Wear mechanism of disc-brake block material for new type of drilling rig

Transcription

1 Front. Mech. Eng. China 2008, 3(1): DOI /s RESEARCH ARTICLE Xinhua WANG, Simin WANG, Siwei ZHANG, Deguo WANG Wear mechanism of disc-brake block material for new type of drilling rig E Higher Education Press and Springer-Verlag 2008 Abstract To improve friction and wear performance and service life of the disc-brake pair material of a drilling rig, a new type of asbestos-free frictional material with better performance for disc-brake blocks is developed, and its wear mechanism is investigated by friction and wear experiments. Topography and elementary components of the brake block s wear surface are analyzed by employing SEM and EDAX patterns, revealing its tribological behaviour and wear mechanism. When the frictional temperature is lower, the surface film of the brake block is thinner, dense, smooth with plasticity, and divided into the mixture area, Feabundant area, carbon-abundant area and spalling area. The mixture area consists of various constituents of frictional pairs without ploughing and rolling trace. The Fe-abundant area mainly consists of iron and other constituents. The carbon-abundant area is the zone where graphite and organic fibre are comparatively gathered, while the spalling area is the zone where the surface film is spalled and its surface is rough and uneven, with a loose and denuded state. During the period of high frictional temperature, the frictional surface is also divided into the mixture area, Feabundant area and spalling area. In this case, the mixture area consists of abrasive dust from friction pairs, and the surface film is distributed with crumby hard granules, exiguous oxide, carbide granules and sheared slender fibre. The Fe-abundant area is mostly an oxide layer of iron with a flaky distribution. Fracture and spalling traces as well as an overlapping structure of multilayer surface films can be easily found on the Received May 16, 2007; accepted August 1, 2007 Xinhua WANG (*), Simin WANG College of Mechanical Engineering & Applied Electronics Technology, Beijing University of Technology, Beijing , China wangxinhua@bjut.edu.cn Siwei ZHANG, Deguo WANG School of Mechanical & Electrical Engineering, China University of Petroleum, Beijing , China surface film. The components of the spalling area are basically the same as that of the matrix. At the beginning of wear, the hard peaks from the friction surface of the disc-brake plough on the surface of the brake block. With increasing frictional temperature, the friction surface begins to soften and expand, and oxidized wear occurs at the same time. During the high-temperature wear period, severely influenced by friction heat, obvious softening and plastic flow can be found on the friction surface of the brake block, its anti-shearing ability is weakened, and adhesive wear is intensified. Thermal decomposition of cohesive material in the brake block is simultaneously strengthened, so that constituents shed due to loss of adhesion. Organic fibre is in a flowing state and obviously generates drawing, shearing, carbonization and oxidization. In addition, thermal cracking, thermal oxidization, carbonization and cyclization of organic substances on the surface of brake block can make the friction surface produce pores or cracks, thus fatigue wear occurs. Keywords disc brake, brake block, friction material, wear mechanism, surface film 1 Introduction The disc-brake system of the drilling rig is a frequent braking system of heavy-duty use, and the frictional work generated in the process of braking is transformed into heat in a short time. This makes the surface temperature rise, and its flash temperature can even reach 1280uC [1 2]. The performance quality of the material of the frictional pair directly influences the reliability of the discbrake equipment and the safety and benefit of the drilling well, especially in deep wells and ultra-deep wells. Specific pressure of the brake and temperature are both higher and the energy load is much larger; more requests on wear resistance and heat fading-resistance performance are thus put forward [3 4]. Hence, the development of the brake disc and brake block material with excellent friction and

2 Wear mechanism of disc-brake block material 11 wear performance and definite anti-thermal fatigue and anti-heat fade performance is the key technique in achieving excellent brake performance, safety and reliability. A new type of disc-brake block material for drilling rigs is developed composed of a friction pair with the new material, and its friction as well as wear performances are tested at variable temperatures. The topography, elementary components and phase structure of the wear surface of the brake block are analyzed by employing SEM, EDAX and XRD analytical apparatus when thermal equilibrium temperature is 150uC and 300uC, respectively; its tribological behavior and wear mechanism are revealed, which provides technical support and theoretical instructions for extending applications of the new brake block material. 2 Experimental material and method The brake block material is a new type of asbestos-free frictional material, and its basic constituents include an organic binder, refractory inorganic adhesives, asbestosfree organic and inorganic fibre, aramid fibre, resin, steel fibre, mineral fibre, graphite, stuffing, and friction material modifier. The characteristics of the new material are as follows: 1) Being non-asbestos, environment pollution is decreased, and the health of drilling workers are protected. 2) The friction coefficient is stable, and its value is within the range of 500uC with low susceptivity to speed and pressure. 3) Adoption of high-carbon constituents decreases matching pair wear and metallic transfer. 4) Adoption of material with many gas cavities decreases brake noise. 5) Adoption of multiple asbestos-free compositions ensures adequate strength and decreases the generation of heat fatigue crack in the reiterative high-temperature brake. The brake disc adopts a surface overlaying technology, and its material has Si, Mo, W, V, and Nb to strengthen and improve build-up welding metallic hardness, antiscuffing and oxidation resistance. The matrix material of the brake disc uses steel 35. The microstructure after surface overlaying is austenite and carbide, an intermetallic compound. The age-hardening effect of the surfacing welding metal is obvious, and its hardness can reach HRC The variable temperature friction and wear characteristics experiment are carried out by using the disc-ring sample mode on the type of MMW-1 multifunctional friction and wear tester, and by friction and wear performance indexes of the brake pairs at different thermal equilibrium temperatures by adopting various combinations of load and sliding speed. The range of the test load is N, and the range of sliding speed is m/s. The topography, elementary components and phase structure after wear are analyzed by employing SEM, EDAX and XRD, respectively. 3 SEM and EDAX analysis of brake block friction surface Wear surface and elementary components of the brake block are analyzed by employing SEM and EDAX when the thermal equilibrium temperatures are at 150uC and 300uC respectively to reveal the wear and failure mechanism. The X-ray energy-dispersive graph of the original surface of the brake block is shown in Fig. 1. The surface element constituents before wear include C, O, Fe, Si, Na, Mg, Al, K, Ca, and Ti, of which the main constituents are C, O, and Fe coming from graphite, phenolic resin, aramid fibre and steel fibre, and other elements coming from mineral fibre, inorganic bond, friction material modifier, and stuffing. 3.1 SEM analysis of wear surface of the brake block at the thermal equilibrium temperature 150uC The SEM topography of the wear surface of the brake block at 150uC is shown in Fig. 2. The characteristics of the wear surface are as follows: 1) Obvious surface film is on the wear surface of the brake block, which is thinner, in a plastic state, and adheres to the friction surface under normal load and friction force. Surface film is dense and smooth with different patterns, some of which are black-and-white in a uniform and continuous distribution without rolling and ploughing trace, and others are grayish-white or grayish-black in a flaky distribution with exiguous scratches. 2) In the spalling area, the surface film does not exist any more but its fracture trace remains. Honeycomb cavity and the header of organic fibre brought about by obvious material shedding appears on the (naked) friction surface where many organic and inorganic particles are scattered. In addition, spalling pits appear on the matrix because of the spalling of clumps of particles. 3) In the spalling area, the spalling trace of the surface film can be found on the friction surface because of the fatigue and meanwhile the fatigue crack extrends from the friction surface to the subsurface due to the friction heat and surface stress. Moreover, re-daubing and plastic flow of the surface film appear during its formation, which results in generating overlapping multilayer surface film.

3 12 Xinhua WANG, et al. Fig. 1 X-ray energy-dispersive graph of the original surface of the brake block 4) Obvious fracture crack can be found on the surface film because of the matrix crack. 5) In the spalling area of the surface film, thermal cracking and thermal oxidization occur on the matrix material influenced by friction heat, which is shown by the matrix shape of the friction surface and analysis of EDAX. EDAX analysis of the friction surface of the brake block at thermal equilibrium temperature 150uC showed the surface divided into four areas: mixture area, Feabundant area, carbon-abundant area, and spalling area. 1) The surface topography of the brake block in mixture area is smooth without ploughing and rolling trace, with a uniform and continuous distribution in black-and-white. Main constituents of the surface film are C, O, F, Nb, Fe, Cr, Al, Si, Ti, Na, Mg, K, Bb, and Ca, which consist of various constituents of the friction pair. The elements C and O come from graphite, aramid fibre and organic cohesive material phenolic resins; elements Fe and Si come from steel fibre, inorganic binder SiO 2, SiC of the brake block as well as friction surface of the brake disc; elements Nb and Cr come from the friction surface of the brake disc; elements Al and Ti come from inorganic binder Al 2 O 3 and TiO 2 of the brake block; and elements Na, Mg, K, Bb, and Ca come from brake block mineral fibre and stuffing. These constituents of the surface film are in flaky, block, strip and granular distribution. Mixture of abrasive dust adheres to the friction surface and smears under the effect of friction heat, forms a surface film and is distributed in bilateral areas in the direction of sliding. Generated in the friction process with surface adhesive energy and cohesive energy, grindings move to the sides of the brake block and discharge and mix sufficiently during the moving process. In addition, because roughness of the friction surface is higher and hardness is relatively lower, grindings can easily be generated, gather and firmly adhere to the friction surface, then smeared onto the smoothing surface film in the succeeding sliding process. 2) Fe-abundant area mainly consists of ferrum and other constituents. Surface film is smooth and grayishwhite, with flaky distribution in the major center area in the direction of gliding. Friction temperature of this area is relatively higher, causing the materials of the brake block to begin to soften, which makes the ferrum on the friction surface adhere to steel fibre in the brake block, and make the ferrum of the brake disc shift towards the friction surface of the brake block. Meanwhile, ferrum with higher hardness and its oxide abrasive dust can be embedded into the friction surface of the brake block with lower hardness under frictional interface stress. Exiguous abrasive dust of ferrum oxides can also be deoxidized to elementary ferrum by reducing atmosphere under high friction temperature. Here the activity of nascent ferrum is higher and is prone to sinter on the friction surface of the brake block [5]. Due to binding, embedding and sintering, a Fe-abundant surface film emerges on the friction surface of the brake block during the friction process. 3) Carbon-abundant area, whose major constituent is carbon, is the zone where graphite and aramid fibre are comparatively gathered. Its surface film is smooth and grayish-black with a breadcrust phenomenon. The formation is attributed to the predisposition of the heading of organic fibre on the friction surface with higher activity to absorb graphite and other carbide granules. Meanwhile, it enhances the possibility that

4 Wear mechanism of disc-brake block material 13 Fig. 2 SEM topography of the wear surface of the brake block at the thermal equilibrium temperature 150uC oxygen diffuses into its interior through fibre, which makes oxidization expand into its depth., Exiguous abrasive grindings form after oxidization film cracking and spalling have a bigger surface energy and incessantly deposit on its surface, which produces a surface film with carbon-abundant structure. Surface film of the carbon-abundant area is smooth with a lubricating effect, which is favorable to improve wear resistance of the brake pair. 4) The components of the spalling area are basically the same as those of original surface of the brake block. Spalling area is also the regeneration zone where the surface film is spalled, and its surface is rough and uneven in a loose and denuded state.

5 14 Xinhua WANG, et al. 3.2 SEM analysis of wear surface of the brake block at thermal equilibrium temperature 300uC The SEM topography graph of the wear surface of the brake block at thermal equilibrium temperature 300uC is shown in Fig. 3. Similarly, the surface film uniformly and continuously distributes on the friction surface at high temperature. The frictional surface is divided into the mixture area, Fe-abundant area, and spalling area by EDAX analysis. 1) The surface film of the mixture area consists of abrasive dust of the friction pair, and it is distributed with hard crumb-like granules, exiguous oxide and carbide granule and sheared slender fibre. The surface topography appears significantly loose, the interface between granules is distinct, and bond strength with the matrix is loose, which could possibly lead to spalling at any moment. 2) Surface film of the Fe-abundant area is mostly an oxide layer of ferrum with a grayish-white and flaky distribution. Fracture and spalling traces and an overlapping structure of multilayer surface films can be easily found on the surface film. 3) The spalling area, of which the components are basically consistent with the matrix components, is the zone where the surface film is variously spalled and appears. Some spalling surfaces are distributed with many spalling pits of hard granules falling away caused by loosening due to fatigue, unspalling hard granules and mineral fibre. Others are in a loose state and distributed, with many incompact granular grindings and cracks due also to fatigue. Moreover, because organic fibre is drawn, sheared and spalled, traces of heading, drawing and shearing can be found on some spalling surfaces. In addition, more severe oxidization, carbonization and thermal decomposition occur on the wear surface of the brake block at a high temperature. 4 Wear mechanism analysis of the wear surface of the brake block According to the above analysis, the wear process of the brake block is as follows: 1) At the beginning of wear, the surface friction temperature of the brake pair is low; surface wear between the brake block and the brake disc is mainly abrasive wear, while the hard granules of the friction surface of the brake block plough on the surface of the brake disc. Hard peaks from the friction surface of the disc-brake also plough on the soft friction surface of the brake block. Collision also occurs between the two hard joints in the friction process. Because the combined strength between the hard granule and the matrix of the brake block is lower than the sheared strength of hard peaks of the brake disc, it may become the third body due to a loose connection and shedding under many collisions, which aggravates the wear of the brake pair accordingly, especially the wear of the brake block. At this stage, no surface film is on the friction surface because of the lower temperature. 2) With the rise of frictional temperature, friction surface of the brake block begins to soften and expand; abrasive wear is intensified; abrasive grindings admix sufficiently and aggregate in the process of grindings transferring to the sides of the brake block and excavating. There is incessant adhering and daubing to the brake block surface and the uniform and continuous surface film is formed under normal pressure. Moreover, because temperature is relatively higher at the central area of the friction surface of the brake block, oxidation is prone to take place at the place where steel fibre congregates, and thus the oxidizing layer is developed. While the activity of headings of organic fibre is relatively higher at the congregating place of organic fibre, it is prone to absorb graphite and carbide granules, leading to the development of the surface film with carbonabundant structure. At the same time, because of the alternate stress of the friction surface of the brake block and loss of adhesion due to thermal decomposition of the cohesive material, the matrix surface of the brake block is prone to crack, which brings about fracture of the surface film, produces more granules, and intensifies abrasive wear. At the spalling area, loss of adhesion causes constituents on the friction surface of the brake block to shed, producing granules and spalling pits on the wear surface. Surface film shedding of the brake block is mainly fatigue spalling at this stage. 3) Wear during the high-temperature period, which is strongly influenced by friction heat, obvious softening and plastic flow can be found on the friction surface of the brake block, its anti-shearing ability is weakened, and adhesive wear is intensified. At the same time, most of the organisms of the friction surface pyrolyze at frictional high temperature and mix with abrasive dusts. This forms the hybrid with bigger surface cohesive energy that adheres not only to the brake-disc surface, but also to the brake-block surface, which forms the surface film. Exiguous Fe-oxide granules can be deoxidized into nascent ferrum with higher activity at high friction temperature, which is prone to adhere to the steel fibreabundant place of the brake block surface, forming the zone of Fe-oxidation [5]. At high temperature, thermal decomposition of cohesive material in the brake block is strengthened; shedding of constituents due to the loss of adhesion intensifies; organic fibre is in a semi-fluid state; and obvious drawing, shearing and carbonization and oxidization of organic substance can be found. Surface film spalling of the brake block is mainly adhesive tear at this stage.

6 Wear mechanism of disc-brake block material 15 Fig. 3 SEM topography of the wear surface of the brake block at the thermal equilibrium temperature 30uC In addition, instantaneous high temperature results in thermal decomposition of organisms on the subsurface of the brake block in the friction process, and escaping gaseous molecules generate high pressure under instantaneous compression and spraying over the friction surface, leading to the appearance of pores or cracks [6 7]. Bigger thermal stress within the material interior is produced due to temperature gradient of the friction surface, which makes minute cracks appear. With the expansion of cracks, the friction surface of the brake block splits and accelerates wear of the brake block accordingly. Wear mechanism of the brake block is shown as a process of generating, spalling and regenerating of the

7 16 Xinhua WANG, et al. surface film, which is the result of abrasive wear, adhesive wear, oxidized wear and fatigue wear. 959 Developing and Programming Project for Science and Technology of China National Petroleum Corporation ( ). 5 Conclusions The friction and wear surface of the brake block is analyzed by employing SEM, EDAX and XRD patterns, revealing its wear forms and failure mechanism. Results show that wearing of the brake block is a dynamic process of generation, spalling and regeneration of the surface film. When the friction temperature is low, the wear mechanism and spalling of the surface film are abrasive wear and fatigue spalling, respectively. When the friction temperature is high, the wear mechanism and spalling of surface film are adhesive wear and adhesive tear, respectively. Moreover, thermal cracking, thermal oxidization, carbonization and cyclization of organic substances can increase wear of the brake block and leave pores or cracks on the friction surface during friction, leading to cracks on the surface of the brake block and eventually fatigue wear. Acknowledgements The research was supported by the Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality, and the References 1. Yi Maozhong, Han Zhihai, Chen Hua, et al. The study on the brake friction characteristic of plasma sprayed fe-ni-co-wc coating. Tribology, 1996, 16(2): Wang Xinhua, Zhang Siwei, Fan Qiyun. Experimental study in screening and matching of disc brake pair materials of drilling rig. Tribology, 2002, 22(3): Zhang Siwei, Wang Xinhua, Fan Qiyun, et al. Investigation of the tribological performance of friction pair for disc brake of drilling rig. Science in China (Series A), 2001, 44(Supplement): Fan Qiyun, Zhang Siwei, Li Weiming. Thermal design, a new concept of the disc brake design. Oil Field Equipment, 1995, 24(3): Jia Xian, Zhou Benlian, Chen Yongtan, et al. Study on worn surface layers of the friction pair vonsisting of semimetallic friction materials and grey cast iron. Tribology, 1995, 15(2): Wang Bozhan, Yao Anyou. Research on meerschaum brakefriction material and analysis of its friction and wear mechanism. In: Proceedings of the 5 th National Symposium on Tribology, Part I, 1992, Zhang Mingzhe, Liu Yongbing, Yang Xiaohong. The progress in the tribologial investigation of automotive friction materials. Tribology, 1999, 19(4):