Synthesis and characterisation of sol-gel prepared Al2O3 coatings on C45 steel

S sol-gel postopkom lahko sintetiziramo čiste in homogene materiale s širokim naborom lastnosti. Na ta način lahko izdelamo materiale v različnih oblikah, kot so prah, vlakna, ploščice ali tanke plasti. Prednosti izdelave materialov po postopku solgel so nizka temperatura sintetiziranja, enostavnost postopka, čistost materialov tankih plasti in nizki stroški. V okviru magistrskega dela smo po postopku sol-gel izdelali tanko plast Al2O3 na nelegiranem jeklu za poboljšanje C45. Na jeklo smo s potapljanjem nanesli pripravljen sól za izdelavo Al2O3. Sledilo je sušenje prevlek pri različnih pogojih. Prevleke smo nato analizirali s svetlobnim in vrstičnim elektronskim mikroskopom (SEM) in EDXS analizatorjem ter mikroskopom na atomsko silo (AFM). Preiskovali smo tudi vpliv prevleke na razogljičenje in oksidacijo jekla pri temperaturah žarjenja jekla pod AC1 (600 °C in 700 °C) in nad AC3 (850 °C in 950 °C) ter časih žarjenja od &#189ure do 4 ure v zračni atmosferi. Prav tako smo izvedli preizkuse korozijske odpornosti v slani komori po standardu ISO 9227 in preiskali vpliv tanke prevleke iz Al2O3 na korozijsko obstojnost jekla C45. Ugotovljeno je bilo, da se prevleke iz Al2O3 dobro oprijemajo jeklene površine in so razmeroma gladke. Kvaliteta prevlek je odvisna od pogojev sušenja, predvsem od temperature. Kompaktne prevleke brez razpok so nekoliko izboljšale korozijsko odpornost jekla. Prevleka na razogljičenje jeklene površine normaliziranih vzorcev ni imela pomembnega vpliva, je pa nekoliko upočasnila razogljičenje kaljenih vzorcev pri pogojih popuščanja. Rezultati nakazujejo, da pri nižjih temperaturah, ko je prevleka iz Al2O3 še kompaktna in zato manj prepustna za pline, upočasni proces razogljičenja. Pri višjih temperaturah pa prevleke nekoliko upočasnijo oksidacijo jekla glede na difuzijo ogljika, kar se v nekaterih primerih odrazi v nekoliko večjem razogljičenju.The sol-gel method leads to pure and homogeneous materials with wide range of properties. This way the materials can be made in various forms such as powder, fibres, plates or in the form of thin layers. Low temperature of the synthesis, the ease of the process, the purity of thin layers and low cost are main advantages of sol-gel coating technics. In the context of Master\u27s Thesis, a thin layer of Al2O3 film was prepared on a non-alloy steel C45 by sol-gel technic. Al2O3 thin film fabricated by dip coating method was dried under various conditions. Coatings were analysed by an optical and scanning electron microscope (SEM) with attached EDXS detector as well as with atomic force microscopy (AFM). The effect of the coatings on the decarburization and oxidation of the steel was investigated during the annealing at temperatures below the AC1 (600 °C and 700 °C) and above the AC3 (850 °C and 950 °C) and at different annealing times of &#189hour to 4 hours in the air atmosphere. Experiments of corrosion resistance in a salt chamber were also carried out in accordance with ISO 9227 and the influence of a thin coating of Al2O3 on the corrosion resistance of steel C45 was investigated. Al2O3 coatings were successfully deposited onto steel C45. It was found that the coatings of Al2O3 tightly cling to the steel surface and are relatively smooth. The quality of coatings depends on the drying conditions, particularly on the temperature. Compact thin film without cracks is effective in improving the corrosion resistance of steel. The effect of the Al2O3 coating on decarburization of normalized steel was not significanthowever, it slightly lowered the carburization rate at tempering conditions in the case of quenched steel. The results indicate that at lower temperatures, when the coating of Al2O3 is still compact and thus less permeable to gases, slows down the process of decarburization. At higher temperatures, the coating slightly slow down the oxidation of the steel according to the carbon diffusion, which in some cases results in a slightly larger decarburization