Quantitative determination of the amorphous phase in plasma sprayed alumina coatings using the Rietveld method Magdalena Lassinantti Gualtieri a, ⁎ , Maria Prudenziati a , Alessandro F. Gualtieri b a Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via G. Campi 213/1, I-41100 Modena, Italy b Dipartimento di Scienza della Terra, Università di Modena e Reggio Emilia, Largo S. Eufemia 19, I-41100 Modena, Italy Received 22 March 2006; accepted in revised form 7 June 2006 Available online 24 July 2006 Abstract Plasma-sprayed alumina coatings mainly consist of γ-alumina with minor amounts of α-alumina due to incorporation of incompletely fused powder. The presence of amorphous materials has also been mentioned in the literature, but not quantified. In this work, X-ray powder diffraction and Rietveld refinements were explored as potential tools for the determination of the amorphous content in plasma sprayed alumina coatings. To cross-check the accuracy of the Rietveld analysis, standard additions of amorphous alumina were performed. Both approaches provided consistent results supporting the validity of the Rietveld method for routine quantification of the amorphous phase in plasma-sprayed alumina. For the as- sprayed coatings studied in the present work, the amount of amorphous alumina was found to be 12.0 ± 0.7 wt.%. © 2006 Elsevier B.V. All rights reserved. PACS: 61.10.-i; 61.43.Dq; 81.15.Rs Keywords: (X) Rietveld method; (C) Plasma spraying; (B) X-ray diffraction; (A) Amorphous; (D) Aluminum oxide 1. Introduction Plasma spray is an additive material process in which molten or semi-molten particle droplets are propelled onto a substrate where they spread, rapidly solidify and form a coating. Distinct features of plasma-sprayed ceramic coatings are their porosity, lamellar microstructure and predominant presence of metastable phase(s) because of the extremely rapid solidification from temp- eratures as high as 10000 K [1]. The best known example is plasma sprayed alumina which consists mostly of γ-alumina rather than α-alumina, the only stable equilibrium phase and the only phase in the starting powder [1]. Only a minor amount of the α-phase is generally observed, which is explained by incorpora- tion of incompletely fused particles [2–4]. Traces of δ and θ alumina may also exist [5,6]. Rapid solidification of molten material may result in the formation of amorphous material. The mobility of the molecules are reduced before they crystallize, thus resulting in a non-ordered state. This is a real possibility in plasma-sprayed coatings, because of the quenching of the material during deposition. In fact, amorphous material was observed in plasma sprayed Ni- and Fe alloy coatings [7], hydroxyapatite coatings [8] and Al 2 O 3 – ZrO 2 coatings [9]. In addition, an amorphous aluminum rich layer between plasma sprayed NiCoCrAlY and ZrO 2 –Y 2 O 3 coatings was observed [10]. Young et al. used Low Pressure Plasma Spray (LPPS) for the deposition of alumina coatings [11]. According to bright-field TEM images and selected area diffraction data, the splats consisted of a polycrystalline core embedded in an amorphous matrix. Shaw et al. also presented selected area diffraction data for plasma sprayed Al 2 O 3 –13 wt.% TiO 2 coatings, showing the presence of an amorphous phase [3]. Sun et al. reported variations in the amount of amorphous phase in plasma sprayed hydroxyapatite coatings [8], based on the observed intensity of the background bump. However, the weight fraction of the amorphous phase was not determined. Appropriate techniques to quantify the amorphous phase in semi-crystalline materials are powder diffraction and the Rietveld method [12]. The Rietveld method is a full pattern analysis method in which all factors contributing to the intensity measured Surface & Coatings Technology 201 (2006) 2984 – 2989 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +39 059 2055293; fax: +39 059 2055235. E-mail address: mgualtieri@unimore.it (M.L. Gualtieri). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.06.009