrix surrounded by single or multi-crystal powder of fillers; the other is meshwork generated by resin base and friction fillings. The superficial structure of friction material after wear test is composed of three layers. The first is surface film that was worn and torn continually during abrasion process as a result of friction force and heat. The second is transition layer with a thickness of about 0.3mm and could be translated into film incessantly. The third layer is friction material keeping the initial state before abrasion test.The BP artificial neural network is adopted to develop an intelligent design system for semi-metallic friction material. For the network of property forecast, the best network architecture is decided as 5-[29]1-2. The train function is trainlm,and logsig is the functions between input and hidden layers, purelin between hidden and output layers. For the network of ingredient forecast, the best network architecture is 2-[300]1-[150]2-4. The train function is trainscg,and logsig, tansig are the functions between input and hidden layers, purelin between hidden and output layers.Test results showed that the trained network can accurately forecast the friction coefficient and wear rate, the error between forecast results and the experimental results is small and sufficient to meet the needs of practical application.The developed stainless/carbon fibers reinforced PEEK friction plates were used in Tunnel Boring Machine and their wear loss was smaller than that of asbestos friction facings. The developed friction materials exhibited stable friction coefficient and lower wear ratio and are suitable for clutch Disc either in normal or in sliding conditions.