TiO-based coatings prepared by plasma spraying in air of Ti + C mixtures
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rimaud and J-C. Labbeb) Laboratoire Sciences des Procédés Céramiques et de Traitements de Surface (SPCTS) UMR-CNRS 6638, 87060 Limoges cedex, France
J-P. Bonnet Groupe d’Etude des Matériaux Hétérogènes (GEMH) AE 3178, 87060 Limoges cedex, France (Received 7 December 2005; accepted 31 March 2006)
This work presents an original way for preparing TiO-based coatings by thermal spraying. As titanium monoxide is oxidized by the mere trace of oxygen contained in hydrogen, it is obvious that plasma spraying of TiO powder does not lead easily to the formation of a titanium monoxide coating. However, thermodynamical calculations show that the conditions necessary for the preparation of TiO can be reached, at the titanium melting temperature (Tm⳱1933 K), when oxygen reacts with liquid metal in the presence of excess carbon. These results have led to experiments in which TiO-based coatings have been prepared by spraying a stoichiometric mixture of graphite and titanium grains onto cast iron in air. In optimal conditions, a gas-tight hard coating (1000 ± 15 HV3) of a TiO-based phase (composition: TiO0.81±0.06C0.04±0.02) has been obtained.
I. INTRODUCTION
Titanium nitride (TiN), titanium carbide (TiC), and titanium monoxide (TiO) are three Ti-based phases presenting a centered-face cubic (cfc) structure. The nitride and the carbide are well known for their hardness,1,2 but gas-tight coatings are difficult to obtain.3,4 The mechanical properties of TiO are less known but are probably lower due to the lower bond energy of TiO (−1374 kJ mol−1 fot TiC, −1265 kJ mol−1 fot TiO) values calculated from thermodynamic data.5,6 Moreover, TiO is a very difficult phase to obtain as a bulk material: it is oxidized by the mere trace of oxygen contained in hydrogen,7 but its low melting temperature could allow the preparation of gas-tight coatings by plasma spraying. Actually, plasma spraying of TiO powder does not lead to the formation of coatings because of oxidation (Fig. 1). One solution would consist in spraying a mixture whose reaction at high temperature during the flight and on the substrate would promote reductive atmosphere to stabilize titanium monoxide. This is the reason for Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2006.0203 1770 J. Mater. Res., Vol. 21, No. 7, Jul 2006 http://journals.cambridge.org Downloaded: 14 Mar 2015
studying the behavior of a mixture of titanium and graphite sprayed by plasma in air. II. THERMODYNAMIC APPROACH
In the presence of oxygen and excess graphite, the composition of the gas phase in equilibrium conditions is controlled by the Boudouard equilibrium CO2共g兲 + C共s兲 ↔ 2CO共g兲 ,
(1)
if PCO, PCO2, and PO2 are respectively the partial pressure of carbon monoxide, carbon dioxide, and dioxygene in equilibrium conditions, at a temperature higher than 1300 K, PCO Ⰷ PCO2 Ⰷ PO2.8 Considering that all the oxygen available in the surrounding atmosphere can locally react with carbon, 0 , 2PCO = PO 2
(2)
P0O2 being the initial
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