Investigation Of Laser Nanocrystallisation On Wear Mechanisms Of Electroless Ni-W-P Coatings

As a ternary alloy,electroless Ni-W-P coatings possess high wearresistance,corrosion resistance, microhardness and thermal stability.All theperformances will be enhanced after crystallization treatment in different degrees. So,electroless Ni-W-P coatings is widely used in the industrial field. In this article,theeffect of laser crystallizing and annealing crystallizing on the wear resistance andmicrostructure of electroless Ni-5.24wt%W-6.71wt%P alloys is investigated. X-raydiffractometry（XRD） coupled withs canning electron microscopy（SEM）,energydispersive spectrum（EDS） is used to anlysis the microstructures,surface morphologyand Chemical Constituents of electroless deposits.In addition,the microstructures, interms of phase type, the mass fraction and grain size of every phase, degree ofcrystallisation and lattice strain of electroless Ni-W-P coatings can be obtained byanalysis the X-ray diffraction pattern with Jade software.The microhardness and wearrate are obtained by means of measurement of microhardness and wear test underunlubrication friction respectively.By analying the effect of different heat treatments onmicrostructure,a contact can be established and the effect of various factors on thehardness and wear resistance of electroless deposits can also be clear.The results indicate that the as-plated Ni-5.24wt%W-6.71wt%P alloy depositsconsist of a mixture of amorphous and microcrystalline nickel phases.The degree ofcrystallization of deposits increased with the decrease of annealing temperature and theincrease of laser scanning velocities.At the temperature of700℃and the scanningvelocity of8mm/s,the degree of crystallization of deposits are95.3%and82.1%respectively.During the heat treatment,the grain size of Ni3P phase in larger than Niphase when Ni3P precipitate from deposits.But,at a higher temperature or a lowerscanning velocity,Ni phase have a larger grain size; The mass fraction of Ni3P phase oflaser crystallizing deposits is between the values of annealing crystallizing deposits at400～500℃; The grain size of laser crystallizing deposits at14⁸mm/s is small than thatof annealing crystallizing deposits at400～700℃. As-plated electroless deposits havethe lagerest lattice strain and the value of lattice strain decrease with the decrease ofannealing temperature and the increase of laser scanning velocities.However,the latticestrain of laser crystallizing deposits is larger relatively.During the heat treatment at200～400℃,the hardness of annealing crystallizingdeposits show increasing trend,and increase up to the maximum value（947.6HV0.1） at 400℃; Between500～700℃, the hardness of annealing crystallizing deposits showdecreasing trend;With the laser scanning velocity decreasing from14mm/s to10mm/s，the hardness of deposits increase and reach to the macimun（958.5HV0.1）;When thescanning velocity decline from10mm/s to8mm/s,there is a small decreasement in thehardness of deposits.As a result of the smaller grain size of Ni3P and Ni phases andespecially for the larger mass fraction of Ni3P,the hardness of laser crystallizing depositsis higher than annealing crystallizing deposits.As-plated deposits and the annealing crystallizing deposits at200℃are almostpolished completely under the experimental conditions that have been set.When theannealing temperature rise from300℃to500℃，the wear rate decrease slowly andreach a maximum value-5.3×10^-13m³/Nm.With the annealing temperature increase from500℃to700℃，the wear rate also increase.The drop of scanning velocity from14mm/s to10mm/s cause the decrease of wear rate.The laser crystallizing depositspresent the best wear resistance at10mm/s(wear rate is4.9×10^-13m³/Nm).The wear rateof laser crystallizing deposits increase suddenly at8mm/s.When the laser crystallizingdeposits and annealing crystallizing deposits have the best wear resistance,micro-adhesive wear is the major wear form,and there is no apparent furrows in thesurface of deposits. Adhesive wear dominates at the crystallizing condition of300℃、8mm/s;The wear feature is the result of the combined effects of adhesive wear andabrasive wear at400℃,600℃and14,12mm/s;The main wear mechanism of annealingcrystallizing deposits is abrasive wear.