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In an effort to decrease the operating cost in the manufacturing industries, industrial knives and large tools in the forming technology should possess characteristics that increase its lifetime. One such properties would be the high surface hardness and a possibility to improve this characteristic would be through boriding. Through this method, whereby boron powder or paste is spread on the tool surface before the workpiece is heated in the oven at 750 – 950 °C for 4 – 6 h, a diffusion process occurs between the boron paste and the substrate. Borides are formed in the workpiece until a depth of approx. 250 µm from the workpiece surface depending on the substrate type and the heat treatment parameters. Genel et al. attained a hardness value of around 1700 HV at 75 µm depth when they borided AISI W1 steel workpiece at 950 °C for 6h[1],[2],[3]. By boriding a nickel superalloy, Inconel 718, at 950°C for 6h, Campos-Silva et al. achieved approx. 2200 HV at 30 µm depth[4]. However this method is disadvantageous in terms of the high energy cost and the long treatment period. As an alternative, hard chrome layer that has a hardness value between 600 to 1200 HV[5],[6] can be electroplated on the workpiece. Besides having an improved wear protection, this chrome layer offers a good corrosion protection and can be produced with a lower cost since it does not need a thermal post-treatment to increase its hardness and hence this method is less energy intensive as boriding. Admittedly the application of chrome-VI ions in the hard chrome electrolyte is without special permission prohibited in the European Union since 2017 according to REACH[7] considering its carcinogenic properties[8],[9],[10]. Another possible method would be through composite coating. Many works have been done on electroless composite nickel coating with boron nitride, silicium carbide, diamonds, aluminium oxide and titanium oxide among others used as dispersoid. After a heat treatment at 400 °C for 1 h, the hardness of most of these coating increases significantly from around 700 HV to approx. 1100 HV[11]. In the present study, the positive aspects of boriding and electroless deposition of nickel-phosphorus layer should be combined, so that a hard surface with a good wear resistant can be produced for industrial tools in a more efficient way. It is expected that lower heat treatment temperature and/or less treatment time is needed to achieve the same effect as boriding since boron particles are already homogenously distributed and have a shorter diffusion path in the thick composite layer. In addition to that, the hardness gradient throughout the heat-treated composite nickel coating should be much lower compared to boriding, especially when a high amount of boron particles is evenly distributed in the layer. The coating thickness, which will then be equated as boride layer thickness after sufficient heat treatment, can be manipulated with coating parameters and electrolyte type. Hence, through this combined method, a thicker boride layer is achievable by depositing a thicker composite nickel layer. Amorph boron powder of different sizes are used as dispersoid in the composite electroless nickel layer. The dependency of incorporated boron content in the composite layer with the particle content in the electrolyte, the particle size and the effect of ultrasound-assisted deposition is investigated. A homogenous particle distribution in the deposited layer and controllable incorporation of the boron particle in the nickel-phosphorus matrix are desirable. Furthermore, the agglomeration behaviour of the boron particle in the electrolyte is examined through laser doppler electrophoresis. The influence of the incorporated boron particle on the coating hardness and its wear resistance before and after heat treatment is studied by measuring it according to Vickers and with Calotester kaloMAX NT II. The effect of the heat treatment parameters with respect to the heat-treating temperature and time to the surface hardness and its wear resistance is investigated. At the conference results of the metallographic examinations, SEM-EDS and XRD analysis will be presented. Figure 1 |
