Erosion resistance of laser textured plasma-sprayed Al₂o₃-13%Tio₂ coatings on mild steel

Highlights

• LST can be utilised in altering surface profile by developing a texture on a surface of coating with minimal/zero defects.

• The micro-grooves on the coating surface enhanced the erosion wear resistance of alumina titania coating.

• The micro-grooves could stop the cracks from propagating to adjacent areas, shortening the overall crack length.

Abstract

A micro-grooved texture was fabricated on the surface of a plasma sprayed alumina-13% titania (Al₂O₃-13%TiO₂) coating using laser surface texturing (LST) method. Aspects of hardness, phase composition and erosion wear resistance of the textured coating were investigated and compared with those of non-textured coating. As for the erosion study, a standard slurry pot erosion test was conducted in silica (SiO₂) slurry at different impact angles of the SiO₂ particles. Microstructural features of both textured and non-textured coatings were characterised with Scanning Electron Microscopy (SEM) analysis to evaluate the failure of coatings due to wear behaviour. The results revealed that the LST technique is indeed an effective way to alter the surface characteristics of plasma-sprayed Al₂O₃-13%TiO₂ coating with minimal effect on mechanical properties and phase composition of the coating. The SEM analysis displayed formations of erosion scar and crack propagation on the coating surface. Based on the impact angle effect, the cumulative mass loss of the coating was greater at 90° when compared to 30°. In this study, the cumulative mass loss of the textured coating decreased up to 45% at the impact angle of 90° with 750 rpm slurry velocity; signifying enhanced coating resistance against erosion by engraving micro-grooved texture onto its surface.

Introduction

Surface texturing refers to a method that modifies the characteristics of a material surface to enhance the properties of the material. In recent years, texturing of ceramic coatings with well-defined surface structures has been performed to maximise the benefits of protective coatings and surface texturing [[1], [2], [3]]. Several surface texturing technologies are currently available, namely chemical etching [4], mechanical machining [5], lithography [6], ion beam texturing [7] and laser surface texturing (LST) [8]. Varied technologies are applied to design texture characteristics in accordance to specific applications. For instance, soft lithography techniques, such as micro-contact printing, micro-moulding, micro-transfer moulding, replica moulding and imprinting, are used extensively for their thermal effect and vast applicability. Nevertheless, these techniques involve multiple steps that demand a specialised master [9,10] and are particularly suitable for microelectronics and biological applications [11]. Another technique, mechanical machining, is a conventional method of processing ceramic materials widely applied across a range of in industries. Nonetheless, the drawbacks of this method are fracture, tool failure, wear and low surface integrity [12,13].

LST refers to a method that has attracted a great deal of attention recently due to its significant advantages, including being a rapid and environmental-friendly technique as well as its capability for exceptional control over pattern, size and shape [14]. The LST can produce textures directly from a digital file, which allows the creation of precise and defect free samples. This condition discards variance that may result in slight differences between the samples produced. Usage of chemical substances also can be avoided by using the laser technique. Nevertheless, processing of ceramic materials using LST is quite challenging due to the considerably high hardness and brittleness of the ceramic materials. Additionally, massive amount of energy (in the laser beam) tends to generate cracks on the surface due to induced thermal stress. The literature depicts that several attempts have been made to reduce surface damage when using laser technology on ceramic materials [15,16]. Jagdheesh [17] discovered that short-pulsed lasers are advantageous in high-quality processing of ceramic materials with minimal surface defect and heat-affected zones. Parry et al., [18] investigated the effects of laser pulse duration on the texturing process of stabilised tetragonal zirconia (Y-TZP) surfaces. The outcomes showed that short-pulsed laser resulted in crack-free surface whereas long-pulsed laser showed signs of microcracks formation on the surface. This was attributed to loss of high energy before the laser pulse interacted with the surface and the occurrence of thermal diffusion. The advantages of using ceramic materials with high-quality surface finish, especially for tribological applications have been reported to be numerous. In enhancing the wear resistance of hard ceramic coatings, a textured surface is necessary to protect the surface from damages, i.e., scratches [19]. Prior studies revealed that surface texture has an impact on wear resistance of ceramic coatings by trapping wear debris, storing lubricants and increasing load-carrying capacity, thus promoting the two-body abrasive wear mechanism during friction process [20].

Erosion wear is a critical problem for numerous marine industrial structures including hydraulic turbines, pumps and pipelines conveying solid particles [21]. In an erosion problem, particles (e.g., sand) are incorporated with fluid flow and exert impact on a component surface, resulting in local damage that escalates operational costs, decreases efficiency, causes premature failure in some components and impairs operational safety [22,23]. Hence, mitigating erosion wear is integral to prevent the adverse effects during service, particularly in marine components and structures. In erosion applications, plasma-sprayed Al₂O₃-13%TiO₂ coating possesses the ability to decrease wear rate due to its excellent properties, which are crucial for tribological and corrosion applications [21]. The extensive use of Al₂O₃-13%TiO₂ coatings as wear and corrosion resistant coatings, mainly in the textile, machinery and printing industries, is due to their properties such as high hardness, low density, low thermal expansion as well as high resistance to oxidation and wear [24,25]. The performance of Al₂O₃-13%TiO₂ coating in these applications can be improved by applying some kind of texture onto its surface. This concept has been inspired by natural environments, wherein most wildlife that possesses textured skin is able to survive for extended period in erosive environments [26,27]. For instance, a bionic specimen that mimics the skin structure of the desert lizard, Laudakin stoliczkana, possesses anti-erosive properties that aid in reducing its erosion rate [28]. However, no study has investigated the effect of textured surfaces on reducing erosion-related issues in slurry conditions. Notably, the presence of texture on a surface can prevent the propagation of cracks to the neighbouring areas.

In this study, micro-grooved texture was produced on Al₂O₃-13%TiO₂ surface by using a short-pulsed laser system via LST. The effect of microgrooves on the erosion behaviour of plasma-sprayed Al₂O₃-13%TiO₂ coating was investigated by using a standard slurry pot erosion tester. Silica (SiO₂) was used as the erosive media in the slurry pot mainly because SiO₂ is a primary erosive element found in marine environment. The factors that influenced the erosion wear behaviour of laser textured Al₂O₃-13%TiO₂ coating were determined and the erosion wear mechanisms were assesses via microstructural analysis of the eroded surfaces.