摘要
用磁控溅射技术获得了晶粒尺寸小于１００ｎｍ的Ｃｏ—３０Ｃｒ—５Ａｌ微晶涂层。对同成分ＣｏＣｒＡｌ合金及其微晶涂层进行１１００℃恒温和１０００℃循环氧化试验，并与ＣｏＣｒＡｌＹ合金进行比较。用声发射技术、划痕法及透射电镜对溅射ＣｏＣｒＡｌ微晶涂层的抗氧化机理进行了研究。结果表明微晶化提高了ＣｏＣｒＡｌ合金表面氧化膜的粘附性，从而大大改善了合金的抗恒温与循环氧化性能。而微晶化提高氧化膜粘附性的原因是：（１）微晶表面氧化膜比铸态合金表面氧化膜的晶粒细得多，细晶氧化膜可通过扩散蠕变释放生长应力和部分热应力；（２）微晶涂层氧化膜的热应力可通过靠近氧化膜的微晶合金的塑性变形而释放；（３）溅射微晶涂层具有柱状结构，沿柱状结构氧化可产生许多“微钉”，从而大大提高了氧化膜的附着力。从氧化膜的附着力和塑性看，微晶化比添加活性元素的作用还大。
A microcrystalline Co-30Cr-5Al coating with grain sizes <0.1μm was obtained using magnetron sputtering technique.Isothermal oxidation at 1100℃ and cyclic oxidation at 1000℃ for cast Co-30Cr-5Al alloy and its sputter deposited microcrystalline coating were carried out,in comparison with cast Co-30Cr-6Al-0.5Y alloy.The scratch test and acoustic emission technique were used to study the oxide adhesion,and the scanning electron microscopy(SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy(XPS) were also adopted to characterize the surface morphology, grain sizes and the compositions of initially formed oxide scales. The results indicated that on the cast Co-30Cr-5Al alloy an Al_2O_3 film could form at the initial stage of oxidation, but it was prone to spall during cooling. After the initially protective Al_2O_3 scale had spalled off, aluminium would further be oxidized forming internal oxides rather than a continual layer because of the relatively high oxygen solubility and diffusivity in the alloy, together with the low interdiffusion coefficients of alloying elements in the CoCrAl system. During further oxidation, Cr_2O_3 scale formed on the alloy surface. Since the Cr2O3 scale was inferior to the Al2O3 scale in protectiveness, the cast Co-30Cr-5Al alloy exihibited very poor oxidation resistance, especially poor cyclic oxidation resistance. However, the sputtered microcrystalline CoCrAl coating possessed much better both isothermal and cyclic oxidation resistance than the cast CoCrAl alloy. Through this investigation, the beneficial effect of microcrystallization on oxidation resistance of the CoCrAl alloy may be explained as follows.(i) Sputter deposited microcrystalline alloy coating possesses columnar-type structures. The grain boundaries are the nucleation sites of oxides during oxidation, and further oxidation along columnar-type grain boundaries into the coating may form micro-pegs which anchor the scales to the coating and enhance the bonding force between the scales and coating. It is believed that this beneficial effect of microcrystallization is similar to that of reactive elements.(ii)The oxide scale formed on microcrystalline alloy coating has finer grain size than those formed on the cast alloy (the former is five times finer than the latter). Therefore.thermal stresses may be released as a result of plastic deformation of both oxide scales and metallic substrate ad.jacent to the scale during cooling so that the scales adhesion is improved. From the results of scratch test and acoustic emission measurement, it can be seen that the oxide scales formed on microcrystalline coating of CoCrAl without reactive elements have better plasticity and adhesion than those on the cast CoCrAl alloy with reactive elements.The beneficial effect of microcrystallization on oxidation resistance of CoCrAlAl alloy is stronger than that of the addition of reactive elements.
出处
《腐蚀科学与防护技术》
CAS
CSCD
1994年第1期7-16,共10页
Corrosion Science and Protection Technology
基金
国家自然科学基金
关键词
溅射沉积
微晶覆层
钴铬铝涂层
CoCrAl-microcrystalline coating,oxidation mechanism,scale adhesion.
