Comportamento tribologico di rivestimenti ceramici avanzati applicati mediante tecniche APS e HVOF [Friction and wear behaviour of APS and HVOF advanced ceramic coatings]

Ceramic and cermet coatings are widely used in many industrial applications due to their friction and wear resistance, high hardness, chemical stability, oxidation-resistance at high temperatures and thermal barrier properties. In particular, it is a generally accepted practise to coat high temperature components in civil and military aero, marine and industrial gas turbines. The coatings can be deposited on the high temperature components such as liner, nozzles, first stage turbine blades and vanes in order to increase the firing temperature and subsequently the efficiency of the turbine. A variety of coatings and coating processes are available for protecting components in gas turbine engines operating in a variety of conditions. With reference to coated turbine blades and vanes the philosophy is that the base component material is developed to possess optimised mechanical properties whilst the coating is selected to achieve maximum protection from the service environment. Accordingly, the properties required by a surface coating system for blade and vane applications are high corrosion, oxidation and erosion resistance, good interfacial adhesion to the metallic substrate, as well as high mechanical and aerodynamic properties. In this paper is reported a research activity carried out in order to evaluate the tribological behaviour of four types of advanced ceramic coatings, Al 2O 3-13TiO 2, Cr 2O 3, WC-12Co and Cr 3C 2-37WC-18Me, deposited onto cemented steel plates. The ceramic coatings are applied by plasma-spraying (APS), while the cermet coatings are deposited by HVOF-spraying technique. Pin-on-disk wear tests are performed by means of a DUCOM tribometer in accordance with ASTM G99-05 standard, using alumina pins as counterpart material. The tests are carried out under different conditions of relative humidity (20% and 70%); other test parameters, such as normal load, test duration, sliding speed and temperature, are maintained constant. Optical Microscope (OM) and Scanning Electron Microscope (SEM) observations of the worn surfaces show the interaction of different wear mechanisms, confirming the importance of relative humidity in relation to the nature of the coatings employed. In particular, the ceramic coatings (Al 2O 3-13TiO 2 and Cr 2O 3) are more sensitive to moisture than the cermet coatings (WC-12Co and Cr 3C 2-37WC-18Me), as highlighted by both the friction coefficient and wear rate results. Moreover, comparing the behaviour of the investigated coatings, the cermets present the best wear resistance. The SEM observations of the Al 2O 3-13TiO 2 worn surface show a non-protective tribolayer and the generation of surface microcracks with the consequent removal of wear debris in the form of flakes. The Cr 2O 3 worn surface appears smoother without evidence of significant wear and the pores remain open in the flattened surface. Many cracks are also propagated in the same direction as the applied normal load. WC-12Co coating is less sensitive to moisture due to the formation of oxide layers acting as a solid lubricant. Finally, the morphology of Cr 3C 2-37WC-18Me does not clarify the influence of relative humidity on the tribological behaviour of this coating. In this case, further analyses by X-ray diffraction (XRD) could provide other information about the involved wear mechanisms.

Nella memoria è presentata una attività di ricerca riguardante la caratterizzazione microstrutturale e la valutazione del comportamento ad usura di quattro diverse tipologie di rivestimento, Al2O3-13TiO2, Cr2O3, WC-Co e Cr3C2-37WC-18Me, che sono stati depositati su dischi in acciaio da cementazione. I primi due rivestimenti sono stati applicati mediante tecnica APS, i restanti mediante tecnica HVOF. Le prove di usura sono state effettuate utilizzando un tribometro DUCOM in configurazione pin-on-disk, secondo normativa ASTM-G99-05, con pin costituito da un agglomerato di allumina. Le prove sono state eseguite in condizioni variabili di umidità relativa, mantenendo costanti tutti i rimanenti parametri quali carico applicato, durata della prova, velocità di rotazione e temperatura. L’analisi delle superfici usurate mediante tecniche di microscopia ottica ed elettronica ha evidenziato l’instaurarsi di differenti meccanismi di danneggiamento, a conferma dell’importanza dell’umidità in relazione alla diversa natura del rivestimento.