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TiN coatings on mild steel substrates with electroless nickel as an interlayer
Surface
and Coatings
Technology,
48 (1991) 163-168
163
TIN coatings on mild steel substrates interlayer Yung-I Department
(Received
Chen
and Jenq-Gong
of Materials
Science
nickel as an
Duh
and Engineering,
March 18, 1991; accepted
with electroless
National
Tsing Hua
University,
Hsinchu
(Taiwan)
March 30, 1991)
Abstract Eiectroless nickel deposits are incorporated as an interlayer between TiN coatings and mild steel substrates. The electroless nickel layer recrystallizes during r.f. sputtering and a coating assembly of TiN/NiJFe is formed. The employment of electroless nickel results in an increase in the surface hardness as well as the adhesion strength. A Knoop surface hardness as high as 2120 HK, (close to the hardness of bulk TiN) can be achieved in the interlayer-modified TIN coating.
1. Introduction
TiN films, which possess high hardness, are widely applied as hard coatings on tool materials [l]. An interlayer, e.g. titanium, between the TiN coating and the underlying substrate appears to be beneficial to adhesion owing to chemical gettering and mechanical effects [2]. The titanium interlayer is expected to be hardened by the presence of oxygen, carbon and nitrogen in solid solution. In addition, the interlayer may act as a compliant region and relax the shear stress at the interface. TiN coatings with a titanium interlayer deposited on low carbon steel by reactive r.f. magnetron sputtering have been investigated in a previous study [3]. The preliminary titanium interlayer reacts with oxygen, carbon and nitrogen during sputtering as the substrate temperature is increased. The adhesion strength estimated by the pull-off test is observed to increase owing to the existence of the titanium interlayer. However, the effect of a soft layer, resulting in a decrease of the surface hardness under the coating, shows up as the load level is raised. Thus a harder interlayer which can support the integrity may be expected to eliminate the effect of the substrate and to enhance the surface hardness. Electroless nickel, which possesses good resistance to corrosion, wear and abrasion and provides uniformity in thickness of the deposit [4], has been selected as the interlayer in this work. The most widely used electroless-nickel-plating solution is a hypophosphitebased bath. For the codeposition of nickel and phosphorus in electroless deposition, the product is thus an alloy instead of pure nickel. The content of phos-
0257-8972/91/$3.50
phorus in the alloy has a prominent effect on the microstructure of the deposit [5, 61. Electroless nickel deposits with low phosphorus content are crystalline. However, if the phosphorus content is high, e.g. larger than 12.5 at.% [7], the as-deposited product is amorphous. After heat treatment, N&P, a b.c.t. phase, will precipitate. As precipitation hardening takes place during heat treatment, the Knoop hardness is increased from 550 to 950 HK [S]. During heat treatment, metastable transition phases such as N&P,, N&P,, Ni,,PS, N&P and N&P, may appear owing to compositional inhomogeneity in the as-deposited state, as revealed by transmission electron microscopy (TEM) observation. Nevertheless, the final stable phases are reported to be N&P and nickel after continued heating for long periods of time [5]. In this study, TiN films are prepared on mild steel with an electroless nickel interlayer by reactive r.f. magnetron sputtering. The heat treatment of the electroless nickel interlayer during sputtering is carried out to investigate the resultant phase transformation. Various Ar:N, flow rate ratios and sputtering times are employed during the sputtering and the effects of these variables on the mechanical properties of the deposited TiN films are investigated.
2. Experimental
procedure
The composition of the mild steel substrates analysed by atomic emission spectrometry was 0.019% Si, 0.21% Mn, 0.011% P, 0.011% S and balance iron. The carbon content was too low to be detected. After grinding with
0 1991 -
Elsevier
Sequoia,
Lausanne
Y-1.
164
Heating
Controller
Chen, J.-G.
Duh / TiN coatings on mild steel substrates
Thermocoupie
_
Substrate
/
/i
I
Controller
I
Turbomolecular
Fig. 1. Sputtering
Pump
set-up in this experiment.
I_
(a)
50 20 Fig. 2. X-ray diffraction patterns of (a) as-deposited electroless nickel and (b)-(d) treated electroless nickel with heating time (b) 2 h, (c) 3 h, (d) 4 h. (The predominant phase is N&P.) 35
40
45
#1200 Sic paper, the substrates were placed in the electroless nickel bath for 1 h. The chemical constituents of the electroless nickel solution employed in this study were 20 g 1-l nickel sulphate hexahydrate, 27 g 1-l sodium hypophosphite and 16 g I-’ sodium succinate hexahydrate. The thickness of the electroless nickel layer was determined with a scanning electron microscope (SEM, Camscan). The composition of the electroless nickel layer was analysed by electron probe Xray microanalysis (EPMA, Joel, JCXA-733). The Xray mappings of iron, nickel, phosphorus and titanium were also evaluated by EPMA to detect the distribution of elements. The sputtering set-up was as shown in Fig. 1. In the sputtering chamber the distance between target and
substrate was fixed at 8 cm. The diameter of the titanium target was 3.8 cm. The argon inlet was near the surface of the target, through a specially designed gas injection ring. This allowed the introduction of argon gas completely around the target area. Two tubular quartz lamps were used as radiant heaters. The substrate with an electroless nickel interlayer was placed in the vacuum chamber after an ultrasonic clean in isopropanol. The chamber was evacuated for 3 h, heated to 300 “C in 30 min and held for 30 min. It usually took 1 h to preheat the substrate in vacuum. The reactive sputtering was carried out at an r.f. power of 200 W and a total pressure of 10 mTorr. Different Ar:N, gas ratios were introduced and the substrate temperature was maintained at 300 “C. After reactive sputtering, the chamber was cooled to below 100 “C in 90 min at a pressure of 10 mTorr. The film structure was analysed with an X-ray diffractometer (Rigaku Dmax-II B) and a scanning transmission electron microscope (STEM, Joel 2000FX). The Knoop hardness of the as-deposited film surface was measured with a microhardness tester (Anton Paar K. G., MHT-4). Adhesion was measured in a pull tester (Sebastian Five Strength Tester, Quad Group) with the aid of epoxy-coated studs. The maximum strength that the epoxy could endure was 700 kgf cme2.
3. Results and discussion 3.1. Composition of electroless nickel deposit For the strengthening of electroless nickel by precipitation of N&P during heat treatment, a deposit with
Y.-l.
TABLE
1. Thickness,
Sample code
E 0 D Pb F C J $ Zd Mild steel
hardness
Chen, J.-G.
and strength
Duh I TiN coalings
16.5
on mild steel substrates
of TiN coatings
Ar:N,
Time (h)
Ni,P
TIN
5g
10 g
20 g
PUll strength (kgf cm-‘)
80:20 80:20 80:20 80:20 50:50 50:50 50:50 so:50 50:50 As-deposited
1 2 3 3 1 2 3 3 3 Ni-P
3.26 3.00 3.94 3.00 3.41 3.00 -
0.35 1.07 1.66 -
_ 2120+220 _
0.31 0.59 1.03 -
1037~60 1268 + 100 1673+176 1093+31 958 f 78 572 + 35 266+13
1351+99 1241+ 12.5 855 f 71 1072+87 1131&30 671+ 48 657+47 506 f 20 207+6
698f66 720 f 83 553 + 39 546+25 564*51 373 + 27 461+ 34 375 + 19 181+6
788” 811” 632’ 800” 813” 180 780” 713” -
Sputtering
conditions
Thickness (pm)
Surface hardness (HK)
“Failure occurred in epoxy side (700 kgf cm-‘). ‘Without electroless nickel interlayer. ‘Local failure. “Without TIN coating.
high phosphorus content is preferred in order to enhance the surface hardness. Since the thickness of electroless nickel layer is only 3-4 pm as observed by scanning electron microscopy (SEM), it is possible that the activation volume analysed by EPMA includes part of the substrate and thus the iron is detectable. In order to eliminate the substrate effect in the microanalysis, the composition of the electroless nickel layer measured by EPMA was evaluated indirectly as illustrated below. (1) First, the nickel and iron contents of the asdeposited samples were analysed with standard specimens by averaging eight analysis points. The composition was found to be 87.49 kO.48 wt.% Ni and 5.13 +0.08 wt.% Fe. (2) Secondly, the total content of nickel, iron and phosphorus was assumed to be 100 wt.% and the phosphorus content is thus 100% - %Ni - %Fe. (3) Finally, the contents of nickel and phosphorus only were normalized to 100 wt.% and the results were 92.2 wt.% Ni and 7.8 wt.% P (13.8 at.% P). It should be pointed out that the results for the nickel and phosphorus contents obtained in this way may deviate somewhat from those obtained by direct measurement on the basis of pure nickel and phosphorus-containing compound standards. However the error in microanalysis is less using pure nickel and iron standards in the indirect approach as compared to pure nickel and phosphorus-containing compound standards in the direct method. Further, the direct approach may intrinsically involve the substrate influence, which introduces additional iron X-map counts in the quantitative evaluation. Thus the indirect method is preferred in the determination of nickel and phosphorus contents. An identical procedure was used to analyse the com-
Fig. 3. Scanning electron micrographs of (a) electroless nickel deposit with uniform thickness on mild steel substrate and (b) TiN/Ni,P/Fe integrity.
position of the heat-treated sample with a heating time of 4 h and the result was 92.0 wt.% Ni and 8.0 wt.% P. With such a high phosphorus content the structure of the as-deposited electroless nickel analysed by its X-ray diffraction pattern was amorphous as shown in Fig. 2(a).
Y.-I. Chen, J.-G.
Fig. 4. Micrographs
4u
for sample
F: (a) SEM
image;
Duh / TiN coatings
(b) titanium
diffraction
map;
(c) iron
X-ray
map;
(d) nickel
X-ray
map.
44
42
Fig. 6. STEM structure.
28 Fig. 5. X-ray revealed.
X-ray
on mild steel substrates
pattern
of sample
0 with TiN(200)
diffraction
ring pattern
of sample
S with an f.c.c.
peak
3.2. Heat treatment on electroless nickel layer
Since the electroless-nickel-coated substrate is preheated for 1 h before reactive r.f. sputtering and the sputtering is carried out at a substrate temperature of
300 “C, the sputtering procedure is in fact a heat treatment process for the electroless nickel layer. As a consequence, the Ni-P alloy will be crystallized during sputtering. To investigate the effect of heat treatment, similar procedures without sputtering were performed
Y.-I. Chen, J.-G.
Fig. 7. (a) STEM micrograph 0.
Duh I TiN coatings
and (b) diffraction pattern of sample
on the electroless nickel deposit. It should be remembered that the heating time is 1 h longer than the period of sputtering. Figures 2(b)-2(d) show the X-ray diffraction patterns of electroless nickel deposited on mild steel for heating times of 2, 3 and 4 h respectively. The N&P phase predominates in the structure of the coating even with a heating time of only 2 h. The surface hardnesses of the electroless nickel deposited on substrates with or without heating are listed in Table 1. Three different loads, 5, 10 and 20 gf, were applied in the measurements. The hardness of the substrate itself without the interlayer coating is also indicated in Table 1. With the as-deposited electroless nickel layer the surface hardness ranges from 266 to 572 HK,. Furthermore, the surface hardness is increased to 958 HK, after heat treatment at 300 “C for 4 h. It is apparent that the existence of the electroless nickel layer increases the surface hardness of the substrate. 3.3. TiN coating on electroless-nickel-coated substrate As N&P precipitates during sputtering of TiN, a TiN/ N&P/Fe construction with uniform thickness is estab-
on mild steel substrates
167
lished as indicated in Fig. 3(a). The thickness of the TIN layers estimated by SEM observation is indicated in Table 1. Figure 3(b) shows a typical SEM image. As the content of reactive gas is decreased at constant total pressure, the deposition rate increases. Figure 4 shows the X-ray mappings of iron, nickel and titanium of sample F, which was sputtered under a 50:50 Ar:N, gas ratio for 1 h. It appears that there exists a uniform distribution of the three elements. In the X-ray diffraction patterns, TiN(lll) and TiN(200) peaks generally appeared. Figure 5 shows a typical X-ray diffraction pattern where a TiN(200) peak is visible. Similar procedures, except for interlayer deposition, were carried out to determine the effect of the interlayer. With a sputtering time of 3 h, samples S and P without interlayers were prepared with Ar:N, gas ratios of 50:50 and SO:20 respectively. Figure 6 shows the diffraction ring pattern of sample S, which exhibits the TiN phase with an f.c.c. structure. Sample P possesses the same diffraction pattern as sample S. The STEM micrograph of sample 0, a TiN-coated sample with an electroless nickel interlayer, is indicated in Fig. 7(a). The crystallite size of N&P is about 0.2 pm. The diffraction ring pattern of sample 0 is shown in Fig. 7(b). The hardness of sample S is 1093 HK,, while that of sample J with an electroless nickel interlayer is 1673 HK,. The elevation of surface hardness is attributed to the introduction of the heat-treated electroless nickel interlayer. For the sample with the thickest TiN layer, i.e. sample D with 1.66 pm TiN, the hardness is 2120 HK,, which is the highest hardness value obtained in this study. In the pull-off test the pull strength of all the samples with an electroless nickel interlayer is higher than 700 kgf cm-‘. In general, the adhesion of the electroless nickel coating to most metals is excellent and the bond strength of the coating to properly cleaned steel substrates has been found to be at least 2800-4200 kgf cm-* [9]. In this work the adhesion between the electroless nickel deposit and the mild steel substrate is larger than 700 kgf cm-‘, the upper limit of this test. Thus the electroless nickel interlayer can support the coating integrity without failure. However, the adhesion strength of samples without an interlayer is rather low and a scatter of measured data is usually found. Since the substrate surface is ground with # 1200 Sic paper only, the adhesion strength between TiN coating and substrate may vary and depend on the degree of roughness of the surface. For example, the failure values of sample S are 180, 385 and 407 kgf cm-‘. In the case of sample P, local failure results in a lower pulloff strength than the interface should have. Thus the adhesion between TiN coating and substrate is enhanced by the presence of the electroless nickel interlayer.
168
Y-I. Chen, J.-G. Duh 1 TiN coarings on mild steel subswares
4. Conclusions
References 1
For the characteristics of hardness, adhesion and uniformity of thickness, electroless nickel possesses the potential to be used in mutilayer coatings to strengthen the substrate. In this work an electroless nickel layer acting as an interlayer has been deposited on mild steel substrates prior to TiN r.f. sputter coating. The electroless nickel layer recrystallizes during r.f. sputtering and a coating structure of TiN/Ni,P/Fe is formed. The introduction of electroless nickel between TiN coating and mild steel substrate is successful both in promoting the surface hardness and strengthening the adhesion of the coating.
2 3 4 5 6
7 8 9
J.-E. Sundgren and H. T. G. Hentzell, J. Vat. Sci. Technol. A, 4 (5) (1986) 2259. D. S. Rickerby, S. J. Bull, T. Robertson and A. Hendry, Su$ Coat. Technoi., 41 (1990) 63. Y. I. Chen and J. G. Duh. Surf: Coal. Technol., 46 (1991) 371-384. L. F. Spencer, Mer. Finirh., (October 1974) 35; (November 1974) 50; (December 1974) 58; (January 1975) 38. R. C. Agarwala and S. Ray, Z.Metallk, 80 (1989) 556. R. J. Keyse and C. Hammond, Manx Sci. Techno/., 3 (1987) 963. M. Erming, L. Shoufu and L. Pengxing, Thin Solid Films, 166 (1988) 273. C. E. Johnson and F. Ogburn, Surf: TechnoL, 4 (1976) 161. W. D. Field, R. N. Duncan and J. R. Zickgraf, in Metals Handbook, Vol. 5, ASM, Metals Park, OH, 9th edn., 1983, p. 225.
and Coatings
Technology,
48 (1991) 163-168
163
TIN coatings on mild steel substrates interlayer Yung-I Department
(Received
Chen
and Jenq-Gong
of Materials
Science
nickel as an
Duh
and Engineering,
March 18, 1991; accepted
with electroless
National
Tsing Hua
University,
Hsinchu
(Taiwan)
March 30, 1991)
Abstract Eiectroless nickel deposits are incorporated as an interlayer between TiN coatings and mild steel substrates. The electroless nickel layer recrystallizes during r.f. sputtering and a coating assembly of TiN/NiJFe is formed. The employment of electroless nickel results in an increase in the surface hardness as well as the adhesion strength. A Knoop surface hardness as high as 2120 HK, (close to the hardness of bulk TiN) can be achieved in the interlayer-modified TIN coating.
1. Introduction
TiN films, which possess high hardness, are widely applied as hard coatings on tool materials [l]. An interlayer, e.g. titanium, between the TiN coating and the underlying substrate appears to be beneficial to adhesion owing to chemical gettering and mechanical effects [2]. The titanium interlayer is expected to be hardened by the presence of oxygen, carbon and nitrogen in solid solution. In addition, the interlayer may act as a compliant region and relax the shear stress at the interface. TiN coatings with a titanium interlayer deposited on low carbon steel by reactive r.f. magnetron sputtering have been investigated in a previous study [3]. The preliminary titanium interlayer reacts with oxygen, carbon and nitrogen during sputtering as the substrate temperature is increased. The adhesion strength estimated by the pull-off test is observed to increase owing to the existence of the titanium interlayer. However, the effect of a soft layer, resulting in a decrease of the surface hardness under the coating, shows up as the load level is raised. Thus a harder interlayer which can support the integrity may be expected to eliminate the effect of the substrate and to enhance the surface hardness. Electroless nickel, which possesses good resistance to corrosion, wear and abrasion and provides uniformity in thickness of the deposit [4], has been selected as the interlayer in this work. The most widely used electroless-nickel-plating solution is a hypophosphitebased bath. For the codeposition of nickel and phosphorus in electroless deposition, the product is thus an alloy instead of pure nickel. The content of phos-
0257-8972/91/$3.50
phorus in the alloy has a prominent effect on the microstructure of the deposit [5, 61. Electroless nickel deposits with low phosphorus content are crystalline. However, if the phosphorus content is high, e.g. larger than 12.5 at.% [7], the as-deposited product is amorphous. After heat treatment, N&P, a b.c.t. phase, will precipitate. As precipitation hardening takes place during heat treatment, the Knoop hardness is increased from 550 to 950 HK [S]. During heat treatment, metastable transition phases such as N&P,, N&P,, Ni,,PS, N&P and N&P, may appear owing to compositional inhomogeneity in the as-deposited state, as revealed by transmission electron microscopy (TEM) observation. Nevertheless, the final stable phases are reported to be N&P and nickel after continued heating for long periods of time [5]. In this study, TiN films are prepared on mild steel with an electroless nickel interlayer by reactive r.f. magnetron sputtering. The heat treatment of the electroless nickel interlayer during sputtering is carried out to investigate the resultant phase transformation. Various Ar:N, flow rate ratios and sputtering times are employed during the sputtering and the effects of these variables on the mechanical properties of the deposited TiN films are investigated.
2. Experimental
procedure
The composition of the mild steel substrates analysed by atomic emission spectrometry was 0.019% Si, 0.21% Mn, 0.011% P, 0.011% S and balance iron. The carbon content was too low to be detected. After grinding with
0 1991 -
Elsevier
Sequoia,
Lausanne
Y-1.
164
Heating
Controller
Chen, J.-G.
Duh / TiN coatings on mild steel substrates
Thermocoupie
_
Substrate
/
/i
I
Controller
I
Turbomolecular
Fig. 1. Sputtering
Pump
set-up in this experiment.
I_
(a)
50 20 Fig. 2. X-ray diffraction patterns of (a) as-deposited electroless nickel and (b)-(d) treated electroless nickel with heating time (b) 2 h, (c) 3 h, (d) 4 h. (The predominant phase is N&P.) 35
40
45
#1200 Sic paper, the substrates were placed in the electroless nickel bath for 1 h. The chemical constituents of the electroless nickel solution employed in this study were 20 g 1-l nickel sulphate hexahydrate, 27 g 1-l sodium hypophosphite and 16 g I-’ sodium succinate hexahydrate. The thickness of the electroless nickel layer was determined with a scanning electron microscope (SEM, Camscan). The composition of the electroless nickel layer was analysed by electron probe Xray microanalysis (EPMA, Joel, JCXA-733). The Xray mappings of iron, nickel, phosphorus and titanium were also evaluated by EPMA to detect the distribution of elements. The sputtering set-up was as shown in Fig. 1. In the sputtering chamber the distance between target and
substrate was fixed at 8 cm. The diameter of the titanium target was 3.8 cm. The argon inlet was near the surface of the target, through a specially designed gas injection ring. This allowed the introduction of argon gas completely around the target area. Two tubular quartz lamps were used as radiant heaters. The substrate with an electroless nickel interlayer was placed in the vacuum chamber after an ultrasonic clean in isopropanol. The chamber was evacuated for 3 h, heated to 300 “C in 30 min and held for 30 min. It usually took 1 h to preheat the substrate in vacuum. The reactive sputtering was carried out at an r.f. power of 200 W and a total pressure of 10 mTorr. Different Ar:N, gas ratios were introduced and the substrate temperature was maintained at 300 “C. After reactive sputtering, the chamber was cooled to below 100 “C in 90 min at a pressure of 10 mTorr. The film structure was analysed with an X-ray diffractometer (Rigaku Dmax-II B) and a scanning transmission electron microscope (STEM, Joel 2000FX). The Knoop hardness of the as-deposited film surface was measured with a microhardness tester (Anton Paar K. G., MHT-4). Adhesion was measured in a pull tester (Sebastian Five Strength Tester, Quad Group) with the aid of epoxy-coated studs. The maximum strength that the epoxy could endure was 700 kgf cme2.
3. Results and discussion 3.1. Composition of electroless nickel deposit For the strengthening of electroless nickel by precipitation of N&P during heat treatment, a deposit with
Y.-l.
TABLE
1. Thickness,
Sample code
E 0 D Pb F C J $ Zd Mild steel
hardness
Chen, J.-G.
and strength
Duh I TiN coalings
16.5
on mild steel substrates
of TiN coatings
Ar:N,
Time (h)
Ni,P
TIN
5g
10 g
20 g
PUll strength (kgf cm-‘)
80:20 80:20 80:20 80:20 50:50 50:50 50:50 so:50 50:50 As-deposited
1 2 3 3 1 2 3 3 3 Ni-P
3.26 3.00 3.94 3.00 3.41 3.00 -
0.35 1.07 1.66 -
_ 2120+220 _
0.31 0.59 1.03 -
1037~60 1268 + 100 1673+176 1093+31 958 f 78 572 + 35 266+13
1351+99 1241+ 12.5 855 f 71 1072+87 1131&30 671+ 48 657+47 506 f 20 207+6
698f66 720 f 83 553 + 39 546+25 564*51 373 + 27 461+ 34 375 + 19 181+6
788” 811” 632’ 800” 813” 180 780” 713” -
Sputtering
conditions
Thickness (pm)
Surface hardness (HK)
“Failure occurred in epoxy side (700 kgf cm-‘). ‘Without electroless nickel interlayer. ‘Local failure. “Without TIN coating.
high phosphorus content is preferred in order to enhance the surface hardness. Since the thickness of electroless nickel layer is only 3-4 pm as observed by scanning electron microscopy (SEM), it is possible that the activation volume analysed by EPMA includes part of the substrate and thus the iron is detectable. In order to eliminate the substrate effect in the microanalysis, the composition of the electroless nickel layer measured by EPMA was evaluated indirectly as illustrated below. (1) First, the nickel and iron contents of the asdeposited samples were analysed with standard specimens by averaging eight analysis points. The composition was found to be 87.49 kO.48 wt.% Ni and 5.13 +0.08 wt.% Fe. (2) Secondly, the total content of nickel, iron and phosphorus was assumed to be 100 wt.% and the phosphorus content is thus 100% - %Ni - %Fe. (3) Finally, the contents of nickel and phosphorus only were normalized to 100 wt.% and the results were 92.2 wt.% Ni and 7.8 wt.% P (13.8 at.% P). It should be pointed out that the results for the nickel and phosphorus contents obtained in this way may deviate somewhat from those obtained by direct measurement on the basis of pure nickel and phosphorus-containing compound standards. However the error in microanalysis is less using pure nickel and iron standards in the indirect approach as compared to pure nickel and phosphorus-containing compound standards in the direct method. Further, the direct approach may intrinsically involve the substrate influence, which introduces additional iron X-map counts in the quantitative evaluation. Thus the indirect method is preferred in the determination of nickel and phosphorus contents. An identical procedure was used to analyse the com-
Fig. 3. Scanning electron micrographs of (a) electroless nickel deposit with uniform thickness on mild steel substrate and (b) TiN/Ni,P/Fe integrity.
position of the heat-treated sample with a heating time of 4 h and the result was 92.0 wt.% Ni and 8.0 wt.% P. With such a high phosphorus content the structure of the as-deposited electroless nickel analysed by its X-ray diffraction pattern was amorphous as shown in Fig. 2(a).
Y.-I. Chen, J.-G.
Fig. 4. Micrographs
4u
for sample
F: (a) SEM
image;
Duh / TiN coatings
(b) titanium
diffraction
map;
(c) iron
X-ray
map;
(d) nickel
X-ray
map.
44
42
Fig. 6. STEM structure.
28 Fig. 5. X-ray revealed.
X-ray
on mild steel substrates
pattern
of sample
0 with TiN(200)
diffraction
ring pattern
of sample
S with an f.c.c.
peak
3.2. Heat treatment on electroless nickel layer
Since the electroless-nickel-coated substrate is preheated for 1 h before reactive r.f. sputtering and the sputtering is carried out at a substrate temperature of
300 “C, the sputtering procedure is in fact a heat treatment process for the electroless nickel layer. As a consequence, the Ni-P alloy will be crystallized during sputtering. To investigate the effect of heat treatment, similar procedures without sputtering were performed
Y.-I. Chen, J.-G.
Fig. 7. (a) STEM micrograph 0.
Duh I TiN coatings
and (b) diffraction pattern of sample
on the electroless nickel deposit. It should be remembered that the heating time is 1 h longer than the period of sputtering. Figures 2(b)-2(d) show the X-ray diffraction patterns of electroless nickel deposited on mild steel for heating times of 2, 3 and 4 h respectively. The N&P phase predominates in the structure of the coating even with a heating time of only 2 h. The surface hardnesses of the electroless nickel deposited on substrates with or without heating are listed in Table 1. Three different loads, 5, 10 and 20 gf, were applied in the measurements. The hardness of the substrate itself without the interlayer coating is also indicated in Table 1. With the as-deposited electroless nickel layer the surface hardness ranges from 266 to 572 HK,. Furthermore, the surface hardness is increased to 958 HK, after heat treatment at 300 “C for 4 h. It is apparent that the existence of the electroless nickel layer increases the surface hardness of the substrate. 3.3. TiN coating on electroless-nickel-coated substrate As N&P precipitates during sputtering of TiN, a TiN/ N&P/Fe construction with uniform thickness is estab-
on mild steel substrates
167
lished as indicated in Fig. 3(a). The thickness of the TIN layers estimated by SEM observation is indicated in Table 1. Figure 3(b) shows a typical SEM image. As the content of reactive gas is decreased at constant total pressure, the deposition rate increases. Figure 4 shows the X-ray mappings of iron, nickel and titanium of sample F, which was sputtered under a 50:50 Ar:N, gas ratio for 1 h. It appears that there exists a uniform distribution of the three elements. In the X-ray diffraction patterns, TiN(lll) and TiN(200) peaks generally appeared. Figure 5 shows a typical X-ray diffraction pattern where a TiN(200) peak is visible. Similar procedures, except for interlayer deposition, were carried out to determine the effect of the interlayer. With a sputtering time of 3 h, samples S and P without interlayers were prepared with Ar:N, gas ratios of 50:50 and SO:20 respectively. Figure 6 shows the diffraction ring pattern of sample S, which exhibits the TiN phase with an f.c.c. structure. Sample P possesses the same diffraction pattern as sample S. The STEM micrograph of sample 0, a TiN-coated sample with an electroless nickel interlayer, is indicated in Fig. 7(a). The crystallite size of N&P is about 0.2 pm. The diffraction ring pattern of sample 0 is shown in Fig. 7(b). The hardness of sample S is 1093 HK,, while that of sample J with an electroless nickel interlayer is 1673 HK,. The elevation of surface hardness is attributed to the introduction of the heat-treated electroless nickel interlayer. For the sample with the thickest TiN layer, i.e. sample D with 1.66 pm TiN, the hardness is 2120 HK,, which is the highest hardness value obtained in this study. In the pull-off test the pull strength of all the samples with an electroless nickel interlayer is higher than 700 kgf cm-‘. In general, the adhesion of the electroless nickel coating to most metals is excellent and the bond strength of the coating to properly cleaned steel substrates has been found to be at least 2800-4200 kgf cm-* [9]. In this work the adhesion between the electroless nickel deposit and the mild steel substrate is larger than 700 kgf cm-‘, the upper limit of this test. Thus the electroless nickel interlayer can support the coating integrity without failure. However, the adhesion strength of samples without an interlayer is rather low and a scatter of measured data is usually found. Since the substrate surface is ground with # 1200 Sic paper only, the adhesion strength between TiN coating and substrate may vary and depend on the degree of roughness of the surface. For example, the failure values of sample S are 180, 385 and 407 kgf cm-‘. In the case of sample P, local failure results in a lower pulloff strength than the interface should have. Thus the adhesion between TiN coating and substrate is enhanced by the presence of the electroless nickel interlayer.
168
Y-I. Chen, J.-G. Duh 1 TiN coarings on mild steel subswares
4. Conclusions
References 1
For the characteristics of hardness, adhesion and uniformity of thickness, electroless nickel possesses the potential to be used in mutilayer coatings to strengthen the substrate. In this work an electroless nickel layer acting as an interlayer has been deposited on mild steel substrates prior to TiN r.f. sputter coating. The electroless nickel layer recrystallizes during r.f. sputtering and a coating structure of TiN/Ni,P/Fe is formed. The introduction of electroless nickel between TiN coating and mild steel substrate is successful both in promoting the surface hardness and strengthening the adhesion of the coating.
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