Zirkonya esaslı seramik kaplamanın fiziksel ve kimyasal özelliklerinin değiştirilmesi

Abstract

ÖZET İleri teknoloji seramikleri içinde zirkonya fiziksel ve kimyasal özelliklerinden dolayı üzerinde en çok araştır ma yapılan malzemelerdir. Bu çalışmada metaller üzerine alevle püskürtme tekniği ile kaplanan kısmen stabilize edilmiş zirkonyan (PSZ) özelliklerinin daha da iyileştirilmesi ele alınmıştır. Bilhassa kaplandığı metali korozyona karşı korumasına engel olan ve kullanım alanını daraltan gözenekli yapısının değiştirilmesi hedeflenmiştir. Kaplanmaları esnasında metal yüzey sıcaklıklarının maksimum 200°C olması nedeniyle alevle püskürtme tekniği tercih edilmiştir. Bu teknikle elde edilen zirkonya kap lamanın %20 ye varan gözenekliliğini ortadan kaldırabilmek için çeşitli metal tuzları denenmiştir. Bu tuzlardan kap lamanın gözenekleri içinde bozunarak dayanıklı oksitler verebilen potasyum-krom şapı, amonyum bikromat ve kobalt nitrat' in doymuş çözeltileri kullanılarak geçirgen olmayan bir yapı oluşturulmaya çalışılmıştır. Bu yeni yapıdaki zirkonya kaplamanın ne gibi özellikler kazandığını tesbit etmek için çeşitli deneyler yapılmıştır. Korozyon direncini ölçmek için asidik ve bazik ortamlardaki davranışları incelenmiştir. Bu deneylerin sonuçları mukayese edilerek yüksek korozyon direnci sağla yan metal tuzu ve gözenek doldurma işlemi tavsiye edilmiş tir. Doymuş çözelti emdirilerek yapılan gözenek doldurma işlemlerinin ne şekilde gerçekleştiği deneysel ve matematiksel olarak ifade edilmiş, aralarındaki bağıntılar araştırılmıştır. Elde edilen yeni seramik yapının sertlik değişimleri de benzer şekilde ele alınmıştır. Termal şoka karşı daya nımlarını etkileyen parametreler ile termal şok esnasında meydana gelen gerilmelerin hesaplanmaları üzerinde çalışılmıştır. Diğer fiziksel özellikleri de dikkate alınarak kaplamanın kullanım alanlarına etkileri incelenmiştir. Bilhassa amonyum bikromat tuzu kullanılarak gözenek lerde Cr203 oluşturulması sonucunda kaplamanın yüksek korozyon direnci sağladığı ve 1400 vickers'e varan sertlik değerlerine çıkıldığı tesbit edilerek, yeni kullanım alanları üzerinde durulmuştur. - vı -

CHANGING PHYSICAL AND CHEMICAL PROPERTIES OF ZIRCONIA BASED CERAMIC COATING SUMMARY In this study, it is intented to remove the porosity of coating which is applied on the cast irons and low carbon steels by zirconia. Zirconia was applied on the metal surfaces by flame spray technique. On coating that carried out by this technique approximately 20% porosity takes place. Porosity increases the thermal barrier of the coating but, decreases the mechanical properties and corrosion resistance of material. Corrosive gas and ions can easily penetrate to the metal through these porous coating. Zirconia was coated on standart samples. Cast iron samples were 105x105x3 mm in size, steel samples prepared were 105x105x1 mm. The surfaces on samples were cleaned with various ways and roughness were attained by sand blasting. The NiCrAlY adherent was applied on the samples first. On it 1 mm thick zirconia was coated. The physical and chemical properties of coatings was determined afterwards. The properties of ceramic dusts used in coating are given below: Chemical composition Zr02 (95%) +CaO (5%) Grain size range `` ' ` Apparent density Macro-hardness approx. Maximum used temperature Particle shape Linear coefficant of expansion 10 -11x10`^ Powder consumption per 0,1 mm coat thickness approx. 0,54 Kg/m2 It was observed that prepared samples contain 12% pores. To fill the pores with resistant oxides, various metal salts were tested. Saturated solutions of metal salts were absorbed. Samples were heated at 10°C/min. heating velocity. Metal salts remained in pores were decomposed and oxide were formed. Maximum furnace temp, was selected as 550°C. To attain necessary binding of oxides to coating samples were held in furnace at given temperature 1 hour. - vix -Various salts were tested to fill the pores. Good results were obtained with KCr (SO.) ~. 12H`0, (NH.)`Cr207 and Co (NO.,) `. 6H`0. These salts were used during the study. The decomposition reactions of these salts are as following 2(KCr(S04)2.12H20 - -«- K2S04+Cr203+ 3SC>3 + 24H20 (NH4)2Cr207 Cr203+ N2 + 4H20 2Co (NOJ2. 6H20 *~ Co203 + 4N02 + 1/2C>2 + 6H20 As can be seen from reactions Cr`0., and Co.,0., are formed. Corrosion resistance of these oxides is hxgh and they show same properties as ceramics have. Saturated solutions of metal salts can penetrate to the open pores. After about 10 dipping, pores are filled. But the number of pores which are not filled completely are increased. The mass amount got in pores can be calcu lated theoretically for each step. Therefore, after each filling step the pore volume is, U = ( a~1 )nU pn a where, Upn is the porosity after operation, U is initial porosity, n is number of filling operation and a is the ratio of solution volume oxide volume. According to the above given equation to get Vp_=0 initate operation is required. Simply, it is impossxble to fill the pores completely. Indeed, many of the pores are closed before filling. Remaining open pores can be found with the following equation, U = u I ( a~1 )n - -5_ ( a`1 )10 I pg I { a ' 10 l a ' I This equation can be used for the salts which 10 steps are sufficient to fill the pores. a=28,57 and a=33,9 ammonium bichromate and cobalt nitrate respectively. The coatings in which pores are filled are tested in acidic and basic medium to determine their corrosion resistance. For acidic medium HNO.,, HC1 and CH^COOH and for basic medium NaOH are choosed. Samples were tested both in boiled medium and vapour phase. For each concentration, corrosion speed was calculated one by one with the equation, V = 24. A m (g/dm2.day) k T.S where A m is reduced mass amount (g) S is the surface area (dm2) T is corrosion time (hour) - viii -Test equipment used for acidic medium is made by heat resistance glass. But test equipment used for basic medium is stainless steel. As it is known it basic medium contacts with glass, glass corrodes either. This affects the corro sion results. The corrosion tests are made under the fol lowing conditions: Corrosion medium Concentration of HC1 Concentration of HNO3 Concentration of CH3COOH Concentration of NaOH Corrosion temperature Corrosion time Corrosion surface : HC1, HNO3, CH3COOH,Na0H : %(5, 10, 20, 30,40) : %(5, 10,20, 30, 40) : %(20,40,60,80,100) : %(1,2,3,4,5) : Solution boiling temperature : Different :50 cm2 Corrosion application type: Liquid, vapour. After the comparison at corrosion test results, it was understood that coatings in which ammonium bichromate pore filling is applied have higher corrosion resistance. The major mass loose was observed in concentrate HC1 solu tion as 0,45 g/dm2day. The resistance of thermal shock of coating are measured under the unfilled pores and filled pores condition. The average thermal shock temperatures are as follows: Material Low C Steel Cast Iron Coating ZrO, ZrO, ZrO. ZrO, ZrO, ZrO, ZrO, ZrO, During the thermal shock, the calculation of stresses in coating, metal and interface was explained. The normal stress in coating is: e. E. $ o = _ 55 s 1 + *. z - XX -where, is relative % elongation between tha coating and ceramic, Em is elastic modulus of metal, $ eaual<5 Sjn ^ Cross section in metal ) Ss ( Cross section in coating ), Em ( Elastic modulus of metal ) Es ( Elastic modulus of coating ) The shear stress between metal and coating can be calcula ted as given below: o. h o. h /-*- -r s s mm x x y ı x The thermal shock resistance of coatings decreased slightly with removing of pores. In addition, thermal expansion values are too much effective. The electrical properties are also changed with the filling pores. Drilling voltage measured by. spark discharge was found as 100 KV. Electrical insulation measured in iso lation test equipment decreased from 100 Mfi to 10-60 MS. Changing of electrical properties directly related with the properties of filling material gathered in pores. The thermal insulating properties of coatings change in siiffilar way. The pore filling material amount and properties reduce the insulation. In the study, metallographic examination of coatings was conducted. The coating, adherent layer and coated metal structure were observed clearly on photomicrographes. How the spherical ceramic particles deform due to heat and impact effect was observed clearly. During the filling of pores processes, the hardness change is too much. The hardness of sand sprayed coatings is too low. Due to porous structure, their hardness meas ured were 150-200 vickers. After each filling step, their hardnesses increased gradually. It is obvious that, the hardness increases with increasing filling material. It is possible to measure the amount of pure filling material after each step. This amount can be counted as given below: nt =l ±_ a-1' ' ~ U Hardness equals k.Gnt and k equals 4.104. Hardness value can be found as given below: - x -h = k I ^ -4- tJ. By this way, it is possible to calculate the hardness after each step. There is good agreement between the meas ured and calculated hardness values. There is a big deviation in increasing hardness values obtained by pore filling operation. This distribution is related with closed pores. The hardness of ammonium bichromate pore filled coatings rises 1400 vickers. This hardness value is in the place of tool steels given in hardness of materials. The changing of hardness of coating does not depend on the type of pore filling material. It depends on the amount of material gets in the pores. The following headings are determined for pore filling operations which affect all properties of coatings: 1. The properties of coating changes with the prop erties of oxide formed in pore. 2. It is possible to fill pores with the large amount at materials if the mentioned materials solubility is high and it gives low density oxide salts. 3. The amount of material get in the pores increases if the ratio of volume of inlet solution to outlet volume at oxide increase. 4. During the removal of the crystal water, slow heating should be applied. 5. To get a good bonding between oxide and ceramic it is useful to hold the material long time at high temperature, 6. It is impossible to fill the pores completely since the gates between the pores are stached. 7. If the filling processes applied are too much, thin layer forms on the coating surface obtained is very smooth. It is found that the pores with ammonium bichromate has good physical and chemical properties. Due to high corrosion resistance, high hardness, good heat insulation, high thermal shock resistance makes ZrC>2 coatings very usefull in the following applications: - The insulation of combustion chamber in internal combustion engines, - The various application in heating, - Various places in chemical industry, - Food industry and medical industry, - Civil engineering, mining, marine industry, - Engineering applications which require high wear resistance, - Clarifying of industrial wastes. - xi -