Nanoscale Visualization of Elastic Inhomogeneities at TiN Coatings Using Ultrasonic Force Microscopy

Abstract

Ultrasonic force microscopy has been applied to the characterization of titanium nitride coatings deposited by physical vapor deposition dc magnetron sputtering on stainless steel substrates. The titanium nitride layers exhibit a rich variety of elastic contrast in the ultrasonic force microscopy images. Nanoscale inhomogeneities in stiffness on the titanium nitride films have been attributed to softer substoichiometric titanium nitride species and/or trapped subsurface gas. The results show that increasing the sputtering power at the Ti cathode increases the elastic homogeneity of the titanium nitride layers on the nanometer scale. Ultrasonic force microscopy elastic mapping on titanium nitride layers demonstrates the capability of the technique to provide information of high value for the engineering of improved coatings.

Keywords: Nanomechanics; PVD nanostructured coatings; Scanning probe microscopy; TiN; Ultrasonic force microscopy.

Figure 1

aSet-up for the UFM measurements;bTypical UFM cantilever response when a modulated ultrasonic excitation of 4 MHz with maximum amplitude A_m = 8 V_ppis applied to the piezo beneath the TiN sample (S-UFM mode), being the initial tip-sample set-point force (in the absence of ultrasound) of ≈70 nN

Figure 2

aXRD patterns (θ-2θBragg–Brentano scan) of TiN deposited on SS304 with different sputtering power (W_S) andbschematic representation of the δ-TiN/α-Ti/SS304 system with the TiN grains growing in a specific direction

Figure 3

SEM (a–a″), AFM (b–b″) topographic images and SEM cross sectional view (c–c″) of the TiN film on SS304 produced withW_S = 100 W (a–c), 150 W (a′–c′) and 200 W (a″–c″). Thegrey-scale range in AFM images (b–b″) is 112, 160 and 125 nm, respectively

Figure 4

TiN film obtained atW_S = 100 W:aTopography in AFM contact mode. Surface area: (5 × 5) 03BCm²;Grey-scale range: 466 nm.bUFM image simultaneously recorded with (a).cTopography in AFM contact mode recorded over the square region in (a). Surface area: (1 × 1) μm²Grey-scale range: 72 nm.dUFM image simultaneously recorded with (c), over the region squared in (b).eTopographic andfelastic profile along thelinesindicated in (c) and (d), respectively

Figure 5

aAFM topographic image. Surface area: (500 × 500) nm²Grey-scale range: 58 nm.bDerivative image of (a).cUFM image simultaneously recorded with (a), over the region near to thedarker grain(iii) in 4d

Figure 6

TiN film obtained atW_S = 150 W:aTopography in AFM contact mode. Surface area: (5 × 5) μm²Grey-scale range: 288 nm.bUFM image simultaneously recorded with (a).cTopography in AFM contact mode. Surface area: (1 × 1) μm².Grey-scale range: 64 nm.dDerivative image of (c).eUFM image over the squared region in (a,b)

Figure 7

TiN film obtained atW_S = 200 W.aTopography in AFM contact mode. Surface area: (5 × 5) μm²Grey-scale range: 119 nm.bUFM image simultaneously recorded with (a). Thearrowsindicate softer UFM regions and their corresponding location in the AFM image