Preparation,Friction And Wear Properties Study Of C/C-SiC Composites By Multi-processes

With a series of outstanding advantages such as low density,high strength,excellent friction and wear properties,and excellent thermo-physical properties,C/C-SiC composites have been widely used in aerospace field such as the thermal structure materials and aircraft panels.They are also used in high-energy braking areas such as brake pads of high-speed train,aircraft and heavy vehicle.The typical preparation methods of such composites include chemical vapor infiltration?CVI?,precursor infiltration and pyrolysis?PIP?and liquid/gaseous silicon infiltration?LSI/GSI?,but the above single method has some limitations and thus,tends to fail to meet specific material requirements.This dissertation combines the processes of CVI and GSI and exploits the advantages of them to prepare C/C-SiC composites.As the CVI and GSI have complementary advantages,it is expected to improve the densification efficiency of composites and their final density and obtain C/C-SiC composites with advanced properties but low-cost.This dissertation is organized as follows.Firstly,we optimize the parameters that densify C/C porous green body via CVI SiC,then study the structure and properties of C/C-SiC composites prepared from the combined process routes of CVI and GSI,and finally investigate the friction and wear properties and mechanisms of such composites fabricated from various processing routes.Firstly,we studied the effects of the deposition temperature,deposition pressure,deposition time,and the specimen placement position on the deposition rate of SiC,and optimize the better processing conditions.They are:the deposition temperature at1100^oC;the deposition pressure lower than 1kPa;with H₂ as the carrier gas at a flow of600ml/min;with argon as the dilution gas with a flow of 400ml/min;and the upper specimen placement position.Secondly,we studied the effect of different C/C-SiC composite densities after the CVI process on the density,phase and composition,microstructure and mechanical and thermo-physical properties of the composites after the combined processes of CVI-GSI.The results show that,prior to the GSI process,the composites consist of C and?-SiC two phases;whereas,they have a third phase,i.e.the residual Si phase after the GSI process.With the increase of the density of the composites after the CVI process,the density of the composites after the combined CVI-GSI process initially increases and then decreases;the carbon content increases;the residual Si content decreases;the SiC content firstly increases and then decreases;the closed pore proportion initially decreases and then increases.CVI SiC particles are relatively small and have a cauliflower-like surface.SiC layer is dense and continuous.After CVI,the density of C/C-SiC composites is still low.GSI SiC particles are of different sizes angular polyhedral morphology.After GSI,the density of C/C-SiC composites is higher.The mechanical properties increase first,which then decrease with the increasing of CVI C/C-SiC composites density.Specifically,when the CVI C/C-SiC composites density is1.42g/cm³,CVI-GSI C/C-SiC composites have the highest flexure strength,flexure modulus and fracture toughness of 247.50MPa,25.63GPa and 10.08MPa·m^1/2,respectively.With the increasing of CVI C/C-SiC composites density,the thermal conductivity and thermal expansion coefficient of CVI-GSI C/C-SiC composites first increase and then decrease.When the CVI C/C-SiC composites density is 1.42g/cm³,CVI-GSI C/C-SiC composites have a maximum thermal conductivity of60.59W·m^-1·K^-1 and have an average thermal expansion coefficient of 3.80×10^-6/K.When the CVI C/C-SiC composites density is 1.63g/cm³,CVI-GSI C/C-SiC composites have a minimum average thermal expansion coefficient of 2.23×10^-6/K and have a thermal conductivity of 39.88W·m^-1·K^-1.Thirdly,we studied the friction and wear properties of C/C-SiC composites prepared by CVI process,GSI process and CVI-GSI process.The friction coefficient of GSI C/C-SiC composites is about 0.196⁰.375 under self-ring condition.GSI C/C-SiC composites have a largest friction coefficient,highest wear rate and worst braking stability.The friction coefficient of CVI C/C-SiC composites is about 0.214⁰.249under self-ring condition.CVI C/C-SiC composites have a minimum friction coefficient and moderate wear rate and braking stability.Friction coefficient of CVI-GSI C/C-SiC composites is 0.285 under self-ring condition.CVI-GSI C/C-SiC composites have a moderate friction coefficient,lowest wear rate and best braking stability.The results show that the friction and wear properties of C/C-SiC composites can be improved by using CVI-GSI process.Finally,we studied the mechanism of friction and wear.The friction of C/C-SiC composites is the result of adhesive effect and plowing effect.In the earlier and the later stage of braking,the plowing effect is the main factor;in the intermediate stage,the adhesive effect is dominant.The main wear patterns of C/C-SiC composites include grain wear,adhesive wear,oxidation wear and fatigue wear.Several forms of wear usually exist at the same time in the actual braking process,and the occurrence of one wear tends to induce other forms of wear.