| With the continuous application of low temperature technology in frontier fields,such as large scientific engineering,space exploration,and medical science,the evaluation of the service performance of key materials in extreme low-temperature environments has been more and more important.The low temperature indentation testing technology has emerged.Advanced thin films show great application potential in the above fields due to their advantages of miniaturization,intelligence,and easy integration.The material mechanical parameters and the characterized behaviors measured by the low temperature indentation testing technology are the vital basis for the design,preparation,life prediction,and reliability evaluation of low temperature service thin films.However,the theories and technologies of low temperature microand nanoindentation test are still immature,which are limited by the single load range and insufficient temperature drift control ability.It will restrict the accurate test and characterization of the mechanical properties of materials at low temperatures.Therefore,there is an urgent need to develop reliable low temperature micro-and nanoindentation testing apparatuses.Since reviewing the current situation of low temperature indentation testing technology,this paper designed and developed a low temperature micro-and nanoindentation testing apparatus that adopts contact-atmosphere hybrid refrigeration and multi-range indentation.Furthermore,the existing correction theories of the thin film indentation testing characterization and calibration technologies of the indentation testing apparatus have been improved.The indentation response tests at room temperature and low temperatures were carried out for advanced thin films with low temperature application potential,and the micro-domain deformation damage mode and failure mechanism were analyzed and investigated.The main research work of this paper is as follows:(1)Considering the effects of film/coating thickness and the pile-up behavior,the modified models for thin film indentation testing were proposed.For ultra-thin films,a novel indentation method was proposed to evaluate the intrinsic mechanical properties of ultra-thin films under the condition of full penetration.To validate the method,indentation tests on commercial indium tin oxide(ITO)thin films were performed.Meanwhile,because of the pile-up phenomenon on thin film indentation,a dimensionless function relationship for the indentation response of thin films with the conical indenter was established based on dimensional analysis and the finite element method(FEM).A method to acquire the intrinsic hardness and elastic modulus of thin film materials was proposed combined with the 10%critical indentation depth rule.In addition,to accurately extract the intrinsic mechanical properties of thin films,a correction model for the measurement of elastoplastic mechanical properties of bulk substrate materials based on the Kalidini-Pathak indentation stress-strain(ISS)model was proposed.To validate the method,cyclic indentation tests using a spherical indenter were carried out on the high Nbcontaining TiAl alloy.(2)A low temperature indentation apparatus was designed and developed.Through the coupling characteristic analysis of the apparatus to be developed,the modular design of low temperature indentation apparatus was carried out.Furthermore,the effects of temperature-dependent material properties at the contact region,testing temperature,and loading conditions on thermal drift rate were systematically studied by using Taguchi orthogonal array design and force-thermal coupled FEM.The optimum control factor level of thermal drift and the range analysis,variance analysis,and contribution rate calculation of the signal-to-noise ratio were used to judge the importance of the above factors.According to the comparative analysis of the thermal drift suppression strategy and instrument optimization method,a contact-atmosphere hybrid refrigeration system was developed,and an independent PID correlation temperature control strategy was adopted to construct a low temperature loading environment with a minimum temperature of-190℃,which solved the technical problem of thermal drift elimination.Through the quick-change mechanical structure design,the drive loading and detection cells were optimized.Combined with the multi-mode contact judgment method,a multi-range indentation testing module was developed,which solved the technical problem of a single load range and broadened the testing object from thin films to bulk materials.(3)The calibration,traceability,and uncertainty assessment were carried out for the self-developed indentation apparatus in this paper.To realize the accurate acquisition of load/displacement measurement values and the calculation of parameters including indentation hardness and reduced modulus,the effect factors,such as load/displacement factor,frame compliance,and indenter area function that affect the measurement accuracy of nanoindentation were calibrated,respectively.The performance of the whole spatulas was verified by the indentation loading closed-loop control test,the thermal drift calibration test at room temperature and low temperatures,and the repeatability test of the multi-range indentation test carried out on the standard fused silica.According to the ISO 14577 standard,the load/displacement factor,noise level,resolution,indication error,measured repeatability,and dispersion of measurement results by factors such as judgment of contact zero,frame compliance,thermal drift,etc.were evaluated.The indentation tests on the standard fused silica were used to evaluate the uncertainties of the indentation hardness and reduced modulus,and to carry out the error analysis of the apparatus.(4)Advanced thin films were investigated on the indentation behaviors at room temperature and low temperatures.For the ITO single-layer thin film and ITO/Ag/ITO(IAI)multilayer thin film by magnetron sputtering,the structural properties and surface morphological characterization were characterized.The indentation tests at room temperature were carried out using the commercial indentation apparatus.The mechanical parameters such as hardness,elastic modulus,energy release rate,and fracture toughness of the ITO thin films were obtained.Furthermore,the low-temperature indentation tests were carried out on the ITO single-layer thin film using the self-developed apparatus.The mechanical properties of ITO thin films at low temperatures were obtained for the first time.Compared with the indentation response at room temperature,the elastic modulus and hardness of ITO thin films increased significantly at low temperatures.However,the picture-frame crack density also increased gradually.the complex stress-temperature coupling effect on the initiation and propagation mechanism of cracks was investigated by comparing the analysis of residual imprints under cyclic thermal loading.In this paper,the modular design of the instrument to be developed and the corrected theories of thin-film indentation testing characterization are investigated.It has made breakthroughs in technologies such as continuous variable temperature loading with low thermal drift and multi-range indentation.A stable and reliable low temperature indentation has been developed.The testing apparatus has important scientific significance and application value for the design and preparation,service performance evaluation,and deformation failure mechanism research of advanced thin films,which service at low temperatures. |
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