Laser irradiated transient thermography inspection of iron-zinc alloy coatings on steel substrates

Abstract

The potential of static Laser Irradiated Transient Thermography (LlTI) as a non-destructive testing (NOT) method for the characterisation of thin binary Fe-Zn alloy coatings on steel substrates is investigated. The ability of static pulsed LlTI to non-destructively distinguish the presence of both underalloyed phases at the alloy coating surface, and overalloyed phases at the alloy coating-substrate interface is demonstrated. The sensitivity of static pulsed LlTI to the thickness of a buried layer of r­ phase Fe-Zn alloy is discussed. 1. Introduction Transient thermography is an extension of the basic infrared imaging technique which since the early 1970's has been used as a surface inspection tool [1]. Some transientthermography methods use a flashlamp as a heating source, and are classified as Flash Lamp Transient Thermography (FL TI) or Pulsed Video Thermography (PVT) [2]. Where the heating source is a pulsed laser the technique may be referred to as Laser Irradiated Transient Thermography (UTI) or Pulsed PhotoThermal Radiometry (PPTR) [3]. If a very short laser pulse is used for heating, the technique is well suited to the characterisation of thin film material systems. UTT may be performed by scanning the laser and detector, measuring the radiometry signal as a function of position on the sample, or in static mode, in which case the entire radiometric transient is measured. In this paper the static pulsed UTI technique is used to characterise thin binary Fe-Zn alloy coatings on steel substrates. Its ability to non-destructively distinguish the presence of different phases in the coating is demonstrated, and the sensitivity of static pulsed UTI to the thickness of a buried layer of r-phase Fe-Zn alloy is discussed. 2. The Fe-Zn binary alloy Fe-Zn binary alloy coatings are utilised to improve the oxidation resistance of steels to harsh environmental conditions. The Fe-Zn phase diagram is shown in Figure 1 . While pure Zn coatings (i.e. the so-called 11 phase of Fe-Zn) provide a degree of oxidation resistance, the 0,­ phase Fe-Zn alloy provides a superior resistance to the environment. The alloy coating process involves the annealing of the substrate after coating with a layer of zinc. Alloying takes place by diffusion of iron from the steel substrate into the zinc. The iron alloy fraction (iron content of the alloy) is therefore lower at the surface and increases with depth down to the coating-substrate interface. The o,-phase is the most desirable alloy phase from the point of view of steel protective coating, while other phases of the Fe-Zn binary alloy system (11 (pure Zn), l, and r ) suffer from brittleness or flaking in harsh environments. In the Fe-Zn alloy, variations in the iron alloy fraction through the coating must be confined within a small window (8-13%) in order to produce the o,-phase exclusively. Underalloying (emergence of the 11 or E, phases) is most likely to occur near the surface of the coating. Overalloying (emergence of the r-phase) is most likely to occur near the coating-substrate interface, and will result in a thin buried layer of the mechanically poor r-phase interposed between the desired o,-phase coating and the interface, impairing the coating performance. Ideally, the alloy fraction (%Fe) will be greater than 8% at the surface, and less than 13% at the interface. A method of non-destructive detection and quantification of both surface underalloying and interfacial overalloying is required. 3. Static pulsed L1TT In transient thermography, the surface under inspection is heated by a laser pulse of short duration (typically between 5 ns and 50 jJs) at a fluence well below the laser ablation threshold for the material. The transient blackbody radiation emission from the heated surface is then monitored with an infrared detector. Static pulsed UTI is therefore a form of pulsed photothermal radiometry [3]. The shape of the observed blackbody radiation transient, in the case of one-dimensional heat flow in the direction normal to the surface, is well known [4] and is related to the ability of the material system to dissipate a heat pulse, characterised by the thermal diffusivity D, and the optical absorption coefficients a. and a.' of the material at the laser