Adhesion Evaluation of TiN and (Ti, Al)N Coatings on Titanium 6A1-4V
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The metallic components of gas turbine engines are continually subjected to hostile atmospheres. Nitride coatings improve the performance of the metallic compressor blades in these engines [3]. In such cases, the adhesion of the coatings becomes the limiting factor in the lifetime of the blade assembly. A variety of methods have been explored for evaluating the adhesion of coatings [4]. Perhaps the most accepted method for evaluating the adhesion of TiN and (Ti, Al)N is the scratch test, for which much data are available [1, 5, 6]. Spallation of coatings has also been demonstrated as an effective method for evaluating adhesion [7-13]. In this method, a compressive shock wave is created in a coated substrate. This triangular compressive wave is initiated at the uncoated surface, travels through the substrate and coating to the free surface of the latter, and is reflected back into the coating as a tensile wave as shown in
Metal
Ceramic
Flyer Plate
Figure 1. Schematic of the stress wave path through
a coated sample during plate impact spallation. 377
Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
Figure 1. The reflected wave passes through the interface, placing it in tension and causing the coating to spall from the substrate. During experiments involving high speed displacements, a laser velocity interferometer (VISAR) can be used to record the velocity of a free surface [14]. In previous experiments, a VISAR signal has been used in conjunction with hydrodynamic computer simulations to determine the spall strength of bulk materials [15]. In addition, Gupta and co-workers have recently calculated interfacial spall strength of metal films on sapphire, incorporating the use of a VISAR to record the free surface velocity of the coating during the spallation experiment [8]. It has been further shown that computer simulation using DYNA2D can be used to calculate free surface velocity and interfacial stress history during a spallation experiment [9, 10]. EXPERIMENTAL PROCEDURE Sample Preparation and Analysis For this research, commercial TiN and (Ti, A1)N coatings were deposited on titanium 6% aluminum 4% vanadium (Ti 6A1-4V), a candidate metal for turbine compressor blades, by an enhanced cathodic arc (CA) deposition process in a MA-500e evaporator. [2, 16] Surface cleaning prior to deposition was accomplished with titanium or titanium-aluminum ion bombardment of the surface. TiN coatings were also deposited in a Balzers triode electronbeam (EB) evaporator. Following an argon ion plasma cleaning, a 20 nm interlayer of titanium was deposited on the substrate surface to improve adhesion. Coating properties are listed in Table 1. Substrates were coupons between 0.2 and 0.4 mm thick and polished to a mirror surface finish using O.ljm diamond. The substrates were kept thin to minimize attenuation of the stress wave as it passed through the sample during subsequent spallation experiments. Table 1. Data for TiN and (Ti, AI)N coatings deposited on Ti 6AI-4V.
Coating
Deposition Method
Thickness (4
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