Reactivity in the chromium oxide-calcium fluoride system: An empirical approach
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Reactivity in the chromium oxide-calcium fluoride system: An empirical approach Frans Vos, Luc Delaey, Marc De Bonte, and Ludo Froyen Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Heverlee 3001, Belgium (Received 20 August 1998; accepted 11 January 1999)
The reaction mechanisms observed when sintering loose Cr2 O3 –CaF2 powder mixtures were analyzed, and the influence of the sintering parameters on the reaction behavior is presented. Using x-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and differential thermal analysis (DTA) measurements, CaCrO4 was shown to be the reaction product when sintering in air. The reaction occurs in two steps: CaF2 transforms to CaO at the Cr2 O3 –CaF2 interface, followed by a CaO–Cr2 O3 interaction, which creates the reaction product. Scanning electron microscopy (SEM) and x-ray fluorescence (XRF) analysis showed an increasing loss of CaF2 with increasing sintering temperature and heating rate, while an opposite evolution of the amount of reaction product was observed.
I. INTRODUCTION
In order to solve the challenging problem of energy losses due to wear phenomena, several research laboratories are developing self-lubricating materials. A dispersion of oil-containing microcapsules1,2 or solid lubricant particles3–5 in a wear resistant material allows the formation of a permanent lubricating layer at the interface with the counterbody. A homogeneous distribution of the lubricant particles (in this study CaF2 ) in the matrix material (in this study Cr2 O3 ) is a prerequisite to obtain a uniform friction coefficient and uniform wear at the coating surface. The research results presented in this paper fit within a project aiming to produce self-lubricating Cr2 O3 –CaF2 coatings by plasma spraying. Feeding the plasma unit with composite particles, containing both the constituents in the desired ratio, would allow the deposition of a homogeneous coating. On the contrary, the use of a loose powder blend would induce a segregation of the components in the plasma plume and thus in the coating.6,7 In previous work the spray-drying technique was used to create the mentioned composite particles,8,9 but the physical Cr2 O3 –CaF2 interactions were not satisfactory to avoid partial decomposition of the “agglomerates.” This decomposition was caused by mechanical forces and thermal stresses acting on the agglomerates during their transport to and through the plasma. As a result coating homogeneity could not be guaranteed. In the present work chemical bonds between the Cr2 O3 and CaF2 particles were created, which give a higher mechanical stability compared to the spraydried agglomerates. As sintering is the most obvious 2518
J. Mater. Res., Vol. 14, No. 6, Jun 1999
method to create chemical bonds in ceramic materials, the sintering behavior of loose Cr2 O3 –CaF2 powder blends was studied. II. SINTERING Cr2 O3 – CaF2 POWDER BLENDS
The sintering behavior of Cr2 O3 –CaF2 powder blends containing 20 and
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