Interface characterization and scratch resistance of plasma sprayed TiO₂-CNTs nanocomposite coating

Highlights

• TiO₂/CNT interface includes chemical reaction and physical bonding.

• Strengthening mechanisms are scratched-out CNTs rolling, CNTs bridging.

Abstract

TiO₂ and TiO₂-CNTs nanostructured coatings were fabricated by plasma spray with agglomerated nanocrystalline powders. The microstructure and interface between TiO₂ and CNT of the nanocomposite coating were observed extensively using scanning and transmission electron microscopy. The scratch resistance of the two coatings was compared under a constant load scratch test. Results showed that both nanostructured coatings consisted of fully melted (FM) regions, partially melted (PM) with solid-state sintered regions, and pores. The addition of CNTs can effectively inhibit the growth of TiO₂ grains and has a fine-grain strengthening effect. Besides, the interface bonding includes good chemical bonding by locally forming carbothermic reduction product in the FM regions and relatively weak physical bonding by van der Waals forces in the PM regions. In the scratch test, the lateral friction force and material removal volume of TiO₂-CNTs coating were reduced remarkably compared to those of un-reinforced coating. The corresponding strengthening mechanisms induced by CNTs are scratched-out CNTs rolling on the scratch track and CNTs bridging inside the nanocomposite coating.

Introduction

Plasma spraying technology possesses many advantages, such as high heat source temperature (∼10,000 K), wide variety of sprayable materials, a high jet velocity and deposition efficiency, as well as low thermal effect on workpieces. Ceramic coatings prepared by the technique have good comprehensive performance and are widely used in the manufacture and repair of parts used in the aerospace field, automobile industry, light industry, and equipment remanufacturing field [1,2]. Among them, TiO₂ coating prepared by this technique has moderate hardness (Vickers hardness of ∼680 HV), good wear resistance and corrosion resistance, and exhibits a certain electrical conductivity and high-temperature self-lubricating property [3]. Hence, it is often used to protect the surfaces of mechanical parts and components under medium loads, such as strengthening and repairing of bearing sleeves, friction end faces of mechanical seals for pumps, and inner walls of engine cylinder [4,5]. During the spraying process, however, the heat of the plasma arc cannot be completely transferred to the interior of TiO₂ powder owing to the super speed of in-flight particle (up to 400 m/s) and a relatively high melting point of the ceramic (∼1843 °C), which will result in the formation of semi-molten regions in the coating. In addition, native defects, such as pores and cracks, in the ceramic coating lead to its brittleness performance and low cohesive strength between the stacking splats, hence limiting the promotion and application of this kind of coating.

Carbon nanotubes (CNTs) have a high aspect ratio and excellent mechanical properties. The average elastic modulus is 1.8 TPa, bending strength is 14.2 GPa, and strain energy storage is as high as 100 keV [6]. In the field of thermal spraying, CNTs are introduced into ceramic coatings as a reinforcing agent to improve the coating quality and mechanical properties, which has been studied to some extent. In the last decade, researchers have successively attempted multiple methods including mechanical ball-milling [7], spray drying [8], in situ chemical vapour deposition [9,10], and heterocoagulation [11] to enhance the dispersibility of CNTs in Al₂O₃ feedstock powder, and have prepared composite coatings with a relatively dense structure, high hardness, and improved toughness.

The interface of composite materials is the main medium that external stress load is transferred from matrix to reinforcing phase. The bonding types of the interface include mechanical bonding, physical bonding, and chemical bonding. The type of interface bonding determines the cohesive strength of coatings, which is directly related to various mechanical properties of coatings under different loading conditions. Agarwal et al. [12] constructed a theoretical model to understand the underlying wettability via ab initio molecular simulation of Al₂O₃-CNT interface. They obtained a very low thermodynamic activity coefficient for Al₄C₃ of only ∼6.89 × 10⁻¹⁹ at 2200 K (near the melting point of Al₂O₃). Therefore, it was difficult for the formation of stable/chemical reaction products of C reaction with Al₂O₃ during the plasma spraying process. Besides, this model also exhibited partial bonding of Al-terminated Al₂O₃ with C, showing improved wetting and stability of the interface. Jambagi et al. [13] observed the interface between Al₂O₃ and CNT using transmission electron microscopy. They found an amorphous layer beside the interface which can withstand viscous strain and reduce interfacial stress. In his follow-up studies, however, the author still can not find obvious reaction layer at the interface of matrix (Al₂O₃, TiO₂) and CNT [14].

In order to obtain a fundamental understanding of damage characteristics and abrasive material removal mechanisms in brittle materials, both static indentation and sliding indentation or scratch tests have been conducted [15,16]. Unlike in (static) indentation tests where only normal loads are imparted, scratch tests, on the other hand, import both normal and tangential loads in lateral direction, thus providing a better insight into abrasion resistance of ceramic material and residual surface damage during grinding [17,18]. Furthermore, scratch test is an effective approach to estimate the cohesion strength of thermally sprayed ceramic coating as the coating is more inclined to failure from its inherent defects (weak inter-splat boundaries, partially melted regions, pre-existing microcracks and micropores). Numerous researchers have revealed the damage mechanisms of several widely used thermal-sprayed ceramic coatings (yttria-stabilized zirconia [19], alumina [20,21], alumina/13%titania [22], and chromic oxide [22]), and found that the common failure modes of such coatings were microfracture within plastic regions and formation of tensile cracks normal to scratch direction. In the recent years, Jambagi et al. [14,23] have made valuable efforts to improve the scratch resistant properties of ceramic coatings by doping micro-sized crushed alumina or titania feedstock powder with CNTs using heterocoagulation method. This, however, might have the drawback that the structure of the CNTs attached on the surface of the crushed powder was prone to be severely destroyed as they were directly exposed to the plasma jet with ultra-high temperature, hence weakening the strengthening effects offered by CNTs. Besides, compared to the conventional coarse-grained ceramic powder used in the above method, the agglomerates of constituent nanopowders could eventually form nanostructured coating with higher scratch resistance owing to its unique bimodal microstructure (partially-melted/solid-state sintered and fully molten phases) [20,24].

In the preliminary work of our research group, spraying dried nano-TiO₂ and CNTs powder was used as feedstock for attaining plasma sprayed nanostructure coating. Then the static indentation and tribology tests were conduct to study the mechanical properties of the coatings [25,26]. Based on it, this paper further analysed the TiO₂/CNT interface phenomena and some other typical microstructure of the reinforced coating which have not been elucidated before. Then, the ability of TiO₂ and TiO₂-CNTs coatings to resist scratch damage was compared using micro-scratch method. To the knowledge of the authors, it is the first attempt to investigate the scratch resistance of TiO₂-CNTs nanostructured coating plasma sprayed from nanocrystalline feedstock powders.