Effect of spinels on the mechanical and tribological behavior of plasma sprayed alumina based coatings

Plasma sprayed ceramic coatings are widely used to protect metallic substrates when high temperature and/or friction are developed in service. In particular, alumina based coatings have been extensively used. Examples of applications are pistons in pumps or internal combustion engines and steam valve spindles. Alumina is hard, it shows a very high oxidation resistance and it is not permeable to aggressive gases. However, the alumina main drawback is its low fracture toughness. Brittle fracture is one of the most common wear mechanisms reported for ceramics. It is well known that the fracture toughness and the operative conditions are the key parameters controlling the crack propagation throughout the material. As a consequence, transition from mild to severe wear in ceramic coatings is mainly controlled by the fracture toughness of the coating. The typical solutions to improve the fracture behavior of alumina coatings are based on mixing alumina powders with other ceramic powders with superior toughness. Alumina – Zircona systems are used in this way. When a mixture of ceramics is thermally sprayed, a very complex microstructure is attained in the coating. Nevertheless, a common feature can be pointed out for different systems: formation of spinels from both oxides is commonly reported. These phases seem to play an important role in the fracture behavior of the coatings and subsequently in their tribological behavior. This paper presents a summary of the work done in the microstructural, mechanical and tribological characterization of plasma sprayed alumina based coatings (Al2O3 – TiO2, Al2O3 – ZrO2 and Al2O3 – Cr2O3). Special attention is paid on the role played by spinels formed during deposition

Bioactive and Tribological Behaviour of Atmospheric Plasma Sprayed Hydroxyapatite Coatings Reinforced by Lanthanum Oxide

Lanthanum oxide (La2O3) reinforced Hydroxyapatite coating was deposited by using unique gas tunnel type plasma spray torch under optimum spraying conditions. The phase and microstructure of the as-prepared powder and coatings were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). In vitro bioactivity of the plasma sprayed lanthanum oxide reinforced hydroxyapatite coatings were investigated by using simulated body fluid solution. Results showed that there was onset of apatite formation on the surface of coatings after 15 days of immersion in SBF, while after 19 days of immersion in SBF it was indicated that a HCAp phase crystallized on their surface. Our studies demonstrate that lanthanum oxide reinforced hydroxyapatite coatings are potentially useful biomaterials with good tribological and bioactive behaviour

Characteristics of TiN Coatings by Gas Tunnel Type Plasma Reactive Spraying

TiN coating and film that have excellent properties, have been already used in the various field of industry. But TiN film has problems in the formation process: i.e., low deposition rate and poor thickness of the film. In order to solve these problems and to obtain much thicker Titanium nitride (TiN) coating speedy, TiN coatings were formed by means of a reactive spraying by using the gas tunnel type plasma jet. In this study, the fundamental characteristics of this method were investigated by measuring the properties of the titanium nitride ( TiN) coatings formed on the traverse stainless steel substrate. Consequently, TiN coatings of 150µm thickness were obtained at P=25kW, t=5s, and the characteristics were discussed