Materials Science Forum Vol. 681 (2011) pp 171-176 Online available since 2011/Mar/28 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.681.171
Determination of residual stresses in thermal and cold sprayed coatings by the hole-drilling method A. Ryabchikov1,a, H. Lille1,b , S. Toropov1,c,J. Kõo1,d, T. Pihl2,e and R. Veinthal3,f 1
Estonian University of Life Sciences, Kreutzwaldi 5, 51014 Tartu, Estonia 2
University of Applied Sciences, Pärnu mnt 62, 10135 Tallinn, Estonia
3
Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia
a
b
c
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected],
[email protected]
Keywords: residual stress, thermal and cold spray coatings, hole-drilling method, curvature method
Abstract. Residual stresses in most coatings have a significant influence on adhesion, on mechanical properties and on tribological performance. Residual stress can not be measured directly, rather strain is measured and stress is calculated. In this study residual stresses were investigated in thick thermal Ni95Al5 and cold sprayed Ni, Cu, NiZn, AlZn powder coatings using the semi-destructive hole-drilling and layer growing curvature methods. Residual stresses obtained for the thermal spray coating were compressive and with a relatively large deviation of the results indicating the nonuniformity of residual stress. Cold sprayed coatings have more uniform structure than thermal sprayed coatings. Residual stresses were compressive, and results obtained by holedrilling and curvature method were comparable. Introduction The laying of coatings is an effective method for modifying different surface properties of materials (fatigue, fracture, corrosion, wear, friction, etc) [1]. In the current study two coating types are investigated. Thermal spray hard metal and cold sprayed coatings are widely used in a variety of applications. The cold spray or cold gas dynamic spraying (CGDS) process is the next progressive step in the development of high kinetic energy coating processes. In the cold spray process, powder particles are accelerated by the supersonic gas jet at a temperature that is always lower than the melting point of the material, resulting in coating formation from particles in the solid state [2]. Thermal spraying is an effective method for improving and modifying the mechanical properties of materials and machine parts. The use of thermally sprayed coatings is limited by cracking, spalling, bulking and delamination [3]. These phenomena are directly related to the magnitude of residual stresses. In this study the Ni-Al-based powder flame sprayed coating (Ni95Al5) was under investigation. Cold spray technology was initially developed in the mid of 1980s at the Institute for Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Sciences in Novosibirsk by A. Papyrin and colleagues [4]. A wide range of pure metals, metallic alloys polymers and composites were deposited onto a variety of substrate materials. Currently extensive cold spray research has been conducted at different Institutions in the U.S.A, Russia, Germany and Japan [5,6,7,8,9]. Two categories of cold spray systems exist today - the high and the low pressure systems acting at gas pressure ranges from 2 to 5 [10] MPa and 0.3-1 MPa [4], respectively. High-pressure systems allow to achieve higher particle velocity compared to low-pressure systems, which in turn provide higher deposition efficiency and a broader range of eligible materials. The major disadvantage of these systems is high operation cost as they use high pressure gas (N2 or He) at high flow rate (120All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 193.49.22.128, Ecole des Mines de Paris, Paris, France-14/08/14,13:18:41)
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220 m3/h). In low pressure systems, such metals as Al, Zn, Cu, Co and Ni [2] can be deposited with good coating properties but with low deposition efficiency. Usually, hard particles, for example, aluminum oxides, are added to the metal starting powder. This improves deposition efficiency 20-30 %, reduces porosity (1-7%) and increases bond strength (40-80 MPa) [2]. Numerous studies have dealt with the fundamentals of the coating buildup process [5, 7, 8] in cold spraying of a mixture of ceramic and ductile powders. However, there is very limited data about the level of residual stresses of deposited coatings. The aim of this study is to investigate residual stresses in cold sprayed Ni, Cu, NiZn, AlZn powder coatings using the hole-drilling and layer growing curvature methods. The hole-drilling technique (semi-destructive method) is the most common method for measuring residual stresses, which is sometimes directly used for coated machine parts. This method requires drilling of a small hole, typically 1-4 mm in diameter, to a depth approximately equal to its diameter. In layer growing curvature method, thin plate substrates with different shapes are usually used for coating deposition. Residual stresses in the coating cause deformation of the substrate. Through measuring deformation of the substrate, it is possible to calculate residual stresses in the coating. In the current study aluminum strips were coated by cold spray coating and residual stresses were calculated by extended Stoney’s formula when the biaxial state of stress is taken into consideration. Experimental techniques A Ni-Al-based flame sprayed powder coating (Ni95Al5) with a thickness of 1.10 mm was deposited on a steel plate substrate with dimensions (80×80×1.52) mm. Before the deposition process the plate was placed onto the device where its edges were motionless. This kind of a fixture prevents deformation of the substrate during deposition of the coating. After deposition of the coating the substrate was released from the device. To produce the supersonic jet of particles, commercial gas dynamic spraying equipment DYMET 404 (Obninsk Center for Powder Spraying, Russia) was used. It includes a spray gun with an air heater and supersonic nozzle, two switched powder feeders, and a control unit [11]. The supersonic nozzle used has throat diameter of 2.5 mm, exit diameter of 5 mm, and diverging part length of 130 mm. The nozzle produces a supersonic jet with a total airflow rate of about 0.3 m3/min at constant pressure of 0.53 MPa and temperature of 35 °C. Total powder feed rate of 0.4 g/s and 10 mm distance from the nozzle exit to the substrate surface were kept in the experiments. The traversing speed of spray gun with respect to substrate was maintained in the range of 1 to 3 cm/s. A series of cold spray coatings were deposited using various metallic single powders and powder mixtures Cu, Ni, NiZn and AlZn which were produced and commercially available in the Obninsk Centre of Powder Deposition. An average particle size of
