Research On Grinding Surface Microstructures And Tribological Characteristics Of Woven Ceramic Matrix Composites

Woven ceramic matrix composites(WCMC)not only inherits the advantages of high specific stiffness,high temperature resistance,corrosion resistance and low density of ceramic materials,but also has excellent performances of high specific strength,high toughness and abrasion resistance due to the reinforcement of woven fibers.Therefore,this kind of materials are widely used in many advanced technical fields.As a new type of materials,WCMC are faced with difficulties and challenges different from traditional materials when mechanically processed and applied to practical engineering scenarios reasonably and reliably.In order to realize the wide application of WCMC in industrial fields,it is necessary to study the processing technology and performance of WCMC.Before that,it is essential to establish an objective and effective WCMC surface evaluation system.However,due to the complex structural composition of WCMC,such materials have obvious anisotropy and heterogeneity.As for the research on surface evaluation technology,grinding technology and tribological characteristics,many traditional theories and models applicable to homogeneous materials cannot be applied in the related researches of WCMC.Therefore,it is urgent to establish a new theoretical system suitable for WCMC to guide the design,processing and application of such materials.This paper tentatively presents a grading surface evaluation system of WCMC.The system consists the levels of fiber,fiber bundle,cell body and whole surface.At the fiber level,the typical forms of fiber damage and their effects on the surface morphology and tribological properties of WCMC were analyzed,which laid a foundation for the measurement and evaluation methods at the next three levels.At each subsequent level,the system proposes a set of surface measurement sampling parameter determination method and surface quality evaluation method based on the principle of statistics.In terms of fiber bundle scale,this paper proposes to use 2D evaluation method for fiber bundle surface evaluation,and demonstrates the influence of sampling direction,sampling length,sampling group number and sampling step on fiber bundle surface measurement results.Four evaluation indexes,namely Ra_ave,Rq_ave,Rsk_ave and Rku_ave,were proposed to evaluate the roughness,damage type and severity of fiber bundle surface.In terms of cell body scale,this paper points out that 3D evaluation method should be used for cell body surface evaluation,and the determination method of maximum allowable sampling step is proposed based on confidence interval theory.On the whole surface scale,this paper demonstrated the relationship between the cell body measurement results and the whole surface morphology,and proposed an evaluation method of the whole surface morphology based on the characteristic fiber area ratio.Taking a typical WCMC-carbon fiber reinforced silicon carbide ceramic matrix composite(C_f/Si C)as an example,the evaluation method is illustrated.Through the example verification,this set of parameters and methods can realize the non-distortion and objective evaluation of C_f/Si C surface.The evaluation results can also directly reflect the real morphology of the surfaces and establish a relationship with the tribological properties of the materials.Through theoretical derivation and experimental verification,this paper demonstrates that the traditional grinding force model based on the maximum undeformed chip thickness theory cannot be used to estimate the grinding force of WCMC.Through mathematical derivation,a grinding force model based on variable specific grinding energy is proposed,and its effectiveness is verified by an example of quartz fiber reinforced silica ceramic matrix composite(Si O_2f/Si O₂).Then,on the basis of single abrasive particle grinding analysis,this paper carried out research on grinding of wheel on the surface of WCMC,established grinding force model of grinding wheel,and studied the influence of different grinding parameters on grinding force and ground surface quality through experiments,so as to determine the optimal combination of grinding parameters and the optimal selection method.Next,this article combines the acoustic emission(AE)technology,surface evaluation technique and scanning electron microscope(SEM)observation,to study the AE signal processing method and frequency characteristics in the process of single abrasive grain and grinding wheel grinding of Si O_2f/Si O₂,and put forward a kind of grinding damage type recognition and damage degree of quantitative evaluation method based on AE characteristic frequency.The relationship between grinding process parameters of Si O_2f/Si O₂ and the workpiece processing damage as well as surface quality after machining is established,so as to provide a theoretical fundament for grinding process planning,machining process status monitoring and machining quality evaluation of WCMC.In this paper,the tribological characteristics of the fiction pairs of the WCMC and ceramics are studied.Firstly,the influence on tribological properties of WCMC processing angle,surface processing parameters and the surface microstructure of the ceramic pairs,was studied under the same scale of external experimental conditions.Then,the influences of friction time,load,speed and temperature on the friction pairs of WCMC and ceramics are studied.The fiction and wear mechanisms are also analyzed.During the researches above,the selected WCMC is C_f/Si C,and the counterpart material is zirconia(Zr O₂)and silicon nitride(Si₃N₄).Aiming at the special case of working condition of high temperature,friction pair is optimized.The friction pairs of carbon fiber reinforced carbon matrix composite(C_f/C)with Zr O₂ and Si₃N₄are selected.The influence of the temperature,microstructure on the tribological properties are studied,and the friction and wear mechanism of the high temperature conditions are analyzed.This paper provides a theoretical basis for the wide application of WCMC in the fields of high temperature friction.The results show that the friction coefficient lower than 0.2 can be obtained in the range of room temperature to 1000?under dry condition,by reasonably selecting the friction pair material and controlling the surface microstructure of the materials.