Self‐Organized Nanostructures in Hard Ceramic Coatings

Abstract

Nanostructures have attracted increasing interest in modern development of hard coatings for wear‐resistant applications. In plasma‐assisted vapor deposited thin films, nanostructures can evolve during growth or a post‐deposition annealing treatment. In this review we demonstrate, using TiB2.4, TiN–TiB2, Ti0.34Al0.66N, and Ti(N,B) as model‐coatings, the development of nanostructures and its influence on the mechanical properties of ceramic thin films. For TiB2.4 and TiN–TiB2 a two‐dimensional and three‐dimensional nanostructure, respectively, organizes itself during growth by segregation driven processes. Growth of Ti0.34Al0.66N and Ti(N,B) results in the formation of a supersaturated TiN based phase, which tends to decompose into its stable constituents during post‐deposition annealing via the formation of nm‐sized domains. As the hardness of a material is determined by resistance to bond distortion and dislocation formation and motion, which depend on the amount and constitution of obstacles provided, there is a direct relation between hardness and nanostructure.

Synthesis and characterization of Cr–B–N coatings deposited by reactive arc evaporation

“…5,8 For similar systems, such as Ti-B-N, B-enriched areas, covering TiN crystals and promoting the nucleation of TiB 2 (which itself has a low solubility for N) were reported. 9 Similar segregation-driven processes occur for N in TiB 2 . These processes periodically interrupt the epitaxial growth of individual crystallites and cause renucleation, leading to extremely small crystallite size of 2 to 3 nm for TiNTiB 2 coatings.…”

“…All the arc-evaporated B-containing coatings deposited (with and without N) exhibit high hardness (22)(23)(24)(25)(26)(27)(28)(29). This behavior could be attributed to boron segregation to the grain boundaries, 9 which causes hindering of dislocation movement and the formation of amorphous Cr 2 B and BN phases. These two mechanisms of hardness improvement are both valid for the coatings deposited at low and high N 2 fractions, respectively.…”

“…These effects have been attributed to the nucleation of boron as well as to boron diffusion and accumulation at grain boundaries. 9 The addition of boron also promotes increased cohesion at the grain boundaries by affecting the local bonding at the interface (assuming that there is no oxygen, chlorine, or hydrogen uptake during deposition that would weaken the interfacial strength). 10,11 Although little is known about Cr-B-N thin films, [4][5][6][7][8] bulk Cr-B-N is a multiphase material with a complex phase diagram, 12 consisting of CrN, Cr 2 N, BN, and various Cr x B y phases (see Fig.…”

“…13 However, in multiphase nanostructured coatings, BN is generally found to be present as an amorphous phase. 7,13,14 For other M-B-N (M stands for transition metal) ternary (or quaternary) systems, such as Ti-B-N, 11 Ti-Al-B-N, 13,14 and Zr-B-N, [15][16][17][18][19] it is generally agreed that improvement of the mechanical properties results from a nanocomposite structure where virtually dislocation-free nanometer sized crystallites are separated by one to two monolayers of a strongly bonded amorphous phase at the grain boundaries. The crystallite size and the amorphous phase fraction can be finely adjusted by varying the target composition.…”

Microstructural Evolution of (Ti,W,Cr)B2 Coatings Deposited on Steel Substrates during Annealing

“…Materials based on titanium diboride are in use as cutting tools, wear resistant parts, protective coatings, diffusion barriers, and for a reinforcement of carbide ceramics [1][2][3][4][5][6][7][8][9][10]. Appropriate deposition techniques like magnetron sputtering, dynamic ion mixing, electron beam evaporation or chemical vapour deposition allow producing TiB 2 thin films or coatings as hard materials [11][12][13][14][15][16][17][18][19][20]. Superhard coatings composed of over-stoichiometric nanocolumnar TiB 2 and synthesized by DC magnetron sputtering show hardness values of 60 GPa [11].…”