Machining of Single?Crystal Sapphire with Polysaccharide?Bonded Abrasive Tool

，，，，，

YUAN Julong2，YAO Weifeng3

1.School of Mechanical Engineering，Hefei University of Technology，Hefei 230009，P.R.China；2.Ultra?precision Machining Center，Zhejiang University of Technology，Hangzhou 310014，P.R.China；3.College of Mechanical and Electrical Engineering，Shaoxing University，Shaoxing 312000，P.R.China

（Received 21 July 2019；revised 21 October 2019；accepted 19 January 2020）

Abstract: A novel polysaccharide-bonded abrasive tool is proposed for the green machining of single-crystal sapphires.The prescription and manufacturing process of the proposed tool is designed，and the gelation property of polysaccharide by microwave treatment is investigated. Abrasive tool samples are fabricated，and a machining experiment on a single-crystal sapphire is performed. It is found that the crystallinity of polysaccharide gel decreases as the proportion of cross-linked polysaccharide increases. Abrasive tool samples with cross-linked polysaccharide present higher surface hardness. With the new abrasive tool，the surface quality of sapphire wafer can be significantly improved. This new tool with an abrasive to binder ratio of 2∶1 attains a material removal rate of 0.68 μm/min. It is found that increasing the abrasive to binder ratio leads to better self-dressing performance but worse material removal ability and greater loss of abrasive tool materials. The validity of polysaccharide as an abrasive tool binder is preliminarily verified.

Key words：polysaccharide binder；microwave treatment；single-crystal sapphire；green manufacturing

0 Introduction

Surface machining，such as the back-grinding and polishing of a single-crystal sapphire substrate，is an important step in the manufacturing process of advanced lighting chips. It requires high machining efficiency and surface quality［1-2］. In response to the rapidly increasing market demand，the productivity in the precision machining of wafer substrates has re?markably increased in recent years；however，it has also introduced negative environmental effects that cannot be ignored［3］. Currently，the realization of high-quality and high-efficiency green machining of semiconductor substrate wafers is one of the key problems in ultra-precision machining［4-5］.

An effective technique for the ultra-precision machining of single-crystal sapphire wafer is chemomechanical grinding （CMG）［6-7］. With this new method，the surface roughness of a machined sap?phire surface can have aRavalue of less than 1 nm，and the material removal rate can reach 0.22 μm/min［7］. By employing the mechano?chemical ef?fect，the material removal with high surface integri?ty can be achieved［8-9］. Currently，the abrasive tool binder in the CMG is typically phenolic resin whose raw materials are toxic oil derivatives. In view of this，the use of phenolic resin in the manufacture of abrasive tools and machining endangers human health and environment［10］. Moreover，because oil is non-renewable，the long-term use of phenolic resin is unreliable. In recent studies，efforts have been made to investigate new types of binders，such as sodium alginate hydrogel［11］and magne?sium oxychloride［12］，for abrasive tools to improve their environmental friendliness and machining per?formance.

Polysaccharide， with hydroxyl functional groups in its molecular structure，is a type of high polymer that can be prepared from natural products.Starch is the most common natural polysaccharide，and various modification processes（cross-linked，oxidation， etc.） can significantly improve its strength and stability. Modified polysaccharide has been widely used in products，such as environmentprotecting tableware，adhesives，and casting，as well as in 3D printing materials［13］.

In this paper，a novel polysaccharide-bonded abrasive tool is proposed. The prescription and man?ufacturing process of this proposed abrasive tool is designed，and the gelation properties of polysaccha?ride by microwave treatment are also investigated.A sample of the abrasive tool is manufactured，and a machining experiment on a single-crystal sapphire using the new abrasive tool is performed. The sur?face textures of abrasive tool samples are observed，and the validity of polysaccharide binder is dis?cussed.

1 Polysaccharide

1.1 Natural polysaccharide

In the natural world，natural polysaccharide（NP）widely exists in plants，animals，and microor?ganisms. The molecular structure of the adopted polysaccharide is a single-glucose chain connected by glycosidic linkages，as shown in Fig.1. There are three alcoholic hydroxyls in a single glucose unit：C6 primary alcoholic hydroxyl，and C2 and C3 sec?ondary alcoholic hydroxyls. Alcoholic hydroxyl has good reactivity and can create hydrogen bonds，which can link neighbouring polysaccharide units and provide the polysaccharide with high cohesive strength.

Fig.1 Polysaccharide from natural product

1.2 Cross?linked polysaccharide

To further improve the abrasive tool strength，cross-linked polysaccharide（CLP）is adopted and mixed with native polysaccharide，which functions as the binder. Cross-linking is a common method used in the preparation of gel materials［14］. By crosslink modification，the alcohol hydroxyls of polysac?charide can react with reagents having dual or multi?ple functional elements. Hydroxyl groups from dif?ferent polysaccharides can therefore connect with each other to form a reticulated structure，which im?proves the abrasive tool’s mechanical strength. The most familiar cross-link agent is epichlorohydrin or sodium trimetaphosphate. For example，when epi?chlorohydrin is employed as the cross-linking agent，the cross-link reactions shown in Fig.2 will occur at pH 10 and 30 ℃.

Fig.2 Cross-linking of polysaccharide by epichlorohydrin[15]

1.3 Gelation of polysaccharide by microwave treatment

To use polysaccharide as binder，it has to be heated in aqueous environment until the micro-parti?cles that swell with its crystalline area disappear（this process is called gelation）. The glucose chains thereafter disperse in water，attach on the abrasive surface，and link with each other through the hydro?gen bond to form a gel network. When the aqueous system’s temperature decreases， free glucose chains regularly cluster through hydrogen bonds and form microcrystallines. This process is known as re?crystallisation.

In this study，the gelation by microwave treat?ment is adopted. Compared with traditional polysac?charide gelation methods，such as the hydrothermal gelation，the microwave treatment is time-saving and results in even gelation. The gelation and re?crystallisation of polysaccharide are crucial to achieve the mechanical properties of polysaccharidebonded abrasive tools. A gelation experiment is ac?cordingly performed with the prescribed design list?ed in Table 1.

Table 1 Designed prescriptions of gelation experiment

For each prescription listed in Table 1，four different microwave treatment parameters are adopt?ed：#1（P100T20），#2（P80T40），#3（P40T80），and #4（P20T100）. Here，the microwave oven out?put power is 900 W，Prepresents the percentage of the full output power， andTis the heating time（s）.

The gelation in the experiment is achieved as follows. Polysaccharide is added to the distilled wa?ter. Heating and mixture gelation are realized by mi?crowave treatment with a working frequency of 2 450 MHz. After heating，the single glucose chains link with each other and form a highly viscous gel.The gel is thereafter exposed to air（ambient tem?perature is 10 ℃）for 24 h and then placed in an incu?bator with a temperature of 3 ℃for 4 h. Finally，the gel is dried by hot air until its weight becomes con?stant. The gel dries into a hard semi-transparent sol?id film.

It is found that gel samples with cross-linked polysaccharides have a higher hardening speed，and the cross-linked polysaccharide improves the strength of polysaccharide gel. On the other hand，the gel crystallinity also affects the strength of the polysaccharide binder. The gel samples are accord?ingly tested by X-ray diffraction（XRD）（DX-2700，Haoyuan Instrument Co.，China；step size：0.02；scan speed：1°/min）. The XRD results of the gel with prescriptionBand the dispersion peak caused by the microcrystalline of polysaccharide are shown in Fig.3. There is a strong peak near the diffraction angle of 17° and two medium peaks near 22° and 24°，indicating that the crystal isB-type form［16］.

Fig.3 XRD spectrum of gel with prescription B

It can be observed in Fig.3 that the gel samples mainly have an amorphous structure with an ex?tremely low crystallinity. Their crystallinity can be calculated by

whereXC，IC，andIAare the crystallinity，cumula?tive diffraction intensity of crystalline，and amor?phous area，respectively［17］. The calculation results are shown in Fig.4.

Fig.4 Gel crystallinity with different prescriptions

Fig.4 shows that the gel crystallinity decreases as the proportion of cross-linked polysaccharide in?creases. This occurs because the cross-linked poly?saccharide can delay the polysaccharide recrystallisa?tion［18］. In the following composition design of poly?saccharide-bonded abrasive tool，the ratio of the nat?ural polysaccharide to cross-linked polysaccharide is accordingly set to be 1∶1. It is also found that sam?ples with P100T20 microwave treatment have the highest crystallinity.

2 Polysaccharide?Bonded Soft Abra?sive Tool

2.1 Composition

The composition details of the new abrasive tool are summarised in Table 2. The abrasive to binder ratios（ABRs）are 2∶1，4∶1，and 6∶1，and the natural/cross-linked polysaccharide proportions are 1∶0 and 1∶1. For each prescription listed in Ta?ble 2，three different microwave treatment parame?ters are adopted：#1（P100T20），#2（P80T40），and #3（P20T100）.

Table 2 Prescription of polysaccharide?bonded abrasive tool

The selected abrasives are #600 SiO2spherical beads with a hardness of 2 on the Mohs’scale（the so-called soft abrasive）. Under a certain contact pressure and relative speed，the mechano?chemical reaction between SiO2and sapphire occurs as fol?lows［19］.

The resultants of the above reactions are mull?ite or cyanite with a hardness of 6―7 on the Mohs’scale that can be removed by SiO2abrasives. Thus，the non-damage material removal of sapphire can be realised.

2.2 Manufacturing process

The manufacturing process of polysaccharidebonded soft abrasive tool is shown in Fig.5.

After full stirring，the distilled water is added to the mixed powder of polysaccharide and abrasive.The mixture has to be heated until the polysaccha?ride swells and becomes single molecules because polysaccharide does not dissolve in water. The heat?ing and gelation are realized by microwave treat?ment with a working frequency of 2 450 MHz. After heating，the single glucose chains link with each oth?er and form a highly viscous gel. A refrigeration step of 3 ℃with a polypropylene（PP）film cover is re?quired. After the refrigeration，the gel is subjected to drying using alcohol displacement and hot air un?til the weight of the gel becomes constant. As shown in Fig.5，all the raw materials used during the manufacturing process are non-toxic，and all the solvents are waterborne. Compared with the manu?facturing process of the abrasive tool using resin binders，such as phenolic resin，the manufacturing process of the new abrasive tool is environmentfriendly.

Fig.5 Manufacturing process of polysaccharide-bonded abrasive tool

The abrasive tool samples with no cross-linked polysaccharides are more severely warped than those with cross-linked ones. The surface hardness of the abrasive tool samples is measured by a LX-D digital shore durometer and the number of sampling points is four. The measurement results（except for sample #c-3 because of its poor forming quality in the boundary area）are presented in Fig.6.

Fig.6 shows that the surface hardness of abra?sive tool samples decreases with the increase in abra?sive?binder ratio，and the samples with cross-linked polysaccharides have a higher surface hardness.This indicates that cross-linked polysaccharides im?prove the abrasive tool’s strength.

Fig.6 Surface hardness of abrasive tool

3 Machining Experiment of Sap?phire Wafer

3.1 Machining experiment setup

To study the machining performance of the new abrasive tool，a machining experiment is per?formed on a single-crystal sapphire wafer using a 10-mm diameter and 0.7 mm thick multi-wire saw. The crystal plane for machining is C plane（0001）. The experimental setup is shown in Fig.7. The sapphire wafer is fixed on an auto-maintain-load platform.The machining parameters are summarized in Table 3. If the sapphire wafer is parallel to the face of abrasive tool，the surface layer of the abrasive tool may be scraped by the edge of the wafer during friction. This can affect the evaluation of the abra?sive tool’s self-sharpening performance. The sap?phire wafer is accordingly set to be slightly tilted to?wards the abrasive tool with a sectional shape fric?tion area. Abrasive tool samples with no crosslinked polysaccharides are not utilized in the machin?ing experiment because of their poor forming quality.

Fig.7 Experiment setup

Table 3 Machining parameters

3.2 Experimental results

After the experiment，the machined surface of sapphire wafer is cleaned with ethanol and distilled water.The machined areas of sapphire wafers are ob?served by optical microscopy（XZJ-2030，Phoenix，China），as shown in Fig.8. In the boundary area ofA-2，the rough surface marks created by wire saw cutting can be observed. The machined area exhibits a considerably smoother surface. Each of the nine samples of the abrasive tool can significantly im?prove the surface quality of sapphire wafer. Grooves with no brittle cracks are formed by the scratching of SiO2abrasives because the sapphire wafer is fixed as the abrasive tool rotates. During this period，the sap?phire material is removed by the mechano?chemical reaction between SiO2abrasives and sapphire. TheC-series samples present the best machining quality among the samples with considerably few grooves.

Fig.8 Surface texture and roughness of machined sapphire wafer

The profile of the abrasion areas on the sap?phire wafer surface is measured by a profilometer（SV3200，Mitutoyo，Japan）. A typical profile of a machining spot is shown in Fig.9，where the remov?al depth of sapphire materials can be determined.The removal depth of different abrasive tools is shown in Fig.10. This indicates that this depth gen?erally decreases as the ABR increases. Among theAandBsamples，those that have been subjected to P80T40 microwave treatment present a lower mate?rial removal depth，which may be related to the crystallisation of polysaccharide binder. The maxi?mum removal depth with a value of 20.51 μm is cre?ated byA-1 abrasive tool. The material removal rate ofA-1 abrasive tool attains 0.68 μm/min over a 30 min machining time. The machining results indi?cate that the application of polysaccharide as an abra?sive tool binder is feasible.

Fig.9 Section profile of machining spot by abrasive tool A-1

Fig.10 Removal depth by different abrasive tools

4 Discussion

Abrasive tool samples after machining are shown in Fig.11. An annular yellowish scorching mark appears in the abrasion area ofA-series sam?ples. Scattered scorching marks are also distributed on the surfaces ofB-series samples. Scorching marks are not found onC-series samples.

Fig.11 Abrasive tool after machining

The surface textures of the abrasive tools are observed by XZJ-2030 microscopy with a magnifica?tion of 160× in polarisation mode. The direct obser?vation of SiO2grits and consolidated polysaccha?rides is difficult because they are all transparent.Here，iodine tincture is utilized because triiodide ion can cause complexities in polysaccharide glucose chains and modify the light-absorbing property of polysaccharide［20］. When the iodine tincture is smeared，the polysaccharide binder becomes pur?ple，and the bonded spherical SiO2abrasives can be distinguished by cross-extinction under polarised light. For the series ofAandBsamples，iodine tinc?ture is smeared on the scorching areas. The observa?tion results of samplesA-1，B-1，andC-1 are shown in Fig.12.

The cross-extinction of spherical SiO2abra?sives is indicated by yellow arrows in Fig.12. The figure shows that before the machining experiment，the distribution density of SiO2abrasive increases with the ABR. After machining，a blocking area with few abrasives is observed on the surface of sampleA-1. Compared with the initial surface，it is observed that sampleB-1 has few blocking areas with a lower distribution density of abrasives.There is practically no change observed on the sur?face texture of sampleC-1. It seems that theC-se?ries samples have the best self-dressing perfor?mance.

The powder produced during the machining process is thereafter collected，as shown in Fig.13.FromAtoC，the amount of powder increases，indi?cating that abrasive tool materials are lost. As the ABR increases，the abrasive holding ability of the polysaccharide binder becomes weaker. This means that the abrasive easily falls off from the tool surface during the machining process. This explains why theC-series samples have the best self-dressing per?formance and lowest material removal ability.

Fig.12 Micro-textures of abrasive tool surface

Fig.13 Powder produced in machining process

FromAtoC，the powder colour also becomes lighter. During the machining process，the friction heat is produced in the frictional region between the polysaccharide binder and the sapphire. The polysac?charide partly converts to furaldehyde，especially 5-hydroxymethylfurfural，by dehydration under high temperatures［21-22］. The furaldehyde thereafter be?comes yellow and further transforms rapidly into a brown resin-like polymer under the action of the heat and oxidation. In Fig.13，the powder colour of samples fromA-1 toC-1 becomes lighter（from brown to greyish white）. The reason lies in that theA-series samples have the best abrasive holding abil?ity，a higher temperature can be reached during the machining，and a larger amount of furaldehyde is generated in the powder.

The powders produced during the machining process with different abrasive tools are observed by field emission scanning electron microscopy（SEM）（FEI Inspect F50，USA），whose results are shown in Fig.14.

Fig.14 SEM results of powder produced during machining process

It can be indicated from Fig.14 that as the ABR increases，there are more dropped abrasives appear?ing in the powder，and the surface of dropped abra?sive becomes smoother. It means that the depth of the polysaccharide binder attached to the SiO2abra?sive surface decreases，and the sectional area of the bonding link between adjacent abrasives also de?creases as the ABR increases. As a result，the bond?ing strength between adjacent abrasives and the abil?ity of the tool to hold the abrasives decrease，thus affecting the machining and self-dressing perfor?mance of abrasive tools.

The SEM image of single abrasive in the pow?der produced by theA-series abrasive tool is shown in Fig.15. It can be observed that the polysaccharide binders are evenly attached to the abrasive surface，indicating that polysaccharides have good compati?bility with SiO2abrasives. It can also be concluded that the abrasives fall off because of cohesive frac?tures between adjacent polysaccharide binders and not the result of adhesives fractures between poly?saccharide binders and SiO2abrasives. The low bonding strength of the polysaccharide limits the ability of the tool to hold the abrasives.

Fig.15 SEM image of single abrasive in powder produced by A?series abrasive tool

To further study the influence of ABR on the machining performance of abrasive tools，the pow?ders produced during the machining process employ?ing different abrasive tools are analysed by energy dispersive spectroscopy（EDS）（QUANTAX 100，BRUKER，USA）. The EDS results are shown in Fig.16.

Fig.16 EDS results of powder produced during machining process

In Fig.16，the two 2 keV peaks are caused by the Au element introduced through the gold-spray?ing process. The Si and O elements mainly belong to the SiO2abrasives that fell off. The Al element is a solid?solid reaction product between SiO2abra?sives and single-crystal sapphire. The carbon ele?ment mainly belongs to the polysaccharide binder at?tached to the abrasives that fell off. As the ABR in?creases，the peak intensity of the Al element de?creases，proving that the increase in the ABR de?creases the abrasive tool’s holding ability. This re?duces not only the heat generated in the friction area between the abrasive and single-crystal sapphire but also the reaction rate of the solid?solid reaction prod?uct between SiO2abrasives and single-crystal sap?phire. The peak intensity of carbon element also de?creases with the ABR increase. This similarly indi?cates that the higher the ABR，the lower the bond?ing strength among abrasives，and the lower the abrasive holding ability of tools.

By considering the micro-texture of a singlecrystal sapphire machined by different abrasive tools，it can be deduced that for abrasive tools with different ABRs，the material removal mechanics dif?fer. TheA-series abrasive tools with an ABR of 2∶1 have better abrasive holding ability，and the materi?als are mainly removed by the grooving process（twin body wear）. TheC-series abrasive tools with an ABR of 1∶6 have worse abrasive holding ability，and the materials are mainly removed by the rolling and micro-grooving of abrasives that fell off（threebody wear）. This explains why theC-series sam?ples present the best quality in the machining of sap?phire wafer among the abrasive tool samples.

5 Conclusions

A novel polysaccharide-bonded abrasive tool for green surface machining of single-crystal sap?phire is proposed. The following conclusions can be drawn.

（1）The crystallinity of polysaccharide gel de?creases as the proportion of cross-linked polysaccha?rides increases. The prescription of the polysaccha?ride-bonded abrasive tool with SiO2abrasives is de?signed，and the manufacturing process of the new abrasive tool is established. The composition and manufacturing process of the new abrasive tool are fully environment-friendly. Abrasive tool samples with various ABRs are made. The surface hardness of abrasive tool samples decreases with the increase in the ABR，and the samples with cross-linked poly?saccharides present a higher surface hardness.

（2）The machining of single-crystal sapphire wafer based on the new abrasive tool is performed，and results show that all the abrasive tool samples can significantly improve the surface quality of sap?phire wafer. The removal depth generally decreases as the ABR increases. The material removal of abra?sive tools with an ABR of 2∶1 attains the rate 0.68 μm/min. The machining results indicate that the ap?plication of polysaccharide as an abrasive tool binder is feasible.

（3）As the ABR increases，the abrasive hold?ing ability of polysaccharide binder becomes weak?er，leading to a better self-dressing performance of the abrasive tool. Moreover，the material removal ability decreases，and the material loss of the abra?sive tool increases.

In the future study，the balance between the material removal and self-dressing abilities of the polysaccharide-bonded abrasive tool should be inves?tigated. Furthermore，the prescription of the new abrasive tool has to be modified to improve its me?chanical strength.