用化学共沉淀法制备平均粒径约为18 nm的Fe3O4水基磁悬液, 并用XRD(X射线衍射)、TEM(透射电镜)、VSM(磁滞回线)测试手段进行了表征。 利用锯床加工、焊接、打磨等工序制作不同厚度带裂纹缺陷涂覆层的人工试块；并分别用纳米Fe3O4水基磁悬液和HR-1型红磁膏配置的水基磁悬液, 对制作的含裂纹缺陷试块进行磁粉检测。结果表明: 随着涂覆层厚度的增大, 使用纳米Fe3O4磁悬液和常规HR-1型红磁膏进行检测所得的磁痕均由密实变得稀疏；涂覆层厚度较小时, 纳米Fe3O4磁悬液的磁痕清晰度稍优于常规HR-1型红磁膏的磁痕清晰度；涂覆层厚度较大时, 外加磁场施加过程中, 纳米Fe3O4磁悬液的磁痕清晰度稍优于常规HR-1型红磁膏的磁痕清晰度；但外加磁场撤离后, 纳米Fe3O4磁悬液的磁痕显示变得松散、不连续, 显示效果不如常规HR-1型红磁膏。

Fe3O4 water-based magnetic fluid of averaged particle diameter 18nm was prepared by chemical co-precipitation and characterized by TEM, VSM and XRD. Test block with a coating artificial crack for different thickness coatings was constructed by machining, welding and grinding processing and so on. We used nano-Fe3O4 magnetic fluids and water-based magnetic HR-1 red magnetic paste that can be bought in the market to the magnetic particle testing of crack defects block, the result shows: with the coating thickness increasing, the magnetic image of both using nano-Fe3O4 magnetic particle and HR-1 magnetic paste was changed as from being dense to being thinning. At the relatively thin coating location, nano-Fe3O4 magnetic image resolution was a bit better than the conventional HR-1 red magnetic paste, but at the thicker location, nano-Fe3O4 magnetic image was slightly superior to the conventional HR-1 red magnetic paste while applying an extra magnetic field, but when the magnetic field was moved away, nano-Fe3O4 magnetic image became loose, discontinuous, so the display effect was no better than conventional HR-1 red magnetic paste.