Influence of Posttreatment on the Microstructural

Authors: O Ali R Ahmed N H Faisal Nayef M Alanazi LM Berger A Kaiser FL Toma E K Polychroniadis M Sall Y O Elakwah M F A Goosen

Publish Date: 2017/01/19

Volume: 65, Issue: 2, Pages: 33-

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Abstract

The potential to improve the tribomechanical performance of HVOFsprayed WC–12Co coatings was studied by using aqueous WC–12Co suspensions as feedstock Both assprayed and hotisostaticpressed HIPed coatings were studied Mathematical models of wear rate based on the structure property relationships even for the conventionally sprayed WC–Co hardmetal coatings are at best based on the semiempirical approach This paper aims to develop these semiempirical mathematical models for suspension sprayed nanocomposite coatings in assprayed and heattreated HIPed conditions Microstructural evaluations included transmission electron microscopy Xray diffraction and scanning electron microscopy equipped with energydispersive Xray spectroscopy The nanohardness and modulus of the coated specimens were investigated using a diamond Berkovich nanoindenter Sliding wear tests were conducted using a ballonflat test rig Results indicated that the HIPing posttreatment resulted in crystallization of amorphous coating phases and increase in elastic modulus and hardness Influence of these changes in the wear mechanisms and wear rate is discussed Results are also compared with conventionally sprayed highvelocity oxyfuel hardmetal WC–Co coatingsAccording to the state of the art wearresistant WC–Co coatings are predominantly obtained by highvelocity oxyfuel spraying HVOF from conventional feedstock powders During spraying of WC–Co powders significant changes in the chemical and phase compositions can occur 1 The past two decades have seen extensive research in optimizing the feedstock powder characteristics process parameters and posttreatments of wearresistant hardmetal coatings 2 3 4 5 6 7 8 9 10 11 12 13 14 Most research however has related to the coatings sprayed from agglomerated and sintered powders with the typical particle size ranging from 10 to 50 μm and WC grain size ranging from 08 to 35 µm 2 3 4 5 6 7 Optimization of these coatings has resulted in coating microstructures with negligible porosity high fracture toughness and minimization of secondary carbide phases 2 3 4 5 6 7 8 9 10 15 16 17 18Nanocomposite hardmetal coatings are still undergoing research and development These coatings have the potential to provide further improvement in their tribomechanical performance Previous researchers have used conventional thermal spray systems to deposit coatings from nanostructured WC–Co feedstocks 9 10 19 20 21 In these previous investigations agglomerated and sintered nanosized particles have been used for the deposition of nanostructured thermal spray coatings generally resulting in a predominantly bimodal coating structure where the coating architecture exhibits micrometersized zones with nanometersized structure 19 Suspension spraying is currently the only viable route capable of depositing nanocomposite coatings directly from nanosized feedstock powder However thermal spraying of these nanocomposite hardmetal coatings especially WC–Co is technologically challenging By this reason so far only few studies on this topic are published in the literature 22 23 24The first investigation of suspension sprayed WC–Co coatings was done by Oberste Berghaus et al 24 They conducted a comprehensive study in order to develop WC–12Co nanocomposite coatings by suspension spraying using a commercial atmospheric plasma spray process APS AXIAL III Northwest Mettech Corp Canada of an ethanolbased suspension with 20 wt solid concentration and an internal injector They indicated that coatings with low porosity were produced which showed a maximum hardness of about 700 HV03 These coatings also showed a pronounced amorphous “hump” in the XRD pattern When a nanostructured agglomerated and sintered powder in an ethanol/ethylene glycol suspension was used the crystallinity of the coating and the hardness about 780 HV03 increased In our previous studies nanostructured suspension sprayed WC–Co HVOF coatings were successfully produced from aqueous suspensions of nanocomposite particles 22 23 Moreover despite a high carbon loss during spraying these SHVOF coatings had hardness values comparable to those of conventional HVOF WC–Co coatingsThermal treatment is one of the most important posttreatment methods for thermal spray coatings 1 Main effect can be from a shift of the phase composition from the asdeposited nonequilibrium toward the equilibrium state but also change in the residual stress state densification by sintering and the crossdiffusion of elements between the coating and the substrate The extent of these changes is dependent upon the composition and microstructure of the starting powder and deposited coating Due to the high oxidation rate above 600 °C the thermal treatment of WC–Co coatings has to be carried out in inert atmosphere or vacuum Microstructural effects appearing in a WC17Co coating during thermal analyses for determination of the thermophysical properties have been described previously 25 These microstructural changes were also studied by mechanical spectroscopy 26 Depending on the chemical composition of the assprayed coating and heat treatment conditions a replacement of the amorphous phase metallic W as well as the nonequilibrium phase W2C by the formation of ηphases was observed eg 27 28 29 30 31In our previous research it was shown that the heat treatment of HVOFsprayed WCbased coatings ie WC–Co and WC–NiCrBSi coatings can be effectively applied to crystallize amorphous phases and hence further improve the tribomechanical performance of coatings 15 16 17 32 33 A better bonding resulting from the heat treatment at the intersplat and interlayer levels and changes in through thickness profile of residual stresses was predominantly responsible for the improvement in fracture toughness hardness and elastic modulus 34 35 As a result sliding wear and rolling contact fatigue performance also improved The extent of these improvements was dependent upon the selection of time and temperature used for vacuum heat treatment and the additional parameter of pressure in HIPing It is also possible to control the diffusion layer at the coating substrate interface by varying the heat treatment parametersWC–Co coatings are highly suitable for service under dry sliding conditions preferably at room temperature as mentioned above Studies have been performed in pairs of different materials for the counterbody such pairs can show total wear rates sum of wear rates of the coating and the counterbody of less than 10−6 mm3/Nm ie comparable to values typically measured under mixed/boundary conditions 36 37The emphasis in the current paper is to consider the influence of a posttreatment of suspension sprayed SHVOF WC–12Co coatings via hot isostatic pressing HIPing on the tribomechanical performance against the previously reported assprayed SHVOF coatings 22 23 In order to support the interpretation of the changes the influence of carbon loss on the expected equilibrium phases was evaluated by thermodynamic calculations This paper also aims to develop semiempirical mathematical models of wear rate based on structure property relationships for suspension sprayed nanocomposite coatings in assprayed and heattreated HIPed conditions Microstructural and tribomechanical investigations included scanning electron microscopy SEM energydispersive Xray spectroscopy EDX transmission electron microscopy TEM Xray diffractometry XRD nanohardness and sliding wear evaluations

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