Processing and Characterization of Fly Ash - Quartz Coatings

Abstract

Fly ash emerges as the major waste material of many thermal power plants. It mainly comprises of oxides of silicon, aluminium, iron, and titanium along with some other minor constituents. The present investigation explores the coating potential of this industrial waste. It envisages the processing and characterization of a series of plasma sprayed coatings made with fly ash premixed with quartz, which is a low cost mineral available in plenty. These materials do not belong to the so-called “plasma sprayable” category. These have been deposited on mild steel and copper substrates by atmospheric plasma spraying. Utilization of such kind of industrial waste as coating material minimizes the cost of plasma spray deposition process, which posed to be the major hindrance to its wide spread application due to high cost of the spray grade powders.

Using atmospheric plasma spraying system coatings are deposited on metal substrates at different operating power levels of the plasma torch ranging from 11kW to 21kW and then characterization of the coatings is carried out. The properties of the coating depend on the materials used, operating condition and the process parameters. The plasma spraying process is controlled by the parameter interdependencies, co-relations and individual effect on coating characteristics. The particle sizes of the raw materials used for coating are characterized using Laser particle size analyzer. Deposition efficiency is an important factor that determines the techno economics of the process. It is evaluated for the deposited coatings. Coating interface bond strength is evaluated using coating pull out method, confirming to ASTM C-633 standard. In view of tribological applications, hardness is one of the most required mechanical properties. Hardness measurement is done on the polished cross section of the samples using Leitz Micro-Hardness Tester. Coating porosity is measured by image analysis technique. To ascertain the phases present and phase changes / transformation taking place during plasma spraying, XRD analysis is made. Coating surface & interface morphology is studied with Scanning Electron Microscope.

To study the suitability of the coatings for wear resistance application, wear properties of these coatings are studied. The erosion wear behaviour of these coatings is evaluated with solid particle erosion tests under various operating conditions. In order to control the wear loss in such a process, one of the challenges is to recognize parameter interdependencies; correlations and their individual effects on wear so that the coating can be useful for tribological application. Statistical analysis of the experimental results using Taguchi experimental design is presented. Spraying parameters such as impact angle, size of the erodent, standoff distance and impact velocity are identified as the significant factors affecting the coating erosion wear. Other statistical techniques like ANN and SYSTAT have also been very much useful in predicting the experimental data with a larger domain. This work establishes that fly ash-quartz mixture can be used as a potential coating material suitable for depositing plasma spray coating. It also opens up a new pathway for value added utilization of this industrial waste and low-grade ore mineral.