Tribology And High Temperature Oxidation Properties Of ZrB₂-SiC Ceramics Synthesized Via PDC Method

Ceramics have become indispensable materials for a wide range of industrial applications due to their excellent properties.However,the traditional preparation of ceramic materials is often time consuming and involves high sintering temperatures.These results in considerable energy consumption and high production costs,which limit their application in some industries.In recent decades,the advent of polymer derived ceramics has enabled the application of ceramics to fibers,composites,coatings and films,mainly due to their excellent design,process and low-temperature ceramic properties.Among all the ceramics,ultra-high temperature ceramics?UHTCs?have been extensively used due to their high melting point?>3000^oC?,good ablation resistance,and excellent mechanical strength at high temperatures.As a result,they are often used in harsh working environments such as re-entry atmospheres and hypersonic flight components.However,with the development of technology and application,application environments became complex.Thus,components in the aerospace industry,such as ball bearings and engine valves often require a combination of antioxidant and anti-friction capabilities to accommodate harsh service environments.In UHTCs,zirconium boride-silicon carbide?ZrB₂-SiC?is widely studied due to its lowest theoretical density of 6.09 g/cm³.As a structural material,ZrB₂-SiC ceramics have been mostly investigated from the perspective of ultra-high temperature performance.Although,research has focused on improving its high temperature oxidation and ablation resistance,not much has been explored from a tribology perspective.Tribo-properties of materials are usually related to their mechanical properties,especially fracture toughness.Compared with the traditional preparation methods,UHTCs prepared by polymer derived methods reduce the oxide impurities in the grain boundary and improve their mechanical properties.Additionally,nano-ceramics can be prepared,where nano-sized particles can effectively reduce the size of grain boundary cracks.The overall mechanical properties of ceramics,especially fracture toughness,can be effectively improved,thereby enhancing their anti-wear ability.Utilizing these benefits,nano-composite ZrB₂-SiC ceramics were prepared by polymer derived ceramics method,and their wear characteristics and antioxidant properties were studied.The research focused on the following preparation methods and characterization of the prepared ceramics:1.ZrB₂-SiC nanocomposite powder was successfully synthesized via a novel one-pot polymeric precursor route.By using the structure design of polymer precursor and regulating the curing-cracking process,a uniform composite phase ceramic powder was prepared,that is,each ceramic particle is composed of two or more kinds of materials.On the one hand,this method effectively improved the dispersion of each phase and reduced the mechanical properties caused by abnormal growth.On the other hand,nanometer-level control of particle size of the powder reduced the interfacial defect size of ceramic after sintering,thereby significantly improving its fracture toughness.The pyrolyzed result revealed that the powder consisted of uniformly distributed spherical Zr B₂ nanoparticles embedded within a SiC matrix,and the mechanism of particle growth followed the Ostwald ripening phenomenon.The outer SiC was found to have an obvious inhibitory effect on the Zr B₂ particle growth.The fracture toughness of ceramic after sintering was improved,and the microstructure analysis revealed an intergranular fracture characteristic for crack propagation when the second phase particle size was small.2.The tribo-properties of ZrB₂-SiC ceramics were studied experimentally,and the wear mechanism of ceramics under different composite ratios was analyzed.The hardness and fracture toughness were the two main variables that influenced the calculation of wear in the common ceramic wear formula.With hardness and fracture toughness known,pressure and distance were the main variables in most studies and calculations.It was observed that the particle sizes of second phase Zr B₂ caused the changes in mechanism of ceramic wear.Considering the pressure as a variable,for small sized second phase grains,a micro-crack wear mechanism was obtained.For larger grain sizes,the second phase in matrix was prone to transgranular fracture.With the changes of wear distance,the damage mode caused on the wear contact surface was mainly determined by the degree of abrasive accumulation.Higher accumulation of abrasive debris led to greater damage of the abrasive contact surface.Beyond a certain level of abrasive debris accumulation,the abrasive wear was the main damage mechanism.3.SiC powders doped with aluminum or yttrium were prepared by the polymer derived ceramic method,and then mixed with commercial Zr B₂ for hot press sintering process to improve ZrB₂-SiC anti-oxidation properties.The doping element was found to reduce the activity of Si O₂ at elevated temperatures,thus greatly improving the oxidation resistance of ZrB₂-SiC ceramics.Compared with aluminum,the diffusion of yttrium to the surface at the initial stage of oxidation resulted in a rapid oxidation.However,once the surface of the yttrium concentration exceeded a certain value,the oxidation rate decreased.Thus,the anti-oxidant properties of ZrB₂-SiC ceramics doped with yttrium were found to be better than aluminum doped ceramics.