APPLICATION OF X-RAY DIFFRACTION IN DEVELOPMENT OF NEW TYPE OF MOULDS | June | 2013 | 396/397 of refractory materials producers practically impossible. The solution which was proposed rises from a similar pattern, but instead of sintered mould plate, cold spray or HVOF coating, much cheaper plasma sprayed ceramic coatings are to be used. Plasma spraying is capable of complete feedstock powder particles melting and, thus, facilitates creation of ceramic coating via their solidification on the substrate which is a distinctively different mechanism than sintering, cold spray or HVOF. The incurred microstructure of plasma sprayed coating can be characterized by the so called “splats” or solidified particles [Sampath 2004], number of inter- and intra-lamellar cracks, pores and regions of incomplete bonding are usually present as illustrated in Fig. 1. In this research, water stabilized plasma spraying (WSP) [Chraska 1992] was involved. Such plasma jet is distinguished by high jet velocities and by high enthalpy. The temperature of the jet can exceed 25 000 K. The parameters of WSP system result in special performance characteristic in plasma spraying, namely high coating material feed rates, typically tens of kg per hour, or spraying of materials with high melting points. Residual stresses in the coatings and in the coating/substrate interface significantly affect their performance. Quenching residual stresses are present within the coating as a consequence of rapid solidification of the impacting molten particles on the substrate or previously deposited splats and generally lead to tensile stresses. A mismatch in thermal expansion of the coating and substrate material can give rise to thermal stresses with maxima usually at the coating substrate interface. Transformation residual stresses in the thermal spray coatings originate from an array of possible phase transformations linked with volume changes. In some cases, relaxation of the residual stresses may take place, which is accompanied with evolution of micro and macro cracks, mutual splat sliding or plastic deformation [Bose 2007]. The crucial phenomena closely intertwined with plasma coating quality are adhesion to the substrate and cohesion between splats which take place because of four mechanisms, namely mechanical, chemical, dispersive and diffusive. The most common adhesion mechanism of thermal spray coating adhesion is mechanical anchorage of the splats to the irregularities of the substrate [Balic 2009]. This mechanism is supported by the substrate surface features, especially by the surface roughness by which any type of bond either chemical, dispersive, diffusive or mechanical will be more effective. The substrate surface area may contain features which allow molten material to flow into, fill and solidify. Therefore, the essential parameter for coating adhesion to the substrate is the substrate’s roughness. 2. Experimental Since the lower limit of substrate surface roughness prior to plasma spraying process is Ra = 7 μm and the blasting of the traditionally used tool steels in case-hardened state on-site of the spraying facility resulted only in the roughness lower than Ra = 4 μm, the technologists of the of Czech grade 12 or 14 which have a lower hardness. 1. Introduction Refractory materials are one of the indispensable essences of the industrial era. No steel works, glass works or even a residential building could exist without them. Their ability to isolate hot air, melted glass or melted iron and at the same time contain them is truly vital. However, their existence comes for a price, most refractory materials are highly abrasive and their manufacturing is a challenge as they cause extreme wear of the contact surfaces during their pressing. The traditional and most common attitude to tackle this issue was to use steels with highest possible hardness, which has been successfully fulfilled by case-hardened tool steels with hardness approaching 65 HRC. Another option would be to employ a wear protective layer, most conveniently in a form of a coating deposited on mould parts exposed to the pressed mixture. There exists a variety of coatings intended for improvement of wear resistance, so far very good records in this field have HVOF (High Velocity Oxygen Fuel) cermet coatings of tungsten carbide grains in either cobalt or chromium matrix [Wayne 1994] or chromium carbide grains in nickel-chromium matrix [Matthews 2004]. The highest abrasion resistance has been reported for sintered WC-Co with hardness about 2400 HV [Stern 1990] or cold spray WC-Co coating with low porosity with 2050 HV hardness [Kim 2005]. Nevertheless, the cost of sintered mould plates or cold spray coatings renders their usage in the fiercely competitive environment APPLICATION OF X-RAY DIFFRACTION IN DEVELOPMENT OF NEW TYPE OF MOULDS FOR PRODUCTION OF REFRACTORY AND HIGHLY ABRASIVE OBJECTS Kamil Kolarik 1 , Zdenek Pala 1 , Libor Beranek 2 , Nikolaj Ganev 1 1 Department of Solid State Engineering, Faculty of Nuclear Sciences and Physical Engineering, CTU in Prague, Prague, Czech Republic 2 Department of Manufacturing Technology, Faculty of Mechanical Engineering, CTU in Prague, Prague, Czech Republic e-mail: kamil.kolarik@email.cz Keywords: ceramic coatings, plasma spraying, roughness, residual stresses, adhesion, phase composition Ceramic materials can be used due to their high hardness and chemical stability as both the wear and corrosion resistant coatings. Their deposition on moulds for manufacturing of objects from refractory and highly abrasive materials is challenging, because they are traditionally made from hard-to-blast case- hardened tool steels with hardness exceeding 60 HRC. The pressing process imposes further stringent requirements on the coating microstructure; especially in respect to its homogeneity, cohesion and wear resistance. Considering results of laboratory analyses of residual stresses and roughness of blasted surfaces, mould plates from low-carbon C45 steel were sprayed with five different ceramic materials by water stabilized plasma gun. The testing of the moulds with plasma coatings was performed on-site in a factory under standard pressing conditions. Figure 1. Typical features of the thermal spray coating microstructure [Herman 1988]