MS-7-P-2112 Characterization of Self-Hardening CrAlN/BN Nanocomposite Coatings

The “self-hardening phenomena” of TiBN and TiCrBN coatings after annealing have been observed in vacuum or inert gas atmospheres. An increase of the indentation hardness of CrAlN/BN nanocomposite coatings after annealing may also be performed in air in the range of 700 to 800 ^oC. This peculiar phenomenon of the CrAlN/BN coatings has been studied by examining the microstructure and microchemistry of the coatings.
Cr₅₀Al₅₀ alloy (99.8%) and h-BN (99.5%) targets were sputtered in a mixture of highly purified argon and nitrogen gases (6N). Total thickness of the film was in the range of 1.8 ~ 2.2 μm. Coating morphology, microstructure and microchemistry were examined with a FEG SEM and a FEG TEM (JEM-2100F and ARM-200F) that were equipped with Oxford and JEOL EDS system, respectively.
Figure 1 illustrates the change of indexed indentation hardness, H_IT (%), of the CrAlN, CrAlN/8vol% BN and CrAlN/18 vol.% BN coatings at different annealing temperatures in ambient air. The hardness of these coatings as-deposited was 42, 43 and 39 GPa, respectively. Self-hardening of the CrAlN/BN coating was evidenced by the increases of indentation hardness that occurred after annealing to 800 ^oC in air. Changes in surface morphology of CrAlN and CrAlN/8 vol.% BN coatings after annealing were observed in SEM. After oxidation at 800 ^oC, scale-like precipitates were strewed over a large area surface of CrAlN coatings (Fig. 1b). The surface morphology of CrAlN/BN coating is similar to that of the as-deposited coating that consisted of granulated particles (Figs.1c/1d). TEM cross-sectional images of CrAlN/18vol%BN coating, both as-deposited and annealed samples, appeared rather similar to each other with columnar structure (Figs. 2a/2b). However, a disruption in the columnar structure by a very thin layer (20 ~ 40 nm) of film was observed in the annealed sample (Figs.2c/2d) but not in the as-deposited sample. HRTEM image revealed that the top most layer is characterized by amorphous materials with embedded nanocrystalline particles (white circles in Fig. 2d). EDS line profiles of cross-sectional samples showed a high concentration of O in the uppermost layer of the annealed sample (Fig. 3a). The O content remained constant at a lower level throughout the film surface, up to a depth of ~100 nm in the as-deposited sample (Fig. 3b). This indicates that the oxidized layer formed near the top surface is likely to be one of the factors responsible for the self-hardening phenomena in CrAlN/BN coatings. Our recent investigation using ARM confirmed that the as deposited CrAlN/BN coatings have nanocomposite structure consisting of CrAlN nanocrystalline grains embedded in amorphous BN phase. This structure probably causes the self-hardening effect.