FANGLILI,XUE LISHA,CHENG BINGLIANG

Abstract  The effect of plating technology and process conditions on the performance of plated-diamond was studied by utilizing different plated-diamonds made at different plating conditions as abrasives to farbricate resin grinding wheels .The parallelism,hardness and abrasive resistance of grinding wheels were assesed .The results demonstrate that the performance of grinding wheels is significantly improved when using plated-diamond abrasives .When the eletroplating process and the coating weight gain rate are both similar,the abrasive resistance of grinding wheels,using grit coated by the vacuum evaporation of titanium,is superior.The abrasive resistance of grinding wheels where the diamond is plated under high current and short time plating technology is enhanced. The coating weight gain rate was found to affect the growth of Ni spikes on the diamond surface.It is also found that the abrasive resistance of grinding wheels decreases if the plated-diamond’s coating weight increases too quickly,

Key words   diamond;plating;resin grinding wheels;technology;performance

With the development of industrial technology,the percentage of resin bond abrasives has reached 60% to 70% of the total.Resin bond diamond grinding wheels are widely used in industrial production and materials processing and they take an important place in diamond grinding tools with the advantages of high grinding efficiency,good self-sharping,low production coat and easy dressing.

The grinding efficiency of resin bond grinding wheels is significantly improved and the service life prolonged when surface coated diamond abrasives is used .Not only is the bonding strength between the abrasive and the enhanced,but the burn effect of abrasives,on the resin,is reduced.The fundamental reason is that the surface of diamond is coated with metal ,which covers defects and cracks on the surface of diamond,weakens the heat transfer between diamond and resin and isolates air impurities so that the diamond is not easily damaged .More importantly,a rough surface metal coating can enhance the mechanical coupling between diamond and bond.Therefore,it is crucial to study the plating technology and process conditions invloved in the surface coating of diamond .

In this paper,the diamond surface were coated using electroless nickel plating technology,Nickel plating technology and vacuum titanium plating technology. Different plated-diamond abrasives were prepared by changing the plating technology and process confitions.By studying the parallelism,hardness and abrasive resistance of the grinding wheels,which were manufactured by using different coated diamonds,the influence of the plating technology and process condidtions on the properties of the diamond grinding wheels were investigated.

1.Preparation and Characterization of Samples.

1.1 Preparation of plated-diamond abrasives

RVD 120/140 Diamond powder were used as the raw material to prepare,different grinding wheels abrasives at different plating conditions.Some conclusions can be drawn by comparing the plating technology and process conditions in Table1. The plating technology is divided into two kinds: electroless nickel plating+nickel plating,and vacuum titanium plating+electroless nickel plating+nickel plating,as used on 2# abrasive and 4# abrasive respectively.There are two kind of plating conditions:one is continuous plating for 24h under 2 A,the other is transitory plating under different conditions.The coating weight gain rate can reach 63% for 2# abrasive and 108% for 5# abrasive.

1.2 Preparation of different abrasive resin bond diamond grinding wheels

Fig.1 shows the specification of the grinding wheels,namely,1A1 79*1*41 RVD 120/140 B25 .

The RVD synthetic diamond content was kept constant for 1#-5# grinding wheels.The type and ratio of resin powder and filler were fixed and the pressing process conditions were as follows: pressing temperature 185℃，mixing time 1h,pressure 19 Mpa,holding time 50min with grinding wheel thickness controlled at 1mm.

1.3 Characterization of grindng wheel

Scanning electron microscopy (SEM) and energy dispersive sepctroscopy( EDS) were used to detect the surface morphology of plated-diamond abrasive grains;a DSG-6 digital display inductance micrometer was used to measure the thickness at 50 points around the grinding wheel periphery;the hardness of 20 points on the grinding wheel were measured by a TMVP-1 Vivtorinox hardness tester and the weight of the grinding wheel was measured by a FA2004 electronic balace,The wear rate was calculated by the following formula(where.m1 is the weight before tesing and m2 is the weight after tesing):

The grinding wheel was dressed on a double side polishing grinding machine,and machining tests were carried out on a UNIPOL-802 series precision grinding and polishing machine.The experimental apparatus is shown in Fig.2. Before and after the test procedure,the grinding wheel was dried then weighed.

2 Results and results analysis

2.1 Parallelism results and analysis

The test results of the standard deviation of the grinding wheel parallelism are shown in Table 2.It could be found that,the standard devitation of the grinding wheel parallelism  varies.However,the range of deviation is less than 10% ,and the flucation is less than 0.1mm.The degree of parallelism varies because the manual wheel fabracation results in an uneven material.

2.2 Hardness results and analysis

The grinding wheel hardness test results are shown in Table3.The average hardness of the grinding wheel is distributed between 1040-1140kg/mm2,which shows that the effect of abrasive on hardness is very small,under the same pressing condition.The main performance of the grinding wheel is its grinding ability,so this study focuses on the influence of different grinding materials on the abrasive resistance of the grinding wheel.

2.3 Abrasive resistance results and analysis

The grinding wheel abrasive resistance test results are shown in Table 4.The grinding weight differs from 0.2g to 0.5g. When the diamond raw materirals were of similar weight,the wear of 1# grinding wheel is the largest with a wear rate of  3.8%.The performance of grinding wheels prepared with coated abrasives is significantly improved.Taking 4# grinding wheel as an example ,the wear rate is reduced to 2.10%.Ni spikes grow on the plated-diamond surface,making the diamond surface rough,thus improving the bond between abrasive grain and grinding wheel.Therefore,the abrasive resistance of the grinding wheel is improved.

Comparing 2# and 4# grinding wheels in Fig.3, where the plating process conditions and the coating weight gain rate are both similar,the abrasive resistance of grinding wheels ,using grit coated by the vacuum evaporation of titanium process,is superior. In Fig.4,the growth of Ni spikes,coated on different plated-diamond surfaces,is shown.Fig4a shows a pure Nickel plated diamond where thick and barbed Ni spikes are mechanically attached to the diamond surface.When grinding,these Ni spikes can easily detach,which means that the diamond particles pull out easily and that the abraion resistance of the grinding wheels is reduced. In Fig. 4c where the plating technology is vacuum titanium plating,before nickel plating,the nickel spikes are rounded.With the vacuum titanium plating technology,titanium carbide forms on the diamond surface as a result of chemical reaction.Both titanium and nickel are metals and have fine webttability.Therefore,abrasive grains do not easily to pull out during the grinding process,which results in less grinding wheel mass loss.

In Fig.3, comparing 2# and 3# grinding wheels,when the plating technology and the coating weight gain rate are both similar,the plating process condition, i.e. the current and plating time,were changed.From Fig. 4a and Fig.4b we can see that,continuous plating,nicle spikes begin to grow on the diamond surface,these are relatively small and low in number.The curve in Fig.3 shows that the abrasion resistance of grinding wheels using diamond plated under high current and short time is superior.According to Literature,with increasing electroplating current,the temperature of the plating bath is increased and the maximum density of Nickel plated diamond was achieved at 50℃ .Therefore,under the condition of high current and short time plating,the nickel coating of high density is better than that using current of 2A and plating time of 24H,because the nickel spikes grows slowly and the coating is not easily worn.

2# and 4# grinding wheels have the same coating weight gain rate but different plating technology.In theory,because the plating process conditions are identical,the abrasive resistance of 5# grinding wheel should be better than that of 3#.But the result is opposite,the weight gain rate of 5# grinding wheel was 108%.The coating is relatively thick and the volume of diamond with the coating is increased.In the case of constant grinding wheel formulation,when the volume ratio of the abrasive grains increases,the specific surface area increases and the bond needs more binder.When the dosage of the binder is constant,the holding force of the bond to the abrasive grains decreases,which leads to abrasive gain pull out,thus the abrasive resistance decreases.

3 Conclusions

1.Nickel spikes are grown on plated-diamond surface,which improves the bond of the binder,so the grinding performance of a diamond grinding wheel is significantly increased.

2. When the electroplating process and the coating weight gain rate are both similar,the abrasive resistance of grinding wheels,using grit coated by vacuum evaporation of titanium,is superior.

3.The nickel coating of the abrasives,prepared by a plating process with high current and short time,has a high density,hence the abrasive resistance of the grinding wheel is improved.

4.The coating weight gain rate affects the growth status of Ni spikes coated on a diamond surface.When the weight gain rate is too high,the holding force of the bond to the abrasive grains decreases.This leads to abrasive grain pull out,thus the abrasive resistance decreases.