Process And Mechanism Of Plasma Electrolytic Oxidation On The Hot-dipping Aluminum Coatings And NiTi Shape Memory Alloys Substrate

Using the methods of plasma electrolytic oxidation (PEO), the structures,the phasecompositons and the properties of the ceramic coatings on Q235 steel, industrial pure Tiand NiTi shape memory alloy (SMA) were studied in this thesis. The main contents areas follows:Firstly, a hybrid method of hot-dipping aluminum (HDA) and PEO was employed tofabricate composite ceramic coatings on the surface of Q235 steel substrate. Thecross-sectional microstructure and surface morphologies were investigated with scanningelectron microscopy (SEM). The X-ray diffraction (XRD) was used to analyze the phasecompositon of the ceramic coatings. Particularly, the transmission electron microscopy(TEM) was used for selected area electron diffraction and to observe the structure of theceramic coatings. The evolutions of PEO behaviors changing with PEO time and currentdensity of the HDA coatings on Q235 steel substrate were analyzed. And on this basis,the ratio of the ingrown and outgrown coatings was investigated, the model of therelationship between the film formation, the surface phenomena of the coatings and thecorresponded voltage region was given. The cross-sectional microhardness, the wearresistance, the porosity and the corrosion resistance of the HDA-PEO treated specimenswere studied. At last, the film formation mechanism during the PEO process of the HDAcoatings on Q235 steel substrate was analyzed. The results indicate that: (1) Thecomposite coatings obtained by this hybrid method consists of three layers from inside tooutside, i.e. Fe-Al alloy layer next to the substrate, aluminum layer between the Fe-Allayer and the ceramic coatings which is as the top exterior layer. Metallurgical bondingwas observed between every of the two layers. There are many micro-pores andmicro-cracks in the PEO coatings. The grain size is biger and biger, the re-melting andsolidification of the grains is severe more and more and the size of the pore in theceramic coatings increases gradually with the increasing of the PEO time prolonged. (2)The phase composition of the ceramic coatings is mainly composed of crystal Al₂O₃oxides and a certain quantity of amorphous phase. The crystal Al₂O₃phase includesκ- Al₂O₃,θ- Al₂O₃andβ- Al₂O₃. Compared with the others, theβ- Al₂O₃content is the least.The amorphous phase convertes to the crystal phase gradually with the PEO timeprolonged. The grain size of theκ- Al₂O₃crystal is quite non-uniform. The smallest oneis less than 100 nm and the largest one is more than 500 nm. One of the largest grains isover the TEM diffraction zone and the single crystal diffraction patterns can be obtained.(3) The total thickness, outgrown thickness and growth rate have maximum values withthe PEO time prolonged. The time when the maximum value appears decreases with thecurrent density increasing. In our experiement conditions, the maximum value of the totalthickness is about 47.5μm and the corresponding current density and the PEO time are1.5A/dm²and 12min respectively. The ingrown thickness of the ceramic coatingsincreases with the PEO time prolonged at any current density. The ingrown dominantturns to outgrown dominant little by little with the current density increasing. (4) Thehardness of the ceramic coatings is about HV1300. When the wearing sliding distance is70m, the wear resistance of the PEO samples is about 2.3 times higher than that of theheat treated 45 steel. The corrosion resistance of the PEO coatings is much better thanthat of the Q235 steel when immersed in the NaCl solution or in the sea water.Secondly, the ceramic coatings was deposited on the surface of the pure industrialtitanium by PEO method. The surface and cross-sectional morphologies of the ceramiccoatings were observed with SEM. The phase composition of the ceramic coatings wasanalyzed with XRD.The influence of several different additions on the corrosionresistance, the surface morphology, the component and the phase compositon of theceramic coatings was investigated. The PEO manufacturability of the pure industrialtitanium was studied. The results show that: (1) The surface of the ceramic coatingsobtained in the mixed electrolyte containing Na2SiO3, NaAlO2and NaH2PO4is even.There are many honeycomb-like and over lap micro-pores and invisible cracks in it. Thebongding of the ceramic coatings and the substrate is strong. (2) The phase compositionof the ceramic coatings obtained in the mixed electrolyte is mainly composed of anatase.(3) The element in the electrolyte takes part in the film formation. Besides large amountof Ti and O elements, there are little amount of Al, Si, P elements and the elements fromthe additons in the electrolyte such as B, Ce, et al. in the ceramic coatings. (4) The addition of Ce makes the contents of the Al, Si, O and P elements increase significantlyin the ceramic coatings, and the corrosion resistance of the ceramic coatings improveremarkablely.Thirdly, the ceramic coatings was obtained on the NiTi SMA substrate with PEOafter the pretreatment of selective delloying to remove the Ni from the surface of NiTiSMA. The PEO surface morphologies and the PEO manufacturability were studied of thesamples which were not pretreated and pretreated by immersed in the solutions of HNO3,H₂O:HNO₃:H₂SO₄=5:4:1 (V) and aqua regia respectively. The influence of thepretreatment technology on the surface morphologies, the ingredient and the phasecompositions of the ceramic coatings were analyzed. The results indicate that: (1) ThePEO manufacturability of the samples not pretreated by selective delloying was very bad.The selective delloying pretreatment technology can improve the PEO manufacturabilityof NiTi SMA prominently.When the dealloying time is 10 min in aqua regia at roomtempreture, the PEO manufacturability of the NiTi SMA is best and the curve ofcurrent-time is most close to that of the pure industrial titanium at the same experimentalconditions. (2) The ingredient of the electrolyte which is suitable for PEO of the NiTiSMA is as follows: NaH₂PO₄5g/l, NaAlO₂5g/l, Na₂SiO₃, TiO₂1g/l and H₂O₂5ml/l. (3)The main elements of the ceramic coatings on NiTi SMA are Ti, Al, O, P and Si. Thephase compositon is mainly composed of anatase. And the quantity of the anataseincreases with the selective delloying time prolonged immersed in aqua regia solutions.(4) The corrosion uniformity of the NiTi SMA is best when the delloying time is 10 minimmersed in aqua regia solutions and the spongy surface morphology can be seen withthe SEM. The texture of the PEO ceramic coatings on the NiTi SMA after delloying isdesired. There are many micro-pores in the honeycomb-like coatings which is beneficialto the adherence of the hydroxylapatite.