A study of bonding mechanisms and corrosion behaviour of cold sprayed coatings.
PhD thesis, University of Nottingham.
Cold gas dynamic spraying (CDGS) is a material deposition technique, in which powder particles are accelerated to speeds of between 300-1200 m/s and upon impact deform plastically and adhere. The overall aims of this research project were to understand the bonding behaviour in cold spraying of copper, aluminium and titanium, and to produce corrosion resistance barrier layer of titanium coatings using cold spraying.
The mechanism of bonding in cold spraying is still a matter of some debate. In this thesis, copper has been cold sprayed onto aluminium alloy substrates, the surfaces of which had been prepared in a variety of ways. The coating - substrate bonding was assessed via a novel intermetallic growth method along with adhesive pull-off testing. The bond strength has been rationalised in terms of a modified composite strength model, with two operative bonding mechanisms, namely (i) metallurgical bonding and (ii) mechanical interlocking of substrate material into the coating. In most cases, mechanical interlocking is able to account for a large proportion of the total bond strength, with metallurgical bonding only contributing significantly when the substrate had been polished and annealed prior to spraying. In addition, grit-blasting has been shown to significantly reduce the bond strength compared to other substrate preparation methods. Aluminium has also been cold sprayed onto copper substrates, the mechanical interlocking of substrate material was not observed and the bond strength was relatively low. Titanium particles have been deposited onto three different steel substrates, namely low carbon steel, an Armco iron, and an austenitic stainless steel. Using the novel intermetallic growth method it was found that a barrier does exist at the interface of the titanium deposited onto the low carbon steel and Armco iron substrates which is not removed in either of the stages of impact or during the heat treatment process. On the other hand, in the case of titanium deposited onto the austenitic stainless steel, the barrier is removed.
Cold spraying is believed to have the potential for the deposition of corrosion resistant barrier coatings. However, to be effective, a barrier coating must not have interconnected porosity. Titanium coatings were sprayed using nitrogen as an accelerant gas at two process gas temperatures of 600 and 800˚C to reduce porosity. A modified in-situ grit blasting was used to improve the coating-substrate adhesion. The mean bond strength of the titanium deposits was ~70 MPa and tensile strength was 250 MPa. Mercury intrusion porosimetry (MIP) was used to characterize the interconnected porosity over a size range of micrometers to nanometers. The MIP results showed that in cold sprayed deposits a significant proportion of the porosity was sub-micron and so could not be reliably measured by optical microscope based image analysis. A set of free standing deposits was also vacuum heat treated to further decrease porosity levels.
The effect of porosity on the corrosion behaviour of titanium coatings onto carbon steels was investigated in 3.5 wt.% NaCl. The electrochemical measurements of the coatings showed significant substrate influence when the interconnected porosity of the coating was 11.3 vol.% but a decreased substrate influence with a porosity level of 5.9 vol.%. Salt spray (fog) tests confirmed these electrochemical findings and showed the formation of corrosion products following 24-h exposure. Laser surface melting (LSM) was used to seal the top ~140 μm of the coating to eliminate any interconnected porosity. The LSM titanium coatings showed no sign of corrosion after 100-h of salt spray tests, and the open circuit potential and passive current density values were similar to those of the bulk titanium.
Thesis (University of Nottingham only)
||Cold spray, Coatings, Titanium, Bonding, Corrosion, Additive manufacturing
||T Technology > TS Manufactures
||UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
||12 Oct 2011 14:54
||28 Oct 2016 10:12
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