Preparation And Friction Behavior Of Nano-micro Composite Ceramics

Advanced ceramics as engineering materials with promising application have attracted extensiveattention, due to their superior properties such as high temperature stability, chemical stability and wearresistance. Of various ceramics, Al2O3-based ceramics as the most widely used essential structural materialshave promising applications in electronic, machinery, and aerospace industries. However, their applicationsand development are highly limited by relatively low fracture toughness, poor thermal shock resistance andpoor creep resistance. To overcome that drawback, many researchers have tried to improve the mechanicalproperties of alumina ceramics though doping polyphase materials. Particularly, alumina ceramicstoughened with phase transformable tetra-ZrO₂as the dispersed phase and ductile phase have been highlyfocused on in the field of ceramics.Bearing those perspectives in mind, and noticing that CN_x:H film, as an excellent antiwear andfriction-reducing protective candidate, has low friction coefficient, good lubricity and good chemicalstability, while alumina ceramics have similar thermal expension coefficient as CNxfavoring to reduce thethermal expension stress of ceramics-CNx:H film thereby increasing the film-substrate bonding strength, inthe present research we design and fabricate ZrO₂/Al2O3composite ceramic powders via asol-coprecipitation route with ZrO₂as the continuous phase and SiO₂additive as the glassy phase forceramic sintering. As-obtained ZrO₂/Al2O3composite ceramic powders are sintered at ambient pressure togenerate ZrO₂/Al2O3multiphase ceramics. Moreover, CN_x:H film is deposited on the surfaces of thesintered composite ceramics so as to remove defects thereon and thereby reducing friction coefficient andincreasing wear resistance of the ceramics. The main contents of this thesis are as follows:1. Preparation of ZrO₂nanoparticles modified by SiO₂and analysis of their morphology and grainsizeZrO₂nanoparticles modified by SiO₂are prepared from ZrOCl₂·8H₂O as the starting material anddiethoxydimethylsilane as the silicon source in the presence of water as the solvent or mixedn-propanol/water as the solvent. The morphology and grain size of resultant ZrO₂nanoparticles as well asthe bonding mode of SiO₂with ZrO₂are analyzed. Results show that when SiO₂is present in the reaction system, as-obtained ZrO₂grains are refined with increasing reaction time, and the agglomerationphenomenon is greatly retarded. In the mixture of n-propanol/water, ZrO₂nanoparticles with sheet-likestructure are obtained under the same experimental condition as that in water. On the one hand, SiO₂acts asthe center of heterogeneous nucleation to favor refining of ZrO2grains, while zirconium inorganic salt iscompletely hydrolyzed and/or alcoholyzed completely in the mixed solvent of n-propanol/water, resultingin ZrO2nanoparticles with a grain size of less than10nm. On the other hand, SiO₂may function as astructural manipulating agent to allow oriented growth of ZrO2nanoparticles. In the meantime, SiO₂ischemically combined with ZrO2via Si―O―Zr bond, benefiting to retard the agglomeration of as-preparedZrO2nanoparticles. In one word, ZrO2nanoparticles can be readily prepared by making use of thealcoholysis of inorganic ZrOCl₂·8H2O, and the approach may be well extended to prepare other similarfunctional inorganic nanomaterials.2. Preparation of ZrO2/Al2O3nano-micro composite ceramics and investigation of their propertiesZrO2/Al2O3nano-micro composite ceramic powders are prepared from readily available ZrOCl2·8H2O oflow cost and commercial α-Al₂O₃as the starting materials via sol-coprecipitation route withn-propanol/water as the mixed solvent. The composition and microstructure of as-obtained ZrO2/Al2O3composite powders have been analyzed. It has been found that ZrO2is the continuous phase and SiO₂is theglassy phase in the composite ceramic powders, and they both well surround micron α-Al₂O₃. With theincrease of the mass ratio of ZrO₂:Al₂O₃(from20:80to30:70,40:60, and50:50), the agglomeration ofZrO₂surrounded by Al2O3tends to be serious. When the mass ratio of ZrO2:Al2O3is20:80, SiO₂-modifiedZrO₂/Al₂O₃obtained in the mixed solvent of n-propanol/water has a very small grain size and possessesgood dispersion capability as well as excellent mechanical properties. Corresponding ZrO2/Al2O3composite ceramics obtained by sintering of the composite ceramic powders at1600℃has a high density(about4.16g/cm3, relative density about99%) and hardness (Vickers hardness is about2123HV, which is40%higher than that of ZTA prepared from commercial ZrO₂/Al₂O₃and50%higher than that of aluminaceramic made from commercial α-Al₂O₃), showing potential application in engineering.3. Preparation of hydrogenated carbon nitride films on silicon substrate and evaluation of theirfriction and wear behaviorEthylenediamine is used as the precursor to prepare hydrogenated-carbon nitride (CN_x:H) films on silicon substrate via CVD route (pyrolysis of ethylenediamine). The microstructure of as-prepared CNx:Hfilms on Si substrate has been analyzed, and their friction and wear behavior has been evaluated. Resultsindicate that carbon in as-prepared CNx:H films exist mainly in the form of C═C. Increasing growthtemperature gives rise to a decrease of N/C ratio and contributes to formation of CNxcomposition. TheCNx:H films obtained at relatively lower temperatures of700oC and800oC are amorphous and have pooradhesion to Si substrate and hence poor wear resistance as sliding against stainless steel counterpart.Although the CNx:H film prepared at900oC has next to the lowest N/C ratio of0.02, it consists ofnanocrystalline and possesses the maximum adhesion to Si substrate, therefore it shows the best wearresistance among the four kinds of tested carbon nitride films. It seems that the incorporation of a smallamount of N atoms into the crystal lattice of graphite forming CNxcompounds in carbon films and resultsin improved wear resistance; and the antiwear life of as-deposited carbon nitride films is highly dependenton their thickness.4. Preparation of hydrogenated carbon nitride films on ceramic substrate and evaluation of theirfriction and wear behaviorHydrogenated-carbon nitride (CN_x:H) films are prepared on ceramic substrate via the pyrolysis ofethylenediamine in a temperature range of900¹000oC. The microstructure of as-prepared CNx:H films onZrO2/Al2O3ceramic substrate has been analyzed, and their friction and wear behavior has been evaluated.Results show that carbon in as-prepared CNx:H films exist mainly in the form of C═C, while N inas-prepared CNx:H films and Al in ZrO2/Al2O3composite ceramic substrate form N―Al bond. Besides, theCNx:H films formed on the composite ceramic substrates with different surface roughness have similarfriction coefficient (about0.20) but different antiwear life. In the meantime, N in the CNx:H films and Al inthe composite ceramic substrate form N―Al bond, which contributes to increase the bonding strengthbetween the films and the ceramic substrate and hence significantly improve wear resistance of the films.This is why the CNx:H films formed on the composite ceramic substrate have better wear resistance thanthe same films formed on Si substrate.