The main mechanisms involved in the wear of cutting tools are reviewed. Evaluation of the different coating properties required for the reduction of the different kinds of wear was also reviewed. The types of coatings and their ranges of applicability are presented and discussed in view of their properties. Various coating processes as well as their advantages and shortcomings are described. Potential future developments in the field of wear-resistant coatings are discussed

Salik, J.

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C+)1
NASA Technical Memorandum 83512
A Review of the Use of Wear-Resistant
Coatings in the Cutting-Tool Industry -*
J. Salik
Lewis Research Center
Cleveland, Ohio
(NASA-T11-83511) A REVIEW OF THE USE OF
	 N84-11295
WEAR-RESISTANT COATINGS IN THE CUTTING-TOOL
INDUSTRY (WASA)	 11 p HC A02/MF A01 CSCL 11C
Unclas
GJ/27 42458
Presented at the
International Workshop on Industrial Applications of Plasma Chemistry
sponsored by the Israeli Research Council
Ashkelon, Israel, April 1981
^ 0
A REVIEW OF
THE USE OF WEAR-RESISTANT COATINGS IN THE CUTTING-TOOL INDUSTRY'k
J. Salik**
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
The main mechanisms involved in the wear of cutting tools are reviewed
and applied for the evaluation of the different coating properties required
for the reduction of the different kinds of wear. The types of coatings
0)	 commonly used today in the cutting-tool industry, their effect on tool life
r0	 and their ranges of applicability are presented and discussed in view of their
properties. Various coating processes as well as their advantages and short-
``'	 comings are described. Potential future developments in the field of wear-
resistant coatings are discussed.
IN-nn000TION
Cutting tools have to satisfy different requirements regarding the bulk
and the surface. The bulk has to be tough in order to prevent the propagation
of cracks which are formed mainly at the surface and also has to be hard in
order to resist plastic deformation. The main requirement from the surface,
which is our main concern here, is resistance to wear. These requirements are
hard to meet using one kind of material and this has led to the development of
coated tools in which the surface material is different from that of the bulk.
The first industrial usage of coatings ii the cutting-tool industry was
inspired by a work done at the Laboratorie Suisse de Recherches Horloge
(ref. 1) where a CVD process for coating moving parts with titanium carbide
was developed with the purpose of red!_lcing their wear rate, and the first
pa.ent for coating cemented carbide cutting tools with TiC was filed in 1969
by Sandvk (ref. 2). Since then the use of coated cemented carbide cutting
tools has grown constantly and today some 50 percent of cemented carbide
cutting tools are coated.
The purpose of this paper is to present and discuss the main considera-
tions involved in selecting the coating materials and the coating process as
well as their advantages, shortcomings, and range of applications.
*Material previously presented at the International Workshop on Industrial
Applications of Plasma Chemistry, Ashkelon, Israel, A pril 1981.
**NRC-NASA Research Associate.
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MECHANISMS OF WEAR IN CUTTING TOOLS
In order to determine the factors affecting the performance of wear
resistant coatings, it is necessary to know the main mechanisms which are
responsible for the wear of cutting tools. An excellent survey of this sub-
ject has been given by Trent (ref. 3) and here a brief review of the various
mechanisms will be presented. These can be divided into the following
categories:
(1) Adhesive wear. When two clean surfaces are in contact with each
other, a bond is formed between them. This phenomenon is also termed often
"cold welding." Experimental and theoretical studies performed by Buckley
(ref. 4) and Ferrante (ref. 5) have shown that in the case of two metallic
materials the interface is stronger than the weaker of the two materials but
weaker than the stronger of two materials and thus when the two surfaces are
parted there is a transfer of the weaker material to the surface of the
stronger material, i.e., wear of the part made of the weaker material occurs.
(2) Abrasive wear. This type of wear occurs when a hard, rough surface
slides against a softer surface, digs into it and plows groovP- z in it. The
material originally in the grooves is normally removed in the form of loose
fragments. This type of abrasive wear is called two-body abrasive wear
(ref. 6). Another type of abrasive wear occurs when hard particles are
present between the sliding surfaces and abrade material off each. The mecha-
nism in this case seems to be that an abrasive grain adheres to one of the
surfaces, or else is embeddea in it, and plows out a groove in the other
(ref. 7). This type of abrasive wear is termed three-body abrasive wear. In
metal cutting this type of wear occurs when the work materials contain parti-
cles of phases which are harder than the tool. For example, in cast iron and
also in steel there are particles of carbides and oxides which are harder than
the carbides used as the basis for cemented carbides, i.e, WC, TiC, and TaC,
and these abrade the cutting tool.
(3) Corrosive wear. This kind of wean is caused by chemical reaction
with the environment. In the case of a friction system the danger of corro-
sive damage is much morE severe than in the usual case since the contact
between the surfaces brings about disruption of protective surface layers
which are formed in many cases and contribute to the corrosion resistance.
Thus, fresh surfaces having an inferior corrosion resistance are continuously
exposed.
(4) Diffusional wear. This type of wear takes place at high tempera-
tures. At these temperatures, diffusion of certain components, mainly carbon,
from one part to the other occurs. This weakens the material thus causing
fracture of fragments.
When examining worn cutting tools one can observe two regions of ;year as
shown in figure 1. Wear occuring at the clearance face is termed flank wear
and wear occurring at the rake face is termed crater wear. Flank wear is
usually due to irregular flow of the work material past the cutting edqe,
causing tearing of fragments off the tool surface. This mechanism which is
called attrition wear is prominent at low cutting speeds. As for the crater
wear, studies conducted by Trente (ref. 8) and by Lolazde (ref. 9) show that
the mechanism involved there is diffusion wear. During metal cutting the
highest temperature is attained on the rake face at a distance of approxi-
mately 1.5 mm from the edge (ref. 14) reaching 1000' C or more.
At these high temperatures carbon has an appreciable solubility in steel
and since the diffusion coefficient also increases sharol y with tMIDPraturP,
U'*fusion of carbon from the tool to the chip takes place thus causing weak—
ening of the latter and resulting in crater wear.
rill*
i
E
P
t
PROPERTIES OF COATING MATERIALS
We are now in a position to assess the desirable properties of the coat-
ings. These are:
(1) Hardness. According to the classical work of Khrushchov and Babich,,v
(ref. 10), the abrasion resistance is proportional to hardness. Thus, it is
clear that the coating material has to be as herd as possible in order to
minimize this severe form of wear. The hardest materials found in nature are
diamond having a hardness of 80 GPa and cubic boron nitride having a hardness
of 45 GPa. It might have therefore been concluded that diamond would be the
ideal coating material. This, however, is not universally true. Diamond
tools are produced by some manufacturers and iZ is found that they have a very
high wear rate when machining ferrous alloys: This proves that a wear mech-
anism other than abrasion is involved in this case. This problem is not en-
countered when machining ferrous alloys with cubic boron nitride. However,
like diamond, it is thermodynamically unstable and no industrial technique for
its deposition as a coating has yet been developed.
(2) Adhesion. This is a very important factor since in many cases fail-
ure of coated inserts results from peeling of the coating. The factors which
affect adhesion, especially between metallic and nonmetallic materials are not
yet well understood. For a long time it was believed that the best adhesion
is obtained when the substrate surface is clean. This is true in many cases
where contaminants prevent the formation of a bond between coating and sub-
strate. On the other hand, in other cases, like the cases of Al, Cr, and Fe
on glass substrates which were the subject of the classical work of Benjarlin
and Weaver (ref. 11), exposure of the glass surface to oxygen enhances adhe-
sion. This was attributed to the formation of an oxide having a stronger bond
than the metal-glass bond at the interface. The development in recent years
of surface analytical tools (AES, ESCA, etc.) made it possible to examine this
hypothesis and studies performed by Pepper (ref. 12) show that exposure of the
surface to very small amounts of oxygen (less than a monolayer) which does not
form any chemical bond with the metal enhances the adhesion significantly.
On the other hand, exposure to Clz reduces adhesion.
(3) Corrosion resistance. Since most coating materials are stable non-
metallic compounds, no serious corrosion problem exists and, in fact, the
coating improves corrosion resistance.
(4) Resistance to diffusion. This property is important mainly at high
temperatures. Since the temperatures encountered in metal cutting are of the
order of 1000' C or higher (ref. 13) where appreciable diffusion takes place
it is of great importance there.
Once this was understood it was rather obvious that one way of reducing
crater wear is the application of a diffusion barrier. The material which was
first picked up for this purpose was TiN which was applied on top of the TiC
coating with an intermediate layer of TiCN for matching purposes. This
reduced crater wear by some 50 percent as can be seen in the machining test
results presented in figures 2 and 3. TiN has another very important charac-
teristic, namoly its golden color. This fact was not ignored by the adver-
tising departments of many manufacturers.
The next development in the field was the introduction of an even better
diffusion barrier - Al204, which in addition is harder than TiN.
(5) Thermal Properties. Because of the high temperatures attained at
the work/tool interface it is intuitively at least clear that the thermal
properties of the tool surface p lay a cAss_antial role in its waar haha;,i ^; .
3
Experimental data (ref. 15) indicate that tools with lower thermal conduc-
tivity have a shorter tool-chip contact length and lower wear rate in both
clearance and rake face. This was explained by Lenz, et al. (ref. 16) in
terms of transient heat condution considerations.
Figure 4 shows the heat conductivity of some wear-resistant materials.
One can see that the thermal conductivities of both TiC and TiN increase with
temperature, that of TiN being substantially lower than that of TiC. The heat
conductivity of Al20^, however, behaves in the opposite manner, i.e., de-
creases with temperature. This is reflected in the wear behavior of tools
coated with this material - whereas at low cutting speeds there is no big
difference between the wear rates of tools coated with TiN and those coated
with Al203, there is an improvement in wear rate at high cutting speeds as
can be seen in figure 5.
Another thermal property of importance is the melting temperature and
even more so the softening temperature defined as 0.47 Tm where Tm is the
absolute melting temperature. The values of these parameters for the three
most common wear-resistant materials are given in table I. Other thermal
properties of the coating materials like the coefficient of thermal expansion
and thermal shock resistance, also affect the wear behavior, but have not as
yet been studied.
COATING PROCESSES
The must common industrial process by which wear-resistant coatings are
applied in chemical vapor deposition (CVD). The reactions are:
H
or TiC	 Ti C1 4 + CH  + xH 2 	?— TiC + 4HC1
H
for TiN	 2 TiCl 4 + N 2 + 2H 2	2 TiN + 4HC1
H
for Al 20 3	2 Al C1 3 + 3H 2O + XH 2 ? - Al 20 3 + 6 HC1
All these processes require temperatures around 1000 ` C. These high
temperatures affect the substrate as reflected by the lower transverse rupture
strength (TRS) of coated cemented carbides as compared with uncoated ones.
Recently some Japanese manufacturers claimed they were using reactive ion
plating for the coating of milling grades which require a high TRS. The pro-
cess is sketched in figure 6. It is done in a reactive gas atmosphere and
consists of evaporation (usually by means of electron beam) of the desirable
metal (titanium or aluminum) and ionization of a fraction of the reactive gas
molecules by means of electric field. This induces chemical reaction between
the evaporated metal and the reactive gas and results in formation of a com-
pound coating. No details of the process as well as of the performance of the
coated tools were given.
FUTURE DEVELOPMENTS
The main R and D effort is currently directed towards finding methods of
low-temperature deposition of coatings. Such methods will enable application
of protective coatings which is limited at the present to cemented carbides,
4
Oto high-speed tools as well. The methods which are explored are reactive ion
plating, sputtering and plasma-assisted chemical vapor deposition (ref. 17)
where the reacting species are excited by means of plasma rather than by high
temperature and organometallic chemical vapor desposition.
Introduction of new coating materials is not to be expected in the near
future unless a breakthrough in deposition of cubic boron nitride and diamond
takes place. In the mean time improvement in formulating the optimal sequence
and thickness of the three basic coatings - TiC, TiN and Al203 will probably
be the main course of progress in the field.
REFERENCES
1. Muenster, Arnold; Ruppert, Wilhelm; and Schwedler, Gottfried: Carbide
Coatings on Tools and Machine Parts. German Patent 1, 170, 218, May 14,
1964.
2. Ekemar, Carl S.: Carbide-Coated Cemented Metal Carbide. German Patent
2,007, 427, Sept. 10, 1970.
3. Trent, E. M.: Wear of Metal-Cutting Tools. Wear, Douglas Scott, eds.,
Treatise on Materials Science and Technology, vol. 13, Academic Press,
1979, pp. 443-489.
4. Buckley, D. H.: Adhesion of Met-a ls to a Clean Iron Surface Studied With
Leed and Auger Emission Spectroscopy. W:ar, vol. 20, 1972, pp. 89-103.
5. Ferrante, J.: Adhesion of a Bimetallic Interface - for Al, Mg, and Zn.
Ph.D. Thesis, Case Western Reserve University, 1978. (See also NASA
TM-78890).
6. Burwell, John T., Jr.: Survey of Possible Wear Mechanisms. Wear,
vol. 1, Aug. 1957-June 1958, pp. 119-141.
7. Rabinowicz, E.: Friction and Wear of Materials. Wiley, 1965.
8. Trent, E. M.: Cutting Steel and Iron With Cemented Cartide Tools,
Part III - The Influence of Metallurgical Factors in the Flow Zone.
J. Iron Steel Inst. Lo.;don, vol. 201, no. 12, Dec. 1963, pp. 1001-1015.
9. Loladze, T. N.: Wear of Cutting Tools, Moscow, 1958.
10. Khrushchov, M. M.; and Babichev, M. A.: Resistance to Abrasive Wear and
the Hardness of Metals. (in Russian), Dokl. Akad. Nauk. SSSR, vol. 88,
1953, 445-448.
11. Benjamin, P.; and Weaver, C.: Adhesion of Metal Films to Glass, Proc. R.
Soc. London, Ser. A, vol. 254A, 1960, pp. 177-183.
12. Pepper, S. V.: Shear Strength of Metal-Sapphire Contacts. J. Appl.
Phys., vol. 47, Mar. 1976, pp. 801-808.
13. Lenz, E.: Int. Cemented Carbide Conf., 1st, SME, Dearborn, 1971.
14. Smart, E. F.; and Trent, E. M.: Temperature Distribution in Tools Used
for Cutting Iron, Titanium and Nickel. Int. J. Prod. Res., vol. 13,
no. 3, May 1975, pp. 265-290.
15. Katz, Z.; and Lenz, E.: A Correlation Between Chip-Tool Contact Length
and Flank-Wear, CIRP, Ann. Int. Jnst. Prod. Eng. Res., 22, 1973, 11.
16. Lenz, E.; Pnueli, D.; and Rozeanu, L.: Tools. Wear, vol. 53, 1979,
pp. 337-344.
r
	
l i,
TAKE I. — PROPERTIES OF MATERIALS
Material Hardness Melting Softening
HVickers temperature, temperature,
C C
WC 2400 2630 1091
TiC 3200 3140 1331
TiN ;950 2950 1242
faC 1800 3880 1679
Nbc 2400 3600 1547
Al 2 03 3000 2000 795
E00 .15
.25
.20
 ^
O TIC COATING
A TIC +TICN +TiN COATING
VB /	 Y
_a
t
qk
10
ORIGINAL PAGE 19
OF POOR QUALITY
CHIP`..
WORK
RAKE FAC
_CUTTINGTOOLI	 ^ ^	 III	 I CLEARANCE FACE
I	 ^ j^i^l CUTTING
DIRECTION
Figure 1. - Wear regions on a cutting tool.
.05	
1	 1	 I	 1	 1	 1	 1	 1	 I	 1	 I	 1
0	 2	 4	 6	 8	 10	 12	 14	 16	 18	 20	 22	 24	 26	 28	 30
TIME, min
Figure 2. - Machining test results. Carbide grade, 150 P40 coated; Indexable insert, TNPnA 432; cutting
speed, 160 m/min; depth of cut, 1.5 mm; feed, 0.36 mm/rev; material, AI51 1045.
M
r'
1
•i
•	 iI
1
J
ORIGINAL PAGE: 3
OF POOR QUALITY
150
0 TIC COATING
	125	 A TIC +TICK +TIN COATING
100
KB KM
r 75x	 KT	 r
	
50	 a —
	
25	
k4
 
_L__ I 	
	0 	 2	 4	 6	 8	 10	 12	 14	 16	 18	 20	 22	 24	 26	 28	 30
TIME, min
Figure 3. - Machining test results. Carbide grade, ISO P40 coated; indexable insert, TNMA 432; cutting
speed, 160 m/min; depth of cut, 2.5 mm; feed, 0.36 mm/rev; material, AISI 1045,
50
TIC
40	
rr TaC
/ r rr rr
r
01
	 r	 NbC
//
//TIN
__. Al203
0	 500	 1000	 1500
TEMPERATURE, oC
Figure 4. - Heat conduction of some wear-resistant
materials.
Y_
Z
30
U
D
z	 !
10
i n-+.
ORIGINAL PAGE M
OF POOR QUALITY
.40
. 
35
.30
E '25
E
9 
.20	 IfVB-
—a
.15
t _
10
k	 ^
Imo_ 1_	 1 I I I I I I I I I I
0	 2	 4	 5	 6	 8	 10	 11	 14	 16	 18	 20	 22	 24	 26	 28
TIME, min
Figure 5. Mach ining test results. Carbide grade, ISO M15 coated; indexable insert, TNMG 433; cutting
speed, 240 mlmin; depth . f
 cut, 2.5 mm; feed, 0.25 mm/rev; mate. ::, AISI 1045.
-SUBSTRATE
r
iiiOWZ11YI&A -V BIAS
+V BIAS
ELECTRODE
MOLTEN
POOL
ELECTRON-
GlJN 
CO
Figure 6. - Schematics of reactive ion
plating.


A review of the use of wear-resistant coatings in the cutting-tool industry

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