Authors: Jothi Sudagar K Venkateswarlu Jainshe Lian
Publish Date: 2009/09/30
Volume: 19, Issue: 6, Pages: 810-818
Abstract
Dry sliding wear behavior of electroless nickelphosphorus EN coating of thickness ~35 μm deposited on a 7075T6 aluminum alloy was studied EN was deposited from a bath with sodium hypophosphite as a reducing agent In asdeposited conditions plating with 69 wt phosphorus has a mixture of amorphous and microcrystalline phase Three pretreatments of Zn zincate Ni strike and absorbed hypophosphite layer were given to the substrate before EN coating to examine their wear performance The surface morphology of the pretreatments was studied by a confocal laser scanning microscope The performances of these pretreatments of EN were evaluated by dry sliding wear studies and followed by SEM studies The results suggest that the wear behavior of EN mostly depends on the pretreatment conditions Heat treatment at temperature of 400 °C can enhance the wear resistance properties for all types of pretreatment conditioned samples and in addition that the average coefficient of friction of 400 °C specimens μav had minimum value as compared to 200 °C specimens Ni strike provided better interlocking adhesion between EN and Al and this pretreatment noticeably improved the wear frictional and hardness behavior of the EN coatings on 7075 Al substrate and further enhanced it by heat treatment of 400 °C/hElectroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions coated to a base substrate without applying an external electric voltage Ref 13 Electroless deposition can also provide a protective coating for common metal surfaces Ref 4 5 In general an electroless nickelphosphorus EN deposit is less porous and more uniform than an equivalent electrolytically plated deposit and thus provides better corrosion and wear protection Ref 6 7 and is considered as the most simplest and economic method to coat on steel aluminum copper plastics and many other materials Suitable heattreatment processes can further enhance the hardness and wear resistance of EN deposits In general heattreatment temperature of 400 °C results optimum hardness and wear resistance properties and are similar to ‘hard’ chromium deposits Ref 8 The performance of EN deposits can also be enhanced through the codeposition of inert particles such as polytetrafluoroethylene PTFE silicon carbide or boron nitride Ref 9 forming a composite coating Ramalho and Miranda Ref 10 have closely examined the influence of PTFE on the sliding and friction properties of NiP coated This investigation showed that the introduction of PTFE particles in the electroless NiP coatings produces a significant rise in the wear resistance Wear properties of electroless NiP coatings are related mainly to their phosphorus content and microstructural details In addition low phosphorous has better wear resistance compared to high phosphorous coatings and other mechanical property Ref 11 But the pretreatment of EN on Al alloys has more influence in wear propertiesThe paramount importance of better mechanical properties depends on the pretreatment of the alloy prior to coating Moreover all aluminum alloys are extremely susceptible to the formation of instantaneous oxide layer and results in an unattractive appearance with inferior mechanical properties However the electroless plating on aluminum alloys meets many challenges in the processing of plating and research on this topic is scanty Attempts have been made to study the dry sliding wear behavior of different pretreated condition of 7075 Al alloy The latter is a very important engineering material and widely employed in aerospace industry for the manufacturing of different components due to its high strengthtoweight ratio and other superior physical and mechanical properties Ref 12The most difficult part of plating on aluminum is to develop an appropriate pretreatment process The most widely used pretreatment processes for plating on Al alloys are nickel strike zinc immersion zincate and adsorbed hypophosphite HP layers Ref 13 The present investigation is aimed to study the dry sliding wear and frictional properties of an EN coating with three different types of pretreatments on a highstrength 7075T6 aluminum alloy The effects of different pretreatments including zinc nickel and hypophosphite immersion on wear resistance have been investigated while the other processing conditions for electroless coating are unchanged A dry sliding pinondisc wear test method ASTM G99 is employed to investigate the wear resistance of the coated samples under various test conditions The 3D surface morphology of pretreatment without EN is studied under a confocal laser scanning microscope LEXTOLS 3000 The wear performance of these coated samples is discussed in the light of SEM/EDAX JEOL JSM 840A studies
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