| This project is a part of National Natural Science Youth Foundation?51405278?.In this paper,the tribological characteristics of Si3N4-hBN ceramic composites?with different hBN content?sliding against metal were investigated under the different conditions?seawater,dry condition and pure water?.The tribological characteristics and lubrication mechanism of the friction pair in seawater are emphatically discussed.The influences of the external factors?speed,load?and internal factor?hBN content?on the tribological performance are studied in this thesis.The aim of this thesis is to reveal the interaction mechanism between external and internal factors of Si3N4-hBN/metal friction pair in seawater.The MMW-1 type universal friction and wear tester was utilized to carry out the fricition test.Scanning Electron Microscope?SEM?,X-ray Diffraction?XRD?,X-ray Photoelectron Spectroscopy?XPS?and Raman Spectroscopy were used to analyze the microstructure and phase composition of the worn surface.The relevant research works are displayed as follows:?1?Frist,the effect of load and velocity on the tribological properties of Si3N4-hBN/GCr15 friction pair was studied.The results show that the increase of the load can not effectively reduce the friction coefficient and the wear rate of the friction pair.When the load was continuously increased to 30 N,the friction coefficient was 1.41 and the wear rate can reach 8.8×10-5mm3/Nm.In addition,with the increase of the speed,the friction coefficient and the wear rate were significantly reduced.On the whole,when the load is 10 N and the speed is 1.73m/s,the Si3N4-hBN/GCr15 friction pair obtain the best tribological characteristics: the friction coefficient is as low as 0.03,and the wear rates of the pin and the disc were 1.4×10-5mm3/Nm or less.When the speed is constant?0.52m/s?and the load is 10 N,the wear mechanism of the friction pair is mainly dominated by the mixture of adhesive wear and corrosion wear.In this case,the friction coefficient of Si3N4-10%hBN/GCr15 can be reduced to 0.2,which is mainly due to tribochemical reaction on the wear surfaces.Then,the tribochemical reaction products composed of B2O3 had a good lubrication effect,and the sliding pairs entered to boundary lubrication state.When the load increased to 20 N and 30 N,an effective protective material cannot be formed on the wear surface,and the friction coefficients were at a range from 0.4 to 1.4.When the load is constant?10N?and the velocity is 0.86 m /s,the friction coefficients of friction pair was at a range form 0.21 to 0.48.When the sliding speed increased to 1.73m/s,the friction coefficient of the friction pair was reduced to within the confines of 0.03 to 0.25,which is mainly attributed to the promotion of tribochemical reaction on surfaces by increaseing in speed.In such a setting,a more complete self-lubricating material was formed on the wear surfaces of sliding pairs,and contributing to the fluid lubrication state.?2?Under the dry condition,the friction coefficient of the Si 3N4-hBN/GCr15 friction pair is about 0.49,and the wears rate of the pin and disc are higher,the wear mechanism of the friction pair is mainly dominated by the mixture of abrasive wear and adhesion wear.In pure water,the friction coefficient of friction pair can decreases to 0.06 and the wear rate of the pin was reduced to 3.23×10-6mm3/Nm in the same load and speed to the dry condition.But,compared with the seawater condition,the friction conefficient and wear rate of firction pair in pure water are higher.?3?The addition of hBN significantly reduces the friction coefficient and wear rate of the Si3N4-hBN/GCr15 friction pair in the pure water and seawater.For example,when the load is 10 N and the sliding speed is 1.73m/s,with the addition of hBN,the friction coefficient decreases from 0.26 of Si3N4 to 0.03 of SN10 in the seawater.This is mainly due to two following aspects:on the one hand,hBN have self-lubricating attribution and play a certain anti-friction role on the wear surfaces;on the other hand,Si3N4 and hBN can react with water in the friction process,It was found that hBN was peeled off form Si3N4-hBN during the friction and wear tests and some spalling pits were formed on the wear surfaces of the Si 3N4-hBN,then some reaction products accumulated in the spalling pits and reacted with seawater,a tribochemical film was covered on the wear surfaces.?4?Si3N4-hBN composite ceramics show more excellent tribological properties in seawater.It has been found that the reaction product SiO2 was present in the form of colloids in the seawater.Because of the absence of Na+,Cl-plasma agglomeration in pure water,SiO2 was easily carried away by water,which results in the formation of very thin lubricating film on the wear surfaces.The thin lubricating film can not effectively lubricate the surfaces.Meanwhile,Na+,Cl-in seawater can aggravate the condensation of SiO2 on the friction surfaces into silica gel,and Ca2+ and Mg2+ can deposit siliceous CaCO3 and Mg?OH?2 on the friction surface of GCr15.So,the friction pair entered to the state of boundary lubrication.Therefore,the seawater is a admixture lubrication for the friction pair,and friction pair sliding in seawater has better tribological properties compared with dry friction and pure water.Results show that Si3N4-hBN/GCr15 friction pair has good tribological properties in seawater.This not only provides technical guidance for the development and application of marine engineering equipment,but also enriches and consummates the ceramic tribology theory.This paper has important engineering application value and scientific research significance. |
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