Study On Technique And Mechanism Of Ultrasound-assisted Electroplating Of Nickel-based Diamond Bit

Electroplated synthetic diamond bit is suitable for drilling medium-hard and hard rock, reinforced concrete,building material,refractory material,ceramic and other hard-brittle nonmetallic materials.As an important kind of electroplated diamond tools,electroplated diamond bit has been widely applied in geological exploration,engineering investigation, machining of building materials,processing of gem,medical care and manufacturing of plastic molds,etc.The manufacturing technique originated from the early 1970's,and now the level has been highly improved through over three decades' development.But some problems still exist, such as long production cycle,poor gauge protection ability,narrow range of application and so on.Especially,with the rapid promotion and expansion of geological work,electroplated diamond bit industry is facing great challenge and opportunity.So it is impending and significant to realize high speed and high quality production of electroplated diamond bit in order to meet the expanding market demand.According to present specific conditions and existing problems,and drawing lessons from applications of ultrasound in electroplating,this paper is to introduce ultrasonic into the manufacture of electroplated Ni-based diamond bit.That is to say,ultrasonic agitation is applied to the electroplating process of diamond bit to realize the high speed and high quality production of electroplated diamond bit based on previous studies of electroplated diamond bit. Electrochemical testing technique,material structure testing technique,mechanical property testing technique were employed to study the effects of ultrasonic agitation on electrodeposition mechanism of nickel,bath performance,microstructure and mechanical property of deposit. According to the above research,technique of ultrasonic-assisted electroplating of Ni-based diamond bit was presented.At last,indoor drilling test and field drilling test of diamond bit were carried out.The effects of ultrasound on electrodeposition mechanism of nickel were studied,which included:ⅠThe effect of ultrasonic agitation on kinetics process of nickel electrodepositing was studied by Tafel curves;ⅡCathodic polarization of Nickel electrodepositing was analyzed by linear scanning curves;ⅢPolarization style of nickel electrodepositing was distinguished by cyclic voltammetry technique;ⅣThe effect of ultrasonic agitation on nickel electrocrystallization was studied by single potential step chronoamperometry;ⅤThe effect of ultrasonic agitation on hydrogen evolution reaction in electroplating was studied by linear scanning technique.In the presence of ultrasound,equilibrium potential of nickel electrodeposition shifted positively about 70mV as showed in Tafel curves.Polarization curves showed that ultrasonic agitation could obviously decrease cathode concentration polarization,but had no effect on electrochemical polarization.Cyclic voltammetric graphs indicated that,ultrasonic agitation didn't change the nickel deposition mechanism when cathode potential was more positive than -1.0V vs.S.C.E,that is,in bulk deposition cathodic process was controlled by electron transfer step and in heteropical deposition cathodic process was controlled by nucleation step;when potential shifted negatively from -1.0V vs.S.C.E,concentration polarization began to occur obviously in the static state,but cathodic process was still controlled by electrochemical polarization in the presence of ultrasound.Single potential step experiments showed that the nucleation mode and growth mode of nickel electrodepositing didn't change with ultrasonic agitation.Nickel electrodeposited with three-dimensional hemisphere growth mode,but the growing rate in direction perpendicular to substrate could be promoted and diffusion coefficient of nickel ion could be increased in the presence of ultrasound.Cathodic potential affected nucleation mode of nickel electrocrystallization obviously,which changed from progressive nucleation to instantaneous nucleation when potential shifted negatively.Polarization curves of hydrogen evolution in blank plating solution indicated that ultrasonic agitation had buffeting action to local pH near cathode and decreased hydrogen permeation and hydroxide inclusions in deposit.Based on requirements of bath performances in electroplating of diamond bit,limited diffusion current density,allowable current density,throwing power,covering power and current efficiency of bath were considered in this paper.Limited diffusion current density was approximately measured by linear scanning method.Allowable current density was primarily evaluated by cyclic voltammetic curves combined with the appearance analysis of deposit after cyclic voltammetic scanning.Throwing power was measured by far-cathode near-cathode method.Bath covering power was measured by inner holes method.Cathodic current efficiency was measured by copper coulomb method.Linear scanning experiments indicated that limited diffusion current density increased by 3.5 times with ultrasonic agitation.In the presence of ultrasound,diffusion coefficient increased and thickness of diffusion layer decreased remarkably.With ultrasonic agitation allowable current density increased from 11A/dm~2 to 34A/dm~2,that meant it increased by 2.1times.Throwing power experiments showed that throwing power of bath with ultrasonic agitation had been enhanced,for example throwing power increased by 19.1%under ultrasound of 1.5W/cm~2. Bath's covering power was improved remarkably with ultrasonic agitation.For instance,the value of coveting power was 28.5%in the static state,but it was 63.7%under ultrasound of 1.5W/cm~2.Cathodic current efficiency slightly decreased in the presence of ultrasound at current density of 2A/dm~2,but cathodic current efficiency obviously increased at 4A/dm~2:.Cathodic current efficiency was 87.2%in the static state at 4A/dm~2,but it was 93.5 in the presence of ultrasound.Macroscopic physical property and Mechanical performance of deposit is dependent on its microstructure.In this paper,crystal orientation,crystal size and surface morphology of deposit were chosen to characterize its microstructure,and crystal orientation and grain size were measured by X-ray diffraction,surface morphology was observed by scanning electron microscope.X-ray diffraction spectrum analysis showed that ultrasonic had no effect.on crystal orientation of deposit at 2A/dm~2,that is,nickel deposit had highly preferred orientation of(200); when current density increased from 2A/dm~2 to 8A/dm~2,the preferred orientation of deposit changed from(200)plane to(220)plane in static state,but deposit tended to have no preferred orientation in the presence of ultrasound.Grain size of deposit decreased when current density increased from 2A/dm~2 to 8A/dm~2,but in a word,ultrasonic agitation had no obvious effect on grain size.Scanning electron microscope graphs of deposit indicated that surface structure of nickel deposit was cone crystals with or without ultrasonic agitation,which also showed that ultrasound didn't change grain size.But the surface structure of nickel deposit was coarse grain of twined shape at 8A/dm~2 in the static state,Compared with the deposit obtained at 2A/cm~2,the crystals obviously became coarse.Deposit microstructure remarkably changed with ultrasonic agitation at 8A/dm~2.The microstructure was hemisphere crystallite and deposit was more compact,smoother and uniform.By comparison of scanning electron microscope graphs of deposit with/without ultrasound,it could be drown that with ultrasonic agitation the grain agglomeration could be obviously reduced and the deposit was refined at high current density.According to practical requirements of matrix for diamond bits in drilling,the effects of ultrasonic agitation on hardness,wearing resistance,internal stress and porosity of Ni electrodeposits were studied.Surface morphology of Ni/diamond composite coating was also studied.Deposits of 150μm thick were used for measuring microhardness at a load of 0.49N. Experimental results indicated that microhardness was enhanced significantly with ultrasonic agitation.For example,microhardness increased from HV305 to HV469 in the presence of 1.2W/dm~2 ultrasound at 6A/dm~2,that is to say it increased about 50%.The hardness of pure nickel by ultrasound-assisted electroplating corresponded with that of Ni-Co matrix used in the production,which could meet the demand of hardness of matrix for electroplated diamond bit. Wearing resistance was weighed by the mass loss of deposit during grinding.With presence of ultrasound,wearing resistance was enhanced.Internal stress was measured by deforming cathode method and deposit porosity was measured by dipping method.The results of tests indicated that ultrasonic agitation could remarkably reduce internal stress and porosity of deposit.From scanning electron microscope graphs of Ni/diamond composite coating,it could be seen that there was certain width concave slot in the interface between coating and diamond,but.the slots became narrower and the diamond periphery swelled with ultrasonic agitation,which improved bonding strength of coating-to-diamond.Through comprehensive analysis on above results,technique of ultrasound-assisted electroplating was determined as ultrasound strength of 0.9W/cm~2 and current density of 6A/dm~2. The technique of composite plating was studied out as follows:composite pre-plating in static state was used firstly after adding diamonds,and when the diamonds were wrapped about 1/5, composite plating with ultrasound-assisted started at 6A/dm~2.Combined above composite plating technique with conventional electroplating process of diamond bit,ultrasound-assisted electroplating technique for diamond bit manufacturing was established.Four layers of diamonds could be wrapped a day by ultrasound-assisted electroplating,so production cycle of electroplated diamond bit was shortened by half.Then,indoor drilling test and field drilling test were carried out to certify established ultrasound-assisted electroplating technique.The result of field drilling test was that average penetration rate increased by 33%and average bit life increased by 54.6%compared with diamond bits by conventional electroplating.So ultrasound-assisted electroplating technique for diamond bit manufacturing has following advantages:shortening production cycle by half,long bit life,high penetration rate and good adaptability,so it possesses great practical value for application.In a word,through experiments and theoretical analysis,the mechanism of ultrasound affecting nickel electrodepositing was revealed to some degree,and ultrasound-assisted electroplating technique for diamond bit manufacturing was determined;Ultrasound was successfully applied to production of electroplated diamond bit,realizing high speed and high quality production of electroplated diamond bit.The expected goal was achieved.At last,some suggestions were put forward for further study.