Theoretical Method Of Low Temperature Indentation Calibration And Experimental Study Of Typical Materials

Micro-nano indentation testing technology has gradually developed into a mainstream technology for testing mechanical properties of material due to its advantages of rapid measurement,high precision and no simple sample preparation.However,with the rapid development of the application field of low temperature environment,higher requirements are put forward for the characteristics of materials in the low temperature and severe environment.The conventional indentation test can not meet the requirements,and the existing low temperature indentation test technology is not mature,which further limits the research work of the indentation response test of materials at low temperature.Therefore,it is of great scientific significance and broad application prospect to develop low temperature micro-nano indentation testing technology,test and control the performance of typical materials at low temperatures,and carry out research on new phenomena and new laws.As a new material testing technology,low temperature indentation technology still has shortcomings in instrument calibration,data analysis and other aspects.In this paper,based on the self-made instrument,the calibration work is carried out under working condition environment,and analysis method of indentation testing error under low temperature environment is proposed,which eliminates the impact of temperature on indentation testing process and post-processing analysis.Compared with the existing low temperature indentation instruments and methods,this paper obtains more accurate indentation test curves and physical properties of testing materials,which provides an important test basis for the analysis,preparation and use of the low temperature performance of materials.In addition,mechanical properties at low temperatures of two typical single crystal materials are obtained and related microscale phenomena are further studied.The main research work of this paper is as follows:(1)The classical data analysis process Oliver-Pharr method is summarized,and then the module composition and mechanical structure of the cryogenic indentation system used in this paper is introduced.The sensors in the testing system are calibrated,and the area function of the indenter is calibrated by the atomic force microscope.The calibration of the instrument under normal temperature and pressure is realized.The displacement sensor and force sensor are tested in vacuum and low temperature environment respectively,and the accuracy of data acquisition in their working environment is confirmed.(2)In terms of hardness,low temperature will lead to the change of indenter area function,and cause the calculated hardness value to be larger.In terms of elastic modulus,low temperature will lead to the increase of elastic modulus of sapphire indenter,and then cause the calculated elastic modulus value decreased.Through adopting sapphire indenter with low thermal conductivity,copper wire for indenter refrigeration and pre-contact before indentation,the contact temperature drift in the process of low-temperature indentation is greatly reduced and eliminated,meanwhile accurate and reliable low-temperature indentation curve is obtained.(3)Room temperature and cryogenic indentation tests are performed on monocrystalline silicon.The error caused by temperature is eliminated by the error analysis and correction method.The hardness and elastic modulus are found slightly increased with temperature decreasing.Compared with the existing data,the accuracy of the instrument is verified.Meanwhile,the probability of occurrence of pop-out phenomenon during unloading process also decreases with temperature decreasing,and the vanished pop-out is replaced by elbow phenomenon.Through molecular dynamics(MD)simulation analysis method,the phase states in the indentation are obtained during the loading process at room temperature and low temperature.The cryogenic environment has no influence on the generation of Si-II phase during loading process,but inhibits the generation of Si-III/XII phase and promotes the generation of a-Si during unload process.Electric current monitoring method is used to realize the in-situ indentation test at low temperatures.It is found that the method has effectively response to the pop-out phenomenon during monocrystalline silicon indentation,and initiation and propagation of crack during loading process with characterized phenomenon “steep drop” of current.(4)Room temperature and cryogenic indentation tests are performed on monocrystalline copper.The hardness and elastic modulus are found increased with temperature decreasing.Inner slip band are observed at 150 K through indentation tests on three crystal faces with the cone indenter.Dislocation and slip are confirmed by transmission electron microscope as the only plastic deformation mode of monocrystalline copper at low temperatures.Through MD simulation analysis,the dislocation generation in the loading section and the elastic recovery in the unloading section are obtained.It is found that the inner slip band phenomenon occurs during loading process at both room temperature and low temperatures.The hardness increase caused by low temperature maintains the inner slip bands at 150 K until the end of unloading process.Meanwhile,the increased elasticity recovery during unloading process resulted in ambiguous and band-like boundaries,and change of maximum dimension direction of indents.