Saturday, May 12, 2007

Thermal Spray with HVOF

Figure 1: Digital picture of the newly designed dual powder feed device.

Design of an Integrated semi-automated Powder Feed Device for HVOF / Plasma Spray Processes to Produce Functionally Graded Materials (FGM) of Ti alloy and HA powders for Biomedical Applications

Md. Kabir Al Mamun, Joseph Stokes
Materials Processing Research Centre, and
National Centre for Plasma Science & Technology, Dublin City University, Ireland

Introduction: Functionally Graded Materials (FGMs) are a growing application area with significant promise for the future production of; (a) improved materials and devices for use in applications subjected to large thermal gradients, (b) lower-cost clad materials for combinations of corrosion and strength or wear resistance, and (c) improved electronic material structures for batteries, fuel cells, and thermoelectric energy conversion devices and (d) biomedical implant devices for enhanced bone-tissue attachment. The most immediate application for FGMs is as Thermal Barrier Coatings (TBCs), where large thermal stresses can be minimised. Component lifetimes are improved by tailoring the coefficients of thermal expansion, thermal conductivity, and oxidation resistance. The application of FGMs is quite difficult, but thermal spray processes like Plasma spray have demonstrated their unique potential in producing graded deposits, where researchers have used twin powder feed systems to mix different proportions of powders. FGMs vary in composition and/or microstructure from one boundary (substrate) to another (top service surface), and innovative characteristics result from the gradient from metals to ceramics or non-metallic to metals.

The present study investigates an innovative modification of a HVOF (High Velocity Oxy-Fuel) thermal spray process to produce functionally graded thick coatings. In order to deposit thick coatings, certain problems have to be overcome. More specifically these problems include minimising residual stress, which causes shape distortion in as-sprayed components. Graded coatings enable gradual variation of the coating composition and/or microstructure, which offers the possibility of reducing residual stress build-up with in coatings.
In order to spray such a coating, modification to a commercial powder feed hopper was required to enable it to deposit two powders simultaneously which allows deposition of different layers of coating with changing chemical compositions, without interruption to the spraying process (Figure 1). Various concepts for this modification were identified and one design was selected, having been validated through use of a process model, developed using ANSYS Flotran Finite Element Analysis. In the current research the mixing of different proportions of powders were controlled by a computer using Lab VIEW software and hardware, which allowed the control and repeatability of the microstructure when producing functionally graded coatings. This research has been carried out on DCU HVOF Feed unit system. However this semi-automated powder feed unit system could be use with Plasma techniques to produce FGM coatings of Ti alloy and HA powders for Biomedical Applications.


1 comment:

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