Research Of Detecting Metal Elements In Lubricating Oils Based On LIBS

The engine is hailed as the heart of the car,and its importance is self-evident.When the engine is running,physical friction produces metal particles of different sizes mixed into the lubricating oil,and chemical corrosion also produces soluble metal organic substances mixed into the lubricating oil.The additive content in the lubricating oil is also attenuated due to chemical reaction,so the elemental composition of the lubricating oil will change.Therefore,the detection of wear metal elements in lubricating oil can provide valuable information for diagnosing engine operating conditions.Laser Induced Breakdown Spectroscopy?LIBS?is a new type of spectral analysis technology that uses high-energy laser pulses to generate high-temperature plasma and collects plasma emission spectra to complete sample detection.It has characteristics such as multi-elements analysis,high measuring speed,real-time detection and simple sample pretreatment.In this thesis,LIBS was used to study the wear metal elements in lubricating oil.The quantitative analysis of four kinds of wear metal elements such as Mg,Ti,Ni and Cr in lubricating oil was carried out.The specific contents are as follows:In this thesis,the physical properties of laser induced lubricating oil plasma were investigated.Mg II 279.55 nm,Ti I 334.94 nm,Ni I 352.45 nm and Cr I 425.44 nm were selected as analytical lines.The effects of different experimental parameters on the experimental results were investigated in all aspects,including sample standing time,average oil film thickness,laser repetition rate,number of spectral accumulations,detection delay,and laser energy.By analyzing the experimental results under different experimental parameters,the general rule is obtained according to the spectral signal intensity,signal-to-back ratio and relative standard deviation of the spectral line,and the optimal experimental conditions are determined.Under the optimal experimental conditions,the physical parameters of the lubricating oil plasma were diagnosed.Two characteristic lines of Mg II 275.55 nm and Mg II 280.27 nm were selected,and the plasma excitation temperature was calculated to be 13784 K by Boltzmann two-line method.The H_?I 656 nm characteristic line is selected,and the full width at half maximum of the Stark broadening of the line is obtained by Lorentz fitting.The electron number density of the plasma is calculated to be 1.27×10¹⁷ cm^-3.The plasma parameters satisfy the local thermodynamic equilibrium conditions.In this thesis,the feasibility of LIBS to quantitatively analyze the wear metal elements in lubricating oil was also verified.Quantitative analysis of four kinds of wear metal elements such as Mg,Ti,Ni and Cr in lubricating oil was carried out.The experiment was carried out with 6 standard oil samples with different concentrations.According to the experimental data,the external standard method and the internal standard method linear calibration model were established.According to the linear calibration model of external standard method,the quantitative detection limits of Mg,Ti,Ni and Cr were calculated to be 3.10?g?g^-1,8.17?g?g^-1,18.79?g?g^-1 and 6.10?g?g^-1,respectively.According to the linear calibration model of external standard method,the concentrations of Mg,Ti,Ni and Cr in the other five standard oil samples with different concentrations were predicted,and the relative errors were 7.43%,8.91%,13.66%and10.40%,respectively;According to the linear calibration model of internal standard method,the concentrations of Mg,Ti,Ni and Cr in the other five standard oil samples with different concentrations were predicted,and the relative errors were 6.59%,8.53%,10.23%and 8.89%,respectively.Comparing the two methods of external method and internal method,it is found that the linear calibration model of the internal method has higher fitting degree than the external method,and the predicted average relative error is smaller,and the prediction data is more realistic and reliable.The results of this thesis will provide a basis for the detection of wear metal elements and early engine diagnosis in engine oils based on LIBS technology.