- Email: [email protected]
Wear and corrosion properties of TiN layers deposited on nitrided high speed steel
Surface and Coatings Technology 72 (1995) 189-195
Wear and corrosion properties of TIN layers deposited on nitrided high speed steel J. Michalski
a, Ellina Lunarska
b, T. Wierzchon
‘, S. AlGhanem
b
a Institute of Precision Mechanics, ul. Duchnicka 3, 00-967 Warsaw, Poland b Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44, 01-224 Warsaw. Poland ’ Warsaw Technical University, Institute of Surface Engineering, ul. Narbutta 85, Warsaw, Poland Received 3 June 1994; accepted
in final form 3 1 August
1994
Abstract The susceptibility of high alloyed tool steel, prenitrided under glow discharge conditions and then covered with plasma-assisted chemical-vapour deposited TIN formed at different gas flow rates, to wear and corrosion has been found to depend on both the structure and properties of the surface TiN layer and the chemical composition and properties of the substrate. Nitrogen from the steel surface assists in the TIN formation and this process affects the structure and hardness of the nitrided layer and the structure of the TIN deposit. Depletion of the steel surface in nitrogen during TIN formation may improve the wear resistance but deteriorates the corrosion resistance. By appropriate combination of nitriding and TIN deposition parameters, the optimum combination of wear and corrosion properties could be achieved. Keywords: PACVD TIN; Composite layer; Wear resistance; Corrosion resistance
1. Introduction
2. Experimental
The fast development of science and technology persistently leads to new assignments in materials and surface engineering. The tendency to increase the exploitation parameters (temperature, pressure, environmental aggressiveness) demands materials with improved mechanical properties and corrosion, erosion and heat resistance. This may be met by the advance of surface engineering, i.e. by producing surface layers with desired properties on less-resistant materials. Especially good prospects seem to be provided by composite surface layers which could maintain the properties of appropriately selected single layers, and hence could allow the extended application of coated metal. Good interphase cohesion of the layers, without deterioration of the substrate microstructure, has been achieved by covering of prenitrided steel with TiN deposited by plasmaassisted physical vapour deposition [ 1,2] and plasmaassisted chemical vapour deposition (PACVD) [ 31 methods: wear- and corrosion-resistant composite layers have been formed on various steels [l-3]. The aim of the present work is to check the effect of composite layers produced by nitriding under glow discharge conditions and then by the PACVD TiN deposition method on the wear and corrosion resistance of high alloyed tool steel.
The composite layers were produced on a substrate of high speed steel (HSS) (Polish SW 18 tool steel containing 0.8% C, 17.5% W, 4.3% Cr, 1.3% Ni and 0.8% MO) in a two-step process carried out in the same reactor. In step 1, steel was nitrided under the glow discharge conditions at 790 K in a gas mixture (H,:N,=3:1) at total gas pressure 300 Pa for 1 or 10 h: after that, in step 2, TiN layers were formed on the steel by the PACVD method in the reactor. The TiN deposition process lasted for 3 h at 840 K in a gas mixture with H,:N, = 6: 1 with the addition of 3 g 1-l TiCl, at reactant gas flow rates u of 0.06 x 10m3, 0.15 x 10e3 and 0.25 x 10m3 m s-l at total gas pressure 300 Pa. The layers were produced in a special laboratory apparatus for plasma treatment with a hot anode, described elsewhere [4]. The specimens were subjected to wear and corrosion resistance tests. Wear resistance was estimated in a “treerollers” friction test [ 51, and the linear wear (wear depth (pm)) at 50 and 400 MPa surface load was the measure of the wear resistance. The corrosion resistance was examined electrochemically by comparing the polarization curves recorded in 20% H,PO, (pH = 1). This
0257-8972/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0257.8972(94)02354-9
J. Michalski et al./Surface and Coatings Technology 72 (1995) 189-195
190
solution was chosen because it allowed us to establish the behaviour of specimens in the acid solution and to estimate the phase composition of the studied surface
the substrate. However, at low gas flow rates, the TiN layer grows faster on steel nitrided for 1 h than that for 10 h.
C61. The surface of specimens in the as-received condition and subjected to electrochemical tests was examined by electron diffraction spectroscopy (EDS). Microscopic examination by optical and scanning microscope as well as measurements of Vickers hardness have been carried out. The mean value from three, four or five hardness measurements has been taken into account; the hardness data scattering was 20-30 HVo,l.
3. Results 3.1. TiN layer deposition kinetics Fig. 1 shows the effect of the flow rate of reactant gases on the deposition rate of TiN on SW 18 tool steel prenitrided for 1 and 10 h. The deposition rate increases linearly with increase in the square-root of the flow rate. It can also be seen that the deposition kinetics do not depend on the layer thickness or on the treatment of 1s A
lhr IO
; 6.0L , __ C
0 u Ln 4.0-’
,
x % I
/
0.8 square
root
1
I
1.2
1.6
flow rate
Vi35
I
x102
Fig. 1. Effect of reactant gas flow rate on estimated deposition TiN on SW 18 tool steel prenitrided for different times.
rate of
3.2. Wear resistance Fig. 2(a) shows the hardness depth profile in a surface layer of SW tool steel nitrided for 1 and 10 h. Micrographs of specimen cross-sections (Figs. 2(b) and 2(c)) show that, in both cases, the layer consists of a diffusion zone only. Similarly to in the case of Refs. [ 1,7] no compact iron nitride layer was formed under the applied nitriding conditions. Nitriding for 10 h causes the increase in thickness and hardness of that zone in comparison with nitriding for 1 h (cf. Fig. 2(a)). Fig. 3 shows the linear wear vs. friction time for steel nitrided for 1 and 10 h as measured at different surface loads, and the appearance of the steel surface. Steel nitrided for 1 h exhibits, in the 50 MPa test, the higher wear resistance (lower linear wear values) and has a less developed surface than steel nitrided for 10 h. At a surface load of 400 MPa, the wear resistance is also higher in steel nitrided for 1 h; however, in both cases the 400 MPa test was broken off after about 40 min. Fig. 4 shows the hardness depth profile for specimens coated with TIN, deposited on prenitrided steel. The deposition of TiN affects the hardness of the substrate. In the case of TIN deposition on steel prenitrided for 1 h, the high surface hardness is accounted for by the presence of TIN. It can be seen, however, that the underneath substrate layer exhibits a hardness lower than that characteristic of the material subjected to the nitriding only. At a distance of about 30 mm from the surface the hardness of the surface-treated metal approaches the hardness of steel (see Fig. 4(a)). In the case of TiN deposited on steel nitrided for 10 h, a decrease in the hardness of the underneath nitrided layer occurs at a flow rate of 0.06 x 10m3 m s-l, whereas at a flow rate of 0.25 x 1O-3 m s-l some increase in hardness of the underneath nitrided layer could be observed (see Fig. 4(b)).
900.
20 40
dtstance Fig. 2. (a) Hardness
60 80 ID0 120 from surface (pm)
depth profile and (b), (c) microstructure
of tool steel nitrided
(b) for 1 h and (c) for 10h.
191
J. Michalski et al./Surface and Coatings Technology 72 ( 1995) 189-195
(a)
/
; 15 0 ,' Lz z 10 II d 5 3
50
20 40 60 80 testduration(mm )
Fig. 3. (a) Linear wear at surface loads of 50 and 400 MPa surface of steel nitrided (b) for 1 h and (c) for 10 h.
vs. friction
for different
times; (b), (c) appearance
of the
Fig. 6 shows the linear wear data for steel covered with a TIN layer deposited at a flow rate of 0.06 x 10e3 m s-i. As follows from the comparison of data presented in Figs. 3 and 6, the wear resistance of the composite layers is higher than that of the nitrided layer. The TiN layer deposited on steel nitrided for 1 h exhibits in the 50 MPa test a higher wear resistance and in the 400 MPa test a lower resistance than those exhibited by specimens prenitrided for 10h.
16Oll
1200 $
time for tool steel nitrided
800
3.3. Electrochemical tests
20 40 60 80 100120(i~m) distance from surface Fig. 4. Hardness depth profile for deposited at different gas flow rates and (b) for 10 h.
tool steel covered on steel prenitrided
with TiN (a) for 1 h
Fig. 5 presents the microstructure of composite layers. The white layer consisting of the coarse needles is seen beneath the TIN deposit in steel nitrided for 1 h (Figs. 5(a) and 5(b)). The TIN deposit itself exhibits the layered structure. In the case of TIN deposition on steel prenitrided for 10 h (Figs. 5(c) and 5(d)) the microstructure of the layer situated beneath the TiN one is similar to that observed for nitrided steel presented in Figs. 2(b) and 2(c). No separate layers could be distinguished within the TIN deposit.
Fig. 7 shows the anodic polarization curves recorded in H,PO, on tool steel, bare and nitrided for different times. The polarization curve for bare steel exhibits the active dissolution and passive regions; in the latter, two peaks marked E, and E, could be distinguished. The extension of the active dissolution region is associated with the chemical composition of the solid solution, whereas the peaks E, and E2 are due to the dissolution of metal (Me) carbides of the MeC and Me& (Fe,W,C) types respectively [6]. This is supported by the EDS data obtained in the present work. The intensity of Ka peaks for W and Cr taken from the electron diffraction spectra taken from the bare steel in the as-received condition and after polarization at constant potential within the active dissolution region and at potential E2 are shown in Fig. 8. Polarization in the active dissolution region causes some decrease in Cr peak intensity and increase in the W peak, in comparison with as-received steel, revealing the depletion in Cr and enrichment in W of the steel surface due to active
J. Michalski et al.ISurface and Coatings Technology 72 (1995) 189-195
192
I!
20
40
test duration
60
(mln
80
)
Fig. 6. Linear wear at surface loads of 50 and 400 MPa vs. friction time for steel covered with TiN deposited on a surface prenitrided for 1 and 10 h.
.-.-lhr ---1Ohrs
(4
Fig. 7. Anodic polarization curves recorded in HaPO, solution for bare steel and steel nitrided for 1 h and for 10 h. Characteristic parameters are marked: i,, anodic dissolution current density; E, and E,, potentials associated with the dissolution of carbides of MeC type and Me& type respectively.
cnts
1800 1400 1000 600
(4
(4
Fig. 5. Microstructure of composite layers formed by PACVD of TiN with flow rates of (a), (c) 0.06 x 10e3 m s-l and (b), (d) 0.25 x lOm3 m s-i on steel prenitrided (a), (b) for 1 h and (c), (d) for 10 h.
dissolution. After polarization at a potential close to E,, the change in W and Cr peak intensities indicates the depletion in W and slight enrichment in Cr of the treated surface. Nitriding of steel does not considerably change the appearance of the polarization curve but it decreases
200 bare active steel dlSOl
E2
Fig. 8. Intensity of W Kcc and Cr Kcc peaks seen in electron diffraction spectra taken for bare steel in the as-received condition and after polarization at constant potential within the active dissolution region and at potential E2 (shown in Fig. 7).
the active dissolution and promotes passlvation. Nitriding for 1 h does not change the position of the carbide peaks, whereas nitriding for 10 h causes the shift
J. Michalski et al./Surface and Coatings Technology 72 (1995) 189-19.5
of these peaks to more noble potentials, revealing the change in their chemistry. A series of polarization curves have been recorded for one specimen after subsequent removal of the surface layer; the change of position of the E2 (Fe,W,C) peak on these curves with the specimen depth is shown in Fig. 9. The shift of the peak and thus the change of the carbide chemistry is pronounced within a depth of 20 urn, but some change might be also observed at depths up to 60 urn. Typical polarization curves recorded
+L_ _A 0
20
40
60
80(pm)
distance from surfa&’ Fig. 9. Position of the potential E, (associated with the Me&-type carbide dissolution) on the polarization curve vs. distance from the surface.
193
for steel covered with TIN layers deposited at different gas flow rates after nitriding for 1 h and 10 h are shown in Fig. 10 and Fig. 11 respectively. The polarization curve for TiN on an A1,03 support is also shown in Fig. 10. Although the current density recorded in the case of composite layers is several orders of magnitude lower than that for nitrided steels, (cf. Fig. 7) the appearance of the polarization curves in almost all cases resembles that for bare or nitrided steel. The polarization curves for steel with composite layers exhibit active dissolution, not observed for TiN/A1203. Prenitriding and the TIN deposition parameters affect the electrochemical behaviour of the coated steel. Deposition of a TIN layer on steel prenitrided for 10 h causes a decrease in current density of l-2 orders of magnitude. In steel nitrided for 10 h the resistance increases with the increase in gas flow rate. For TiN deposited at a gas flow rate of 0.25 x lop3 m sir, the recorded polarization curve is similar to that for TiN/A1203 (Fig. 11(a)). The difference in TiN surface topography seen in Figs. 10 and 11, especially in nodule size and distribution, is affected not only by the different gas flow rates at TIN deposition, but also by the different times of prenitriding.
I
(b)
(4 Fig. 10. (a) Anodic polarization curves recorded in HsPO, for steel covered with TiN deposited by PACVD at different gas flow rates (marked values are the mean times lo3 m s-r) on steel prenitrided for 1 h and for TiN deposited on Al,O,. (b)-(d) Surface topography of TiN deposited at (b) 0.06 x 10e3 m s-r, (c) 1.5 x 10-a m s-r and (d) 0.25 x 10m3 m S-IS.
J. Michalskiet al./Surfaceand CoatingsTechnology72 (1995) 189-195
194
went density[m /\ f \ \
.I
2
QO6
\
I 0
I
I
I
I
I
I
0.8 1.2 p&ntial [VI
/
I
I
1.6
(4 Fig. 11. (a) Anodic polarization curves recorded in H,PO, solution for steel covered with TiN deposited at different gas flow rates (marked values are the mean times lo3 m s-l) on steel prenitrided for 10 h. (b)-(d) Surface topography of TiN deposited at (b) 0.06 x 10e3 m SC’, (c) 0.15 x 10m3m s-l and (d) 0.25 x 10e3 m s-l.
4. Discussion
4.2. TiN deposition
4.1. Nitriding
According to the results presented in Fig. 1, the TIN deposition reaction is pressumably controlled by the mass transport of the reactant gases. If the reacting gases are thoroughly mixed and their transport to the surface occurs by diffusion, the flux of gases J,, might be expressed as follows [9,10]:
The applied nitriding conditions allow the formation of a diffusion zone on steel; however, on long nitriding, some changes occur in the nitrided zone. After 10 h steel exhibits a thicker nitrided zone of higher hardness and more developed surface than that after 1 h of nitriding. Although no nitrides have been found in the microstructure after 10 h, the change in carbide chemistry has been revealed by the electrochemical test. The shift of dissolution potential E, in the more noble direction suggests the transformation of carbides to carbonitrides due to nitriding, since carbonitrides are known to be more chemically stable [ 81. The observed increase in hardness of steel nitrided for 10 h might be a result of this change of the precipitates chemistry. The higher wear resistance observed for steel nitrided for 1 h in comparison with that nitrided for 10 h might be accounted for by the less developed surface promoting better friction or by the lower hardness. Duration of nitriding does not affect the dissolution of the tool steel matrix in the studied electrolyte.
JD=D,---
Pb
-p,
(1)
6RT
where D, is the diffusivity of the reactant gases; P,_,and the partial pressures of the reactant gases in the stream bulk and at the substrate respectively; R is the gas constant; T is the temperature over the substrate; and 6 is the boundary layer thickness given by P, are
0.5
(>
6=iL 2
with L the dimension of the substrate, flow rate, and ,u and p the gas viscosity Since D,, T, Pb, P,, p and p remain experiment, the following expression for
(2) u the total gas and density. constant in the gas flux may be
J. Michalski
et al./Surface
and Coatings
derived from Eqs. ( 1) and (2): J D = KJJ0.5
(3)
If the deposition reaction is controlled by mass transport of reactant gases, the deposition rate of TiN is proportional to the mass flux & [9,10]. Therefore, according to Eq. (3) the deposition rate should be proportional to the square root of the gas flow rate, which has indeed been estimated (see Fig. 1). Prenitriding of the steel does not change the TIN deposition kinetics, but does affect the growth rate of TiN and the structure of both the TiN deposit and the substrate. The effect of prenitriding on TIN formation and structure may be traced from the difference in distribution and size of nodules, seen on the surface of TIN formed on steel prenitrided for 1 and 10 h (see Figs. 10(b) and 10(c) and 11(b)-11(d)). The obtained results show that the nitrogen from the substrate is involved in the formation of TiN. In the case of nitriding for 1 h no change in carbide chemistry has been observed and, during the subsequent TIN deposition, nitrogen from the solid solution sinks to the surface participating in the TIN layer formation. As a result, the nitrogen-depleted zone (white zone) of needle structure and low hardness is formed in the subsurface of steel beneath the TiN layer. The layered structure of the TIN deposit seen in this case (Figs. 5(a) and 5(b)) may also reflect the assistance of nitrogen from the substrate in the TIN formation. In the case of steel prenitrided for 10 h, the transformation of carbides takes place during nitriding. In this case, deposition of TIN also involves the nitrogen from the substrate, but the drainage does not cause the change in the microstructure. In the case of TIN deposition at low gas flow rates, the decrease in nitrided layer thickness without change in its microstructure is observed (see Fig. 5(c)). Participation of substrate constitutents in the surface layer formation should cause the improvement of its adhesion. The wear resistivity of coated metal depends on the properties and microstructure of the deposit and substrate as well as the interphase cohesion, since the substrate metal participates in the wear process. Similarly to in the case of corrosion behaviour of coated metal, the deposited layers and the substrate metal are involved. As observed in the electrochemical tests, the “transparency” of the TIN layers (Figs. 10 and 11) indicates that the main process in the corrosion of TiNcoated steel is the dissolution of support metal due to the permeation of environment solution and corrosion products through the coating. Therefore the effect of both the TIN layer porosity and electrochemical properties and the underneath metal electrochemical behaviour should be expected. The same layer-support composite could not necessarily provide good behaviour under wear and aggressive
Technology
72 (1995)
189-195
195
environment conditions, since different parameters affect either the corrosion or the wear process in different ways. However, the deposition and treatment parameters can be selected to maintain the optimum mechanical, adhesion and corrosion properties of the composite. Prenitriding for 1 h and TIN deposition promote the softening of the nitrided layer and thus the better wear resistance, at least as measured in the 50 MPa test (Fig. 6). However, the same composite layer exhibits lower resistance to the environment, reflected by the higher dissolution rate (Fig. 10). This might be explained by the effect of depletion of the prenitrided layer in nitrogen during the TIN deposition. As can be seen from Fig. 7, nitriding reduces the metal dissolution; it might be expected therefore that a decrease in the nitrogen in the metal surface due to TIN formation deteriorates the corrosion behaviour of the composite. Prenitriding could also affect the corrosion resistance of TiN-covered steel by the modification of the structure of the TIN deposit (Figs. 10 and 11) and thus by providing easy paths for permeation of solution and corrosion products through the TIN layer. 5. Conclusions (1) The TIN deposition reaction under the studied conditions is controlled by mass transport. (2) In the course of long-term nitriding under glow discharge conditions, the transformation of carbides to carbonitrides within the nitrided zone occurs. (3) Nitrogen from the steel surface is involved in the formation of the TiN layer, changing the structure and hardness of the nitrided layer and the structure of the TIN. (4) The wear and corrosion resistance of steel depends on the structure of the TIN layer and on the structure and properties of the support. The desired wear and corrosion of the composite layers can be achieved by the appropriate combination of nitriding and TIN deposition parameters.
References 111 Y. Sun and T. Bell, Mater. Sci. Eng.. A140 (1991) 419. VI M. Zlatanovic, T. Gredic, N. Popovic and Z. Bogdanov, Vacuum, 44 (1993) 83. J. Michalski, J. Rudnicki, B. Kulakowska and c31 T. Wierzchon, W. Zyrnicki, J. Mater. Sci., 27 (1992) 771. and T. Wierzchon, Polish Patent Application [41 J. Michalski W-36560, 1988. 1983. [51 Polish Standard PN-83/H-04302, C61 E.T. Shatalov, L.I. Baranova and G. Zekcer, Electrokhimicheskie metody u metallovedenii ifazouom analize, Metallurgia, Moscow, 1988 (in Russian). [71 T. Sone and K. Matsui, Mater. Sci. Eng., A140 (1991) 486. Termicheskaia obrabotka stali i chuguna, 181 I. Kontorovich, Metallurgizdat, Moscow, 1950 (in Russian). Crystal Growth Theory and Techniques, c91 C.H.L. Goodman, Vol. 1, Plenum, London, 1974. CIOI J.-O. Carlsson, Thin Solid Films. 230 (1985) 261.
Wear and corrosion properties of TIN layers deposited on nitrided high speed steel J. Michalski
a, Ellina Lunarska
b, T. Wierzchon
‘, S. AlGhanem
b
a Institute of Precision Mechanics, ul. Duchnicka 3, 00-967 Warsaw, Poland b Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44, 01-224 Warsaw. Poland ’ Warsaw Technical University, Institute of Surface Engineering, ul. Narbutta 85, Warsaw, Poland Received 3 June 1994; accepted
in final form 3 1 August
1994
Abstract The susceptibility of high alloyed tool steel, prenitrided under glow discharge conditions and then covered with plasma-assisted chemical-vapour deposited TIN formed at different gas flow rates, to wear and corrosion has been found to depend on both the structure and properties of the surface TiN layer and the chemical composition and properties of the substrate. Nitrogen from the steel surface assists in the TIN formation and this process affects the structure and hardness of the nitrided layer and the structure of the TIN deposit. Depletion of the steel surface in nitrogen during TIN formation may improve the wear resistance but deteriorates the corrosion resistance. By appropriate combination of nitriding and TIN deposition parameters, the optimum combination of wear and corrosion properties could be achieved. Keywords: PACVD TIN; Composite layer; Wear resistance; Corrosion resistance
1. Introduction
2. Experimental
The fast development of science and technology persistently leads to new assignments in materials and surface engineering. The tendency to increase the exploitation parameters (temperature, pressure, environmental aggressiveness) demands materials with improved mechanical properties and corrosion, erosion and heat resistance. This may be met by the advance of surface engineering, i.e. by producing surface layers with desired properties on less-resistant materials. Especially good prospects seem to be provided by composite surface layers which could maintain the properties of appropriately selected single layers, and hence could allow the extended application of coated metal. Good interphase cohesion of the layers, without deterioration of the substrate microstructure, has been achieved by covering of prenitrided steel with TiN deposited by plasmaassisted physical vapour deposition [ 1,2] and plasmaassisted chemical vapour deposition (PACVD) [ 31 methods: wear- and corrosion-resistant composite layers have been formed on various steels [l-3]. The aim of the present work is to check the effect of composite layers produced by nitriding under glow discharge conditions and then by the PACVD TiN deposition method on the wear and corrosion resistance of high alloyed tool steel.
The composite layers were produced on a substrate of high speed steel (HSS) (Polish SW 18 tool steel containing 0.8% C, 17.5% W, 4.3% Cr, 1.3% Ni and 0.8% MO) in a two-step process carried out in the same reactor. In step 1, steel was nitrided under the glow discharge conditions at 790 K in a gas mixture (H,:N,=3:1) at total gas pressure 300 Pa for 1 or 10 h: after that, in step 2, TiN layers were formed on the steel by the PACVD method in the reactor. The TiN deposition process lasted for 3 h at 840 K in a gas mixture with H,:N, = 6: 1 with the addition of 3 g 1-l TiCl, at reactant gas flow rates u of 0.06 x 10m3, 0.15 x 10e3 and 0.25 x 10m3 m s-l at total gas pressure 300 Pa. The layers were produced in a special laboratory apparatus for plasma treatment with a hot anode, described elsewhere [4]. The specimens were subjected to wear and corrosion resistance tests. Wear resistance was estimated in a “treerollers” friction test [ 51, and the linear wear (wear depth (pm)) at 50 and 400 MPa surface load was the measure of the wear resistance. The corrosion resistance was examined electrochemically by comparing the polarization curves recorded in 20% H,PO, (pH = 1). This
0257-8972/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0257.8972(94)02354-9
J. Michalski et al./Surface and Coatings Technology 72 (1995) 189-195
190
solution was chosen because it allowed us to establish the behaviour of specimens in the acid solution and to estimate the phase composition of the studied surface
the substrate. However, at low gas flow rates, the TiN layer grows faster on steel nitrided for 1 h than that for 10 h.
C61. The surface of specimens in the as-received condition and subjected to electrochemical tests was examined by electron diffraction spectroscopy (EDS). Microscopic examination by optical and scanning microscope as well as measurements of Vickers hardness have been carried out. The mean value from three, four or five hardness measurements has been taken into account; the hardness data scattering was 20-30 HVo,l.
3. Results 3.1. TiN layer deposition kinetics Fig. 1 shows the effect of the flow rate of reactant gases on the deposition rate of TiN on SW 18 tool steel prenitrided for 1 and 10 h. The deposition rate increases linearly with increase in the square-root of the flow rate. It can also be seen that the deposition kinetics do not depend on the layer thickness or on the treatment of 1s A
lhr IO
; 6.0L , __ C
0 u Ln 4.0-’
,
x % I
/
0.8 square
root
1
I
1.2
1.6
flow rate
Vi35
I
x102
Fig. 1. Effect of reactant gas flow rate on estimated deposition TiN on SW 18 tool steel prenitrided for different times.
rate of
3.2. Wear resistance Fig. 2(a) shows the hardness depth profile in a surface layer of SW tool steel nitrided for 1 and 10 h. Micrographs of specimen cross-sections (Figs. 2(b) and 2(c)) show that, in both cases, the layer consists of a diffusion zone only. Similarly to in the case of Refs. [ 1,7] no compact iron nitride layer was formed under the applied nitriding conditions. Nitriding for 10 h causes the increase in thickness and hardness of that zone in comparison with nitriding for 1 h (cf. Fig. 2(a)). Fig. 3 shows the linear wear vs. friction time for steel nitrided for 1 and 10 h as measured at different surface loads, and the appearance of the steel surface. Steel nitrided for 1 h exhibits, in the 50 MPa test, the higher wear resistance (lower linear wear values) and has a less developed surface than steel nitrided for 10 h. At a surface load of 400 MPa, the wear resistance is also higher in steel nitrided for 1 h; however, in both cases the 400 MPa test was broken off after about 40 min. Fig. 4 shows the hardness depth profile for specimens coated with TIN, deposited on prenitrided steel. The deposition of TiN affects the hardness of the substrate. In the case of TIN deposition on steel prenitrided for 1 h, the high surface hardness is accounted for by the presence of TIN. It can be seen, however, that the underneath substrate layer exhibits a hardness lower than that characteristic of the material subjected to the nitriding only. At a distance of about 30 mm from the surface the hardness of the surface-treated metal approaches the hardness of steel (see Fig. 4(a)). In the case of TiN deposited on steel nitrided for 10 h, a decrease in the hardness of the underneath nitrided layer occurs at a flow rate of 0.06 x 10m3 m s-l, whereas at a flow rate of 0.25 x 1O-3 m s-l some increase in hardness of the underneath nitrided layer could be observed (see Fig. 4(b)).
900.
20 40
dtstance Fig. 2. (a) Hardness
60 80 ID0 120 from surface (pm)
depth profile and (b), (c) microstructure
of tool steel nitrided
(b) for 1 h and (c) for 10h.
191
J. Michalski et al./Surface and Coatings Technology 72 ( 1995) 189-195
(a)
/
; 15 0 ,' Lz z 10 II d 5 3
50
20 40 60 80 testduration(mm )
Fig. 3. (a) Linear wear at surface loads of 50 and 400 MPa surface of steel nitrided (b) for 1 h and (c) for 10 h.
vs. friction
for different
times; (b), (c) appearance
of the
Fig. 6 shows the linear wear data for steel covered with a TIN layer deposited at a flow rate of 0.06 x 10e3 m s-i. As follows from the comparison of data presented in Figs. 3 and 6, the wear resistance of the composite layers is higher than that of the nitrided layer. The TiN layer deposited on steel nitrided for 1 h exhibits in the 50 MPa test a higher wear resistance and in the 400 MPa test a lower resistance than those exhibited by specimens prenitrided for 10h.
16Oll
1200 $
time for tool steel nitrided
800
3.3. Electrochemical tests
20 40 60 80 100120(i~m) distance from surface Fig. 4. Hardness depth profile for deposited at different gas flow rates and (b) for 10 h.
tool steel covered on steel prenitrided
with TiN (a) for 1 h
Fig. 5 presents the microstructure of composite layers. The white layer consisting of the coarse needles is seen beneath the TIN deposit in steel nitrided for 1 h (Figs. 5(a) and 5(b)). The TIN deposit itself exhibits the layered structure. In the case of TIN deposition on steel prenitrided for 10 h (Figs. 5(c) and 5(d)) the microstructure of the layer situated beneath the TiN one is similar to that observed for nitrided steel presented in Figs. 2(b) and 2(c). No separate layers could be distinguished within the TIN deposit.
Fig. 7 shows the anodic polarization curves recorded in H,PO, on tool steel, bare and nitrided for different times. The polarization curve for bare steel exhibits the active dissolution and passive regions; in the latter, two peaks marked E, and E, could be distinguished. The extension of the active dissolution region is associated with the chemical composition of the solid solution, whereas the peaks E, and E2 are due to the dissolution of metal (Me) carbides of the MeC and Me& (Fe,W,C) types respectively [6]. This is supported by the EDS data obtained in the present work. The intensity of Ka peaks for W and Cr taken from the electron diffraction spectra taken from the bare steel in the as-received condition and after polarization at constant potential within the active dissolution region and at potential E2 are shown in Fig. 8. Polarization in the active dissolution region causes some decrease in Cr peak intensity and increase in the W peak, in comparison with as-received steel, revealing the depletion in Cr and enrichment in W of the steel surface due to active
J. Michalski et al.ISurface and Coatings Technology 72 (1995) 189-195
192
I!
20
40
test duration
60
(mln
80
)
Fig. 6. Linear wear at surface loads of 50 and 400 MPa vs. friction time for steel covered with TiN deposited on a surface prenitrided for 1 and 10 h.
.-.-lhr ---1Ohrs
(4
Fig. 7. Anodic polarization curves recorded in HaPO, solution for bare steel and steel nitrided for 1 h and for 10 h. Characteristic parameters are marked: i,, anodic dissolution current density; E, and E,, potentials associated with the dissolution of carbides of MeC type and Me& type respectively.
cnts
1800 1400 1000 600
(4
(4
Fig. 5. Microstructure of composite layers formed by PACVD of TiN with flow rates of (a), (c) 0.06 x 10e3 m s-l and (b), (d) 0.25 x lOm3 m s-i on steel prenitrided (a), (b) for 1 h and (c), (d) for 10 h.
dissolution. After polarization at a potential close to E,, the change in W and Cr peak intensities indicates the depletion in W and slight enrichment in Cr of the treated surface. Nitriding of steel does not considerably change the appearance of the polarization curve but it decreases
200 bare active steel dlSOl
E2
Fig. 8. Intensity of W Kcc and Cr Kcc peaks seen in electron diffraction spectra taken for bare steel in the as-received condition and after polarization at constant potential within the active dissolution region and at potential E2 (shown in Fig. 7).
the active dissolution and promotes passlvation. Nitriding for 1 h does not change the position of the carbide peaks, whereas nitriding for 10 h causes the shift
J. Michalski et al./Surface and Coatings Technology 72 (1995) 189-19.5
of these peaks to more noble potentials, revealing the change in their chemistry. A series of polarization curves have been recorded for one specimen after subsequent removal of the surface layer; the change of position of the E2 (Fe,W,C) peak on these curves with the specimen depth is shown in Fig. 9. The shift of the peak and thus the change of the carbide chemistry is pronounced within a depth of 20 urn, but some change might be also observed at depths up to 60 urn. Typical polarization curves recorded
+L_ _A 0
20
40
60
80(pm)
distance from surfa&’ Fig. 9. Position of the potential E, (associated with the Me&-type carbide dissolution) on the polarization curve vs. distance from the surface.
193
for steel covered with TIN layers deposited at different gas flow rates after nitriding for 1 h and 10 h are shown in Fig. 10 and Fig. 11 respectively. The polarization curve for TiN on an A1,03 support is also shown in Fig. 10. Although the current density recorded in the case of composite layers is several orders of magnitude lower than that for nitrided steels, (cf. Fig. 7) the appearance of the polarization curves in almost all cases resembles that for bare or nitrided steel. The polarization curves for steel with composite layers exhibit active dissolution, not observed for TiN/A1203. Prenitriding and the TIN deposition parameters affect the electrochemical behaviour of the coated steel. Deposition of a TIN layer on steel prenitrided for 10 h causes a decrease in current density of l-2 orders of magnitude. In steel nitrided for 10 h the resistance increases with the increase in gas flow rate. For TiN deposited at a gas flow rate of 0.25 x lop3 m sir, the recorded polarization curve is similar to that for TiN/A1203 (Fig. 11(a)). The difference in TiN surface topography seen in Figs. 10 and 11, especially in nodule size and distribution, is affected not only by the different gas flow rates at TIN deposition, but also by the different times of prenitriding.
I
(b)
(4 Fig. 10. (a) Anodic polarization curves recorded in HsPO, for steel covered with TiN deposited by PACVD at different gas flow rates (marked values are the mean times lo3 m s-r) on steel prenitrided for 1 h and for TiN deposited on Al,O,. (b)-(d) Surface topography of TiN deposited at (b) 0.06 x 10e3 m s-r, (c) 1.5 x 10-a m s-r and (d) 0.25 x 10m3 m S-IS.
J. Michalskiet al./Surfaceand CoatingsTechnology72 (1995) 189-195
194
went density[m /\ f \ \
.I
2
QO6
\
I 0
I
I
I
I
I
I
0.8 1.2 p&ntial [VI
/
I
I
1.6
(4 Fig. 11. (a) Anodic polarization curves recorded in H,PO, solution for steel covered with TiN deposited at different gas flow rates (marked values are the mean times lo3 m s-l) on steel prenitrided for 10 h. (b)-(d) Surface topography of TiN deposited at (b) 0.06 x 10e3 m SC’, (c) 0.15 x 10m3m s-l and (d) 0.25 x 10e3 m s-l.
4. Discussion
4.2. TiN deposition
4.1. Nitriding
According to the results presented in Fig. 1, the TIN deposition reaction is pressumably controlled by the mass transport of the reactant gases. If the reacting gases are thoroughly mixed and their transport to the surface occurs by diffusion, the flux of gases J,, might be expressed as follows [9,10]:
The applied nitriding conditions allow the formation of a diffusion zone on steel; however, on long nitriding, some changes occur in the nitrided zone. After 10 h steel exhibits a thicker nitrided zone of higher hardness and more developed surface than that after 1 h of nitriding. Although no nitrides have been found in the microstructure after 10 h, the change in carbide chemistry has been revealed by the electrochemical test. The shift of dissolution potential E, in the more noble direction suggests the transformation of carbides to carbonitrides due to nitriding, since carbonitrides are known to be more chemically stable [ 81. The observed increase in hardness of steel nitrided for 10 h might be a result of this change of the precipitates chemistry. The higher wear resistance observed for steel nitrided for 1 h in comparison with that nitrided for 10 h might be accounted for by the less developed surface promoting better friction or by the lower hardness. Duration of nitriding does not affect the dissolution of the tool steel matrix in the studied electrolyte.
JD=D,---
Pb
-p,
(1)
6RT
where D, is the diffusivity of the reactant gases; P,_,and the partial pressures of the reactant gases in the stream bulk and at the substrate respectively; R is the gas constant; T is the temperature over the substrate; and 6 is the boundary layer thickness given by P, are
0.5
(>
6=iL 2
with L the dimension of the substrate, flow rate, and ,u and p the gas viscosity Since D,, T, Pb, P,, p and p remain experiment, the following expression for
(2) u the total gas and density. constant in the gas flux may be
J. Michalski
et al./Surface
and Coatings
derived from Eqs. ( 1) and (2): J D = KJJ0.5
(3)
If the deposition reaction is controlled by mass transport of reactant gases, the deposition rate of TiN is proportional to the mass flux & [9,10]. Therefore, according to Eq. (3) the deposition rate should be proportional to the square root of the gas flow rate, which has indeed been estimated (see Fig. 1). Prenitriding of the steel does not change the TIN deposition kinetics, but does affect the growth rate of TiN and the structure of both the TiN deposit and the substrate. The effect of prenitriding on TIN formation and structure may be traced from the difference in distribution and size of nodules, seen on the surface of TIN formed on steel prenitrided for 1 and 10 h (see Figs. 10(b) and 10(c) and 11(b)-11(d)). The obtained results show that the nitrogen from the substrate is involved in the formation of TiN. In the case of nitriding for 1 h no change in carbide chemistry has been observed and, during the subsequent TIN deposition, nitrogen from the solid solution sinks to the surface participating in the TIN layer formation. As a result, the nitrogen-depleted zone (white zone) of needle structure and low hardness is formed in the subsurface of steel beneath the TiN layer. The layered structure of the TIN deposit seen in this case (Figs. 5(a) and 5(b)) may also reflect the assistance of nitrogen from the substrate in the TIN formation. In the case of steel prenitrided for 10 h, the transformation of carbides takes place during nitriding. In this case, deposition of TIN also involves the nitrogen from the substrate, but the drainage does not cause the change in the microstructure. In the case of TIN deposition at low gas flow rates, the decrease in nitrided layer thickness without change in its microstructure is observed (see Fig. 5(c)). Participation of substrate constitutents in the surface layer formation should cause the improvement of its adhesion. The wear resistivity of coated metal depends on the properties and microstructure of the deposit and substrate as well as the interphase cohesion, since the substrate metal participates in the wear process. Similarly to in the case of corrosion behaviour of coated metal, the deposited layers and the substrate metal are involved. As observed in the electrochemical tests, the “transparency” of the TIN layers (Figs. 10 and 11) indicates that the main process in the corrosion of TiNcoated steel is the dissolution of support metal due to the permeation of environment solution and corrosion products through the coating. Therefore the effect of both the TIN layer porosity and electrochemical properties and the underneath metal electrochemical behaviour should be expected. The same layer-support composite could not necessarily provide good behaviour under wear and aggressive
Technology
72 (1995)
189-195
195
environment conditions, since different parameters affect either the corrosion or the wear process in different ways. However, the deposition and treatment parameters can be selected to maintain the optimum mechanical, adhesion and corrosion properties of the composite. Prenitriding for 1 h and TIN deposition promote the softening of the nitrided layer and thus the better wear resistance, at least as measured in the 50 MPa test (Fig. 6). However, the same composite layer exhibits lower resistance to the environment, reflected by the higher dissolution rate (Fig. 10). This might be explained by the effect of depletion of the prenitrided layer in nitrogen during the TIN deposition. As can be seen from Fig. 7, nitriding reduces the metal dissolution; it might be expected therefore that a decrease in the nitrogen in the metal surface due to TIN formation deteriorates the corrosion behaviour of the composite. Prenitriding could also affect the corrosion resistance of TiN-covered steel by the modification of the structure of the TIN deposit (Figs. 10 and 11) and thus by providing easy paths for permeation of solution and corrosion products through the TIN layer. 5. Conclusions (1) The TIN deposition reaction under the studied conditions is controlled by mass transport. (2) In the course of long-term nitriding under glow discharge conditions, the transformation of carbides to carbonitrides within the nitrided zone occurs. (3) Nitrogen from the steel surface is involved in the formation of the TiN layer, changing the structure and hardness of the nitrided layer and the structure of the TIN. (4) The wear and corrosion resistance of steel depends on the structure of the TIN layer and on the structure and properties of the support. The desired wear and corrosion of the composite layers can be achieved by the appropriate combination of nitriding and TIN deposition parameters.
References 111 Y. Sun and T. Bell, Mater. Sci. Eng.. A140 (1991) 419. VI M. Zlatanovic, T. Gredic, N. Popovic and Z. Bogdanov, Vacuum, 44 (1993) 83. J. Michalski, J. Rudnicki, B. Kulakowska and c31 T. Wierzchon, W. Zyrnicki, J. Mater. Sci., 27 (1992) 771. and T. Wierzchon, Polish Patent Application [41 J. Michalski W-36560, 1988. 1983. [51 Polish Standard PN-83/H-04302, C61 E.T. Shatalov, L.I. Baranova and G. Zekcer, Electrokhimicheskie metody u metallovedenii ifazouom analize, Metallurgia, Moscow, 1988 (in Russian). [71 T. Sone and K. Matsui, Mater. Sci. Eng., A140 (1991) 486. Termicheskaia obrabotka stali i chuguna, 181 I. Kontorovich, Metallurgizdat, Moscow, 1950 (in Russian). Crystal Growth Theory and Techniques, c91 C.H.L. Goodman, Vol. 1, Plenum, London, 1974. CIOI J.-O. Carlsson, Thin Solid Films. 230 (1985) 261.