Recent progress in the development and industrialization of superhard and tough nanocomposite coatings, which consist of hard transition metal nitride nanocrystals “glued together” by about 1 monolayer of silicon nitride, is summarized and documented by selected illustrative examples. It is shown that already the presently available superhard nanocomposites applied on machining, stamping and forming tools significantly increase their lifetime and the cutting speed, and consequently also the overall productivity of the machining operations. Further improvement of the presently available and newly developed nanocomposite coatings will push the machining technology towards new horizons. Besides of the superhard nanocomposites, also medium hard, but very tough thick coatings with low internal stress applied to forming tools increase their lifetime by at least one order of magnitude.

Industrial applications of superhard nanocomposite coatings

Approximately 1.5 μm thick CrN and CrAlN coatings were deposited on silicon and mild steel substrates by reactive direct current (DC) magnetron sputtering. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The bonding structure of the coatings was characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the coatings was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD data showed that the CrN and CrAlN coatings exhibited B1 NaCl structure. Nanoindentation measurements showed that as-deposited CrN and CrAlN coatings exhibited a hardness of 18 and 33 GPa, respectively. Results of the surface analysis of the as-deposited coatings using SEM and AFM showed a more compact and dense microstructure for CrAlN coatings. The thermal stability of the coatings was studied by heating the coatings in air from 400 to 900 °C. The structural changes as a result of heating were studied using micro-Raman spectroscopy. The Raman data revealed that CrN coatings got oxidized at 600 °C, whereas in the case of CrAlN coatings, no detectable oxides were formed even at 800 °C. After annealing up to 700 °C, the CrN coatings displayed a hardness of only about 7.5 GPa as compared to CrAlN coatings, which exhibited hardness as high as 22.5 GPa. The potentiodynamic polarization measurements in 3.5% NaCl solution indicated that the CrAlN coatings exhibited superior corrosion resistance as compared to CrN coatings.

A comparative study of reactive direct current magnetron sputtered CrAlN and CrN coatings

Modern, state-of-the-art, PVD coatings are required to fulfill a variety of different applications. Each metal cutting operation requires an optimal combination of various film parameters to achieve a high end cutting performance. Especially, Al-based coatings such as AlTiN- and AlCrN-coatings show very good results in high performance metal cutting applications. Wear resistance, thermal stability such as oxidation resistance and hardness at elevated temperatures are key issues within these cutting operations. In this paper the influence of these properties on Al-based nitride coatings in relation to metal cutting tests such as milling and drilling will be discussed.

Modern coatings in high performance cutting applications

Four nitride coatings, TiN, TiAlN, AlTiN and CrAlN were deposited on YG6 (WC + 6 wt.% Co) cemented carbide by cathodic arc-evaporation technique. The friction and wear properties were investigated and compared using ball-on-disc method at high speed with SiC ball as a counter material. The tests were evaluated by scanning electron microscopy, X-ray diffractometer, energy dispersive X-ray, micro hardness tester and an optical profilometer. The results showed that TiN and TiAlN coatings presented lower friction coefficient and lower wear rate, and that high Al content AlTiN and CrAlN coatings didn't present better anti-wear properties in this test. Oxidation and abrasive wear were the main wear mechanism of TiN coating. In spite of the observation of micro-grooves and partial fractures, TiAlN possessed perfect tribological properties compared with the other coatings. High Al content increased the chemical reactivity and aroused severe adhesive wear of AlTiN coating. CrAlN coating presented better properties of anti-spalling and anti-adhesion, but abundant accumulated debris accelerated wear of the coating under this enclosed wear environment.

Friction and wear properties of TiN, TiAlN, AlTiN and CrAlN PVD nitride coatings

An excellent adhesion of hard coatings to steel substrates is paramount in practically all application areas. Conventional methods utilize Ar glow etching or cathodic arc discharge pretreatments that have the disadvantage of producing weak interfaces or adding droplets, respectively. One tool for interface engineering is high power impulse magnetron sputtering (HIPIMS). HIPIMS is based on conventional sputtering with extremely high peak power densities reaching 3kWcm−2 at current densities of >2Acm−2. HIPIMS of Cr and Nb was used to prepare interfaces on 304 stainless steel and M2 high speed steel (HSS). During the pretreatment, the substrates were biased to Ubias=−600V and Ubias=−1000V in the environment of a HIPIMS of Cr and Nb plasma. The bombarding flux density reached peak values of 300mAcm−2 and consisted of highly ionized metal plasma containing a high proportion of Cr1+ and Nb1+. Pretreatments were also carried out with Ar glow discharge and filtered cathodic arc as comparison. The adhesion was ev...

Interface microstructure engineering by high power impulse magnetron sputtering for the enhancement of adhesion

Cr1−XAlXN films were synthesized on mirror-polished stainless steel substrates by the arc ion plating method using Cr1−XAlX alloy targets with diffent Al contents. Oxidation resistance of films was estimated by heating substrates in air at 800, 900 and 1000 °C and subsequent analysis by the X-ray diffraction method (XRD). The XRD peaks from Ti0.7Al0.3N films, annealed at 800 °C for 14 h, disappeared and the peaks from iron oxides consequently appeared. The oxidation resistance of Ti1−XAlXN films improved with increasing Al content X. On the other hand, the peaks from Cr1−XAlXN films which were annealed at 800 °C did not change at all, but Cr1−XAlXN films were slightly oxidized over 900 °C. It is considered that the oxidation resistance of Cr1−XAlXN films was superior to that of Ti1−XAlXN films.

Oxidation resistance of Cr1-XAlXN and Ti1-XAlXN films

Ti1−XAlXN and Cr1−XAlXN films were synthesized by the arc ion plating method differing in the second metal contents X and investigated on their crystal structures, lattice parameters and microstructures. X-ray diffraction patterns showed that crystal structures of both Ti1−XAlXN and Cr1−XAlXN films changed from the NaCl-type (cubic structure; c-) to wurtzite-type (hexagonal structure; h-) at Al contents X=0.6–0.7. Lattice parameters of c-Ti1−XAlXN and c-Cr1−XAlXN with Al contents X⩽0.6 uniformly decreased with respect to all lattice spacings with increasing X values. On the other hand, for h-Ti1−XAlXN and h-Cr1−XAlXN films with X⩾0.7, anisotropic lattice expansion and shrinkage were observed with the a- and c-axes. The lattice spacing expanded in the a-direction and shrank in the c-direction by adding Ti and Cr atoms with larger atomic radius into AlN structure, respectively.

Anisotropic lattice expansion and shrinkage of hexagonal TiAlN and CrAlN films

Microhardness and lattice parameter of Cr1−xAlxN films

Cr 1− X Al X N and Zr 1− X Al X N (0≤×≤1.0) films were synthesized by the cathodic arc method using alloy and metal targets. The X-ray diffraction patterns showed that the Cr 1− X Al X N changed from cubic structure at X =0.6 to hexagonal structure at X =0.7. The peaks of Zr 1− X Al X N showed the NaCl structure at X =0.37 and changed to wurtzite structure at X =0.50. The lattice parameter of Cr 1− X Al X N films changed from 0.416 nm ( X =0) to 0.413 nm ( X =1.0), and that of Zr 1− X Al X N changed from 0.458 nm ( X =0) to 0.444 nm ( X =0.37). The maximum microhardness of Cr 1− X Al X N and Zr 1− X Al X N films was 27 GPa ( X =0.6) and 29 GPa ( X =0.37), respectively. Changes in microstructures-related transition from the cubic to hexagonal were studied, and corresponding hardness changes were discussed in Cr 1− X Al X N and Zr 1− X Al X N.

Effects of Al contents on microstructures of Cr1−XAlXN and Zr1−XAlXN films synthesized by cathodic arc method

Tungsten nitride films (WNX) were deposited by the cathodic arc method in argon–nitrogen gas mixture changing nitrogen partial pressures. When the N2/Ar+N2 ratios were less than 0.1, the films showed α-W with hexagonal structures. On the other hand, the crystal structure changed to cubic W2N with the ratios between 0.1 and 0.35. When the ratios exceeded 0.35, the films were hexagonal WN. The micro-hardness of α-W, cubic W2N and hexagonal WN phase showed approximately 13, 24 and 28 GPa, respectively. The surface roughness of WNX films were smooth with approximately 10 nm and, correspondingly, the grains are all small with 10–50 nm in size. In this paper, the changes in crystal structures of WNX were investigated as a function of nitrogen flow and their mechanical properties were studied.

Masahiro Kawate

Papers

Oxidation resistance of Cr1-XAlXN and Ti1-XAlXN films

Anisotropic lattice expansion and shrinkage of hexagonal TiAlN and CrAlN films

Microhardness and lattice parameter of Cr1−xAlxN films

Effects of Al contents on microstructures of Cr1−XAlXN and Zr1−XAlXN films synthesized by cathodic arc method

Effects of nitrogen concentration on microstructures of WNX films synthesized by cathodic arc method