| 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. |
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