Materials to be investigated to develop adherent, wear resistant coatings using the plasma spray process are listed in Table 6. PS200 coating exhibits good friction and wear at elevated temperatures, however, the need for diamond grinding may prove uneconomical to manufacture. Work with sputter coated MoB? has shown interesting friction results but adherence was poor. The MoFeB to be used in this program was developed by Caterpillar and has shown excellent adherence with laser fusing. Combinations of molybdenum and cobalt will be used in the binders for the carbides and borides in an effort to achieve an adherent self-lubricating oxide film.

Two types of thin film coating processes will be evaluated - PVD and CVD. There are certain advantages with each process. The PVD process has a lower working temperature (400-500 °C) than the CVD process (900-1000 °C). Due to the high process temperature of the CVD process, the substrate usually needs to be reheat treated after coating to obtain surface hardnesses of Rockwell (RKW) C 55 min, required for acceptable wear resistance. By using an air hardening substrate material, hardening takes place during cooling from the coating process temperature. This avoids reheat treatment which can induce undesirable distortion in the coated piece part. With the PVD process, post coating heat treatment is not necessary.

The main advantage of the CVD process is that thicker coatings can be applied compared to PVD, 10 microns vs 5-7 microns. Also, layered composite coatings can be applied with CVD whereas only single layer coatings are commercially available for PVD. Adhesion is generally better for CVD coatings due to the high process temperature.


0 0

Post a comment