Application of diamond-coated cutting tools *

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1 ELSEVER Surface and Coatings Technology 73 (1995) Application of diamond-coated cutting tools * SURfACE &COATNiS HCHNOLOGY K. Kanda a, S. Takehana a, S. Yoshida a, R. Watanabe a, S. Takano a, H. Ando b, F. Shimakura b a Research and Development Division, Nachi-Fujikoshi Corp., 2 shigane, Toyama City, Toyama 93, Japan b Tools Division, Nachi-Fujikoshi Corp., 2shigane, Toyama City, Toyama 93, Japan Received 29 April 1994; accepted in final form 3 September 1994 Abstract Today, diamond-coated cutting tools are used primarily for machining non-ferrous materials such as aluminium-silicon alloys, copper alloys, fibre-reinforced polymers, green ceramics and graphite. The tool life of cemented carbide cutting tools is greatly improved by diamond coating, and typically more than 1 times the tool life is obtained. n this report we will present cutting performances of diamond-coated inserts, twist drills, square end mills and ball nose end mills. Diamond films are usually deposited more than 1 J.U11 thick to make tool life longer, since tool life is directly related to film thickness. However, increased film thickness caused many problems with cutting performance. The most severe problem was the decrease in the transverse rupture strength of the diamond-coated substrates. Because of this effect, the diamond-coated insert used under high speed and intermittent cutting conditions was supposed to suffer chipping easily. Keywords: Diamond-coated cutting tool; nsert; Twist drill; End mill; Cutting performance 1. ntroduction Diamond deposition on various substrates was origi nally made feasible by the development of practical chemical vapour deposition (CVD) diamond synthesis techniques [1,2]. Since then diamond coating of cutting tools has been one of the primary targets of CVD diamond applications [3-5], because of its high hard ness. Diamond is the best cutting tool material for processing non-ferrous materials because of its high hardness. However, it is not applicable for heavy duty processing of ferrous materials because of its low oxida tion temperature and tendency to dissolve carbon into the surface of ferrous materials. The demand for diamond-coated cutting tools is increasing for two reasons. One reason is the utilization of new materials such as hypereutectic aluminium silicon alloys and fibre-reinforced polymers (FRPs) in automobiles and electrical products, since these materials are extremely abrasive and wear resistant. Another is the never-ending demand for new cutting tools for high precision and high productivity processing. The major technical problem to be solved when using CVD diamond in cutting tool applications was to obtain * Presented at the 21st nternational Conference on Metallurgical Coatings and Thin Films, San Diego, CA, April high adherence to the substrate material. This problem was overcome at several companies and now many kinds of diamond-coated cutting tools with sufficient adher ence properties acceptable for practical use have become commercially available [6]. n this study, the adhesion strength of the diamond film was improved by the following three methods. The first was the dissolution ofthe metallic cobalt in the surface layer ofthe cemented carbide substrate, because the cobalt enhances the for mation of a graphite layer which lowers the adhesion strength. The second was the scratching the substrate surface with diamond powder. By this treatment, the nucleation density of the diamond particles was greatly enhanced and adhesion strength was increased. The third was the choice of substrate materials. n this study, the cemented carbide which did not contain titanium carbide was chosen, since the titanium carbide which is often added in the cemented carbide decreases the adhe sion strength of the diamond film. The composition of the cemented carbide used in this study was tungsten carbide and 6% cobalt. n this study, cutting tests were carried out with many kinds of workpiece materials using diamond-coated inserts, twist drills, square end mills and ball nose end mills, which were deposited by hot filament CVD and microwave plasma CVD processes. The typical composi tion of the source gas used in these CVD processes was /95/$ Elsevier Science SA All rights reserved SSDl (94)237-

2 116 K Kanda et al.jsurface and Coatings Technology 73 (1995) % methane and 99% hydrogen, and the maximum deposition temperature was approximately 173 K. The performance of these diamond-coated cutting tools was compared with that of conventional tools. n this report we will present these results and discuss the problems related to the utilization ofdiamond-coated cutting tools. 2. Performance ofdiamond-coated cutting tools 2.1. nserts nserts were the first cutting tool used as a substrate for diamond coating, because the shape was relatively simple and therefore it was easy to deposit diamond film on them. The simple shape of the inserts also lent itself to evaluating cutting performances and other film properties. The cutting performance of diamond-coated inserts was tested by face milling and turning of A39, which is a hypereutectic aluminium alloy containing 18% by weight of silicon. Although A39 has characteris tics of high strength and low specific gravity, it is highly abrasive owing to dispersed silicon particles in its struc ture. Face milling was carried out on the A39 material with the diamond-coated cemented carbide insert. The type of insert was SPGN1238, without its cutting edge chamfered. The thickness of the diamond film around the cutting edges was about 25 Jlm. The size of the upper surface of the workpiece was 95 mm x 5 mm. Two slots of 1 mm width were cut lengthwise in the work piece to simulate intermittent cutting. Cutting parame ters were as follows: cutting speed, 33 m min-1; depth of cut,.5 mm; feed rate,.5 mm rev-; dry cutting. The diameter of the milling cutter was 16 mm with one diamond-coated insert on it. For c!j1parison, face milling was also carried out with the same type of high pressure polycrystalline diamond (PCD) insert, since PCD inserts are thought to be the strongest competitor for diamond-coated inserts. The result of the face milling test is shown in Fig. 1. From this figure, the wear rate of the diamond-coated insert is found to be comparable with that of the ped insert. The flank wear of the diamond-coated insert became large compared with that of the PCD insert at the beginning of the cut. This was due to the increased roundness of the coated edge, since scars became large at the edge radius and flank wear was scaled by measur ing scars that appeared on the cutting edge. A turning test was also carried out on the A39 material with the same type of diamond-coated insert and a fine-grained PCD insert. The initial radius of the A39 workpiece was 2 mm with four slots cut of 1 mm width along the axial direction to simulate intermittent cutting. Cutting parameters were as follows: cutting speed, 4 m min-; depth of cut,.2 mm; feed rate,.15 mm rev-; dry cutting. This test lasted 24 min without any trouble. The result showed approximately the same wear tendency as that of face milling. Although results are not shown in Fig. 1, the uncoated cemented carbide inserts were also tested by face milling and turning, but their flank wear exceeded 15 Jlm after a 1 min cutting period. n the case of the diamond-coated insert, which has 11 clearance angle and 25 Jlm film thickness, substrate material is exposed after the flank wear exceeded approx imately 4 Jlm. After that the end of tool life will come soon, since the substrate material is worn away quickly for A39 alloy. Therefore, the tool life of a diamond_ coated insert with ordinary film thickness will never be greater than that of PCD. We believe the superiority of 6 5 Work : A39 Cut speed : 33 m/min Depth of cut: n.5 mm Feed rate : n.15 mm/rev Tip: SPGN238 Blade: 1 pc Dry cut 2 1 o o r....a nd coat.\illng_-<::r-_-o--::: Diatn O J ;.b- - -.t:r ;o --- -A--- A_;O _...t:r-- l' CD 15 Cutting lime (min) 2 25 Fig. 1. Wear characteristics of the diamond-coated and the PCD inserts.

3 K Kanda et al./surface and Coatings Technology 73 (1995) the diamond-coated insert over the PCD is in inserts which have chip breakers or those with multiple comers Twist drills Diamond-coated twist drills were used for processing aluminium alloys, copper alloys, glass fibre reinforced polymers (GFRPs), carbon fibre reinforced polymers, graphite and semisintered ceramics with nominally more than ten times the tool life of cemented carbide twist drills. Typical data obtained from customers and our testing are summarized in Table 1. Cutting performances of the diamond-coated tools were compared with the same type of uncoated tools. n these tests, water-soluble emulsions were used as coolant for processing the cast aluminium alloy ADC12 which contain 12% Si in its matrix and the A39 alloy, and others were processed without coolant. As shown in Table 1, better improve ment in tool life was obtained for processing workpiece materials which contain hard particles in their matrix. High stability of size and quality of processed holes is one of the advantages ofdiamond-coated twist drills. Variations in diameter and surface roughness R max of drilled holes are shown in Figs. 2 and 3 respectively. The diameter of the drill used was 3. mm and the processed work material was A39. The vertical scale of Fig. 2 shows the difference between the diameters of e ::t ,, Cemenled carbide,,,, 1 Work P'_ : Drill di.meler: A39 3. mm Revolution : 425 rpm Feed rate :.1 mmlrev Hole depth : 1 mm Coolant : ""ter-soluble emulsion Diamond coating 2 3 Number of processed holes 4 5 Fig. 2. Variation in the difference lid between the diameter of processed holes and the tool diameter. e 1 ::t.. " 6 "e 12' r---,r---' ,...--r-,.---r--, 8... ',, \ 2 Cemenled carbide 1 2 Workpi_ : Drill dllmeter : Revolution Feed rale Hole depth Coolant 3 A39 3.mm 425 rpm 1 mmjrev : 1mm : water-soluble emulsion Diamond coating Number of processed holes 4 Fig. 3. Variation in surface roughness of the processed holes. - 5 twist drills and processed holes. The diameter of the holes processed with the uncoated cemented carbide twist drill changed about 15 lm during its short life of 525 holes, whereas the diameter change of the diamond coated twist drill was only 3 lm over 5 holes. The surface roughness of holes processed with the diamond coated twist drill was better than that with the uncoated tool. t is believed that these characteristics come from the high wear resistance and low adhesion properties of diamond. Photographs of the principal cutting edge of the diamond-coated and the uncoated twist drills after the cutting test are shown in Fig. 4. The principal cutting edge of the diamond-coated twist drills after processing 21 holes had not changed, whereas that of the uncoated drill was worn away and some of the work material was adhered on the surface. The cutting force of the diamond-coated twist drill was expected to increase, because the cutting edge became rounder with the added film thickness. To prove this, torque and thrust force were measured during processing of the A39 material by changing the feed rate from.1 to.25 mm rev. -1. Other cutting parame ters were as follows: revolution, 636 rev min -1; depth of hole, 2 mm; coolant, water-soluble emulsion. The results are shown in Fig. 5. n contrast to our expectation, the cutting force of the diamond-coated twist drill was not higher than that of the uncoated cemented carbide. This result may be Table Comparison of the tool life between diamond-coated and uncoated cemented carbide twist drills Diameter Workpiece Thickness Tool life Cutting parameter (mm) material or depth (mm) WC-Co Diamond coat Revolution Feed rate (holes) (holes) (rev min -1) (mm rev-) 2.5 ADC2 5 ooסס 2 25S ooסס 2 O.OS 6. A S GFRP 3 3 ooסס 3 ooסס Semisintered WC-Co

118 K. Kanda et al.fsurface and Coatings Technology 73 ( 1995) 115-/2 Fig. 4.

6.,..,. E,,..r:: f-o.4.2.1.15.2.25 Feed rate (mm/rev) 1.8 1.6 (b) - Diamond coating - - - Cemented carbide 1.4 1.2 b 1. E ---., ".8.6 explained as follows.

Therefore, the increase in cutting force due to the roundness of the cutting edge may be negated by the decrease in material adherence.

4 118 K. Kanda et al.fsurface and Coatings Technology 73 ( 1995) 115-/2 Fig. 4. Cutting edges of (a) the diamond-coated twist drill after 21 holes and (b) the uncoated cemented carbide twist drill after 525 holes. 1.2 _ Diamond coating (a) Cemented carbide Z,.,---.8.,. c.,..,..,..,., Vi.6.,..,. E,,..r:: f-o Feed rate (mm/rev) (b) - Diamond coating Cemented carbide b 1. E ---., ".8.6 explained as follows. The higher cutting force of the uncoated twist drill may come from adhesion of the aluminium, whereas adhesion does not tend to OCcur with the diamond-coated twist drill. Therefore, the increase in cutting force due to the roundness of the cutting edge may be negated by the decrease in material adherence. The drilling performance with glass fibre was eval uated for GFRP processing, since the GFRP for electric circuit boards requires high precision holes to secure electrical conductivity through holes. Excellent fibre cutting performance was obtained with the as-deposited diamond-coated twist drill; however, the burr height exceeded the limitation of 25 11m with an average value of 55 flm. To overcome this problem, about 5 m of diamond film on the cutting edges was ground off to recover the sharpness of the cutting edge after 25 m of diamond film had been deposited. Photographs of the outer cutting blades before and after grinding process are shown in Fig. 6. The average burr height of the GFRP board processed by the resharpened drill decreased to 2 flm. The tool life of the diamond-coated twist drill listed in Table 1 for GFRP processing was obtained with this type of resharpened twist drill Square end mills The square end mill is one of the most difficult cutting tools on which to deposit the diamond film, since its shape is very complicated and the cutting blades on which a uniform diamond film should be deposited are long. However, we could overcome this difficulty by utilizing the hot filament CVD method and the rotating square end mill placed between the suitably arranged hot filaments. The peripheral milling test was carried out for the A39 material to evaluate the cutting performance of a square end mill. The diameter of the end mill was 8 mm and cutting parameters were as follows: revolution, 8 rev min - 1; feed rate, 2. m min - 1; axial depth of cut, 14 mm; radial depth of cut,.2 mm; dry cutting. The cutting test, which was performed over 4 m of total cutting length, concluded without any trouble and flank wear of the outer cutting edge was 16 flm, whereas the Feed rate (mm/rev) Fig. 5. Cutting force of the diamond-coated and the uncoated cemented carbide twist drills: (a) thrust; (b) torque. Fig. 6. Photographs of the outer cutting edge: (a) before and (b) after grinding the diamond films.

5 K Kanda et al.jsurface and Coatings Technology 73 (1995) Workpiece A39 6 Diameter 8mm Revolution 8 rpm Feed rate 2m1mon e 5 Axial d.o.c. 14mm Radil.l d.d.c. : O.2mm ::. Coolant dry 4 :l... 3 [ij i: 2 1 r " ' carbide flank wear of the uncoated cemented carbide became 3 11m after.3 m of cut. n Fig. 7, the wear characteris tics ofthe diamond-coated square end mill are compared with those of the uncoated mill. The flank wear of the cutting edge was measured at the centre of the cutting depth. From this figure, it is clear that the wear rate of the diamond-coated square end mill is extremely low compared with that of the uncoated cemented carbide. Therefore, a tool life more than 1 times longer can be expected for the diamond-coated square end mill com pared with the uncoated mill for A39 processing Ball nose end mill Diamond coaling Cutting distance (m) Fig. 7. Comparison of the wear characteristics of the diamond-coated and the uncoated square end mills. A peripheral milling test of the diamond-coated ball nose end mill was also carried out on graphite, since ball nose end mills are frequently used for processing of graphite electrodes, which are used for electrodischarge machining processing of dies. The nose radius of the mill was 6. mm and the cutting parameters were as follows: revolution, 6 rev min-1; feed rate, 1.8 m min -1; axial depth of cut, 3.5 mm; radial depth of cut,.5 mm; dry cutting. The wear characteristics of the diamond-coated ball nose end mill are compared with those of the uncoated cemented carbide in Fig. 8. Photographs of the cutting edges after the cutting test are shown in Fig. 9. The flank wear of the uncoated ball nose end mill became approximately 3 11m after a total cutting length of 2 m, whereas that of the diamond coated tool was lower than 15llm after 15 m of processing. From this result, it can be said that the diamond-coated ball nose end mill has a tool life more than 1 times longer than that of the uncoated tool for graphite processing. 3. Problems with the utilization of diamond-coated cutting tools Diamond films are usually deposited more than 1 11m thick to make tool life longer, since the tool life is directly related to the film thickness. However, increased film thickness causes many problems with cutting perfor mance. One of the problems, as mentioned above, is the increase in cutting force due to the increased roundness of the cutting edge. The smaller diameter, approximately under 1 mm, of diamond-coated twist drills will be more likely to be broken under heavy cutting conditions such as high speed and deep hole processing as a result of this effect. e 3 :::t 2 '-'.. u -"l l;j ti: 1 Cernenled carbide Workpiece Nose radius Revolution Axial d.o.c. Radial d.o.c. Graphite (Hs 65) 6mm 1.8m/min 3.5mm O.5mm OL----L._..._'----L._..._'----L._..._'----L._...L.._'----L._...L.._'--... o 5 1 Cutting distance (rn) 15 Fig. 8. Wear characteristics of the diamond-coated and the uncoated ball nose end mills.

12 K Kanda et al./surface and Coatings Technology 73 (1995) 115-12 Fig. 9.

nose end mill. The second problem comes from the roughness of the diamond-coated surface.

6 12 K Kanda et al./surface and Coatings Technology 73 (1995) Fig. 9. Cutting edge of ball nose end mills after milling test of graphite: (a) rake face and (b) flank face of the diamond-coated ball nose end mill; (c) rake face and (d) flank face of the uncoated ball nose end mill. The second problem comes from the roughness of the diamond-coated surface. Usually the surface of the dia mond film becomes rough unless any deposition tech niques are used to make the diamond grainsfine. Because of this roughness, the surface roughness ofthe processed workpiece increases. The chip removal ability also decreases because of this roughness and leads to an increase in torque of the twist drills. To avoid these problems, the diamond deposition process should be developed to make fine-grained diamond films, or otherwise some surface grinding technique should be developed. The third problem is the decreased strength of the diamond-coated substrates. As shown in Fig. 1, the transverse rupture strength of the diamond-coated cemented carbide decreases with the increased film thickness. These data were obtained by the three-point 2.4 Po. 2.2 f c: : f-oo " 2. - oo E 1.8 f- - 2 "" " 1.6 f- - " '" O 1.4 f- a> Film thickness (tl m) Fig. 1. Relationship between the transverse rupture strength and thickness of the diamond film. bending test with diamond-coated specimens of size 4 mm x 8 mm x 3 mm. The decrease in transverse rup ture strength with film thickness was well explained by assuming a stress concentration at the tip of a crack with depth equal to the film thickness. From this fact we concluded that the initial crack is formed in th brittle diamond film as a result of tensile stress, and then the crack propagates into the substrate when the stress concentration at the crack tip exceeds the critical strength of the substrate material. More details are shown in our previous paper [7]. Because of this effect diamond-coated inserts used under high speed and intermittent cutting conditions suffer chipping easily. These problems can be overcome by lowering the film thickness and changing the shape of cutting tool. 4. Conclusions The tool life of cemented carbide cutting tools was greatly improved by applying a diamond coating for processing non-ferrous materials. Because of the pro longed tool life, high precision processing also became feasible. Therefore, the demand for diamond-coated cut ting tools is gradually increasing. Diamond-coated tools are finding a niche as processing tools for aluminium alloys, copper alloys, FRP, semisintered ceramics and graphite. However, problems such as increased round ness of the cutting edge due to film thickness, surface roughness of the diamond film and decrease in the transverse rupture strength of the diamond-coated sub strates must be solved for widespread use of the dia mond-coated cutting tools. Diamond coating techniques for complicated shapes of cutting tools may also be required, since such tools are often used in factories to minimize the processing stage. References [1] S. Matsumoto, Y. Sato, M. Kamo and N. Setaka, lpn. J. Appl. Phys., 21 (1982) Ll83. [2] M. Kamo, Y. Sato, S. Matsumoto and N. Setaka, J. Cryst. Growth, 62 (1983) 642. [3] M. Murakawa, S. Takeuchi, H. Miyazawa and Y. Hirose, Surf Coat. Technol., 36 (1988) 33. [4] N. Kikuchi and H. Yoshimura, New Diamond, (1988) 42. [5] J. Oakes, XX Pan, R. Haubner and B. Lux, Surf. Coat. Technol 47(1991)6.., [ 6] K. Kanda, S. Takehana, S. Yoshida, F. Shimakura and K. shigane, Proc. 2nd nt. Conf. on the Application of Diamond Films and Related Materials, Ohmiya, MYU, Tokyo, 1993, p [7] S. Takehana, S. Tsukao and K. Kanda, Proc. 2nd nt. Conf on the Application of Diamond Films and Related Materials, Ohmiya, MYU, Tokyo, 1993, p. 571.

Workshop Practice TA 102 Lec 6 & 7 :Theory of Metal Cutting. By Prof.A.Chandrashekhar

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