Design And Experimental Research Of Nanoindentation Testing Device For Cryogenic Temperature

The nanoindentation testing technique conducts data analysis based on the load-depth curve, and it has irreplaceable advantages such as rich information obtaining of materials’ micro mechanical properties, simple preparation of sample and high resolution of measuring when compared with other testing techniques. Since the mechanical properties of materials are closely related to its practical use conditions, it’s obviously unscientific to guide the design, manufacture and use of materials or structures that served at cryogenic temperature with mechanical property parameters of the materials measured at room temperature. However, traditional nanoindentation testing instrument has not yet had the ability to assess the mechanical properties of materials at cryogenic temperature, it is quite important to develop nanoindentation testing technology and testing device for cryogenic temperature.In this paper, considering with the current testing background of mechanical properties of materials, the research status at home and abroad on nanoindentation testing technique for room and cryogenic temperatures was summarized, current problems in the research of nanoindentation test at cryogenic temperature were analyzed, and the process of data analysis using Oliver-Pharr analysis method was mainly introduced. We analyzed and compared the advantages and disadvantages of cryogenic refrigeration methods and commonly used precision driving methods and precision detection methods in nanoindentation.Combining the above work, a testing device that working in high vacuum for continuous temperature indentation test was designed in this paper and it could accomplish the indentation test at temperatures from 100 K to room temperature(293K). To verify the feasibility of the design scheme, virtue prototyping model of the testing device was built and the static and dynamic characteristics analysis work of the key mechanical components and the testing device were carried out. We proposed three solutions to realize cryogenic temperature, the advantages of “integral vacuum-temperature change by contacting” method were pointed out and the solution to the problem of heat insulation of electrical components was presented.Based on the above work, the prototype and test system of nanoindentation testing device for cryogenic temperature were built. We carried out a lot of testing work on the performance of the testing device, such as the calibrating of the sensors, the measuring of compliance of the device, the output performance testing of the precision pressing unit, the testing of closed loop control characteristics, the output characteristic testing of displacement and the repeatability testing of indentation curve at different temperatures. The testing results indicated that the precision pressing unit that we designed had quick response speed; the test system had two kinds of closed loop control mode, which are load control and depth control, and could achieve indentation test with the maximum indentation depth of 20nm~30μm; the indentation curve of the calibrated device could be perfectly coincidence with that of commercial indentation tester under the same conditions and had good reproducibility, and the hardness and elastic modulus measured was only 1.42% and 0.89% off the standard. Meanwhile, pumping speed testing and air leakage testing of the vacuum system were also carried out. We could know from the thermal insulation performance testing of the heat insulating pressure rod and the cooling-heating time history testing of the extended stage that the heat insulating pressure rod designed in this paper could meet the desire of heat insulation for nanoindentation test at cryogenic temperature; when liquid nitrogen was used for cooling, temperature difference about 10 K could always be found between the extended stage and the cryostat and it was not exist while heating, the lowest temperature of the extended stage could be 87.3K.With the nanoindentation testing device developed for cryogenic temperature here, we studied how the change of the vacuum degree and the curing time of the sample adhesive could affect the test result. Nanoindentation test on fused silica at cryogenic temperature was taken and the hardness, elastic modulus and indentation curve of fused silica were obtained. It was shown that when the vacuum degree changed from 10-1Pa to 105Pa(1atm), its influence on the indentation curve and the mechanical parameters measured could be ignored. With the decrease of temperature from 293 K to 100 K, the indentation curve of fused silica showed obvious regularity which was the increase in maximum indentation depth and the decrease of contact stiffness. The elastic modulus and hardness obtained was 60.36 GPa and 9.10 GPa at 293 K, while that was 49.75 GPa and 7.25 GPa at 100 K, both the elastic modulus and the hardness had a decreasing trend when temperature decreased.