SEM Investigations of TiB2 and TiBN Coatings Produced by DC Magnetron Sputtering

Abstract

Binary nitrides (TiN and CrN) and ternary nitrides (TiAlN and CrAlN) have become excellent coatings for engineering applications, where protective coatings are required to increase the lifetime and performance of cutting tools [1][2][3]. Research conducted on the development of nanostructured ternary nitride coatings found that with a nanograin structure, the coatings were of much higher hardness than those of the coarser grain size and this was influenced by the nitrogen pressure during deposition [4][5]. More recent research has shown that TiN and TiAlN are excellent coatings for machining ferrous products, whereas CrN and CrAlN have been found to exhibit unique properties for machining non-ferrous alloys. For this reason, alternative surface coatings need to be investigated.Other materials that have been studied for coating purposes are TiB 2 and TiBN. The combination of high hardness and chemical resistance make these coatings good for applications such as die casting and aluminium machining [6][7][8]. Although TiB 2 has numerous outstanding properties, there are still a few problems yet to be overcome. Being very hard, the coating is extremely brittle. This in turn can lead to the chipping of the coating, which is no use for corrosion and wear resistant coatings. TiB 2 also exhibits very poor adhesion properties when coated on most substrates and has been reported to be soluble in iron and consequently would not be ideal for engineering applications where iron and steel (ferrous materials) are the products to be machined. This work has investigated a variety of ways to overcome the adhesion problems of TiB 2 and TiBN to substrates. The use of; a) different interlayers between the substrate and the coating, b) multilayers of TiN/TiB 2 , TiN/TiBN, and TiN/TiB 2 /TiBN, c) varying the bias across the substrate, d) varying the substrate temperature, and e) gradually changing the composition of the individual layers, are methods that have been addressed with the aim of improving the bonding/adhesion properties of the coatings. This paper will report the results on the production of TiB 2 and TiBN coatings as well as their multilayer counterparts by reactive DC magnetron co-sputtering. Figure 1 shows the surface morphology of TiB 2 and TiBN. The single layer TiB 2 and TiBN coatings have a similar morphology consisting of a cauliflower structure that has a finer grain structure within. The TiBN has a more uniform, fine grain structure with a surface grain size around 20nm. However the TiB 2 has a more densified structure and consists of grains around 40nm in size and a micro-hardness above 3300HV 2g .The two coatings exhibited some adhesion problems with respect to the substrate. When only using a Ti interlayer for the TiB 2 coatings, there appeared to be a lack of interaction between the Ti and TiB 2 (Figure 2a). The introduction of a second interlayer of TiN improved bonding between the Ti and TiB 2 (Figure 2b). TiB 2 was also investigated as an interlayer for TiBN coatings (Figure 2c). As can be seen, cracking b...

Structural Characterization of Boro-Titanized AISI 1040 Steel

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