Self-lubrication Behavior of Enamel Coatings with in situ Precipitated Nano CaF2 Crystals

The work aims to control the precipitated nano CaF2 crystals to make the enamel coating has self-lubrication at normal temperature. Three kinds of enamel coatings were prepared on 304 stainless steel substrate, i.e. the one without fluoride addition (Base), the one blended with 3.5wt.% CaF2 by ball-milling (MJ3.5), and the one with 3.5wt.% CaF2 added during the glaze smelting procedure (RJ3.5). Through the friction and wear experiment, the softening point and Vickers hardness tests, wear performances, thermal and mechanical properties of enamel were evaluated. By scanning electron microscopy (SEM) and electron probe microanalysis (EPMA), the crystallization of enamel, morphology and elements distribution at wear scar were investigated, and the self-lubricating mechanisms were finally discussed. For the enamel coating with CaF2 particles blended by ball-milling (MJ3.5), its fluoride crystals were of large size and uneven distribution. However, when CaF2 was introduced during the smelting procedure of enamel, the fluoride can take part in the construction of enamel network and precipitated out as nano crystals at the firing stage of the enamel coating. In this case, the crystals had uniform size of 132 nm and distributed evenly in the enamel coating RJ3.5. Friction coefficient was decreased from 0.57 to 0.37 by adding 3.5wt.% CaF2 (Base vs. RJ3.5), and the wear rate also decreased by two order of magnitude, i.e. decreased from 1.76×10–4 mm3/(N·m) to 5.03×10–6 mm3/(N·m), both friction coefficient and wear rate decreased significantly. In comparison, friction coefficient of the enamel coating MJ3.5 was slightly decreased from 0.57 to merely 0.49, and the wear rate was still kept at the order of 10–5 mm3/(N·m) as the enamel coating of Base, the improvement of tribological properties was not too much. After in-depth analysis of the worn morphologies of RJ3.5 enamel coating, it can be concluded that the lubrication mechanism of the RJ3.5 coating was as follows. The in situ precipitated nano crystals of CaF2 which added during the glaze smelting procedure (RJ3.5) had excellent bonding performance with the enamel itself, made the whole enamel coating have certain deformation ability. Therefore, no severe brittle fracture phenomenon occurred during the friction process, only small wear debris with nano CaF2 crystals were produced. The wear debris existed between the friction pair and the coating surface. Due to the small size and deformability of nano CaF2 crystals, the wear debris was easy to be compacted. After continuous friction and wear, CaF2 accumulated on the grinding crack surface and formed a lubricating layer which was rich in CaF2. The lubrication layer can separate the friction pair from the coating surface and protect the coating, so that RJ3.5 coating can also has excellent self-lubricating properties at normal temperature. RJ3.5 enamel coating was innovatively different from MJ3.5 enamel coating through different addition method. Apparently, only through this different method, RJ3 enamel coating shows far better performance than MJ3.5 enamel coating in crystal distribution, bonding ability, friction coefficient, wear rate and other aspects, this way of adding CaF2 to enamel coating provides a possibility and new idea for CaF2` to produce lubrication effect at normal temperature. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.