Nanostructured surfaces

Factors affecting nanograin formation during mechanical-pulse treatment
Maksymiv O. V.1 , Kyryliv V. I.1 , Zvirko O. I.1, Chaikovs’kyi B. P.2, Nykyforchyn H. M.1
1 Karpenko Physico-Mechanical Institute of the NAS of Ukraine,
5, Naukova St., Lviv 79060, Ukraine.
okyryliv@yahoo.com

2 Stepan Gzytskyi National University of Veterinary Medicine and Biotechnologies,
50, Pekarska St., Lviv 79010, Ukraine.

One of the known methods of formation of surface nanocrystalline structures (NCS) is mechanical-pulse treatment (MPT) which used high-speed friction for severe plastic deformation of the surface layer. Surface-grinding and treated-turning machines after insignificant modification can be used for MPT. Thermostressed state of a metal in the friction contact (FC) zone is influenced by variation of treatment regime of MPT. Hence, the parameters of the treated layer, namely grain’s size on the surface and its thickness can be changed in that way.
MPT of 65Г steel was carried out at different treatment regimes to study their influence on the parameters of the strengthened surface layer. The treatment regimes were changed in the following ranges: specimen rotation velocity – 0.12 – 0.29 m/s, longitudinal feed of the strengthened tool relative to the specimen – 0.8 – 1.2 mm/rev., specific pressure in the FC zone – 0.48 – 0.80 GPa. Tool rotation velocity was equal 50 m/s and industrial oil 1-12A GOST 20799–88 was used as cooling technological medium for all MPT regimes.
Consequently, NCS with the grain size range 16 – 40 nm were formed on 65Г steel’s surface due to variation of treatment regime of MPT. At that time, the surface microhardness decreased from 9.8 to 7.7 GPa accordingly. It was also established that increment of the thickness of strengthened layer and decreasing its grain size led to increasing of contact fatigue (number of cycles before destruction) of treated steel.
Based on carried out researches, the following main regularities were determined: i) increment of the specimen rotating velocity resulted in raising the grain size and decreasing the thickness of the layer ii) due to raising of the pressure in the FC zone, the grain size decreased and the thickness of the layer increased; iii) due to raising of the longitudinal feed of the tool relative to the specimen the grain size slightly decreased and the thickness of the layer slightly raised.
Therefore, the technological regimes of MPT significantly influenced on formation of the surface layers with NCS and then on their properties.