低温低压条件下石墨/MoS_2对金刚石钻头胎体磨损性能影响的试验研究
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摘要
人类对太空资源的开发已经日渐被提上日程,如何在太空中长期高效率的钻进是太空资源开发首先要解决的问题。长期的地球钻探实践表明,金刚石钻进是一种快速有效的钻进方法,然而在太空中无法找到合适的液体作为冲洗介质,钻进过程中必然产生大量的摩擦热造成金刚石热损伤。针对该问题,本文提出了自润滑孕镶金刚石钻头的设想,即在钻头胎体中添加固体润滑剂,使其在钻进岩石的过程中降低摩擦系数,减少摩擦热的产生,从而降低金刚石的热损伤。考虑到月球和火星表面的温度压力特征及玄武岩在月球和火星上分布的广泛性,本课题主要研究在低温低压条件下含固体润滑剂的金刚石钻头胎体材料在干钻进玄武岩过程中的摩擦磨损特性及机理,并初步建立自润滑孕镶金刚石钻头的设计理论。课题的研究不仅可为月球勘探、火星勘探等太空开发计划中的金刚石钻进工艺作技术准备,而且有助于提高地质勘探中孕镶金刚石钻头的使用性能,提高我国自产的丰富的中低档金刚石的应用范围和利用率,减少对进口高档金刚石的依赖。
本文为了设计自润滑孕镶金刚石钻头胎体的配方,首先根据自润滑孕镶金刚石钻头的制作工艺以及低温低压(真空)的使用环境条件选取合适的固体润滑剂。综合比较各类固体润滑剂性能,石墨是一种比较理想和普遍的固体润滑剂材料,在热压烧结过程中不会氧化,也不会与胎体材料发生反应。另外,MoS：的摩擦系数在真空中与空气中没有多大差别,甚至更小一些,且不需要气体和水蒸气来保持润滑,所以特别适宜于宇航应用。因此,本文选择石墨和二硫化钼两种润滑剂,并且针对其不同的性能,采用热压法制造石墨复合胎体材料,而为了避免二硫化铝在高温烧结时丧失其润滑性则采用复合电镀法制造二硫化钼复合胎体材料。
本文在基于前人关于固体润滑剂增强胎体复合材料的研究基础之上,进行石墨/MoS2自润滑孕镶金刚石钻头胎体材料与岩石对磨时的干摩擦学性能研究,主要研究内容包括四个方面：(1)低温低压(真空)摩擦磨损试验机的研制；(2)石墨固体润滑剂对钻头胎体物理力学性能的影响；(3)石墨对钻头胎体在常温常压和低温低压条件下摩擦学性能的影响及其机理研究；(4)MoS2/Ni复合镀层的摩擦性能及其磨损机理研究。下面将阐述具体研究内容与成果。
为了模拟太空钻探环境,开展低温低压(真空)条件下的摩擦磨损试验,并且由于市场上没有合适的摩擦磨损试验机可供购买,本课题组通过对市面现有摩擦磨损试验机类型与结构进行调研,并结合火星表面低温低压具体条件(600Pa,-5℃),进行了低温低压(真空)摩擦磨损试验机的开发研制。所研制的低温低压(真空)摩擦磨损试验机由真空腔室、机台总成、控制柜和工业计算机四大部分组成,以液压为动力(回转,给进),由计算机采集摩擦系数、温度、扭矩等参数,人机交互界面友好。可在低温、真空条件下对扭矩(摩擦系数)、试块温度进行实时检测,并可实时显示、输出检测结果。试验机主要技术参数：(1)上位驱动液压马达转速200-900r/min(无极调速)；(2)钻压范围0-5MPa(无极调速)；(3)试验腔环境温度：常温～-20℃；(4)试验腔环境压力≥600Pa；(5)钻进行程200mm；(6)摩擦阻力及检测精度：0～100kg±1.5。
采用热压烧结的方法制备石墨+胎体复合材料试样,本试验选用的烧结工艺为：烧结温度945℃,成型压力15MPa,保温时间3mmin。采用单因素试验方法研究了石墨含量和粒度对胎体材料的物理力学性能(硬度、抗弯强度)的影响,石墨含量包括：0、2.5%、5%、7.5%和10%(体积浓度),粒度包括：40/60目、60/80目、80/100目和100/120目,抗弯强度测试采用三点弯曲法,硬度测试为HRB洛氏硬度。试验结果发现：随着胎体中石墨含量的增加,胎体的硬度以及抗弯强度均有所降低,分别降低了8.7%和39.3%；而随着胎体中石墨粒度的细化,胎体的孔隙率升高,胎体的硬度有所升高,抗弯强度有所降低。石墨在胎体中属于软质相,且和胎体结合不牢,对胎体有产生孔隙和引起裂纹的作用,石墨含量的增加相当于增加了胎体中的孔隙,因而使其抗弯强度和硬度降低。硬度反应了胎体抵抗局部塑性变形的能力,压痕范围内石墨颗粒越大,压痕深度就会越大,所测得的洛氏硬度值也就会越低；而当石墨含量一定时,粒度越小其颗粒数日就越多,引入的裂纹和孔隙也就越多,胎体试样承受外载荷的受力面积就越小,抗弯强度也就会越低。
在常温常压条件下进行了摩擦磨损试验,研究石墨润滑剂在胎体中的含量、粒度对胎体与j岩石对磨时的摩擦系数、磨损量的影响,并使用环境扫描电子显微镜对胎体材料的摩擦表面进行形貌分析,研究其磨损机理。由于难以购买到合适的幺武岩,本文中选用与玄武岩性质相近的花岗岩作为试样的对磨件。常温常压下磨损试验结果表明：石墨在摩擦磨损过程中受到热应力以及机械应力的作用涂覆在岩石表面形成润滑膜,降低摩擦副的摩擦系数,润滑膜工作一段时间后在摩擦表面复杂应力的作用下发生疲劳损坏,胎体继续磨损,又在岩石表面形成新的润滑膜；随着胎体中石墨含量的增加,岩石表面的润滑膜更加完整,且润滑膜再生的速度提高,因而摩擦系数不断降低,但是石墨含量的增加增大了胎体孔隙率,降低了胎体与岩石的有效接触面积,增大了压强,因而随着石墨含量的增加,胎体的磨损量先减小后增大；随着胎体中石墨粒度的细化,摩擦副的摩擦系数降低,但是胎体的孔隙率升高,减小了胎体与岩石的有效接触面积,增大了压强,因而增大了胎体的磨损量。石墨的添加降低了摩擦表面的温度,减少了胎体的塑性变形。
在低温低压条件下进行了石墨+胎体复合材料的摩擦磨损试验,研究石墨含量、粒度对胎体材料的摩擦学行为及机理的影响,并且对比分析了环境条件对石墨润滑剂增强胎体复合材料的摩擦磨损性能的影响。试验结果表明,在低温低压条件下,石墨含量、粒度对胎体磨损性能的影响规律基本上上与常温常压条件下一致,即：随着石墨含量的增加摩擦系数降低,磨损量先减少,但当石墨含量超过一定值后磨损量开始上升；随着石墨粒度变细,摩擦系数降低,磨损量增加；添加石墨改变了胎体的麿损机理,由粘着磨损逐渐转变为磨粒磨损为主导。在常温低压条件下,由于在真空条件下、而环境温度相对较高时,磨损腔室内无空气介质散热,磨损端面的热无法扩散,使磨损端面温度迅速升高而导致烧样；磨损过程中摩擦系数急剧抖动,平均摩擦系数与磨损量较大,试样发生严重粘着磨损。相较于常温常压条件,胎体在低温低压环境下进行磨损试验时,没有空气介质,则能避免摩擦副表面的氧化反应,从而相应降低摩擦系数和磨损量。
采用复合电镀法制备了MoS2/Ni自润滑复合镀层材料试样,采用单因素试验方法研究了复合电镀工艺、镀液中MoS2的加入量对复合镀层硬度的影响。本文选择镀液中MoS2含量、镀液PH值和阴极电流密度三个参数作为试验因子,MoS2含量实验范围为0～0.6g/l,PH值范围为2.0～4.5,阴极电流密度范围为1.0～3.5A/dm2。试验结果表明：随着二硫化钼含量的增加,胎体材料的显微硬度不断降低,二硫化钼的添加使胎体材料的显微硬度下降,一定程度降低了胎体材料的力学性能；不同电镀工艺下所获得的MoS2/Ni复合镀层的显微硬度不同,这是电镀工艺对二硫化铝在镀层中含量的影响和对镀层性质影响的共同作用的结果；随着镀液中PH值的增加,胎体硬度不断减小；随着阴极电流密度的增大,镀层硬度先迅速减小后逐渐增大,并趋于稳定,在电流密度为2.5A/dm2有最小值。
进行了低温低压条件下MoS2/Ni复合镀层的摩擦学性能(摩擦系数、磨损率)研究,并对比分析环境条件对MoS2/Ni复合镀层摩擦磨损性能的影响。研究结果表明,在低温低压条件下,二硫化钼在胎体材料中起到了润滑减摩的作用,表现了良好的自润滑性。未添加二硫化铝的试样有着较高的摩擦系数和磨损量,磨损严重,随着二硫化钼含量的不断增加,摩擦副的摩擦系数和胎体材料的磨损量不断减小。电镀工艺对胎体材料的摩擦性能也有一定程度的影响,随着阴极电流密度的增大,摩擦副的摩擦系数先减小后增大,摩擦系数在电流密度为2.5A/dm2时出现最小值0.539,而胎体材料的磨损量随之不断减少,可以改善胎体的耐磨性。)通过常温常压与低温低压的试验比较,在低温低压下表现为更大的摩擦系数和磨损量,对钻头胎体材料的损耗更大。在低温低压下二硫化钼仍然保持良好的自润滑性,提高胎体的耐磨性,但同时二硫化钼对钻头胎体材料摩擦磨损性能的影响趋势并不受低温低压环境的太大影响。磨损形貌分析表明在低温低压条件下,磨损表面的磨痕更深,并有严重的塑性变形和片状剥落,磨损形式主要表现为疲劳磨损及粘着磨损,常温常压以磨粒磨损为主。
总的来说,本文通过大量试验和理论分析,研究了石墨含量、粒度对钻头胎体力学性能的影响,探讨了石墨+胎体复合材料在低温低压条件下的摩擦学行为,分析了MoS2对镀层硬度、磨损性能的影响,也就石墨和MoS2两种固体润滑剂对胎体材料的磨损性能影响及作用机理进行了一定程度的揭示,但在石墨和MoS2对金刚石钻头性能的影响与作用方面还有待于进一步的深入研究。
Exploitation of space resources has increasingly been put on the agenda, of which to run a long-term effective drilling in space has ranked top. Diamond drilling is one of the highly efficient methods, indicated through long-term earth drilling practice. Without appropriate liquid as cooling fluid in the outer space, however, diamond thermal damage will be created as a result of the massive friction heat generated from drilling process. In view of the above-mentioned problem, a project of impregnated diamond bit with self-lubrication ability is proposed in this paper. Based on the assumption, solid lubricant is added to the matrix to cause reductions both in the friction coefficient and friction heat of the drilling process, which will lead to a reduction in diamond thermal damage. Considering the temperature and pressure properties as well as wide distribution of basalt on the moon and Mars, the project mainly studies the tribological properties and mechanism of the impregnated diamond bit with self-lubricating matrix when dry-drilling into basalt at low temperature and low pressure. Thus, a design theory of the matrix of the self-lubricating impregnated diamond bit with will be basically established. The study of the subject, on the one hand, provides technical preparationto diamond drilling technology in space development programs such as lunar exploration, Mars exploration etc.; on the other hand contributes to the improvement of the impregnated diamond bit usability in geological prospecting, which will extend the application scope and availability of abundant home-grown middle-and low-end diamond resource, so as to reduce the dependence on high-end foreign diamond.
To devise a formula for self-lubricating impregnated diamond bit matrix, first of all appropriate solid lubricant was selected according to the manufacturing process and working atmosphere, i.e. low temperature and low pressure (vacuum), of the impregnated diamond bit with self-lubrication ability. This article plans to obtain the diamond bit by hot pressing sintering method, and the sintering temperature is945℃. And the diamond bit would be used at low temperature and low pressure (600pa,-5℃). According to comparative analysis of several common solid lubricant, we found that, compared to other solid lubricants, graphite still had good lubricity in low temperature environment, and it was cheaper than others. Furthermore, graphite has neither been oxidized nor reacted on matrix in the sintering process. Thus graphite is the suitable solid lubricant. Through synthetical comparison of various solid lubricants, graphite is a relatively ideal and prevalent one because of its incapability of oxidation or any chemical reaction with matrix in the sintering process. In addition, there is scarcely any difference between the friction coefficient of MoS2in vacuum and that in the air; with even smaller friction coefficient in vacuum, and no gaseous fluid or vapor for maintaining lubricating, MoS2is particularly suitable for astronavigation application. Hence, graphite and MoS2are selected as the solid lubricants here. For the two materials, distinct manufacturing methods are adopted due to their different properties:hot pressing sintering method for graphite composite matrix; composite electroplating for MoS2composite matrix so as to maintain MoS2's lubrication in high-heat sintering.
On the basis of the predecessors' research work on solid lubricant's enhancing matrix composite, the study aims at the dry tribological characteristics, which will be observed in the process that graphite/MoS2self-lubricating impregnated diamond bit matrix grinding with rocks. Four aspects will be covered:(1) the design and manufacture of a low-temperature and low-pressure vacuum wear tester;(2) the effects of graphite(as a solid lubricant) on the physical mechanics properties of bit matrix;(3) research on effects and mechanism of graphite on tribological properties of bit's matrix at NPT and low pressure and temperature respectively;(4) research on tribological properties and wear mechanism of MoS2/Ni composite coating. Elaboration of research contents and results are as follows:
The research aims to carry out low pressure and ternperature(vacuum) friction-wear tests in simulated space drilling conditions. For lack of appropriate friction-wear testers in the market, testers available in the market are surveyed in terms of their types and structure--based on the survey and simulated actual conditions(600Pa,-5℃)on the surface of Mars, low pressure and temperature(vacuum) friction-wear testers are developed. The testers mainly comprise four parts:a vacuum cavity, a machine assembly, a control cabinet, and an industrial computer. Powered by hydraulic pressure(gyration, feeding), parameters like friction coefficients, temperature, torque gathered by the computer, the testers create a user-friendly interface. These testers allow real-time detection in torque(friction coefficient) and temperature of the briquette at a low temperature and in vacuum, with which the test results can be displayed and output in real time. Main technical parameters of the testers are listed:(1) Rotational speed of the upper driving hydraulic motor:200-900r/min (stepless speed regulation);(2) scope of bit pressure:0～5MPa (stepless speed regulation);(3) ambient temperature of the experimental cavity:normal atmospheric temperature～-20℃;(4) ambient pressure:≥600Pa;(5) drilling depth:200mm;(6) frictional drag and detection precision:0～100kg±1.5kg.
Hot pressed sinteringmethod is introduced here in the manufacture of the composite of graphite&matrix samples. The sintering process here involves a sintering temperature of945℃, briquetting pressureof15MPa, and3min of soaking time. Single-factor test was adopted in investigating effects of content and granularity of graphite on physical mechanics properties(hardness and bending strength) of the matrix. Respectively, the content of graphite were0,2.5%,5%,7.5%, and10%(volume concentration) in each test; granularity were40/60mesh,60/80mesh,80/100mesh, and100/120mesh. Three-point bending test was developed in bending strength, and HRB in hardness test. The tests results showed that as the content of graphite increased, there was a decrease in both hardness and bending strength of the matrix,8.7%and39.3%respectively. With the refining of graphite granularity there was an increase in porosity and hardness of the matrix, but a drop in bending strength. Graphite belongs to soft phase, and the interphase between graphite and the matrix was not strong, which caused pores and cracks in the matrix. An increase of graphite equals an increase of pores in the matrix, which gives rise to a decrease in bending strength and hardness. Hardness is a reflection of the matrix's capability of resisting local plastic deformation. Within the scope of indentation, the depth increases as graphite particles expand, and test values of HRB decrease; when content of graphite is fixed, the smaller the granularity is, the more the particles, cracks and pores are, and the smaller the force bearing area from external load to matrix sample is. Thus its bending strength becomes lower.
Friction-wear tests were carried out at normal pressure and temperatureto study the effects of graphite lubricant's content and granularity in matrix on friction coefficient and abrasion losswhen the matrix is grinding with rocks. Also, environmental scanning electron microscope (ESEM) was used to analyze the microstructure on the friction surface of the matrix, and study the wear mechanism. For lack of appropriate basalt, granite, with similar nature to that of basalt, was selected as the grinder to the matrix sample. Results of the tests at normal pressure and temperatureindicated that, subjected to thermal stress and mechanical stress, graphite in the matrix was daubed on the surface of the granite, which could reduce the friction coefficient. Working in the involute stress for a period of time, the lubrication film would be subjected to fatigue damage, and then the sample was abraded, so there would be new graphite daubed on the granite. With increase of graphite particles, the lubrication film was more intact, and the regeneration speed of the lubricating film was higher, so the friction coefficient decreased. But the increase of graphite augmented porosity of the matrix, and then reduced the effective contact area between matrix and granite, so the pressure was enlarged, and the wearing capacity first decreased and then increased. With decrease of the granularity of graphite particles, the friction coefficient decreased, but the porosity of the matrix increased, so the wearing capacity increased. Adding graphite reduced the temperature of the friction surface and plastic deformation of the matrix.
Another friction-wear test of graphite&matrix composite at low pressure and temperaturewere implemented to study the effects of graphite content and granularity on the tribological behaviors and mechanism of the matrix. Also, contrastive analysis of effects of ambient condition on the friction and wear performance of graphite lubricant's enhancing composite was involved. Test results showed that at low pressure and temperature, effects of graphite content and granularity on matrix's wear properties are basically in accordance with that of normal pressure and temperature, i.e. with an increase in graphite content, friction coefficient decreases, and abrasion loss first decreases and then increases, yet starts to rise when the content of graphite exceeds a certain amount; with the diminution of graphite granularity, friction coefficient decreases, and abrasion loss increases; adding graphite altered the wear mechanism of the matrix, of which the dominant gradually switched from adhesive wear to abrasive wear. At normal pressure and temperature, in a vacuum yet comparatively higher ambient temperature, without air dielectric for heat dissipation in the wear cavity, heat of the wear end face failed to dissipate, which led to a rapid rise on the end face and burnt the samples; during wear process friction coefficient jittered, and there was severe adhesive wear due to greater abrasion loss and average friction coefficient. Compared with condition in normal pressure and temperature, without air dielectric, wear tests of matrix at low pressure and temperature can avoid oxidizing reaction on the surface of the friction pairs, thereby reduced friction coefficient and abrasion loss.
MoS2/Ni self-lubricating composite coating material sample was manufactured through composite platingmethod; effects of the quantity of MoS2added in composite plating and plating bath on the hardness of composite plating was studied through single-factor tests. The content of MoS2in plating bath, PH value of plating bath, and cathode-current density were chosen as three parametersin the tests:the scope of content of MoS2is0-0.6g/1, PH value2.0-4.5, and cathode-current density1.0-3.5A/dm2. Test results indicated that with the increase of the content of MoS2microhardness of the matrix continuously decreased. The adding of MoS2caused a decrease in microhardness of the matrix, which reduced mechanical property of the matrix, to a certain extent; different electroplating processes generated different microhardness of MoS2/Ni composite coating, which was the outcome of the effects of electroplating processes on both the MoS2content in plating and that of properties of coating; with the increase in PH value of the plating bath, hardness of the matrix continuously decreased; with the increase in cathode-current density, hardness of the coat sharply reduced and then continuously increased until it achieved stability, when the electric current density reached a minimum value of2.5A/dm2.
The study of the MoS2/Ni composite plating's tribological properties (friction coefficient and wear rate) at low pressure and temperature was carried out, and contrastive analysis of effects of ambient condition on the friction and wear performance of MoS2/Ni composite plating was involved. The research resultsshowed that at low pressure and temperature, MoS2played the role of antifriction in the material of matrix, which suggested great self-lubricating ability. The sample without the adding of MoS2had higher friction coefficient, abrasion loss, and attrition rate; with the increase in the content of MoS2, friction coefficient of friction pairs and abrasion loss of the matrix decreased. Electroplating process, to some degree, also had impact on the friction performance of the matrix:with the increase in cathode-current density, friction coefficient of friction pairs first decreased and then increased; when the electric current density was2.5A/dm2the friction coefficient reached a minimum value of0.539, thus the abrasion loss decreased, which improved abrasion performance of the matrix. Through comparative tests at both normal and low pressure and temperature, conclusion can be drawn that at low pressure and temperature, friction coefficient and abrasion loss are higher, which leads to greater loss of the matrix bit. MoS2maintained good self-lubricating ability at low pressure and temperature, which enhanced the abrasive resistance of the matrix. Meanwhile, effects of MoS2on the wear resistance of bit's matrix was not subject to low pressure and low temperature. The analysis of wear appearance indicated that at low pressure and temperature, grinding crack on the surface was deeper, with severe plastic deformation and exfoliation. The attrition mainly took in the form of fatigue wear and adhesive wear at low pressure and temperature, and abrasive wear at normal pressure and temperature.
In summary, through lots of experiments and theoretical analyses, the effects of content and granularity of graphite particles on the mechanical propertyof the matrix were studied; tribological behaviors of graphite-matrix composite at low pressure and temperature were discussed; impact of MoS2on the hardness and wear performance of the cladding material were investigated. Moreover, mechanism of action plus effects of the two solid lubricants i.e. graphite and MoS2,on tribological properties of bit matrix were revealed, to a certain degree. Further study of the effects of graphite and MoS2on the properties of diamond bit, however, must be involved.
本文为了设计自润滑孕镶金刚石钻头胎体的配方,首先根据自润滑孕镶金刚石钻头的制作工艺以及低温低压(真空)的使用环境条件选取合适的固体润滑剂。综合比较各类固体润滑剂性能,石墨是一种比较理想和普遍的固体润滑剂材料,在热压烧结过程中不会氧化,也不会与胎体材料发生反应。另外,MoS：的摩擦系数在真空中与空气中没有多大差别,甚至更小一些,且不需要气体和水蒸气来保持润滑,所以特别适宜于宇航应用。因此,本文选择石墨和二硫化钼两种润滑剂,并且针对其不同的性能,采用热压法制造石墨复合胎体材料,而为了避免二硫化铝在高温烧结时丧失其润滑性则采用复合电镀法制造二硫化钼复合胎体材料。
本文在基于前人关于固体润滑剂增强胎体复合材料的研究基础之上,进行石墨/MoS2自润滑孕镶金刚石钻头胎体材料与岩石对磨时的干摩擦学性能研究,主要研究内容包括四个方面：(1)低温低压(真空)摩擦磨损试验机的研制；(2)石墨固体润滑剂对钻头胎体物理力学性能的影响；(3)石墨对钻头胎体在常温常压和低温低压条件下摩擦学性能的影响及其机理研究；(4)MoS2/Ni复合镀层的摩擦性能及其磨损机理研究。下面将阐述具体研究内容与成果。
为了模拟太空钻探环境,开展低温低压(真空)条件下的摩擦磨损试验,并且由于市场上没有合适的摩擦磨损试验机可供购买,本课题组通过对市面现有摩擦磨损试验机类型与结构进行调研,并结合火星表面低温低压具体条件(600Pa,-5℃),进行了低温低压(真空)摩擦磨损试验机的开发研制。所研制的低温低压(真空)摩擦磨损试验机由真空腔室、机台总成、控制柜和工业计算机四大部分组成,以液压为动力(回转,给进),由计算机采集摩擦系数、温度、扭矩等参数,人机交互界面友好。可在低温、真空条件下对扭矩(摩擦系数)、试块温度进行实时检测,并可实时显示、输出检测结果。试验机主要技术参数：(1)上位驱动液压马达转速200-900r/min(无极调速)；(2)钻压范围0-5MPa(无极调速)；(3)试验腔环境温度：常温～-20℃；(4)试验腔环境压力≥600Pa；(5)钻进行程200mm；(6)摩擦阻力及检测精度：0～100kg±1.5。
采用热压烧结的方法制备石墨+胎体复合材料试样,本试验选用的烧结工艺为：烧结温度945℃,成型压力15MPa,保温时间3mmin。采用单因素试验方法研究了石墨含量和粒度对胎体材料的物理力学性能(硬度、抗弯强度)的影响,石墨含量包括：0、2.5%、5%、7.5%和10%(体积浓度),粒度包括：40/60目、60/80目、80/100目和100/120目,抗弯强度测试采用三点弯曲法,硬度测试为HRB洛氏硬度。试验结果发现：随着胎体中石墨含量的增加,胎体的硬度以及抗弯强度均有所降低,分别降低了8.7%和39.3%；而随着胎体中石墨粒度的细化,胎体的孔隙率升高,胎体的硬度有所升高,抗弯强度有所降低。石墨在胎体中属于软质相,且和胎体结合不牢,对胎体有产生孔隙和引起裂纹的作用,石墨含量的增加相当于增加了胎体中的孔隙,因而使其抗弯强度和硬度降低。硬度反应了胎体抵抗局部塑性变形的能力,压痕范围内石墨颗粒越大,压痕深度就会越大,所测得的洛氏硬度值也就会越低；而当石墨含量一定时,粒度越小其颗粒数日就越多,引入的裂纹和孔隙也就越多,胎体试样承受外载荷的受力面积就越小,抗弯强度也就会越低。
在常温常压条件下进行了摩擦磨损试验,研究石墨润滑剂在胎体中的含量、粒度对胎体与j岩石对磨时的摩擦系数、磨损量的影响,并使用环境扫描电子显微镜对胎体材料的摩擦表面进行形貌分析,研究其磨损机理。由于难以购买到合适的幺武岩,本文中选用与玄武岩性质相近的花岗岩作为试样的对磨件。常温常压下磨损试验结果表明：石墨在摩擦磨损过程中受到热应力以及机械应力的作用涂覆在岩石表面形成润滑膜,降低摩擦副的摩擦系数,润滑膜工作一段时间后在摩擦表面复杂应力的作用下发生疲劳损坏,胎体继续磨损,又在岩石表面形成新的润滑膜；随着胎体中石墨含量的增加,岩石表面的润滑膜更加完整,且润滑膜再生的速度提高,因而摩擦系数不断降低,但是石墨含量的增加增大了胎体孔隙率,降低了胎体与岩石的有效接触面积,增大了压强,因而随着石墨含量的增加,胎体的磨损量先减小后增大；随着胎体中石墨粒度的细化,摩擦副的摩擦系数降低,但是胎体的孔隙率升高,减小了胎体与岩石的有效接触面积,增大了压强,因而增大了胎体的磨损量。石墨的添加降低了摩擦表面的温度,减少了胎体的塑性变形。
在低温低压条件下进行了石墨+胎体复合材料的摩擦磨损试验,研究石墨含量、粒度对胎体材料的摩擦学行为及机理的影响,并且对比分析了环境条件对石墨润滑剂增强胎体复合材料的摩擦磨损性能的影响。试验结果表明,在低温低压条件下,石墨含量、粒度对胎体磨损性能的影响规律基本上上与常温常压条件下一致,即：随着石墨含量的增加摩擦系数降低,磨损量先减少,但当石墨含量超过一定值后磨损量开始上升；随着石墨粒度变细,摩擦系数降低,磨损量增加；添加石墨改变了胎体的麿损机理,由粘着磨损逐渐转变为磨粒磨损为主导。在常温低压条件下,由于在真空条件下、而环境温度相对较高时,磨损腔室内无空气介质散热,磨损端面的热无法扩散,使磨损端面温度迅速升高而导致烧样；磨损过程中摩擦系数急剧抖动,平均摩擦系数与磨损量较大,试样发生严重粘着磨损。相较于常温常压条件,胎体在低温低压环境下进行磨损试验时,没有空气介质,则能避免摩擦副表面的氧化反应,从而相应降低摩擦系数和磨损量。
采用复合电镀法制备了MoS2/Ni自润滑复合镀层材料试样,采用单因素试验方法研究了复合电镀工艺、镀液中MoS2的加入量对复合镀层硬度的影响。本文选择镀液中MoS2含量、镀液PH值和阴极电流密度三个参数作为试验因子,MoS2含量实验范围为0～0.6g/l,PH值范围为2.0～4.5,阴极电流密度范围为1.0～3.5A/dm2。试验结果表明：随着二硫化钼含量的增加,胎体材料的显微硬度不断降低,二硫化钼的添加使胎体材料的显微硬度下降,一定程度降低了胎体材料的力学性能；不同电镀工艺下所获得的MoS2/Ni复合镀层的显微硬度不同,这是电镀工艺对二硫化铝在镀层中含量的影响和对镀层性质影响的共同作用的结果；随着镀液中PH值的增加,胎体硬度不断减小；随着阴极电流密度的增大,镀层硬度先迅速减小后逐渐增大,并趋于稳定,在电流密度为2.5A/dm2有最小值。
进行了低温低压条件下MoS2/Ni复合镀层的摩擦学性能(摩擦系数、磨损率)研究,并对比分析环境条件对MoS2/Ni复合镀层摩擦磨损性能的影响。研究结果表明,在低温低压条件下,二硫化钼在胎体材料中起到了润滑减摩的作用,表现了良好的自润滑性。未添加二硫化铝的试样有着较高的摩擦系数和磨损量,磨损严重,随着二硫化钼含量的不断增加,摩擦副的摩擦系数和胎体材料的磨损量不断减小。电镀工艺对胎体材料的摩擦性能也有一定程度的影响,随着阴极电流密度的增大,摩擦副的摩擦系数先减小后增大,摩擦系数在电流密度为2.5A/dm2时出现最小值0.539,而胎体材料的磨损量随之不断减少,可以改善胎体的耐磨性。)通过常温常压与低温低压的试验比较,在低温低压下表现为更大的摩擦系数和磨损量,对钻头胎体材料的损耗更大。在低温低压下二硫化钼仍然保持良好的自润滑性,提高胎体的耐磨性,但同时二硫化钼对钻头胎体材料摩擦磨损性能的影响趋势并不受低温低压环境的太大影响。磨损形貌分析表明在低温低压条件下,磨损表面的磨痕更深,并有严重的塑性变形和片状剥落,磨损形式主要表现为疲劳磨损及粘着磨损,常温常压以磨粒磨损为主。
总的来说,本文通过大量试验和理论分析,研究了石墨含量、粒度对钻头胎体力学性能的影响,探讨了石墨+胎体复合材料在低温低压条件下的摩擦学行为,分析了MoS2对镀层硬度、磨损性能的影响,也就石墨和MoS2两种固体润滑剂对胎体材料的磨损性能影响及作用机理进行了一定程度的揭示,但在石墨和MoS2对金刚石钻头性能的影响与作用方面还有待于进一步的深入研究。
Exploitation of space resources has increasingly been put on the agenda, of which to run a long-term effective drilling in space has ranked top. Diamond drilling is one of the highly efficient methods, indicated through long-term earth drilling practice. Without appropriate liquid as cooling fluid in the outer space, however, diamond thermal damage will be created as a result of the massive friction heat generated from drilling process. In view of the above-mentioned problem, a project of impregnated diamond bit with self-lubrication ability is proposed in this paper. Based on the assumption, solid lubricant is added to the matrix to cause reductions both in the friction coefficient and friction heat of the drilling process, which will lead to a reduction in diamond thermal damage. Considering the temperature and pressure properties as well as wide distribution of basalt on the moon and Mars, the project mainly studies the tribological properties and mechanism of the impregnated diamond bit with self-lubricating matrix when dry-drilling into basalt at low temperature and low pressure. Thus, a design theory of the matrix of the self-lubricating impregnated diamond bit with will be basically established. The study of the subject, on the one hand, provides technical preparationto diamond drilling technology in space development programs such as lunar exploration, Mars exploration etc.; on the other hand contributes to the improvement of the impregnated diamond bit usability in geological prospecting, which will extend the application scope and availability of abundant home-grown middle-and low-end diamond resource, so as to reduce the dependence on high-end foreign diamond.
To devise a formula for self-lubricating impregnated diamond bit matrix, first of all appropriate solid lubricant was selected according to the manufacturing process and working atmosphere, i.e. low temperature and low pressure (vacuum), of the impregnated diamond bit with self-lubrication ability. This article plans to obtain the diamond bit by hot pressing sintering method, and the sintering temperature is945℃. And the diamond bit would be used at low temperature and low pressure (600pa,-5℃). According to comparative analysis of several common solid lubricant, we found that, compared to other solid lubricants, graphite still had good lubricity in low temperature environment, and it was cheaper than others. Furthermore, graphite has neither been oxidized nor reacted on matrix in the sintering process. Thus graphite is the suitable solid lubricant. Through synthetical comparison of various solid lubricants, graphite is a relatively ideal and prevalent one because of its incapability of oxidation or any chemical reaction with matrix in the sintering process. In addition, there is scarcely any difference between the friction coefficient of MoS2in vacuum and that in the air; with even smaller friction coefficient in vacuum, and no gaseous fluid or vapor for maintaining lubricating, MoS2is particularly suitable for astronavigation application. Hence, graphite and MoS2are selected as the solid lubricants here. For the two materials, distinct manufacturing methods are adopted due to their different properties:hot pressing sintering method for graphite composite matrix; composite electroplating for MoS2composite matrix so as to maintain MoS2's lubrication in high-heat sintering.
On the basis of the predecessors' research work on solid lubricant's enhancing matrix composite, the study aims at the dry tribological characteristics, which will be observed in the process that graphite/MoS2self-lubricating impregnated diamond bit matrix grinding with rocks. Four aspects will be covered:(1) the design and manufacture of a low-temperature and low-pressure vacuum wear tester;(2) the effects of graphite(as a solid lubricant) on the physical mechanics properties of bit matrix;(3) research on effects and mechanism of graphite on tribological properties of bit's matrix at NPT and low pressure and temperature respectively;(4) research on tribological properties and wear mechanism of MoS2/Ni composite coating. Elaboration of research contents and results are as follows:
The research aims to carry out low pressure and ternperature(vacuum) friction-wear tests in simulated space drilling conditions. For lack of appropriate friction-wear testers in the market, testers available in the market are surveyed in terms of their types and structure--based on the survey and simulated actual conditions(600Pa,-5℃)on the surface of Mars, low pressure and temperature(vacuum) friction-wear testers are developed. The testers mainly comprise four parts:a vacuum cavity, a machine assembly, a control cabinet, and an industrial computer. Powered by hydraulic pressure(gyration, feeding), parameters like friction coefficients, temperature, torque gathered by the computer, the testers create a user-friendly interface. These testers allow real-time detection in torque(friction coefficient) and temperature of the briquette at a low temperature and in vacuum, with which the test results can be displayed and output in real time. Main technical parameters of the testers are listed:(1) Rotational speed of the upper driving hydraulic motor:200-900r/min (stepless speed regulation);(2) scope of bit pressure:0～5MPa (stepless speed regulation);(3) ambient temperature of the experimental cavity:normal atmospheric temperature～-20℃;(4) ambient pressure:≥600Pa;(5) drilling depth:200mm;(6) frictional drag and detection precision:0～100kg±1.5kg.
Hot pressed sinteringmethod is introduced here in the manufacture of the composite of graphite&matrix samples. The sintering process here involves a sintering temperature of945℃, briquetting pressureof15MPa, and3min of soaking time. Single-factor test was adopted in investigating effects of content and granularity of graphite on physical mechanics properties(hardness and bending strength) of the matrix. Respectively, the content of graphite were0,2.5%,5%,7.5%, and10%(volume concentration) in each test; granularity were40/60mesh,60/80mesh,80/100mesh, and100/120mesh. Three-point bending test was developed in bending strength, and HRB in hardness test. The tests results showed that as the content of graphite increased, there was a decrease in both hardness and bending strength of the matrix,8.7%and39.3%respectively. With the refining of graphite granularity there was an increase in porosity and hardness of the matrix, but a drop in bending strength. Graphite belongs to soft phase, and the interphase between graphite and the matrix was not strong, which caused pores and cracks in the matrix. An increase of graphite equals an increase of pores in the matrix, which gives rise to a decrease in bending strength and hardness. Hardness is a reflection of the matrix's capability of resisting local plastic deformation. Within the scope of indentation, the depth increases as graphite particles expand, and test values of HRB decrease; when content of graphite is fixed, the smaller the granularity is, the more the particles, cracks and pores are, and the smaller the force bearing area from external load to matrix sample is. Thus its bending strength becomes lower.
Friction-wear tests were carried out at normal pressure and temperatureto study the effects of graphite lubricant's content and granularity in matrix on friction coefficient and abrasion losswhen the matrix is grinding with rocks. Also, environmental scanning electron microscope (ESEM) was used to analyze the microstructure on the friction surface of the matrix, and study the wear mechanism. For lack of appropriate basalt, granite, with similar nature to that of basalt, was selected as the grinder to the matrix sample. Results of the tests at normal pressure and temperatureindicated that, subjected to thermal stress and mechanical stress, graphite in the matrix was daubed on the surface of the granite, which could reduce the friction coefficient. Working in the involute stress for a period of time, the lubrication film would be subjected to fatigue damage, and then the sample was abraded, so there would be new graphite daubed on the granite. With increase of graphite particles, the lubrication film was more intact, and the regeneration speed of the lubricating film was higher, so the friction coefficient decreased. But the increase of graphite augmented porosity of the matrix, and then reduced the effective contact area between matrix and granite, so the pressure was enlarged, and the wearing capacity first decreased and then increased. With decrease of the granularity of graphite particles, the friction coefficient decreased, but the porosity of the matrix increased, so the wearing capacity increased. Adding graphite reduced the temperature of the friction surface and plastic deformation of the matrix.
Another friction-wear test of graphite&matrix composite at low pressure and temperaturewere implemented to study the effects of graphite content and granularity on the tribological behaviors and mechanism of the matrix. Also, contrastive analysis of effects of ambient condition on the friction and wear performance of graphite lubricant's enhancing composite was involved. Test results showed that at low pressure and temperature, effects of graphite content and granularity on matrix's wear properties are basically in accordance with that of normal pressure and temperature, i.e. with an increase in graphite content, friction coefficient decreases, and abrasion loss first decreases and then increases, yet starts to rise when the content of graphite exceeds a certain amount; with the diminution of graphite granularity, friction coefficient decreases, and abrasion loss increases; adding graphite altered the wear mechanism of the matrix, of which the dominant gradually switched from adhesive wear to abrasive wear. At normal pressure and temperature, in a vacuum yet comparatively higher ambient temperature, without air dielectric for heat dissipation in the wear cavity, heat of the wear end face failed to dissipate, which led to a rapid rise on the end face and burnt the samples; during wear process friction coefficient jittered, and there was severe adhesive wear due to greater abrasion loss and average friction coefficient. Compared with condition in normal pressure and temperature, without air dielectric, wear tests of matrix at low pressure and temperature can avoid oxidizing reaction on the surface of the friction pairs, thereby reduced friction coefficient and abrasion loss.
MoS2/Ni self-lubricating composite coating material sample was manufactured through composite platingmethod; effects of the quantity of MoS2added in composite plating and plating bath on the hardness of composite plating was studied through single-factor tests. The content of MoS2in plating bath, PH value of plating bath, and cathode-current density were chosen as three parametersin the tests:the scope of content of MoS2is0-0.6g/1, PH value2.0-4.5, and cathode-current density1.0-3.5A/dm2. Test results indicated that with the increase of the content of MoS2microhardness of the matrix continuously decreased. The adding of MoS2caused a decrease in microhardness of the matrix, which reduced mechanical property of the matrix, to a certain extent; different electroplating processes generated different microhardness of MoS2/Ni composite coating, which was the outcome of the effects of electroplating processes on both the MoS2content in plating and that of properties of coating; with the increase in PH value of the plating bath, hardness of the matrix continuously decreased; with the increase in cathode-current density, hardness of the coat sharply reduced and then continuously increased until it achieved stability, when the electric current density reached a minimum value of2.5A/dm2.
The study of the MoS2/Ni composite plating's tribological properties (friction coefficient and wear rate) at low pressure and temperature was carried out, and contrastive analysis of effects of ambient condition on the friction and wear performance of MoS2/Ni composite plating was involved. The research resultsshowed that at low pressure and temperature, MoS2played the role of antifriction in the material of matrix, which suggested great self-lubricating ability. The sample without the adding of MoS2had higher friction coefficient, abrasion loss, and attrition rate; with the increase in the content of MoS2, friction coefficient of friction pairs and abrasion loss of the matrix decreased. Electroplating process, to some degree, also had impact on the friction performance of the matrix:with the increase in cathode-current density, friction coefficient of friction pairs first decreased and then increased; when the electric current density was2.5A/dm2the friction coefficient reached a minimum value of0.539, thus the abrasion loss decreased, which improved abrasion performance of the matrix. Through comparative tests at both normal and low pressure and temperature, conclusion can be drawn that at low pressure and temperature, friction coefficient and abrasion loss are higher, which leads to greater loss of the matrix bit. MoS2maintained good self-lubricating ability at low pressure and temperature, which enhanced the abrasive resistance of the matrix. Meanwhile, effects of MoS2on the wear resistance of bit's matrix was not subject to low pressure and low temperature. The analysis of wear appearance indicated that at low pressure and temperature, grinding crack on the surface was deeper, with severe plastic deformation and exfoliation. The attrition mainly took in the form of fatigue wear and adhesive wear at low pressure and temperature, and abrasive wear at normal pressure and temperature.
In summary, through lots of experiments and theoretical analyses, the effects of content and granularity of graphite particles on the mechanical propertyof the matrix were studied; tribological behaviors of graphite-matrix composite at low pressure and temperature were discussed; impact of MoS2on the hardness and wear performance of the cladding material were investigated. Moreover, mechanism of action plus effects of the two solid lubricants i.e. graphite and MoS2,on tribological properties of bit matrix were revealed, to a certain degree. Further study of the effects of graphite and MoS2on the properties of diamond bit, however, must be involved.
引文
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