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1540 |
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Effect of thermal history on properties of WC-10Co4Cr HVOF coatings |
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Tommi Varis* / VTT Technical Research Centre of Finland, Finland Tomi Suhonen / VTT Technical research center of Finland, Finland Kimmo Ruusuvuori/ VTT Technical research center of Finland, Finland Erja Turunen/ VTT Technical research center of Finland, Finland |
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Despite the extensive applicability of HVOF WC-Co coatings, the effect of processing conditions during spraying is still not well understood. Coating performance may vary significantly between different process conditions even if similar powder and spray parameters were used. Solubility of wolfram and carbon to the Co or CoCr matrix during HVOF spraying of WC-Co or WC-CoCr -coatings is well demonstrated. As a consequence of W and C enrichment and due to rapid cooling the matrix is not crystallized as an equilibrium structure. Amorphous structures and crystalline secondary carbides were usually formed on splat boundaries and matrix. The properties of matrix are strongly dependent on the phases formed during rapid quenching of spray particles. Typically WC-CoCr coating are sprayed between 80 - 200 °C substrate temperatures depending on size of work piece. Thermal history of the coating layers can be affected also by gun traverse speed and pitch of the traverse. It is also clear that thermal history of the first and the last coating layer is not similar. However, the effect of thermal history of coating layers on the phase structure and coating properties are not systematically studied. The motivation of the study is to understand the effect of temperature history on phases formed through the coating thickness. Phases formed during coating process were determined by XRD-analysis. The amounts of amorphous and crystallized secondary carbides were compared. The effect of different structures on the coating properties such as hardness, wear resistance, elastic modulus and fracture toughness were determined. The possibility to control the coating structure and residual stresses by altering the thermal history is discussed. |