3 ㎛-thick CrN coatings were deposited on high speed steel by d.c. magnetron sputtering. The coating process was interrupted several times for an intermediate plasma etching process (-1000V, 5min). (Ti,Al)N coatings were deposited on M2 high speed steel by inductively coupled plasma (ICP) assisted d.c. magnetron sputtering. When (Ti,Al)N coatings were deposited, the applied negative substrate bias voltage was varied from 0V to 100V. Microstructures were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). A Knoop hardness indenter with a load of 10g determined the coating hardness. The adhesion property of coatings was investigated using a CSEM scratch tester(REVESTER) as well as a Rockwell C indenter. In case of CrN coatings, the corrosion behavior was electrochemically measured using the potentiodynamic and potentiostatic method.
CrN coatings showed a mono-phase CrN to have a NaCl FCC structure and CrN (200) preferred orientation was more increased in the CrN coatings with 16 IE. Bright-field TEM images showed that all coatings had the columnar structure. The coating with no IE had the defects like pores and pinholes. However, the coating with 16 IE had few defects and had the dense structure and the steps. The grain size was decreased with increasing the number of IE. From the HRTEM results, amorphous layers were formed at the step by IE. The thickness of the amorphous layer was 3~5 nm. There was no difference of the crystallographic orientation in the top and the bottom grains of amorphous layers. Boundaries between neighboring columnar grains were semi-coherent grain boundary. The corrosion resistance and hardness were increased with increasing the number of intermediate etching process.
Intermediate etching decreased the number of the defects like pores and pinholes, and grain size. Therefore, the dense microstructure by IE is expected to improve corrosion resistance and hardness.
(Ti,Al)N coatings were a single phase NaCl-type and the preferred orientation was changed from TiN (111) to TiN (220) with increasing a substrate bias voltage. TEM images showed that all films had the columnar structure. Without the bias voltage, film had a columnar structure exhibiting a high intercolumn, intragrain, and porosity and the coating was grown with facet-shaped grains at the surface. As increasing substrate bias voltage, the decrease of the void density was observed, the facet-shaped grains disappeared, and only smaller grains remained. Therefore, (Ti,Al)N films became the denser columnar structure. The hardness was increased with increasing a substrate bias voltage.
The high hardness is caused by the microstructure change from the columnar structure with facet-shaped grains to the denser columnar structure with small and round-shaped grains.
