Near-isotropic air plasma sprayed titania R.S. Lima * , B.R. Marple Industrial Materials Institute, National Research Council of Canada, 75 de Mortagne Blvd., Boucherville, QC J4B 6Y4, Canada Received 13 October 2003; received in revised form 29 October 2003; accepted 5 November 2003 Abstract A titania feedstock was air plasma sprayed on low carbon steel substrates. In-flight particle temperature, velocity and diameter were monitored in order to find a parameter set that resulted in high particle temperature and velocity. Coatings were produced using the chosen parameter set, and certain mechanical properties (Vickers microhardness, Knoop microhardness and elastic modulus) of these deposits were measured on the cross-section and in-plane (top surface). The microstructure was evaluated using image analysis (porosity) and scanning electron microscopy. Thermal spray coatings are widely known for their anisotropic character, however, in this work, the mechanical properties of the titania coatings exhibited very similar values on the cross-section and in-plane regions. The reasons why this near-isotropic character is present are suggested and discussed. Crown copyright Ó 2003 Published by Elsevier Ltd on behalf Acta Materialia Inc. All rights reserved. Keywords: Plasma spraying; Titania; Microindentation; Elastic behavior; Hardness 1. Introduction In previous work [1,2], the relationships between in- flight particle characteristics, coating microstructural features and mechanical properties (microhardness and elastic modulus) of high velocity oxy-fuel (HVOF) sprayed titania coatings were investigated. The values of microhardness and elastic modulus were determined in the planes parallel and perpendicular to the deposition surface. It was observed that the microhardness and elastic modulus of these coatings exhibited near-isotro- pic behavior, i.e., their respective values on the cross- section and in-plane regions were very similar. This finding was considered very interesting due to the fact that thermal spray coatings are known for ex- hibiting an oriented microstructure [3–8] and aniso- tropic behavior of mechanical properties [9–13]. It was hypothesized that the near-isotropic behavior of the HVOF-sprayed titania coatings was due to the uniform microstructural features and high densities exhibited by those coatings. It was also suggested that in order to produce these coating characteristics the use of a ther- mal spray feedstock having a fine particle size and a narrow particle size range was extremely important. A feedstock having these characteristics tends to have a uniform particle heating in the thermal spray jet, which is a key factor for obtaining a uniform coating micro- structure [1,2]. Based on these principles, air plasma spray (APS) was applied with success for producing near-isotropic titania coatings [14]. In order to produce dense, uniform and near-isotropic coatings via APS, high particle temperatures and velocities had to be achieved. Recently, the fundamental nature of the anisotropic behavior of thermal spray coatings has been studied using indentation techniques [10,13], modeling [9,15,16] and small-angle neutron scattering [17–19]. These stud- ies are evidence of a growing interest in the design and engineering of thermal spray microstructures. The goal is to predict results based on a fundamental under- standing of the effects of the various parameters, as opposed to using a trial-and-error approach to eventu- ally arrive at a desired structure. It is believed that the introduction of the near-isotropic coatings discussed in this paper and earlier work will enhance the discussion and broaden the base of knowledge related to the fun- damental nature of thermal spray microstructures. Acta Materialia 52 (2004) 1163–1170 www.actamat-journals.com * Corresponding author. Tel.:+1-450-641-5150; fax: +1-450-641- 5105. E-mail address: rogerio.lima@cnrc-nrc.gc.ca (R.S. Lima). 1359-6454/$30.00 Crown copyright Ó Published by Elsevier Ltd on behalf Acta Materialia Inc. All rights reserved. doi:10.1016/j.actamat.2003.11.002