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Corrosion Resistance And High Temperature Properties Of Thermal Spraying WC-12Co-6Cr And WC-12Cr3C2-6Ni Coatings

Posted on:2016-03-13Degree:MasterType:Thesis
Country:ChinaCandidate:F ZhangFull Text:PDF
GTID:2271330509450838Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
WC-based and Cr3C2-based coatings thermally sprayed by high velocity oxygen-fuel flame(HVOF) are widely used in surface protection of high wear-resistant parts like corrugating roll, engineering machinery cutter, roller bit, etc, to extend the component service life due to their high hardness and excellent abrasion resistance. However, for the study of the coatings in such severer service conditions as high temperature and different acid, alkali and salt environment are rarely reported, and high temperature abrasion resistance and corrosion mechanism of the coatings after sealing are reported even fewer. Hence, this paper focused on the WC-Co-Cr and WC-Cr3C2-Ni coatings and their each sealing coatings about four aspects,such as high temperature sliding wear behavior respectively at 5 different temperatures(25℃,450℃, 600℃, 750℃ and 800℃), electrochemical corrosion behavior respectively in 1mol/L HCl, 1mol/L NaOH, and 3.5%NaCl, immersion corrosion behavior in 3.5%Na2SO4 and 35℃± 2 ℃ and anti thermal shock performance at 700 ℃. Comparative study on failure mechanism of four kinds of coatings was in order to provide a reference for the research of WC-based and Cr3C2-based metal ceramic coatings. Taking 42 CrMo steel as base material,WC-12%Co-6%Cr and WC-12%Cr3C2-6%Ni coatings prepared by supersonic flame spraying technology were marked as W1 and T1; Then the preparation of sol-gel aluminium phosphate inorganic sealing agent for the sealing treatment and obtained two another kinds of coatings were denoted as W2 and T2. Four kinds of coatings structures, mechanical properties, phase compositions and microstructures were all analyzed, and the following conclusions were that:(1) Aluminium phosphate sealing treatment significantly reduced the porosity of the coatings, and porosities of W2 and T2 coatings were both lower than 2% after sealing, which significantly increased the density of the coating, meanwhile made the hardness of the coatings increased.(2) With the increase of wear temperature, the wear rate of the coatings showed a rising trend, and loss reduction effect of phosphate sealing gradually weakened, which appeared the highest antifriction and loss reduction service temperature around 450℃. In addition, at room temperature WC-Co-Cr coating showed better wear-resistant than WC-Cr3C2-Ni coating,which showed worse wear-resistant than WC-Cr3C2-Ni coating at more than 300℃。(3) It was relatively weak for sealing treatment to improve the coatings in 2h high temperature oxidation resistance, and WC-Cr3C2-Ni coating showed weaker anti high temperature oxidation than WC-Co-Cr coatings.(4) Corrosion resistance of the coatings in 3.5% NaCl solution were characterized by:W2>T2>W1>T1, while in 1mol/L NaOH and 1mol/L HCl solution were both characterized by: T2>W2>W1>T1, while immersed in 3.5% Na2SO4 at 35℃±2℃ were characterized by:T2>W2>W1>T1; Aluminium phosphate sealing treatment could improve the corrosion resistance of the coatings, and with the immersion corrosion going on, the different oxide film were formed on the four kinds of coatings surfaces to from passive behavior, and the formed passivation film was in a state of dynamic equilibrium between damage and generation to slow down corrosion along the coating surface to the matrix, which all resulted no obvious corrosion morphologies at coating/substrate interface.(5) Phosphate sealing treatment could improve resistance to cyclic thermal shock cracking of the coatings. Meanwhile, it obtained that the WC-Co-Cr coating showed better thermal shock resistance than WC-Cr3C2-Ni coating. The generation and propagation of thermal fatigue crack of coating is governed by many factors, namely, thermal stress formed inside of coating, coating internal defects, cyclic alternating stress formed the interface between binder phase and WC bulk inner coating, the difference of the thermal expansion coefficient between coating and substrate, and so on.
Keywords/Search Tags:HVOF, 42CrMo, Sliding wear, Hardmetal coatings, High temperature, Wear resistance, Corrosion resistance, Sealing, Thermal shock resistance
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