【作者】 何迪；

【导师】 刘晓鹏；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2014， 博士

【摘要】 在储氢、太阳能、核聚变等涉氢领域中,不锈钢是一种常见的结构材料。在使用过程中,不锈钢常与高温、高压氢或其同位素直接接触,同时,氢及其同位素在不锈钢中以间隙原子形式扩散,具有较高的渗透率,会造成诸如氢脆、放射性污染等危害。为了降低氢及其同位素的渗透率,在结构材料表面制备金属陶瓷阻氢渗透涂层是一种行之有效的解决方案。本课题提出利用MOCVD (Metal-oganic Chemical Vapor Deposition,有机金属化学气相沉积)在316L奥氏体不锈钢表面制备Cr203/A1203陶瓷梯度阻氢渗透涂层,通过梯度涂层的制备,可以有效缓解涂层与基底间的热膨胀系数失配,提高涂层高温稳定性。为了更好地研究Cr203/A1203涂层,首先优化了MOCVD工艺,并对A1203和Cr203单层涂层进行了氢渗透方面的研究,重点探讨了诸如基底粗糙度、热膨胀系数失配、基底析出相、涂层微观结构以及涂层生长取向等因素对涂层阻氢渗透性能的影响规律。在此基础上,研究了Cr203/Al203涂层中出现的热膨胀系数梯度、界面效应、晶格诱导效应、晶型转变以及结晶质量等因素对复合涂层阻氢渗透性能的影响规律。获得如下研究成果：(1)利用MOCVD制备氧化铝涂层,详细研究了气体组分、沉积时间、沉积温度、反应源温度、热处理等因素对涂层微观结构及成分的影响规律。结果表明,使用H2载水沉积A1203涂层可以有效去除涂层中的C元素；涂层微观结构与沉积温度密切相关,由于反应源分子的吸附和脱附,在基底表面迁移和成核等步骤会受到基底温度的显著影响,因此随着沉积温度升高(400℃至600℃),涂层表面开始出现越来越明显的3D岛形貌,沉积温度过高时(高于600℃)涂层表面出现裂纹甚至疏松；在沉积时间1h-8h之内,涂层厚度与沉积时间之间满足线性关系d=-19.4+5.5t,厚度过高会导致涂层结构不稳定、致密度变差；反应源温度提高导致沉积速率加快,过快的沉积速率会导致涂层结构变得不稳定,使得涂层表面致密度下降；涂层物相与退火温度有关,非晶相氧化铝经过700℃退火后为非晶氧化铝,900℃退火后由非晶和γ-A12O3构成,1100℃退火后由γ-Al2O3.θ-A1203和α-Al203构成。900℃以上的高温退火导致氧化铝涂层发生开裂,这与退火过程中产生的热应力和氧化铝晶型转变造成的体积收缩有关。(2)在316L基底表面制备厚度为243nm的非晶相氧化铝涂层,涂层在600℃-700℃下的表观氢渗透率为P=8.68x10-7exp(.107606/RT)mol/m·s·Pa05,对316L不锈钢的渗透阻挡因子(Pemeation Reduction Factors,PRF)为42.0-75.8,在一定程度上提高了基底的阻氢渗透性能。(3)基底粗糙度以及热冲击影响涂层的阻氢渗透性能。基底粗糙度由8.71nm升高至65.3nm,涂层PRF由41-106降低至23-45。同时,较高的基底粗糙度导致涂层在受到700℃/30次热冲击之后性能出现更加明显的降低。(4)涂层经过900℃退火之后由非晶相转变为γ相氧化铝,表面出现大量呈网状分布的MnCr2O4尖晶石结构。MnCr2O4尖晶石造成的“短路效应”和热膨胀系数失配导致的涂层剥落造成涂层阻氢渗透性能大幅降低,涂层经过900℃退火后PRF由65.3-138.7降低至5.6-11.1。(5)利用MOCVD制备了沿(110)择优取向的六方结构结晶态Cr2O3涂层,涂层满足化学计量比,厚度均匀,无缺陷。厚度为366nm的Cr2O3涂层在550℃-700℃的表观氢渗透率为P=2.66x1O-6exp(-113522/RT) mol/m·s·pa0.5,对316L不锈钢的PRF为24-117。(6)氧化铬涂层厚度由222nm增加至904nm,涂层PRF由13.1-36.1增加至42.2-165.4,涂层的阻氢渗透性能随厚度增加而增大。但是,涂层厚度过高会导致其内应力过高,在高温测试时涂层会出现裂纹、剥落等失效行为,厚度为1820nm的涂层650℃-700℃的PRF仅为3.1-4.6。(7)择优取向生长的氧化铬涂层在600℃,650℃和700℃的PRF分别为38.2、21.1和13.1,而随机取向生长的氧化铬涂层仅为2.6、1.9和1.5,PRF低于前者约10倍。择优取向生长的涂层,由于晶粒排列规则有序,可以形成较为致密的结构；而随机取向生长的涂层,由于晶粒杂乱无序排列,易在涂层中产生大量通孔等缺陷,导致涂层阻氢渗透性能严重下降。(8)制备了Cr2O3/Al2O3复合涂层,550℃-700℃的PRF为229.8-543.5,远高于同等厚度的Al2O3(94.7-246.9)和Cr2O3(24.1-116.5)。原因有三点：①通过Cr2O3缓冲层的加入,形成热膨胀系数梯度结构,降低了涂层中的内应力,使得涂层的高温稳定性得到提升。②内应力降低使得涂层致密度得到提升,通过折光率计算得到的单层Al2O3和复合层中Al2O3的相对致密度分别为88.7%和94.3%。③复合涂层的制备引入了Cr2O3-Al2O3界面,复合陶瓷界面层降低了可供与H原子键合的O原子活性,降低了H在陶瓷界面的扩散迁移率。三因素综合作用,使得Cr2O3/Al2O3复合涂层的阻氢渗透性能得到显著提升。(9)Cr2O3缓冲层的加入形成了热膨胀系数梯度结构复合涂层,缓解了Al2O3在900℃退火时的体积收缩和高应力,避免了非晶相Al2O3转变为γ-Al2O3时的失效行为。此外,由于晶格诱导效应,在Cr2O3和Al2O3界面处出现了少量α-Al2O3.但是与非晶相Al2O3复合涂层相比,结晶态Al2O3复合涂层的阻氢渗透性能没有提高,反而出现小幅下降,非晶相Al2O3复合涂层与结晶态Al2O3复合涂层的PRF分别为108.1-389.1和130.9-208.3。研究表明,较差的结晶质量导致涂层中微裂纹数量增加是氢渗透降低的主要原因。更多还原

【Abstract】 Stainless steel is a common structural material in fields of hydrogen storage, vacuum solar receivers and fusion reactors etc. Stainless steel has a high hydrogen permeation rate at elevated temperatures and pressure, because hydrogen permeates through steel in the form of interstitial atom which would resulted in hydrogen embrittlement and radiological hazards. It is widely recognized that a thin ceramic coating on stainless steel could efficiently suppress the hydrogen permeation. In this study, Cr2O3/Al2O3gradient coating was prepared by MOCVD (Metal-oganic Chemical Vapor Deposition) on the surface of stainless steel to reduce the mismatch of the coefficient of thermal expansion (CTE) and improve the high temperature stability. We first optimized the technology of MOCVD and studied the hydrogen permeation resistance performance of Al2O3and Cr2O3coating respectively. Then the influences of the substrate roughness, mismatch of the CTE, precipitated phase, microstructure and the growth orientation on the hydrogen permeation resistance performance of the coatings were discussed. After that, influences of the CTE gradient, interfacial effect, lattice induced effect, crystal transition and the crystallization quality on the hydrogen permeation resistance performance of the Cr2O3/Al2O3coatings were studied. The research findings were reported below.(1) The influences of the gas composition, deposition time, deposition temperature, precursor temperature and thermal treatment on the microstructure and composition of the Al2O3coating prepared by MOCVD were studied. The results suggest that, the carbon could be effectively eliminated by using water vapor as the reaction gas. The coating microstructure was related to deposition temperature, the migration and nucleation of the precursor molecule on the substrate surface were remarkable influenced by the deposition temperature due to the adsorption and desorption of the precursor molecule. Therefore, as the deposition temperature rise (400℃-600℃), the3D island morphology on the surface of the coating was increasingly obvious. When the deposition temperature was above600℃, crack was found on the coating. In the deposition time1h-8h, the relationship of the coating thickness and deposition time exhibited a linear relationship following the equation d=-19.4+5.5t. Excessive thickness could cause a structural instability and depravation of the coating compactness. The deposition rate was accelerated as the precursor temperature increase, excessive growth rate could also cause a structural instability and depravation of the coating compactness. The crystalline phase of the coating was related to the annealing temperature. The Al2O3was amorphous after700℃annealing, amorphous and γ-Al2O3after900℃annealing, y-A12O3、θ-Al2O3and α-Al2O3after1100℃annealing. Cracks were observed on the Al2O3coating after annealing above900℃, which was attributed to the thermal stress and volume shrinkage in the crystal transition process.(2) Amorphous Al2O3coating with243nm thickness was prepared on316L substrate, the apparent hydrogen permeability of the coating was P=8.68x10-7exp(-107606/RT) mol/m·s·Pa05at600℃-700℃and the PRF (Permeation Reduction Factors) of the coating was42.0-75.8. The alumina coating has offered a certain extent hydrogen permeation suppression performance.(3) The hydrogen permeation performance could be affected by the substrate roughness and thermal shock test. The PRF of the coating was reduced from41-106to23-45as the substrate roughness raised from8.71nm to65.3nm. Meanwhile, a higher substrate roughness could lead to a more reduction extent after700℃/30time thermal shock tests.(4) The Al2O3transformed from amorphous to y-phase. Besides, spinel MnCr2O4was observed and it formed a network on the coating surface.’Short-circuiting effect’caused by the spinel MnCr2O4and coating spalling due to the CTE mismatch resulted in a sharply reduction of the hydrogen permeation performance. The PRF of the900℃annealed coating was reduced from65.3-138.7to5.6-11.1.(5) The Cr2O3coating on316L was prepared via MOCVD route. The coating has a corundum structure with strong preferred orientation of (110). The coating was corresponds well with the stoichiometric Cr2O3and was dense, crack-free. The apparent hydrogen permeability of Cr2O3coating with a thickness of366nm was P=2.66x10-6exp(-113522/RT) mol/m·s·Pa05and the PRF of the coating was24-117.(6) The PRF of the Cr2O3coating raised from13.1-36.1to42.2-165.4as the thickness raised from222nm to904nm. The hydrogen permeation performance increased as the thickness of the coating increase. However, excessive thickness causes an excessive inner stress. This could result in failure behaviors such as crack or spalling of the coating. The PRF of a1820nm thick Cr2O3coating was only3.1-4.6at650℃-700℃.(7) The PRF of the Cr2O3coating with preferred orientation was38.2,21.1and13.1at600℃,650℃and700℃， respectively, while the random orientation coating was only2.6,1.9and1.5,10times lower than the former one. The crystalline grain of the preferred orientation coating was regularly arranged, so the structure of the coating was compact. Whereas the random orientation coating possesses a crystalline grain of messy arrangement, defects like through-hole were easily generated in the coating resulted in a serious decline of the hydrogen permeation performance.(8) Cr2O3/Al2O3composit coating was prepared on316L. The PRF of the coating was 229.8-543.5at550℃-700℃, much higher than the A12O3coating and Cr2O3coating with similar thickness which PRF were94.7-246.9and24.1-116.5respectively. There were three reasons for it. Firstly, preparation of the Cr2O3buffer layer has formed a CTE gradient structure, reduced the inner stress and improved the high temperature stability of the coating. Secondly, the compactness of the coating was improved as the inner stress reduced. The relative density of the Al2O3coating and Al2O3in Cr2O3/Al2O3composit coating was88.7%and94.3respectively. Thirdly, preparation of the Cr2O3/Al2O3composit coating generated a Cr2O3-Al2O3interface. The migration rate of the hydrogen atom was low at the interface due to the low activity of the oxygen atom. For the above reasons, the hydrogen permeation performance of the Cr2O3/Al2O3composit coating has been significantly improved.(9) Preparation of the Cr2O3buffer layer has formed a CTE gradient structure, reduced the inner stress and volume shrinkage of the Al2O3coating when annealed at900℃. Therefore, invalidation behavior of the Al2O3coating was avoided when it transformed from amorphous to γ-Al2O3. Moreover, due to the’lattice induced effect’, a little α-Al2O3emerged in the interface of Cr2P3and Al2O3. However, compared with the amorphous Al2O3in Cr2O3/Al2O3coating, the hydrogen permeation performance of the crystalline Al2O3in Cr2O3/Al2O3coating has not been improved, but slightly decreased. The PRF of amorphous and crystalline Al2O3in Cr2O3/Al2O3coating was108.1-389.1and130.9-208.3respectively.The studies have shown that, the increase of the micro-crack numbers caused by the poor crystallization quality was the main reasons for the high permeability.更多还原