海工水力机械非晶纳米晶防护涂层微观组织控制及空蚀-腐蚀耦合损伤行为研究

The material degradation of vane hydraulic machinery under coupling effect of cavitation erosion and corrosion in ocean engineering is a significant factor to restrict ocean development. The exploration of this kind of issue is a basic theoretic subject on development and application of cavitation erosion resistance materials in corrosion environment. Fe-based amorphous/nanocrystalline coatings have excellent corrosion and cavitation erosion resistance, respectively. Nevertheless, the research on damage behaviour and mechanism of the coating under coupling effect of cavitation erosion and corrosion is shortage that limits industrial applications of the cost effective coatings in severe environment. In present project, preparation and microstructure control of the amorphous/nanocrystalline coatings to cavitation erosion resistance in corrosion environment by high velocity oxy-fuel (HVOF) thermal spraying are achieved through composition design of the feeding powder and rotatary chemical vapor deposition(CVD)surface modification of the powder, in order to improve the amorphous formation ability of the Fe-based alloy, and cooperating with post heat treatment to increase the content of nanocrystalline. Interaction law, cavitation erosion-corrosion behaviour and cavitation erosion dynamical mechanisms of the coating in corrosion environment are revealed by cavitation erosion-corrosion coupling test. Gradual change law of crack initiation, propagation and microstructural change of cavitation erosion process in corrosion environment are explored from multiscale (macroscopical and microcosmic angle) and multi-view (surface and cross-section). Cavitation erosion damage mechanism are clarified and physical model of damage are attempted to be established . The results of this proposed project are potential to produce fundamental theortical and technical backup for application of the amorphous/nanocrystalline composite material in vane hydraulic machinery of ocean engineering.

空蚀-腐蚀耦合作用导致的海工水力机械的破坏是制约海洋开发的重要因素，探讨这类损伤问题是腐蚀环境中抗空蚀材料研发、应用的重要基础理论课题。Fe基非晶纳米晶复合涂层的耐腐蚀性、抗空蚀性等性能优良，但微观组织的控制和涂层在空蚀-腐蚀耦合作用下损伤行为与机理的研究较为匮乏，制约了这类高性价比材料在恶劣环境中的应用。本项目通过喂料粉末的成分设计和粉末旋转化学气相沉积（CVD）表面改性，提高Fe基合金的非晶形成能力，配合涂层后续热处理，实现腐蚀环境下抗空蚀非晶纳米晶涂层的超音速火焰（HVOF）喷涂制备及微观组织控制；对涂层进行空蚀-腐蚀耦合实验研究，探索涂层的空蚀-腐蚀行为及其交互作用规律，揭示涂层在腐蚀环境下的空蚀动力学机制；多尺度（宏观、微观）、多视角（表面、截面）探明涂层在腐蚀介质中空蚀裂纹萌生、扩展及微观组织的变化规律，阐明空蚀损伤机理；为非晶纳米晶复合材料在海工水力机械上的应用提供理论支持。

本项目通过成分设计，制备了三种具有高非晶形成的新型Fe基非晶粉末，并采用超音速火焰（HVOF）喷涂工艺将其制备成涂层。在喷涂过程中，采用Taguchi正交试验设计方法，对喷涂距离、氧气流量和煤油流量三个喷涂工艺参数进行了优化，获得了最优的喷涂工艺参数为喷涂距离330 mm、氧气流量1840 scfh、煤油流量6.8 gph。对喷涂态涂层及热处理后涂层的组织结构、物相及显微硬度进行表征发现：当涂层在较低温度下进行热处理时，非晶涂层中的纳米晶含量较多，非晶/纳米晶复合涂层的显微硬度较高；而当涂层的热处理温度过高时，随着复合涂层中晶相的晶粒尺寸增加，涂层的显微硬度反而下降。此外，研究发现：当非晶涂层中形成较多的纳米晶时，非晶/纳米晶复合涂层的抗纯空蚀性能提高，耐腐蚀性能下降，且其抗空蚀-腐蚀耦合的性能也下降；而当非晶涂层中形成的晶体晶粒增加时，其抗空蚀性能与耐腐蚀性能同时下降，且其抗空蚀-腐蚀耦合性能进一步下降。涂层的空蚀-腐蚀耦合破坏机理主要为空蚀坑沿涂层中孔隙、裂纹或贫Cr、Ni等合金元素的薄弱区域优先形成，并最终扩展到整个涂层面的过程。该项目的研究有利于指导新型抗空蚀-腐蚀耦合材料的制备，并突破现有研究对空蚀-腐蚀耦合损伤行为与机理认识的不足，进而为新材料在水轮机叶片等水力机械过流部件上的应用提供理论与技术支持。