1. Introduction
Plasma sprayed alumina-13% titania (AT-13) coating is one of the most important coatings for many industrial applications [1-7]. They provide a dense and hard surface coating which are resistant to abrasion, corrosion, cavitation, oxidation and erosion and are therefore regularly used for wear resistance, electrical insulation, thermal barrier applications etc. A number of papers reported that the Al2O3-TiO2 coating containing 13 wt% of TiO2 showed the most excellent wear resistance among the AT-13 ones [3-5].
AT-13 coating is a typical ceramic coating with relatively high degree of porosity and the properties of these coatings, such as high temperature corrosion resistance, toughness and abrasive resistance, may thereby be reduced. To improve these properties, various methods have been proposed, such as impregnation with polymers or ceramics, seal sintering with liquid alloys and postlaser irradiation [8,11-21]. Laser surface treatment is one novel method that has potential for eliminating porosity and producing a homogeneous surface layer. Unfortunately, there is a very limited research on the effect of laser irradiation on the surface morphology and microstructural of AT-13 coatings. Previous research has shown that CW-CO2 and Nd:YAG lasers, with the appropriate parameters can improve the microstructural and morphological characteristics of AT-13 or Alumina coatings [13-15]. The formation of columnar grains was observed on the laser-treated zones. Chemical composition and thermal conductivity affected the way the materials behaved during laser processing.
Excimer pulsed lasers are characterized by relatively short pulse duration (~25 ns), and wavelengths in the ultra-violet which result in very shallow treatment depths, of the order of a few hundred nanometers [15-17]. Pulsed laser irradiation can be used to melt or soften very thin surface layer of metal or ceramic that resolidifies, due to the high thermal gradients. Excimer lasers present certain distinct advantages for material processing applications in comparison to the other types of lasers. This is due to the fact that Excimer lasers operate in the ultraviolet region of the spectrum at wavelengths from 190 nm to 310 nm. At these short wavelengths the reflectivity of most metals and ceramics is lower than at longer wavelengths and the absorptivity is higher [20].
In this investigation, Excimer laser was used for the surface annealing of free-standing AT-13 samples that were manufactured with a plasma spray gun. The effect of the Excimer laser annealing on the main features of the coated surface was evaluated in terms of surface modifications, microstructural and mechanical properties. A detailed parametric study was performed to investigate the effects of several parameters such as laser energy density (fluence), pulse repetition rate (PRR), number of pulses on the mechanical properties, surface morphology, and microstructure of the coatings.
2. Experimental Materials and Procedures
Free-standing AT-13 coatings were produced by a water-stabilized plasma (WSP) spray gun to obtain a thickness of 5 mm; Table 1 lists the spray parameters. The coating was sprayed on mild steel substrates. The substrates were grit blasted and then a thin layer of aluminum was arc sprayed before spraying the AT-13 coating. The thin aluminum layer was dissolved using hydrochloric acid so that free-standing alumina-titania plates were obtained. All specimens were mechanically polished to a mirror surface in the present study, which results in a surface roughness of 0.7 μm prior to laser treatment. This operation also facilitates the characterization of surface topological evolutions. The procedure consisted of successive grinding by silicon carbide papers and a final cloth polishing with a 0.25 μm diamond particle suspension.
Excimer laser pulses was generated from a Lambda Physic Compex 205 system having Krypton Fluoride (KrF) as the lasing gas, resulting in a laser wavelength of 248 nm, a bandwidth of 300 pm and a pulse duration of 24 ns. Table 2 lists the laser processing parameters employed in this study.
Surface morphology and microstructure of the coatings were investigated before and after laser treatment by optical microscopy (OM) and a LEO field emission scanning electron microscopy (SEM). Mitutoyo surface roughness and Vickers microhardness testers were used to measure the surface roughness and hardness of the treated and untreated surfaces. The Vickers hardness number (VHN) measurements were conducted under 300 gm load over 15 sec duel time.
Table 1. Spray parameters for water-stabilized plasma.
Table 2. Laser processing parameters and surface properties of the coating.
The porosity of the coatings was estimated with quantitative image analysis on as polished and as laser treated samples. Five SEM images were analyzed using Image-J software from NIH (National Institute of Health, Bethesda, MD, USA). The phase composition of the coatings before and after the laser treatment were determined by X-ray diffraction (XRD) using a Philips X-ray diffractometer (Philips APD 3520).
3. Results and Discussion
3.1. Phase Composition
The XRD profiles of the as-sprayed and laser treated coatings are illustrated in Figure 1. The analysis of the coatings indicated the presence of one distinct diffraction peak of the metastable γ-Al2O3 phase even though the starting powder was mainly α-Al2O3 phase. This is consistent with what was observed in earlier studies on the plasma sprayed AT-13 coatings [3-7]. The formation of metastable phase is generally attributed to the large kinetic undercooling generated in the melt that favors nucleation of the metastable phase over the stable phase α-Al2O3. The microstructure evolution during rapid solidification depends on the interplay between undercooling and solidification velocity [16]. The XRD profile of the laser treated surface matched well with that of the coating as shown in Figure 1. This result indicates that a very high cooling rate was achieved with the nano-second (ns) pulsed laser, which suppressed the transformation of γ-Al2O3 to α-Al2O3.
3.2. Microstructure of As-Sprayed Coatings
Figure 2 shows the typical morphology of as-sprayed