Study On The Preparation, Performances And Mechanism Of The SiO_x Film Deposited On Aluminum And Aluminum Alloy

Ceramic film coated on metallic substrate, widely used in aerospace, aviation, electronics and electrics, has high toughness, intensity and conductibility of the metal and high hardness, good wear resistance, heat resistance and corrosion resistance of the ceramic. The silicon rich oxide (SRO) has good properties of hardness, insulation, thermal conduction, etc; and many studies of it have been carried out on optical, electronics, mechanics and engineering performances. This accommodate the SRO a widely use in photo electricity apparatus, integrate circuit, barrier materials, etc. Having many potential applications, this research have not been widely studied with a technical difficulty.In this dissertation, a new kind of SRO film is deposited on Al and Al alloy substrate by low temperature APCVD approach. The pretreatment and the process parameters are studied. The results show that, with the increase of temperature, there is an optimal point at about 400°C. The flux rate of the diluted silane and air are fixed at 0.2 L/min and 0.3 L/min optimally. The growth rate of the SRO film increases with the flux rate of nitrogen carrier, which is fixed at 2 L/min. The thickness of the film increases with the deposition time increasing. SEM results demonstrate the various morphologies of the SRO film in different conditions. The film surface is stacked and packed up by a number of SRO cell units with some gaps. The growth process is supposed to be followed: After silane and oxygen chemical reacted and nucleated on the surface, the nascent nucleus grow up and present a spherical or equaxial shape cell packed on the surface. The cells press each other and stack up with some gaps. With the increase of deposition time or the proceeding of anneal, the cells coalescence and grow up with a contraction of the gaps.The SRO film cured and developed after the anneal process has been investigated in this dissertation. SRO cell units grow up and coalescence in anneal process, formed in chain-like or layer-like morphology. With the increase of anneal temperature and heat preservation time, much more cell units coalescence and grow up. Besides, due to the Al, Si and 0 atoms inter diffusion between the SRO film and the substrate, the film thickness increases in the anneal process.Thermal dynamics and kinetics analysis shows that: the APCVD reaction of the SRO film is spontaneous under the optimal process condition; the Al-O-Si complicated crystalline structure is formed by the inter-diffusion of the Al, Si, 0 atoms. The velocity of nucleation and film growth is dependent of pressure, temperature, interfacial energy, interfacial contact angel, diffusional activation energy of the deposited surface, nucleation energy barrier of the vapor reaction, surface desorption energy, etc. Because the atmospheric pressure is higher than usual CVD and the nucleation energy barrier of the vapor reaction is low, the reaction in this experiment is very acute. This leads to a high nucleation rate and a non-crystalline microstructure. Besides, high reaction rate increases the Si dangling bonds and the density of defects in SRO film.XPS results show that the O/Si ratio of the different region in the SRO film stabilized at 2.2 - 2.4. It is indicated that there are many Si-OH bonds in the film and the composition does nothing with the deposition time. There are much more Si-OH bonds in the near surface region than in the interior, because of the saturation of the OH in vapor phase and absent of the dehydration inside the film. The results also show that there is a widely transition layer between the SRO film and the substrate, indicated the inter-diffusion of the Al atoms and Si, 0 atoms. The Al and Si atoms supposed to be formed an Al-O-Si bonding structure and the 0 atoms play a role of bridge oxygen. A complicated spinel like crystalline structure is formed in some oxygen rich region and shows a great bonding ability to the substrate. XRD results show that a crystalline SiO₂ microstructure is also appeared in some oxygen rich region with the absent of Al atoms.The TEM, TED and HRTEM results show that the SRO film is not totally non-crystalline but with a little crystalline structure in some oxygen rich region. This is from the beginning of the film growth; the surface of the substrate is coated with a layer of alumina and offers an oxygen rich environment; so in the nascent SRO film, the silicon dangling bonds are very easy to be saturated by the oxygen in the alumina and some of them grow up in crystalline structure with atoms migration or diffusion. The experiments prove that the formation of the crystalline structure is independent of the deposition time.TEM results show that the full growth process supposed to be a series of nucleation and growth: After silane and oxygen reacted on the surface, the nucleus grow up with the continuously supply of feed gas and formed an island-like micro structure; Because of the strong bonding between the SRO, the wetting ability of the interface is enhanced and bring out a result of that the growth rate of X-Y direction is faster than that of Z direction; The island-like micro structures develop and construct a laminar structure when they contact. With the repeat of the nucleation, growth and coalescence, the laminar structures stack up and show a SEM morphology of SRO cells.PL and IR results show that, the microstructure of SRO film is constructed of many Si-O-Si irregular ring nets with a lot of Si dangling bonds, Si-Si bonds and Si-OH bonds in it. Because of the bridge oxygen vacancies in the Si-O-Si bond, the Si dangling bonds are formed; some neighbors of them connect and form Si-Si bonds; a few of them form Si-OH bonds or Si-H bonds by saturated of the OH group or H atoms in the vapor phase. The PL results show that the impurities in the SRO film increase the density of the defects. This consequence can induce the photoluminescence intensity. The results also show that the film microstructure is more complicated with the increase of deposition time.In this dissertation, the bonding property, mechanics, wear and optical absorption performances of the SRO film deposited on Al or Al alloy substrates are studied, respectively. The nick test shows that the SRO film is still not scaled out of the substrate in the condition of an 80 N load and a 1.24 GPa shear stress. It is definitely that this good bonding performance bases on the strong bonding ability between Al-O and Si-O. The micro Vicker's hardness measurement and the impress observation shows that the hardness of Al and Al alloy is obviously increased by deposited a coating of SRO film on it. Due to the brittle and porous property, the SRO film is very easy to be collapsed and fragmentized. This results in a irregular shape of the impress. With the increase of the load, the SRO film is plastic deformed with the substrate deformation. The results of wear experiment show that the wear resistant property of the Al or Al alloy is effectively enhanced by deposited a coating of SRO film. The wear mechanism indicates that the SRO cell units are easy to be collapsed and press to fill the gaps. The tiny granules formed in this process play a role of abrasive grains and wear the substrate. The wear loss increases linearly with the increase of wear time and the wear rate decreases with the increase of deposition time.UV-VIS and IR reflection results show that the SRO film deposited on Al or Al alloy present an excellent optical absorption in the wave length scale from ultraviolet to middle infrared. The reflection rates are basically below 30%. The reasons of the absorption are: Firstly, the ray is scattered reflected because of the surface fluctuation; Secondly, the gaps among the packed SRO cell units; Then, the energy of the ray is consumed due to the increase of transmission distance; Besides, much ray of different wave length is absorbed due to the dangling bonds, oxygen vacancies and other defects. PL results show that the SRO film is photo luminescent in large scale under laser activation. The mechanism is the transition of electrons in defects under laser activation. The results also show that the photo luminescent intensity of Al alloy is great than that of Al, because there are much more defects formed in the SRO film due to the diffusion of other element.A heat spray process of PE has also been carried out after SRO film deposition. The results show that the sticky PE enter the gaps among the SRO cell units and consequently increases the adhesion of the PE coatings to the SRO film.