Phase formation and microstructure of boron nitride thin layers deposited using Nd:YAG and KrF lasers

Abstract

Boron nitride (BN) thin coatings were produced by means of the pulsed laser deposition technique from hexagonal BN (h-BN) target. Two types of laser i.e. Nd:YAG with Q-switch as well as KrF coupled with RF generator were used. Deposition performed by Nd:YAG laser on steel buffered with Ti substrates led to the formation of nanocrystalline strongly texturized h-BN layers. Substrate temperature or type of background gas comprising nitrogen, argon or a mixture of nitrogen and argon did not influence the surface morphology and phase composition. The deposition of BN on Ti6Al4V substrate covered with titanium nitride by means of the KrF laser enhanced by the RF generator produced layers with nano-composite structure, consisting of wurtzite (w-BN) phase particles located in a turbostratic (t-BN) and hexagonal (h-BN) matrix. Infrared absorption spectroscopy examinations showed that increasing the substrate temperature from 20 to 700 °C caused the raise in content of sp 2-phase which was also manifested in variation of film surface morphology examined using AFM. The increase of the substrate temperature and decrease of the nitrogen pressure led to the raise of the thickness of BN layers deposited by KrF laser combined with RF generator on Ti6Al4V substrate without any buffer layers. The obtained structure consisted of the fine-grained matrix and the half-spherical-shaped conglomerates. Lower fluence resulted in the formation of larger and better-shaped crystallites and increase in the rms roughness. The morphology of the same sample varied in the lateral position. The layers near the edges of the sample grew slower and therefore the crystallites were larger and had smoother surfaces. In the central part of the sample, crystallites were a few times smaller with accumulation of deposited material in the crystallite boundaries.