Microstructure And Mechanical Properties Of Cold Sprayed Nanostructured CBN-NiCrAl Cermet Coatings

Cermet coatings have been widely used in mining,machining industries etc.for their excellent wear resistance and corrosion resistance.With conventional thermal spray processes,melting of the binder materials always results in bonding material oxidation,brittle phase formation,reinforce phase decomposition and other problems and thus deteriorates the coating properties.Cold spraying is an emerging coating deposition technique.In this process,a coating is deposited through plastic deformation of spray particles in completely solid state via high velocity impact.Consequently,problems inherent to the conventional thermal spray can be minimized or eliminated,which makes cold spray a potential process to deposit high quality cermet coatings.However,some fundamental aspects in cold spray deposition of cermet were not well established so far.In this study,cubic Boron Nitride(cBN)and NiCrAl alloy was selected as strengthening material and binder matrix material,respectively,for cermet coating.Mechanical alloying(MA)was employed to prepare cBN-NiCrAl composite powders.Cold spray was used to deposit composite coatings.Effects of particle velocity and microstructure on particle deposition behavior were investigated.Microstructure and mechanical properties of the sprayed coatings were examined.Annealing treatment was carried out to modify unbonded inter-particle boundaries and cBN/NiCrAl interface.Abrasive wear behavior and wear mechanisms of the cold sprayed cBN-NiCrAl coatings were examined by a pin-on-disk test using Si C abrasive.In conventional MA method,the composite powders are easily fractured into small fragments when high proportion of ceramic components is introduced.To solve this problem,a ?step-fashion? MA process was proposed.After milling for 40 h by this method,cBN-NiCrAl composite powders with cBN particles uniformly distributed of an average size of 22 mm were obtained.NiCrAl grains were refined to be ~27 nm.cBN particle exhibited a multiple size distribution ranging from nano-scale to submicro-scale.By coating NiCrAl on the grinding medium surface and the ?step-fashion? MA method,the iron contamination arising from grinding medium can be decreased from 10.9wt.% to 2.2wt.%.20vol.%,40 vol.% cBN-NiCrAl coatings with dense microstructures were deposited by cold spray.By using a mixture of 40 vol.% cBN-NiCrAl powders and micro-sized cBN particles,cBN-NiCrAl coatings with cBN proportion of 60 vol.% were prepared.Phase transformation and grain growth of the nanocrystalline NiCrAl was not detected for the sprayed coatings Metalstable microstructures including nano-scale thick amorphous layers,preferred orientation NiCrAl nanocrystals with high density of dislocations were firstly observed at cBN/NiCrAl interfaces resulting from high velocity impact of the spray powders.The deposition behavior of the composite powders is significantly influenced by powder microstructures.An enhanced scattering by numerous grain boundaries and cBN/NiCrAl interfaces in nanostructured cBN-NiCrAl to the elastic waves during the high velocity impact effectively reduces the rebound energy of spray particle and thus results in a surprising increase of deposition efficiency with the increasing powder hardness.Vickers hardness test yielded hardness values of 1055 HV,1172 HV and 1270 HV for 20 vol.%,40 vol.% and 60 vol.% composite coatings,respectively.In indentation fracture toughness test,a high load of 20 kg did not induce cracks in 20 vol.% and 40 vol.% cBN-NiCrAl coatings.The test yielded a fracture toughness of 20.4 MPa m-0.5 for 60 vol.% cBN-NiCrAl coatings which is much higher than HVOF sprayed WC-12 Co coating with comparable hardness.Thermodynamics of cBN/NiCrAl interfacial reaction is greatly enhanced by the metastable microstructures at cBN/NiCrAl interfaces.Nano and submicro-sized cBN reacted with NiCrAl matrix at 825 ?.Micro-sized cBN reacted with NiCrAl at ~1000? which is much lower than the reproted value of 1300-1400 ?.After annealing at 825 ? for 5 h,the unbonded inter-particle boundaries were healed by atom diffusion.A 5-25 nm thick reaction layer consisting of Cr2 N and Ni3 B was formed at cBN/NiCrAl interface.Meanwhile,the nanocrystalline NiCrAl could be remained due to the relatively low temperature.The wear resistance of the cBN-NiCrAl coatings increases with the increasing cBN content.20 vol.% and 40 vol.% cBN-NiCrAl composite coatings annealed at 750 ? for 5 h show the highest wear resistance followed by the as-sprayed coatings and 825?/5 h annealed coatings.The wear resistance for 60 vol.% cBN-NiCrAl composite coatings increases with the annealing temperature increase from 750 to 825 ?.The dominant wear mechanism of 20 vol.% and 40 vol.% cBN-NiCrAl coatings was micro-ploughing.For 60 vol.% cBN-NiCrAl composite coatings,protruding micro-sized cBN particles,resulted from the preferential abrasion of the matrix between micro-sized cBN particles,effectively prevent Si C abrasives from indenting into the binder material and therefore yield a high abrasive resistance.For as-prayed coatings,peeling off of some weakly bonded particles was also observed.The wear rate of 60 vol.% cBN-NiCrAl is half that for HVOF WC-12 Co coating.