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Jour nal o f Mater ia l s Process ing Technology, 39 (1993)
165-177 165Elsevier
T i N c o a t i n g o f t o o l s t e e l s: a r e v i e
wShanyong ZhangSchool of M echan ical an d Produ ct ion Eng
ineering, Nan yan g Technological Universi ty ,Singapore
2263Weiguang ZhuMicroelectronics Center, .School of Electrica l an
d Electronic En gine ering , Na ny an gTechnological Un iversi ty,
Singa pore 2263(Received March 27, 1992; accepted December 28,
1992)
I n d u s t r i a l S u m m a r yTitanium ni tride (TIN) has
been used in the coating of tool steels since the mid-sixties.
Thereasons to coat cutt ing tools in a production situat ion are to
increase tool life, to improve the
surface quality of the product, and to increase the p roduction
rate. The advantages of TiNcoating include high hardness and
adhesion, good ductility, excellent lubricity, high chem-ical
stabilit y and tough resistance to wear, corrosion and
temperature.In this paper, the principles, advantages and
limitations of various TiN coating processesare summarized, the
microstructures and mechanica l properties of TiN coatings on tool
steelsubstrates are reviewed and new developments in the property
design of TiN coatings arepresented. It is concluded that TiN
coating of tool steels is a proven way of success inboosting
production and cur tai ling cost. For HSS applications, however,
PVD processes aremore approp riate than CVD processes although PVD
has its own limita tions, which need tobe addressed in research
into and the development of coating processes in the future.
Withthe growing popularity of TiN-coated tools and new development
of coating process, andthus better property control, the market of
TiN-coated tool steels promises a prosperousfuture.
1. I n t r o d u c t i o n
Sod ium ch lo r ide s t ruc tu red T iN i s a go lden ye l low
re f rac to ry compound o flow de ns i ty (5 .22 g/cm 3) and hig h
me lt i ng poi nt (2930 C). Ti ta ni um ni t r id e asa coa t in g
fo r t oo l s t ee ls has been ava i l ab le wide ly s ince the l
as t decade and i sen joy in g inc re as ing a t t e n t io n and a
pp l i c a t ion in too l i ndus t r i es . The r easonsare s imp
le ye t impor t an t : t he adva n tag es o f T iN coa t ings o f t
oo l s tee l s
Correspondence to: Dr. S.Y. Zhang, School of Mechanical and
Production Engineering,Nanyang Technological University, Singapore
2263.0924-0136/93/$06.00 1993 Elsevier Science Publishers B.V. All
rights reserved.
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166 S.Y. Zh an g and W .G. Z hu /T iN coating of
tool~steelsTable 1Life increase of TiN-coated tools [3,7]Tool name
Substrate Workpiece Life increase (%)END MILL M7 1022 STEEL, RC35
269END MILL M7 6061-T6 Al-alloy 804END MILL M3 7075T Al-alloy
489GEAR HOB M2 8620 Steel 100BROACH M3 303 Stainless steel
200BROACH M2 48 % Nickel alloy 1600BROACH M2 410 Stainless steel
158 210PIPE TAP M2 Gray iron 200TAP M2 1050 Steel, RC30-33 971
1233FORM TOOL T15 303 Stainless steel 220DRILL 12L14 Steel 275DRILL
304 Stainless steel 2067DRILL 316 Stainless steel 2357DRILL 4140
Steel 714DRILL Alloy steels 200 400DRILL Aluminum bronze 2850DRILL
Carbon steels 300 500DRILL Cast irons 600-800DRILL Copper alloys
1900DRILL H-13 Tool steel 300DRILL Hastelloy 417DRILL Stainless
steel 400 1100DRILL Ti alloy 856DRILL Tool steels 400-600
include a noble appearance, excel lent adhesion to substrates,
high chemicalinertness, resistance to elevated temperatures, hard
surfaces (2400HV) toreduce abrasi ve wear, a low coefficient of
friction with most wor kpie ce mate-rials which increases lubricity
and results in excellent surface finish anddecrease of horsepo wer
requirements, improved abi li ty to hold tolerances andhigh
temperature stabi l i ty and low maintenance cost and high
productivi ty[1-8]. In practice, the degree of extended tool life
and/or increas ed p rod uct ivi tyattained with coated tools
depends primarily on the tool and its application,the workpiece
material and the operat ing parameters. Keeping all these condi-t
ions equivalent , tool l i fe improvement can be evaluated by
comparingthe increase in number of workpieces m achined by a
TiN-coated tool withthe numb er of workpieces mach ined by an un
coate d tool. Some resul tsare i llu str ate d in Table 1. It is
seen th at the life inc rea se is over 1000%(tenfold) in some
cases, while the cost of coat ing is usua lly 20 30% of the
basepric e of the tool, or as little as a 15% rise in th e t ot al
pric e [6,7]. The to ol lifeenh anc emen t is found to remain even
with the re sharp ening of tools in whichthe TiN coat ing on the
flank surfaces of high-speed steel (HSS) drills isremoved
[3,9].
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168 S.Y. Zh an g and W .G. Zh u /T iN coating of tool~steels
A r ~ _ _N2 ~ , m
I Arc supply
To anode
A rc supply ~ To allude
& A rc s o u r c e/1\Ti a toms
W or k ~ ie ce sW o rk p ie ceo lde r II _ _Bias supp ly
To anode
+Arc supp ly J
To vacuum system
Fig . 1. P r inc ip l e s o f t he m ul t i - a rc eva po ra t
ion sy s t em fo r T iN co a t ing .
A r ~ _ _N2 ~ -
~ Ca thode
Workp ieces[ W orkp iece ho lde r ]
1, ; g u s y s , e m600C
To vacuum sys tem
Fig . 2 . P r inc ip l e s o f t he rea c t ive spu t t e r ing
o f T iN co a t ing .
f r o m it , t h u s c r e a t i n g a u n i f o r m i t y p r o
b l e m , w h i c h c a n b e o v e r c o m e b y u s i n ga m u l
t i - a r c s y s t e m , a s i l l u s t r a t e d i n F ig .
1.
S p u t t e r i n g i s a n o l d t e c h n i q u e , m o r e w
i d e l y u s e d in e l e c t r o n i c s , t h a t a p p l i e sa
h i g h v o l t a g e ( 50 0 -5 0 00 V , g e n e r a l l y d i r e
c t c u r r e n t f o r m e t a l s a n d r a d i of r e q u e n c
y f o r n o n - c o n d u c t i v e t a r g e t s ) , b e t w e e n
a n a n o d e ( t h e s u b s t r a t e ) a n d
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S. . Zha ng and W .G. Zh u /T iN coating o f tool~steels 169a
cathode t hat supplies coating ions liberated by the bombardment of
inert gasions. For coating TiN, solid titanium is used at the
cathode, and nitrogen isint rodu ced as the reac tive gas (Fig. 2).
The inert gas atmosphere (such as Ar ata pressure of 10- 3-10-1
Torr) is used to avoid chemical reactions a t the ta rge tand
substrate. This process is termed "reactive sp utter ing" since
reactive gasis involved. The disadvan tages of the sput tering
process are a low depositionrate and a high thermal load on the
substrates due to bombardment bysecondar y electrons. Addition of a
properly shaped magne tic field can speed upthe deposition [2], in
which case the technique becomes "reacti ve magne tronsputterin g".
The essential disadvantage of magne tron sp uttering is the
strongdecrease of the substrate ion current with increasing
distance of substratesfrom the magnet ron target: this can be
corrected by additional gas ionizationor magnetic confinement of
the plasma [10].The most commonly used ion-plating techniques for
TiN coating are reactiveion plating and sputtering ion plating.
Reactive ion plating of TiN involvesreactive gas N2 and a molten Ti
target (thus in the physical set-up, Ti iscont ained in a crucible
and placed at the bottom of the reaction chamber). Theadvantages of
the reactive ion plating over evaporation include improvedadhesion,
uniform coating distribution and a more homogeneous film struc
turewith h igh dens ity [2], greater hardn ess (>2300 HV), a
very fine grain size(0.2-0.5 pm [11] ), stress r el ief of the base
mater ia l (due to slow cooling from thecoating temperature), and
chemical and thermal stability, thus sui tabilit y forelevated
temperat ure and corrosive environment s [4]. Sputterin g ion
platingdiffers from reactive ion plating in th at the subs trate is
at a lower voltage tha nthe anode, so that whilst deposition takes
place on the subs trate there is stilla limited sputtering action
between the anode and the substrate; thus, thesubstra te is contin
uousl y polished during the operation and a better adhesionis
achieved. However, the rat e of depos ition is low, of the order of
1 gm/h [12].
The advantages of PVD include a lower deposition temperature ,
thus post-coating t rea tment is avoided, no dimensional change in
precision tooling, thepossibility of tailorin g the coating
properties by careful control of depositionconditions, excellent
adhesion due to the high arrival energy of the coatingmaterial and
the ability to clean the surface prior to coating very thoroughlyby
a sputtering stage in the cycle, a good surface finish that in some
systemsequals th at of the substrate, the elimination of finish
machining, no effluents orpollutants as a result of the process, no
hydrogen embrittlement, dense struc-tures, controllable and
repeatable stoichiometry and crystallographic struc-ture, a wide
range of coatings and substrate materials, including metals,
alloysand ceramics, the possibility of multiple coatings, and
greater productivi ty andmajor cost savings. The limitations of PVD
processes arise because in a PVDprocess it is necessary to rotate
the substrate in order to achieve uniformity,the deposition
temperatures are still not low enough for some applications,holes
in a substrate should not be more than twice as deep as their d
iameter ifcoating is needed to the full depth, although holes can
be 4-5 times thediameter in depth.
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170 S.Y . Zha ng a nd W.G . Zh u/ T i N coat ing o f
tool~steelsMost recently, MOCVD has been used successfully in
coating TiN on toolsteels at around 500C; an elaboration of this
process is given in Section 4.
3. M i c r o s t r u c t u r e s a n d m e c h a n i c a l p r o
p e r t ie s o f T iN c o a t i n g sThe mechan ical properties of
TiN coatings are a fu nction of the microstruc-
ture, the morphology, the density, and the stoichiometry. The
most importantphysical and mechanical properties of a coating for
tooling applications arethe coat ing thickness, density (or
porosity), hardness, adhesion, tem peratu reresistance (i.e., hot
hardness), wear, corrosion and oxidation resistance, etc.
The coating thickness is usually determined thro ugh SEM, Auger
composi-tion profile or even metallographic microscope. There are
also examples ofusing methods involving X-ray dif fraction
intensiti es [13,14]. The coat ingthickness of TiN film on tool
steels is generally between 2 and 10 pm: the CVDcoatings tend to be
thicker (usually 7 9 pm) while the PVD coatings areth inner
(usually 3-5 ~m). The thicknes s of the coat ing has a significant
effecton tool life: in planing tests, a coating of thickness 2-3 pm
was found to give thelongest tool life, but in turning tests,
however, the thicker the coating, thelonger the tool life
[15].Microscopically, TiN films deposited by the PVD method can be
character-ized as being composed of columnar grains elongated along
the growth direc-tion while those by the CVD method are composed of
fine grains at the init ialstage of growth, after which a preferred
orien tati on forms in conju nction withthe fo rmati on of elonga
ted grains along the dir ection of growth [16 21]. InPVD TiN, the
leng th of the columnar crystals is usua lly in the range 1-3
gm,which means that many crystalli tes extend thro ugh the whole
thickness of thecoating. Depending on the coating conditions,
either equiaxed or crystallo-graphically aligned column ar grains
or some mixture of the two can occur withvarying levels of porosity
both in the coating and along the subs trat e-coa tinginterface.
Columnar structures have been found to exhibit pronounced hard-ness
anisotropy between plane and profile sections and are characterized
byhigh porosi ty which, in turn, adversely effects the mechan ical
properties of thefilm. Film porosity can be measured directly using
the ferroxyl test method[11] : A piece of filter paper is applied
to the coating and dampened with 10 gK3 [Fe(CN)6] + 30 g NH4CI + 60
g NaC1 + H20 solution, taking care to keep thepaper in close
contact wi th t he surface to be tested for 10 min. It is
thenremoved, washed and dried, any pores showing themselves as blue
spots on thetest paper.Another aspect of the micr ostruc ture of
TiN coating is its stoichiometry onwhich the deposition process pa
rameters have a signifi cant effect. As reportedby Stanislav et al.
[22], the color coordinates of TiN layers are a function of
thenitrogen flow rate and bias potential. With increasing nitrogen
flow the redand yellow coordinates are increased step-wise. The
bias increases the propor-tion of the yellow color in the layer s
and the layers gain a golden tint. In a truesto ichiomet ry, the
atomic rat io of Ti : N is uni ty, or in TiNx where x = 1, the
re
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S.Y. Zha ng and W .G. Zh u/ T iN co ating of tool~steels 171
~ 14
~ _6
~ 2>
B 0 1 D 3 M 2 M 4I I I
1000 g rams500 g rams200 g rams
50 g rams190000 700 10CO 1300 1800
Substrate Hardness (kg/m m 2)Fig. 3. Vickers indenter
penetration plotted as a function of load and substrate hardness.At
low loads, the penetration hardness is essentially independent of
the substrate and isdominated by the coating [17].
is a steep increase in the color coordinates and the coating has
a golden color;when it is under-stoichiometric (x 1), a copper
color. Varia tion of the stoichiometry of TiNcreates differences in
mechanical properties, of which microhardness andadhesion strength
are the most commonly measured. For observed values ofx rang ing
from 0.5 to 1.11, the reported microhar dness ranges from 22 to 69
GPaand the modulus of the elasticity ranges from 350 to 550 MPa
[23].
The coating hardness is usually tested on a microhardness tester
(load Hs (1)Ho=vHf+ V
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172 S.Y. Zhang and W.G. Zh u/ Ti N coat ing of tool~steels4
0
2 pm T iN u / / ~M2 too steel 8 ) // -35 V 0 / 3 0 -3 6V / . A .
IA -9o v ~ / = . .
oo 2 0 - - O130V / ~
o
1 0
I I I I I I I I1 2 3 4 5 6 7 8 10d 2/~t [x]0 6 n3/2
Fig. 4. Variation in L c with d 2 / ~ t for 2 pm TiN coatings
deposited onto M2 tool steel ata range of bias voltages [24].
wher e Hf is the h ardn ess of the film itself, Hs is that of
the sub stra te and c is theinterf ace parameter, expected to be a
function of the mismatch between theplastic zone radii in the
coating and that in the substrate.
Adhesion is anoth er imp ortan t mechanical pr opert y of
engineeri ng coatings,especially for use in tribological
environments. Adhesion is a sensitive func-tion of interfacial
bonding, cleanliness and topography, etc. The adhesionstren gth of
a coating depends on the ex tent of both chemical and
physicalinteractions between the coating and substrate materials
and the microstruc-ture of the inte rface region. Accordingly,
interface struct ure plays a veryimpo rtan t role in determining
the adhesive stren gth of coatings. Poor adhesionmay be attributed
to a low degree of chemical bonding, poor interfacialcontact, low
frac ture tough ness and high i nter nal stresses. Adhesion is
usuallytested on th e scr atch test er whe re a loaded diamond tip
(of radius 0.1-0.2 mm[25, 26] or 0.01 mm [21] ) makes a scratch on
the film surface. The load at themom ent of film det ach men t is
called the cri tical load and gives a compar ablepa ram ete r of
film adhesion. In a scrat ch test, the coati ng is exposed to
combina-tions of spalling, plo ughin g and thr oug h-t hick nes s
fra ctu re [17, 24]; a merecritical load expression is therefo re
much too simplified to repr esent the realprocess. In the case
where there is a sharp transition to coating interfacialfailur e at
a fixed load, Bull and Ricker by [24] proposed th at the coati
ngdetac hes from the s ubst rate so as to minimize its stored
elastic ener gy and thusrelated the critical load L to the work W
thr oug h
d 2L = K, tl/2 (2)where KI-(2EW) 1/2 n/8 is defined as the
interfacial toughness, d is the trackwidth, E the Young's modulus
of the c oatin g and t is the coat ing thickness .
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S.Y. Zhan g an d W .G. Zh u/ Ti N coat ing of tool~steels
173Table 2Increase of adhesion achieved by ion bombardment of the
substrate during coating [21](Tested on 0.02 mm diamond Critical
load (g)tip)Substrate A1 Ni Si MoReactive sputtering 1.35 3.80 5.00
10.45Ion-assisted sputtering 3.15 5.90 10.45 12.70
Plott ing Lc agains t d 2/ t 1/2 gives rise to the inter racial
toughn ess K, as the slopeof a st rai ght line (Fig. 4). Altho ugh
the work W may be calcu lated from theKx expression, the values
obtained may not be correct owing to the variationsof the effective
Young's modulus E with the deposition type and condition.Since the
interfacial toughness (KI) can be determined for any specimen
fromparameters that are easy to measure with some accuracy and as
it enablesfactors such as the varia tion in t he critical load (Lc)
with int ernal stress to besubtracted, Bull and Rickerby [24]
recommended tha t the interfacial toug hnessK~ be used in place of
critical load L in characterization of the adhesionstrength of a
coating.
Deposition temper ature has a profound influence on the microstr
ucture andthus on the mechanical properties of a coating. During
reactive magne tronsputtering deposition of structural steel, TiN
film had a dense columnarstructure at deposition temperatures below
200C, a uniform structure ataround 400C, and a columnar st ructure
again a t 700C [27, 28]; therefore, bothhardnes s and adhesion
reached maxima at about 400C. Grain g rowth at hi gherdeposition
tempe ratur e also damages the mechanica l properties of a
coating,as demonstrated on ASP 23 HSS [29].
Aside from temperature, different methods of prepara tion create
significantdifferences in the mechanical properties of titanium
nitride coatings, thedifferences usually being attributed to
variations in coating morphology andthe exten t of lattice
distortions creat ed during prepara tion [19]. Generally,energetic
particle bombardment is known to suppress columnar growth
struc-ture and to induce compressive film stress which gives rise
to bette r adhesion.As reported by Bieli et al. [21], ion-assisted
sputte ring results dist ingui shedadhesion enhancement from that
of simple reactive sputtering (see Table 2).Application of a sub
strate voltage bias dur ing the deposition of a coa ting hasalso a
profound effect on the growth and result ant m icros tructu re of
PVD films[30]. The unbia sed coat ing shows an open col umnar s
truct ure which results inlow hardness and poor wear resistance,
while the biased film becomes muchmore dense and the indiv idual
columns are less well defined, resu lting in highhardness and
excellent wear resistance.
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174 S.Y. Zh an g and W .G. Z h u/ T iN coating o f
tool~steels
H 2 - ~ " - ~ ~ - ~ M e ta llo -o rg a n ic~ c o m p o u n ds o
l u t i o n
N 2
Heat ing
@@
s y s t e m , ~@- pFig. 5. Principles of the MOCVD process of
coating TiN.
WorkpiecesA AA A
0101@1@11
T ow a t e rbott le- - - l b . -
4. N e w d e v e l o p m e n t s in T iN c o a t i n gNew
developments in TiN coating can be summarized in three aspects:
processing, composition, and structure. The trend in process
development ofTiN coating is to lower the process temperature and
thus minimize the nega-tive effect of high- tempe ratur e exposure
on the substr ate proper and expandthe list of the "coa tabl e" mat
erials and workpieces. However, because of thetribological nature
of the tooling applications, all successful new processdevelopments
will have to address the adhesion dilemma: the lower the
temper-ature, the poorer the adhesion tends to be. Good adhesion
and lower depositiontemper ature is therefore roughly the direction
of process developments.Plasma-enhanced chemical vapor deposition
(PECVD) or plasma-assistedchemical vapor deposition (PACVD) is a
combination of CVD and PVD pro-cesses where CVD is operated in
conjunct ion with a plasma, part of the energyrequired for reaction
being supplied electrically, raising the ions to a hightemperature
and thus reducing the need for thermal energy to be supplied;hence,
enabling a reducti on in the working tempera ture to a level tha t
wouldbe considered too low from thermodynamic considerations. Aside
from thelower temperatur e requirement, other advantag es of the
PACVD process in-clude the good throwing power of the coatings to
the substrate due to the highworking pressure of 0.1-10Torr, the
simplicity of the apparatus and goodadhesion strength of the
coatings. Details of PACVD can be found in [12,
31,32].Ion-beam-enhanced deposition (IBED) or ion-beam-assisted
deposition(IBAD) is one of the newest techniques for film
formation, in which ionimplantation and vacuum deposition proceed
simultaneously. It has the ad-vantages of improved adhesion to the
substrate, easier control of compositionand thickness of the film,
the possibility of synthesizing compound films andgrowing films at
low temperature [26].
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S.Y . Zha ng and W.G . Zh u/ T i N coat ing o f tool~steels 175T
h e m e t a l l o - o r g a n i c c h e m i c a l v a p o r d e p o
s i t i o n ( M O C V D ) p r o c e s s i s a v e r y
n e w p r o c e s s f o r t h e a p p l i c a t i o n o f T i N
c o a t i n g o n t o o l s t e e l s [ 33, 34 ], i n w h i c hh y
d r o g e n g a s is a l l o w e d t o p a ss t h r o u g h a t i t
a n i u m - c o n t a i n i n g o r g a n i c l i qu id ,t h e r e
b y c a r r y in g t h e m e t al l o -o r g a n ic s u b s t a n c
e t o th e r e a c t i o n c h a m b e r w h e r et h e m e t a l l
o - o r g a n i c c o m p o u n d d e c o m p o s e s a n d / o r r
e a c t s w i t h n i t r o g e n g a s t of o r m T i N o r T i (
C , N ) o n t h e s u b s t r a t e ( F ig . 5). Z h o u e t a l. [
3 3 ] f o u n dt h a t b e t w e e n 50 0C a n d 5 75 C , t i t a n
i u m t e t r a k i s d i e t h y l a m i d e ( T i ( N E t : ) 4
)d e c o m p o s e s s e r i a l l y a n d r e d u c e s t o T i N
: T i ( N E t E ) 4 - - * T i ( N E t : ) 3 - - * T i ( N E t : )
:~T i ( N E t : ) -* T i N . I n th i s p r o c e s s t h e p a r t
ia l p r e s s u r e o f n i t r o g e n g a s h a sn o i n f lu e
n c e o n th e d e p o s i t i o n r a te , w h i le a t te m p e r
a t u r e s h i g h e r t h a n5 75 C , T i ( N E t : ) 4 d e c o m
p o s e s f u l l y i n t o T i a t o m s , n i t r o g e n c o m p
o u n d a n dc a r b o n c o m p o u n d ; t h e n t h e T i a t o
m s r e a c t w i t h N a n d C in t h e a t m o s p h e r e t of o
r m T i( C ,N ) : T i (N E t E ) 4 -~ N c o m p o u n d C c o m p o
u n d --* T i (C , N ). I n t h i s p r o-c e ss , t h e p a r t i
a l p r e s s u r e o f n i t r o g e n h a s a p o s i t i v e e f
fe c t o n t h e d e p o s i t i o nr a t e .M o d i f i c a t io n
o f t h e c o m p o s i t i o n o f t h e T i N c o a t i n g i s a
n o t h e r i m p o r t a n ta s p e c t a im i n g a t p r o p e r
t y i m p r o v e m e n t . W h i l e a n n e a l i n g T i N c o a
t i n g i n a ir a t7 00 C , P a g e a n d K n i g h t [1 7] f o u
n d t h a t t h e o x i d a t i o n o f T i N c o a t i n g t o o k
p l a c ea s 2 T i N + 202 --* 2 T i O : + N : g i v i n g r i s e
t o T i O : a n d n i t r o g e n b u b b l e s . S o m eh a v e r
e p o r t e d t h a t t h e o x i d a t i o n r e s i s t a n c e o
f T i N i s u p t o 60 0 C [ 4] , w h i l s to t h e r s h a v e r
e p o r t e d t h a t i t is a s lo w a s 4 5 0 C [ 3 5]. T o c o p
e w i t h t h e o x i d a t i o np r o b l e m , i n v e s t i g a
t o r s h a v e b e g u n t o a l l o y T i N c o a t i n g w i t h
A 1 t o f o r m (T i,A 1 )N c o a t i n g [ 2 ]. T h e a l u m i n
u m a t o m s a r e s m a l l ( r = 1 .4 3 , ,) a n d c a n f i t i
n t o t h eT i N c r y s t a l s t r u c t u r e a t s u b s t i t
u t i o n a l s i te s o r i n t e r s t i t i a l s i te s . F o r
th i sr e a s o n , a m o n g v a r i o u s c o m p e t i t i v e a
l l o y i n g e l e m e n t s , a l u m i n u m h a s b e e ns t u
d i e d a s a s e c o n d e l e m e n t i n th e T i N s y s t e m
. A l l o y i n g A 1 i n t o T i N c o a t i n gh a s a n o t h e
r a d v a n t a g e , i .e ., r e s i s t a n c e t o i n t e r f a
c i a l d i f f u s io n . A t t h e l e a d i n ge d g e o f a c u
t t i n g t o o l , s u f f ic i e n tl y h i g h t e m p e r a t u
r e s f o r d i f f u s i o n o f t h e c u t t i n gm a t e r i a
l s i n t o t h e p r o t e c t i v e c o a t i n g a r e o b s e r
v e d , w h i c h d e c r e a s e s t o o l l i f e .A l u m i n u
m c a n fo r m a s t a b l e a n d d en s e o x id e w h e n h e a
t e d d u r i n g t h e c u t t i n go p e r a t i o n a n d t h u
s r e d u c e t h i s u n w a n t e d d i f f u s i o n . A s a r e
s u l t , h i g h h o th a r d n e s s t h r o u g h o u t t h e h
ig h e r t e m p e r a t u r e r a n g e m e a s u r e d u p t o
1000 C h a sb e e n o b t a i n e d [ 3 6 ] .T o e x t e n d t h e
T i - A 1 - N s y s t e m a n d t o a c h i e v e e v e n h i g h e
r h a r d n e s s , b e t t e rw e a r r e si s t an c e , a n d b
e t t e r a d h e s i o n a n d c o r r o s i o n p e r f o r m a n
c e , s o m e re -s e a r c h e rs h a v e a l r ea d y b e g u n t
h e i n v e s t i g a ti o n o f m u l t ic o m p o n e n t s y s
te m sT i - A 1 - V - N a n d T i - A 1 - V - C - N o r h a v e i n
t r o d u c e d r a r e e a r t h e l e m e n t s in t o th eT i N
c o a t in g . I t h a s b e e n s h o w n [ 36 ] t h a t a d d i t
i o n o f v a n a d i u m i n c r e a s e s t h ew e a r r e s is t
a n c e , b u t t e n d s t o i n c r e a s e t h e b r i t t l e n
e s s o f t h e m a t e r i a l. B yc o n t r a s t , c o a t i n g
s w i t h a d d e d a l u m i n u m e x h i b i t g o o d w e a r r
e s i s t a n c e e v e n o nt h e t o o l fl a n k . ( T i, A1, V
) N c o a t i n g s p o s s e s s g o o d h a r d n e s s , g o o d
a d h e s i o n a n da d e q u a t e t h e r m a l s t a b il i ty
, e v e n a f t e r l e n g t h y h i g h - t e m p e r a t u r e a
n n e a l in g .B a r r e l l a n d R i c k e r b y [ 2 ] s h o w e
d t h a t i t i s p o s s i b l e t o o v e r c o m e t h e h o th
a r d n e s s d e c r e a s e o f T i N s a t h ig h t e m p e r a
t u r e b y a d d i n g W T i C o n t o t h e T i Nc o a t i n g
.
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176 S.Y. Zh an g and W.G . Z hu /T iN coating of
tool~steelsOther systems such as T i-C-N are also und er study.
Ertfi rk et al. [37] foundthat the mic rohardness of Ti(C,N)
coatings increases wi th inc reasing C/(C + N)
ratio, optimized coating properties for wear protection being
achieved whenthis ratio falls between 0.3 and 0.5. These Ti(C,N)
coatings with a thin TiNintermediate layer show excellent adhesive
properties on the tool and on thesubstrates: the critical load
value in the scratch test is between 60 and 70 N.
Addition of rare ear th elements in the TiN coating was found
[11] to enrichthe interface between the coating and the substrate
(A3 steel and 1Cr18Ni8stainless steel) and resulted in enormous
enhanc ement of interfacial adhesion.The authors explained that
although the yttrium introduced into the coatingdid not show any
detectable effect on the chemical composition or phaseconst ituen t
of the top surface of the coating, it st ayed at the interface in
theform of Y6Fe23, yttrium and YN phases, and contributed to the
excellentcorrosion performance of Ti(Y) N coating.
In structure, a t hin layer of Ti at the interface changes the
mic rostru cture ofthe inte rface and result s in a subs tan tial
improvement in adhesion [38, 39], andin improvement in corrosion
resistance in sulfuric acid and sodium chloridesolutions [40]. It
is believed that the t ita niu m int erl aye r modifies the TiN
layerstructure and forms a passive film (TiO2) with high resistance
to localizedattack. Some researchers suggest that the Ti interlayer
acts as a gradedinterface that avoids the abrupt change in
composition at the sharp interfacebetween a coating and metal
substrate. The titanium interlayer reacts withoxygen, carbon and
nitrogen; thus, TiN, TiO, TiC and Ti2N phases may appearin the
buffer region, depending on the coating conditions. The o rient
atio n ofthe out er TiN layer is affected by the existence of the t
itan ium int erlayer. Oneof the significant benefits of the Ti
interlayer is that it reduces the internalstresses dram ati cal ly
[38]. The bette r adhesion of a TiN layer in the presence ofan
intermediate titanium layer can be explained on the basis of a
favorablecombination of a number of effects: the better bonding
between the titaniuminter laye r and the substrate, the better
bonding between the titani um layerand the TiN coating, and the
absence of a yFe4N phase in the top layer of thesubstrate [41].5 .
C o n c l u s i o n s
The TiN coati ng of tool steels is a proven way of success in
boosting pro ductionand curtailing cost. For HSS applications,
however, PVD processes are moreappropriate than CVD processes,
although PVD processes have their ownlimitations of component
complexity, the need for workpiece rotation toachieve uniformity
and coating temperatures still being too high for somematerials.
These and other limitations need to be addressed in research
anddevelopment in TiN coating processes, which are far from
complete. With thegrowing popularity of TiN-coated tools and new
developments of the coatingprocess, and thus better proper ty
control, the m ark et of TiN-coated tool steelsis bound to
grow.
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S.Y. Zhang and W.G. Zhu/TiN coating of tool/steels 17 7R e f e r
e n c e s
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